Co-reporter:Jaime W. DuMont, Amy E. Marquardt, Austin M. Cano, and Steven M. George
ACS Applied Materials & Interfaces March 22, 2017 Volume 9(Issue 11) pp:10296-10296
Publication Date(Web):February 27, 2017
DOI:10.1021/acsami.7b01259
The thermal atomic layer etching (ALE) of SiO2 was performed using sequential reactions of trimethylaluminum (TMA) and hydrogen fluoride (HF) at 300 °C. Ex situ X-ray reflectivity (XRR) measurements revealed that the etch rate during SiO2 ALE was dependent on reactant pressure. SiO2 etch rates of 0.027, 0.15, 0.20, and 0.31 Å/cycle were observed at static reactant pressures of 0.1, 0.5, 1.0, and 4.0 Torr, respectively. Ex situ spectroscopic ellipsometry (SE) measurements were in agreement with these etch rates versus reactant pressure. In situ Fourier transform infrared (FTIR) spectroscopy investigations also observed SiO2 etching that was dependent on the static reactant pressures. The FTIR studies showed that the TMA and HF reactions displayed self-limiting behavior at the various reactant pressures. In addition, the FTIR spectra revealed that an Al2O3/aluminosilicate intermediate was present after the TMA exposures. The Al2O3/aluminosilicate intermediate is consistent with a “conversion-etch” mechanism where SiO2 is converted by TMA to Al2O3, aluminosilicates, and reduced silicon species following a family of reactions represented by 3SiO2 + 4Al(CH3)3 → 2Al2O3 + 3Si(CH3)4. Ex situ X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction of silicon species after TMA exposures. Following the conversion reactions, HF can fluorinate the Al2O3 and aluminosilicates to species such as AlF3 and SiOxFy. Subsequently, TMA can remove the AlF3 and SiOxFy species by ligand-exchange transmetalation reactions and then convert additional SiO2 to Al2O3. The pressure-dependent conversion reaction of SiO2 to Al2O3 and aluminosilicates by TMA is critical for thermal SiO2 ALE. The “conversion-etch” mechanism may also provide pathways for additional materials to be etched using thermal ALE.Keywords: Al2O3; aluminosilicate; atomic layer etching; hydrogen fluoride; SiO2; trimethylaluminum;
Co-reporter:Younghee Lee, Huaxing Sun, Matthias J. Young, and Steven M. George
Chemistry of Materials 2016 Volume 28(Issue 7) pp:2022
Publication Date(Web):March 1, 2016
DOI:10.1021/acs.chemmater.5b04360
The atomic layer deposition (ALD) of a variety of metal fluorides including ZrF4, MnF2, HfF4, MgF2, and ZnF2 was demonstrated using HF from a HF–pyridine solution. In situ quartz crystal microbalance (QCM) studies were utilized to examine the growth of these metal fluorides. ZrF4 ALD using tetrakis(ethylmethylamido) zirconium and HF as the reactants was studied as a model system. The QCM measurements determined a mass gain per cycle (MGPC) of 35.5 ng/(cm2 cycle) for ZrF4 ALD at 150 °C. This MGPC was equivalent to a growth rate of 0.9 Å/cycle at 150 °C. MnF2, HfF4, MgF2, ZnF2, and additional ZrF4 were also grown using bis(ethylcyclopentadienyl) manganese, tetrakis(dimethylamido) hafnium, bis(ethylcyclopentadienyl) magnesium, diethylzinc, and zirconium tetra-tert-butoxide as the metal precursors, respectively. The growth rates for MnF2, HfF4, MgF2, ZnF2, and ZrF4 ALD were 0.4, 0.8, 0.4, 0.7, and 0.6 Å/cycle, respectively, at 150 °C. All of these metal fluoride ALD systems displayed self-limiting reactions. Ex situ measurements of the growth rates using X-ray reflectivity and spectroscopic ellipsometry analysis agreed with the in situ QCM measurements. Analysis of the QCM mass changes after the individual metal precursor and HF exposures quantified HF adsorption after the HF reaction. The ZrF4 and HfF4 films acted as strong Lewis acids and adsorbed an average of two HF per deposited MFy species after the HF reaction. In contrast, the MnF2, MgF2, and ZnF2 films all behaved as weak Lewis acids and did not adsorb HF after the HF reaction. The refractive indices of the metal fluoride films were in agreement with previous optical measurements. Most of the metal fluoride films were crystalline as measured by X-ray diffraction studies. The majority of the metal fluoride films also had high purity as established by X-ray photoelectron spectroscopy studies. This pathway for metal fluoride ALD using HF–pyridine as the fluorine precursor will be useful for many applications of metal fluoride films such as optical coatings in the ultraviolet wavelength region.
Co-reporter:Younghee Lee, Jaime W. DuMont, and Steven M. George
Chemistry of Materials 2016 Volume 28(Issue 9) pp:2994
Publication Date(Web):April 11, 2016
DOI:10.1021/acs.chemmater.6b00111
Trimethylaluminum (TMA, Al(CH3)3) was used as the metal precursor, together with HF, for the atomic layer etching (ALE) of Al2O3 using sequential, self-limiting thermal reactions. Al2O3 ALE using TMA demonstrates that other metal precursors, in addition to Sn(acac)2, can be employed for Al2O3 ALE. The use of TMA for Al2O3 ALE is especially interesting because TMA can also be used for Al2O3 atomic layer deposition (ALD). Quartz crystal microbalance (QCM) experiments monitored Al2O3 ALE at temperatures from 250 to 325 °C. The Al2O3 ALE was linear versus the number of HF and TMA reaction cycles. The QCM studies showed that the sequential HF and TMA reactions were self-limiting versus reactant exposure. The Al2O3 etching rates increased at higher temperatures. The QCM analysis measured mass change per cycle (MCPC) values that varied from −4.2 ng/(cm2 cycle) at 250 °C to −23.3 ng/(cm2 cycle) at 325 °C. These MCPCs correspond to Al2O3 etch rates from 0.14 Å/cycle at 250 °C to 0.75 Å/cycle at 325 °C. X-ray reflectivity and spectroscopic ellipsometry analyses confirmed the linear removal of Al2O3 and etching rates. Fourier transform infrared spectroscopy measurements monitored Al2O3 ALE by observing the loss of infrared absorbance from Al–O stretching vibrations. Surface intermediates were also identified after the HF and TMA exposures. Al2O3 ALE with TMA is believed to occur by the reaction Al2O3 + 4Al(CH3)3 + 6HF → 6AlF(CH3)2 + 3H2O. The proposed mechanism involves fluorination and ligand-exchange reactions. The HF exposure fluorinates the Al2O3 and forms an AlF3 surface layer and H2O as a volatile reaction product. During the ligand-exchange transmetalation reaction, TMA accepts F from the AlF3 surface layer and donates CH3 to produce volatile AlF(CH3)2 reaction products. The QCM measurements were consistent with an AlF3 surface layer thickness of 3.0 Å on Al2O3 after the HF exposures. The larger etch rates at higher temperatures were attributed to the removal of a larger fraction of the AlF3 surface layer by TMA exposures at higher temperatures.
Co-reporter:Younghee Lee, Craig Huffman, and Steven M. George
Chemistry of Materials 2016 Volume 28(Issue 21) pp:7657
Publication Date(Web):October 4, 2016
DOI:10.1021/acs.chemmater.6b02543
Atomic layer etching (ALE) can result from sequential, self-limiting thermal reactions. The reactions during thermal ALE are defined by fluorination followed by ligand exchange using metal precursors. The metal precursors introduce various ligands that may transfer during ligand exchange. If the transferred ligands produce stable and volatile metal products, then the metal products may leave the surface and produce etching. In this work, selectivity in thermal ALE was examined by exploring tin(II) acetylacetonate (Sn(acac)2), trimethylaluminum (TMA), dimethylaluminum chloride (DMAC), and SiCl4 as the metal precursors. These metal precursors provide acac, methyl, and chloride ligands for ligand exchange. HF-pyridine was employed as the fluorination reagent. Spectroscopic ellipsometry was used to measure the etch rates of Al2O3, HfO2, ZrO2, SiO2, Si3N4, and TiN thin films on silicon wafers. The spectroscopic ellipsometry measurements revealed that HfO2 was etched by all of the metal precursors. Al2O3 was etched by all of the metal precursors except SiCl4. ZrO2 was etched by all of the metal precursors except TMA. In contrast, SiO2, Si3N4, and TiN were not etched by any of the metal precursors. These results can be explained by the stability and volatility of the possible reaction products. Temperature can also be used to obtain selective thermal ALE. The temperature dependence of ZrO2, HfO2, and Al2O3 ALE was examined using SiCl4 as the metal precursor. Higher temperatures can discriminate between the etching of ZrO2, HfO2, and Al2O3. The temperature dependence of Al2O3 ALE was also examined using Sn(acac)2, TMA, and DMAC as the metal precursors. Sn(acac)2 etched Al2O3 at temperatures ≥150 °C. DMAC etched Al2O3 at higher temperatures ≥225 °C. TMA etched Al2O3 at even higher temperatures ≥250 °C. The combination of different metal precursors with various ligands and different temperatures can provide multiple pathways for selective thermal ALE.
Co-reporter:Steven M. George and Younghee Lee
ACS Nano 2016 Volume 10(Issue 5) pp:4889
Publication Date(Web):May 24, 2016
DOI:10.1021/acsnano.6b02991
Thermal atomic layer etching (ALE) of Al2O3 and HfO2 using sequential, self-limiting fluorination and ligand-exchange reactions was recently demonstrated using HF and tin acetylacetonate (Sn(acac)2) as the reactants. This new thermal pathway for ALE represents the reverse of atomic layer deposition (ALD) and should lead to isotropic etching. Atomic layer deposition and ALE can together define the atomic layer growth and removal steps required for advanced semiconductor fabrication. The thermal ALE of many materials should be possible using fluorination and ligand-exchange reactions. The chemical details of ligand-exchange can lead to selective ALE between various materials. Thermal ALE could produce conformal etching in high-aspect-ratio structures. Thermal ALE could also yield ultrasmooth thin films based on deposit/etch-back methods. Enhancement of ALE rates and possible anisotropic ALE could be achieved using radicals or ions together with thermal ALE.
Co-reporter:Younghee Lee, Jaime W. DuMont, and Steven M. George
Chemistry of Materials 2015 Volume 27(Issue 10) pp:3648
Publication Date(Web):April 24, 2015
DOI:10.1021/acs.chemmater.5b00300
Thermal Al2O3 atomic layer etching (ALE) can be performed using sequential, self-limiting reactions with tin(II) acetylacetonate (Sn(acac)2) and HF as the reactants. To understand the reaction mechanism, in situ quartz crystal microbalance (QCM) and Fourier transform infrared (FTIR) measurements were conducted versus temperature. The mass change per cycle (MCPC) increased with temperature from −4.1 ng/(cm2 cycle) at 150 °C to −18.3 ng/(cm2 cycle) at 250 °C. Arrhenius analysis of the temperature-dependent MCPC values yielded an activation barrier for Al2O3 ALE of E = 6.6 ± 0.4 kcal/mol. The mass changes after the individual Sn(acac)2 and HF exposures also varied with temperature. The mass changes after the Sn(acac)2 exposures were consistent with more Sn(acac)2 surface reaction products remaining at lower temperatures. The mass changes after the HF exposures were consistent with more AlF3 species remaining at higher temperatures. The FTIR spectroscopic analysis observed Al2O3 etching by measuring the loss of absorbance of Al–O stretching vibrations in the Al2O3 film. The infrared absorbance of the acetylacetonate vibrational features from Sn(acac)2 surface reaction products was also smaller at higher temperatures. The correlation between the MCPC values and the acetylacetonate infrared absorbance suggested that the Al2O3 ALE rate is inversely dependent on the acetylacetonate surface coverage. In addition, the QCM and FTIR measurements explored the nucleation of the Al2O3 ALE. A large mass gain and loss of infrared absorbance of Al–O stretching vibrations after the initial HF exposure on the Al2O3 film was consistent with the conversion of Al2O3 to AlF3. FTIR experiments also observed the formation of AlF3 after the initial HF exposure and the presence of AlF3 on the surface after each HF exposure during Al2O3 ALE. In the proposed reaction mechanism, AlF3 is the key reaction intermediate during Al2O3 ALE. HF converts Al2O3 to AlF3 prior to removal of AlF3 by Sn(acac)2.
Co-reporter:Matthias J. Young, Charles B. Musgrave, and Steven M. George
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 22) pp:12030
Publication Date(Web):May 12, 2015
DOI:10.1021/acsami.5b02167
The growth of Al2O3 films by atomic layer deposition (ALD) on model sp2-graphitic carbon substrates was evaluated following a nitrogen dioxide (NO2) and trimethylaluminum (TMA) pretreatment to deposit an Al2O3 adhesion layer. Al2O3 ALD using TMA and water (H2O) as the reactants was used to grow Al2O3 films on exfoliated highly ordered pyrolitic graphite (HOPG) at 150 °C with and without the pretreatment procedure consisting of five NO2/TMA cycles. The Al2O3 films on HOPG substrates were evaluated using spectroscopic ellipsometry and electrochemical analysis to determine film thickness and quality. These experiments revealed that five NO2/TMA cycles at 150 °C deposited an Al2O3 adhesion layer with a thickness of 5.7 ± 3.6 Å on the HOPG substrate. A larger number of NO2/TMA cycles at 150 °C deposited thicker Al2O3 films until reaching a limiting thickness of ∼80 Å. Electrochemical impedance spectroscopy (EIS) measurements revealed that five cycles of NO2/TMA pretreatment enabled the growth of high quality insulating Al2O3 films with high charge-transfer resistance after only 20 TMA/H2O Al2O3 ALD cycles. In contrast, with no NO2/TMA pretreatment, EIS measurements indicated that 100 TMA/H2O Al2O3 ALD cycles were necessary to produce an insulating Al2O3 film with high charge-transfer resistance. Al2O3 films grown after the NO2/TMA pretreatment at 150 °C were also demonstrated to have better resistance to dissolution in an aqueous environment.Keywords: charge-transfer resistance; dissolution; electrochemistry; nucleation; porosity; surface chemistry; thin film;
Co-reporter:Younghee Lee and Steven M. George
ACS Nano 2015 Volume 9(Issue 2) pp:2061
Publication Date(Web):January 20, 2015
DOI:10.1021/nn507277f
The atomic layer etching (ALE) of Al2O3 was demonstrated using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac)2) and hydrogen fluoride (HF) as the reactants. The Al2O3 samples were Al2O3 atomic layer deposition (ALD) films grown using trimethylaluminum and H2O. The HF source was HF-pyridine. Al2O3 was etched linearly with atomic level precision versus number of reactant cycles. The Al2O3 ALE was monitored at temperatures from 150 to 250 °C. Quartz crystal microbalance (QCM) studies revealed that the sequential Sn(acac)2 and HF reactions were self-limiting versus reactant exposure. QCM measurements also determined that the mass change per cycle (MCPC) increased with temperature from −4.1 ng/(cm2 cycle) at 150 °C to −18.3 ng/(cm2 cycle) at 250 °C. These MCPC values correspond to etch rates from 0.14 Å/cycle at 150 °C to 0.61 Å/cycle at 250 °C based on the Al2O3 ALD film density of 3.0 g/cm3. X-ray reflectivity (XRR) analysis confirmed the linear removal of Al2O3 and measured an Al2O3 ALE etch rate of 0.27 Å/cycle at 200 °C. The XRR measurements also indicated that the Al2O3 films were smoothed by Al2O3 ALE. The overall etching reaction is believed to follow the reaction Al2O3 + 6Sn(acac)2 + 6HF → 2Al(acac)3 + 6SnF(acac) + 3H2O. In the proposed reaction mechanism, the Sn(acac)2 reactant donates acac to the substrate to produce Al(acac)3. The HF reactant allows SnF(acac) and H2O to leave as reaction products. The thermal ALE of many other metal oxides using Sn(acac)2 or other metal β-diketonates, together with HF, should be possible by a similar mechanism. This thermal ALE mechanism may also be applicable to other materials such as metal nitrides, metal phosphides, metal sulfides and metal arsenides.Keywords: Al2O3; atomic layer deposition; atomic layer etching; quartz crystal microbalance; sequential exposures; thermal reactions; X-ray reflectivity;
Co-reporter:Younghee Lee
The Journal of Physical Chemistry C 2015 Volume 119(Issue 25) pp:14185-14194
Publication Date(Web):May 27, 2015
DOI:10.1021/acs.jpcc.5b02625
The atomic layer deposition (ALD) of AlF3 was demonstrated using trimethylaluminum (TMA) and hydrogen fluoride (HF). The HF source was HF-pyridine. In situ quartz crystal microbalance (QCM), quadrupole mass spectrometer (QMS), and Fourier transform infrared (FTIR) spectroscopy measurements were used to study AlF3 ALD. The AlF3 ALD film growth was examined at temperatures from 75 to 300 °C. Both the TMA and HF reactions displayed self-limiting behavior. The maximum mass gain per cycle (MGPC) of 44 ng/(cm2 cycle) for AlF3 ALD occurred at 100 °C. The MGPC values decreased at higher temperatures. The MGPC values were negative at T > 250 °C when TMA and HF were able to etch the AlF3 films. Film thicknesses were also determined using ex situ X-ray reflectivity (XRR) and spectroscopic ellipsometry (SE) measurements. The AlF3 ALD growth rate determined by the ex situ analysis was 1.43 Å/cycle at 100 °C. These ex situ measurements were in excellent agreement with the in situ QCM measurements. FTIR analysis monitored the growth of infrared absorbance from Al–F stretching vibrations at 500–900 cm–1 during AlF3 ALD. In addition, absorption peaks were observed that were consistent with AlF(CH3)2 and HF species on the surface after the TMA and HF exposures, respectively. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectrometry (RBS) measurements revealed that the deposited films were nearly stoichiometric AlF3 with an oxygen impurity of only ∼2 at %. AlF3 ALD may be useful for a number of applications such as ultraviolet optical films, protective coatings for the electrodes of Li ion batteries, and Lewis acid catalytic films.
Co-reporter:Jaime W. DuMont
The Journal of Physical Chemistry C 2015 Volume 119(Issue 26) pp:14603-14612
Publication Date(Web):February 20, 2015
DOI:10.1021/jp512074n
The pyrolysis of alucone molecular layer deposition (MLD) films was studied in vacuum using in situ transmission Fourier transform infrared spectroscopy. The initial alucone MLD films were grown using trimethylaluminum (TMA) and either ethylene glycol (EG) (HO–(CH2)2–OH) or hydroquinone (HQ) (HO–C6H4–OH) at 150 °C. The alucone MLD films were then pyrolyzed in vacuum at temperatures ranging from 400 to 750 °C. The absorbance features for the C–H, C–C, and C–O stretching vibrations were observed to be lost at pyrolysis temperatures from 350 to 500 °C. For the alucone films grown using TMA and EG, the loss of these absorbance features was coupled to an increase in carboxylate (R-COO–) absorbance features. The carboxylate absorbance features reached their peak at a pyrolysis temperature of 450 °C and then decreased slowly with higher pyrolysis temperatures. The carboxylate absorbance features are consistent with an Al2O3/carbon composite material with Al3+/COO– species at the interface. In addition, the presence of carbon in the Al2O3/carbon composite led to an increase in the background infrared absorbance for the pyrolyzed alucone films grown using HQ containing six carbons. This background infrared absorbance is linked to electrical conductance in a network of carbon domains in the pyrolyzed alucone films, as described by Drude–Zener theory. In contrast, the alucone films grown using EG containing two carbons did not display an increase in the background infrared absorbance. This absence of background infrared absorbance is consistent with less carbon in the Al2O3/carbon composite grown using EG. The pyrolysis of the alucone films on ZrO2 particles led to very conformal Al2O3/carbon composite films, as observed by transmission electron microscopy images.
Co-reporter:Byoung H. Lee, Virginia R. Anderson, and Steven M. George
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 19) pp:16880
Publication Date(Web):September 9, 2014
DOI:10.1021/am504341r
Molecular layer deposition (MLD) of the hafnium alkoxide polymer known as “hafnicone” was grown using sequential exposures of tetrakis(dimethylamido) hafnium (TDMAH) and ethylene glycol (EG) as the reactants. In situ quartz crystal microbalance (QCM) experiments demonstrated self-limiting reactions and linear growth versus the number of TDMAH/EG reaction cycles. Ex situ X-ray reflectivity (XRR) analysis confirmed linear growth and measured the density of the hafnicone films. The hafnicone growth rates were temperature-dependent and decreased from 1.2 Å per cycle at 105 °C to 0.4 Å per cycle at 205 °C. The measured density was ∼3.0 g/cm3 for the hafnicone films at all temperatures. Transmission electron microscopy images revealed very uniform and conformal hafnicone films. The XRR studies also showed that the hafnicone films were very stable with time. Nanoindentation measurements determined that the elastic modulus and hardness of the hafnicone films were 47 ± 2 and 2.6 ± 0.2 GPa, respectively. HfO2/hafnicone nanolaminate films also were fabricated using HfO2 atomic layer deposition (ALD) and hafnicone MLD at 145 °C. The in situ QCM measurements revealed that HfO2 ALD nucleation on the hafnicone MLD surface required at least 18 TDMAH/H2O cycles. Hafnicone alloys were also fabricated by combining HfO2 ALD and hafnicone MLD at 145 °C. The composition of the hafnicone alloy was varied by adjusting the relative number of TDMAH/H2O ALD cycles and TDMAH/EG MLD cycles in the reaction sequence. The electron density changed continuously from 8.2 × 1023 e–/cm3 for pure hafnicone MLD films to 2.4 × 1024 e–/cm3 for pure HfO2 ALD films. These hafnicone films and the HfO2/hafnicone nanolaminates and alloys may be useful for flexible thin-film devices.Keywords: alloys; atomic layer deposition; hafnicone; HfO2; molecular layer deposition; nanolaminates
Co-reporter:Virginia R. Anderson ; Noemi Leick ; Joel W. Clancey ; Katherine E. Hurst ; Kim M. Jones ; Anne C. Dillon
The Journal of Physical Chemistry C 2014 Volume 118(Issue 17) pp:8960-8970
Publication Date(Web):April 3, 2014
DOI:10.1021/jp412539y
Pt nanoparticles were grown on titanium oxide and tungsten oxide at 200 °C by Pt atomic layer deposition (ALD) using platinum(II) hexafluoroacetylacetonate [Pt(hfac)2] and formalin as the reactants. The Pt ALD surface chemistry and Pt nanoparticles were examined using in situ Fourier transform infrared (FTIR) vibrational spectroscopy and ex situ transmission electron microscopy (TEM). The FTIR spectra identified the surface species after the Pt(hfac)2 and formalin exposures on TiO2. An infrared feature at ∼2100 cm–1 in the FTIR spectrum after Pt(hfac)2 and formalin exposures on TiO2 was consistent with CO on Pt, revealing that Pt(hfac)2 and formalin exposures led to the formation of Pt nanoparticles. The FTIR spectrum of Pt(hfac)2 on TiO2 was very similar to the FTIR spectrum of hexafluoroacetylacetone (hfacH) on TiO2. The FTIR spectra also revealed that hfacH blocked the adsorption of Pt(hfac)2 on TiO2. The coverage of the Pt nanoparticles could be reduced by preadsorbing hfacH on TiO2 prior to Pt(hfac)2 adsorption. Time-dependent FTIR spectra showed that the coverage of hfacH and its adsorption products were reduced versus time following hfacH exposure. Pt ALD on WOx at 200 °C led to the growth of Pt nanoparticles that were fairly similar to the Pt nanoparticles from Pt ALD on TiO2. The TEM images revealed that the size of the Pt nanoparticles on WOx could be adjusted by varying the number of Pt ALD cycles. Because of site-blocking by the hfac ligands, the Pt(hfac)2 and formalin reactants required many more ALD cycles for nucleation and growth compared with other Pt ALD surface chemistries.
Co-reporter:Daniel J. Higgs ; Matthias J. Young ; Jacob A. Bertrand
The Journal of Physical Chemistry C 2014 Volume 118(Issue 50) pp:29322-29332
Publication Date(Web):August 13, 2014
DOI:10.1021/jp505508c
The oxidation of calcium films by water vapor is the basis of the “Ca test” used to measure low water vapor transmission rates (WVTRs) through gas diffusion barriers. The Ca test assumes that the Ca film oxidation rate is linear with H2O flux transmitted through the barrier. However, lag times are often observed during WVTR measurements that could indicate that the Ca film oxidation rate is not linear with H2O flux. To explore the oxidation kinetics of Ca films by H2O vapor, a new Ca test was developed based on quartz crystal microbalance (QCM) measurements. The QCM measures the mass gain that occurs as the Ca film oxidizes according to the reaction: Ca + 2H2O → Ca(OH)2 + H2. The QCM measurements observed a long lag time before the Ca films began to oxidize. The Ca film oxidation was studied using a number of experimental configurations. In all cases, the Ca film oxidation was not linear with H2O flux to the Ca film. These results suggest that the nonlinearity of Ca film oxidation in many WVTR experiments may result from the nonlinear oxidation of the Ca film itself. Additional measurements of the times required for complete Ca film oxidation versus H2O flux at constant temperatures were consistent with oxidation kinetics that were second-order in H2O flux. The nonlinear Ca film oxidation raises doubts about the validity of WVTR measurements using the Ca test. However, modeling based on the measured nonlinear Ca film oxidation kinetics suggests that the Ca test can yield reliable WVTR measurements under special circumstances. These circumstances occur when the accumulation volume between the barrier and the Ca film is small enough to allow for steady state pressure conditions where the H2O flux transmitted through the barrier is equivalent to the H2O flux removed by oxidation of the Ca film. The lag time is associated with the time required to obtain the steady state pressure in the accumulation volume.
Co-reporter:Byoung H. Lee;Byunghoon Yoon;Aziz I. Abdulagatov;Robert A. Hall
Advanced Functional Materials 2013 Volume 23( Issue 5) pp:532-546
Publication Date(Web):
DOI:10.1002/adfm.201200370
Abstract
Molecular layer deposition (MLD) is a useful technique for fabricating hybrid organic-inorganic thin films. MLD allows for the growth of ultrathin and conformal films using sequential, self-limiting reactions. This article focuses on the MLD of hybrid organic-inorganic films grown using metal precursors and various organic alcohols that yield metal alkoxide films. This family of metal alkoxides can be described as “metalcones”. Many metalcones are possible, such as the “alucones” and “zincones” based on the reaction of trimethylaluminum and diethylzinc, respectively, with various organic diols such as ethylene glycol. Alloys of the various metalcones with their parent metal oxide atomic layer deposition (ALD) films can also be fabricated that have an organic-inorganic composition that can be adjusted by controlling the relative number of ALD and MLD cycles. These metalcone alloys have tunable chemical, optical, mechanical, and electrical properties that may be useful for designing various functional films. The metalcone hybrid organic-inorganic materials offer a new tool set for engineering thin film properties.
Co-reporter:Shih-Hui Jen and Steven M. George, Robert S. McLean and Peter F. Carcia
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 3) pp:1165
Publication Date(Web):December 28, 2012
DOI:10.1021/am303077x
Alucone films were employed as interlayers to minimize stress caused by thermal expansion mismatch between Al2O3 films grown by atomic layer deposition (ALD) and Teflon fluorinated ethylene propylene (FEP) substrates. The alucone films were grown by molecular layer deposition (MLD) using trimethylaluminum (TMA), ethylene glycol (EG), and H2O. Without the alucone interlayer, the Al2O3 films were susceptible to cracking resulting from the high coefficient of thermal expansion (CTE) mismatch between the Al2O3 film and the Teflon FEP substrate. Cracking was observed by field emission scanning electron microscopy (FE-SEM) images of Al2O3 films grown directly on Teflon FEP substrates at temperatures from 100 to 160 °C and then cooled to room temperature. With an alucone interlayer, the Al2O3 film had a crack density that was reduced progressively versus alucone interlayer thickness. For Al2O3 film thicknesses of 48 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >60 nm on 50 μm thick Teflon FEP substrates. For thinner Al2O3 film thicknesses of 21 nm deposited at 135 °C, no cracks were observed for alucone interlayer thicknesses >40 nm on 50 μm thick Teflon FEP substrates. Slightly higher alucone interlayer thicknesses were required to prevent cracking on thicker Teflon FEP substrates with a thickness of 125 μm. The alucone interlayer linearly reduced the compressive stress on the Al2O3 film caused by the thermal expansion mismatch between the Al2O3 coating and the Teflon FEP substrate. The average compressive stress reduction per thickness of the alucone interlayer was determined to be 8.5 ± 2.3 MPa/nm. Comparison of critical tensile strains for alucone films on Teflon FEP and HSPEN substrates revealed that residual compressive stress in the alucone film on Teflon FEP could help offset applied tensile stress and lead to the attainment of much higher critical tensile strains.Keywords: Al2O3; alucone; atomic layer deposition; molecular layer deposition; Teflon; thermal stress;
Co-reporter:Byoung H. Lee;Virginia R. Anderson
Chemical Vapor Deposition 2013 Volume 19( Issue 4-6) pp:204-212
Publication Date(Web):
DOI:10.1002/cvde.201207045
Abstract
Hybrid organic/inorganic polymer films based on zirconium are grown using molecular layer deposition (MLD) techniques. The zirconium alkoxide films, known as “zircones”, are grown using sequential exposures of zirconium tert-butoxide (ZTB) and ethylene glycol (EG) as the reactants at temperatures from 105 to 195°C. In-situ quartz crystal microbalance (QCM) and ex-situ X-ray reflectivity (XRR) experiments confirm linear growth versus the number of reaction cycles. The growth rates decrease versus temperature from 1.6 Å per cycle at 105°C to 0.3 Å per cycle at 195°C. The measured density is ∼2.17 g cm−3 for all the growth temperatures. Transmission electron microscopy (TEM) images reveal very uniform and conformal zircone films. ZrO2/zircone alloys are also fabricated by combining ZrO2 atomic layer deposition (ALD) and zircone MLD at 145°C. The composition of the ZrO2/zircone alloy is varied by adjusting the relative number of ZrO2 ALD and zircone MLD cycles in the reaction sequence. The ZrO2/zircone alloys display varying density, refractive index, elastic modulus, and hardness. The refractive index and elastic modulus change progressively from n = 1.63 and E= 27 ± 0.6 GPa for pure zircone MLD films, to n = 1.86 and E= 97 ± 5 GPa for pure ZrO2 ALD films, respectively. In capacitor structures, the zircone films display low leakage currents and a dielectric constant of ∼6.7. The zircone films are also utilized as the dielectric layer in pentacene-based thin film transistors (TFTs), which display a high field effect mobility of 2.11 cm2 V−1 s−1 operating at −3 V with an on/off current ratio of ∼103. The zircone and ZrO2/zircone alloy films provide a new class of hybrid organic/inorganic polymer films for many functional film applications.
Co-reporter:J. A. Bertrand, D. J. Higgs, M. J. Young, and S. M. George
The Journal of Physical Chemistry A 2013 Volume 117(Issue 46) pp:12026-12034
Publication Date(Web):July 30, 2013
DOI:10.1021/jp4043057
The electrical Ca test was used to measure H2O vapor transmission through polyethylene naphthalate (PEN) polymer with a thickness of 200 μm. On the basis of the time required for the normalized conductance of the Ca film to reach zero, the H2O vapor transmission rate was determined versus H2O flux, temperature, and saturation of the PEN polymer with H2O. The H2O vapor transmission rate was proportional to the H2O flux and only weakly dependent on temperature at constant H2O flux. The transmission coefficient, Γ, for H2O through the PEN polymer at 70 °C was Γ ∼ 3.2 × 10–10. The corresponding water vapor transmission rate (WVTR) at 70 °C/80% RH was 0.65 g/(m2 day). The temperature dependence of the H2O vapor transmission rate through PEN at constant H2O flux yielded an activation barrier of E = 12.4 kJ/mol. There was no observable reservoir effect for H2O in the PEN polymer. The H2O vapor transmission rates for initially dry or H2O-saturated PEN polymer substrates were nearly identical at various temperatures. Although the time required for the normalized conductance of the Ca film to reach zero was inversely proportional to the H2O flux, the Ca film conductance did not decrease linearly versus H2O exposure. The Ca film conductance changed very little during initial H2O exposure. This behavior may be caused by the nonlinear oxidation kinetics of the Ca film.
Co-reporter:Aziz I. Abdulagatov, Kalvis E. Terauds, Jonathan J. Travis, Andrew S. Cavanagh, Rishi Raj, and Steven M. George
The Journal of Physical Chemistry C 2013 Volume 117(Issue 34) pp:17442-17450
Publication Date(Web):July 30, 2013
DOI:10.1021/jp4051947
Titanium alkoxide films known as “titanicones” were grown using molecular layer deposition (MLD) techniques using the sequential exposure of TiCl4 and glycerol. These titanicone MLD films were then pyrolyzed under argon to yield conducting TiO2/carbon composite films. The Raman spectra of the pyrolyzed titanicone films revealed the characteristic “D” and “G” peaks associated with sp2-graphitic carbon. X-ray diffraction analysis of the pyrolyzed titanicone films displayed the signatures for anatase and rutile TiO2 after heating to 600 °C and then only rutile TiO2 after heating to 900 °C. X-ray photoelectron depth profiling of the pyrolyzed titanicone films showed that the carbon was distributed throughout the film and began to segregate to the surface after heating to 900 °C. The sheet resistance of the pyrolyzed titanicone films dropped dramatically versus pyrolysis temperature and reached a minimum sheet resistance of 2.2 × 104 Ω/□ after heating to 800 °C. On the basis of the measured film thickness of 88 nm, the resistivity of the pyrolyzed titanicone film after heating to 800 °C was ρ = 0.19 Ω cm. Segregation of other hybrid organic–inorganic films into sp2-graphitic carbon and metal oxide domains after pyrolysis under argon was also observed for alucone films and various metalcone films based on Zn, Zr, Hf, and Mn. The conducting TiO2/carbon composite films and other metal oxide/carbon composite films could have important electrochemical applications as electrodes for Li ion batteries or pseudocapacitance supercapacitors.
Co-reporter:Xiang Sun, Ming Xie, Jonathan J. Travis, Gongkai Wang, Hongtao Sun, Jie Lian, and Steven M. George
The Journal of Physical Chemistry C 2013 Volume 117(Issue 44) pp:22497-22508
Publication Date(Web):October 2, 2013
DOI:10.1021/jp4066955
Amorphous TiO2 thin films were conformally coated onto the surface of both graphene (G) and multiwalled carbon nanotube (CNT) samples using atomic layer deposition (ALD). An ultrathin Al2O3 adhesion layer was employed to obtain the conformal TiO2 ALD films. Using 1 M KOH as the electrolyte, the electrochemical characteristics of TiO2 ALD films grown using 25 and 50 TiO2 ALD cycles were then determined using cyclic voltammetry, galvanostatic charge/discharge curves, and electrochemical impedance spectroscopy. Because the TiO2 ALD films were ultrathin, the poor electrical conductivity and low ionic diffusivity of TiO2 did not limit the ability of the TiO2 ALD films to display high specific capacitance. The specific capacitances of the TiO2 ALD-coated G and CNT samples after 50 TiO2 ALD cycles were 97.5 and 135 F/g, respectively, at 1 A/g. The pseudocapacitance of the TiO2 ALD films greatly exceeded the electric double layer capacitance of the uncoated G and CNT samples. The galvanostatic charge/discharge experiments also revealed that the charge storage was dependent on the thickness of the TiO2 ALD film. This observation argues that the pseudocapacitance is derived largely from the TiO2 bulk and is not limited to the TiO2 surface. The molar ratio of stored charge to TiO2 was estimated to be in the range of 0.03–0.08 (mol stored charge/mol TiO2) for the various TiO2 ALD-coated G and CNT samples. An optimized asymmetric cell was also developed based on TiO2 ALD-coated CNT as the positive electrode and uncoated CNT as the negative electrode. This energy storage device could be reversibly operated over a wide voltage range of 0–1.5 V in the aqueous 1 M KOH electrolyte. An energy density of 4.47 W·h/kg was achieved on the basis of the total weight of both electrodes. This energy density was ∼4 times higher than the symmetric CNT cell. The TiO2 ALD-coated G and CNT electrodes and the asymmetric cell based on the TiO2 ALD-coated electrode exhibited excellent stability over >1000 cycles. The results of this study demonstrate that metal oxide ALD on high surface area conducting carbon substrates can be used to fabricate high energy storage supercapacitors.
Co-reporter:Aziz I. Abdulagatov, Robert A. Hall, Jackson L. Sutherland, Byoung H. Lee, Andrew S. Cavanagh, and Steven M. George
Chemistry of Materials 2012 Volume 24(Issue 15) pp:2854
Publication Date(Web):June 29, 2012
DOI:10.1021/cm300162v
Molecular layer deposition (MLD) techniques were used to grow titanium-containing hybrid organic–inorganic films known as “titanicones” using titanium tetrachloride (TiCl4) and either ethylene glycol (EG) or glycerol (GL). The surface chemistry for titanicone MLD was self-limiting versus TiCl4 and either EG or GL exposures. Quartz crystal microbalance (QCM) measurements observed a film growth rate of ∼83 ng/cm2/cycle using TiCl4 and EG from 90 to 115 °C. The growth rate then decreased significantly at 135 °C. X-ray reflectivity (XRR) studies yielded a growth rate of ∼4.5 Å/cycle with a constant density of ∼1.8 g/cm3 from 90 to 115 °C. The growth rate measured using XRR also decreased to 1.5 Å/cycle at 135 °C. Titanicone films were grown using TiCl4 and GL at higher temperatures between 130 and 210 °C. GL should increase the bridging between the polymer chains in the titanicone film and change film properties and improve film stability. The film growth rates decreased with temperature from 49 ng/cm2/cycle at 130 °C to 34 ng/cm2/cycle at 210 °C. XRR studies were consistent with a temperature-dependent film growth and measured growth rates of 2.8 Å/cycle at 130 °C and 2.1 Å/cycle at 210 °C. Nanoindentation experiments revealed that the elastic modulus and hardness of the titanicone films grown using GL were much higher than titanicone films grown using EG. Annealing the titanicone films to 600 °C in air removed the carbon constituents and yielded TiO2 films with a density of ∼3.3 g/cm3 that is slightly higher than the density of TiO2 ALD films grown at 115 °C. The titanicone films absorbed light in the ultraviolet, and the absorption threshold was consistent with an optical bandgap of ∼3.6 eV. Prolonged ultraviolet exposures on the titanicone films produced TiO2 films with a low density of 2.7 g/cm3.Keywords: atomic layer deposition; hybrid organic-inorganic; molecular layer deposition; thin films; titanium oxide;
Co-reporter:Byunghoon Yoon, Byoung H. Lee, and Steven M. George
The Journal of Physical Chemistry C 2012 Volume 116(Issue 46) pp:24784-24791
Publication Date(Web):October 25, 2012
DOI:10.1021/jp3057477
Highly conducting and transparent hybrid organic–inorganic thin films were grown using atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques. The conducting films were grown at 150 °C by combining ZnO ALD and zincone MLD processes. ZnO ALD was performed using diethylzinc (DEZ) and water (H2O). Zincone MLD was performed using DEZ and hydroquinone (HQ). The ALD:MLD alloys were deposited by controlling the relative number of ALD and MLD cycles in the reaction sequence. The growth of the ALD:MLD alloys was examined using in situ quartz crystal microbalance and Fourier transform infrared spectroscopy studies. The surface reactions during alloy growth were self-limiting, and the ALD and MLD reaction sequences produced linear growth rates. The alloys exhibited exceptional conductivity relative to ZnO ALD films grown at 150 °C. The highest conductivities were obtained from alloys with ALD:MLD cycle ratios of 1:1 and 2:2. The 1:1 and 2:2 alloy films yielded conductivities of 116 and 170 S/cm, respectively. In comparison, ZnO ALD films displayed a conductivity of 14 S/cm. The high conductivities for the zincone alloys may result from the modulated structure in the alloy that provides high charge carrier densities and high mobilities. The alloy films were also transparent and displayed high transmission until their band-gap absorption at <400 nm. In addition, nanoindentation studies revealed that the elastic modulus and hardness of the alloys both increased with the fraction of ZnO ALD in the alloy. These hybrid organic–inorganic alloys may have the potential to replace indium tin oxide (ITO) as a conducting transparent film.
Co-reporter:D. Seghete, G.B. Rayner Jr., A.S. Cavanagh, V.R. Anderson, and S.M. George
Chemistry of Materials 2011 Volume 23(Issue 7) pp:1668
Publication Date(Web):March 11, 2011
DOI:10.1021/cm101673u
Mo ALD has been demonstrated by fluorosilane elimination chemistry using MoF6 and Si2H6 as the reactants. The nucleation and growth characteristics of Mo ALD were investigated using a variety of in situ and ex situ techniques in both high vacuum and viscous flow reactors. Quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) investigations showed that Mo ALD has significant growth rate of 500−600 ng/cm2 per cycle or 6−7 Å per cycle for temperatures between 90 and 150 °C. The large growth rates could result from extra Mo deposition by MoF6 → Mo + 3F2 that may be facilitated by the very exothermic reaction of MoF6 with silicon-containing surface species. The QCM studies revealed that the Mo ALD surface chemistry is self-limiting. The QCM and Auger electron spectroscopy (AES) studies indicated that Mo ALD nucleates very rapidly on Al2O3 ALD surfaces and reaches the linear growth regime after only 4−5 ALD cycles. Oscillatory behavior for the total mass gain and individual mass gains was observed versus ALD cycle number during the nucleation region. The AES studies revealed that Mo films grown in a high vacuum reactor do not contain silicon impurities. In contrast to the AES results, Rutherford backscattering spectroscopy (RBS) analysis showed that Mo ALD films grown in a viscous flow reactor contain ∼16 at % Si impurities. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of silicon and showed that varying temperature, precursor dose and purge parameters did not lower the Si impurities significantly. Glancing incidence X-ray diffraction (GIXRD) studies indicated that Mo ALD films were nanocrystalline. The Si impurities may exist at grain boundaries or amorphous Mo silicides as a result of Si2H6 decomposition during the highly exothermic fluorosilane elimination reaction. Fourier transform infrared (FTIR) analysis revealed that MoFx surface species are reduced to metallic Mo during the Si2H6 exposure. Because of its rapid nucleation rate, Mo ALD films could serve as ultrathin continuous conducting films or as adhesion layers for other metal ALD systems on oxide surfaces.Keywords: atomic layer deposition; disilane; fluorosilane elimination; molybdenum; molybdenum hexafluoride; nucleation;
Co-reporter:A. I. Abdulagatov, Y. Yan, J. R. Cooper, Y. Zhang, Z. M. Gibbs, A. S. Cavanagh, R. G. Yang, Y. C. Lee, and S. M. George
ACS Applied Materials & Interfaces 2011 Volume 3(Issue 12) pp:4593
Publication Date(Web):October 27, 2011
DOI:10.1021/am2009579
Al2O3 and TiO2 atomic layer deposition (ALD) were employed to develop an ultrathin barrier film on copper to prevent water corrosion. The strategy was to utilize Al2O3 ALD as a pinhole-free barrier and to protect the Al2O3 ALD using TiO2 ALD. An initial set of experiments was performed at 177 °C to establish that Al2O3 ALD could nucleate on copper and produce a high-quality Al2O3 film. In situ quartz crystal microbalance (QCM) measurements verified that Al2O3 ALD nucleated and grew efficiently on copper-plated quartz crystals at 177 °C using trimethylaluminum (TMA) and water as the reactants. An electroplating technique also established that the Al2O3 ALD films had a low defect density. A second set of experiments was performed for ALD at 120 °C to study the ability of ALD films to prevent copper corrosion. These experiments revealed that an Al2O3 ALD film alone was insufficient to prevent copper corrosion because of the dissolution of the Al2O3 film in water. Subsequently, TiO2 ALD was explored on copper at 120 °C using TiCl4 and water as the reactants. The resulting TiO2 films also did not prevent the water corrosion of copper. Fortunately, Al2O3 films with a TiO2 capping layer were much more resilient to dissolution in water and prevented the water corrosion of copper. Optical microscopy images revealed that TiO2 capping layers as thin as 200 Å on Al2O3 adhesion layers could prevent copper corrosion in water at 90 °C for ∼80 days. In contrast, the copper corroded almost immediately in water at 90 °C for Al2O3 and ZnO films by themselves on copper. Ellipsometer measurements revealed that Al2O3 films with a thickness of ∼200 Å and ZnO films with a thickness of ∼250 Å dissolved in water at 90 °C in ∼10 days. In contrast, the ellipsometer measurements confirmed that the TiO2 capping layers with thicknesses of ∼200 Å on the Al2O3 adhesion layers protected the copper for ∼80 days in water at 90 °C. The TiO2 ALD coatings were also hydrophilic and facilitated H2O wetting to copper wire mesh substrates.Keywords: Al2O3; atomic layer deposition; copper; corrosion; TiO2; water;
Co-reporter:Dragos Seghete, Francois H. Fabreguette, Steven M. George
Thin Solid Films 2011 Volume 519(Issue 11) pp:3612-3618
Publication Date(Web):31 March 2011
DOI:10.1016/j.tsf.2011.01.285
A special slit doser is used to form near unit steps in the spatial profile of an Al2O3 ALD film thickness. The unit step is formed as the Al2O3 ALD occurs mainly downstream from the slit doser because the trimethylaluminum and H2O reactants are entrained in a viscous flow carrier gas. Spectroscopic ellipsometry measurements yielded thickness profiles of Al2O3 ALD on samples placed at different locations relative to the exit of the slit doser and the ALD growth zone. The effects of carrier gas flow rate, reactor pressure, and reactant dose and purge times on the Al2O3 ALD film profile provided details about the gas dynamics around the slit doser. Experimental indications of gas turbulence were observed at the exit of the slit doser. Lateral gradients in the Al2O3 ALD film thickness were also formed by linear translation of the sample relative to the slit doser during ALD. Lateral gradients of various desired pitches ranging from 119 Å/in to 444 Å/in were achieved as a result of accurate control of the Al2O3 ALD film thickness and small sample translation steps relative to the slit doser.
Co-reporter:Younghee Lee, Byunghoon Yoon, Andrew S. Cavanagh, and Steven M. George
Langmuir 2011 Volume 27(Issue 24) pp:15155-15164
Publication Date(Web):October 26, 2011
DOI:10.1021/la202391h
Molecular layer deposition (MLD) of aluminum alkoxide polymer films was examined using trimethlyaluminum (TMA) and glycidol (GLY) as the reactants. Glycidol is a high vapor pressure heterobifunctional reactant with both hydroxyl and epoxy chemical functionalites. These two different functionalities help avoid “double reactions” that are common with homobifuctional reactants. A variety of techniques, including in situ Fourier transform infrared (FTIR) spectroscopy and quartz crystal microbalance (QCM) measurements, were employed to study the film growth. FTIR measurements at 100 and 125 °C observed the selective reaction of the GLY hydroxyl group with the AlCH3 surface species during GLY exposure. Epoxy ring-opening and methyl transfer from TMA to the surface epoxy species were then monitored during TMA exposure. This epoxy ring-opening reaction is dependent on strong Lewis acid–base interactions between aluminum and oxygen. The QCM experiments observed linear growth with self-limiting surface reactions at 100–175 °C under certain growth conditions. With a sufficient purge time of 20 s after TMA and GLY exposures at 125 °C, the mass gain per cycle (MGPC) was 19.8 ng/cm2-cycle. The individual mass gains after the TMA and GLY exposures were also consistent with a TMA/GLY stoichiometry of 4:3 in the MLD film. This TMA/GLY stoichiometry suggests the presence of Al2O2 dimeric core species. The MLD films resulting from these TMA and GLY exposures also evolved with annealing temperature to form thinner conformal porous films with increased density. Non-self-limiting growth was a problem at shorter purge times and lower temperatures. With shorter purge times of 10 s at 125 °C, the MPGC increased dramatically to 134 ng/cm2-cycle. The individual mass gains after the TMA and GLY exposures in the CVD regime were consistent with a TMA/GLY stoichiometry of 1:1. The MGPC decreased progressively versus purge time. This behavior was attributed to the removal of reactants that could lead to CVD and the instability of the surface species after the reactant exposures. These results reveal that the TMA and GLY reaction displays much complexity and must be carefully controlled to be a useful MLD process.
Co-reporter:D.N. Goldstein, S.M. George
Thin Solid Films 2011 Volume 519(Issue 16) pp:5339-5347
Publication Date(Web):1 June 2011
DOI:10.1016/j.tsf.2011.02.037
Palladium (Pd) atomic layer deposition (ALD) can be performed with Pd(hfac)2 (hfac = hexafluoroacetyl-acetone) and formalin as the reactants. For Pd ALD on oxide surfaces, the nucleation of Pd ALD has been observed to require between 20 and 100 ALD cycles. To understand the long nucleation periods, this study explored the surface reactions occurring during Pd ALD nucleation and growth on hydroxylated Al2O3 substrates. In situ Fourier transform infrared (FTIR) spectroscopy on high surface area nanopowders was used to observe the surface species. The adsorption of Pd(hfac)2 on hydroxylated Al2O3 substrates was found to yield both Pd(hfac)* and Al(hfac)* surface species. The identity of the Al(hfac)* species was confirmed by separate FTIR studies of hfacH adsorption on the hydroxylated Al2O3 substrates. Isothermal loss of the Al(hfac)* species revealed second-order kinetics at 448–523 K with an activation barrier of Ed = 39.4 kcal/mol. The lack of correlation between Al(hfac)* and AlOH* species during the loss of Al(hfac)* species suggested that the Al(hfac)* species may desorb as Al(hfac)3. After Pd(hfac)2 exposure and the subsequent formalin exposure on hydroxylated Al2O3 substrates, only hfac ligands from Pd(hfac)* species were removed from the surface. In addition, the formalin exposure added formate species. The Al(hfac)* species was identified as the cause of the long nucleation period because Al(hfac)* behaves as a site blocker. The surface poisoning by Al(hfac)* species was corroborated by adsorbing hfacH prior to the Pd(hfac)2 exposures. The amount of Pd(hfac)* species after Pd(hfac)2 exposures decreased progressively versus the previous hfacH exposure. Pd ALD occurred gradually during the subsequent Pd ALD cycles as the Al(hfac)* species were slowly removed from the Al2O3 surface. Ex situ transmission electron microscopy analysis revealed Pd nanoclusters that grew in size and dispersion with increasing number of Pd ALD cycles. These nanoclusters eventually coalesced to form a continuous Pd ALD film. Surface poisoning by the hfac ligands may help to explain the nucleation difficulties for metal ALD on oxide substrates using β-diketonate reactants.
Co-reporter:Shaibal K. Sarkar, Jin Young Kim, David N. Goldstein, Nathan R. Neale, Kai Zhu, C. Michael Elliott, Arthur J. Frank and Steven M. George
The Journal of Physical Chemistry C 2010 Volume 114(Issue 17) pp:8032-8039
Publication Date(Web):April 13, 2010
DOI:10.1021/jp9086943
In2S3 atomic layer deposition (ALD) with indium acetylacetonate (In(acac)3) and H2S was studied with quartz crystal microbalance (QCM), X-ray reflectivity (XRR), and Fourier transform infrared (FTIR) spectroscopy techniques. Subsequent In2S3 ALD on TiO2 nanotube arrays defined a model semiconductor sensitized solar cell. For In2S3 ALD on initial Al2O3 ALD surfaces, the In2S3 ALD displayed a nucleation period of ∼60−70 cycles followed by a linear growth region. These results were obtained under ALD conditions that were not completely self-limiting for the In(acac)3 exposure because of the low In(acac)3 vapor pressure. The growth per cycle decreased at higher temperature over the temperature range from 130 to 170 °C at these same reactant conditions. The growth per cycle was 0.30−0.35 Å per cycle at 150 °C as determined by QCM and XRR measurements at higher In(acac)3 exposures where the surface reactions were self-limiting chemistry versus In(acac)3 and H2S exposures. The FTIR examinations revealed that the nucleation period on Al2O3 ALD surfaces may be related to the formation of Al(acac)* species that act to poison the initial Al2O3 ALD surface. X-ray diffraction investigations revealed β-In2S3 ALD films and X-ray photoelectron measurements were consistent with In2S3 films. The In2S3 ALD was employed as a semiconductor sensitizer on TiO2 nanotube arrays for solar conversion. Scanning electron microscopy and energy dispersive X-ray analysis imaging revealed In2S3 over the full length of the TiO2 nanotube array after 175 cycles of In2S3 ALD at 150 °C at reactant exposure conditions that were self-limiting on flat substrates. The photoelectrochemical properties of these In2S3 ALD-sensitized TiO2 nanotube arrays with a Co2+/Co3+ electrolyte were then characterized by measuring the photocurrent density versus voltage and the external quantum efficiency versus photon energy. A small quantum efficiency of ∼10% was observed that can be attributed to charge recombination losses and charge injection/collection processes.
Co-reporter:Jay Yu Kim
The Journal of Physical Chemistry C 2010 Volume 114(Issue 41) pp:17597-17603
Publication Date(Web):September 23, 2010
DOI:10.1021/jp9120244
Tin monosulfide (SnS) was grown by atomic layer deposition (ALD) using sequential exposures of tin(II) 2,4-pentanedionate (Sn(acac)2) and hydrogen sulfide (H2S). In situ quartz crystal microbalance (QCM) studies showed that the SnS ALD mass gain per cycle was 11−12 ng/cm2 at 175 °C on a gold-covered QCM sensor. Using a film density of 5.07 g/cm3 determined by X-ray reflectivity measurements, these mass gains are equivalent to SnS ALD growth rates of 0.22−0.24 Å/cycle. The ratio of the mass loss and mass gain (|Δm2/Δm1|) from the H2S and Sn(acac)2 reactions was |Δm2/Δm1| ∼ 0.32 at 175 °C. This measured ratio is close to the predicted ratio from the proposed surface chemistry for SnS ALD. The SnS ALD was self-limiting versus the Sn(acac)2 and H2S exposures. The SnS ALD growth rate was also independent of substrate temperature from 125 to 225 °C. The SnS ALD growth on Al2O3 ALD substrates displayed nucleation problems and smaller growth rates. These differences may be caused by site blocking by the Al(acac)* surface species. X-ray fluorescence studies confirmed a Sn/S atomic ratio of ∼1.0 for the SnS ALD films. X-ray photoelectron spectroscopy measurements revealed that the SnS ALD films contained oxygen impurities at 15−20 atom % after air exposure. These oxygen-containing SnS ALD films displayed a band gap of ∼1.87 eV that is higher than the SnS bulk value of ∼1.3 eV. In addition, these SnS ALD films produced very weak photoluminescence at room temperature. SnS ALD may be useful to fabricate photovoltaic or solar conversion devices.
Co-reporter:R. A. Wind and S. M. George
The Journal of Physical Chemistry A 2010 Volume 114(Issue 3) pp:1281-1289
Publication Date(Web):September 16, 2009
DOI:10.1021/jp9049268
Al2O3 atomic layer deposition (ALD) growth with Al(CH3)3 (trimethylaluminum (TMA)) and H2O as the reactants was examined at the relatively low temperature of 125 °C using quartz crystal microbalance (QCM) measurements. The total Al2O3 ALD mass gain per cycle (MGPC) and MGPCs during the individual TMA and H2O reactions were measured versus TMA and H2O exposures. The Al2O3 MGPC increased with increasing H2O and TMA exposures at fixed TMA and H2O exposures, respectively. However, the TMA and H2O reactions were not completely self-limiting. The slower surface reaction kinetics at lower temperature may require very long exposures for the reactions to reach completion. The Al2O3 MGPCs increased quickly versus H2O exposure and slowly reached limiting values that were only weakly dependent on the TMA doses. Small TMA exposures were also sufficient for the Al2O3 MGPCs to reach different limiting values for different H2O doses. The TMA MGPCs increased for higher TMA exposures at all H2O exposures. In contrast, the H2O MGPCs decreased for higher TMA exposures at all H2O exposures. This decrease may occur from more dehydroxylation at larger hydroxyl coverages after the H2O exposures. The hydroxyl coverage after the H2O exposure was dependent only on the H2O exposure. The Al2O3 MGPC was also linearly dependent on the hydroxyl coverage after the H2O dose. Both the observed hydroxyl coverage versus H2O exposure and the Al2O3 ALD growth versus H2O and TMA exposures were fit using modified Langmuir adsorption isotherm expressions where the pressures are replaced with exposures. These results should be useful for understanding low-temperature Al2O3 ALD, which is important for coating organic, polymeric, and biological substrates.
Co-reporter:Steven M. George, Byunghoon Yoon and Arrelaine A. Dameron
Accounts of Chemical Research 2009 Volume 42(Issue 4) pp:498
Publication Date(Web):February 27, 2009
DOI:10.1021/ar800105q
The fabrication of many devices in modern technology requires techniques for growing thin films. As devices miniaturize, manufacturers will need to control thin film growth at the atomic level. Because many devices have challenging morphologies, thin films must be able to coat conformally on structures with high aspect ratios. Techniques based on atomic layer deposition (ALD), a special type of chemical vapor deposition, allow for the growth of ultra-thin and conformal films of inorganic materials using sequential, self-limiting reactions. Molecular layer deposition (MLD) methods extend this strategy to include organic and hybrid organic−inorganic polymeric materials. In this Account, we provide an overview of the surface chemistry for the MLD of organic and hybrid organic−inorganic polymers and examine a variety of surface chemistry strategies for growing polymer thin films. Previously, surface chemistry for the MLD of organic polymers such as polyamides and polyimides has used two-step AB reaction cycles using homo-bifunctional reactants. However, these reagents can react twice and eliminate active sites on the growing polymer surface. To avoid this problem, we can employ alternative precursors for MLD based on hetero-bifunctional reactants and ring-opening reactions. We can also use surface activation or protected chemical functional groups. In addition, we can combine the reactants for ALD and MLD to grow hybrid organic−inorganic polymers that should display interesting properties. For example, using trimethylaluminum (TMA) and various diols as reactants, we can achieve the MLD of alucone organic−inorganic polymers. We can alter the chemical and physical properties of these organic−inorganic polymers by varying the organic constituent in the diol or blending the alucone MLD films with purely inorganic ALD films to build a nanocomposite or nanolaminate. The combination of ALD and MLD reactants enlarges the number of possible sequential self-limiting surface reactions for film growth. Extensions to three-step ABC reaction cycles also offer many advantages to avoid the use of homo-bifunctional reactants and incorporate new functionality in the thin film. The advances in ALD have helped technological development in many areas, including semiconductor processing and magnetic disk-drive manufacturing. We expect that the advances in MLD will lead to innovations in polymeric thin-film products. Although there are remaining challenges, effective surface chemistry strategies are being developed for MLD that offer the opportunity for future advances in materials and device fabrication.
Co-reporter:Byunghoon Yoon;Jennifer L. O'Patchen;Dragos Seghete;Andrew S. Cavanagh
Chemical Vapor Deposition 2009 Volume 15( Issue 4-6) pp:112-121
Publication Date(Web):
DOI:10.1002/cvde.200806756
Abstract
The molecular layer deposition (MLD) of a hybrid organic-inorganic polymer based on zinc is demonstrated using sequential exposures of diethyl zinc (DEZ, Zn(CH2CH3)2) and ethylene glycol (EG, HOCH2CH2OH). This polymer is representative of a class of zinc alkoxide polymers with an approximate formula of (ZnORO)n that can be called “zincones”. The film growth and surface chemistry during zincone MLD is studied using in-situ Fourier transform infrared (FTIR) measurements. The absorbance of the infrared features of the zincone film increase progressively versus the number of MLD cycles. The FTIR spectra after the DEZ and EG exposures are consistent with the gain and loss of absorbance from CH, OH, CO, and ZnO stretching vibrations. FTIR studies also confirm the self-limiting nature of the surface reactions and monitor the temperature dependence of the film growth. Transmission electron microscope (TEM) images of ZrO2 nanoparticles show very conformal zincone films and determine that the growth rate varies from 4.0 Å per MLD cycle at 90 °C to 0.25 Å per MLD cycle at 170 °C. Quartz crystal microbalance (QCM) and X-ray reflectivity (XRR) measurements show linear zincone growth versus the number of MLD cycles. XRR studies on silicon wafers are consistent with a growth rate of 0.7 Å per MLD cycle at 130 °C. The higher growth rate on the ZrO2 nanoparticles is attributed to the lower gas conductance and possible CVD reactions in the ZrO2 nanoparticles. The reaction mechanism for zincone MLD is dependent on temperature. At higher temperatures, there is evidence for “double” reactions of EG because no free hydroxyl groups are observed in the FTIR spectrum after the EG exposures. The zincone film can grow in the absence of free hydroxyl groups if DEZ can diffuse into the zincone film and react during the subsequent EG exposure. The zincone films initially adsorb H2O upon exposure to air and then are very stable with time.
Co-reporter:B.B. Burton, F.H. Fabreguette, S.M. George
Thin Solid Films 2009 Volume 517(Issue 19) pp:5658-5665
Publication Date(Web):3 August 2009
DOI:10.1016/j.tsf.2009.02.050
Manganese oxide (MnO) atomic layer deposition (ALD) was accomplished using sequential exposures of bis(ethylcyclopentadienyl)manganese (Mn(CpEt)2) and H2O. Rutherford backscattering analysis revealed a nearly 1:1 atomic ratio for Mn:O in the MnO ALD films. X-ray diffraction determined that the films were crystalline and consistent with the cubic phase of MnO. Quartz crystal microbalance (QCM) measurements monitored the mass deposition rate during MnO ALD and verified self-limiting reactions for each reactant. Extremely efficient reactions were observed that required reactant exposures of only 3 × 104 L (1 L = 1.33 × 10− 4 Pa s). X-ray reflectivity (XRR) studies were used to confirm the QCM measurements and determine the film density and film thicknesses. The MnO ALD film density was 5.23 g/cm3. The growth per cycle was investigated from 100–300 °C. The largest MnO ALD growth per cycle was 1.2 Å/cycle at 100 °C and the growth per cycle decreased at higher temperatures. Transmission electron microscopy images observed the conformality of MnO films on ZrO2 nanoparticles and confirmed the growth per cycle observed by the XRR studies. Fourier transform infrared spectroscopy was used to study the –CpEt⁎ and –OH⁎ surface species during MnO ALD and also monitored the bulk vibrational modes of the growing MnO films. The results allowed a growth mechanism to be established for MnO ALD using Mn(CpEt)2 and H2O. Only 54% of the Mn sites are observed to retain the –CpEt⁎ surface species after the Mn(CpEt)2 exposure. Efficient MnO ALD using Mn(CpEt)2 and H2O should be useful for a variety of applications where metal oxides are required that can easily change their oxidation states.
Co-reporter:A. A. Dameron, D. Seghete, B. B. Burton, S. D. Davidson, A. S. Cavanagh, J. A. Bertrand and S. M. George
Chemistry of Materials 2008 Volume 20(Issue 10) pp:3315
Publication Date(Web):April 29, 2008
DOI:10.1021/cm7032977
Polymeric films can be grown by a sequential, self-limiting surface chemistry process known as molecular layer deposition (MLD). The MLD reactants are typically bifunctional monomers for stepwise condensation polymerization and can yield completely organic films. The MLD of organic–inorganic hybrid polymers can also be accomplished using a bifunctional organic monomer and a multifunctional inorganic monomer. In this work, the growth of a poly(aluminum ethylene glycol) polymer is demonstrated using the sequential exposures of trimethylaluminum (TMA) and ethylene glycol (EG). These hybrid polymers, known as alucones, were grown over a wide range of temperatures from 85 to 175 °C. In situ quartz crystal microbalance and ex situ X-ray reflectivity experiments confirmed linear growth of the alucone film versus number of TMA/EG reaction cycles at all temperatures. The alucone growth rates decreased at higher temperatures. Growth rates varied from 4.0 Å per cycle at 85 °C to 0.4 Å per cycle at 175 °C. In situ Fourier transform infrared spectroscopy was used to monitor the surface reactions during alucone MLD. Ex situ FTIR spectroscopy, X-ray photoelectron spectroscopy, and X-ray reflectivity measurements were also employed to determine the chemical composition, thickness, and density of the alucone films. These ex situ studies revealed that the alucone films grown on Al2O3 ALD surfaces evolved under ambient conditions before reaching a stable state. Alucone films capped with rapid SiO2 ALD displayed much more stability than alucone films grown on Al2O3 ALD surfaces. These results indicated that H2O may facilitate the chemical transformation of the alucone MLD films. The alucone films represent a new class of organic–inorganic hybrid polymers. Modification of this basic alucone MLD chemistry with use of other diols or other bifunctional monomers can produce different alucone polymers with variable properties.
Co-reporter:Russell Cooper, Hari P. Upadhyaya, Timothy K. Minton, Michael R. Berman, Xiaohua Du, Steven M. George
Thin Solid Films 2008 Volume 516(Issue 12) pp:4036-4039
Publication Date(Web):30 April 2008
DOI:10.1016/j.tsf.2007.07.150
Thin films of Al2O3 grown using atomic layer deposition (ALD) techniques can protect polymers from erosion by oxygen atoms. To quantify this protection, polyimide substrates with the same chemical repeat unit as Kapton® were applied to quartz crystal microbalance (QCM) sensors. Al2O3 ALD films with varying thicknesses were grown on the polyimide substrates. The ALD-coated polyimide materials were then exposed to a hyperthermal atomic-oxygen beam. The mass loss versus oxygen-atom exposure time was measured in situ by the QCM. Al2O3 ALD film thicknesses of ∼ 35 Å were found to protect the polymer from erosion.
Co-reporter:X. Du, S.M. George
Sensors and Actuators B: Chemical 2008 Volume 135(Issue 1) pp:152-160
Publication Date(Web):10 December 2008
DOI:10.1016/j.snb.2008.08.015
Ultrathin tin oxide films were deposited on flat hotplate templates using atomic layer deposition (ALD) techniques with SnCl4 and H2O2 as the reactants. The resistance of the SnOx ALD films across an electrode gap on the hotplate template was observed to oscillate and decrease versus the number of sequential SnCl4 and H2O2 reactions at 250 °C. The resistance also varied with exposure to O2 and CO pressure at 300 °C and 325 °C. A wide range of SnOx ALD film thicknesses between 15.9 Å and 58.7 Å was prepared by varying the number of sequential, self-limiting SnCl4 and H2O2 reactions. The CO gas sensor response was then measured for these SnOx ALD film thicknesses at 300 °C. The CO gas sensor response increased for increasing thicknesses between 15.9 Å and 26.2 Å and decreased for increasing thicknesses between 26.2 Å and 58.7 Å. The results were interpreted in terms of the Debye length and resistance for the SnOx ALD films. The Debye length is comparable with the SnOx ALD film thickness of 26.2 Å corresponding to the maximum responsivity for CO gas sensing. For film thicknesses >26.2 Å, the responsivity decrease was explained by a larger fraction of the film with thickness greater than the Debye length that is not affected by the O2 and CO chemisorption. For film thicknesses <26.2 Å, the responsivity decrease was attributed to the increasing resistance of the SnOx ALD film. The gas sensor response was temperature dependent and displayed its highest responsivity at temperatures between 250 °C and 325 °C. The response times of the SnOx ALD gas sensors were also faster at the higher temperatures >260 °C.
Co-reporter:C.A. Wilson, J.A. McCormick, A.S. Cavanagh, D.N. Goldstein, A.W. Weimer, S.M. George
Thin Solid Films 2008 Volume 516(Issue 18) pp:6175-6185
Publication Date(Web):31 July 2008
DOI:10.1016/j.tsf.2007.11.086
Tungsten (W) atomic layer deposition (ALD) was investigated on a variety of polymer films and polymer particles. These polymers included polyethylene, polyvinyl chloride, polystyrene, polycarbonate, polypropylene and polymethylmethacrylate. The W ALD was performed at 80 °C using WF6 and Si2H6 as the gas phase reactants. W ALD on flat polymer films can eventually nucleate and grow after more than 60 AB cycles. X-ray photoelectron spectroscopy studies of W ALD on polystyrene after 50 AB cycles suggested that tungsten nanoclusters are present in the W ALD nucleation regime. The W ALD nucleation is greatly facilitated by a few cycles of Al2O3 ALD. W ALD films were grown at 80 °C on spin-coated polymers on silicon wafers after 10 AB cycles of Al2O3 ALD. The W ALD film was observed to grow linearly with a growth rate of 3.9 Å per AB cycle on the polymer films treated with the Al2O3 ALD seed layer. The W ALD films displayed an excellent, mirror-like optical reflectivity. The resistivity was 100–400 µΩ cm for W ALD films with thicknesses from 95–845 Å. W ALD was also observed on polymer particles after W ALD in a rotary reactor. Without the Al2O3 ALD seed layer, the nucleation of W ALD directly on the polymer particles at 80 °C required > 50 AB cycles. In contrast, the polymer particles treated with only 5 AB cycles of Al2O3 ALD were observed to blacken after 25 AB cycles of W ALD. W ALD on polymers may have applications for flexible optical mirrors, electromagnetic interference shielding and gas diffusion barriers.
Co-reporter:J. A. McCormick;K. P. Rice;D. F. Paul;A. W. Weimer;S. M. George
Chemical Vapor Deposition 2007 Volume 13(Issue 9) pp:
Publication Date(Web):10 SEP 2007
DOI:10.1002/cvde.200606563
Al2O3 atomic layer deposition (ALD) is analyzed on ZrO2 nanoparticles in a rotary reactor. This rotary reactor allows for static exposures and efficiently utilizes the reactants for ALD on high surface area nanoparticles. The Al2O3 ALD is performed using exposures to Al(CH3)3 and H2O reactants. The pressure transients during these exposures are examined using a sequence of reactant micropulses. These micropulses are less than the required exposures for the ALD surface chemistry to reach completion. The pressure transients during identical sequential Al(CH3)3 and H2O micropulses change as the surface chemistry progresses to completion. These pressure transients allow the required saturation reactant exposure to be determined to maximize reactant usage. The ZrO2 nanoparticles are coated using various numbers of Al(CH3)3 and H2O reactant exposures. The Al2O3 ALD-coated ZrO2 nanoparticles are subsequently analyzed using a number of techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES), scanning AES (SAES), and X-ray photoelectron spectroscopy (XPS). The TEM images reveal very conformal Al2O3 ALD on the ZrO2 nanoparticles. The Al2O3 ALD thicknesses versus number of Al(CH3)3 and H2O reactant exposures yielded an Al2O3 ALD growth rate of 2.0 Å per reactant cycle. The AES and XPS results are consistent with an Al2O3 ALD film that completely and conformally covered the underlying ZrO2 nanoparticle. The SAES measurements show that the Al2O3 ALD films are continuous and homogeneous on the ZrO2 nanoparticles. These results demonstrate that a rotary reactor can successfully perform ALD with high reactant efficiency on high surface area nanoparticles.
Co-reporter:Francois H. Fabreguette, Steven M. George
Thin Solid Films 2007 Volume 515(Issue 18) pp:7177-7180
Publication Date(Web):25 June 2007
DOI:10.1016/j.tsf.2007.03.044
Atomic layer deposition (ALD) techniques were used to fabricate W/Al2O3 superlattices with high X-ray reflectivity on flexible Kapton® polyimide and polyethylene naphthalate (PEN) polymer substrates. Reflectivities of 78% and 74% at λ = 1.54 Å were measured for 6-bilayer W/Al2O3 superlattices on Kapton® polyimide and PEN, respectively. These excellent X-ray reflectivities are attributed to precise bilayer thicknesses and ultrasmooth interfaces obtained by ALD and smoothing of the initial polymer surface by an Al2O3 ALD layer. The conformal ALD film growth also produces correlated roughness that enhances the reflectivity. These W/Al2O3 superlattices on flexible polymers should be useful for ultralight and adjustable radius of curvature X-ray mirrors.
Co-reporter:J.D. Ferguson, K.J. Buechler, A.W. Weimer, S.M. George
Powder Technology 2005 Volume 156(2–3) pp:154-163
Publication Date(Web):23 August 2005
DOI:10.1016/j.powtec.2005.04.009
Thermite mixtures with improved contact between the fuel and oxidizer can provide increased reaction rates compared with traditional thermite mixtures. One technique to create thermite mixtures with improved contact is to deposit the oxidizer directly onto nanometer-sized fuel particles. This study investigates the atomic layer deposition (ALD) of SnO2 onto nanoparticles using SnCl4 and H2O2 reactants. The nanoparticle ALD was performed in a small, hot wall, vertical fluidized bed reactor. The SnO2 ALD was first demonstrated on ZrO2 nanoparticles. Auger electron spectroscopy, inductively coupled plasma-atomic emission spectroscopy (ICP-AES), transmission electron microscopy (TEM) and particle size distribution analysis were used to characterize the SnO2-coated ZrO2 nanoparticles. Subsequently, SnO2 ALD was performed on Al nanoparticles. The SnO2-coated Al nanoparticles were analyzed using ICP-AES and TEM. The SnO2-coated Al and the uncoated Al particles were also ignited and filmed with a digital video recorder. Although the SnO2-coated Al particles were far from stoichiometric thermite composites, the SnO2-coated Al particles reacted much more quickly and violently than the uncoated Al particles. The lower than expected Sn percent by mass observed on the SnO2-coated Al nanoparticles highlighted a major difficulty with coating nanoparticles. The nanoparticles have an extremely high surface area and the required reactant exposures are large even when assuming 100% reactant efficiency. These results illustrate the utility of ALD techniques to coat oxidizers on fuel nanoparticles to create enhanced thermite materials.
Co-reporter:F.H. Fabreguette, Z.A. Sechrist, J.W. Elam, S.M. George
Thin Solid Films 2005 Volume 488(1–2) pp:103-110
Publication Date(Web):22 September 2005
DOI:10.1016/j.tsf.2005.04.114
The atomic layer deposition (ALD) of tungsten can be accomplished using sequential exposures of WF6 and Si2H6 (disilane). In this paper, W ALD is explored using in situ quartz crystal microbalance (QCM) measurements as a function of the reactant exposure and deposition temperature. The QCM measurements revealed that the WF6 reaction is self-limiting. In contrast, W ALD growth rates exhibited a slow and continual increase for disilane exposures > 4 × 104 L. The W ALD growth rate was also weakly temperature-dependent with an activation energy of 1.5 ± 0.1 kcal/mol at T < 250 °C and a lower activation energy of 0.6 ± 0.3 kcal/mol at T > 275 °C. The QCM results and previous Auger results for W ALD yield the relationship between the silicon coverage deposited during the Si2H6 exposure and the tungsten coverage deposited during the WF6 exposure. The W/Si atomic ratio of ∼ 1:1 is consistent with earlier Auger investigations of the surface chemistry during W ALD at 200 °C. The QCM measurements are also consistent with silicon coverages of 1.7–2.1 monolayers after the Si2H6 exposures. These high silicon coverages are believed to result by silylene insertion from Si2H6 into surface Si–H bonds.
Co-reporter:Y. Du, X. Du, S.M. George
Thin Solid Films 2005 Volume 491(1–2) pp:43-53
Publication Date(Web):22 November 2005
DOI:10.1016/j.tsf.2005.05.051
SiO2 thin films were grown by catalyzed atomic layer deposition (ALD) at low temperatures in a viscous flow reactor using sequential SiCl4 and H2O exposures. Pyridine (C5H5N) was used as a catalyst for both reactant exposures. Micropulsing was employed to avoid possible film contamination by pyridinium chloride. Quartz crystal microbalance experiments measured a SiO2 film growth rate of 1.35 Å per SiCl4/H2O AB cycle at 305 K with SiCl4, H2O and pyridine exposures of 103–104 Langmuir (1 Langmuir = 10− 6 Torr s). This SiO2 ALD growth rate was verified by ex situ X-ray reflectivity, spectroscopic ellipsometry and surface profilometry measurements. The SiO2 ALD film growth rate decreased dramatically at higher temperatures. Transmission Fourier transform infrared spectroscopy studies revealed that the hydrogen-bonded pyridine coverage was correlated with the catalyzed SiO2 ALD growth rates from 305 to 360 K. Larger pyridine pressures were needed for catalyzed SiO2 ALD at higher temperatures to offset the larger pyridine desorption rates. The SiO2 ALD films contained negligible C, N, or Cl impurities as determined by X-ray photoelectron spectrometry. The deposited SiO2 films were also extremely smooth with a surface roughness identical to the initial Si(100) substrate.
Co-reporter:J.D. Ferguson, A.R. Yoder, A.W. Weimer, S.M. George
Applied Surface Science 2004 Volume 226(Issue 4) pp:393-404
Publication Date(Web):30 March 2004
DOI:10.1016/j.apsusc.2003.10.053
Abstract
TiO2 was deposited with atomic layer control on ZrO2 particles using alternating exposures of TiCl4 and H2O. Transmission Fourier transform infrared (FTIR) spectroscopy was used to monitor the sequential surface chemistry in vacuum. The ZrO2 particles initially displayed vibrational modes consistent with ZrOH* surface species. TiCl4 exposure at 600 K removed the surface hydroxyls and subsequent H2O exposure at 600 K produced TiOH* surface species. Repeating the TiCl4 and H2O exposures in an ABAB… reaction sequence deposited TiO2 with atomic layer control. The intensity of the bulk vibrational modes for TiO2 increased with the number of AB reaction cycles. The ZrO2 particles after TiO2 deposition were examined with transmission electron microscopy (TEM). The TEM images revealed ZrO2 particles encapsulated by conformal TiO2 films with a thickness of ∼16 Å after 40 AB reaction cycles. These TEM images are consistent with a TiO2 atomic layer deposition (ALD) growth rate at 600 K of 0.4 Å/AB cycle.
Co-reporter:R.K. Grubbs, C.E. Nelson, N.J. Steinmetz, S.M. George
Thin Solid Films 2004 Volume 467(1–2) pp:16-27
Publication Date(Web):22 November 2004
DOI:10.1016/j.tsf.2004.02.099
Nucleation and growth are critical during the atomic layer deposition (ALD) of ultra thin films and nanolaminates. This study examined the nucleation and growth during tungsten (W) ALD on aluminum oxide (Al2O3) surfaces and Al2O3 ALD on W surfaces using Auger electron spectroscopy (AES). W ALD was performed using alternating exposures of WF6 and Si2H6. Al2O3 ALD was performed using alternating exposures of Al(CH3)3 and H2O. AES signals were measured after each WF6 and Si2H6 exposure during W ALD on Al2O3 and after each Al(CH3)3 and H2O exposure during Al2O3 ALD on W. The AES measurements revealed that 3 WF6/Si2H6 reaction cycles were required to nucleate the W ALD film on Al2O3 surfaces at 473 K. Subsequently, the W ALD film grew linearly at a rate of 2.6–3.5 Å per WF6/Si2H6 reaction cycle. The AES measurements also revealed that only one H2O/Al(CH3)3 cycle was needed to nucleate Al2O3 ALD on W at 450 K. Subsequently, the Al2O3 ALD film grew linearly at the rate of 1.0 Å per Al(CH3)3/H2O reaction cycle. As expected from the W ALD surface chemistry, the W and Si AES signals oscillated dramatically during the sequential WF6 and Si2H6 exposures. Many parameters were varied to determine their effect on the W ALD nucleation period. The WF6 surface reaction was surprisingly insensitive to the Al2O3 substrate temperature and the initial hydroxyl coverage on the Al2O3 surface. These results for the nucleation and growth during W ALD on Al2O3 and Al2O3 ALD on W are relevant to the growth of W/Al2O3 nanolaminates that have potential as X-ray mirrors, thermal barrier coatings and tribological films.
Co-reporter:J.W Elam, M Schuisky, J.D Ferguson, S.M George
Thin Solid Films 2003 Volume 436(Issue 2) pp:145-156
Publication Date(Web):31 July 2003
DOI:10.1016/S0040-6090(03)00533-9
Surface chemistry and film growth were examined during titanium nitride (TiN) atomic layer deposition (ALD) using sequential exposures of tetrakis-dimethylamino titanium (TDMAT) and NH3. This ALD system is shown to be far from ideal and illustrates many potential problems that may affect ALD processing. These studies were performed using in situ Fourier transform infrared (FTIR) techniques and quartz crystal microbalance (QCM) measurements. Ex situ measurements also analyzed the properties of the TiN ALD films. The FTIR studies revealed that TDMAT reacts with NHx* species on the TiN surface following NH3 exposures to deposit new Ti(N(CH3)2)x* species. Subsequent NH3 exposure consumes the dimethylamino species and regenerates the NHx* species. These observations are consistent with transamination exchange reactions during the TDMAT and NH3 exposures. QCM studies determined that the TDMAT and NH3 reactions are nearly self-limiting. However, slow continual growth occurs with long TDMAT exposures. In addition, the TiN ALD growth rate increases progressively with growth temperature. The resistivities of the TiN ALD films were ⩾104 μΩ cm and the densities were ⩽3 g/cm3 corresponding to a porosity of ∼40%. The high porosity allows facile oxidation of the TiN films and lowers the film resistivities. These high film porosities will seriously impair the use of these TiN ALD films as diffusion barriers.
Co-reporter:J.W. Elam, Z.A. Sechrist, S.M. George
Thin Solid Films 2002 Volume 414(Issue 1) pp:43-55
Publication Date(Web):1 July 2002
DOI:10.1016/S0040-6090(02)00427-3
Nanolaminates are unique nanocomposites that allow various thin film properties to be tuned by changing the composition and interfacial density. ZnO/Al2O3 nanolaminates allow the surface roughness to be controlled because ZnO is crystalline and Al2O3 is amorphous at low deposition temperatures. ZnO/Al2O3 nanolaminates were grown using atomic layer deposition (ALD) methods. The ZnO and Al2O3 films were deposited at 450 K using alternating diethyl zinc/H2O exposures and trimethyl aluminum/H2O exposures, respectively. The growth rate and surface topography of the pure oxide films were examined using ex situ ellipsometry, stylus profilometry and atomic force microscopy (AFM) techniques. The ZnO ALD films grew at 2.01 Å/cycle and roughened significantly versus film thickness because of the presence of ZnO nanocrystals. In contrast, the Al2O3 ALD films grew at 1.29 Å/cycle and remained remarkably smooth versus film thickness because of their amorphous structure. In situ quartz crystal microbalance measurements were employed to study the ZnO/Al2O3 nanolaminate growth. The individual ZnO and Al2O3 nanolayers nucleated and grew easily on each other with minimal interfacial effects. ZnO/Al2O3 nanolaminate films were prepared where the number of ZnO/Al2O3 interfaces was varied but the total thickness remained constant at ∼1250 Å. Using AFM techniques, the RMS roughness of the nanolaminates was observed to decrease substantially with increasing ZnO/Al2O3 interfacial density. The Al2O3 nanolayer interrupts the ZnO crystal growth and forces the ZnO nanolayer to renucleate on the Al2O3 surface. AFM measurements revealed that a single trimethyl aluminum/H2O reaction cycle was sufficient to reduce markedly the surface roughness of the ZnO/Al2O3 nanolaminate films. ZnO/Al2O3 nanolaminates should be useful to fabricate a surface topography with a controlled surface roughness.
Co-reporter:J.D Ferguson, A.W Weimer, S.M George
Thin Solid Films 2002 Volume 413(1–2) pp:16-25
Publication Date(Web):24 June 2002
DOI:10.1016/S0040-6090(02)00431-5
The atomic layer deposition (ALD) of boron nitride (BN) was demonstrated on ZrO2 particles. The BN ALD was accomplished by splitting the binary chemical vapor deposition reaction, BCl3+NH3→BN+3HCl, into BCl3 and NH3 half-reactions. BCl3 and NH3 were alternately applied in an ABAB… reaction sequence at 500 K. Fourier transform infrared (FTIR) spectroscopy observed that the OH stretching vibration of the ZrOH* surface species on the initial ZrO2 particles was removed by the first BCl3 exposure. NH2 asymmetric and symmetric stretching vibrations attributed to BNH2* dihydride species and NH stretching vibrations assigned to B2NH* monohydride species were observed after the subsequent NH3 exposure. The BNH2* and B2NH* species were removed and added after the BCl3 and NH3 exposures, respectively. The surface species were monitored during the first 26 AB cycles. FTIR spectroscopy was also used to monitor the bulk BN vibrational feature that grew progressively throughout the 26 AB cycles. After the 26 AB cycles at 500 K, transmission electron microscopy studies revealed uniform and conformal BN films with a thickness of ∼25 Å on the ZrO2 particles.
Co-reporter:M.D. Groner, J.W. Elam, F.H. Fabreguette, S.M. George
Thin Solid Films 2002 Volume 413(1–2) pp:186-197
Publication Date(Web):24 June 2002
DOI:10.1016/S0040-6090(02)00438-8
Al2O3 films with thicknesses ranging from 30 to 3540 Å were grown in a viscous flow reactor using atomic layer deposition (ALD) with trimethylaluminum and water as the reactants. Growth temperatures ranged from 125 to 425 °C. The Al2O3 ALD films were deposited successfully on a variety of substrates including Au, Co, Cr, Cu, Mo, Ni, NiFe, NiMn, Pt, PtMn, Si, stainless steel, W, and ZnO. Electrical properties were characterized by current–voltage and capacitance–voltage measurements using a mercury probe. These measurements focused mainly on Al2O3 ALD films deposited on n-type Si(1 0 0) and on Mo-coated Si(1 0 0) substrates. Excellent insulating properties were observed for nearly all of the Al2O3 films. For a typical Al2O3 ALD film with a 120 Å thickness, leakage currents of <1 nA/cm2 were observed at an applied electric field of 2 MV/cm. Fowler–Nordheim tunneling was observed at high electric fields and dielectric breakdown occurred only at ⩾5 MV/cm. Dielectric constants of k∼7.6 were measured for thick Al2O3 ALD films. The measured dielectric constant decreased with decreasing Al2O3 film thickness and suggested the presence of a thin interfacial oxide layer. For Al2O3 ALD films grown on n-type Si(1 0 0), capacitance measurements were consistent with an interfacial layer with a SiO2 equivalent oxide thickness of 11 Å. Spectroscopic ellipsometry investigations also were in agreement with a SiO2 interfacial layer with a 13 Å thickness.
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