Low temperature atomic layer growth of aluminum nitride on Si(100) using dimethylethylamine alane and 1,1-dimethylhydrazine

Thin Solid Films 372 Ž2000. 10᎐24

Low temperature atomic layer growth of aluminum nitride on SiŽ100. using dimethylethylamine alane and 1,1-dimethylhydrazine David W. Robinson, J.W. Rogers Jr.1,U Department of Chemical Engineering, Box 351750, Uni¨ ersity of Washington, Seattle, WA 98195-1750, USA Received 17 July 1999; received in revised form 29 March 2000; accepted 17 May 2000

Abstract Surface chemistry investigations of 1,1-dimethylhydrazine ŽDMHy. and dimethylethylamine alane ŽDMEAA. on SiŽ100. and DMHy on aluminum were conducted using X-ray photoelectron spectroscopy ŽXPS. and temperature programmed desorption ŽTPD.. DMHy and DMEAA both showed adsorption and decomposition behavior under certain conditions on SiŽ100. that was promising for aluminum nitride ŽAlN. deposition. For DMHy, a temperature window existed between approximately 600᎐720 K where adsorption of only NHx Ž xs 0᎐2. species occurred. DMEAA adsorption was self-limiting below approximately 420 K with the predominant species at 420 K being AlHy Ž y s 0᎐3.. DMHy also showed promising adsorption and decomposition behavior under certain conditions on an aluminum film deposited on SiŽ100.. At 660 K, nitridation of aluminum is observed with carbon contamination at the noise level. Using these results, a growth strategy was developed for the deposition of AlN thin films at low temperature employing a temperature modulated atomic layer growth ŽALG. process. Exposure temperatures of 420 K for the DMEAA and 660 K for DMHy were selected. The growth strategy was implemented in an investigation of the first few cycles of ALG, and AlN was successfully deposited with carbon contamination at the noise level. No distinct advantage was observed for beginning the cycle with either DMEAA or DMHy. In either sequence, the growth proceeded primarily through the dehydrogenation of AlHy NHx , where regeneration of these species occurred in each cycle. This was also found to be the primary mechanism for ALG of AlN under similar processing conditions using DMEAA and ammonia. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Atomic layer growth; Dimethylethylamine alane; Dimethylhydrazine; Aluminum nitride; SiŽ100.; X-Ray photoelectron spectroscopy; Temperature programmed desorption; Chemical vapor deposition; Deposition process

1. Introduction AlN is a direct wide bandgap Ž6.2 eV. material with excellent physical properties that make it useful for a wide variety of solid state device applications. For many U

Corresponding author. Tel.: q1-506-376-1833; fax: q1-509-3765106. E-mail address: [email protected] ŽJ.W. Rogers Jr... 1 Permanent address: William R. Wiley Environmental Molecular Sciences Laboratory, MSIN: K8-93, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA.

of these applications, deposition of the nitride films is commonly completed by delivering the sources Žprecursors. to the growth surface through the gas phase. The most commonly used nitrogen source is ammonia w1᎐4x. The major disadvantage of ammonia is its low dissociation efficiency on the growth surface, which is attributed to the strength of the N᎐H bond Ž435 kJ moly1 w5x.. To overcome the low dissociation efficiency, high growth temperatures and a high overpressure of ammonia are typically used. Alternate nitrogen sources have been sought in applications: Ži. where high temperature processing steps cannot be tolerated; Žii. where

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D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

high overpressures of ammonia lead to undesired gas phase reactions w2,3,6x; or Žiii. where the low dissociation efficiency of ammonia does not result in sufficient nitrogen incorporation w7᎐9x. One possible alternate nitrogen source is 1,1-dimethylhydrazine ŽDMHy.. DMHy is a liquid at room temperature with a vapor pressure of approximately 157 torr w10x. It is much more stable than hydrazine, which is an explosive, and DMHy is commercially available in semiconductor grade purity. DMHy is an attractive nitrogen source because of its potential to dissociate with higher efficiency on the growth surface compared to ammonia. This is due to the strength of the N᎐N bond, which has a dissociation energy of 264 kJ moly1 w5x. DMHy has been utilized as the nitrogen source in the deposition of thin films of GaN on GaAs w11᎐16x, sapphire w17x and SiŽ111. w18x substrates, GaAsN on GaAs w7᎐9x and GaInNAs on GaAs w19x. As a result of the carbon and oxygen contamination and high growth temperatures associated with the trialkylaluminum precursors, alternate aluminum precursors have been sought. Alane would be an ideal aluminum precursor due to the elimination of the Al᎐C bond, but it is unstable and polymerizes into an involatile polymeric aluminum trihydride Žpolyalane. w20x. Alane, which is a strong Lewis acid, can be stabilized by an electron donor ŽLewis base.. Basestabilization of alane with an amine has resulted in the synthesis of a number amine alane precursors including dimethylethylamine alane ŽDMEAA.. These precursors contain no Al᎐C bond and react with water and oxygen to form involatile products. Most importantly, the low dissociation energy of the amine-alane covalently-coordinated bond and the stability of the volatile amine provides an attractive mechanism for facile desorption of the carbon containing amine from the growth surface at relatively low temperatures. DMEAA also has the advantage of being a liquid at room temperature, and it is available in higher purity compared to the other amine alanes because its synthesis does not involve the use of oxygenated solvents Žonly alkane solvents.. Synthesis of other amine alanes such as trimethylamine alane ŽTMAA. requires ether that may be transported to the film surface and result in oxygen incorporation w21,22x. The disadvantage of DMEAA is that it is not thermally robust. It slowly decomposes at room temperature with the evolution of hydrogen ŽH2 . that may result in dangerous pressure levels in large samples w23x. DMEAA has been used successfully in the growth of III᎐V compounds including AlAs w21,24x, AlGaAs w21x, AlSb w25x and AlAsSb w25,26x. Kidder et al. have deposited AlN by atomic layer growth using DMEAA and ammonia w27᎐29x. Films deposited at 613 K had the best properties. One promising variation of a typical metalorganic chemical vapor deposition process ŽMOCVD. process

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is the alternate introduction of the precursors into the reactor. This type of process is referred to as atomic layer growth ŽALG.. In an atmospheric pressure ALG process, an inert gas purge step is inserted in each cycle between the precursor pulses. This prevents gas phase mixing of the precursors and, therefore, prevents undesired gas phase reactions. In a lower pressure ALG process, inert gas purge steps in each cycle may be utilized or a pump-out step Žtime. may be added to the process between precursor exposures. In an ideal ALG process, each precursor exposure results in self-limiting adsorption and reaction of the precursor at a single layer. Under certain conditions, self-limiting adsorption of a single precursor is adequate for ALG. Growing a film layer-by-layer should allow for simple and accurate thickness control, reproducibility, and good film stoichiometry and uniformity. These properties have been demonstrated in group II᎐VI and III᎐V compounds w30᎐35x. Lower growth temperatures may also be possible with ALG because of a more active surface andror increased time for the species to migrate over the surface. Lower growth temperatures have been demonstrated for AlN and other group III-nitrides w6,27, 36᎐39x. This paper begins with a presentation of results from adsorption and decomposition studies of DMHy and DMEAA on SiŽ100. and DMHy on aluminum. From these results, a growth strategy for the ALG of AlN on SiŽ100. using DMHy and DMEAA was developed. The growth strategy and the results from its implementation are presented and discussed, and these results are compared with those where ammonia was used as the nitrogen source. 2. Experiment The experiments were conducted in a stainless steel ultrahigh vacuum ŽUHV. chamber which has been described previously w40x. The UHV chamber has a base pressure of ; 1 = 10y1 0 torr. In this investigation, a non-monochromatized MgŽK ␣ . X-ray source and a hemispherical sector analyzer ŽHSA. were used for X-ray photoelectron spectroscopy ŽXPS., a quadrupole mass spectrometer was used for temperature programmed desorption ŽTPD., and an ion gun and controller was used for inert ion sputtering. The silicon sample Ž14 mm= 18 mm. was cut from a p-type boron doped SiŽ100. Ž0.5᎐1 ⍀-cm. wafer. The sample was mounted via a sample holder to the feedthrough on the end of a manipulator rod. The sample was secured to the sample holder by tantalum foils that were spot welded to tantalum clips that sandwiched the sample on the corners. The sample temperature was measured with type K Žchromelralumel. thermocouple wire spot welded to the tantalum clips. A DC power supply, which was remotely controlled with a tempera-

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D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

ture controller, was used for resistive heating of the sample. The sample temperature was periodically calibrated by exposing a clean SiŽ100. surface to ammonia at 310 K. Molecular hydrogen ŽH2 . desorption was then monitored in a TPD experiment. H2 recombinative desorption from the monohydride state on a SiŽ100. surface is known to occur at ; 780 K w41᎐45x. The SiŽ100. sample was rinsed with methanol prior to its insertion into the UHV chamber. Before each experiment, the SiŽ100. sample was cleaned by sputtering with 1 keV Arq bombardment followed by annealing at ; 1140 K for ; 10 min. The sample was then cooled to ; 310 K, and its cleanliness verified by XPS. This type of cleaning procedure Žsputterrcleanrcool. is known to result in a Ž2 = 1. reconstructed SiŽ100. surface w41,42,46,47x. After cleaning the SiŽ100. sample, a very low level Ža few percent. nickel impurity was always observed. Nickel contamination is commonly observed after sputter cleaning a silicon substrate w48x. It may result from a number of sources including the tantalum clips, thermocouple wire and the sputter gun. XPS was performed at 10 kV, 30 mA with the sample near room temperature Ž; 310 K.. To increase surface sensitivity, the sample was placed at an angle of ; 65⬚ between the surface normal and the direction of the analyzer. The adjustable window size on the HSA and the sample position were fixed throughout all of the experiments. The high resolution scans had a span of 15 eV for each element, and the experiments were performed in fixed analyzer transmission ŽFAT. mode with a retarding energy of 22 eV that gave a maximum resolution of approximately 1.0 eV. The work function of the analyzer was determined using a gold sample ŽAuŽ4f.7r2 s 84.00 eV., and it was found to be 4.55 eV. All XPS binding energies are referenced to the Fermi level, and the SiŽ2p. binding energy of clean SiŽ100. was found to be 99.15" 0.05 eV. The XPS curve fits assume a 75% Gaussian᎐25% Lorentzian line shape where a Shirley-type w49x background subtraction was used prior to the fit. Semiconductor grade DMEAA ŽSchumacher. and DMHy ŽMorton International . were received in standard two valve stainless bubblers. A carrier gas was not employed, and the bubblers were operated as single valve, high vacuum evaporators where the outlet valve was opened for a few seconds prior to an exposure to allow the source vapor to expand into the gas handling system. An exposure consisted of opening a leak valve to the UHV chamber until the chamber pressured reached a desired value where it was held for a measured time period. All exposures are reported in Langmuirs Ž1 L s 1 = 10y6 torr-s. where the pressure is defined in terms of the background pressure at the time of the exposure.

Fig. 1. NŽ1s.rSiŽ2p. ratios as a function of the SiŽ100. temperature during an exposure to DMHy. A saturation exposure of ; 1800 L was used at temperatures between 310 and 780 K and an exposure of ; 3600 L at temperatures between 840 and 1140 K.

3. Results and discussion 3.1. DMHy r Si(100) We have previously investigated the adsorption and decomposition of DMHy on SiŽ100. to assess its potential as a nitridant of SiŽ100. and as a nitrogen source for nitride films w50x. The study included a series of isothermal adsorption experiments. In these experiments, dosages between 1800 and 3600 L were employed. A typical dosage of 3600 L consisted of exposing the clean SiŽ100. held at the desired temperature to a background DMHy pressure of ; 6 = 10y6 torr for 10 min. In this range of exposure, DMHy adsorption appeared to saturate as judged by the SiŽ2p. and NŽ1s. XPS signals. The saturation of nitridation has been observed previously in the case of ammonia w46,51᎐53x, hydrazine w51,54x and DMHy w55x. Following the exposure, the sample was cooled to ; 310 K, and the XPS spectra were acquired. Figs. 1 and 2 summarize some of the key results in this investigation that are pertinent towards developing a growth strategy for ALG of AlN on SiŽ100.. Fig. 1 shows the NŽ1s. peak area which has been normalized using the clean SiŽ2p. area acquired prior to the exposure. At temperatures below 720 K, the SiŽ2p. substrate signal Žnot shown. was found to be negligibly attenuated. In this temperature range Ž310᎐660 K., the nitrogen species were confined to the surface or near surface region, and the adsorption was self-limiting. That is, multilayer adsorption was not observed. When the substrate signal is negligibly atten-

D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

Fig. 2. The atomic percent carbon calculated from the XPS peak areas as a function of the SiŽ100. temperature during an exposure to DMHy. A saturation exposure of ; 1800 L was used at temperatures between 310 and 780 K and an exposure of ; 3600 L at temperatures between 840 and 1140 K.

uated, the NŽ1s.rSiŽ2p. ratio is directly proportional to the coverage of nitrogen species w46x. The decrease in the coverage of nitrogen species from 310 to ; 600 K was attributed to an increase in the decomposition of the DMHy. The decrease in coverage was concomitant with the dehydrogenation of Si᎐NH2 species, therefore, the passivation of active sites by hydrogen ŽSi᎐H. appeared to be occurring. At temperatures above 720 K, which was the temperature where the onset of H2 desorption was observed in TPD experiments Žresults not shown., the nitrogen area increased dramatically. The increase was due to diffusion of nitrogen atoms into the silicon resulting in SiNx . The quantification of the carbon contamination in the isothermal exposure experiments is shown in Fig. 2. At low temperatures Ž310᎐420 K., the atomic percent carbon was found to be at a maximum due to the adsorption of N-dimethyl containing species. Carbon

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species adsorption decreased substantially between 420 and 600 K. This was in the temperature range where dimethylamine desorption was observed in TPD experiments Žresults not shown.. Between 600 and 720 K, the atomic percent carbon is below XPS detection. In this temperature window, nitrogen was delivered to the SiŽ100. surface as NHx Ž xs 0᎐2. species. Above 720 K, where nitridation of the silicon to SiNx was occurring, carbon was again observed. The carbon resulted from N᎐C bond cleavage and the transfer of methyl species to the silicon surface. At exposure temperatures above ; 900 K, the methyl species were completely dehydrogenated to form silicon carbide. While there was some silicon carbide present at high temperatures, it appears that the percent of carbon remained relatively constant Ž6%. and did not continue to increase with temperature. Therefore, it appears that temperatures greater than 720 K were necessary to cleave the N᎐C bond, and carbon contamination near 6 at.% can be expected in SiNx films deposited by direct thermal nitridation of SiŽ100. using DMHy. 3.2. DMEAAr Si(100) A series of isothermal adsorption experiments were conducted to determine the adsorption behavior of DMEAA on SiŽ100. at temperatures that are of interest for aluminum-containing film growth. In these experiments, a clean SiŽ100. substrate held at the desired temperature was exposed to DMEAA at a pressure of ; 1.0= 10y5 torr for 20 min. An exposure of this magnitude was more than sufficient to saturate the surface at 310 K. Following the exposure, the sample was cooled to ; 310 K, and XPS spectra were acquired. Fig. 3 shows a series of AlŽ2p. and CŽ1s. XPS spectra acquired following the DMEAA exposure at the stated temperature. Following the DMEAA exposure at 310 K, an asymmetric AlŽ2p. signal is observed at 73.8 eV with a FWHM of ; 1.8 eV. The asymmetry and FWHM

Fig. 3. High resolution XPS spectra in the Ža. AlŽ2p. and Žb. CŽ1s. regions acquired following exposure of SiŽ100., held at the stated temperatures, to DMEAA. The exposure was for 20 min at a pressure of ; 1.0= 10y5 torr.

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D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

of the peak suggest that it is composed of more than one feature. If fit as two sub-peaks with the FWHM constrained to 1.3 eV, peaks at 74.2 eV and 73.3 eV result. The constraint is based on the FWHM found for aluminum metal Žsee below.. Aluminum metal has a reported binding energy between 72.7 and 72.8 eV w33,56᎐59x. TMAA condensed on gold has a binding energy of 73.7 eV w60x. Liu has reported that TMAA adsorbs both molecularly and dissociatively on SiŽ100. at 320 K w61x. The binding energy of the molecularly adsorbed TMAA is 74.3 eV, and the dissociated component wAlHy Ž y s 1᎐3.x has a binding energy of 73.2 eV. Therefore, it appears that DMEAA also adsorbs both molecularly and dissociatively on SiŽ100. at 310 K. The 74.2 eV peak is assigned to molecularly adsorbed DMEAA and the 73.3 eV peak to AlHy Ž y s 1᎐3.. The exposure at 420 K results in an identical peak area, binding energy, and FWHM. At 435 K, the peak shifts to lower binding energy and the area has increased. By 480 K, a large AlŽ2p. peak has developed with a binding energy of 72.8 eV and FWHM of ; 1.2 eV. There is also a small shoulder to higher binding energy at ; 73.8 eV. The peak at 72.8 eV is assigned to aluminum, and the shoulder is assigned to AlHy Ž y s 1᎐3.. The CŽ1s. signal shows dramatic changes at temperatures above 400 K compared to 310 K. The CŽ1s. is just above noise level at 420 K, at noise level at 435 K, and below detection by 480 K. The NŽ1s. signal Žnot shown. is below detection at temperatures above 400 K. It is interesting that there was no appreciable peak shift in the AlŽ2p. at 400 K while the CŽ1s. and NŽ1s. signals dropped drastically. This supports the experimental and theoretical work by Fauquet et al. w60x. They found that the bond between the alane moiety and the nitrogen in TMAA did not extensively change the charge density on the TMA ligand. It appears that the DMEA ligand, which is present at most at a few atomic percent at 400 K, also does not extensively change the charge density of the alane moiety. Fig. 4 shows the normalized AlŽ2p. peak area from the results presented in Fig. 3, as well as a number of other isothermal adsorption experiments using the same experimental procedure. The AlŽ2p. signal is normalized using the clean SiŽ2p. signal measured prior to the exposure. As Fig. 3 showed, the AlŽ2p. signal has increased at ; 435 K compared to lower temperatures. It is evident in Fig. 4 that an exposure temperature of 435 K is on the leading edge of a sharp increase in the AlŽ2p. signal. The AlŽ2p. area increases dramatically from 420 to 480 K. Above 480 K, the signal levels off. The leveling off is attributed to aluminum desorption that will compete with aluminum adsorption at these exposure temperatures. In the case of atomic aluminum deposition on SiŽ100., Zhu et al. found that re-evaporation was no longer negligible at temperatures greater

Fig. 4. The normalized AlŽ2p. area as a function of the SiŽ100. temperature during a 20-min DMEAA exposure at a pressure of ; 1.0= 10y5 torr. The SiŽ2p. area acquired from the clean SiŽ100. substrate prior to the exposure was used for the normalization.

than 400 K, and the probability for re-evaporation increases with increasing temperature w62x. The adsorption of TMAA has been shown to be self-limiting on SiŽ111. w63x and SiŽ100. w64x surfaces at temperatures below ; 400 K. Figs. 3 and 4 show that at temperatures below ; 420 K, DMEAA adsorption is self-limiting with the predominant aluminum species being AlHy . Aluminum metal becomes the predominant aluminum species at temperatures above ; 435 K. By 480 K, aluminum film deposition is occurring as observed by the large increase in the AlŽ2p. signal and large attenuation in the SiŽ2p. substrate peak Žnot ˚ which was shown.. The film has a thickness of ; 7 A calculated using the SiŽ2p. attenuation and a simple attenuation model based on Beer’s law w65x. Following the saturation exposure of dimethylethylamine alane ŽDMEAA. on a clean SiŽ100. surface at 310 K, a TPD experiment was conducted. Three desorption products were observed. Fig. 5 shows the TPD spectra for mrzs 2, 26 and 58 acquired during the experiment. Dimethylethylamine Ž mrz s 58., which desorbed at ; 415 K, was the first species detected. This was the same temperature where molecular dimethylethylamine ŽDMEA. desorption was observed following a DMEA exposure on SiŽ100. at 310 K Ždata not shown.. The second desorption product detected was ethylene Ž mrzs 26.. As Fig. 5 shows, ethylene desorption occurred at ; 575 K. This is also the same temperature ethylene desorption was observed following a DMEA exposure. Ethylene desorption has also been observed in this temperature range Ž550᎐640 K. following exposing SiŽ100. to either ethylene with or without atomic hydrogen w66x, ethylbromide w67x, or

D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

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3. the dehydrogenation of AlHy species; and 4. the desorption of H2 .

Fig. 5. TPD spectra acquired for mrzs 2, 26 and 58 following a saturation exposure of DMEAA on SiŽ100. at 310 K. The heating rate was 2 K sy1 .

TMAA w61x. A low temperature peak was also detected in the mrzs 26 spectrum. This peak is attributed to a cracking fragment of DMEA in the mass spectrometer. All of the expected cracking fragments for DMEA and ethylene were also observed, but mrzs 58 and 26 are presented due to their strong intensity. The final desorption product detected was H2 Ž mrzs 2.. H2 desorption was observed at both low and high temperatures. The high temperature peak at ; 780 K is characteristic of H2 recombinative desorption from the monohydride state ŽSi᎐H. on a SiŽ100. surface. The decomposition of a number of hydrogen containing molecules on a SiŽ100. surface including ammonia w41᎐45x, ethylbromide w67x, dimethylhydrazine w4x, hydrazoic acid w68x and hydrazine w69,70x have resulted in H2 desorption at a similar temperature, and in all the cases, the desorption was attributed to H2 recombinative desorption from the monohydride state. The low temperature H2 desorption is attributed to the dehydrogenation of alane. H2 desorption due to the decomposition of alane to aluminum metal at low temperature Ž; 330᎐420 K. has been observed following adsorption and decomposition of DMEAA w71x and trimethylamine alane ŽTMAA. w63x on AlŽ111. and TMAA w61x on SiŽ100.. Unfortunately, it is difficult to differentiate the mrzs 2 signal due to H2 desorption from the surface from that due to the cracking of DMEA and ethylene in the mass spectrometer. Therefore, the peak desorption temperature is not known. There appear to be four predominant reactions leading to aluminum metal deposition. They are: 1. the cleavage of the amine-alane bond; 2. the desorption of DMEA from the SiŽ100. surface;

It is important to distinguish the two different pathways toward hydrogen desorption. Recombinative desorption of hydrogen from the monohydride state on a SiŽ100. surface typically occurs at ; 780 K. The temperature may vary slightly depending on whether the source of hydrogen is from atomic hydrogen or the decomposition of a hydrogen containing molecule on the SiŽ100. surface w42x. Recombinative desorption on an aluminum surface is observed at ; 330 K w72᎐74x. Hydrogen desorption from an approximately three monolayer aluminum film deposited on SiŽ111. has been observed at ; 345 K w75x. Therefore, if we observe low temperature hydrogen desorption, it involves a reaction between Al᎐H species rather than Si᎐H species. In the case of aluminum deposition from the adsorption of DMEAA on SiŽ100., the rate-limiting reaction during the initial stages of deposition appears to be either the dehydrogenation of the AlHy species or the desorption of DMEA. This assumes that recombinative desorption of the hydrogen can occur at temperatures below 400 K as observed on aluminum films. Based on the XPS results at 420 K Žsee Fig. 3., which show that the predominant species on the surface is AlHy , it appears that the dehydrogenation of AlHy is rate-limiting. Further investigation is needed to verify this conclusion. Figs. 3 and 4 show that DMEAA is very good source for delivery aluminum metal to a silicon surface with minimal contamination. At 480 K, aluminum metal was deposited with carbon and nitrogen concentrations below XPS detection. Hayama et al. have shown that deposition of aluminum films using DMEAA results in smaller carbon contamination levels compared to those grown by other alkyl aluminum sources such as trimethylaluminum and triisobutylaluminum w76x. To gain more insight into the nucleation and growth of aluminum films on SiŽ100. using DMEAA, it is useful to review the current understanding of the initial stages of growth. There are very few investigations of the growth behavior of aluminum on SiŽ100.-2 = 1 surface using molecular aluminum sources. From XPS data, Liu et al. speculated that TMAA decomposition results in a layer-by-layer growth mode at lower temperatures Ž; 445 K. and a three-dimensional island growth mode at 985 K w61x. Dimethylaluminum hydride ŽDMAH. has been observed to adsorb on SiŽ100. as uniformly distributed aluminum dimer rows that fill into aluminum clusters w77x. The low temperature Ž298᎐373 K. nucleation and growth of aluminum films using atomic aluminum as a source has been extensively investigated. Zhu et al. give a very good review w78x. Upon adsorption, aluminum atoms are found to adsorb as dimers that orient in rows running perpen-

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Fig. 6. High resolution XPS spectra of the Ža. AlŽ2p., Žb. CŽ1s. and Žc. NŽ1s. regions acquired following exposure of an aluminum film, which was held at the stated temperatures, to DMHy at a pressure of ; 1.0= 10y5 torr for 20 min. The aluminum was deposited on SiŽ100. by exposing a clean surface at 480 K to DMEAA at a pressure of ; 1.0= 10y5 torr for 20 min.

dicular to the underlying silicon dimer rows. Each dimer has a periodicity of two silicon unit cells along the row. The growth initially proceeds in a layer-by-layer fashion with a well-ordered monoatomic Al-2 = 2 layer being completed at 0.5 monolayers ŽML.. A monolayer is defined as the silicon atom density on the SiŽ100. surface Ž1 ML s 6.8= 1014 cmy2 .. At coverages greater than 0.5 ML, cluster formation is observed with a disruption and disappearance of the Al-2 = 2 phase. As the growth proceeds, the pseudomorphic adlayers relax and highly oriented islands form with the prominent crystal orientation of the aluminum films being AlŽ110.. Growth of aluminum films on SiŽ100. using DMEAA has also been found to have a crystalline orientation of AlŽ110. w76x. Therefore, we can speculate that the initial stages of growth using DMEAA may also proceed by a similar mechanism as found for the atomic aluminum source. That is, the initial nucleation results in a monoatomic Žno island formation. ordered aluminum layer. This is very promising for growth of binary compounds such as AlN by an ALG process since a layer by layer growth mode would be preserved.

3.3. Nitridation of Al r Si(100)

Aluminum films were deposited on SiŽ100. by exposing clean surfaces at 480 K to DMEAA for 20 min at a pressure of ; 1.0= 10y5 torr. Based on the SiŽ2p. ˚ Next, attenuation, the films had thicknesses of ; 7 A. each substrate was ramped to a desired temperature where it was held during a 20-min DMHy exposure at a pressure of ; 1.0= 10y5 torr. Finally, the substrate was cooled to ; 310 K, and XPS spectra were acquired. Fig. 6 shows the AlŽ2p., CŽ1s., and NŽ1s. spectra acquired following exposure of an aluminum film, which was held at either 310, 480, or 660 K, to DMHy. AlŽ2p., CŽ1s. and NŽ1s. spectra of a typical aluminum film prior to a DMHy exposure are also given for comparison. At 660 K, nitridation of the aluminum is observed with carbon contamination at the noise level. Using the binding energy and FWHM observed for the aluminum metal film as a constraint, the AlŽ2p. spectrum at 660 K is fit as two sub-peaks assigned to aluminum metal at

D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

72.8 eV and Al᎐NHx Ž xs 0᎐2. at 73.8 eV. The fit suggests that ; 73% of the aluminum metal has been nitrided. The NŽ1s. spectrum at 660 K is also fit as two sub-peaks. The main peak at 397.0 eV is assigned to AlN, and the broad ŽFWHM of ; 2.1 eV. high binding energy shoulder at 398.8 eV is assigned to Al᎐NHx Ž xs 1᎐2. species. These assignments are consistent with the reported binding energy for these type of species. The AlŽ2p. and NŽ1s. binding energies of AlN are between 73.8 and 74.4 eV and 396.4 and 397.5 eV w57,59,79᎐84x. Al᎐NHx type species have a wide range of reported NŽ1s. binding energies. The binding energy of Al᎐NH3 is ; 400.0 eV w85,86x, and Al᎐NHx Ž xs 1᎐2. species have binding energies between 398.5 and 399.9 eV w61,86x. The SiŽ2p. spectrum at 660 K Žnot shown. has a very small shoulder at ; 100.6 eV, which was not observed at the lower exposure temperatures; this is assigned to SiNx . As the exposure Žnitridation. temperature was lowered, Fig. 6 shows an increase in carbon species adsorption and a reduction in the nitridation of the aluminum. At 310 K, the NŽ1s. signal is very broad ŽFWHM of ; 3.5 eV. with a binding energy of ; 398.8 eV. The CŽ1s. signal is also broad ŽFWHM of ; 1.9 eV. with a binding energy of ; 286.7 eV. These binding energies are consistent with species that contain N᎐H ŽNHx species., N᎐N and N᎐C bonds w4,69,87x. From these results, a simple nitridation mechanism can be proposed based on the similarity of the nitridation of SiŽ100. using DMHy w50x. At 310 K, partial decomposition of DMHy may occur, but the dimethylamido ŽNMe2 . species still exist on the surface based on the CŽ1s. and NŽ1s. binding energies. By 660 K, the lack of a CŽ1s. signal shows that the carbon is efficiently removed from the surface. This is the same temperature range where dimethylamine desorption results in minimal carbon contamination on a SiŽ100. surface exposed to DMHy w50x. Therefore, dimethylamine is the likely desorption product responsible for the removal of the methyl species in this case too. The large shoulder in the NŽ1s. signal at 660 K that was assigned to Al᎐NHx species is speculated to be a result of the incomplete dehydrogenation of NHx species that reside on the surface. The small shoulder observed on the SiŽ2p. at 660 K, which was assigned to SiNx , appears to result from nitrogen atoms diffusing through the aluminum film. It is interesting to compare DMHy to ammonia. In the case of an identical exposure of ammonia on a thinner ˚ ., no shoulder was observed aluminum film Ž; 4.4 A Žresults not shown.. The ability of nitrogen to diffuse through the film in the case of DMHy may be a result of differences in the surface reaction Ždehydrogenation. rates of DMHy vs. ammonia. Assuming the diffusion is mitigated by a surface reaction, faster generation of nitrogen atoms on the surface would allow more

17

time for diffusion. Kinetic studies have reported that the nitridation rate of hydrogen terminated SiŽ111. using DMHy is five times shorter than that with ammonia w55x. There is also a larger concentration Žlarger NŽ1s. area. of nitrogen species in the case of DMHy which favors nitrogen diffusion. Therefore, it is very possible that the nitridation rates of aluminum using DMHy are faster than ammonia. Kinetic studies are needed to verify this assertion. 3.4. ALG growth strategy During the growth of a film by ALG, a number of process parameters are under the control of the film grower. These include: 1. the substrate temperature during each exposure; 2. the order in which the precursors are exposed to the surface; and 3. the pressure and duration of the exposure Žflux.. The substrate temperature will ideally be selected so that self-limiting adsorption and reaction of the precursor on the surface results in a desired intermediate that allows the growth to proceed in a layer-by-layer mode. If self-limiting adsorption occurs, the pressure and duration of the exposure are simply selected so that saturation occurs. The selection of which precursor is exposed to the clean substrate may also have an effect on the film growth. This will depend on the adsorption and decomposition behavior of the precursor on the substrate. In the ALG of AlN using DMEAA and DMHy, a few specific issues need to be addressed. First, the exposure temperatureŽs. chosen must lead to self-limiting adsorption of DMEAA and DMHy with intermediates that eventually result in AlN. Therefore, the exposure temperatureŽs. selected will ideally induce surface reactions that result in the complete removal of the carbon containing ligands and the complete dehydrogenation of both precursors. Second, the adsorption and decomposition of DMEAA and DMHy on SiŽ100. should be considered in the selection of which precursor is first exposed to the clean substrate. For example, the adsorption of carbon species on the silicon surface due to the precursor decomposition may be important. These issues will be discussed in more detail below. The surface chemistry studies of DMHy on SiŽ100. and DMEAA on SiŽ100. and aluminum presented in Sections 3.1, 3.2 and 3.3 can be utilized in the selection of the exposure temperatureŽs.. In the case of DMHy, self-limiting adsorption on SiŽ100. as NHx Ž xs 0᎐2. species was observed in a temperature window between approximately 600 and 720 K. The nitridation of an aluminum film to AlN with carbon contamination at the noise level occurred at 660 K. Therefore, it appears

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that at 660 K, self-limiting adsorption of DMHy with minimal carbon contamination should occur, and complete dehydrogenation to AlN with minimal carbon should be possible. In the case of DMEAA, self-limiting adsorption was observed below ; 420 K with the predominant adsorbed species on the surface at 420 K being AlHy . Therefore, it appears that 420 K should be a good temperature for self-limiting adsorption of DMEAA with minimal carbon contamination. The selection of which precursor is first exposed to the clean SiŽ100. has been investigated previously in the case of trimethylamine alane ŽTMAA. and ammonia w64x and DMEAA and ammonia w88x. In both investigations, the adsorption of carbon species on the silicon interface due to trimethylamine ŽTMA. or dimethylethylamine ŽDMEA. lead the investigators to begin the ALG cycle with ammonia. By exposing the clean SiŽ100. surface to ammonia, carbon contamination at the substrate-film interface was prevented. An additional advantage of ammonia Ža hard base. was its ability to displace a weaker base ŽDMEA or TMA.. This resulted in carbon-levels below XPS detection during the first few cycles of film growth. In this case, both DMEAA and DMHy have carbon-containing ligands, therefore, both can be a source of carbon contamination. But, we found that both species will adsorb on SiŽ100. with self-limitation and minimal carbon contamination at certain temperatures. Therefore, carbon contamination at the silicon interface will be minimal with an exposure of either DMEAA or DMHy if the adsorption temperature is carefully selected. Investigations using both DMHyrDMEAA and DMEAAr DMHy exposure sequences should show if there is a clear advantage for using one over the other. 3.5. ALG of AlN using DMEAA and DMHy The experimental procedure for the ALG experiments consisted of the following steps. The clean SiŽ100. surface was exposed to either the nitrogen source, DMHy, or the aluminum source, DMEAA, at a temperature selected using the growth strategy discussed above. The surface was then exposed to the other source to complete one cycle. The saturation exposures were selected based on the results found in Sections 3.1 and 3.2. Twenty-minute exposures of DMEAA at ; 1 = 10y5 torr and 10-min exposures of DMHy at 1 = 10y5 torr were selected. After each exposure, the sample was cooled to ; 310 K, and XPS spectra were acquired. This also allowed time to purge Žpump out. the chamber of residual DMEAA or DMHy before the next exposure. This reduced the probability of gas phase interactionsrreactions between the precursors. A typical experiment monitored the growth for two or three cycles. Fig. 7 shows a series of AlŽ2p., NŽ1s. and CŽ1s.

spectra following each exposure in a three-cycle experiment. For each cycle, DMHy was exposed first at 660 K. The DMEAA was then exposed at 420 K to complete the cycle. This will be referred to as a DMHyrDMEAA sequence. After the first DMHy exposure, there is a NŽ1s. peak with a binding energy of 397.9 eV and no CŽ1s. peak. Earlier Žsee Section 3.1., we found that the surface is derivatized with Si᎐NHx Ž xs 0᎐2. and Si᎐H species. Upon exposure of DMEAA, the AlŽ2p. region shows that aluminum species have adsorbed. There is also a shift in the NŽ1s. binding energy, and the CŽ1s. signal again shows no evidence of adsorption of carbon-containing species. The AlŽ2p. signal has a binding energy of 74.4 eV. This binding energy is in the range reported for AlN Ž73.8᎐74.4 eV; w57,59,79᎐84x. and AlH2 ND2 species Ž74.6 eV; w61x.. Since an exposure of DMEAA on SiŽ100. at 420 K results in the adsorption of AlHx species Žsee Section 3.2., the peak is assigned to AlHy NHx Ž y s 1᎐3, x s 0᎐2. species. The adsorption of aluminum species on Si᎐H sites is not likely because both atomic aluminum w89x and dimethylaluminum hydride ŽDMAH. w77x have shown resistance towards adsorption on hydrogen-terminated SiŽ100. surface sites. The NŽ1s. peak has broadened and shifted to lower binding energy. The peak is fit as two sub-peaks of FWHM of ; 1.7 eV that are located at 397.5 and 399.0 eV. The peak at 397.5 eV is assigned to AlN. AlN has a reported binding energy in the range of 396.4᎐397.5 eV w57,79,80,82x. Since the binding energy is shifted to slightly higher binding energy compared to that found in the earlier nitridation studies of aluminum, the peak shift is attributed to some hydrogenation in the AlN. That is, some Al᎐H and N᎐H bonds likely exist. The peak at 399.0 eV is assigned to AlHy NHx Ž xs 1᎐2, y s 1᎐2. species. Al᎐NHx Ž xs 1᎐2. and AlHy NHx species have reported binding energies between 398.5 and 399.9 eV w61,86x. The AlrN ratio was calculated using standard sensitivity factors w56x and found to be ; 0.65. Since an AlrN ratio of less than one indicates that not all of the nitrogen species are bonded to aluminum species, some of the NŽ1s. signal is also due to the remaining Si᎐NHx that exist on the surface. In an ideal ALG process, stoichiometric growth would result in an AlrN ratio of 1.0. The nitrogen-rich environment existing after one cycle may be a result of an insufficient exposure for saturation. As mentioned above, the exposure time was selected based on conditions that result in saturation on a SiŽ100. surface. If the derivatized surface is less reactive Žsmaller sticking probability. towards DMEAA, which appears to be the case, the exposure may have simply been insufficient for saturation. Since the surface contains more than one type of nitrogen species ŽSi᎐NHx , x s 0᎐2., there also may be a difference in the reactivity of each type

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Fig. 7. High resolution XPS spectra in the Ža. AlŽ2p., Žb. NŽ1s. and Žc. CŽ1s. regions acquired after each processing step in the ALG growth of AlN on SiŽ100. using DMHy and DMEAA. Each cycle Ž1᎐3. had sequential saturation exposures of DMHy and DMEAA where the substrate was held at 660 K during the DMHy exposure and 420 K during the DMEAA.

of site. The least reactive sites on the surface are probably the ones that are occupied by the completely dehydrogenated Si᎐NHx species Ži.e. xs 0.. This would be expected since SiNx has been used as a mask material for selective area growth of IIIrV materials deposited in UHV w90᎐92x. The reactivity of the surface will be discussed in more detail in the next section. The second exposure of DMHy resulted in small changes in the AlŽ2p. and NŽ1s. binding energies. The AlŽ2p. peak remained the same except for a very slight Ž0.2 eV. shift to lower binding energy. The NŽ1s. peak area doubled compared with the first DMHy exposure, and it was fit as two sub-peaks at 397.3 eV and 398.6 eV. The shifting in both the AlŽ2p. and NŽ1s. peaks is attributed to the dehydrogenation of the AlHy NHx species to AlN. Similar behavior in the NŽ1s. binding energy has been observed in an investigation of the ALG of AlN using ammonia and TMAA w61x. The NŽ1s. peak at 398.6 eV shows that in addition to AlN Žat 397.3 eV., Al᎐NHx species are present. This is a result of the second DMHy exposure contributing additional NHx species to the surface. The continued

absence of a CŽ1s. peak shows that no carbon-containing species exist in the growing film. Cycle two is complete by the next DMEAA exposure. The AlŽ2p. signal is approximately double in area and shifts slightly to higher binding energy. The binding energy, which is 74.4 eV, is the same as observed after the first DMEAA exposure. Since a CŽ1s. peak is still absent, which would indicate the adsorption of carboncontaining species, the shifting of the AlŽ2p. binding energy back to 74.4 eV is attributed to the formation of new AlHy NHx species on the surface by the addition of AlHy . The NŽ1s. signal is the same area as before the exposure, but the peak has broadened and shifted in binding energy slightly. The peak is fit as two sub-peaks at 397.4 eV with a FWHM of ; 1.7 eV and at 398.6 eV with a FWHM of ; 1.8 eV. The low binding energy peak Ž397.4 eV., which is nearly identical in binding energy, FWHM, and area as found before the exposure, is assigned to AlN. The 398.6 eV peak, which has broadened considerably since before the DMEAA exposure, is assigned to AlHy NHx species. The SiŽ2p. peak Žnot shown. has a very small shoulder at high

20

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Fig. 8. AlrN ratio as a function of processing stage during the ALG growth of AlN on SiŽ100. using DMHy and DMEAA. The process begins on the left with the DMHy exposure which results in an AlrN ratio of zero. Each cycle had sequential saturation exposures of DMHy and DMEAA where the substrate was held at 660 K during the DMHy exposure and 420 K during the DMEAA.

binding energy after this cycle. The shoulder is assigned to SiNx and is attributed to diffusion of nitrogen into the silicon. Since nitrogen-rich conditions ŽAlrN ratio of ; 0.65. were observed upon completion of cycle one, nitrogen is available for diffusion. Cycle three shows similar trends to cycle two. The AlŽ2p. peak shifts from 74.2 eV after the DMHy exposure back to 74.4 eV after the DMEAA exposure. The shifting to 74.2 eV is a result of the dehydrogenation of the AlHy NHx species when the substrate is ramped to 660 K and exposed to DMHy, and the shift back to 74.4 eV is due to the generation of new AlHy NHx species on the surface when it is cooled to 420 K and exposed to DMEAA. The NŽ1s. signal is again fit as two subpeaks. The low binding energy peak Ž397.3" 0.2 eV., which is assigned to AlN, increases in intensity after the DMHy exposures. As stated above, this is due to the dehydrogenation of AlHy NHx species to AlN. The high binding energy peak Ž398.7" 0.2 eV. is assigned to AlHy NHx species. The AlHy NHx species will exist during each cycle because they are being regenerated by the exposures. The CŽ1s. signal still remains near the noise level after three cycles. Therefore, the growth strategy has been effective. It has resulted in the delivery of nitrogen and aluminum species to the surface with the desorption of the carbon-containing ligands and the dehydrogenation of the remaining species to AlN. After each exposure, we calculated the AlrN ratio. Fig. 8 shows the AlrN ratio for the three processing cycles. If a stoichiometric film was growing, a value of 0.5 would be expected after the second and third DMHy exposures and a value of 1.0 after each DMEAA exposure. As mentioned earlier, a value of ; 0.65 was

found upon completion of cycle one. Since stoichiometry was not achieved in the first exposure, stoichiometric film growth is unlikely after cycles two and three. Fig. 8 does show that the film’s stoichiometry is improving after each cycle. At the end of cycle three, an AlrN ratio of nearly 0.8 is observed. Since the reactivity Žadsorption probability. of the NHx derivatized silicon surface towards DMEAA appeared relatively low, the sequence of exposures was reversed so that the SiŽ100. surface was first exposed to DMEAA. Comparisons could then be made between the importance of starting the cycle with either DMEAA or DMHy. After the DMEAA exposure, we found that the surface was derivatized predominately with AlHy Ž xs 1᎐3. species Žsee Section 3.2.. A comparison of the AlŽ2p. peak area showed that it was ; 1.2 times larger than that found upon completion of cycle one in Fig. 7. A NŽ1s. signal below noise level and a CŽ1s. signal just above noise level at ; 286.6 eV showed that only a small amount of molecular DMEAA or DMEA was adsorbed. Next, the surface was exposed to DMHy. A shift in the AlŽ2p. binding energy occurred, a strong NŽ1s. signal was observed, and the small CŽ1s. signal that was observed previously disappeared below noise level. The AlŽ2p. and NŽ1s. peak binding energies were found to be nearly identical to those observed in Fig. 7 and were assigned to similar species ŽAl᎐NHx and AlN.. Using the AlŽ2p. and NŽ1s. areas, we calculated the AlrN ratio to be ; 0.40. This value showed that a nitrogen rich environment exited after the first cycle. It did not appear that the NHx species selectively adsorbed on only Al sites. The SiŽ2p. peak showed evidence of a very small shoulder at high binding energy characteristic of SiNx . It appeared that open sites still existed on the silicon surface, and NHx species adsorbed on these sites. Therefore, there was no advantage in terms of achieving more stoichiometric adsorption of aluminum and nitrogen species by using a DMEAArDMHy sequence. 3.6. A comparison study: ALG of AlN using DMEAA and ammonia For comparison, the ALG of AlN using DMEAA and deuterated ammonia was also investigated. Isotopically labeled ammonia was used simply because of its availability. We refer to deuterated ammonia as ammonia in this section. Fig. 9 shows a series of AlŽ2p., NŽ1s. and CŽ1s. spectra for the first two exposure cycles using a ammoniarDMEAA sequence. In each cycle, ammonia was exposed first at 660 K. The DMEAA was then exposed at 420 K. Identical exposures Žtime and background chamber pressure. were used for the aluminum and nitrogen sources as were employed in the DMHyrDMEAA experiments. Therefore, the results

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Fig. 9. High resolution XPS spectra in the Ža. AlŽ2p., Žb. NŽ1s. and Žc. CŽ1s. regions acquired after each processing step in the ALG growth of AlN on SiŽ100. using ammonia and DMEAA. Each cycle Ž1᎐2. had sequential saturation exposures of ammonia and DMEAA where the substrate was held at 660 K during the ammonia exposure and 420 K during the DMEAA.

presented in Fig. 9 are comparable with the results presented in Fig. 7 for the DMHyrDMEAA ALG process. After the first ammonia exposure, the NŽ1s. peak has a binding energy of 398.0 eV. The binding energy of the peak is nearly identical to that found in Fig. 7 following an exposure of DMHy, and it is characteristic of Si᎐NHx Ž xs 0᎐2. species. The only noticeable difference in the NŽ1s. signal after a clean SiŽ100. surface is exposed to either DMHy or ammonia is in the degree of dehydrogenation of the NHx species. More dehydrogenation was observed when DMHy was the nitridant due to the desorption of dimethylamine w50x. After an exposure of DMEAA, the NŽ1s. signal has broadened and shifted to lower binding energy. The AlŽ2p. region shows that aluminum species have adsorbed, and the CŽ1s. region shows no evidence of adsorbed carbon-containing species. The NŽ1s. peak is nearly identical to that found after one cycle using DMHy and DMEAA. The peak is fit as two sub-peaks of FWHM of ; 1.6 eV that are located at 397.5 eV and 399.0 eV. As previously assigned, the 397.5 eV peak is associated with AlN, and the peak at 399.0 eV is assigned to

AlHy NHx . Since the AlN binding energy is shifted to slightly higher binding energy compared to that found in the earlier nitridation studies of aluminum, the shifting is attributed to some hydrogenation in the AlN Žsome Al᎐H and N᎐H bonds.. The AlŽ2p. signal is rather broad and has an easily observed asymmetry in its line shape. The peak is fit as two sub-peaks at 72.8 eV and 74.4 eV. Both peaks are at binding energies which are characteristic of previous assigned peaks; the low binding energy peak Ž72.8 eV. is characteristic of aluminum metal and the high binding energy peak Ž74.4 eV. is assigned to AlHy NHx species. It is interesting that aluminum metal is observed, which was not detected with DMHy and DMEAA Žsee Fig. 7.. The aluminum metal is most likely not bonded to the AlHy NHx species since hydrogen ŽAl᎐H. inhibits adsorption of additional aluminum species, therefore, the aluminum is probably bonded to the silicon surface. The presence of aluminum metal is surprising since DMEAA adsorbs on silicon as AlHy species at this temperature. We calculated the AlrN ratio, and it was found to be ; 1.4. Slightly Al-rich conditions are expected since aluminum metal was observed.

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After a single cycle of ammonia and DMEAA, the AlrN ratio is ; 1.4 compared with ; 0.65 after a cycle of DMHy and DMEAA. This supports the previous observation that there is a difference in reactivity in the different Si᎐NHx species. We have previously observed a difference in the NŽ1s. line shape following an exposure of DMHy vs. ammonia on a SiŽ100. surface at 660 K w50x. When the peak was fit as two sub-peaks, the surface exposed to ammonia had more hydrogenated species ŽSi᎐NHx , xs 1᎐2. compared with the DMHy-exposed surface. Therefore, it appears that a surface containing more hydrogenated Si᎐NHx species is more reactive. We speculate that the dehydrogenated species are diffusing into a subsurface region. These sites would not be as easily accessible for adsorption as the hydrogenated sites. The hydrogenated sites may be more reactive because they remain at a location on the surface where they are more available for adsorption. They may have a bonding configuration similar to that found for an Si᎐NH2 species where the Si᎐N bond is directed at an angle of 35⬚ from the normal w42x. The XPS results of cycle two are similar to those found for DMHy and DMEAA in Fig. 7. Only subtle differences are observed. After the second ammonia exposure, the AlŽ2p. peak shifted to a binding energy of 74.3 eV. It then shifted to 74.6 eV after the second DMEAA exposure. After both exposures, a very small shoulder Žslightly above noise level. was also observed at 72.8 eV that is assigned to aluminum metal. The binding energy and peak shift of the main peak are similar to that found in the case of DMHy and DMEAA. Again, the binding energies are assigned to AlHy NHx Ž y s 0᎐3, xs 0᎐2. species, and the small peak shift to higher binding energy is attributed to the generation of new AlHy NHx species. After both the ammonia and the DMEAA exposures, the NŽ1s. peak is fit as two peaks. A low binding energy peak at 397.4" 0.1 eV with a FWHM of ; 1.7 eV and a high binding energy peak at 399.0" 0.1 eV with a FWHM of ; 2.1 eV. As observed in cycle one and also in the DMHyrDMEAA process, the low binding energy peak is assigned to AlN, and the broad high binding energy peak is assigned to all the hydrogenated species ŽAlHy NHx .. The high binding energy peak is located at a slightly higher binding energy than observed in Fig. 7 which is attributed to the presence of more hydrogenated species. This is understandable since the NHx species should be more hydrogenated in this case because dimethylamine desorption is not occurring. Dimethylamine desorption is an additional getter for hydrogen in the DMHyrDMEAA process. A final subtle difference that is observable in the XPS results is in the SiŽ2p. peak Žnot shown.. When DMHy was used, a small shoulder at high binding energy due to SiNx was observable, but this shoulder was not present when

Fig. 10. Atomic layer growth model for AlN using DMEAA and either DMHy or ammonia.

ammonia was the nitrogen source. As was mentioned in Section 3.3, the nitridation of aluminum using DMHy also resulted in evidence of SiNx while an identical ammonia exposure did not. The dehydrogenation kinetics must be playing a role. A nitrogen-rich environment Žhigher nitrogen concentration ., which exists after DMHyrDMEAA cycles, also provides a driving force for nitrogen diffusion. This does not exist after ammoniarDMEAA cycles as discussed below. We calculated the AlrN ratio for cycle two, and it found to be ; 0.75 after the ammonia exposure and ; 1.2 upon completion of the cycle. The film is slightly aluminum-rich after two cycles compared to nitrogen rich Žsee Fig. 8. in the case of DMHyrDMEAA. After two cycles of ALG of AlN using TMAA and ammonia, an AlrN ratio near 1.2 was also observed w64x. Therefore, aluminum incorporation during the early stages of film growth using amine alanes appears more favorable when ammonia is the nitrogen source. This is not well understood, but we expect the more hydrogenated NHx species following an ammonia exposure are playing a role. In the ALG of AlN using DMEAA and either DMHy or ammonia, the initial decomposition of the sources on the surface results in the delivery of the aluminum and nitrogen as AlHy and NHx species. Fig. 10 summarizes the ALG growth mechanism by showing a simplified growth model where only the predominant species that were observed by XPS are shown. Upon exposure

D.W. Robinson, J.W. Rogers Jr. r Thin Solid Films 372 (2000) 10᎐24

of the nitrogen source at 660 K, NHx species are adsorbed on the surface. After the adsorption and decomposition of DMEAA on the surface at 420 K, AlHy NHx species are generated where some of the species are completely dehydrogenated Ž x s y s 0.. Ramping the substrate temperature to 660 K results in the complete dehydrogenation of all AlHy NHx species to AlN, and the next nitrogen source exposure followed by DMEAA exposure at 420 K results in the generation of new AlHy NHx species. 4. Conclusions From the investigations of the adsorption and decomposition of DMHy and DMEAA on SiŽ100. and the nitridation of aluminum using DMHy, a growth strategy was developed for the low temperature deposition of AlN on SiŽ100. using a temperature-modulated ALG process. For DMHy, an exposure temperature of 660 K was selected because self-limiting adsorption with minimal carbon contamination should occur and complete dehydrogenation to AlN with minimal carbon should be possible. In the case of DMEAA, self-limiting adsorption was observed below ; 420 K with the predominant adsorbed species on the surface at 420 K being AlHx . Therefore, 420 K was selected as the DMEAA exposure temperature. The growth strategy was implemented in an investigation of the first two or three cycles of ALG, and AlN was successfully deposited with carbon contamination at the noise level for XPS. No distinct advantage was observed for beginning the cycle with either DMEAA or DMHy. In either sequence, the growth proceeded primarily through the dehydrogenation of AlHy NHx species which were regenerated in each cycle. This was also found to be the primary mechanism for ALG of AlN using DMEAA and ammonia. Acknowledgements The authors gratefully acknowledge the National Science Foundation and the United States Air Force Office of Scientific Research for supporting this research under grant number DMR 96-32166, Sharp Microelectronics, Inc., the University of Washington Center for Nanotechnology, and the Ford Motor Company. References w1x H.O. Pierson, Handbook of Chemical Vapor Deposition ŽCVD.: Principles, Technology and Applications, Noyes Publications, Park Ridge, NJ, 1992. w2x D.A. Neumayer, J.G. Ekerdt, Chem. Mater. 8 Ž1996. 9. w3x R.D. Dupuis, J. Cryst. Growth 178 Ž1997. 56. w4x J.L. Armstrong, Y.M. Sun, J.M. White, Appl. Surf. Sci. 120 Ž1997. 299.

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