e
0 Copyright 1990 American Chemical Society
AUGUST 1990 VOLUME 6, NUMBER 8
Articles Infrared Study of AlN Films Prepared by Ion Beam Deposition. 1. Effects of Film Thickness, Aging, and Moisture Ursula Mazur Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 Received August 28,1989. In Final Form: February 28,1990 Specular reflectance FT-IFt spectroscopywas employedto study the changes in the chemicalcomposition of ion beam deposited A1N thin films as a function of film thickness, aging under air and vacuum, and exposure to liquid water. Aluminum nitride films 10, 20, and 40 nm thick were prepared by utilizing a pure aluminum metal target and both pure and 25% hydrogen enriched nitrogen. All films were deposited on gold substrates. It was found that the concentration of the Al-N2 species present in the A1N films strongly depends on the film thickness and on the gas mixture used for film fabrication. We also observed that the AlN films fabricated with pure N1 plasma are less prone to hydrolysis (when exposed to room air) than are the films prepared with H2-N2 plasma. Prolonged vacuum environment had no significant effect on the spectra of the differently prepared AlN films. Exposure to liquid water promoted hydrolysis equally well in N2 and H2-N2 fabricated A1N films.
Introduction A1N is an extensively studied nitride system due to ita desirable optical, electrical, dielectric, and acoustical properties. AlN has a wide band gap (6.3 ev) and possesses a high refractive index along with a high chemical and thermal stability. AlN is used for optical coatings, passivation, and encapsulation of semiconductor surfaces, as well as thin-film transducers, surface acoustic wave devices, and piezoelectric systems. Although there have been many studies addressing the electrical, optical, acoustical, and morphologicalpropertie~l-~~ of A1N thin films, the available ~~
~~~~
(1) Cox, C. A.; Cummins, D. 0.;Kawaba, K.; Tredgold, R. H. J. Phys. Chem. Solids 1967,28,543. (2) Kubiak, C.J. G.; Aita, C. R.; Hinkernell, F. S.;Joseph, S.J. Mater. Res. soc. SvmD. h o c . 1985.47. 75. (3) Nettkield, R. P.; Mhle;, K. H.; McKenzie, D. R.; Goonan, M. J.; Martin, P. J. J . Appl. Phys. 1988,63,760. (4) Hentzell, H. T. G.; Harper, J. M. E.; Cuomo, J. J. J . Appl. Phys. 1985.50. - .- -, .., 556. - - -.
(5) Xinjiao, L.; Zechuan, X.; Ziyou, H.; Wuda, S.;Zhongcai, C.; Feng, Z.; Enguang, W. Thin solid F i l m 1986, 139, 261. (6) Xinjiao, L.; Zechuan, X.; Ziyou, H.; Huazhe, C.;Wuda, S.;Zhongcai, C.; Feng, Z.; Li, S. ZEEE 35th Electr. Comp. Conj. 1985, 442. (7) Fathimulla, A.; Lakhani, A. A. J . Appl. Phys. 1983,54,4586.
molecular vibrational data for this material are not e x t e n s i ~ e . ~We ~ - have ~ ~ recently reported on t h e vibrational structure of AlN films deposited on aluminum substrates.20 These A N films were fabricated by reactive ion beam sputtering utilizing a pure aluminum target and both pure and 25% hydrogen enriched nitrogen. Inelastic electron tunneling spectroscopy, IETS, and FT-IR were (8) Weisamantel, C. J . Vac. Sci. Technol. 1981,18, 179. (9) Sibran, C.; Blanchet, R.; Garriguee, M.; Viktorovich, P. Thin Solid F i l m 1983,103,211. (10) Bhat, S.; Ashok, 5.;Fonash, S. J.; Tongeon, L. J. Electron. Mater. 1985, 14, 405. (11) Weisamantel, C . Thin Solid Films 1981,92,55. (12) Grill, A. Vacuum 1983,33,329. (13) Taylor, J. A.; Rabalais, J. W. J . Chem. Phys. 1981, 75,1735. (14) Hydis, M. J. Appl. Phys. 1973,44,1489. (15) Liu, M. B.; Gruen, D. M.; Kraues, A. R.; Reis, A. H., Jr.; Peterson, S.W . High Temp. Sci. 1978,10, 53. (16) Demiryont,H.;Thompson, R. L.; Collins, G. J. J . Appl. Phys.1986, 59, 3235. (17) Hasegawa, F.; Takahashi, T.; Kubo, K.; Nannichi, Y. Jpn. J . Appl. Phys. 1987,26, 1555. (18) Duchene, J. Thin Solid Films 1971,8,69. (19) Gerova, E. V.; Ivanov, N. A.; Kirov, K. I. Thin Solid Film 1981, 81,201. (20) Mazur, U.; Cuneo Cleary, A. J. Phys. Chem. 1990,94,189.
0743-7463/90/2406-1331$02.50/0 0 1990 American Chemical Society
1332 Langmuir, Vol. 6, No. 8, 1990
employed in analyzing the A1N film structures. We found that the principal component of the depositad nitride films was AlN. AI-H, Al-NZ, and NH, were also present as minor components. In addition, we demonstrated that the ion beam deposited AlN barriers yielded satisfactory AlAlN-Pb IET devices with low noise tunneling spectra. To our knowledge, this was the first detailed vibrational work on AIN thin films. In this study, we report on the changes in the chemical composition of AlN thin films as a function of film thickness, aging under air and vacuum, and exposure to liquid water. To this end, we have employed specular reflectance FT-IR spectroscopy. Whereas in our previous workmwe deposited aluminum nitride layers on aluminum, in this work we utilize gold films as substrates for the deposited nitride films. The use of gold as substrate material was prompted by our concerns that (1)aluminum reacts easily with 0 2 and HzO and that (2) the nitrogen, nitrogen-hydrogen plasma, and/or HzO may react with the aluminum substrate during AlN deposition. These substrate-related side reactions would greatly affect our nitride aging results as well as film thickness measurements. For example, water and oxygen diffusing through the A1N film’s microstructure would not only react with the nitride itself but also oxidize the aluminum underlayers. It would be difficult to estimate the relative amounts of oxide formed from the hydrolysis of the nitride and from the oxidation of the aluminum substrate. Consequently, our A1N film aging study would have little significance. The plasma and/or water reactions with the aluminum substrate during the AlN deposition can produce errors in the nitride film thickness measurements.20 The film thickness monitor registers only the net amount of AlN deposited, while the amount of material formed due to substrate oxidation and to beamsubstrate reaction is not measured. The above difficulties should not occur with gold. The motivation for examining the reactivity of AlN thin films in the presence of air and moisture was to access the effect of varying the gas mixtures used in fabricating AlN films on the rate of the nitride hydrolysis and the chemical composition of the resulting films. This is of importance to optoelectronic technology, where freshly deposited coatings may be exposed to the atmosphere. I t is frequently found that a coating’s optical and electronic properties may vary in an unpredictable and irreversible fashion depending on the relative humidity.21 That is, water absorbs into the microstructure and changes the film’s refractive index as well as its resistivity. Of special interest are the A1N films prepared with Hz-Nz plasma since previous studies indicate that the presence of hydrogen ions in the plasma improves the bulk and interface properties of the f i l m ~ . ~ 2 , ~ 3 Experimental Section Film Deposition. Film fabrication was carried out in a deposition system described earlier.20.24 Specimens for the specular reflectance measurements were prepared as follows. First, Au/Al composite substrates were prepared by resistively depositing a 100-nm-thick Au film onto a 200-nm-thick aluminum base film. Metal films were deposited on a precleaned Pyrex microscope slide at 1 X 10-7 Torr base pressure. The AlN films, (21) Martin, P. J. J.Mater. Sci. 1986, 21, 25. (22) Bhat, S.; Ashok, S.; Fonash, S. J.; Tongson, L. J. Electron. Mater. 1985, 14, 405. (23) Taylor, J. A.; Rabalis, J. W. J. Chem. Phys. 1981, 75, 1735. (24) Mazur, U.; Konkin, M. in Atomic and Molecular Processing of Electronic and Ceramic Materials: Preparation, Characterization, and
Properties. Mater. Res. Soc. Conf.Proc. Aksay, I. A., McVay, G. L., Stoebe, T. G., Wagner, J. F., Eds.; 1988, p 47.
Mazur 10-40 nm thick, were deposited at an average deposition rate of 0.01 nm/s onto unheated Au/M substrates by reactive ion beam sputtering. The ion gun was a 2.5-cm low-energy source purchased from Ion Tech Inc. An Edwards FTM4 film thickness monitor was used to measure the deposition rate as well as thickness of the sputtered films. Both pure N2 and 25% H2 enriched Nz gas (premixed) were employed at 2 X 10-4 Torr total pressure for film fabrication. All gasses used were ultrapure grade. The target employed was a 3-in. aluminum disk (99.999 3‘% pure) purchased from Cerac, Inc. The experimental ion beam voltage and current were lo00 eV and 9.0 mA, respectively. Aging and Moisture Effects Studies. AlN films (20 nm thick) prepared with pure and hydrogen-enriched nitrogen were exposed to room atmosphere for different time lengths. The average relative humidity of air during the aging experiments was 60 7%. Reflectance spectra were acquired immediately upon sample withdrawal from the deposition chamber and after 05, 1.5-, and 3.0-h exposure intervals. AlN films used to study the effects of moisture were treated with deionized liquid water for 15 s following the removal from the chamber and then spun dry. Spectra of the water-treated films were collected subsequently. Appropriate references were employed in all studies. FT-IR.Vibrational spectra were measured by using an IR/ 98 FT-IR spectrometer. The specular reflectance of A1N films was measured with 80° grazing incidence radiation. The incident light was not polarized. The diffuse reflectance spectrum of bulk A1N (99.99% pure from Cerac, Inc.) was obtained from a powdered nitride sample mixed in a 1:3 ratio with KBr placed in a microcup. The A1N sample was prepared and stored under NOuntil the measurement. All FT-IR data were acquired with 8-cm-1 resolution and are the result of 1024 scans. A helicoil source, a germaniumcoated KBr beam splitter, and a nitrogen-cooled MCT detector were employed. All spectra were obtained a t room temperature. In some cases, a low-order polynomial was used to remove a sloping background. This procedure yields flatter base lines. Electron Microprobe. The stoichiometry of the AlN films was determined by using the Camebax electron microprobe from Cameca. Duplicate samples consisted of 50-nm AlN films deposited on gold substrates according to procedures described above. The results were compared with a bulk AlN standard. AlN films prepared with pure Nz were stoichiometric with 49.5 f 1.0 mol % A1 and 50.5 f 1.0 mol % N. Films synthesized with 25% Hz enriched Nz revealed a small deviation from stoichiometry with 46.5 f 1.0 mol % A1 and 53.5 f 1.0 mol % N.
Results and Discussion The vibrational results presented and discussed below are representative of the spectroscopic data collected from duplicate (or more) samples. Figure 1 displays the reflectance spectra of stoichiometric AlN fims, 10 and 40 nm thick, fabricated with pure Nz. Collected in Table I are the peak position and assignments of the vibrational modes present in that figure. The weak band near 2140 cm-l present in the spectrum of the 10-nm-thick film is associated with the Al-NZ species in which the nitrogen molecule is coordinatively bound to an aluminum atom or ion. The position of this band is in good agreement with the MN=N stretching vibration observed in metal dinitrogen complexes,25as well as in NZ absorbed on clean metal surfaces26926 and metals on oxide s ~ p p o r t s . We ~~-~~ have observed this transition earlier both in the reflectance and the absorbance spectra of AlN films deposited on aluminum mirrors.20 Although we do not believe that it (25) Murray, R.; Smith, D. C. Coord. Chem. Rev. 1968,3,429. (26) Slavnikov. V. S.: Petukhov, P. A.: Turbistsw. A. M.: Usynina, N. A. Sou. Phys. J. 1974,17,1189. (27) Oh-Kita, M.; Aika, K.; Urabe, K.; Ozaki, A. J. Catal. 1976,44,460. (28) Oh-Kita.. M.:. Aika.. K.:. Midorikawa, H.: Ozaki, A. J. Catal. 1981, 70,‘384. (29) Kinishita, N.; Kido, K.; Domen, K.; Aika, K.; Onishi, T. J. Chem. Soc., Faraday Trans. 1 1986,82,2269.
Langmuir, Vol. 6, No. 8,1990 1333
ZR Study of A1N Films Prepared by Zon Beam Deposition h
0
--
F l -
0
I O E
u 0
P Fl
N V C U
o+ 0
0
4000
2000 Energy
(cm-1)
Figure 1. Vibrational spectra of AIN films prepared with pure Nz: (a) 10-nm-thickfilm (absorbance maximum = 0.045); (b) 40nm-thick film (absorbancemaximum = 0.147). Table 1. Specular Reflectance Data (em-') for 20-nm-Thick AlN Films. 25% Hz in NZ 660 sh 840 s
pure N2
660 sh
(T0)AlNb 6 or v(A1-N2)E.d
910 s
(LO)AINb 6- or v(Al--Nz)c.d AIN combination bandd 6(NH1 in NHa coordinated to A l b ~ ( N Hin ) NHI+~ or ~ ( N Hin ) AI-NH' A1N combination bandd 6(NH) in NH3 coordinated to Alb 6(NH) in NH3 coordinated to Alb 6(NH) in NH3 coordinated to Alb or nNH3.HlOe v(Al-H)b Y (Al N N )b u(CH)b ~(0-He-N) in NH3.HzOe ~(0-H-N) in 2NHrHzOe 4NH) in NH3 coordinated to A l b v(NH) in NHa coordinated to Al,b NH,+,b or nNHa.HzOl u(NH) in NH3 coordinated to A1,b NH,+) or nNHa-HzOe
950 s 1288 w 1304 m 1385 m 1446 w 1520 m 1590 1612 mf 1831 vw 2141 m
1836 vw 2145 w 2904 vw
2900 shf 2970 wf 3220 wf 3230 m 3255 mf
assi-rrnmenta
a Uncertainty in the optical values is 5 cm-'. References 20, 38, and 39. e Tentative assignment. This work. e Reference 42. f Transitions associated with aged or wet specimens. 8 References 44 and 45.
is very likely,20 another possibility for the 2140-cm-' transition might be the u(CN) mode. The most likely source of the CN fragment would be the reaction between the graphite grids in the ion gun and the ionized nitrogen gas. The residual hydrocarbon contamination in the vacuum chamber can be discounted as a possible carbon supply since there is no substantial evidence for the u(CH) in our IR data. Further, in the spectrum of the 40-nmthick A1N film, Figure l b , the 2140-cm-1 feature is nearly absent. Figure 2 plots the variation in the concentration of the covalently bound nitrogen with changing AlN film thickness for films synthesized with pure N2 and H2-N2 plasma. For the A1N films produced with pure N2, the concentration of the Al-NZ complex reaches a maximum in the films 20+ nm thick and then drops off to negligible amounts in films 140 nm thick. Reported absorbance spectra of A1N films I100 nm thick, prepared either with
25 50 F i l m t h i c k n e s s (nm) Figure 2. Absorbance maximum of the 2140-cm-l transition plotted as a function of AIN film thickness for films prepared with different gas mixtures. Circles represent the data for AIN 0
films produced with 25 % Hz enriched N2, while the crosses are associated with data for films made with pure Nz.
NzAr or N2plasma, show no transitions above lo00 cm-I.l8 In the A1N films made with H2-N2, the A1-N2 concentration rises almost linearly as the film thickness increases. It appears, therefore, that the concentration of the AlN2 species present in the film depends significantly on the gas mixture used for film fabrication. T h e LO A1N phonon near 910 cm-l is the most prominent feature in both traces of Figure 1. This frequency has been associated with the crystalline form of AlN.3*33 Our earlier IETS and FT-IR data on ion beam deposited A1N films support the existence of crystallites in these films.20 We are presently performing scanning tunneling microscopy, STM, experiments in order to learn more about the morphological structure changes in our films as a function of film growth. In the spectrum of the 40-nm-thick A1N film, the LO mode is shifted slightly to lower energy. Works dealing with the analysis of vibrational data obtained from thin-f'ii materials typically attribute shifts in the expected band positions to particular physical and/or chemical properties of the films. For example, the shifts in the maxima of the A1N vibrational bands have been associated with increasing amorphous character.30 Other reported reasons for small frequency variations in the assigned AIN phonons are changes in grain size of the nitride samples and the variation in the dielectric properties of the support medium used in spectral meas~rements.~'Thickness-dependent frequency shifts have also been observed for the Si-0 stretching band in Si02 films. The causes cited for this observation are chemical bonding character and film porosity and den~ity.3~~~5 The broad low-intensity features at 1288 and 1446 cm-1 present in Figure 1are assigned as phonon combination bands. The rationale for this assignment is based on the comparison of energies of these transitions with previously assigned lattice vibrations in crystalline AlN.30 Figure 3 (30) Collins, A. T.; Lightowlers, E. C.; Dean, P. J. Phys. Reu. 1967,258, 833. (31) Carlone, C.; Lakin, K.; Shanks,H. R. J.Appl. Phys. 1984,55,4010. (32) Pastemak, J.; Hejda, B. Phys. Lett. 1969,29A, 314. (33) Brafman, 0.;Lengyel, G.;Mitra, S. S.Solid State Commun.1968, 6, 523. (34) Boyd, I. W.; Wilson, J. I. B. J. Appl. Phys. 1982,53, 4166. (35) Boyd, I. W.; Wilson, J. I. B. Appl. Phys. Lett. 1987, 50, 320.
1334 Langmuir, Vol. 6, No. 8,1990
Mazur r 795
a,
a,
C
U C
n
n
n
D
U
m
m
L
L 0 01
0 01 4
Q
I N 500
,\ w
,\
1
I
1000
1500
2000
Energy (cm-1) Figure 3. Vibrational spectrum of a 20-nm-thick AlN film
prepared with pure N2 with an expanded inset on the 10002000-cm-1 region. Absorbance maximum equals 0.069.
contains an expanded 1000-2000-cm-' region of Figure lb, and Figure 4 displays a similar diffuse reflectance spectrum of bulk AlN. Note that the 91Q-, 1288-, and 1446-cm-I bands are shifted by 115 cm-' to higher energies relative to their 795-, 1173-, and 1331-cm-I equivalents appearing in the diffuse reflectance spectrum of bulk A1N. (The bands appearing in the 1000-2000-cm-' region in Figure 4 have been previously assigned a s AlN phonon combination bands.30) The 115-cm-l difference in energy between the bands observed in the diffuse vs the specular reflectance data can be related to the methods used in obtaining the respective vibrational spectra The diffuse reflectance, DR, method provides transmission-likespectra of powders and rough samples while the specular reflectance, SR, technique is used in obtaining spectra of thin films on mirror surfaces. In SR, the sensitivity depends on the polarization of the incident radiation with selective excitation of vibrations with transition dipole components perpendicular to the metal surface. Thus, the SR spectra may look different from transmission spectra in many ways; e.g., bands may be shifted to higher wavenumbers and some bands may not be observed at T h e primary reason for t h e difference in the A1N refledance spectra in Figures 3 and 4 is the orientational/ polarization dependence in specular reflection. Our assignment of t h e 1288- and 1446-cm-' bands a s combination bands is further substantiated by the vibrational study of AlN f i prepared with pure nitrogen and aged in room atmosphere for variable periods of time. Results of AlN film aging are presented in Figure 5. The spectra were recorded after the immediate removal of the film from the deposition chamber ( t = 0) and after 0.5-, 1.5, and 3.0-h exposure periods. Because large sections of the amassed data overlapped, only the spectra acquired at t = 0 and 1.5 h are presented here. The absorbance maximumof the AlN stretching band measured 0.069 upon immediate removal of the A1N sample from the chamber. This value stayed unchanged as the sample aged in air over a period of 3 h. (The AlN LO phonon is not depicted in Figure 5 because of its disproportionately large intensity relative to intensities of the remaining transitions in the spectra of the aged film.) The number and the intensities (36) Greenler, R. G.; Rhan, R. R.; Shwartz, J. J. Catol. 1971,23, 42. (37) Handke, H.; Paluszkiewicz, C. Infrared Phys. 1984,24, 121.
500
1000
1500
2000
Energy (cm-1) Figure 4. Diffuse reflectance spectrum of powder AlN mixed
with KBr.
I
1000
2000
Energy
3000
4000
(cm-1)
Figure 5. Spectra of a 20-nm-thick AIN film fabricated with a pure N2 plasma following different periods of exposure to room air. The solid trace corresponds to data acquired immediately after sample removal from the deposition chamber (t = 0). The broken curve corresponds to data collected after a 1.5-hexposure to air.
of features found in the 600-2500-~m-~ region remained constant throughout the aging study. The intensity of the 3200-cm-1 band, which is attributed to an NH stretching motion, approximately doubled after a 0.5-h exposure and doubled again after a 1.5-h exposure. No further increase in the intensity of the 3220-cm-' transition was observed after 3.0 h. The observation of v(NH) is a direct consequence of a hydrolysis reaction occurring in the nitride film in the presence of air. This reaction is understood to be 2A1N + 3Hz0 A1203 + 2NH3 The N-H stretching frequency in the spectra of the aged films is, therefore, due to the NH3 molecules present on the nitride surface as species coordinated to aluminum ions. The intensity of the NH stretch is about the same as that of the A1N combination bands (Figure 5) and not very significant when compared to the very strong A1N stretching motion (Figure 1). Consequently,the associated b(NH3) modes which are expected to be weaker then v(NH)
Langmuir, Vol. 6, No. 8,1990 1335
IR Study of A1N Films Prepared by Ion Beam Deposition have no observable intensity in Figure 5. In the Al(NH3L3+complexes (z = l, 3,5, and 6), these transitions are reported to lie in the 1230-1357-cm-1 and 1600-1640cm-l regions, for the symmetric and asymmetric NH deformation modes, respectively.38*39In Figure 5, any NH deformation bands are expected to lie under the A1N combination bands. The extent of hydrolysis of the AlN films prepared with pure NZis not very substantial, -0% ,based on the lack of any significant loss in the intensity of the AlN stretching band in the spectra of the films aged for 3 h in room air. A possible reason for the high stability of the AlN film is that the hydrolysis product, aluminum oxide, actually passivates the nitride and protects it from further reaction with atmospheric water. This hypothesis requires further experimentation. There is no discernible evidence for the presence of A1203 in the spectra of the aged AlN films.The A1-0 stretch in A1203 is located a t 950 cm-l.= Any A1-0 intensity due to the very small amount of A1203 formed during the aging of the AlN films would be found under the LO ALN envelope. A 24-h aging of 20-nm AlN films under vacuum a t Torr did not produce any significant changes in the spectra of those films. The spectra of the vacuum aged films were essentially the same as the spectra of films obtained immediately after fabrication (t = 0). There are three conclusions that can be drawn from our A1N film aging studies. Firstly, the observation that the intensities of the AlN LO phonon as well as the 1288- and 1446-cm-1 bands did not change during the aging study supports our assignment of the latter transitions as A1N combination bands. Secondly, the Al-NZ species appear to be air stable for prolonged periods of time. The third conclusion is that A1N films prepared with pure Nz are at least as air-stable as bulk AlN, which hydrolyzes very slowly under those condition^.^^ Bulk A1N is known to react with 02 a n d d r y HC1 b u t only a t elevated temperatures, M O O 0C.34 An IR study examining the effects of high-temperature treatment of thin A1N films reports no observable changes in the absorption spectra of films annealed in oxygen or nitrogen gas a t 400 0C.5 Although the A1N films prepared in pure NZremained stable in the atmosphere for times on the order of hours, they did suffer partial hydrolysis immediately upon exposure to liquid water. The effect of moisture on these films is depicted in Figure 6. Bands associated with the AlN transitions located in the 600-1800-~m-~ region of the spectrum of the HzO exposed film (Figure 6, broken line) show -20% decrease in the absorbance signal compared to the absorbance of the same bands found in the spectrum of the dry reference film (solid curve). The 20% decrease in absorbance signal is equivalent to a 4-nm decrease in the nitride thickness upon contact between the 20-nmthick AlN film and liquid water. The spectrum of the water-treated film reveals an -30% absorbance loss associated with the Al-NZ species and the appearance of a new feature near 3220 cm-l due to the NH stretch. The NH stretching band is essentially absent in the dry reference spectrum. It appears that direct water contact causes an immediate surface hydrolysis of the top A1N (38) Sipachev, V. A.; Grigor'ev, A. I. Rrurs. J. Inorg. Chem. 1970,15, 905. (39) Schmidt, K.H.; Muller, A. Coord. Chem. Rev. 1976,19,41. (40) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds, 3rd ed.; Wiley: New York, 1978; p 301. (41) C o m p , J.; Hortley, C. C.; Oxley, D. P.; Prichard, R. G.; Tegg, J. L.J. Adhesion 1891, 12, 171. (42) Hoch, M.; Vemardakis, T.; Nair, K. M. Sci. Ceram. 1980,10,227. (43) Bauer, G. Handbook of Preparative Inorganic Chemistry 1965, 2,827.
1
1000
2000
3000
4000
E n e r g y (cm-1) Figure 6. Spectra of 20-nm AlN films prepared with pure Nz. The solid trace was acquired promptly after specimen removal from the deposition chamber. The broken curve is the spectrum of the film exposed to water.
a,
0
2000
4000
E n e r g y (cm-1) Figure 7. Vibrational spectra of AlN films synthesized with 25% Hz enriched Nz: (a) 10-nm-thickfulm (absorbance maximum = 0.0%); (b)40-nm-thickfilm (absorbance maximum = 0.076). Base line correction was performed on both spectra.
layers, which are subsequently washed away. I t is interesting to note that although the Al-NZ species appears to be stable in air it does not survive well the reaction with H20. Whereas the electron microprobe analysis of our A1N films prepared with pure NZ verified stoichiometry, the same analysis of films prepared with concomitantly added HZrevealed an excess N content. The approximate AkN ratio in AlN films synthesized with 25 % HZenriched NZ calculates a t 1:1.15. The measured stoichiometric imbalance in these films is reflected in their vibrational spectra, Figure 7, which are more complicated than the spectra of stoichiometric A1N films synthesized with pure NP,Figure 1. The A1-N2 vibration is quite strong in both traces of Figure 7, especially in trace b, the spectrum on the 40-nm-thick AlN film. Several of the other bands present in Figure 7 have been tentatively assigned by us as originating from NH, type species.20 A review of peak positions and assignments of the vibrational modes present in Figure 7 is presented in Table I.
Mazur
1336 Langmuir, Vol. 6, No. 8,1990 The bands a t 1304 and 1520 cm-1 are due to NH deformation modes in a NH3 molecule coordinated to an A1 atom or ion.20 The band at 1385 cm-l was assigned previously as a NH deformation mode originating from an NH4+ surface ion which is formed from the interaction of a NH3 molecule with a surface-bound H ion.20 Another possibility is that the 1385-cm-I transition is due to an AlNH deformation. Weak bands at 1385and 1370 cm-l were identified as u(M-NH) in the IR spectra of ultrafine powders of alumina and silica heated in NH3 gas a t elevated temperatures, respectively.35 Unfortunately, no related NH stretching motions were reported in that work. In Figure 7, the NH bend near 1600 cm-1 and NH stretch near 3230 cm-' are associated with all the NH, species.20 In that same figure, a very weak AI-H stretch is observed near 1820 cm-1.20 The A1N stretching region in Figure 7 is somewhat different from what we observed in our earlier vibrational work with A1N films on aluminum mirrors.20 The reflectance spectra of A1N films prepared with 25 % H2/ N2 and deposited on aluminum showed a single band near 910 cm-I with a high-energy shoulder near 960 cm-l. In Figure 7b, we observe bands near 840 and 950 cm-l in the spectrum of the 40-nm-thick A1N film. It is important to mention that the envelopes of the AlN phonons occupy the same energy region in the spectra of A1N films on A1 and Au. The maximum at 953 cm-l in Figure 7b shifts from 945, 950, to 953 cm-I as films increase in thickness from 10,20,to 40 nm, respectively. Note this was not the case for the LO phonon in the spectra of A1N films made with pure Nz, Figure 1. In that figure, the LO phonon moved to lower energies with increasing film thickness. The transitions near 840 and 950 cm-I in Figure 7 may be combination bands and/or surface modes. Low-intensity 746-, 794-, 838-, and 950-cm-1 surface modes have been identified as phonon combination bands or surface modes in the IR absorbance spectra of A1N crystalline films.29.30 It is likely, however, that the 840-950-~m-~ region in our FT-IR data is perturbed by the presence of stretching and deformation motions of the Al-Ng complex. The observed shifts in energy, or alternatively rise in intensity, of the 840- and the 950-cm-I bands with increasing film thickness can be correlated with the rise in intensity of the 2140cm-1 band, the u(AlN=N), in the thicker AlN films. In Figure 7, the 2140-cm-l band is about twice as intense in the spectrum of the 40-nm-thick AlN film as it is in the spectrum of the 10-nm-thick film. One would expect, therefore, an increased contribution in the intensity of u(AlN2) and G(Al-N=N) with increase in the concentration of the Al-N2 complex. Unfortunately, no literature data are available concerning u(Al-N2) and G(Al-NEN) modes in Al-Nz, and very little has been reported about lowenergy u(M-N2) and 6(M-N2) motions in general.35 In the osmium and ruthenium complexes of the type [M(Nd(NH3)5In+,the u(M-NZ) is quoted in the 450-550-cm-I An alternate explanation for the 950-cm-1 band in Figure 7 is that it is an A1N LO phonon whose location is sensitive to changes in the regional chemical environment produced by the excess nitrogen content present in the A1N films fabricated with H2-Nz. Finally, we discount the possibility that the 950-cm-l band is u(Al0). This is due to the fact that A1N films prepared with concomitantly added H2 have very low alumina content because of the oxygen-gathering effect of the hydrogen ions.1° Hydrogen ions in plasmas dramatically reduce (by orders of magnitude) residual oxygen content.10 We have already discussed A1N film thickness influence on the concentration of the covalently bound nitrogen in
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1000
I
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Energy
(cm-1) Figure 8. Spectra of a 20-nm-thick AlN film, fabricated with 25% Hz enriched N2, exposed to room atmosphere for different time intervals. The solid trace correspond to data collected immediately after film removal from the deposition chamber ( t = 0). The single break curve corresponds to data obtained after a 1.5-h exposure. The multiple-break trace was acquired after a 3.0-h exposure period.
films prepared with H2-N2 in the earlier parts of this section. Refer to Figure 2. In Figure 7, we also observe that the bands in the 1000-2500-~m-~ region of the spectra increase as film thickness increases. Only the NH stretch near 3230 cm-I appears with an equal relative intensity in both traces. The results of a complementary aging study of an A1N film synthesized with 25% H2 enriched N2 are given in Figure 8. Spectra of the aged film were obtained following the same time intervals as discussed above. The data presented here were collected at t = 0,1.5, and 3.0 h. The spectrum measured after a 0.5-h exposure to air is omitted. The absorbance maximum of the AlN phonons measured 0.064 upon immediate removal of the sample from the chamber, decreased to 0.054 after 0.5 h of aging, and thereafter remained nearly constant. The band at 950 cm-I in the spectrum recorded at t = 0 shifted to 937 cm-I in the spectrum collected after 0.5 h of aging. No further shifts were detected in that transition with successive aging measurements. Similarly, the band near 2140 cm-I in Figure 8 experienced maximum intensity loss followingthe first 0.5 h of aging, and its intensity did not change in the subsequent measurements. Although the number of bands in the 1000-2500-~m-~ region remained constant throughout the aging study, their intensities decreased uniformly as a function of air exposure time, except for the NH bending deformation near 1620 cm-1. This transition did not appear to lose much intensity and actually developed from a weak shoulder originally located at 1590 cm-1 in the reference spectrum acquired at t = 0 (also see Figure 7) into a well-defined band maximizing at 1616 cm-1 in the spectra of the aged ( t > 0) film. This shift may perhaps be related to the increase in the total concentration of NH, species upon hydrolysis. The increase in the number of NH3 species enlarges the ratio of the NH, coordinated to the individual A1 atoms or ions. In the chloride complexes of A1(NHdx3+,where x equaled 1,3,5, and 6 , the G(NH3) was observed to exhibit a small but steady upward move in energy from 1596 cm-l ( x = 1)to 1611cm-l (x = 6) with the increase in the number of coordinated a m m o n i a ~ Other . ~ ~ possible contributors
IR Study of A1N Films Prepared by Ion Beam Deposition -2141
0 ul N
I
1000
I
I
I
I
2000
3000
Energy
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(Cm-1)
Figure 9. Spectra of 20-nm AlN films prepared with 25% HP enriched N2. The solid trace corresponds to data collected immediately after the sample was withdrawn from the deposition chamber. The broken curve is the spectrum of the film treated with water.
Langmuir, Vol. 6, No. 8, 1990 1337 reference film. The A1N phonon intensity decreased from an absorbance maximum value of 0.064 to 0.052 upon water treating the film. This is equivalent to approximately 20% loss of the nitride resulting from the A1N films' exposure to liquid water. Both the aged and the water-treated 20-nm-thick A1N films fabricated with concomitantly added HZ suffered approximately 20% or 4.0 nm loss of the nitride. The observed structural modifications included an increase in the concentration of ammonia and the formation of hydrolyzed forms of NH3. The results of the film aging and direct exposure to HzO studies have an important bearing on the optical technology of A1N films. Although the presence of hydrogen in the plasma is reported to produce harder and denser films,=S these films appear to react with H 2 0 much more rapidly than A1N films prepared with pure N2. Further work is necessary to evaluate the optical properties of AlN films prepared with different gas mixtures.
Conclusions The vibrational results obtained from AlN films prepared by reactive ion beam sputtering and deposited on gold substrates for the most part corroborate our earlier spectroscopicwork involving AlN deposition on aluminum substrates.Z0 Aluminum nitride thin films deposited on gold substrates utilizing pure N2 are stoichiometric films, and their reflectance spectra indicate the presence of the A1N stretching modes as well as two combination bands. The assignment of the AlN combination bands at 1288and 1446 cm-' was aided by the film aging study performed in this
to the intensity of the 1616-cm-' band result from the interaction of the NH, species with H20. In hydrated complexes of ammonia, nNHsqH20 (n = 1or 2), a medium intensity vibration was reported near 1630 cm-1.44,45 The intensities of the transitions found in the 25004000-cm-' region in Figure 8 approximately doubled after each successive aging measurement. The new band near 2920 cm-' may be due to an 0-H--N stretch. This type of IR transition was reported for the 2NHrH20 and the work. NHrH20 adducts at 2975 and 2910 cm-l, r e s p e c t i ~ e l y . ~ ? ~ ~ A1N films prepared with 25% H2 enriched N2 and The shift of the 3230-cm-l band in Figure 7 to 3250 cm-' deposited on gold substrates possess excess nitrogen, with in traces of Figure 8 also may be caused by a broadening an A1:N ratio of 1:1.15. The reflectance spectra of these of the 4 N H ) region due to the interaction of atmospheric films are similar to those obtained from A1N films Hz0 with the NH,. In the IR spectra of the NHSvH20 deposited on aluminum substrates.20 AlN films prepared complex, the u(O-H--O) motion due to the hydrogenwith concomitantly added H2 exhibited a growth in the bonded water molecules appeared as a very strong feature concentration of the Al-NZ and NH, species with increasing at 3190 cm-l.44945 The v(NH) modes in the mono- and difilm thickness. hydrate complexes were located in the 3270-3400- and The spectroscopic results of the film aging and H20 3370-3400-cm-l regions, exposure studies show that the A1N films suffered either The above spectroscopic data on aged films prepared 0% or
