AlN thin films deposited by pulsed laser ablation, sputtering and filtered arc techniques

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1 Thin Solid Films (001) AlN thin films deposited by pulsed laser ablation, sputtering and filtered arc techniques a a, a a a b Ravi Bathe, R.D. Vispute *, Dan Habersat, R.P. Sharma, T. Venkatesan, C.J. Scozzie, b b b c c Matt Ervin, B.R. Geil, A.J. Lelis, S.J. Dikshit, R. Bhattacharya a CSR, Department of Physics, University of Maryland, College Park, MD 074, USA b US Army Research Laboratory, Adelphi, MD 0783, USA c UES, Inc. 4401, Dayton-Xenia Road, Dayton, OH 4543, USA Abstract The present work describes the growth and characterization of AlN thin films deposited by pulsed laser deposition (PLD), DC magnetron sputtering and filtered arc techniques. The focus of this paper is not only on the optimization of process parameters for the production of device quality thin AlN films, but also to investigate deposition techniques that could provide stable and reliable dielectric films wide band-gap power device. We investigated the working gas pressure dependence of the deposition of AlN on the vertical walls of the etchedypatterned silicon for device passivation studies. Under high-pressure ( Pa) conditions, high-density plasma was achieved which produced AlN passivation of a vertically etched Si wall. The films were characterized by X-ray diffraction (XRD), Rutherford backscattering and ion channeling spectrometry, atomic force microscopy (AFM), UV-visible spectroscopy, scanning electron microscopy (SEM). 001 Elsevier Science B.V. All rights reserved. Keywords: AlN films; Pulsed laser deposition; Sputtering; Arc-deposition; Conformal deposition; High-temperature high-power electronics 1. Introduction Future high-temperature electronics need the implementation of wide band-gap (WBG) materials such as SiC and III V nitrides (AlN, GaN, AlGaN) due to their wide band gap (energy gap )3 ev), superior electrical properties and high thermal conductivity w1,x. Ability to dope SiC with n- as well as p-type dopants make it the first choice for high temperature electronics and power semiconductor devices w3x. Although SiC and related materials are progressing towards advancement of the device technologies, robust high-temperature gate dielectric and passivation materials are required in order to fabricate the analog power switches and inverter circuit using current-controlled SiC gate turn-off thyristors (GTOs). Although thermally grown and depositedreoxidized SiO dielectrics are under development w4x, * Corresponding author. Tel.: q ; fax: q address: vispute@squid.umd.edu (R.D. Vispute). they have not yet been demonstrated to provide stable and reliable MOSFET operation even at a temperature of 3008C and with a gate field of 1 MVycm. Therefore, other insulators, especially those with high dielectric constants (resulting in a lower field in the insulator), are being investigated for high-temperature device applications. Thin films of AlN seem to be the most promising dielectric material wband gap, 6. ev, Wurtzite crystal structure, high thermal conductivity with thermal expansion coefficient closely matched to that of SiC, highly insulating, higher dielectric constant (8.) than SiO (3)x for use in fabrication of SiC based metal insulator semiconductor structures. An additional advantage of AlNySiC materials system is the small lattice mismatch between AlN and SiC (;1%), which may improve the electronic properties compared to SiO dielectrics. We have recently demonstrated that pulsed laser deposited AlN dielectric with low leakage currents in the temperature range of C w4 6x. The ultimate goal is to provide passivation and gate dielectric films that enable stable and reliable WBG /01/$ - see front matter 001 Elsevier Science B.V. All rights reserved. PII: S Ž

2 576 R. Bathe et al. / Thin Solid Films (001) power device operation at junction temperatures up to 3508C under operating fields of at least MVycm. In this paper, we discuss the preliminary investigation of conformal deposition process by physical vapor deposition techniques such as pulsed laser deposition, sputtering and arc-techniques for passivation of semiconductor devices. For the growth of AlN thin films, in addition to parameters such as substrate temperature, background gas pressure, substrate orientation and target to substrate distance, we have optimized the parameters unique to each technique: for PLD, laser fluence and pulse repetition rate; for sputtering, discharge DC power and N y (N qar) pressure ratio; for filtered arcing, bias voltage and arc current. After optimizing the growth conditions and understanding the dependence on the operating parameters for AlN thin films, we investigate sidewall deposition on the patterned substrates for electrical device passivation.. Experimental.1. Pulsed laser deposition Details of our pulsed laser deposition technique and process can be found elsewhere w7x. In brief, a stainlesssteel vacuum chamber was evacuated by turbomolecular y6 pump to a base pressure 4=10 Pa. A KrF excimer laser (ls48 nm, ts5 ns) was used for ablation of a polycrystalline, stoichiometric AlN target (99.99% purity) at an energy density of ; Jycm. A strong absorption of the 48-nm laser radiation by the target produced an intense plasma plume in front of the target surface upon laser irradiation. For optimization of crystalline quality, AlN films were grown on Al O and SiC 3 substrates, whereas for conformal deposition studies, etched patterns of Si substrates were used. Before deposition, the substrates were ultrasonically cleaned in trichroethylene, acetone and methanol. Si substrates were etched in a HOyCH5OHyHFs9:1:9 solution and then rinsed in deionized water and blow-dried in N before loading into the chamber. The NH back- 3 y4 ground gas pressure was varied from 10 to 10 Pa. The pulse repetition rate (5 10 Hz) and total deposition time (30 60 min) were used to control the deposition rate and the film thickness. With the existing set-up, uniform films (5% uniformity) can be grown on cm diameter Si, SiC and sapphire wafers by utilizing laser scanning and substrate rotation mechanism... DC magnetron sputtering The DC magnetron sputtering system employed a chamber with a water-cooled pure aluminum target disk (99.99%) of 5.08-cm diameter. The substrates were precleaned by the method described above. The base vacuum of the deposition system was stabilized at y5 approximately 6.67=10 Pa. Once the desired back- ground pressure was reached, the target was pre-sputtered at 50 W DC power in 0.4 Pa Ar pressure for 10 0 min to clean and equilibrate the target surface prior to film deposition. The sputtering was done in a reactive N and Ar gas mixture with various ratios of N gas to Ar gas. The discharge power was varied between 50 and 50 W, the process pressure was varied between and Pa, and substrate temperature was varied between room temperature to 9008C. The substrate to target distance was kept constant at approximately 5 cm..3. Filtered arc techniques Deposition of AlN was pursued using an industrial cathodic arc system, large area filtered arc deposition (LAFAD). Prior to loading, silicon samples were cleaned in a dilute solution of HFqHNO3 to remove any surface formations of oxide. These Si and Al O 3 substrates were then cleaned in an ultrasonic bath of alcohol. Once the samples were loaded, the chamber was evacuated by a diffusion and rotary pump system y3 to a background pressure of 5.334=10 Pa. Low- pressure argon was then introduced and the samples were plasma cleaned. Next, nitrogen gas was vented into the chamber and the substrates were heated to the desired temperature prior to deposition. A y40 V bias was placed on the substrate with an RF source and a constant current of 50 A was run through a filtered aluminum arc source to generate the plasma and deposit AlN for 0 min. The trajectory of the plasma was altered by an external magnetic field to insure proper orientation upon the substrates..4. Characterization The films were characterized by four-circle X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV-visible spectroscopy. A mechanical stylus and Dectak II were used for thickness measurement. The quantitative analysis of the crystalline quality, composition and interface structure of the films was determined by Rutherford backscattering spectrometry (RBS) and ion channeling techniques. 3. Results and discussion We have optimized the growth conditions of AlN films on sapphire and SiC substrates by pulsed laser deposition technique. The typical growth conditions y were 1.333=10 Pa NH 3, 9008C substrate temperature andjycm laser energy density. The AlN films grown under these conditions were epitaxial on sapphire and

3 R. Bathe et al. / Thin Solid Films (001) out-growth features is associated with the morphology on the etched surface on the Si vertical wall as shown in Fig. 3. The etched Si samples patterned in our laboratory have heavily scalloped side walls (Fig. 3a), which are associated with the bosch process w8x. The commercially etched sample has smooth sidewalls as shown in Fig. 3b. On such samples, we can clearly see the uniform deposition on the vertical wall in high Fig. 1. Laser-induced plasma plume of AlN for conformal deposition process. Note that the substrate is normal to the principal axis of plasma plume expansion. silicon carbide substrates with v-rocking FWHM of 6 arc-min and smooth surface morphology (1 nm rms roughness). The ion channeling minimum yield near the surface region for the PLD AlN films on sapphire and SiC were 5 and 3%, respectively, indicating a high degree of crystallinity. The electrical properties of these films were measured using epitaxial structures of TiNy AlNySiC capacitors. Metal insulator semiconductor y4 (MIS) capacitors with gate areas of 4=10 cm were fabricated, and high-temperature current voltage (I V) characteristics were studied up to 4508C. We measured y8 leakage current densities in the 10 Aycm order at y3 room temperature and 10 Aycm order at 4508C under MVycm field w7x. In order to study the conformal deposition characteristics of the pulsed laser deposition process, AlN films were deposited on vertically etched patterned Si samples. The deposition geometry is shown in Fig. 1. Fig. shows cross-sectional SEM of the pulsed laser deposited AlN samples as a function of background NH pressure. 3 Fig. a shows the AlN film grown under 1.333=10 y Pa of NH 3. The cross-sectional SEM image clearly indicates the deposition of AlN on the basal plane of the substrate and no evidence of deposition on the vertical sidewall. This indicates that the laser deposition y under 1.333=10 Pa is a line of sight process, which is a typical characteristic of a physical vapor deposition. As the process pressure increased in the range of Pa, out growth on the vertical wall is clearly seen, Fig. b and Fig. c. The occurrence of the Fig.. SEM cross-sectional images of pulsed laser deposited vertical wall of the Si etched samples at NH3 process pressure of (a) y 1.333=10 Pa, (b) Pa and (c) Pa.

4 578 R. Bathe et al. / Thin Solid Films (001) reactive sputtering was Pa total gas pressure (ArqN ) with 75% N, 30 W discharge power and 6008C substrate temperature, whereas in the case of arcdeposition, the films were grown in the temperature range of C, and (ArqN ) pressure of Pa. As seen in these micrographs, we were able to produce conformal deposition covering the sidewalls both by sputtering as well as arc-deposition technique, Fig. 3. Cross-sectional SEM micrographs of the (a) etched patterns to produce vertical sidewalls, (b) commercially available etched Si patterns. pressure pulsed laser deposition process (Fig. 4a). Itis interesting to note that the aspect ratio (thickness of the side wall film to the base film) under high pressure laser deposition process is close to 1:. This indicates that the thin film deposition can take place if the ablated species inside the laser plume have a significant velocity component perpendicular to their main expansion direction. Thus, the growth on the side wall is because of the high deposition pressure of approximately Pa, where the mean-free path of the NH3 molecules is approximately mm resulting in a high impact probability with the ablated particles of the AlN target. For the atomic species predominantly present in the laser plume, these impacts result in a Brownian-like motion of the particles leading to a deposition on the vertical wall. Now it is interesting to compare the results of the pulsed laser deposition process with those of conventional physical vapor deposition such as sputtering and arc techniques. Fig. 4b,c show cross-sectional SEM of the DC sputtered and cathodic arc deposited AlN samples, respectively. A typical growth condition for DC Fig. 4. Cross-sectional SEM micrographs of the conformal deposition of AlN thin films by (a) PLD at Pa (b) sputtering and (c) arcdeposition techniques.

5 R. Bathe et al. / Thin Solid Films (001) optical transmission in the visible and UV range. The PLD film shows a sharp drop in the optical transmission at 00 nm corresponding to a band-gap of 6. ev, which is close to the bulk AlN value. The sputtered and cathodic arc films did not drop as sharply as the PLD film and the band-gap was slightly lower ( ev). The broad optical absorption edge and the lower values of the band-gaps of the sputtered and arc-deposited films indicate that the films contain significant amount of defects, which is consistent with the XRD and ion channeling experiments. Though the conformal coating is possible using sputtering and arc-depositions, defects in the films may pose problems in dielectric characteristics of the passivation layer. It is also important to note that the quality (crystalline, optical and morphology) of the passivation layers may improve on SiC device structures due to the close lattice match between AlN and SiC (1%) as compared to the Si substrate. The conformal growth, structural and electrical characterization of the laterally grown AlN on SiC are under present investigation and the results will be published elsewhere. 4. Conclusion We have studied the pulsed laser deposition, DC magnetron sputtering and cathodic arc techniques for Fig. 5. XRD spectra of AlN thin films grown by (a) PLD and (b) sputtering. allowing for a greater flexibility in device passivation. The XRD results of AlN thin films deposited on sapphire (0001) by sputtering are compared with that of PLD AlN films, and the results are shown in Fig. 5. The XRD patterns in both cases clearly show {000l} family planes of wurtzite AlN and sapphire, indicating highly oriented AlN films. However, major differences were found in the crystalline quality that was measured using ion channeling technique. The ion channeling studies indicate that the film grown by sputtering is poorly aligned to the substrate (x min)60%) as compared to laser deposited films substrate (x min;3 4%). These structural features of the sputtered AlN films on Al O 3 and Si are consistent with that of films reported earlier w9 13x. In the case of cathodic arc deposited AlN films, we did not see any XRD peak which is an indication of either an amorphous or fine grained AlN film. Fig. 6 shows the optical transmission spectra for the PLD, sputter and cathodic arc AlN films on sapphire substrates. The films exhibit approximately 80 85% Fig. 6. UV-Visible transmittance of the PLD, sputtered and arc-deposited AlN thin films.

6 580 R. Bathe et al. / Thin Solid Films (001) the conformal growth of AlN films. After optimizing process pressure, we were able to demonstrate film deposition on the vertical walls of etched silicon. This result was obtained by the increase of the process pressure, leading particularly in pulsed laser deposition technique to high-density plasma conditions, which enhanced the diffusion of the ablated species in the lateral direction (normal to the plasma plume). AlN films grown by PLD were epitaxial on sapphire and silicon substrates, AlN films grown by sputtering and arc techniques were poorly crystalline and amorphous, respectively. The conformal coating by AlN thin films can be promising for passivation of SiC power devices operating at high temperature and high electric field density. Acknowledgements Funding support from ARL, Adelphi, MD, is acknowledged. Ravi Bathe acknowledges support from Maryland Industry partnerships (MIPS grant -9419). The authors would like to thank Dr Noor Mohammad, and Asif Mohammad of Howard University, Washington DC, and Varun Sarin of University of Maryland for helpful discussions and experimental help, respectively. References w1x S. Strite, H. Morkoc, J. Vac. Sci. Technol. B 10 (199) 137. wx H. Morkoc et al., J. Appl. Phys. 76 (1994) 1363, and references therein. w3x C.H. Carter Jr., R.P. Devaty, G.R. Rohrer (Eds.), Silicon Carbide and Related Materials: 1999, Part I II, ICSCRM 1999, 1999, and references therein. w4x R.D. Vispute, A. Patel, K. Baynes, B. Ming, R.P. Sharma, T. Venkatesan, Mater. Res. Soc. Sump. 595 (000) W w5x C.J. Scozzie, A.J. Lelis, B.F. McLean, R.D. Vispute, A. Patel, R.P. Sharma, T. Venkatesan, J. Appl. Phys. 86 (1999) 405. w6x A.J. Lelis, C.J. Scozzie, B.F. McLean, B.R. Geil, R.D. Vispute, T. Venkatesan, in: C.H. Carter Jr., R.P. Devaty, G.R. Rohrer (Eds.), Silicon Carbide and Related Materials 1999, Part I II, ICSCRM 1999, 1999, p w7x R.D. Vispute, S. Choopun, R. Enck, A. Patel, V. Talyansky, R.P. Sharma, T. Venkatesan, W.L. Sarney, L. Salamanca-Riba, S.N. Andronescu, A.A. Iliadis, K.A. Jones, J. Elect. Mater. 8 (1999) 75. w8x S. Franssila, J. Kiihamaki, J. Karttunen, Microsys. Technol. 6 (000) 141. w9x C.-C. Cheng, Y.-C. Chen, H.-J. Wang, W.-R. Chen, J. Vac. Sci. Technol. A 14 (1996) 38. w10x R.S. Naik, R. Reif, J.J. Lutsky, C.G. Sodini, J. Elec. Soc. 146 (1999) 691. w11x J.S. Morgan, W.A. Bryden, T.J. Kistenmacher, S.A. Ecelberger, T.O. Poehler, J. Mater. Res. 5 (1990) 677. w1x H. Okano, Y. Takahashi, T. Tanaka, K. Shibata, S. Nakano, Jpn. J. Appl. Phys. 31 (199) w13x F. Engelmark, G. Fucntes, I.V. Katardjiev, A. Harsta, U. Smith, S. Berg, J. Vac. Sci. Technol. A 18 (000) 1609.