Aluminum nitride films synthesized by dual ion beam sputtering

Transcription

1 Aluminum nitride films synthesized by dual ion beam sputtering Sheng Han Department of Finance, National Taichung Institute of Technology, Taichung, Taiwan 404, Republic of China Hong-Ying Chen Department of Applied Life Science, Taichung Healthcare and Management University, Taichung County, Taiwan 413, Republic of China Chih-Hsuan Cheng Department of Materials Engineering, National Chung Hsing University, Taichung, Taiwan 402, Republic of China Jian-Hong Lin Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China Han C. Shih a) Department of Materials Engineering, National Chung Hsing University, Taichung, Taiwan 402, Republic of China; and Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China (Received 28 May 2004; accepted 13 August 2004) Aluminum nitride films were deposited by varying the voltages of argon ion beams from 400 to 1200 V in dual ion beam sputtering. The crystal structure, microstructure, and elemental distributions of the aluminum nitride films were analyzed by x-ray diffraction, field emission scanning electron microscopy, and secondary ion mass spectroscopy, respectively. The aluminum nitride films exhibited the 002 preferred orientation at an optimal ion beam voltage of 800 V. The orientation changed to a mixture of {100} and {002} planes above 800 V, accounting for radiation damage. The thickness of the film increases with increasing ion beam voltage, reaching a steady state value of 210 nm at an ion beam voltage of 1200 V. Under optimal condition (800 V), the c-axis orientation of the aluminum nitride 002 film was obtained with a dense and high-quality crystal structure. I. INTRODUCTION Aluminum nitride (AlN) is a III-V semiconductor compound with a hexagonal wurtzite crystal lattice structure and is a promising material for use in microelectronics packaging owing to its high thermal conductivity, a moderate dielectric constant and a thermal expansion coefficient that matches that of silicon. 1 3 Additionally, c- axis-oriented AlN thin films have potential applications in high-frequency surface-acoustic-wave devices because they have a large electromechanical coupling constant and high acoustic velocity. 4 AlN films are grown by chemical vapor deposition, 5,6 a) Address all correspondence to this author. hcshih@mse.nthu.edu.tw DOI: /JMR reactive sputtering deposition, 7 plasma-assisted molecularbeam epitaxy, 8 laser ablation deposition, 9 and ion-beamassisted deposition. 2,10 Dual ion beam sputtering (DIBS) uses metallic or compound targets to produce a sputterdeposited film, while the growing film is concurrently bombarded during deposition with a second ion beam. 10,11 The bombardment process can strongly modify the structural and chemical properties of the resulting film and the incident particles may also become part of the growing film during deposition. Although the variation of reactive nitrogen ion-beam energy/voltage in the DIBS technique has been investigated, 10,11 the variation of argon ion-beam energy has rarely been examined. In this study, the properties of AlN films deposited using a DIBS system, an aluminum target with an inert argon ion-beam and reactive nitrogen ions for bombarding the growing film were examined by x-ray diffraction J. Mater. Res., Vol. 19, No. 12, Dec Materials Research Society 3521

2 (XRD), field-emission scanning electron microscopy (FESEM), and secondary-ion mass spectrometry (SIMS). This study investigates the effect of various ion beam voltages used to bombard the aluminum target on the crystal structure and check the orientation of the AlN films. II. EXPERIMENTAL DETAILS A. Film deposition The AlN film was prepared on a p-type (100) Si wafer (Toshiba Ceramics Co., Ltd., Japan) in a DIBS system, schemati-cally shown in Fig. 1. The system was equipped with two ion sources. The argon gas (purity %) was fed into a Kaufmann-type ion gun (Commonwealth Scientific Corporation, Alexandria, VA), which was operated at V and 7 ma with a current density of 1 ma/cm 2. The ionized argon ions sputtered the aluminum target (99.999% purity) to generate the aluminum atoms. The nitrogen gas (purity %) was passed through a Mark I gridless end-hall ion gun (Commonwealth Scientific Corporation, Alexandria, VA), and the typical ratio of the ionized nitrogen/neutral nitrogen was 15 25%. The operating voltage and current were 130 V and 0.5 ma, respectively, with a corresponding energy of 80 ev. The chamber was pumped down to a base pressure of Torr prior to deposition, before the chamber pressure was raised to Torr when the reactive gases were presenting, which were presenting a controlled flow ratio of Ar/N 2 1/3, such as at 3 sccm for Ar and 10 sccm for N 2. The substrates were heated to 450 C during the deposition for 90 min. The Si substrate was sputtered clean using an end-hall ion gun before the AlN films were deposited. interval of 0.02 /2 was used. The texture coefficient of AlN 002 determined from the diffraction spectra, was defined as AlN 002 = I AlN 002 I AlN I AlN 100, (1) where I is the integrated intensity of the corresponding Bragg peak. The surface and cross sectional morphology were observed using FESEM (Hitachi S-4100, Japan) operated at 10 kv. The elemental depth profile of aluminum, nitrogen, oxygen and silicon in AlN films were analyzed using the SIMS technique. A Cameca IMS-4f (France) secondary ion mass spectrometer with a Cs + primary beam having impact energy of 14.5 kev and a primary current of 50 na was used. B. Characterization The crystal structure and the preferred orientation of the AlN films on Si substrates were characterized by XRD. A Mac Science MXP3 (Japan) diffractometer using Cu K radiation ( nm) with a collection FIG. 1. The schematic diagram of the dual ion beam sputtering system (DIBS) used in this study. FIG. 2. (a) XRD patterns and (b) the (002) texture coefficients of the AlN films prepared at various argon ion beam voltages J. Mater. Res., Vol. 19, No. 12, Dec 2004

3 III. RESULTS AND DISCUSSION A. Crystallographic analysis Figure 2(a) shows the XRD pattern of AlN films prepared on an Si (100) substrate at various ion beam voltages, matching the results of the JCPDS File. 12 No AlN diffraction peak was found when the argon ion beam voltage was less than 400 V. For voltages from 600 to 1200 V, the AlN {100} and {002} planes were both evident in the XRD pattern. Notably, the {101} plane also contributed to the strong peak at 800 V. Nevertheless, the diffraction peaks, showing pronounced broadening revealed the fine grain structure and/or residual stress in the AlN film. Moreover, the film had a 002 preferential orientation at 800 V might be due to the residual compressive stress in the film at 800 V is much than those of 100 and 1200 V. Figure 2(b) depicts the integrated intensity of AlN 002 with various argon ion beam voltages. No preferential orientation of 002 was obtained at argon ion beam voltages that were either too low or too high. When the ion beam voltage was 800 V, AlN 002 was the preferred orientation with a texture coefficient of 0.76, which is double 0.38, calculated in Ref. 12. Lower ion beam energies result in non-reactive energetic ions or neutral atoms bombarding a growing film and transferring momentum. Therefore, the mobility of ions and/or atoms deposited on the substrate is low and results in poor crystallites of the AlN film. The simultaneous use of moderate ion beam energy enhances crystallization. The films again exhibit a dominant {002} plane because this plane is the closest packing plane and has low-energy the hexagonal structure. 7 However, when the ion beam energy is too high, radiation damage 13,14 can lead to disordering, and energetic ions and/or AlN particles impinge on the film s surface, their momentum to the {002} planes making them less densely packed, like the {100} plane whose c axis is parallel to the substrate. Restated, the film s structure changes from oriented (as the {002} plane is) to randomized (as a mixture of {100} and {002} planes are) since the crystallites are agitated by the kinetic energy of the ions and particles. Moreover, the ion beam energy is proportional to the ion beam voltage when the other process parameters are fixed. A moderate ion beam voltage, such as 800 V enhances the 002 preferred orientation and changes to a mixture of {100} and {002} planes because increasing the ion beam voltage (above 800 V for example) causes radiation damage. B. Morphology Figure 3 shows the cross-sectional SEM micrographs of the AlN films prepared at various ion beam voltages. The film grown at 800 V has a columnar structure. Figure 4 shows the variation of the film thickness versus FIG. 3. Cross-sectional SEM micrographs of AlN films prepared at (a) 600 V, (b) 800 V, (c) 1000 V, and (d) 1200 V. J. Mater. Res., Vol. 19, No. 12, Dec

4 the ion beam voltage. When the ion beam energy is low, the less energetic aluminum atoms are sputtered from the target. Therefore, the deposition rate is low so the film is thin at low ion beam energy. The film thickness increases with the increasing ion beam voltage. However, the slope of the curve whose ion beam voltages less than 800 V is greater than those of the curves obtained at voltages of V, which indicate a steady state. The film thickness reaches a maximum of 210 nm at 1200 V (Fig. 4). Re-sputtering causes the input kinetic energy and momentum to be so high that the deposited material is again vaporized by sputtering, reducing the growth velocity and the final thickness of the film. 13,14 As steady state of growth continues when the ion impingement rate equals the sputtering rate. FIG. 4. Thickness variation of AlN films prepared at various argon ion beam voltages. C. Elemental distributions Figure 5 shows elemental depth profiles of aluminum, nitrogen, oxygen, and silicon versus sputtering time, prepared at various argon ion beam voltages. The depth profile of aluminum and nitrogen with the silicon signal background reveals that the concentration in the AlN films over argon ion beam voltages from 600 to 1200 V is uniform. Moreover, the interface between the AlN films and the Si substrate was observed after sputtering times of 250 s at 600 V, 550 s at 800 V, and 600 s at both 1000 and 1200 V. The change in the sputtering time with the interface is consistent with the cross-sectional FESEM micrographs, shown in Fig. 4. The oxygen within the AlN films is probably from the oxygen and H 2 O as impurities in both the nitrogen and the argon gases, or is from residual gas, such as oxygen and H 2 O, FIG. 5. Elemental depth profiles of Al, N, Si, and O at various argon ion beam voltages: (a) 600 V, (b) 800 V, (c) 1000 V, and (d) 1200 V J. Mater. Res., Vol. 19, No. 12, Dec 2004

5 in the chamber. The silicon and oxygen signals abruptly peaked at the interface between the films and the substrate, mainly because of the native oxide of the silicon wafer. Further work in this are is required. Additionally, x-ray photoelectron spectroscopy (XPS) was used to investigate Al-2p 3/2, N-1s, and O-1s of AlN films prepared at V. The spectra reveal that the Al-2p 3/2 and N-1s are centered at 74.2 and ev, consistent with results for AlN films. 15 Furthermore, the weakness of the oxygen peaks spectra at approximately ev are attributed to the adsorbed water. The SIMS and XPS results reveal that high-quality AlN films were prepared. IV. CONCLUSIONS AlN films were successfully deposited on a silicon wafer using a DIBS system at argon ion beam voltages of V. The characterizations of their crystal structure by XRD, of the microstructure by FESEM, and of the elemental concentration profiles by SIMS lead to the following conclusions. (1) AlN films exhabit a preferred orientation of the c-axis 002 plane and exist as columnar structures, when formed at an optimal ion beam voltage of 800 V. The changes to a mixture of 100 and 002 planes account for the radiation damage where the ion beam voltage exceeds 800 V. (2) The thickness of AlN films increases with the ion beam voltage and approaches a steady value of 210 nm at approximately 1200 V. (3) The concentrations of aluminum and nitrogen, with the silicon background, was examined by SIMS, revealing that excellent uniformity is obtained over argon ion beam voltages from 600 to 1200 V. The trend in the interface position is also consistent with the FESEM observations. REFERENCES 1. S. Strite and H. Morkoc: GaN, AlN and InN a review. J. Vac. Sci. Technol. B 10, 1237 (1992). 2. X. Wang, A. Kolitsch, F. Prokert, and W. Möller: Ion-beamassisted deposition of AlN monolithic films and Al/AlN multilayers: A comparative study. Surf Coat. Technol. 103/104, 334 (1998). 3. F. Engelmark, G. Fucntes, I.V. Katardjiev, A. Harsta, U. Smith, and S. Berg: Synthesis of highly oriented piezoelectric AlN films by reactive sputter deposition. J. Vac. Sci. Technol. A 18, 1609 (2000). 4. M.T. Wauk and D.K. Winslow: Vacuum deposition of AlN acoustic transducers. Appl. Phys. Lett. 13, 286 (1968). 5. W.M. Yim, E.J. Stofko, P.J. Zanzucchi, J.I. Pankove, M. Ettenberg, and S.L. Gilbert: Epitaxially grown AlN and its optical band gap. J. Appl. Phys. 44, 292 (1973). 6. Y. Someno, M. Sasaki, and T. Hirai: Low temperature growth of AlN films by microwave plasma chemical vapour deposition using an AlBr 3 -H 2 -N 2 gas system. Thin Solid Films 202, 333 (1991). 7. F.S. Ohuchi and P.E. Russell: AlN thin films with controlled crystallographic orientations and their microstructure. J. Vac. Sci. Technol. A 5, 1630 (1987). 8. L.B. Rowland, R.S. Kern, S. Tanaka, and R.F. Davis: Epitaxial growth of AlN by plasma-assisted, gas-source molecular beam epitaxy. J. Mater. Res. 8, 2310 (1993). 9. M.G. Norton, P.G. Kotula, and C.B. Carter: Oriented aluminum nitride thin films deposited by pulsed-laser ablation. J. Appl. Phys. 70, 2871 (1991). 10. J.M.E. Harper, J.J. Cuomo, and H.T.G. Hentzell: Quantitative ion beam process for the deposition of compound thin films. Appl. Phys. Lett. 43, 547 (1983). 11. H. Oechsner: Ion and plasma beam assisted thin film deposition. Thin Solid Films 175, 119 (1989). 12. Joint Committee on Powder Diffraction Standards, Powder Diffraction File Card , ASTM, Philadelphia, PA, W. Ensinger: Low energy ion assist during deposition An effective tool for controlling thin film microstructure. Nucl. Instrum. Meth. B 127/128, 796 (1997). 14. W. Ensinger: On the mechanism of crystal growth orientation of ion beam assisted deposited thin films. Nucl. Instrum. Meth. B 106, 142 (1995). 15. H-Y. Chen, S. Han, and H.C. Shih: Effect of argon ion beam voltages on the microstructure of aluminum nitride films prepared at room temperature by a dual ion beam sputtering system. Appl. Surf. Sci. 228, 128 (2004). J. Mater. Res., Vol. 19, No. 12, Dec