Electrical properties of AlN thin films prepared by ion beam enhanced deposition

Transcription

1 Surface & Coatings Technology 196 (2005) Electrical properties of AlN thin films prepared by ion beam enhanced deposition Zhenghua An a,b, Chuanling Men b, Zhengkui Xu a, Paul K. Chu a, *, Chenglu Lin b a Department of Physics and Material Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong b State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai , China Available online 3 October 2004 Abstract Aluminum nitride (AlN) thin films prepared by ion beam enhanced deposition (IBED) have desirable properties as the buried dielectric in silicon-on-insulator substrates. In this work, the electrical properties of IBED deposited AlN films were investigated. Our results show that the electrical properties of AlN films deteriorate with increasing Al evaporation rate. The film deposited at a deposition rate of 0.05 nm/s exhibits good insulating properties and its breakdown field is 2.1 MV/cm. After thermal treatment, the breakdown field exceeds 4 MV/cm and the leakage current is reduced about 60 times. Capacitance Voltage (C V) results corroborate that the AlN film possesses an extremely low density of trapped charges. However, the density of the interfacial states inside the bandgap is relatively high in the non-abrupt AlN/Si interface region but they can be partially reduced by high-temperature annealing. D 2004 Elsevier B.V. All rights reserved. Keywords: Aluminum nitride (AlN); Electrical property; Ion beam enhanced deposition (IBED) 1. Introduction Aluminum nitride (AlN) having a wide bandgap of 6.3 ev is a promising dielectric material. For instance, it can serve as the gate dielectric in high voltage, high power electronic devices made on SiC substrates [1,2]. Aluminum nitride is also being investigated as a substitute for the silicon dioxide buried layer in silicon-on-insulator (SOI) substrates due to its low thermal expansion coefficient, high breakdown dielectric strength, and high chemical and thermal stability [3,4]. In particular, the thermal conductivity of AlN is almost 200 times higher than that of SiO 2 (3.2 W cm 1 K 1 for AlN compared to W cm 1 K 1 for silicon dioxide). The buried silicon dioxide layer (BOX) in traditional SOI structures can lead to severe self-heating effects (SHE) [5], * Corresponding author. Tel.: ; fax: / address: paul.chu@cityu.edu.hk (P.K. Chu). especially for applications where high work voltages and/or high power are required. The excellent thermal conductivity of AlN layer can greatly reduce SHE and AlN-SOI is thus more superior in high voltage, high power applications. In our previous work [4], SOI structures with buried AlN insulating layers were fabricated by means of the ion-cut process [6]. The starting AlN thin films were prepared by ion-beam enhanced deposition (IBED). However, the electrical properties of the AlN films prepared by IBED have not yet been examined and there have been few reports on the electrical properties of the sandwiched AlN films in SOI structures. Previous works on electrical properties of AlN primarily focused on AlN films prepared by magnetron sputtering [7] and chemical vapor deposition (CVD) [8]. In the work reported here, the electrical properties of AlN films prepared by IBED are systematically studied. This particular study is important because IBED that is a common technique for many other thin films may result in different film microstructures and consequently different electrical properties /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.surfcoat

2 Z. An et al. / Surface & Coatings Technology 196 (2005) Experimental One hundred-millimeter diameter p-type Si (100) (1 3 V) wafers were first cleaned with a standard RCA procedure. AlN films were deposited on these wafers by electron beam evaporation of Al and simultaneous bombardment by nitrogen ions. In our IBED system (Eaton Z- 2000), an electron beam was employed to evaporate high purity Al (99.999%). The evaporation rate of Al could be adjusted from 0.05 to 0.5 nm/s. A Kaufman ion source was employed to produce a nitrogen ion beam composed of roughly 60% N + 2 and 40% N +. The bombarding energy was 20 kev and ion flux density was 25 AA/cm 2. Prior to deposition, the vacuum chamber was pumped to a base pressure of about Pa. During the IBED process, nitrogen gas was introduced into the chamber to a working pressure of about Pa and the wafers were maintained at 700 8C. The combination of Al evaporation and N ion implantation produced the AlN thin films. After deposition, the samples were cut into small pieces for analysis and some samples also underwent annealing at temperatures of 800, 900, 1000 and C, respectively, under nitrogen for 60 min. The annealing temperatures were chosen according to the requirements of device fabrication in microelectronics, and the highest one, C, is a typical temperature used to enhance the bonding strength of SOI substrates produced by the ion-cut process. Spreading resistance profiling (SRP) was utilized to examine the insulating properties of the AlN films. For the electrical measurements, Al dots (12 mm 2 ) were evaporated onto the AlN films in vacuum using a shadow mask technique. For the backside contact, Al was evaporated immediately after an HF clean to remove the native oxide. Current voltage (I V), capacitance voltage (C V) and capacitance frequency (C F) tests were performed. 3. Results and discussion The SRP results acquired from the AlN films deposited at different evaporation rates are depicted in Fig. 1. A spreading resistance higher than 10 8 V (exceeding the measurement range of the SRP apparatus) is found from the surface to a depth of around 250 nm for the film deposited at a rate of 0.05 nm/s [curve (a) in Fig. 1], indicating the best insulating properties among the group. The spreading resistance drops to 10 3 V after a depth of 250 nm corresponding to the Si substrate. Curve (b) in Fig. 1 shows the SRP of the AlN film deposited at 0.1 nm/s and is similar to that of the one deposited at 0.05 nm/s, but its surface spreading resistance is smaller by one to two orders of magnitude. Curve (c) in Fig. 2, the SRP of the film deposited at 0.25 nm/s, exhibits a reversed trend. The surface spreading resistance is two to three orders of magnitude lower than that of the Si substrate. Our results indicate clearly that the spreading resistance of AlN films Fig. 1. Spreading resistance profiles of AlN films deposited using different evaporation rates Al: (a) 0.05 nm/s; (b) 0.1 nm/s; (c) 0.25 nm/s. decreases with increasing Al evaporation rate. This can be correlated to the Al/N ratio in the films. Previous studies [9] show that the Al/N ratio increases with the Al evaporation rate. In the film deposited at the highest rate [curve (c)], Al enrichment renders the film more conductive. The deterioration of the spreading resistance with increasing Al evaporation rate shows that a small Al evaporation rate is more appropriate for AlN fabrication as far as the electrical properties are concerned in spite of the low deposition rate. The electrical breakdown property influences the long term device reliability and lifetime. The I V characteristics of the AlN films deposited at 0.05 and 0.1 nm/s are displayed in Fig. 2(a) and (b), respectively. The breakdown voltage of the former film is about 53 V and the breakdown field is calculated to be 2.1 MV/cm. This breakdown field compares reasonably well with previously reported results. For instance, AlN films deposited by metal organic chemical vapor deposition exhibit breakdown fields in the range of 1 2 MV/cm [10], whereas RF magnetron sputtered films typically have a breakdown field as high as 6 MV/cm [11,12]. However, for the film deposited at 0.1 nm/s, the calculated breakdown field is only about 0.3 MV/cm. The poorer performance is consistent with the SRP results aforementioned and our previous study on the Al/N atomic ratio [9]. Fig. 3 displays the I V characteristics of the AlN film deposited at 0.05 nm/s and subsequently annealed. No abrupt current increase is observed for all the annealed samples and it suggests the AlN films have not been broken down during the test. In other words, the breakdown voltage is larger than 100 V (exceeding the measurement range of our apparatus), and thus the breakdown field is definitely larger than 4 MV/cm that approaches the value achieved by RF magnetron sputtered AlN films [11]. The leakage currents at 40 V for the as-deposited and annealed AlN films are comparable. The leakage current measured from the as-deposited AlN film at 40 V is about A, and it decreases to ~110 6 A after annealing. Thus, the high-

3 132 Z. An et al. / Surface & Coatings Technology 196 (2005) Fig. 2. I V characteristics of AlN films deposited at: (a) 0.05 nm/s, (b) 0.1 nm/s. temperature annealing process reduces the leakage current by about 60 times. The large leakage current in the asdeposited AlN film could be due to the high defects/damage density arising from high energy ion bombardment and these defects can be partially removed by the heat treatment. The C V characteristics of the as-deposited AlN film (0.05 nm/s deposition rate) and the same film after 800 8C annealing are exhibited in Fig. 4(a) and (b), respectively. The solid lines and dotted ones indicate different sweep directions. They are different from the ideal curve expected from a standard metal/insulator/semiconductor (MIS) structure. The curve shape changes with different frequencies used in the measurement. It may be due to the complicated Fig. 3. I V characteristics of AlN films deposited at 0.05 nm/s and annealed at 800, 900, 1000 and C, respectively, under flowing N 2 for 60 min. Fig. 4. C V characteristics of: (a) the AlN film deposited at 0.05 nm/s (acquired at 10 khz) and (b) same sample after annealing at 800 8C (acquired at 3 khz). interfacial region that will be discussed later in this paper. As a matter of fact, in previous reports on the C V characteristics of AlN films, other researchers also fail to fully explain their results [7,8,13]. However, no clear hysteresis can be identified in our C V results and it is different from AlN films deposited by MOCVD [8] and sputtering [13]. Our results suggest that the trapped charge density in our films is extremely low. It can be inferred that the densities of both the interfacial trapped charges and the nitride equivalent of fixed oxide charges are low in our films since they are the main trapped charges causing C V hysteresis [14]. This advantage leads us to believe that IBED may be the better method with respect to the performance of the finished electronic devices. The C F characteristics of the as-deposited AlN film (0.05 nm/s deposition rate) and that after 800 8C annealing films at +3 V are shown in Fig. 5(a) and (b), respectively. The capacitance at low frequencies is considerably higher than that at high frequencies. The capacitance versus frequency curves exhibit well-defined plateaus at both low and high frequencies. In order to interpret the C V and C F results, high-resolution transmission electron microscopy (HRTEM) was performed and Fig. 6 shows the HRTEM image of the interfacial region between the AlN and Si substrate. It can be observed that the interface is not sharp at all, and an interfacial region with a thickness of several nanometers exists. This is actually not surprising since

4 Z. An et al. / Surface & Coatings Technology 196 (2005) Fig. 5. C F characteristics of: (a) AlN film deposited at 0.05 nm/s and (b) same film after annealing at 800 8C. The applied voltage is +3 V. energetic nitrogen ion bombardment is used during deposition to enhance ion mixing and film adhesion. This is an inevitable consequence of IBED. The interfacial region between the amorphous AlN layer and single crystalline Si substrate is almost amorphous but contains certain orderly sub-structures that appear to originate from the original Si lattice (lower side of the TEM micrograph). This interfacial structure has a definite influence on the interfacial defect density and consequently the electrical properties, and as a result, our experimental results deviate from that of an ideal MIS structure. Sakhaf and Schmeits [15] have reported on the C F characteristics of semiconductors hetero-junctions with continuous distribution of the interfacial states inside the gap, and their theory can be applied to this study as well. In Fig. 5, the two plateaus with different capacitance values indicate the presence of interfacial defect states. The constant value up to 10 4 Hz is usually labeled C 0 in the literature and the value that reaches around Hz is labeled as C inf when bulk defects are studied [16]. The line shape exhibited in Fig. 5(a) can be explained as follows. Up to 10 4 Hz, the interfacial states that are responsible for the transitions between the valence or conduction bands follow the modulation of the applied voltage. Above 10 4 Hz, their response is considerably reduced and beyond 10 6 Hz, none of the defect state is able to react to the applied voltage. The cutoff frequency, F ct [defined by C( F ct ) C inf =(C 0 C inf )/2] in this case is 10 5 Hz. For the annealed samples [Fig. 5(b)], different C 0 values are obtained but the C inf values are almost unchanged. This is because unlike C 0, C inf is independent of the interfacial states. The cutoff at 10 7 Hz corresponds to the RC cutoff frequency of the junction. These curves approach the ideal one for a defect-free Al/Si structure that resembles a straight line with constant capacitance up to the RC cutoff frequency. Our data indicate that the interfacial states are partially removed by annealing. The decreasing C 0 values with increasing annealing temperature show that more defect states are Fig. 6. High-resolution transmission electron microscopy (HRTEM) image of the interface between the IBED AlN film and Si substrate.

5 134 Z. An et al. / Surface & Coatings Technology 196 (2005) removed by annealing at higher temperature whereas the shift of F ct may be caused by the evolution of the remaining interfacial states. 4. Conclusion In this work, the electrical properties of AlN films deposited by ion beam enhanced deposition were investigated. The insulating properties of the AlN films deteriorate with increasing Al evaporation rate. The film deposited at an Al evaporation rate of 0.05 nm/s exhibits the best insulating property and its breakdown field is 2.1 MV/cm. After annealing, the breakdown field exceeds 4 MV/cm and the leakage current is reduced by about 60 times. Capacitance voltage (C V) results show that the trapped charges density in the AlN film deposited at 0.05 nm/s are extremely low. Therefore, a low Al evaporation rate combined with high-temperature annealing is the preferred condition for AlN films used as the dielectric layer in silicon-on-insulator substrates for high power, high voltage microelectronic devices. However, owing to ion mixing, the AlN/Si interface is broadened and the interfacial defect states must be controlled carefully. Acknowledgements This work was jointly supported by Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grants (CERG) No. CityU 1137/03E City University of Hong Kong Strategic Research Grant (SRG) # , the special Funds for major state Basic Research projects No. G , as well as the National Nature Science Foundation of China under grant No and References [1] M.O. Aboelfotoh, R.S. Kern, S. Tanaka, R.F. Davis, C.I. Harris, Appl. Phys. Lett. 69 (1996) [2] K.S. Stevens, M. Kinniburgh, A.F. Schwartzman, A. Ohtani, R. Beresford, Appl. Phys. Lett. 66 (1995) [3] S. Bengtsson, M. Bergh, M. Choumas, C. Olesen, K.O. Jeppson, Jpn. J. Appl. Phys. 35 (1996) [4] C.L. Men, Z. Xu, Z.H. An, P.K. Chu, Q. Wan, X.Y. Xie, C.L. Lin, Appl. Surf. Sci. 199 (2002) 287. [5] R. Awadallah, J.S. Yuan, Int. J. Electron. 86 (1999) 707. [6] X. Lu, S.S.K. Iyer, C.M. Hu, N.W. Cheung, J. Min, Z.N. Fan, P.K. Chu, Appl. Phys. Lett. 71 (1997) [7] I.C. Oliveira, M. Massi, S.G. Sanos, C. Otani, H.S. Maciel, R.D. Mansano, Diamond Relat. Mater. 10 (2001) [8] C.L. Aardahl, J.W. Rogers Jr., H.K. Yun, Y. Ono, D.J. Tweet, S.-T. Hsu, Thin Solid Films 346 (1999) 174. [9] C.L. Men, Z. Xu, Z.H. Zheng, X.Z. Duo, M. Zhang, C.L. Lin, Chin. Phys. Lett. 18 (2001) [10] K. Tsubouchi, N. Mikoshiba, IEEE Trans. Sonics Ultrason. 32 (1985) 634. [11] E.V. Gerova, N.A. Ivanov, K.I. Kirov, Thin Solid Films 81 (1981) 201. [12] A. Fathimulla, A.A. Lakhani, J. Appl. Phys. 54 (1983) [13] T. Adam, J. Kolodzey, C.P. Swann, M.W. Tsao, J.F. Rabolt, Appl. Surf. Sci (2001) 428. [14] S.M. Sze, Physics of Semiconductor Devices, Wiley-Interscience, New York, 1981, p [15] M. Sakhaf, M. Schmeits, J. Appl. Phys. 80 (1996) [16] W.G. Oldham, S.S. Naik, Solid-State Electron. 15 (1972) 1085.