Materials Science and Engineering B 135 (2006) 289–293
Ta–Si contacts to n-SiC for high temperatures devices M. Guziewicz a,∗ , A. Piotrowska a , E. Kaminska a , K. Grasza b,c , R. Diduszko b,c , A. Stonert d , A. Turos b,d , M. Sochacki e , J. Szmidt e a Institute of Electron Technology, Al. Lonikow 32/46, 02-668 Warsaw, Poland Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland c Institute of Physics, Polish Academy of Science, Al. Lonikow 32/46, 02-668 Warsaw, Poland d Institute of Nuclear Problems, Hoza 24, 00-662 Warsaw, Poland e Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland b
Abstract The properties of Ta–Si contact to n-SiC have been investigated by complementary use of 2 MeV He+ Rutherford backscattering spectroscopy and X-ray diffraction measurements. Electrical properties were characterized by current–voltage characteristics and specific contact resistance. Contact metallization was deposited on 4H- and 6H-SiC (0 0 0 1) wafers by RF magnetron sputtering the 100 nm thick Ta silicide film. The Ni(100 nm) film was sputtered on reference wafers for comparative studies electrical characteristics of the contacts. The I–V characteristics of Ta–Si and Ni contact are linear after rapid thermal annealing (RTA) at 850 and 750 ◦ C, for 3 min, respectively. The specific contact resistance of 4 × 10−5 cm2 for Ta–Si contact annealed at 950 ◦ C is achieved on the 4H-SiC with donor concentration of (6–8) × 1018 cm−3 . For the reference contact, the contact resistivity is 2 × 10−5 cm2 . The amorphous microstructure of Ta–Si contacts is stable up to 900 ◦ C and no significant reaction at SiC/Ta–Si interface is observed at this temperature. After annealing at 1000 ◦ C a reaction at the interface is revealed but limited to very narrow region. The phases of Ta5 Si3 , Ta, -Ta, Ta2 C and Ta(O) are detected in amorphous matrix of the film. Surface morphology of the Ta–Si contact is stable up to 1000 ◦ C, therefore, this kind of contact has great advantage over Ni-silicides contacts. Even at 1100 ◦ C, Ta–Si contact has still abrupt interface with SiC and featureless surface. © 2006 Elsevier B.V. All rights reserved. Keywords: Ohmic contact; Silicon carbide; Contact metallurgy; Specific contact resistance; Tantalum silicide
1. Introduction Rapid progress in SiC technology opens a new window for high temperature electronics. The processing of SiC semiconductor into device structures imposes a number of challenges. Formation of low resistive and thermally stable ohmic contact to this wide band gap semiconductor is crucial for successful work under high power and high temperature (up to 600 ◦ C) operational stresses without any contact degradation. The contact material should be selected for low resistivity, low reactivity and compatibility to the SiC process technology. Currently, ohmic contacts to n-SiC are formed by depositing single metal layer or bi-layers on high-doped SiC and subsequent annealing to about 1000 ◦ C. Unfortunately, interfacial reaction results in rough surface, poor thermal stability of metal/SiC ohmic contacts and nonuniform interface [1,2]. Numerous refractory metals such as
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[email protected] (M. Guziewicz).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.08.021
Ti, W, Ni, Mo, Ta, were studied as materials for contact metallization, but Ni is commonly applied. As a result of thermally activated reaction at Ni/SiC interface, Ni silicides form yielding low resistivity contact. Moreover, carbon-rich phases or carbon inclusions are formed [3]. On the other hand, metal silicides and carbides [4] deposited on SiC give contacts with improved morphology and interface, and some ternary phase involving silicon and carbon could ensure stable and homogeneous metallization [5]. In this regard, tantalum based materials are potentially promising metals for ohmic contacts. They have quite low resistivity and are very inert with respect to reaction with SiC under high temperature. In the present work Ta–silicide was chosen as a promising ohmic contact to n-SiC. The decision was made taking into account superior thermal stability of SiC/Ta–Si contact [6], as well as our experience gained during development of thermally stable metallization schemes for GaAs and GaN [7–9]. The effect of thermal processing of SiC/Ta–Si contact on its resistivity, morphology and structure was investigated. Lowresistivity ohmic contact with excellent surface is presented. For
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comparison purposes, the properties of SiC/Ni contacts were studied. 2. Experimental SiC substrates used for this study were: (i) n-type 4H-SiC, with donor concentration of (6–8) × 1018 cm−3 ; (ii) 6H-SiC, with donor concentration of (1–2) × 1018 cm−3 and (iii) high resistive 6H-SiC with resistivity above of 100 cm. Net doping of SiC substrates was determined by Hg-probe CV measurement. Samples were (0 0 0 1) oriented Si-face with 4◦ of-axis and on-axis for 4H-SiC and 6H-SiC, respectively. The substrates were shaped as squares with the side length of 10 mm. Before metal deposition, the samples were cleaned in boiling trichloroethane, acetone, and methanol, followed by a rinse in deionized water. Next SiC surface was chemically etched sequentially in hot solutions of NH4 OH:H2 O2 :H2 O = 1:1:5 for 10 min, H2 O2 :HCl:H2 O = 1:1:5 for 10 min at 70 ◦ C, BOE for 2 min, and rinsed in deionized water. Ni and TaSi metallizations were deposited by RF magnetron targets, respectively. Heat treatment experiments were carried out at temperatures ranging from 600 to 1100 ◦ C under Ar flow in the RTP system. The annealing time for 3 min was chosen expecting detectable changes in metallization and at interface
metal/SiC. Electrical characterisation of metal/semiconductor contacts involved measurements of specific contact resistance and current–voltage (I–V) characteristics using a Keithley Source Meter 2400. Contact resistivity was evaluated using the circular transmission line method [10]. CTLM patterns were defined by lift off photolithography. The internal circles with the diameter of 100 m were separated from the outer metal area by spacing of 10, 20, 30, 45 and 60 m. A Veeco fourpoint probe was used to measure electrical resistivity of the film deposited on bare high resistive SiC wafers. Microstructure of contacts was analyzed by XRD, morphology was observed by optical and SEM microscopes, and surface roughness measured by ␣-Step® Tencor profiler. Composition profile of contacts was measured by 2 MeV He+ RBS spectroscopy. 3. Results and discussion 3.1. Composition and microstructure of the metallization RBS spectra for SiC/TaSi(100 nm) samples before and after annealing at 900, 1000 and 1100 ◦ C are shown in Fig. 1. Because of the low intensity of RBS signals for carbon only Ta and Si depth profiles could be analyzed. In the case of Si this concerns both Si in the substrate and in the TaSi layer. The RUMP code
Fig. 1. 2 MeV He+ backscattering spectra of the 6H (0 0 0 1)SiC/TaSi(100 nm) metallization before and after TRA annealing in Ar at 900, 1000 and 1100 ◦ C for 3 min: total spectra (a), high-energy Si edge (b) and the Ta signal (c).
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Fig. 2. X ray diffraction spectra for Cu K␣ radiation of the 6H (0 0 0 1)SiC/TaSi(100 nm) contact metallization before and after annealing in Ar for 3 min at 900 ◦ C (a), 1000 ◦ C (b) and 1100 ◦ C (c).
simulation of the spectra for the TaSi film yields the atomic ratio of Ta:Si as above 3 thus indicating that its composition is richer in Ta than in Ta5 Si3 compound used as the target material. The spectrum for the sample annealed at 750 ◦ C has not shown any difference when compared to the spectrum for asdeposited sample, hence, is not included in the figures. After annealing the sample at 900 ◦ C for 3 min a small decrease of the Ta signal height is visible. Such a reduction of the height indicates the presence of another element in the film. The most probable explanation is that Si and C have penetrated from the SiC substrate into the TaSi film. According to the RUMP simulation evaluated here Si concentration is increased about 2 at.%. However, no shift of the Si high-energy edge has been observed. The RBS spectra for the sample annealed at 1000 ◦ C revealed more pronounced changes in Ta and Si depth profiles. The width of the Ta signal has increased by about 9%, and the peak height was further decreased. This is apparently due to the Si diffusion from the SiC substrate into the film. Such diffusion produces changes in the Si distribution and small shift of high-energy Si edge of the Si towards lower energies (cf. Fig. 2b). One notes that the Si signal is enhanced at the region near the SiC/TaSi interface. The simulation can only be used for determination of Si distribution. The density of Si in the film was increased near
the interface. The possible C atoms diffusion can be indirectly deduced. It seems that also carbon atoms became mobile under annealing. Taking into account very high diffusion coefficient of C in bulk Ta, it could be supposed that C released from SiC surface penetrated into the layer too. The number of Si atoms diffused into the film may be evaluated as above 4% of the total number of atoms in the as-deposited film. Upon annealing at 1100 ◦ C the spectrum has revealed a difference in the profile of the Ta peak and the Si signal. Basing on RUMP simulation one can speculate that the top shape of the Ta signal is modified by the enhanced Si concentration in the near surface region accompanied by the similar C-distribution in the vicinity of substrate/film interface. It should be also pointed out that the sharp SiC/TaSi interface indicates that there is no penetration of Ta into the SiC substrate. The X-ray diffraction spectra of the SiC/TaSi metallization are shown in Fig. 2. The spectrum of as-deposited film has increased intensity in the region of 32–45◦ , where main Ta peaks might be expected. This proves amorphous structure of the films. The spectra of film annealed at 900 ◦ C for 3 min reveal only three small peaks of Ta5 Si3 . For the film annealed at 1000 ◦ C more peaks were detected indicating crystal grains of Ta5 Si3 , Ta, -Ta, Ta(O) and Ta2 C embedded in amorphous matrix. The presence
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Table 1 The structure of the Ti–Si metallization formed via RTA processing of 6H (0 0 0 1)SiC/TaSi(100 nm) at Ar flow for 3 min Temperature (◦ C)
Phases of Ta–Si metallization
As-deposited 750 900 1000 1100
Amorphous Amorphous Amorphous matrix, Ta5 Si3 Amorphous matrix, Ta5 Si3 , Ta2 C, Ta, Ta(O), -Ta Ta5 Si3 , Ta2 C, Ta(O), Ta, -Ta, a some amorphous phase
of unreacted Ta grains is consistent with the previous evaluation that the film is rich with tantalum. After annealing at 1100 ◦ C the Ta5 Si3 phase is more crystallized as indicated by peak intensities (ratio of peaks intensity/background is increased). Positions of some of the Ta5 Si3 peaks are moved towards lower angles suggesting increased lattice spacing. This shift may have originated from nonstoichiomery of phase and/or point defects. The Ta2 C peaks seem to have intensity comparable to those of the film annealed at 1000 ◦ C, but Ta peaks are less represented. The two peaks were identified as those for the less known Ta(O) phase. An increased signal at 38.3◦ in Fig. 2c, where Ta2 C peak is in position close to the Ta5 Si3 peak, indicates most probably more crystallized Ta2 C and Ta5 Si3 phases. It is worthy to mention that crystal phases still coexist with amorphous matrix in the metallization. Carbon is lightly represented by weak Ta2 C peak. Therefore, it may influences the lattice spacings of the Ta5 Si3 phase. For that reason the considered layer after annealing could be assigned as the Ta–Si–C metallization and the as-deposited layer as the Ta–Si metallization. The influence of annealing temperature on the structure of Ta–Si metallization is summarized in Table 1. 3.2. Morphology and electrical characteristics of the Ta–Si and Ni contacts The 100 nm tick Ta–Si film is characterized by electrical resistivity of 265 cm this can be acceptable for using as metallization. As-deposited SiC/Ta–Si contacts show Schottky type behavior on both 4H-SiC and 6H-SiC substrates. After annealing at 750 ◦ C in Ar for 3 min, the I–V characteristics become more symmetric than rectifying for 4H-SiC substrate,
and specific contact resistance is estimated in the range of the decade of 10−3 cm2 . After further annealing at 850 ◦ C, the I–V characteristic becomes linear and the contact resistivity decreases to 9 × 10−4 cm2 . Subsequent annealing at T = 950 ◦ C reduces contact resistivity to 4 × 10−5 cm2 . Reference Ni contact deposited on 4H-SiC wafer was ohmic after annealing at 750 ◦ C. Annealing at 950 ◦ C results in contact resistivity of 2 × 10−5 cm2 . I–V characteristics of contacts on 6H-SiC substrate become linear after annealing at 900 ◦ C. However, no reaction on SiC/Ta–Si contact annealed at this temperature was detected, but unreacted Ta could react with SiC at interface as it was observed on -SiC/Ta contact [6] annealed at the same temperature. Therefore, we believe that the interface had to be changed and density of interface states was reduced to form a low resistive ohmic contact. As a results of annealing at 950 and 1000 ◦ C, contact resistivity decreased to (1–2) × 10−4 cm2 . The surface morphology of Ta–Si metallization seems to be intact up to 900 ◦ C, while morphology of Ni film becomes worse with increasing annealing temperature. These behaviors can be directly correlated with thermal and chemical stability of metallizations. The SiC/Ni contact surface becomes rough because of strong solid-state reaction [1]. The surface of the SiC/Ta–Si contact is smooth and featureless even after annealing at 1100 ◦ C. This should serve as an advantage for this contact over the SiC/Ta contact formed at similar temperature [6]. The characteristics of Ni and Ta–Si metallization are summarized in Table 2. Taking into account that the 4H-SiC substrates are doped to higher level ((6–8) × 1018 cm−3 ) than 6H-SiC substrates ((1–2) × 1018 cm−3 ) this could explain the outcome for lower specific contact resistance on 4H-SiC than on 6HSiC. The Ni surface becomes rough at temperature before ohmic contact is thermally formed. Only a small increase in roughness was noted on the SiC/Ta–Si contact annealed above 900 ◦ C, but it is a few times lower than roughness of the SiC/Ni contact. The contact resistivity of Ta–Si is in satisfactory range ((0.4–1) × 10−4 cm2 ) as can be obtained on similarly doped n-SiC substrates. Optimizing Ta–Si film thickness and timetemperature conditions an improvement in low contact resistivity could be probably attained. The above study shows a great chance to apply the Ta–Si contact in high temperature electronic devices.
Table 2 Specific contact resistance RC of SiC/Ta–Si(100 nm) and SiC/Ni(100 nm), and surface roughness(*) of metallizations vs. annealing temperature Temperature (◦ C)
RC of 4H-SiC/Ta–Si ( cm2 )
RC of 4H-SiC/Ni ( cm2 )
RC of 6H-SiC/Ta–Si ( cm2 )
Roughness of 6H-SiC/Ta–Si (nm)
Roughness of 6H-SiC/Ni (nm)
As-deposited 750 850 900 950 1000
NL QL 9 × 10−4 – 4 × 10−5 –
NL L – – 2 × 10−5 –
NL NL QL 3 × 10−4 1 × 10−4 2 × 10−4
3 3 3 3.5 6 6
4 17 27 30 35 –
RTA processing at Ar, 3 min; n-type 4H-SiC with donor concentration of (1–2) × 1018 cm−3 , 6H-SiC with donor concentration of (6–8) × 1018 cm−3 ; NL: nonlinear; L: linear; QL: quasi-linear; (*) roughness measured on the 20 m scan length at horizontal sampling step of 40 nm and at vertical resolution of 0.5 nm.
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4. Conclusions
References
Comparative study of the Ta–Si and Ni contacts to n-SiC has been performed. Both metallizations allow formation of ohmic contacts to n-SiC after annealing in temperature range of 850–1000 ◦ C. The specific contact resistance of 4 × 10−5 cm2 for Ta–Si contact and of 2 × 10−5 cm2 for Ni-based contact annealed at 950 ◦ C is achieved on the 4H-SiC with donor concentration of 7 × 1018 cm−3 . The surface morphology of the Ta–Si contact is excellent as compared to rough Ni-based metallization. The amorphous microstructure of Ta–Si metallization as well as the SiC/Ta–Si interface is stable up to 900 ◦ C. A reaction at interface takes place after annealing at 1000 and 1100 ◦ C, but this is limited to a very narrow region. The phases of Ta5 Si3 , Ta, -Ta, Ta2 C and Ta(O) are present in amorphous matrix of the Ta–Si film. Abrupt SiC/Ta–Si interface and a featureless surface are observed after annealing at 1100 ◦ C.
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Acknowledgements This work was partially supported by the Ministry of Science and Information Technologies, Poland trough the grant No. 3 T11B 042 30.
