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Low resistivity ohmic contacts on 4H-silicon carbide for high power and high temperature device applications
Microelectronic Engineering 60 (2002) 261–268 www.elsevier.com / locate / mee
Low resistivity ohmic contacts on 4H-silicon carbide for high power and high temperature device applications a ¨ S.-K. Lee a , *, C.-M. Zetterling a , M. Ostling , J.-P. Palmquist b , U. Jansson b a
Department of Microelectronics and Information Technology, KTH, Royal Institute of Technology, Electrum 229, S-164 40, Kista, Sweden b˚ ¨ Laboratory, Department of Inorganic Chemistry, Uppsala University, S-755 21, Uppsala, Sweden Angstrom
Abstract We investigated titanium based ohmic contacts using co-evaporated epitaxial titanium carbide (TiC) on highly doped n 1 - and p 1 -type epilayers as well as Al ion implanted layers for high power and high temperature device application. Epitaxially grown TiC ohmic contacts on epilayers as well as Al implanted layers of 4H-SiC were formed by UHV co-evaporation with Ti and C 60 at low substrate temperature. The specific contact resistance ( rC ) was as low as 5 3 10 26 , 2 3 10 25 , and 2 3 10 25 Vcm 2 for TiC contacts on n 1 , on p 1 epilayer, and on Al implanted layer, respectively, using a linear TLM measurement. In addition to TiC, we also investigated TiW (weight ratio 30:70) ohmic contacts to p- and n-type 4H-SiC for the purpose of long-term reliability tests at high temperature. The average rC of sputtered TiW contacts was 4 3 10 25 for p 1 and n 1 epilayer. We also found that an evaporated top layer (Au or Pt) helps to protect from degradation of the contacts under long-term reliability tests with temperatures of up to 6008C in a vacuum chamber. 2002 Elsevier Science B.V. All rights reserved. Keywords: Ohmic contacts; Ohmic contact resistance; Power device; 4H-SiC
1. Introduction Silicon carbide has received considerable attention during the last decade as one of the promising device materials for especially high temperature, high power, and high frequency applications under which conventional semiconductors can barely perform. At the present time, full performance of silicon carbide (SiC) devices such as unipolar and bipolar SiC devices is limited by material quality and the fabrication of high temperature stable and low resistivity ohmic contacts. Many research reports have been published on silicon carbide concerning both Schottky and ohmic electrical * Corresponding author. Tel.: 1 46-8-752-1300; fax: 1 46-8-752-7850. E-mail address: [email protected] (S.-K. Lee). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 01 )00603-7
2002 Elsevier Science B.V. All rights reserved.
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contacts. Lower specific contact resistance can usually be obtained to n-type 4H- and 6H-SiC ( | 10 24 to 10 26 Vcm 22 ) than to p-type 4H- and 6H-SiC ( | 10 23 to 10 25 Vcm 22 ) [1,2]. Ion implantation is one of the alternative solutions to increase the number of donors and acceptors for the formation of low resistivity ohmic contacts. Interest in ion implantation is increasing in SiC process technology due to the various advantages despite difficulties such as a high sheet resistivity, high temperature annealing, and low activation efficiency [3]. Device application also requires high performance of the ohmic contacts under high power and high temperature (up to 6008C) operational stresses without any contact degradation or wire bond failure. The contact material should be selected for low resistivity, reaction properties and compatibility to the process for SiC device application. In this regard, titanium based metals (titanium carbide and titanium tungsten) are potentially promising metals for both ohmic and Schottky contacts. They have quite low resistivity, can be wet etched, and are very inert with respect to reaction with SiC under high temperature. We also demonstrated that titanium tungsten (TiW) Schottky contacts have excellent rectifying characteristics after annealing at 5008C with a thermally stable ideality factor of 1.08 and a Schottky barrier height of 1.22 and 1.93 eV for n- and p-type 4H-SiC, respectively, in the temperature range of 24–3008C [4]. In this paper we investigated low resistivity ohmic contacts of titanium based contacts (TiC and TiW) to 4H-SiC for high power and high temperature device application. We will discuss ohmic contact behavior in detail with annealing effect and high temperature measurement. We also investigated the long-term reliability of ohmic contacts with a cap layer (in our case e-beam evaporated gold and platinum) under high temperature (500 and 6008C in a vacuum furnace). Some of the material characterization (LEED, RBS, XPS, and XRD) is also reported.
2. Experimental details
2.1. Ohmic contacts on epi-layer of 4 H-SiC Both highly doped n- and p-type epilayers (0.5–1 mm thick) with a doping of 1.1 to 1.3 3 10 19 cm 23 for n 1 and various doping densities (6 3 10 18 , 1.3 3 10 19 , and . 10 20 cm 23 ) for p 1 epilayer on the Si-face (0001) 4H-SiC were used for the experiments. We cleaned starting wafers in two sequential cleaning processes with a mixture of H 2 SO 4 :H 2 O 2 (2.4:1) at a temperature . 808C for 5 min and H 2 O:CH 3 CH(OH)CH 3 :HF (100:3:1) for 100 s. After each cleaning process, the wafers were rinsed with deionized (DI) water and blown dry in N 2 gas. Prior to the mesa etching, all samples were ˚ / h) was etched subjected to sacrificial oxidation at 12508C. Thermally grown silicon dioxide ( ¯ 530 A away with HF(5%) solution for 5 min prior to metal deposition. Inductively coupled plasma (ICP) etching was used in a mixture of SF 6 and Ar at 600 W RF forward power, 50 W platen power, and 5.0 mTorr base pressure with an evaporated aluminum etch mask (0.25 mm thick), resulting in an etching rate of 0.16 mm / min. After mesa-structure etching, the epitaxial growth of titanium carbide contacts was performed in a UHV chamber comprising an electron beam evaporator for Ti and a Knudsen ˚ / min. For effusion cell for C 60 . TiC films were deposited at 5008C with a deposition rate of 30–35 A TiW contacts, the sputtering was performed by two different dc magnetron sputterers under deposition conditions of | 75 sccm Ar gas flow, room temperature or 2008C substrate temperature, 5 3 10 27 Torr base pressure, and 5 3 10 23 Torr deposition pressure using a TiW target with weight ratio of
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30:70. Transmission line method (TLM) structure consisting of five contact pads separated by 5–25 mm was defined by a lithographic process. TiC and TiW were wet etched using a mixture of 25% ˚ / min. The annealing was performed by diluted NH 3 :H 2 O 2 (1:5) with an etching rate of 300–2000 A 26 rapid thermal annealing (RTA) with 10% H 2 in Ar or vacuum furnace with base pressure of 2 3 10 Torr. For electrical characteristics, current–voltage (I–V ) measurements were performed in a Cascade probe station connected to an HP 4156A semiconductor parameter analyzer for as-deposited and annealed contacts at 500, 700, and up to 9508C. TiC contacts were also characterized by in situ low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ex situ X-ray diffraction (XRD), and Rutherford backscattering spectroscopy (RBS). Secondary ion mass spectroscopy (SIMS) measurement was also employed on several different regions of the TLM structure for sputtered TiW ohmic contacts in order to investigate the dependence of the specific contact resistance on the concentration.
2.2. Ohmic contacts on implanted layer of 4 H-SiC For ohmic contact formation on implanted layers, we used a nitrogen doped n-type epilayer (10 mm ¨ ¨ thick, | 9 3 10 15 cm 23 ) grown (IFM, Linkoping University, Linkoping, Sweden) on a Si-face (0001) n 1 4H-SiC wafer which was purchased from CREE (CREE Research, Durham, NC, USA). Prior to ˚ thickness was deposited using the Al ion implantation a silicon nitride (Si 3 N 4 ) layer of 1400-A plasma enhanced chemical vapor deposition (PECVD) on the SiC surface. This was done in order to reach the highest surface concentration (which was determined by TRIM-97 simulation [5]). After ion implantation the silicon nitride layer was removed. The samples were implanted by Al at 7008C with a dose of 3 3 10 14 cm 22 and energy of 180 keV and annealed at 17008C for 30 min in Ar ambient with silane for the activation of acceptors and the recrystallization of SiC (ABB Corporate Research, Kista-Stockholm, Sweden). The epitaxial TiC films were deposited and the SiC was patterned by exactly the same procedure as for TiC ohmic contacts on epilayer of 4H-SiC.
3. Results and discussion
3.1. Epitaxially grown TiC ohmic contacts 3.1.1. TiC metallization on Al implanted 4 H-SiC Fig. 1 shows the theoretical depth profile, which is generated by TRIM-97 simulation, for implanted Al in SiC with a dose of 3 3 10 14 cm 22 and energy of 180 keV, and SIMS results after Al implantation and post-implantation annealing. As shown in Fig. 1, the peak Al concentration at the SiC surface from the SIMS results was ¯ 2 3 10 19 cm 23 , which is consistent with the TRIM simulation using a Si 3 N 4 sacrificial layer even though the peak of Al concentration was slightly shifted. However, the TRIM simulation does not include information about the diffusion and crystal structure (channeling and orientation) of SiC. The summarized results of TLM measurements for TiC ohmic contacts to Al ion implanted 4H-SiC as a function of various annealing temperatures (as-deposited, 500, 700 and 8508C) and measurement temperature are shown in Fig. 2. We observed that TLM structures had different values of sheet resistance (R S ), varying from 0.6 to 6.3 kV / h for as-deposited contacts in
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Fig. 1. Experimental (SIMS result) and simulated (TRIM result) depth profiles for Al implanted into SiC. The dose and energy were 3 3 10 14 cm 22 and 180 keV, respectively.
the temperature range 25–3008C. Even the as-deposited contacts showed good ohmic behavior from I–V characteristics. For as deposited contacts, the specific contact resistance ( rC ) was 4.8 3 10 25 2 25 2 Vcm at 3008C. After annealing at 5008C, rC reached the lowest value of 2.0 3 10 Vcm at 258C. Finally after sequential annealing at 700 and 8508C, we observed that rC did not improve further. These results of the annealing temperature dependence indicated that epitaxially grown TiC contacts on Al ion implanted 4H-SiC would not require higher ( . 5008C) annealing temperature reduce rC . We also believe that the combination of epitaxial TiC contacts and a sacrificial silicon nitride layer to
Fig. 2. The specific contact resistance ( rC ) of TiC ohmic contacts (TLM1) to 4H-SiC versus measurement temperature and annealing temperature. The annealing was performed by RTA in a 10% H 2 /Ar mixture for 180 s.
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reach a high Al dopant concentration at the surface causes the lower specific contact resistance on Al implanted 4H-SiC epilayers.
3.1.2. TiC metallization on highly doped epilayer Fig. 3 shows an X-ray diffractogram of a typical TiC film with clear hexagonal LEED pattern indicating epitaxial growth of the film. The diffractogram shows only reflections of the h111j-type from the film, which suggests a highly textured or an epitaxial growth with the relationship TiC(111) / / SiC(0001). The lattice misfit for this relationship can be estimated to be | 0.6% [6]. XRD and RBS results also showed no indication of interfacial reactions after high temperature annealing (700 and 9508C RTA). For TiC contacts to p-type 4H-SiC, we found that rC was 1 3 10 24 Vcm 2 at 258C. After annealing at 9508C, rC reached its lowest value of 2 3 10 25 Vcm 2 at 3008C. Contrary to p-type TiC contacts, the results of n-type TiC contacts showed different behavior in the specific contact resistance between as-deposited and annealed contacts. The rC value was 5 3 10 26 to 4 3 10 25 Vcm 2 for as-deposited and 9508C annealed contacts. A detailed study of the microstructure at the interface after high temperature annealing and carbon composition variation dependence on the specific contact resistance is required to fully understand the mechanism at the interface. 3.2. Sputtered titanium tungsten ohmic contacts 3.2.1. p- and n-type ohmic contact characterization For the p-type contact study, we used sputtered TiW on two different epilayers (P1 and P2 were doped 1.3 3 10 19 and 6 3 10 18 cm 23 , respectively) on n-type 4H-SiC. After 9808C RTA, the average specific contact resistance ( rC ) was as low as 1.2 3 10 24 and 4.0 3 10 25 Vcm 2 for samples P1 and P2, respectively. Secondary ion mass spectroscopy (SIMS) measurement was employed to determine
˚ thick blanket TiC 0.7 film deposited on 4H-SiC. The inset shows the hexagonal Fig. 3. X-ray diffraction spectra of a 900 A LEED pattern from the same sample.
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Fig. 4. SIMS depth profile for sample P2 for different TLM structures (1, 2, 3, and 4) with specific contact resistance (inset).
the exact doping concentration. From Fig. 4, sample P2 had a large variation of rC and the figure shows that the specific contact resistance strongly depends on the doping concentration [7]. Fig. 5 shows the specific contact resistance as a function of the temperature up to 3008C for n-type TiW ohmic contacts to 4H-SiC. The results of n-type sputtered TiW are summarized in Table 1. The rC
Fig. 5. Specific contact resistance distribution as a function of the temperature from 25 to 3008C for n-type TiW ohmic contacts.
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Table 1 Summary of ohmic contact resistivity for different titanium based contacts (TiC and TiW) to n- and p-type 4H-SiC Epi-type
1
n 4H-SiC p 1 4H-SiC Al ion implanted
Specific contact resistance ( 3 10 25 Vcm 2 ) TiC
TiW
0.5–4 2–20 2–10
2–6 0.4–12 Not performed
was 2–6 3 10 25 Vcm 2 for samples N1 and N2. We also found that substrate heating (in our case 2008C) makes the TiW film dense, which is confirmed by wet etching tests.
3.2.2. Long-term reliability with TiW contacts For the long-term reliability study, we evaporated cap layers (in our case platinum and gold with an ˚ thick titanium) on annealed TiW ohmic contacts. The thickness of the top layer adhesion layer, 300 A ˚ was 2500 A. We tested five different samples called N1-1 (TiW), N1-2 (Au / Ti / TiW), N3 (Pt / Ti / TiW), N2-1 (TiW), and N2-2 (Au / Ti / TiW) to n-type 4H-SiC. The difference between the sample groups (N1-1, N1-2, and N3) and (N2-1 and N2-2) is the substrate temperature during TiW deposition (2008C and room temperature, respectively) made in different sputtering systems. Another difference is that the samples are selected from different regions on the same wafer, which could influence the specific contact resistance for each sample due to the concentration difference. The samples were annealed under high temperature (500 and 6008C) in vacuum furnace (Pbase ¯ 2 3 10 26 Torr). Fig. 6 shows the results of the specific contact resistance as a function of the high temperature annealing time up to 308 h for each sample, N1-1, N1-2, N3, N2-1, and N2-2. After 150 h, we accelerated the test with a higher temperature (6008C) in the same vacuum furnace. As shown in Fig. 6, we clearly
Fig. 6. Specific contact resistance of sample N1-1 (TiW), N1-2 (Au / Ti / TiW), N3 (Pt / Ti / TiW), N2-1 (TiW), and N2-2 (Au / Ti / TiW) as a function of annealing time up to 308 h in the vacuum furnace at 500 and 6008C (legend indicates deposition temperature).
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see that evaporated Au and Pt top layers help to protect from the degradation of the contacts under the long-term reliability test up to 308 h.
4. Conclusions We demonstrated low temperature contact formation and electrical characteristics of epitaxially grown TiC ohmic contacts to 4H-SiC using a novel combination of co-evaporation and a sacrificial silicon nitride for ion implantation. In addition, sputtered TiW ohmic contacts to both n- and p-type 4H-SiC contact were also investigated with electrical characterization and long-term reliability for high power and high temperature device applications. From our linear TLM measurement, the specific contact resistance ( rC ) was as low as 5 3 10 26 , 2 3 10 25 , and 2 3 10 25 Vcm 2 for TiC contacts on n 1 , on p 1 epilayer, and on Al implanted layer, respectively (Table 1). We found some variation of the specific contact resistance and the sheet resistance from our TLM measurement for sputtered TiW ohmic contacts to p-type 4H-SiC indicating that rC strongly depends on the doping concentration of epitaxial layers from SIMS measurement. We also found that a protective cap layer (Pt or Au) on contact metals could help ohmic contacts to operate for a much longer time under high temperature environment. The capping layers, Pt or Au, are also compatible with wire bonding. A more detailed study of long-term reliability of contacts under harsh environment such as O 2 , NOx , and higher temperature (up to 8008C) is being initiated for automotive device applications.
Acknowledgements This work has been supported by the Swedish Foundation for Strategic Research (SSF) in one of the research projects in SiCEP. The author acknowledges Dr Henry Bleichner in ABB Corporate Research AB for assistance.
References [1] L.M. Porter, R.F. Davis, Mater. Sci. Eng. B34 (1995) 83–105. [2] J. Crofton, L. Beyer, J.R. Williams, E.D. Luckowski, S.E. Mohney, J.M. Delucca, Solid State Electron. 41 (1997) 1725–1729. [3] T. Kimoto, A. Itoh, H. Matsunami, T. Nakata, M. Watanabe, J. Electron. Mater. 25 (1996) 879–884. ¨ [4] S.-K. Lee, C.-M. Zetterling, M. Ostling, J. Appl. Phys. 87 (2000) 8039–8044. [5] J.F. Ziegler, TRIM-97, Yorktown, NY, USA. [6] L. Norin, S. McGinnis, U. Jansson, J.O. Carlsson, J. Vac. Sci. Technol. A15 (1997) 3082–3085. [7] E.H. Rhoderick, R.H. Williams, 2nd Edition, Metal-Semiconductor Contacts, Vol. 19, Clarendon Press, Oxford, 1988.
Low resistivity ohmic contacts on 4H-silicon carbide for high power and high temperature device applications a ¨ S.-K. Lee a , *, C.-M. Zetterling a , M. Ostling , J.-P. Palmquist b , U. Jansson b a
Department of Microelectronics and Information Technology, KTH, Royal Institute of Technology, Electrum 229, S-164 40, Kista, Sweden b˚ ¨ Laboratory, Department of Inorganic Chemistry, Uppsala University, S-755 21, Uppsala, Sweden Angstrom
Abstract We investigated titanium based ohmic contacts using co-evaporated epitaxial titanium carbide (TiC) on highly doped n 1 - and p 1 -type epilayers as well as Al ion implanted layers for high power and high temperature device application. Epitaxially grown TiC ohmic contacts on epilayers as well as Al implanted layers of 4H-SiC were formed by UHV co-evaporation with Ti and C 60 at low substrate temperature. The specific contact resistance ( rC ) was as low as 5 3 10 26 , 2 3 10 25 , and 2 3 10 25 Vcm 2 for TiC contacts on n 1 , on p 1 epilayer, and on Al implanted layer, respectively, using a linear TLM measurement. In addition to TiC, we also investigated TiW (weight ratio 30:70) ohmic contacts to p- and n-type 4H-SiC for the purpose of long-term reliability tests at high temperature. The average rC of sputtered TiW contacts was 4 3 10 25 for p 1 and n 1 epilayer. We also found that an evaporated top layer (Au or Pt) helps to protect from degradation of the contacts under long-term reliability tests with temperatures of up to 6008C in a vacuum chamber. 2002 Elsevier Science B.V. All rights reserved. Keywords: Ohmic contacts; Ohmic contact resistance; Power device; 4H-SiC
1. Introduction Silicon carbide has received considerable attention during the last decade as one of the promising device materials for especially high temperature, high power, and high frequency applications under which conventional semiconductors can barely perform. At the present time, full performance of silicon carbide (SiC) devices such as unipolar and bipolar SiC devices is limited by material quality and the fabrication of high temperature stable and low resistivity ohmic contacts. Many research reports have been published on silicon carbide concerning both Schottky and ohmic electrical * Corresponding author. Tel.: 1 46-8-752-1300; fax: 1 46-8-752-7850. E-mail address: [email protected] (S.-K. Lee). 0167-9317 / 02 / $ – see front matter PII: S0167-9317( 01 )00603-7
2002 Elsevier Science B.V. All rights reserved.
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contacts. Lower specific contact resistance can usually be obtained to n-type 4H- and 6H-SiC ( | 10 24 to 10 26 Vcm 22 ) than to p-type 4H- and 6H-SiC ( | 10 23 to 10 25 Vcm 22 ) [1,2]. Ion implantation is one of the alternative solutions to increase the number of donors and acceptors for the formation of low resistivity ohmic contacts. Interest in ion implantation is increasing in SiC process technology due to the various advantages despite difficulties such as a high sheet resistivity, high temperature annealing, and low activation efficiency [3]. Device application also requires high performance of the ohmic contacts under high power and high temperature (up to 6008C) operational stresses without any contact degradation or wire bond failure. The contact material should be selected for low resistivity, reaction properties and compatibility to the process for SiC device application. In this regard, titanium based metals (titanium carbide and titanium tungsten) are potentially promising metals for both ohmic and Schottky contacts. They have quite low resistivity, can be wet etched, and are very inert with respect to reaction with SiC under high temperature. We also demonstrated that titanium tungsten (TiW) Schottky contacts have excellent rectifying characteristics after annealing at 5008C with a thermally stable ideality factor of 1.08 and a Schottky barrier height of 1.22 and 1.93 eV for n- and p-type 4H-SiC, respectively, in the temperature range of 24–3008C [4]. In this paper we investigated low resistivity ohmic contacts of titanium based contacts (TiC and TiW) to 4H-SiC for high power and high temperature device application. We will discuss ohmic contact behavior in detail with annealing effect and high temperature measurement. We also investigated the long-term reliability of ohmic contacts with a cap layer (in our case e-beam evaporated gold and platinum) under high temperature (500 and 6008C in a vacuum furnace). Some of the material characterization (LEED, RBS, XPS, and XRD) is also reported.
2. Experimental details
2.1. Ohmic contacts on epi-layer of 4 H-SiC Both highly doped n- and p-type epilayers (0.5–1 mm thick) with a doping of 1.1 to 1.3 3 10 19 cm 23 for n 1 and various doping densities (6 3 10 18 , 1.3 3 10 19 , and . 10 20 cm 23 ) for p 1 epilayer on the Si-face (0001) 4H-SiC were used for the experiments. We cleaned starting wafers in two sequential cleaning processes with a mixture of H 2 SO 4 :H 2 O 2 (2.4:1) at a temperature . 808C for 5 min and H 2 O:CH 3 CH(OH)CH 3 :HF (100:3:1) for 100 s. After each cleaning process, the wafers were rinsed with deionized (DI) water and blown dry in N 2 gas. Prior to the mesa etching, all samples were ˚ / h) was etched subjected to sacrificial oxidation at 12508C. Thermally grown silicon dioxide ( ¯ 530 A away with HF(5%) solution for 5 min prior to metal deposition. Inductively coupled plasma (ICP) etching was used in a mixture of SF 6 and Ar at 600 W RF forward power, 50 W platen power, and 5.0 mTorr base pressure with an evaporated aluminum etch mask (0.25 mm thick), resulting in an etching rate of 0.16 mm / min. After mesa-structure etching, the epitaxial growth of titanium carbide contacts was performed in a UHV chamber comprising an electron beam evaporator for Ti and a Knudsen ˚ / min. For effusion cell for C 60 . TiC films were deposited at 5008C with a deposition rate of 30–35 A TiW contacts, the sputtering was performed by two different dc magnetron sputterers under deposition conditions of | 75 sccm Ar gas flow, room temperature or 2008C substrate temperature, 5 3 10 27 Torr base pressure, and 5 3 10 23 Torr deposition pressure using a TiW target with weight ratio of
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30:70. Transmission line method (TLM) structure consisting of five contact pads separated by 5–25 mm was defined by a lithographic process. TiC and TiW were wet etched using a mixture of 25% ˚ / min. The annealing was performed by diluted NH 3 :H 2 O 2 (1:5) with an etching rate of 300–2000 A 26 rapid thermal annealing (RTA) with 10% H 2 in Ar or vacuum furnace with base pressure of 2 3 10 Torr. For electrical characteristics, current–voltage (I–V ) measurements were performed in a Cascade probe station connected to an HP 4156A semiconductor parameter analyzer for as-deposited and annealed contacts at 500, 700, and up to 9508C. TiC contacts were also characterized by in situ low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ex situ X-ray diffraction (XRD), and Rutherford backscattering spectroscopy (RBS). Secondary ion mass spectroscopy (SIMS) measurement was also employed on several different regions of the TLM structure for sputtered TiW ohmic contacts in order to investigate the dependence of the specific contact resistance on the concentration.
2.2. Ohmic contacts on implanted layer of 4 H-SiC For ohmic contact formation on implanted layers, we used a nitrogen doped n-type epilayer (10 mm ¨ ¨ thick, | 9 3 10 15 cm 23 ) grown (IFM, Linkoping University, Linkoping, Sweden) on a Si-face (0001) n 1 4H-SiC wafer which was purchased from CREE (CREE Research, Durham, NC, USA). Prior to ˚ thickness was deposited using the Al ion implantation a silicon nitride (Si 3 N 4 ) layer of 1400-A plasma enhanced chemical vapor deposition (PECVD) on the SiC surface. This was done in order to reach the highest surface concentration (which was determined by TRIM-97 simulation [5]). After ion implantation the silicon nitride layer was removed. The samples were implanted by Al at 7008C with a dose of 3 3 10 14 cm 22 and energy of 180 keV and annealed at 17008C for 30 min in Ar ambient with silane for the activation of acceptors and the recrystallization of SiC (ABB Corporate Research, Kista-Stockholm, Sweden). The epitaxial TiC films were deposited and the SiC was patterned by exactly the same procedure as for TiC ohmic contacts on epilayer of 4H-SiC.
3. Results and discussion
3.1. Epitaxially grown TiC ohmic contacts 3.1.1. TiC metallization on Al implanted 4 H-SiC Fig. 1 shows the theoretical depth profile, which is generated by TRIM-97 simulation, for implanted Al in SiC with a dose of 3 3 10 14 cm 22 and energy of 180 keV, and SIMS results after Al implantation and post-implantation annealing. As shown in Fig. 1, the peak Al concentration at the SiC surface from the SIMS results was ¯ 2 3 10 19 cm 23 , which is consistent with the TRIM simulation using a Si 3 N 4 sacrificial layer even though the peak of Al concentration was slightly shifted. However, the TRIM simulation does not include information about the diffusion and crystal structure (channeling and orientation) of SiC. The summarized results of TLM measurements for TiC ohmic contacts to Al ion implanted 4H-SiC as a function of various annealing temperatures (as-deposited, 500, 700 and 8508C) and measurement temperature are shown in Fig. 2. We observed that TLM structures had different values of sheet resistance (R S ), varying from 0.6 to 6.3 kV / h for as-deposited contacts in
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Fig. 1. Experimental (SIMS result) and simulated (TRIM result) depth profiles for Al implanted into SiC. The dose and energy were 3 3 10 14 cm 22 and 180 keV, respectively.
the temperature range 25–3008C. Even the as-deposited contacts showed good ohmic behavior from I–V characteristics. For as deposited contacts, the specific contact resistance ( rC ) was 4.8 3 10 25 2 25 2 Vcm at 3008C. After annealing at 5008C, rC reached the lowest value of 2.0 3 10 Vcm at 258C. Finally after sequential annealing at 700 and 8508C, we observed that rC did not improve further. These results of the annealing temperature dependence indicated that epitaxially grown TiC contacts on Al ion implanted 4H-SiC would not require higher ( . 5008C) annealing temperature reduce rC . We also believe that the combination of epitaxial TiC contacts and a sacrificial silicon nitride layer to
Fig. 2. The specific contact resistance ( rC ) of TiC ohmic contacts (TLM1) to 4H-SiC versus measurement temperature and annealing temperature. The annealing was performed by RTA in a 10% H 2 /Ar mixture for 180 s.
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reach a high Al dopant concentration at the surface causes the lower specific contact resistance on Al implanted 4H-SiC epilayers.
3.1.2. TiC metallization on highly doped epilayer Fig. 3 shows an X-ray diffractogram of a typical TiC film with clear hexagonal LEED pattern indicating epitaxial growth of the film. The diffractogram shows only reflections of the h111j-type from the film, which suggests a highly textured or an epitaxial growth with the relationship TiC(111) / / SiC(0001). The lattice misfit for this relationship can be estimated to be | 0.6% [6]. XRD and RBS results also showed no indication of interfacial reactions after high temperature annealing (700 and 9508C RTA). For TiC contacts to p-type 4H-SiC, we found that rC was 1 3 10 24 Vcm 2 at 258C. After annealing at 9508C, rC reached its lowest value of 2 3 10 25 Vcm 2 at 3008C. Contrary to p-type TiC contacts, the results of n-type TiC contacts showed different behavior in the specific contact resistance between as-deposited and annealed contacts. The rC value was 5 3 10 26 to 4 3 10 25 Vcm 2 for as-deposited and 9508C annealed contacts. A detailed study of the microstructure at the interface after high temperature annealing and carbon composition variation dependence on the specific contact resistance is required to fully understand the mechanism at the interface. 3.2. Sputtered titanium tungsten ohmic contacts 3.2.1. p- and n-type ohmic contact characterization For the p-type contact study, we used sputtered TiW on two different epilayers (P1 and P2 were doped 1.3 3 10 19 and 6 3 10 18 cm 23 , respectively) on n-type 4H-SiC. After 9808C RTA, the average specific contact resistance ( rC ) was as low as 1.2 3 10 24 and 4.0 3 10 25 Vcm 2 for samples P1 and P2, respectively. Secondary ion mass spectroscopy (SIMS) measurement was employed to determine
˚ thick blanket TiC 0.7 film deposited on 4H-SiC. The inset shows the hexagonal Fig. 3. X-ray diffraction spectra of a 900 A LEED pattern from the same sample.
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Fig. 4. SIMS depth profile for sample P2 for different TLM structures (1, 2, 3, and 4) with specific contact resistance (inset).
the exact doping concentration. From Fig. 4, sample P2 had a large variation of rC and the figure shows that the specific contact resistance strongly depends on the doping concentration [7]. Fig. 5 shows the specific contact resistance as a function of the temperature up to 3008C for n-type TiW ohmic contacts to 4H-SiC. The results of n-type sputtered TiW are summarized in Table 1. The rC
Fig. 5. Specific contact resistance distribution as a function of the temperature from 25 to 3008C for n-type TiW ohmic contacts.
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Table 1 Summary of ohmic contact resistivity for different titanium based contacts (TiC and TiW) to n- and p-type 4H-SiC Epi-type
1
n 4H-SiC p 1 4H-SiC Al ion implanted
Specific contact resistance ( 3 10 25 Vcm 2 ) TiC
TiW
0.5–4 2–20 2–10
2–6 0.4–12 Not performed
was 2–6 3 10 25 Vcm 2 for samples N1 and N2. We also found that substrate heating (in our case 2008C) makes the TiW film dense, which is confirmed by wet etching tests.
3.2.2. Long-term reliability with TiW contacts For the long-term reliability study, we evaporated cap layers (in our case platinum and gold with an ˚ thick titanium) on annealed TiW ohmic contacts. The thickness of the top layer adhesion layer, 300 A ˚ was 2500 A. We tested five different samples called N1-1 (TiW), N1-2 (Au / Ti / TiW), N3 (Pt / Ti / TiW), N2-1 (TiW), and N2-2 (Au / Ti / TiW) to n-type 4H-SiC. The difference between the sample groups (N1-1, N1-2, and N3) and (N2-1 and N2-2) is the substrate temperature during TiW deposition (2008C and room temperature, respectively) made in different sputtering systems. Another difference is that the samples are selected from different regions on the same wafer, which could influence the specific contact resistance for each sample due to the concentration difference. The samples were annealed under high temperature (500 and 6008C) in vacuum furnace (Pbase ¯ 2 3 10 26 Torr). Fig. 6 shows the results of the specific contact resistance as a function of the high temperature annealing time up to 308 h for each sample, N1-1, N1-2, N3, N2-1, and N2-2. After 150 h, we accelerated the test with a higher temperature (6008C) in the same vacuum furnace. As shown in Fig. 6, we clearly
Fig. 6. Specific contact resistance of sample N1-1 (TiW), N1-2 (Au / Ti / TiW), N3 (Pt / Ti / TiW), N2-1 (TiW), and N2-2 (Au / Ti / TiW) as a function of annealing time up to 308 h in the vacuum furnace at 500 and 6008C (legend indicates deposition temperature).
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see that evaporated Au and Pt top layers help to protect from the degradation of the contacts under the long-term reliability test up to 308 h.
4. Conclusions We demonstrated low temperature contact formation and electrical characteristics of epitaxially grown TiC ohmic contacts to 4H-SiC using a novel combination of co-evaporation and a sacrificial silicon nitride for ion implantation. In addition, sputtered TiW ohmic contacts to both n- and p-type 4H-SiC contact were also investigated with electrical characterization and long-term reliability for high power and high temperature device applications. From our linear TLM measurement, the specific contact resistance ( rC ) was as low as 5 3 10 26 , 2 3 10 25 , and 2 3 10 25 Vcm 2 for TiC contacts on n 1 , on p 1 epilayer, and on Al implanted layer, respectively (Table 1). We found some variation of the specific contact resistance and the sheet resistance from our TLM measurement for sputtered TiW ohmic contacts to p-type 4H-SiC indicating that rC strongly depends on the doping concentration of epitaxial layers from SIMS measurement. We also found that a protective cap layer (Pt or Au) on contact metals could help ohmic contacts to operate for a much longer time under high temperature environment. The capping layers, Pt or Au, are also compatible with wire bonding. A more detailed study of long-term reliability of contacts under harsh environment such as O 2 , NOx , and higher temperature (up to 8008C) is being initiated for automotive device applications.
Acknowledgements This work has been supported by the Swedish Foundation for Strategic Research (SSF) in one of the research projects in SiCEP. The author acknowledges Dr Henry Bleichner in ABB Corporate Research AB for assistance.
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