A Materials Investigation of Nickel Based Contacts to n-SiC Subjected to Operational Thermal Stresses Characteristic of
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devices it is important to assess the performance of device components under the influence of their potential operational stress regimes. This is particularly critical for pulsed power device applications, namely palpitated high power switching, where the operational environment is dominated by acute cyclic pulsed power actions which ultimately translate into severe thermal, electrical and mechanical cyclic stresses in the device materials. In order to fully explore SiC's utilization for pulsed power switching applications it is necessary to determine the effects of such cyclic stress regimes, both individually and as combined effects, on the fundamental pulsed power device components. In this paper we report results of a unidimensional, that is, non-combined effects, investigation which evaluated the reliability of Ni-SiC ohmic contact device components in response to acute cyclic thermal loading. Our results demonstrate that the electrical, compositional and structural integrity of the metal-SiC interface strongly influences the reliability of the Ni-SiC ohmic contacts under acute cyclic thermal stress. It is well documented that device performance is often limited by the electrical and materials integrity of the ohmic contacts [4,5]. Since ohmic contacts are a fundamental component of all pulsed power devices the ohmic contact-SiC device structure was selected for cyclic thermal testing. A number of different metals have been proposed as suitable ohmic contacts to n-SiC. Specifically, metals such as Ni, A1/Ni/AI, Cr, Al, Au-Ta, TaSi 2 , W, Ta, Ti, Ti/Au, TiSi 2 , Co and WSi have been studied with the Ni based metallization systems suggested as superior candidates due to their low specific contact resistance, (p,), less than 5.0x10-6 ohms-cm2 [6-9]. Additionally, published data suggests that annealed Ni contacts. which react with SiC to form Ni-Si. exhibit excellent static thermal stability at temperatures as high as 500'C [7]. Based on this information Ni was selected as the contact metallization for cyclic thermal fatigue testing. 93 Mat. Res. Soc. Symp. Proc. Vol. 572 ©1999 Materials Research Society
EXPERIMENTAL 200 nm of Ni was deposited on 4H n-type SiC substrates purchased from CREE Research. The SiC substrates were non-research grade with a micropipe density of greater than 100 cm-2 . The substrates were Si faced and the donor density was 2.0 x 1019 cm-3 . Prior to the metal deposition the wafers were cleaned in warm electronic grade trichoroethane (TCA), boiling acetone and methanol followed by a rinse in deionized water. The Ni deposition was accomplished via electron beam evaporation with a base pressure of 5x10- 7 Torr. The Ni on SiC samples, were annealed at 950'C for 5 min. in a N 2 ambient in order to produce ohmic behavior. Cyclic thermal fatigue experiments were conducted using a 10.6 Pim IR pulsed CO 2 laser. The pulsed thermal fatigue design was configured for a 3 second heating interval followed by a 60 second cooling interval. The heating and cooling intervals were chosen to mimic military
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