High Current Density Stability of Ohmic Contacts to Silicon Carbide

Open Access
Author:
Downey, Brian P
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 11, 2011
Committee Members:
  • Suzanne E Mohney, Dissertation Advisor
  • Suzanne E Mohney, Committee Chair
  • Douglas Edward Wolfe, Committee Member
  • Patrick M Lenahan, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • high current
  • electromigration
  • Pd
  • contact
  • ohmic
  • SiC
  • stability
Abstract:
The materials properties of SiC, such as wide bandgap, high breakdown electric field, and good thermal conductivity, make it an appealing option for high temperature and high power applications. The replacement of Si devices with SiC components could lead to a reduction in device size, weight, complexity, and cooling requirements along with an increase in device efficiency. One area of concern under high temperature or high current operation is the stability of the ohmic contacts. Ohmic contact degradation can cause an increase in parasitic resistance, which can diminish device performance. While contact studies have primarily focused on the high temperature stability of ohmic contacts to SiC, different failure mechanisms may arise under high current density stressing due to the influence of electromigration. In addition, preferential degradation may occur at the anode or cathode due to the directionality of current flow, known as a polarity effect. The failure mechanisms of ohmic contacts to p type SiC under high current density stressing are explored. Complementary materials characterization techniques were used to analyze contact degradation, particularly the use of cross sections prepared by focused ion beam for imaging using field emission scanning electron microscopy and elemental analysis using Auger electron spectroscopy. Initially the degradation of commonly studied Ni and Al based contacts was investigated under continuous DC current. The contact metallization included a bond pad consisting of a TiW diffusion barrier and thick Au overlayer. The Ni contacts were found to degrade due to the growth of voids within the ohmic contact layer, which were initially produced during the high temperature Ni/SiC ohmic contact anneal. The Al based contacts degraded due to the movement of Al from the ohmic contact layer to the surface of the Au bond pad, and the movement of Au into the ohmic contact layer from the bond pad. The inequality of Al and Au fluxes generated voiding within the ohmic contact layer causing a large increase in contact resistance. A bottom to top approach was used to develop a more robust contact structure based on the failure mechanisms of the Ni and Al based contacts. Contacts utilizing a Pd layer contacting the SiC were found to provide a lower specific contact resistance (ρc) and improved stability under current stressing. A Pd/Ti contact was introduced that when annealed under a N2 atmosphere produced a robust TiN layer at the surface of the contact. The ρc of the Pd/Ti contact was (4.7±1.7)x10-6 Ω cm2, compared to the Ni and Al based contacts, all of which had a ρc of greater than 10-5 Ω cm2. The Pd/Ti contacts were able to withstand higher currents than the Ni or Al based contacts under continuous DC current stressing. The degradation mechanism of the Pd/Ti contacts depended on whether the current was pulsed or continuous. Under continuous DC stressing, Au from the bond pad diffused through the TiW barrier and into the ohmic contact region leading to severe intermixing and voiding. Under pulsed DC stressing, voiding at the Au/TiW interface occurred caused by the electromigration of Au. The different degradation mechanisms were related to the temperature during stressing, as the temperature of the continuous DC stressed contacts exceeded 649 ˚C at failure, while the peak temperature of the pulsed DC contacts was between 316 ˚C and 371 ˚C, using 5 µs pulses and a 10% duty cycle. Finally, the lowest ρc, (1.4±0.6)x10-6 Ω cm2, was attained with a Pd/Ti/Pt contact, which also possessed a very smooth surface morphology, especially compared to the conventional Ti/Al contact. The Pd/Ti/Pt contacts were also shown to be more stable under continuous DC lateral current stressing than the Ti/Al contacts. A polarity effect on temperature was observed during stressing with the temperature of the cathode being higher than the anode, likely due to carrier recombination at the cathode for the p type material. The increased temperature caused preferential degradation of the cathode of both the Pd/Ti/Pt and Ti/Al contacts. Under continuous DC stressing, degradation of the Pd/Ti/Pt contacts was characterized by voiding and intermixing at the leading edge of the cathode. The temperature of the Pd/Ti/Pt contacts during stressing was reduced under pulsed DC current, and voiding was observed between the Au bond pad and the ohmic contact. Mechanical stresses and thermal cycling were suspected to have produced the voiding under pulsed DC stressing.