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In-Situ Characterization of Ohmic Contacts to N-Type SiC Under High Temperature and Current


We report the results of the in-situ characterization of Ti/TaSi2/Pt ohmic contacts to 6H-SiC during high temperature and current accelerated stressing. Several 6H-SiC four-point probe test structures are exposed to a constant current of 1 mA and temperature up to 600 °C in air ambient at intervals of 100 hrs up to 400 hrs. The insertion of SiC devices into the critical sections of the propulsion systems and on-board electronics and sensors for planetary missions to harsh planets (i.e., Venus) and other cosmic bodies will require that these devices survive and operate reliably during the entire mission. The temperature in these environments is sometimes greater than 500 °C and with high radiation values. Thus, the goal of this work is to evaluate the long term stability and reliability of Ti/TaSi2/Pt ohmic contact metallization to 6H-SiC under simulated constant operating conditions at high temperature. The current-voltage (I-V) characteristics, the series resistance, and the specific contact resistance (SCR) of the test structures are measured in-situ under the above environmental conditions. After every 100 hrs of heating at 600 °C in air and at 1 mA, the samples are cooled and similar measurements are repeated. The results are then compared to determine deviations from the initial values. Unstable voltage offsets that precede the eventual failure of SiC devices have been largely attributed to the electrical instability at the semiconductor metallurgical junction. Another marker of premature failure is associated with the irreversible drift of the device reading during operation. One source of drift is likely due to microstructural changes in the material after extended current stressing. Before the contact resistance is measured, the ohmic behavior of the test structure is verified. The device is heated in the atmosphere from room temperature to 600 °C. The temperature effect on the resistance shows a non-linear behavior, starting with a high resistance at room temperature and gradually decreasing. From about 250 °C, the resistance begins an upward swing and continues to 600 °C. This behavior is explained by the dominance of impurity and phonon scattering mechanisms on current transport at temperature below 250 °C and above, respectively. The plots of the resistance versus temperature during the various 100 hr cyclic soaking period do not track perfectly as is expected under ideal conditions, thus suggesting that microstructural changes are occurring either within the metal contact (e.g., phase transformation), at the metallurgical junction (e.g., chemical reaction), or within the crystal. With regard to the SCR, it is initially measured to be 4 x 10-4 cm2 and remains constant over the temperature range after the first 100 hrs. However, a factor of two increase is observed after 400 hrs of soak at 600 °C in air, even as the SCR remains relatively constant at the new values. At the talk, auger analysis and scanning electron microscopy will be used in conjunction with the I-V characteristics to discuss the failure mechanisms. 

D. DeAngelis and R. S. Okojie. In-Situ Characterization of Ohmic Contacts to N-Type SiC Under High Temperature and Current. In the
Proceedings of the 46th Electronic Materials Conference (EMC04); Notre Dame, IN; June 23-25, 2004; p. 37.