Ohmic Contacts for High Temperature Integrated Circuits in Silicon Carbide

Sammanfattning: In electrical devices and integrated circuits, ohmic contacts are necessary and a prerequisite for the current transport over the metal-semiconductor junctions. At the same time, a desired property of the ohmic contacts is to not add resistance or in other way disturb the performance. For high temperature electronics, the material demands are high regarding functionality and stability at elevated working temperatures, during and after temperature cycling and during long time of use. Silicon carbide, SiC, is a suitable substrate material for fabrication of high temperature electronics with a large band gap, good thermal conductivity and chemical inertness. The large bandgap causes however difficulties in finding a common material for ohmic contacts to both n- and p-type SiC. The goal of this project is to find a suitable metallization for n- as well as p-type contacts to epitaxially doped SiC for use in integrated circuits. The circuit technology developed and used in KTH for SiC is built on bipolar junction transistors. The p-doped base layer in the transistors is relatively low doped; 5 x 1017 – 1 x 1018 cm-3. P-type SiC is more difficult to form ohmic contacts to than n-type SiC, with such a low doping concentration it is even more complicated. Today, the metallizations used in the contacts consists of Ni for n-type contacts and Ni/Ti/Al to p-type SiC. Experiments of the specific contact resistivity ?c and the sheet resistance Rsh of these contacts, characterized to two types of samples, are presented for the extreme temperature range of -40 °C – 500 °C. Ohmic contact test chips with transfer length method structures have been compared to similar test structures on samples from the KTH SiC integrated circuits. The metals were also tested on the opposite doping polarity. Sputter deposition of Co and Si in layers to form CoSi2 contacts was made to chips from the same contact test wafer for comparison with the behavior of the presently used contacts. Since this was a new metallization, the process development of the high temperature anneal and contact formation made an important part of the work. Out of the tested contacts, Ni has been the best for n-SiC and Ni/Ti/Al for p-SiC. None of the three metallizations formed ohmic contacts to both n- and p-SiC. However, the CoSi2 contacts showed specific contact resistivity ?c <4 x 10-6 ?cm2 at 25 °C and non-linear I-V characteristics but without barriers to p-SiC after voltage sweeps of ± 75V.