TCR Control of Ni/Cr Resistors

A detailed study was made to identify the important deposition parameters and to determine how they interact to affect the TCR and sheet resistance of Ni/Cr films deposited on alumina. Wire-fed flash evaporation is described as a simple method for the control of composition, deposition rate, and thickness. The Ni/Cr ratio, substrate temperature, amount of Au in the Ni/Cr, and the film thickness have major effects on TCR. Deposition rate and pressure and the effect of a Ni barrier layer between the Ni/Cr and the Au conductor were also investigated. The use of a Ni barrier layer degrades the resistor stability somewhat. Approximately 50 percent Cr is necessary to get the TCR on alumina below about +100 ppm/°C, but variation from 50 to 70 percent has little further effect, while Au in the Ni/Cr depresses the TCR, and can make it negative. Reduced substrate temperature during film condensation and increased film thickness also reduce TCR. Sheet resistance is affected by thickness, Au content, and substrate temperature. A particular value for each of the above parameters was found that when used together produced 'zero' TCR and 180 \Omega/square sheet resistance.