Improvement of reproducibility and resistance scattering of high ohmic CrSi thin films

CrSi resistive layers are commonly used for the production of high ohmic thin film resistor devices. The highest resistivities with a required temperature coefficient of resistivity (TCR) near zero ppm/K [roh(indTCR-greater than 0)] are obtainable by incorporation of oxygen into the CrSi layer. Using partial reactive sputtering as a deposition process, resistivities [roh(indTCR-greater than 0)] from 10(exp3) micro-Ohm-cm to 10(exp5) micro Ohm cm can be obtained with the same target composition. However, the reproducibility of the CrSi-O deposition process decreases and the resistance scattering of the devices increases with increasing partial pressure of oxygen. Thus, the maximum obtainable sheet resistance for technical processes is limited to about 10 k-Ohm/square when oxygen is used as the reactive gas. To overcome these problems depositions with various target compositions and oxygen-containing reactive gases such as carbon dioxide (C02) or nitrous oxide (N20) were performed using flat ceramics as substrates. Compared to depositions with 02 as reactive gas an improved process reproducibility and less resistance scattering has been observed, particularly when C02 is used. This can be explained by the lower reactivity of the oxygen-containing gases. The best results were achieved with C02. Electrical measurements of the annealed layers showed that values for [roh(indTCR-greater than 0)] of greater than 10(exp5) micro-Ohm-cm are possible. These trends could be confirmed with CrSi-CO layers on cylindrical ceramic substrates (MELF) which were coated in a bulk deposition process using C02 as reactive gas. Sheet resistivities of more than 20 k-Ohm/square have been obtained with a corresponding scattering of about 20 - 30%. Almost 90% of the samples showed an increased TCR in the range of plus/minus 100 ppm/K after annealing for 6 hours at 480 deg. Cel in air.