Thermal thin-film sensors for r.m.s. Value measurements

Abstract

Abstract Because of their principle of operation, r.m.s. thermal converters work at elevated temperatures. Time stability in such conditions is one of the major concerns behind the choice of materials. Reproducibility and time stability have been achieved here by, using a thin-film microelectronic approach. Such a solution also has the following advantages: relatively fast time response (small dimensions), simplicity of fabrication in a standard process and low cost. Two types of r.m.s. converters (linear and circular) have been developed. In both types the heater is made of a NiCr resistive layer and the temperature sensor consists of NiCr Ni thermocouples. Corning 0.1 mm thick glass foil is used as a substrate. The whole surface is protected by a 5μm thick SiOx layer to improve the stability at elevated temperatures. The converter chips are mounted on a standard TO-5 packages. The sensitivity of the r.m.s. converters is proportional to the thermal resistance and to the sensitivity of the thermocouples, which depends on the thicknesses and microstructures of the Ni and NiCr layers. For NiCr and Ni layers having resistivities below 5 Ω/ □ and 0.5 Ω/ □ respectively, the sensitivity of the thermocouple is about 20μV/K. The thermal resistance of a chip mounted close to the package base mainly depends on the thermal conductivity of the gas (air) and the applied materials. The low conductivity of the glass foil is an advantage here. A thermal resistivity of 2.5 K/mW and r.m.s. conversion sensitivity of 0.5 V/W have been achieved for our sensors.