Reducing Multijunction Thermal Converter Output Resistance to Reduce Measurement Noise

摘要

This paper describes the effects on device performance caused by modifications to thermocouple design and fabrication. Such modifications are intended to lower the output resistance of multijunction thermal converters (MJTCs) fabricated on fused silica substrates. Device component metals were deposited by physical vapor deposition and fabricated into device features by either wet chemical etching or metal lift-off. The low-temperature-coefficient-of-resistance Ni75Cr20Al2.5Cu2.5alloy heater is thermally isolated from the rest of the chip by a $25 mu mathrm{m}$ fused silica membrane produced by bead blasting the back side of the substrate. Thermocouples were shorted along 50% and 25% of their length for a set of test devices; control devices were left unmodified. MJTC output resistances were decreased from about $13.2 mathrm{k}Omega$ for the non-shorted thermocouples to about $10.5 mathrm{k}Omega$ for the 50% shorted thermocouples. At 3 V input voltage, measured ac-dc differences of the devices for frequencies between 10 Hz and 1 MHz showed no significant impact on ac-dc difference due to thermocouple shorting. However, we observed indications that shorting the thermocouples results in a reduction in the signal-to-noise ratio of the measurement of the output voltage.