RuO₂ Thermometer for Ultra-Low Temperatures

A small, high-resolution, low-power thermometer has been developed for use in ultra-low temperatures that uses multiple RuO₂ chip resistors. The use of commercially available thickfilm RuO₂ chip resistors for measuring cryogenic temperatures is well known due to their low cost, long-term stability, and large resistance change. To measure the resistance, a small excitation is applied across the sensor and the resistance is measured. At increased currents, a greater output signal is achieved, resulting in better sensitivity Problems including lowered resolution and sensitivity are found because of selfheating at temperatures below 100 mK when a single chip is used.

A new thermometer design uses multiple RuO₂ chip resistors and off-theshelf bobbins. The chips would be configured in an array to spread the heat over a greater area during excitation. A technique was developed to connect the chips together, first in a 2×2 array, and finally in a 3×3 array. The 3×3 array configuration of the RuO2 chips allows better internal heat distribution than a single chip, thereby reducing self-heating. The uniqueness of this design is in the array configuration, which allows greater sensitivity at ultralow temperatures while keeping a small package footprint [about 0.4 in. (10 mm)]. The device uses a standard round bobbin with a #4 screw through-hole.

This work was done by Thomas Hait, Peter J. Shirron, and Michael DiPirro of Goddard Space Flight Center. For further information, contact the Goddard Innovative Partnerships Office at (301) 286-5810. GSC-15690-1