Effects of Packaging on Thermal Resistance below 1 K for Cryogenic Temperature Sensors

Cryogenic thermometry utilizing resistive temperature sensors presents a challenge in that simply performing a measurement can cause an error due to self-heating of the resistive element. Increases in resolution and accuracy can be obtained by maximizing the sensor signal. The limiting factor is the thermal connection of the sensor to its environment, and the quantitative measure of this property is the thermal resistance of the sensor which relates the excitation power of the sensor to the subsequent temperature rise due to self-heating. The thermal resistance is directly dependent upon the temperature and design details of the package. Greater thermal connection of the sensor element to the environment results in a lower thermal resistance, and subsequently allows a greater excitation level during operation before encountering severe self-heating problems. In this research, twenty-three CernoxTM resistance temperature sensors were fabricated using three different packaging techniques. Self-heating was measured at eleven temperatures ranging from 0.1 K to 1.2 K allowing calculation of the thermal resistance at these temperatures. Data for the three packaging techniques are intercompared. The experiment, data, and analysis are detailed in this paper.