Micromachined 30 K Joule-Thomson cryogenic cooler

For many electronic devices, colder is better. At lower temperatures, electronic devices such as infrared detectors and low-noise amplifiers operate with a higher signal-to-noise ratio and better overall performance than they do at room temperature. Superconducting devices such as superconducting quantum interference devices need extremely cold temperatures to operate. However, exising cryogenic coolers are very large compared to sizes of these devices to be cooled and mismatch the small cooling power requirements of these devices. In order to allow more widespread use of these electronic devices, micro-sized cryogenic coolers need to become cheaper and more reliable. Addressing this challenge, this thesis focuses on the design, optimization and fabrication of a 30 K (-243 °C) micro-sized cryocooler. The microcooler is fabricated using only micromachining technology. This technology offers high fabrication accuracy and the possibility of batch processing, which opens the opportunity of mass production. The application potential of the microcooler coupled with electronic devices has been demonstrated by cooling an yttrium barium copper oxide film through its superconducting phase transition. The utilization of the microcooler in cooling a low-noise amplifier is investigated by using a 115 K single-stage microcooler. A major hurdle in the long-term operation of microcoolers is the clogging caused by the deposition of water that is present as impurity in the working fluid. It is found that the position and the rate at which the water molecules deposit mainly depends on the inlet partial pressure of water and the temperature profile along the microcooler. The clogging rate can be reduced by decreasing the inlet partial pressure of water using a getter filter and changing the temperature profile along the microcooler using a piece of silicon. The microcooler requires a vacuum environment to reduce the parasitic loss that is due to the heat flow from the warm environment via the surrounding gas. The possible sources of gas, the evolution mechanisms and their corresponding effects on the vacuum prewssure have been discussed theoretically and experimentally.