Micromachined Heat Exchanger for a Cryosurgical Probe

This paper describes a lithography-based microfabrication process developed for a recuperative heat exchanger intended for use in a cryosurgical probe. The probe, which uses the Joule-Thomson (JT) cooling cycle, must achieve a temperature < -50°C, with a freeze rate of 25-50°C/min. The heat exchanger must maintain high stream-to-stream thermal conductance while restricting parasitic stream-wise (axial) conduction losses. It has a footprint of 6x1.5cm and 2.5mm thickness, and is fabricated using a five-mask process. Rows of fins composed of high conductivity silicon are bonded onto a 100μm base plate composed of low conductivity Pyrex glass. The fabrication process involves anodic bonding, deep reactive ion dry etching, glass frit bonding, electrochemical drilling and several other steps, along with special features that compensate for manufacturing tolerances. The paper describes the fabrication challenges and how they constrain dimensional variables.