Miniaturized chip calorimeter for high-pressure cells at low temperature

Abstract

Heat capacity measurements under high pressure places high demands on the calorimeter. Here we describe the development of a miniaturized nanocalorimeter for high-pressure heat capacity measurements at low temperature. The device, fabricated on a silicon substrate, employs a high-frequency AC calorimetry technique and features a design with an outer diameter of 300 $μ$m and thickness of 25-40 $μ$m, small enough to fit into high pressure diamond anvil cells. Miniaturization is achieved by stacking all components, including thermometer and heaters, within a central area. The thin-film calorimeter thermometer measures 40 $μ$m square and maintains the sensitivity and properties of larger thermometers. The fabrication process uses controlled anisotropic etch to produce calorimeter chips with a balance between robustness and thickness, suitable for experiments at high pressures and low temperatures. The calorimeter operates at a relatively high characteristic frequency between 10 Hz and 1 kHz, constraining the thermal oscillation to an effective volume dominated by the sample, thereby avoiding the use of a suspended membrane that is the basis for conventional nanocalorimeters.