Thermal and thermomechanical study of micro-refrigerators on a chip based on semiconductor heterostructures

We present results from optical characterization of active solid-state SiGe/Si thermionic micro coolers with sizes ranging from 40×40 up to 100×100 micron square. These devices have achieved 7K cooling at 100°C ambient temperature. These micro refrigerators can be used to remove hot spots in IC chips and achieve localized temperature control. Transient thermoreflectance measurements have shown that the cooling speed of these thin film coolers is on the order of 20-30 microseconds, 10⁴ times faster than the commercial Bi₂Te₃ thermoelectric coolers. We characterized several micro-refrigerators devices by various optical non-contact methods such as interferometry or thermoreflectance. Maximum surface temperature and displacement was measured for a variety of devices sizes. The contribution of Peltier/thermoionic effect at interfaces and Joule heating inside the structure were separated by studying their different current dependence. Cooling is proportional to the current while Joule heating is proportional to the square of the current. We found that these two terms have different device size area dependence. This was explained by the fact that cooling occurs on top of the device and thus the cooling temperature is proportional to the sum of the device and substrate thermal resistances while the temperature rise due to Joule heating is only proportional to the substrate thermal resistance. This shows that the dominant source of heat is in the buffer layer below the device or in the substrate itself.