The Impact of BEOL Stress on SiGe HBTs at Cryogenic Temperatures

To support the use of SiGe heterojunction bipolar transistors (UBTs) in cryogenic environments such as space or quantum computing readout, the temperature dependence of back-end-of-line (BEOL) induced mechanical stress effects was investigated from 300 K to 13 K. Test structures with 9 metal layers placed directly above the device were used to introduce BEOL stress in the SiGe HBT. Measurements and simulations show a substantial increase in sensitivity to BEOL stress at cryogenic temperatures. This increase is attributed to a combination of the higher slope of the Gummel characteristics with cooling and higher overall stress caused by thermal mismatch. Increasing the distance of the metal stack from the active device was simulated to help determine the region in which BEOL stress effects are significant. For circuits intended to operate at cryogenic temperatures, designers should ensure identical BEOL routing around devices sensitive to mismatch.