Effect of seed layers and rapid thermal annealing on the temperature coefficient of resistance of Ni-Cr thin films

In this research, we studied the integration of radio frequency (RF)-sputtered Ni-Cr resistors to improve the resistor behavior so that the resistance can be independent of temperature, or the temperature coefficient of resistance (TCR). The deposited Ni-Cr (80/20 at.%) films were annealed with or without a seed layer, such as Ti, Cr, and Ni, by rapid thermal annealing (RTA) in the temperature range from 550 degrees C to 850 degrees C with 150 degrees C intervals for 90 s in nitrogen ambient to get obtain films with higher thermal stability. The TCR was dramatically increased after RTA at a higher temperature of 700 degrees C. The higher RTA temperature showed a larger grain size of the Ni-Cr film and more diffusion of Cr and Ni atoms. The Ni-Cr film deposited on the seed layers showed a lower TCR than that without the seed layer after the RTA process. The TCR values of the 10-nm-thick Cr/100-nm-thick Ni-Cr (top) film were 35.8 ppm/K (at 125 degrees C) and 56.2 ppm/K (460 degrees C), respectively, before and after RTA at a temperature of 550 degrees C for 90 s. This shows the potential of sputtered Ni-Cr film as the resistor for a pressure sensor in CMOS applications considering a back-end-of-line (BEOL) annealing temperature.