A study into the electrical trim of silicon chromium thin film resistors

Silicon Chromium (SiCr) Thin Film Resistors (TFRs) have been developed and integrated into CMOS ICs for over twenty years. Current state of the art sputtering technology achieves integrated resistors with temperature coefficient of resistance (TCR) of -25 ppm/ᵒC and absolute resistance values post LASER trim of 0.01%. Discrete passive resistors however are now stating TCR figures of 0.05 ppm/ᵒC which vastly improves the stability of the resistor over temperature. Discrete resistors achieve these accuracies by designing a specific substrate that compensates for the TCR of the resistive film. This dissertation details the implementation of a novel TCR and absolute resistance post processing trimming technique (iTrim). This trimming method results in TCRs and absolute resistance values equivalent to discrete resistors but integrated into a standard CMOS IC stack. iTrim is a controlled Joule heating technique, generated using high power densities to precision anneal the SiCr film post processing. The energy generated from Joule heating induces a physical change to a portion of the resistive film. The altered film is Chromium dominant which results in a lower resistance and positive TCR. By balancing the ratio of standard negative TCR material and new positive TCR material, a near zero TCR resistor can be generated. The ratio of positive TCR material to negative TCR material is measured and controlled using a power sweep across the resistor. From this power sweep, the power coefficient of resistance (PCR) can be extracted. The PCR value has been shown to provide an efficient approximation of the TCR value while only requiring single ambient temperature testing. In the first section of this thesis an overview is given of current integrated resistor technologies. In the second section a detailed study is carried out on pre and post trim SiCr resistors. The final section reviews a test chip with an integrated 10 kΩ resistor that uses iTrim to trim the TCR to less than 3 ppm/ᵒC and the absolute resistance to 0.02%.