Processing high-purity silicon used for sensor applications

Abstract

The suitability of silicon for integrated electronic circuitry and as a sensor material renders it a likely candidate for the integration of both on a single chip, a so-called smart sensor. The demands on the silicon, however, are quite different for the two parts, and are often not compatible. An investigation was made into the behaviour of high-purity silicon (HP-Si) during the fabrication of radiation detectors, which are, in fact, simply pn-junction diodes. The results are presented here. The resistivity of the silicon used was 2 – 5 kω cm. The investigation centred on three standard bipolar processes, one diffusion and two ion implantations, with additional combinations thereof. It was found that all processing steps needed to integrate electronics with a detector can be applied on HP-Si, thereby not affecting the detector characteristics. However, the sequence of applying the different processing steps is of major importance. Comparing the processes used, the best diode characteristics were achieved when an implantation together with an anneal of 600 °C was used, whereas the worst resulted from an implantation and a 900 °C anneal (which is advisable for the electronics processing). The diffused samples yielded an intermediate case. It appeared, however, that a high-temperature treatment (above 600 °C) following the implantation and 600 °C annealing step severely deteriorated the diode characteristics, whereas it had no influence when it preceded the implantation. Thus it is shown that if a smart sensor is required, then the implantation must be the final processing step.