Reduction of ultraviolet-radiation damage in SiO2 using pulse-time-modulated plasma and its application to charge coupled 44 device image sensor processes

Abstract

We found that vacuum-ultra/violet (VUV) light emitted by plasmas causes dark current in charge coupled device (CCD) image sensors. When a CCD was irradiated with inductively coupled plasmas using helium (He), argon (Ar) or oxygen (O2) gas, the He plasma caused higher plasma-induced dark current in the CCD than the Ar or O2 plasmas. To investigate the influence of VUV radiation in He plasma, the plasma-induced electric current in SiO2 was measured using two types of on-wafer monitoring devices to separate the effects of He ions and He VUV radiation. One monitoring device has an aluminum filter that only allows He VUV 58.4 nm to pass through, and the other has no filter. We performed time-resolved-measurement experiments using the two types of devices in pulse-time-modulated He plasma (He-TM plasma). With the filter device, the decay of the VUV intensity after plasma off corresponded completely to the decay of the plasma-induced current curve. On the other hand, in the no-filter device, decay curve of the plasma-induced current coincided with that of the electron density in the plasma. In other words, the TM plasma does not reduce the ion-induced current, but can completely reduce the photoinduced current in SiO2. In the CCD image sensor we found that the TM plasma dramatically suppresses plasma-induced dark current. Consequently, the effect of TM plasma on the CCD dark current is due to the influence of reducing the photons in the plasma.