Monitoring system for silicon coating thickness of disk

In batch ion implanter equipment, wafers are set on a rotating wafer-disk which has a metal surface exposed to ion beams. In order to reduce metal contamination from the disk, the metal surface is usually coated with silicon. As a result of the disk surface being ion implanted as well as wafers, the surface is sputtered and the silicon coated layer becomes thinner. When the silicon coated layer is removed by sputtering, the base disk material (usually aluminum alloy) is exposed to the ion beam, and thus metal contamination increases. Therefore, it is necessary to monitor the thickness of the silicon coated layer and to estimate the remaining thickness. This is achieved using a charge sensor on the wafer-disk. This system consists of a sensor on the surface of disk and a detection circuit which measures the charging voltages during implantation. It is known that the output of the charge sensor depends on a material on the sensor surface. After coating the surface of the sensor by silicon, correlation between the thickness of the silicon coated layer of the sensor and the output of the sensor was measured. It is found that this in-situ monitoring system can detect the marginal thickness of silicon coated layer precisely