DETECTION OF DEPOSITED PARTICLES FROM THE BACKSIDE OF A GLASS PLATE

Microcontamination of product surfaces by deposited particles is an im portant problem in clean technologies. A most sensitive product to con tamination by particles is a wafer during chip production. Therefore, methods for monitoring particle deposition on wafer surfaces have been developed in the last decade. A wafer with an unstructured and reflec ting surface is inserted into the process equipment. After some time, depending on the process, this wafer is removed from the process equip ment and is analysed with respect to the number of deposited particles using a wafer scanner. However, in situ particle detection in a proce ss chamber is not possible with this technique. This would be possible if, instead of a monitor wafer, a transparent glass plate is mounted, e.g. in the housing of the process equipment. Then the illuminating a nd scattered light detection equipment can be mounted outside the proc ess equipment. Since both the illuminating laser beam and the scattere d light have to be transmitted through the glass plate, losses will oc cur, which will reduce the lower limit of detection with respect to pa rticle size. In this article we estimate the detection possibilities t heoretically and experimentally. A simple model based on Mie and vecto r scattering theory has been developed to describe the light-scatterin g behavior of a single spherical particle on a glass plate with random surface irregularities. The scattered light of individual particles o f four particle sizes (1.03, 1.6, 2.92 and 4.23 mu m) on the same glas s surface and from the uncontaminated area of the glass plate was meas ured for unpolarized and normally incident light. The values of the sc attered light from this model were compared with the experimental resu lts. The comparison shows a reasonable agreement of the angular distri bution of the scattered light. The developed model is used to predict the lower limit of detection for particles on a transparent surface. T he theoretical estimations show that it should be possible to detect p articles of a diameter down to 0.2 mu m with the described measurement technique.