| Abstract: | Microcontamination of product surfaces by deposited particles is an important problem in clean technologies. A most sensitive product to contamination 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 reflecting surface is inserted into the process equipment. After some time, depending on the process, this wafer is removed from the process equipment and is analysed with respect to the number of deposited particles using a wafer scanner. However, in situ particle detection in a process 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 and scattered light detection equipment can be mounted outside the process equipment. Since both the illuminating laser beam and the scattered light have to be transmitted through the glass plate, losses will occur, which will reduce the lower limit of detection with respect to particle size. In this article we estimate the detection possibilities theoretically and experimentally. A simple model based on Mie and vector scattering theory has been developed to describe the light-scattering behavior of a single spherical particle on a glass plate with random surface irregularities. The scattered light of individual particles of four particle sizes (1.03, 1.6, 2.92 and 4.23 μm) on the same glass surface and from the uncontaminated area of the glass plate was measured for unpolarized and normally incident light. The values of the scattered light from this model were compared with the experimental results. The comparison shows a reasonable agreement of the angular distribution of the scattered light. The developed model is used to predict the lower limit of detection for particles on a transparent surface. The theoretical estimations show that it should be possible to detect particles of a diameter down to 0.2 μm with the described measurement technique. |
