Optimizing the detection efficiency of a low pressure, in-situ particle monitor using aerodynamic focusing lenses

The past few years have witnessed a growing trend in the semiconductor industry to use in-situ particle detectors to monitor contamination levels in processing tools used for device fabrication. Such in-situ particle monitors (ISPM) generally detect particles by laser light scattering and are typically mounted on reactor pumping lines. In some designs, the laser illuminates only a small portion of the flow passage, resulting in low particle detection efficiencies (0-25%). This paper presents a method for enhancing and optimizing the detection efficiency of these devices by using aerodynamic focusing lenses placed immediately upstream of the detector. These lenses consist of one or more thin-plate orifice elements concentric with the flow passage. The highly concentrated, narrow particle beams produced by this technique may be directed through the center of the laser viewing volume to increase the detection efficiency to near 100%. This has been experimentally verified for a TSI Model 7340 vacuum particle detector with laboratory generated aerosols over a range of pressures (0.2-10 torr) typical of many semiconductor fabrication processes.