Application of Aerodynamic Lenses to In-Situ Particle Monitors (ISPM) for Higher Reliability in Semiconductor Fabrication Process

摘要

In-Situ Particle Monitors (ISPMs) are widely used today to monitor particles within and downstream of semiconductor processing equipment, for they can detect particles under vacuum conditions on real-time basis. However, ISPMs have some limitations. Firstly, since only a small fraction of the flow passes through the laser beam, the counting efficiency of ISPMs is quite low. Also, particles are not uniformly distributed in low pressure flows and this results in the poor correlation between ISPM counts and the actual particle concentration. Secondly, the intensity of the laser light in the viewing volume is not uniform, but instead has a profile that is typically Gaussian. This leads to a broad distribution of scattered intensities for particles of a given size. Therefore, ISPMs cannot offer accurate sizing resolution. In this experimental study, we improved ISPM performance by using aerodynamic lenses to focus particles through the center of the illuminated optical viewing volume. Aerodynamic lenses consist of a series of circular orifices in a cylindrical tube. The flow converges and then diverges as it flows through these orifices. Because of their inertia, particles do not perfectly follow the flow stream lines. Particles smaller than a critical size concentrate along the centerline of the flow, while particles larger than this critical size are lost by inertial deposition. Therefore, it is possible to design a lens system that focuses all the particles through the viewing volume. We found that by using aerodynamic lenses we were able to increase the counting efficiency of an HYT Model 70 ISPM from 0-10% to near 100% for particles of 0.494 μm in diameter at pressures ranging from 0.1 to 10 torr. Also, uniform illumination of particles in the center region of the viewing volume led to a more uniform pulse height distribution of scattered light, which implies the possibility that ISPM can be used as a particle sizer.