Wafer Inspection System: Calibration and Measurement
Wafer surface scanners are widely used in semiconductor industries to detect particle contamination on wafers. The performance characteristics of a PMS SAS-3600 wafer surface scanner have been evaluated using ideal polystyrene latex (PSL). It was also used to measure irregularly shaped silicon particles. Three sizes of PSL spheres (0.199μm, 0.3μm, and 0.426μm) were used to study the sizing accuracy and counting efficiency of this wafer scanner. The results show that this scanner sizes 0.199μm and 0.3μm PSL reasonably accurate. But it cannot size 0.426μm PSL very well. The background noise lever was so high that this scanner could not measure particles near its nominal lower detection limit: 0.1μm. It over-counted Si particles by 28%. An electrostatic enhanced wafer deposition chamber was used to prepare the test wafers. Both the theoretical calculations and experimental results show that electrostatic force increases the deposition velocity by 2 orders of magnitudes for particles smaller than 1μm. The sedimentation is the dominant mechanism for particles larger than 1μm.
Particle Deposition on Semiconductor Wafers
The deposition of aerosol particles on semiconductor wafers in the typical manufacturing environment of the clean room has been calculated using the equations of convective diffusion and sedimentation. The result shows that the deposition velocity decreases with increasing particle size in the diffusion regime and increases with increasing particle size in the sedimentation regime, with a minimum deposition velocity occurring in the vicinity of 0.2 μm. The minimum deposition velocity varies from approximately 0.2 × 10−3 to 0.7 × 10−3 cm/s, depending on the size of the wafer, the airflow velocity, and whether the wafer is freestanding or placed on top of a workbench.
Generation of monodisperse aerosols in the 0.1-1.0-μm diameter range using a mobility classification-inertial impaction technique
A new technique to generate monodisperse aerosols in the 0.1–1.0-μm diameter range has been developed and tested. This technique involves the use of a differential mobility analyzer to classify “equal mobility” aerosol particles and a micro-orifice impactor to remove undesirable, multiply charged particles of larger size. The system is capable of reducing the multiply charged particles to 6% or less of the primary-size, singly charged particles. Details of the selection of the system components and experimental procedures are provided.