User's Guide

A.1
Appendix A
Large Particle Detection
The WaferSense APS3 and APS-FPD sensor are capable of large particle measurements.
General information about large particle detection: In some circumstances it is desirable to sense
only large particles and not detect small particles. This may be the case in certain photolithography
tools. The APS3, with the large particle, has two detector channels. Each has separate optics and
amplifiers. One channel is for small particles and another for large particles.
In the normal small particle mode, the APS3 uses a high gain detector channel. When the large particle
mode is selected, a second detector channel is enabled that has much lower gain and is optimized and
calibrated for particles larger than 2 microns. The two different modes cannot be viewed at the same
time. Also, since large particles are typically counted at a much lower rate than small particles, the large
particle mode only counts particles in frequency mode.
Because the large particle scale covers a wider range of particle sizes, there are 4 bins. The bin sizes are
for particles larger than 2, 5, 10 and 30 microns.
The APS3 large particle mode is calibrated by measuring dry glass micro-spheres dispersed via a venturi
aspiration technique. As with all airborne particle detectors, the size is calibrated to an “equivalent” test
particle, in this case glass micro-spheres. Real world contamination particles may have non-spherical
shapes and may respond differently than glass micro-spheres.