3 Solutions for Semiconductor Wafer Presence Detection

The first type of sensor that can be used to detect wafers is a dual mode laser distance sensor mounted through a window glass.  Laser sensors with dual mode capability like the Q4X from Banner detect not only changes in distance but also changes in light intensity from a stable background condition.   This allows the Q4X to detect challenging targets such as clear and reflective wafers with a high level of accuracy.  

A laser distance sensor with dual mode can detect not only when the target is present within a certain distance, but also when it returns a certain amount of light to the receiver.  For this to work, a stable reference surface must be taught, and the distance and intensity of the reference surface are recorded and used as a baseline.

The presence of a transparent or reflective wafer entering the sensing range of the beam alters the perceived distance and light intensity from the background condition.   The Q4X from Banner is also able to detect through a glass window without interference. 

Another sensor that accurately detects challenging wafers targets is a retro-reflective photoelectric sensor with a coaxial optical design.  The QS18 with Clear Object Detection emits and receive light along a single narrow axis. This optimizes the detection of transparent wafers by dramatically reducing the risk of false detection from reflected light.

Additionally, because light emitted and received travels on a single narrow beam, these sensors can see through a small aperture, allowing them to be used through a glass window for vacuum chamber applications. Photoelectric sensors with polarizing filters will emit and detect light waves of a specific polarization, ignoring all other light waves. This further reduces the risk of false detection caused by reflected light.

Learn more about polarized coaxial optics for detecting transparent objects. 

Finally, a fiber optic solution that is rated for use in a vacuum chamber can also be used to detect transparent or reflective wafers.  

With a fiber optic solution, the fiber amplifier is mounted outside of the vacuum chamber.  A special pass-through fitting with a vacuum-rated glass fiber mounted inside brings fiber optic light into the chamber.

The fiber is the positioned so that the light shines towards a vacuum-rated glass reflector, detecting both glass and reflective wafers that pass between the fiber and the reflector.