During the processing of semiconductor wafers, wafer misalignment and wafer movement after placement on a wafer holder (e.g., susceptor) can cause the wafer to be outside the holder's wafer pocket, which is designed to hold the wafer in the correct position as the wafer is rotated. If a wafer is not fully seated in the pocket, the resulting temperature gradients across the wafer can make it unusable. Thus, it is of great importance to detect whether wafers are being processed while they are misplaced or out-of-pocket, in order to take corrective action before the wafer and/or equipment are damaged. In addition, due to the extremely harsh environment inside the process chamber, simplicity and resistance to interference are both highly desirable characteristics, especially with regard to those components of a wafer detection system which are located in the process chamber. Also, reducing the cost of wafer fabrication machines is a major concern in an industry where fabrication processes are machine intensive.
In attempts to solve these problems, the prior art employs sensors for detecting whether a wafer sits flat or inclined within a susceptor pocket. Many of the prior art detection systems employ optical sensors which are subject to interference from the intense and varying light produced by the process chamber heat lamps. Also, the prior art systems are unnecessarily complex and expensive. In addition, conventional detection system components are subjected to the harsh environment, which consequently further increases replacement costs as compared with a simpler system.
Accordingly, a simpler, more accurate, and less expensive wafer placement detection system is needed.