Wafer edge-defect detection and capacitive probe therefor
A wafer edge-defect detection system with a probe assembly having one or more capacitive plates conforming in edge shape to an edge shape of a wafer; and processing electronics for electronically driving the one or more capacitive plates and for sensing an electrical signal representing capacitance between each one or more plates and the wafer. Filtering and demodulation techniques enhance the signal to noise ratio.
1. A wafer edge-defect detection system comprising:
a probe assembly having one or more capacitive plates forming a gap, the probe assembly for receiving a wafer in the gap;
the one or more capacitive plates conforming to an edge shape of the wafer by extending from a top surface of the wafer, around an edge of the wafer, to a bottom surface of the wafer;
processing electronics for electronically driving the one or more capacitive plates and for sensing an electrical signal representing capacitance between each one or more plates and the wafer.
2. The detection system of claim 1 wherein said probe assembly has one capacitive plate and one or more guard plates.
3. The detection system of claim 1 wherein said probe assembly has two or more capacitive plates separated one from the other in a vertical direction referenced to the wafer height.
4. The detection system of claim 3 wherein said two or more plates are separated from one another in a circumferential direction around said wafer.
5. The detection system of claim 1 wherein said processing electronics includes an AC capacitive plate drive and a demodulator for the sensed electrical signal producing an output reflecting capacitance variation.
6. The detection system of claim 1 wherein said processing electronics includes a means for differing the sensed signals from two or more plates.
7. The detection system of claim 2 wherein said processing electronics provides an output indication of defects in the crown of the wafer.
8. The detection system of claim 2 wherein said processing electronics is responsive to sensed capacitance provides an indication of edge thickness.
9. The detection system of claim 1 further including a servo system responsive to sensed capacitance for maintaining a probe to wafer edge distance.
10. The detection system of claim 1 further including means for correlating wafer position with sensed capacitance.
11. The detection system of claim 10 wherein said correlating means includes one of spinning the wafer past said probe a constant rate and encoding wafer position.
12. The detection system of claim 1 wherein said processing electronics includes parallel A/D converters one of which is of low resolution and determines incremental scale factor information therefrom.
13. The detection system of claim 1 further including a reference probe for measuring runout error and shape information on the wafer when moving.
14. The detection system of claim 1 , wherein said processing electronics includes means for providing one or more of edge/defect discrimination, match and/or noise filtration, shape and stage signature removal, thickness determination, pit and bump classification and incremental scale factor determination.