'CMOS Compatible Alignment Marks for the SCALPEL Proof of Lithography Tool
01 May 1999
The SCALPEL proof of lithography (SPOL) tool uses alignment marks for many applications ranging from periodic setup and calibration to routine alignment and registration for achieving level to level overlay. A mark detection system was developed that uses the backscattered electron (BSE) signal generated from scanning the aerial image of a mask mark over a corresponding wafer mark. We performed an analysis of the mark material requirements assuming a SCALPEL tool operating under high throughput conditions. The result is that, to achieve a detectable signal with a signal to noise of at lease 3, the ratio of the mark BSE coefficient to the substrate must be a minimum of 0.01. Therefore, a large range of materials and geometries are acceptable as marks for SCALPEL. We have fabricated an alignment device for tool setup and calibration using silicon that was coated with 500 nm of tungsten. Patterns were written and transferred into the tungsten using 3-beam lithography and plasma etching.