Inventors:
David A. Cathey - Boise ID
Brett Rolfson - Boise ID
Assignee:
Micron Technology, Inc. - Boise ID
International Classification:
G03F 900
Abstract:
A method of preventing null formation is performed on a phase shifted photomask including a clear quartz substrate, dark chrome feature features, and alternating clear phase shifters raised from the substrate. The phase shifter features are terminated in a transmissive, optically clear edge. To prevent null formation and consequent formation of stringers on the surface of the integrated circuit, the substantially vertical edge of the optically clear end of the phase shifter is tapered. The slope at any point along the tapered edge between the photomask substrate and the phase shifter is set to an angle, typically less than forty-five degrees, shallow enough that the point spread function does not produce an image. The point spread function of the imaging system spreads out the null, which is therefore not printed into the photoresist layer on the integrated circuit. The tapered edge of the phase shifter is created by either discrete or continuous etching methods.