Process for etching a transparent workpiece including backside endpoint detection steps

A method of defining a pattern on a mask comprising a transparent substrate of quartz having top and bottom surfaces and an opaque layer on said top surface, comprising:

• defining a photoresist pattern on said opaque layer, said pattern including a periodic structure having a periodic spacing between elements of the structure;
• placing said mask on a support pedestal in a plasma reactor chamber and generating a first plasma in said chamber to etch said opaque layer through openings in said photoresist pattern;
• sensing through openings in said support pedestal light transmitted through said transparent substrate, and monitoring said light for a change that indicates completion of etching of said opaque layer, and halting the etching of said opaque layer in response to said change;
• generating a second plasma in said chamber to etch said transparent substrate through openings in said opaque layer;
• transmitting light through said pedestal and through said bottom surface of said transparent substrate;
• viewing through said support pedestal light reflected from a periodic structure in said substrate corresponding to said periodic structure of said photoresist and detecting:
• (a) an interference pattern in said reflected light and (b) an image of said periodic structure in said substrate;
• and
• determining from said interference pattern a depth to which periodic structure has been etched in said top surface and determining from said image of said periodic structure in said substrate a line width dimension.