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Recent studies in our laboratory have shown that the diffuse reflectivity of sputtered Al /1% Si films plays a significant role in the lot to lot control of photoresist linewidth for 2.0 m features on our 16K SRAM device.The exposure energy required to produce 2.0 m resist lines on product wafers at this level was monitored and correlated with both the total and diffuse reflectivity as measured on control wafers included with the product wafers during Al /Si deposition.Fourteen product lots were monitored over a period of several months.The results show a strong correlation between changes in the diffuse reflectivity and the exposure energy. An increase of 64% in the diffuse reflectivity of the Al /Si film is accompanied by an increase of 80% in the exposure energy required to produce 2.0 m lines in positive resist.For these same lots, the total reflectivity varies by less than +3 %, usually taken as an indication of good process control at metal deposition, with no apparent correlation to the exposure energy. Several different resist processes were investigated to control the effects of changes in the diffuse reflectivity. The most promising of these has been a new dyed photoresist developed at Sandia. The unbleachable dye has been selected for optimal performance at the exposure and alignment wavelengths used in the Ultratech stepper.The diffuse reflectivity of Al /Si films coated with dyed resist is diminished by a factor of eight over the reflectivity of films coated with our standard resist.The total reflectivity is diminished by a factor of five.

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