Application of Matching Structures to Identify the Source of Systematic Dimensional Offsets in GHOST Proximity Corrected Photomasks

The effects of the GHOST proximity correction process on chrome-on-quartz photomasks can prove difficult to quantify and so they are not routinely characterised. This paper presents a methodology for addressing this issue using electrical test structures designed to measure dimensional mismatch. In the past these have been used successfully to characterise standard GHOSTed photomasks, which displayed systematic offsets that were not seen on an unGHOSTed mask using the same design. In order to investigate this further, a second mask was fabricated using a variation of the GHOST process which increased the resolution of the secondary exposure to be the same as the primary pattern. This enabled the source of the previously observed systematic offset to be determined as test structures on the new mask did not show the same overall dimensional bias. However, the range of mismatch in some of the structures was increased as a result of the new process.