A new photomask develop tool designed by Tokyo Electron Limited (TEL) with wafer puddle technology was evaluated at the Advanced Mask Technology Center (AMTC) in Dresden, Germany. Parameters selected for this evaluation were resist dark loss uniformity, critical dimension (CD) uniformity, loading, linearity, resist cross sectional images, and defects using chemically amplified resists (CARs) exposed with DUV (λ=257nm) and 50KeV e-beam pattern generators. Implementing wafer puddle technology to photomask developing was not a simple, straightforward process. Standard CAR puddle recipes for wafer developing were inadequate to match CDU requirements for photomasks at the 130nm technology node using DUV exposure. While the results were disappointing, the TEL alpha develop tool cannot be held entirely responsible. Other, non-develop tool related factors such as resist, substrate, coating bake temperature and time, lithography tool, and post exposure bake temperature and time, all contributed to the final post develop results. Indeed, other CAR/substrate combinations exposed at 50keV e-beam and processed on the TEL alpha develop tool were markedly better in CD performance when compared to DUV results. The AMTC has recently taken delivery of a full scale, production worthy, TEL photomask develop tool for use at future technology nodes.
Critical dimensions (CD) measured in resist are key to understanding the CD distribution on photomasks. Vital to this understanding is the separation of spatially random and systematic contributions to the CD distribution. Random contributions will not appear in post etch CD measurements (final) whereas systematic contributions will strongly impact final CDs. Resist CD signatures and their variations drive final CD distributions, thus an understanding of the mechanisms influencing the resist CD signature and its variation play a pivotal role in CD distribution improvements. Current technological demands require strict control of reticle critical dimension uniformity (CDU) and the Advanced Mask Technology Center (AMTC) has found significant reductions in reticle CDU are enabled through the statistical analysis of large data sets. To this end, we employ Principle Component Analysis (PCA) -a methodology well established at the AMTC 1 -to show how different portions of the lithographic process contribute to CD variations. These portions include photomask blank preparation as well as a correction parameter in the front end process. CD variations were markedly changed by modulating these two lithographic portions, leading to improved final CDU on test reticles in two different chemically amplified resist (CAR) processes.
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