Proceedings Article | 12 February 1997
KEYWORDS: Coating, Photoresist processing, Reticles, Photomasks, Electron beam lithography, Polymers, Etching, Optical proximity correction, Critical dimension metrology, Phase shifts
Advanced reticle fabrication, including phase-shifting mask (PSM) and optical proximity correction (OPC) reticle, has created a growing needs for a resist system with wider process latitude, superior linearity and fidelity, which lead to better CD uniformity. Some of aqueous-based DNQ-novolak resist systems, including chemical amplified one, have been proposed, however, their feasibility has not reached a practical level yet. Accordingly there seem to be many activities, in the industry at present, in re-establishing a new process technique and condition with conventional high-molecular polymer resists such as PBSTM particularly in utilizing a 'thin resist' coated blanks. Figure 1a shows e-beam resist trend of leading-edge 6' square and 250 mils thick (6025TM) photomask blanks shipment in '96 world-wide from our photomask blanks factory. High molecular polymer resist is still the major, and the top five resists have roughly 90% share, which are PBS, EBR-9TM, EBR-9 HS31TM, ZEP810TM and ZEP7000TM. Figure 1b shows 6025 PBS blanks shipment per coating thickness in '96. Thin PBS was started utilizing from '91, and had 17% share out of all 6025 PBS blanks shipment in '96. In order to reduce reticle CD error sources due to blanks, we have been doing exploratory study to determine the optimal resist process conditions in coating thickness and post-spin baking (PSB), exposure, dose as well, which enables us to appreciate real advantages of 'thin resist.' And we reported the results and its efficacy in patterning performance at Photomask Japan '97. In the report, 250 nm thick was selected particularly for PBS since it was a kind of standard as thin PBS already in the industry, while 300 nm thick was selected for the other four resists as our investigation results. Our study also resulted that only PBS had an inferiority in CD linearity as compared with the other four resists, but, exposure was done at 10 kV for PBS and 20 kV for the others. Then, we looked for an optimal PBS coating thickness again for suitable pattern resolution and risk of clear defect quality. We also tried to see an impact of e-beam accelerating voltage especially for PBS linearity and fidelity improvement this time. This paper describes our investigation results on optimum conditions in coating thickness, post-spin baking and exposure dose, as well as comparison results on patterning performance of thin resist coating between the top five conventional resists, PBS, EBR-9, EBR-9 HS31, ZEP-810 and ZEP-7000, for advanced e-beam reticle fabrication. An impact of e-beam accelerating voltage up on PBS patterning performance is also reported in this paper.
