Update on Resolution Enhancement Technologies

OPC has been used in IC manufacturing for the past few years, while weak PSMs — also called embedded attenuated or half-tone masks — are just beginning to appear in advanced production devices, primarily at the contact and via levels, according to Steve Carlson, senior vice president of technology at Photronics (Allen, Texas). Though the attenuated masks comprise only 1-4 masks of a 25-mask high-end reticle set, they will be an increasing part of the 0.18 µm shrink and beyond. For 193 nm masks, Photronics has received its first development requests for the 0.13 µm technology node, also for contact holes and vias. These devices are targeted for 2002.

Though the impact of strong shifters like alternating PSMs is impressive, they have had slower industry acceptance. According to Dr. Frank Schellenberg, strategic marketing manager for the Calibre Business Unit at Mentor Graphics (San Jose, Calif.), OPC has more than 20 times the volume of alternating PSMs; and weak, attenuating PSMs have more than 2-3 times the volume. The lack of an effective mask repair technology takes a devastating toll on reticle yield, making alternating PSM suitable primarily for high-volume products like DRAMs and microprocessors. By applying strong PSM techniques to the 120 nm gate regions of polysilicon layers, it is possible to pattern both isolated and dense gates at critical dimensions as small as 100 nm in resist using 248 nm tools, noted Dr. Christopher Spence, AMD fellow. And to ensure gate resolution and control as well as eliminate pattern distortions, it is necessary to combine PSM with OPC techniques (Figure), he added. Software packages such as those provided by Numerical Technologies (NumeriTech, San Jose, Calif.) and Mentor Graphics take an original layout and make the modifications required for OPC or PSM before the final version is sent to the mask manufacturer. Phenomena like mask error enhancement factor have been added to the programs to further fine-tune the final image. According to Schellenberg, what is new in these packages is reflected in shorter integration times and fewer errors. In achieving results such as 1GHz microprocessor performance, Mentor Graphics' Calibre software has been used to generate the initial PSM layers, define the gate poly layer using a PSM operation, define the field poly using a binary mask, and perform model-based OPC all in a single run.