Near Half-l Contacts with 248 nm
Ruth DeJule, Associate Editor -- Semiconductor International, 5/1/1999
Dense arrays of 140 nm contacts using 248 nm illumination have been printed by MicroUnity (Sunnyvale, Calif.) in collaboration with National Semiconductor Corp. (NSC). Expanding on work by International SEMATECH's DELPHI project, near half-wavelength contact hole and via formation combine high-transmission ternary attenuated phase shift mask (T-APSM) and OPC technologies to create "Bessel" contacts.
Attenuated materials on PSMs are known to improve resolution, depth of focus and exposure latitude. Typically, low-transmission APSMs with less than 8% transmission have been used in production of quarter micron contacts. Pushing the technology, printing contacts below 0.18 µm require simulations to determine optimal conditions. Subsequent studies performed at NSC indicated that materials with transmissions as high as 18% can improve resolutions to 140 nm with good DOF. However, the higher transmission materials were shown to also heighten proximity effects, limiting their application to isolated contacts. Combining opaque chrome, attenuated phase shift background and a central quartz hole creates a Bessel function in the image plane with the effect of improved image log slope and increased DOF. But again, severe proximity effects were seen in simulation studies as Bessel contacts were placed closer together. Clearly, to make any contact technology viable for production, the ability to print dense arrays of contacts is mandatory.
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Fig. 1. The three toned attenuated PSM represents a general layout for Bessel contact design on a reticle. (Source: MicroUnity) |
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Fig. 2. Cross-sectional SEM of a resist image indicates 140 nm contact holes on 500 nm pitch. (Source: National Semiconductor) |
The 18% transmission APSM mask is more than a solution for contacts, it is also a practical extension for poly and metal masks, noted MicroUnity's Roger Caldwell. This generalized solution, however, is not without manufacturing challenges. Though test masks have been successfully produced and tools such as the phase-shifted defect sensitivity monitors are available, blank manufacturing, inspection and repair and software for random logic remain major barriers. Today, software to support poly and metal T-APSM applications is used in production. MicroUnity has launched co-development efforts with NSC and four other companies to develop software that supports production-level random logic Bessel contact masks by year's end.
