07/01/1998
"Manufacturable" 100-nm features from 248-nm lithography
The combination of optical proximity correction (OPC) design features with alternating phase-shift photomasks (PSM) has been shown to have real potential for a manufacturable process window at less than half the wavelength of light. Reported results from SEMATECH`s Delphi Project cites good critical dimension (CD) control of 100-nm features on the wafer using 248-nm deep ultra-violet wavelength exposure on 4? lithography systems (see figure).
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Resist CD measurements with and without OPC: Target CD = 100 nm, KrF exposure = 16.06 mJ/cm2.
The Delphi Project targets determination of the practical limits of optical microlithography. The reported work comes from a year-long team research effort by maskmaker Photronics, IC manufacturer National Semiconductor, and OPC software supplier MicroUnity. The work was presented at the Photomask Japan `98 Conference held at Kanagawa Science Park; the conference is sponsored by SPIE, BACUS, and Photomask Japan.
John S. Petersen, SEMATECH fellow and DELPHI project leader, says, "We are extremely pleased with results on the printed layers. The data clearly indicates that OPC combined with alternating phase-shift can overcome proximity related CD error, while taking full advantage of the resolution improvement of the PSM technique. Ultimately, demonstrating the feasibility of OPC to correct for these effects may be considered one of the milestones on the road to implementation of alternating PSM into sub-wavelength production."
The team of engineers designed test reticles containing a myriad of 180? phase-shifted structures combined with MicroUnity`s subresolution OPC "scattering bar" features and fine selective biasing, the latter done with MicroUnity`s MaskRigger software. They simulated the entire imaging process with PROLITH 3D from FINLE Technologies; this program incorporates the latest resist models used at SEMATECH. These simulations fine tuned the placement and size of the OPC structures needed to obtain the desired image. They used TEMPEST, a simulation program from the University of California at Berkeley that simulates the electromagnetic field at the mask, to minimize diffraction-related feature placement problems and to further improve process latitude. TEMPEST provided the best mask topography for shaping the projected image. The final development step validated the alternating PSM and topographical design with non-OPC test masks designed by Benchmark Technologies.
Photronics fabricated the OPC reticles using its UltraRes process and phase-shift fabrication techniques, which achieve resolution of features on the reticle down to 250 nm. SEMATECH used these highly complex reticles to print wafers using a 0.53-NA 248-nm exposure tool from Integrated Solutions Inc. Cross-section scanning electron microscopy (SEM) provided by Charles Evans & Associates was used to make thousands of measurements of the tiny resist features. The measurements were then compiled and analyzed by MicroUnity and SEMATECH to determine optimal process conditions.
The conclusion is that this work presents the first clear picture of how to solve the proximity problems encountered with deep sub-wavelength phase-shift processing. It is also significant that production-worthy phase shift masks were produced and that the design process worked around coherence-enhanced PSM-OPC problems. Robert Socha, National Semiconductor senior engineer assigned to DELPHI, says, "Now, we are optimistic that our 248-nm DUV capital infrastructure can survive a few more generations." - P.B.