100 nm Features Using DUV

 Combined technologies may ultimately improve DOF and exposure latitude.

Ruth DeJule, Associate Editor -- Semiconductor International, 6/1/1998

Combining optical proximity correction (OPC) and alternating phase-shift photomask (PSM) technologies has provided the potential for a manufacturable process window at less than half the wavelength of light. The teaming of SEMATECH (International SEMATECH, Austin, Texas), Photronics (Brookfield, Conn.), National Semiconductor (Santa Clara, Calif.) and MicroUnity (Sunnyvale, Calif.), part of SEMATECH's DELPHI project, has resulted in critical dimension (CD) control of 100 nm features using 248 nm DUV, 4X lithography (see figure). The work was presented at the SPIE-sponsored Pho tomask Japan '98 Conference held at Kanagawa Science Park in April.

The DELPHI project is part of SEMATECH's risk management strategy for maintaining the SIA lithography roadmap and has been designed to determine the practical limits of optical microlithography. The roadmap currently seeks a 193 nm optical or a non-optical solution for 100 nm imaging and non-optical solutions beyond 100 nm. The significance of this result is that the combined technologies 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 subwavelength production," noted John S. Petersen, International SEMATECH Fellow and DELPHI project leader. However, Petersen cautioned, "While I am very optimistic with our results, there are other issues that must be solved prior to making this technology a practical production solution at the 130 nm SIA technology node."

The collaboration began about a year ago when SEMATECH joined with MicroUnity to provide photomask design and process support to help address the proximity effect problems seen with phase-shift reticles. As features approach one another, their diffraction patterns change, causing CDs to vary widely from the initial design. The CD variation caused by these proximity effects can significantly impact the yield for advanced ICs. The proximity effect problem and the resulting yield loss worsen as circuit features shrink below the wavelength of light used to print them. This situation is exacerbated by strengthening the effect of phase-shifting using highly coherent light to illuminate the PSM.

The work presented at Photomask Japan is "the first clear picture of how to solve the proximity problems encountered with deep subwavelength phase-shift processing," said Roger Caldwell, vice president of silicon technology at MicroUnity. This teaming has meant participating in fundamental engineering work that will ultimately define the requirements for photomask manufacturing. Those requirements will then be implemented into highly automated software suitable for production of full-sized microprocessor and ASIC circuits.

Engineers from SEMATECH, National Semiconductor, Photronics and MicroUnity designed test reticles containing a myriad of 180° phase-shifted structures that were combined with MicroUnity's subresolution scattering bar features and fine selective biasing.

Prior to reticle fabrication, two types of simulations were done. First, the entire imaging process was simulated with PROLITH 3D, a simulation program provided by FINLE Technologies (Austin, Texas), and then the placement and size of the OPC structures were fine-tuned. Second, to minimize diffraction-related feature placement problems and to further improve process latitude, TEMPEST, a simulation program from the University of California at Berkeley that simulates the electromagnetic field at the mask, was used to design the best mask topography for shaping the projected image. Then, in the case of alternating PSM, topographical design was validated with non-OPC test masks designed by Benchmark Technologies (Lynnfield, Mass.) and manufactured by Photronics.

Upon completion of the design phase, Photronics fabricated the OPC reticles using its proprietary UltraRes process and phase-shift fabrication techniques, which allow the company to achieve resolution of features on the reticle down to 0.25 µm (250 nm). SEMATECH then used these complex reticles to print wafers using a 248 nm exposure tool with a numerical aperture of 0.53 from Integrated Solutions (Austin, Texas). Cross-section SEMs provided by Charles Evans & Associates were 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. This impressive collaborative effort has resulted in the capacity for off-pitch sub-100 nm features using DUV, said Petersen, and the potential for a production-worthy optical process.