Brion debuts tools for dual dipole and custom illumination

by M. David Levenson, Editor-in-Chief, Microlithography World

Brion Technologies, the computational lithography wing of lithography toolmaker ASML, is announcing two new advances at this week’s BACUS Symposium on Photomask Technology and Management (Sept. 18, Monterey, CA). To permit chip designs to approach more closely to the limit of single resist exposure technology, Brion’s Tachyon system will now do the mask transformations needed for dual dipole lithography (DDL), according to Neal Callan, VP of product operations. Also, to enhance the process window for repetitive structures that occur in memory, Brion’s LithoCruiser 2.2 wavefront engineering system can now co-optimize the diffractive optical element (DOE) needed for custom illumination along with the mask pattern.

The absolute resolution limit for single-resist-layer lithography corresponds to a k1 factor of 0.25 or 36nm for ASML’s most advanced immersion exposure tool. Approaching that limit without the use of alternating aperture PSMs requires aggressive polarized dipole illumination and two critical exposures, one for horizontally oriented lines and another for vertically oriented features. Thus, there two masks for each level must be printed in the dual dipole lithography system, with each mask corrected for optical proximity effects and made opaque where features with the wrong orientation occur. Brion’s Tachyon OPC+ system (version 7.06) now can fracture a design into the two masks needed, emplace the opaque shielding and apply the OPC corrections needed by the specific illumination scheme.

According to Callan, the calculation on the Tachyon dedicated litho computer requires only twice the time of a correction for a single mask. Tool time is also roughly doubled in production, as the wafer remains on the wafer stage while the reticle and illumination are changed and the second exposure performed, he told WaferNEWS. However, overlay challenges are reduced in DDL, compared to double processing technology (DPT) where intermediate development, etch and coating steps must take place outside the exposure tool. The DDL system enabled by ASML and Brion was reported to demonstrate k1=0.3 performance after two exposures, which would correspond to 43nm half-pitch for an ASML 1900i exposure tool.

DPT, of course, can break the k1=0.25 resolution barrier and is expected to be used for 32nm lithography, but with lower throughput and higher cost. Jim Wiley, senior technical director at Brion, envisions the DDL system as a cost-effective step towards future lithography schemes. He noted that quickly bringing the computational lithography needed for ASML’s DDL option to market is an example of the synergies of the Brion/ASML combination.

Not all devices and markets require the ultimate resolution or will tolerate the lowered throughput of DDL. Manufacturers of DRAM and flash memory must push their single-exposure systems to the limit while maintaining adequate yield and high throughput. Fortunately, the most challenging elements of these circuits are repetitive arrays that can benefit from customized illumination — just the thing that LithoCruiser, originally an ASML MaskTools product, was designed to facilitate. While illumination was customized in the early days with apertures in the condenser optics, modern methods incorporate a diffractive optical element (DOE) which sends most of the light in the directions needed, increasing throughput.

The Source-Mask Optimizer (SMO) option of LithoCruiser is a system that co-optimizes the illumination and reticle pattern for a specific design fragment (clip), while obeying the constraints of the DOE and mask fabrication processes, Callan explained. The optimization process uses a given source shape as input for the model based OPC mask revision and then cycles the process until the combination giving the optimum focus exposure condition appears. Optimization metrics include the overlapping process window for multiple clips and mask error enhancement factor (MEEF).

Wiley noted that the ASML supply chain could provide customers with the custom DOE “for a small fraction of the cost of a mask set.” Callan reported that this version of LithoCruiser has gotten “good traction” in the memory markets, and is beginning to penetrate in logic where geometries are more random. — M.D.L.

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