FEP-171 resist is commonly used both together with 50 keV VSB and DUV laser mask writers. To improve resolution and other lithographic parameters, the industry has strived towards thinner resist and absorber films on the mask blank. The chrome thickness and etch resistance limit how thin the resist can be. The NTAR7 (730Å) chrome was optimized for binary masks for 193 nm lithography, while NTAR5 (590Å) chrome is used for attenuated PSM blanks with a MoSi absorber beneath the chrome film.
Resolution and lithographic performance can be improved further by integrating improved processes, including PEB, development and dry-etch. Micronic has in a series of papers described improvements to the FEP-171 process in combination with different chrome films and the SLM-based DUV (248 nm) Sigma7300 mask writer. The thickness of FEP-171 for Sigma7300 has been optimized for NTAR7 chrome and improvements have been described for the PEB and dry-etch process of the FEP-171/NTAR7 blanks.
In this paper we describe the FEP-171 process development further. We have investigated improvements to the develop process for FEP-171/NTAR7 blanks using Design of Experiments (DOE) and a Steag Hamatech ASP-5000. Improved performance on mask, especially for CD linearity and clear-field/dark-field deviation, was achieved using the resulting development recipe together with the Sigma7300. Better than 5 nm (range) CD linearity in chrome was demonstrated for isolated spaces in the range 200-1400 nm.
This work also covers a process study of FEP-171 on NTAR5 chrome. The resist thickness was optimized to 3200Å for the Sigma7300 and the performance was tested in terms of resolution, resist profile, CD linearity and CD uniformity. Resolution of 120 nm isolated lines and 140 nm isolated spaces was demonstrated, as well as 4 nm (range/2) global CD uniformity.
A continuing improvement in resist process is a necessity for high-end photomask fabrication. In advanced chemically amplified resist systems the lithographic performance is strongly influenced by diffusion of acid and acid quencher (i.e. bases). Beside the resist properties, e.g. size and volatility of the photoacid, the process conditions play important roles for the diffusion control. Understanding and managing these properties influences lithographic characteristics on the photomask such as CD uniformity, CD and pitch linearity, resolution, substrate contamination, clear-dark bias and iso-dense bias. In this paper we have investigated effects on the lithographic characteristics with respect to post exposure bake conditions, when using the chemically amplified resist FEP-171. We used commercially available mask blanks from the Hoya Mask Blank Division with NTAR7 chrome and an optimized resist thickness for the 248 nm laser tool at 3200Å. The photomasks were exposed on the optical DUV (248nm) Sigma7300 pattern generator. Additionally, we investigated the image stability between exposure and post exposure bake. Unlike in wafer fabrication, photomask writing requires several hours, making the resist susceptible to image blur and acid latent image degradation.
Chrome and resist thickness are limiting factors for final resolution on mask. The trend in mask manufacturing is consequently moving towards thinner chrome and resist films.
The Sigma7300 is a 248nm DUV laser pattern generator with optical resolution approaching 100nm. The earlier standard mask blank for the mask writer had 1030Å thick AR8 chrome together with 4000Å FEP-171 resist. To fully benefit from the resolution capability of the mask writer, this study aimed to investigate the 730Å thick NTAR7 chrome together with thinner FEP-171 resist. The dry etch characteristics of thin chrome and thin resist were also studied.
As a first step, a set of plates with varying resist thickness was exposed to extract the swing curve. The resist thickness ranged from 3050Å - 3600Å in steps of 50Å. The fitted curve based on the dose required to break through the resist (dose-to-clear) for different thicknesses showed a maximum at approximately 3200Å. A resolution improvement of about 10nm was achieved in this resist thickness compared to the earlier 4000Å film.
Design of Experiments (DoE) was used to perform a screening of the dry etch process on NTAR7 and the 3200Å resist. All plates were exposed using the Sigma7300. Etching was performed on a UNAXIS Gen III Mask Etcher with standard Cl2/O2/He gas mixture. The dry etch process developed from the DoE responses was used to characterize the lithographic performance on mask from the Sigma7300 together with the new optimized blanks.
CD linearity <10nm (range) was demonstrated both for clear and dark isolated lines down to 180nm line width. Global CD uniformity <6nm (3s) was achieved and very well defined chrome profiles for 150nm isolated clear lines and 130 nm isolated dark lines were demonstrated.
With each new technology generation, photomask manufacturing faces increasing complexity due to shrinking designs and accelerating use of reticle enhancement techniques. Denser and more complex patterns on the mask result in lower yields and long write and turn-around times, important factors for the rapidly increasing mask related costs in IC manufacturing. Laser pattern generators operating at DUV wavelengths were recently introduced to provide cost effective alternatives to electron-beam systems for printing of high-end photomasks. DUV wavelengths provide the required resolution and pattern fidelity. Optical tools that use raster writing principles and massively parallel printing ensure short and predictable write times for photomasks almost independent of pattern complexity.
One such high-volume production system, the Sigma7300, uses spatial light modulator (SLM) technology and a 248 nm excimer laser for printing. Partially coherent imaging and multi-pass printing as in a lithography scanner further increases resolution and pattern accuracy. With four-pass printing the system provides resolution and pattern accuracy meeting mask requirements for critical layers at the 90-nm node and sub-critical layers at the 65-nm node and beyond.
The paper discusses how mask layout can be optimized to take full advantage of the speed potential provided by the SLM-based writer. It shows how flexible use of the writing principle can provide cost effective writing solutions for many layers in high-end mask sets. Resolution and pattern accuracy results from the Sigma7300 will be presented together with write times for different types of designs.
This paper treats a for the semiconductor industry somewhat different application: The first-ever manufacture of Diffractive Optical Elements (DOE’s) as directly written multilevel diffractive micro-reliefs using the DUV SLM-based Sigma7300 Mask. The reliefs were manufactured in the DUV Chemically Amplified Resist (CAR) FEP-171. This particular application is of direct interest since DOE’s are already incorporated in the Sigma7300 system. The design and manufacture are demonstrated with (1.) A Fan-out element and (2.) A logotype generator. The first attempts, reported here, resulted in a Fan-out element with diffraction efficiency of 64% compared to the theoretical design of 88%.
Critical dimension control is becoming more and more critical in the mask making industry as the exposure wavelength goes down. For laser pattern generators, the move from traditional DNQ/Novolak based towards DUV chemically amplified resist processing was initially troublesome. The relative long total exposure time of pattern generators in contrast to wafer steppers, in combination with thick quartz substrates with relatively low heat capacity, may result in
reduced lithographic performance due to excessive diffusion of photogenerated acid. The photoresist polymer architecture play a large role in determining the acid diffusion characteristics and thereby also the image fidelity and resolution. In the Sigma7300 laser pattern generator the image is created by the spatial light modulator, which acts as a reflective computer-controlled reticle. By adopting a proper writing strategy, the negative effects of acid diffusion could be reduced. One component in the Sigma writing strategy is to expose the pattern in several passes that allows for dose compensation as well as averaging schemes to reduce CD errors. By adjusting the dose per pass and by keeping track of the delay times between each shot as well as the exposure path, a better control of the linewidth may be achieved for certain photoresist chemistry. In this study we present results from investigations of AZ DX 1100P and FEP-171 resists using different writing strategies.
For many years, laser pattern generation has been printing on i-line resists. As features sizes continue to shrink, laser pattern generation is moving to DUV laser wavelengths, and a production worthy resist process is needed. Characteristics such as standing waves, resist foot and CD drift under and after exposure have previously challenged efforts to migrate 248nm stepper chemically amplified resists (CARs) to mask making applications. In this study the performance of a commercially available 248nm laser/e-beam resist solution is examined in the Sigma7000 series laser pattern generators. To achieve virtually no resist foot as well as tight CD control the optimum process conditions for DUV laser applications were determined. Cross-sectional and top-down scanning electron microscopy analysis was performed to evaluate the resist and dry etch processes. A comparison is made with the resist DX1100P, used in initial stages for DUV pattern generators development. The new resist also benefits from being well established in mask making e-beam mask writers.
This paper presents the properties of a second-generation DUV laser pattern generator based on spatial light modulator technology and designed to meet the requirements of the 90-nm to 65-nm technology nodes. The system, named Sigma7300, is described and major changes compared to its predecessor are pointed out. These changes result in improved pattern accuracy and fidelity as well as system reliability and maintenance. This improved performance is accompanied with greatly reduced writing times of typically 3 Hrs. per mask. Performance data is presented that shows the system meets the resolution requirement of 260 nm with CD linearity of 10 nm and assist line resolution of 140 nm. CD uniformity data and registration data are also presented that indicates that the system meets the requirements for most layers at the 90-nm and 65-nm nodes.
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