Optical proximity correction (OPC) is widely used in wafer lithography to produce a printed image that best matches the
design intent while optimizing CD control. OPC software applies corrections to the mask pattern data, but in general it
does not directly compensate for the mask writer and mask process characteristics. The Sigma7500 deep-ultraviolet
(DUV) mask writer projects the image of a programmable spatial light modulator (SLM) onto the mask using partially
coherent optics similar to wafer steppers, and the residual optical proximity effects of the mask writer are in principle
correctable with established OPC methods.
To enhance mask patterning, an embedded OPC function called LinearityEqualizerTM has been developed for the
Sigma7500 that is transparent to the user and which does not degrade mask throughput. It employs the Mentor Graphics
Calibre OPC engine, selected for the computational speed necessary for mask run-time execution. A multi-node cluster
computer applies optimized table-based CD corrections to polygonized pattern data, which is then refractured into a
standard writer format for subsequent data processing. This short-range proximity correction works in conjunction with
ProcessEqualizerTM, a previously developed print-time function that reduces long-range process-related CD errors. OPC
flattens the linearity behavior for all linewidths and pitches, which should improve the total CD uniformity on
production photomasks. Along with better resolution of assist features, this further extends the application space of DUV
mask writing. Testing shows up to a 4x reduction in the range of systematic CD deviations for a broad array of feature
sizes and pitches, and dark assist features are reliably printed down to 120 nm at mask scale.
Managing the total CD error in advanced mask manufacturing requires that error contributions from writing, process and
metrology are minimized. This paper describes how both the writing and process contributions have been addressed in
the Sigma7500 DUV laser pattern generator, which prints masks by imaging a programmable spatial light modulator
(SLM). System enhancements have reduced the writing contribution to global CD uniformity to 5 nm (3s). Process-related
CD error sources, such as the signatures from mask developing and etching can be significant contributors to the
total CD error in mask manufacturing. These errors are classified as being either pattern-independent or pattern-dependent,
and the effects of both can be reduced using the ProcessEqualizer feature of the Sigma7500. This software
tool performs CD sizing during writing based on pattern density maps derived during mask data preparation, along with
tunable parameters that are determined experimentally. The CD sizing function has no effect on system throughput and
does not require flattening and re-fracturing of the pattern data.
Photomask CD control requirements continue to tighten due to inevitable device scaling, in addition to the increasing
mask error enhancement factor (MEEF). Managing the total CD error on production masks requires not only that the error contributions from the pattern generator should be minimized, but also that there is a way to handle errors due to
the mask process. This paper addresses process-related contributions such as the CD signatures from mask developing
and etching, and how their impact on the total CD error range can be reduced. This is accomplished using the on-line
ProcessEquilizerTM function in the Sigma7500 DUV laser pattern generator.
Long range and medium range process CD errors are major contributors to the total CD error range. These errors can be
classified as being either pattern-independent or pattern-dependent. Pattern-independent errors may occur in the bake,
develop and etch process steps. These errors are by definition static from mask to mask, and can therefore be mapped
and compensated by local sizing of the mask pattern data. Pattern-dependent errors typically originate from loading
effects, for example, in plasma etching. If such errors can be predicted from the pattern density variation across the
mask, then they can also be corrected for with local sizing.
The on-line ProcessEqualizerTM function performs local sizing, and offers a significant advantage over off-line solutions
that have the drawback of requiring flattening and refracturing of the pattern data. Local sizing is performed in parallel
with writing in the Sigma7500, and has no effect on throughput. In this paper the ProcessEqualizerTM function is
described, including how it is operated in the maskshop. Results are presented demonstrating the performance of the
ProcessEqualizerTM for handling global CD error signatures.
Lithographic exposure tools in the deep-ultraviolet (DUV) region face challenges with contamination. Airborne molecular contamination is generally recognized as a severe threat in high-volume production of integrated circuits (ICs), and has recently also become of a concern in patterning of masks. When using high-energy photons at 248nm wavelength or lower, the risk of contamination may increase due to higher potential of breaking molecular bonds of organic species in the ambient of the optics. Especially resist outgassing during exposure may result in a build-up on the surface of the lens. The
photodissociated molecules may readily deposit on the optics depending on the interaction between the contaminants and the
lens surface and possibly cause a loss of transmission of light with time. Eventually the growth of the deposit will severely impact the throughput of the exposure tool, and in the worst case, necessitate a replacement of lens elements. Contamination control is therefore crucial for cost-effective DUV wafer and mask manufacturing. Trustworthy measurement methods as well as deep understanding of the mechanisms involved are of vital importance in order to understand and prevent molecular contamination. This paper discusses some of the factors influencing the deposition of hydrocarbon contaminants and also simulation work related to investigation of resist outgassing and contamination issues in the Sigma7300 laser pattern generator.
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.
The push for smaller linewidths and tighter critical dimension (CD) budgets forced manufacturers of optical pattern generators to move from traditional i-line to deep ultraviolet (DUV) resist processing. Entering the DUV area was not without pain. The process conditions, especially exposure times of a few hours, put very tough demands on the resist material itself. However, today 248nm laser writers are fully operating using a resist process that exhibits the requested resolution, CD uniformity and environmental stability. The continuous demands of CD performance made Micronic to investigate suitable resist candidate materials for the next generation optical writer using 193nm excimer laser exposure. This paper reports on resist benchmarking of one commercial as well as several newly developed resists. The resists were investigated using a wafer scanner. The data obtained illustrate the current performance of 193nm photoresists, and further demonstrate that despite good progress in resist formulation optimization, the status is still a bit from the required lithographic performance.
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.
Airborne molecular contamination (AMC) in the form of bases, acids and condensable organic and inorganic substances threaten both costly and sensitive optics and mask pattern formation in the chemically amplified resists (CAR) used for both E-beam and laser lithography. This is particularly so for mask pattern generators due to the relatively long writing times. In the development work of the SLM-based DUV-laser mask pattern generator Sigma7300, AMC aspects have been taken into consideration from an early stage. That includes e.g. analysis and selection of construction materials and development of handling methods as well as application of chemical filtering systems. Tool manufacturer and filter supplier have together specified and designed efficient hybrid filtration systems for use in Sigma7300. This paper describes AMC aspects specific for mask pattern generators, the successful design actions of the Sigma7300 and verifying analyses of the processes.
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%.
One of the sub-functions in the Micronic Sigma 7300 mask writer is the 2:nd layer alignment system for writing of phase shift masks. The strategy chosen for performing PSM alignment is to use the DUV writing laser together with the spatial light modulator (SLM) to create a light stamp image, which is reflected on the first layer alignment marks. The reflected image is captured and measured with a DUV-sensitive CCD camera. Using the writing laser has many benefits since there is no position offsets coming from misalignment of multiple laser sources. The anti-reflection (AR) function in chemically amplified resists (CAR), bottom anti-reflex coatings (BARC) and top anti-reflex coatings (TARC) reduces reflectance for 248 nm incoming light. This could reduce the signal strength and accuracy of the alignment system as the 248 nm laser is used for the alignment. The paper focuses mainly on two issues, image contrast at different resist thicknesses and image contrast when AR coatings are used. The algorithm measuring the fist layer alignment mark positions is also described. The studies of this and results of the final PSM alignment system show that Micronic has found an efficient way of dealing with these issues.
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.
The recently installed Sigma7100 laser pattern generator brings a new concept into photomask manufacturing. The spatial light modulator (SLM) technology enables 2D patterning using commercially available 248 nm lasers. This wavelength shift from the 413 nm wavelength of the Omega6000 scanning laser pattern generators facilitates the high resolution needed for 100 nm mask production. In addition, the partially coherence of the 2D patterning further enhances CD linearity and edge acuity. The rapidly increasing mask costs are partially attributed to increasing photomask writing times. These tend to increase as feature density increases with the roadmap, which is a challenge for any pattern generator with a limited number of writing beams. Instead, the SLM technology relies on the massive parallelism of one million micromirrors in combination with gray-scale control for fine addressing. A real-time FPGA-based data-rendering engine matches the speed. The result is pattern generation with high resolution at manageable mask writing times
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