Electron beam resists are critical to photomask production and have significant impacts on advanced semiconductor manufacturing. In this paper, we’ll discuss current and future challenges in electron beam resist development. These materials face many of the same issues as EUV resists, especially in their tradeoffs between resolution, dose and LER. However, electron beam exposure creates unique complications associated with backscattered electrons and charging. We’ll investigate these effects and the requirements and challenges that result.
As photomask minimum feature size requirements continue to shrink, resist resolution limitations and their tradeoffs
with exposure dose are critical factors. Recently, nearly every node needs a new electron beam resist, customized for
exposure dose requirements while simultaneously meeting resolution specifications. Intel Mask Operations has an active
program focused on screening new electron beam resists and processes. We discuss the performance metrics we use to
evaluate materials and discuss the relative capabilities of the latest resists. We present fundamental resist metrics
(resolution, LER and dose) as well as manufacturing process sensitivities.
High resolution sub resolution assist features (SRAFs) are challenging to pattern, especially on photomasks with pattern
density variations and beam corrections. This paper presents analysis techniques of SRAF resist resolution performance
and manufacturing robustness. Electron beam proximity effects and their correction methods impact aerial image
quality. Resist resolution and LER depend strongly on the aerial image, and these effects will be looked at theoretically
and experimentally with CDSEM and reflected die-to-die inspection techniques. A quantitative understanding of
resolution process latitude is important in SRAF patterning, especially when one considers beam corrections that are
used to compensate for effects like electron fogging and etch loading.
Pixelated phase masks rendered from computational lithography techniques demand one generation-ahead mask
technology development. In this paper, we reveal the accomplishment of fabricating Cr-less, full field, defect-free
pixilated phase masks, including integration of tapeout, front-end patterning and backend defect inspection, repair,
disposition and clean. This work was part of a comprehensive program within Intel which demonstrated microprocessor
To pattern mask pixels with lateral sizes <100nm and vertical depth of 170nm, tapeout data management, ebeam write
time management, aggressive pattern resolution scaling, etch improvement, new tool insertion and process integration
were co-optimized to ensure good linearity of lateral, vertical dimensions and sidewall angle of glass pixels of arbitrary
pixelated layout, including singlets, doublets, triplets, touch-corners and larger scale features of structural tones
including pit/trench and pillar/mesa. The final residual systematic mask patterning imperfections were corrected and
integrated upstream in the optical model and design layout.
The volume of 100nm phase pixels on a full field reticle is on the order tera-scale magnitude. Multiple breakthroughs in
backend mask technology were required to achieve a defect free full field mask. Specifically, integration of aerial
image-based defect inspection, 3D optical model-based high resolution ebeam repair and disposition were introduced.
Significant reduction of pixel mask specific defect modes, such as electro static discharge and glass pattern collapse,
were executed to drive defect level down to single digit before attempt of repair. The defect printability and repair yield
were verified downstream through silicon wafer print test to validate defect free mask performance.
Chemically amplified resist (CAR) performance can be greatly influenced by post apply bake (PAB) and post exposure bake (PEB) conditions. The difficulty with optimizing these conditions for photomask process is cost and time. In typical wafer CAR resist development, multiple wafer splits and skews can be rapidly processed with relatively low cost and fast turn around time, whereas in photomask processing each ebeam-written mask with a set of DOE conditions can be expensive and time consuming to produce.
This paper discusses a novel mask design and testing methodology that allow for many combinations of PEB and PAB conditions to be evaluated with one mask. In brief, this methodology employs orthogonal PAB and PEB thermal gradients across a plate. Some thermal profile, darkloss, resist top down critical dimensions (CD), and SEM cross section image results will be shared and discussed.
This paper includes an empirical determination of the relative CD error as a function of in-vacuum post exposure delay (PED). The effects of local pattern density and the impact of reticle proximity effect correction on the in-vacuum PED CD bias error are also considered. Results of dose compensation to improve CD uniformity on both artifact and production reticles are reviewed. The results show that by applying an exposure time dependent dose correction, the CD bias dependency upon in-vacuum PED is effectively compensated. In addition, the results show that dose compensation is effective at correcting for the in-vacuum PED dependency of local pattern density proximity errors. Finally, the paper concludes with a brief discussion of the relationship between existing reticle CD correction techniques for errors including electron beam fogging, etch loading, stable reticle process spatial CD non-uniformities and the new time dependent dose correction.
As photomask complexity has increased, mask manufacturing has become significantly more challenging. Tightening specs on defect performance, resolution, and CD control have pushed mask manufacturing to achieve levels that nearly match wafer capabilities. To meet wafer manufacturing needs, mask production requires high yield and quick turn-around time, resulting in an increased demand for very high equipment reliability. In-line resist coating capability is important to meet these demands; both for robust 2nd level phase-shift coating processes, and the enablement of advanced 1st-level process development with new resists and new resist process conditions. Intel Corporation worked with Tokyo Electron Ltd (TEL) to bring one of the first CLEAN TRACK ACT M (ACT M) units through design, acceptance tests and into manufacturing. TEL's CLEAN TRACK ACT M is a resist coating tool based on the CLEAN TRACK ACT12 (ACT 12) wafer manufacturing platform, and contains multiple mask-specific modules including advanced softbake oven units, edge-bead removal modules, and cleaning systems. After setup and optimization, the tool shows impressive performance, (for example, within-plate thickness uniformity of < 8A (3s) for certain processes). The motivation of the tool layout is discussed thoroughly. Elements of the module designs and their performance are shown. The acceptance testing performance is presented and includes: cleaning capabilities, oven performance, thickness performance, coating defect levels and edge bead removal capabilities. Finally, there is a limited discussion of manufacturing performance.
Line edge roughness (LER) and intrinsic bias of 193-nm photoresist based on two methacrylate polymers are evaluated over a range of base concentration. Roughness is characterized as a function of the image log slope of the aerial image, the gradient in photoacid concentration, and the gradient in polymer protecting groups. Use of the polymer protection gradient as a characteristic roughness metric accounts for the effects of base concentration. Results demonstrate that a methacrylate terpolymer exhibits an advantage over the copolymer resist by achieving lower roughness at smaller values for the polymer protection gradient, resulting in lower LER for patterning. Intrinsic bias is found to be a function of the concentration of base. Process window analysis demonstrates that a greater depth of focus can be achieved for resists with low intrinsic bias. However, a tradeoff in depth of focus with LER is found. Spectral analysis indicates resists with greater intrinsic bias exhibit greater correlation lengths. Systems with greater intrinsic bias demonstrate lesser roughness for patterned features, with a minimum roughness achieved at maximum intrinsic bias. Kinetics of deprotection are modeled to calculate the chemical contrast of each resist. Resists exhibiting the greatest chemical contrast are identified as materials that generate the least roughness.
Computer simulators are ideal tools to study complex process spaces,
but current lithography simulators are based on empirically-derived
continuum approximations and thus are unsuited for investigating
properties like line edge roughness (LER) because they do not incorporate molecular level details. A "mesoscale" simulation is
described that enables molecular level effects to be captured. This
technique is a compromise between accurate, but slow, atomic-level
simulations and the less accurate, but fast, continuum models. The
modeling of stochastic processes that lead to LER is enabled via use
of Monte Carlo techniques. Mesoscale simulation was used to study
the effects of added base quencher to overall photoresist performance. Simulations of acid/base kinetics with quencher loadings ranging from 0 to 20% show good qualitative agreement with
experimental data. Results show that decreasing aerial image quality
increases the root-mean-square (RMS) roughness, whereas increasing
base quencher loading improves LER, up to approximately 50% base. A
mechanism that explains line edge roughness stemming from acid gradients is proposed. This mechanism is supported by simulations
showing that the catalytic chain length varies inversely with acid
concentration. Simulation results show that base effectively limits
the influence of acid in low concentration regions. A critical drawback of using base additives is significantly reduced photospeed.
A hydrogen silisesquioxane (HSQ) bilayer process and a top surface imaging (TSI) process are investigated for application as low-voltage electron beam resist systems. Namatsu, van Delft, and others have reported printing exceptionally small features using high-voltage electron beam exposure of HSQ at high-exposure doses (~2000 µC/cm2 at 100 kV). The shallow penetration depth of low-voltage electrons results in greatly reduced dose requirements, and smooth, high-resolution images are generated at 1 kV with an exposure dose of less than 60 µC/cm2. HSQ's high silicon content enable it to be used in a bilayer form utilizing reactive ion etching with an oxygen plasma, thus generating high aspect ratio images. TSI has been studied in the past by numerous researchers at low voltages using various TSI schemes. We investigate the use of a chemically amplified TSI resist process based on poly (t-BOC-hydroxystyrene). The effect of base quencher loading in the resist formulation on line edge roughness and resolution is investigated, and is found to have a dramatic influence. High-resolution, high aspect ratio images are printed down to 40 nm, and exhibit only moderate levels of line edge roughness. Furthermore, proximity effects at 1, 2, and 3 kV are examined and compared to simulation.
As critical dimensions in microlithography become ever smaller and the importance of line edge roughness becomes
more pronounced, it is becoming increasingly important to gain a fundamental understanding of how the chemical
composition of modern photoresists influences resist performance. Modern resists contain four basic components:
polymer, photoacid generator, dissolution inhibitor, and base quencher. Of these four components, the one that is least
understood is the base quencher. This paper examines the influence of base additives on line edge roughness, contrast,
photospeed, and isofocal critical dimension (CD). A mathematical model describing the tradeoff between contrast and
photospeed is developed, line edge roughness values for different base types and loadings are reported, and isofocal CD
is shown for various photoacid types as well as for different base types and loadings.
Recent work on Step and Flash Imprint Lithography (SFIL) has been focused on process and materials fundamentals and demonstration of resolution capability. Etch transfer rpocesses have been developed that are capable of transferring imprinted images though 150 nm of residual etch barrier, yielding sub 50 nm lines with aspect ratios greater than 8:1. A model has been developed for the photoinitiated, free radical curing of the acrylate etch barrier materials that have been used in the SFIL process. This model includes the effects of oxygen transport on the kinetics of the reaction and yields a deeper understanding of the importance of oxygen inhibition, and the resulting impact of that process on throughput and defect generation. This understanding has motivated investigation of etch barrier materials such as vinyl ethers that are cured by a cationic mechanism, which does not exhibit these same effects. Initial work on statistical defect analysis has is reported and it does not reveal pathological trends.
Namatsu, van Delft, and others have reported printing exceptionally small features using high voltage (>50kV) electron beam exposure of hydrogen silsesquioxane (HSQ). They also reported that HSQ has very high exposure dose requirements (~2000(mu) C/cm2 at 100kV). We have explored the utility of HSQ as a resist for low-voltage electron beam lithography. Because low energy electrons have a very limited penetration depth, a thin film imaging technique must be employed in conjunction with anisotropic oxygen reactive ion etching to generate the high aspect-ratio features required to provide adequate etch resistance for subsequent image transfer steps. HSQ's exceptionally low oxygen plasma etch rate makes it an excellent top layer for a bilayer process of this sort. High resolution, high aspect ratio images were printed with this system using 1kV electrons with an imaging dose of less than 60 (mu) C/cm2. The resulting features have very smooth sidewalls. Monte Carlo simulations have been performed for the exposure process and compared to experimental results.
Top surface imaging (TSI) has had an interesting history. This process showed great promise in the late 1980's and several attempts were made to introduce it to full-scale manufacturing. Unfortunately, defect density problems limited the process and it fell from favor. TSI emerged again as an important part of the EUV and 193 nm strategies in the early stages of those programs because it offered a solution to the high opacity of common resist materials at both wavelengths. A flurry of research in both areas identified the seemingly insurmountable problem of line edge roughness than typical single layer resist systems. This has largely been due to the development of polymers specifically tailored for this end use. The optimum materials must be moderately transparent and have high Tg's in the silylated state. The 157nm program has much in common with the early stages of the 193nm program. The optical density of even 193nm resist materials at 157nm is far too high to allow their use in single layer applications. The less stringent optical density of even 193nm resist materials at 157nm is far too high to allow their use in single layer applications. The less stringent optical density requirements of TSI make it a potentially viable imaging scheme for use at 157nm. Various TSI materials, including the traditional poly(t-BOC- hydroxystyrene), as well as novel aliphatic cyclic polymers bearing bis-trifluoromethyl carbinol substituents, have been investigated for use at 157 nm, and smooth high-resolution images have been generated.