This paper shows the latest challenges facing mask blank evolution to support leading-edge lithography processes. ArF immersion lithography has been employing multi-pass exposures to exceed the physical diffraction limit. These photomasks demand very accurate overlay, higher NILS and best CD uniformity for wider process window. The subject was considered from two perspectives from a mask blank producer, which are the mask-making perspective and the wafer lithography perspective. To improve the overlay, we introduced the dedicated CDL (Charge Dissipation Layer) for improving mask registration error. From the lithography resolution perspective, we have developed a high-transmittance phase-shifter film for higher NILS. CDU stability point of view, we described “Superior pattern fidelity CAR”, “High ArF durability SiN phase-shifter” and “Transparent etching stopper”. The industry decided to move to EUV lithography. But there are still many challenges for optical lithography.
To make photomasks with high overlay accuracy, “Charge Dissipation Layer (CDL)” materials have been developed. Commercialized CDL materials can reduce electro-static charging on the surface of resist during electron beam exposure. However, some side effects are introduced to the mask-making process. The resolution performance of chemically amplified resist (CAR) is degraded owing to acid diffusion from the CDL components to the resist surface. A newly developed CDL solved this problem by controlling the acid diffusion. A positive-tone CAR with the CDL showed no resolution degradation, and performance was maintained for over 30 days after coating CDL and resist. Furthermore, the CDL has been evaluated on a negative-tone CAR which is more sensitive to CDL.
A novel adhesion promoting material has been developed to prevent very small resist patterns from collapse. One target for the development of the material is to make an advanced negative-tone mask with 40 nm sub-resolution assist features (SRAF). The SRAF on photomasks has become shorter and shorter as well as narrowing. The 2-dimensional resist patterns easily collapse during the resist developing process. Resist under-coating material controlled the surface condition on a chrome absorber film, and it improved the resolution of the SRAF. As a result, SRAFs of 46nm width and 200nm length were achieved using the material. A negative-tone resist on the under-coating layer demonstrated 35 nm isolated line patterns on a mask without pattern collapse.
The proportion of mask fabrication in the total cost budget for IC production is increasing, particularly for the double
patterning generation. Prolonging mask lifetime is very effective in reducing the total mask cost. The factors shortening the
mask lifetime principally damage by cleaning and by 193nm excimer laser irradiation during wafer exposure. In order to
solve these issues, Advanced Binary Film (ABF) was developed that is more durable against 193nm irradiation during
wafer exposure, and has superior cleaning durability. We confirmed the dry etching characteristics of the ABF, using
100nm thick Chemically Amplified Resist and exposure by 50keV EB tool. We obtained impressive results from the ABF
evaluation, through cycle cleaning tests (simulating cleaning during pellicle re-mounting), ArF irradiation damage and the
effects on Critical Dimension changes.
The resolution of photomask patterns were improved with a hardmask (HM) system. The system which is thin Sicompounds
layer is easily etched by the hyper-thin resist (below 100nm thickness). The HM material has sufficient
etching selectivity against the chrome-compounds which is the second layer chrome absorber for the phase-shifter. This
hardmask layer has been completely removed during the phase-shifter etching. It means that the conventional phase-shit
mask (PSM) has been made with the ultimately high-resolution without configuration changes. Below 50nm resolution
of PSM was made with 90nm thickness resist on HM layer in this paper. The CD bias between a resist feature CD and a
chrome feature CD was almost zero (below 1nm) in the optimized etching condition. We confirmed that the mask
performances were the equal to COMS (Cr-HM on MoSi binary mask) in resolution and CD linearity. The performances
of hardmask blanks will be defined by resist performance because of almost zero bias.
Double-patterning generation at 32-nm node and beyond raises many subjects for photomask blanks. We especially focus on the resolution improvement by hyper-thin resist combined with the hardmask process called the hyper-thin resist system (HTRS). Cr-hardmask has been specially developed for the HTRS, and this Cr material shows an extremely high etching rate. Additionally, we confirmed that a 55-nm resist thickness was available to etch the Cr-hardmask and last then the resolution of MoSi-absorber patterns was improved by HTRS, such as 45-nm LS, 60-nm isolated line and hole, and 35-nm isolated space. Moreover, the Cr-hardmask showed almost no film stress, which is necessary to achieve the image placement accuracy required for the double patterning. MoSi-binary with HTRS meets the photomask technology requirements for 32-nm node and beyond.
193nm-immersion lithography is the most promising technology for 32nm-node device fabrication. At the 32nm
technology-node, the performance of photomasks, not only phase-shift masks but also binary masks, needs to be improved,
especially in "resolution" and "CD accuracy". To meet sub-100nm resolution with high precision, further thinning of resist
thickness will be needed.
To improve CD performance, we have designed a new Cr-on-glass (COG) blank for binary applications, having OD-3
at 193nm. This simple Cr structure can obtain superior performance with the conventional mask-making process. Since the
hardmask concept is one of the alternative solutions, we have also designed a multilayered binary blank.
The new COG blank (NTARC) was fully dry-etched with over 25% shorter etching time than NTAR7, which is a
conventional COG blank. Thinner resist (up to 200nm) was possible for NTARC. NTARC with 200nm-thick resist showed
superior resolution and CD linearity in all pattern categories.
On the other hand, the multilayered binary stack gives us a wide etching margin for several etching steps. Super thin
resist (up to 100nm) was suitable by using a Cr-hardmask on a MoSi-absorber structure (COMS). The COMS blanks
showed superior performance, especially in tiny clear patterns, such as the isolated hole pattern.
We confirmed that these new photomask blanks, NTARC and COMS, will meet the requirements for 32nm-node and
beyond, for all aspects of mask-making.
193nm-immersion lithography is the most promising technology for 32nm-node device fabrication. A new Cr absorber
(TFC) for 193-nm attenuated phase-shift blanks was developed to meet the photomask requirements without any additional
process step, such as hardmask etching.
TFC was introduced with a design concept of the vertical profile for shorter etching time, the over etching time
reduction. As a result, the dry-etching time was dramatically improved by more than 20% shorter than the conventional
Cr absorber (TF11) without any process changes. We confirmed that 150nm-resist thickness was possible by TFC. The
32nm technology-node requirement is fully supported by TFC with thinner CAR, such as resolution and CD
Recently, extremely-high-quality-quartz substrates have been demanded for advancing ArF-lithography. HOYA has
developed a novel inspection method for interior defects as well as surface defects. The total internal reflection of the
substrate is employed to produce an ideal dark field illumination. The novel inspection method can detect a "nano-pit" of
12nm-EDS, the Equivalent of the Diameter of a Sphere (EDS). It will meet the sensitivity for 32nm node and beyond.
Moreover, a type of unique defect is detected, which induces Serious Transmittance Error for Arf-LiTHography. We call it
the "STEALTH" defect. It is a killer defect in wafer printing; but it cannot be detected with any conventional inspection in
the mask-making process so far.
In this paper, the performance of the novel inspection method for quartz substrates and the investigation of "STEALTH" are reported.
The CD requirements for the 45nm-node will become tighter so as it will be difficult to achieve with 65nm node
technologies. In this paper, a method to improve resolution by using DRECE (Dry-etching Resistance Enhancement
bottom-Coating for Eb) will be described. After all, DRECE has five times as high dry-etch resistance than the EB resist,
and this enables to accept higher anisotropic dry etching condition. By optimizing dry etching conditions, the CD
iso-dense bias dropped to 1/3 and the CD shift was reduced to 1/2. Also, there was no negative effect to CD uniformity.
From these results, we propose the use of DRECE for the 45nm-node technology.
The mask-making process for 45nm-node and beyond demands higher resolution and CD accuracy. To meet the requirements, the multi-layer resist system is developed as one of the solutions. BIL (Bottom Insulating Layer) can correct the profile of CAR (Chemically Amplified Resist). CAR shows profile degradation by photo-acid loss at the boundary of chrome and resist. The photo-acid loss induces excess footing in positive-tone CAR and under-cutting in negative-tone CAR. BIL reduced the profile degradation to less than half of the conventional resist system. BIL requires no extra mask process steps. Final CD linearity of isolated lines was improved by BIL. It is very beneficial for the patterning of sub-resolution assist features. Moreover, BIL with a hard-mask layer showed superior dry-etching bias performance.
For advanced reticle fabrication, a resist thinning technique continues a promising trend of the resolution enhancement. To bring out thin resist performances, a new chrome absorber has been developed for the second layer of 193nm att-PSM. The new chrome absorber is thinner and has a higher dry-etch rate than our current products, such as NTAR5. This new chrome absorber can utilize a super thin resist application because of a reduction in dry-etching time. Additionally, a technique of film stress reduction was also developed to reduce placement shift by film stress relaxation. The new chrome absorber with super thin resist (TF blanks) exceeds current products in the mask-making metrics of resolution and CD performance. This performance will meet the requirements of 65nm-node and beyond.
DUV (Deep Ultra-Violet) laser reticle writers were released to the market for advanced reticle fabrication in 2002, AZ-DX1100P resist (for KrF lithography) has historically been employed for these tools. To respond to further high-end requirements, a new resist more friendly to DUV reticle fabrication is needed. FEP171 is a positive-type CAR (Chemically amplified resist) developed for EB reticle fabrication, which is sensitive to DUV as well. In this paper, we have investigated the applicability of FEP171-coated blanks for DUV reticle fabrication. As the results show, FEP171 could achieve 200 nm patterns by DUV exposure. FEP171 blanks showed superior performance in resolution and profile as compared to AZ-DX1100P. FEP171 blanks are promising for DUV reticle fabrication as well as EB reticle fabrication.
A negative-CAR for EB reticle fabrication beyond 100 nm node is needed, which is superior in performance such as resolution, sensitivity, pattern quality, CD movement by process delays (PCD, PED) and process latitudes. We started preliminary screening on negative-CARs, and more than 10 resists out of 3 suppliers were examined including some that were still under development. Then, three CARs (A-2, B-1 and C-3) was selected as candidates, and those candidates were evaluated in 'resolution and sensitivity', 'pattern quality', 'CD movement due to process delays' and 'process latitudes'. B-1 turned out to be the best choice in total performance. In addition, thinning coating thickness was investigated for resolution improvement. A negative-CAR for EB reticle fabrication beyond 100 nm node is needed, which is superior in performance such as resolution, sensitivity, pattern quality, CD movement by process delays (PCD, PED) and process latitudes. We started preliminary screening on negative-CARs, and more than 10 resists out of 3 suppliers were examined including some that were still under development. Then, three CARs (A-2, B-1 and C-3) was selected as candidates, and those candidates were evaluated in 'resolution and sensitivity', 'pattern quality', 'CD movement due to process delays' and 'process latitudes'. B-1 turned out to be the best choice in total performance. In addition, thinning coating thickness was investigated for resolution improvement.
In order to provide a platform for the industry, we have been working on positive-working CAR screening by joint-works with resist-makers. In this paper, firstly, we did validation of a technique 'dark erosion analysis by excess develop' on our latest benchmark CAR C-2, to re-introduce our baking optimization technique to all the blanks users. The baking condition described by the technique (the sweet spot) certainly provided us a superior pattern profile, a longer blanks life, and better sensitivity stability for PEB time. To find a CAR that exceeds C-2 performance, we continued CAR screening. A CAR with 'less-footing' and 'high-sensitivity' was found, however, it had still a difficulty of terrible 'spray-damages' due to insufficient remaining thickness after develop. Desiccant (silica-gel) worked significantly to extend CAR blanks life, however, we found that it was not available for practical use unfortunately due to excess particle contamination during shipment. As a technique to solve 'spray-develop damage' issue, it was certainly effective to form a protection layer, and aqueous TAR over-coating layer could be an option to prevent resists film from the spray-damages. Finally, we reported negative-working CARs screening results. And some negative CARs were superior to the most popular one today for sensitivity or pattern profile, however, they had a difficulty of excess-undercut.
For advanced EB reticle fabrication with CAR blanks, we have been trying blanks life improvement and resolution enhancement. This paper describes several options to extend CAR blanks life by shipping package and storage manners. Dry-N<SUB>2</SUB> purged shipping package and desiccant showed efficiency for PCD improvement. However, desiccant is not available for products since it is not investigated yet if particle contamination could be happened. To extend blanks life after blanks package opened, we blanks maker really recommend the fresh dry-N<SUB>2</SUB> purged box (overflowed) as an ideal storage manner, or regular shipping box storage at least. We tried a BARC as isolator and thinning CAR for resolution enhancement, and found that a neutral-BARC showed an improvement. However, we needs further study about its feasibility, especially for coating uniformity and defect quality control. Thin resist showed almost no improvement for CAR resolution enhancement. To improve CAR performance further, we continuously need the resist-makers cooperation to design and develop a CAR that provides us footing-less pattern profile and long-life after coating.
For advanced EB reticle fabrication, we have been studying chemically amplified resist (CAR) blanks feasibility as a mission of blanks supplier, since the mask-makers are likely to procure resist-coated blanks even with CARs for the future as they have been. In our study, we found several difficulties to utilize CARs for mask-making, such as CD movement due to delays in process, excess footing, CAR poor adhesion to chrome, spray-develop damage defects and so on. Then, we are working on solutions by joint-work with some resist makers, by new CARs development and process optimization. In addition, we did CARs performance comparison, by a practical 50keV-exposure tool, between a newly developed CAR and Tokyo Ohka Kogyo EP-009 as a benchmark, which was previously selected. This paper describes our findings how to use CARs for mask making as well as our comparative evaluation results between EP-009 and a newly developed CAR. 1. INTRODUCTION
The rise of a high acceleration voltage E-beam exposure tool has created a growing need for a chemically amplified resist system with high sensitivity, high contrast, superior resolution, superior PCD and PED stability. While mask- makers have been procuring resist coated blanks, it is generally considered that CARs must be spun on just prior to exposure due to its very short life after coating. However, it is general intention in the industry to stay in the same manner even with CARs for the next generation, which is the mission of blanks supplier also. In order to study feasibility for CARs coated blanks supply, we started screening CARs that were commercially available at present by patterning evaluation especially for PCD stability. We first tried to establish PSB and PEB optimization technique for CARs by using a benchmark resist of RE-5153P. We also tried to establish a stress tests for PCD stability by using NH<SUB>3</SUB> gas and dry-N<SUB>2</SUB> gas mixture. Then, we did comparative evaluation in patterning performance such as sensitivity, contrast, resolution, process latitude, PED and PCD stability among RE-5153P, EP-009, TLE-011, UVIIHS and others. ZEP7000 was also examined as another benchmark, which was the most popular resist at present for advanced EB reticle fabrication in the industry. This paper describes chemically amplified resist feasibility study result especially for blanks supply for the next generation e-beam reticle fabrication.
Appearance of e-beam exposure tools with a high acceleration voltage has created a growing need for a chemically amplified resist (CAR) system with high sensitivity, high contrast, superior resolution and superior post-coating delay stability. While mask-makers have been procuring resist-coated photomask blanks, it is generally considered that CARs must be spin-coated just prior to exposure due to its short shelf-life after coating. However, the industry would like to continue procuring pre-CAR-coated blanks from a blanks supplier, who also intends to support mask-makers in this way. This paper describes the results of a feasibility study on mask blanks coated with CARs for advanced e-beam reticle fabrication.