In the ArF lithography for sub-100nm, PSM (Phase Shift Mask) has been considered as one of the basic RETs (Resolution Enhancement Techniques). Nowadays, besides attenuated PSM, alternating PSM and CPL (Chromeless Phase Lithography) containing Cr patch is widely studied for targeting sub-100nm. Since 2nd process using 365nm laser tools for Cr patch has been a wide gap between the reality and the demands, various candidates using 254nm laser or e-beam exposure tool have been presented to overcome the current 2nd process limitation. And, the Cr patch operate as an assist pattern to control the transmittance of mask, therefore, the CPL mask with Cr patch have advantages of improving process margin such as dose margin and its applicable flexibility for various layers, dense or isolated pattern in the memory and logic device. In this paper, we scrutinize the feasibility of 2nd alignment using 10keV e-beam. Process issues such as the charging effects caused by 2nd e-beam exposure on the 1st Cr etched substrate were evaluated as well.
Recently, there are lots of interest in using chemical amplification (CA) on electron beam lithography for application to photo mask fabrication, direct writing, and projection printing. E-beam resists introducing chemically amplification concepts provide superior lithographic performance in comparison with traditional non CA E-Beam resist in particular high resolution and sensitivity. In first approach, we applied CA concepts to acetyl polymer based E-beam resist (resist thickness: 4,000Å), which can print fine images (<100nm), meet sensitivity (10μC/cm2), and have stability against post exposure delay (PED)(>10hrs) using 50KeV E-beam exposure tool. But, there is vacuum delay problem (40nm CD shrinkage/5hrs) due to thermally unstable blocking group in polymer. To prevent this vacuum delay problem due to polymer-inherent thermal instability in low-activation-energy-acetal polymer, we newly designed various poly(hydroxystyrene-acrylate) copolymer derivatives that contained thermally stable (acrylate) acid-blocking group. In this presentation, first we will discuss the chemistry of newly designed copolymer derivatives, and second, vacuum delay effects and other lithographic performances (resolution, sensitivity, line edge roughness) of these resist systems.
Chromeless Phase Lithography (CPL) is one of the promising RETs for low K1 optical lithography. However, there are remained issues in CPL mask manufacturing, such as phase defect, which can be generated during quartz dry etching process. In CPL mask technology, the traditional defect printability specification is no longer adequate. This paper investigates to understand the tolerance of the CPL in view of phase defect specification. We studied to find out specifications for phase defect in CPL mask. Three-dimensional topography is used in the phase defect simulation. Based on the simulation results, programmed defect mask is made to evaluate phase defect printability by measuring aerial images with AIMS. Also the inspection sensitivity for quartz phase defect was evaluated with current inspection tool.
The demands for shrinking critical dimension (CD) and for tight control of CD uniformity on photomask are rapidly increasing. To keep pace with the demands, optical pattern generators using i-line resist is expanding its capability by continuously modifying hardware and writing strategies. Aside from their advantage of high throughput and layer-to- layer alignability for phase-shifting masks (PSM), this optical process has proven to show a good capability in view of the mean-to-target (MTT) control. In this paper, we investigate the extended capability of a laser writing tool for 180nm node and below in view of process-induced limitations and systematic errors. We also discuss the effects of pattern density and of writing strategy on Cd accuracy and pattern placement error. Since MTT and CD uniformity error for 180 nm generation device are critical for wafer printing, we scrutinize process-induced limitations and systematic errors. The process-induced limitations, related to pattern density and shape, are discussed along with the football effect and iso-dense bias.
As the design rule is rapidly decreased, tighter critical dimension (CD) control is highly requested. Considering the mask error enchantment factor, higher mask quality below 8nm should be guaranteed for 0.10micrometers generation devices. Among a number of actors causing CD errors in e-beam mask fabrication, dry etching plays an important role. Therefore, it is necessary to reduce loading effect for accurate CD control. As the loading effect in the dry etching is closely related to the selectivity of Cr to resist, a clue to reduce the loading effect is to reduce loading. In this paper, we will clarify the relation mechanism between the selectivity and loading effect. We will investigate the degree of loading effect by quantifying the selectivity with different etch processes.
Chemically amplified resists (CAR) which are widely used in KrF lithography are recently employed to e-beam mask making for its high sensitivity and high performance. The high sensitivity of CAR is attractive in a 50keV variable shaped e-beam system for reducing heating effect and improving throughput problems. As the device shrinkage is accelerated, superior mask process has highly been requested in the 10keV system as well. To cope with these requests, the feasibility of a CAR in a 10keV e-beam system has been investigated through comparison with a typical resist for 10keV, PBS. The difference of each resist potentiality between CAR and PBS results in a contrast superiority. As CAR uses an aqueous developer, the development induced error can be reduced owing to its good develop stability and a high evaporation heat. As a result, more accurate CD control can be achieved. The CD linearity and dose margin with the CAR are comparable to or better than those with the PBS. It is concluded that the CAR has various advantages over PBS and can supersede the PBS in a 10keV e-beam system.
Resolution comparison of a CAR (positive resist) and ZEP- 7000 was investigated for 50 kV e-beam machine and dry etching process. The CAR is superior to ZEP-7000 in view of resist profile, while it is inferior in view of CD variation, after Cr dry etching. The etching results were improved using thin resist, optimizing the etching condition and process effect correction. The best performance was obtained form e-beam proximity correction. It is difficult to apply this model to a real device since it has model errors and inconvenience in data handling. Among the activities for the improvements, etch condition optimization is the most effective. A pattern fidelity issue such as edge roughness and line-end shortening remains even with a CD linearity improvement.
In spite of the advantages of low cost and resistance, dual damascene process has some problems. When contact holes are patterned within the trench patterns, the contact holes are frequently found to be unopen and are bent toward trench side wall (TSW). These cause CD variation and small depth of focus. We can explain this phenomenon in view of limited resolution of photoresist (PR) and the light reflected from the TSW. The deeper the trench depth is, the thicker the thickness of the photoresist for contact hole patterns is, which leads to decreased resolution. And the light reflected off the TSW makes the contact hole's profile bent toward TSW. This reflected light influences on both sides. One is helpful in defining the contact holes near the TSW, and the other causes CD variations according to distance between the contact holes and TSW. If the contact holes and trench patterns are exactly the same sizes, it is possible to decrease the CD variation and to prevent PR contact holes from unopening within the trench patterns. Also it is of help to improve resolution at the bottom of the PR.
Delay effects were evaluated for various chemically amplified resist (CAR) types in view of exposure conditions, vacuum and atmosphere. Since the mask is exposed in the vacuum chamber for a long period of time, unexpected phenomenon has been emerging in CAR such as pattern degradation, line width variation owing to vacuum delay effect (VDE). In the acetal resist based on ethyl vinyl ether (EVE), the VDE emerges as space CD decrease, while post exposure delay (PED) in an optical process shows space CD increase. Acrylate resist and modified acetal resist are superior in VDE as well as PED to EVE resist. VDE seems to be caused by out-gassing. It can be overcome by choosing out-gassing free chemistry such as acrylate and modified acetal. An over-coating method was evaluated to prevent any volatile materials in CAR from being evaporated in the vacuum, but it is disclosed ineffective to VDE. CAR linearity reaches to 0.2micrometers , and its resist and Cr pattern as well as OPC was equivalent to current e-beam resist, ZEP7000. Finally, we can have achieve 8.3nm CD non- uniformity in 3(sigma) in 135*135 mm2 area that allows beyond 0.13micrometers device mask application.
We have experimentally studied a possibility of chemically amplified (CA) resist process for mask production in various aspects. The pattern fidelity of CA resist for small patterns such as serifs and scattering bars was compared to that of ZEP7000, the most frequently used e-beam resist. We elucidated the design of delay effect in vacuum during a long e-beam writing time. It proved that critical-dimension (CD) change occurred with an acetal type resist compared to an acrylate type resist. We have achieved CD uniformity of < 10 nm in 3(sigma) within 135 X 135 mm2 field showing a high possibility for CAR process to be applied to the mask production for device generations beyond 180 nm.
A possibility of chemically amplified resist (CAR) process for mask production has been studied experimentally to solve the low throughput problem with a high voltage variable shaped e-beam system. The pattern fidelity of CAR for small patterns like serifs and scattering bars is comparable to the ZEP7000 resist that is the most popular e-beam resist nowadays. We estimated the amount of delay effect in vacuum that can be generated during a long writing time in the vacuum state. And it proved that critical dimension (CD) change occurred with an acetal type resist rather than an acrylate type resist. The effect of temperature fluctuation during post exposure baking (PEB) was also evaluated by measuring line-widths and line- width variations across the mask plate. Through our work, we have achieved CD non-uniformity of < 10nm in ?3 within 135 X 135 mm2 field showing a high possibility for CAR process to be applied to the mask production for device generations beyond 180nm.
Optical lithography is the most fundamental technology for the development of 1 Gbit DRAM device. As a current status, KrF lithography is a powerful candidate for 180 nm generation because of relatively high cost of ArF lithography and its untimely applicability to mass production. In this paper, we showed that the optimized OAI system with large quadrupole offset and small opening could improve the resolution and process margin in the photo process of 180 nm level DRAM devices. We also demonstrated what the effect of CD amplification factor ((alpha) ) was related to the mask CD control and resist tone under the optimized OAI system. The result shows that the combination of the optimized OAI system and positive tone resist can give rise to the reduction of (alpha) from 4.5 to almost 1 and provide a reasonable margin.
KrF extendibility to 180 nm and 150 nm L/S patterns and optimized NA were investigated by simulation. Mask CD error and exposure dose error are very important factor in photo process of device manufacture. We took 2 level of expected mask quality and dose control. The mask CD error of plus or minus 15 nm and dose error of plus or minus 4% are very tight but possible level in near future, and plus or minus 10 nm and 3% as extremely tight level but expected to be achieved in sometime. 0.6 NA and quadrupole illumination (pole offset 0.75, diameter 0.1) shows 0.8 micrometer depth of focus (DOF) with mask CD error of plus or minus 15 nm and dose error of plus or minus 4% for 180 nm patterns and bigger in our simulation. This shows that the 0.6 NA KrF exposure tool could be applied to 180 nm devices with acceptable mask and dose errors, but there are still problems of illumination uniformity and throughput caused by extreme off axis condition. Including 150 nm pattern, only 0.7 NA shows 0.6 micrometer DOF with mask CD error of plus or minus 10 nm and dose error of plus or minus 3% which is extremely tight condition.
A new class of photodefinable polymer based on di-tert-butyl malonate protecting group was developed. A novel alkyl malonated copolymer was synthesized by copolymerization of di- tert-butyl malonylmethyl styrene (DBMST) with 4-acetoxystyrene (AST), and the subsequent deprotection of acetoxy group. Exposure of the material to deep-UV light followed by postbaking results in significant changes in solubility and polarity due to the formation of carboxylic functions which were produced on the polymer chain through the photogenerated acid catalyst (chemical amplification). This resist resolved 0.24 micrometer line-and-space patterns, formulated from di-tert-butyl malonate-protected polyhydroxystyrene (PHS) and triphenylsulfonium (TPS) triflate, with the aqueous base development using a KrF excimer laser stepper (NA 0.45) with a dose of 44 mJ/cm2.
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