Photolithography is a key component of the industry's ability to continue to shrink devices according to Moore's Law, and any improvements immediately translate into large economic benefits for the industry. The relative contribution of the wetware, i.e., the photoresists and other chemicals used in the photolithography step, vs. the contribution of the exposure tool development is investigated by expressing the resist performance improvement as a numerical aperture (NA). Using this equivalent NA concept, it is shown that the contribution from resist improvements has historically outpaced that from stepper NA improvements. However, R&D expenditures for hardware and wetware have consistently been at greatly different levels. While part of that difference is probably intrinsic in the cost structure of the respective development efforts, it is suggested that the relative amount of spending on resist R&D has historically not been at a level that optimizes the overall return for the industry.

Spatially addressable immobilization of biomolecules and cells on patterned surfaces and microfabricated topographies has gained momentum in recent years. One major driving force behind this new impetus was the emergence of combinatorial DNA, protein and -soon- cell-based chips. The recent advances in New Generation Lithographies (NGL) as members with full rights in the panoply of microlithography techniques, automatically opened enormously large opportunities for microlithography engineering in the area of microarrays, which can be now seen as patterned chips. Despite all of these developments and related -yet to be fulfilled- opportunities, many other remained almost entirely unexplored. For instance, the intrinsic capability of the microlithographic materials -resists- to manipulate the surface properties via exposure to light has not been used for the patterning of biomolecules or cells. Also, other areas of classical microlithographic expertise, e.g. yield management, registration and alignment, and resolution enhancement are in many senses undiscovered by the bio-techniques for biomolecular and cellular patterning. This contribution will explore these hidden opportunities review the state of the art of biomolecular and cellular patterning through the glasses of a microlithographic engineer and it will explore the future areas where microlithography can contribute with the highest scientific, technical and commercial return-on-investment.

Our primary platform for 157 nm positive resists is built on a copolymer of t-butyl 2-trifluoromethylacrylate (TBTFMA) and norbornene bearing hexafluoroisopropanol (NBHFA) as an acid group, which is prepared by radical copolymerization. The radical copolymerization of 2-trifluoromethylacrylic monomers with norbornene derivatives has been found through reactivity ratio determination and in situ ¹H NMR analysis of kinetics to deviate from the terminal model but to follow the penultimate model. These copolymers typically contain >50 mol% TBTFMA, are lipophilic, and fail to provide good imaging due to poor wettability. Blending a homopolymer of NBHFA (optical density (OD)=1.7/micrometers at 157 nm) into the copolymers (OD=2.5-2.7/micrometers ) increases the hydrophilicity and reduces OD to 2.2-2.0/micrometers , providing high resolution images. Another platform we have identified is a copolymer of TBTFMA with vinyl ethers, which can be prepared by using a common radical initiator. Some of the vinyl ether copolymers are also homogeneously miscible with the NBHFA homopolymer and thus their OD and aqueous base development can be improved by blending.

Lithography with 157 nm fluorine lasers is rapidly emerging as the next evolutionary step in optical lithography and is clearly seen as the likely successor to 193 nm lithography. In fact, it may become the technology of choice for the sub-100-nm node features. The photoresists used for this technology will be required to be extendable to less than 70 nm. As has been demonstrated with the transition to shorter wavelengths in the past, the photoresist materials that were developed for the longer wavelength applications are too absorbent for practical use as high-resolution single layer resist with 157 nm radiation. This high absorbency will force the coated resist thicknesses to be well under 100 nm. Fluorine containing polymers have been demonstrated to be more transparent in this spectral region than pure hydrocarbon polymers. We have developed and evaluated a number of unique 4-hexafluoroisopropanol styrene based polymer systems which we previously termed FESCAP resists and have developed new acetal partially blocked 4-hexafluoroisopropanol styrene based copolymers. These resists can have absorbencies of under 3 micrometers ^-1 at 157 nm which could allow imaging to thicknesses of 150 nm. Our recent resist designs are shown to have imaging capability down to 70 nm with a 0.60 NA microstepper.

Several fluoro-substituted polymers consisting of acid cleavable methacryoic or cinnamic acid tert.-butyl ester compounds copolymerized with maleic acid anhydride derivatives were synthesized by radical copolymerization. Vacuum ultraviolet transmission measurements of the samples reveal absorbances down to 5micrometers ^-1 despite of the strongly absorbing anhydride moiety which serves as silylation anchor for the application of the Chemical Amplification of Resist Lines (CARL) process, one of the promising approaches for sub-90nm pattern fabrication. Some of the samples exhibit resolutions down to 110nm dense at 157nm exposure using an alternating phase shift mask. The feasibility of the CARL principle including the silylation reaction after development has been demonstrated with selected fluorinated polymer samples.

This paper describes characterization and lithographic results for one class of low absorbance fluoropolymers that were developed for use in 157 nm lithography. We discuss basic resist properties such as absorbance, hydrophobicity, thickness, resolution and profile for dense 1:1 and semi- dense 1:1.5-10 L/S features, reflection control and plasma etching resistance as a function of composition. Lithographic results were obtained on two types of substrates, silicon and SiON hardmask anti-reflectant. The results on the anti-reflectant were compared to those obtained from simulations using PROLITH. Some of the conclusions of this investigation are: Lower absorbance resists have higher hydrophobicity and better resolution; Resists with high hydrophobicity have very poor adhesion on SiOn, but have very good adhesion on Si and organic anti-reflectants; Only inorganic anti-reflectants have sufficient absorption to provide very low reflectance in <30nm thick films; 100 nm 1:1 L/S resolution is attained in 205 nm thick resist on Si at a resist absorption of 2.2/micrometers . The profile is tapered due to absorption; Adhesion to SiON has been achieved by polymer modification.

The synthesis and characterization of several new fluoropolymers designed for use in the formulation of photoresists for exposure at 157 nm will be described. The design of these resist platforms is based on learning from previously reported fluorine-containing materials. We have continued to explore anionic polymerizations, free radical polymerizations, metal-catalyzed addition polymerizations and metal-catalyzed copolymerizations with carbon monoxide in theses studies. The monomers were characterized by vacuum-UV (VUV) spectrometry and polymers characterized by variable angle spectroscopic ellipsometry (VASE). Resist formulations based on these polymers were exposed at the 157 nm wavelength to produce high-resolution images. The synthesis and structures of these new materials and the details of their processing will be presented.

Fluorocarbon based polymers have been identified as promising resist candidates for 157nm material design because of their relatively high transparency at this wavelength. This paper reports our recent progress toward developing 157nm resist materials based on transparent dissolution inhibitors. These 2 component resist systems have been prepared and preliminary imaging studies at 157nm are described. Several new approaches to incorporating these transparent monomers into functional polymers have been investigated and are described. The lithographic performance of some of these polymers is discussed.

Novel fluoropolymers having partially fluorinated monocyclic (5-membered and 6-membered ring) structure have been synthesized with radical cyclo-polymerization, which have C- F bond in the polymer main chain and also possess fluorocontaining acidic alcohol group. These polymers have excellent transparency lower than 1.0 μm^-1 at 157nm wave length. The number-average molecular weight (Mn) of the polymers is 4000 to 20000, the glass transition temperature (Tg) is 130 to 155 °C and the decomposition temperature (Td) is about 400 °C. Copolymerization reaction with the other monomers (ex. fluoroolefins,(meth)acrylates and vinyl esters) were also examined. The introduction of protecting group (ex. methoxylmethly, and t-butoxycarbonyl group) to alcohol units of the polymer can be applied before or after polymerization reaction. We also evaluated fundamental resist performances. These have excellent transparency of 0.5 to 1.5 μm^-1, good solubility in the standard alkaline solution (0.26N N-tetramethylammonium hydroxide aqueous solution) and relatively high sensitivities below than 10mJ/cm². The imaging results of the above fluoropolymer based positive- working resists are presented. Under 100-nm line and space pattern are delineated in 200-nm thick film by using the phase shift mask.

Hexafluoroisopropyl alcohol-functionalized acrylate monomers and their (co)polymers were prepared as photoresist platforms for 157 nm imaging. In order to balance transparency with other desirable traits such as etch resistance, we developed several copolymer systems. One is using 2-methyl adamantyl methacrylate as a comonomer, and the copolymer system showed better dissolution contrast compared to the copolymer with tetrahydropyranyl methacrylate without sacrificing transparency. To further improve the absorption properties at 157 nm, monomers having (alpha) -trifluoromethyl group were prepared and polymerized in anionic mechanism. The product polymer was unexpectedly transparent at 157 nm (A = 1.6 micrometers ^-1) in spite that all the monomers contain carbonyl group. The second system is the copolymer with p-t-butoxy-tetrafluorostyrene. p-Hydroxy-tetrafluorostyrene and p-t-butoxy-tetrafluorostyrene were polymerized radically using AIBN in good yield, and the two resulting polymers showed distinctive solubility differences in aqueous base solution. Finally, this paper describes the synthesis of new monomers having fluorine (e.g CF₃- group) in the vicinity of the double bond to improve transparency at 157 nm. Due to the lower electron density of the double bond, these monomers can be copolymerized with electron-rich vinyl monomers using radical initiators.

The design of 157 nm photoresists is a daunting task since air, water, and most organic compounds are opaque at this wavelength. Spectroscopic studies1 led to the observation that fluorinated hydrocarbons and siloxanes offer the best hope for the transparency that is necessary for the design of an effective 157nm photoresist, and these classes of materials have quickly become the prominent platforms for a variety of research activities in this field. There have been a number of authors that have suggested that negative resists have unique attributes for specific device applications. Numerous authors have discussed negative photoresists over the years. There are many uses for such materials at various levels in a semiconductor device. One such use is with complementary phase shift mask thus eliminating the need for a second exposure step. This paper reports our recent progress toward developing a negative 157nm resist materials based on fluoropolymers with crosslinkers that are transparent at 157nm. The authors will report on the synthesis of the polymers used in this work along with the crosslinkers and other additives used in the formulation of the photoresist. Imaging experiments at practical film thicknesses at 157nm with binary and strong phase shifting masks will be shown demonstrating imaging capabilities. Spectroscopic data demonstrating chemical mechanisms and material absorbance will be shown along with other process related information.

The copolymerization reaction between methyl cyanoacrylate (MCA) and a variety of cycloolefins (CO) was investigated. Cycololefin/cyanoacrylate (COCA) copolymers were obtained in good yields and with lithographically interesting molecular weights for all cycoolefins studied. Anionic MCA homopolymerization could be largely suppressed using acetic acid. Based on NMR data, the copolymerization may tend to a 1:1 CO:MCA incorporation ratio but further work with better suppression of the anionic component is needed to confirm this. Lithographic tests on copolymers of appropriately substituted norbornenes and MCA showed semi-dense and isolated line performance down to 90 nm.

The present paper describes a novel class of norbornene- based copolymers containing 7,7-dimethyloxepan-2-one acid labile groups. Poly(3-(bicyclo[2.2.1]hept-5-en-2- ylhydroxymethyl)-7,7-dimethyloxepan-2-one-co-5-((2- decahydronaphth-yl)oxycarbonyl)-norbornene-co-5-norbornene- 2-carboxxylic acid-co-maleic anhydride) was synthesized and evaluated as a potential chemically amplified resist for ArF lithography. The 7,7-dimethyloxepan-2-one group of the matrix polymer was readily cleaved and the carboxylic acid functionality was formed by acid-catalyzed ring-opening reaction in the exposed region after post-exposure bake. 0.12micrometers line and space patterns were obtained at a dose of 10 mJ cm^-2 with a conventional developer, 2.38 wt% tetramethylammonium hydroxide aqueous solution, using an ArF excimer laser stepper.

We have investigated new photoresist materials based on modified cycloolefin-maleic anhydride resin for 193nm lithography. Compared to COMA resin, the new resin offers several good properties such as UV transmittance, taper in pattern profile, PED stability and storage stability. The resist using the new resin showed good lithographic performances on ArF exposure tool. So, we obtained good hole patterns below 90nm with condition of exposure: PAS 5500/900 full field scanner, resist thickness: 320nm, development: 2.38% TMAH, for 40sec., Illumination: 0.61NA, conventional SOB: 110 degree(s)C/90sec, PEB:130 degree(s)C/90sec, on BARC. In this paper we will report on the properties of new polymer and will show the contact hole pattern that demonstrate progress in these areas.

A high performance 193 nm resist has been developed from a novel hybrid copolymer based on a cycloolefin-maleic anhydride and methacrylate (COMA/Methacrylate) polymer system. A variety of copolymers have been synthesized from t-butyl norbornene carboxylate (BNC), t-butyl tetracyclo [4.4.0.1.^2,61^7,10]dodec-8-ene-3-carboxylate (BTCDC), and 5-[2-trifluoromethyl-2,1,1-trifluoro-2-hydroxypropyl]-2- norbornene (F1) with different types of methacrylate monomers and maleic anhydride (MA). The effect of the monomers and the ratio of monomers in the copolymer on lithographic performance and etch rate has been studied. Lithographic evaluation of some of these polymers has shown resolution down to 80 nm using conventional 193 nm illumination and standard development conditions, particularly for semi and fully isolated lines. This paper will report the chemistry of the polymer platform and relative advantages of having certain monomers in terms of lithographic performance and line edge roughness.

Crown ether derivatives are composed of multi-ethyleneoxy units and have an electron rich cavity that can accommodate a proton. We have broadly investigated the effect of lone pair electrons of accumulated oxygen. First, we studied whether these crown compounds can control acid diffusion or not. Second, we synthesized monomers containing cyclic multi-ethyleneoxy units and studied their effect in polymers. Finally, we compared them with amines. Crown either, 18-crown-6, has a proper cavity to capture a proton by hydrogen bonding and actually had enough basicity to control acid diffusion. These studies show that crown ether derivatives can replace amines as a bases to restrain acid diffusion.

ArF lithography is the current ramp-up technology for next generation devices. However, some manufacturing issues still remain when considering the resist design for the most advanced processes. Several polymer platforms have been proposed, among them, Methacrylate, CycloOlefin-alt Maleic Anhydride, and even pure Cyclo-Olefin. More recently, Vinyl-Ether Maleic Anhydride (VEMA) polymers have demonstrated potential in terms of both lithographic properties and etch capabilities. In this paper, the evaluation of some advanced samples of VEMA resists for 120nm and sub-120 nm gate applications will be discussed. The various criteria investigated for this study were; focus and exposure latitude for 120 and 100 nm lines (1/1.5 L/S to isolated lines), Iso-Dense bias, Line End Shortening (LES), Line Edge Roughness (LER), masking linearity, BARC compatibility, sensitivity to PEB temperature and electron beam, and finally etch resistance. Additionally some process optimizations were tested in order to minimize Iso-Dense Bias and the LER of the resists (See figure 1). In fact, this latter parameter has been a major focus of this work in improving the VEMA resist chemistry since its introduction and preparing it for device manufacture. The results obtained when varying parameters such as resist formulation, development conditions will be reported and so will demonstrate the current maturity of the most advanced VEMA samples.

We have studied 193 nm contact hole resists in view of resist components, process conditions and optical settings. Sidewall roughness was improved by optimizing photoacid generators. Side lobes were eliminated by applying higher post exposure bake temperature or modification of polymers. The influence of optical settings, types of masks and mask bias was discussed with simulation and lithographic results and guidelines for better resolution and iso-dense bias were proposed. The optimized formulation, AZAX1050P has a high resolution combined with a large depth of focus and an iso- dense overlap window (130 nm(NA=0.63) DOF 0.38micrometers Exposure latitude 10%).

As resist feature sizes have decreased and the performance demands on chemically amplified photoresists have increased the role of the photoacid generator (PAG) in determining overall resist performance has become increasingly apparent. Over the past 20 years a variety of different types of PAG's have been introduced as researchers have sought to optimize properties such as acid strength, acid volatility, diffusion length, wavelength response, solubility etc. PAGs that produce very strong organic acids are widely used, in part because of requirements for high photospeed resists. Most of these acid generators are based on perfluoroalkyl sulfonic acid based onium salts. In an effort to identify and characterize alternative PAGs we have investigated the performance of a variety of photoacid generators that are not based on sulfonic acids. In this report we will describe the relative reactivities of these PAGs under a variety of exposure wavelengths and processing conditions including acid diffusion proprieties and photospeed measurements.

The lithographic performance of a new class of onium type photo acid generators (PAGs) developed by 3M Company has been evaluated using standard hybrid/acetal and ESCAP type DUV (248 nm) photoresist formulations. The new PAGs produce perfluoroalkyl sulfonylimides or sulfonylmethides with superacidic properties matching or exceeding the acid strength of commonly known perfluoroalkyl sulfonic acids, such as trifluoromethane sulfonic acid. The direct comparison of near commercial photoresist formulations containing onium perfluoroalkyl sulfonates with identical materials using equimolar amounts of the new PAGs revealed that the new 3M PAGs give rise to almost identical lithographic properties, such as resolution, DOF, and exposure latitude. In addition, secondary properties such as photoresist stability, delay stability, or line edge roughness are almost equivalent, and thus the new PAGs are considered as suitable candidates used for the manufacturing of high volume production photoresists.

The advent of 193nm lithography has stimulated the search for novel photocid generators (PAGs) with increased transparency at 193nm. This need for more transparency stems from the use of phenyl groups in the classic 248nm PAGs, such as triphenylsulfonium salts. Unfortunately for 193nm resist development, the phenyl group is highly absorbing at that wavelength, thereby severely restricting the PAG formulation space. It has not yet been pointed out, however, that photoacid generation efficiency has also become more important for 193nm systems. The reason is that 193 polymers cannot sensitize the PAG, as is the case for 248nm phenolic systems, i.e. the light absorbed by the polymer does not contribute to acid generation. Furthermore, the photoacid generation efficiency of sulfonium PAGs drops considerably when going from 248nm to 193nm. Thus, the ability to quantify the photoefficiency of a new PAG quickly becomes key to successful new 193nm PAG development. While there are many ways to determine the quantum yield of photoacid generation ((Phi) ) of PAGs, they are usually time and labor-intensive. Therefore, in the first part of this paper, we describe the P-parameter as a fast, simple and more practical way than the quantum yield, (Phi) to characterize the photoefficiency of a PAG. Results for a number of sulfonium PAGs at both 248 and 193nm are determined this way, and the observed trends are discussed. In the second part of this work we investigate how lithographic performance is impacted by those two parameters. A transparency/efficiency matrix of nine PAGs is evaluated in a 193nm standard resist formulation. The results indicate that the PAGs with a combination of high efficiency and transparency yield the best lithographic results, underlining the importance of efficiency as a key parameter in 193nm PAG design.

Fluoropolymers are key materials for single-layer resists in 157-nm lithography. We investigated main-chain fluorinated polymers and found that the incorporation of fluorine atoms into polymer backbones such as that in tetrafluoroethylene and monocyclic monomers reduced their absorption coefficients to less than 1 micrometers ^-1 at 157 nm, while side-chain fluorinated polymers had absorption coefficients of 2 to 3 micrometers ^-1. The main-chain fluorinated polymers also showed good solubility in a standard alkaline developer and their dry-etching resistance was comparable to that of a ArF resist. Prototype positive-tone resists had good sensitivities of less than 10 mJ/cm², and they exhibited fine imaging resolution with 80-nm dense patterns. The resists can be used to obtain 300-nm-thick films.

We designed platforms for a 157-nm resist polymer that have both high transparency and high etch resistance. We measured the etching rate of various fluoropolymers under the conditions of poly etch and oxide etch processes. Most of the fluoropolymers showed good correlation with an etching rate ratio and a value of (N_C-N_O-N_F) / N_T (N_T, N_C, N_O, and N_F was the number of total atoms, carbon, oxygen, and fluorine atom in a monomer unit, respectively). This index, (N_C-N_O-N_F) / N_T was named IERF (index for etch resistance of fluoropolymers). It became easy to design a fluoropolymer that has high etch resistance using this index. Moreover, the acrylic polymer that has a particular kind of fluorinated alicyclic compound in its side chain was not in the above-mentioned correlation, however, it was found to have higher etch resistance. Base upon these results, two platforms were built, one of the copolymer type with polyacrylate (PA) and polystyrene (PS), the other of the copolymer type with PA and polycycloolefin (PC). Resists belonging to the two platforms were made and a high resolution of 85 nm or less was obtained by using the Microstepper (NA0.6, (sigma) 0.3, alternating phase shift mask). These resists showed a slightly higher etch resistance as compared with a COMA (cycloolefin-maleic anhydride) type resist with comparatively high etch resistance even during the ArF resist.

To accomplish minimizing feature size to sub 70nm, 157nm photolithography becomes a strong candidate as a new lithographic technology. However, there is a strong need for new photoresists, which are transparent to 157nm light sources. To have a transparency for 157nm light source, fluorinated organic polymers are studied intensively. As a result, there are some of polymers that have absorbance of 2/micrometers . However, in spite of this low absorbance of 2micrometers , resist profile simulation tells us bulk slope problems. TO obtain more than 85 degree of resist pattern profile, the absorbance of resist must be 1.2micrometers . The absorbance of 1.2micrometers is very difficult target to accomplish. To overcome this light absorption problem, we have developed amine gradient resist process (AGRP) which gives an amine gradient in photoresist and can make a vertical profile though the resist has poor transparency to light source. By adding chromophore that absorbs 193nm wavelength, we made model ArF resists of which absorbances were from 1.2micrometers to 4micrometers . By patterning experiment using these model resists and 193nm scanner, we could confirm that the resist absorbance should be lower than 1.2micrometers to obtain vertical profile pattern at the resist thickness of 150nm. But if we use AGRP, the absorbance of 2.5micrometers will be enough value for the vertical profile pattern. So we could conclude that by combining 157nm resists and amine gradient process, resist absorbance problem in 157nm photolithography could be solved. We also studied for the resist properties that were suitable for AGRP.

We describe and evaluate three kinds of pattern transfer processes that are suitable for 157-nm lithography. These transfer processes are 1) a hard mask (HM) process using SiO as a HM material, 2) a HM process using an organic bottom anti-reflecting coating (BARC)/SiN structure, and 3) a bi- layer process using a silicon-containing resist and an organic film as the bottom layer. In all of these processes, the underlayer fo the resist acts as an anti-reflecting layer. For the HM processes, we patterned a newly developed fluorine-containing resist using a 157-nm microstepper, and transferred the resist patterns to the hard mask by reactive ion etching (RIE) with minimal critical dimension shift. Using the HM pattern, we then fabricated a 65nm Wsi/poly-Si gate pattern using a high-NA microstepper (NA=0.85). With the bi-layer process, we transferred a 60nm 1:1 lines and spaces pattern of a newly developed silicon-containing resist to a 300nm-thick organic film by RIE. The fabrication of a 65nm 1:1 gate pattern and 60nm 1:1 organic film patten clearly demonstrated that 157-nm lithography is the best candidate for fabricating sub-70nm node devices.

A fabrication technique recently developed at Georgia Institute of Technology involving thermally sacrificial polymeric materials allows for fabrication of microfluidic devices with greater degrees of functionality (i.e., fully integrated, complex, multi-level fluidic systems with functional valves, pumping systems, and other micro- electromechanical system [MEMS] components). In this method, thermally sacrificial polymers are coated onto a substrate and patterned into the shape of the desired channels and devices. These polymeric structures are then over-coated with a permanent structural material such as an inorganic glass or polymer. These steps can be repeated to produce complex, three-dimensional systems. Once the device build-up is complete, the structure is heated to a temperature at which the sacrificial polymer slowly decomposes, thus leaving behind the desired open-channeled structures. This process was first developed using functionalized polynorbornenes that decompose at temperatures in the range of 425 degree(s)C. In order to make this approach compatible with a wider range of substrates and structural materials, polymers with lower decomposition temperatures were desired. Polycarbonates were identified as a class of polymers with the desired lower decomposition temperatures (200- 300 degree(s)C). A disadvantage of commercially available polycarbonates, like poly (propylene carbonate), is that they have low glass transition temperatures (T_gapproximately equals 40 degree(s)C). This introduces several problems, including pattern deformation at elevated processing temperatures. Also, a significant number of process steps are required to pattern the channel structures using simple polymer materials. Therefore, this paper will describe recent results of work on photodefinable polycarbonates with improved thermal properties. Utilizing a polymer that can be patterned directly by conventional lithography greatly simplifies the fabrication process and eliminates the need for the plasma etch steps required in the original process. Results of the synthesis and characterization of polycarbonates patterned with the use of a photoacid generator, thus exploiting the acid-catalyzed thermolysis of polycarbonates, will be presented.

Alkaline soluble maleimide based tetrapolymers which were prepared by free radical copolymerization are found to be suitable for use as non-imaginable resists in multilevel photolithographic processing. These materials offer widely variable undercut rates because of the ability to vary the composition during free radical polymerization and show little or no scumming when used in lift-off resist. This new polymer system may also be used as the base soluble resin in a photoresist formulation or a base soluble antiflective coating composition as well as used in making lift-off resists.

An ever increasing need exists for thick resist layers in the processing of MEMS and for advanced packaging. Applications in the MEMS field include bulk micromachining, surface micromachining, and the actual creating of active device structures. For advanced packaging, the applications are in redistribution and passivation layers, and micromolds for metal bumps. The various applications can require resist layer thicknesses up to and exceeding 1000 micrometers . In order to properly achieve these thicknesses, appropriate coating materials were developed by manufacturers. These materials include AZ P4620, Shipley SPR220, AZ PLP100XT, JSR THB 611P, and SU-8. Finally, equipment was developed to handle these materials, in the form of specialized coating equipment and contact/proximity aligners.

Low-k₁ imaging, high-NA optics, pattern collapse, and the absorption of resist materials in 157-nm and EUV lithographies are driving down the thickness of the photoresist layer in integrated circuit fabrication processes. Although devices and test structures have been successfully fabricated with resist films thinner than 160 nm on various levels, the fabrication of working devices with high yield using ultrathin resist (UTR) integrations on multiple device layers has yet to be demonstrated. In the present work, gates have been patterned with 140-nm thick resist films with 10-15 defects per wafer, none of which are specific to the UTR process. Similar UTR gates were also patterned over 80-nm steps with no defects associated with the topography. The UTR NMOS transistors in this work have 10 pA/micrometers leakage and 400 (mu) A/micrometers drive currents, but the PMOS transistors do not perform as well. The line-edge roughness (LER) is 5-8 nm 3(sigma) depending upon exposure mask (binary vs. PSM) and substrate. Etching into 100 nm of crystalline Si reduces the LER to 4-7 nm 3(sigma) . The power spectral densities of the roughness have a Lorentzian shape, and most of the roughness occurs over length scales larger than 100 nm. Contact chains with electrical characteristics comparable to standard processes were fabricated with 120-nm thick resist films. Polysilicon as thick as 150 nm was etched successfully with 80-nm thick resist films and hardmasks.

The lithographic performance of a single layer 193nm resist platform, Sumitomo PAR707, was evaluated for 100nm node patterns for thicknesses ranging from 313nm to 60nm. We first demonstrated that the standard resist formulation could not be used for sub-200nm thicknesses because of unacceptable line edge roughness (LER). We then evaluated the influence of the concentration of photo acid generator (PAG) in the resist formulation of LER over the thickness range of 313nm to 60nm. High PAG loading was found to decrease LER significantly for sub-200nm thicknesses. Using the optimal formulation for minimal LER for a given thickness, process latitudes for 100nm node patterns were determined. The overall dose-focus latitudes were found to remain very close for all thicknesses, with slightly larger latitude for thicker imaging layers.

Investigations of the dissolution behavior of the IBM V2 193 nm methacrylate photoresist polymer and its formulations using the recently developed impedance quartz-crystal microbalance methodology unequivocally confirm the presence of an interfacial gel layer during dissolution. All aspects of V2 dissolution behavior are strong functions of resist formulation, developer concentration, and resist film thermal history. The dissolution process is characterized by (a) a complex gel layer evolution regime; (b) a steady-state dissolution regime; and (c) a final gel layer decay regime upon depletion of solid resist film. The gel layer evolution regime exhibits a linear increase in mass and development of a lossy film component (interfacial gel) associated with developer uptake. The transition from this regime to the steady-state dissolution regime is accompanied by reproducible correlated excursions in frequency shift and film resistance measurements. The steady-state dissolution regime exhibits a linear decrease in mass and fairly constant gel-layer; upon depletion of the solid resist film, the gel layer decays. Absolute and relative metrics for phenomena occurring in these regimes can be extracted from the dissolution data and are useful for analyzing the influences of formulation components on dissolution and gelation behaviors. Formulation components explored include a photoacid generator, bis(t-butylphenyliodonium) perfluorobutanesulfonate, and common dissolution modifiers, t-butyl lithocholate and lithocholic acid. Film thermal history impacts dissolution phenomena strongly and appears to reflect both polymer annealing and residual solvent evaporation effects.

The use of chemically amplified photoresists for the fabrication of sub-100 nm features will require spatial control with nanometer level resolution. To reach this goal, a detailed understanding of the complex reaction-diffusion mechanisms at these length scales is needed and will require high spatial resolution measurements. In particular, few experimental methods can directly measure the spatial evolution of the deprotection reaction front and correlate it with the developed structure. In this work, we demonstrate the complementary use of neutron (NR) and x-ray (XR) reflectometry to measure the reaction front profile with nanometer resolution. Using a bilayer geometry with a lower deuterium-substituted poly(tert-butoxycarboxystyrene) (d-PBOCSt) layer and an upper poly(hydroxystyrene) (PHOSt) layer loaded with a photoacid generator (PAG), we directly measure the spatial evolution of the reaction front. We show that the reaction front profile is broader than the initial interface after a post-exposure bake and the compositional profile changes upon development in an aqueous base solution. We also directly correlate the final developed structure with the reaction front profile. The spatial detail enabled by this general methodology can be used to differentiate between and evaluate quantitatively reaction-diffusion models.

Water is known to play a key role in the solubility switching reaction of novolac-diazonaphthoquinone photoresists and certain chemically amplified resists. In order to quantitatively study these effects, an environmental chamber was built in which the % RH could be controlled while the extent of acid catalyzed deprotection was monitored during the post exposure bake by reflectance FTIR spectroscopy. The extent of acid catalyzed deprotection of tBOC, KRS-XE, UV6, and a tBOC-poly(hydroxystyrene) copolymer have been measured as a function of time over a range of 0-60 % RH. For tBOC, the deprotection reaction rate was found to slow considerably as the %RH was increased. Also, the relative film shrinkage varied considerably with varying % RH. Several possible mechanisms for the dependence of the reaction rate and thickness loss on % RH were investigated. For KRS-XE, the deprotection reaction kinetics were found to increase as the % RH was increased, which was an expected trend. For UV6 and the tBOC-PHOST copolymer, no change in deprotection reaction rate was observed with changes in %RH.

Surface properties of photo resist lines are of particular interest in future semiconductor applications. Current hot topics as pattern collapse (PC) and line edge roughness (LER) are closely related to surface properties of the resist. The surface origin on PC and LER based on the interaction of the rinse liquid with the resist and the interaction between the resist surface and the developer, respectively. In this paper we present a characterization of a reference surface prepared by an open frame exposure. Utilizing scanning force microscopy (SFM) the surface topography is analyzed. From the SFM images vertical structures (root mean square roughness) as well as lateral structures (most prominent in-plane lengths) are extracted. Advancing and receding contact angles were measured to characterize changes in hydrophilic/hydrophobic surface properties of the photo resist after development. With increasing thickness loss of the initial film the surface roughness and the most prominent in-plane length scales increase while the contact angle decreases. An analogy of the prepared surfaces with phase separated structures is presented.

Using Small Angle Neutron Scattering (SANS), we present the first quantitative measurements of the 3-dimensional conformation of macromolecules in thin polymer films of D < R_G,Bulk, where D is the film thickness and R_G,Bulk is the bulk radius of gyration. For D approximately equals 0.5 R_G,Bulk, the molecular size along the film normal is less than 0.5 R_G,Bulk, while relatively small changes are observed parallel to the surface. The observed changes in molecular size agree with predictions of molecular dynamics simulations of polymers confined in thin films, resulting from increased molecular orientation rather than chain distortion. Segregation of molecular centers of mass to the film center facilitates molecular packing while minimizing chain distortion. Estimates of length scales relating to the onset of chain distortion and chain orientation in thin film resists are D approximately equals 0.1 R_G,Bulk and D approximately equals N_(sigma ) respectively, where D is the film thickness, N is the number of chain segments, and (sigma) is the persistence length.

We have extended the recent work on high numerical aperture (NA) effects in novolac photoresists to investigate the influences of s- and p-polarization at Brewster's angle on the final developable image of a chemically amplified (CA) resist. Using our interferometric lithography tool, resist films (Shipley UVII-HS) were exposed using either s- or p-polarization at both Brewster's angle (146 nm pitch), and at a small angle of incidence (400 nm pitch), where polarization effects are considered negligible. Cleaved and decorated samples were analyzed with a scanning electron microscope (SEM) which provides a measure of the spatial distribution of energy deposition in the film convolved with the image transformation that occurs during PEB. The experimental images were compared to simulated latent images using a PEB model that mimics UVII-HS behavior and a more simplified model, a blurred acid image. Our results indicate that the degraded aerial image contrast at high NA caused by the absence of an antireflection coating (ARC) leads to complex developable images. This degraded contrast can increase line-edge roughness, and affect critical dimension variation. This work reveals the need to understand the details of resist response to high NA aerial images to predict structures, and the importance of a well-designed ARC to maintain control over image formation.

The effect of aerial image contrast upon line edge roughness (LER) of four different commercial resists-UV210, SEPR-463, Apex-E, and UVII-HS-is examined via scanning electron microscopy (SEM) and atomic force microscopy (AFM). Image contrast is varied by exposing both a detailed foreground pattern and a large open area background pattern on the same wafer location. Contrast varies from 0.37 to 1.0. In the case of UV210, LER measured via AFM is 2.55 nm at 1.0 contrast, 2.16 nm at 0.54 contrast, and 3.6 nm at 0.37. Results from other resists follow this trend, wherein no consistent correlation between LER and aerial image contrast can be drawn. SEM measurements also demonstrate no significant correlation between LER and image contrast in any resist type.

Lithographic performance is controlled by both the projected aerial image and the photoresist process. It is usually easier to calculate the output of the optical system than to construct an accurate predictive model for the photoresist process. This point, coupled with the trend toward extremely high contrast photoresists where the resist starts to act like an ideal image threshold detector, means that aerial images alone are increasingly useful for predicting performance. In this work, discrepancies between resist processes with finite imaging contrast, and aerial image predictions are explored. The method used is to construct several lithographic problems (various line and hole patterns with high NA lenses) and to directly calculate differences for important responses such as exposure latitude, dense:isolated bias, and isofocal curvature. The expected performance range for resist processes can be found with a simulation program and systematically varied resist models, producing hundreds of trial cases. The results show that the resist process predictions deviate from the aerial image predictions in a consistent way; interpretable as a resist vector whose magnitude varies depending on the details of the resist process, but whose direction does not vary much. Knowledge of the vector provides a route for extrapolating aerial image calculations to what actually prints on the wafer. Physical explanations based on finite dissolution contrast and non-zero diffusion are offered. Where possible, experimental data is used to support the arguments.

A mesoscale computer simulation for positive tone chemically amplified resists is reported. The simulation is based upon detailed models of resist function, and the required simulation inputs are fundamental quantities that can be measured independently of lithographic performance. These input parameters include resist formulation variables such as polymer molecular weight, photoacid generator loading, and activation energy of the deprotection reaction, as well as processing variables such as temperature and duration of bake steps and developer concentration. The simulation can model all major resist processing steps: spincoat and post apply bake, exposure, post exposure bake, dissolution, and drying. The general approach involves representing the photoresist film on a three dimensional lattice. Some lattice sites represent acid or solvent molecules and other lattice sites are strung together to form polymer chains. Each lattice cell is considered individually during the simulation, and each behaves in a manner consistent with its chemical identity. The mesoscale nature of the model allows investigation of stochastic effects that lead to line edge roughness, such as shot noise during exposure and the finite size of resist molecules. The model is based upon extensive experimental studies, and has now yielded results that are qualitatively correct for all major lithographic processing steps.

An extended 248nm lithography process with alternating phase-shift masks (PSMs) and etch-trimming techniques has produced 50nm gate critical dimensions (CDs). Well-controlled CD uniformity and line-end edge roughness (LER) are also demonstrated in this work. The primary factor in the improvement of through-pitch proximity bias was phase shifter width. With optimum shifter width, where pitches are greater than 600 nm, a through-pitch proximity bias of less than 10 nm can be obtained. Photoresist also has a significant role in implementing alternating PSMs, requiring high activation energy and suitable thickness. Soft-bake and post-bake temperatures were determined to enlarge the depth-of-focus (DOF). The forbidden pitch effect was enhanced to constrain the common process window after optical proximity correction (OPC) had been implemented. However, proximity bias can be kept constant with changeable diffusion length due to the through-pitch CD profiles, all isolated like. Improvement of through-focus CD at the forbidden pitch was examined by optimizing diffusion length; the common process window can be improved by 25%. All process results, including line-end roughness, DOF, exposure latitude, well-controlled CD uniformity, and through-pitch proximity bias, showed that 248 nm photolithography with alternating phase shifting mask could meet the requirements of 100nm node application.

As 193 nm resist moves into production with minimum feature sizes approaching 100nm, bilayer resist is being evaluated more closely for certain applications. Our polymer design has been evolving to meet tighter outgassing requirements. Optical density, etch resistance and dissolution behavior are other considerations. The protecting group used in our 248 nm bilayer is not useful for 193 nm lithography because of the high optical density contribution from Si-Si linkage. Silicon was incorporated into a COMA platform for the first generation polymer. Maleic anhydride is used to modulate dissolution characteristics. The first generation 193 nm bilayer was optimized to print 120 nm L/S patterns with an attenuated PSM on a 0.6 NA Nikon S302. We will describe next generation platforms that address silicon outgassing concern. The lithographic performance of these resists was evaluated on a 0.6 NA Nikon S302 with a dark field mask. Results for 280nm pitch (1:1 L/S) and 245 nm pitch (105 nm L, 140 nm S) lithography are presented. Also shown is result for a 245 nm pitch (1:1 L/S) and 210 nm pitch (1:1 L/S) on a 0.75 NA ASML PAS 5500/1100. Outgassing data generated at MIT Lincoln Laboratory will be discussed.

Carbon dioxide (CO₂) based microlithography (dry microlithography) represents an immense opportunity for the semiconductor industry to keep pace with Moore's Law while reducing its environmental impact. Currently, rinsing <130nm developed images with supercritical CO₂ is the only method which can prevent image collapse at this resolution. In this article, we will discuss CO₂'s ability to improve lithographic performance as we demonstrate its potential to replace the most solvent intensive steps of the microlithography process; spin coating, developing, and stripping. During these steps, semiconductor manufacturers produce vast amounts of organic and aqueous waste, which are detrimental to our ecosystem. However, before CO₂ can replace conventional solvents, photoresist systems must be designed and synthesized to be compatible with CO₂. These photoresists must be soluble in liquid CO₂ to insure that uniform thin-films can be produced by spin coating while maintaining characteristics of conventional resist systems such as low absorbance, high sensitivity, solubility contrast, good resolution, and etch resistance. Using our CO₂ compatible resist system, we will show the ability of CO₂ to spin coat uniform thin-films which (after exposing and PEB) can be developed using scCO₂ to produce lithography features that may be stripped in CO₂. Thus, revealing the enormous potential of CO₂ to provide the microlithography industry an opportunity to escape its water and organic solvent dependence.

A dissolution rate monitor (DRM) was successfully constructed to study the behavior of thin photoresist films undergoing the dissolution process in supercritical carbon dioxide (SCCO₂). The DRM is based on the principles of interferometry but requires special modifications to the processing vessel to allow for the passage of transmitted and reflected He-Ne laser light. Dissolution rates obtained agree well with independent profilometric measurements of film thickness loss. We found that for block and random copolymers of THPMA-F7MA, dissolution rates vary with film thickness, slowing down considerably towards the silicon surface. This behavior was also observed in TBMA-F7MA random copolymers.

Recently, there is significant interest in using chemically amplified (CA) resists for electron beam (E-Beam) applications including mask making, direct write, and projection printing. CA resists provide superior lithographic performance in comparison to traditional non CA E-beam resists in particular high contrast, resolution, and sensitivity. Due to the electron scattering effect and the image collapse problem, thinner imaging layer is desirable. Sufficient etch selectivity is needed to compensate reduced resist thickness. Therefore, there is a need to have a high etch resistant resist system which can survive Cr etch (Cl₂/O₂RIE etchant) process in mask making. For device making, the thin film bilayer approach needs a resist that can withstand O₂ etch for image transfer to the underlayer. We have found Si-O containing polymer has the etch characteristics for both applications. In the first approach, using a blend of KRS-XE and silsesquioxane polymer, we have been able to resolve resist images down to 50nm with etch rate 20% slower than conventional novolak I- line resist systems. In the second approach, we have investigated the copolymer of vinyl phenol and acrylate siloxy silane systems. Superior litho performance and etch properties have been observed. In this presentation, we will discuss the chemistry, the miscibility in blends, etch characteristics and lithographic performance of these resist systems.

Over the years for enhanced resolution, electron beam exposure tools have migrated to higher accelerating potentials. High kV exposure requires faster resist to offset the loss of absorbed radiation. In addition, in order to maintain throughput in mask writing tools, photoresist sensitivities of the order of 5(mu) C/cm² at 50 kV are needed and likewise 5 (mu) C/cm² at 100 kV for projection exposure tools. To meet this sensitivity requirement there is a keen interest in using chemically amplified resists (CAR). The CAR resists offer additional benefits such as high contrast, improved resolution, larger process windows, and dry resistance. This study is focused on enhancing the sensitivity of a low activation ketal protected polyhydroxystyrene. One such resist, which has been reported previously, is insensitive to the post apply bake temperature and does not require a post exposure bake. To enhance the resolution, process latitudes, and chemical contrast, base quenchers are usually added to limit the diffusion of the acid after exposure. To improve the sensitivity of the formulation, the effect of the acid generator (AGX), type of base, base concentrations and ratios in combination with the polymer protection level has been studied. Yields of acid formed were measured by titration and NMR. It was also found that both the AGX and base has an inhibition effect on dissolution. By increasing the among of AGX and decreasing the polymer protection level, the sensitivity of the formulation was improved while maintaining the resist contrast, resolution, vacuum stability, and bake latitudes. In addition, the sensitivity of the resist can be altered by quantity and type of added base. We have also investigated the effect of Pk_a of the base on the sensitivity, and process exposure latitude. The ionic onium acid generators or non ionic hydroxyimides provide adequate acid when coupled with adjusted concentrations of base to achieve 5 (mu) C/cm² at 100 kV.

The goal of this work was to develop a resist material for high-keV (>=10 keV) electron beam patterning with: (1) high sensitivity (~10 (mu) C/cm² 50 keV), (2) high contrast, and (3) etch resistance comparable to current optical resists. In this work, an approach was desired that used multi-component resist design to (1) decouple sensitivity from etch performance and to improve resist contrast but (2) avoided environmental stability problems. Therefore, a resist design first proposed by researchers at IBM in the late 1980s that uses an electron beam sensitive polymeric dissolution inhibitor (poly (2-methyl-1-pentene co 2-ethoxyethyl-methallyl ether sulfone)) in conjunction with novolac resins was explored in this work. Significant advancements have been made in high performance novolac resins used for advanced I-line resist materials since the original work performed at IBM. Thus, one of the primary reasons for reinvestigating this resist design is to evaluate the performance of resists formulated using polysulfone dissolution inhibitors in conjunction with such advanced novolac resins. In this work, the basic lithographic performance of sulfone-novolac system (SNS) resists formulated using modern novolac resins was evaluated and optimized by varying the resist film preparation and development conditions.

Fluorinated dissolution inhibitors (DIs) for 157 nm lithography were designed and synthesized as part of an ongoing study on the structure/property relationships of photoresist additives. The problem of volatilization of small DI candidates was observed from matrices such as poly(methyl methacrylate) (PMMA) and poly(hexafluorohydroxy-isopropyl styrene) (PHFHIPS) during post-apply bake cycles using Fourier Transform Infrared Spectroscopy (FT-IR). To avoid this problem, low volatility fluorinated inhibitors were designed and synthesized. Three fluorinated DIs, perfluorosuberic acid bis-(2,2,2,-trifluoro-1-phenyl-1-trifluoromethyl-ethyl) ester (PFSE1), perfluorosuberic acid bis-[1-(4-trifluoromethyl-phenyl)-ethyl] ester (PFSE2) and a fluorinated phenylmethanediol diester (FPMD1), largely remained in a PHFHIPS film during the post-apply bake. The dissolution behavior of the two fluorinated diesters was studied and found to slow down the dissolution rate of PHFHIPS with inhibition factors of 1.9 and 1.6, respectively. The absorbance of PHFHIPS films containing 10 wt% of the diester inhibitors is 3.6 AU/micron compared with an absorbance of 3.3 AU/micron for the polymer itself. The absorbance of 10% FPMD1 in PHFHIPS was measured as 3.5 AU/micron compared with an absorbance of 3.4 AU/micron for the polymer itself. Thus, the non-volatility and transparency of the fluorinated inhibitors at 157 nm as well as their ability to reduce the development rate of fluorinated polymers make them suitable for use in a 157 nm resist system.

The reactive ion etch (RIE) properties of fluorine funtionalized polymers in which fluorine atoms were incorporated in the main chain were examined. There was a tendency that the etching rates of these polymers were higher as lower the fluorine contents. The existing four models such as the Ohnishi model, the Kunz model, the Ohfuji model and the Kishimura model were applied to explain the correlation between the etching rates and the polymer compositions or structures, but the errors were too large to explain the relationship. A new model has developed to explain the effect of the fluorine incorporation to the dry etch resistance. The model assumed that there would be a correlation between the number of main chain fluorine atoms and the dry etch resistance, and the main chain fluorine incorporation would increase the dry etch resistance. The model could explain the dry etch resistance of the main chain fluorine incorporated polymers with adequate accuracy.

Various polymer structures for 157-nm lithography have been vigorously developed for post 193-nm lithography. Polymers with a fluorinated main chain can improve high transparency at the 157-nm wavelength; conversely, norbornene copolymers with fluoride pendant groups do not achieve so high transparency at the 157-nm wavelength. We have developed a novel 157-nm chemically amplified resist. It is composed of a fluorinated monocyclic main-chain polymer, which shows high transparency and can be used for single layer resists. This new resist is referred to as a Small Absorbance Fluorine contAining ResIst (SAFARI). The SAFARI resist exhibited high-resolution patterns of 65-nm line and space patterns using a 0.85 NA microstepper, good sensitivity of 4 mJ/cm², and high transparency when applied to 250-nm film thickness.

We have designed and synthesized a number of unique polymer systems composed of acrylate and styrene even though it had moderate transparency. Our first model of 157nm photoresist was based on a (alpha) trifluoromethylacrylate and styrene bearing a pendent hexafluoroisopropanol with pentafluoroisopropyl t-butyl carbonate (PFITBC) as the transparent enhancer and acid labile compound. PFITBC was obtained from perfluorinated enolate with di-t-butyl carbonate with high yield. All of the absorbance of our system ranged over 3.0~3.4micrometers ^-1 for base resin, which corresponded to a resist thickness of 110~125 nm at the optical density of 0.4. We have formulated several resists based on these polymers and these formulations have shown high resolution and contrast at 248 nm. We were able to obtain 240nm 1:1 image when exposed at 248 nm by a Nikon stepper with 0.45NA. To overcome the weak etch resistance with thin thickness film, we investigated the vapor phase silylation treatment (SILYAL) in which the treated pattern was more persistent against the O₂ plasma and turned to smoother surface. DMSMDA with Bi(DMA)MS of 30-40 wt% showed not only good control resist flow but also the improvement of line-edge roughness. Our results suggested that a facile approach to fluorine incorporated resin with SILYAL process can accelerate the 157nm lithography.

In future lithography the 157 nm wavelength is expected to succeed the 193 nm wavelength in 2004. So an early CARL resist for sub 100 nm resolution was developed at Infineon Technologies within the German BMBF project Laserbasierte Ultraprazisionstechnik - 157 nm Lithographie. Common 248 and 193 nm resist materials have a high absorbance (7- 12 μm^-1) A main challenge at this short exposure wavelength is the development of a transparent base polymer or the imaging has to be done alternatively with ultra thin films. In contrast to a high transparency of the polymer a high quantum yield for the photo chemicals is required. CARL is a bilayer resist system developed by Siemens/Infineon Technologies. A modified CARL version is presented here for exposures at 157 nm, consisting of a silicon free top resist (Si free CARL) as thin imaging layer. A separate silylation step of the structured top resist after exposure and wet development provides a high etch resistance in the dry development step and allows imaging of ultra-thin films with a film thickness of ca. 50 nm. An oxygen plasma is taken to transfer these top resist structures into the up to 300 nm thick underlying Novolac type bottom resist. In dry development. The bottom resist itself provides high etch resistance also for aggressive substrate etch processes.

The data presented is from resist materials exposed on the 157-nm Microstepper at SEMATECH. Photoresist suppliers provided the first samples of materials in early 2001. During the remainder of 2001, several improved formulations were supplied and tested. Over the last year, the lithographic performance of thick chemistries has improved, but exposure latitude is still inferior compared to Ultra Thin Resist (UTR) samples. Recently, more transparent samples have appeared demonstrating performance that is close to that of UTRs in terms of processing latitudes. Although sub 100-nm imaging has been shown, limited 70-nm performance with the alternating PSM indicates that significant progress still has to be made before the resists are capable of the targeted 70-nm node with a high NA tool.