Lithographic technology for 64Mb DRAMs is one of the largest concerns of the semiconductor industry. The minimum feature size of 64M DRAMs will be at around 0.3-0.4μm. This size is almost the same as the wavelength of the light on conventional steppers. To delineate such fine patterns, the use of excimar laser light and phase shifting technology are intensively evaluated for use in optical lithography and other lithography technologies such as fast electron beam lithography and X-ray lithography are also intensively evaluated. These recent lithography developments are reviewed in this paper.

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Continued advances in microelectronic fabrication are trying the limits of conventional lithographic technologies [1]. The traditional diazonaphthoquinone/novolac resist materials are not suitable for use with the deep-UV exposure tools both from the absorption and sensitivity considerations [2]. New resist materials and processes are necessary to fully utilize the potential of deep-UV microlithography, a more promising technology for the production of sub-0.5μm geometries. The advent of resist materials based on chemical amplification principle [3, 4], where one photochemical event leads to a cascade of subsequent events causing a solubility differential, exhibit high resolution, excellent sensitivity and good etch selectivity required for deep-UV lithography.
While process sequence for chemically amplified materials is similar to conventional positive resist process sequence, post-exposure bake (PEB) assumes the role of effecting a solubility differential by means of thermally activating the acid catalyzed crosslinking or deblocking reaction. The PEB parameters, the delay time between exposure and post exposure bake (post exposure delay time, PED) and the inherent chemistry of the resist system greatly effect the process performance.
This paper summarizes the process characteristics of select positive and negative deep-UV resists, and attempts to correlate the results to the chemical design of the individual resists.

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Chemical amplification resists based on the reverse polarity change from a polar to a nonpolar state have been designed. The imaging mechanism relies on the pinacol-pinacolone rearrangement, wherein vic-diols (pinacols) are converted to ketones or aldehydes with photochemically generated acid as a catalyst. A polymeric pinacol has been synthesized and has demonstrated very efficient and clean rearrangement to provide negative images upon development with methanol. Small pinacol compounds have been also utilized in three-component, aqueous base developable, negative resist systems, where the rearrangement products inhibit the dissolution of the phenolic matrix resin in the exposed regions.

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Methodology was developed for the synthesis of novel 2- substituted o-nitrobenzyl chromophores which were derivatized as the tosylate esters. The thermal stabilities of these new tosylates fell within the range predicted from an empirical model. Lithographic sensitivities of deep UV resists made from these new esters and poly(4-(tert-butoxycarbonyloxy)styrene-sulfone) ranged from 45mJ/cm² to 90 mJ/cm².

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New KrF excimer laser resist was developed. The chemically amplified positive resist, named ASKA, attained high sensitivity (30mJ/cm²), high resolution (0.3micron) and high stability. This resist was successfully applied to the fabrication of 64M DRAM with KrF excimer laser stepper (N.A 0.42). This paper describes the chemistry and lithographic characteristics of ASKA, followed by the demonstration of actual pattern fabrication of 64M DRAM.

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Four prototype projection lenses have been designed to study the feasibility of ArF excimer laser lithography. In view of the fact that there are currently technological limits to the further bandwidth narrowing of ArF excimer laser and to achieving more accuracy in optical surface fabrication, a partially achromatized lens is found to offer the best solution. 0.18-μm resolution is confirmed using the fabricated prototype lens combined with the 10-pm bandwidth laser and Ag/Se-Ge inorganic resist. A quarter-micron line-and-space pattern is obtained using a tri-layer resist system.

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This paper describes further developments of Acid Hardened Resist (AHR) systems, with particular focus on the DUV and i-line AHR resist performance. Results regarding the time delay between exposure and post-exposure bake are given for Megaposit®SNR™248 photoresist over a 23 hour time delay, with no effect observed on the 0.5μm linewidths. Lithographic performance for the i-line and DUV resists is also presented. Using i-line phase-shift mask technology 0.35μm lines and spaces are printed with the i-line AHR resist. Finally, results are presented comparing the relative solubility of crosslinked resist films of SNR248 in tetrahydrofuran vs. various normality levels of tetramethylammonium hydroxide (TMAH). These results lead us to the conclusion that phenolic hydroxyl site blocking plays a role in dissolution inhibition in TMAH developers.

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Chemically amplified photoresist systems consist of a photoacid generator (PAG), which interacts with the actinic radiation, and at least one acid sensitive compound, which changes the material's solubility properties under acidic conditions. The PAG is not only the coupler between the applied exposure tools and the photoresist systems, but also determines the nature of the acid produced, which in turn controls the process and performance latitudes of the resist materials. The selection of the optimum PAG is a critical issue in the preparation of production-worthy materials. This paper reviews several new classes of photoacid generating compounds with respect to some of their chemical and physical properties and their lithographic behaviour. The compounds investigated include new 4, 6-bis(trichloromethyl)-1, 3, 5-triazenes, α, α-bis-(sulfonyl)diazomethanes, α-carbonyl-α-sulfonyldiazomethanes, and certain sulfonic esters. Their impact on the lithographic performance of chemically amplified positive and negative tone resists for deep- and near-UV as well as e-beam applications is discussed.

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Negative resistsystem, consisting of organic polyhalides and poly(hydroxystyrene), has been examined for KrF excimer laser lithography. Several trichloromethylated 1, 3, 5-triazines and trichloromethylated benzenes are suitable to the photoactive compounds for this system. The resist using trichloromethylated benzene has a sensitivity of 66mJ/cm² to KrF excimer laser and a resolution of 0.38μm.
The insolubilization mechanism of this system has been clarified. Hydroxyl groups of poly(hydroxysyrene) are protected with the acyl groups derived from trichloromethyl groups of photoactive polyhalides under deep UV irradiation. The polarity change caused by the conversion of poly(hydroxystyrene) to the esterified derivative. Insolubilization in an alkaline solution of the exposed area of the resist film is based on such phenol protection reaction.

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Diphenyltetrazole (DPT) is decomposed by UV light and the decomposed product is highly reactive with phenol group. Therefore, combination of DPT and polyvinylphenol (PVP) gives negative type UV resist for aqueous alkali development. The PVP-DPT resist is able to apply to KrF excimer laser lithography.
The characteristic curve for KrF excimer shows 86mJ/cm² for sensitivity and 2.9 for gamma. SEM picture given by direct con tact printing indicated resolution less than 0.5μ. Spectral analysis on the DPT resist was carried out and the hardening mechanism discussed.

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t-Butyloxycarbonyl (t-BOC) blocked compounds based on the protection of phenolic groups, e.g. poly-4-hydroxystyrene derivatives, Bisphenol A type dissolution inhibitors, or onium salt photoacid generators, have found widespread research interest for photoresist systems with excellent photosensitivity and high resolution power. We have made an extension of this approach using new phenol type polymers. This contribution presents first details on the chemistry of these systems and results of their lithographic evaluation as positive tone photoresists for UV4, UV3 and especially, UV2 applications.

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Four kinds of new t-BOC protected maleimide copolymers P(t-BOCMI/St), P(t-BOCMI/SiSt), P(t-BOCMI/t-BOCSt) and P(t-BOCOPMI/SiSt) were investigated as thermally stable, deep UV resists in high sensitivity. The resist polymers are composed of N-(t-butyloxycarbonyl)maleimide (t-BOCMI) or N-[p-(t-butyloxycarbonyloxy)phenyl]maleimide (t-BOCOPMI) and styrene derivatives (X-St) such as styrene (St), p-(t-butoxycarbonyloxy)styrene (t-BOCSt) and p-trimethylsilylstyrene (SiSt). The t-BOC protected copolymers have alternating structures and are readily deprotected by thermolysis and acidolysis to the corresponding polar maleimide (MI) and phenolic OH functions from t-BOCMI and t-BOC-oxyphenyl, respectively. The deprotected polymers P(MI/St), P(MI/SiSt), P(MI/HOSt) and P(HOPMI/SiSt) exhibit quite high T_g's above 245°C and high oxygen RIE resistance. In particular, the both silicon containing polymers P(MI/SiSt) and P(HOPMI/SiSt) are superior to a novolac photoresist in oxygen RIE resistance. The RIE resistance of the resist polymers in Cl₂/He and CHF₃/C₂F₆ gases are found to be comparable to the novolac resist. The three t-BOCMI copolymers have very low absorbance no more than 0.2 per μm at 248nm wavelength. The polymers P(t-BOCMI/St), P(t-BOCMI/SiSt), P(t-BOCMI/t-BOCSt) and P(t-BOCOPMI/SiSt) were compounded with 10wt% of triphenylsulfonium hexafluoroantimonate as a photoacid generator and the resists are named as MIST, MISIX, BMIST and PMISIX, respectively. The resists films were exposed to deep UV or KrF excimer laser and post-exposure baked at 100°C followed by development with aqueous alkaline solutions for positive image-making to get sub-micron patterns.In the case of MISIX and PMISIX resists only negative tone images were possible by organic developing. Thus the t-BOC protected maleimide copolymers combine capability of high resolution with high sensitivity in deep UV region and thermal stability above 200°C along with high oxygen RIE resistance.

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Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, 3-and 4-phenylthiobiphenyl isomers by an in-cage fragmentation-recombination mechanism, and the cage-escape sulfide product diphenylsulfide. Both the in-cage and cage-escape reactions result in acid production. Formation of the in-cage products is favored in viscous solutions, polymer films and in the solid state photolysis of triphenylsulfonium salts. However sensitization by photo-induced electron transfer, or triplet energy transfer, gives only the cage-escape sulfide product via the triphenylsulfur radical, or triplet phenyl radical - diphenylsulfinyl radical cation pair, respectively. In-cage and cage-escape products are also found from photolysis of diphenylhalonium salts. The importance of‘cage versus escape’ reactivity of onium salts in acid production and in the mechanistic interpretation of their photochemistry, in solution and polymer films, is discussed.

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A new simple method was developed to estimate the diffusion range of photogenerated acid in chemical amplification resist systems. The acid mobility was investigated for various process conditions. It was found that prebake and post-exposure-bake conditions have strong influence on the mobility of acid. The diffusion range of acid is much larger than values estimated from the catalytic volume. No acid diffusion could be detected at room temperature over a period of 2 days. Acid from an onium salt acid generator showed a stronger diffusion than acid from a methanesulfonic acid ester.

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KrF excimer laser lithography is a major candidate for sub-half micron VLSI technology, although it has many problems. The main topics of the present paper is not the excimer laser lithography, but a lithographic application of excimer laser photoablation, and surface modifications of polymer films, including silylation and metallization induced by excimer laser irradiation. Because the main application of excimer laser ablation is contact hole fabrication, photoablation of Teflon or Teflon like materials is important. We carried out photoablation of Teflon AF (a copolymer of tetrafluoroethylene and 2, 2-bis(trifluoromethyl)-4, 5-difluoro-1, 3-dioxolene) using a dopant at 193 and 248nm. High temperature resistant polymers like polyimides and polyphenylquinoxaline show imagewise wetting after excimer laser exposure in air, giving rise to direct surface silylation of these materials. Although we have not carried out image transfer experiments yet, XPS data clearly showed silicon incorporation. Imagewise metallization of these polymer surfaces after ArF excimer laser exposure in air showed some interfacial compound formations between exposed polymer surfaces and metal deposited. The XPS data of gold on PPQ exposed in air will be discussed.
Effects of excimer laser exposures on polymer surfaces are repoorted, which include photoablation of Teflon AF by ArF and KrF excimer laser pulses, and surface modifications of some unsolved questions of the nature of interactions between deposited metals and laser exposed polymer surfaces. The study on these problems are in progress.

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We present a novel pattern profile improvement method for chemically amplified negative photoresist. Based on the mechanism of chemical amplification systems, direct amending of the "acid latent image" prior to post exposure baking can bring drastic pattern profile improvement by using acid trap reagent soaking on the resist surface. We concluded that the acid trapping method combined with highly absorbant chemically amplified resist is one of the promising ways in KrF excimer lithography.

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Bi-level resist systems are very promising with deep-UV lithography for obtaining high-aspect patterns on high-step substrates. Three-dimensional polysilphnylenesiloxane (TSPS), a negative deep-UV bi-level resist we developed, has high resolution and high sensitivity. The TSPS molecule is structured as a rigid three-dimensional mesh consisting of a silphenylenesiloxane core surrounded by functional groups. The rigid silphenylene core provides low swelling, high thermal stability, high O₂-RIE resistance, and high deep-UV absorption for the initiation of crosslinkage. We found that adding the photoinitiator 2, 2-dimethoxy-2-phenylacetophenon (DMPA) to TSPS raises sensitivity to about 20 times that of TSPS alone. TSPS sensitivity to deep-UV light is 20mJ/cm² at Dg^0.5. A 0.25μm pattern can be delineated with TSPS/MP-1300 bi-level resist systems using a KrF-excimer laser stepper.

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This paper reports work on two chemically amplified resist systems that can be developed in an oxygen plasma environment. These systems are based on the catalytic photogeneration of functional groups within the resist film which react, in a subsequent step, with a silylating agent that is delivered in the gas phase. As a result, the organosilicon species is selectively and covalently incorporated into the exposed regions of the resist film. When the silylated film is exposed to an oxygen plasma, the regions that do not contain the organosilicon species are etched to substrate, while those areas containing silicon are not etched. This generates a negative tone relief image. The chemical mechanisms and lithographic properties of these systems will he discussed.

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The surface imaging processes using germylating agents are investigated. The removability of the resist is expected to be improved by using germylation instead of silylation. The reactivity of several germylating agents with phenolic OH group is evaluated. And germylamine compounds are found to exhibit the highest reactivity among them. Three different types of surface imaging processes are demonstrated and submicron patterns are obtained with each process. Bis(triethylgermyl)amine (BGA) as the germylating agent and PLASMASK 200G-C are used for these processes. The reactivity of BGA is higher than that of HMDS.
In the deep UV exposure process, the differences between the germylation and the silylation are investigated on the oxygen dry etching characteristics and the diffusion depth of the reaction. The process using the germylation by BGA is found to have higher oxygen dry etching resistance and lower swelling which corresponds to the diffusion depth in germylated area as compared with the silylation by HMDS. From these results the surface imaging process is expected to have high controllability for linewidth by using the germylation.

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A positive deep-UV resist consisting of silylated polyhydroxystyrene and photoacid generator was investigated. Optimization of the polymer structure has been achieved from the points of silylating substituent and silylation ratio. The resist is capable of imaging 0.3μm line and space pairs in a 1.05μm thick film by using a KrF excimer laser stepper (NA=0.42) with practical sensitivity of 20mJ/cm². It has been also shown that the resist is workable as a top layer in a bi-layer process.

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Novel sensitizers, pyridine N-oxides, were investigated in negative deep-UV resists. Pyridine N-oxide, three isomers of methylpyridine N-oxides and aliphatic N-oxide compound were tested for their bleaching characteristics and resist characteristics in resist films, which contained these compounds as sensitizers and polyvinylphenol as a matrix polymer. 2-methyl isomer was the most sensitive in these compounds, while aliphatic N-oxide compound had very low sensitivity. Relationships between sensitizer concentration and bleaching characteristic were measured and results agreed with the Dill's model. Photoreaction of pyridine N-oxide was investigated in resist film. Pyridine was found and molecular weight of polyvinylphenol was not changed after UV irradiation. 0.4μm line and space patterns with 1.0μm resist film thickness were resolved, using a KrF excimer stepper(NA=0.42).

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A DNQ/novolac-based positive resist design for KrF excimer laser lithography is described. Both the DNQ-PAC and novolac have strong absorptions at 248nm, which result in a significant tapering in the resist profile and prevente the use of this chemistry to deep UV application. A resist design focused on the absorption reduction was made with the following approaches. (1) DNQ-PAC molecular design effective to the unbleachable absorption reduction as well as the PAC loading level reduction. (2) Developer optimization both for dissolution discrimination enhance, which is also relevant to the PAC loading level reduction, and for gaining a surface induction effect for profile improvement. The resulting resist exhibited a good profile, high resolution down to 0.30μm and fast photospeed, 65mJ/cm² at 0.5μm resist thickness, and thereby suggested a feasibility to be usable in practice unless a thicker film is not used.

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