The chemical amplification concept proposed in 1982 to support high resolution short wavelength lithographic technologies that demand highly sensitive advanced resist systems is based on acid-catalyzed crosslinking, deprotection, and depolymerization reactions. Each approach has shown a tremendous progress. In this paper is reviewed the depolymerization concept for the design of sensitive positive resist systems formulated with triarylsulfonium salt acid generators. Three modes of the initiation of acid-catalyzed depolymerization are discussed; (1) attack of acids onto backbone oxygens as exemplified by depolymerization of polyphthalaldehyde (PPA), (2) scission of pendant ester groups followed by depolymerization with poly(α-acetoxystyrene) (PACOST) as an example, and (3) unzipping from the polymer ends as demonstrated by depolymerization of cationically obtained poly(p-hydroxy-α-methylstyrene) (pPHOMS). The positive resist systems based on depolymerization of PPA derivatives include a thermally- evelopable O₂ reactive ion etch (RIE) barrier resist for use in the bilayer scheme and the use of PPA as a polymeric dissolution inhibitor for novolac resins. The deesterification of PACOST results in formation of poly(phenylacetylene) (PPA) and simultaneous depolymerization to provide positive images upon development with xylenes, which is briefly discussed. Cationically prepared pPHOMS exhibits 80% thickness loss at ca. 1mJ/cm² upon postbake at 130°C whereas anionic pPHOMS and cationic meta-PHOST are very stable toward acidolysis, indicating that the depolymerization propagates from the termination side of the chain.

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The design of chemically amplified imaging systems based on acid catalyzed thermolytic cleavage of polymer main-chains has been exploited with a variety of structures containing tertiary, allylic, or benzylic ester groups. In all cases, the esters have been designed for their ability to undergo facile thermolytic elimination. This report outlines the preparation, study, and imaging of several polyesters containing thermolytically active moieties. These new acid labile polyesters have been incorporated into two-component imaging materials for which radiation sensitivity is due to the presence of compounds which produce acid upon irradiation. The photogenerated acid is then used to catalyze thermolytic cleavage of the polyester main-chain in a process which does not consume the acid and therefore provides for chemical amplification. Imaging is possible using a variety of radiation sources, from deep-UV to X-rays. The concept of imaging through multiple main-chain cleavage is important as it can lead to self- or dry developing images. In the present study, only partial self-development was observed as one of the diacid liberated by the chain cleavage process was not volatile. A second family of radiation sensitive imaging materials has also been designed based on the catalyzed electrophilic crosslinking of polymers containing aromatic rings. While this new approach is only illustrated here for systems in which crosslinking involves polymer pendant groups, the same design is also applicable to polymer main-chains. This family of crosslinkable, chemically amplified, resist materials shows very high sensitivities of ca. 1mJ/cm² for exposure in the deep-UV, ca. 1μC/cm² for E-beam exposure, and ca. 12-15mJ/cm² for X-ray synchrotron radiation.

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A new mechanism which is simple and common for all opium salts were proposed on the photochemical proton generation process in resist systems, from the theoretical aspect based on quantum chemical calculations. The primary product generated from the photochemical reaction is an aryl cation Ar⁺, i.e., for aryldiazonium salts [Figs. 2 and 3]:
(ArN₂)⁺X^-→<hv>Ar⁺+N₂+X^-, for diaryliodonium salts: (Ar₂I)⁺X^-→<hv>Ar⁺+ArI+X^-, for triarylsulfonium salts [Table 1]: (Ar₃S)⁺X^-→<hv>Ar⁺+Ar₂S+X^- where X^- is SbF₆^-, AsF₆^-, PF₆^-, BF₆^-, CF₃SO₃^-, etc. The secondary proton generation reactions are [Figs. 4, 5 and 6],
Ar⁺+H₂O[ROH]→(ArOH₂) ⁺[(ArOHR)⁺] (ArOH₂)⁺[(ArOHR)⁺]→ArOH[ArOR]+H⁺X^- where H₂O and ROH are a water molecule at the air interface and such protic species as novolak, poly(vinylphenol), poly(vinylalcohol), etc., respectively. Since the chemical amplification in resist systems originates from the photochemically generated acid H⁺X^-, we can conclude from this research that the sensitivity of an onium salt as a photo-initiator is determined by the species of its anionic part X^-. This conclusion is consistent with experiments where the sensitivity decreases in the order SbF₆^->PF₆^->BF₆^-.

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A variety of new 2, 6-dinitrobenzyl sulfonate esters were synthesized and evaluated in terms of thermal stability, quantum yield, and absorbance. The lithographic performance of these novel esters was evaluated in poly(4-(t-butyloxycarbonyloxy)styrene sulfone) copolymer. It was found that the lithographic sensitivity of these resist formulation was substantially better than those based on 2, 6- dinitrobenzyl tosylate and approaches that achieved with conventional onium salt based resists. In addition, they have the advantage of not containing metallic or ionic components.

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The novel photo-acid-generator tri(methanesulfonyloxy)benzene was investigated in a positive deep-UV resist, containing bisphenol A protected with t-butoxycarbonyl groups as inhibitor compound in a novolak matrix polymer. The deprotection reaction by the generated methanesulfonic acid was studied using UV- spectroscopy and IR-spectroscopy. The amount of generated acid was determined by comparison with model films containing known amounts of methanesulfonic acid. The deprotection degree, the catalytic chain length of the deprotection reaction, and the quantum yield of the acid generation were determined. The amount of photogenerated acid was unexpectedly high, and the quantum yield was much higher than the theoretical maximum. No photosensitivity was observed in control experiments with model films in which the acid generator was mixed in cellulose acetate as a non-absorbing matrix polymer, whereas the addition of a phenol-type model compound to this formulation resulted in formation of acid after exposure to deep-UV light. This shows that the acid generator is sensitized by a phenolic resin. The sensitivity of the resist as a function of exposure wavelength was in good correlation with the absorption spectrum of the resist.

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The current state of the art g-line and i-line photolithography using conventional novolac based resists will be able to pattern 0.5-0.6micron devices adequately, however beyond 0.5micron, an alternative lithography will be required. KrF excimer laser lithography is one of the leading candidates for sub-half micron VLSI lithography. This technology will be able to take advantage of the higher resolution and wider depth of focus possible with the smaller wavelength. One of the major technical barriers for application of this technology to mass production is the low sensitivity of commercially available resists.
New non-novolac based resist have been introduced with high sensitivity to meet this requirement. By using chemical means, Ito [1] was able to achieve quantum efficiencies much greater than one resulting in very sensitive resist systems. In this paper we describe our work to develop ASKA, Alkaline Soluble Kinetics using Acid Generator, positive resist. This resist has sensitivity greater than 25mJ/cm² which is sufficient for a controllable manufacturing process. ASKA resist also has very good lithographic characteristics with resolution to 0.40micron and near vertical resist profiles.
Process characterization of ASKA resist is also reported. Post exposure bake (PEB) is a critical step that controls performance of this resist. ASKA resist has excellent stability and linewidth control and is not dependent on the time between exposure and PEB. The resist has wide exposure latitude up to +-15% for 10% variation in linewidth. ASKA has mask linearity down to 0.40micron lines where at 0.35micron the linewidth begins to deviate from linearity. Depth of focus for 0.40micron lines is about +-1.0micron. Etch resistance is evaluated and reported for silicon dioxide, nitride and polysilicon layers. ASKA was successfully applied to actual device wafers. KrF excimer laser lithography using ASKA resist can fabricate sub-half micron patterns for 64M DRAM.

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This paper reviews the initial lithographic performance of the Megaposit^® SNR™ 248-1.0 Photo Resist during the imaging of 0.35, 0.40, 0.45, 0.50 and 0.55μm line/space pairs on a 0.42NA/0.5 coherence/ KrF stepper. The test results show that process window overlap can be maintained. to a normalized geometry size of 0.59 k₁ for line/space pairs. This degree of overlap is shown to be comparable to a normalized value of 0.76 k₁ during the simultaneous imaging of line/space pairs, isolated lines and isolated spaces. This result appears to be consistent with the g-line results published previously¹. It is also shown that at the exposure dose required to size the target geometry, the resist induced bias, due to lateral development is negligible and provides the possibility to extend the working resolution to smaller geometries if the intensity minimum of the aerial image can be suppressed. Finally, an additional screening experiment in which soft bake temperat re, post exposure bake temperature, resist thickness and develop time were varied, This latter study suggests that detailed optimization studies should examine lower PEB temperatures, 140C and less, and shorter develop times, 90s and less, at both E_MIN and E_MAX thicknesses.

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A series of pyrogallol-terpenic series (PTR) have been synthesized by the reaction of pyrogallol with terpenic series compounds, such as rosin, α-pinene and limonene. The conditions of the synthesis reactions and the characteristics of products have been investigated.
In order to improve the properties of PTR, the reactions of PTR with formalin have been carried out in acidic condition resulting in a series of pyrogallol-terpene-formalin resis (PTFR-A) which show high softening points. PTR and PTFR-A can be applied to:
1) Practical binder resins of general positive resists.
2) Raw resins to be esterified by 2, 1, 5-naphthoquinone diazide sulfonate.
3) Acidolysis type positive chemical amplification resists after cyclohexyl chloroformation have been introduced into them.
In alkalinous condition the reaction between PTR and formalin produces a series of resins (PTFR-B) having hydroxymethyls, for example pyrogallol-limonene-formalin resin. They can be used as negative chemical amplification resists, because they are cured after first irradiation to UV light under existence of photoinduced acid precursor and then heated.

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Diaryliodonium and triarylsulfonium salts with non-nucleophilic metal halide anions are well known photoinitiators for cationic polymerization as well as photosensitive super acid generators, and play an important role in the chemical amplificated resists. Generally, photosensitizer is required to enhance the spectral response of resists or polymerization systems into the longer wavelength region, due to the low absorptivity of the simple diphenyliodonium salts at above 300nm. In order to circumvent the need for a second component, auxochromic or other groups were incorporated into the iodonium salt molecule which can increase or shift the absorptivity to longer wavelengths. Therefore, a number of new unsymmetrical iodonium salts of general structure(I) were prepared by us by modifying a method already reported.
The photoactivity of these iodonium salts were investigated by gel time study. The results indicated that the iodonium salts incorporated with auxochromic groups exhibited higher photoactivity than that of diphenyliodonium salt. The 2-phenylsulfur methhyl 2′, 4′dimethyl diphenyliodonium salt showed the highest activity and an intramolecular electron transfer mechanism was proposed to explain the unexpectedly high activity.

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α-Acetoxystyrene (ACOST) undergoes radical polymerization in contrast to α-methylstyrene. However, the polymerization is a slow equilibrium process with a ceiling temperature (T_c) of 47°C at 1mol/L. Poly(α-acetoxystyrene) (PACOST) possesses a tertiary benzylic carbon adjacent to an ester oxygen in its backbone and is converted upon heating to ca. 220°C to poly(phenylacetylene) and acetic acid. The temperature of deesterification involving the polymer main chain can be lowered by generating a strong acid in the film. However, the acid-catalyzed deesterification of PACOST is accompanied by simultaneous depolymerization. Due to the solubility alteration through the structural modification and the molecular weight reduction by depolymerization, PACOST containing triphenylsulfonium hexafluoroantimonate functions as a sensitive positive resist incorporating "chemical amplification." PACOST is very transparent in the deep UV region and the resist containing 4.8wt% of P ₃SSbF₆ has an optical density (OD) of 0.3/μm at 248nm. The resist has a sensitivity of ca. 5mJ/cm² and a contrast (γ) of 4.3 when postbaked at 130°C for 5min and developed with xylenes. Owing to its high glass transition temperature (T_g>200°C), the positive images do not show any thermal flow at 180°C and owing to the aromatic nature of the matrix polymer, the resist is as stable as novolac-based resist systems in CF₄ plasma. Unfortunately, however, the resist film tends to suffer from solvent-induced cracking during development due to the stiff polymer chain.

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Aliphatic esters, like malonates, β-ketoesters and acetone dicarboxylic acid esters as well as meldrums acid derivatives, which decompose upon acid catalysis to volatile compounds were evaluated as dissolution inhibitors and modulators for chemical amplification resist systems. The retention of these compounds in resist films under prebake conditions, their interaction with the resin as well as the acid catalyzed decomposition mechanism was investigated. Our studied have shown that these compounds function well as dissolution inhibitors in novolac matrices, but their ability to inhibit the dissolution of poly-vinylphenol is not very effective and therefore the process window is very narrow.
However, select esters can be used as dissolution modulators in combination with special resins, for instance poly-vinylphenol protected partially with tetrahydropyranyl groups, where the resin performs the function of inhibition. The dissolution modulators just act in order to increases the discrimination between ex osed and unexposed parts, since they remains nearly uneffected in unexposed areas, but contribute to enhanced alkaline solubility of exposed areas, due to the formation of carboxylic acids upon photoinduced acid catalyzed decomposition.
Such chemical amplified deep UV resist systems exhibit high sensitivity (20-40mJ/cm2) and resolution (0.5μm L/S).

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Imageable thin films that contain phenolic polymers, acid activated crosslinking agents, and Bronsted acid generators have been developed into a variety of high resolution, high sensitivity resists suitable for use in semiconductor fabrication. Formulations have been optimized for use as DUV photoresists, electron beam resists and X-ray resists. Resists for g- and i-line exposure require the addition of a photosensitizer. Negative imaging versions of these resists contain radiation activated acid generators. Positive imaging versions contain radiation activated base generators and thermally activated acid generators. In either case the imaged resist is crosslinked and stable to temperatures in excess of 200°C.

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A major photoproduct from the irradiation of triphenylsulfonium salts is a Brönsted acid. This photochemical process has been used in several polymer film applications where the acid is used for crosslinking of films or to initiate some other acid catalyzed reaction. Despite the widespread applications of these materials, little has been published about the efficiency of acid generation in polymeric films or catalytic chain length associated with chemically amplified systems. This talk will describe the use of a merocyanine dye technique to analyze for acid production in polymer films. The amount of acid produced on irradiation (UV or E-beam) has been determined by this method and the catalytic chain length has been measured for one chemically amplified resist system.

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A Chemical Amplification system using silylether and silylester polymers as an acid labile compound was investigated. The effects of their structural parameters were examined in terms of their acid catalyzed decomposition rates. A bulkiness of the substituents on Si atom was the dominant parameter for these silicone polymers. The results supported the proposed mechanism of the acid decomposition that S_N2 type nucleophilic attack of water to Si atom is involved as the rate determining step. Disulphone compounds were examined for their usefulness as a photoacid generator to be combined with these silicone polymers. The disulphones desirably exhibited a rapid absorption bleaching with acid generation upon deep UV irradiation. A three components chemical amplification composition, utilizing the disulphones, the silicone polymers as a dissolution inhibitor and a novolac resin, was tested lithographically, and was proved to work as a deep UV positive photoresist.

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DUV ANR photoresists are evaluated as a function of the molar ratio of aromatic hydroxyl sites on the phenolic resin to the melamine methoxy sites in the crosslinking agent. This ratio, χ, which is proportional to [ArOH]: [OCH₃], is vital to appropriate resist function. The melamine content strongly affects the absorbance properties, dissolution rate, crosslinking efficiency, and the imaging properties of DUV ANR photoresists. The χ ratio is studied at low (110°C) and high (140°C) post-exposure bake temperatures. At low χ, the resists can function as dyed resists for reflective substrates. There is an upper limit on χ which defines the transparency limit for these resists.

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Photochemical reactions of the thymine containing polymers were studied and were applied as Deep-UV photoresists materials, which gave high resolution pattern of 0.3mμ.¹ Pyrimidine polymers can also be applied as chemical amplification resists for microlithography. One of the pyrimidine polymers is a polycarbonate containing both thymine photodimer and tertialy alcohol units in the polymer main chain. Another polymer is a polyether containing acid sensitive alkoxypyrimidine units in the main chain. This paper deals with preparation of the acid sensitive polymer containing pyrimidine units, and with photosensitive acid catalytic reactions.

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