Semiconductor device fabrication costs are dramatically escalating each year. Almost lithographers suffer from a paradox; "Which lithography will best meet the technical and economic needs of VLSI manufacturing in the drive toward below 0.25 micron design rules?". This paper gives an outline of the possibility and strategy of optical lithography with productivity and cost efficiency for the development and production of 0.25micron design rule devices and beyond on the basis of our recent development results.
To control costs, lithography will continue to push the extension of optical lithography as far as it can go. In the lithography for 256MbDRAMs production, a capability of 0.20micron lithography resolution is required. We have developed a new chemically amplified positive resist with high stability and an excimer laser based stepper with high alignment accuracy and off-axis illumination for KrF excimer laser lithography as a state-of-the art on shorter wavelength. The solution of 256MbDRAMs lithography is a complementary strategy employing both i-line for non-critical layers and KrF excimer laser for critical layers in order to achieve the lowest manufacturing cost.
Sub-quarter micron feature size is required to lithography for the development of 1Gbit DRAMS. In order to break-through below 0.20micron barrier of optical lithography, ArF excimer laser lithography, which is a super state-of-the-art, has been challenged and developed. The challenges are constructed of the projection system installing a refractive achromatic aspherical 4X lens and a suitable resist material with high transparency. It is expected not only as the next significant technology but also as the final optical lithography to produce next generation VLSI. The strategy of optical lithography will be a endless development for semiconductor industries.

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Recent developments in a wide range of technologies, involving compact electron storage rings, exposure systems, masks, and resist materials, have moved synchrotron radiation x-ray lithography much closer to a viable method for manufacturing sub-quartermicron large-scale integrated circuits. This paper demonstrates NTT's recent progress in synchrotron radiation lithography and presents the successful results of sub-quartermicron Bi-CMOS device fabrication. An overview of the lithography system and resist process is presented with emphasis on patterning and overlay performance.

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A review of recent efforts to develop photoresist materials and processes for 193-nm (ArF excimer laser) photolithography is reported. Three categories of resist processes are discussed: (1) single-layer resists based on acrylate polymers, (2) silicon-polymer bilayer resists, and (3) surface-imaged positive-tone silylation resist processes. To date, materials have been developed for each process which exhibit resolution to better than 0.20μm with sensitivities less than 50mJ/cm².

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Chemical amplification positive resists using partially tetrahydropyranyl-protected polyvinylphenol (THP-M) were investigated for deep UV lithography. A resist formulated from 20% THP-protected polyvinylphenol and bis(tert-butylphenyl) iodonium triflate resolved 0.30-μm line-and-space patterns with the aqueous base development using a KrF excimer laser stepper with a dose of 46mJ/cm². Post-exposure delay effect of this resist system was studied. The deprotection reaction of THP-M for lower dose during the holding time at room temperature and incomplete deprotection reaction for higher dose were found to deteriorate the exposure characteristics.

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A new simple method of measuring diffusion range of acid in chemical amplification resist has been designed. With this method, PEB temperature and PEB time dependence of diffusion range were investigated, and apparent activation energy of diffusion was estimated. Comparing this and activation energy of decomposition of inhibitor, decomposition was revealed to be rate-determining reaction during PEB. This is thought to be the reason for PEB dependence on the pattern profile of the resist.

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Photo-acid generators (PAC's) are vital components in the formulation of deep-UV chemically amplified resists. A new series of α-substituted-2-nitrobenzyl arylsulfonate PAG's was synthesized. Because the thermal stability of these PAG's correlates with the upper limit of the post-exposure bake (PEB) temperature of resists formulated with these, a study of the thermal stability was done looking at the effect of varying the α-substituent. The best thermal stabilities were obtained by having a bulky electron withdrawing group situated at both the o- and α-positions ofthe 2-nitrobenzyl chromophore. This substitution pattern enhances thermal stabilityby suppressing the nucleophilic displacement reaction of the sulfonate moiety by the nitro group oxygen. When used in resist formulations, PAG's based on these new chromophores conveyed higher thermal stability than was observed for similar PAG's based on the 2-(trifluoromethyl)-6-nitrobenzyl chromophore. Through a study done on the lithographic sensitivities of a series of resists formulated with poly(4-(t-butyoxycarbonyloxy)styrene-sulfone) (PTBOCSS) and the new PAG's, it was found that the quantum yield of acid decreased as the electron withdrawing power of the PAG's α-substituent increased.

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The minimum resolution and acceptable depth of focus (DOF) in the case of finite excimer laser bandwidth with high numerical aperture (NA) chromatic lens design for the application of 0.25μm lithography were studied by simulation. Since the chromatic aberration effect in excimer laser stepper is more pronounced for high NA lens, different excimer laser spectral profiles and different lens chromatic aberration coefficients were used to investigate amount of influence of each parameter to the final optical aerial images. As a result of chromatic aberration, the stepper resolution at larger NA is basically limited by the laser spectral profile and lens chromatic aberration coefficient rather than NA of the lens. The DOF is limited by laser spectral profile, lens chromatic aberration coefficient, and NA. Therefore, very narrowed laser spectral profile is required for high NA excimer laser stepper.

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The susceptibility of most chemically amplified resists to performance degradation caused by low levels of airborne basic chemical substances presents a significant barrier to their practical implementation. We have carried out a series of systematic studies intended to improve our understanding of the mechanism of such degradation. We summarize here our investigation of how the contaminant/resist interaction is influenced by resist film composition. Our results suggest that contamination effects can be minimized by targeting specific ranges of physical properties when designing the matrix polymer.

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A molecular design for stabilization of chemical amplification resists toward airborne contamination is described, which is based on our observation that glass transition temperatures (T_g) primarily govern the absorption of N-methylpyrrolidone (NMP) vapor in thin polymer films. Employing the meta-isomer (T_g=85°C) of poly(t-butoxycarbonyloxystyrene) (PBOCST) to replace the para-isomer (T_g=130°C) in the IBM′s tBOC resist has proved the validity of our T_g theory in NMP uptake measurements as well as in negative lithographic imaging. Furthermore, the use of a meta-isomer to lower T_g for better annealing has been shown to be also useful in decreasing NMP absorption in aqueous base developable, . positive copolymer resists, providing increased contamination resistance. We believe that annealing of resist films to minimize free volume plays a key role in environmental stabilization of chemical amplification resists.

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It is important to understand the forming mechanism of insoluble surface residues in chemically amplified positive resists, which is affected by process conditions. In order to clarify the forming mechanism, we investigated the diffusing behavior of hexamethyldisilazane (HMDS) in resist. A novel pattern formation technique is proposed, using the insoluble surface residues which are formed by exposure the HMDS vapor. Several factors in regard to the formation mechanism of the insoluble surface residues are indicated by the dissolution rate, and the FT-IR and XPS studies with variable process parameters. Resist sensitivities are affected by quenching acids which are generated by exposure. The forming mechanism of insoluble surface residues after exposure to HMDS vapor can be recognized by the forming of Si-O bonds and decreasing of the hydrogen bonds. A refined pattern profile with high contrast can be obtained by using the novel pattern formation technique.

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The quest for a high performance positive chemically amplified (CA) resist is a significant challenge. I n this paper we will discuss two approaches to chemically amplified positive resists involving the use of a new and versatile class of acid-labile methacrylate polymers. Methacrylate terpolymers originally designed as chemically amplified positive resists for printed circuit board technology were found to have excellent optical transmission at 193nm. These materials serve as the basis for a high resolution single layer resist for ArF; imaging. In addition, these terpolymers form stable, one-phase mixtures with a variety of phenolic resins and strongly inhibit the dissolution of phenolics in aqueous base. The new dissolution inhibitors based on MMA-TBMA-MAA terpolymers have unusual and useful properties, including excellent optical transmission at 248nm, high glass transition temperatures, and dissolution inhibition/promotion power which can be tailored to accommodate the dissolution properties of a wide variety of phenolic resins.

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At present one of the most promising production-worthy 193nm resist approaches is a single layer, dry developed, surface imaging, silylation process. An evaluation of commercially available developmental resists shows excellent process versatility and lithographic performance. An evaluation of silylation conditions shows more versatility than previously demonstrated. No flow is seen in silylated features, leading to excellent dimensional control and edge profiles. Preliminary results indicate better than 0.18μm resolution and linearity to 0.20μm without the use of phase shift masks. Geometries of 0.1μm have been demonstrated in a resist process which is available for evaluation of 0.20μm devices processes as well as 256MB DRAM circuits.

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Plasma technology is imperative for the successful application of 0.25 micron x-ray lithography. Many of the more conventional processing steps must be developed and optimized for this range of operation and new and improved margins for error must be achieved. In general, exposure of a substrate to a plasma results in a number of processes taking place simultaneously. These include etching, deposition, polymerization, implantation and sputtering. Each processing step requires the optimization of the desired process with concurrent minimization of undesired processing and damage effects.

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Steeply profiled high-aspect-ratio Ta patterns for X-ray masks have been fabricated using low-temperature ECR plasma etching with SF₆ gas. Side etching of the Ta patterns was restrained without using any additional gas to form a sidewall protection polymeric film. This X-ray mask technology has been applied to sub-half-micrometer pattern fabrication using an X-ray lithography system based on a compact synchrotron. N-channel MOS transistors with 0.3-μm gates have been fabricated using the X-ray lithography for all four mask levels.

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A new etching method of semiconductor materials using ionization of CF₄ gas by synchrotron radiation (SR) and acceleration of SR-generated ions by external electric field is proposed. The SR is irradiated parallel to the substrate and the electric filed is applied perpendicular to the substrate. The dominant quasi-stable gas-phase ion is CF₃⁺ which is measured by photoionization mass analysis. This method is applied to etch silicon substrate having resist mask and fine pattern of 0.1μm line and space is successfully fabricated.

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To investigate the way to achieve 250nm photolithography, aerial image simulations are performed. Simulation results show that expected depth of focus will be very shallow even using KrF(248nm) wavelength with conventional illumination system. To enlarge the depth of focus, progress of resist performances, resolution enhancement technologies and mask biases are required. In this paper, we will outline the importance of resist performance and mask bias, and explain the influence of Resolution Enhencement Technologies.

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To reduce critical dimension (CD) variation due to multiple interference effect, anti-reflective coating1) and over coating processes2) have been proposed and investigated. In this paper, we discuss the problem of the over coating processes and report a new and simple process, called OCT (Optical-path Control Technique), for reducing the multiple interference effect that does not require new material and equipment. CD variation can be reduced to 30% using OCT process. Furthermore, we discuss its application to an experimental 64MDRAM process.

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This work describes the evolutionary development of Shipley's positive DUV resists. Two methods for improving resolution are discussed: first, increasing the blocking level on the polymer, and second, adding an Acid-Labile Ester (ALE) as a third component. Although effective at improving resolution, higher blocking level leads to slower photospeed and an increased tendency for capping at the resist surface. These negative effects can be controlled by reducing tan(φ), the tangent of the log-log plot of dissolution rate vs. exposure, to values less than 4. A three-component, ALE-containing resist is shown to have a lower activation energy for deblocking, E_a=26kcal/mole, than does the simpler, two-component resist, for which E_a=38kcal/mole. Two- and three-component resists with identical tan(φ) values of 3.2, Megaposit® Photo Resists XP-3035 and XP-3036, have been developed. XP-3035 resist exhibited resolution to 0.30μm at an energy of 64mJ/sq.cm. XP-3036 resist was linear to 0.275μm at an energy of 86mJ/sq.cm. The XP-3036 resist is superior to XP-3035 due to its higher resolution capability, its reduced tendency for capping, and its greater exposure latitude.

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Significant advances have recently occurred in the development of optical wavelength engineering (Modified Illumination, Phase Shifting Masks (PSMs), Spatial Filters) allowing the extension of Optical Lithography to 0.25μm design rules and below. This paper will discuss recent progress in the development of PSM technology for 0.25μm lithography using I-line or DUV exposure systems.

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