Novel positive deep UV resist for KrF excimer laser lithography

A novel positive deep UV resist for KrF excimer laser lithography has been developed. The resist is composed of 1,7-bis(4-chlorosulfonyl phenyl)-4-diazo-3,5-heptanedione as the alkaline dissolution inhibitor and an alkali-soluble sytrene polymer as the main-polymer. 1,7-bis(4-chlorosulfonyl phenyl)-4-diazo-3,5-heptanedione has great capability of alkaline dissolution inhibition. High thermal stability and excellent photobleachability at 248 nm of the compound are also characterized. The alkali-soluble styrene polymer has a high transmittance of 70% in 1.0 μm thickness at 248 nm. The novel positive resist had an excellent property for dissolution kinetics and photobleaching. We achieved high aspect ratio half-micron pattern fabrication in 1.0 μm thickness using the new resist.     

Poly(t-butyl-3α-(5-norbornene-2-carbonyloxy)-7α,12α-dihydroxy-5β-cholan-24-oate-co-maleic anhydride) for a 193-nm photoresist

A copolymer of t-butyl-3α-(5-norbornene-2-carbonyloxy)-7α,12α-dihydroxy-5β-cholan-24-oate and maleic anhydride was synthesized as a matrix polymer for ArF excimer laser lithography. The polymer has an excellent transmittance at 193 nm and possesses good thermal stability up to 255°C. The resist formulated with the polymer showed better dry-etching resistance than the conventional KrF excimer laser resist for chlorine and oxygen mixed gas. A 0.15 μm line and space patterns were obtained at a dose of 18 mJ cm⁻² using an ArF excimer laser stepper.     

Ultrafast deep UV lithography using excimer lasers

Various excimer laser wavelengths have been used, both in mid- and deep-UV regions—308 nanometers (nm) from XeCl, 249 nm from KrF and 222 nm from KrCl–to delineate images in a number of resists. The quality of the images obtained with the laser exposures, the absence of speckle, and the insignificant loss of resist reciprocity make excimer laser lithography an attractive technique. The ultrafast exposures possible with this method significantly lessen the sensitivity requirements on deep UV resists, making the choice of the resist more flexible.     

Synthesis and lithographic evaluation of poly[(methacrylic acid tert-butyl cholate ester)-co-(γ-butyrolactone-2-yl methacrylate)]

Poly[(methacrylic acid tert-butyl cholate ester)-co-(γ-butyrolactone-2-yl methacrylate)] was synthesized and evaluated as a new 193-nm chemically amplified photoresist. This polymer showed good thermal stability up to 240 °C and had a good transmittance at 193 nm. This material showed good resistance to CF₄-reactive ion etching. The resist patterns of 0.15 μm feature size were obtained at a dose of 11 mJ cm⁻² using an argon fluoride excimer laser stepper.     

Positive excimer laser resists prepared with aliphatic diazoketones

A new class of alkali-developable positive excimer laser (KrF) resists is described. Novel α-diazoacetoacetates derived from aliphatic polyfunctional alcohols were synthesized. These compounds undergo photolysis upon deep UV exposure to yield carboxylic acids, and exhibit excellent bleaching effects. Some of them, especially those having steroid skeletons, act as effective dissolution inhibitors. The composites prepared from these compounds and poly(p-hydroxybenzylsilsesquioxane) were used as alkali-developable positive deep UV resists, whose sensitivities depend on the number of photoactive groups in one photoactive molecule. Imaging results of KrF excimer laser projection printing are presented.     

Submicron lithography in Japan

1Mbit D-RAM. the most advanced VLSI device, is realized by a high performance stepper with the conventional optical techniques. However, as the optical lithography has an inherent limit of resolution, new technologies are being developed rapidly for the development of new generation VLSI devices (4M-16Mbit D-RAM) using shorter wavelength photons of i-line. Electron beam (EB) lithography is already in practical mask making products, but the resolution limit is about 0.5 μm because of proximity effects. In order to make higher resolution and higher precision masks, high voltage EB technique is being developed to minimize the proximity effect, and fabricated 0.25 μm line and space by a single scan at 50kV. X-ray technology 1; becoming practical after a long laboratory-level study, using high performance X-ray resists (CPMS: chlorinated polymetylstylene). Focused Ion Beam (FIB) technology has been anticipated for its capability of submicron lithography due to a reduced proximity effect. High speed submicron Si MOSFET and GaAs MESFET with 0.25 μm gate have been fabricated using FIB technology. Activities of submicron lithography technology in Japan (optical stepper, EB, X-ray, and FIB) are described.     

A high resolution negative working resist,LMR-UV,for g- and i-line lithography

LMR-UV (“low molecular weight resist for uv lithography”), a naphthoquinone-diazide sulfonic acid ester of a novolak resin, is a negative working resist. The mechanism of insolubilization of LMR-UV is based on the facts that the naphthoquinone-diazide moiety is decomposed to indenecarboxylic acid (polar compound) by photolysis upon UV irradiation and that the irradiated resist film insolubilizes in a non-polar developer. LMR-UV reliably forms 0.6 μm lines and spaces over a reflective substrate with steps by using a g-line stepper having a 0.35 NA lens. 0.6 μm-wide aluminum patterns over topography are obtained by use of g-line exposure and reactive ion etching. By use of an i-line aligner (NA = 0.42), LMR-UV resolves 0.25 μm space patterns with overhang profiles. The profiles are due to the large absorption coefficient of 3.8 μm^?1. 0.3 μm wide aluminum patterns are formed by i-line exposure and lift-off metallization.     

Tetrahydropyranyl- and furanyl-protected polyhydroxystyrene in chemical amplification systems

Tetrahydropyranyl- (THP) and furanyl- (THF) protected polyhydroxystyrene (PHS) polymers have been investigated for their potential use in conjunction with onium salt acid precursors to yield high-sensitivity resist systems. The synthesized polymers have high transmittance at 248 nm (the wavelength used in next-generation excimer laser, KrF exposure tools). At 248 nm the transmittance for a 1-μm thick film is ～ 80% (Abs = 0.097 μm^?1). The acid sensitivity of the acetal functionality at room temperature is high, requiring careful handling of all materials to prevent any premature deprotection of the hydroxy group. The highest lithographic sensitivities obtained so far with a system consisting of poly(p-tetrahydropyranyl-oxy-styrene) base resin and 1 mol % of bis (p-tert-butyl phenyl) iodinium triflate (TBIT) was ～ 2 mJ / cm². High-resolution line and space patterns (0.35 μm) were obtained with a system comprising PHS-p-THP and an acid precursor, using an excimer laser step and repeat exposure at 248 nm.     

A negative resist for KrF excimer laser lithography

The design and preparation of a series of negative resists for KrF excimer laser lithography are described. Each resist is composed of poly(hydroxystyrene) and an aromatic azide. The base resin shows high transmittance of 62%/μm at 248 nm, when p-ethylphenyl p-azidophenylsulfonate. 4-azido-4α-methoxy-chalcone, 1-(4 azidobenzylidene)-3-(α-hydroxy-4-azidobenzyl)-indene, 4,4α-diazido-3,3α-dimethoxybiphenyl, or 1-(4-azidostyryl)-5, 5-dimethyl-2-cyclohexen-1-one is employed as a sensitizer. These azides are obtained by red-shifting the absorption maxima to lower energy regions than the exposing wavelength of 248 nm. Transmittance of resists can be controlled from 10 to 30%. The resist is exposed with a KrF excimer laser stepper and developed in an alkaline solution. Sensitivities of about 15 mJ/cm² are observed. A good, subhalf-micron resist profile is achieved. The photochemical reaction mechanisms of poly(hydroxystyrene) and 4,4α-diazido-3,3α-dimethoxybiphenyl were studied at 248 nm and 313 nm exposure. Quantum yield for photodecomposition at 248 nm is seven times larger than that at 313 nm, but dissolution-inhibition effects are larger at 313 nm exposure. Consequently, the resist shows higher sensitivity at 313 nm than at 248 nm.     

Aerial image formation with a KrF excimer laser stepper

We have studied the imaging characteristics of an excimer laser stepper as a function of laser wavelength. Focus, magnification, distortion, CD process window, and laser performance were all examined, and an operating wavelength was selected. The self-metrology capabilities of both the laser and stepper were tested to insure that the system would set itself to the correct wavelength upon initialization, and maintain that setting during volume production activity. The use of an on-line stepper image sensor to evaluate the quality of the aerial image, coupled with the built-in wavelength tracking system as part of the laser optics, was found to provide the required closed loop system for wavelength and focus control. Based on these experiments, we have determined that a quick aerial image check before each production lot should suffice to insure focus control of better than 0.25 microns and magnification errors less than 15 nm during the entire time required to expose the lot.     

Arylazophosphonate containing polymers designed for XeCl excimer laser ablation

Applying the N‐P coupling technique, fifteen new poly(arylazophosphonate)s were synthesized by interfacial polycondensation from bifunctional diazonium salts and bifunctional phosphoric diesters. Because of their excellent film forming properties and absorption behaviour poly(arylazophosphonate)s are suitable materials for laser ablation experiments with XeCl excimer lasers (308 nm). A variety of ablated structures have been generated by irradiation of the polymer films with a commercially available pulsed XeCl excimer laser. Macro experiments indicate remarkable structures with sharp edges, clear contours, and flat bottoms. Moreover, various patterns with μm dimensions were generated by micro experiments and were characterized by means of scanning electron microscopy (SEM). The resolution of these structures indicate that poly(arylazophosphonate)s are suitable materials for applications in microtechnology.     

Progress in deep-UV resists using CARL technology

Three positive working Si-CARL resists for bilayer applications with oxygen-RIE pattern transfer were investigated, and their lithographic performance at deep-UV exposure was compared. With all three we obtained good focus latitudes for 0.35 μm lines and spaces ranging from 1.6 to 2.2 μm exposed with an 0.37 NA KrF-excimer laser stepper. The zero bias exposure dose required for resist R1, of the diazodiketone type, is relatively high (70 mJ/cm²), but R1 has the advantage of not suffering from linewidth fluctuations caused by post-exposure delay time effects. Processing for resists R2 and R3, which are based on acid catalyzed deprotection of t-BOC-imide and t-butylester, respectively, had to be optimized to avoid bridging of isolated spaces after Chemical Amplification of Resist Lines (CARL). This bridging is caused indirectly by evaporation of triflate acid during PEB. Resist R2 needs only 8.5 mJ/cm² for exposure but has a poor linewidth increase after CARL, which seems to be the reason for eroded 0.25 μm patterns after oxygen-RIE. Resist R3 shows the steepest resist slopes and the best overall performance. The ultimate resolution for resists R1 and R3 is 0.25 μm, which, according to the Rayleigh equation for resolution, corresponds to a k-factor of 0.37.     

Excimer laser projection patterning with and without resists

Projection imaging with the deep-UV (193 nm) and VUV (157 nm) output of an excimer laser has been applied to submicrometer patterning of thin films by injected-defect, surface-chemical, and solid-transformation processing. The methods have been designed to take advantage of the short-wavelength, high-peak-intensity pulsed radiation from these sources. Examples are described of pattern definition by exposure of multilayer organic resists, by maskless etching and doping of solids in reactive vapors, and by solid-state chemical transformations in inorganic Al/0 films. Well-resolved 0.4-μm lines and spaces have been achieved. Required doses, between 0.04 and 1 J/cm², are compatible with single- or multi-pulse step-and-repeat projection patterning with a small excimer laser.     

A negative resist,LMR (low molecular weight resist), for deep UV lithography

A negative deep UV resist, LMR, has been developed. LMR is a diazonaphthoquinonesulfonate of cresol novolac resin and possesses a high sensitivity of 15 mJ/cm² and a high resolution up to 0.2 μm by contact printing. An important feature of LMR is the production of negative images with overhung profiles. Such profiles are attributed to LMR possessing a large absorbance in the deep UV region (12 μm^?1 at 250 nm), which is of great advantage to the lift-off process in the fabrication of GaAs-FET, surface acoustic wave (SAW) devices, and so on. Furthermore, because of the good thermal stability of LMR, it can liftoff hard metals such as Ni and Mo. In order to elucidate the phenomenon that deep UV irradiation retards the dissolution rate of LMR, a mechanistic study has been carried out. In both LMR and its analog lacking diazoquinone moiety, their molecular weights Increase upon deep UV irradiation, which suggests that crosslinking occurs in the base resin. This is supported by 13C NMR analyses of photochemical reaction products and by detection of radical species produced from the base resin. The photochemical reaction of the base resin is important to understand the overall lithographic performance of LMR.     

Excimer Laser Induced Removal of Particles from Hydrophilic Silicon Surfaces

A pulsed KrF excimer laser was used to remove several types of submicron-sized particles from silicon surfaces. Polystyrene latex particles, 0.1 μm and larger, were removed from silicon surfaces by dry laser cleaning (no water layer condensed on the surface) but SiO₂ particles could not be so removed. However, during steam laser cleaning, in which a thin film of water is deposited on the surface as both an energy transfer medium and an adhesion force reduction agent, these 0.1–0.2 μm SiO₂ particles were almost entirely removed. Calculations of the various forces contributing to adhesion indicate that hydrogen bonds are the major contributor to the adhesion of inorganic particles to substrate surfaces. Photoacoustic detection, using piezoelectric transducers, monitored the surface vibrations induced by the laser pulses.     

UV curing and matting of acrylate nanocomposite coatings by 172 nm excimer irradiation

For UV-curable acrylate coatings reinforced by silica nanoparticles, the effect of 172 nm excimer irradiation on the surface roughness has been studied. A dual UV lamp set-up consisting of a 172 nm excimer lamp and a mercury arc lamp allowed obtaining gloss levels down to 0.5 units (at 60°) depending on the acrylate formulation and curing conditions. Moreover, UV matt-finished sample showed enhanced surface hardness and increased chemical resistance. It is assumed that 172 nm excimer irradiation resulted in a higher network density via additional cross-linking reactions.To study the depth profile of acrylate conversion for coatings cured by the combination of a 172 nm excimer lamp (accountable for surface curing) and a mercury arc lamp (responsible for through curing), FTIR microscopy as well as (Ge)ATR-FTIR having an IR penetration depth of less than 0.5 μm have been applied. Providing the presence of a photoinitiator as well as the absence of oxygen inhibition, similar degrees of double bond conversion of about 90% were observed on the entire area of the cross-section of the coating, i.e. the wavelength of UV irradiation was found to have no significant impact on acrylate conversion.     

Dry-developed organosilicon resists for 193-nm excimer laser lithography

Irradiation of certain organosilicon polymers with a 193-nm excimer laser forms a latent image that contains increased amounts of oxygen. Patterning is achieved by dry development in an HBr plasma, where the oxidized polymer etches more slowly than the unexposed areas. With these polymers as the top layer in a bilayer resist scheme, 0.2 μm resolution has been demonsrated and resist sensitivities less than 50 mJ/cm² have been achieved. Three classes of organosilicon layers have been investigated: polysilynes; polysilanes, in particular poly(phenylmethyl) silane; and a plasma-deposited polymer derived from tetramethylsilane (PPTMS). The PPTMS, when used with plasma-deposited planarizing layers, opens the possibility of an all-dry, cluster-tool-compatible lithographic cycle.     

Comprehensive study of polymer fiber surface modifications Part 1: high‐fluence UV‐excimer‐laser‐induced structures

Recently, there has been great interest in physico‐chemical surface treatments for modifying polymer surfaces. Ultraviolet (UV)‐excimer‐laser irradiation of polymers is of particular interest. In this study, polyamide was irradiated by a 193 nm excimer laser with a fluence above its ablation threshold (high‐fluence). Morphological changes of the resulting samples were characterized by scanning electron microscopy (SEM) and tapping mode atomic force microscopy (TM‐AFM). Chemical modifications by laser treatment were studied by X‐ray photoelectron spectroscopy (XPS), time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and chemical force microscopy (CFM). Topographical results indicated that ‘ripple‐like’ structures of micrometer size were formed after laser irradiation. XPS and Tof‐SIMS results showed that bond scission occurred on the polymer surface under the action of high‐fluence. Changes in surface chemical properties of the laser‐irradiated polyamide were supported by CFM experiments. Copyright © 2004 Society of Chemical Industry     

Isomerization and decomposition of a poly(alkylaryldiazosulfide): Comparison of thermolytically and photolytically induced reactions

The poly(alkylaryldiazosulfide) compound I has been synthesized to yield preferably the thermodynamically instable Z-isomer, in a mixture of E- and Z-isomers with respect to the N=N bond. Thermolytically and photolytically induced isomerization as well as decomposition of I are monitored by means of infrared and UV/Vis absorption spectroscopy. The activation energy of the Z/E-isomerization is determined to be around 103 kJ mol^–1, independent of the matrix used. The activation energy of the thermolytically induced release of nitrogen is 153 kJ mol^–1. The photolytic decomposition of I is induced by excimer laser pulses at 308 nm. UV/Vis absorption spectra indicate a preferred consumption of the E-isomer. There is evidence for a photochemical decomposition including a rearrangement of the E-isomer into the Z-form as an intermediate reaction step.     

Poly(arylazophosphonates): New Arylazophosphonate‐Containing Polyurethanes and Polyesters for Laser Ablation Structuring

In this article we report the preparation of new arylazophosphonate‐containing polymers, poly(arylazophosphonates), via polycondensation reactions of bifunctional hydroxy functionalized arylazophosphonate‐containing monomers with isocyanate or acid chloride comonomers. The polyurethanes and polyesters were characterized by common methods such as ¹H, ¹³C, ³¹P NMR, FTIR, UV/visible spectroscopy, DSC, and GPC measurements. In addition, a number of polymer structuring experiments (macro and micro experiments) by means of laser ablation with a XeCl excimer laser were performed. In macro experiments, pulse number and fluence were investigated systematically and their influence on the ablation depth was discovered. In micro experiments, structures of high quality were obtained which possess clear contours without any debris on the polymer surface. Structures in the sub micron range were created by applying a phase mask. These high quality results indicate that the poly(arylazophosphonates) are well suited materials for laser ablation structuring with XeCl excimer lasers (e.g. for microstructuring or microlithography).

SEM image of sub micro structures of polymer APU 5 generated in experiments using a phase mask (line width: 0.3–0.5 μm, trench width: 0.3–0.5 μm, fluence: 3 J/cm²).