Excimer Laser Stepper Research Articles

Chemically amplified negative resists using acid-catalyzed etherification of carbinol

Acid-catalyzed etherification of carbinol has been used in the design of an alkali-developable, highly sensitive negative resist. The resist consists of a carbinol compound, an acid generator, and a phenolic resin. The resist using diphenylcarbinol (DPC), m, p-cresol novolak resin, and diphenyliodonium triflate (DIT) shows the best lithographic performance in terms of sensitivity and resolution in the carbinol resists evaluated in this study. In the lightly exposed region (below 2.5 mJ/cm 2) bimolecular etherification of DPC occurs while in the heavily exposed region (above 10 mJ/cm 2) DPC reacts with novolak resin to form o-diphenylmethyl novolak resin. Therefore the solubility of the DPC resist film in alkaline developers decreases upon exposure to deep-UV radiation and subsequent heating. Line and space patterns of 0.3 μm are obtained using the DPC resist in conjunction with KrF excimer laser stepper.

0.35 μm rule device pattern fabrication using high absorption-type novolac photoresist in single layer deep ultraviolet lithography: Surface image transfer for contact hole fabrication

0.35 μm design rule device patterns were fabricated using high absorption-type positive and negative photoresist with a KrF excimer laser stepper. The main reason for using high absorption-type photoresist was to minimize the thin film interference effect caused by high reflectivity of the substrate in the deep ultraviolet region. The positive photoresist was FH-EX1 (Fuji-Hunt) and negative photoresist was SAL601. Both contain novolac resin as the base polymer. The positive photoresist was mainly used for poly-Si and W–Si layer pattern fabrication and the negative photoresist was used for the contact hole pattern fabrication. In the contact hole fabrication a surface image transfer technique was used. This technique relies on the direct transfer of the surface negative photoresist image to the insulating layer with highly anisotropic etching and is completely different from the so-called ‘‘surface imaging technique’’ using gas phase silylation and successive dry resist development (O2 reactive ion etching). Although the process was a tentative one, critical dimension controllability of this approach satisfied research and development level device fabrication requirements.

Chemically amplified negative resist system using acid-catalyzed etherification of carbinol

An acid-catalyzed etherification of carbinol has been used in the design of an alkali developable, highly sensitive negative resist. The resist consists of diphenylcarbinol (DPC), an acid generator, and a novolak resin shows the best lithographic performances in terms of the resist sensitivity and resolution among the resist using carbinol. In the exposed resist film, DPC undergoes bimolecular condensation during post exposure baking to form diphenylmethyl ether which has less polarity than DPC. Therefore the solubility of the DPC resist in alkaline developers decreases upon exposure to deep-UV radiation and subsequent heating. As the bimolecular condensation proceeds catalytically, the resist exhibits high sensitivity. Line and space patterns of 0.3 μm are obtained using the DPC resist in conjunction with a KrF excimer laser stepper.

Tetrahydropyranyl protected polyhydroxy-styrene for a chemically amplified deep u.v. resist

Tetrahydropyranyl (TH) protected poly(p-hydroxystyrene) (PHS) was synthesized and its acid-catalysed thermal deprotection utilized in the design of an alkali developable, positive photoresist system incorporating chemical amplification. The solubility of THP-PHS films mixed with the appropriate onium salt photo acid generators in alkaline developers increases upon exposure to deep u.v. radiation and subsequent heating. A resist composed of THP-PHS and bis(tert-butylphenyl)iodonium triflate resolves 0.35 μm line-and-space patterns using a KrF excimer laser (248 nm) stepper (20 mJ cm−2).

Sub-0.5 µm Lithography Using an Excimer Laser at 248 nm

The advent of deep ultra-violet (DUV) lithography using an excimer laser stepper at 248 nm has extended the capability of photolithography in achieving sub-0.5 µm resolution required for the fabrication of 64MB DRAM and 16MB SRAM. This paper reports on the progress of our DUV R&D efforts in working toward this goal. To qualify the stepper, electrical wafer probing was found effective in providing a large volume of data with high sensitivity, accuracy, and precision in the measurements. Advanced resist materials based on the chemical amplification principle demonstrated good sub-0.5 µm resolution, excellent sensitivity, and good etch selectivity required for DUV lithography. However, resist processes for both the positive and negative tone materials still require further optimization incorporating the use of anti-reflective coatings (ARC). Spin-on organic ARC has proven feasible in eliminating both thin-film interference effects and reflective notching. DUV lithography promises to be a strong contender for the next generation of ULSI fabrication, where the limiting factor may lie in the high cost of advanced resist and ARC materials.

Highly sensitive positive deep-UV resist utilizing a sulfonate acid generator and a tetrahydropyranyl inhibitor

A new positive deep-UV resist composed of the sulfonic acid ester MeSB as acid generator and a tetrahydropyranyl inhibitor in a novolak matrix resin was investigated. As the compound MeSB is sensitized by the strong absorbing novolak matrix resin, the acid generation by MeSB is very effective. Therefore the resist is very sensitive ( D < 10 mJ/cm 2). It shows no negative-tone behavior at high exposure dose, while a resist with onium salts shows a negative tone behavior. The best pattern profile was achieved with prebaking at high temperature and post-exposure-bake at lower temperature for a short time. With optimized prebaking and post-exposure-bake conditions, 0.35 μm features with good line profile could be resolved by excimer laser stepper exposure.

Highly sensitive positive deep UV resist utilizing a sulfonate acid generator and a tetrahydropyranyl inhibitor

A new positive deep UV resist composed of the sulfonic acid ester MeSB as the acid generator and a tetrahydropyranyl inhibitor in a novolak matrix resin was investigated. The resist is very sensitive (D<10mJ/cm2) and exhibits no negative tone behavior at high exposure doses. With optimized prebaking and post-exposure-bake conditions 0.35μm features with good line profiles are easily resolved with excimer laser stepper exposure.

New approach to resolution limit and advanced image formation techniques in optical lithography

Practical resolution, the minimum feature size with a depth of focus (DOF) required for LSI fabrication process, is analyzed. Dependence of practical resolution on various factors, such as optical system parameters (exposure wavelength lambda , and numerical aperture NA), resist processes, and required DOF, is investigated. It is shown that practical resolution in the sub-halfmicrometer region is not improved, and may even be degraded, with increasing NA. Furthermore, resolution improvement by increasing NA becomes less effective as lambda becomes shorter. This means that the high-resolution capability of high-NA/short-wavelength optics cannot be utilized to create fine-pattern LSIs. In order to overcome this limitation, the effectiveness of advanced image formation techniques, the phase-shifting method and the FLEX method, in practical resolution enhancement is investigated. It is experimentally verified, using a phase-shifting mask and the excimer laser stepper, that a pattern feature size less than 0.2 mu m can be clearly delineated with sufficient focus latitude. These advanced techniques make it possible to overcome the resolution limitation of conventional optical lithography. >

An aqueous base developable novel deep-UV resist for KrF excimer laser lithography.

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.

ArF quarter-micron projection lithography with an aspherical lens system

A 5X aspherical refractive lens with 60% transmittance at 193 nm wavelength was successfully designed by decreasing the thickness of the lens material. An ArF excimer laser lithographic tool was constructed by the same concept as KrF excimer laser stepper except an aspherical lens was used to demonstrate 0.25 μ m resolution. 0.25 μ m line and space patterns could be resolved using 0.5 μ m thick PMMA.

Application of photoacid generating chemistry to photobleachable deep-ultraviolet resist

The high optical density of the conventional positive resist in the deep UV region (190 nm to 300 nm) prevents its use in the fabrication of high contrast patterns. The photobleachable deep UV resist composed of a 1,3‐dicarbonyl‐2‐diazo compound as the alkaline dissolution inhibitor and an alkaline‐soluble styrene polymer as the resin matrix, which we developed, is one promising approach to resolve these problems. Using this resist, high contrast 0.5 μm patterns were obtained with a KrF excimer laser stepper. In this paper, we have applied photoacid generators to such photobleachable resists. The photoacid generator used was triphenylsulfonium hexafluoroarsenate, and it greatly enhanced the sensitivity and contrast of the resist. The decomposition of diazo compound in the resist was significantly enhanced (and the dissolution characteristics improved) in the presence of the onium salt. This novel positive resist utilizing chemical amplification more than meets the requirements for KrF excimer laser lithog...

Sulfonamide-Phenolic Resin Negative Resist for KrF Excimer Laser Lithography

It was found that aromatic sulfonamides offer a novel category of photosensitive compounds for negative resists. A photosensitive layer consisting of m-cresol novolak resin and an aromatic sulfonamide such as p-(tolyl sulfonyl)-aniline, benzene sulfonyl aniline, p-toluene sulfonamide or 4,4'-methylene-di(p-tolyl sulfonyl)-aniline is insolubilized in an aqueous base by exposure to deep-UV lights involving KrF excimer laser light. The negative resists exhibited the sensitivities (Dg0.5) from 25 mJ/cm2 to 80 mJ/cm2 depending on the structure of sulfonamide. By exposure with a KrF excimer laser stepper 0.8 and 0.35 µm line patterns were transferred. The mechanism of the photoinsolubilization is also discussed.

Polysilanes: Photochemistry and deep UV lithography

AbstractThe photochemistry of poly(cyclohexylmethylsilane) (PCHMS) has been studied, and PCHMS homopolymer has been shown to be useful for high resolution, deep UV pattern generation either by using a commercial 1:1 projection printer operating in the deep UV or by an excimer laser stepper at 248 nm. In the case of the excimer laser stepper, images 0.5 μm and smaller have been printed, which represents an improvement over the 0.8 μm limit previously reported for polysilane copolymers.

A negative resist for KrF excimer laser lithography

AbstractThe 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.

Polyladder organo siloxane bi-level resist.

We studied the use of a ladder-structure silicone, polyvinylsilsesquioxane introduced phenyl groups(PVSS-P), as a negative bi-level resist for deep-UV lithography.We synthesized PVSS-P by hydrolysis of vinyltrichlorosilane and phenyltrichlorosilane followed by dehydrized condensation copolymerization. The residual active silanol groups in the polymer were terminated with trimethylchlorosilane.We found that the PVSS-P is highly sensitive to deep-UV light, Dg0.5=75mJ/cm2, because the phenyl group generates radicals to cross-link vinyl groups when exposed to deep-UV light, and no oxygen-inhibitation effect when exposed to air.PUSS-P is highly resistive to reactive ion etching in oxygen plasma (0z-RIE) because of its high silicon content. The etch rate of PVSS-P is about one hundredth as large as that of hard-baked MP1300 resist, used as a planarizing layer.PVSS-P also forms good pattern profiles because of its high thermal stability. A 0.4μm pattern can be delineated with PVSS-P/MP-1300 bi-level resist systems using a KrF excimer laser stepper.

Advanced KrF Excimer Laser Stepper for Half Micron Lithography

An excimer laser stepper operating at 248.4 nm was developed for use in 0.5 μm pattern fabrication. The excimer laser stepper consists of a new compact excimer laser, whose oscillation spectrum has been line‐narrowed (FWHM 0.007 nm), and a monochromatic fused silica lens. The 5 times projection lens has a field size of and a numerical aperture (NA) of 0.36. A pattern exposure of 0.5 μm features without speckle is possible using this apparatus, and resolution of 0.4 μm line and space patterns were obtained by using commercially available deep UV resist. A high throughput, equivalent to that of a conventional stepper, is further obtained. Since it is expected that the mask alignment accuracy of ±0.1 μm will be obtained, and some suitable resist for the excimer laser stepper will be developed in the near future, excimer laser lithography will become an important technique for the fabrication of semiconductor devices with 0.5 μm design rules such as 16 Mbit DRAM's.

A KrF Excimer Laser Lithography for Half Micron Devices

An excimer laser stepper has been developed for use in 0.5 µm pattern fabrication. The excimer laser stepper is characterized using a 5× monochromatic quartz lens (exposure area: 10×10 mm2) and a new KrF excimer laser whose oscillation spectrum is limited to a narrow band (FWHM=0.008 nm) by using an etalon. A speckle-free pattern exposure with half-micron order was possible using this apparatus. A resolution of 0.4 µm line and space pattern was obtained by single-layer resist process using a commercial deep UV resist, although its pattern profile was poor. A high-aspect ratio of 0.5 µm line and space pattern was obtained by a trilayer resist process. With this apparatus, a high throughput equivalent to that of a conventional g-line stepper can be expected.

The Way To One-Half Micrometer Photolithography

To investigate the way to achieve 0.5 µm photolithography, experiments have been performed with a high-numerical-aperture lens, with multilayer and contrast-enhancement-layer resist processes, and with an excimer laser deep-UV stepper. The 0.6 N.A. lens is for the g-line and has a 5 mm x 5 mm field size. Single-layer resist exposures show good profiles at 0.6 µm line/space (L/S), with no effects from highly oblique illumination, and a depth of focus of 1.25 µm. Multilayer resists using spin-on-glass and contrast-enhancement layers improve the resolution to 0.375 µm with a large N.A. lens and to 0.5 µm with an i-line lens of N.A. = 0.35. Because the large numerical aperture alone cannot reach 0.5 µm and because a large field, large N.A. lens is difficult to manufacture, an i-line lens with moderate N.A. and large field appears to be a better approach. As a more advanced experiment, a KrF excimer laser stepper with an achromatic quartz/fluorite lens of N.A. = 0.37 shows no effect of speckle and has produced a 0.35 µm L/S in PMMA for a k of 0.5. The resolution with MP2400 photoresist is only 0.4 µm because of deep-UV absorption. This shows the need for more work on practical deep-UV photoresists.