ArF Excimer Laser Lithography Research Articles

Development of Photoresists for ArF Excimer Laser Lithography

Development of Photoresists for ArF Excimer Laser Lithography

Impacts of CF+, CF2+, CF3+, and Ar Ion Beam Bombardment with Energies of 100 and 400 eV on Surface Modification of Photoresist

Photoresists used in advanced ArF-excimer laser lithography are not tolerant enough for plasma etching processes. Degradation of photoresists during etching processes might cause not only low selectivity, but also line edge roughness (LER) on the sidewalls of etched patterns. For a highly accurate processing, it is necessary to understand the mechanisms of etching photoresists and to construct a new plasma chemistry that realizes a nano scale precise pattern definition. In this study, the modified layers formed on the surface of a photoresist by the bombardment of fluorocarbon ions of CF+, CF2+, and CF3+, and argon (Ar) ions were analyzed by X-ray photoelectron spectroscopy (XPS). The etching yield of the modified steady-state surface was almost dependent on the mass of incident ion species. The surface composition was modified with increasing dosage of each ion species, and reached a specific steady state that was dependent on the ion species. The bombardment of F-rich ion species such as CF2+ and CF3+ resulted in the formation of not only fluorocarbon layers, but also graphite like structures on the surface. On the basis of these results, the surface reaction for the ion-beam-induced modification was discussed.

Impacts of CF+, CF2+, CF3+, and Ar Ion Beam Bombardment with Energies of 100 and 400 eV on Surface Modification of Photoresist

Photoresists used in advanced ArF-excimer laser lithography are not tolerant enough for plasma etching processes. Degradation of photoresists during etching processes might cause not only low selectivity, but also line edge roughness (LER) on the sidewalls of etched patterns. For a highly accurate processing, it is necessary to understand the mechanisms of etching photoresists and to construct a new plasma chemistry that realizes a nano scale precise pattern definition. In this study, the modified layers formed on the surface of a photoresist by the bombardment of fluorocarbon ions of CF+, CF2+, and CF3+, and argon (Ar) ions were analyzed by X-ray photoelectron spectroscopy (XPS). The etching yield of the modified steady-state surface was almost dependent on the mass of incident ion species. The surface composition was modified with increasing dosage of each ion species, and reached a specific steady state that was dependent on the ion species. The bombardment of F-rich ion species such as CF2+ and CF3+ resulted in the formation of not only fluorocarbon layers, but also graphite like structures on the surface. On the basis of these results, the surface reaction for the ion-beam-induced modification was discussed.

Mechanism for low-etching resistance and surface roughness of ArF photoresist during plasma irradiation

ArF excimer laser lithography was introduced to fabricate nanometer-scale devices and uses chemically amplified photoresist polymers including photoacid generators (PAGs). Because plasma-etching processes cause serious problems related to the use of ArF photoresists, such as line-edge roughness and low etching selectivity, we have to understand the interaction between plasma and ArF photoresist polymers. Investigating the effects of surface temperature and the irradiation species from plasma, we have found that ion irradiation by itself did not drastically increase the roughness or etching rate of ArF photoresist films unless it was combined with ultraviolet/vacuum ultraviolet (UV/VUV) photon irradiation. The structures of ArF photoresist polymers were largely unchanged by ion irradiation alone but were destroyed by combinations of ion and UV/VUV-photon irradiation. Our results suggested that PAG-mediated deprotection induced by UV/VUV-photon irradiation was amplified at surface temperatures above 100 °C. The etching rate and surface roughness of plasma-etched ArF photoresists are affected by the irradiation species and surface temperature during plasma etching. UV/VUV-photon irradiation plays a particularly important role in the interaction between plasma and ArF photoresist polymers.

ラクトン骨格を有するポリマーの研究開発動向

Development of polymers having lactone-unit(s) for recent 25 years is reviewed. Their application to resists for advanced semiconductor lithography has made a great advance recently and lactone-containing polymers have been widely used to manufacture semiconductor chips with 90 nm and 65 nm and even 45 nm scale micro-fabrication. Polymers having norbornanelactone unit developed by the reviewers are highlighted due to their potency as base-polymers for resists of ArF excimer laser lithography.

A new vacuum pin chuck for ArF laser lithography

Abstract To meet the demands of next-generation ArF excimer laser lithography—a site flatness better than 0.03 μm in a 30-mm square field—a new vacuum pin chuck was developed. A 2-mm wide static-pressure seal, which extends beyond the wafer edge, makes it possible to almost completely flatten the wafer periphery. Since the wafer back-surface contacts only the tips of the pins, the chuck can significantly reduce the influence of dust. In addition, conductive pore-free ceramics and a coating of SiC, as well as round pin corners and small roughness on the pin tops, prevent dust adhesion. It was found experimentally and theoretically that the flattening ability of the new chuck around the wafer periphery is several times higher than that of an ordinary ring-seal chuck. Such superior flattening was achieved by expanding the vacuum region by the wafer edge. Calculations showed that the new chuck can flatten an 8-inch concave wafer with a bow of 200 μm to a residual bow of less than 0.03 μm.

Analysis of pattern collapse of ArF excimer laser resist by direct peeling method with atomic force microscope tip

Quantitative analysis of the collapse property of dot resist pattern formed by ArF excimer laser lithography ranging from 141 to 405 nm diameter and 360 nm height is demonstrated experimentally. By directly applying a load to a top corner of the resist pattern with a microcantilever tip, a resist dot pattern adhering on a substrate can be collapsed easily accompanying slight residue formation. The load required for pattern collapse decreases with decreasing pattern diameter. In combination with an analysis of the internal stress distribution in the resist pattern, destruction mechanisms and the diameter dependency of pattern collapse can be clarified.

Molecular design and development of photoresists for ArF excimer laser lithography

Positive-tone and negative-tone resists for ArF excimer laser lithography are developed from novel polymers, which are called functionally-integrated polymers having polar alicyclic units. The fundamental unit of the polymers is an acrylate having a tetracyclododecane group with a carboxyl end group, which satisfies a lot of the requirements of ArF resists. A typical monomer is carboxy-tetracyclo[4.4.0.12,5.17,10]dodecyl methacrylate. In 1996, we established 0.15-µm “positive” line-and-space patterns at 21.8 mJ/cm2 using an ArF exposure system (0.55 numerical aperture), which meet the design rule requirement for fabricating 1-gigabit dynamic random access memories (DRAMs). This fundamental polymer unit has also been used to develop negative-tone resists for ArF lithography. In 1999, we also established 0.13-µm “negative” isolated line pattern at 7.1 mJ/cm2 using an ArF exposure system which fulfills the design rule requirement for logic circuits combined with gigabit DRAMs. Copyright © 2000 John Wiley & Sons, Ltd.

Adhesion-promoted copolymers based on norbornene derivatives and maleic anhydride for 193-nm photoresists

A copolymer of t-butyl 5-norbornene-2-carboxylate, 2-(2-methoxyethoxy)ethyl 5-norbornene-2-carboxylate, norbornene, and maleic anhydride was synthesized as a matrix polymer for ArF excimer laser lithography. Hydrophilic 2-(2-methoxyethoxy)ethyl ester groups are introduced into side chains of the matrix polymer in order to improve adhesion to a silicon substrate without causing cross-linking during a lithographic process. The resist formulated with the polymer shows better adhesion to a silicon substrate as the mole fraction of 2-(2-methoxyethoxy)ethyl 5-norbornene-2-carboxylate increases. 0.15 μm line and space patterns were obtained at a dose of 10.5 mJ cm −2 using an ArF excimer laser stepper.

Stabilization of ZrSixOy Films by Irradiation with an ArF Excimer Laser

ZrSixOy is a candidate material for fabricating attenuated phase-shift masks used in ArF excimer laser lithography. To investigate the durability mechanism of ZrSixOy films following irradiation with an ArF excimer laser, we analyzed the films chemically and physically. The phase shift and the transmittance of a transmissive film with a high oxygen concentration increases and decreases, respectively, with irradiation. This is because of an increase of Si-O-Zr bond structures. The phase shift and the transmittance of an absorptive film with a low oxygen concentration decreases and increases, respectively, with irradiation because of photooxidation at the surface. The oxidation leads to the formation of Si-O-Zr and Si-O-Si bond structures in the films. Both films are stabilized by irradiation with a large irradiation energy. We interpret that both films are stabilized by the formation of Si-O-Zr bond structures, such as zircon, with saturation of oxygen concentrations at the surface.

A novel bottom antireflective coating working for both KrF and ArF excimer laser lithography

Hexamethyldisiloxane (HMDSO) is used as coating material in a conventional ECR-PECVD process. By simply adjusting the gas flow rate ratio, the material can be varied to have suitable optical constants for making bottom antireflective coating (BARC) layers working at both 248 nm and 193 nm wavelengths. The measured reflectances lower than 1 % on a silicon crystal substrate at both wavelengths have been achieved. The swing effect in the resist is significantly reduced. We also show that an HMDSO-based multi-layer structure can be used for broadband AR coating in deep ultraviolet regimes.

Molecular design and development of photoresists for ArF excimer laser lithography

Positive-tone and negative-tone resists for ArF excimer laser lithography are developed from novel polymers, which are called functionally-integrated polymers having polar alicyclic units. The fundamental unit of the polymers is an acrylate having a tetracyclododecane group with a carboxyl end group, which satisfies a lot of the requirements of ArF resists. A typical monomer is carboxy-tetracyclo[4.4.0.12,5.17,10]dodecyl methacrylate. In 1996, we established 0.15-µm “positive” line-and-space patterns at 21.8 mJ/cm2 using an ArF exposure system (0.55 numerical aperture), which meet the design rule requirement for fabricating 1-gigabit dynamic random access memories (DRAMs). This fundamental polymer unit has also been used to develop negative-tone resists for ArF lithography. In 1999, we also established 0.13-µm “negative” isolated line pattern at 7.1 mJ/cm2 using an ArF exposure system which fulfills the design rule requirement for logic circuits combined with gigabit DRAMs. Copyright © 2000 John Wiley & Sons, Ltd.

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 193nm 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 0.15μm line and space patterns were obtained at a dose of 18mJcm−2 using an ArF excimer laser stepper.

Hexamethyldisiloxane film as the bottom antireflective coating layer for ArF excimer laser lithography

We demonstrate a new bottom antireflective coating (BARC) layer for ArF excimer laser lithography. The antireflective layer is composed of hexamethyldisiloxane (HMDSO) film, which is deposited by the conventional electron cyclotron resonance-plasma-enhanced chemical-vapor deposition process. We obtain the appropriate HMDSO films for BARC layers by varying the gas-flow rate ratio of oxygen to HMDSO. Such a process has several advantages: high deposition rate, low process temperature, easy film removal, and reduced cost. Measured reflectances of less than 0.5% on both Al-Si and silicon crystal substrates have been achieved and agree well with the simulated reflectances. The swing effect is shown to be significantly reduced by addition of the HMDSO-based BARC layer.

Measurement of Parameters for Simulation of 193 nm Lithography Using Fourier Transform Infrared Baking System

By incorporating baking equipment into a Fourier transform infrared (FT-IR) spectrometer, a deprotection reaction parameter measurement system that can be used with chemically amplified resists has been developed. This system allows us to study new models for chemically amplified (CA) resists by including into a conventional deprotection reaction model an initial delay effect and a quencher effect. This model is also used to measure deprotection reaction parameters. The parameters thus obtained are inputted into a lithography simulator PROLITH/2 to perform profile simulations. The results are compared with those of scanning electron microscope (SEM) observations. Although the simulation results and SEM observations are not in complete agreement, the general trends observed are in adequate agreement. These results confirm the applicability of the proposed model to CA resists for ArF excimer laser lithography and verify the usefulness of the measurement system.

Swelling Analysis of Methacrylate Polymers in Aqueous Alkaline Developer

The dissolution behavior of a methacrylate polymer was analyzed by the Quartz Crystal Microbalance (QCM) method in order to clarify the dissolution mechanism of ArF resists and control thereof. The QCM analysis provides information on the dissolution rate and the swelling rate of the polymer film by measuring the weight change in polymer films during the development. Poly(isobornylmethacrylate-co-methacrylic acid)s were studied as a model polymer for the resist in ArF excimer laser lithography by the QCM method. It was found that an increase in the bake temperature for the film preparation decreases the dissolution rates and the swelling rates in an alkaline solution, that an increase in methacrylic acid units produces the characteristic surface and interface layers which do not depend upon the bake temperature, and that an increase in isobornylmethacrylate units produces swelling during the development.

Improvement of Post-Exposure Delay Stability in Alicyclic ArF Excimer Photoresists

This paper reports work toward designing environmentally stable alicyclic polymer-based photoresists for ArF excimer laser lithography. A design concept for improving post-exposure delay stability is suggested in this paper. The polymers described here were prepared from dinorbornene derivatives and maleic anhydride by free radical polymerization. The relationship among polymer structure, glass transition temperature (Tg) and post-exposure delay stability was studied. This new polymer design offers a pathway towards good post-exposure delay stability while maintaining high resolution.

Study of the Bottom Antireflective Coating Process Using a High-Transparency Resist for ArF Excimer Laser Lithography

We have investigated the effect of substrate reflectivity and resist transparency on resist performance in ArF excimer laser lithography with bottom antireflective coating (BARC). Both depth-of-focus (DOF) and exposure latitude in low-transparency (49% per 0.5 µm thickness) resist were improved as the substrate reflectivity was decreased. However, the DOF and the exposure latitude decreased, since the resist profile was tapered on the extremely low-reflectivity (∼0%) substrate. On the other hand, the DOF of high-transparency (67% per 0.5 µm thickness) resist was wider than that of low-transparency resist and was independent of substrate reflectivity. The exposure latitude also improved with decreasing reflectivity. High-transparency resist achieved a 0.75 µm DOF at 0.15 µm lines and spaces (L/S) on BARC using a 0.60-NA lens without any resolution enhancement techniques. Good critical dimensional control at 0.15-µm L/S patterns over the topography with 200-nm step was obtained without significant linewidth variation. Gate etching was successfully achieved down to 0.11 µm L/S with the etching conditions developed for KrF resists.

Synthesis of Silicon-Containing Photoresists for ArF Excimer Laser Lithography

Two new positive-type photoresists which are terpolymers of methyl methacrylate (MMA), 1,3-bis(trimethylsilyl)isopropyl methacrylate (BPMA) and either methacrylic acid (MAA) or maleic anhydride (MA) have been synthesized for ArF excimer laser lithography. Terpolymers have a silicon containing acid labile protecting group which can be deprotected by photo acid generator (PAG). Terpolymers have a good thermal stability up to 140°C and the glass transition temperature (Tg) of poly(MMAx-BPMAy-MAz) was 119°C. The minimum feature resolution with 0.17 µm L/S pattern was achieved with poly(MMA5.9-BPMA2.1-MAA2.0) by using an ArF exposure system with 17 mJ/cm2 energy dose.

Maleic anhydride type ArF photoresist

ArF excimer laser (λ = 193 nm) lithography is a potentially applicable technology for the sub-0.18 μm design rule. However, in practical use of ArF technology, many problems must be solved for the properties of photoresist. To solve these problems, we synthesized ArF polymer resin, poly (2-hydroxyethyl acrylate/2-( t-butoxycarbonyl)-5-norbornene/5-norbornene-2-carboxylic acid/maleic anhydride). 2-hydroxyethyl acrylate is very suitable adhesion promoter to the silicon wafer substrate. Using this resist, 0.14 μm L/S pattern was obtained with 12 mJ cm −2 exposure by ArF stepper on the conventional develop condition, 2.38 wt% tetramethyl ammonium hydroxide aqueous solution.