KrF Excimer Laser Lithography Research Articles

Dynamics of Radical Cation of Poly(4-hydroxystyrene) and Its Copolymer for Extreme Ultraviolet and Electron Beam Resists

The use of ionizing radiation such as extreme ultraviolet (EUV) radiation is expected in next-generation lithography. Poly(4-hydroxystyrene) (PHS), which has been used in KrF excimer laser (248 nm) lithography as a backbone polymer, is a promising material for chemically amplified EUV and electron beam resists. In this study, the dynamics of the radical cation of poly(styrene-ran-4-hydroxystyrene) [P(S-HS)] was investigated. It was found that the hole transfer reaction in the matrix plays an important role in the sensitization of the resist. The hole transfer range is estimated to be ∼2.5 units (∼1.0 nm).

Positive resist for KrF excimer laser lithography

In order to develop a new positive-tone, chemically amplified photoresist for 248 nm lithography, a copolymer containing acrylic silicon moiety was synthesized. The copolymer of 1,3-bis(trimethylsilyl)isopropyl methacrylate and 4-hydroxystyrene was prepared by free radical polymerization. The polymer structure, properties and acid catalyzed deprotection were evaluated by 1H nuclear magnetic resonance, Fourier transform infrared, UV, thermogravimetric analysis and gel permeation chromatography. This polymer is thermally stable up to 150 °C and is suitably transparent at the KrF laser output wavelength (248 nm). The lithographic evaluation shows the capability of 0.28 μm resolution using a KrF excimer laser stepper.

Advanced Micro-Lithography Process for i-line Lithography

In our previous paper [Jpn. J. Appl. Phys. 40 (2001) 419], we reported the development of an advanced micro-lithographic process for producing 0.1 µm contact holes by KrF excimer laser (248 nm) lithography. This chemical shrinkage technology, called resolution enhancement lithography assisted by chemical shrink (RELACS), utilizes the cross-linking reaction catalyzed by the acid component remaining in a predefined resist pattern. We report herein the results of the application of RELACS to i-line (365 nm) lithography. The properties of RELACS for i-line lithography were very different from those for KrF lithography. This is due to the difference in chemical mechanism between i-line and KrF resists. The characteristics of the application of RELACS to i-line lithography were studied by conducting basic experiments on the addition of a photo-acid generator (PAG) to an i-line resist and investigating the property of the cross-linking reactions involved in the pre-doping of various acids to RELACS film. Finally, we optimized RELACS materials to match i-line resist and realized the fabrication of contact holes less than 0.2 µm diameter by i-line lithography.

Estimation of Young's Modulus of Resist Pattern by using Atomic Force Microscope.

By applying a certain load directly with a micro cantilever tip, a elastic deformation of resist micro pattern adhering on a substrate can be characterized quantitatively. By combining with finite element analysis of resist pattern deformation, Young's modulus of micro resist pattern of 190nm width and 770nm height formed by KrF excimer laser lithography can be obtained. Young's modulus of resist pattern is estimated in the range of 4 to 5 GPa, which is similar to those of typical polymer materials.

Postexposure Delay Effect on Linewidth Variation in Base Added Chemically Amplified Resist

To elucidate the linewidth variation caused by postexposure delay (PED) in resist films, the distribution of photogenerated acid, the role of additional base component, and the effect of exposure energy were investigated in tert‐butoxycarbonyl protected‐type chemically amplified positive deep ultraviolet resist. The resist system included an N‐methyl pyrrolidone organic base which was evaluated via KrF excimer laser lithography. Using various line‐and‐space patterns formed with a KrF scanner, this work also investigated the change of linewidth caused by the delay time between exposure and postexposure bake. Experimental results indicate that the linewidth broadened immediately following exposure and became a constant value rather than continuously expanding for various pattern sizes. Based on the mechanism of neutralizing organic base and photogenerated acid, a model was established to describe the linewidth according to PED time. Moreover, the effect of exposure energy on linewidth variation was investigated to not only assess the influence of exposure energy but also clarify the relationship between linewidth broadening and delay time. Experimental analysis demonstrates that the exposure latitude and depth of focus can be improved by employing PED. © 2000 The Electrochemical Society. All rights reserved.

A New Positive Resist Based on Poly(4-Hydroxystyrene) for KrF Excimer Laser Lithography

A positive acrylic silicon containing polymer was synthesized with 1,3-bis(trimethylsilyl)isopropyl methacrylate (BPMA) and 4-hydroxy styrene (4-HST) for KrF lithography. Lithographic evaluation shows the capability of 0.2 μm line and space with 2 wt% PAG concentration.

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.

Imaging Characteristics of 0.12 µm Dynamic Random Access Memory Pattern by KrF Excimer Laser Lithography

Imaging characteristics of a 0.12 µm dynamic random access memory (DRAM) cell pattern by KrF excimer laser lithography are investigated by optical image calculations. Using modified illuminations optimized for specific patterns and an attenuating phase shift mask (atten-PSM), depth of focus (DOF) can be achieved up to ∼0.8 µm with high numerical aperture (NA) of 0.70–0.75. However, the image quality is significantly degraded by lens aberration of ∼0.06λ in root-mean-square optical pass difference (RMS-OPD), which is observed in a stepper for 0.25 µm generation devices. Accordingly, the development of a low aberration lens is necessary to realize 0.12 µm DRAM cell pattern formation by KrF excimer laser lithography.

Phantom Exposure of Chemically Amplified Resist in KrF Excimer Laser Lithography

The phantom exposure of a chemically amplified resist in KrF excimer laser lithography can be attributed to acids generated in the ambient air exposed to KrF excimer laser light. These acids diffuse into the resist film and act as catalytic acids. Phantom exposure expanded to more than 10 mm outside the exposure field, and degrades the patterning characteristics. The critical dimension (CD) uniformity can be improved by using an overcoat to protect the resist from the acid.

0.10 µm Dense Hole Pattern Formation by Double Exposure Utilizing Alternating Phase Shift Mask Using KrF Excimer Laser as Exposure Light

Dense 0.10 µm hole pattern formation is achieved by optical lithography with a KrF excimer laser. A Double exposure utilizing two alternating phase shift masks (PSMs) of the line-and-space (L/S) pattern laid out in different directions produces a dense and small hole image in bright field with large focus and exposure latitude. Applying this method with a KrF excimer laser stepper and a chemically amplified negative-tone resist, a two-dimensional (2-D) 0.10 µm hole array with 0.40 µm pitch is resolved with a 0.6 µm depth of focus (DOF). The hole diameter and pitch in the resolution limit seem to be less than 0.10 µm and 0.28 µm, respectively. Also, 2-D hole arrays with different pitches in the x and y directions are easily formed using masks with different pitches in each direction. The hole pattern of an actual DRAM cell is successfully formed by this method. Because of the excellent patterning performance, this method will enable the fabrication of multi giga bit DRAMs by KrF excimer laser lithography.

Atomic Force Microscopy Study on the Dissolution Processes of Chemically Amplified Resists for KrF Excimer Laser Lithography

The surface images of chemically amplified KrF excimer laser resists after development were measured by atomic force microscopy (AFM). From the AFM images at various exposure doses, the dissolution behavior of the polyvinylphenol-based KrF resists differed from that of novolak-diazonaphthoquinone (DNQ) resists. An explanation for the different dissolution behavior has been proposed by considering the difference in the mechanisms of formation of soluble sites between chemically amplified resists and novolak-DNQ resists (non-chemically amplified resists). In addition, AFM topographic images of the resists dissolving layer by layer were observed clearly in the resist spin-coated on a Si wafer. The step heights of the layered structure observed by AFM are in good agreement with the computed value of the distance between two adjacent nodes of standing waves formed by KrF excimer laser exposure. The layered dissolution may be applied to the processing of thin polymer films.

Study of Resist Pattern Roughness on 0.15 .MU.m KrF Lithography.

Resist pattern edge roughness of a chemically amplified resist for 0.15μm KrF excimer laser lithography has been studied. The linewidth 3σ critical dimension (CD) dispersion of the chemically amplified positive working resist increases significantly from 3.0nm to 18.5nm upon reduction of the film thickness for the 0.15μm isolated line pattern, and shows pattern size dependence. After the gate etching process, the pattern edge roughness was almost the same as the linewidth 3σ CD dispersion on the resist film before the etching. From gas chromatography-mass spectrometry (GC-MS) analysis of the residual solvent in the resist film, the solvent density was found to decrease as the film thickness decreased. The relationship between residual solvent density and linewidth 3σ CD dispersion for the 0.15μm pattern before and after the etching process was observed. From the process condition dependence investigation, it was determined that the resist pattern edge roughness could be reduced by optimizing the baking conditions mainly determined by the prebake temperature.

Investigations on Reaction Mechanisms in a Surface Modification Resist Process Using Chemical Vapor Deposition of Polysiloxane

A surface modification resist process (SMR), in which polysiloxane is formed on the exposed resist surface by chemical vapor deposition, was studied for subquarter micron pattern fabrication. This paper describes the microscopic and macroscopic reaction mechanisms in SMR when methyltriethoxysilane is used as the chemical vapor deposition gas. Hydrolysis and condensation reactions occurring in acid‐catalyzed polysiloxane formation were investigated through Fourier transform infrared spectroscopy. It was demonstrated that the hydrolysis is the dominant reaction rather than the condensation, and is dependent on the generated acid catalyst and sorbed water. Polysiloxane layer growth at the near resist surface was observed by Rutherford backscattering spectroscopy and from cross‐sectional views of the polysiloxane layer. The polysiloxane layer initially grows in the resist film because of the dominant diffusion of methyltriethoxysilane molecules. Afterward the diffusion is prevented due to the excessive networking of the polysiloxane so that the polysiloxane layer grows on the resist surface. Furthermore, 0.20 μm pattern fabrication was demonstrated by applying SMR to KrF excimer laser lithography with an alternating phase‐shifting mask.

Fluorinated silicon nitride film for the bottom antireflective layer in quarter micron optical lithography

Fluorinated silicon nitride thin film as a bottom antireflective layer (BARL) material, being suitable for line-patterning in KrF excimer laser (248 nm) lithography, has been studied by film fabrication/characterization and computer simulation. Three-dimensional reflectance simulation process suggests that the 0% reflectance between photoresist (PR) and BARL can be achieved by selecting proper combinations of film optical properties such as refractive index (n), extinction coefficient (k) and thickness (d). For the PR/300 Å BARL/c-Si or PR/300 Å BARL/W-Si structure at a wavelength of 248 nm, the simulation process reveals that nearly 0% reflectance could be obtained when the n and k values of the film are 2.109 and 0.68 or 2.052 and 0.592 respectively. The fluorinated silicon nitride films prepared by ICP enhanced CVD have been evaluated with the variations of flow rates under the two conditions of and 3:20 (sccm). The film n and k values at 248 nm vary in the ranges of 1.665 - 2.352 and 0.007 - 0.695 respectively, depending on gas flow ratio. As it is very sensitive to the film thickness, the reflectance could be reduced, using compoter simulation, to almost 0% by changing the film thickness. Furthermore, the ARL performance for line/space processed by the KrF excimer laser stepper and the stripping ability/selectivity show this material to be a superior candidate for deep-UV microlithography applications.

Fabrication of 0.2 µm Hole Patterns in KrF Excimer Laser Lithography

This paper describes the fabrication of 0.2 µ m hole resist patterns in KrF excimer laser lithography. By using a SiNx single-layer halftone phase-shifting mask (PSM) and an in-house chemically amplified positive resist, 0.2 µ m hole resist patterns can be obtained with sufficient depth of focus (DOF). Furthermore, a 0.15 µ m hole resist pattern can also be fabricated by using the PSM.

Titanium oxide film for the bottom antireflective layer in deep ultraviolet lithography

Titanium oxide thin film, fabricated with tetraisopropyltitanate and oxygen by electron cyclotron resonance-plasma-enhanced chemical vapor deposition, is investigated as a potential candidate for the antireflective layer in KrF excimer laser (248-nm) lithography. The oxygen flow-rate dependence of the optical properties such as the refractive index (n) and the extinction coefficient (k) of the film at the 248-nm wavelength has been characterized, and the films with the expected combinations of n and k values for the antireflective layer have been deposited. Simulation results indicate that reflectance values of less than 4% and as low as 1.2% can be reached at the interface between the photoresist and the film postulating the structures of the photoresist/300-A TiO(x) film/c-Si substrate and the W-Si substrate, respectively, by selected proper combinations of n and k values. Moreover the reflectance can be further reduced to almost zero by changing the film thickness. Thus it is found that titanium oxide thin films can be used as the bottom antireflective layer in KrF excimer laser lithography.

The present position and future status of electron beam lithography for VLSI fabrication

Electron beam lithography is available in fabrication for ASIC-LSIs in order to contribute QTAT process and cost saving for optical reticle making. On the other hand, the optical lithography is facing difficulty in the resolution for isolated pattern arrangement under quarter micron region even if KrF excimer laser lithography is used. It is expected that the electron beam lithography will play a complementary role for the optical lithography. The mix-and-match strategy will be open the door for the large volume production-use of the electron beam lithography.

KrF excimer laser lithography with a phase‐shifting mask for gigabit‐scale ultra large scale integration

Resolution-enhancement technologies such as alternatingtype phase-shifting masks (PSMs), half-tone PSMs, and the off-axis illumination method in optical lithography are necessary for manufacturing gigabit-scale ultra large scale integration (ULSI) devices. Because an alternating-type PSM is the most effective way to enhance resolution, we examine the resolution capabilities of KrF excimer laser lithography combined with the use of an alternating-type PSM through simulations. Our goal is to apply this technique to attain pattern delineation smaller than 200 nm. We simulate light intensity profiles for various types of PSMs in terms of the 3-D mask structure, and find that a PSM structure with a spin-on glass (SOG) phase shifter on a Cr layer that is thinner than in a conventional mask is one of the best choices for KrF excimer laser lithography. We examine potential problems such as the durability of the SOG phase shifters to KrF excimer laser irradiation exposure, and phase angle error due to the surface topography of the Cr aperture patterns. From our experimental results, we confirm that the optical characteristics of the PSM are not degraded, and the phase angle can be controlled with an accuracy sufficient for gigabit-scale ULSI device fabrication. Improved PSMs with a thin Cr layer and SOG phase shifters were successfully used to fabricate several layers of experimental 1-Gbit dynamic random access memory (DRAM) devices with sufficient resolution capability.

The Applicability of Fluorinated Silicon Nitride Film As Bottom Antireflective Layer In Deep Ultraviolet Lithography

AbstractThis study describes the use of fluorinated silicon nitride film as a bottom antireflective layer (BARL) material suitable for line-patterning in quarter-micron KrF excimer laser lithography. For the structures of photoresist/BARL (300Å)/c-Si and photoresist/BARL (300 Å )/W-Si at a wavelength of 248nm, 0% reflectance could be achieved when the refractive index (n) and extinction coefficient (k) values of the film are 2.11 and 0.68 or 2.05 and 0.59, respectively. The fluorinated silicon nitride thin films on p-type (100) Si substrates obtained by inductively coupled plasma enhanced CVD have been evaluated with the variations of NF3 flow rates under the two conditions of SiH4:N2=2:15 and 3:20 (seem). The films optical constants and reflectance were investigated by spectroscopic ellipsometry combined with a reflectance simulation program. The film n and k values at 248nm vary in the ranges of 1.67~2.35 and 0.01~0.69, respectively, depending on gas flow ratio of SiH4:N2:NF3. Low reflectance of below 5% can be obtained from reflectance simulation for two deposition conditions with a BARL thickness of 300Å. In addition, the reflectance could be reduced to almost 0% by controlling film thickness. Finally, the antireflective layer performance was investigated using KrF excimer laser lithography.

Negative Resists Using Acid-Catalyzed Dehydration of Phenylcarbinols: Correlation between Chemical Structure and Resist Sensitivity

A series of phenylcarbinols have been evaluated as components of chemically amplified resists used for KrF excimer laser lithography. The character of the carbon atom to which the hydroxyl group is bonded to determines the insolubilization reactions, leading to differences in sensitivity. Resists using tertiary phenylcarbinol, which converts into polymeric compounds, exhibits the highest sensitivity while the resists using primary phenylcarbinol, which cross-links the phenolic resin, has the lowest sensitivity. Properties, such as sensitivity and chemistry, of the resist containing the secondary phenylcarbinol groups lie between those of the primary and tertiary phenylcarbinol containing resists.