Takeshi Ohfuji

Positive chemically amplified resist for ArF excimer laser lithography composed of a novel transparent photoacid generator and an alicyclic terpolymer

A new positive chemically amplified resist which consists of a novel photoacid generator NEALS (new alkylsulfonium salt) and methacrylate terpolymer with tricyclodecanyl group (TCDA) has been developed. NEALS shows good thermal stability up to 151 degree(s)C, high transparency and high acid generation efficiency for an ArF excimer laser exposure. A new base polymer with TCDA groups demonstrated high transparency (69.3%/micrometers ), high thermal stability up to 141 degree(s)C, a good dry-etching resistance and high solubility for an aqueous base (tetramethylammonium hydroxide) developer. A 0.20 micrometers lines and spaces pattern has been resolved using an ArF excimer laser exposure system (NA equals 0.55).

Novel alkaline-soluble alicyclic polymer poly(TCDMACOOH) for ArF chemically amplified positive resists

We have developed a novel ArF resist polymer poly[carboxy- tricyclo(5.3.2.0)decanylmethyl methacrylate] [poly(TCDMACOOH)], which has a carboxyl substituent on the tricyclodecanyl group. This polymer exhibited good solubility (1.93 micrometer/sec) in a 2.38% TMAH solution, high transparency (70%/micrometer) at 193 nm, and a good dry-etching resistance for CF4 gas (1.2 times the etching rate of novolac resin). Furthermore, the resist composed of the partially protected copolymer poly(TCDMACOOH60-TCDMACOOtBu40) with a photoacid generator exhibits a resolution of 0.18 micrometer L/S using an ArF exposure system (NA equals 0.55).

Dry etching resistance of methacrylate polymers for ArF excimer laser lithography

We have investigated dry-etching resistance of methacrylate polymers for use as ArF chemically amplified resists and proposed a new etching model that can predict the etching rate very accurately. The examined polymers were methacrylate polymers with alicyclic groups. The polymers were dry etched using a LAM TCP-9400 machine under the chlorine-based gas conditions used for poly-silicon etching. The obtained etching rate was explained in term of a carbon- atom-density parameter known as the ohnishi parameter. However, the fitting accuracy is not good enough especially for alicyclic polymers (R equals 0.87). And a ring parameter model also resulted in a similar fitting accuracy (R equals 0.86). Hence, we proposed a new model that introduced polymer-structure dependence into the carbon-atom-density model. The new model gives excellent agreement with measured data (R equals 0.99). And it is very useful in designing ArF resist polymers and predicting etching resistance of future ArF resists.

Analysis of molecular diffusion in resist polymer films simulated by molecular dynamics

The diffusion process of acids plays important roles in chemically amplified resists. Polymer matrices from the diffusion path, and the structure significantly influences the behavior of the acid diffusion. We have simulated the diffusions of molecules in polymer matrices by molecular dynamics in order to analyze the diffusion mechanism in chemically amplified resist syste. To represent bulk state conditions of the polymer film, we prepared the molecular structures under the 3D periodic boundary conditions utilizing the molecular simulation. This amorphous cell contained three chains of methacrylate polymers such as poly(methacrylate), poly(tert-butylmethacrylate), poly(isobornylmethacrylate) and one diffusion molecule such as oxygen and methanesulfonic acid. The structure was energy-minimized and equilibrated under stable conditions. The free volumes in the system were estimated as the volumes enclosed by the iso-potential surfaces around the polymer using the Gusev-Suter method. The average size of the free volumes in the poly(methylmethacrylate) system was obtained as 3.7 angstrom3 with large standard deviation of 11.1 angstrom3, which indicates the large width of the size- distribution of free volumes scattered at random in the system. Molecular diffusion in the energy-minimized cell was simulated for 50 picoseconds by the molecular dynamics. The time dependence of the mean-square displacements of diffusing molecules was obtained from the dynamics treatments and it determined the diffusion constant in the resist systems. It is shown that the molecules do not always rapidly diffuse with larger free volumes, but the diffusions also depend upon the interaction with the polymer, and that the computer simulation tools provide the potentia for the molecular level study of resists chemistry.

Prospect and challenges of ArF excimer laser lithography processes and materials

Practical chemically amplified resists for 193 nm lithography will be realized from newly developed polar alicyclic base polymers. Their resist performance is likely to be equal to that of resists used for 248 nm lithography, since a 193 nm resist basically follows the same physical rules as a conventional resist. Top surface imaging with a combination of 193 nm exposure and bi‐functional silyation reagent B(DMA)MS was used to produce 0.12 μm L&S patterns. This resolution corresponds to a k1 factor of 0.34. Therefore, the use of dry‐developed resists, which provide excellent resolution and a generous process margin, will help extend the use of 193 nm lithography. However, resolution enhancement techniques must still be developed to improve the process window.

Negative-type chemically amplified resists for ArF excimer laser lithography

A new negative resist consisting of an anhydride, an acrylic acid, an epoxy crosslinker and a photoacid generator is introduced. In the exposed area, the epoxy groups of the crosslinker react with anhydride groups and/or carboxylic acids in the polymer under existence of photogenerated acid as a catalyst during post exposure baking. A 0.20 micrometers pattern was resolved by an ArF exposure at a dose of 28 mJ/cm2.

Chromium Fluoride Attenuated Phase-Shifting Mask for Argon Fluoride Excimer Laser Lithography

Chromium fluoride (CrF) film was chosen as the phase shifter for attenuated phase-shifting masks because of its high transmittance at 193 nm. The refractive index n and absorption coefficient k was derived by using the reflectance-transmittance (RT) method. Using argon fluoride (ArF) excimer laser exposure equipment and a chemically amplified positive resist, fine hole patterns were resolved. Depth of focus (DOFs) with 0.24 µm were extended to 30%. The resolution limit was 0.20 µm, while it was 0.22 µm with a conventional mask.

Dissolution behavior of alicyclic polymers designed for ArF excimer laser lithography

We have analyzed the dissolution rate of alicyclic polymer resists designed for ArF excimer laser lithography with an in-house laser dissolution rate monitor. The analyzed polymers were methacrylate terpolymer with a tricyclodecanyl group and carboxy-tricyclodecanylmethyl methacrylate copolymer which has a polar carboxylic acid in alicyclic groups. We found that the alicyclic polymer dissolution rate was affected very little by developing conditions such as developer concentration or kind of developer. Furthermore, when we measure the dissolution rate as a function of the polar molecule content, we found that the dissolution rate of the alicyclic polymer is similar to that of conventional polyvinylphenol (PVP) polymers. We also demonstrate that the polymer dissolution rate can be predicted from the percolation theory for both conventional PVP polymer and alicyclic polymers. Moreover, the percolation theory can be applied to chemically amplified positive resists while taking the contribution of the protective group to polarity into consideration. These findings should be very useful for theoretical resist design and material development.

Characterization And Modeling Of High Resolution Positive Photoresists

In this paper, dissolution rate characteristics for high resolution positive photoresists are studied. The dissolution mechanism investigated by changing the concentration of photoactive compound, PAC, in the photoresist is also presented. Based on these dissolution rate characteristics, a new dissolution model and an analytic rate function are proposed. Dissolution rate, R, was measured as a function of relative PAC concentration, M, by changing exposure dose. It is found that R(M) is divided into three regions, depending on M values. These characteristics are commonly observed in high resolution positive photoresists which can resolve 0.5μm L/S with a high NA (0.45) g-line steppiere These behaviors cannot be explained by conventional dissolution model proposed so far.I To investigate this dissolution mechanism, dissolution rates, RI(M), of non-exposed resist (PAC+novolak) and ,RE(P), of fully-exposed resist (PPA+novolak) were measured. Log RE is almost proportional to PPA concentration, P. On the other hand , log RI shows a drastic change at the medium PAC concentration. R(M) expressed as a product of RI(M) and RE(P), agreed with measured dissolution rate. From these results,it is considered that the distinctive dissolution behavior of high resolution positive photoresist is mainly attributed to the PAC inhibitation effect. Based on these dissolultion rate behaviors, a new analytic dissolution rate function is proposed. SAMPLE simulation results show good agreement with the experimental pattern profiles.

Polyvinyl Alcohol Film Coating Effect on Novolac-Based Positive Electron Resist Sensitivity in X-Ray Lithography in an Atmospheric Environment

The polyvinyl alcohol (PVA) film coating effect on novolac-based positive electron resist (NPR) has been studied in X-ray lithography using a Si X-ray source in an atmospheric environment. Extremely striking deterioration in NPR sensitivity was observed in an atmospheric environment compared with in vaccum. It was also found that the sensitivity deterioration was to a greater extent improved by using the PVA/NPR scheme.