Norihiko Samoto

Changes of Volume and Surface Compositions of Polymethylmethacrylate under Electron Beam Irradiation in Lithography

The change of the content of oxygen and carbon in Polymethylmethacrylate (PMMA) under electron beam irradiation is investigated by Auger electron spectroscopy (AES) whereby we estimate the characteristic energy deposition of the decrement of oxygen to be 0.8–1.2×103 eV/nm3. It is also confirmed that cooling of PMMA considerably reduces the elimination rate of oxygen and that the decrement of the thickness of PMMA film under electron beam irradiation is closely related to the change of oxygen content. Moreover, it is shown that the PMMA alters from the positive resist to the negative resist as determined by observing thickness changes.

A study of acid diffusion in chemically amplified deep ultraviolet resist

Postexposure bake (PEB) dependence of photogenerated acid diffusion was investigated in a chemically amplified deep ultraviolet positive resist. The resist consisted of a tert‐butoxycarbonyl protected polystyrene as base resin and 2,4‐dimethylbenzenesulfonic acid derivative as photoacid generator. The diffusion length of photoacid increased with increasing PEB temperature or its time. Moreover, the activation energy of acid diffusion reaction within the resist film became smaller, with increased exposure dose. It is considered that hydrophilic OH sites of the base resin generated by the deprotection of hydrophobic protecting groups has a role as one of the diffusion paths in the polymer matrix. Furthermore, it was found that the diffusion coefficient under high PEB conditions was affected by the acid reduction. Based on the analysis of diffusion characteristics, clear correlation between acid diffusion in the resist film and PEB conditions was obtained. These results are useful for improving both resoluti...

A novel electron‐beam exposure technique for 0.1‐μm T‐shaped gate fabrication

This article reports on a novel fabrication technique for T‐shaped short gates using double‐layer electron‐beam(EB) resist system, which provides uniform gate length across 2‐in. wafers. In the proposed novel fabrication technique, a 1‐μm PMMA(I)/0.25‐μm PMMA(II) (HI/LOW MW) double‐layer resist system is adopted to fabricate T‐shaped gates. To avoid the influence of scattered electrons, the bottom PMMA(II) resist is EB‐exposed through the top opening with an acceleration of 50 or 25 kV and a single‐path line dose of 0.8–2.0 nC/cm. The top opening has been formed with a 25‐kV Gaussian electron beam at a 120 μC/cm2 dosage. The developed technique has accomplished T‐shaped gates with 70‐nm minimum footprint and 0.1–0.2‐μm T‐shaped gates, whose yield is over 80% on wafers.

Polymer structure effect on dissolution characteristics and acid diffusion in chemically amplified deep ultraviolet resists

The role of molecular weight dispersion (Mw/Mn) of base phenolic resin was investigated in chemically amplified positive KrF resists. The resists consisted of tert-butoxycarbonyl (t-BOC) blocked polystyrene as base resin and 2,4-dimethylbenzenesulfonic acid derivative as photoacid generator, and the Mw/Mn value was changed as 1.2, 4.0, and 9.0. Not only dissolution rate contrast, but also the slope m of log(dissolution rate)-log(exposure dose) plots increased with decreasing Mw/Mn. In the case of low Mw/Mn, the molecular structure becomes more homogeneous, so resist film could be dissolved uniformly via constant developer penetration into film. This uniform dissolution characteristics may be attributable to the origin of dispersion effects mentioned above. Furthermore, higher acid diffusion property and smooth pattern side wall were obtained without side wall roughness in lower Mw/Mn. This fact indicates that acid diffusion length is also uniform within resist film in the case of low Mw/Mn. Based on the e...

Relationship between Remaining Solvent and Acid Diffusion in Chemically Amplified Deep Ultraviolet Resists

For clarification of the diffusion path of photogenerated acid in the resist film, the prebake temperature dependence and tert-butoxycarbonyl ( t-BOC) blocking level dependence on acid diffusion were investigated for chemically amplified deep ultraviolet (DUV) positive resists. The resists consisted of a t-BOC protected polystyrene base resin and a 2,4-dimethylbenzenesulfonic acid derivative as a photoacid generator (PAG). The concentration of remaining solvent in the resist film decreased as the prebake temperature or t-BOC blocking level increased. The acid diffusion coefficient was almost independent of exposure dose; however, it decreased with increasing prebake temperature or blocking level. Therefore, it was considered that the concentration of remaining solvent in the resist film corresponds to one of the acid diffusion paths. Moreover, hydrophilic OH sites, whose concentration was the reciprocal of the blocking level, were also considered to correspond to a diffusion path in the polymer matrix, based on the fact that the acid diffusion length increased gradually with decreasing t-BOC blocking level, even when the solvent concentration remained constant. Based on the experimental analysis results, the existence of two diffusion paths, as well as of explicit relationships among the remaining solvent, t-BOC blocking level and acid diffusion length was confirmed.

Molecular‐beam epitaxial growth of InAs/GaAs superlattices on GaAs substrates and its application to a superlattice channel modulation‐doped field‐effect transistor

The molecular‐beam epitaxial growth conditions for (InAs)m(GaAs)n short period superlattices (SPSs) on GaAs substrates have been optimized by monitoring reflection high‐energy electron diffraction (RHEED) intensity oscillations. The RHEED oscillation measurements enable understanding InAs growth behavior on a 7% lattice‐mismatch GaAs substrate. Within one monolayer InAs deposition with lower than 560 °C growth temperature can give high SPS crystalline quality. The SPS periodic structure and the monolayer InAs formation, embedded in GaAs layers, have been confirmed by x‐ray diffraction and transmission electron microscopy measurements. The obtained thickness controllability for the SPSs is less than±6% for InAs and ±3% for GaAs. The electron Hall mobilities for modulation‐doped structures having an (InAs)1(GaAs)n SPS as an electron channel, whose layer index of n varied from 3 to 6, have been compared with those with a pseudomorphic InGaAs random alloy channel which has the equivalent In composition. The S...

High performance 60-GHz coplanar MMIC LNA using InP heterojunction FETs with AlAs/InAs superlattice layer

We describe a 60-GHz coplanar MMIC low-noise amplifier (LNA) using 0.1 /spl mu/m-gate-length InP heterojunction FETs (HJFETs). An optimum gate width of 80 /spl mu/m was determined for the first stage FET by using a small signal model including accurate scaling of the gate resistance. On-wafer noise measurements demonstrated a noise figure of 2.2 dB and a gain of 22.8 dB at 60 GHz.

Gate length scaling for Al/sub 0.2/Ga/sub 0.8/N/GaN HJFETs: two-dimensional full band Monte Carlo simulation including polarization effect

Two-dimensional self-consistent full band Monte Carlo (FBMC) simulator was developed for electron transport in wurtzite phase AlGaN/GaN heterojunction (HJ) FET. Recessed gate Al/sub 0.2/Ga/sub 0.8/N/GaN HJFET structures with an undoped cap layer were simulated, where the spontaneous and piezoelectric polarization effects were taken into account. The polarization effect was shown to not only increase the current density, but also improve the carrier confinement, and hence improve the transconductance. An off-state drain breakdown voltage (BV/sub ds/) of 300 V and a maximum linear output power (P/sub max/) of 46 W/mm were predicted for a 0.9-/spl mu/m gate device. For a 0.1-/spl mu/m gate device, 60 V BV/sub ds/, 20 W/mm P/sub max/, and 160 GHz current-gain cutoff frequency were predicted. Although there is considerable uncertainty due to lack of information on the band structure, scattering rates, and surface conditions, the present results indicate a wide margin for improvements over current performance of AlGaN/GaN HJFETs in the future. To our knowledge, this is the first report on the FBMC simulation for AlGaN/GaN HJFETs.

A study of dissolution characteristics and acid diffusion in chemically amplified DUV resist

The effects of dissolution characteristics and acid diffusion behavior on lithographic performance were evaluated in tert-butoxycarbonyl (t-BOC)-protected chemically amplified positive deep-ultraviolet (DUV) resists. The resists consisted of t-BOC-protected polyhydroxystyrene as a base resin and 2,4-dimethylbenzenesulfonic acid derivative as a photoacid generator (PAG). In particular, the line width difference between an isolated line and a dense line (iso/dense bias) was investigated by changing the post-exposure bake (PEB) temperature. As a result, clear relationships among dissolution characteristics, acid diffusion length, and iso/dense bias were obtained. Moreover, suitable dissolution characteristics and acid diffusion length for reducing iso/dense bias were clarified.

Electron focusing with multiparallel GaAs‐AlGaAs wires defined by damageless processing

Magnetoresistance modulation, resulting from electron focusing, is investigated with multiparallel GaAs‐AlGaAs wires, which are defined by electron beam lithography and damageless wet‐chemical etching. Distinct focusing peaks in magnetoresistance are observed, although the samples have wires longer than a ballistic mean free path, which is derived from the focusing peak intensity. Specularity coefficient p for the etched region boundary is also obtained as p≊1 from the focusing peak ratio. The mechanism limiting ballistic transport in the electron focusing condition is discussed, based on the temperature and the electron density dependence of the peak intensity.