Yoshio Yamashita

Radiation-induced reactions of chemically amplified x-ray and electron-beam resists based on deprotection of t-butoxycarbonyl groups

Radiation-induced reactions in chemically amplified resists based on deprotection of t-butoxycarbonyl groups have been investigated by both time-resolved (the pulse radiolysis methods) and steady-state optical absorption spectroscopy. Upon exposure of a partially tBOC-protected novolak by electron and synchrotron radiation beams, the yields of the intermediates contributing to the acid generation (phenoxyl radical and proton adducts of base resin) decreased with increasing the protection ratio of hydroxyl groups. Therefore, the efficiency of the acid generation is closely related with the protection ratio. The relation of the acid generation mechanism with the protection ratio was discussed.

Improvement of Phase-Shifter Edge Line Mask Method

Two kinds of improved phase-shifter edge line (PEL) masks have been developed for critical dimension control. One new mask consists of a chrome line and partially overlapped shifter film. In this mask, exposed pattern sizes are controlled by adjusting the chrome linewidth. It has been found that a set of repeated fine-chrome lines (blind) acts as a uniform light attenuator. The other new mask (blind PEL mask) has been devised by combining the blind and shifter pattern. These new masks make it possible to delineate different-sized fine patterns (≥0.2 µm) at the same exposure dose. With the blind PEL-mask, unnecessary shifter edge patterns can be eliminated by only one-time exposure.

0.2 mu m or less i-line lithography by phase-shifting-mask technology

A novel phase-shifting technology has been developed for improving resolution of isolated patterns. A phase-shifter edge line (PEL) is used as an opaque mask instead of a chrome pattern. The intensity distribution of the PEL is sharper than those of chrome patterns under the same illumination conditions. 0.15 mu m wide space patterns in LMR-UV (negative resist) are delineated in the focus range of more than 1.5 mu m by using a PEL-mask and an i-line stepper. In the same manner, 0.15 mu m wide line patterns are clearly formed by use of PFR-TT15 (positive resist). In addition, a new modification adapted to hole or pillar patterns has been developed by applying double exposure with a pair of PEL-masks. By this method 0.2 mu m hole patterns in LMR-UV are successfully delineated in a wide focus latitude of 1.5 mu m.<<ETX>>

Sub-Half-Micron i-Line Lithography by Use of LMR-UV Resist

Negative photoresist, LMR-UV, has been developed for i-line lithography. It resolves isolated 0.3 µm space and 0.35 µm hole patterns of 1.0 µm thickness by using a 0.42-numerical-aperture i-line reduction projection aligner. LMR-UV gives overhung profiles because of its large absorption coefficient of 3.8 µm-1 at the i-line. New phase-shifting mask patterns which are adapted to isolated space and hole patterns for negative resist have been developed. By use of this mask, LMR-UV clearly resolves 0.25 µm space and 0.3 µm hole patterns by using the i-line aligner. This phase-shifting mask improves both resolution and focus margin.

Subhalf‐micron patterning of negative working resist by using new phase‐shifting masks

A lithographic technique using phase‐shifting masks combined with i‐line reduction projection printing in negative working results has been evaluated for subhalf‐micron patterning. The combination of a ‘‘standard’’ phase‐shifting mask and image reversal resist, AZ 5214E (from Hoechst), makes it possible to resolve 0.3‐μm line and space patterns with practically enough focus latitude on a 0.42 numerical aperture i‐line aligner (σ:0.5). New phase‐shifting masks which are adapted to isolated spaces and holes for negative working resist have been developed. Of these new masks, a mask whose patterns consist of only shifter film gives excellent results. By using this mask and the i‐line aligner, 0.3‐μm hole patterns are clearly resolved in LMR‐UV (low‐molecular‐weight resist for UV lithography resist) film.

Sub-Quarter-Micron Gate Fabrication Process Using Phase-Shifting Mask for Microwave GaAs Devices

A phase-shifting mask technique using an i-line stepper was applied to the gate formation process of AlGaAs/GaAs high electron mobility transistors (HEMTs). To obtain a fine gate pattern of less than a quarter micron, the phase-shifter edge-line mask, which has the highest resolution and wide focus margin for an isolated pattern, was used and the double exposure process was developed to form a real gate pattern. The controllable gate length was in the range of 0.50-0.15 µm. By using this technique, 0.18 µm-gate HEMTs with good and uniform microwave performances in a 3-inch wafer were obtained. This technique has great advantages for applications to microwave GaAs devices and ultrahigh-speed GaAs LSIs.

Acid generation process by radiation‐induced reaction in chemically amplified resist films

A new interpretation of the acid generation process in novolak resin systems is reported. To investigate the acid generation process in chemically amplified resist, a novolak resin system with an acid generator (PAG) was employed. In order to analyze the acid generation process, the visible absorption characteristics from a conventional spectrophotometer, and nanosecond and millisecond pulse radiolysis systems, were analyzed. For m‐cresol novolak with triphenylsulfonium triflate, it was hypothesized that acid would be generated during exposure; however, it is asserted that a protonated intermediate was generated during exposure and that acid was generated during exposure, and more acid was formed during PEB. The yield of the acid during PEB, rather than that of the protonated intermediate during exposure, was strongly influenced by the presence of triphenylamine. A comparison of the experimental result for m‐cresol novolak which contains PAG with that for p‐cresol novolak which also contains PAG indicates...

Fabrication of X-Ray Mask Using W-CVD for Forming Absorber Pattern

Tungsten chemical vapor deposition (W-CVD) using WF6 and H2 as reactants was applied to forming absorbers of X-ray masks for synchrotron radiation (SR) lithography. For this purpose, the properties of deposited W (CVD-W), such as stress, density and thermal stability, were examined. The stress can be minimized reproducibly to less than 1×108 dyn/cm2 by controlling the flow rate of WF6 at various substrate temperatures. This W film was thermally stable up to 200°C, and the density was 18.5 g/cm3. From these results, CVD-W was found to be suitable as an absorber material. For the formation of absorber patterns, filling SiO2 grooves with stress-reduced CVD-W was examined. The 0.2-µm-wide grooves were filled with CVD-W and absorber patterns were formed by etch-back. The X-ray mask was successfully fabricated by this developed process.

Proton Transfer to Melamine Crosslinkers in X-Ray Chemically Amplified Negative Resists Studied by Time-Resolved and Steady-State Optical Absorption Measurement

Proton transfer to melamine crosslinkers has been analyzed in three X-ray chemically amplified (CA) negative resists. One is an in-house fabricated resist composed of m-cresol novolac, 2,4,6-tris (trichloromethyl)-1,3,5-triazine (TCT), and hexamethoxymethylmelamine (HMMM). The other two are commercially available resists: SAL-601 (Shipley Co.) and AZ-PN 100 (Hoechst Co.). The behavior of protons induced by X-ray exposure was analyzed using both conventional steady-state optical absorption spectroscopy and nanosecond time-resolved absorption spectroscopy (pulse radiolysis). The results indicate that the protons are transferred from the proton adducts (acid precursors) of matrix polymers to the melamine crosslinker in a very early step. This proton transfer was much faster than that from the proton adducts of matrix polymers to dissolution inhibitors in CA positive resists.

Study on x-ray irradiation stability of absorber materials for x-ray masks by stress measurement

We fabricated photomasks by means of a surface imaging technique using electron beams. For this purpose, a bi-level resist process was used. The resist consisted of three- dimensional polysilphenylenesiloxane (TSPS), a negative resist containing silicon, at the top and sulfonated polyaniline (SPAn), an electrically conductive polymer film, at the bottom. TSPS is known to be sensitive to electron beams. It swells slightly during development and it is easily oxidized. These features together make TSPS a good top coat substance. It is also know that the bottom layer substance SPAn reduces substrate charging because it is electrically conductive. By taking advantage of this resist system, we used O2-RIE (oxygen reactive ion etching) for transferring surface images from the top to bottom layer. This paper discusses the performance of photomasks which we fabricated by combining this bi-level resist process with variable-shaped electron-beam lithography system (acceleration voltage of 20 keV and current density of 10 A/cm2). By using this process, we succeeded in obtaining 0.2 micrometer line/space patterns. In addition since the multilayer resist system could reduce proximity effect, it was possible to produce reticles for 1-Gbit DRAM devices without any data correction.