Haruo Iwasaki

A 2.5 ns clock access 250 MHz 256 Mb SDRAM with a synchronous mirror delay

A 245.7 mm/sup 2/ 256 Mb SDRAM uses: (1) 60.2% cell-occupancy ratio array, (2) prefetched pipeline using first-in first-out buffer with parallel/serial converter, (3) synchronous mirror delay circuit.

Dissolution kinetics analysis for chemically amplified deep ultraviolet resist

Dissolution kinetics of a chemically amplified deep-ultraviolet (DUV) positive resist, which consists of tert-butoxycarbonyl (t-BOC)-protected phenolic resin, benzenesulfonic acid derivative as a photoacid generator (PAG) and an additional dissolution inhibitor, has been investigated by focusing on the t-BOC-protected phenolic resin structure (t-BOC protection ratio/molecular weight M w) and postexposure bake (PEB) temperature. Based on the analysis of the dissolution rate curve and Arrhenius plots, it was concluded that only one mechanism, namely, the penetration of tetramethylammonium hydroxide (TMAH) developer into hydrophobic t-BOC resin, rules the dissolution kinetics. It was also found that a steep slope of the dissolution rate curve is very effective for improving resolution capability. Polymer structure, as well as the PEB condition, was optimized from the correlation between dissolution kinetics and resist performance, and good resist performance was realized.

High-transmittance rim-type attenuated phase-shift masks for sub-0.2-μm hole patterns

The lithographic performance of high- and standard- transmittance attenuated phase shift masks (PSMs) was investigated in order to determine the suitability of applying attenuated PSMs to the fabrication of 0.15-micrometers hole patterns. Both PSMs had rim structures to eliminate side lobes, and they have two layers on the quartz substrate: a chromium-fluoride attenuated phase shifter layer and an opaque chromium layer. Both PSMs had similar lithographic performances that were high enough for 0.15 micrometers hole patterns.

Attenuated phase shift masks reducing side lobe effect in DRAM peripheral circuit region

We applied deep UV attenuated phase shift masks (PSMs) to the quarter micron level contact hole pattern layer of a DRAM. There were two different hole sizes: 0.26 micrometer hole in the memory cell region, and 0.35 micrometer in the peripheral circuit region. We examined two methods to reduce the side lobe effects in the peripheral circuit region. The first method was a chrome (Cr) shield method: the peripheral circuit region was covered by Cr. The second method was a mask bias method: large mask bias was added to contact hole patterns in memory cells. Both methods sufficiently reduced the side lobe effect in the peripheral circuit region.

Acid size effect of chemically amplified negative resist on lithographic performance

The chemically amplified negative resist, composed of polyvinylphenol derivative as a base resin, melamine derivative as a crosslinker and alkyl sulfonic acid derivative which has several number of carbon atoms (n equals 3, 4 and 8) as a photoacid generator PAG, has been investigated. The absorbance of this resist system remains constant even in the introduction of different alkyl sulfonic acid type PAG materials. The resolution, as well as the depth of focus, was not so different from each other within the carbon number from n equals 3 (n- and iso-propyl) to n equals 4 (n-butyl). In the case of n equals 8 (n-octyl), however, the resolution capability was degraded drastically and the depth of focus was also reduced. Furthermore, the carbon number of alkyl sulfonic acid affected the resist profile, that is, rounded profile (n equals 3) became rectangular (n equals 4) or overhang (n equals 8) profile. Especially, very heavy overhang profile was observed in the case of n equals 8, therefore, good resolution could not be achieved. And, the standing wave effect at the side wall increased with increasing carbon number. Relationship between lithographic performance and acid size was investigated by evaluating the dissolution rate characteristics. The fact that the dissolution kinetics were almost the same in the cases of n equals 3 and n equals 4 but degraded in the n equals 8, was consistent with lithographic performance as mentioned above. Based on these results, the most suitable acid size for this chemically amplified negative resist could be determined.

Optimization of dissolution-rate characteristics of chemically amplified positive resist

The dissolution kinetics of a 3-component chemically amplified DUV positive resist (base resin; t-BOC protected polyhydroxystyrene, PAG; benzenesulfonic acid derivative, dissolution inhibitor; t-BOC protected bisphenol A;) was investigated under various conditions. Particularly, the effects of the dissolution inhibitor on the dissolution rate characteristics together with the effect of the t-BOC protection ratio of the base resin were studied. Moreover, in order to obtain ideal dissolution characteristics, a resist profile simulation analysis was carried out. The dissolution rate contrast as well as the slope N of log(dissolution rate)-log(exposure dose) plots increased with increasing inhibitor concentrations. However, a very high inhibitor concentration induced a severe standing wave effect and T-topping profile, resulting in the deterioration of resist performance. Therefore, it was discovered that inhibitor concentration has an optimum value in this resist system. These results are similar to those of t-BOC protection ratio in base resin. According to resist simulation, the slope N was closely related to resist performance, but the dissolution rate contrast was not. A slope value of more than 15 was necessary for 0.25 micrometers patterning (focus margin > 1.3 micrometers ). In addition, the acid diffusion length was one of the key factors: suitable diffusion length was essential for achieving a good resist profile; short diffusion length induces a severe standing wave profile and a long diffusion length deteriorates resolution capability. Based on the analysis of the dissolution characteristics and resist profile simulation, it was concluded that the optimization of the base resin structure and inhibitor concentration is effective for modifying the dissolution parameter. Moreover, it was also found that a steep slope (> 15), moderate dissolution contrast (approximately 10,000) and adequate acid diffusion length (approximately 35) can bring about ideal dissolution characteristics leading to 0.25 micrometers pattern formation.

Fabrication and evaluation of chromium/phase-shifter/quartz structure phase-shift mask

The deviation of phase shift angle from 180° seriously deteriorates the focus latitude. In order to obtain the expected performance of phase shift mask, a Chromium(Cr)/Phase-Shifter/Quartz(Qz) structure is investigated. In this phase shift mask structure, the shifter thickness i.e., phase shift angle, can be precisely controlled, compared with a conventional Shifter/Cr/Qz structure. Spin-on-grass(SOG) is used as the phase shifter material because of its excellent thickness uniformity. Alternating phase shift mask that has the Cr/SOG/Qz structure was fabricated using Ar-laser writing method, and evaluated using a NA=0.45, 6=0.3-0.5,I-line stepper. Obtained results show that this phase shift mask structure is very promising for the subhalfmicron pattern formation.

Fabrication of the 70-nm line patterns with ArF chromeless phase-shift masks

The problems of chromeless phase shift masks (CL-PSMs), which cannot fabricate large patterns, can be overcome by using CL-PSMs that have opaque chrome (Cr). This paper presents evaluation results for these enhanced CL-PSMs. We exposed with an ArF scanner of 0.60 numerical aperture with annular illumination and we used a positive chemically amplified ArF resist 0.21 μm thick. We did not use assist bars. For it was difficult to make assist bars which were smaller than sub-70-nm main patterns. We obtained good critical dimension controlled patterns with bias optical proximity correction. The mask error enhancement factors were about 1 for >300-nm pitch patterns. The resist pattern profiles were good. The depth of focus of isolated line patterns was about 0.3 μm. We could fabricate random logic patterns that had various pattern widths from 70 nm to more than 100 nm.

Fabricating 0.10-μm line patterns using attenuated phase-shift masks

On fabricating the next generation logic-devices, printing 0.10-um gate patterns is a critical issue. So one suggests the application of the some strong phase-shift masks (PSMs) to print 0.10-um lines. However, those processes have some difficult problems: the defect inspection, the phase shift error repair, etc.. Accordingly, we evaluated the application of the ArF attenuated PSM to printing the 0.10-um lines. We made the ArF attenuated PSM with 6% transmittance. In addition, that mask was exposed with an ArF scanner, NA = 0.60. A 0.4-um thick chemically amplified positive resist was used. We evaluated the performance and the optical proximity effect under these conditions. We adjusted an exposure dose to yield 0.10-um-line width for a 0.10-um line and 0.15-um space designed pattern. The critical dimension (CD) variation for the target CD is 23 nm from 0.10-um line and 0.15-um space to 0.1-um isolated line. In that region, we could suppress the CD variation with biasing. A common depth of focus (DOF) between 0.10-um line and 0.15-um space and 0.1-um isolated line was 0.4 um. This DOF value is large enough to apply the ArF attenuated PSM to the logic gate. In conclusion, we could fabricate 0.10-um lines with the ArF attenuated PSM in more than 0.15-um space region. The satisfactory DOF was obtained in that region. These results were obtained using annular illumination. We are going to refer to the performance comparison between annular and conventional illumination in our paper.

Dissolution characteristics optimization for chemically amplified positive resist

Dissolution kinetics of a 3-component chemically amplified positive resist, which consists of t-BOC protected phenolic resin, benzenesulfonic acid derivative as a PAG and an additional dissolution inhibitor, have been investigated under various conditions. Especially, the effects of t-BOC protection ratio and molecular weight of the base resin have been studied. In a previous paper, we reported that the dissolution rate R of a 2- component positive resist was determined by one rate determining step, i.e., developer penetration into hydrophobic t-BOC protected phenolic resin. Rapid formation of surface insoluble layer which induced T-toping profiles deteriorated its original resist performance. In this paper, we evaluated both t-BOC protection ratio and the molecular weight dependencies of the dissolution characteristics. The 3-component resist evaluated did not produce distinct T-topping profile when time duration between exposure and PEB was within 30 minutes.