Man-Hyoung Ryoo

Control of line edge roughness of ultrathin resist films subjected to EUV exposure

The line edge roughness (LER) of ultrathin chemically amplified (CA) KrF-resist-based films was investigated using exposure to extreme ultraviolet (EUV) radiation (13.5nm). For the films between 0.09micrometers and 0.13micrometers thick, the LER was about 5~7% for a target critical dimension (CD) of 70nm and exposure to coherent illumination ((sigma) =0.01). The LER was found to be smaller in samples containing a relatively strong-acid photo-acid generator (PAG). The use of baking conditions producing greater acid diffusivity and the use of weak developer were very effective in reducing the LER of thin resist films. Atomic force microscope (AFM) observations showed the surface morphology of samples with a small LER to be very uniform. These results suggest that the use of a high-sensitivity resist and a weak developer may help to create an environment promoting uniform dissolution, thus resulting in a smaller LER in thin resist films.

Spin-on hard mask with dual-BARC property for 50-nm devices

ArF lithography is in the early stage of mass production and also is going to be further extended to 40nm generation with the aid of immersion lithography. Therefore, it is important to make ArF process production-friendly and extendible for the continuous shrinkage of design rule. Development of ArF process has proceeded with the increase of numerical aperture (NA) and the decrease of resist thickness, which are causing several problems both in mass production and development stage. NA is going to exceed unity in immersion, which necessitates the use of dual bottom antireflective coating (BARC) with increased process complexity and cost. Resist thickness, on the other hand, is expected to further decrease below 100 nm. Therefore, it is inevitable to use additional hard masks, which increases production cost due to chemical vapor deposition (CVD) process. Here we disclose our novel spin-on hard mask system with dual BARC property to overcome both problems aforementioned. Spin-on hard mask composed of two layers of siloxane and carbon materials shows high etch selectivity between thin resist and several substrates. Composition and etch chemistries of two layers are intensively studied to give CVD-comparable step-by-step etch selectivity to transfer various patterns of thin resist including line/space and contact holes to the various substrates. In addition, optical properties of two layers are finely designed from comprehensive optical simulation to be applied to various generation of ArF lithography from dry to immersion process. Such designed optical properties are incorporated to the above two layers of spin-on hard mask. This novel system is under extensive optimization to be applied to various generation of ArF lithography from mass production to the most pioneering semiconductor devices utilizing immersion lithography.

Characteristics of the Ru buffer layer for EUVL mask patterning

The characteristics of Ru film were examined to determine its suitability as a buffer layer for EUV mask patterning. When etched in an O2/Cl2 gas mixture with a high Cl2 content at a low total gas flow rate, Ru exhibited a high etching selectivity with respect to a-Si, the otp layer of a Mo/Si multilayer mirror. This could enable use of a simpler mask patterning process without any damage to the multilayer. The patterning of a mask with a TaN absorber layer and a Ru buffer layer was demonstrated. Etching the TaN with an Ar/Cl2 gas mixture yielded a high etching selectivity with respect to Ru of over 30:1. In addition, the use of Ru rather than SiO2 for the buffer layer improved the DUV inspection contrast of TaN mask patterns before and after buffer layer etching. Finally, Ru is etched more slowly than SiO2 by a focused ion beam, which makes it more suitable as a sacrificial layer during repair.

Approach to Patterning of Extreme Ultraviolet Lithography Masks using Ru Buffer Layer

The properties of Ru film were examined to determine its suitability as a repair buffer layer for the patterning of extreme ultraviolet lithography (EUVL) masks. Ru is etched more slowly than a conventional SiO2 buffer layer by a focused ion beam (FIB) for mask repair, which makes it more suitable as a sacrificial layer during repair. When etched in an O2/Cl2 gas mixture with a high Cl2 content at a low total gas flow rate, Ru exhibited a high etching selectivity with respect to Si, the top layer of a Mo/Si multilayer reflector. This could enable use of a simpler mask patterning process without any damage to the multilayer. In addition, the use of Ru rather than SiO2 for the buffer layer improved the deep ultraviolet (DUV) inspection contrast before and after buffer layer etching. The patterning of a mask with a TaN absorber layer and a Ru buffer layer was demonstrated.

Converging lithography by combination of electrostatic layer-by-layer self-assembly and 193nm photolithography: Top-down meets bottom-up

Photolithography is a part of the top-down approach that forms the basis of various processes in the semiconductor industry, which has followed Moore’s law for new generations of devices. However, as feature sizes are scaled to the nanometer regime, the bottom-up approach is being touted as a means to solve problems arising due to size reduction. Here the authors report a result based on the convergence of 193nm lithography and electrostatic layer-by-layer assembly, which overcomes the resolution limit of photolithography and also assists self-assembly to form complex patterns. Their result shows that hurdles associated with top-down approaches to further device scaling can be overcome by introducing bottom-up approaches.

Imaging results for resist films exposed to EUV radiation

The lithographic performance of DUV-based TSI, bi-layer and single-layer CA resists imaged at the EUV wavelength is reported and compared. The images exhibit a very high aspect ratio and good profiles. This means that a DUV-based CA resist process can be used directly in EUVL without modification. However, the TSI resist has poor sensitivity and the bi-layer resist has a large line edge roughness. In EUV imaging using a single-layer resist, fine images were obtained with a resist thickness of over 0.2 μm; and the maximum thickness for EUV imaging was limited to around 0.3 μm, at which the calculated transmittance for polyvinylphenol resin at the EUV wavelength is about 30%.

Pattern asymmetry correction using assist patterns

The trend of critical dimension (CD) asymmetry due to coma aberration of exposure tool and the effectiveness of assist pattern (AP) to minimize the CD asymmetry are investigated in line and space patterns by simulation and experiment. The optimum space of AP form main bar pattern to correct the CD asymmetry is about 0.3-0.4 um and seems to be insensitive to the target CD of main bar, NA and degree of coherence of lithography tool, and AP size. The results, in case of the application of AP with optical proximity correction rules achieved by simulation and experiment to logic device with 0.18 micrometers gate length, show the CD asymmetry of metrology error level and about 50 percent in-field CD uniformity improvement, as compared with those obtained before Ap application. Therefore, it is evident that the use of AP is very effective and useful to correct the CD uniformity and the CD asymmetry simultaneously.

Top barrier coating materials for immersion lithography and beyond

Immersion barrier coats were formulated and evaluated on ArF photoresist in view of interaction between photoresist and top coats. Acrylate polymers having an acid-labile protecting group, an acid group, and a polar group were synthesized to realize water barrier property and developability. To compensate the insufficient developability, thermal acid generator was included as an additive that can enhance the developability of the acrylate top coats by post exposure bake. In the course of the material evaluation, it became evident that carboxyl acid group in the top coat base polymers has great influence on photoresist profiles, and this result was fedback to a new acid group, deuterated carboxyl acid, that is suitable for both ArF wavelength and EUV wavelength. When top coat materials having deuterated carboxyl acid were applied on ArF photoresist, fine pattern profiles were confirmed. Further, an extension of barrier coating concept to EUV lithography as outgas barrier coats was examined on an EUV photoresists test sample. These outgas barrier coat materials do not include fluorine atoms, therefore, achieves good transparency at EUV wavelength.

Novel chemical shrinkage material for small contact hole and small space patterning

It is becoming difficult for the lithography progress to keep pace with the acceleration of design rule shrinkage and high integration of memory devices. In order to retain the acceleration, low k1 processes beyond the limitation of wavelength are required. Various resolution enhancement techniques have been suggested for this purpose. Especially, chemical shrinkage process utilizing an additional chemical treatment upon patterned photoresist to make patterns finer has been turned out to be effective. The current chemical shrinkage materials are, however, suffering from small attachment amounts or pattern deformation. In this paper, a novel chemical shrinkage material causing large attachments without pattern deformation is suggested. The material is an aqueous solution of two kinds of polymers and its shrinkage mechanism is based on inter-polymer complex formation and gelation principle. Compositions, shrinkage properties, and application studies to contact hole patterns are presented.

In-line chemical shrink process for 70 nm contact hole patterns by the room-temperature electrostatic self-assembly

Electrostatic self-assembly (ESA) is combined with optical lithography to develop a novel process to form 70 nm space patterns to overcome the resolution limit of ArF lithography with numerical aperture (NA) of 0.75. It is proven that patterned photo resist are useful template with specific topography to undergo the subsequent ESA. Weak polyelectrolytes are shown to control the attachment amount by adjusting pH. Puddle-assembly is applied instead of spin- or dip-assembly considering pattern profile and practicality to be used in the real FAB environment. With optimized composition and assembly method, it is successful to form 70 nm spaces patterns by ESA-induced chemical attachment above 45 nm, combined with ArF lithography of 0.75 NA. Since it works at room temperature without extra process unit after exposure and development, it overcomes the disadvantages of the conventional chemical shrink processes such as thickness loss, dependence on pattern and photo resist, and throughput lowering. In addition, in-wafer uniformities are comparable to that of forming 120 nm spaces patterns with only ArF lithography, which proves that the combination of ESA and optical lithography can be a potentially and practically alternative way of forming uniform 70 nm spaces patterns over 200 nm substrates. It also means that now it is time for top-down and bottom-up approaches to meet together to access nano world.