Yasuhiko Sato

Application of Zn-Compound Buffer Layer for Polycrystalline CuInSe2-Based Thin-Film Solar Cells

In order to meet the requirement for the development of a more environment-friendly device structure for polycrystalline CuInSe2-based thin-film solar cells, Zn compound buffer layers for use as an alternative to CdS buffer layers have been fabricated by a chemical-bath deposition (CBD) method which is believed to be cost-effective and applicable to large-area deposition. CIS-based thin-film solar cells with a Zn(O, OH)x buffer layer showed a relatively low efficiency of about 10% because of the difficulty in the reduction of the amount of OH- ions in the buffer layer. By adding a sulfur source to the CBD solution as a new approach to reduce the amount of OH- ions, a dramatic decrease in the content of hydroxide in the buffer layer and the formation of a better heterointerface between the CIS-based thin-film absorber and the ZnO window layer were achieved simultaneously for the first time. The application of Zn(O, S, OH)x as a buffer layer to CIS-based thin-film solar cells led to the higher efficiency of 12.8% on the active area of 3.2 cm2. Zn(O, S, OH)x fabricated by a CBD method was demonstrated to be a promising alternative to CdS and a more environment-friendly buffer layer, although further investigation is required to understand the mechanisms of the irradiation effect.

Novel antireflective layer using polysilane for deep ultraviolet lithography

A bottom antireflective layer (ARL) is essential for deep ultraviolet (UV) lithography. We have just developed a polysilane antireflective layer (PSARL) process. PSARL can act as an antireflective layer due to the Si–Si bond which has absorption in the deep UV region, and it can be etched with high selectivity to resist under etch conditions that use a chlorine plasma which is a suitable inorganic silicon etchant. Similar to the conventional organic ARL process, PSARL can be simply spin coated onto the substrate, and removed with an asher using an O2/CF4 gas mixture. The partially cross-linked diphenylsilane copolymer was evaluated with a view to its application as an ARL material and the results obtained are presented in this article.

Application of polysilanes for deep-UV antireflective coating

Application of polysilanes for a deep UV (DUV) bottom anti- reflective coating (BARC), in order to resolve the problem posed by the insufficient anti-reflection with thin conventional organic BARC applied on transparent dielectric film, is described. The newly developed polysilane anti- reflective coating has the real part of refractive index, n equals 2.00, and the imaginary part, k equals 0.23 at 248 nm. The polysilane coating is immiscible with a chemically amplified photoresist, and is not removable during normal wet development of photoresist. Etching rate of the polysilane is 2 times faster than that of DUV resist during BARC etching, and lower than that of DUV resist during dielectric film etching. The polysilane layer is easily removed by ashing using O2 gas process. Using thick polysilane coating, it can realize both the suppression of the interface reflection between the resist and BARC and good critical dimension control on dielectric film.

A New Stacked-Mask Process Utilizing Spun-on Carbon Film for Sub-130-nm Etching

A novel stacked-mask process (S-MAP) was developed for sub-130-nm etching. The S-MAP consists of a tri-layer with a top layer of thin resist for patterning, a spin-on-glass (SOG) interlayer, and a newly developed spun-on carbon-film bottom layer. The spun-on carbon film was synthesized, in particular to have high carbon and low oxygen contents, and showed an etching resistivity 1.3 times higher than that of conventional resist films. An S-MAP utilizing this spun-on carbon film provides improved critical dimension (CD) control and enables etching of high-aspect-ratio holes. Via first dual damascene (DD) formation employing S-MAP eliminated the undesirable SiO2 fence around the top of the via holes, whereas the conventional process does not.

Characterization of Microbridges Generated on Negative Resist Patterns

We analyze the mechanism of microbridge formation on chemically amplified negative resist patterns through direct observation of progressive development in alkali-developers. The microbridge formation was attributed to contact occurring between adjacent resist patterns due to swelling of the resist. These swollen resist patterns contracted upon immersion in deionized water; however, bridge structures remained as microbridges between these patterns. The swelling of the resist films occurred more easily at defocus positions than at a focus center, and the microbridges localized near the top of the pattern at defocus positions and delocalized at a focus center. An optical simulation suggested that both the crosslinking density and its distribution determine the number and position of the microbridges. A scaling law governing microbridge formation is observed as a function of focus position and exposure dose.

Optimization of polysilane structure as fast-etching bottom antireflective coating for deep ultraviolet lithography

A bottom antireflective coating (BARC) is essential for deep ultraviolet lithography. We have already reported the BARC composed of polysilanes which can be spin coated and etched faster than resists. In this study, polysilane structures are optimized in order to set the etch selectivity against resists to a higher level than that previously reported without losing the antireflection performance. The results indicate that the networked polysilanes structure is the most suitable. Poly(methylhydrosilane) whose Si–H is partially cross-linked and has the highest silicon content of 64.8 wt% yields the optimal results. The resist profile is achieved on it without footing and residue. The refractive index at the wavelength of KrF excimer (248 nm) is n=1.93, k=0.32, and the polymer reduces multi-reflection in both resists and in transparent substrates. The etch selectivities are 4.8 under Cl2 plasma and 6.6 under HBr plasma, which are much higher than that of an organic BARC, namely, about 1.

Spun-on carbon antireflective layer with etch resistance for deep and vacuum ultraviolet lithography processes

Dry etch resistance and antireflective performance were studied for a film containing a high amount of carbon (83.4 wt %), which was named the spun-on carbon film. The film was formed by using a carbon cluster precursor synthesized by reductive coupling of a mixture of carbon tetrabromide and phenylbromide. The refractive indices of the spun-on carbon film at the exposure wavelengths of excimer lasers are n=1.72, k=0.35 (KrF), n=1.46, k=0.67 (ArF), and n=1.37, k=0.14 (F2). A bilayer bottom antireflective coating system composed of upper spun-on glass (SOG) and lower spun-on carbon was evaluated. By optimizing the SOG thickness, the reflectivity is reduced to 0.2% (KrF), 3.3% (ArF), and 0.5% (F2). Remarkable improvement is observed at the KrF and F2 wavelengths. Resist profiles are obtained without any footing, residue, or standing wave using the KrF and ArF scanning steppers. The etch resistance of the spun-on carbon film is 1.34 times greater than that of the thermally oxidized novolak film (i.e., a conv...

Electrical Characteristics in Al/ZnO/SiO2/Si Structure

Al/ZnO/SiO2/Si structure is experimentally studied for application to SAW devices and other progressive devices. The good agreement between the experimental and theoretical value of Cac/Cinv shows that the high density interface states do not exist at the SiO2-Si interface of the Al/ZnO/SiO2/Si (MZOS) structure. It is found that hysteresis characteristics of the MZOS?C versus the gate bias depends on the voltage sweep rate and the resistivity of ZnO film. It is confirmed that the quasi-stable flat band voltage of the MZOS-C depends on the ZnO thickness and sputtering rf power.

Application of polysilazane to etch mask in pattern transfer processes for deep and vacuum ultraviolet lithography

A polysilazane is investigated for a spin-on glass (SOG) used for a middle layer in a trilevel resist system. Higher film density is required for the middle layer to obtain higher etch resistance during the underlayer etching. The compositions of the polysilazane baked at 200°C and 300°C are Si0.42O0.34C0.40N0.20 and Si0.29O0.65C0.10N0.05, respectively, which are determined by x-ray photoelectron spectroscopy. The polysilazane is converted to silicon-oxide-like compositions by baking at 300°C. The film density and etch rate of SOG made from polysilazane are compared with that of polysiloxsane on condition that both films are baked at 300°C. The film density of the SOG made from the polysilazane is 2.07 g/cm3, which is higher than 1.87 g/cm3 of the conventional SOG made from a polysiloxsan. The etch resistance of the SOG made from the polysilazane is improved by 90% compared with that of the SOG made from the polysiloxsane due to the increased film density.

Application of electron beam cured spin-on glass to tri-level resist system for deep and vacuum ultraviolet lithography

A positive chemically amplified (CA) resist can cause footing because the acid in the CA resist diffuses into the spin-on glass (SOG) film due to its porous structure. Densifying the SOG film with electron beams (EB) is proposed in order to solve this problem. This method not only avoids footing but also increases the etch resistance of SOG for etching organic underlayers precisely.