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In this paper, we show a novel method to obtain small size textures usable in crystalline silicon (c-Si) solar cells. SiO2-based glass microparticles are mixed with a conventional KOH-based alkaline solution for making the textures. Using this mixing method, the texture size can be drastically reduced from 10 to ≤2 µm (0.3–2 µm). In addition, the process time and c-Si loss during the texture formation are reduced from 25 to 2 min and from 20 to 2 µm, respectively. Thus, the process is applicable to c-Si with thickness down to 50 µm. High-quality passivation showing the effective minority carrier lifetimes (τeff) larger than several ms and effective antireflection coating are possible on the new textures. The process is named “microparticle-assisted texturing (MPAT) process”, and its features are also demonstrated.
Phosphorus (P) doped ultra thin n+-layer is formed on crystalline silicon (c-Si) at low substrate temperatures of 80 – 350 °C using radicals generated by the catalytic reaction of phosphine (PH3) with a tungsten catalyzer heated at 1300 °C. The sheet carrier concentration obtained by Hall effect is in the range between 3×1012cm-2 and 8×1012cm-2. The distribution of P atoms obtained by secondary ion mass spectrometry (SIMS) indicates that P atoms locate within the depth of 4 nm from surface and the profile has almost the same distribution independent of any doping conditions such as substrate temperature or radical exposure time. The sheet carrier concentration is 1.15 – 2.12% of the amount of P atoms incorporated through the radical doping. The ratio of activated donors increases with substrate temperature during the radical doping, suggesting that P-related species bonded on the c-Si surface require thermal energy for their activation. Using the n+-layer formed by radical doping, the reduction of surface recombination velocity for n-type c-Si wafer is attempted. The effective minority carrier lifetime of the n-type c-Si sample covered with 6-nm-thick intrinsic amorphous Si (i-a-Si) layers on both side increases from 32 μs to1576 μs by the radical doping of P atoms to n-type c-Si surface, suggesting that the radical doping can be utilized for the formation of passivation layers on a-Si/ n-c-Si hetero-interface.
A novel method to make low-resistivity metal lines in assembled silicon (Si) integrated circuit (IC) chips or other semiconductor chips with high-speed and low-cost is demonstrated. In the method, functional silver (Ag)-liquid (Ag-ink) which contains Ag nanoparticles (NPs) in organic solution is used to draw metal-lines in trenches formed on a plastic substrate by imprint technology. Surface energy of trenches is modified by exposing the substrate to ultra-violet (UV) light with the purpose of concentrating the functional Ag-ink into trenches by capillary effect in order to connect with electrodes of Si chips. The resistivity of such metal-lines can be lowered to 4×10-6 Ωcm by exposing the Ag metal-lines to hydrogen (H) atoms generated by catalytic cracking reaction with a heated tungsten catalyzer. X-ray photoelectron spectroscopy (XPS) proves that H atoms can remove organic compounds surrounding Ag NPs, resulting in the low-temperature sintering of NPs as confirmed by scanning electron microscopy (SEM). The method is promising for low-cost fabricating of IC cards or other electronic devices utilizing assemble of many semiconductor chips.
Properties of p-type μc-Si prepared by Cat-CVD (Catalytic Chemical Vapor Deposition), often called Hot-Wire CVD, are studied for possible application to window layer of a-Si solar cells. Electrical, structural and optical properties are investigated. It is concluded that Cat-CVD p-type μc-Si is a suitable material as a window layer for Cat-CVD a-Si solar cells.
Rapid progresses are achieved in catalytic CVD (Cat-CVD), often called hot-wire CVD, in the past 3-years NEDO national project in Japan. Cat-CVD technology presents many advantages in thin-film formation processes; high-efficiency of gas use, large-area deposition, no ion bombardment and low-temperature deposition even below 200°C. All of the elemental techniques for the industrially applicable Cat-CVD apparatuses, such as the suppression of the metal contamination, the precise control of the substrate temperature, the life extension of the catalyzer, 1-m size uniform deposition and the chamber cleaning, have been completely developed. Sophisticatedly designed substrate holder with electrostatic chuck and showerhead equipped with catalyzers are both key technologies for these achievements. High reproducibility for film properties is also obtained by controlling the reaction between high-density radicals and chamber walls. Prototype mass-production apparatus for SiNx passivation films in GaAs devices has been already developed and this will be probably the first application of Cat-CVD in industry. These recent movements appear to promise the drastic revolution in semiconductor and flat-panel display industries by introducing Cat-CVD in very near future.
Large grain-size polycrystalline Si (poly-Si) films are obtained on glass substrate by newly developed catalytic chemical sputtering method at low temperatures around 400 C. Si films are also epitaxially grown on (100) single-crystalline Si substrates. In the method Si films are deposited by the chemical transport of SiH4 species generated by the reaction between solid Si target and catalytically generated H atoms. Efficient deposition is realized using the remarkable difference in the etch rate depending on Si target temperatures. That is, SiH4 species are efficiently generated on cooled Si target by atomic-H etching and deposited on substrates with suppressed etching phenomena by heating. Full-width at half maximum of transverse-optical Raman signals originating from crystalline phase for the obtained poly-Si films is narrower than that for poly-Si prepared by excimer-laser annealing. It was noticeable that the grain size exceeds 1 m for the films with a thickness of about 1 m. Growth mode of poly-Si films especially in the initial stage is remarkably changed with a difference in the substrate material. It was found that formation of seed layer enhances the growth of poly-Si films on glass substrate.
This paper reports structural and electrical properties of catalytic-nitrided silicon dioxide (SiO2) films. The surface of SiO2/Si(100) was nitrided at temperatures below 573 K. It was found that the incorporated N atoms are bound to Si atoms and O atoms and located on the top-surface of SiO2. Catalytic-nitrided SiO2 films have small amounts of Si-OH bonds and adequate resistance to boron (B) penetration.
Thermal influence on film preparation in catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD, is investigated since heat radiation from catalyzer and thermal transport by introduced gases are one of the dominant factors to determine the film properties. It was found that catalyzer-surface area is a key parameter determining not only the deposition rate but also the thermal influence. “Catalytic platE” instead of the conventional wire was proposed in order to suppress the heat radiation with keeping the catalyzer-surface area. Cat-CVD method employing the catalytic plate is superior to conventional “hot-wirE” CVD method.
Gas-phase and surface reactions of transported species decomposed on the catalyzer were investigated in catalytic CVD (Cat-CVD), often called hot-wire CVD, using a specially designed reactor tube. The phenomena were comparatively studied using H2- or He-diluted SiH4. It turned out that the control of gas flow through the catalyzer between the gas showerhead and the substrate is a key factor to obtain high uniformity in not only the film thickness but also the crystallinity for Si films prepared by Cat-CVD using the gas pressure above about 10 Pa and the gas-flow velocity faster than several m/s.
Feasibility of SiNx passivation films at low substrate temperatures prepared by catalytic chemical vapor deposition (Cat-CVD) is studied for ferroelectric nonvolatile random access memories (FRAMs). SiNx films were prepared at low substrate temperatures of 100 °C, 175 °C and 200 °C using Cat-CVD. Adjusting on flow rate ratio of SiH4/NH3, the refractive index of SiNx film, deposited at 175 °C and 200 °C, measured by ellipsometry is controlled around 2.0. SiNx films, with the refractive index around 2.0, deposited at only 200 °C show the following properties. 1) No oxidation during air exposure for 3 months was observed for the films. 2) Etching rate of the films in buffered HF is 20 nm/min. The dense SiNx film, which is resistive for oxidation in air exposure and dissolution in buffered HF, is prepared at 200 °C and the film is suitable to the passivation of ferroelectric capacitors.
This paper reports a procedure for low-temperature nitridation of silicon dioxide (SiO2) surfaces using species produced by catalytic decomposition of NH3 on heated tungsten in catalytic chemical vapor deposition (Cat-CVD) system. The surface of SiO2/Si(100) was nitrided at temperatures as low as 200°C. X-ray photoelectron spectroscopy measurements revealed that incorporated N atoms are bound to Si atoms and O atoms and located top-surface of SiO2.
This is to review the present understanding on Cat-CVD (catalytic chemical vapor deposition) or hot wire CVD. Firstly, the deposition mechanism in Cat-CVD process is briefly mentioned along with key issues such as the effect of heat radiation and a method to avoid contamination from the catalyzer. Secondly, the properties of Cat-CVD Si-based thin films such as amorphous silicon (a-Si), polycrystalline silicon (poly-Si) and silicon nitride (SiNx) films are demonstrated, and finally, the feasibility of such films for industrial application is discussed.
This is to report the feasibility of ultra-thin silicon nitride (SiNx) films, prepared by catalytic chemical vapor deposition (Cat-CVD) method, as an ultra-thin gate insulator. In the Cat-CVD method, the deposition gases such as a gaseous mixture of silane (SiH4) and ammonia (NH 3) are decomposed by catalytic cracking reactions with a heated tungsten catalyzer placed near substrates, and SiNx films are formed at substrate temperatures around 300°C without using plasma. In the paper, additionally the effect of post-deposited treatments by using NH3-decomposed species or hydrogen (H2)-decomposed species formed by catalytic cracking of NH3 and H2 are also studied. It is found that a small hysteresis loop is seen in the C-V curve of as-deposited Cat-CVD SiNx films and that the leakage currents with thickness of 3nm equivalent oxide thickness (EOT) is slightly larger than that in the conventional thermal SiO2 of similar EOT. However, it is also found that the properties of Cat-CVD SiNx films are drastically improved by the post-deposited H2 or NH3 treatments, that is, the hysteresis loop disappears and the leakage current decreases by three orders of magnitude.
Ultra-thin silicon dioxide films can be formed at temperatures as low as 220°C by direct oxidation of Si, using active oxygen species generated by tungsten catalytic reaction in a catalytic chemical vapor deposition (Cat-CVD) system. The structural and electrical properties of such a films are investigated. It is found that the density of Si atoms in intermediate oxidation states and the density of films determined from etch rate in dilute HF solution were comparable to those of the films by a conventional thermal oxidation at 900°C. The electrical properties, breakdown electric field and leakage current were also comparable to those of thermally oxidized films.
This paper reports a feasibility of Cat-CVD system for improvement in characteristics of ultra thin gate dielectrics. Particularly, the effects of post deposition catalytic anneal (Catanneal) by using hydrogen (H2)-decomposed species or NH3-decomposed species produced by catalytic cracking of H2 or NH3, are investigated. The C-V characteristics are measured by MIS diode for the 4.5nm-thick Cat-CVD SiNx and 8nm-thick sputtered SiO2 for comparison. The small hysteresis loop is seen in the C-V curve of both SiNx and SiO2 films as deposition. However, it is improved by the Cat-anneal using H2 or NH3, and the hysteresis loop completely disappears from the C-V curves for both films. This result demonstrates that the Cat-anneal is a powerful technique to improve quality of insulating films, such as Cat-CVD SiNx and even sputtered SiO2 films. In addition, the leakage current of SiNx, films with 2.8nm equivalent oxide thickness is decreased by several orders of magnitude than that of the conventional thermal SiO2 of similar EOT and the breakdown field is increased several MV/cm by Cat-anneal at 300°C.
A novel method to prepare nanometer-size patterns by using currently available massproduction technology is proposed. In this study, a contact pattern-mask with nanometersize slit is fabricated by combination of photolithography and anodic oxidation of metal. The slit width of the pattern-mask can be controlled in the order of nano-meters by anodic voltage during oxidation of side-wall of the metal. 10nm width trench is formed in Si substrate by using such nanometer slit-mask. It is suggested that the technique can be utilized as fabrication process of the nano-scale devices.
We investigated the effects of exposure to active ammonia (NH3) gas generated by catalytic chemical vapor deposition (Cat-CVD) apparatus on ferroelectric Pb(Zr0.52Ti0.48)O3 (PZT) capacitors. It is very important to know these effects in order to apply Cat-CVD SiNx films to passivation films for ferroelectric FRAMs. The exposure to active NH3 was carried out for PZT film capacitors with two types of bottom electrodes on Si wafer at various substrate temperatures. The capacitor with Pt/IrO2 bottom electrode peeled off from substrate during exposure over 200°C. On the other hand, the ferroelectricity of the capacitors with IrO2 bottom electrodes gradually degraded from 200°C to 300°C. As a result, it is found that no degradation of the ferroelectricity is detected for exposure below 200°C. It is concluded that the Cat-CVD method is a promising candidate for preparation of the SiNx passivation film on ferroelectrics, since it is a low stressed film with low hydrogen content.
Polycrystalline silicon (poly-Si) films are obtained at temperatures lower than 400°C by catalytic chemical vapor deposition (Catalytic CVD = Cat-CVD) method, often called hot-wire CVD method. Structural properties of the Cat-CVD poly-Si films, deposited with various gas pressures, are studied by Raman scattering spectroscopy and X-ray diffraction technique. It is found that there are two recipes for obtaining device quality poly-Si films, that is, such poly-Si films are obtained at low gas pressure around 1 mTorr or less as already reported, and also at high gas pressure around 0.1 to 1 Torr. It is also found that, in addition to catalyzer temperature, the gas pressure is a key factors to obtain device quality poly-Si films at high deposition rates.
Silicon nitride (SiNx) films have been successfully synthesized by the catalytic chemical vapor deposition (cat-CVD) method using a gaseous mixture of silane (SiH4) and ammonia (NH3). In the method, the deposition gases are decomposed by catalytic cracking reactions with a high temperature (1700°C) catalyzer near the substrates, and SiNx films can be deposited at substrate temperatures lower than 400°C without using plasma or photochemical excitation. Nearly stoichiometric Si3N4 films are formed when the flow ratio of NH3 exceeds over 100 times of that of SiH4. These cat-CVD SiNx films show excellent properties. That is, the resistivity, the breakdown voltage, the chemical etch resistance and hydrogen content in the films are almost equivalent to those of high-temperature thermal CVD films. In addition, the surface diffusion length of depositing species is about several-tens μπι and step-coverage itself is conformai. Thus, the cat-CVD SiNx films are regarded not only as a new device passivation films superior to the conventional plasma-CVD films but as a gate insulator for electon devices due to their high quality.
Polycrystalline silicon (poly-Si) films are deposited at temperatures lower than 300–400°C by the cat-CVD method. In the method, a SiH4 and H2 gas-mixture is decomposed by catalytic cracking reactions with a heated tungsten catalyzer placed near substrates. Carrier transport, optical and structural properties are investigated for this cat-CVD poly-Si. The films show both large carrier mobility and large optical absorption for particular deposition conditions. The cat-CVD poly-Si films are found to be one of the useful materials for thin film transistors and thin film solar cells.