Tsukasa Hayashi

Large-Area and High-Speed Deposition of Microcrystalline Silicon Film by Inductive Coupled Plasma using Internal Low-Inductance Antenna

A novel inductively-coupled RF plasma source with internal low-inductance antenna (LIA) units was developed to synthesize microcrystalline silicon (µc-Si) film on a large glass substrate. A film thickness profile on a 600×720 mm2 glass substrate was achieved with high plasma uniformity and a variation of less than ±5% without a standing-wave effect. Raman and transmission electron microscope (TEM) analysis revealed that highly crystallized µc-Si films, which were directly deposited on a glass substrate, were synthesized without an amorphous-phase incubation layer at the substrate interface. A bottom-gate thin-film transistor (BG-TFT) was fabricated employing an optimized µc-Si layer and exhibited a field-effect mobility of 3 cm2/(Vs), which is one order higher than that of a typical amorphous silicon TFT.

Effects of Antenna Size and Configurations in Large-Area RF Plasma Production with Internal Low-Inductance Antenna Units

Recent trends of liquid crystal display (LCD) fabrication toward a significant enlargement of glass substrates require large-area plasma sources with a scale length exceeding 1 m. To meet this requirement, large-area plasma sources with internal low-inductance antenna (LIA) units have been developed for uniform processes, in which design principles for selecting antenna size and configurations in the multiple installation of the LIA units are established. In this study, the effects of antenna size were examined in terms of plasma production characteristics indicating small increase in plasma density with a decrease in antenna size (or antenna impedance). Furthermore, plasma density distributions with the LIA units were investigated to understand the nature of plasma diffusion, which can be utilized for designing plasma profiles with multiple LIA units. First, it was shown that the plasma density distributions followed exponential decay as a function of distance from the antenna. Secondly, the measured plasma density profiles with multiple LIA units were shown to agree well with those obtained by superposing those described by exponential functions, which can be utilized for prediction.

Enhancing memory efficiency of Si nanocrystal floating gate memories with high-κ gate oxides

High-performance floating gate memory devices of Si nanocrystal (NC) dots on HfO2 gate oxide were fabricated at temperatures below 400°C. A large counterclockwise hysteresis of 5.2V, at an applied voltage of +6V, and a stored charge density of 6×1012cm−2 were observed. Moreover, the smaller band offset of the high-κ tunneling layer resulted in higher charge tunneling probabilities towards the Si NC dots than those observed with a SiO2 tunneling layer. Advantages in terms of scaling for a high-performance and stable reliability memory device are confirmed.

Study of Deposition Process in Modulated RF Silane Plasma

The influences of plasma parameters on the deposition of a-Si:H film and particle growth have been studied with silane discharge using amplitude-modulated RF methods. Plasma parameters have been measured with the Langmuir probe system and optical emission spectrometer. Behaviors and generation processes of particles have been observed by the laser scattering method. The deposited thin film has been characterized by various techniques such as Fourier-transform infrared (FT-IR) spectrometry, ESR and the constant photocurrent method (CPM). High deposition rate with low particle density as well as high film quality has been realized for a-Si:H film by amplitude-modulated RF methods.

Nanometer‐scale characterization of SiO2/Si with a scanning capacitance microscope

A novel scanning capacitance microscope (SCaM) is developed using a tungsten stylus as a cantilever. The deflection of the cantilever is detected by projecting the cantilever onto a photodetector. A combined SCaM, atomic force microscope (AFM), and scanning tunneling microscope has been constructed. The spatial resolution for SCaM measurements is estimated to be less than 100 nm. Local electrical properties of a SiO2/Si system are investigated using the SCaM and AFM functions of the combined microscope. A lateral p‐n junction is formed, by ion implantation of P into a lightly B‐doped Si wafer, the structure is then annealed and thermally oxidized. It is demonstrated that (1) the p‐n junction can be observed even through the oxide layer, (2) the local impurity concentration can be estimated from capacitance–voltage (C–V) characteristics, and (3) P diffuses into the B‐doped region during the thermal processes.

Determination of Band Discontinuity in Amorphous Silicon Heterojunctions

A new procedure of determining the band offset in semiconductor heterojunctions has been developed. The valence band spectra for ultrathin amorphous silicon nitride (a-Si1-xNx:H) or silicon carbide (a-Si1-xCx:H) deposited on hydrogenated amorphous silicon (a-Si:H) have been measured by X-ray photoelectron spectroscopy. It is demonstrated that the empirical deconvolution of measured spectra yields the valence band discontinuity in a-Si1-xNx:H/a-Si:H and a-Si1-xCx:H/a-Si:H systems.

Experimental investigation of tunnel oxide thickness on charge transport through Si nanocrystal dot floating gate memories

The effect of tunnel layer thicknesses on the charging/discharging mechanism and data retention of Si nanocrystal dot floating gate devices was studied. Floating gate memories of Si nanocrystals dots with three different SiO2 tunnel thicknesses were fabricated, the key variable being tunnel oxide thickness. Other parameters which can affect memory properties were carefully controlled. The mechanism of electron discharging is discussed based on differences in tunnel SiO2 thickness. Direct tunneling was found to predominate in the cases of 3- and 5-nm-thick SiO2 tunnel layers. However, Fowler-Nordheim tunneling affects the electron discharging characteristics with thicker SiO2 tunnel layers. Clear characteristics in discharging peak differences could be observed in capacitance-voltage measurements on metal-oxide semiconductors with Si floating nanodot devices. Memory properties also depended strongly on tunnel oxide thickness.

Early stage of silicon oxidation studied by in situ X-ray photoelectron spectroscopy

In this study, a clean silicon surface is oxidized in a UHV chamber and the surface suboxide compositions are analyzed using in situ X-ray photoelectron spectroscopy. It is found that the predominant suboxides are Si2O3 and SiO irrespective of crystallographic orientations in the early stages of oxidation. This is interpreted in terms of a significant number of atomic steps existing on the clean Si surface. The observed chemical shift of O(1s) core level signal is explained by the partial charge transfer from the first and second nearest-neighbor silicon atoms to oxygen.

Low temperature polycrystalline silicon thin film transistors flash memory with silicon nanocrystal dot

We fabricated a flash memory by embedding a Si nanocrystal dot as the floating node for a low temperature polycrystalline silicon (poly-Si) thin film transistor (LTPS-TFT). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses revealed that the average Si dot size and density were approximately 5 nm and 8.5×1011 cm-2, respectively. Electron charging and discharging were clearly confirmed at room temperature by the transient behavior of the transfer curve. Furthermore, we evaluated the electronic behavior by varying the bias condition. For an improvement of the retention time and the electronic properties, high-pressure vapor annealing (HPVA), which exhibits even better performance for the fabrication of high-quality flash memory, was employed.

Effect of SiO2 tunnel oxide thickness on electron tunneling mechanism in Si nanocrystal dots floating-gate memories

The effect of tunnel oxide thickness on the charging/discharging mechanism and data retention of Si nanocrystal dot floating-gate devices was studied. The key point here is the difference in tunnel oxide thickness. Other parameters that can affect memory properties were carefully controlled. The mechanism of electron discharging was discussed on the basis of the difference in tunnel SiO2 thickness. Direct tunneling was found to dominate in the 3- and 5-nm-thick SiO2 tunnel oxides. However, Fowler–Nordheim (FN) tunneling affects the electron discharging characteristics of a thicker SiO2 tunnel oxide. It was found that memory properties also strongly depend on tunnel oxide thickness.