Yutaka Sakakibara

New model of electron free path in multiple layers for Monte Carlo simulation

A new model of an electron free path in multiple layers is proposed for Monte Carlo simulation of electron trajectories. In this model, the free path is calculated taking into account not only the scattering probability in the layer involving the initial scattering point but also that in the layers along the scattering direction. The result, simulated with the new model, agrees with the experimental result much better than results obtained with conventional models for backscattered electron intensity. It is also suggested that the simulation accuracy for the electron beam lithography is improved using the new model.

Si Surface Cleaning by Si2H6–H2 Gas Etching and Its Effects on Solid-Phase Epitaxy

Si surface cleaning with Si2H6–H2 was studied for lateral solid-phase epitaxy (L-SPE) of CVD a-Si films. The cleaning was performed at 850°C, which is considerably lower than the previously-reported H2-cleaning temperature (1100°C). This is because etching of native oxide is enhanced by a reaction between oxide and Si-atoms dissociated from Si2H6. The effects of Si2H6-cleaning on L-SPE were studied using a Si (100) wafer with SiO2 stripes. After Si2H6-cleaning or H2-cleaning at 850°C, CVD a-Si films are deposited on the wafers, then crystallized by annealing (575°C). An H2-cleaned sample showed imperfect L-SPE because of a residual native-oxide layer at the a-Si / substrate interface. In the Si2H6-cleaned sample, the native-oxide layer was removed, and L-SPE growth was observed uniformly in the wafer. Si2H6-cleaning at 850°C is applicable for substrates with abrupt impurity distribution.

Oxygen-Doped Si Epitaxial Film (OXSEF)

We propose a new wide-gap material, an oxygen-doped Si epitaxial film (OXSEF), for applications to Si heterobipolar transistors (HBTs). OXSEF containing several tens of at.% of oxygen can be grown on a Si substrate by depositing Si in about 10-6 Torr O2. OXSEF is literally almost a single crystal with an identical crystalline structure to Si, although it includes {111} twins as defects caused by oxygen. Temperature and O2 pressure dependences of oxygen concentration in OXSEF are dominated by oxygen adsorption on the Si surface. Oxygen atoms in OXSEF segregate to some extent toform incomplete oxides like SiO1.5, but complete phase separation as a mixture of Si and SiO2. regions does not occur, probably bcause of non-equilibrium conditions in MBE growth. Valence band discontinuity at the Si/OXSEF interface is deduced to be 0.26 eV based on photoelectrical measurements.

Stepped Electrode Transistor: SET

SET can achieve high performance without precise photolithography and metallization techniques. An arsenic doped polycrystalline silicon is used as a part of the emitter electrode in the SET structure. It is processed to have an inverse trapezoid shape. Procedure to make the inverse trapezoid shape uses a difference of etching rates between double layers of polycrystalline silicon. Base contact windows are opened through the ion-implantation process followed by chemical etching. The spacing between the emitter diffused layer and the base contact is as short as 0.4 µm or less. The cut off frequency of |S21e| is about 8.4 GHz. This frequency is higher than that of the conventional planar transistors with equal size emitter by 2 GHz. The rise time is 150 psec in the integrated SET.

Segregation and defect termination of fluorine at SiO2/Si interfaces

Segregation of fluorine in a SiO2/Si structure has been examined with respect to depth profile and paramagnetic defects at the SiO2/Si interface. The fluorine distribution was found to be restricted to a narrow (less than 35 A) interface region on the oxide side. Furthermore, a greater amount of fluorine segregation was observed for (111) than for (100) interfaces. These observations suggest that fluorine segregation is caused by intrinsic defects in the interface region and that fluorine atoms terminate the defects. In fact, electron spin resonance measurements provide direct confirmation that fluorine atoms terminate the trivalent Si dangling bonds at the interface.

I-V Characteristics of oxygen-doped Si epitaxial film (OXSEF)/Si heterojunctions

We have fabricated oxygen-doped Si epitaxial film (OX-SEF)/Si heterodiodes to examine device capabilities of the new wide-gap material, OXSEF. Various locations of the p-n junction with respect to the OXSEF/Si interface are achieved by changing the annealing time for arsenic diffusion from the implanted poly-Si layer on top of the OXSEF. When the p-n junction is located at the heterointerface, the diode n-value is 1.4-1.5 after H2annealing. This can be reduced to 1.1-1.2 by pushing the p-n junction into a Si substrate of about 600 Å.

Determination of Hydrogen Concentration in PCVD Silicon Nitride Films by Elastic Recoil Detection Analysis

The hydrogen concentrations of PCVD silicon nitride films were measured by Elastic Recoil Detection (ERD) analysis using 2.5 MeV He+ ions. For annealed films, H concentrations measured by ERD analysis are smaller than those measured by an infrared absorption analysis. These results indicate that absorption cross sections of Si-hydrogen and N-hydrogen bonds are changed by annealing.

Effects of Substrate-Surface Cleaning on Solid Phase Epitaxial Si Films

The effects of two substrate-surface cleaning procedures were studied for solid-phase epitaxy of chemical vapor deposited amorphous Si (a-Si) films with respect to crystalline quality. These a-Si films were deposited on Si (100) wafers after surface cleaning by HF-dipping or in-reactor gas etching. The a-Si films were then crystallized by furnace annealing. The film quality was estimated using several techniques, such as etch-pit density measurements. For the samples etched by HF-dipping, the film crystallinity became worse with increasing film thickness owing to a residual oxide layer at the deposited-film/substrate interface. As a result, the electron mobility decreases with increasing deposited film thickness. For in-reactor gas-etched samples, the residual at the interface was negligible, and the crystallinity of a SPE film was as good as that of bulk Si.

Evaluation of hot-hole-induced interface traps at the Tunnel-SiO2 (3.5 nm)/Si interface by the conductance technique

Hot-hole-induced interface degradation in ultrathin (3.5 nm) oxides has been evaluated by the small-signal AC conductance technique. To degrade the tunnel-SiO2/Si interface, holes are injected by the avalanche technique. The conductance technique makes it possible to estimate the number of interface traps even when there are a large intrinsic tunneling leakage before injection and a large stress-induced leakage current after injection. Experimental results show that holes can create acceptor-like interface traps at the tunnel-SiO2/Si interface and that the conventional reliability test comprising carrier injection and interface-trap estimation is still applicable to oxides as thin as 3.5 nm.

New resist technologies for 0.25-μm wiring pattern fabrication with KrF lithography

A new resist system composed of a chemically amplified positive resist and an organic base designed to be compatible with an acid generator has been developed to improve resolution on TiN films. It was clarified that the non-uniformity of the organic base like the acid generator in the resist film significantly contributes to high resist performance. A new hardening method has also been developed to improve the thermal stability of resist patterns for metal etching. Using the new technologies, 0.25-µ m line and space (l&s) resolution on TiN films and highly accurate metal etching have been achieved.