Noriaki Nakayama

Homogeneous hydrogen‐terminated Si(111) surface formed using aqueous HF solution and water

Aqueous HF etching of silicon surface removes surface oxide, leaving a silicon surface terminated by atomic hydrogen. We studied the effect of the immersion in water, following HF etching, on the surface hydride structure and flatness, by measuring Si‐H stretching vibration using infrared absorption spectroscopy. Immersion at 20 °C flattens the Si(111) surface, which is atomically rough just after etching, to some extent. Boiling water (100 °C) produces an atomically flat surface homogeneously covered with silicon monohydride (—SiH) normal to the surface and free of oxidation. The surface has a low defect density of less than 0.5%.

Photoluminescence of porous Si, oxidized then deoxidized chemically

We examined the change in photoluminescence spectra of porous Si when it is oxidized then deoxidized chemically. After both steps, photoluminescence shifted to higher frequencies and increased in intensity. These shifts to higher frequencies indicate the photoluminescence is a result of the quantum size effect. Moreover, the increase in photoluminescence intensity after oxidation suggests that termination by hydrogen on the porous Si surface does not always play a key role in the photoluminescence mechanism.

Base transit time of shallow-base bipolar transistors considering velocity saturation at base-collector junction

The authors studied the influence of the velocity saturation in the base-collector depletion layer and compared the injected electron concentration profile, collector current density, and base transit time with velocity saturation to those without. The collector current with velocity saturation is only a little smaller than the current without saturation, but the injection electron concentration profile changes substantially when comparing currents with and without velocity saturation. The base transit time is increased by velocity saturation, and the ratio of base transit time with velocity saturation to that without velocity saturation increases as the base width decreases. Thus, velocity saturation must be considered in order to evaluate the base transit time of shallow-base bipolar transistors. The dependence of the base transit time on the doping profile was also analyzed, revealing that the base transit time of a box doping profile is increased more than that of a Gaussian doping profile. >

Abnormal solid solution and activation behavior in Ga‐implanted Si(100)

Rapid thermal annealing (RTA) and furnace annealing of Ga‐implanted Si were studied. Ga atoms implanted into Si are located on substitutional lattice sites in concentration above the solid solubility limit after short‐time and low‐temperature annealing. Low‐resistivity shallow p+ junctions can be fabricated using this metastable layer. However, precipitation and redistribution of the Ga atoms were observed after high‐temperature or longer time annealing. Shallow p+ junctions fabricated by Ga implantation and RTA are suitable for submicron complementary‐metal‐oxide‐semiconductor devices.

Transient analysis of stored charge in neutral base region

The authors studied the transient relationship between stored charge in the neutral base region and electron current flowing through emitter and collector terminals. Stored charge flows not only through an emitter terminal but also through a collector terminal when emitter-base junction voltage decreases from the switch-on voltage to zero. The ratio of net charge flowing through the emitter terminal to that flowing through the collector terminal is 2:1 once a steady state has been reached. No charge accumulates through the collector terminal when the emitter-base junction voltage increases from zero to the switch-on voltage, however, so the charge partition ratio depends on the sign of the time gradient of the emitter-base voltage. >

Comments on Transient analysis of stored charge in neutral base region [with reply]

The commenter addresses questions raised in the above-titled paper by K. Suzuki et al. (ibid., vol.39, p.1164-9, May 1992) concerning the validity of bipolar transistor models using the partitioned-charge (PC) approach for transient simulations. Suzuki et al. assert that the concept of charge partitioning applies only to discharge transients and thus the charge-partition ratio depends on the sign of the emitter-base voltage gradient, which, if true, would greatly reduce the utility of PC-based models. The commenter shows that this is not the case and that their conclusion is due only to a misinterpretation of the PC model. The authors reply. >

Proximity-effect correction for VLSI patterns in electron-beam lithography

Abstract A proximity-effect correction method for VLSI patterns has been developed. In this method, a dose ratio has been introduced as a control parameter for the negative- resist thickness after development, in addition to the proximity parameters. A new technique has been used to obtain the proximity parameters. By using the dose ratio and the proximity parameters, both the exposure dose and the size of the irradiated shape are easily determined. A pattern accuracy of ±0.1 μm and a uniform resist of the desired thickness were obtained. The computation time is proportional to 1.2 power of pattern density, and is 100 seconds on a 1.5-MIPS computer when correcting for 10 4 shapes in a pattern whose pattern density is 10 4 .

Enhanced proximity-effect correction for VLSI patterns in electron-beam lithography

When applying a proximity-effect correction to VLSI patterning, the major challenge is to obtain a highly accurate pattern without excessive computation. A fast correction program that has two new techniques for the proximity effect has been developed for highly accurate VLSI patterning on a negative resist. The first technique is the effective definition of sample points where energy intensity is calculated to obtain a highly accurate pattern. The second is the use of the exposure-intensity reduction rate which corresponds to a change in the pattern dimensions to reduce the computation time. A pattern accuracy of ±0.1 µm and a uniform resist of the desired thickness were obtained. The computation time is proportional to the 1.2 power of pattern density.

Test structure for determining boron diffusion coefficient in tungsten silicide

In order to study impurity diffusion in silicide, the authors patterned tungsten silicide into a line 2 mm long and 30 mu m wide and selectively implanted boron into a 40- mu m-long window. They modeled lateral diffusion profiles in this test structure and determined the diffusion coefficients of boron from SIMS (secondary ion mass spectrometry) line-scan profiles as 4.0*10/sup -10/ cm/sup 2//s at 650 degrees C, 9.5*10/sup -9/ cm/sup 2//s at 800 degrees C, and 2.0*10/sup -8/ cm/sup 2//s at 900 degrees C. This is more than 10/sup 6/ times larger than that in silicon. The results obtained make it possible to design a thermal process for half-micron dual-gate CMOS.<<ETX>>

Analysis of pattern accuracy in submicrometer electron-beam direct writing

To create submicrometer patterns with high accuracy on thick single-layer negative resist, error factors that degrade pattern accuracy have been investigated. Pattern accuracy was analyzed using a new evaluation method based on the difference between the resist development energy and the exposure energy at points on the edge of each shape. By introducing a new evaluation parameter, we were able to clarify error factors from the exposure conditions, the proximity effect correction method, and the machine exposure fluctuation. The evaluation parameterKisQ/Q_{0}whereQis the exposure dose appropriate for the desired resist thickness and Q0is the interface gel dose. It was found that the resist resolution and the rounding error of the exposure dose were serious error factors, especially in delineation on submicrometer patterns. To achieve 0.5-µm patterns with ±0.1-µm accuracy on 1-µm-thick negative resist, the resist evaluation parameterKmust be less than 2, the rounding error of the exposure dose must be less than 2.5 percent of the dose, and the beam addressing unit (LSB) must be less than 0.025 µm.