Hidefumi Nakata

Loading Effect and Temperature Dependence of Etch Rate in CF4 Plasma

Plasma etching of silicon, silicon dioxide, and silicon nitride in CF4 and CF4+O2 (up to 20%) was evaluated concerning exposed area dependence and temperature dependence of the etch rate with a modified barrel type plasma reactor having a temperature-controlled sample holder which enabled samples to be etched one by one. Experimental results related to the exposed area dependence of the etch rate showed that the etching mode was divided into two regions; the loading effect region, and the intrinsic region. The behavior of the etch rate, the activation energy for the etch rate, and the etch rate ratio for silicon and silicon nitride over silicon dioxide were obtained. The pattern undercut with excess etch time was also examined, and it became evident that suppression of the loading effect and improvement of the pattern undercut could be achieved by lowering the sample temperature.

Gas plasma etching of ion implanted chromium films

The etching characteristics of ion implanted chromium films in gas plasmas have been studied. It was found that in the case of Sb+ and As+ the etching rate of implanted chromium films decreased with increasing ion dose. It was also found that the etching rate depends on the surface impurity concentration, not on the total implanted dose of ions in the film. Using chromium film samples implanted with 20 keV ions at a dose of 1×1016 cm−2, reversely etched LSI photomask patterns have been obtained with good edge profiles. It is speculated that oxides of impurities are responsible for the reversal gas plasma etching.

A Three-Dimensional Study of the Absorbed Energy Density in Electron-Resist Films on Substrates

A computer program has been developed for the three-dimensional calculation of the absorbed energy density in polymer films on substrates in electron beam lithography. In this calculation the Monte Carlo results have been used for the radial energy intensity distribution for a point source electron beam. The program is based on the reciprocity principle proposed by Chang. Some exposure experiments have been conducted with an electron resist of PMMA (polymethyl methacrylate) for isolated patterns in the from of a line of finite length (8.1 µm) as well as of a rectangle (3.1×8.1 µm2) in order to check the reliability of the calculations. Operating beam voltages used for the investigation are 14 and 20 keV. The electron resist thickness is 8000 A. Relatively good agreement has been obtained between the calculated and the experimental results. This program is applicable to an arbitraty pattern, and therefore it will be useful for investigations of the proximity effect in electron beam lithography.

Experimental and theoretical study of cross‐sectional profiles of resist patterns in electron‐beam lithography

Experimental and theoretical study was carried out on cross sectional profiles of resist patterns for line and area exposures. Experimental work was performed using an electron beam exposure system, where a PMMA film of 6000 A thickness was used as an electron resist and the incident electron energy was 20 keV. The electron‐beam probe size was measured from a line spectrum of the secondary electron signal when the probe was scanned over a fine gold wire. The diameter of the probe was 5500 A. The developer was a 1:1 solution of MIBK and IPA. The time evolution of the cross sectional profile was obtained from the SEM observation at various development times. Theoretical evaluation of the time evolution was conducted by a combination of Monte Carlo calculation of electron scattering and a well‐known equation for the solubility rate. Monte Carlo calculation was performed with a new model based on the Spencer and Fano theory for electron energy loss. For assumed parameters in the solubility rate equation a rea...

Metal‐oxide‐semiconductor field‐effect transistors and a ring oscillator fabricated in laser‐recrystallized polycrystalline silicon islands

Directions of grain boundaries in laser‐recrystallized polycrystalline silicon (polysilicon) islands are found to be arranged along with the laser scan direction, and are connected to the electrical characteristics of metal‐oxide‐semiconductor field‐effect transistors (MOSFET’s) fabricated in polysilicon islands. When the laser scan is parallel to the channel direction, grain boundaries work as fast‐diffusion paths of arsenic from source and drain into the channel and a decrease of effective channel length of the MOSFET results. But when the laser scan is perpendicular to the channel direction, few grain boundaries are contributed to diffusion paths of arsenic. A maximum electron mobility of 590 cm2/V s approaching that of a single crystalline silicon can be obtained in devices with a channel length of 3 μm, though grain boundaries work as additional potential barriers for carriers. A nine‐stage ring oscillator is fabricated by applying these results. A minimum propagation delay of 38 ns is obtained.

Electron beam testing of VLSI circuits assisted by focused ion beam etching

Abstract Electron beam testing assisted by focused ion beam etching was examined. Before electron beam testing (EB testing), a small window was made in the passivation film by focused ion beam etching (FIB etching). EB testing was performed through this window. This method was useful because charge buildup on the passivation film is avoided during EB testing. The threshold voltage shift caused by FIB etching was permitted until the residual film thickness on the gate electrode became 0.5μm. This technique was applied to measure the internal voltage waveform of the 256K bit dynamic RAM and confirmed that it was effective for functional testing and failure analysis of VLSI circuits.

Gas Plasma Etching of Chromium Films

The etching of Cr film was investigated employing mixtures of CCl4 with CO, CO2, or O2. The highest etch rate ratio of the Cr film to the photoresist was given by the combination of CCl4 with CO2, and the Cr film could be etched successfully without serious loss in the pattern width using this CCl4/CO2 mixture. It is speculated that O(3P) contributes to the etching reactions of chromium films. Spectrometry revealed that singlet oxygen radicals contribute to the decomposition reactions of the resist films in the plasmas. Using this novel gas plasma etching technique, photomasks for MOS LSI were made with good pattern width control.

Lateral impurity transport in silicon films on insulators during laser recrystallization

The lateral transport of dopants in silicon films on insulators during laser recrystallization is investigated. The dopants implanted locally in silicon films on insulators are found to be transported in the forward direction of the laser scan as well as the backward direction. Both transport lengths from the originally implanted region are measured as a function of the laser scan velocity. The transport mechanism is explained by taking into account a liquid phase diffusion and a segregation of impurities depending on the crystallization speed. The diffusion coefficients of (1.2±0.2)×10−4 and (1.3±0.4)×10−4 cm2/s for arsenic and boron, respectively, in molten silicon are obtained.

The Role of a Photoresist Film on Reverse Gas Plasma Etching of Chromium Films

The role of a photoresist film on reverse gas plasma etching of chromium photomask plates has been studied. The variation of etching profiles has been observed using SEM techniques. It is speculated that a WO3 layer on the chromium film forms a masking layer to the etching and that the WO3 layer can be removed by decomposition of the photoresist film in the plasma. A study of the relation between photoresist thickness and etching time has shown that there is an optimum photoresist thickness for each WO3 concentration in the chromium film.

A New Transistor Structure for High Speed Bipolar LSI

A new transistor structure for high speed bipolar LSI, named PEE (Polycrystalline Electrode with Epitaxy) transistor, has been developed especially with respect to base and collector geometry. The basic features of the new structure are ; (1) polycrystalline silicon electrodes for a base and a collector and also polycrystalline silicon resistors, and (2) two-step oxide isolation which enables the bottom side isolation of an extrinsic base to the collector. The new structure can minimize parasitic capacitance, for instance, CTC can be reduced to about half of the conventional oxide isolated structure. This results in a high cutoff frequency of 6.0 GHz. Gate delay time and power-delay product measured with five-stage ECL ring oscillators ranged from 0.74 nsec, 2.8 pJ to 1.3 nsec, 1.8 pJ, and the performance was improved approximately 20% compared with the conventional devices.