Akihiro Tsuji

Control of plasma profile in microwave discharges via inverse-problem approach

In the manufacturing process of semiconductors, plasma processing is an essential technology, and the plasma used in the process is required to be of high density, low temperature, large diameter, and high uniformity. This research focuses on the microwave-excited plasma that meets these needs, and the research target is a spatial profile control. Two novel techniques are introduced to control the uniformity; one is a segmented slot antenna that can change radial distribution of the radiated field during operation, and the other is a hyper simulator that can predict microwave power distribution necessary for a desired radial density profile. The control system including these techniques provides a method of controlling radial profiles of the microwave plasma via inverse-problem approach, and is investigated numerically and experimentally.

Studies on plasma direct energy converters for thermal and fusion-produced ions using slanted cusp magnetic and distributed electric fields

Two types of direct energy converters, cusp direct energy converter (CUSPDEC) and travelling-wave (TW) DEC, used to produce electricity from thermal ions and fusion products in an advanced fuelled fusion, are investigated using small-scale devices. In CUSPDEC, magnetized electrons are deflected along the field lines of the cusp magnetic field to the line cusp region and collected by an electron collector, while weakly magnetized ions can traverse the separatrix and enter into the point cusp region. Thus, ions are separated from electrons, and flow into an ion collector to produce dc power. Efficiencies of energy conversion of separated ions with large thermal spread of energy are measured to be ~55%. An additional lateral electrode, together with the existing collector, constitutes a two-stage ion collector that provides distributed ion-decelerating fields. From the measured voltage–current characteristics, the efficiency of this collector is estimated to be improved to 65–70%, which is consistent with the calculation. Fusion-produced fast ions enter into TWDEC and are velocity-modulated by RF fields, bunched and then decelerated by RF travelling-wave fields on the decelerator to produce RF power. The TWDEC device has shown that the energies of ions of 3–6 keV can be decreased by 10–15% for a one-wavelength decelerator. This would give a total efficiency of 60–70% for a full-length decelerator. A novel system is being investigated for further improvement, in which the incoming ions are deflected transversely, according to each energy, to form a fan-shaped beam and a distributed electrode array for modulation and deceleration generates travelling-waves appropriate to each ion path depending on the energy.

Benefit of precise control of surface reaction by new patterning technique for small-contact etching with TiN hard mask

We introduce state-of-the art small-contact etching by a new patterning technique using atomic layer etching (ALE) for sub-5 nm technology generation. In small-contact etching, SiO2 is etched by using a TiN hard mask with the progress of the miniaturization process. However, when applying the conventional method to small-contact etching with a TiN mask, etch stop is caused by excess deposition on the SiO2 film. From the results of surface analysis by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy–energy-dispersive X-ray spectroscopy (TEM–EDX), it is considered that the deposition is formed by the reaction of fluorocarbon (FC) plasma and TiN. To solve this problem, we have developed a quasi-ALE technique to improve the ALE process to make it more suitable for SiO2 etching. By adopting this technique to small-contact etching with a TiN hard mask, etch stop was significantly reduced. Quasi-ALE precisely controls the surface reaction by controlling the radical flux and ion flux independently. Therefore, the reaction of FC plasma and TiN leading to etch stop can be minimized. Quasi-ALE can resolve the etch-stop issue due to the TiN mask used in the conventional method.

New Simulation Approach to Controlling Plasma Uniformities

Plasma simulations have never been used for tool tuning in the field of semiconductor manufacturing because existing plasma simulations cannot inversely calculate input parameters such as power and gas distributions from output parameters such as the distribution of electron density ne. One of the solutions is to reconstruct the framework of simulations as an inverse problem. A new simulation system has been developed as the first step. It has two key points. One is to introduce a power coupling coefficient ap as an index of the tool tuning, and the other is to add some functions to inversely calculate ap from the target ne distribution. In the verification of a two-dimensional model, it is shown that the error between the distribution of ne calculated by a check simulation and the target is sufficiently small. Therefore, this approach can be one of the solutions to control plasma uniformities.

Three-dimensional fluid simulations on a line-shaped microwave plasma

A three-dimensional (3D) fluid simulation code has been developed to investigate the discharge characteristics and plasma distribution in a line-shaped microwave plasma device, which consists of a rectangular waveguide with a variable width a, a long quartz discharge tube with a U-shaped cross section partly inserted into the waveguide and the plasma diffusion chamber. The line-shaped plasmas are mainly produced just under the discharge tube surrounded by the complex structure. Experiments have revealed that the axial profile of the plasma density ne can be changed when a and the insertion depth di of the discharge tube into the waveguide are varied. 3D numerical analyses show that the axial distribution of power absorption Pabs depends on a and di and concentrates near the single side of the discharge tube when the high-ne plasma is inserted deeply into the waveguide. 3D simulations show that ne has peaks near the inner edges of the discharge tube and the axial distribution of ne depends on the waveform of the electric fields formed in the waveguide. The change in density profiles due to the variation of a and di observed in the experiment is mostly reproduced qualitatively by the present 3D simulation.

Simple Model to Inclusively Understand a Radiated Wave Field in Slot-Excited Microwave Plasmas

A simple model to inclusively understand radiated wave fields in slot-excited microwave plasmas has been developed. It is shown that the result of the simple model has a good agreement with that of a full wave model from the viewpoint of the mode spectra. The calculation cost of the simple model is considerably lower than that of a commonly used full wave model. In addition, it is also shown that the mode spectra exist in the broad region in the case of a few slots and in the narrow region in the case of many slots. The comparison with the experimental results supports that the physical phenomena such as mode jumps and hysteresis behavior observed in experiments are understood in terms of the mode spectra. Therefore, the simple model is useful to inclusively understand the antenna system and to design an antenna system with the complex slot geometry.