Shigeyuki Takagi

Development of key components and technologies for a high repetition rate and high-power excimer laser

Key components and technologies have been developed for an ultrahigh repetition rate XeCl excimer laser of 5 kHz to be used for industrial applications. A compact axial blower having a revolution rate of up to 10 000 min−1 and a maximum pressure of 16.2 kPa in air was made with a canned magnetic coupler to circulate a laser gas at a flow velocity of over 150 m/s in a discharge region. Materials constituting a laser chamber were tested to prevent discharge instability by gas contamination to enable long time operation. The dominant cause of the instability was investigated by a simple simulation. For a preionization, a novel sealed-off x-ray tube was developed to compare the suitability in a high repetition rate operation with that of conventional UV preionization. The gas due to the shock and acoustic waves generated by discharge pulses was measured to design the damper, which enabled the suppression of the gas turbulence by around a tenth. To simplify cumbersome laser maintenance, a new power supply prov...

Study on Surface Modification of Indium Tin Oxide and Resist Surfaces Using CF4/O2 Plasma for Manufacturing Organic Light-Emitting Diodes by Inkjet Printing

We studied a surface modification technique for indium tin oxide (ITO) anodes without precleaning and resist banks for manufacturing organic light-emitting diodes (OLEDs) by inkjet printing. The ITO surface modified by inductively coupled plasma (ICP) with an optimized CF4/O2 (7:3) gas mixture improved both its hydrophilicity and its work function, while the resist surface treated by the plasma became hydrophobic. The resist and ITO surfaces treated by plasmas of various gas mixtures (i.e., CF4, CF4/Ar (1:2), CF4/O2 (x:1; x=1, 7/3, 4, and 9) were analyzed by X-ray photoelectron spectroscopy (XPS) of the C 1s, F 1s, O 1s, and In 3d5/2 core levels. On the uncleaned ITO surfaces modified by CF4/O2 plasmas, organic contaminants were removed more efficiently and the deposition of CFx on the remaining contaminants decreased with increasing oxygen. The amount of F in the form of InFx increased using the CF4/O2 (7:3) plasma in comparison with that using the CF4/Ar and CF4 plasmas. We investigated the effect of adding oxygen to CF4 on the change in gaseous species produced in the plasma chamber by mass spectrometry. In the CF4/O2 (7:3) plasma, the peak intensities of F+, HF+, F2+, O+, and O2+ were higher than those in the CF4 plasma. The results suggest that In2O3 was generated by the oxidation of indium with O, and InFx was generated by the fluoridation of indium with HF. By introducing InFx onto ITO surfaces using the CF4/O2 plasma, the hole-injection energy barrier could be reduced.

Statistical study of subthreshold characteristics in polycrystalline silicon thin-film transistors

We investigate the influence of grain size variations on device properties of polycrystalline silicon thin-film transistors (poly-Si TFTs) by drift–diffusion device simulations. Employing our grain boundary model which represents the intrinsic variations of grain size [Jpn. J. Appl. Phys., 42, L634 (2003)], the subthreshold characteristics are simulated for the various configurations of grains in the channel region so that the statistical fluctuation of device characteristics is investigated. It is shown that the variations in subthreshold characteristics are caused mainly by the number of grains included in the channel because the grain boundaries act as highly resistive regions. However, as the channel length shrinks, the grain boundary located close to the drain critically determines the channel resistance of poly-Si TFTs and the device characteristics could differ even if the number of grains included in the channel region is the same.

Multiscale analysis of silicon low-pressure chemical vapor deposition

We consistently performed computer fluid dynamics (CFD) analysis in a reactor (macroscale analysis) and deposition profile analysis on a submicron hole (microscale analysis) for Si low-pressure chemical vapor deposition (LPCVD). For the gaseous phase and the surface reaction of the SiH4 source gas, we adopted the dominant reaction model, which involved two intermediates, SiH2 and Si2H6, and was based on the Kleijn Model. We analyzed the fluid flow, heat transfer and chemical reactions throughout the entire batch-type reactor, and estimated the Si growth rate, gaseous species concentration, and relative contributions of SiH4, SiH2 and Si2H6 to Si growth. Moreover, the Si-filling profile on a submicron hole was predicted by topography simulation in which the parameters were the growth rate, the relative contribution and the sticking coefficient of each species. The relationship between the relative contribution of SiH2, which has a high sticking coefficient, to Si growth and the hole-filling capability was quantitatively clarified from the results of a combination of the two analyses. The hole-filling capability at the wafer edge was deteriorated by the influence of SiH2 gas produced in the decomposition of Si2H6 gas, which was diffused from outside the wafer. This effect became considerable with increasing temperature. Reducing the wafer pitch will be effective in improving the hole-filling capability because both the SiH2 generation reaction in the region between wafers and SiH2 gas diffusion from outside the wafer will be inhibited.

Electron Density Measurements in UV-Preionized XeCl and CO2 Laser Gas Mixtures

A Langmuir probe technique has been used to measure electron densities and temperatures in UV-preionized XeCl excimer and CO2 laser gas mixtures in a laser tube. For this experiment, only pin electrodes (preionization sparks) were operated with no discharge between the main electrodes. The measured electron densities were about 108 cm-3 in both the excimer and CO2 laser gases, compared with 1010 cm-3 in pure He gas. The electron density was found to increase due to the proximity of the main electrodes. The coefficients of absorption for excimer and CO2 laser gas were obtained from the characteristics of the electron densities vs the distance from the UV source. Based on the absorption coefficient for XeCl, 0.9 cm-1 atm-1, we propose pin-electrode arrangements for spatially uniform preionization.

Topography Simulation of Reactive Ion Etching Combined with Plasma Simulation, Sheath Model, and Surface Reaction Model

We developed an oxide-film reactive ion etching (RIE) topography simulation which consists of a plasma simulation, a sheath model, and a surface reaction model. In the plasma simulation, the plasma parameters were calculated in two dimensions using the particle-in-cell/Monte Carlo collision (PIC/MCC) method. In the surface reaction model, the motion of particles in the etching trench was simulated by the Monte Carlo method. This topography simulation was applied to the etching by a capacitively coupled plasma (CCP). Etching conditions were as follows:gas pressure 5.3–10.6 Pa and RF power 1.1–1.7 kW in the gas mixture of C4F8, CO, O2, and Ar. The calibration method for such simulation parameters as the ion reflection ratio, the etch rate and the polymer etch rate was established based on the experimental results. As a result, the change of the etching profile was reproduced according to the change of the gas pressure and RF power with high accuracy. Furthermore, it was shown that the simulator can predict the profile change corresponding to the process change.

Design concept and performance considerations for fast high power semiconductor switching for high repetition rate and high power excimer laser

A semiconductor switching power supply has been developed, in which a novel structure semiconductor device, metal-oxide-semiconductor assisted gate-triggered thyristor (MAGT) was incorporated with a single stage magnetic pulse compression circuit (MPC). The MAGT was specially designed to directly replace thyratrons in a power supply for a high repetition rate laser. Compared with conventional high power semiconductor switching devices, it was designed to enable a fast switching, retaining a high blocking voltage and to extremely reduce the transient turn-on power losses, enduring a higher peak current. A maximum peak current density of 32 kA/cm2 and a current density risetime rate di/dt of 142 kA/(cm2×μs) were obtained at the chip area with an applied anode voltage of 1.5 kV. A MAGT switching unit connecting 32 MAGTs in series was capable of switching on more than 25 kV–300 A at a repetition rate of 5 kHz, which, coupled with the MPC, was equivalent to the capability of a high power thyratron. A high repe...

Gap-Fill Process of Shallow Trench Isolation for 0.13 µm Technologies

Gap-fill technology using high-density plasma chemical vapor deposition (HDP-CVD) is one of the leading technologies in 0.13 µm generation semiconductor device processing. The analysis of the dependence of HDP-CVD filling characteristics on the processes used revealed that film deposition under an increased plasma power and low-pressure conditions is effective for stable gap filling. By optimizing these process parameters, we were able to implement shallow trench isolation (STI) of space width 0.13 µm and aspect ratio 3.9. Furthermore, we demonstrated that the angular dependence of sputter yield and the ionic deposition mechanism are important factors when performing filling by means of HDP-CVD. The filling characteristics can be improved by increasing the maximum sputter yield angle and the amount of ionic deposition component. Based on these mechanisms, we constructed a topography simulation model which enables accurate expression of the topography of the HDP-CVD film.

Characterization of Carbon Nano-Gap for Surface Conduction Electron Emitters

Field-emission displays (FEDs) retain the advantages of cathode-ray tubes (CRTs), namely, high color reproducibility, fast response, wide viewing angle, and high contrast level. The emitters used in FEDs are based on the principle that electrons are emitted by a strong electric field. It is necessary to obtain an electric field that is sufficiently strong to induce a tunnel current, even at a low voltage, by forming nanometer-sized gaps (nano-gaps) in the emitters. Previously, we formed a carbon nano-gap by our original method and reported that electrons can be stably emitted at a low voltage. In this study, we succeeded in forming a nano-gap with a uniform width by controlling the externally applied activation voltage by the same method. In addition, the strength of the electric field caused by the external voltage (Vf), β×Vf, was found to be constant. In this paper, we discuss the mechanism underlying the formation of a carbon nano-gap and report electrical characteristics, temperature measurement data, and the results of observing cross-sectional images of the carbon nano-gaps obtained by scanning electron microscopy (SEM). In addition, the mechanism underlying the formation of the carbon nano-gap is modeled.

Predictable topography simulation of SiO2 etching by C5F8 gas combined with a plasma simulation, sheath model and chemical reaction model

We have developed a simulation for predicting reactive ion etching (RIE) topography, which is a combination of plasma simulation, the gas reaction model, the sheath model and the surface reaction model. The simulation is applied to the SiO2 etching process of a high-aspect-ratio contact hole using C5 F8 gas. A capacitively coupled plasma (CCP) reactor of an 8-in. wafer was used in the etching experiments. The baseline conditions are RF power of 1500 W and gas pressure of 4.0 Pa in a gas mixture of Ar, O2 and C5F8. The plasma simulation reproduces the tendency that CF2 radical density increases rapidly and the electron density decreases gradually with increasing gas flow rate of C5F8. In the RIE topography simulation, the etching profiles such as bowing and taper shape at the bottom are reproduced in deep holes with aspect ratios greater than 19. Moreover, the etching profile, the dependence of the etch depth on the etching time, and the bottom diameter can be predicted by this simulation.