Tsuneaki Ohta

AlGaAs/GaAs Buried Multiquantum Well Lasers with a Reactive Ion Etched Window Facet

The reactive-ion etching (RIE) process in fabricating mirror facets for AlGaAs/GaAs buried multiquantum well (BM-QW) lasers has been developed. The lasers have a window structure where a mirror is formed by RIE. Window and BM-QW structures are fabricated by Zn-diffusion-induced disordering. Because of no carrier injection, the damage caused by RIE does not affect the characteristics of the lasers. The lasers having one facet formed by RIE and the other by cleaving operate in the fundamental transverse mode. A low threshold current (25 mA) and high external quantum efficiency are comparable to those of cleaved lasers.

AlGaAs/GaAs Multiquantum Well Lasers with Buried Multiquantum Well Optical Guide

Transverse mode stabilized AlGaAs/GaAs multiquantum well (MQW) lasers with buried MQW optical guide (MQW-BOG) have been fabricated utilizing Zn-diffusion-induced disordering of AlGaAs/GaAs MQW. Lasers with such an optical guide structure operate in the fundamental transverse mode up to an output power as high as 30 mW. A small kink is observed in the L-I curve at an output power exceeding 30 mW. Above the kink, a twin peak appears in the far-field pattern while a single peak appears in the near-field pattern. From these results, it is confirmed that the MQW-BOG laser acts as an index guided laser with partial gain-guided effect.

AlGaAs Window Stripe Buried Multiquantum Well Lasers

Transverse mode controlled high power AlGaAs lasers with a window stripe buried multiquantum well structure have been developed using the simple and reliable Zn-diffusion-induced disordering process. The maximum pulsed light output of the laser is 240 mW. The lasers operate in a fundamental transverse mode up to 100 mW. The threshold current and external differential quantum efficiency depend on the total window region length, because of large free carrier absorption in the Zn diffused window region. By controlling the diffusion depth and window region length, the lasers with low threshold current and high external differential quantum efficiency are realized.

X-Ray Mask Distortion Induced in Back-Etching Preceding Subtractive Fabrication: Resist and Absorber Stress Effect

The influence of resist and absorber stress distributions on X-ray mask distortion induced during back-etching preceding subtractive fabrication is analyzed experimentally and simulated. The stress distribution (gradient) in a resist and/or that in an absorber film causes larger pattern displacement rather than the film average stress. Some resists have considerably high stress after coating on a wafer, and this stress also changes during exposure, causing local pattern displacements. However, use of chemically amplified resist systems, in which the reaction after exposure is limited to a small amount acid generation, might solve this problem. Low-stress positive-tone resists should thus be developed.

High temperature deposition of SiN films using low pressure chemical vapor deposition system for x‐ray mask application

SiN films for x‐ray mask membranes were prepared using a low pressure chemical vapor deposition system designed for high temperature deposition and low impurity incorporation. The physical and optical properties of the films such as stress, uniformity, optical transmittance, and absorption were investigated. Film composition and impurities were also evaluated. The SiN film deposited at a substrate temperature of 1000 °C showed suitable properties for x‐ray mask membrane, such as well controlled tensile stress of about 5×107 Pa, high optical transmittance over 95% at 500 to 800 nm, and low impurity concentration.

Cross Sectional Transmission Electron Microscopy of Zn Diffusion Induced Disordering of GaAlAs-GaAs Multiquantum-Well Structures

Zn diffusion induced disordering of a GaAlAs-GaAs multiquantum-well (MQW) structure has been studied by cross sectional transmission electron microscopy. The disordering is found to occur uniformly along the lateral direction within a transition region of about 0.2 µm. The narrow width of the transition region is explained by the steep Zn concentration gradient of the diffusion front.

Fabrication of X-Ray Mask Using W-CVD for Forming Absorber Pattern

Tungsten chemical vapor deposition (W-CVD) using WF6 and H2 as reactants was applied to forming absorbers of X-ray masks for synchrotron radiation (SR) lithography. For this purpose, the properties of deposited W (CVD-W), such as stress, density and thermal stability, were examined. The stress can be minimized reproducibly to less than 1×108 dyn/cm2 by controlling the flow rate of WF6 at various substrate temperatures. This W film was thermally stable up to 200°C, and the density was 18.5 g/cm3. From these results, CVD-W was found to be suitable as an absorber material. For the formation of absorber patterns, filling SiO2 grooves with stress-reduced CVD-W was examined. The 0.2-µm-wide grooves were filled with CVD-W and absorber patterns were formed by etch-back. The X-ray mask was successfully fabricated by this developed process.

SiC/SiN Multilayer Membrane for X-Ray Mask Deposited by Low Pressure Chemical Vapor Deposition

SiC/SiN multilayer membranes consisting of SiC and thin SiN layers were prepared using hot-wall, low-pressure chemical vapor deposition (LPCVD) for synchrotron radiation (SR) lithography. The SiC/SiN multilayers had advantages such as uniformity in the controlled film stress, good surface morphology and optical transmittance of over 60% at film thicknesses from 1.7 to 2.5 µ m. The optical transmittance of over 80% with an antireflection coating (ARC) was obtained. X-ray masks were fabricated using the SiC/SiN multilayer membranes and W films as absorber. The 80-nm-feature patterns on the resist films were obtained by electron beam (EB) lithography. SR durability of the X-ray masks using the SiC/SiN multilayer was discussed assuming a stress change in the thin SiN layers.

Fabrication of reliable x-ray mask using high-temperature deposited SiN membrane by low-pressure chemical vapor deposition system

The SiN membranes were deposited by using high temperature LPCVD system. The SiN films deposited over 1000 degree(s)C showed the suitable properties for X-ray mask, such as well- controlled tensile stress of 5 X 107 Pa, high optical transmittance over 95% and low impurity concentrations. The high optical transmittance of the SiN films deposited over 1000 degree(s)C was related to the high N/Si. The X-ray masks fabricated by using the SiN membranes deposited at 1000 degree(s)C showed the high optical transmittance of about 92% and X-ray durability. The pattern position displacement induced by SR irradiation was simulated using FEM calculation with qualitative agreement.

Simulation of x-ray mask displacement by absorber and membrane stress

X-ray mask displacements were simulated using finite element method in order to estimate the requirements for the stress and stress distribution of the absorber and membranes. The structure of X-ray mask in this simulation was as follows: the substrate was 3inch φ and 2mm thick Si wafer, the membranes were 2μm thick SiN and SiC, absorber thickness was 0.5μm, and window area was 25mm square. The simulations were focused on the film stress, various absorber patterns, such as half pattern of window, line and space patterns, and the influence of backetch.