Akira Ono

Aspherical mirror testing using a CGH with small errors

A method for reducing errors in aspherical mirror testing using a computer-generated hologram (CGH) is described. By using a modified filtering method the carrier frequency in the CGH can be reduced by two-thirds, and the resulting error due to distortion is only one-half of that of a conventional CGH. By adopting a Fizeau-type optical setup, only the surface quality of the reference affects the measured results.

Plotting errors measurement of CGH using an improved interferometric method

An improved interferometric method is described for measuring plotting errors of desk-top computer plotters used to make computer-generated holograms. The plotting errors are measured using moire fringes formed using Young’s fringes and straight lines drawn by the plotters. The Young’s fringes are produced by laser beams originating from two single-mode optical fibers. Using this method, plotting errors of Hewlett-Packard 7225A and 7470A plotters are measured.

Highly efficient operation of diode laser transversely pumped high-power Nd:YAG rod laser

We are developing rod-type all solid-state laser with average power more than 10 kW as a tool for high speed and highly precise material processing such as cutting and welding. We developed a rod-type all solid-state laser system with average power of 3 kW level and succeeded in attaining an average power of 3.3 kW with multi head configuration with combined mode of CW and QCW operation. We also obtained electrical- optical efficiency of 19% in CW operation and that of 17% in QCW operation in the region of average power of 0.8 kW to 1.0 kW.

Mask defect inspection method by database comparison with 0.25-0.35 μm sensitivity

Photomasks used in the fabrication of ultra-LSI (ULSI) circuits must be inspected for defects. For 256 Mbit dynamic random access read write memory (DRAM), it is necessary to inspect defects as small as 0.15 µ m. Moreover, inspection of defects of phase-shift masks is becoming an important task of an inspection system. This paper describes an automated mask inspection system (MC-100) based on die-to-database comparison, and a defect inspection method with 0.15 µ m sensitivity for edge and pindot defects. System configuration and the defect inspection method are discussed in detail, including the difficulties of defect detection in an attenuated phase-shift mask.

High-resolution DUV inspection system for 150-nm generation masks

In order to perform mask inspection with the high reliability for 150 nm-rule and below devices, the inspection system with high resolution is indispensable. The phase shift masks like DUV HT masks must also be inspected with high sensitivity. A next generation mask inspection system MC-3000 which used DUV optics has been developed, in order to achieve these requirement. The wavelength of this optics is 257 nm that is shorter than that of current UV inspection systems, and is nearly equal to that of current DUV lithography systems. Short wavelength light and high NA optics obtain high resolution, so the defect detection of 130 nm or less is attained. The special issues for the DUV optics were solved by several new techniques. This paper reports the system configuration, basic characteristics for defect detection and inspection performances.

Optical aspheric surface profiler using phase shift interferometry

The optical aspheric surface profiler was developed. In the profiler a phase shift interferometric technique is used to measure an optical wavefront precisely and modified to be applied for an aspheric wavefront measurement. The profiler makes it easier to measure aspherical surfaces than the profiler using an optical probe or a stylus. The intensity distributions of fringe patterns from a Twyman-Green interferometer are obtained by a linear image sensor while a phase shifter driven by a piezoelectric transducer (PZT) moves. The charge coupled device (CCD) linear image sensor, which has smaller pixels than an area image sensor, detects fine fringe patterns from aspherical surfaces. The fringe data are transmitted into a computer and calculated into the wavefront phase data. In order to analyze the phase data, a non-linear least squares phase fitting method is used to eliminate the effects of the setting error of a tested surface. The profile error from the designed surface, whose parameters are previously saved into the computer, is calculated. The measurement results for the aspherical mirrors were compared to another profiler using a stylus. Finally, the new approach to analyze the phase data is proposed to solve the problem for this profiler and simulated for some data.

Video high-density disk surface testing with a laser diode and a position sensor

A surface tester was developed for the VHD (video high-density disk) using an optical technique. A 10-microm diam beam from a laser diode irradiates the disk surface, and a photosensor detects the angle and the intensity of a reflected beam. Defects on the surface are found by observing changes in the photosensor signal. A microscopic examination of the disk surface revealed that the tester can detect defects larger than 10 microm. The tester classifies defects by their category and size and displays a defect map on a monitor screen. The data obtained are utilized for improving the VHD manufacturing process.

Measuring Apparatus for Setting Errors of VTR Magnetic Heads

Measuring apparatus for VTR magnetic head setting errors on a rotating disk in a cylinder unit has been developed. Two or four heads on a rotating disk are used to record and playback video and audio signals. Magnetic head setting errors affect reconstructed image qualities. Three extremely important positioning errors are:n 1) Height difference between two or four heads along a rotating disk shaft 2) Distances head tips protrude from a rotating disk edge 3) Setting angle error for two symmetrically positioned heads. The height difference is measured statically using an ITV camera with ± 1 μm resolution. At the rated rotational speed, the protruded distance and the angle error are measured using focused laser beams for detecting intensity change and diffraction from the tips of the moving heads. Their resolutions are ±1 μm and ±3.8″, respectively. Necessary measuring time is only 20 seconds per cylinder unit. To assure reliable measurement, spatial filters and polarizing optical components are employed in the apparatus to block unwanted scattered light.

Phase measurement using Hg-Xe lamp and KrF excimer laser

We have developed a new system to measure the phase shift caused by the phase shift mask (PSM) for a deep-UV stepper. The system uses two types of 248 nm wavelength deep-UV light sources. One is an Hg-Xe lamp, and the other is a KrF excimer laser. These are interchangeable to match the illumination light source of the stepper. The phase is measured with a polarizing shearing interferometer. It has two wollaston prisms, a condenser lens, and an objective. The objective has a long working distance of more than 7 mm. This is necessary in order to measure the PSM with a pellicle. A light beam is separated into two orthogonal polarized light beams and sheared on the PSM, and then recombined on a sensor. The transmission beams through a shifter and a substrate interfere each other. Then the phase difference from the shifter is obtained from the difference between the two beams. We evaluated phase measurement repeatability and the measurable minimum pattern width. In the measurement of a halftone PSM, the repeatability was +/- 1 degree over several months, and the minimum pattern width was 2 micrometers or less.

Development of an advanced mask and its fabrication system

Masks and their fabrication technologies are keys to the further advancement of optical lithography. A stable SiNx single layer attenuated masks for DUV have been developed. A 0.2 micrometers contact hole pattern was fabricated using a KrF stepper with the SiNx attenuated mask. Toshiba mask fabrication system, including an electron beam writing system, a data base inspection system, and a data conversion system, has been developed for 64 Mbit DRAM class. Required mask improvements for increasing optical lithography resolution include better critical dimension (CD) uniformity, higher mask writing system resolution, and automatic shifter patten generation of alternating phase shifting masks. In addition, improved mask pattern positioning accuracy is also required. In this paper, experimental CD uniformity and resolution improvements, automatic phase shifter assignment method, and improvement in positioning accuracy, are described. The future development of masks will incorporate these key technologies.