Ryuichi Funatsu

Focusing and leveling based on wafer surface profile detection with interferometry for optical lithography

A new concept of shot-by-shot leveling for high resolution stepper systems, profile-based- leveling, is presented. This detects the wafer surface profile using laser interferometry. From the detected profile, this system determines where on the LSI chip to focus, and controls the wafer stage for focusing and leveling. With an experimental setup, a profile detection repeatability of +/- 0.02 micrometers , a tilt measurement repeatability of +/- 0.24 (mu) rad and a tilt measurement linearity of +/- 1.4 (mu) rad were obtained.

X-Ray Exposure System With Plasma Source For Microlithography

An x-ray exposure system with a plasma source has been developed to be applied in R & D of a deep-submicron device fabrication process. This system features high accuracy align-ment using imaging optics with high-speed signal processing, precision proximity gap control by means of a wafer surface flattening mechanism, and fine pattern replication by incorporating a plasma focus soft x-ray source. The imaging optics are arranged to be diagonally symmetric with respect to the x-ray exposure axis to perform continuous pattern detection (lateral displacement and gap) directly in an exposure field. A unique wafer chuck capable of flattening a wafer surface to within ±0.5 μm by piezoelectric actuators, supported by a six-axis, micro-motion mechanism, makes it possible to align the wafer to the mask (resolution of 0.01 μm) with a uniform gap (15=11 μm). The plasma focus source which emits x-rays with wavelengths in the range of 10 ~ 14 Å from high-temperature neon plasma using the pinch effect induced by a pulse current, has been newly developed. The pattern replication performance is thoroughly examined, showing an alignment accuracy of within ±0.1 μm (2σ), and a fine pattern replication of 0.3 μm.

Shot leveling and focusing with interferometry for optical lithography of sub-half-micron LSI

This paper presents a shot-by-shot leveling applied to an i-line stepper system. This shot leveler utilizes laser interferometry with a laser beam that has S-polarization and a large incident angle to the exposure surface of the LSI wafer. Thereby leveling and focusing accuracy is preserved regardless of the wafer surface condition under the photoresist. This system acquires interference fringe data with charge-coupled device (CCD) line image detector, and extracts the fringe frequency and phase information with a fast-Fourier-transformation (FFT). These correspond to the tilt and height information of the photoresist surface. The performance was evaluated with 4M-DRAM process wafers. Tilt and height measurement linearity was proved to be ±2 µrad and ±0.3 µm respectively, and total leveling control accuracy was proved to be better than ±7 µrad.

A Novel Method of Beryllium Window Protection in a Plasma Focus Soft X-Ray Source

A significant problem of plasma X-ray sources such as a plasma focus device is how to protect the very thin beryllium window from damage and contamination caused by the bombardment of plasma particles. Improvement of the protection of the beryllium window has been made by inserting a magnetic deflector between the focused plasma and the window and by inverting the polarity of the applied voltage to the electrodes. As a result, a 13-µm-thick beryllium membrane can be used safely. The measured X-ray intensity is about 5 mW/cm2 30 cm from the source operated at 3.5 kJ discharge energy. The achieved intensity is suitable for R&D application in X-ray lithography.