Yasuhiro Yoshitake

Defect control using an automatic killer defect selection method

In LSI mass production lines, it is very important to improve the detection accuracy of fatal excursions in defect trend charts in order to ramp up quickly and maintain good yield. It is for this reason that killer defects which could cause device faults should be monitored. We have developed an automatic killer defect selection (AKIDS) method to assist and improve the accuracy of this process. This method judges defects as killer or nonkiller by comparing defect sizes against killer defect judgement criteria. We applied AKIDS in the defect QC and sampling review processes, and found it to be effective in both detecting fatal excursions and selecting good review points for identifying the source of device faults.

Multispot scanning exposure system for excimer laser stepper

A new type of spot scanning exposure system has been developed for an excimer laser stepper. To optimize exposure parameters, the authors developed a SIPSE (Simulator for Spot Scanning Exposure) and performed experiments with a He-Cd laser and a g-line lens. Applying the results, a prototype of an excimer laser exposure system was developed. A lens evaluation system has also been developed to adjust the lens elements in the projection lens. Using these systems, 0.35 micron line and space patterns were clearly resolved. SIPSE will be usable to optimize a phase-shifting exposure system with a potential for 0.25 micron patterns.

Broadband alignment scheme for a stepper system using combinations of diffractive and refractive lenses

By using broadband illumination, a stepper alignment system is less sensitive to asymmetric coverage of photoresist around alignment targets and realizes high accuracy. However, a projection lens causes chromatic aberrations at the wavelengths in the broadband light. To correct the chromatic aberrations, we employed a Schupmann system composed of a projection lens, an achromat lens, and a refractive-diffractive hybrid lens. With the proposed system the spread of image planes and the magnification difference in the spectral range from 550 to 650 nm can be reduced to 50 µm and 0.05%, whereas without the correction system these values are 11.28 mm and 1.18%, respectively.

Resolution enhancement of stepper by complementary conjugate spatial filter

An ultra-high resolution stepper is proposed, which combines annular illumination and a complementary conjugate spatial filter. This technique reproduces deep submicron complex non-repeated patterns by relatively emphasizing the diffracted light from high spatial-frequency patterns. Images of several patterns are simulated for a conventional stepper, a stepper with phase shifting, and for the proposed technique. The results indicate that this technique has the potential to pattern 0.3-|im images using an i-line stepper with 0.5 N.A. with conventional masks. It should enable stepper ranges to be extended to the next generation of DRAMs.