Issey Tanaka

Excimer Laser Stepper for Sub-half Micron Lithography

An excimer laser stepper having a flat image field was developed. Using a frequency stabilized, narrow band KrF 248nm laser (FWHM≈3μm), this stepper can print uniform patterns under 0.5 μm over a 15 mm square field. When the excimer laser output power is 2W, illumination intensity is more than 40 mW/cm2. Illumination uniformity is within ±2.5% over the field and exposure energy density is controlled within the precision of ±1.596, using an integrator sensor and a pulse number controller. A chromatic projection lens is adjusted so finely that aberrations are removed. In order to get imaging stability, a lens controller is used. An alignment system is off-axis, using He-Ne laser light spots and diffraction light detectors. By employing the EGA (Enhanced Global Alignment) method, overlay accuracy better than 0.18μm was obtained. Because the alignment detection light is insensitive to the resist, any types of resist may be used. The developed system has all the basic functions of a stepper. It can be used to improve DUV process and to fabricate sub-half micron devices in a laboratory.