JUNR Supply OSRAM120V-1000W Hg-lamp FOR Nikon stepper G6-I14
The all-reflecting 1:1 projection system was first reported by Moller in 1973. The optical and scanning configuration of such a system uses reflective spherical mirror surfaces to project images of the mask onto the wafer substrate. The illuminating radiation generated from a high-pressure mercury lamp passes through a condenser and a slit of a few mili- meters in width. A set of spherical mirrors is used to generate a narrow, ring-shaped aberration-free arc of light. The slit or arc of radiation is imaged by primary and secondary mirrors to cover the entire width of the mask.
Wafers are scanned with this arc to produce an image as the mask is moved synchronously in the object plane. This scanning approach is designed to minimize distortion and aberration of the optical system by keeping the imaging illumination always within the zone of good optical correction.
A schematic representation of a reduction projection system .The system projects a mask image onto a limited area of wafer, consisting of one or two chips of DRAM devices. In order to expose the entire area of a wafer, the wafer is stepped with precise mechanical movement to a new position before each exposure. A series of exposures involves repeating of stepping, alignment and exposure, which is called as a step-and-repeat exposure system, or a stepper for short.
A high pressure mercury lamp is used as a light source for g-line (436 nm) and i-line (365 nm) steppers. One of the characteristic emission lines from the mercury lamp is selected by a combination of a mirror and a filter, since the chromatic aberration prevents the construction of projection optics for all the emission lines. Illumination optics is designed for uniform exposure intensity at the mask and the wafer plane. A series of masks of chromium patterns on quartz substrates are usually fabricated by electron beam lithography.
The higher numerical aperture of projection optics offers a higher resolution capability. The design of the projection optics determines the performance of the stepper (such as resolution capability, exposure field size, depth of focus, etc.). Although only one element for an objective lens, the lens for the projection optics consists of more than ten elements to optimize chromatic aberration and other aberrations such as spherical aberration, coma, astigmatism, curvature of field and distortion. Alignment accuracy becomes important for shrinking design rule of LSI because large process latitude should be incorporated in LSI design for poor overlay accuracy.
