Excimer-based step-and-repeat scanner achieves 0.25-µm resolution
Excimer-based step-and-repeat scanner achieves 0.25-µm resolution
New wafer-stepper technology for the production of 256-Mbit DRAMs and next-generation microprocessors has been announced by Nikon Precision Inc. (NPI, Belmont, CA). A 248-nm KrF excimer laser serves as the light source for the novel step-and-scan wafer-exposure system, which reportedly achieves a resolution of better than 0.25 µm. The laser, in combination with a scanning refractive optical arrangement, simultaneously scans the reticle and wafer stages, allowing the exposure light to illuminate only a portion of the reticle and image it onto the wafer through the "sweet spot" of the lens; using only this spot of the lens minimizes lens-induced distortion and focal deviation. Conventional i-line steppers typically illuminate the entire reticle at once and consequently need a larger lens. The new method also has a 4X image-reduction ratio, which means, for a given reticle, the image on the wafer can be larger than in conventional systems that use a 5X reduction ratio.
The wafer throughput of the step-and-scan technique is comparable to that of existing i-line steppers. "This technology is the first to achieve the 0.25-µm design rules required for production of 256-Mbit DRAMs--we`re really in a class of our own," commented NPI president David Huchital. "Semiconductor manufacturers will now be able to use this technology to do 0.25-µm production and even 0.18-µm development work." NPI expects to start shipping products incorporating step-and-scan technology in mid-1995.