Photolithographic process engineers are frequently tasked with developing new processes on older exposure tools, this can be challenging due to the tool's degradation due to age. CD uniformity across the exposure field is one such challenge. Gray filters are a corrective filters designed to improve the illumination uniformity and the CD uniformity.

ASML 5500 Yield Improvement: Improving Illumination Uniformity: Digital Gray Filters

Photolithographic process engineers are frequently tasked with developing new processes on older exposure tools, this can be challenging due to the tool's degradation due to age. CD uniformity across the exposure field is one such challenge. Gray filters are a corrective filters designed to improve the illumination uniformity and the CD uniformity.

What is a Gray Filter? It is a neutral density filter with a pattern that increases or decreases the irradiance [mw/cm2] across the object plane [reticle] designed to optimize [reduce] the illumination uniformity across this full field area. It is customized for a specific exposure tool. It compensates for the exposure tool's optical component degradation that occurs over time causing poor illumination uniformity conditions. It is more cost effective and significantly reduces the downtime than replacing optical components.

What is a Digital Gray Filter? It is a gray filter with a pattern defined as pixels in chrome. The pattern is modeled from the existing illumination uniformity condition (no gray filter).

   Simax has developed a custom Digital Gray Filter program for ASML 5500/xxx steppers to significantly improve CD uniformity. Improving illumination uniformity has been shown to improve CD uniformity and increase yields by 5% and reduce reworks by 3%. (These results are product and process sensitive.)

  







            Pre Gray Filter 3.59%                   SIMAX Digital Gray Filter: 0.54%

Digital Gray Filter wafer plane Irradiance results on ASML 5500 Stepper exposure tool:
Source: Simax

This e-mail address is being protected from spambots. You need JavaScript enabled to view it. Simax Applications Engineering for assistance in improving CD uniformity.


Steve Brainerd is a Senior Applications Engineer with Simax Lithography. With over 30 years “hands-on” wafer fabrication experience concentrated in semiconductor wafer, MEMs, thick photoresist, solar, and LCD fabrication manufacturing processes, Steve uses his strong analytical skills to develop robust processes for customers. His knowledge and experience include process-equipment startup, process development, process modeling, anti-reflection coating design/modeling, cost reduction, yield improvement, and capacity increases.