Cleaning optics

Cleaning watefersteppers

During the period of 1993, the company O3systems get a call from the development department, of ASM Lithography in the Netherlands. They had a problem with the lenses in their waver steppers.

The lens encountered in their "printing process " can be imagined as a telephoto lens which consists of 28 glass lenses. Each lens is very carefully balanced and so responsible for a very high resolution printing of the images of the electric circuit. This is printed on silicon wafers. After the exposure, the silicon wafers are developed, etched and again made ready for the next floor. This cycle is repeated approximately 28-36 times. For example, a memory chips, processor. is build as a “sky carper” containing 28-36 floors (levels) which are also very precisely linked.

The lenses were previously treated with ultra pure Helium. Despite these precautions, the lens went dirty, there was an oil film. This meant that production dramatically collapsed, and many chips were rejected and the resolution was not reached. The company ASM Lithography was at that time on the brink of bankruptcy.

The defective lenses had to be removed and replaced by other lenses. Is curious to note that these lenses were hardly subject to vibration so because otherwise the system again became unbalanced. The lens should again be balanced out. That alone takes 3 months!

At that time at Carl Zeiss in Oberkochen a team of about 40 specialists try to solve this problem.

The solution to this problem was very simple. To flush the lens with ozone gas. This “simple” operation had in a very short time a dramatic and positive impact. The lens was -as I predicted- even better than if Carl Zeiss could make! It was no longer necessary to transport defective lens over the world

Due to transport and contamination the % transmission decreased gradually up to 40%.

After an old dead a lens 2 million Euro got a second life and a better life, this process had been first applied to the MICRON company in America. Micron produced 256 K DRAM chips. At this location there were 35 machines virtually none of their was on specification. After the management had finally agreed to clean a machine with this new cleaner the staff was flabbergasted and confident of the result. I quote the management "I Want to Have All Those waver steppers Cleaned the critical and non citical's"

Despite several uncertainties related to the then guarantees and yet unforeseen effects that could be found in the future the management demanded that all units would receive this treatment (rinsing with ozone into oxygen gas). Prompt the following year MIRCRON was the best and most efficient producing semiconductor company in the world.

The ozone treatment has the effect that there is at least four improvements in four areas namely: at least 30% faster exposure time, 30% better transmission, 30% less product failure, 30% better resolution. Math what the benefit and impact on prices of microprocessors.

The above applies to the result in the cause of ASM Lithography they are still the the leader in this field. Meanwhile the gas quality has been changed (there is now a smart

method using oxygen) Each waverstepper made by ASM has now somewhere a small ozone generator that can switched on and of at times . To get the lens

cleaned or reconditioned

Epilogue ASML project.

It appeared that the very fast results improvement of the % transmission better than that of the virgin lenses was caused to the fact that the "organic" layer was bleached not removed!

After longer treatment periods 2 a 3 day's with ozone the relative humidity increased which led to water condensation in the last sections of the lens system. Lowering Ozone concentration and optimizing cleaning intervals led to complete removal of the contaminant.

Writhing this I realize now that this (NEW?) system and measurement principle can in a simplified way used in a laboratory environment for studying reaction rates etc. of several (organic) components with ozone. Use a set of contaminated Quarts or glass sheets and a (UV A/B/C) light source and spectrofotometer.