Contributing to the development and evolution of IC steppers and scanners at Nikon
Contributing to the development of IC steppers and scanners at Nikon
What did you do at Research Section II, where you were first assigned at Nikon?
My first task there was to design optics for a focal-length measurement instrument, which measures the focal length of ophthalmic lenses automatically. I had to create the blueprint for it immediately after I joined Nikon, and failed. I should have designed it using the actual dimensions of test plates, but I made the drawings using nominal numbers. This taught me the difference between theory and practice.
Following this, I participated in developing a measuring method for an instrument that measures lens thicknesses employing an interferometer. This method, based on Mr. Tsuruta's ideas, achieved extremely precise measurement and was very advanced, so we presented it at an international conference. We couldn't put it to practical use. But later, we were able to develop an easy-to-use, lens thickness measurement system, employing a laser interferometer.
After that, you worked on a project which later contributed to the development of IC steppers and scanners at Nikon.
Nikon entered the IC stepper and scanner business prompted by the development of a proximity exposure IC stepper*. I wrote software with which we could simulate the most appropriate distribution of light sources for the system. IC steppers and scanners print IC photomask patterns onto wafers. Today's machines etch a miniaturized IC pattern by exposing it through projection lenses. Unlike this, machines at that time etched an IC pattern at the same size, because patterns were simpler. In proximity photolithography, the machine maintains a small gap between the photomask and wafer, to avoid photomask damage caused by bringing them into contact. But this gap causes diffraction patterns, which require several light sources on the machine to erase. We needed to study and determine the most effective locations for the light sources.
Did you face many problems?
Optical design, using software like that, requires vast computation. Although our laboratory was equipped with the latest computer of the time, it took a lot of time for us to obtain results. Data processing which today's computers can handle in a mere second, took a whole day. Once I had started processing data I had to continually occupy the computer, blocking my colleagues' access to it. So I came to work on weekends to carry out my data processing.
- *Proximity photolithography: a noncontact photolithography method that maintains a gap of from a few to several dozen micrometers between the photomask and wafer during exposure.
Establishing a measuring method for aspherical lenses.
Later, when you were at the Optical Technology Development Department, Core Technology Center, what projects were you involved in?
As the lines of circuit patterns became thinner, larger projection lenses were needed for IC steppers and scanners. Eventually, the size of lenses we required became so large that they were impossible for us to fabricate. Employing aspherical lenses was an inevitable step in order to reduce the lens size while improving the performance. We launched an aspherical lens project with three teams. I became the leader of the metrology team, establishing a measuring method for aspherical lenses.
You contributed to the further evolution of IC steppers and scanners, that is clear. Is measuring aspherical lenses more complicated than the spherical lens process?
Yes. Both kinds of measurement employ an interferometer. When evaluating the precision of a lens in such measurement, we study interference patterns which appear when fitting the lens with the test plate, made of glass. In this process, the precision of the test plate is crucial. Examining the precision of a spherical lens test plate is relatively simple, because it has a fixed curvature throughout the surface. But we couldn't confirm the precision of aspherical lens test plates, so created a problem. What's more, precision has to be measured at the sub-nanometer scale.
Nikon was the only Japanese optical manufacturer that successfully created an aspherical lens measuring method. Without this technology, we couldn't have developed IC steppers and scanners that can stay abreast of the increasing intricacy of circuit patterns.
