Survival of the Fittest: Lenses Versus Concave Mirrors

"A mirror projection aligner is related to a stepper, which is used in semiconductor device production. Canon initially built steppers incorporating a reflective mirror. As semiconductor integration levels increased, however, we began to realize the limit of systems using reflective mirrors. We then shifted to lenses, and after much research this shift began to bear fruit in the field of producing liquid crystal displays." (Ito)

"It sure is Canon Heavy Industries!" A crane overhead with a humming motor and yellow flashing lamps plies its way back and forth across the manufacturing line.
Haruna Kawashima, in charge of illumination system design, once again looks over this world-leading manufacturing line, which appears like a shipbuilding yard. Soon after joining Canon, he was assigned to work on liquid crystal substrate development, after which he shifted to designing steppers for LSI fabrication. Six years ago, he returned to liquid crystal substrate development. Around that time, Canon began to achieve dramatic increases in the performance of its mirror projection aligners, allowing manufacture of systems with larger and larger exposure size.

The world of LSI fabrication steppers is characterized by fierce technical competition as manufacturers raise the level of integration and narrow the linewidth of circuit patterns. The current focus is on developing special light sources that emit short wavelengths, and on designing lenses compatible with such light sources. Mirror projection aligners are similar to LSI steppers in that a minute circuit pattern is exposed onto the substrate with high precision. However, one point of difference is that mirror projection aligners have gotten larger. Kawashima explains the reasons for this difference.

"One could say that LSI steppers and mirror projection aligners are related.
However, they have evolved in different directions. Steppers have become more compact, with narrower circuit patterns. But with mirror projection aligners, the degree of intricacy only needs to satisfy the human eye, and they have evolved continually, based on the aim of enabling efficient volume exposure onto glass substrates. Since the substrates have increased in size, mirror projection aligners have grown larger and larger as well."

Although both types of equipment are the same with respect to "exposure" processes, the exposure size of a stepper is 20-30 mm square at most. The mask (reticle) pattern is scaled down to one-quarter of its size and exposed repeatedly onto a wafer measuring 300 mm in diameter. Liquid crystal substrates, by contrast, now measure up to 47" across. The exposure area is the same size as the mask. The larger the exposure area, the better for achieving efficient production.

In a LCD television screen, red, green and blue color filters and a liquid crystal layer are sandwiched between substrates, onto which transparent electrodes have been exposed. The vivid color images that appear on an LCD are produced by illuminating and dimming red, green and blue in each tiny pixel and instantaneously displaying a succession of various colors---like mixing colors on a painter's palette. The role of the mirror projection aligner is to expose onto the glass substrate the transparent patterns that send electrical signals to each and every one of the tiny pixels crammed into it.

In the LCD television market, screens are growing larger and larger thanks to technological innovations that began at the turn of the 21st century. Screens have now reached 47" across. What technologies have made this possible?