|Home : Volume 13 : Spring 1987 :|
|X-ray lithography for chips: What It is, why It's Important|
New technology from Wisconsin
First of a series
The UW-Madison is making available to U.S. microelectronics companies a unique research facility that may help the industry in its struggles with foreign competition. The facility permits research in X-ray lithography, a technology that could shrink the smallest features of integrated circuits to one-quarter or one-tenth their present size.
This facility is Aladdin, a synchrotron electron storage ring that produces X-rays ideal for lithography. It is one of two such sources in the U.S., and the only one with multiple ports available for microelectronics work.
The following story gives an overview of this new technology.
X-ray lithography promises to be an important tool for producing some of the microelectronic chips at the heart of all modern electronic equipment, from $5 calculators to television sets, medical electronic devices, defense systems, and the largest computers.
The technology could also produce three-dimensional microscopic devices which are impossible to make with current production technology and which would have a wide range of uses.
Chip-making, whether using X-rays or not, starts with a pure silicon wafer three inches or more in diameter. It ends with the wafer sliced into chips one-quarter inch or less on each side, each enclosed in a small plastic case with wires coming out for attachment to other electrical components.
Between start and finish of this process may be hundreds of steps to add layers of precisely formulated materials and to remove certain materials in an exacting series of patterns. The result is an intricate, three-dimensional network of transistors and other devices on each chip.
While some of today's most complex chips may contain one million transistors, those produced with X-rays could contain as many as 100 million, according to electrical and computer engineering Professor Henry Guckel.
This would mean much smaller, faster, and more powerful chips which would be especially valuable in defense systems, supercomputers, and other demanding applications.
In addition to Guckel, ker faculty members in this work at Wisconsin are chemistry Professor James Taylor, chairman of the materials science program; Associate Dean John Wiley; Professor Franco Cerrina, electrical and computer engineering and physics Professor David Huber, head of the Synchrotron Radiation Center.
In general, "lithography" refers to processes, used in printing and some other applications, involving chemical treatment and exposure to light. The microscopic patterns that make up today's microelectronic circuits are reproduced by passing light through stencil-like masks and developing the pattern in much the same way photographs are developed in a darkroom.
The wavelength of the exposing light itself limits smallest features of a pattern to about a micron. (The diameter of a human hair is about 100 microns.) X-rays, with a shorter wavelength, could reduce circuit features to one-quarter or even one-tenth micron.
Despite the great promise of X-ray lithography, there are several obstacles to its use. One is that the features of masks used in X-ray lithography are too small to be examined and checked for errors by light microscopes. The process requires time on expensive electron microscopes. The mask's complexity and small size of its features also make it difficult to repair defects.
Reducing heat problems
Another concern is that heat generated in opaque areas during X-ray exposure distorts the mask. In addition, for the technology to be applied in industry, X-ray sources would have to be designed and built for production runs rather than primarily for research.
Guckel said a technique that uses an electron beam to draw microcircuit patterns is another practical way to produce more-compact chips, but this is a relatively slow process for drawing circuits directly on large chips. With a combination of one set of masks created by the electron beam method and X-rays to expose chips through these masks, high-volume production runs may be possible. X-ray lithography also may offer advantages over other methods in the manufacture of more common types of chips, and in other applications, in the years ahead:
This greater depth of field with X-rays could eliminate chip processing steps now necessary to recreate flat surfaces at some stages of manufacture. It also could facilitate production of very large chips.
X-ray technology has special capabilities for the manufacture of microscopic mechanical devices, some of which would be connected to electronic circuits and some of which would not. These devices could include tiny nozzles for engine fuel injection and other purposes; sensors integrated with electronic circuits on a chip to measure pressure, acceleration, flow, sound, or other quantities; and microscopic probes or surgical instruments for work on individual living cells. Microscopic valves and even motors may be feasible.
X-ray lithography has capabilities for this work that far exceed those of visible or ultraviolet light because X-rays have great depth of field and the ability to penetrate far into certain materials used in chips.
As a result, microstructures are possible which are only a fraction of a micron wide but hundreds of microns high. Sensors, probes, and other devices can be constructed with X-ray technology which would be impossible to build by any other known means.
This mask is used like a stencil to map integrated circuit patterns on silicon wafers.
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