By Mark K. Anderson
Sometimes a typewriter is better than a laser printer. This viewpoint, long held by cranky writers and technological traditionalists, has now been at least partly verified.
A group of engineers at Princeton unveiled a method Thursday that can make smaller, faster and cheaper computer chips than currently possible. This is done by physically imprinting the chip -- analogous to striking raised type against a sheet of paper.
Currently most computer chips are made by shining ultraviolet light through a template and onto a silicon wafer coated with a surface film called a resist. The resist is developed like a photograph and the silicon is etched following this photographic guide. The resist is then stripped away, leaving the channels in the wafer that will become the byways for the chip's millions of wires, bits and logic circuits.
Laser printers -- which use laser light to electrically charge a surface that picks up toner and then imprints the toner onto paper -- use an analogous optical two-step process.
However, photolithography can only make chip features that are as small as the wavelength of the light being used -- typically 193 nanometers. And these intrinsic limits are fast approaching as chip-size scales continue to diminish.
"With optical tricks you can get (chip) feature sizes down to maybe one-quarter of the wavelength of the light," said Tsu-Jae King of the University of California at Berkeley. "So it's better to use physical imprinting to achieve small feature sizes."
As she pointed out, some chips now coming off the assembly line have 65-nanometer-sized features. So chip manufacturers are already near the edge of photolithography's physical limit.
Photolithography is also a time-consuming and expensive process.
"We're talking about six or seven steps, and each step takes minutes," said Stephen Y. Chou of Princeton.
"In our case, everything happens at once. You put your flat wafer in and then in a fraction of a second, the pattern will form."
Chou, whose team's work is published in Thursday's edition of the journal Nature, developed the precursor to his current chip assembly process in 1996.
In this early version, the raised quartz printing surface was pressed against a silicon wafer coated with a resist. Then the chip was etched and the resist scraped away, as in the traditional method.
However, the method he announced Thursday cuts out the resist and the developing and etching steps altogether. It simply presses a raised-quartz printing surface against the silicon and -- after a nanosecond-long laser burst through the quartz and onto the silicon -- the chip is done. The laser pulse melts the top layer of silicon, which then expands to fill the mold.
Chou's method created chip features as small as 10 nm, and he estimates that it would be 10 times cheaper than photolithography. It also eliminates the need for the resist and developing and etching chemicals -- some of which have raised environmental concerns.
"Here it's dry. You have no chemicals," Chou said. "This is a completely physical process."
Chou even predicted that his method could be used to someday create the chip structures necessary to house single-molecule transistors such as the ones that were announced last week.
King noted, however, that the next hurdle Chou's team will face is aligning the high-resolution features on a silicon surface that typically gets "printed" on more than once. Poor alignment would result in the equivalent of an off-register color print -- as seen in newspapers plagued by snafus at the printing plant.
"If you don't align (the features) very well, that can affect the transistor's performance," she said.
King predicted that chip manufacturers will almost certainly be using direct imprinting techniques like Chou's in the future. But, she added, the computer industry is also unlikely to give up the photolithography ghost anytime soon.
"The industry usually likes to avoid change," she said. "In the future, it's possible that they'll use direct imprint for the critical layers and optical lithography for the others."
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