each of the contenders uses a different method to carve silicon, but they share a basic method: use of a stencil-like mask and basic etching techniques. Chips are now made with a gadget much like a large, highly accurate slide projector. Instead of blowing up your vacation pictures on the living room wall, however, an optical lithography machine shines light through an exquisitely crafted mask of the circuit pattern and images this on a layer of organic film, called a photoresist, that covers a silicon wafer. The organic film hardens on exposure to light; in the areas not exposed, the photoresist is washed off by solvents. This leaves regions of bare silicon that can either be etched to form channels or have other materials deposited on top to create the logic or memory elements in the integrated circuit.
The shorter the wavelength of light that is projected through the mask, the smaller the structures you can make on the chip. Leading chip makers, like Intel, have already moved deep into the ultraviolet region of light and use the 248-nanometer- wavelength light from a krypton fluoride excimer laser. This “deep UV” technology makes possible the etching of features as small as 200 nanometers and it is now being employed to crank out Intel’s latest microprocessors, the Pentium IIs, each packing about 7.5 million transistors.
the winning technology at the December meeting, EUV lithography, is based on a simple premise: make the wavelengths even shorter, and the feature sizes will shrink in tandem. That, however, is easier to envision than to execute. For one thing, the wavelengths of EUV light-40 nanometers down to 1-aren’t transmitted by any known materials, so conventional lenses are useless; the entire system must be made from reflective optics in order to focus the EUV light. There’s no smoke, but it does rely on an extremely complex arrangement of very accurate mirrors.
“The mirrors have to have an unprecedented degree of perfection,” explains John Bjorkholm, principal scientist at Intel’s advanced lithography department. The surface roughness cannot exceed the thickness of a few atoms. For a mirror 2.5 centimeters in diameter, this is like plowing the United States flat from New York to San Francisco, making sure no bumps more than 4 centimeters high remain. And once that problem is licked, the mirrors must be coated-bare metal or glass surfaces would absorb too much radiation. But, says Bjorkholm, “there has been exceptional progress” in manufacturing the mirrors.
The show-stopper for EUV, however, could be the mask. It’s not that masks can’t be constructed at this scale, but once they’re made, there is no known scheme for fixing the defects that inevitably turn up in them. In commercial lithography schemes, designers routinely tweak individual elements in the mask to eliminate blemishes. For the reflective masks needed in EUV lithography, however, nobody knows how to repair these delicate, multilayer coatings. “Reducing these defects is the number one challenge,” says Gardini.
EUV technology has been embraced by a consortium comprised of Intel, AMD and Motorola. These heavyweights have formed an entity called the EUV Limited Liability Company, which in turn has teamed up with three national laboratories in the United States to form a “virtual national lab” to develop EUV techniques. The group has already built a test system that can produce lines in a photoresist down to about 80 nanometers, and it has designs that should go down to 50 nanometers. The plan is to have a working prototype ready in the fall of this year.