June 4, 2002
Berkeley Lab Science Beat Berkeley Lab Science Beat
The race to make the next generation of semiconductor chips: extreme ultraviolet lithography leads the field
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It has been an exciting horse race. But thanks in part to help from Berkeley Lab's Center for X-Ray Optics (CXRO), researchers dedicated to making extreme ultraviolet lithography (EUVL) the tool of choice for the next generation of semiconductor chip manufacturers have left virtually all competitors in the dust.

Neville Smith, scientific director of the Advanced Light Source, makes a point of taking visitors to see the beamlines where optics for the prototype EUVL chip printer are rigorously tested. The "at-wavelength" EUV interferometer at beamline 12.0.1 is arguably the most accurate wavefront measuring device in the world; lately, this device has morphed into a mechanism that can print test wafers by itself.

The extreme-ultraviolet interferometer at the Advanced Light Source's beamline 12.0.1 was converted to a subfield exposure station to test printing with EUV optics.

"EUVL research is a perfect example of the maxim that 'for every $100 invested in research there is a return on average of $30 per annum in perpetuity,'" says Smith, quoting David King, chief scientific adviser to the British government. "Many research projects lead nowhere, of course, so it's the really big winners like this one that keep the average high."

Smith estimates that in terms of ultimate impact on the U.S. economy, EUVL research and development alone will have "paid for the ALS and maybe the entire U.S. synchrotron program."

The micro-crunch
For thirty years the industry has been packing ever smaller, ever more numerous electronic devices into microscopic integrated circuits, doubling chip density roughly every eighteen months by using photolithography. But no printer can craft features smaller than half the wavelength of its light, and at wavelengths shorter than deep ultraviolet, refractive lenses don't work: instead of bending light they absorb it.

In 1994 a consortium of semiconductor manufacturers launched a search for the best way to use shorter wavelength radiation in chip lithography. Four technologies entered the race: x-rays, electron beams, ion beams, and extreme ultraviolet. In October 2001 these manufacturers winnowed the field, choosing EUVL as the most likely technology to be able to reach the 32 nm "node" in the industry's technology roadmap -- which in practice means the ability to manufacture finished chips with individual lines as narrow as 13 nanometers (billionths of a meter) by 2009.

EUVL has been championed by a group of companies first assembled in 1997, now including Intel, Motorola, Advanced Microdevices, Micron Technology, Infineon Technologies, and IBM, who partnered with the Department of Energy's "Virtual National Laboratory," consisting of teams from Lawrence Berkeley, Lawrence Livermore, and Sandia National Laboratories.

Sandia is responsible for assembling the Engineering Test Stand (ETS), the collaboration's "alpha" camera and chip-printing tool. Livermore employs layers of precision coatings to create optics and masks and uses advanced optical testing methods and defect-inspection technologies.

Berkeley Lab's role, spearheaded by EUV program manager and former CXRO head David Attwood, has been to use ultraviolet light from the ALS in applying the world's highest standards for measuring reflectivity and uniformity to test components of the ETS camera.

Multilayer mirrors
Because the chosen 13 nm wavelength EUV is absorbed by glass or quartz lenses (indeed by all materials, including air), the beams must be focused by curved mirrors instead. The mirrors, made by Livermore and Tinsley Laboratories, are built up of dozens of layers of silicon and molybenum, each a few atoms thick.

A small fraction of the light reaching each layer is reflected, but each little reflection constructively interferes and adds up to some 70 percent of the total falling on the mirror. Their critical uniformity is measured by Eric Gullikson and his CXRO colleagues at ALS beamline 6.3.2.

Not only the focusing optics but the flat masks themselves must be made of multiple layers, with opaque regions on the reflective surface forming the printing patterns. Even the tiniest flaw in a mask can damage a circuit printed from it, and flaws can occur on the substrate or in any of the mask's dozens of layers. Masks are inspected for defects at ALS beamline 11.3.2.

Focusing mirrors are tested with the at-wavelength interferometer at beamline 12.0.1, whose coherent radiation, like light from a laser, is ideal for measuring optical components. The incoming beam is split in two, with one beam passing through and acquiring the aberrations of the optical system, while the other beam passes through a small pinhole, forming a nearly perfect spherical reference wave. The interference pattern of the two beams is recorded on a CCD camera.

The at-wavelength interferometer tests multilayer coated optics by generating interferograms, which measure departures from the ideal reflected wavefront.

If the optics were perfect, interference between the two beams would constitute a perfectly regular array of fringes; in the real world, aberrations displace the fringes from their ideal location.

A CXRO team led by Ken Goldberg, Patrick Nalleau, and Jeffrey Bokor tested two sets of four mirrors for the Engineering Test Stand (ETS), the alpha printer at Sandia, with the at-wavelength interferometer, measuring tolerances as fine as .04 nm -- less than the radius of a hydrogen atom! In April 2001, using the first set of optics, Sandia's ETS demonstrated its capabilities as the first full-scale EUVL prototype step-printer.

The race for extreme ultraviolet lithography, part 2