HOW do you accurately measure the surface of a mirror to check for high or low spots that are no larger than a few atoms? Until recently, you couldn't. But that has changed, thanks to a team at Lawrence Livermore. Their new Absolute Interferometer can measure large surfaces to find uneven spots less than 1 nanometer (billionth of a meter) higher or lower than the rest of the surface.
Optical interferometers are instruments that can make very precise measurements of objects using the interference pattern of two waves of light. One wave interacts with the object being measured, and the other does not; their interference when they encounter one another allows measurements to within one-thousandth of the wavelength being used. Very small distances and thicknesses can be measured, including extremely small surface irregularities in optical devices such as mirrors. In astronomy, interferometers are used to measure the distances between stars and the diameters of stars.
For your bathroom mirror, such perfection is hardly necessary. But for high-end optical applications, accuracy is essential. The semiconductor industry will find the new interferometer indispensable as the demand for ever more powerful microchips necessitates a change in chip printing methods.
As the tiny circuits printed on microchips are made smaller, more circuits and hence more information can be included. Today, the binary-circuit patterns are projected onto a resist-coated silicon wafer. The size of the features on the chip is limited by the shortest wavelength that the lenses in the projector will transmit. When the wavelength gets down to about 180 nanometers, no lens can transmit it.
To make the chips' features smaller, mirrors can be used to reflect rather than transmit the light, allowing the use of light with wavelengths as short as 13 nanometers. This new process is known as extreme ultraviolet (EUV) lithography because the light used is in the far edge of the ultraviolet range of the spectrum. With it, microprocessor features can be made as small as 0.1 nanometers, which is about 1,000 times smaller than the width of a human hair. With current lithographic methods, the smallest achievable feature size is 0.18 nanometers. Today, the smallest features in production are 0.35 nanometers.
To reflect such short wavelengths, mirrors must have high and low spots (known as surface figure errors) less than approximately 0.25 nanometers. Fabricating such a mirror requires surface measuring systems that border on perfection. This is where the Absolute Interferometer comes in. The brainchild of physicists Gary Sommargren, Donald W. Phillion, and Eugene Campbell and designer Franklyn Snell, the new interferometer represents a 100-fold improvement in accuracy for measuring surface shapes of optical components and removes one of the blocks to furthering the development of EUV lithography.

Adding Diffraction
Like all interferometers, this one uses the interference pattern of two waves of light to measure objects or phenomena. These light waves are usually imperfect because they are dependent on the condition of the surface or lens from which they emanated, and this imperfection introduces error into the measurements. To correct this problem and produce a nearly perfect spherical wavefront, Livermore's new interferometer incorporates diffraction, which is the breaking up of light as it passes around an object or through a hole. The core of an optical fiber acts as an aperture through which the light beam passes. There are other optical interferometers that incorporate diffraction, but this one is different in that the two wavefronts are generated independently. Their relative amplitude and phase can be controlled, yielding the contrast adjustment and phase-shifting capability necessary for the highest possible accuracy.
A frequency-doubled, Nd:YAG (neodymium-doped ytterbium-aluminum-garnet) laser operating at a 532-nanometer wavelength launches light into two single-mode optical fibers. As shown in the figure (below), the light diffracts on exit from the fibers, forming spherical wavefronts. The "measurement" wavefront passes through the optical system being tested, which induces aberrations in the wavefront and causes it to focus on the endface of the other fiber. Here the wavefront reflects off a semitransparent metallic film on the fiber endface and interferes with the "reference" wavefront to generate an interference pattern. The pattern is recorded by a charge-coupled device camera. Associated software describes the magnitude and spatial distribution of errors so that correction strategies can be devised.






The design is simple, containing only the optic being tested and the two optical fibers that generate the two wavefronts. This design makes the interferometer versatile for measuring optical components and systems and allows it to measure in a single-pass transmission, unlike conventional interferometers. The critical component is the fiber endface coated with a semitransparent film. It must have a flatness comparable to the desired accuracy of the measurement, but only over a very small area around the fiber core. By embedding the fiber in a glass substrate and superpolishing the entire assembly, achieving the surface finish is easy. To ensure stability and ease of mounting, the fiber remains embedded in the substrate during use. Typical fiber cores have a diameter of 3 micrometers. The critical, carefully polished region has a diameter of about 1 millimeter.
The system's design assures its accuracy in at least two ways. The fibers act as spatial filters, correcting for any lack of quality in the wavefronts as they pass through the fibers. And before the two wavefronts interfere, they encounter no other optical components that can degrade accuracy, except for the one fiber endface.

Improved Optics
The Absolute Interferometer opens the door to fabrication of optical components for EUV lithography with surface errors within approximately 0.25 nanometers. Fabrication of these mirrors requires real-time, surface-measurement feedback during the polishing process at a level of accuracy that conventional interferometers cannot provide.
Livermore's new interferometer can be used to test any optical component or system where extreme accuracy is required. Perhaps more important, it can also be used to create higher-quality reference surfaces for use in conventional interferometers. These reference surfaces would advance the accuracy of the large base of interferometers already installed in the optical industry. Small companies would then be able to increase their capabilities without having to purchase new interferometers.
The usefulness of any optical system is limited by its inability to produce perfect images. As we move to extreme ultraviolet and soft-x-ray wavelengths for microscopic medical imaging or deep-space astronomical imaging, advances in optical fabrication and testing must keep pace. The Absolute Interferometer brings new accuracy to these applications.

--Katie Walter

Key Words: extreme ultraviolet (EUV) lithography, fiber optics, integrated circuit manufacturing, interferometry, optics, R&D 100 Award.

For further information contact Gary Sommargren (510) 423-8599 (gesommargren@llnl.gov).


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