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.
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.
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 (firstname.lastname@example.org).