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  • Excimer Laser Powers Phase-Shifting Interferometer

Photonics Spectra
May 2004
Jôrg Schwartz

An interferometer that promises to improve the characterization of phase-shift masks for use in semiconductor lithography has been developed by researchers in Germany. The instrument, which uses an ArF laser as an illumination source, takes several pictures of the mask with a different relative phase shift and calculates the phase distribution.

The continuing trend toward more powerful integrated electronic circuits requires manufacturers to increase feature densities on a chip of a given size by decreasing the size of the circuit structures. To enable them to do so, engineers are developing resolution enhancement techniques such as the application of masks that employ controlled phase shifts to improve the pattern created on a substrate. Interference produces intensity values of zero between structures that would be inseparable if amplitude masks had been used.

This requires that the phase shifts creating the interference patterns be generated and measured very accurately, typically expressed in terms of the phase difference introduced between adjacent details of the mask. Heretofore, this has been done indirectly, using atomic force microscopy and other profiling techniques that measure variations in the thickness of the mask.

The new method, devised by scientists at Friedrich Alexander University of Erlangen-Nuremberg in Erlangen, in collaboration with TuiLaser AG of Munich, both in Germany, takes the direct approach. The optical setup consists of a UV microscope operated in transmission mode with two surface relief gratings -- Ronchi gratings -- in the imaging path. A common-path arrangement is used, rather than separate object and reference paths, to ease issues of stability, vibration and air turbulence and to minimize losses due to absorption by other components in the optical path. The gratings generate two laterally sheared wavefronts that carry phase information to the object being characterized.

The introduced shear depends on the distance between the phase gratings, and to introduce large shears, a complex coherence is prepared. This is done using a structured illumination source, in which a pattern is added to the lateral beam profile of the laser source. The technique reduces unwanted interferences and allows for a high contrast ratio and minimal light loss. Under these conditions, the phase step introduced by a phase-shift mask structure can be measured with a high degree of accuracy.

At the short wavelengths used, a high degree of stability and sensitivity is critical. At 193 nm, for example, the phase shift induced by a feature with a height of 1 nm results in an intensity change of only 1.65 percent.

The performance of the illumination source in the system also is critical. The laser must display a homogeneous spatial beam profile, low shot-to-shot noise and high long-term stability in both continuous and burst modes. TuiLaser's ExciStar S-500, an industrial-grade 193-nm ArF laser optimized for photolithography applications, uses solid-state switching and corona preionization of the discharge to meet these requirements.

Another design consideration is the need for reliable synchronization between the laser and the CCD that collects the phase information returning from the mask. In the interferometer, this is done through a software driver that ensures that the same number of laser pulses is averaged for each CCD frame. On the hardware side, this means that the laser trigger must display very low jitter.

Tests of the inspection system confirm that direct phase measurement offers accurate results because the measurements are performed at the design wavelength, avoiding mask and wavelength conversion errors. It provides an absolute accuracy of the phase measurement of better than 2π/280 (i.e., <0.7 nm at 193 nm) and a reproducibility of 2π/1150 rms (i.e., <0.17 nm at 193 nm).

The technique is used as part of Carl Zeiss lithography systems and as an extension for 157-nm applications to support enhancement of phase-shift mask resolution for even smaller structures. It also may find application outside semiconductor manufacturing.

An instrument that employs the interference of lightwaves to measure the accuracy of optical surfaces; it can measure a length in terms of the length of a wave of light by using interference phenomena based on the wave characteristics of light. Interferometers are used extensively for testing optical elements during manufacture. Typical designs include the Michelson, Twyman-Green and Fizeau interferometers. The basic interferometer components are a light source, a beamsplitter, a reference...
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