New Breakthrough Cryogenic X-ray Microcalorimeter Improves Microanalysis Tenfold

The Commerce Department's National Institute of Standards and Technology has developed a revolutionary new X-ray micro- calorimeter that offers tremendous promise for more precise materials analysis, especially in the semiconductor industry.

The new microcalorimeter fits easily onto a commercially available scanning electron microscope (known as an SEM), conveniently operates even if the sensor is cooled to near absolute zero and achieves X-ray energy resolution that is at least 10 times better than conventional products. It was developed by a team of scientists including John M. Martinis, Gene C. Hilton, Kent D. Irwin and David A. Wollman at NIST's Boulder, Colo., laboratories.

Already, the microcalorimeter has received positive reviews from a technical working group at SEMATECH, the semiconductor industry's manufacturing research consortium in Austin, Texas. ... The SEMATECH Analytical Laboratory Managers Working Group believes that microcalorimeter detectors have the potential of becoming the X-ray detector of choice for semiconductor characterization applications," says Alain C. Diebold of SEMATECH.

Currently, semiconductor energy dispersive spectrometer (or EDS) detectors are the most commonly used X-ray spectrometers for microanalysis; they typically count 3,000 X-ray photons per second and measure the energy of each X-ray to within 100 electron volts. Better energy resolution between 10 and 20 electron volts is obtained by another commercial detector, the wavelength dispersive spectrometer (or WDS). However, this device is difficult to use and records X-rays only within one narrow energy band at a time. The new NIST detector combines the excellent energy resolution of a WDS with the operational ease of a semiconductor EDS.

In a microcalorimeter, the energy of an X-ray is converted into heat, and a measurement of the resulting temporary rise in temperature gives the deposited photon energy. NIST's micro- calorimeter consists of a normal-metal X-ray absorber in contact with a thermometer that uses the rapid change in resistance of a superconductor near its transition temperature to provide high sensitivity. When biased by a voltage source,this transition- edge-sensor (or TES) thermometer receives electrothermal feedback (or ETF) that stabilizes its operating temperature in the transition region and also reduces its response time.

The NIST detector presently achieves an X-ray energy resolution of 10 electron volts at a count rate of 100 photons per second. Researchers believe that the device eventually can exceed both of these limits.

Such ability is critically important to the semiconductor industry because a high-resolution system such as the NIST microcalorimeter fully resolves the nearly overlapping X-ray lines of silicon and tungsten. Therefore, manufacturers can make the definitive identification of the tungsten silicide they need to fabricate integrated circuits.

Two additional advanced technologies developed at NIST allow the ETF-TES microcalorimeter to be incorporated into a fully practical microanalysis system. The required 0.1 K operating temperature is provided by a compact liquid-helium cryostat that incorporates a novel two-stage adiabatic demagnetization refrigerator. The ADR is unique because it attains a 0.05 K base temperature working directly from the 4 K temperature of liquid helium.

The microanalysis system also uses a sensitive SQUID (superconducting quantum interference device) developed at NIST as a preamplifier for the TES thermometer output. The SQUID preamplifier provides the otherwise unattainable high output, high bandwidth and low noise required to utilize the micro-calorimeter fully.

When mated to a commercially available SEM, the ETF-TES microanalysis system has recorded X-ray fluorescence spectra that cannot be obtained easily by other methods. Of particular importance to the semiconductor industry, the high-energy resolution of the NIST system fully resolves the nearly overlapping X-ray lines of silicon and tungsten, allowing unambiguous identification of the tungsten silicide commonly used in integrated circuits.

More generally, the improved energy resolution of the NIST detector is expected to translate into a broad range of new capabilities in microanalysis, including greater sensitivity, higher spatial resolution and improved identification of light elements. NIST is currently looking for industrial partners to share in the commercial development of the ETF-TES microcalorimeter. Patents have been or are being obtained on many of the significant components; licensing arrangements are encouraged. For more information on industrial collaboration, contact John Martinis, Div. 814.03, NIST, Boulder, Colo. 80303-3323, (303) 497-3597, e-mail: john.martinis [at] nist.gov (john[dot]martinis[at]nist[dot]gov).

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.