Norman F. Bergren

Microfabricated transition-edge X-ray detectors

We are developing high performance X-ray detectors based on superconducting transition-edge sensors (TES) for application in materials analysis and astronomy. Using our recently developed fully lithographic TES fabrication process, we have made devices with an energy resolution of 4.5/spl plusmn/0.1 eV for 5.9 keV X-rays, the best reported energy resolution for any energy dispersive detectors in this energy range. These detectors utilize micromachined thermal isolation structures and transition-edge sensors fabricated from Mo/Cu bilayers with normal-metal boundary conditions. We have found the normal-metal boundary conditions to be critical to stable and reproducible low noise operation. In this paper we present details of fabrication and performance of these devices.

Precision Measurements Using a 300 mV Josephson Arbitrary Waveform Synthesizer

We have developed a Josephson digital-to-analog converter, otherwise know as a Josephson arbitrary waveform synthesizer, with 300 mV peak output voltage. This is the first system of its kind with demonstrated quantum accuracy. We show precision synthesized waveforms from dc to 100 kHz with measured distortion and harmonic content below 115 dBc (dB below the carrier or fundamental). The heart of the system is a superconducting microwave integrated circuit with two Josephson junction arrays biased in parallel for the microwave drive and connected in series to double the voltage for the audio frequency output waveforms. New superconducting integrated circuits with twice as many stacked junctions and improved microwave circuits have allowed us to more than double the output voltage of our recent system. We also demonstrate that quantum-based synthesized arbitrary waveforms (not just sine waves) can be used for precision measurements of a high-performance commercial analog-to-digital converter.

Broadband Lumped-Element Integrated

This paper presents a monolithically integrated broadband lumped-element Wilkinson power divider centered at 20 GHz, which was designed and fabricated to uniformly distribute power to arrays of Josephson junctions (JJs) for superconducting voltage standards. This solution achieves a fourfold decrease in chip area, and a twofold increase in bandwidth (BW) when compared to the previous narrowband distributed circuit. A single Wilkinson divider demonstrates 0.4-dB maximum insertion loss (IL), a 10-dB match BW of 10-24.5 GHz, and a 10-dB isolation BW of 13-30 GHz. A 16-way four-level binary Wilkinson power divider network is characterized in a divider/attenuator/combiner back-to-back measurement configuration with a 10-dB match BW of 10-25 GHz. In the 15-22-GHz band of interest, the maximum IL for the 16-way divider network is 0.5 dB, with an average of 0.2 dB. The amplitude balance of the divider at 15, 19, and 22 GHz is measured to be plusmn1.0 dB utilizing 16 arrays of 15 600 JJs as on-chip power detectors.

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We describe the current performance of the prototype microcalorimeter energy‐dispersive spectrometer (µcal EDS) developed at NIST for X‐ray microanalysis. We show that the low‐energy µcal EDS, designed for operation in the energy range 0.2–2 keV, offers significant advantages for low‐beam‐energy microanalysis. We present several examples in which the prototype µcal EDS has been used to solve problems in low‐voltage microanalysis, including the analysis of tungsten silicide (WSi2), titanium nitride (TiN) and barium titanate (BaTiO3) and the measurement of chemical shifts in Fe and C compounds.

-Way Power Dividers for Voltage Standards

The voltage from a single, microwave-biased Josephson junction is a small quantity; thus useful voltages are generated only through series arrays of many thousands of junctions. Arrays of superconductor-normal metal-superconductor junctions have been fabricated and tested with as many as 16,500 junctions per array. The arrays are optimized for the highest voltage operation with the largest operating margins for the current bias. Measurements show that these arrays, driven with 20 GHz microwaves, generate a dc voltage greater than 680 mV per array with a dc bias margin over 1 mA. To increase the microwave uniformity across the array, the transmission line impedance has been tapered. By use of this technique, ac Josephson voltages over 110 mVrms per array have been generated, also with over 1 mA dc bias margin.

Microcalorimeter energy-dispersive spectrometry using a low voltage scanning electron microscope

We have developed high-performance X-ray microcalorimeters based on superconducting transition-edge sensors. These superconducting detectors, which are cooled by a compact adiabatic demagnetization refrigerator mounted on a scanning electron microscope, provide significant new capabilities for X-ray microanalysis. The performance characteristics of these detectors are nearly ideal for many applications in X-ray microanalysis, attaining an energy resolution of 3-4 eV at a counting rate of 500 s/sup -/ and an effective collection area of 4 mm/sup 2/. The excellent energy resolution enables measurements of chemical shifts in X-ray spectra caused by changes in electron binding energy due to chemical bonding. Another important application of these detectors is analysis of contaminant particles and defects for the semiconductor industry. We present data demonstrating the analysis of particles on Si wafers, including 0.3 /spl mu/m tungsten particles and 0.1 /spl mu/m alumina particles.

Design of SNS Josephson Arrays for High Voltage Applications

A high-resolution energy-dispersive X-ray spectrometer (EDS) based on cryogenic microcalorimeter X-ray detectors has been developed for use in X-ray microanalysis. With an energy resolution of 3 eV at 1.5 keV, count rate of ∼500 s−1, and an effective collection area of ∼5 mm2 (using polycapillary X-ray optics), the microcalorimeter EDS combines many of the favorable qualities of commercially-available wavelength dispersive spectrometers (WDS) and semiconductor EDS. After describing the spectrometer system, we present several applications of microcalorimeter EDS to important microanalysis problems.

Superconducting transition-edge microcalorimeters for X-ray microanalysis

We have developed a high-resolution microcalorimeter energy-dispersive spectrometer (EDS) at NIST that provides improved x-ray microanalysis of contaminant particles and defects important to the semiconductor industry. Using our microcalorimeter EDS mounted on a scanning electron microscope (SEM), we have analyzed a variety of specific sized particles on Si wafers, including 0.3 μm diameter W particles and 0.1 μm diameter Al2O3 particles. To compare the particle analysis capabilities of microcalorimeter EDS to that of semiconductor EDS and Auger electron spectroscopy (AES), we report measurements of the Al-Kα/Si-Kα x-ray peak intensity ratio for 0.3 μm diameter Al2O3 particles on Si as a function of electron beam energy. We also demonstrate the capability of microcalorimeter EDS for chemical shift measurements.

The Approaching Revolution in X-Ray Microanalysis: The Microcalorimeter Energy Dispersive Spectrometer

Abstract We describe two magnetic techniques that may be used to determine the transport critical current density Jct of granular superconductors by measuring the intergranular magnetization of a sample. In the first method, magnetization critical current density Jcm is used to estimate Jct by isolating the intergranular magnetization and applying the critical state model. In the second method, magnetic detection is used to measure Jct directly: intergranular magnetization hysteresis loops are obtained while increasing a transport current through a sample. The critical current density Jct is that value of transport current density which causes the intergranular magnetization to collapse at a given magnetic field and temperature. Both methods give values of Jct in fair agreement with values obtained from conventional transport measurements of Jct. Magnetization was measured with both extraction and Hall probe magnetometers.

High-resolution microcalorimeter energy-dispersive spectrometer for x-ray microanalysis and particle analysis

We present the current performance of the prototype high-resolution microcalorimeter energy-dispersive spectrometer (μcal EDS) developed at NIST for x-ray microanalysis. In particular, the low-energy μcal EDS, designed for operation in the energy range from 0.2 keV to 2 keV, offers significant advantages for low-beam-voltage (high-spatial-resolution) x-ray micro-analysis for critical applications in the semiconductor industry such as particle analysis. We present several examples in which the low-energy μcal EDS has successfully solved problems important to the semiconductor industry, including analyses of small contaminant particles (Al, Al oxide, Cu oxide) and thin films (TiN, Ti oxide, 0.5% by weight Cu in Al). We also describe the future development of a large-format microcalorimeter array with significantly improved total effective area and total count rate.