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Multichannel, Chemically Precise X-ray Pulse Processor


Energy dispersive x-ray spectroscopy is a widely used materials analysis technique in industries such as metallurgy and semiconductor manufacturing. In particular, it is used to determine elemental compositions including trace components (e.g. yield-destroying, processed-wafer defects in large semiconductor fabrication lines). In recent years, energy dispersive x-ray spectroscopy using cryogenic microcalorimeter sensors has become commercially viable. Microcalorimeter sensors provide more than 10x better spectral resolving power than previous semiconducting detectors. This additional resolving power can be used to separate overlapping x-ray features, improve peak-to-background ratios for trace constituents, and observe so-called ‘chemical shifts’, x-ray features that identify the chemical state of a particular element. While commercial x-ray pulse processors exist for semiconducting detectors, there is an unmet need for a pulse processor compatible with the electrical signals produced by emerging arrays of superconducting microcalorimeter x-ray sensors. A complete microcalorimeter x-ray spectrometer consists of a cryostat, the sensors, their readout electronics, and a pulse processor. At the present time, all of these components are commercially available except an optimized pulse processor. Hence, the commercial development of a pulse processor will enable the widespread dissemination of microcalorimeters to the large analytical community that already uses energy dispersive x-ray spectroscopy. NIST has a strong interest in the development of such a pulse processor because of its connections to precision industrial analysis, especially semiconductor manufacturing, its ability to extend precision x-ray measurements to chemical composition, and NIST’s previous role in the commercialization of other aspects of microcalorimeter technology. The ability to more precisely quantify chemical composition via this technology is expected to enable analytical applications in a wide range of U.S. industries.

The central goal of this subtopic is the development of pulse processing electronics with the specifications described below:

1. Accept analog input signals spanning range: 0 Volts to 10 Volts.

2. Digitize analog input signals with sampling speeds >= 200 kHz and >=12 bits of sampling.

3. Filter digitized input signals to extract pulse heights with the highest possible signal-to-noise ratio. Pulse processing should produce energy resolutions better than 3 eV at 6 keV and better μthan 2 eV at 1.5 keV from sensors with sufficient intrinsic signal-to-noise to achieve those resolutions. Extracted pulse heights should be independent of DC level of pulse signals.

4. Accept pulse rise times in range 20 μs to 100 μs and pulse fall times in range 100 μs to 500 μs. Pulse decays are expected to be well modeled by a single exponential time constant.

5. Algorithms to extract accurate pulse heights in the presence of pulse pile-up are encouraged.

6. Algorithms to correct pulse heights for slow drifts in system response are encouraged. An example would be the application of a correction based on a previously measured correlation between baseline value and pulse height.

7. Pulse heights measured in electrical units shall be converted into absolute energy by means of a user-supplied non-linear calibration curve. The calibration curve is expected to be a 2nd order polynomial or even more complicated functional form. Provisions for a separate operating mode to determine the calibration curve from spectra of a reference material or materials are encouraged.

8. Processed, calibrated pulse heights shall be output in a format that is accepted by the software environment of one or more vendors of tools for energy dispersive x-ray spectroscopy on scanning electron microscopes. When possible, the number of output pulse height channels shall reflect the improved spectral resolution provided by microcalorimeter sensors. For example, more than 3000 output channels are needed over the range 0 eV to 10 keV to avoid discarding useful energy information. Pulse heights shall be accompanied by time tags or other information needed to correlate x-ray results with the position of the microscope’s electron beam so as to generate x-ray maps of spatially varying materials.

9. Pulse processing shall be performed for at least 16 independent sensor channels.

10. User shall have the ability to look at a single co-added master spectrum or 16 individual spectra. User shall have the ability to exclude user-selected channels from the co-added master spectrum.

11. The pulse processing must be performed in real-time but a short latency is acceptable (< 3 sec).

12. Pulse processing shall be compatible with total input count rates > 10 kHz across 16 sensor channels.

13. Pulse processor shall compute and report deadtime.

14. Pulse processing architectures that can be scaled in the future to larger numbers of independent sensor channels (>16) are preferred. In this future vision, input signals might come in a single interleaved timestream as is produced by time-division SQUID multiplexing.

15. Applicant should either be a vendor of x-ray microcalorimeter spectrometers or should collaborate with a vendor of x-ray microcalorimeter spectrometers in order to be sure that the pulse processor is optimally matched to these instruments.

16. Algorithms that detect the loss of flux lock in the SQUID readout electronics and that can rapidly reset such electronics or signal that such a reset is needed are encouraged.

Phase I expected results:
A full design study of the desired pulse processor is expected for Phase I. In addition, partial demonstrations of pulse processing functionality with actual microcalorimeter sensors or with previously digitized and recorded data from microcalorimeter sensors are encouraged. These demonstrations can be performed at the single channel level.

Phase II expected results:
Construction and a full demonstration of the desired pulse processor at the 16 channel scale is expected for Phase II. This demonstration shall be performed with a microcalorimeter x-ray spectrometer. This demonstration shall include seamless integration of the pulse processor with a software environment for energy dispersive x-ray spectroscopy on electron microscopes.

NIST may be available to assist the awardee in consultation on desired specifications and candidate algorithms for filtering, calibration, addressing pileup, and addressing drift; providing previously digitized microcalorimeter pulse data; and exchange of site visits for technical discussions and system demonstrations.

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