L. R. Canfield

Stability and quantum efficiency performance of silicon photodiode detectors in the far ultraviolet

Recent improvements in silicon photodiode fabrication technology have resulted in the production of photodiodes which are stable after prolonged exposure to short wavelength radiation and which have efficiencies in the far ultraviolet close to those predicted using a value of 3.63 eV for electron-hole pair production in Si. Quantum efficiency and stability data are presented in the 6-124-eV region for several variations on the basic successful design and on devices with extremely thin silicon dioxide antireflecting/passivating layers. The results indicate that the oxide is dominant in determining many of the performance parameters and that a stable efficient far ultraviolet diode can be fabricated by careful control of the Si-SiO(2) interface quality.

A Method for Measuring Polarization in the Vacuum Ultraviolet

A simple experimental method for determining the degree of plane polarization of a far ultraviolet monochromator was developed and tested. This determination requires only a measurement of the reflectance at 45° angle of incidence with two or more orientations of the reflector about the optic axis. This procedure works at all wavelengths without requiring a detailed knowledge of the optical constants of the reflector material. The degree of plane polarization has been determined for a 0.5-m Seya-Namioka monochromator. The polarization has been found to depend strongly on wavelength and on both the grating and the grating overcoating.

NBS Detector Standards for the Far Ultraviolet

A program at NBS leading to the realization of practical, stable transfer detector standards for the far ultraviolet is reviewed. Three basic detector types, one covering the region of 584-1216 A and the other two covering the region of 1164-2537 A, are described. Examples of these detectors have been calibrated at NBS and distributed to laboratories throughout the United States and Europe, where they are being used as primary radiometric calibration references in a variety of far-uv experiments.

New far UV detector calibration facility at the National Bureau of Standards

A description is given of a new facility, located at the NBS (Gaithersburg) Synchrotron Ultraviolet Radiation Facility (SURF-II), in which the absolute calibration of working standard detectors for the 5-50-nm spectral region, and the subsequent calibration of outgoing transfer standard detectors is performed. A dual toroidal grating monochromator, with diffraction gratings optimized for 3-13 and 12-52 nm, disperses synchrotron radiation from the electron storage ring into tandem experimental chambers in which a rare gas ionization chamber determines the absolute magnitude of the incident radiant flux and hence the efficiency of the photoemissive photodiodes.

Reflective scattering from substrates and evaporated films in the far ultraviolet.

Measurements of radiation reflectively scattered from mirror surfaces have been made at the wavelengths 1216 A and 584 A. Several glass and fused silica substrates with differing degrees of surface roughness have been studied, as well as evaporated films of aluminum and gold as a function of film thickness. A relatively small area detector was scanned in angle about the sample, and the detected energy integrated over the scattering angle. The results indicate that fused silica can be polished to a smoother surface than glass, that a smooth substrate is significantly advantageous in obtaining evaporated films having surfaces with low scatter, and that gold films are considerably smoother than aluminum films of equal thickness.

Silicon photodiodes with integrated thin-film filters for selective bandpasses in the extreme ultraviolet

Silicon photodiodes which operate satisfactorily in the extreme ultraviolet (EUV) have been commercially available for the past few years. These photodiodes also inherently respond to radiation extending from the x-ray region to the near infrared, a property which is undesirable in many EUV applications. The addition of a thin film of a suitable filtering material to the surface of such a photodiode can accomplish the restriction of the sensitivity of the silicon to a much narrower band, or bands, in the EUV. This results in a rugged, yet sensitive photometer for applications in which dominant out-of-band radiation is present. Applications include plasma diagnostics, solar physics, x-ray lithography, x-ray microscopy, and materials science. Previous attempts to produce such devices have resulted in degraded shunt resistance with a corresponding increase in background noise. Prototype detectors have now been fabricated using directly deposited films of aluminum, aluminum/carbon, aluminum/carbon/scandium, silver, tin, and titanium, without degradation of the noise characteristics of the uncoated photodiodes. Measured and theoretical sensitivity data are presented, as well as a discussion of relatively simple methods to reduce the x-ray response of such filtered detectors.

Silicon photodiodes optimized for the EUV and soft x-ray regions

Available photodiodes are reviewed with attention given to the performance parameters, temporal stability, and appropriateness for narrow bandpass applications and certain photon energies. The configuration of XUV photodiodes for use in the EUV and soft X-ray regions is delineated, and the measured parameters are outlined. The photodiodes have stable efficiencies that vary linearly with photon energy and exceed 1 electron/incident photon for photon energies of at least 10 eV. The silicon photodiodes are found to be suitable for EUV and soft X-ray applications and are stable, very highly efficient, and are unaffected by operation under high gas pressures. The silicon dioxide outer surface can be coated with thin films to develop narrow bandpass applications. The present XUV silicon detectors have active areas of 1 or 3 sq cm and can be used with an instrument for measuring photocurrent without external power supplies.

Comparison of an Ionization Chamber and a Thermopile as Absolute Detectors in the Extreme Ultraviolet

A comparison has been made between a calibrated thermopile and an argon ionization chamber as absolute detectors of radiant flux of wavelengths 584 A and 735 A. Corrections were applied to the data in order to account for the absorption due to gases in the monochromator, the energy carried away from the thermopile by photoejected electrons, nonuniformity of response across the surface of the thermopile, and uneven illumination of the thermopile by the flux from the monochromator. The two detectors were found to agree within the 3% estimated probable error in the measurements.

NIST programs for calibrations in the far ultraviolet spectral region

The National Institute of Standards and Technology (NIST) serves the growing ultraviolet user community by providing calibration services throughout the spectral range from 2 nm to 400 nm. In this paper we describe the far ultraviolet transfer standard detector program, the NASA-supported Spectrometer Calibration Beamline at the Synchrotron Ultraviolet Radiation Facility, SURF III, and the recent upgrade of the SURF electron storage ring. Several types of transfer standard detectors are issued by NIST in the spectral range from 5 nm to 254 nm; Al2O3 windowless photoemissive devices, CsTe photoemissive devices with integrated MgF2 windows, and radiation-hardened, semiconductive Si photodiodes. The Spectrometer Calibration Beamline makes use of the cathode, undispersed synchrotron radiation from SURF III as a standard of spectral irradiance from 2 nm to 400 nm. The upgrade of SURF has greatly improved the accuracy of calibrations based on SURF, as well as extending the useful spectral range to shorter wavelengths. Taken together, the transfer standard detector program and the calibration beamline at SURF III offer a unique calibration resource for scientists and engineers working in the far ultraviolet spectral region.

Intercomparison of radiometric irradiance scales in the 90-250-nm wavelength range.

The work described in this paper covers an intercomparison of absolute irradiance calibration methods applied at the U.S. National Bureau of Standards (NBS), the SRC Appleton Laboratory, Astrophysics Research Division (ARD), the Max-Planck-Institut fuer Extraterrestrische Physik (MPE), and the Physikalisch- Technische Bundesanstalt (PTB). This was done by determining the MPE and PTB quantum efficiencies of two different calibrated transfer standards developed by the NBS and ARD. The comparison shows that the calibrations agreed within the estimated bounds of uncertainty and also suggests that the uncertainty in the measured electron temperature required in the argon arc source technique may be less than was estimated.