Present status of radiometric quality silicon photodiodes

Abstract: Evaluation of five types of silicon photodiode was undertaken to verify their suitability for absolute radiometry and also for their use as transfer standards in the spectral region from 1 to 1100 nm. Four types of photodiode were fabricated for this study; these were the p-on-n photodiode, n-on-p photodiodes with silicon dioxide front windows and n-on-p photodiodes with a metal-silicide front window. Fabrication of photodiodes with 100% internal quantum efficiency is demonstrated and their necessity for making absolute radiometric measurements with the lowest possible uncertainty is pointed out. The linearity characteristics of these devices, as measured by the ac/dc method, are far superior to those of the p-on-n diodes especially fabricated for this work and also to those exhibited by p-on-n diodes widely used at present by the radiometric community. Results on the stability of the quantum efficiency of the fabricated diodes after exposure to intense radiation of 13,120,157,193 and 254 nm radiation will also be presented. Photodiodes with a metal-silicide front window were the only devices stable when exposed to the intense beams of third generation synchrotrons and UV excimer lasers.

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Abstract: The performance characteristics of a silicon photodiode (type AXUV100) were determined using the beamline X24C at the National Synchrotron Light Source. The diode sensitivity was measured in the temperature range of -92 ^o C to +41 ^o C and in the wavelength region of 3.0 nm to 88.2 nm. This work is important for understanding variations of the diode sensitivity in environments that are colder or hotter than ambient room temperature, such as on a spacecraft or under intense synchrotron or laser irradiation. In addition, the sensitivity of a AVUV100 diode with a multilayer interference coating was measured as a function of the polarization of the incident radiation and the angle of incidence. The Mo/Si multilayer coating, when operating at an angle of incidence of 45 ^o, was designed to selectively transmit the P polarized radiation for detection by the underlying diode and to efficiently reflect the S polarization. The results demonstrate the ability to accurately measure the polarization of radiation within the reflectance profile of the multilayer centered at 13.5 nm. By optimizing the transmittance and reflectance profiles of the multilayer coating for the desired wavelength range and angle of incidence, this new multilayer-coated photodiode technique can be used to measure the polarization of incident radiation from solar, astrophysical, synchrotron, or other laboratory sources over a wide range of extreme ultraviolet and soft x-ray wavelengths.

Abstract: Silicon photodiode detectors with multilayer coatings were characterized using synchrotron radiation. The coatings were composed of thin layers of metals and other materials and were designed to provide wavelength bandpasses in the 17-150 A wavelength region. The measured transmittances of the multilayer coatings are in good agreement with the calculated transmittances. The modeling accounts for the transmittance of the multilayer coating and the deposition of the radiation energy in the underlying silicon photodiode. Detectors with the following layer materials (and wavelength bandpasses) were characterized: Fe/Al (17-30 A), Mn/Al (19-30 A), V/Al (24-35 A), Ti/C (27-40 A), Pd/Ti (27-50 A), Ti/Zr/Al (27-50 A), Ag/CaF2/Al (36-50 A), and Ti/Mo/C (50-150 A).

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Abstract: Stable silicon photodiodes with 100% internal quantum efficiency have been developed for the vacuum ultraviolet and soft x-ray regions. It is demonstrated that the response of these detectors can be reasonably well represented by a simple model for photon energies above 40 eV. The measured efficiency is consistent with a constant electron-hole pair creation energy for Si above 40 eV. Radiation damage is demonstrated to result in loss of carriers to recombination at the front surface. The uniformity of the diodes is shown to be better than 0.1% RMS at 110 eV.

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Abstract: Silicon n-on-p photodiodes with 100% internal efficiency have been studied in the 160 nm to 254 nm range. Preliminary values for the quantum yield of silicon, a fundamental material property, are determined. Using these values, a trap detector for absolute flux measurement in this region is presented. The stability under intense 193 nm irradiation, a property of importance in lithography and in refractive keratectomy, has been measured, and the diodes tested were found to be several orders of magnitude more stable than p-on-n diodes tested by other investigators at this wavelength. Spatial non-uniformities of the n-on-p diodes were found to be less than one percent at 254 nm and 161 nm wavelengths.

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Abstract: 100% internal quantum efficiency silicon photodiodes with 4 to 8 nm passivating silicon dioxide have been fabricated by rapid thermal nitridation in nitrous oxide and ammonia ambients with the aim of increasing their radiation hardness. The fabricated diodes were exposed to 10.2 eV photons using a hydrogen plasma light source and a normal incidence monochromator. The measured quantum efficiency degradation indicates that the interface trap area density (Nit) increases sublinearly with dose up to a measured dose of one Gigarad. No noticeable change in quantum efficiency over the range of 50 to 250 nm was observed after exposure to 100% relative humidity. This suggests that the nitrided Si-SiO2 interface is practically insensitive to moisture.

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Abstract: 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 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/carbon, aluminum/carbon/scandium, silver, tin, titanium, titanium/yttrium/carbon, and titanium/zirconium/carbon without degradation of the noise characteristics of the uncoated photodiodes. Measured and theoretical sensitivity data will be presented, as will a discussion of relatively simple methods to reduce the x-ray response of such filtered detectors.

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Abstract: Recent advances in solid-state detector (SSD) technology have demonstrated the detection of ions and electrons down to 1 keV. However, ions at keV energies lose a substantial amount of energy D_N in a SSD through Coulombic interactions with target nuclei rather than through interactions that contribute to the SSD output pulse, whose magnitude is a measure of the ion's incident energy. Because D_N depends on the ion species, detector material, and interaction physics, it represents a fundamental limitation of the output pulse magnitude of the detector. Using 100% quantum collection efficiency silicon photodiodes with a thin (40-60 Å) SiO2 passivation layer, we accurately quantify D_N for incident 1-120 keV ions and, therefore, evaluate the detection limits of keV ions using silicon detectors. © 2004 American Institute of Physics.

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Abstract: Electron irradiation of 100% internal quantum efficiency silicon photodiodes having a thin (60 Å) SiO2 dead layer results in measured responsivities ranging from 0.056 A/W at an incident electron energy Eo =0.2 keV to 0.24 A/W at E0 =40 keV. A theoretical electron-hole pair creation energy of 3.71 eV, in close agreement with other studies, is derived using a Monte Carlo simulation of electron interactions with the photodiode over an energy range of 1 to 40 keV. Analysis of electron energy lost to processes that do not contribute to electron-hole pair creation shows that the energy lost in the SiO2 dead layer is dominant for E0 < 1.5 keV, whereas the energy removed by back scattered electrons is dominant for E0 > 1.5 keV. At E0 =300 eV, the Monte Carlo simulation results show that the electron projected range is significantly less than the dead layer thickness even though the measured response is 0.082 A/W, indicating that electron-hole pairs generated in the oxide dead layer are collected by the junction.

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Abstract: We measure the response of silicon photodiodes to irradiation by H+, He +, C +, N +, O +, Ne +, and Ar+ ions with energies up to 60 keV. The unique properties of these photodiodes, including an ultrathin SiO2 dead layer and 100% internal carrier collection efficiency, allow direct measurement of the total energy lost to nuclear (nonionizing) and electronic (ionizing) energy loss processes, which are important for quantifying effects such as damage and charge deposition. When plotted as a function of E/mZ^1/2 , where E, m, Z and are the incident ion energy, mass, and atomic number, respectively, the responsivity is found to follow a single curve that represents all ion species and energies used in this study. This enables rapid, accurate estimation of damage and charge deposition by an ion as a function of penetration depth in silicon. A comparison of the measurements with the stopping and range of ions in matter (SRIM) Monte Carlo simulation code shows that SRIM significantly overestimates the fraction of the incident energy lost to electronic stopping processes for E/mZ^1/2 < 2 keV/amu.

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Abstract: We measure the change in the response of 100% internal carrier collection efficiency silicon photodiodes having 60 Å SiO₂ passivation layers due to the damage induced by bombardment with 10-60 keV ions of H, He, N, Ne, and Ar. We find an initially exponential decrease in responsivity with increasing ion fluence Φ and use this to define a damage constant β. The correlation of β with the nuclear stopping power of the incident ion instead of with the total energy lost to nuclear stopping indicates that damage in a channel lying within the n-type silicon near the Si-SiO₂ interface dominates the radiation-induced change in the photodiode response. We use a fluid model of electron transport in the channel to derive a universal curve to describe the damage as a function of ion fluence and to show that the damage constant β is proportional to the damage cross section. Over the energy range of this study, damage cross sections of N⁺, Ne⁺, and Ar⁺ are 10-100 times that of He⁺, and ~1000 times that of H

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Optical Engineering / December 1993 / Vol. 32 No. 12 / 3121-3125

Abstract: New developments in transmission grating and photodiode technology now make it possible to realize spectrometers in the extreme ultraviolet (EUV) spectral region ( ), which are expected to be virtually constant in their diffraction and detector properties. Time-dependent effects associated with reflection gratings are eliminated through the use of free-standing transmission gratings. These gratings together with recently developed and highly stable EUV photodiodes have been utilized to construct a highly stable normal incidence spectro-photometer to monitor the variability and absolute intensity of the solar 304- line. Owing to its low weight and compactness, such a spectrometer will be a valuable tool for providing absolute solar irradiance throughout the EUV. This novel instrument will also be useful for cross-calibrating other EUV flight instruments and will be flown on a series of Hitchhiker shuttle flights and on SOHO. A preliminary version of this instrument has been fabricated and characterized, and the results are described.

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Abstract: The responsivity of an extreme-ultraviolet transmission grating spectrometer with silicon photodiode detectors was measured with synchrotron radiation. The spectrometer was designed to record the absolute radiation flux in a wavelength bandpass centered at 30 nm. The transmission grating had a period of 200 nm and relatively high efficiencies in the +1 and the -1 diffraction orders that were dispersed on either side of the zero-order beam. Three photodiodes were positioned to measure the signals in the zero order and in the +1 and the -1 orders. The photodiodes had aluminum overcoatings that passed the desired wavelength bandpass centered at 30 nm and attenuated higher-order radiation and wavelengths longer than approximately 80 nm. The spectrometer's responsivity, the ratio of the photodiode current to the incident radiation power, was determined as a function of the incident wavelength and the angle of the spectrometer with respect to the incident radiation beam. The spectrometer's responsivity was consistent with the product of the photodiode responsivity and the grating efficiency, both of which were separately measured while removed from the spectrometer.

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Journal of Geophysical Research, Vol. 105, No. A12, P. 27,179-27,194, 2000

Abstract: Beginning on March 11, 1998, the Student Nitric Oxide Explorer (SNOE) satellite has made daily observations of the solar soft X-ray irradiance. These measurements are carried out by a multichannel photometer system. The spectral range between 2 and 20 nm is covered by three channels with bandpasses of 2-7 nm, 6-19 nm, and 17-20 nm respectively. Absolute sensitivities were measured preflight using the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology. The results of the first 1.5 years of SNOE solar measurements are presented. During this time period the F10.7 solar index varied between 80 and 250×10-22?W?m-2?Hz-1 and the 81-day average of the F10.7 solar index varied between 100 and 175×10-22?W?m-2?Hz-1. The solar irradiances in the 2-7 nm interval varied between 0.3 and 2.5 mW m-2, while the irradiances in the 6-19 and 17-20 nm intervals varied between 0.5 and 3.5 and 1.0 and 3.5 mW m-2, respectively. The measured irradiances are correlated with the F10.7 solar index with a correlation coefficient of ~0.9 in all three bandpasses. For the levels of activity observed so far the SNOE measurements are typically a factor of 4.0 larger than the irradiances predicted by the Hinteregger et al. [1981] empirical model (hereafter the Hinteregger model). This fact and a long-term trend in the ratio of SNOE measurements to Hinteregger model predictions show that the Hinteregger model underpredicts the long-term variability in the solar soft X-ray irradiance. It is shown that other empirical models provide a reasonable representation of the 27-day variability but also underpredict the magnitude and long term variability. A sounding rocket measurement made on November 2, 1998, by the Thermosphere Ionosphere Mesosphere Energetics and Dynamics Solar EUV Experiment prototype instrument using the same technique measured the solar irradiance in similar wavelength bands and produced results that are in good agreement with the SNOE measurements. © 2000 American Geophysical Union

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Solar Physics, Vol. 186: 243-257, 1999

Abstract: Measurements of the solar soft x-ray (XUV: 2 nm to 30 nm) irradiance were performed from a sounding rocket payload flown from White Sands Missile Range, New Mexico on October 4, 1993 and again on November 3, 1994. The soft x-ray instrumentation comprised of silicon photodiodes with thin films deposited directly onto their active areas. The deposited material and its thickness in conjunction with the sensitivity of a bare diode determine the passband and the sensitivity of these photometric devices. The measurements are interpreted in terms of appropriate SERF 1 (Hinteregger et al., 1981) model solar spectra. It is found that the data are consistent with a solar spectrum that is on average approximately a factor of two times the model solar spectra. It is shown that the measured irradiances are in reasonable agreement with other experiments.

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Abstract: Trapezoidal detectors with 28 cm< active area have been fabricated on > 2500 cm, 4 inch diameter n-type silicon wafers. Instead of the commonly used ion implantation method, low-cost, high volume solid state diffusion technology along with phosphosilicate-glass and TCA gettering was adopted for boron and phosphorus doping. Typically the diode dark current was 15 µA @ 100 volts. Efforts are being made to obtain a finished device yield of 80% to meet the $2/cm< price goal of SSC semiconductor detector group.

Abstract: The Multi-Angle Imaging SpectroRadiometer (MISR) is to be launched in 1998 as part of NASA's Earth Observing System. The 3% (one sigma) absolute radiometric calibration requirement for this instrument is considered challenging, particularly since it must be maintained through the five-year mission life. To meet this specification MISR will rely on detector-based calibration techniques, which are primarily founded on High Quantum Efficiency (HQE) detector technology. Filtered HQE photodiodes will be used to characterize solar-reflected light from a diffuse calibration target during the mission. In addition, radiation-hard photodiodes, which have an extended lifetime over the HQE detectors, will be utilized as part of the on-board calibrator. To date, flight photodiodes and filters have been fabricated, along with components of the photodiode flight package, and the photodiodes have undergone performance and stability testing. This paper gives a status report on these new in-flight photodiode standards, with primary emphasis on the photodiode performance measurements taken to date.

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Abstract —Responsivity of silicon photodiodes was measured from -100 C to +50 C in the 3 to 250 nm wavelength range using synchrotron and laboratory radiation sources. Two types of silicon photodiodes were studied, the AXUV series having a thin nitrided silicon dioxide surface layer and the SXUV series having a thin metal silicide surface layer. Depending on the wavelength, the responsivity increases with temperature with the rates 0.013%/C to 0.053%/C for the AXUV photodiode and 0.020%/C to 0.084%/C for the SXUV photodiode. The increase in responsivity is consistent with the decrease in the silicon bandgap energy which causes adecrease in the pair creation energy. These results are particularly important for dose measurement in extreme ultraviolet (EUV) lithography steppers and sources since the detector temperatureoften increases because of the high EUV intensities involved.

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Keywords: photodiode, soft x-ray, extreme ultraviolet, multilayer coating, filter, detector