Silicon Charge-coupled Device Research Articles

Optical upconverter with integrated heterojunction phototransistor and light-emitting diode

We report an optical upconversion device that converts input 1.5μm light to output 0.87μm light with a built-in gain mechanism. The device consists of an InGaAs∕InP heterojunction phototransistor (HPT) integrated with a GaAs∕AlGaAs light-emitting diode (LED) by wafer fusion process. Incoming 1.5μm optical radiation is absorbed by the HPT, generating an amplified photocurrent. The resultant photocurrent drives the LED that emits at 0.87μm, which could be detected by a conventional silicon charge-coupled device. Upconversion is demonstrated at room temperature with a gain of 20 from the HPT and an overall external upconversion efficiency of 0.07W∕W.

Raman spectrometry of explosives with a no-moving-parts fiber coupled spectrometer: A comparison of excitation wavelength

In this study Stokes Raman spectra measured with 785 and 830 nm excitation, as well as 1064 nm anti-Stokes Raman spectra, of 23 explosive materials were obtained using fiber-optic sampling and a no-moving parts spectrograph. By using a silicon charge-coupled device (CCD) detector, 830 nm Stokes Raman spectra were collected over the range 100–2400 cm −1. 830 nm excitation offers slightly better fluorescence rejection than 785 nm, which is key for the analysis of some fluorescent explosives. Even with a high-powered 1064 nm laser, anti-Stokes measurements did not yield spectra of as high quality as 785 and 830 nm Stokes Raman measurements. Fourier transform (FT)-Raman remains the preferred method for fluorescence-free analysis of these explosive materials in the laboratory, but 830 nm excitation is preferred for a field-portable instrument.

Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors

Silicon charge-coupled devices (CCDs) covered with a scintillating film are now available on the market for use in digital medical imaging. However, these devices could still be improved in terms of sensitivity and especially spatial resolution by coating the CCD with an array of scintillating waveguides. In this paper, such waveguides were fabricated by first etching pores in silicon, then performing metallization or oxidation of the pore walls and finally filling the pores with CsI(Tl). The resulting structures were observed using scanning electron microscopy and tested under X-ray exposure. Theoretical efficiencies of macropore arrays filled with CsI(Tl) were also calculated, indicating that the optimal pore depth for metallized macropore arrays is about 80 /spl mu/m while it is around 350 /spl mu/m for oxidized ones. This result, together with the roughness of the metal coating, explains why lower SNR values were measured with the metallized macropores. Indeed, the macropore arrays had depths in the range of 210-390 /spl mu/m, which is favorable to oxidized structures.

RADIATION EFFECTS IN CHARGE-COUPLED DEVICE (CCD) IMAGERS AND CMOS ACTIVE PIXEL SENSORS

This review concerns radiation effects in silicon Charge-Coupled Devices (CCDs) and CMOS active pixel sensors (APSs), both of which are used as imagers in the visible region. Permanent effects, due to total ionizing dose and displacement damage, are discussed in detail, with a particular emphasis on the space environment. In addition, transient effects are briefly summarized. Implications for ground testing, effects mitigation and device hardening are also considered. The review is illustrated with results from recent ground testing.

Anti-Stokes Raman Spectrometry with 1064-nm Excitation: An Effective Instrumental Approach for Field Detection of Explosives

Anti-Stokes Raman spectra of 28 explosive materials were obtained with 1064-nm excitation using fiber-optic sampling and a dispersive spectrograph equipped with a charge-coupled device (CCD) array detector. By using a silicon CCD detector, anti-Stokes features could clearly be observed for the majority of samples from -250 to -1650 cm(-1). Using the fiber-optic probe, spectra were routinely obtained from samples positioned up to twelve meters from the spectrograph within 240 s. The utility of an anti-Stokes correction routine is demonstrated, which routine allowed anti-Stokes spectra measured with 1064-nm excitation to be successfully searched and identified against libraries of Stokes spectra obtained using a Fourier transform (FT) Raman system equipped with a 1064-nm Nd:YAG laser.

Proton radiation damage in p-channel CCDs fabricated on high-resistivity silicon

P-channel backside illuminated silicon charge-coupled devices (CCDs) were developed and fabricated on high-resistivity n-type silicon. The devices have been exposed up to 1 /spl times/ 10/sup 11/ protons/cm/sup 2/ at 12 MeV. The charge transfer efficiency and dark current were measured as a function of radiation dose. These CCDs were found to be significantly more radiation tolerant than conventional n-channel devices. This could prove to be a major benefit for space missions of long duration.

Substrate preparation and low-temperature boron doped silicon growth on wafer-scale charge-coupled devices by molecular beam epitaxy

Silicon charge-coupled devices (CCDs) are extensively used for commercial and scientific imaging in the visible to near-infrared wavelength range of 450 to 850 nm. Ground-based astronomers require large-scale high-performance CCDs with high sensitivity at wavelengths from the 320 nm atmospheric cutoff to 900 nm. We report on wafer-scale low-temperature silicon molecular beam epitaxy to enable ultraviolet (UV) detection utilizing silicon-fabrication-facility-compatible surface preparation. Characterization of the UV response-enhanced backside-illuminated CCDs fabricated with this technique show near 100% internal quantum efficiency in the wavelength range of 200 to 900 nm.

Visible reflectance hyperspectral imaging: characterization of a noninvasive, in vivo system for determining tissue perfusion.

We characterize a visible reflectance hyperspectral imaging system for noninvasive, in vivo, quantitative analysis of human tissue in a clinical environment. The subject area is illuminated with a quartz-tungsten-halogen light source, and the reflected light is spectrally discriminated by a liquid crystal tunable filter (LCTF) and imaged onto a silicon charge-coupled device detector. The LCTF is continuously tunable within its useful visible spectral range (525-725 nm) with an average spectral full width at half-height bandwidth of 0.38 nm and an average transmittance of 10.0%. A standard resolution target placed 5.5 ft from the system results in a field of view with a 17-cm diameter and an optimal spatial resolution of 0.45 mm. The measured reflectance spectra are quantified in terms of apparent absorbance and formatted as a hyperspectral image cube. As a clinical example, we examine a model of vascular dysfunction involving both ischemia and reactive hyperemia during tissue reperfusion. In this model, spectral images, based upon oxyhemoglobin and deoxyhemoblobin signals in the 525-645-nm region, are deconvoluted using a multivariate least-squares regression analysis to visualize the spatial distribution of the percentages of oxyhemoglobin and deoxyhemoglobin in specific skin tissue areas.

Manipulation of elementary charge in a silicon charge-coupled device.

The ultimate limit in the operation of an electronic device is the manipulation of a single charge. Such a limit has been achieved in single-electron tunnelling devices. However, these devices are based on multiple tunnel barriers and conductive islands, which are complex structures to fabricate. Here we demonstrate another type of device that can also manipulate elementary charge, but which is more suitable for large-scale integration. The device consists of two closely packed silicon wire-MOSFETs, which are commonly used building blocks of electronic circuits. We have developed a scheme to generate and store holes in the channels of either of these MOSFETs. Subsequently, holes can be transferred between the two MOSFETs at the level of an elementary charge, and their exact position can be monitored. This single-charge transfer device, which is operated at 25 K, is in effect a charge-coupled device. This is also the first realization of a silicon-based device that manipulates elementary charge.

The impact of space protons on X-ray sensing with charge coupled devices (CCDs)

The latest, large-geometry silicon charge-coupled device technology for X-ray astronomy has been characterized with respect to the impact of space protons. Extensive radiation damage of physical interest include a better understanding of the roles of VP, VO and VV defects in the prediction of performance in orbit are proposed. The importance of photon hit rate and X-ray energy in determining the CTI damage constant was confirmed. In CCD manufacture, improvements of tolerance to particle damage can be made. We recommend a lower operating temperature in orbit (-110 or lower rather than -90/spl deg/C).

Proton effects in charge-coupled devices

Basic mechanisms and ground-test data for radiation effects in solid-state imagers are reviewed, with a special emphasis on proton-induced effects on silicon charge-coupled devices (CCDs). For the proton fluxes encountered in the space environment, both transient ionization and displacement damage effects arise from single-particle interactions. In the former case, individual proton tracks will be seen; in the latter, dark-current spikes (or hot pixels) and trapping states that cause degradation in charge-transfer efficiency will be observed. Proton-induced displacement damage effects on dark current and charge transfer are considered in detail, and the practical implications for shielding, device hardening, and ground testing are discussed.

CCD Quantum Efficiency at the Theoretical Limit with Molecular Beam Epitaxy

A variety of biological, medical, astronomical and materials science measurements require real-time imaging of phenomena which produce low light levels. Charge-coupled devices (CCDs), which have high resolution and linear response over a large dynamic range, are often used as imaging sensors in conjunction with image intensifiers or phosphors to improve the signal-to-noise at low light levels, particularly in the blue and UV regions of the spectrum. Conventional CCDs typically have quantum efficiency (QE) as low as ~10% in the range of 300-500 nm. This region of the spectrum is very important in biological experiments which require detecting the fluorescence of dyes, in astronomical observations of spectral lines in the blue and near UV, and in a variety of applications which use image intensifiers to amplify the signal. Improvement of CCD quantum efficiency with respect to the conventional technology is critical for most of these experiments. The detection of blue and UV photons in a silicon CCD is challenging due to the short absorption length of these photons in silicon (e.g., 4 nm absorption length at 270 nm) and the existence of a backside potential well (caused by positive charge at the interface of Si and SiO2).

The X-ray polarisation sensitivity of CCDs

The regular array of pixels in a silicon Charge Coupled Device (CCD) may be used to measure photoelectron emission directions following the absorption of high energy X-rays ( E > 15 keV). CCDs offer, therefore, the possibility of combining X-ray imaging and spectroscopy with measurement of the linear polarisation of the incident beam. We describe a simple model of electron transport in CCDs which leads to an estimate of the energy-dependent modulation factor M( E) — the parameter determining polarisation sensitivity — for arbitrary pixel geometries. The predictions of the model are in good agreement with published measurements. The sensitivity of an optimised CCD “pixel polarimeter” for cosmic X-ray astronomy is assessed.

High-Fidelity Raman Imaging Spectrometry: A Rapid Method Using an Acousto-Optic Tunable Filter

In this communication, we describe a technique for obtaining high-fidelity Raman images and Raman spectra. The instrumentation provides the ability to rapidly collect large-format images with the number of image pixels limited only by the number of detector elements in the silicon charge-coupled device (CCD). Wavelength selection is achieved with an acousto-optic tunable filter (AOTF), which maintains image fidelity while providing spectral selectivity. Under computer control the AOTF is capable of µs tuning speeds within the operating range of the filter (400–1900 nm). The AOTF is integrated with the CCD and holographic Raman filters to comprise an entirely solid-state Raman imager containing no moving parts. In operation, the AOTF is placed in front of the CCD and tuned over the desired spectral interval. The two-dimensional CCD detector is employed as a true imaging camera, providing a full multichannel advantage over competitive Raman imaging techniques. Images and spectra are presented of a mixture of dipalmitoylphosphatidylcholine (DPPC) and L-asparagine, which serves as a model system for the study of both lipid/peptide and lipid/protein interactions in intact biological materials. The Raman images are collected in only several seconds and indicate the efficacy of this rapid technique for discriminating between multiple components in complex matrices. Additionally, high-quality Raman spectra of the spatially resolved microscopic regions are easily obtained.

Device Degradation on a Full-Frame CCD Image Sensor with a Transparent Gate Electrode

ABSTRACTThe electrical and optical properties of Indium-Tin-Oxide (ITO) films, deposited by radio frequency (r.f.) magnetron sputtering, were studied. ITO films, when deposited using optimum sputtering conditions, were reproducibly prepared with resistivity as low as 1.5 × 10−4 Ω-cm and optical transmissivity higher than 80% over the wavelength range of interest. Device stability when ITO is used as a replacement for polysilicon as a gate electrode in silicon charge-coupled device (CCD) image sensors was also studied. After an anneal process at 950 °C in N2 the device degraded. The degradation can be attributed to the generation of oxide charge and interface states in the ITO/SiO2/Si system.

Effects of Deliberate Metal Contamination on CCD Imagers

ABSTRACTThe effects of intentional metal contamination on silicon charge-coupled device imagers are reported. Such imagers are both sensitive to and provide sensitive measures of the presence of metals in the fabrication process. High-purity iron, cobalt, nickel, copper, palladium, and gold were deliberately introduced into the device wafers just before the last high temperature step. Metals were found to cause both electrical defects and distinctive imaging defects.We find that transition metals can be effectively removed from device regions by internal gettering, but that this gettering can be defeated by a fast cool-down. Gold, however, is poorly gettered.

Tunable near-infrared Raman spectroscopy

Approaches to generating fluorescence-free Raman spectra using excitation in the near-infrared (near-IR) region of the spectrum are not limited to the use of Fourier transform (FT) Raman spectroscopy. Although FT-Raman does introduce the well-established advantages of Fourier transform methods to the field of Raman spectroscopy, it is clearly not the only alternative to obtaining a Raman spectrum with near-IR excitation. The coupling of laser excitation at wavelengths longer than 700 nm with low noise multichannel detection can also produce fluorescence-free Raman spectra. In this paper we describe the capabilities and use of a slow scan camera system employing a silicon charge-coupled device (CCD) specifically fabricated with enhanced performance in the near-IR region of the spectrum as a detector for near-IR Raman spectroscopy. This highly sensitive detection method provides a convenient means to rapidly generate high-quality and nearly fluorescence-free spectra using a variety of excitation sources including diode lasers tuned beyond 800 nm. Examples of spectra generated with CCD detection and the FT-Raman technique are compared. A discussion of the appropriate conditions for application of the two approaches is also included.

Imaging charge-coupled device (CCD) transient response to 17 and 50 MeV proton and heavy-ion irradiation

The results of irradiating a high-resolution, large-area, silicon charge-coupled device (CCD) imaging array (Kodak KAF-1400) with controlled low fluxes of collimated monoenergetic (17- and 50-MeV) protons and selected heavy ions are presented. The CCD response was measured at several angles of incidence, from normal to 70 degrees off normal, and at several azimuthal angles. The transient response events are recorded and analyzed to infer the effective charge collection depth of the CCD. Selected individual proton-induced events are analyzed for their two-dimensional spatial amplitude, and the results are compared to a charge collection model which included contributions from both the pixel depletion and diffusion volumes for the geometry (pixel size and spacing) and thickness (depletion depth and epitaxial layer thickness) of this CCD. >

Progress in low-light-level charge-coupled device imaging in astronomy

Techniques for data acquisition and processing, which permit accurate photometry at a level of 10−4 of the background night-sky level, are described. High quantum efficiency, excellent linearity, and low noise make frame-transfer silicon charge-coupled devices (CCD’s) nearly ideal imagers at ultralow-light levels in the UV to 900-nm range. Scientific-grade CCD’s are now available with less than 10-electron noise from several manufacturers. In astronomy these CCD’s permit studies of objects that are 100 times fainter than is possible by using photographic or video-camera techniques. Detection levels of 2 × 10−5 photons m−2 nm−1 sec−1 pixel−1 have been achieved. The efficiency of telescopes of all apertures has increased to the point at which observations that were unthinkable a decade ago are now becoming routine. New techniques for ultradeep imaging with CCD’s are described. The telescope is moved randomly between exposures, producing 30–100 disregistered images. Automated image processing on a workstation yields the final image, corrected for CCD imperfections and sky + optics systematics. Faint galaxies and stars of 30th magnitude are detected in 10-h integration on a 4-m telescope, corresponding to 0.02 photon sec−1 pixel−1. These techniques may be applied to longer-wavelength imagers with similar problems of bad pixels, bad lines or columns, background variations, etc. Application to faint-galaxy photometry is reviewed, and mosaic CCD imagers are discussed.

Calibration and X-ray spectroscopy with silicon CCDs

Abstract It is shown that the novel mode of operation of charge coupled devices (CCDs) allows a new method of measuring the X-ray energy-to-charge conversion factor, without requiring externally calibrated circuits etc., thereby reducing possible contributions to systematic uncertainties in this measurement. In addition, the low noise operation of CCDs is shown to lead to possibilities for measuring the Fano factor in silicon with improved precision. Advances in CCD X-ray detection performance are described, including energy resolutions of 80 eV FWHM at a temperature of 180 K, and detection efficiencies of greater than 90%. Such improvements are shown to have potential benefit for various X-ray spectroscopic applications.