Back-illuminated CCDs Research Articles

Charge Diffusion Variations in Pan-STARRS1 CCDs

Thick back-illuminated deep-depletion CCDs have superior quantum efficiency over previous generations of thinned and traditional thick CCDs. As a result, they are being used for wide-field imaging cameras in several major projects. We use observations from the Pan-STARRS 3π survey to characterize the behavior of the deep-depletion devices used in the Pan-STARRS 1 Gigapixel Camera. We have identified systematic spatial variations in the photometric measurements and stellar profiles that are similar in pattern to the so-called “tree rings” identified in devices used by other wide-field cameras (e.g., DECam and Hypersuprime Camera). The tree-ring features identified in these other cameras result from lateral electric fields that displace the electrons as they are transported in the silicon to the pixel location. In contrast, we show that the photometric and morphological modifications observed in the GPC1 detectors are caused by variations in the vertical charge transportation rate and resulting charge diffusion variations.

A Novel Method to Remove Fringes for Dispersive Hyperspectral VNIR Imagers Using Back-Illuminated CCDs

Dispersive hyperspectral VNIR (visible and near-infrared) imagers using back-illuminated CCDs will suffer from interference fringes in near-infrared bands, which can cause a sensitivity modulation as high as 40% or more when the spectral resolution gets higher than 5 nm. In addition to the interference fringes that will change with time, there is fixed-pattern non-uniformity between pixels in the spatial dimension due to the small-scale roughness of the imager’s entrance slit, creating a much more complicated problem. A two-step method to remove fringes for dispersive hyperspectral VNIR imagers is proposed and evaluated. It first uses a ridge regression model to suppress the spectral fringes, and then computes spatial correction coefficients from the object data to correct the spatial fringes. In order to evaluate its effectiveness, the method was used to remove fringes for both the calibration data and object data collected from two VNIR grating-based hyperspectral imagers. Results show that the proposed method can preserve the original spectral shape, improve the image quality, and reduce the fringe amplitude in the 700–1000 nm region from about ±23% (10.7% RMSE) to about ±4% (1.9% RMSE). This method is particularly useful for spectra taken through a slit with a grating and shows flexible adaptability to object data, which suffer from time-varying interference fringes.

Focus on lasers and imaging

Focus on lasers and imaging

Back-illuminated CCD for coherent soft X-ray imaging

Direct-illumination CCDs find increasing use in coherent X-ray imaging due to their good spatial resolution and signal-to-noise ratio. These CCDs offer a high quantum efficiency for soft X-rays near the 3d transition metal L edges. We compare front- and back-illuminated CCDs and describe a method to obtain images with a large dynamic range (five orders of magnitude) and high spatial resolution. Back-illuminated CCDs are resistant to radiation damage at the soft X-ray energies and provide a true 13.5 μm pixel resolution. High quality diffraction images are obtained in magnetic studies from metallic multilayer structures in reflection geometry.

Focus on Photonics and Imaging

Focus on Photonics and Imaging

Quantum efficiency of back-illuminated CCD detectors in the VUV region (30–200 nm)

Charge-coupled devices (CCD’s) are the detectors of choice for spectroscopy of highly charged ions due to their good linearity, spatial resolution, and high quantum efficiency (QE) over a wide wavelength range going from the γ-rays to the far infrared. However, the efficiency in the UV region of back-illuminated CCDs is not well known because of the lack of commercially available calibrated light sources in that wavelength range. By using the beam-foil light source, which produces well controlled line rich spectrum all over the XUV and VUV spectra, we have been able to measure the QE of our CCD over a large wavelength range. Our results are in accordance with the data gathered from the literature and allow us to propose a “universal” curve for the QE efficiency of CCDs in the 30–200nm region.

Measurement of lateral charge diffusion in thick, fully depleted, back-illuminated CCDs

Lateral charge diffusion in back-illuminated CCDs directly affects the point spread function (PSF) and spatial resolution of an imaging device. This can be of particular concern in thick, back-illuminated CCDs. We describe a technique of measuring this diffusion and present PSF measurements for an 800/spl times/1100, 15 /spl mu/m pixel, 280 /spl mu/m thick, back-illuminated, p-channel CCD that can be over-depleted. The PSF is measured over a wavelength range of 450 nm to 650 nm and at substrate bias voltages between 6 V and 80 V.

Mesh experiment for back-illuminated CCDs in improvement of position resolution

We have carried out a mesh experiment with back-illuminated (BI) CCDs. BI CCDs possess the same structure to those of front-illuminated (FI) CCDs. X-ray photons entering from the back surface of a CCD produce a primary charge cloud far from the electrodes. The cloud expands by diffusion process until it reaches the potential well just below the electrodes. Since the diffusion time of the charge cloud is longer than in the FI CCD, a larger charge cloud is expected. The mesh experiment enables us to specify the X-ray point of interaction with a subpixel resolution. We measured the charge cloud shape in the BI CCD. We found that there are two components of the charge cloud: a narrow component and a broad component. The size of the narrow component is 2.8– 5.7 μm and strongly depends on the attenuation length in Si of the incident X-rays. The shorter the attenuation length, the larger the charge cloud. This result is qualitatively consistent with a model of diffusion inside the CCD. On the other hand, the size of the broad component is independent of X-ray energy of ∼13 μm . We conclude that the narrow component is originated in the depletion region whereas the broad component is in the field-free region. Based on the charge cloud shape obtained, we determined the X-ray point of interaction within a pixel and found that the position accuracy of our method could be achieved up to 1.1±0.1 μm . Moreover, the fraction of events used to improve the position resolution can be increased by an order of magnitude. We can thus develop a high-throughput superfine X-ray resolution imager.

Modeling charge transfer inefficiency in the Chandra Advanced CCD Imaging Spectrometer

The front-illuminated (FI) CCDs in the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory ( Chandra) were damaged in the extreme environment of the Earth's radiation belts, causing charge traps that result in enhanced charge transfer inefficiency (CTI) during parallel readout. This causes row-dependent gain, event grade ‘morphing’ (spatial redistribution of charge) and energy resolution degradation. The ACIS back-illuminated (BI) CCDs also exhibit pronounced CTI due to their manufacturing. It is mild enough that position-dependent energy resolution is not seen, but it is present in both parallel and serial registers. This CTI also changes the gain and event grades, in a spatially complicated way as parallel and serial CTI interact. Given these realities, we have developed and tuned a phenomenological model of CTI for both FI and BI CCDs and incorporated it into our Monte Carlo simulations of the ACIS CCDs. It models charge loss and the spatial redistribution of charge (trailing), thus reproducing the spatially dependent gain and grade distribution seen in all ACIS CCDs and the row-dependent energy resolution seen in the FI devices. Here we explore the evidence for CTI, compare our simulations to data, and present a technique for CTI correction based on forward modeling.

Synchrotron calibration and response modelling of back-illuminated XMM-RGS CCDs

Back-illuminated CCDs with high quantum efficiency for 0.35–2.5 keV X-rays in combination with low efficiency at optical wavelengths have been developed for XMM-RGS. As part of the calibration programme, a systematic study of the CCD response in this energy range has been performed at the Synchrotron Radiation Source in Daresbury, UK. These measurements show the good quantum efficiency of the CCDs. The results are consistently described by a Monte Carlo model.

Absolute measurement of oxygen edge structure in the quantum efficiency of X-ray CCDs

As part of the program to calibrate X-ray CCD detectors for the AXAF mission we have performed high-spectral resolution, absolute measurements of CCD quantum efficiency using the synchrotron radiation and grating monochromator facilities of the PTB radiometry laboratory at BESSY. We present here detailed measurements of the absolute quantum efficiency in the spectral region surrounding the oxygen K absorption edge for both front- and back-illuminated MIT Lincoln Laboratory CCDs. The absolute scale of the measurements is established by reference to an Electrical Substitution Radiometer. Near-edge absorption structure produces variations as large as 40% in the detection efficiency of the front-illuminated detectors. We find structure at energies immediately below the K absorption edge which we attribute to resonant absorption. The amplitude of edge structures in the back-illuminated devices is, as expected, much smaller. Finally, we demonstrate that edge structure can be reliably detected in CCD response using an electron impact source and a grating monochromator.

Back-illuminated CCDs made by gas immersion laser doping

Back-illuminated CCDs with high quantum efficiency in the soft X-ray range have been developed by EEV in collaboration with the Space Research Organisation Netherlands. By using Gas Immersion Laser Doping (GILD) for producing the backside accumulation layer very shallow doping profiles can easily be achieved. Additionally the GILD process does not affect the silicon behind the p + layer in contrast to the commonly used ion implantation process. This implies that only the electrons generated in, or reaching the very small accumulation layer will have a probability to recombine at the surface or in the accumulation layer itself. Therefore only a small fraction of the electron clouds produced by the absorbed soft X-rays will suffer charge loss, resulting in a high quantum efficiency. X-ray measurements of back-illuminated CCDs with doping profiles of 50 and 100 nm depth are presented and shown to be consistent with calculations based on minority carrier transport.

Response analysis in the 300- to 2500-(angstrom) spectral range of ultraviolet-enhanced charge-coupled devices

The measurements of the ultraviolet response by some UV-enhanced CCDs have been realized. In particular, the quantum efficiency of coronene- and lumigen-coated and of back-illuminated ion-implanted CCDs have been measured in the 300 to 2500-A spectral range. Very interesting performances have been found, mainly for one ion-implanted CCD with quantum efficiency values of more than 60% at 304 A. Also some measurements of the CCD uniformity response and spatial resolution capability in this spectral region have been performed. The obtained results are very promising for the possibility of using these semiconductor detectors in the ultraviolet spectral region also with very good performance.

Tabulated responses of X-ray CCDs to optical astronomical sources

We calculate the response of both front-illuminated and back-illuminated X-ray CCDs to various astronomical optical inputs. The results are given in tabular form and are useful when calculating the expected optical yields encountered during an observation or in designing optical filters for soft X-ray telescopes.

Ultraviolet And Extreme Ultraviolet Response Of Charge-Coupled-Device Detectors

We present results of a program to enhance the ultraviolet and extreme ultraviolet response of charge-coupled devices. The ultimate goal of our program is to develop a large format device with both high and stable quantum efficiency from 100 to 3000 A that can be used as a windowless imaging detector in a space environment. Ultraviolet quantum efficiency measurements have been made for several ion-implanted and laser-annealed test CCDs specially fabricated by Tektronix for this program. Quantum efficiencies as high as 22% at 2500 A , where the absorption depth in silicon is -55 A, have been observed in one such test CCD. Quantum efficiency measurements of standard back-illuminated RCA and Tektronix CCDs are also presented.

Antireflection Coatings For Silicon Charge-Coupled Devices

With the increasing use of charge-coupled devices for astronomical observations, it is vital to obtain the highest quantum efficiency possible due to the limited amount of observing time available on large telescopes. One method of increasing the response of back-illuminated CCDs is to apply antireflection coatings. Because silicon's index of refraction is extremely high in the ultra-violet, a gain of more than 100% in quantum efficiency can be achieved at a wavelength of 0.37 Am with the proper choice of dielectric thin-film materials. We present a study of materials suitable for silicon CCD antireflection coatings for use in the 0.32 to 1.0 Am spectral range. Constraints on these materials, such as refractive index, absorption coefficient, application method, tempera-ture requirements, internal stress, and radioactivity, are discussed. We find hafnium oxide, lead fluoride, and aluminum oxide to be among the most suitable thin-film materials available. We also present a number of single- and multiple-layer antireflection coating designs optimized for CCDs, with particular emphasis on the blue and ultraviolet.