William F. Kolbe

The effects of charge transfer inefficiency (CTI) on galaxy shape measurements

We examine the effects of charge transfer inefficiency (CTI) during CCD readout on the demanding galaxy shape measurements required by studies of weak gravitational lensing. We simulate a CCD readout with CTI such as that caused by charged particle radiation damage in space-based detectors. We verify our simulations on real data from fully depleted p-channel CCDs that have been deliberately irradiated in a laboratory. We show that only charge traps with time constants of the same order as the time between row transfers during readout affect galaxy shape measurements. We simulate deep astronomical images and the process of CCD readout, characterizing the effects of CTI on various galaxy populations. Our code and methods are general and can be applied to any CCDs, once the density and characteristic release times of their charge trap species are known. We baseline our study around p-channel CCDs that have been shown to have charge transfer efficiency up to an order of magnitude better than several models of n-channel CCDs designed for space applications. We predict that for galaxies furthest from the readout registers, bias in the measurement of galaxy shapes, Δe, will increase at a rate of (2.65 ± 0.02) × 10^(-4) yr^(-1) at L2 for accumulated radiation exposure averaged over the solar cycle. If uncorrected, this will consume the entire shape measurement error budget of a dark energy mission surveying the entire extragalactic sky within about 4 yr of accumulated radiation damage. However, software mitigation techniques demonstrated elsewhere can reduce this by a factor of ~10, bringing the effect well below mission requirements. This conclusion is valid only for the p-channel CCDs we have modeled; CCDs with higher CTI will fare worse and may not meet the requirements of future dark energy missions. We also discuss additional ways in which hardware could be designed to further minimize the impact of CTI.

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.

Proton radiation damage in high-resistivity n-type silicon CCDs

A new type of p-channel CCD constructed on high-resistivity n-type silicon was exposed to 12 MeV protons at doses up to 1 X 1011 protons/cm2. The charge transfer efficiency was measured as a function of radiation dose and temperature. We previously reported that these CCDs are significantly more tolerant to radiation damage than conventional n-channel devices. In the work reported here, we used pocket pumping techniques and charge transfer efficiency measurements to determine the identity and concentrations of radiation induced traps present in the damaged devices.

Charge trap identification for proton-irradiated p+ channel CCDs

Charge trapping in bulk silicon lattice structures is a source of charge transfer inefficiency (CTI) in CCDs. These traps can be introduced into the lattice by low-energy proton radiation in the space environment, decreasing the performance of the CCD detectors over time. Detailed knowledge of the inherent trap properties, including energy level and cross section, is important for understanding the impact of the defects on charge transfer as a function of operating parameters such as temperature and clocking speeds. This understanding is also important for mitigation of charge transfer inefficiency through annealing, software correction, or improved device fabrication techniques. In this paper, we measure the bulk trap properties created by 12.5 MeV proton irradiation on p+ channel, full-depletion CCDs developed at LBNL. Using the pocket pumping technique, we identify the majority trap populations responsible for CTI in both the parallel and serial transfer processes. We find the dominant parallel transfer trap properties are well described by the silicon lattice divacancy trap, in agreement with other studies. While the properties of the defects responsible for CTI in the serial transfer are more difficult to measure, we conclude that divacancy-oxygen defect centers would be efficient at our serial clocking rate and exhibit properties consistent with our serial pocket pumping data.

Radiation Tolerance of Fully-Depleted P-Channel CCDs Designed for the SNAP Satellite

Thick, fully depleted p-channel charge-coupled devices (CCDs) have been developed at the Lawrence Berkeley National Laboratory (LBNL). These CCDs have several advantages over conventional thin, n-channel CCDs, including enhanced quantum efficiency and reduced fringing at near-infrared wavelengths and improved radiation tolerance. Here we report results from the irradiation of CCDs with 12.5 and 55 MeV protons at the LBNL 88-Inch Cyclotron and with 0.1-1 MeV electrons at the LBNL 60Co source. These studies indicate that the LBNL CCDs perform well after irradiation, even in the parameters in which significant degradation is observed in other CCDs: charge transfer efficiency, dark current, and isolated hot pixels. Modeling the radiation exposure over a six-year mission lifetime with no annealing, we expect an increase in dark current of 20 e-/pixel/hr, and a degradation of charge transfer efficiency in the parallel direction of 3 times 10-6 and 1 times 10-6 in the serial direction. The dark current is observed to improve with an annealing cycle, while the parallel CTE is relatively unaffected and the serial CTE is somewhat degraded. As expected, the radiation tolerance of the p-channel LBNL CCDs is significantly improved over the conventional n-channel CCDs that are currently employed in space-based telescopes such as the Hubble Space Telescope.

Device Design for a 12.3-Megapixel, Fully Depleted, Back-Illuminated, High-Voltage Compatible Charge-Coupled Device

A 12.3-megapixel charge-coupled device (CCD) that can be operated at high substrate-bias voltages has been developed in support of a proposal to study dark energy. The pixel size is 10.5 mum, and the format is 3512 rows by 3508 columns. The CCD is nominally 200 mum thick and is fabricated on high-resistivity n-type silicon that allows for fully depleted operation with the application of a substrate-bias voltage. The CCD is required to have high quantum efficiency (QE) at near-infrared wavelengths, low noise and dark current, and an rms spatial resolution of less than 4 mum. In order to optimize the spatial resolution and QE, requirements that have conflicting dependences on the substrate thickness, it is necessary to operate the CCD at large substrate-bias voltages. In this paper, we describe the features of the CCD, summarize the performance, and discuss in detail the device-design techniques used to realize 200-mum-thick CCDs that can be operated at substrate-bias voltages in excess of 100 V.

Development of fully depleted back-illuminated charge-coupled devices

The status of CCD development efforts at Lawrence Berkeley National Laboratory is reviewed. Fabrication technologies for the production of back-illuminated, fully depleted CCDs on 150 mm diameter wafers are described. In addition, preliminary performance results for high-voltage compatible CCDs, including a 3512 x 3512, 10.5 μm pixel CCD for the proposed SuperNova Acceleration Probe project, are presented.

High-voltage-compatable, fully depleted CCDs

We describe charge-coupled device (CCD) development activities at the Lawrence Berkeley National Laboratory (LBNL). Back-illuminated CCDs fabricated on 200-300 μm thick, fully depleted, high-resistivity silicon substrates are produced in partnership with a commercial CCD foundry. The CCDs are fully depleted by the application of a substrate bias voltage. Spatial resolution considerations require operation of thick, fully depleted CCDs at high substrate bias voltages. We have developed CCDs that are compatible with substrate bias voltages of at least 200V. This improves spatial resolution for a given thickness, and allows for full depletion of thicker CCDs than previously considered. We have demonstrated full depletion of 650-675 μm thick CCDs, with potential applications in direct x-ray detection. In this work we discuss the issues related to high-voltage operation of fully depleted CCDs, as well as experimental results on high-voltage-compatible CCDs.

Ultra-fast voltage comparators for transient waveform analysis

The time at which an input signal crosses the reference level of a voltage comparator can be used in the analog-to-digital conversion of fast single waveform transients. In such a converter, an array of identical comparators, properly biased, provides stop inputs to a picosecond-resolution multistop time digitizer. Each stop represents a point in the voltage-time reconstruction of the measured waveform. A number of state-of-the-art comparators, bipolar and GaAs, have been evaluated for this application to determine the differences in their time propagation as a function of the input signal overdrive and risetime. Normalized data are presented to assist in the correction of a digitizers measurement errors. A picosecond time resolution measurement system used in the tests is described. >

Near-100% Quantum Efficiency of Delta Doped Large-Format UV-NIR Silicon Imagers

We have demonstrated a back surface process for back-illuminated high-purity p-channel charge-coupled devices (CCDs), enabling broadband coverage from the ultraviolet to near infrared (NIR). The process consists of the formation of a delta layer followed by a double layer antireflection (AR) coating. The process is performed below 450degC and is applied to fully fabricated CCDs with aluminum metallization. The delta doping process was demonstrated on 1 k times 1 k and 2 k times 4 k CCDs, which were found to exhibit low dark current and near reflection-limited quantum efficiency. Two broadband AR coatings were developed to cover the UV-visible and visible-NIR bands. These coatings consist of a double layer of SixNy and SiOx deposited by plasma enhanced chemical vapor deposition onto the back surface of a delta doped CCD. The thicknesses of the coating layers are adjusted for the desired bandpass.