Radiation-Induced Deep-Level Traps in CCD Image Sensors
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0994-F12-07
Radiation-Induced Deep-Level Traps in CCD Image Sensors Cristian Tivarus, and William C. McColgin Eastman Kodak Company, 1999 Lake Avenue, Rochester, NY, 14650-2008
ABSTRACT Dark current spectroscopy (DCS) is used to study deep-level traps corresponding to the bright pixels that form the histogram ìtailsî of irradiated charge-coupled devices (CCD). We found four distinct traps, among which the double vacancy (V2) and the vacancyñphosphorous (VP) have the highest concentrations and generation rates. We show that DCS can be used to examine the annealing mechanisms of silicon defects to concentrations as low as 2 × 107 cm-3. INTRODUCTION The influence of particle irradiation on the CCD properties is well known from previous studies and includes among other effects the formation of a wide range of bright pixels [1]. Dark current histograms collected after irradiation with high-energy particles showed pronounced bright point tails that increase with irradiation time and dose [1−3]. While most of the previous work has concentrated on the properties of the ìhotî pixels that end these tails and have very high generation rates, little is known about the origin of the defects inside the bright pixels that form the bulk of the tails. Furthermore, these tails are detrimental to device performance since they are responsible for the increase in CCDís dark current. Usually, the microscopic origin of the defects generating the bright pixels is inferred from measurements of CCDsí macroscopic parameters or deep-level trap studies performed using bare silicon wafers. In this study, we use DCS to directly determine the origin and the properties of irradiation-induced traps in CCD image sensors [4−6]. We also show that DCS can be used to study annealing mechanisms of intrinsic silicon defects, such as impurity-assisted annealing.
EXPERIMENT The irradiation experiments were performed at room temperature using Eastman Kodak KAF-8300 Image Sensors and a 210Po alpha particle source (~5.3 MeV) with an estimated emission rate of about ~2 x 103 alpha particles/s. After irradiation, the image sensors were tested on a wafer probe station with a temperature-controlled chuck. The method chosen to investigate the deep-level traps produced by irradiation was DCS. Dark images were collected using 4 s of integration at 60∞C and 32 image averages in order to reduce temporal noise. Isothermal annealing studies were performed using a hot plate that allows
temperature control to within one degree. So far, DCS has been successfully used to identify metallic contaminants in image sensors at concentrations as low as ~107 cm-3. The method is based on the fact that if any traps are present in the active volume of the pixels, they generate dark current [4−6]. Histograms of the pixelsí generation rate obtained from images taken in the dark and using long integration times show characteristic peaks that each correspond to a given deep level trap [4−6]. These peaks enable one to count the number of traps in a pixel as well as the total number of t
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