William C. McColgin

Probing Metal Defects in CCD Image Sensors

We have investigated the role of heavy metals in causing visible pixel defects in Charge Coupled Device (CCD) image sensors. Using a technique we call dark current spectroscopy, we can probe for deep-level traps in the active areas of completed image sensors with a sensitivity of 1 × 10 9 traps/cm 3 or better. Analysis of histograms of dark current images from these sensors shows that the presence of traps causes quantization in the dark current. Different metal traps have characteristic dark current generation rates that can identify the contaminant trap. By examining the temperature dependence of the dark current generation, we have calculated the energy levels and generation cross sections for gold, iron, nickel, and cobalt. Our results show the relationship of these traps to the “white spot” defects reported for image sensors.

Polymethine dye lasers

Merocyanine dyes which contain a pyran nucleus as part of the intercyclic chain are useful as laser dyes. These dyes are used in solution with a non-interfering solvent to form lasing media useful in dye lasers. Such lasers generally include a reservoir for containing the laser dye solution and a pumping energy source operably associated therewith for producing stimulated emission of the laser dye solution.

Bright-Pixel Defects in Irradiated CCD Image Sensors

We have examined environmental radiation sources for digital cameras to find the origins of bright-pixel defects known to accumulate with time. We show that beta and gamma emissions from camera parts and lenses cause image transients, but permanent damage can occur with alpha particles from the CCD cover glass. Our experiments with 14-MeV- and thermal-neutron beams confirm that cosmic rays are the primary cause of new imager bright points.

Dark Current Spectroscopy Of Metals In Silicon

Dark current spectroscopy (DCS) is used to identify the signature of metals that generate dark or leakage current in silicon image sensors. Individual metal atoms or defects are detected by DCS on a pixel-by-pixel basis. DCS is applied here to show how the number of electrically active iron atoms in a pixel changes with light and with low-temperature anneals. The measurements explore the dissociation and association of iron-boron pairs and the diffusion of iron near room temperature.

Effects of Deliberate Metal Contamination on CCD Imagers

The 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.

Silicon-Added Bilayer Resist (SABRE) System

A new silylated resist process has been developed that provides the advantages of trilayer photoresist processing in a simpler bilayer format. In this new process, silicon for oxygen plasma etch resistance is added to a conventional photoresist layer after it has been exposed and developed. This silylated resist image provides an etch mask for 02 RIE etching of an underlying organic planarizing layer with 15:1 selectivity, vertical profiles, and high resolution. The process is positive-working, uses conventional materials, and improves the thermal stability of the resist image.

Defect Engineering in CCD Image Sensors

Defect engineering principles are integral to the design and manufacture of high-quality CCD image sensors. As examples, we describe the use of epitaxial silicon for defect control, hydrogen passivation of interface defects, and several forms of impurity gettering. The high sensitivity of image sensors to contaminants reveals that boron segregation gettering of iron dominates gettering by oxygen precipitates for both fast and slow cooling cycles. We estimate that the gettering efficiency for iron is 99.95%.