A Charge Coupled Device Readout System for Electron Microscopy

Abstract

We have investigated the possibility of using a charge coupled device (CCD) as a direct, electron-sensitive readout device for a CTEM. Two-dimensional imaging CCD's, developed as normal television camera elements, are semiconductor devices in which an image is formed on an array of photo-sensitive sites, causing the accumulation of an electric charge proportional to the incident flux. The video signal is generated by sequentially transferring the charges for each element of a line, in bucket-brigade fashion, to the input of the video amplifier. Sensitivity of the CCD to electrons has been demonstrated by the successful application in photocathode tubes, where the photoelectrons are accelerated to an energy up to 15 keV onto the image sensing area of the CCD. The application of the device in a 100 keV transmission electron microscope (Ferrier and Chapman, private communication), with the device in vacuo at the image plane, seems to have promising possibilities for image intensifier, electron counting, and computer input devices. A CCD readout system should have several advantages over previously designed video readout systems, including elimination of the phosphor, fiber optic or lens coupling, and intermediate image intensifier stages. The high gain and low noise of the device should allow detection of single electrons with a detective quantum efficiency near unity.