Performance Characterization and Improvement for CCD Cameras in Transmission Electron Microscopy

K H Downing
2005 Microscopy and Microanalysis  
The development of charge coupled device (CCD) cameras for the consumer market has been dramatic over the last several years. A significant fraction of the population now uses inexpensive cell phones that have built-in cameras with better resolution than was recently available for electron microscopy at hundreds of times the price. While there have been tremendous advances in the technology for producing many types of CCD chips, the requirements for effective performance in microscopy have so
more » ... r made it impractical to take advantage of the mass produced chips. Still, CCDs are now in routine use for a number of TEM applications, particularly where large series of images are needed such as in tomography and wavefront reconstruction from focus series, and where the benefits of rapid, digital data recording outweigh any of the disadvantages in CCD performance compared to other data recording media. We discuss here some of the performance issues that relate particularly to high-end cameras and how limitations in the performance might be overcome. All CCD cameras used with electron microscope currently use a scintillator to convert the electron beam energy to photons which are then passed to the light-sensitive CCD chip. The signal is read out through a series of amplifiers. CCDs have the very favorable property of a highly linear response -the output signal is proportional to the electron intensity over a wide rang of intensities. Ideally this linearity would extend to very low intensities so that every single electron would be clearly detected. A number of factors combine to make the signal from a single-electron somewhat difficult to detect. The electron-to-photon conversion generally utilizes only a fraction of the electron energy. Some fraction of the resultant photons is lost before reaching the CCD. The conversion of photons to signal in the CCD is less than 100% efficient. Finally there is always some noise introduced in the readout process. The amount of noise in an electronic circuit decreases as the bandwidth of the circuit decreases. In the context of CCD readout, this means that we can greatly reduce the readout noise by reading the signal from the CCD slowly. Thus the CCDs used for most high quality data collection are "slow scan" CCDs. The slow readout also improves the efficiency of the internal charge transfers involved in extracting the signal. As for the light conversion efficiency of the CCD, manufacturers have steadily been making improvements that lead to better sensitivity and spectral response. Developments in these areas have been effectively driven at least as much by astronomers as by electron microscopists; while the number of high-end telescopes is much smaller than for electron microscopes, the total costs involved are much greater, providing a funding source for the very expensive developments in the technology. The interaction of the electrons with the scintillator and subsequent coupling to the CCD has been an area of very active development by the microscopy community. The scintillator is most commonly attached to a fiber optic plate that is in turn bonded to the CCD chip. Tapered fiber optics can change the size scale or magnification between the scintillator and the CCD pixels over a 608
doi:10.1017/s1431927605510225 fatcat:5fyimq2rcjcxfi3tagkyahttgi