In a longstanding effort to overcome limits of film and the charge coupled device (CCD) systems in electron microscopy, we have developed a radiation-tolerant system that can withstand direct electron bombardment. A prototype Direct Detection Device (DDD) detector based on an Active Pixel Sensor (APS) has delivered unprecedented performance with an excellent signal-to-noise ratio (approximately 5/1 for a single incident electron in the range of 200-400 keV) and a very high spatial resolution. This intermediate size prototype features a 512×550 pixel format of 5&mgr;m pitch. The detector response to uniform beam illumination and to single electron hits is reported. Radiation tolerance with high-energy electron exposure is also impressive, especially with cooling to -15 °C. Stable performance has been demonstrated, even after a total dose of 3.3×106 electrons/pixel. The characteristics of this new detector have exciting implications for transmission electron microscopy, especially for cryo-EM as applied to biological macromolecules.
High resolution electron imaging is very important in nanotechnology and biotechnology fields. For example, Cryogenic Electron-Microscopy is a promising method to obtain 3-D structures of large protein complexes and viruses. We report on the design and measurements of a new CMOS direct-detection camera system for electron imaging. The
active pixel sensor array that we report on includes 512 by 550 pixels, each 5 by 5 μm in size, with an ~8 μm epitaxial layer to achieve an effective fill factor of 100%. Spatial resolution of 2.3 μm for a single incident e- has been measured. Electron microscope tests have been performed with 200 and 300 keV beams, and the first recorded Electron Microscope image is presented.
There is an urgent need to replace film and CCD cameras as recording instruments for transmission electron
microscopy (TEM). Film is too cumbersome to process and CCD cameras have low resolution, marginal to poor
signal-to-noise ratio for single electron detection and high spatial distortion. To find a replacement device, we have
tested a high sensitivity active pixel sensor (APS) array currently being developed for nuclear physics. The tests were
done at 120 keV in a JEOL 1200 electron microscope. At this energy, each electron produced on average a signal-tonoise
ratio about 20/1. The spatial resolution was also excellent with the full width at half maximum (FWHM) about
20 microns. Since it is very radiation tolerant and has almost no spatial distortion, the above tests showed that a high
sensitivity CMOS APS array holds great promise as a direct detection device for electron microscopy.