Future directions for camera systems in electron microscopy.

We are working to develop an optimal, direct digital replacement for electron microscopy film. Our efforts are based on a longstanding effort to push the limits of charge-coupled devices (CCDs), which has motivated us to pioneer technologies for direct electron detection. Here we review our current efforts to scale up our own lens-coupled CCD camera and make a system capable of exceeding the spatial resolution of film, while maintaining single-electron sensitivity. This 8k x 8k lens-coupled CCD system represents the current state-of-the-art in CCD-based systems, but it also demonstrates the great engineering effort required to achieve these key performance benchmarks when the detector is based on a resolution-limiting scintillation screen. We have therefore developed a parallel effort to produce a radiation-tolerant system that can withstand direct electron bombardment. We briefly describe our early efforts to adapt the pixel array detector (PAD) that is commonly used in X-ray diffraction. We then describe our subsequent development of a groundbreaking prototype system based on an active pixel sensor (APS). This early implementation of an APS-based direct detection detector (DDD) has already delivered unprecedented performance-in many areas exceeding the fundamental capabilities of CCD-based systems. This implementation features a 512 x 550 pixel format of 5 mu m pitch with an excellent signal-to-noise ratio (similar to 13/1 for a single incident electron in the range of 200-400 keV) and a very high spatial resolution (2-4 mu m). Radiation tolerance with high-energy exposure is also impressive, especially with cooling to - 15 degrees C. Stable performance has been demonstrated over a period of several months of continuous use before a thermal annealing process is required to restore the device. The characteristics of this new detector have exciting implications for transmission electron microscopy, especially for cryo-electron microscopy (cryo-EM) as applied to biomolecular structures. Due to its excellent spatial resolution, this DDD is capable of distinguishirig macromolecule structures at very high resolution using a magnification of only 50,000x. As such, this system has the potential to deliver cryo-EM three-dimensional (3D) reconstructions with 3 angstrom resolution.