Imaging

There are a number of options and technologies available for digital imaging in transmission electron microscopy (TEM) applications today. Traditionally, high energy electrons could not be directly exposed to a sensor without excessively damaging the detector. As a consequence, conventional TEM cameras first expose the incoming electron beam to a scintillating film that converts the electrons into light (photons). These photons are then transferred to the sensor, either through a series of optical lenses or a coupled fiber optic plate. Finally, the light is collected by a sensor where the image is created pixel-by-pixel based on the amount of light detected at each position in the sensor.

Step 1) Convert electrons to signal

Gatan uses proprietary phosphor scintillators to optimize signal conversion that enhances detector sensitivity (SENS) and resolution. When you select a scintillator, it is appropriate to know the performance trade-offs between SENS and resolution.

• Sensitivity (signal): Ideal for dose-sensitive use cases where you need to generate more photons per incoming electron (e.g., cryo-tomography, beam-sensitive materials)
• Resolution (spatial detail): Favorable for applications where you require more information to resolve details, but you can increase the dose (signal) without harming the sample (e.g., semiconductors and other less-sensitive materials)

Step 2) Transfer signal

Various coupling (lens- and fiber-coupled) mechanisms are available to optimize signal transfer and meet cost or performance targets for a given detector.

Step 3) Detect the signal with the sensor

Sensor type (CCD vs. CMOS) offers significant trade-offs for TEM camera performance as there are fundamental differences in architecture.

• Charge-coupled device (CCD): Charge transfers between neighboring cells, and read-out (e.g., noise) is seen at the final stage; binning minimizes the impact of read-out noise

• Complementary metal–oxide–semiconductor (CMOS): Charge immediately converts to voltage (read-out with digital output); supports high frame rates, low overall electronics noise

   

Both technologies possess inherent advantages, so the question arises about what unique performance characteristics arise from each choice. CCDs can have a 100% fill factor that captures all incoming light, whereas part of the CMOS sensor is occupied by transistors and metal wiring associated with each pixel.  Historically, CCDs provided higher-quality images with low noise at affordable prices. Recent design advancements and processing techniques now advance CMOS sensor performance so it is a viable choice for some applications. Note that CCDs still maintain an advantage for binning in terms of signal-to-noise. However, CMOS chips can scale the number of read-out ports and achieve very high frame rates.

Step 4) Transfer signal and read-out image

When a charge converts to voltage, you typically generate noise

• CCD: Transfer data out of the serial register
• CMOS: Converts to voltage per pixel

It is very important to optimize read-out noise (higher voltages) and speed (multi-port and fast read times) for CCDs.

• Optimize controller for low read-noise; leverage multi-port read-outs for faster speed
• Interline CCDs with binning have the fastest readouts (fps) – up to 30 fps due to 100% duty cycle

CMOS typically is seen as a fast sensor because you can run in rolling shutter mode verses the slow global shutter mode.