Well, you do not have to worry about this anymore. The new ORIUS SC200D CCD camera is designed to allow users to record electron diffractions patterns without being worried about damaging the CCD and even more the camera can record diffraction patterns free from the common streaking artifact (figure 1). Figure 2a shows the SAED (selected area electron diffraction pattern) from LaCO₃ crystal recorded by the SC200D CCD camera. Figure 2b shows the same pattern with contrast reversed. From these images it is clear that the streaking has been successfully eliminated.

ORIUS^® SC200D CCD Camera

The function of any scintillator is to convert high energy electrons into photons. If the phosphor coating is on a glass or fiber optics substrate, all the electrons are stopped by the substrate, and most of the electron energy is deposited in a thin layer on top of the substrate, where it can lead to damage to the phosphor or even the substrate itself. With a thin transmission type substrate, only little energy is deposited in the support film when the electrons pass through, hence it takes a much higher electron flux to damage the scintillator. The SC200D scintillator uses a film supported phosphor, where the film is thin enough for electrons to pass through.

A second advantage for the transmission scintillator is the very limited generation of (hard) x-rays, since the electrons are not stopped in the substrate. Finally, the electrons can only scatter over a limited volume before exiting the substrate again, yielding better resolution than a bulk substrate especially at higher accelerating voltages. Most of the damage to the scintillator occurs during its prolonged exposure to an intense electron beam. The main damage mechanism is the heat trapped in the scintillator.

From our tests for beam damage we used calibrated screen measurements to estimate beam current at the scintillator. It was found that the transmission scintillator can withstand up to an electron dose of 11nA/cm² for up to 10 minutes using a 200kV FEG TEM (large condenser apt, spot size #4, gun lens 4) without suffering apparent structural damage. This is more than enough time for performing diffraction analysis where the transmitted beam is focused onto the scintillator in diffraction mode of the TEM. This amount of time allows for in-situ recording while in diffraction mode without fear of damaging the camera system. After subjecting the camera system to a high intensity transmitted electron beam, light microscopy was used to evaluate the surface of the substrate and the phosphor coating for any signs of physical damage. Upon inspection there was no evidence of deformed substrate or a blistered phosphor.

Compared with fiber-optical coupled CCD cameras, many lens-coupled cameras, using standard “off-the-shelf” lens, suffer problems of image distortion, vignetting and low sensitivity. In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. In microscopy this is represented by the formula

where n is refraction index of the medium in which the lens is working (1.0 for air, 1.33 for pure water, and up to 1.56 for oils), and θ is the half-angle of the maximum cone of light that can enter or exit the lens. In general, this is the angle of the real marginal ray in the system. The angular aperture of the lens is approximately twice this value (within the paraxial approximation). The NA is generally measured with respect to a particular object or image point and will vary as that point is moved.

In microscopy, NA is important because it indicates the resolving power of a lens.

The custom designed lens assembly has a value of NA of 0.15 or ~ 8.6° half-opening angle. As a reference, in a conventional light microscope using an objective of 1x magnification a typical NA is 0.04. Since the optics used in the SC200D is a 1:1 magnification its larger NA value ensures the sensitivity and light collecting efficiency of the SC200D to be close to our fiber-optical coupled cameras. SC200D is about 50% in sensitivity of fiber-optical coupled CCD, e.g. SC1000 at 200kV. This high sensitivity value allows SC200D to record weak intensities in electron diffraction patterns with high efficiency.

The high quality lens assembly design in the SC200D results in a large uniform illumination across the field of view (FOV) of the CCD sensor. Figure 4a is an unprocessed image of vacuum space taken with the SC200D. The intensity profile across the FOV (figure 4b) shows only 2% change in intensity. This means that the vignetting effect is negligible. Such a small change can be easily corrected by gain normalization procedure. Figure 5a shows the gain normalized image and figure 5b is the intensity profile, where uniform illumination has been achieved. The geometric distortion in the custom designed lens is less than 15µm (e.g. 2 CCD pixels).

      

The diffraction pattern shown in figure 2 is free of the “streaking” artifact commonly observed in the recorded diffraction patterns by a CCD camera. To eliminate the streaking, advanced CCD electronics has been successfully developed to handle the large amount of “excessive” charge in the saturated CCD pixels by effectively “trapping” and “draining” the charges. This has resulted in a patented technology in producing “streak-free” diffraction patterns.

It is worth pointing out that the ability to record “streak-free” diffraction patterns depends on sample thickness and TEM beam setting conditions.

To put it in context familiar to TEM users, we used a commercially available cross grating replica (2160 lines per mm commonly used for magnification calibration) and recorded selected area electron diffraction patterns (SAED) under various beam conditions. Figure 6 shows a pair of patterns observed in the live View mode (full CCD and 4x binning) for spot size #1 (6a) and #4 (6b) on a 200kV TEM with parallel beam and a large SAED aperture (800µ). In both cases, although the central spot is saturated, no streaking is observed in the vertical direction. Figure 7 shows the corresponding diffraction patterns recorded in full resolution (full CCD and 1x binning).

(a)

(b)

Although rapid progress has been made in digital imaging technology, recording electron diffraction patterns (especially streak-free) with CCD cameras remains a big challenge. With the birth of the new ORIUS^® SC200D this challenge has been met with a robust and easy to use CCD camera, freeing the user from concerns of damage and artifacts when recording diffraction patterns.

APPENDIX

SC200D uses the same high speed controller as the other ORIUS^® cameras. As a result, it can output LIVE images at 30 fps (full CCD 4x binning). When in diffraction mode, due to the overhead in producing the patented “streak-free” diffraction patterns, the frame rate is reduced to about 25 fps, still the TV rate.

Field of view

Since SC200D is mounted in the 35mm port (aka wide angle port) of a TEM, it has a larger field of view than the same CCD mounted in the bottom port. Figure 8 shows the field of view comparison of SC200D to film. The rectangular blue frame is the FOV of photographic film.

The large FOV of SC200D makes it ideal for recording electron diffraction patterns and viewing sample areas during LIVE search.

SC200D can be easily inserted or retracted by a simple click of mouse in the software. Together with its high frame rate, SC200D is the ideal choice for any remote microscopy application.

The ORIUS® family now comprises the very successful SC1000 (4008x2672), the SC600 (2672x2672) the SC200 (2048x2048) and now the SC200D (2048x2048). Each of the ORIUS cameras have as part of their design the capability of fast frame acquisition ranging from 14fps of the SC1000 and SC600 models to the 30fps of the new SC200 models.