New Performance for SEM with an Annular Switchable Gain BSE Detector Diode

Abstract text (incl. figure legends and references)

Using the whole energy spectrum of the SEM has been always an issue by installation of just one Back Scattered Electron Detector (BSD) to get high quality images in each scanning speed and energy range for every application. Despite of the state-of-the-art BSDs with all premium features as a GCE up to 85%, a DCE up to 55% at 1 keV (see Fig. 1), and a high signal to noise (S/N) ratio, there is still lack of a feature which closes the gap between low and high keV range when high scan speed and high throughput are needed. This applies specially for life science and bio science applications where the sensitive samples need to be scanned at low keV conditions but at the same time with high image acquisition.

In order to be able to maintain the scan speed relatively high at very low energies, one needs to increase the gain of the BSD collecting as much signal as possible to optimize the signal to noise (S/N) ratio. A typical BSD is not capable of changing the gain once the higher value is needed since the gain is already fixed in the preamplifier. There is no more need to exchange the BSD while working under this situation. Instead with PNDetector"s new Switchable Gain Advanced BSD (SWG-BSD) one can overcome this issue and switch the gain manually to a higher value in a fraction of a second while scanning the sample so that the scan speed is not sacrificed and significantly reduced to get high resolution images. Here we show how the new detector enables this technique.

The SWG-BSD is equipped with two preamplification gains in the preamplifier integrated into a compact vacuum compatible housing shown in Fig. 2. The gain values are adjusted to and V/A. The lower value is called "high speed" mode and the higher value is called "high gain" mode. With switching between these two modes one is capable of adjusting the gain while changing the energy in order to maintain the desired scan speed and optimize the S/N. When the SWG-BSD is used in high speed mode the typical rise time achieved by the detector is down to 30ns (the real TV speed) and very high resolution single frame images without smearing can be taken under 0.1 sec with 0.8 MP once the sample is scanned at high energies such as 30keV. The typical example of such application is in semiconductor industry to surf the samples very fast and search for hot spots. Once the spot is found and focused it is now the time to switch to high gain mode and zoom the spot for detail inspection to get an optimized image while reducing the scan speed not that low as it is normally used with traditional BSDs. The SWG-BSD enables the possibility of maintaining the scan speed relatively high under 1000ns rise times and the result will be a superior SEM image of the spot in e.g. 31 sec at 1 keV (see Fig. 3). Other common application for SWG-BSD is observation of sensitive samples like tissues for which very low energies under 1keV are needed to ensure minimal interaction between the electron and the sample and reduce damage and charging.

The two gain settings adjusted for SWG-BSD is used in combination with the main amplifier with a Python based GUI software enabling the switch action by a single button designed in the software. The SWG-BSD comes also with the complete motorized retractable system shown in Fig. 2 with all vacuum compatible parts to be installed directly in the SEM.

Figure 1.

Figure 2.

Figure 3. Two SEM images taken by SWG-BSD in (left) high speed and (right) high gain modes.