WO2005041243A1

WO2005041243A1 - Electron detection system for a scanning electron microscope

Info

Publication number: WO2005041243A1
Application number: PCT/PL2004/000079
Authority: WO
Inventors: Witold Slowko
Original assignee: Politechnika Wroclawska
Priority date: 2003-10-27
Filing date: 2004-10-06
Publication date: 2005-05-06
Also published as: EP1678734A1; PL208151B1; PL363131A1

Abstract

The electron detection system for a scanning electron microscope is mounted in the head body (1), made of teflon. In the lower part of the head body (1), the lower throttling aperture (2) of the form of the metal plate with a small hole at the axis of the electron beam WE is placed. Above the lower throttling aperture (2), the micro-porous plate (3) is located. The micro-porous plate (3) has a hole at the axis of the electron beam WE, in which the screen pipe (4) is fastened by means of the teflon sealing. Over the micro-porous plate (3), scintillators (5) are disposed symmetrically around the electron beam WE axis. Scintillators (5) are connected with the light pipes (6) that lead to photomultipliers.

Description

Electron detection system for a scanning electron microscope

The subject of the invention is the electron detection system for a scanning electron microscope, destined particularly for the work at pressures of order 100 Pa in the sample chamber. There are solutions of the electron detection systems known from the patent applications: nr P329339, entitled "Higςh pressure scanning electron microscope", and nr P359748, entitled "Secondary electron detector system for a scanning electron microscope", which are composed of a microporous plate, advantageously of the micro sphere type, and a secondary electron detector of the scintillation type. In the both solutions, the stream of secondary electrons, irrespectively of their emission directions, is bring in the hole in the lower wall of the intermediate chamber, which is biased with a proper voltage. The hole plays simultaneously the role of the lower throttling aperture, whch limits gas flow from the sample chamber to the intermediate chamber. Secondary electrons that came into the intermediate chamber impinge the input surface of the microporous plate, which is placed at the electron optical axis. The electrons passing across the microporous plate are multiplied, and at the output side they are attracted by the scintillator biased with a high voltage. Here they are detected finally, that means converted into a light signal that is sent by the right pipe to the photo- multiplier to be ones more converted into the electric signal. The signal does depend substantially on the directions of the secondary electrons emission that are detected. The subject of the invention is the electron detection system for a scanning electron microscope equipped with the lower throttling aperture, the microporous plate with the screen pipe and scintillators connected with light pipes and photo- multipliers. The substance of the invention consists in the fact, that at least two scintillators are placed at the output side of the microporous plate. It is advantageous when each scintillator is positively biased against the output of the of the microporous plate with high voltage of the value over 1000 V. It is also advantageous when the screen pipe placed in the hole of the microporous plate is negatively biased against the lower throttling aperture. The main advantage of the electron detector system, according to the invention, is the possibility of the directional detection of secondary electrons in order to obtain output signals that are dependent on a local azimuth and slope angle of the surface examined. The output signals may be processed to obtain three-dimensional reconstruction of the surface. The subject of the invention is shown in the Figure, which displays the secondary electron detector unit for a scanning electron microscope in the cross- section. The electron detection system for a scanning electron microscope is mounted in the head body (1), made of teflon. In the lower part of the head body (1), the lower throttling aperture (2) of the form of the metal plate with a small hole at the axis of the electron beam WE is placed. Above the lower throttling aperture (2), the micro-porous plate (3) is located. The micro-porous plate (3) has a hole at the axis of the electron beam WE, in which the screen pipe (4) is fastened by means of the teflon sealing. Over the micro-porous plate (3), four scintillators (5) are disposed symmetrically around the electron beam WE axis. Scintillators (5) are connected with the light pipes (6) that lead to photomultipliers. The electron detection system for a scanning electron microscope arranged in the described way works as follow. Secondary electrons EW generated from the sample stage (7) are attracted by the lower throttling aperture (2) biased positively, and pass through the hole in the electrode. At the other side of the lower throttling aperture (2), the stream of the secondary electrons EW encounter an electric retarding field created by the screen pipe (4) negatively biased with respect to the lower throttling aperture (2). As the result, in the lower throttling aperture (2) region a diverging electron lens arises and the flow of the secondary electrons EW from the sample stage (7) to the micro-porous plate (3) is laminar. Then, the secondary electrons EW impinge the sector of the micro-porous plate (3) adequately to the direction of their emission. The secondary electrons EW pass through microscopic channels in the micro-porous plate (3) of the micro sphere kind supplied with a voltage of order lkV, and they are multiplied thanks to secondary emission from its walls. The multiplied stream of secondary electrons EW escapes the micro-porous plate (3) at the output side, where four scintillators (5) are disposed symmetrically around the axis of the electron beam WE. The scintillators (5) are positively biased with respect to the micro-porous plate (3) with a voltage about 12kV, thus they are trapping secondary electrons EW that escape adjacent sectors of the microporous plate (3) respectively. There, the electron signals is converted into light signals transmitted through the light pipes (6) to corresponding photo-multipliers where they are converted into output signals finally. Amplitudes of the signals in each of the four channels depend of the number of secondary electrons EW emitted toward the sector corresponding to a particular scintillator (5), then the detection system has directional properties. The properties may be disturbed by a scattering of the secondary electrons EW are scattered in collision with gas molecules that fill the sample chamber to a relatively high pressure Pi The gas pressure P₂ in the space between the micro-porous plate (3) and the lower throttling aperture (2) that constitutes the intermediate chamber, is two orders of magnitude less and does not cause serious scattering of the secondary electrons EW. In order to minimize effects of the electron scattering, a distance of the sample stage (7) from the lower throttling aperture (2) should be comparable to a diameter of the lower ImOttling aperture (2). The four signals obtained from the electron detection system for a scanning electron microscope, can be processed in the way that leads to the synthesis of the three dimensional image.

Claims

1. Electron detection system for a scanning electron microscope equipped with the lower throttling aperture, the microporous plate with the screen pipe, and scintillators connected with bight pipes and photomultipliers characterized in that at least two scintillators (5) are placed at the output side of the microporous plate (3).

2. Electron detection system of claim 1 characterized in that each scintillator (5) is positively biased against the output of the of the microporous plate (3) with high voltage of the value over 1000 V.

3. Electron detection system of claim 1 characterized in that the screen pipe (4) placed in the hole of the microporous plate (3) is negatively biased against the lower throttling aperture (2)