Electron beam scanning

Transcription

1 Electron beam scanning The Electron beam scanning operates through an electro-optical system which has the task of deflecting the beam Synchronously with cathode ray tube which create the image, beam moves along parallel equidistant lines for covering a rectangular area from the sample. L specime L CRT n A correspondence is established between each region of the investigated sample and a region of the screen to obtain an enlarged image with magnification factor M = L CRT /Lspecimen.

2 Electron beam scanning The scanning system is normally constituted by a series of magnetic coils (eg, A and B, in the figure) placed close to the optical axis, usually inserted next to the objective lens. z x According to the scheme of the figure, the deflection occurs along the x axis. To scan an area xy need two pairs of coils placed at perpendicular direction introducing the scanning along y. Each pair of coils produces a uniform magnetic field in proximity of the optical axis: the first deflects the beam of θ A ; the second pair deflects the beam in the opposite direction, so as to pass the beam at the center of the final opening. In this way the aberrations of the objective lens are minimized.

3 Electron beam scanning The coils system described takes the name of pre-lens double deflection system (figure a). Usually electronic circuits are inserted in the column to automatically compensate for any variations in the acceleration voltage and the focal length, introduced by the deflection system during the observation. Particular attention should be paid also in the design of the system to minimize the interference of the magnetic fields generated by the objective lens. An alternative to the pre-lens deflection are the post-lens single deflection (b) and pre- and post-lens combined systems with single or double deflection (c, d).

4 Vacuum system The electron microscope requires a vacuum level better than 10-4 Pa (even up to 10-8 Pa). WHY? 1. The vacuum preserves sources (a) from the danger of oxidation, i.e. the chemical reaction at high temperature (for the to thermionic emission sources) with oxidizing gases (H 2 O, CO 2, O 2, ) (b) by adsorption of contaminants (for sources with field emission) which reduces the efficiency 2. The electrons of the beam must have a free mean path exceeds the size of the column of the microscope, in order to make negligible the number of collisions of the electrons with the molecules of the residual gas. Example: for an electron beam energy of 20 kev, in a column in which it was made a vacuum of about 10-5 Pa, it can be expected that only 1 out of 10,000 electron collides with the molecules of the residual gas, in the path of 50 cm, from the electron gun to the surface of the sample, while all the others join, undisturbed, the sample. Therefore, better is the vacuum level less degradation of the beam is obtained. Furthermore, due to the interactions with the electrons, the molecules can be ionized and consequently knock against the source (ion bombardment) and damage it.

5 Sample preparation in a SEM The samples do not require special care in preparation. However, if the sample is not conductive generates a charge density that change the trajectory of the electrons with the result of a distorted image. Solutions 1. Metallization of the surface with Au, Au-Pd, C 2. Analysis in low vacuum In any case, even if the sample is conductive, it has to be well connected to the sample holder. Therefore, sticky conductive diskettes (C) are used to fix the sample to the sample holder.

6 Sample preparation in a SEM To increase the production of Secondary Electrons (SE) emitted from the sample, the surface is coated with thin film (~ 10 nm) of heavy metal. The metallization has a dual purpose: 1. Reduction of the surface charging which induces beam distortion. 2. Increase of the SE when analyses of sample at low Z (atomic mass) and in particular made of C (for example biological samples) are performed.

7 Variable pressure microscopes or Low Vacuum Scanning Electron Microscopy (LVSEM) The instrument has the same characteristics of a conventional SEM with the advantage that the pressure can be adjusted to reduce or eliminate the effects of sample loading.

8 Environmental scanning electron microscope (ESEM) This microscope allows the analysis of samples in their natural state without the need to dry them previously. It is possible to control both the pressure and the temperature and the relative humidity. This allows to analyze at high / low temperature, dry and / or wet samples