A Sputter Source for Electron Microscopic Preparation

pscCoating electron microscopic specimens by sputtering yields particulary fine grained coatings of strong adherence. Two methods have been applied up to now. Their difference lies in the arrangement of target and specimen. If, in one case both, target and specimen are arranged inside the discharge chamber, damage by the discharge of sensitive specimens is possible. If, in the other case both, target and specimen are arranged outside the discharge chamber, focussing of the ions is necessary, above all for shadowing. Simple ion guns of low energy (some keV) however, yield a relatively low ion current, so that coatings with materials which are difficult to sputter, such as tungsten, can hardly be produced. With ions of higher energies the gain of sputtering particles would certainly increase. However, the danger arises that ions scattered at the target and neutral particles of high energy could damage the specimen.

The basic idea of the PSC1a sputter source consists of arranging the target inside the discharge chamber but the specimen outside of it. An essential requirement of this design is, that the mean free path of the sputtered particles must be greater than the distance between target and aperture in the opposite electrode in order to avoid collisions of the particles with the discharge gas. Therefore, a Penning type low pressure gas discharge was chosen.

Due to the constricting effect of the field of a permanent magnet the electrons of the discharge oscillate between the electrodes (target-cathode and opposite cathode with aperture), thereby providing a great probability of ionization. In this design, however, both electrodes are hit by ions. In order to prevent penetration of argon ions through the aperture in the outlet-electrode, deflecting electrodes are provided; the voltage of which is simply the operation voltage of the discharge. Used for coating are only those sputtered target atoms that can pass through the aperture. Target and electrode can be exchanged. The cylindrical anode together with the magnetic field effects a sharp focussing of the ions so that the area of emission of the sputtered particles is very small. Therefore it is possible to get excellent shadow effects at TEM micrographs.

 

The described Neutral Particle Gun was especially designed for connection with the EPA101 preparation unit, but it is also possible to drive it on other suitable vacuum units using appropriate adaptor flanges.