Integrating a Scanning Tunneling Microscope into a Scanning Electron Microscope

The obtainable resolution of optical positioning is of order 1 $\mu$m. An order of magnitude in resolution can be gained for conducting samples when using an Scanning Electron Microscope (SEM)[80,96] (See figure 4.279). There are two crucial points to be observed: first, the sample and tip have to be arranged such that they are well visible from the electron gun of the SEM. The STM has to be connected rigidly to the SEM to ensure a good resolution of the SEM. The vibration isolation has to be located outside the SEM vacuum system. Secondly, the SEM has to have an ultra high vacuum chamber. The rest gas in ordinary high vacuum contains hydrocarbons which are cracked by the electron beam at the sample surface. The resulting carbon film is poorly conducting and can render an STM inoperable (A hydrocarbon rich rest gas together with an electron beam is used to write patterns on integrated circuits. The hydrocarbon film is known as ``contamination resist'').

Abbildung 4.279: A schematic drawing showing the integration of a STM into a scanning electron microscope. This figure is taken from Ch. Gerber et al.[80] and reproduced with permission of the American Institute of Physics.

The tunneling current in the STM is affected by the electron beam. If the electron beam current becomes comparable to the tunneling current it might cause the feedback loop of the STM to become unstable. The SEM should always be operated at currents lower than the tunneling current, unless the STM is not in the tunneling regime.

One advantage of the SEM is its huge depth of view. The tip and the sample can be seen sharp simultaneously. This allows a very precise positioning of the tip, which must have a small opening angle to not obscure the sample. If the SEM is equipped with Auger electron analysis it can determine the chemical composition of the sample surface on submicrometer scale.