Surface imaging by an ion probe scanning

We describe the realization of an ultrahigh vacuum scanning ion beam appara tus. The ion gun is based on the principle of field emission from a liquid gallium film wetting a sharp needle. The beam can be focused to a probe spo t with a diameter less than 40 nm. The gun is fixed to an ultrahigh vacuum chamber, and is driven by a digital scanning device that works at variable frequency. The video image is built from the outgoing electrons resulting f rom the ion beam impacts. We describe the setup and discuss the relation be tween the ion probe current and the scanning speed. Specific applications t o surface imaging are presented.