When the atomic force microscope (AFM) was invented in 1986, it was anticipated that the technique should be able to produce atomically resolved images with aquality similar to what was routinely obtained with the scanning tunnelling microscope (STM). Even though the AFM quickly became a very popular technique, the development toward ultra-high resolution imaging turned out to be slow due to problems of both fundamental and technical nature. The last years have, however, seen a breakthrough in true atomic resolution which is a result of the refinement of the ac-mode technique where the tip is oscillated as it interacts with the surface. The work presented in this thesis is part of this effort to obtain and understand atomically resolved AFM data obtained in ac mode.

The first part of the thesis describes the construction of the instrument, a combined atomic force/scanning tunneling microscope (AFM/STM) operating in ultrahigh vacuum using a fiber-optic laser interferometer to detect the lever deflection. As force microscope it operates in ac and de mode with commercial (Si, Si3 N4) or individually made (W) cantilevers. Samples and cantilevers can be inserted without breaking the vacuum using a load-lock system. The force sensor includes a novel three-dimensional micropositioner based on the piezoelectric slider principle. The system includes standard surface analytical techniques (low-energy electron diffraction/Auger, prepared for x-ray photoelectron spectroscopy) and is equipped for mass spectroscopic detection of reaction products from catalytic surfaces at elevated temperature. Tips are cleaned in situ using electron bombardment. By using tungsten cantilevers with a high spring constant (k = 100-200 N/m) it is possible to switch directly between STM and AFM operation.

The second part of the thesis deals with the application of the technique to high resolution imaging, and investigations of the imaging mechanism. Atomically resolved images of the Si(111)7x7 reconstruction are obtained using tungsten cantilevers, in STM mode as well as in ac-mode AFM. In addition, the ac-mode AFM image show a contrast between inequivalent adatoms in the 7x7 unit cell which differ from the known STM contrast associated with the variations of the electronic states of the adatoms. This was the first time that inequivalent atoms ofthe same species were discriminated in an AFM. In order to understand the image mechanism for ac-mode AFM we compare the experimental cantilever resonance curves with simulated curves. For the simulation we use a model for the tip-surface interaction which include a van der Waals force, an interatomic force, and treats the surface elasticity as a linear spring. These simulations show that the tip probes only the attractive part of the interaction, and that interatomic binding energy variations on the order of 1 eV can be resolved. This indicates that the AFM has a potential for chemical selectivity.

For precise measurement of surface topography using AFM it is necessary to know to which extent the tip shape influences the obtained data. We present a method, where an electrostatic force interaction is used for characterising tip shapes. We show that the analytical expression for the electrostatic force obtained by approximating the tip as a sphere works well for small tip-sample separations and the radii that we obtain are consistent with data from scanning electron microscopy data. The method also gives the possibility to measure the van der Waals force and obtain a value of the Hamaker constant.