Atomic Resolution Imaging on Fluorides
Fluorides are an important class of inorganic materials whose bulk properties have been intensively studied for several decades [1 . Among crystals with the fluorite structure, CaF2 is the most prominent prototype crystal that recently gained tremendous i
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Atomic Resolution Imaging on Fluorides
Michael Reichling and Clemens Barth
Fluorides are an important class of inorganic materials whose bulk properties have been intensively studied for several decades [1]. Among crystals with the fluorite structure, CaF 2 is the most prominent prototype crystal that recently gained tremendous interest as a vacuum ultraviolet optical material. Components made from CaF 2 are needed to develop future generations of laser lithography, a key technology for the semiconductor industry [2]. Lithography optics requires materials with structural perfection and utmost purity of bulk and surface. In this context, high resolution and sensitivity imaging on fluorides for structural characteri:6ation and surface defect detection has become an important issue and is a major driving force behind further development. Beside this specific application, fluorides are important test materials for the development of atomic resolution force microscopy on insulators. Surfaces of fluorite-type crystals prepared by cleavage along the (Ill) plane are atomically flat over large areas and very stable under ultra-high vacuum conditions. Their structure is simple but bears more detail than the structure of halides with NaCl structure (see Chap. 5). These details provide an excellent test for the resolving power of the scanning force probe and also for theoretical predictions concerning scanning results. As will be discussed in Chap. 17, scanning force imaging on the CaF 2(111) surface has been modelled by theoretical simulations and this is presently the best understood insulator surface in terms of a quantitative interpretation of atomic scale contrast formation. Both, the ease of preparation and the detailed understanding of atomic contrast predestines the CaF2(1l1) surface as a standard for atomic resolution scanning force measurements and for calibrating tips. In this chapter we review the state of the art in atomic resolution imaging of fluoride surfaces and describe the main features and peculiarities while all quantitative aspects of imaging are discussed in Chap. 17. In the following sections we will briefly introduce details of our experimental techniques, then address the important issue of tip structure and tip instability, and finally discuss atomic contrast on flat surfaces and at step edges and kinks. An important feature of the experiments introduced here is that measurements are often not run in the standard topography mode, where the cantilever resonance frequency detuning is kept constant, but rather in the constant height mode, where the detuning is the primary output signal. The two modes are contrasted and we discuss which details can best be deduced from each method. S. Morita et al. (eds.), Noncontact Atomic Force Microscopy © Springer-Verlag Berlin Heidelberg 2002
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Michael Reichling and Clemens Barth
6.1
Experimental Techniques
All experiments reported here were performed with a scanning force microscope based on the design by Howald et al. [3J operating in an ultra-high vacuu
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