Scanning Tunneling Microscopy

Scanning Tunneling Microscopy (STM) is a technique that can produce atomic resolution topographic images of both metal and semiconductor surfaces. STM relies on a quantum mechanical tunnel current that runs between a surface and a sharp metallic tip which are in close proximity. Since the tunnel current is very strongly dependent on the distance between the tip and the surface, use of a feedback loop can control the distance between tip and surface enabling imaging of the surface and individual ad-atoms or ad-molecules.

An electronic schematic of the apparatus is given below.

The versatile modular electronics and software are capable of many different data taking modes, e.g. conventional constant current topographs, current-voltage (IV) or voltage-distance (ZV) characteristics. IV curves can distinguish between different adsorbates due to their different local density of states whereas ZV curves can provide information about the local work function and/or bound image states on metal surfaces.

The STM is designed for room temperature scanning only, and is situated in an ultra high vacuum (UHV) system. The STM also incorporates a reverse-view LEED and has the following additional facilities; (a) sample transfer (load lock), (b) sample heating and annealing (outside of STM), (c) ion sputtering, (d) gas dosing and (e) metal evaporation with associated film thickness monitoring. See the following schematic of the apparatus.

We also use a quadrapole mass spectrometer for thermal desorption spectroscopy (TDS) and mass resolution of the metal-organic CVD by-products. In addition, the incorporation of an effusive He atom source will allow simultaneous in-situ specular He reflectivity (HR) measurements to be performed during deposition of materials. This enables us to characterize defect densities, adsorbate densities, and their lateral distributions to quantify total exposures.

In the past, atomic resolution has been displayed with this instrument on semiconductor surfaces and, more recently, on a Cu(100) substrate. An example of an image obtained using this STM is shown at the top of this page. It is a constant current topographic image taken from a copper silicide layer grown by CVD (from SiH₄) on Cu(100). We highlight here some of the particularly interesting chemical and structural features of growth in this silane on copper system. They arise from the unique density and symmetry changes that occur in Cu₂Si overlayer growth. Si is initially miscible in the top layer, but at a critical silane exposure the 2-D Cu₂Si compound segregates. A cubic to hex reconstruction is accompanied by a surface density change. A saturated surface is achieved after 6-8L exposure which is inert to further silane adsorption and decomposition. At low growth temperatures islands have a fractional area coverage of 20% or less.

The following figure also shows the strongly-anisotropic single-stripe domain growth which occurs during growth below room temperature.

We have investigated this striped domain structure and have developed a microscopic model for anisotropic domain growth. We have also extracted domain size information from saturated surfaces, that is not accessible from diffraction techniques. We have demonstrated that the observed domain shapes must be meta-stable and have established that the prior existence of strongly anisotropic domain growth can be implied even from images of the saturated surface.

We have also developed a computer based routine for 2-D island evaluation, and have investigated the island size distributions as a function deposition temperature. We have interpreted temperature independent island shapes in terms of anisotropic island attachment rates. In addition, we have shown that size dependent island aspect ratios are a feature common to all kinetically limited anisotropic island growth mechanisms.

Our long term aim is to investigate the growth of materials, especially metals, under realistic conditions of semiconductor device metalization. We plan to characterize both equilibrium and non-equilibrium growth kinetics and to demonstrate the clear influence of site specific chemistries in chemical vapor deposition (CVD). Experiments on Cu CVD are currently under way.

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