Contact Size

we analysed the influence of the top metal contact size on the switehing behaviour. We used Au as the contact metal to exclude the possible influenee of an unintentionally formed oxide interlayer between the metal and the Cu(TCNQ). 

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Perhaps the most significant complication in the interpretation of nanoscale adhesion and mechanical properties measurements is the fact that the contact sizes are below the optical limit ( 1 t,im). Macroscopic adhesion studies and mechanical property measurements often rely on optical observations of the contact, and many of the contact mechanics models are formulated around direct measurement of the contact area or radius as a function of experimentally controlled parameters, such as load or displacement. In studies of colloids, scanning electron microscopy (SEM) has been used to view particle/surface contact sizes from the side to measure contact radius [3]. However, such a configuration is not easily employed in AFM and nanoindentation studies, and undesirable surface interactions from charging or contamination may arise. For adhesion studies (e.g. Johnson-Kendall-Roberts (JKR) [4] and probe-tack tests [5,6]), the probe/sample contact area is monitored as a function of load or displacement. This allows evaluation of load/area or even stress/strain response [7] as well as comparison to and development of contact mechanics theories. Area measurements are also important in traditional indentation experiments, where hardness is determined by measuring the residual contact area of the deformation optically [8J. For micro- and nanoscale studies, the dimensions of both the contact and residual deformation (if any) are below the optical limit. 

The above measurements all rely on force and displacement data to evaluate adhesion and mechanical properties. As mentioned in the introduction, a very useful piece of information to have about a nanoscale contact would be its area (or radius). Since the scale of the contacts is below the optical limit, the techniques available are somewhat limited. Electrical resistance has been used in early contact studies on clean metal surfaces [62], but is limited to conducting interfaces. Recently, Enachescu et al. [63] used conductance measurements to examine adhesion in an ideally hard contact (diamond vs. tungsten carbide). In the limit of contact size below the electronic mean free path, but above that of quantized conductance, the contact area scales linearly with contact conductance. They used these measurements to demonstrate that friction was proportional to contact area, and the area vs. load data were best-fit to a DMT model. 

Indentation has been used for over 100 years to determine hardness of materials [8J. For a given indenter geometry (e.g. spherical or pyramidal), hardness is determined by the ratio of the applied load to the projected area of contact, which was determined optically after indentation. For low loads and contacts with small dimensionality (e.g. when indenting thin films or composites), a new way to determine the contact size was needed. Depth-sensing nanoindentation [2] was developed to eliminate the need to visualize the indents, and resulted in the added capability of measuring properties like elastic modulus and creep. 

Tsuruta, D., and Jones, J. G. (2003). The vimentin cytoskeleton regulates focal contact size and adhesion of endothelial cells subjected to shear stress. J. Cell Sci. 116, 4977-4984. 

The question may arise whether the self-energy effects are important in the normal state. These are known to be smaller than the inelastic backscattering nonlinearities in the ballistic regime [18]. If we decrease the contact size d or the elastic mean free path li in order to make the inelastic contribution negligible, the latter parameters become comparable to the Fermi wave length of charge carriers and the strong nonlinearities connected with localization occur, which masks the desired phonon structure [19]. 

It cannot be emphasized enough that the determination of the contact resistance is not a trivial matter. First, a decision about what measuring structure must be made (four terminal Kelvin, sheet end or other structures) and what correction factors for the current crowding will have to be used. Then extreme care should be taken such that no over etching of the contact down into the silicon occurs and that the correct contact size is... 

Contact Diameter In principle, selective tungsten has almost no limitations as to the contact size. However, especially in the SiH4/WF6 case, the local growth rate can drop when the contact size is too large, when the contact density is very high, or when the scribe lines are open. Blanket tungsten has, as discussed in chapter II, an upper limit for the contact size. 

In summary, simulations with finite reservoirs have the distinct advantage that TDDFT algorithms, propagation methods and computer codes are well established for isolated systems and can be used for transport calculations without major changes. Computational artifacts due to the finite reservoirs can and need to be kept under control by a systematic enlargement of the contact sizes. 

Figure 4 is a scale drawing of a spreading resistance probe tip and the nominal contact size for a 10 gram probe load. The probe tips are a hard tungsten-osmium alloy they have a... 

Figure 27.2 I-V characteristics of a Cu/Cu(TCNQ)/Au device. The Au contact size is larger than a 1 mm. First signs of formation of a slight non-linear branch for the low conducting state.

The two-layer solution can be used to obtain the solution for a single layer of thermal conductivity fcj and thickness t, in perfect contact with a flux tube of thermal conductivity k2. In this case the dimensionless constriction resistance yi )ayer depends on the relative contact size e, the conductivity ratio k21, and the relative layer thickness x, ... 

The contact deformation of these thermoplastic polymers was studied experimentally by pressing polymeric balls (of 4 mm diameter) with continuously increasing load (0.6 N/s) against an optically smooth glass surface and measuring both contact deformation displacement and contact size under load as described above (see Figure 1). The polymer balls had a mean peak-to-valley roughness of 0.6 - 1.0 im and a c.l.a. 

When tensile loads were applied to puU the spheres apart, an equilibrium contact spot size could be obtained as the load was reduced, but below a certain contact size, equilibrium could no longer be found and the surfaces then came apart rather quickly at a load given by... 

Such behavior is important to the understanding of frictional phenomena. It has been known since Amontons ° in the 17th century that friction, the force required to slide one solid over another, increases in proportion to the load pressing the solids together. This is clearly inconsistent with the Hertz equation for contact area. In the Hertz case, the contact diameter increases with thus the contact area increases with and therefore the friction force should also increase with, at odds with the Amontons observations. However, the behavior shown in Fig. 9.9(b), after sliding causes scratching of the surfaces, is consistent with Amonton s Law. The contact size increases with so that the contact area, and therefore friction, is then proportional to load... 

Figure 9.21. (a) Hexagonal structure of coalesced film (b) contact size measuremenl of latex pailicles (c) electron microscope and optical measuiements confinniitg /KR theory fot latex. 

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