Metallic films, sheet resistance

In situ measurements of electrical sheet resistance provide another clue to the composition of the films. A plot of the sheet resistance as a function of dose is shown in Figure 8. The decrease in sheet resistance trails the loss of carbon and oxygen by a factor of 10 in terms of ion dose and reaches a limiting value of 2xl04 uto-cm. This is two orders of magnitude greater than the lowest value measured in the laser-exposed material, at least qualitatively consistent with the relative purity of the metals in each case. 

Four-point probe is the most commonly used method to measure sheet resistance for metal films in silicon processing. Given known resistivities of each material, the film thickness can be obtained. Table VI lists the resistivities for the films often used in the back-end interconnect processes. The theory in four-point prohe originally comes from Ohm s law ... 

If a single metal film is deposited on an oxide, the sheet resistance measurement results can by easily interpreted and converted to the thickness. In practice, however, this is not usually the case. For example, in W CVD, the tungsten is not directly deposited on oxide due to high residual stress and unreliable adhesion. A titanium (Ti) layer must be first deposited as a glue layer. In addition, to prevent the fluorine in the CVD-precursor WFg from directly reacting with Ti (a strong catalytic reaction will occur), a barrier layer of titanium nitride (TiN) must be deposited on top of the Ti. As a result, we have a trilayer film of W on TiN on Ti on oxide, as shown schematically in Fig. 21. This poses some problems in accuracy in the four-point probe measurements. Based on the resistivities in Table VI, the... 

Ar" backsputter as a standard pre-metal clean or Incomplete resist and/or developer removal In wet patterning results In an ohmlcally conducting surface of sheet resistivity 1.5 x 10 Q/q. Otherwise polylmlde film surfaces are not measurably conductive. 

Formerly, Bradley and Hammes [152] studied dc conductivity phenomma on a number of metal-free and some metal-containing plasma polymers. They foimd plots of Igdc resistivity versus 1/T to be generally linear and concluded that a single activation process is responsible, with activation energies of 1.36 eV for metal-free polymers and < 1 eV for metal-doped polymers. For evaporated Cu/poly-ethylene films [129] the Ig sheet resistance revealed a linear dependence on The conduction process was explained in terms of hopping of electrons between traps in localized states dose to the Fermi level. 

Yang and co-workers used the separated metallic SWNTs in a systematic evaluation on transparent conductive films, which also included nanotubes from different sources coupled with various fabrication conditions. It is worth noting that the films were on a flexible substrate (PET), for which dip coating was used. Again, those films from the separated metallic SWNTs exhibited sheet resistance down to 130 Q sq for 80% optical transmittance at 550 nm (Table 6.1). The comparison of nanotube films with ITO coatings on the... 

In another study, Maeda et al fabricated transparent conductive films by air spraying enriched metallic SWNTs (from the amine-assisted post-production separation ) on to both quartz and PET substrates. The sheet resistances in the films on PET were 690 Q sq at an optical transmittance of 81% (550 nm) and 9000 Q sq at 97% (550 nm), which represented reductions by a factor of 20 in comparison with the performance in films of non-separated SWNTs. The same group also found that the effectiveness in the separation for metallic SWNTs is determined by the use of different amines at various concentrations. The proportion of metallic SWNTs against semiconducting SWNTs in the... 

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