Sheet resistivity

Diffusivities of various elements ate determined experimentally. Dopant profiles can be determined. The junction depth can be measured by chemically staining an angle-lapped sample with an HE/HNO mixture. The -type region of the junction stains darker than the n-ty e region. The sheet resistivity can also be measured using a four-point probe measurement. These two techniques ate used for process monitoring. 

The transparent top contact is deposited last of all, which imposes restrictions on the process temperature. Thermally evaporated ITO and ZnO deposited by metal-organic CVD (MOCVD) are most suitable. At a typical thickness of 70 nm the ITO serves as a good antireflection coating as well. Due to the somewhat high sheet resistance, a metal (Ag) grid is necessary to reduce the series resistance [11]. 

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. 

Figure 8. Sheet resistance of 0.90 fim palladium acetate film as a function of 2 MeV He+ and Ne+ ion dose.

Figure 6.17. I-V curve for CuGaS2 thin film. The linear region below 3.0 x 10 5 amps was used to calculate sheet resistance.

Sheet resistance (Rs) is defined by p = Rst, where p is resistivity and t is film thickness. For CuGaS2 thin-film samples, Rs was determined using the four-point probe method53 and calculated from current-voltage data (Fig. 6.17) using... 

Hoefer, U. Steiner, K. Wagner, E. Contact and sheet resistances of Sn02 thin films from transmission line model measurements. Sensors and Actuators B (1995), p. 59-63. 

FIGURE 6.2 Sheet resistance and resistivity of ITO films as functions of hydrogen partial pressure. 

Rt = R]R2/(R + R2), where Rt and R2 are defined as the sheet resistances for a low-conductivity surface layer of thickness x and a conductive bulk ITO layer of thickness d-x as shown in Figure 6.11b. 

The combination of low optical absorbance and high electrical conductivity has attracted a lot of interest for transparent conductor applications. When coupled with its flexibility, it is widely seen as a possible replacement for indium-doped tin oxide (ITO), which has a sheet resistance of 100 Q/cm at 90 % transparency. By growing graphene on copper foils, sheet resistances of 125 Q/cm at 97.4% transparency have been achieved [19]. This has been improved by combining four layers with doping of the graphene, giving resistance of 30 Q/cm at 90% transparency, all done on 30-inch roll-to-roll production scale. 

Fig. 4.14 (a) Optical transmittance of graphene on a polyethylene terephthalate (PET) flexible substrate [19]. Optical and electrical data for PEDOT PSS-based composites with SWCNT (b) transmittance at 550 nm, (c) sheet resistance, (d) DC conductivity and (e) ratio of DC to optical conductivity. 

Measurement of electrical sheet resistance (two- or four-point method). The simplest technique for measurement of sheet resistance of polymer samples is... 

Yoshioka Y, Calvert PD, Jabbour GE (2005) Simple modification of sheet resistivity of conducting polymeric anodes via combinatorial ink-jet printing techniques. Macromol Rapid Commun 26 238-246... 

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 ... 

A schematic for the four-point probe measurements is shown in Fig. 22. For an infinite sheet wafer (film on an unbounded wafer), the sheet resistance is... 

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... 

Due to the lack of a shallow acceptor level in 4H-SiC and a relatively low hole mobility, the intrinsic base resistance of a 4H-SiC BJT is very high for example, a base epilayer sheet resistance of 46 kQ per square was reported for a 4H-SiC BJT in [6]. The high intrinsic base resistance in 4H-SiC BJTs results in a very severe emitter current crowding effect [7], which limits most of the current flow to within 1 jum from the edge of the emitter regions [8]. This indicates that tight cell pitches are necessary to achieve a low specific on-resistance, as shown in Figure 6.2(a) [6]. 

Base contact resistance per unit length in ohm-cm. Intrinsic base sheet resistance in ohm/sq. 

---