7.4 Contact Resistance Analysis

Contacts and vias are often the weak point in integrated circuits. Their electrical characteristics must be studied in order to determine under what circumstances they impact circuit performance or reliability. Our tools can be used to analyze vias and the ohmic contact between metal or metal-silicide thin films and single-crystal silicon.

We present two contact structures, where a titanium nitride barrier layer is used to insulate metallurgically the metal from the semiconductor. In Figure 7.9 we show SEM pictures after a TiN deposition into a 1.0$\mu m$ circular hole located 260mm below the center of the sputter target disk (see Figure 7.8-(i)) and when the deposition of TiN is made at a position 90mm off the wafer center (see Figure 7.8-(ii)). The picturesures in Figure 7.10 are cuts from the simulated three-dimensional structures and agree well with the upper SEMs.

Figure 7.8: Contact hole positions: (i) In the axis of the sputtering target disk. (ii) 90mm off-axis.

Figure 7.9: Contact simulation: SEM picture (after TiN sputtering).

Figure 7.10: Contact simulation: Result from topography simulation (a cut is shown).

Figure 7.11: Contact simulation: Current density.

In figure 7.11 we show the current density in the contacts for both cases, after being filled with aluminium. The effect of the asymmetry in the TiN layer in the current density is clearly seen. In case (ii) the current flows mainly in unusual areas, namely in the trailing edge of the contact, whereas in the other case we have the known current crowding effect at the opposite edge.

As electromigration problems are worsened by current crowding effects [90], our tools can be used to understand and minimize this unwanted phenomena. The resistance values obtained were 0.77$\Omega$ and 0.89$\Omega$ for the upper and lower cases respectively, and agree well with the experiments.