7.5 CMOS Inverter

A more complicated structure which consists of two transistors is shown in Fig. 7.24. The NMOS transistor and the PMOS transistor form a typical complementary MOS (CMOS) device. While the geometrical structures of the two transistors cannot be distinguished from each other (Fig. 7.24), it is the doping profile which differs. The blue material segments denote the two poly-silicon gate regions. The green segment of the structure is the silicon which is separated from the gate by a thin oxide layer. This oxide layer covers a shallow trench around all four sides of each transistor to ensure a proper isolation (shallow trench isolation, STI). The mesh density in the oxide layer is of no relevance. However in the silicon region the mesh density is crucial to resolve the doping profile and the electrical current density. The source and drain contacts penetrate the oxide layer to reach the silicon.

The red segments are silicon-nitride spacers which help to isolate the gate from the source and drain regions and which prevent adversary effects due to a source-gate or drain-gate capacitor. From a process technology point of view the spacers allow a further separation between the deep drain/source implant regions and the gate. The source/drain extensions with a shallow doping profile reach under the spacer.

The metallization as shown in the figure consists of the Aluminum conductors which connect the two gates and the two drain contacts. The passive oxide layers which cover the entire device and which form the isolation between the metal layers are stripped to make the interconnects visible in the figure. A quite coarse mesh is depicted in Fig. 7.25. It consists of 1016 and 1005 tetrahedra for the two gate regions, 3199 tetrahedra for the four spacers, 13620 tetrahedra in the oxide layer, 15511 tetrahedra for the silicon, and 1283 tetrahedra for the metallization.

Figure 7.24: Typical CMOS inverter structure with two transistors.

Figure 7.25: Initial coarse mesh of the CMOS device.