2.3.1.2 Semiconductors as Conducting Materials

Determining the optimum layout of interconnect lines is a quite complicated task since the thermal properties of the underlying materials and devices have to be considered. Therefore, and because the deposition and pattering processes for metals operate at elevated temperatures, the thermal budget is the fundamental constraint. If, for instance, the thermal budget is exceeded, the temperature profile causes thermally induced diffusion processes which alter the underlying device structures. For instance the doping profile can change its shape or certain materials can diffuse into other materials at contacts or at protective interface layers. To overcome these types of problems associated with metals, semiconductors can be used instead as contacts and interfacing materials between the lowest level where the semiconductors device structures are located and the first metalization level. The use of semiconducting materials at the interfacial layer includes for instance contacts to semiconductors regions via ${\mathrm{polySi}}$ or silicides ( ${\mathrm{Co}}$ , ${\mathrm{Ni}}$ , and ${\mathrm{Ti}}$ silicides) for gate contacts or interconnect lines made of ${\mathrm{polySi}}$ . This is possible since the properties of semiconducting materials can be adjusted within a wide range.

Semiconducting materials are nearly ideal insulators at very low absolute temperatures and show a drastically decreased resistivity due to thermal activation of electrons of the semiconductor material itself and of impurities [65,66,136,137,138] at moderate temperatures (room temperature 300 K. Hence, these materials offer also an interesting alternative for conductors for certain applications where the current load is not too high. To further increase the conductivity, impurities can be implanted into the crystal lattice to shift the FERMI level according to the demands. Since the conductivity can be adjusted over a wide range, conductors and resistance elements can be realized.