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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
or silicides (
,
, and
silicides) for gate contacts or interconnect lines made of
.
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.