Interconnection paths that possess low resistivities and the ability to
withstand subsequent high temperature processes are critical to VLSI
manufacturing. The resistivity of Al is low enough for VLSI
interconnection purposes, but its low melting and eutectic temperatures
restrict subsequent processes to operating
less than 500 deg C. Thus, instead of using Al, such low-resistivity
interconnections are usually fabricated using materials known as
(MSix), which can handle much higher processing temperatures
of refractory metal silicides (such as WSi2,
and TaSi2) in VLSI circuits can generally be accomplished in
ways: 1) by deposition of the pure metal onto an Si layer (which
can be the single-crystal substrate or poly-crystalline Si); 2)
simultaneous evaporation of the silicon and the refractory metal from
two sources (or 'co-evaporation'); 3) sputter-deposition of the
silicide, either from a composite target or by co-sputtering; and 4)
chemical vapor deposition (CVD).
formation technique of directly depositing a refractory metal on a
silicon surface to form the required silicide layer employs the process
After the metal is deposited on the silicon, the wafer is exposed to
high temperatures that promote the chemical reactions between the metal
and the silicon needed to form the silicide.
In such a
metallurgical reaction, metal-rich silicides generally form first, and
continue to grow until all the metal is consumed. When the metal has
been consumed, silicides of lower metal content start appearing, which
can continue to grow simply by consuming the metal-rich silicides.
To illustrate this with Ti as the metal, studies conducted by experts
show that TiSi would be the first silicide to form on Si, typically
appearing at a temperature above 500 deg C and peaking at 700 deg C.
only starts to appear at 600 deg C and peaks at 800 deg C. Beyond
800 deg C, TiSi would be fully converted into TiSi2,
at which point the system attains stability.
say, silicide formation by direct metallurgical reaction consumes
silicon from the substrate onto which the metal was placed. Thus, it is
important that enough silicon is available when this technique is
employed to form the silicide layers.
another technique for silicide formation, consists of the simultaneous
deposition of the metal and the silicon under high vacuum conditions.
The metal and silicon are vaporized through one of several possible
heating techniques: with an electron beam, by rf induction, with a
laser, or by resistive heating. However, e-beam heating is the preferred
technique because the refractory metals (Ti, Ta, Mo, W) have very high
melting points (1670-2996 deg C) while silicon has a low vapor pressure.
With the use of 2 e-beam guns whose power supplies may be individually
adjusted, the proper metal-to-Si ratio can be achieved.
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aspects of an evaporation process that will ensure the deposition of
films with repeatable properties include: 1) the evaporation base
pressure (must be < 1 microtorr); 2) evaporation rates; 3) purity of the
elements; and 4) residual gases present in the evaporation chamber.
technique for silicide formation is
Sputtering is basically a deposition
wherein atoms or molecules are ejected from a target material by
high-energy particle bombardment so that the ejected atoms or molecules
can condense on a substrate as a thin film.
As in co-evaporation, the correct sputtering rates of the metal and Si
must be carefully determined and applied to ensure proper deposition of
the silicide film.
The step coverage of co-sputtered films are generally superior to that
of evaporated films.
Sputter-deposition of silicides come in various forms. For instance,
sputtering from two targets using multi-pass sputtering systems
achieves the appropriate mixture of metal and Si in a layered structure.
Sintering then completes the chemical reaction between the metal and Si
to form the silicide. Sputtering from a composite target (MSix)
can also be done, allowing better compositional control, but vulnerable
to contamination issues associated with composite targets.
Chemical vapor deposition
of silicides, the fourth technique for silicide formation and which
involves chemical reactions between vapors to form the silicide film,
offers distinct advantages over the other techniques, namely, better
step coverage, higher purity of the deposited films, and higher
throughput. However, the availability of gas reactants whose
chemical reactions will produce the desired silicide is necessary for
the employment of CVD in silicide formation.
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