Silicide Formation    

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 temperatures of less than 500 deg C. Thus, instead of using Al, such low-resistivity interconnections are usually fabricated using materials known as refractory metal silicides (MSi_x), which can handle much higher processing temperatures than Al.

The formation of refractory metal silicides (such as WSi2, TiSi2, MoSi2, and TaSi2)  in VLSI circuits can generally be accomplished in four 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).

The silicide formation technique of directly depositing a refractory metal on a silicon surface to form the required silicide layer employs the process of direct metallurgical reaction.  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. TiSi2 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.    

Needless to 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. 

Co-evaporation, 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.

Critical 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.

The third technique for silicide formation is sputter deposition.  Sputtering is basically a deposition process 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.