7.4.3 Chemical vapour deposition (CVD)
If step coverage or equipment cost is more critical than purity, then PVD is supplanted by CVD.
There are many variants on the chemical vapour deposition technique, but the concept is simple: gases adsorb onto the wafer surface where a chemical reaction forms a solid product. Any other products are gases, or at least volatile liquids, and are pumped away.
There is one obvious restriction: the wafer surface must be the only place where the reaction can occur. If it is not, particles spontaneously form within the gas and a thick coating forms on the chamber walls, eventually flaking off as more particles and contaminating the process. To isolate the reaction region the wafer surface is heated: if the reaction can occur only at high temperature, then it can occur only on the wafer. The film is then deposited everywhere on the surface, with no preference for horizontal over vertical walls, so near-perfect conformal coverage can be achieved, and the hot surface also helps with crystallisation.
Having established this technique, all that remains is the choice of chemistry. The examples given in Table 3 illustrate the range of materials that can be used with the technique.
Table 3 Examples of CVD chemistry
|Solid product||Chemical reaction|
|Polycrystalline silicon||SiH4||→ Si(s) + 2H2|
|Silicon nitride||3SiCl2H2 + 4NH3||→ Si3N4(s) + 6HCl + 6H2|
|Silicon oxide (LPCVD route)||SiH4 + O2||→ SiO2(s) + 2H2|
|Silicon oxide (TEOS route)||Si(OC2H5)4||→ SiO2(s) + 4C2H4 + 2H2O|
|Silicon oxide (flowfill route)||SiH4 + 4H2O2||→ SiO2(s) + 6H2O|
|Tungsten||WF6 + 3H2||→ W(s) + 6HF|
|MOCVD GaN||Ga(CH3)3 + NH3||→ GaN(s) + 3CH4|
CVD can therefore be used to deposit oxides, nitrides, metals, and both silicon and III-V semiconductors. For a particular material, there may be several chemical routes to the same result. There are chamber architectures for single-wafer processing and batches, with hot walls or cold walls, and many designs of nozzles and showerheads for delivering the gases. Depending on the operating regime the technique might be known as APCVD (atmospheric pressure), LPCVD (low pressure), flowfill (producing a flowable aqueous suspension, which solidifies only when dried) or MOCVD (metal organic). Whatever the name there is a common factor: these CVD reactions are initiated simply by the heat of contact with the wafer surface.