Homoepitaxy

From ArticleWorld

Homoepitaxy is a method of epitaxy that refers to a thin-film technique of deposition in which materials of the same kind are involved. It is used to grow a crystalline film on a substrate or film of the same semiconducting material, if applied in semiconductor technology.

The benefit that is obtained from this techonology is that layers which possess different doping levels can be manufactured. In situations where film more purified than the substrate is to be grown, this technology becomes indispensable.

Applications of homoepitaxy

Layers of higher quality semiconducting silicon crystals are used extensively in manufacturing microchips; this is possible because of the technique of homoepitaxy. A wide variety of electrical engineering applications are possible because of homoepitaxy.

Steps in making semiconductor devices

The first and foremost step in making semiconductor devices is to start with a semiconductor wafer. Wafers are obtained by slicing large single crystals. Epitaxial growth is initiated over the substrate material by a number of means, prominent among them being molecular beam epitaxy (MBE), metal-organic chemical vapour deposition (MOCVD) and wafer bonding. The principle of operation in the first case is i.e. MBE is that gases are bound to cold surfaces by means of cryocondensation. Epitaxial growth starts with the heating of the compound required for the addition of a certain atomic species into the vacuum chamber. In an advancement of this method, room temperature can be used to carry out the appropriate reactions. Inside the ultra-high vacuum, the free atoms have a long mean-free path and collisions with other atoms are not very frequent. Everything is remotely controlled by a computer, thus facilitating the beams of atoms from the so-called “effusion cells” to attach to the substrate material and allow an epitaxial layer to form. The MOCVD is another widely used method of preparing epitaxial layers on a substrate by carefully controlled deposition of atoms. The substrate wafer to be layered is kept on a susceptor made of graphite, inside a reaction chamber which is heated by a RF induction heater. The temperature maintained may be between 500°C and 700°C depending on the compounds used. A medium pressure metal-organic gas is kept in the reaction chamber. The growth precursors like arsine (AsH3¬¬¬) for group V, trimethyl gallium Ga(CH3)3 or TMG for group III decompose on contact with the heated substrate, resulting in epitaxial layers being formed. Wafer bonding, another method, can be done to fuse two semiconductor materials at the atomic level. This method is useful when bonding materials have different lattice constants. Quartz reactors are used for this purpose. The wafers should necessarily be flat and free of any contaminants.