CHEMICAL VAPOUR DEPOSITION

• Deposition of a solid material onto a heated substrate via decomposition or channel reaction of compounds contained in the gas passing over the substrate

• Many materials such as silicon nitride silicon dioxide ,polycrystalline silicon silicon ,and single crystal silicon ,can be deposited via CVD

• Aspecial case of CVD is called epitaxy or epitaxial layer deposition or vapour phase epitaxy(VPE) , in which case the deposited layer is in single crystal form

• In CVD process the materials to be deposited enter a reaction chamber in the gaseous or vapour phase and react on or near the surface of the substrates ,which are at some elevated temperatures

• The chemical reaction that occurs produces the atoms or molecules that are deposited on the substrate surface

A number of different materials can be deposited by the CVD process

1. Silicon epitaxial layer on a single-crystal silicon substrate
2. Silicon epitaxial layer deposition on a sapphire
3. Silicon dioxide deposition
4. Silicon nitride deposition

EPITAXIAL DEPOSITION

• The word “epitaxy” is a Greek word ‘epi’ means ‘upon’ and ‘taxy’ means ‘arranged’

• ie, it is an arrangement of atoms upon a crystal substrate ,so that the resulting added layer structure is an exact extension of the crystal structure

• There are a number of different chemical reactions that can be used for the deposition of epitaxial layers

Four silicon sources have been used for growing epitaxial silicon–
Silicontetra chloride(SiCl4), dichlorosilane(SiH2Cl2),trichlorosilane(SiHCl3) and silane(SiH4)

Some important Reactions for Epitaxial layer Deposition

• Silicon tetrachloride has been the most studied and has seen the widest industrial use.The overall reaction can be classed as a hydrogen reduction of a gas

SiCl4(gas) + 2H2(gas) --> Si(solid)+4HCl(gas)

• The epitaxial layer deposition takes place in a chamber called an epitaxial reactor

• Horizontal reactor ,Vertical reactor, Cylindrical reactor

• Horizontal reactors offer lowest cost construction,however controlling the deposition process over the entire susceptor length is a problem

• Vertical reactors are capable of very uniform deposition, but suffer from mechanical complexity

• Cylindrical reactors are also capable of uniform deposition due to employment of radiant heating,but are not suited for extended operation at temperature above 1200c

• The process is carried out in a reaction chamber consisting of a long cylindrical quartz tube encircled by an RF induction coil

• The silicon wafers are placed on a rectangular graphite rod called a boat

• This boat is then placed in the reaction chamber where the graphite is heated inductively to a temperature 1200c

• The various gases required for the growth of desired epitaxial layers are introduced into the system through a control console

• The basic chemical reaction used for the epitaxial growth of pure silicon is the hydrogen reduction of silicon tetrachloride
  

SiCl4+2H2 <--> Si+4HCl

• For producing doped p or n type epitaxial layers, a number of gases can be metered into the reactor tube, including some very small amounts of doping gases,such as B2H6(diborane) for boron doping and PH3(Phosphine) for phosphorous doping of the epitaxial layer

Advantages

• SiCl4 is non-toxic,inexpensive and easy to purify

• The reaction making silicon from SiCl4 takes place only at surface and not on the boat or reaction chamber walls

Disadvantages

• The growth process is accompanied by the diffusion phenomenon which causes an exchange of impurities between silicon wafer and growing film.This prevents the fabrication of an ideal step junction

• SiCl4 process requires higher temperature and also has slower growth rate