Epitaxy vs CVD: Crystal Quality, Substrate, Temperature and Uses Compared
Epitaxial growth grows a single-crystal film on a single-crystal substrate with the same crystal orientation as the substrate. Most silicon epitaxy is done by CVD (chemical vapor deposition), so the two are not opposites: epitaxy can be seen as CVD that continues the substrate's crystal and grows single crystal (there are also non-CVD methods such as MBE). This table compares epitaxial growth of single crystal with ordinary CVD that deposits non-single-crystal films such as polysilicon, amorphous silicon, oxide and nitride.
| Aspect | Epitaxial growth | CVD (ordinary deposition) |
|---|---|---|
| Resulting crystal | Single crystal that continues the arrangement and orientation of the underlying crystal | Polycrystalline (polysilicon) or amorphous; silicon oxide and nitride films are amorphous too |
| Substrate requirement | Needs a clean single-crystal surface to copy. In heteroepitaxy of a different material, mismatch in lattice constant and thermal expansion becomes an issue | Need not be single crystal: films deposit on oxide, metal, patterned topography or any surface the gas can reach |
| Surface preparation before growth | Strict. Native oxide about 1 nm thick or carbon left on the surface seeds defects such as stacking faults. For silicon, an HF clean is usually followed by a hydrogen bake at about 1,100 °C that removes the oxide | No crystal has to connect to the underlying layer, so oxide removal is generally not as strict as for epitaxy |
| Temperature tendency | For silicon it depends on the precursor: about 1,100–1,150 °C with trichlorosilane, about 1,000–1,100 °C with dichlorosilane, and roughly 600–900 °C with silane. SiC uses about 1,500–1,650 °C | Wide range by method: LPCVD about 600–800 °C, plasma CVD about 300–400 °C. Silicon from silane is amorphous below about 580 °C and polycrystalline above it |
| Typical films | Silicon epitaxial layers, SiGe, SiC, and compound-semiconductor layers such as GaN and GaAs | Polysilicon, amorphous silicon, silicon oxide (from TEOS and others), silicon nitride, tungsten |
| Doping control | In-situ doping with phosphine or diborane during growth lets each layer have its own concentration and gives abrupt depth profiles. Autodoping from heavily doped substrates must be controlled | Polysilicon can be made conductive by in-situ doping; for insulating films, doping is not the main purpose |
| Main uses | Lightly doped drift layers in power devices, epitaxial wafers for CMOS (such as p/p+), substrates for CMOS image sensors, raised transistor source/drain, SiGe HBT bases, multilayer structures in LEDs and lasers | Interlayer dielectrics, STI fill, spacers, hard masks, polysilicon gates and resistors, tungsten plugs |
| What quality means | Crystal defects such as stacking faults and dislocations, epi thickness and resistivity (doping). In heteroepitaxy, exceeding the critical thickness introduces misfit dislocations | Thickness uniformity, step coverage, film density and impurities such as hydrogen, film stress |
| Equipment | For silicon on 300 mm wafers, lamp-heated single-wafer reactors are mainstream. Compound semiconductors widely use MOCVD, and MBE is also used, for example in research | LPCVD that processes many wafers at once in a vertical furnace, plasma CVD, HDP-CVD for gap fill and others |
Figures are typical orders of magnitude and vary by process generation and product.