Page 86 - Sami Franssila Introduction to Microfabrication

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Epitaxy

Epitaxial deposition is a very special case of thin- be converted to epitaxial films by annealing. Of course,
film deposition. Epitaxy means the growth of a single all the limitations of clean surfaces, matching lattice and
crystalline layer on top of a single crystalline substrate. so on still apply. Epitaxy from liquid phase (LPE) is
The growing layer registers the crystalline information also possible: both saturated solutions and melts can
from the layer below. In order to do so properly, be used as sources for epitaxial growth. LPE was the
the crystal lattices of the two layers must be closely dominant technology in the early days of III-V semi-
matching. Because crystal information is ‘transmitted’ conductor laser and LED fabrication, but it has largely
across the substrate–film interface, surface quality of the been superseded by gas-phase and vacuum systems.
starting wafers is of paramount importance. Defects, be In homoepitaxy, the substrate and the growing film
they native oxide, crystal defects (dislocations, stacking are the same material. Silicon epitaxy on silicon
faults) or metal impurities, can destroy epitaxial growth. enables freedom in doping level and doping type
Epitaxy is a delicate process, and high quality epitaxial tailoring. Epitaxial wafers account for some 20%
films are difficult to make. Epitaxy can fail partially of all wafers sold. A lightly doped epitaxial p-
and result in a defective single crystalline material, or type layer (10 ohm-cm) can be grown on a heavily
it can fail completely, and result in a polycrystalline p-doped substrate wafer (0.2 ohm-cm). This is the
film. Whether the defective material is usable for devices material for advanced microprocessors and other high-
depends on the density and location of those defects: if performance logic circuits. n-Silicon on p-substrate is
defects are confined to the substrate–epi interface and used in many micromechanical devices because of
the epilayer is mostly defect-free, the material is usable; electrochemical etch stop. The number and thickness
but this depends on the device operating principle, and of layers is practically unlimited: in IGBT (Insulated
engineering judgement is needed to decide on acceptable
defect levels. Gate Bipolar Transistor) power transistors a moderately
Epitaxy has nothing to do in particular with sil- doped n-layer is grown first, followed by a thicker lightly
icon or semiconductors: epitaxy is a phenomenon doped layer. In semiconductor laser structures, there
that is seen in many classes of solids. However, can be hundreds of epitaxial layers. Another benefit of
semiconductor-on-semiconductor epitaxy, both Si/Si and epitaxy is the absence of oxygen and carbon, which are
GaAs/Al x Ga 1−x As, has been, and remains, the most always present in CZ-silicon. Uniformity of epitaxial
voluminous industrial application of epitaxial deposi- layers is good, for both thickness and resistivity, and
tion. Insulators like calcium fluoride (CaF 2 ) and yttrium if very tight resistivity specification is needed, epitaxial
oxide (Y 2 O 3 ) can be grown epitaxially on silicon, and wafers override bulk silicon wafers.
so can cobalt silicide (CoSi 2 ). Epitaxial silicon can be Hardware for epitaxial deposition is varied: in
grown on sapphire (crystalline aluminum oxide, Al 2 O 3 ) principle, almost any deposition system can be used
and epitaxial cerium oxide, CeO 2 , can be grown on sili- for epitaxial deposition under some conditions but there
con, and epitaxial YBCO superconductor can be grown are a couple of established technologies for epitaxial
on CeO 2 . deposition. CVD epitaxy of silicon with SiH 4−x Cl x
In solid phase epitaxy (SPE), the film regis- (0 ≤ x ≤ 4) source gases is the standard method. In
ters the crystalline structure from the underlying the compound semiconductor field, MOCVD (Metal
single-crystalline substrate. Amorphous films can thus Organic CVD; also known as MOVPE for Vapour Phase

Introduction to Microfabrication Sami Franssila
 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85105-8 (HB); 0-470-85106-6 (PB)