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Thin film can be synthesized by various methods such as chemical-vapor deposition, physical
vapor deposition, molecular-beam epitaxy, electrochemical deposition. Each method produces films
with certain compositional, structural, and mechanical characteristics. The choice of deposition
method depends on the film properties required for specific applications. Chemical vapor deposition
is one of the frequently employed techniques for dielectric and semiconductor thin films with high
deposition rate. Physical vapor deposition is mostly used for metallic films as well as dielectric film
deposition. Molecular beam epitaxy is a highly controlled deposition to produce single-crystal films,
but it is not cost effective. Electrochemical deposition is mostly used for thick metallic films. Back
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Chemical vapor deposition (CVD) is a thin film synthesis process extensively employed in semiconductor
industry. It can produce amorphous, polycrystalline, eptaxial and uniaxially oriented polycrystalline films with
high purity. In this process, the precursor gas mixtures, often diluted with an inert carrier gas, of constituents
for desired film composition are introduced into the reactor and the reactions take place on substrate with
an elevated temperature. The growth rate and quality of thin film depend on (1) substrate temperature,
(2) total flow rate, (3) gas phase composition, and (4) precursor temperature. The substrate temperature is
the most important controlled parameter. Higher substrate temperatures usually will reduce the deposition
while lower substrate temperature will lead to a higher deposition rate. Back
1.1 Spray pyrolysis
a hot substrate. The reagent then decomposes or reacts with oxygen to deposit a stable residue. This
process produces thick films and is very cost effective. Antireflection coatings, compound semiconductors
for solar cells and sensor coatings are some examples of the applications.
1.2 Atmospheric pressure chemical vapor deposition
Atmospheric pressure chemical vapor deposition (APCVD) is a conventional CVD approach. It operated
at a pressure from 10 to 100 kPa. APCVD is used for low-temperature oxide with high deposition rate and
usually it operates at a low temperature (350 to 400 ºC). However, it has a poor coverage for stepped surface
and particle contamination is also a concern.
1.3 Plasma enhanced chemical vapor deposition
Plasma enhanced chemical vapor deposition (PECVD), also called plasma assisted chemical vapor deposition
(PACVD), uses an electrical discharge to break chemical bonds and to initiate chemical reactions. The electron
temperature in the discharge is very high, while the gas temperature remains near ambient. Films deposited by
PECVD have a low substrate temperature, good adhesion to substrate and good step coverage. Chemical
(such as hydrogen) and particulate contamination are concerns. It is often used for low-temperature insulators
over metals and for passivation film (nitride) deposition.
1.4 Low pressure chemical vapor deposition
Low pressure chemical vapor deposition (LPCVD) operates at a pressure below 10 Pa. This surface-
reaction controlled process enables uniformly deposition of large quantity of samples at the same time
owing to large diffusion coefficient at low pressure. It also has excellent purity, conformable step coverage
and better particulate contamination control. Oxide, nitride, silicate, polysilicon films are produced
frequently via LPCVD.
1.5 Metallorganic chemical vapor deposition
Metallorganic chemical vapor deposition (MOCVD), also named as organo-metallic vapor-phase
epitaxy (OMVPE), provides the process of film thickness control within one atomic layer on large surface
areas via the flow of gases past samples placed in the stream. It is a cost effective approach for compound
semiconductors for solar cells, III-V laser, light emission devices, and quantum walls. Safety sometimes
is a concern.
1.6 Other chemical vapor depositions
Other non-conventional chemical vapor deposition processes include very low pressure CVD operating at
about 1 Pa for epitaxial growth of single crystal deposition, high temperature CVD running around 800 oC,
medium temperature CVD operating around 600 oC and low temperature CVD is running 400 oC or lower.
Alternatively oxide films can be formed by thermal growth in an oxygen environments.
Physical vapor deposition (PVD) include thermal evaporation, sputtering, molecular beam epitaxy,
laser ablation, ion plating and cluster beam deposition. Evaporation and sputtering are most commonly
used techniques. For evaporation, atoms are removed from the source by thermal energy, whereas
for sputtering atoms are ejected from target surface through bombardment of energetic gaseous ions.
PVD reactors may use a solid, liquid or vapor raw materials. Back
2.1
Resistance-heated evaporation is one of the oldest techniques of thermal evaporation. Various types of filaments,
boats and crucibles are used as evaporation sources. Both metal, inorganic compounds can be prepared with
this approach. For most of inorganic compounds the vapor composition is different from that of the original
solid or liquid source. Reactive gas species can be introduced during deposition to compensate the loss of
constituents for stoichiometric film. This process is usually called Reactive Evaporation. Disadvantages of
resistively evaporation sources include possible contamination by crucibles, heaters, and support materials
and limitation of relatively low input levels.
Electron-beam evaporation is another widely used thermal evaporation techniques for films with high
purity. Electrons are thermionically emitted from heated filament. Due to effective shielding, contamination
from the heated source is minimized. Electron-beam evaporation can deposit virtually all materials at
almost any rate. It evaporates materials from a point source and hence it requires a long distance between
source and
substrate to achieve good film uniformity.
2.2
Sputtering deposition
Compared to the thermal evaporation, sputter deposition can generally provide a better surface coverage,
better adhesive to substrate, easier control of film composition, uniformity and other film properties. It also
has easier scalability and almost unlimited choices of deposition source materials. Hence, it has a much
wider spectrum of industrial applications as compared to the thermal evaporation. However, it has some ion
bombardment induced surface defects.
DC sputtering deposition For this technique a constant electric field is applied between the metal target
(cathode) and substrate (anode). Process gas such as argon is introduced and serves as the medium in which
a discharge is initiated and sustained between electrodes. The electric field causes electrons to be accelerated.
These electrons collide with the introduced process gas, resulting in ionization and the generation of ions and
secondary electrons. The ions produced are accelerated in the electric field and sputter off the materials at the
target and films are then deposited on the substrate. A magnetic field usually is installed parallel to the target
surface to provide high density plasma and high deposition rate. Other sputtering deposition techniques such
as cathodic sputtering deposition, diode sputtering deposition bear the sample principles.
Radio-frequency
reactive sputtering deposition
is used as the cathode electrode (target) to be deposited by a glow discharge in a vacuum chamber. An oscillatory
electric field with radio frequency (RF) is applied between the target (cathode) and substrate (anode). Higher sputtering
rates are possible by adding magnetron facilities to the system. At very high RF powers, applied voltage tends to
rise rapidly resulting in decrease of sputtering process efficiency. Practically most large RF discharge systems
operate best at an applied power below 5 kW.
AC sputtering deposition AC reactive sputtering usually employs in dielectric film deposition and has a
configuration similar to RF sputtering deposition except power source frequency. The frequency in its power
supply ranges from 25 kHz to 250 kHz. The deposition rate for AC sputtering usually is higher than RF sputtering.
2.3
Ion beam deposition
Ion beam deposition (IBD) is a low-temperature (< 200 ºC) thin film deposition technique, offering advantages
of independent control of ion energy and flux and better vacuum levels compared to sputtering deposition approaches.
IBD deposited films can have the highest purity and avoid complex processes by plasma interaction at both the target
and the surface of the growing film. In this process, ion energy, current density and angle of incidence as well as the
total pressure in the chamber can be varied independently over wide ranges. However, IBD usually has a very low
deposition rate.
2.4 Ion beam assisted deposition
Ion beam assisted deposition (IAD) combines ion beam bombardment and sputtering or evaporation. They can
operate independently in the same chamber. For examples, for the ion beam assisted evaporation, the deposition rate is
controlled by evaporation process, while structure and properties of deposited films are determined by ion beam
process. IAD deposition has the advantages of obtaining high density film, better adhesion, depositing meatastable
films, easier control for deposition species at the substrate surface. Compound films with stoichiometric composition
are easier to achieve.
2.5
Ion plating
Ion plating is an evaporation processes while the substrate is exposed to a flux of high-energy ions or plasma.
The ions or plasma are capable of causing appreciable sputtering before and during film formation. In most cases,
the substrate is subjected to ion bombardment resulting in contamination removal from the substrate surface. Source
evaporation then follows without interrupting the bombardment. Once the interface between deposited film and
substrate has formed, ion bombardment may or may not be continued. The advantage of ion plating is the ability to
promote extremely good adhesion between the film and substrate and achieve high deposition rate.
2.6 Molecular beam epitaxy
Molecular beam epitaxy (MBE) is the important approach to obtain single crystal film with high purity on single
crystal substrates. The deposition process is highly controlled. It requires ultrahigh-vacuum system (10-10 Torr)
and relatively high temperature. In return, MBE deposition rate is relative low (~ 0.2 nm/s). Compared to CVD
expitaxy, MBE is performed at a lower temperature reducing the diffusion and auto-doping effects. Quantum walls,
superlattice, and compound semiconductor devices are usually achieved by MBE.
2.7 Laser ablation deposition
Laser ablation is also named as laser sputter deposition. This technique employs intense laser radiation to erode
a target and deposit the corresponding materials onto the substrate. Compared to the electron beam evaporation,
this technique avoids x-ray damage to the substrate and decomposition of the target materials, particularly for
complex compounds. The deposited films are usually amorphous requiring substrate heating for a crystalline
film. Large area deposition is usually not feasible.
2.8 Cluster-beam deposition
This technique requires a special evaporation configuration enabling 100 to 1000 atoms with single charge. The
deposited films usually have high purity, excellent adhesion and low defects. The requirement for the substrate
conductivity is also low as the cluster beam usually will not cause any charge buildup effects.
Electrochemical deposition is one of the oldest techniques for metal and metal alloy film deposition. It is a
wet process and relatively cost effective. It enables metal film replication of high-aspect-ratio resist molds with
high precision. It also has a very high deposition rate but may be limited by the chemical reaction. Back
3.1 Electroless deposition
Electroless metal deposition is a metal displacement process where the a less noble metal exchanges charge
with metal ions in solution. This process has two independent electrode reactions: the cathodic partial
reaction and the anodic partial reaction. This chemical potential driving process will stop only when the reaction
sites are eliminated or full reaction surface is covered. The deposited film thickness ranges 1-3µm. By varying the
electroless solution, a wide range of materials can be grown and catalyst is often used for nonactive surfaces.
Examples are Au, NiP, Teflon, polycarbonate.
3.2 Electrodeposition
Electrodeposition includes electroplating and anodization. Electroplating takes place in an electrolytic cell
where an imposed bias leads to current flow and electrochemical reactions. The materials plated are contributed
from the ions in the solution. Anodization is an oxidation process taking place in an electrolytic cell. The
material to be anodized turns into anode while noble metal is the cathode. Usually the materials in a water solution
is generally porous while an organic electrolytes will produce dense films.