Deposition

Deposition or Growth refers to the controlled synthesis, growth, or transfer of materials as thin films on a substrate. A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. Based on the growth dynamics which prevail during the deposition, the resulting material can be amorphous, polycrystalline, or crystalline. Deposition techniques that result in crystalline material are often referred to as epitaxial growth.

Technologies

Typical technologies include atomic layer deposition (ALD), chemical vapor deposition (CVD), electrodeposition/ electroplating or electrochemical deposition (ECD), physical vapor deposition (PVD), and molecular beam epitaxy (MBE). Selection of deposition technique depends on material deposited, desired film characteristics, and substrate temperature tolerance:

Deposition Method	Materials	Deposition Rate	Substrate Temperature	Conformality / Sidewall Coverage	Film Density	Impurity Levels	Uniformity	Grain Size	Primarily Used for:
Evaporation	Metals and Dielectrics	1–15 Å/sec	10–100ºC from dep. 300ºC with heater	Poor–None	Poor	Low	Poor	10–100 nm	Thicker blanket metal films. Low substrate temp. Directionality means it works well with lift-off patterning
Sputter deposition	Metals and dielectrics	0.1–10 Å/sec	50–300ºC	OK	Good	Low	Good	5–20 nm	More conformal metal and dielectric thin film deposition. Better than evaporation for maintaining stoichiometry of compounds
CVD Parylene Deposition	Parylene	25–50 Å/sec	20ºC	Good	Good	Very Low	Good	unknown	Thick (0.8–75 μm) encapsulation and insulation; Biocompatible
Plasma Enhanced CVD (PECVD)	Mainly Dielectrics	5–200 Å/sec	200–400ºC	OK	Good	Very Low	Good	10–100 nm	Lower temp oxide/nitride deposition
Low Pressure CVD (LPCVD)	Mainly Dielectrics	10–100 Å/sec	600–1200ºC	Very Good	Very Good	Very Low	Very Good	1–10 nm	Better quality oxide/nitride dep where substrate can handle higher temp
Thermal oxidation	Oxide on Silicon	0.1–100 Å/sec	900–1200ºC	Very Good	Very Good	Very Low	Very Good	1–10 nm	Best quality oxide when substrate can handle higher temp and slower deposition rate
ECD/Plating	Conductive Materials	Depends on process	0–100ºC	Very Good	Good	Depends on process	Depends on process	Depends on Process	Thicker films deposition with good conformality
Atomic Layer Deposition (ALD)	Metals, metal oxides and nitrides	0.1–3 Å/cycle. 5–200 sec cycle	50–300ºC	Very Good	Good	Low	Very good	10–100 nm	Very thin, very conformal films such as gate dieletrics, barriers, encapsulation

Chemical vapor deposition (CVD)

In chemical vapor deposition (CVD), a substrate is typically heated and exposed to one or more gaseous precursors, which decompose and react on the substrate surface to produce the desired thin-film material. CVD can be used to grow high quality and uniform thin films of various materials (mostly insulating or semiconducting).

CVD can be subdivided into classifications based on pressure requirements (atmospheric (APCVD), low-pressure (LPCVD), and ultra-high vacuum (UHCVD)). LPCVD is used in the LNF to deposit silicon dioxide, silicon nitride, and doped and undoped polysilicon. It can also be classified based on the mechanism used to decompose the source gas: plasma-enhanced CVD (PECVD) breaks apart gas molecules by application of ionizing voltage, whereas LPCVD and APCVD use elevated temperatures to cause the source gas to decompose. PECVD is used in the LNF to deposit silicon dioxide, silicon nitride, and amorphous silicon (a-Si:H). Catalytic CVD refers to CVD where the surface reaction is facilitated by the presence of a catalyst material on the substrate, or where the substrate itself is a catalyst for the growth reaction. Carbon nanotubes and graphene can be grown by catalytic CVD. Another type of CVD is metalorganic CVD, which uses organometallic gas precursors to grow III-V and II-VI compound semiconductors such as InP, GaN, AlGaAs, etc. Other example of CVD is the deposition of Parylene, in this case the solid Parylene dimer is evaporated and separated into the monomer by heat and deposited in uniform, pin hole free thin films.

Electroplating

Electroplating (electrodeposition, electrochemical deposition (ECD), plating) is the technique recommended when metal layers of more than a micron of thickness are needed. It is only available on conductive substrates and for conductive films. It is also the technique of choice when there is no line of sight with the surface to be deposited, for example, the filling of vias in semiconductor processing. The principle is simple: positive ions are attracted to the negative electrode (anode which is the sample in the case of metal deposition), and negative ions travel towards the cathode or positive electrode. ECD is an electrochemical cell, which consists of a cathode, anode, and electrolyte that contains the ion to be deposited. Electrodeposition does not require a vacuum environment and can be done in batch processes, thus making it relatively inexpensive. It creates a thick, durable film whose surface finish can be tailored depending on the requirements.

Physical vapor deposition (PVD)

Physical vapor deposition (PVD) Physical vapor deposition (PVD) is a type of deposition where source materials are transformed into a vapor or plasma using a physical process (typically heating or bombardment.) The vapor then moves towards a substrate, usually in a vacuum or inert gas environment, where it condenses on the substrate surface.

Thermal Oxidation

Thermal oxidation is used to grow very high-quality silicon dioxide on silicon. By exposing silicon to oxygen at very high temperatures (~1000 C), the silicon and oxygen react and form silicon dioxide. Thermal oxidation is typically used to grow silicon dioxide for MOS transistor gates.

Figures of merit

Deposition rate

The deposition rate, usually expressed in Å/sec, is measured at the substrate using various methods depending on the type of film deposited. It is in measured real-time in the evaporators and after run completion for other techniques.

Film Composition

Also known as stoichiometry and usually expressed in units of atomic % or weight %. The film composition affects film behavior, optical constants, stress, etch rates, and other physical properties like melting point, vapor pressure, etc.

Refractive index

It defines the optical properties of a given material for a specific frequency or wavelength of light. Also known as the index of refraction, or n. The refractive index of a film can be measured using Ellipsometry and also gives clues as to the density, dielectric constant, and stoichiometry of the film ^[1].

Conformality or Step Coverage

Step coverage is the measure of how much coating is on the bottom/sidewall of a feature vs. how much coating is on the top/field areas. It is highly dependent on the geometry of the features and the type of deposition chosen. ALD, TEOS, HTO, and thermal oxide are very conformal. LTO, PECVD, sputtering, and evaporation are much less conformal.

Film Stress

It is the elastic mismatch between the thin film deposited and the substrate that results in a change in substrate curvature. Residual stress is typically defined by a unit of measurement (MPa) across a given area.

Thermal budget

It is defined as the amount of thermal energy that is transferred to the substrate. The thermal budget is determined by temperature, time-at-temperature and heat transfer to the substrate. Higher-temperature (and higher-kinetic-energy) methods of deposition tend to have better step coverage/conformality, lower defects, better optical properties and may have lower stress. However, lower temperatures methods may be needed to account for temperature limitations of the substrate, previously deposited layers and patterning/sacrificial layers.

References

↑ Handbook of Thin Film Deposition: Processes and Technologies