Chemical vapor deposition

Chemical vapor deposition (CVD) is used to deposit solid material onto a substrate. This involves the reaction or decomposition of one or more precursor gases in a chamber containing one or more heated objects to be coated. The reactions occur on and near the hot surfaces, resulting in the deposition of a thin film on the surface. The chemical by-products or unreacted gases are then eliminated from the reactor chamber via the exhausting system. CVD must take place under vacuum to avoid the inclusion in the film, or creation of side products from the reaction of the ambient components with the precursor gases.

Technologies

While all CVD processes involve a chemical reaction near the substrate surface to deposit the film, there are many ways to achieve this. In particular, there are different ways to decompose the precursors. Several of these methods that are used in the LNF are described below.

Atomic layer deposition (ALD)

Main article: Atomic layer deposition

Atomic Layer Deposition (ALD) is a technique which allows the deposition of ultra-thin films, a few nanometers thick, highly conformal and self limiting to be deposited in a precisely controlled way. These characteristics offer many benefits in semiconductor engineering, MEMS, catalysis and other nanotechnology applications. In ALD the precursor gas or gases are introduced, one at time, into the reactor and made react with the surface until all reactive sites are occupied and the reaction stops. The precursor gases are pulsed, alternatively, never present simultaneously in the chamber. These type of deposition is slow and requires highly pure substrates to obtained the desired films.

Low pressure chemical vapor deposition (LPCVD)

Main article: Low pressure chemical vapor deposition

Low pressure chemical vapor deposition uses heat to initiate a reaction of a precursor gas(es) on the substrate surface. This reaction at the surface is what forms the solid phase material. Low pressure is used to decrease any unwanted gas phase reactions, and also increases the uniformity across the substrate.

Parylene deposition

Main article: Parylene deposition

Parylene deposition is a method for depositing parylene, a thin, transparent polymer coating that is conformal, usually pinhole free, has high dielectric strength, high surface and volume resistivity, and resists moisture, acids, alkalis, petroleum products and solvents. Parylene is also bio-compatible which means it can be used to protect medical devices and implantable electronics.

Plasma enhanced chemical vapor deposition (PECVD)

Main article: Plasma enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD) is a technology that utilizes a plasma to provide some of the energy for the deposition reaction to take place. This provides an advantage of lower temperature processing compared with purely thermal processing methods like low pressure chemical vapor deposition (LPCVD). PECVD processing temperatures range between 200-400°C. LPCVD processes range between 425-900°C.

Carbon nanotube and graphene growth

Main article: Carbon Nanotubes and Graphene

Both carbon nanotubes (CNTs) and graphene can be grown via catalytic CVD. In catalytic CVD, a metal catalyst is used to cause the precursor gas to react at the substrate. This allows CNTs or graphene to grow at a much lower temperature than would be possible without the catalyst.

Applications

CVD is used for depositing thin layers of material. Frequently these are insulating/dielectric layers. They can range from single atomic layers to a few microns thick depending on the technology used.

• carbon nanotube growth
• trench refill (TEOS, HTO)

Figures of merit

The following properties of CVD films can be tuned. Frequently there is a trade off between these properties and process temperature. High process temps usually have lower Hydrofloric Acid (HF) etch rates, set indexes of refraction, and lowered pin hole densities. As you go to lower temperature process the Hydrofloric Acid Etch rate will general increase, there are more options for changing the index of refraction, and pinhole density's may increase.

• Define film quality
• Film composition
• Refer back to deposition page FoM

Refractive index

The Refractive Index is an optical property of the film measured using Ellipsometry that also gives clues to the density, dielectric constant, and stoichiometry of the film . Silicon Nitride has a value from 1.8-2.2 with 2.0 being idea. Variation from this indicate a silicon rich film (>2.0)less than <2.0 usually indicates the presence of oxygen . Silicon oxide is varies from 1.44-1.47 depending on deposition technique and film purity.

Etch rate

• Difference between wet and plasma
• Common examples (like HF for oxide)

This test gives information about the quality of the HF film. The etch rate is related to the density and amount of SiO₂ in the film. The slower the etch rate the higher the density and amount of SiO₂. A slower HF etch rate also correlates to an increased dielectric constant. Thermal oxide (the hightest quality oxide) has an etch rate of 230Å/min in 10:1 HF:H₂O.

See also

Other related wiki pages

Further reading

• LNF Tech Talk for CVD is Coming Soon!