Additive nanofabrication processes

This is an accordion element with a series of buttons that open and close related content panels.

Additional information

Layers of different materials are often deposited onto substrates, as an essential step in the nanofabrication of useful structures. Additive processes start with a substrate which may or may not already have previous material layers or patterned structures on its surface. The newly added layer is usually a thin film – that is, on the order of one micron or less in thickness.

Additive processes can allow precise control over many important properties of the film, including composition, thickness, uniformity, stress, density, and others.

WCNT has the ability to form thin films of the majority of materials in practical use for nanofabrication.

The three main methods (outlined below) used in additive processes are chemical vapor deposition, sputter deposition, and evaporation. Sputtering and evaporation are examples of physical vapor deposition.

Sputter deposition

Sputter deposition is a form of physical vapor deposition (CVD).

This is an accordion element with a series of buttons that open and close related content panels.

What it is used to do

Sputter deposition allows the formation of thin films on a substrate, with good control over the characteristics of the film.

How it works

In sputtering processes, the sample to be coated is placed in a vacuum chamber along with a solid target composed of the material to be deposited. The chamber is backfilled with a low pressure of inert gas, such as argon. The gas is then ionized and the ions are accelerated towards the target. Material from the target is ejected from its surface, or sputtered, and some of this material arrives at the sample, where it adheres as a thin film.

The voltage applied to the gas in the chamber can be DC or AC. DC sputtering is suitable for conductive target materials, such as most metals. However, for dielectric target materials, the target will acquire a net charge during DC sputtering, terminating the ejection of material from its surface. This situation can be overcome by applying an AC voltage at RF frequencies to the working gas, which gives the target an opportunity to lose electrical charge once per AC cycle. RF sputtering can be used for metal targets as well as for nonconducting materials.

Instrumentation Available

Strengths

  • An important advantage of sputter deposition is that even materials with very high melting points may be easily sputtered, whereas evaporation of these materials in a resistance or ebeam evaporator is problematic or impossible.
  • Sputter deposited films have a composition close to that of the target material. Differences in composition between target and film may occur due to differences in how light or heavy elements are deflected by the working gas, but this difference is constant.
  • Sputtered films typically have a better adhesion on the substrate than evaporated films.
  • A sputtering target contains a large amount of material, and is maintenance-free, making the technique suited for ultrahigh vacuum applications.
  • Sputtering sources contain no hot parts (to avoid heating they are typically water cooled) and are compatible with reactive gases such as oxygen.
  • Sputtering can be performed top-down while evaporation must be performed bottom-up.
  • Advanced processes such as epitaxial growth are possible.

Limitations

  • During prolonged ion or plasma bombardment of a substrate, sputtering processes can lead to significant erosion of materials. However, since sputtering is commonly used for thin-film deposition, most sputtering processes are of short enough duration that this is not an issue.
  • Sputtering may not be the best additive technique to use for liftoff work, since the diffuse transport characteristic of sputtering may make a full shadow impossible.
  • Inert sputtering gases may become entrained in the deposited film as impurities.

Additional Reading

Chemical vapor deposition (CVD)

Includes low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD).

This is an accordion element with a series of buttons that open and close related content panels.

What it is used to do

Chemical vapor deposition (CVD) can be used to produce high-performance thin films of solid material on substrates.

How it works

CVD is usually carried out under vacuum and at elevated temperatures.  The substrate to be coated is exposed to one or more volatile precursors, which react or decompose on the substrate surface to produce the desired deposit.

Instrumentation Available

Strengths

  • Microfabrication processes widely use CVD to deposit materials in various forms, including: monocrystalline, polycrystalline, amorphous, and epitaxial.
  • A wide range of materials can be deposited by CVD, including silicon (dioxide, carbide, nitride, oxynitride), carbon (fiber, nanofibers, nanotubes, diamond and graphene), fluorocarbons, filaments, tungsten, titanium nitride, and various high-k dielectrics.
  • CVD is commonly used to deposit conformal films and augment substrate surfaces in ways that more traditional surface modification techniques are not capable of.

Limitations

Applications

Additional Reading

Chemical vapor deposition

Evaporation

Evaporation is a form of physical vapor deposition (PVD).

This is an accordion element with a series of buttons that open and close related content panels.

What it is used to do

Evaporative deposition allows the formation of thin films on a substrate, with good control over the characteristics of the film.

How it works

In evaporative deposition processes, the sample to be coated is placed in a vacuum chamber along with a source of the material to be deposited, typically in the form of a solid piece or grains. The chamber is pumped to a vacuum level low enough that the mean free path is greater than the dimensions of the chamber, so that atoms leaving the source can reach the sample and adhere to it.

The source material is heated either by being placed in a holder which his heated by electrical resistance, or by bombardment from a beam of accelerated electrons. Controlling the heating of the source material controls its vapor pressure and hence the rate of deposition.

Instrumentation Available

Strengths

  • Can be excellent for liftoff processes, due to good shadow production
  • PVD coatings may be harder and more corrosion resistant than coatings applied by the electroplating process
  • Can theoretically apply virtually any type of inorganic and some organic coating materials onto an equally diverse group of substrates and surfaces

Limitations

  • Specific technologies can impose constraints: for example, line-of-sight transfer is typical of most PVD coating techniques. However there are methods that allow fuller coverage of complex geometries.
  • Some PVD techniques operate at high temperatures and vacuums, requiring special attention by operating personnel

Additional Reading

Spin coating

This is an accordion element with a series of buttons that open and close related content panels.

What it is used to do

Spin coating, in the context of nanofabrication, often refers to methods used for producing uniform thin films of photoresist for use in lithographic patterning processes.

Spin coating may also be used to form thin films of other materials, for example spin-on dopants or spin-on glass.

How it works

To produce thin films of photosensitive polymer for lithography, liquid photresist is applied to a flat substrate. The substrate is then spun at a controlled speed. Liquid photoresist spreads into a film and flows off the edges of the substrate until an equilibrium is reached between cetripetal force and forces due to tension in the film, viscosity of the fluid, and adhesion to the substrate.

Instrumentation Available

Strengths

Precise control of spin speed, coupled with high quality control by photoresist manufacturers, allows precise control of photoresist film thickness.

Limitations

Spincoating works best on whole, round wafers.

Additional Reading