Additive nanofabrication processes

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

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.

• 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.

• 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.

ALD can produce very thin, conformal films with control of the thickness and composition of the films possible at the atomic level.

During atomic layer deposition a film is grown on a substrate by exposing its surface to alternate gaseous species (typically referred to as precursors). The precursors are generally not present simultaneously in the reactor, but are inserted as a series of pulses. In each of these pulses the precursor molecules react with the surface in a self-limiting way, so that the reaction terminates once all the reactive sites on the surface are consumed. Consequently, the maximum amount of material deposited on the surface after a single exposure to all of the precursors (one ALD cycle) is determined by the nature of the precursor-surface interaction. By varying the number of cycles it is possible to grow materials uniformly and with high precision on arbitrarily complex and large substrates.

Veeco Fiji G2 ALD

• The system is for depositing non-toxic materials.
• Maximum film dep thickness is 40nm.
• This instrument has material restrictions. Consult the allowed materials list for this instrument in FOM.

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

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.

• 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.

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

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.

• 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

• 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

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.

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.

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

Spincoating works best on whole, round wafers.