So far, we’ve discussed top-down nanofabrication techniques for patterning and removing material at a nanoscale such as photolithography and etching, as well as techniques for measuring our work. The question at hand today is what if we want to add new materials? In top-down nanofabrication we add materials by a process known as thin-film deposition.
Deposition is a term most commonly used in geology when referring to the soil and sediments that attach to landmasses to form layers of built-up coating. A similar analogy can be used to describe thin-film deposition, however in a much more controlled fashion. Rather than the random deposition of material within geological processes, specific materials are chosen based on their physical and chemical properties to construct the desired films or nanostructures.
There are several different methods to deposit thin-films, with no one-size-fits-all technique. Most thin-film techniques fall into one of two categories, either chemical vapor deposition (CVD) or physical vapor deposition (PVD). CVD is a process that uses heat or plasma to induce chemical reactions at the surface of a heated substrate, with reagents supplied in gaseous form. PVD is a process where a solid material is vaporized in a vacuum environment and deposited on substrates as a pure material or alloy composition coating.
PVD is a highly diverse technique with several different variations; however, all require vacuum chambers capable of creating very low-pressure environments, a high energy source to vaporize the solid source material, and ideally control substrate temperature to tune the end material’s properties. The two main ways to vaporize the solid source material are thermal heating and sputtering. Thermally evaporated materials are commonly heated by either induction, electron beam, resistive heating elements, or lasers. Sputtered source materials are bombarded with ions created from inert plasmas, to create vaporized atoms. In both techniques, these vaporized atoms travel through the reaction chamber, eventually condensing onto the substrate. This technique allows for the deposition of a wide range of materials, giving engineers and scientists significant flexibility in choosing a thin-film with the desired properties.
CVD is the method of choice when creating the highest-quality thin-film is required. Some of these high-quality characteristics included finer grain structures, higher crystallinity, higher purity films, and better structural integrity. While there are several different types of CVD, they all work with the same basic principle of utilizing controlled vapor phase chemical reactions to create solid films. CVD processes occur in varying degrees of a vacuum environment, depending on the application, and use volatile precursor materials as the source of chemical components that make the end materials. The chemical reactions are aided by an external energy source, most commonly heat or plasma, but sometimes can be laser induced. The chemical reactions occur on the substrate surface, producing the desired film and volatile by-products that are subsequently swept away by the continued pumping and gas flow of incoming precursors.
Within the CVD family exists a special technique known as atomic layer deposition or ALD. As its name states, ALD is capable of depositing a single monolayer of film at a time. It also is a highly uniform and conformal process, allowing for deposition on complex shapes and curved surfaces. ALD uses two different precursors to build a thin-film in sequential order, meaning the precursors are never in the reaction chamber at the same time. Each time a pulse of precursor is introduced to the substrate, a self-limiting chemical reaction occurs, building one part of the thin-film. The process cycles between the two precursors, giving the engineer ultimate control over the film thickness at the atomic level.
As with every process step in nanofabrication, there are several aspects to consider in thin-film deposition. As an example, the substrate surface needs some capacity to adhere to the molecules that are being deposited. Mismatches in thermal expansion coefficients or lattice constants between the deposited material and substate can cause residual stress, which in its extreme can result in cracking of films. Choosing the correct film and technique for the end device or application is critical. The good news is engineers and scientist have many techniques and materials to choose from!