One of the more commonly used chemical feed stocks in the plasma treating process is Hexamethyldisiloxane (HMDSO).
Primarily used for barrier coatings, adhesion promotion, and to help protect a coating post treatment. HMDSO dispensed with argon into the vacuum chamber, is a big part of the hydrophilic coating process. Though, this chemical can be used to produce hydrophobic processes as well.
By adjusting the ratio of HMDSO and Oxygen that will be in the plasma chamber, a thin film can be produced with customized film hardness by setting the mix of oxygen and HMDSO. Even after time, the film will hold its strength to protect the surface. The coating is known for changing the contact angle of a surface by changing chemistry.
To learn more about the use of plasma in manufacturing, please read our eBook titled "Manufacturer’s Surface Activation Guide for Improved Adhesion."
Plasma Processes with Hexamethyldisiloxane
Hexamethyldisiloxane or HDMSO is a colorless liquid solvent, which is a member of the organosilicon compounds. It is very commonly used for organic synthesis of silyl esters and silyl ethers. However, in plasma deposition applications it is used as a precursor for obtaining silicon containing films. Such thin films act very frequently as a corrosion barrier on different substrates. Plasma enhanced chemical vapor deposition (PECVD) was successfully used on substrates, such as steel, aluminium, silicon, plastics, or even wood. In the case of wood, however, the primary goal is not to achieve corrosion resistant films but to increase hydrophobicity. This creates waterproof surfaces without the necessity of using any kind of paint or varnish.
Trimethylsilyl (TMS) is actually not a chemical compound on its own, it is rather an organic functional group that is included in several plasma processes as part of a precursor. Examples of such precursors are bis(trimethylsilyl)carbodiimide, which has been successfully used to obtain hard silicon carbonitride films on steel or trimethylsilyl acetate that can form different varieties of coatings and is also heavily used for plasma polymerization. Each of these coatings has different properties like hardness, wettability, morphology, etc… , which depend heavily on the chemical composition of the precursor, the gas flow rate, and other process parameters.
There are a lot of novel areas in which plasma processes with PDMS are applied, either in vacuum vessels or at atmospheric pressure, commonly with a radiofrequency discharge. Just like HMDSO or TMS plasma this precursor can form thin films and coatings such as SiOx or other ceramics or dielectric films on various substrates. Thus, it is used to manufacture microfluidic chips or state-of-the-art membranes.
Optoelectronics: Hexamethyldisiloxane in combination with other gases such as nitrogen can be used to create layered materials with different refractive index for each of these layers. Generally speaking, the use of HMDSO and other gases as precursors allows to custom-tailor the band gap and, hence, the refractive index of the deposited thin films.
Automotive and tool-making industry: The ability of HMDSO plasma coatings to form strong Si-O-Si bonds on different materials (especially metals) leads to an improved corrosion resistance of the treated surfaces. The same effect can be achieved with TMS based plasma but with the difference that TMS forms polymer films on the treated surface. Recent studies have also shown that PDMS coatings in connection with diamond-like carbon (DLC) can significantly decrease wear rates and, thus, improve tribological properties.
High-end photo catalysts: This is a very new field that is still in its infancy but it has been suggested that HMDSO plasma enables the embedding of metal oxide particles into organosilicon films. Such films could be used for revolutionary photo catalysts in the near future.
Biomedicine: HMDSO has also been successfully used to deposit thin films of organosilicon-metal composites, which have advantageous anti-microbial properties. This is also the case for certain films obtained with TMS on substrates like stainless steel and titanium. Those coatings decreased the ability of microorganisms to attach themselves on the surface. This is particularly advantageous for implants or intravascular stents. Recent experimental studies also showed that plasma polymerized thin films on a titanium substrate has a positive impact on the absorption behaviour of protein molecules.
High-Tech Glass: On glass items such as windows or displays, plasma deposited TMS was successfully used to enhance the hydrophobicity and even the mechanical strength of the glass surface. PDMS has also been used for smart coatings on glass substrates – for example, for the creation of hydrophilic nanochannels, which can be used for biomedical sampling as well.
Microfluidics: The primary aim of microfluidics is to control fluids on very small scales, usually less than a millimetre. For this purpose, plasma polymerization of PDMS has been used to create hydrophilic surfaces for microfluidic devices. Similar experiments were also successfully carried out with TMS and HMDSO as precursors.
Nanotechnology: PDMS based plasma processes can be used to enhance the thermal conductivity of AlN nanoparticles.
Plasma polymerization is a process, which is used to create long-chained hydrocarbons directly from the gas phase. Plasma is thereby used to provide energy for the formation of ions and radicals of monomers. Plasma polymerization is usually carried out in low-pressure glow discharges in which the free electrons gain kinetic energy from electric fields and collide with the monomer precursor gas particles. They then transfer some of their kinetic energy to these monomers. This enables the formation of more complex polymer structures, such as hexamethyldisiloxane, acetylene , styrene or something similar.
Hexamethyldisiloxane can be used for a variety of plasma enhanced chemical vapor deposition (PECVD) techniques on different materials. One prominent example is plasma polymerization, which uses plasma to initiate polymerization from the gas phase.
Trimethylsilyl (TMS) can be used for a wide variety of thin film coatings due to its ability to form films with very different physical properties, depending on the plasma parameters that are used in the process. It can be utilised for very soft films with high elasticity mostly in the form of plasma polymerization - but also for films with high hardness values. Especially the latter one is important for protective coatings on various forms of metal substrates, such as steel.
HMDSO is highly flammable and its vapours become explosive when they mix with air. It is also hazardous to marine life. Hence, it should be handled with care and kept away from open flames. However, since hexamethyldisiloxane is usually used in low pressure plasma processes, the amount that is needed for depositions is very small and there is no risk of ignition in a vacuum chamber at low process presures.
Hexamethyldisiloxane has a density of 0.764 g/cm³.
Dimethicone or polydimethylsiloxane (PDMS) is also an organosilicon compound (just like HMDSO and TMS) and has the chemical formula CH3[Si(CH3)2O]nSi(CH3)3, where n is the number of the [SiO(CH3)2] monomer group, which is repeating.