Plasma Technology and Development of Efficient Plasma Processes

The plasma atmosphere consists of free electrons, radicals, ions, UV radiation and a large number of differently excited particles, depending on which gas is used. 

In the plasma chamber, various reactive species are generated that interact with the substrate surface and clean, modify or coat it, depending on the process parameters that have been set.

Furthermore, plasma treatment can be carried out with different process control:

1. The substrates are treated directly in the plasma zone.
   
2. The samples are positioned outside the plasma zone; this process is called remote process.
3. The substrates are activated in the plasma and a grafting reaction is carried out immediately afterwards.
4. The substrates are previously treated with a polymer solution or a polymer gas and this is fixed or polymerized by the subsequent plasma.
   

High-energy and reactive particles from the plasma gas phase bombard all materials in contact with them. They can, depending on the process control,

• remove the surface,
  
• create chemical functions on the surface or
  
• separate layers.

Ablation, functionalization and deposition take place simultaneously as elementary processes in every plasma treatment. Thus plasma processes can be used for

• etching, cleaning, activation,
  
• chemical functionalization and
  
• coating.

Which of the processes determines the net result of the treatment – whether etching or coating is ultimately preferred – depends on various parameters in process control. For each task, we determine the optimum parameters for the desired change in surface properties.

The ions, molecules and atoms in electronically excited states, UV and light emissions, and high kinetic energy particles (especially ions) activate and/or etch surfaces, induce polymerization of many (especially organic) substances in the gas phase and/or on surfaces, and lead to film building on the substrate surface.

The plasmachemical reactions pathways and products differ appreciably from those with "conventional" chemistry. Plasmas can induce chemical reactions even with gases and surfaces which are totally inert under normal conditions – saturated hydrocarbons, nitrogen and inert gases are all activated or ionized in plasmas and participate in chemical reactions.

The following table illustrates some of the practically important forms of plasma-substrate interaction:

The animation demonstrates only one of the possible reaction pathways during plasmachemical oxidation of polyolefins. The reaction is initiated with atomic oxygen, which abstracts an hydrogen atom from the surface. This result in a free radical on the surface, which in turn can react with an oxygen molecule to produce a hydroperoxide group, which can decay into a keton group.

Besides keton groups, there also formed aldehyd, carboxyl, hydroperoxide, hydroxide, and ester groups (the picture above). The plasma oxidation results furthermore in breaking the polymer chains and material removal. A more or less specific surface functionalisation is still possible under precise control of the reaction conditions.

Literature: F.Clouet and M.K.Shi, J. Appl. Polym.Sci. , v.46, p.1955 (1992)

A typical plasma system consists of five modules or functions: vacuum system, energy supply, tuning unit, reaction chamber and control units.

• Vacuum system: Low-pressure plasma treatment works in a pressure range between 0.1 mbar and 1 mbar with a continuous gas flow into the reaction chamber. In some cases, however, it is necessary to lower the base pressure to values below 0.1 mbar before treatment.
• Power supply: It provides the electrical energy necessary to ignite the plasma. The power varies between 10 and 5000 watts, depending on the size of the reactor and the desired treatment. A wide range of frequencies, from DC to microwaves, can be used for excitation.
  
• Reaction chamber: This is the “heart” of the system and can be adapted to the process. The material to be treated can be treated in a batch, semi-continuous or air-to-air manner. The latter is very expensive due to the airlock technology required.
• Control unit: This controls all process variables: gas type, pressure, gas flow, power and process time.

Example: Physical parameters of a system at Fraunhofer IGB in Stuttgart

• Frequency: 13.56 MHz
• Plasma power: 10-1000 W
• Base pressure: 10-3 mbar
• Working pressure: 0.01-1 mbar
• Speed: 0.2-20 m/min
• Maximum width: 18 cm

In low-pressure plasmas that work at reduced pressure, all solids that are vacuum-compatible can be treated:

• Metals
• Most polymers
• Biomaterials and many other organic and inorganic substances

The advantages of the low-pressure technology are the unparalleled layer homogeneity as well as the extremely low consumption of chemicals. With plasma processes, in particular low-pressure methods, even chemically inert materials such as Teflon® can be modified and made accessible to further processing (e.g. bonding).

However, there are material related limits when materials are corroded too strongly in the plasma, whether chemically or by (UV) radiation as in the case of the plastic polyoxymethylene (POM).

At the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB, surfaces are first of all fully characterized with the aim of selectively modifying their properties, and in a second step, they are functionalized using various modification and coating technologies.

• Process development for the plasma modification of surfaces (powders, fibers, surfaces and shaped bodies)
• Layer development
  • Scratch protection, abrasion protection layers
  • Production of adhesive or dehesive agents
  • Corrosion protection layers
  • Barrier layers (e.g. impervious to oxygen and water vapor)
• Functionalization of surfaces
  • Biofunctionalization
  • Chemical functionalization
• Development of plasma cleaning processes
• Development of plasma sterilization processes
• Surface and layer characterization
  • Geometry, morphology, roughness
  • Chemical composition, biological properties
  • Interfacial energy, adhesion
• Development of processes and plants
• Upscaling of the laboratory process
• Consultations, evaluation und feasibility studies to establish plasma methods as a technological alternative
• Patent and literature research on subjects relating to plasma technology

Sample coatings

Do you need a sample coating? We can provide coatings on a laboratory scale on a surface area of up to DIN A3.

We have at our disposal a series of plants to carry out and further develop various plasma-chemical and -physical processes mainly in the low-pressure and subatmospheric pressure plasmas (0.01 to 300 mbar). If appropriate, we also work with plasma processes at atmospheric pressure. Besides commercially available plants (some of them modified), we have plants of our own design. For special specimen geometries and process requirements, we can build suitable reactors and combine them with existing plant components (process gas, flow and pressure controls, vacuum components, high-frequency generators) to make laboratory or test facility plants.

• Vacuum surface treatment equipment
• Surface analytics

Plasma technology is considered a key technology and makes important contributions in a wide range of application fields - from plastics and metal processing, packaging, automotive, electronics, optics and energy technology to medical technology, biotechnology and diagnostics. Plasma processes provide new and better solutions for many materials-related problems. Applications in the field of nanotechnology are also becoming increasingly important.

Plasma treatment of polymeric materials that have been processed into textiles, membranes, films, fleeces, composites, etc. is able to optimize a wide range of interesting surface properties. The following are examples of some of the results from the very large number described in the literature.

Mechanical properties

• Improved softness with consistent tensile strength
  
  • Material: e.g. cotton, other cellulosic materials
  • Treatment: e.g. oxygen plasma
• Reduced felting
  • Material: e.g. wool
  • Treatment: e.g. oxygen plasma
• Crease resistance
  • Material: e.g. wool, cotton, silk
  • Treatment: e.g. dipping in DMSO followed by a nitrogen plasma
    

Electrical properties

• Antistatic treatment
  • Materials: e.g. rayon
  • Treatment: e.g. plasma of chloro(chloromethyl)disilane

Wetting

• Improvement of wetting
  • Materials: e.g. PA, PE, PP, PET, PTFE, etc.
  • Treatment: e.g. O₂, air, NH₃ plasmas
    
• Hydrophilic treatment also serves to repel dirt and to provide an antistatic finish
• Hydrophobic treatment
  • Materials: e.g. cotton, cotton/polyester,
  • Treatment: e.g. siloxane or perfluorocarbon plasmas

Oleophobic treatment

• Materials: e.g. cotton/polyester
  
• Treatment: seeding with perfluoracrylates

Dyeing, printing

• Improved capillarity
  • Materials: e.g. wool, cotton
  • Treatment: e.g. oxygen plasma
• Improved dyeing
  • Material: e.g. polyester
  • Treatment: e.g. SiCl₄ plasma
• Increasing color depth
  
  • Materials: e.g. polyamide
  • Treatment: e.g. Ar plasma
    

Other properties

• Bleaching
  • Materials: e.g. wool
  • Treatment: oxygen plasma
• UV protection
  • Materials: e.g. dyed cotton/polyester
  • Treatment: e.g. HMDSO plasma
• Flame retardancy
  • Materials: e.g. PAN, rayon, cotton
  • Treatment: e.g. phosphorus monomers

• Preferred cell growth on (also biodegradable) fabrics for
  • Cell culture tests
  • Fermentation
  • Implants (blood vessels, skin)

• Prevention of cell growth for
  • Catheters
  • Membranes in fermenters
• Enzyme immobilization

• Sterilization

• Gas separation
  • e.g. oxygen enrichment
• Solution-diffusion membranes
  • e.g. alcohol enrichment
• UF/MF membranes
  • Improvement of selectivity
  • Antifouling equipment
  • Hydrophilization of pore interiors
• Functionalized membranes
  • Affinity membranes
• Charged membranes
  • Bipolar membranes

Frequently, supposedly high acquisition costs are put forward as a counter-argument against plasma technology. But even low-pressure processes are far less expensive than they may have been many decades ago. As a result of the versatile and increasing use of vacuum technology, not only in the semiconductor industry but also in many other branches of industry, the price for the creation of a vacuum has dropped substantially once again in past years. This applies both to the acquisition and provision of the plants and also – as a result of high technical quality and high efficiency – the running costs.

The financial costs of acquisition, installation and operation of a plasma plant are set against the high running costs of wet-chemical processes. Merely dispensing with processes involving various baths – besides the regular exchange and disposal of media there are also high costs for waste disposal – results in savings. In addition, wet-chemical methods can also cause high acquisition and maintenance costs – for often the chemical media have to be constantly monitored during use to check their quality. This requires the appropriate maintenance of the plant technology and sensors.

The outcome of all this is that plasma technology – in spite of substantially higher initial investment costs – on the basis of lower operational costs (e.g. disposal costs for baths) at same or better quality for example in the coating of contact lenses – has in the meantime replaced wet-chemical methods.

Fine-cleaning of metals

Here we can mention as an example the otherwise solvent-intensive fine-cleaning of metals, for which water plasmas are occasionally to be preferred. When cleaning in smaller plants (40 l volume), one mol (18 g water) is sufficient for several cleaning cycles. This is possible because during the discharge, highly reactive particles are produced that attack dirt accordingly. The concentration of aggressive particles is in the meantime much lower than in liquid cleaning agents. This is not detrimental to the cleaning work as in low-pressure gas discharges, the mobility of the particles is higher than in liquids by several orders of magnitude. Also hardly any hazardous waste substances are produced. When the discharge is switched off, the active particles react, for example by recombining. Plasma cleaning in production plants can thus replace solvent-based cleaning methods.

Sterilization of thermolabile plastics

Thermostabile plastics are not suitable for conventional vapor sterilization. Conventional methods of low-temperature sterilization work with toxic or carcinogenic substances such as formaldehyde, ethylene oxide or peroxy acetic acid. With low-pressure plasmas a sterile surface can be obtained just by using a special mixture of oxygen and nitrogen at a power density of several mW/ m².

Chemical activation of plastics

The chemical activation of plastics for the purpose of overcoating or bonding generally requires harsh conditions. Thus chromic-sulfuric acid for ABS-plastics and sodium naphthalenide in tetrahydrofuran for fluorocarbons are used as activators. However, these substances are flammable or toxic and must not be released! These treatment methods can be replaced by the use of various plasma processes.

Also the anti-felting of wool based on chloric compounds can be substituted by a more environmentally compatible plasma treatment. Moreover, plasma technology can also reduce environmental pollution in existing industrial processes by decomposing undesired (e.g. nasty smelling) or noxious waste gases by an appropriate plasma waste gas cleaning stage. This can also be applied to engine exhaust fumes.