The cleaning power of plasma

The biggest issue in cleanrooms is particles landing on surfaces. Mike Hopkins, Impedans CEO, explains how the electric charge in plasma can be used to remove or reduce particles and even eliminate biological contamination on surfaces and equipment

Advanced production sites need to avoid contamination due to particles, and hence, there is a need for rooms where particles are reduced to a very low level so that production processes are not contaminated. Particles can be inert, chemical or biological.

The size of the particles can vary from the thickness of a human hair at 50 microns down to the size of a piece of DNA; several thousand times smaller. Chemically active compounds can alter the nature of a material or pollute highly pure products. Naturally, chemical and biologically active particles are a problem in medical devices.

The obvious solution is to strip the air in the room of its normal contents using highly efficient air filters. The process can include a method to kill biological components. In principle, this should leave the environment clean and avoid contamination of equipment, processes and products. But, equipment, processes and products generate particles, thus the filtering system needs to recirculate the air at a rate that continually removes newly created particles.

Particles in the air are not the problem; the real issue occurs when particles land on surfaces.

It might be assumed that the surface density of particles is directly related to the particle density in the air. This, however, ignores the role played by electrical charge.

Electrical forces

Many materials in cleanrooms and product substrates found in cleanroom production sites are electrical insulators such as plastics, glass, and silicon. These surfaces can hold electrical charges for a long time. Due to the electrical insulation of the surfaces, it takes electrical charges a long time to drain away to earth.

This electrical behaviour makes the removal and control of particles, especially on the most sensitive surfaces, a critical problem in cleanroom management

Particles are attracted to the charged surfaces, and there are strong interactive bonding forces between the particles and the surfaces. These attractive forces determine how quickly particles arrive at a surface and how easily the surface can be cleaned.

The forces can be due to static charges, but also due to ionic charges at the surface. Static charges are due to electrons in the air or positive or negative ions being created by rubbing a surface. These are called static because they are still and not moving or flowing as would happen if an electrical current were flowing.

Charges can also appear due to ionic bonds at the surfaces of materials. These ionic charges will determine if a surface is wettable, hydrophilic or hydrophobic.

This electrical behaviour makes the removal and control of particles, especially on the most sensitive surfaces, a critical problem in cleanroom management. Here, however, is where plasma can play an important role.

The making of plasma

Plasma is formed when an electrical current is passed through a gas. The electrical field causes any free electrons in the gas to accelerate and these hot electrons knock-off further electrons from the gas atoms leaving behind positive- charged ions.

The cloud of free electrons created is accelerated in the electrical field and causes a current of electrons to flow out of the gas and leave behind the positive ions; the environment charges up to control the further flow of electrons.

This space charge is the principal mechanism behind the formation of a plasma where charged particles, ions and electrons begin to behave collectively.

The plasma also has high densities of ions and radicals, which attack biological entities and reduce them to harmless gas products. Indeed, plasma can sterilise the air

Once the space charge appears, the plasma density can grow rapidly, filling up the room with charged particles. The ions are usually at room temperature, but the electrons see the applied electrical field and appear hot, often close to a 100k Celsius, but because the electrons are light and only make up << .01% of the gas, it can appear cool. However, the electrons control what the chemistry does, hence the gas chemistry is that of a very hot gas. The corona is where the electron density is not quite high enough to create a full space charge, therefore it is much less dense than a full plasma.

The first use of plasma, and the lower density corona, is in charging the air particles so that they can be removed electrically. There are many free electrons in the plasma; any particle will be hit by electrons and charge negatively, a positive electrode will then collect these particles. Plasma can filter much smaller particles than mechanical filters. The plasma also has high densities of ions and radicals, which attack biological entities and reduce them to harmless gas products. Indeed, plasma can sterilise the air.

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A drawback is that the plasma environment is ideal for particle creation. The high electrical fields used to produce the plasma can also sputter materials. Ions striking the electrodes have high energy and they knock off tiny pieces of the electrode.

Plasma also charges up particles, which make them attractive to those with a natural positive ionic charge and they come together. This process is called agglomeration. Tiny particles formed in the plasma can grow in size to become larger and often produce more killer particles for some applications. Careful design of the plasma is needed to minimise plasma-produced particles becoming a serious issue.

Particles can be driven by either electrical force or by temperature gradients. In plasma, a cold surface will attract particles no matter what their charge. Gravity is the dominant force in many situations, but with small particles, the mass is very small and gravity has little impact on their motion.

Cleaning prowess

The second significant contribution from plasma is the cleaning of surfaces and equipment. Hydrocarbon contamination is a common problem in many situations, where polymers can form and affect product or process performance. Removing hydrocarbons can be problematic because polymers bond with the surface and are difficult to remove.

Plasma cleaning tools have developed from plasma ashers, aggressively removing all organic material from a surface

Plasma can be used to clean surfaces very effectively. It produces electron and ion bombardment of the surface, which breaks up the surface bonds. The presence of free radicals in the plasma produces very reactive surface reactions creating volatile by-products that outgas and are pumped away leaving the surface free of carbonaceous material.

Plasma cleaning tools have developed from plasma ashers, aggressively removing all organic material from a surface.

Primary cleaning systems require the surface to be placed inside the plasma chamber; an argon or argon with oxygen plasma is formed where plasma particles, including oxygen radicals, attack polymers on the surface. To be useful, the system requires the equipment, or product, to be disassembled and placed in the reactor.

These primary systems do not address carbon polymers forming inside surfaces of the cleanroom equipment. To address this, scientists are developing products based on secondary or downstream plasma cleaning. Such systems are already routinely deployed in electron microscopes and other sensitive metrology equipment.

The secondary systems consist of a chamber in which a plasma is formed and generates ions and free radicals that flow into the instrument chamber attacking and removing layers, such as polymers, from the walls and other exposed surfaces. Plasma-based cleaning of SEM microscopes is now a recommended procedure for many demanding applications in nanotechnology.

A key parameter to monitor in plasma is the impedance,
to inform the user if a plasma has deviated from normal operation

Measurement and monitoring

Engineering plasma to remove or reduce particles, and eliminate biological contamination, is now possible. But, plasma can be a source of particles if they are not monitored and controlled. A growing area of research is in better measuring the plasma so that a better understanding of the underlying processes will lead to better- engineered systems that make plasma ever more effective in cleaning processes.

A key parameter to monitor in plasma is the plasma impedance and this can be done in real-time. An expert system can monitor changes in impedance and inform the user if a plasma has deviated from normal operation.

Expert systems are being developed by several companies and will enable online, in situ plasma monitoring for better control of cleanrooms, including particles production, volatile organic compounds, odour and infection control.

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