Cleaning with low-pressure plasma

1. How does cleaning with plasma work?

Plasma cleaning is an important process in plasma surface technology. Through a chemical reaction with the ionized gas, particles of dirt are removed, converted into the gas phase and carried away by the continuous flow of gas via the vacuum pump. The purity levels thus achieved are extremely high.

For copper oxide reduction, copper oxides are exposed to a hydrogen plasma, and thus the oxides are chemically reduced and water is formed, which is sucked away by the pump.

Low-pressure plasma system: cleaning with a low-frequency or high-frequency generator
Low-pressure plasma system: cleaning with a microwave generator

1.1 Plasma cleaning

2. How does plasma function as a cleaning agent?

2.1 Plasma cleaning of metals

Some items for treatment are covered with fats, oils, wax, silicone (not PWIS-free - PWIS=paint-wettingimpairment substances) and other organic and inorganic contamination (also oxide layers).

For certain applications, it may be necessary to achieve absolutely clean and oxide-free surfaces e.g.

  • before sputtering
  • before painting
  • before gluing
  • before printing
  • before PVD and CVD coating
  • for special medical applications
  • in analytical sensors
  • prior to bonding
  • prior to the soldering of printed circuit boards
  • for switches etc.

The plasma acts here in two ways:

1. It removes organic coatings

  • These are chemically attacked by e.g. oxygen and air (see sketches).
  • The impurities partially evaporate due to the low pressure and surface heating.
  • Impurities are converted into smaller, stable molecules by the energetic particles in the plasma and can therefore be extracted by suction.
  • UV radiation can also destroy impurities and cause their detachment from the surface.

The impurities may be only a few microns thick, since the plasma is only capable of removing a few nm/s.

Fats contain lithium compounds for example. Only the organic components can be removed from them. The same applies to fingerprints.

2. Reduction of oxides

  • The metal oxide reacts chemically with the process gas. The process gas is pure hydrogen, or a mixture with argon or nitrogen.

It is also possible to drive the process in two stages. For example, goods to be treated are oxidized for 5 minutes with oxygen; then they are reduced for 5 minutes with argon-hydrogen (e.g. a mixture of 90% argon and 10% hydrogen).

2.2 Plasma cleaning of plastics

Plasma cleaning of plastics is always accompanied by activation of the plastic. If a plastic really needs to be just cleaned and not activated, the process parameters must simply be reduced until the desired effect is achieved. You will need to consider whether merely cleaning a workpiece is sufficient for subsequent processes.

The process gas is usually industrial oxygen, but ambient air is often sufficient. The plasma treatment can be repeated and no toxic fumes are produced.

The principle is the same as for plasma cleaning of metals.

2.3 Plasma cleaning of glass and ceramics

The cleaning of glass and ceramics is carried out in the same manner as the cleaning of metals. Process gases recommended for cleaning glass include argon or oxygen.

In general, cleaning is usually performed with oxygen plasma.

The other parameters (pressure, generator power, gas flow, duration of treatment) may vary depending on the sensitivity of the workpieces to be treated.

3. Is there a measurable weight loss?

Yes, using the homogeneity test the etching rate is determined by the weight loss.

For this purpose, object holders are covered with PE tape and weighed before and after plasma treatment. The difference can then provide information about the etching rate.

Weighing must take place on an analytical balance, because the weight loss is only very small.

4. How can a PWIS-free state be achieved?

Through paint-wetting impairment substances - or PWIS - clearly visible defects appear in the final product, since uniform wetting of the surface to be painted is prevented. Funnel-shaped defects and crater formations occur on the coating layer. Such substances may be silicones, fluorine-containing (PTFE) substances, certain oils and fats.

The plasma method permanently removes all paint-wetting impairment substances from the surface and from the elastomer itself.

Components from a very wide range of materials can be cleaned, such as PVC-U, PVC-C, PP, PE, ABS and PVDF, as well as metallic components.

After cleaning, the parts are treated for up to an hour with the plasma according to the degree of contamination present. To confirm the success of the treatment, and thus that it is PWIS-free, a PWIS test is carried out following plasma treatment in accordance with the Volkswagen inspection standard PV 3.10.7, which provides a rapid method for detecting silicon residues.

All that is needed is a clean glass plate, acetone and a commercially available spray paint, which must of course be free of silicone. The colour white has proven particularly suitable for this. For the test, the material to be tested is placed on the glass plate and washed with acetone. After evaporation of the acetone, the glass plate is sprayed with the spray paint in a cross shape. After the paint has dried, it can be clearly determined whether or not silicone residues are present on the surface. At these points the paint does not wet and forms a so-called crater.

Plasma cleaning can be used with special processes for the treatment of silicone materials. A PWIS-free surface can be achieved, even with silicone rubber.

The removal of PWIS substances from component surfaces to be coated ensures a noticeable problem has been solved through the use of innovative and environmentally conscious low-pressure plasma technology. The benefits of plasma cleaning in the production chain include:

  • The reduction of rework rates
  • The reduction of rejection rates
  • The avoidance of complaints
  • Increased production reliability

Diener electronic also offers these processes as a surface treatment service. Several plasma systems as well as skilled, experienced employees are available for this. Thus, we can ensure an optimum surface quality for your parts and components.

5. Which applications are possible?

For more information, see "Applications".

6. Which generators for LF, RF, MW?

We use generators with the following frequencies as standard:

LFLow frequency40 kHz100 W, 200 W, 300 W, 1000 W, 1500 W, 2500 W
RFRadio frequency13.56 MHz100 W, 300 W, 600 W, 1000 W, ...
MWMicrowave2,45 GHz300 W, 850 W, 1,2 kW, ...

7. What frequency is the best frequency?

This question can not be answered in general terms, but must be decided from case to case. The trend at the moment, however, is towards kHz generators. About 90% of all our customers parts can be treated with kHz machines.

This overview is intended as a decision-making aid:

7.1 Frequency 40 kHz

Inexpensive solutionMore power is required for the same etching rate than at 13.56 MHz
RobustOnly passivated semiconductors can be cleaned before bonding
Due to potential-free coupling, the highest homogeneities of all three
frequencies are achieved
Metal drums can be used easily
RIE operation is possible (high etching rate)
The impedance matching requires the use of no interference-sensitive mechanical components
Efficiency: approx. 90%
Electrodes/product carriers with 10 or more levels are constructed
in order to obtain a very high throughput
Well suited for semiconductor back-end processes
Lower deposition rates for plasma polymerization processes
Generators are easy to repair

7.2 Frequency 13.56 MHz

RIE operation possibleExpensive
Homogeneity is better than at 2.45 GHzRelatively sensitive to interference
Etching rate is higher for the same power than at 40 kHzImpedance matching with mechanically moving parts
Metal rotary drums can be easily usedHF system consists of a generator and matching
Electrodes/product carriers can be constructed, but the balancing
of the electrodes is very laborious
Efficiency approx. 50%
Good for front-end and back-end semiconductor processesHigh wiring costs
High deposition rates for plasma polymerization processesGenerator repairs are expensive

7.3 Frequency 2.45 GHz

InexpensiveHF system consists of power supply and coupling
Relatively robustHigh wiring costs
Highest etch rate at a given powerSilicon wafer must be cooled
Efficiency approx. 60%Glass and ceramic components
ECR operation is possible (high etching rate)Magnetron requires a voltage of 4500 V
Well suited for semiconductor front-end and back-end processesMetal rotary drums can only be used conditionally
Very high deposition rates for plasma polymerization processes Plasma is non-homogeneous due to the small wavelength (12 cm)
Generators are easy to repair