Plasma Technology

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Plasma Technology

Plasma is a gas, in which the energy of the atoms is so high, that the electrons are separated from the nucleus. Positive ions and electrons exist both together. Plasma is electrically conductive. In total plasma is not charged, because positive and negative charges are compensated.

Light Emmission

Atoms in a plasma emit electrons and catch them again. It is a kind of equilibrium. When electrons move from a higher energy state to a lower one, photons are emitted and the plasma glows. The wavelength of the emitted photons depends on the elements. Oxygen plasmas glow red or green, while nitrogen plasmas glow blue or violet. It is possible to use the emission spectrum in order to analyse which elements are present in a plasma. This method can be used also for end point detection during a plasma etching process.

Fragmentation of Molecules

Molecules fragment in a plasma and form reactive radicals. Compounds which are not or little reactive can me transferred this way into reactive fragments. This fact can be used in order to synthesize special layers.

Plasma Temperature

Temperature is defined by kinetic energy of particles and because the energy of particles in a plasma is very high, the calculated temperature of a plasma is also very high. However particle density is not so high, if the plasma is generated in a low pressure (vacuum) environment, and therefore a plasma can still be used for gentle and low temperature removal or deposition of layers. This processes are called plasma etching or plasma enhanced chemical vapor deposition PECVD.

It is also possible to generate plasma under atmosheric pressure conditions e.g. in an electric arc between two electrodes. In this case, the high temperature of the plasma can be used for cutting steel or for the complete fragmentation and decomposition of chemical compounds in an exhaust gas cleaner. The high temperature is especially useful for the decomposition of very stable chloro-fluoro-hydrocarbons.

Plasma Generation

Plasma can be formed by the insertion of much energy. This is technically done by coupling of electromagnetive waves, using a plasma generator, working at 13,56 MHz. In semiconductor technology, coupling of the energy is done either via two electrodes in a parallel plate reactor, or by external plasma generation in an inductive coupled plasma ICP-unit or by using microwaves ECR.

Plasma and plasma reactors

Reactive Ion Etching RIE

If the ions from a plasma are accelerated towards a substrate via an electrical field, then these ions can react with substrate atoms and cause etching or deposition of layers. A material removal process can be anisotropic, because of the mono-directional particle movement. This process is called plasma etching or dry etching (in comparison with wet etching, using chemicals in an etching bath). Depending on the used gas, different materials (e.g. nitrid or oxide) etch with different etching speed. Using oxygen plasma, also ashing of organic layers or fotoresists can be done.

Inductive Coupled Plasma ICP

In conventional RIE the plasma density is limited by the method of coupling RF energy into the plasma. This limits the rates at which certain materials can be etched or deposited. This problem becomes more serious at reduced pressures, where the plasma density can become pretty low. The utilization of an inductively coupled plasma source allows higher plasma densities when the power is transferred into the bulk plasma via the magnetic field resultant from inductively coupling. Processing at lower pressure has some significant benefits. It allows increased anisotropy and higher aspect ratio structures can be reached.

Microwave source ECR

The external plasma can also be generated by microwave excitation. In this case, a Magnetron source is used for the generation of microwaves at 2,45 GHz and a magnet is used to deflect the electrons generated by the inserted energy in a circular path. If the microwave frequency and the magnetic flow are adjusted correctly, electron cyclotron resonance (ECR) can be reached. This method results in a spirally trajectory of the electrons and continuous increase of kinetic energy. The number of crashes between particles and therefore plasma density increases. The main advantages of this technology compared to ICP generated plasma is the higher plasma density and the low self bias and charging of the substrate. The result is a very effective and damage free plasma, which can be used very well for plasma ashing of photo resist, where anisotropy and high aspect ratios are not so important.

Layer deposition from a plasma

As mentioned before already, the acceleration of ionized molecule fragments from a plasma towards a substrate can also result in the deposition of special layers. This methode is called plasma enhanced chemical vapor deposition PECVD. In semiconductor technology silicon nitride layers can be formed from silane and ammonia
3 SiH4    +     4 NH3    →    Si3N4     +    24 H2
or silicon oxide layers can be generated from TEOS (Tetraethoxysilan)
Si(OC2H5)4    →     SiO2    +    decomposition products
as well as graphene or carbo-nanotubes (cnt), which can be prepared from plasma with injection of hydrocarbon gases like methane.

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