The Process of Plasma Etching

The process of dry etching and plasma etching

Etching is the process of removing a material from the surface of another material. There are several methods of plasma treatment, but two main types of etching.

One is wet etching and the second is dry etching, otherwise known as plasma etching or simply plasma etch.

When a chemical or etchant is used to remove a substrate material in the etching process, it is called wet etching. On the other hand, plasma etching uses plasmas or etchant gases for the removal of substrate materials. It is also used to fabricate an integrated circuit or a monolithic integrated circuit.

Plasma etching is a tool that’s universally used for structural etching since 1985. As compared to other etching techniques that go into chip manufacturing, plasma etch was unheard of outside the microelectronic community before 1980.

It was during this time that new processes of etching were being explored and introduced. The relatively high success rate of plasma etching has made it the preferred kind of etching for a wide range of manufacturers.

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What is Plasma Etching or Dry Etching?

Simply put, plasma or dry etching is the etching process performed with plasma instead of the liquid etchant.

The set-up for this is a lot like sputtering. In the process, you do not really have to deposit a layer, but rather, etch the surface of the material at the same time.

The main challenge with plasma etching – or dry etching – lies with producing the right type of plasma that’s somewhere between the electrode and the wafer that has to be etched.

When done right, the wafer will get etched the right way. For the plasma etching to occur, the pressure chamber has to be at a pressure that’s less than 100 pa. Ionization occurs only with a glow charge. The resulted excitation occurs by an external source, which can deliver up to 30 kW, along with frequencies ranging from 50 Hz (DC) to 5 – 10 HZ (Pulsed DC), and a radio and microwave frequency (MHz-GHz).

Types of Dry Etching

The dry etching process can be further divided into two types: with the first being microwave plasma etching, which occurs with an excitation in the frequency of the microwave, which lies between MHz and GHz. The second being hydrogen plasma etching, which is a variation of the plasma etching process that uses gas as plasma. Both processes are currently being used to process semiconducting materials, used in the fabrication of electronics.

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Oxygen Plasma Etching Explained

The process of oxygen plasma etching is carried out by using low-pressure plasma. The addition of oxygen is used as a precursor gas that is channeled into a vacuum chamber with a wafer.

High power radio waves are then applied into the chamber. The radio waves coupled with the pressure in the vacuum chamber results in the ionization of oxygen molecules which, in turn, form plasma. The oxygen plasma then etches the photoresist by turning it into ash.

To ensure that the surface remains clear of foreign objects, the ash is then removed with the help of a high-pressure vacuum pump.

This is also one of the reasons why oxygen plasma etching is usually referred to as “ashing”.

Plasma Etching Advantages

It has been found that plasma etching can lead to significant improvements in the quality of the fabrication of integrated circuits. The following are some of the benefits of using plasma etching:

  • Unlike acid etchants, a plasma etchant is an excellent cleaner and can remove any unwanted organic residues from metal surfaces.
  • Plasma etching can stick two surfaces much better when compared to other etchants.
  • Plasma etching is considered to be less risky than traditional acid etching.
  • The use of plasma cleaning improves the physical properties of the etched material.
  • Plasma etching improves the chemical and physical properties of metals.
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How does plasma treatment work?

Plasma consists of electrons, molecules or neutral gas atoms, positive ions, UV light along with excited gas molecules and atoms and carries a high amount of internal energy. When all these molecules, ions and atoms come together and interact with a particular surface, plasma treatment is initiated.

A plasma treatment is usually performed in a chamber or enclosure that is evacuated, but can be done atmospherically. The air within the chamber or enclosure is pumped out prior to letting gas in. The gas then flows in the enclosure at a low pressure. This is done before any electrical energy is applied.

The effects of plasma treatment upon any surface can be precisely tuned by selecting a special gas mixture, pressure, or power.

What are the main effects of Plasma treatment?

The major effects of a Plasma treatment are:The major effects of a Plasma treatment are:

  • Plasma cleaning: Treatment with plasma removes any foreign contaminants present on the surface of a material leaving an ultra-clean surface and making it more suitable for further processing.
  • Surface activation: A plasma treatment increases the surface energy of low-energy surfaces, which improves their wettability and adhesive properties.
  • Surface characteristics: Other surface characteristics, such as liquid repellency or low friction, can be introduced, if required.

What is plasma cleaning used for?

Plasma treatment is used to modify the surface of an object or plasma cleaning of the object.

Many solid materials have a low surface energy, which results in a poor wettability and poor adhesive properties. A plasma treatment can be used to prepare these materials for further processing by increasing their surface energy while simultaneously removing contaminations from the production process. This increases the quality and lifespan of any coating or printing applied to the surface and improves its adhesive properties.

A plasma modification only modifies the surface of an object and does not change the object in any other way.

Conclusion

Because of its many advantages, it’s easy to see how dry etching and plasma etch is going to remain an important technique for etching microsystems and integrated circuits for many years to come.

Furthermore, for many applications, using capacitively coupled RF plasma will be the best option. For other more specific applications, especially where a high aspect ratio is needed, using low-pressure plasma can provide a better and more efficient solution.

ECR plasmas have some limitations when it comes to being used for large substrates, but they can be the ideal choice for small samples.

On the other hand, inductively coupled plasma systems that use a planar coil and extra bias at the substrate holder have proven to be extremely versatile.

They have been able to deliver excellent results when it comes to manufacturing or integrated circuits and microsystems.

FAQ for process of plasma etching

Why increase the surface energy of an object?

When you glue, print, paint or bond a surface, this surface meets a liquid. If the molecules of this liquid are more attracted to each other than to the surface, the liquid does not wet the entire surface evenly but instead, it forms beads. This leads to a poor adhesion. 

For a proper bond to exist between a liquid and a substrate surface, the substrate’s surface energy should exceed the liquid’s tension by about 2-10 mN/m. By plasma treatment, you can adjust the surface’s characteristics exactly to your demands, and the bond between the materials will be stronger and more durable.

How does a plasma treatment increase surface energy?

Plasma can interact with different materials in various ways. When a plasma is put in contact with a polymer surface, for instance, it breaks the existing molecular bonds and creates polar functional groups on the surface, which increases the surface energy, improves the material’s wettability and its adhesive properties.

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