The Process of Plasma Etching

Etching is the process of removing a material from the surface of another material. There are two main types of etching. One is wet etching and the second is dry etching, otherwise known as plasma etching. 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 etching 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 manufacturers.

What is Plasma 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 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 Plasma Etching

The plasma 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.

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 improves the physical properties of the etched material.
  • Plasma etching improves the chemical and physical properties of metals.

Conclusion

Because of its many advantages, it’s easy to see how plasma etching 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.