Different types of Plasma Dry Etching process:

• Plasma Etching (PE)
• Reactive Ion Etching (RIE)
• Chemical Downstream Etching (CDE)
• Combination of CDE / RIE

Plasma Etching (PE)

Plasma Processes are for etching and deposition of surfaces. As all these vacuum processes they have been developed on a wide bases during the 1^st half of the 20^th century when sufficient vacuum technology was available. The first applications were for removal of fingerprints from electron microscopically samples where oxygen (O₂) was made reactive. It reached to carbon dioxide (1C + 2O  Þ 1CO₂) with the carbon (C) from the fat of the fingerprints. The next step came already closer to semiconductor manufacturing: the problem of photoresist (PR) removal from silicon wafers was also solved by activated oxygen since PR consists mainly of carbon.

Plasma etching (PE) is a method of removal of surface layers by establishing conditions so that gas can react with this surface layer such that reaction product is volatile and can be pumped away. Plasma etching in the conventional way means to select a configuration with 2 electrodes in a vacuum chamber where the upper electrode is connected to a radio frequency (RF) voltage and the lower electrode is grounded. This will lead to glow discharge as it can be seen in a luminescence tube. This glow discharge forms a shining column of ions (charged particles), radicals (dissociated parts of a molecular such as O₂ Þ O,  O and SF₆ Þ S, F, F, .... F) and electromagnetic radiation with a high short wave portion (UV radiation).

Plasma Etching (PE)

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Reactive Ion Etching (RIE)

Plasma etching, however, can be done in today usual way of reactive ion etching (RIE). The same vacuum chamber can be used, but in this case the upper electrode is grounded and the lower electrode connected to the RF voltage, as it can be seen in the schematic below: -

The dotted lines in both sketches stand for the field lines connecting the two electrodes with the different potentials (~ 500V at the "hot" electrode which is connected to the RF generator and 0V at the grounded electrode). Charged particles are now accelerated in the direction of the field lines towards the wafer (thick line) and cause an isotropic (with one preferred direction) etching. This etching has two components:

• F ions (e.g. dissociated from SF₆) get to the silicon (Si) surface and react according to the formula 1Si + 4F Þ  SiF₄. 
• These F ions are accelerated in the electric field, gain energy and knock Si atoms out of the wafer.

Both PE and RIE processes do an isotropic etching.

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Chemical Downstream Etching (CDE)

Different from PE and RIE processes, CDE does isotropic etching which means equally in all directions. Here the plasma is generated in a radical generator outside the vacuum reaction chamber. Only radicals can get out of this radical generator while the ions remain trapped inside. The reaction of the radicals with the surface is called CDE. If the surface is not protected it will be etched equally. The processes gases are dissociated in the radical generator and distributed equally in the process chamber. This causes equal removal from all surfaces attacked by the radicals.

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Combination of CDE / RIE

A short investigation of the electric conditions in a RIE process chamber shows that 80 - 90% of the applied RF energy are used for molecule dissociation and only 10 - 20% of energy remain for the vertical (an isotropic) acceleration towards the surface. The wafer, however, sees the full voltage (typically ~ 600 V) and is facing the danger of damage by high energetic ions. 

In a combined CDE / RIE process, the process gases get into the process chamber already dissociated. Applying a RF voltage to the electrode (RIE mode) will allow to use the full voltage for ionization. Despite the reduced voltage, we will get more ion current relative to pure RIE. The such generated low energy ions will not cause damage of the crystal lattice.

The ration between isotropic and an isotropic etching can be varied continuously. With full power of the radical generator and without RIE-RF power, the etching will be isotropic. With increasing RIE-RF power and decreasing power of the radical generator, the an isotropic part of the etching will grow until full RIE characteristics will be reached. Such gradient technology is useful for etching of tulip shaped vias, for smoothing of edges and as damage free end of RIE processes.

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Last modified: Oktober 28, 2004