Plasma etching

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Plasma etching
Rieoperation.svg
Technology Details
Other Names Dry etching
Technology Etching
Equipment List of plasma etching equipment

Plasma etching is a form of plasma processing designed to remove material from a sample using plasma discharges. It is highly controllable and can be used to etch a wide variety of materials. The most commonly used form of plasma etching is referred to in the microfabrication world as reactive ion etching (RIE). However, there are other types of plasma etching, including plasma ashing and ion milling. For a detailed overview of plasma etching in the LNF, please review the technology workshop

Method of operation

Parameters

Parameters that affect plasma etching characteristics include pressure, gas composition, and, plasma generation method, and generator power.

Gas composition

Main article: Plasma etching/Gases

The type and ratio of gases used in a plasma etch is chosen depending on the material being etched, the masking material, and the etch stop material in order to achieve high selectivity.

Pressure

Chamber pressure varies depending on the system and material being etched but typically ranges from 5 mTorr to 300 mTorr. Typically, more physical etches and etches designed to be very vertical or to have high aspect ratios run at lower pressures, while more reactive etches will use a higher pressure to increase the density of the reactive gases. Most plasma etching systems control the pressure in the chamber using a throttle valve on the exhaust port, allowing it to be accurately set regardless of the gases chosen.

Plasma source and power

The plasma generation source is critical to the function of the etch. Capacitively and inductively coupled RF plasmas are very common, particularly in RIE, but certain applications may use microwave sources, ECR sources, etc. Additionally, while in RIE the sample is typically placed directly under the source, sometimes the sample may be placed more indirectly from the source, such as in plasma ashing.

Sample temperature

Some plasma etching systems allow for the temperature of the sample to be controlled. For most processes, the sample is kept between 20-50°C and are actively cooled to prevent damage to the mask, which is often photoresist. A liquid nitrogen cooled chuck, such as on the Oxford ICP RIE can be used to perform cryogenic etching. Also, some etch processes (e.g. polymer etching) benefit from a heated chuck (can be up to 200°C).

Reactive ion etching

Main article: Reactive ion etching

Reactive ion etching (RIE) is one of the most common forms of plasma etching. It typically uses a combination of chemically reactive elements and energetic ions to etch the desired material. One major advantage to RIE over other forms of etching is that the process can be designed to be highly anisotropic, allowing for much finer resolution and higher aspect ratios.

Plasma ashing

Main article: Plasma ashing

Plasma ashing typically refers to the removal of organics, particularly photoresist from a sample using a plasma discharge. These processes typically use oxygen as the main etch gas and sometimes require a high temperature to enhance the reactivity.

YES-CV200RFS(E)

The YES-CV200RFS(E) (YES Plasma Stripper) is the primary plasma ashing equipment in the main cleanroom of the LNF. Its primary uses are low-temperature plasma descum and high-temperature stripping of photoresist but can also be used to etch a wide variety of polymers as well as for sample cleaning and surface activation.

Ion milling

Main article: Ion milling

Ion milling uses an ion beam (typically argon) to sputter material from the surface of the sample. It has very low selectivity (typically 1:1) but can be used on any material, even inert metals. The LNF currently does not have any ion milling capabilities.

Equipment

For a complete list of plasma etching equipment available at the LNF, please see list of plasma etching equipment or the specific plasma etching category, above.

See also


Further reading