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 Tech Talk
|Other Names||Dry etching|
|Equipment||List of plasma etching equipment|
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.
The P5000 is a 3 chamber tool designed for production etching. Chambers A and B are configured for SiO2 and Si3N4 etching, where as chamber C is configured for polysilicon and amorphous silicon etching. Chambers B and C are restricted to CMOS clean devices where as chamber A is open to semi-clean devices.
STS APS DGRIE
LAM 9400 SE
The LAM 9400 SE is an ICP etcher configured with a wide range of gas chemistries. It is mainly used to etch polysilicon but can also etch SiO2, Si3N4, compound semiconductors, some metals, and organic materials.
Oxford Plasmalab System 100
The oxford is another ICP etcher that also has a cryogenic chuck. By cooling the sample down to -150°C nearly vertical etches are possible in certain materials.
The Plasmatherm is configured with a variety of gases so that it can etch a wide array of materials. Most recipes tend to have slow etch rates on the order of 200 Å/min which is ideal for very thin films. The tool also has few material restrictions to allow it to process as many things as possible.
Deep reactive ion etching
Deep reactive ion etching (DRIE) is a term used to describe RIE processes that are designed to achieve high (>10:1) aspect ratios or etch depths greater than several microns. One common process is the Bosch process.
The STS Pegasus is a tool that utilizes the Bosch process for high aspect ratio etching of silicon. It uses SF6 for the etch step and C4F8 for passivation. It can achieve etch rates of up to 20 μm/min and aspect ratios up to 50:1.
STS Glass Etcher
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.
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.
Nanoquest II Ion Mill
The Nanoquest II Ion Mill is a 22 cm ion source for milling up to 150 mm wafers. It can etch most materials including metals, ceramics, semiconductors, dielectrics, etc.
Method of operation
Parameters that affect plasma etching characteristics include pressure, gas composition, and, plasma generation method, and generator power.
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.
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).
Complete tool list
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.