|Equipment||List of Etching equipment|
Etching is any mechanism which removes a material from the surface of a sample or from the sample substrate itself. Typically, the material is masked from the etchant to create the desired pattern. Photoresist is a common masking material, but some situations require a more durable mask, such as silicon dioxide or some metals.
Etching technologies are often grouped into categories based on the phase of the reactant. Wet or chemical etching uses a liquid reactant, gaseous etching uses a gas or vaporized reactant, and plasma etching uses a plasma generated with gaseous reactants. Within these categories, there are several types of etching. When determining what type of etch to use, it is important to consider two things:
- Selectivity: will the etch erode the mask or attack the layer below the material to be etched
- Isotropy: Isotropic etches will etch in all directions, and anisotropic etches will etch directionally
Most liquid and gas chemical etches have reasonably high selectivity but are typically isotropic and the etch rate can be difficult to control. Plasma etching is very controllable and can be tuned to be very anisotropic, but it is often more challenging to obtain high selectivity.
Wet etching is subjecting the sample to a liquid chemical that will preferentially etch the material that you want to removed from the sample.
Wet etching is often chosen for its selectivity. Many wet etches are very selective to different materials. This allows thinner masks, and the ability to stop etching abruptly on the layer beneath the layer being etched. The disadvantage of most wet etches is that they tend to be isotropic, and therefore undercut the mask, making features larger than the mask. There are some materials such as Si that can be anisotropically etched.
Plasma etching involves loading the sample into a vacuum chamber which is then injected with a reactive gas mixture that is ignited using a high power source. The resulting plasma reacts chemically and/or physically with the sample to remove the desired material.
Plasma etching has several advantages over wet etching. In particular, the process can be tuned very finely using several different parameters. In many cases, this allows for an anisotropic etch, which is difficult or impossible to achieve with most liquid-based etches. This allows for much finer feature sizes (down to several nm, limited mainly by the lithography used to define the mask) and much higher aspect ratios (in many cases > 10:1). Additionally, it does not require the sample to be immersed in any liquid, which can cause failure of suspended mechanical devices, e.g. stiction. However, it has the disadvantage that it typically cannot achieve as high selectivities as with wet etching. Another disadvantage is that ion bombardment can cause surface damage.
Figures of merit
This is how fast a material is removed from the surface exposed to the etchant. It is typically expressed as a ratio of length to time (e.g. Å/min) but is occasionally expressed in terms of cycles. Etch rate may be dependent on the geometry, such as feature size or exposed area. In some cases, the etch may progress faster in one direction than another, often due to crystal orientation (see Isotropy).
Selectivity is either expressed as a rate of etch for other materials, or as a ratio of (rate of etch for primary material):(rate of etch of secondary material).
Critical dimension (CD) in etching is typically defined as the smallest feature that can be etched using a given process. This can be limited by the ability of the reactant to reach the surface within a mask opening or by the amount of undercut. For instance, an isotropic process will typically have a larger CD than an anisotropic one, as the etch will undercut the mask resulting in a larger feature than was originally patterned.
The aspect ratio of a feature is the height or depth of the feature divided by the width. Most etching processes have a limit to the aspect ratio achievable by the process.
The etch profile refers to the shape of the cavities that are produced during the etching process. For a dry etching process, it is common that recipes are tuned to produce vertical walls. The Bosch dry etching process typically produces ripples (or scallops) on the walls. The wet etching process is typically isotropic and produces rounded cavities.