Etching is any mechanism that removes 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.
|Equipment||List of Etching equipment|
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 etching to use, it is important to consider two things:
- Selectivity: it refers to the etching rate of the mask versus the etching rate of the material to be removed
- Isotropy: Isotropic etching has the same etching rate in all directions. Anisotropic etching removes with a preferred direction
Most liquid and gas chemical etches have reasonably high selectivity but are typically isotropic, and the etch rate can be challenging 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 preferentially etches the material that you want to remove from the sample.
Wet etching is often chosen for its selectivity. Many wet etches are very selective to different materials. A high selectivity 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 finely tuned 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. The anisotropic etching allows for much finer feature sizes (down to several nanometers, 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 the failure of suspended mechanical devices, e.g., stiction. However, it has the disadvantage that it typically cannot achieve as high selectivity as with wet etching. Another disadvantage is that ion bombardment can cause surface damage.
Figures of merit
The etching rate of a material refers to how fast it is removed when 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 etching for the mask materials or as a ratio of [rate of etching for the primary material]:[rate of etching for a secondary material].
The Critical dimension (CD) in an etching process is typically defined as the smallest feature that can be etched. The CD 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 typically has a larger CD than an anisotropic one; an isotropic etching undercuts the mask resulting in larger features than the initially 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.