If you’ve been following along with our blog series, then you may know that etching is one of the major steps in almost any nanofabrication process. In our previous post, we described photolithography as a means of patterning a surface to create areas of a substrate that are covered with the photoresist polymer, known as masked areas, and areas that are open or exposed. After photolithography, an etching agent can be exposed to surfaces to create the desired three-dimensional nanostructures. The etching agent will wear off the unmasked areas while the areas that are covered by the resist will remain protected, this process in realm of nanofabrication is referred to as etching.
Etching is a crucial step used in nanofabrication to remove substrate or excess material that has been previously laid down onto the surface. The word originates from the Dutch “etsen” which means to engrave material surfaces with acids and from the German “ätzen” which means to eat away. Although it is difficult to say who was the first to use etching as a technique, jewelry was decorated this way during the time of Queen Cleopatra (69-30 BC) of Egypt. Centuries later, Swiss artist Urs Graf, became known as an early pioneer of etching, printing from iron plates in 1513. Today, scientists and engineers have applied the technique to create nano-scale patterns that make the many devices that enhance our daily lives.
Within nanofabrication, there are two main types of etching: wet and dry etching. While both techniques only differ in terms of the etching agent used whether this is “liquid-phase” in wet etching or “plasma-phase” in dry etching, both types of etching involve the removal of layers from a masked surface to create three-dimensional transfer of nanostructures.
Wet etching relies purely on chemical reactions, as the substrate is submerged in the liquid etchant and can be sensitive to temperature and concentration of the etchant in the solution. The technique is easily scalable as all that is needed is more chemical etchant to accommodate larger or several substrates. Given that the process is driven by chemical reactions, the wet etchants etch isotopically, or in all directions at equal rates. The resultant profiles then are generally rounded, etching both vertically and horizontally. Unfortunately, this horizontal etching sacrifices some feature resolution, however process engineers at times utilize this isotropic nature to undercut features by design.
Dry etching is the process that uses beams or etchant gases to wear off material. Dry etching techniques are split into three categories: physical dry etching, chemical dry etching, and physical-chemical dry etching. Physical dry etching techniques use the kinetic energy generated by particle beams (ion, electron, and photon beams) to knock out atoms from the substrate’s surface, which subsequently evaporate and are easily removed. Although this method is suitable for almost all surfaces, not being limited to a specific chemical reaction, the reflected ions can wear off nearby surfaces also known as trench effects. Additionally, ions remove mask materials at a high rate, which can limit the depth one can etch with this technique. Secondly, chemical dry etching uses etchant gases to degrade the substrate’s surface. Gaseous by-products are desired since solid by-products can deposit on the surface and protect from further reaction. Physical-chemical dry etching, also known as reactive ion etching (RIE), is a combination of the two methods described previously. Etchant gases are excited into an ionic state and deflected perpendicularly onto the substrate’s surface under low pressure and a magnetic field. As opposed to wet etching, RIE can be more controlled, leading to etch sidewalls that are vertical or anisotropic in nature. Each etch technique results in slightly different sidewall profiles and varying etch rates for different materials.
Speaking of etch rates, what is it? The etch rate is the rate of material removed by an etchant. It is imperative to control the amount of material removed, in order to maintain design specifications of a device process. Without knowing the rate of removal, exposing the etching agent to the substrate for too long will result in wearing off too much material while leaving the etching agent for too short will result in too little material being removed. It’s important to know the etch rates for all the materials in the system, especially for the materials you’re not looking to remove (the masking layers) so that you can estimate the maximum time you can expose the material to the etching agent before damage can occur to these regions.
Selectivity is the ratio of the etch rate of the different materials involved in the reaction (initial material and the film/photoresist being used as the masking layer). Selectivity will determine how deep the etching agent can go in the material. High etch selectivity, when the etch rate of the mask is lower when compared to the film, allows the film exposed to be worn away at a higher rate than the mask that is covering it and this is normally desired.
Considering these parameters, it’s not that surprising that not all materials are suitable to be used in this technique. According to the Center for Nanoscale Systems at Harvard University, silicon, diamond, and category II-V compounds have properties suitable for dry etching, in which case etchant agents can be electron, ion, or photon beams that can dissolve the unmasked surfaces. For wet etching processes, several acids such as hydrofluoric acid, nitric acid, and acetic acid are commonly used as the etchant agent for silicon.
As mentioned previously, directionality of an etch is a critical parameter in selecting which etch technique to use. There are times when a perfectly anisotropic or vertical etch profile is required and times when an undercut of secondary material is desired. In the first case, one would choose a dry technique, either using a beam of ions or a version of the reactive ion etching technique mentioned. More advanced RIE techniques have been developed over the years, such as deep reactive ion etching (DRIE), to yield incredibly high aspect ratio features. In the second case, one might choose to use a purely chemical process to etch, as with a wet etch or a dry chemical process to purposely etch both vertically and horizontally.
Nanofabrication scientists and engineers have a multitude of etching techniques that are available to choose from to produce the most desired etch profile for given material and device requirements.