Etching is used in micro fabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete. As etching plays an extremely important role is in micro fabrication process there are few very important things which need to be taken care of during the etching process, and those things are as follows
- Etching Rate
While etching rate can be controlled using the etching time, we can optimize the whole process depending on the material which is being used and the method of etching and type of etchant. Selectivity and Isotropy is an important figure of merit which need to be taken care of and on the basis of that we decide whether we will go for dry etching and wet etching. In any etching process it is very important that etching must entirely remove the top layer of a multilayer structure, without damaging the underlying or masking layers.
Depending on the process there are two main types of process of etching, and those are as follows
- Wet Etching
- Dry Etching
Wet Etching is an etching process that utilizes liquid chemicals or etchants to remove materials from the wafer, usually in specific patterns defined by photoresist masks on the wafer. Materials not covered by these masks are ‘etched away’ by the chemicals while those covered by the masks are left almost intact. These masks were deposited on the wafer in an earlier wafer fab step known as ‘lithography.’
A simple wet etching process may just consist of dissolution of the material to be removed in a liquid solvent, without changing the chemical nature of the dissolved material. In general, however, a wet etching process involves one or more chemical reactions that consume the original reactants and produce new species.
A basic wet etching process may be broken down into three basic steps:
- Diffusion of the etchant to the surface for removal
- Reaction between the etchant and the material being removed
- Diffusion of the reaction by-products from the reacted surface.
Advantages of Wet Etching
Advantages of wet etching are as follows
- Wet etching costs very low and it is easy to implement.
- High Etching Rate.
- Good Selectivity for most of the materials.
Disadvantages of Wet Etching
Disadvantages of wet etching are as follows
- This is inadequate for defining feature size < 1 micron.
- Leads to some chemical bad effects.
- Wafer contamination.
The dry etching technology can split in three separate classes called reactive ion etching (RIE), sputter etching, and vapour phase etching.
In RIE, the substrate is placed inside a reactor in which several gases are introduced. Plasma is struck in the gas mixture using an RF power source, breaking the gas molecules into ions. The ions are accelerated towards, and react at, the surface of the material being etched, forming another gaseous material. This is known as the chemical part of reactive ion etching. There is also a physical part which is similar in nature to the sputtering deposition process. If the ions have high enough energy, they can knock atoms out of the material to be etched without a chemical reaction. It is a very complex task to develop dry etches processes that balance chemical and physical etching, since there are many parameters to adjust. By changing the balance it is possible to influence the anisotropy of the etching, since the chemical part is isotropic and the physical part highly anisotropic the combination can form sidewalls that have shapes from rounded to vertical.
Sputter etching is essentially RIE without reactive ions. The systems used are very similar in principle to sputtering deposition systems. The big difference is that substrate is now subjected to the ion bombardment instead of the material target used in sputter deposition.
Vapour phase etching is another dry etching method, which can be done with simpler equipment than what RIE requires. In this process the wafer to be etched is placed inside a chamber, in which one or more gases are introduced. The material to be etched is dissolved at the surface in a chemical reaction with the gas molecules. The two most common vapour phase etching technologies are silicon dioxide etching using hydrogen fluoride (HF) and silicon etching using xenon diflouride (XeF2), both of which are isotropic in nature. Usually, care must be taken in the design of a vapour phase process to not have bi-products form in the chemical reaction that condense on the surface and interfere with the etching process.
Advantages of Dry Etching
- Capable of defining very small feature size, < 100nm.
- Anisotropic etching can be achieved.
- Better purity (contamination free) then wet etching.
Disadvantages of Dry Etching
- High cost and hard to implement.
- Low throughput.
- Poor Selectivity.
- Potential radiation damage.
Deep Reactive Ion Etching (DRIE)
Deep reactive-ion etching (DRIE) is a highly anisotropic etch process used to create deep penetration, steep-sided holes and trenches in wafers/substrates, typically with high aspect ratios. It was developed for micro electro mechanical systems (MEMS), which require these features, but is also used to excavate trenches for high-density capacitors for DRAM and more recently for creating through silicon via (TSVs) in advanced 3D wafer level packaging technology.
There are two main processes for DRIE, and those are as follows
- Cryogenic Process Typical Parallel plate reactive ion etching system
- Bosch Process
In cryogenic-DRIE, the wafer is chilled to −110 °C (163 K). The low temperature slows down the chemical reaction that produces isotropic etching. However, ions continue to bombard upward-facing surfaces and etch them away. This process produces trenches with highly vertical sidewalls. The primary issues with Cryo-DRIE is that the standard masks on substrates crack under the extreme cold, plus etch by-products have a tendency of depositing on the nearest cold surface, i.e. the substrate or electrode.
The Bosch process, named after the German company “Bosch” which patented the process, also known as pulsed or time-multiplexed etching, alternates repeatedly between two modes to achieve nearly vertical structures:
- Standard, nearly isotropic plasma etch. The plasma contains some ions, which attack the wafer from a nearly vertical direction. Sulphar hexafluoride [SF6] is often used for silicon.
- Deposition of a chemically inert passivation layer. (For instance, Octafluorocyclobutane [C4F8] source gas yields a substance similar to Teflon.)
Each phase lasts for several seconds. The passivation layer protects the entire substrate from further chemical attack and prevents further etching. However, during the etching phase, the directional ions that bombard the substrate attack the passivation layer at the bottom of the trench (but not along the sides). They collide with it and sputter it off, exposing the substrate to the chemical etchant.
Applications of DRIE
- In DRAM memory circuits, capacitor trenches may be 10–20 μm deep.
- In MEMS, DRIE is used for anything from a few micrometres to 0.5 mm.
- In irregular chip dicing, DRIE is used with a novel hybrid soft/hard mask to achieve sub-millimetre etching to dice silicon dies into lego-like pieces with irregular shapes.
- In flexible electronics, DRIE is used to make traditional monolithic CMOS devices flexible by reducing the thickness of silicon substrates too few to tens of micrometres.