As for etch... there are two main types, wet and dry. The general purpose of an etch step is the controlled selective removal of material from the wafer. Etch can be used for planarization, thickness control, complete material removal, and feature definition.
Let's start with some basic terminology. In almost any etch step there is what is referred to as a masking layer. This is a layer of material that has a different chemical/physical composition than the material you wish to etch and therefore will not react to the chemistry being used in the same manner. This can be any material, the most common of which are: photoresist, silicon nitride, silicon dioxide, and various metals (W, Ti, TiN, Al, Cu). An important term you'll hear about in regards to etch is isotropy. This describes how directional the etch is. An isotropic etch will etch in all directions at an equal rate, an anisotropic etch will etch much faster in one direction than the others. Selectivity, another term, is a ratio of the etch rates (distance/time) of two different materials. Often we speak of the selectivity of the film we wish to etch and the underlying or masking layer. General rule of thumb is you need a minimum of 10:1 selectivity for a process to work, 100:1 is desired.
Wet etch is the oldest and simpler of the techniques. Traditionally the wafers are processed in a batch process of 2 lots at a time (50 8" wafers). In the 300mm generation some wet processes are moving to single wafer processing for increased control and flexibility. In the batch process the wafers are placed by robot (or human) into a bath of chemical. The processes are almost exclusively timed, which means an etch rate is determined and the wafers are left in the chemical for the time necessary to remove the desired thickness. The chemical used is chosen for its selectivity to remove the desired film. Cleaning chemistry often include an oxidizing agent such as hydrogen peroxide to oxidize the surface and then a reduction agent to remove the newly oxidized surface. This allows the cleans to work on a variety of materials.
Common wet etchants include:
Phosphoric Acid (H3PO4) ? Used for removal of Silicon Nitride (Si3N4)
Hydrofluoric Acid (HF) ? Used for removal of Silicon Dioxide (SiO2)
Potassium Hydroxide (KOH) ? Used for removal of Silicon
Sulfuric Acid (H2SO4) ? Used for the removal of organic materials such as photoresist and organic anti-reflective coatings
SC1/APM (Ammonium-Peroxide-Water mixture) ? Used to remove organic contaminants in clean steps
SC2/HPM (Hydrochloric acid-Peroxide-Water mixture) ? Used to remove metallic contaminants in clean steps
Al Etch (phosphoric, nitric, and acetic acids) ? Used to etch aluminum, acetic and nitric are also used to etch chrome for photomasks
Dry etch is the newer and more advanced technique. Referred to by a variety of names you might see it called plasma etch or RIE (reactive ion etch). Most dry etch processes are single wafer performed in a chamber platform system. If you?ve never seen a chamber system before the basic description is a common load port where a cassette of wafers is placed. Directly behind the load port is a common robotic transfer stage/platform which takes wafers off the cassette and places them inside of individual process chambers located around the transfer stage. These are called cluster tools. In each chamber the wafer is placed on a chuck. The entire chamber is evacuated to a vacuum state so the chuck used to hold the wafer is often electrostatic since obviously you can?t use a vacuum to hold a wafer in place within a vacuum. A large potential difference using an RF power source is used in the presence of reactive and inert gases. At sufficient potential and under vacuum these gases will be excited into the plasma state of matter. This plasma ionizes the gases by stripping off electrons.
By negatively biasing the chuck of the wafer these positive ions can be induced to accelerate towards the wafer surface. Large inert ions such as argon serve to physically remove material from the surface. This is primarily a physical process and is very anisotropic. This allows for deep narrow trenches and contact holes (vias) to be formed. The reactive ions such as fluorine will react in a combination of chemical/physical manner. Chemically they will combine with the surface material to form volatile species which can be evacuated through the vacuum. This process is more isotropic in nature but much more selective than the physical bombardment. The lower the vacuum reached the higher the mean free path of the ions which allows more of the ions to reach the surface and increases your etch rate. It will also increase your anisotropy by reducing ion collisions above the wafer surface. Likewise the flow rate of your gases will determine the amount of ions readily available and change your etch rate.
Temperature also plays a role both in the chemical reactions occurring at the surface as well as eliminating flaking from your chamber walls. Remember that all the material you are removing has to go somewhere, when it?s physically sputtered off the wafer it gets deposited either on the chamber walls or falls back onto the wafer itself creating defects. Eventually the material on the walls of the chamber will start to delaminate and flake off onto your wafers leading to large yield problems. It?s for this reason that every chamber is taken down after so many processing hours and wet cleaned by technicians.
A common use in fabs of dry etch is for the removal of photoresist, called plasma strip. In that process the reactive ion used is oxygen. The basics of dry etch in conjunction with the right process conditions and gases can be used to strip a variety of materials and also perform dry cleans to remove residual material left on wafers after process steps. The gases used for each process are much more varied than those of wet etch and particular to each company/process so I can?t tell you particular ones.
The main advantages of dry etch is lower usage of raw materials compared to wet etch and increased control. As the ions in the plasma react with the material on the wafer different spectra of light are given off depending on the material being etched. Using this phenomena we can implement endpoint detection where the intensity of a wavelength of interest can be used as a trigger to end the etch process. Alternatively using knowledge gained by incoming measurements a timed etch can be performed.
That?s my brain dump on the subject, hope some of it is new or helpful. I should point out i've never worked in Etch professionaly, so most of my knowledge is from working with engineers in those departments and my educational research background.
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