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Etching is a microfabrication process that differs depending on the use of the silicon wafer. For solar cells with too much glare, unwanted material can be etched off the wafer. Or some sidewalls may need to be increased vertically or smoothed out to obtain a desired pattern. Physical incising and chemical baths are the prime way to achieve this. To achieve a desired etching a dry or wet etching must take place. Dry etchings, sometimes referred to as “plasma” or “reactive-ion” etchings remove unwanted material by utilizing reactive chemicals, gases, beam-induction, plasma and physical bombardment. It may also be a combination of physical bombardment and chemical reactions. The most common method of etching is wet etching. This capitalizes on the wafer being immersed in a liquid solution. The solution’s goal is to remove layers of unwanted material from the silicon wafer while the protective layer remains intact. Silicon wafers may be etched isotropically and anisotropically as well. Isotropic etching will have the same etch rate in all directions and anisotropic etches will have different vertical and lateral rates. Silicon wafers can be purchased in various quantities online and some websites will offer prompt feedback on any etching concerns you may have.
Sometimes referred to as Buffered HF or BHF this wet etchant is used for thin films of Silicon and Silicon Dioxide that need to be etched. To achieve this, aluminum fluoride or another buffering agent as well as hydrofluoric acid must be in place. This allows for a repeatable process with consistent results. Buffered Oxide Etching is also photoresist compatible, won’t contaminate or stain silicon, and wont undercut masked oxide. Plasma etching can also be performed for BOE as it’s more precise and the elimination of hydrofluoric acid solutions make it much safer.
To conduct electric signals, metallic connections must be developed on microelectronic devices. To achieve this, silicon wafers will have deposits of copper, gold, or aluminum as these materials are highly conductive. Metals will have to be etched during the silicon wafer process. This can be challenging, as each one requires different chemicals based on its process. However it is relatively safer as it doesn’t require the highest of temperatures. Most commonly, metal etching is performed in a vacuum to ensure hydrogen bubbles are instantly removed so snow and bridging reactions do not occur. A vacuum metal etcher will immediately pump and evacuate any hydrogen bubbles. For gold, a strong oxidiser is required for separating unpaired valence electrons and a complexing agent is needed for suppressing oxidised gold reassembly. Luckily, gold is more commonly used for contacts and protection and it has high chemical stability. Copper is more commonly used however for it’s low cost and high conductivity in microelectronics. A combination of electroplating and a wet-chemical method needs to be used however because it can’t be achieved through plasma dry-etching.
For fabricating silicon nanostructures a wet Potassium Hydroxide wet etching process must be performed. It creates cavities in your silicon wafer and allows for optimal precision. DI water and thermal regulation is used with pH > 12 or another corrosive alkaline compound. Consistency through the nano chemical process will assure precision of the Si wafer etching as the etch rate is limited. However, since this process ensures optimization, it has become one of the most popular methods because it can be automated. This increases the efficiency of the etch and greatly reduces the cost. This also makes it prime for batch etches and is amongst the most safe.
Mixtures of sulfuric acid and hydrogen peroxide are used in the piranha etching solution. This allows silicon wafers to be clean during the process of semiconductor manufacturing. This will ensure organic compounds found on substrates are cleaned and most metals will be oxidized. Photoresist can also be removed. However this process, although very effective, is not used often as it’s very dangerous. It uses highly corrosive and powerful oxidizers so great safety measures must be taken throughout. It should also never be stored and used immediately as it generates toxic gases. Safe disposal is also strongly encouraged.
In semiconductor manufacturing silicon nitride works as a masking material. A thin film of silicon nitride is placed directly on the silicon wafer and phosphoric acid is used to complete the etching. This is done in a nitride bath with a hot phosphoric acid strip at 180c and DI water. A bath of nitride will ensure temperature precision and the deionized water to phosphoric acid ratio will be regulated. Keep in mind that adding water to this solution can increase risk, as too much water at once will cancel the boiling and leave a film of water above the acid. This in turn will cause the acid to boil and an explosive reaction will occur. Only add small amounts of water at a time and make sure your silicon nitride etching environment is accurately controlled.
Never etch with chemicals you’re unsure about as it puts you and others at risk. Contact a certified professional on the matter and ensure you have all your questions about silicon wafers answered before you perform any etching process.