What Is Etching

Depending on what you’re using your Silicon for, you may need to put it through microfabrication etching after the silicon wafer manufacturing process . You may want to remove unwanted material from your wafer (for example some solar cells may have too much glare) or to create a desired pattern in your wafer (making the sidewalls smoother or more vertical). This can be achieved through either a chemical bath or a physical incising. There are processes to consider when etching depending on the coveted result. Wet etching is the immersion of the wafer in a liquid solution which will remove the layer of unwanted material yet keep the protective layer beneath intact. Dry etching (or depending on the method “plasma” or “reactive-ion” etching) removes materials by using chemically reactive gases, plasma, beam-induction, physical bombardment or a combination of both chemical and physical. Silicon wafer can also be etched isotropically with the same etch rate in all directions or anisotropically with differing lateral and vertical rates. There are five main processes to achieve the necessary etch and each one has the ability to be improved upon. 

KOH Etching

This wet etching process arguably allows for optimal precision when fabrication silicon wafers as it can be automated to increase efficiency and reduce cost. KOH Etching creates cavities in silicon are created using a solution of Potassium Hydroxide. For this etch to be successful a highly corrosive alkaline is used alongside deionized water and regulated thermally. KOH is performed in Teflon tanks which increases flexibility and control. However, this process can be greatly impacted by temperature, KOH percentage, anisotropy, and the presence of atomic defects or miscellaneous impurities in the Silicon wafer itself. It is however one of the safest and easiest methods, and is able to yield high batches with repetitive and fast high quality results. As this process is generally unassailable a tighter control on the etch rate can improve the precision of the Silicon Wafer etching. 

Buffered Oxide Etch

This wet etchant is most effective in etching thin films to remove Silicon dioxide or Silicon nitride. In this process highly concentrated hydrofluoric acid and a buffering agent (commonly ammonium fluoride). On it’s own HF etches silicon incredibly quickly and can peel photoresist, therefore a buffering agent is added to control and exact the process and result. This process is also incredibly user friendly as active regions can be exposed during the beginning or contact holes can be defined at the end. As well, the temperature of the solution can be gradually increased to boost the rate of the etching. Stirring the solution continuously during the process will homogenous it and aid in removing etched material. This process is selective and will not etch further than the silicon wafer itself. The good news is that this process is already incredibly consistent and won’t contaminate your staub silicon when diffused. To improve this process ensure the entirety of it is under strict control. As HF is incredibly acidic it can pose great harm if used incorrectly or haphazardly, so having two or more eyes on the continual stirring and overall safety of the environment is the best approach for improvement. 

Silicon Nitride Etching

Silicon Nitride Etching is one of the most fundamental etching process. For this Silicon Nitride (a compound of silicon and nitrogen) is used as masking material and is the most thermodynamically stable. A thin film of the Si3N4 is placed on the silicon wafer and hot phosphoric is poured onto it to perform the etching process. This is performed in a nitride etch bath with a hot phosphoric acid trip at 180° C. There isn’t much that can be done to improve this process, but controlling the acid concentration and temperature with incremental additions of deionized water will successfully give you the results you desire.

Piranha Etching

An etching solution using hydrogen peroxide and sulfuric acid during the manufacturing of semiconductors is used to clean silicon wafers. For Piranha etching a ratio of 3 parts sulfuric acid and 30% hydrogen peroxide is used. This is done so organic material is removed and unwanted metals are oxidized, and even removes photoresist. To improve on this process the mixture must be ensured to be exact and must be controlled diligently with avid attention. 

Copper, Gold and Aluminium Etching

If any microelectronic devices are developed on silicon wafer then metallic connections must be put in place to conduct electric signals. Therefore in this etching process copper, gold, or aluminum is deposited onto the silicon wafer to be etched. Bubbles of hydrogen can often can often occuring during this etching process so it is implemented in a vacuum to immediately remove them for the wafer’s surface. Each metal will have to use a different mixture to complete the etching but it often a combination of nitric acid and hydrochloric acid and whatever else is conducive to the appropriate metal. Metal etching is an all around challenging process but can be improved upon by using an exact amount of the chemicals related to your chosen method. 

Conclusion

The silicon etching process doesn’t have to be complicated, and there are many websites and reliable resources that can help you navigate equipment and chemicals. All etching processes can be improved upon by proper control of the material you are working with, a clean environment, and close attention to your etchings.