Wet etching

From Home Semiconductor Fabrication
Jump to: navigation, search

Etching Silicon Dioxide[edit]

SiO2 can be wet etched in a home chip lab environment easily with ~2% HF from Whink Rust Stain remover, which can be purchased on amazon. It can take 15+ minutes at times to etch all the way through a field oxide at room temperature, but can be greatly sped up if you heat the etch solution. Also note that the etch solutions have a very short life time due to the low HF content, you will usually see small bubbles in an active HF etch solution during the first few etch cycles and will not be present after that. Then you will notice that the etch rate greatly decreases and then stops, at which point you will need to get more Whink. It should also go without saying that extreme care should be exercised while working with HF, including all PPE, thick gloves, mask, gown, etc. You should also have HF skin treatment cream nearby, such as Calgonate .

From Sam's "Semiconductor Fabrication Basics Background Theory" video: "Last topic is etching, so I mentioned earlier that you can etch SiO2 layers and stuff with HF acid, but this actually leads to something called isotropic etching, the HF solution will etch sideways in unwanted direction as fast as it etches down in the wanted direction, so ideally you want anisotropic etching, but thats not the case with these liquid etching sources. So this is why ion beam etching and milling and plasma etching has come into play, so you can actually get anisotropic etching. This is not a problem where you are making large devices, but as soon as you try to get things smaller and smaller you have to be very careful of your critical dimensions, and you have to allow for this tolerancing, another thing is every time you put your wafer in the furnace, your dopants will laterally diffuse as well, so you have to take into account unwanted lateral diffusion, and isotropic etching when you're designing a device. And you may think "Oh! I can make a .1 micron device!", well... make sure you take this into consideration, because theres a lot of wasted space in chips, especially older chips before plasma etching and stuff was very common, there was a lot of space that was wasted because of isotropic etching and such. A way around that to get a more anisotropic etch, is something like a reactive ion etching machine, so it consists of a vacuum chamber and mass flow controller where we can put a precise amount of reactive gas inside of it. and then we have this platform down here, which is sometimes spinning -not very fast I think- and thats grounded, and then we have an electrode in the top which ionizes the reactive gas into a plasma, and thats high energy RF, normally at 13.56MHz -the specific reason for using that is just because it's ISM band, and the harmonics don't screw with other electronics and stuff- and that RF excites and ignites the plasma, and it allows you to do a very controlled etch that's more anisotropic. This is a reactive ion etching machine, even more anisotropic than this would be an ion milling machine, is even more directional and better."

Etching Silicon Nitride[edit]

Silicon Nitride can be etched with hot concentrated Phosphoric Acid. Read more about this here .

Etching Bulk Silicon (MEMS, etc)[edit]

KOH and TMAH can be used to etch silicon surfaces with reasonable rates. Read more about this here .

Etch Mask[edit]

Commonly, exposed and developed photoresists are used as etch masks to pattern wafers but for a home chip lab, electrical tape or adhesive vinyl sign material can be used as a quick mask to make diodes and transistors. The vinyl sheets can be cut by hand with a knife to make individual transistors or on a laser for more complicated or smaller designs.