Published on August 7th, 2015 | by ryankapsar1
Working For A Chipmaker: Part II
In this series we take a look at what it’s like to work in the semiconductor industry. These articles reflect the opinion and experiences of the author and do not reflect the opinions of KBMOD.
In my last article I talked about some of the manufacturing capacities of modern Chip Fabs. I also talked about one of the major groups responsible for making a wafer. That group was Photo, where the design of a layer is transferred via laser onto the photo resist residing on top of a wafer, thus preparing the wafer for Etching. The great thing about Photo, from a defect perspective, is that you can clear the developed photo resist off the wafer and start over. This is awesome because if a particle gets on the chuck that the wafer rests on while the laser is developing the photo resist, there will be a “hot spot” of out of focus resist, which will basically kill any chip that has the hotspot.
When a wafer finishes in Photo, it moves on to a group called Dry Etch, or Etch. This group creates an electromagnetic plasma which then bombards the wafer with particles. The areas where the photo resist remains block the particles while the developed areas are open for particles to hit the wafer and material is blasted away. This is a very similar process to power washing where a layer of material is removed at a consistent rate, but because the pressure isn’t high enough the power washer cannot cut the wood or cement you are cleaning. On the other hand Laser etching can and does. Check out this example Gif of laser etching.
The way this works is there is a mixture of chemicals that are pumped in through the “Gas Inlet,” as seen in the picture, which maintains the plasma as well as strikes the wafer. Once the particles react at the surface of the wafer, particles are ejected and are measured at a detector (not shown in image). This gas flow is carefully designed by the Process Engineers to create a consistent etch rate. This ensures uniform etching across the wafer and an expected etch completion time. Each layer being etched has a clear chemical composition. Whenever this composition begins to change, because you’re moving into a different layer, the detector notes this change and. based on rules designed by the Process Engineer, stops flowing gas into the system and removes the wafer from the tool. These wafers are extremely hot and must be cooled by Helium, which is a reason why Helium balloons are so expensive now. If the He system fails, the wafer is ruined and the tool will have to go down for repair and there will be arcing, effectively lightning within the tool, going from the chamber wall to the wafer. click here for an image of it.
At this point, whatever was done to the wafer is permanent, so any defects from Photo are etched into the wafer. If any particles flaked from the Etch Reactor and fell onto the wafer during the etch process, there will likely be “etch bridges” where the etching process was blocked by the particle.
The next step in the process is called Wet Etch, Wets, or Regen, depending on the company. I’m going to call it Wets since this is how we typically referred to the group at Samsung Austin Semiconductor. This group is probably the most terrifying group in the Fab. While Photo might have resist that causes mild headaches periodically and potentially causing sterility if you inhale the material in an enclosed space, in Wet Etch you’re dealing with acids that will literally kill you. Furthermore, these tools are much more open access where you are able to see what’s happening through the whole process and unless you “interlock” (lock the tool from doing anything), you could put your head into the tool and have a robot swing by and hit you.
In some cases you are dealing with fairly high molarity Hydrfluoric Acid (HF), in others you are dealing with HydroChloric Acid or a range of others, to remove material from the wafer. HF is dangerous because you don’t feel it eating away your skin so you have to be EXTREMELY careful whenever you work in Wets. However, loading wafers into the tool is safe and you are not at risk of being exposed to the material unless you open the tool while it is running. The fortunate thing about Wets is that you can simply change the molarity of the HF by adding more water to the mixture and running the result through a filter and it’s good to drink.
With the level of danger in having this material around, why is it used in Semiconductor manufacturing? It eats through other materials, too. Since it’s a liquid it has unique properties compared to dry etch that allows it to etch down deep holes that need to be flared out. For example, capacitors are extremely deep holes and because of the aspect ratio of the circumference of the top of the capacitor and the length (think about a half yard of beer and that is similar in aspect ratios needed for a capacitor) HF is needed to reach the bottom to flare it out. Wets is also extremely important immediately after the Etch process because it removes any remaining photo resist and particles from the etching process.
Similar to Dry Etch, Wet Etch really needs a lot of chemical engineers designing the processes and reaction between the chemical baths and the wafer. This is to prevent over etching and destroying the wafer and to ensure the proper level of etching. The best part is that some of these acids don’t react with come chemical mixtures so there are cases where it’s OK to just leave the wafer in the wet bench without causing any issues. Wet Etch is one of the quieter groups, because the group doesn’t cause many defects and really only cause problems when rust falls into the wet bench.
Both of these groups are vital to the success of a fab and each has their strengths and weaknesses. Both groups are kept alive by their techs and need mechanical engineers to help maintain the equipment. While most groups are served best by engineers, it’s possible for Chemists to work in both of these groups as they also have a strong understanding of chemical reactions.
In my next post I’ll talk about the groups the build deposit material onto the wafers filling in the trenches that Etch cuts.