Backside Etch Preparation
1. All SOIs have been frontside-etched. You need to do litho ONLY for the SOIs to prepare for next week lab.
2. The order of the process should be like this
PART I (Estimated time: 2.5 hours)
- Put the SOI wafers in YES Oven.
- Coat 7 um of SPR 220 on the frontside of the wafers (check which program you need - no Edge Bead Removal), use the appropriate svgcoat bake program for this resist (check which program you need). Make sure you check all the "events" in the programs.
- Put the wafers in 90C oven (it's next to the 110C oven) for 1 hour (stand the cassette up on its end, like the svgcaot or svgdev, so that the wafer is horizontal, resist side facing up).
- Then run the wafers in svgcoat again. This time we'll be coating the backside, so make sure you have the right orientation of the wafers in the upstream cassette. Coat 10 um of SPR 200 resist (check which program you need), do NOT use the coater prebake program (skip it by setting the bake program to #9!). Note: If you prebake the wafer after coating, you will burn the resist on the frontside.
- Put the wafers in 90C oven for 30 minutes (wafer horizontal, newly-coated side faces up).
- Take the wafers out and put them in the box. Before you do part II, wait at least 2 hours. If you have time for 24 hours, it is recommended to wait for a full day. If not, at least 2 hours. Please store your box inside the litho room, on top of the set of drawers where the staff bin (F98) is located. NEVER take them outside of the litho room before you finish part II.
PART II (Estimated time: 2 hours, but a bit trickier since you're using "backside alignment" for the first time)
- Next is to expose the wafers using Karlsuss. Make sure you have the reservation for Karlsuss (2 hours should be enough). Please get the mask called "Backside Etch" from bin F98. Use the backside alignment instruction sheet available either on top of Karlsuss 1 or Karlsuss 2 or somewhere nearby (you can also find the instruction sheet here). This process is a bit tricky and it needs a few extra steps. However, as long as you follow the instructions, you should be fine. Last year, nobody made any mistakes, so we're sure you can do this too. Remember to orient your wafer correctly. We're patterning the backside of the wafer now, so when you put the wafer into the machine, the backside should be facing upwards. Check the parameters: Exposure time should be around 14s. Gap is 40 um and it should be in HARD contact mode.
- Develop. Choose the program for 7um SPR 220 , although you're actually developing 10um resist on the backside of your wafer. Make sure you put the wafers in the correct orientation. We want to develop the backside, so it should face up. SKIP THE BAKING step on the developer! Why? Because you have resist on both sides. You don't want to burn your frontside resist by baking it on the svgdev bake plate.
- Check for alignment. This is hard to do since you can't really see both front side-backside of your wafers at the same time. Don't worry too much. As long as you do litho correctly, it should be fine. Just make sure you
don't make obvious mistakes like patterning the backside upside down, etc. You can flip the wafer back and forth to see if everything looks alright.
- Postbake in the 110C oven for 45 minutes.
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Backside Etch and Final Release
After backside etch of the wafers, the wafers are very fragile. Because of this, we have to change how we would normally handle the wafers at the wetbenches. Be very careful handling the wafers. This lab assignment shouldn't take more than 1.5 hours.
- When etching in 6:1 BOE (Check the logbook, make sure the bath is 6:1, and not 20:1), dip the wafers briefly in the dump rinser water to wet the surface, then put it in the BOE, dunking it a few times to make sure that any air bubbles that get trapped in the trenches are removed.
- Do NOT overtech in BOE (Etch for 6 minutes only). You could use the wetbench timer.
- To rinse the wafers, do either one of the following methods:
a. Put the cassette in the dump rinser, but instead of hitting the start button like normal, hit the "low flow" button. This will allow the water to recirculate gently around the wafer. Leave the cassette in the bath for about 10 min, dunking the cassette up and down in the bath every once in awhile.
b. Put the cassette in the dump rinser, but don't run anything! Just leave it there for 1 minute, take it out, refresh the water (dump and fill it up again), put the cassette back in the dump rinser, leave it there for 1 minute... do this cycle 6 times.
- Do NOT dry the cassette in the spin rinse dryer. Instead, take some cleanroom wipes and place your wafer on it to first dry one side. Turn the wafer over to dry the other.
- Once all visible droplets are off the surface (and trenches) of the wafer, put the wafer in the 110C post bake over for about 20-30 min to make sure the wafer is dry before putting resist stripping in the PRX127. Wafers need to be DRY before they get in PRX127.
- Resist strip as usual in PRX127 and rinse and dry the wafers the same way as above (#3,4,5)
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