Silicon Wafer Bonding Process
This page contains the basic steps for the successful bonding of silicon wafers.
Cleaning Process for Silicon Wafers
1. Grow wet oxide layer on two clean wafers in bruce furnace- at least 200nm.
2. Using plastic tweezers, gently place wafers into two separate containers of H2SO4/H2O2 solution (Pirahna/Nanostrip). Leave on burner for 20-30 min. @90 C.
3. Carefully remove wafers from nanostrip using plastic tweezers and place into separate wafer holders. Place the wafer holders into a cascade rinser.
4. Cascade rinse the wafers for 5-10 minutes. If you do not have access to a cascade rinser, rinse wafers thoroughly with water.
Preparing the Silicon Wafer for Bonding
1. Pull one wafer holder out, take it to the nearest spray rinser, and thoroughly rinse the oxidized side of the wafer. Place the wafer back into the cascade rinse. If no cascade rinser is available, place the wafer in a container of water.
2. Pull out the second wafer holder and perform the same rinsing process on the oxidized side of the wafer. Place the wafer and holder back into cascade rinser or water container. Then, use plastic tweezers to remove the wafer from the wafer holder. Carefully turn the wafer over. Align the flats of the two wafers, then carefully place the second wafer on top of the first inside of the wafer holder of the first wafer (This must be done underwater to prevent air bubbles between the wafers).
3. Remove the wafers from the water. At this point, the wafers will probably be sliding around on top of each other. Press them together with tweezers and make sure they stay aligned on the edge by maintaining pressure on the wafers with your fingers. As long as you do not touch exposed oxide, using your hands here is fine. Bring the wafers over to the bruce furnace immediately.
4. Place the wet, pressed-together wafers in the quartz wafer boat for the oxidation tube of the bruce furnace. Make sure that they stay aligned when placed into the wafer boat slot. This is important because the wafers can easily become mis-aligned in this step. When you are satisfied with the alignment of the wafers, pull out the rack from the pre-heated furnace, place the boat on the rack, and start the furnace cycle. Make sure to do this step IMMEDIATELY after cleaning, as the bonding will not work as well or at all if the wafers are allowed to dry before starting the annealing process. If the wafers do not fit vertically in a wafer boat, they must be laid on some sort of surface horizontally. Something must also be placed on top of them to both hold them in place and create enough pressure for proper bonding.
Annealing Directions for Wafer Bonding
1. Right after starting furnace cycle, turn on oxygen flow to furnace
2. Starting temperature of the furnace: 850 C
3. Push in wafers at 15% speed for 4 minutes at 850 C
4. Heat furnace up to 1200 C @ 6 degrees/min - Approx 1 hr
5. Keep furnace temperature steady at 1200 C for 3 hours
6. Cool furnace back to 850 C @ 3.3 degrees/min - Approx 2 hours
7. Pull wafers at 15% speed for 4 minutes, then turn off furnace tube heat
8. Allow wafer to cool before testing bond integrity
Testing Silicon Wafer Bond Strength and Integrity
Here are some basic suggestions for verifying that silicon wafers are successfully bonded:
-Dip the bonded wafers in buffered HF for up to an hour to see if they stay bonded
-Cleave the bonded silicon wafer pair with a diamond scribe- You will be able to tell by how easily and how cleanly they cleave together if they have bonded. If they cleave separately, there are voids in the bond.
-Insert a razor edge into the space at the edge of the wafer pair and press until they break apart
-After cleaving, inspect the edges of the bonded area under a microscope to look for voids or unbonded areas
-Additionally, a dicing saw may be used to cut the wafers. After dicing, voids are readily apparent. This may be the most surefire way of checking for bonding. Dicing prevents the awkward angles and shininess that result when cleaving, making it much easier to observe voids.
Notes About the Silicon Wafer Bonding Process
-Try to avoid using tweezers as much as possible- Only necessary for placing and removing wafers from wafer boat for furnace, also for removal of wafer from nanostrip
-Any particles on the wafers WILL create voids in the bonding- Keep the wafers as clean as possible at all times, and move immediately to the next step of the process right after cleaning steps
-Avoid letting anything touch the oxidized surface of the wafer at all costs. When laying the wafer down, always lay the non-oxidized side down. When using the wafer holder, have the oxidized side OUT and keep that side up at all times
-Make sure the bruce furnace is pre-heated by the time the cleaning process is done so that the wet wafers can be placed straight into the furnace.
-Wafers are pushed and pulled at 850 C at the recommendation of Jim Fraser. I do not know if the wafers or the machine would be damaged by pushing and pulling at 1200 C, since I never tried it. However, it would be worth trying, if possible. This would cut a substantial amount of time off of the bonding process, as it would eliminate the heating and cooling steps.
-This process still has room for improvement. Feel free to manipulate steps and see if the bonding works better.
-When the wafers have bonded with this process, expect the edges to not be very-well bonded. After about 1cm around the edge, though, the wafers should bond very well and should cleave almost perfectly.