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Using Crystalbond in Plasma Etchers

Crystalbond wash-away adhesive is used to temporarily bond wafers or pieces of wafers to a carrier wafer for etching. Materials are attached while the Crystalbond is heated and detaching is achieved by heating the samples. Samples may then be soaked or sprayed with acetone to remove any residual Crystalbond. It has good thermal properties making it a good bonding material for etchers with back side gas cooling. Please Note; Crystalbond may only be used with full wafers. It should not be exposed to the plasma.

Purpose:

To bond full wafers to a carrier wafers for plasma etching.

 

Equipment:

Mortal and pestle, headway2, hotplates.

 

Time of Execution:

  • Set up time = 30 mins
  • Per wafer = 15-20 mins
  • Cure time = 30 mins

 

Step Summary:

  1. Mix up Crystalbond.
  2. Coat carrier wafer.
  3. Evaporate acetone.
  4. Heat carrier wafer.
  5. Align device wafer onto heated carrier wafer.
  6. Cool wafer stack.
  7. Check for voids.
  8. Ready to etch!

 

Detailed Procedure:

 

Before etching:

 

1. Crush Crystalbond 509 into powder (in wafer saw room at the bench) and mix it with acetone into a solution 80 parts acetone to 20 parts Crystalbond by weight. This solution can be stored in a closed bottle for later use as well.  (Let's help with the conversion; Acetone density = 0.791 g/ml at 25C => 80g = 101ml.  The mixing ratio is 20g CrystalBond tp 101ml acetone).  Put the ground (powdery) CrystalBond in a glass container before adding acetone.  Use the rolling mixer to dissolve the mixture.

2.  Use headway2 at 1500 rpm to spin coat it on your carrier wafer. Dispense it onto the wafer, slowly rotating the chuck by hand so that it covers the whole wafer. Surface tension prevents that it goes over the edge of the wafer. After spin you should see a nice coating similar in appearance to resist.

3. Wait 5 min after spin so that acetone can evaporate. You can see a slight color change in the film.

4.  Heat the wafer on hotplate 90°C for one min to evaporate acetone and preheat the wafer.

5.  Heat the wafer on second hotplate at 125°C for 3 min.  This will melt the Crystalbond.

6.  Place your device wafer on top and gentle push in the center first then at the edges of the wafer stack. You can slide around the wafer because CB is liquid at that temperature. Be sure to line up the flats of the wafers.  You can use a weight, but it is not necessary.  Wait for 4 min to allow good contact.

7.  Remove wafer stack from hotplate and wait 30 min before further processing.

8.  Check under IR camera for air pockets.

 

After etching:

 

1.  Before separating perform your polymer removal step in O2 plasma.

2.  Wafers can be soaked in acetone to separate them, but this will take a long time (typically overnight.) 

3.  Alternatively, heat the stack on a hotplate at 150C.

               - Use tweezers to gently separate vertically.

               - Or slide the top wafer right off the bottom. 

4.  Clean thoroughly in acetone. 

5.  Clean as usual in piranha.

 

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