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Using PMMA

This page describes two methods of using PMMA to attach samples to carrier wafers. A Wet Mounting procedure and a Heated Mounting procedure. These procedures are courtesy of labmembers Jim Kruger and Jan Petykiewicz.

 Pro and Cons

 

Pros

 

 

Cons

Wet Mounting

 

From Jan Petykiewicz

I've had success with both 5% and 2% 950K PMMA when mounting for p5000. The procedure is pretty 
straightforward: 

-Start with a clean Si wafer 

-Place a drop of PMMA near the center -- the plastic end of the foam swabs supplied in the litho area works 
well for making a suitably small drop, just wet it and tap once on a hard surface to get rid of excess, 
then tap on the wafer. 

-Carefully place the chip on top of the droplet -- some excess PMMA can accumulate on the sides, but usually 
not much. 

-At this point I usually pick up the wafer, turn it vertically and tap the edge on a wipe to make sure the 
chip doesn't slide around; if the resist layer is thin enough then it stays in place, otherwise it slides 
down slightly, thinning down the PMMA layer. This isn't entirely necessary (and I avoid it when mounting for 
applications where exposed resist is undesirable), but I think a thinner layer helps with thermal contact. 

-Bake at 90C for two minutes to harden the resist.

 

 

 

Heated Mounting

 

 

From Jim Kruger

PMMA mounting

Not good for e=beam resist patterned sample

1. Clean carrier wafer, especially particles that may affect bonding.

2. Spin onto carrier wafer PMMA (495K, 11% in anisole) at 1800 rpm for 1 minute (thickness ~1.6 micron). 
When dispensing, filter PMMA with a syringe through a 1 micron filter for best results. 

3. Bake PMMA-coated carrier wafer at 150C for 1 minute to dry the film, then remove from heat and allow 
to cool.

4. Clean piece of particles with isopropanol soak, rinse and blow dry. 

5. Place piece onto center of PMMA-coated carrier wafer, then return to hot plate at 150C. Allow to heat 
up for 1 minute.

6. Now gently press down on piece to squeeze out and expel air bubbles between the piece and PMMA. Place 
weight on top of piece, using a teflon sheet to protect surface of piece from the weight.

7. Perform patterning and etching. Be wary of spin processes that accelerate the wafer to more than 2000 rpm 
as they may throw off the piece if it is not well centered. Be wary of heating the wafer beyond about 120C as 
PMMA can start flowing above this temperature.

8. To debond, heat up wafer to 400-500C for 10 minutes in air on a hotplate. The piece should slide off the 
wafer easily, leaving no residue. Alternatively, soak in acetone to dissolve the PMMA, but this may take up 
to several days if the piece is larger than about 1"x1".

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