520/580.495 MIcrofabrication Laboratory

580.495/520.495 MICROFABRICATION LABORATORY

LAB SESSION #5

During last week's session, we diffused boron into the n-type silicon wafer to p-type piezoresistors. The doping step was followed by a wet oxidation to form an insulating layer of silicon dioxide on the wafer surface. This week, we'll etch openings in the oxide on the back of the wafer, then anisotropically etch the exposed silicon to form the membrane under the piezoresistors.

I. Cleaning

1. Dip wafers into 50:1 HF for 15 seconds, then rinse in deionized water. Rinse and dry in the spin/rinse/dryer.

2. Load the wafers in the quartz boat and place in the Blue-M oven at 200°C for 30 minutes. (Boat must be placed diagonally on the rack of the oven so that it doesn't capsize! Use narrow green teflon tweezers to grip front bar of the boat.)

II. Photolithography

1. Adjust the temperature of the oven to 150°C. Prepare developer in 1000 ml beaker, 160 ml of KTI 809 developer and 640 ml of water.

2. Spin photoresist on the front side of the wafer, 4000 rpm for 60 seconds, acceleration and deceleration set at 2 o'clock. The front side of the wafer is the one that we have formed the pizoresistors. Return to quartz boat.

3. As this is simply a protective layer, we do not need to prebake, expose or develop it. Bake in the oven at 150°C for 30 minutes.

4. Spin photoresist on the back side of the wafer, 3000 rpm for 60 seconds.

5. Prebake in the Blue-M oven 90°C for 15 minutes.

6. Align wafers to "NO" mask. Align the flat of the wafer to the outer edge of the rectangular hole in the mask closest to you, and the right edge of the wafer should be tangent to the outer edge of the other alignment rectangle.

7. Expose for 17 seconds. Develop for 18 seconds, or until the openings rectangles clear.

8. Postbake for 60 seconds at 130° on the hotplate.

III. Oxide etching

1. Etch wafer in buffered HF with gentle agitation until oxide is removed at diaphragm locations. This should take 4 to 5 minutes. After 4 minutes, lift wafers out of etchant every 30 seconds and observe if etchant no longer wets, indicating etching is complete.

2. Strip photoresist by immersing the carrier in acetone. Use a wash bottle of acetone to rinse the wafers when removing them from the beaker.

IV. Anisotropic etching

The next step in the process will be the anisotropic etching of the silicon membranes. This will be done in two groups. One group will use KOH and the other EDP. Further instructions on this will be given in the lab.