520.495/580.495 Microfabrication Laboratory

Laboratory Assignment #7

This week we'll metallize our wafers. This will involve, first, opening contact cuts in the silicon dioxide to expose the p-type silicon of the piezoresistors, then removing the photoresist used as a mask for the etching step. Next, an aluminum film will be evaporated over the entire wafer. Finally, we'll spin photoresist on the fresh aluminum film, and pattern the resist with the connecting lead patterns.

I. Contact cut etching

1. Fill a 1000 ml plastic beaker with the prepared buffered HF solution. Also fill two 2000 ml beakers with deionized water for use as rinse.

2. Load wafers into a carrier and immerse in the BHF for approximately 6 minutes. The oxide on the back side of the wafer should be removed after about 4 minutes, as determined by lifting the wafers out of the etch and noting whether or not the acid wets the surface. (Water will not bare clean silicon.) Why is the oxide thicker on the front side of the wafer?

3. Rinse, dry and inspect the wafers.

II. Cleaning aluminum

1. Prepare PAN etchant in 200 ml beaker. (75 ml phosphoric acid, 3 ml nitric acid, 15 ml acetic acid, 5 ml water).

2. Handling the wire with gloves, clean 0.5 g of aluminum wire by soaking for 5 minutes in ethanol, 5 minutes in PAN etch, then rinse with water, acetone and blow dry with nitrogen. Clip into 1 inch pieces on clean piece of aluminum foil, and cover until loaded into evaporator.

III. Cleaning wafers

1. Prepare "Piranha" cleaning solution in 1000 ml beaker, by adding 600 ml of H2SO4, into 150 ml of H2O2.

2. Prepare "RCA 1" organic cleaning solution in 1000 ml beaker.

3. Remove photoresist in acetone, by immersing for 5 minutes.

4. Clean wafers 10 minutes in "Piranha" etch. Rinse twice in DI.

5. Clean wafers 10 minutes in the RCA organic clean, rinsing twice in DI water.

6. Dip wafers into 50:1 HF for 15 seconds, then rinse in deionized water. Rinse and dry the wafers with N2 gun.

IV. Evaporation

1. Place wafers onto the stage of the evaporator.

2. Load 1/2" pieces of Al wire into the "boat".

2. Pump down evaporator following instructions provided.

3. When a pressure of 8 x 10-7 torr is reached, use the variac to increase the current through the boat. Aluminum should melt, and a coating

V. Photolithography

1. Prepare developer in 1000 ml beaker, 160 ml of KTI 809 developer and 640 ml of water. Prepare DI water rinse in 2000 ml beaker.

2. Spin photoresist on the front side of the wafer, 4000 rpm for 60 seconds, acceleration and deceleration set at 2 o'clock.

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

6. Align wafers to "NM" mask.

  1. Expose for 17 seconds. Develop for 30 seconds. Postbake for 60 seconds at 130° on the hotplate.
  2. Store your wafers in their containers until next week.