This week we will do lithography on the deposited thin film of aluminum to define the electrode patterns .  We will first place the wafers and mask in the mask aligner and expose the desirable pattern. We will then develop the wafers and etch the aluminum in the areas not covered by photoresist. A thin layer of passivating material will then be spin coated on the wafers.

Preliminaries:

1. All the cleaning procedures (except using spin/rinse/dryer) should be done in the hood. Aprons, protective sleeves, gloves, face shield, lab coat, and goggles must be worn during cleaning procedures. Wear plastic disposable gloves at all times.

2. Transfer wafers with tweezers;  try to grasp the wafer at the same place each time, usually at the flat edge.

3. Using the spin/rinse/dryer: Turn on nitrogen gas (50 psi). Test the spin/rinse/dryer to make sure that the door seals after pressing start. Line the wafers in the blue wafer holder so that the wafers are far apart. After rinsing and drying, do not open washer door until wafer holder has automatically turned right side up.

1. In a schematic diagram show today's processing steps -draw a cross-section of the wafer for each step showing the desirable effect.

II. Lab Work:

A.  Photolithography using metal (M) mask

1. Prepare a solution of H₂O (640 ml) and photoresist developer (160 ml) in a 1000 ml beaker. Fill a 2000 ml beaker with 1500 ml of de-ionized water for rinsing the wafers following development.
2. Set wafer on to the chuck of the photo mask aligner, and align the wafer with the mask. Set exposure timer at 15 seconds. Turn away from the aligner during exposure.
3. Develop the photoresist by immersing the wafer in the developer for 15 seconds. (IMPORTANT: Do not over develop. Rinse the wafer by first immersing the wafer in the beaker of DI water, then again under running de-ionized water at the sink. Dry the wafer using the filtered nitrogen gun, and inspect the wafer under the microscope.
4. Post bake the wafer on the hot plate for two minutes at 130°C. Let the wafers cool down for 5 minutes in their carriers but in the laminar flow hood.

B.  Aluminum etching

1. Fill a 90 mm crystallization dish with 50 ml of PAN etch (prepared last week).  Also fill two 2000 ml beakers with deionized water for use as rinse.
2.  Immerse a wafer into the acid in the crystallization dish.  Gently swirl the dish to insure the etching is uniform.  After approximately 10 minutes, the aluminum should clear at one edge of the wafer, revealing the patterns.  Continue to etch until the field is clear across the entire wafer.  Remove from the acid, and immerse in DI water to rinse.
3. Rinse, dry and inspect the wafers.

C.  Photoresist removal

1. Fill a 1000 ml beaker with approximately 700 ml of acetone.  Fill another with 700 ml of isopropanol

2. Immerse the wafers in the acetone for 10 minutes, then for 10 minutes in the isopropanol.  Rinse with deionized water and dry with nitrogen.

D. Aluminum annealing

1. Insure oven is at 450°C, and nitrogen is flowing (100 on flowmeter scale).

2. Load wafers onto quartz carrier.

3. Anneal for 30 minutes.

4. Using the alpha profilometer, measure the thickness of the aluminum layer deposited last week. Make five measurements (one in the middle and four at the corners of the wafer).

III. Postlab Work:

1. Why do we have to anneal the aluminum after deposition?
2. Compare the thickness of the aluminum film measured today with the estimate that you did last week. Discuss any reasons for discrepancy. Discuss your experimental findings about the uniformity of the deposited aluminum film.