520.495/580.495 Microfabrication Laboratory

Laboratory Assignment #8

This week we'll finish up the processing of our wafers. The first step will be to etch the aluminum which is not protected by the photoresist layer we patterned following the evaporation step last week. The resist will be removed by soaking the wafers in acetone. Finally, the wafers will be annealed in the furnace to insure a low resistance electrical contact between the aluminum.

Prelab:

Design test methods to characterize electrically the sheet resistance of the aluminium wires that we have deposited, the sheet resistance of the p-diffusion and the sheet resistance of the contacts. Given the processing steps that we have followed derive first order theoretical predictions for the sheet resistances.

Laboratory work:

I. 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.

II. 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.

III. Annealing

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

2. Load wafers onto quartz carrier.

3. Anneal for 30 minutes.

IV. Probing and Electrical Characterization

  1. Use a digital multimeter to probe the large resistors around the periphery of the wafer. Based on your answer, what do you estimate the sheet resistance of the boron diffusion to be?
  2. Measure the sheet resistance of the Aluminium wires
  3. Measure the sheet resistance of the contacts between Aluminium and the p-diffusion regions
  4. Make a wafer map of such sheet resistance measurements.

Postlab:

Compare the theoretical numbers for sheet resistance with the measured ones and comment.