580.495/520.495 MICROFABRICATION LAB

LAB SESSION 4

AN IC PIEZORESISTIVE PRESSURE SENSOR FOR BIOMEDICAL INSTRUMENTATION

This week, the piezoresistors are formed by diffusing boron through openings

etched in the silicon dioxide. In the first step, the photoresist patterned last week serves as a mask to protect the silicon dioxide from etching when the wafer is immersed in a buffered

hydrofluoric acid solution.

I. Etch Diffusion Openings in Oxide

1. Make up buffered oxide etch in a 1000 ml plastic beaker. Weigh out 296 g NH₄F, and add to 425 ml of deionized water with stirring. Then add to this mixture 106 ml HF. Fill a 2000 ml beaker with deionized water for rinsing.

2. Carefully pour 700 ml of buffered oxide etch into a 1000 ml plastic beaker. Fill a 2000 ml beaker with deionized water for rinsing.

3. Load wafers into carrier, and immerse into etchant, with gentle agitation, for 5 minutes. Lift carrier out of etch and observe the solution running off the back side of the wafer. If the etch has removed the unprotected silicon dioxide, the etchant will not wet the exposed silicon regions.

4. Inspect wafers under microscope.

II. Cleaning

1. Fill a 1000 ml beaker with 750 ml acetone. Fill a second 1000 ml beaker with 750 ml of isopropanol. Fill a 2000 ml beaker with 1500 ml of deionized water.

2. Load wafers into carrier, and immerse into acetone. Gently agitate for a few minutes until the photoresist is removed. Transfer wafers into the isopropanol and rinse for 5 minutes. Finally, transfer the wafers into the deionized water and rinse.

3. Dry a wafer using the filtered nitrogen gun, and use the profilometer to measure the thickness of the oxide layer. Record this for future reference.

4. Transfer the acetone and isopropanol to waste bottles. Clean the beakers in detergent and rinse thoroughly in hot tap water, then deionized water.

5a. Prepare the "RCA 1" cleaning solution by sequentially adding deionized H₂O (600 ml), NH₄OH (120 ml), and H₂O₂ (120 ml), to a 1000 ml beaker.

5b. Fill a 1000 ml plastic beaker with 50:1 DI H₂0:HF solution. Fill two 2000 ml beaker with 1500 ml deionized H₂O for rinsing the wafers following cleaning.

6. Load the wafers into white carrier, and immerse carrier in the cleaning solution for 15 minutes.

7. Carefully remove the carrier from the cleaning solution and rinse the wafers in the deionized H₂O for 5 minutes with gentle agitation.

8. Dip the carrier into the dilute HF solution for 15 seconds, then transfer to the second beaker of rinse water.

9. Transfer the wafers from the rinse water to the white wafer carrier. (This should be done quickly to prevent the wafers from drying.) Load the carrier into the spin/rinse/dryer. Rinse for 120 seconds, and dry for 300 seconds.

III. Boron Diffusion

1. Spin Boron-A dopant onto wafers. Using 1 ml plastic droppers to dispense 1 ml Boron onto wafer. This should be done as quickly as possible to minimize evaporation. Spin for 60 seconds at 3000 RPM with acceleration at the 2 o'clock position, deceleration at 3 o'clock position. Transfer wafers into quartz boat. (It is normal for wafers to have a streaky appearance.)

2. Disconnect nitrogen from the spin/rinse/dryer and hook up the flowmeter to the furnace. Flush the furnace tube with nitrogen (flow set to 100 on the flow meter).

3. Place wafers into furnace at 1100 C for 60 minutes with nitrogen flow (turn flowmeter down to 20 after wafers are in.)

4. After 60 minutes, turn off N₂ and introduce steam into furnace (wet oxidation). Wafers should remain in furnace for an additional 40 minutes.

5. Remove wafers from furnace, place into storage containers when cool.