LAB SESSION 3

AN IC PIEZORESISTIVE PRESSURE SENSOR

FOR BIOMEDICAL INSTRUMENTATION

This week we'll begin the process of fabricating the piezoresistive pressure sensor by first cleaning the wafers, forming an insulating layer of silicon dioxide on the wafers in a high temperature furnace, then using photoresist to define the regions where the strain-sensing piezoresistors will be formed. The cleaning procedures are described in Section 2.1.1 of the text. Since we're not making transistors and have a limited amount of time, we are only doing the first step of the cleaning process, which removes organic contaminants such as grease and oils from the wafer surface. The oxidation of silicon is covered in Chapter 3, and photolithography in Chapter 2.

The wafers that we will use come with a nominan thickness between 350 and 400 micron. To find out the exact thickness of your wafer, use the micro-caliper in the lab. Exercise great caution in doing this as it does not take much to break a wafer. This is something you ought to remember throughout the lab sessions.

I. Cleaning

1. 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. Fill a 2000 ml beaker with 1500 ml deionized H₂O for rinsing the wafers following cleaning.

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

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

4. Transfer the wafers from the rinse water to the blue 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.

II. Oxidation

1. Before oxidizing, make-sure that:

the furnace is at 1100° and all 3 sections in the furnace are stable

the water temperature inside heating mantle is near 100°C (rheostat set at 20)

the oxygen is turned on and the float set at 20

2. Transfer wafers from the blue carrier into the quartz wafer "boat". Remove the end cap of the furnace and slide the boat into the furnace with the quartz push rod. (Don't touch any part of the rod that will go in the furnace so as not to introduce any contamination.) To prevent the wafers from breaking due to a rapid temperature change, push the boat in slowly (approx 5 inches every minute for 5 minutes, use red tape marks on the floor). Oxidize for an 60 minutes.

3. Remove the wafers from the furnace with the quartz push rod (again over 5 minutes time) and set wafers under hood until completely cooled.

III. Photolithography

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 deionized water for rinsing the wafers following development.

3. Using a transfer pipette, carefully withdraw approximately 1 ml of positive photoresist (OCG 820 27CS) from the bottle into the pipette. Again, to minimize contamination, do not touch any part of syringe that is going into the photoresist bottle, and use a new pipette each time.

4. Dispense photoresist on to the wafer slowly, and try not to create any air bubbles.

5. Double check vacuum and cover the spinner. Now start spinning for 60 seconds at 4000 R.P.M. Don't forget to hit reset before each spinning.

6. Prebake wafers on hot plate at approximately 110° for 45 seconds.

7. 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.

8. Develop the photoresist by immersing the wafer in the developer for 15 seconds. (IMPORTANT: Do not overdevelop. Rinse the wafer by first immersing the wafer in the beaker of DI water, then again under running deionized water at the sink. Dry the wafer using the filtered nitrogen gun, and inspect the wafer under the microscope.

9. Post bake the wafer on the hot plate for two minutes at 130°C. Store the wafers until next week in the wafer carrier.

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.