Laboratory Assignment #2

520.495/580.495 Microfabrication Laboratory

Laboratory Assignment #2

Objectives:

This week we'll begin the process of fabricating the microelectrode array by first cleaning the wafers and then forming an insulating layer of silicon dioxide on the wafers in a high temperature furnace. 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 of the text.

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.

4. Turn on the water supply to cooling coils of the furnace before turning it on. Turn it off when done and after the temperature has dropped to below 200°C.

I. Prelab Assignment:

1. Why do we need to grow silicon dioxide on the silicon wafers? Why don't we just deposit the metal lines on the silicon itself.

2. Review the safety instructions in dealing with NH₄OH and H₂O₂.

II. Lab Work:

Task #1: Wafer thickness measurement.

The wafers that we will use come with a nominal 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 silicon wafer. This is something you ought to remember throughout the lab sessions.

Task #2: Wafer cleaning.

1. Prepare the "RCA 1" cleaning solution by sequentially adding de-ionized H₂O (600 ml), NH₄OH (120 ml), and H₂O₂ (120 ml), to a 1000 ml beaker. Fill a 2000 ml beaker with 1500 ml de-ionized 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 de-ionized 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. Turn off the nitrogen supply to the dryer when you are finished.

Task #3: Wafer oxidation.

1. Before performing the wet oxidation step, 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) and water is boiling.
the tube from the boiling water flask is indeed connected to the inlet of the oxidation furnace.
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 60 minutes.

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

4. When wafers cooled place carefully in plastic wafer carriers and mark them with your section and names.

III. Postlab Assignment:

1. Note the colors of the wafers before and after oxidation. Can we estimate the thickness of the oxide from the change in color. Explain why? What is your estimate for the thickness of the oxide.

2. Based on the amount of time and oxidation temperature, estimate how much oxide was deposited on the wafers. Justify your answer with a table, graph or calculation.