520.495/580.495 Microfabrication Laboratory

Laboratory Assignment #5

Objectives:

I) Deposit thin-film insulating material (polysiloxane) using a spin-coat process and cure it.

II) Spin-coat polysiloxane with negative photoresist KTFR.

NOTE: We will do just TWO student wafers per section  (saving the others for the individual final projects).

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.

I. Prelab Work:

None; grade credit is given to postlab work.

II. Lab Work:

A. 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.

B. Polysiloxane deposition

  1. Set the photoresist spinner  rotational speed at 3000  RPM (Revolutions Per Minute) with acceleration in approximately 5 second and the timer to 20 second for spinning.
  2. Make sure that the vacuum is on after centering the wafer on the chuck. You can do that by gently trying to push the wafer off the chuck using the tweezers.
  3. Using a transfer pipette, carefully withdraw approximately 2 ml of polysiloxane (PS220) from the bottle into the pipette. Again, to minimize contamination, do not touch any part of syringe that is going into the polysiloxane bottle, and use a new pipette each time.
  4. Dispense polysiloxane on to the wafer slowly, and try not to create any air bubbles.
  5. Make sure that the spinner is flat and the the solution does not run off the wafer.
  6. Double check that vacuum is on and cover the spinner. Now start spinning (20 seconds at 2500 RPM) Don't forget to hit reset before each spinning. The wafer will be coated with the polysiloxane but you will see some streaks due to the solids suspended in the solvent.

C. Polysiloxane curing

  1. Insure oven is at 300°C, and nitrogen is flowing (100 on flowmeter scale).
  2. Cure the wafers for 30 minutes.
  3. Carefully, get the wafer out of the furnace, and let them cool down (about five minutes).

D. Photoresist deposition

  1. Set the photoresist spinner  rotational speed at 2500  RPM (Revolutions Per Minute) with acceleration in approximately 5 second and the timer to 20 seconds for spinning.
  2. Make sure that the vacuum is on after centering the wafer on the chuck. You can do that by gently trying to push the wafer off the chuck using the tweezers.
  3. Using a transfer pipette, carefully withdraw approximately 1 ml of NEGATIVE KTFR photoresist  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 that vacuum is on and cover the spinner. Now start spinning (20 seconds at 2500 RPM ) Don't forget to hit reset before each spinning.
  6. Prebake wafer at approximately 90°C  for 120 minutes in the Blue M oven. This "soft" prebake -as opposed to the hot plate- is more desirable when doing the photolithography of the small feature polysiloxane cuts at the electrode sites.
  7. Carefully get the wafer out of the Blue M oven, let them cool down for 5 minutes and place them in their carriage to be stored (in a dark place) until next week.

III. Postlab Work:

  1. What is polysiloxane? Identify manufacturers of the material (Hint: use the WWW to search for it). Is there an alternative to the PS220 polysiloxane material that we have used in the lab? If you think so please explain why.
  2. Why did we use the Negative photoresist today?
  3. How do we "etch" cured polysiloxane? Why is it a good passivating material?