Photolithography Level 3

Mask 3 Photolithography will define the areas of SiO₂ that will be etched to bare silicon.

The areas defined will ultimately become the gate oxide for the MOS devices

Equipment

• Bake out hotplate
• PR syringe
• PR spinner
• Softbake hotplate
• UT 1000 WF stepper
• Developing fume hood
• Microscopes
• Hardbake hotplate

Supplies

• HMDS: 6 drops
• Shipley 1813 positive photoresist (PR): 40-50 drops
• Shipley MF319 developer
• Reticle 1, Layer 1

Operating parameters

• Bake out hotplate: 210° C
• PR spinner: 3000rpm for 30 seconds
• Softbake hotplate: 115° C
• UT 1000 WF stepper: 150 mJ/cm²
• Hardbake hotplate: 115° C

Equipment/controls/tools locations

All equipment is located in rm50J (lithography lab)

• PR spinning hood:
  • hotplates
  • PR syringe
  • HMDS
  • PR spinner
• Developing hood
  • developer
  • rinses

Operating precautions

High temperatures

Use caution when working with the hotplates. They will burn you if you touch them.

Chemical hazard

PR and HMDS are volatile. Keep away from flame. Possible biological hazards: only use in a fume hood.

Developer is caustic. Treat exposure as described in the safety sections.

UV light hazard

Do not look directly into the stepper's light source while exposing. Potential long-term vision problems upon exposure.

Operating procedure

Degrease tweezers and wafer

Cleanliness is extremely important. Tweezers and wafers should (almost) always be degreased at the beginning of a processing session. Appendix B of the paper version describes the degreasing procedure and is posted on the developing hood.

Note: although degreasing is an important step, it must be modified from what is posted in the Vapor degrease instructions. TCA is no longer available for sale, and a suitable substitute is being evaluated. You will only need to degrease using the Acetone, IPA, DI, IPA, N2 dry procedure.

Coat wafer with PR

Before beginning, record the conditions of the PR room and equipment by making entries in your notebook. The wafer surface should be scrupulously clean before beginning this process.

1. Drive any moisture out of your wafer with a 120 second bake on the bakeout hotplate
   • if relative humidity >60% then bakeout 180 seconds
   • check that the spinner is set for a 30 second duration
2. Cool the wafer on the "cool block" for 20 seconds.
   • Wipe off the spinner chuck with a kimwipe while waiting.
3. Center the wafer on the spinner chuck
4. Start the spinner by momentarily pressing the front of the foot switch.
   • check the indicator to make sure it is spinning at 3000rpm
5. While the wafer is spinning, spray it with nitrogen from the N₂ arm.
   • Vacuum is applied to the chuck only while spinning. You may stop the spinner by pressing the back of the foot switch, but don't continue blowing nitrogen on it when it stops. The spinner will automatically stop after the preset time.
6. While the spinner is spinning the wafer, place 6 drops of hexamethyldisilazane (HMDS) onto the center of the wafer and stop the spinner as soon as the appearance of the wafer remains constant. HMDS behaves as a surfactant, a wetting agent. It helps the photoresist adhere better. Think of it much like a detergent. In this case, the organic end (hexamethyl) of the molecule is similar to and binds well to the organic PR. The silazane end, being silicon based, sticks well to the wafer/oxide surfaces (this is a simplistic description of the mechanism - see if you can find the actual mechanism in Grainger). It even seems to help adhesion on aluminum as well.
7. Immediately place 40-50 drops of Shipley 1813 positive resist on and around the center of the wafer using the filtered syringe. A pattern like that below works well for the first 9 drops. Use the rest to fill in any voids between the drops. Use extra drops if necessary to fill all interior dry spots within the PR puddle.
   
   
8. Wait at least 5 seconds after the drops completely flow together, then start the spinner. This time, let the spinner stop by itself.
9. Complete coverage of the wafer is critical to the operation of the steppers. There are targets on the wafer which must not be etched away. If your wafer is not completely covered, ask your TA since there are places which can be etched while still allowing the stepper to perform.
10. Check the uniformity of the resist by looking for a bull’s-eye effect - if you see it, it is not uniform (which is not critical to the ece444 process - what problems can nonuniformity cause?). The thickness of the PR is 1.2 μm, and exhibits thin film interference effects, much like oxide. The color of the film is altered by varying thickness.
11. Bake for 45 seconds on the Softbake hotplate.
12. Allow the wafer to cool for a few seconds on the "cool" plate.

Expose PR coated wafer

The photoresist is now ready for exposure to ultraviolet light through a mask. Refer to the instructions for the mask aligners (Appendix G) and steppers (Appendix H).

After exposure, develop until pattern is sharp (see Development procedure to be posted on developer hood.)

Prepare for etch

Finally, complete the preparation of the PR for use as an etch stop by performing a 60 second hardbake on the hardbake hotplate (the same as the softbake hotplate). Cool the PR for a few seconds on the "cool" plate before putting it in the wafer carrier. The photoresist is now prepared for the etch.