EE 410 / Saraswat Handout #3 - Stanford Nanofabrication Facility

EE410 / Saraswat Handout #3
• Postbake wafers in the 110°C oven for 30 min to assure resist has predictable undercut
behavior. Cool in carrier 30 min.
• Descum the resist in Drytek2 for 30 seconds. This is IMPORTANT for good etching since
it makes resist hydrophilic.
• Etch wafers in fresh BOE 6:1 until backsides clear plus 10%.
1. Set 6:1 BOE timer to 420 seconds.
2. Start down counter, at T=415 submerge in DI.
3. At T=400 transfer to 6:1 BOE
4. Agitate every 20 seconds. Begin peeking at T=300 sec.
5. Expect wafer backs to clear (hydrophobic) at T=50 sec.
6. Add 10% (about 40 seconds) after clear.
7. Quickly dump rinse then spin.
• Microscope examination: The etch should look slightly undercut, about 0.5 microns. The
resist should be in good condition.
• Check etched areas with the Nanospec to insure that etching is complete.
4C. Week #2: Preparation
8. Sacrificial Oxidation (STEPS 1.250-1.340)
Photoresist Removal:
• Use the Standard Hard Resist Strip process to remove resist.
• After the resist removal, inspect the wafers to make sure that all of the resist is gone.
Sacrificial Oxidation:
The Sacrificial Oxide protects the active silicon from contamination through the well implant
steps. Old versions of this process used this oxide as the gate oxide; we now regrow a fresh and
clean gate oxide just prior to poly deposition.
• Process wafers through Standard Pre-Diffusion Clean.
• Use tylan1 or tylan 2.
• Program "GATEOX" in Tylan.
• Process summary:
o Slow push at 800°C.
o Ramp to 900°C in 2.4 L/min N2 and 0.1 L/min.O2 (20 min).
o Oxidize: dry O2 (5 min), steam (12 min), dry O2 (5 min).
o Ramp down in N2 (15 min).
o Slow pull.
• The process takes ~1 hr and 30 min of furnace time.
• The target sacrificial oxide thickness is ~40 nm.
Field Oxide Thickness (After Sacrificial Oxidation):
The field oxide will be a bit thicker from the sacrificial oxide grown, but also thinner from a bit
of HF exposure. It’s a good time to check it.
Revised Jan. 11, 2008 Page 18/47