EE 410 / Saraswat Handout #3 - Stanford Nanofabrication Facility

EE410 / Saraswat Handout #3
4F. Week #3: Lab Section
14. Polysilicon Etch (STEPS 3.2000-3.240)
Postbake: 30 min @ 110°C in the oven
Because the wafers have been potentially been sitting for several days since the lithography was
finished, the resist may have absorbed some water from the ambient air. To drive off any
moisture, we do a 30 minute postbake @ 110°C in the standard post-bake oven.
Plasma Poly Etch:
This etch defines the gates and poly conductors. As was the case with poly lithography, it is very
important that this etch be well controlled to produce uniform gate lengths. With care the EE410
process produces working 2 micron parts—this is where one exercises that care. The Drytek2
can use a He-Ne laser to look at the top wafer in the machine. As the poly is etched, interference
minima and maxima are seen by the laser. This signal can be used for endpoint detection of the
etching process. However, it is more accurate to spot the endpoint visually by watching the
“bulls eye” as the wafer etches.
• Clean Drytek2 with the O2 plasma descum program for 30 min.
• Season with the standard poly etch program for 30 min.
• DRYTEK 100 - poly etch conditions:
Pressure: 150 mT
SF6 flow: 117 sccm
F22 flow: 51 sccm
RF power: 500 W
• Chose an electrode pair on which to etch all wafers. Electrode #3 or #4 is convenient.
• Etch T wafer to calibrate time to fully clear by visual inspection (watch the interference
ring close and disappear). This should take approx. 120 to 150 seconds. Add 25% overetch
and manually stop the etch.
• Time to test-wafer clear: ____________ sec.
• Time with overetch: ____________ sec.
• Electrode number: ____________ sec.
• Set the timer to the chosen etch time and run the rest of the wafers one-at-a-time in the
same electrode. Keep an eye on the flow rates, pressure, power and clearing time. Adjust
the etch time if needed. The typical time to load, etch and unload is 5 minutes per wafer.
• Visual inspection: The wafers should be rust-orange where there is resist and may be light
lavender where they are etched. The lavender is the field oxide and etch-produced
polymers.
• Microscope inspection: Etch should look clean with minimal undercut. Active areas should
be well cleared, although a tint of brownish polymer is acceptable. No poly stringers should
be visible. Check this carefully on each wafer.
Photoresist Removal:
The resist is covered with a tough fluorinated polymer. It comes off in fresh hot piranha, but not
old piranha. New piranha is a must.
• The wafers need to be cleaned using Gasonics, Recipe 013.
Revised Jan. 11, 2008 Page 23/47