EE 410 / Saraswat Handout #3 - Stanford Nanofabrication Facility

EE410 / Saraswat Handout #3
N-Source/Drain Implant:
Species: Arsenic
Energy: 180 keV
Dose: 5 1015 cm-2
Beam Current: < 40 µAmp
Implant Angle: 7°
Photoresist Removal and Clean:
The implant hardens the resist so that piranha alone cannot clear the wafers. Tests have shown
that even extended times in fresh piranha are not sufficient. Therefore the resist MUST be ashed.
• Process using Standard Hard Resist Strip.
P Blanket Implant:
The active silicon and poly not implanted with arsenic must be doped with boron. Since the
arsenic went down at 5 15, this dose of 1 x 1015 will not reverse the arsenic, but is enough to
make good p-type contacts and okay poly.
Species: BF2
Energy: 80 KeV
Dose: 1 1015 cm-2
Beam Current: no limit
Implant Angle: 7°
4H. Week #4: Lab Section
17. LTO Deposition (STEPS 4.280-4.300)
LTO (Glass) Deposition:
This step deposits and flows PSG, while annealing the n+ and p+ implants. While higher
temperatures could better flow the PSG, they would over-drive the boron. IMPORTANT: Do
these steps without interruption. A diff-clean between PSG deposition and flow causes reentrant
step profiles and severely impairs the metal step coverage.
• Process wafers through Standard Hard Resist Strip.
• Use tylabbpsg
• Program "LTOPSG40" in Tylan.
• Process summary:
o Slow push at 400°C.
o Pump down to base pressure and perform a leak check by closing the gate valve
and making sure that the system pressure doesn't rise above 100 mT.
o Re-pump and flow 115 sccm of O2.
o Start SiH4 flow of 100 sccm for enough time to deposit 50 nm of undoped SiO2.
o • Begin the doped deposition by flowing 14 sccm of SiH4 and 86 sccm of
PH3/SiH4. The PH3/SiH4 is a mixture of 15% PH3 in SiH4 which is designed to
reduce the hazard of handling pure PH3. A leak of PH3/SiH4 will burn which is
less hazardous from a toxic standpoint than a leak of unreacted PH3.
o Purge and backfill with N2 to atmospheric pressure.
Revised Jan. 11, 2008 Page 25/47