Removing Contaminants From Si Wafers Using an O 2 Plasma Ross Robinson Aaron Jackson

Methodology Use Ellipsometry to measure apparent optical thickness. Modeled as thick Si with layer of SiO 2 of unknown thickness. Ellipsometry can not quantitatively differentiate between SiO 2 and contaminants

A known evil is better A monolayer of polydiallyldimethyl- ammonium chloride (DADMAC) is applied. Well known polymer. Self assembles a monolayer Ellipsometry typically reported 8 Å increase in apparent thickness

2 Plasma Sources Used “LFE” Simple to use Gas flow controlled by needle valve RF Power controlled by a POT “Matrix” Complicated Microprocessor control Gases regulated by 3 mass flow controllers Digital RF power control Designed to handle wafer in a production environment

LFE Run at 250 W RF power ~ 0.1 Torr Pressure Initial test with 8 fragments of a wafer

“Matrix” System Torr Pressure 250W RF (max 350) 0.75 SCCM O 2 flow No extra heat applied Test with 38 fragments Loaded by placing fragments on top of 4” Si wafer. RF

Conclusions O 2 Plasma can remove all the polymer! The matrix system works much faster than anticipated. Contaminants accumulate very rapidly.

For Further Investigation Is it really SiO 2 that builds up? Further tests with short exposures. Can the rate be slowed by reducing the O 2 partial pressure? Selectivity?

Putting It All Together Wafer cleaned with Opticlean to remove bulk contaminants Å residue left on surface 1:20 in O 2 plasma Plasma removed Å