EE 410 / Saraswat Handout #3 - Stanford Nanofabrication Facility

EE410 / Saraswat Handout #3
clean, to remove any residual plasticizers or other contaminants which may adhere to wafers
which have been sitting around for a while. Another variation of the standard resist strip process
for metal-containing wafers may include a 10 minute PRX-127 dip between the gasonics clean
and PRX-1000 dip. PRX-127 is another commercially available, alkaline-based stripper which
has added selectivity for removal of the residual side-wall polymer which accumulates during
plasma metal etch. It is absolutely critical that wafers be perfectly dry (especially if sequential
dips are used) since trace moisture yields a highly alkaline solution which will corrode metal.
When to use: For resist removal from "semiclean" substrates.
How to process:
• Process in gasonics, program 013.
• At wbmetal: 10 minutes in PRX-1000 at 40 C.
• Rinse in the quick dump rinse tank for 6 cycles (about 4 minutes)
• Spin rinse/dry in the Semitool spinner (about 5 minutes)
• Visually inspect to ensure complete resist removal
3A. 4. Standard Pre-Diffusion Clean: wbdiff
Purpose: To decontaminate wafers before furnace processing.
Method: The process used at SNF is a modified "RCA" clean, originally developed at RCA in
the 1960's as a way to decontaminate wafers before furnace or deposition steps. The RCA clean
consists of an RC-1 mixture of ammonia, hydrogen peroxide and water; this removes particles
and surface contaminants by simultaneously mildly oxidizing the silicon and dissolving the
resulting oxide to undercut the contaminant. Although very effective, the process margin is
narrow and can easily lead to silicon pitting and plating of metal ions. This is followed by an
RC-2 mixture consisting of hydrochloric acid, hydrogen peroxide, and water, which is effective
at remove trace metals. Instead of the RC-1, at SNF we use a piranha dip (which mildly oxidizes
the silicon surface) followed by an HF dip (which dissolves the resulting oxide), a sequence
which has a broader process window.
When to use: For "clean" before processing in any furnace. Wafers must be processed
through piranha clean at wbnonmetal prior to pre-diffusion cleaning at wbdiff.
How to process:
• At wbdiff, dip for 10 minutes in 4:1 concentrated sulfuric acid: 30% hydrogen peroxide at
90 C.
• Rinse in the quick dump rinse tank for 6 cycles (about 4 minutes)
• Dip for 30 second in 50:1 HF
• Rinse in the quick dump rinse tank for 6 cycles (about 4 minutes)
• Dip for 10 minutes in 5:1:1 water: concentrated hydrochloric acid: 30% hydrogen
peroxide.
• Rinse in the quick dump rinse tank for 6 cycles (about 4 minutes)
Revised Jan. 11, 2008 Page 8/47