Surface Chemistry Modification for PEG
The goal of this project is to determine the viability of PEG as a structural support for large area nano porous membranes. This would enable lift-off vs. the current method of etching through the wafer. The ideal properties are that the structure can withstand an aqueous solution for many days without peeling off or dissolving, it must also be resistant to the BOE used to separate the membrane from the substrate. It should have minimal shrinkage/expansion as this will destroy the porous membranes. It must also be bio-compatible.
Piranha Etch: Used to clean surface, consumes organic matter.
– Sulfuric Acid : Hydrogen Peroxide [4:1] gently agitated for 90 seconds
– rinse in DI water
– stored in Isopropyl Alcohol until use
TPM Treatment: When glass is treated, PEG adheres better due to dangling acrylate groups.
– 5.1mM TPM [3-(Trichlorosilyl)propyl methacrylate] in Toluene ( 100ml Toluene w/ 0.13 ml TPM) for 15 minutes
– Toluene rinse
– air dry
PEG Process: Based off various papers, Dr. Gaborski’s notebook, and Alex’s Experiments.
– Poly (ethylene glycol) diacrylate (PEGDA) : DI water, [1:1] ratio with 3% 2-Hydroxy-2-methylpropionphenone (photosensitizer)
– deposited by pipette, thickness determined by viscosity / thickness after spreading 60 ul across ~1.4 inches squared
– exposed on microscope using 10x magnification for 100ms in dapi using the Field Diaphragm O4 (~ 3mm diameter circle)
– unexposed PEG removed in DI water ~90 seconds
BOE: Required for the crucial lift-off step, PEG must be resistant to removal in BOE to be a viable candidate.
– standard 10:1 BOE w/ surfactant (Water : Hydrofluoric Acid with Ammonium Fluoride)
– two samples of each: Si wafer with native Oxide, and Nitride wafer
– cleaned with Piranha, treated with TPM, then multiple sites exposed with above process
– (control group) one sample of each is placed in water and checked for adhesion integrity (poked with tweezers to see if still attached) periodically for 1 hour
– samples to be tested in BOE were made in Dr. Gaborski’s lab, then transported to the clean room in DI water to prevent drying (~20 minutes)
– (experimental group) the remaining two samples are placed in the BOE and checked in the same manner
Results:
In water, both the Si and SiN samples retained the PEG for the duration of the test (1hour). However, in BOE the PEG structures on the SiN sample began to peel off within 10 seconds and were completely removed within 30 seconds. The PEG structures on the Si adhered better in BOE, but the actual time is unknown since a ring of debris formed around the structures, these made it look like the PEG structure was still there after it had fallen off/dissolved. This happened somewhere between 1-5 minutes (estimated based on slight visual change that was not considered significant at first). The samples were in the BOE for a total of 30 minutes.
Next:
– determine if a longer exposure will help the SiN bonding, try same for Si.
– determine if PEG is attacked by BOE, or if its just the bond ( after the PEG fell of, did not see it in the BOE)
– try different surface chemistry if BOE does not dissolve PEG