Silicone Detection on Tape Using XPS Analysis

XPS analysis is a powerful tool for detecting silicone contamination in the outer 10 nm of a surface.  We used it to examine two 3M film tapes with acrylic adhesive on polyimide backing film to see if they were suitable for an application requiring that the surface be free of silicones or any other chemistry involving the element silicon.

The first of these tapes is called Polyimide Film Electrical Tape 1205 with thermosetting acrylic pressure-sensitive adhesive in the 3M Data Sheet dated February 2017.  It can be used at temperatures up to 155°C.

The second tape is called 3M Low Static Polyimide Film Tape 7419 on its Technical Data sheet dated August 2014.  It has an acrylic adhesive and is used for PCB solder masking and other high temperature applications.  The Technical Data sheet says it stays firmly in place during processing and removes cleanly from electronic substrates even after 10 minutes at 260°C.  It says it is a non-silicone adhesive formulation which lessens the potential for silicone contamination.


Silicone detection results of the XPS analysis of the 3M 1205 film tape:

Elemental XPS Spectrum of 3M 1205 Film Tape Adhesive Surface

Starting from the low binding energy end of the spectrum the peaks are Si 2p, Si 2s, C 1s, O 1s, and O Auger (about 975 eV).  The elemental composition of the surface in atomic percent is:  Carbon 82.21 at.%, Oxygen 15.49 at.%, and Silicon 2.30 at.%.  The acrylic adhesive should probably only have carbon and oxygen.  It is not expected to have silicon in it.  So what is the chemistry of the silicon?

Carbon 1s Spectrum of the Surface

The carbon 1s high energy resolution XPS spectrum has the two higher binding energy peaks of nearly the same areas expected of an acrylic adhesive, with the higher binding energy peak having a bit more area due to some acrylic acid.

Oxygen 1s XPS Spectrum of the Acrylic Adhesive Surface

The two oxygen atoms in the acrylic adhesive molecule are seen as expected in the oxygen 1s XPS spectrum.

The Silicon 2p high energy resolution XPS spectrum of the surface of the acrylic adhesive

Upon compensating for the 1.05 downward shift in the binding energy caused by surface charging, the Si 2p binding energy is found to be 102.15 eV, which is that of a very short chain length dimethyl siloxane.  Dimethyl siloxane is the most common type of silicone and the short chain length dimethyl siloxane is the most common type of airborne silicone contamination.  Such contamination causes decreased surface wetting and degrades adhesive bonding strength at interfaces.  To be sure, the 2.3 at.% silicon in the form of PDMS is a bit less than the 3 at.% usually found at the interface of failed adhesive bonds.  The 3M 1205 film tape was found to be entirely unsuitable as a means of testing surfaces for silicone contamination as a contact silicone transfer test material to be returned to our laboratory for XPS analysis to determine whether a surface in a customer’s facility was contaminated with silicone or not.


The results of the XPS analysis of the 3M 17419 film tape:

XPS elemental survey spectrum of the surface of the 3M 7419 film tape

Sad to say, the Si 2p and Si 2s peaks are gigantic in the surface of this non-silicone acrylic adhesive formulation.  The atomic percentage composition is 42.99 at.% carbon, 29.89 at.% oxygen, and 27.12 at.% silicon in the outer 10 nm of the adhesive surface.  This is close to the expected 2:1:1 ratios of carbon:oxygen:silicon expected of a poly (dimethyl siloxane).  It immediately implies that the outer 10 nm of the surface is composed of nothing but poly (dimethyl siloxane) (PDMS).  The high energy resolution XPS spectra of the carbon 1s, oxygen 1s, and silicon 2p peaks confirms this and also explain why the carbon is slightly depressed in the ratio and the oxygen is slightly enhanced in the ratio expected for poly (dimethyl siloxane).

The high energy resolution carbon 1s XPS spectrum of the 3M 7419 adhesive surface

The two higher binding energy components found in an acrylic adhesive are entirely missing, so there is no acrylic adhesive in the outer 10 nm of this adhesive surface.

The high energy resolution XPS spectrum of the oxygen 1s photoelectron peak of the adhesive surface

The double oxygen peak of the acrylic adhesive is also missing.

The XPS high energy resolution silicon 2p photoelectron peak for the surface of the adhesive

After correcting for the 1.17 eV charging shift, the lower binding energy and the main Si 2p peak here has a binding energy of 102.55 eV.  This is characteristic of a mid-chain length PDMS, such as one commonly finds in pressure-sensitive silicone adhesives.  The smaller higher binding energy component is likely due to a rather severely heat-degraded PDMS material which becomes carbon-deficient and enriched in oxygen or somewhat less likely it may be due to a very fine silica powder.  This explains why the elemental ratio is not more exactly 2:1:1 for carbon:oxygen:silicon as one expects for a mid-chain length dimethyl siloxane.

Once again, the effort to find a truly silicon-free and silicone-free adhesive tape proved to be a non-trivial exercise.  Such a tape has been found for use in our tape transfer silicone detection and measurement test kits, but we continue to search for alternatives.

Please contact us with your silicone contamination concerns!