Rules of Polyimide Use
Polyimides are a class of polymers which are versatile, robust, and used in a wide variety of applications. They can be highly resistant to variety of chemicals and temperatures up to 400 C, depending on the formulation. Polyimides are generally used at SNF in either of two forms.
The ubiquitous "Kapton" tape is an adhesive-backed, cured polyimide film. "Kapton" is the DuPont brand version and similar tapes are available under other trade names. Various types of tape are available and selection should depend on your application. In general, the main considerations are usually the temperature and vacuum requirements. Make sure the tape selected is rated for the conditions your substrates will be subjected to. Not only are different polyimide films rates for different temperatures and vacuum applications, but more importantly, so are the adhesives used. An ordinary acrylic adhesive, for example, will char and become nearly impossible to remove, if subjected to the kind of high temperatures that polyimides can usually withstand. Many adhesives contain low molecular weight organic volatiles which can off-gas - and this can interfere with vacuum processing. Silicone-based adhesives are generally rated for higher temperature and some vaccum applications.
Kapton and polyimide tapes are considered to be in the same class as photoresist -- and so are generally banned from use in tools in which resist-coated substrates are not normally allowed.
When in doubt about whether Kapton tapes are allowed in certain tools, contact the Special Materials Committee at specmat@snf.
Spin Coated Films
Polyimides can also be applied to substrates through spin-coating, followed by curing. A wide variety of coating formulations are available. In general, they vary by viscosity (allowing you to target different thicknesses), solvent used (at SNF, we prefer NMP or other less hazardous chemical solvents), and functionality (for example, photopatternable polyimide or KOH-etch resistant polyimide.)
To bring in a new polyimide formulation, obtain an MSDS and send it to specmat@snf. Cured polyimide films are allowed in tools which normally allow wafers with photoresist. (If you plan to process polyimide films in a tool which does not normally allow processing of wafers with photoresist, you should in addition submit your process flow as part of your SpecMat request.) Polyimide coating may be done in the litho area as follows:
- Uncured polyimide is soluble in NMP but not very soluble in acetone. Excess acetone can be used to clean up polyimide which may, for example, have dripped onto a benchtop, but be aware that polyimide can clump up in acetone, plugging vacuum lines. Dried polyimide is difficult to remove -- it is not dissolved in the usual solvents. So take care to clean up all surfaces and labware after processing.
- Spin coating may be done only on headway2. Spin chuck and parts should be cleaned up
as soon as possible after coating (see warning about acetone above). Personal or dedicated wafer handling
tools must be used (do NOT used the general, shared litho cassettes.)
- The Blue M oven should be used for curing the resin (do NOT use the singe or YES ovens.) Except where specifically allowed, do not use the hot plates for curing resins at temperatures above 200C.
- Only cured polyimide is allowed in tools in the white area of the lab. A specmat request must be submitted to process wafers with cured polyimide in tools other than gold-contaminated equipment.