Improved Poly(Methacryloxyethyl Amic Esters), Photosensitive Precursors of Polyimides, with Low Levels of Chlorine

Improved Poly(Methacryloxyethyl Amic Esters), Photosensitive Precursors of Polyimides, with Low Levels of Chlorine

       The synthesis of photosensitive polyimide precursors
based on methacrylate esters of PMDA-ODA was described by Siemens AG
in the mid 1970's.  In this process the anhydride, PMDA, was
converted into the pyromellitic acid diesters and then subsequently
converted to the pyromellitic acid chlorides with thionyl chloride
and pyridine at -10 C.  Finally, ODA was added to form the
photoreactive polyimide precursor.  The amount of ionogenic chloride
in the polymer prepared by this method amounted to <15 ppm.  No
mention is made by Siemens as to the amount of nonionic or chemically
bound chlorine in the polymer.  Unfortunately, the use of thionyl
chloride leads to sulfur contamination in the polymer which is
undesirable for microelectronic applications and therefore oxalyl
chloride at 40-50oC can be used in place of thionyl chloride to give
a cleaner product.  However, applying this type of approach to the
synthesis of the photosensitive ester precursors of polyimides, the
amounts of total chlorine in the polymer become very high, greater
than 2000 ppm.  In addition, the photosensitive polymers prepared
using the warm oxalyl chloride reactions have a high molecular weight
component visible in Gel Permeation Chromatography (GPC) and the
shelf life and lithographic windows are small as a result.

      In order to avoid this high temperature preparation of the
photoreactive monomer, the diacid/diesters can be first converted to
diacid salts by the addition of a tertiary amine, such as pyridine or
a trialkyl amine.  The salt ca...