This invention relates to an improved method of preparing high purity polyamic acids, to be transformed into polyimide resins which, in turn, are used in the processing of circuitry in the microelectronics industry. The improvement relates to increased yield.
Due in part to such characteristics as high tensile properties, desirable electric properties and excellent stability in the face of heat and water, polyimide resins have been widely used in the processing of microelectronic circuitry. However, the electrical properties of microelectronic circuits can be impaired by the presence of ionic impurities, hence it is vital that polyamic acids and polyimides prepared from them, be ionically "pure". The necessity of using polyamic acids with reduced ionic impurities, when working in the electronics field, is well-recognized. For example, see U.S. Pat. No. 4,225,702, issued Sept. 30, 1980, to Mikino et al.
Mikino et al. disclose a method of preparing a polyamic acid with reduced ionic impurities. It involves reacting a diamine or diaminoamide compound monomer with a polycarboxylic acid dianhydride monomer, wherein said monomer compounds have been purified by a recrystallization process so that they have reduced ionic impurities.
While recrystallization is an acceptable method of reducing impurities, it has the disadvantage of yielding purified monomers in an amount equivalent to only about 80 to 85% by weight of the unpurified monomer.
The present invention provides an improved method of preparing high purity polyamic acids that provides an increased yield of the purified monomers.