POLYIMIDES DERIVED FROM 2,2'-BIS(TRIFLUOROMETHYL)-4,4'-DIAMINOBIPHENYL .4. OPTICAL-PROPERTIES OF FLUORINATED POLYIMIDES FOR OPTOELECTRONIC COMPONENTS

Optical loss and refractive indices of fluorinated polyimides and copolyimides are studied for application to optoelectronic components. These polyimides and copolyimides are prepared from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), pyromellitic dianhydride (PMDA), and 2,2'bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB). The polyimide 6FDA/TFDB prepared from GFDA and TFDB cured at 350 degrees C has an optical loss of 0.7 dB/cm at 0.63 mu m, and this is the lowest of all polyimides cured at high temperature. Direct measurement of the optical loss of a polyimide at 1.3 mu m was achieved for the first time by using a 6FDA/TFDB block with unequal length, width, and thickness. The optical loss was also low, 0.3 dB/cm. 6FDA/TFDB, PMDA/TFDB, and their copolyimides cured on a silicon substrate at 350 degrees C have two different refractive indices, an in-plane refractive index (n(TE)) and an out-of-plane refractive index (n(TM)). The n(TE) of the copolyimide at 1.3 mu m can be controlled between 1.523 (6FDA/TFDB) and 1.614 (PMDA/TFDB) by changing the 6FDA/TFDB content. It decreases monotonically with increasing 6FDA/ TFDB content. The n(TM), on the other hand, can be controlled between 1.514 (6FDA/TFDB) and 1.521 (copolyimides with 60 mol % 6FDA/TFDB), but it has a maximum value at a 6FDA/TFDB content of 60 mol %. These polyimides and copolyimides prepared on a silicon substrate have birefringences between 0.008 (6FDA/TFDB) and 0.123 (PMDA/TFDB). Low optical loss and precise refractive index control are very important characteristics for optoelectronic components, especially at the telecommunication wavelength of 1.3 mu m.