Crosslinkable Sul-containing Fluorinated Polyimides For Optical Waveguide Device

The continual trend toward high transmission speed, data capacity, and data density in integrated circuits demands a solution to the bottleneck resulting from the limited data rate of electrical interconnects. One approach to this problem is the use of optical interconnection operating with polymer waveguides. Polymer optical waveguides have attracted considerable attention for their possible application as optical components in optical interconnects and optical communication systems because of their potential ease of manufacture at low temperature, and the low cost of processing.The key issues on the polymer waveguide materials include four aspects: (1) low propagation losses at the optical communication wavelengths, (2) high thermal stability to provide compatibility with high-performance electronic device fabrication, (3) low birefringence in optical materials is an important issue which can reduce the polarization dependent loss (PDL) (4) controllability of refractive index for the easy control of the waveguide dimension to match the mode size with fibers, and good adhesion to the silicon substrate. However, hydrocarbon polymers have a high optical loss in the infrared communication region due to carbon–hydrogen (C–H) bondvibrational absorption. By modifying a molecule via the substitution of fluorine or deuterium for hydrogen in the C–H greatly reduces optical loss. Many organic polymers such as deuterated or fluorinated poly(methyl methacrylate) (PMMA), polystyrene (PS), and poly(carbonate) (PC) are used as materials for a variety of optical components. However, these polymers do not have sufficient thermal stability at high temperature, for the fabrication temperature of the optical devices is 260 ?C, and the short time temperature is up to 400 ?C. Polyimides are more accessible than these polymers because of their molecular structure. In addition, they have good thermooxidative stability, outstanding mechanical properties, fire resistance and so on. However, conventional polyimide materials possess bad solubility, high moisture adsorption and relatively high dielectric constant, which limit their availability in many field. Many soluble fluorinated polyimides have been synthesized in recent years. Further sul-containing aromatic polymers have great thermal stability for the existence of 3d orbit sulfur atom.In this study, we introduced the sulfur atom to the famous fluorinated polyimide monomer 2,2'-bis-(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB), and hope the new material can keep the advantage of the famous polyimide. Novel sul-containing fluorinated polyimides have been synthesized by the reaction of 2,2'-bis-(trifluoromethyl)-4,4_-diaminodiphenyl sulfide (TFDAS) with 1,4-bis-(3,4-dicarboxyphenoxy)benzene dianhydride (HQDPA), 2,2'-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 4,4'-oxydiphthalicanhydride (ODPA) or 3,4,3',4'-biphenyl-tetracarboxylic acid dianhydride (s-BPDA). The sulfide groups in polymers were introduced to improve adhesion to the Si substrates. The fluorinated polyimides, prepared by a one-step polycondensation procedure, have good solubility in many solvents, such as N-methyl-2-pyrrolidinone (NMP), dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), cyclohexanone, tetrahydrofuran (THF) and m-cresol. The molecular weights (Mn's) and polydispersities (Mn/Mw's) of polyimides were in the range of 1.24×105 to 3.21×105 and 1.59–2.20, respectively. The polymers exhibit excellent thermal stabilities, with glass-transition temperatures (Tg) at 221–275 ?C and the 5% weight-loss temperature are above 531 ?C.The sul-containing fluorinated polyimides can be heated to crosslink. Considering the thermal decomposition, we choose the lower temperature (400 oC) as crosslinking temperature. The probable crosslinking reaction is the reaction between sulfur and sulfur after the polymer cured above a special temperature. After crosslinking, these polymers show higher thermal stability. Furthermore, these polymers are insoluble in common solvent, which can satisfy the need of the interbedded technology of the devices. The sulfide crosslinking will not introduce C–H bond compared to the other common crosslinking groups, thus it can decrease the C–H bond vibrational absorption and lower optical loss will be expected.The films of polymers have high optical transparency. The novel sul-containing fluorinated polyimides also have low absorption at both 1310 and 1550 nm wavelength windows. The reduction of the refractive indices of the polymers can attribute to the increasing of the fluorine and accretion of the free volume, so it can control the refractive indices of the polymers in the range of 1.5626–1.638 at 1550 nm. They also have a low water absorption rate about 0.8%. TheΔn less than 0.011 was measured in all the polymers. This result is lower (or the same) compared to the conventional fluorinated polyimides. The lowest birefringence of 0.0047 was measured for polyimide 2 film. This may be due to the existence of sulfur atoms and more CF3 groups which decreased the moleculer alignment. Rib-type optical waveguide device was fabricated using the fluorinated polyimides and the near-field mode pattern of the waveguide was demonstrated.