Polyimides, additives Moisture

In order for a polyimide to be useful as an interlevel dielectric or protective overcoat (passivant), additional demanding property requirements must be met In the case of the passivant, the material must be an excellent electrical insulator, must adhere well to the substrate, and must provide a barrier for transport of chemical species that could attack the underlying device. It has been demonstrated that polyimide filrns can be excellent bulk barriers to contaminant ion motion (such as sodium) [10], but polyimides do absorb moisture [11,12], and if the absorbed moisture affects adhesion to the substrate, then reliability problems can result at sites where adhesion fails. However, in the absence of adhesion failure, the bulk electrical resistance of the polyimide at ordinary device operating temperatures and voltages appears to be high enough to prevent electrochemical corrosion [13]. 

When used as an interlevel dielectric, even greater demands are placed on the polyimide. Because integrated circuit processing includes as a final step a metal sinter at 400 C, the interlevel insulator film must withstand such exposures without degradation of electrical, chemical, or mechanical properties. In addition, the deposition, cure, and etch process must provide for reliable interconnection between the metal layers above and beneath the film (the "via contact") [8]. Issues of ion motion, moisture uptake, and electrical conduction both in bulk and at interfaces must also be considered carefully. 

The 6FDA/BDAF polyimide was modified using PPD in an effort to "stiffen" the polymer backbone and improve thermal performance (Tg). A better overall property balance was achieved in several of these 6FDA/BDAF/PPD copolyimides. A series of random copolymers was prepared in which the level of PPD was varied from 0% to 100% based on the total moles of diamine. The incorporation of PPD had little effect on the dielectric constant but did result in improved thermal performance and was accompanied by increased moisture uptake (Figures 1,2, and 3). This behavior is consistent with the overall reduction in the amount of bound fluorine in the polymer backbone however, additional work is required to establish a direct correlation. A reasonable property balance was realized over a range of 40 to 60 mole% PPD which displayed dielectric constants from 2.85 to 2.90, moisture absorption from 1.5% to 2.0%, and Tg from 280°C to 290°C. In addition, the 6FDA/BDAF/PPD copolyimides displayed somewhat less solvent sensitivity than the 6FDA/BDAF homopolymer as described above. 

Other properties of Pis were also improved by the diimide additives [135]. The dielectric constant, e of PI from BDSDA and 2,2 -bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF) reduced appreciably by additives shown in Fig. 44. The results are as listed in Table 5. Furthermore, the addition of 10 wt% AN-BDSDA-AN into BDSDA-ODA m-PDA polyimide caused a decrease in the saturation moisture content from 1.52% for the neat PI to 1.27%. Positron annihilation spectroscopy proved that a decreased free volume is responsible for the reduced water uptake. Also, a slightly increased in modulus and a decreased CTE were caused by the 5% addition of AN-6FDA-A into LARC-TPI in both the undrawn and drawn states. 

Polyimides are compatible with IC processes and have both chemical stability and long-term stability in a presence of moisture and heat, in addition to the good hygroscopic and... 

A comparison of these base materials is shown in Table 63.8. Although these new polyimide materials have lower moisture absorption and CTEs than traditional polyimide films, their dimensional change is still not zero. In addition, process variations of 0.02 to 0.05 percent are common from batch to batch during film production. 

The most widely used and least expensive polymer resins are the polyesters and vinyl esters. These matrix materials are used primarily for glass fiber-reinforced composites. A large number of resin formulations provide a wide range of properties for these polymers. The epoxies are more expensive and, in addition to commercial applications, are also used extensively in PMCs for aerospace applications they have better mechanical properties and resistance to moisture than the polyesters and vinyl resins. For high-temperature applications, polyimide resins are employed their continuous-use, upper-temperature limit is approximately 230°C (450 F). Finally, high-temperature thermoplastic resins offer the potential to be used in future aerospace applications such materials include polyetheretherketone (PEEK), poly(phenylene sulfide) (PPS), and polyetherimide (PEI). 

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