Kazuhiro Yamanaka

Novel low-dielectric constant photodefinable polyimides for low-temperature polymer processing

Current photosensitive polyimide formulations that can be developed in aqueous alkaline developers are based on the use of either (1) soluble poly(amic acid) precursor polymers or (2) polyimides functionalized with hydrophilic groups (e.g. phenol). The use of poly(amic acid) polymers requires the subsequent high temperature thermal cyclization of the polymer after imaging to produce the desired polyimide which can prevent ue of such materials in many applications. However, the use of pre-imidized poliyimides by imparting solubility with hydrophilic groups also is problematic since the presence of such groups in the polymer generally degrades the dielectric constant and water uptake performance of such materials. The goal of the work in this paper was to overcome these problems by developing new low dielectric constant polyimides that can be formulated into photo-definable materials and processed at low temperatures. In this work the use of a novel hexafluoroisopropanol (HFA)-substituted diamine to synthesize novel poly(amic-acid) and polyimide polymers is reported. The addition of HFA to the polymers is shown to produce polyimides which are soluble in both common casting solvents and 0.26 N TMAH alkaline developers. A photosentitive polyimide composition based on formulation of the HFA-subtituted polyimide with 20 wt% of a DNQ inhibitor is shown to produce high resolution patterns with a sensitivity of 170 mJ/cm2 and a contrast of 1.32 using I-line exposure. The HFA groups in the polymers are contained on a substituent group attached to the main chain by an ester linkage. It is shown that these HFA-substituent side-groups can be easily removed from the polymer after development of the patterned image by thermal treatment of the polymer at temperatures above 280 oC. The cleavage of the HFA side groups produces a polymer which does not swell and is insoluble in aqueous alkaline developers. Polyimide film properties including dissolution rate, dielectric constant, thermal expansion coefficient (CTE), water absorption have been measured and are shown to be strongly dependent on the presence of the HFA side groups. In particular, the dielectric constant of the polyimide is shown to decrease dramatically from 3.20 to 2.60 after thermal cleavage of the HFA side groups using a thermal cure at 350 oC for 30 minutes. In general, the strategy of using solubility enhancing functional groups that can be cleaved from the polymer during or after imaging and development appears to be a very promising strategy for developing photo-definable low dielectric constant polymers that can be processed at low temperatures.

Novel photodefinable low-k dielectric polymers based on polybenzoxazines

The development of spin-on photodefinable dielectrics for use in microelectronic device fabrication and packaging has traditionally focused mainly on the use of soluble polymer precursors because the desired final dielectric polymer structure is generally insoluble in any appropriate casting solvent. One major drawback to this approach however is that high temperature processes (often >300 °C) after imaging and development are generally required to convert the imaged precursor polymer into the desired low dielectric constant material. These high processing temperatures make such materials impractical for a variety of applications. The goal of the work reported in this paper was to overcome this problem by developing new low dielectric constant polymers that can be formulated into photodefinable materials and processed at lower temperatures. In this work the use of a novel hexafluoroisopropanol (HFA)-substituted diamine to synthesize novel polybenzoxazines is reported. Whereas polybenzoxazoles form a five membered ring by situating an alcohol ortho to an amide, the benzoxazine rings in the polymer backbone described in this work are formed by the dehydration of a hexafluoroalcohol (HFA) group situated ortho to the amide to form a six membered ring. Of this general class of new polybenzoxazine amide alcohol precursor polymers, it will be shown that selected polymers exhibit good solubility in developer solutions and can be formulated into photosensitive compositions by addition of a DNQ inhibitor. Polybenzoxazine film properties including dielectric constant, thermal expansion coefficient, glass transition temperature, water absorption and dissolution rate have been measured. In particular, the dielectric constant for polybenzoxazine is reported to be as low as 2.2. In contrast to polybenzoxazoles which are known to cyclize at temperatures well above 280 °C, the new polymers reported here can be cyclized at temperatures as low as 210 °C. This significantly lower thermal cyclization temperature greatly alleviates many problems with other photodefinable dielectric polymers such as thermal stress build-up during curing and also allows these materials to be integrated with a wider variety of materials that would not survive the significantly higher processing temperature required with most current polyimides and polybenzoxazoles.