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The application of photosensitive polyimides in electrical and optical interconnection technology is discussed. Critical properties of two photosensitive polyimide formulations with significantly different structural chemistries have been compared. Polyamic ester based formulations such as Selectilux® HTR3 show significant distortions in the photopatterned features upon high-temperature baking, which can be ascribed to anisotropic shrinkage. A BTDA-alkylated diamine based preimidized formulation (Probimide®), on the other hand, shows a more uniform shrinkage of the photopatterned features. HTR3 films interacts strongly with metals such as copper, which adversely affect the photopatternability of these materials. Planarization behavior in multiple layer structures has also been investigated. We explored the viability of these materials for applications in optical interconnection. Probimide® materials exhibit low scattering losses and appear to be promising candidates for the development of a MCM-compatible optical interconnection technology. We also describe here a novel technique for optical recording of refractive index patterns in Probimide® films.

We have investigated interface formation and degradation at the junction of preimidized polyimide and Cu by means of scanning Auger and secondary electron microscopies, and x‐ray photoelectron spectroscopy. Our approach has been to spin on ultrathin (tens of A) polyimide overlayers onto Cu substrates, cure the polyimide while in contact with the Cu in an inert atmosphere, and then expose the interface to different environments. We found that only a very small amount of oxidation occurs during the curing process itself. This result is expected on the basis of the preimidization step prior to contact with Cu. The limited amount of oxidation appears to be due to the formation of a Cu–amine complex at the interface. Storage in vacuum leaves the interface abrupt and no further oxidation occurs. However, exposure to room‐temperature air or high humidity at 85 °C causes more extensive Cu oxidation to occur. In the case of exposure to high humidity at 85 °C, metal oxidation is accompanied by Cu outdiffusion and p...

Multilayer high-density thin-film interconnection structures with a wiring density of up to 1000 lines per inch per signal layer were fabricated using a photosensitive polyimide as dielectric and electroplated copper conductor lines. Lithographic considerations associated with the patterning of high-aspect-ratio (>1.0) polyimide precursor features and their distortions during high-temperature annealing steps were investigated. An anisotropic shrinkage causing inward skewing of the feature profiles was observed. This phenomenon strongly depended on the UV exposure conditions and appears to be related to nonuniformity in the precursor cross-link density. The present approach of electroplating small-geometry copper features in patterned polyimide trenches provides excellent planarization in interconnect structures containing high-aspect-ratio features. Transmission characteristics of high-frequency signals in these lossy interconnect lines showed minimal degradation in the pulse shape or attenuation in the pulse amplitude. A high-density memory module, consisting of two orthogonal X-Y layers of 25- mu m-wide copper lines fabricated at a pitch of 80 mu m over a silicon substrate, was fabricated. The two layers of conductor lines were interconnected with electroplated copper vias. Epoxy-based die-bonding and gold wire bonding techniques were used for the assembly of the populated module in a metal flat pack. Integrity of the fabricated copper-polyimide multichip modules under moisture and thermal stressing conditions was demonstrated. >

We report here, for the first time, a novel ultraviolet (UV) defined selective in‐diffusion technique for refractive index alterations in a BTDA‐alkylated diamine type of preimidized polyimide film. This has been achieved by in‐diffusing disperse red 1 (DR1) dye into the polyimide film from an organic solvent‐based solution. UV photocrosslinking of the polyimide chains was observed to significantly suppress the in‐diffusion of DR1 and provided the means for selective doping of this dye in the polyimide film. This technique is amenable to a laser direct write maskless approach for applications in integrated optics. We have demonstrated this technique by fabricating a low‐loss (

A novel phenomenon, leading to the planarization of topologically prominent features, in partially cured BTDA‐alkylated diamine based preimidized photosensitive polyimide films is reported. This phenomenon appears to be related to solvent interdiffusion between two polyimide layers in intimate contact leading to uniform shrinkage of the two layers. A novel approach for the fabrication of high‐density copper/polyimide multichip interconnect structures has been developed based on the observed planarization phenomenon.

A photosensitive benzophenone tetracarboxylic dianhyride-alkylated diamine polyimide formulation has been evaluated for application in an optical interconnection area. The refractive-index patterns in this material were optically recorded by UV-assisted photodoping of sensitizers. The polyimide films were selectively doped with benzoin-type photosensitizers such as benzildimethylketal and benzoin ethyl ether, which cause a decrease in the refractive index. High-dose UV irradiation that causes cross linking of the polyimide chains was also employed for augmenting the refractive-index difference to 0.017 between the doped and undoped regions. Refractive-index variations and lightguiding properties were investigated as a function of doping concentrations and other processing conditions. The author utilized this technique for the fabrication of embedded polyimide channel waveguides. The two photosensitizers have different effects on the waveguiding characteristics of the polyimide films. Losses for benzoin ethyl ether remained low whereas doping with benzildimethylketal caused significant increase in the waveguiding loss at high doping concentrations. Near-field imaging of the output from such waveguides shows good confinement of 815-nm light.

We have used x‐ray photoelectron spectroscopy in a normal emission configuration to investigate the chemistry of formation of the buried polyimide on Cu interface. We find that oxidation of the Cu surface begins within an hour of the curing process, and that Cu atoms in contact with the polyimide overlayer are completely oxidized to CuO after being stored in air for several days. This conclusion is in stark contrast with that drawn for the inverse system, Cu on polyimide, which has been shown to be essentially unreactive in vacuum over a time scale of several hours. We suggest that the key difference between the two systems is a very slow corrosion reaction perhaps involving trace amounts of chloride ion, water trapped in the polyimide overlayer, and the Cu surface. In contrast, the same experiment involving a Au substrate shows no evidence of reaction over a time frame of weeks.

A comparative investigation of polyimide/Cu interface degradation has been carried out for ultrathin photosensitive, non-photosensitive, and preimidized polyimide precursor films cured while in contact with a Cu substrate. The role of curing byproducts and environmental conditions on interface degradation has been elucidated by means of X-ray photoelectron spectroscopy. Immediately after curing, we observed some oxidation of the Cu in contact with non-photosensitive and photosensitive polyimide overlayers. On the other hand, only negligible oxidation was observed for the preimidized polyimide/Cu interface. Experiments in which samples were stored in vacuum, air and a humidity chamber show a dependence of the oxidation kinetics on air/moisture exposure. Preimidized and photosensitive polyimide/Cu interfaces, stored in air, became more extensively oxidized with time relative to identical samples stored in vacuum. Moreover, all three polyimide/Cu interfaces exhibited significantly more oxidation after 3 days in a humidity chamber than after 19 days of storage in air. Taken together, these data clearly demonstrate that absorbed water and its interaction with curing byproducts are key factors in the extent of Cu oxidation at the interface.