Copper has 44% higher conductivity than aluminum, making copper an attractive interconnect metal for advanced multilevel interconnections in integrated circuits (IC) and multichip modules (MCM). Since sputter deposition is widely used in IC and MCM fabrication, the properties of sputtered copper films are investigated in this study. In particular, MCM technology requires relatively thick coatings (5–8 μm), which can result in high stress. Thin film stress can lead to dielectric cracking, wafer bow, and reduced operating lifetime. This study reports the residual stress, morphology, and electrical resistivity of annealed copper coatings as a function of the thin film crystal orientation determined by x-ray diffraction. We found that the stress and electrical resistivity of copper thin films can be significantly reduced by controlling the orientation of the copper crystallites.

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A photosensitive polyimide is evaluated as a dielectric material in high density thin film copper-polyimide interconnect structures. Factors affecting the photolithographic fabrication of structures with small feature size and high aspect ratio have been investigated. The shrinkage characteristics of the precursor film were observed to depend strongly on the UV exposure conditions. Dimensional changes taking place during curing of the polyimide structures show anisotropic shrinkage, the degree of which depends upon the feature size. The photosensitive polyimide precursor is observed to interact strongly with copper and results in residue formation during patterning. Based on this photosensitive polyimide process, high density thin film copper-polyimide interconnect structures with wiring density close to 800 lines per centimeter were fabricated.Continuous advancement in the speed and complexity of integrated circuits used in high performance systems have created a demand for the development of an interconnect technology which offers a high wiring density, good electrical characteristics for the propagation of high speed signals, and good thermal performance. Multilayer interconnection schemes with fine line conductors and associated ground planes have been proposed for applications in high performance systems (1, 2). As evidenced by the large number of publications, thin film hybrid-wafer-scaleintegration has become an exceedingly intense field of research and development activity [Ref.(3) and references therein]. In the interconnection technology intended for high performance systems, the electrical performance considerations dictate the use of materials with low dielectric constants and conductors with controlled circuit parameters such as characteristic impedance, signal cross-talk, and propagation delay. To achieve these conditions it is necessary to have tight tolerances on the geometrical dimensions. High density interconnect structures fabricated using our processing approach meet the necessary electrical and thermal requirements for use in high performance systems.The conventional approach of fabricating high density copper-polyimide interconnect structures is based on photolithographically defining copper conductor lines several microns thick either by a subtractive or additive approach followed by polyimide dielectric deposition. The polyimide dielectric is deposited by spin coating or spraying of a polyamic acid based nonphotosensitive precursor solution on the substrate containing the copper conductor lines (4). High temperature baking steps induce imidization of the precursor molecules and formation of polyimide films. The curing process is also accompanied by significant shrinkage in the film thickness, which necessitates multiple spin coating steps before a satisfactory degree of planarization is achieved. Fabrication of the interconnecting vias requires reactive ion etching of the dielectric layer through a metal or silicon dioxide mask followed by metal deposition. The overall process is tedious ...

Issues relating to the fabrication of complex multichip modules were investigated from a manufacturability perspective. The use of a preimidized photosensitive polyimide reduces the number of processing steps, thus making it a desirable dielectric material. Factors influencing the polyimide resolution, stress, and feature profile were studied both by experimentation and modeling. Thermal cure cycles for polyimide baking were optimized for solvent resistance, polyimide mechanical properties, and process throughput. Studies were also done on polyimide interactions with metal depositions, including adhesion and polyimide surface damage. Results of this research are shown as embodied in a fabricated 1.4 Gbit/s optical transceiver multichip module.