Photosensitive Polyimide as a Dielectric in High Density Thin Film Copper‐Polyimide Interconnect Structures

Abstract

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 ...

Early-stage phase separation in polyimide precursor blends: an atomic force microscopy study

“…These materials will significantly reduce the processing cost of constructing multilayer structures for chip packaging if successfully engineered. 18,19 The importance of polyimide is further augmented by its application as a matrix material in constructing high-performance composites in the aerospace industry.20 Polyimide copolymers have also been constructed for application as structural adhesives in the aerospace and microelectronics industries. 21 The length scales probed in the present study fall in the 101-103 nm range; this is complementary to techniques using visible light (such as light scattering and optical microscopy) and neutron (or X-ray) scattering.…”

Selection Criteria for Multichip Module Dielectrics

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