Low-Stress Ultra-low Dielectric Porous Polymer for High Density Applications: A Review

Two major issues that prevented organic substrates and interposers from achieving high VOs at fine pitch and dimensional stability are thermal expansion mismatches between material layers and requirement of low dielectric constant to improve signal transmission. Thermal dimensional stability is dependent on the difference in thermal expansion between the various materials in the package, the temperature, and the elastic modulus, when stresses are considered. The dielectric constant of a material can be modified on a molecularly level, or by injection of pores into the material. In this study, the development of a new class of low stress coating that has ultra-low dielectric (ultra low-k) constant is reviewed. Using new low temperature vapor induced separation process, pores were injected into polyimide to form a coating that is dense on the surface, but is porous in the core. Thermal mechanical testing showed that the introduction of pores did not affect the glass transition temperature, which indicated that the molecular entanglement and free volume of the polymer in the struts remained unchanged. However, tests showed that the elastic modulus was dramatically lowered, and the dielectric constant was lowered to 1.65. To demonstrate the process compatibility of the new material with conventional circuitization procedure, circuits were built using metallized porous polyimide. Performance test data showed that the material is compatible with wet etching process, and signal speeds in transmission lines built on porous polyimide were significantly faster than lines build on dense polyimide. Combined with its high mechanical compliance, low-stress porous polyimide is an excellent alternative for high speed substrates.