Next generation thin film polymers for WLP applications and their mechanical and electrical characterization
Authors: Woehrmann Markus , Toepper M. , Walter H. , Lang K.-D. 
Published: October 2015

Thin film polymers, like PI, PBO and BCB are used in every wafer level packaging device. The improvement of the reliability of wafer-level packages and chip I/Os consider the choice of the polymer, which is used as dielectric on the chip, as a minor point. Because the production lines are normally fixed on one polymer and the high investments to evaluate the processing of an alternative polymer formulation in combination with costly reliability test seems to be not attractive till today. But the increased demands of advanced WLP and 3-D-Integration, which includes thin chips, chips stacking and higher routing densities, leads to reaching the limits of the common used material system combinations. The demand of better polymer films becomes evident by the fact that dozens of “next generation polymers” have entered the marked in the last years, which are tailored to get higher mechanical toughness and electrical performance aside of a nearly unchanged resolution capacity.

The challenge for new polymer formulation is the evaluation of the processing and the generation of a reliable material property data base, which set the basics for any benchmarking to the already used polymer materials. The processing evaluation is done typically by the material supplier or the fab himself, where no special equipment is needed. The material property generation is a quite more complex topic because you need special equipment and partly the material need to be free standing without any substrate. This is also a handling issue, if we talk about thin films in the range of 5 to 20μm.

This paper presents the reliable thin film polymer properties characterization of mechanical and electrical values. The measurements of the mechanical properties include the estimation of parameters like young's modulus, tensile strength, elongation at break, coefficient of thermal expansion, stress and time-temperature related effects. The evident topic of warpage related impacts by “new generation polymers” will be presented and discussed.

Measurement structures on wafer-level are developed for the estimation of the electrical parameters, which allows a high accuracy and a device relevant value estimation. Parameters like break down voltage, leakage current, dielectric constant, loss factor are measured related to frequencies by MIM and resonator structures. We demonstrate with analyzing of the time-dependent dielectric breakdown (TDDB) of thin film polymers that there is an exponential linkage between field strength and the time till the breakthrough occurs.

The mechanical and electrical properties were also investigated related to aging effects, when the application is running on elevated temperature. We examine a degradation of the mechanical and electrical performance, which should be taken into account for the mechanical system reliability and also for impedance controlled HF-application.

This paper present advanced material characterization of thin film polymers which gives a guideline for the decision of the polymer related to the demands of the application.