Moisture Uptake of Bisbenzocyclobutene (BCB) Films for Electronic Packaging Applications

Abstract

Electronic packaging has been changing.rapidly to meet the often conflicting demands of higher speed and lower cost. In the multichip module (MCM), an array of semiconductor chips is mounted on a substrate and interconnected via a network of multilevel metal lines with polymer layers used as a dielectric between orthogonal metal signal paths. A potential reliability problem that must be investigated is the absorption of moisture by the dielectric in the MCM package. Moisture absorption may lead to long term problems such as loss of adhesion, corrosion of the metal interconnect and increased insulator conductivity.Bisbenzocyclobutene (BCB) is an insulating polymer that is being used in the MCM package as a dielectric material. In this work, the moisture uptake in BCB is investigated gravimetrically using a Cahn-1000 microbalance with a resolution of I pgram. The BCB films are spincoated onto silicon substrates and cured yielding film thickness ranging from 7 to 20µm. The moisture uptake of the BCB film is monitored in situ and non destructively to reveal the performance of the film as used in an integrated circuit environment. The moisture uptake of the films was monitored at different ambient relative humidity (RIH) values between 20% and 80% at 23°C. The transient behavior of the moisture absorption in BCB films has been characterized. Moisture uptake in BCB films increases as the relative humidity is increased, and reaches a maximum of less than 0.2% by weight at 80% RH. By comparison, this moisture uptake is significantly lower than that of fully cured PMDA-ODA polyimide, which is about 2.5% by weight under similar conditions. The transient data show that the equilibration time for a 22.8 µ.m film is approximately 220 seconds, yielding a diffusion coefficient of about 4.5µm2/s.

Water Solubility and Diffusivity in BCB Resins used in Microelectronic Packaging and Sensor Applications

“…The error on the measurement of diffusivity 0.7 µm 2 .s -1 is a little higher than the error reported in [1] ( 0.5 µm 2 .s -1 ). Table II.…”

“…Its permittivity is close to 2.65 when it is dry. Its diffusivity should be close to that of Cyclotene TM 3022, estimated to 4.5 0.5 m 2 .s -1 at 23°C, with a gravimetric method, using a 1 µg readability microbalance [1]. The measurements performed on the structures showed a water uptake of 0.090% by weight at 64%RH and 23°C.…”

“…Its distinctive features are low loss in the RF range and one of the lowest available water sorption [1,2]. Although low, it does not ensure the fabrication of hermetic packages as needed for MEMS.…”

Accurate modeling of high-Q spiral inductors in thin-film multilayer technology for wireless telecommunication applications

“…The top copper layer is coated with NiAu in order to allow easy mounting of other active devices (flip chip, wire bonding). The uncovered BCB at the air interface also serves as a passivation layer since it is very stable, very corrosion hard, and has a very low moisture absorption [30].…”

Photosensitive benzocyclobutene for stress-buffer and passivation applications (one mask manufacturing process)

“…Detailed electrical analyses of CdBCB interconnect structures have been published by several authors [15-161. In these referenced papers, the electrical properties of BCB have been shown to be insensitive to both temperature and frequency. The low dielectric constant and dissipation factors for Photo-BCB, coupled with the low moisture uptake (<0.25%) [17] provide the circuit designers with a large degree of latitude in varying line widths and dielectric thicknesses in order to meet higher performance criteria.…”