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Comparing Crosslinking Effects in the Low Temperature Curing of PBOs
Keywords: PBO, crosslinking, microwaves
The use of Polybenzoxazole (PBO) chemistries for wafer level polymer dielectric layers has the advantages of lower cure temperatures and aqueous development. To retain the high Tg values of the polyimide family, it is necessary to include some level of chain extension and crosslinking into the PBO precursor resin reaction scheme. The “cure” to PBOs now involves both cyclization reactions and crosslinking reactions (Figure 1). The cyclization (ring closure) reaction is similar in PBOs and polyimides (PI) and has been thoroughly studied. The crosslinking of PBOs is more complicated and has not been reported as extensively before. It may be possible that mechanical and thermal properties of the final dielectric films could be impacted by differences in these multiple chemical reactions with different heating mechanisms and process profiles. Besides the reversible cyclization reaction, there are at least two possible reactions of a PBO precursor resin at, or next to, its phenol functional group with a crosslinking additive. A potential crosslinking reaction to form an ether linkage and a rearrangement product are described. The rearrangement product can continue to cyclize to a PBO, but the ether intermediate cannot cyclize. This probable combination of reaction products is the source of previously suspected “competition” between cyclization and crosslinking in PBO resin curing. High extent of cyclization provides good dielectric constant and high breakdown voltage as well as resistance to chemical and thermal degradation. Since the starting resins are moderate molecular weight polymers, the “crosslinking” reactions most likely produce more chain extensions at the ends rather than linkages between the middles of chains. The extent and type of crosslinking would be expected to affect the mechanical and thermal properties of the final cured films. Further complications to the understanding of final film properties include the polymerization of, as well as the breakdown and removal of, the crosslinking agent at temperatures above 150°C. As previously reported in the literature, the use of variable frequency microwave (VFM) curing produces high levels of extent of cyclization of several PBO precursors at cure temperatures from 175-200°C with three hour cycle times or less. Extent of crosslinking has been estimated to be high from this cure process. In these PBO films, measurement of Tg may not be an accurate indicator of extent of crosslinking, so we have looked for an alternate analytical method to determine the extent of crosslinking. In a joint project between HDM and Lambda, the extent of cyclization and crosslinking of the PBO precursor resin HD-8930 was measured directly in two sets of experiments using FTIR spectroscopy to quantify the absorption of specific functional groups as they change during the reaction. These absorptions were standardized by comparison to functional group absorptions that do not change during either reaction. Narrow range emissivity wafers were coated with HD-8930 and soft-baked. The oven curing was done at HDM and the VFM curing was done at Lambda. FTIR measurements were done at both locations with identical methods. Elongation, modulus, CTE, stress, and Tg were measured and compared between the oven and VFM cured films. The cyclization of HD8930 is more advanced at lower temperatures and effectively completed (>95%) by curing at 200°C for thirty minutes with VFM. Films cured by oven (one hour soak) reached complete cyclization at 250°C. The chain extension/crosslinking is substantially higher at all temperatures with VFM curing with a maximum reached at 200°C that was double that in the oven. Tensile, thermal and thermo-mechanical properties of the cured films show subtle changes over cure temperatures from 200 to 250°C. We will discuss the relation between these property changes and the underlying chemical changes during cure.
Robert L. Hubbard, Director, Technology Development
Lambda Technologies
Morrisville, North Carolina

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