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Photopatternable Laminate BCB-based Dielectric for Advanced Packaging
Keywords: Dielectric, Film, Benzocyclobutene
As packaging trends in microelectronics evolve to necessitate higher densities through miniaturization, new package designs and substrates must be considered. To satisfy these trends, novel dielectric materials with specific performance characteristics will become critical components of the advanced technologies. Laminates based on Benzocyclobutene (BCB) polymers are uniquely qualified to satisfy the demands of next generation packaging. BCB-based dielectrics have been used in high volume manufacturing for many years, included under Dow’s CYCLOTENETM series of Advanced Electronic Resins. Traditionally applied as spin-on coatings, BCB-based polymers may also be utilized as the backbone of dry films or laminates applied by hot roll or vacuum lamination. Modifications to the formulation allow for the increased flexibility necessary in a laminate product while also enabling patterning through various methods. Laminates are particularly suited to applications such as Fan-Out Panel Level Packaging (FOPLP) as well as advanced substrates with smaller line spacing for mobile applications. Advantages include the ability to coat large area panels, the ability to tent and protect through vias, use as a TSV via filling material, and the avoidance of thinning over pad corners or copper RDL while maintaining good conformal performance. The laminate coating thickness can also be optimized over a wide range, generally 2µm to 50µm with good uniformity. Depending on the application, the laminate may be based on Divinylsiloxane-bis-Benzocyclobutene (DVS-bis-BCB) polymer or Aqueous-Developable BCB (AD-BCB) polymer. The two versions of photopatternable Benzocyclobutene-based laminate are negative tone and may be patterned utilizing various means. Aspect ratios of 1:1 are approached by patterning with a broadband exposure tool such as the Süss MicroTec Mask Aligner, ASML i-line stepper, or through laser direct imaging (LDI) with a tool such as the Orbotech Ultra 200. Post-patterning, the DVS-bis-BCB laminate is developed using a solvent blend while the AD-BCB laminate may be developed using a standard TMAH-based aqueous developer. Alternatively laser ablation, such as with a Süss Photonics 248nm excimer laser and photomask, can produce aspect ratios approaching 4:1. A post-rinse to clean ablation debris may be necessary in place of solvent or aqueous development. BCB-based laminate properties including low dielectric constant (Dk), low loss (Df), high breakdown voltage (VBD), high thermal stability, and excellent moisture and copper barrier properties are viewed as necessary attributes in next generation packaging. The laminate based on DVS-bis-BCB has a low Dk, <2.65 across a wide range of frequencies evaluated from 1MHz – 15GHz. Coupled with the low loss value of 0.001, this laminate is a strong candidate for high frequency packaging applications. Initial RF test vehicles have shown low transmission loss and low insertion loss at high frequencies. Transmission of 90% has been achieved at 40GHz. The AD-BCB-based laminate has a Dk value of 3.2 and a loss value of 0.02, making it suitable for fan-out and RDL dielectric applications or applications that require aqueous development. Low curing temperatures, <250°C, are another desirable property of BCB-based laminates when considering molded wafers or organic substrates. Finally, initial component level reliability results also indicate that the DVS-bis-BCB-based laminate passes MSL-1 testing without electrical failure or delamination and thermal cycling (TCT) testing has exceeded 750 cycles without failure. The combination of these properties in a film dielectric format is seen as an enabling technology for next generation packaging solutions.
Corey O'Connor, Scientist
Dow Electronic Materials
Marlborough, MA

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