Progression in a Novel Low Loss Photodielectric for Wafer Level Packaging (WLP)

The electronics industry relies on the integration of separately manufactured components into higher-level assemblies to enable enhanced functionality and operations. Heterogeneous integration of wireless and mixed signal devices, bio-chips, power devices, optoelectronics and microelectromechanical systems in a single package continue to advance. The evolution of heterogenous packaging designs creates new material requirements these diverse components are interconnected in new packaging architectures for the multitude of industries where electronics are used. Novel materials are required to meet the fabrication goals at the foundry and OSAT level. Opportunity arises for new dielectric materials that can mitigate some of the challenges found with the incumbent materials, while maintaining reliability specifications. The performance of the end device becomes highly dependent on the dielectric material. In this paper, we present our progressive research on the material properties and reliability of PRL-29 to demonstrate device fabrication and reliability performance. PRL-29, is a novel, block co-polymer exhibiting promising attributes for passivation and redistribution dielectric layers. PRL-29 ranges between 5–15 microns in film thicknesses and experiences less than 5% shrinkage after hard bake allowing for a wide photolithographic processing window. Photolithographic performance has been evaluated for a range of film thicknesses using a mask aligner system with and without a 360nm long pass filter and i-line stepper system. Shear strength adhesion was measured on various substrates and layers using a DAGE 4000 stylus. Additionally, PRL-29 shows strong adhesion after pressure cooker test (PCT). Passing TCT is vital to assure reliability performance of the dielectric material. PRL-29 has passed 2000 cycles of temperature cycling from −55°C to 125°C, replicating automotive standards. Furthermore, any change in the inherent film properties should be noted post reliability. PRL-29 mechanical properties were measured after PCT, TCT, and a series of high temperature storage testing where the integrity of the material was not deteriorated. Dielectric properties (Dk and Df values) were assessed after curing at 150°C, 200°C, and 250°C before and after PCT and high temperature aging. The Dk and Df were found to average 2.5 and 0.0025 in the frequency range of 10GHz to 50GHz. Thermogravimetric (TGA) analysis identifies cured material stability with increasing temperatures. Outgassing of the material was assessed to understand the potential effects during solder bump reflow processing through gas chromatography mass – spectrometry (GC-MS). TGA-GCMS was performed after PRL-29 was cured at 200°C to determine the outgassing components and decomposition mass loss up to 400°C. Breakdown voltage and volume resistivity were measured to indicate when PRL-29 becomes no longer resistant to leakage current through its body. Finally, optical properties of the dielectric material were assessed. The refractive index (n) and extinction coefficient (k), as well as thickness, must be understood and controlled for repeatability in a manufacturing environment. The Cauchy constants have been calculated by fitting the equation. Characterization of the bulk film and ongoing reliability studies of PRL-29 continue to demonstrate its robust ability as an option for a photo dielectric layer in future heterogeneous advanced package designs.