Development of New Concept Photo Imageable Dielectric Materials for Next-Generation Advanced Packaging

The demand of photo imageable dielectric materials performance for next-generation packaging requires various properties, not only good electrical property but also high lithography resolution and low stress of the film. We have developed polyimide materials, that meet each of those new requirements. This paper reports three kinds of new negative tone photosensitive polyimide (PSPI) which have excellent thermal and mechanical properties: 1) PSPI with good lithography resolution for micro-via formation. 2) PSPI with low dielectric loss tangent (Df) for high-frequency devices such as antenna-in-packaging for 5G communication. 3) Low stress PSPI which could be applied to large and core-less multilayer structure formation such as RDL interposer. Low cure temperature and small post cure shrinkage are the key to enabling those kinds of structures. These newly developed negative tone PSPI can be used as an interlayer dielectric film as following process flow: (1) Spin-coat PSPI on a substrate cover on RDLs. (2) Exposure with an i-line (365 nm) stepper or aligner. (3) Negative tone patterning by developing with an organic solvent such as cyclopentanone. High resolution PSPI were designed with high solubility polymer structure and increased polymer's photo reactivity for high solubility contrast by improved solubility of unexposed area (Rmax) and decreased solubility of exposed area (Rmin). Resolution of below 5 μm via diameter was then achieved. This micro via size enables the next generation scaling of advanced packaging (fine pitch RDL Line and Space below 2 / 2 μm). Next, lowering transmission loss is a key parameters to realize the next generation of high-frequency devices with lower Dk and Df numbers for PSPI materials being desired. Therefore, we have developed a PSPI that achieves a dielectric constant (Dk) of 2.8 and a dielectric loss tangent (Df) of 0.01 at a frequency of 36 GHz. This material was designed to control polarity and molecular mobility in the polymer to realize dielectric property. Lastly, larger packaging and multilayer RDL are also of high interest in the industry. Low stress type PSPI has been developed for this application. This low stress type polyimide can be processed at lower temperatures, around 200 °C, compared to a conventional polyimide requiring 230 °C curing temperature. This material also indicates a lower shrink rate around 10% after cure step. These three types of PSPI showed excellent thermal and mechanical properties and good reliability test results without short-circuit in the bias HAST test at 121 °C and 85 RH% for 96 hours. These results were described in detail in this paper.