Die-first and RDL-first FOWLP Processing for Flexible Hybrid Electronics
Authors: Fukushima Takafumi (Tak) , Susumago Yuki , Takahashi Noriyuki , Kino Hisashi , Tanaka Tetsu 
Published: January 2025

A new flexible hybrid electronics (FHE) based on die-first FOWLP is studied for integrating high-performance and scalable flexible biomedical systems such as photoplethysmographic (PPG) and percutaneous oxygen saturation (SpO2) sensos. The unique structure is consisting of an elastomer PDMS as a flexible substrate in which micro-LEDs, passive devices, and Si LSI chiplets having photodiode and micro-LED driver circuits etc. are embedded. Embedding of 100-microns-thick 1-mm-sqaure Si chiplets or a 200-microns-thick 2.5-mm-square biomedical sensor chiplets having photodiode and micro-LED driver circuits in a biomedical grade PDMS were implemented with two carrier Si wafers. These chiplets were gently placed on the 1st carrier wafer in a face-down configuration. Then, the PDMS monomers were poured on the chiplet-on-wafer structure, followed by vacuum defoaming and the subsequent compression molding with the 2nd carrier. After debonding of the 1st carrier, the chiplets are embedded on the 2nd carrier and planarized without any mechanical processes. Prior to the following metallization processes, a stress buffer layer (SBL) or two SBLs are coated on the PDMS/chiplets. By using standard photolithography with metal sputtering/wet etching, 200-nm-thick Au wirings were formed on the SBL(s) at the wafer-level. Finally, the FHE systems were debonded from the 2nd carrier. A key material is SBL inserted between inter-chiplet fan-out wirings and the substrate to enhance wire reliability. The impact of the SBL properties on the bendability of the FHE systems is described in this work. In addition, we evaluate the electrical properties of the embedded micro-LED and photodiode circuits between before and after bending for comparison. On the other hand, another new FHE based on RDL-first FOWLP with hydrogel substrates is studied for integrating highly biocompatible medical systems such as a UV sterilization patch for COVID-19 etc. Hydrogels mainly consisting of water have high adaptability, substance permeability, and breathability in addition to excellent biocompatibility, and they are expected for biomedical applications. A hydrogel ‘Wizard gel’ used in this study was from Yushiro Chemical Industry Co., Ltd. First, a laser-assist removable temporary adhesive as a sacrificial layer was spin-coated on a Si carrier wafer. Then, Ti/Au was deposited by sputtering, and Au wirings were formed by wet etching. The subsequent process is microbump formation with Cu/Sn electroplating. After that, mini-LED are flip-chip bonded on the carrier wafer through the microbump. After capillary underfilling, 1-micron-thick parylene as an insulator between the metal and hydrogel was vapor-deposited on the wirings, and then, a silane coupling treatment was performed after the surface modification with a 172-nm excimer ramp. Thereafter, the hydrogel was compression-molded while being thermally cured. After that, the temporary adhesive was debonded from the carrier wafer by visible laser irradiation. Finally, the Au wirings were transferred to the hydrogel substrate from the Si wafer. The adhesion between the parylene and hydrogel was evaluated by a cross-cut test (ASTM D 3359-87 Method B). 4-point probe patterns were used for the resistance evaluation and mini-LED operation. In this work, we integrate mini-LED embedded in a hydrogel substrate on which fan-out interconnections are formed in wafer-level processing and characterize their electrical properties for biomedical application. Our FHE system embedding an LSI chiplets in PDMS by die-first FOWLP were successfully fabricated using a hard-type (high Young’s modulus) SBL. 10-microns-width Au wirings are formed on PDMS with the hard SBLs and 40-microns-width Au wirings exhibit high bendability with a bending cycle of beyond 1,000. This heterogeneous integration scheme enables high-performance and scalable FHE to create highly-integrated wearable systems. On the other hand, we demonstrate a new heterogeneous integration by RDL-first FOWLP with hydrogel-based FHE for biomedical patch application. Inter-chip Au wirings were photolithographically formed on a hydrogel and relatively low resistances were obtained by the advanced WLP technologies. Good bendability beyond 100 cycle bending was also obtained when the bending radius was 40 mm. The embedded mini-LED in hydrogel was well operated by the RDL-first flexible FOWLP methodology. This paper also compares these die-first and RDL-first FOWLP feature for FHE application.