Flexible electronic-photonic 3D integration from ultrathin polymer chiplets

By Yunxiang Huang, Gen Li, Tianyu Bai, Yieljae Shin, Xiaoxin Wang, Alexander Ian More, Pierre Boucher, Chandramouli Chandrasekaran, Jifeng Liu & Hui Fang 

Abstract

Integrating flexible electronics and photonics can create revolutionary technologies, but combining these components on a single polymer device has been difficult, particularly for high-volume manufacturing. Here, we present a robust chiplet-level heterogeneous integration of polymer-based circuits (CHIP), where ultrathin polymer electronic and optoelectronic chiplets are vertically bonded at room temperature and shaped into application-specific forms with monolithic Input/Output (I/O). This process was used to develop a flexible 3D integrated optrode with high-density microelectrodes for electrical recording, micro light-emitting diodes (μLEDs) for optogenetic stimulation, temperature sensors for bio-safe operations, and shielding designs to prevent optoelectronic artifacts. CHIP enables simple, high-yield, and scalable 3D integration, double-sided area utilization, and miniaturization of connection I/O. Systematic characterization demonstrated the scheme’s success and also identified frequency-dependent origins of optoelectronic artifacts. We envision CHIP being applied to numerous polymer-based devices for a wide range of applications.

Introduction

Integrated electronic-photonic systems have a wide range of applications, such as optical wireless communications^1,2 and biosensing^3,4. In polymer-based, flexible systems, integrated electronic-photonic devices have emerged as strong contenders for optogenetic neurostimulation^5,6,7,8, photoplethysmography sensing^9,10,11, and continuous glucose monitoring^12,13 due to their biocompatibility and tissue-conforming ability^14,15,16. Given that for many applications, there are space constraints, the interfacing devices typically cannot be made from standard printed circuit board (PCB) manufacturing^17,18,19. Moreover, integrated electronic-photonic devices require combining various electronic and photonic components such as electrodes, sensors, and light sources, each of which relies on separate fabrication, necessitating complex manufacturing. As an example, integrating these components into a single chip within semiconductor manufacturing is conventionally achieved in a cumbersome sequential fabrication process with different metallization layers^8,14,20,21 and also using specialized silicon wafers²² or through epilayer transfer of III-V photonic materials²³, which can lead to low yield and high cost. To address the mismatch between Si and III-V semiconductors, tremendous efforts have established micro-transfer printing of fully formed device elements^7,24 as a practical way of integrating photonic components. This establishment is especially true for flexible integrated systems, where epitaxy cannot be performed on polymer substrates. Building on this success, micro-scale stacking has enabled the integration of different flexible device layers, though generally after the device micro-profile definition^{7,25,26,27,28,29}. As a result, it is challenging to achieve device miniaturization or volume production. Overcoming these challenges would enable advanced device capabilities such as minimally invasiveness in medical diagnostics and therapies, as well as cost-effective device manufacturing for affordable and equitable technology accessibility.

Here, we report a robust, scalable 3D-integration scheme - chiplet-level heterogeneous integration of polymer-based circuits (CHIP) - for fabricating advanced, flexible electronic-photonic integrated devices. This scheme first leverages co-design and parallel fabrication of polymeric chiplet layers with different device functions. After the chiplet fabrication, vertically aligning and bonding of post-fabricated chiplet layers using ultrathin biocompatible adhesives at room temperature achieve 3D function integration with monolithic I/Os. To illustrate this process, we demonstrated a proof-of-concept 3D-integrated optrode system with high-density arrays of μLEDs and microelectrodes. Using CHIP, for the first time, we achieved (i) chiplet-level 3D integration of all needed functions, including temperature sensing and optoelectronic shielding capabilities for a thermal safe, low noise, and artifact-free bio-optrode; (ii) simplified double-sided area utilization, which would otherwise be not achievable without backside lithography and is highly challenging for Si-based devices; and (iii) miniaturization of the connection for the final 3D-integrated device. Systematic experimental studies and simulations revealed the integrated system’s mechanical flexibility, high performance of electrical, optical, and thermal functions, and remarkable reliability. In addition to multifunctional incorporation, the new 3D-integrated platform also enabled device physics studies in flexible electronic-photonic systems and revealed frequency-dependent origins of optoelectronic artifacts on polymeric substrates, where low-frequency-band artifacts are light-induced, and high-frequency-band ones are from electrostatic effects, a key insight useful for future device design and system integration. We envision the CHIP approach here to be widely generalizable and produce a wide variety of advanced device technology with the integration of unprecedented multifunctionalities while with a strong ability of miniaturization and excellent manufacturability.

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