**On the Feasibility of Fan-Out Wafer-Level Packaging of Capacitive Micromachined Ultrasound Transducers (CMUT) by Using Inkjet-Printed Redistribution Layers**


Received: 4 May 2020; Accepted: 30 May 2020; Published: 31 May 2020

**Abstract:** Fan-out wafer-level packaging (FOWLP) is an interesting platform for Microelectromechanical systems (MEMS) sensor packaging. Employing FOWLP for MEMS sensor packaging has some unique challenges, while some originate merely from the fabrication of redistribution layers (RDL). For instance, it is crucial to protect the delicate structures and fragile membranes during RDL formation. Thus, additive manufacturing (AM) for RDL formation seems to be an auspicious approach, as those challenges are conquered by principle. In this study, by exploiting the benefits of AM, RDLs for fan-out packaging of capacitive micromachined ultrasound transducers (CMUT) were realized via drop-on-demand inkjet printing technology. The long-term reliability of the printed tracks was assessed via temperature cycling tests. The effects of multilayering and implementation of an insulating ramp on the reliability of the conductive tracks were identified. Packaging-induced stresses on CMUT dies were further investigated via laser-Doppler vibrometry (LDV) measurements and the corresponding resonance frequency shift. Conclusively, the bottlenecks of the inkjet-printed RDLs for FOWLP were discussed in detail.

**Keywords:** microelectromechanical systems (MEMS) packaging; inkjet printing; redistribution layers; capacitive micromachined ultrasound transducers (CMUT); fan-out wafer-level packaging (FOWLP)

#### **1. Introduction**

Fan-out wafer-level packaging (FOWLP) has spurred increasing interest due to significant cost advantages over competitive technologies, increased interconnect density, as well as enhanced electrical and thermal package performance. With its roots in integrated circuit (IC) manufacturing technology, FOWLP has also recently gained a lot of attention for microelectromechanical systems (MEMS) and sensors packaging. Some examples for FOWLP of sensors including MEMS-based acceleration

and pressure sensors, capacitive micromachined ultrasonic transducers (CMUTs), gas sensors and biomedical sensors were recently realized and reported [1–7]. acceleration and pressure sensors, capacitive micromachined ultrasonic transducers (CMUTs), gas sensors and biomedical sensors were recently realized and reported [1–7].

Employing FOWLP for MEMS sensor packaging creates some unique challenges [2]. For instance, the thin and sensitive parts of MEMS components are incompatible with FOWLP processes such as laminating, molding, back-grinding and dicing. This leads to deterioration of component performance, i.e., a resonance frequency shift of a MEMS microphone, or membrane rupture, which are issues with mold encapsulation per se [7]. Employing FOWLP for MEMS sensor packaging creates some unique challenges [2]. For instance, the thin and sensitive parts of MEMS components are incompatible with FOWLP processes such as laminating, molding, back-grinding and dicing. This leads to deterioration of component performance, i.e., a resonance frequency shift of a MEMS microphone, or membrane rupture, which are issues with mold encapsulation per se [7].

Moreover, some issues are emerging from the typical fabrication processes of redistribution layers (RDL) for fan-out packages, a combination of sputtering, photolithographic processes, etching, as well as electroplating. RDLs are typically metal interconnects used to provide power supply and route signals within the package and towards its periphery [8]. To ensure proper functionality of MEMS, it is essential to temporarily protect delicate sensing areas during RDL processing and ensure that temporary protection is entirely removed afterward. This complex procedure for the protection of thin film is also known as keep-out-zones (KOZ) processing on RDLs [2]. Moreover, some issues are emerging from the typical fabrication processes of redistribution layers (RDL) for fan-out packages, a combination of sputtering, photolithographic processes, etching, as well as electroplating. RDLs are typically metal interconnects used to provide power supply and route signals within the package and towards its periphery [8]. To ensure proper functionality of MEMS, it is essential to temporarily protect delicate sensing areas during RDL processing and ensure that temporary protection is entirely removed afterward. This complex procedure for the protection of thin film is also known as keep-out-zones (KOZ) processing on RDLs [2].

In this study, an alternative FOWLP concept by implementing additively manufactured RDLs for CMUT array packaging was proposed. The printed RDLs served as an interconnect between capacitive microphones and speakers, operating in the ultrasonic domain, with corresponding application-specific integrated circuits (ASICs), which allow features such as touchless activation or control using gestures [9,10]. As schematically illustrated in Figure 1, metallic and dielectric structures can be deposited selectively and in a controlled volume via a drop-on-demand printing technology (e.g., inkjet, aerosol, electrostatic, electrohydrodynamic, etc.). The concept of inkjet-printed RDLs for FOWLP was introduced in our previous work [11]. Inkjet-printed circuitry was also evaluated for the fabrication of low-cost silicon [12] and organic interposers [13], which showed great potential for rapid prototyping and signal probing. Aerosol printed RDLs for 3D smart devices were also recently reported by Serpelloni et al. [14] as well as screen printed RDLs by Chia-Yen et al. [15]. In this study, an alternative FOWLP concept by implementing additively manufactured RDLs for CMUT array packaging was proposed. The printed RDLs served as an interconnect between capacitive microphones and speakers, operating in the ultrasonic domain, with corresponding application-specific integrated circuits (ASICs), which allow features such as touchless activation or control using gestures [9,10]. As schematically illustrated in Figure 1, metallic and dielectric structures can be deposited selectively and in a controlled volume via a drop-on-demand printing technology (e.g., inkjet, aerosol, electrostatic, electrohydrodynamic, etc.). The concept of inkjetprinted RDLs for FOWLP was introduced in our previous work [11]. Inkjet-printed circuitry was also evaluated for the fabrication of low-cost silicon [12] and organic interposers [13], which showed great potential for rapid prototyping and signal probing. Aerosol printed RDLs for 3D smart devices were also recently reported by Serpelloni et al. [14] as well as screen printed RDLs by Chia-Yen et al. [15].

**Figure 1.** Schematic illustration of the fabrication process of redistribution layers in FOWLP by using drop-on-demand inkjet-printing. **Figure 1.** Schematic illustration of the fabrication process of redistribution layers in FOWLP by using drop-on-demand inkjet-printing.

Inkjet printing technology avoids long lithography procedures including global resist coating and sputtering, thus lower thermo-mechanical stresses are expected to be applied to the sensitive MEMS structures which will be assessed here. Additionally, long-term reliability of these printed RDLs for FOWLP will also be investigated and discussed. Inkjet printing technology avoids long lithography procedures including global resist coating and sputtering, thus lower thermo-mechanical stresses are expected to be applied to the sensitive MEMS structures which will be assessed here. Additionally, long-term reliability of these printed RDLs for FOWLP will also be investigated and discussed.
