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

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.

Printed Electronics Technologies for Additive Manufacturing of Hybrid Electronic Sensor Systems

“…[15][16][17][18] One particularly appealing field for combining silicon-based electronics with printing technologies is the back-end packaging. As an example, Roshanghias et al [19] recently studied the feasibility of fan-out wafer-level packaging of capacitive micromachined ultrasound transducers using inkjet-printing. Thin and sensitive parts of MEMS are generally prone to deterioration when being processed using standard methods, such as laminating, molding, back-grinding, and dicing, that may result, for example, in a resonance frequency shift of a MEMS microphone, or even a membrane rupture.…”

Classifications and Applications of Inkjet Printing Technology: A Review

“…Roshanghias et al [245] inkjet-printed redistribution layers (RDLs) and a metal route used to connect the MEMS microphone pads to application-specific integrated circuits (ASICs) and fan-out the signals via solder balls. In another study, they inkjet-printed the RDLs of Ag-ink nanoparticles for the fan-out packaging of capacitive micromachined ultrasound transducers [246]. Other studies have demonstrated the synthesis of adhesives by inkjet printing [247] [248].…”

Die level thinning after bonding on the substrate in 3D packaging

“…The existing packaging process was executed by die thinning first and then bonding on a substrate. This process is difficult to control when the intermediate die becomes thinner to a certain level during die bonding, resulting in a defect in the suction step of lifting the die [7][8][9]. In addition, this method is limited because the size of the die varies for heterogeneously integrated packaging.…”