  Citation: Al Farisi, M.S.; Tsukamoto, T.; Tanaka, S. Electroplated Al Press Marking for Wafer-Level Bonding. Micromachines 2022, 13, 1221. https://doi.org/10.3390/ mi13081221 Academic Editor: Margarita Tecpoyotl-Torres, Jesus J. Alcantar Peña and Sanchez Mondragon Received: 12 July 2022 Accepted: 28 July 2022 Published: 30 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). micromachines Article Electroplated Al Press Marking for Wafer-Level Bonding Muhammad Salman Al Farisi 1,2, * , Takashiro Tsukamoto 1 and Shuji Tanaka 1,3 1 Department of Robotics, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan; takashiro.tsukamoto@tohoku.ac.jp (T.T.); mems@tohoku.ac.jp (S.T.) 2 Department of Biomedical Information Sciences, Hiroshima City University, Hiroshima 731-3194, Japan 3 Micro System Integration Center (μSIC), Tohoku University, Sendai 980-8579, Japan * Correspondence: alfarisi@hiroshima-cu.ac.jp Abstract: Heterogeneous integration of micro-electro mechanical systems (MEMS) and complemen- tary metal oxide semiconductor (CMOS) integrated circuits (ICs) by 3D stacking or wafer bonding is an emerging approach to advance the functionality of microdevices. Aluminum (Al) has been of interest as one of the wafer bonding materials due to its low cost and compatibility with CMOS processes. However, Al wafer bonding typically requires a high temperature of 450 ◦ C or more due to the stable native oxide which presents on the Al surface. In this study, a wafer bonding technique for heterogeneous integration using electroplated Al bonding frame is demonstrated. The bonding mechanism relies on the mechanical deformation of the electroplated Al bonding frame through a localized bonding pressure by the groove structures on the counter wafer, i.e., press marking. The native oxide on the surface was removed and a fresh Al surface at the bonding interface was released through such a large mechanical deformation. The wafer bonding was demonstrated at the bonding temperatures of 250–450 ◦ C. The influence of the bonding temperature to the quality of the bonded substrates was investigated. The bonding shear strength of 8–100 MPa was obtained, which is comparable with the other Al bonding techniques requiring high bonding temperature. Keywords: wafer bonding; aluminum; electroplating; press marking; MEMS 1. Introduction Micro-electro mechanical systems (MEMS) is a class of transducers which is able to sense or manipulate physical, optical, or chemical quantities. To bridge the physical and cyber world, MEMS transducers are often coupled with complementary metal oxide semiconductor (CMOS) integrated circuits (ICs) which enables the electrical signal trans- mission with an adequate intelligence [1]. Most of the signal processing functions, such as amplification, temperature compensation, filtering, analog-to-digital conversion, storage, and communication are often performed by ICs in the integrated microsystems. The advancement of the smart society driven by the internet of things (IoT) has enhanced the development of microfabricated devices, in particular for ubiquitous sensing to gather as much information as possible from the environment. Sensors, actuators, and ICs are demanded in a higher volume than ever. To date, the size of these devices have been scaled down in accordance to the Moore’s law. Such a miniaturization scheme has reduced the production cost of each chip. However, the miniaturization scheme is currently approaching its limit in the technological and economical aspects [2,3]. Heterogeneous integration by 3D stacking of microdevices is a promising alternative route when the device miniaturization according to the Moore’s law is approaching its limit [4]. Wafer bonding enables not only the integration of multiple devices in a small volume, but also device packaging at the same time [5]. In addition, each device can be fabricated separately at the wafer-level within the batch process before integrated at the final step, which maximizes the degree of freedom in the fabrication technique of each device. Micromachines 2022, 13, 1221. https://doi.org/10.3390/mi13081221 https://www.mdpi.com/journal/micromachines