Sensors and Actuators A 135 (2007) 10–15 Post-processing techniques for locally self-assembled silicon nanowires Ongi Englander a,∗ , Dane Christensen a , Jongbaeg Kim b , Liwei Lin a a Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720, USA b School of Mechanical Engineering, Yonsei University, Seoul, South Korea Received 23 February 2006; received in revised form 8 July 2006; accepted 11 August 2006 Available online 15 September 2006 Abstract Post-processing techniques are applied after the integration and assembly of nanostructures and Microelectromechanical Systems (MEMS) to realize integrated Nanoelectromechanical Systems (NEMS). Experimentation is focused specifically on the application of post-processing steps to a locally self-assembled micro-to-nano system comprising of suspended silicon nanowires between two MEMS bridges. Local contact metallization, global metallization for rapid system functionalization and the application of aqueous treatment to the NEMS are among the post- processing techniques studied. These techniques are evaluated for their effectiveness and compatibility with integrated NEMS and traditional MEMS processes. It is found that local and global contact metallization techniques effectively alleviate inherent problems at the nano-to-micro contact and the aqueous treatment study confirms the effectiveness of the super critical drying process for nanostructures. © 2006 Elsevier B.V. All rights reserved. Keywords: NEMS; Silicon nanowires; Integration; Post-processing 1. Introduction Nanoscale and NEMS based applications and devices have shown significant promise for a wide range of applications with improved performance and reduced costs when compared to their microscale counterparts [1,2]. Post-processing techniques are indispensable steps toward the functionalization, realization and operation of NEMS devices, such as chemical and mechan- ical sensors, biomedical detectors, electrical interconnects and switches. We explore various post-processing techniques for self-assembled and suspended silicon nanowires between two MEMS bridges, including (1) local contact metallization, (2) global metallization for rapid system functionalization and (3) aqueous treatment [3]. The self-assembled NEMS devices were previously developed using localized synthesis and electric-field assisted self-assembly of silicon nanowires with MEMS struc- tures to yield a two-terminal NEMS device [4,5]. In contrast to more traditional approaches for integrated nanoscale appli- cations [1,6,7], the utilization of localized heating and thus localized nanostructure synthesis permits the formation of the nano-to-micro contact in situ yielding a CMOS compatible ∗ Corresponding author. Tel.: +1 510 393 4667. E-mail address: oenglander@gmail.com (O. Englander). platform for NEMS fabrication and functionalization. Here we present our NEMS’ responses to various post-processing steps where the selection of these processes focused on minimizing the complexity of the post-processing step and seeking compatibly with conventional processes. The benefits and impact of these processes are evaluated and hydrogen sensing capabilities using this post-processed NEMS are presented as a demonstration. 2. Experimental The NEMS studied in this work is fabricated following a previously presented process [4,5]. The two-terminal, self- assembled silicon nanowires based system is schematically illus- trated in Fig. 1. Briefly, two suspended MEMS bridges are positioned in close proximity to each other and the synthesis of silicon nanowires using the VLS growth mechanism is initiated locally as a result of the resistive heating of the hot growth bridge. In the presence of a locally acting electric-field, constructed solely between the two bridges, the silicon nanowires follow electric-field lines and link together the two MEMS bridges. The first bridge on which the nanowire synthesis process originates is designated as the growth structure while a second bridge, located nearby, is designated as the bias structure. Post-processing tech- niques are required to enable and improve the functionality of this NEMS device. 0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.08.004