Functionalization and Environmental Stabilization of ZnO Nanobridge Sensors Fabricated using Carbonized Photoresist
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Functionalization and Environmental Stabilization of ZnO Nanobridge Sensors Fabricated using Carbonized Photoresist Ashley D. Mason1, Chien-Chih Huang1, Myra T. Koesdjojo2, Nate D. Stephon1, Vincent T. Remcho2 and John F. Conley, Jr.1 1 School of Electrical Engineering and Computer Science, 2Department of Chemistry, Oregon State University, Corvallis, OR 97331, U.S.A. ABSTRACT ZnO nanobridge sensors were selectively grown and electrically connected using lithographically patterned carbonized photoresist (C-PR). As ZnO is known to dissolve in deionized (DI) water, parylene-A was investigated as a moisture barrier and potential functionalization platform. A chemical vapor deposition (CVD) process for parylene-A was developed and it was demonstrated that parylene-A coated ZnO NWs do not show any signs of dissolution after 24 hours in an aqueous solution. The impact of the parylene-A coating on the electrical performance and sensitivity of ZnO nanobridge devices was investigated. A comparison of UV, O2, and H2O sensitivity between uncoated and coated devices revealed that a thin coating of parylene-A attenuated all of these responses, suggesting the ability to modulate environmental sensitivity using this non-covalently bound polymer passivation layer. INTRODUCTION A major challenge for creating a viable nanowire (NW) based sensor is the integration of nanowires into electrically accessible device [1-5]. A novel method for the selective growth and directed integration of ZnO NWs was achieved using a lithographically patterned carbonized photoresist layer (C-PR) [6]. Unlike other approaches, this C-PR method avoids the use of metal catalysts, seed layers, and additional patterning process steps. Growth and electrical connection of NWs take place simultaneously for many devices. Although there have been many demonstrations of highly sensitive NW based sensors [7, 8], the effects of activation layers on the sensor response to UV, O2 and H2O has not yet been presented. In this work we examine the effects of surface functionalization on the interactions between the ambient environment and the NW surface. EXPERIMENTAL DETAILS ZnO nanobridge sensor devices were fabricated on Si/SiO2 substrates. After direct patterning, the photoresist (Shipley 1818) was carbonized at 900 °C for 60 min and 5 Torr in a reducing atmosphere (95% Ar, 5% H2) [6]. On top of the C-PR, a 150 nm thick layer of molybdenum was sputtered at 100 W, 3.5 mTorr Ar from a 3″ target. Vapor–solid growth of ZnO NWs was performed using a mixed 1:1 ratio of ZnO (99.99%, Sigma–Aldrich) and graphite powders (99.9995%, Alpha Aesar) [9]. This mixture was placed in a tube furnace at 920 °C and was carried downstream to the substrate (770 °C) with 150 sccm N2. To aid in nanobridge growth 1–2 sccm of O2 was introduced 2.5 cm upstream from the substrate.
The procedure developed for CVD coating of parylene-A is illustrated in figure 1. In a single vacuum chamber, 0.1 g of amino-[2,2]paracyclophane (dix-A, Kisco) was vaporized at 150 °C, pyrolized at 690 °C, and then deposi
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