Micro- and Nano-link Ultra-Low Power Heaters for Sensors A. W. Groenland, E. Vereshchagina, A. Y. Kovalgin, R. A. M. Wolters, J.G.E. Gardeniers, and J. Schmitz MESA+ Institute for Nanotechnology, University of Twente, Enschede, The Netherlands email: a.y.kovalgin@utwente.nl Abstract—A new microfabricated device for heating and sensing in gases is presented. It is based on the resistive heating of a micro- or nano-metric hollow cylinder of titanium nitride, and measurement of its (temperature-dependent) resistance. This article presents the fabrication and temperature calibration of the device, and illustrates its function as flow meter and thermal conductivity meter. A temperature of 280 ºC is achieved at a power consumption of only 5.5 μW, orders of magnitude less than existing commercial hotplate devices. The thermal time constant can be as low as 60-120 microseconds. I. INTRODUCTION Contemporary micro hotplates can be electrically heated to 100-600 ºC with small time constants (~100 ȝs). Such micro hotplates are commonly used in chemical sensors [1] and mass flow meters [2] based on temperature changes. Because of the elevated operating temperatures, the devices can also behave as chemical actuators, i.e., in micro-reactors providing energy in the form of heat to initiate thermo-activated chemical reactions [3]. When coated with catalytically active surface coatings, hotplates can be used as catalytic microreactors or catalytic combustion sensors such as Pellistors [1, 4] for the detection of hydrocarbons. One significant drawback of conventional hotplates is their relatively high power consumption. Commercially available platinum wire based (‘classic’) devices require a power around 100 mW [4]. Other types of hotplates, that are fabricated in the last few decades using micro-technology, are based on a suspended membrane with metal thin film resistors and still require 10-40 mW [5, 6]. As the heat losses scale with the hot plate size, a smaller resistive heater can yield a better power-temperature balance. A new generation of micro hotplate devices was recently reported, with a power consumption of only a few milliwatts [7, 8]. In that approach, the heat was generated by a conductive nano-link, sandwiched between two polysilicon electrodes. The link was formed by antifusing (breakdown) a thin silicon dioxide (SiO 2 ) dielectric between the electrodes, followed by controllable electrical programming. While these devices proved the viability of power reduction by downsizing, the approach is difficult to industrialize due to the antifusing / programming sequence as well as limited predictability and reliability of these hot plates. In this work, we present a new fabrication approach to realize the link, aiming to overcome these limitations. The conductive link is formed directly by microfabrication, by first etching a hole in a SiO 2 layer on top of the first electrode, and filling this hole in with the second electrode material (TiN) via Atomic Layer Deposition (ALD). The device design and the fabrication procedure are detailed in Section II. In Section III, we show the electrical and thermal characterization of these hotplates with an outer link diameter of 2-6 ȝm (‘micro-link’) and 100 nm (‘nano-link’). The work is summarized in the Conclusion. II. DESIGN AND FABRICATION A cross-section and a three-dimensional schematic of the device design are shown in Figure 1. The width W of the electrodes is 10 ȝm. The link is realized in the center of a suspended membrane of 40×40 ȝm. For the device fabrication, standard 4” boron-doped (10 15 atoms/cm 3 ) <100> silicon wafers were processed as described in ref. [9]. Briefly, this included 1) deposition of a 100 nm low stress silicon nitride (SiRN) layer, 2) sputtering and further patterning a layer of 100-nm- thick TiN to make the bottom electrode (see Fig. 1), 3) deposition of a 100-nm-thick SiO 2 by plasma enhanced chemical vapor deposition (PECVD), 4) sputtering and patterning a 100-nm-thick TiN layer to form the top electrode, 5) etching a hole in the SiO 2 layer for the micro- or nano- link by UV or e-beam lithography, respectively, and 6) ALD of 7 nm (micro-link) or 15 nm (nano-link) of TiN to form the link between the electrodes. The fabrication of 1-ȝm-thick aluminum (Al) contact pads and release of the SiRN membrane in a KOH solution finalized the structure. A cross-section (made by the focused ion beam (FIB) in combination with high-resolution scanning 978-1-4673-1708-5/12/$31.00 ©2012 IEEE 169