Sensors and Actuators A 136 (2007) 291–298
Microcantilever hotplates: Design, fabrication, and characterization Jungchul Lee, William P. King ∗ Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405, United States Received 12 September 2006; received in revised form 10 October 2006; accepted 22 October 2006 Available online 4 December 2006
Abstract This paper describes design, fabrication, and characterization of microhotplates based on microcantilevers. Six different hotplate cantilever designs are proposed to investigate heating response time and temperature uniformity. Devices were fabricated and their electrical, thermal, and mechanical characteristics were measured. The cantilevers have electrical resistance in the 1 k range, consume 1–100 mW power, have a heating and cooling time constant 1000 ◦ C. The temperature uniformity was characterized and is excellent for all of the devices. These cantilevers are targeted for use where both microhotplates and microcantilevers would be useful sensing platforms. © 2006 Elsevier B.V. All rights reserved. Keywords: Microcantilever; Microhotplates; Chemical sensing; Thermal sensors
1. Introduction Microcantilevers offer outstanding opportunities for bio/ chemical sensors, as they can be highly sensitive to specific bio/chemical analytes [1,2], are relatively easy to fabricate and use, and can interface with existing laboratory equipment and integrated microfluidic handling systems. In addition, microhotplates have been shown to be extremely useful for calorimetry [3,4] and chemical sensing [5]. While several studies have shown that microcantilevers can be fabricated with internal resistive heaters [6,7], little work has been done to converge microcantilevers with microhotplates for sensing applications. This paper describes microcantilever heaters for microhotplate applications having well-characterized temperature uniformity and sub-ms response time. Microfabricated hotplates have previously been used for various sensing applications, including as a Pirani gauge [8], gas sensor [9], and a flow-rate sensor [10]. In some cases, the method or materials of microsensor fabrication limited its performance. The main design considerations for microhotplates are thermal isolation and temperature uniformity that can be achieved through free standing heatable microstructures,
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[email protected] (W.P. King).
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which are either bridges or cantilevers. To further minimize heat conduction through mechanical links, porous silicon has been introduced for low power microhotplate arrays [11]. Microhotplates made from thin-film platinum heater-thermometers [12] could not be integrated with on-chip circuitry since platinum is not compatible conventional silicon microelectronics fabrication. Microhotplates made from polysilicon [13] have poor long-term stability at high temperature since the grain boundary of polysilicon is highly reactive. Microcantilever heaters made from doped single-crystal silicon overcome these drawbacks, as integrated electronics could be produced in the same silicon layer, and because they can be cycled many times to temperatures above 800 ◦ C [14]. Remarkably, microcantilever heaters made from doped single-crystal silicon have a temperature coefficient of electrical resistance (TCR) that can exceed that of platinum by a factor of 2 [14], and so their temperature sensitivity can exceed that of platinum thermometers. Microcantilevers with internal heaters have been extensively studied for their applications to thermomechanical data storage [15,16], nanomanufacturing [17,18], and fundamental thermophysical measurements [19,20]. Silicon cantilevers are capable of temperature that exceeds 1000 ◦ C [14] and heating time on the order of 10–50 s [6,14]. Silicon cantilevers capable of high temperature heating have been shown to control local growth of carbon nanostructures [21] and enable new thermal analysis measurements on novel materials [22]. However, the cantilevers
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J. Lee, W.P. King / Sensors and Actuators A 136 (2007) 291–298
Table 1 Electrical, thermal, and mechanical design requirements Electrical resistance, R (k) Power consumption, P (mW) Resonance frequency, f (kHz) Spring constant, k (N/m) Time constant, τ (ms) Maximum temperature, Tmax (◦ C)
