* Corresponding author. Tel.: #45-4525-5727; fax: #45- 4588-7762. E-mail address: boisen@mic.dtu.dk (A. Boisen) Ultramicroscopy 82 (2000) 11}16 Environmental sensors based on micromachined cantilevers with integrated read-out A. Boisen*, J. Thaysen, H. Jensenius, O. Hansen Mikroelektronik Centret, Technical University of Denmark, Bldg. 345e, DK-2800 Lyngby, Denmark Received 31 May 1999; received in revised form 18 August 1999 Abstract An AFM probe with integrated piezoresistive read-out has been developed and applied as a cantilever-based environmental sensor. The probe has a built-in reference cantilever, which makes it possible to subtract background drift directly in the measurement. Moreover, the integrated read-out facilitates measurements in liquid. The probe has been successfully implemented in gaseous as well as in liquid experiments. For example, the probe has been used as an accurate and minute thermal sensor and as a humidity sensor. In liquid, the probe has been used to detect the presence of alcohol in water.  2000 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Probe; Cantilever based sensor; Piezoresistive; Integrated read-out; Thermal sensor; Humidity sensor; Alcohol sensor in water; Symmetrical Wheatstone bridge 1. Introduction Micrometer-sized cantilevers fabricated by micromachining are commonly used for atomic force microscopy (AFM) imaging. Recently, AFM probes have been applied as chemical sensors [1}3]. No imaging is involved in these applications and thus it is not necessary to have a tip placed at the end of the cantilever. Basically, a change in the surface stress or temperature of a material on the cantilever surface can be monitored as a cantilever de#ection [3]. Moreover, a change in mass can be detected as a change in the resonant frequency of the cantilever [4]. Normally, the cantilever de#ec- tion is recorded by optical leverage, which can detect cantilever de#ections in the sub-angstrom regime. However, for many applications it would be advantageous to have a read-out mechanism requiring less adjustments and alignment. Also, a probe with integrated de#ection read-out may greatly facilitate measurements in liquid. For canti- lever-based sensors a reference cantilever is crucial, in order to reduce background noise such as ther- mal drift and gas turbulence [5]. AFM probes suitable for cantilever-based sen- sors and with integrated piezoresistive read-out have been developed [6]. In the probe design we have placed a full Wheatstone bridge symmetrical on chip, with two resistors placed on cantilevers and two resistors on the substrate, see Fig. 1. This design makes it possible to perform di!erential measurements where the signals from the two can- tilevers are subtracted. The relative resistance 0304-3991/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 9 9 1 ( 9 9 ) 0 0 1 4 8 - 5