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Environmental sensors based on micromachined cantilevers with integrated read-out
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-
* Corresponding author. Tel.: #45-4525-5727; fax: #454588-7762. E-mail address: [email protected] (A. Boisen)
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 cantilever-based sensors a reference cantilever is crucial, in order to reduce background noise such as thermal drift and gas turbulence [5]. AFM probes suitable for cantilever-based sensors 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 cantilevers 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
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Fig. 1. Schematic drawing and microscope image of AFM probe with integrated piezoresistive read-out. The cantilevers are approximately 50 lm wide, 200 lm long and 1.5 lm thick.
change of the piezoresistors (*R/R) is detected as an output voltage (< ) from the Wheatstone bridge 0 with a supply voltage (<). The output voltage can be written as < "1/4<*R/R. 0 The cantilevers are layered silicon/silicon oxide beams with integrated polysilicon resistors. The cantilevers are 200 lm long, 50 lm wide and approximately 1.5 lm thick. The de#ection sensitivity is measured to be approximately *R/R"10~6 nm~1 and for a Wheatstone bridge supply voltage of 2 V and for a measurement bandwidth of 10 Hz the minimum detectable cantilever de#ection is estimated to 0.3 As [6]. Here we demonstrate the possibility of using the AFM probe with integrated read-out as a versatile cantileverbased sensor. The probe has been successfully applied as a thermal sensor and as a humidity sensor. Moreover, the probe has been operated in liquid, where it has been used for detection of alcohol in water. The Wheatstone bridge has in all experiments been operated at a supply voltage of 4 V.
mal spectroscopy [2]. Moreover, commercially available piezoresistive cantilevers have been used for thermogravimetry [8]. We have investigated the stability and the sensitivity of our piezoresistive probe when applied as a thermal sensor. Since the used cantilevers are bilayered structures the cantilevers will bend when heated due to di!erences in the thermal expansion coe$cient of the silicon and the silicon oxide layer. Therefore, the probe can be directly used as a thermal sensor. Due to the symmetrical Wheatstone bridge con"guration the probe registers the di!erence in the temperature of the two cantilevers. This e!ect can, as earlier demonstrated for cantilevers with optical read-out [1], be used to estimate the amount of irradiation power absorbed by the cantilever. As the cantilevers are inherently sensitive to temperature changes this could cause an unintended drift in the read-out. However, the symmetrical Wheatstone bridge will eliminate such e!ects, if the two cantilevers are exposed to the same thermal environment.
2. Thermal sensor
2.1. Experiment
Cantilevers have previously been utilised as micromechanical calorimeters, by exploiting the bimaterial e!ect. Cantilevers with optical read-out have been used in for example investigations of phase transitions in alkanes [7] and in photother-
To investigate the performance of the probe as a thermal sensor, one of the two cantilevers in the Wheatstone bridge has been locally heated by focusing a continuous wave argon ion laser at the cantilever apex. A schematic cross-sectional
A. Boisen et al. / Ultramicroscopy 82 (2000) 11}16
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Fig. 3. Cantilever response as a function of laser power. The cantilever signal is seen to depend linearly on the irradiation power.
Fig. 2. Schematic drawing of silicon oxide/silicon cantilever with integrated polysilicon piezoresistor. A laser beam is focused at the cantilever apex.
drawing of the cantilever and the laser position is shown in Fig. 2. The spot diameter of the focused laser beam is approximately 1 lm, and the laser has been operated at a wavelength of 488 nm. The cantilever is placed below the focused laser spot by high-resolution dc motor stages and a CCD camera monitors the laser spot and the cantilever position. 2.2. Results The power of the laser, positioned at the cantilever apex, has been varied and the corresponding cantilever de#ection has been measured. The cantilever de#ection is registered as the output voltage from the Wheatstone bridge. As shown in Fig. 3 a linear relationship between laser power and cantilever de#ection is observed. Similar results have previously been observed for an aluminium-coated silicon cantilever with optical read-out [1]. From the slope of the linear relation a sensitivity of 1.6 lV/lW is deduced, which for the used cantilever
corresponds to a de#ection sensitivity of 1.6 nm/ lW. The minimum detectable cantilever de#ection is on the order of 1 As , which gives a minimum detectable laser power of approximately 50 nW. This value is approximately 100 times larger than estimated for the aluminium-coated silicon cantilever experiment [1]. The di!erence in thermal sensitivity is primarily caused by di!erences in the cantilever dimensions and materials. The piezoresistive probe can be optimised for thermal sensing by using cantilever materials with large di!erences in thermal expansion coe$cients. However, for other applications it might be advantageous to minimise the thermal sensitivity by fabricating cantilevers with a symmetrical cross section, such that the di!erent cantilever layers are deposited in equal thickness on both sides of the cantilever. The cantilevers are multi-layered structures. However, a simple two-layered cantilever model [9] has been used to estimate the cantilever de#ection (z) as a function of irradiation power (P): l3 t #t 2 z"5 (a !a ) 1 P, 4 1 2 t2 k (j t #j t )w 1 1 2 2 2
(1)
where a is the thermal expansion coe$cient, t is the cantilever layer thickness, l and w are the length and the width of the cantilever and j is the thermal conductivity. The indices refer to the two cantilever materials. The equation contains a geometrical
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factor k [9], which for our cantilever composition equals 36. Inserting the material parameters for our cantilever and a silicon thickness of 0.5 lm and a silicon oxide thickness of 1 lm, we obtain a theoretical de#ection sensitivity of 0.6 nm/lW, which is a factor of 2.7 lower than the experimentally observed sensitivity. The di!erence is probably due to the use of a simple two-layered cantilever model. As the laser heats the cantilever also the temperature of the piezoresistor will increase. This might result in a resistance change of the piezoresistor, which is not related to the bimaterial e!ect. However, the resistance change is expected to depend quadratically on the temperature change and the polysilicon resistors have been developed and have been measured to have a low-temperature dependence [10]. The observed linear relation between the laser power and the cantilever response suggests that the cantilever response is primarily related to the temperature-induced cantilever bending.
3. Humidity sensor Micromachined cantilevers have been applied as humidity sensors [8,11]. Basically, one side of the cantilever is treated with a material, which absorbs water. The water absorption can be measured as a mass change of the cantilever or as a change in the surface stress. For example one side of a silicon oxide cantilever has been coated with gold, whereas the other side is left unprotected. Since gold is inert to water vapour, the water absorption by the exposed oxide face of the cantilever will cause the cantilever to bend [11]. The mentioned humidity experiments have been carried out using optical leverage for cantilever read-out. A reference signal has been obtained by addressing closely spaced cantilevers sequentially by a multiplexing technology. We have applied the piezoresistive probe as a humidity sensor with built-in reference cantilever by placing a water absorbing polymer layer on one of the cantilevers. 3.1. Experiment To make the piezoresistive probe sensitive to humidity a UV sensitive resist [12] is placed on one
of the cantilevers with a glass capillary and a micromanipulator. The resulting polymer layer is approximately 10 lm thick. The polymer is know to swell when exposed to water and it is therefore expected that the cantilever will bend away from the polymer side when the humidity is increased. That is, a compressive stress in the polymer is expected. The cantilever chip is placed in an environmental chamber where the humidity can be controlled by mixing of dry and wet nitrogen #ow. A conventional humidity sensor based on capacitive read-out monitors the chamber humidity. 3.2. Results The humidity in the environmental chamber has been varied from 2% to 60% and the cantilever response has been recorded simultaneously. In Fig. 4, the output voltage from the probe is plotted as a function of the humidity in the chamber. The polymer-coated cantilever responds strongly to the humidity in the chamber and at a humidity of 60% the cantilever de#ection is approximately 10 lm. The saturation of the response appearing at approximately 58% humidity could be due to a plastic deformation of the resist. The cantilever response is very reproducible and when measured by the Wheatstone bridge the background drift has already been subtracted, yielding very stable
Fig. 4. Cantilever response as a function of humidity. One of the two cantilevers on the piezoresistive probe has been coated with a water-absorbing polymer. The water induced swelling of the polymer causes the cantilever to bend.
A. Boisen et al. / Ultramicroscopy 82 (2000) 11}16
measurement conditions. However, there is not a linear relation between humidity and cantilever response, which would be ideal for sensor application. The delay in the cantilever response probably re#ects the time it takes for water molecules to reach equilibrium conditions in the polymer "lm. The non-linearity can be minimised by optimising the thickness and the quality of the polymer coating.
4. Alcohol in water Several micromachined cantilevers with integrated piezoresistive read-out have been developed since the beginning of the 1990s for scanning probe microscopy [13}15]. However, to our knowledge none of these devices have been operated in liquid. We demonstrate a "rst application of our piezoresistive AFM probe as a cantilever-based alcohol sensor in water. 4.1. Experiment For measurements in liquid the metal wires on the AFM probe as well as the bonding wires to a ceramic substrate on which the probe is mounted have to be protected. For this purpose the probe and the ceramic substrate have been coated with a vacuum sealing wax [16] which is liquid at approximately 1003C. The wax is melted and the chip and the ceramic substrate are immersed into the liquid wax with a micromanipulator. Thereby, it is possible to carefully protect bonding wires as well as the wires on the probe. With this protective coating the cantilevers have been tested for drift and stability in water. By a micromanipulator the cantilever is immersed into a 1.4 ml liquid container. During 24 h of measurements in water a drift of approximately 1% was observed and no instabilities occurred. The noise level is comparable to the noise level for measurements in air. 4.2. Results A probe with one polymer-coated cantilever has been inserted in a 1.4 ml water container. The poly-
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mer and cantilever preparation is the same as described for the humidity measurements. As the probe is immersed in the water the polymer starts to swell and the cantilever bends. After approximately 1 min the cantilever de#ection stabilises. At this point the polymer is probably completely saturated with water. When the cantilever de#ection is stable, liquid ethanol is placed with a syringe on the water surface, close to the cantilever. In Fig. 5 the cantilever response is shown as a function of time for three amounts of ethanol. The injection of alcohol is marked with arrows. It is seen that the cantilever responds immediately to the injected alcohol and the cantilever de#ection corresponds to a contraction of the polymer "lm. Maybe, the contraction is caused by alcohol entering the polymer and expelling the water from the "lm. Moreover, the magnitude of the cantilever response is related to the amount of alcohol injected. It is plausible that the response re#ects the local concentration of alcohol in the polymer. Thus, the probe might be able to detect the presence and the amount of alcohol in water. The cantilever signal decreases as a function of time, and for 60 ll of ethanol, the cantilever has reached its initial position after approximately 8 min. Since the liquid container is not completely sealed, alcohol can evaporate from the water surface. Thus, the decreasing signal is probably a picture of a local decrease in the alcohol concentration which
Fig. 5. Cantilever response as a function of time for piezoresistive probe in water. Di!erent amounts of ethanol have been injected into the water (indicated by the arrows) which causes the cantilever to bend.
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is caused by a combination of alcohol evaporating from the surface, and alcohol being diluted in the water. These preliminary alcohol measurements show that it is possible to operate piezoresistive cantilevers in liquid and demonstrate a simple example of a sensor application.
5. Conclusions In conclusion, we have demonstrated the use of micromachined cantilevers with integrated piezoresistive read-out as environmental sensors. The piezoresistive read-out o!ers a simple, fast and compact read-out scheme for the cantilever and the integrated reference cantilever allows us to subtract background drift directly in the measurement. The cantilever-based sensor has been applied as a laser power meter and as a humidity sensor. Finally, the piezoresistive cantilevers have been operated successfully in liquid and have been used for detection of ethanol in water. Integrated read-out greatly facilitates measurements in liquid, and we see the piezoresistive probe as a very promising tool for future cantilever based sensor experiments in liquid.
Acknowledgements The authors would like to thank Esben Friis for helpful suggestions on protective coating of the AFM probes. This work has been supported by the FREJA programme.
References [1] J.K. Gimzewski, Ch. Gerber, E. Meyer, R.R. Schlitter, Chem. Phys. Lett. 217(5,6) (1994) 589.
[2] J.R. Barnes, R.J. Stephenson, M.E. Welland, Ch. Gerber, J.K. Gimzewski, Nature 372 (1993) 79. [3] R. Berger, E. Delamarche, H.P. Lang, Ch. Gerber, J.K. Gimzewski, E. Meyer, H.J. GuK ntherodt, Science 276 (1997) 2021. [4] L. Scandella, G. Binder, T. Mezzacasa, J. Gobrecht, R. Berger, H.P. Lang, Ch. Gerber, J.K. Gimzewski, J.H. Koegler, J.C. Jansen, Microporous Mesoporous Materials 21 (1998) 403. [5] H.P. Lang, R. Berger, F. Battiston, J.-P. Ramseyer, E. Meyer, C. Andreoli, J. Brugger, P. Vettiger, M. Despont, T. Mezzacasa, L. Scandella, H.-J. GuK ntherodt, Ch. Gerber, J.K. Gimzewski, Appl. Phys. A 66 (1998) s61. [6] J. Thaysen, A. Boisen, O. Hansen, S. Bouwstra, Digest of technical papers, Transducers 99, The 10th International Conference on Solid-state Sensors and Actuators, Sendai, Japan, June 7}10, 1999, pp. 1852}1855. [7] R. Berger, Ch. Gerber, J.K. Gimzewski, E. Meyer, H.J. GuK ntherodt, Appl. Phys. Lett. 69 (1) (1996) 40. [8] R. Berger, H.P. Lang, Ch. Gerber, J.K. Gimzewski, J.H. Fabian, L. Scandella, E. Meyer, H.J. GuK ntherodt, Chem. Phys. Lett. 294 (1998) 363. [9] A.M. Moulin, R.J. Stephenson, M.E. Welland, J. Vac. Sci. Technol. B. 15 (3) (1997) 590. [10] J. Thaysen, A. Boisen, O. Hansen, S. Bouwstra, Sensors Actuators (1999), submitted for publication. [11] H.P. Lang, M.K. Baller, F.M. Battiston, J. Fritz, R. Berger, J.-P. Ramseyer, P. Fornaro, E. Meyer, H.-J. GuK ntherodt, J. Brugger, U. Drechsler, H. Rothuizen, M. Despont, P. Vettiger, Ch. Gerber, J.K. Gimzewski, in: Technical Digest of MEMS'99, 12th IEEE International Micro Electro Mechanical Systems Conference, January 17}21, 1999, Orlando, FL, USA, 1999, pp. 9}13. [12] AZ 5200 E, Hoechst Celanese Corp., Chatham, NJ 07928, USA [13] M. Tortonese, H. Yamada, R.C. Barret, C.F. Quate, IEEE 91CH2815-5 (1991) 448. [14] R. Linnemann, T. Gotszalk, I.W. Rangelow, P. Dumania, E. Oesterschulze, J. Vac. Sci. Technol. B 14 (2) (1996) 856. [15] B. W. Chui, T.D. Stowe, Y.S. Ju, H.T. Soh, S.C. Minne, K.E. Goodson, C.F. Quate, T. Kenny, Solid State Sensor and Actuator Workshop, Hilton Head, South Carolina, 1996, p. 219. [16] Apiezon wax W, Ax-Lab A/S, Strandboulevarden 64, DK2100, K+benhavn ", Denmark.
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-
* Corresponding author. Tel.: #45-4525-5727; fax: #454588-7762. E-mail address: [email protected] (A. Boisen)
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 cantilever-based sensors a reference cantilever is crucial, in order to reduce background noise such as thermal drift and gas turbulence [5]. AFM probes suitable for cantilever-based sensors 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 cantilevers 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
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Fig. 1. Schematic drawing and microscope image of AFM probe with integrated piezoresistive read-out. The cantilevers are approximately 50 lm wide, 200 lm long and 1.5 lm thick.
change of the piezoresistors (*R/R) is detected as an output voltage (< ) from the Wheatstone bridge 0 with a supply voltage (<). The output voltage can be written as < "1/4<*R/R. 0 The cantilevers are layered silicon/silicon oxide beams with integrated polysilicon resistors. The cantilevers are 200 lm long, 50 lm wide and approximately 1.5 lm thick. The de#ection sensitivity is measured to be approximately *R/R"10~6 nm~1 and for a Wheatstone bridge supply voltage of 2 V and for a measurement bandwidth of 10 Hz the minimum detectable cantilever de#ection is estimated to 0.3 As [6]. Here we demonstrate the possibility of using the AFM probe with integrated read-out as a versatile cantileverbased sensor. The probe has been successfully applied as a thermal sensor and as a humidity sensor. Moreover, the probe has been operated in liquid, where it has been used for detection of alcohol in water. The Wheatstone bridge has in all experiments been operated at a supply voltage of 4 V.
mal spectroscopy [2]. Moreover, commercially available piezoresistive cantilevers have been used for thermogravimetry [8]. We have investigated the stability and the sensitivity of our piezoresistive probe when applied as a thermal sensor. Since the used cantilevers are bilayered structures the cantilevers will bend when heated due to di!erences in the thermal expansion coe$cient of the silicon and the silicon oxide layer. Therefore, the probe can be directly used as a thermal sensor. Due to the symmetrical Wheatstone bridge con"guration the probe registers the di!erence in the temperature of the two cantilevers. This e!ect can, as earlier demonstrated for cantilevers with optical read-out [1], be used to estimate the amount of irradiation power absorbed by the cantilever. As the cantilevers are inherently sensitive to temperature changes this could cause an unintended drift in the read-out. However, the symmetrical Wheatstone bridge will eliminate such e!ects, if the two cantilevers are exposed to the same thermal environment.
2. Thermal sensor
2.1. Experiment
Cantilevers have previously been utilised as micromechanical calorimeters, by exploiting the bimaterial e!ect. Cantilevers with optical read-out have been used in for example investigations of phase transitions in alkanes [7] and in photother-
To investigate the performance of the probe as a thermal sensor, one of the two cantilevers in the Wheatstone bridge has been locally heated by focusing a continuous wave argon ion laser at the cantilever apex. A schematic cross-sectional
A. Boisen et al. / Ultramicroscopy 82 (2000) 11}16
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Fig. 3. Cantilever response as a function of laser power. The cantilever signal is seen to depend linearly on the irradiation power.
Fig. 2. Schematic drawing of silicon oxide/silicon cantilever with integrated polysilicon piezoresistor. A laser beam is focused at the cantilever apex.
drawing of the cantilever and the laser position is shown in Fig. 2. The spot diameter of the focused laser beam is approximately 1 lm, and the laser has been operated at a wavelength of 488 nm. The cantilever is placed below the focused laser spot by high-resolution dc motor stages and a CCD camera monitors the laser spot and the cantilever position. 2.2. Results The power of the laser, positioned at the cantilever apex, has been varied and the corresponding cantilever de#ection has been measured. The cantilever de#ection is registered as the output voltage from the Wheatstone bridge. As shown in Fig. 3 a linear relationship between laser power and cantilever de#ection is observed. Similar results have previously been observed for an aluminium-coated silicon cantilever with optical read-out [1]. From the slope of the linear relation a sensitivity of 1.6 lV/lW is deduced, which for the used cantilever
corresponds to a de#ection sensitivity of 1.6 nm/ lW. The minimum detectable cantilever de#ection is on the order of 1 As , which gives a minimum detectable laser power of approximately 50 nW. This value is approximately 100 times larger than estimated for the aluminium-coated silicon cantilever experiment [1]. The di!erence in thermal sensitivity is primarily caused by di!erences in the cantilever dimensions and materials. The piezoresistive probe can be optimised for thermal sensing by using cantilever materials with large di!erences in thermal expansion coe$cients. However, for other applications it might be advantageous to minimise the thermal sensitivity by fabricating cantilevers with a symmetrical cross section, such that the di!erent cantilever layers are deposited in equal thickness on both sides of the cantilever. The cantilevers are multi-layered structures. However, a simple two-layered cantilever model [9] has been used to estimate the cantilever de#ection (z) as a function of irradiation power (P): l3 t #t 2 z"5 (a !a ) 1 P, 4 1 2 t2 k (j t #j t )w 1 1 2 2 2
(1)
where a is the thermal expansion coe$cient, t is the cantilever layer thickness, l and w are the length and the width of the cantilever and j is the thermal conductivity. The indices refer to the two cantilever materials. The equation contains a geometrical
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A. Boisen et al. / Ultramicroscopy 82 (2000) 11}16
factor k [9], which for our cantilever composition equals 36. Inserting the material parameters for our cantilever and a silicon thickness of 0.5 lm and a silicon oxide thickness of 1 lm, we obtain a theoretical de#ection sensitivity of 0.6 nm/lW, which is a factor of 2.7 lower than the experimentally observed sensitivity. The di!erence is probably due to the use of a simple two-layered cantilever model. As the laser heats the cantilever also the temperature of the piezoresistor will increase. This might result in a resistance change of the piezoresistor, which is not related to the bimaterial e!ect. However, the resistance change is expected to depend quadratically on the temperature change and the polysilicon resistors have been developed and have been measured to have a low-temperature dependence [10]. The observed linear relation between the laser power and the cantilever response suggests that the cantilever response is primarily related to the temperature-induced cantilever bending.
3. Humidity sensor Micromachined cantilevers have been applied as humidity sensors [8,11]. Basically, one side of the cantilever is treated with a material, which absorbs water. The water absorption can be measured as a mass change of the cantilever or as a change in the surface stress. For example one side of a silicon oxide cantilever has been coated with gold, whereas the other side is left unprotected. Since gold is inert to water vapour, the water absorption by the exposed oxide face of the cantilever will cause the cantilever to bend [11]. The mentioned humidity experiments have been carried out using optical leverage for cantilever read-out. A reference signal has been obtained by addressing closely spaced cantilevers sequentially by a multiplexing technology. We have applied the piezoresistive probe as a humidity sensor with built-in reference cantilever by placing a water absorbing polymer layer on one of the cantilevers. 3.1. Experiment To make the piezoresistive probe sensitive to humidity a UV sensitive resist [12] is placed on one
of the cantilevers with a glass capillary and a micromanipulator. The resulting polymer layer is approximately 10 lm thick. The polymer is know to swell when exposed to water and it is therefore expected that the cantilever will bend away from the polymer side when the humidity is increased. That is, a compressive stress in the polymer is expected. The cantilever chip is placed in an environmental chamber where the humidity can be controlled by mixing of dry and wet nitrogen #ow. A conventional humidity sensor based on capacitive read-out monitors the chamber humidity. 3.2. Results The humidity in the environmental chamber has been varied from 2% to 60% and the cantilever response has been recorded simultaneously. In Fig. 4, the output voltage from the probe is plotted as a function of the humidity in the chamber. The polymer-coated cantilever responds strongly to the humidity in the chamber and at a humidity of 60% the cantilever de#ection is approximately 10 lm. The saturation of the response appearing at approximately 58% humidity could be due to a plastic deformation of the resist. The cantilever response is very reproducible and when measured by the Wheatstone bridge the background drift has already been subtracted, yielding very stable
Fig. 4. Cantilever response as a function of humidity. One of the two cantilevers on the piezoresistive probe has been coated with a water-absorbing polymer. The water induced swelling of the polymer causes the cantilever to bend.
A. Boisen et al. / Ultramicroscopy 82 (2000) 11}16
measurement conditions. However, there is not a linear relation between humidity and cantilever response, which would be ideal for sensor application. The delay in the cantilever response probably re#ects the time it takes for water molecules to reach equilibrium conditions in the polymer "lm. The non-linearity can be minimised by optimising the thickness and the quality of the polymer coating.
4. Alcohol in water Several micromachined cantilevers with integrated piezoresistive read-out have been developed since the beginning of the 1990s for scanning probe microscopy [13}15]. However, to our knowledge none of these devices have been operated in liquid. We demonstrate a "rst application of our piezoresistive AFM probe as a cantilever-based alcohol sensor in water. 4.1. Experiment For measurements in liquid the metal wires on the AFM probe as well as the bonding wires to a ceramic substrate on which the probe is mounted have to be protected. For this purpose the probe and the ceramic substrate have been coated with a vacuum sealing wax [16] which is liquid at approximately 1003C. The wax is melted and the chip and the ceramic substrate are immersed into the liquid wax with a micromanipulator. Thereby, it is possible to carefully protect bonding wires as well as the wires on the probe. With this protective coating the cantilevers have been tested for drift and stability in water. By a micromanipulator the cantilever is immersed into a 1.4 ml liquid container. During 24 h of measurements in water a drift of approximately 1% was observed and no instabilities occurred. The noise level is comparable to the noise level for measurements in air. 4.2. Results A probe with one polymer-coated cantilever has been inserted in a 1.4 ml water container. The poly-
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mer and cantilever preparation is the same as described for the humidity measurements. As the probe is immersed in the water the polymer starts to swell and the cantilever bends. After approximately 1 min the cantilever de#ection stabilises. At this point the polymer is probably completely saturated with water. When the cantilever de#ection is stable, liquid ethanol is placed with a syringe on the water surface, close to the cantilever. In Fig. 5 the cantilever response is shown as a function of time for three amounts of ethanol. The injection of alcohol is marked with arrows. It is seen that the cantilever responds immediately to the injected alcohol and the cantilever de#ection corresponds to a contraction of the polymer "lm. Maybe, the contraction is caused by alcohol entering the polymer and expelling the water from the "lm. Moreover, the magnitude of the cantilever response is related to the amount of alcohol injected. It is plausible that the response re#ects the local concentration of alcohol in the polymer. Thus, the probe might be able to detect the presence and the amount of alcohol in water. The cantilever signal decreases as a function of time, and for 60 ll of ethanol, the cantilever has reached its initial position after approximately 8 min. Since the liquid container is not completely sealed, alcohol can evaporate from the water surface. Thus, the decreasing signal is probably a picture of a local decrease in the alcohol concentration which
Fig. 5. Cantilever response as a function of time for piezoresistive probe in water. Di!erent amounts of ethanol have been injected into the water (indicated by the arrows) which causes the cantilever to bend.
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A. Boisen et al. / Ultramicroscopy 82 (2000) 11}16
is caused by a combination of alcohol evaporating from the surface, and alcohol being diluted in the water. These preliminary alcohol measurements show that it is possible to operate piezoresistive cantilevers in liquid and demonstrate a simple example of a sensor application.
5. Conclusions In conclusion, we have demonstrated the use of micromachined cantilevers with integrated piezoresistive read-out as environmental sensors. The piezoresistive read-out o!ers a simple, fast and compact read-out scheme for the cantilever and the integrated reference cantilever allows us to subtract background drift directly in the measurement. The cantilever-based sensor has been applied as a laser power meter and as a humidity sensor. Finally, the piezoresistive cantilevers have been operated successfully in liquid and have been used for detection of ethanol in water. Integrated read-out greatly facilitates measurements in liquid, and we see the piezoresistive probe as a very promising tool for future cantilever based sensor experiments in liquid.
Acknowledgements The authors would like to thank Esben Friis for helpful suggestions on protective coating of the AFM probes. This work has been supported by the FREJA programme.
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