Closed-loop surface stress compensation with an electromagnetically actuated microcantilever

Sensors and Actuators B 213 (2015) 566–573

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Closed-loop surface stress compensation with an electromagnetically actuated microcantilever夽 Daniel Kopiec a,∗ , Piotr Pałetko a , Konrad Nieradka a,1 , Wojciech Majstrzyk a , a ´ Piotr Kunicki a , Andrzej Sierakowski b , Grzegorz Józwiak , Teodor Gotszalk a a b

Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wrocław 50-372, Poland Division of Silicon Microsystem and Nanostructure Technology, Institute of Electron Technology, Warsaw 02-668, Poland

a r t i c l e

i n f o

Article history: Available online 9 March 2015 Keywords: Electromagnetically actuated cantilever Lorentz force Surface stress Halbach array Static mode

a b s t r a c t In this paper, we present a new method for bending and stress measurements using an electromagnetically actuated cantilever. The proposed method is based on compensation of the cantilever’s bending resulting from induced stress, which can be translated into the difference in surface stress, a measure of intermolecular interactions on the cantilever’s surface. In our method, the Lorentz force acts as balancing force restoring the cantilever to its original position. Optical beam deflection (OBD) scheme is used to measure the cantilever’s bending. The error signal of the feedback loop is used to maintain the cantilever in a fixed position by controlling the Lorentz force. The Lorentz force is hence the measure of the debalancing force, e.g. surface stress. The laser spot on the detector may be maintained in the zero position where the OBD method has the highest sensitivity and nearly linear response. Thus, we provide linear working range of the photodetector. In addition, the actuator current loop on the cantilever acts as a thermal sensitive resistor for the cantilever and its immediate surroundings; thus, trends in temperature can be measured in parallel to the surface stress measurement and accounted for in the results. The new method was successfully applied to monitoring the self-assembly process during adsorption of thiophenol on the gold cantilever surface. The results demonstrate that the new single electromagnetically actuated cantilever working in static mode has great potential for sensor applications. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Microcantilever sensors are popular devices for chemical and biochemical sensing [1,2] as they allow fast and reliable detection of small amounts of analytes in the gas phase and liquids. The cantilever sensors have been successfully applied in studies DNA interactions [3], to measure pH changes [4], to determine surface stress associated with molecular adsorption [5,6], in cancer research [7], and in the detection of viruses and bacteria [8]. These type of sensors can operate in two measurement modes. In the static measurement mode the cantilever is bent due to the surface stress occurring during the adsorption of molecules [9]. Whereas in the dynamic mode the cantilever resonance frequency changes,

夽 Selected papers presented at EUROSENSORS 2014, the XXVIII edition of the conference series, Brescia, Italy, September 7–10, 2014. ∗ Corresponding author. Tel.: +48 713203651. E-mail address: [email protected] (D. Kopiec). 1 Present address: Fraunhofer IIS, Project Group Wireless Distribution Systems/Digital Broadcasting, 98693 Ilmenau, Germany. http://dx.doi.org/10.1016/j.snb.2015.03.001 0925-4005/© 2015 Elsevier B.V. All rights reserved.

when the sensor is mass loaded [10]. The nanoscale deflection is caused by variation in the cantilever surface stress due to molecular interactions. The top and bottom surfaces of the sensor can be coated with different functional layers; one serves as a sensing layer that reacts to the presence of target species, and the other one is insensitive to them or gives the opposite response. In most of the cantilever based systems the bending of the sensing cantilever is measured with respect to the reference cantilever, which serves as a reference sensor to detect cantilever deflection resulting from temperature variations and/or uncontrolled chemical reactions [11–13]. In this case the reference cantilever must exhibit the same parameters e.g. spring constants, dimensions and should be placed very close to the sensing cantilever. Additionally, the detection system should measure cantilever bending simultaneously. In this paper, we present a novel method and our homemade equipment for the measurement of surface stress which is compensated by an integrated Lorentz force actuator. The Lorentz force-actuated cantilevers are the new group of microand nanotools first proposed by Buguin et al. [14] for atomic force microscopy. In our case, we employed the Lorentz force actuated

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573

567

cantilever for measurements of the surface stress induced by adsorption of thiophenol molecules on cantilever gold surface. Moreover, to measure and compensate thermally induced cantilever bending the actuator current loop was additionally used as a temperature sensing resistor. In our setup, we used the electromagnetically actuated cantilever placed inside the Halbach array and an optical beam deflection (OBD) technique was applied to monitor the microcantilevers’ deflections measurements with sub-angstrom resolution. The novelty of our method is primarily based on the fact that the cantilever bending under surface stress is directly compensated in the closed loop feedback. The cantilever and reflected spot position is held constant by acting static Lorentz force playing the role of a balancing interaction. Moreover, in the proposed technique the position of the laser spot on the split-photodiode detector may be maintained on the border between sections. Since the sensitivityposition characteristic of this type of detector is a reflection of the optical power distribution in the laser beam, when focused on this border, the optical setup operates at its highest sensitivity, which ensures the highest resolution of the stress compensation. At the same time the nonlinearities introduced by the optical power distribution have no effect on the response characteristic of the proposed method, which is in wide range linear. The presented method allows the direct measurement of the stress on the cantilever surface, where the resulting surface stress depends largely on the balancing force and geometric dimensions of the cantilever.

planarized with polyimide film to control the mechanical stress of the structures during the release of plasma. Finally, the microcantilevers were released in a back-side dry silicon etching process, and the protective polyimide layer was removed. Fig. 1 shows three types of fabricated microcantilevers with integrated current loops; additional fabrication details are described elsewhere [15]. In our experiments we used rectangular tip-less cantilevers shown in Fig. 1a. Length and width of the cantilever measured using a scanning electron microscope were 575 ␮m × 190 ␮m, respectively. The cantilevers thickness of 2.4 ␮m was calculated using ´ method described by Józwiak et al. [16]. The resonance frequency and resulting spring constant were 7059.6 Hz and 0.39 N/m, respectively. The resistance of gold line, acting as Lorentz force actuator, was approximately 70.4 . The length of the active part of Lorentz actuator was 135 ␮m.

2. Materials and methods

 =

2.1. Fabrication of cantilever sensor

where t is the thickness of the cantilever, R curvature radius of the cantilever surface, E and  are its material’s Young’s modulus and Poisson ratio, respectively. The Young’s modulus must be known exactly in order to determine the surface stress. However, the precise determination of the Young’s modulus is extremely difficult for a multilayer cantilever, due to the stress between the particular layers. Therefore, in our investigations we used formula proposed by Godin et al. [19], which stems from the energy calculations done for a spring beam. In this approach stress  is a function of beam geometry and cantilever deflection z:

The cantilever sensor was fabricated based on the double-sided micromachining concept. As an input substrate, 4-inch DSP silicon wafers with (1 0 0) crystallographic orientation, thicknesses of 400 ␮m, n-type conductivities, and resistivities of 3–5  cm were used. The first step consisted of wet oxidation and a low pressure chemical vapor deposition (LPCVD) coating of the wafer with an Si3 N4 film followed by photolithography and KOH etching to form a 50-␮m thick membrane. Next, the front of the wafer was coated with a 200-nm thick gold film by magnetron sputtering, The current lines, the reflective apex, and the self-assembled monolayer (SAM) deposition region were defined by photolithography and the wet etching process. The next step consisted of the photolithography and etching of the cantilever shape using deep reactive ion etching (DRIE) to obtain a ‘relief’ defining the desired thickness of the cantilever (3–5 ␮m). The thickness of the cantilever is a crucial parameter for the mechanical parameters of the microprobe, including stiffness, stress distribution and resonance frequency. The relief technique may be applied down to the 3–4 ␮m range with satisfactory microprobe yield. The front-side of the wafer was

2.2. Static mode The source of surface stress is the interaction between the surface and adsorbates [4], and/or the interactions between a surface and its ambient environment [17]. The differential surface stress created by molecular adsorption results in the cantilever bending. The measurement of bending is the basis for the static mode operation of the cantilever sensor. Stoney’s equation relates the difference in surface stress  between the chemically modified surface and the untreated surface to the cantilever bending [18]:

 =

Et 2 6R(1 − )

4 1 lC kz 3 (1 − ) wt

(1)

(2)

where lC , w, and k are the length, width and spring constant of the cantilever, respectively. In the case of a molecular adsorption induced surface stress, we must account for the fact that the cantilever deflection is the result of an isotropic surface stress [20], which acts over the entire surface of the cantilever. In Eq. (2) the factor 4/3 takes into account the different cantilever beam curvatures resulting from a uniform

Fig. 1. SEM images of three types of electromagnetically actuated cantilevers fabricated at the Institute of Electron Technology in Warsaw. Their top sides are coated with gold, which serves as a mirror for the readout laser beam, a substrate for self-assembled monolayers (SAMs), and a current loop for Lorentz force actuation. The difference in construction provides different spring constant and sensitivity to stress. In the described experiments we used only cantilever type (a).

568

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573

surface stress, as opposed to a concentrated load applied at the end of the cantilever. This factor can be derived by comparing the strain energies of a cantilever that is deflected by a concentrated load and one deflected by a surface stress [19]. According to Eq. (2) relation between Hooke’s law and surface stress can be written as: F=

3 wt  (1 − ) 4 lC

(3)

By using the above formula, Young’s modulus of the cantilever, which often carries a high level of uncertainty, does not need to be known. Whereas, the geometric parameters of the cantilever can be measured using optical microscopy or scanning electron microscopy (SEM). The microcantilever deflections z is monitored using the OBD technique. The cantilever’s displacement z is transformed into the laser spot’s displacement y on the position sensitive detector (PSD). Under the assumption of circular deformation of the cantilever the relation between the displacements takes form of: z =

lC y 4s

(4)

where y vertical displacement of the laser spot on the PSD, s the distance between the cantilever and the PSD. Thus, with use of Eqs. (2) and (4) the microcantilever deflection signal can be converted into a surface stress value. The OBD setup should be calibrated, to ensure accurate quantitative surface stress measurements. The sensitivity and noise level of the readout system determine the detection threshold of investigated phenomena, such as static bending, vibrations, or the thermomechanical noise of the cantilevers. In our investigations we calibrated the OBD system using a reference cantilever whose stiffness and thermomechanical noise amplitude were determined precisely using a laser vibrometer. Thermomechanical noise of the reference cantilever was used to calibrate the sensitivity of the applied OBD measurements head. In this way the deflection sensitivity of 15.8 mV/nm was determined. 2.3. Lorentz force actuated cantilever In our setup, stress induced by the adsorption of molecules is compensated by the Lorentz force. Fig. 1(b) shows a schematic of the cantilever actuated by the Lorentz force. In static mode when a direct current i passes through the U-shaped current loop, and the cantilever is placed in static magnetic field B the Lorentz force FEL causes the cantilever to bend. This mode is characterized by the up and down movements of the cantilever in the z-direction. It should be also noted that the Lorentz force can be used for both static and dynamic actuation. In the static operation the direction of the deflection depends on the current and the magnetic field vector directions. The acting Lorentz force can be described as: FEL = iL × B

(5)

where L is a vector whose magnitude is the length of the active part of the current loop and direction is along the active part of the loop, aligned with the direction of the current flow, i is the current in the loop, and B is the magnetic field vector. According to Eq. (5) the force acting on the cantilever can be induced by control of the current flowing through the loop. The Lorentz force acting on the whole beam is equal to the sum of the Lorentz force acting on each segment of the current loop. If the magnetic field is uniform and perpendicular to the active part of the cantilever, the Lorentz force of the parallel legs of actuator is nearly zero. The cancellation of the force from the cantilever legs causes that the Lorentz force is concentrated at the cantilever end. In case of non-uniform distribution of the magnetic field the gradient may result in torque of the cantilever. On the other hand the

force induced by a magnetic dipole generated by the closed current loop is negligible. Thus the force acting on the cantilever is dominated by the magnetic field, and not by the magnetic field gradient. In our setup as a source of the magnetic field we have used NdFeB permanent magnets forming a Halbach array to augment the magnetic field and uniformity [21,22]. Due to the setup symmetry the Halbach arrays ensure high density magnetic field and enable control of its direction. Since the value of the magnetic field inside the Halbach array depends on the lateral position it is necessary to carry out the calibration procedure in the presence of magnetic field with a precisely known value. In order to perform quantitative Lorentz force measurements, we conducted the calibration procedure of actuation sensitivity of the Lorentz force actuator. The aim of the calibration is to determine the effect of the magnetic field and current on the cantilever deflection and the quantitative representation in nm/␮AT. In this way, assuming that one of the actuation sensitivity factors is constant it is straightforward to derive the other. The calibration of an electromagnetic actuator were performed inside the Helmholtz coils serving as the precise reference source of the magnetic field [23]. It should be noted that actuation sensitivity can be performed in the static mode or dynamic mode [20]. The choice of method is depends on the cantilever spring constant and the value of the magnetic field of Helmholtz coil. Although the magnetic field of Helmholtz coils is uniform its value does not exceed several mT. Additionally, the loop current should be kept as small as possible in order to avoid parasitic thermal effects. Therefore, in general a soft cantilever (of spring constant less than 0.1 N/m) should be applied in the static mode experiments, whereas a stiffer sensor should be used in dynamic mode investigations. During the calibration procedure the cantilever deflection/vibration were measured by laser vibrometer SP-S series. Finally, the actuation sensitivity measured in dynamic mode was determined as a change of the cantilever oscillation amplitude due to the change of magnetic field for constant value of alternating current in the loop. The actuation sensitivity coefficient is true for actuation close to the resonance frequency, whereas for operation in static mode one should take into account that actuation sensitivity is Q-fold lower, where Q is the quality factor of the cantilever. Fig. 2 shows the directions of the magnetic field lines. The magnetic field strength measured inside the center Halbach array cylinder was 330 mT. A simple calculation according to Eq. (5) can be made if the magnetic field is oriented perpendicular to the active part of the current loop, otherwise torque of the cantilever may occur. In order to avoid these phenomena before the experiment the actuator loop of the cantilever is powered with white noise signal and the averaged power spectral densities of deflection signal is analyzed. If any torsional resonance in the spectrum is observed the Halbach array is rotated relatively to the cantilever mounted in the OBD head and its orientation should be adjusted. In this way, we ensure that the magnetic field is oriented perpendicularly to the active part of the Lorentz loop. 2.4. Thermal drift Thermal drift is one of the important parasitic phenomena influencing cantilever bending and may distort the measurement results. If the cantilever has a sandwich-like structure consisting of materials with different thermal expansion coefficients, the sensor can work as a very sensitive thermometer or calorimeter [24]. The resulting bending signal can come from transduction coefficients for differential surface stress and temperature [25]. The sensor bending caused by changes in temperature may be assumed to be proportional to the change in temperature. In addition to changes

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573

569

Fig. 2. Halbach array, used to generate a magnetic field. The matrix dimensions are as follows: outer diameter, 100 mm; inner diameter, 60 mm; height, 30 mm. Iron filings indicate the propagation of the magnetic field lines.

in environmental temperature, the flow rate of the medium also affects the effectiveness of cantilever cooling or heating. In our experiments, we used a current loop as a reference thermal resistor for measurement changes in temperature of the cantilever and its environment. Fig. 3 shows the block diagram of the proposed setup. The current loop acting as a Lorentz force actuator and temperature sensing resistor (RLOOP ) is connected in a full Wheatstone bridge. The proposed method is derived from thermal scanning microscopy [26]. The measurement bridge is driven from a voltage-controlled current source using a signal consisting of two components: the AC measurement signal at 10 kHz with an amplitude of 12 ␮A, and the DC signal responsible for compensating the beam deflection. The AC current was kept small in order to avoid cantilever self-heating and additional bending. Before started the measurement, the bridge had to be balanced so that the ac component was zero. Temperature changes caused the bridge to unbalance and change the amplitude of the ac signal, which we measured using an AMETEK SR7280 lock-in amplifier. This enables to perform measurement of the temperature changes of the cantilever caused by exothermic adsorption of chemicals on its surfaces [27], the environment, and the self-heating of the cantilever.

3. Surface stress measurement by the balancing force The diagram of experimental setup is depicted in Fig. 3. In the proposed method, the Lorentz force is a balancing force to restore the cantilever and reflected spot to its original position, regardless of the resulting surface stress. It should be noted, that the split-photodiode detector has its highest sensitivity on the border between sections. When the laser spot is shifted from the center, the deflection sensitivity is reduced significantly [28,29] and nonlinearities appear in the response function. In case of a conventional OBD system increasing the sensitivity of surface stress measurements usually has a negative result in decreasing the measurement range. However, in our setup the cantilever deflection is controlled in closed-loop and the laser spot does not leave the central position. We are able to measure the cantilever surface stress within wide range and with maximum sensitivity achievable by OBD method at the same time. The useful signal is derived from the feedback loop, the acting force is proportional to the current flowing through the resistance R1 and RLOOP (see Fig. 3). The measurements of surface stress were performed in a home-made system, as shown in Fig. 4. The electromagnetically actuated cantilever was placed inside the Halbach

Fig. 3. Block diagram of the proposed method using Lorentz force as a balancing force to restore the cantilever to its original position. The optical beam deflection technique is used to monitor the induced bending of the cantilever. The displacement of the reflected spot, which is proportional to the cantilever deflection, is monitored using a position-sensitive photodetector. The actuation loop is also used as a thermal reference resistor, giving information about the trend in temperature. The letters on the diagram indicate the useful signals: A – deflection signal from photodetector; B – feedback control signal, which can be expressed as current in the loop; and C – signal that corresponds to the change in temperature.

570

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573

Fig. 4. Experimental setup for surface stress measurements using measurements of balancing force. The system includes a home-made OBD head, PID regulator and front-end electronics; current source along with a summing amplifier, preamplifier, commercial lock-in amplifier (SR7270), syringe infusion pump, and Halbach array as a source of the magnetic field.

array, serving also as a gas cell with a capacity of about 8 cm3 . Thiophenol vapor was injected in 2-ml portions by a syringe infusion pump at 3-min intervals. Cantilever bending was monitored by the OBD head, and its position was kept constant by the closed loop, the proportional-integral-derivative (PID) regulator controlling the current flowing through the current loop. The ac signal responsible for the measurement of temperature change was recorded using a SR7280 lock-in amplifier. The resulting signals were digitalised using a digital acquisition card and recorded on a PC with dedicated LabVIEW software.

3.1. Cantilever surface preparation Gold is one of the commonly used surfaces for the immobilization of bioreceptors in cantilever-based biosensors that measure bending due to surface stress difference between opposite sides of the cantilever. In such sensors, the high affinity of sulphur for gold is harnessed to form SAMs of thiol ( SH)-derivatized molecules with useful chemical or biological functionalities on the coated cantilever surface [5,30]. Since the substrate morphology plays a crucial role in the response of microcantilever chemical sensors [31], before the deposition of SAMs from the vapor phase, the gold surface on the cantilever shown in Fig. 1(a) was cleaned by immersing it in piranha solution at 60 ◦ C for 30 s and then thoroughly rinsed with water and ethanol. This step cleared the gold surface from unwanted adsorbates. All steps were conducted under atmospheric pressure and at room temperature.

4. Results and discussion 4.1. Calibrating and closed loop stress compensating The calibration results of the actuation sensitivity of the Lorentz force actuator placed inside the Helmholtz coils are presented in Fig. 5. The results were obtained for dynamic mode, in which the cantilever was excited on its resonance frequency. The dynamic actuation sensitivity was obtained as a ratio of changes in the cantilever oscillation amplitude to the change the current and static magnetic field. The sensitivity of the electromagnetic actuation was estimated as 26.04 nm/␮AT. In this way, it is possible to use a electromagnetically actuated cantilever as a magnetic field sensor. It should be noted, that the actuation sensitivity in static mode is Q-times smaller than obtained in the dynamic mode. In order to estimate magnetic and thermal components of the static deflection we measured cantilever deflection in response to pseudo-static (low frequency, below 100 Hz) current through in the loop. During the experiment cantilever was immersed in the strong magnetic field generated by the Halbach array. Results are presented in Fig. 6. The magnetic actuation coefficients were established as 0.21 nm/␮AT and 0.17 × 10−4 nm/␮A2 for the parabolic thermal component [15].

3.2. Chemical reagents We used the thiophenol, which consists of aromatic thiol molecules, to test the system performance. Due to the presence of the sulphur in thiophenol molecule, formation of SAM on the cantilever surface was enabled. The presence of an aromatic ring results in a strong interaction between these molecules and the newly formed SAM layer. Additionally, thiophenol has high molecular weight (110.19 g/mol) and vapor pressure (1.9 hPa at 20 ◦ C), allowing SAM to be deposited on the gold cantilever surface from the gas phase. One drop of thiophenol was placed in a 20-ml airfilled syringe and allowed to stand for 1 h to form thiophenol vapors.

Fig. 5. Actuation sensitivity obtained in dynamic mode. Cantilever oscillation amplitude vs. magnetic field generated by Helmholtz coil. AC current through in the loop as a parameter.

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573

Fig. 6. Cantilever static deflection in response to a pseudo-static (low frequency) actuation. Force was calculated according to Eq. (5). The calculations take into account the current loop length (l = 135 ␮m) and magnetic field strength (B = 330 mT).

The linear electromagnetic actuation was accompanied by parasitic thermal actuation, which had a quadratic characteristic. The parasitic thermal effect stems from the Joule heat generated by the current flowing through the loop and from mismatch of thermal expansion coefficients for silicon and gold; both give rise to biomorphic behavior of the cantilever beam. Fig. 7 shows the operation of the cantilever closed-loop. To keep the cantilever deflection in the fixed position, we used an PID analog controller to provide current in the loop. The current value depends on the initial set point value of cantilever deflections and the interactions on the cantilever surfaces. Benefits of closed-loop are possibility of making immediate adjustments as needed, improved stability thanks to which the closed loop systems are less affected by noise. The static deflection noise during the bending compensation (presented in Fig. 8) of 78 pm was achieved, which corresponds to a minimum detectable force of 32 pN and surface stress 4.04 × 10−5 N/m. 4.2. Thermal trend measurements Fig. 8 shows the trend in temperature when the current loop is used as a thermal sensing resistor. The temperature change can result from the changing temperature of the environment, exothermic adsorption of chemicals, or from the self-heating generated by the flow of current through the loop. Knowledge of the temperature trend helps in normalization of the temperature balancing force.

571

Fig. 8. The temperature trends measured using a current loop as a thermal sensitivity resistor. The black line shows the constant cantilever deflection held in a fixed position by the feedback loop, the blue line presents the current on the loop, and the red dots show the trend in temperature measured with the current loop. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

During the adsorption measurements, the feedback loop also responds to the deflection due to changes in temperature; therefore, it is important to know the trend in these changes. In contrast to another experiments in which two reference cantilever was used to trace temperature changes, we track the thermal effects using the Lorentz actuation loop. In order to subtract the temperature impact from the regulator signal we assumed the following signal model: s(n) = ˛T (n) + v(n)

(6)

where s(n) is a regulator output, T(n) is a temperature related signal,

v(n) is the part independent on the temperature change. ˛ is an unknown calibration constant. Since the v(n) is independent, it may be computed by the subtraction of the projection of the regulator output signal s(n) onto the measured temperature signal T(n). It may be done using the following equation: vˆ = s −

s·T T T·T

(7)

where s and T are column vectors of values of signals s(n) and T(n), respectively. This procedure is sensitive to an incidental correlation between a chemical signal v(n) and a temperature signal T(n). In such a case it is impossible to distinguish between T(n) and this correlated part of v(n) without a precise calibration of a ˛ constant. However, if there is no correlation between a chemical signal v(n) and a temperature signal T(n), it is possible to remove the impact of bimorphic effect caused by temperature variations. 4.3. Self-assembly monolayer formation

Fig. 7. Feedback signals of electromagnetically actuated cantilever working in the closed-loop.

The result of cantilever functionalization with thiophenol injected in the measurement chamber is presented in Fig. 9. After preliminary period of stabilizing the air-filled chamber thiophenol vapors were injected causing the cantilever to bend toward the gold layer. In the performed experiments negative value of balancing force indicates beam bending toward the gold layer due to tensile stress in the forming monolayer. The system reacts to the thiophenol assembly by increasing the force needed to keep the cantilever deflection at the defined level. This process is related to the attraction of thiophenol molecules bonded to a gold surface. The stress increased until the entire gold surface was covered by SAMs. After 30 min of thiophenol deposition, the cantilever surface was saturated. Additional portions of thiol vapors did not cause significant changes in the cantilever bending. The presented data reflecting the adsorption trend of the

572

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573

References

Fig. 9. Balancing force, cantilever bending and surface stress as a function of exposure time during the deposition of thiophenol molecules on the gold cantilever surface. The temperature drift has been removed from the force graph according to the temperature measured using the current loop as a thermal sensitivity resistor.

SAM is in good agreement with data presented by Berger et al. [5]. The surface stress and balancing force values were obtained according to Eqs. (2) and (5), respectively. The total uncertainty in the calculated surface stress is a function of the uncertainties in all the variables stated in Eq. (2). In this way, the change in the Lorentz force caused by thiophenol deposition on the cantilever was 4 ± 0.48 nN, which corresponds to the surface stress of approximately 8 ± 1.24 mN/m.

5. Conclusion We have demonstrated the application of an electromagnetically actuated cantilever working in static mode as a surface stress sensor. We have used a single cantilever with an integrated actuator as a current loop that is actuated by the Lorentz force. We used this technique to monitor a self-assembly process on the cantilever surface. The proposed method compensates for cantilever bending induced under stress or mass loading. The Lorentz force is the balancing force to restore the cantilever to the defined position. Since, the reflected laser spot is kept in the center of the splitphotodiode detector the maximum sensitivity and linear system response are maintained while measuring surface stress in wide range. Additionally, the loop integrated on the cantilever acts as a thermally sensitive resistor for the cantilever and its immediate surroundings. The conducted experiments have demonstrated that the electromagnetically actuated cantilever working in the static mode has potential applications for surface stress measurements along with the simultaneous measurement of temperature trends using an integrated cantilever current loop. We also see potential for use of the presented method for monitoring formation of various SAM layer in liquid environment.

Acknowledgements The research was funded by the Foundation For Polish Science within the Mistrz (Master) grant no. 4/2012 and within the TEAM Programme “High-resolution force and mass metrology using actuated MEMS/NEMS devices – FoMaMet” (Grant No. TEAM/2012-9/3) co-financed by the European Regional Development Fund resources within the framework of Operational Program Innovative Economy. The electromagnetically actuated cantilevers were fabricated within the EMAGTOOL NCN OPUS project.

[1] L.G. Carrascosa, M. Moreno, M. Álvarez, L.M. Lechuga, Nanomechanical biosensors: a new sensing tool, TrAC Trends Anal. Chem. 25 (3) (2006) 196–206. [2] P.S. Waggoner, H.G. Craighead, Micro-and nanomechanical sensors for environmental, chemical, and biological detection, Lab Chip 7 (10) (2007) 1238–1255. [3] F. Huber, M. Hegner, C. Gerber, H.-J. Güntherodt, H.P. Lang, Label free analysis of transcription factors using microcantilever arrays, Biosens. Bioelectron. 21 (8) (2006) 1599–1605. [4] J. Fritz, M. Baller, H. Lang, T. Strunz, E. Meyer, H.-J. Güntherodt, E. Delamarche, C. Gerber, J. Gimzewski, Stress at the solid–liquid interface of self-assembled monolayers on gold investigated with a nanomechanical sensor, Langmuir 16 (25) (2000) 9694–9696. [5] R. Berger, E. Delamarche, H.P. Lang, C. Gerber, J.K. Gimzewski, E. Meyer, H.-J. Güntherodt, Surface stress in the self-assembly of alkanethiols on gold, Science 276 (5321) (1997) 2021–2024. [6] E. Finot, A. Fabre, A. Passian, T. Thundat, Dynamic and static manifestation of molecular absorption in thin films probed by a microcantilever, Phys. Rev. Appl. 1 (2) (2014) 024001. [7] C. Ricciardi, S. Fiorilli, S. Bianco, G. Canavese, R. Castagna, I. Ferrante, G. Digregorio, S.L. Marasso, L. Napione, F. Bussolino, Development of microcantilever-based biosensor array to detect angiopoietin-1, a marker of tumor angiogenesis, Biosens. Bioelectron. 25 (5) (2010) 1193–1198. ´ ´ [8] K. Nieradka, K. Kapczynska, J. Rybka, T. Lipinski, P. Grabiec, M. Skowicki, T. Gotszalk, Microcantilever array biosensors for detection and recognition of gram-negative bacterial endotoxins, Sens. Actuators B: Chem. 198 (2014) 114–124. [9] K.M. Hansen, T. Thundat, Microcantilever biosensors, Methods 37 (1) (2005) 57–64. [10] P. Lu, H. Lee, C. Lu, S. O’shea, Surface stress effects on the resonance properties of cantilever sensors, Phys. Rev. B 72 (8) (2005) 085405. [11] R. Raiteri, M. Grattarola, H.-J. Butt, P. Skládal, Micromechanical cantilever-based biosensors, Sens. Actuators B: Chem. 79 (2) (2001) 115–126. [12] H. Jensenius, J. Thaysen, A.A. Rasmussen, L.H. Veje, O. Hansen, A. Boisen, A microcantilever-based alcohol vapor sensor-application and response model, Appl. Phys. Lett. 76 (18) (2000) 2615–2617. [13] M. Godin, O. Laroche, V. Tabard-Cossa, L. Beaulieu, P. Grütter, P. Williams, Combined in situ micromechanical cantilever-based sensing and ellipsometry, Rev. Sci. Instrum. 74 (11) (2003) 4902–4907. [14] A. Buguin, O. Du Roure, P. Silberzan, Active atomic force microscopy cantilevers for imaging in liquids, Appl. Phys. Lett. 78 (19) (2001) 2982–2984. ´ Z. Kowalska, P. Grabiec, P. Janus, [15] K. Nieradka, D. Kopiec, G. Małozie˛ c, A. Sierakowski, K. Domanski, T. Gotszalk, Fabrication and characterization of electromagnetically actuated microcantilevers for biochemical sensing, parallel afm and nanomanipulation, Microelectron. Eng. 98 (2012) 676–679. ´ [16] G. Józwiak, D. Kopiec, P. Zawierucha, T. Gotszalk, P. Janus, P. Grabiec, I. Rangelow, The spring constant calibration of the piezoresistive cantilever based biosensor, Sens. Actuators B: Chem. vol. 170 (2012) 201–206. [17] R.R. Syms, E.M. Yeatman, V.M. Bright, G.M. Whitesides, Surface tensionpowered self-assembly of microstructures-the state-of-the-art, J. Microelectromech. Syst. 12 (4) (2003) 387–417. [18] G.G. Stoney, The tension of metallic films deposited by electrolysis, Proc. R. Soc. Lond. Ser. A 82 (553) (1909) 172–175. [19] M. Godin, V. Tabard-Cossa, P. Grütter, P. Williams, Quantitative surface stress measurements using a microcantilever, Appl. Phys. Lett. 79 (4) (2001) 551–553. [20] T. Miyatani, M. Fujihira, Calibration of surface stress measurements with atomic force microscopy, J. Appl. Phys. 81 (11) (1997) 7099–7115. [21] K. Halbach, Design of permanent multipole magnets with oriented rare earth cobalt material, Nucl. Instrum. Methods 169 (1) (1980) 1–10. [22] B. Hills, K. Wright, D. Gillies, A low-field, low-cost Halbach magnet array for open-access NMR, J. Magn. Reson. 175 (2) (2005) 336–339. [23] R. Beiranvand, Analyzing the uniformity of the generated magnetic field by a practical one-dimensional Helmholtz coils system, Rev. Sci. Instrum. 84 (7) (2013) 075109. [24] J. Barnes, R. Stephenson, C. Woodburn, S. O’shea, M. Welland, T. Rayment, J. Gimzewski, C. Gerber, A femtojoule calorimeter using micromechanical sensors, Rev. Sci. Instrum. 65 (12) (1994) 3793–3798. [25] A.N. Sohi, P.M. Nieva, Thermal sensitivity analysis of curved bi-material microcantilevers, J. Micromech. Microeng. 24 (11) (2014) 115004. [26] G. Wielgoszewski, P. Sulecki, T. Gotszalk, P. Janus, P. Grabiec, M. Hecker, Y. Ritz, E. Zschech, Scanning thermal microscopy: a nanoprobe technique for studying the thermal properties of nanocomponents, Phys. Status Solidi B 248 (2) (2011) 370–374. [27] R.G. Nuzzo, L.H. Dubois, D.L. Allara, Fundamental studies of microscopic wetting on organic surfaces. 1. Formation and structural characterization of a selfconsistent series of polyfunctional organic monolayers, J. Am. Chem. Soc. 112 (2) (1990) 558–569. [28] T.E. Schaffer, P.K. Hansma, Characterization and optimization of the detection sensitivity of an atomic force microscope for small cantilevers, J. Appl. Phys. 84 (9) (1998) 4661–4666. ´ [29] K. Nieradka, G. Józwiak, D. Kopiec, P. Grabiec, P. Janus, A. Sierakowski, T. Gotszalk, A method for linearization of split photodiode position detectors response, Procedia Eng. 25 (2011) 358–361.

D. Kopiec et al. / Sensors and Actuators B 213 (2015) 566–573 [30] C. Vericat, M. Vela, G. Benitez, P. Carro, R. Salvarezza, Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system, Chem. Soc. Rev. 39 (5) (2010) 1805–1834. [31] N.V. Lavrik, C.A. Tipple, M.J. Sepaniak, P.G. Datskos, Gold nano-structures for transduction of biomolecular interactions into micrometer scale movements, Biomed. Microdevices 3 (1) (2001) 35–44.

Biographies ˙ Daniel Kopiec was born in 1984 in Zory, Poland. In 2009, he received the MSc degree from Faculty of Microsystem Electronics and Photonics of Wroclaw University of Technology. His scientific interests are: design of analog and digital electronic circuits for micro- and nanomechanical devices. Currently he is pursuing his PhD degree at Wrocław University of Technology, focusing on application of electromagnetically actuated cantilever in SPM technique and sensor application. Piotr Pałetko was born in 1984 in Nysa, Poland. In 2009, he received the MSc degree from Faculty of Chemistry of Wroclaw University of Technology. Since then he is a PhD student at the Faculty of Microsystem Electronics and Photonics of Wroclaw University of Technology. His scientific interests focus on surface functionalization for micro- and nanomechanical devices, especially for the SPM cantilever and silicon nanowires sensors. Konrad Nieradka was born in 1982 in Staszów, Poland. He graduated from the Faculty of Microsystems Electronics and Photonics of Wrocław University of Technology in 2007, receiving the MSc degree in electronics and telecommunications. For several years he worked at Siemens and Nokia Siemens Networks in the field of mobile communications. He conducted research at Wrocław and Ilmenau Universities of Technology, developing micromechanical biosensors and novel scanning probe microscopy and nanolithography techniques. Currently he is working at Fraunhofer Institute for Integrated Circuits in Ilmenau, conducting research on advanced mobile telecommunication systems. He is also a PhD candidate at Wrocław University of Technology. ˙ Poland in 1989. In 2013 he received the Wojciech Majstrzyk was born in Zarów, MSc degree from Faculty of Microsystem Electronics and Photonics of Wroclaw

573

University of Technology. Currently he is pursuing his PhD degree at Wrocław University of Technology, focusing on application of cantilevers array for force and mass measurements. Piotr Kunicki was born in 1990 in Wrocław, Poland. In 2014 he received the MSc degree from Faculty of Microsystem Electronics and Photonics of Wroclaw University of Technology. His scientific interests are: focued ion and electron beam technologies and analog electronics. Currently he is working at Wrocław University of Technology as an SEM/FIB operator. Andrzej Sierakowski was born in 1976 in Piaseczno, Poland. He received the MSc degrees from the Faculty of Mechatronics of Warsaw University of Technology in 2001. He is a technical engineering senior specialist at the Institute of Electron Technology, Division of Silicon Microsystem and Nanostructure Technology in Warsaw. He is responsible for photolithography processing, masks designing and fabrication for semiconductor technology and MEMS/MOEMS devices based on silicon micromachining and other applications requiring high pattern precision and resolution. Currently he is pursuing his PhD degree at the Institute of Electron Technology, his research presently focuses on developing an atomic force microscopy measurement for photolithography application. Grzegorz Józwiak was born in Zwolen, Poland. He received the MSc degree from Faculty of Electronics of Wroclaw University of Technology in 2002. In 2005 he received the PhD degree from Faculty of Microsystems Electronics and Photonics of Wroclaw University of Technology. His current research interests are noise analysis in MEMS NEMS devices and application of digital signal processing techniques to micro- and nanometrology. He is the author of 24 scientific publications. Teodor Paweł Gotszalk was born in Wrocław, Poland. He received the MSc degrees from the Faculties of Electronics and of Electrical Engineering of Wrocław University of Technology in 1989 and 1991, respectively. In 1996, he received the PhD degree from the Institute of Electronic Technology of the Wrocław University of Technology. He has been honored with Siemens Research Award (2000) and the prize of Polish Science Foundation FNP (1997) for his scientific work. He is the head of the Division of Micro- and Nanostructures Metrology at the Faculty of Microsystems Electronics and Photonics of Wrocław University of Technology. He has authored over 100 scientific publications.