Direct-reading Resonant Silicon Cantilever for ... - Science Direct

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ScienceDirect Procedia Engineering 168 (2016) 658 – 661

30th Eurosensors Conference, EUROSENSORS 2016

Direct-reading resonant silicon cantilever for probing of surface deposits Shuo Zhanga, Yichao Dinga, Wenze Wua, Maik Bertkea, Hutomo Suryo Wasistoa, Lutz Doeringb, Uwe Brandb, Erwin Peinera,* a

Technische Universität Braunschweig, Institute of Semiconductor Technology (IHT) and Laboratory for Emerging Nanometrology (LENA), Hans-Sommer-Str. 66, 38106 Braunschweig, Germany b Physikalisch-Technische Bundesanstalt (PTB), Department 5.1 Surface Metrology, Bundesallee 100, 38116 Braunschweig, Germany

Abstract Contact resonance spectroscopy measurements on thin film deposits based on a tactile piezoresistive silicon microcantilever probe are described. Direct-reading capability during linescans across selectively thin-film-deposited areas of an artefact surface is demonstrated using a phase-locked-loop circuit (PLL). Good agreement found with frequency-sweep measurements followed by line-shape analysis (LSA) including non-linear damping confirm the high potential of the proposed method for nondestructive inline manufacturing-process metrology. © 2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: thin-film metrology; viscoelasticity; contact resonance method; MEMS technology;

1. Introduction Worldwide regulations of combustion engines towards reduced exhaust emissions have been drawing attention to internal and external deposits, which form in modern car engines due to high combustion temperatures, fuel additives, etc. [1]. Even thin deposit layers of few μm in thickness can adversely affect an engine’s performance owing to the small clearances, which are necessary to manage the high pressures in a common-rail system [2]. Due to geometrical restrictions (e.g., of the spray holes of a 100 μm in diameter and a 1 mm in length), non-destructive detection/characterization of such deposits is a non-solved challenge. Recently, we showed that a slender

* Corresponding author. Tel.: +49-531-391-3761; fax: +49-531-391-5844. E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference

doi:10.1016/j.proeng.2016.11.241

Shuo Zhang et al. / Procedia Engineering 168 (2016) 658 – 661

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piezoresistive silicon cantilever with a tiny tip at its free end is appropriate for tactile probing of high-aspect-ratio geometries (Fig. 1). Furthermore, by operating it in the second bending mode surface deposits can be characterized via the shift of resonance frequency when the tip is brought into contact [3]. However, frequency sweep measurement about the resonance mode followed by line-shape analysis (LSA) is time-consuming and requires bulky instrumentation. For in-line measurements with workpieces manufactured in an industrial process adaptive measurement tools offering direct-reading capability are inevitable, which can be achieved by self-oscillating or phase-locked-loop (PLL) circuits [4,5]. Nomenclature f2 Q2 LSA PLL

frequency of second out-of-plane cantilever bending mode quality factor of second out-of-plane cantilever bending mode line-shape analysis phase locked loop

Fig. 1. Piezoresistive tactile cantilever probe with a wet-etched probing tip.

2. Experimental We employed a tactile piezoresistive silicon microcantilever probe (5 mm u 200 μm u 61 μm; CAN50-2-5; CiS Forschungsinstitut für Mikrosensorik GmbH; www.cismst.org) driven into its second out-of-plane bending resonance using small multilayer piezo actuator (5 u 5 u 2 mm3; PL 055.30 PICMA® Chip Actuator; www.piceramic.de) operated at 5 V(ac) from a waveform generator (HP 33120A). Sensor output at the piezoresistive Wheatstone bridge was measured at a supply voltage of 3.3 V by a multimeter (HP 34401A). Using resolution: 0.2 nm; linear positioning systems (P-621.1CD and P-518.ZCD; range: 100 μm, www.physikinstrumente.de) the probing tip was brought into contact to the surface of a sample at defined x-position and contact force (z-position). Scans across surface areas with and without deposits were controlled using a LabVIEW program. For online tracking of the frequency frequency a PLL circuit was developed and operated simultaneous to the offline measurements. 3. Results Figure 2(a) shows frequency-sweep measurements about the second deflection mode of a piezoresistive silicon microcantilever with an area-selectively photo-resist-deposited silicon wafer (AZ 5214E, 1.58 μm). The resonance peaks show sharply raising flanks, which indicate non-linear softening behaviour owing to a spring constant and

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damping added by the surface contact of the probing tip. Obviously, the peak frequency is shifted by a 1 kHz when moving the tip from the bare silicon to the photoresist surface (Fig. 2(b)), which is significant regarding the uncertainties of 0.06 kHz and 0.1 kHz calculated as the standard deviations of ten repeated measurements on silicon and photoresist, respectively.

Fig. 2. (a) Second bending mode with tip in contact to surface area-selectively covered with photo resist; (b) second-mode frequency profile across the transition from deposited to bare silicon surface.

A photograph of a homebuilt PLL circuit is depicted in Fig. 3(a) consisting of an instrumental amplifier, a bandpass filter, an amplifier, a PLL chip, a square-to-sine wave converter and an output amplifier. Figure 3(b) exhibits contact resonance line scans across the area-selectively photo-resist-deposited silicon wafer measured using the cantilever probe operated using the developed PLL circuit in comparison with the peak frequencies. We found fair matching, i. e. deviations almost within the uncertainty limits thus proving the feasibility of the direct-reading concept.

Fig. 3. (a) Photograph of PCB of realized PLL-based cantilever read-out circuit; (b) second-mode frequency profiles tracked using the PLL circuit (online) and determined as the peak frequency from upward frequency-sweep measurements (offline), respectively.

The value shifts of f2 measured offline and online, respectively, can be explained if the non-linear behaviour of the cantilever in contact to the surface is taken into account, i.e. the PLL detects the phase shift at resonance, which is lower than the peak frequencies measured under both upward (f+max) and downward (f-max) frequency sweeps


(Fig. 4(a)). By LSA using an analytical approach including non-linear damping [6], we were able to improve the fit to our measurements performed at upward frequency sweep (Fig. 4(b)). However, full description will require downward frequency-sweep measurements as well, which are in progress. Then, in addition to the frequency, an analysis of the quality factor will be possible. Further measurements are necessary and will be presented at different frequency sweep directions as well as varied static and dynamic contact force levels on photoresist and other deposit materials of different thicknesses.

Fig. 4. (a) Non-linear resonance curve with the measured resonance frequencies of the PLL (f2) and LSA after upward (f+max) and downward (f-max) frequency sweep; (b) LSA including non-linear damping with a contact resonance curve measured at upward frequency sweep.

4. Conclusions Offline and online contact resonance spectroscopy measurements have been described on bare silicon and silicon coated with a thin photo resist layer showing significant resonance shifts. By using a PLL circuit the resonance frequency can be tracked enabling scanning of thin deposits in real time. Fair agreement of online and offline measurements was found which will be improved by LSA including non-linear damping and frequency-sweep measurements in both upward and downward directions.

Acknowledgements We thank Mrs. Juliane Breitfelder, Mrs. Doris Rümmler, and Mr. Uili Wobeto-Reinheimer for their valuable technical support during sensor fabrication and measurements. This work was funded by the German Federal Ministry of Education and Research (BMBF) in the frame of the collaborative project “High-speed micro tactile sensor for high-aspect ratio metrology (HmtS)” under Project No. 03V0409. References [1] H. Xu, C. Wang, X. Ma, A. K. Sarangi, A. Weall, J. Krueger-Venus, Fuel injector deposits in direct-injection spark-ignition engines, Prog. Energy Combust. Sci. 50 (2015) 63-80. [2] W. Urzędowska, Z. Stępień, Prediction of threats caused by high FAME diesel fuel blend stability for engine injector operation, Fuel Processing Technology 142 (2016) 403–410. [3] H. S. Wasisto, R. Dang, L. Doering, U. Brand, E. Peiner, Microtactile cantilever resonators for characterizing surface deposits, Proc. Eng. 120 (2015) 861–864. [4] D. O. Uribe, R. Stroop, J. Wallaschek, Piezoelectric Self-Sensing System for Tactile Intraoperative Brain Tumor Delineation in Neurosurgery, Proc. 31st Ann. Intern. Conf. of the IEEE EMBS Minneapolis, Minnesota, USA, September 2-6, 2009, pp. 737-740. [5] H. S. Wasisto, Q. Zhang, S. Merzsch, A. Waag, E. Peiner, A phase-locked loop frequency tracking system for portable microelectromechanical piezoresistive cantilever mass sensors. Microsyst. Technol. 20 (2014) 559–569. [6] F. Tajaddodianfar, M. R. H. Yazdi, H. N. Pishkenari, Nonlinear dynamics of MEMS/NEMS resonators: analytical solution by the homotopy analysis method, Microsyst. Technol., (2016), doi:10.1007/s00542-016-2947-7.

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