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Thermally driven piezoresistive cantilevers for shear-force microscopy
Microelectronic Engineering 86 (2009) 1212–1215
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Thermally driven piezoresistive cantilevers for shear-force microscopy M. Woszczyna a,*, T. Gotszalk a, P. Zawierucha a, M. Zielony a, Tzv. Ivanow b, K. Ivanowa b, Y. Sarov b, N. Nikolov c, J. Mielczarski d, E. Mielczarska d, I.W. Rangelow b a
Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, ul. Janiszewskeigo 11/17, 50-372 Wrocław, Poland Ilmenau University of Technology, Institute of Micro- and Nanoelectronicsystems, Gustav Kirchoffstr. 1, 98-693 Ilmenau, Germany c Microsystems Ltd., Bul. Levski, Computer Centre Risk Electronik, 9010 Varna, Bulgaria d Inst. Nat. Polytechnique de Lorraine, LEM, INPL/CNRS, 15 avenue du Charmois, Vandoeuvre lés Nancy, 54501, France b
a r t i c l e
i n f o
Article history: Received 6 November 2008 Received in revised form 16 January 2009 Accepted 17 January 2009 Available online 24 January 2009 Keywords: Shear-force microscopy Thermal actuation Piezoresistive deflection detection Silicon cantilever
a b s t r a c t In this article we will present architecture, properties and application of a novel scanning probe microscopy (SPM) piezoresistive cantilever with thermal deflection actuator. This microprobe with an integrated planar microtip is utilized in shear-force microscopy (ShFM) in which the cantilever microtip vibrates parallel to the surface. We will describe the calibration procedure of sensitivity of cantilever piezoresistive deflection detection, thermal actuation and experimental setup of the developed measurement system. We will also present results of topography measurements of highly oriented pyrolytic graphite (HOPG) and Hafnia alvei bacteria culture surface using developed ShFM system. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction SPM, of which atomic force microscopy (AFM) is one of examples, measure the interaction between the surface and a microtip integrated on a soft cantilever. In general two methods can be applied in SPM surface measurements. In the first method the investigated sample is scanned using a micromechanical lever with a microtip integrated vertically on the beam. In the second method, so called ShFM, the microprobe is mounted perpendicularly to the sample and oscillates parallel to the surface. There are various techniques detecting the microtip deflection in ShFM, which are based on piezoelectrical tuning forks [1–3] or optical sensors [4– 5]. In comparison with piezoelectrical and optical tip deflection methods application of self-actuated piezoresistive cantilevers simplifies the architecture of the scanning probe microscope head. In this way utilization of complicated optical and not always reliable systems is avoided. In addition the piezoresistive detection scheme enables to perform quantitive force and microprobe deflection methods, which is crucial for molecular investigation of tip-sample interactions (see Fig. 1). In this article we present the piezoresistive cantilever with integrated planar microtip and integrated thermal deflection actuator. In this technique the metrological analysis of the thermally excited tip oscillation and piezoresistively detected interaction acting on the probe is performed. This analysis is the key factor that allows * Corresponding author. Tel.: +886 3 5916736; fax: +886 3 5826104. E-mail address: [email protected] (M. Woszczyna). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.043
more reliable application of the proposed novel SPM technique in versatile surface investigation. 2. Cantilever theory of operation The fabrication process used in this work is a modification of a double side silicon micromachining process developed for manufacturing of piezoresistive AFM sensors [6]. Double-side polished h1 0 0i-oriented, 3–7 X/cm silicon wafers are used as the starting material. Next, standard CMOS processing like oxidation, phosphorus and boron diffusion, ion implantation, dry and wet etching, insulator and aluminium film deposition, and photolithography are sequentially applied to form piezoresistors, p+ diffusion connecting paths, contact windows, metallic connections, and the resistive microheater at the front side of the wafer. Aluminum micro-heater with the resistance of 25.6 X was deposited and connected to the golden pad, which can be used in further experiments as biochemical platform. Taking into consideration that the highest stress caused by bending of the cantilever concentrates on the surface, we employ a low voltage 20 keV boron implantation step and rapid thermal annealing at 800 °C for 30 s. In this way, very shallow resistors are fabricated [7]. In the following back side processing sequence a corner compensated membrane pattern is created by a two-side photolithography process and anisotropic deep etching with electrochemical etch stop of silicon in 10% tetramethyl ammonium hydroxides hTMAHi solution at 60 °C, to create a 3 lm thick silicon membrane where the parameters of the cantilever will be defined [8]. Finally, the
M. Woszczyna et al. / Microelectronic Engineering 86 (2009) 1212–1215
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Fig. 2. Cantilever vibration spectrum observed by frequency modulation of the microheater signal.
Fig. 1. Thermally driven piezoresistive ShFM cantilever.
cantilever shape is defined in the membrane by a last photolithographic step at the top side of the wafer and silicon dry etching using inductively coupled plasma with gas chopping of SF6 and C4F8 gases. 5-lm thick photoresist AZ4562 was used to mask the piezoresistive circuit and the resistive microheater during the dry etching. The microtip was formed at the cantilever apex along the length of the spring beam. The measured cantilever resonance frequency was 29770.5 Hz. We calculated the cantilever spring constant based on the calibration procedure developed for the piezoresistive cantilevers with rectangular cross-section [9]. In this case the spring constant can be calculated in accordance with: 3
k ¼ 2p3 Ewl f03
q3=2 E
ð1Þ
where E – Si Young’s modulus, l – the cantilever length, w – the cantilever width, q – Si density, fr – the beam resonance frequency. In this case, the determined spring constant was 7.49 ± 0.02 N/m. When the heat is dissipated in the microheater the cantilever is deflected due to the different thermal expansion coefficients of the Si/SiO2 and Al layers. The thermal conductivity of silicon beam and metal layers is high enough to conduct the energy from the microheater to the bulk sensor body in a few microseconds [10]. In this way the excitation of cantilever resonance vibration is possible. It allows to dissipate heat periodically in order to excite the cantilever in its mechanical resonance [11]. The power dissipated in the deflection actuator can be calculated on the basis of the formula:
PðxÞ ¼ R½Idc þ Iac sinðxtÞ2 1 1 ¼ R I2dc þ I2ac þ 2Idc Iac sinðxtÞ I2ac cosð2xtÞ 2 2
only with the doubled driving frequency. In order to excite resonance cantilever vibration the microheater supply signal frequency should be the half of the mechanical frequency. It should also be noted, that it is possible to excite the resonance cantilever oscillation by the microheater supplied with signal whose frequency is equal to the sensor mechanical resonance. However, in this case the vibration amplitude depends linearly on the microheater current offset. Fig. 2 presents the spectrum of the thermally driven cantilever vibration when the frequency of the heating current with the amplitude and offset of 2 mA and 1 mA was modulated. The resonance curves at first and second eigenfrequencies of 29770.5 Hz and 185262.1 Hz were observed when the cantilever mechanical resonance frequency was equal to the single frequency and doubled frequency of the driving signal. 3. Experimental setup Optical fibre Fabry-Perot interferometer was used to investigate the thermal efficiency of the cantilever deflection actuation [12]. In order to obtain thermally induced bending of the cantilever we designed a precision, wide bandwidth voltage-controlled current source with output current and bandwidth of 50 mA and 1 MHz. A precision, low-noise voltage preamplifier was designed to amplify Wheatstone bridge signals corresponding with the cantilever
ð2Þ
where Idc – the offset current, Iac – the amplitude of the supply signal and R – the microheater resistance. In this equation three power components can be distinguished: constant dissipation power, which caused static cantilever deflection, dissipation power with the frequency equal to the driving signal frequency and dissipation power with the doubled driving signal frequency. When the microheater is supplied only with sinusoidal signal the cantilever vibrates
Fig. 3. Setup for calibration of actuation and detection of cantilever displacement sensitivity.
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M. Woszczyna et al. / Microelectronic Engineering 86 (2009) 1212–1215
Fig. 4. Cantilever vibration amplitude vs. microheater power; driving frequency equal to the half of the first cantilever mechanical resonance frequency.
Fig. 5. Cantilever vibration amplitude vs. AC microheater current with offset current as a parameter; driving frequency equal to the first mechanical resonance frequency.
Fig. 6. ShFM image of HOPG (left) and Hafnia alvei bacteria culture (right) measured using thermally driven piezoresistive cantilever.
deflection. Using the designed measurement setup it was possible to record simultaneously the interferometric signals, which quantitatively described the thermally actuated sensor deflections, and Wheatstone bridge output signal (see Fig. 3). 4. Results and discussion In Fig. 4 we present the cantilever resonance vibration amplitude dependence on power which was dissipated in the cantilever microheater. In this experiment the cantilever was thermally excited with the actuation signal, whose frequency was equal to the half of the cantilever mechanical resonance frequency. The vibration amplitude increased linearly with the dissipation power, which is in an agreement with Eq. (2). The sensitivity of the thermal deflection actuation estimated in this way was 1.9 nm/lW. Fig. 5 shows relationship between the cantilever vibration amplitude and a current excitation amplitude with the frequency equal to the mechanical resonance frequency and offset heating current as a parameter. In our experiments we applied the presented microprobe in the open architecture homebuilt atomic force microscope. In order to test the resolution possibilities of the entire measurement system we observed the topography of freshly cleaved HOPG sample. In Fig. 6 we present the topography image, which was recorded at the first resonance frequency of 29770.5 Hz and by tip vibration
of 50 nm, which was detected by the piezoresitive read-out. Atomic steps of 1 nm height can be clearly seen, which prove the high resolution possibilities of the fabricated sensors. Additionally in order to test the performance of our probe on biological samples we recorded the surface of Hafnia alvei bacteria. In this case, topography of higher structures was revealed, which required reduction of the tip vibration to 20 nm. 5. Conclusion The thermally driven piezoresistive cantilever with planar tip was developed, calibrated and applied in ShFM high resolution surface investigations. The architecture and properties of the fabricated sensor enable its simply application in almost all SPM systems. In addition the possibility of precise calibration of sensor parameters creates a perfect metrological tool. The obtained experimental resolution of surface measurements proved high performance of the fabricated system, which enables its further application in investigations of technological and biological samples. Acknowledgment This work was supported by the EU project IST-2004-516865 STREP ’’TASNANO’’.
M. Woszczyna et al. / Microelectronic Engineering 86 (2009) 1212–1215
References [1] [2] [3] [4] [5]
K. Karrai et al., Phys. Rev. B 62 (29) (2000) 13173–13181. M. Antognozzi et al., Appl. Phys. Lett. 82 (17) (2003) 2761–2763. F. Giessibl, Rev. Modern Phys. 75 (2003) 949–983. R. Levy et al., Nanotechnology 13 (2002) 33–37. H. Holscher et al., Surf. Sci. 579 (2005) 21–26.
[6] [7] [8] [9] [10] [11] [12]
I.W. Rangelow et al., Proc. SPIE 2879 (1996) 56. I.W. Rangelow, Microelectron. Eng. 84 (2007) 1260–1264. K. Ivanova et al., J. Vac. Sci. Technol. B 26 (6) (2008) 2367–2373. T. Gotszalk et al., Ultramicroscopy 97 (2003) 385–389. T. Ivanov et al., Microelectron. Eng. 67–68 (2003) 550–556. E. Pedrak et al., J. Vac. Sci. Technol. B 21 (6) (2003) 3102–3107. M. Woszczyna et al., Vacuum 82 (10) (2008) 982–987.
1215
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Thermally driven piezoresistive cantilevers for shear-force microscopy M. Woszczyna a,*, T. Gotszalk a, P. Zawierucha a, M. Zielony a, Tzv. Ivanow b, K. Ivanowa b, Y. Sarov b, N. Nikolov c, J. Mielczarski d, E. Mielczarska d, I.W. Rangelow b a
Wrocław University of Technology, Faculty of Microsystem Electronics and Photonics, ul. Janiszewskeigo 11/17, 50-372 Wrocław, Poland Ilmenau University of Technology, Institute of Micro- and Nanoelectronicsystems, Gustav Kirchoffstr. 1, 98-693 Ilmenau, Germany c Microsystems Ltd., Bul. Levski, Computer Centre Risk Electronik, 9010 Varna, Bulgaria d Inst. Nat. Polytechnique de Lorraine, LEM, INPL/CNRS, 15 avenue du Charmois, Vandoeuvre lés Nancy, 54501, France b
a r t i c l e
i n f o
Article history: Received 6 November 2008 Received in revised form 16 January 2009 Accepted 17 January 2009 Available online 24 January 2009 Keywords: Shear-force microscopy Thermal actuation Piezoresistive deflection detection Silicon cantilever
a b s t r a c t In this article we will present architecture, properties and application of a novel scanning probe microscopy (SPM) piezoresistive cantilever with thermal deflection actuator. This microprobe with an integrated planar microtip is utilized in shear-force microscopy (ShFM) in which the cantilever microtip vibrates parallel to the surface. We will describe the calibration procedure of sensitivity of cantilever piezoresistive deflection detection, thermal actuation and experimental setup of the developed measurement system. We will also present results of topography measurements of highly oriented pyrolytic graphite (HOPG) and Hafnia alvei bacteria culture surface using developed ShFM system. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction SPM, of which atomic force microscopy (AFM) is one of examples, measure the interaction between the surface and a microtip integrated on a soft cantilever. In general two methods can be applied in SPM surface measurements. In the first method the investigated sample is scanned using a micromechanical lever with a microtip integrated vertically on the beam. In the second method, so called ShFM, the microprobe is mounted perpendicularly to the sample and oscillates parallel to the surface. There are various techniques detecting the microtip deflection in ShFM, which are based on piezoelectrical tuning forks [1–3] or optical sensors [4– 5]. In comparison with piezoelectrical and optical tip deflection methods application of self-actuated piezoresistive cantilevers simplifies the architecture of the scanning probe microscope head. In this way utilization of complicated optical and not always reliable systems is avoided. In addition the piezoresistive detection scheme enables to perform quantitive force and microprobe deflection methods, which is crucial for molecular investigation of tip-sample interactions (see Fig. 1). In this article we present the piezoresistive cantilever with integrated planar microtip and integrated thermal deflection actuator. In this technique the metrological analysis of the thermally excited tip oscillation and piezoresistively detected interaction acting on the probe is performed. This analysis is the key factor that allows * Corresponding author. Tel.: +886 3 5916736; fax: +886 3 5826104. E-mail address: [email protected] (M. Woszczyna). 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.01.043
more reliable application of the proposed novel SPM technique in versatile surface investigation. 2. Cantilever theory of operation The fabrication process used in this work is a modification of a double side silicon micromachining process developed for manufacturing of piezoresistive AFM sensors [6]. Double-side polished h1 0 0i-oriented, 3–7 X/cm silicon wafers are used as the starting material. Next, standard CMOS processing like oxidation, phosphorus and boron diffusion, ion implantation, dry and wet etching, insulator and aluminium film deposition, and photolithography are sequentially applied to form piezoresistors, p+ diffusion connecting paths, contact windows, metallic connections, and the resistive microheater at the front side of the wafer. Aluminum micro-heater with the resistance of 25.6 X was deposited and connected to the golden pad, which can be used in further experiments as biochemical platform. Taking into consideration that the highest stress caused by bending of the cantilever concentrates on the surface, we employ a low voltage 20 keV boron implantation step and rapid thermal annealing at 800 °C for 30 s. In this way, very shallow resistors are fabricated [7]. In the following back side processing sequence a corner compensated membrane pattern is created by a two-side photolithography process and anisotropic deep etching with electrochemical etch stop of silicon in 10% tetramethyl ammonium hydroxides hTMAHi solution at 60 °C, to create a 3 lm thick silicon membrane where the parameters of the cantilever will be defined [8]. Finally, the
M. Woszczyna et al. / Microelectronic Engineering 86 (2009) 1212–1215
1213
Fig. 2. Cantilever vibration spectrum observed by frequency modulation of the microheater signal.
Fig. 1. Thermally driven piezoresistive ShFM cantilever.
cantilever shape is defined in the membrane by a last photolithographic step at the top side of the wafer and silicon dry etching using inductively coupled plasma with gas chopping of SF6 and C4F8 gases. 5-lm thick photoresist AZ4562 was used to mask the piezoresistive circuit and the resistive microheater during the dry etching. The microtip was formed at the cantilever apex along the length of the spring beam. The measured cantilever resonance frequency was 29770.5 Hz. We calculated the cantilever spring constant based on the calibration procedure developed for the piezoresistive cantilevers with rectangular cross-section [9]. In this case the spring constant can be calculated in accordance with: 3
k ¼ 2p3 Ewl f03
q3=2 E
ð1Þ
where E – Si Young’s modulus, l – the cantilever length, w – the cantilever width, q – Si density, fr – the beam resonance frequency. In this case, the determined spring constant was 7.49 ± 0.02 N/m. When the heat is dissipated in the microheater the cantilever is deflected due to the different thermal expansion coefficients of the Si/SiO2 and Al layers. The thermal conductivity of silicon beam and metal layers is high enough to conduct the energy from the microheater to the bulk sensor body in a few microseconds [10]. In this way the excitation of cantilever resonance vibration is possible. It allows to dissipate heat periodically in order to excite the cantilever in its mechanical resonance [11]. The power dissipated in the deflection actuator can be calculated on the basis of the formula:
PðxÞ ¼ R½Idc þ Iac sinðxtÞ2 1 1 ¼ R I2dc þ I2ac þ 2Idc Iac sinðxtÞ I2ac cosð2xtÞ 2 2
only with the doubled driving frequency. In order to excite resonance cantilever vibration the microheater supply signal frequency should be the half of the mechanical frequency. It should also be noted, that it is possible to excite the resonance cantilever oscillation by the microheater supplied with signal whose frequency is equal to the sensor mechanical resonance. However, in this case the vibration amplitude depends linearly on the microheater current offset. Fig. 2 presents the spectrum of the thermally driven cantilever vibration when the frequency of the heating current with the amplitude and offset of 2 mA and 1 mA was modulated. The resonance curves at first and second eigenfrequencies of 29770.5 Hz and 185262.1 Hz were observed when the cantilever mechanical resonance frequency was equal to the single frequency and doubled frequency of the driving signal. 3. Experimental setup Optical fibre Fabry-Perot interferometer was used to investigate the thermal efficiency of the cantilever deflection actuation [12]. In order to obtain thermally induced bending of the cantilever we designed a precision, wide bandwidth voltage-controlled current source with output current and bandwidth of 50 mA and 1 MHz. A precision, low-noise voltage preamplifier was designed to amplify Wheatstone bridge signals corresponding with the cantilever
ð2Þ
where Idc – the offset current, Iac – the amplitude of the supply signal and R – the microheater resistance. In this equation three power components can be distinguished: constant dissipation power, which caused static cantilever deflection, dissipation power with the frequency equal to the driving signal frequency and dissipation power with the doubled driving signal frequency. When the microheater is supplied only with sinusoidal signal the cantilever vibrates
Fig. 3. Setup for calibration of actuation and detection of cantilever displacement sensitivity.
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M. Woszczyna et al. / Microelectronic Engineering 86 (2009) 1212–1215
Fig. 4. Cantilever vibration amplitude vs. microheater power; driving frequency equal to the half of the first cantilever mechanical resonance frequency.
Fig. 5. Cantilever vibration amplitude vs. AC microheater current with offset current as a parameter; driving frequency equal to the first mechanical resonance frequency.
Fig. 6. ShFM image of HOPG (left) and Hafnia alvei bacteria culture (right) measured using thermally driven piezoresistive cantilever.
deflection. Using the designed measurement setup it was possible to record simultaneously the interferometric signals, which quantitatively described the thermally actuated sensor deflections, and Wheatstone bridge output signal (see Fig. 3). 4. Results and discussion In Fig. 4 we present the cantilever resonance vibration amplitude dependence on power which was dissipated in the cantilever microheater. In this experiment the cantilever was thermally excited with the actuation signal, whose frequency was equal to the half of the cantilever mechanical resonance frequency. The vibration amplitude increased linearly with the dissipation power, which is in an agreement with Eq. (2). The sensitivity of the thermal deflection actuation estimated in this way was 1.9 nm/lW. Fig. 5 shows relationship between the cantilever vibration amplitude and a current excitation amplitude with the frequency equal to the mechanical resonance frequency and offset heating current as a parameter. In our experiments we applied the presented microprobe in the open architecture homebuilt atomic force microscope. In order to test the resolution possibilities of the entire measurement system we observed the topography of freshly cleaved HOPG sample. In Fig. 6 we present the topography image, which was recorded at the first resonance frequency of 29770.5 Hz and by tip vibration
of 50 nm, which was detected by the piezoresitive read-out. Atomic steps of 1 nm height can be clearly seen, which prove the high resolution possibilities of the fabricated sensors. Additionally in order to test the performance of our probe on biological samples we recorded the surface of Hafnia alvei bacteria. In this case, topography of higher structures was revealed, which required reduction of the tip vibration to 20 nm. 5. Conclusion The thermally driven piezoresistive cantilever with planar tip was developed, calibrated and applied in ShFM high resolution surface investigations. The architecture and properties of the fabricated sensor enable its simply application in almost all SPM systems. In addition the possibility of precise calibration of sensor parameters creates a perfect metrological tool. The obtained experimental resolution of surface measurements proved high performance of the fabricated system, which enables its further application in investigations of technological and biological samples. Acknowledgment This work was supported by the EU project IST-2004-516865 STREP ’’TASNANO’’.
M. Woszczyna et al. / Microelectronic Engineering 86 (2009) 1212–1215
References [1] [2] [3] [4] [5]
K. Karrai et al., Phys. Rev. B 62 (29) (2000) 13173–13181. M. Antognozzi et al., Appl. Phys. Lett. 82 (17) (2003) 2761–2763. F. Giessibl, Rev. Modern Phys. 75 (2003) 949–983. R. Levy et al., Nanotechnology 13 (2002) 33–37. H. Holscher et al., Surf. Sci. 579 (2005) 21–26.
[6] [7] [8] [9] [10] [11] [12]
I.W. Rangelow et al., Proc. SPIE 2879 (1996) 56. I.W. Rangelow, Microelectron. Eng. 84 (2007) 1260–1264. K. Ivanova et al., J. Vac. Sci. Technol. B 26 (6) (2008) 2367–2373. T. Gotszalk et al., Ultramicroscopy 97 (2003) 385–389. T. Ivanov et al., Microelectron. Eng. 67–68 (2003) 550–556. E. Pedrak et al., J. Vac. Sci. Technol. B 21 (6) (2003) 3102–3107. M. Woszczyna et al., Vacuum 82 (10) (2008) 982–987.
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