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Fabrication and characterization of SiO2 microcantilever for microsensor application
Sensors and Actuators B 97 (2004) 109–113
Fabrication and characterization of SiO2 microcantilever for microsensor application Yanjun Tang, Ji Fang∗ , Xiaodong Yan, Hai-Feng Ji∗ Chemistry Program and Institute for Micromanufacturing, Louisiana Tech University, 600 W. Arizona Avenue, Ruston, LA 71270, USA Received 1 May 2003; received in revised form 2 August 2003; accepted 4 August 2003
Abstract SiO2 microcantilevers were developed to improve the sensitivity of microcantilever sensors. Silicon plasma dry etching technique was employed to release SiO2 cantilever from bulk silicon at high rate using an ordinary 2 m thickness of SiO2 covered silicon wafer. The V-shaped microcantilever is 200 m long, 25 m wide, and 2 m thick. The spring constant of the cantilevers was measured to be 0.104 N/m. Significant deflection amplitude of the microcantilever was observed upon exposure of low concentration of aminoethanethiol due to its low spring constant. © 2003 Elsevier B.V. All rights reserved. Keywords: SiO2 cantilever; Plasma dry etching; Spring constant; Chemical sensors; Significant amplitude
1. Introduction Microcantilevers have recently attracted considerable interests in the development of a wide range of novel physical [1], chemical [2], and biological [3] sensors. Microcantilever deflection is one unique transduction mechanism of microcantilever sensors. The technology is based upon changes in the deflection property induced by environmental factors in the medium in which a microcantilever is immersed. It has been observed that microcantilevers bend upon the specific binding of species in the environment due to an adsorption induced surface stress change [4]. This concept has already been used to demonstrate the feasibility of chemical detection of a number of vapor phase analytes as well as highly sensitive detection of chemical and biological species in solutions. Surface modification by self-assembled monolayer (SAM) has proven to be a successful method for surface modification of microcantilever for sensing applications [5,6]. In these microcantilever sensors, molecular recognition agents containing thiol compounds or silanes have been synthesized for the preparation of SAM on the microcantilever. Nanometer scale microcantilever bending was observed in most of SAM modified microcantilever sensors developed recently [3,6]. A 200 m-long silicon microcan∗ Corresponding authors. Tel.: +1-318-2575125; fax: +1-318-2575104. E-mail addresses: [email protected] (J. Fang), [email protected] (H.-F. Ji).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.08.003
tilever modified with self-assembled monolayers generally bends 10–200 nm upon binding of analytes in solutions with monolayer receptors on the microcantilever surface. At such nano-scale bending, especially for 10 nm bending amplitude, noise (1–5 nm in general) is a serious problem for accurate concentration measurement. The weak microcantilever bending amplitudes are due to both weak surface stress change and large spring constant of the microcantilever. First, most of the adsorption induced forces on the SAM modified microcantilevers were not significant enough to produce a large microcantilever bending amplitude. Specific molecular bindings on the SAM modified microcantilevers normally produce surface stress at nano Newton (nN) levels. Recently, surface modification chemistries that could generate significant surface stress changes are under development in order to improve the cantilever bending amplitude [7]. Second, most microcantilever chemical and biological sensors developed recently were based on silicon microcantilevers. Silicon material has a relative large spring constant that resists microcantilever bending, especially when the surface stress change is rather small. It is anticipated that a microcantilever made of a more elastic material with a smaller spring constant, such as silicon dioxide, could give larger bending amplitude under the same condition as silicon cantilevers. The Young’s module of silicon dioxide is 76.5–97.2 GPa for SiO2 , which is much smaller than that of silicon at 155.8 GPa [8]. In this work, we report the fabrication and characterization of silicon dioxide microcantilevers. Although other elastic
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materials can also be used, silicon dioxide material is of special interest for developing microcantilever sensors because of the capability to form silane based self-assembled film [9] on silicon dioxide surface.
2. Experiments 2.1. Materials Commercially available silicon microcantilevers were purchased from Veeco Instruments, CA, USA. The dimensions of the V-shaped silicon microcantilevers were 180 m length, 25 m width, and 1 m thickness. One side of the cantilever had a thin film of chromium (3 nm) followed by a 20 nm layer of gold deposited by e-beam evaporation. Another side of the microcantilever was made of silicon with a thin, naturally grown oxide layer. Aminoethanethiol hydrochloride was used as received from Aldrich. High-purity de-ionized water was obtained with a Milli-Q water system (Millipore). 2.2. Fabrication procedure The dimensions of the designed V-shaped SiO2 microcantilevers were 200 m in length, 25 m in width, and 2 m in thickness. The width at the root was 150 m. A round tip, which has 25 m radius, was specifically designed at the tip of cantilever to reflect the incident laser. In order to lower the noise level, 2 m thickness was designed to give better mechanical strength.
The fabrication process has two steps: first, pattern the cantilever beam on SiO2 layer by photolithography and buffered oxide etching (BOE); second, release the cantilever from bulk silicon by dry plasma etching. The fabrication process of SiO2 cantilever is depicted in Fig. 1. Shipley 1813 positive tone photoresist was spun on the surface of a 500 m thick silicon wafer with 2 m thick SiO2 layer (Fig. 1a). A microcantilever beam pattern was transferred to the photoresist layer on the front side of wafer by standard photolithography process and then the SiO2 cantilever beams were formed by etching with buffered oxidation etchant (BOE HF:HNO3 = 1:6). In the mean time, the entire SiO2 layer on back side was etched off (Fig. 1b). The photoresist was then cleaned by acetone and DI water (Fig. 1c). A 20 m thickness of photoresist AZ9260 was spun on the backside of wafer and then the back side wafer was patterned with photolithography process (Fig. 1d). The thick photoresist pattern served as a mask for deep silicon plasma etching. The wafer was etched off by inductive coupling plasma (ICP) process to release microcantilever beams from bulk silicon (Fig. 1e). In our system, the plasma source was generated by an inductively coupled plasma source manufactured by Alcatel Comptech Inc. 2.3. Deflection measurement The deflection experiments were performed in a flowthrough glass cell (Digital Instruments, CA, USA), such as those used in atomic force microscopy. The microcantilever was immersed in distilled water. Initially, water was circulated through the cell using a syringe pump. A schematic
Fig. 1. Fabrication process of a SiO2 microcantilever.
Y. Tang et al. / Sensors and Actuators B 97 (2004) 109–113
diagram of the apparatus used in this study was reported before [5]. A constant flow rate of 4 ml/h was maintained during the entire experiment. Aminoethanethiol solution was injected directly into the fluid flow via a low pressure injection port/sample loop arrangement. This arrangement allowed for continuous exposure of the cantilever to the desired solution without disturbing the flow cell or the flow rate. The deflection measurements were carried out with an AFM photodiode. The bending of the cantilever was measured by monitoring the position of a laser beam reflected from the cantilever onto a four-quadrant photodiode. The cantilever was immersed in water until a stable base line was obtained and the voltage of the position sensitive detector was set as background corresponding to 0 nm.
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Fig. 2. Scanning electron micrograph of SiO2 microcantilevers.
3.2. Characterization 3. Results and discussion 3.1. Fabrication Wet etching was initially used in our experiment to fabricate SiO2 microcantilevers. The fabrication process was unsuccessful. Due to the low selectivity of low SiO2 :Si etching selectivity using either KOH or EDP etchants, the processing conditions were hard to control in order to selectively etch off 500 m thickness of silicon while keeping 2 m thickness of SiO2 unaffected. Dry plasma etching offers many advantages over wet chemical etching methods, including high etch rate, compatibility with traditional IC processing [10], high Si:SiO2 etching selectivity [11] (up to 350:1). The high selectivity is a distinct advantage in releasing SiO2 cantilever beams from bulk silicon. Deep dry plasma etching achieves the high etching rate and straight sidewall. This process simplifies the device design and fabrication processing. The plasma had high energy in the vertical direction, so the passivation layer (photoresist) was removed from the horizontal surface much faster than the vertical direction. After the passivation layer (photoresist) was removed from the horizontal surface, the bulk silicon was exposed and etched by the high-energy plasma and SiO2 microcantilever beams were successfully released from bulk silicon (Fig. 2).
The spring constant of the SiO2 cantilevers was measured to estimate their performance in chemical sensing. Methods for microcantilever spring constant measurement have been well illustrated before [12–15]. Using a cantilever of known spring constant as reference was also commonly used [14,15]. In our work, a commercially available calibrated silicon cantilever [10] was used as a reference cantilever and its spring constant was known to be 0.26 N/m. The silicon cantilever was horizontally placed on a fixed substrate. Fabricated SiO2 cantilever was horizontally placed on the same substrate while having a different altitude level from that of silicon cantilever. The SiO2 cantilever beam was placed in contact with the silicon cantilever beam (Fig. 3) Both cantilevers deflected due to the contact force against each other. The forces applied on the two cantilever beams were the same while their directions were opposite, and the cantilever deflection was inversely proportional to its spring constant, 1 y= F (1) κ where y is the deflection of the cantilever at the end and F is the force applied at the end. As shown in Fig. 3, the deflection of silicon cantilever tip was about 40 m and the deflection of SiO2 cantilever was 101 m. Therefore, the
Fig. 3. Scanning electron micrographs of silicon and SiO2 cantilevers.
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Fig. 4. SEM pictures of a SiO2 cantilever (left) and a silicon cantilever (right).
spring constant of SiO2 cantilever was calculated to be
whereZ is the observed deflection at the end of the cantilever, ν is Poisson’s ratio, t is the thickness of the cantilever, L is the length of the cantilever, and ␦s is the differential stress on the cantilever. And, the spring constant for a cantilever is
40 m × 0.26 N/m = 0.104 N/m 101 m The shape and mechanical properties of both cantilevers are shown in Fig. 4 and Table 1, respectively. The resonance frequency of the microcantilever beam was measured to be 22.5 kHz. The resonance frequency of both SiO2 and silicon microcantilever are also compared in Table 1. Although the thickness of the SiO2 microcantilever was double that of silicon microcantilever, the spring constant of the SiO2 cantilevers was 2/5 of that of the commercially available silicon cantilevers, suggesting that a SiO2 cantilever has the potential to give stronger bending amplitude than silicon cantilever as chemical or biological sensors. Microcantilever bendings of silicon and SiO2 microcantilevers to the same concentration of aminoethanethiol solution was used to compare the bending amplitudes of the silicon and SiO2 cantilevers. Both cantilevers were coated with a thin layer of gold (20 nm) on one side. It was known that thiol compounds, such as aminoethanethiol, form a monolayer on the gold surface that bends the microcantilever [2c]. As shown in Fig. 5, the deflection of silicon cantilever upon exposure to 10−5 M of aminothenethiol solution was approximately 90 nm, while that for SiO2 cantilever was approximately 510 nm. The 2 m SiO2 microcantilever bent 5.6 times of that of 1 m silicon microcantilever upon exposure to the same concentration of an aminoethanethiol solution. Since the surface stresses generated on the two cantilevers were the same, these results suggest that SiO2 is a better material for microcantilever chemical and biological sensing application. It has been shown that the V-shaped cantilevers can be approximated by two rectangular beams in parallel [16], the deflection at the end of the rectangular cantilever can be quantitatively expressed as [17] 3(1 − ν)L2 Z = s (2) Et2
κ=
Ewt3 4L3
(3)
where w is width of a cantilever, and κ is spring constant of a cantilever. The Eq. (2) is then derived as 1 3(1 − ν)wt 3(1 − ν)wt Z = s= s (4) 3 3 4L 4Lκ (Ewt /4L ) The average (1 − ν)SiO2 /(1 − ν)Si is 1.15 [18–21]. Since the two cantilevers have very close geometry shapes and dimensions, it is expected that the surface stresses induced on the two cantilevers are the same (sSiO2 = sSi ). Thus ZSiO2 tSiO2 κSi LSi = 1.15 = 5.18 ZSi κSiO2 tSi LSiO2
Fig. 5. The bending responses of the silicon and SiO2 microcantilevers upon exposure to a 10−5 M solution of aminoethanethiol in ethanol.
Table 1 Comparison of mechanical properties of SiO2 cantilever and silicon cantilever Cantilever type
Length (m)
Width (m)
Thickness (m)
Spring constant (N/m)
Resonance frequency (kHz)
SiO2 cantilever Commercial silicon cantilever
200 180
25 25
2 1
0.104 0.26
22.5 40
Y. Tang et al. / Sensors and Actuators B 97 (2004) 109–113
This analysis fits well with our observation of microcantilever bending amplitude upon exposure to 1 × 10−5 M aminoethanethiol solution.
4. Conclusion In this paper, we report the fabrication process and characterization of SiO2 microcantilevers. Silicon plasma dry etching was used to release SiO2 cantilever beam from bulk silicon. The SiO2 cantilevers fabricated have lower spring constant compared to commercially available silicon microcantilevers. The fabricated SiO2 cantilevers could be used for the development of microcantilever chemical and biological sensors with much higher sensitivity than standard silicon microcantilevers.
References [1] (a) T. Thundat, P.I. Oden, R.J. Warmack, Microcantilever sensors, Microscale Thermophys. Eng. 1 (1997) 185–199; (b) H.-J. Butt, A sensitive method to measure changes in the surface stress of solids, J. Colloid Interface Sci. 180 (1996) 251–260. [2] (a) H.P. Lang, M.K. Baller, R. Berger, Ch. Gerber, J.K. Gimzewski, F.M. Battiston, P. Fornaro, J.P. Ramseyer, E. Meyer, H.J. Guntherodt, An artificial nose based on a micromechanical cantilever array, Anal. Chim. Acta 393 (1999) 59–65; (b) X. Xu, T.G. Thundat, G.M. Brown, H.-F. Ji, Detection of Hg2+ using microcantilever sensors, Anal. Chem. 74 (2002) 3611– 3615; (c) H.-F. Ji, T. Thundat, R. Dabestani, G.M. Brown, P.F. Britt, P.V. Bonnesen, Ultrasenstivite detection of CrO4 2− using a microcantilever sensor, Anal. Chem. 73 (2001) 1572–1576. [3] (a) J. Fritz, M.K. Baller, H.P. Lang, H. Rothuizen, P. Vettiger, E. Meyer, H.-J. Güntherodt, Ch. Gerber, J.K. Gimzewski, Translating biomolecular recognition into nanomechanics, Science 288 (2000) 316–318; (b) K.M. Hansen, H.-F. Ji, G. Wu, R. Datar, R. Cote, A. Majumdar, T. Thundat, Cantilever-based optical deflection assay for discrimination of DNA single nucleotide mismatches, Anal. Chem. 73 (7) (2001) 1567–1571; (c) G. Wu, H.-F. Ji, K. Hansen, T. Thundat, R. Datar, R. Cote, M.F. Hagan, A.K. Chakraborty, A. Majumdar, Nanomechanical signatures of biomolecular recognition and interactions, Proc. Natl. Acad. Sci. U.S.A. 98 (2001) 1560–1564.
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[4] 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 (1997) 2021. [5] (a) H.F. Ji, R. Dabestani, E. Finot, T. Thundat, G.M. Brown, P.F. Britt, A novel self-assembled monolayer coated microcantilever for low level cesium detection, Chem. Commun. (2000), 457–458; (b) Y. Yang, H.-F. Ji, T. Thundat, J. Am. Chem. Soc. 125 (2003) 1124. [6] H.-F. Ji, T.G. Thundat, Trace Ca2+ microcantilever sensor, Biosens. Bioelectron. 17 (2002) 337–343. [7] Y. Zhang, H.-F. Ji, G. Brown, T. Thundat, Ultra sensitive detection of CrO4 2− using a hydrogel swelling microcantilever sensor, Anal. Chem., in press. [8] W.A. Brantley, Calculated elastic constants for stress problems associated with semiconductor devices, J. Appl. Phys. 44 (1973) 534, http://www.korth.de/material/sio2.htm. [9] S.R. Wasserman, Y.-T. Tao, G.M. Whitesides, Structure and reactivity of alkylsiloxane monolayers formed by reaction of alkyltrichlorosilanes on silicon substrates, Langmuir 5 (1989) 1074. [10] Veeco Instruments, See probes under http://store.veeco.com. [11] T. Pandhumsopom, M. Feldbaum, P. Gadgil, High etch rate, anisotropic deep silicon plasma etching for the fabrication of microsensors, SPIE 2879 (1994) 94. [12] J.P. Cleveland, S. Manne, D. Bocek, P.K. Hansma, A nondestructive method for determining the spring constant of cantilevers for scanning force microscopy, Rev. Sci. Instrum. 64 (1993) 403–405. [13] J.G.E. Harris, D.D. Awschalom, Fabrication and characterization of 100-nm-thick GaAs Cantilever, Rev. Sci. Instrum. 67 (10) (1996) 3591–3593. [14] D. Saya, K. Fukushima, H. Toshiyyosh, G. Hashiguch, H. Fujita, H. Kawakatsu, Fabrication of single-crystal Si cantilever array, Sens. Actuators A 95 (2002) 281–287. [15] M. Tortones, M. Kirk, Characterization of application specific probes for SPMs, SPIE Proc. 3009 (1997) 53–60; Micromachining and Imaging, T.A. Michalske (Ed.), ISBN: 0-81942420-x. [16] H.-J. Butt, P. Siedle, K. Seifert, T. Seeger, K. Fendler, E. Bamberg, K. Goldie, A. Engel, Scan speed limit in atomic force microscopy, J. Microsc. 169 (1993) 75–84. [17] (a) F.J. Pressing, J. Appl. Phys. 66 (1989) 4262; (b) P.R. Muller, R. Kern, R. Surf. Sci. 301 (1994) 386. [18] M.T. Kim, Influent of substrates on the elastic reaction of films for the microindentation tests, Thin Solid Films 283 (1996) 12–16. [19] J. Dolbow, M. Gosz, Effect of out-of-plane properties of a polyimide film on the stress fields in microelectronic structures, Mech. Mater. 23 (1996) 311–321. [20] P. Hess, Laser diagnostics of mechanical and elastic properties of silicon and carbon films, Appl. Surf. Sci. 106 (1996) 429–437. [21] L. Gan, B. Ben-Nissan, A. Ben-David, Modelling and finite element analysis of ultra-microhardness indentation of thin films, Thin Solid Films 290–291 (1996) 362–366.
Fabrication and characterization of SiO2 microcantilever for microsensor application Yanjun Tang, Ji Fang∗ , Xiaodong Yan, Hai-Feng Ji∗ Chemistry Program and Institute for Micromanufacturing, Louisiana Tech University, 600 W. Arizona Avenue, Ruston, LA 71270, USA Received 1 May 2003; received in revised form 2 August 2003; accepted 4 August 2003
Abstract SiO2 microcantilevers were developed to improve the sensitivity of microcantilever sensors. Silicon plasma dry etching technique was employed to release SiO2 cantilever from bulk silicon at high rate using an ordinary 2 m thickness of SiO2 covered silicon wafer. The V-shaped microcantilever is 200 m long, 25 m wide, and 2 m thick. The spring constant of the cantilevers was measured to be 0.104 N/m. Significant deflection amplitude of the microcantilever was observed upon exposure of low concentration of aminoethanethiol due to its low spring constant. © 2003 Elsevier B.V. All rights reserved. Keywords: SiO2 cantilever; Plasma dry etching; Spring constant; Chemical sensors; Significant amplitude
1. Introduction Microcantilevers have recently attracted considerable interests in the development of a wide range of novel physical [1], chemical [2], and biological [3] sensors. Microcantilever deflection is one unique transduction mechanism of microcantilever sensors. The technology is based upon changes in the deflection property induced by environmental factors in the medium in which a microcantilever is immersed. It has been observed that microcantilevers bend upon the specific binding of species in the environment due to an adsorption induced surface stress change [4]. This concept has already been used to demonstrate the feasibility of chemical detection of a number of vapor phase analytes as well as highly sensitive detection of chemical and biological species in solutions. Surface modification by self-assembled monolayer (SAM) has proven to be a successful method for surface modification of microcantilever for sensing applications [5,6]. In these microcantilever sensors, molecular recognition agents containing thiol compounds or silanes have been synthesized for the preparation of SAM on the microcantilever. Nanometer scale microcantilever bending was observed in most of SAM modified microcantilever sensors developed recently [3,6]. A 200 m-long silicon microcan∗ Corresponding authors. Tel.: +1-318-2575125; fax: +1-318-2575104. E-mail addresses: [email protected] (J. Fang), [email protected] (H.-F. Ji).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.08.003
tilever modified with self-assembled monolayers generally bends 10–200 nm upon binding of analytes in solutions with monolayer receptors on the microcantilever surface. At such nano-scale bending, especially for 10 nm bending amplitude, noise (1–5 nm in general) is a serious problem for accurate concentration measurement. The weak microcantilever bending amplitudes are due to both weak surface stress change and large spring constant of the microcantilever. First, most of the adsorption induced forces on the SAM modified microcantilevers were not significant enough to produce a large microcantilever bending amplitude. Specific molecular bindings on the SAM modified microcantilevers normally produce surface stress at nano Newton (nN) levels. Recently, surface modification chemistries that could generate significant surface stress changes are under development in order to improve the cantilever bending amplitude [7]. Second, most microcantilever chemical and biological sensors developed recently were based on silicon microcantilevers. Silicon material has a relative large spring constant that resists microcantilever bending, especially when the surface stress change is rather small. It is anticipated that a microcantilever made of a more elastic material with a smaller spring constant, such as silicon dioxide, could give larger bending amplitude under the same condition as silicon cantilevers. The Young’s module of silicon dioxide is 76.5–97.2 GPa for SiO2 , which is much smaller than that of silicon at 155.8 GPa [8]. In this work, we report the fabrication and characterization of silicon dioxide microcantilevers. Although other elastic
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materials can also be used, silicon dioxide material is of special interest for developing microcantilever sensors because of the capability to form silane based self-assembled film [9] on silicon dioxide surface.
2. Experiments 2.1. Materials Commercially available silicon microcantilevers were purchased from Veeco Instruments, CA, USA. The dimensions of the V-shaped silicon microcantilevers were 180 m length, 25 m width, and 1 m thickness. One side of the cantilever had a thin film of chromium (3 nm) followed by a 20 nm layer of gold deposited by e-beam evaporation. Another side of the microcantilever was made of silicon with a thin, naturally grown oxide layer. Aminoethanethiol hydrochloride was used as received from Aldrich. High-purity de-ionized water was obtained with a Milli-Q water system (Millipore). 2.2. Fabrication procedure The dimensions of the designed V-shaped SiO2 microcantilevers were 200 m in length, 25 m in width, and 2 m in thickness. The width at the root was 150 m. A round tip, which has 25 m radius, was specifically designed at the tip of cantilever to reflect the incident laser. In order to lower the noise level, 2 m thickness was designed to give better mechanical strength.
The fabrication process has two steps: first, pattern the cantilever beam on SiO2 layer by photolithography and buffered oxide etching (BOE); second, release the cantilever from bulk silicon by dry plasma etching. The fabrication process of SiO2 cantilever is depicted in Fig. 1. Shipley 1813 positive tone photoresist was spun on the surface of a 500 m thick silicon wafer with 2 m thick SiO2 layer (Fig. 1a). A microcantilever beam pattern was transferred to the photoresist layer on the front side of wafer by standard photolithography process and then the SiO2 cantilever beams were formed by etching with buffered oxidation etchant (BOE HF:HNO3 = 1:6). In the mean time, the entire SiO2 layer on back side was etched off (Fig. 1b). The photoresist was then cleaned by acetone and DI water (Fig. 1c). A 20 m thickness of photoresist AZ9260 was spun on the backside of wafer and then the back side wafer was patterned with photolithography process (Fig. 1d). The thick photoresist pattern served as a mask for deep silicon plasma etching. The wafer was etched off by inductive coupling plasma (ICP) process to release microcantilever beams from bulk silicon (Fig. 1e). In our system, the plasma source was generated by an inductively coupled plasma source manufactured by Alcatel Comptech Inc. 2.3. Deflection measurement The deflection experiments were performed in a flowthrough glass cell (Digital Instruments, CA, USA), such as those used in atomic force microscopy. The microcantilever was immersed in distilled water. Initially, water was circulated through the cell using a syringe pump. A schematic
Fig. 1. Fabrication process of a SiO2 microcantilever.
Y. Tang et al. / Sensors and Actuators B 97 (2004) 109–113
diagram of the apparatus used in this study was reported before [5]. A constant flow rate of 4 ml/h was maintained during the entire experiment. Aminoethanethiol solution was injected directly into the fluid flow via a low pressure injection port/sample loop arrangement. This arrangement allowed for continuous exposure of the cantilever to the desired solution without disturbing the flow cell or the flow rate. The deflection measurements were carried out with an AFM photodiode. The bending of the cantilever was measured by monitoring the position of a laser beam reflected from the cantilever onto a four-quadrant photodiode. The cantilever was immersed in water until a stable base line was obtained and the voltage of the position sensitive detector was set as background corresponding to 0 nm.
111
Fig. 2. Scanning electron micrograph of SiO2 microcantilevers.
3.2. Characterization 3. Results and discussion 3.1. Fabrication Wet etching was initially used in our experiment to fabricate SiO2 microcantilevers. The fabrication process was unsuccessful. Due to the low selectivity of low SiO2 :Si etching selectivity using either KOH or EDP etchants, the processing conditions were hard to control in order to selectively etch off 500 m thickness of silicon while keeping 2 m thickness of SiO2 unaffected. Dry plasma etching offers many advantages over wet chemical etching methods, including high etch rate, compatibility with traditional IC processing [10], high Si:SiO2 etching selectivity [11] (up to 350:1). The high selectivity is a distinct advantage in releasing SiO2 cantilever beams from bulk silicon. Deep dry plasma etching achieves the high etching rate and straight sidewall. This process simplifies the device design and fabrication processing. The plasma had high energy in the vertical direction, so the passivation layer (photoresist) was removed from the horizontal surface much faster than the vertical direction. After the passivation layer (photoresist) was removed from the horizontal surface, the bulk silicon was exposed and etched by the high-energy plasma and SiO2 microcantilever beams were successfully released from bulk silicon (Fig. 2).
The spring constant of the SiO2 cantilevers was measured to estimate their performance in chemical sensing. Methods for microcantilever spring constant measurement have been well illustrated before [12–15]. Using a cantilever of known spring constant as reference was also commonly used [14,15]. In our work, a commercially available calibrated silicon cantilever [10] was used as a reference cantilever and its spring constant was known to be 0.26 N/m. The silicon cantilever was horizontally placed on a fixed substrate. Fabricated SiO2 cantilever was horizontally placed on the same substrate while having a different altitude level from that of silicon cantilever. The SiO2 cantilever beam was placed in contact with the silicon cantilever beam (Fig. 3) Both cantilevers deflected due to the contact force against each other. The forces applied on the two cantilever beams were the same while their directions were opposite, and the cantilever deflection was inversely proportional to its spring constant, 1 y= F (1) κ where y is the deflection of the cantilever at the end and F is the force applied at the end. As shown in Fig. 3, the deflection of silicon cantilever tip was about 40 m and the deflection of SiO2 cantilever was 101 m. Therefore, the
Fig. 3. Scanning electron micrographs of silicon and SiO2 cantilevers.
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Fig. 4. SEM pictures of a SiO2 cantilever (left) and a silicon cantilever (right).
spring constant of SiO2 cantilever was calculated to be
whereZ is the observed deflection at the end of the cantilever, ν is Poisson’s ratio, t is the thickness of the cantilever, L is the length of the cantilever, and ␦s is the differential stress on the cantilever. And, the spring constant for a cantilever is
40 m × 0.26 N/m = 0.104 N/m 101 m The shape and mechanical properties of both cantilevers are shown in Fig. 4 and Table 1, respectively. The resonance frequency of the microcantilever beam was measured to be 22.5 kHz. The resonance frequency of both SiO2 and silicon microcantilever are also compared in Table 1. Although the thickness of the SiO2 microcantilever was double that of silicon microcantilever, the spring constant of the SiO2 cantilevers was 2/5 of that of the commercially available silicon cantilevers, suggesting that a SiO2 cantilever has the potential to give stronger bending amplitude than silicon cantilever as chemical or biological sensors. Microcantilever bendings of silicon and SiO2 microcantilevers to the same concentration of aminoethanethiol solution was used to compare the bending amplitudes of the silicon and SiO2 cantilevers. Both cantilevers were coated with a thin layer of gold (20 nm) on one side. It was known that thiol compounds, such as aminoethanethiol, form a monolayer on the gold surface that bends the microcantilever [2c]. As shown in Fig. 5, the deflection of silicon cantilever upon exposure to 10−5 M of aminothenethiol solution was approximately 90 nm, while that for SiO2 cantilever was approximately 510 nm. The 2 m SiO2 microcantilever bent 5.6 times of that of 1 m silicon microcantilever upon exposure to the same concentration of an aminoethanethiol solution. Since the surface stresses generated on the two cantilevers were the same, these results suggest that SiO2 is a better material for microcantilever chemical and biological sensing application. It has been shown that the V-shaped cantilevers can be approximated by two rectangular beams in parallel [16], the deflection at the end of the rectangular cantilever can be quantitatively expressed as [17] 3(1 − ν)L2 Z = s (2) Et2
κ=
Ewt3 4L3
(3)
where w is width of a cantilever, and κ is spring constant of a cantilever. The Eq. (2) is then derived as 1 3(1 − ν)wt 3(1 − ν)wt Z = s= s (4) 3 3 4L 4Lκ (Ewt /4L ) The average (1 − ν)SiO2 /(1 − ν)Si is 1.15 [18–21]. Since the two cantilevers have very close geometry shapes and dimensions, it is expected that the surface stresses induced on the two cantilevers are the same (sSiO2 = sSi ). Thus ZSiO2 tSiO2 κSi LSi = 1.15 = 5.18 ZSi κSiO2 tSi LSiO2
Fig. 5. The bending responses of the silicon and SiO2 microcantilevers upon exposure to a 10−5 M solution of aminoethanethiol in ethanol.
Table 1 Comparison of mechanical properties of SiO2 cantilever and silicon cantilever Cantilever type
Length (m)
Width (m)
Thickness (m)
Spring constant (N/m)
Resonance frequency (kHz)
SiO2 cantilever Commercial silicon cantilever
200 180
25 25
2 1
0.104 0.26
22.5 40
Y. Tang et al. / Sensors and Actuators B 97 (2004) 109–113
This analysis fits well with our observation of microcantilever bending amplitude upon exposure to 1 × 10−5 M aminoethanethiol solution.
4. Conclusion In this paper, we report the fabrication process and characterization of SiO2 microcantilevers. Silicon plasma dry etching was used to release SiO2 cantilever beam from bulk silicon. The SiO2 cantilevers fabricated have lower spring constant compared to commercially available silicon microcantilevers. The fabricated SiO2 cantilevers could be used for the development of microcantilever chemical and biological sensors with much higher sensitivity than standard silicon microcantilevers.
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