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Chemisorption sensing and analysis using silicon cantilever sensor based on n-type metal–oxide–semiconductor transistor
Microelectronic Engineering 88 (2011) 1019–1023
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Chemisorption sensing and analysis using silicon cantilever sensor based on n-type metal–oxide–semiconductor transistor Jian Wang, Bo Feng, Wengang Wu ⇑, Ying Huang National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, PR China
a r t i c l e
i n f o
Article history: Received 7 September 2010 Received in revised form 10 December 2010 Accepted 25 January 2011 Available online 4 February 2011 Keywords: Cantilever sensor MOS transistor Chemisorption Surface stress Molecular interaction mechanism
a b s t r a c t This paper presents a silicon cantilever sensor based on n-type metal–oxide–semiconductor transistor for chemical sensing and analysis using the chemisorption-induced surface stress sensing principle. The cantilever is along the h1 0 0i crystal orientation of the (1 0 0) silicon, and the transistor channel is parallel to as well as located at the rear part of the cantilever to obtain high stress sensitivity. The gold film deposited on the bottom surfaces of cantilevers is chemically functionalized with a self-assembled monolayer of 4-mercaptobenzoic acid via the Au–SH covalent bonding. The vapor phase chemical sensing experiments with acetone, ethanol, nitroethane and water vapor as targets are performed. The observed response differentiation implies that the molecular interaction mechanisms between different chemical molecules are different. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The chemical sensing and analysis technology for gas or vapor phase targets has been applied in many fields including process control, military and homeland security, meteorology, agriculture, medical equipments and environmental monitoring, etc. [1–3]. Conventional analytical chemistry methods such as gas chromatography, ion mobility spectrometry, mass spectrometry, and quartz microbalance have been applied for analyzing or measuring the target chemicals, with the merits of high sensitivity, high selectivity and high analyzing speed [4–8]. However, they suffer from the need for complicated equipment and long sample preparation time. To fulfill the requirement for high sensitive, rapid, label-free, nondestructive in situ sensing and analysis, cantilever sensors based on structure deflection induced by surface stress from chemical molecule or biomolecular interactions have been extensively researched [9–13]. As for the measurement of cantilever deflection, optical and piezoresistive detection technologies are generally used. Although optical detection method is very sensitive, it is difficult in system manipulation such as light-beam focusing and multi-cantilever operating [14]. The system is also complex, of a high cost and is unportable. Compared with optical method, piezoresistive detection is very simple. However, the stress measurement based on the piezoresistors is not localized, because the piezoresistors usually cover a large length in cantilevers and require high doping lev⇑ Corresponding author. Tel.: +86 10 62757163x21; fax: +86 10 62751789. E-mail address: [email protected] (W. Wu). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.069
els and doping depths to obtain the appropriate resistance value [15]. Cantilever sensor based on MOS (metal–oxide–semiconductor) transistor is an alternative technology as an effective electromechanical sensing and analysis platform for chemical molecules or biomolecular interaction [16]. When the MOS transistor channel is subjected to strain induced by the cantilever deflection as a result of chemisorption-induced surface stress, carrier mobility in the channel will change and results in the change in source–drain current. The sensitivity of the MOS transistor can be improved compared with that of piezoresistor, because the transistor channel region usually has a light doping [17,18] and is very shallow (about 10 nm) near the surface [19], which can be controlled by gate voltage. Akiyama et al. [19] have applied the cantilever sensor based on the h1 1 0i pMOSFET in scanning force microscopy. Shekhawat et al. [20] have employed h1 1 0i nMOSFET-embedded microcantilevers to measure the structure deflection induced by biomolecular interaction. In this paper, we report a silicon cantilever sensor along the h1 0 0i crystal orientation based on nMOS (n-type MOS) transistor for chemisorption sensing and analysis using the surface stress sensing principle, in which the nMOS transistor channel is parallel to the cantilever. The devices are fabricated on a silicon-on-insulator (SOI) wafer based upon the Post-CMOS process. They are believed to be more sensitive than those along other orientations, because the piezoresistive coefficient of the n-type inversion layer along the h1 0 0i orientation is higher than that along other silicon crystal orientations [21]. It is shown that the responses of the cantilever sensor induced by different chemical molecule interactions on the surfaces of cantilevers coated with 4-mercaptobenzoic acid
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(4-MBA) self-assembled monolayer (SAM) are different, which correspond to different interaction mechanisms.
2. Design and fabrication 2.1. Device design Fig. 1 shows the schematic drawings of the cantilever sensor. A rectangle cantilever structure is chosen because the structure is more sensitive to surface stress [22]. An nMOS transistor is configured at the base of the rectangle cantilever which is aligned with the h1 0 0i crystal orientation of the (1 0 0) silicon device layer, and the channel of the transistor is parallel to the cantilever. The transistor’s width/length ratio is designed to be 50/10 to achieve high trans conductance as well as low conductance fluctuation noise (1/f noise). Two types of structure dimensions of the cantilevers (type A: length 450 lm, width 200 lm; type B: length 450 lm, width 100 lm) are designed to assure that the channel length of the transistor is short enough compared to the cantilever length, and the channel width is as long as possible in the high stress concentration region. All these are beneficial to high sensitivity of the sensor. Surface stress change deflects the cantilever, and the resulting longitudinal concentrated stress in the base of the cantilever induces a change in the source–drain current I of the MOS transistor. Neglecting the small transversal stress [14], the relative change in the source–drain current of the transistor under stress is a function of the piezoresistive coefficient p of the n-type channel inversion layer and the longitudinal concentrated stress rc induced by the surface stress change, rs, of the cantilever, [23]
The sensitivity of this structure can be improved compared with the MOSFET-embedded cantilevers along other crystal orientations, because the longitudinal piezoresistive coefficient of an ntype inversion layer in the h1 0 0i crystal orientation of a (1 0 0) silicon material is 8.4 1010 Pa1, which is highest in all crystal orientations [21].
2.2. Device fabrication The cantilever sensors were fabricated on SOI wafers based upon Post-CMOS and etch-stop technology. Fig. 2 shows the schematic drawings of the process steps. The SOI wafers have a 2 lm device layer and a 2 lm buried oxide layer (Fig. 2(a)). The nMOS transistors are fabricated firstly by the standard LOCOS (local oxidation of silicon) CMOS process. The thick LOCOS oxide layer is not grown on the area where cantilevers should be formed (Fig. 2(b)). The gate oxide, gate polysilicon and passivation oxide layer is fabricated as thin as possible for higher stress sensitivity (Fig. 2(c)). The passivation oxide layer on the cantilever area is also etched to obtain the complete silicon cantilever in the etching process of the contact hole (Fig. 2(d)). Aluminum metal is sputtered for ohmic contact (Fig. 2(e)). After the transistor fabrication, the silicon cantilever shapes are defined by inductive-coupled-plasma (ICP) etching from the top (Fig. 2(f)). Then, the SOI wafer is etched from the backside using ICP anisotropic etching, in which the 2 lm buried oxide serves as an etch-stop layer (Fig. 2(g)). At last, the 2 lm buried oxide is removed to release the cantilevers from the backside, as shown in Fig. 2(h).
2
DI 3rs l ¼ prc ¼ p ; I wts
ð1Þ
where l, w and t are the length, width and thickness of the cantilever, respectively.
Fig. 1. Schematic drawing of the cantilever sensor based on nMOS transistor: (a) top view, (b) cross-sectional view.
Fig. 2. Microfabrication process flow for the cantilever sensor.
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Fig. 3 illustrates the scanning-electron-microscopy (SEM) images of the fabricated cantilever sensors. The sensing cantilevers were coated with thin gold film (30 nm) using titanium (8 nm) as an adhesion layer on their bottom sides by electron-beam evaporation. X-ray diffraction (XRD) analysis reveals that the gold film on the cantilevers exhibits Au (1 1 1) texture (inset in Fig. 3(d)), which is suitable for self-assembly of mercapto-compound. The gold film was deposited on the bottom sides of the cantilevers to effectively avoid the influence of gold on the MOS transistors.
3. Experiments 3.1. Cantilever coating To elucidate the mechanism between different chemical molecule interaction using the cantilever sensor, we observed the chemical molecule interaction between acetone, ethanol, nitroethane, or water vapor and thiol, respectively. The cantilever sensors were cleaned in acetone, absolute ethanol, deionized water and at last oxygen plasma for 5 min at 250 W. Then, the gold surface of the cantilevers was functionalized with 4-MBA SAM by immersing the cantilevers into the thiol solution (6 mM; with absolute ethanol as solvent) for 24 h. The 4-MBA [SHC6H4COOH] with carboxyl group (–COOH) is acidic and hydrophilic (water contact angle is 34°) and can dissociate to give –COO group.
3.2. Chemical vapor experiment results and discussion The cantilever sensors (type A) were placed in a small chamber. Droplets of 100 lL acetone, ethanol and nitroethane were injected
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into the chamber to generate chemical vapors for sensing and analysis experiments, respectively. Fig. 4 shows the response of the cantilever sensors to acetone, ethanol and nitroethane vapor as a function of time. Each of the measured response curves (the surface-stress curves) follows the Langmuir adsorption isotherm model (LM) [24], h / 1 expðktÞ, where h is the surface stress, t is the adsorption time and k is the reaction rate. LM describes the chemisorption between probe molecules, which is usually used to functionalize cantilever surface and analytes. The measured stress curves follow the LM characteristics and permit us to conclude that the surface stress change is proportional to the number of molecules adsorbed. The change in output voltages or in other words the surface stress is maximum for ethanol (13.9 mN/m), minimum for acetone (3.7 mN/m) and in-between for nitroethane (5.7 mN/m). The output voltage decrease, which reveals that the surface stress induced by the chemical molecule interactions is tensile stress leads to the downward bending of the measuring cantilever. The conclusion can be inferred from the relationship between the change in the output voltage (DVout) of the common-source stage circuit (inset in Fig. 3(c)) and the deflection-induced piezoresistive response of the cantilever sensor. As for acetone, since it is an organic solvent and inert to polar molecules, it will interact via van der Waals with the 4-MBA SAM coating on the gold film [25]. This leads to the following intermolecular interaction mechanism as shown in Fig. 5. Due to the general attractive characteristics of van der Waals intermolecular interaction, it will bring thiol molecules close to acetone molecule. This in turn will bring thiol molecules closer to each other, inducing shrinkage in the gold layer. It might be aided by probable p–p interaction between acetone molecules. This results in downward deflection of the cantilevers and decrease in output voltage as shown in Fig. 4.
Fig. 3. (a) SEM images of the cantilever sensor array (type A). (b) One of the cantilever sensors. The nMOS transistor location at the rear part of the cantilever where the stress is most concentrated. (c) Details of the nMOS transistor. The inset displays the common-source stage circuit with resistive load, which was used as the measurement circuit in our experiments. The resistive load is chosen as 2 KX, and the VDD voltage and gate voltage Vg are chosen as 5.0 V and 3.0 V, respectively. Under these conditions, the nMOS transistors work in their saturated region, and the change in the output voltage (DVout) as a function of surface stress acted on the cantilever is linear, where DV out ¼ R DI. (d) Backside of the released cantilevers with gold film. The inset shows the XRD spectra of the gold film. The structure of type B is similar with type A except the width of cantilevers.
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Fig. 4. Response of the cantilever sensors to ethanol, nitroethane and acetone vapor as a function of time.
Fig. 7. Response of the sensors to various relative humidity levels.
Fig. 8. Molecular interaction between 4-MBA molecules and water molecules. Fig. 5. Molecular interaction between 4-MBA molecules and acetone vapor phase molecules.
In case of ethanol and nitroethane vapor, it is likely that ethanol or nitroethane molecules will try to form hydrogen bonds with the –COOH end group of 4-MBA [13]. The hydrogen bonds between ethanol and –COOH group of 4-MBA are stronger than those between nitroethane and –COOH group of 4-MBA, due to their higher polarity, and they are all stronger than the van der Waals interaction between acetone and –COOH group of 4-MBA. Ethanol or nitroethane, being a polar solvent, has also a tendency to form hydrogen bonds with themselves as well as other polar molecules. They might screen the repulsion between electronegative oxygen atoms. Thus, it would lead to the shrinkage in the gold film and downward deflection of the cantilevers. The mechanism is shown in Fig. 6. The response of the cantilever sensors to water vapor with various relative humidity levels is opposite to ethanol, nitroethane and acetone vapor, which is observed by placing the sensors (type
B) in a humidity-adjustable chamber. In the chamber, the relative humidity level can be controlled by mixing dry and wet air flows. The output voltage of the cantilever sensor as a function of defined relative humidity level is shown in Fig. 7. The positive voltage response of the sensor reveals that the hydrogen bonding, between the water molecule and –COOH group, results in a compressive surface stress, and leads to an upward bending of the cantilever. Apparently, the above-mentioned mechanisms inferred from the interaction between ethanol, nitroethane or acetone vapor and –COOH group of 4-MBA SAM coating cannot be used to explain the phenomenon. Although the inner mechanism is not completely understood, the most possible mechanism is that water molecules may flow in between the thiol strands and push the thiol strands apart in order to overcome the steric hindrance, as shown in Fig. 8. In these cases, the gold film will experience expansion and the cantilevers would bend upward. The above-mentioned experiments show that we can learn chemical molecule interaction mechanisms based upon the responses (usually nanoscale motions) of the cantilever sensors for various target vapors when using proper sensor coating materials. The cantilever deflection or in other words the surface stress induced by different chemical molecules interaction is different, even opposite, which shows that the interaction mechanisms between different chemical molecules are different. 4. Conclusion
Fig. 6. Molecular interaction between 4-MBA molecules and ethanol or nitroethane vapor phase molecules.
We have developed a silicon cantilever sensor along h1 0 0i crystal orientation based on nMOS transistor with its channel parallel to the cantilever for chemical sensing and analysis. The sensor employs the transistor to detect the cantilever deflection induced by the chemisorption-based surface stress. The functionalized cantilever sensors fulfill the observation and analysis of the reaction process and surface-stress differentiation between different inter-
J. Wang et al. / Microelectronic Engineering 88 (2011) 1019–1023
molecular interactions. This study shows that the cantilever sensors can be used to understand the chemisorption reaction mechanism of chemical molecule or biomolecular interactions. The sensors can also lead to the development of a portable detection device for chemical or biological traces.
[9] [10] [11] [12] [13] [14]
Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 50730009 and 91023045), and the National Basic Research Program of China (973 Program, Grant No. 2009CB320300).
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Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Chemisorption sensing and analysis using silicon cantilever sensor based on n-type metal–oxide–semiconductor transistor Jian Wang, Bo Feng, Wengang Wu ⇑, Ying Huang National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University, Beijing 100871, PR China
a r t i c l e
i n f o
Article history: Received 7 September 2010 Received in revised form 10 December 2010 Accepted 25 January 2011 Available online 4 February 2011 Keywords: Cantilever sensor MOS transistor Chemisorption Surface stress Molecular interaction mechanism
a b s t r a c t This paper presents a silicon cantilever sensor based on n-type metal–oxide–semiconductor transistor for chemical sensing and analysis using the chemisorption-induced surface stress sensing principle. The cantilever is along the h1 0 0i crystal orientation of the (1 0 0) silicon, and the transistor channel is parallel to as well as located at the rear part of the cantilever to obtain high stress sensitivity. The gold film deposited on the bottom surfaces of cantilevers is chemically functionalized with a self-assembled monolayer of 4-mercaptobenzoic acid via the Au–SH covalent bonding. The vapor phase chemical sensing experiments with acetone, ethanol, nitroethane and water vapor as targets are performed. The observed response differentiation implies that the molecular interaction mechanisms between different chemical molecules are different. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction The chemical sensing and analysis technology for gas or vapor phase targets has been applied in many fields including process control, military and homeland security, meteorology, agriculture, medical equipments and environmental monitoring, etc. [1–3]. Conventional analytical chemistry methods such as gas chromatography, ion mobility spectrometry, mass spectrometry, and quartz microbalance have been applied for analyzing or measuring the target chemicals, with the merits of high sensitivity, high selectivity and high analyzing speed [4–8]. However, they suffer from the need for complicated equipment and long sample preparation time. To fulfill the requirement for high sensitive, rapid, label-free, nondestructive in situ sensing and analysis, cantilever sensors based on structure deflection induced by surface stress from chemical molecule or biomolecular interactions have been extensively researched [9–13]. As for the measurement of cantilever deflection, optical and piezoresistive detection technologies are generally used. Although optical detection method is very sensitive, it is difficult in system manipulation such as light-beam focusing and multi-cantilever operating [14]. The system is also complex, of a high cost and is unportable. Compared with optical method, piezoresistive detection is very simple. However, the stress measurement based on the piezoresistors is not localized, because the piezoresistors usually cover a large length in cantilevers and require high doping lev⇑ Corresponding author. Tel.: +86 10 62757163x21; fax: +86 10 62751789. E-mail address: [email protected] (W. Wu). 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.01.069
els and doping depths to obtain the appropriate resistance value [15]. Cantilever sensor based on MOS (metal–oxide–semiconductor) transistor is an alternative technology as an effective electromechanical sensing and analysis platform for chemical molecules or biomolecular interaction [16]. When the MOS transistor channel is subjected to strain induced by the cantilever deflection as a result of chemisorption-induced surface stress, carrier mobility in the channel will change and results in the change in source–drain current. The sensitivity of the MOS transistor can be improved compared with that of piezoresistor, because the transistor channel region usually has a light doping [17,18] and is very shallow (about 10 nm) near the surface [19], which can be controlled by gate voltage. Akiyama et al. [19] have applied the cantilever sensor based on the h1 1 0i pMOSFET in scanning force microscopy. Shekhawat et al. [20] have employed h1 1 0i nMOSFET-embedded microcantilevers to measure the structure deflection induced by biomolecular interaction. In this paper, we report a silicon cantilever sensor along the h1 0 0i crystal orientation based on nMOS (n-type MOS) transistor for chemisorption sensing and analysis using the surface stress sensing principle, in which the nMOS transistor channel is parallel to the cantilever. The devices are fabricated on a silicon-on-insulator (SOI) wafer based upon the Post-CMOS process. They are believed to be more sensitive than those along other orientations, because the piezoresistive coefficient of the n-type inversion layer along the h1 0 0i orientation is higher than that along other silicon crystal orientations [21]. It is shown that the responses of the cantilever sensor induced by different chemical molecule interactions on the surfaces of cantilevers coated with 4-mercaptobenzoic acid
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(4-MBA) self-assembled monolayer (SAM) are different, which correspond to different interaction mechanisms.
2. Design and fabrication 2.1. Device design Fig. 1 shows the schematic drawings of the cantilever sensor. A rectangle cantilever structure is chosen because the structure is more sensitive to surface stress [22]. An nMOS transistor is configured at the base of the rectangle cantilever which is aligned with the h1 0 0i crystal orientation of the (1 0 0) silicon device layer, and the channel of the transistor is parallel to the cantilever. The transistor’s width/length ratio is designed to be 50/10 to achieve high trans conductance as well as low conductance fluctuation noise (1/f noise). Two types of structure dimensions of the cantilevers (type A: length 450 lm, width 200 lm; type B: length 450 lm, width 100 lm) are designed to assure that the channel length of the transistor is short enough compared to the cantilever length, and the channel width is as long as possible in the high stress concentration region. All these are beneficial to high sensitivity of the sensor. Surface stress change deflects the cantilever, and the resulting longitudinal concentrated stress in the base of the cantilever induces a change in the source–drain current I of the MOS transistor. Neglecting the small transversal stress [14], the relative change in the source–drain current of the transistor under stress is a function of the piezoresistive coefficient p of the n-type channel inversion layer and the longitudinal concentrated stress rc induced by the surface stress change, rs, of the cantilever, [23]
The sensitivity of this structure can be improved compared with the MOSFET-embedded cantilevers along other crystal orientations, because the longitudinal piezoresistive coefficient of an ntype inversion layer in the h1 0 0i crystal orientation of a (1 0 0) silicon material is 8.4 1010 Pa1, which is highest in all crystal orientations [21].
2.2. Device fabrication The cantilever sensors were fabricated on SOI wafers based upon Post-CMOS and etch-stop technology. Fig. 2 shows the schematic drawings of the process steps. The SOI wafers have a 2 lm device layer and a 2 lm buried oxide layer (Fig. 2(a)). The nMOS transistors are fabricated firstly by the standard LOCOS (local oxidation of silicon) CMOS process. The thick LOCOS oxide layer is not grown on the area where cantilevers should be formed (Fig. 2(b)). The gate oxide, gate polysilicon and passivation oxide layer is fabricated as thin as possible for higher stress sensitivity (Fig. 2(c)). The passivation oxide layer on the cantilever area is also etched to obtain the complete silicon cantilever in the etching process of the contact hole (Fig. 2(d)). Aluminum metal is sputtered for ohmic contact (Fig. 2(e)). After the transistor fabrication, the silicon cantilever shapes are defined by inductive-coupled-plasma (ICP) etching from the top (Fig. 2(f)). Then, the SOI wafer is etched from the backside using ICP anisotropic etching, in which the 2 lm buried oxide serves as an etch-stop layer (Fig. 2(g)). At last, the 2 lm buried oxide is removed to release the cantilevers from the backside, as shown in Fig. 2(h).
2
DI 3rs l ¼ prc ¼ p ; I wts
ð1Þ
where l, w and t are the length, width and thickness of the cantilever, respectively.
Fig. 1. Schematic drawing of the cantilever sensor based on nMOS transistor: (a) top view, (b) cross-sectional view.
Fig. 2. Microfabrication process flow for the cantilever sensor.
J. Wang et al. / Microelectronic Engineering 88 (2011) 1019–1023
Fig. 3 illustrates the scanning-electron-microscopy (SEM) images of the fabricated cantilever sensors. The sensing cantilevers were coated with thin gold film (30 nm) using titanium (8 nm) as an adhesion layer on their bottom sides by electron-beam evaporation. X-ray diffraction (XRD) analysis reveals that the gold film on the cantilevers exhibits Au (1 1 1) texture (inset in Fig. 3(d)), which is suitable for self-assembly of mercapto-compound. The gold film was deposited on the bottom sides of the cantilevers to effectively avoid the influence of gold on the MOS transistors.
3. Experiments 3.1. Cantilever coating To elucidate the mechanism between different chemical molecule interaction using the cantilever sensor, we observed the chemical molecule interaction between acetone, ethanol, nitroethane, or water vapor and thiol, respectively. The cantilever sensors were cleaned in acetone, absolute ethanol, deionized water and at last oxygen plasma for 5 min at 250 W. Then, the gold surface of the cantilevers was functionalized with 4-MBA SAM by immersing the cantilevers into the thiol solution (6 mM; with absolute ethanol as solvent) for 24 h. The 4-MBA [SHC6H4COOH] with carboxyl group (–COOH) is acidic and hydrophilic (water contact angle is 34°) and can dissociate to give –COO group.
3.2. Chemical vapor experiment results and discussion The cantilever sensors (type A) were placed in a small chamber. Droplets of 100 lL acetone, ethanol and nitroethane were injected
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into the chamber to generate chemical vapors for sensing and analysis experiments, respectively. Fig. 4 shows the response of the cantilever sensors to acetone, ethanol and nitroethane vapor as a function of time. Each of the measured response curves (the surface-stress curves) follows the Langmuir adsorption isotherm model (LM) [24], h / 1 expðktÞ, where h is the surface stress, t is the adsorption time and k is the reaction rate. LM describes the chemisorption between probe molecules, which is usually used to functionalize cantilever surface and analytes. The measured stress curves follow the LM characteristics and permit us to conclude that the surface stress change is proportional to the number of molecules adsorbed. The change in output voltages or in other words the surface stress is maximum for ethanol (13.9 mN/m), minimum for acetone (3.7 mN/m) and in-between for nitroethane (5.7 mN/m). The output voltage decrease, which reveals that the surface stress induced by the chemical molecule interactions is tensile stress leads to the downward bending of the measuring cantilever. The conclusion can be inferred from the relationship between the change in the output voltage (DVout) of the common-source stage circuit (inset in Fig. 3(c)) and the deflection-induced piezoresistive response of the cantilever sensor. As for acetone, since it is an organic solvent and inert to polar molecules, it will interact via van der Waals with the 4-MBA SAM coating on the gold film [25]. This leads to the following intermolecular interaction mechanism as shown in Fig. 5. Due to the general attractive characteristics of van der Waals intermolecular interaction, it will bring thiol molecules close to acetone molecule. This in turn will bring thiol molecules closer to each other, inducing shrinkage in the gold layer. It might be aided by probable p–p interaction between acetone molecules. This results in downward deflection of the cantilevers and decrease in output voltage as shown in Fig. 4.
Fig. 3. (a) SEM images of the cantilever sensor array (type A). (b) One of the cantilever sensors. The nMOS transistor location at the rear part of the cantilever where the stress is most concentrated. (c) Details of the nMOS transistor. The inset displays the common-source stage circuit with resistive load, which was used as the measurement circuit in our experiments. The resistive load is chosen as 2 KX, and the VDD voltage and gate voltage Vg are chosen as 5.0 V and 3.0 V, respectively. Under these conditions, the nMOS transistors work in their saturated region, and the change in the output voltage (DVout) as a function of surface stress acted on the cantilever is linear, where DV out ¼ R DI. (d) Backside of the released cantilevers with gold film. The inset shows the XRD spectra of the gold film. The structure of type B is similar with type A except the width of cantilevers.
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Fig. 4. Response of the cantilever sensors to ethanol, nitroethane and acetone vapor as a function of time.
Fig. 7. Response of the sensors to various relative humidity levels.
Fig. 8. Molecular interaction between 4-MBA molecules and water molecules. Fig. 5. Molecular interaction between 4-MBA molecules and acetone vapor phase molecules.
In case of ethanol and nitroethane vapor, it is likely that ethanol or nitroethane molecules will try to form hydrogen bonds with the –COOH end group of 4-MBA [13]. The hydrogen bonds between ethanol and –COOH group of 4-MBA are stronger than those between nitroethane and –COOH group of 4-MBA, due to their higher polarity, and they are all stronger than the van der Waals interaction between acetone and –COOH group of 4-MBA. Ethanol or nitroethane, being a polar solvent, has also a tendency to form hydrogen bonds with themselves as well as other polar molecules. They might screen the repulsion between electronegative oxygen atoms. Thus, it would lead to the shrinkage in the gold film and downward deflection of the cantilevers. The mechanism is shown in Fig. 6. The response of the cantilever sensors to water vapor with various relative humidity levels is opposite to ethanol, nitroethane and acetone vapor, which is observed by placing the sensors (type
B) in a humidity-adjustable chamber. In the chamber, the relative humidity level can be controlled by mixing dry and wet air flows. The output voltage of the cantilever sensor as a function of defined relative humidity level is shown in Fig. 7. The positive voltage response of the sensor reveals that the hydrogen bonding, between the water molecule and –COOH group, results in a compressive surface stress, and leads to an upward bending of the cantilever. Apparently, the above-mentioned mechanisms inferred from the interaction between ethanol, nitroethane or acetone vapor and –COOH group of 4-MBA SAM coating cannot be used to explain the phenomenon. Although the inner mechanism is not completely understood, the most possible mechanism is that water molecules may flow in between the thiol strands and push the thiol strands apart in order to overcome the steric hindrance, as shown in Fig. 8. In these cases, the gold film will experience expansion and the cantilevers would bend upward. The above-mentioned experiments show that we can learn chemical molecule interaction mechanisms based upon the responses (usually nanoscale motions) of the cantilever sensors for various target vapors when using proper sensor coating materials. The cantilever deflection or in other words the surface stress induced by different chemical molecules interaction is different, even opposite, which shows that the interaction mechanisms between different chemical molecules are different. 4. Conclusion
Fig. 6. Molecular interaction between 4-MBA molecules and ethanol or nitroethane vapor phase molecules.
We have developed a silicon cantilever sensor along h1 0 0i crystal orientation based on nMOS transistor with its channel parallel to the cantilever for chemical sensing and analysis. The sensor employs the transistor to detect the cantilever deflection induced by the chemisorption-based surface stress. The functionalized cantilever sensors fulfill the observation and analysis of the reaction process and surface-stress differentiation between different inter-
J. Wang et al. / Microelectronic Engineering 88 (2011) 1019–1023
molecular interactions. This study shows that the cantilever sensors can be used to understand the chemisorption reaction mechanism of chemical molecule or biomolecular interactions. The sensors can also lead to the development of a portable detection device for chemical or biological traces.
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Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 50730009 and 91023045), and the National Basic Research Program of China (973 Program, Grant No. 2009CB320300).
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