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Piezoresistive Microcantilever-Based Gas Sensor using Dynamic Mode Measurement

Nuning Aisah, Lia Aprilia, Ratno Nuryadi

Center for Materials Technology Agency for the Assessment and Application of Technology

South Tangerang, Indonesia nuning.aisah@bppt.go.id, lia.aprilia@ui.ac.id, ratno.nuryadi@bppt.go.id

Abstract— The purpose of this paper is to investigate an

application of a piezoresistive microcantilever for gas sensor using a dynamic mode operation. The working principle of the microcantilever sensor is based on the measurement of microcantilever deflection or resonance frequency change due to the objects attached on the microcantilever surface. The measurement was performed by using Wheatstone bridge circuit, which is constructed by two piezoresistors in the microcantilever and two external resistors, in order to measure the resonance frequency shift of the microcantilever vibration. The result shows that the voltage of oscillation peak-to-peak from the output of Wheatstone bridge circuit, which represents the microcantilever vibrations, decreases with the time due to the gas detection. This occurs due to the resonance frequency shift caused by the addition of gas molecules mass on the microcantilever surface. This result indicates that the developed system can be used as gas sensor.

Keywords— microcantilever; piezoresistive; gas sensor; dynamic mode; amplitude

I. INTRODUCTION In a last decade, a development of a

microelectromechanical system (MEMS) technology has promoted an innovative microcantilever-based sensors. Such sensors have big potential to replace many conventional sensor systems because of a relatively low cost of production, high sensitive, rapid response, and a reduced size of the active area (typically 10-6 cm²) [1-3]. Moreover, the microcantilever sensors have been investigated in the fields of environment, medicine, chemistry, physics, and biology. Basic principle of the microcantilever sensor is a detection of the microcantilever bending, so called as deflection, due to the attachment of the objects (bacteria, virus, glucose, molecule gas, etc.) on the microcantilever surface.

Gas sensor is one of important issue in the industry. For many industries, monitoring and controlling gas are needed for efficient and savety of the production process. A rapid and accurate measurement of the gas have become a challenge for a long time. Recently, MEMS technology-based microcantilever sensor is very popular to miniaturize various kinds of sensors, including the gas sensor. However, to our knowledge, few experiment studies have been devoted on the detection of the gas [4-6]. Especially, Liquefied Petroleum Gas (LPG) detection has not been reported. So far, our group

have studied the detection of humidity using piezoresistive microcantilever and the design of the microcantilever for chemical object detection [7,8]. In this work, we investigate the possibility of piezoresistive microcantilever as the gas sensor using the dynamic mode measurements. Here, a Wheatstone bridge circuit is constructed in order to detect a change of piezoresistor in the microcantilever due to the gas molecules attached on the microcantilever surface.

II. DYNAMIC MODE MEASUREMENT The microcantilever sensor has two operation modes, i.e.,

static mode and dynamic mode. The static mode operation directly measures the deflection of the microcantilever. For the dynamic mode operation, the deflection of the microcantilever is not directly measured. The dynamic mode one measures the resonance frequency shift of the microcantilever vibration due to the object detection. In the experimental setup, the microcantilever is usually placed on the piezoelectic. When the microcantilever is vibrated by giving a signal generator to the piezoelectric, the resonance frequency of the microcantilever can be calculated by equation (1) [9].

(1) where k is the spring constanta and M is the effective mass of the microcantilever.

Next, the addition of mass on the microcantilever surface will cause a decrease in the resonance frequency. Therefore, mass changes Δm can be obtained from the resonance frequency shift caused by molecular adsorption according to the equation (2) [9].

∆ 2 ∆ (2) From the equation above, a mass sensitivity S of the microcantilever-based sensors is as follow,

∆∆

2 (3)

Figure 1(a) shows illustrations of the microcantilever vibration before (1) and after (2) the target molecules attach on the microcantilever surface. Before the molecules attach on the microcantilever surface (1), the microcantilever has the deflection of δ0 and vibrates with a certain frequency of f0 [10]. The microcantilever then captures the target molecules (2), resulting in the change in both resonance frequency due to

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the additional mass (Δm) of the microcantilever, as shown in Fig. 1(b). The resonance frequency shifts to lower value, after the target molecules are captured on the microcantilever surface.

(a)

(b)

Figure 1. (a) Microcantilever vibration before and after molecules detection, (b) Resonance frequency shift due to the molecules detection

III. EXPERIMENTAL SETUP In the experimental setup, the microcantilever sensor

system consists of function generator, Wheatstone bridge, Op-Amp and oscilloscope, as shown in the Fig. 2 [7]. The Wheatstone bridge circuit was constructed by two piezoresistors in the microcantilever and two external resistors. The piezoresistive microcantilever used here is commercially manufactured by Seiko Instrument Inc., Japan. Even the microcantilever was designed to be an atomic force microscope application, we suggest that the sensor system with same microcantilever structure in Fig. 2 can be used for chemical sensor, including gas detection. A long microcantilever has a length of 110 μ m, a width of 50 μm, area 9.80 x 10-5 cm2 (both sides), weight 46 ng, spring constant 40 N/m and piezoresistor R1 of about 630 Ω [11]. The

scanning electron microscope (SEM) image of this microcantilever can be seen in Fig. 3. It is noted that the short microcantilever with piezoresistor R2 of about 630 Ω is a reference cantilever, which was designed to be unchangeable during the measurement. Such reference cantilever is crucial in order to reduce background noise, such as thermal drift and gas turbulence [12].

Figure 2. Block diagram of the whole sensor system

The output of Wheatstone bridge (∆V) in Fig. 2 could be written as ∆ ⁄ . Such ∆V will be modified if the value of piezoresistor R1 changes due to the gas detection. The ∆V is then amplified by amplifier instrument, resulting in Vout, which is monitored by oscilloscope. It is noted that the gas molecules attached on the microcantilever surface causes the microcantilever deflection, so that the stress on the base of the microcantilever increases. This results in the change in the piezoresistor, which is located in the base of microcantilever.

Figure 3. SEM image of the piezoresistive cantilever

In the measurement based on the dynamic mode operation,

the activation of the piezoelectric will vibrate the microcantilever with a certain frequency. In our work, the sensor system is firstly placed inside the box, and the resonance frequency of the microcantilever vibration was measured. Next, Liquefied Petroleum Gas (LPG) is introduced in box, and then the resonance frequency shift was measured by the oscilloscope.

IV. RESULTS AND DISCUSSION In present experiment, we recorded the ∆V oscillation

peak-to-peak (Vpp) which represents the microcantilever vibration change due to the gas flow into the box. The ∆V

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oscillation is then analyzed by a Fast Fourier Transform (FFT). Figure 4(a) shows the FFT graph before LPG gas is introduced into the box. It is found that the resonance frequency of the microcantilever vibration is about 318 kHz.

(a)

(b)

Figure 4. (a) FFT analysis of the initial resonance frequency and (b) change of peak-to-peak ∆V oscillation due to gas detection

Next, the gas is introduced and then the change of ∆V oscillation peak-to-peak (Vpp) is recorded, as shown in Fig. 4(b). It can be seen that, before the gas is introduced, the Vpp is almost constant of about 1400 mV. When gas enters the box at the time of 155 s, the Vpp abruptly decreases. After the time of 200 s, the Vpp slowly decreases, and then there is no significant change in Vpp. We suggest that the decrease in the Vpp due to the gas flow corresponds to the resonance frequency shift, as our illustration in Fig. 5. In present experiment, since we measured the ∆V oscillation in the fixed frequency of 318 kHz, the detected Vpp is not always at the peak of the resonance frequency. The value of Vpp, which is indicated by A-F points (see Fig. 5(a)), changes may be due to the resonance frequency shift to the left (see Fig. 5(b)). When the resonance frequency shifts to the lower value, the detected Vpp reduces. After the time of 200 s, the attached gas molecules are saturated so that there is no Vpp change. We suggest that, since the value Vpp in Fig. 4(b) reached a constant after about 250 s, the resonance frequency ahift is about 57.14 kHz, such shift represents the mass change Δm of about 16.5 ng due to the gas molecule on the microcantilever surface.

(a)

(b)

Figure 5. Illustration of relationship between the decrease in the Vpp and the resonance frequency shift

V. CONCLUSION We have investigated the possibility of the piezoresistive

microcantilever as the LPG gas sensor using the dynamic mode measurements. The Wheatstone bridge circuit was built in order to detect the resonance frequency shift due to the detection of the gas. The gas detection is indicated by the decrease in the microcantilever vibration amplitude, which represents the resonance frequency shift. Even the linearity of gas-cantilever response relation and the coating of the microcantilever surface need to be clarified, we believe that such results open up the possibility of the gas detection using piezoresistive microcantilever-based sensor.

ACKNOLEDGEMENT This work was partially supported by incentive research

grant from Indonesia State Ministry of Research and Technology, and by Indonesia Toray Science Foundation (ITSF) Science and Technology Research Grant.

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