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High Sensitive Mass Detection using GaAs Coupled Micro Resonators
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1  FEMTO-ST institute

Abstract: This work demonstrates the improvement of mass detection sensitivity and time response using a quite simple structure of sensor. Indeed, complicated technological processes are often required to reach high sensitivity when we want to detect specific molecules in biological fields. These developments constitute an obstacle to the early diagnosis of diseases. An alternative is the design of coupled structures. The device is based on the piezoelectric excitation and detection of two GaAs micro structures vibrating on antisymmetric modes. GaAs is a piezoelectric material which has the advantage to be micromachined easily using clean room processes. Moreover, we showed its high potential in direct biofunctionalisation to be used in biological field1. A specific design of electrodes (three electrodes) was performed to improve the detection at low mass and an original detection method has been developed. The principle is to exploit the variation in amplitude at a fixed frequency when we are concerned by a weak frequency shift of the resonance peak. The three electrodes are geometrically identical. We excited the device at the resonance frequency, corresponding to maximum voltage of the initialization electrode. Thus we noted the voltage on the measuring electrode, which had, in the vicinity of weak added mass, the greatest slope. Therefore, we get a very good resolution for an infinitely weak mass: relative voltage variation of 8%/1fg. The analysis given in this paper is based on results obtained by finite element modeling.     1A. Bienaime et al, Materials 2013, 6, 4946-4966
Keywords: GaAs piezoelectric transducer, acoustic coupling, high resolution
Comments on this paper
Wei Chen
some questions about the micro-mass detecting system
dear therese,
i have some questions about the system. for i do not major in this filed, maybe the questions are simple even navie....

1, i want to know whether the location of the micro-mass placed on the cantilever would affect the result?
2, how is the vibration frequency of the system related with the output voltages of Vce and Voce?  and how the position of the electrodes on the micro-cantilever is decided?

3, is the 10v-AC source used to excite the vibartion of the cantilever?
best regards,
Therese Leblois
Dear Wei,
question 1: It is clear that the location of the micro mass added on the cantilever affects the experimental measurement. If it is located near the fixed area of the device the sensitivity will be lower than if it is located near the free end of the cantilever. So, to limit this phenomena, self assembled monolayers composed of several thiols with different tail groups on GaAs surface are immobilized at the free end of the cantilever to modify the surface tension. The surface becomes more hydrophilic and then the biologic molecules (which constitute the added mass) are located near the free end of the cantilever. The frequency and amplitude responses to added mass are then more reproducible.

question 2: the sizes of the electrodes depend on technological constraints. The initialization electrode must be located at the center of the device.
Vce and Voce signals are observed at the  resonance or near the resonance frequency. As seen on figure 3(a), teh Vce electrode gives a response with an even function. The off center electrode (Voce signal) provides an odd function at the same frequency. This last response allows an increase of sensitivity for very weak added masses.
question 3: Yes, the 10V AC source is used to excite the cantilever vibration.

Best regards,
Th leblois