A novel T-shaped piezoelectric ZnO cantilever sensor for chem/bio-detection is designed and fabricated with MEMS technology. By using Rayleigh-Ritz method, the fundamental resonant frequency formula of T-shaped cantilevers is deduced for the first time and is validated by simulation results and experimental results. From this formula, we can easily find the superiority of adopting T-shape for the cantilevers. The complete process of the cantilever sensor is then successfully developed. The cantilever sensor is actuated by a layer of high-quality ZnO film with preferred (002) orientation, which is evaluated by SEM and XRD. The key step of the process is protecting the ZnO film from KOH etching by a novel and effective method, which has rarely appeared in the literature. Finally, this cantilever sensor is measured by a network analyzer, and it has a fundamental resonant frequency of 24.60 kHz. The cantilever sensor developed in this study illustrates the feasibility and potential for many miniaturized sensor applications.
There is a dramatic and urgent demand to develop inexpensive, microfabricated, and extremely sensitive sensors for chem/bio-sensing. As mass sensitive sensors, microcantilever sensors have many advantages over other transducers. For example, these sensors have been shown to exhibit over two orders of magnitude greater absolute sensitivity such as quartz crystal microbalance (QCM), surface acoustic wave (SAW) devices, acoustic plate mode (APM) devices, chemiresistors, and flexural plate wave (FPW) oscillators [
Another popular method without huge optical systems is by using piezoresistive layer on the cantilever itself. Any changes in the surface stress due to bending will cause an electrical output from the piezoresistive electrodes. The disadvantage of piezoresistive technique is that it requires passing a current through the cantilever for displacement measurements. This results in electronic noises and thermal drift in cantilever deflection.
In this study, we choose piezoelectric method for the purpose of simplifying and minimizing the sensors. To date, lead zirconate titanate (PZT) is one of the most widely exploited and extensively used piezoelectric materials. For example, Lee et al. [
Recently, ZnO has attracted a lot of attention because it is one of the pollution-free piezoelectric materials which do not suffer from the mentioned problems of PZT. ZnO finds applications in MEMS due to its unique combination of electrical, optical, and piezoelectric properties [
Fortunately, we have developed a novel, simple, and practical method to protect the ZnO film from alkaline etching with a protective coating of black wax (Apiezon-W). Hence, the selected piezoelectric layer in this study is a
The sensitivity of resonant microcantilever sensors is directly proportional to the resonant frequency [
As shown in Figure
Schematic drawing of the sensor.
Slide view
Top view
Figure
Schematic drawing of a rectangular cantilever.
When applying a normal force
As one end of the cantilever is fixed, the corresponding boundary conditions are
The solution of (
This is the deflection function along the length direction, where
In this section, we derive the fundamental resonant frequency of T-shaped cantilevers using Rayleigh-Ritz method.
Figure
Schematic drawing of a T-shaped cantilever.
The deflection function of (
The kinetic energy of the system is
Therefore, the maximum kinetic energy of the system is
The potential energy of the system is
Therefore, the maximum potential energy of the system is
According to conservation law of mechanical energy, the maximum kinetic energy equals to the maximum potential energy:
For calculation convenience, it is reasonable to define the width ratio
Hence, the fundamental resonant frequency can be obtained by substituting (
In order to represent the relationship between the fundamental resonant frequency and the two ratios
Thus, the fundamental resonant frequency of T-shaped cantilever sensor can be written as
Figure
The function image of
Consider an n-layered T-shaped cantilever of density
An n-layered T-shaped cantilever and its equivalent monolayered T-shaped cantilever.
Equivalent density:
Equivalent Young’s modulus:
Equivalent thickness:
With (
In order to validate (
Geometric and material data of the T-shaped piezoelectric ZnO cantilever.
Layer | Density | Young’s modulus | Thickness |
---|---|---|---|
(Kg/m3) | (GPa) | ( |
|
ZnO | 5676 [10] | 209.7 [10] | 2 |
Si3N4 | 3170 [10] | 270 [10] | 0.5 |
Hence, the necessary parameters are listed below:
Substituting these parameters into (
In order to assess the accuracy of (
According to experimental results discussed in Section
On the other hand, according to the finite-element method (FEM) simulation results, the resonant frequency of this device is
Deformed shape of the first vibration mode.
It can be noted from these statistics that the relative error is very small, and the formula we have deduced is reasonably accurate.
The piezoelectric ZnO cantilever sensor is fabricated by MEMS technique depicted in Figure
Fabrication processes of the sensor.
It is important to emphasize the difficulty for MEMS researchers to find a way to protect electric circuits and other delicate devices during deep silicon wet-etch processes. The challenge exists in choosing an appropriate protective coating that could meet these criteria:
Figure
SEM image of the cross-section of the ZnO film.
XRD pattern of the ZnO film on Au.
Fabricated piezoelectric ZnO cantilever.
The fundamental resonant frequency of the piezoelectric ZnO cantilever sensor is measured by an Agilent E5100A network analyzer. From the frequency response curve shown in Figure
Frequency response of the piezoelectric ZnO cantilever sensor (
Using Rayleigh-Ritz method, the fundamental resonant frequency formula of T-shaped cantilevers is deduced for the first time, is validated by simulation results and experimental results, and can be widely used in design and optimization of T-shaped cantilevers. This formula illustrates that, comparing T-shaped cantilevers and rectangular cantilevers with the same dimensions and same materials, the former can lead to higher sensitivity. Then, a novel T-shaped piezoelectric ZnO cantilever sensor is designed and successfully fabricated by MEMS technique and finally tested by a network analyzer. The key step is protecting the ZnO film from KOH etching with a protective coating of black wax (Apiezon-W). This is one important technique that has shown considerable success. With this technique, many other delicate functional materials such as special ceramics, metals, polymers, and organic molecules will come into the use in the MEMS field. Experimental results show that this novel piezoelectric ZnO cantilever sensor has excellent dynamic response and can provide a new platform for chem/bio-sensing.