**3. Fabrication**

The substrate is the (100) silicon wafer with a 300 nm SiO2 layer. The bottom electrode is Mo, with 300 nm thick. The (002) oriented AlN film is deposited on the Mo layer by the magnetron sputtering process. It is 1.3 μm thick. The piezoelectric coefficient d33 of the AlN film is 5.09 pm/V tested by the piezoresponse force microscopy [21]. A composite top electrode layer 150 nm Ti/Pt is deposited by the electron beam evaporation. The normal MEMS processes have been used to fabricate this accelerometer. The AlN film is etched by the ICP process with Cl2/BCl3/Ar [22]. The entire structure is released by the ICP isotropic etching process with SF6.

Figure 6a,b are the scanning electron microscope (SEM) pictures of the fabricated accelerometer. Figure 6a is the top view of the structure. Figure 6b is the side view of the structure. The structure bends along the +z-axis because of the residual stress.

**Figure 6.** The micrographs of the AlN MEMS accelerometer. (**a**) Top view; (**b**) side view.

In order to achieve a better performance, a vacuum package process—the discharge welding—was used. In this process, the vacuum is 0.16 Torr.

#### **4. Results and Discussion**

The characteristics of this AlN accelerometer were tested by the dynamic signal analyzer 35,670 A. The source signal, which comes from the 35,670 A, applies on the drive electrode to excite the structure. The amplitude of the input signal is 0.2 Vpk. The GND electrodes are the bottom electrode. It connects the ground. In addition, the resonant signal is detected from the sense electrode. Figure 7 shows the

schematics of an electrical test of this accelerometer. The resonance frequency is 24.66 kHz at a static state at room temperature. The quality factor is 1868 as shown in Figure 8.

**Figure 7.** The schematics of the electrical test.

**Figure 8.** The resonant characteristics of the AlN accelerometer at a static state.

For the sensitivity test, the accelerometer was placed on the rotating platform vertically, as shown in Figure 9. A centripetal acceleration, which comes from the circular motion (i.e., *a* = ω2*r*), would apply on the AlN resonant accelerometer, and lead to a frequency shift. The rates of the rotating platform range from 0~1000◦/s. The accelerometer is placed at the position 20.5 cm away from the center. In this test, the rate was set from zero to 1000◦/s with a step of 100◦/s. The room temperature is 25 ◦C.

**Figure 9.** The sensitivity test by the rotating platform.

In the rotation test, the centripetal acceleration is perpendicular to the plane of the accelerometer. To avoid the Coriolis force, the MEMS chip is placed on the support vertically, which the direction of the vibration velocity →*v* is parallel with the rate →ω. Therefore, the Coriolis force is null. Figure 10 shows the experiment results of the relationship between the accelerations and resonance frequencies. The sensitivity of the AlN resonant accelerometer is 8.53 Hz/g, tested from −5 g to +5 g. The relative sensitivity (i.e., Δ*f f*0/*g*, where *f0* is the resonance frequency) is 346 ppm/g at the base frequency

of 24.66 kHz. The linearity of the accelerometer is 0.9988. With the increase of the acceleration, the resonance frequency increases. As shown in Figure 10b, the amplitude of each resonant peak is nearly equivalent.

**Figure 10.** (**a**) Sensitivity of the accelerometer; (**b**) resonant peaks at different accelerations.

The cross error of this MEMS accelerometer was characterized by the rotation test. Figure 11a,b show the test results. The output frequencies scatter around 24.66 kHz. The x-axis sensitivity is 0.039 Hz/g, and the y-axis sensitivity is 0.52 Hz/g. Compared with the z-axis sensitivity, the cross error of the x-axis is 0.46% and the y-axis is 6.1%. These results prove that this MEMS accelerometer is insensitive to the in-plane accelerations.

**Figure 11.** Cross axis sensitivities of the AlN MEMS accelerometer. (**a**) X-axis sensitivity; (**b**) Y-axis sensitivity.

The temperature characteristic of the AlN accelerometer is tested. The temperatures range from −40 ◦C to 85 ◦C with a step of 5 ◦C. The temperature coefficient of frequency (TCF) of this accelerometer is −2.628 Hz/◦C (i.e., −106 ppm/◦C). Figure 12 shows the temperature characteristic of this accelerometer. Due to the negative temperature coefficient of AlN, a higher temperature will lead to a lower Young's modulus. The resonance frequency is proportional to -*E*ρ . Where *E* is the Young's modulus. And ρ is the density.

$$f\_0 \propto \sqrt{\frac{E}{\rho}}\tag{3}$$

Therefore, the resonance frequency will decrease with the increase of the temperature. In order to reduce the TCF, an optimized temperature compensation layer (SiO2 layer) or a temperature compensation circuit will be used.

**Figure 12.** The temperature characteristic of the AlN accelerometer tested from −40 ◦C to 85 ◦C.
