**4.5 Dynamic characterization**

#### **4.5.1 In-plane motion measurement**

A 20× objective was used in the dynamic experiments. The stimulating signal was sine waveform with 10V amplitude, 20V offset voltage. The exposing time of CCD was 20ms for LED illumination and 100μs for LD illumination. The strobed pulse percent was 100%. The strobed phase number was 8 for in-plane motion measurement and 16 for out-of-plane motion measurement.

In order to get the resonant frequency of the tested device, the sweep-frequency measurement in a large range was firstly needed. The frequency increases in the logarithm mode, so the range of resonant frequency can be decided through one time frequency

MEMS Characterization Based on Optical Measuring Methods 105

In order to study the relationship between the stimulating voltage and the movement, amplitude sweeping experiments were performed. The stimulating frequency was 23 kHz with the starting amplitude 0% and the ending amplitude 100% (10 V). The sweeping number was 21 and the amplitude increased linearly. The experimental results were shown in figure 20. The curves show that the relationship between the moving amplitude and the stimulating voltage is linear which is consistent with the computer simulated results. It draws the conclusion that the design and fabrication process of the device are valid and the

(a) Amplitude-scale curve (b) Phase-scale curve Fig. 20. Sweep-amplitude measurement of in-plane motion. The blue curve is the result in x

A region of interest (ROI, see in figure 21) on the device was tested to study the dynamic profile change at a certain stimulating signal. The dynamic profiles were shown in figure 22.

device has good dynamic behaviors.

direction. The yellow curve is the result in y direction.

**4.5.2 Dynamic profile measurement** 

Fig. 21. Choice of ROI

sweeping measurement. Then another frequency sweeping measurement can be done in a small range. In summary, smaller range can achieve more times measurement, and the gotten resonant frequency will be more accurate. Figure 18 shows the experimental results in the range from 100 Hz to 100 kHz, the sweeping number is 21. After that, the frequency sweeping range can be decreased to 22 kHz~25 kHz, the sweeping number was 100 (see in figure 19). From that, the resonant frequency of the microstructure can be obtained. From the amplitude-frequency and phase-frequency curves, the device can be fitted into a second order system, the resonant frequency of the device is about 23.41 kHz, the maximum moving amplitude is around 764.52 nm, so the quality factor can be calculated as Q=23.41/(24.04-22.87)=20. Because of the air damp during the moving process of the device, the quality factor is not high. All data will be fed back to the designer and the quality factor of the device can be increased through improving the structure of the micro-resonator. Then five times of experiments without stimulating signal were done. The noise floor was calculated as 0.56 nm.

(a) Amplitude-frequency curve (b) Phase-frequency curve

Fig. 18. Large range sweep-frequency measurement of in-plane motion. The blue curve is the result in x direction. The yellow curve is the result in y direction.

Fig. 19. Small range sweep-frequency measurement of in-plane motion. The blue curve is the result in x direction. The yellow curve is the result in y direction.

sweeping measurement. Then another frequency sweeping measurement can be done in a small range. In summary, smaller range can achieve more times measurement, and the gotten resonant frequency will be more accurate. Figure 18 shows the experimental results in the range from 100 Hz to 100 kHz, the sweeping number is 21. After that, the frequency sweeping range can be decreased to 22 kHz~25 kHz, the sweeping number was 100 (see in figure 19). From that, the resonant frequency of the microstructure can be obtained. From the amplitude-frequency and phase-frequency curves, the device can be fitted into a second order system, the resonant frequency of the device is about 23.41 kHz, the maximum moving amplitude is around 764.52 nm, so the quality factor can be calculated as Q=23.41/(24.04-22.87)=20. Because of the air damp during the moving process of the device, the quality factor is not high. All data will be fed back to the designer and the quality factor of the device can be increased through improving the structure of the micro-resonator. Then five times of experiments without stimulating signal were done. The noise floor was

(a) Amplitude-frequency curve (b) Phase-frequency curve

(a) Amplitude-frequency curve (b) Phase-frequency curve Fig. 19. Small range sweep-frequency measurement of in-plane motion. The blue curve is the

Fig. 18. Large range sweep-frequency measurement of in-plane motion. The blue curve is

the result in x direction. The yellow curve is the result in y direction.

result in x direction. The yellow curve is the result in y direction.

calculated as 0.56 nm.

In order to study the relationship between the stimulating voltage and the movement, amplitude sweeping experiments were performed. The stimulating frequency was 23 kHz with the starting amplitude 0% and the ending amplitude 100% (10 V). The sweeping number was 21 and the amplitude increased linearly. The experimental results were shown in figure 20. The curves show that the relationship between the moving amplitude and the stimulating voltage is linear which is consistent with the computer simulated results. It draws the conclusion that the design and fabrication process of the device are valid and the device has good dynamic behaviors.

Fig. 20. Sweep-amplitude measurement of in-plane motion. The blue curve is the result in x direction. The yellow curve is the result in y direction.
