## 3 Results and Discussions
### Output Characteristics under PI Protection  
The output characteristics under high pressure range of the PI protected micro pressure sensor were tested. The device with a PI protection layer was pressure from 0 to 5 MPa. In order to compare the influence of the protective layer on the output characteristics, V-P curves of the devices with and without the top surface PI layer are presented in Figure 7.  
The output curves show that the PI protection layer has almost no effect on the output characteristics of the micro sensor. A further experiment was designed to test the anticorrosive performance of the polyimide layer. Both devices, with and without a polyimide protection layer, were immersed in seawater and heated to 80 \({}^{\circ}\)C in an oven to accelerate the corrosion of the devices. The devices were removed from the seawater and dried before a resistance test every 30 min. Normally, the resistance of the Wheatstone bridge was between 1.7 and 2.0 k\(\Omega\); if the output resistance was not in the normal range, the device was considered out of operation. The experiment results are shown in Table 3.  
Results show that the devices with the polyimide protective layer could still work after 2 h immersion in seawater, while the devices without PI layer were out of operation after 30 min immersion. It can be inferred that the PI protective layer possesses an exceptional anticorrosive performance. Therefore, the PI protection layer has a broad prospect on the micro pressure sensor for harsh environment applications, such as gas pressure measurement on ships or other marine engineering applications.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameters** & **Units** &
\begin{tabular}{c} **Fabricated** \\ **Devices** \\ \end{tabular} & **MSD700-ASO 1** & [14] & [31] \\ \hline Thickness of & & & & & \\ Diaphragm & \(\mu\)m & 50 & 40 & – & 40 & 1.2 \\ Chip Size & mm & \(1.5\times 1.5\times 0.82\) & \(0.95\times 0.95\times 0.6\) & \(1.5\times 1.5\) & \(1.6\times 1.6\times 0.9\) \\ Excitation Voltage & V & 3.3 & 3.3 & 5 & 10 & 3 \\ Pressure Range & MPa & 0-5 & 0-5 & 0-0.7 & 0-1 & 0-0.55 \\ Full-Scale Output & mv & 117.1 & 167.87 & 100 & 105 & 45.99 \\ Sensitivity & mv/V/MPa & 7.85 & 10.01 & 28.57 & 10.5 & 27.87 \\ Nonlinearity & F.S.\% & 0.41 & 0.48 & 0.3 & – & 0.34 \\ \hline \hline \end{tabular}  
* Product code: MSD700-ASO, MEMSensing Microsystems Co., Ltd., Suzhou, China.  
\end{table}
Table 2: Comparison among our devices, a commercial device, and others described in literature.  
Figure 7: Output voltage of the devices with and without the PI protection layer.  
In order to compare the stability of PI protection layer and traditional silicone rubber encapsulation under instantaneous violent pressure variation, a dynamic pressure test was designed. Tested pressure sensors were covered with a PI protection layer and silicone rubber, nonencapsulated sensors served as a control group. The start pressure was set at 3, 4, and 5 MPa. After each instantaneous decompression to normal atmospheric pressure, the output resistance was measured and compared with the control group. The process was repeated three times; if the output resistance was abnormal, the process was discontinued. Test results are presented in Table 4.  
It can be concluded from the table that the polyimide coat exhibits better stability than traditional silicone rubber encapsulation. Due to less deformation under high pressure compared with silicone rubber, the PI protection layer leads to less potential for abnormal output resistance caused by a fracture of the bonding wires. In conclusion, the polyimide protection layer has reliable corrosion resistance and high stability under violent pressure variation. Thus, it has great potential for application on micro sensors designed for harsh environments and high-pressure applications.