# Measurement of Structural Loads Using a Novel MEMS Extrinsic Fabry-Perot Strain Sensor
## 3 Sensor Fabrication  
Figure 2 depicts the schematic diagram of configuration of the extrinsic fiber FP strain sensor in this paper, which is mainly developed through an FP optical interference cavity, fiber collimator support, and fiber collimator. The cavity is composed of two high reflectivity parallel planes that are made by MEMS microprocessing technologies. One plane, \(S_{1}\), is the high reflection film on the right face of the fixed glass, and the other plane, \(S_{2}\), is the left side of the bottom layer of silicon of the SOI (silicon-on-insulator wafers) strain beam. The strain beam is bonded on the fixed glass by the silicon-glass anode bonding. Then, an anti-reflection coating is deposited on the face of the fixed glass that is not bonded, and an anti-reflection layer and a passivation layer are deposited on the other surface of the SOI strain beam to reduce the reflected light and weaken the error signal. When the coherent beam is incident into the optical interference cavity along the collimated beam expanding fiber, the incident light beam is reflected multiple times between the \(S_{1}\) and \(S_{2}\) planes to form multiple-beam interference, and it returns to the collimated beam, expanding the fiber along the original path. The interference output signal is related to the length of the microcavity between the \(S_{1}\) and \(S_{2}\) planes. The manufacturing process of wafers is illustrated in Figure 3. (1) The first step is to carry out the SOI wafer used as the silicon strain beam with a buried silicon dioxide layer, in which lithography was performed on the top side of the silicon wafer (Figure 3a). (2) Then, the SOI wafer is etched via deep reactive ion etching (DRIE) until the buried silicon dioxide layer emerged as shown in Figure 3b to form a cylindrical hole. (3) The intermediate oxide layer exposed in wafer is removed by wet etching or dry etching to obtain a hole on the intermediate oxide layer (Figure 3c). (4) A high reflective film \(S_{2}\) patterned by lift-off method is deposited on the silicon wafer by physical vapor deposition (Figure 3d). (5) The same technique is used to deposit another high reflection film \(S_{1}\) (Figure 3e) on the top of the fixed glass, and the high reflection film image processing is completed. (6) The SOI silicon wafers obtained in steps (1)-(4) are bonded to the glass obtained in step (5) by silicon-glass anode bonding (Figure 3f). (7) The anti-reflection film is deposited on the top face of the bonded glass; an anti-reflection coating and a passivation layer are successively deposited on the lower face of the silicon strain beam, and the anti-reflection coating and the passivation layer are all patterned by lift-off method (Figure 3g,h). Finally, the strain-sensitive probe is fabricated, an optical fiber is then bonded with the sensor chip to form the fiber-optical F-P sensor, as depicted in Figure 3.  
Figure 2: Schematic of miniaturization of the extrinsic fiber Fabry–Perot (EFFP) strain sensor. The measuring gauge length of the packaged sensor is \(L\). It will increase or decrease when the sensor is subjected to external strain, which will result in variation of the length of the microcavity between the two planes. Furthermore, the optical signal intensity, wavelength, or phase corresponding to the cavity length will also shift. By using demodulation technology [30], the spectral change of light is converted into a digital signal that can be recorded by a computer, and the sensor can accurately measure the external physical quantities through precision calibration. According to the principle of elasticity, the measured strain [31] can be calculated as  
\[\varepsilon=\frac{\Delta d}{L}. \tag{9}\]  
The variation of the sensor cavity length \(\Delta d\) is obtained by measuring the reflected output spectrum.