## Materials and Methods
### Test Structure and Extraction Model  
The schematic illustration of the test structure for measuring the thickness and sidewall etch angle of MEMS thin films is shown in Figure 3. It consists of three units, which deposited horizontally and at an angle of \(\varphi\) (\(0^{\circ}<\varphi<90^{\circ}\)) and \(\psi\) (\(0^{\circ}<\psi<90^{\circ}\), \(\varphi\
eq\psi\)), respectively. The first unit is composed of a Greek cross structure and a four-probe bridge containing several continuous step structures (see Figure 3a). All step structures have the same dimension. The widths of the cross and the bridge are identical, and the bridge is deposited horizontally. The second unit consists of two four-probe bridges deposited at an angle of \(\varphi\) (see Figure 3b). The left bridge is comprised of several continuous step structures, while the right bridge is deposited on the flat surface of the PSG layer and shares two probes with the left bridge. The third unit is similar to the second unit, with the only difference of the angle of four-probe bridges (see Figure 3c). The dimensions of these three units are illustrated in Figure 3.  
Figure 1: Schematic diagrams of a step structure: (**a**) 3-D view; (**b**) Cross section view. Figure 2: Resistance of corners: (**a**) Various patterns of corners; (**b**) SEM photo for the cross-section view of a step structure. Greek cross structure is used to evaluate the sheet resistance [26]. Based on [PERSON] theory, the sheet resistance of polysilicon thin film can be obtained by [27]  
\[R_{S}=\frac{\pi}{\ln 2}\frac{(V_{S1S2}+V_{S2S3})}{2I}, \tag{4}\]  
where \(I\) is the applied current, \(V_{SIS2}\) is the potential different between pads S1 and S2 when the current is passed between pads S4 and S3, \(V_{S2S3}\) is the potential different between pads S2 and S3 when the current is forced between pads S1 and S4. The width of the bridges can be estimated as [28]  
Figure 3: Schematic illustration of the test structure for measuring geometrical parameters of surface micromachined thin films: (**a**) Unit 1; (**b**) Unit 2; (**c**) Unit 3. \[w=\frac{R_{S}l_{1}I}{V_{a}}, \tag{5}\]  
in which \(l_{1}\) is the length between contacts S3 and A1, \(V_{a}\) is the potential different between pads A1 and S3 when the current \(I\) is applied between pads C1 and S1. To use the test structure to obtain the thickness and sidewall etch angle of thin films, a current \(I\) is passed between contacts B1-A1, B2-A2, B3-A3, F2-D2, and F3-D3, while the potential different between pads D1-C1, D2-C2, D3-C3, E2-B2, and E3-B3 are measured as \(V_{1}\), \(V_{2}\), \(V_{3}\), \(V_{b}\), and \(V_{c}\) (see Figure 3). The resistances of continuous step structures of the three units can be derived from Equations (1)-(3)  
\[R_{1}=n\bigg{\{}\frac{2R_{S}l_{a}}{w}+\frac{R_{S}}{w}\bigg{[}l_{b}-\frac{2(l_{ 2}-t_{1})\cos\theta+2t_{1}}{\sin\theta}\bigg{]}+\frac{2R_{S}(t_{2}-t_{1})}{w \sin\theta}+\frac{\gamma R_{S}t_{1}}{w}\bigg{\}}, \tag{6}\]  
\[R_{2}=n\cdot\bigg{\{}\frac{2R_{\varphi}l_{a}}{l_{2}\cos\varphi}+\frac{R_{ \varphi}}{l_{2}}\bigg{[}\frac{l_{b}}{\cos\varphi}-\frac{2(t_{2}-t_{1})\cos \theta+2t_{1}}{\sin\theta}\bigg{]}+\frac{2R_{\varphi}(t_{2}-t_{1})}{l_{2}\cos \varphi\sin\theta}+\frac{\gamma R_{S}t_{1}}{w}\bigg{\}}, \tag{7}\]  
\[R_{3}=n\cdot\bigg{\{}\frac{2R_{\varphi}l_{a}}{l_{3}\cos\psi}+\frac{R_{\varphi} }{l_{3}}\bigg{[}\frac{l_{b}}{\cos\psi}-\frac{2(t_{2}-t_{1})\cos\theta+2t_{1}} {\sin\theta}\bigg{]}+\frac{2R_{\varphi}(t_{2}-t_{1})}{l_{3}\cos\psi\sin\theta }+\frac{\gamma R_{S}t_{1}}{w}\bigg{\}}, \tag{8}\]  
where \(R_{1}=V_{1}/I\), \(R_{2}=V_{2}/I\), \(R_{3}=V_{3}/I\), \(R_{\varphi}=V_{b}/I\), \(R_{\varphi}=V_{c}/I\), \(l_{2}\) and \(l_{3}\) are the lengths of the right bridges of Unit 2 and Unit 3, respectively, \(n\) is the number of the step structures in each unit, \(\gamma\) is the total correction factor for the four corners of each step structure (\(\gamma=2\times(0.559+0.583)=2.284\)).