# W-Band 4 th Order Waveguide Filter Based on Double Layer SU8 Microfabrication
## 3 Experimental Results
### Difficulties and Solutions in the Fabricating
#### 3.2.2 Underetching on Sidewalls  
Steepness refers to an angle of deviation from a vertical wall. Steepness is another important factor affecting device performance in the SU-8 process. The Steepness mainly depends on the optimal exposure dose, the closeness of the contact between the silicon wafer and the mask, the wavelength of the light source, and the SU-8 photoresist thickness. A typical sidewall profile of a small cavity obtained using the SU-8 process is shown in Figure 6a. The sidewall profile exhibits an inverted isosceles trapezoid due to the fact that the actual exposure dose is larger than the optimal exposure. This reduces the effective size of the resonant cavity, which causes the self-resonant frequency of the cavity to shift to high frequencies. Deviations in the resonant frequency of a single cavity may cause severe response detuning. Therefore, the optimal exposure dose will effectively improve the steepness. In this paper, the same thickness of SU-8 photoresist is exposed to ultraviolet light with different exposure doses, and the optimal exposure dose is determined by observing the steepness. Figure 6b shows that the photoresist thickness of 249.7 \(\upmu\)m corresponds to a steepness of 0.72\({}^{\circ}\) as measured by a confocal lens. Another method for determining the steepness is to calculate it more accurately by the tangent formula mentioned in Section 5 of this paper.  
Since the main factor affecting the frequency offset and bandwidth variation of the device is caused by the steepness of the resonators and the coupling irises between them, the sensitivity of their steepness is analyzed in this paper, as shown in Figure 7. It shows a maximum steepness tolerance of 4\({}^{\circ}\) that can be tolerated for performance while the device is guaranteed to operate properly.  
Figure 6: (**a**) Sidewall profile of the fabricated filter and (**b**) h = 249.7 \(\upmu\)m, \(\theta\) = 0.72\({}^{\circ}\) (i.e., 90–89.278\({}^{\circ}\)). A typical sidewall profile of a small cavity obtained using the SU-8 process is shown in Figure 6a. The sidewall profile exhibits an inverted isosceles trapezoid due to the fact that the actual exposure dose is larger than the optimal exposure. This reduces the effective size of the resonant cavity, which causes the self-resonant frequency of the cavity to shift to high frequencies. Deviations in the resonant frequency of a single cavity may cause severe response detuning. Therefore, the optimal exposure dose will effectively improve the steepness. In this paper, the same thickness of SU-8 photoresist is exposed to ultraviolet light with different exposure doses, and the optimal exposure dose is determined by observing the steepness. Figure 6b shows that the photoresist thickness of 249.7 \(\upmu\)m corresponds to a steepness of 0.72\({}^{\circ}\) as measured by a confocal lens. Another method for determining the steepness is to calculate it more accurately by the tangent formula mentioned in Section 5 of this paper.  
Figure 7: The sensitivity analysis of the steepness of the designed filter is based on simulations.