Future work

First, improvements on the device fabrication should be noticed. In the current fabrication process, which is an expensive one, two DRIE steps and a SOI wafer are needed. Overall, this process is unnecessary to some extent. While SOI wafers are chosen to achieve electrical isolation and structure support, backside DRIE is used to remove partial SOI handle layer and eliminate parasitic capacitance. A new process utilizing a quartz wafer and only one DRIE step is introduced as shown in Figure 5.1.

In a quartz wafer, pits are etched by HF solution to allow the moving mass to glide above them. A prime wafer is then bonded to the quartz wafer by an anodic bond.

Once the wafers are securely bonded, the structure is defined by DRIE. Eventually, the quartz wafer contributes to no parasitic capacitance and provides electrical isolation and structure support.

(b)

(c) Handle wafer Device wafer

Pyrex Silicon

Figure 5.1 Improved device fabrication: (a) pit holes etched in a Pyrex wafer, (b) anoidc bond and (c) structure defined by DRIE

(a) Pit

Second, the energy conversion process needs an external DC voltage source such as a battery. This limits the applications of the converter. Advanced method using pre-charged electret as the initialize polarizing voltage source has been reported [38].

As shown in Figure 5.2, the variable capacitor is polarized by an electret. The variation of the capacitance results in a current through a load circuit. With the electret included, there will be no need of external battery and thus extend the application potential.

Regarding the fabrication processes, since the feature size shrinkage during device fabrication always exists, the compensation for the reduced variable capacitance should be done. Generally, the shrinkage of spring width reduces the spring constant and thus larger displacement of the device can be achieved. In other words, the increased minimum finger spacing (due to finger width shrinkage) and the reduced maximum capacitance are compensated. In the future, a more flexible design that tolerate wider range of reduced springs constants can be designed.

Figure 5.2 Principle of electret-based generator [38]

m d

k

z(t)

y(t)

C(z)

Load

Polarisation sources electret

Finally, more efforts should be made on the device robustness. When the devices are subject to large acceleration, they are often broken because the center plate collides the mechanical stops. If the current minimum gap (0.1 μm) controlled by the mechanical stops can be reduced to 0 μm, the movable fingers will touch the fixed fingers each time the center plate collides the mechanical stops. The soft, insulated fingers act like a buffer that prevent the direct collision and thus enhance the device robustness. However, the stability of the device is still an issue when the bumps occur.

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