Figure 3.6 Scanning electron microscope image of silicon dioxide mask lines after isotropic etching in 2 min.
The scanning electron microscope (SEM) image of oxide mask lines after this step is shown in figure 3.6. As we see, the width of second mask line is 1.02 àm, slightly smaller than Chromium mask (1.1 àm). This may be due to diffraction in photolithography or by the fluctuation of room temperature changing etching rate of BHF. At micro scale, this error is acceptable.
Figure 3.7 displays the SEM image of second mask line after etching time is 8
can estimate the average rate of undercut etching is about 60nm per minute. This value is smaller than the etching rate in first step.
In fact, it is hard for me to reduce the size of oxide mask line to less than 100nm.
The cause is when increasing the under distance, the reaction products is more difficult to move out. It prevents the etchant to attack the sidewall of oxide mask lines due to decreasing etch rate over time. Moreover, if the sample is embedded in
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Figure 3.7 Scanning electron microscope image of dioxide mask lines after isotropic etching in 8 minutes.
BHF solution so long, the photoresist layer can be destroyed. Of course, the oxide mask will be destroyed rapidly then. For reaction products move out more easily, we can increase the thickness of oxide layer. However, the deposited rate of dry
We can use wet oxidation for depositing the thick oxide mask, but the oxide which forms by this technique is porous and low quality.
After the time etching of 2 minutes, the size of top surface is 244nm (Fig. 3.8 a) and after 6 minutes, the size is down to 91nm (Fig. 3.8 b). Based on these values, we estimate the etching rate in (111) plane is about 16nm per minute and anisotropy ratio is about 90. By calculating the time etching, we can reduce the size of top surface to zero and form a triangular waveguide.
In summary, we have presented a top-down fabrication method for producing single-crystal silicon waveguides with high throughput based on the standard photolithography technique and KOH wet-bulk micromachining. The fabricated single-crystal silicon waveguides are molds for forming the surface plasmonic waveguides after depositing metal layer.
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Figure 3.8 The SEM images of silicon waveguide after anisotropic in 2 min.
(a) and 6 minute (b).
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CONCLUSIONS
In this thesis, we have proposed wedge plasmon polariton waveguide structures that are based on the etching properties of single crystal silicon in Potassium Hydroxide solution.
The thesis also has carried out simulation analysis on the influence of structural parameters on the transmission characteristic of waveguide.
Electromagnetic wave is confined strongly in a size at nano scale on the wedge and top surface of structures. When decreasing the wedge corner, the distance of two tip or increasing the refractive index of surrounding medium, the confinement is stronger but the attenuation also increased. In the case of is very thin metal layer, WPP can couple with the high order mode in silicon dielectric waveguide leading to the variation of propagation characteristics.
We also describe a simple and lowcost method to fabricate waveguide structures based on wet bulk micromachining. The size of structure is controlled at nano scale. By using this method, we have successfully fabricated a number of structures that are selected from the simulation results.
SUGGESTED FURTURE WORKS
- Establish a measurement setup to verified the obtained results from simulation
- Explore a possible solution for integrating the waveguides into optic circuits - Develop coupling mechanism for the waveguide and optical sensors based on
the waveguide.
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