Suggestion for future work

Design of PC waveguide: For the future it is worthwhile to design a single mode PC waveguide and investigate how it improves the transmission efficiency. Both the sensitivity region and the sensitivity of the pressure sensor can be tuned by the adjustment of the period and the shape of the microcavity. The design can be changed according to its

future application.

Fabrication: Each step in the process flow is critical and the final structure can be easily affected by any change. Good control of each step is essential for a good device.

The domed shape with higher centre part than the solid wafer is not fully explained yet.

Either a better way of fabrication is required or a more suitable wafer on which the device is realized is needed. To complete the entire device for optical test, photolithography and e-beam lithography is ready to be combined. The mask design for the experiment verification was shown in Figure 6.1. The die consists of four sets of conventional waveguides. The top and bottom waveguides are used to align the light source and detectors to the die. The two centre waveguides is the device region, and consists of conventional waveguide coupled to PC waveguide.

Figure 6.1 Mask design for experiment verification.

Sensor: We plan to combine AFM with optical set up including tunable laser and detector. The preliminary experiment using AFM force measurements showed positive results. The optical setup will provide light source coupling the light from the conventional waveguide to PC waveguide, Figure 6.2. As the pressure offered by AFM

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cantilever is applied onto the PC bridge, the distance changes between the silicon layer and the substrate which induces the coupling of the light transmitting in the waveguide into the substrate. Hence the transmission property was influenced accordingly and detected by the photo-detector at the other end of the PC waveguide. Experimental verification of the PC waveguide based pressure sensor is in progress.

Piezoelectric Acuator

Figure 6.2 Schematic optical measurement set up.

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Appendix A: E-Beam Lithography

Zep520A D.R = 1.4 is dispensed onto the wafer, and the wafer is spun rapidly to produce a 200nm uniformly thick layer, 300rpm for 3 sec and 3500rpm for 2min.

Compared to PMMA, it has an advantage of 3-4 times faster and has good dry etch resistance. It has the disadvantages of poor adhesion and normal exposure does result in re-entrant pattern profiles. The photoresist-coated wafer is then prebaked to drive off excess solvent, typically 180C for 3 minutes.

For high resolution lithography, writing conditions of e-beam lithography needs to be optimized. To write the PC waveguides coupled to conventional waveguide, the

6) Centre to centre distance: 20nm

7) Dose: Varied from 57pC/cm² to 78pC/cm2

Table Appendix A-l: Optimized E-beam Lithography parameters

The accelerating voltage, aperture size and magnification were selected to achieve

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high resolution. Line spacing and point spacing were selected to get desired filled holes.

Appendix B: Recipe for Bosch etching

The 40 second etching process for individual sample consisted of two alternating steps: ¹ second plasma etch using SF⁶ and ² seconds passivation by C⁴F⁸.

Table below shows the optimized parameters for ICP Alcatel system to achieving high etch rate and high aspect ratio.

~IC P Etch 1) RF power for Plasma generation: 1000 Watts 2) Substrate RF power: 50 Watt

3) Chamber Pressure: 1.3E-3mBar to 9E-2mBar 4) Chamber Temperature: 20°C

5) Helium Pressure: lE+lmBar

6) SF⁶ Flow: lOOsccm 7) C⁴F⁸ Flow: lOOsccm

Table Appendix B-l: Recipe for ICP etch

Appendix C: Recipe for Cryogenic etching

This appendix gives the recipe used for cryogenic etching process. 3-step etching process was created, starting with temperature stabilization, then pressure stabilization and etching.

Recipe for Cryogenic etching 1) r = -125°C

2) Pressure = 1 x 10“2 mBar 3) DC substrate power = 40 W

4) SF6-02 ratio = 80SCCM:10SCCM 5) Etching time - 7min

6) Stabilization time for temperature = 2min 7) Stabilization time for pressure = lOsec 8) RF power = 200W

9) Helium pressure = 1 x 10“3 mbar

Table Appendix C-l: Recipe for cryo-etching

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Appendix D: Hydrofluoric Acid Etch for Silicon dioxide

This appendix gives the recipe used for wet etch of Silicon dioxide buried (BOX) under device layer, Silicon. Buffered hydrofluoric acid (BHF) was used for removing BOX. Etch rate of BHF at room temperature is typically O.lpm/minute for the recipe and process flow given in Table Appendix D-l.

Recipe for HF etch

1) BHF was made in Teflon beaker by mixing:

i) 6:1 NH4F(40%):HF(48%) 2) Temperature: Room temperature

3) Etched sample wafer in BHF for lOminutes 4) Rinsed wafer in running water.

Table Appendix D-l: Recipe for HF etch