Photolithography, Metallization and lift-off of PPBGs.StepDescriptionDetails1Ash30 s in reactive-ion etcher (RIE) at 100 W with O2 flow of 100 sccm2Deposition of HMDSPlace in HMDS oven at 97 °C3Deposition of LORSpin coat of Microchem PMGi SFG 2S4Deposition of PRSpin coat of Microposit S18055Exposure12 s exposure with Karl Suss MA 6 contact mask aligner6DevelopmentDevelop for 1 min 40 s in Microposit MF-321 with aggressive agitation7RinseFlush rinse with de-ionized (DI) water8MetallizationThermal deposition of 35 nm Au9Lift-off5 min static dip in Microposit 1165 followed by 10 s ultrasonic. Repeat once. Static IPA and DI water baths for 10 min each. Finally, dry in N210DehydrationLeave on hotplate at 95 °C for 15 minFull-size tableTable optionsView in workspaceDownload as CSVThe PPBG steps-in-width are extremely fine and tax the limits of optical lithographic resolution. E-beam writing is available but was avoided because of the high cost associated with this process and the difficulties of writing on thick Cytop (which is insulating). A technique that could produce many waveguides in parallel was desired therefore we selected optical contact lithography and attempted to refine the WYE-687 of features as much as possible.PPBGs of varying dimensions were attempted in order to have a broad design space. This would allow for analysis of the relationship between design and performance. A typical fabricated PPBG is shown in Fig. 3. The design duty cycle of the gratings is 50%, the inner stripe width (w2) should be 3 μm, the outer stripe width (w1) should be 8 μm and the period (Λ) should be 1.8 μm. From optical microscope inspections, the design targets seem to have been achieved in this structure except for w2 which was fabricated as 4.5 μm. Slight rounding of the features is also noted due to the limited resolution of the lithography process.Fig. 3. Microscope image of a PPBG. The dimensions of the grating are Λ = 1.8 μm, w1 = 8 μm, w2 = 4.5 μm.Figure optionsDownload full-size imageDownload as PowerPoint slide3.3. Top claddingPrevious fabrication of the top-cladding produced a smooth cladding which minimally deformed the waveguides. However, the top-cladding remained tacky because it retained too much solvent [23] and [24]. On the other hand, if the solvent content is too low, the dielectric becomes stiff and prone to cracking during the channel etch. To address these concerns a new top cladding procedure was developed which minimizes the degree of waveguide deformation, Cytop tackiness, and crack formation. The procedure for baking the Cytop top cladding is presented in Table 3. The top cladding consists of 6 layers, each spun at 1000 rpm for 10 s followed by 4000 rpm for 20 s to produce layers 1.35 μm thick.Table 3.
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