This paper discusses the two-step fabrication of a novel in-plane Si-air linear variable optical filter (LVOF). LVOF has alternating quarter-wave stack layers of high refractive and low refractive index materials sandwiching a tapered cavity. Different passbands can be observed at various positions along the length of the filter. Challenges of LVOF fabrication include depositing consistent thickness of quarter-wave stacks and precise control of the taper angle to be in the range of milli-degrees. In many instances, due to the limitations of thin film deposition systems, surface roughness and deposition thickness vary across entire wafer surface. Such deviations could result in different LVOFs possessing varying response to input signal. <p> </p>Electron-beam lithography (EBL) was utilized for accurate patterning of Si pillars and taper angle which are difficult to achieve using traditional fabrication methods. In the absence of hardmask, SU-8 was used for pattern transfer with Si:SU-8 etch selectivity as high as 60:1. By optimizing SF<sub>6</sub> and C<sub>4</sub>F<sub>8</sub> gas flow and time parameters, aspect ratio of 10:1 and almost- 90° pillars were deep etched into Si with scallop depth <30 nm. High Bragg contrast mirrors were obtained with [HLH]-wedge-[HLH] configuration. <p> </p>This LVOF operates in free space with continuous tuning from 3.1-3.8 μm. FWHM of 95 nm is observed at 3.3 μm. Simulation and other characterization results are discussed. Finally, the proposed LVOF can be wafer-level packaged with normal incidence detector array, suitable light source and other essential optical elements.
Graphene has been well studied to be an excellent thermoelectric (TE) material of choice for thermal detection. It is widely considered a key enabler for next-in-class infrared (IR) detectors given its superb carrier mobility, sensitivities and broadband absorption in far-IR range surpassing that of current thermopiles. Normally, TE studies are conducted using graphene exfoliated from graphite crystal. It is then transferred onto Si/SiO<sub>2</sub> substrate and fabricated into Hall bar configuration with microheater at one end. A gate voltage (Vg) is passed through the substrate and the response is examined in vacuum condition. By tuning the Vg, one can possibly obtain different thermoelectric power (TEP) values. The challenge is to maintain optimum Vg for the TE device to function which requires higher power consumption. This translate to the need for additional power supply. In this report, we proposed CVDG as TE material. Typically, CVDG are synthesized on Cu film and eventually transferred onto Si/SiO<sub>2</sub> substrate. The benefit of CVDG is that it is large area, relatively inexpensive and does not require a Vg with associated circuitry. For the first time, CVDG system was extended to nonvacuum condition to simulate open detector system where detector is exposed to sensing environment. Average TEP was measured to be 168μV/K at 298K. Moreover, CVDG is tested to be stable in air over several months with little or no decrease in performance. A comprehensive characterization between exfoliated and CVDG will be presented. In addition, measurement results for vacuum and non-vacuum detector mode will be compared as well.