Fabrication of surface plasmon waveguides and integrated components on ultrathin freestanding membranes
(Adapted from Refs. 27 and 28) (a) Thin metal stripe of width and thickness on an adhesion layer of thickness , supported by a dielectric membrane of thickness , surrounded by a gaseous or liquid environment of refractive index . (b) Implementation as a freestanding membrane clamped around its perimeter to an underlying Si substrate. The dashed (hidden) lines show a rectangular cavity etched below the membrane. (c) Computed contours of of the mode supported by the structure of (a) for , , , , and . Au and are the materials of the stripe and membrane, respectively, and is the environment . The field distribution is normalized such that .
Fabrication process flow summarized in Table I.
Images of various freestanding thick (nominal) membranes. (a) Stitched microscope image of a wide and long membrane. (b) Scanning electron microscope image of a cleaved sample showing membranes and portions of etched cavities. (c) Image of the left end of a membrane cut using a focused ion beam.
(a) Optical parameters measured via ellipsometry (FilmTek 4000 EM DUV, Scientific Computing International) near the centre of the layer prior to membrane release. (b) Thickness of a freestanding membrane measured at 31 locations along the longitudinal centre (Filmtek 2000 PAR, Scientific Computing International).
Scanning electron microscope image of a junction formed from a pair of wide (nominal) stripes.
AFM scans of a nominally wide thick stripe post lift-off. (a) Scan over a area; the vertical axis ranges from . [(b) and (c)] Traces across a stripe.
(a) Stitched microscope image of an electrically contacted Mach-Zehnder interferometer. The metal stripes are (nominally) wide and thick ( ). The membrane is thick, and about wide by long. (b) High-magnification microscope image of a region of part (a) showing a contact pad, a contact arm, and an isolation gap. The contact pad is and the isolation gap is long. (c) Membrane waveguide cut using a focused ion beam with a portion of the structure broken off. The membrane is thick , and the metal stripe is wide and thick ( ). (d) Sketch of the membrane showing the dotted circular curve followed by the focused ion beam. The red dot indicates the approximate location where a probe was lowered to destroy that section of the membrane.
Microscope images of a framed triple output equal arm Mach-Zehnder interferometer on a membrane. The metal stripes are wide and thick ( ). The membrane is thick and wide by long. The Au frame surrounding the structure is thick. (a) Stitched image of the whole structure, (b) high-magnification image of the input portion of the output coupler, (c) high-magnification image of the -junction input splitter, (d) high-magnification image of the output portion of the output coupler.
(a) Output region of a Mach-Zehnder interferometer showing a tipless AFM probe contacting the membrane. The membrane is thick and the metal stripes are wide and thick ( ). (b) Longitudinal cross-sectional sketch showing the tipless AFM probe contacting the metal stripe on the membrane, as the membrane is raised (or equivalently as the AFM probe is lowered) by a distance . (c) Deflection and vertical forces and , respectively, acting on the probe as the membrane is raised.
Flow of fabrication process steps keyed to the sketches of Fig. 2.
Metal stripe lift-off lithography process.
thickness and refractive index prior to membrane release as measured via ellipsometry.
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