3D fabrication by stacking prepatterned, rigidly held membranes
(Color online) Schematic of the membrane-stacking approach to 3D structures using membranes attached to a rigid frame by means of tethers. (a) Top view and (b) side view. The pattern within the membrane is depicted as uniform and suitable for 3D photonic crystals.
(Color online) Finite-element analysis of stress distribution in a tether of the tapered lag design. Note the high concentration at the narrowest width of the tether. Modeling is used to determine how a load is distributed among tethers and the effect of radius of curvature on stress concentration.
(Color online) (a) Photograph of a quarter of an Si wafer with four windows, each containing a low-stress silicon-nitride membrane patterned with a 600 nm pitch, fourfold-symmetric array of holes. (b) Micrograph of the membrane with tethers along the perimeter. The inset is an electron micrograph of the array of holes.
(Color online) (a) Photograph of the frame held on a glass adapter within a custom-built alignment apparatus with six-axis control (not shown). The frame contains membranes in three of its four windows. Membrane 1 is in close proximity (separation of less than 5 μm) to the mesa on the substrate beneath it, as indicated by the interference fringes under monochromatic illumination. (b) Photograph of membrane 1 bonded to the mesa. The mesa is the key feature that enables repeatable bonding.
(a) Optical micrograph of a stack of three-layer structure (patterned mesa with two sequentially bonded membranes). The 2D periodic pattern of about 60 μm period is a moiré effect, indicative of an azimuthal misalignment between the top two membranes of about 10 mrad. Straightness of the fringes indicated absence of in-plane distortion in the bonded membranes. At the edge of the substrate on the right, the membrane cleave is determined by the tethers. There is no evidence of debris generated from the separation. (b) Zoom view of edge. The cleaved boundaries of two membrane layers, which overhang the mesa, are observable. A lower contrast moiré effect between the mesa (patterned with a 1D grating) and the first membrane is observed as a grating beat pattern with 60° orientation.
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