Three-beam pattern-integrated interference exposure system (PIIES).28 (a) A ray trace depicts the propagation of k 1, k 2, and k 3 through the PIIES optical configuration, implementing (b) a wavevector configuration required to produce a square-lattice interference pattern with collimated beams at the sample plane. (c) A functional-element amplitude mask is placed at the mask plane with features sizes of d = a sq/|m|, where m is the magnification due to the compound objective lens. (d) The result is an optical-intensity distribution of an integrated non-periodic functional element in an all-surrounding, high-spatial-frequency periodic square lattice, enabling single-exposure fabrication of a functional device, such as a PC waveguide coupler.32
PIIES configuration. The prototype system is arranged as a 6f configuration using large-diameter aspheric lenses. Expander lenses are added to ensure that the multiple beams are focused at the front focal plane of objective lens 1, providing collimated interfering beams at the sample plane and coherent illumination of a mask located at the mask plane. The incidence angle at the sample plane, θ S , may be increased or decreased by adjusting the radial distance of the individual beams, d beam, from the optical axis.
The change in the axial lengths, Δd EL-CL, Δd OL1-OL2, and Δd total are plotted as a function of the incidence angle, θ S , at the sample plane.
PIIES alignment flowchart. After all of the alignment tests are successfully passed, the system is ready for a PIIL exposure.
PIIES alignment and build sequence. (a) The optical axis is established with an alignment card and a diffractive mask element centered on the axis beam. (b) The two objective lenses are centered on the optical axis using the diffraction pattern from the mask element. (c) The condenser lens is positioned and the interfering beams are aligned to the required positions on the alignment card. (d) Expander lenses are added to provide collimated beams at the sample plane and coherent illumination of the pattern mask.
Diffractive mask feature. (a) A Greek cross on the pattern mask is centered on the axis beam to create a unique diffraction pattern. (b) The resulting crosshair pattern is used to establish the optical axis of the system and align the objective lenses.
Alignment cards for (a) a square and (b) a hexagonal lattice.
Experimental focusing of PIIES. CCD image captures using an Olympus NC60 microscope depict fabricated pattern-integrated images of a Greek cross at z SP0 + 7.5, z SP0 + 5.0, z SP0 + 2.5, z SP0, z SP0 – 2.5, z SP0 – 5.0, and z SP0 – 7.5 μm.
PIIES single-exposure fabrication results. (a) A pattern-mask feature of a 600.0 × 600.0 μm Greek cross is projected to a size of 172.2 × 172.2 μm. (b) A SEM image depicts the resulting single-exposure PIIES optical-intensity distribution of the projected cross and interference pattern. (c) A magnified SEM view of a corner of the fabricated corss depicts a well-defined corner produced by the projected Greek cross surrounded by the interferomatrically defined square PC lattice with a periodicity of a = 1.0 μm.
Demonstration of PIIL single-exposure PC waveguide fabrication. (a) A pattern-mask feature of a 2.0 × 20.0 μm line segment is projected to a size of 0.6 × 5.8 μm. (b) A SEM image depicts the resulting single-exposure PIIES optical-intensity distribution of the projected line segment and square-lattice PC. (c) A magnified SEM view of the line segment depicts the selective elimination of a single row of lattice points in the surrounding periodic lattice, demonstrating the ability of the PIIL to fabricate a PC waveguide, the fundamental element of most PC devices.
Simulated PIIES interference pattern performance.
PIIES lens data.
PIIES aspheric lens equation data.
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