Micromachined Polarization Beam Splitters for the Visible Spectrum

We have built and characterized batch-processed polarization beam splitters (PBS), important optical components to separate the orthogonal TE and TM components of light. The devices were fabricated from thin-film, low-stress silicon nitride membranes and showed excellent performance. By stacking membranes, and a triple-layer PBS produced extinction ratios of 21 and 16dB for reflected and transmitted light rays, respectively. In order to obtain the optimal performance for a thin-film PBS, the film thickness must be accurately controlled. For 635-nm light and a thin-film membrane of silicon nitride (refractive index = 2.1), the thickness should be an integral multiple of 83.5 nm [2]. In order to obtain reasonable yield in our processing, we aimed for a desired thickness of 417.5nm (five times the minimum thickness) trading off transmission through the film with membrane strength. A disadvantage of a thin-film PBS is that it shows a low extinction ratio for the transmitted TM mode since transmitted light still contains some TE mode. This disadvantage can be alleviated by employing multi-layer thin-film PBS to filter out more of the remaining TE mode in the transmitted light. The fabrication steps are shown in Figure 1. Using low-pressure chemical-vapor deposition (LPCVD), we deposited a low-stress silicon nitride layer on p-type silicon wafers, aiming for a thickness slightly greater than the target value. Then, we reduced the nitride thickness down to the target value in a 160°C, phosphoric acid bath (Figure 1a). Next, we created etch windows by photolithographic-patterning and dry-etching the nitride layer on the backside of the wafers (Figure 1b) [3]. To open the window cavity (4.25mm square), we used 80°C KOH to etch through 530-μm (+/-5 μm) Si, leaving only the nitride membrane over the cavity (Figure 1c). Measurements using NANO Deep-UV System showed that the final thickness of the membranes varied from 418.8 to 419.5 nm. Using a WYKO NT3300, we measured the radius-of-curvature of a typical nitride membrane to be 51 m; the membranes are virtually flat! Figure 2 shows several of the fabricated membranes: single-, double-, and triple-layer nitride membrane PBS. For a multi-layered PBS, membranes are stacked at an angle to clear the optical path for transmitted light. Figure 3 indicates a possible design to integrate our thin-film PBS into an optical-MEMS by employing silicon nitride windows in pop-up structures. Such a system could be fabricated in an SOI process. To calibrate our silicon nitride membrane PBS, a 635-nm beam from a laser diode was directed at the surfaces of both singleand stacked-PBS devices at the film Brewster angle (64.5°). The transmitted and reflected rays from the test structures were then passed through two Tech SpecTM 15mm High-Efficiency Polarizing-Cube Beamsplitters (reflection efficiency > 99.5%, transmission efficiency > 95%) using the system shown in Figure 4. The intensities of both TE and TM components were then measured using a photo detector. The insertion losses and extinction ratios derived from these measurements are listed in Table 1. Very good performance is demonstrated by the new MEMS PBS structures: extinction ratios (for reflected and transmitted light) of (21dB, 10dB), (21dB, 14dB), and (21dB, 16dB) for single-, double-, and triple-layer systems, respectively with corresponding insertion losses of 3, 10, and 13% . The stacked PBS devices clearly exhibit the expected improvements over single-layer splitters in the transmitted extinction ratios. A literature search led us to 1998 work by Pu, Zhu and Lo who investigated a MEMS-compatible surface-micromachined PBS made using thin-film polycrystalline silicon. They achieved extinction ratios of 21 and 10dB for reflected and transmitted light with an insertion loss of ~50%. For visible light, this polysilicon PBS is excessively lossy, and clearly not suitable for a stacked PBS.