Figure 1: (a) Propagation loss as a function of waveguide width for wafers with and without thermal processing representative of the full electronics process. Solid fit lines estimating the confinement factor scaling for the indicated bulk material losses overlay measured data points. Inset of electric field contours show confinement change from core to cladding material for narrow waveguide widths. (b) Propagation loss as a function of wavelength and waveguide width for the thermally processed wafer.

Low propagation loss deposited waveguides are an important component to enable photonic integration in the majority of high-volume electronics processes. Polycrystalline silicon waveguides are desirable in this role as propagation losses below 10 dB/cm are achievable ^{[1] [2] [3]} using materials already common in such processes. However, previous demonstrations of low-loss poly-Si waveguides have utilized layer thicknesses of 200 nm or greater and reduced index-contrast oxynitride claddings to achieve such results. Further, the low-loss performance was not verified to withstand the high-temperature steps present in electronics processes.

To provide a suitable process integration test platform, 120-nm thick poly-Si waveguides were deposited and patterned on 300- mm wafers within the surrounding dielectric stack-up of a state-of-the-art memory process. To measure the impact of the thermal processing, the wafers were split with and without the all of the high-temperature anneals present in the full process for comparison. To minimize the impact of this thermal processing on top surface roughness, the waveguide core was first deposited at low-temperature by LPCVD to form an amorphous film and crystallized to form poly-Si with a ~950 °C 20-second anneal.

To measure the resulting waveguide loss, “paperclip” structures were patterned in which straight waveguide sections of varying propagation lengths are connected by identical input and output bends and vertical grating couplers. The resulting losses at 1550 nm as a function of waveguide width for wafers with and without thermal processing are shown in Figure 1a and are shown as a function of wavelength for the thermally processed wafers in Figure 1b. Although the waveguide loss increases slightly after thermal processing, both the as-deposited and end-of-line waveguide losses are below 10 dB/cm for 350-nm waveguide widths at 1550 nm. The narrow waveguides enable long-distance, on-chip routing to be enabled with 6.2 dB/cm loss.

1. L. Liao et al., “Optical transmission losses in polycrystalline silicon strip waveguides: effects of waveguide dimensions, thermal treatment, hydrogen passivation, and wavelength,” J. Electron. Mater. vol. 29, pp. 1380-1386, 2000.
2. Q. Fang et al., “Low loss (~6.45 dB/cm) sub-micron polycrystalline silicon waveguide integrated with efficient SiON waveguide coupler,” Opt. Express, vol. 16, pp. 6425-6432, 2008.
3. C. W. Holzwarth et al., “Localized substrate removal technique enabling strong-confinement microphotonics in bulk Si CMOS processes,” in Conference on Lasers and Electro-Optics, Technical Digest (Optical Society of America, 2008), paper CThKK5.

Figure 1: (a) Through transmission of 40 µm radius ring with 200 nm coupling gap for four different in-waveguide input powers. (b) Measured through transmissions (circles) on resonance normalized to off-resonance transmission, and fits (lines) for 20 µm rings, and (c) for 40 µm rings with various coupling gaps g; along with the gap, the fitted β is labeled for each sample.

Nonlinear optics comprises a rich body of physics that could enable a number of interesting functionalities in integrated silicon photonics ^[1] . We are currently interested in the phenomenon of two-photon absorption (TPA) as a means to achieving sub-bandgap photodetection in silicon. Though TPA is a relatively weak absorption mechanism in crystalline silicon, the fact that absorbed power scales quadratically with intensity implies that its strength can be increased for device applications by concentrating a large amount of light energy in a small mode-volume resonator, for example in photonic crystal microcavities ^{[2] [3]} .

Further enhancement of TPA may be expected when polycrystalline Si is used instead of crystalline Si as the detector material, due to a larger TPA coefficient (β) resulting from mid-gap electronic states at grain boundaries. Though β has been fairly well characterized for crystalline Si ^[4] , an understanding of two-photon absorption in the polycrystalline phase is lacking. We have therefore measured β in polycrystalline Si waveguide devices by monitoring the peak through transmission dip in ring resonator devices as a function of input power and fitting the results to a model with β as a fitting parameter; data from such measurements are shown in Figure 1. These measurements indicate β = 310 ± 70 cm/GW, a value over two orders of magnitude larger than that in crystalline Si at 1550 nm and on the same order of magnitude as the value observed in amorphous silicon ^[5] . As polysilicon is widely available in advanced CMOS processes, these results suggest that high-density, integrated devices utilizing nonlinear absorption at relatively low powers should be achievable.

1. J. Leuthold, C. Koos, and W. Freude, “Nonlinear silicon photonics.” Nature Photonics, vol. 4, pp. 535-544, July 2010.
2. J. Bravo-Abad, E.P. Ippen, and M. Soljacic. “Ultrafast photodetection in an all-silicon chip enabled by two-photon absorption.” Appl. Phys. Lett., vol. 94, no. 241103, pp. 1-3, June 2009.
3. T. Tanabe, H. Sumikura, H. Taniyama, A. Shinya, and M. Notomi. “All-silicon sub-Gb/s telecom detector with low dark current and high quantum efficiency on chip,” Appl. Phys. Lett., vol. 96, no. 101103, pp. 1-3, March 2010.
4. M. Dinu, F. Quochi, and H. Garcia. “Third-order nonlinearities in silicon at telecom wavelengths,” Appl. Phys. Lett., vol. 82, pp. 2954-2956, May 2003.
5. K. Ikeda,Y. Shen and Y. Fainman. “Enhanced optical nonlinearity in amorphous silicon and its application to waveguide devices,” Opt. Express, vol. 15, 17761-17771, December 2003.