Optical transmission losses in polycrystalline silicon strip waveguides: Effects of waveguide dimensions, thermal treatment, hydrogen passivation, and wavelength

Signal propagation delays dominate over gate delays in the ever-shrinking u ltra large scale integrated (ULSI) circuits. Consequently, silicon-based mo nolithic optoelectronic circuits (SMOE) with their Light speed signal propa gation can provide unique advantages for future generations of microprocess ors. For such SMOE circuits, we need optical interconnects compatible with silicon technology. Strip waveguides consisting of polycrystalline silicon (polySi) clad with SiO2 offer excellent optical confinement and ease of fab rication that are ideal for such interconnect applications. One major chall enge with using this material system, however, is its insertion loss. Tn th is paper we provide techniques for minimizing optical transmission losses i n polySi strip waveguides. Our previous work using polySi strip waveguides, showed an optical transmission loss of 15 dB/cm at lambda = 1.55 mum, whic h is a communication wavelength of choice in optical fibers because it repr esents an absorption minimum. Similar measurements in crystalline silicon s trip waveguides' yielded transmission losses of less than 1 dB/cm. Hitherto , in decreasing loss from 77 dB/cm to 15 dB/cm, we had minimized loss from surface scattering by improving the film surface morphology, and decreased bulk absorption with hydrogen passivation. In this paper we report a furthe r reduction in the residual bulk loss from 15 dB/cm to 9 dB/cm. By experime nting with different waveguide core dimensions, we find that the contributi on of bulk loss towards net transmission loss decreases with waveguide core thickness. Additionally, high temperature treatment provides strain relief in the polySi, decreasing transmission loss. Annealing in an oxygen ambien t is not recommended because it always increases transmission loss. Hydroge n passivation improves transmission, attributable to passivation of light-a bsorbing dangling bond defect sites present at polySi grain boundaries. Tog ether, these methods have resulted in the lowest measured loss value of 9 d B/cm at lambda = 1.55 mum. Since integrated SiGe and Ge photodetectors are more efficient at shorter wavelengths like lambda = 1.32 mum, transmission loss is also measured at lambda = 1.32 mum. Losses at the two wavelengths ( 1.32 mum and 1.55 mum) are similar when defects and stress in the waveguide s are minimized.