Polysilicon Waveguides for Silicon Photonics
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mal crosstalk, low power dissipation and high speed data transfer. Since crosstalk and power consumption are greatly reduced, higher interconnection densities are achievable. Small propagation delays and load independent design allow minimization of clock skew. Polycrystalline silicon (polySi) is a part of VLSI silicon processing technology being used for gate, source and drain contacts in CMOS structures and resistors and bipolar transistor emitters. Additionally, polySi has been considered for several applications where single crystallinity may be desirable, for example, Thin Film Transistors (TFTs) [1, 2, 3]. The main reason for the use of polySi is its ease of deposition in Low Pressure Chemical Vapor Deposition (LPCVD) furnaces on a variety of substrates. Here we propose the use of polySi waveguides for optical interconnects. In comparison with Bonded and Etched Back Silicon On Insulator (BESOI) or Separation by IMplantation of OXygen (SIMOX), polySi waveguides permit more design flexibility since they are easier to fabricate and allow for multilevels of interconnection. They also offer the possibility of a wider range of cladding and core thicknesses. For example, one can adjust the SiO 2 cladding thickness easily unlike in SIMOX waveguides and one can vary the polySi core thickness easily unlike in BESOI waveguides. Therefore, although strip-waveguide structures using siliconon-insulator (SOI) technology yield low cutback losses of 1dB/cm [4] and Mach-Zehnder 327 Mat. Res. Soc. Symp. Proc. Vol. 403 0 1996 Materials Research Society
Waveguide Geometry Sit
=lRm
w = 84m Figure 1: The experimental strip waveguide structure used to measure transmission losses at A=1.54pm in polySi. waveguide interferometers [51 in SOI material with insertion losses of 4.81dB have been demonstrated, polySi is preferred for the implementation of silicon optical interconnections. In comparison with the Si 3 N4 /SiO 2 waveguiding system, the polySi/SiO 2 system confines light better because of its higher dielectric contrast. Single-mode waveguides require much smaller cross-sectional dimensions (1sm2 compared to 1000•im 2 ) and become possible due to the higher dielectric contrast of the polySi/Si0 2 system. However, just as carrier scattering and recombination contribute to losses in polySi electronic devices (for example TFTs), thereby leading to lower mobilities and gain, photon scattering and absorption losses limit the performance of photonic devices such as waveguides. In this work we evaluate the material properties of polySi in an attempt to lower the optical transmission losses measured in polySi strip waveguides. Materials characterization tools have been applied to define the contribution of the grains and grain boundaries to bulk scattering and absorption losses and to investigate the impact of film roughness on surface scattering losses. EXPERIMENT LPCVD polySi films were deposited at 625°C and amorphous silicon films were deposited at 560°C and 580°C. The films were lm thick and were deposited on 3/.sm Low Temperatu
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