Research : Optical Integrated Circuits on LSIs

Appearances of a high-capacity and high quality media, such as 3D-movies and videos have led higher data transaction capability of LSIs. The required clock frequency and total data stream for LSI chips are expected to exceed 5 GHz and several Tera-bits/sec by 2019. For achieving the requirement, miniaturization technology of the complementary metal oxide semiconductor (CMOS) transistors can enhance the operation speed of the local wire layer. On the other hand, the signal delay in the global metallic wires, which connect between blocks in a LSI-chip, is limiting the total LSI performance due to the skin effect. As one of the long-term solutions, introduction of optical circuits on the LSIs is proposed as well as 3D-LSI chip technology. And for the optical circuits on the LSIs, silicon photonics technology have evolved drastically during the past ten years due to the superiority in the integration of electronics and photonics, which are both based on the same silicon-platform. Our research group has mainly challenged to the following two topics.

Ⅰ. Athermal wavelength filters toward optical interconnection to LSIs

Our research group targets the monolithic integration of optical circuit with electronic LSI circuits. Then, one of the problems to be solved is the wavelength shift to long wavelength side caused by the thermal diffusion from the logic layer in Si-LSI because of the large positive temperature coefficient of refractive index of Si. This effect is wrong to the wavelength filter for Wavelength Division Multiplexing (WDM) systems. For the solution, I proposed an athermal Si slot waveguides embedded with benzocyclobutene (BCB), which is the low-k polymer material for electronics. The slot waveguide can confine the light in the central low index area (Fig.1). By controlling the widths of the waveguide and slot-gap, the temperature coefficient of equivalent index could be zero.

The fabricated athermal ring resonator and MZI have the temperature dependence of the drop wavelengths was as small as -0.6 and 0.9 pm/K, respectively, which are small enough to use as wavelength filters with compare to a conventional long haul 100GHz-WDM system. As a next stage, wavelength trimming technology with deep ultra-violet (DUV) irradiation has been investigated.

Ⅱ. Amorphous-silicon multilayer waveguides vertically stacked on SOI substrate

Silicon is the main material for the optical circuit. To integrate the optical components on a LSI chip, all the fabrication processes for the optical components should be regulated to below 400°C in order to avoid damages to the electrical circuits. Amorphous silicon is the most promising material for optical circuits since it can be deposited under low-temperature conditions. Moreover, by stacking amorphous silicon and cladding material alternatively, high-density 3-dimensional optical circuits can thereby be realized on the LSI. Fig 3(b) illustrates the vertical signal transmission using grating coupler between two layers. Using metal reflector under the grating coupler, more than 80% of layer-to-layer coupling efficiency was obtained with 3D Finite Difference Time Domain (FDTD) method.

List of reports

Journal Papers

International Conferences

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Nishiyama Laboratory
School of Engineering Dept. of Electrical and Electronic Engineering, Institute of Science Tokyo

7F, S9-1, 2-12-1 O-okayama, Meguro-ku Tokyo 152-8552, Japan +81-3-5734-2555 ee.e titechnishiyama

Nishiyama lab. Student's room : South Bldg. 9 #701, #706, #707 | Measurement room : South Bldg. 9 #604, #502, #201 |
Clean room : South Bldg. 9 #202, B1F Exposure house | Research Laboratory of Ultra-High Speed Electronics