Design and fabrication of integrated quad and data photodiodes for board-to-board free-space high speed computer and server optical communication.

摘要

In computer and server communications, cables are currently used for data transferring. However, because copper wires have significant disadvantages such as limited transmission time, high noise level, unstable electrical properties (shape dependence) and high power consuming on cooling, optical interconnect technology is researched to replace the cables with light. In the free space optical interconnect system, an array of laser lights with 850nm wavelength is generated by vertical-cavity surface-emitting laser (VCSEL) pass through two lenses for focusing purpose, and then arrive the photiodiodes array.;Speed and position adjustment (due to server vibration) are the two main factors in the receiver approach for optical interconnect system, therefore we designed a high speed GaAs PIN photodiode array integrated with quad photodiodes and data transferring photodiodes. Both 4 x 4 and 9 x 9 arrays are designed for characterization purpose. Each array consists of four quad photodiodes at the corners, and the rest are all high speed data photodiodes, all the photodiodes have a pitch distance of 250 um. Data photodiodes will be fabricated on a InGaP/GaAs wafer, with size of 50um x 50um to achieve 10Gbps speed. Similar techniques are applied also to the quad photodiodes (QPD), which tracks the laser spot, and information will be sent through a closed loop circuit for position adjustment. Performance of a Si base pin quad photodiode with reference to a one dimensional position sensitive detector (PSD). Results showing that 90% of the spots are similar for quad photodiodes and PSD, which guarantees the quad photodiode adjust the position of the light misalign due to vibration in optical interconnect system. Regarding to the fact that QPD has its limitations on accuracy and unstable steps for alignment for its scan and search nature, a new beam tracking mechanism is introduced with its principle, algorithm and simulations.;The device is currently being fabricated, and it will be characterized thus more data will be provided in the future. However, we did measurements of contact resistance and PN junction I-V characteristics on the current device, which fabricated up the N-mesa step. Data shows the annealing effects on the contact resistance. This finding matches the principle that P type material has a lower energy barrier height which is easier to accomplish by lower temperature, thus ohmic contact is observed in contrast of N type material does not. Also simulation results show that the new mechanism is preferred over QPD scan and searching base for beam tracking with a lot better performance in terms of searching steps but comparable accuracy and device absorption percentage.;Keywords. High speed, Board-to-board Free Space Optical Communication, Photodiode, Quad Photodiode, Strip Quad Photodiode.