Abstract
For hybrid integration of an optical chip with an electronic chip containing photo-diodes and processing electronics, light must be coupled from the optical to the electronic chip. This paper presents a method to fabricate quasi-total-internal-reflecting mirrors on an optical chip, placed at an angle of 45° with the chip surface, that enable 90° out-of-plane light coupling between flip-chip bonded chips. The fabrication method utilizes a metal-free, parallel process and is fully compatible with conventional fabrication of optical chips. The mirrors are created using anisotropic etching of 45° facets in a Si substrate, followed by fabrication of the optical structures. After removal of the mirror-defining Si structures by isotropic etching, the obtained interfaces between optical structure and air direct the output from optical waveguides to out-of-plane photo-detectors on the electronic chip, which is aimed to be flip-chip mounted on the optical chip. For transverse-electric (transverse-magnetic) polarization simulations predict a functional loss of 7% (15%), while 7% (18%) is measured.
Highlights
In recent years, waveguide (WG) based integrated optical devices have been used in many applications, such as telecommunication, optical spectroscopy, biological sensing, and signal processing in medical imaging [1,2,3]
For spectroscopy the signals from a large number of different channels must be measured simultaneously, which can be done most conveniently with 2D arrays of PDs. Monolithic integration of the latter to the Si substrate of an optical chip has several disadvantages: (i) the PDs and their electronic processing circuitry must be fabricated first, implying that the temperature budget to fabricate the optical structures is limited to about 400 °C, whereas higher-temperature process steps are often needed, such as low-pressure chemical vapor deposition (LPCVD) and reflow of deposited layers; (ii) fabrication of PDs and their electronic processing circuitry on each optical chip would be far more expensive than processing a CMOS chip that includes densely packed PDs and electronics which could be flip-chip mounted on an optical die due to the large area occupied by the optical chip compared to the CMOS chip
This paper describes a method to fabricate 45° mirrors in optical chips that enable highly efficient 90° out-of-plane light coupling to a flip-chip mounted electronic chip holding 2D PD arrays with corresponding processing electronics [7,8,9]
Summary
Waveguide (WG) based integrated optical devices have been used in many applications, such as telecommunication, optical spectroscopy, biological sensing, and signal processing in medical imaging [1,2,3]. A number of techniques have been reported to provide a connection between optical WGs and electronic PDs on two individual, flip-chip mounted chips One of these techniques uses a focusing grating coupler on top of a WG to focus the light onto the PD that is placed above the WG [4]. High-volume production of such a grating could be expensive owing to its relatively small period Another technique uses focused ion beam (FIB) milling for fabricating total-internal-reflection (TIR) mirrors at the end of silica WGs [5]. This technique is not suitable for high-volume production due to the relatively low speed of FIB milling and the associated high cost. The results of structural device characterization and experimental performance of the fabricated devices are presented
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