Silicon-on-Insulator Microspectrometer J. Brouckaert, S.Selvaraja, W. Bogaerts, G. Roelkens, M.Y. Ling, J. Allaert, P. Dumon, D. Van Thourhout and R. Baets Ghent University – IMEC, Dept. of Information Technology, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium Joost.Brouckaert@intec.ugent.be Miniature optical spectrometers have huge potential for application in mobile sensors which are capable of real-time environmental monitoring at any time, in any place. In this paper, we demonstrate that the heterogeneous integration of InGaAs photodetectors onto very compact silicon-on-insulator echelle grating demultiplexers is an efficient approach for the fabrication of highly integrated, cost-effective and performant spectrometers-on-a-chip. Introduction Near-infrared spectroscopy is a technique which is widely used for highly sensitive measurements of the composition of unknown organic samples by exciting overtones and combinations of molecular vibrations. Typical applications include pharmaceutical, agricultural and biological analysis. Conventional photospectrometers in the labs are typically large and expensive and have a performance that often exceeds the requirements for typical industrial applications. For industrial applications, what counts are the cost, size, robustness, sample volume, measurement time, … of the spectrometer. For these applications, miniaturized spectrometers [1] that can be mass fabricated are better suited and using these devices, a new range of applications such as real-time, mobile sensing becomes possible. In this paper, we demonstrate that silicon (Si-) photonics is an interesting technology platform for these applications. We fabricated a very compact near-infrared spectrometer-on-a-chip without any moving parts. The heart of the spectrometer is a silicon-on-insulator (SOI) echelle grating demultiplexer, fabricated on 200mm SOI wafers using CMOS-compatible waferscale processes. The high index contrast of this material system allows to drastically reduce the size making it possible to produce hundreds of chips on a single wafer. For measuring the optical signal in the different wavelength channels, near-infrared InGaAs photodetectors are heterogeneously integrated. These detectors are lithographically aligned onto the Si chips and can be processed on a waferscale. Wavelength demultiplexer The demultiplexer is a 30-channel planar concave grating (PCG) or echelle grating with a 3.2nm channel spacing and a 120nm free spectral range. It is fabricated on a 200mm SOI wafer with a 220nm thick Si top layer using 193nm deep-UV lithography in combination with ICP-RIE etching. The fabricated device is shown in figure 1. The design is based on the Rowland geometry [2]: the input and output waveguides are positioned on a circle with a radius of 554µm and the curved grating sits on an 1108µm radius circle. The order of diffraction is 10 and the entrance and exit waveguides are 2µm wide with a spacing of 5µm between the centers of the output waveguides along