Integrated Silicon Infrared Microspectrometers
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Integrated Silicon Infrared Microspectrometers S.-H. Kong, G. de Graaf and R.F. Wolffenbuttel Delft University of Technology, Faculty ITS, Dept. for Micro-electronics, Mekelweg 4, 2628 CD Delft, The Netherlands Phone +31 15 278 4707, Fax. +31 15 278 5755
Abstract Design, fabrication and performance of a microspectrometer for operation in the infrared spectral range between 1 and 8 µm and fabricated in silicon are presented. The opto-electrical system is composed of two bonded silicon wafers, which have been subjected to microelectronic process compatible micromachining to enable co-integration of the optical components (an aluminum based grating, an optical path in crystalline silicon and an array of integrated polysilicon thermocouples) with readout circuits in silicon. The FWHM is smaller than 0.5 µm at λ= 5 µm. The performance is compared to alternatives and directions for improving the selectivity are given. 1. Introduction Analysis of an optical spectrum is a well-established technique in physics, chemistry and biology. Spectroscopic measurement of the emission and absorption spectra of a particular atom provides detailed information about its energy band structure. The technique is also used to identify the energies associated with the chemical bonds in a molecule [1]. In chemical analysis the fluorescence spectrum is widely used to identify the components in a sample solution and to measure their concentrations [2]. Similarly, in chromatography the wavelength dependent absorption of the chemical constituent between a light source and the entrance slit of a spectrometer is measured in both the visible and IR part of the spectrum [3, 4]. Fluorescence signals are also investigated for monitoring of photosynthesis in vegetation [5]. Based on such measurements, it might be possible to monitor plant condition and use this information for online control of the illumination conditions in such a way that photosynthesis is maximized, without causing plant stress. Available high-performance multiple-grating macroscopic spectrometers feature an impressive spectral resolution, R= ∆λ/λ, that exceeds R= 106, where ∆λ denotes the -3dB power bandwidth (also referred to as the full-width half-maximum-FWHM) at a particular wavelength setting, λ [6]. However, these are bulky and expensive. Such a resolution specification is required in e.g. astronomy, but often exceeds by far what is required in industrial applications, where issues such as costs, sample volume to be chemically analyzed and measurement time prevail. Microspectrometers satisfy these additional requirements better. These are small, lightweight and some are fabricated using silicon process compatible technologies, thus featuring the possibility for realizing an intelligent opto-electronic system on a chip by co-integration of optics with microelectronics [7].
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