Micromirrors for integrated tunable mid-infrared detectors and emitters
The mid-infrared range is of interest in spectroscopic applications, due to the large number of organic compounds that exhibit characteristic absorption bands in this spectral region. Semiconductor technology, however, has been developed mainly for the near-infrared, for applications in telecom, as well as in the far-infrared, mostly driven by the interest in thermographic applications. In this thesis, micromirrors for novel integrated narrow-band tunable mid- infrared detectors and emitters are presented. Both such detectors and emitters can be fabricated by placing an active layer inside a resonant microcavity, which consists of two reflectors facing each other. Both narrowband mid-infrared sensors and lasers can be fabricated using epitaxial growth of lead-chalcogenides on silicon substrates. With the same materials, high reflectivity distributed Bragg reflectors can be fabricated, which can be used as resonant cavity mirrors. The resonance wavelength for sensors and for lasers depends on the cavity length. Length and finesse of the resonant cavity determine the resonance wavelength and the detection (emission) ￼linewidth. One possibility to make the sensor or the laser tunable over a certain wavelength region is to vary the cavity length. This can be achieved through a mechanically moving micromirror at one end of the resonant cavity. The challenges for such mechanically moving micromirrors lie in the fabrication of high quality reflective surfaces with a movement perpendicular to the wafer surface in the range of the detection wavelength, i.e. of some micrometers for the mid-infrared. The suspensions have to be designed sufficiently soft for obtaining reasonably low actuation voltages for the required displacements, but sufficiently stiff in order to retain adequate mechanical stability. Reflectivity, curvature and parallelism of the movable micromirrors in the resonant cavity have to be controlled in order to adapt the resonant cavity layout and finesse to the needs of the relevant application. Within the scope of this thesis, two variants of micromirrors for integrated tunable detectors and emitters have been investigated, including design, process development and fabrication. The micromirrors were fabricated in the device layer of a Silicon-On-Insulator wafer using Deep Reactive Ion Etching and standard microfabrication technology. Using these fabrication methods, very compact integrated systems can be manufactured. Both micromirror variants are equally suited for implementation in tunable detectors and tunable emitters. In both variants, the displacement vertical to the wafer surface is obtained by electrostatic actuation, on the one hand with actuation electrodes in a parallel plate configuration, and on the other hand with comb drive actuators. The layout includes the design of the micromirror, the configuration of the suspension beams and the electrostatic actuator design in order to obtain the desired displacements. The typical micromirror square length is between 300 mm and 600 mm, thickness 10 mm, and the suspensions are accommodated in a square frame with a typical length ranging from 0.8 mm to 1.5 mm. However, there remains room for further minimizing the footprint. The micromirrors fabricated in this work showed displacements around 3 mm at 30 V actuation voltage, depending on the geometry. High mirror reflectivity was achieved by a 60 nm thin gold coating. The micromirrors’ radii of curvature have been adjusted precisely in the range of centimeters by applying an additional chrome thin film on the mirror, and separated actuation electrodes allowed tilting of the micromirrors in order to achieve a parallel alignment inside the resonant cavity. The mechanical resonance frequencies occur above a few kHz and may be adjusted by the design of the mirror geometry and the suspensions. At atmospheric pressure, the mechanical resonances are strongly degraded by squeeze-film air damping, reducing ￼Q-factors typically below 100. At low pressures, below 1 mbar, Q-factors increase typically over 3’000. In collaboration with the Thin Film Physics group at ETH Zürich, fabricated micromirrors of both types have been joined successfully with photodiodes to form tunable resonance cavity enhanced detectors. During assembly, temperatures are kept below 110°C in order to avoid diffusion processes which could deteriorate the device performance. The use of the micromirrors allowed the realization of very compact tunable detector systems in the mid-infrared. These are the first tunable resonant cavity enhanced detectors in the mid-infrared that have been presented to date. Using parallel plate actuated micromirrors, detectors with a single mode tuning range from 4.85 mm to 5.15 mm have been presented, and using comb drive actuated micromirrors, a wide wavelength tuning range from 4.7 mm to 5.4 mm has been achieved. The detector performance was limited by the broadened linewidth due to finesse degradation, among others influenced by reflectivity, curvature and departure from parallelism. The detector linewidth was about 0.1 mm. A narrower linewidth can be obtained with a higher initial cavity length, thus a higher operating resonance mode, however, to the cost of a reduced free spectral range, i.e. a reduced tuning range. Alternatively, linewidth reduction can be achieved by increasing the cavity finesse which can be obtained by fabrication process improvements. Finally, a spectroscopic application has successfully been demonstrated using a tunable mid-infrared resonant cavity enhanced detector, which was used to detect carbon monoxide in a 5 cm long gas cell filled with a carbon monoxide partial pressure of 250 mbar. In a further step, implementation of the micromirrors in vertical external cavity surface emitting lasers is envisioned. The developed micromirrors are suitable for these applications without modification in the fabrication process; the expected narrow emission linewidth of a few nanometers make them among others potentially interesting for spectroscopic applications.