Mikko Tuohiniemi

Optical transmission performance of a surface-micromachined Fabry–Pérot interferometer for thermal infrared

We developed a surface-micromachined tunable Fabry?P?rot interferometer for the thermal infrared spectral range of wavelengths 7?12??m. In this paper, we present the device performance in terms of the optical transmission and the tunability. The device represents the first layout that proved successful in terms of the manufacturing process yield (about 80%). The optical transmission over the wavelengths from 3 to 20??m is presented with the emphasis on analysing the first-order transmission peak. The transmission band width and the peak height are compared using the existing theory for this type of an interferometer. The deviation from an ideal performance is resolved and partly explained through the known structural unidealities.

Surface-micromachined silicon air-gap Bragg reflector for thermal infrared

We present a MEMS-based distributed Bragg reflector for thermal infrared (TIR) wavelengths 7 µm < λ < 12 µm. Surface micro-machining process flow was developed for [poly-Si/air-gap/poly-Si] λ/4-mirror using low-pressure chemical-vapour deposited SiO2 as intra-mirror and mirror-to-substrate sacrificial layers. Circular 3 mm diameter mirrors with theoretical reflectance exceeding 99% were designed. Poly-Si layers of the mirror were anchored for retaining constant mutual distance. Anchoring density and mirror-to-substrate gap were varied among samples. We utilized scanning-electron microscope (SEM) imaging for qualitative estimation of processing result success as well as for layer-thickness measurements. We characterized the mirror topography and mechanical response under local point loading by scanning with a stylus profilometer. Fourier-transform IR (FT-IR) spectroscopy was utilized in studies of a reflectance spectrum. A one-dimensional computer simulation was allowed to fit model parameters to FT-IR data. Best-fit thicknesses of air gaps and poly-Si layers were compared with design parameters and with SEM measurements in order to verify the final structure corresponding to the design.

Hydrocarbon gas detection with microelectromechanical Fabry-Perot interferometer

VTT Technical Research Centre of Finland has developed microelectromechanical (MEMS) Fabry-Perot interferometer (FPI) for hydrocarbon measurements. Fabry-Perot interferometer is a structure where is two highly reflective surfaces separated by a tunable air gap. The MEMS FPI is a monolithic device, i.e. it is made entirely on one substrate in a batch process, without assembling separate pieces together. The gap is adjusted by moving the upper mirror with electrostatic force, so there are no actual moving parts. The manufactured MEMS FPIs have been characterized. The tuning wavelength range of the MEMS FPI is 2.8-3.5 μm and its spectral resolution is 50-60 nm. VTT has designed and manufactured a handheld size demonstrator device based on the technology presented in this abstract. This device demonstrates gas detecting by measuring cigarette lighter gas and various plastic materials transmission spectra. The demonstrator contains light source, gas cell, MEMS FPI, detector and control electronics. It is connected to a laptop by USB connection, additional power supply or connection is not needed.

Micro-machined Fabry-Pérot interferometer for thermal infrared

We designed and demonstrated a surface micro-machined tunable Fabry-Pérot interferometer for the thermal infrared wavelengths 7.6 μm <; λ <; 10.3 μm. The device is based on two poly-Si/air distributed Bragg reflectors, separated by an electrostatically adjustable gap. Our work introduces the first micro-machined single-wafer solution for this wavelength range with the resolution adequate for the spectroscopic applications in pharmaceutics and biotechnology. The optical performance under the electrostatic tuning is presented and analyzed. Analytical model and numerical computation, including electromechanical FEM simulation, are exploited for evaluating the spectral behavior of the measured transmission peak width, height, and location.

MOEMS Fabry–Pérot interferometer with point-anchored Si-air mirrors for middle infrared

We studied how a micromachined Fabry–Perot interferometer, realized with wide point-anchored Si/air-gap reflectors, performs at the middle-infrared. A computational analysis of the anchor mechanical behavior is also presented. Compared with solid-film reflectors, this technology features better index contrast, which enables a wider stop band and potentially higher resolution. In this work, we investigate whether the performance is improved according to the index-contrast benefit, or whether the mechanical differences play a role. For comparison, we manufactured and characterized another design that applies solid-film reflectors of Si/SiO2 structure. This data is exploited as a reference for a middle-infrared interferometer and as a template for mapping the performance from the simulation results to the measured data. The novel Si/air-gap device was realized as a non-tunable proof-of-concept version. The measured data is mapped into an estimate of the achievable performance of a tunable version. We present the measured transmission and resolution data and compare the simulation models that reproduce the data. The prediction for the tunable middle-infrared Si/air-gap device is then presented. The results indicate that the interferometer’s resolution is expected to have improved twofold and have a much wider stop band compared with the prior art.

Infrared Spectroscopy Study of a Poly-Silicon Film for Optimizing the Boron-Implanting Dose

An experiment was carried out for recording the infrared transmission between 3 and 16 μm for a free-standing poly-silicon (Si) thin film as a function of the boron-ion implanting. The optical constants were extracted in order to optimize the postdeposition implanting of the membranes in a surface micromachined Fabry-Perot interferometer for the thermal infrared. The free-carrier concentration must not degrade the refractive index n significantly since a high and constant value over the application wavelength range is desired. Moreover, an increase in the extinction coefficient k is detrimental for the interferometer performance. On the other hand, poly-Si electrical conductivity must be doped high enough to avoid static charging and to prevent any layout-dependent distribution of the applied control voltage. We applied the variable-angle transmission spectrometry for recording the data of suspended poly-Si membranes, doped with different levels of implanting dose. The Drude model dispersion formula was exploited for extracting the optical constants n(λ) and k(λ). The optical constants are presented as a function of the dopant concentration and the electrical resistivity.

MEMS Fabry-Perot interferometer with Si-air mirrors for mid- and thermal infrared

We studied how a surface-micromachined Fabry-Perot interferometer, realized with Si / air-gap distributed Bragg reflectors, would perform at the middle-infrared wavelengths. Compared with traditional solid-film pairs, this Si-FPI technology features better index contrast, which enables wider stop band and potentially higher resolution. Four different designs of interferometers were prepared and compared. Two designs apply the solid-film reflectors of Si/SiO2 structure. Their data is exploited as a reference of a middle-infrared interferometer and, as a template for mapping the performance from the simulation results to the measured data. The third design operates at the thermal infrared and it was our first embodiment with the Si/air-gap mirrors. The performance, reported earlier, is here referred to for estimating the technology scalability down to shorter wavelengths. Finally, we realized a non-tunable proof-of-concept version of the Si/air-gap technology for middle infrared. The measured data is mapped into an estimate of the achievable performance of a tunable version. We present the transmission and resolution data and argument the simulation models that reproduce the data. The prediction for the tunable middle-infrared Si-FPI is then presented. The results indicate that such a device is expected to have two-fold better resolution and a clearly wider stop band, compared with the prior art.

MEMS Fabry-Perot interferometer-based spectrometer demonstrator for 7.5 μm to 9.5 μm wavelength range

VTT Technical research centre of Finland has developed a MEMS Fabry-Perot interferometer (FPI) for the wavelength range from 7.5 μm to 9.5 μm. The device consists of two Distributed Bragg Reflectors (DBR) manufactured with MEMS processing techniques. The full width half maximum of the transmission peak is 150nm. This transmission peak can be tuned from 7.5 μm to 9.5 μm by applying a control voltage from 0 V to 30 V. A laboratory demonstrator has been put together to show the use of this module as a part of a spectral measurement setup. Several gas samples have been measured with the setup and compared against measurement results found in literature.

Miniaturized MOEMS spectrometer technology for gas sensing

VTT has developed a miniaturized mid-IR spectrometer system for gas sensing utilizing a surface micromachined tunable MOEMS Fabry-Perot Interferometer (FPI). The MOEMS FPI enables high-quality spectral measurements using a single-pixel detector, thus avoiding expensive linear array detectors. This measurement scheme increases sensor specificity and robustness compared to conventional Nondispersive Infrared (NDIR) sensors.

Study on Al2O3 Film in Anhydrous HF Vapor

HF vapor etching using thin Al2O3 film as etch stop material was studied. It was found that behavior of Al2O3 film in HF vapor can not be predicted from the blanket film studies only but it is important to use samples that resemble closely real process conditions instead. Resistivity of Al2O3 against HF vapor depends on whether a SiO2 was etched on top of it or not. Other affecting factors are the thickness of the Al2O3 film, HF vapor process, stress of the oxide on top of it as well as topography under it. Even very thin 3 nm Al2O3 film resist HF vapor if no SiO2 film is etched on top of it and etch process is slow enough.