Martti Blomberg

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.

MOEMS miniature spectrometers using tuneable Fabry-Perot interferometers

New tuneable MOEMS filters have been developed to cover the spectral range from 400 to 750 nm. Compared with previous MEMS based visible light filters, these Fabry-Perot Interferometers (FPIs) have increased transmission (90%), spectral resolution of ∼ 4 to 9 nm, and larger aperture diameter (2 mm), which allows them to be used in spectral imaging devices. We present the fabrication process and characterization of tuneable MOEMS FPIs for central wavelengths of λ = 420  nm and λ = 670  nm. Miniature imaging spectrometers have potential novel applications in diagnostics and health care, bioprocess, and environmental monitoring, process analytical instrumentation, and water-quality analysis.

A silicon microsystem-miniaturised infrared spectrometer

A miniaturised multipurpose infrared analyser has been developed. This comprises an electrically modulatable infra-red source, an electrically tunable Fabry-Perot pass band filter, an integrated circuit and a photodetector, all silicon microfabricated and assembled on the same silicon substrate. The infrared source and the detector are monolithically integrated in the substrate. The Fabry-Perot interferometer is flip-chip bonded and the IC-circuit wire-bonded. The whole spectrometer assembly is in a DIL package, having holes for the incoming and outgoing radiation. The dimensions of the spectrometer silicon substrate are 5/spl times/15 mm/sup 2/.

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.

MEMS and piezo actuator-based Fabry-Perot interferometer technologies and applications at VTT

Miniaturized spectrometers covering spectral regions from UV to thermal IR are of interest for several applications. For these purposes VTT has for many years been developing tuneable MEMS-based and more recently piezo-actuated Fabry-Perot Interferometers (FPIs). Lately several inventions have been made to enter new wavelengths in the VIS range and enlarge apertures of MEMS devices and also extending the wavelength range of piezo-actuated FPIs. In this paper the background and the latest FPI technologies at VTT are reviewed and new results on components and system level demonstrators are presented. The two FPI technologies are compared from performance and application point of view. Finally insight is given to the further development of next generation devices.

Low-temperature processes for MEMS device fabrication

The high temperatures typical in semiconductor and conventional MEMS fabrication limit the material choices in MEMS structures. This paper reviews some of the low-temperature processes and techniques available for MEMS fabrication and describes some characteristics of these techniques and practical process examples. The techniques described are plasma-enhanced chemical vapour deposition, atomic layer deposition, reactive sputtering, vapour phase hydrofluoric acid etching of low-temperature oxides, and low-temperature wafer bonding. As a practical example of the use of these techniques, the basic characteristics of a MEMS switch and other devices fabricated at VTT are presented.

Novel tunable laser diode arrangement with a micromachined silicon filter: feasibility

Abstract We demonstrate the workability of a new arrangement of a tunable high-power laser diode. This novel composite is realized with the use of a micro-optoelectro-mechanical system—a tunable silicon micromachined interferometric filter—a standard edge-emitting laser diode. In current extremely-short-external-cavity configuration, the quasi-continuous tuning over a 4-nm range with output power of tens of mW was obtained. Power and etalon characteristics of a considered hybrid laser are discussed. The advantage of proposed configuration is in its potentially low cost (due to the use of commercially available components) and loose alignment tolerances.

The Kirk effect in the LIGBT devices

A study is made of the DC current-voltage characteristics of several different LIGBT (lateral insulated-gate bipolar transistor) devices both theoretically and experimentally. The theoretical part is based on the analytical expressions and includes some improvements as compared to the earlier circuit simulation models for the LIGBTs. This work extends the range of validity of these models. The fit between the theory and experiments is satisfactory also in the saturated regime of the current-voltage curves. The importance of the Kirk effect is emphasized especially for devices with short distances between the anode and cathode. >

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.

Main characteristics of a miniaturized multipurpose infrared spectrometer

This paper describes the main characteristics of a miniaturized multipurpose IR spectrometer. The miniaturized spectrometer comprise of three silicon micromachined devices: an electrically modulated thermal IR emitter, and electrically tunable Fabry-Perot interferometer and a photodetector. The IR emitter and the detector are monolithically integrated into a silicon substrate. In addition, the silicon substrate carries an integrated circuit die-bonded and wire-bonded on the silicon substrate. The whole spectrometer assembly is packaged in a DIL package having holes for the incoming and outgoing radiation. The dimensions of the package are 12 mm X 23 mm X 5 mm. This concept enables the realization of a miniaturized spectrometer for high-volume and low-cost products. In the miniaturization, the critical optical characteristic is the throughput of a system. In addition, the S/N-ratio and crosstalk of the module are the main electrical characteristics to be considered in the miniaturization. In this paper, the performance of the spectrometer module is presented via measurements including the radiometric analysis, S/N-ratio analysis and crosstalk analysis.