Karla Hiller

Tunable infrared detector with integrated micromachined Fabry-Perot filter

We report the design, fabrication, and test results of a tunable pyroelectric detector with an integrated micromachined Fabry-Perot FP filter for gas analysis in the mid-wave infrared MWIR. The new approach is based on a bulk micromachined Fabry-Perot interferometer with an air cavity, which is electrostatically tuned. Various types of movable reflectors and spring configurations are fabricated to determine the optimum solution with respect to maximum tuning range, low gravity influence on center wavelength, and suitable filter bandwidth. Short and long cavity filters are designed for the spectral ranges of 3 to 4.3 m and 3.7 to 5.0 m, respectively. The tunable filter is arranged on top of a current mode pyroelectric detector with a flat spectral response. It is shown that the main challenge is to achieve a high finesse in spite of nonperfect parallelism, mirror curvature, and the additional phase shift caused by the Bragg reflectors.

Bonding and deep RIE: a powerful combination for high-aspect-ratio sensors and actuators

In this paper we present the very promising results for two methods of the so-called Bonding and Deep RIE (BDRIE) technology, characterised by bonding of two wafers with pre-patterned vertical gaps and subsequent RIE trench etching of the active layer. In case of the anodically bonded silicon-glass compound detection electrodes for vertical movement are integrated. The silicon layer contains the movable structure as well as drive and detection electrodes for lateral movement. It is advantageous that finally the mechanical active elements consist of single crystalline silicon without any additional layers. The BDRIE approach allows a great variation of parameters. The active layer thickness can be defined due to application issues. Our examples show active layers thickness ranging from 30 up to 200 μm, patterned by dry etching steps with maximum aspect ratio between 20:1 and 30:1. Structures with trench width variations of more than 50 (widest/smallest trench) have been fabricated successfully. Methods and results of preventing notching and backside etching of the active layer are presented as well. The size of the vertical gap can be as small as 1.5 μm for a very sensitive detection or several tens or hundreds of microns in order to reduce damping and parasitic capacitance. Holes for release in the movable structure are not necessary and will therefore not restrict the design. However, restrictions are given by the minimum size of bond area and the relation between layer thickness, free standing area above the groove and bond pressure, which are discussed within the paper. Applications of BDRIE are inertial sensors like gyroscopes, step-by-step switchgears as well as micro mirrors.

Characterization of low-temperature wafer bonding by infrared spectroscopy

We present the results of an infrared (IR) spectroscopic investigation of interfaces between two hydrophilic Si wafers bonded at low temperature. Multiple internal transmission IR spectra were recorded of the bonds, with different chemical pretreatments of Si surfaces employed before bonding. The analysis of IR spectra shows that the number of O–H and H–Si–Ox species at the interface depends strongly on the chemical pretreatment type, which determines the bonding energy. The annealing procedure used in the bonding process leads to dissociation of water molecules, oxidation of silicon at the interfaces, and diffusion of hydrogen into silicon oxide layer formed at the interface. The difference in bonding processes is discussed.

Low-temperature approaches for fabrication of high-frequency microscanners

Within this paper novel applications of low temperature silicon wafer bonding technologies for the fabrication of high frequency silicon microscanners are presented. Two technological approaches are discussed, both using low temperature bonding as a key technological step. Results of the integration of a special low temperature bonding process within the bulk technology approach are shown. Micromirror arrays fabricated with this technology are presented and show promising results for optical applications.

Widely tunable Fabry-Perot filter based MWIR and LWIR microspectrometers

As is generally known, miniature infrared spectrometers have great potential, e. g. for process and environmental analytics or in medical applications. Many efforts are being made to shrink conventional spectrometers, such as FTIR or grating based devices. A more rigorous approach for miniaturization is the use of MEMS technologies. Based on an established design for the MWIR new MEMS Fabry-Perot filters and sensors with expanded spectral ranges in the LWIR have been developed. The range 5.5 - 8 μm is particularly suited for the analysis of liquids. A dual-band sensor, which can be simultaneously tuned from 4 - 5 μm and 8 - 11 μm for the measurement of anesthetics and carbon dioxide has also been developed. A new material system is used to reduce internal stress in the reflector layer stack. Good results in terms of finesse (≤ 60) and transmittance (≤ 80 %) could be demonstrated. The hybrid integration of the filter in a pyroelectric detector results in very compact, robust and cost effective microspectrometers. FP filters with two moveable reflectors instead of only one reduce significantly the acceleration sensitivity and actuation voltage.

Infrared spectroscopic investigations of the buried interface in silicon bonded wafers

Non-destructive multiple internal transmission and multiple internal reflection infrared (IR) measurements were used to investigate the silicon wafer bonding process. IR measurements performed ex situ and in situ reveal the chemical reactions which take place during annealing at the interface of silicon bonded wafers with thin native (Si/Si) or thick thermally grown interfacial oxide (Si–SiO2/Si). A comparative analysis of the IR response of the buried interface in low temperature silicon bonded wafers prepared using different surface activation treatments is presented. The evolution with annealing temperature of the chemical species at the interface is used to explain the bonding mechanism of Si wafers in the temperature range of 30–400 °C. Very good bonding (~3000 mJ m−2) at 200 °C was obtained between pairs of Si wafers covered with native and thermally grown oxide in the case when the wafers were treated by reactive ion etching oxygen plasma.

Tunable Fabry-Perot-Interferometer for 3-5 μm wavelength with bulk micromachined reflector carrier

This contribution deals with design, fabrication and test of a micromachined first order Fabry-Perot-Interferometer (FPI) usable as tunable infrared filter in a spectrometer. The approach discussed here minimizes mirror curvature by using relative thick (300 μm Si ) mirror carriers for the fixed and the movable mirror of the FPI. We use thermally grown λ/4 thick SiO2 for antireflection layer at the mirror back side and for the first low refractive layer followed by a λ/4 thick polycrystalline silicon high refractive layer. Second and third λ/4 layer pairs of SiO2 and polycrystalline silicon complete the mirrors. The cavity size is electrostically tuned and capacitively detected by a closed loop control.

Testing microcomponents by speckle interferometry

Design, manufacturing and test of microcomponents generate new challenges for measurement techniques in general. The non- contacting operation of optical metrology makes it attractive to solve the task of measuring geometric quantities of microparts. So far, speckle interferometry (ESPI) is well established as a measuring tool for analyzing deformation, vibration and strain on a macroscopic level. This paper deals with possibilities and application limits of ESPI in the case of scaling down the object size below one millimeter. In a first part, both spatial resolution and displacement sensitivity of the technique are discussed. Theoretical considerations are shown together with experimental verification. Secondly, a micro speckle interferometer will be presented that has been built for the use with different microscopes. Its capabilities are demonstrated by a practical application. The microcomponent under investigation is a bulk micromachined gyroscope, a demanding object with respect to its multilayer design. Developments aim at increasing the spatial resolution step by step and results obtained with different field of view will demonstrate the progress. Finally, the deformation behavior of an X-shaped torsional spring with a width of 100 micrometer could be characterized.

Recent advances in expanding the spectral range of MEMS Fabry-Perot filters

First results of a MEMS Fabry-Perot filter (FPF) for the spectral range of 8-11 μm are presented. Broad bandwidth and a low phase dispersion could be achieved by using new dielectric materials with an enhanced refractive index contrast and relatively simple reflector designs with a low number of layers. Different designs of the Bragg reflectors and complete FPF devices have been developed and samples have been fabricated and tested. Especially tilting and in certain cases bowing of the reflectors lowers the finesse of the fabricated filters. The bandwidth of the FPF is in the range of 170- 250 nm and the peak transmittance has been determined to be up to 60 %. Simulation and measurements agree very well if the effect of the defective finesse is taken into considerations.

Novel MWIR microspectrometer based on a tunable detector

In this paper we present basic designs, operation concepts and some application examples of a novel microspectrometer for the spectral range of 3-5μm, which is based on a pyroelectric detector with an integrated micromachined Fabry-Perot filter (FPF). We discuss the influence of different optical setups on the spectral resolution and the signal-to-noise ratio of the microspectrometer. Two basic operation modes, step scan mode and continuous sweep mode are demonstrated. Such a device has a large potential in the field of infrared absorption spectroscopy, particularly if multicomponent mixtures have to be analyzed.