A novel micromachined thermal emitter for fast transient temperature operation is presented. Compared to most
commercial available thermal emitters, the one here presented, is able to operate in a pulsed mode. This allows the use of
lock-in techniques or pyrodetectors in the data acquisition without the use of an optical chopper for light modulation.
Therefore, these types of thermal emitters are very important for small filter photometers. Several spider type hotplate
concepts were studied in order to find a design with excellent mechanical stability and high thermal decoupling. The
thermal emitters are fabricated using silicon on insulator (SOI) technology and KOH-etching. The emitters are heated
with Pt-meanders. For temperature determination an additional Pt-structure is deposited onto the hotplates. The emitters
are mounted in TO-5 housings using a ceramic adhesive and gold wire bonding. The used operation temperature is
750°C. In pulsed operation it's important to have a large modulation depth in terms of thermal radiation intensity in the
needed spectral range. The maximal reachable modulation depth ranges from ambient temperature to steady state
temperature. A modulation frequency of 5 Hz still allows using nearly the maximum modulation depth.
Sensitive and fast detection of explosives remains a challenge in many threat scenarios. Fraunhofer IPM works on two
different detection methods using mid-infrared absorption spectroscopy in combination with quantum cascade lasers
(QCL). 1. stand-off detection for a spatial distance of several meters and 2. contactless extractive sampling for short
The extractive method is based on a hollow fiber that works as gas cell and optical waveguide for the QCL light. The
samples are membranes contaminated with the explosives and real background. The low vapor pressure of TNT requires
a thermal desorbtion to introduce gaseous TNT and TATP into the heated fiber. The advantage of the hollow fiber setup
is the resulting small sample volume. This enables a fast gas exchange rate and fast detection in the second range. The
presented measurement setup achieves a detection limit of around 58 ng TNT and 26 ng TATP for 1 m hollow fiber.
TATP - an explosive with a very high vapor pressure in comparison to TNT or other explosives - shows potential for an
adequate concentration in gas phase under normal ambient conditions and thus the possibility of an explosive detection
using open path absorption of TATP at 8 μm wavelength. In order to lower the cross sensitivities or interferents with
substances with an absorption in the wavelength range of the TATP absorption the probe volume is checked
synchronously by a second QCL emitting beside the target absorption wavelength. In laboratory measurements a
detection limit of 5 ppm*m TATP are achieved.
Stand-off and extractive explosive detection methods for short distances are investigated using mid-infrared laser spectroscopy. A
quantum cascade laser (QCL) system for TATP-detection by open path absorption spectroscopy in the gas phase was developed. In
laboratory measurements a detection limit of 5 ppm*m was achieved. For explosives with lower vapor pressure an extractive hollow
fiber based measurement system was investigated. By thermal desorption gaseous TATP or TNT is introduced into a heated fiber.
The small sample volume and a fast gas exchange rate enable fast detection. TNT and TATP detection levels below 100 ng are
feasible even in samples with a realistic contaminant background.
We present an overview of the current status of laser diodes used in remote sensing application including novel
laser types such as single mode emitting DFB lasers operating at wavelengths up to 3 μm and quantum cascade
lasers for mid infrared absorption spectroscopy. In particular we will focus on applications of these devices in the
frame of safeguard measures and home security.
A grey body emitter based on a microcavity array with Pt-heater on the backside is presented. The microcavity array is
made by electro-chemical etching of silicon. It has been shown in a previous work, that this emitter has especially in the
spectral region >8 μm significantly higher emissivity than commercial available emitters.
Due to the thin-film technology of MEMS-based emitters, these types can be typically operated with a maximum
temperature of 700°C to 800°C. Higher temperature causes degradation of the heater. But higher temperatures also mean
an enhancement in radiation power and thus open a wider area of application.
The presented work shows a temperature enhanced thermal emitter with a ceramic heater passivation. Short time tests
show the possibility of a maximum temperature of 1000°C.
The part of light emitted by the microcavities in comparison to the whole device as well as the influence of the pore size
concerning the emitted spectral range is investigated. The results are the basis for a redesign of the microcavity array for
an enhancement of the geometry tuned emissivity.
In various fruit storage applications precise and continuous ethylene detection is needed. The aim of this work is the
development of a miniaturised mid-infrared filter spectrometer for ethylene detection at 10.6 &mgr;m wavelength. For this
reason optical components and signal processing electronics were developed, tested and integrated in a compact
measurement system. The present article describes the optical components, the integration of the optical system,
electronics and results of gas measurements. Next to a Silicon-based macroporous IR-emitter, a miniaturised absorption
cell and a detector module for the simultaneous measurement at four channels for ethylene, two interfering gases and the
reference signal were integrated in the optical system. Optical filters were attached to fourfold thermopile-arrays by flip-chip-
technology. Silicon-based Fresnel multilenses were processed and attached to the cap of the detector housing.
Because of the high reflection losses at the silicon-air surface the Fresnel lenses were coated with Antireflection layers
made of Zinc sulphide. For the signal processing electronics a preamplification stage and a Lock-in-board has been
developed. First ethylene measurements with the optical system with miniaturised gas cell, Silicon-based IR-emitter, a
commercial thermopile detector and the self-developed system electronics showed a detection limit of smaller than 20ppm.
Diffractive Fresnel Lenses (FL) were designed, fabricated and tested. The lens aims for increasing the sensitivity of a Non-Dispersive InfraRed (NDIR) silicon based optical gas system, focusing as much radiation as possible onto the detector. The studied wavelengths are 10.6μm and 3.4μm, which are the main absorption lines for ethylene and ethanol respectively. The lens diameter (5mm) and the focal length (4mm) are fixed by the detector package. Those diffractive lenses are compatible with the planar nature of silicon microtechnology. A theoretical study about the global lens efficiency as a function of the technological constrains and the process complexity has been carried out. Using only three photolithographic masks, eight quantization steps can be etched and a theoretical lens efficiency of 95% can be achieved. Once the devices were fabricated, the focal length and the spot size have been measured.
Precise and continuous ethylene detection is needed in various fruit storage applications. The aim of this work is the development of a miniaturised mid-infrared filter spectrometer for ethylene detection at 10.6 μm wavelength. For this reason optical components and signal processing electronics need to be developed, tested and integrated in a compact measurement system. The present article describes the proposed system set-up, the status of the development of component prototypes and results of gas measurements performed using a first system set-up. Next to a microstructured IR-emitter, a miniaturised multi-reflection cell and a thermopile-array with integrated optical filters and microstructured Fresnel lenses for the measurement of ethylene, two interfering gases and one reference channel are proposed. Recently a miniaturised White cell as absorption path is tested with various commercial and a self-developed thermal emitter. First ethylene measurements have been performed with commercial twofold thermopile detectors and a Lock-in-amplifier. These showed significant absorption at an ethylene concentration of 100ppm. For the detection module different types of thermopiles were tested, first prototypes of Fresnel lenses have been fabricated and characterised and the parameters of the optical filters were specified. Furthermore a compact system electronics for signal processing containing a preamplification stage and Lock-in-technique is in development.
There are several micro sized thermal emitters commercially available, but compared with an ideal black body radiator, their emissivity and thus the emitted radiation is moderate. This was the motivation to develop a novel type of micromachined thermal IR-emitters. The main difference compared with common thermal micro emitters is the use of 2D structured bulk silicon. The regular ordered macropores of the emitters are obtained by electrochemical etching of prepatterend silicon substrates. Typical pore diameter of the fabricated photonic-crystal-like structures are in the range of 2.5 μm to 30 μm. The macroporous silicon shows a black-body-like emission profile for a wide wavelength range.
Metal oxide gas sensors are widely used for different applications and operate normally at high temperatures between 300°C and 600°C. Such high temperatures are mainly needed to speed up the desorption of molecules from the gas sensor surface. Goal of the reported investigations is the reduction of the operating temperatures of such devices by the influence of radiation on the gas adsorption/desorption process. Therefore, the influences of radiation on the gas sensing mechanisms at surfaces of different metal oxides (SnO<sub>2</sub>, ZnO, WO<sub>3</sub> and Cr<sub>2-x</sub>Ti<sub>x</sub>O<sub>3+z</sub>) have been studied for different wavelengths. The experiments were carried out at an operating temperature of 130°C as well as at room temperature. As radiation sources LEDs emitting at different wavelength were used. The sensor response to NO<sub>2</sub>, CO, NH<sub>3</sub> and H<sub>2</sub> has been measured with and without illumination. The investigations have shown that light mainly influences the photo-activation of electron-hole pairs, which results in an increasing of the electrical conductivity of the illuminated metal oxide. The observed influences of photoadsorption and photocatalytic effects are small compared to the photoelectric effect. Only a weak increase of the NO<sub>2</sub> sensitivity during illumination has been measured.
We present a novel approach to produce a micromachined low cost hotplate gas sensor with reduced number of technology steps. The basic idea was to realize a simple device on common silicon substrates using conventional photolithography, sputtering and evaporation techniques. Two main performance parameters were targeted: the power consumption should not exceed 200 mW for an operation at 350 degree(s)C-400 degree(s)C and the thermal response time should be faster than 1 second. Fast thermal time constants allows the operation of device in temperature pulse mode. The first step of the development was the theoretical determination of the power consumption of the micromachined substrates, even temperature distribution on the sensitive area and sufficient mechanical stability. For this we build models describing the thermal behavior of the devices by means of the finite element method (FEM) and corresponding resistance-capacitor-networks (RC-network). Then we developed technological processes to fabricate sensor structures according to the optimal geometry resulting from the model calculations. A first prototype is introduced in this publication.