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.
A micro component for a non-selective NDIR (non dispersive infrared) gas detection system is presented in this work. This device consist of an IR detection module composed of a thermopile and a thin film filter array. The thermopile arrays (up to 4x4) are built on a silicon substrate by bulk micro-machining processes. The whole matrix is built on a thin freestanding silicon oxide/silicon nitride membrane of 2100x2100μm2 defined by anisotropic wet etching. To ensure the existence of hot and cold junctions for each detector we define on the insulating membrane absorbers and ribs, 6μm thick, by heavy boron doping of the silicon underneath. The ribs crisscross the membrane contacting the silicon bulk acting as a heat sink. Absorbers are located in the centre of each individual pseudo-membrane defined by the ribs intersection. Incident radiation heats up the absorber creating a temperature difference that is measured by the thermocouples that are placed between the absorber and the ribs.
On a second chip, the elements of the filter array are fabricated in a matching configuration. The filters are built on a silicon substrate alternating thin films of different refraction index acting like a Fabry-Perot structure with 2-8μm silicon oxide cores. The transmitted filter peaks are not tuned for the detection of any specific substance: they configure a non selective general purpose filter array (400-4000 cm<sup>-1</sup>), making signal processing and pattern recognition techniques necessary.
Both dies have been fabricated and characterized and have been successfully attached using flip-chip techniques.
The measurements on these devices have been used to build an optical simulation tool that allows the assessment of the whole NDIR system behaviour in operating conditions.
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.