This paper will discuss recent results obtained when applying a photoconductive linear MCT array in a demonstration spectrometer designed for the NIR wavelength range from 1300 to 2500 nm. A new 128x1 element MCT sensor was developed specifically for spectroscopy, i.e. with "tall", rectangular pixels in order to optimize both wavelength resolution and optical throughput. Also new read-out electronics was developed using multilayer LTCC (Low Temperature Co-Fired Ceramics) techniques, which is integrated into the package and realizes synchronous ("lock in") detection for each of the 128 channels. Advantages of this current-detection scheme include compatibility with chopped light sources (insensitivity to ambient stray light) and elimination of read-out noise (affecting charge-detection amplifiers). The first test results reported here confirm spectrometer operation and present encouraging performance, even though the system is not yet optimized. The spectrometer is very fast, with minimum integration time of 1.2 ms, while photometric noise will reduce with longer integration times. There is no fundamental limit in the maximal length of the integration time. Testing with integration times of 1.2, 12, 120 and 1200 ms resulted in absorbance noise levels of approximately 2500, 330, 94 and 49 μA units. Demonstration spectra were measured from lactose and copying paper samples. Thanks to high speed and parallel spectral recording of 128 wavelengths, MCT array technology appears highly potential for developing powerful on-line spectrometers for process analytical applications not only in the near infrared (NIR) but also for the lower mid-IR wavelengths, up to approximately 6 μm.
The optical analyzers used in on-line and off-line process measurements set a large variety of special demands for optical detectors, i.e. customized detectors are needed. Both the detector (array) and its read-out circuit are affected. The typical volume for process analyzers is so low that fully customized ASICs easily become too expensive for signal recovery solutions in optical detectors. This is due to high fixed costs in ASIC development.
Low Temperature Co-fired Ceramics (LTCC) substrates allow high integration grade and the smart packaging solutions needed in optical detectors. The ceramic substrate is suitable for hermetic packaging. The high integration grade is possible thanks to multilayer capability, with narrow metal strips as well as blind and buried vias which can be placed directly underneath the solder pads. Standard connection methods can be used, i.e. soldering, gluing and wire bonding for components and detector chip assembly. By using small passive components and bare chip or chip scale packaged active components, one is able to integrate the read-out functions in a small enough space.
This paper briefly presents three different cases where infrared photoconductive detector arrays are attached to read-out circuitry which is made on LTCC substrate. The detector read-out hybrids are packaged in hermetic metal packages. The packages have been either non-cooled or thermoelectrically cooled. 24 element PbS, 4 by 4 element PbS and 128 element MCT cases are handled. Also an example of an integrated infrared light emitting diode array for an LED-spectrometer is presented. General feasibility analysis, including some electrical test results and the management of substrate dimensional tolerances, is given.