Deliverables
The infrared snapshot camera is the second product of the TECH TIR project. The technology utilised for its production is developed by the consortium; the main characteristics are the thermoelectric sensor, the piezo motor, the HDPE objective, the design assembly. The image is obtained by the linear scan of a two columns TE array.
The movement of the array is guaranteed by the piezo motor and the image is properly collected onto the pixels by an HDPE objective. The design assembly guarantees the proper collocation of all the components and the external package suitable for various applications. In fact, the main applications of the product are the surveillance and the security market, which involves the commercial, residential and industry fields.
Conclusions
A spectrum of applications was fully analysed and outlined and the system specifications were drawn. Regarding the components, a low-cost optics was designed and manufactured; a compact actuator motion module was designed, manufactured and demonstrated.
The front-end electronics was designed, manufactured and tested, the single element TE sensor was successfully tested and expected performance was validated. The vacuum package was designed and validated with a reference sensor. The TE sensor array was designed, manufactured and tested. However a problem related to the VLSI design was found, which prevented the reading-out the sensors signal. The problem was fully analysed and corrective actions were determined.
Some image reconstruction algorithms were developed, implemented and tested and several camera packages were designed using virtual prototyping tools to fit different product configurations.
The piezo motor was designed considering that the FPA is linearly scanned. The motor developed is very small with dimensions 7x2x1 mm. The mount for the motor is a novel flexure approach. The mount enables better transfer of mechanical power and relaxes the straightness requirements of the stage. The dynamic performance of the motor is good as well as the precision control one. The piezo-motor requires a position feedback for the close servo loop. The type of sensor adopted is a rotary one, which has the advantage to achieve higher encoder resolution
Micro-machined thermal sensors are revolutionizing IR imaging with uncooled thermal sensors. This is achieved by the realization of 2D arrays of low-cost micro-machined sensor pixels with very small thermal conductance as well as thermal capacitance that yield high sensitivity and response fast enough for imaging. A comprehensive comparison between the different thermal sensors, i.e. bolometers, pyroelectric and thermoelectric sensors, is beyond the scope of this summary.
However, thermocouples have some inherent advantages, including low power consumption, no 1/f noise and less stringent requirements of controlling the chip operating temperature. The thermoelectric sensors are constructed of one or more thermocouples that respond with spontaneous voltage to temperature differences induced by absorbed IR radiation. In order to allow temperature differences to develop, the "hot" contact of the thermocouples has to be thermally isolated from the "cold" contact heat sink. Such thermal isolation can be obtained in thin-film thermocouples fabricated on silicon substrates by micro machining of the substrate below the "hot" contacts. If CMOS circuits are also realized on the same substrate, whole micro-systems can be fabricated monolithically.
Since thermoelectric sensors can be realized using standard CMOS layers, they have the best compatibility with CMOS technology and therefore have the potential of achieving low-cost uncooled thermal imagers. Traditional designs of CMOS compatible thermoelectric sensors suffered from two main shortcomings. One disadvantage is the relatively small signal. In order to increase the signal several thermocouples are connected in series to form a thermopile. This, however, reduces the achievable thermal resistance and thus performance.
The second disadvantage is the problematic realization of 2D arrays of sensors. As opposed to traditional micro-machining methods that used wet anisotropic etching processes for silicon bulk micro-machining, the silicon bulk is used as a sacrificial layer and structures made of CMOS process thin films constitute the sensors. This technique allows better yield and better thermal isolation in small pixels. Uncooled IR sensors have been realized integrated in standard CMOS chips using this technique.
Conclusions
The building block for the motion detector were developed and manufactured. The lab-scale post-processing provides a yield of 95%. Nevertheless the current design is very sensitive to manufacturing yield. For that reason an off-the shelf pyro-sensor was demonstrated.
Due to the fact that the design specifications were higher than the actual application needs and that the size of the unitary elements was decided based on the snapshot camera, many technical improvements can be done. That is, the yield can be improved by making bigger elements and by moving from a serial to parallel-serial electronics architecture. A balance between the size of the unitary element, bandwidth and responsivity can be achieved.
The front-end electronics was designed, manufactured and tested, the single element TE sensor was successfully tested and expected performance was validated. The vacuum package was designed and validated with a reference sensor. The TE sensor array was designed, manufactured and tested. However a problem related to the VLSI design was found, which prevented the reading-out the sensors signal. The problem was fully analysed and corrective actions were determined.
Two optical designs were investigated upon the functional input parameters. The first one is a very low cost solution based on injection moulded optical parts, and particularly Fresnel lenses made of high-density polyethylene (HDPE). HDPE has found extensive use as an optical material for low-end non-imaging applications; extension to high-end imaging applications has so far been prevented by its poor transmittance compared to semiconductor crystals or chalcogenide glasses; this fact makes it necessary to implement HDPE lenses as thin Fresnel elements (<1mm), so as to maximise the radiation throughput. HDPE Fresnel lenses can be manufactured by hot pressing, particularly suited to flat lenses, or injection moulding, required for curved lenses; the former allows a good moulding accuracy, while the accuracy of the latter is limited by the shrinkage of the material (>2-3%), which leads to rounding of the microrelieves and deformation of the substrate.
Lower-risk solutions based on hot-pressed chalcogenide glasses was also considered; In particular the low cost solution is based on a doublet of chalcogenide material. As is known, chalcogenide lenses are less expensive than lenses based on crystalline semiconductors, thanks to the lower cost of the raw materials as well as the cost-effective manufacturability (through moulding).
The HDPE objective was, finally, successfully designed, tested and manufactured.
The development of a TE sensor in CMOS technology allows the possibility to integrate, on the same sensor's chip, also the main parts of the electronics necessary to realize an IR motion detector. Using the traditional pyroelectric (PIR) sensor, largely applied on the traditional motion detectors, such a possibility doesn't exist, and the related electronics is necessarily made through external discrete components usually mounted on a printed circuit board.
The CMOS TE technology would then enclose inside the same chip and sensor housing most of the electronic circuits needed to achieve the motion detection function: amplifiers, choppers, filters, etc.
Only few and "bulky" components, like the alarm relay and the electrolytic capacitors, might be discrete and mounted outside on the printed circuit board (p.c.b.).
An IR motion detector realized on such a way would give a certain number of advantages in comparison with the traditional PIR detectors as follows:
- Much smaller dimensions of the p.c.b., and then smaller size of the overall detector
- Lower energy consumption due to the full CMOS electronics
- Higher electromagnetic immunity due to the on-chip integration of the electronics housed inside the same shielding metallic case of the TE sensor
- Higher detection capability due to the wider bandwidth of the TE sensor
Conclusions
The building block for the motion detector were developed and manufactured. The lab-scale post-processing had provided a yield of 95%. Nevertheless the current design was very sensitive to manufacturing yield. For that reason an off-the shelf pyro-sensor was demonstrated
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