Uncooled infrared bolometer arrays have become the technology of choice for low-cost infrared imaging systems used in
applications such as thermography, firefighting, driver night vision, security and surveillance. Uncooled infrared
bolometer arrays are reaching performance levels which previously only were possible with cooled infrared photon
detectors. With a continuously increasing market volume (> 100 000 units per year to date), the cost for uncooled
infrared imaging chips are decreasing accordingly. In this paper we give an overview of the historical development of
uncooled infrared bolometer technology and present the most important bolometer performance parameters. The
different technology concepts, bolometer design approaches and bolometer materials (including vanadium oxide,
amorphous silicon, silicon diodes, silicon-germanium and metals) are discussed in detail. This is followed by an analysis
of the current state-of-the-art infrared bolometer technologies, the status of the infrared industry and the latest technology
trends.
A new low-cost long-wavelength infrared bolometer camera system is under development. It is designed for use with an
automatic vision algorithm system as a sensor to detect vulnerable road users in traffic. Looking 15 m in front of the
vehicle it can in case of an unavoidable impact activate a brake assist system or other deployable protection system. To
achieve our cost target below €100 for the sensor system we evaluate the required performance and can reduce the
sensitivity to 150 mK and pixel resolution to 80 x 30. We address all the main cost drivers as sensor size and production
yield along with vacuum packaging, optical components and large volume manufacturing technologies.
The detector array is based on a new type of high performance thermistor material. Very thin Si/SiGe single crystal
multi-layers are grown epitaxially. Due to the resulting valence barriers a high temperature coefficient of resistance is
achieved (3.3%/K). Simultaneously, the high quality crystalline material provides very low 1/f-noise characteristics and
uniform material properties. The thermistor material is transferred from the original substrate wafer to the read-out
circuit using adhesive wafer bonding and subsequent thinning. Bolometer arrays can then be fabricated using industry
standard MEMS process and materials. The inherently good detector performance allows us to reduce the vacuum
requirement and we can implement wafer level vacuum packaging technology used in established automotive sensor
fabrication. The optical design is reduced to a single lens camera. We develop a low cost molding process using a novel
chalcogenide glass (GASIR®3) and integrate anti-reflective and anti-erosion properties using diamond like carbon
coating.
Novel single crystalline high-performance temperature sensing materials (quantum well structures) have been developed for the manufacturing of uncooled infrared bolometers. SiGe/Si and AlGaAs/GaAs quantum wells are grown epitaxially on standard Si and GaAs substrates respectively. The former use holes as charge carriers utilizing the discontinuities in the valence band structure, whereas the latter operate in a similar manner with electrons in the conduction band. By optimizing parameters such as the barrier height (by variation of the germanium/aluminium content respectively) and the fermi level Ef (by variation of the quantum well width and doping level) these materials provide the potential to engineer layer structures with a very high temperature coefficient of resistance, TCR, as compared with conventional thin film materials such as vanadium oxide and amorphous silicon. In addition, the high quality crystalline material promises very low 1/f-noise characteristics promoting an outstanding signal to noise ratio and well defined and uniform material properties, A comparison between the two (SiGe/Si and AlGaAs/GaAs) quantum well structures and their fundamental theoretical limits are discussed and compared to experimental results. A TCR of 2.0%/K and 4.5%/K have been obtained experimentally for SiGe/Si and AlGaAs/GaAs respectively. The noise level for both materials is measured as being several orders of magnitude lower than that of a-Si and VOx. These uncooled thermistor materials can be hybridized with read out circuits by using conventional flip-chip assembly or wafer level adhesion bonding. The increased bolometer performance so obtained can either be exploited for increasing the imaging system performance, i. e. obtaining a low NETD, or to reduce the vacuum packaging requirements for low cost applications (e.g. automotive).
Pedestrian fatalities are around 15% of the traffic fatalities in Europe. A proposed EU regulation requires the automotive industry to develop technologies that will substantially decrease the risk for Vulnerable Road Users when hit by a vehicle. Automatic Brake Assist systems, activated by a suitable sensor, will reduce the speed of the vehicle before the impact, independent of any driver interaction. Long Wavelength Infrared technology is an ideal candidate for such sensors, but requires a significant cost reduction. The target necessary for automotive serial applications are well below the cost of systems available today. Uncooled bolometer arrays are the most mature technology for Long Wave Infrared with low-cost potential. Analyses show that sensor size and production yield along with vacuum packaging and the optical components are the main cost drivers. A project has been started to design a new Long Wave Infrared system with a ten times cost reduction potential, optimized for the pedestrian protection requirement. It will take advantage of the progress in Micro Electro-Mechanical Systems and Long Wave Infrared optics to keep the cost down. Deployable and pre-impact braking systems can become effective alternatives to passive impact protection systems solutions fulfilling the EU pedestrian protection regulation. Low-cost Long Wave Infrared sensors will be an important enabler to make such systems cost competitive, allowing high market penetration.
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