Most of today's commercial solutions for un-cooled IR imaging sensors are based on resistive bolometers using either
Vanadium oxide (VOx) or amorphous Silicon (a-Si) as the thermistor material. Despite the long history for both
concepts, market penetration outside high-end applications is still limited. By allowing actors in adjacent fields, such as
those from the MEMS industry, to enter the market, this situation could change. This requires, however, that
technologies fitting their tools and processes are developed. Heterogeneous integration of Si/SiGe quantum well
bolometers on standard CMOS read out circuits is one approach that could easily be adopted by the MEMS industry.
Due to its mono crystalline nature, the Si/SiGe thermistor material has excellent noise properties that result in a state-ofthe-
art signal-to-noise ratio. The material is also stable at temperatures well above 450°C which offers great flexibility
for both sensor integration and novel vacuum packaging concepts. We have previously reported on heterogeneous
integration of Si/SiGe quantum well bolometers with pitches of 40μm x 40μm and 25μm x 25μm. The technology scales
well to smaller pixel pitches and in this paper, we will report on our work on developing heterogeneous integration for
Si/SiGe QW bolometers with a pixel pitch of 17μm x 17μm.
To address the growing needs of the automotive industry for low cost solutions to far infrared imaging, a silicon - silicon
germanium (Si/SiGe) quantum well resistive bolometer technology is presented. The Si/SiGe thermistor structure is
epitaxially grown and combines a high temperature coefficient of resistance (TCR) with low flicker noise. A TCR of
approximately 3%/K for a Ge fraction of 32% is demonstrated. Quantum mechanical calculations show that a minimum
SiGe layer thickness of 8 nm is needed to avoid degradation caused by ground state shift due to carrier confinement in
the SiGe potential wells. In contrast to most of today's bolometer designs, the optical quarter wave cavity needed to
achieve high absorption of radiation is an integral part of the quantum well thermistor structure. Optimization of the full
bolometer design is made where the interaction between optical absorption, heat capacity and electrical properties is
considered and a design approach targeting the lowest noise equivalent temperature difference is presented. As part of
the optimization, it was found that for the best overall solution, optical absorption can be sacrificed in favor for a smaller
Cost efficient integration technologies and materials for manufacturing of uncooled infrared bolometer focal plane arrays
(FPA) are presented. The technology platform enables 320x240 pixel resolution with a pitch down to 20 μm and very
A heterogeneous 3D MEMS integration technology called SOIC (Silicon-On-Integrated-Circuit) is used to combine high
performance Si/SiGe bolometers with state-of-the-art electronic read-out-integrated-circuits.
The SOIC integration process consists of: (a) Separate fabrication of the CMOS wafer and the MEMS wafer. (b)
Adhesive wafer bonding. (c) Sacrificial removal of the MEMS handle wafer. (d) Via-hole etching. (e) Via formation and
MEMS device definition. (f) Sacrificial etching of the polymer adhesive. We will present an optimized process flow that
only contains dry etch processes for the critical process steps. Thus, extremely small, sub-micrometer feature sizes and
vias can be implemented for the infrared bolometer arrays.
The Si/SiGe thermistor is grown epitaxially, forming a mono-crystalline multi layer structure. The temperature
coefficient of resistance (TCR) is primarily controlled by the concentration of Ge present in the strained SiGe layers.
TCR values of more than 3%/K can be achieved with a low signal-to-noise ratio due to the mono-crystalline nature of the
material. In addition to its excellent electrical properties, the thermistor material is thermally stable up to temperatures
above 600 °C, thus enabling the novel integration and packaging techniques described in this paper.
Vacuum sealing at the wafer level reduces the overall costs compared to encapsulation after die singulation. Wafer
bonding is performed using a Cu-Sn based metallic bonding process followed by getter activation at ≥350 °C achieving a
pressure in the 0.001 mbar range. After assembling, the final metal phases are stable and fully compatible with hightemperature
processes. Hermeticity over the product lifetime is accomplished by well-controlled electro-deposition of
metal layers, optimized bonding parameters and a suitable bond frame design.
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
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).