Uncooled FIR-imagers decreased in pixel pitch from latest state-of-the-art value of 17 µm to 10 µm. Following this trend of a reduction of pixel size, Fraunhofer IMS provides a manufacturing process for FIR-imagers (IRFPAs) based on a scalable microbolometer technology. Beside conventional approaches of a thermal isolation of microbolometer membranes realized by lateral legs, Fraunhofer IMS developed a manufacturing process for a thermal isolation realized by nanotubes. To demonstrate the scalability of the nanotube-microbolometers the nanotube contact is applied to microbolometer membranes with 12, 10, 8 and 6 µm pixel size on top of a 17 µm digital readout integrated circuit (ROIC). The arrays are sealed by a chip-scale vacuum package to evaluate the microbolometers’ performance by means of a complete IRFPA. Quantitative measurement results for the responsivity as well as qualitative test pictures of the 12, 10 and 8 µm nanotube-microbolometers will be presented. A direct visual comparison in a test scene demonstrates no obvious decrease in sensitivity between 12 and 8 µm. Only at 6 µm pixel size a reduced sensitivity is observed. In summary, a fully working uncooled IRFPA with QQVGA resolution based on a 6 µm nanotube-microbolometer technology is presented here. The scalability of the nanotube-microbolometer technology from state-of-the-art pixel sizes down to 6 µm is demonstrated.
For some applications, a reliable detection of the distance of objects is necessary, even under strong environmental conditions. Commonly this includes sunlight, but fog, rain and snow cause interferences as well. For fast and reliable threedimensional monitoring of the environment, LiDAR is a key sensor technology. A light source, often in the near-infrared, emits a short light pulse and the time-of-flight of the photons reflected by an object is measured. This allows to calculate the distance by using the speed of light. In order to be able to ensure reliable detection despite possible interferences, we have set up a new measurement concept based on the existing time-gating. Thus, an area is covered by step-wise shifting of the measuring window. By accumulating different delays, the true distance to the object can be determined. An advantage of the method is that no information about the approximate position of the object has to be known in advance. In this paper we present measurement results with this method, which were taken in different environmental conditions. The method can be implemented in addition to already existing concepts and can therefore supplement them
A fast and reliable three dimensional monitoring of the environment is indispensable for robotics, automation engineering or autonomous driving. For these applications LiDAR is a key sensor technology. Normally a light source in the near infrared range is used, which is invisible to human eyes. High ambient light compared to the laser source intensity is a major problem for these systems. Therefore, a measurement concept to reduce the impact of ambient light is necessary. In this paper we present a measurement concept in which the full distance range is scanned and the probability to detect events from far objects is improved. The general problem is that a photon of the background illumination can be detected instead of the reflected laser signal which stops the measurement. The concept allows us to detect the received laser pulse buried in the superimposed background light easier and improve the measurement quality. This is possible due to the delayed start of the measurement and thus the selection of different measurement windows in which an earlier detection of the laser generated events is accessible. In consequence, the probability for receiving an unwanted ambient photon is reduced. For this technique no prior information about the object conditions or its rough distance is required and it can be applied in all situations of the direct time-of-flight measurement to cope with high ambient light. Hence it allows a reliable distance measurement at various ambient and target conditions.
This paper presents the results of high-performance infrared detectors (IRFPA – InfraRed Focal Plane Array) based on uncooled microbolometers with 17 μm and 12 μm pixel pitch and a chip-scale-package as the vacuum package developed and fabricated by Fraunhofer-IMS. Like CMOS image sensor IRFPAs also have been following the trend of reducing the pixel size in order to reduce the costs and increase the optical resolution. For microbolometer based uncooled IRFPA the pixel pitch has been reduced from 35 μm pixel pitch ten years ago via 25 μm and 17 μm down to 12 μm. Fraunhofer IMS has developed digital IRFPAs featuring a direct conversion of the microbolometer’s resistance into a 16 bit value by the use of massively parallel on-chip Sigma-Delta-ADCs achieving a high scene temperature dynamic range of more than 300 K and a very low NETD-value below 50 mK. Due to a broad-band antireflection coating the digital IRFPAs achieve a high sensitivity in the LWIR (wavelength 8 μm to 14 μm) and MWIR (wavelength 3 μm to 5 μm) range. In this paper the microbolometer, the vacuum-packaging, the architecture of the readout electronics, and the electro-optical performance characterization will be presented.
This paper presents the results of a high-performance digital QVGA-IRFPA based on uncooled microbolometers with a pixel-pitch of 17 μm and a chip-scale-package as the vacuum package developed and fabricated by Fraunhofer-IMS. Due to a direct conversion of the microbolometer’s resistance into a 16 bit value by the use of massively parallel on-chip Sigma-Delta-ADCs a high scene temperature dynamic range of more than 300 K and a very low NETD-value below 50 mK is achieved. Due to a broad-band antireflection coating the digital 17 μm QVGA-IRFPA achieves a high sensitivity in the LWIR (wavelength 8 μm to 14 μm) and MWIR (wavelength 3 μm to 5 μm) range. In this paper the microbolometer, the vacuum-packaging, the architecture of the readout electronics, and the electro-optical performance characterization will be presented.
The integration of silicon photomultiplier (SiPM) and frontend electronics in a suitable optoelectronic CMOS
process is a promising approach to increase the versatility of single-photon avalanche diode (SPAD)-based singlephoton detectors. By integrating readout amplifiers, the device output capacitance can be reduced to minimize the waveform tail, which is especially important for large area detectors (>10 × 10mm2). Possible architectures include a single readout amplifier for the whole detector, which reduces the output capacitance to 1:1 pF at minimal reduction in detector active area. On the other hand, including a readout amplifier in every SiPM cell would greatly improve the total output capacitance by minimizing the influence of metal routing parasitic capacitance, but requiring a prohibitive amount of detector area. As tradeoff, the proposed detector features one readout amplifier for each column of the detector matrix to allow for a moderate reduction in output capacitance while allowing the electronics to be placed in the periphery of the active detector area. The presented detector with a total size of 1.7 ♦ 1.0mm2 features 400 cells with a 50 μm pitch, where the signal of each column of 20 SiPM cells is summed in a readout channel. The 20 readout channels are subsequently summed into one output channel, to allow the device to be used as a drop-in replacement for commonly used analog SiPMs.
In this paper a novel concept for the fabrication of highly sensitive uncooled microbolometers is presented. The approach
is based on the realization of thermal isolation and simultaneous electrical contacting of the microbolometers by means
of sufficiently long and thin coated nanotubes, which can be fabricated by post processing on top of CMOS wafers
comprising the ROIC. Thus, the effective area of the absorption layer is maximized at a given pixel size, as lateral legs,
which have been the main component of the thermal isolation commonly, are completely omitted. The resulting thermal
conductivity can be tuned independently from the pixel size by varying the geometry and structuring of the nanotubes.
Based on test structures the nanotube microbolometers are characterized with respect to electro-optical and mechanical
properties. The focus in this paper is on nanotube microbolometers with a pixel size of 12 μm.
This paper deals with the characteristics of circular shaped polysilicon pressure sensor diaphragms operating in the non-tactile mode. Using a phase shifting interferometer the main characteristics of diaphragms were investigated under applied pressure with respect to sensitivity, initial deflection and cavity height. Diaphragms with a thickness of 1 μm and a diameter of 96 μm were investigated in an intended pressure range of applied pressure of about 700 – 2000 hPa. Process parameters with major impact on performance and yield limitations were identified. These include the variance in diaphragm sensitivity and the impact of the variance of the sacrificial oxide layer defining the diaphragm cavity height on the contact pressure point. The sensitivity of these diaphragms including the variance was found to be - 19.8 ± 1.3 nm per 100 hPa. The impact of variance in the cavity height on the contact pressure point was found to be about 3.7 ± 0.5 hPa per nm. Summarizing both impacts a maximum variation of the contact pressure point of more than 450 hPa is possible to occur considering a nominal deflection of 300 nm. By optimizing the process of diaphragm deposition the variance in the sensitivity of the diaphragm was decreased by a factor of 2. A semi – empirical formula was evaluated that describes the deflection including initial deflection due to intrinsic stress and the process variations. A validation to the experimental obtained deflection lines showed a good agreement with deviations of less than 2 % for radial ranges of maximum deflection.
This contribution describes the modeling of CMOS image sensors employed in time-of-flight (ToF) sensor systems for 3D ranging applications. Our model relies on the theoretical description of photo-generation, charge transfer including diffusion, fringing field, and self-induced drift (SID). This method makes it possible to calculate the time-dependent charge carrier generation, transfer, and distribution. The employed approach allows elimination not only of irradiance-dependent charge transfer, but also of undesired reflectance effects, and the influence of ambient light through an in-pixel background measurement. Since the sensor is operated with very short integration times it is crucial to accomplish a fast transfer of the generated charge from the photodetector to the sense node, and speedy conversion into an electrical signal at its output. In our case, we employed a lateral drift field photodetector (LDPD), which is basically a pinned photodiode with a built-in drift field formed by a doping gradient. A novel pixel structure is presented which is optimized for a fast charge transfer by the appliance of the shown model. Numerical calculations predict a two times faster charge collection.
This paper presents the results of an advanced digital IRFPA-family developed by Fraunhofer IMS. The IRFPA-family compromises the two different optical resolutions VGA (640 ×480 pixel) and QVGA (320 × 240 pixel) by using a pin-compatible detector board. The uncooled IRFPAs are designed for thermal imaging applications in the LWIR (8 .. 14μm) range with a full-frame frequency of 30 Hz and a high thermal sensitivity. The microbolometer with a pixel-pitch of 17μm consists of amorphous silicon as the sensing layer. By scaling and optimizing our previous microbolometer technology with a pixel-pitch of 25μm we enhance the thermal sensitivity of the microbolometer. The microbolometers are read out by a novel readout architecture which utilizes massively parallel on-chip Sigma-Delta-ADCs. This results in a direct digital conversion of the resistance change of the microbolometer induced by incident infrared radiation. To reduce production costs a chip-scale-package is used as vacuum package. This vacuum package consists of an IR-transparent window with an antireflection coating and a soldering frame which is fixed by a wafer-to-chip process directly on top of the CMOS-substrate. The chip-scale-package is placed onto a detector board by a chip-on-board technique. The IRFPAs are completely fabricated at Fraunhofer IMS on 8” CMOS wafers with an additional surface micromachining process. In this paper the architecture of the readout electronics, the packaging, and the electro-optical performance characterization are presented.
The performance of a fabricated CMOS line sensor based on the lateral drift-field photodiode (LDPD)1 concept is described. A new pixel structure was designed to decrease the charge transfer time across the photoactive area. Synopsys TCAD simulations were performed to design a proper intrinsic lateral drift-field within the pixel. The line sensor was fabricated in the 0.35 μm CMOS technology, and further characterized using a tailored photon-transfer method2 and the EMVA 1288 standard3. The basic parameters such as spectral responsivity, photo-response non-uniformity and dark current were measured at fabricated sensor samples. A special attention was paid to charge transfer time characterization4 and the evaluation of crosstalk between neighboring pixels – two major concerns attained during the development. It is shown that the electro-optical characteristics of the developed line sensor are comparable to those delivered by CCD line sensors available on the market, which are normally superior in performance compared to their CMOS based counterparts, but offering additional features such as the possibility of time gating, non-destructive readout, and charge accumulation over several cycles: approaches used to enhance the signal-to-noise ratio (SNR) of the sensor output.
This paper presents the improvements of an advanced digital VGA-IRFPA developed by Fraunhofer-IMS. The uncooled
IRFPA is designed for thermal imaging applications in the LWIR (8 .. 14 μm) range with a full-frame frequency of
30 Hz and a high sensitivity with NETD < 100 mK @ f/1. The microbolometer with a pixel-pitch of 25 μm consists of
amorphous silicon as the sensing layer. The structure of the microbolometer has been optimized for a better performance
compared to the 1st generation IRFPA1. The thermal isolation has been doubled by increasing the length and by
decreasing the width of the legs. To increase the fill-factor the contact areas have been reduced. The microbolometers are
read out by a novel readout architecture which utilizes massively parallel on-chip Sigma-Delta-ADCs. This results in a
direct digital conversion of the resistance change of the microbolometer induced by incident infrared radiation. Two
different solutions for the vacuum package have been developed. To reduce production costs a chip-scale-package is
used. This vacuum package consists of an IR-transparent window with antireflection coating and a soldering frame
which is fixed by a wafer-to-chip process directly on top of the read substrate. An alternative solution based on the use of
a standard ceramic package is utilized as a vacuum package. This packaging solution is used for high performance
applications. The IRFPAs are completely fabricated at Fraunhofer-IMS on 8" CMOS wafers with an additional surface
micromachining process.
This paper introduces a simple vacuum packaging method which is based on a Chip-to-Wafer process. The MEMS-device
is provided with an electroplated solder frame. A Si-lid with the same solder frame is mounted on each die of the
wafer using a flip chip process. The same materials for lid and substrate are used in order to reduce the mechanical stress
due to the same thermal coefficients of expansion. The resulting cavity between die and lid can be evacuated and
hermetically sealed with an eutectic soldering process. The feasibility of the method is demonstrated with an infrared
focal plane array (IR-FPA). In this case, the Si-lid acts as an optical window and contains an anti reflective layer for the
8-14 μm wavelength area on both sides. The long-term vacuum stability is supported by a getter film inside the package.
This method simplifies the sawing process and has the additional cost benefit that it is possible to package only known
good dies.
This paper presents an advanced 640 x 480 (VGA) IRFPA based on uncooled microbolometers with a pixel-pitch of
25μm developed by Fraunhofer-IMS. The IRFPA is designed for thermal imaging applications in the LWIR (8 .. 14μm)
range with a full-frame frequency of 30 Hz and a high sensitivity with NETD < 100 mK @ f/1. A novel readout
architecture which utilizes massively parallel on-chip Sigma-Delta-ADCs located under the microbolometer array results
in a high performance digital readout. Sigma-Delta-ADCs are inherently linear. A high resolution of 16 bit for a secondorder
Sigma-Delta-modulator followed by a third-order digital sinc-filter can be obtained. In addition to several thousand
Sigma-Delta-ADCs the readout circuit consists of a configurable sequencer for controlling the readout clocking signals
and a temperature sensor for measuring the temperature of the IRFPA. Since packaging is a significant part of IRFPA's
price Fraunhofer-IMS uses a chip-scaled package consisting of an IR-transparent window with antireflection coating and
a soldering frame for maintaining the vacuum. The IRFPAs are completely fabricated at Fraunhofer-IMS on 8" CMOS
wafers with an additional surface micromachining process. In this paper the architecture of the readout electronics, the
packaging, and the electro-optical performance characterization are presented.
In this paper we discuss methods to improve the geometric design of microbolometer pixels in uncooled focal plane
arrays. For cost reduction reasons, the pixel pitch of these microbolometer elements should be reduced as much as
possible while keeping the same level of performance. This becomes increasingly difficult once the dimensions of the
microbolometer elements reach a critical value of about 25 micrometers, mainly because the available space limits the
thermal isolation and the available area for IR absorption. For these reasons it is essential to optimize not only the
material properties but also the geometric aspects of the microbolometer structure to get the maximum performance for a
given size of the elements. Extending the work of Liddiard, in the first part of this paper we discuss the design of the
optical cavity, focussing mainly on the influence of the sacrificial layer thickness, which defines the properties of the
resulting Fabry Perot resonator. In the second part of this paper we concentrate on the geometry of the absorbing
membrane itself and give estimates for optimum film thickness and lateral dimensions.
Thermal IR imagers (bolometer arrays with resistive, ferroelectric or diode detector elements) require sophisticated circuitry to extract the signal out of the noisy background. Suitable models for circuit optimization with simulation tools like SPICE or SPECTRE are therefore inevitable. SPICE has the capability to model electrical and thermal circuits in the same model description. The models described here have a common thermal section, but differ in their electrical description. The thermal SPICE model uses a capacitor to model the thermal capacity of the sensing element, resistors for heat conductance due to radiation and along the supporting legs. The incoming radiation injects a current, as does the power dissipated in the sensor layer, resulting in a temperature rise of the sensor. Electrically the bolometer resistor is modeled via a non-linear dependent current source, changing with temperature, and emitting heat during readout. Noise is injected via dependant noise current sources, including white resistive and 1/f excess noise of the detector resistor and band limited thermal conductance noise of the detector. In the diode bolometer a non-linear temperature controlled diode model replaces the resistor. Shot and flicker noise sources are added. The pyroelectric detector is described by a non linear temperature dependant capacitor and a parallel resistor caused by dielectric losses. A chopper modulating the incoming radiation is required for signal detection.
This paper reports the results of the modeling silicon microsystem flow sensor based on Thermal Time-Of-Flight (TTOF) mode. The basic heat transfer equations and the modeling approach are first presented. The problem domain is decomposed into two subdomains which represent the fluid and the sensor chip structure, respectively. The thermal boundary layer where the interaction between the two subdomains is taking place is modeled using flow-dependent equivalent thermal resistance elements. The two subdomains and the boundary layer are subsequently implemented using the combination of SPICE and analog HDL. An experimental chip of silicon thermal flow sensor is used to validate the present model. The model has been used to predict the behavior of the flow sensor in free-running TTOF mode and also in Thermal-Convection Delay-Line Oscillator (TC-DLO) mode. Both the agreement and discrepancy found between the model and the experiments are shown and discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.