Dirk Weiler

Theoretical and practical minimum of the power Consumption of 3 ADCs in SC technique

In this paper the theoretical and practical minimum of the power consumption is investigated for 3 ADC architectures, which are predestined for low conversion speed and applications in low-power sensor readout circuits. The analysis is made for the switched capacitor versions of the SAR ADC, cyclic ADC and sigma-delta modulator. The theoretical limit of the power efficiency is investigated by a commonly used figure of merit (FOM). As a case study two low-power ADCs were designed and fabricated. In order to compare ADCs fabricated in different technologies a technology-independent FOM is introduced.

Uncooled digital IRFPA-family with 17μm pixel-pitch based on amorphous silicon with massively parallel Sigma-Delta-ADC readout

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.

A Very Low-Power CMOS 11-b Cyclic A/D Converter with Mismatch Compensation

A 11-b CMOS cyclic A/D converter with very low-power consumption is presented. A method to reduce the effect of component mismatch is introduced, which significantly improves the accuracy. The improvement has been confirmed by simulations

High-performance uncooled digital 17 μm QVGA-IRFPA-using microbolometer based on amorphous silicon with massively parallel Sigma-Delta-ADC readout

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.

Digital uncooled IRFPAs based on microbolometers with 17 µm and 12 µm pixel pitch

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.

Automating wafer-level test of uncooled infrared detectors using wafer-prober

Fraunhofer IMS develops and fabricates far-infrared focal plane arrays (IRFPA) using microbolometers with a pixel pitch of 17μm technology realized on top of a 0.35 μm CMOS readout integrated circuit (ROIC). The microbolometers are encapsulated by a Chip-Scale-Package (CSP) to ensure a high quality vacuum level. The CSP is realized by placing an infrared transparent lid above a solder frame surrounding the microbolometer array. To concept a wafer-level test it is very challenging to implement highly accurate electrical stimuli and a far infrared radiation source (black body) while affecting the wafer-prober handling by a non-flat wafer surface, due to the infrared transparent lids of the CSP. Accordingly, wafer-level test has been developed based on a PC which controls, by using a test program, the wafer handling of a prober, the electrical stimuli of a test hardware, and the far-infrared radiation such as the optical stimuli. Thus, the most important electro-optical parameters of IRFPAs will be measured at wafer-level: Noise Equivalent Temperature Difference (NETD), responsivity, and the percentage of the defective pixels.

Ungekühlte Mikrobolometer-Arrays mit einer Pixelgröße von 12 µm basierend auf einer neuartigen thermisch isolierenden Struktur

Zusammenfassung In diesem Paper wird ein innovatives Konzept zur Herstellung von hochempfindlichen ungekühlten Mikrobolometern, zur Detektion von langwelliger Infrarotstrahlung (IR-Strahlung) in einem Wellenlängenbereich von 8 μm–14 μm, beschrieben. Der Ansatz basiert auf der Realisierung der thermischen Isolierung und gleichzeitiger elektrischer Kontaktierung der Mikrobolometer mit Hilfe von ausreichend langen und dünnbeschichteten Hohlröhrchen (hier als Nanotubes bezeichnet), welche mit Technologien und Prozessen aus der Mikrosystemtechnik hergestellt werden können. Somit wird der relative Flächenanteil des Absorbers bei einer gegebenen Pixelgröße maximiert, da laterale Stege, welche bislang Hauptbestandteil der thermischen Isolierung waren, komplett entfallen. Der resultierende thermische Leitwert kann über die einzelnen Schichtdicken, Grundradius und Länge der Nanotubes flexibel und unabhängig von der Pixelgröße eingestellt werden. Die gefertigten Nanotube-Mikrobolometer werden zunächst anhand von Teststrukturen im Hinblick auf die elektro-optischen und mechanischen Eigenschaften grundlegend charakterisiert. Der Fokus liegt in dieser Arbeit auf Pixelgrößen von 12 μm.

Measurement results of a 12 μm pixel size microbolometer array based on a novel thermally isolating structure using a 17 μm ROIC

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.

Simulation method for LWIR radiation distribution using a visual ray-tracer

Infrared cameras with passive, uncooled sensor chips utilize the longwave-infrared (LWIR) range of the electromagnetic spectrum with wavelength between 8 and 14μm for image generation. The reason for this is that every object is self-luminous at room temperature at that wavelength. Therefore, every surface acts as a source of radiation in an LWIR scenario. To gain an impression and to model the effects and circumstances in an infrared scenario, a simulation method is required. In the visual domain this task is accomplished by ray-tracing software, which allows the generation of synthetic images as well as the analysis of irradiance distribution in a given scene. In this paper we demonstrate a way to apply one of such ray-tracers to a LWIR scenario. We will also demonstrate an application of the proposed simulation method.

A Far Infrared VGA Detector Based on Uncooled Microbolometers for Automotive Applications

Warm bodies like humans or animals emit radiation in the long-wave infrared band (8 to 14 μm) which can be used for pedestrian detection in an automotive application. Fraunhofer-IMS has developed an advanced 640 x 480 (VGA) IR detector (IRFPA=infrared focal plane array) based on uncooled micro bolometers with a pixel-pitch of 25μm. The IRFPA is designed for thermal imaging applications with a full-frame frequency of 30 Hz and a high sensitivity with a NETD < 100 mK @ f/1. The microbolometer as the sensing element is based on amorphous silicon as the sensing layer. A novel readout architecture which utilizes massively parallel on-chip Sigma-Delta-ADCs located under the microbolometer array re sults in a high performance digital readout. Since packaging is a significant part of a 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.