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.
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