Patent application title: UNCOOLED MICROBOLOMETER DETECTOR AND ARRAY FOR TERAHERTZ DETECTION

Inventors: 
Assignees:  Institut National D'Optique 
IPC8 Class: AG01J520FI 
USPC Class: 2503384 
Class name: Invisible radiant energy responsive electric signalling infrared responsive semiconducting type 
Publication date: 2014-06-19 
Patent application number: 20140166882

Read more: http://www.faqs.org/patents/app/20140166882#ixzz35CxuyrW6

An uncooled bolometer detector that includes a substrate, a platform held above the substrate by a support structure, at least one thermistor provided on the platform, and an optical absorber. The optical absorber includes at least one electrically conductive layer extending on the platform over and in thermal contact with the at least one thermistor and patterned to form a resonant structure defining an absorption spectrum of the uncooled microbolometer detector. The optical absorber is exposed to electromagnetic radiation and absorbs the electromagnetic radiation according to the absorption spectrum. A microbolometer array including a plurality of uncooled microbolometer detectors arranged in a two-dimensional array is also provided. Advantageously, these embodiments allow extending the absorption spectrum of conventional infrared uncooled microbolometer detectors to the terahertz region of the electromagnetic spectrum.

Claims:

1. An uncooled microbolometer detector comprising: a substrate; a platform held above the substrate by a support structure; at least one thermistor provided on the platform; and an optical absorber comprising at least one electrically conductive layer extending on the platform over and in thermal contact with the at least one thermistor and patterned to form a resonant structure defining an absorption spectrum of the uncooled microbolometer detector, the optical absorber being exposed to electromagnetic radiation and absorbing the electromagnetic radiation according to the absorption spectrum. 

2. The uncooled microbolometer detector according to claim 1, wherein the at least one electrically conductive layer comprises a first electrically conductive layer patterned to form a capacitive structure and a second electrically conductive layer patterned to form an inductive structure. 

3. The uncooled microbolometer detector according to claim 2, wherein the optical absorber further comprises an electrically insulating layer disposed between the first and second electrically conductive layers. 

4. The uncooled microbolometer detector according to claim 2, wherein: the capacitive structure comprises a cross-shaped slot patterned through the first electrically conductive layer; and the inductive structure comprises inductive elements patterned in the second electrically conductive layer, the inductive elements being electromagnetically coupled to the cross-shaped slot. 

5. The uncooled microbolometer detector according to claim 4, wherein the cross-shaped slot has four arms extending outwardly from a center thereof, with adjacent arms extending at right angles relative to each other, and wherein each inductive element extends substantially orthogonally across a corresponding one of the four arms of the cross-shaped slot. 

6. The uncooled microbolometer detector according to claim 1, wherein each of the least one thermistor comprises a thin film of one of vanadium oxide and amorphous silicon. 

7. The uncooled microbolometer detector according to claim 1, wherein the optical absorber is configured to absorb the electromagnetic radiation in a wavelength range of between about 30 and 3000 micrometers. 

8. The uncooled microbolometer detector according to claim 1, wherein the absorption spectrum of the uncooled microbolometer detector comprises a plurality of absorption bands. 

9. The uncooled microbolometer detector according to claim 1, further comprising a radiation reflecting mirror provided on the substrate and disposed under the platform. 

10. The uncooled microbolometer detector according to claim 1, further comprising a spectral filter supported above the platform in a spaced relationship therewith and configured to pre-filter the electromagnetic radiation before the electromagnetic radiation impinges onto the optical absorber. 

11. The uncooled microbolometer detector according to claim 10, wherein the spectral filter comprises a low-pass filter. 

12. The uncooled microbolometer detector according to claim 11, wherein the low-pass filter is a capacitive filter. 

13. A microbolometer array comprising a plurality of uncooled microbolometer detectors according to claim 1, wherein the plurality of microbolometer detectors is arranged in a two-dimensional array. 

14. The microbolometer array according to claim 13, wherein the plurality of uncooled microbolometer detectors is divided in a plurality of subsets of uncooled microbolometer detectors, the absorption spectrum of the uncooled microbolometer detectors of each subset being different from one another.

Description:

FIELD OF THE INVENTION 

[0001] The present invention relates to the field of uncooled microbolometer detectors, and more particularly concerns an uncooled microbolometer detector suitable for absorption and detection of terahertz radiation, and an array including a plurality of the same. 

BACKGROUND OF THE INVENTION 

[0002] Thermal detectors operate by absorbing energy from electromagnetic radiation incident thereonto and by converting the heat thus generated into an electrical signal representative of the amount of absorbed radiation. Perhaps the most prominent type of thermal detectors currently available is uncooled microbolometer detectors, usually shortened as microbolometers. A microbolometer is typically based on a suspended platform or bridge structure having a low thermal mass and on which is disposed a material having a temperature-dependent electrical resistance. The platform is generally held above and thermally insulated from a substrate by a support structure, and is provided with a thermistor, which is the resistive element whose electrical resistance changes in response to temperature variations caused by the absorbed radiation. The thermistor may, for example, be composed of a material having a high temperature coefficient of resistance (TCR) such as vanadium oxide and amorphous silicon. 

[0003] Microbolometers are capable of operating at room temperature. Because they do not require cryogenic cooling, may be integrated within compact and robust devices that are often less expensive and more reliable than those based on cooled detectors. 

[0004] Arrays of uncooled microbolometer detectors may be fabricated on a substrate using common integrated circuit fabrication techniques. Such arrays are often referred to as focal plane arrays (FPAs). In most current applications, arrays of uncooled microbolometers are used to sense radiation in the infrared portion of the electromagnetic spectrum, usually in the mid-wave infrared, encompassing wavelengths of between about 3 and 5 μm (micrometers), or in the long-wave infrared, encompassing wavelengths of between about 8 and 14 μm. 

[0005] Such arrays are often integrated in uncooled thermal cameras for sensing incoming infrared radiation from a target scene. Each microbolometer detector of the array absorbs some infrared radiation resulting in a corresponding change in the microbolometer detector temperature, which produces a corresponding change in electrical resistance. A two-dimensional pixelated thermal image representative of the infrared radiation incident from the scene can be generated by converting the changes in electrical resistance of each microbolometer detector of the array into an electrical signal that can be displayed on a screen or stored for later viewing or processing. By way of example, state-of-the-art arrays of infrared uncooled microbolometer detectors now include 1024 by 768 pixel arrays with a 17-μm pixel pitch. 

[0006] In the last decade, there has been a growing interest toward extending uncooled microbolometer spectroscopy and sensing applications beyond the traditional infrared range, namely in the far-infrared and terahertz (or sub-millimeter) spectral regions. As known in the art, these regions of the electromagnetic spectrum have long been relatively unused for industrial and technological purposes due to the lack of efficient techniques for detection and generation of radiation in this frequency range. 

[0007] In this context, extending the absorption spectrum of uncooled microbolometers beyond 30 μm is not straightforward, since the materials used to fabricate the detectors absorb predominantly in the infrared, and because the pitch of terahertz-sensitive pixels is typically larger than that of infrared-sensitive pixels to avoid diffraction effects. In addition, to maximize radiation absorption in the desired spectral band, conventional infrared microbolometer detectors generally include a reflector deposited on the underlying substrate to form a quarter-wavelength Fabry-Perot optical resonant cavity with the suspended platform. However, forming such a quarter-wavelength resonant cavity is generally not practical from the point of view of surface micromachining techniques used in the microfabrication of uncooled microbolometer detecting electromagnetic radiation at wavelengths longer than 10 μm. 

[0008] Therefore, there remains a need in the art for an uncooled microbolometer detector capable of absorbing electromagnetic radiation in the terahertz and far-infrared regions, while retaining at least some of the advantages of infrared detector technology in terms of cost, reliability, ease of fabrication, and maturity of the field. 

SUMMARY OF THE INVENTION 

[0009] According to an aspect of the invention, there is provided an uncooled microbolometer detector. The uncooled microbolometer includes: 

[0010] a substrate;

[0011] a platform held above the substrate by a support structure;

[0012] at least one thermistor provided on the platform; and

[0013] an optical absorber including at least one electrically conductive layer extending on the platform over and in thermal contact with the at least one thermistor and patterned to form a resonant structure defining an absorption spectrum of the uncooled microbolometer detector, the optical absorber being exposed to electromagnetic radiation and absorbing the electromagnetic radiation according to the absorption spectrum. 

[0014] In some embodiments, the at least one electrically conductive layer includes a first electrically conductive layer patterned to form a capacitive structure and a second electrically conductive layer patterned to form an inductive structure. In such embodiments, the absorption spectrum of the uncooled microbolometer detector can be controlled by adjusting the geometric properties of the pattern defined in the electrically conductive layers and the materials composing the same. 

[0015] In some embodiments, the optical absorber is preferably configured to absorb the electromagnetic radiation in a wavelength range from about 30 to 3000 micrometers, corresponding to the terahertz region of the electromagnetic spectrum. 

[0016] In some embodiments, the uncooled microbolometer detector further includes a spectral filter supported above the platform in a spaced relationship therewith. In such embodiments, the spectral filter is configured to pre-filter the electromagnetic radiation before the electromagnetic radiation impinges onto the optical absorber. In some embodiments, the spectral filter comprises a low-pass filter, preferably a capacitive structure, which prevents electromagnetic radiation with frequencies above a certain cutoff frequency from reaching the optical absorber. 

[0017] According to another aspect of the invention, there is provided a microbolometer array including a plurality of uncooled microbolometer detectors as described above, wherein the plurality of uncooled microbolometer detectors is arranged in a two-dimensional array. 

[0018] In some embodiments, the plurality of uncooled microbolometer detectors is divided in a number of subsets of uncooled microbolometer detectors, the absorption spectrum of the uncooled microbolometer detectors of each subset being different from one another. 

[0019] Other features and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings. 


Read more: http://www.faqs.org/patents/app/20140166882#ixzz35CxGaxeJ