Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-06-14
2001-10-23
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
Reexamination Certificate
active
06307202
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to thermal sensors for detecting thermal radiation, and in particular to thermal sensors using bimorph spirals for uncooled imaging of thermal radiation.
BACKGROUND AND DESCRIPTION OF PRIOR ART
Recent advances in uncooled detectors for thermal radiation or infrared (IR) radiation have resulted in thermal imaging systems with excellent properties. The most recent systems achieve a sensitivity that approaches the thermal limit and operate at speeds of 60 frames per second. More information about these systems can be found in P.W. Kruse, Proceedings 1996, SPIE Infrared Detectors and Focal Plane Arrays IV (1996), p. 34; C. Marshall et al., Proceedings 1996 SPIE Infrared Detectors and Focal Plane Arrays IV (1996), p. 23; and W. Radford et al. Proceedings 1996 SPIE Infrared Detectors End Focal Plane Arrays IV (1996), p. 82.
In these prior art systems images with high spatial resolution are obtained from arrays containing as many as 400 pixels per mm
2
by reading electronic signals generated at each thermal radiation detector. In order to process this information, multiple transistors are integrated into each pixel and a separate device is used to display the image.
The prior art also teaches a direct view system where the thermal radiation is converted to a visible image thus eliminating the need for a complex readout system. Most advanced direct view systems use cantilever bimorph elements which deform in response to thermal radiation. Specifically, a microfabricated bimorph cantilever beam is constructed of two materials that have different coefficients of thermal expansion (CTE). A change in temperature causes the materials to expand or contract by different amounts causing the beam to bend or deform. This deformation can be observed by reflecting visible light, diffracting light from a number cantilevers or by employing any other well-known technique, e.g., from among the ones used in the field of Atomic Force Microscopy. Temperature changes on the order of 10
−5
K. can be detected using bimorph cantilevers for such photothermal spectroscopy. Information about the basic concepts of photothermal spectroscopy using bimorph cantilevers is found in J. K. Gimzewski et al., Chemical Physics Letters, No. 217, 1994, pp. 589.
Further advances in bimorph cantilevers for photothermal spectroscopy are described in publications by J. R. Barnes et al., Nature, No. 372, (1994), pp. 79; P. G. Datskos et al., Applied Physics Letter, Vol. 69, (1996), pp. 2986 and P. I. Oden et al., Uncooled Thermal Imaging Using a Piezoresistive Microcantilever”, Applied Physics Letters, Vol. 69, (1996), pp. 3277. These teachings detail how microfabricated cantilevers can be coated with metal to form a bimorph and used for photothermal spectroscopy with a power resolution of 1 nW/Hz
−½
. Furthermore, Oden et al. teach that two dimensional arrays of heat sensitive cantilevers can serve as thermal imaging devices. This photothermal technique has proved effective in measuring the power of radiation ranging from ultra-violet (UV) to IR with high sensitivity.
One of the most important parameters relating to thermal sensitivity in cantilever-based photothermal spectroscopy is the length of the beam. In most prior art devices the beam length ranges between 200-400 &mgr;m. When such beams are placed in a two-dimensional array for imaging applications, the density of beams in the direction of the beam is limited to only a few per millimeter. It is possible to obtain a reasonable density of beams in the direction perpendicular to the cantilever. Unfortunately, developing a sensitive detection system for closely spaced sensors is difficult.
OBJECTS AND ADVANTAGES OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide bimorph elements for photothermal spectroscopy which overcome the limitations of cantilever beams. In particular, the bimorph elements of the invention can be arranged in a dense two-dimensional array and yield the same resolution in both directions.
It is a further object of the invention to provide bimorph elements whose deformation can be easily and accurately measured by photometric techniques, thus providing for high measurement sensitivity.
It is yet another object of the invention to ensure that such bimorph elements can be easily manufactured in arrays and to provide an efficient apparatus for detecting and measuring thermal radiation using these bimorph elements.
The above objects and advantages, as well as numerous improvements attained by the apparatus and method of the invention are pointed out below.
SUMMARY
The objects and advantages are secured by a bimorph spiral which exhibits a shape-altering response to thermal radiation. The bimorph spiral is dimensioned to have a focussing effect on a light, such as a visible light. In fact, the bimorph spiral acts almost like a Fresnel element—it is a quasi-Fresnel element—or a Fresnel zone plate with respect to the light. The focussing effect varies as the shape of the bimorph spiral changes due to absorption of thermal radiation.
In order to improve the efficiency of the bimorph spiral it is preferable to add a heat absorbing material to the spiral. In this manner the absorption of the thermal radiation is facilitated. The heat absorbing material can be deposited on a surface of the spiral, e.g., the surface directly exposed to the thermal radiation.
In an apparatus employing an array of bimorph spirals for photothermal spectroscopy it is advantageous to include an electrically conducting material in each bimorph spiral, e.g., the conducting material can be one of the bimorph materials. The conducting material is used to pass a current through the bimorph spiral or to apply a voltage between the bimorph spiral and its support structure. By doing this a bias force F
b
can be applied to the bimorph spiral to further improve its efficiency in detecting the thermal radiation.
In one embodiment of an apparatus for photothermal spectroscopy the bimorph spirals are illuminated by light generated by a light source, e.g., a coherent light source such as a laser. The apparatus also has a detector for detecting the light. The detector can be a charge-coupled device (CCD) or any other suitable photodetector. Additional light filtering devices, such as spatial light modulators or aperture plates can be positioned in the path of the light to perform filtering and other optical functions.
During measurement each of the bimorph spirals is exposed to the thermal radiation. Depending on the quantity of radiation absorbed, the spiral changes its shape and hence alters its focussing effect on the light by which it is illuminated. The detector can detect either the light which is transmitted by the bimorph spiral or reflected from it. In measuring reflected light, the detector can also measure light which is reflected from the spiral's support structure and passes through it.
The particulars of the invention and its various embodiments are described in detail in the detailed description section with reference to the attached drawing figures.
REFERENCES:
patent: 3781955 (1974-01-01), Lavrinenko et al.
patent: 4355898 (1982-10-01), Dakin
patent: 07307500-A (1995-11-01), None
P. I. Oden et al., Uncooled Thermal Imaging Using a Piezoresistive Microcantilever, Applied Physics Letters vol. 69, (Nov. 1996), pp. 3277-3279.
Paul W. Kruse,The Design of Uncooled Infrared Imaging Arrays, Proceedings-SPIE-The International Society for Optical Engineering, Infrared Detectors and Focal Plane Arrays, vol. 2746, Jun. 1996, pp. 34-37.
Charles Marshall, Neal Butler, Richard Blackwell, Robert Murphy, Tom Breen,Uncooled Infrared Sensor with Digital Focal Plane Array, Proceedings-SPIE-The International Society for Optical Engineering, Infrared Detectors and Focal Plane Arrays, Jun. 1996, vol. 2746, pp. 23-31.
W. Radford, D. Murphy, M. Ray, S. Propst, A. Kennedy, J. Kojiro, J. Woolaway, K. Soch, et al., 320X240Silicon Microbolometer Uncooled IRFPAs with On-Chip Offset Correction,
Manalis Scott R.
Minne Stephen C.
Quate Calvin F.
Hannaher Constantine
Lumen Intellectual Property Services Inc.
The Board of Trustees of the Leland Stanford Junior University
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