Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1998-12-10
2001-01-23
Ham, Seungsook (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S370080, C250S338400
Reexamination Certificate
active
06177673
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention is related to a structure for manufacturing and using a fabrication technique of micro-electronics so as to include an optically black surface adapted to function as an absorber or emitter over a predetermined wavelength range.
An optically black surface can be used as an absorber of a limited wavelength range in detectors of optical radiation and an emitting surface in thermal emitters of the optical wavelength range. Particularly in infrared detectors of the bolometer and thermopile type, there is a need for a surface capable of efficiently absorbing radiation over a wide optical wavelength spectrum. Correspondingly, infrared radiation emitters require a surface with a high emissivity. The principle of reciprocity says that a good absorber also is a good emitter, thus making the same surface suitable for use in both applications.
In the context of the present invention, a black surface functioning over a predetermined wavelength range must be understood to refer to a surface of high absorptance over said wavelength range. Respectively, a white surface functioning over a predetermined wavelength range must be understood to refer to a surface of high reflectance over said wavelength range. When such an optically black surface is desired to be used in a detector functioning over said wavelength range, the ideal surface of the detector behaves as a black surface over said wavelength range and as a white or transparent surface outside said wavelength range. Thus, wavelengths falling outside said wavelength range cannot disturb the measurement to be performed.
In conventional techniques, the manufacture of broadband absorbers has been carried out using polymers. This kind of absorber has a thin polymer film deposited on, e.g., a bismuth layer. Additionally, the base polymer could have been blended with absorption-improving agents such as carbon black particles. While polymer-based absorbers have been easy and economical to manufacture, they have also been hampered by a number of drawbacks. The polymers used as the absorber layers have been sensitive to their operating environment, particularly to moisture, and the performance of polymer absorbers have remained far from perfect. Furthermore, the thermal mass of detectors has been relatively large, thus making detectors of the polymer-based absorber type relatively slow by their response speed. An additional disadvantage of polymer films has been their poor high-temperature performance, which excludes their use as an emitting surface in heatable IR emitters.
Also absorber and emitter components are known based on semiconductor technology. In a paper written by K. C. Liddiard in the publication Infrared Physics, 1993, Vol. 34, 4, p. 379 ff., is described a multilayer film structure in which the uppermost layer is a semitransparent metallic thin film is provided with, thereunder a lossless dielectric layer and a lowermost metallic thin film acting as an infrared mirror. The multilayer structure is grown on an unthinned glass substrate. The basic disadvantage of this structure is its slow response and low sensitivity, both resulting from its relatively large thermal mass. The structure is further characterized by a substantially high loss of heat by conduction into the substrate. The semitransparent metallic thin film is difficult to produce to a correct thickness, and moreover, is readily destroyed when serving as the outer surface of the detector.
In a paper published by L. Dobrzanski et al. in the publication Proc. Euro-sensors X, 1996, Löwen, p. 1433 ff., is described a structure further developed from the above-described type by having the absorber deposited on a 100-200 &mgr;m thick silicon wafer. In the structure, there is first deposited on a silicon wafer a 0.2-1.5 &mgr;m thick, lossless film of silicon nitride, and next thereon, a 0.1-1.5 &mgr;m thick, lossy film of doped polycrystalline silicon. The reason for selecting polycrystalline silicon as the top layer material is because of its good performance at elevated temperatures and relatively high temperature coefficient of resistivity. Under the silicon and silicon nitride layers is produced an infrared-reflective mirror by sputtering a layer of tungsten or a nickel-chromium alloy from below via openings made into the substrate.
The above-described structure has a number of disadvantages. The bottomside metallization layer of the component permits high lateral conductivity of heat into the substrate. Since the metallization layer has no protective film thereon, the structure is also unsuitable for use in thermal emitters. Moreover, simultaneous thermal and optical optimization of layer thicknesses in the structure is impossible.
SUMMARY AND OBJECTS OF THE INVENTION
It is an object of the present invention to overcome the drawbacks of the above-described techniques and to provide an entirely novel type of optically black surface for emitting and absorbing components, as well as a method for manufacturing said surface.
The goal of the invention is achieved by optically matching a lossy metallic surface over a desired wavelength range to its surrounding medium by means of a multilayer structure formed by a doped semiconductor material layer such as a silicon or germanium layer with a dielectric layer or layers. Thus, the metallic mirror surface is prevented from acting as a mirror over said desired wavelength range. Hence, the invention is also characterized in that a metallic mirror surface, which is optically matched to its surrounding medium over a desired wavelength range, is made to act as an optically black surface. Then, radiation imposed over said wavelength range on said metallic mirror is optically matched with said optically black surface and will be absorbed almost completely in the mirror material and the lossy material used as the matching layer. Reversely, the invention may also be utilized to emit radiation over a desired wavelength band. The invention is further characterized in that the location of the absorption band of said optically black surface can be shifted along the wavelength axis by altering the doping of said semiconductor layer.
In absorber use, the invention is different from prior-art components based on semiconductor techniques therein that, by virtue of the optical matching, the device according to the invention permits a major fraction of the radiation energy to be absorbed to reach the metallic mirror. This kind of optical matching can be accomplished already by using a single layer of optically functional thickness. By contrast, the above-described absorber disclosed by Liddiard is based on the use of a reversed antireflection coating, whereby the fraction of radiation passing through the semitransparent mirror layer is reflected back from the second metallic mirror layer and, by virtue of a phase shift in the lossy layer, at least partially cancels the radiation reflected from the semitransparent mirror layer. The absorber disclosed by Dobrzanski is optically different from Liddiard's absorber in that the semitransparent mirror forming the top layer in Liddiard's structure is replaced in Dobrzanski's absorber by an 0.1-1.5 &mgr;m thick, doped polycrystalline silicon layer. However, the principle of antireflection is the same as used by Liddiard, and also in this absorber the absorption is achieved in a different manner from that of the present invention by being chiefly concentrated in layers deposited above the metallic mirror layer of the device bottom surface.
More specifically, the structure according to the invention includes a metallic layer that is positioned on an upper surface of a support layer. A lossy layer is made from a doped semiconductor material. The thickness and doping of the lossy layer are proportioned to each other so that the structure formed by the metallic mirror layer with its overlying multilayer structure will be optically matched over the predetermined wavelength range of absorption or emission to the medium surrounding the structu
Blomberg Martti Juhani
Lehto Ari
Torkkeli Altti Kaleva
Birch & Stewart Kolasch & Birch, LLP
Ham Seungsook
Hanig Richard
Valtion teknillinen tutkimuskeskus
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