Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-10-07
2001-12-11
Dang, Hung Xuan (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S338400, C338S014000, C338S018000
Reexamination Certificate
active
06329655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to the field of radiation sensors. More particularly, the present invention relates to high sensitivity electromagnetic radiation sensors useful as passive imagers in the millimeter wave or microwave regimes.
2. Description of the Related Art
Radiation sensors are generally known which convert electromagnetic radiation energy absorbed by a radiation sensitive material into an electrical signal. Conventional thermal detection has included temperature sensitive capacitors or temperature sensitive resistors (also called bolometers) arranged in an array where a detector signal is coupled into a signal multiplexer. The amount of radiation received by a thermal detector is relatively small, which is critical, particularly when the detector is an uncooled type, and measures have had to be taken to decrease the heat capacity of the detector and prevent dissipation of the received radiation. To accomplish this, the prior art has used semiconductor fine patterning or micro-machining techniques to fabricate a radiation sensor having a thermally-isolated, thermally sensitive resistive or capacitive material portion suspended as a microbridge structure over a substrate which keeps the thermally sensitive material out of direct contact with the substrate. (See, for example, U.S. Pat. Nos. 4,574,263 and 5,302,933.)
Thermally isolated thermal detectors have been fabricated as an array of microbridges with a thermoresistive (thermo-capacitive) element in each microbridge. In the following, the reference to the “thermoresistive” phenomena or devices will also be understood to include thermo-capacitive phenomena or devices. The resistive microbolometers optimally have a high thermal coefficient of resistance and low thermal conductance between the absorbing area and a readout circuit that multiplexes the radiation signal. As each detector pixel absorbs the radiation being detected or monitored, the microbridge temperature changes accordingly and the elemental resistance is altered. For these arrangements, standard photolithographic techniques with selective etching have been used to pattern the thin film to form detectors for individual pixels of the array of detectors.
However, a problem associated with conventional microbolometer architectures involving thermally isolated detectors is the difficulty in achieving and maintaining an efficient coupling of some bands of electromagnetic radiation, such as millimeterwave (mm-wave) and microwave radiation, to a thermal detector and, in particular, to an array of thermal detectors, while maintaining the thermal isolation of the detector(s) needed for high sensitivity. U.S. Pat. No. 5,450,053, the teachings of which are incorporated herein by reference, describes incorporating antennas in microbolometer detector architectures used for IR/mm-wave detection to provide a mm-wave energy coupling apparatus. In one embodiment, U.S. Pat. No. 5,450,053 describes use of “bow-tie” microantenna designs formed on the backside of silicon substrate while IR sensitive microdetector arrays are formed on the opposite frontside of the substrate, such that incident radiation is collected by the antennas after passing through the substrate.
While U.S. Pat. No. 5,450,053 addresses the issue, a need remains in the art for a radiation sensor with increased sensitivity relative to prior designs.
SUMMARY OF THE INVENTION
The need in the art is addressed by the radiation sensor of the present invention. The inventive sensor comprises a thermally sensitive detector element that is efficiently coupled to an electromagnetic radiation field, via a receiver (e.g., an antenna), in a manner that endows the sensor with increased sensitivity.
The radiation sensor of the invention has a two-level detector structure formed on a substrate. A thermal detector element is suspended over the substrate as a microbridge structure. The detector has a sandwich structure of a heater metal film, a dielectric layer, and a thin film thermoresistive material (i.e., a thermally responsive resistive material). The heater metal film is maintained out of physical contact with a receiver of electromagnetic radiation. The receiver is provided on the same side of the substrate in a manner which efficiently couples the radiation field to the thermal detector element.
In one embodiment, the inventive radiation sensor has a receiver of electromagnetic radiation that is an antenna having a unique and improved shape for coupling a radiation field to the thermal detector element. Namely, the improved antenna shape of the antenna is defined by two constituent micro-antennas which are orthogonally-oriented to one another in a manner enabling interception of electromagnetic radiation of both polarities. Preferably, the antenna shape is represented by a “crossed bowtie” configuration in which each constituent bowtie micro-antenna of the antenna has first and second conductive arms terminating at inner (output) ends separated from each other by a lateral gap. Thus, each constituent bowtie micro-antenna of the antenna is missing a “knot”, so to speak. This is done so that the radiation sensor can be capacitively coupled to the radiation field by arranging the inner ends of the pair of constituent microantennas comprising each antenna as underlapping part of a heater metal layer provided on the underside (or, alternatively, above or on both sides) of the thermal detector element. The resistance of the heater metal layer and the capacitance from the antenna underlap are selected so that the total impedance of the series capacitance-resistance-capacitance is matched as much as possible to the antenna impedance for efficient coupling, and the capacitive impedance is preferably smaller than the dissipative metal impedance on the bridge. Other symmetrical crisscrossed antenna shapes besides the crossed bowties, such as a logarithmic-curve side profile and so forth, are also contemplated for practice of this invention that provides the coupling function. Preferably, the antenna is formed as planar, thin conductive film upon a dielectric layer that rests on a semiconductor body.
The performance of the inventive sensor as passive mm-wave sensor is significantly enhanced by such a crossed bowtie antenna shape and the like specifically in case of receiving natural passive radiation which is randomly polarized, i.e., has equal components in both polarization directions. The sensor devices of this invention are especially well-suited for implementation in the mm-wave (e.g., 3 mm at 94 GHz) regime. Also, the inventive sensors have versatile implementation capabilities as they can be implemented in either a capacitively coupled mode, a resistively coupled mode, or an inductively coupled mode.
As will be apparent from the descriptions herein, this invention provides an enhanced sensor architecture and geometry compatible with semiconductor VLSI processing which enables high efficiency non-contact coupling between a receiver of electromagnetic radiation, for example, a thin film antenna, and a thermal detector, for example, a bolometer, pyroelectric or thermopile. Simultaneously, this invention provides for efficient dissipation of the coupled energy within the thermally isolated bridge. The inventive sensor can be used as an individual thermal pixel or in linear or two-dimensional arrays thereof. The invention also is directed to a method of fabricating such a radiation sensor.
REFERENCES:
patent: 4654622 (1987-03-01), Foss et al.
Dolezal Franklin A.
Fetterman Harold
Grinberg Jan
Jack Michael D.
Ray Michael
Dang Hung Xuan
Lenzen, Jr. Glenn H.
Raytheon Company
Schubert William C.
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