Microbolometer optical cavity tuning and calibration systems and methods

Systems and methods are disclosed herein, as an example, to provide microbolometer resonant cavity tuning techniques and calibration techniques in accordance with one or more embodiments of the present invention. For example, in accordance with one embodiment, a method of operating an array of microbolometers on a substrate of an infrared camera system includes filtering infrared radiation to pass a first infrared radiation wavelength and to block a second infrared radiation wavelength, wherein the first infrared radiation wavelength is different than the second infrared radiation wavelength; setting a spacing between the microbolometers and the substrate to approximately tune the microbolometers to the second infrared radiation wavelength which is blocked by the filtering; and determining calibration data for the microbolometers.

TECHNICAL FIELD

The present invention relates generally to infrared cameras and, more particularly, to techniques for tuning microbolometer optical cavities and calibrating for infrared camera applications.

BACKGROUND

The microbolometer (or bolometer) operates on the principle that the electrical resistance of the microbolometer material changes with respect to the microbolometer temperature, which in turn changes in response to the quantity of absorbed incident infrared radiation. These characteristics may be exploited to measure incident infrared radiation on the microbolometer by sensing the resulting change in its resistance.

Modern microbolometer structures are typically fabricated on monolithic silicon substrates to form an array of microbolometers, with each microbolometer functioning as a pixel to produce a two-dimensional image. The change in resistance of each microbolometer is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC), which is typically formed within the silicon substrate upon which the microbolometer array is fabricated. The combination of the ROIC and the microbolometer array is commonly known as a microbolometer focal plane array (FPA) or microbolometer infrared FPA.

The microbolometer is generally thermally isolated from its supporting substrate or surroundings by forming an air-bridge structure (microbolometer bridge or microbridge) to allow the absorbed incident infrared radiation to generate a temperature change in the microbolometer material. For example, a conventional microbolometer array structure may be formed as a two-dimensional array of closely spaced air-bridge structures that are coated with a temperature sensitive resistive material, such as vanadium oxide, that absorbs infrared radiation.

The conventional air-bridge structure (which may refer to and be implemented as a vacuum-gap structure) provides good thermal isolation between the microbolometer and the silicon substrate and also forms a resonant cavity structure for improved infrared absorption, with the silicon substrate typically coated with a reflective material to reflect the infrared radiation back to the microbolometer. Thus, the air-gap thickness (i.e., the approximate distance between the reflective material and the air-bridge structure) may determine to a substantial degree the infrared absorption characteristics and spectral response of the microbolometers.

A drawback of a conventional microbolometer FPA is that the air-gap thickness is fixed during the manufacturing process and, consequently, the spectral response of the microbolometer array is limited by the resonant cavity structure formed during the manufacturing process. Another drawback, for example, of a conventional microbolometer FPA is that a mechanical shutter is typically required to calibrate the microbolometer FPA, with the mechanical shutter often being relatively difficult to manufacture, with certain labor intensive and expensive manufacturing processes. Furthermore, the mechanical shutter may be viewed as being a slow mechanism relative to the electronics and microbolometer FPA capabilities, with the mechanical shutter typically having a number of mechanical components that may degrade the reliability, may increase power consumption, and possibly reduce the performance of an infrared camera incorporating the microbolometer FPA.

As a result, there is a need for improved techniques for detecting infrared radiation with microbolometer FPAs and, for example, calibrating the microbolometer FPA.

SUMMARY

Systems and methods are disclosed herein, as an example, to provide microbolometer resonant cavity tuning and calibration techniques in accordance with one or more embodiments of the present invention. As an example for an embodiment, a microbolometer resonant cavity may be tuned to calibrate the microbolometer, with the calibration process performed without the use of a conventional mechanical shutter.

For example, in accordance with an embodiment of the present invention, an infrared camera system includes an optical element adapted to pass infrared frequencies of a first range of wavelengths and block infrared frequencies of a second range of wavelengths; a substrate; a plurality of microbolometers coupled to the substrate to form corresponding microbolometer resonant cavities, wherein the microbolometers are disposed to receive the first range of wavelengths passed by the optical element; a plurality of reflective layers disposed on the substrate and corresponding to the plurality of microbolometers; and at least one voltage source adapted to provide a variable voltage potential to vary a spacing of the microbolometer resonant cavities between the reflective layers and the corresponding microbolometers, wherein the at least one voltage source is adapted to provide at least a first voltage potential during a calibration process of the microbolometers to set the spacing corresponding to the microbolometers detecting the second range of wavelengths.

In accordance with another embodiment of the present invention, an infrared camera system includes an optical element adapted to pass an infrared frequency of a first wavelength and block an infrared frequency of a second wavelength; a microbolometer focal plane array comprising: a substrate; and an array of microbolometers disposed on the substrate and forming a corresponding cavity between each of the microbolometers and the substrate; and means for varying a dimension of the cavity for each of the microbolometers to vary spectral absorption properties of the microbolometers, wherein the varying means sets the dimension of the cavities to tune the spectral absorption properties of the microbolometers to the second wavelength during calibration.

In accordance with another embodiment of the present invention, a method of operating an array of microbolometers on a substrate of an infrared camera system includes filtering infrared radiation to pass a first infrared radiation wavelength and to block a second infrared radiation wavelength, wherein the first infrared radiation wavelength is different than the second infrared radiation wavelength; setting a spacing between the microbolometers and the substrate to approximately tune the microbolometers to the second infrared radiation wavelength which is blocked by the filtering; and determining calibration data for the microbolometers.

In accordance with another embodiment of the present invention, a microbolometer focal plane array includes a substrate; a plurality of microbolometers coupled to the substrate and elevated above the substrate to form corresponding microbolometer resonant cavities; a plurality of reflective layers disposed on the substrate and corresponding to the plurality of microbolometers; and a voltage source coupled to at least one of the reflective layers and configured to provide a variable voltage bias to the reflective layer to change a spacing between the reflective layer and the corresponding microbolometer.

In accordance with another embodiment of the present invention, an infrared camera system includes a microbolometer focal plane array that includes a substrate; and an array of microbolometers disposed on the substrate and forming a corresponding cavity between each of the microbolometers and the substrate. The infrared camera system further includes interface system electronics coupled to the microbolometer focal plane array that includes means for varying a dimension of the cavity for each of the microbolometers to vary spectral absorption properties of the microbolometers.

In accordance with another embodiment of the present invention, a method of varying a response of a microbolometer includes providing a first voltage level to a corresponding substrate of the microbolometer, wherein the first voltage level determines a first spacing between the substrate and the microbolometer; and providing a second voltage level to the corresponding substrate of the microbolometer, wherein the second voltage level determines a second spacing between the substrate and the microbolometer, wherein the second spacing is different than the first spacing.

In accordance with another embodiment of the present invention, a method of tuning an array of microbolometers on a substrate of an infrared camera system includes moving each of the microbolometers to a corresponding first spacing from the substrate to provide a first response over a first range of wavelengths; and moving each of the microbolometers to a corresponding second spacing from the substrate to provide a second response over a second range of wavelengths, wherein the first spacing is different than the second spacing.

In accordance with another embodiment of the present invention, a microbolometer focal plane array includes a substrate; a plurality of microbolometers coupled to the substrate and elevated above the substrate to form corresponding microbolometer resonant cavities, wherein each of the microbolometers includes a biasing layer configured to provide an approximately uniform voltage reference plane; a plurality of reflective layers disposed on the substrate and corresponding to the plurality of microbolometers; and a voltage source coupled to at least one of the reflective layers and configured to provide a variable voltage bias to the reflective layer, relative to the voltage reference plane of the biasing layer, to change a spacing between the reflective layer and the corresponding microbolometer.

In accordance with another embodiment of the present invention, an infrared camera system includes a microbolometer focal plane array having a substrate; and an array of microbolometers disposed on the substrate and forming a corresponding cavity between each of the microbolometers and the substrate, wherein each microbolometer includes a conductive layer corresponding to a reflective layer on the substrate; and interface system electronics coupled to the microbolometer focal plane array and having means for varying a dimension of the cavity for each of the microbolometers, based on a voltage potential difference between the conductive layer and the reflective layer associated with each of the microbolometers, to vary spectral absorption properties of the microbolometers.

In accordance with another embodiment of the present invention, a method of varying a response of a microbolometer includes providing a first voltage potential difference between a biasing layer of the microbolometer and a corresponding portion of substrate which forms a resonant cavity for the microbolometer, wherein the first voltage potential difference determines a first spacing between the substrate and the microbolometer; and providing a second voltage potential difference between the biasing layer of the microbolometer and the corresponding portion of substrate, wherein the second voltage potential difference determines a second spacing between the substrate and the microbolometer, wherein the second spacing is different than the first spacing.

In accordance with another embodiment of the present invention, a method of tuning an array of microbolometers on a substrate of an infrared camera system includes moving each of the microbolometers having a biasing layer to a corresponding first spacing from the substrate to provide a first response over a first range of wavelengths; and moving each of the microbolometers having the biasing layer to a corresponding second spacing from the substrate to provide a second response over a second range of wavelengths, wherein the first spacing is different than the second spacing.

DETAILED DESCRIPTION

FIG. 1shows a perspective view illustrating a microbolometer array100in accordance with an embodiment of the present invention. Microbolometer array100includes a substrate110and nine microbolometers120(e.g., a three-by-three array of microbolometers120) to receive infrared radiation (indicated generally by an arrow160). In general, microbolometer array100is shown as an exemplary three-by-three microbolometer FPA, but this is not limiting as the array may be of any desired size (e.g., from one microbolometer to a 512 by 512 array or larger), depending for example upon the application requirements.

Microbolometers120are elevated above substrate110by legs130(to form the air-bridge structure or vacuum-gap), which provide structural support and electrically connect microbolometers120with substrate110. Substrate110(e.g., a silicon substrate) may include unit cell circuitry and may also include the ROIC for microbolometer array100(e.g., microbolometer FPA) as would be understood by one skilled in the art.

As an example, microbolometer120includes a resistive material layer having a high temperature coefficient of resistivity (TCR) material (e.g., vanadium oxide (VOx) or amorphous silicon), with legs130coupling the resistive material of microbolometer120(e.g., via a leg metal to resistive metal contact) to contacts132on substrate110. However, this is not limiting and, in general, microbolometers120may be constructed in a conventional manner with various types of conventional materials.

In general, a theoretical model of a microbolometer array (microbridge array) would use a material that is a perfect blackbody absorber (i.e., equally absorbing all wavelengths of infrared radiation). However, because a microbridge has to be quite thin (e.g., approximately 500 nm) to have low thermal mass (and provide a sensor with a short time constant), all conventional process-compatible materials (e.g., silicon and the resistance material deposited on the silicon) are generally quite transparent. Consequently, due to the optical properties of the microbridge, the first pass of incident radiation is generally not efficiently absorbed by the microbridge.

For example, the “raw” absorption may be about fifteen percent, which is typically far too low to make a useful microbolometer sensor for the desired performance and with typical optics (e.g., f/1.4 to f/2). However, the resonant cavity structure of the microbolometer (the microbridge and the highly reflective substrate) greatly enhances infrared radiation absorption by providing multiple passes of the incident radiation. The operation may be viewed as behaving similar to a Fabry-Perot interferometer having a semi-transparent boundary condition (e.g., the microbridge) and a highly reflective boundary (e.g., the substrate) spaced about one-quarter wavelength away.

As an example, the microbridge may be made of silicon with a layer of vanadium oxide deposited on the silicon to form a resistor with a high TCR. However in accordance with one or more embodiments of the present invention, it should be understood that this example of a microbridge implementation is not limiting and that the microbolometer and microbridge structure may be implemented using various types of conventional materials (e.g., silicon dioxide, silicon nitride, amorphous silicon, titanium nitride, and/or vanadium oxide) and that the principles disclosed herein may be applied to various types of microbolometer structures made with various conventional materials, as would be understood by one skilled in the art.

The microbridge absorbs infrared radiation that is imaged onto it by the detector's optics. Any light that passes through the microbridge reflects off the substrate back towards the optics. If the wavelength of the incident light is exactly four times the cavity spacing, and the incident light has the correct phase relative to the microbridge, then incident light has a node at the substrate and the reflected wave reaches an antinode at the microbridge, as illustrated inFIG. 2. An antinode may represent a peak in the electric field of a light wave, and thus there is maximum absorption of photons if an absorbing material is located at an antinode.

Any reflected component from the microbridge returns to the substrate again, is reflected, and again forms an antinode at the microbridge. This process repeats until the incident wave energy is absorbed by the microbridge (the dominant mechanism), absorbed by the substrate (e.g., less than ten percent), or reradiated back towards the optics (e.g., less than ten percent). In general, the microbridge absorption may reach ninety percent at the peak wavelength, where the peak wavelength is approximately equal to4d, and “d” is the spacing between the microbridge and the substrate (as shown inFIG. 2). As an example, d may be set to approximately 2.5 micrometers, which in a classical interference model leads to a peak absorption at approximately ten micrometers wavelength (e.g., approximately the peak of the Planck spectral radiance curve (in power units) at terrestrial temperatures of 300 K).

In more specific terms, the phenomena of resonant absorption may be considered in terms of wave phenomena, with the cavity model also taking into account phase relationships. For example, the microbridge and the substrate may be viewed as forming a resonant cavity which has a high Q at the resonance wavelength of4d. The damping mechanism in this resonant system is the microbolometer structure itself, which ends up absorbing the bulk of the radiation, leading to measurable absorption efficiencies as high as ninety percent or more, as noted above.

In typical applications, the silicon itself may have intrinsic lattice absorption that begins around six micrometers, which results in a deviation from a classical two-boundary resonant cavity model to a more complex spectral absorption curve that is often found in actual infrared sensor arrays. For example,FIG. 3shows the transmission curves for five millimeter thick samples of two forms of silicon, with the absorption at a given wavelength, by conservation of energy, equal to one minus the transmission and minus the reflectance.

For this example, the reflectance of an incident light wave is due generally to Fresnel reflection (e.g., the change of refractive index experienced by the light wave as it passes from the vacuum to the material or out again). As a specific example, for a five hundred nanometer thick microbridge, the absorption will be much lower thanFIG. 3may indicate, simply because there is far less material in the optical path. In general,FIG. 3may be viewed as illustrating that the silicon material has complex optical properties, and therefore, generally may not be treated as a greybody absorber in a typical microbridge cavity model.

In accordance with one or more embodiments of the present invention, if the resonant cavity spacing is modified (e.g., by mechanical movement of the microbridge), then the spectral properties of the resonant cavity are modified. For example, a larger spacing (i.e., “d” increased) will enhance longer wavelength absorption, while a smaller spacing (i.e., “d” decreased) will enhance shorter wavelength absorption. As a specific implementation example, the resonant cavity spacing may be varied by placing an electric charge on the microbridge (e.g., suspended by legs with a relatively weak spring constant) and varying an electrical bias on the substrate (e.g., the reflective layer on the substrate).

Thus, the microbridge will move away or towards the substrate by varying the potential difference (e.g., the voltage difference and/or polarity difference) between the electrically charged substrate and the electrically charged microbridge, which will result in a variation of the spectral response of the microbolometer. As an example, a microbolometer designed for a spectral response peak at ten micrometers, may provide an enhanced spectral response between seven and nine micrometers when the microbridge is drawn closer to the substrate (due to the decreased resonant cavity spacing). Consequently in accordance with one or more embodiments of the present invention, tunable microbolometers may be provided for various multi-spectral applications as discussed further herein.

As an example,FIG. 4shows a perspective view illustrating a microbolometer array400in accordance with an embodiment of the present invention. Microbolometer array400may represent a portion of microbolometer array100(FIG. 1), with only one microbolometer120and associated substrate110shown for clarity to illustrate certain aspects of an embodiment of the present invention.

Microbolometer array400includes a reflective layer140on substrate110and below microbolometer120. Reflective layer140may represent a conventional reflective layer, such as a thin film metal layer (e.g., made of aluminum or platinum), which is deposited on substrate110and forms the reflective surface of the resonant cavity for microbolometer120.

In accordance with an embodiment of the present invention, a voltage source150is provided for reflective layer140. Voltage source150may be used to vary the voltage potential difference between reflective layer140and microbolometer120, which as discussed herein, results in a change in the distance (d) between reflective layer140and microbolometer120and thus, a change in the spectral response of microbolometer120. Although voltage source150is disclosed for varying the voltage potential, any number of alternative techniques may be implemented, as would be understood by one skilled in the art, for varying a bias or voltage potential difference between microbolometer120and substrate110(e.g., reflective layer140) in accordance with one or more embodiments of the present invention.

FIGS. 5-7bshow side views illustrating voltage biasing of microbolometer array400in accordance with an embodiment of the present invention.FIG. 5may represent a neutral or preset voltage biasing (or no voltage biasing) by voltage source150to provide a certain spacing (labeled “d” and indicated generally by an arrow152) between microbolometer120and reflective layer140.

FIG. 6illustrates a change (e.g., increase) in the voltage biasing by voltage source150to provide a decrease in the spacing (d) between microbolometer120and reflective layer140as compared to the spacing (d) of the preset voltage biasing ofFIG. 5. Consequently, microbolometer120may provide improved spectral response performance with respect to shorter wavelengths (i.e., improved shorter wavelength absorption).

FIG. 7aillustrates the increase in the spacing (d) between microbolometer120and reflective layer140due to a change (e.g., decrease) in the voltage biasing provided by voltage source150as compared to the voltage biasing ofFIG. 5. Consequently, microbolometer120may provide improved spectral response performance with respect to longer wavelengths (i.e., improved longer wavelength absorption) due to the increased spacing (d) relative to the spacing (d) ofFIG. 5.

It should be understood that the voltage potential difference may or may not be uniform between reflective layer140and microbolometer120and in general may be viewed as an average voltage potential difference. For example as illustrated inFIG. 7b, the voltage potential difference may vary due to the voltage drop across microbolometer120resulting from a bias provided in a conventional fashion via legs130to measure the change in resistance of microbolometer120to incident infrared radiation detection. Specifically for example this is illustrated symbolically inFIG. 7b, with microbolometer120represented by a resistor (R) having a voltage drop (e.g., a voltage gradient) across the resistor R from a voltage (V1) to a lesser voltage (V2).

The voltage drop across the resistor R (i.e., microbolometer120) may result in non-uniform forces (e.g., represented by different electrostatic force values (f1and f2)), which may result in non-uniform distances of the spacing (d) between portions of reflective layer140and corresponding portions of microbolometer120and thus, non-uniform infrared performance by microbolometer120. However, the voltage drop across microbolometer120may be insignificant relative to the voltage potential difference from the voltage biasing provided by voltage source150and, consequently, the spacing between reflective layer140and microbolometer120will remain approximately uniform.

Alternatively, the voltage biasing provided by voltage source150may be varied across reflective layer140(or a resistance across reflective layer140may be varied) to compensate for the expected differences in the voltage potential difference between reflective layer140and microbolometer120. It should also be understood that, rather than providing the voltage biasing to reflective layer140, a variable voltage bias (e.g., voltage source150) may be provided to microbolometer120to provide the voltage potential difference between reflective layer140and microbolometer120. In addition, variable voltage biasing may be provided to both reflective layer140and microbolometer120, if desired, to provide the required voltage potential difference and vary the spacing for the desired applications.

Alternatively in accordance with one or more embodiments of the present invention, microbolometer120may further include a separate biasing layer to maintain a uniform voltage potential relative to reflective layer140and provide the appropriate uniform spacing (d) for the desired application. For example,FIGS. 7c-7eshow side views illustrating voltage biasing of a microbolometer array700in accordance with an embodiment of the present invention. Microbolometer array700is similar to microbolometer array400(e.g.,FIG. 4), but further includes a conductive layer170that may be used as a reference plane and be coupled to microbolometer120to form a microbolometer720.

Specifically for example this is illustrated symbolically inFIG. 7c, with microbolometer120represented by the resistor (R) and conductive layer170(e.g., an absorptive conductive film below the resistor (R)) coupled to the voltage (V1) (but not to the voltage (V2)) to provide a constant voltage plane (e.g., at a value of the voltage (V1)) relative to a voltage applied by the voltage source150to reflective layer140. Thus, uniform forces (e.g., represented by the same electrostatic force values (f1)) may be exerted on microbolometer720and the spacing (d) may be uniform between microbolometer720and reflective layer140(e.g., microbolometer720does not tilt or otherwise deform due to non-uniform forces applied) so that microbolometer720may provide the desired infrared performance.

Conductive layer170may also provide certain additional advantages. For example, conductive layer170may aid in the absorption of infrared energy of microbolometer120by proper selection of resistance (e.g., sheet resistance) of conductive layer170. As a specific example, conductive layer170having a determined sheet resistance (e.g., 350-850Ω/) for a specific microbolometer and infrared detection application may provide increased and more uniform infrared absorption (e.g., free carrier absorption or impedance matching of free space), as would be understood by one skilled in the art.

Microbolometer720may be formed using conventional techniques as would be understood by one skilled in the art, but with conductive layer170coupled to a voltage source (e.g., any type of source to provide a voltage, bias, and/or electrical charge onto conductive layer170) in accordance with one or more embodiments of the present invention. For example,FIG. 7dillustrates an example of a cross-section of microbolometer720in accordance with an embodiment of the present invention.

Specifically as an example, microbolometer720may include (but not limited to) a resistive film layer (e.g., vanadium oxide (VOx)) between oxide layers (e.g., silicon dioxide), with conductive layer170(e.g., titanium nitride) below and coupled to one of the oxide layers. For example, the resistive film layer and the oxide layers may represent a specific example of a cross-section of microbolometer120, while the addition of conductive layer170to microbolometer120forms a specific example of a cross-section of microbolometer720. In general, microbolometers120and720may be formed using various known processing techniques and various types of conventional materials (e.g., such as with materials and processing techniques as disclosed in U.S. Pat. No. 5,021,663 or other conventional techniques), as would be understood by one skilled in the art.

Conductive layer170may be coupled to one of legs130(e.g., leg metal) to receive a voltage bias via one of contacts132(e.g., as shown inFIG. 1) on substrate110. For example, conductive layer170may be coupled to the same voltage bias as provided to microbolometer120(e.g., but at only one leg130), such as via a separate contact provided for this voltage bias to conductive layer170or by coupling conductive layer170to the same contact (e.g., on the microbridge or near the leg metal/microbridge interface) as used by the resistive film layer of microbolometer720or by using other conventional contact techniques to provide a suitable voltage bias to conductive layer170, as would be understood by one skilled in the art.

Alternatively for example, conductive layer170may be coupled to a different voltage bias, than provided to microbolometer120, via a separate voltage path through one of legs130, as would be understood by one skilled in the art. As an example, if conductive layer170is provided with a separate voltage bias, this voltage bias may be used to control the spacing (d) between conductive layer170and reflective layer140, while reflective layer140maintains a reference voltage (e.g., ground plane), or the spacing (d) may be controlled by varying the voltage biases on both conductive layer170and reflective layer140. As a further example, conductive layer170may be provided with a constant reference voltage to provide a uniform voltage potential across a general area of microbolometer720and relative to a variable voltage bias provided to reflective layer140.

Consequently, the spacing (d) may be varied by varying the voltage potential between reflective layer140and microbolometer120or720(e.g., as discussed previously in general in reference toFIGS. 4-7e) to modify the spectral properties of the resonant cavity associated with microbolometer120or720. For example, a larger spacing (i.e., “d” increased) will enhance longer wavelength absorption (e.g., as illustrated inFIG. 7e), while a smaller spacing (i.e., “d” decreased) will enhance shorter wavelength absorption (e.g., as illustrated inFIG. 7d). Thus, microbolometer120or720will move away or towards reflective layer140by varying the potential difference (e.g., the voltage difference and/or polarity difference) between the electrically charged reflective layer140and the electrically charged microbolometer120or720, which will result in a variation of the spectral response of microbolometer120or720.

As noted previously, a conventional microbolometer FPA typically employs a mechanical shutter to perform a calibration process for the microbolometer FPA (e.g., to obtain desired calibration data, which may include correction terms, offsets, biases, and/or gain factors), as would be understood by one skilled in the art. However, the use of a mechanical shutter during the calibration process may have certain drawbacks, such as for example in terms of being a slow and imprecise mechanism relative to the electronics and microbolometer FPA capabilities. Furthermore, a mechanical shutter generally adds to the manufacturing costs of the infrared camera incorporating the microbolometer FPA, with the mechanical shutter typically having a number of mechanical components that may degrade the reliability, may increase power consumption, and possibly reduce or limit the overall performance of the infrared camera.

In contrast in accordance with one or more embodiments of the present invention, techniques disclosed herein may allow a calibration process to be performed for the microbolometer FPA (e.g., microbolometer array400or microbolometer array700) without the use of a mechanical shutter. For example,FIGS. 7fand7gshow side views illustrating techniques for calibrating microbolometer array700in accordance with an embodiment of the present invention. However, it should be understood that the calibration techniques disclosed herein for microbolometer array700would also apply for microbolometer array400, as would be understood by one skilled in the art.

Specifically,FIG. 7fillustrates microbolometer array700detecting infrared radiation (e.g., during normal infrared camera operation, with infrared radiation indicated generally by arrow160) and, as discussed previously herein, with the spacing (d) adjusted appropriately (e.g., by adjusting a voltage potential between reflective layer140and conductive layer170) to detect the desired infrared wavelengths. For example, an example of an infrared wavelength is shown symbolically as infrared wavelength730, with the spacing (d) tuned such that the antinode (e.g., one quarter wavelength of infrared wavelength730) occurs approximately at microbolometer720(e.g., as discussed previously in reference toFIG. 2) to provide maximum absorption of infrared radiation by microbolometer720.

However in accordance with an embodiment, if it is desired to perform a calibration operation for microbolometer720, the spacing (d) may be tuned (e.g., adjusted as discussed herein) such that the node (e.g., one half wavelength of infrared wavelength730) occurs approximately at microbolometer720to minimize absorption of infrared radiation by microbolometer720. Furthermore, an optical element740may be provided (e.g., as part of microbolometer array700or incorporated into an infrared camera system that includes microbolometer array700) to block infrared radiation wavelengths that normally would be detected by microbolometer720at this spacing (d) during the calibration process. Consequently in accordance with an embodiment, with microbolometer array700tuned to infrared wavelengths that are not passed by optical element740(e.g., outside of the passband or within a stopband), a conventional calibration process may be performed without requiring a conventional mechanical shutter (e.g., to block infrared radiation and provide a desired reference image).

Optical element740may represent a conventional infrared window, infrared filter, and/or optics to pass desired infrared wavelengths and block undesired infrared wavelengths. As a specific example, optical element740may represent an infrared window with filtering functionality to provide a desired infrared radiation passband, such as for a desired infrared camera application.

The calibration process (e.g., during which the spacing (d) of microbolometer720adjusts to an infrared wavelength range outside of the passband of optical element740) may be viewed, for example, as using an electronic blinking operation (e.g., an electronic version of the mechanical shutter by tuning the spacing (d) to a one half wavelength or “null” region). Thus, this calibration process in accordance with an embodiment may provide certain advantages over a conventional calibration process using a mechanical shutter, such as the elimination of slow, mechanical moving parts associated with the mechanical shutter and the ability to provide higher-speed and possibly more frequent electronic calibration operations. For example, on-FPA “null” and non-uniformity correction (NUC) operations may be performed electronically (e.g., including changing the spacing (d) at a relatively high speed) as needed, which may eliminate or reduce the need for a more time-consuming formal calibration process that would typically be required upon start-up or periodically during infrared camera operation.

In general in accordance with an embodiment, a calibration process is disclosed that detunes the optical cavity of the microbolometer (e.g., of microbolometer array400or microbolometer array700) rather than moving a mechanical shutter into position to block the infrared radiation and provide a uniform reference image (e.g., a blackbody or “no signal” reference). Furthermore the microbolometer may use the optical element740as a virtual shutter due to the optical cavity being detuned in accordance with an embodiment such that optical element740may appear opaque (e.g., during the calibration process).

FIG. 8shows a block diagram800illustrating a microbolometer focal plane array802and interface system electronics818, with uniformity-correction circuitry, in accordance with an embodiment of the present invention. For example, block diagram800may represent an infrared camera system utilizing the techniques disclosed herein to provide selectable spectrum performance.

Microbolometer focal plane array802includes a microbolometer array (labeled unit cell array) and a readout integrated circuit (ROIC) having control circuitry, timing circuitry, bias circuitry, row and column addressing circuitry, column amplifiers, and associated electronics to provide output signals that are digitized by an analog-to-digital (A/D) converter804. The microbolometer array (unit cell array) of microbolometer focal plane array802may be formed by microbolometers as described in reference toFIGS. 1,2, and4-7g(e.g., an array of microbolometers120or an array of microbolometers720). The ROIC of microbolometer focal plane array802may be employed to select the desired microbolometers for obtaining the desired output signals and may be constructed in a conventional manner.

The A/D converter804may be located on or off the ROIC. The output signals from A/D converter804are stored in a frame memory806. The data in frame memory806is then available to image display electronics808and a data processor812, which also has a data processor memory810. A timing generator814provides system timing.

Data processor812generates uniformity-correction data words, which are loaded into a data register load circuitry816that provides the interface to load the correction data into the ROIC. In this fashion the digital-to-analog converters, and other variable circuitry, which control voltage levels, biasing, circuit element values, etc., are controlled by data processor812to provide the desired output signals from microbolometer focal plane array802.

Data processor812may also generate the desired voltage biasing values for reflective layer140and/or conductive layer170to control the spacing (d) of the resonant cavity of corresponding microbolometers120or720of the unit cell array of microbolometer focal plane array802. For example, data processor812via data register load circuitry816may control the spacing (d) of microbolometers120or720on a pixel-by-pixel basis (e.g., based upon calibration data) or on a global basis (e.g., by providing a global voltage bias value or corresponding global voltage bias values to all microbolometers120or720). As another example, data processor812may also control the spacing (d) of the resonant cavity of corresponding microbolometers120or720to perform a calibration process (e.g., as discussed in reference toFIGS. 7fand7g).

For the pixel-by-pixel basis, the performance may be tuned by calibrating each microbolometer120or720relative to voltage biasing, as would be understood by one skilled in the art, to determine the performance of each microbolometer120or720as the voltage bias varies. If a global signal for voltage biasing is provided to microbolometers120or720, calibration may still be performed to calibrate the overall performance of microbolometers120or720(as a unit cell array) relative to voltage bias values. Consequently, the proper voltage bias may be applied, for example, individually to each microbolometer120or720or uniformly to the unit cell array to obtain the desired performance for the wavelength range or region of interest.

For example, a checkerboard pattern or other type of pattern may be provided based on the pixel-by-pixel approach to provide the desired wavelength responses of microbolometers120or720within microbolometer focal plane array802. As a specific example, first, second, and third groups of microbolometers120or720may be designated to detect first, second, and third wavelengths, respectively, for a designated image at a designated timeframe. The groups may be formed in any fashion across microbolometer focal plane array802, such as in a checkerboard-like repetitive pattern (e.g., first group pixel next to second group pixel next to third group pixel next to first group pixel in a repetitive pattern row after row) or in a stripe-like pattern of stripes of groups (e.g., first group pixels covering a first row or column, second group pixels covering a second row or column, third group pixels covering a third row or column, first group pixels covering a fourth row or column in a repetitive pattern) or in any other desired pattern of pixels having different spectrum responses. Thus, microbolometers120or microbolometers720may be controlled on a pixel-by-pixel basis to provide the desired tailored spectrum response, which may be used, for example, to provide the desired composite image data.

FIG. 9shows a block diagram900illustrating a microbolometer focal plane array902and interface system electronics918, with uniformity-correction circuitry, in accordance with an embodiment of the present invention. Block diagram900(e.g., an infrared camera system) is similar to block diagram800, but includes additional techniques for providing, for example, temperature compensation and/or correction for various non-uniformities, as described in further detail for example in U.S. Pat. No. 6,812,465, issued Nov. 2, 2004, which is incorporated herein by reference in its entirety.

Microbolometer focal plane array902includes a microbolometer focal plane array (labeled unit cell array) and a readout integrated circuit (ROIC) having control circuitry, timing circuitry, bias circuitry, row and column addressing circuitry, column amplifiers, and associated electronics to provide output signals that are digitized by analog-to-digital (A/D) converter804. The microbolometer array (unit cell array) of microbolometer focal plane array902may be formed by microbolometers as described in reference toFIGS. 1,2, and4-7g(e.g., an array of microbolometers120or an array of microbolometers720). The ROIC of microbolometer focal plane array902may be employed to select the desired microbolometers for obtaining the desired output signals and may be constructed in a conventional manner.

The output signals from A/D converter804are adjusted by a non-uniformity correction circuit (NUC)904, which applies temperature dependent compensation (e.g., Lagrange Offset, Temperature Dependent Gain, and additional Offset). After processing by NUC904, the output signals are stored in a frame memory806. The data in frame memory806is then available to image display electronics808and data processor812, which also has data processor memory910.

Data processor812generates uniformity-correction data words, which are loaded into a correction coefficient memory906. As discussed similarly in reference toFIG. 8for example, data processor812may also generate the desired voltage biasing for each reflective layer140and/or conductive layer170to control the spacing (d) of the resonant cavity of corresponding associated microbolometers120or microbolometers720of the unit cell array of microbolometer focal plane array902(e.g., on an individual basis or on a single global signal basis). Furthermore as another example, data processor812may also control the spacing (d) of the resonant cavity of corresponding microbolometers120or720to perform a calibration process (e.g., as discussed in reference toFIGS. 7fand7g), such as for example to determine the appropriate NUC calibration terms (e.g., NUC gain and/or offsets).

The voltage biasing may be provided to correction coefficient memory906or directly to data register load circuitry816, which provides the interface to load the correction data into readout integrated circuit902. In this fashion the variable resistors, digital-to-analog converters, and other variable circuitry, which control voltage levels, biasing, circuit element values, etc., may be controlled by data processor812so that the output signals from the readout integrated circuit are uniform over a wide temperature range and over the desired wavelength regions of interest.

It should be understood that block diagrams800and900provide examples of infrared camera systems and that the various techniques disclosed herein in accordance with one or more embodiments are not limited to these specific infrared camera systems, as would be understood by one skilled in the art. In general, for example in accordance with an embodiment, an infrared camera system may include a microbolometer FPA, a logic device (e.g., a processor, an application specific integrated circuit, or a programmable logic device), a memory (e.g., either discrete or within the logic device), and optionally a display for viewing by a user of the infrared camera system. Thus, for example, block diagrams800and900may represent specific implementation examples of this more general system, with various elements of block diagram800or block diagram900combinable, as would be understood by one skilled in the art.

Systems and methods are disclosed herein to provide microbolometer resonant cavity tuning techniques and calibration techniques in accordance with embodiments of the present invention. For example, in accordance with an embodiment of the present invention, a tunable microbolometer resonant cavity is disclosed that may be used to exploit the substantial degree of variability of absorptance relative to air gap for the microbolometer structure. As disclosed herein, by varying the resonant cavity dimensions (e.g., by moving the microbolometer away from or towards the associated substrate), the microbolometer's spectral absorption properties are changed, which results in a change in the spectrum range being imaged.

As a specific example in accordance with an embodiment, a variable bias (e.g., a variable voltage) was provided to the substrate's reflective metal layer and/or conductive layer (if implemented) to control the distance between the microbolometer and the reflective metal layer (e.g., provide a tuned resonant cavity (e.g., one-quarter wavelength optical cavity) that determines peak wavelength of the microbolometer's response). The resonant cavity dimensions (e.g., cavity resonance) may be varied rapidly (e.g., the variable voltage change may be performed in the kilohertz frequency range—a voltage change in milliseconds or less) to measure the desired infrared spectrum ranges.

Thus for example, by controlling the variable bias or biases to control the microbolometer's resonant cavity (e.g., tunable microbolometers), uncooled color performance over a selective range of wavelengths may be provided, such as for multi-spectral applications. For example, the microbolometer's peak wavelength response may be varied for various detection applications, such as for gas detection or discrimination, emissivity determination for thermography applications, image differencing applications (e.g., imaging a scene over different wavelengths for comparison, contrasting, and other differencing applications). Consequently, the multi-spectral applications may include, for example, various conventional techniques, which previously required multiple infrared cameras and/or filters.

As another specific example in accordance with an embodiment, the microbolometer resonant cavity may be tuned to perform a calibration operation, without the use of a conventional mechanical shutter. For example, the microbolometer resonant cavity within an infrared camera may be tuned to a wavelength range that is outside of the passband of the infrared camera's optics (e.g., filter or window) to perform a calibration operation. Consequently as an example in accordance with an embodiment, a conventional calibration process may be performed without requiring the use of a conventional mechanical shutter, which may provide certain advantages in terms of speed, reliability, and manufacturing costs relative to conventional infrared camera systems.