Thermal actuator for an infrared sensor

In one embodiment, an infrared (IR) sensor has a flexible beam connected between two anchors supported on a substrate. The beam is mechanically coupled to a plate that has an IR-absorbing layer and is adapted to transfer the IR-induced heat to the beam. The heat transfer causes the beam to deform and move the plate with respect to the substrate. The motion of the plate is detected electrically or optically to quantify the amount of IR radiation received by the plate. The beam, anchors, and plate are formed from a planar layer of material that is supported at a specified offset distance from the substrate. During fabrication, certain portions of the planar layer are removed to define the beam, anchors, and plate.

CROSS-REFERENCE TO RELATED APPLICATIONS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detectors of infrared radiation and infrared imaging systems.

2. Description of the Related Art

Infrared (IR) detectors are classified into two categories: photonic and thermal. In a photonic IR detector, infrared photons are absorbed to excite electronic transitions and/or generate photocurrent within an IR absorber, usually a semiconductor material having an appropriate bandgap. The excitation changes material properties of the IR absorber such as, for example, electrical conductivity. This change is measured to quantify the amount of absorbed IR radiation. Photonic IR detectors usually operate at very low temperatures, e.g., about 78 K, to suppress thermally induced electronic transitions and/or thermal “dark” current in the IR absorber. As such, photonic IR detectors often utilize cryostats and/or complex cooling systems, which make these detectors heavy, bulky, and relatively expensive.

In a thermal IR detector, the energy of absorbed infrared photons is converted into heat, which causes a temperature increase within the detector. This temperature increase is converted into a mechanical or electrical response, which is measured to quantify the amount of absorbed IR radiation. A sensor employed in a thermal IR detector typically has (i) a resistive bolometer, whose electrical resistance changes with temperature, (ii) a pyroelectric material, which exhibits a spontaneous electric polarization change with temperature, (iii) a thermocouple, whose voltage depends on the thermocouple's temperature differential, and/or (iv) a bi-material (also referred to as bimorph) cantilever, whose shape is sensitive to temperature changes.

Unlike photonic IR detectors, thermal IR detectors typically (i) do not use cooling and (ii) can operate at temperatures normally present in the environment, e.g., about 300 K. As a result, thermal IR detectors can be light, compact, and relatively inexpensive. Accordingly, thermal IR detectors and infrared imaging systems employing such detectors are being actively developed.

SUMMARY OF THE INVENTION

In one embodiment, an infrared (IR) sensor of the invention has a flexible beam connected between two anchors supported on a substrate. The beam is mechanically coupled to a plate that has an IR-absorbing layer and is adapted to transfer the IR-induced heat to the beam. The heat transfer causes the beam to deform and move the plate with respect to the substrate. The motion of the plate is detected electrically or optically to quantify the amount of IR radiation received by the plate. The beam, anchors, and plate are formed from a planar layer of material that is supported at a specified offset distance from the substrate. During fabrication, certain portions of the planar layer are removed to define the beam, anchors, and plate. Advantageously, the IR sensor is relatively insensitive to variations in ambient temperature, has a relatively high sensitivity to IR radiation, and lends itself to incorporation into a sensor array suitable for detection of IR images.

According to one embodiment, a device of the invention has a substrate and a first flexible beam connected between two anchors supported on the substrate. The device further has a plate mechanically coupled to the first flexible beam and adapted to absorb incident radiation. The plate is adapted to transfer heat generated due to said absorption to the first flexible beam to cause the first flexible beam to deform and move the plate with respect to the substrate. The first flexible beam and at least a portion of each of the two anchors have been formed from a common layer of material.

According to another embodiment, a method of detecting incident radiation has the step of exposing a plate to the incident radiation, wherein the plate is adapted to absorb at least a portion of said radiation. The method further has the step of transferring heat generated in the plate due to said absorption to a first flexible beam to cause the first flexible beam to deform and move the plate with respect to the substrate. The first flexible beam is connected between two anchors supported on a substrate. The plate is mechanically coupled to the first flexible beam. The first flexible beam and at least a portion of each of the two anchors have been formed from a common layer of material. The method further has the step of detecting the motion of the plate.

DETAILED DESCRIPTION

FIGS. 1A-Bshow cross-sectional side views of a thermal actuator100according to one embodiment of the invention. Actuator100has a flexible beam110that is attached between two anchors120a-baffixed to a substrate102. At temperature T, beam110has a first shape, e.g., a straight shape shown inFIG. 1A. If the temperature of beam110is elevated by ΔT, then the length of beam110increases due to thermal expansion. If substrate102remains at temperature T, then the substrate does not expand and the distance between anchors120a-bremains unchanged. At a relatively large ΔT, the thermal expansion of beam110causes the beam to buckle, e.g., as shown inFIG. 1B, and adopt a second shape. The buckling of beam110generates a displacement of the beam's midsection, which enables actuator100to convert heat into mechanical movement. The magnitude of the displacement (Δx, seeFIG. 1B) is related to ΔT.

In one embodiment, beam110is placed in thermal contact with an IR absorber (not explicitly shown inFIG. 1). Provided that anchors120a-bare designed to conduct heat relatively poorly, the heat generated by the absorption of IR radiation in the IR absorber will produce temperature difference ΔT between beam110and substrate102. The resulting displacement of the midsection of beam110can then be measured to quantify the amount of IR radiation received by the IR absorber.

FIG. 2shows a three-dimensional perspective view of an IR sensor200according to one embodiment of the invention. Sensor200has two thermal actuators analogous to thermal actuator100ofFIG. 1. More specifically, each of the two thermal actuators of sensor200includes a flexible beam210, which is attached between two anchors220affixed to a substrate202. However, one difference between beam210of sensor200and beam110of actuator100is that beam210has a slightly arched shape at the intended operating temperature even in the absence of IR radiation. The arched shape of beam210removes an uncertainty with respect to the buckling direction inherent to the straight shape of beam110and causes beam210to buckle outward with respect to substrate202.

Sensor200further has a plate212connected to beams210a-bby supports218a-b, respectively. In one embodiment, plate212includes two layers of material: an IR-absorbing layer214and a base layer216. When layer214is subjected to IR irradiation, the temperature of plate212rises. Due to the thermal contact between plate212and beams210a-bprovided by supports218a-b, heat is transferred from the plate to the beams, which causes the beams to buckle and move the plate.

To detect motion of plate212, sensor200has an electrode204attached to substrate202and electrically insulated from the substrate by a dielectric layer206. Electrode204and base layer216of plate212form a parallel-plate capacitor208whose capacitance depends on the distance between the plate and the electrode. As such, change in the position of plate212can be measured by measuring the changing capacitance of capacitor208. The measured capacitance change can then be related to the temperature of plate212and/or amount of IR radiation received by the plate. For additional details on actuator100and sensor200, the reader is referred to the above-cited U.S. patent application Ser. No. 11/036,264.

FIGS. 3A-Bshow three-dimensional perspective views of an IR sensor300according to another embodiment of the invention. Sensor300is generally analogous to sensor200(FIG. 2) and analogous elements of the two sensors are designated with labels having the same last two digits. However, there are certain differences between sensors300and200, which are explained in more detail below.

Sensor300differs from sensor200in that beams310a-b, anchors320, fasteners330, base layer316of plate312, and supports318a-bare all formed using the same layer of material, which layer is labeled322. Standoff pillars334support layer322at a specified offset distance from substrate302. In an undeformed state, layer322is a substantially planar layer that is parallel to substrate302. During fabrication, certain portions of layer322are removed to define the corresponding elements of sensor300. For example, four cutouts324in layer322define supports318a-band the edges of plate312to which those supports are attached. Cutouts324together with slots326a-betched through layer322further define beams310a-b, fasteners330a-b, and four anchors320. Each fastener330is a stripe of material that ties two respective anchors320and is attached to the corresponding standoff pillar334.

FIG. 3Ashows sensor300when there are no temperature gradients in the sensor. In particular, beams310a-band fasteners330a-bare at the same temperature, which results in layer322being substantially flat and the various structural elements defined in that layer lying in a single plane. If IR-absorbing layer314of plate312is heated by impinging IR radiation, then the heat flow from the plate, through supports318a-b, to beams310a-bcauses the temperature of the beams to rise. At the same time, the topology of beams310a-b(which have a relatively small cross-section and a relatively large length) and the presence of slots326a-binhibit significant heat flow from the beams to fasteners330a-b,which leads to a temperature difference between the beams and the fasteners. The higher temperature of beams310compared to that of fasteners330causes the beams to buckle, e.g., as shown inFIG. 3B. The buckling of beams310a-bmoves plate312with respect to substrate302. If beams310a-bhave similar mechanical properties, then the motion of plate312is a translation along a Z direction, during which the plate remains substantially parallel to substrate302.

As already explained above, the initial straight shape of beam310can create an uncertainty regarding the buckling direction of the beam. However, a small stress gradient that can be built into layer322, as known in the art, can remove that uncertainty and favor one buckling direction over the other. For example, if layer322is manufactured so that the built-in stress across that layer increases in the positive Z direction, then IR radiation received by plate312will cause beams310a-bto buckle in the positive Z direction as well (as shown inFIG. 3B). On the other hand, if layer322is manufactured so that the built-in stress across layer322decreases in the positive Z direction, then IR radiation received by plate312will cause beams310a-bto buckle in the negative Z direction.

Alternatively, layer322can be fabricated to be substantially stress-gradient free. Then, an additional thin layer of a different material (not explicitly shown inFIG. 3) can be deposited over the portion of layer322corresponding to beam310. It is known in the art that differences in thermal expansion coefficients typically cause a contact region between two dissimilar materials to have a stress gradient. This contact-region stress gradient will remove the buckling-direction uncertainty and favor one buckling direction over the other in a manner similar to the above-described case of layer322fabricated with a built-in stress gradient.

In one embodiment, sensor300incorporates an electrode (not explicitly shown) similar to electrode204of sensor200to enable electrical detection of changes in the position of plate312with respect to substrate302. In an alternative embodiment, sensor300can be adapted for optical interrogation to enable optical detection of the same. More specifically, the motion and/or displacement of plate312can be used to impart a corresponding phase shift onto an interrogating optical beam that is reflected from the plate. This phase shift can then be detected, as known in the art, and used to quantify the displacement of plate312with respect to a reference position. More details on optical interrogation of IR sensors can be found, e.g., in the above-cited U.S. patent application Ser. Nos. 11/531,011, 11/766,414, and 11/766,430.

The above-described features of sensor300enable certain embodiments of the sensor to be relatively insensitive to ambient temperature fluctuations and to respond substantially only to a temperature difference between beam310and the corresponding fastener330. Thus, sensor300might be used in applications where the sensor can be subjected to a relatively wide range of ambient temperatures.

In one embodiment, sensor300can be fabricated using the following set of materials: (i) amorphous hydrogenated silicon carbide for substrate302, layer322, and standoff pillars334; and (ii) Ti/W alloy for layer314. One skilled in the art will appreciate that other appropriate materials can similarly be used.

In one embodiment, sensor300has the following dimensions: (i) between about 10 to a few hundred microns for the length and width of plate312and the length of beam310; (ii) between about 1 and 5 micron for the width of beam310; (iii) about 0.5 micron for the gap between substrate302and layer322; (iv) between about 0.1 and 0.5 micron for the thickness of beam310; (v) about 0.1 micron for the thickness of layer314; and (vi) about 0.5 micron for the thickness of plate312.

FIG. 4shows a top view of an arrayed IR sensor400according to one embodiment of the invention. Array400is illustratively shown as having nine sensors300′ arranged in three rows and three columns. One skilled in the art will appreciate that a different number of sensors300′ can similarly be arrayed to form a relatively large (e.g., ˜1000-pixel) array suitable for detection of IR images.

Each of sensors300′ is similar to sensor300ofFIG. 3, except that sensors300′ belonging to the same row, e.g., row402, share some structural elements. For example, two adjacent sensors300′ in row402have a common standoff pillar434, which is otherwise analogous to standoff pillar334ofFIG. 3. Two adjacent sensors300′ in row402further have a common fastener430, which is otherwise analogous to fastener330ofFIG. 3. Advantageously, the structural-element sharing implemented in array400can be used to reduce the overall complexity of the array.

FIGS. 5A-Bshow an IR sensor500according to yet another embodiment of the invention. More specifically,FIG. 5Ashows a top view of sensor500.FIG. 5Bshows a three-dimensional perspective view of sensor500when the sensor is heated by IR radiation.

Sensor500has a flexible beam510, which is attached between two anchors520a-baffixed to a substrate502. Sensor500further has a plate512connected to beam510by a heat-conducting bridge518. An extension538of bridge518further connects plate512to a torsion rod542that is attached between two anchors540a-b. Similar to anchors520, anchors540are affixed to substrate502. In one embodiment, plate512, bridge518, extension538, rod542, and top portions of anchors520and540are all formed from the same layer of material that is analogous to layer322of sensor300(seeFIG. 3).

If plate512is subjected to IR irradiation, then the heat generated therein flows from the plate, through bridge518, to beam510, thereby creating a temperature gradient between the beam and substrate502. The temperature gradient causes beam510to buckle, which in turn causes extension538to pivot about torsion rod542. Bridge518transfers the pivoting motion of extension538to plate512to tilt the plate as shown inFIG. 5B.

The position of plate512with respect to substrate502is detected electrically, e.g., by measuring the capacitance of a capacitor508formed by the plate and a C-shaped electrode504disposed on substrate502(seeFIG. 5B). Electrode504generally follows the outline of the far (with respect to beam510) side of plate512and is located beneath the free end of the lever formed by extension538, bridge518, and plate512. The lever amplifies displacement Δx of the midsection of beam510(see alsoFIG. 1B) at the lever's free end. As a result, capacitor508is capable of producing higher relative capacitance changes than, e.g., capacitor208(FIG. 2), under otherwise comparable conditions. This property of sensor500can advantageously be used to implement a system having a relatively high sensitivity to IR radiation.

FIG. 6shows a top view of an arrayed IR sensor600according to another embodiment of the invention. Array600is illustratively shown as having twelve sensors500′ arranged in three rows and four columns. Each of sensors500′ is similar to sensor500ofFIG. 5, except that sensors500′ share some structural elements. For example, anchor520of one sensor500′ might also serve as anchor540for another sensor500′ (see alsoFIG. 5). For example, anchor5201of sensor5001′ serves as anchor5402for another sensor5002′ (seeFIG. 6).

Inspection ofFIG. 6reveals that array600has the following features. Sensors500′ belonging to the same column, e.g., column604, are arranged so that plates512of different sensors500′ form a substantially contiguous segmented plate having independently movable segments. This feature enables array600to have a relatively high (e.g., >95%) IR-radiation fill factor. Both the distance between two anchors520corresponding to the same beam510and the length of that beam are greater (e.g., by a factor of about two or more) than the linear size of plate512. One skilled in the art will appreciate that the relatively large lengths of beams510enable array600to produce relatively large displacements Δx (see alsoFIG. 1B), which is beneficial in terms of sensitivity to IR radiation. Plates520of sensors500′ belonging to adjacent rows pivot in opposite directions. Plates512, bridges518, extensions538, rods542, and top portions of anchors520and540corresponding to different sensors500′ can all be formed from the same layer of material supported on a common substrate. This feature helps to simplify the fabrication process for array600.

Sensors and arrays of the invention can be implemented as MEMS devices and fabricated, e.g., using layered wafers as described in commonly owned U.S. Pat. Nos. 6,850,354 and 6,924,581, which are incorporated herein by reference in their entirety. Various layers of material can be deposited onto a wafer using, e.g., chemical vapor deposition. Various parts of the devices can be mapped onto the corresponding layers using lithography. Additional description of various fabrication steps can be found, e.g., in U.S. Pat. Nos. 6,201,631, 5,629,790, and 5,501,893, all of which are incorporated herein by reference in their entirety. Representative fabrication-process flows can be found, e.g., in U.S. Pat. Nos. 6,667,823, 6,876,484, 6,980,339, 6,995,895, and 7,099,063 and U.S. patent application Ser. No. 11/095,071 (filed on Mar. 31, 2005), all of which are incorporated herein by reference in their entirety.

As used in this specification, the term infrared radiation covers all of the following spectral bands: (1) visible to near IR, wavelengths from about 400 nm to about 1 μm; (2) short-wave IR, wavelengths from about 1 μm to about 3 μm; (3) midwave IR, wavelengths from about 3 μm to about 7 μm; and (4) long-wave IR, wavelengths from about 7 μm to about 14 μm.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Although sensors of the invention were described in reference to IR radiation, one skilled in the art will appreciate that these sensors can also be adapted to detect other types of radiation, e.g., ultraviolet or corpuscular, provided that the radiation can be converted into heat upon absorption in the plate of the sensor. Various surfaces may be modified, e.g., by metal deposition for enhanced reflectivity, IR absorption, and/or electrical conductivity, or by ion implantation for enhanced mechanical strength. Differently shaped levers, anchors, plates, pillars, posts, supports, bridges, extensions, flexible beams, and/or electrodes may be implemented without departing from the scope and principle of the invention. Sensors of the invention can be variously arrayed to form linear or two-dimensional arrays. Interrogating light can be of any suitable wavelength, e.g., from the near-infrared region, and is not necessarily limited to the visible spectrum. Devices of the invention can be formed using one, two or more wafers secured together. Various modifications of the described embodiments, as well as other embodiments of the invention, which are apparent to persons skilled in the art to which the invention pertains are deemed to lie within the principle and scope of the invention as expressed in the following claims.

Throughout the detailed description, the drawings, which are not to scale, are illustrative only and are used in order to explain, rather than limit the invention. The use of terms such as height, length, width, top, bottom, is strictly to facilitate the description of the invention and is not intended to limit the invention to a specific orientation. Similarly, while many figures show the different structural layers as horizontal layers, such orientation is for descriptive purpose only and not to be construed as a limitation.