BACKGROUND OF THE INVENTION

Technical Field

[0001] The invention relates most generally to infrared (IR) sensors, and more particularly to coated IR sensors, and still more particularly to an improved IR sensor including a stress mitigating layer and an optical coating stack, including, among others, an anti-reflective optical stack deposited atop an IR sensor surface.

Background Art

[0002] Infrared (IR) sensors are well known in commerce, industry, and in government applications. They come in many varieties. There are IR detectors that operate on thermal principles and others that are photonic, i.e., photoconductive or photovoltaic. A common class of photoconductive IR sensors is made from lead salts, namely lead selenide (PbSe) and lead sulfide (PbS). Lead sulfide IR sensors detect IR radiation in the 1 to 2.5 micron range, while lead selenide is sensitive to IR from approximately 1.5 to 5.2 microns. Indium antimonide and indium arsenide are two other common photovoltaic IR detection materials.

[0003] Lead sulfide is one of the earliest materials used in IR detector fabrication. Lead sulfide detectors work in one of two ways: either the change in resistance or the photocurrent generated by the IR radiation can be measured. With lead selenide, the change in

conductivity of a thin film of the material is measured when IR radiation is incident upon it. Indium antimonide and indium arsenide are photovoltaic photodiodes, generating electric current when exposed to IR radiation.

[0004] Several things reduce the efficiency of an IR sensor. To being with, the index of refraction for the detector materials is high, commonly around four, meaning that about forty percent of the incident radiation is lost due to surface reflection. Second, the methods commonly used for depositing IR sensors are chemical bath deposition (CBD) or epitaxial growth. These low energy deposition processes result in low density films with a porous structure. The films are easily stressed and are also vulnerable to moisture contamination. Exposure to the ambient atmosphere is often sufficient to cause the films to take up moisture and degrade the performance of the sensor by reducing the signal response to incident infrared radiation. Finally, the materials are soft, with a low Mohs hardness rating, and are thus susceptible to damage.

[0005] A common solution to these problems is to encapsulate the IR detector in glass to protect it from the surrounding environment and to prevent moisture contamination and damage. However, a glass enclosure increases the loss of incident radiation due to surface reflection, makes the detector unit larger and heavier, and adds cost due to increased labor and materials in production. For sensors that marginally meet performance specifications, the increase in surface reflection from the encapsulating may render their performance unacceptable and thereby decreases production yield.

[0006] The foregoing patents reflect the current state of the art of which the present inventor is aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein.

DESCRIPTION OF EMBODFMENTS

Disclosure of Invention

[0007] The instant invention solves the above-described problems by providing an IR sensor with a thin film deposition of a stress mitigating layer and an anti-reflecting optical stack on top of the sensor substrate.

[0008] The anti -reflective (AR) coating works by decreasing the surface reflection of the substrate, thereby allowing more light to be transmitted to the IR sensor and improving its performance. For example, a simple single layer AR film is formed by depositing a material that is transparent to IR radiation and has an index of refraction matched as closely as possible to the square root of the substrate index of refraction.

[0009] Anti-reflective coatings are well known in the thin film and optical coating industries. Accordingly, it might seem obvious to apply an AR coating to an IR sensor in order to reduce the surface reflection and result in more IR radiation reaching the sensor, thereby increasing the signal response. However, when an AR coating is applied to an IR sensor, the results are entirely contrary to what one would expect: Rather than improving the signal response, the signal is degraded and the sensor performs significantly worse than the uncoated sensor.

[0010] This unexpected result is due to stress. The crystalline structure of these sensors is crucial to their performance and depositing a denser, less porous AR coating on them causes a stress mismatch between the sensor and the AR film and disrupts the electrical properties of the sensor and decreases its performance. Research has shown that the sensors are very sensitive to this stress mismatch.

[0011] The instant invention solves this difficulty by providing an IR sensor having a stress mitigating layer deposited between the IR sensor and the AR thin film. The stress mitigating layer can be composed of many different materials and can comprise more than one layer. The key features of this mitigating layer are that it is optically inert and less dense and more porous than the AR thin film deposited on top of it. Keeping it optically inert prevents the stress mitigating layer from interfering with the decrease in surface reflection provided by the AR coating. Making the layer less dense and more porous results in it being more "flexible." It can be deposited on top of the sensor without disrupting its electrical performance and protect it from the denser, less flexible AR coating.

[0012] A further advantage of depositing an AR coating on an IR sensor which is more dense and less porous than a stress mitigating layer deposited below it is that the denser films provide environmental protection as well, forming a hermetic seal that prevents moisture contamination, and it creates a harder, more durable surface. This eliminates the need to encapsulate the sensors in glass, which reduces labor costs, material costs, production time, increases yield, and prevents loss of incident radiation due to surface reflection from the glass. If the basic AR coating design is comprised of materials that are not environmentally durable, an optically neutral, environmentally durable layer may be added to the design to hermetically seal the sensor. This seal may include one or more layers and may include multiple materials.

[0013] Different materials and coating processes can be used to produce a stress mitigating layer and an AR coating tuned to a sensor's specific wavelength response range. For example, the coatings may be deposited by evaporation, sputtering, chemical vapor deposition (CVD), or other commonly known processes. In addition, different types of processes may be combined to produce the optimal sensor performance.

[0014] In an embodiment of the invention an evaporated aluminum (III) oxide coating is deposited on a lead salt sensor as the stress mitigating layer. An evaporation process may be used for the deposition because it typically produces lower stress, more porous films. After this layer is formed, a simple one layer AR composed of sputtered zirconium dioxide may be deposited to reduce surface reflection. In this embodiment, the stress mitigating layer prevents the signal degradation observed when the zirconium dioxide layer is deposited directly onto the sensor, and the signal response improves more than an optical model predicts. The zirconium dioxide also provides a durable, wear- resistant coating that is highly resistant to moisture contamination.

[0015] While this first example involves a single layer AR, more complex multilayer coatings can also be deposited in order to form broader AR coatings that reduce reflection over a larger range or that have other desired optical effects. Multilayer stress mitigating designs may also be used.

[0016] In an embodiment, the IR sensor of the present invention may include an active IR substrate, a stress mitigating layer deposited atop the IR substrate, one or more AR layers deposited atop the stress mitigating layer, and one or more optical filter layers deposited atop the AR layer.

[0017] In another embodiment, the IR sensor of the present invention may include an active IR substrate, a stress mitigating layer deposited atop the IR substrate, one or more AR layers deposited atop the stress mitigating layer, one or more optical filter layers deposited atop the AR layer, and an environmental durability layer deposited atop the optical filter layers.

[0018] In still another embodiment, the IR sensor may include an active IR substrate, a stress mitigating layer deposited atop the IR substrate, and one or more optical filter layers deposited atop the stress mitigating layer in lieu of the AR layer.

[0019] In another embodiment, the IR sensor may include an active IR substrate, a stress mitigating layer deposited atop the IR substrate, one or more optical filter layers deposited atop the stress mitigating layer in lieu of the AR layer, and an environmental durability layer deposited atop the optical filter layers.

[0020] In another embodiment, the IR sensor may include an active IR substrate, a stress mitigating layer deposited atop the IR substrate, and an environmental durability layer deposited atop the stress mitigating layer.

[0021] In summary, as will be clear from the foregoing, the invention described and claimed herein improves the performance of an IR sensor by increasing signal response and durability. It also obviates the need to encapsulate the sensor in glass, thereby eliminating increased surface reflection of incident radiation, additional detector volume, and increased labor costs.

[0022] The foregoing summary broadly sets out the more important features of the present invention so that the detailed description that follows may be better understood, and so that the present contributions to the art may be better appreciated. There are additional features of the invention that will be described in the detailed description of the preferred embodiments of the invention which will form the subject matter of the claims appended hereto.

Brief Description of the Drawings

[0023] The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

[0024] FIG. 1 is a highly schematic cross-sectional side view in elevation showing a sensor with a stress mitigating layer and an anti -reflective coating.

[0025] FIG. 2 is a schematic cross- sectional side view in elevation showing a sensor with a stress mitigating layer, an anti -reflective coating, and a hermetic sealing layer.

[0026] FIG. 3 is a graph comparing signal responses of a first IR sensor having an AR coating without a stress mitigating later with a second IR sensor having a stress mitigating layer.

[0027] FIG. 4 is a schematic cross-sectional side view in elevation showing an IR sensor having optimal filter layers disposed atop an anti -reflective layer. [0028] FIG. 5 is a schematic cross- sectional side view in elevation showing an IR sensor having optical filter layers disposed atop a stress mitigating layer in lieu of one or more AR layers.

Best Mode for Carrying Out the Invention

[0029] Referring first to FIGS. 1 and 2, there is illustrated therein a new and improved infrared sensor. The invention is an improved IR sensor that has increased signal response and does not need to be encased in a protective covering to protect it from environmental degradation. The current state of the art IR sensor has limited performance for many reasons. The high index of refraction of common sensor materials means that up to forty percent of the incident infrared radiation is lost due to surface reflection. Sensors are also commonly made by chemical bath deposition or epitaxial growth. These low-energy processes result in porous, low density films that are susceptible to moisture contamination and are easily damaged. The current solution is to encase the sensors in glass, which is labor intensive, costly, decreases yield and increases the size of the sensor, rendering it less suitable for many applications.

[0030] As noted above, photoconductive and photovoltaic IR sensors are well-known in the sensor industry, as are anti -reflection coatings, yet AR coatings have not been commonly used with these IR sensors due to such coatings unexpectedly degrading the performance of the sensors. A sensor's response to IR radiation is highly dependent on its crystalline structure and research showed that the denser, less porous anti -reflective coatings caused a stress mismatch that disrupted the crystalline structure and therefore its response to infrared radiation. The instant invention uses a stress-mitigating layer made of material which is softer and more porous than the AR coating material deposited on the stress mitigating layer. This combination of material characteristics does not negatively affect sensor performance.

[0031] The stress mitigating layer shields the sensor from the stress mismatch of the anti- reflective coating. This permits the anti -reflective coating to serve its intended purpose of decreasing surface reflection and increasing the amount of incident radiation that reaches the sensor. Experiments show that depositing a stress mitigating layer before the anti-reflective coating not only improves sensor performance, it improves it more than would be predicted by the AR coating alone, which was unexpected. [0032] An embodiment of the invention is illustrated in FIG. 1. Here it is seen that the IR sensor 10 includes an IR substrate (the IR sensitive or active layer) 12 with an upper (or top) surface 14 having a stress mitigating layer 16 deposited on the upper surface. The stress mitigating layer may be comprised of one or more layers and may be made of multiple materials. It may be deposited using a variety of techniques, including sputtering,

evaporation, ion beam deposition, or CVD. An anti-reflective coating 18 is then deposited on upper surface 20 of the stress mitigating layer. The anti -reflective coating may be a single layer of one material, or it may be a more complex design consisting of multiple layers and multiple materials, depending on what wavelength range the AR coating is required to cover. The AR coating may be deposited using the same variety of techniques available for the stress mitigating layer. Alternatively, a different process may be used for each layer. In an embodiment the AR coating may also provide environmental durability, depending on the materials chosen.

[0033] In another embodiment, shown in FIG. 2, an IR sensor 30 again includes an IR substrate (IR active layer) 32, a stress mitigating layer 34 deposited on the upper surface 36 of the IR substrate. The stress mitigating layer may be comprised of one or more layer and may be made of multiple materials. It may be deposited using a variety of techniques, including sputtering, evaporation, ion beam deposition, or CVD. An anti-reflective coating 38 is then deposited on the upper surface 40 of the stress mitigating layer. The anti-reflective coating may be a single layer comprising one material or a more complex design consisting of multiple layers and multiple materials, depending on what wavelength range the AR coating is required to cover. The AR coating may be deposited using the same variety of techniques available for the stress mitigating layer. A different process may be used for each layer. In this embodiment, the AR coating does not provide environmental durability, a characteristic that may occur when a good design for the desired AR performance uses less environmentally durable materials. In this instance, a durability layer 42 that provides protection from moisture and other environmental degradation can be deposited on the upper surface 44 of the AR coating. The durability layer may be comprised of one or more layers and may be made from one or more materials. The durability layer may be deposited using the same techniques available for the stress mitigating and AR layers. The same deposition technique may be used for each layer, or different deposition processes may be used. [0034] In tests analyzing the performance of an embodiment of the invention, an aluminum oxide (III) (A1 ₂ 0 ₃ ) stress mitigating layer and a single layer zirconium dioxide (Zr0 ₂ ) anti -reflective coating were deposited on a group of lead sulfide sensors. Before the coatings were applied, sensor performance was measured.

[0035] The A1 ₂ 0 ₃ coating was applied using evaporation techniques in a vacuum coater. The chamber was pumped down to a range of 10-5 torr and 3/8 inches of 99.99% aluminum wire 1/8 inch in diameter was placed in a tungsten boat. Oxygen was added to the chamber and the material was evaporated at a pressure of approximately 3.5 x 10-4 torr. The aluminum wire was evaporated in the oxygen atmosphere for six minutes at approximately 2.6 V dc, depositing a layer of aluminum (III) oxide on the infrared sensors.

[0036] After the aluminum (III) oxide layer was deposited, the evaporation system was shut off and a single layer of zirconium dioxide was sputtered on top of the evaporated aluminum (III) oxide layer. The zirconium dioxide layer was designed to provide both anti- reflective optical properties and environmental durability. The zirconium dioxide was sputtered at a pressure of 4.2 mT in a combined atmosphere of argon and oxygen, using a standard dc power supply operating at 3 kW.

[0037] After the two coatings were applied, the sensors were measured and their performance compared to sensors that had been coated with the same zirconium dioxide anti- reflective coating but which lacked the stress mitigating layer. The results are shown in the graph 50 of FIG. 3.

[0038] The lower line 52 (connecting square data points) shows sensors that were coated with an anti -reflective coating but lacked an aluminum (III) oxide stress mitigating layer underneath the anti -reflective layer. The upper line 54 (connecting diamond data points) shows identical sensors but including a stress-mitigating layer disposed between the IR substrate and the anti -reflective layer. The results compare the performances of uncoated sensors with those of coated sensors and show the differences as a percentage. That is, a data point at the 20 percent line for a coated sensor has a performance (signal response) twenty percent that of the uncoated sensor. A sensor with a reading of 140 percent has a signal response forty percent greater than that of the uncoated sensor.

[0039] The graph of FIG. 3 clearly shows that without a stress mitigating layer, the coated sensors had a performance significantly lower than coated sensors, while sensors having a stress mitigating layer applied before the anti-reflective coating showed a significantly improved signal response - as high as fifty percent higher.

[0040] Note that the anti-reflective coating greatly improves environmental durability. An uncoated sensor is highly sensitive to moisture contamination, so much so that simply leaving the uncoated sensor out in a regular environment can cause it to take up moisture and degrade its performance, necessitating the labor intensive and costly solution of

encapsulating the sensor in glass. With an anti -reflective coating, the sensors can be placed in a high moisture environment, such as a humidity chamber, and still have an unchanged performance, indicating they are no longer sensitive to moisture contamination.

[0041] The foregoing tests cover only one embodiment of the invention, yet there are many ways the inventive IR sensor could be implemented, many obvious to one ordinarily skilled in the art. For example, the stress mitigating layer may be made of different materials, such as oxides, nitrides, or mixed materials. The stress mitigating layer may be deposited by evaporation, sputtering, CVD or any other deposition method. The best choice will be determined by the sensor material. The stress mitigating layer may also be composed of a multilayer coating if desired.

[0042] The anti -reflective coating may also be deposited using different coating methods and different materials. As shown in FIG. 4, if an IR sensor 60 having broader anti -reflective range is desired, the sensor may include a sensor substrate 62 having an upper surface 64, a stress mitigating layer 66 deposited on the upper surface of the sensor substrate and itself having an upper surface 68, and an anti-reflective coating composed of multiple layers 70a, 70b, the uppermost 70b having an upper surface 72, designed to give the desired optical performance. If further optical properties are desired, single or multilayer filters 74a, 74b may be deposited on the upper surface 72 of the upper anti -reflective coating layer 70b.

[0043] In still another embodiment, 80, FIG. 5, wherein an anti -reflective coating is not needed, an IR substrate 72 may have a stress mitigating layer 74 deposited on its upper surface 76, and a single or multilayer optical filter coating 78a, 78b, may be placed directly on the upper surface 80 of the stress mitigating layer 74. Also, if the desired optical properties are best acquired using less durable materials, a layer of a material 82 different from the rest of the design may be deposited on the upper surface 84 of the upper optical filter coating 78b as the top (or outer) layer of the design in order to maintain environmental durability.

[0044] The instant invention is also applicable for many different types of sensor materials in addition to the lead sulfide and lead selenide IR sensors discussed above. These materials include, but are not limited to, InAs/GasInSb, HgTe/CdTe, Si:As, Si:Ga, Si:Sb, Ge:Hg, Ge:Ga, GaAs/AlGaAs, PtSi, IrSi, InAs/GaAs, PbTe, PbSe, PbS, CdS, CdSe, PbCdS, PbxSnl-xTe, PbySnl-ySe, HgCdTe, InSb, InAs, QWIPs, QDIPs.

[0045] The improved sensor overcomes persisting problems by providing an IR sensor having an anti -reflective coating deposited on top of an IR sensor with a stress mitigating layer disposed between the sensor substrate and the AR layer. The stress mitigating layer prevents the anti -reflective coating from disrupting the crystalline structure of the sensor, and therefore its electrical properties, due to stress. This allows the AR to perform its intended function of decreasing surface reflection and allows more incident radiation to reach the sensor, thereby improving sensor performance. The anti -reflective coating also serves to improve the environmental durability of the sensor, decreasing its susceptibility to moisture contamination, and obviating the need to encapsulate the sensor in glass, with corresponding decreases in production time and expense, as well as a reduction in sensor size.

[0046] The above disclosure is sufficient to enable one of ordinary skill in the art to practice the invention, and provides the best mode of practicing the invention presently contemplated by the inventor. While there is provided herein a full and complete disclosure of the preferred embodiments of the invention, it is not desired to limit the invention to the exact construction, dimensional relationships, and operation shown and described. Various modifications, alternative constructions, changes and equivalents will readily occur to those skilled in the art and may be employed, as suitable, without departing from the true spirit and scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like.

[0047] Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims.