US20100133536A1

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Publication number: US20100133536A1
Application number: US11/498,939
Authority: US
Inventors: Althanasios J. Syllaios; Thomas R. Schimert; Michael F. Taylor
Original assignee: Individual
Current assignee: DRS Network and Imaging Systems LLC
Priority date: 2006-08-03
Filing date: 2006-08-03
Publication date: 2010-06-03
Also published as: US7718965B1

Abstract

Microbolometer infrared detector elements that may be formed and implemented by varying type/s of precursors used to form amorphous silicon-based microbolometer membrane material/s and/or by varying composition of the final amorphous silicon-based microbolometer membrane material/s (e.g., by adjusting alloy composition) to vary the material properties such as activation energy and carrier mobility. The amorphous silicon-based microbolometer membrane material/s materials may include varying amounts of one or more additional and optional materials, including hydrogen, fluorine, germanium, n-type dopants and p-type dopants.

Description

FIELD OF THE INVENTION

This invention relates in general to infrared detectors and more particularly to microbolometer infrared detector elements and methods for forming the same.

BACKGROUND OF THE INVENTION

Infrared (IR) detectors are often utilized to detect fires, overheating machinery, planes, vehicles, people, and any other objects that emit thermal radiation. Infrared detectors are unaffected by ambient light conditions or particulate matter in the air such as smoke or fog. Thus, infrared detectors have potential use in night vision and when poor vision conditions exist, such as when normal vision is obscured by smoke or fog. IR detectors are also used in non-imaging applications such as radiometers, gas detectors, and other IR sensors.

Infrared detectors generally operate by detecting the differences in thermal radiance of various objects in a scene. That difference is converted into an electrical signal which is then processed. Microbolometers are infrared radiation detector elements that are fabricated on a substrate material using traditional integrated circuit fabrication techniques. Microbolometer detector arrays consist of thin, low thermal mass, thermally isolated, temperature-dependent resistive membrane structures. They are suspended over silicon readout integrated circuit (ROIC) wafers by long thermal isolation legs in a resonant absorbing quarter-wave cavity design.

Conventional infrared detector arrays and imagers operating at ambient temperature include microbolometer arrays made of thin films of hydrogenated amorphous silicon (a-Si:H) or amorphous vandium oxide (VOx). Other materials used for microbolometer arrays include films of various metal (e.g., titanium) and high temperature superconductors. For an array based on amorphous silicon, the detector pixel membrane is generally comprised of an ultra-thin (˜2000 Å) a-SiN_x/a-Si:H/a-SiN_xstructure. The membrane is deposited at a low temperature nominally below 400° C. using silane (SiH₄) and ammonia (NH₃) precursors for the amorphous silicon nitride (a-SiN_x) layers, and using silane for the hydrogenated amorphous silicon (a-Si:H) layer. Hydrogen atoms from silane (SiH₄) molecules are the source of hydrogen content in the a-Si:H layer. A thin absorbing metal layer such as Titanium (Ti), Titanium-Aluminum alloy (TiAl), Nichrome (NiCr), black gold, or other material absorbing in the infrared band of interest, (e.g., at wavelength range of 1 micron to 14 micron), is inserted in the membrane to enhance infrared absorptance. Contact between the a-Si:H detector electrodes and the interconnect pads on a complementary metal oxide semiconductor (CMOS) signal processor of the ROIC is accomplished by thick aluminum tab metal interconnects.

After fabrication, microbolometers are generally placed in vacuum packages to provide an optimal environment for the sensing device. Conventional microbolometers measure the change in resistance of a detector element after the microbolometer is exposed to thermal radiation. Microbolometers have applications in gas detectors, night vision, and many other situations.

The primary factors affecting response time and sensitivity of microbolometers are thermal mass and thermal isolation. Microbolometer response time is the time necessary for a detector element to absorb sufficient infrared radiation to alter an electrical property, such as resistance, of the detector element and to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer sensitivity is determined by the amount of infrared radiation required to cause a sufficient change in an electrical property of the microbolometer detector element. Microbolometer response time is inversely proportional to both thermal mass and thermal isolation. Thus, as thermal mass increases, response time becomes slower since more infrared energy is needed to sufficiently heat the additional thermal mass in order to obtain a measurable change in an electrical property of the microbolometer detector element. As thermal isolation increases, response time becomes slower since a longer period of time is necessary to dissipate the heat resulting from the absorption of the infrared radiation. Microbolometer operating frequency is inversely proportional to response time. However, microbolometer sensitivity is proportional to thermal isolation. Therefore, if a specific application requires high sensitivity and does not require high operating frequency, the microbolometer would have maximum thermal isolation and minimal thermal mass. If an application requires a higher operating frequency, a faster microbolometer may be obtained by reducing the thermal isolation which will also result in a reduction in sensitivity.

SUMMARY OF THE INVENTION

Disclosed herein are microbolometer infrared detector elements and methods for forming the same. The disclosed microbolometer infrared detector elements may be formed and implemented by varying type/s of precursors used to form amorphous silicon-based microbolometer membrane material/s and/or by varying composition of the final amorphous silicon-based microbolometer membrane material/s (e.g., by adjusting alloy composition) to vary the material properties such as activation energy and carrier mobility. Advantageously, by so varying precursor types and/or material properties of the microbolometer membrane material/s, it is possible to control and optimize device parameters including, but not limited to, resistance, thermal coefficient of resistance (TCR), electrical noise, and combinations of such device parameters.

By adjusting precursors and alloy composition of the membrane structure, the disclosed microbolometer material systems may be implemented to provide microbolometer devices having improved stability and performance compared to conventional microbolometer devices by independent control of resistance and TCR. In one exemplary embodiment, low doped material may be employed to increase TCR which results in higher microbolometer device responsivity. For example, in one exemplary embodiment low doping levels may be selected in order to obtain TCR values of from about 2% per ° C. to about 5% per ° C., and resitivity values of from about 1 ohm-centimeter to about 10,000 ohm-centimeters, although greater and lesser doping levels are also possible. Low doped material results in high resistance which is difficult to match with the input impedance of ROICs operating at ambient temperature.

The disclosed microbolometer infrared detector elements may include membrane structures formed from thin films of amorphous silicon-based materials that include varying amounts of one or more additional and optional materials, including hydrogen, fluorine, germanium, n-type dopants and p-type dopants. In this regard, the disclosed microbolometer infrared detector elements may include membrane structures formed from amorphous silicon-based materials such as fluorinated amorphous silicon-based materials that include at least fluorine and silicon constituents, amorphous silicon germanium-based materials that include at least silicon and germanium constituents, amorphous germanium-based materials that include at least germanium, and/or hydrogenated amorphous silicon-based materials that include at least amorphous silicon and that have a hydrogen content of greater than about 4 atomic percent, or a combination thereof. It will be understood that such amorphous silicon-based materials may optionally included additional constituents, for example, other elements, p-type or n-type dopants, etc. Advantageously, material properties of microbolometer membrane structures may be varied to optimize device parameters by varying the amount of hydrogen, germanium, fluorine, n-type dopants, p-type dopants, etc. within a given membrane material.

Specific examples of such amorphous silicon-based materials include, but are not limited to, undoped or doped (p-type or n-type) hydrogenated amorphous silicon (a-Si:H); undoped or doped (p-type or n-type) fluorinated amorphous silicon-based materials such as fluorinated amorphous silicon (a-Si:F), hydrogenated fluorinated amorphous silicon (a-Si:H:F) and hydrogenated fluorinated amorphous silicon germanium (a-Si_1-xGe_x:H:F); and undoped or doped (p-type or n-type) amorphous silicon germanium-based materials such as amorphous silicon germanium (a-Si_1-xGe_x), hydrogenated amorphous silicon germanium (a-Si_1-xGe_x:H), and hydrogenated fluorinated amorphous silicon germanium (a-Si_1-xGe_x:H:F), where “x” is the Ge content relative to silicon content of a-Si_1-xGe_xor an alloy of a-Si_1-xGe_x:H. It is noted that in the case where the value of “x” is equal to zero, a-Si_1-xGe_xrepresents amorphous silicon (a-Si), and where “x” is equal to one, a-Si_1-xGe_xrepresents amorphous germanium (a-Ge). Further, amorphous silicon germanium-based materials having the formula a-Si_1-xGe_x(with or without additional constituents) are represented by the case where the value of “x” is greater than zero but less than one.

In one embodiment, the disclosed microbolometer infrared detector elements may be formed using a hydrogen-dilution process in which hydrogen (H₂) precursor is used to dilute other precursor materials, including those described elsewhere herein, to form amorphous silicon-based microbolometer membrane structures having improved stability and performance characteristics. Although not wishing to be bound by theory, it is believed that this improved stability and performance results from the effects that H₂has on reaction and/or reaction kinetics of the amorphous silicon-based material formation process. Hydrogen dilution may be employed in the formation of thin films of amorphous silicon-based materials that include varying amounts of one or more additional materials, such as fluorine, germanium, n-type dopants, p-type dopants, etc. As an example, a microbolometer membrane structure including a-Si:H may be formed by the addition of hydrogen gas to a chemical vapor deposition (CVD) reactor, such as PECVD reactor, to dilute silane gas during membrane fabrication. Advantageously, hydrogen dilution of silane results in the growth of hydrogenated amorphous silicon (a-Si:H) which exhibits improved properties such as lower electrical noise, higher temperature coefficient of resistance (TCR) and more stable atomic configuration. In one exemplary embodiment, hydrogenated amorphous silicon having a thickness of about 600 Angstrom may be formed in a PECVD reactor using a dilution ratio of about 10:1 (i.e., about 10 parts hydrogen gas to about one part silane gas) to fabricate a microbolometer membrane structure that exhibits a reduction in noise by a factor of greater than about three (alternatively by a factor of greater than about four), in comparison to a amorphous silicon microbolometer membrane structure similarly formed but with no hydrogen dilution.

In one exemplary embodiment, hydrogen dilution may be employed to result in an amorphous silicon-based material that includes a hydrogen content that is enhanced as compared to amorphous silicon-based materials formed from SiH₄precursor without hydrogen dilution. For example, hydrogen content of an amorphous silicon-based material formed with hydrogen dilution may be greater than about 4 atomic percent (%), alternatively greater than or equal to about 5 atomic %, alternatively greater than or equal to about 9 atomic %, alternatively greater than or equal to about 10 atomic %, alternatively from about 9 atomic % to about 11 atomic %.

In another embodiment, a source of germanium may be added during amorphous silicon-based material film growth to form an amorphous silicon germanium-based alloy. Examples of suitable precursors for this embodiment include, but are not limited to, precursors such as silane or silicon tetrafluoride (SiF₄) for the silicon source, and precursors such as germanium tetrafluoride (GeF₄) or Germane (GeH₄) as the Ge source. In this embodiment, the amount of Ge may be varied to form a a-Si_1-xGe_x-based film having an amount of Ge, represented by “x”, that may vary from 0 for pure silicon to 1 for pure germanium. The amount of Ge may be so varied to result in silicon germanium alloy-based films with an extended range of electrical properties such as resistivity, TCR and noise. As previously described, hydrogen may be added to dilute the precursor gases (in this case silicon and germanium precursor gases). For example, hydrogenated silicon germanium (a-Si_1-xGe_x:H) may be grown from hydrogen, silane and germane precursors. In another example, hydrogenated and fluorinated amorphous silicon germanium films (a-Si_1-xGe_x:H:F) may be grown by also adding a source of fluorine (e.g., BF₃dopant and/or fluorine-based precursor such as SiF₄and or GeF₄) to the hydrogen, silicon and germanium precursor gases.

In another embodiment, a p-type dopant may be added during the growth of amorphous silicon-based material film to introduce boron in a growing amorphous silicon-based film of a microbolometer membrane structure. Examples of suitable p-type dopants include, but are not limited to, aluminum, gallium, indium and boron, e.g., from boron sources such as boron trifluoride (BF₃), diborane (B₂H₆), Trimethyl Boron (B(CH₃)₃(TMB), Boron Trichloride (BCl₃), etc. In one exemplary embodiment, BF₃may be utilized as a source of both boron p-type dopant and fluorine atoms for forming a fluorinated amorphous silicon-based material. Advantageously fluorine atoms promote stable atomic configuration of the amorphous silicon film by preferentially etching and removing weakly bonded silicon atoms. During film growth, some of the fluorine atoms are incorporated in the film and the resulting doped film is also fluorinated.

In yet another embodiment, a n-type dopant may be added during amorphous silicon-based material film growth to dope a film with n-type dopant. Examples of suitable n-type dopants include, but are not limited to, sources of phosphorous such as phosphine (PH₃) that may be added during film growth. For example, silicon germanium films may be doped with n-type dopant by adding such a n-type dopant during growth of the film.

As previously described, a fluorinated amorphous silicon-based material may be formed by adding at least one fluorine source may during the growth of amorphous silicon-based material film to introduce fluorine in a growing amorphous silicon-based film of a microbolometer membrane structure. Examples of suitable precursors for this embodiment include, but are not limited to, precursors such silicon tetrafluoride (SiF₄), germanium tetrafluoride (GeF₄) and boron trifluoride (BF₃).

In another embodiment, amorphous silicon-based material films may be grown near the amorphous-crystalline phase transition to result in a stable amorphous atomic structure, e.g., an amorphous silicon-based material structure that contains an amount of microcrystallites that is less than or equal to about 10% by volume of the amorphous silicon-based material (as determined by Raman spectroscopy) and that are from about 1 nanometers to about 10 nanometers in size (as determined by x-ray diffraction). However, it will be understood that amorphous silicon-based material films may contain an amount of microcrystallites greater than about 10% by volume of the amorphous silicon-based material, and/or that have a size of less than about 1 nanometer or greater than about 10 nanometers.

Examples of amorphous silicon-based materials disclosed herein include, but are not limited to, undoped or doped (p-type or n-type) hydrogenated amorphous silicon (a-Si:H), fluorinated amorphous silicon (a-Si:F), hydrogenated fluorinated amorphous silicon (a-Si:H:F), amorphous silicon germanium (a-Si_1-xGe_x), hydrogenated amorphous silicon germanium (a-Si_1-xGe_x:H), and hydrogenated fluorinated amorphous silicon germanium (a-Si_1-xGe_x:H:F). Amorphous silicon-based material films may be grown in one exemplary embodiment by chemical vapor deposition (CVD), such as plasma enhanced chemical vapor deposition (PECVD), using silane and optionally one or more other precursors. For example, silane, hydrogen and germanium tetrafluoride (GeF₄) precursors may be used to form an a-Si_1-xGe_x:H alloy with specific composition “x” by adjusting the ratio of H₂, SiH₄and GeF₄flow rates. In such a case, growth parameters such as temperature, growth rate and addition of H₂may be selected so that a silicon germanium alloy structure is amorphous, but near the amorphous-to-crystalline phase transition. In other embodiments, amorphous silicon-based materials may be formed by any other technique that is suitable for forming the same such as sputtering, molecular beam epitaxy, etc. For example, amorphous silicon germanium (a-Si_1-xGe_x) may be formed by sputtering of silicon and germanium or by molecular beam epitaxy using silicon and germanium.

In addition to PECVD, any other method/s for growing films of amorphous silicon-based materials may be employed, for example, to grow and form a-Silicon Nitride/a-Silicon-based material/a-Silicon Nitride microbolometer pixel membranes or any other configuration of microbolometer pixel membrane that includes amorphous silicon-based material. Examples of such alternative growth techniques include, but are not limited to, Hot Wire Chemical Vapor Deposition (HWCVD), Electron Cyclotron Resonance Chemical Vapor Deposition (ECR-CVD), and Microwave CVD.

Using the methods and materials disclosed herein, a microbolometer infrared detector element pixel may be provided that in one embodiment includes a suspended membrane of amorphous silicon-based material that is disposed between two layers of amorphous-silicon nitride that support and adjust the overall stress of the membrane, although any other suitable microbolometer membrane configuration may be employed. As disclosed herein, such microbolometer infrared detector elements may be provided in one embodiment in the form of microbolometer bridge structures that may be used to form large arrays. Further, the microbolometer pixel optical design of a microbolometer infrared detector element pixel may be of any suitable configuration, for example, a refractive resonant cavity or a diffractive resonant cavity (DRC).

In one respect, disclosed herein is an infrared detector element including: a substrate; and an infrared detector membrane disposed in spaced relationship above the substrate. The infrared detector membrane may include an amorphous silicon-based material, the amorphous silicon-based material including at least one of a fluorinated amorphous silicon-based material, an amorphous silicon germanium-based material, an amorphous germanium-based material, a hydrogenated amorphous silicon-based material having a hydrogen content of greater than about 4 atomic percent, or a combination thereof.

In another respect, disclosed herein is a focal plane array assembly, including: a substrate; and a plurality of infrared detector elements, each of the plurality of infrared detector elements including an infrared detector membrane disposed in spaced relationship above the substrate, and read out integrated circuitry (ROIC) electrically coupled to the infrared detector membrane. The infrared detector membrane of each of the plurality of infrared detector elements may include an amorphous silicon-based material, the amorphous silicon-based material including at least one of a fluorinated amorphous silicon-based material, an amorphous silicon germanium-based material, an amorphous germanium-based material, a hydrogenated amorphous silicon-based material having a hydrogen content of greater than about 4 atomic percent, or a combination thereof.

In another respect, disclosed herein is a method for making an infrared detector element, the method including: providing a substrate; and forming an infrared detector membrane in spaced relationship above a surface of the substrate. The infrared detector membrane may include an amorphous silicon-based material, the amorphous silicon-based material including at least one of a fluorinated amorphous silicon-based material, an amorphous silicon germanium-based material, an amorphous germanium-based material, a hydrogenated amorphous silicon-based material having a hydrogen content of greater than about 4 atomic percent, or a combination thereof.

In another respect, disclosed herein is a method of making a focal plane array assembly, the method including: providing a substrate; and forming a plurality of infrared detector elements, each of the plurality of infrared detector elements including an infrared detector membrane disposed in spaced relationship above the substrate, and read out integrated circuitry (ROIC) electrically coupled to the infrared detector membrane. The infrared detector membrane may include an amorphous silicon-based material, the amorphous silicon-based material including at least one of a fluorinated amorphous silicon-based material, an amorphous silicon germanium-based material, an amorphous germanium-based material, a hydrogenated amorphous silicon-based material having a hydrogen content of greater than about 4 atomic percent, or a combination thereof.

In another respect, disclosed herein is a method for making an infrared detector element, the method including: providing a substrate; forming an infrared detector membrane in spaced relationship above a surface of the substrate, the infrared detector membrane including amorphous silicon-based material; and forming the amorphous silicon-based material from precursors including at least one source of silicon and boron trifluoride (BF₃).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of an infrared detector according to one embodiment disclosed herein.

FIG. 1B is a perspective of a microbolometer infrared detector element formed on a substrate in accordance with one embodiment disclosed herein.

FIG. 2 is a cross-section illustration of a partially formed microbolometer of one embodiment disclosed herein.

FIG. 3 is a diagram illustrating the partially fabricated microbolometer after completing the steps illustrated inFIG. 2.

FIG. 4 is a cross-section illustration of a method of forming the microbolometer of one embodiment disclosed herein.

FIG. 5 is a diagram illustrating a partially fabricated microbolometer after completion of the steps illustrated inFIG. 4.

FIG. 6 is a cross-section illustration of a method of forming the microbolometer of one embodiment disclosed herein.

FIG. 7 is a diagram illustrating the microbolometer of one embodiment disclosed herein after etching to define a final form of the microbolometer.

FIG. 8 is a cross-section diagram illustrating deposition of a post and thermal shunting device.

FIG. 9 is a cross-section schematic illustration of the microbolometer of one embodiment disclosed herein prior to removal of a polyimide layer.

FIG. 10 is a cross-section illustration of the completed microbolometer of one embodiment disclosed herein.

FIG. 11 is an illustration of a microbolometer with spiral legs.

FIG. 12 is a flow diagram illustrating the formation of the microbolometer of one embodiment disclosed herein.

FIG. 13A is an illustration of a configuration of microbolometers in accordance with one embodiment disclosed herein wherein non-imaging pixels are connected electrically in parallel.

FIG. 13B illustrates an array of microbolometers in accordance with one embodiment disclosed herein wherein non-imaging pixels are connected in an electrically series-parallel circuit.

FIG. 13C schematically illustrates an electrical series-parallel configuration of non-imaging pixels for a large array.

FIG. 14A is a schematic illustration of a linear non-imaging pixel array with shared thermal isolation legs for adjacent microbolometers of one embodiment disclosed herein.

FIG. 14B is a schematic illustration of an array of spiral leg pixels connected electrically in parallel for large non-imaging arrays for maximized fill factor; and

FIG. 15 illustrates another embodiment of a microbolometer formed in accordance with one embodiment disclosed herein for maximizing the fill factor and minimizing space between adjacent microbolometers.

FIG. 16 is a perspective of a microbolometer infrared detector element formed on a substrate in accordance with one embodiment disclosed herein.

FIG. 17 is a perspective of a microbolometer infrared detector element formed on a substrate in accordance with one embodiment disclosed herein.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1A is a diagrammatic perspective view of one embodiment of aninfrared detector1 which is capable of sensing thermal energy and of outputting electrical signals representative of a two-dimensional image of that sensed thermal energy. Theinfrared detector1 includes afocal plane array2 disposed on asubstrate11. Thesubstrate11 may include an integrated circuit of a type which is commonly known as a read out integrated circuit (ROIC). The ROIC reads out the thermal information gathered by thefocal plane array12.

Athermal element17 may be optionally provided on the side of thesubstrate16 opposite from thefocal plane array2, in order to serve as a form of controlled heat sink which maintains theintegrated circuit substrate11 at a substantially constant temperature which is predefined. The constant temperature prevents ambient or internally generated temperature gradients from affecting operation of thefocal plane2, and thus provides a baseline with which the thermal energy impinging on thefocal plane array2 may be accurately measured.

Thefocal plane2 includes a plurality of thermal sensors or detector elements, one of which is designated byreference numeral10. The detector elements are arranged in a two-dimensional array, and eachdetector element10 corresponds to a respective pixel in each image detected by theinfrared detector1. Thefocal plane array2 of the disclosed embodiment includes 76,800detector elements10, which are arranged in a 320 by 240 array. For clarity, however,FIG. 1A diagrammatically depicts only about 140 detector elements. It will be recognized that the total number ofdetector elements10 in thefocal plane array2 could be larger or smaller. Further, even thoughFIG. 1A shows thedetector elements10 arranged in a two-dimensional array, they could alternatively be arranged in a one-dimensional array, or could be provided at arbitrary locations that do not conform to a specific pattern. With reference toFIG. 1B, one of thedetector elements10 ofFIG. 1A will be described in more detail.

More specifically,FIG. 1B is a diagrammatic fragmentary perspective view of a portion of theintegrated circuit substrate11, which has one of thedetector elements10 on it that is formed in the configuration of a microbolometer infrared detector element in accordance with one exemplary embodiment disclosed herein. In the disclosed embodiment, the detector elements all have the same structural configuration, and therefore only one of them is illustrated and described here in detail. In this embodiment, microbolometerinfrared detector element10 is formed on asubstrate11.Substrate11 may be any suitable substrate material including a monocrystalline silicon wafer or a silicon wafer containing a readout integrated circuit (ROIC). In this embodiment, microbolometerinfrared detector element10 is a sensor that is operable to detect infrared radiation.

Still referring toFIG. 1B, microbolometerinfrared detector element10 includesthermal isolation legs14 coupled to adetector membrane12. Infrared radiation sensed by thedetector membrane12 results in a measurable change in the resistance of the material comprising the detector.Detector membrane12 is suspended over the surface ofsubstrate11 bythermal isolation legs14. Construction of thedetector membrane12 may be in several layers of various materials as discussed in further detail below. In this exemplary embodiment, twothermal isolation legs14 are coupled along one side ofdetector membrane12 and proceed unattached along a second, adjacent side to anelectrode terminal end15. Apost16 is coupled to theelectrode terminal end15 ofthermal isolation leg14.Post16 provides structural support and electrical connection formicrobolometer10. Electrical circuitry connected to electrode terminal ends15 provides a constant voltage across thethermal isolation legs14 and senses a change in electrical current flowing throughdetector membrane12. The magnitude of the change in electrical current varies with the amount of infrared radiation detected. In an alternate embodiment, the electrical circuitry may provide a constant electrical current flowing throughdetector membrane12 and senses a change in the voltage acrossthermal isolation legs14.

The thermal mass ofmicrobolometer10 affects the thermal isolation, response time, operating frequency, and sensitivity. By fabricating a microbolometer with minimal thermal mass, high sensitivity and high operating frequency may be realized. Thermal isolation ofmicrobolometer10 fromsubstrate11 also affects the operating frequency and sensitivity. Thermal isolation ofdetector membrane12 fromsubstrate11 increases the sensitivity ofmicrobolometer10 since less infrared radiation energy is necessary to raise the temperature ofdetector membrane12. Thermal isolation also affects the operating frequency and response time ofmicrobolometer10 since it affects the cooling rate ofdetector membrane12. An increase in thermal isolation results in a corresponding decrease in cooling rate ofdetector membrane12 and, thus, a corresponding decrease in operating frequency ofmicrobolometer10.

In one embodiment, a single step in the fabrication ofmicrobolometer10 may be modified to place athermal shunt18 onthermal isolation legs14 coupled toposts16 to decrease the thermal isolation ofmicrobolometer10. Placing athermal shunt18 onthermal isolation leg14 will increase the operating frequency ofmicrobolometer10 since the cooling rate ofdetector membrane12 is increased.Thermal shunt18 onthermal isolation legs14 also results in decreased sensitivity since more thermal coupling betweendetector membrane12 andsubstrate11 exists. Thus, an increased amount of infrared radiation energy is necessary to increase the temperature ofdetector membrane12 resulting in a corresponding change in the electrical resistance of the detector. By varying the length ofthermal shunt18, and thus the amount of thermal shunt material deposited onthermal isolation legs14, amicrobolometer10 with differing operating frequency and sensitivity characteristics may be fabricated.

As shown for the exemplary embodiment ofFIG. 1B, beneathdetector membrane12 is an antireflective structure andresonant cavity20.Antireflective structure20 functions to minimize the amount of infrared radiation unabsorbed bydetector membrane12 and may be present in one embodiment to enhance absorption of infrared radiation.Detector membrane12 is suspended above the surface ofsubstrate11 at a height of approximately one-quarter wavelength of the infrared radiation to be detected bymicrobolometer10. The one-quarter wavelength height causes infrared energy waves unabsorbed bydetector membrane12 to be reflected byreflector22 and trapped inantireflective structure20 until the infrared radiation is absorbed bydetector membrane12.Antireflective structure20 creates a moreefficient microbolometer10 since the amount of infrared radiation absorbed bydetector membrane12 is maximized.

Referring now toFIG. 2, semiconductor substrate or integratedcircuit11 provides the base for the formation ofmicrobolometer10. Asilicon dioxide layer30 is formed onsubstrate11. A thin layer oftitanium32 is next formed onsilicon dioxide layer30 followed by a thin layer ofaluminum34.Aluminum layer34 andtitanium layer32 are patterned using a photoresist and etch process to formconnection pads40 for providing electrical connections to other electrical circuitry formicrobolometer10. In addition,aluminum layer34 andtitanium layer32 are patterned to formreflector22 for providing a reflective surface within antireflective structure and theresonant cavity20 as shown inFIG. 1B. In one exemplary embodiment,microbolometer10 is formed as a part of a readout integrated circuit (ROIC). Oneconnection pad40 ofmicrobolometer10 passes through the surface dielectric layer of thesubstrate11 to make contact with the underlying electrical circuitry. Theother connection pad40 ofmicrobolometer10 is coupled to a common bus formed from thealuminum layer34 on the surface ofsubstrate11.FIG. 3 illustrates inpart aluminum layer34 after patterning by the photoresist and etch technique.

As shown inFIG. 2, apolyimide layer36 is deposited in this exemplary embodiment over the entire structure to a depth on the order of one-quarter wavelength of the infrared radiation to be detected. A one-quarter wavelength depth provides the proper spacing betweenreflector22 ofantireflective structure20 and the bottom surface ofdetector membrane12. Thepolyimide36 is an organic material. Openings are etched inpolyimide layer36 to exposealuminum connection pads40 to definepost receptors38.Post receptors38 are holes in electrode terminal ends15 that will eventually contain an aluminum post providing structural support and electrical connections formicrobolometer10.Post receptors38 may be formed using a photoresist and etch technique.FIG. 3 illustrates in part the location ofpost receptors38.

Referring now toFIG. 4, a first lowstress dielectric film50 is formed on the surface of the existing structure, e.g., to a depth of from about 50 Angstroms (Å) to about Angstroms (Å), alternatively to a depth of about 300 Angstroms (Å), although depths of less than about 50 Angstroms (Å) and greater than about 1000 Angstroms (Å) are also possible. In one embodiment, first lowstress dielectric film50 may be an amorphous silicon nitride material but may be any other suitable dielectric material, e.g., silicon oxide. First lowstress dielectric film50 may be formed using, for example, PECVD or other suitable method. Next,detector element layer52 is formed to obtain a resistive layer to function as the detector element inmicrobolometer10. In one embodiment, the deposition may take place at a temperature just below that which will degrade polyimide layer36 (e.g., from about 360° C. to about 450° C., alternatively at about 365° C.). Further information on deposition ofdetector element layer52 is given below in relation to Tables 1-6. A second lowstress dielectric film54 is then deposited ondetector element layer52 to a depth of from about 50 Angstroms (Å) to about 1000 Angstroms (Å), alternatively to a depth of about 250 Angstroms (Å), although depths of less than about 50 Angstroms (Å) and greater than about 1000 Angstroms (Å) are also possible.

Adetector element layer52 ofdetector membrane12 is next formed on the surface of the structure in contact with first lowstress dielectric film50, e.g., to a depth of from about 500 to about 1,000 Angstroms (Å), although depths of less than about 500 Angstroms (Å) and greater than about 1000 Angstroms (Å) are also possible.Detector element layer52 is resistive and may be an amorphous silicon-based material that includes varying amounts of one or more additional materials, including hydrogen, fluorine and/or germanium. In one exemplary embodiment,detector element layer52 may include an amorphous silicon-based material such as undoped or doped (p-type or n-type) hydrogenated amorphous silicon (a-Si:H), hydrogenated fluorinated amorphous silicon (a-Si:H:F), amorphous silicon germanium (a-Si_1-xGe_x), hydrogenated amorphous silicon germanium (a-Si_1-xGe_x:H), or hydrogenated fluorinated amorphous silicon germanium (a-Si_1-xGe_x:H:F).

An amorphous silicon-based material may be grown asdetector element layer52 using PECVD or other suitable method, e.g., Hot Wire Chemical Vapor Deposition (HWCVD), Electron Cyclotron Resonance Chemical Vapor Deposition (ECR-CVD), Microwave CVD, etc. Tables 1-6 below list exemplary parameters for growth in a PECVD reactor of selected amorphous silicon-based materials. In each of the tables below, the second column provides exemplary parameter ranges in within which one or more parameters may be varied to achieve desired material properties, e.g., flow rate of silane may be varied relative to flow rate of germane to achieve the desired value of “x” in an a-Si_1-xGe_x-based material.

In each of the embodiments described below, an amorphous silicon-based material may be optionally grown as an amorphous silicon-based material structure that contains an amount of microcrystallites that is less than or equal to about 10% by volume of the amorphous silicon-based material (as determined by Raman spectroscopy) and that are from about 1 nanometers to about 10 nanometers in size (as determined by x-ray diffraction). In this regard, the substrate temperature, plasma power (when using PECVD reactor) and hydrogen dilution ratio may be optionally set to drive the silicon or silicon germanium structure near the amorphous to crystalline transition where the material has stable (low energy) configuration. However, it will be understood that growth parameters may be varied to achieve desired amorphous silicon-based materials, including amorphous silicon-based materials containing microcrystallites in an amount greater than about 10% by volume of the amorphous silicon-based material, and/or that have a size of less than about 1 nanometer or greater than about 10 nanometers. For example, higher substrate temperature, higher plasma power, and greater amounts of hydrogen dilution tend to increase the microcrystallite amount.

In addition, it will also be understood that the parameter ranges and parameter values given in each table are exemplary only, and that any other combination of parameter ranges or parameter values suitable for formation of the amorphous silicon-based materials described herein may be employed. In this regard, the values of the growth parameters, such as the precursor gas flow rates, may be determined and set appropriately for the various desired final material composition, e.g., such as Ge composition “x” value. Further, it will be understood that PECVD reactor parameters (e.g., LF power density duty cycle may vary), and that the amount of precursors (e.g., SiH₄and/or H₂) that may be provided in argon or other suitable diluent may be varied (e.g., greater than about 5% or less than about 5%) as needed or desired to form the indicated material of each table.

In one exemplary embodimentdetector element layer52 may be grown as undoped a-Si:H with or without hydrogen dilution. Table 1 below lists exemplary parameters for growth in a PECVD reactor of an undoped a-Si:H material, with third column giving exemplary parameters for growth of undoped a-Si:H material film grown with hydrogen dilution to have a hydrogen content of 10 atomic %, although hydrogen contents of greater or less than about 10 atomic % are also possible.

TABLE 1

Parameters for PECVD growth of undoped a-Si:H Using Hydrogen Dilution

		Exemplary Parameter
		Value for Hydrogen
Parameter	Exemplary Parameter Range	Content = 10 Atomic %

Substrate Temperature	about 50° C. to about 450° C.	about 365° C.
Total Pressure	about 500 mTorr to about	about 1800 mTorr
	1800 mTorr
High Frequency (HF) Power	about 5 Watts to about 25	about 9 Watts
	Watts
HF Power density	about 0.005 Watts/cm²to	about 0.011 Watts/cm²
	about 0.03 Watts/cm²
Low Frequency (LF) Power	about 50 Watts to about	about 150 Watts
(50% duty cycle)	150 Watts
LF Power density	about 0.03 Watts/cm²to	about 0.088 Watts/cm²
	about 0.1 Watts/cm²
5% SiH₄in Argon (Ar) Flow	about 60 standard cubic	about 120 sccm
rate	centimeter per minute
	(sccm) to about 480sccm
6% H₂in Argon (Ar) Flow rate	About 120 sccm to about	about 1200 sccm
	1800 sccm
H₂dilution (ratio of hydrogen	about 1:1 to about 100:1	about 10:1
flow rate to SiH₄flow rate)
Typical Growth rate	about 0.5 Å/second to	about 0.8 Å/second
	about 2.0 Å/second

In another exemplary embodimentdetector element layer52 may be grown as a p-doped a-Si:H:F with or without hydrogen dilution. Table 2 below lists exemplary parameters for growth in a PECVD reactor of p-doped a-Si:H:F material with the third column giving exemplary parameters for growth of p-doped a-Si:H:F material film having fluorine content of 1 atomic % and grown with hydrogen dilution to have a hydrogen content of 10 atomic %. In one exemplary embodiment, p-doped a-Si:H:F may be formed having a fluorine content of from about 1 atomic % to about 10 atomic %, it being understood that fluorine contents greater than about 10 atomic % and less than about 1 atomic % are also possible, as are hydrogen contents of greater or less than about 10 atomic %.

TABLE 2

Parameters for PECVD growth of p-doped a-Si:H:F
with or without Hydrogen Dilution

		Exemplary Parameter Value
		for Fluorine Content =
		1 atomic % and Hydrogen
Parameter	Exemplary Parameter Range	Content = 10 atomic %

Substrate Temperature	about 50° C. to about 450° C.	about 365° C.
Total Pressure	about 500 mTorr to about	about 1800 mTorr
	1800 mTorr
High Frequency (HF) Power	about 5 Watts to about 25	about 9 Watts
	Watts
HF Power density	about 0.005 Watts/cm²to	about 0.011 Watts/cm²
	about 0.03 Watts/cm²
Low Frequency (LF) Power	about 50 Watts to about	about 150 Watts
(50% duty cycle)	150 Watts
LF Power density	about 0.03 Watts/cm²to	about 0.088 Watts/cm²
	about 0.1 Watts/cm²
5% SiH₄in Argon (Ar) Flow	about 60 standard cubic	about 120 sccm
rate	centimeter per minute
	(sccm) to about 480 sccm
BF₃Flow rate	about 0.4 sccm to about	about 1sccm
	10sccm
6% H₂in Ar Flow rate	0 sccm to about 1800	about 1200 sccm
	sccm
H₂dilution (ratio of hydrogen	about 1:1 to about 100:1	about 10:1
flow rate to SiH₄flow rate)
Typical Growth rate	about 0.5 Å/second to	about 0.8 Å/second
	about 2.0 Å/second

Table 2 illustrates formation of a p-doped a-Si:H:F material using BF₃as both source of boron p-dopant and a source of fluorine atoms. Advantageously, BF₃has a boiling temperature that is relatively low compared to the boiling temperature of BCl₃, meaning that the potential for film-damaging condensation from the p-type dopant source is reduced. However, it will be understood that other p-type dopants and combinations of p-type dopants may be employed to dope an amorphous silicon-based material in the formation of membrane structures for the disclosed microbolometer infrared detector elements. Examples of other types of p-type dopants that may be employed include, but are not limited to, aluminum, gallium, indium and boron, e.g., from boron sources such as diborane (B₂H₆), Trimethyl Boron (B(CH₃)₃(TMB), Boron Trichloride (BCl₃), and combinations thereof. Such p-type dopant sources do not contain fluorine atoms and therefore may be employed instead of BF₃to form non-fluorinated p-doped a-Si:H, with or without hydrogen dilution. In this regard, any one or more p-type dopant sources may be substituted for BF₃in Table 2 at the same gaseous flow rates as indicated for BF₃in Table 2, or may be separately added in combination with BF₃.

It will also be understood that other fluorine sources and combinations of fluorine sources may be employed to incorporate fluorine into an amorphous silicon-based material in the formation of membrane structures for the disclosed microbolometer infrared detector elements. Examples of other sources of fluorine include, but are not limited to, a combined silicon and fluorine source such as SiF₄. In one embodiment, SiF₄may be substituted for BF₃in Table 2 at the same gaseous flow rates indicated for BF₃in Table 2 to form an undoped a-Si:H:F with or without hydrogen dilution. Alternatively SiF₄may be substituted for SiH₄at the same gaseous flow rates indicated for SiH₄in Table 2, and in combination with BF₃, to form a p-doped a-Si:H:F with or without hydrogen dilution. In yet another alternative, SiF₄may be substituted for SiH₄at the same gaseous flow rates indicated for SiH₄in Table 2, and without the presence of another source of fluorine (e.g., without BF₃) to form undoped a-Si:H:F.

In another exemplary embodimentdetector element layer52 may be grown as an undoped a-Si_1-xGe_x:H with or without hydrogen dilution. The second column of Table 3 below lists exemplary parameter ranges for growth in a PECVD reactor of undoped a-Si_1-xGe_x:H material. The third column of Table 3 gives exemplary parameters for growth of undoped a-Si_1-xGe_x:H material film having germanium content “x” of 0.5 relative to silicon and grown with hydrogen dilution to have a hydrogen content of 10 atomic %, although hydrogen contents of greater or less than about 10 atomic % are also possible. In one exemplary embodiment, undoped a-Si_1-xGe_x:H material may be grown having a germanium content “x” that is from about 0 to about 1, with undoped a-Ge:H material being grown in one exemplary embodiment where the value of “x” is equal to 1.

TABLE 3

Parameters for PECVD growth of undoped
a-Si_1−xGe_x:H with or without Hydrogen Dilution

		Exemplary Parameter
		Value for Hydrogen
		Content = 10 Atomic %,
Parameter	Exemplary Parameter Range	and “x” = about 0.5

Substrate Temperature	about 50° C. to about 450° C.	about 365° C.
Total Pressure	about 500 mTorr to about	about 1800 mTorr
	1800 mTorr
High Frequency (HF) Power	about 5 Watts to about 25	about 9 Watts
	Watts
HF Power density	about 0.005 Watts/cm²to	about 0.011 Watts/cm²
	about 0.03 Watts/cm²
Low Frequency (LF) Power	about 50 Watts to about	about 150 Watts
(50% duty cycle)	150 Watts
LF Power density	about 0.03 Watts/cm²to	about 0.088 Watts/cm²
	about 0.1 Watts/cm²
5% SiH₄in Argon (Ar) Flow	about 60 standard cubic	about 120 sccm
rate	centimeter per minute
	(sccm) to about 480 sccm
GeH₄undiluted Flow rate	about 0.4 sccm to about 5.0	About 1.0 sccm
	sccm
6% H₂in Ar Flow rate	0 sccm to about 1800 sccm	about 1200 sccm
H₂dilution (ratio of hydrogen	about 1:1 to about 100:1	about 10:1
flow rate to SiH₄+ GeH₄flow
rate)
Typical Growth rate	about 0.5 Å/second to	about 0.8 Å/second
	about 2.0 Å/second

It will be understood that other germanium sources and combinations of germanium sources may be employed to incorporate germanium into an amorphous silicon-based material in the formation of membrane structures for the disclosed microbolometer infrared detector elements. Examples of other sources of germanium include, but are not limited to germanium tetrafluoride (GeF₄). In one embodiment, GeF₄may be substituted for germane (GeH₄) in Table 3 at the same gaseous flow rates indicated for GeH₄in Table 3 to form an undoped a-Si:Ge:H:F with or without hydrogen dilution. In another exemplary embodiment, GeF₄may be substituted for GeH₄in Table 3 at the same gaseous flow rates indicated for GeH₄, and SiF₄may be substituted for SiH₄in Table 3 at the same gaseous flow rates indicated for SiH₄in Table 3 to form an undoped a-Si:Ge:F, or which may optionally be formed as a-Si:Ge:H:F with the addition of hydrogen dilution.

In another exemplary embodimentdetector element layer52 may be grown as a p-doped a-Si_1-xGe_x:H:F with or without hydrogen dilution. The second column of Table 4 below lists exemplary parameter ranges for growth in a PECVD reactor of p-doped a-Si_1-xGe_x:H:F material. The third column of Table 3 gives exemplary parameters for growth of p-doped a-Si_1-xGe_x:H:F material film having germanium content “x” of 0.5, fluorine content of 1% and grown with hydrogen dilution to have a hydrogen content of 10 atomic %. In one exemplary embodiment, p-doped a-Si_1-xGe_x:H:F material film may be grown having germanium content of “x” that is from about 0 to about 1, and having a fluorine content of from about 0.5 to about 2 atomic %, it being understood that fluorine contents greater than about 2 atomic % and less than about 0.5 atomic % are also possible, as are hydrogen contents of greater or less than about 10 atomic %.

TABLE 4

Parameters for PECVD growth of p-doped
a-Si_1−xGe_x:H:F with or without Hydrogen Dilution

		Exemplary Parameter Value for
		Fluorine Content = 1 Atomic %,
		Hydrogen Content = 10 Atomic %,
Parameter	Exemplary Parameter Range	and “x” = about 0.5

Substrate Temperature	about 50° C. to about 450° C.	about 365° C.
Total Pressure	about 500 mTorr to about	about 1800 mTorr
	1800 mTorr
High Frequency (HF) Power	about 5 Watts to about 25	about 9 Watts
	Watts
HF Power density	about 0.005 Watts/cm²to	about 0.011 Watts/cm²
	about 0.03 Watts/cm²
Low Frequency (LF) Power	about 50 Watts to about	about 150 Watts
(50% duty cycle)	150 Watts
LF Power density	about 0.03 Watts/cm²to	about 0.088 Watts/cm²
	about 0.1 Watts/cm²
5% SiH₄in Argon (Ar) Flow	about 60 standard cubic	about 120 sccm
rate	centimeter per minute
	(sccm) to about 480 sccm
GeF₄undiluted Flow rate	about 0.4 sccm to about 5.0	About 1.6 sccm
	sccm
BF₃undiluted Flow rate	about 0.4 sccm to about 10	about 3 sccm
	sccm
6% H₂in Ar Flow rate	0 sccm to about 1800 sccm	about 1200 sccm
H₂dilution (ratio of	about 1:1 to about 100:1	about 10:1
hydrogen flow rate to SiH₄+
GeF₄flow rate)
Typical Growth rate	about 0.5 Å/second to	about 0.8 Å/second
	about 2.0 Å/second

Table 4 illustrates formation of a p-doped a-Si_1-xGe_x:H:F material using GeF₄as both a source of germanium and fluorine atoms in combination with BF₃which acts as both source of boron p-dopant and a source of fluorine atoms. However, as previously described in relation to Table 2, other p-type dopants and combinations of p-type dopants may be employed including, but not limited to, aluminum, gallium, indium and boron from other sources of boron such as diborane (B₂H₆), Trimethyl Boron (B(CH₃)₃(TMB), Boron Trichloride (BCl₃), and combinations thereof. In this regard, any one or more p-type dopant sources may be substituted for BF₃in Table 4 at the same gaseous flow rates indicated in Table 4, or may be separately added in combination with BF₃to result in p-doped a-Si_1-xGe_x:H:F material.

As further described previously in relation to Table 2, it will be understood that other fluorine sources and combinations of fluorine sources may be employed including, but not limited to, a combined silicon and fluorine source such as SiF₄. In one embodiment, SiF₄may be substituted for BF₃in Table 4 at the same gaseous flow rates indicated for BF₃in Table 4 to form an undoped a-Si_1-xGe_x:H:F with or without hydrogen dilution. Alternatively SiF₄may be substituted for SiH₄at the same gaseous flow rates indicated for SiH₄in Table 4, and in combination with BF₃, BCl₃or other p-type dopant source to form a p-doped a-Si_1-xGe_x:H:F with or without hydrogen dilution.

In another exemplary embodimentdetector element layer52 may be grown as a p-doped a-Si_1-xGe_x:H with or without hydrogen dilution. The second column of Table 5 below lists exemplary parameter ranges for growth in a PECVD reactor of p-doped a-Si_1-xGe_x:H material. The third column of Table 5 gives exemplary parameters for growth of p-doped a-Si_1-xGe_x:H material film having germanium content “x” of 0.5 and grown with hydrogen dilution to have a hydrogen content of 10 atomic %, although hydrogen contents of greater or less than about 10 atomic % are also possible. In one exemplary embodiment, p-doped a-Si_1-xGe_x:H material film may be grown having germanium content “x” that is from about 0 to about 1.

TABLE 5

Parameters for PECVD growth of p-doped
a-Si_1−xGe_x:H with or without Hydrogen Dilution

		Exemplary Parameter
		Value for Hydrogen
		Content = 10 Atomic %,
Parameter	Exemplary Parameter Range	and “x” = about 0.5

Substrate Temperature	about 50° C. to about 450° C.	about 365° C.
Total Pressure	about 500 mTorr to about	about 1800 mTorr
	1800 mTorr
High Frequency (HF) Power	about 5 Watts to about 25	about 9 Watts
	Watts
HF Power density	about 0.005 Watts/cm²to	about 0.011 Watts/cm²
	about 0.03 Watts/cm²
Low Frequency (LF) Power	about 50 Watts to about	about 150 Watts
(50% duty cycle)	150 Watts
LF Power density	about 0.03 Watts/cm²to	about 0.088 Watts/cm²
	about 0.1 Watts/cm²
5% SiH₄in Argon (Ar) Flow	about 60 standard cubic	about 120 sccm
rate	centimeter per minute
	(sccm) to about 480 sccm
GeH₄undiluted Flow rate	about 0.4 sccm to about 5.0	About 1.0 sccm
	sccm
BCl₃undiluted Flow rate	about 0.4 sccm to about 10	about 1 sccm
	sccm
6% H₂in Argon (Ar) Flow rate	About 120 sccm to about	about 1200 sccm
	1800 sccm
H₂dilution (ratio of hydrogen	about 1:1 to about 100:1	about 10:1
flow rate to SiH₄+ GeH₄flow
rate)
Typical Growth rate	about 0.5 Å/second to	about 0.8 Å/second
	about 2.0 Å/second

Table 5 illustrates formation of a p-doped a-Si_1-xGe_x:H material using GeH₄as source of germanium in combination with BCl₃which acts as source of boron p-dopant. However, as previously described in relation to Table 2, other p-type dopants and combinations of p-type dopants may be employed including, but not limited to, aluminum, gallium, indium, and boron, e.g., from boron sources such as diborane (B₂H₆), Trimethyl Boron (B(CH₃)₃(TMB), Boron Trichloride (BCl₃), and combinations thereof. In this regard, any one or more p-type dopant sources may be substituted for BCl₃in Table 5 at the same gaseous flow rates indicated in Table 5, or may be separately added in combination with BCl₃to result in p-doped a-Si_1-xGe_x:H material.

In another exemplary embodimentdetector element layer52 may be grown as a n-doped a-Si_1-xGe_x:H with or without hydrogen dilution. The second column of Table 6 below lists exemplary parameter ranges for growth in a PECVD reactor of n-doped a-Si_1-xGe_x:H material. The third column of Table 6 gives exemplary parameters for growth of n-doped a-Si_1-xGe_x:H material film having germanium content “x” of about 0.5 and grown with hydrogen dilution to have a hydrogen content of 10 atomic %, although hydrogen contents of greater or less than about 10 atomic % are also possible. In one exemplary embodiment n-doped a-Si_1-xGe_x:H may be grown having germanium content “x” that is from about 0 to about 1.

TABLE 6

Parameters for PECVD growth of n-doped
a-Si_1−xGe_x:H with or without Hydrogen Dilution

		Exemplary Parameter
		Value for Hydrogen
		Content = 10 Atomic %,
Parameter	Exemplary Parameter Range	and “x” = about 0.5

Substrate Temperature	about 50° C. to about 450° C.	about 365° C.
Total Pressure	about 500 mTorr to about	about 1800 mTorr
	1800 mTorr
High Frequency (HF) Power	about 5 Watts to about 25	about 9 Watts
	Watts
HF Power density	about 0.005 Watts/cm²to	about 0.011 Watts/cm²
	about 0.03 Watts/cm²
Low Frequency (LF) Power	about 50 Watts to about	about 150 Watts
(50% duty cycle)	150 Watts
LF Power density	about 0.03 Watts/cm²to	about 0.088 Watts/cm²
	about 0.1 Watts/cm²
5% SiH₄in Argon (Ar) Flow	about 60 standard cubic	about 120 sccm
rate	centimeter per minute
	(sccm) to about 480 sccm
GeH₄undiluted Flow rate	about 0.4 sccm to about 5.0	About 1.0 sccm
	sccm
PH₃	About 0.1 sccm to about 10	about 1 sccm
	sccm
6% H₂in Argon (Ar) Flow rate	About 120 sccm to about	about 1200 sccm
	1800 sccm
H₂dilution (ratio of hydrogen	about 1:1 to about 100:1	about 10:1
flow rate to SiH₄+ GeH₄flow
rate)
Typical Growth rate	about 0.5 Å/second to	about 0.8 Å/second
	about 2.0 Å/second

Table 6 illustrates formation of n-doped a-Si_1-xGe_x:H material using PH₃as both source of n-dopant. However, it will be understood that other n-type dopants and combinations of n-type dopants may be employed to dope an amorphous silicon-based material in the formation of membrane structures for the disclosed microbolometer infrared detector elements. Examples of other types of n-type dopants that may be employed include, but are not limited to, nitrogen, arsenic, antimony, and combinations thereof. In this regard, any one or more n-type dopants may be substituted for PH₃in Table 6 at the same gaseous flow rates indicated in Table 6, or may be separately added in combination with PH₃. It will also be understood that one or more n-type dopants may be used to form n-doped a-Si:H (e.g., with hydrogen dilution), n-doped a-Si:H:F (e.g., with or without hydrogen dilution), as well as fluorinated n-doped amorphous silicon-based materials such as n-doped a-Si_1-xGe_x:H and n-doped a-Si_1-xGe_x:H:F. For example, n-type dopant/s may be employed in the formation of other types of amorphous silicon-based material membrane structures for the disclosed microbolometer infrared detector elements by substituting n-type dopant source/s for p-type dopant source/s in previous Tables 1-5, or by adding n-type dopant source/s where no p-type dopant source/s is present in previous Tables 1-5, in each case at the same n-type gaseous flow rates indicated in Table 6. It is also possible that n-type dopants and p-type dopants may be employed together in the formation of amorphous silicon-based material membrane structures with properties of the amorphous silicon-based material membrane structures being adjusted by varying relative amount of n-type to p-type dopant, e.g., to form compensated semiconductor material.

Sincedetector element layer52 is transparent to infrared radiation, a material sensitive to infrared radiation is used to thermally transfer energy absorbed from the infrared radiation. A thinmetal absorber film56 is deposited on second lowstress dielectric film54 to a depth of from about 50 to about 150 Angstroms (Å), although depths less than about 50 Angstroms (Å) and greater than about 150 Angstroms (Å) are also possible. In one embodiment, thinmetal absorber film56 may be titanium but alternatively may be any other suitable material that will absorb infrared radiation. Thinmetal absorber film56 is patterned to leave an absorber area ondetector membrane12.Absorber56 may be patterned using a photoresist and etch technique, or other available techniques such as by a photoresist liftoff method.

FIG. 5 illustrates in part the location ofabsorber56 in relation to the structure ofmicrobolometer10 for this exemplary embodiment.Absorber56 absorbs heat from infrared radiation and transfers the heat todetector element layer52. Although second lowstress dielectric film54 provides electrical insulation fordetector element layer52, it does not thermally isolatedetector element layer52 fromabsorber56. Thus,detector element layer52 is thermally coupled toabsorber56 resulting in the transfer of thermal energy fromabsorber56 todetector element layer52. Asdetector element layer52 increases in temperature, the electrical resistance ofdetector element layer52 changes. The change in electrical resistance is measured and processed to yield a quantity of infrared radiation present in the detection area. Any infrared radiant energy not absorbed byabsorber56 passes through the structure, reflects offreflector22, and becomes trapped inantireflective structure20 such thatabsorber56 absorbs the trapped infrared radiant energy. Therefore,absorber56 absorbs infrared radiant energy both as it passes throughdetector membrane12 and after it becomes trapped inantireflective structure20.

Referring toFIG. 5,absorber56 is shown in relation tomicrobolometer10 formed onsubstrate11. In the illustrated embodiment, the outer surface of second lowstress dielectric film54 is patterned and openings are etched to expose portions of the outer surface ofdetector element layer52 to define thermalisolation leg channels60. In this embodiment, the second low stressdielectric film layer54 may be patterned and etched, for example, using a photoresist and etch technique.

Referring now toFIG. 6, a thinelectrode metal layer70 is deposited in thermalisolation leg channels60 to a depth of from about 50 Angstroms (Å) to about 250 Angstroms (Å), and alternatively to a depth of about 200 Angstroms (Å), although depths less than about 50 Angstroms (Å) and greater than about 250 Angstroms (Å) are also possible. In one embodiment,electrode metal layer70 may be titanium or nickel and may be deposited using a photoresist and lift-off technique.Electrode metal layer70 may be in direct contact withdetector element layer52 to provide a low resistance electrical connection between the detector element of detector membrane12 (i.e., detector element layer52) and electrical circuitry to measure the change in resistance ofdetector membrane12 in response to absorbing infrared radiation. A third lowstress dielectric film72 is deposited on the surface of the structure to a depth of about 100 Angstroms (Å), in order to provide a final layer of protection formicrobolometer10, although depths less than or greater than about 100 Angstroms (Å) are also possible.

In an alternate embodiment of the process for fabricatingmicrobolometer10, the deposition of a thinmetal absorber film56 may form bothabsorber56 andelectrode metal layer70. In this alternate embodiment, after second lowstress dielectric film54 is deposited, the outer surface of second lowstress dielectric film54 may be patterned and openings may be etched to expose portions of the outer surface ofdetector element layer52 to define thermalisolation leg channels60, e.g., using a photoresist and etch technique. Thinmetal absorber film56 may be deposited over the structure to a depth of from about 50 to about 150 Angstroms (Å), although depths less than about 50 Angstroms (Å) and greater than about 150 Angstroms (Å) are also possible. Thinmetal absorber film56 may be patterned using a photoresist and etch technique to leaveabsorber56 andelectrode metal layer70. The process of this alternate embodiment eliminates a separate step for deposition ofelectrode metal layer70.

Referring now toFIG. 7, a photoresist and etch technique may be used to pattern the structure to formmicrobolometer10. Theareas surrounding microbolometer10 may be etched down to thepolyimide layer36 and postreceptors38 may be etched down to thealuminum layer34. At this point,microbolometer10 includes several layers of material stacked on top of apolyimide layer36. In this embodiment,polyimide layer36 will be removed in a later step to create a space betweensubstrate11 and bothdetector membrane12 andthermal isolation legs14. In order to supportdetector membrane12 andthermal isolation legs14 above the surface ofsubstrate12, posts may be formed inpost receptors38 to provide both structural support and electrical connections formicrobolometer10.Post receptors38 may be formed in electrode terminal ends15 by removing the previously deposited layers of first lowstress dielectric film50,detector element layer52, second lowstress dielectric film54, and third lowstress dielectric film72 thereby exposingconnection pads40.Post receptors38 may be formed using a photoresist and etch technique simultaneously with defining thebolometer10. In this embodiment, the base layer ofpost receptor38 isconnection pad40 and the top layers ofpost receptor38 iselectrode metal layer70. Therefore, an electrically conductive material may be used to electrically couplethermal isolation legs14 withconnection pads40.

In an alternate exemplary embodiment of the process for fabrication ofmicrobolometer10,post receptors38 may not be etched inpolyimide layer36 immediately afterpolyimide layer36 is formed. In addition, the photoresist and etch step to form the structure ofmicrobolometer10 does not etch andreform post receptors38. Instead, a separate photoresist and etch step may be added to remove all layers aboveconnection pads40.

Referring now toFIG. 8, third lowstress dielectric film72 may be removed fromthermal isolation legs14 in the area to receive apost80 andthermal shunt18. Third lowstress dielectric film72 may be removed using a photoresist and etch technique to exposeelectrode metal layer70. A thin layer oftitanium82 and a thick layer ofaluminum84 may be deposited inpost receptor38 and on electrodeterminal end15.Titanium layer82 and thealuminum layer84 may be deposited in sequence and patterned at the same time, e.g., by a liftoff or by an etching technique.Titanium layer82 andaluminum layer84 also formthermal shunt18 onthermal isolation leg14.Titanium layer82 may be deposited to a depth of from about 250 Angstroms (Å) to about 550 Angstroms (Å) thick, alternatively to a depth of about 1,000 Angstroms (Å), and thealuminum layer84 may be deposited to a depth of from about 10,000 to about 30,000 Angstroms (Å) thick, althoughtitanium layer82 depths less than about 250 Angstroms (Å) and greater than about 550 Angstroms (Å) are also possible, as arealuminum layer84 depths of less than about 10,000 Angstroms (Å) and greater than about 30,000 Angstroms (Å).

Still referring toFIG. 8, post80 andthermal shunt18 comprisetitanium layer82 andaluminum layer84 deposited in and aroundpost receptor38.Titanium layer82 andaluminum layer84 comprisingpost80 may be deposited using a sputtered film process and patterned using a etching technique or a photoresist and lift off technique. Althoughpost80 is described as comprising titanium and aluminum layers, any suitable metal, metal layers, or metal alloys may be used such as nickel in combination with titanium and aluminum.Post80 provides both structural support formicrobolometer10 by suspendingdetector membrane12 above the surface ofsubstrate11 and electrical connection betweenthermal isolation leg14 andconnection pads40.Post80 is formed in electrode terminal ends15. Therefore, in one exemplary embodiment, eachmicrobolometer10 will have twoposts80, one on each of two opposite corners.

In addition to providing structural support and electrical connections formicrobolometer10, posts80 of the illustrated embodiment also provide thermal shunting formicrobolometer10. By increasing the length ofthermal shunt18 overelectrode metal layer70, the thermal isolation ofmicrobolometer10 may be reduced. This results in a microbolometer with increased operating frequency and decreased sensitivity as previously described.

Referring toFIG. 9, a cross-section ofmicrobolometer10 is shown. Thermalisolation leg gaps90 illustrate that all layers abovepolyimide layer36 have been removed in the areas where there is nomicrobolometer10 structure.

Referring toFIG. 10,polyimide layer36 may be removed by exposing the structure to an oxygen plasma dry etch. The byproduct of this etching process is carbon dioxide eliminating the need to specially dispose of the byproduct of etching.

Referring toFIG. 13A andFIG. 14A

several microbolometers

Referring toFIG. 13A the electrically parallel array embodiment provides a technical advantage for parallel groups of long narrow detector lines, such as for a spectrometer. The electrically series-parallel configuration ofFIG. 13B is useful and provides technical advantages for large rectangular arrays of detector elements functioning as a single detector.

Several microbolometers

10 may be formed and placed in a single vacuum package to form a pixel array structure for thermal imaging. In this embodiment, themicrobolometers10 are discrete devices detecting thermal energy in a specific portion of a target (scene) area. For example, microbolometers10 may be formed on a device wafer and then sealingly assembled with a lid wafer that is at least partially transmissive of infrared radiation (i.e., having at least some infrared radiation transmission characteristics) in the presence of a vacuum to sealingly contain a vacuum therebetween, although non-vacuum packaged device wafer/lid wafer package combinations are also possible.

In a thermal imaging array embodiment, select microbolometers within the microbolometer array structure may have an infrared shield deposited on the upper surface of the microbolometer and/or thethermal shunt18 may be extended to thedetector membrane12 to provide reference detector elements that are non-responsive to incident radiation. These infrared shield depositions provide an ambient temperature reference resistance for comparison with the resistance of the detector pixel. These reference pixels are thermally isolated from the substrate and therefore respond to the joule heating by bias current as do the detector pixels.

Referring toFIG. 11, an alternate embodiment ofmicrobolometer10 is illustrated and includesspiral legs100.Spiral legs100 are equivalent tothermal isolation legs14 as previously described. The spiral leg pixel configuration has utility both in imaging arrays and non-imaging arrays. It may be a desirable configuration for non-imaging arrays because the spiral leg configuration provides a higher fill factor and provides a more stressed-tolerant microbolometer. In the spiral leg configuration the detector membrane may be essentially a continuous sheet with openings for the spiral legs with the membrane in contact to the substrate as illustrated inFIG. 14B. The electrode70 (seeFIG. 10) may have a thickness equal to theabsorber56 and therefore also contributes to the absorption IR energy. As shown inFIG. 14B there is an array of 16 spiral leg pixels connected electrically in parallel. The spiral legs and pixels ofFIG. 14B are as described previously with reference toFIG. 11.

A spiral leg array such as illustrated inFIG. 14B may be configured in an electrically parallel connection as shown inFIG. 13A or in a series-parallel connection as illustrated inFIGS. 13B and 13C. The spiral leg design may also have an IR shield deposition on the upper surface to form reference pixels as previously described. Further, the spiral leg configuration may have metal deposition as a thermal shunt on the spiral leg as previously described for thethermal isolation leg14. For an imaging array configuration the spiral leg design may be employed to provide a larger detector for a given surface area (higher fill factor) on a substrate and to provide a more stress-tolerant microbolometer.Spiral legs100 may be formed using the same process asthermal isolation legs14 as earlier described.

FIG. 15 illustrates an exemplary embodiment havingthermal isolation legs14 formed between the substrate11 (not shown inFIG. 15) and abolometer10. This provides the technical advantage of a maximized fill factor since a relatively small absorbing surface area is sacrificed for supporting legs and spaced between adjacent pixels. In the embodiment ofFIG. 15 the thermal isolation leg14 (only one shown) is spaced below thebolometer10 with theconnection pads40 on the surface of the supporting substrate as illustrated inFIGS. 1 and 2.

Referring now toFIG. 12, a flow diagram summarizing the formation ofmicrobolometer10 according to one exemplary embodiment. The method begins atstep200 wheresilicon dioxide layer30 is formed onsubstrate11. The method proceeds to step202 wheretitanium layer32 is deposited onsilicon dioxide layer30. The method proceeds to step204 wherealuminum layer34 is deposited ontitanium layer32. The method proceeds to step206 wheretitanium layer32 andaluminum layer34 are patterned using a photoresist and etch process to formconnection pads40 andreflector20.

The method proceeds to step208 wherepolyimide layer36 is deposited over the entire structure to a depth on the order of one-quarter wave length of the infrared radiation to be detected. The method proceeds to step210 wherepost receptors38 are formed by removing a portion ofpolyimide layer36 thereby exposingconnection pads40. The method proceeds to step212 where the first lowstress dielectric film50 is formed on the surface of the existing structure. The method proceeds to step214 where amorphous silicon-based materialdetector element layer52 is formed on first lowstress dielectric film50. As described elsewhere herein, amorphous silicon-based material ofdetector element layer52 may be an undoped or doped (p-type or n-type) hydrogenated amorphous silicon (a-Si:H), hydrogenated fluorinated amorphous silicon (a-Si:H:F), amorphous silicon germanium (a-Si_1-xGe_x), hydrogenated amorphous silicon germanium (a-Si_1-xGe_x:H), or hydrogenated fluorinated amorphous silicon germanium (a-Si_1-xGe_x:H:F). An amorphous silicon-based material film may be grown instep214, for example, by chemical vapor deposition (CVD), such as plasma enhanced chemical vapor deposition (PECVD), using silane and optionally one or more other precursors.

The method proceeds to step216 where second lowstress dielectric film54 is deposited ondetector element layer52. The method proceeds to step218 where a thinmetal absorber film56 is deposited on second lowstress dielectric film54. The method proceeds to step220 where thinmetal absorber film56 is patterned leavingabsorber56.

The method proceeds to step222 where second lowstress dielectric film54 is patterned with openings etched to expose portions of the outer surface ofdetector element layer52 to define thermalisolation leg channels60. The method proceeds to step224 where thinelectrode metal layer70 is deposited in thermalisolation leg channels60.

The method proceeds to step226 where a third lowstress dielectric film72 is deposited on the surface of the structure. The method proceeds to step228 where a photoresist and etch technique is used to pattern the structure to formmicrobolometer10 by removing previously deposited layers down topolyimide layer36. The method proceeds to step230 wherepost receptors38 are formed by removing previously deposited layers thereby exposingconnection pads40.

The method proceeds to step232 where third lowstress dielectric film72 is removed fromthermal isolation legs14 in the area to receivepost80 andthermal shunt18. The method proceeds to step234 wheretitanium layer82 andaluminum layer84 are formed and patterned leavingthin titanium layer82 andaluminum layer84 inpost receptor38 and on electrodeterminal end15. The method proceeds to step236 wherepolyimide layer36 is removed by exposing the structure to an oxygen plasma dry etch. At the conclusion ofstep236,microbolometer10 is complete and suspended abovereflector22 bythermal isolation legs14 and posts16.

It will be understood that the methodology ofFIG. 12 is exemplary only, and that other steps, combinations of steps, and/or sequence of steps are possible that are suitable for forming a microbolometer having a desired configuration and amorphous silicon-based material construction are possible. Furthermore, the various structures depicted and described herein in relation toFIGS. 1-11 and13-15 are exemplary. In this regard, the disclosed amorphous silicon-based materials may be employed to form microbolometer membrane structures for microbolometer infrared detector elements having any other configuration suitable for detection of infrared radiation.

The disclosed amorphous silicon-based materials may be employed, for example, with microbolometer infrared detector structures and methods for forming the same that are illustrated in described in U.S. Pat. No. 6,777,681 and U.S. Pat. No. 6,690,014, each of which is incorporated herein by reference. In this regard the disclosed amorphous silicon-based materials may be employed in the place of amorphous silicon layers of microbolometer infrared detector structures described in these references. For example,FIGS. 16 and 17 herein illustrate alternative exemplary embodiments of microbolometerinfrared detector elements10 havingmembranes12 that may include the disclosed amorphous silicon-based materials. Such microbolometer detector element configurations are described in further detail in U.S. Pat. No. 6,777,681. In particular,FIG. 17 illustrates a microbolometerinfrared detector element10 having diffractive resonance cavity structure in whichmembrane12 has 16openings149 provided through it, in a cross-grating pattern which forms a four-by-four array (grid) of openings. Due to the presence of the openings, themembrane12 may be viewed as having five spaced and parallel strips151-155 that extend in one direction, and three spaced and parallel strips156-158 that extend from thestrip151 to thestrip155 in a direction perpendicular to the strips151-155. It will be understood thatFIGS. 16 and 17 are exemplary only, and that the disclosed amorphous silicon-based materials may be employed as layers in microbolometer infrared detector structures of other configurations, e.g., employed in the place of conventional amorphous silicon layers in microbolometer infrared detector structures.

Additionally, microbolometer infrared detector elements formed from the disclosed amorphous silicon-based materials may be employed to form focal plane arrays that may be packaged, for example, using vacuum packaging techniques described and illustrated in U.S. Pat. No. 6,586,831, U.S. Pat. No. 6,521,477, U.S. Pat. No. 6,479,320, United States Patent Publication number 2004/0219704, and U.S. patent application Ser. No. 11/141,356, each of which is incorporated herein by reference.

While the invention may be adaptable to various modifications and alternative forms, specific embodiments have been shown by way of example and described herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. Moreover, the different aspects of the disclosed structures, systems and methods may be utilized in various combinations and/or independently. Thus the invention is not limited to only those combinations shown herein, but rather may include other combinations.