Infrared radiation sensors and methods of manufacturing infrared radiation sensors

An infrared radiation sensor comprises a substrate, a membrane formed in or at the substrate, a first counter electrode, a second counter electrode, and a composite comprising at least two layers of materials having different coefficients of thermal expansion. At least a portion of the membrane forms a deflectable electrode and the deflectable electrode is electrically floating. A first capacitance is formed between the deflectable electrode and the first counter electrode, and a second capacitance is formed between the deflectable electrode and the second counter electrode. The membrane comprises the composite or is supported at the substrate by the composite. The membrane comprises an absorption region configured to cause deformation of the composite by absorbing infrared radiation, the deformation resulting in a deflection of the deflectable electrode, which causes a change of the first and second capacitances.

RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 to German Patent Application No. 102017221076.1, filed on Nov. 24, 2017 the content of which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to infrared radiation sensors and methods of manufacturing infrared radiation sensors. In particular, the present disclosure relates to MEMS (Micro-Electro Mechanical Systems) infrared radiation sensors and methods of manufacturing infrared radiation sensors using MEMS technologies.

BACKGROUND

Infrared imaging has a number of well-established industrial, commercial and military applications, such as security, law enforcement surveillance and industrial and environmental monitoring. The ability of detecting infrared radiation establishes a broad spectrum of applications, beginning with temperature measurements up to a precise characterization of all kinds of inorganic and organic substances by spectroscopic methods, such as IR and Raman spectroscopy.

In infrared cameras, bolometers may be used as radiation detectors. In such bolometers, detection takes place indirectly by converting absorbed light energy into heat. The heat results in an intensification of scattering of freely movable electrons within a solid state body. Thereby, an increase of the electric resistance may be detected as a signal. Highly-doped silicon, both amorphous and polycrystalline, in the form of a membrane may be used as a heat sensitive resistive element. The surface area and the thickness of the membrane may be adapted or optimized to achieve a maximum efficient radiation reception. Bolometers may have a pixel size of 17×17 μm2and a thermal contrast (NETD=Noise Equivalent Temperature Difference) of 30 mK to 50 mK.

Generally, bolometers may comprise two electrical contacts which are connected to the membrane at two opposite sides thereof so as to operate the membrane structure as a resistor. This may result in a relatively strong heat dissipation through the contact structures, which may be implemented using metal studs, and, therefore, the sensitivity may be reduced. Miniaturization of bolometers is directly linked with a reduction of the active surface area of the membrane.

Other infrared imagers may use MEMS differential capacitive infrared sensors within a sensor array. Such differential capacitive infrared sensors may include a bimaterial deflectable electrode element anchored to a top surface of an integrated circuit substrate and a surface electrode fabricated on the top surface of the integrated circuit substrate and positioned below the deflectable electrode element. The surface electrode and the deflectable electrode element may be separated by a gap to form a first variable capacitor having a first capacitance value. In addition, an infrared transparent sealing cap electrode may be provided. A first bias voltage may be applied to the surface electrode and a second bias voltage may be applied to the sealing cap electrode. A differential capacitance monitor is physically coupled to the bimaterial deflectable electrode element, to the surface electrode and to the sealing cap electrode. The differential capacitance monitor is to monitor a magnitude of the differential between the first capacitance value of the first variable capacitor and the second capacitance value of the second variable capacitor.

Infrared capacitance sensors may be composed of a bimaterial strip which changes the position of one plate of a sensing capacitor in response to temperature changes due to absorbed incident thermal radiation. The bimaterial strip may be composed of two materials with a large difference in thermal expansion coefficients. The plates of the sensing capacitor face each other in parallel to a substrate plane and are electrically connected to a detection circuit.

SUMMARY

Examples of the present disclosure provide an infrared radiation sensor comprising a substrate; a membrane formed in or at the substrate, at least a portion of the membrane forming a deflectable electrode, the deflectable electrode being electrically floating; a first counter electrode; a second counter electrode; and a composite comprising at least two layers of materials having different coefficients of thermal expansion. A first capacitance is formed between the deflectable electrode and the first counter electrode. A second capacitance is formed between the deflectable electrode and the second counter electrode. The membrane comprises the composite or is supported at the substrate by the composite. The membrane comprises an absorption region configured to cause deformation of the composite by absorbing infrared radiation, the deformation resulting in a deflection of the deflectable electrode, which causes a change of the first and second capacitances.

Examples of the present disclosure provide an infrared radiation sensor comprising a substrate having a main surface defining a substrate plane; a membrane formed in or at the substrate, at least a portion of the membrane forming a deflectable electrode; and a counter electrode. The deflectable electrode and the counter electrode face each other laterally with respect to the substrate plane, wherein a capacitance is formed between the deflectable electrode and the counter electrode. The membrane comprises a composite comprising at least two layers of materials having different coefficients of thermal expansion. The composite comprises an absorption region configured to cause deformation of the opposite by absorbing infrared radiation, the deformation resulting in a deflection of the deflectable electrode relative to the counter electrode, which causes a change of the capacitance.

Examples of the present disclosure provide methods of manufacturing such infrared radiation sensors.

DETAILED DESCRIPTION

In the following, examples of the present disclosure will be described in detail using the accompanying drawings. It is to be pointed out that the same elements or elements having the same functionality are provided with the same or similar reference numbers and that a repeated description of elements provided with the same or similar reference numbers is typically omitted. Hence, descriptions provided for elements having the same or similar reference numbers are mutually exchangeable. In the following description, a plurality of details is set forth to provide a more thorough explanation of examples of the disclosure. However, it will be apparent to those skilled in the art that other examples may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagrams form rather than in detail in order to avoid obscuring examples described herein. In addition, features of the different examples described hereinafter may be combined with each other, unless specifically noted otherwise.

Examples of the present disclosure relate to infrared radiation sensors comprising a substrate. Generally, the substrate may include one or two main surfaces, i.e. surfaces having a larger square dimension than the other surfaces thereof. The main surface of the substrate may define a substrate plane, i.e. a substrate plane may be laid through the main surface of the substrate even if the surface comprises irregularities. Generally, the substrate may include two main surfaces and the substrate plane may be parallel to the two main surfaces of the substrate. A length direction and a width direction may be parallel to the subject plane, while a thickness direction may be vertical (or perpendicular) to the subject plane. Generally, the terms “vertical” and “lateral” as used herein may be vertical and lateral with respect to the substrate plane.

Examples of the present disclosure provide infrared radiation sensors which may be manufactured using MEMS technologies in a cost effective manner. Examples provide MEMS infrared radiation sensors. Examples permit miniaturization and integration of infrared detectors in microelectronic, wherein CMOS (Complementary Metal-Oxide Semiconductor) compatible materials and processes may be used in manufacturing the sensors. Examples of the present disclosure permit low-energy infrared radiation to be received and converted into a useful signal with high efficiency.

According to examples of the present disclosure, infrared radiation is detected indirectly by measuring an electrical capacitance. In examples of the present disclosure, less electrical contacts to the sensitive element of the sensor are provided. In examples, the area of the electrical contact can be reduced for a better thermal isolation. In examples, a radiation sensitive membrane may be arranged within an evacuated cavity so that thermal losses due to convective flow of heat and heat dissipation through the atmosphere may be reduced. In examples, infrared sensors may be implemented with a high thermal contrast with an NETD up to 1 mK.

In examples of the present disclosure, the sensitive element is a composite comprising two layers of materials having different coefficients of thermal expansion, which are arranged on top of each other in direct mechanical contact with each other. In examples, the sensitive element comprises a material combination of silicon and silicon oxide or silicon dioxide. The sensitive element may be supported at one position thereof and may be movable at other positions thereof. Thus, the sensitive element may deflect in case of temperature changes due to the different coefficients of thermal expansion of the layers of the composite. In examples, the composite is a bimaterial composite comprising a first layer of a material having a first coefficient of thermal conductivity and a second layer of a material having a second coefficient of thermal conductivity.

FIGS. 1A and 1Bshow an example of an infrared radiation sensor comprising a membrane10. The membrane10comprises a semiconductor layer12and an oxide layer14formed on top of the silicon layer12. The semiconductor layer may be a silicon layer and the oxide layer may be a SiO2layer. The membrane forms a sensitive element is fixed at one side10athereof to a substrate24and freely movable at the other side10bthereof. Thus, the membrane10forms a cantilever beam and the semiconductor layer12forms a deflectable electrode. The free end10bhas a lateral surface facing a lateral surface16bof a stator16. The stator16comprises a semiconductor layer17forming a stationary electrode. The stator16may be formed in the same material layers as the sensitive element, but is fixed to the substrate24and does not comprise a movable portion. In examples, the stator does not comprises a movable portion since the extension of the stator in the direction of the longitudinal extension of the cantilever beam is substantially less than the extension of the cantilever beam in this direction. The semiconductor materials12,17of the cantilever beam10and the stator16may be doped to be electrically conductive enough to form electrodes of a capacitor. To be more specific, the free end10band the lateral side face16bof stator16face each other laterally via an air gap18and, therefore, the deflectable electrode and the stationary electrode form a capacitor. A contact20for contacting the deflectable electrode12and a contact22for contacting the stationary electrode16may be provided.

FIG. 1Ashows the sensor element in a state in which the deflectable electrode is not deflected, which may be a state of equilibrium or quiescent state.FIG. 1Bshows the sensor element under the influence of heat.FIG. 1Cshows an electric equivalent circuit of the system with a variable capacitance C between the laterally facing surfaces of the semiconductor layers12and17and the electric resistance Rmof the semiconductor layers.

Incident infrared radiation may be absorbed within the semiconductor layer12by interaction with free charge carriers within the doped semiconductor layer. Since the charge carriers aim to assume a state of lower energy, they are relaxed due to interaction with photons in the solid state body. Thus, the membrane is heated. Since the membrane comprises a combination of layers of materials having different coefficients of thermal expansion, such as Si and SiO2, which are in direct contact with each other, heating the membrane results in a deformation and, therefore, reflection of the membrane, as shown schematically inFIG. 1B.

For example, silicon has a coefficient of thermal expansion of about 26×10−7K−1and SiO2has a coefficient of thermal expansion of about 5×10−7K−1. Thus, the coefficient of silicon is about five times higher than that of SiO2.

Connecting a voltage source to the contacts20and22results in the equivalent circuit shown inFIG. 1C. Since the thermal expansion is a reversible process, the deflection of the sensitive element10varies with varying radiation intensity. Thereby, the capacitance between cantilever beam10and stator16varies and the capacitance may be read out electrically. To be more specific, the capacitance may change since the overlapping area of the deflectable electrode and the stationary electrode when viewed in the lateral direction changes. In the example shown inFIGS. 1A to 1C, the overlapping area is maximum in the state ofFIG. 1A.

In examples, the membrane may be pre-deflected so that the capacitance between the deflectable electrode (i.e. the sensitive element) and the stator is not maximum in the state of equilibrium. Thus, read out may take place in a more linear region of the capacitance variation. Pre-deflection may be achieved by depositing an oxide layer of the membrane to the semiconductor layer at increased temperatures so that mechanical stress develops between the layers when cooling down to room temperature. A pre-deflection of the membrane may be provided with respect to all examples of the present disclosure.

The semiconductor layer12represents an absorption region of the membrane and may be doped with phosphor or boron in order to allow for absorption of the infrared radiation. In order to increase interaction of the incident photons with free charge carriers within the semiconductor a high concentration of the dopants is desired.

Accordingly,FIGS. 1A to 1Cshow an example of an infrared radiation sensor, in which a membrane is formed in or at a substrate24. The semiconductor layer12of the membrane10forms a deflectable electrode, and the semiconductor layer17of the stator16forms a counter electrode. The semiconductor layer or region17of the stator16is electrically isolated from the semiconductor layer12. The semiconductor layer12and semiconductor layer17face each other laterally with respect to the substrate plane, wherein a capacitance C is formed between the same and, therefore, between the deflectable electrode12and the counter electrode16. The membrane comprises a composite comprising the semiconductor layer12and the oxide layer14in direct mechanical contact with each other. The semiconductor layer12of the cantilever beam10represents an absorption region configured to cause deformation of the composite by absorbing infrared radiation, the deformation resulting in a deflection of the deflectable electrode12relative to the counter electrode17, which causes a change of the capacitance C.

According to the example, the capacitance or capacitor is formed by electrodes facing each other laterally. Thus, a vertical distance between the deflectable electrode and a bottom of a cavity above which the deflectable electrode may be formed is not decisive for the capacitance. Thus, the height of the cavity in the thickness direction of the substrate may be increased and, therefore, the risk that pull-in effects occur may be reduced.

Generally, the layer of the composite, through which the infrared radiation is incident into the absorption region, may act as an antireflection layer in addition to the purpose of allowing for deformation upon heating. For example, the oxide layer14may also serve as an antireflection layer and, to this end the thickness of the antireflection layer may be adapted to be in conformity with the antireflection condition:

d=k⁢λ02
wherein k=2n−1, wherein d is the thickness of the layer, n is a natural number equal ≥1 and λ0is the wavelength of the incident infrared radiation.

According to the example shown inFIGS. 1A to 1C, the membrane and, therefore, the flexible electrode10, is in the form of a cantilever beam fixed at one end10athereof. One side face of the cantilever beam, such as lateral side face10bmay laterally face a lateral side face of a stationary electrode, such as side face16b. In examples, plural lateral side faces of the flexible electrode may laterally face side faces of one or more stationary electrodes. For example, stator16may be provided to substantially surround membrane10so that the front side and the back side of the flexible electrode shown inFIGS. 1A to 1Balso laterally face side faces of the stator16, which act as stationary electrodes. Thus, change of capacitance in case of deflection may be increased.

In examples, a counter electrode laterally faces the deflectable electrode at several lateral margins of the deflectable electrode, wherein a membrane support is coupled to the membrane in a region spaced from the lateral margins of the membrane when viewed in plan view onto the substrate plane. In examples, the membrane support may be coupled to the membrane in a region spaced from all lateral margins of the membrane when viewed in plan view onto the substrate. In examples, the membrane support may be coupled to the membrane in a central region of the membrane in plan view onto the substrate plane.

FIG. 2Ashows an example of an infrared radiation sensor comprising a membrane supported at a central portion thereof. The sensor comprises a membrane110. The membrane110comprises a doped semiconductor layer112and an oxide layer114in direct mechanical contact with the doped semiconductor layer112. The doped semiconductor layer112and the oxide layer114provide a composite comprising two layers of materials having different coefficients of thermal expansion as explained above. The sensor further comprises a stationary electrode116forming a stator. The membrane110and the stator116are separated from each other by a narrow trench118. In examples, the width of the trench and, therefore, the distance between opposing lateral surfaces of the membrane110and the stationary electrode116is at most 500 nm. Thus, a capacitor structure is formed between opposing lateral surfaces of the doped semiconductor regions of the membrane110and the stationary electrode116. As shown inFIG. 2B, the stationary electrode116and the membrane112may be formed in the same semiconductor layer.

The membrane110and the stationary electrode116are electrically decoupled from a substrate120in which the same are formed. To be more specific, the stator116and the membrane110are formed above a cavity122. A trench124laterally surrounds the stator116to electrically decouple the stator116from the substrate120. The membrane110and the stator116are mounted to a lid126. The lid126is not shown inFIG. 2Ain order not to hide the below structures. In addition,FIG. 2Aonly shows the structures within the outer margin of the stator116. The cavity122may have a depth of about 1 μm. Above the membrane110a further cavity128may be formed within the lid126. The membrane110is fixed to the lid126via a membrane support. The membrane support is formed by an oxide cylinder130extending between the lid126and the oxide layer114. A contact132for contacting the doped semiconductor layer112of the membrane110extends through the oxide cylinder130. A further contact134for contacting the doped semiconductor region of the stationary electrode116extends through the lid126and the oxide layer114. As shown inFIG. 2A, the contact134may be split into a plurality of contacts.

In the example shown inFIGS. 2A and 2B, the membrane110is supported and electrically contacted in a center portion thereof, i.e. a portion separated from all lateral margins of the membrane. In examples, the portion at which the membrane is supported may be the geometrical center of the membrane in a top view onto the substrate plane.

In the example shown, pylons136are provided to stabilize the lid126. The pylons136extend through openings138formed in the membrane110and the stator116. The pylons136may be formed of oxide columns mechanically connecting the lid126to the bottom of the cavity122. The pylons136are electrically and mechanically decoupled from the membrane110and the stator116by trenches surrounding the pylons due to the larger dimensions of the openings138when compared to the pylons136.

In order to increase the active area of the capacitor between the stator116and the membrane110, the trench between them may be meander-shaped in plan view. In examples, the trench may be formed in a meander shape by etching in an easy manner. Lid126may seal the sensitive structure of the sensor with respect to the outside atmosphere. Thus, the pressure within the cavities122and128may be adjusted by means of oxide deposition processes. In order to achieve a better thermal isolation, in examples, a low pressure may be adjusted within the cavities. In examples, the cavities may be evacuated. Generally, the lid126and the oxide layer114may be transparent for infrared radiation.

The portions of the membrane110spaced apart from the fixed portion thereof represent a deflectable electrode. The doped semiconductor layer112represents an absorption region.

As explained above, incident infrared radiation causes a rise of temperature of the composite of the membrane (doped semiconductor layer112and oxide layer114) resulting in a deformation of the composite, wherein the deformation results in a deflection of the deflectable electrode. To be more specific, the margins of membrane110will bend up in case of a rise of temperature and, therefore, the capacitance between the deflectable electrode and the stator electrode116will change. This change in capacitance may be detected as a measure for the incident infrared radiation.

Since the only mechanical contact to the membrane110is via the membrane support130and the contact132, heat dissipation from the membrane110may be low. Thus, sensitivity of the sensor may be increased.

In examples, additional trenches may be provided in the membrane to reduce a thermal influence of the region coupled to the membrane support on other regions of the membrane. One such example is shown inFIGS. 3A and 3B. As shown inFIG. 3A, trenches150are provided in the region where the membrane is supported by the membrane support130. As shown inFIG. 3A, trenches150are provided to form lands or bars152, via which the portion of the membrane, which is coupled to the membrane support130, is mechanically connected to other portions of the membrane. The trenches150may penetrate the membrane and are not shown inFIG. 3Bfor clarity reasons. The bars152may include first and second bars extending from the membrane support130in opposite directions. The bars152may include a third bar extending from the distal end of the first bar perpendicular to the first bar in opposite directions. The bars152may include a fourth bar extending from the distal end of the second bar perpendicularly to the second bar in opposite directions. To form such bars, the trenches150may include four trenches as shown inFIG. 3A. In other words, the trenches150may be etched into the membrane to provide a spring structure surrounding the membrane support and extending to the sides of the membrane. Such a spring structure may be formed by etching trenches and may serve to increase the thermal resistance of the membrane.

In examples, the pressure within the infrared radiation sensor, such as within the cavities surrounding the membrane, is kept low in order to reduce the thermal conductivity within the cavity. Thus, heat dissipation from the sensitive structure can be decreased and sensitivity can be increased.

Operation of such a sensor has been simulated using finite element analysis and showed a substantial deflection of the margins of the membrane supported at a central portion thereof. Such a deflection can be detected as a change in capacitance between the deflectable electrode formed by portions of membrane110and the stator electrode116. The change of capacitance can be detected using an appropriate detector connected to the terminals132and134.

FIG. 4shows a schematic cross-sectional view of a modification of the sensor shown inFIG. 3B. As shown inFIG. 4, an optical reflector160is provided at the backside of substrate120, i.e. on the side facing away from the side on which lid126is provided. Moreover, an optical filter162is provided on lid126. The optical filter162may be an infrared filter blocking wavelengths different from infrared. The optical reflector160may be reflective for infrared radiation so that infrared radiation passing through the absorption region and not being absorbed therein will be reflected back into the absorption region. Thus, efficiency may be improved using the optical reflector. Moreover, sensitivity may be improved using the optical filter162. Corresponding optical reflectors and filters may be provided in each of the examples described herein. Corresponding filters and reflectors may be provided in all examples of the present disclosure.

As described above, in examples of the present disclosure, at least a portion of the membrane forms a deflectable electrode and the deflectable electrode is provided with an electrical contact to permit detection of the capacitance between the deflectable electrode and the counter electrode. In other examples of the present disclosure, the deflectable electrode is electrically floating. In the context of this disclosure, “electrically floating” means that the deflectable electrode is not provided with a galvanic connection to any circuitry outside the deflectable electrode. In such examples, the infrared radiation sensor may comprise a first counter electrode and a second counter electrode, wherein a first capacitance is formed between the deflectable electrode and the first counter electrode and a second capacitance is formed between the deflectable electrode and the second counter electrode.

An example of an infrared radiation sensor comprising a floating deflectable electrode and lateral capacitances between the deflectable electrode and two counter electrodes is shown inFIG. 5. The sensor shown inFIG. 5is similar to the sensor shown inFIG. 4with the following exceptions. There is no galvanic electric contact to the silicon layer112of membrane110. The membrane support130is formed by a solid oxide cylinder in this case. There is no contact extending through the membrane support130. Rather, two separate contacts134aand134bare provided for two separate counter electrodes116aand116b. Counter electrodes116aand116bmay be electrically isolated from each other by additional trenches formed within the semiconductor layer in which the membrane110and the counter electrodes116aand116bare formed. In examples, such trenches may separate the counter electrode116shown inFIGS. 2A and 3Ainto two halves. Accordingly, a first capacitance is formed between a lateral side face of the deflectable electrode110and the counter electrode116aand a second capacitance is formed between another lateral side face of the deflectable electrode110and the second counter electrode116b. The first and second capacitances are connected in series through the doped silicon layer112. A detector may be coupled to the contacts134aand134bto detect the capacitance of this series connection of series capacitances and/or changes thereof. Thus, incident infrared radiation may be detected by a detector connected to the contacts134aand134b.

In examples, in which the deflectable electrode is electrically floating, thermal isolation of the deflectable electrode may be improved and, therefore, sensitivity may be increased.

In the examples described above referring toFIGS. 1-5, lateral capacitances are formed between lateral side faces of a deflectable electrode and one or more stationary counter electrodes. In such examples, the area of the capacitor electrodes facing each other may be varied an increased in an easy manner by shaping the trench separating the electrodes. An example is the meander shaped trench shown inFIGS. 2A and 3A.

In other examples of the present disclosure, vertical capacitances are formed between surfaces of a deflectable electrode and surfaces of first and second counter electrodes facing each other in a vertical direction.

FIG. 6shows a schematic cross-sectional view of an infrared radiation sensor comprising a membrane210formed in a substrate220. In examples, the membrane210is separated from the substrate220by a cavity222and trenches224. The membrane210may be supported at a central portion thereof using a membrane support130as described above. There is no other mechanical or electrical connection between the membrane210and the substrate220. The membrane210comprises a composite of a doped semiconductor layer112and an oxide layer114. Thus, incident infrared radiation will cause a deformation of the membrane210, portions of which form a deflectable electrode. The sensor comprises a first counter electrode226and a second counter electrode228. A first vertical capacitance is formed between portions of the membrane210forming a deflectable electrode and the first counter electrode226, and a second vertical capacitance is formed between other portions of the membrane210forming a deflectable electrode and the second counter electrode228. The first and second counter electrodes226,228may be electrically connected to a detection circuit230as indicated inFIG. 6in broken lines.

A deformation of the deflectable electrode210causes a change of the first and second capacitances. Again, there is a series connection between the first capacitance and the second capacitance via the doped semiconductor layer112. In the example shown inFIG. 6, the oxide layer114contributes to the capacitances. In other examples, the oxide layer114may be arranged on the side of the membrane112facing away from the counter electrodes226,228.

FIG. 7shows a schematic cross-sectional view of another example of an infrared radiation sensor with vertical readout. A membrane210′ in the form of a cantilever beam is formed in a substrate220and separated from the substrate by a cavity222and a trench224. Again, the membrane210′ may include a doped silicon layer112and an oxide layer114and, therefore, comprises a layer composite having at least two layers of materials with different coefficients of thermal expansion. The infrared radiation sensor further includes a first counter electrode236and a second counter electrode238, which are provided on the bottom of the cavity222. The first and second counter electrodes236,238face respective portions of the membrane210′ in a vertical direction. The first and second counter electrodes236,238may be connected to a detection circuit230as indicated inFIG. 7in broken lines. As explained above in detail, infrared radiation incident in doped semiconductor layer112will result in a deformation of the semiconductor layer. Such deformation results in a deflection of the membrane210′ and, therefore, in a change of the first and second capacitances. Such change may be detected by the detection circuit230via a change of the series capacitance formed by the first and second capacitances, which are coupled through the doped semiconductor layer112.

In examples, the portion of the membrane forming the deflectable electrode is deflectable vertically with respect to the substrate plane, wherein the deformation of the composite causes a vertical deflection of the portion of the deflectable electrode relative to the first counter electrode and the second counter electrode. In examples, the deflectable electrode and the first counter electrode face each other laterally with respect to the substrate plane and the deflectable electrode and the second counter electrode face each other laterally with respect to the substrate plane. In other examples, the deflectable electrode and the first counter electrode face each other vertically with respect to the substrate plane and the deflectable electrode and the second counter electrode face each other vertically with respect to the substrate plane.

In the above examples, portions of the membrane form a deflectable electrode and the membrane comprises the composite. In other examples, the membrane is supported at the substrate by the composite. In such examples, the membrane may be formed of a single layer and may be coupled to the substrate via the composite, which comprises at least two layers of materials having different coefficients of thermal expansion. In such examples, the composite may comprise a first cantilever beam and a second cantilever beam, wherein the first cantilever beam and the second cantilever beam bear the membrane, wherein a free end of the first cantilever beam is coupled to a first side of the membrane and a free end of the second cantilever beam is coupled to a second side of the membrane, which is opposite to the first side in a lateral direction with respect to the substrate plane. One such example is now described referring toFIGS. 8 to 16B.

As shown inFIGS. 8 to 11, an infrared radiation sensor comprises a substrate220. A cavity222is formed in the substrate220and defines a semiconductor layer of the substrate. First and second cantilever beams302,304and a membrane310are structured in this semiconductor layer by trenches306, which penetrate the semiconductor layer. Each cantilever beam302,304comprises a composite of a doped semiconductor layer312and an oxide layer314. The membrane310includes the doped silicon layer312but, in this example, does not include the oxide layer314.

The sensor further comprises a first counter electrode320and a second counter electrode322. The first and second counter electrodes320,322are formed in a lid326of the infrared radiation sensor, which seals the cavity in which the membrane310and the cantilever beams302,304are formed. Contacts324and326as shown inFIG. 10may be provided to connect the counter electrodes320,322to a detection circuit. In the schematic perspective view ofFIG. 10, lid326is shown transparent in order not to hide the structures below the same.

FIG. 9shows a schematic top view of the semiconductor layer, in which the cantilever beams302,304and the membrane310are formed. First ends302aand304aof the cantilever beams may be fixed to the substrate220. A second end302bof the cantilever beam302is mechanically connected to the membrane310via a spring structure330and a second end304bof the cantilever beam304is mechanically connected to an opposite end of the membrane310via a spring structure332. The first counter electrode320is facing a portion of the deflectable membrane310in a vertical direction and the second counter electrode322is facing another portion of the deflectable membrane310in a vertical direction. Accordingly, a first capacitance is formed between the first counter electrode320and the deflectable electrode310and a second capacitance is formed between the second counter electrode322and the deflectable electrode310. Again, there is a series connection between the first and second capacitances through the conductive doped silicon layer312forming the deflectable electrode.

One end302a,304aof the cantilever beams302may be mechanically fixed to the substrate220. In examples, the ends302a,304amay be mechanically fixed via oxide bridges or oxide pylons. The other sides302b,304bof the bimaterial cantilevers are attached to different sides of the membrane310. Mechanical rotational springs330and332may be provided to insure maximum deformation of the cantilever beams by decoupling with respect to rotation the ends302b,304bfrom the membrane310, which forms a stiffer middle part. As can be imagined fromFIG. 11, due to the rotational springs330,332, the membrane310may have a flat profile which may be optimum for readout and which may contribute to a highest possible output signal. In other words, the rotational springs330,332hold the metal membrane310while allowing the whole structure to relax thus encouraging greater swing and flatter profile.

The membrane310is formed of heavily doped semiconductor, such as silicon, and is used as a movable floating contact for sensing, readout and IR absorption. The cantilever beams302,304comprise in this example the composite of two layers of materials having different coefficients of thermal expansion to convert temperature changes to mechanical deformations. In examples, the cantilever beams may be bimaterial cantilever beams comprising a silicon layer and a deposited oxide layer. Infrared radiation incident into the membrane310may result in a rise of temperature of the membrane310, which, in turn, may result in a rise of temperature of the cantilever beams302,304and, therefore, in a mechanical deformation thereof. This deformation of the cantilever beams302,304results in a deflection of the membrane310as shown inFIG. 11. This deflection results in a change of capacitance, which may be detected by a detection circuit connected to the counter electrodes320,322. Accordingly, the cantilever beams may be regarded as representing a bimaterial pair configured to convert heat generated in the membrane to a vertical displacement thereof.

The lid326may be a silicon oxide lid and may seal the cavity in which the deflectable electrode is arranged. Thus, a vacuum cavity may be formed to ensure minimum thermal losses. The cavity222may be formed by a Venezia cavity allowing electrical and thermal insulation of the membrane from the substrate.

FIGS. 12A to 12Cshow the sensor ofFIGS. 8 to 11in different operational states. InFIG. 12A, the membrane is in the non-deflected state, such as the equilibrium state. InFIG. 12B, the membrane in deflected upwardly when the membrane is heated. InFIG. 12C, the membrane in deflected downwardly. This state may be achieved when the membrane is cooled down starting from the non-deflected state.FIG. 13Ashows an equivalent circuit including a variable capacitance Cn between the counter electrode320and the membrane310, a variable capacitance Cr between the counter electrode322and the membrane310and a variable capacitance Cb between the membrane and the bulk of the substrate below the membrane. The contact324of the first counter electrode320, the contact326of the second counter electrode322and a bulk contact350are also shown inFIG. 13A.FIG. 13Bshows a corresponding equivalent circuit, wherein the membrane310, which is formed of highly doped conductive semiconductor material, is collapsed to a circuit node.

Since the membrane310is floating, the overall charge within the membrane remains the same and, therefore, electrostatic forces will be balanced at any time. Thus, the risk that pull-in effects will occur may be reduced.

FIGS. 14A to 16Bshow schematic views and equivalent circuits to explain examples of possible detection circuits. In all examples, the bulk contact is connected to ground.

According toFIG. 14A, a supply voltage Vdd is applied to contact324and, therefore, the first counter electrode320, and an output voltage Vout is detected at contact326. The output voltage326depends on the deflection of the membrane310and, therefore, represents a measure for the incident infrared radiation. Accordingly,FIGS. 14A to 14Bmay be regarded as showing a single-ended readout with no pull-in.

FIGS. 15A to 15Bshow a readout using reference pixel offset cancellation and pull-in cancellation for symmetric supply. An infrared radiation sensor400comprises a structure as described above with respect toFIGS. 8 to 11. A reference structure402comprises a similar structure. However, a shielding layer404is provided so that there is no infrared radiation incident onto the membrane of the reference structure402. A supply voltage Vdd is applied to one counter electrode of the sensor structure400and a supply voltage Vss is applied to one counter electrode of the reference structure402. The other counter electrodes are connected to each other and to an output contact410. The output voltage Vout is detected at the output contact410. As shown inFIG. 15B, the capacitances of the sensor structure400are variable depending on the infrared radiation and the capacitances of the reference structure do not change due to the shielding layer404.

FIGS. 16A to 16Bshow a direct sensor and reference integration into an inverting amplifier with capacitive feedback. The supply voltage Vdd is applied to the first counter electrode320via contact324. The voltage at the second counter electrode is applied to a first input of a differential amplifier420. A reference voltage is applied to a second input of the differential amplifier420. The output voltage Vout of the amplifier420represents a measure for the infrared radiation incident on the sensor. The output of the differential amplifier is connected to the first input of the differential amplifier420via a feedback capacitor Cf. The output of the differential amplifier depends on the relationship between the feedback capacitor Cf and the capacitance at the first input of the differential amplifier420and, therefore, is a measure for the incident infrared radiation.

WhileFIGS. 14A to 16Bhave been described in connection with an infrared radiation sensor as shown inFIGS. 8-11, it is clear that a corresponding readout may also take place with the other examples of infrared radiation sensors described herein. In addition, any other circuitry for detecting the respective capacitance or changes in the respective capacitance may be used.

In examples of the present disclosure, thermal insulation of the deflectable electrode may be improved since thermally insulating materials may be used to mechanically contact the sensing membrane. In examples, mechanical springs may encourage maximum displacement of cantilever beams and a maximum possible average displacement of a conducting membrane. In examples, using an electrically insulated and “floating” structure may eliminate the effect of pull-in, since the top and bottom electrostatic forces may always be balanced. Spring systems as described in the context of examples of the present disclosure may work most efficiently when all electrostatic effects are compensated, so that there is a reduced pull-in thread.

In examples, the sensor structure as described herein may be integrated in a substrate or a chip along with a readout circuit. Examples of the present disclosure relate to infrared radiation sensors that may be implemented in MEMS technology using CMOS compatible materials and processes.

Examples relate to methods of making an infrared radiation sensor, in which a layer arranged over a cavity in a substrate is produced, wherein the layer comprises an absorber layer. A deflectable electrode or a deflectable electrode and a support structure for the deflectable electrode are produced by structuring the layer. A composite comprising at least two layers of materials having different coefficients of thermal expansion is produced on the deflectable electrode or the support structure before or after structuring the layer. A first counter electrode facing the deflectable electrode is produced so that a first capacitance is formed between the first counter electrode and the deflectable electrode. A second counter electrode facing the deflectable electrode is produced so that a second capacitance is formed between the second counter electrode and the deflectable electrode. The deflectable electrode is floating, i.e. there is not provided any galvanic connection to the deflectable electrode.

Examples provide a method of making an infrared radiation sensor in which a layer arranged over a cavity in a substrate is produced. A deflectable electrode is produced by structuring the layer, wherein at least one lateral face of the layer surrounding the deflectable electrode forms a counter electrode, which faces a lateral face of the deflectable electrode. A lateral capacitance is formed between the lateral faces which face each other laterally. A composite comprising at least two layers of materials having different coefficients of thermal expansion is produced on the deflectable electrode before or after structuring the layer.

Examples of methods for manufacturing infrared radiation sensors as shown inFIGS. 3B and 5are now described referring toFIGS. 17A to 17L. It goes without saying that corresponding steps may also be conducted in manufacturing infrared radiation sensors according to other examples described herein.

As shown inFIG. 17A, a semiconductor substrate500comprising a cavity502so that a semiconductor layer504is arranged above the cavity502is prepared. At least the semiconductor layer504may be highly doped. The cavity502may be formed by a so-called Venezia process. The Venezia process may comprise etching trenches in a surface of the semiconductor substrate, annealing the semiconductor substrate in a H atmosphere in order to cause the semiconductor material to reflow so that the bottoms of the trenches are joined and the top portions of the trenches are closed so as to form a buried cavity beneath the surface of the semiconductor substrate. In other examples, the cavity502may be formed using a sacrificial layer process. The substrate500may be a silicon substrate.

As shown inFIG. 17B, an oxide layer506is generated on the substrate500to cover the semiconductor layer504. The oxide layer500may be deposited using chemical vapor deposition at increased temperatures of 400 to 500° C. so that a pre-deflection of a membrane to be generated may take place after cooling down to room temperature. Thereupon, a first deep trench etch takes place to generate a first deep trench508. Thereafter, a sacrificial layer510is applied and structured, seeFIG. 17C. The sacrificial layer is removed later to form the bottom cavity above the deflectable electrode. In examples, the sacrificial layer may be a carbon layer. Thereupon, a further oxide layer512is deposited, such as by chemical vapor deposition. Planarization may take place to remove unevenness of the surface of oxide layer512. For example, chemical-mechanical polishing may be performed to reduce or remove surface unevenness. Thereupon, a second deep trench etch takes place as shown inFIG. 17D. Thus, further deep trenches514are formed to structure the semiconductor layer504to achieve the membrane structures520and counter electrode structures522within the semiconductor layer. In the second deep trench etch, the semiconductor layer504may be structured to obtain a single counter electrode116(as shown inFIG. 3B) or separate counter electrodes116a,116b, as shown inFIG. 5.

FIGS. 17E to 17Hrelate to a process for manufacturing an infrared radiation sensor as shown inFIG. 3B, in which the membrane110comprises an external contact. To this end vias are etched reaching to the counter electrode116and the membrane110. The vias may be filled with conductive material, such as tungsten. Thereafter, a metallization may take place to form a contact134for the counter electrode116and a contact132contacting the membrane110. WhileFIGS. 17E to 17Hshow a symmetric structure with two counter electrode contacts134,134′, a single contact may be sufficient. During this process, an additional oxide layer524may be formed on top of oxide layer512.

Thereupon, openings530reaching to the sacrificial layer510are formed through the oxide layers512and524, as shown inFIG. 17F. The sacrificial layer510is removed through the openings530to generate the cavity128above the membrane. Thereupon, an additional oxide layer532is applied to seal the cavities128and502. Finally, an optical filter562may be applied on the oxide layer532, and an optical reflector560may be applied to the backside of the substrate500.

The contacts132,134may be coupled to external circuitry.

FIGS. 171 to 17Lshow part of a process suitable in manufacturing an infrared radiation sensor as shown inFIG. 5, which follows the process shown inFIGS. 17A-17D. The process shown inFIGS. 171-17Lis different from the process shown inFIGS. 17E to 17Hin that a contact to the membrane110is not formed. Rather, two separate contacts134aand134bare formed to both counter electrodes116aand116b. For the rest, the process is similar to the process described referring toFIGS. 17E to 17H.

In examples, the composite is a layer composite comprising at least two layers of materials having different coefficients of thermal expansions and being in direct mechanical contact to each other. In examples, one layer may be formed by a doped semiconductor layer, such as silicon or germanium. In examples, the other layer may be formed by an oxide layer, such as a silicon oxide, silicon dioxide or germanium oxide. In examples, the first layer is doped silicon and the second layer is silicon dioxide, wherein the coefficient of thermal expansion of silicon is almost five times higher than that of silicon oxide or silicon dioxide. Thus, a substantial deflection can be achieved in case of a rise in temperature.

Examples provide an infrared camera comprising an array of infrared radiation sensors as disclosed herein. Generally, the size of the membrane will influence the sensitivity of the sensor element since the larger the size of the membrane, the larger the change of the active area of the capacitor due to a mechanical deformation of the membrane will be. Accordingly, examples may be directed to sensors having a high sensitivity. Such sensors may be useful as sensor elements in CO2sensors, in which a sensitivity of 1 mK is desired. In other applications, the sensor elements may be downscaled to a reduced size in order to implement small infrared cameras having a high resolution.

In examples, the composite comprises two layers of materials having different coefficients of thermal expansions. In other examples, the composite may have more than two layers of materials having different coefficients of thermal expansion, wherein such examples may have an increased sensitivity.

In examples described herein, an oxide layer of the composite may act as a filter stack deposition on the semiconductor layer. In examples, bimaterial parts are only in the cantilever beams and do not contribute to optical performance. In such examples, a filter stack may be applied on top of the lid. In examples, an oxide layer may be deposited over the whole semiconductor membrane and may be used as an anti-reflection coating. In examples, in which oxide is deposited on the cantilevers only and the surface of the membrane is not covered with an oxide layer, optical components may be included in the lid. In examples, a Venezia process may be used to generate the semiconductor layer, in which the membrane is formed. The Venezia process may permit membranes having little or no charges to be formed. Otherwise, additional charges trapped in deposited layers may be a source of misbalance. Accordingly, using the Venezia process may help in creating a balanced system and may help in reducing mechanical stress so that the overall mechanical performance may be improved.

In examples, the deflectable electrode is deflectable vertically with respect to the substrate plane. In such examples, a membrane plane defined by main surfaces of the membrane in a non-deflected state may be arranged in parallel to the substrate plane. In other examples, the deflectable electrode may be deflectable laterally with respect to the substrate plane. In such examples, a membrane plane defined by main surfaces of the membrane may be vertical to the substrate plane.

Although some aspects have been described as features in the context of an apparatus, it is clear that such a description may also be regarded as a description of corresponding features of a method, such as a method for manufacturing such an apparatus or a method for operating such an apparatus. Although some aspects have been described as features in the context of a method, it is clear that such a description may also be regarded as a description of corresponding features concerning the functionality of an apparatus.

In the detailed description, it can be seen that various features are grouped together in examples for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed examples require more features than are expressly recited in each claim. Rather, as the following claims reflect, an inventive subject-matter may lie in less than all features of a single disclosed example. Thus, the following claims are hereby incorporated into the detailed description, where each claim may stand on its own as a separate example. While each claim may stand on its own as a separate example, it is to be noted that, although a dependent claim may refer in the claims to a specific combination with one or more other claims, other examples may also include a combination of the dependent claim with the subject-matter of each other dependent claim or a combination of each feature with other dependent or independent claims. Such combinations are proposed herein unless it is stated that a specific combination is not intended. Furthermore, it is intended to include also features of a claim to any other independent claim even if this claim is not directly made dependent to the dependent claim.

The above-described examples are merely illustrative for the principles of the present disclosure. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to those skilled in the art. It is the intent, therefore, to be limited by the scope of the impeding patent claims and not by the specific details presented by way of description and explanation of the examples herein.