EP4673715A1

EP4673715A1 - Material for sensor applications

Info

Publication number: EP4673715A1
Application number: EP24764597.1A
Authority: EP
Inventors: Sean ANDREWS
Original assignee: Obsidian Sensors Inc
Current assignee: Obsidian Sensors Inc
Priority date: 2023-03-01
Filing date: 2024-02-29
Publication date: 2026-01-07
Also published as: CN121420178A; WO2024182607A1; KR20250156166A; JP2026511382A; TW202511710A

Abstract

A thermo-resistive sensor, including: a substrate; a structure coupled to the substrate; and a layer of niobium oxide coupled to the structure.

Description

MATERIAL FOR SENSOR APPLICATIONS

BACKGROUND

Field

[0001] The present disclosure relates to material for sensors, and more specifically, to using niobium oxide (NbOx) as the sensor material .

Background

[0002] Uncooled thermo-resistive sensors, including microbolometers, thermistors, and passive infrared (PIR) motion sensors, may use resistive transduction of heat to current or voltage. Performance and stability of these devices may be dictated by the sensor material . These resistive materials may exhibit a higher temperature coefficient of resistance (TCR) while being limited by excess noise (e.g. , 1/f in character) . These devices may be used to convert a fixed bias voltage to a current output (or vice versa) wherein changes in the output stem from changes in the resistor's temperature.

SUMMARY

[0003] The present disclosure discloses thermo-resistive sensors such as microbolometers, thermistors, and PIR motion sensors.

[0004] In one implementation, a thermo-resistive sensor is disclosed. The sensor includes: a substrate; a structure coupled to the substrate; and a layer of niobium oxide coupled to the structure.

[0005] In another implementation, a microbolometer sensor for thermal imaging is disclosed. The microbolometer sensor includes: a substrate; a structure coupled to the substrate but positioned to thermally isolate the structure from the substrate; and a pixel coupled to the structure, wherein the pixel includes a layer of niobium oxide

[0006] In a further implementation, a microbolometer sensor for thermal imaging is disclosed. The microbolometer sensor includes: a niobium oxide material having a metal-to-insulator transition (MIT) temperature above 400°C and a temperature coefficient of resistance (TCR) of 2-5% at room temperature.

[0007] Other features and advantages should be apparent from the present description which illustrates, by way of example, aspects of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The details of the present disclosure, both as to its structure and operation, may be gleaned in part by study of the appended drawings, in which like reference numerals refer to like parts, and in which:

[0009] FIG. 1 shows exemplary transition temperatures of different materials, including NbO₂ and VO₂ ;

[0010] FIG. 2 illustrates an exemplary sensor in accordance with an implementation;

[0011] FIG. 3 shows noise measurement data and associated Hooge parameter for an exemplary NbO_x sensor;

[0012] FIG. 4 shows TCR data for an exemplary NbO_x sensor which remains unchanged over several weeks;

[0013] FIG. 5 shows noise measurement data and associated Hooge parameter for an exemplary Ti -doped NbO_x sensor; [0014] FIG. 6 shows TCR data for an exemplary Ti-doped- NbO_x sensor;

[0015] FIG. 7 shows signal-to-noise ratio data for exemplary NbO_x and Ti- doped NbO_x sensors;

[0016] FIG. 8 shows exemplary dependence of the resistivity on the partial pressure of O₂ during reactive sputtering; and

[0017] FIG. 9 shows performance comparisons between exemplary sensors including different materials.

DETAILED DESCRIPTION

[0018] As described above, conventional resistive sensor materials for microbolometers may exhibit a higher TCR while being limited by excess noise (e.g., 1/f in character) . These microbolometers may use amorphous silicon, which may be limited in performance due to higher 1/f noise and a more limiting temperature stability window. When the noise is lowered by crystallizing part of the amorphous network or by lowering resistivity (e.g. , by increasing crystallinity or doping) , the TCR may drop beyond an acceptable level . Some uncooled bolometers use vanadium oxide (Vox or VO₂) , which may have better noise metrics while maintaining higher TCR values. However, depending on phase, the metal-to-insulator transition (MIT) for VOx may be approximately 65-85C. This may limit the operational temperature of devices utilizing VOx near or below the transition temperature. While materials such as quantum wells, graphene, and carbon nanotubes may be used for thermal sensing, they may not be as suitable when considering manufacturability. [0019] Certain implementations of the present disclosure provide for microbolometers using niobium oxide (NbOx) as sensor material . In one implementation, NbOx may refer to niobium monoxide (NbO) , niobium pentoxide (Nb₂O₅) , or preferably niobium dioxide (NbO₂ ) . Although the preferred target is NbO₂ , based on the f ilm properties desired and the physical non- stoichiometric nature of non- crystalline materials , the actual Nb/O ratio may not be exactly 2 , but rather between 1 . 8 and 2 . 2 .

[0020] In one implementation, the transition temperature for NbO₂ is above 400°C , which may make it a less suitable material for switching applications operating near room temperature . However , some microbolometer implementations may benef it from a transition temperature that is substantially higher than the temperature of operation . In one implementation, the higher transition temperature allows a higher dynamic range of operation for some thermosensitive applications . For example , high temperature obj ects in a scene may not push the pixel temperature past the transition in a microbolometer that uses NbO₂ .

[0021] After reading the below descriptions , it will become apparent how to implement the disclosure in various implementations and applications . Although various implementations of the present disclosure will be described herein, it is understood that these implementations are presented by way of example only, and not limitation . As such, the detailed description of various implementations should not be construed to limit the scope or breadth of the present disclosure .

[0022 ] As described above , there are several factors that go into a good candidate for use in a microbolometer . The factors include high TCR, low 1/f noise , process compatibility, manuf cturing potential into a thin f ilm, appropriate resistivity, and high transition temperature .

[0023 ] FIG . 1 shows exemplary transition temperatures of dif ferent materials , including NbO₂ and V0₂ . FIG . 1 clearly shows that NbO₂ is a good candidate based on a high transition temperature while satisfying other factors mentioned above .

[0024] In one implementation, the sensor includes an uncooled microbolometer sensor for thermal imaging in Long-Wave Infrared (LWIR) , Mid-Wave Infrared (MWIR) , or Short -Wave Infrared ( SWIR) . In another implementation, the sensor includes an antenna- coupled uncooled microbolometer sensor for terahertz sensors . In another implementation, the sensor includes a low noise thermistor . In another implementation, the sensor includes a passive infrared ( PIR) sensor . In one implementation, the sensor has an operational temperature range of -40°C to 345°C . In one implementation, the sensor may not be damaged when exposed to or imaging high temperature sources , such as the sun .

[0025] In one implementation, the upper end of the operating temperature (ambient ) of a microbolometer using the NbOx sensor may be established as follows . A microbolometer that is imaged onto an obj ect in the f ield of view may have a temperature transfer ratio of 1/100 . That is , if the obj ect surface temperature is T_obj ect , the pixel temperature may increase from the ambient temperature by T_obj ect/100 . The operating temperature T_op adds directly to the pixel temperature so the pixel temperature may become T_op + T_obj ect/100 . If the obj ect is the sun whose surface temperature is 5500°C , then the requirement that the pixel temperature remain below the material ' s transition temperature (400C) translates to an upper bound for the operating temperature T_op < 345°C . In practice , the operating temperature may be substantially below this .

[0026] In one implementation, the sensor has a range of resistivities of 1 to 100 ohm- cm at room temperature , which may enable a desired noise equivalent temperature dif ference (NETD) , as described below . The range of resistivities may be set based on pixel geometric factors , such as pixel pitch and thickness , and requirements of an associated readout circuit . In one implementation, the resistivity range is achieved via intrinsic non- stoichiometries , wherein a lower x results in lower resistivity . In one implementation, the resistivity range is achieved using extrinsic doping with at least one of titanium (Ti ) , molybdenum (Mo) , and other suitable dopants , such as Chromium (Cr) .

[0027 ] In one implementation, the sensor includes an LWIR, MWIR, or SWIR microbolometer sensor , and a complex refractive index of the sensor provides suf f icient absorption for transducing radiation input to heat .

[0028] In one implementation, the sensor advantageously achieves a desired pixel resistance , 1/f noise power , TCR, and material stability . In one implementation, the performance of the sensor is expressed in NETD , and the desired sensor parameters may achieve a desired NETD ( lower numbers are better) . The NETD may be expressed as a function of bolometer and camera parameters as follows : wherein, K is the 1/f noise parameter that is determined by the resistive sensor material (K is unitless since it represents the f luctuations in a normalized quantity) ,

F is the F/# or focal ratio = focal length/diameter of the lens ,

5L/ (dTtarget) is the dif ferential radiance (per temperature dif ference) at the target or obj ect , integrated across the LWIR band ( 8 - 14 |lm) ,

A_p is the area of the pixel ,

Gth is the thermal conductance of the pixel , and f_H and f_L are the high frequency and low frequency limits of integration to have a standard measure of 1/f noise energy .

[0029] In one implementation, the pixel ' s thermal isolation capability is characterized by Gth (unit = W/ °K) , the thermal conductance of the pixel which is controlled primarily by the geometry (width, thickness , length) of the hinges that suspend the pixel over the substrate , and the material used in that hinge and the vacuum level . The lower the Gth, the larger temperature rise for a given energy absorbed . In terms of the pixel design and fabrication, the hinges need to be narrower and/or thinner to make up for the shorter hinge resulting from the reduced pixel pitch . The Gth scaling may also help with the fact that the pixel has become smaller and the ability for the lens to become faster ( smaller F/# ) is limited .

[0030] In one implementation, the sensor advantageously exhibits lower 1/f noise , as quantif ied by the Hooge parameter (i.e., K*V < 5*e^-22 cm³, where K is the measured 1/f noise shown in Equation [1] that is determined by the resistive sensor material and V is the volume of a sample) . The Hooge parameter is a material -dependent parameter that allows for noise comparison between materials. Since the noise of the device decreases as the size of the device increases, the Hooge parameter is used as a means to normalize the size effects.

[0031] FIG. 3 shows noise measurement data and associated Hooge parameter for an exemplary NbO_x sensor. FIG. 3 is a graph of the noise as a function of frequency, wherein the dependence is linear on a log- log scale. This graph shows that, once normalized to the applied voltage, all of the noise values for the device directly overlay.

Thus, this graph provides confidence in the value measured for the noise. The Hooge parameter is then calculated by multiplying the noise (i.e. , K value which is y-axis value at 1 Hz) measured here times the volume of the sensor in the device (not given in the graph) .

[0032] FIG. 4 shows TCR data for an exemplary NbO_x sensor. In one implementation, the TCR data advantageously remains unchanged over several weeks. The TCR value obtained from this graph may be used in Equation [1] . Thus, the graph of FIG. 4 shows that this material (i.e. , NbO_x) is suitable for temperature sensing applications .

[0033] Although there are other materials that exhibit low noise (i.e. , small Hooge parameter) , a substantial number of them are metals which exhibit (a) very low TCR values and (b) very low resistivities for any practical applications. Also, there are other materials that exhibit very high TCR values, but a substantial number of them are (a) extremely resistive and (b) very noisy (i.e., high Hooge parameter) . It has been shown that NbOx provides both low noise and high TCR value at the same time .

[0034] FIG. 5 shows noise measurement data and associated Hooge parameter for an exemplary Ti-NbO_x sensor. In FIG.

5, NbOx is doped with titanium dopants (Ti-) . FIG. 5 shows that even when doped/alloyed with another metal that results in a change in resistivity, the noise remains low.

[0035] FIG. 6 shows TCR data for an exemplary Ti-NbO_x sensor, and shows that the TCR value, like the resistivity, may be tuned with external doping/alloying . However, it should be noted that the resistivity and TCR may be controlled using different methods such as by controlling the Nb/O ratio directly.

[0036] FIG. 7 shows signal-to-noise ratio data for exemplary NbO_x and Ti-NbO_x sensors. The graph of FIG. 7 shows that the signal-to-noise ratio (SNR) , defined here TCR as , — , is higher for Ti-doped films. K*V

[0037] In one implementation, NbOx without doping may improve signal-to-noise ratio by lowering resistivity and/or TCR to an unsuitable range for some applications. In one implementation, the addition of Ti advantageously allows the TCR to increase while reducing noise. In one implementation, Cr is added to reduce resistivity for NbOx material having a higher oxygen content .

[0038] In one implementation, a sensor includes amorphous or polycrystalline thin films of NbOx. For example, the sensor includes Amorphous or polycrystalline thin films of NbO_x. In one implementation, a sensor is configured to not change phase (e.g. , from metal to insulator) within a range of temperature, for example, -40 to 345°C. In one implementation, the range of temperature is associated with excursion during packaging process. In one implementation, the range of temperature is associated with temperature excursion during sensor operation.

[0039] In one implementation, the NbO_x is compatible with silicon processing and/or glass substrate semiconductor processing (e.g. , PVD or CVD) . In one implementation, the NbO_x has a TCR of 2-5% (a preferred TCR number is between 2 and 3%) , which may advantageously satisfy TCR requirements for achieving a desired NETD, and can be controlled by deposition and anneal processes during manufacturing of the sensor. In one implementation, the deposition includes one or more of reactive sputter, molecular-beam epitaxy (MBE) , metal organic chemical vapor deposition (MOCVD) , atomic layer deposition (ALD) , physical vapor deposition (PVD) , electrochemical deposition, sol-gel techniques, and other thin-film deposition techniques.

[0040] FIG. 8 shows exemplary dependence of the resistivity on the partial pressure of O₂ during reactive sputtering. The graph of FIG. 8 shows resistivity as a function of both oxygen content and thermal process and not dominated by either, but rather scales. The graph shows that conductive oxides may not be too sensitive to subsequent processes, such as high temperature steps, and may not limit the allowable properties of a film. Thus, this means that NbOx may be tuned in resistivity without the need of an external dopant. The graph also shows that the subsequent thermal processing, like the type required for vacuum packaging, has a small but deterministic effect .

[0041] FIG. 9 is a graph showing performance comparisons io between sensors with dif ferent materials . In particular , the graph shows that the disclosed NbOx-based sensors may achieve similar or better performance as VOx with the described benef its .

[0042 ] FIG . 2 is a block diagram of a thermo- resistive sensor 200 in accordance with one implementation of the present disclosure . In one implementation, sensor 200 includes a substrate 230 , a structure 220 coupled to substrate 230 , and a pixel 210 coupled to the structure 220 .

[0043 ] In one implementation, the structure 220 is positioned to thermally isolate the structure 220 from the substrate 230 . In one implementation, the pixel 210 includes a layer of niobium oxide . In one implementation, sensor 200 experiences a voltage dif ference and generates a current or charge based on the received radiation (e . g . , the resistance between two terminals of the sensor changes in response to exposure to LWIR or MWIR radiation) .

[0044] In one implementation, the thermo- resistive sensor is a microbolometer and the structure is thermally isolated from the substrate . In one implementation, the thermo- resistive sensor is a thermistor . In one implementation, the thermo- resistive sensor is a passive infrared ( PIR) motion sensor . In one implementation, the niobium oxide is NbO₂ . In one implementation, the niobium oxide is NbOx . In one implementation, the niobium oxide is titanium doped, Ti -NbOx . In one implementation, the layer of niobium oxide is a pixel sensor .

[0045] In one particular implementation, a microbolometer sensor for imaging is disclosed . The microbolometer sensor includes : a substrate ; a structure coupled to the substrate ; and a pixel coupled to the structure , wherein the pixel includes a layer of niobium oxide .

[0046] In one implementation, the sensor is used for thermal imaging in an infrared ( IR) range . In one implementation, the IR range includes at least one of Long-Wave Infrared (LWIR) , Mid-Wave Infrared (MWIR) , and Short -Wave Infrared ( SWIR) . In one implementation, the sensor has an operation range of -40°C to 345°C . In one implementation, the sensor further includes a readout circuit . In one implementation, the sensor has a range of resistivities that is set based on geometric factors of the pixel and the requirements of the readout circuit . In one implementation, the geometric factors of the pixel include pixel pitch and thickness . In one implementation, the range of resistivities is achieved using extrinsic doping with at least one of titanium (Ti ) , molybdenum (Mo) , and Chromium (Cr) . In one implementation, the sensor has the range of resistivities of 1 to 100 ohm- cm at room temperature . In one implementation, the structure is a set of hinges that thermally isolates the pixel from the substrate .

[0047 ] In another particular implementation, a microbolometer sensor for thermal imaging is disclosed . The microbolometer sensor includes a niobium oxide material having a metal - to- insulator transition (MIT) temperature above 400°C and a temperature coef f icient of resistance (TCR) of 2 - 5% at room temperature .

[0048] In one implementation, the niobium oxide material has a resistivity of 1 to 100 ohm- cm at room temperature . In one implementation, the microbolometer sensor further includes : a substrate ; and a structure coupled to the substrate , wherein the structure is a set of hinges that thermally isolates the niobium oxide material from the substrate .

[0049] In one implementation, a sensor includes a glass substrate, a structure manufactured from any of the methods described herein and coupled to the glass substrate, and a pixel coupled to the structure. The pixel includes a material described herein.

[0050] In one implementation, a sensor includes a MEMS or NEMS device manufactured by an LCD-TFT manufacturing process and a structure manufactured by any of the methods described herein.

[0051] By way of examples, sensors can include resistive sensors. Bolometers (or microbolometers) can be used in a variety of applications. For example, long wave infra-red (LWIR, wavelength of approximately 8-12 pm) bolometers can be used in the automotive and commercial security industries. For example, LWIR bolometers with QVGA, VGA, and other resolution. Terahertz (THz, wavelength of approximately 1.0 -0.1 mm) bolometers can be used in security (e.g. , airport passenger security screening) and medical (medical imaging) . For example, THz bolometers with QVGA resolution and other resolutions. Some electronic or electrooptical systems can include X-Ray sensors or camera systems. Similarly, LWIR and THz sensors are used in camera systems . Some electromechanical systems are applied in medical imaging, such as endoscopes and exoscopes. X-ray sensors include direct and indirect sensing configurations.

[0052] Those skilled in the art will recognize that the implementations described herein are representative, and deviations from the explicitly disclosed implementations are within the scope of the disclosure. For example, one implementation includes using different dopants than described. As another example, manufacturing temperature (e.g., growth temperature) may be controlled to control morphology and/or dopant efficiency. As another example, after deposition, additional processing may be performed on the material, such as annealing and/or laser- induced crystallization .

[0053] Although the disclosed implementations have been fully described with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the disclosed implementations as defined by the appended claims .

[0054] The terminology used in the description of the various described implementations herein is for the purpose of describing particular implementations only and is not intended to be limiting. As used in the description of the various described implementations and the appended claims, the singular forms "a", "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," and/or "including," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof . It will be further understood that the term "exemplary, " when used in this specif ica ion, refers to serving as an example , instance , or illustration rather than to commendable or serving as a pattern .

[0055] All features of each of the above-discussed examples are not necessarily required in a particular implementation of the present disclosure . Further , it is to be understood that the description and drawings presented herein are representative of the subj ect matter which is broadly contemplated by the present disclosure . It is further understood that the scope of the present disclosure fully encompasses other implementations that may become obvious to those skilled in the art and that the scope of the present disclosure is accordingly limited by nothing other than the appended claims .

Claims

1. A thermo-resistive sensor, comprising: a substrate; a structure coupled to the substrate; and a layer of niobium oxide coupled to the structure.

2. The sensor of claim 1, wherein the thermo- resistive sensor is a microbolometer and the structure is thermally isolated from the substrate.

3. The sensor of claim 1, wherein the thermo- resistive sensor is a thermistor.

4. The sensor of claim 1, wherein the thermo- resistive sensor is a passive infrared (PIR) motion sensor .

5. The sensor of claim 1, wherein the niobium oxide is NbO_x.

6. The sensor of claim 1, wherein the niobium oxide is titanium doped, Ti-NbO_x.

7. The sensor of claim 1, wherein the layer of niobium oxide is a pixel sensor.

8. A microbolometer sensor for imaging, the microbolometer sensor comprising: a substrate; a structure coupled to the substrate; and a pixel coupled to the structure, wherein the pixel includes a layer of niobium oxide.

9. The microbolometer sensor of claim 8, wherein the sensor is used for thermal imaging in an infrared (IR) range .

10. The microbolometer sensor of claim 9, wherein the IR range includes at least one of Long-Wave Infrared (LWIR) , Mid-Wave Infrared (MWIR) , and Short-Wave Infrared (SWIR) .

11. The microbolometer sensor of claim 8, wherein the sensor has an operation range of -40°C to 345°C.

12. The microbolometer sensor of claim 8, further comprising a readout circuit.

13. The microbolometer sensor of claim 12, wherein the sensor has a range of resistivities that is set based on geometric factors of the pixel and the requirements of the readout circuit.

14. The microbolometer sensor of claim 13, wherein the geometric factors of the pixel comprise pixel pitch and thickness.

15. The microbolometer sensor of claim 13, wherein the range of resistivities is achieved using extrinsic doping with at least one of titanium (Ti) , molybdenum (Mo) , and Chromium (Cr) .

16. The microbolometer sensor of claim 13, wherein the sensor has the range of resistivities of 1 to 100 ohm- cm at room temperature.

17. The microbolometer sensor of claim 10, wherein the structure is a set of hinges that thermally isolates the pixel from the substrate.

18. A microbolometer sensor for thermal imaging, the microbolometer sensor comprising a niobium oxide material having a metal-to-insulator transition (MIT) temperature above 400C and a temperature coefficient of resistance (TCR) of 2-5% at room temperature .

19. The microbolometer sensor of claim 18, wherein the niobium oxide material has a resistivity of 1 to 100 ohm- cm at room temperature.

20. The microbolometer sensor of claim 18, further comprising : a substrate; and a structure coupled to the substrate, wherein the structure is a set of hinges that thermally isolates the niobium oxide material from the substrate .