Hydrogen gas sensor and method of forming the same

Abstract

Various embodiments may relate to a hydrogen gas sensor. The hydrogen gas sensor may include a first electrode for sensing hydrogen gas. The hydrogen gas sensor may also include a second electrode. The hydrogen gas sensor may further include a dielectric layer between the first electrode and the second electrode. The dielectric layer may include or consist of tantalum pentoxide (Ta2O5). The hydrogen gas sensor may additionally include an interlayer between the first electrode and the dielectric layer, the interlayer including or consisting of titanium (Ti). The first electrode may be configured to dissociate hydrogen gas into hydrogen atoms. The interlayer may be configured to form bonds with the hydrogen atoms.

Description

HYDROGEN GAS SENSOR AND METHOD OF FORMING THE SAMETECHNICAL FIELD

[0001] Various embodiments of this disclosure may relate to a hydrogen gas sensor. Various embodiments of this disclosure may relate to a method of forming a hydrogen gas sensor.BACKGROUND

[0002] Recently, hydrogen (Hz) has drawn great attention as a clean and renewable energy source to reduce carbon footprint. However, as H2 is flammable at concentrations of 4% to 76% and explosive at concentrations of 18.3% to 59%, a reliable H2 leakage sensor is required for early detection and risk mitigation. Among various types of H2 leakage sensors, resistive random access memory (RRAM)-based sensors are quite promising owing to their advantageous features.

[0003] Currently, conventional H2 sensors are constrained by complex buried structure, high thermal budget (> 400 °C), and are only able to sense H2 of a relatively high concentration levels (concentrations > 0.1 %). In addition, they require a high operating temperature (i.e., above room temperature).SUMMARY

[0004] Various embodiments may relate to a hydrogen gas sensor. The hydrogen gas sensor may include a first electrode for sensing hydrogen gas. The hydrogen gas sensor may also include a second electrode. The hydrogen gas sensor may further include a dielectric layer between the first electrode and the second electrode. The dielectric layer may include or consist of tantalum pentoxide (Ta2O5). The hydrogen gas sensor may additionally include an interlayerbetween the first electrode and the dielectric layer, the interlayer including or consisting of titanium (Ti). The first electrode may be configured to dissociate hydrogen gas into hydrogen atoms The interlayer may be configured to form bonds with the hydrogen atoms.

[0005] Various embodiments may relate to a method of forming a hydrogen gas sensor. The method may include forming a first electrode for sensing hydrogen gas. The method may also include forming a second electrode. The method may further include forming a dielectric layer between the first electrode and the second electrode, the dielectric layer including or consisting of tantalum pentoxide ( Ta2O5). The method may additionally include forming an interlayer between the first electrode and the dielectric layer, the interlayer including or consisting of titanium (Ti). The first electrode may be configured to dissociate hydrogen gas into hydrogen atoms The interlayer may be configured to form bonds with the hydrogen atoms.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows a general illustration of a hydrogen gas sensor according to various embodiments. FIG. 2 shows a general illustration of a method of forming a hydrogen gas sensor according to various embodiments.FIG. 3 A shows a perspective view of a hydrogen gas sensor according to various embodiments. FIG. 3B shows a cross-sectional view of the hydrogen gas sensor according to various embodiments along line A-A’ in Fig. 3A.FTG. 3C shows a cross-sectional view of the hydrogen gas sensor according to various embodiments along line B-B’ in Fig. 3A (but without including the first contact pad).FIG. 3D shows a cross-sectional high resolution transmission electron microscopy image (HRTEM) image of the hydrogen gas sensor according to various embodiments.FIG. 4A shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) hysteresis characteristics of a pristine cross-point resistive random access memory (RRAM)-based hydrogen gas sensor according to various embodiments.FIG. 4B shows a plot of resistance (in ohms or Ω1) as function of retention (in seconds or sec) illustrating the stable data retention characteristics of the hydrogen gas sensor according to various embodiments at a current compliance (CC) of 10 mA.FIG. 5A shows a plot of sensing current (in Amperes or A) as a function of time (in seconds or sec) illustrating the hydrogen gas (H2) response current-time (I-t) characteristics of the cross- point resistive random-access memory (RRAM)-based hydrogen gas sensor according to various embodiments at room temperature.FIG. 5B shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the successive direct current (DC) sweeping to resolve the drift issue of the hydrogen gas sensor according to various embodiments after hydrogen gas (H2) exposure.FIG. 6 shows a plot of sensing current (in Amperes or A) as a function of time (in seconds or sec) illustrating the hydrogen gas (H2) response characteristics of the cross-point resistive random-access memory (RRAM)-based hydrogen gas sensor according to various embodiments at H2 concentration levels of 20 parts per million (ppm), 50 ppm and 100 ppm.FIG. 7 shows a plot of current (in Amperes or A) as a function of exposure time (in seconds or sec) illustrating the cross-sensitivity test results of the cross-point resistive random-accessmemory (RRAM)-based hydrogen gas sensor according to various embodiments at room temperature.FIG. 8A shows a cross-sectional side view of another hydrogen gas sensor according to various embodiments.FIG. 8B shows a cross-sectional traverse view of the hydrogen gas sensor shown in FIG. 8A according to various embodimentsFIG. 9A shows a conventional one transistor-one resistor (1T1R) type hydrogen gas sensor with a buried resistive random-access memory (RRAM) structure.FIG. 9B shows a conventional single layer one resistor (1R) hydrogen gas sensor using platinum (Pt) as a sensing electrode.FIG. 9C shows a conventional one transistor (IT)-one metal -insulator-metal (MIM) capacitive type hydrogen gas sensor.DESCRIPTION

[0007] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0008] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / orcombinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0009] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0010] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e g. within 10% of the specified value.

[0011] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0012] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising” Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0013] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0014] Embodiments described in the context of one of the hydrogen gas sensors are analogously valid for the other hydrogen gas sensors, embodiments described in the context of a method are analogously valid for a hydrogen gas sensor, and vice versa.

[0015] Various embodiments may provide a H2 leakage sensor that can detect H2 at room temperature at < 0.1 % H2 concentration. Various embodiments may include a titanium (Ti) layer (e g., beneath the catalytic metal layer). Various embodiments may be based on an unburied cross-point RRAM structure.

[0016] Various embodiments may relate to a back-end-of-line (BEOL) and complementary metal oxide semiconductor (CMOS) compatible unburied cross-point resistive H2 leakagesensor Various embodiments may enable detecting H2 at as low as 20 parts per million (ppm) concentration level at room temperature.

[0017] FIG. 1 shows a general illustration of a hydrogen gas sensor according to various embodiments. The hydrogen gas sensor may also be referred to as a hydrogen gas leakage sensor, a H2 leakage sensor, or simply a hydrogen sensor. The hydrogen gas sensor may include a first electrode 102 for sensing hydrogen gas. The hydrogen gas sensor may also include a second electrode 104. The hydrogen gas sensor may further include a dielectric layer 106 between the first electrode 102 and the second electrode 104. The dielectric layer 106 may include or consist of tantalum pentoxide (Ta2O5). The hydrogen gas sensor may additionally include an interlayer 108 between the first electrode 102 and the dielectric layer 106, the interlayer 108 including or consisting of titanium (Ti). The first electrode 102 may be configured to dissociate hydrogen gas into hydrogen atoms. The interlayer 108 may be configured to form bonds with the hydrogen atoms.

[0018] In other words, the hydrogen gas sensor may be a structure including four layers, with the dielectric layer 106 and the interlayer 108 between the first electrode 102 and the second electrode 104. The interlayer 108 may be between the first electrode 102 and the dielectric layer 106. In various embodiments, the bottommost layer may be the second electrode 104, with the dielectric layer 106 on or over the second electrode 104, the interlayer 108 on or over the dielectric layer 106, and the first electrode 102 on or over the interlayer 108.

[0019] For avoidance of doubt, FIG. 1 seeks to illustrate some features of a hydrogen gas sensor according to various embodiments, and is not intended to limit for instance, the dimensions, shapes, orientations etc. of the various features. For instance, while FIG. 1 shows the layers 102, 104, 106, 108 to have the same thickness, the thickness of the interlayer 108 may be smaller than the thicknesses of the other layers 102, 104, 106.

[0020] Tn various embodiments, the hydrogen gas sensor may have a cross-point structure. The hydrogen gas sensor may include a first contact pad in contact with the first electrode 102. The hydrogen gas sensor may also include a second contact pad in contact with the second electrode 104.

[0021] The first electrode 102 may include an interconnect portion extending in a first direction to be in contact with the first contact pad. The interconnect portion of the first electrode 102 may extend from a planar or pad portion of the first electrode 102, a width of the interconnect portion being smaller than a width of the planar or pad portion of the first electrode 102. The second electrode 104 may include an interconnect portion extending in a second direction to be in contact with the second contact pad, the second direction substantially perpendicular to the first direction. The interconnect portion of the second electrode 104 may extend from a planar or pad portion of the second electrode 104, a width of the interconnect portion being smaller than a width of the planar or pad portion of the second electrode 104.

[0022] In various other embodiments, the first contact pad may have an interconnect portion extending in a first direction to be in contact with the first electrode 102. The interconnect portion of the first contact pad may extend from a planar or pad portion of the first contact pad, a width of the interconnect portion being smaller than a width of the planar or pad portion of the first contact pad. Likewise, the second contact pad may have an interconnect portion extending in a second direction to be in contact with the second electrode 104, the second direction substantially perpendicular to the first direction. The interconnect portion of the second contact pad may extend from a planar or pad portion of the second contact pad, a width of the interconnect portion being smaller than a width of the planar or pad portion of the second contact pad

[0023] Generally speaking, the first electrode 102 or the first contact pad may have an interconnect portion extending in a first direction, while the second electrode 104 or the secondcontact pad may have an interconnect portion extending in the second direction. Tn various embodiments, the first direction may be substantially perpendicular to the second direction, e.g., 89° to 91°. This may advantageously result in the hydrogen gas sensor having improved sensitivity. Nevertheless, in various other embodiments, the first direction may be at any suitable angle to the second direction, e.g., 80° or 110°.

[0024] Tn various embodiments, the first contact pad and / or the second contact pad may include any electrically conductive metal, e.g., gold (Au) or aluminum (Al).

[0025] In various embodiments, the hydrogen gas sensor may be adapted or configured to have improved sensitivity. For instance, the hydrogen gas sensor may be unburied. The hydrogen gas sensor may include exposed sidewalls. In other words, at least a portion of each of the first electrode 102, the second electrode 104, the dielectric layer 106 and the interlayer 108 may be exposed. This may increase a surface area of the device for exposure to hydrogen gas. In various embodiments, the first electrode 102 may be patterned. For instance, the first electrode 102 may include grooves for increasing a surface area of the first electrode 102. In various other embodiments, the hydrogen gas sensor may have fm-FET like structure.

[0026] Tn various embodiments, the interlayer 108 may be of a thickness selected from a range from 0.1 nm to 4 nm, e.g., from 0.1 nm to 3.5 nm, e.g. from 0.1 nm to 2 nm. In various embodiments, the interlayer 108 may be of a thickness of less than 4 nm. In various embodiments, the first electrode 102 may have a thickness selected from a range from 20 nm to 100 nm. In various embodiments, the second electrode 104 may have a thickness selected from a range from 20 nm to 100 nm Tn various embodiments, the dielectric layer 106 may have a thickness selected from a range from 3 nm to 30 nm.

[0027] In various embodiments, the interlayer 108 may be an adhesion layer. The interlayer 108 may have a first side in contact with the first electrode 102 and a second side, the second side opposite the first side, in contact with the dielectric layer 106.

[0028] Tn various embodiments, the interlayer 108 may be configured to scavenge oxygen from the dielectric layer 106, thereby forming oxygen vacancies in the dielectric layer 106. This may also help to obtain forming-free resistive switching of the pristine device, thereby enabling low -power consumption.

[0029] In various embodiments, the titanium included in the interlayer 108 may be in the form of nanocrystals. Tn other words, the interlayer 108 may include or consist of titanium nanocrystals. In various other embodiments, the titanium included in the interlayer 108 may form a single crystalline layer or may form an amorphous layer.

[0030] In various embodiments, the first electrode 102 may include or consist of any material that can sense and / or dissociate hydrogen gas at room temperature (25°C). For instance, the first electrode 102 may include or consist of palladium (Pd), iridium (Ir), palladium (Pd) with titanium (Ti), palladium (Pd) with nickel (Ni), palladium (Pd) with niobium (Nb) or palladium (Pd) with vanadium (V). In various embodiments, the first electrode 102 may include or consist of Pd metal, iridium (Ir), palladium (Pd) with titanium (Ti), palladium (Pd) with nickel (Ni), palladium (Pd) with niobium (Nb) or palladium (Pd) with vanadium (V).

[0031] Tn various embodiments, the second electrode 104 may include or consist of any material that is non-reactive to oxygen gas and / or hydrogen gas. For instance, the second electrode may include or consist of titanium tungsten (TiW), molybdenum (Mo), ruthenium (Ru), tantalum nitride (TaN) or tungsten (W).

[0032] In various embodiments, the hydrogen gas sensor may be configured to detect a hydrogen gas concentration of 20 ppm at room temperature (25 °C).

[0033] In various embodiments, the hydrogen gas sensor may be a resistive random access memory (RRAM)-based hydrogen gas sensor.

[0034] Various embodiments may include an array including a plurality of hydrogen gas sensors as described herein. The array may be a periodic array. The array may include a processor or controller in electrical connection to the plurality of hydrogen gas sensors

[0035] FIG. 2 shows a general illustration of a method of forming a hydrogen gas sensor according to various embodiments. The method may include, in 202, forming a first electrode for sensing hydrogen gas. The method may also include, in 204, forming a second electrode. The method may further include, in 206, forming a dielectric layer between the first electrode and the second electrode, the dielectric layer including or consisting of tantalum pentoxide (Ta2O5). The method may additionally include, in 208, forming an interlayer between the first electrode and the dielectric layer, the interlayer including or consisting of titanium (Ti). The first electrode may be configured to dissociate hydrogen gas into hydrogen atoms The interlayer may be configured to form bonds with the hydrogen atoms.

[0036] For avoidance of doubt, FIG. 2 is not intended to limit the sequence of the various steps. In various embodiments, the second electrode may be formed first, followed by forming of the dielectric layer. The interlayer may then be formed, followed by forming the first electrode.

[0037] In various embodiments, the second electrode may be formed on a substrate. The dielectric layer may be formed on the second electrode. The interlayer may be formed on the dielectric layer. The first electrode may be formed on the interlayer. In various embodiments, the substrate may include any suitable semiconductor material, e.g. silicon (Si). For instance, the substrate may be silicon wafer, such as a p-doped or n-doped silicon wafer In various embodiments, the substrate may include any suitable insulator layer including an insulator material, e g. silicon dioxide (SiCh), silicon nitride or aluminum oxide (AI2O3), over the silicon wafer.

[0038] In various embodiments, the method may further include forming a first contact pad and / or a second contact pad. In various embodiments, the method may further include forming the first contact pad in contact with the first electrode The method may also include forming the second contact pad in contact with the second electrode. In various embodiments, the first electrode or the first contact pad may include an interconnect portion extending in a first direction The second electrode or the second contact pad may include an interconnect portion extending in a second direction. In various embodiments, the second direction may be substantially perpendicular to the first direction.

[0039] In various embodiments, the method may be backend-of-line (BEOL)-compatible. Forming the first electrode, the second electrode, the dielectric layer and the interlayer may be carried out at temperatures below 400 °C.

[0040] In various embodiments, forming the first electrode and the adhesion may be carried out without breaking chamber vacuum.

[0041] In various embodiments, the method may be complementary metal oxide semiconductor (CMOS)- compatible.

[0042] In various embodiments, the hydrogen gas sensor may have a cross-point structure. In various other embodiments, the first electrode may be patterned.

[0043] In yet various other embodiments, the hydrogen gas sensor may have fin-FET like structure.

[0044] In various embodiments, the method may require 5 mask layers. The first mask layer may be to create alignment marks on the substrate. The second mask may be used to form the second electrode. The third mask may be used to form the dielectric layer. The fourth mask may be used to form the interlayer (including titanium) and the first electrode. The fifth mask may be used to form the first contact pad and the second contact pad.

[0045] Tn various embodiments, the interlayer may be of a thickness selected from a range from 0.1 nm to 4 nm, e.g., from 0.1 nm to 3.5 nm, e.g. from 0.1 nm to 2 nm.

[0046] In various embodiments, the first electrode may include or consist of any material that can sense and / or dissociate hydrogen gas at room temperature (25°C). For instance, the first electrode may include or consist of palladium (Pd), iridium (Ir), palladium (Pd) with titanium (Ti), palladium (Pd) with nickel (Ni), palladium (Pd) with niobium (Nb) or palladium (Pd) with vanadium (V).

[0047] In various embodiments, the second electrode may include or consist of any material that is non-reactive to oxygen gas and / or hydrogen gas. For instance, the second electrode may include or consist of titanium tungsten (TiW), molybdenum (Mo), ruthenium (Ru), tantalum nitride (TaN) or tungsten (W).

[0048] Various embodiments may include a method of forming an array including a plurality of hydrogen gas sensors as described herein.

[0049] FIG. 3A shows a perspective view of a hydrogen gas sensor according to various embodiments. FIG. 3B shows a cross-sectional view of the hydrogen gas sensor according to various embodiments corresponding to along line A-A’ in Fig. 3A. FIG. 3C shows a cross- sectional view of the hydrogen gas sensor according to various embodiments corresponding to along line B-B’ in Fig. 3 A (but without including the first contact pad 312). The hydrogen gas sensor may include 4 layers with 2 vertical sidewalls. As shown in FIGS. 3A - C, the second electrode 304 (including e.g., TiW) may be on or over a silicon dioxide (SiO2) layer 310 of a substrate. At least a portion of the dielectric layer 306 (including e.g., Ta2O5) may be on or over the second electrode 304, the interlayer 308 (including e.g., Ti) may be on or over the dielectric layer 306, and at least a portion of the first electrode 302 (including e g , Pd) may be on or over the interlayer 308. The dielectric layer 306 may separate and isolate the first electrode 302 from the second electrode 304, while the interlayer 308 may have a first side in contactwith the first electrode 302 and a second side (opposite the first side) in contact with the dielectric layer 306. The hydrogen gas sensor may also include a first contact pad 312 (including e.g., Al or Au) in contact with the first electrode layer 302, and a second contact pad314 (including e.g., Al or Au) in contact with the second electrode layer 304. In various embodiments, the first contact pad 312 and the second contact pad 314 may be contact pads of the same metal (e g., Al or Au). As shown in FIG. 3 A and FIG 3C, an interconnection portion 302a of the first electrode layer 302 may be in contact with the first contact pad 312, while an interconnection portion 304a of the second electrode 304 may be in contact with the second contact pad 314.

[0050] The unburied and vertical sidewalls of the hydrogen gas sensor may enhance H2 exposure area without increasing the device area. The first or top electrode 302 may act as a H2 sensing layer, and may include palladium (Pd). The interlayer 308 may include titanium (Ti) metal of a thickness of 0.1 nm to 4 nm. The interlayer 308 may enhance the adhesivity of the first electrode 302 on the substrate 310. The dielectric layer 306 may include a thermo- chemically stable high-A dielectric (Ta2O5). The interlayer 308 may also play the role of an oxygen scavenger, and may take oxygen from the dielectric layer 306, while leaving oxygen vacancies (Vo) in the dielectric layer 306. Additionally, the interlayer 308 may create a strong Ti-H bond at room temperature when the sensor device is exposed to H2, resulting in a change in device resistance at a lower concentration (sub-300 ppm) of H2. The second or bottom electrode 304 may include titanium tungsten (TiW), which may be non-reactive to O2 and H2. As shown in FIG 3A, the interconnection portion 302a of the first electrode 302 and the interconnection portion 304a of the second electrode 304 may be aligned with an angle of 90° to each other The hydrogen gas sensor may be BEOL-compatible, and the maximum process temperature may be below 400 °C. A total of 5 masks may be used for the fabrication of the hydrogen gas sensor. During fabrication, the first electrode 302 and the interlayer 308 may bedeposited without breaking the chamber vacuum. The minimum area of the diced chip may be 3 mm x 3 mm. FIG. 3D shows a cross-sectional high resolution transmission electron microscopy image (HRTEM) image of the hydrogen gas sensor according to various embodiments.

[0051] All the electrical testing is carried out at room temperature. FIG. 4A shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) hysteresis characteristics of a pristine cross-point resistive random access memory (RRAM)-based hydrogen gas sensor according to various embodiments. The paths (a) - (d) shown in FIG. 4A indicate the first forming-free cycle, while the paths (e) - (f) indicate the second hysteresis I-V cycle. The pristine 2-terminal cross-point hydrogen gas sensor may not need any additional forming bias, which may alleviate power consumption. The hydrogen gas sensor may require a set voltage (VSET) of -1 ,3 V and a reset voltage (VRESET) of 2 V at a current compliance (CC) of 10 mA. FIG. 4B shows a plot of resistance (in ohms or Q) as function of retention (in seconds or sec) illustrating the stable data retention characteristics of the hydrogen gas sensor according to various embodiments at a current compliance (CC) of 10 mA. FIG. 4B demonstrates the stable data retention for 3 hours with min. memory window of > 10 at a read voltage Vrcad of 0.5 V at room temperature. No significant variation is observed for both resistance states. This indicates the non-volatile aspect of the RRAM-based hydrogen gas sensor. The interlayer may act as an oxygen scavenger. Consequently, intrinsic defects may become high in the Ta2O5 layer, resulting in forming free current-voltage characteristics of the pristine RRAM-based hydrogen gas sensor For IF gas sensing, after successive direct current (DC) biased I-V cycles, a high resistance state (HRS) may be considered as reference state of the hydrogen gas sensor.

[0052] FIG. 5A shows a plot of sensing current (in Amperes or A) as a function of time (in seconds or sec) illustrating the hydrogen gas (H2) response current-time (I-t) characteristics ofthe cross-point resistive random-access memory (RRAM)-based hydrogen gas sensor according to various embodiments at room temperature. The hydrogen gas sensor shows good I-t characteristic at 500 ppm H2 concentration with response time (at t90%) of ~ 40 sec. tyo% represents the time taken for the hydrogen gas sensor to reach 90% of the maximum sensing current. FIG. 5B shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the successive direct current (DC) sweeping to resolve the drift issue of the hydrogen gas sensor according to various embodiments after hydrogen gas (H2) exposure. As shown in FIG. 5B, the drift issue may be addressed by leveraging on RRAM I-V sweeping.

[0053] Further, response characteristics are tested at a lower hydrogen gas concentration, i.e., sub-200 ppm concentration levels. FIG. 6 shows a plot of sensing current (in Amperes or A) as a function of time (in seconds or sec) illustrating the hydrogen gas (H2) response characteristics of the cross-point resistive random-access memory (RRAM)-based hydrogen gas sensor according to various embodiments at H2 concentration levels of 20 parts per million (ppm), 50 ppm and 100 ppm. It is shown that the hydrogen gas sensor may detect H2 at the concentration level of 20 ppm. No significant drift issue is observed at the lower concentration levels of H2. The interlayer including titanium (Ti) may enable detection of H2 at the concentration level of 20 ppm. During exposure to H2, the catalytic palladium (Pd) included in the first electrode may dissociate H2 into H atoms, which create strong bonds with Ti and oxygen vacancies at the Ti / Ta2O5 interface, resulting in change in electrical conductivity of the hydrogen gas sensor. Therefore, the interlayer including Ti may play a very important role here. Cross-sensitivity is a very common issue for gas sensors. To verify that, the hydrogen gas sensor may be tested in different gas environments at room temperature ambient as shown in FIG. 7. FIG. 7 shows a plot of current (in Amperes or A) as a function of exposure time (in seconds or sec) illustrating the cross-sensitivity test results of the cross-point resistive random access memory (RRAM)-based hydrogen gas sensor according to various embodiments at roomtemperature. The results indicate that the hydrogen gas sensor may be highly immune to carbon dioxide (CO2), methane (CH4), sulfur hexafluoride (SFR), helium (He), and synthetic air.

[0054] Various embodiments may relate to the design and arrangement of a BEOL- compatible, unburied, forming-free cross-point resistive hydrogen gas sensor. The hydrogen gas sensor may be able to detect H2 at a very low concentration level of 20 ppm. This may be attributed to the Ti layer underneath the sensing electrode. With increasing H2 concentrations, the conductivity of the hydrogen gas sensor may increase. At higher H2 concentration levels, the drift issue may be solved by applying a reset voltage to the hydrogen gas sensor. The hydrogen gas sensor may be demonstrated to be repeatable and reliable. In addition, the hydrogen gas sensor may be demonstrated to be highly immune to CO2, CH4, SFe, He, and synthetic air.

[0055] FIG. 8A shows a cross-sectional side view of another hydrogen gas sensor according to various embodiments. FIG. 8B shows a cross-sectional traverse view of the hydrogen gas sensor shown in FIG. 8A according to various embodiments. The hydrogen gas sensor may have a fin-FET like structure. The hydrogen gas sensor may include a second electrode 804 on the substrate 810. The hydrogen gas sensor may also include a dielectric layer 806 such that the substrate 810 and the dielectric layer 806 surround or enclose a middle portion of the second electrode 804. The hydrogen gas sensor may also include an interlayer 808 such that the interlayer 808 and the substrate 810 surround or enclose the dielectric layer 806. The hydrogen gas sensor may additionally include a first electrode 802 such that the first electrode 802 and the substrate 810 surround or enclose the interlayer 808.

[0056] FIG. 9A shows a conventional one transistor-one resistor (1T1R) type hydrogen gas sensor with a buried resistive random access memory (RRAM) structure. The sensor includes a bi-layer TaOx. The sensor shown in FIG. 9A is complex and requires forming voltages. Moreover, the conventional sensor is only able to detect H2 at concentrations of 10,000 ppm.Also, the sensor shown in FIG. 9A does not have a cross-point structure, and does not have a titanium interlayer as well as a Pd first electrode as a sensing layer. FIG. 9B shows a conventional single layer one resistor (1R) hydrogen gas sensor using platinum (Pt) as a sensing electrode. The conventional sensor requires a high thermal budget of 600°C / 800°C. There is a need to oxidize the polycrystalline metal-oxide prior to H2 sensing. The conventional device is only able to detect high concentrations of H2. FIG. 9C shows a conventional one transistor (1 T)- one metal-insulator-metal (MIM) capacitive type hydrogen gas sensor. The hydrogen gas sensor has a complex 3 terminal structure and uses silicon dioxide (Si02) / (aluminum oxide (AI2O3). The sensor shown in FIG. 9B does not have a titanium interlayer as well as a Pd first electrode as a sensing layer. In addition, the sensor does not have a Ta2O5 dielectric layer. The hydrogen gas sensor uses capacitive sensing approach, and may also only able to detect high concentrations of H2.

[0057] In contrast, various embodiments may relate to a BEOL- and CMOS -compatible hydrogen gas sensor which is relatively easier to fabricate, and which is able to detect much lower levels of H2. Various embodiments may detect hydrogen gas at 20 ppm at room temperature.

Claims

Claims1. A hydrogen gas sensor comprising: a first electrode for sensing hydrogen gas; a second electrode; a dielectric layer between the first electrode and the second electrode, the dielectric layer comprising tantalum pentoxide (TajOs); and an interlayer between the first electrode and the dielectric layer, the interlayer comprising titanium (Ti); wherein the first electrode is configured to dissociate hydrogen gas into hydrogen atoms; and wherein the interlayer is configured to form bonds with the hydrogen atoms.

2. The hydrogen gas sensor according to claim 1, wherein the hydrogen gas sensor has a cross-point structure.

3. The hydrogen gas sensor according to claim 1, further comprising: a first contact pad in contact with the first electrode; a second contact pad in contact with the second electrode; wherein the first electrode comprises an interconnect portion extending in a first direction to be in contact with the first contact pad; and wherein the second electrode comprises an interconnect portion extending in a second direction to be in contact with the second contact pad, the second direction substantially perpendicular to the first direction.

4. The hydrogen gas sensor according to claim 1, wherein the hydrogen gas sensor comprises exposed sidewalls.

5. The hydrogen gas sensor according to claim 1, wherein the interlayer is of a thickness selected from a range from 0.1 nm to 3.5 nm.

6. The hydrogen gas sensor according to claim 1, wherein the interlayer has a first side in contact with the first electrode and a second side, the second side opposite the first side, in contact with the dielectric layer.

7. The hydrogen gas sensor according to claim 1, wherein the interlayer is configured to scavenge oxygen from the dielectric layer, thereby forming oxygen vacancies in the dielectric layer.

8. The hydrogen gas sensor according to claim 1, wherein the first electrode comprises palladium (Pd), iridium (Ir), palladium (Pd) with titanium (Ti), palladium (Pd) with nickel (Ni), palladium (Pd) with niobium (Nb) or palladium (Pd) with vanadium (V).

9. The hydrogen gas sensor according to claim 1, wherein the second electrode comprises titanium tungsten (TiW), molybdenum (Mo), ruthenium (Ru), tantalum nitride (TaN) or tungsten (W).

10. The hydrogen gas sensor according to claim 1,wherein the hydrogen gas sensor is configured to detect a hydrogen gas concentration of 20 ppm at 25 °C.

11. A method of forming a hydrogen gas sensor, the method comprising: forming a first electrode for sensing hydrogen gas; forming a second electrode; forming a dielectric layer between the first electrode and the second electrode, the dielectric layer comprising tantalum pentoxide ( Ta2O5); and forming an interlayer between the first electrode and the dielectric layer, the interlayer comprising titanium (Ti); wherein the first electrode is configured to dissociate hydrogen gas into hydrogen atoms; and wherein the interlayer is configured to form bonds with the hydrogen atoms12. The method according to claim 11, wherein the second electrode is formed on a substrate; wherein the dielectric layer is formed on the second electrode; wherein the interlayer is formed on the dielectric layer; and wherein the first electrode is formed on the interlayer.

13. The method according to claim 11, wherein forming the first electrode, the second electrode, the dielectric layer and the interlayer is carried out at temperatures below 400 °C.

14. The method according to claim 11,wherein forming the first electrode and the interlayer is carried out without breaking chamber vacuum.

15. The method according to claim 11, wherein the method is complementary metal oxide semiconductor (CMOS)- compatible.

16. The method according to claim 11, wherein the hydrogen gas sensor has a cross-point structure.

17. The method according to claim 11, further comprising: forming a first contact pad in contact with the first electrode; forming a second contact pad in contact with the second electrode; wherein the first electrode comprises an interconnect portion extending in a first direction to be in contact with the first contact pad; and wherein the second electrode comprises an interconnect portion extending in a second direction to be in contact with the second contact pad, the second direction substantially perpendicular to the first direction.

18. The method according to claim 11, wherein the interlayer is of a thickness selected from a range from 0.1 nm to 3.5 nm.

19. The method according to claim 11, wherein the first electrode comprises palladium (Pd), iridium (Ir), palladium (Pd) with titanium (Ti), palladium (Pd) with nickel (Ni), palladium (Pd) with niobium (Nb) or palladium (Pd) with vanadium (V).

20. The method according to claim 11, wherein the second electrode comprises titanium tungsten (TiW), molybdenum(Mo), ruthenium (Ru), tantalum nitride (TaN) or tungsten (W).T1

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