SILICON CARBIDE BASED FIELD EFFECT GAS SENSOR FOR HIGH TEMPERATURE APPLICATIONS

Abstract

A field effect gas sensor, for detecting a presence of a gaseous substance in a gas mixture, the field effect gas sensor comprising: a SiC semiconductor structure; an electron insulating layer covering a first portion of the SiC semiconductor structure; a first contact structure at least partly separated from the SiC semiconductor structure by the electron insulating layer; and a second contact structure conductively connected to a second portion of the SiC semiconductor structure, wherein at least one of the electron insulating layer and the first contact structure is configured to interact with the gaseous substance to change an electrical property of the SiC semiconductor structure; and wherein the second contact structure comprises: an ohmic contact layer in direct contact with the second portion of the SiC semiconductor structure; and a barrier layer formed by an electrically conducting mid-transition-metal oxide covering the ohmic contact layer.

Claims

1. A field effect gas sensor, for detecting a presence of a gaseous substance in a gas mixture, said field effect gas sensor comprising: a silicon carbide (SiC) semiconductor structure; an electron insulating layer covering a first portion of said SiC semiconductor structure; a first contact structure at least partly separated from said SiC semiconductor structure by said electron insulating layer; and a second contact structure conductively connected to a second portion of said SiC semiconductor structure, different from said first portion, wherein at least one of said electron insulating layer and said first contact structure is configured to interact with said gaseous substance to change an electrical property of said SiC semiconductor structure; and wherein said second contact structure comprises: an ohmic contact layer in direct contact with the second portion of said SiC semiconductor structure; and a barrier layer covering said ohmic contact layer, said barrier layer being formed by an electrically conducting mid-transition-metal oxide.

2. The field effect gas sensor according to claim 1, wherein said electrically conducting mid-transition-metal oxide is selected from the group consisting of iridium oxide and rhodium oxide.

3. The field effect gas sensor according to claim 1, wherein said second portion of the SiC semiconductor structure is doped.

4. The field effect gas sensor according to claim 3, further comprising a third contact structure conductively connected to a third portion of said SiC semiconductor structure, different from said first portion and said second portion, wherein said third contact structure comprises: an ohmic contact layer in direct contact with the third portion of said SiC semiconductor structure; and a barrier layer covering said ohmic contact layer, said barrier layer being formed by an electrically conducting mid-transition-metal oxide; said third portion of the SiC semiconductor structure is doped; and said first portion of the SiC semiconductor structure is arranged between said second portion and said third portion to form a field effect transistor structure.

5. The field effect gas sensor according to claim 1, wherein said ohmic contact layer includes a metal.

6. The field effect gas sensor according to claim 5, wherein said metal is selected from the group consisting of nickel, chromium, titanium, aluminum, tantalum, tungsten, and molybdenum.

7. The field effect gas sensor according to claim 1, wherein each of the barrier layer of said second contact structure and the barrier layer of said third contact structure is at least partly covered by an insulating passivation layer.

8. The field effect gas sensor according to claim 7, wherein at least a portion of at least one of said electron insulating layer and said first contact structure is uncovered by said insulating passivation layer, to allow direct contact by said gas mixture to said portion.

9. A method of manufacturing a field effect gas sensor for detecting a presence of a gaseous substance in a gas mixture, said method comprising: providing a silicon carbide (SiC) semiconductor structure; forming an electron insulating layer on a first portion of said SiC semiconductor structure; depositing a first contact layer on said electron insulating layer; depositing an ohmic contact layer on a second portion of said SiC semiconductor structure; and depositing a barrier layer formed by an electrically conducting mid-transition-metal oxide on said ohmic contact layer to cover said ohmic contact layer.

10. The method according to claim 9, wherein said barrier layer is deposited using a deposition method selected from the group consisting of sputtering and pulsed laser deposition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

[0036] FIG. 1 illustrates a field effect gas sensor of the MOSFET/MISFET type according to an embodiment of the present invention;

[0037] FIG. 2 illustrates a field effect gas sensor of the MOS capacitor type according to an embodiment of the present invention;

[0038] FIG. 3 illustrates a field effect gas sensor of the Schottky diode type according to an embodiment of the present invention;

[0039] FIGS. 4A, 4B, and 4C illustrate an example of a suitable means for electrically connecting and heating the field effect gas sensor according to an embodiment of the present invention; FIG. 4A shows a front view, FIG. 4B shows a backside view and FIG. 4C shows a side view, in which a field effect gas sensor of the present invention is mounted to the suitable means for electrically connecting and heating;

[0040] FIG. 5 illustrates an example of an encapsulated field effect gas sensor according to an embodiment of the present invention;

[0041] FIG. 6 illustrates an example of a configuration for detection of a gaseous substance in a gas flow using the field effect gas sensor according to an embodiment of the present invention;

[0042] FIGS. 7a-b illustrate the temperature stability of exemplary SiC-based field effect transistor gas sensors with conventional barrier layers on the ohmic contact layers; and

[0043] FIG. 8 illustrates the temperature stability of a SiC-based field effect transistor gas sensors according to an example embodiment of the present invention with barrier layers made of IrO.sub.2.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0044] FIG. 1 displays an example of a field effect gas sensor of the MOSFET/MISFET type 1 according to an embodiment of the present disclosure. The field effect gas sensor of the MOSFET/MISFET type 1 comprises a semiconductor layer 2 of e.g. n-type doped SiC. On the semiconductor layer 2, an epilayer 3 (also of SiC), of p-type (doping concentration 5−10.sup.15/cm.sup.3) is grown to a thickness of approximately 10 μm. In the epilayer, 3 doped regions are created e.g. by ion implantation to form a drain region 4 of n-type, a source region 5 of n-type and a substrate region 6 of p-type (doping concentration approximately −10.sup.20/cm.sup.3). On top of the epilayer 3 an electron insulating layer 7 is grown, consisting of e.g. a thermally grown SiO.sub.2 layer to an approximate thickness of 500 Å, and an LPCVD deposited layer of silicon nitride (Si.sub.3N.sub.4) of approximate thickness 250 Å, which is densified to create a thin layer of silicon dioxide on top of the nitride, typically 50 Å.

[0045] Three contact structures 8a-c to the source 5, drain 4 and substrate regions 6 of the epilayer 3, respectively, are then created. The contact structures 8a-c may be processed by first etching the electron insulating layer 7 (e.g. using standard photo-lithographic patterning and wet etching techniques or dry etching techniques such as reactive ion etching) over the drain region 4 of n-type, the source region 5 of n-type and over the substrate region 6. Onto the implanted areas where the electron insulating layer has been removed the contact structures 8a-c may then be created by the following process:

[0046] First, the ohmic contact layer 9 is formed by, for example, deposition of Nickel (Ni) to an approximate thickness of 500 Å followed by rapid thermal annealing in argon at 950° C. and then deposition of approximately 50 Å titanium (Ti).

[0047] Thereafter, a barrier layer 10 is deposited to completely cover the ohmic contact layer 9, to protect the ohmic contact layer 9 from oxidation at high operating temperatures (such as above 500° C.). This barrier layer 10 may be configured so as to also cover part of the electron insulating layer 7. The protective oxygen diffusion barrier materials that may be used for the barrier layer 10 is selected from the group of metal oxides consisting of IrO.sub.2, RuO.sub.2, RhO.sub.2, and ReO.sub.3, preferably from one of IrO.sub.2 and RhO.sub.2, and may also be arranged as a layered combination, as a composite or any other kind of mixture of said materials. At least part of the oxygen diffusion barrier layer may also include a layer composed of a metal such as Pt or Au. The oxygen diffusion barrier materials can be processed/fabricated in the preferred thin-film layout and structure by a number of different methods, including both CVD (Chemical Vapor Deposition) based methods, such as ordinary CVD, MBE (Molecular Beam Epitaxy) and ALD (Atomic Layer Deposition), and PVD (Physical Vapor Deposition) based methods, such as thermal/e-beam evaporation, RF/DC magnetron sputtering, and Pulsed Laser Deposition (PLD). The currently preferred methods are RF/DC magnetron sputtering and Pulsed Laser Deposition, in both cases by using a metal or metal oxide target and running the process in presence of a certain partial pressure of oxygen added to the vacuum deposition chamber.

[0048] On top of the barrier layer 10, except where it is intended to electrically contact the contact structures 8a-c through various bonding techniques, a conventional passivation layer 11 may be applied using methods known to one of ordinary skill in the art.

[0049] Onto at least a part of the electron insulating layer 7, an electrical contact 12, which may be a gate contact when the field effect gas sensor is of MOSFET/MISFET type, is created, comprising a thin film of at least one material including (but not limited to) metals such as Au, Pt, Ir, and Rh, binary metal oxides, such as FeO.sub.x, IrO.sub.x and RuO.sub.x, binary sulfides and selenides such as MoS.sub.2, MoSe.sub.2, and WS.sub.2, ternary compounds such as SrTiO.sub.3, BaCoO.sub.3, and LaMnO.sub.3, and any material with the general formula ABO.sub.3, specifically of the perovskite type, as well as any combinations or mixtures of these materials, where at least one of the materials is electrically conductive. At least a part of the electrical contact 12 may be deposited by sputtering, in the case of oxide materials in an oxygen ambient, or evaporation to a thickness of up to 500 Å. On top of the electrical gate contact 12 a thin, discontinuous layer of a catalytic or otherwise promoter material, e.g. 25 Å Pt, may be deposited. Part of the electrical gate contact 12 may be in contact with a contact layer 13 comprising a double layer of Ti/Pt films of a thickness of approximately 25 and 200 Å, respectively. Adsorption of the one or more gaseous substance(s) of interest on the electrical gate contact 12 induces, either directly or through reactions with adsorbed oxygen anions, a change in the gate to semiconductor electric field and thus a change in conductance in the channel between the source 5 and drain 4 regions. The voltage over the field effect gas sensor of the MOSFET/MISFET type when keeping a constant current through the gas sensor thus reflects the presence and/or ambient concentration of the gaseous substance to be detected.

[0050] FIG. 2 displays an example of a field effect gas sensor of MOS capacitor type 20 according to an embodiment of the present disclosure. The field effect gas sensor of MOS capacitor type 20 has a semiconductor layer 2 of SiC, being of n-type semi-insulating material, onto which an epilayer 3 of n-type and of approximately 5 μm thickness, is grown. On top of the epilayer 3 an electron insulating layer 7 is created. The electron insulating layer 7 comprises a stack of three insulators 7a, 7b and 7c consisting of a thermally grown oxide (SiO.sub.2) 7a and an LPCVD deposited and densified silicon nitride (Si.sub.3N.sub.4) 7b, the latter also resulting in a thin silicon dioxide film 7c on top of the nitride, to an approximate total thickness of the electron insulating layer 7 of 800 Å.

[0051] Further, a backside contact structure 14, is created on the semiconductor layer through the following process:

[0052] First, the ohmic contact layer 9 is formed by, for example, deposition of Nickel (Ni) to an approximate thickness of 500 Å followed by rapid thermal annealing in argon at 950° C. and then deposition of, approximately 500 Å tantalum silicide (TaSi.sub.2) and 4000 Å platinum (Pt) or optionally 50 Å titanium (Ti) and 4000 Å platinum (Pt)

[0053] Thereafter, a barrier layer 10 is deposited to completely cover the ohmic contact layer 9, as well as a part of a first passivation layer 15, to protect the ohmic contact layer 9 from oxidation at high operating temperatures (such as above 500° C.). The barrier layer 10 may be configured so as to also cover part of the electron insulating layer 7. The protective oxygen diffusion barrier materials that may be used for the barrier layer 10 is selected from the group of metal oxides consisting of IrO.sub.2, RuO.sub.2, RhO.sub.2, and ReO.sub.3, preferably from one of IrO.sub.2 and RhO.sub.2, and may also be arranged as a layered combination, as a composite or any other kind of mixture of said materials. At least part of the oxygen diffusion barrier layer may also include a layer composed of a metal such as Pt or Au. The oxygen diffusion barrier materials can be processed/fabricated in the preferred thin-film layout and structure by a number of different methods, including both CVD (Chemical Vapor Deposition) based methods, such as ordinary CVD, MBE (Molecular Beam Epitaxy) and ALD (Atomic Layer Deposition), and PVD (Physical Vapor Deposition) based methods, such as thermal/e-beam evaporation, RF/DC magnetron sputtering, and Pulsed Laser Deposition (PLD). The currently preferred methods are RF/DC magnetron sputtering and Pulsed Laser Deposition, in both cases by using a metal or metal oxide target and run the process in presence of a certain partial pressure of oxygen added to the vacuum deposition chamber.

[0054] On top of the barrier layer 10, except where it is intended to electrically contact the backside contact structure 14 through various bonding techniques, a conventional second passivation structure 11, comprising of one or more materials/layers may be applied using methods known to one of ordinary skill in the art.

[0055] Onto at least a part of the electron insulating layer 7, an electrical contact 12, which may be a gate contact when the field effect gas sensor is of MOSFET/MISFET type, is created, comprising a thin film of at least one material including (but not limited to) metals such as Au, Pt, Ir, and Rh, binary metal oxides, such as FeO.sub.x, IrO.sub.x and RuO.sub.x, binary sulfides and selenides such as MoS.sub.2, MoSe.sub.2, and WS.sub.2, ternary compounds such as SrTiO.sub.3, BaCoO.sub.3, and LaMnO.sub.3, and any material with the general formula ABO.sub.3, specifically of the perovskite type, as well as any combinations or mixtures of these materials, where at least one of the materials is electrically conductive. At least a part of the electrical contact 12 may be deposited by sputtering, in the case of oxide materials in an oxygen ambient, or evaporation to a thickness of up to 500 Å. On top of the electrical gate contact 12 a thin, discontinuous layer of a catalytic or otherwise promoter material, e.g. 25 Å Pt, may be deposited. Part of the electrical gate contact 12 may be in contact with a contact layer 13 comprising a double layer of Ti/Pt films of a thickness of approximately 25 and 200 Å, respectively. Adsorption of the one or more gaseous substance(s) of interest on the electrical contact 12 induces, either directly or through chemical reactions e.g. with adsorbed oxygen anions, a change in material properties and/or a change in the gate to semiconductor electric field, thus changing the capacitance-voltage characteristics of the field effect gas sensor of MOS capacitor type. The bias voltage over the field effect gas sensor when keeping a constant capacitance over the sensor thus reflects the presence and/or ambient concentration of the one or more gaseous substance(s) of interest.

[0056] FIG. 3 displays an example of a field effect gas sensor of Schottky diode type 30 according to an embodiment of the present disclosure. The field effect gas sensor of Schottky diode type 30 has a semiconductor layer 2 of e.g. n-doped SiC. Onto the semiconductor layer 2, an epilayer 3 of n-type (e.g. doping concentration 3×10.sup.16/cm.sup.3) is grown to a thickness of approximately 10 μm. On top of the epilayer 3 an electron insulating layer 7 is created, consisting of a thermally grown oxide (SiO.sub.2) layer to an approximate total thickness of approximately 800 Å.

[0057] Further, a backside contact structure 14, is created on the semiconductor layer through the following process:

[0058] First, the ohmic contact layer 9 is formed by, for example, deposition of Nickel (Ni) to an approximate thickness of 500 Å followed by rapid thermal annealing in argon at 950° C. and then deposition of, approximately 500 Å tantalum silicide (TaSi.sub.2) and 4000 Å platinum (Pt) or optionally 50 Å titanium (Ti) and 4000 Å platinum (Pt).

[0059] Thereafter, a barrier layer 10 is deposited to completely cover the ohmic contact layer 9, as well as a part of a first passivation layer 15, to protect the ohmic contact layer 9 from oxidation at high operating temperatures (such as above 500° C.). The barrier layer 10 may be configured so as to also cover part of the electron insulating layer 7. The protective oxygen diffusion barrier materials that may be used for the barrier layer 10 is selected from the group of metal oxides consisting of IrO.sub.2, RuO.sub.2, RhO.sub.2, and ReO.sub.3, preferably from one of IrO.sub.2 and RhO.sub.2, and may also be arranged as a layered combination, as a composite or any other kind of mixture of said materials. At least part of the oxygen diffusion barrier layer may also include a layer composed of a metal such as Pt or Au. The oxygen diffusion barrier materials can be processed/fabricated in the preferred thin-film layout and structure by a number of different methods, including both CVD (Chemical Vapor Deposition) based methods, such as ordinary CVD, MBE (Molecular Beam Epitaxy) and ALD (Atomic Layer Deposition), and PVD (Physical Vapor Deposition) based methods, such as thermal/e-beam evaporation, RF/DC magnetron sputtering, and Pulsed Laser Deposition (PLD). The currently preferred methods are RF/DC magnetron sputtering and Pulsed Laser Deposition, in both cases by using a metal or metal oxide target and run the process in presence of a certain partial pressure of oxygen added to the vacuum deposition chamber.

[0060] On top of the barrier layer 10, except where it is intended to electrically contact the backside contact structure 14 through various bonding techniques, a conventional second passivation layer 11 may be applied using methods known to one of ordinary skill in the art.

[0061] The electron insulating layer 7 may be patterned by conventional photolithographic methods and wet etched in 50 percent HF.

[0062] Onto at least a part of the electron insulating layer 7, an electrical contact 12, which may be a gate contact when the field effect gas sensor is of MOSFET/MISFET type, is created, comprising a thin film of at least one material including (but not limited to) metals such as Au, Pt, Ir, and Rh, binary metal oxides, such as FeO.sub.x, IrO.sub.x and RuO.sub.x, binary sulfides and selenides such as MoS.sub.2, MoSe.sub.2, and WS.sub.2, ternary compounds such as SrTiO.sub.3, BaCoO.sub.3, and LaMnO.sub.3, and any material with the general formula ABO.sub.3, specifically of the perovskite type, as well as any combinations or mixtures of these materials, where at least one of the materials is electrically conductive. At least a part of the electrical contact 12 may be deposited by sputtering, in the case of oxide materials in an oxygen ambient, or evaporation to a thickness of up to 500 Å. On top of the electrical gate contact 12 a thin, discontinuous layer of a catalytic or otherwise promoter material, e.g. 25 Å Pt, may be deposited. Part of the electrical gate contact 12 may be in contact with a contact layer 13 comprising a double layer of Ti/Pt films of a thickness of approximately 25 and 200 Å, respectively. The contact layer 13 may also cover a part of the electron insulating layer 7. Adsorption of the gaseous substance of interest on the electrical contact 12 induces, either directly or through reactions with adsorbed oxygen anions, a change in the Schottky barrier, thus changing the current of the field effect gas sensor of Schottky diode type. The bias voltage over the field effect gas sensor when keeping a constant current over the sensor thus reflects the presence and/or ambient concentration of the gaseous substance of interest.

[0063] FIG. 4 displays an example of a suitable means 40 for electrically connecting and heating the field effect gas sensor of the present disclosure. An alumina substrate 42 (or a substrate of some other suitable material) has connector lines 46 and contact pads 45 printed on the front side and a resistive-type heater line 44 on the backside. The field effect gas sensor 41 is flipped upside-down and bumps 43 of e.g. gold or platinum connect the field effect gas sensor 41 to the contact pads 45 and connector lines 46 printed on the alumina substrate. An opening 47 is created in the alumina substrate just above the electrical contact (the gate contact in transistor devices) of the field effect gas sensor 41 to allow the ambient gas mixture to reach the electrical contact of the field effect gas sensor 41. The resistor structure 44 is printed on the backside of the alumina substrate 42 to facilitate heating of the sensor device. All connector lines 46 are printed in such a way that they can be easily contacted at the end of the alumina substrate by e.g. a clamp contact.

[0064] FIG. 5 displays an example of a field effect gas sensor of the present disclosure comprising means for encapsulation 50. The semiconductor layer 2, the epilayer 3, and the electron insulating layer Tare covered with an encapsulation layer 51 of a suitable material, e.g. Si.sub.3N.sub.4 or SiO.sub.2. The electrical contact 12 is however in contact with the ambient to facilitate detection of at least one substance of interest in a gas mixture.

[0065] FIG. 6 displays an example of a configuration for detection of a gaseous substance in a gas mixture flow using a field effect gas sensor 60 according to an embodiment of the present invention. The configuration comprising the field effect gas sensor 60 is mounted in the gas flow of interest, e.g. in a tail pipe, a flue gas channel, a chimney etc. The field effect gas sensor 60 is placed inside an outer tube 61 a short distance from the end of an inner tube 62. The inner tube 62 is of smaller diameter than the outer tube 61 and disposed within the outer tube 61 such that there is a gap between the inner 62 and the outer 61 tube. Furthermore, the inner tube 62 extends outside the outer tube 61 at the end opposite to the location of the field effect gas sensor 60. In between the end of the inner tube 62 and the field effect gas sensor 60 a coarse filter 65 is applied such that it spans the cross section of the outer tube 61. The outer 61 and inner 62 tubes are assembled such that the gas mixture of interest can pass in through the outer tube opening 64, come into contact with the field effect gas sensor 60 and exit through the opening of the inner tube 63. The outer tube 61 is also supplied with a gas-tight thermal barrier 66 and means for electrically connecting the sensor device 67 as well as a thread for screwing it into place.

[0066] In the following, the improvement in temperature stability of SiC-based field effect sensors according to embodiments of the present invention will be illustrated with reference to FIGS. 7a-b and FIG. 8.

[0067] FIGS. 7a-b show the current-voltage-characteristics (I/V-characteristics) of the same kind of ohmic contact—Ti.sub.3SiC.sub.2—before and after 100 hours of operation at 600° C. when applying platinum (FIG. 7a) and iridium (FIG. 7b) as the respective conductive ohmic contact protective (capping) layer. As can be seen, the Pt protective layer (which otherwise has been quite commonly used as an oxygen diffusion barrier in devices for operation up to approximately 450° C.) does not prevent the fairly rapid in-diffusion of oxygen and subsequent oxidation of the ohmic contact layer, turning the contact into an insulating oxide (preventing any current to pass for at least low voltages). Also Ir-capped ohmic contacts degrade over time as can be seen from the no longer linear I/V-characteristics after 100 hours of operation at 600° C.

[0068] FIG. 8 shows the I/V-characteristics of the same kind of Ti.sub.3SiC.sub.2 ohmic contact as in FIGS. 7a-b, but when an IrO.sub.2 layer is used as barrier layer for 600° C. operation. As can be seen in FIG. 8, the performance was actually improved over the course of the experiment; the contact resistance decreased (the slope of the linear I/V-characteristics increasing) with time, at least for the 1000 hours recorded here.

[0069] The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

[0070] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.