Semiconductor cold cathode electron sources for ionization

Abstract

An ionizer comprising a semiconductor chip having a substrate; a cold cathode unitary with the substrate, and having a surface with chemically and optionally lithographically introduced nanoscale to microscale features; an anode; and a voltage source or signal generator that produces a first non-zero voltage across the cathode and the anode. The ionization source relies on field emission of electrons as opposed to thermionic emission, and it is resilient against degradation as a result of a series of redundant emission points, hierarchical field enhancement, and the inert, refractory nature of the material. Unlike hot electron sources used for ionization, cold cathode electron sources can emit at any temperature, but can function whether cold, or even when heated. The field emission-based ionizer can comprise individual gated or ungated electron emitter arrays, or any combination of such arrays that can present in a matrix format or multi-electrode configuration within a single device.

Description

SEMICONDUCTOR COLD CATHODE ELECTRON SOURCES FOR IONIZATIONField of the Invention

[0001] The field of the invention is an on-chip, monolithic and porous silicon carbide or semiconductor field electron emitter, and particularly a gated or ungated cold cathode field electron emitter, for applications or devices that employ electron sources for ionization.Background

[0002] Electron emitters, or “cold cathode” electron sources, based on field emission have been continuously researched for decades. Extensive effort has been motivated by the significant technological applications enabled by field-extracted cold electrons in comparison with heat- induced (thermal, or “thermionic”) electron emission from materials, including exotic metals and composites. Aside from improvements in electron source technology, attributes such as reliability (long lifetime), energy efficiency, on-off source switching, and precise control of emission from low- to high-energies would allow for further advances in critical applications that fundamentally rely on and employ electron sources. Specifically, applications of cold cathode electron emitters for ionization include mass spectrometry, gas sensing, pressure gauging (barometry), equipment sterilization, and air purification. The unique features of semiconductorbased cold sources should lead to superior performance in ionization performance, as well as new functionalities and modalities in sensing technology for environmental monitoring and security.

[0003] Applications requiring stable emitters capable of moderate to high emission currents remained firmly in the realm of thermal sources until over a decade ago, when a cold cathode source exhibiting sustained emission at practical current densities was demonstrated at the National Institute of Standards and Technology (NIST). By 2012, NIST scientists invented a variety of patterned structures that produced significant electric field enhancement, leading to current densities and lifetimes comparable to standard, high-power thermal sources. Various methods of formation of these structures are described in the prior art. [PCT / US2023 / 033152 to Cannara et al. (2023), Feenstra (2008); Kang (2013); Sharifi (2014), (2016), (2017)]. US Patents Nos. 8,907,553, 9,324,534 and 9,558,907 to Sharifi et al. from NIST and Julia Jean, LLC andNIST joint provisional PCT / US 2023 / 033152 application to Cannara et al. disclose some techniques for creating such emitters.

[0004] Nanoporous silicon carbide microstructures are effective cold cathode field emitters (“cold” electron sources), reaching current densities comparable to thermal electron sources. Thermomechanical stability of these structures was of paramount importance prerequisite to their invention, as other highly promising cold source candidates — mainly carbon nanotubes (CNTs) and gated molybdenum cones (“Spindt” emitters) — had been known to fail by interfacial delamination and geometric degradation, respectively. Some of these other solutions continue to be researched or have found limited use in systems with well-matched performance specifications. However, the successes of these earlier field emitter arrays, along with their constraints, highlight the need for scalable, microfabricated nanostructures with high stability and lifetime.

[0005] To that end, the NIST emitters are designed to be structurally monolithic and refractory to avoid failure via weak materials interfaces or plastic deformation of emitter tips. Silicon carbide’s (and other semiconductors’) wide bandgap produces an emission channel from the upper band with a low, inherent, effective work function, eliminating the need for specialized coatings or heterostructures. Moreover, the presence of many neighboring tips formed through the nanoscale porosity of electrochemically modified silicon carbide provides redundancy between emission points: If one tip fails, its emission is compensated by the immediate, automatic activation of emission from a neighboring tip.

[0006] The method of Cannara et al. also allows for metal gate electrodes to be introduced on either surface of the wafer, using various lithography methods. This flexible and scalable approach also provides for varying charge transport to tune the emission properties (density and direction of the electrons as they exit the material), through recess-gating, or by gating the emission current from the underside of the semiconductor chip.

[0007] In the field of ionization, cold cathode electron sources (e.g., U.S. Patent No. 3,665,241 to Spindt et al.) compete with hot filament electron sources in applications that employ charged moieties (e.g., electrons and ionized atoms or molecules), including gas sensors based on mass spectrometry such as residual gas analyzers and ion traps (e.g., U.S. Patent No. 11,764,051 toRellergert et al.), ionization gauges that use one or more electrodes to induce or detect charge, or even ion propulsion systems that achieve thrust based on electric field acceleration of metal plasmas (e.g., U.S. Patent No. 11,644,016 to Krishnan et al.) or ionic liquids (e.g., U.S. Patent No. 11,545,351 to Perna et al.) Equipment sterilization and air purification technologies often rely on gas ionization. A typical electron source that is used for ionization consists of a hot filament comprising a metal wire or strip that is heated to boil off electrons in a gas flow chamber or cell to form the requisite ionized species.

[0008] Unlike these hot sources, which typically require careful turn-on procedures due to significant internal stresses during activation and thermal cycling, cold electron sources may be switched on and off at will. Barring economic considerations, a cold source replacement should therefore provide an effective substitute for hot sources in applications that would benefit from improved longevity and energy efficiency. Indeed, the practicality and sustainable operation of devices that currently rely on hot ionization sources is undermined by the limited lifetime and reliability of the ordinary metal filament. Moreover, for any such devices that currently rely on hot sources to be successfully deployed in remote settings, their performance must improve dramatically. Accordingly, the silicon carbide-based emitters described herein for ionization demonstrate the longevity and emission properties that prove them uniquely capable of meeting or exceeding the requirements that grant feasibility to otherwise out-of-reach, advanced applications.

[0009] What is still needed is the successful integration of cold cathode electron sources as ionization sources in a working device at the system level. Until now, cold sources have been limited to high-vacuum applications, such as vacuum electronic devices used for x-ray generation and microwave signal amplification, among other uses. Materials sensitivity of the emission source to ion back-bombardment typically precludes the use of cold sources in devices that require a reliable supply of electron currents under harsh environments, such as the moderate vacuum pressures present in a typical gas analyzer. The superior refractory nature of bulk silicon carbide and the built-in redundancy provided by numerous nanoscale tips on the emission surface uniquely position porous silicon carbide field emitters as the first class of cold source materials capable of replacing hot sources in ionization-based systems.Summary of The Invention

[0010] All publications herein mentioned are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0011] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein, and ranges include their endpoints.

[0012] In some embodiments, the numbers expressing quantities of properties used to describe and claim certain embodiments of the invention arc to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0013] The use of any and all examples, or exemplary language (e.g., “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention. Unless a contrary meaning is explicitly stated, all ranges are inclusive of their endpoints, and open-ended ranges are to be interpreted as bounded on the open end by commercially feasible embodiments.

[0014] As used herein, the term “features” includes structures and spaces. As applied to the height and width of a feature, the term "microscale" means 1 - 100 pm, and the term “nanoscalc” means 0.1 nm to 1 pm.

[0015] The inventive subject matter provides apparatus, systems, and methods in which a device uses a cold cathode electron emitter as an ionization source. Specifically, a new approach to ionization using a refractory semiconductor cold cathode electron source is described that provides a scalable, on-chip, distributable, energy-efficient, long-lived, and reliable solution in comparison with standard hot ionization sources.Brief Description of the Drawings

[0016] FIGURE 1: Example cross-sectional side view (100) of a substrate (110, solid) of a semiconductor chip with a porous layer (120, striped); the porous surface has sharp features (122, inset), which arc shown as uniform vertical tips for simplicity but should be interpreted as pore walls of variable thickness, spacing (124) and morphology.

[0017] FIGURE 2: Example cross-sectional view (200) of a substrate with a porous layer (120) that has also been patterned (before, after, or without introducing porosity) to have microscale features.

[0018] FIGURE 3: Example side view of an ionization device (300) in a diode configuration, showing the applied cathode-anode voltage V (350), and emitted electrons (360), between the substrate (110) and at least first anode (370).

[0019] FIGURE 4: Example side view (400) of a multi-electrode geometry, forming two emitter structures (120), three independently addressable gate electrodes (472, 474, 476), electrically isolated by electrically insulating layers 405 and 415.

[0020] FIGURE 5: Example side view (500) of an ionization device in a triode configuration with a voltage V (350) between the cathode and the first anode (370) and a voltage VT (550) between the second intermediate anode (572) and the first anode.

[0021] FIGURE 6: A scanning electron microscope image of an example 2 mm-diameter, un- pattemed, porous silicon carbide emitter structure (600) with nanoscale and microscale features.

[0022] FIGURE 7: Performance data (700) from emitter (600) have so far demonstrated over 150 hours of stable, continuous-wave (de) operation at more than 540 microamps (pA), without degradation. Breaks in the data (700) are a result of intermittent testing, for which the results are beyond the scope of this invention.

[0023] The cold cathode ionization sources described herein are components that can be fabricated from various compounds and polytypes of the emitter material using electrochemical techniques, leading to a range of chemistries and conditions that result in a correspondingly wide range of porosities and pore properties. All possible methods of forming porous silicon carbide structures with nanoscale and microscale features are thus incorporated, as represented schematically by the diagrams in Figs. 1 and 2.

[0024] Figs. 1 and 2 represent un-pattemed and patterned porous emitters, respectively, that produce distinct, desired performance characteristics, according to the chosen design. Such diode electron sources can serve as ionizer elements, either individually or as a combination of multiple components in a device, providing a counter electrode, as in the system represented by Fig. 3. Such a counter-electrode (Fig. 3 diode), or additional desired counter-electrodes (Figs. 4 and 5 triodes to pentodes, etc.), may be introduced by introducing electrically isolated metal electrode(s) directly onto, or in the vicinity of, the electron source.

[0025] Electrodes can be formed by mechanically mounting metal plates or grids, or by lithography. On a top surface of the cathode, an electrically isolated gate electrode may thus be introduced or deposited and stacked with or without other electrically insulated conducting layers to form a multiple-gated structure of arbitrary stack height (see Fig. 4).

[0026] Fig. 3 provides a side-view example of a simplified, standard ionizer in which a negative (or relatively low or ground) voltage is applied to the cathode, and a positive (or relatively higher) voltage applied to the anode, thereby producing an electric field that drives electrons from the cathode to the anode. The cold cathode emitter may thus be configured as a simple diode. The emitter may also be itself a triode or other multi-electrode configuration, as exemplified by the pentode in Fig. 3.

[0027] In Fig. 5, a representative multi-electrode structure is portrayed that may then he repeated and to form a matrix of ionizers in a single system. Additionally, the matrix of emitter arrays may comprise a heterogeneous combination of surface patterns formed on a single semiconductor chip. Such a matrix may thus be populated by multiple arrays with similar or different ionization properties to achieve simultaneous ionization over a desired range of ionization energies.

[0028] Multiple independent electrodes within single or multiple planes can thus be used to vary response across a single emitter array. The simplest embodiment of this concept is a diode configuration with a single emitter morphology (combination of nanoscale and microscale features) and performance characteristic, wherein emission is gated locally by a series of independent metal plates or grids. Discrete (insulated) metal gates can be placed on the same or different planes at some non-zero, insulated distance above the emission plane, as in Fig. 4.

[0029] In preferred embodiments, an ionizer comprises a cold cathode having a semiconductor substrate having a surface with nanoscale to microscale features, at least a first anode, and a voltage source or signal generator that produces a first non-zero voltage across the cathode and the at least first anode.

[0030] In preferred embodiments the cold cathode is unitary with the semiconductor substrate, lithographically or chemically formed from the substrate.

[0031] In preferred embodiments, the features are structures that protrude from a bulk of the substrate to a height of 1 - 100 pm. The features can advantageously be created using lithography and / or electrochemistry.

[0032] In preferred embodiments, the features are nanoscale spaces disposed within a depth of 0.1 nm to 1 pm from a surface of the substrate. In preferred embodiments, the features present in a pattern to provide a desired electric field enhancement through geometry and resulting hierarchical structure. For example, the patterns can include honeycombs, hexagons, lines, rectangles, rhombic or cubic geometries, or a combination thereof.

[0033] In preferred embodiments, the substrate comprises a composition selected from the group of wide band gap semiconductors that includes silicon carbide, gallium nitride, and aluminum nitride.

[0034] In preferred embodiments, the first non-zero voltage is selected to produce electrons emitted from the cathode with energies between 1 electron volt (eV) and 50 eV.

[0035] In preferred embodiments, the first non-zero voltage and subsequent non-zero voltages are selected to produce electrons emitted from the cathode with energies between 100 eV and 1000 eV.

[0036] In preferred embodiments, the first non-zero voltage and subsequent non-zero voltages are selected to produce electric fields between 0.1 - 10 V / p m.

[0037] In preferred embodiments, the first non-zero voltage is between 1 - 1000 V.

[0038] In preferred embodiments, a surface of the cathode is distanced from the at least first anode by a non-zero distance of no more than 1 - 10 pm.

[0039] In preferred embodiments, a surface of the cathode is distanced from the at least first anode by a non-zero distance of no more than 10 - 1000 pm.

[0040] In preferred embodiments, the device further comprises a second anode, wherein the first non-zero voltage across the cathode and the at least first anode is different from a second nonzero voltage across the cathode and the second anode.

[0041] In preferred embodiments, the device comprising an at least second electrode further comprises a controller (signal generator) configured to separately control the first non-zero voltage and a voltage between the cathode and the at least second electrode (e.g., anode, gate, grid, or extraction grid).

[0042] In preferred embodiments, the cathode and the at least first anode are contained within a flow chamber.

[0043] In another embodiment, the present subject matter provides a gated or ungated porous silicon carbide field emitter having a plurality of discrete emission projections extending from an emission face of the field emitter. The field emitter is monolithic and homogenous in a direction transverse to the emission face of the field emitter.

[0044] In still another embodiment, the present subject matter provides a gated or ungated cold cathode silicon carbide field emitter with an emission face having a collection of discrete emission projections. In some embodiments that electron source is a field emitter. Preferred substrates include SiC, AIN, GaN semiconductors and similar materials.

[0045] In sum, the invented component device is an ionizer based on an on-chip, cold cathode semiconductor electron source. The ionization source relies on field emission of electrons as opposed to thermionic emission, and it is resilient against degradation as a result of a series of redundant emission points, hierarchical field enhancement, and the inert, refractory nature of the substrate material. Unlike hot electron sources used for ionization, cold cathode electron sources can emit at any temperature, but can function whether cold, or even when heated (e.g., by external or self-heating). Further, emitters made from silicon carbide are capable of operating under harsh conditions, such as oxidizing or high temperature environments (e.g., at hundreds of degrees). Thermal resilience is important, because electrons extracted by thermionic emission have highly varied Boltzmann-distributed energies; by contrast, field emitted electrons have narrow energy distributions as determined by the applied electric field. Cold cathodes are also energy-efficient, as field emitters can be switched on and off at will and do not require a heater power supply. The field emission-based (cold cathode) ionizer described herein is a scalable, distributable on-chip source comprising either individual gated or ungated electron emitter arrays, or any combination of such arrays that can present in a matrix format or multi-electrode configuration within a single device. In contrast to other cold cathodes, it provides stable continuous emission over a minimum of days or weeks without significant degradation.

[0046] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.Example

[0047] The example below serves as a general embodiment within a range of similar devices and provides one method of fabricating emitters that can then be incorporated into devices that act as ionization sources. Specifically, what follows is a general, exemplary method of making porous silicon carbide emitters in a diode (two-electrode) configuration, as in Fig. 3:

[0048] Deposit a few hundred nanometer-thick metal layer on the surface of a silicon carbide substrate of known carrier concentration (e.g., 10A18 cmA-3). As this metal layer will mask the surface to form a pattern through an ion etch process, a metal should be chosen that is significantly more resistant to ion etching than the silicon carbide. Such a metal may be deposited through various means, including thermal evaporation, sputtering, or electrodeposition.

[0049] Next, optical lithography may be used to imprint a typically microscale pattern onto the metal layer. Example metal mask patterns include hexagonal, striped or dotted arrays, which produce hexagonal, lined, cubic, rectangular or rhombic pillars.

[0050] The anodization step that forms a porous morphology in the substrate surface can be performed before or after the above dry etching, using electrochemistry conditions that depend on the desired porosity.Results

[0051] There is no restriction on the design, and different patterns may be used to control hierarchical field enhancement and, thus, the emission properties. Cold cathode emitters can be made to perform at the level of existing hot ionization sources. By further improving emission properties through controlled field enhancement or extending lifetime by operating in pulsed mode (e.g., at 20% duty cycle), these cold sources can readily achieve an order of magnitude increase in longevity and current density in comparison with standard hot sources.

[0052] Fig. 6 shows a simple 2 mm-diameter, un-pattemed, porous silicon carbide emitter structure with nanoscale and microscale features. Performance data from this emitter have so far yielded over 150 hours of stable continuous- wave (de) operation at more than 540 pA, without significant degradation, as shown in Fig. 7. Breaks in the Fig. 7 data correspond to intermittent testing of the voltage response of the emitter at varying distances to the anode; however, theemission current always remained between 300 - 600 p A for the entire lifetime of the emitter. The average voltage during the constant currcnt-controllcd longevity measurement was 6.9 kV over a nominal gap of 1 mm, producing an electric field of approximately 7 V / pm.

[0053] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specifications and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprise” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refer to at least one of something selected from the group consisting of A, B, C .... and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.

Claims

CLAIMSWhat is claimed is:

1. An ionizer comprising: a semiconductor chip having a substrate; a cold cathode unitary with the substrate, and having a surface with chemically and optionally lithographically introduced nanoscale to microscale features; at least a first anode; and a voltage source or signal generator that produces a first non-zcro voltage across the cathode and the at least first anode.

2. The ionizer of claim 1, wherein the features are structures that protrude from a bulk of the substrate to a height of 1 - 10 pm.

3. The ionizer of claim 1, wherein the features are structures that protrude from a bulk of the substrate to a height of 10 - 100 pm.

4. The ionizer of claim 1, wherein the features are nanoscale spaces disposed within a depth of 0.1 nm to 1 pm from a surface of the substrate.

5. The ionizer of claim 1, wherein the features present in a pattern comprising at least one of a honeycomb, hexagonal, linear, rectangular, rhombic, and cubic arrangement.

6. The ionizer of claim 1, wherein substrate comprises a composition selected from the group of wide band gap semiconductors that include SiC, GaN, and AIN.

7. The ionizer of claim 1, wherein the first non-zero voltage is selected to produce electrons emitted from the cathode with energies between 1 eV and 100 eV.

8. The ionizer of claim 1, wherein the first non-zero voltage is selected to produce electrons emitted from the cathode with energies between 100 eV to 1000 eV.

9. The ionizer of claim 1, wherein the first non-zero voltage is between 1 V and 1 kV.

10. The ionizer of claim 1, wherein a surface of the cathode is distanced from the at least first anode by a non-zero distance of no more than 1 - 10 pm.11 . The ionizer of claim 1 , wherein a surface of the cathode is distanced from the at least first anode by a non-zero distance of no more than 10 - 1000 m12. The ionizer of claim 1, further comprising a second anode, wherein the first non-zero voltage across the cathode and the at least first anode is different from a second non-zero voltage across the cathode and the second anode.

13. The ionizer of claim 1, further comprising a second electrode and a controller configured to separately control the first non-zero voltage and a voltage between the cathode and the second electrode.

14. The ionizer of claim 1, wherein the cathode and the at least first anode are contained within a flow chamber.

15. The ionizer of claim 1, wherein the cathode, the at least first anode, and additional electrodes are contained within a flow chamber.

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