Apparatus and method for radical species detection via paramagnetic gas properties

Abstract

An apparatus comprises a transport tube configured to transport a plasma gas flow, a sample tube connected to the transport tube at a first end and a second end, the sample tube configured to receive a sampled gas flow from the transport tube, a magnet positioned adjacent to the sample tube between the first end and the second end of the sample tube, the magnet configured to generate a magnetic field into an interior of the sample tube, and a sensor arranged in operable proximity to the sample tube in a region where the magnet is positioned, the sensor configured to detect changes in properties of the sampled gas flowing through the magnetic field generated by the magnet.

Description

APPARATUS AND METHOD FOR RADICAL SPECIES DETECTION VIA PARAMAGNETIC GAS PROPERTIESCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provisional Patent Application Ser. No. 63 / 673,196, filed on July 19, 2024, the contents of which are incorporated by reference in their entirety.TECHNICAL FIELD

[0002] Embodiments of the present invention relate generally to detection of radical species and, more particularly, to detection of radical species by sensing paramagnetic properties of a gas.BACKGROUND

[0003] Semiconductor manufacturing processes often utilize plasma sources to generate reactive species for etching, deposition, and chamber cleaning applications. Precise control over the composition and delivery of these reactive species is crucial for achieving consistent and effective processing results. However, as reactive species and gases are transported from remote generation sources to processing chambers, complex reactions and recombination can occur, altering the relative concentrations of reactive and stable species. This compositional evolution during transport presents challenges for maintaining reproducible plasma processing conditions.

[0004] Techniques for monitoring and characterizing reactive gas compositions, particularly the concentrations of radical species, or radicals, can provide valuable insights for optimizing semiconductor manufacturing processes. Various approaches have been explored for detecting radical species, including optical emission spectroscopy, mass spectrometry, and paramagnetic sensing methods. Each technique offers different advantages and limitations in terms of sensitivity, selectivity, and ease of implementation in industrial settings. Continued development of robust and practical methods for radical species detection remains an active area of research and development in the semiconductor industry.SUMMARY

[0005] The present application discloses embodiments of an apparatus and a method that have been conceived and developed aiming to provide solutions to the above stated objective technical needs, as will be evidenced in the following description.

[0006] To address the above enumerated problems, in accordance with one embodiment, the present application discloses an apparatus, comprising a transport tube configured to transport a reactive flow, a sample tube connected to the transport tube at a first end and a second end, the sample tube configured to receive a sampled gas flow from the transport tube, a magnet positioned adjacent to the sample tube between the first end and the second end of the sample tube, the magnet configured to generate a magnetic field into an interior of the sample tube, and a sensor arranged in operable proximity to the sample tube in a region where the magnet is positioned, the sensor configured to detect changes in properties of the sample reactive gas flowing through the magnetic field generated by the magnet.

[0007] In accordance with another one embodiment, the present application discloses an apparatus comprising a hollow toroidal cell core configured to receive a sampled gas flow, a plurality of windings wound around the toroidal cell core, the plurality of windings configured to generate a magnetic field within the toroidal cell core, and a controller configured to detect changes in properties of the sampled gas flowing within the toroidal cell core based on interactions between the magnetic field and paramagnetic species in the sampled gas flow.

[0008] In accordance with yet another embodiment, the present application discloses a method of detecting radical species comprising transporting a reactive gas flow through a transport tube, diverting a portion of the reactive gas flow into a sample tube as a sampled gas flow, generating a magnetic field in a region of the sample tube using a magnet, detecting, using a sensor, changes in the properties of the sampled reactive gas flowing through the magnetic field generated by the magnet, and returning the sampled gas flow to the transport tube as a return gas flow.

[0009] More detailed explanations regarding these and other aspects and advantages of the various embodiments of the apparatus and method are provided herewith.BRIEF DESCRIPTION OF DRAWINGS

[0010] The above and other aspects, features and advantages of the toroidal plasma source will become more apparent from the subsequent description thereof, presented in conjunction with the following drawings, wherein:

[0011] FIG. 1 is an orthogonal view of a radical species detection apparatus, in accordance with an aspect of the present disclosure;

[0012] FIG. 2 is a schematic view of a magnetic permeability-type radical species detection apparatus, in accordance with an aspect of the present disclosure;

[0013] FIG. 3 is a section view of a magnetic permeability detection apparatus, in accordance with an aspect of the present disclosure;

[0014] FIG. 3A is a cross-sectional view taken along line IIIA-IIIA of FIG. 3, in accordance with an aspect of the present disclosure;

[0015] FIG. 4 is a section view of another magnetic permeability-type radical species detection apparatus, in accordance with an aspect of the present disclosure;

[0016] FIG. 5 is a schematic view of a split hollow toroidal magnetic permeability-type radical species detection apparatus, in accordance with an aspect of the present disclosure;

[0017] FIG. 6 is a schematic diagram of a magnetic permeability-type radical species detection apparatus;

[0018] FIG. 7 is a schematic diagram of a measurement apparatus, in accordance with an aspect of the present disclosure; and

[0019] FIG. 8 is a flow chart representation of a method in accordance with the present invention.DETAILED DESCRIPTION

[0020] Example embodiments are described herein with reference to the accompanying FIGS. Unless otherwise expressly stated, in the drawings the sizes, positions, etc., of components, features, elements, etc., as well as any distances therebetween, are not necessarily to scale, but are exaggerated for clarity.

[0021] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It should be recognized that the terms "comprises" and / or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range, as well as any sub-ranges therebetween. Unless indicatedotherwise, terms such as "first," "second," etc., are only used to distinguish one element from another. For example, one node could be termed a "first node" and similarly, another node could be termed a "second node", or vice versa. The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0022] Unless indicated otherwise, the term "about," "thereabout," "substantially," etc., means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and / or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art.

[0023] Spatially relative terms, such as "below," "beneath," "lower," "above," and "upper," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element or feature, as illustrated in the FIGS. It should be recognized that the spatially relative terms are intended to encompass different orientations in addition to the orientation depicted in the FIGS. For example, if an object in the FIGS, is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "below" can encompass both an orientation of above and below. An object may be otherwise oriented (e.g., rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may be interpreted accordingly.

[0024] Like numbers refer to like elements throughout. Thus, the same or similar numbers may be described with reference to other drawings even if they are neither mentioned nor described in the corresponding drawing. Also, even elements that are not denoted by reference numbers may be described with reference to other drawings.

[0025] It will be appreciated that many different forms and embodiments are possible without deviating from the spirit and teachings of this disclosure, and so this disclosure should not be construed as limited to the example embodiments set forth herein. Rather, these examples and embodiments are provided so that this disclosure will be thorough and complete, and will convey the scope of the disclosure to those skilled in the art.

[0026] Embodiments of the present invention can be generally characterized as relating to techniques for monitoring the amount, concentration or presence of radicals within a reactivegas flow generated by a remote plasma source, by monitoring the paramagnetic properties of the gas.

[0027] As is known, paramagnetic species have a small, positive magnetic susceptibility (x) and are weakly attracted to an externally applied magnetic field. This paramagnetism arises from the presence of an unpaired electron in the species, which may be an atom, ion, or molecule. In contrast, diamagnetic species have no unpaired electrons and thus exhibit a small, negative susceptibility to (and are weakly repelled by) an externally applied magnetic field. The following table, Table 1, describes the paramagnetic / diamagnetic nature of radicals and their parent molecules of some elemental species that are commonly used in semiconductor processes:Table 1

[0028] From Table 1, it is evident that radical and the parent molecule from which it is generated have opposite magnetic properties. That is, for the same elemental species, the parent molecule (whether as present precursor material used in the generation of the reactive gas or a by-product present in the reactive gas) is diamagnetic whereas the radical is paramagnetic, and vice-versa. The devices and methods disclosed herein exploit these divergent magnetic properties of a common elemental species to provide radical species detection apparatuses and methods of measuring, monitoring or otherwise detecting radical species within a reactive gas. Such radical species detection apparatuses (and associated methods of measuring, monitoring, or detecting) can be categorized as being of a "magnetic force" type or a "magnetic permeability" type.

[0029] Atoms and molecules in the gas phase differ from those in condensed phases in that they are free or almost free to perform translational motion. The pressure of a gas (including areactive gas) depends on the gas density and its temperature as described by the ideal gas law. When a magnetic field is imposed on a gas (including a reactive gas) composed of a paramagnetic species (for example, a molecular paramagnetic species such as O2), magnetic alignment forces (for example, alignment of magnetic moments) experienced by the paramagnetic species restrict the free motion of the paramagnetic species and influence properties of the gas, including (but not restricted to) pressure, density, flow rate, thermal conductivity, index of refraction, and speed of sound within the gas. The influence is quantifiable and can be utilized to detect (or measure) the presence (or concentration or amount) of a paramagnetic constituent within a gas (for example, within the aforementioned reactive gas).

[0030] FIG. 1 shows an orthogonal view of a radical species detection apparatus, in accordance with an aspect of the present disclosure. The figure illustrates a magnetic force-type radical species detection apparatus 100 adapted to sample a portion of a reactive gas flowing through a transport tube 101. The apparatus 100 includes: a sample tube 102 connected to the transport tube 101 at first and second ends 102a and 102b, a magnet 104 arranged in the proximity to the sample tube 102, and a sensor 106 arranged in operable proximity to the sampled reactive gas flowing through the sample tube 102.

[0031] FIG. 1 also shows reactive gas 103 flowing through the transport tube 101 (indicated by arrow 103), a portion of the reactive gas 103 flowing into the sample tube 102 at the first end 102a (the reactive gas flow indicated by arrow 105), and the sampled reactive gas reentering the transport tube 101 from the second end 102b of the sample tube 102 (the reactive gas flow indicated by arrow 107). In one embodiment, the sample tube 102 is formed of any suitable material(s), such as quartz, one or more known or suitable ceramic materials, such as aluminum oxide, etc., materials that do not react, or are relatively resistant to reacting, with the sampled reactive gas, and which transmit or are permeable to an externally-generated magnetic field.The magnet 104 is positioned to generate a magnetic field that is transmitted into the interior of the sample tube 102. In the illustrated embodiment, the magnet 104 comprises an electromagnet, coupled to a controller (not shown in the figure) operative to selectively activate the magnet 104, and generating a magnetic field having a magnetic flux intensity at or above a predetermined threshold value or deactivate the magnet 104, thereby preventing a magnetic field from being generated, or generating a magnetic field having a magnetic flux intensity below the predetermined threshold value. The sensor 106 is configured to measure or detect changes in properties of the sampled reactive gas flowing through the magnetic field generated by the magnet 104.

[0032] More specifically, the magnetic force-type radical species detection apparatus 100 is adapted to sample a portion of a reactive gas (that may be generated by a remote plasma source not shown in FIG.1) flowing through a transport tube 101. The transport tube 101 may be in fluid communication with an output of the remote plasma source and an input port of a semiconductor processing chamber (not shown in Fig. 1). The reactive gas can include one or more radical species of a material such as fluorine, oxygen, nitrogen, hydrogen, or the like or any compounds thereof. As illustrated, the magnetic force-type radical species detection apparatus 100 includes a sample tube 102, a magnet 104 and a sensor 106.

[0033] Referring again to FIG. 1, the sensor 106 can be provided as one or more selected from the group consisting of a pressure sensor, such as a membrane-type pressure sensor, a thermal conductivity-type pressure sensor, a speed of sound sensor, an index of refraction sensor, such as a fixed length optical cavity sensor, or the like or any combination thereof, or any other sensor known in the art. The sensor 106 is arranged in any known or otherwise suitable manner so as to be in operable proximity to the sampled reactive flowing through the sample tube 102, so that the sensor 106 can be operated to measure (or otherwise detect a change in) one or more of the aforementioned properties of the sampled reactive gas flowing in the sample tube 102 and through the magnetic field generated by the magnet 104. The output of the sensor 106 may be communicatively coupled, by one or more wired or wireless links, to an input of a controller (not shown in the figure). The controller may thus be operative to interpret or otherwise process the input sensor signals and determine whether a paramagnetic constituent is present within the sampled reactive gas, the amount or concentration of the paramagnetic constituent within the sampled reactive gas, or the like or any combination thereof. In one embodiment, the controller may have access to information identifying the paramagnetic constituent(s) in the reactive gas flowing through the transport tube and such information - taken with the measurement signals - may optionally be used to facilitate a determination of the amount or concentration of the paramagnetic constituent within the sampled reactive gas.

[0034] The magnetic flux density, B, of the magnetic field generated by the magnet 104 may be set or otherwise vary depending on the type of reactive gas flowing through the transport tube 101, the temperature of the reactive gas flowing through the transport tube 101, the pressure of the reactive gas flowing through the transport tube 101, the paramagnetic constituent sought to be measured or detected, or the like or any combination thereof. For example, when the reactive gas includes oxygen, flowing through the transport tube 101 at a pressure of 1 atm and temperature of 298 K, a magnetic field generated by the magnet 104having a magnetic flux density, B, of 10 kG (or thereabout) can generate a pressure change, measurable by the sensor 106 of about 0.7 Pa (5E-3 Torr).

[0035] When an oscillating magnetic field is applied to a gas (including a reactive gas) contained within a hollow core cell (e.g. See FIG. 2, hollow toroidal cell core 202; FIG. 5, reference cell core 502, sample cell core 504), by a primary winding wound around the hollow core cell, the magnetic field strength, H, of the magnetic field within the core cell is constant, indicating that it is insensitive to the gas present within the core cell. However, the magnetically-induced magnetic flux density, B, of the magnetic field is proportional to the permeability of the gas inside core cell. B = p*H, where p is the relative magnetic permeability of the gas in the core cell. The magnetic flux density, B, will have one value if the core cell is at vacuum and, if filled with a gas, will have another value depending on the presence, concentrations or amounts of paramagnetic / diamagnetic constituents within the gas. If a secondary winding is wound around the core cell, then the secondary winding can be inductively coupled to the primary winding via the magnetic field transmitted through the cell core. In the presence of gas, as indicated by the magnetic flux density, B, the magnetic field in the gas of the core cell can influence the voltage level induced in the secondary winding. The influence is quantifiable and can be utilized to detect (or measure) the presence (or concentration or amount) of a paramagnetic constituent within a gas (i.e., within the aforementioned reactive gas), according to embodiments of the present invention exemplarily described below.Concerning these embodiments, it should be understood that the reactive gas referred to therein, can include one or more species of a material such as fluorine, oxygen, nitrogen, hydrogen, or the like or any radicals or compounds thereof.

[0036] Therefore, to summarize, the sensors in the described apparatus detect changes in the properties of the sampled reactive gas flowing through the magnetic field at least in the following ways:

[0037] By using pressure sensors, changes are detected in gas pressure caused by the magnetic field restricting the motion of paramagnetic species;

[0038] By using speed of sound sensors, changes are measured in the speed of sound through the gas, which can be affected by the magnetic field's interaction with paramagnetic species;

[0039] By using index of refraction sensors, changes are detected in the optical properties of the gas caused by the magnetic field's influence on paramagnetic species;

[0040] By using magnetic permeability sensors, changes are measured in the magnetic permeability of the gas flowing through a toroidal cell core with windings. The interactions between the magnetic field generated by the windings and paramagnetic species in the gas alter the inductance, which is detected as changes in voltage in the secondary winding;

[0041] By using a lock-in circuit configured to perform synchronous measurements, thereby allowing detection of small changes in magnetic permeability by analyzing signals at a specific frequency of operation. These sensor types exploit the fact that paramagnetic species in the reactive gas interact with the applied magnetic field, causing measurable changes in various physical properties of the gas. By detecting these changes, the sensors can indirectly measure the presence and concentration of radical species in the reactive gas flow.

[0042] FIG. 2 shows a schematic view of a magnetic permeability-type radical species detection apparatus. As shown, a magnetic permeability-type radical species detection apparatus 200 is adapted to receive a sampled portion of a reactive gas. The apparatus 200 includes a hollow toroidal cell core 202 connected to a sample tube 102, between its first and second ends 102a and 102b, and a plurality of windings 204 wound around the cell core 202. The sample tube 102, between its first and second ends 102a and 102b, is not illustrated in FIG. 2, and is identical with the one illustrated in FIG. 1. A portion of sampled reactive gas flows through the sample tube 102 from the first end 102a (indicated by arrow 105) into the interior of the cell core 202, and sampled reactive gas flowing through the cell core 202 and then back into the sample tube 102, exiting from the second end 102b (indicated by arrow 107).

[0043] Referring again to FIG 2, the windings 204 are provided as a set of primary and secondary windings. The primary winding is configured to generate a magnetic field within the cell core 202 when energized by an AC source (not shown in the figure). The secondary winding is inductively coupled to the primary winding and is designed to detect changes in the magnetic field caused by the presence of paramagnetic constituents in the sampled reactive gas flowing through the cell core 202. More specifically, the magnetic permeability-type radical species detection apparatus 200 is adapted to receive a sampled portion of a reactive gas (for example, generated by a remote plasma source, not shown) flowing through the aforementioned transport tube 101 (not shown). As exemplarily illustrated, the magnetic permeability-type radical species detection apparatus 200 is in fluid communication with the aforementioned sample tube 102, and may be coupled to the transport tube 101 as discussed above with respect to FIG. 1 and includes a hollow toroidal cell core 202 and a plurality of windings 204 wound around the cell core 202. Generally, the windings 204 are provided as a set of primary andsecondary windings, as is known in the art. The primary winding is electrically connected to an AC source (not shown in the figure) and, when energized by the AC source, is configured to generate a magnetic field within the cell core 202. The secondary winding can be inductively coupled to the primary winding as described above and is electrically connected to a controller (not shown in the figure) operative to receive the induced voltage in the secondary winding (also conceptually considered herein as a "sensor signal"). The controller may thus be operative to interpret or otherwise process the sensor signal and determine whether a paramagnetic constituent is present within the sampled reactive gas, the amount or concentration of the paramagnetic constituent within the reactive gas, or the like or any combination thereof. In one embodiment, the controller may have access to information identifying the paramagnetic constituent(s) in the reactive gas flowing through the transport tube and such information - taken with the measurement signals - may optionally be used to facilitate a determination of the amount or concentration of the paramagnetic constituent within the sampled reactive gas.

[0044] As shown in FIG. 2, the cell core 202 is connected to the sample tube 102 between the first and second ends 102a and 102b, respectively, thereof. Accordingly, a portion of sampled reactive gas flowing through the sample tube 102 from the first end 102a, as schematically shown by arrow 105, can flow into the interior of the cell core 202. Sampled reactive gas flows through the cell core 202 and then flows into the sample tube 102 and back into the transport tube 101 from the second end 102b, as schematically shown by arrow 107. Generally, the core cell 202 is formed of any suitable material(s), such as quartz, one or more known or suitable ceramic materials, such as aluminum oxide, etc., materials which do not react or are relatively resistant to reacting with the sampled reactive gas and which transmit an externally-generated magnetic field.

[0045] Although FIG. 2 illustrates the magnetic permeability-type radical species detection apparatus 200 as being in fluid communication with the sample tube 102, it will be appreciated that the magnetic permeability-type radical species detection apparatus 200 can be in direct fluid communication with the interior of the semiconductor processing chamber, to allow sensing of radical concentration that is delivered into the semiconductor processing chamber after any recombination losses that may have be incurred in the transport tube 101, from the remote plasma source.

[0046] FIG. 3 shows a sectional view of a magnetic permeability detection apparatus, in accordance with an aspect of the present disclosure. As shown, the magnetic permeability-type radical species detection apparatus 300 may be adapted to sample a portion of a reactive gasflowing through a transport tube 101 (not illustrated in the figure and described above in connection with FIG. 1). The apparatus 300 includes a hollow cylindrical reference cell core 302, a hollow cylindrical sample cell core 304, a primary winding 306, a secondary winding 308, and secondary winding magnetic cores 310.

[0047] As illustrated, the sample cell core 304 is coupled to a sample tube 102, allowing a portion of sampled reactive gas to flow through it, the flow being indicated by arrows 105 and 107. The reference cell core 302 may be at vacuum, or contain a gas with a composition similar to the precursor gas used to generate the plasma. The primary winding 306 is wound around the reference cell core 302, and the secondary winding 308 is wound around the sample cell core 304. The magnetic cores 310 are arranged between the reference cell core 302 and the sample cell core 304, directing magnetic field lines generated by the primary winding 306 into the secondary winding 308.

[0048] The primary winding 306 is electrically connected to an AC source (not shown in the figure), while the secondary winding 308 is inductively coupled to the primary winding 306 via the magnetic cores 310. This arrangement allows for the detection of changes in magnetic permeability caused by paramagnetic constituents in the sampled reactive gas flowing through the sample cell core 304. In one embodiment, the magnetic permeability-type radical species detection apparatus 300 is adapted to sample a portion of a reactive gas, generated by a remote plasma source, (not shown in the figure) flowing through a transport tube 101 which may be in fluid communication with an output of the remote plasma source and an input port of a semiconductor processing chamber (not shown in FIG. 3). The magnetic permeability-type radical species detection apparatus 300 includes the hollow cylindrical reference cell core 302, the hollow cylindrical sample cell core 304, the primary winding 306, the secondary winding 308 and magnetic cores 310. In FIG. 3, the reference cell core 302 and the sample cell core 304 are shown in cross-section.

[0049] The sample cell core 304 can be coupled to the aforementioned sample tube 102 (not shown, which is to be coupled to the transport tube 101 as discussed above with respect to any of FIGS. 1 to 2). Accordingly, a portion of sampled reactive gas flowing through the sample tube 102 from the first end 102a (e.g., as schematically shown by arrow 105) can flow into the interior of the sample cell core 304. Sampled reactive gas flowing through the sample cell core 304 then flows into the sample tube 102 and back into the transport tube 101 from the second end 102b (e.g., as schematically shown by arrow 107). In another embodiment, the sample cellcore 304 can be the aforementioned sample tube 102. In another embodiment, the sample cell core 304 can be the aforementioned transport tube 101.

[0050] The reference cell core 302 may be at vacuum or may contain a gas that has at least substantially the same composition as the precursor gas that was used by the remote plasma source to generate the reactive gas. In the illustrated embodiment, the reference cell core 302 is closed. In another embodiment, however, the reference cell core 302 may be in fluid communication with an outlet of one or more precursor gas sources (not shown) supplying precursor(s) to the remote plasma source to generate the reactive gas. The reference cell core 302 and the sample cell core 304 may be formed of the same or similar materials, such as, quartz, one or more known or suitable ceramic materials, such as aluminum oxide, etc., which do not react or are relatively resistant to reacting with the sampled reactive gas and which transmits an externally-generated magnetic field. Likewise, the reference cell core 302 and the sample cell core 304 may have the same dimensions, as measured within the plane shown in FIG. 3A. FIG. 3A represents a schematic cross-section of the magnetic permeability-type radical species detection apparatus 300 taken along line IIIA-IIIA, shown in FIG. 3.

[0051] Referring again to FIG 3, the primary winding 306 is electrically connected to an AC source (not shown) and is wound around the reference cell core 302. The secondary winding 308 is wound around the sample cell core 304 and is inductively coupled to the primary winding 306 by the magnetic cores 310. Thus, the magnetic cores 310 are arranged between the reference cell core 302 and the sample cell core 304 to direct magnetic field lines generated by the primary winding 306 into the secondary winding 308. Further, the secondary winding 308 is electrically connected to a controller (not shown, but which may be provided as exemplarily described above in the section describing FIG. 1) operative to receive the induced voltage in the secondary winding, also conceptually considered herein as a "sensor signal". As best shown in FIG. 3A, the magnetic cores 310 can have cylindrical cutouts to partially enclose the reference cell core 302 and the sample cell core 304, such that there is desirably a small gap between the windings and the magnetic cores 310. The construction and arrangement of the primary and secondary windings 306 and 308, respectively, is configured to encourage the magnetic field lines to form a magnetic field approximately toroidal in form. Containing the magnetic field lines in this approximately toroidal form reduces sensitivity to external magnetic disturbances and noise sources. Additional electromagnetic shielding can be implemented to reduce unwanted noise.

[0052] When the sample cell core 304 is provided as the transport tube 101, the magnetic permeability-type radical species detection apparatus 300 can advantageously measure the full flow of the reactive gas as it is delivered to the semiconductor processing chamber. This can provide a faster response time and allow a larger winding diameter, which can increase the inductance and therefore the signal output from the secondary winding 308 to the controller.

[0053] FIG. 4 shows a perspective view of an embodiments of a magnetic permeabilitytype radical species detection apparatus 400, which may be used with the apparatus shown in FIG. 3. As shown, the apparatus 400 includes a sample cell core 304, which is coupled to a sample tube to allow reactive gas to flow through it. Like the previous embodiments, the apparatus 400 also comprises a magnetic core 402, which replaces the magnetic cores 310 from the embodiment of FIG. 3 and further includes a primary winding 306 (illustrated by the solid line), and a secondary winding 308.1 In contrast to the magnetic permeability-type radical species detection apparatus 300 illustrated in FIG. 3, in the magnetic permeability-type radical species detection apparatus 400 the reference cell core 302 has been omitted, the primary winding 306 is wound around both the magnetic core 402 and the sample cell core 304, and the secondary winding 308 is wound around the sample cell core 304 and the magnetic core 402 in the return path. The apparatus 400 requires fewer winding turns around the magnetic core 402 to establish the same inductance between the primary and secondary windings compared to the embodiment illustrated in FIG. 3. The apparatus is designed to detect changes in magnetic permeability caused by paramagnetic constituents in the sampled reactive gas flowing through the sample cell core 304.

[0054] In more detail, referring to FIG. 4, a magnetic permeability-type radical species detection apparatus 400 is provided as exemplarily described with respect to the magnetic permeability-type radical species detection apparatus 300, but the reference cell core 302 is omitted and the magnetic cores 310 are replaced by magnetic core 402. In this embodiment, the primary winding 306 (shown by the solid line) is wound around the magnetic core 402 and the sample cell core 304 whereas the secondary winding 308 is wound around the sample cell core 304 and the magnetic core 402 in the return path. Compared with the magnetic permeability-type radical species detection apparatus 300, the magnetic permeability-type radical species detection apparatus 400 requires fewer winding turns around the magnetic core 402 in order to establish the same inductance between the primary and secondary windings.

[0055] FIG. 5 illustrates a magnetic permeability-type radical species detection apparatus500 featuring a split hollow toroidal cell core structure. The apparatus 500 includes a referencecell core 502, that may exemplarily be one half of the split toroid, a sample cell core 504 that may exemplarily be the other half of the split toroid, a primary winding 508, a secondary winding 510 and a secondary winding a reference tube 506.

[0056] As shown in FIG. 5, the sample cell core 504 is coupled to a sample tube 102, allowing a portion of sampled reactive gas to flow through it, flow indicated by arrows 105 and 107. The reference cell core 502 is connected to the reference tube 506, which can be in fluid communication with precursor gas sources. Precursor gas is flowing into the reference cell core 502 (as indicated by arrow 501) and exits the reference cell core 502 (as indicated by arrow 503). The primary winding 508 is wound around both the reference cell core 502 and the sample cell core 504. The secondary winding 510 is also wound around both the reference cell core 502 and the sample cell core 504.

[0057] The embodiment of FIG. 5 allows for a direct comparison between the reference gas in the reference cell core 502 and the sampled reactive gas in the sample cell core 504. The apparatus is configured to detect changes in magnetic permeability caused by paramagnetic constituents in the sampled reactive gas by comparing it to the reference gas. The primary winding 508 generates the magnetic field, while the secondary winding 510 detects changes in the field due to differences in magnetic permeability between the two cell cores. The magnetic permeability-type radical species detection apparatus 500 includes a split hollow toroidal cell core structure having a two hollow half-toroid cell cores which are fluidly isolated from each other so that gas in either half-toroid cell core cannot enter into the other half-toroid cell core. In the embodiment illustrated in FIG. 5, one of the half-toroid cell cores is a reference cell core 502 and the other of the half-toroid cell cores is a sample cell core 504.

[0058] Generally, the sample cell core 504 can be coupled to the aforementioned sample tube 102 (which is to be coupled to the transport tube 101 as discussed above with respect to any of FIGS. 1 to 3). Accordingly, a portion of sampled reactive gas flowing through the sample tube 102 from the first end 102a (as schematically shown by arrow 105) can flow into the interior of the sample cell core 504. Sampled reactive gas flowing through the sample cell core 504 then flows into the sample tube 102 and back into the transport tube 101 from the second end 102b (as schematically shown by arrow 107).

[0059] The reference cell core 502 can be coupled to a reference tube 506 that, in turn, is in fluid communication with an outlet of one or more precursor sources (not shown) supplying precursor(s) to the remote plasma source to generate the reactive gas. Accordingly, a portion of the precursor gas(es) from which the plasma gas is generated can flow into the interior of thereference cell core 502 (e.g., as schematically shown by arrow 501). The precursor gas(es) exits the reference cell core 502 into the reference tube 506 (as schematically shown by arrow 503). In another embodiment, the reference cell core 502 may be sealed and maintained at vacuum or may contain a gas that has at least substantially the same composition as the precursor gas(es) used by the remote plasma source to generate the reactive gas. In yet another embodiment, the reference cell core 502 is open to a reference tube or sealed - in either case, as discussed above - but receives or contains a gas having a composition corresponding to the reactive gas that is intended to be generated by the remote plasma source, that is expected to be present within the transport tube 101 or that is expected to be ultimately delivered into the semiconductor processing chamber.

[0060] The reference cell core 502 and the sample cell core 504 may be formed of the same or similar materials, for example quartz, one or more known or suitable ceramic materials, such as aluminum oxide, etc., which do not react or are relatively resistant to reacting with the sampled reactive gas and which transmit an externally-generated magnetic field. Likewise, the reference cell core 502 and the sample cell core 504 may have the same dimensions, in terms of core wall thickness, inner diameter, and outer diameter.

[0061] Referring still to FIG. 5, the magnetic permeability-type radical species detection apparatus 500 also includes a primary winding 508 and a secondary winding 510. The primary winding 508 is electrically connected to an AC source (not shown in the figure) and is wound around the reference cell core 502 and the sample cell core 504. The secondary winding 510 is wound around the reference cell core 502 and the sample cell core 504 and is electrically connected to a controller (not shown in the figure, but which may be provided as exemplarily described above in connection with the embodiment of FIG. 1). The secondary winding 510 can be inductively coupled to primary winding 508 as described above and is also electrically connected to a controller (not shown in the figure) operative to receive the induced volage in the secondary winding 508.

[0062] Although FIG. 5 illustrates a single secondary winding 510 wound around the reference cell core 502 and the sample cell core 504, in another embodiment, the single secondary winding 510 can be replaced with two secondary windings 510 that are electrically connected to each other but wound in opposite directions around the reference cell core 502 and the sample cell core 504. For example, a first one of the secondary windings 510, i.e., a "reference secondary winding," is wound around the reference cell core 502 and a second one of the secondary windings 510 i.e., a "sample secondary winding," is wound around the samplecell core 504. In this case, the direction around which the first one of the secondary windings 510 is wound around the reference cell core 502 can be the same as the direction around which the primary winding 508 is wound around the reference cell core 502 and the direction around which the second one of the secondary windings 510 is wound around the sample cell core 504 is opposite to the direction around which the primary winding 508 is wound around the sample cell core 504. Alternatively, the direction around which the first one of the secondary windings 510 is wound around the reference cell core 502 can be opposite to the direction around which the primary winding 508 is wound around the reference cell core 502 and the direction around which the second one of the secondary windings 510 is wound around the sample cell core 504 is the same as the direction around which the primary winding 508 is wound around the sample cell core 504.

[0063] In embodiments in which the reference cell core 502 is not maintained at vacuum, the induced voltage in the reference and sample secondary windings 510 cancels out (i.e., | AV | = VREF - VSAM = 0) when the gas in the reference cell core 502 has the same magnetic permeability as the gas in the sample cell core 504. However, if the magnetic permeability of the gas in the reference cell core 502 (PREF) is different from that of the gas in the sample cell core 504 (PSAM ), then induced voltage in the secondary winding is no longer zero (i.e., | AV | * 0) and the magnitude of the differential voltage AV is proportional to the degree to which the sampled reactive gas is paramagnetic or diamagnetic. An equivalent circuit diagram explaining this arrangement of primary and secondary windings is shown in FIG. 6.

[0064] Referring now to FIG. 6, the drawing shows a schematic diagram 600 of a magnetic permeability-type radical species detection apparatus. The apparatus consists of three main sections a primary 602, a toroid 604 and a secondary 606. The primary section 602 comprises a transformer coil with an AC voltage source (shown by a sine wave symbol) labeled VPR| . The toroid 604 comprises the two components labeled "pREF" and "pSAM", respectively the reference cell core 502 and the sample cell core 504. The secondary section 606 comprises another coil component to an amplifier or comparator that outputs a signal AV. The components are connected in series. The electrical circuit diagram 600 shows how the magnetic permeability measurement apparatus processes signals from the primary input through the toroidal sensor to generate a differential voltage output. If the primary winding 508 is driven with a constant AC voltage, and the gas permeability is different from vacuum, then the induced voltage output by the secondary winding will be reduced. Therefore, in a preferred alternative embodiment, theprimary winding 508 is driven with a constant AC current instead of a constant AC voltage, depending on how the voltage at the secondary is measured.

[0065] If each primary and secondary winding could be on or proximal to a plane that includes the axis of the toroid axis of the split hollow toroidal cell core structure, then zero or very little magnetic field would leak outside of the toroid. But the windings have to be at an angle as they pass around the toroid. As such, the magnetic fields generated within the cell cores are actually the vector sum of the field from a perfect toroid, with all fields inside, plus the field from a single loop that runs in a circle around the inside of the toroid. The field that escapes from a toroid is called the field from the "once-around loop" which, optionally, can be mostly cancelled by putting a shorting loop adjacent to the toroid.

[0066] In many cases, the relative magnetic permeabilities for gases are very small values and the controller should be configured with adequate detection limit capabilities. If the primary winding 508 is operated at a fixed AC frequency, the controller can be configured to perform a synchronous measurement via a lock-in circuit, which would permit the sensor signal to be analyzed at the specific frequency of operation and provide both phase and amplitude information for the transformer output relative to the input. A lock-in circuit can measure signals down to nanovolt (nV) levels, which can support low detection limits. An equivalent circuit corresponding to the description above is shown in FIG. 7.

[0067] Referring now to FIG. 7, the drawing shows a schematic diagram of a measurement apparatus 700 that includes a voltage source 702 (VPR| ) connected in series to a magnetic permeability-type radical species detection apparatus 500 in series. The output from this arrangement connects to a controller with lock-in circuit 704 through two inputs, labeled "AV" and "VREF , respectively. The controller 704 has two output indicators or terminals, labeled as "Amplitude Information" and "Phase Information". The diagram illustrates a circuit configuration for measuring and analyzing voltage differences, with the lock-in circuit controller 704 being a key component for processing the signals. While the controller 704 in the apparatus of FIG. 7 is a specific type of controller, in general, in connection with all the embodiments of the present invention, the controller detects changes and analyzes information at least in the following ways:

[0068] Receives sensor signals: The controller receives signals from sensors such as pressure sensors, speed of sound sensors, index of refraction sensors, or magnetic permeability sensors;

[0069] Interprets sensor data: It processes and interprets the received sensor signals to determine changes in gas properties caused by interactions between the magnetic field and paramagnetic species;

[0070] Synchronous measurements: The controller may employ a lock-in circuit to perform synchronous measurements, allowing detection of very small changes in magnetic permeability by analyzing signals at a specific frequency of operation;

[0071] Comparison to reference data: The controller may compare detected changes in magnetic permeability to reference data associated with known concentrations of radical species;

[0072] Concentration determination: Based on the comparison to reference data, the controller can determine the concentration of radical species in the sampled gas flow;

[0073] Selective magnet activation: For electromagnet-based systems, the controller can selectively activate and deactivate the magnet, generating magnetic fields with specific flux intensities;

[0074] Differential analysis: In split toroidal core designs, the controller may analyze differential signals between reference and sample gas flows to detect changes in magnetic permeability;

[0075] Signal processing: The controller may employ various signal processing techniques to filter noise, amplify small changes, and extract relevant information from sensor signals;

[0076] Calibration and nulling: The controller may include functionality to calibrate the apparatus and null out any baseline mismatches between reference and sample measurements;

[0077] Data output: The controller processes the analyzed data to provide information on the presence, amount, or concentration of paramagnetic constituents within the sampled reactive gas.

[0078] In an alternate implementation, the reference gas tube may be replaced with a reference winding around the ferrite core in the return path. Fewer windings around the ferrite would be needed in order to establish an equal inductance, as compared to the sample tube.

[0079] In connection with the above presented example embodiments, the following assumptions were made: the outer diameter of the toroid is about or equal to 0.10 m, the toroid's inner diameter is about or equal to 0.06 m, and the magnet wire is about or equal to 28American Wire Gauge (AWG) heavy build insulation. The insulation's relative permittivity is about or equal to 3.0. Therefore, the magnet wire copper diameter is about or equal to 3.20E-4 m, and the magnet wire insulation diameter is about or equal to 3.73E-4 m. The maximum possible number of windings on the toroid is appx. 505. The primary inductance is approximately 500 uH. The dominant capacitance across the primary is the capacitance between the two ends of the coil (not between individual winding loops). Approximately 5% of the toroid perimeter, between the two primary leads, has no windings to allow this gap. Then the capacitance between the ends of the primary is appx. 0.47 pF. An estimate of the primary resonant frequency is 10.4 MHz. In one embodiment, the resonant circuit is operated at the resonant frequency of the toroid divided by 10, at 1.04 M Hz (e.g., the frequency of the resonant circuit is approximately 10.4 MHz). The primary can be driven with 100 kHz to make the signal relatively easy to process with a controller formed of common components. The primary impedance will be appx. 310 O at 100 kHz. The primary can be driven with 10 Vpk (peak voltage) and the coil currents will be safe and convenient. In this case, the voltage across each secondary will be 5 Vpk. If the permeability of one side of the toroid changes by 1 ppm, the differential voltage across the two secondaries will be approximately 5 uVpk. The above signal is small, but detectable and measurable with lock-in detection of a high precision voltmeter.

[0080] Concerning the presence of electrostatic shielding, it is of note that with signals output by the secondary, electrostatic shielding may be provided between the primary and secondary to prevent capacitive coupling between the primary and secondary windings. Otherwise, the undesired signal capacitive coupling between the primary and secondary will dwarf the real signal that is inductively coupled. The shielding can be copper tape, it is wound between two windings, and it is grounded. A few layers of Kapton tape may positioned between the shield and each winding to increase separation and reduce capacitance.

[0081] Due to minor irregularities in the apparatus 500, like variation in the windings and tubes, the induced voltage in the reference and sample secondary windings may vary when filled with identical gas. The mismatch could be greater than 0.1% and maybe as large as a few percent. If each secondary winding has a nominal voltage of 5.0 Vpk, the null signal might be around 50 mVpk. Accordingly, the controller may be equipped with any known or suitable means to calibrate / null the apparatus 500, such as means permitting to dial-in the null, and not relying completely on the secondaries being balanced well enough. One approach is to take the two secondary voltages out separately and subtract them in a differential amplifier - with one signal having adjustable gain. Another approach is to take the differential secondary signal, andthen add or subtract a small fraction of the primary voltage to null it. Another approach is to add one more secondary winding with a small number of turns to pick up a small fraction of the primary voltage and be in phase with the other secondaries discussed above (that are making the measurements); then take the induced voltage from this extra secondary, divide it with a pot, and put in in series with the aforementioned measurement secondaries. In this approach, the innermost windings would be the measurement secondaries, then the primary winding would be wound over the measurement windings and the extra secondary would be wound over the primary winding.

[0082] The greatest analog risk comes from the presence of undesirable capacitance between primary and secondary windings. In a typical power transformer, the manufacturer will usually wind the primary directly on top of the secondary. It may be desirable to measure a precision signal; undesirable capacitance between the two windings can cause more problems than in a power application. That capacitance should be reduced. Layers of an insulator, such as Kapton tape, etc., could be provided between primary and secondary for this purpose. The insulator thickness should be at least one outer diameter (and maybe a few outer diameters) of the wire used in the windings. The magnet is selectively activated and deactivated using a controller. The magnet is used to generate a magnetic field having a magnetic flux intensity at or above a predetermined threshold value. A synchronous measurement is performed to detect small changes in magnetic permeability of the sampled gas flow.

[0083] The present application also discloses a method for radical species detection via paramagnetic gas properties. More specifically, a reactive gas flow is transported through a transport tube. A portion of the reactive gas flow is diverted from the transport tube into a sample tube as a sampled gas flow. A magnetic field is created in a region of the sample tube using a magnet. Changes are detected in the properties of the sampled reactive gas flowing through the magnetic field using a sensor. The sampled gas flow returns to the transport tube as a return gas flow.

[0084] Alternatively, the method for radical species detection via paramagnetic gas properties of the present invention comprises receiving a sampled gas flow into a hollow toroidal cell core, generating a magnetic field within the toroidal cell core using a plurality of magnetic windings wound around the cell core, detecting changes in magnetic permeability of the sampled gas flow within the toroidal cell core based on interactions between the magnetic field and paramagnetic species, determining a concentration of radical species in the sampled gas flow based on the detected changes in magnetic permeability, performing a synchronousmeasurement using a lock-in circuit to detect small changes in magnetic permeability, providing electrostatic shielding between magnetic windings and the hollow toroidal cell core to prevent undesired capacitive coupling, comparing detected changes in magnetic permeability to reference data associated with known concentrations of radical species, driving the primary winding with a constant AC current instead of a constant AC voltage, and using a controller to interpret or process sensor signals and determine the presence, amount, or concentration of paramagnetic constituents within the sampled reactive gas.

[0085] FIG. 8 is a flow chart representation of a method in accordance with the present invention. As illustrated in the figure, a method of detecting radical species in a reactive gas, may comprise transporting 802 a reactive gas flow through a transport tube, diverting 804 a portion of the reactive gas flow 103 into a sample tube 102 as a sampled gas flow 105, generating 806 a magnetic field in a region of the sample tube 102 using a magnet 104, detecting 808, using a sensor 106, changes in the properties of the sampled reactive gas flowing through the magnetic field generated by the magnet 104, and returning 810 the sampled gas flow 105 to the transport tube 101 as a return gas flow 107.

[0086] The magnet 104 is an electromagnet, and the method further comprises selectively activating and deactivating the magnet 104 using a controller. Selectively activating the magnet 104 comprises generating a magnetic field having a magnetic flux intensity at or above a predetermined threshold value. The sensor 106 is selected from the group consisting of a pressure sensor, a speed of sound sensor, and an index of refraction sensor. The method further comprises performing a synchronous measurement to detect small changes in magnetic permeability of the sampled gas flow 105.

[0087] Generally, the controller includes one or more processors operative to generate the aforementioned commands and control signals (e.g., upon executing one or more instructions). A processor can be provided as a programmable processor (e.g., including one or more general purpose computer processors, microprocessors, digital signal processors, or any other suitable form of circuitry including programmable logic devices (PLDs), central processing units (CPUs), graphics processing units (GPUs), accelerated processing units (APUs), real-time processing units (RPUs), field-programmable gate arrays (FPGAs), field-programmable object arrays (FPOAs), application-specific integrated circuits (ASICs) - including digital, analog and mixed analog / digital circuitry - or the like, or any combination thereof) operative to execute the instructions.Execution of instructions can be performed on one processor, distributed among multipleprocessors, made parallel across processors within a device or across a network of devices, or the like or any combination thereof.

[0088] Generally, the instructions may be embodied as software (e.g., an executable code, file, library file, or the like or any combination thereof), hardware configuration (e.g., in the case of FPGAs, ASICs, etc.), or the like or any combination thereof, which can be readily specified by artisans, from the descriptions provided herein (e.g., written in C, C++, Visual Basic, Java, Python, Tel, Perl, Scheme, Ruby, assembly language, hardware description language such as LUCID, VHDL or VERILOG, etc.). Software is commonly stored in one or more data structures conveyed by tangible media such as computer memory, which is accessible (e.g., via one or more wired or wireless communications links) by a processor. Examples of tangible media include magnetic media (e.g., magnetic tape, hard disk drive, etc.), optical discs, volatile or non-volatile semiconductor memory (e.g., RAM, ROM, NAND-type flash memory, NOR-type flash memory, SONOS memory, etc.), or the like or any combination thereof, and may be accessed locally, remotely (e.g., across a network), or any combination thereof.

[0089] The foregoing is illustrative of embodiments and examples of the invention and is not to be construed as limiting thereof. Although a few specific embodiments and examples have been described with reference to the drawings, those skilled in the art will readily appreciate that many modifications to the disclosed embodiments and examples, as well as other embodiments, are possible without materially departing from the novel teachings and advantages of the invention. Accordingly, all such modifications are intended to be included within the scope of the invention as defined in the claims. For example, skilled persons will appreciate that the subject matter of any sentence, paragraph, example or embodiment can be combined with subject matter of some or all of the other sentences, paragraphs, examples or embodiments, except where such combinations are mutually exclusive. The scope of the present invention should, therefore, be determined by the following claims, with equivalents of the claims to be included therein.

Claims

1. WHAT IS CLAIMED IS:

1. An apparatus, comprising: a transport tube configured to transport a reactive gas flow; a sample tube connected to the transport tube at a first end and a second end, the sample tube configured to receive a sampled gas flow from the transport tube; a magnet positioned adjacent to the sample tube between the first end and the second end of the sample tube, the magnet configured to generate a magnetic field into an interior of the sample tube; and a sensor arranged in operable proximity to the sample tube in a region where the magnet is positioned, the sensor configured to detect changes in properties of the sampled gas flowing through the magnetic field generated by the magnet.

2. The apparatus of claim 1, wherein the sample tube is formed of a material selected from the group consisting of quartz and ceramic.

3. The apparatus of claim 1, wherein the magnet is an electromagnet coupled to a controller operative to selectively activate and deactivate the magnet.

4. The apparatus of claim 3, wherein the controller is configured to activate the magnet to generate a magnetic field having a magnetic flux intensity at or above a predetermined threshold value.

5. The apparatus of claim 4, wherein the sensor is selected from the group consisting of a pressure sensor, a speed of sound sensor, and an index of refraction sensor.

6. A method of detecting radical species in a reactive gas, comprising: transporting a reactive gas flow through a transport tube; diverting a portion of the reactive gas flow into a sample tube as a sampled gas flow; generating a magnetic field in a region of the sample tube using a magnet; detecting, using a sensor, changes in properties of the sampled gas flowing through the magnetic field generated by the magnet; and returning the sampled gas flow to the transport tube as a return gas flow.

7. The method of claim 6, wherein the magnet is an electromagnet, and the method further comprising selectively activating and deactivating the magnet using a controller.

8. The method of claim 7 , wherein selectively activating the magnet comprises generating a magnetic field having a magnetic flux intensity at or above a predetermined threshold value.

9. The method of claim 8, wherein the sensor is selected from the group consisting of a pressure sensor, a speed of sound sensor, and an index of refraction sensor.

10. The method of claim 9, further comprising performing a synchronous measurement to detect small changes in magnetic permeability of the sampled gas flow.

11. An apparatus, comprising: a hollow toroidal cell core configured to receive a sampled gas flow; a plurality of windings wound around the toroidal cell core, the plurality of windings configured to generate a magnetic field within the toroidal cell core; and a controller configured to detect changes in properties of the sampled gas flow within the toroidal cell core based on interactions between the magnetic field and paramagnetic species in the sampled gas flow.

12. The apparatus of claim 11, wherein the hollow toroidal cell core is formed of a material selected from the group consisting of quartz and ceramic.

13. The apparatus of claim 11, wherein the controller is configured to perform a synchronous measurement circuit to detect small changes in magnetic permeability of the sampled gas flow.

14. The apparatus of claim 13, further comprising electrostatic shielding between the plurality of windings and the hollow toroidal cell core to prevent undesired capacitive coupling between the windings and the core.

15. The apparatus of claim 14, wherein the controller is further configured to compare detected changes in magnetic permeability to data associated with known concentrations of radical species to determine a concentration of radical species in the sampled gas flow.

16. The apparatus of claim 11, wherein the toroidal cell core is a split hollow toroidal cell core structure.

17. The apparatus of claim 16, further comprising: a reference tube connected to the toroidal cell core and configured to be in fluid communication with a precursor gas source.

18. The apparatus of claim 11, further comprising: electrostatic shielding between the plurality of windings, and the cell core.

19. The apparatus of claim 11, wherein the controller is configured to compare detected changes in magnetic permeability to reference data associated with known concentrations of radical species to determine a concentration of radical species in the sampled gas flow.

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