Patent Description:
This application generally relates to in-situ deposition thickness monitoring, and, more particularly, to an in-situ deposition thickness monitoring system that determines the thickness of a wear-resistant coating on a substrate, such as an extrusion die, while the substrate is disposed in a deposition chamber.

Wear-resistant coatings may be used in manufacturing processes, for example, in order to extend the life of a tool, such as an extrusion die in a ceramic honeycomb body manufacturing process. Processes to apply such coatings, such as chemical vapor deposition processes, may require high temperatures and harsh environments that make it infeasible to measure the thickness of a coating material deposited on a tool or other substrate.

<CIT> discloses a film thickness monitoring device including a capacitor composed of rotary electrode and a fixed electrode. <CIT>, <CIT>, <CIT> and <CIT> provide additional prior art.

All examples and features mentioned below can be combined in any technically possible way.

The invention provides a method for in-situ measurement of a thickness of a coating deposited by a deposition process according to claim <NUM>. The method includes the steps of: initiating deposition within a deposition chamber such that a first coating forms on an outer surface of a probe disposed in the deposition chamber, wherein the probe comprises a coil assembly including at least one coil, wherein the probe is separated by a distance from a substrate disposed within the deposition chamber; exciting the coil assembly with a first alternating current to produce a first time-varying magnetic field, the first time-varying magnetic field generating an eddy current in the first coating; determining a metric related to an inductance or resistance of the coil assembly, wherein a value of the metric is related to a first thickness of the first coating and results at least partially from an eddy current magnetic field produced by an eddy current in the coating; and correlating the first thickness of the first coating to a second thickness of a second coating deposited on a surface of the substrate.

In an example, the metric is an impedance or transimpedance of the coil assembly.

In an example, the metric is a frequency at which a resonant peak occurs.

In an example, the metric is a coefficient of a polynomial fit to a curve of the impedance or transimpedance of the coil assembly at a plurality of frequencies.

In an example, the probe comprises a housing having a body and a cap, wherein the outer surface of the probe is an outer surface of the housing, wherein the coil assembly is disposed in the housing.

In an example, the at least one coil is operatively coupled to at least one capacitor to form a resonant circuit.

In an example, the distance is at least three times a radius of the at least one coil.

In an example, the at least one coil is wound about a threaded mandrel such that the at least one coil is restrained in an axial direction.

In an example, the coil assembly comprises a first coil and a second coil.

In an example, the first coil and the second coil are wound about an axis, wherein the first coil is spaced apart from the second coil to define a gap therebetween, wherein a third coil is disposed in the gap between the first coil and the second coil.

In an example, the method further includes the step of exciting the second coil with a second alternating current, the second alternating current flowing in a direction opposite to the first alternating current to produce a second time-varying magnetic field that at least reduces the first time-varying magnetic field in the gap between first coil and the second coil, wherein the first alternating current excites the first coil.

In an example, the first coil is wound about the axis in a first direction and the second coil is wound about the axis in a second direction.

In an example, the first coil and the second coil are formed from the same conducting wire such that first alternating current and the second alternating current are the same.

According to another aspect not covered by the scope of the claims, an electrical probe for measuring the thickness of a coating, includes: a first coil being wound about an axis in a first direction and having a first number of turns; a second coil being wound about the axis in a second direction and having a second number of turns, wherein the first coil is spaced apart from the second coil to define a gap therebetween, wherein the first direction is opposite the second direction such that a first time-varying magnetic field generated by a first alternating current flowing through the first coil reduces, in the gap, a second time-varying magnetic field generated by a second alternating current flowing through the second coil; and a third coil disposed in the gap between the first coil and the second coil, wherein a coating deposited on an outer surface of the electrical probe measurably changes a metric related to a mutual inductance between the third coil and the first coil and the second coil during operation.

In an example, the first coil and the second coil are formed by the same wire such that the first alternating current and the second alternating current are the same.

In an example, the first number of turns are the same as the second number of turns.

In an example, at least one of the first coil, the second coil, or the third coil is operatively connected to at least one capacitor to form a resonant circuit.

In an example, the first coil, the second coil, and the third coil are housed in a housing comprising a body and a cap.

In an example, the cap is removable from the body.

In an example, the first coil, the second coil, and the third coil are wound about a threaded mandrel such that the first coil, the second coil, and the third coil are restrained in an axial direction.

Further, the invention provides a deposition system according to claim <NUM> providing in-situ measurement of deposition thickness, includes: a deposition chamber dimensioned to receive a substrate and having an inlet that receives a chemical vapor to apply a coating to a surface of the substrate; an electrical probe positioned in the chemical vapor deposition chamber such that an outer surface of the electrical probe is coated by the chemical vapor, wherein the electrical probe comprises a coil assembly including at least one coil, wherein the probe is separated by a distance from the substrate; and a signal generator supplying an alternating current signal to the coil assembly to generate a first time-varying magnetic field such that eddy currents are generated in the coating on the surface of the electrical probe.

In an example, the probe comprises a housing, wherein the outer surface of the probe is an outer surface of the housing, wherein the coil assembly is disposed on the housing.

In an example, the housing comprises a body and a cap.

In an example, the probe comprises a first coil and a second coil.

In an example, the first coil and the second coil are wound about an axis, wherein the first coil is spaced apart from the second coil to define a gap therebetween, wherein a third coil is disposed in the gap between the first coil and the second coil, wherein the coating deposited on the outer surface of the electrical probe measurably changes a metric related to mutual inductance between the third coil and the first coil and the second coil.

Other features, objects, and advantages will be apparent from the description and the drawings, and from the claims.

Ceramic honeycomb bodies, such as those used in catalytic converters and/or particulate filters, can be produced by extruding a ceramic-forming batch mixture through a die. Based on the raw materials in the ceramic-forming batch mixture the extrusion process may be highly abrasive, in which case it may be advantageous to coat the die with a wear resistant coating. For example, a coating can be applied using a chemical vapor deposition process (CVD) with a coating material such as titanium nitride (TiN), titanium carbon nitride (TiCN), or other nitrides or abrasion-resistant materials, to protect the die from abrasion, maintain dimensional accuracy throughout the extrusion process, and extend the life of the die. In addition to protecting the die from abrasion, the coating also serves to fine-tune (i.e., narrow) the slot widths of the die through which the ceramic batch material is extruded. The slot widths are tuned to a highly exacting thickness-the applied coating is typically only less than <NUM> in thickness-and so the thickness of the coating must be monitored closely during the deposition process.

However, it is not feasible to measure the thickness of the coating during the deposition process, for example due to the high temperature and harsh conditions during coating. In lieu of measuring the thickness, the coating may be applied over a set time period that has previously been correlated to an approximate thickness based on approximated deposition rates. The thickness of the coating can be measured optically once the die has been removed from the deposition chamber and, e.g., allowed to cool to room temperature.

<FIG> is schematic view of a deposition chamber <NUM> with a probe <NUM> for in-situ measurement of the coating thickness on a substrate <NUM> disposed in the deposition chamber <NUM>. Deposition chamber <NUM>, in the example of <FIG>, is a chemical vapor deposition chamber, and, as such, includes an inlet <NUM> for receiving a chemical vapor and an outlet <NUM> for exhausting the vapor during the deposition process. The probe <NUM> is particularly suited for chemical vapor deposition processes, but the probe <NUM> can be used with other coating processes, e.g., requiring high temperatures or harsh environments, that make it infeasible to measure the thickness of a coating during the coating process. Other such deposition processes include, e.g., sputtering and evaporation. Furthermore, in one example, substrate <NUM> is an extrusion die for extruding a green ceramic body, e.g., a green honeycomb body, although in alternative examples, substrate <NUM> can any substrate, e.g., tool or workpiece, receiving a deposited coating.

Probe <NUM> is at least partially positioned within chamber <NUM> so as to also receive a coating during the deposition process. The thickness of the coating deposited on probe <NUM> itself will be related to the thickness of the coating deposited on substrate <NUM>, and thereby can serve as a proxy for measuring the thickness of a coating disposed on substrate <NUM>. In this way, the in-situ measurement of the thickness of the coating on substrate <NUM> is determined by measuring a thickness of a coating on probe <NUM>.

<FIG> shows a schematic magnified view of coatings on the surfaces of probe <NUM> and substrate <NUM>(which have been illustrated in close proximity to each other for the sake of comparison of the surfaces of probe <NUM> and substrate <NUM>). As shown, during the deposition process probe <NUM> receives coating C<NUM> with a thickness T<NUM> and substrate <NUM> receives coating C<NUM> with a thickness T<NUM>. The thickness T<NUM> is related to thickness T<NUM>, due to the probe <NUM> also being located within deposition chamber <NUM>, and thus the thickness T<NUM> can be effectively monitored during the deposition process by monitoring the thickness T<NUM>. For example, thickness T<NUM> can be related to thickness T<NUM> by a scaling factor, through the addition of a constant, or a linear or nonlinear function, which can be, e.g., determined experimentally or via modeling, based on the particular geometries of the chamber <NUM>, the substrate <NUM>, the probe <NUM>, the positioning of the substrate <NUM> and the probe <NUM> within the chamber <NUM>, and/or the deposition parameters, such as coating material type, temperature, flow rate, etc..

According to some embodiments, probe <NUM> is an electrically reactive probe, including at least one inductive element and/or at least one capacitive element. As the coating C<NUM> is deposited on the surface of probe <NUM>, a reactance of the at least one capacitive element and/or the at least one inductive element changes as a result of a magnetic or electrical field generated by the coating C<NUM> in response to an alternating current flowing through the probe <NUM>. The change in the reactance will be related to a change in thickness of the coating deposited on the surface of the probe, and thus the thickness of the coating disposed on substrate <NUM> can be approximated by monitoring the change in reactance of probe <NUM> during the deposition process. As will be described in more detail below, the thickness can be determined according to various metrics related to the reactance or resistance of probe <NUM>. These metrics can be, in various examples, an impedance or transimpedance, a frequency at which a resonant peak occurs, or at least one coefficient of a polynomial fit to a curve of the impedance or transimpedance of probe <NUM> over a range of frequencies; however, it is contemplated that other metrics could be used. For the purposes of this disclosure, a metric related to a reactance or resistance of the probe is one that reproducibly (and thus predictably) changes with reactance and from which the thickness of the coating can be inferred.

A signal generator <NUM> can provide the alternating current through probe <NUM> that induces the magnetic or electric field in the coating and that, in turn, changes the reactance of probe <NUM>. Any signal generator suitable for delivering an alternating current to probe <NUM> can be used. Furthermore, any suitable measurement device <NUM> for measuring one or more parameters from which the metric related to the reactance or resistance of probe <NUM> can be determined can be used. In one example, signal generator <NUM> and measurement device <NUM> can together be provided by a vector network analyzer <NUM>. However, one of ordinary skill in the art, in conjunction with a review of this disclosure, will appreciate that there a myriad of ways of implementing a signal generator and for measuring the parameters from which the metric can be determined. For example, signal generator <NUM> can alternatively be implemented by a local oscillator. Likewise, measurement device <NUM> can be implemented as an impedance bridge circuit or a lock-in amplifier.

As shown, probe <NUM> can be inserted into deposition chamber <NUM> by way of a feedthrough <NUM>, e.g., a hermetic feedthrough. The feedthrough <NUM> permits probe <NUM> to be inserted into deposition chamber <NUM> while maintaining the vacuum created in deposition chamber <NUM>. In one example, the feedthrough <NUM> is a KF fitting, although CF fittings, or any O-ring that can withstand the temperatures existing within deposition chamber <NUM> can be used, e.g., to maintain a hermetic seal.

As shown in <FIG>, probe <NUM> is disposed remote from substrate <NUM> such that a distance d exists between the probe <NUM> and substrate <NUM>. Locating the probe <NUM> remote from substrate <NUM> can be useful to reduce the perturbation of the deposition on substrate <NUM>. For example, where the deposition process is accomplished through chemical vapor deposition, additional surface area within deposition chamber <NUM> may "steal" away chemical vapor from substrate <NUM>, lowering the concentration of chemical vapor within deposition chamber <NUM> and lowering the deposition rate. Positioning another object, e.g., the probe <NUM>, close to the substrate <NUM> may cause boundary flow effects, or otherwise impede, block, alter, or disrupt flow patterns near the substrate <NUM>, and thus coating thickness at various locations on the substrate <NUM>, Positioning probe <NUM> remote from substrate <NUM> reduces such perturbation, thus helping to maintain the concentration of chemical of chemical vapor at substrate <NUM> and maintain a consistent deposition rate across the entirety of the substrate <NUM>. In one example, distance d is at least three times the radius r (or otherwise, the width, in examples in which probe <NUM> is not cylindrical) of probe <NUM>.

To further reduce the perturbance of the flow of chemical vapor, probe <NUM> can be shaped to have a longitudinal axis A, shown in the example cross-section view of probe <NUM> in <FIG>, oriented parallel to the flow of vapor. Further, the radius r of probe <NUM> (or otherwise, the width, in examples in which probe <NUM> is not cylindrical) can be minimized, resulting in a geometric profile that minimally impacts the flow of chemical vapor.

<FIG> shows a cross-section view of one example of probe <NUM>. In this example, probe <NUM> is an inductive probe featuring a housing <NUM> in which a coil assembly <NUM> is disposed. In alternative examples, a capacitive assembly, rather than or in addition to a coil assembly <NUM>, can be disposed in the housing. During the deposition process, an outer surface <NUM> of housing <NUM> is the surface of probe <NUM> coated during the deposition process. Housing <NUM> can comprise a body <NUM> and a cap <NUM> fit over coil assembly <NUM> and removably attached to body <NUM>. The removable attachment of cap <NUM> permits rapid turnover time between applications. For example, after a deposition process, cap <NUM> can be quickly removed and either cleaned (e.g., removing coating C<NUM> using an etching bath, physical force, or other stripping treatment) and reattached to housing <NUM> or a different cap <NUM> (e.g., new or previously cleaned), can be attached to housing <NUM>. In an example, cap <NUM> can be attached to body <NUM> to way of a threaded connection, although in various alternative examples any mechanical connection that can be maintained through the deposition process can be used.

Housing <NUM>, cap <NUM>, and other parts of probe <NUM>, not including the inductive or capacitive elements, can be formed of ceramic or other material having low coefficient of thermal expansion, as these perform well in the high-temperature environment of deposition chamber <NUM>.

Maintaining a fixed coil geometry can be beneficial to maintain accuracy during and across tests, e.g., particularly where high temperatures are utilized during the deposition process. As shown in <FIG>, the coil(s) of coil assembly <NUM> can be wound about a threaded mandrel <NUM>, in which at least one turn of the coil is seated in a respective thread, thus restraining coil assembly <NUM> from axial movement (i.e., movement along the longitudinal axis A) during the deposition process. Because the deposition chamber <NUM> can reach as much as <NUM>° C, the turns of coil assembly <NUM> are prone to drift as a result of thermal expansion. Threaded mandrel <NUM> eliminates or otherwise reduces this axial drift and thus retains the distance between respective turns of coils assembly <NUM>. Furthermore, housing <NUM> (e.g., cap <NUM>) can be tightly fitted over coil assembly <NUM> to eliminate or reduce radial expansion of coil assembly <NUM>. In an alternative example, coil assembly <NUM> can be potted in a potting material (e.g., a thermosetting plastic or epoxy) in order to restrain coils from drifting due to thermal expansion.

<FIG> show several alternative example coil topologies of coil assembly <NUM>. The topologies of <FIG> shown removed from housing <NUM> and threaded mandrel <NUM> so that the topologies can be easily observed. Turning first to <FIG>, there is shown an example single coil topology comprising a first coil <NUM>. In this example, the alternating current flowing through first coil <NUM> generates a time-varying magnetic field that creates eddy currents in the coating C<NUM> forming on outer surface <NUM>. The eddy currents, in turn, create an opposing magnetic field that alters the self-inductance of first coil <NUM>. The magnitude of the eddy currents and of the magnetic field they create will be determined at least in part by the thickness of the coating forming on outer surface <NUM>, and thus the thickness T<NUM> of coating C<NUM> can be monitored by determining the change in a metric related to self-inductance or resistance of first coil <NUM> (such as impedance).

<FIG> shows an alternative example featuring two coils, a first coil <NUM>' and a second coil <NUM>. When an alternating current is flowing through one of first coil <NUM>' and second coil <NUM>, the resulting time-varying magnetic field will generate a voltage across the other coil <NUM>', <NUM>. Thus, for example, if an alternating current is flowing through first coil <NUM>', the resulting time-varying magnetic field will cause a voltage to form across second coil <NUM>, which will induce a current through second coil <NUM>, forming a second time-varying magnetic field. The eddy currents in coating C<NUM> that ultimately form from the either first time-varying magnetic field or the second will alter the self-inductance of the coils <NUM>', <NUM> individually and the mutual inductance between the coils <NUM>', <NUM> because the eddy current magnetic field steals away or adds to the time-varying magnetic fields that would otherwise couple first coil <NUM>' or second coil <NUM>. Thus, the thickness T<NUM> of coating C<NUM> can be monitoring by monitoring a metric related the self-inductance or resistance of the coils <NUM>', <NUM> individually (e.g., impedance) or the mutual inductance between coils <NUM>', <NUM> (e.g., transimpedance).

In the example of <FIG>, first coil <NUM>' and second coil <NUM> are both shown disposed along the same longitudinal axis A; however, in alternative examples, first coil <NUM>' and second coil <NUM> can be disposed along separate axes. For example, in an alternative example, first coil <NUM>' and second coil <NUM> can be disposed along parallel axes.

<FIG> depicts another example of a topology of coil assembly <NUM> in which a first coil <NUM>" and second coil <NUM>' are separated by some distance forming a gap g in which a third coil <NUM> is disposed. The first coil <NUM>" and second coil <NUM>', as shown in this example, are formed from the same conductive wire, so that the same current flows through each. The coils in this embodiment are wound in opposite directions in order to produce opposing magnetic fields in the gap. The opposing magnetic fields will reduce the effect of the time-varying magnetic fields produced by the first coil <NUM>" and second coil <NUM>' within gap g, and, thus, on third coil <NUM>, which is formed of a separate conductive wire, thereby canceling (or reducing) the mutual inductance between third coil <NUM> and first coil <NUM>" and second coil <NUM>'. Because the mutual inductance is canceled at third coil <NUM> when no coating is present, even minute changes to the metrics related to self-inductance or resitance of third coil <NUM> (e.g., impedance) or by metrics related to the mutual inductance between third coil <NUM> and first coil <NUM>" and second coil <NUM>' (e.g., transimpedance) will be readily detectable.

The topology of <FIG> is only one example of a method for canceling or reducing the mutual inductance at third coil <NUM>. In an alternative example, rather than using counter wound coils that are formed from a common conductive wire, first coil <NUM>" and second coil <NUM>' can be formed from separate conductive wires, and be wound in the same direction, but receive current flowing in opposing directions (i.e., the alternating current flowing through the first coil <NUM>" is approximately <NUM>° out of phase with alternating current flowing second coil <NUM>'). Assuming that the currents are approximately <NUM>° (±<NUM>°) out of phase and of equal magnitude, the effects of the time-varying magnetic fields at third coil <NUM> will be reduced, similar to the example of <FIG>. In various alternative examples, the currents through first coil <NUM>" and second coil <NUM>' can be out of phase by some value besides approximately <NUM>° and/or need not receive currents of equal magnitude. Further, in yet another example, first coil <NUM>" and second coil <NUM>' can be formed of independent conductive wires and be counter wound, so that current of the same magnitude and phase flowing through each will similarly cause canceling or reduction of mutual inductance at third coil <NUM>.

The geometry of the conductive wires that connect probe <NUM> to signal generator <NUM> and to measurement device <NUM> can be held in a fixed geometry. An example is shown in <FIG>, which depicts a cross-section view of probe <NUM>, taken along line B-B. As shown, first conductive wire <NUM>, second conductive wire <NUM>, third conductive wire <NUM>, and fourth conductive wire <NUM>, which are formed from the leads of first coil <NUM> and second coil <NUM> (e.g., arranged in the geometries of <FIG>) are respectively arranged in channels <NUM> extending parallel to the longitudinal axis A of stalk <NUM> of probe <NUM>. First conductive wire <NUM> and second conductive wire <NUM> form input and output leads of first coil <NUM>, while third conductive wire <NUM> and fourth conductive wire <NUM> form input and output leads of second coil <NUM>. Arranged in the manner shown in <FIG>, background transmissive coupling is substantially reduced, as the magnetic fields generated by the transmission of the alternating currents to first coil <NUM> or second coil <NUM> are effectively canceled by the return of the same current in an opposing wire. Thus, any time-varying magnetic field generated by the alternating current input to first coil <NUM> by first conductive wire <NUM>, is effectively canceled at third conductive wire <NUM> and fourth conductive wire <NUM> by the time-varying magnetic field generated by the same current returning in second conductive wire <NUM>. Likewise, the time-varying magnetic field generated by an alternating current input to third conductive wire <NUM> is effectively canceled at first conductive wire <NUM> and second conductive wire <NUM> by the same alternating current returning in the fourth conductive wire <NUM>. The cancelling is maintained by the fixed geometry in which the conductive wires are held via channels <NUM>. Furthermore, the stalk <NUM> of probe <NUM> can be configured to be of a length such that the canceling effect is maintained for a distance enough to reduce or minimize uncancelled background transmissive coupling effects on probe <NUM>. Thus, in one example, stalk <NUM> is at least a half a meter in length, although other lengths that enable or enhance such cancelling are within the scope of this disclosure.

As mentioned above, the temperature inside the deposition chamber can reach as much as <NUM>° C. As such, the long stalk <NUM> length (that is at least greater than half a meter) further helps to manage the thermal gradient in the hot deposition chamber <NUM>.

Turning now to determining a metric related to a reactance or resistance of probe <NUM>. As described above, for the purpose of this disclosure, the metric related to a reactance (e.g., inductance, such as self-inductance or mutual inductance, or capacitance) or resistance of the probe is any metric that predictably changes with reactance or resistance and from which the thickness of the coating can be inferred. The reactance can be a capacitive reactance (for a capacitive probe), an inductive reactive (for an inductive probe), or some combination of capacitive or inductive reactance (e.g., an LC circuit).

While any number of metrics can be used, three metrics were shown to reliably determine the thickness T<NUM> of coating Ci: (<NUM>) the change in impedance or transimpedance over time, (<NUM>) the frequency at which a resonant peak occurs, and (<NUM>) the curvature of the impedance or transimpedance of the probe over frequency.

Turning first to the change in impedance or transimpedance or time. As shown in <FIG>, the change in impedance of a coil or the transimpedance between two coils is correlated to coating thickness. Such a change can be seen in <FIG> which plots the impedance of a single coil over a period of approximately <NUM> minutes during a deposition process. During this time frame, the impedance changes from approximately <NUM>Ω to approximately <NUM>Ω, according to the change in coating thickness.

Turning to the next metric, probe <NUM> will exhibit some resonance, manifesting in a resonant peak at a particular frequency that changes depending on the thickness of the coating on probe <NUM>. To understand this, the angular resonant frequency of an LC circuit, which probe <NUM> can be roughly modeled as, is given by <MAT>, and so the frequency at which the resonant peak occurs will change with the reactance (either inductance or capacitance) of probe <NUM>. Thus, as the thickness of coating increases, altering the reactance of probe <NUM>, the resonant peak will shift in a related manner.

More specifically, as the thickness T<NUM> of coating C<NUM> increases over time, the frequency fp at which the resonant peak occurs will increase. As an example of this, <FIG> depicts an three overlaid resonant curves taken at separate times throughout the deposition process: a first curve at time t=<NUM>, representing the resonant curve at a time before deposition begins and having a resonant peak occurring at approximately <NUM>, a second curve at time t=<NUM>, representing the resonant curve at some time (approximately <NUM> minutes) after deposition has started and having a resonant peak occurring at frequency approximately <NUM>, and a third resonant curve taken at time t=<NUM>, representing the resonant curve toward the end of the deposition process (approximately <NUM> minutes after the second resonant curve) and having a resonant peak occurring at frequency <NUM>. As shown, the resonant peak frequency of the third resonant curve is higher than the resonant peak frequency of the first and second resonant curves as the resonant peak has shifted due to the change in thickness of the coating deposited on probe <NUM>. Accordingly, the frequency at which the resonant peak occurs will shift in time in a manner related to the thickness of the coating C<NUM>. Experimentally, the resonant frequency tends to change about <NUM> per minute for a coating rate of about <NUM> per minute.

The frequency at which the resonant peak occurs can be determined, in one example, by exciting probe <NUM> over a range of frequencies in which the resonant peak is expected to occur and recording the frequency at which the peak is measured. This frequency sweep can be repeated multiple times over the course of the deposition process to track the change in thickness of coating C<NUM> over time. Alternatively, probe <NUM> can be excited with an alternating current of a single frequency. For example, as long as the frequency at which probe <NUM> is excited resides in the resonant curve, the resulting change in impedance can be measured and related to its relative location within the curve. Because the width of the resonant curve will remain largely unchanged as the thickness of coating C<NUM> increases, a change in measured in impedance can be easily related to a relative position within the resonant curve, and, accordingly, to a corresponding thickness of the coating C<NUM>.

In either instance, a lookup table can be used to relate the frequency at which the resonant peak occurs fp to a thickness T<NUM> of coating C<NUM>. Alternatively, a different part of the resonant curve (e.g., the <NUM> dB point), rather than the peak, can be correlated to thickness T<NUM>. In the example in which a single frequency is used instead of a frequency sweep, the measured impedance, representative of a position within the resonant curve, can be used as the input parameter to the lookup table to determine the thickness T<NUM> directly.

In general, following Lenz's law, higher frequency time-varying magnetic fields create higher electromotive force. Thus, as frequency increases, more current is induced the coating, resulting in greater change in the reactance of probe <NUM> resulting in better sensitivity for higher frequency values. However, skin depth δ, (i.e., the penetration depth of the time-varying magnetic field into coating C<NUM>) is given by <MAT>, where µ represents and thus the penetration of the time-varying magnetic field decreases as the frequency increases. If the time-varying magnetic field fails to penetrate the entire depth of the coating, the change in the thickness will not induce a continued change in the reactance of probe <NUM>. Accordingly, the higher sensitivity at higher frequencies needs to be balanced against the diminished skin depth at the same frequencies. Empirically, the range of suitable frequencies was found to be between <NUM> and <NUM>, while a frequency of approximately <NUM> was found to achieve the best sensitivity while retaining a sufficient skin depth. The above values are only given as examples and the frequency range can be selected depending on the particular geometries of probe <NUM>, coating materials, and/or other operational parameters.

In order to ensure that the resonant peak occurs approximately at a desirable frequency (i.e., one that maintains sufficient skin depth, such as on or about <NUM>), a matched network, such as matched network <NUM> shown in the circuit schematic of <FIG> can be employed. Matched network <NUM> comprises an LC circuit formed of elements C1, C2, and C3, which effectively shifts the resonant peak of probe to approximately <NUM> by varying the total reactance seen by measurement device <NUM>. Although matched network <NUM> is a T-matching network, any suitable matching network for changing the resonant peak to a desired value can be used. Furthermore, although the matched network is shown attached to first coil <NUM>' in the coil assembly example described in connection with <FIG>, it should be understood that the matched network can be suitably attached to any coil of the coil assembly. For example, in the coil assembly example of <FIG> the matched network can operatively be connected to any of first coil <NUM>", second coil <NUM>', or third coil <NUM>. (In this example, the connection to the first coil <NUM>" and second coil <NUM>' is electrically equivalent because they are formed from the same wire.

In a second example, the thickness T<NUM> can be determined from the shape of a curve that represents the impedance or transimpedance measured against a set of frequencies that includes a frequency in which coupling between probe <NUM> and coating C<NUM> begins. At low frequencies (including DC currents) the current through probe <NUM> will be insufficient cause measurable coupling between probe <NUM> and coating C<NUM>. As these frequencies increase, coupling will begin as the time-varying magnetic field reaches and penetrates coating C<NUM>. Furthermore, as the excitation frequency changes, the impedance or transimpedance, measured against frequency, will change as coupling increases due to the higher generated electromotive force (under Lenz's law, as described above). The impedance plotted against frequency will form a curve from which the thickness can be determined. The shape of this curve, measured over a frequency band including the frequency at which the coupling begins, will vary as the thickness increases. Thus, over the course of the deposition process, multiple frequency sweeps can yield a curve of the impedance over frequency, the shape of each curve being a metric representative a reactance, and thus the thickness of the coating deposited, on probe <NUM>.

<FIG> depicts a plot of overlaid impedance curves (<FIG> is the same as <FIG>, except showing a greater frequency range). As shown, a first curve represents the impedance curve of probe <NUM> at time t=<NUM>, prior to the deposition process, a second curve represents the impedance curve of probe <NUM> at time t=<NUM>, at a first point during the deposition process, and third curve represents the impedance curve of prior <NUM> at time t=<NUM>, at a second point during the deposition process.

The shape of the curve can be represented as a set of coefficients of a polynomial representing a line-of-best fit to the curve. For example, the coefficients of a polynomial regression can be used to represent the curve; although any suitable method for fitting a polynomial to a dataset can be used. In one example, the coefficients can be stored in a lookup table and referenced to determine, for a given set of polynomial coefficients, the thickness at the time of coating C<NUM> at the time of measurement.

As mentioned above, the curve of <FIG> plots an impedance or a transimpedance. An impedance measurement measures the opposition of current through a particular element or set of elements of probe <NUM>. For example, the impedance measurement can measure the impedance of an inductive or capacitive element of probe <NUM>. By contrast, the transimpedance measures a voltage at one port resulting from a current introduced at a different port. This is particularly useful for measuring changes in mutual inductance or mutual capacitance. For example, the examples of probe <NUM> shown in <FIG>, the mutual inductance between first coil <NUM> and second coil <NUM> or third coil <NUM> can be measured by the transimpedance between the first coil <NUM> and second coil <NUM> or third coil <NUM>. The impedance or transimpedance can be measured any suitable way. For example, using the network analyzer of <FIG>, the S-Parameters of probe <NUM> can be measured and converted to Z-parameters. In this example, the transimpedance between coils will be represented as Z12 or Z21. However, this is only provided as an example and any number of ways for measuring the impedance or the transimpedance can be used.

As described above, the thickness T<NUM> of coating C<NUM> on probe <NUM> will be related to the thickness T<NUM> of coating C<NUM> on substrate <NUM> by a known function such as a scaling factor or an added constant. The thickness T<NUM> can thus be determined by transforming the determined thickness T<NUM> to thickness T<NUM> by the known function. This can be accomplished by way of a mathematical operation or through a lookup table that relates thickness T<NUM> to thickness T<NUM>. Alternatively, to the extent that a lookup table is used to relate a metric related to reactance (e.g., the frequency at which a resonant curve occurs or to the coefficients of an impedance curve) or resistance, the lookup table incorporate the relation between thickness T<NUM> and thickness T<NUM> so that the input parameter is directly related to thickness T<NUM>.

Claim 1:
A method for in-situ measurement of a thickness of a coating deposited by a deposition process, comprising the steps of:
initiating deposition within a deposition chamber (<NUM>) such that a first coating (C<NUM>) forms on an outer surface of a probe (<NUM>) disposed in the deposition chamber (<NUM>), wherein the probe (<NUM>) comprises a coil assembly (<NUM>) including at least one coil (<NUM>, <NUM>'; <NUM>, <NUM>'; <NUM>), wherein the probe is separated by a distance (d) from a substrate (<NUM>) disposed within the deposition chamber (<NUM>);
exciting the coil assembly (<NUM>) with a first alternating current to produce a first time-varying magnetic field, the first time-varying magnetic field generating an eddy current in the first coating;
determining a metric related to an inductance or resistance of the coil assembly (<NUM>), wherein a value of the metric is related to a first thickness (T<NUM>) of the first coating (C<NUM>) and results at least partially from an eddy current magnetic field produced by an eddy current in the coating; and
correlating the first thickness (T<NUM>) of the first coating (C<NUM>) to a second thickness (T<NUM>) of a second coating (C<NUM>) deposited on a surface of the substrate (<NUM>).