Patent Grant
7097537
The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for monitoring a layer during chemical mechanical polishing.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. One fabrication step involves depositing a filler layer over a non-planar surface, and planarizing the filler layer until the non-planar surface is exposed. For example, a conductive filler layer can be deposited on a patterned insulative layer to fill the trenches or holes in the insulative layer. The filler layer is then polished until the raised pattern of the insulative layer is exposed. After planarization, the portions of the conductive layer remaining between the raised pattern of the insulative layer form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization is needed to planarize the substrate surface for photolithography.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing disk pad or belt pad. The polishing pad can be either a “standard” pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, such as a slurry with abrasive particles, is supplied to the surface of the polishing pad.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness, or when a desired amount of material has been removed. Overpolishing (removing too much) of a conductive layer or film leads to increased circuit resistance. On the other hand, under-polishing (removing too little) of a conductive layer leads to electrical shorting. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, and the load on the substrate can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One way to determine the polishing endpoint is to remove the substrate from the polishing surface and examine it. For example, the substrate can be transferred to a metrology station where the thickness of a substrate layer is measured, e.g., with a profilometer or a resistivity measurement. If the desired specifications are not met, the substrate is reloaded into the CMP apparatus for further processing. This is a time-consuming procedure that reduces the throughput of the CMP apparatus. Alternatively, the examination might reveal that an excessive amount of material has been removed, rendering the substrate unusable.
More recently, in-situ monitoring of the substrate has been performed, e.g., with optical or capacitance sensors, in order to detect the polishing endpoint. Other proposed endpoint detection techniques have involved measurements of friction, motor current, slurry chemistry, acoustics and conductivity. One detection technique that has been considered is to induce an eddy current in the metal layer and measure the change in the eddy current as the metal layer is removed.
In one aspect, the invention is directed to a method of polishing that includes bringing a surface of a substrate into contact with a polishing pad, causing relative motion between the substrate and the polishing pad, using one or more in-situ monitoring sensors to generate a series of measurements of one or more properties of the substrate, associating each measurement of the series with information indicating a time when the measurement was made, generating a first measurement of a position of a carrier head holding the substrate, using the first measurement of the position of the carrier head and a sinusoidal first function to define a second function that associates measurements from the series with positions on the substrate, and for each measurement in the series, using the second function to determine a position on the substrate where the measurement was taken.
Implementations of the invention may include one or more of the following features.
The position of the carrier head may be measured with an encoder. Defining the second function may include adjusting the sinusoidal function based on the first measurement. A plurality of positions of the carrier head may be measured with the encoder. Defining the second function may include curve fitting the sinusoidal function to the plurality of encoder measured positions. The first function may be updated based on a second measurement of the position of the carrier head made after the first measurement, e.g., by calculating a phase shift. The encoder may generate position measurements with a frequency greater than 100/millisecond, e.g., about 256/millisecond. The second function may include a phase correction representing lag resulting from a processing delay in generating the first measurement of the position of the carrier head, and may include a phase shift representing variations in carrier head sweep frequency from a target sweep frequency. Measurements may be associated with positions on the substrate corresponding to an edge of the substrate. The in-situ monitoring sensor may be an eddy current sensor.
In another aspect, the invention is directed to a method of polishing that includes bringing a surface of a substrate into contact with a polishing pad, causing relative motion between the substrate and the polishing pad, using one or more in-situ monitoring sensors to generate a measurement of a substrate property, associating the measurement of the substrate property with information indicating a time when the measurement of the substrate property was made, generating a measurement of a position of a carrier head holding the substrate, and using the first measurement of the position of the carrier head, the time when the measurement of the substrate property was made, and a phase correction representing lag resulting from a processing delay in generating the measurement of the position of the carrier head in determining a position on the substrate where the measurement of the substrate property was taken.
The invention includes computer program products, tangibly stored on machine-readable medium, for operating a polishing apparatus, the product comprising instructions operable to cause a processor to perform the steps set forth above.
Possible advantages of implementations of the invention can include one or more of the following.
The optical and eddy current monitoring systems can monitor essentially the same spot on the substrate. Implementations can provide accurate conversion of time domain data to the position domain in systems using optical and non-optical (e.g., magnetic) monitoring systems. The optical monitoring system can sample relatively small zones on the substrate surface (e.g., one millimeter or less) and can determine the edge of the substrate to relatively high accuracy.
In some embodiments, the apparatus and methods may improve wafer edge detection resolution and accuracy, despite a possible decrease in the signal to noise ratio of the optical monitoring system.
The thickness of the conductive layer can be measured during bulk polishing. The thickness of a polishing pad used to polish the substrate can also be measured during polishing. The pressure profile applied by the carrier head can be adjusted to compensate for non-uniform polishing rates and non-uniform thickness of the incoming substrate. Polishing can be stopped with high accuracy. Over-polishing and under-polishing can be reduced, as can dishing and erosion, thereby improving yield and throughput.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.
Like reference symbols in the various drawings indicate like elements.
Referring to
Each polishing station includes a rotatable platen 24 on which is placed a polishing pad 30. The first and second stations 22a and 22b can include a two-layer polishing pad with a hard durable outer surface or a fixed-abrasive pad with embedded abrasive particles. The final polishing station 22c can include a relatively soft pad or a two-layer pad. Each polishing station can also include a pad conditioner apparatus 28 to maintain the condition of the polishing pad so that it will effectively polish substrates.
Referring to
During a polishing step, a polishing liquid 38, such as an abrasive slurry or abrasive-free solution can be supplied to the surface of the polishing pad 30 by a slurry supply port or combined slurry/rinse arm 39. The same slurry solution may be used at the first and second polishing stations, whereas another slurry solution may be used at the third polishing station.
Returning to
Each carrier head 70 is connected by a carrier drive shaft 74 to a carrier head rotation motor 76 (shown by the removal of one quarter of cover 68) so that each carrier head can independently rotate about it own axis. In addition, each carrier head 70 independently laterally oscillates in a radial slot 72 formed in carousel support plate 66. A description of a suitable carrier head 70 can be found in U.S. Pat. Nos. 6,422,927 and 6,450,868, and in U.S. patent application Ser. No. 09/712,389, filed Nov. 13, 2000, the entire disclosures of which are incorporated by reference. In operation, the platen is rotated about its central axis, and the carrier head is rotated about its central axis and translated laterally across the surface of the polishing pad.
Referring to
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As shown by
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As shown in
After polishing, the patterned underlying layers will provide metal features, e.g., vias, pads and interconnects. However, prior to polishing the bulk of conductive layer 16 is initially relatively thick and continuous and has a low resistivity, and relatively strong eddy currents can be generated in the conductive layer 16. As previously mentioned, the eddy currents cause the metal layer to function as an impedance source in parallel with the coil 44.
Referring to
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Referring also to
The spot size of a beam can be defined as the beam diameter within which, e.g., 80% of the beam power is contained. Generally, spot size depends on the wavelength of the beam, and the nature of the focusing optic. For example, where the focusing optic is a lens, the fraction of a beam's power, P, in a beam with a Gaussian profile within a diameter D is given by
where F is the lens focal length and a is the unfocused beam's radius. In some implementations, where the light beam has a wavelength between about 400 nanometers and 800 nanometers (e.g., 633 nanometers or 670 nanometers) the beam spot size is less than about two millimeters (e.g., less than about one millimeters, 0.5 millimeters, 0.2 millimeters).
Referring now specifically to
In some embodiments, focusing optic 1301 and collimating optic 1310 are lenses with similar focal lengths (e.g., with identical focal lengths). More generally, focusing optic 1301 and/or collimating optic 1301 can include any optical component or combination of optical components that focus the light beam to reduce the spot size of the beam at surface 36A of transparent section 36. Such optical components include refractive optical components (e.g., lenses), reflective optical components (e.g., focusing mirrors), diffractive optical components (e.g., gratings), and/or holographic optical components (e.g., holographic gratings).
In
In some embodiments, optics can be integrated with the window. For example, one or more of the optics can be bonded to surface 36B of the window (e.g., using an optical adhesive). Another example of integrated components are where the focusing and/or collimating optics are formed in the window from a monolithic piece of the window material. Such an embodiment is shown in
Referring again to
Although the optical monitoring system described above includes collimating optic 1310, other embodiments can have no collimating optic between the window and the detector.
Referring to
Returning to
The information provided by the position sensor can be useful in various aspects of CMP control. For example, the duration that the optical signal is interrupted and/or the time between sweeps provides the CMP apparatus with information about the angular velocity, ωp, of the platen. Specifically, if the flag 82 is of a known angular arc, Φ, and the optical signal is interrupted for a duration Tinterrupt, then the angular velocity can be calculated as Φ/Tinterrupt. Similarly, if the time between the start of subsequent optical interruptions is Tsweep, then the angular velocity can be calculated as 1/Tsweep. The calculated angular velocity can be compared against the target angular velocity set by the polishing recipe and used for closed loop control of the platen rotation velocity, or compared against the angular velocity as determined from an encoder attached to the platen drive system and used to correct for drift or inaccuracy in the encoder measurements. The angular velocity can also be used in calculations of the measurement positions, as discussed below.
Optionally, the high resolution position sensor can provide information to a computer (for example the one described below), which can use the information to provide real time process control. As an alternative or in addition to the described optical position sensor, the CMP apparatus can include an encoder to determine the angular position of platen.
A general purpose programmable digital computer 90 receives the signals from the eddy current sensing system and the optical monitoring system. The printed circuit board 160 can include circuitry, such as a general purpose microprocessor or an application-specific integrated circuit, to convert the signals from the eddy current sensing system and optical monitoring system into digital data. This digital data can be assembled into discrete packets which are sent to computer 90 via a serial communication channel, e.g., RS-232. So long as both printed circuit board 160 and computer 90 use the same packet format, computer 90 can extract and use the intensity and phase shift measurements in the endpoint or process control algorithm. For example, each packet can include five bytes, of which two bytes are optical signal data, two bytes are either amplitude or phase difference data for the eddy current signal, one bit indicates whether the packet includes amplitude or phase shift data, and the remaining bits include flags for whether window section 36 is beneath the substrate, check-sum bits, and the like.
Since the monitoring systems sweep beneath the substrate with each rotation of the platen, information on the metal layer thickness and exposure of the underlying layer is accumulated in-situ and on a continuous real-time basis (once per platen rotation). The computer 90 can be programmed to sample measurements from the monitoring system when the substrate generally overlies transparent section 36 (e.g., as determined by the position sensor). As polishing progresses, the reflectivity or thickness of the metal layer changes, and the sampled signals vary with time. The time varying sampled signals may be referred to as traces. The measurements from the monitoring systems can be displayed in real time (or near real time) on an output device 94 during polishing to permit the operator of the device to visually monitor the progress of the polishing operation. (The display can also indicate detected errors and polishing parameters such as, for example, pressures, slurry flow, temperature, platen rotation speed.) The traces may be used to control the polishing process and determine the end-point of the metal layer polishing operation, as will be described below.
In operation, CMP apparatus 20 uses eddy current monitoring system 40 and optical monitoring system 140 to determine when the bulk of the filler layer has been removed and to determine when the underlying stop layer has been substantially exposed. The computer 90 applies process control and endpoint detection logic to the sampled signals to determine when to change process parameter and to detect the polishing endpoint. Possible process control and endpoint criteria for the detector logic include local minima or maxima, changes in slope, threshold values in amplitude or slope, or combinations thereof.
In addition, computer 90 can be programmed to associate each measurement from eddy current monitoring system 40 and optical monitoring system 140 from each sweep beneath the substrate with a radial position on the substrate, as described in U.S. Pat. Nos. 6,159,073, and 6,280,289, the entire disclosures of which are incorporated herein by references. Once the measurements are associated with radial positions, computer 90 can be programmed to sort the measurements into radial ranges, to determine minimum, maximum and average measurements for each sampling zone, and to use multiple radial ranges to determine the polishing endpoint, as discussed in U.S. Pat. No. 6,399,501, the entirety of which is incorporated herein by reference.
To associate the measurements with radial positions on the substrate surface, computer 90 first collects the data (e.g., eddy current or light intensity values) as a function of time, t, from a complete scan across the retaining ring and substrate from both optical monitoring system 140 and eddy current monitoring system 40. The computer determines, for each data point collected (i.e., each current or intensity value measured), the radial position of the sensor relative to the center of the wafer according to the following algorithm, which is described with reference to
x″(t)=X0−ΔX cos(ωwt+C) (Equ. 1)
where X0=(Xmax+Xmin)/2 and ΔX=(Xmax−Xmin)/2, ωw is the head sweep frequency, and C is a correction term. As the platen rotates, the position of a sensor 1430, e.g., the eddy current sensor or the optical sensor, located a distance R from the platen rotation axis, is given by
x′(t)=R cos ωpt
y′(t)=R sin ωpt′ (Equ. 2)
where ωp is the platen angular velocity. The platen angular velocity ωp can be taken from the polishing recipe, or derived from data collected by the position sensor as described above.
The radial coordinate in the position domain is then given by
r(t)=√{square root over ((x′(t)−x″(t))2+y′(t)2)}{square root over ((x′(t)−x″(t))2+y′(t)2)}{square root over ((x′(t)−x″(t))2+y′(t)2)}.
This data provides a mapping from time domain to position domain, allowing the system user to associate intensity measurements and corresponding eddy current sensor measurements with a radial position on the wafer.
Returning to the determination of the head position, the above described function (i.e., Equation 1) can be used in conjunction with discrete encoder-measured head positions, for example, by curve fitting, to provide an accurate mapping between time and position domains. The curve fit can be updated as each encoder-measured head position is collected. To map a time associated with an eddy current and/or light intensity measurement, the computer inputs the measurement time and the head sweep frequency into Equation 1. The head sweep frequency ωw, head position offset X0 and head sweep ΔX can be taken from the polishing recipe.
The foregoing algorithm assumes constant ωv and ωp during each sweep of the optical monitoring system relative to the substrate. The correction term, C, is optionally included to correct for offsets between the wafer position calculated based on the head sweep frequency, ωw, and the head position as determined from a position encoder coupled to the polishing head. (The later measures and indicates the measured position of the wafer center along the x-axis described above in reference to
For example, each time a new head position measurement is obtained from the encoder, the correction term C can be updated. For example, the correction value Ci for calculations of the head position measurement x″(t) after time ti can be calculated as
where M(ti-1) is the most recent encoder-measured head position, and x″(ti-1) is the head position as calculated using the previous version of x″(t) (i.e., using Ci-1) at time ti-1.
The correction term, C, can have other functional dependences on x″(t) and/or M(t), for example, C can depend on the ratio of these values or functions of these values. The correction term can depend on higher order derivatives of x″(t) or on derivatives of M(t). The function form of the correction term can be determined empirically or theoretically.
In one implementation, the system accounts for a processing delay that causes an error in the time that is attributed to each encoder-measured head position separately from the curve fitting correction term C. Specifically, the processing delay causes the attributed time to include a lag, and the actual time of measurement occurs earlier than the attributed time. To correct for this lag, a phase correction, φ, is defined so that the above described function for calculating head position is phase shifted to the left to accommodate the lag, i.e.,
x″(t)=X0−ΔX cos(ωwt+C+φ)
Note that, instead of phase shifting the function, the time inputted into above described function of Equation 1 to calculate head position can be adjusted to account for the lag. In this case, the computer calculates head position for measurement at time ti as a function of (ti+Δt). As described above, the lag can be determined empirically. Specifically, the value of the correction term (φ or Δt) is adjusted until a trace in the time domain correctly indicates the edge position. For example, given a 300 mm wafer, the trace should have one edge at the −150 mm position and another at the +150 mm position (assuming the coordinate system of
The computer can further reduce inaccuracies in the position data by identifying reflection measurements associated with the edge of the substrate, and rescaling the calculated positions based on the known size of the substrate. For example, for a 300 mm wafer, the two edge measurements are associated with the 150 mm radial position. Similarly, for a 200 mm diameter wafer, the two edge measurements are associated with the 100 mm radial position. The computer compares the calculated positions for measurements corresponding to the substrate edge and scales each of the calculated intermediate positions proportionally so that the edge measurements correspond to the substrate's known radius. Thus, each scaled radial measurement r′(t) for a measurement taken at time t can be calculated as r′(t)=r(t)*[R/r(Tedge)], where R is the substrate radius and Tedge is the time of one of the edge measurements, e.g., the closer edge.
Alternatively, the computer can apply techniques other than the above described one to scale the calculated positions. For example, the computer can calculate a length delimited by the first and last calculated positions and a length delimited by the two reflection measurements associated with the substrate edges. The computer can the scale the calculated positions according to a ratio of the two lengths.
In order to identify the reflection measurements associated with the edge of the substrate, the computer looks at the variation in detected intensity for adjacent measurements. Typically, the reflection measurements from the substrate edge correspond to two sudden changes in the intensity where the light beam transitions from to reflecting from the retaining ring of the carrier head to reflecting from the substrate. For oxide polishing, for example, because the retaining ring surface is typically highly reflective, the reflections from the retaining ring correspond to the highest intensity reflection measurements. Thus, the initial sudden transition from a high intensity to a low intensity should indicate the leading edge of the substrate, whereas the later sudden transition from a low intensity to a high intensity should indicate the trailing edge of the substrate. Of course, the reverse may be true (particularly for metal polishing), as the relative reflectivity of the retaining ring and substrate depend on their material properties and the polishing process. Measurements of intermediate reflectance acquired between the retaining ring measurements correspond to the substrate surface.
In some embodiments, the intensity of light reflected from the retaining ring is more than about 20% greater than that reflected from the substrate (e.g., more than about 30%, such as about 40% or more). Based on the intensity change from the retaining ring to the wafer surface, a user can define a threshold intensity or intensity ratio to allow the system to identify measurements corresponding to the edge of the wafer. This threshold and/or intensity ratio can be adjusted to account for detector sensitivity, light source intensity, signal to noise ratio, etc.
The above described scaling technique can also be implemented by using measurements from eddy current sensors. Specifically, the eddy current sensors can detect the presence of a retaining ring, which usually includes a metal backing ring. As the substrate is held inside the inner diameter of the retaining ring, the computer can use retaining ring edge information to identify substrate edges and scale calculated positions as described above.
More generally, the scaling technique can be performed based on a determination of the substrate edge using the same sensor that generated the data being scaled, or based on a determination of the substrate edge using a different sensor from the sensor that generated the data being scaled. Moreover, the scaling technique is applicable to both oxide polishing and conductive polishing, e.g., data from either an optical sensor or an eddy current sensor can be scaled. In particular, for oxide polishing, the eddy current sensor can be used to find the retaining ring edge, and the optical data could be scaled accordingly. On the other hand, where there is a sharp difference in reflectivity between the substrate and retaining ring (e.g., typical for metal polishing, but also possible for oxide polishing), the optical system can be used to find the wafer edge by detecting the retaining ring edge.
Using the eddy current sensor to identify eddy sensor measurements associated with substrate edges can avoid problems typically present when using an optical sensor. One problem, for example, is that the optical sensor is typically not situated at the exact same spatial position as is the eddy current sensor. The eddy current measurement consequently is taken at a position on the substrate that does not exactly correspond to the position measured by the optical sensor, and there is thus an in inherent systematic error in the computer's calculation. Furthermore, the difference between the two sensors can vary from one in-situ monitoring module to another.
The foregoing paragraphs describe one algorithm for mapping time domain measurements to the position domain. Other mapping algorithms can also be used. For example, in some embodiments, a linear mapping can be used to transform the time domain measurements to position domain. In a linear mapping algorithm, to associate the remaining measurements the computer can simply assume a linear relationship between the time domain and the position domain. Thus, the position P(t) can be calculated as a linear interpolation
where D is the substrate diameter, t is the time of the particular measurement, T1 is the measurement time for the initial edge and T2 is the measurement time for the trailing edge.
Each measurement by the monitoring systems covers an associated sampling zone on the substrate. Due to focusing the light beam of the monitoring system to reduce its spot size on the surface of substrate 10, the size of the sampling zones is reduced compared to a substantially similar system that does not focus the light beam. The size of the sampling zone is the distance the beam traverses along the beam path direction during the acquisition of one reflection measurement data point. The reduction in sampling zone size provides a corresponding increase in resolution in the reflection measurements made by the system using the optical monitoring system. Improved resolution may be particularly advantageous in embodiments where the optical measurements are used to identify the position of the wafer edges in a scan because, e.g., the portion of the substrate surface probed by the eddy current sensor can be determined to greater accuracy using the time domain to position domain conversion described above.
In addition to beam spot size on the substrate surface, sampling zone size depends on the acquisition rate of the detector and the rotational velocity of the platen. In embodiments, the sampling zone size may be less than about two millimeters in length (e.g., less than about one millimeter, 0.5 millimeters, 0.2 millimeters). The data acquisition rate for the optical monitoring system and/or eddy current sensor can be greater than 500 Hz (e.g., greater than about 1,000 Hz, such as up to 5,000 Hz). In general, for a light beam of constant intensity, and where the reflectance of the substrate surface does not dramatically change, the detector signal will be reduced at higher acquisition rates. The detector signal is reduced due to the corresponding reduction of detector integration time at these higher acquisition rates, which leads to reduced detected intensity for each data point. Thus, in order for the optical monitoring system to acquire data at higher acquisition rates, more sensitive detectors or more intense light sources may be used. In some embodiments, the data acquisition rate can be a variable parameter that can be selected by a user of the CMP apparatus. In such cases, the sensitivity of the detector and/or intensity of the light source may be adjustable parameters as well in order to accommodate varying acquisition rates. In such implementations, these parameters can be adjusted by the system operator, or can be adjusted based on a feedback signal derived from, e.g., the detector signal.
Computer 90 may also be connected to the pressure mechanisms that control the pressure applied by carrier head 70, to carrier head rotation motor 76 to control the carrier head rotation rate, to the platen rotation motor (not shown) to control the platen rotation rate, or to slurry distribution system 39 to control the slurry composition supplied to the polishing pad. Specifically, after sorting the measurements into radial ranges, information on the metal film thickness can be fed in real-time into a closed-loop controller to periodically or continuously modify the polishing pressure profile applied by a carrier head, as discussed in U.S. patent application Ser. No. 09/609,426, filed Jul. 5, 2000, the entirety of which is incorporated herein by reference. For example, the computer could determine that the endpoint criteria have been satisfied for the outer radial ranges but not for the inner radial ranges. This would indicate that the underlying layer has been exposed in an annular outer area but not in an inner area of the substrate. In this case, the computer could reduce the diameter of the area in which pressure is applied so that pressure is applied only to the inner area of the substrate, thereby reducing dishing and erosion on the outer area of the substrate.
The eddy current and optical monitoring systems can be used in a variety of polishing systems. Either the polishing pad, or the carrier head, or both can move to provide relative motion between the polishing surface and the substrate. The polishing pad can be a circular (or some other shape) pad secured to the platen, a tape extending between supply and take-up rollers, or a continuous belt. The polishing pad can be affixed on a platen, incrementally advanced over a platen between polishing operations, or driven continuously over the platen during polishing. The pad can be secured to the platen during polishing, or there could be a fluid bearing between the platen and polishing pad during polishing. The polishing pad can be a standard (e.g., polyurethane with or without fillers) rough pad, a soft pad, or a fixed-abrasive pad. Rather than tuning when the substrate is absent, the drive frequency of the oscillator can be tuned to a resonant frequency with a polished or unpolished substrate present (with or without the carrier head), or to some other reference.
Although illustrated as positioned in the same hole, optical monitoring system 140 could be positioned at a different location on the platen than eddy current monitoring system 40. For example, optical monitoring system 140 and eddy current monitoring system 40 could be positioned on opposite sides of the platen, so that they alternately scan the substrate surface.
Various aspects of the invention, such as placement of the coil on a side of the polishing surface opposite the substrate or the measurement of a phase difference, still apply if the eddy current sensor uses a single coil. In a single coil system, both the oscillator and the sense capacitor (and other sensor circuitry) are connected to the same coil.
Although in the foregoing embodiment the optical monitoring system is used in conjunction with an eddy current sensor, the optical monitoring can also be used with other non-optical monitoring systems, such as, e.g., thermal sensors, electric sensors, pressure sensors.
The present invention has been described in terms of a preferred embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the appended claims.
This application claims priority to U.S. Application Ser. No. 60/496,311, filed on Aug. 18, 2003, the entire disclosure of which is incorporated herein by reference.
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