COMPENSATION FOR SUBSTRATE DOPING IN EDGE RECONSTRUCTION FOR IN-SITU ELECTROMAGNETIC INDUCTIVE MONITORING

A method of compensating for a contribution of conductivity of the semiconductor wafer to a measured trace by an in-situ electromagnetic induction monitoring system includes storing or generating a modified reference trace. The modified reference trace represents measurements of a bare doped reference semiconductor wafer by an in-situ electromagnetic induction monitoring system as modified by a neutral network. The substrate is monitored with an in-situ electromagnetic induction monitoring system to generate a measured trace that depends on a thickness of the conductive layer, and at least a portion of the measured trace is applied to a neural network to generate a modified measured trace. An adjusted trace is generated, including subtracting the modified reference trace from the modified measured trace.

TECHNICAL FIELD

The present disclosure relates to chemical mechanical polishing, and more specifically to monitoring of a conductive layer during chemical mechanical polishing.

BACKGROUND

An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon wafer. A variety of fabrication processes require planarization of a layer on the substrate. For example, one fabrication step involves depositing a filler layer over a non-planar surface and planarizing the filler layer. For certain applications, the filler layer is planarized until the top surface of a patterned layer is exposed. For example, a metal layer can be deposited on a patterned insulative layer to fill the trenches and holes in the insulative layer. After planarization, the remaining portions of the metal in the trenches and holes of the patterned layer form vias, plugs, and lines to provide conductive paths between thin film circuits on the substrate.

Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier head. The exposed surface of the substrate is typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. Polishing slurry with abrasive particles is typically 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. Variations in the slurry composition, the polishing pad condition, the relative speed between the polishing pad and the substrate, the initial thickness of the substrate layer, 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, determining the polishing endpoint merely as a function of polishing time can lead to non-uniformity within a wafer or from wafer to wafer.

In some systems, a substrate is monitored in-situ during polishing, e.g., through the polishing pad. One monitoring technique is to induce an eddy current in the conductive layer and detect the change in the eddy current as the conductive layer is removed.

SUMMARY

In one aspect, a method of compensating for a contribution of conductivity of the semiconductor wafer to a measured trace by an in-situ electromagnetic induction monitoring system includes storing or generating a modified reference trace representing measurements of a bare doped reference semiconductor wafer by an in-situ electromagnetic induction monitoring system as modified by a neutral network, monitoring the substrate with an in-situ electromagnetic induction monitoring system as the conductive layer to generate a measured trace that depends on a thickness of the conductive layer, applying at least a portion of the measured trace to a neural network to generate a modified measured trace, and generating an adjusted trace, including subtracting the modified reference trace from the modified measured trace.

In one aspect, a method of polishing a substrate includes storing or generating a modified reference trace representing measurements of a bare doped reference semiconductor wafer by an in-situ electromagnetic induction monitoring system as modified by a neutral network, bringing a substrate having a conductive layer disposed over a semiconductor wafer into contact with a polishing pad, generating relative motion between the substrate and the polishing pad, monitoring the substrate with the in-situ electromagnetic induction monitoring system as the conductive layer is polished to generate a measured trace that depends on a thickness of the conductive layer, applying at least a portion of the measured trace to a neural network to generate a modified measured trace, generating an adjusted trace to at least partially compensate for a contribution of conductivity of the semiconductor wafer to the measured trace including subtracting the modified reference trace from the modified measured trace, and at least one of halting polishing or modifying a polishing parameter based on the adjusted trace.

Each of these aspects may also be applied as a computer program product, tangibly encoded on a computer readable media including instructions to cause a computer system to carry out appropriate operations (e.g., storing or generating the modified reference trace, applying the measured trace, and generating the adjusted trace), or as a polishing system including a controller configured to carry out appropriate operations.

Implementations of the methods, the computer program products, and/or the systems may include one or more of the following features.

The modified reference trace may include a sequence of equivalent thickness values, and the modified measured trace may include a sequence of actual thickness values. At least a portion of an initial reference trace may be applied to the neural network to generate the modified reference trace. Raw signal values in a preliminary reference trace may be converted to thickness values to generate the initial reference trace. User input may be received selecting a reference trace from a plurality of reference traces. Generating the modified reference trace may include scanning a sensor of an in-situ electromagnetic induction monitoring system across the bare doped reference semiconductor wafer.

Generating the adjusted trace may include scaling a difference between the modified reference trace and the modified measured trace. The adjusted trace A(x) may be calculated such that A(x)=(T(x)−S(x)−b)/k where T(x) is the modified measured trace, S(x) is the modified reference trace, and b and k are constants. The constants b and k according to a configuration of the sensor of the in-situ monitoring system.

The at least a portion of the measured trace applied to the neural network may include a portion corresponding to an edge region of the substrate. The at least a portion of the measured trace applied to the neural network need not includes a portion corresponding to a central region of the substrate. The neural network may be trained with a plurality of training traces representing measurements of one or more training substrates having a conductive layer on an undoped semiconductor wafer with different training traces corresponding to different thickness of the conductive layer and different edge profiles.

Implementations may include one or more of the following advantages. During monitoring of processing, e.g., polishing, of a substrate, possible inaccuracy of the correlation between a measured eddy current signal and a conductive layer thickness caused by doping of an underlying semiconductor wafer can be mitigated, particularly at the edge of the substrate. An adjusted eddy current signal or an adjusted conductive layer thickness using the compensating processes can be more accurate. The system can compensate for distortions in a portion of the signal that corresponds to the substrate edge. The adjusted eddy current signal and/or the adjusted conductive layer can be used for determining control parameters during a polishing process and/or determining an endpoint for the polishing process. Reliability of the control parameter determination and endpoint detection can be improved, wafer under-polish can be avoided, and within-wafer non-uniformity can be reduced.

DETAILED DESCRIPTION

One monitoring technique for a polishing operation is to induce currents in a conductive layer on a substrate. The induced currents can be measured by an inductive monitoring system in-situ during polishing to generate a signal. Assuming the outermost layer undergoing polishing is a conductive layer, then the signal from the sensor should be dependent on the thickness of the conductive layer. Based on the monitoring, control parameters for the polishing can be adjusted, e.g., so that the locations of the layer are substantially the same thickness after polishing or so that polishing of the locations of the layer completes at about the same time. Such profile control can be referred to as real time profile control (RTPC). In addition, the polishing operation can terminate based on an indication that the monitored thickness has reached a desired endpoint thickness.

An in-situ monitoring system can be subject to signal distortion for measurements at locations close to the substrate edge. For example, the inductive monitoring system can generate a magnetic field. Near the substrate edge, the signal can be artificially low because the magnetic field only partially overlaps the conductive layer of the substrate. Various techniques can be used to compensate for distortions. For example, the signal can be fed into an artificial neural network to generate a modified signal.

In practice, the magnetic field generated by the eddy current sensor does not stop within the conductive layer, but can extend into the underlying substrate. Without being limited to any particular theory, the skin depth in these magnetic permeable materials for the electromagnetic frequency employed in eddy current sensor can be larger than thickness of the conductive layer and the underlying semiconductor wafer. As a result, the signal generated by the eddy current sensor can depend on the conductivity of the semiconductor wafer.

If the semiconductor wafer is not doped, e.g., as typically used in “blank” wafers used for system calibration and basic substrate wafers, the electrical resistance of the wafer can be sufficiently high that the presence of the wafer does not have detectable influence on the eddy current signal. However, for actual device fabrication the wafers will typically be doped, e.g., highly doped, for various purposes. In this situation, the signal generated by the eddy current sensor can have significant contribution from the wafer, depending on the conductivity of the semiconductor wafer. As such, thickness measurement based on signals captured by the eddy current sensor can be inaccurate. Techniques can be used to compensate for this inaccuracy, e.g., by taking into account the contribution to the signal from the semiconductor wafer. However, such compensations can introduce additional errors at the substrate edge when edge reconstruction techniques are utilized.

However, a trace from the substrate and a trace from a doped wafer can be run separately through the edge reconstruction algorithm. The modified doped wafer trace can be subtracted from the modified measured substrate trace; the resulting difference will be closer to the actual thickness of the layer being polished. In addition, the difference can be scaled to compensate for sensor configurations.

FIGS. 1 and 2illustrate an example of a polishing station20of a chemical mechanical polishing system. The polishing station20includes a rotatable disk-shaped platen24on which a polishing pad30is situated. The platen24is operable to rotate about an axis25. For example, a motor22can turn a drive shaft28to rotate the platen24. The polishing pad30can be a two-layer polishing pad with an outer polishing layer34and a softer backing layer32.

The polishing station20can include a supply port or a combined supply-rinse arm39to dispense a polishing liquid38, such as an abrasive slurry, onto the polishing pad30. The polishing station20can include a pad conditioner apparatus with a conditioning disk to maintain the surface roughness of the polishing pad.

A carrier head70is operable to hold a substrate10against the polishing pad30. The carrier head70is suspended from a support structure72, e.g., a carousel or a track, and is connected by a drive shaft74to a carrier head rotation motor76so that the carrier head can rotate about an axis71. Optionally, the carrier head70can oscillate laterally, e.g., on sliders on the carousel, by movement along the track, or by rotational oscillation of the carousel itself.

The carrier head70can include a retaining ring84to hold the substrate. In some implementations, the retaining ring84may include a highly conductive portion, e.g., the carrier ring can include a thin lower plastic portion86that contacts the polishing pad, and a thick upper conductive portion88. In some implementations, the highly conductive portion is a metal, e.g., the same metal as the layer being polished, e.g., copper.

In operation, the platen is rotated about its central axis25, and the carrier head is rotated about its central axis71and translated laterally across the top surface of the polishing pad30. Where there are multiple carrier heads, each carrier head70can have independent control of its polishing parameters, for example each carrier head can independently control the pressure applied to each respective substrate.

The carrier head70can include a flexible membrane80having a substrate mounting surface to contact the back side of the substrate10, and a plurality of pressurizable chambers82to apply different pressures to different zones, e.g., different radial zones, on the substrate10.

In some implementations, the polishing station20includes a temperature sensor64to monitor a temperature in the polishing station or a component of/in the polishing station. Although illustrated inFIG. 1as positioned to monitor the temperature of the polishing pad30and/or slurry38on the pad30, the temperature sensor64could be positioned inside the carrier head70to measure the temperature of the substrate10. The temperature sensor64can be in direct contact (i.e., a contacting sensor) with the polishing pad or the outermost layer of the substrate10, which can be a conductive layer, to accurately monitor the temperature of the polishing pad or the outmost layer of the substrate. The temperature sensor can also be a non-contacting sensor (e.g., an infrared sensor). In some implementations, multiple temperature sensors are included in the polishing station22, e.g., to measure temperatures of different components of/in the polishing station. The temperature(s) can be measured in real time, e.g., periodically and/or in association with the real-time measurements made by the eddy current system. The monitored temperature(s) can be used in adjusting the eddy current measurements in-situ.

Referring toFIG. 3A, the polishing system can be used to polish a substrate10that includes a conductive material overlying and/or inlaid in a patterned dielectric layer. For example, the substrate10can include a layer of conductive material16, e.g., a metal, e.g., copper, aluminum, cobalt or titanium, that overlies and fills trenches in a dielectric layer14, e.g., silicon oxide or a high-k dielectric. Optionally a barrier layer18, e.g., tantalum or tantalum nitride, can line the trenches and separate the conductive material16from the dielectric layer14. The conductive material16in the trenches can provide vias, pads and/or interconnects in a completed integrated circuit. Although the dielectric layer14is illustrated as deposited directly on a semiconductor wafer12, one or more other layers can be interposed between the dielectric layer14and the wafer12.

The semiconductor wafer12can be a silicon wafer, e.g., single crystalline silicon, although other semiconductor materials are possible. In addition, the semiconductor wafer12can be doped, e.g., with p-type or n-type doping. The doping can be uniform laterally across the wafer, or the wafer can be selectively doped, e.g., as appropriate for fabrication of transistors in integrated circuits using the semiconductor wafer.

Initially, the conductive material16overlies the entire dielectric layer14. As polishing progresses, the bulk of the conductive material16is removed, exposing the barrier layer18(seeFIG. 3B). Continued polishing then exposes the patterned top surface of the dielectric layer14(seeFIG. 3C). Additional polishing can then be used to control the depth of the trenches that contain the conductive material16.

In some implementations, a polishing system includes additional polishing stations. For example, a polishing system can include two or three polishing stations. For example, the polishing system can include a first polishing station with a first electromagnetic induction monitoring system and a second polishing station with a second electromagnetic induction current monitoring system.

For example, in operation, bulk polishing of the conductive layer on the substrate can be performed at the first polishing station, and polishing can be halted when a target thickness of the conductive layer remains on the substrate. The substrate is then transferred to the second polishing station, and the substrate can be polished until an underlying layer, e.g., a patterned dielectric layer.

Returning toFIG. 1, the polishing system includes an in-situ electromagnetic induction monitoring system100which can be coupled to or be considered to include a controller90. A rotary coupler29can be used to electrically connect components in the rotatable platen24, e.g., the sensors of the in-situ monitoring systems, to components outside the platen, e.g., drive and sense circuitry or the controller90.

The in-situ electromagnetic induction monitoring system100is configured to generate a signal that depends on a depth of the conductive material16, e.g., the metal. The electromagnetic induction monitoring system can operate either by generation of eddy currents in the sheet of conductive material that overlies the dielectric layer, or generation of current in a conductive loop formed in a trench in the dielectric layer on the substrate.

As an eddy current monitoring system, the electromagnetic induction monitoring system100can be used to monitor the thickness of a conductive layer by inducing eddy currents in the conductive sheet. Alternatively, as an inductive monitoring system, the electromagnetic induction monitoring system can operate by inductively generating a current in a conductive loop formed in the dielectric layer14of the substrate10for the purpose of monitoring, e.g., as described in U.S. Patent Publication No. 2015-0371907.

In operation, the polishing system can use the in-situ monitoring system100to determine when the conductive layer has reached a target thickness, e.g., a target depth for metal in a trench or a target thickness for a metal layer overlying the dielectric layer, and then halts polishing. Alternatively or in addition, the polishing system can use the in-situ monitoring system100to determine differences in thickness of the conductive material16across the substrate10, and use this information to adjust the pressure in one or more chambers82in the carrier head80during polishing in order to reduce polishing non-uniformity.

A recess26can be formed in the platen24, and optionally a thin section36can be formed in the polishing pad30overlying the recess26. The recess26and thin section36can be positioned such that regardless of the translational position of the carrier head they pass beneath substrate10during a portion of the platen rotation. Assuming that the polishing pad30is a two-layer pad, the thin section36can be constructed by removing a portion of the backing layer32, and optionally by forming a recess in the bottom of the polishing layer34. The thin section can optionally be optically transmissive, e.g., if an in-situ optical monitoring system is integrated into the platen24.

The in-situ monitoring system100can include a sensor102installed in the recess26. The sensor102can include a magnetic core104positioned at least partially in the recess26, and at least one coil106wound around a portion of the core104. The drive and sense circuitry108is electrically connected to the coil106. The drive and sense circuitry108generates a signal that can be sent to the controller90. Although illustrated as outside the platen24, some or all of the drive and sense circuitry108can be installed in the platen24.

Referring toFIGS. 1 and 4, the drive and sense circuitry108applies an AC current to the coil106, which generates a magnetic field150between two poles152aand152bof the core104. AlthoughFIG. 4illustrates a C-shaped core, other cores are possible, e.g., E-shaped, I-shaped, etc. In operation, when the substrate10intermittently overlies the sensor102, a portion of the magnetic field150extends into the substrate10.

The circuitry108can include a capacitor connected in parallel with the coil106. Together the coil106and the capacitor can form an LC resonant tank.

If monitoring of the thickness of a conductive layer on the substrate is desired, then when the magnetic field150reaches the conductive layer, the magnetic field150can pass through and generate a current (if the target is a loop) or create an eddy-current (if the target is a sheet). This modifies the effective impedance of the LC circuit.

However, the magnetic field150can also penetrate into the semiconductor substrate12. As such, the effective impedance of the LC circuit, and thus the signal from the drive and sense circuitry108, can also depend on the doping and resultant conductivity of the semiconductor substrate12.

The drive and sense circuitry108can include a marginal oscillator coupled to a combined drive/sense coil106, and the output signal can be a current required to maintain the peak to peak amplitude of the sinusoidal oscillation at a constant value, e.g., as described in U.S. Pat. No. 7,112,960. Other configurations are possible for the drive and sense circuitry108. For example, separate drive and sense coils could be wound around the core. The drive and sense circuitry108can apply current at a fixed frequency, and the signal from the drive and sense circuitry108can be the phase shift of the current in the sense coil relative to the drive coil, or an amplitude of the sensed current, e.g., as described in U.S. Pat. No. 6,975,107.

Referring toFIG. 2, as the platen24rotates, the sensor102sweeps below the substrate10. By sampling the signal from the circuitry108at a particular frequency, the circuitry108generates measurements at a sequence of sampling zones94across the substrate10. For each sweep, measurements at one or more of the sampling zones94can be selected or combined. Thus, over multiple sweeps, the selected or combined measurements provide the time-varying sequence of values.

The polishing station20can also include a position sensor96, such as an optical interrupter, to sense when the sensor102is underneath the substrate10and when the sensor102is off the substrate. For example, the position sensor96can be mounted at a fixed location opposite the carrier head70. A flag98can be attached to the periphery of the platen24. The point of attachment and length of the flag98is selected so that it can signal the position sensor96when the sensor102sweeps underneath the substrate10.

Alternately or in addition, the polishing station20can include an encoder to determine the angular position of the platen24.

Returning toFIG. 1, a controller90, e.g., a general purpose programmable digital computer, receives the signals from sensor102of the in-situ monitoring system100. Since the sensor102sweeps beneath the substrate10with each rotation of the platen24, information on the depth of the conductive layer, e.g., the bulk layer or conductive material in the trenches, is accumulated in-situ (once per platen rotation). The controller90can be programmed to sample measurements from the in-situ monitoring system100when the substrate10generally overlies the sensor102.

In addition, the controller90can be programmed to calculate the radial position of each measurement, and to sort the measurements into radial ranges. By arranging the measurements into radial ranges, the data on the conductive film thickness of each radial range can be fed into a controller (e.g., the controller90) to adjust the polishing pressure profile applied by a carrier head. The controller90can also be programmed to apply endpoint detection logic to the sequence of measurements generated by the in-situ monitoring system100signals and detect a polishing endpoint.

Since the sensor102sweeps underneath the substrate10with each rotation of the platen24, information on the conductive layer thickness is being accumulated in-situ and on a continuous real-time basis. During polishing, the measurements from the sensor102can be displayed on an output device to permit an operator of the polishing station to visually monitor the progress of the polishing operation.

Referring toFIGS. 2 and 5, changes in the position of the sensor head with respect to the substrate10can result in a change in the signal from the in-situ monitoring system100. That is, as the sensor head scans across the substrate10, the in-situ monitoring system100will make measurements for multiple regions94, e.g., measurement spots211, at different locations on the substrate10. The regions94can be partially overlapping.

FIG. 6illustrates a graph that shows a signal220from the in-situ monitoring system100during a single pass of the sensor102below the substrate10. This signal220can be termed a “trace” across the substrate. The signal220is composed of a series of individual measurements from the sensor head as it sweeps below the substrate. The graph can be a function of measurement time or of position, e.g., radial position, of the measurement on the substrate. In either case, different portions of the signal220correspond to measurement spots211at different locations on the substrate10scanned by the sensor102. Thus, the graph depicts, for a given location of the substrate scanned by the sensor head, a corresponding measured signal value from the signal220.

Referring toFIGS. 5 and 6, the signal220includes a first portion222that corresponds to locations in an edge region203of the substrate10when the sensor102crosses a leading edge of the substrate10, a second portion224that corresponds to locations in a central region201of the substrate10, and a third portion226that corresponds to locations in edge region203when the sensor102crosses a trailing edge of the substrate10. The signal can also include portions228that correspond to off-substrate measurements, i.e., signals generated when the sensor head scans areas beyond the edge204of the substrate10inFIG. 5.

The edge region203can correspond to a portion of the substrate where measurement spots211of the sensor head overlap the substrate edge204. The central region201can include an annular anchor region202that is adjacent the edge region203, and an inner region205that is surrounded by the anchor region202. The sensor head may scan these regions on its path210and generate a sequence of measurements that correspond to a sequence of locations along the path210.

In the first portion222, the signal intensity ramps up from an initial intensity (typically the signal resulting when no substrate and no carrier head is present) to a higher intensity. This is caused by the transition of the monitoring location from initially only slightly overlapping the substrate at the edge204of the substrate (generating the initial lower values) to the monitoring location nearly entirely overlapping the substrate (generating the higher values). Similarly, in the third portion226, the signal intensity ramps down when the monitoring location transitions to the edge204of the substrate.

Although the second portion224is illustrated as flat, this is for simplicity, and a real signal in the second portion224would likely include fluctuations due both to noise and to variations in the layer thickness. The second portion234corresponds to the monitoring location scanning the central region201. The second portion224includes two sub-portions230and232that are caused by the monitoring location scanning the anchor region202of the central region201, and sub-portion234that is caused by the monitoring location scanning the inner region205of the central region201.

As noted above, the variation in the signal intensity in the regions222,226is caused in part by measurement region of the sensor106overlapping the substrate edge, rather than an intrinsic variation in the thickness or conductivity of the layer being monitored. Consequently, this distortion in the signal220can cause errors in the calculating of a characterizing value for the substrate, e.g., the thickness of the layer, near the substrate edge. To address this problem, the controller90can include a neural network, e.g., neural network300ofFIG. 7, to generate a modified signal corresponding to one or more locations of the substrate10based on the measured signals corresponding to those locations.

Referring now toFIG. 7, the neural network300is configured to, when trained appropriately, generate modified signals that reduce and/or remove the distortion of computed signal values near the substrate edge. The neural network300receives a group of inputs304and processes the inputs304through one or more neural network layers to generate a group of outputs350. The layers of the neural network300include an input layer310, an output layer330, and one or more hidden layers320.

Each layer of the neural network300includes one or more neural network nodes. Each neural network node in a neural network layer receives one or more node input values (from the inputs304to the neural network300or from the output of one or more nodes of a preceding neural network layer), processes the node input values in accordance with one or more parameter values to generate an activation value, and optionally applies a non-linear transformation function (e.g., a sigmoid or tan h function) to the activation value to generate an output for the neural network node.

Each node in the input layer310receives as a node input value one of the inputs304to the neural network300.

The inputs304to the neural network include measured signal values from the in-situ monitoring system100for multiple different spots211on the substrate10, such as a first measured signal value301, a second measured signal value302, through an nth measured signal value303. The measured signal values can be individual values of the sequence of values in the signal220.

In general, the multiple different locations include locations in the edge region203and, optionally, the anchor region202of the substrate10. In some implementations, the multiple different locations are only in the edge region203and the anchor region202. In other implementations, the multiple different locations span all regions of the substrate.

These measured signal values are received at signal input nodes344. Optionally, the input nodes304of the neural network300can also include one or more state input nodes316that receive one or more process state signals304, e.g., a measure of wear of the pad30of the polishing apparatus20.

The nodes of the hidden layers320and output layer330are illustrated as receiving inputs from every node of a preceding layer. This is the case in a fully-connected, feedforward neural network. However, the neural network300may be a non-fully-connected feedforward neural network or a non-feedforward neural network. Moreover, the neural network300may include at least one of one or more fully-connected, feedforward layers; one or more non-fully-connected feedforward layers; and one or more non-feedforward layers.

The neural network generates a group of modified signal values350at the nodes of the output layer330, i.e., “output nodes”350. In some implementations, there is an output node350for each measured signal from the in-situ monitoring system that is fed to the neural network300. In this case, the number of output nodes350can correspond to the number of signal input nodes304of the input layer310.

For example, the number of signal input nodes344can equal the number of measurements in the edge region203and the anchor region202, and there can be an equal number of output nodes350. Thus, each output node350generates a modified signal that corresponds to a respective measured signal supplied as an input to a signal input node344, e.g., the first modified signal351for the first measured signal301, the second modified signal352for the second measured signal302, and the nth modified signal353for the nth measured signal303.

In some implementations, the number of output nodes350is smaller than the number of input nodes304. In some implementations, the number of output nodes350is smaller than the number of signal input nodes344. For example, the number of signal input nodes344can equal the number of measurements in the edge region203, or equal to the number of measurements in the edge region203and anchor region202. Again, each output node350of the output layer330generates a modified signal that corresponds to a respective measured signal supplied as a signal input node304, e.g., the first modified signal351for the first measured signal301, but only for the signal input nodes354that receive signals from the edge region203.

The polishing apparatus100can use the neural network300to generate modified signals. The modified signals can then be used to determine a thickness for each location in a first group of locations of a substrate, e.g., the locations in the edge region (and possibly the anchor region). For example, referring back toFIG. 6, the modified signal values for the edge region can provide a modified portion230of the signal220.

In some implementations, for a modified signal value that corresponds to a given measurement location, the neural network500can be configured such that only input signal values from measurement locations within a predetermined distance of that given location are used in determining the modified signal value.

To train the neural network, the sensor102of the in-situ monitoring system100can be used to generate a profiles of reference substrates. In addition, ground truth measures of thickness of the reference substrates can be obtained; these measurements can be performed for locations that are to be processed by the neural network. The system can generate the ground truth measures of thickness using an electrical impedance measuring method, such as a four-points probe method. The signal values from the reference substrate are applied to the inputs304while the ground truth measurements are applied to the outputs350and the system is run in a training mode, e.g., gradient descent with backpropagation.

The reference substrates can include blank undoped wafers on which a uniform thickness of a conductive material is deposited. The amount of conductive material can be selected to simulate the presence of a doped wafer.

The reference substrates can also include sample device substrates at an equivalent stage of processing as the device substrate for which the in-situ monitoring system is to be used for controlling of polishing, e.g., substrates with layers having different edge profiles.

As noted above, the signal generated by the in-situ monitoring system also includes the contribution from the doped wafer. If not handled properly, attempts to compensate for the contribution to the signal from the doped wafer can introduce additional errors, e.g., at the substrate edge when edge reconstruction techniques are utilized.

Referring toFIG. 8, a reference trace420across a blank doped wafer is generated. This reference trace420is generated prior to polishing of the substrate. The blank doped wafer has the same doping profile as the wafers to be used in the device substrate to be polished. In some implementations, the reference trace is generated by scanning a sample blank doped wafer, e.g., a sacrificial wafer, with the sensor102of the in-situ monitoring system100. For example, the reference trace could be generated by fab operator. Alternatively, the system manufacturer could generate reference traces for wafers having a variety of different dopings (e.g., concentrations and/or doping materials), and these traces can be stored in a library. The operator can then select one of the references traces from the library, e.g., from a drop-down menu or similar user interface, that corresponds most closely to the doping of the wafer in the device substrates to be polished.

The raw signal values in the reference trace420from the sensor102can be converted to thickness values (represented by reference trace420′) using a correlation curve.

FIG. 9shows a correlation curve510, for a given resistivity, between the thickness of a conductive layer of the given resistivity and the signal from the electromagnetic induction monitoring system100. DSTARTrepresents the initial thickness of the conductive layer, SSTARTis the desired signal value corresponding to the initial thickness DSTART; DFINALrepresents the final thickness of the conductive layer, and SFINALis the desired signal value correspond to the final thickness; and K is a constant representing a value of the signal for zero conductive layer thickness.

The relationship curve510can be represented in the controller90by a function, e.g., a polynomial function, e.g., a second order function, a third order function, or a higher order function. The correlation between the signal X(x) and the thickness D(x) can be represented by the equation:

where W1, W2, and W3are real number coefficients. Thus, the controller can store the values of the coefficients of the function, e.g., W1, W2, and W3, as well as the resistivity ρ0for which the relationship curve510applies. In addition, the relationship could be represented with a linear function, a Bezier curve, or a non-polynomial function, e.g., exponential or logarithmic.

The relationship curve510can be used to convert the signal values in the raw signal420from the reference wafer to “equivalent” thickness measurements. That is, although there is no conductive layer on top of the doped reference wafer, the measurement can be represented as a thickness values. These are “equivalent” thickness values because each is a thickness of an equivalent conductive layer on an undoped wafer that would generate the same signal as the doped reference wafer.

Returning toFIG. 8, the reference trace420′ is then processed by the neural network as if it were a normal signal to perform the edge reconstruction algorithm on the reference trace. This generates a modified reference trace450with a portion having modified signal values430.

In some implementations, the conversion to thickness is performed in advance, and what is stored in the library (and selected by the operator) is the reference trace420′ with thickness values. In some implementations, the thickness conversion and edge reconstruction are performed in advance, and what is stored in the library (and selected by the operator) is a modified reference trace450.

During the polishing operation, the substrate10is monitored by the in-situ monitoring system, and the measured trace220for the substrate10is generated for each sweep of the sensor102across the substrate10. This measured trace220can also be termed a “total” trace or signal, as it includes contributions from both the conductive layer being polished and the underlying doped wafer.

The relationship curve510(seeFIG. 9) can be used to convert the signal values in the signal220from the substrate being polished to thickness measurements (represented by measured trace220′).

Each measured trace220′ is processed by the neural network, as discussed above, to generate a modified measured trace250with a portion having modified values230.

In some implementations, the conversion from raw signal to thickness can be performed for both the reference wafer and the substrate being polished after the edge reconstruction is performed.

To compensate for the wafer doping, the controller190can generate an adjusted trace480. Generating the adjusted trace includes subtracting the modified reference trace450from the modified measured trace250. Assuming the modified reference trace450is represented by S(x), and the modified measured trace250is represented by T(x), with x being a radial position, then T(s)−S(x) provides an apparent thickness trace.

For some configurations of the sensor102, the contribution from the doped wafer and the substrate to the trace are not a simple superposition. Rather, the apparent thickness of the conductive layer can be somewhat smaller than the actual thickness. This problem can become more pronounced at higher driving frequencies.

However, any particular sensor configuration (e.g., driving frequency, shape and dimensions of core, location and number of winding s of coil, etc.) does appear to have a generally linear relationship between the actual thickness and the apparent thickness. This relationship is illustrated inFIG. 10. A function520that relates the apparent thickness to the actual thickness can be expressed as a linear function with a slope of k and a y-intercept (where the thickness should be zero) of b. These values k and b can be determined empirically by testing, and will vary between different sensor configurations. The value of k tends to be less than or equal to 1, e.g., a value from 0.7 to 1.

Thus, an adjusted film thickness profile, A(x), of the conductive layer on the substrate can be calculated according to A(x)=(T(x)−S(x)−b)/k.

Endpoint can be called when the adjusted thickness value A(x) reaches a target thickness value DTARGET. Similarly, the adjusted thickness values A′(x) can be used for control of the polishing parameters, e.g., for calculation of polishing pressures to reduce non-uniformity.

In some cases, the relationship between the apparent thickness and the actual thickness for a particular sensor configuration may not be linear. In such a case, a more complex equation, e.g., a polynomial, may be used to calculate the actual thickness.

In some implementations, the raw signal is normalized before conversion to thickness values. This technique is applicable to both the reference trace420and the substrate trace220. For example, a calibrated signal X′(x) can be generated according to

where G is a gain and AK is an offset, but determined experimentally for the in-situ monitoring system using a blank wafer having a conductive layer of known thickness and conductivity. X(x) represents the raw signal values, e.g., from either the reference trace420or the substrate trace220, as appropriate for processing of the respective traces. The calibrated signal X′(x) is then used for the correlation curve, e.g., in place of X(x) in Equation 1 above, to determine the thickness values.

In addition, during conversion of the raw signal values to thickness values, the resistivity of the layer can be taken into account. For example, the thickness value calculated using the correlation curve, e.g., Equation 1 above, can be adjusted based on the resistivity of the layer to provide a corrected thickness value. This technique can be used for both the reference trace420and the substrate trace220.

The corrected thickness values D′(x) can be calculated as follows:

where ρXis the resistivity of the conductive layer, and ρ0is the resistivity for which the relationship curve410(and the values W1, W2, W3) applies, and where D(x) represents the initial thickness values calculated using the correlation curve (from either the reference trace420or the substrate trace220, as appropriate). The edge reconstruction algorithm can be applied to the corrected thickness values D′(x) instead of initial thickness values D(x).

In addition to the substrate-to-substrate variations in resistivity, changes in temperature of the layer can result in a change in the resistance of the conductive layer. For example, the conductive layer may become hotter as polishing progresses, and thus more conductive (lower resistivity). In particular, the controller carrying out the process can also calculate a resistivity ρTof the conductive layer at the real time temperature T(t). The real time temperature T(t) can be determined from the temperature sensor64. In some implementations, the adjusted resistivity ρTis calculated based on the following equation:

where Tiniis the initial temperature of the conductive layer when the polishing process starts. The adjusted resistivity ρTis then used in place of the resistivity ρX, e.g., in Equation 3 above (or in calculation of the gain and offset in Equation 2).

In situations where the polishing process is carried out under room temperature, Tinican take the approximate value of 20° C. ρXis the resistivity of the conductive layer at Tini, which can be room temperature. Typically, α is a known value that can be found in literature or can be obtained from experiment. Although the raw signal220includes a contribution from the underlying doped wafer, the value α of the conductive layer can be used as a first approximation in calculation of the thickness values for the trace220′.

In some implementations, the temperatures T and Tiniused in adjusting the measured eddy current signal are the temperature of the conductive layer, e.g., as measured by a temperature sensor in the carrier head. In some implementations, the temperatures T and Tinican be the temperatures of the polishing pad or the temperatures of the slurry instead of the temperatures of the conductive layer.

The above described polishing apparatus and methods can be applied in a variety of polishing systems. Either the polishing pad, or the carrier heads, or both can move to provide relative motion between the polishing surface and the substrate. For example, the platen may orbit rather than rotate. The polishing pad can be a circular (or some other shape) pad secured to the platen. Some aspects of the endpoint detection system may be applicable to linear polishing systems, e.g., where the polishing pad is a continuous or a reel-to-reel belt that moves linearly. The polishing layer can be a standard (for example, polyurethane with or without fillers) polishing material, a soft material, or a fixed-abrasive material. Terms of relative positioning are used to refer to relative positioning within the system or substrate; it should be understood that the polishing surface and substrate can be held in a vertical orientation or some other orientation during the polishing operation.

Functional operations of the controller90can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, and structural equivalents thereof, or in combinations of them. The computer software can be implemented as one or more computer program products, i.e., one or more computer programs tangibly embodied in a non-transitory computer readable storage media, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple processors or computers. A computer program (also known as a program, software, software application, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file. A program can be stored in a portion of a file that holds other programs or data, in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, although the description above has focused on chemical mechanical polishing, the control system can be adapted to other semiconductor processing techniques, e.g., etching or deposition, e.g., chemical vapor deposition. In addition, the technique can be applied to an in-line or stand-alone metrology system rather than in-situ monitoring. Accordingly, other embodiments are within the scope of the following claims.