Chemical mechanical polishing control system and method

A system, method, and computer program product for chemical mechanical polishing a substrate in which initially a plurality of predetermined pressures are applied to a plurality of regions of the substrate. A plurality of portions of the substrate are monitored during polishing with an in-situ monitoring system. If the difference in thickness between two portions of the substrate exceeds a predetermined threshold, a plurality of adjusted pressures are calculated in a closed-loop control system, and the plurality of adjusted pressures are applied to the plurality of regions of the substrate. The predetermined threshold includes an initial threshold for the start of the polishing process and a second threshold for a period of polishing after the start of the polishing process.

BACKGROUND

The present invention relates generally to chemical mechanical polishing of substrates, and more particularly to methods and apparatus for controlling a chemical mechanical polishing system.

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 slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, 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, underpolishing (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.

Another reoccurring problem in CMP is that polishing rates may vary across the substrate as the substrate is being polished. Thus, it is desirable to use data from the in-situ monitoring system to adjust the polishing parameters (such as the pressure applied to different portions of the substrate) to ensure that the substrate is polished to a uniform flatness.

SUMMARY

In one aspect, the invention is directed to a method of chemical mechanical polishing a substrate having a layer. In the method, the substrate is polished, and a plurality of predetermined pressures are applied to a plurality of regions of the substrate. A plurality of portions of the substrate are monitored during polishing with an in-situ monitoring system. A plurality of thicknesses of the layer are determined using data from the in-situ monitoring system. The plurality of thicknesses include a first thickness of the layer in a first portion of the substrate and a second thickness of the layer in a second portion of the substrate. A determination is made as to whether a difference between the first thickness and the second thickness is outside a predetermined threshold, where the predetermined threshold includes an initial threshold for a start of the polishing process and a second threshold for a period of polishing after the start of the polishing process. If the difference between the first thickness and the second thickness is outside the threshold, a plurality of adjusted pressures are calculated in a closed-loop control system, and the plurality of adjusted pressures are applied to the plurality of regions of the substrate.

Implementations of the invention may include one or more of the following features. The system may continue to determine the plurality of thicknesses and recalculate the plurality of adjusted pressures continuously or periodically in the closed-loop control system. The system may continue to determine whether a difference between the first thickness and the second thickness is outside the predetermined threshold. The step of recalculating may halt if the difference between the first thickness and the second thickness returns to a value within the threshold. A most recently calculated plurality of adjusted pressures may be applied to the plurality of regions of the substrate. The second threshold may be narrower than the first threshold. The predetermined threshold may be a function, e.g., a step function, of at least one of the plurality of thicknesses or a function of time. A ratio between the initial threshold and the second threshold may be between about 2:1 and 10:1. The predetermined threshold may include an upper limit and a lower limit, and an absolute value of the upper limit may be different than an absolute value of the lower limit. The first portion and the second portion may correspond to a first region and a second region from the plurality of regions. The plurality of regions of the substrate may comprise concentric regions, e.g., an inner region, a middle region and an outer region. The first region may be the inner region, and the second region may be the middle region or the outer region. Calculating the plurality of adjusted pressures in the closed-loop control system may include calculating a plurality of adjusted pressures to achieve a target thickness profile. The in-situ monitoring system may be an eddy current monitoring system, and the layer may be a metal. The in-situ monitoring system may be an optical monitoring system, and the layer may be a dielectric.

The invention also includes computer program products and polishing systems that carry out these methods.

Possible advantages of implementations of the invention can include one or more of the following. During bulk polishing of the metal layer, preselected pressures can be applied to different regions of the substrate in order to compensate for non-uniform polishing rates and non-uniform thickness of the incoming substrate. By avoiding unnecessary modifications to the polishing process parameters, the risk of unexpected polishing results is reduced. In addition, by using larger error thresholds at the beginning of the polishing process, there is time for the polishing process to stabilize and for the copper polishing process to initiate. Thus, the closed loop system is not activated while the behavior of the polishing system is chaotic. However, once the process has had time to stabilize, should the polishing process deviate from the expected, the pressure profile applied by the carrier head can be adjusted to ensure that the substrate is planarized to a desired flatness uniformity.

Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.

DETAILED DESCRIPTION

Referring toFIG. 1, one or more substrates10can be polished by a CMP apparatus20. A description of a similar polishing apparatus20can be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. Polishing apparatus20includes a series of polishing stations22a,22band22c, and a transfer station23. The transfer station23transfers the substrates between the carrier heads and a loading apparatus.

Each polishing station includes a rotatable platen24on which is placed a polishing pad30. The first and second polishing stations22aand22bcan include a two-layer polishing pad with a hard durable outer surface or a fixed-abrasive pad with embedded abrasive particles. The final polishing station22ccan include a relatively soft pad or a two-layer pad. Each polishing station can also include a pad conditioner apparatus28to maintain the condition of the polishing pad so that it will effectively polish substrates.

Referring toFIG. 2, a two-layer polishing pad30typically has a backing layer32which abuts the surface of the platen24and a covering layer34which is used to polish the substrate10. The covering layer34is typically harder than the backing layer32. However, some pads have only a covering layer and no backing layer. The covering layer34can be composed of foamed or cast polyurethane, possibly with fillers, e.g., hollow microspheres, and/or a grooved surface. The backing layer32can be composed of compressed felt fibers leached with urethane. A two-layer polishing pad, with the covering layer composed of IC-1000 and the backing layer composed of SUBA-4, is available from Rodel, Inc., of Newark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.).

During a polishing step, a polishing liquid38, such as an abrasive slurry or abrasive-free solution can be supplied to the surface of the polishing pad30by a slurry supply port or combined slurry/rinse arm39. The same slurry solution may be used at the first and second polishing stations22aand22b, whereas another slurry solution may be used at the final polishing station22c.

Returning toFIG. 1, a rotatable multi-head carousel60supports four carrier heads70. The carousel is rotated by a central post62about a carousel axis64by a carousel motor assembly (not shown) to orbit the carrier head systems and the substrates attached thereto between the polishing stations22a–22cand the transfer station23. Three of the carrier head systems receive and hold substrates and polish them by pressing them against the polishing pads. Meanwhile, one of the carrier head systems delivers a polished substrate to the transfer station23and receives an unpolished substrate from the transfer station23.

Each carrier head70is connected by a carrier drive shaft74to a carrier head rotation motor76(shown by the removal of one quarter of cover68) so that each carrier head can independently rotate about its own axis. In addition, each carrier head70independently laterally oscillates in a radial slot72formed in carousel support plate66. 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.

Descriptions of a suitable carrier head70can be found in U.S. Pat. No. 6,422,927, and in U.S. patent application Ser. No. 09/712,389, filed Nov. 13, 2000, the entire disclosures of which are incorporated by reference. Referring toFIGS. 2 and 3, the carrier head70can independently apply different pressures to different radial zones of the substrate10. For example, the carrier head70may include a flexible membrane with a substrate receiving surface and three independently pressurizable concentric chambers50,52and54behind the membrane. Thus, the inner circular chamber50will apply a pressure to an inner circular region50aof the substrate10, the middle annular chamber52will apply a pressure to a middle annular region52aof the substrate10, and the outer annular chamber54will apply a pressure to an outer annular region54aof the substrate10.

Referring again toFIG. 2, a recess26is formed in the platen24, and a transparent section36is formed in the polishing pad30overlying the recess26. The transparent section36is positioned such that it passes beneath the substrate10during a portion of the platen's rotation, regardless of the translational position of the carrier head70. Assuming that the polishing pad30is a two-layer pad, the transparent section36can be constructed by cutting an aperture in the backing layer32, and by replacing a section of the cover layer34with a transparent plug. The plug can be a relatively pure polymer or polyurethane, e.g., formed without fillers. In general, the material of the transparent section36should be non-magnetic and non-conductive.

At least one of the polishing stations, e.g., the first polishing station22aor the second polishing station22b, includes an in-situ eddy current monitoring system40and an optical monitoring system140. The eddy current monitoring system40and optical monitoring system140can function as a polishing process control and endpoint detection system. The first polishing station22acan include just an eddy current monitoring system40, and the final polishing station22ccan include just an optical monitoring system140, although either may include both monitoring systems, only an eddy current monitoring system, or only an optical monitoring system.

As shown byFIG. 3, the sensor assembly of the monitoring system is embedded in the platen24and sweeps beneath the substrate10with each rotation of the platen24. Each time the sensor assembly sweeps beneath the substrate10, data can be collected from the eddy current monitoring system40and optical monitoring system140. Specifically, as the sensor assemblies sweep in a path96across the substrate10, the monitoring systems will make a series of measurements, e.g., at measurement points98. Each measurement point98can be associated 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, for endpoint or process control. Measurements can be taken more frequently than shown by measurement points98inFIG. 3. The number of measurement points98shown inFIG. 3has been limited for clarity.

Returning toFIG. 2, the eddy current monitoring system40induces and senses eddy currents in a metal layer on the substrate10. The sensor assembly for the eddy current monitoring system40includes a core42positioned in the recess26to rotate with the platen, and a coil44wound around the core42. The coil44is connected to a control system, some components of which can be located on a printed circuit board58inside the recess26. A suitable control system is described in U.S. patent application Ser. No. 10/633,276, filed Jul. 31, 2003, by G. Laurie Miller et al., entitled “Eddy Current System for In-Situ Profile Measurement,” the entire disclosure of which is incorporated herein by reference. A computer90can be coupled to the components in the platen, including the printed circuit board58, through a rotary electrical union92.

Referring toFIG. 4, the core42can be a U-shaped body formed of a non-conductive material with a relatively high magnetic permeability. The exact winding configuration, core composition and shape, and an associated capacitor size can be determined experimentally. As shown, the lower surface of the transparent section36may include indentations29, and the two prongs42aand42bof the core42may extend into the indentations so as to be positioned closer to the substrate10. The system can also include a transparent cover27e.g., of glass or a hard plastic, over the recess26. The top surface of the transparent cover27can be flush with the top surface of the platen24. In one implementation, the core42only extends into the transparent cover27and not into the polishing pad30.

In operation, an oscillator in a controller drives the coil44to generate an oscillating magnetic field48that extends through the body of the core42and into the gap between the two poles42aand42bof the core42. At least a portion of the oscillating magnetic field48extends through the polishing pad30and into the substrate10. If a conductive layer16, e.g., a metal such as copper, is present on the substrate10, the oscillating magnetic field48generates eddy currents in the conductive layer16. The eddy currents cause the conductive layer16to act as an impedance source that is coupled to sense circuitry in the controller and to coil44. As the thickness of the conductive layer16changes, the impedance changes. By detecting this change, the sense circuitry can sense the change in the strength of the eddy currents, and thus the change in thickness of the conductive layer16.

As shown inFIGS. 5A and 5B, for a polishing operation, the substrate10is placed in contact with the polishing pad30. The substrate10can include a silicon wafer12and the conductive layer16disposed over one or more patterned underlying layers14, which can be semiconductor, conductor and/or insulator layers. A barrier layer18, such as tantalum or tantalum nitride, may separate the conductive layer16from the patterned underlying layers14.

After polishing, the conductive layer16will provide metal features, e.g., vias, pads and interconnects. However, prior to polishing, the bulk of conductive layer16is relatively thick and continuous and has a low resistivity, and relatively strong eddy currents can be generated in the conductive layer16. As previously mentioned, the eddy currents cause the conductive layer16to function as an impedance source in parallel with the coil44.

Referring toFIG. 5B, as the substrate10is polished, the bulk portion of the conductive layer16is thinned. As the conductive layer16thins, its sheet resistivity increases, and the eddy currents in the metal layer become dampened. Consequently, the coupling between conductive layer16and the sense circuitry is reduced (i.e., increasing the resistivity of the virtual impedance source).

Referring toFIG. 5C, eventually the bulk portion of the conductive layer16is removed, exposing the barrier layer18and leaving conductive interconnects16′ in the trenches between the underlying patterned layers14. At this point, the coupling between the conductive portions in the substrate, which are generally small and generally non-continuous, and the sense circuitry reaches a minimum.

Referring toFIG. 5D, continued polishing removes the barrier layer18and exposes the underlying patterned layers14, leaving conductive interconnects16′ and buried barrier layer films18′ in the trenches between the underlying patterned layers14.

Returning toFIG. 2, the optical monitoring system140, which can function as a reflectometer or interferometer, can be secured to the platen24in the recess26adjacent the eddy current monitoring system40. Thus, the optical monitoring system140can measure the reflectivity of substantially the same location on the substrate10as is being monitored by the eddy current monitoring system40. Specifically, the optical monitoring system140can be positioned to measure a portion of the substrate10at the same radial distance from the axis of rotation of the platen24as the eddy current monitoring system40. Thus, the optical monitoring system140can sweep across the substrate in the same path as the eddy current monitoring system40.

The optical monitoring system140includes a light source144and a detector146. The light source generates a light beam142which propagates through the transparent section36and the slurry to impinge upon the exposed surface of the substrate10. For example, the light source144may be a laser and the light beam142may be a collimated laser beam. The light beam142can be projected from the light source144at an angle α from an axis normal to the surface of the substrate10. The light source144can be configured so that the light beam142impinges upon a point at the center of the region on the substrate10monitored by the eddy current monitoring system40. In addition, if the recess26and the transparent section36are elongated, a beam expander (not illustrated) may be positioned in the path of the light beam142to expand the light beam142along the elongated axis of the transparent section36.

Referring toFIGS. 2 and 3, the polishing station22acan also include a position sensor80, such as an optical interrupter, to sense when the core42and the light source144are beneath the substrate10. For example, the position sensor80could be mounted at a fixed point opposite carrier head70. A flag82is attached to the periphery of the platen24. The point of attachment and length of the flag82is selected so that it interrupts the optical signal of the position sensor80while the transparent section36sweeps beneath the substrate10. Alternately, the polishing station22acan include an encoder to determine the angular position of platen24.

A general purpose programmable digital computer90receives signals from the eddy current monitoring system40and the optical monitoring system140. Since the sensor assembly sweeps beneath the substrate10with each rotation of the platen24, information on the conductive layer thickness and exposure of the underlying layer is accumulated in-situ and on a continuous real-time basis (once per platen rotation). The computer90can be programmed to sample measurements from the monitoring systems when the substrate10generally overlies the transparent section36(e.g., as determined by the position sensor80). As polishing progresses, the reflectivity or thickness of the conductive layer16changes, 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 on an output device94during polishing to permit an operator of the device to visually monitor the progress of the polishing operation. In addition, as discussed below, the traces may be used to control the polishing process and determine the end-point of the metal layer polishing operation.

In operation, the polishing station22auses the eddy current monitoring system40and optical monitoring system140to determine when the bulk of the conductive layer16(seeFIGS. 5A–5D) has been removed and to determine when the underlying barrier layer18has been substantially exposed. The computer90applies process control and endpoint detection logic to the sampled signals to determine when to change process parameters 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, the computer90can be programmed to divide the measurements from both the eddy current monitoring system40and the optical monitoring system140from each sweep beneath the substrate10into a plurality of measurement points98. The computer90can calculate the radial position on the substrate10for each measurement point, can sort the measurements into radial ranges, to determine minimum, maximum, and average measurements for each radial range, and can 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. In one implementation, the computer90averages measurements (e.g., measurements corresponding to the radial ranges) over time during multiple sweeps beneath the substrate10. A user can specify over how many sweeps the measurements should be averaged (e.g., 5). A rolling average weighted by time can be used, so that more recent measurements are weighted more heavily than older measurements.

The computer90may also be connected to the pressure mechanisms that control the pressure applied by the carrier head70, to the carrier head rotation motor76(seeFIG. 1) to control the carrier head rotation rate, to the platen rotation motor (not shown) to control the platen rotation rate, or to the slurry distribution system39to control the slurry composition supplied to the polishing pad30. Specifically, after sorting the measurements into radial ranges, information on the conductive layer thickness can be fed in real-time into a closed-loop controller to periodically or continuously modify the polishing pressure profile applied by the carrier head70, 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 computer90could 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 patterned layers14have been exposed in the outer annular region54abut not in the inner circular region50aof the substrate10. In this case, the computer90could reduce the pressure applied to an outer area of the substrate10. In one implementation, a user specifies how frequently the polishing pressure profile can be modified (e.g., to be a multiple of the rotation rate of platen24, such as sixty times per minute).

A method of polishing a metal layer, such as a copper layer, is shown in flowchart form inFIG. 6. First, a substrate10is polished at the first polishing station22a(seeFIG. 1) to remove the bulk of the metal layer (step160). When a predetermined thickness, e.g., 2000 to 4000 Angstroms (Å), of the copper layer remains over an underlying barrier layer, as determined using data from the eddy current monitoring system40, the polishing process is halted and the substrate10is transferred to the second polishing station22b. At the second polishing station22b, the substrate10is initially polished at a high polishing rate (step162). However, when the metal layer in a particular region of the substrate10is near clearing or is becoming cleared, as determined using data from the eddy current or optical monitoring system, the polishing parameters are modified to reduce the polishing rate for at least that particular region (step164), e.g., by reducing the pressure on the particular region. Once the metal layer has been removed at the second polishing station22b, as determined using data from the optical monitoring system140, the substrate is transferred to the third polishing station22cfor removal of the barrier layer (step166).

In an alternative method, all of the polishing of the metal layer is performed at the first polishing station22a. Removal of the barrier layer is performed at the second polishing station22b, and a buffing step is performed at the final polishing station22c.

During the bulk polishing steps at the first and second polishing stations22aand22b, the polishing process can be monitored by the eddy current monitoring system40. As polishing progresses, the radial thickness information from the eddy current monitoring system40can be fed into a closed-loop feedback system to control the pressure on different regions of the substrate (or to control the size of the loading area). The closed-loop control system calculates pressures to apply to the different regions of the substrate. This permits the carrier head to compensate for the non-uniformity in the polishing rate or for non-uniformity in the thickness of the metal layer of the incoming substrate. The closed-loop control system may be configured to accept user input setting maximum and minimum pressures that will be output to control the polisher. These minimum and maximum pressures can be based on physical limitations of the carrier head. In the event that the closed-loop control system calculates a desired pressure outside the range, it instead outputs a pressure at the edge of the permitted range. The pressure of the retaining ring on the polishing pad may also be adjusted to adjust the polishing rate. In one implementation, the pressure applied to the retaining ring chamber is varied as a function of the pressures applied to the substrate in order to keep the pressure of the retaining ring on the polishing pad substantially constant. For example, the pressure applied to the retaining ring chamber can be a fixed amount greater than the pressure applied to the substrate. As more pressure is applied to the substrate, the retaining ring may tend to lift away from the polishing pad for certain carrier head designs. Increasing the pressure applied to the retaining ring chamber can keep the pressure of the retaining ring on the polishing pad constant. The closed-loop feedback system need only be activated when differences in thickness across the substrate exceed a threshold.

The polishing system using the closed-loop feedback system is illustrated as a block diagram inFIG. 7. The polishing parameters150that control the polishing system are stored in a memory. The polishing parameters of particular interest include the individual chamber pressures. The chamber pressures will determine the pressures on the different regions50a–50con the substrate. Initially, the polishing pressures are preset (e.g., default settings) values determined experimentally or theoretically to provide a target thickness profile. Other parameters include platen and carrier head rotation rates, retaining ring pressure, and slurry flow rate.

The polishing parameters150stored in memory are fed to pressure controllers86, such as pressure regulators, that control the pressure in the chambers of the carrier head70. The carrier head loads the substrate10against the polishing pad. Both the platen and carrier head70rotate to create relative motion between the polishing pad and substrate10, resulting in polishing.

During polishing, the in-situ eddy current monitoring system40monitors the metal layer on the substrate10and generates a signal that is passed to a signal processing system152. The signal processing system152sorts the measurements from the in-situ eddy current monitoring system40into radial regions and calculates a metal layer thickness profile, i.e., the metal layer thickness as a function of radial position on the substrate. In other words, the layer thickness is calculated at multiple positions from the substrate center to the substrate edge. The calculated layer thicknesses may represent, for example, an average thickness for each of a plurality of radial concentric portions of the substrate. These concentric portions can correspond to the regions50a–54aof the substrate10corresponding to the independently controllable chambers50–54in the carrier head70(seeFIGS. 2 and 3).

The metal layer thicknesses are passed to an error threshold system154which is configured to determine whether the difference in thickness between the various regions on the substrate exceed a predetermined threshold. In one implementation, the error threshold system154calculates a thickness difference for various pairs of the regions50a–54aof the substrate. Specifically, the error threshold system154can calculate a thickness difference between the inner region50aand the middle region52a, and between the inner region50aand the outer region54a. Each pair of regions can have its own error threshold.

If the thickness difference exceeds the predetermined threshold, then the error threshold system154activates the closed-loop feedback control system156. The feedback control system156receives the metal layer thickness profile and a target thickness profile158that is stored in memory. In response to the metal thickness profile (and possibly other sensor measurements and/or the machine history), the feedback control system156calculates adjusted pressures for the carrier head70. For example, if the substrate edge is being polished faster than the substrate center, then the pressure applied by the carrier head to the edge of the substrate can be reduced. In one implementation, the pressure applied to the inner circular chamber50of carrier head70is fixed and only the pressures applied to the middle annular chamber52and the outer annular chamber54are varied. The feedback control system156can modify the stored polishing parameters150to match the calculated pressures. If the thickness difference falls below the threshold, then the system returns to the open-loop control using the last values calculated by the feedback control system156.

Although the signal processing system152, error threshold system154and feedback control system156are illustrated as separate components, some or all of the components can part of a single controller, such as the computer90. Such a controller can include one or more general purpose or special purpose processors to perform the functions of the systems152–156, and instructions for causing the controller to perform these functions can be stored on a computer-readable medium.

Referring toFIG. 8, the process performed by the polishing system begins by setting the polishing parameters, including the pressure values, to experimentally or theoretically determined preset values that should provide a target thickness profile (step170). There can be a different pressure for each independently controllable chamber50–54of the carrier head. The substrate is polished using these preset pressures from the system memory (step172).

As polishing progresses, the error threshold system monitors the thickness differences between the various portions of the substrate. As described above, these portions of the substrate can be the regions50a–54athat correspond to the independently controllable chambers50–54of the carrier head. If one or more of the thickness differences exceed a predetermined threshold (step174), the error threshold system activates the feedback control system (step176). The predetermined threshold may be in the range of 50 to 1000 Å, e.g., 100, 200, or 700 Å. The thickness difference can be calculated as the difference between the thickness of the inner region50aand the thickness of the outer region54a, or as the difference between the thickness of the inner region50aand the thickness of the middle region52a.

Referring toFIGS. 9A and 9B, different error thresholds190and192can be used for different periods during the polishing process. For example, while a preselected region of the substrate, e.g., the inner region50a, is thicker than a predetermined thickness (in the “early window”) the error threshold can have a first value, e.g., 700 Å (the thicknesses of the different regions of the substrate are calculated by the signal processing system152as discussed above). On the other hand, when the preselected region of the substrate falls below the predetermined thickness (in the “late window”), the error threshold can be lowered to a different value, e.g., 100 Å. The predetermined thickness may be about 5000 to 10000 Å, e.g., 7500 Å. In one implementation, the predetermined thickness is a function of an initial layer thickness. For example, the predetermined thickness can be 3000 Å less than the initial average thickness of the layer. The ratio between the first threshold and the second threshold can be between 2:1 and 10:1, e.g., 7:1 or 3.5:1. Of course, far more complex error threshold functions are possible. In addition, the function may depend on thickness of multiple regions of the substrate, or on an average thickness of the layer across the substrate. In particular, the feedback control system156can be activated if the difference between the inner substrate region and either the middle region or the outer region exceeds the threshold. Alternatively, the error threshold may be a function of time rather than layer thickness.

In addition, an upper threshold190can be different from a lower threshold192. Assuming that the error E is calculated as E=Tinner−Touter, where Tinneris the thickness of the inner region, and Touteris the thickness of a region surrounding the inner region, then the positive error threshold can be smaller than the negative error threshold. For example, during the “late window”, the upper threshold can be 100 Å and the lower threshold can be −200 Å.

Returning toFIG. 8, once the error threshold system is activated, it calculates adjusted pressures (there can be a new pressure for each independently controllable zone of the carrier head) to apply to the substrate (step178), and the substrate is polished using these adjusted pressures (step180).

The polishing feedback system remains activated and continuously or periodically recalculates the polishing pressures as long as the thickness difference on the substrate is outside the threshold limits. However, once the thickness difference is inside the threshold limits, the closed-loop feedback control system is deactivated, the pressure parameters are set to the last values calculated by the feedback control system (step184), and polishing returns to using pressure values that are not updated (step172).

Without being limited to any particular theory, the behavior of a polishing system (in terms of reaction of the polishing rate to input variables such as the pressure) can be chaotic during the initial start-up of polishing. This may be due to factors such as stabilization of temperature or, particularly for copper polishing, initiation of the polishing mechanism (e.g., oxidization of the metal layer). By using larger error thresholds at the beginning of the polishing process, the closed loop system is not activated while the behavior of the polishing system is chaotic and unlikely to respond properly to the closed-loop control system. In addition, by avoiding unnecessary modifications to the polishing process parameters, the risk of unexpected polishing results is reduced.

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.

Although the invention has been described in the context of metal polishing, which uses the eddy current monitoring system to generate thickness measurements of the metal layer, the invention would also be applicable to oxide polishing, which could use an interferometer or spectrometer to generate thickness measurements of the oxide layer.

Although illustrated as positioned in the same hole, the optical monitoring system140could be positioned at a different location on the platen than the eddy current monitoring system40. For example, the optical monitoring system140and eddy current monitoring system40could be positioned on opposite sides of the platen, so that they alternately scan the substrate surface. Various aspects of the invention still apply if the eddy current sensor uses separate drive and sense coils instead of a single coil.

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.