In-SITU monitoring system with substrate held by carrier in chemical mechanical polishing

Abstract

A technique includes: while chucking a substrate to a carrier head to prevent rotation of the substrate relative to the carrier head, rotating the carrier head and moving a platen; bringing the substrate into contact with a polishing pad; while the substrate is chucked to the carrier head and in contact with the polishing pad, monitoring the substrate by sweeping a sensor of an in-situ monitoring system to generate a sequence of traces with each respective trace corresponding to a respective sweep by the sensor, each respective trace including a sequence of signal values; tracking an angular orientation of the carrier head; for each respective signal value, determining, using the angular orientation of the carrier head, an angular and radial position of the substrate for the respective signal value; and constructing, using the thickness values, a two-dimensional map of a thickness of a layer on the substrate.

Description

[0001]Attorney Docket No. 44023977WO01; 05542-1628WO1 IN-SITU MONITORING SYSTEM WITH SUBSTRATE HELD BY CARRIER IN CHEMICAL MECHANICAL POLISHING TECHNICAL FIELD The present disclosure relates to in-situ monitoring during polishing of a substrate. BACKGROUND An integrated circuit is typically formed on a substrate (e.g., a semiconductor substrate) by the sequential deposition of conductive, semiconductive, or insulative layers on a silicon substrate and by the subsequent processing of the layers. 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 may be used to planarize the substrate surface for lithography. 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 placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. A polishing liquid, such as slurry with abrasive particles, is supplied to the surface of the polishing pad. SUMMARY In one aspect, a method includes: while chucking a substrate to a carrier head to prevent rotation of the substrate relative to the carrier head, rotating the carrier head and moving a platen supporting a polishing pad; bringing the substrate into contact with the polishing pad on the platen; while the substrate is chucked to the carrier head and in contact with the polishing pad, monitoring the substrate by sweeping a sensor of an in-situ monitoring system across the substrate to generate a sequence of traces with each respective trace of the sequence of traces corresponding to a respective sweep of a sequence of sweeps by the sensor, each respective trace including a sequence of signal values; tracking an angular Attorney Docket No. 44023977WO01; 05542-1628WO1 orientation of the carrier head; for each respective signal value of each respective trace of the sequence of traces, determining, using the angular orientation of the carrier head, an angular and radial position of the substrate for the respective signal value; and constructing, using the thickness values, a two-dimensional map of a thickness of a layer on the substrate. In another aspect, apparatus for chemical mechanical polishing includes: a platen supporting a polishing pad; a carrier head suspended from a support structure and configured to chuck a substrate comprising a notch to prevent rotation of the substrate when the substrate is in contact with the polishing pad; and an in-situ monitoring system comprising a sensor and control circuitry and configured to construct a two-dimensional map of a thickness of a layer on the substrate. In some implementations, the method further includes: while the substrate is retained by a retaining ring of the carrier head and in contact with the polishing pad, applying a positive pressure to the substrate to chemically mechanically polish the substrate while the substrate is free to rotate relative to the carrier head. In some implementations, monitoring the substrate while the substrate is chucked to the carrier head to prevent rotation of the substrate relative to the carrier head occurs before the chemical mechanical polish of the substrate. In some implementations, monitoring the substrate while the substrate is chucked to the carrier head to prevent rotation of the substrate relative to the carrier head occurs after the chemical mechanical polish of the substrate. In some implementations, chucking the substrate includes applying a negative pressure to a center of the substrate and a positive pressure to an edge of the substrate. In some implementations, the method further includes: measuring an angular orientation of a notch in the substrate before bringing the substrate into contact with the polishing pad. In some implementations, tracking the angular orientation of the carrier head includes receiving a motor encoder signal indicating the angular orientation. In some implementations, determining the angular position includes using the angular orientation of the notch and the angular orientation of the carrier head. In some implementations, the method further includes: prealigning, before chucking the substrate to the carrier head, the notch to have a same singular coordinate as a flag attached to a periphery of the platen. Attorney Docket No. 44023977WO01; 05542-1628WO1 In some implementations, the method further includes: for each signal value of the sequence of signal values from a respective trace, determining a polar coordinate on the substrate for the signal value. In some implementations, the sensor is supported on the platen. Moving the platen can include rotating the platen such that the sensor sweeps along an arcuate path across the substrate. In some implementations, bringing the substrate into contact with the polishing pad includes lowering a membrane assembly relative to a housing of the carrier head that is attached to a drive shaft. The membrane assembly can include a membrane support and a flexible membrane that is secured to and extends below the membrane support to form a lower pressurizable chamber below the membrane support. In some implementations, lowering the membrane assembly includes expanding an upper chamber disposed between the membrane support and the housing. In some implementations, the situ monitoring system is located below the carrier head. In some implementations, the situ monitoring system includes: a light source configured to generate a light beam to be reflected by the substrate; and a light detector positioned to receive a reflected light beam from the substrate. The control circuitry can be configured to send and receive signals between the light source and light detector and use the signals to detect the notch on the substrate. In some implementations, constructing the two-dimensional map of the thickness of the layer on the substrate includes: sweeping the sensor across the substrate to generate a sequence of traces, track an angular orientation of the carrier head, for each respective trace of the sequence of traces, determining, using the angular orientation of the carrier head, an angular position of the substrate for the respective trace, converting each signal value of the sequence of traces to respective thickness values, and constructing, using the thickness values, the two-dimensional map of the thickness of the layer on the substrate. In some implementations, the apparatus further includes: an optical sensor; a position sensor disposed adjacent to the platen and configured to sense when the optical sensor is beneath the substrate; and a flag attached to a periphery of the platen and configured to interrupt an optical signal of the optical sensor when the position sensor sweeps beneath the substrate. The control circuitry can be configured to determine an orientation of the platen by determining when the optical signal is interrupted. Attorney Docket No. 44023977WO01; 05542-1628WO1 Certain implementations can include one or more of the following advantages. Scanning of a substrate by a sensor of an in-situ carrier head can be conducted while the substrate is still chucked to the carrier head. This prevents rotation of the substrate relative to the carrier head. As a consequence, multiple scans of the substrate by the sensor can be used to generate a two-dimensional map of the thickness of a layer on the substrate. Such a map can be generated more quickly than with conventional stand-alone metrology techniques. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other aspects, features and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF DRAWINGS FIG.1 is a schematic plan view of an example of a polishing apparatus. FIG.2 is a schematic side view, partially cross-sectional, of an example chemical mechanical polishing station that includes an in-situ monitoring system. FIG.3 is a schematic cross-sectional view of an implementation of an example carrier head. FIG.4 is a schematic side view of an example of an inter-platen notch finder. FIG.5A is a schematic top view of a substrate being scanned by a sensor head of a polishing apparatus for a single pass. FIG.5B is a schematic top view of a substrate being scanned by a sensor head of a polishing apparatus for multiple passes. FIG.5C is a schematic top view of relative angular positions of a substrate and a platen. FIG.6 is a schematic of a scan path of a sensor head of a polishing apparatus for a substrate with a notch. FIG.7 is a schematic illustrating an example two-dimensional (2D) reconstruction of a substrate map. Like reference symbols in the various drawings indicate like elements. DETAILED DESCRIPTION Generating a “full wafer” measurement thickness map, e.g., a two-dimensional map in polar coordinates of thickness of a layer on a substrate, using conventional stand-alone metrology techniques can be time and resource intensive. On the other hand, a typical in-situ monitoring system scans the substrate when it is being pressed by the carrier head against a Attorney Docket No. 44023977WO01; 05542-1628WO1 polishing pad. In this situation, due to the downward pressure on the substrate, the substrate is no longer chucked to the carrier head and may be free to rotate relative to the carrier head; this behavior can be termed “precession. Without knowledge of the angular orientation of the substrate relative to the carrier head, determining the angular coordinate of a substrate thickness measurement in addition to the radial coordinate with arcuate scans can be challenging or impossible. The present disclosure provides a carrier head configured to hold the substrate in a chucked state while lowering the substrate onto a polishing pad supported by a platen, thereby eliminating uncertainty regarding the orientation of the substrate when the substrate lands on the polishing pad. Assuming the angular position of a fiducial, e.g., a notch, is measured in advanced and assuming the substrate does not rotate relative to the carrier head, the system can keep track of the angular orientation of the substrate, e.g., the angular location of the fiducial, by tracking the angular orientation of the carrier head. Consequently, when the platen rotates to sweep the sensor below the substrate, the angular and radial position of the sensor on the substrate can be calculated reliably. This enables quick generation of “full wafer” measurement maps by an in-situ monitoring system, e.g., instead of performing a full two-dimensional scan by a stand-alone metrology system. Polishing Station FIG.1 is a plan view of an example chemical mechanical polishing apparatus 100 for processing one or more substrates. The polishing apparatus 100 includes a plurality of polishing stations 110. For example, the polishing apparatus can include three polishing stations 110a, 110b, and 110c. The polishing apparatus 100 also includes at least one carrier head 140, e.g., four carrier heads 140. The polishing apparatus 100 also includes a transfer station 104 for loading and unloading substrates from the carrier heads 140. The stations of the polishing apparatus 100, including the transfer station 104 and the polishing stations 110, can be positioned at substantially equal angular intervals around the center of the platform. Referring to FIG.2, each polishing station 110 includes a polishing pad 130 supported on a rotatable platen 120. The polishing pad 130 can be a two-layer polishing pad with an outer polishing layer 132 and a softer backing layer 133. A top surface of the polishing layer 132 can provide a polishing surface 136. Returning to FIG.1, for a polishing operation, one carrier head 140 can be positioned at each polishing station 110. An additional carrier head 140 can be positioned in the transfer Attorney Docket No. 44023977WO01; 05542-1628WO1 station 104 to exchange a polished substrate for an unpolished substrate while the other substrates are being polished at the polishing stations 110. The carrier heads 140 are held by a support structure, e.g., a rotatable carousel or a carriage suspended from a track, that can cause the carrier head to move along a path 106 that passes, in order, each polishing station 110a-110c and the transfer station 104. Referring to FIGS.1 and 2, each polishing station 110 can include a port 165, e.g., at the end of an arm 167, to dispense polishing liquid 169, such as abrasive slurry, onto the polishing pad 130. Each polishing station 110 of the polishing apparatus 100 can also include pad conditioning apparatus 170 to abrade the polishing pad 130 to maintain the polishing surface 136 in a consistent abrasive state. For example, the conditioning apparatus can include a conditioning head 172 with a conditioning disk at the end of an arm 174. Each platen 120 is operable to rotate about a central axis 122. For example, a motor 124 can turn a drive shaft 126 to rotate the platen 120. Each motor 124 can include an encoder 125 that measures the angular position or rotation rate of the associated drive shaft 126. The associated drive shafts can have respective reference angular positions that are recognized by the encoder 125 to measure the number of revolutions of the drive shafts. In other words, the angular orientation of the platen 120 can be determined from the encoder 125. Each carrier head 140 is operable to hold a substrate 10 against the polishing pad 130. Each carrier head 140 can include a retaining ring 142 to retain the substrate 10. Each carrier head 140 can also include a plurality of independently controllable pressurizable chambers 146 defined by the membrane, as will be discussed with reference to FIG.3. For example, the carrier head 140 can apply a positive pressure to a center of the substrate 10 to chemically mechanically polish the substrate 10 while the substrate 10 is free to rotate relative to the carrier head 140, e.g., not in a chucked state. An in-situ monitoring system 180 installed in the platen 120 monitors the progress of the polishing operation and / or measures a thickness of the layer on the substrate 10 that is being polished. The in-situ monitoring system 180 can use an optical sensor, e.g., a spectrometer, an eddy current sensor, a capacitive sensor, a friction sensor, etc. A controller 190, such as a programmable computer, is connected to each motor 124, 156 to independently control the rotation rate of the platen 120 and the carrier heads 140. For example, each motor 156 can include an encoder 158 that measures the angular position or rotation rate of the associated drive shaft 154. The associated drive shafts can have Attorney Docket No. 44023977WO01; 05542-1628WO1 respective reference angular positions that are recognized by the encoder 158 to measure the number of revolutions of the drive shafts. Consequently, the controller 190 can determine the angular orientation of the carrier head 140 using data collected by the encoder 158. The controller 190 can include a central processing unit (CPU) 192, a memory 194, and support circuits 196, e.g., input / output circuitry, power supplies, clock circuits, cache, and the like. The memory is connected to the CPU 192. The memory is a non-transitory computer readable medium and can be one or more readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk, hard disk, or other form of digital storage. In addition, although illustrated as a single computer, the controller 190 could be a distributed system, e.g., including multiple independently operating processors and memories. Each carrier head 140 is suspended from the support structure 150 and is connected by a drive shaft 154 to a carrier head rotation motor 156 so that the carrier head can rotate about a central axis 152. Optionally each carrier head 140 can oscillate laterally, e.g., driven by a carriage on the track, by a motor that oscillates the carrier head radially, or by rotational oscillation of the carousel itself. In operation, the platen is rotated about its central axis 122, and each carrier head is rotated about its central axis 152 and translated laterally across the top surface of the polishing pad. In-Situ Monitoring System The in-situ-monitoring system 180 includes a sensor head 181 disposed in a recess 184 to rotate with the platen and drive and sense circuitry 186 connected by wiring 187 to the sensor head 181. The drive and sense circuitry 186 can be located in the same recess 183 or a different portion of the platen 120 or could be located outside the platen 120 and be coupled to the components in the platen through a motor 124. Alternatively or in addition, an optical monitoring system, which can function as a reflectometer or interferometer, can be secured to the platen 120 in the recess 184. The polishing stations 110 can also include a position sensor 188, such as an optical interrupter, to sense when the sensor head 181 is beneath the substrate 10. For example, the optical interrupter could be mounted at a fixed point opposite the carrier head 140. A flag 189 is attached to the periphery of the platen for platen indexing. The point of attachment and length of flag 189 is selected so that it interrupts the optical signal of sensor 188 while Attorney Docket No. 44023977WO01; 05542-1628WO1 the sensor head 181 sweeps beneath substrate 10. By monitoring when the optical signal is interrupted, the in-situ monitoring system 180 can keep track of the platen orientation. The signals can pass from the in-situ monitoring system 180 to the controller 190 through the rotary electrical union 123. Alternatively, the circuitry 186 could communicate with the controller 190 by a wireless signal. The sensor head 181 is supported on the platen 120, so moving the platen 120 causes the platen 120 to rotate. Accordingly, the sensor head 181 sweeps along an arcuate path across the substrate 10. Since the sensor head 181 sweeps beneath the substrate with each rotation of the platen, information on the conductive layer thickness is accumulated in-situ and on a continuous real-time basis (once per platen rotation). The controller 190 can be programmed to sample measurements from the monitoring system 180 when the substrate 10 generally overlies the sensor head 181 (as determined by the position sensor). The measurements from the monitoring systems can be displayed on the output device 193 during polishing to permit the operator of the device to visually monitor the progress of the polishing operation, although this is not required. Assuming the thickness of the layer varies across the substrate, the change in the position of the sensor head with respect to the substrate 10 can result in a change in the signal from the in-situ monitoring system 180. The time varying sampled signal resulting from a single sweep of the sensor below the substrate may be referred to as a trace. Variation in the signal across a trace can indicate variation in the layer thickness across the substrate. In addition, as polishing progresses, the thickness of the conductive layer changes, and the sampled signals vary with time. Thus, trace-to-trace differences can indicate variation in the layer thickness over time. In operation, the polishing stations 110 can use the in-situ monitoring system 180 to determine when the bulk of the filler layer has been removed and / or to determine when an underlying layer has been substantially exposed. 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. using Carrier Head with Membrane Assembly FIG.3 depicts an example of the carrier head 140’ that allows for a substrate to be brought into contact with the polishing pad 130 while in a chucked state. In some implementations, a “chucked” state refers to when the chambers, e.g., chambers 146a, 146b, Attorney Docket No. 44023977WO01; 05542-1628WO1 and 146c, are configured to apply pressure as a function of radial distance on the substrate 10 to maintain the substrate being in contact with the membrane 144. For example, the chambers can apply a negative pressure to the center of the substrate 10 by controlling central chambers, e.g., central chamber 146c, apply a positive pressure to the edges of the substrate 10 by controlling edge chambers, e.g., chamber 146a, or simultaneously applying negative pressure to central chambers and positive pressure to edge chambers. In some implementations, chemical mechanical polishing can occur before or after the substrate is in a chucked state, e.g., pre- or post-polishing. The controller 190 is also connected to pressure regulators to control the pressures in the chambers 146a-146c. In particular, the controller 190 can be configured to receive thickness measurements from the in-situ monitoring system and control the pressures in the chambers 146a-146c to provide improved polishing uniformity. While the substrate is chucked, the carrier head 140’ can continue to rotate, and the platen 120 supporting the polishing pad 130 can move. For example, bringing the substrate 10 into contact with the polishing pad 130 can include lowering a membrane assembly 400 relative to the housing 102 of the carrier head 140’. The polishing pad 130 can already be rotating by the time the substrate 10 contacts the polishing pad 130. Consequently, angular rotation of the substrate 10 relative to the carrier head 140’ can be avoided. Similarly to carrier head 140 shown in FIG.2, the carrier head 140’ can be suspended from a support structure 150, e.g., a carousel or track. Optionally each carrier head 140’ can oscillate laterally, e.g., on sliders on the support structure 150, or by rotational oscillation of the carousel itself, or by sliding along the track. In typical operation, the platen is rotated about its central axis 122, and each carrier head is rotated about its central axis 152 and translated laterally across the top surface of the polishing pad. The carrier head 140’ includes a housing 102’ with an upper carrier body 204 and lower carrier body 206, a gimbal mechanism 108 (which may be considered part of the lower carrier body 206), a retaining ring 142’ connected to the housing 102’ (e.g., connected the upper carrier body 204 or the lower carrier body 206), and the membrane assembly 400. A volume between the lower carrier body 206 and the upper carrier body 204 can be sealed by an upper flexible seal 164 to provide a loading chamber 111. This upper flexible seal 164 can flex to accommodate the change in vertical position between the upper carrier body 204 and the lower carrier body 206. In some implementations, the upper carrier body 204 and the lower carrier body 206 are replaced by a single unitary body. Attorney Docket No. 44023977WO01; 05542-1628WO1 The upper carrier body 204 can generally be circular in shape. There may be passages extending through the upper carrier body 204 for pneumatic control of the carrier head 140’. The lower carrier body 206 is located beneath the upper carrier body 204, and vertically movable relative to the upper carrier body 204. The loading chamber 111 is located between the upper carrier body 204 and the lower carrier body 206 to apply a load, e.g., a downward pressure or weight, to the lower carrier body 206. The vertical position of the lower carrier body 206 relative to a polishing pad is also controlled by the loading chamber 111. In some implementations, the vertical position of the lower carrier body 206 relative to the upper carrier body 204 is controlled by an actuator. The gimbal mechanism 108 permits the lower carrier body 206 to gimbal and vertically move relative to the upper carrier body 204 while preventing lateral motion of the lower carrier body 206 relative to the upper carrier body 204. However, in some implementations, there is no gimbal. The substrate 10 is held beneath the carrier head 140’ by the retaining ring 142’, which retains the substrate 10 against lateral motion. The retaining ring 142’ can also provide active edge process control and control of pressure on the polishing pad in a region outside the substrate but adjacent the substrate edge can affect polishing rates at the substrate edge. Some implementations can include an outer ring, which can provide positioning or referencing of the carrier head to the surface of the polishing pad. A volume between the lower carrier body 206 and the membrane assembly 400 can be sealed by a lower flexible seal 162 to form a pressurizable upper chamber 134. This lower flexible seal 162 can flex to accommodate the change in vertical position between the lower carrier body 206 and the membrane assembly 400. Pressure in the pressurizable upper chamber can control the downward load on the membrane assembly 400 and / or the vertical position of the membrane assembly 400 relative to the housing. For example, expanding the pressure article upper chamber 134 by lowering the pressure can cause the membrane assembly 400 to decrease the vertical position of the membrane assembly 400. Each chamber in the carrier head 140’ can be fluidly coupled by passages through the upper carrier body 204 and the lower carrier body 206 to an associated pressure source (e.g., a pressure source 922), such as a pump or pressure or vacuum line. There can be one or more passages for the loading chamber 111, for the pressurizable upper chamber 134, and for each of the individually pressurizable lower chambers 146a-146n. One or more passages from the lower carrier body 206 can be linked to passages in the upper carrier body 204 by flexible Attorney Docket No. 44023977WO01; 05542-1628WO1 tubing that extends inside the loading chamber 111 or outside the carrier head 140’. Pressurization of each chamber can be independently controlled with respect to other chambers, e.g., pressurization of the chambers can be individually controlled. In particular, pressurization of each lower chamber 146a-146n can be independently controlled. This permits different pressures to be applied to different radial regions of the substrate 10 during polishing, thereby compensating for non-uniform polishing rates. The membrane assembly 400 can include a membrane support 138 and the flexible membrane 144. The flexible membrane 144 is secured to and extends below the membrane support 138. The flexible membrane 144 has a circular lower portion 171 having a lower outer surface 175 that provides a mounting surface for the substrate 10. The flexible membrane 144 also has a plurality of flaps 173, e.g., annular flaps, which extend from the inner surface 176 of the lower portion 171 to define the individually controllable pressurizable lower chambers 146a-146n. For example, the ends of the flaps 173, e.g., the ends adjacent to the lower portion 138a, can be clamped to the membrane support 138. The membrane support 138 can include a disk-shaped lower portion 138a and an annular upper portion 138b that extends upwardly from the disk-shaped lower portion 138a at an outer edge thereof. The flaps 173 of the flexible membrane 144 can be clamped to the disk-shaped lower portion 138a. The membrane support 138 can be formed of a material that is more rigid than the membrane 144, e.g., a metal, ceramic, or hard plastic. The membrane support 138 can be considered inflexible under the pressure regimes typically occurring in the polishing process. The pressurizable upper chamber 134 is configured to extend across the top of the membrane support 138 and be contained by the lower flexible seal 162. In particular, the pressurizable upper chamber 134 can be bounded on the bottom by the top surface 139 of the disk-shaped lower-portion 138a, on the sides by the inner surface 141 of the annular upper portion 138b, and on the top by the bottom surface 161 of the lower flexible seal 162, and the bottom surface 107 of the lower carrier body 206. The lower carrier body 206 can be connected to the membrane assembly 400 by a flexure 160. The flexure 160 is more rigid than the lower flexible seal 162, but less rigid than the membrane support 138. The membrane assembly 400 is suspended from the upper carrier body 206 by the flexure 160. In addition, the flexure 160 acts as a spring, urging the membrane assembly 400 toward a “neutral” position while still permitting the membrane assembly 400 to move vertically relative to the housing 102 based on the pressure in the Attorney Docket No. 44023977WO01; 05542-1628WO1 pressurizable upper chamber 134. The flexure 160 can be connected to the lower carrier body 206 and the membrane assembly 400 using fasteners, e.g., adhesive, screw, bolt, clamp, or by interlocking, to name a few examples. The flexure 160 extends through the pressurizable upper chamber 134 between the inner surface 141 of the upper portion 138b of the membrane assembly 400 and a sidewall 163 of a downward projection 191 of the lower carrier body 206. The projection 191 extends downward toward the top surface 139 of the lower portion 138a of the membrane support 138. Thus, the pressurizable upper chamber 134 can include a lower portion 134a below the flexure 160, and an upper portion 134b above the flexure 160. The lower portion 134a and upper portion 134b are fluidically coupled to stay at the same pressure. In some implementations, the flexure 160 has gaps or apertures to permit the flow of gas. For example, the flexure 160 can be an annular body that includes multiple apertures therethrough. As another example, the flexure 160 can be provided by multiple arcuate pieces that are separated by gaps. The upper flexible seal 164 and the lower flexible seal 162 can be composed of a flexible material such as a rubber, e.g., silicone rubber, ethylene propylene diene terpolymer (EPDM), or a fluoroelastomer, or a plastic film, e.g., polyethylene terephthalate (PET) or polyoxymethylene. The flexure 160 is sufficiently stiff to resist lateral motion to keep the membrane assembly 400 centered below the lower carrier body 206. However, the flexure 160 can be sufficiently vertically flexible to permit vertical motion of the membrane assembly 400 relative to the housing 102. The flexure 160 permits the assembly 400 to vertically move relative to the lower carrier body 206 by flexing, e.g., bendably deflecting. An advantage of the configuration with the flexure 160 extending through the upper chamber 134 is that the pressure in the pressurizable upper chamber 134 can be adjusted without inducing pressure differentials across the flexure 160, thus avoiding reacting out pressure through the flexure, which can skew pressure distribution across the membrane 144, e.g., against the substrate 10. An advantage of pressure on both sides of the flexure 160 is to reduce stresses that would be created by bending of the flexure 160 by applying pressure on only one side. The controller 190 regulates the pressure of the various chambers of the carrier head 140’. The controller 190 is coupled to a plurality of pressure sources, e.g., pressure source 922, pressure source 924, and pressure source 926. The pressure sources 922, 924, and 926 Attorney Docket No. 44023977WO01; 05542-1628WO1 can be, for example, a pump, a facilities gas line, and controllable valve, etc. Each of the pressure sources 922, 924, and 926 can be individually connected to a pressurizable chamber. In this example, pressure source 922 connects to loading chamber 111, pressure source 924 is connected to the pressurizable lower chambers 146a-146n, and pressure source 926 is connected to the pressurizable upper chamber 134. Although only one pressure source 924 is shown for ease of illustration, there can be one per lower chamber 146a-146n so that each lower chamber 146a-146c can be individually controlled. One or more sensors 930 measure the pressure(s) applied by the pressure sources 922, 924, and 926, e.g., the pressures in the individually pressurizable lower chambers 146a-146n, the pressurizable upper chamber 134, and the loading chamber 111. The sensor 930 communicates the measured pressure(s) to the controller 190. The controller 190 causes the pressure sources 922, 924, 926 to increase and / or decrease the pressure in the pressurizable lower chambers 146a-146n, the pressurizable upper chamber 134, and / or the loading chamber 111. Use of In-Situ Monitoring System for Generation of Thickness Map Returning to FIG.2, although the in-situ monitoring system 180 can be used during a polishing operation to track the thickness of a layer on the substrate 10 during polishing, as noted above, it is also possible to use the in-situ monitoring system to generate a thickness map of the substrate. The carrier head 140 can place the substrate 10 in contact with the polishing pad 130 and the carrier head 140 and platen 120 can rotate in a scanning operation to repeatedly sweep the sensor head 181 across the substrate 10 in order to obtain thickness data at multiple different locations on the substrate. In order for a thickness map to be accurate, the measurements for various locations on the substrate 10 need to be acquired without changing the thickness of the layer at previously measured locations. In short, polishing is to be avoided during the scans by the sensor head 181 for generating the thickness map. The upper chamber 134 can be vented to atmosphere so that the carrier head 140’ does not apply positive pressure on the substrate 10 and the substrate simply rests on the polishing pad 130. In addition, there should not be any polishing liquid, e.g., abrasive slurry, on the polishing pad 130 during the scanning operation. The polishing pad 130 can be rinsed with a cleaning fluid, e.g., deionized water, before and / or during the scanning operation. Attorney Docket No. 44023977WO01; 05542-1628WO1 The thickness map can be generated before or after a polishing process. During the polishing operation, positive pressure is applied to the substrate 10 and thus the substrate is not chucked to the carrier head 140 and is free undergo precession relative to the carrier head 140. In contrast, in the scanning operation, the substrate is chucked to the carrier head 140 and does not undergo precession relative to the carrier head 140. Pre-Aligning In some implementations, the notch position is identified using pre-alignment methods prior to loading the substrate 10 on the carrier head 140. In general, pre-alignment methods alone, e.g., without chucking the substrate 10 to the carrier head 140 during loading, can allow for improved estimates of the angular orientation of the substrate 10 during scans of the sensor head 181. However, due to the substrate 10 undergoing precession when the carrier head applies positive pressure to the substrate 10 to keep the substrate in contact with the carrier head 140, and due to precession models not being perfect, there is still uncertainty, e.g., 1° to 2° of error, regarding the orientation of the substrate 10 relative to the carrier head. For example, precession can include the substrate 10 rotating relative to the retaining ring 142 of the carrier head 140 at a different rate. Further, the substrate 10 can slightly shift to not be centered on the center of the retaining ring 142. Combining pre-alignment methods with chucking the substrate 10 during scanning by the sensor head 181 can remove this uncertainty. With reference to FIG.5A, the angular orientation of a fiducial 509, e.g., a notch or flat, of the substrate 10 is measured relative to a reference point 185 of the carrier head 140, prior to bringing the substrate in contact with the polishing pad 130, e.g., head-indexing. In some implementations, pre-aligning includes aligning the fiducial 509 of the substrate to have the same angular coordinate as the flag 189 (see FIG.2) before chucking the substrate 10 to the carrier head 140. If the controller 190 can determine the location of the platen, e.g., through platen indexing using the encoder 125, the angular coordinates of a scan can be determined, and the controller 190 can reconstruct a full, 2D map of a substrate property. Notch Finder Station In some implementations, the notch position is identified using notch finding hardware. Referring to FIGS.1 and 4, the polishing apparatus 100 can also include one or more notch-finding stations 200. In some implementations, a notch-finding station 200a is positioned on the path 106 at a spot between the transfer station 104 and a first polishing Attorney Docket No. 44023977WO01; 05542-1628WO1 station, e.g., polishing station 110a. One or more notch-finding stations 200b, 200c can be positioned on the path 106 traveled by the carrier heads 140 between two polishing stations 110. In some implementations, the polishing system includes two inter-platen notch- finding stations 200b, 200c. The two inter-platen notch-finding stations 200b, 200c could be on the path 106 on opposite sides of a polishing station, e.g., the second polishing station 110b. The inter-platen notch-finding stations 200a, 200b, and 200c can be integrated into the transfer station 104. In some modes of operation, the substrate orientation is measured at a notch-finding station 200 located before a polishing station 110 along the path 106, and then moved forward along the path 106 to the polishing station 110 and polished at that polishing station 110. However, in some modes of operation, the substrate orientation can be measured at a notch-finding station 200 located after a polishing station 110 along the path 106, and then moved backward along the path 106 to the polishing station 110 and polished at that polishing station 110. The substrate can then be moved forward again along the path 106 to the next polishing station, optionally stopping at the notch-finding station 200 before being polished at the next polishing station. FIG.4 illustrates an implementation of an example notch-finding station 200 positioned between two platens 120 of two polishing stations 110 that are adjacent along the path. This implementation of the notch finding station 200 has an optical notch detector 210 that includes a light source 212, a light detector 214, and circuitry 216 for sending and receiving signals between the controller 190 and the light source 212 and light detector 214. The optical notch detector 210 can also include some of the functionality implemented by software in the controller 190. The light source 212 is located such that a carrier head 140 can be positioned on the path 106 where the substrate 10 can be scanned by the optical notch detector 210. In particular, the light source 212 generates a light beam 220 that can be reflected by the substrate 10, and the light detector 214 is positioned to receive a reflected light beam 222 from the substrate 10. The light detector 214 can be positioned where the light beam 220 and reflected light beam 222 have equal incidence angles, e.g., such that the light detector 214 is receiving reflected (rather than scattered) light. Although FIG.4 illustrates the light beam 220 traveling in a straight line to the substrate 10, one or more mirrors can be positioned in the optical path of the light beam 220. Attorney Docket No. 44023977WO01; 05542-1628WO1 During operation, the carrier head 140 is positioned in the notch-finding station 200 with the substrate 10 positioned above and spaced apart from the optical components of the optical notch detector 210. In particular, the carrier head 140 is positioned such that the light beam 220 passes through an aperture 232 in a cover of the notch-finding station 200 and impinges the substrate 10 in an impingement spot 224 that contacts or overlaps the substrate edge 16. The carrier head 140 rotates so that the substrate 10 rotates, thus sweeping the light beam along the circumference of the substrate 10. Since the membrane 144 will have a different reflectivity than the substrate 10, the signal from the detector 214 should change when the notch position passes across the light beam 220. The notch position can thus be detected based on variation of the intensity signal from the detector 214. For some processes, it is useful to have each substrate oriented to a consistent angular position before polishing commences. The substrate notch can be oriented by rotating a respective carrier head. If the polishing operation has some inherent angular variation, e.g., due to a pattern of features on the substrate, then a consistent starting angular position can improve the ability to compensate for such variation, e.g., by application of varying pressure by the chambers inside the carrier head. In addition, a consistent starting angular position can improve the likelihood that the in-situ monitoring system 180 traces a consistent series of paths across the substrate on a substrate-to-substrate basis, thus making processing of the signal from the in-situ monitoring system more reliable. Signal Processing FIGS.5A-5D depict schematic top views of the substrate 10 being scanned by the sensor head while the substrate 10 is in a chucked position, e.g., the substrate 10 is not rotating relative to the carrier head 140. FIG.5A shows a schematic top view of a substrate being scanned by a sensor head of a polishing apparatus for one scan path 510. The scan path starts at position 512 on the leading edge of the substrate 10 and ends at position 513 on the trailing edge of the substrate 10. As noted above, changes in the position of the sensor head with respect to the substrate 10 can result in a change in the signal from the in-situ monitoring system 180. That is, as the sensor head 181 scans across the substrate 10, the in-situ monitoring system 180 will make measurements for multiple regions, e.g., measurement spots 511, at different locations on the substrate 10. The in-situ monitoring system 180 generates a signal during a single pass of the sensor head below the substrate 10. For a given trace, the signal can be captured (and thus Attorney Docket No. 44023977WO01; 05542-1628WO1 the graph can represent the signal) as a function of measurement time or of position, e.g., radial or diameter position, of the measurement on the substrate. In either case, different portions of the signal correspond to measurement spots 511 at different locations on the substrate 10 scanned by the sensor head. A signal can include a first portion that corresponds to locations in an edge region 503 of the substrate 10 when the sensor head crosses a leading edge of the substrate 10, a second portion that corresponds to locations in a central region 501 of the substrate 10, and a third portion that corresponds to locations in edge region 503 when the sensor head crosses a trailing edge of the substrate 10. The signal can also include portions that correspond to off- substrate measurements, i.e., signals generated when the sensor head scans areas beyond the edge 504 of the substrate 10 in FIG.5A. The edge region 503 can correspond to a portion of the substrate where measurement spots 511 of the sensor head overlap the substrate edge 504. The sensor head may scan these regions on its path 510 and generate a sequence of measurements that correspond to a sequence of locations along the path 510. The sensor conducts multiple scans, with each scan measuring the thickness at various radial positions across the substrate. Due to the differing rotation rate of the carrier head 140 and the platen 120, and due to the lateral oscillation of the carrier head, the scan paths are not consistent. As illustrated in FIG.5B, the substrate 10 is shown having been scanned by a sensor head along five distinct paths, 510, 520, 530, 540, and 550. However, the number of scans can be far larger, e.g., 20 to 200 scans. In some implementations, the polishing system 100 has at least one notch-finding station 200 to measure an angular orientation of the substrate 10 prior to polishing. By measuring the angular orientation of the substrate prior to polishing, the substrate 10 substrate can be rotated to a specified angular position. Consequently, the positioning of the substrate notch or flat is consistent before each polishing process commences. Referring to FIG.5C, which is a schematic top view of a CMP polishing station, the controller causes the carrier head to rotate the substrate 10 prior to polishing an angular orientation where the substrate notch or flat, e.g., fiducial 509, is positioned at a particular predetermined angular position in the stationary frame of reference, e.g., the frame of reference of the base that holds the substrates. Using the fidicual 509, the in-situ monitoring system can, for each trace, track an angular position of the carrier head and thereby an angular position of the substrate 10 while the substrate is in a chucked state. Attorney Docket No. 44023977WO01; 05542-1628WO1 For example, the predetermined angle 1, can be measured relative to an axis 195 that passes through both the central axis 122 of rotation of the platen 120 and the center 197 of the carrier head 140. In some implementations, this axis can pass through the fixed-position sensor 188. The controller 190 can be configured such that the notch or flat of each substrate to be polished is consistently aligned at this specified angular position before the polishing process begins for that substrate. The controller 190 is also configured to rotate the platen 120 to a specified angular position before commencing polishing of the substrate 10. The sensor head 181 can be fixed on the rotating platen 120 such that the sensor head 181 rotates with the platen 120. This predetermined angular position, 2, as depicted in FIG.5C, can be relative to the axis 195. With both the platen and the substrate consistently beginning at specific angular positions, the sensor head 181 should follow a consistent series of paths across the substrate on a substrate-to-substrate basis. Returning to FIG.5B, provided that the relative angular position between the substrate 10 and the sensor head remains consistent from one substrate to another, the initial scan path 510 can consistently commence at the path start position 512 and conclude at the path ending position 513 for each substrate, while for the second scan, the scan path 520 can consistently commence at the path start position 522 and conclude at the path ending position 523 for each substrate. Likewise, for the third, fourth and subsequent scans, the starting and ending positions of each path would also remain consistent from one substrate to another. Consequently, for equivalent positions within the sequence of scans, the sensor's path exhibits repeatability from substrate to substrate. As noted above, the polishing apparatus 100 can include three polishing stations 110. The substrate notch alignment can be done at each notch-finding station 200a, 200b, and 200c prior to polishing at the next polishing station 110a, 110b, and 110c, respectively (see FIG.1). Similarly, each polishing station 110 can have an encoder 125 that measures the angular position or rotation rate of the associated drive shaft 126, and a motor 124 that can turn a drive shaft 126 to rotate the platen 120 to a specified angular position prior to polishing (see FIG.2). The specified angular position can vary among different polishing stations 110 and notch-finding station 200. Specifically, three polishing stations 110a, 110b, and 110c, may each employ a distinct angular alignment for the substrate 10 and the platen 120. Nevertheless, the angular positioning remains consistent from one substrate to another for the same polishing station 110. Attorney Docket No. 44023977WO01; 05542-1628WO1 Full Map Reconstruction Without using the methods for monitoring the angular position described above, the full, two-dimensional location for data collected along a scan path cannot be determined with satisfactory accuracy. With reference to FIG.6, for a particular scan path 515, the radial coordinate, e.g., the distance from the center 602 of the substrate 10 to the two-dimensional location for a corresponding portion of the signal, can be determined. For example, the controller 190 can use data related to the time of the scan and rate of the platen to determine the radial coordinate. Just knowing the radial coordinate determines a circle 604 with a radius 606 equal to the radial coordinate. However, the two-dimensional location could have any angular coordinate, which is less precise than knowing both the angular and radial coordinates. The disclosed methods, however, remove the uncertainty related to the angular position of the substrate 10. By using the angular orientation of the notch and the angular orientation of the carrier head, the controller 190 can determine the angular coordinate, e.g., 3 in FIG.6. The sensor head 181 can collect multiple signals over multiple traces and determine an appropriate polar coordinate on the substrate for each signal value. With reference to FIG. 7, using the multiple signals and their associated polar coordinates, the apparatus can construct using thickness values from a plurality of sweeps, a two-dimensional map 700 of the thickness of a layer on the substrate 10. In FIG.7, the pattern of an area within the circle refers to the thickness of measurement spots within that region. For example, the region 710 refers to measurements within 20-25 micron, the regions 720 refers to measurements within 25-30 micron, and the region 730 refers to measurements within 30-35 micron. Having a map like that in FIG.7 would allow for targeted polishing to achieve a more uniform or other type of desired thickness profile. Integration with Polishing need to add that this is before or after polishingA number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications Attorney Docket No. 44023977WO01; 05542-1628WO1 may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

Attorney Docket No. 44023977WO01; 05542-1628WO1 CLAIMS What is claimed is:

1. A method, comprising: while chucking a substrate to a carrier head to prevent rotation of the substrate relative to the carrier head, rotating the carrier head and moving a platen supporting a polishing pad; bringing the substrate into contact with the polishing pad on the platen; while the substrate is chucked to the carrier head and in contact with the polishing pad, monitoring the substrate by sweeping a sensor of an in-situ monitoring system across the substrate to generate a sequence of traces with each respective trace of the sequence of traces corresponding to a respective sweep of a sequence of sweeps by the sensor, each respective trace including a sequence of signal values; tracking an angular orientation of the carrier head; for each respective signal value of each respective trace of the sequence of traces, determining, using the angular orientation of the carrier head, an angular and radial position of the substrate for the respective signal value; and using the sequence of traces, determine thickness values, constructing, using the thickness values and the angular and radial positions, a two- dimensional map of a thickness of a layer on the substrate.

2. The method of claim 1, comprising, while the substrate is retained by a retaining ring of the carrier head and in contact with the polishing pad, applying a positive pressure to the substrate to chemically mechanically polish the substrate while the substrate is free to rotate relative to the carrier head.

3. The method of claim 2, wherein monitoring the substrate while the substrate is chucked to the carrier head to prevent rotation of the substrate relative to the carrier head occurs before the chemical mechanical polish of the substrate.Attorney Docket No. 44023977WO01; 05542-1628WO1 4. The method of claim 2, wherein monitoring the substrate while the substrate is chucked to the carrier head to prevent rotation of the substrate relative to the carrier head occurs after the chemical mechanical polish of the substrate.

5. The method of claim 1, wherein chucking the substrate comprises applying a negative pressure to a center of the substrate and a positive pressure to an edge of the substrate.

6. The method of claim 1, comprising measuring an angular orientation of a notch in the substrate before bringing the substrate into contact with the polishing pad.

7. The method of claim 6, wherein tracking the angular orientation of the carrier head includes receiving a motor encoder signal indicating the angular orientation.

8. The method of claim 7, wherein determining the angular position includes using the angular orientation of the notch and the angular orientation of the carrier head.

9. The method of claim 6, further comprising prealigning, before chucking the substrate to the carrier head, the notch to have a same singular coordinate as a flag attached to a periphery of the platen.

10. The method of claim 1, comprising for each signal value of the sequence of signal values from a respective trace, determining a polar coordinate on the substrate for the signal value.

11. The method of claim 1, wherein the sensor is supported on the platen, and wherein moving the platen comprises rotating the platen such that the sensor sweeps along an arcuate path across the substrate.

12. The method of claim 1, wherein bringing the substrate into contact with the polishing pad comprises lowering a membrane assembly relative to a housing of the carrier head that is attached to a drive shaft,Attorney Docket No. 44023977WO01; 05542-1628WO1 wherein the membrane assembly includes a membrane support and a flexible membrane that is secured to and extends below the membrane support to form a lower pressurizable chamber below the membrane support.

13. The method of claim 12, wherein lowering the membrane assembly comprises expanding an upper chamber disposed between the membrane support and the housing.

14. An apparatus for chemical mechanical polishing comprising: a platen supporting a polishing pad; a carrier head suspended from a support structure and configured to chuck a substrate to prevent rotation of the substrate when the substrate is in contact with the polishing pad; and an in-situ monitoring system comprising a sensor and control circuitry and configured to construct a two-dimensional map of a thickness of a layer on the substrate.

15. The apparatus of claim 14, wherein the situ monitoring system is located below the carrier head.

16. The apparatus of claim 14, wherein the situ monitoring system comprises: a light source configured to generate a light beam to be reflected by the substrate; and a light detector positioned to receive a reflected light beam from the substrate, wherein the control circuitry is configured to send and receive signals between the light source and light detector and use the signals to detect a notch on the substrate.

17. The apparatus of claim 14, wherein constructing the two-dimensional map of the thickness of the layer on the substrate comprises: sweeping the sensor across the substrate to generate a sequence of traces; tracking an angular orientation of the carrier head; for each respective trace of the sequence of traces, determining, using the angular orientation of the carrier head, an angular position of the substrate for the respective trace; converting each signal value of the sequence of traces to respective thickness values; andAttorney Docket No. 44023977WO01; 05542-1628WO1 constructing, using the thickness values, the two-dimensional map of the thickness of the layer on the substrate.

18. The apparatus of claim 14, further comprising: an optical sensor; a position sensor disposed adjacent to the platen and configured to sense when the optical sensor is beneath the substrate; and a flag attached to a periphery of the platen and configured to interrupt an optical signal of the optical sensor when the position sensor sweeps beneath the substrate, wherein the control circuitry is configured to determine an orientation of the platen by determining when the optical signal is interrupted.

19. A method of operating a polishing system, comprising: polishing a layer of a substrate by pressing a substrate held in a carrier head against a polishing pad while rotating the carrier head and rotating a platen supporting the polishing pad, wherein the substrate is free to rotate relative to the carrier head during polishing of the layer; during polishing, monitoring a thickness of the layer with an in-situ monitoring system; and before or after polishing, acquiring thickness measurements of the layer at a plurality of different positions on the substrate by holding the substrate in the carrier head to contact the polishing pad and rotating the platen and carrier head so as to repeatedly sweep a sensor of the in-situ monitoring system across the substrate while the substrate is chucked to carrier head to prevent rotation of the substrate relative to the carrier head.

20. The method of claim 19, wherein polishing comprises supplying an abrasive polishing liquid to the polishing pad, and further comprising rinsing the polishing pad to remove the abrasive polishing liquid before, during, or both before and during the monitoring thickness measurements of the layer.

21. The method of claim 19, wherein polishing the layer and acquiring thickness measurements of the layer are performed without removing the substrate from the polishing pad.

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