Apparatus for measuring substrate thickness using acoustic energy and related methods

Abstract

Embodiments of the present disclosure generally relate to chemical mechanical polishing (CMP) systems. More particularly, embodiments of the present disclosure relate to an in situ acoustic monitoring system that can measure the thickness of a substrate as it is being planarized, and related methods. In one or more embodiments, a method of determining a thickness of a substrate includes emitting an acoustic pulse towards a substrate and receiving a first reflection of the acoustic pulse from the substrate at a first time. The method further includes receiving a second reflection of the acoustic pulse at a second time and measuring a difference between the first time and the second time to determine a time of flight. The method further includes determining a thickness of the substrate based at least in part on the time of flight.

Description

PATENTAttorney Docket No.: 44025185WO01APPARATUS FOR MEASURING SUBSTRATE THICKNESS USING ACOUSTIC ENERGY AND RELATED METHODSBACKGROUNDField

[0001] Embodiments of the present disclosure generally relate to chemical mechanical polishing (CMP) systems. More particularly, embodiments of the present disclosure relate to an in situ acoustic monitoring system that can measure the thickness of a substrate as it is being planarized, and related methods.Description of the Related Art

[0002] 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 planarizing the one or more layers deposited over the substrate. For certain applications, substrate is planarized until a desired thickness is achieved. 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 typically placed against a rotating polishing pad. The carrier head provides a controllable load on the substrate to push it against the polishing pad. An abrasive polishing slurry is typically supplied to the surface of the polishing pad.

[0003] One problem in CMP is determining whether the polishing process is complete, such as when a polishing endpoint has been reached, e.g., whether a substrate layer has been planarized to a desired flatness or thickness, when a desired amount of material has been removed, or when an underlying layer has been exposed. Variations in the slurry distribution, 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, as well as variations in the initial thickness of the substrate layer cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint usually cannot be determined merely as a function of polishing time.PATENTAttorney Docket No.: 44025185WO01

[0004] For the foregoing reasons, there is a need for a system and related methods that can measure the thickness of a substrate in real time as it is being planarized.SUMMARY

[0005] Embodiments of the present disclosure generally relate to chemical mechanical polishing (CMP) systems. More particularly, embodiments of the present disclosure relate to an in situ acoustic monitoring system that can measure the thickness of a substrate as it is being planarized, and related methods.

[0006] In one or more embodiments, a method of determining a thickness of a substrate includes emitting an acoustic pulse towards a substrate and receiving a first reflection of the acoustic pulse from the substrate at a first time. The method further includes receiving a second reflection of the acoustic pulse at a second time and measuring a difference between the first time and the second time to determine a time of flight. The method further includes determining a thickness of the substrate based at least in part on the time of flight.

[0007] In one or more embodiments, a chemical mechanical polishing (CMP) apparatus includes a platen comprising a recess. A polishing pad is disposed on the platen. An acoustic monitoring system is at least partially disposed within the recess. The acoustic monitoring system includes an acoustic transducer, an acoustic window, and a pulser.

[0008] In one or more embodiments, a non-transitory computer-readable medium storing instructions that, when executed by a processor, cause a computer system to perform a method. The method includes emitting, via an acoustic monitoring system disposed at least partially in a platen, an acoustic pulse towards a substrate disposed in a carrier head and receiving, via the acoustic monitoring system, a first reflection of the acoustic pulse from the substrate at a first time. The method further includes receiving, via the acoustic monitoring system, a second reflection of the acoustic pulse at a second time and measuring a difference between the first time and the second time to determine a time of flight. The method further includes determining a thickness of the substrate based at least in part on the time of flight.PATENTAttorney Docket No.: 44025185WO01BRIEF DESCRIPTION OF THE DRAWINGS

[0009] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of the disclosure and are therefore not to be considered limiting of its scope, as the disclosure may admit to other equally effective embodiments.

[0010] Figure 1 is a schematic cross-sectional view of a polishing apparatus, according to one or more embodiments.

[0011] Figures 2A, 2B, and 2C are close-up schematic cross-sectional views of the acoustic monitoring system, according to embodiments.

[0012] Figure 3 is a schematic cross-sectional view of an acoustic pulse reflecting off a substrate, according to one or more embodiments.

[0013] Figure 4 is a schematic graphical representation of pulses of acoustic energy emitted by the acoustic transducer during a measuring process, according to one or more embodiments.

[0014] Figure 5 is a schematic graphical representation of the first pulse and the plurality of echoes measured within the acoustic window after the first pulse, according to one or more embodiments.

[0015] Figure 6 is a schematic graphical representation of the first echo, according to one or more embodiments.

[0016] Figure 7 is a schematic block diagram view of a method 700 of determining a thickness of a substrate, according to one or more embodiments

[0017] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation.PATENTAttorney Docket No.: 44025185WO01DETAILED DESCRIPTION

[0018] Embodiments of the present disclosure generally relate to chemical mechanical polishing (CMP) systems. More particularly, embodiments of the present disclosure relate to an in situ acoustic monitoring system that can measure the thickness of a substrate as it is being planarized, and related methods.

[0019] Figure 1 is a schematic cross-sectional view of a polishing apparatus 100, according to one or more embodiments. The polishing apparatus 100 includes a rotatable disk-shaped platen 120 on which a polishing pad 110 is situated. In one or more embodiments, the polishing pad 110 is a two-layer polishing pad with an outer polishing layer and a softer backing layer. The platen 120 is operable to rotate about an axis 125. For example, a motor 121 , e.g., a DC induction motor, can turn a drive shaft 124 to rotate the platen 120.

[0020] In one or more embodiments, the polishing apparatus 100 includes a port 130 to dispense polishing liquid 132, such as abrasive slurry, onto the polishing pad 110 to the pad. The polishing apparatus can also include a polishing pad conditioner to abrade the polishing pad 110 to maintain the polishing pad 110 in a consistent abrasive state.

[0021] The polishing apparatus 100 includes at least one carrier head 140. The carrier head 140 is operable to hold a substrate 10 against the polishing pad 110. Each carrier head 140 can have independent control of the polishing parameters, for example pressure, associated with each respective substrate.

[0022] In one or more embodiments, the carrier head 140 includes a retaining ring 142 to retain the substrate 10 below a flexible membrane 144. The carrier head 140 also includes one or more independently controllable pressurizable chambers defined by the membrane, e.g., three chambers 146a, 146b, 146c, which can apply independently controllable pressures to associated zones of the flexible membrane 144 and thus on the substrate 10 (see FIG. 1 ). Although only three chambers are illustrated in FIG. 1 for ease of illustration, there could be one or two chambers, or four or more chambers, e.g., five chambers.PATENTAttorney Docket No.: 44025185WO01

[0023] The carrier head 140 is suspended from a support structure 150, e.g., a carousel or track, and is connected by a drive shaft 152 to a carrier head rotation motor 154, e.g., a DC induction motor, so that the carrier head can rotate about an axis 155. Optionally each carrier head 140 can oscillate laterally, e.g., on sliders on the support structure 150, or by rotational oscillation of the support structure 150 itself, or by sliding along the track. In typical operation, the platen 120 is rotated about its central axis 125, and each carrier head is rotated about its central axis 155 and translated laterally across the top surface of the polishing pad 110.

[0024] A controller 190, such as a programmable computer, is connected to the motors 121 , 154 to control the rotation rate of the platen 120 and carrier head 140. For example, each motor can include an encoder that measures the rotation rate of the associated drive shaft. A feedback control circuit, which could be in the motor itself, part of the controller, or a separate circuit, receives the measured rotation rate from the encoder and adjusts the current supplied to the motor to ensure that the rotation rate of the drive shaft matches at a rotation rate received from the controller.

[0025] The polishing apparatus 100 includes at least one acoustic monitoring system 160. The acoustic monitoring system 160 includes one or more acoustic transducers 162. The one or more acoustic transducers 162 include one or more acoustic signal generators that are configured to transmit acoustic energy towards a side of the substrate 10 closer to the polishing pad 110 and one or more acoustic signal sensors configured to detect an acoustic signal. Each acoustic signal sensor or acoustic signal generator can be installed at one or more locations on the platen 120. In particular, the acoustic monitoring system 160 can be configured to detect acoustic emissions caused by stress energy when the material of the substrate 10 undergoes deformation and, in implementations where acoustic signal generators are included, to detect the reflection of actively generated acoustic signals from the surface of the substrate 10. In one or more embodiments, the acoustic monitoring system 160 is an in-situ acoustic monitoring system which can measure the thickness of the substrate 10 in real time during a planarizing process.

[0026] A position sensor (not shown), e.g., an optical interrupter connected to the rim of the platen or a rotary encoder, can be used to sense the angular position ofPATENTAttorney Docket No.: 44025185WO01 the platen 120. This permits only portions of the signal measured when the acoustic transducer 162 is in proximity to the substrate, e.g., when the acoustic transducer 162 is below the carrier head or substrate, to be used in endpoint detection.

[0027] In one or more embodiments, the acoustic transducer 162 is positioned in a recess 164 in the platen 120 and is positioned to receive acoustic signals through an acoustic window 118. The acoustic transducer 162 can be connected by circuitry 168 to a pulser 166 through a rotary coupling, e.g., a mercury slip ring. The pulser 166 can be connected in turn to the controller 190, which can be additionally configured to control the magnitude or frequency of the acoustic energy transmitted by the pulser 166, e.g., by variably increasing or decreasing the current supply to the pulser 166. In one or more embodiments, the pulser 166 is connected to an oscilloscope 167. The oscilloscope 167 measures the acoustic energy emitted and received by the acoustic transducer 162. The pulser 166 is configured to emit an electronic pulse to the acoustic transducer 162. The acoustic transducer 162 converts the electronic pulse into an acoustic pulse. The acoustic pulse has a period. In one or more embodiments, the period of the acoustic pulse is from about 0.002ps to about 0.2 ps.

[0028] In one or more embodiments, the acoustic window 118 includes a volume at least partially filled with a fluid. In one or more embodiments, the fluid includes deionized water. The acoustic window 118 includes a fluid inlet 171 and a fluid outlet 172. The fluid inlet 171 and the fluid outlet 172 are fluidly connected to a pump 170. The pump 170 is configured to flow the fluid from a fluid supply connected to the pump 170 into the acoustic window 118 through the fluid inlet 171. The fluid is then pumped out of the acoustic window 118 through the fluid outlet 172. In one or more embodiments, the pump 170 is connected to the controller 190 which can be additionally configured to control the pump 170. The fluid helps maintain the acoustic pulse as it travels through the acoustic window 118.

[0029] Figures 2A - 2C are close-up schematic cross-sectional views of the acoustic monitoring system 160, according to embodiments. Figure 2A shows the acoustic monitoring system 160, according to one or more embodiments. The acoustic monitoring system 160 includes the acoustic window 118. The acousticPATENTAttorney Docket No.: 44025185WO01 window 118 extends through the recess 164 so that an uppermost surface 201 of the acoustic window 118 is in contact with a lower surface 203 of the polishing pad 110. Therefore, when the acoustic transducer 162 emits a pulse of acoustic energy, the pulse of acoustic energy travels from the acoustic transducer 162 through the acoustic window 118 and through the polishing pad 110. The pulse of acoustic energy then travels through the polishing pad 110 and reflects off the substrate 10. The pulse of acoustic energy reflects off the substrate 10 and then travels back through the polishing pad 110 and the acoustic window 118 where the reflection of the pulse of acoustic energy is received by the one or more acoustic sensors within the acoustic transducer 162.

[0030] Figure 2B shows the acoustic monitoring system 160, according to one or more embodiments. The acoustic monitoring system 160 includes the acoustic window 118. The acoustic window 118 extends through the recess 164 and the polishing pad 110 so that the uppermost surface 201 of the acoustic window 118 is in contact with a lower surface of the substrate 10. Therefore, when the acoustic transducer 162 emits a pulse of acoustic energy, the pulse of acoustic energy travels from the acoustic transducer 162 through the acoustic window 118 and reflects off the substrate 10. The pulse of acoustic energy reflects off the substrate 10 and then travels back through the acoustic window 118 where the reflection of the pulse of acoustic energy is received by the one or more acoustic sensors within the acoustic transducer 162.

[0031] Figure 2C shows the acoustic monitoring system 160, according to one or more embodiments. The acoustic monitoring system 160 includes the acoustic transducer 162. The acoustic transducer extends through the recess 164 and the polishing pad 110 so that an upper surface 205 of the acoustic transducer 162 is in contact with a lower surface of the substrate 10. Therefore, when the acoustic transducer 162 emits a pulse of acoustic energy, the pulse of acoustic energy travels from the acoustic transducer 162 and reflects off the substrate 10. The pulse of acoustic energy reflects off the substrate 10 and then travels back to the acoustic transducer 162 where the reflection of the pulse of acoustic energy is received by the one or more acoustic sensors.PATENTAttorney Docket No.: 44025185WO01

[0032] Figure 3 is a schematic cross-sectional view of an acoustic pulse P1 reflecting off a substrate 10, according to one or more embodiments. The acoustic pulse P1 is emitted by the acoustic transducer 162. The acoustic pulse P1 travels through the acoustic window 118 until it reaches the substrate 10. Once the acoustic pulse P1 reaches a first surface 301 of the substrate 10, a first portion of the acoustic pulse P1 bounces off the first surface 301 as a first reflection R1 . A second portion of the first pulse P1 continues to travel through the substrate 10 to a second surface 302 of the substrate 10. The second portion of the first pulse P1 bounces off the second surface 302 as a second reflection R2. The first reflection R1 travels back through the acoustic window 118 to the acoustic transducer 162 where the one or more acoustic sensors measure the first reflection R1 at a first time. The second reflection R2 travels back through the substrate 10 and the acoustic window 118 to the acoustic transducer 162 where the one or more acoustic sensors measure the second reflection R2 at a second time. The substrate 10 has a thickness T. The difference between the first time and the second time are used to calculate the thickness T of the substrate 10. During a CMP process, the thickness T of the substrate 10 is decreased as the substrate 10 gets planarized. As the thickness T decreases the distance between the first time and the second time decreases. The thickness T is decreased until a desired difference between the first time and the second time is reached.

[0033] In one or more embodiments, the acoustic pulse P1 has a wavelength from about 5MHz to about 500 MHz. The acoustic pulse P1 has a period of about 0.002 ps to about 0.2 ps. The range of the thickness T detectable by the acoustic pulse P1 depends on the wavelength of the acoustic pulse P1 and the material of the substrate 10. For example, if the acoustic pulse has a wavelength of 5MHz, then the minimum detectable thickness of a silicon dioxide substrate is about 119 pm. If the acoustic pulse has a wavelength of 500MHz, then the minimum detectable thickness of a silicon dioxide substrate is about 1 pm. The minimum thickness of the substrate 10 is about 0.1 pm. In one or more embodiments, the acoustic pulse P1 is used to measure a thickness of layers disposed over a substrate.

[0034] Figure 4 is a schematic graphical representation of pulses of acoustic energy emitted by the acoustic transducer 162 during a measuring process,PATENTAttorney Docket No.: 44025185WO01 according to one or more embodiments. As seen in the graph 400 the acoustic transducer 162 emits a plurality of acoustic pulses, P1 , P2, P3 during a measuring process. In one or more embodiments, each acoustic pulse P1 , P2, P3 has a period PR of about 0.2 ps. In one or embodiments, each pulse P1 , P2, P3 is separated by a length of time.

[0035] Figure 5 is a schematic graphical representation of the first pulse P1 and the plurality of echoes measured within the acoustic window 118 after the first pulse P1 , according to one or more embodiments. During a measuring process a first pulse P1 is emitted by the acoustic transducer 162 towards the substrate 10. After a first length of time, a reflection of the first pulse P1 is reflected from the substrate 10 back to the acoustic transducer 162. The one or more acoustic sensors within the acoustic transducer 162 measures the first reflection of the first pulse P1 as a first echo E1. The first echo E1 bounces off the acoustic transducer 162 back to the substrate 10. After a second length of time a reflection of the first echo E1 is reflected from the substrate 10 back to the acoustic transducer 162. The one or more acoustic sensors within the acoustic transducer 162 measures the reflection of the first echo E1 as a second echo E2. The second echo E2 bounces off the acoustic transducer 162 back to the substrate 10. After a third length of time a reflection of the second echo E2 is reflected from the substrate 10 back to the acoustic transducer 162. The one or more acoustic sensors within the acoustic transducer 162 measures the reflection of the second echo E2 as a third echo E3. In one or more embodiments, the third echo is reflected back by the substrate to form additional echoes.

[0036] Figure 6 is a schematic graphical representation of the first echo E1 , according to one or more embodiments. The graph 600 is a zoomed in representation of the echo E1 shown in the graph 500. The first echo E1 includes two peaks of high sound intensity. These peaks are the first reflection R1 and the second reflection R2. During a measuring process the acoustic transducer 162 emits a first pulse P1 of acoustic energy. The first pulse P1 travels through the acoustic window 118 to the substrate 10. As shown in Figure 3, a portion of the first pulse P1 is reflected of the first surface 301 of the substrate 10 as the first reflection R1. A portion of the second pulse P2 is reflected of the second surface 302 of thePATENTAttorney Docket No.: 44025185WO01 substrate 10 as the second reflection R2. As seen in the graph 600, at the first time T1 the first reflection R1 hits the one or more sensors within the acoustic transducer 162. At the second time T2 the second reflection R2 hits the one or more sensors within the acoustic transducer 162. The second time T2 is greater than the first time T1 because the first reflection R2 needs to travel a greater distance than the first reflection R1 to reach the acoustic transducer 162. The thickness T of the substrate can be determined by measuring the difference between the first time T1 and the second time T2. As the substrate 10 gets planarized, the thickness T decreases and the difference between the first time T1 and the second time T2 decreases.

[0037] Figure 7 is a schematic block diagram view of a method 700 of determining a thickness of a substrate, according to one or more embodiments. In one or more embodiments, the method 700 is performed using one or more components of the polishing apparatus 100 described herein.

[0038] Operation 702 of the method 700 includes positioning a substrate over a polishing pad on a polishing apparatus. In one or more embodiments, the polishing apparatus 100 includes the acoustic monitoring system 160. The substrate 10 is positioned on the polishing pad 110 so that at least a portion of the substrate 10 is positioned over the acoustic window 118 of the acoustic monitoring system 160. In one or more embodiments, the controller 190 determines the position of the substrate 10.

[0039] Operation 704 of the method 700 includes emitting a first acoustic pulse from an acoustic transducer. In one or more embodiments, the pulser 166 sends an electronic pulse to the acoustic transducer 162. In turn, the acoustic transducer 162 emits a first acoustic pulse P1. In one or more embodiments, the first acoustic pulse P1 travels through the acoustic window 118 to the substrate 10. In one or more embodiments, the acoustic transducer 162 is in contact with the substrate 10. In one or more embodiments, the first pulse P1 travels through the polishing pad 110 before hitting the substrate 10.

[0040] Once the first pulse P1 reaches the substrate 10, a first portion of the first pulse P1 is reflected off the first surface 301 of the substrate 10 back towards the acoustic transducer 162 as the first reflection R1. A second portion of the first pulsePATENTAttorney Docket No.: 44025185WO01P1 travels through the substrate 10 and is reflected off of the second surface 302 of the substrate back towards the acoustic transducer 162 as the second reflection R2.

[0041] Operation 706 of the method 700 includes receiving the first reflection of the first acoustic pulse from the substrate. In one or more embodiments, the first reflection R1 is reflected off the first surface 301 of the substrate 10 back into the acoustic window 118. The first reflection R1 travels back to the acoustic transducer 162 where one or more sensors receive the first reflection R1 at a first time T1 .

[0042] Operation 708 of the method 700 includes receiving the second reflection of the first acoustic pulse from the substrate. In one or more embodiments, the second reflection R2 is reflected off the second surface 302 of the substrate 10 back into the acoustic window 118. The second reflection R2 travels back through the substrate 10 and travels to the acoustic transducer 162 where one or more sensors receive the second reflection R2 at a second time T2. The second reflection R2 is received after the first reflection R1 because the second reflection R2 has to travel a greater distance than the first reflection R1 .

[0043] Operation 710 of the method 700 includes measuring the difference between the first time T1 when the first reflection R1 is detected by the acoustic transducer 162 and the second time T2 where the second reflection R2 is detected by the acoustic transducer 162. The difference between the first time T1 and the second time T2 determines amount of time the second reflection R2 traveled through the substrate 10. The difference between the first time T1 and the second time T2 is also referred to as the time of flight.

[0044] Operation 712 of the method 700 includes determining the thickness of the substrate using the time of flight. The controller 190 calculates the thickness of the substrate 10 by determining the time of flight as described in operation 710 and empirical data stored within the controller 190. The empirical data includes the material of the substrate 10, the material of the polishing pad 110, the material of the acoustic window 118, the distance of the acoustic transducer 162 to the substrate 10, the frequency of the first pulse P1 emitted by acoustic transducer 162, and the speed of the first pulse P1 as it travels through the acoustic window 118 and the substrate 10. The controller 190 calculates the thickness of the substrate 10.PATENTAttorney Docket No.: 44025185WO01

[0045] In one or more embodiments operations 704-712 are repeated until the substrate 10 reaches a desired thickness. In one or more embodiments a second acoustic pulse is emitted towards the substrate 10. A first portion of the second acoustic pulse reflects of the first surface 301 of the substrate 10 to form a third reflection. A second portion of the second acoustic pulse reflects of the second surface 302 of the substrate 10 to form a fourth reflection. The acoustic transducer 162 receives the third reflection at a third time and the fourth reflection at a fourth time. The difference between the third time and the fourth time is measured to determine a second time of flight. The second time of flight is used to determine a second thickness of the substrate 10. As the polishing process continues, the thickness of the substrate 10 decreases.

[0046] Benefits of the present disclosure include a polishing apparatus 100 having an acoustic monitoring system 160 that can measure the thickness of a substrate 10 in real time during a processing operation. Additional benefits include accurate measurements of substrate thickness, decreased production time, increased device performance, decreased cost, and decreased maintenance.

[0047] It is contemplated that one or more aspects disclosed herein may be combined. As an example, one or more aspects, features, components, operations and / or properties of the polishing apparatus 100, the platen 120, the polishing pad 110, the port 130, the carrier head 140, the substrate 10, the controller 190, the acoustic monitoring system 160, the acoustic transducer 162, the pulser 166, the acoustic window 118, the pump 170, and / or the method 700 may be combined. Moreover, it is contemplated that one or more aspects disclosed herein may include some or all of the aforementioned benefits.

[0048] While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

PATENTAttorney Docket No.: 44025185WO01What is claimed is:1 . A method of determining a thickness of a substrate, comprising: emitting an acoustic pulse towards a substrate; receiving a first reflection of the acoustic pulse from the substrate at a first time; receiving a second reflection of the acoustic pulse at a second time; measuring a difference between the first time and the second time to determine a time of flight; and determining a thickness of the substrate based at least in part on the time of flight.

2. The method of claim 1 , further comprising positioning a substrate over a polishing pad.

3. The method of claim 1 , further comprising: emitting a second acoustic pulse towards a substrate; receiving a third reflection of the second acoustic pulse from the substrate at a third time; receiving a fourth reflection of the second acoustic pulse at a fourth time; measuring a difference between the third time and the fourth time to determine a second time of flight; and determining a second thickness of the substrate using the second time of flight.

4. The method of claim 1 , wherein the acoustic pulse is from about 5MHz to about 500 MHz.

5. The method of claim 1 , wherein the acoustic pulse has a period.PATENTAttorney Docket No.: 44025185WO016. The method of claim 5, wherein the period is from about 0.002 ps to about 0.2 ps..

7. A chemical mechanical polishing (CMP) apparatus, comprising: a platen comprising a recess; a polishing pad disposed on the platen; and an acoustic monitoring system at least partially disposed within the recess, the acoustic monitoring system comprising: an acoustic transducer; an acoustic window; and a pulser.

8. The CMP apparatus of claim 7, further comprising: a carrier head configured to hold a substrate against the polishing pad; and a motor to generate relative motion between the platen and the carrier head so as to polish the substrate.

9. The CMP apparatus of claim 7, wherein the acoustic monitoring system further comprises: a fluid inlet in fluid communication with the acoustic window; a fluid outlet in fluid communication with the acoustic window; and a pump in fluid communication with the fluid inlet and configured to pump a fluid into the acoustic window via the fluid inlet.

10. The CMP apparatus of claim 9, wherein the fluid includes deionized water.11 . The CMP apparatus of claim 7, wherein the acoustic window extends through the recess to an upper surface of the platen.

12. The CMP apparatus of claim 7, wherein the acoustic window extends through the recess and the polishing pad to an upper surface of the polishing pad.PATENTAttorney Docket No.: 44025185WO0113. The CMP apparatus of claim 7, wherein the acoustic transducer extends through the acoustic window.

14. The CMP apparatus of claim 7, wherein the acoustic transducer emits an acoustic pulse from about 5MHz to about 500 MHz.

15. A non-transitory computer-readable medium storing instructions that, when executed by a processor, cause a computer system to perform a method comprising: emitting, via an acoustic monitoring system disposed at least partially in a platen, an acoustic pulse towards a substrate disposed in a carrier head; receiving, via the acoustic monitoring system, a first reflection of the acoustic pulse from the substrate at a first time; receiving, via the acoustic monitoring system, a second reflection of the acoustic pulse at a second time; measuring a difference between the first time and the second time to determine a time of flight; and determining a thickness of the substrate based at least in part on the time of flight.

16. The medium of claim 15, wherein the method further comprises positioning a substrate over a polishing pad, disposed on the platen, using the carrier head.

17. The medium of claim 15, wherein the method further comprises: emitting, via the acoustic monitoring system, a second acoustic pulse towards a substrate; receiving, via the acoustic monitoring system, a third reflection of the second acoustic pulse from the substrate at a third time; receiving, via the acoustic monitoring system, a fourth reflection of the second acoustic pulse at a fourth time; measuring a difference between the third time and the fourth time to determine a second time of flight; andPATENTAttorney Docket No.: 44025185WO01 determining a second thickness of the substrate using the second time of flight.

18. The medium of claim 15, wherein the acoustic pulse is from about 5MHz to about 500 MHz.

19. The medium of claim 15, wherein the acoustic pulse has a period.

20. The medium of claim 19, wherein the period is from about 0.002 ps to about 0.2 ps.

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