WO2011142975A2

WO2011142975A2 - Pad window insert

Info

Publication number: WO2011142975A2
Application number: PCT/US2011/034215
Authority: WO
Inventors: Boguslaw A. Swedek; Dominic J. Benvegnu
Original assignee: Applied Materials, Inc.
Priority date: 2010-05-12
Filing date: 2011-04-27
Publication date: 2011-11-17
Also published as: CN102893377A; TWI611866B; KR20130103674A; JP2013526420A; TWI663021B; KR20170088444A; TW201817541A; KR101762936B1; TWI663020B; US20110281510A1; KR101956848B1; WO2011142975A3; TW201742705A; TW201217104A; CN102893377B

Abstract

A polishing pad includes a polishing layer having a polishing surface, an adhesive layer on a side of the polishing layer opposite the polishing layer, and a solid light-transmitting window extending through and molded to the polishing layer. The solid light-transmitting window has an upper portion with a first lateral dimension and a lower portion with a second lateral dimension that is smaller than the first lateral dimension. A top surface of the solid light-transmitting window coplanar with the polishing surface and a bottom surface of the solid light-transmitting window coplanar with a lower surface of the adhesive layer.

Description

PAD WINDOW INSERT

TECHNICAL FIELD

[0001] A polishing pad with a window, a system containing such a polishing pad, and a process for making and using such a polishing pad are described.

BACKGROUND

[0002] In the process of fabricating modern semiconductor integrated circuits (IC), it is often necessary to planarize the outer surface of the substrate. For example, planarization may be needed to polish away a conductive filler layer until the top surface of an underlying layer is exposed, leaving the conductive material between the raised pattern of the insulative layer to form vias, plugs and lines that provide conductive paths between thin film circuits on the substrate. In addition, planarization may be needed to flatten and thin an oxide layer to provide a flat surface suitable for photolithography.

[0003] One method for achieving semiconductor substrate planarization or topography removal is chemical mechanical polishing (CMP). A conventional chemical mechanical polishing (CMP) process involves pressing a substrate against a rotating polishing pad in the presence of an abrasive slurry.

[0004] In general, there is a need to detect when the desired surface planarity or layer thickness has been reached or when an underlying layer has been exposed in order to determine whether to stop polishing. Several techniques have been developed for the in- situ detection of endpoints during the CMP process. For example, an optical monitoring system for in-situ measuring of uniformity of a layer on a substrate during polishing of the layer has been employed. The optical monitoring system can include a light source that directs a light beam toward the substrate during polishing, a detector that measures light reflected from the substrate, and a computer that analyzes a signal from the detector and calculates whether the endpoint has been detected. In some CMP systems, the light beam is directed toward the substrate through a window in the polishing pad. SUMMARY

[0005] A window can be attached to the underside of a polishing pad such that a portion of the window rests in a recess in a platen. This can permit a large surface area contact between the window and the pad so as to increase bonding strength between the window and the polishing pad.

[0006] In one aspect, a polishing apparatus includes a platen having a planar upper surface, a recess formed in the upper surface, the recess having a bottom surface, and a passage connected to the lower surface of the recess, as well as a polishing pad comprising a polishing layer, a polishing surface, and underside and an aperture therethrough, the aperture having a smaller lateral dimension than the recess, the aperture aligned with the passage. A solid light-transmitting window has a first portion positioned at least partially in the aperture in the polishing pad and a second portion positioned at least partially in the recess in the platen, the second portion having a larger lateral dimension than the first portion and extending below the polishing layer, the second portion of the window adhesively attached to an underside of the polishing pad.

[0007] Implementations can include one or more of the following features. The first portion of the window can plug the aperture in the polishing pad. A top surface of the first portion of the window can be coplanar with the upper surface of the platen. The bottom surface of the recess can be parallel with the upper surface of the platen. A lower surface of the second portion of window can contact the lower surface of the recess. The lower surface of the second portion of window may not be adhered to the lower surface of the recess. The polishing apparatus can also include an adhesive layer spanning the polishing layer. The adhesive layer can include a double-sided adhesive tape. The adhesive layer can abut the polishing layer. The underside of the polishing pad can be adhesively attached to the upper surface of the platen by the adhesive layer. A top surface of the second portion of the window can be adhesively attached to underside of the polishing pad by the adhesive layer. A top surface of the second portion of the window can be adhesively attached to underside of the polishing pad. The polishing pad can include the polishing layer. The polishing pad can include the polishing layer and a lower layer that is less compressible than the polishing layer. The second portion can have a lateral dimension between two and ten times larger, e.g., about eight times larger, than the first portion. The second portion of the window can laterally fill the recess in the platen. The polishing pad can have a thickness less than 1 mm. The polishing apparatus can also include an optical fiber in the passage and positioned to direct or receive light through the first portion of the window. The optical fiber can be wider than the first portion of the window. The sides of the recess can be sloped and sides of the second portion of the window can be sloped.

[0008] In another aspect, method of assembling a window for a polishing apparatus includes forming an aperture through a polishing pad, the polishing pad comprising a polishing layer having a polishing surface and underside, forming a solid light-transmitting window having a first portion and a second portion having a larger lateral dimension than the first portion, inserting the first portion of the window into the aperture of the pad, adhering a top surface of the second portion of the window to the underside of the polishing pad, and positioning the polishing pad and window on a platen such that the second portion of the window fits into a recess in a planar upper surface of the platen and the underside of the polishing pad is adhered to the planar upper surface of the platen.

[0009] Implementations can include one or more of the following features. A layer of adhesive can be formed on the bottom of the polishing layer and a liner covers the adhesive, a portion of the liner can be removed around the aperture, and the top surface of the second portion of the window can contact the adhesive in the removed portion of the liner.

[0010] Implementations may include the following potential advantages. A strong bond can be formed between the window and a thin polishing pad, reducing the likelihood of slurry leakage and reducing the likelihood of the window being pulled from the pad due to shear force from the substrate being polished. In addition, the polishing pad can improve wafer-to-wafer uniformity of spectrum reflected from the substrate, particularly at short wavelengths.

[0011] The details of one or more embodiments 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. DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a cross-sectional view of a CMP apparatus containing a polishing pad.

[0013] FIG. 2 is a top view of an embodiment of a polishing pad with a window.

[0014] FIG. 3 A is a cross-sectional view of the polishing pad of FIG. 2 installed on a platen.

[0015] FIG. 3B is a cross-sectional view of the polishing pad of FIG. 2.

[0016] FIGS. 4-7 illustrate a method of forming a polishing pad.

[0017] FIG. 8 is a cross-sectional view of another implementation the polishing pad installed on a platen.

[0018] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0019] A window can be attached to the underside of a polishing pad such that a portion of the window rests in a recess in a platen. This can permit a large surface area contact between the window and the pad so as to increase bonding strength between the window and the polishing pad.

[0020] As shown in FIG. 1, the CMP apparatus 10 includes a polishing head 12 for holding a semiconductor substrate 14 against a polishing pad 18 on a platen 16. The CMP apparatus may be constructed as described in U.S. Patent No. 5,738,574, the entire disclosure of which is incorporated herein by reference.

[0021] The substrate can be, for example, a product substrate (e.g., which includes multiple memory or processor dies), a test substrate, a bare substrate, and a gating substrate. The substrate can be at various stages of integrated circuit fabrication, e.g., the substrate can be a bare wafer, or it can include one or more deposited and/or patterned layers. The term substrate can include circular disks and rectangular sheets.

[0022] The effective portion of the polishing pad 18 can include a polishing layer 20 with a polishing surface 24 to contact the substrate and a bottom surface 22 secured to the platen 16 by an adhesive layer 28, e.g., an adhesive tape. The adhesive 28 can be a pressure-sensitive adhesive. Other than the adhesive tape and any liner, the polishing pad can be, e.g., consist of, a single-layer pad, with the polishing layer 20 formed of a thin durable material suitable for a chemical mechanical polishing process. Thus, the layers of the polishing pad can consist of the single-layer polishing layer 20 and the adhesive layer 28 (and optionally a liner, which would be removed when the pad is installed on the polishing platen).

[0023] The polishing layer 20 can be, e.g., consist of, a foamed polyurethane, with at least some open pores on the polishing surface 24. The adhesive layer 28 can be a double-sided adhesive tape, e.g., a thin layer of polyethylene terephthalate (PET), e.g., Mylar®, with adhesive, e.g., pressure-sensitive adhesive, on both sides. Such a polishing pad is available under the trade name H7000FFN from Fujibo in Tokyo, Japan.

[0024] Referring to FIG. 2, in some implementations the polishing pad 18 has a radius R of 15.0 (381.00 mm) to 15.5 inches (393.70 mm), with a corresponding diameter of 30 to 31 inches. In some implementations, the polishing pad 18 can have a radius of 21.0 (533.4 mm) to 21.5 inches (546.1 mm), with corresponding diameter of 42 to 43 inches.

[0025] Referring to FIG. 3A, in some implementations, grooves 26 can be formed in the polishing surface 24. The grooves can be in a "waffle" pattern, e.g., a cross-hatched pattern of perpendicular grooves with sloped side walls that divide the polishing surface into rectangular, e.g., square, areas.

[0026] Returning to FIG. 1, typically the polishing pad material is wetted with the chemical polishing liquid 30, which can include abrasive particles. For example, the slurry can include KOH (potassium hydroxide) and fumed-silica particles. However, some polishing processes are "abrasive-free".

[0027] The polishing head 12 applies pressure to the substrate 14 against the polishing pad 18 as the platen rotates about its central axis. In addition, the polishing head 12 is usually rotated about its central axis, and translated across the surface of the platen 16 via a drive shaft or translation arm 32. The pressure and relative motion between the substrate and the polishing surface, in conjunction with the polishing solution, result in polishing of the substrate.

[0028] An optical aperture 34 is formed in the top surface of the platen 16. An optical monitoring system, including a light source 36, such as a laser, and a detector 38, such as a photodetector, can be located below the top surface of the platen 16. For example, the optical monitoring system can be located in a chamber inside the platen 16 that is in optical communication with the optical aperture 34, and can rotate with the platen. One or more optical fibers 50 can carry light from the light source 36 to the substrate, and from the substrate to the detector 38. For example, the optical fiber 50 can be a bifurcated optical fiber, with a trunk 52 in proximity, e.g., abutting, the window 40 in the polishing pad, a first leg 54 connected to the light source 36, and a second leg 56 connected to the detector 38.

[0029] The optical aperture 34 can be filled with a transparent solid piece, such as a quartz block (in which case the fiber would not abut the window 40 but could abut the solid piece in the optical aperture), or it can be an empty hole. In one

implementation, the optical monitoring system and optical aperture are formed as part of a module that fits into a corresponding recess in the platen. Alternatively, the optical monitoring system could be a stationary system located below the platen, and the optical aperture could extend through the platen. The light source 36 can employ a wavelength anywhere from the far infrared to ultraviolet, such as red light, although a broadband spectrum, e.g., white light, can also be used, and the detector 38 can be a spectrometer.

[0030] A solid window 40 is formed in the overlying polishing pad 18 and aligned with the optical aperture 34 in the platen. The window 40 and aperture 34 can be positioned such that they have a view of the substrate 14 held by the polishing head 12 during at least a portion of the platen's rotation, regardless of the translational position of the head 12.

[0031] The light source 36 projects a light beam through the aperture 34 and the window 40 to impinge the surface of the overlying substrate 14 at least during a time when the window 40 is adjacent the substrate 14. Light reflected from the substrate forms a resultant beam that is detected by the detector 38. The light source and the detector are coupled to an unillustrated computer that receives the measured light intensity from the detector and uses it to determine the polishing endpoint, e.g., by detecting a sudden change in the reflectivity of the substrate that indicates the exposure of a new layer, by calculating the thickness removed from of the outer layer (such as a transparent oxide layer) using interferometric principles, by monitoring the spectrum of the reflected light and detecting a target spectrum, by matching a sequence of measured spectra to reference spectra from a library and determining where a linear function fit to index values of the reference spectrum reaches a target value, or by otherwise monitoring the signal for predetermined endpoint criteria.

[0032] One problem with placement of a normal large rectangular window (e.g., a 2.25 by 0.75 inch window) into a very thin polishing layer is delamination during polishing. In particular, the lateral frictional force from the substrate during polishing can be greater than the adhesive force of the molding of the window to the sidewall of the pad.

[0033] Returning to FIG. 2, the window 40 can be small, e.g., less than about 3 mm in diameter, e.g., so as to reduce the frictional force applied by the substrate during polishing. For example, the upper portion of the window 40 can be a circular area about 3 mm wide centered a distance D of about 7.5 inches (190.50 mm) from the center of a 30 to 31 inch diameter polishing pad 18, or centered a distance D of about 9 to 11 inches from the center of a 42 to 43 inch diameter polishing pad 18.

[0034] The window 40 can have an approximately circular shape (other shapes are possible, such as rectangular). If the window is elongated, its longer dimension can be substantially parallel to the radius of the polishing pad that passes through the center of the window. The window 40 can have a ragged perimeter 42, e.g., the perimeter can be longer than a perimeter of a similarly shaped circle or rectangle, e.g., a zig-zag or other meandering pattern (in a top view). This increases the surface area for contact of the window to the sidewall of the polishing pad, and can thereby improve adhesion of the window to the polishing pad.

[0035] Referring to FIG. 3A, the window 40 includes an upper portion 40a and a lower portion 40b. The window 40, including the upper portion 40a and lower portion 40b, can be a unitary single-piece body of homogeneous material. The upper portion 40a is vertically aligned with the lower portion 40b but is laterally smaller (i.e., in one or both directions parallel to the polishing surface) than the lower portion 40b. Thus, a portion of the polishing layer 20 projects above the lower portion 40b so that the rim of the lower portion 40b that projects beyond the upper portion 40a forms a ledge 49. The lower portion 40b can project laterally beyond the upper portion 40a on all sides of the window 40, or optionally the lower portion 40b can project laterally beyond the upper portion 40a on two opposing sides of the window 40 but be aligned along other sides of the window 40. The upper surface of the lower portion 40b that projects beyond the upper portion 40a can be a substantially planar surface. The upper portion 40a can be located in the center of, e.g., be concentric with, the lower portion 40b. The lower portion 40b can have a lateral dimension 2 to 10 times, e.g., about 8 times, as large as the lateral dimension of the upper portion 40a. For example, if the window 40 is circular, the upper portion 40a can have a diameter of 3 mm, and the lower portion 40b can have a diameter of 25 mm.

[0036] The upper portion 40a can be about the same thickness as the lower portion 40b. Alternatively, the upper portion 40a can be thicker than, or be thinner than, the lower portion 40b.

[0037] The upper portion 40a of the window 40 can project into an aperture in the adhesive layer 28. The edge of the adhesive layer 28, e.g., adhesive tape, can abut the sides of the upper portion 40a of the window 40. The lower portion 40b of the window can project into a recess 78 in the top surface 76 of the platen 16.

[0038] The upper portion 40a of the window is as thick as the combination of the polishing layer 20 and the adhesive layer 28. A top surface 44 of the upper portion 40a of the window 40 is coplanar with the polishing surface 24. A bottom of the upper portion 40a of the window 40 can be coplanar with a bottom surface of the adhesive layer 28.

[0039] The upper surface of the lower portion 40b is secured to the underside of the polishing layer 20 by a portion of the adhesive layer 28. Optionally the perimeter of the upper portion 40a of the window 40 can be secured to the inner sidewall edges 48 of the polishing layer 20, e.g., by additional adhesive.

[0040] The increased surface area of connection between the window 40 and the polishing layer 20 provided by the connection on the ledge 49 can provide a strong bond, reducing the likelihood of slurry leakage and reducing the likelihood of the window 40 being pulled from the polishing pad 18 due to shear force from the substrate being polished. The trunk 52 of the optical fiber abuts or nearly abuts the lower portion 40b. In some implementations, the trunk 52 may be wider than the upper portion 40a of the window 40. [0041] The bottom surface of the lower portion 40b of the window can abut, e.g., rests on, without being adhesively or otherwise secured to the bottom of the recess 78 in the upper surface 76 of the platen 16. In some implementations, the lower portion 40b of the window fills the recess 78.

[0042] Referring to FIG. 3B , before installation on the platen 16, the polishing pad 18 can also include a liner 70 that spans the adhesive layer 28 on the bottom surface 22 of the polishing pad 18, except in the region covered by the adhesive lower portion 40b of the window 40. The liner 70 can be a thin flexible material, e.g., paper, with a release coating so that it can be peeled away from the adhesive 28. In some

implementations, the liner can be an incompressible and generally fluid-impermeable layer, for example, polyethylene terephthalate (PET), e.g., Mylar®. In use, the liner 70 is manually peeled from the adhesive layer 28, and the polishing layer 20 is applied to the platen 16 with the adhesive layer 28. The liner 70, however, does not span the window 40, but is removed in and immediately around the region of the lower portion 40b of the window 40, e.g., in a region about 25 cm across, to form a hole 72 into which the lower portion 40b of the window 40 fits.

[0043] The polishing pad 18 is very thin, e.g., less than 2 mm, e.g., less than 1 mm. For example, the total thickness of the polishing layer 20, adhesive 28 and liner 70 can be about 0.8 or 0.9 mm. The polishing layer 20 can be about 0.7 or 0.8 mm thick, with the adhesive 28 and the liner 70 providing about another 0.1 mm. The grooves 26 can be about half the depth of the polishing pad, e.g., roughly 0.5 mm.

[0044] To manufacture the polishing pad, initially the polishing layer 20 is formed and the bottom surface of the polishing layer 20 is covered with the pressure sensitive adhesive 28 and a liner 70, as shown by FIG. 4. Grooves 26 can be formed in the polishing layer 20 as part of a pad molding process before attachment of the pressure sensitive adhesive 28, or cut into the polishing layer 20 after the pad is formed. The grooves 26 can be formed before or after the liner 70 is attached.

[0045] Referring now to FIG. 5, in some implementations the window 40 may be formed by casting and curing a polymer in the shape of the window 40. In one implementation, the polymer is a mixture of 2 parts Calthane A 2300 and 3 parts

Calthane B 2300 (available from Cal Polymers, Inc. of Long Beach, California). The liquid polymer mixture can be degassed, e.g., for 15-30 minutes, before being placed into the aperture. The polymer can be cured at room temperature for about 24 hours, or a heat lamp or oven can be used to decrease cure time. In some implementations, the polymer can be poured into a mold and cured or otherwise solidified to form the window 40 in its final shape. In some implementations, the window 40 can cured in a large solid block, and then can be formed by machining the solid block of polymer to form the final shape of the window 40.

[0046] In some implementations, a sidewall 84 of the lower portion 40b may be substantially perpendicular to the bottom surface 46 of the window 40. In some implementations, the sidewall 84 may be formed at an angle to the bottom surface 46, as will be discussed further in the description of FIG. 7.

[0047] A hole 82 is punched through the entire polishing pad 18, including the polishing layer 20, the adhesive 28 and the liner 70. The hole 82 is sized to

accommodate the upper portion 40a of the window 40. In some implementations, the upper portion 40a substantially plugs the hole 82 of the polishing pad 18. The hole 82 can be punched from the top (i.e., the side with the polishing surface) of the pad, e.g., by a machine press. This permits the position of the hole 82 to be positioned and sized with a high degree of accuracy and repeatability.

[0048] A portion 72 of the liner 70 is peeled away or otherwise removed from the adhesive layer 28. The liner 70 need not be peeled of the polishing pad 18 entirely at this time. The portion 72 of the liner 70 that is peeled away exposes a portion of a bottom surface 22 of the adhesive layer 28 around the hole 82. The portion 72 that is peeled off can also be cut away, e.g., in a region sized to accommodate the ledge 49 of the bottom portion 40b of the window 40, although this step can be performed at a later time.

[0049] Referring to FIGS. 5 and 6, the window 40 is secured to the polishing pad 18 such that the upper portion 40a extends into the hole 82, and the upper surface of the bottom portion 40b (e.g., the ledge 49) contacts the adhesive layer 28. In some embodiments, the upper portion 40a is sized to extend substantially through and substantially fill the hole 82 such that the upper surface 44 is coplanar with the polishing surface 24 of the polishing layer 20 when the ledge 49 is adhered to the adhesive layer 28.

[0050] In addition to the liner 70, an optional window backing piece 74 can be placed to span the window 40. For example, the window backing piece 74 can be secured to a portion of the adhesive layer 28 immediately around the window 40. The backing piece 74 can be the same thickness as the liner 70, or thinner than the liner 70. The backing piece 74 can be polytetrafluoroethylene (PTFE), e.g., Teflon®, or another non-stick material. The combined polishing pad 18 and window 40 can then be readied for shipment to the customer, e.g., in a sealed plastic bag.

[0051] Referring now to FIG. 7, when the customer receives the combined polishing pad 18 and window 40, the customer can remove the liner 70 (and window backing piece 74 if present) and then attach the polishing pad 18 on the platen 16 using the adhesive layer 28. The lower portion 40b of the window 40 is inserted into a recess 78 in the upper surface 76 of the platen 16. In some methods, the liner 70 can be peeled away partially in an area around the window 40, the lower portion 40b of the window 40 is inserted into the recess, and then the remainder of the liner is peeled away and the rest of the polishing pad is secured to the platen 16.

[0052] The lower portion 40b can be shaped and sized to substantially fill the recess 78, for example, the sidewall 84 of the lower portion 40b can contact substantially all of a sidewall 86 of the recess 78, and the bottom surface 46 of the window 40 substantially contacts a floor 88 of the recess 78 while the upper surface 76 of the platen 16 contacts the adhesive layer 28.

[0053] In some implementations, the floor 88 of the recess 78 may be substantially parallel with the upper surface 76 of the platen 16. In some

implementations, the sidewall 84 of the lower portion 40b is perpendicular to the bottom surface 46, and the sidewall 86 of the recess 78 is perpendicular to the polishing surface 75. Referring to FIG. 8, in some implementations, the sidewall 84 of the lower portion 40b may be formed at a non-perpendicular angle, e.g., between 20° and 80°, e.g., 45°, to the bottom surface 46, and the sidewall 86 of the recess 78 may be formed at a substantially similar angle such that the sidewall 84 and the sidewall 86 substantially contact each other when the lower portion 40b is inserted into the recess 78. For example, the sidewall 84 may slope inward from the ledge 49 to the bottom surface 46, such that the lower portion 40b forms a conic section. Similarly, the sidewall 86 may be formed to mate with the conic section. As such, the sloped sidewall 84 may cause the window 40 and the polishing pad 18 to exhibit a self-centering property as the window 40 is inserted into the recess 78 within the sloped sidewall 86.

[0054] As such, the polishing pad 18 is adhered to the platen 16 by the adhesive layer 28, thereby retaining the window 40 within the recess 78 in the platen 16. The window 40 can be vertically supported by the floor 88 of the recess 78 and can be retained laterally by the sidewalls 86 of the recess 78. The window 40 can be adhered to the polishing pad by the contact of the top surface of the ledge 49 to the same adhesive layer that secures the underside of the polishing pad to the platen 16.

[0055] While certain embodiments have been described, the invention is not so limited. For example, although a window with a simple circular shape is described, the window could be more complex, such as a rectangle, oval or star. The top portion of the window can project past one or more sides of the bottom portion. It will be understood that various other modifications 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

WHAT IS CLAIMED IS:

1. A polishing apparatus, comprising:

a platen having a planar upper surface, a recess formed in the upper surface, the recess having a bottom surface, and a passage connected to the lower surface of the recess;

a polishing pad comprising a polishing layer a polishing surface, and underside and an aperture therethrough, the aperture having a smaller lateral dimension than the recess, the aperture aligned with the passage; and

a solid light-transmitting window having a first portion positioned at least partially in the aperture in the polishing pad and a second portion positioned at least partially in the recess in the platen, the second portion having a larger lateral dimension than the first portion and extending below the polishing layer, the second portion of the window adhesively attached to an underside of the polishing pad.

2. The polishing apparatus of claim 1, further comprising an adhesive layer spanning the polishing layer.

3. The polishing apparatus of claim 2, wherein the underside of the polishing pad is adhesively attached to the upper surface of the platen by the adhesive layer.

4. The polishing apparatus of claim 3, wherein a top surface of the second portion of the window is adhesively attached to underside of the polishing pad by the adhesive layer.

5. The polishing apparatus of claim 1, wherein the first portion of the window plugs the aperture in the polishing pad.

6. The polishing apparatus of claim 5, wherein a top surface of the first portion of the window is coplanar with the upper surface of the platen.

7. The polishing apparatus of claim 1, wherein a lower surface of the second portion of window contacts the lower surface of the recess.

8. The polishing apparatus of claim 7, wherein the lower surface of the second portion of window is not adhered to the lower surface of the recess.

9. The polishing apparatus of claim 1, wherein the second portion has a lateral dimension between two and ten times larger than the first portion.

10. The polishing apparatus of claim 1, wherein the polishing pad has a thickness less than 1 mm.

11. The polishing apparatus of claim 1 , further comprising an optical fiber in the passage and positioned to direct or receive light through the first portion of the window.

12. The polishing apparatus of claim 11, wherein the optical fiber is wider than the first portion of the window.

13. A method of assembling a window for a polishing apparatus, comprising: forming an aperture through a polishing pad, the polishing pad comprising a polishing layer having a polishing surface and underside;

forming a solid light-transmitting window having a first portion and a second portion having a larger lateral dimension than the first portion;

inserting the first portion of the window into the aperture of the pad;

adhering a top surface of the second portion of the window to the underside of the polishing pad; and

positioning the polishing pad and window on a platen such that the second portion of the window fits into a recess in a planar upper surface of the platen and the underside of the polishing pad is adhered to the planar upper surface of the platen.

14. The method of claim 13, wherein a layer of adhesive is formed on the bottom of the polishing layer and a liner covers the adhesive, a portion of the liner is removed around the aperture, and the top surface of the second portion of the window contacts the adhesive in the removed portion of the liner.

15. The method of claim 14, further comprising removing a remainder of the liner before positioning the polishing pad on the platen such that the adhesive adheres the underside of the polishing pad to the planar upper surface of the platen.