WO2001063201A2 - Optical endpoint detection system for chemical mechanical polishing - Google Patents

Abstract

An apparatus for monitoring the thickness of a film on a substrate includes a housing having a first end and a second end with a well at the first end. The housing also has a bore which extends through the housing to the well, and a fluid channel which is in fluid communication with the well. The apparatus also includes an optical detector assembly positioned within the bore to optically detect a substrate being polished. The apparatus may further include a fluid discharge canal having a first end in fluid communication with the well and extending from the well to a surface of the housing.

Description

OPTICAL ENDPOINT DETECTION SYSTEM FOR CHEMICAL MECHANICAL POLISHING

FIELD OF THE INVENTION

The present inv ention generally relates to a chemical mechanical polishing apparatus, and more particularly to the technique of optical endpomt detection The invention provides an apparatus for optical endpoint detection that avoids resultant endpoint errors

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U S Provisional Application No. 60/184,884, filed February 25, 2000

BACKGROUND The production of semiconductor devices begins with the creation of high quality semiconductor wafers Because of the high precision required in the production of these semiconductor devices, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface Chemical mechanical planaπzation (CMP) is often used to remove material from the surface of the wafer or workpiece to provide a relatively flat surface

Such polishing is well known in the art and generally includes placing one side of the workpiece in contact against a flat polishing surface, and moving the workpiece and the polishing surface relativ e to each other A slurry, including abrasive particles and/or chemicals that react with the material on the workpiece surface to dissolve the material, may also be placed in contact with the workpiece surface to assist removing a portion of the material During the polishing or planaπzation process, the workpiece is typically held by a workpiece carrier and pressed against the polishing pad while the pad rotates In addition, to improve the polishing effectiveness, the workpiece may also rotate and oscillate back and forth over the surface of the polishing pad

During the planaπzation process, it is desirable to gather data on the condition of the wafer's surface The data may then be used to optimize the planaπzation process or to determine when the planaπzation process should be terminated (referred to as endpoint) It is generally preferred that endpoint detection (EPD) systems be m-situ systems to provide monitoring during the polishing process Numerous m-situ EPD systems have been proposed, but few have been successful in a manufacturing environment and even fewer are sufficiently robust for routine production use.

One group of prior art in-situ EPD techniques involves the electrical measurement of changes in the capacitance, the impedance, or the conductivity of the overlying films on a wafer and calculation of the endpoint based on an analysis of this data. To date, these particular electrically-based approaches to EPD are not commercially viable.

Another electrical approach that has proved production worthy is to sense changes in the friction between the wafer being polished and the polish pad. Such measurements are done by sensing changes in the wafer rotation motor drive current. These systems use a global approach, i.e., the measured signal assesses the entire wafer surface. Thus, these systems do not obtain specific data about localized regions. Further, this method works best for EPD for tungsten CMP because of the dissimilar coefficient of friction between the polish pad and the tungsten-titanium nitride-titanium film stack versus the polish pad and the dielectric underneath the metal. However, with advanced interconnection conductors, such as copper (Cu), the associated barrier metals, e.g., tantalum or tantalum nitride, may have a coefficient of friction that is similar to the underlying dielectric. The motor current approach relies on detecting the copper-tantalum nitride transition, then adding an overpolish time. Intrinsic process variations in the thickness and composition of the remaining film stack layer mean that the final endpoint trigger time may be less precise than is desirable. Another group of methods uses acoustic approaches. In a first acoustic approach, an acoustic transducer generates an acoustic signal that propagates through the surface layer(s) of the wafer being polished. Some reflection occurs at the interface between the layers, and a sensor positioned to detect the reflected signals can be used to determine the thickness of the topmost layer as it is polished. In a second acoustic approach, an acoustical sensor is used to detect the acoustical signals generated during CMP. Such signals have spectral and amplitude content that evolves during the course of the polish cycle. However, these acoustic approaches are not yet commercially viable.

Finally, the present invention falls within the group of optical EPD systems. In one approach for optical EPD systems, the carrier is positioned on the edge of the platen so as to expose a portion of the wafer. A fiber optic-based apparatus is used to direct light at the surface of the wafer, and spectral reflectance methods are used to analyze the signal. The drawback of this approach is that with the wafer positioned over the edge of the platen, the wafer is subject to effects associated with the edge of the polish pad going across the wafer while a portion of the wafer is completely exposed An example of this type of approach is described in PCT application WO 98/05066

In another approach, the wafer is lifted off of the pad a small amount, and a light beam is directed betw een the wafer and the slurry-coated pad The light beam is incident at a small angle so that multiple reflections occur The irregular topography on the wafer causes scattering, but if sufficient polishing is done prior to raising the carrier, then the wafer surface will be essentially flat and there will be very little scattering due to the topography on the wafer. An example of this type of approach is disclosed in U S Patent No 5,413,941 The difficulty with this type of approach is that the normal process cycle must be interrupted to make the measurement

Another approach entails monitoring absorption of particular wavelengths in the infrared spectrum of a beam incident upon the backside of a wafer being polished so that the beam passes through the wafer from the non-polished side of the wafer Changes in the absorption w ithin narrow, well-defined spectral windows correspond to changing thickness of specific types of films This approach has the disadvantage that, as multiple metal layers are added to the wafer, the sensitivity of the signal decreases rapidly One example of this type of approach is disclosed in U S Patent No 5,643,046

One of the techniques for detecting when a silicon wafer surface has been polished to the extent required is the use of in-situ optical endpoint detectors wherein the endpoint detectors are situated w ithin the polishing platen of a CMP apparatus One example of this approach is of the type disclosed in U S Patent No 5,433.651 to Lustig et al in which a window is coupled to a light reflectance measurement device to detect endpoint Another approach is of the type disclosed in European application EP 0 824 995 Al, which uses a transparent indow in the actual polish pad itself A similar approach for rotational polishers is of the type disclosed in European application EP 0 738 561 Al, in which a pad with an optical window is used to transmit light used for EPD In these approaches, the window complicates the CMP process because it presents to the wafer an mhomogeneity in the polish pad Such a region can also accumulate slurry and polish debris, which tends to scatter light traveling through it, thus attenuating the light emitted from the endpoint detector The problems inherent in prior art optical endpoint detection systems may be better explained with reference to the attached Figs 1, 1 A, and 2 Fig 1 is a schematic representation, not to scale, of a common chemical mechanical polishing assembly, including a platen 10 to which is mounted a central spindle 12 allowing rotation of the platen The top surface of the substantially circular platen 10 is covered with a polishing pad 14 that is attached by conventional means such as adhesives As shown in Fig 2, the polishing pad 14 is often scored with grooves running in the "x" and "y" directions to form a grid with parallel x-direction grooves 16 and crossing perpendicular groov es 18 Alternatively, the polishing pad may have radial or circumferential grooves These grooves are typically shallow and narrow and assist in the distribution of the chemical slurry during chemical mechanical polishing

When an optical endpoint detector is used in the chemical mechanical polishing apparatus, an optical fiber 20 is inserted through a bore in the platen 10 and through a registering bore in pad 14 so that the distal tip of the fiber is flush with the lower end of a groove 16 and thus slightly spaced from the top surface of pad 14 by the groove depth, as schematically shown in Fig 1A Ordinaπl} two optical fibers 20 are used - one to act as a "send fiber," and the other a "receive fiber "

It has been found that the abov e-descπbed prior an optical endpoint detection system is subject to interference from contaminants resulting from workpiece polishing, and air bubbles that form in the chemical slurry and that intensify light received through the "receive fiber" 20, sometimes resulting in noise observ ed by the optical detector (not shown) As might be expected, during chemical mechanical polishing, especially of semiconductor wafers that include copper-based circuitry, copper particulates and reaction products of these particulates come into contact with optical fibers 20, resulting in fouling and contamination of these fibers As a result, precise optical endpoint detection is adversely affected

Accordingly, there exists a need for an optical endpoint detection system that is simple and self-cleaning, that may be retrofitted to existing chemical mechanical cleaning apparatus, and that provides for the easy replacement of consumables, such as polishing pads

SUMMARY OF THE INVENTION

This summary of invention section is intended to introduce the reader to aspects of the invention and is not a complete description of the invention Particular aspects of the invention are pointed out in other sections herembelow . and the invention is set forth in the appended claims which alone demarcate its scope The invention provides an apparatus for monitoring the thickness of a film on a substrate The apparatus includes a housing having a first end having a well, a bore which extends through the housing to the w ell and a fluid channel which is in fluid communication with the well The apparatus also includes an optical detector assembly, which is positioned within the bore to optically detect a substrate being polished

In accordance with an embodiment of the invention, the optical detector assembly includes a fiber optic probe and a fiber optic cable connected at a first end to the fiber optic probe and connected at a second end to a light source In accordance with another embodiment of the invention, the optical detector assembly has a first end having a tip surface and a second end connected to the fiber optic cable The first end of the optical detector assembly is positioned within the well and the tip surface is flush with a surface of the first end of the housing

In a further embodiment of the invention, the apparatus includes a fluid pump which urges a fluid from a fluid source through the fluid channel and into the well

In yet another embodiment of the invention, the housing also includes a fluid discharge canal having a first end in fluid communication with the well The fluid discharge canal extends from the well to a surface of the housing

In accordance with yet a further embodiment of the invention, a method for optically detecting an endpoint of a chemical mechanical polishing process includes polishing a workpiece surface with a polishing pad, monitoring the workpiece with an optical detector assembly, providing a housing for the optical detector assembly wherein the housing includes a well adjacent the optical detector assembly, urging a fluid into the well of the housing, and stopping the polishing when an endpoint is detected through the monitoring These and other aspects of the present invention are described in the following description, claims and appended drawings

BRIEF DESCRIPTION OF THE DRAWINGS Exemplary embodiments of the present invention will hereafter be described in conjunction with the appended dra ing figures, wherein like designations denote like elements, and

Figure 1 is a schematic, illustrative depiction of a prior art chemical mechanical polishing pad mounted to a rotatable platen,

Figure 1A is a magnified view of the optical fiber insertion to the pπor art apparatus of Figure 1,

Figure 2 is a schematic representation of the top surface of a polishing pad (prior art), showing a grooved rectangular matrix,

Figure 3 is a schematic side view representation of an exemplary embodiment of the present invention;

Figure 4 is a schematic representation of an optical endpoint detection system of the present invention;

Figure 5 is a top view representation of an exemplary embodiment of the present invention.

Figure 6 is a schematic side view representation of another exemplary embodiment of the present invention.

Figure 7 is a schematic side view representation of yet another exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION The following description is of exemplary embodiments only and is not intended to limit the scope, applicability or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described without departing from the scope of the invention as set forth in the appended claims.

The invention provides a significant advancement in the art of endpoint detection in chemical mechanical polishing. More particularly, the invention provides an optical endpoint detection system that is less susceptible to fouling by contaminants and interference from bubbles formed in chemical slurry.

A schematic representation of the endpoint detection apparatus of the present invention is shown in Fig. 3. As seen, a wafer carrier 100 holds a wafer W that is to be polished. The wafer carrier 100 preferably rotates about a vertical axis 102. A polishing pad 104 is mounted onto a polishing platen 106. The hardness and density of polishing pad 104 are selected based on the type of material to be planarized. Blown polyurethane pads, such as the IC and GS series of pads available from Rodel Products Corporation of Scottsdale, Arizona, may be advantageously utilized by the CMP system, although it will be appreciated that any suitable polishing pad may be used. A polishing slurry containing an abrasive medium, such as silica or alumina, may be deposited through a conduit (not shown) onto the surface of the polishing pad 104. Polishing platen 106 is connected to a driver or motor assembly (not shown) that is operative to move polishing platen 106 and polishing pad 104 in an orbital motion or to rotate polishing platen 106 about a vertical axis (not shown). A hole 108 is formed in the platen 106 Hole 108 is positioned such that it has a view of wafer W held by wafer carrier 100 during a portion of the platen's 106 movement Hole 108 may have a stepped diameter, thus forming a shoulder 1 10 Shoulder 110 is used to contain and hold a probe assembly 90 Probe assembly 90 includes a probe housing 1 12 A top surface 114 of probe housing 112 is preferably configured to be flush with a top surface 1 16 of polishing platen 106 One or more annular shims 1 18 may be positioned adjacent shoulder 1 10 to raise probe housing 1 12 so that top surface 1 14 of probe housing 1 12 is flush with top surface 1 16 of polishing platen 106 In an alternate embodiment, probe housing 1 12 may have a constant, uniform diameter and hole 108 similarly may have a constant, uniform diameter for receiving probe assembly 90 Probe housing 1 12 may be secured into hole 108 by a locking mechanism 124, which may be a lock nut or any other locking device suitably configured to secure probe housing 1 12 in hole 108 One or more annular shims may be positioned between platen 106 and locking mechanism 124 so that top surface 1 14 of probe housing 1 12 is flush with top surface 1 16 of polishing platen 106 Probe housing 1 12 has at least one annular groove 120 on its outside surface to receive an O-πng 122 O-πng 122 suitably seals the interface between probe housing 112 and polishing platen 106 so that chemical slurry (not shown) finding its way between polishing pad 104 and probe housing 1 12 cannot leak through to the bottom of platen 106 While Fig. 3 shows two O-πngs 122 seated in two annular grooves 120, it will be appreciated that one, three or more O-rmgs 122 and annular grooves 120 may be used Probe housing 1 12 is secured into hole 108 by a locking mechanism 124 Locking mechanism 124 may be a lock nut or any other locking device suitably configured to secure probe housing 1 12 m hole 108

Probe housing 112 includes a bore 132 which is situated parallel to a longitudinal axis 134 and extends from a bottom surface 136 of probe housing 1 12 to terminate at a well 126 While bore 132 is shown in Fig 3 to he concentrically with longitudinal axis 134, bore 132 may be situated along any axis parallel to longitudinal axis 134 An optical detector assembly, such as a broadband spectrum reflectometry sensor assembly, is received within bore 132 In the embodiment shown in Fig 3, the broadband spectrum reflectometry sensor assembly includes a fiber optic cable assembly 138, although it will be appreciated that any suitable light reflectance apparatus may be used Fiber optic cable assembly 138 includes a fiber optic probe 140 operatively connected to a fiber optic cable 142 Flange 144 is connected to fiber optic probe 140 and is configured to contain and hold fiber optic probe 140 within bore 132 While flange 144 is shown in Fig 3 to be integrally connected to fiber optic probe 140, flange 144 may be connected to fiber optic probe 140 by any suitable mechanism, such as by screwing on to a threaded portion of fiber optic probe 140 A locking device 146 is used to secure fiber optic probe 140 to probe housing 1 12 In the present embodiment, locking device 146 comprises a screw which is received by a pre-dπlled hole in probe housing 112, however, locking dev ice 146 may include or utilize any appropriate fastener or connection mechanism, including balls, pins, and the like

Polishing pad 104 includes a through-hole 148 that is coincident and communicates with well 126 The diameter of through-hole 148 is preferably smaller than the diameter of well 126, although it should be large enough so as not to interfere ith light transmission from fiber optic probe 140 Although through-hole 148 is shown in Fig 3 as smaller in diameter than well 126, it will be appreciated that diameter of through-hole 148 may be equal to the diameter of well 126 The surface of a tip 156 of fiber optic probe 140 is preferably positioned within well 126 and flush w ith surface 1 14 of probe housing 1 12 It will be appreciated, ho ever, that tip 156 may extend beyond surface 154 into through-hole 148. although tip 156 should not extend so far as to contact wafer W Alternatively, tip 156 could be positioned below surface 1 14 and within well 126 or tip 156 could be flush with a bottom surface 158 of well 126

Probe housing 1 12 also includes a fluid canal 128 which is situated parallel to longitudinal axis 134 and extends from bottom surface 136 of probe housing 1 12 to side-leg canal 150 While Fig 3 shows side-leg canal 150 at a 90 degree angle to fluid canal 128, it will be appreciated that side-leg canal 150 may be situated at any suitable angle to fluid canal 128 at any suitable point along fluid canal 128 so that side-leg canal 150 results in fluid communication between well 126 and fluid canal 128

Fig 5 shows a top view of probe housing 1 12 w ith fiber optic probe 140 positioned therethrough In an exemplary embodiment of the in ention, side-leg canal 150 is positioned in probe housing 112 so that an output orifice 152 of side-leg canal 150 directs fluid at a point in well 126 offset from fiber optic probe 140 With output orifice 152 directing fluid flow to a point beside fiber optic probe 140, fluid is permitted to flow around and abov e fiber optic probe 140 in a turbulent fashion to enhance the removal of slurry and other contaminants from fiber optic probe 140 Referring back to Fig 3, fluid canal 128 is connected to fluid line 130 at an end opposite to that end connected to side-leg canal 150 Fluid line 130 is operatively connected to a fluid source (not shown) and a fluid pump (not shown) which cooperatively pump fluid through fluid line 130 and fluid canal 128 to side-leg canal 150, where it exits into well 126 In an exemplary embodiment of the invention, the fluid is deiomzed water Alternatively, the fluid may include any suitable cleaning solution that permits light from fiber optic probe 140 to be transmitted therethrough without interference from solution particulates For example, ammonia or potassium hydroxide may be added to deiomzed water so that the pH of the fluid may more closely match the pH of the slurrv being used in the CMP process When the fluid is pumped through fluid line 130 and fluid canal 128, the fluid pump causes the fluid to enter well 126 at a positive pressure In an exemplary embodiment of the present invention, the fluid pump may urge the fluid through fluid canal 128 at a pressure in the range of from about 1.0 psi to about 2.0 psi It will be appreciated, however, that the fluid may enter well 126 at any suitable pressure that results in the slurry and other contaminants being washed from the area between tip 156 of fiber optic probe 140 and wafer W With the diameter of through-hole 148 smaller than the diameter of well 126, fluid is urged into the well 126 and into through-hole 148 to prevent slurry from entering through-hole 148 and thus interfeπng with the measurements of fiber optic probe 140 A functional representation of the overall system of the invention is shown in Fig 4

Fiber optic cable 142 leads from the fiber optic probe 140 to an optical coupler 200 that receives light from a light source 202 via a fiber optic cable 204 The optical coupler 200 also outputs a reflected light signal to a light sensor 206 via a fiber optic cable 208 The reflected light signal is generated in accordance with the present invention, as described below A computer 210 provides a control signal 212 to light source 202 that directs the emission of light from the light source 202. The light source 202 is a broadband light source, preferably with a spectrum of light between 200 and 2000 nm in wavelength (ultraviolet to near-infrared light) A tungsten bulb is suitable for use as the light source 202 Computer 210 also recei es a start signal 214 that will activate the light source 202 and the EPD methodology The computer also provides an endpoint trigger 216 when it is determined that the endpoint of the polishing has been reached

A position sensor 218 provides the position of the pad/platen assembly while a rotary position sensor 220 of wafer carrier 100 provides the angular position of the wafer carrier 100 to the computer 210, respectivelv Computer 210 can synchronize the trigger of the data collection to the positional information from the sensors

In operation, soon after the CMP process has begun, the start signal 214 is provided to the computer 210 to initiate the monitoring process Computer 210 then directs light source 202 to transmit light from the light source 202 via fiber optic cable 204 to optical coupler 200 This light in turn is routed through fiber optic cable 142 to be incident on the surface of the wafer W through through-hole 148 in the polishing pad 104.

Reflected light from the surface of the wafer W is captured by the fiber optic probe 140 and transmitted through fiber optic cable 142 and routed back to the optical coupler 200. Although in the preferred embodiment the reflected light is relayed using fiber optic probe 140, it will be appreciated that a separate dedicated fiber optic probe and separate fiber optic cable connected thereto may be used to collect the reflected light.

The optical coupler 200 relays this reflected light signal through fiber optic cable 208 to light sensor 206. Light sensor 206 is operative to provide reflected spectral data 222, referred to herein as the reflected spectral data 222, of the reflected light to computer 210.

After a specified or predetermined integration time by the light sensor 206, the reflected spectral data 222 is transmitted to the computer 210, which analyzes the reflected spectral data 222. One result of the analysis by computer 210 is an endpoint signal 224 that is displayed on a monitor 226. Preferably, computer 210 automatically compares endpoint signal 224 to predetermined criteria and outputs an endpoint trigger 216 as a function of this comparison. Alternatively, an operator can monitor the endpoint signal 224 and select an endpoint based on the operator's interpretation of the endpoint signal 224. The endpoint trigger 216 causes the CMP apparatus to advance to the next step.

One advantage provided by the fiber optic cable assembly 138 is that rapid replacement of the polishing pad 104 is possible while retaining the capability of endpoint detection on subsequent wafers. Because the fiber optic cable assembly 138 is not physically connected to polishing pad 104, a new polishing pad 104 may be installed by removing the used polishing pad and placing a new polishing pad in its stead. Another advantage provided by the fiber optic cable assembly 138 is that conventional polishing pads may be used in the CMP process. Extensive modifications do not need to be made to polishing pad 104.

In another exemplary embodiment of the present invention, as shown in Fig. 6, probe housing 112 further includes a fluid discharge canal 300 which is configured parallel to longitudinal axis 134 and extends from bottom surface 136 of probe housing 112 to well 126. Fluid discharge canal 300 is connected to a fluid discharge line 302. Fluid discharge line 302 is preferably connected to a vacuum pump (not shown) which creates a negative pressure at a discharge orifice 304. The negative pressure at discharge orifice 304 causes the fluid introduced into well 126 at output orifice 152 to be aspirated out of well 126. Removal of the fluid from well 126 through discharge canal 300 prevents fluid in well 126 from accumulating on the surface of the polishing pad 104 to such an extent that the abrasive action of the slurry is compromised However, the negative pressure at discharge orifice 304 is preferably maintained at a level below that of the pressure of the fluid introduced at output orifice 152 so that the fluid is permitted to accumulate to an appropriate extent that the slurry is prevented from enteπng through-hole 148 While the above embodiment of the present invention is descπbed as using a vacuum pump connected to fluid discharge line 302. alternatively, the fluid in well 126 may be permitted to drain through fluid discharge canal 300 under the effects of gravity

In et another exemplary embodiment of the present invention, as shown in Fig 7, probe housing includes a fluid canal 400 which is situated parallel to longitudinal axis 134 and extends from bottom surface 136 to well 126 Fluid canal 400 is connected to fluid line 130. Fluid line 130 is operatively connected to a fluid source (not shown) and a fluid pump (not shown) w hich pumps fluid through fluid line 130 and fluid canal 400 where it exits into well 126 Probe housing further includes fluid discharge canal 300 which extends from bottom surface 136 of probe housing 112 to well 126 Fluid discharge canal 300 is connected to fluid discharge line 302 Fluid discharge line 302 is preferably connected to a vacuum pump (not shown) which creates a negative pressure at discharge orifice 304 The negative pressure at discharge orifice 304 causes the fluid introduced into well 126 at an output orifice 402 of fluid canal 400 to be aspirated out of well 126 The negative pressure at discharge orifice 304 is preferably maintained at a level below that of the pressure of the fluid introduced at output orifice 402 so that the fluid is permitted to accumulate to an appropriate extent that slurry is prevented from entering through-hole 148 While the above embodiment of the present invention is described as using a vacuum pump connected to fluid discharge line 302, alternatively, the fluid in well 126 may be permitted to drain through fluid discharge canal 300 under the effects of gravity While the above detailed description describes one probe assembly seated within a polishing platen, any suitable number of probe assemblies may be seated within a polishing platen, although it will be appreciated that the fluid introduced into the planaπzation process may increase with an increase m the number of probe assemblies used Further, while the above detailed description describes a CMP process in which the polishing pad/platen "faces up" and the wafer "faces down," the present inv ention could be used in a reversed process where the wafer "faces up" and the polishing pad/platen "faces down "

From the foregoing Figures, it is apparent that the invention provides significant advantages ov er the prior art The invention substantially reduces any interference effects caused by air bubbles in chemical slurry and any normalized reflectance due to accumulation of polished debris on the sensing end of the optical detection assembly.

Although the subject invention has been described herein in conjunction with the appended drawing Figures, it will be appreciated that the scope of the invention is not so limited. Various modifications in the arrangement of the components discussed and the steps described herein for using the subject dev ice may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

We claim:

1. An apparatus for monitoring the thickness change in a film on a substrate, comprising: a) a housing having a first end and a second end, wherein said housing comprises a well at said first end, a bore which extends through said housing to said well, and a fluid channel which is in fluid communication with said well; and b) an optical detector assembly positioned within said bore to optically detect a substrate being polished.

2. The apparatus of claim 1, wherein said optical detector assembly comprises a fiber optic probe, a light source, and a fiber optic cable connected at a first end to said fiber optic probe and connected at a second end to said light source.

3. The apparatus of claim 2 wherein said fiber optic probe has a first end having a tip surface and a second end connected to said fiber optic cable, and wherein said first end is positioned within said well and said tip surface is flush with a surface of said first end of said housing.

4. The apparatus of claim 1 further comprising a fluid pump, wherein said fluid pump urges a fluid from a fluid source through said fluid channel into said well.

5. The apparatus of claim 4 wherein said fluid pump urges said fluid through said fluid channel at a pressure in the range of from about 1.0 psi to about 2.0 psi.

6. The apparatus of claim 1 , wherein said housing further comprises a fluid discharge canal in fluid communication with said well, and wherein said fluid discharge canal extends from said well to a surface of said housing.

7. The apparatus of claim 6, further comprising a fluid discharge line which is in fluid communication with said fluid discharge canal.

8. The apparatus of claim 7, wherein said fluid discharge line is operatively connected to a vacuum source.

9. An apparatus for optical detection of endpoint in a chemical mechanical process, the apparatus comprising: a) a platen; b) a polishing pad mounted to a surface of the platen; c) a housing having a proximate end and a distal end, wherein said housing is positioned within said platen and comprises a well at said proximate end, a bore which extends through said housing to said well and a fluid channel which is in fluid communication with said well; and d) a reflectometry sensor assembly having a first end and a second end, wherein said reflectometry sensor assembly is positioned within said bore with said first end positioned within said well.

10. The apparatus of claim 9 wherein said polishing pad comprises an orifice positioned adjacent said first end of said reflectometry sensor assembly.

11. The apparatus of claim 9, wherein said reflectometry sensor assembly comprises a fiber optic probe, and a fiber optic cable connected at a first end to said fiber optic probe and connected at a second end to a light source.

12. The apparatus of claim 9 further comprising a fluid pump, wherein said fluid pump urges a fluid from a fluid source through said fluid channel into said well.

13. The apparatus of claim 13 wherein said fluid pump urges said fluid through said fluid channel at a pressure in the range of from about 1.0 psi to about 2.0 psi.

14. The apparatus of claim 9, wherein said housing further comprises a fluid discharge canal in fluid communication with said well, and wherein said fluid discharge canal extends from said well to a surface of said housing.

15. The apparatus of claim 14, further comprising a fluid discharge line which is in fluid communication with said fluid discharge canal.

16. The apparatus of claim 15, wherein said fluid discharge line is operatively connected to a vacuum source.

17. A method of optically detecting an endpoint of a chemical mechanical polishing process, the method comprising: a) polishing a workpiece surface with a polishing pad; b) monitoring the workpiece with an optical detector assembly; c) providing a housing for said optical detector assembly, wherein said housing comprises a well adjacent said optical detector assembly; d) urging a fluid into said well of said housing, and e) stopping the polishing when an endpoint is detected through the monitoring.An apparatus for creating a dynamic fluid interface, which is largely optically transparent, between an in-situ sensor and a substrate, comprising: f) a housing having a first end and a second end, wherein said housing comprises a well at said first end, a bore which extends through said housing to said well, and a fluid channel which is in fluid communication with said well; and g) an optical detector assembly positioned within said bore to optically detect a substrate The apparatus of claim 18, wherein said optical detector assembl comprises a fiber optic probe, a light source, and a fiber optic cable connected at a first end to said fiber optic probe and connected at a second end to said light source The apparatus of claim 19 wherein said fiber optic probe has a first end having a tip surface and a second end connected to said fiber optic cable, and wherein said first end is positioned within said well and said tip surface is flush with a surface of said first end of said housing The apparatus of claim 18 further comprising a fluid pump, wherein said fluid pump urges a fluid from a fluid source through said fluid channel into said w ell The apparatus of claim 21 wherein said fluid pump urges said fluid through said fluid channel at a pressure in the range of from about 1 0 psi to about 2 0 psi The apparatus of claim 18, wherein said housing further comprises a fluid discharge canal in fluid communication with said well, and wherein said fluid discharge canal extends from said well to a surface of said housing The apparatus of claim 23, further comprising a fluid discharge line which is in fluid communication with said fluid discharge canal The apparatus of claim 24, wherein said fluid discharge line is operatively connected to a vacuum source