WO2000067951A1 - Optical endpoint detection during chemical mechanical planarization - Google Patents

Abstract

An apparatus is provided for use with a tool for polishing thin films on a semiconductor wafer surface that detects an endpoint of a polishing process. In one embodiment, the apparatus includes a polish pad (109) having a through-hole (112), a light source (117), a fluid source (116), a fiber optic cable assembly (113, 115, 118, 122), a light sensor (119), and a computer (121). The fiber optic cable (113) propagates the light through the through-hole (112) to illuminate the wafer surface during the polishing process. The light sensor (119) receives reflected light and generates data (218) corresponding to the spectrum of the reflected light. The computer receives the reflected spectral data (218) and generates an endpoint signal (124).

Description

OPTICAL ENDPOINT DETECTION DURING CHEMICAL MECHANICAL

PLANARIZATION

Field of the Invention The present invention relates to chemical mechanical polishing (CMP), and more particularly, to optical endpoint detection during a CMP process

Background Information Chemical mechanical polishing (CMP) has emerged as a crucial semiconductor technology, particularly for devices with critical dimensions smaller than 0 5 micron One important aspect of CMP is endpoint detection (EPD), l e , determining during the polishing process when to terminate the polishing

Many users prefer EPD systems that are "in situ EPD systems", which provide EPD during the polishing process Numerous m situ EPD methods have been proposed, but few have been successfully demonstrated in a manufacturing environment and even few er have proved 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 wafer and calculating the endpoint based on an analysis of this data To date, these particular electrically based approaches to EPD are not commercially available One other 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 motor 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 m the thickness and composition of the remammg film stack layer mean that the final endpoint trigger time may be less precise than is desirable

Another group of methods uses an acoustic approach 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 duπng the course of the polish cycle How e\ er, to date there has been no commercially available in situ endpoint detection system using acoustic methods to determine endpoint

Finally, the present invention falls within the group of optical EPD systems One approach for optical EPD systems is of the type disclosed in U S Patent No 5,433,651 to Lustig et al in which a window in the platen of a rotatmg CMP tool Light reflected from the wafer surface back through the window is used to detect end pomt However, the window complicates the CMP process because it presents to the wafer an lnhomogeneity in the polish pad Such a region can also accumulate slurry and polish debris

Another approach is of the type disclosed in European application EP 0 824 995 Al, which uses a transparent window 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 both of these approaches, v arious means for implementing a transparent window m a pad are discussed, but making measurements without a window were not considered The methods and apparatuses disclosed in these patents require sensors to indicate the presence of a wafer m the field of view Furthermore, integration tunes for data acquisition are constrained to the amount of time the window in the pad is under the wafer

In another type of approach, 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 the process must be mterrupted to position the wafer in such a way as to allow the optical signal to be gathered In so doing, with the wafer positioned over the edge of the platen, the wafer is subjected to edge effects associated with the edge of the polish pad gomg across the wafer while the remaining 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 between 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

Yet 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 nonpo shed side of the wafer Changes in the absorption within 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 Each of these abo\ e methods has drawbacks of one sort or another What is needed is a new method for m situ EPD that provides noise immunity, can work w ith multiple underlying metal layers, can measure dielectric layers, and can be easily used in the manufacturing environment

Summary An apparatus is provided for use with a tool for polishing thin films on a semiconductor wafer surface that detects an endpoint of a polishing process In one embodiment, the apparatus includes a polish pad having a through-hole, a light source, a fluid source, a fiber optic cable assembly, a light sensor, and a computer The light source provides light withm a predetermined bandwidth The light passes through a fiber optic cable, through the through-hole to illuminate the wafer surface during the polishing process The light sensor receives reflected light from the surface through the fiber optic cable and generates data corresponding to the spectrum of the reflected light The fluid source concurrently provides fluid to flush the through-hole to reduce optical signal degradation caused by polish debris and accumulated slurry in the light path between the wafer surface and the end of the fiber optic cable The computer receives the reflected spectral data and generates an endpoint signal as a function of the reflected spectral data In a metal film polishing application, the endpoint signal is a function of the intensities of at least two individual wavelength bands selected from the predetermined bandwidth In a dielectric film polishing application, the endpoint signal is based upon fitting of the reflected spectrum to an optical reflectance model to determine remaining film thickness The computer compares the endpoint signal to predetermined cπteria and stops the polishing process when the endpoint signal meets the predetermined criteπa Unlike prior art optical endpoint detection systems, an apparatus according to the present invention, together with the endpoint detection methodology, advantageously allows for accuracy and reliability in the presence of slurry and polishing debris This robustness makes the apparatus suitable for in situ EPD in a production environment In particular, the fluid flushing of the through-hole advantageously improves the performance of the system when using slumes that cause a relatively large amount of light scattering and in systems that require the end of the fiber optic cable to be relatively far from the wafer surface

In another aspect of the present invention, the fiber optic cable is replaced with a liquid waveguide assembly The fluid source provides the fluid for the liquid waveguide assembly, which is used to propagate light to the wafer surface from the light source and to propagate reflected light from the wafer surface to the light sensor In addition, the fluid serves to flush the through-hole as described above

Brief Description of the Drawings The forgoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the drawings listed below

FIGURE 1 is a diagram schematically illustrating one embodiment of an apparatus in accordance with the present invention, adapted for use with an orbital CMP machme FIGURE 1A is a diagram illustrating in more detail the fluid source fitting of FIGURE 1 FIGURE 2 is a schematic diagram of a light sensor for use in the apparatus of FIGURE 1 FIGURE 2A is a diagram illustrating reflected spectral data

FIGURE 3 is a top view of the pad assembly for use in the apparatus of FIGURE 1 FIGURE 4 illustrates an example trajectory for a given point on the pad showing the annular region that is traversed on the wafer when the wafer rotates and the pad orbits

FIGURES 5A-5F are diagrams illustrating the effects of applying various noise-reducmg methodologies to the reflected spectral data, in accordance with the present invention

FIGURES 5G-5K are diagrams illustrating the formation of one endpoint signal (EPS) from the spectral data of the reflected light signal and show transition points in the polishing process, m accordance with one embodiment of the present invention

FIGURE 6 is a flow diagram illustrating the analysis of the reflectance signal m accordance with the present invention

FIGURE 7 is a diagram schematically illustrating another embodiment of an apparatus usmg a liquid waveguide assembly, in accordance with the present invention

Detailed Description

The present invention, in general, relates to methods of optical EPD and of processmg the optical data during EPD CMP machines typically include a means of holding a wafer or substrate to be polished Such holding means are sometimes refeπed to as a earner, but the holdmg means of the present invention is referred to herein as a "wafer chuck " CMP machines also typically include a polishing pad and a means to support the pad Such pad support means are sometimes referred to as a polishing table or platen, but the pad support means of the present invention is refeπed to herem as a "pad backer" Slurry is required for polishing and is delivered either directly to the surface of the pad or through holes and grooves in the pad directly to the surface of the wafer The control system on the CMP machine causes the surface of the wafer to be pressed against the pad surface with a prescribed amount of force The motion of the wafer is arbitrary, but is rotational about its center around an axis perpendicular to the plane of the wafer in a prefeπed embodiment

As will be described below, the motion of the polishing pad in one embodiment is non- rotational to enable a short length of fiber optic cable to be inserted into the pad without breakmg Instead of bemg rotational, the motion of the pad is "orbital" in a prefeπed embodiment In other words, each point on the pad undergoes circular motion about its individual axis, which is parallel to the wafer chuck's axis Further, it is to be understood that other elements of the CMP tool not specifically shown or described may take various forms known to persons of ordinary skill m the art For example, the present invention can be adapted for use in the CMP tool disclosed in U S Patent No 5,554,064

In particular, an in situ optical thin film measurement system is described that uses fluid flowing between optical components of the EPD system and the surface being polished The flowing fluid helps to keep the optical path between the optical components and the surface being polished clear of artifacts (e g , slurry particles, polishing debris, bubbles in the slurry) that can interfere with the propagation of light between the optical components and the surface being polished

Optic Fiber With Fluid Flushing A schematic representation of the overall system of the present invention is shown in FIGURE 1 As seen, a wafer chuck 101 holds a wafer 103 that is to be polished The wafer chuck 101 preferably rotates about its vertical axis 105 A pad assembly 107 includes a polishing pad 109 mounted onto a pad backer 120 The pad backer 120 is in turn mounted onto a pad backmg plate 140 In a prefeπed embodiment, the pad backer 120 is composed of urethane and the pad backing plate 140 is stainless steel Other embodiments may use other suitable materials for the pad backer and pad backmg Further, the pad backing plate 140 is secured to a driver or motor means (not shown) that is operative to move the pad assembly 107 m the prefeπed orbital motion In alternative embodiments, a sub pad (not shown) may be disposed between polishing pad 109 and pad backer 120

Polishing pad 109 includes a through-hole 112 that is coincident and communicates with a pinhole opening 11 1 in pad backer 120 Further, a canal 104 is formed in the side of pad backer 120 adjacent pad backing plate 140 Canal 104 leads from the exterior side 110 of pad backer 120 to pinhole opening 111 In a prefeπed embodiment, a fiber optic cable assembly including a fiber optic cable 113 is inserted in pad backer 120, with one end of fiber optic cable 113 extending through the top surface of pad backer 120 and partially into through-hole 112 Fiber optic cable 113 can be embedded in pad backer 120 so as to form a watertight seal with the pad backer 120, but a watertight seal is not necessary to practice the invention Further, m contrast to conventional systems as exemplified by U S Patent No 5,433,651 to Lustig et al that use a platen with a wmdow of quartz or urethane, the present invention does not include such a window Rather, pinhole opening 111 is merely an orifice in the pad backer m which fiber optic cable 113 may be placed Thus, in the present invention, fiber optic cable 113 is not sealed to pad backer 120 Moreover, because of the use of pinhole opening 111, fiber optic cable 113 may even be placed within one of the existmg holes m the pad backer and polishing pad used for the delivery of slurry without adversely affectmg the CMP process As an additional difference, polishing pad 109 has a simple through-hole 112

Fiber optic cable 113, through a fluid source fitting 114, leads to an optical coupler 115 that receives light from a light source 117 via a fiber optic cable 118 The optical coupler 115 also outputs a reflected light signal to a light sensor 119 via fiber optic cable 122 The reflected light signal is generated in accordance with the present invention, as described below

Fluid source fitting 114 also has an inlet for receiving a fluid source 116 that provides fluid at a selectable flow rate In one embodiment, the flow rate is about ^lA to about two millihters per minute The fluid preferably does not attenuate light m the desired wavelength bands and is pH compatible with the slurry For example, the fluid may be pH adjusted de-ionized water Alternatively, the fluid may be the solution used m the slurry (I e , the solution in which the slurry particles are suspended but without the abrasive particles) Preferably, the fluid will not adversely the CMP process In this embodiment fluid source 1 14 may be incorporated m the slurry delivery system (not shown) of the CMP machme Fluid source fitting 114 also has an outlet for providmg the pressurized fluid into canal 104 Thus, the fluid flows out of pinhole openmg 1 1 1 and through- hole 112 This fluid flow serves to flush slurry and polishing debris from the holes 11 1 and 112, thereby significantly reducmg the scattermg of light by polishing debris and slurry particles In addition, the fluid flow may flush away bubbles that can be present in the slurry For example, some shinies (e g , those containing hydrogen peroxide) can form gas bubbles in the slurry that can cause signal excursions Bubbles can arise m the slurry from other sources as well

A computer 121 provides a control signal 183 to light source 117 that directs the emission of light from the light source 117 The light source 117 is a broadband light source, preferably with a spectrum of light between 200 nm and 1000 ran m wavelength, and more preferably w ith a spectrum of light between 400 nm and 900 nm in wavelength A tungsten bulb is suitable for use as the light source 117 Computer 121 also receives a start signal 123 that will activate the light source 117 and the EPD methodology The computer also provides an endpoint trigger 125 when, through the analysis of the present invention, it is determined that the endpoint of the polishing has been reached

Orbital position sensor 143 provides the orbital position of the pad assembly while the wafer chuck's rotary position sensor 142 provides the angular position of the wafer chuck to the computer 121, respectively Computer 121 can synchronize the trigger of the data collection to the positional information from the sensors The orbital sensor identifies which radius the data is coming from and the combination of the orbital sensor and the rotary sensor determine which point These position sensors may be implemented with rotary optical encoders available from Renco Encoders, Inc , Goleta, California

In operation, soon after the CMP process has begun, the start signal 123 is provided to the computer 121 to initiate the monitoπng process Computer 121 then directs light source 117 to transmit light from light source 117 via fiber optic cable 118 to optical coupler 115 This light in turn is routed through fiber optic cable 113 to be incident on the surface of the wafer 103 through pinhole opening 111 and the through-hole 112 in the polishing pad 109

Reflected light from the surface of the wafer 103 is captured by the fiber optic cable 113 and routed back to the optical coupler 115 Although m the prefeπed embodiment the reflected light is relayed using the fiber optic cable 113, it will be appreciated that a separate dedicated fiber optic cable (not shown) may be used to collect the reflected light The return fiber optic cable would then preferably share the canal 104 with the fiber optic cable 113 in a single fiber optic cable assembly

The optical coupler 115 relays this reflected light signal through fiber optic cable 122 to light sensor 119 Light sensor 1 19 is operative to provide reflected spectral data 218, refeπed to herein as the reflected spectral data 218, of the reflected light to computer 121

One advantage provided by the optical coupler 115 is that rapid replacement of the pad assembly 107 is possible while retammg the capability of endpoint detection on subsequent wafers In other words, the fiber optic cable 1 13 may simply be detached from the optical coupler 115 and a new pad assembly 107 may be installed (complete with a new fiber optic cable 113) For example, this feature is advantageously utilized m replacing used polishing pads m the polisher A spare pad backer assembly having a fresh polishing pad is used to replace the pad backer assembly in the polisher The used polishing pad from the removed pad backer assembly is then replaced with a fresh polishing pad for subsequent use

After a specified or predetermined integration time by the light sensor 119, the reflected spectral data 218 is read out of the detector aπay and transmitted to the computer 121, which analyzes the reflected spectral data 218 The integration time typically ranges from 5 ms to 150 ms, with the integration time being 15 ms m an embodiment using a photodiode as a light source Alternative embodiments using a different light detection source (e g , a CCD aπay) with a greater or lesser sensitivity allows a decrease or increase in the integration time Further, the use of different light sources (e g , bulb type) allow different ranges of integration time One result of the analysis by computer 121 is an endpoint signal 124 that is displayed on monitor 127 Preferably, computer 121 automatically compares endpoint signal 124 to predetermined cπteria and outputs an endpoint trigger 125 as a function of this comparison Alternatively, an operator can monitor the endpoint signal 124 and select an endpoint based on the operator's interpretation of the endpomt signal 124 The endpomt trigger 125 causes the CMP machine to advance to the next process step

To facilitate understanding of this disclosure, the same reference numbers are used among the drawings to indicate features having the same or similar function or structure In FIGURE 1A, one embodiment of fluid source fitting 114 is shown in more detail Fluid source fittmg 114 mcludes a fittmg body 114 A, which is fitted to the opening of canal 104 so as to form an essentially watertight seal Fitting body 114A includes a passage for a fluid conduit 114B and a passage for fiber optic cable 113 Fluid is provided from the fluid source 116 (FIGURE 1) into canal 104 through conduit 114B In this embodiment, these passages when fitted with conduit 114B and fiber optic cable 113 are essentially watertight Thus, ideally, no fluid can leak from fitting body 114A, though some fluid leakage can of course be tolerated In one embodiment, fittmg body is implemented usmg a machined block of urethane and fitted to the opening of canal 104 using standard threaded, swedge or cold fit techniques, while conduit 114B is implemented usmg a stainless steel tube fitted mto fittmg body 114A Of course, other materials and configurations may be used in alternative embodiments Ideally, fluid source fittmg 114 will form a watertight seal with canal 104 and include a fluid mlet to received fluid from fluid source 116 (FIGURE 1), a fluid outlet communicating with canal 104, and a passage for fiber optic cable 113

As previously described, the fluid flows through canal 104 and out holes 111 and 112 (FIGURE 1 ) to reduce the occuπence of bubbles, polishing debris and slurry particles in the optical path between the end of fiber optic cable 1 13 and the wafer surface, thereby improving the signal-to- noise ratio performance of the system The relatively clear optical path advantageously allows the end of fiber optic cable 113 to be placed further from the wafer surface while remammg within the noise tolerance of the system That is, the scattering caused by the slurry tends to mcrease as the optical path through the slurry increases In addition to the other noise sources descπbed above, optical noise can also be caused by light reflections from the sidewalls of the holes 111 and 112 (FIGURE 1), as light propagates to and from the afer surface These noise sources tend to become more significant as the end of fiber optic cable 1 13 is placed further from the wafer surface However, the improved performance of this system allows the end to be moved further from the wafer surface while keeping the signal-to-noise ratio within tolerance Moving the end of fiber optic cable 113 from the wafer surface helps minimize the risk that the end of fiber optic cable 113 will scratch or otherwise undesirably influence the polishing process

Turning to FIGURE 2, the light sensor 1 19 contains a spectrometer 201 that disperses the light according to wavelength onto a detector aπay 203 that includes a plurality of light-sensitive elements 205 The spectrometer 201 uses a grating to spectrally separate the reflected light The reflected light incident upon the light-sensitive elements 205 generates a signal in each light-sensitive element (or "pixel") that is proportional to the intensity of light in the naπow wavelength region incident upon said pixel The magnitude of the signal is also proportional to the integration time Following the integration time, reflected spectral data 218 indicative of the spectral distribution of the reflected light is output to computer 121 as illustrated in FIGURE 2 A

In light of this disclosure, it will be appreciated that, by varying the number of pixels 205, the resolution of the reflected spectral data 218 may be varied For example, if the light source 117 has a total bandwidth of between 200 nm to 1000 nm, and if there are 980 pixels 205, then each pixel 205 provides a signal indicative of a wavelength band spanning 10 nm (9800 nm divided by 980 pixels) By increasing the number of pixels 205, the width of each wavelength band sensed by each pixel may be proportionally naπowed In a prefeπed embodiment, detector aπay 203 contams 1024 pixels 205 Likewise, changing the dispersive properties of the spectrometer can allow one to choose the number of pixels covering a specified wavelength range FIGURE 3 shows a top view of the pad assembly 107 The pad backing plate 140 has a pad backer 120 (not shown in FIGURE 3) secured to its top surface Atop the pad backer 120 is secured the polishing pad 109 Pinhole opening 111 and through-hole 1 12 are shown near a pomt m the middle of the polishing pad 109, though any point in the polishing pad 109 can be used The fiber optic cable 113 extends through the body of the pad backer 120 and emerges m pinhole opening 111 Further, clamping mechanisms 301 are used to hold the fiber optic cable 113 in fixed relation to the pad assembly 107 Clamping mechanisms do not extend beyond the plane of interface between the pad backer 120 and the polishing pad 120 The pmhole is preferably located in a groove in the polish pad

With a rotating wafer chuck 101 and an orbiting pad assembly 107, any given pomt on the polishing pad 109 will follow spirographic trajectories, with the entire trajectory lymg mside an annulus centered about the center of the wafer An example of such trajectory is shown m FIGURE 4 The wafer 103 rotates about its center axis 105 while the polish pad 109 orbits Shown m FIGURE 4 is an annulus with an outer limit 250, an inner limit 260, and an example trajectory 270 In the example shown, the platen orbit speed is 16 times the wafer chuck 101 rotation speed but such a ratio is not critical to the operation of the EPD system described here

In a prefeπed embodiment of the present invention, the location of the orbital motion of through-hole 112 is contained entirely within the area circumscribed by the perimeter of the wafer 103 In other words, the outer limit 250 is equal to or less than the radius of wafer 103 As a result, the wafer 103 is illuminated continuously, and reflectance data can be sampled continuously In this embodiment, an endpomt signal is generated at least once per second, with a prefeπed integration time of light sensor 119 (FIGURE 1) being 15 ms When properly synchronized, any particular point w ithin the sample annulus can be detected repeatedly Furthermore, by sampling twice during the orbit cycle of the pad, at the farthest point in the orbit from the wafer center and the nearest point, the reflectance at the inner and outer radii can be detected Thus, w ith a smgle sensor one can measure uniformity at two radial points For stable production processes, measurmg uniformity at tw o radial points can be sufficient for assuring that a deviation from a stable process is detected when the deviation occurs Orbital position sensor 143 provides the orbital position of the pad assembly while the wafer chuck's rotary position sensor 142 provides the angular position of the wafer chuck to the computer 121, respectively The computer 121 can then synchronize the tπgger of the data collection to the positional information from these sensors The orbital sensor identifies which radius the data are coming from and the combination of the orbital sensor and the rotary sensor determine which point Using this synchronization method, any particular point within the sample annulus can be detected repeatedly

With additional sensors m the pad backer 120 and polishing pad 109, each sampled with proper synchronous triggering, any desired measurement pattern can be obtamed, such as radial scans, diameter scans, multipoint polar maps, 52-sιte Cartesian maps, or any other calculable pattern These patterns can be used to assess the quality of the polishing process For example, one of the standard CMP measurements of quality is the standard deviation of the thicknesses of the material removed, divided by the mean of thicknesses of the material removed, measured over the number of sample sites If the sampling within any of the annuh is done randomly or asynchronously, the entire annulus can be sampled, thus allowing measurements around the wafer Although m this embodiment the capability of sensing the entire wafer is achieved by adding more sensors, alternate approaches can be used to obtam the same result

For example, enlarging the orbit of the pad assembly increases the area a smgle sensor can cover If the orbit diameter is one-half of the wafer radius, the entire wafer will be scanned, provided that the inner limit of the annulus coincides with the wafer center In addition, the fiber optic end may be translated within a canal 104 to stop at multiple positions by means of another moving assembly In light of this disclosure, one of ordinary skill m the art can implement alternative approaches that achieve the same result without undue experimentation Simply collecting the reflected spectral data 218 is generally insufficient to allow the EPD system to be robust, since the amplitude of the signal fluctuates considerably, even when polishmg uniform films The present invention further provides methods for analyzing the spectral data to process EPD information to detect more accurately the endpoint The amplitude of the reflected spectral data 218 collected during CMP can \ ary by as much as an order of magnitude, thus addmg "noise" to the signal and complicating analysis The amplitude "noise" can vary due to the amount of slurry between the wafer and the end of the fiber optic cable, the variation in distance between the end of the fiber optic cable and the wafer (e g , this distance variation can be caused by pad wear or vibration), changes in the composition of the slurry as it is consumed in the process, changes in surface roughness of the wafer as it undergoes polishmg, and other physical and/or electronic sources of noise

Several signal processmg techniques can be used for reducing the noise in the reflected spectral data 218a-218f, as shown m FIGURES 5A-5F For example, a technique of single-spectrum wavelength averaging can be used as illustrated m FIGURE 5A In this technique, the amplitudes of a given number of pixels within the smgle spectrum and centered about a central pixel are combined mathematically to produce a wavelength-smoothed data spectrum 240 For example, the data may be combined by simple average, boxcar average, median filter, gaussian filter, or other standard mathematical means when calculated pixel by pixel over the reflected spectral data 218a The smoothed data spectrum 240 is shown m FIGURE 5 A as a plot of amplitude vs wavelength Alternatively, a time-averagmg technique may be used on the spectral data from two or more scans (such as the reflected spectral data 218a and 218b representing data taken at two different tunes) as illustrated in FIGURE 5B In this technique, the spectral data of the scans are combmed by averaging the coπesponding pixels from each spectrum, resulting in a smoother spectrum 241

In another technique illustrated m FIGURE 5C, the amplitude ratio of wavelength bands of reflected spectral data 218c are calculated usmg at least two separate bands consistmg of one or more pixels In particular, the average amplitude in each band is computed and then the ratio of the two bands is calculated The bands are identified for reflected spectral data 218g in FIGURE 5C as 520 and 530, respectively This technique tends to automatically reduce amplitude variation effects smce the amplitude of each band is generally affected in the same way while the ratio of the amplitudes m the bands removes the variation This amplitude ratio results in the single data point 242 on the ratio vs time plot of FIGURE 5C

FIGURE 5D illustrates a technique that can be used for amplitude compensanon while polishing metal layers on a semiconductor wafer For metal layers formed from tungsten (W), aluminum (Al), copper (Cu), or other metal, it is known that, after a short delay of 10 to 60 seconds after the initial startup of the CMP metal process, the reflected spectral data 218d are substantially constant Any changes in the reflected spectral data 218d amplitude would be due to noise as described above After the short delay, to compensate for amplitude variation noise, several sequential scans (e g , 5 to 10 m a prefeπed embodiment) are averaged to produce a reference spectral data signal, in an identical way that spectrum 241 was generated Furthermore, the amplitude of each pixel is summed for the reference spectral signal to determine a reference amplitude for the entire 512 aπay of pixels present Each subsequent reflected spectral data scan is then "normalized" by (l) summing up all of the pixels for the entire aπay of pixels present to obtam the mtegrated sample amplitude, and then (n) multiplying each pixel of the reflected spectral data by the ratio of the reference amplitude to the sample amplitude to calculate the amplitude-compensated spectra 243

In addition to the amplitude variation, the reflected spectral data, m general, also contain the instrument function response For example, the spectral illumination of the light source 117 (FIGURE 1), the absorption characteristics of the various optical fibers and the coupler, and the inherent interference effects within the fiber optic cables, all undesirably appear m the signal As illustrated in FIGURE 5F, it is possible to remove this instrument function response by normalizing the reflected spectral data 218f by dividing the reflected spectral data 218f by the reflected signal obtained when a "standard" reflector is placed on the pad 109 (FIGURE 1) The "standard" reflector is typically a first surface of a highly reflective plate (e g , a metallized plate or a partially polished metallized semiconductor wafer) The instrument-normalized spectrum 244 is shown as a relatively flat line with some noise present

In view of the present disclosure, one of ordinary skill in the art may employ other means to process reflected spectral data 218f to obtain the smooth data result shown as spectra 245 For example, the aforementioned techniques of amplitude compensation, instrument function normalization, spectral wavelength averaging, time averaging, amplitude ratio determination, or other noise reduction techniques known to one of ordinary skill in the art, can be used individually or in combination to produce a smooth signal

It is possible to use the amplitude ratio of wavelength bands to generate an endpomt signal 124 directly Further processing on a spectra-by-spectra basis may be required in some cases For example, this further processing may include determining the standard deviation of the amplitude ratio of the wavelength bands, further tune averaging of the amplitude ratio to smooth out noise, or other noise-reducing signal processing techniques that are known to one of ordinary skill m the art

FIGURES 5G-5J illustrate the endpoint signal 124 generated by apply mg the amplitude ratio of wavelength bands technique described in conjunction with FIGURE 5C to the sequential reflected spectral data 218g, 218h, and 218ι during the polishing of a metallized semiconductor wafer havmg metal over a baπier layer and a dielectric layer The wavelength bands 520 and 530 were selected by looking for particularly strong reflectance values in the spectral range This averaging process provides additional noise reduction Moreover, it was found that the amplitude ratio of wavelength bands changed as the material exposed to the slurry and polish pad changed Plotting the ratio of reflectance at these specific wavelengths versus time shows distinct regions that coπespond to the various layers being polished Of course, the points coπesponding to FIGURES 5G-5I are only three points of the plot, as illustrated in FIGURE 5 J In practice, as illustrated m FIGURE 5K, the transition above a threshold value 501 mdicates the transition from a bulk metal layer 503 to the barπer layer 505, and the subsequent lowering of the level below threshold 507 after the peak 51 1 indicates the transition to the dielectric layer 509 Wavelength bands 520 and 530 are selected from the bands 450 nm to 475 nm, 525 nm to 550 nm, or 625 nm to 650 nm in prefeπed embodiments for polishmg tungsten (W), titanium nitride (TiN), or titanium (Ti) films formed on silicon dioxide (Sι02) As described previously, these wavelength bands can be different for different matenals and different CMP processes, and typically would be determined empirically

In the present invention, integration times may be increased to cover larger areas of the wafer with each scan In addition, any portion of the wafer within the annulus of a sensor trajectory can be sensed, and with a plurality of sensors or other techniques pre\ιously discussed, the entire wafer can be measured

Although a robust and practical optical endpoint detection methodology is described above, other types of optical endpoint detection methodologies can be used in alternative embodiments For example, the optical endpoint detection algorithm disclosed in U S Patent Application Senal No 09/271,729 entitled "Method and Apparatus For Endpoint Detecting For Chemical Mechanical Polishmg" and filed March 18, 1999 can be used

For a metal polish process, the specific method of determining the plot of FIGURE 5 is illustrated in the flow diagram of FIGURE 6 The process of FIGURE 6 is implemented by computer 121 properly programmed to carry out the process of FIGURE 6 First, at a box 601, a start command is received from the CMP apparatus After the start command has been received, at box 603, a tuner is set to zero The timer is used to measure the amount of time required from the start of the CMP process until the endpomt of the CMP process has been detected This tuner is advantageously used to provide a fail-safe endpomt method If a proper endpoint signal is not detected by a certain time, the endpoint system issues a stop polishing command based solely on total polish tune In effect, if the timeout is set properly, no wafer will be overpolished and thereby damaged However, some wafers may be underpohshed and have to undergo a touchup polish if the endpomt system fails, but these wafers will not be damaged The timer can also be advantageously used to determine total polish time so that statistical process control data may be accumulated and subsequently analyzed Next, at box 605, the computer 121 acquires the reflected spectral data 218 provided by the light sensor 119 This acquisition of the reflected spectral data 218 can be accomplished as fast as the computer 121 will allow, be synchronized to the timer for a prefeπed acquisition time of every 1 second, be synchronized to the rotary position sensor 142, and/or be synchronized to the orbital position sensor 143 The reflected spectral data 218 consist of a reflectance value for each of the plurality of pixel elements 205 of the detector aπay 203 Thus, the form of the reflected spectral data 218 will be a vector of wavelength bands Rwb_j, where l ranges from one to NPg, with NPp- representmg the number of pixel elements 205 The prefeπed sampling time is to acquire a reflected spectral data 218 scan approximately every 1 second In one embodiment, the mtegration time is 33 milliseconds In light of the present disclosure, those skilled in the art will appreciate that each wavelength band Rwb, represents a finite wavelength band as previously descπbed in conjunction with FIGURE 2

Next at box 607, the desired noise reduction technique or combination of techniques is applied to the reflected spectral data 218 to produce a reduced noise signal At box 607, the desired noise reduction technique for metal polishmg is to calculate the amphmde ratio of wavelength bands The reflectance of a first preselected wavelength band 520 (Rwb_χ) is measured and the amplitude stored in memory Similarly, the reflectance of the second preselected wavelength band 530 (Rwb_v) is measured and its amplitude stored in memory The amplitude of the first preselected wavelength band (Rwb_j) is divided by the amphmde of the second preselected wavelength band (Rwb2) to form a smgle value ratio that is one data entry vs time and forms part of the endpoint signal (EPS) 124

Next at box 609, the endpoint signal 124 is extracted from the noise-reduced signal produced in box 607 For metal polishing, the noise-reduced signal is also already the endpoint signal 124 For dielectric processmg, the prefeπed endpoint signal is derived from fitting the reduced-noise signal from box 607 to a set of optical equations to determine the film stack thickness remaining, as one of ordinary skill in the art can accomplish Such techniques are well known m the art. For example, see MacLeod, Thm Film Optical Filters (out of pπnt), and Born et al , Prmciples of Optics Electronic Theory of Propagation, Interference and Diffraction of Light, Cambridge University Press, 1998 Next, at box 611, the endpoint signal 124 is exammed using predetermined criteria to determine if the endpoint has been reached The predetermined criteria are generally determined from empirical or experimental methods

For metal polishing, a prefeπed endpoint signal 124 over time in exemplary form is shown m FIGURE 5 by reference numeral 124. As seen, as the CMP process progresses, the EPS varies and shows distinct variation The signal is first tested against threshold level 501 When it exceeds level 501 before the timer has timed out, the computer then compares the endpoint signal to level 507 If the endpoint signal is below 507 before the timer has timed out, then the transition to oxide has been detected The computer then adds on a predetermined fixed amount of time and subsequently issues a stop polish command If the timer tunes out before any of the threshold signals, then a stop polish command is issued The threshold values are determined by polishmg several wafers and determining at what values the transitions take place

For dielectric polishing, a prefeπed endpomt signal results in a plot of remammg thickness vs. time The signal is first tested agamst a minimum remaining thickness threshold level. If the signal is equal to or lower than the minimum thickness threshold before the tuner has tuned out, the computer then adds on a predetermined fixed amount of time and subsequently issues a stop polish command If the timer times out before the threshold signal, then a stop polish command is issued The threshold value is determined by polishing several wafers, then measuring remammg thickness with industry-standard tools and selecting the minimum thickness threshold The specific criteria for any other metal/baπier/dielectπc layer wafer system are determined by polishing sufficient numbers of test wafers, generally 2 to 10 and analyzing the reflected signal data 218, finding the best noise reduction technique, and then processing the resultmg spectra on a spectra-by-spectra basis in time to generate a unique endpoint signal that may be analyzed by simple threshold analysis In many cases, the simplest approach works best In the case of dielectnc polishing or shallow trench isolation dielectric polishing, a more complicated approach w ill generally be waπanted

Next, at box 613, a determination is made as to whether or not the EPS satisfies the predetermined endpoint criteria If so, then at box 615, the endpoint trigger signal 125 is transmitted to the CMP apparatus and the CMP process is stopped If the EPS does not satisfy the predetermined endpoint criteria, the process goes to box 617 where the timer is tested to determine if a timeout has occuπed If no timeout has occuπed, the process returns to box 605 where another reflected data spectrum is acquired If the timer has timed out, the endpoint trigger signal 125 is transmitted to the CMP apparatus and the CMP process is stopped Additionally, it is desired that a CMP process should provide the same quality of polishmg results across the entire wafer, a measure of the removal rate, and the same remo\ al rate from wafer to wafer In other words, the polish rate at the center of the wafer should be the same as at the edge of the wafer, and the results for a first wafer should be the same as the results for a second wafer The present invention may be advantageously used to measure the quality and removal rate withm a wafer, and the removal rate from wafer to wafer for the CMP process For the data provided by an apparatus according to the present invention, the quality of the CMP process is defined as the standard deviation of the tune to endpoint for all of the sample pomts divided by the mean of the set of sample pomts In mathematical terms, the quality measure (designated by Q) is

^Q = ? x ⁽i) Computer 121, suitably programmed, may accomplish the calculation of Q The parameter of quality Q, although not useful for terminating the CMP process, is useful for determining whether or not the CMP process is effective

The removal rate (RR) of the CMP process is defined as the known starting thickness of the film minus the thicker of said film at the end of CMP, or the amount removed plus polish time, divided by the time to endpomt The wafer-to-wafer removal rate is the standard deviation of the RR divided by the average RR from the set of wafers polished

Liquid Waveguide Although a fiber optic waveguide system is described above, the present invention can be implemented with a liquid waveguide FIGURE 7 is a block diagram illustrative of an optical endpoint detector system 700 using a liquid waveguide, according to one embodiment of the present invention System 700 is essentially the same as system 100(FIGURE 1), except that fiber optic cable 113 (FIGURE 1) is deleted and the fluid outlet of fluid source fittmg 114 is implemented with a fluid delivery tube 702 In one embodiment, fluid delivery tube 702 is available from Polymicrotechnologies Inc Phoenix, Arizona In general, fluid delivery mbe 702 can be similar to those used in chromatography technology Preferably, fluid deliver ' tube 702 has an index of refraction that is less than the fluid from fluid source 1 16 (FIGURE 1) which allows the tube to function as a waveguide In addition, fluid delivery tube 702 is made of material that can tolerate the pH environment caused by the slurry and fluid Alternatively, fluid delivery mbe 702 may be implemented with a think-walled tube (can be either flexible or rigid) having its inner surface coated with a protectiv e layer Preferably, the protective coating would have an mdex of refraction that is less than the mdex of refraction of the fluid

System 700 essentially operates as descnbed above for system 100 except that the fluid flowing in fluid delivery tube 702 functions as both a waveguide for propagating light to and from the wafer surface and for flushing bubbles, polishing debris and sluπy particles from the optical path near the wafer surface Thus, this embodiment avoids problems that can be caused by the end of a fiber optic cable being too close to the wafer surface

In an alternative embodiment, two fluid delivery tubes may be used, with optical coupler 1 15 (FIGURE 1 ) being deleted One mbe provides a light path between light source 117 and holes 11 1 and 112 (FIGURE 1), whereas the other mbe provides a light path between light sensor 119 (FIGURE 1) and holes 111 and 112 (FIGURE 1)

The embodiments of the optical EPD system described abov e are illustrative of the principles of the present invention and are not intended to limit the invention to the particular embodiments described For example, in light of the present disclosure, those skilled in the art can devise without undue experimentation embodiments using different light sources or spectrometers other than those descnbed Further, other embodiments may be implemented m which the fluid flows across the surface of the optic fiber In addition to polishing wafers, other embodiments of the present invention can be adapted for use in sensing any type of workpiece For example, a workpiece may be a semiconductor wafer, a bare silicon or other semiconductor substrate with or without active devices or circuitry, a partially processed wafer, a silicon on insulator, a hybrid assembly, a flat panel display, a Micro Electromechanical Sensor (MEMS), a wafer, a disk for a hard drive memory, or any other material that would benefit from planaπzation Other embodiments of the present invention can be adapted for use in grinding and lapping systems other than the described CMP polishmg applications Accordmgly, while the prefeπed embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein w ithout departing from the spirit and scope of the invention

Claims

We claim

1 An optical signal transmission apparatus for use in a chemical mechanical polishing system, the chemical mechanical polishmg system being configured to cause duπng a polishing process a relative motion between a surface to be polished and a polishing pad, the apparatus compπsmg a light source configured to provide light, a light propagating medium havmg a first end and a second end, wherem the light propagating medium is configured so that the first end of the light propagating medium receives light from the light source and the second end of the light propagating medium is proximate to the surface being polished, a passage having a first opening and a second opening, wherein the passage is configured so that the second opening of the passage is disposed proximate to the surface to be polished; and a fluid source coupled to the first opening of the passage, wherein the fluid source is configured to provide fluid to the passage, the fluid being capable of propagating light from the light source, wherein the fluid flows out of the second opening while light propagating medium propagates light form the light source provides light to the surface to be polished

2. The apparatus of Claim 1 wherem light reflected from the surface bemg polished is received by the light propagating medium.

3 The apparatus of Claim 1 wherein the light propagating medium comprises an optic fiber.

4. The apparatus of Claim 1, wherein the light propagating medium is disposed within the passage

5 The apparatus of Claim 1 wherein the fluid has substantially the same pH as slurry being used in polishing the surface being polished

6. The apparatus of Claim 5 wherem the fluid comprises de-ionized water.

7. The apparatus of Claim 5, wherein the fluid compπses a solution essentially the same as a solution used in the slurry.

8 The apparatus of Claim 1, wherem the light propagating medium compπses the fluid flowing in the passage

9 The apparatus of Claim 8, wherein the light propagatmg medium further compπses a fluid delivery mbe 10 The apparatus of Claim 9, wherein the fluid delivery mbe has an mdex of refraction that is less than that of the fluid

11 A chemical mechanical polishing system for polishing a surface of a workpiece, the system comprising a pad assembly having a polishmg pad with a through-hole, a wafer chuck configured to hold the workpiece against the polishing pad duπng a polishing process, a light source configured to provide light, a light propagatmg medium having a first opening and a second opening wherein the light propagating medium is configured so that the first opening of the light propagatmg medium receives light from the light source and the second opening of the light propagatmg medium is proximate to the surface of the workpiece, a passage, disposed in the pad assembly, having a first opening and a second openmg, wherein the passage is configured so that the second opening of the passage is aligned with the through-hole, a fluid source coupled to the first opening of the passage, wherem the fluid source is configured to provide fluid to the passage, the fluid being capable of propagatmg light provided by the light source, wherein the fluid flows out of the second opening and out of the through-hole, a light sensor coupled to the second opening of the light propagating medium, the light sensor being aπanged to receive light reflected from the surface of the workpiece through the light propagating medium, wherem the light sensor is configured to generate data coπespondmg to a spectrum of the reflected light, and a computer coupled to the light sensor, wherem the computer is configured to monitor the polishing process using the data from the light sensor

12 The system of Claim 11 wherein light reflected from the surface of the workpiece is received by the light propagating medium

13 The system of Claim 11 wherem the light propagatmg medium comprises an optic fiber

14 The system of Claim 11 wherein the light propagating medium is disposed within the passage

15 The system of Claim 11 wherein the fluid has substantially the same pH as slurry being used in polishmg the surface bemg polished

16 The system of Claim 15 wherem the fluid comprises de-iomzed water 17 The system of Claim 15 wherein the fluid comprises solution essentially the same as a solution used in the slurry

18 The system of Claim 11 wherein the light propagating medium comprises the fluid flowing m the passage

19 The system of Claim 18, wherein the light propagating medium further comprises a fluid delivery mbe

20 The system of Claim 19, wherein the fluid delivery mbe has an index of refraction that is less than that of the fluid

21 A method of momtoπng a chemical mechanical polishing process of a surface of a workpiece, the method compπsmg illuminating at least a portion of the surface of the workpiece with light from a light source while the surface of the workpiece is being polished, wherein the light is propagated through a through-hole in a polishmg pad of a pad assembly, providing a fluid to flow through a through-hole in the polishing pad, the fluid bemg capable of propagatmg light provided by the light source, receiving light reflected from the surface of the workpiece the fluid is flowing through the through-hole, generating data coπespondmg to the received reflected light, and determining a parameter of the chemical mechanical polishing process using the data

22 The method of Claim 21 wherein an optic fiber is used to propagate light from the light source to illummate the surface of the workpiece, the optic fiber disposed m the pad assembly and having an end positioned to propagate light to and from the through-hole

23 The method of Claim 21 wherein the fluid flowing through the through-hole also flows through a passage disposed in the pad assembly, wherem the flowing fluid forms at least part of a light propagating medium to illuminate the surface of the workpiece

24 The method of Claim 23 wherein the fluid flows through a fluid delivery mbe disposed in the polishmg pad assembly and aligned with the through-hole

25 The method of Claim 21 wherem the fluid has substantially the same pH as that of a slurry that is used in polishing the workpiece

26 The method of Claim 25 wherein the fluid compπses a solution that is essentially the same as a solution used m the slurry 27 An apparatus for monitoring a chemical mechanical polishing process of a surface of a workpiece. the apparatus compπsmg means foi illuminating at least a portion of the surface of the workpiece with light while the surface of the workpiece is bemg polished, wherein the light is propagated through a through-hole m a polishing pad of a pad assembly, means for providmg a fluid to flow through the through-hole in the polishing pad, the fluid being capable of propagating light provided by the means for illuminating, means for receiving light reflected from the surface of the workpiece and the fluid is flowing through the through-hole, means for generating data coπesponding to the received reflected light, and means for determining a parameter of the chemical mechanical polishing process usmg the data

28 The apparatus of Claim 27 wherem the means for illuminating comprises an optic fiber, the optic fiber disposed in the pad assembly and having an end positioned to propagate light to and from the through-hole

29 The apparatus of Claim 27 wherein the means for providing fluid comprises a passage disposed in the pad assembly, wherein the flowing fluid forms at least part of a light propagating medium to illuminate the surface of the workpiece

30 The apparatus of Claim 29 wherem the means for providing fluid comprises a fluid delivery tube disposed in the polishing pad assembly and aligned with the through-hole

31 An improved method of tn-situ optical thin film measurement for use during a chemical mechanical polishing process of a surface of a workpiece, the chemical mechanical polishing process using a polishing pad placed in contact with the surface of the workpiece and using an optics unit to illuminate the surface of the workpiece with light and receive light reflected from the surface of the workpiece while the surface of the workpiece is being polished, the polishing pad havmg a through-hole operatively aligned with the optics unit, the improvement compπsmg providing a fluid to flow between the optics unit and the surface of the workpiece, the fluid bemg capable of propagating light provided by the optics unit

32 The method of Claim 31 wherein the fluid flows substantially normal to a surface of the optics unit, the surface of the optics unit bemg configured to emit light to the surface of the workpiece and to receive reflected light from the surface of the workpiece

33 The method of Claim 31 wherein the fluid flows substantially across a surface of the optics unit, the surface of the optics unit bemg configured to emit light to the surface of the workpiece and to receive reflected light from the surface of the workpiece 34 The method of Claim 31 wherein the polishing pad is mounted on a pad backer havmg a passage formed therein, the through-hole being aligned with the passage, wherein the fluid is provided so as to flow through the passage

35 The method of Claim 31 wherem the fluid has a pH that is compatible with the pH of a slurry used in the chemical mechanical polishmg process

36 The method of Claim 34 wherein the fluid comprises a solution that is essentially the same as a solution used in the slurry

37 The method of Claim 31 wherem the fluid is used to propagate light from a light source to the surface of the workpiece

38 The method of Claim 37 wherein the optics unit includes the light source

39 The method of Claim 31 wherem the fluid is used to propagate light reflected from the surface of the workpiece to a light detector

40 The method of Claim 39 wherein the optics unit includes the light detector