WO2000058716A1

WO2000058716A1 - Optical endpoint detection system for rotational chemical mechanical polishing

Info

Publication number: WO2000058716A1
Application number: PCT/US2000/008084
Authority: WO
Inventors: Thomas Frederick Allen Bibby, Jr.; John A. Adams; Mark Anthony Meloni; Christopher E. Barns
Original assignee: Speedfam-Ipec Corporation
Priority date: 1999-03-26
Filing date: 2000-03-24
Publication date: 2000-10-05
Also published as: AU4034200A

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 having a through-hole, a light source, a fiber optic cable assembly, a light sensor, and a computer. The fiber optic cable propagates the light 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 computer receives the reflected spectral data and generates an endpoint signal as a function of the reflected spectral data. The light source, fiber optic cable assembly and light sensor are disposed within a platen of a chemical mechanical polishing machine. The computer is located external to the platen. A wireless link is used to communicate the reflected spectral data to the computer. Alternatively, the light source and light sensor are disposed external to the platen and a rotary union is included to allow the fiber optic cable assembly to be used in a rotary chemical mechanical polishing machine. A through-cylinder is used to provide a protected passage through the platen for the fiber optic cable as the fiber optic cable extends into the through-hole in the polishing pad.

Description

OPTICAL ENDPOINT DETECTION SYSTEM FOR ROTATIONAL CHEMICAL MECHANICAL POLISHING

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), i.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 in situ EPD methods have been proposed, but few have been successfully demonstrated in a manufacturing environment and even fewer 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 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 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 during the course of the polish cycle. However, 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 rotating CMP tool. Light reflected from the wafer surface back through the window is used to detect end point. However, the window complicates the CMP process because it presents to the wafer an inhomogeneity 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, various means for implementing a transparent window in 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 in the field of view. Furthermore, integration times 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 interrupted 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 going 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 nonpolished 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 above methods has drawbacks of one sort or another. What is needed is a new method for in situ EPD that provides noise immunity, can work with 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 fiber optic cable assembly, a light sensor, and a computer. The light source provides light within 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 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 criteria and stops the polishing process when the endpoint signal meets the predetermined criteria. 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 accumulated slurry and polishing debris. This robustness makes the apparatus suitable for in situ EPD in a production environment.

In another aspect of the present invention, the light source, fiber optic cable assembly and light sensor are attached to a platen of a chemical mechanical polishing machine. The computer is located external to the platen, along with a detector. A wireless link is used to communicate the reflected spectral data to the computer. This aspect of the present invention can be advantageously used in rotary chemical mechanical polishing machine in which a fiber optic or wired link between the light sensor and the computer would be difficult to implement.

In yet another aspect of the present invention, a rotary union is included to allow the fiber optic cable assembly to be used in a rotary chemical mechanical polishing machine. A through-cylinder is used to provide a protected passage through the platen for the fiber optic cable as the fiber optic cable extends into the through-hole in the polishing pad. The through-cylinder also prevents cooling fluid within the platen from leaking out, thereby preventing loss of cooling fluid and contamination of the polishing pad and the surface being polished.

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 an apparatus in accordance with the present invention, adapted for use with an orbital CMP machine. 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-reducing 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, in accordance with one embodiment of the present invention. FIGURE 6 is a flow diagram illustrating the analysis of the reflectance signal in accordance with the present invention.

FIGURE 7 is a diagram schematically illustrating an apparatus in accordance with the present invention adapted for use with a rotary CMP machine. FIGURES 7A-7H are diagrams illustrating various embodiments of an optics unit, in accordance with the present invention.

FIGURES 7I-7M are diagrams illustrating various off-platen-coupling embodiments, in accordance with the present invention. FIGURES 8A and 8B are diagrams illustrating the location of sensors on a platen, in accordance with different embodiments of the apparatus of FIGURE 7.

FIGURE 9 is a diagram schematically illustrating an alternative apparatus in accordance with the present invention adapted for use with a rotary CMP machine.

FIGURES 10A and 10B are diagrams illustrating various embodiments of off-platen rotational EPD systems, in accordance with the present invention.

Detailed Description

The present invention relates to a method of EPD using optical means and also to a method of processing the optical data. CMP machines typically include a means of holding a wafer or substrate to be polished. Such holding means are sometimes referred to as a carrier, but the holding 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 referred to herein 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 preferred embodiment.

EPD System: Orbital CMP

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 breaking. Instead of being rotational, the motion of the pad is "orbital" in a preferred 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. In a preferred embodiment, the orbit diameter is 1.25 inches. 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 in 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, which is incorporated herein by reference.

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 backing plate 140. In a preferred 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 backing. 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 in the preferred orbital motion.

Polishing pad 109 includes a through-hole 112 that is coincident and communicates with a pinhole opening 111 in the pad backer 120. Further, a canal 104 is formed in the side of the pad backer 120 adjacent the backing plate. The canal 104 leads from the exterior side 110 of the pad backer 120 to the pinhole opening 111. In a preferred embodiment, a fiber optic cable assembly including a fiber optic cable 113 is inserted in the pad backer 120 of pad assembly 107, 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, in contrast to conventional systems as exemplified by U.S. Patent No. 5,433,651 to Lustig et al. that use a platen with a window of quartz or urethane, the present invention does not include such a window. Rather, the pinhole opening 111 is merely an orifice in the pad backer in which fiber optic cable 113 may be placed. Thus, in the present invention, the fiber optic cable 113 is not sealed to the pad backer 120. Moreover, because of the use of a pinhole opening 111, the fiber optic cable 113 may even be placed within one of the existing holes in the pad backer and polishing pad used for the delivery of slurry without adversely affecting the CMP process. As an additional difference, the polishing pad 109 has a simple through-hole 112.

Fiber optic cable 113 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.

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 nm in wavelength, and more preferably with 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. In operation, soon after the CMP process has begun, the start signal 123 is provided to the computer 121 to initiate the monitoring 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 in the preferred 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 119 is operative to provide reflected spectral data 218, referred 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 retaining the capability of endpoint detection on subsequent wafers. In other words, the fiber optic cable 113 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 in replacing used polishing pads in 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 array 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 in an embodiment using a photodiode as a light source. Alternative embodiments using a different light detection source (e.g., a CCD array) 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 criteria 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 endpoint signal 124. The endpoint 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. Turning to FIGURE 2, the light sensor 119 contains a spectrometer 201 that disperses the light according to wavelength onto a detector array 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 narrow 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 2A. 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 narrowed. In a preferred embodiment, detector array 203 contains 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 1 1 1 and through-hole 1 12 are shown near a point in 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 in 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 pinhole is preferably located in a groove in the polish pad.

With a rotating wafer chuck 101 and an orbiting pad assembly 107, any given point on the polishing pad 109 will follow spirographic trajectories, with the entire trajectory lying inside an annulus centered about the center of the wafer. An example of such trajectory is shown in FIGURE 4. The wafer 103 rotates about its center axis 105 while the polish pad 109 orbits. Shown in 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 preferred 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 endpoint signal is generated at least once per second, with a preferred integration time of light sensor 119 (FIGURE 1) being 15 ms. When properly synchronized, any particular point within 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, with a single sensor one can measure uniformity at two radial points. For stable production processes, measuring uniformity at two 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 trigger 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 in the pad backer 120 and polishing pad 109, each sampled with proper synchronous triggering, any desired measurement pattern can be obtained, such as radial scans, diameter scans, multipoint polar maps, 52-site 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 annuli is done randomly or asynchronously, the entire annulus can be sampled, thus allowing measurements around the wafer. Although in this embodiment the capability of sensing the entire wafer is achieved by adding more sensors, alternate approaches can be used to obtain the same result.

For example, enlarging the orbit of the pad assembly increases the area a single 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 in 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 polishing 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 vary by as much as an order of magnitude, thus adding "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 polishing; and other physical and/or electronic sources of noise.

Several signal processing techniques can be used for reducing the noise in the reflected spectral data 218a-218f, as shown in FIGURES 5A-5F. For example, a technique of single-spectrum wavelength averaging can be used as illustrated in FIGURE 5 A. In this technique, the amplitudes of a given number of pixels within the single 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 in FIGURE 5 A as a plot of amplitude vs. wavelength.

Alternatively, a time-averaging 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 times) as illustrated in FIGURE 5B. In this technique, the spectral data of the scans are combined by averaging the corresponding pixels from each spectrum, resulting in a smoother spectrum 241.

In another technique illustrated in FIGURE 5C, the amplitude ratio of wavelength bands of reflected spectral data 218c are calculated using at least two separate bands consisting 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 since the amplitude of each band is generally affected in the same way while the ratio of the amplitudes in 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 compensation 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 in a preferred 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 array of pixels present. Each subsequent reflected spectral data scan is then "normalized" by (i) summing up all of the pixels for the entire array of pixels present to obtain the integrated sample amplitude, and then (ii) 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, in 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 in 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 endpoint 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 time averaging of the amplitude ratio to smooth out noise, or other noise- reducing signal processing techniques that are known to one of ordinary skill in the art.

FIGURES 5G-5J illustrate the endpoint signal 124 generated by applying the amplitude ratio of wavelength bands technique described in conjunction with FIGURE 5C to the sequential reflected spectral data 218g, 218h, and 218i during the polishing of a metallized semiconductor wafer having metal over a barrier 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 correspond to the various layers being polished. Of course, the points corresponding to FIGURES 5G-5I are only three points of the plot, as illustrated in FIGURE 5J. In practice, as illustrated in FIGURE 5K, the transition above a threshold value 501 indicates the transition from a bulk metal layer 503 to the barrier layer 505, and the subsequent lowering of the level below threshold 507 after the peak 511 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 preferred embodiments for polishing tungsten (W), titanium nitride (TiN), or titanium (Ti) films formed on silicon dioxide (SiO₂). As described previously, these wavelength bands can be different for different materials 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 previously discussed, the entire wafer can be measured.

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 timer is set to zero. The timer is used to measure the amount of time required from the start of the CMP process until the endpoint of the CMP process has been detected. This timer is advantageously used to provide a fail-safe endpoint 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 time. In effect, if the timeout is set properly, no wafer will be overpolished and thereby damaged. However, some wafers may be underpolished and have to undergo a touchup polish if the endpoint 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 preferred 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 array 203. Thus, the form of the reflected spectral data 218 will be a vector of wavelength bands Rwbj, where i ranges from one to NPg, with NPg representing the number of pixel elements 205. The preferred sampling time is to acquire a reflected spectral data 218 scan approximately every 1 second. In one embodiment, the integration time is 33 milliseconds. In light of the present disclosure, those skilled in the art will appreciate that each wavelength band Rwbj represents a finite wavelength band as previously described 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 polishing is to calculate the amplitude ratio of wavelength bands. The reflectance of a first preselected wavelength band 520 (Rwb_x) is measured and the amplitude stored in memory. Similarly, the reflectance of the second preselected wavelength band 530 (Rwby) is measured and its amplitude stored in memory. The amplitude of the first preselected wavelength band (Rwb}) is divided by the amplitude of the second preselected wavelength band (Rwb2) to form a single 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 processing, the preferred 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 in the art. For example, see MacLeod, Thin Film Optical Filters (out of print), and Born et al., Principles of Optics: Electronic Theory of Propagation, Interference and Diffraction of Light, Cambridge University Press, 1998.

Next, at box 611, the endpoint signal 124 is examined 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 preferred endpoint signal 124 over time in exemplary form is shown in 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 times out before any of the threshold signals, then a stop polish command is issued. The threshold values are determined by polishing several wafers and determining at what values the transitions take place.

For dielectric polishing, a preferred endpoint signal results in a plot of remaining thickness vs. time. The signal is first tested against a minimum remaining thickness threshold level. If the signal is equal to or lower than the minimum thickness threshold before the timer has timed 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 remaining thickness with industry-standard tools and selecting the minimum thickness threshold.

The specific criteria for any other metal/barrier/dielectric 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 resulting 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 dielectric polishing or shallow trench isolation dielectric polishing, a more complicated approach will generally be warranted.

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 occurred. If no timeout has occurred, 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 polishing results across the entire wafer, a measure of the removal rate, and the same removal 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 within 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 time to endpoint for all of the sample points divided by the mean of the set of sample points. In mathematical terms, the quality measure (designated by Q) is:

^Q = ? x CD 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 thickness of the film at the end of the CMP process (or the thickness removed plus polish time) divided by the total polish time. The wafer-to-wafer removal rate is the standard deviation of the RR divided by the average RR from the set of wafers polished. EPD System: Rotational CMP

Although an orbital system is described above, the present invention can be used in a rotational CMP tool as well. In one embodiment, the carrier rotates about the center of the wafer and about an axis perpendicular to the plane of the wafer. Likewise, the platen rotates about its centerpoint, and about an axis perpendicular to the plane of the platen. The axis of rotation of the carrier is displaced from the center of the platen by a prescribed amount.

As previously mentioned, one difficulty in using optical EPD systems in a rotational CMP tool is that the rotation of the platen tends to complicate the process of getting light to and from the platen. In accordance with the present invention, one approach is to attach an optics unit onto the platen, thereby allowing the optics unit to rotate along with the platen. This approach helps simplify getting light to and from the platen. FIGURE 7 is a block diagram illustrative of an optical endpoint detector system 700 according to one embodiment of the present invention adapted for use with rotating (as opposed to orbital) CMP tools. System 700 includes an optics unit 701, an optic fiber 703, a fiber optic output line 705, a receiver 707 as well as computer 121. Optics unit 701 includes light source 117, light sensor 119 and optical coupler 115, which are described above in conjunction with FIGURE 1. Optics unit 701, optic fiber 703 and fiber optic output line 705 are mounted in a platen 709 of a rotating CMP tool such as, for example, a model Auriga/CMPV available from SpeedFAM, Phoenix, Arizona. Because optics unit 701 is placed within platen 709, platen 709 is balanced to ensure proper rotation. System 700 also includes wafer chuck rotary position sensor 142 and monitor 127 (as in FIGURE 1), and a platen rotary position sensor 710. Platen rotary position sensor 710 is similar to wafer chuck rotary position sensor 142 but is configured to provide the angular position of platen 709. These rotary position sensors may be implemented using rotary optical encoders available from Renco Encoders, Inc., Goleta, California. System 700 is interconnected as follows. An end 711 of optic fiber 703 is disposed in a hole in a polishing pad 712 mounted on a polishing surface 721 of platen 709, in a manner similar to that described above for the embodiment adapted for an orbital CMP tool. As previously described for the orbital embodiment, there is no need for a window with this system. The other end of optic fiber 703 is connected to optics unit 701, which is disposed within platen 709. Optics unit 701 receives power through a power line run through a hollow spindle (not shown) that is used to rotate platen 709 through a rotating contact. Fiber optic output line 705 has one end connected to optics unit 701 and another end 713 exposed at the non- polishing surface 723 (i.e., the bottom exterior surface in this example) of platen 709. Receiver 707 is disposed beneath platen 709 at the same radial distance from the center of platen 709 as end 713 of output line 705. Thus, as platen 709 rotates, end 713 of output line 705 is periodically aligned with receiver 707. In this embodiment, receiver 707 is implemented with a large-diameter fiber optic cable available from Edmund Scientific Company, Industrial Optics Division, Barrington, New Jersey. Alternatively, receiver 707 can be implemented using a banner group ring light guide, also available from Edmund Scientific. Computer 121 is connected to receiver 707, wafer chuck rotary position sensor 142, monitor 127, and platen rotary position sensor 710.

Although an embodiment with optics unit 701 within platen 709 is described, in light of the present disclosure, those skilled in the art can implement alternative embodiments without undue experimentation. For example, an embodiment with optics unit 701 attached to the bottom surface of platen 709, or partially embedded into platen 709 (e.g., fitting optics unit 701 into a cavity or compartment in platen 709). Still further, other embodiments may have optics unit 701 off platen, as described in conjunction with FIGURES 9-10B below.

In practice, end 711 (i.e., the sensor) of optic fiber 705 is configured to extend through the surface of platen 709 and through the polishing pad (not shown) during a polishing operation. In this embodiment, a standard polishing pad can be used, unlike some conventional optical systems that require a transparent pad. A simple hole-punch tool can be used to form a hole in the standard polishing pad through which the sensor can be exposed to the surface being polished. This feature advantageously avoids the need to align a pre-made hole in the polishing pad with the sensor.

During a polishing operation, system 700 detects the endpoint (or layer thickness, depending on the application) of the polishing process as follows. Optics unit 701 provides light to the surface being polished through optic fiber 703. Light is reflected back from the surface being polished, which optic fiber 703 propagates to optics unit 701. Optics unit 701 derives the spectral data of the received reflected light, which optics unit 701 provides through fiber optic output line 705. When aligned with end 713 of output line 705, receiver 707 optically receives the spectral data from optics unit 710. In one embodiment, receiver 707 is positioned so as to be aligned with end 713 of output line 705 when the surface being polished is aligned with end 711 of optic fiber 703. In this way, spectral data generated by optics unit 701 (in response to the light reflected from the surface being polished) is provided to computer 121. In addition, wafer chuck rotary position sensor 142 and platen rotary position sensor 710 respectively provide the angular positions of the wafer chuck (FIGURE 1) and platen 709 to computer 121, which then determines whether the endpoint has been reached (or the thickness of the layer being polished, depending on the application and algorithm) as described above for an orbital embodiment. The embodiment of FIGURE 7 advantageously allows end 713 of output line 705 to be placed near the axis of rotation of platen 709 independently of the position of end 711 of optic fiber 703. By placing end 713 near the axis of rotation, the relative velocity between end 713 and receiver 707 is relatively low, thereby facilitating proper alignment of end 713 with receiver 707 when the surface being polished is aligned with end 711 of optic fiber 703. A further advantage of this embodiment is that because optics unit 701 is disposed within platen 709, the use of rotational couplers (which would generally be required if the optics unit were outside of the platen in a rotational CMP machine) can be avoided while providing a simple and reliable mechanism to direct light to and receive reflected light from the surface being polished.

Alternatively, receiver 707 and end 713 of output line 705 can be positioned so as to be aligned after end 711 of optic fiber 703 is aligned with the surface being polished. For example, optics unit may incur a delay in providing the reflected signal data to output line 705, in which case end 713 of output line 705 and receiver 707 are aligned when the reflected light data is provided on output line 705. For example, optics unit 701 can include a processing circuit that provides a digital representation of the spectrum of the reflected light, which optics unit 701 then provides as optical pulses through output line 705. In this alternative embodiment, end 713 and optics unit 707 can be positioned for alignment after optics unit 701 receives the reflected light and generates the digital representation of the reflected spectral data, preferably as soon as optics unit 701 can provide the digital data on output line 705 so as to achieve real-time in-situ monitoring.

FIGURES 7A-7H illustrate portions of platen 709, showing in more detail various configurations of optics unit 701, according to the present invention. In the embodiment of FIGURE 7A, optics unit 701 includes light source 117, with optic fiber 703 providing light from light source 117 to illuminate the surface being polished via end 711, as previously described. In this embodiment, output line 705 is also an optic fiber, with one end positioned near end 711 to receive light reflected from the surface being polished. Output line 705 provides the unprocessed reflected light to end 713, which is located at bottom surface 723 of platen 709. An off-platen optical coupling (not shown) is used to receive the reflected light from end 713 and provide the reflected light to light sensor 119 (not shown). The optical coupling may be an optic fiber, or a fiber light ring or arc (described below in conjunction with FIGURES 71 and 7J).

The embodiment of FIGURE 7B is similar to that of FIGURE 7A except that optics unit 701 includes a single optic fiber and optical coupler 115 that are used to provide the light from light source 117 to the surface being polished and to receive reflected light, as previously described. The reflected light is then provided to the off-platen optical coupling located below platen 709 via end 713 of output line 705.

FIGURES 7C and 7D respectively illustrate embodiments similar to the embodiments of FIGURES 7A and 7B, except that the off-platen optical coupling is located above platen 709. Thus, end 713 of output line 705 is located at the top of platen 709. A through-hole is also required for end 713. End 713 is located on platen 709 so that the surface being illuminated does not cover end 713. In addition, the off-platen optical coupling is preferably positioned centrally upon the axis of rotation of the platen so that it is always aligned with end 733. If not central, the off- platen optical coupling is positioned so as to be at least periodically aligned with end 713 so that the reflected light can be communicated off-platen. These embodiments are advantageously used rotational CMP tools in which the radius of the platen is grater than the diameter of the wafer, which is positioned by the carrier so that a center portion of the platen remains uncovered during the polishing process.

FIGURES 7E and 7F are respectively similar to the embodiments of FIGURES 7B and 7D, except that light sensor 119 is included in optics unit 701. In these embodiments, processed reflected light signals (e.g., the spectrum of the reflected light) is then provided off-platen via an end 733 of an output line 735. The processed reflected light signals can be in the form of encoded optical signals. However, because the light is processed, the off-platen coupling need not be optical. For example, the off-platen coupling may be implemented using conventional commutators, electrical brush contacts or RF transmitters (e.g., a LMX-3162 device available from National Semiconductor, Santa Clara, California).

FIGURES 7G and 7H are respectively similar to the embodiments of FIGURES 7E and 7F, except that optical coupler 115 is deleted and separate optic fiber is used to receive the reflected light. This separate cable propagates the reflected light to light sensor 119, which then processes the reflected light and provides the processed reflected light signals off-platen via end 733 of output line 735 as described above in conjunction with FIGURES 7E and 7F.

FIGURES 7I-7M schematically illustrate several embodiments of off-platen optical couplings implemented with light rings or light arcs, according to the present invention. In the embodiment of FIGURES 71, end 713 of output line 705 is ring- shaped with its center being coaxial with the axis of rotation of platen 709 (FIGURE 7). In this embodiment, ring-shaped end 713 lies in a plane that is essentially parallel to bottom surface 723 of platen 709 (FIGURE 7). Receiver 707 is aligned with ring-shaped end 713 (i.e., the same radial distance from the axis of rotation as ring-shaped end 713. Consequently, receiver 707 is aligned with ring- shaped end 713 throughout the rotation of platen 709 (FIGURE 7). In one embodiment, ring-shaped end 713 is implemented with a fiber optic ring light guide available from Edmund Scientific Company, Industrial Optics Division, Barrington, New Jersey. This embodiment can also be used with the embodiments of FIGURES 7A and 7B, and with the embodiments of FIGURES 7C and 7D by placing end 713 and detector 707 in a plane above platen 709. FIGURE 7J schematically illustrates an off-platen optical coupling similar to the embodiment of FIGURE 71, except in this embodiment receiver 707 is also implemented with a fiber optic light ring. FIGURE 7K illustrates an embodiment in which end 713 of output line 705 is not ring-shaped, but receiver 707 is ring-shaped. FIGURE 7L illustrates an embodiment similar to the embodiment of FIGURE 7J, except that the end 713 and receiver 707 are concentric ring-shaped couplings configured so that the light is propagated in essentially the same plane containing end 713 and receiver 707. Although FIGURE 7L shows that receiver 707 is within end 713, in other embodiments, end 713 may be the interior ring. In a further refinement, end 713 and receiver 707 may be sealed, with a fluid (including air) between them. Although the embodiments of FIGURES 7I-7L provide coupling below platen 709 (FIGURE 7), those skilled in the art can implement embodiments that provide coupling above platen 709. FIGURE 7M shows an embodiment using fiber optic light guides that are shaped as arcs instead of rings. Although in this embodiment end 713 and receiver 707 are not aligned throughout an entire rotation of platen 709, the arc-shapes allow them to be aligned for a relatively large portion of a revolution compared to embodiments that use only the end of a standard optic fiber.

In yet another alternative embodiment, system 700 is adapted to use electrical (as opposed to optical) signals to provide the reflected spectral data to computer 121. More particularly, output line 705 is implemented with a standard electrically conductive wire, and receiver 707 is replaced with a conventional rotating commutator (not shown) for induction coupling. Optics unit 701 is modified to provide an electrical signal containing the reflected spectral data to output line 705. The electrical signal is then received by the conventional rotating commutator and provided to computer 121 via induction coupling for processing as described above.

FIGURES 8A and 8B are diagrams illustrating the locations of sensors on the platen surface, in accordance with different embodiments of the present invention. System 700 (FIGURE 7) has one sensor extending through the platen; however, in these alternative embodiments, multiple sensors are used to, in effect, increase the sampling rate of the light signals reflected from the surface being polished. In addition, multiple sensors may be used to sample different points of the surface.

FIGURE 8A illustrates sensors 801 ,-801₄. In particular, sensors 801 ,-801₄ are formed from four optic fibers. Each of these optic fibers has one end connected to an optics unit. The other end of each fiber serves as one of sensors 801 ,-801₄, each end being similar to end 711 of optic fiber 703 as described above in conjunction with FIGURE 7. Further, sensors 801 _r801₄ are positioned along a radius of platen 709. In this embodiment, sensors 801 ,-801₄ are approximately uniformly spaced between the minimum radius 803 of wafer motion and maximum radius 805 of wafer motion. In particular, minimum radius 803 and maximum radius 805 define an annulus within which wafer 103, being rotated by wafer chuck 101 (FIGURE 1), "sweeps through" while platen 709 rotates. Thus, in effect, the center of wafer 103 has a trajectory 807 above platen 709 in the form of a circle having a radius being the mean of minimum and maximum radii 803 and 805.

In this embodiment, optics unit 701 (FIGURE 7) would be modified to provide light to and receive reflected light from sensors 801 ,-801₄. This embodiment advantageously allows for simultaneously monitoring the polishing process along a radius of wafer 103. For example, optics unit 701 can include a light sensor (similar to light sensor 119 in FIGURE 1) that can be configured to have four output channels, such as model F76 available from IPEC Planar, Phoenix. This multi-channel light sensor can use a suitable modulation scheme to provide multiple channels of data (i.e., one for each sensor) to receiver 707 (FIGURE 7). In one embodiment, the multi-channel light sensor latches data from sensors 801 ,-801₄ and then sequentially reads the data out to provide the reflected spectral data to receiver 707 (FIGURE 7). Accordingly, the wafer is sampled at the platen rotation rate, generating four samples at once. Computer 121 is configured to receive the four channels of reflected spectral data. The data from each channel can be buffered to allow computer 121 to process each channel of data as described above for the orbital embodiment. Alternatively, optics unit 701 can have four output lines to be used in conjunction with four properly positioned optical detectors (each being substantially similar to receiver 707). More particularly, in one embodiment, optics unit 701 includes a spectrometer and a detector array as described above in conjunction with FIGURE 2. In a manner similar to that of light sensor 119 (FIGURE 2), the spectrometer and the detector array provide the spectrum of the reflected light received by sensors 801,-801₄ and generates output signals representing each spectrum. An output signal representing a spectrum is then used to modulate an optical signal that is transmitted over output line 735 to receiver 707. In this embodiment, the output signal representing the spectrum is sampled by an analog-to-digital converter (not shown) and used to digitally modulate the optical signal. Optical detector receives the optical signal and provides an output electrical signal containing the spectral data to computer 121. Computer 121 is configured to extract and process the spectral data as described above for the orbital embodiment. In alternative embodiments, optics unit 701 may provide an analog signal to receiver 707, with the analog-to- digital conversion occurring at computer 121.

FIGURE 8B illustrates an alternative sensor arrangement on platen 709. This embodiment is similar to that shown in FIGURE 8A except that instead of having linearly arranged sensors 801,-801₄, this embodiment has spirographically arranged sensors 809_r809₇. In particular, sensors 809,-809₇ are arranged in a spiral pattern from slightly outside minimum radius 803 to slightly inside maximum radius 805 with an approximately linearly increasing radius. In addition, each of sensors 809,-809₇ is angularly separated from neighboring sensors by approximately 60 degrees. This spirographic arrangement of sensors 809,-809₇ allows the optical EPD system to operate continuously on the reflected spectral data as they are sampled and provided to computer 121 (FIGURE 7). This arrangement also allows for sampling of a wafer at six times the platen rotation rate.

FIGURE 9 is a diagram schematically illustrating an optical EPD system 900 adapted for use with a rotary CMP machine, in accordance with an alternative embodiment of the present invention. This embodiment is similar to the embodiment of FIGURE 7 except that optics unit 701 is disposed below platen 709 and uses a fiber optic cable assembly 902 to physically connect the sensor to optics unit 701. Fiber optic cable assembly 902 includes a rotary union 904 and a through- cylinder 906 and a fiber optic cable 908. Unlike in the previously described orbital and optically coupled embodiments, EPD system 900 uses rotary union 904 because the rotation of platen 709 would twist fiber optic cable 902, which would very likely cause severe damage to fiber optic cable assembly 902 or possibly damage other parts of the rotating CMP machine. Through-cylinder 906 is used provide a protected passage for fiber optic cable 908 to be threaded or sleeved through platen 709 with an end 910 extending into the polishing pad (not shown). More particularly, in some rotating CMP machines (e.g., an Auriga CMPV available from SpeedFAM, Phoenix, Arizona), platen 709 contains cooling fluid. As will be appreciated by those skilled in the art, the cooling fluid, in general, should not be allowed to come in contact with the polishing pad or allowed to leak out of platen 709. In this embodiment, through-cylinder 906 is formed from stainless steel and is fitted into platen 709 using standard threaded, swedge or cold fit techniques. Through-cylinder 906 is fitted into platen 709 so as to seal the interfaces between platen 709 and through-cylinder 908 to prevent leakage of the cooling fluid within platen 709. Through-cylinder 906 also includes a passage through which fiber optic cable 908 is fitted so that end 910 extends into the polishing pad (not shown). Although a stainless steel through-cylinder is described, other suitable materials may be used in alternative embodiments.

In this embodiment, the rotating CMP machine has a hollow spindle (not shown) that is used to rotate platen 709. Rotary union 904 is attached to the internal section within the hollow spindle (not shown). Fiber optic cable 908 is run from through-cylinder 906, along the bottom surface of platen 709, and is attached to one port of rotary union 904. In this embodiment, the opposite port of rotary union 904 is connected to optics unit 701 via fiber optic cable. In this embodiment, rotary union 904 is implemented with a Model 215 Fiber Optic Rotary Joint available from Focal Technologies Inc., Dartmouth, Nova Scotia, Canada. Of course, other rotary unions may be used in alternative embodiments. Computer 121 processes the reflected spectral data as previously described to determine the endpoint of the polishing process (or the layer thickness, depending on the application). In an alternative embodiment, the rotary union may have two optic channels and a fluid coupling (both input and output). This embodiment may be implemented using the aforementioned Focal Technologies Model 215 and a standard type Deublin union.

FIGURES 10A and 10B schematically illustrate alternative embodiments that use optical coupling techniques different from rotary union 904. FIGURE 10A shows an embodiment in which the off-platen coupling is implemented using fiber optic ring lights. This embodiment uses a set of fiber optic light rings for the path from light source 117 to end 711 and another set of fiber optic light rings for the path for the reflected light to light sensor 119. In particular, the on-platen fiber optic light ring guide for the forward path (i.e., the path bringing light from light source 117 to the surface being polished) and the on-platen fiber optic light ring guide for the return path (i.e., the path bringing reflected light to light sensor 119) are essentially coaxial with the center of platen 709. The light in both paths propagate between the off-platen fiber optic light ring guides substantially in parallel with the axis of rotation.

The embodiment of FIGURE 10B is similar to the embodiment of FIGURE 10A except that the forward path fiber optic light ring guides are positioned so as to be concentric and, similarly, the return path fiber optic light ring guides are positioned so as to be concentric. In this embodiment, the light in both paths propagates essentially perpendicularly with the axis of rotation of platen 709.

The embodiments of the optical EPD system described above 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 described. 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 Electro- Mechanical Sensor (MEMS), a wafer, a disk for a hard drive memory, or any other material that would benefit from planarization. Other embodiments of the present invention can be adapted for use in grinding and lapping systems other than the described CMP polishing applications. Accordingly, while the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.

Claims

We claim:

1. An apparatus for use in a chemical mechanical polishing system to generate an endpoint signal in polishing a surface, the chemical mechanical polishing system being configured to cause during a polishing process a relative motion between the surface and a polishing pad mounted on a platen, the apparatus comprising: an optics unit attached to the platen, the optics unit including: a light source configured to generate light of a predetermined bandwidth, a fiber optic cable assembly having a first end and a second end, wherein the fiber optic cable is configured to propagate light from the light source to the surface through a through-hole in the polishing pad, the first end of the fiber optic cable assembly extending partially into a through-hole in the polishing pad, an output line having an input end and an output end, and a light sensor coupled to the second end of the fiber optic cable assembly, wherein the light sensor is configured to receive light reflected from the surface through the fiber optic cable assembly and generate data corresponding to a spectrum of the reflected light, the light sensor being further configured to output the data onto the input end of the output line; a detector disposed external to the platen, the detector being configured to remotely receive the data from output end of the output line; and a computer coupled to the detector, wherein the computer is configured to generate the endpoint signal as a function of the data from the light sensor.

2. The apparatus of Claim 1, wherein the light sensor is configured to provide the data using an optical signal, and wherein the detector comprises an optical detector.

3. The apparatus of Claim 1, wherein the light sensor is configured to provide the data using a radio frequency signal, and wherein the detector comprises a radio frequency receiver.

4. The apparatus of Claim 1 wherein the optics unit further includes a plurality of fiber optic cable assemblies configured to propagate light from the light source to the surface through a corresponding through-hole of a plurality of through- holes in the polishing pad, each fiber optic cable assembly of the plurality of fiber optic cable assemblies having a first end and a second end, each first end of the plurality of fiber optic cables extending partially into its corresponding through-hole of the plurality of through-holes.

5. The apparatus of Claim 4 wherein the plurality of through-holes are spirographically arranged.

6. The apparatus of Claim 4 wherein the plurality of through-holes are arranged along a radius of the platen.

7. The apparatus of Claim 4 wherein for each fiber optic cable assembly of the plurality of fiber optic cable assemblies, the light sensor is configured to provide data corresponding to a spectrum of reflected light received by the fiber optic cable assembly and further configured to output the data on the output line.

8. The apparatus of Claim 4 further comprising a plurality of output lines, each output line of the plurality of output lines having a corresponding fiber optic cable assembly of the plurality of fiber optic cable assemblies, wherein for each fiber optic cable assembly of the plurality of fiber optic cable assemblies, the light sensor is configured to provide data corresponding to a spectrum of reflected light received by the fiber optic cable assembly and further configured to output the data on the fiber optic cable assembly's corresponding output line.

9. The apparatus of Claim 1 , wherein the output end of the output line is located at a radial distance from the center of the platen different from the radial distance of the through-hole.

10. The apparatus of Claim 1, further comprising a commutator coupled to the output line of the light sensor of the optics unit, wherein the detector is configured to receive the data from the light sensor through the commutator.

11. A chemical mechanical polishing system for polishing a surface of a wafer, the system comprising: a polishing pad having a through-hole; a platen configured to hold the polishing pad; a rotatable wafer chuck configured to hold the wafer against the polishing pad during a polishing process; an optics unit attached to the platen, the optics unit including: a light source configured to generate light of a predetermined bandwidth, a fiber optic cable assembly having a first end and a second end, wherein the fiber optic cable is configured to propagate light from the light source to the surface through a through-hole in the polishing pad, the first end of the fiber optic cable assembly extending partially into a through-hole in the polishing pad, an output line having an input end and an output end, and a light sensor coupled to the second end of the fiber optic cable assembly, wherein the light sensor is configured to receive light reflected from the surface through the fiber optic cable assembly and generate data corresponding to a spectrum of the reflected light, the light sensor being further configured to output the data onto the input end of the output line, a detector disposed external to the platen, the detector being configured to remotely receive the data from the output line of the light sensor, and, a computer coupled to the detector, wherein the computer is configured to generate the endpoint signal as a function of the data from the light sensor.

12. The system of Claim 11, wherein the light sensor is configured to provide the data using an optical signal, and wherein the detector comprises an optical detector.

13. The system of Claim 11, wherein the light sensor is configured to provide the data using a radio frequency signal, and wherein the detector comprises a radio frequency receiver.

14. The system of Claim 11 wherein the optics unit further includes a plurality of fiber optic cable assemblies configured to propagate light from the light source to the surface through a corresponding through-hole of a plurality of through- holes in the polishing pad, each fiber optic cable assembly of the plurality of fiber optic cable assemblies having a first end and a second end, each first end of the plurality of fiber optic cables extending partially into its corresponding through-hole of the plurality of through-holes.

15. The system of Claim 14 wherein the plurality of through-holes are spirographically arranged.

16. The system of Claim 14 wherein the plurality of through-holes are arranged along a radius of the platen.

17. The system of Claim 14, wherein for each fiber optic cable assembly of the plurality of fiber optic cable assemblies, the light sensor is configured to provide data corresponding to a spectrum of reflected light received by the fiber optic cable assembly and further configured to output the data on the output line.

18. The system of Claim 14 further comprising a plurality of output lines, each output line of the plurality of output lines having a corresponding fiber optic cable assembly of the plurality of fiber optic cable assemblies, wherein for each fiber optic cable assembly of the plurality of fiber optic cable assemblies, the light sensor is configured to provide data corresponding to a spectrum of reflected light received by the fiber optic cable assembly and further configured to output the data on the fiber optic cable assembly's corresponding output line.

19. The system of Claim 11 wherein the output end of the output line is located at a radial distance from the center of the platen different from the radial distance of the through-hole.

20. The system of Claim 11 further comprising a commutator coupled to the output line of the light sensor of the optics unit, wherein the detector is configured to receive the data from the light sensor through the commutator.

21. The system of Claim 11 wherein the platen is rotated about a center axis of the platen during a polishing operation.

22. A method of detecting an endpoint during chemical mechanical polishing of a wafer surface, the method comprising: providing a relative rotation between the wafer surface and a pad mounted on a platen, the pad contacting the surface during a polishing process of the wafer surface; illuminating at least a portion of the wafer surface with light using a light source attached to the platen; receiving light reflected from the wafer surface while the wafer surface is being polished using a light sensor disposed within the platen; generating data corresponding to a spectrum of the received reflected light; and determining whether the endpoint has been reached as a function of the data.

23. The method of Claim 22 further comprising providing an optical signal depending on the received reflected light to an optical detector located external to the platen.

24. The method of Claim 22 further comprising providing a radio frequency signal depending on the received reflected light to a radio frequency receiver located external to the platen.

25. The method of Claim 22 further comprising providing an electrical signal depending on the received reflected light to a detector located external to the platen using a commutator.

26. The method of Claim 22 wherein the light source illuminates a portion of the wafer surface using an optic fiber having an end extending into a through-hole in the pad.

27. The method of Claim 22 wherein a spectrometer and a detector array, disposed within the platen, are used in generating the data.

28. The method of Claim 27 wherein an output signal of the detector array is digitized in generating the data.

29. An apparatus for detecting an endpoint during polishing of a wafer surface, the apparatus comprising: means for providing a relative rotation between the wafer surface and a pad mounted on a platen, the pad contacting the surface during a polishing process of the wafer surface; means, attached to the platen, for illuminating at least a portion of the wafer surface with light ; means, disposed within the platen, for receiving light reflected from the wafer surface while the wafer surface is being polished; means for generating data corresponding to a spectrum of the received reflected light; and means for determining whether the endpoint has been reached as a function of the data.

30. The apparatus of Claim 29 further comprising means, located external to the platen, for providing an optical signal depending on the received reflected light to an optical detector.

31. The apparatus of Claim 29 further comprising: means, attached to the platen, for providing a radio frequency signal depending on the data; and means located external to the platen for receiving the radio frequency signal.

32. The apparatus of Claim 29 wherein the means for receiving light further comprises an optic fiber having an end extending into a through-hole in the pad.

33. The apparatus of Claim 29 wherein the means for generating data comprises a spectrometer and a detector array.

34. The apparatus of Claim 33 wherein the means for generating data further comprises an analog-to-digital converter coupled to the detector array.

35. An apparatus for use in a chemical mechanical polishing system to generate an endpoint signal in the polishing a surface, the chemical mechanical polishing system being configured to cause during a polishing process a relative motion between the surface and a polishing pad mounted on a first surface of a platen, the polishing pad including a through-hole intermittently covered by the surface during the polishing process, the apparatus comprising: a light source configured to provide light; a through-cylinder disposed within the platen, wherein the through-cylinder provides a passage between the first surface of the platen and a second surface of the platen; a fiber optic cable assembly having a first end and a second end, a portion of the fiber optic cable assembly being disposed within the passage of the through- cylinder, the fiber optic cable assembly further having a rotary union with a first end and a second end wherein the rotary union is configured to allow the first end of the rotary union to rotate relative to the second end of the rotary union, the first end of the fiber optic cable extends out of the passage and into the through-hole, wherein the fiber optic cable is configured to propagate light from the light source from the second end of the fiber optical cable to the first end of the fiber optic cable; a light sensor coupled to the second end of the fiber optic cable assembly, wherein the light sensor is configured to receive light reflected from the surface through the fiber optic cable assembly and generate data corresponding to a spectrum of the reflected light; a detector disposed external to the platen, the detector being configured to remotely receive the data from output end of the output line; and a computer coupled to the detector, wherein the computer is configured to generate the endpoint signal as a function of the data from the light sensor.

36. The apparatus of Claim 35 wherein the through-cylinder is configured to prevent cooling fluid from leaking out of the platen.

37. The apparatus of Claim 35 wherein the apparatus is part of rotary chemical mechanical polishing machine.