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WO2000067951A9 - Detection optique de point terminal au cours d'une planarisation chimico-mecanique - Google Patents

Detection optique de point terminal au cours d'une planarisation chimico-mecanique

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Publication number
WO2000067951A9
WO2000067951A9 PCT/US2000/012776 US0012776W WO0067951A9 WO 2000067951 A9 WO2000067951 A9 WO 2000067951A9 US 0012776 W US0012776 W US 0012776W WO 0067951 A9 WO0067951 A9 WO 0067951A9
Authority
WO
WIPO (PCT)
Prior art keywords
light
fluid
workpiece
passage
propagating medium
Prior art date
Application number
PCT/US2000/012776
Other languages
English (en)
Other versions
WO2000067951A1 (fr
Inventor
John A Adams
Thomas Frederick Allen J Bibby
Original Assignee
Speedfam Ipec Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Speedfam Ipec Corp filed Critical Speedfam Ipec Corp
Priority to AU49996/00A priority Critical patent/AU4999600A/en
Publication of WO2000067951A1 publication Critical patent/WO2000067951A1/fr
Publication of WO2000067951A9 publication Critical patent/WO2000067951A9/fr

Links

Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B37/00—Lapping machines or devices; Accessories
    • B24B37/005—Control means for lapping machines or devices
    • B24B37/013—Devices or means for detecting lapping completion
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B37/00—Lapping machines or devices; Accessories
    • B24B37/04—Lapping machines or devices; Accessories designed for working plane surfaces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
    • B24B49/00—Measuring or gauging equipment for controlling the feed movement of the grinding tool or work; Arrangements of indicating or measuring equipment, e.g. for indicating the start of the grinding operation
    • B24B49/12—Measuring or gauging equipment for controlling the feed movement of the grinding tool or work; Arrangements of indicating or measuring equipment, e.g. for indicating the start of the grinding operation involving optical means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L22/00—Testing or measuring during manufacture or treatment; Reliability measurements, i.e. testing of parts without further processing to modify the parts as such; Structural arrangements therefor
    • H01L22/20—Sequence of activities consisting of a plurality of measurements, corrections, marking or sorting steps
    • H01L22/26—Acting in response to an ongoing measurement without interruption of processing, e.g. endpoint detection, in-situ thickness measurement

Definitions

  • the present invention relates to chemical mechanical polishing (CMP), and more particularly, to optical endpoint detection during a CMP process.
  • CMP chemical mechanical polishing
  • CMP Chemical mechanical polishing
  • 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.
  • 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.
  • advanced interconnection conductors such as copper (Cu)
  • the associated barrier metals e.g., tantalum or tantalum nitride
  • 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.
  • an acoustic transducer 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.
  • 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.
  • optical EPD systems fall 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • U.S. Patent No. 5,643,046 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.
  • 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 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 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.
  • the endpoint signal is a function of the intensities of at least two individual wavelength bands selected from the predetermined bandwidth.
  • 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.
  • an apparatus according to the present invention 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.
  • the fluid flushing of the through-hole advantageously improves the performance of the system when using slurries 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.
  • 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.
  • the fluid serves to flush the through-hole as described above.
  • 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 machine.
  • 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-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.
  • EPS endpoint signal
  • 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 another embodiment of an apparatus using a liquid waveguide assembly, in accordance with the present invention.
  • 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 macliines 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.
  • 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.
  • the motion of the pad is "orbital" in a preferred embodiment.
  • each point on the pad undergoes circular motion about its individual axis, which is parallel to the wafer chuck's axis.
  • the present invention can be adapted for use in the CMP tool disclosed in U.S. Patent No. 5,554,064.
  • an in situ optical thin film measurement system 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.
  • artifacts e.g., slurry particles, polishing debris, bubbles in the slurry
  • FIGURE 1 A schematic representation of the overall system of the present invention is shown in FIGURE 1.
  • 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.
  • 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.
  • 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.
  • 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 111 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.
  • 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.
  • pinhole opening 111 is merely an orifice in the pad backer in which fiber optic cable 113 may be placed.
  • fiber optic cable 113 is not sealed to pad backer 120.
  • 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.
  • the flow rate is about l A to about two milliliters per minute.
  • the fluid preferably does not attenuate light in the desired wavelength bands and is pH compatible with the slurry.
  • the fluid may be pH adjusted de-ionized water.
  • the fluid may be the solution used in the slurry (i.e., the solution in which the slurry particles are suspended but without the abrasive particles).
  • the fluid will not adversely the CMP process.
  • fluid source 114 may be incorporated in the slurry delivery system (not shown) of the CMP machine.
  • Fluid source fitting 114 also has an outlet for providing the pressurized fluid into canal 104.
  • the fluid flows out of pinhole opening 111 and through- hole 112.
  • This fluid flow serves to flush slurry and polishing debris from the holes 111 and 112, thereby significantly reducing the scattering of light by polishing debris and slurry particles.
  • the fluid flow may flush away bubbles that can be present in the slurry.
  • some slurries e.g., those containing hydrogen peroxide
  • Bubbles can arise in 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 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.
  • These position sensors may be implemented with rotary optical encoders available from Renco Encoders, Inc., Goleta, California.
  • 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.
  • optical coupler 115 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.
  • 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).
  • 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.
  • 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
  • a different light detection source e.g., a CCD array
  • a greater or lesser sensitivity allows a decrease or increase in the integration time.
  • the use of different light sources 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.
  • computer 121 automatically compares endpoint signal 124 to predetermined criteria and outputs an endpoint trigger 125 as a function of this comparison.
  • 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.
  • Fluid source fitting 114 includes a fitting body 114A, 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.
  • these passages when fitted with conduit 114B and fiber optic cable 113 are essentially watertight.
  • no fluid can leak from fitting body 114A, though some fluid leakage can of course be tolerated.
  • fitting body is implemented using 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 using a stainless steel tube fitted into fitting body 114A.
  • fluid source fitting 114 will form a watertight seal with canal 104 and include a fluid inlet to received fluid from fluid source 116 (FIGURE 1), a fluid outlet communicating with canal 104, and a passage for fiber optic cable 113.
  • the fluid flows through canal 104 and out holes 111 and 112 (FIGURE 1) to reduce the occurrence of bubbles, polishing debris and slurry particles in the optical path between the end of fiber optic cable 113 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 remaining within the noise tolerance of the system. That is, the scattering caused by the slurry tends to increase as the optical path through the slurry increases.
  • 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 wafer surface.
  • 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.
  • reflected spectral data 218 indicative of the spectral distribution of the reflected light is output to computer 121 as illustrated in FIGURE 2A.
  • 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 111 and through-hole 112 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.
  • 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.
  • 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.
  • the location of the orbital motion of through-hole 112 is contained entirely within the area circumscribed by the perimeter of the wafer 103.
  • the outer limit 250 is equal to or less than the radius of wafer 103.
  • the wafer 103 is illuminated continuously, and reflectance data can be sampled continuously.
  • an endpoint signal is generated at least once per second, with a preferred integration time of light sensor 119 (FIGURE 1) being 15 ms.
  • 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 corning 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.
  • 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.
  • 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.
  • FIGURES 5A-5F Several signal processing techniques can be used for reducing the noise in the reflected spectral data 218a-218f, as shown in FIGURES 5A-5F.
  • a technique of single-spectrum wavelength averaging can be used as illustrated in FIGURE 5A.
  • 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.
  • 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 5A as a plot of amplitude vs. wavelength.
  • 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.
  • the spectral data of the scans are combined by averaging the corresponding pixels from each spectrum, resulting in a smoother spectrum 241.
  • 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.
  • 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.
  • 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.
  • several sequential scans e.g., 5 to 10 in a preferred embodiment
  • each pixel is summed for the reference spectral signal to determine a reference amplitude for the entire 512 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.
  • the reflected spectral data in general, also contain the instrument function response.
  • 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.
  • 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.
  • 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.
  • 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.
  • amplitude ratio of wavelength bands 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.
  • 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.
  • 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 (Si02). As described previously, these wavelength bands can be different for different materials and different CMP processes, and typically would be determined empirically.
  • integration times may be increased to cover larger areas of the wafer with each scan.
  • 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.
  • optical endpoint detection algorithm disclosed in U.S. Patent Application Serial No. 09/271,729 entitled “Method and Apparatus For Endpoint Detecting For Chemical Mechanical Polishing” and filed March 18, 1999 can be used.
  • a start command is received from the CMP apparatus.
  • 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.
  • the timeout 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.
  • 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.
  • 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.
  • each wavelength band Rwbj represents a finite wavelength band as previously described in conjunction with FIGURE 2.
  • the desired noise reduction technique or combination of techniques is applied to the reflected spectral data 218 to produce a reduced noise signal.
  • 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.
  • 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 (Rwbi ) 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.
  • EPS endpoint signal
  • the endpoint signal 124 is extracted from the noise-reduced signal produced in box 607.
  • the noise-reduced signal is also already the endpoint signal 124.
  • 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.
  • 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.
  • a preferred endpoint signal 124 over time in exemplary form is shown in FIGURE 5 by reference numeral 124.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • the quality measure (designated by Q) is:
  • Computer 121 may accomplish the calculation of Q.
  • the parameter of quality Q although not useful for temiinating 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 endpoint.
  • the wafer-to-wafer removal rate is the standard deviation of the RR divided by the average RR from the set of wafers polished.
  • 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 fitting 114 is implemented with a fluid delivery tube 702.
  • fluid delivery tube 702 is available from Polymicrotechnologies Inc., Phoenix, Arizona. In general, fluid delivery tube 702 can be similar to those used in chromatography technology.
  • fluid delivery tube 702 has an index of refraction that is less than the fluid from fluid source 116 (FIGURE 1), which allows the tube to function as a waveguide.
  • fluid delivery tube 702 is made of material that can tolerate the pH environment caused by the slurry and fluid.
  • fluid delivery tube 702 may be implemented with a think-walled tube (can be either flexible or rigid) having its inner surface coated with a protective layer.
  • the protective coating would have an index of refraction that is less than the index of refraction of the fluid.
  • System 700 essentially operates as described 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 slurry particles from the optical path near the wafer surface.
  • this embodiment avoids problems that can be caused by the end of a fiber optic cable being too close to the wafer surface.
  • two fluid delivery tubes may be used, with optical coupler 115 (FIGURE 1) being deleted.
  • One tube provides a light path between light source 117 and holes 111 and 112 (FIGURE 1), whereas the other tube provides a light path between light sensor 119 (FIGURE 1) and holes 111 and 112 (FIGURE 1).
  • 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.
  • those skilled in the art can devise without undue experimentation embodiments using different light sources or spectrometers other than those described.
  • other embodiments may be implemented in which the fluid flows across the surface of the optic fiber.
  • other embodiments of the present invention can be adapted for use in sensing any type of workpiece.
  • 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 planarization.
  • MEMS Micro Electromechanical Sensor
  • 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.

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  • General Physics & Mathematics (AREA)
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Abstract

La présente invention concerne un appareil qui est utilisé avec un outil polissant les couches minces sur la surface d'une plaquette de semi-conducteurs afin de détecter le point terminal d'un processus de polissage. Dans un mode de réalisation, l'appareil comprend un tampon de polissage (109) percé d'un trou traversant (112), une source de lumière (117), une source de fluide (116), un ensemble de câbles optiques (113,115,118,122), un capteur de lumière (119) et un ordinateur (121). Le câble à fibres optiques (113) propage la lumière à travers le trou traversant (112) afin d'illuminer la surface de la plaquette pendant le processus de polissage. Le capteur de lumière (119) reçoit la lumière réfléchie et produit des données (218) correspondant au spectre de la lumière réfléchie. L'ordinateur reçoit les données du spectre réfléchi (218) et produit un signal de point terminal (124).
PCT/US2000/012776 1999-05-10 2000-05-10 Detection optique de point terminal au cours d'une planarisation chimico-mecanique WO2000067951A1 (fr)

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