Patent Publication Number: US-6709314-B2

Title: Chemical mechanical polishing endpoinat detection

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to chemical mechanical polishing (CMP). In particular, embodiments of the invention relate to detection of endpoints in CMP processes. 
     Polishing of semiconductor wafers by CMP during fabrication of integrated circuits is an accepted practice in the semiconductor industry. Typically, a wafer to be polished is secured to a head, and then placed into contact with a polishing pad in combination with a slurry. 
     In certain CMP processes, it is desirable to remove one or more layers of material on the wafer, and then to stop the polishing process on an underlying layer of a different material. For example, in a damascene process copper may be formed within a silicon oxide trench featuring a tantalum liner. A CMP step to remove copper and tantalum outside of the trench may end upon encountering oxide on surfaces adjacent to the trench. 
     Conventionally, endpoint of CMP processes is identified as a function of time during process development. During actual processing, the CMP step is timed, and endpoint determined indirectly, in order to produce desired polishing results. 
     However, polishing rates can vary depending upon the actual parameters of the CMP step, such as rotation rate, loading force, and the precise composition and identity of the slurry. Accordingly, conventional timed polishing techniques may result in removal of excessive amounts of material, or may result in too little material being removed. Either result is undesirable from a process repeatability standpoint. 
     Other conventional techniques for determining CMP endpoint include monitoring frictional coefficient between the polishing pad and the wafer, with a change in frictional coefficient indicating a transition in polishing between layers. While effective, this approach to CMP endpoint detection is dependent upon the precise composition and identity of the slurry used in the polishing step. Use of a different slurry, or even use of the same slurry at slightly different mixtures, can have a significant effect upon the frictional coefficient. 
     Therefore, structures and methods that accomplish accurate and reliable detection of the endpoint of chemical-mechanical polishing processes are desirable. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide methods and apparatuses for detecting endpoint in a CMP process. Specifically, acoustical emission information produced by sliding contact between the polishing pad and different material layers on the wafer is monitored using an acoustic information sensor. This acoustic information is resolved into a frequency spectrum utilizing such techniques as fast Fourier transformation. Characteristic changes in the acoustic frequency spectrum reveal any transition in polishing between different material layers. The CMP endpoint indicated by changes in the acoustic frequency spectrum is validated by correlation with other sensed properties, including but not limited to changes in the amplitude of acoustic energy over time, and a change in the measured frictional coefficient between wafer and pad. CMP endpoint can also be validated by comparison with characteristic AE frequency spectra obtained at endpoints of prior CMP operational runs. 
     An embodiment of a method for detecting transition between polishing of material layers during a chemical mechanical polishing process comprises sensing acoustical energy generated by contact between a chemical mechanical polishing pad and a semiconductor wafer. The sensed acoustical energy is converted into an electrical signal, and a low frequency component of the electrical signal is filtered. The filtered electrical signal is resolved into a frequency spectrum. A difference between the frequency spectrum and a previously obtained acoustic emission frequency spectrum is identified. The difference is correlated with a transition in polishing between layers of material on the semiconductor wafer, and the transition is validated with reference to a change in a separate indicia from the CMP process. 
     An embodiment of a method for detecting endpoint of a CMP process comprises sensing a first acoustical energy generated by contact between a chemical mechanical polishing pad and a first semiconductor wafer at a transition between a first material and a second material during a first CMP operational run. The first acoustical energy is resolved into a characteristic transition frequency spectrum. The characteristic transition frequency spectrum is stored in a memory. A second acoustical energy generated by contact between the chemical mechanical polishing pad and a second semiconductor wafer during a second CMP operational run is sensed. The second acoustical energy is resolved into a sensed transition frequency spectrum. The characteristic transition frequency spectrum is compared with the sensed transition frequency spectrum to identify a CMP endpoint during the second operational run. The CMP endpoint is validated with reference to a change in a separate indicia from the second CMP operational run. 
     An embodiment of an apparatus for detecting an endpoint of a chemical mechanical polishing process in accordance with the present invention comprises an acoustic emission sensor positioned proximate to a chemical mechanical polishing pad. The sensor includes a transducer configured to convert acoustical energy generated by contact between the pad and a semiconductor wafer into an electrical signal. A second sensor is configured to detect non-acoustic information from the process. A memory is configured to store a previously obtained acoustic emission frequency spectrum. A low frequency filter is in electrical communication with the transducer. A computer is in electrical communication with the filter, the second sensor, and the memory, the computer configured to resolve the electrical signal into a frequency spectrum and to identify differences between the frequency spectrum and the previously obtained acoustic emission frequency spectrum in order to determine a transition between polishing of different materials, the transition corresponding to an endpoint. 
     These and other embodiments of the present invention, as well as its features and some potential advantages are described in more detail in conjunction with the text below and attached figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart showing the steps of an embodiment of a method in accordance with the present invention. 
     FIG. 2A is an exploded perspective view of one embodiment of a chemical mechanical polishing apparatus in accordance with the present invention. 
     FIG. 2B is a cross-sectional view of the chemical mechanical polishing apparatus of FIG.  2 A. 
     FIG. 3 plots acoustic emission root-mean-square (RMS) versus time for polishing of successive copper, tantalum, and oxide layers of a wafer during CMP. 
     FIG. 4A plots power spectral density versus frequency for polishing of the copper layer during the CMP process of FIG.  3 . 
     FIG. 4B plots power spectral density versus frequency for polishing of an oxide layer during the same CMP process of FIG.  3 . 
    
    
     DESCRIPTION OF THE SPECIFIC EMBODIMENTS 
     Embodiments of the present invention include methods and apparatuses that allow detection of endpoint in CMP processes. Specifically, acoustical emission information produced by sliding contact between the polishing pad and different material layers on the wafer is monitored using an acoustic information sensor. The sensed acoustic information is resolved into a frequency spectrum utilizing such techniques as fast Fourier transformation. Characteristic changes in the acoustic frequency spectrum reveal transition of the pad polishing as portions of different underlying material layers are exposed. CMP endpoint indicated by changes in the acoustic frequency spectrum can be validated by correlation with other sensed properties, including but not limited to changes over time in acoustic energy, and changes over time in measured frictional coefficient. CMP endpoint indicated by a change in the acoustic frequency spectrum can also be validated by correlation with characteristic frequency spectra obtained at transitions of prior CMP operational runs. 
     FIG. 1 is a flowchart showing steps of a method for detecting transition between polishing of material layers during a chemical mechanical polishing process. As shown in FIG. 1, method  8  begins by sensing acoustical energy generated by contact between a chemical mechanical polishing pad and a semiconductor wafer (step  1 ). The sensed acoustical energy is then converted into an electrical signal (step  2 ). Low frequency components of the electrical signal are then filtered (step  3 ). 
     Next, the filtered electrical signal is resolved into a frequency spectrum (step  4 ). In the next step, a difference between the frequency spectrum and a previously obtained acoustic emission frequency spectrum is identified (step  5 ). The difference between the spectra is then correlated with an endpoint in polishing between layers of material on the semiconductor wafer (step  6 ). Finally, the endpoint just indicated may be validated based upon additional information received from the CMP apparatus (step  7 ). 
     FIGS. 2A and 2B show exploded and cross-sectional views, respectively, of one embodiment of a chemical mechanical polishing apparatus in accordance with the present invention. One or more substrates  10  may be polished by a CMP apparatus  20 . A description of a similar polishing apparatus  20  may be found in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference. Polishing apparatus  20  includes a series of polishing stations  22  and a transfer station  23 . Transfer station  23  serves multiple functions, including receiving individual substrates  10  from a loading apparatus (not shown), washing the substrates, loading the substrates into carrier heads, receiving the substrates from the carrier heads, washing the substrates again, and finally, transferring the substrates back to the loading apparatus. 
     Each polishing station includes a rotatable platen  24  on which is placed a polishing pad  30 . The first and second stations may include a two-layer polishing pad with a hard durable outer surface, whereas the final polishing station may include a relatively soft pad. If substrate  10  is an “eight-inch” (200 millimeter) or “twelve-inch” (300 millimeter) diameter disk, then the platens and polishing pads will be about twenty inches or thirty inches in diameter, respectively. Each platen  24  may be connected to a platen drive motor (not shown). For most polishing processes, the platen drive motor rotates platen  24  at about thirty to two hundred revolutions per minute, although lower or higher rotational speeds may be used. Each polishing station may also include a pad conditioner apparatus  28  to maintain the condition of the polishing pad so that it will effectively polish substrates. 
     Polishing pad  30  typically has a backing layer  32  which abuts the surface of platen  24  and a covering layer  34  which is used to polish substrate  10 . Covering layer  34  is typically harder than backing layer  32 . However, some pads have only a covering layer and no backing layer. Covering layer  34  may be composed of an open cell foamed polyurethane or a sheet of polyurethane with a grooved surface. Backing layer  32  may be composed of compressed felt fibers leached with urethane. A two-layer polishing pad, with the covering layer composed of IC-1000 and the backing layer composed of SUBA-4, is available from Rodel, Inc., of Newark, Del. (IC-1000 and SUBA-4 are product names of Rodel, Inc.). 
     A rotatable multi-head carousel  60  is supported by a center post  62  and is rotated thereon about a carousel axis  64  by a carousel motor assembly (not shown). Center post  62  supports a carousel support plate  66  and a cover  68 . Carousel  60  includes four carrier head systems  70 . Center post  62  allows the carousel motor to rotate carousel support plate  66  and to orbit the carrier head systems and the substrates attached thereto about carousel axis  64 . Three of the carrier head systems receive and hold substrates, and polish them by pressing them against the polishing pads. Meanwhile, one of the carrier head systems receives a substrate from and delivers a substrate to transfer station  23 . 
     Each carrier head system includes a carrier or carrier head  80 . A carrier drive shaft  74  connects a carrier head rotation motor  76  (shown by the removal of one quarter of cover  68 ) to each carrier head  80  so that each carrier head can independently rotate about it own axis. There is one carrier drive shaft and motor for each head. In addition, each carrier head  80  independently laterally oscillates in a radial slot  72  formed in carousel support plate  66 . A slider (not shown) supports each drive shaft in its associated radial slot. A radial drive motor (not shown) may move the slider to laterally oscillate the carrier head. 
     The carrier head  80  performs several mechanical functions. Generally, the carrier head holds the substrate against the polishing pad, evenly distributes a downward pressure across the back surface of the substrate, transfers torque from the drive shaft to the substrate, and ensures that the substrate does not slip out from beneath the carrier head during polishing operations. 
     Carrier head  80  may include a flexible membrane  82  that provides a mounting surface for substrate  10 , and a retaining ring  84  to retain the substrate beneath the mounting surface. 
     Pressurization of a chamber  86  defined by flexible membrane  82  forces the substrate against the polishing pad. Retaining ring  84  may be formed of a highly reflective material, or it may be coated with a reflective layer to provide it with a reflective lower surface  88 . A description of a similar carrier head  80  may be found in U.S. patent application Ser. No. 08/745,679, entitled a CARRIER HEAD WITH a FLEXIBLE MEMBRANE FOR a CHEMICAL MECHANICAL POLISHING SYSTEM, filed Nov. 8, 1996, by Steven M. Zuniga et al., assigned to the assignee of the present invention, the entire disclosure of which is incorporated herein by reference. 
     A slurry  38  containing a reactive agent (e.g., deionized water for oxide polishing) and a chemically-reactive catalyzer (e.g., potassium hydroxide for oxide polishing) may be supplied to the surface of polishing pad  30  by a slurry supply port or combined slurry/rinse arm  39 . If polishing pad  30  is a standard pad, slurry  38  may also include abrasive particles (e.g., silicon dioxide for oxide polishing). 
     In operation, the platen is rotated about its central axis  25 , and the carrier head is rotated about its central axis  81  and translated laterally across the surface of the polishing pad. In order to detect transitions between polishing of different material layers, embodiments of methods and apparatuses in accordance with the present invention take advantage of the fact that sliding motion between different materials generates unique sets of acoustic emission signals. 
     Accordingly, the chemical mechanical polishing apparatus of FIGS. 2A and 2B further includes acoustic emission (AE) sensor  100  (see FIG. 2B) positioned in contact with membrane  82 . AE sensor  100  includes a transducer configured to detect vibrational mechanical energy emitted as polishing pad  30  comes into physical contact and rubs against wafer  10 . Acoustic emission signals received by sensor  100  are converted to an electrical signal and then communicated in electronic form to computer  48  via filter  120 . 
     Filter  120  is configured to remove low frequency components of the electronic signal. Specifically, acoustic energy detected by sensor  100  may include such extraneous information as the mechanical vibration of the polishing apparatus itself, or environmental acoustic energy attributable to the operation of nearby fans or other mechanical equipment. However, the frequency of such extraneous information is generally low, such that filtering acoustic information below a threshold value, for example below about 20 kHz, will eliminate substantial noise from the signal. This noise reduction will enhance the ability of the system to recognize changes in AE characteristic of polishing transitions. 
     Computer  48 , which includes associated display  49 , may resolve the acoustic emission information into a variety of different forms. One form of the acoustic emission information is an expression of the change in amplitude of receive acoustic information over time. This is shown in FIG. 3, which plots the root-mean-square (RMS) of acoustic emission amplitude versus time for polishing of successive copper, tantalum, and oxide layers of a wafer, as may be useful in a damascene process. While FIG. 3 does show some difference in RMS as the polishing pad progresses through the various material layers, the RMS difference is relatively minor and can readily be affected by other CMP operational parameters, including but not limited to pad rotation speed, pad wear, and loading force. 
     Accordingly, computer  48  is further capable of resolving AE information received from sensor  100  into a frequency spectrum. Such frequency-based resolution may be obtained through a fast Fourier transformation (FFT) of the electronic signals. This is shown in FIGS. 4A and 4B, which plots power spectral density (in dB/Hz) versus frequency (in Hz) for polishing of the copper and oxide layers respectively, during the CMP process of FIG.  3 . 
     FIGS. 4A and 4B show that polishing different material layers (copper vs. oxide) results in the output of distinctly different AE frequency spectra. For example, the frequency spectrum for polishing copper shown in FIG. 4A exhibits a sharp and small peak centered around 3.76×10 4  Hz. By contrast, the frequency spectrum for polishing oxide shown in FIG. 4B exhibits a broad peak centered around 3.79×10 4  Hz, a difference that is distinct from the location of the peak of the copper polishing. 
     The difference in frequency spectrum observed between Cu and oxide may be attributable to the fact that Cu is a softer material than oxide, which in turn gives rise to different mechanical vibrations and hence acoustic emissions during polishing. This difference in frequency spectra can be exploited to reveal a transition or endpoint of CMP. 
     Specifically, returning to FIG. 2B, computer  48  is in communication with memory  102 . Memory  102  is configured to store frequency spectra corresponding to prior polishing. By comparing the instant AE frequency spectrum with AE frequency spectra information stored memory  102  earlier in the operational run of the tool, it is possible to identify differences revealing transition in polishing between one material layer and the next. 
     As shown in FIGS. 4A and 4B, the change in AE frequency between different material layers may be relatively subtle. Accordingly, a polishing apparatus in accordance with embodiments of the present invention includes non-acoustic sensors for collecting other CMP process information for validating an endpoint identified through a change in AE frequency spectra. Examples of these physical changes that can be monitored include frictional coefficient as determined by a torque sensor or the current draw from a rotational motor, and also changes in resistance and capacitance of the wafer. 
     Accordingly, embodiments of apparatuses and methods of the present invention validate an endpoint indicated by changes in AE frequency spectra with data relating to changes in frictional coefficient, capacitance, and/or resistance. This is shown in FIG. 2B, wherein torque sensor  104 , capacitance sensor  106 , and resistance sensor  108 , are each in communication with computer  48  to communicate coefficient of friction information, capacitance information, and resistance information, respectively. This information may be transmitted to memory  102  for storage and future reference by computer  48 . 
     Embodiments of methods and apparatuses in accordance with the present invention offer a number of advantages over conventional endpoint detection approaches. 
     For example, an AE sensor may pick up acoustic emissions attributable to mechanical vibration of the tool rather than acoustic emissions resulting from contact between the pad and the wafer. However, one advantage of endpoint detection in accordance with embodiments of the present invention is that AE information attributable to tool vibration should be present both before and after a transition has taken place, thereby eliminating this information from consideration. The random nature of vibration of the tool may also result in this AE information being reduced to the level of noise in the frequency spectrum resulting from the FFT operation, thereby allowing each different polished layer to exhibit a readily identifiable frequency spectrum “fingerprint”. 
     Moreover, embodiments in accordance with the present invention reduce the effect of noise in the endpoint analysis through filtering. Low frequency components of the electrical signal from the AE transducer are removed by filtering prior to performance of the frequency analysis. This filtering serves to eliminate low frequency noise that may mask the higher frequency changes attributable to polishing transitions or endpoint. 
     Only certain embodiments of the present invention have been shown and described in the instant disclosure. One should understand that the present invention is capable of use in various other combinations and environments and is capable of changes and modification within the scope of the inventive concept expressed herein. 
     Thus while the above has described apparatuses and methods in accordance with the present invention for detecting CMP endpoint through identification of changes in an acoustic emission frequency spectrum exhibited during a single operational run, a CMP endpoint determination in accordance with embodiments of the present invention can be validated with reference to other indicia. 
     For example, in certain embodiments in accordance with the present invention an AE emission frequency spectrum “fingerprint” can be matched with similar “fingerprints” detected during prior CMP operational runs. Where a change in AE emission spectrum indicates a probable endpoint, this conclusion can be validated by comparison of the spectrum with others obtained during prior operational runs that are known to indicate polishing transitions. Pattern recognition software could be employed to assist in this comparison process. 
     Moreover, while the above discussion has focused upon monitoring changes in acoustic emission frequency spectra to reveal polishing endpoint, the invention is not necessarily limited to detecting endpoint per se. The progression of chemical mechanical polishing through successive material layers could also be monitored for purposes of quality control utilizing apparatuses and methods in accordance with embodiments of the present invention. 
     Given the above detailed description of the present invention and the variety of embodiments described therein, these equivalents and alternatives along with the understood obvious changes and modifications are intended to be included within the scope of the present invention.