Abstract:
The apparatus for polishing control with signal peak analysis consists of two major units: a polishing machine and a polishing process control and monitoring system which has sensors for sensing changes on the surface and/or inside the object during polishing. In response to the above changes the sensors generate operating data signals. The control and monitoring system, which also contains a signal conditioning unit and a control unit, amplifies the operating data signals and sends them to a signal analyzer, which determines average values and peaks of the conditioned signals. The analyzer also determines a ratio of the peak signal values to an average signal values and compares the obtained ratio with a preliminarily determined reference value optimized with regard to the specific CMP process carried out on the polishing machine. When the measured signal ratio reflects abnormal conditions, polishing conditions are adjusted back to normal by means of a control signal generated on the basis of the aforementioned ratio. The apparatus of the invention also provides quantitative evaluation of changes in the CMP process and automatic control of the CMP process in a manner that optimizes the process and prevents occurrence of defects that might be caused by the controlled process.

Description:
REFERENCE TO RELATED CASES  
       [0001]    The present patent application is a continuation-in-part application of U.S. patent application Ser. No. 091859,062 filed on May 17, 2001, now pending. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a method and apparatus for control of a polishing process based on a signal peak analysis, in particular to a method and apparatus for monitoring a polishing process, such as chemical mechanical polishing, in the manufacture of semiconductor wafers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Polishing processes play significant role in modern technologies, in particular in semiconductor fabrication. For example, at certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. In a polishing process, known as mechanical polishing, a polishing pad with abrasive particles repetitively passes over the surface of the substrate. Polishing may also be performed with chemically active abrasive slurry. A polishing system that uses the chemical slurry is commonly known as a chemical mechanical polishing (CMP) system. In contrast with mechanical polishing, the slurry in a CMP system provides an increased removal rate of a substrate material. Additionally, by selecting particular chemicals, chemical slurry can be used to selectively polish certain films on a semiconductor substrate.  
           [0004]    Wafer polishing typically requires that a substrate be mounted in a wafer head or carrier, with the surface of the substrate to be polished exposed. The substrate supported by the head is then placed against a moving polishing pad mounted on a platen. The head holding the substrate may also move, e.g., rotate, to provide additional motion between the substrate and the polishing pad surface. In a CMP process, a polishing slurry (typically including abrasive particles and at least one chemically reactive agent, which is selected to enhance the polishing of the topmost film layer of the substrate) is supplied to the pad to provide an abrasive chemical solution at the interface between the pad and the substrate.  
           [0005]    Both the CMP and traditional mechanical abrasion polishing are difficult to control. The CMP process is frequently carried out without comprehensive information about current polishing conditions, and its control is performed just by using empirical polishing rates and timing. Since such polishing control methods are inaccurate, the polishing process results in serious yield drops and waste of expensive wafers. Therefore, accurate control of polishing based on reliable real-time information is an important issue for automation of such processes.  
           [0006]    One of known methods used to control polishing is based on measurements of sound waves or audible acoustic noise generated in the interface between the wafer and polishing material (e.g., three U.S. Pat. No. 5,222,329 issued on Jun. 29, 1993, U.S. Pat. No 5,245,794 issued on Sep. 21, 1993, and U.S. Pat. No. 5,439,551 issued on Aug. 8, 1995).  
           [0007]    This control method is based on the fact that changes in the polishing process are accompanied by noticeable changes in the amplitude and spectrum of an acoustic signal. The acoustic signals are detected with the use of an acoustic signal receiver, such as a microphone, located in the vicinity, but outside of the polishing zone of a wafer-pad contact. The signal is then recorded in a manner known in the art and used for a process analysis. This method, however, is limited to measuring low frequency signals, which reduces its sensitivity, and to monitoring their average characteristics, which does not allow for detection of local polishing defects such as delamination and scratches.  
           [0008]    U.S. Pat. No. 5,876,265 issued on Mar. 2, 1999 to T. Kojima describes a polishing apparatus with end point detection based on analysis of acoustic signals involving peak detection. The polisher includes a vibration detecting device attached to a wafer carrier, an amplifier, a gain indicator with gain adjuster to adjust a gain of such amplifier, an end-point analyzer based on the change in said adjusted signal, and a controller to stop the polishing motion upon the end-point signal. The suitability of the amplifier gain and need for its adjustment is determined based on whether it&#39;s either amplitude or maximum value resides in a preset reference range. The gain indicator may include a peak detector for detecting peaks of signal waveform and making gain adjustments based on a peak signal output when a magnitude is not suitable. These method and apparatus, however, measure only low-frequency signals, which makes them less sensitive to changes in the polishing process. Also, they do not allow making a distinction between regular acoustic signals and those generated due to abnormal polishing effects, such as scratches and delamination.  
           [0009]    Another example of polishing control based on detection of acoustic signals is disclosed in U.S. patent application Ser. No. 09/859,062 filed by the same applicants on May 17, 2001. The present patent application is a continuation-in-part of the aforementioned patent application.  
           [0010]    The above patent application describes an apparatus for controlling a CMP process, comprising a rotating or orbiting platen with a pad, a rotating head that supports an object to be treated, e.g., a semiconductor wafer, and performs radial movements with respect to the platen, and a polishing process control system comprising a plurality of groups of various sensing devices for detecting changes that occur during polishing. One group of the sensing devices is a group of high-frequency acoustic emission sensors built on various levels into components of the rotating head. Another group of sensing devices is represented by force/torque sensors connected with various elements of the rotating head and the platen, respectively, and intended for direct measurement of compression force and friction response (force or torque) between the head and the platen and a coefficient of friction between the wafer and the polishing pad. All groups of sensors work simultaneously and their measurement data is processed and analyzed by a control unit for obtaining accurate and reliable results. The results are then used for adjusting the process.  
           [0011]    One disadvantage of all known polishing control methods and apparatus is that they do not provide quantitative evaluation of changes in a polishing process in a manner required for optimization of the process and for preventing occurrence of defects that might be caused by the polishing process itself. Another disadvantage consists in that known polish control methods just establish the fact of occurrence of the changes and do not distinctly distinguish between regular peaks of signals and extraordinary peaks. Such extraordinary peaks may be caused by undesired events such as delamination of sublayers within a semiconductor wafer resulting, e.g., from an excessive normal force, with which the polishing pad is pressed to the surface of the wafer being treated.  
         OBJECTS OF THE INVENTION  
         [0012]    It is an object of the present invention to provide a method and apparatus for polishing control with quantitative evaluation of changes in the polishing process. Another object is to provide a method and apparatus for automatic analysis of acoustic or other signals and for controlling the process in a manner that allows to optimize polishing conditions and thus to prevent occurrence of defects during polishing. Another object is to provide a method and apparatus capable of separating extraordinary signal peaks from regular signal peaks. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a general schematic view of the apparatus of the invention for polishing control with signal peak analysis,  
         [0014]    [0014]FIG. 2 is an example of registration of electroacoustic signals obtained during polishing of a laminated structure consisting of layers with different physical properties.  
         [0015]    [0015]FIGS. 3 and 4 are graphs illustrating electroacoustic signals registered under different polishing conditions.  
         [0016]    [0016]FIG. 5 is a block diagram of a control and monitoring system of the apparatus of the invention.  
         [0017]    [0017]FIG. 6 is an example of a block diagram of a comparator suitable for use in the control and monitoring system of the invention.  
         [0018]    [0018]FIG. 7 is a flowchart of signals used for control of the polishing in the apparatus of the invention. 
     
    
     SUMMARY OF THE INVENTION  
       [0019]    The apparatus of the invention for polishing control with signal peak analysis consists of two major units: a polishing machine and a polishing process control and monitoring system. The polishing machine may be a CMP machine for polishing semiconductor wafers, which has a moving polishing pad and a rotating object-holding means such as a polishing head that supports an object to be treated. The control and monitoring system contains sensors for sensing changes on the surface and/or inside the object during polishing and for generating operating data signals in response to these changes. The control and monitoring system also contains a signal conditioning unit for processing the operating data signals and a control unit for generating a control signal for controlling a drive of the polishing machine. The signal conditioning unit amplifies the operating data signals and sends them to a signal analyzer, which determines both average values and peaks of the conditioned signals, as well as a ratio of the peak signal values to the average signal values. The signal analyzer also compares the obtained ratio and signal values with preliminarily determined reference values optimized with regard to the specific polishing process carried out on the polishing machine. When the measured signal ratio reflects abnormal conditions, polishing conditions are adjusted to normal by means of a control signal generated in the control unit. The apparatus of the invention also provides quantitative evaluation of changes in the CMP process and automatic control of the CMP process in a manner that optimizes the process and prevents occurrence of defects that might be caused by the controlled process.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    A general schematic view of the apparatus of the invention for polishing control with signal peak analysis is shown in FIG. 1. The apparatus consists of a polishing machine  20 , e.g., a CMP machine intended for polishing a semiconductor wafer W, and a control and monitoring system, which in FIG. 1 is shown in the form of a single box  22 . The control and monitoring system contains a control unit  23 , which may be, e.g., a stand-along controller, a computer, or the like. The polishing machine  20  and the control and monitoring system  22  are interconnected via a data line L 1  and a feedback line L 2 .  
         [0021]    The polishing machine  20  may have a configuration different from the one shown in FIG. 1, and the CMP machine shown in FIG. 1 may have different arrangement. In any case, however, the CMP machine has an object-holding means, e.g., a polishing head  24  which supports an object to be polished, such as a semiconductor wafer W, and may be driven into rotation, e.g., by an electric motor  26  with a power source  28 .  
         [0022]    The polishing machine has a polishing tool such as a polishing pad  30 , which in the illustrated embodiment is located under the polishing head  24  and has a diameter greater than that of the object being treated. The polishing pad  30  is driven into rotation, e.g., by an electric motor  32  connected to a power source  34 . In different arrangements, the polishing pad  30  may perform an orbital or linear motion. Also, the polishing head  24  may perform a linear reciprocating motion; in the embodiment shown in FIG. 1 this motion is performed with the use of a carriage  36  that slides in guides  38 .  
         [0023]    A force with which the semiconductor wafer W is pressed against the polishing pad  30  is provided by means of a vertical loading mechanism shown in the form of a lead screw  40  driven into rotation by a motor  42  with a power supply  44 . The lead screw  40  engages a nut  45  secured to the carriage  36 . In different arrangements, the loading mechanism can include a hydraulic or air cylinder either instead of or in addition to the lead screw.  
         [0024]    To this point, the above description related to a conventional CMP machine. A part of the control system is also known and consists of sensors and measuring devices described in aforementioned U.S. patent application Ser. No. 09/859,062. More specifically, reference numerals  46  and  48  designate electroacoustic sensors, e.g., piezoelectric transducers, embedded into different components of the polishing head  24  or polishing pad  30 . In the context of the present invention the term “electroacoustic sensor” means any sensor that converts either contact acoustic emission signals, or mechanical waves (vibrations) of both high and low frequency, or contact electrical impedance or capacitance or resistance into electric signals. In the apparatus of the invention these sensors detect contact acoustic emission generated by interactions between the contacting surfaces of the polishing pad  30  and the semiconductor wafer W and convert it into electroacoustic data signals AE.  
         [0025]    [0025]FIG. 2 is an example of registration of electroacoustic signals obtained during polishing of a laminated structure consisting of layers I, II and III of materials with different physical properties. For convenience of presentation the layers I, II and III are shown under the graph aligned with its respective portions. The graph shown in FIG. 2 illustrates variation of the electroacoustic signal AE (the ordinate axis) with polishing time (the abscissa axis). The abscissa axis also reflects the thickness of the object being polished, since the layers I, II and III are polished out sequentially in time.  
         [0026]    It has been found experimentally that polishing of a homogeneous material generates acoustic signals of certain regular amplitude with an extended acoustic spectrum. An RMS (root mean square) value of such an electroacoustic signal is represented by a layer  1 , which has very small peaks of amplitude. A drop of the signal level on the portion C of the electroacoustic signal curve of FIG. 2 corresponds to a transition from layer I to layer II. It can be assumed that the layer II consists of an acoustically softer material (for example, with the Hook constant lower than that in the layer I). The high positive peaks P 1 , P 2  and P 3  on the section of the graph corresponding to the layer II may be caused, e.g., by hard inclusions In 1 , In 2 , and In 3  in the material of the layer II, while the low peak P 4  may be caused, e.g., by a void V 4  or by local delamination of the material in the layer II. The upwardly concave curve on the portion of the graph corresponding to the layer III may lead to an assumption that this last layer has non-uniformity in either hardness or material removal.  
         [0027]    The above example shows that defects of different physical nature can be reflected on the changes in the spectrum of signals. This is true not only for electroacoustic signals but for other electric signals, e.g. resulting from measuring electrical resistance or capacitance in contact between the polishing pad and the wafer during polishing.  
         [0028]    It is understood that defects in the semiconductor wafer can be of two types: inherent defects pre-existing in the material or on the surfaces of the layers (inclusions, voids, etc.) and process defects that occur during polishing and caused by the polishing (scratches, cracks, delaminations, etc.). The term “changes in the object” covers changes both on the surface of the object and in its inner material.  
         [0029]    The applicants have found that the average value of an electroacoustic signal represents the intensity of surface interaction between the polishing pad  30  and the object being treated, e.g., the wafer W. In particular, the average electroacoustic signal generated by an electroacoustic sensor during polishing represents intensity of polishing and the rate of material removal. On the other hand, irregular peaks in an electroacoustic signal and the amplitude of these peaks, such as peaks P 1 , P 2 , P 3 , and P 4  shown in FIG. 2, represent abnormalities in the polishing process. Examples of such abnormalities are aforementioned delaminations, voids, micro-scratches, chipping, flaking, inclusions, micro-cracks, etc.  
         [0030]    The applicants have found that an absolute value of the aforementioned peaks in the electroacoustic signal alone is not a sufficient criterion for characterization of a polishing process. Indeed, in some cases even at a relatively high average level A 0 , the electroacoustic signal does not have peaks. This case is shown in FIG. 3, where the ordinate axis corresponds to the amplitude of the electroacoustic signal. Such a condition would testify to the fact that polishing is carried out with high intensity, but without abnormalities. On the other hand, cases are possible when, as shown in FIG. 4, the average level of electroacoustic signals A 1  is lower than the average level of the acoustic signal AO of the case shown in FIG. 3, but the peaks are present and have the maximum amplitude A 2 , which can be lower than A 0 . Thus, the case of FIG. 3 may be considered acceptable, while the case of FIG. 4 may be considered unacceptable, even though the maximum level of the electroacoustic signal is lower than the absolute value of the average signal of the acceptable case.  
         [0031]    Based on these considerations and on the results of multiple experiments, the applicants came to a conclusion that it would be more appropriate to monitor and control a polishing process by using a ratio of the maximum peak value of an acoustic signal to an average value of this signal as a criterion of the polishing process quality.  
         [0032]    For realization of the aforementioned method the applicants have developed the control and monitoring system  22  shown schematically in FIG. 1 in the form of a single block. The aforementioned threshold of optimized values is used as an input data loaded into this system. The system  22  of the apparatus of the invention is shown in more detail in FIG. 5. It contains aforementioned electroacoustic sensors  46  and  48 , generating electroacoustic signals S 1  and S 2 , respectively, connected to inputs of respective wide-band amplifiers  50  and  52 , both intended for preliminary amplification of the electroacoustic signals. Outputs of wide-band amplifiers  50  and  52  may be connected to inputs of respective signal filters  54  and  56 , such as high-pass or band-pass filters. The filters may have amplifiers  58  and  60  for amplification of the filtered signals. The filtered signals are added in an adding unit  62 , based, e.g., on an operational amplifier, and the resulting signal is then sent to an RMS converter  64 . Such a converter is a standard unit commercially produced, e.g., by Analog Devices, Inc. (Norwood, Mass.). The RMS converter converts the resulting filtered high-frequency signal S 3  into a low-frequency RMS signal S 4 , representing an average intensity of the electroacoustic signal.  
         [0033]    For simplification of the drawings, two of such sensors  46  and  48  are shown in FIGS. 1 and 5. It is understood, however, that either one or more than two electroacoustic sensors may be used. If, for example, only one electroacoustic sensor  46  is used in the system with the respective wide-band amplifier  50 , filter  54 , and amplifier  58 , then the adding unit  62  is not needed.  
         [0034]    Until now, the description was related to a known part of the control and monitoring system  22 , similar to the one described in the aforementioned U.S. Pat. No. 5,245,794, which in the following description will be referred to as a signal conditioning unit  65  (FIG. 5).  
         [0035]    The distinguishing part of the control and monitoring system  22  of the invention is shown on the right side of FIG. 5 in the form of a signal analyzer  66  (dotted line). The signal analyzer  66  measures all aforementioned parameters of the electroacoustic signal, such as the average value, the maximum peak value, and the ratio of the maximum peak value to the average value. More specifically, the signal analyzer  66  contains a peak detector  68  with a timing reset unit  70 . The peak detector  68  is connected to the output of the RMS converter  64  and generates a peak signal V 1  (FIG. 5). The signal analyzer  66  further contains an integrating unit  72 , which is also connected to the output of the RMS converter  64  and generates an average-value signal V 2 ; and a comparator  74  for generating a measured ratio signal V proportional to a ratio of the maximum peak value V 1  to the average value V 2  of the electroacoustic signal. The respective inputs of the comparator  74  are connected to the outputs of the peak detector  68  and of the integrating unit  72 .  
         [0036]    An example of a comparator  74  is shown schematically in FIG. 6, where reference numerals  76  and  78  designate logarithmic amplifiers,  80  designates a subtracting unit which subtracts one input signal from another, and  82  designates an exponential amplifier for generating a signal of a measured ratio of the aforementioned signals V 1  to V 2 .  
         [0037]    The apparatus of the invention operates as follows.  
         [0038]    A semiconductor wafer W with multiple layers, the outermost of which has to be polished, is attached to the polishing head  24  so that it faces a polishing pad  30 . The control and monitoring system  22  is activated, the polishing head  24  with the wafer W and the polishing pad  30  are brought into rotation from respective electric motors  26  and  32  connected to power sources  28  and  34 . The polishing head  24  is then fed towards the polishing pad  30  by means of the loading mechanism formed by the lead screw  40  with the nut  45  attached to the carriage  36 . The lead screw  40  is driven from the motor  42 , which is fed from the power supply  44 . As a result, the semiconductor wafer W comes into contact with the polishing pad  30  with a predetermined contact pressure. Simultaneously with rotation, the polishing head  24  may perform linear oscillating motions in the radial direction of the pad  30  by means of the carriage  36 , moveable in the guides  38 .  
         [0039]    The surface of the polishing pad  30  is being covered with polishing slurry SL, supplied to the polishing surface of the pad from a slurry supply system  47 . It is known to a person skilled in the art that the properties of the slurry and the flow rate of the slurry supply may affect the results of polishing and, therefore, shall be controlled during polishing.  
         [0040]    The polishing process is accompanied by high-frequency contact acoustic emission, generated in the interface of the contacting surfaces of the semiconductor wafer W and the polishing pad  30 . The emission reflects the changes in the object during polishing and is detected as electroacoustic signals by electroacoustic sensors  46  and  48  built into the polishing head  24  and/or polishing pad  30 . The sensors  46  and  48  generate on their outputs operating electroacoustic data signals S 1  and S 2 , respectively. These signals may be of the type shown in FIGS.  2 - 4  and correspond to intensity of the polishing process. FIG. 7 is a flowchart of signals used for control of the polishing process in the apparatus of the invention. The operating electroacoustic data signals S 1  and S 2  (FIGS. 5 and 7) are sent to respective wideband amplifiers  50  and  52  and then to respective filters  54  and  56 , which may be of high-pass or band-pass types. These filters cut off low-frequency components, which correspond to mechanical, electrical, and environmental noise and vibrations. In other words, only high-frequency acoustic components of a desirable bandwidth pass through the filters. After amplification in amplifiers  58  and  60 , the signals are summarized in the adding unit  62 , and the resulting signal  83  is sent to the RMS converter  64 . In the case of a single acoustic sensor, the amplified signal is sent directly from the amplifier to the RMS converter. The RMS converter  64  generates a conditioned signal S 4  (FIG. 5 and FIG. 7), which is sent as an input signal to the signal analyzer  66 . The use of the RMS converter is optional, and after amplification the resulting signal S 3  can be sent directly to the signal analyzer  66 .  
         [0041]    As mentioned above, the electroacoustic signals may have high-amplitude random peaks of the type shown in FIGS. 2 and 4, reflecting abnormalities of the polishing process. Examples of such abnormalities are defects in the material being treated, such as aforementioned delaminations, voids, micro-scratches, chipping, flaking, inclusions, micro-cracks, etc.  
         [0042]    The function of the signal analyzer  66  is to detect such abnormalities and to generate a ratio signal V for controlling the operation of drive motors  26 ,  32 , and  42  of the polishing machine  20  and the slurry supply system  47 . More specifically, the motor  42  controls contact pressure between the wafer W and the polishing pad  30 , while the motors  26  and  32  control rotation of the polishing head  24  and of the polishing pad  30 , respectively.  
         [0043]    In the signal analyzer  66 , signal S 4  is supplied to both the peak detector  68  and the integrating unit  72 . The peak detector  68  generates a signal V 1 , which corresponds to the maximum amplitude of the peak of the conditioned electroacoustic signal S 4 , while the integrating unit  72  generates a signal V 2 , which corresponds to the average level of the same signal S 4 . The peak detector  68  and the integrating unit  72  may be connected to the timing reset unit  70 , which synchronizes operation of the aforementioned devices  68  and  72 .  
         [0044]    Signals V 1  and V 2  are sent to the inputs of the comparator  74  which, in turn, generates the aforementioned ratio signal V used by the control unit  23  for controlling operation of the polishing machine  20 . The control unit  23  compares the ratio signal V with an optimized reference value, stored, e.g., in a memory (not shown) of the control unit  23 , and generates a control signal S 5  (FIG. 7). The control signal S 5  is sent via the feedback line L 2  to the polishing machine, i.e., to motors  26 ,  32 ,  42 , and the slurry supply system  47 .  
         [0045]    When a polishing process is stable, which corresponds to the acoustic signal without high peaks (FIG. 3), the ratio of the maximum peak amplitude to the average value of the electroacoustic signal is about 1, and the ratio signal V has a low level.  
         [0046]    When, however, a polishing process is accompanied by abnormalities characterized by the presence of high peaks, as shown in FIG. 4, the aforementioned ratio of the maximum peak amplitude to the average value of the electroacoustic signal significantly exceeds 1, and the ratio signal V has a high level. The difference in the levels of the aforementioned ratio signal V can be used for controlling, via a feedback control signal S 5  (FIG. 7), the operation of actuating units of the polishing machine  20 , for automatically adjusting parameters of the polishing process to eliminate abnormalities.  
         [0047]    Thus, it has been shown that the method and apparatus of the invention provide polishing control with signal peak analysis and quantitative evaluation of changes and defects in the polishing, e.g. CMP, process. The apparatus is capable of separating the extraordinary signal peaks from regular or random signal peaks. The method and apparatus provide automatic monitoring and control of a polishing process in a manner that optimizes the process and prevents occurrence of surface defects that may be caused by polishing.  
         [0048]    Although the invention has been shown and described with reference to specific practical examples, it is understood that these examples were given only for illustration purposes and that any other changes and modifications, including various combinations of the parts and units of the invention, are possible provided that these changes and modifications do not depart from the scope of the appended claims.  
         [0049]    For example, the comparator of the signal analyzer can be embodied in different forms known in the art. Although in the embodiment illustrated in the description of the invention both the head and the pad perform rotational motions, they may perform orbital or linear movements. The operation of the apparatus was illustrated with two signals S 1  and S 2  on the outputs of the sensors  46  and  48 ; however, only one such signal or more than two signals can be generated by one or by many electroacoustic sensors, respectively. The integrating unit  72  can be made in the form of a low-pass filter, an analog integrator, or as a part of control software. The peak detector  68  and the comparator  74  also can be embodied as a part of the control software.