Abstract:
A method is described for noise reduction in a CMP endpoint detection system employing torque measurement. The torque signals are acquired using an adjustable sampling rate and sample size, and averaged using a moving array of adjustable size. By introducing these three adjustable quantities in the torque-based endpoint control algorithm and properly setting their values in the endpoint detection recipe, periodic noise associated with carrier rotation and carrier oscillation can be effectively removed. This in turn permits reliable, closed-loop control of the CMP process.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to semiconductor processing, and more particularly to noise reduction in the detection of the endpoint for removal of a film by chemical-mechanical polishing.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the manufacture of integrated circuits, the selective formation and removal of films on an underlying substrate are critical steps. Chemical-mechanical polishing (CMP) has become a widely used process for selective film removal and for planarizing a structure where a patterned film overlies another film.  
           [0003]    In film removal processes such as CMP, it is extremely important to stop the process when the correct film thickness has been removed (that is, when the endpoint has been reached). In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. Overpolishing (removing too much) of a film renders the wafer unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Underpolishing may sometimes go unnoticed, which also results in yield loss.  
           [0004]    [0004]FIG. 1 shows a typical CMP apparatus  10  in which a workpiece  100  (such as a silicon wafer) is held face down by a wafer carrier  11  and polished using a polishing pad  12  located on a polishing table or platen  13 ; the workpiece is in contact with slurry  14 . The wafer carrier  11  is rotated by a shaft  15  driven by a motor  16 .  
           [0005]    An example of an important CMP process is shown in FIGS. 2A and 2B. This process involves removal of a polycrystalline silicon (poly-Si) film overlying a patterned film of silicon dioxide (SiO 2 ) or silicon nitride (Si 3 N 4 ); after removal of a blanket layer of poly-Si, a surface having partly poly-Si and partly SiO 2  or Si 3 N 4  is be exposed. In FIG. 2A, a patterned oxide layer  102  is covered by a layer  104  of poly-Si. Generally, it is necessary to remove the target film of poly-Si down to a level  105  so as to completely expose the oxide pattern, while leaving the oxide layer itself essentially intact (FIG. 2B). A successful endpoint detection scheme must detect exposure of the patterned layer with very high sensitivity, and automatically stop the CMP process within a few seconds after that layer becomes exposed. The endpoint detection scheme should also be effective regardless of the pattern factor of the wafer (that is, even if the area of the exposed underlying pattern is a small portion of the total wafer area).  
           [0006]    One widely used approach to monitor and control a CMP process is to monitor a change in the motor current associated with a change in friction between (a) the top surface of the polishing pad  12  and (b) the slurry  14  and the surface being polished (such as the surface of wafer  100 ). This method is satisfactory when there is a significant change in friction as the underlying layer is exposed. However, for many applications, including the poly-Si polishing process described just above, the change in friction associated with the interface between layers is too small to result in a motor current change sufficient to be a reliable indicator of CMP process endpoint. This problem is aggravated by a large noise component in the motor current associated with the typical feedback servo current used to drive the wafer carrier at a constant rotational speed. In addition, a small pattern factor (that is, a relatively small area of the underlying patterned layer, compared with the area of the target layer) causes only a small change in friction as the endpoint is reached, limiting the useful signal.  
           [0007]    A convenient and highly sensitive method of endpoint detection, applicable to CMP equipment such as shown in FIG. 1, is described in U.S. application Ser. No. 09/689,361, “Real-time control of chemical-mechanical polishing processes using a shaft distortion measurement,” the disclosure of which is incorporated herein by reference. According to this method, changes in friction between the surface of the wafer  100  and the polishing pad  12 , in the presence of the slurry  14 , are monitored by directly monitoring the deformation of the carrier shaft  15 . During a polishing process, the shaft  15  driving the wafer carrier  11  can experience changes in torque, bending, thrust and tension. Torque on the shaft (for example, due to rotation by motor  16  in direction  31  being opposed by frictional forces in direction  32 ) will induce deformation of the shaft, as shown schematically in FIG. 3. The degree of deformation depends on the diameter of the shaft, with smaller-diameter shafts being more susceptible to deformation. Such deformations can be measured with extremely high sensitivity at reasonable cost.  
           [0008]    When an underlying film of a different material is exposed during the CMP process (for example, when the polishing of layer  104  exposes surface  105 ; see FIG. 2B), the accompanying change in friction results in a change in torque experienced by the shaft  15 . The change in torque induces a change in deformation of the shaft, which is measured by a strain gauge  201  bonded to (or embedded in) the shaft, as shown in FIG. 4. Strain gauge  201  is connected to a transmitter  202  which broadcasts a signal  203  to a detector  210 . The signal  203  indicates strain caused by deformation of the shaft  15 , which in turn is directly related to torque experienced by the shaft. This arrangement therefore generates a signal indicating changes in friction between the polishing pad  12  and slurry  14  and the wafer  100 . Signals acquired by detector  210  are then processed and analyzed in signal processing unit  220 , which produces an endpoint signal. Processing unit  220  typically includes a computer with a storage medium, the storage medium having software stored therein for performing the endpoint detection algorithm. The endpoint signal may be fed to a control unit  250  to stop the CMP process.  
           [0009]    [0009]FIG. 5A shows an example of a detected torque signal  51  acquired and processed during polishing of a poly-Si layer. The sharp change in the signal indicates that the interface between layers has been reached. The actual amount of torque on the shaft may vary from one polishing process to the next, so that a specific value of torque indicating the endpoint cannot be fixed. It therefore is preferable to detect the CMP endpoint in accordance with a change in the torque, as opposed to a predetermined value of torque. This is done by calculating the time derivative  52  of the torque signal (see FIG. 5B); the peak of the derivative is used to indicate the process endpoint. It is noteworthy that this technique provides real-time, in situ endpoint monitoring and permits closed-loop control of the CMP process.  
           [0010]    Since the endpoint signal is based on measurement of the change in torque associated with interaction among the wafer  100 , slurry  14  and pad  12 , the endpoint signal varies with the rotation and oscillation of the wafer carrier  11 . As shown in FIG. 6A, shaft  15  causes wafer carrier  11  to rotate with respect to pad  12  (fixed to platen  13 ) while carrier  11  oscillates across the pad surface. FIG. 6B is a top view of the apparatus of FIG. 6A, showing the platen  13  and wafer carrier  11 . The wafer carrier rotates about shaft  15  in direction  61 , and oscillates along a radius of the platen in directions  62   a  and  62   b . At a given point in time, the amount of torque on the shaft  15  depends on the angular position of the shaft and on the location of the carrier  11  on its radial trajectory. The torque thus varies periodically according to the separate rotation and oscillation periods. These periodic variations in the torque can be great enough to cause false endpoint signals. This noise cannot be eliminated by using a low/high pass filter or a band pass filter, since the noise generally is in the same frequency region as the true endpoint signal.  
           [0011]    It is possible to remove the noise associated with carrier rotation by using a phase-sensitive detection scheme, using timing signals from additional sensors embedded in shaft  15 . However, this approach adds cost and complexity to the endpoint detection equipment, and may introduce sources of additional noise. Introducing hardware to reduce the noise associated with carrier oscillation leads to similar difficulties. (Noise associated with rotation  65  of the platen  13  has not been found to be significant.)  
           [0012]    There remains a need for a noise reduction technique applicable to a torque-based CMP endpoint detection scheme. It is desirable that such a technique minimize added complexity in the endpoint detection apparatus, and preferably not add any hardware to the apparatus.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention addresses the above-described need by providing a noise reduction method for CMP endpoint detection, including an adjustable sampling rate, sample size, and moving array size for analyzing torque signals. By introducing these three adjustable quantities in the torque-based endpoint control algorithm and properly setting their values in the endpoint detection recipe, the periodic noises associated with carrier rotation and carrier oscillation can be effectively removed. This in turn permits reliable, closed-loop control of the CMP process.  
           [0014]    According to a first aspect of the invention, a method is provided for reducing noise in a CMP endpoint detection system where measurements associated with friction between the polishing pad and the workpiece are performed. In a first computing step, a plurality of values are computed at a predetermined time interval t, given by t=t s +t p  where t s  and t p  are data sampling and processing times respectively. Each of these computed values is an average of a plurality of measurements performed in time period t s , approximately equal to a rotation period of the workpiece carrier. An array is then formed which includes successive values computed in the first computing step, and which includes the most recently computed value. The array has a maximum size given by a moving array size, determined by approximating the product of the moving array size and the time interval to an integral multiple of an oscillation period of the carrier. In a second computing step, an average of the values in the array is computed, to obtain an array average at each successive time interval.  
           [0015]    The measurements may be characterized by a sampling rate; after the rotation period is established, at least one of the sampling rate and a number of the measurements is set so that a sampling time for performing the measurements approximates the rotation period. The array includes all of the computed values when the number of computed values does not exceed the moving array size.  
           [0016]    In accordance with the invention, a CMP endpoint signal may be obtained as follows: A plurality of successive array averages are computed and plotted to obtain a function of time. The derivative of this function with respect to time is then calculated, to yield an endpoint signal.  
           [0017]    The above-described method is applicable to measurements of torque on the shaft connected to the wafer carrier. Alternatively, the method may be applied to measurements of current in the motor used to drive the shaft.  
           [0018]    According to another aspect of the invention, the CMP apparatus includes a computer-readable storage medium; the medium has stored thereon instructions for performing a method as described above.  
           [0019]    The noise reduction method of the present invention is effective in removing noise associated with carrier rotation and oscillation, without requiring any additional hardware. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a general view of a typical chemical-mechanical polishing (CMP) arrangement to which the present invention may be advantageously applied.  
         [0021]    [0021]FIG. 2A shows an arrangement of polycrystalline silicon and silicon dioxide films where film removal by CMP is to be performed.  
         [0022]    [0022]FIG. 2B shows a desired result of CMP processing of the film arrangement of FIG. 2A.  
         [0023]    [0023]FIG. 3 is a schematic illustration of torque-induced deformation of a shaft.  
         [0024]    [0024]FIG. 4 shows a torque-based arrangement for monitoring the endpoint of a CMP process, using a strain gauge mounted on the shaft rotating the wafer carrier, on which the present invention may advantageously be practiced.  
         [0025]    [0025]FIG. 5A shows an example of a signal acquired during a CMP process, indicating the process endpoint.  
         [0026]    [0026]FIG. 5B shows the time derivative of the signal of FIG. 5A.  
         [0027]    [0027]FIG. 6A is a schematic illustration of rotation and oscillation of the wafer carrier.  
         [0028]    [0028]FIG. 6B is a top view of FIG. 6A, showing the oscillation of the wafer carrier in a radial direction with respect to the platen.  
         [0029]    [0029]FIG. 7 illustrates acquisition of a number N of data points during time t s  at a sampling rate SR.  
         [0030]    [0030]FIG. 8A shows a plot of averaged data points in moving arrays, in accordance with an embodiment of the present invention.  
         [0031]    [0031]FIG. 8B shows a plot of the data points of FIG. 8A, after averaging using an adjustable moving array size, in accordance with an embodiment of the present invention.  
         [0032]    [0032]FIG. 9 is a flowchart summarizing steps in a method for noise reduction according to the present invention.  
         [0033]    [0033]FIG. 10A is an example of an endpoint detection trace, where the moving array size N A  is set to 13.  
         [0034]    [0034]FIG. 10B shows an endpoint detection trace using the same raw data as in FIG. 10A, where the moving array size N A  is set to 9. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    The noise associated with rotation and oscillation of the wafer carrier  11  may be effectively eliminated by averaging out the noise over the respective periods of rotation and oscillation T rot  and T osc . This is done by introducing tunable parameters into the endpoint detection algorithm and adjusting those parameters in accordance with the actual polishing conditions. It should be noted that this involves modification only of software in the data analysis unit  220 ; the CMP equipment and the endpoint detection hardware (the actual sources of the noise) are not affected.  
         [0036]    Rotation Noise: Tunable Sampling Rate and Sample Size  
         [0037]    The raw endpoint detection data from the CMP apparatus (in this embodiment, torque signals  203  from the strain gauge  201 , converted to voltage signals by detector  210 ) is sampled at a sampling rate SR by the data analysis unit  220 . As shown schematically in FIG. 7, N data points are acquired in a sampling time t s  at a sampling rate SR: 
         
       N=SR*t 
       s 
     
         [0038]    The values of the N data points, s i , are then averaged to yield a single data point X:  
       X   =       (     1   /   N     )            ∑     i   =   1     N                     s   i                               
 
         [0039]    A typical sample size N is approximately 8000 points.  
         [0040]    The noise associated with rotation of the carrier  11  can be effectively eliminated by tuning the sampling rate SR and sample size N so that t s  is approximately equal to the rotational period T rot : 
           N/SR=t   s   ≈T   rot   (1) 
         [0041]    In practice, t s ≈T rot  if t s  differs from T rot  by 10% or less; that is, if 0.9 T rot ≦t s ≦1.1 T rot .  
         [0042]    If equation (1) is satisfied, the N data points are acquired during a time period approximating one complete rotation of the carrier  11 . The average value X of these data points therefore remains constant if no additional change to the torque is introduced (that is, if there is no actual change in friction due to polishing of the layer on the wafer surface during the rotation).  
         [0043]    Oscillation Noise: Tunable Moving Array Size  
         [0044]    The average values (values of X) may be plotted as shown in FIG. 8A (points  811 - 816  etc.). Each point in FIG. 8A represents an average of N (about 8000) raw data points. The time interval between points in FIG. 8A (e.g. between points  811  and  812 ) is t, given by 
         
       t=t 
       s 
       +t 
       p 
     
         [0045]    where t p  is the time required for processing the N points (averaging, plotting, etc.), and is a small fraction of t s .  
         [0046]    The noise due to oscillation of the carrier (which generally is greater than noise due to rotation) is averaged out by averaging over the period of oscillation T osc , using a moving array with a tunable array size N A . Points  821 - 826  are plotted in FIG. 8B from points  811 - 816 , using a moving array technique. In this example, the moving array size N A  is 5. The first point  821  in FIG. 8B represents the same value as point  811 . The second point  822  represents the average of  811  and  812 ; the third point  823  represents the average of  811 ,  812  and  813 ; the fourth point  824  represents the average of  811 ,  812 ,  813  and  814 .  
         [0047]    After 5 intervals of time t have elapsed, 5 points  811 - 815  (array  81 ) are available for averaging. Point  825  represents the average of the 5 points in array  81 . After the next time interval, point  816  is plotted and a new array  82  is formed which includes the 5 points  812 - 816 . Point  826  represents the average of array  82 . In general, after N A  time intervals have elapsed, the array of N A  points moves with the calculation of a new value of X after each time interval, so that the array includes the N A  most recent values of X. (The array may be thought of as including all the values, if fewer than N A  have yet been calculated, and growing to a maximum size N A ). Variations in the values of X (such as those plotted in FIG. 8A) are therefore smoothed out by averaging the N A  most recent values of X, and then plotting the result (as in FIG. 8B).  
         [0048]    It will be appreciated that an array of size N A  represents data acquired over a time period N A *t. The noise associated with the periodic oscillation of the carrier (period T osc ) may therefore be averaged out by setting this time period approximately equal to (generally, within 10% of) an integer multiple m of T osc : 
           T   osc   *m≈N   A   t   (2) 
         [0049]    Since t≈t s =N/SR≈T rot , Equation (2) may be expressed in terms of the adjustable parameters as follows: 
           T   osc ≈(N A   /m )( N/SR ) 
         [0050]    In practice, T rot  and T osc  may usually be adjusted within narrow limits (about 10%) without affecting the performance of the CMP process. This permits the approximations in Equations (1) and (2) to be still more closely satisfied. If N is changed to adjust t, in order to better satisfy Equation (2), then Trot should be changed to ensure that Equation (1) is still satisfied. In a typical data analysis scheme, N is adjusted while SR remains constant. N A  is generally in the range of about 9 to about 36. If N A  is set to too small a value, inadequate smoothing of the data results; if N A  is set too large, the endpoint detection system responds too slowly to an actual change in the signal, and the resulting endpoint trace (time derivative of the average data values plotted as a function of time) is distorted.  
         [0051]    Steps in a method for implementing the above-described noise reduction technique are summarized in the flowchart of FIG. 9. The rotational period T rot  of the carrier is established, either by measuring the rotation in an existing process or setting Trot to a convenient value (step  901 ). The sample size N and/or sampling rate SR are adjusted so that the sampling time t s =N/SR approximately matches the rotational period T rot  (step  902 ). The carrier oscillation period T osc  is then established (step  903 ), and the moving array size N A  and integer m are chosen so that (N A *t)/m approximately matches T osc  (step  904 ). The data is then averaged using moving arrays of size N A  (step  905 ; see FIGS. 8A and 8B). Finally, as noted above, the time derivative is calculated and the endpoint trace is plotted to give a convenient process control signal which is inputted to control unit  250  (step  906 ).  
       EXAMPLE  
       [0052]    The above-described technique may be illustrated using parameters from a CMP process presently in use. The rotation period of the carrier is  0 . 8  sec and the oscillation period is 2.09 sec. The sampling rate SR is 10000 points/sec; N is therefore set to 8000, so that t s =N/SR=0.8 sec=T rot . From an examination of the data file, it is found that t=0.93 sec. Accordingly, N A /m should be chosen to approximate T osc /t=2.247. This may be done by setting N A =9, m=4.  
         [0053]    [0053]FIG. 10A and 10B each show an endpoint trace using data from this example, with different values of the array size N A . In FIG. 10A, N A =13, so that noise due to carrier oscillation is not averaged out; periodic variations in the endpoint signal may be easily seen. In FIG. 10B, N A =9, so that noise due to carrier oscillation is effectively removed.  
         [0054]    It is noteworthy that, as shown in this example, a larger value of the moving array size does not generally give better smoothing of the endpoint trace. This is an unexpected result in view of the general rule that the signal-to-noise ratio increases according to the square root of the array size.  
         [0055]    This noise reduction technique is also applicable to CMP endpoint detection units using motor current measurements, since periodic variations in polishing friction will lead to periodic variations in the motor current.  
         [0056]    While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.