Abstract:
A method of detecting an end point improved to enable proper detection of end points of polishing so as to improve throughput and stability of the entire polishing process is provided. A semiconductor polishing apparatus is provided in which a wafer held by a polishing head is brought into contact with a polishing pad adhered on a polishing table, with a prescribed pressure. The apparatus includes a dresser for dressing the polishing pad arranged opposing to the polishing pad, and a sound sensor for detecting sound generated by the friction between the polishing head and the wafer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a semiconductor polishing apparatus. More specifically, the present invention relates to a semiconductor polishing apparatus improved to enable proper detection of an end point of polishing. The present invention also relates to a method of detecting an end point of semiconductor polishing improved to enable proper detection of an end point of polishing.  
           [0003]    2. Description of the Background Art  
           [0004]    As the degree of integration of LSIs (large-scale integrated circuits) has been increased, structures have been made smaller and the number of interconnection layers has been increased. Accordingly, depth of focus of a stepper used in the manufacturing process becomes shallower, and exposure margin becomes severer. Consequently, planarization of a surface of a semiconductor wafer having recesses and protrusions by chemical mechanical polishing (hereinafter referred to as CMP) has become an important problem. According to a general process procedure for CMP, at first, a monitor wafer for the film as the object of polishing is polished, and by off-line film thickness measurement before and after polishing, the rate and uniformity of polishing are evaluated. By such evaluation, it is determined whether the state of the apparatus is satisfactory for polishing, and polishing conditions (particularly, polishing time) are determined pattern by pattern for the product wafer, based on the polishing rate of the film as the object of polishing, thickness of the product wafer before polishing and the target thickness after polishing, and then the product wafer is polished.  
           [0005]    In the present polishing process, however, the polishing rate varies widely, because of variations of polishing pads and slurry as polishing liquid. Therefore, in some apparatuses, it is necessary to frequently polish and measure the monitor wafer for the film as the object of polishing, which leads to lower throughput of the polishing step.  
           [0006]    Though the film thickness after polishing of the product wafer is measured off-line to confirm the result of prescribed processing, the prescribed polishing state may not be attained because of the above described variation in polishing rate, change in patterns or the variation in the polishing rate caused by variation in the quality of the polished film. If polishing of the product wafer is insufficient, it is necessary to perform polishing again.  
           [0007]    In the case of a slight over polishing, the process is returned to the preceding step, in which the film is deposited again and polishing is performed again. In this case also, the throughput decreases.  
           [0008]    In the case of excessive over polishing, the wafer must be wastefully abandoned, if over polishing occurs in the step of forming a prescribed shape.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention was made to solve the above described problems; and its object is to provide a semiconductor polishing apparatus improved to absorb variation in polishing process and to provide products in a prescribed polishing state.  
           [0010]    Another object of the present invention is to provide a semiconductor polishing apparatus improved for higher throughput and reduced wafer loss.  
           [0011]    A further object of the present invention is to provide a method of detecting an end point of semiconductor polishing improved to absorb variation in polishing process and to provide products having a prescribed polishing state.  
           [0012]    The semiconductor polishing apparatus in accordance with a first aspect of the present invention is for polishing a wafer held by a polishing head, by bringing into contact the wafer to a polishing pad adhered on a polishing table, with a prescribed pressure. The apparatus includes a polishing table. The polishing pad is adhered on the polishing table. The polishing head is arranged opposing to the polishing pad. A dresser for dressing the polishing pad is arranged opposing to the polishing pad. A sound generated by the friction between the polishing head and the wafer is detected by sound detecting means.  
           [0013]    According to a second aspect, the method of detecting an end point of semiconductor polishing is directed to the method of polishing a wafer held by a polishing head by bringing the wafer into contact with a polishing pad adhered on a polishing table, with a prescribed pressure. A sound generated by friction between the polishing head and the wafer is detected by a sound sensor, and an end point of polishing is determined from the change in intensity of the sound signal.  
           [0014]    The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic illustration representing the method of detecting an end point in accordance with the first embodiment.  
         [0016]    [0016]FIG. 2 shows another example of the position of attaching a sound sensor.  
         [0017]    [0017]FIG. 3 is a schematic illustration representing determination of an end point of polishing in accordance with a third embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    In the semiconductor polishing apparatus in accordance with an embodiment of the present invention, a polishing pad is adhered on a polishing table, and a polishing liquid is supplied to the polishing pad. A wafer rotates held by the polishing head, and the wafer surface is brought into contact with the polishing pad with a prescribed pressure. The surface of the polishing pad is dressed by a dresser.  
         [0019]    Sounds generated by the friction between the polishing pad and the wafer surface at the time of polishing, which sounds are propagated to a peripheral component, are detected by a small sound sensor provided directly outside or inside the component, and based on the change in intensity of the sound signal, end point of polishing.  
         [0020]    According to the present invention, the sound generated by the friction between the polishing pad and the wafer surface and propagated to a peripheral component at the time of polishing the wafer is detected by a small sound sensor provided directly outside or inside the component. When change in intensity of the sound signal ceases and the signal becomes constant, it is understood that the state of the polishing pad surface has attained a prescribed state. Here, the signal detected by the sound sensor is amplified by an amplifier, converted to frequency spectra by a signal processing unit, and the end point of polishing may be determined from the change in intensity of a frequency spectra related to polishing, among the frequency spectra.  
         [0021]    In the following, embodiments of the present invention will be described with reference to the figures.  
         [0022]    First Embodiment  
         [0023]    [0023]FIG. 1 is a schematic illustration representing the method of detecting an end point in accordance with the first embodiment.  
         [0024]    Referring to FIG. 1, a polishing pad  3  is adhered on a polishing table  1 . Polishing Table  1  rotates about a rotary shaft  2  of the polishing table. Polishing liquid  7  is supplied to polishing pad  3  through a polishing liquid supply tube  8 . A wafer  4  is rotated held by a polishing head  5 , with the wafer surface brought into contact with polishing pad  3  with a prescribed pressure.  
         [0025]    Referring to FIG. 1, a small sound sensor  11   a  is provided directly outside of polishing head  3 . An amplifier  12  for amplifying a measured signal is connected to sound sensor  11   a . A signal processing unit  13  for processing the amplified signal is connected to amplifier  12 , and the amplified signal is converted to frequency spectra.  
         [0026]    Sound sensor  11   a  is desirably provided directly outside a metal component or ceramics component of polishing head  5 , through which sound propagates easily.  
         [0027]    While wafer  4  is being polished, sound sensor  11   a  measures the sound generated by the friction between polishing pad  3  and the surface of wafer  4  and propagated to polishing head  5 . The surface of wafer  4  before planarization by polishing has recesses and protrusions. Therefore, there is much friction between the polishing pad  3  and the surface of the wafer  4 . As the polishing proceeds, the recesses and protrusions are reduced and becomes smaller. When the recesses and protrusions are eliminated and the surface becomes flat, friction becomes constant. Therefore, the sound generated by the friction between polishing pad  3  and the surface of wafer  4  and propagated to polishing head  5  decreases in proportion to the friction between polishing pad  3  and the surface of the wafer  4 , and eventually, becomes constant.  
         [0028]    More specifically, by measuring the sound generated by the friction between polishing pad  3  and the surface of wafer  4  and propagated to polishing head  5  by sound sensor  11   a  provided directly on the outside of polishing head  5 , it is possible to detect from the sound signal when the protrusions and recesses on the surface of the wafer  4  are decreased and eventually eliminated, that is, when planarization is completed.  
         [0029]    Therefore, even when the polishing rate changes significantly because of variation of polishing pad  3  or slurry as the polishing liquid  7 , or even when the polishing rate changes significantly because of change in patterns or variation of the quality of the film to be polished, it is possible to detect the time point when the recesses and protrusions on the surface of the wafer  4  are eliminated, as the end point of polishing. Therefore, even when frequent polishing and measurement of monitor wafer for the film as the object of polishing are necessary in some apparatuses, it becomes unnecessary to grasp the polishing rate. Therefore, throughput of the polishing step can be improved.  
         [0030]    Further, insufficient polishing or over polishing resulting from deviation from the prescribed polishing state of the product wafers can significantly be reduced.  
         [0031]    Accordingly, re-polishing performed in the case of insufficient polishing, or re-deposition and re-polishing of the film of the preceding step performed in the case of over polishing can be avoided, and here again, the throughput can be improved.  
         [0032]    Further, the loss of the wafer abandoned for excessive over polishing or over polishing in the polishing step for forming prescribed shape can be decreased.  
         [0033]    The sound signal detected by sound sensor  11   a  is amplified by amplifier  12  and transmitted to signal processing unit  13 . The signal processing unit  13  converts the sound signal to frequency spectra. By monitoring the most changing component of the signal intensity having the frequency of a few k to 20 kHz, that is the component of the sound generated by the friction between polishing pad  3  and the wafer of wafer  4  and propagated to polishing head  5  among frequency spectra, the end point of polishing can more accurately be determined.  
         [0034]    The small sound sensor may be provided directly inside ( 11   b ) of a metal part or a ceramics component of polishing head  5 , through which the sound easily propagates. Alternatively, the sensor may be directly provided outside ( 11   c ) or inside ( 11   d ) of rotary shaft  6  of the polishing head. In either case, it is possible to detect the end point of polishing, from the change in the sound signal as described above. In FIG. 2, component that correspond to those of FIG. 1 are denoted by the same reference characters and description thereof will not be repeated.  
         [0035]    Second Embodiment  
         [0036]    [0036]FIG. 3 is schematic illustration representing the method of detecting an end point in accordance with the third embodiment. In FIGS. 5 and 6, components corresponding to those of FIG. 1 are denoted by the same reference characters and description thereof will not be repeated.  
         [0037]    A small sound sensor is provided directly outside ( 11   i ) or inside ( 11   j ) (only one is sufficient) of polishing table  1 . An amplifier  12  for amplifying a measured signal is connected to sound sensor  11   i  or  11   j,  a signal processing unit  13  for processing the amplified signal is connected to amplifier  12 , and the amplified signal is converted to frequency spectra. It is desirable that sound sensor  11   i  or  11   j  is provided directly outside or inside a metal component or a ceramics component of polishing table  1  through which sound propagates easily.  
         [0038]    Referring to FIG. 3, while wafer  4  is being polished, sound sensor  11   i  or  11   j  measures the sound generated by the friction between polishing pad  3  and wafer  4  and propagated to polishing table  1 . The method of detecting an end point of the polishing process in accordance with the present embodiment is the same as the first embodiment.  
         [0039]    According to the method of detecting an end point of semiconductor polishing of the present invention, the end point of polishing can properly be detected. Therefore, regardless of the variation in polishing process, insufficient polishing or over polishing of the product wafer can be prevented and the throughput of the polishing process can be improved. Further, the wafer loss caused by abandoning the wafer because of excessive over polishing can be reduced.  
         [0040]    Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.