Abstract:
A polishing solution is dispensed onto a polishing pad that has a polishing surface, a substrate is brought into contact with the polishing surface, relative motion is created between the substrate and the polishing pad, a light beam is directed through a window in the polishing pad to impinge the substrate, and an intensity of a reflected light beam from the substrate is monitored. The polishing solution has a first refractive index, and the window has a second index of refraction that is approximately equal to the first index of refraction.

Description:
BACKGROUND  
         [0001]    This invention relates generally to semiconductor device manufacture, and more particularly to a window in a polishing pad for use in chemical mechanical polishing (CMP).  
           [0002]    In the process of fabricating modem semiconductor integrated circuits (IC), it is necessary to form various material layers and structures over previously formed layers and structures. However, the underlying features can leave the top surface topography of an in-process substrate highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These irregularities cause problems in the photolithographic process. Consequently, it is desirable to effect some type of planarization of the substrate.  
           [0003]    One method for achieving semiconductor substrate planarization or topography removal is chemical mechanical polishing (CMP). A conventional chemical mechanical polishing (CMP) process involves pressing a substrate against a rotating polishing pad in the presence of an abrasive slurry.  
           [0004]    In general, there is a need to detect when the desired surface planarity or layer thickness has been reached or when an underlying layer has been exposed in order to determine whether to stop polishing. Several techniques have been developed for the in-situ detection of endpoints during the CMP process. For example, an optical monitoring system for in-situ measuring of uniformity of a layer on a substrate during polishing of the layer has been employed. The optical monitoring system can include a light source that directs a light beam toward the substrate during polishing, a detector that measures light reflected from the substrate, and a computer that analyzes a signal from the detector and calculates whether the endpoint has been detected. In some CMP systems, the light beam is directed toward the substrate through a window in the polishing pad. A layer of slurry is typically present between the substrate and an upper surface of the window.  
         SUMMARY  
         [0005]    In one aspect, the invention is directed to a system for polishing a substrate. The system has a polishing pad with a polishing surface, a polishing head to hold the substrate against the polishing pad during polishing, a layer of slurry on the polishing pad, a window formed in the polishing pad, and an optical monitoring system including a light source and a detector. The slurry has a first refractive index, and the window has a second refractive index close to the first refractive index of the slurry. The optical monitoring system is capable of generating a light beam and is arranged to direct the light beam during at least part of the polishing operation through the window to impinge on the substrate.  
           [0006]    Implementations of the invention may include one or more of the following features. The second refractive index may be sufficiently close to the first refractive index that scratches on the window&#39;s upper surface do not increase reflection or scattering of the light beam at the interface with the slurry. The second refractive index may be within about 0.07 of the first refractive index, or within about 0.045 of the first refractive index, or within about 0.03 of the first refractive index, or within about 0.01 of the first refractive index. The second refractive index may be in the range of 1.26 to 1.4. The second refractive index may be within 5.5% of the first refractive index, e.g., within 1.0% of the first refractive index. The window may be comprised of an optically clear material with negligible diffusing capabilities. The material may be silicone. The may be a flouropolymer, such as poly(pentadecafluorooctylacrylate), poly(tetrafluoroethylene), poly(undecafluororexylacrylate), poly(nonafluropentylacrylate), poly(hepta-fluorobutylacrylate), or poly(trifluorovinylacetate). The polishing pad may have an upper portion and a lower portion, and the window may be formed in the upper portion of the polishing pad. A base window may be formed in the lower portion of the polishing pad directly beneath the window. The base window may be made of glass. The window may be tapered to have dimensions that increase away from the polishing surface.  
           [0007]    In another aspect, the invention is directed to a polishing pad. The polishing pad has a layer with a polishing surface and a window formed in the layer that has a refractive index close to a refractive index of a polishing solution.  
           [0008]    In another aspect, the invention is directed to a method of polishing a substrate. The method includes dispensing a polishing solution onto a polishing pad that has a polishing surface, bringing a substrate into contact with the polishing surface, creating relative motion between the substrate and the polishing pad, directing a light beam through a window in the polishing pad to impinge the substrate, and monitoring an intensity of a reflected light beam from the substrate. The polishing solution has a first refractive index, and the window has a second index of refraction that is approximately equal to the first index of refraction.  
           [0009]    Potential advantages of the invention may include one or more of the following. The window may be formed out of an optically clear material with negligible diffusing capabilities with improved transparency. Scattering and reflecting of the light beam at the upper surface of the window due to scratches and irregularities may be reduced. Furthermore, reflection of the light beam at the interface between the window and the slurry may be reduced. Consequently the window may improve the signal-to-noise ratio in the signal from the detector. In addition, slurry leakage around the perimeter of the window is minimized by the configuration of the window in the polishing pad.  
           [0010]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIG. 1 is a cross-sectional side view of a chemical mechanical polishing apparatus with an optical monitoring system for endpoint detection.  
         [0012]    [0012]FIG. 2 is a simplified cross-sectional view of a portion of the apparatus of FIG. 1.  
         [0013]    [0013]FIG. 3 is a simplified schematic view showing components of a light beam impinging on and reflecting off a substrate.  
         [0014]    [0014]FIG. 4A is a simplified cross-sectional view of a window in a polishing pad.  
         [0015]    [0015]FIG. 4B is a simplified cross-sectional view of a window in a polishing pad from the apparatus of FIG. 1, constructed in accordance with the present invention.  
         [0016]    [0016]FIG. 5 is a simplified cross-sectional view of another implementation of a window in a polishing pad.  
         [0017]    [0017]FIG. 6 is a simplified cross-sectional view of another implementation of a window in a polishing pad.  
         [0018]    [0018]FIG. 7 is a simplified cross-sectional view of another implementation of a window in a polishing pad.  
         [0019]    Like reference symbols in the various drawings indicate like elements. 
     
    
     DETAILED DESCRIPTION  
       [0020]    As shown in FIGS. 1 and 2, the CMP apparatus  10  includes a polishing head  12  for holding a semiconductor substrate  14  against a polishing pad  18  on a platen  16 . The CMP apparatus may be constructed as described in U.S. Pat. No. 5,738,574, the entire disclosure of which is incorporated herein by reference.  
         [0021]    This polishing pad  18  can be a two-layer pad with a backing layer  20  that interfaces with the surface of the platen  16  and a covering layer  22  with a polishing surface to contact the substrate. The covering layer  22  can be a durable rough layer (e.g., Rodel IC-1000), whereas the backing layer can be a more compressible layer (e.g., Rodel Suba-IV). However, some pads have only a covering layer and no backing layer. Alternatively, the polishing pad can be a fixed-abrasive pad with abrasive particles held in a containment media.  
         [0022]    Typically the polishing pad material is wetted with the chemical polishing solution or slurry with a chemically reactive agent, and, assuming a “standard” polishing pad, abrasive particles. For example, the slurry can include KOH (Potassium Hydroxide) and fumed-silica particles. However, some polishing processes are “abrasiveless”.  
         [0023]    The polishing head  12  applies pressure to the substrate  14  against the polishing pad  18  as the platen rotates about its central axis  24 . In addition, the polishing head  12  is usually rotated about its central axis  26 , and translated across the surface of the platen  16  via a translation arm  28 . However, it is also possible for the polishing system to use a linear belt, for just the polishing pad or the substrate to move, or for the polishing surface or the substrate to undergo different types of motion. The pressure and relative motion between the substrate and the polishing surface, in conduction with the polishing solution, result in polishing of the substrate.  
         [0024]    A hole  30  is formed in the top surface of the platen  16  and is aligned with a window  36  formed in the overlying polishing pad  18 . At least part of the hole  30  can be filled with a transparent solid piece  31 , such as a quartz block. The hole  30  and the window  36  are positioned such that they have a view of the substrate  14  held by the polishing head  12  during a portion of the platen&#39;s rotation, regardless of the translational position of the head  12 .  
         [0025]    An optical monitoring system, including a light source  32 , such as a laser, and a detector  42 , such as a photodetector, is fixed below the top surface of the platen  16 . For example, the optical monitoring system can be located in a recess or space  17  inside the platen  16  and can rotate with the platen. Alternatively, the optical monitoring system could be a stationary system located below the platen. The light source  32  projects a light beam  34  through the aperture  30  and the window  36  in the polishing pad  18  to strike the surface of the overlying substrate  14  at least during a time when the window  36  is adjacent the substrate  14 . Light reflected from the substrate forms a resultant beam  60  that is detected by the detector  42 . An unillustrated computer receives the measured light intensity from the detector  42  and uses it to determine the polishing endpoint, e.g., by detecting a sudden change in the reflectivity of the substrate that indicates the exposure of a new layer, by calculating the thickness removed from of the outer layer (such as a transparent oxide layer) using interferometric principles, or by monitoring the signal for predetermined endpoint criteria.  
         [0026]    Slurry applied to the polishing pad  18  during the polishing operation can form a layer  38  between the substrate  14  and the polishing pad  18 , including the upper surface of the window  36 . However, the interface between the window  36  and the polishing pad  18  is sealed, so that the slurry  38  cannot leak through to the platen  16 .  
         [0027]    Due to the proximity of the upper surface of the window  36  to the substrate and carrier head  12 , scratches and other irregularities tend to accumulate on the upper surface during the life of the window. The scratches and irregularities on the upper surface cause scattering and reflection at the window-slurry interface, thus attenuating the light beam and increasing the signal-to-noise ratio in the signal from the detector  42 . Accordingly, although this system works, there is still room for improvement of the signal-to-noise ratio. This may be particularly true where the monitoring system uses small changes in the intensity of the reflected light beam, such as where the monitoring system functions as an interferometer.  
         [0028]    Referring to FIG. 3, assuming that oxide polishing is being performed, the substrate  14  as will include a silicon wafer  50  and an overlying oxide layer  52  (other intervening layers may also be present, but are omitted for simplicity). The portion of the light beam  34  that impinges on the substrate  14  will be partially reflected at the surface of the oxide layer  52  to form a first reflected beam  54 . However, a portion of the light will also be transmitted in beam  56  through the oxide layer  52  and reflect from the underlying layer or wafer  50  to form a second reflected beam  58 . The first and second reflected beams  54 ,  58  interfere with each other constructively or destructively depending on their phase relationship, to form the resultant beam  60 , where the phase relationship is primarily a function of the thickness of the oxide layer  52 . The intensity of the resultant beam  60  is analyzed to determine the thickness of the oxide layer  52  using techniques known in the art. In one implementation the optical monitoring system comprises an interferometer capable of generating a collimated light beam and an interference signal, as described in U.S. Pat. No. 5,964,643, the entire disclosure of which is incorporated herein by reference.  
         [0029]    Without being limited to any particular theory, one possible source of attenuation is scattering of the light beam at the interface between the window  36  and the slurry  38 . As shown in FIG. 4A, if the window  36  has scratches or surface roughness, both the outgoing light beam  34  and the incoming light beam  60  can be scattered at the window-slurry interface  40 . This scattering can increase the signal-to-noise ratio.  
         [0030]    Refraction is the bending of light as the light passes from one medium to another when there is a difference in the index of refraction between the two mediums. When the two refractive indices of two mediums are equal the light passes from the first medium to the second medium without refraction.  
         [0031]    Referring to FIG. 4B, the window  36  can be formed from a material having a refractive index equal to, or nearly equal to, the refractive index of the slurry  38 , at the wavelengths of interest to the optical monitoring system (e.g., if the light source  32  is a laser, then at the wavelength of the beam  34  emitted by the laser). Thus, the light beam  34  can pass from the window  36  into the slurry  38  without refraction. Accordingly, the window  36  and the slurry  38  essentially behave as a single medium for the purpose of transmitting the light beam  34  from the light source  32  to the overlying substrate  14 . As a result, irregularities on the surface of the window  36 , including scratches, do not tend to scatter the light beam  34  at the window-slurry interface  40 .  
         [0032]    As shown, if the window  36  and the slurry  38  have equal, or close to equal, refractive indices, the light beam  34  propagates through the slurry  38  without refraction, is reflected off the surface of the substrate  14  and again propagates through the slurry  38  and into the window  36  without refraction. Because the window-slurry interface  40  is non-existent from the perspective of the light beam  34 , irregularities on the upper surface of the window  36  do not promote scattering of the light beam  34  upon exiting or entering the window  36 . As a result, the signal-to-noise ratio is improved, thus improving the accuracy of the optical monitoring system.  
         [0033]    In one implementation the light source  32  can be an interferometer and the light beam  34  can be a laser beam. It is feasible to employ a wavelength anywhere from the far infrared to ultraviolet. Typically, a laser that emits red light is used. A shorter wavelength results in an increase in the amount of scattering. However, longer wavelengths result in more of the oxide layer being removed per period of the interference signal. It is desirable to remove as little of the material as possible during each period so that optical monitoring system has a high precision and the possibility of any excess material being removed is minimized. It is believed these two competing factors in the choice of wavelength are balanced if a red light laser beam is chosen. Red light offers an acceptable degreed of scattering without an unmanageable amount of material being removed per cycle.  
         [0034]    Typical slurry used in a CMP operation is comprised largely of water and has a refractive index of approximately 1.33 in the visible spectrum. For example, the refractive index of a typical slurry when using a red light is approximately 1.331. Accordingly, the material selected to form the window  36  should have a refractive index equal to, or nearly equal to, 1.331 when using a red light. The material should also be optically clear with negligible diffusing capabilities to allow optimal transmission of the laser beam. In addition, the material needs to be chemically compatible with the slurry and substrate composition.  
         [0035]    In one implementation the window  36  can be formed from silicone.  
         [0036]    In another implementation the window  36  can be formed from a fluorothermoplastic having a refractive index within 0.03 of the refractive index of the slurry. The following fluorothermoplastics manufactured by Dyneon™ LLC of Oakdale, Minn., have a refractive index of about 1.34 and are therefore potential materials to form a window for use with a typical slurry having a refractive index of 1.33: FEP X 6301, FEP X 6303, FEP X 6307 and FEP X 6322, PFA 6502 N, PFA 6505 N, PFA 6510 N and PFA 6515 N. FEP is a polymer of tetrafluoroethylene and hexafluoropropylene and PFA is a polymer of tetrafluoroethylene and perfluorovinylether.  
         [0037]    In another implementation, the window  36  can be formed of a polymer having a refractive index within 0.045 of the refractive index of the slurry. The following polymers are potential candidates to form a window for use with a typical slurry having a refractive index of 1.33:  
                                                       Poly (pentadecafluorooctylacrylate)   refractive index = 1.339           Poly tetrafluoroethylene   refractive index = 1.350           Poly (undecafluororexylacrylate)   refractive index = 1.356           Poly (nonafluropentylacrylate)   refractive index = 1.360           Poly (heptafluorobutylacrylate)   refractive index = 1.367           Poly (trifluorovinylacetate)   refractive index = 1.375                      
 
         [0038]    In addition to the refractive index and optical clarity, the hardness and flexibility of the material selected to form the window  36  can be important characteristics. The material should be hard enough to resist scratching and flexible enough to resist breakage under the frictional and compressive forces applied by the substrate.  
         [0039]    Still referring to FIG. 4B, the window  36  can be a cylindrical or rectangular plug formed in the covering layer  22 , and an aperture  48  can be formed in the backing layer  20 . For example, a two-part aperture can be cut into the polishing pad, with the dimensions of the aperture in the backing layer  20  being smaller than the dimensions of the aperture in the covering layer  22 . The window  36  can be secured in the portion of the aperture in the covering layer  22 , e.g., with adhesive, leaving a gap in the backing layer  20 . Alternatively, it may be possible for the window  36  to be integrally molded into the covering layer  22 .  
         [0040]    [0040]FIG. 5 shows another implementation in which a base window  48  is formed in the backing layer  20  of the polishing pad  18 . The base window  48  also can be a cylindrical or rectangular plug, and can have dimensions greater than the dimensions of the window  36 . Thus, the base window  48  can form a base to hold the window  36  in place in the covering layer  22  of the polishing pad  18 . The base window  48  can be formed from glass. Since the base window  48  is not exposed to slurry, it should not be scratched, and consequently it does not need to match the refractive index of the slurry  38 . Optionally, the bottom surface of the base window  48  can have a diffuse lower surface. In yet another implementation, the base window  48  can be a projection from the transparent block  31  in the platen  16 .  
         [0041]    [0041]FIG. 6 shows another implementation in which the window  36 ′ is tapered so that its dimensions (length and width if it is a rectangular plug, or diameter if it is a cylindrical plug) increases with the distance from the window-slurry interface  40 . An advantage of this implementation is that it reduces the surface area of the window  36 ′ in contact with the substrate, so that the window  36 ′ undergoes less abrasion and is less likely to break. This implementation can be used with a polishing pad  18  having a backing layer  20  and a covering layer  22 , and with a base window  48  formed in the backing layer  20 .  
         [0042]    [0042]FIG. 7 shows another implementation in which the window  36 ″ includes a tapered section  70  in the covering layer  22  and a flat section  72  that extends into the aperture in the backing layer  20 . This implementation can also include a base window  48 ″, although this base window  48 ″ may be thinner than the backing layer  20 .  
         [0043]    Although the above-described embodiment employs a silicon substrate with a single oxide layer, those skilled in the art will recognize that the interference process would also occur with other substrates and other layers. The key for an interference process is that a layer partially reflects and partially transmit the impinging beam. In addition, the invention may also be useful for purely reflective monitoring, e.g., of metal layers.  
         [0044]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.