Abstract:
A method for reducing dishing effects is provided. The method is applied to polish a surface of a wafer containing a silicate film thereon. The method comprises using a polishing slurry containing organic alkyl or aryl compound with at least one hydroxyl group (i.e. ROH compound) during the process of polishing the silicate film. An organic hydrophobic layer created over the silicate film in contact with the ROH compound thus alleviates the undesirable dishing effects. The organic hydrophobic layer is thereafter cleaned using ozone-containing deionized water.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a chemical mechanical polishing (CMP) process, and more particularly, relates to a method for reducing dishing effects during a chemical mechanical polishing process.  
           [0003]    2. Description of the Prior Art  
           [0004]    As integrated circuit device dimensions have decreased, it has become increasingly common within advanced integrated circuits to employ trench isolation methods, such as shallow trench isolation (STI) methods, to form trench isolation regions nominally coplanar with adjoining active semiconductor regions of semiconductor substrates. Such STI methods typically employ a chemical mechanical polishing (CMP) method to provide a nominally planarized surface to a trench fill dielectric layer formed within the trench. Trench isolation regions nominally coplanar with active semiconductor regions within semiconductor substrates are desirable since they optimize, when subsequently forming patterned layers upon those nominally coplanar trench isolation regions and active semiconductor regions, the limited depth-of-focus (DOF) typically achievable with advanced exposure tooling.  
           [0005]    CMP is a process used to create smooth, planar layers on wafers, which essentially provides for polishing a wafer by rubbing a polishing pad against the wafer to grind the surface layer. The polishing pad is saturated with an abrasive slurry solution that may aid the planarization. A common slurry used for polishing silicate film is colloidal silica in an aqueous KOH solution. CMP tools are well known in the art. Such tools are disclosed in U.S. Pat. Nos. 3,841,031 and 4,193,226. The tools include a polishing wheel with the wafer attached to the wheel. As the wheel rotates, the wafer is forced against a wetted polishing surface and the surface is planarized.  
           [0006]    During a CMP process applied on a silicon dioxide layer, there is a thin hydrated silica gel layer created over the surface of the silicon dioxide layer. The reaction may be described by the following formulas:  
           Si-O-Si+H 2 O→2SiOH  
           (SiO 2 ) x +2H 2 O→(SiO 2 ) x−1 +Si(OH) 4    
           [0007]    According to the above equations, a global planarization is achieved by rapidly and repeatedly removing the readily formed hydrated silica gel layers. In an aqueous environment, the surface undergoes a hydration reaction with H 2 O to produce a surface network of hydroxylated Si molecules. Dissolution of this network generally occurs above a pH of 9.0 because of the solubility of the reaction product at high pH. Typically, the formation of Si(OH) 4  is accelerated in a more alkaline environment. A most common composition of slurry for polishing silicate films includes abrasive, for example, fumed silica, deionized water, alkaline solvent, and buffer solution. A normal pH for this composition is usually between 10.5 and 11.5.  
           [0008]    While trench isolation methods employing CMP planarizing of trench fill dielectric layers formed into isolation trenches are therefore quite desirable and common in the art of advanced integrated circuit fabrication, such trench isolation methods are nevertheless not entirely without problems. In particular, it is known in the art that trench isolation methods employing CMP planarizing of trench fill dielectric layers formed into isolation trenches often, when the isolation trenches are comparatively wide, provide isolation trenches where a planarized trench fill dielectric layer formed therein is dished.  
           [0009]    In order to totally remove the trench fill dielectric layer above a silicon nitride stop layer, the silicon nitride stop layer is strategically over-polished. Since the hardness of silicon nitride is higher than that of the trench fill dielectric layer, the trench fill dielectric layer is polished away with a higher rate to cause a dishing top surface of STI. The dishing phenomenon affects the performance of the device.  
           [0010]    Since dishing within planarized trench fill layers formed within trenches within integrated circuits is generally undesirable, it is therefore in general towards forming, without dishing, planarized trench fill layers within trenches within integrated circuits that the present invention is directed. Methods for preventing dishing during a CMP process are known in the art of integrated circuit fabrication. For example, Bose et at., in U.S. Pat. No. 5,492,858 discloses a method for forming a planarized trench fill dielectric layer within an isolation trench within an integrated circuit. The method employs, within the isolation trench, a barrier layer that allows for densifying, through steam annealing, a conformal silicon oxide trench fill dielectric layer, which may then subsequently be planarized, while avoiding dishing, through a chemical mechanical polish (CMP) planarizing method.  
           [0011]    In addition, Homma et at., in U.S. Pat. No. 5,420,075 discloses a method for selectively depositing insulator layers into inter-metal spacings within patterned metal layers within integrated circuits. The method provides for treating a portion of a patterned metal layer with a fluorine containing reactive ion etch (RIE) plasma to inhibit formation of a chemical vapor deposited (CVD) silicon oxide layer upon that portion of the patterned metal layer.  
         SUMMARY OF THE INVENTION  
         [0012]    It is an object of this invention to provide an effective and simple method for reducing dishing of a polished silicate film during a CMP process.  
           [0013]    It is a further object of this invention to provide a method for reducing dishing during an isolating trench formation process by using a polishing slurry to which is added a quantity of alcohol and/or phenol before the polishing end point.  
           [0014]    According to one aspect of the present invention, a method for reducing dishing effects is provided. The method is applied to polish a surface of a wafer containing a silicate film thereon. The method comprises using a polishing slurry containing an organic alkyl or aryl compound with at least one hydroxyl group (i.e. ROH compound) during the processing of polishing the silicate film. An organic hydrophobic layer is created over the silicate film in contact with the ROH compound, thus alleviating the undesirable dishing effects. The organic hydrophobic layer is thereafter cleaned using ozone-containing deionized water.  
           [0015]    According to a further aspect of the present invention, the present invention comprises the following essential chemical mechanical polishing steps:  
           [0016]    (1) starting a chemical mechanical polishing process, at a polishing time=0 second, by using an alkaline slurry containing water, abrasives and soluble inorganic salts;  
           [0017]    (2) at a polishing time=t1 second, adding a predetermined quantity of alcohol and/or phenol into the alkaline slurry;  
           [0018]    (3) at a polishing time=t2 second, detecting an end point of the CMP process, wherein t2&gt;t1; and  
           [0019]    (4) terminating the CMP process.  
           [0020]    To promote the formation of the hydrophobic layer, the pH of the slurry is controlled to between 7 and 13. It is advantageous that by addition of the ROH compound into the slurry during the CMP process, a hydrophobic layer is created over a silicate film, thereby reducing dishing of the silicate film in a wide trench. In addition, the organic hydrophobic layer is easily removed by ozone-containing water.  
           [0021]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The objects, features and advantages of the present invention are understood within the context of the Description of the Preferred Embodiments, as set forth below. The Description of the Preferred Embodiments is understood within the context of the accompanying drawings, which forma material part of this disclosure, wherein:  
         [0023]    [0023]FIG. 1 is a cross-sectional schematic diagram showing a wafer to-be-polished before a CMP process.  
         [0024]    [0024]FIG. 2 is a flow chart showing the method for reducing dishing during a CMP process according to the present invention.  
         [0025]    [0025]FIG. 3 is a cross-sectional schematic diagram showing a wafer positioned against a polishing pad after polishing for a time period t1.  
         [0026]    [0026]FIG. 4 is a cross-sectional schematic diagram showing a wafer positioned against a polishing pad after the addition of ROH compound into the slurry.  
         [0027]    [0027]FIG. 5 is a schematic diagram showing the structure of a hydrophobic layer formed over a silicate film.  
         [0028]    [0028]FIG. 6 is a cross-sectional schematic diagram showing a wafer positioned against a polishing pad after performing an over-polish step.  
         [0029]    [0029]FIG. 7 is a cross-sectional schematic diagram showing a wafer after performing an ozone-containing water cleaning step. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]    Please refer to FIG. 1. FIG. 1 is a cross-sectional schematic diagram showing a wafer to-be-polished before a CMP process.  
         [0031]    As shown in FIG. 1, semiconductor wafer  100  comprises a silicon substrate  102  with a wide trench  104  formed thereon, a polishing stop layer  106  formed outside the trench  104 , and a trench-fill dielectric  108  filling the trench  104  and covering the polishing stop layer  106 . In the preferred embodiment of the present invention, the trench-fill dielectric  108  is silicon dioxide. However, other doped silicate glasses such as phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or fluorinated silicate glass (FSG) are also suitable for the present invention. The polishing stop layer  106  is a silicon nitride layer.  
         [0032]    A flow  50  utilizing chemical and mechanical abrasion to planarize and remove surface irregularities of a topological surface, according to the present invention, is shown in FIG. 2. A CMP process starts by placing a semiconductor wafer face-down on a polishing pad which is fixedly attached to a rotatable table or platen (step  52 ). Elevationally extending features of the semiconductor wafer are positioned such that they contact the slurry attributed to the polishing pad. During the CMP process, the polishing pad and semiconductor wafer may be rotated while a carrier holding the wafer applies a downward force upon the polishing pad. An abrasive slurry is deposited upon the surface of polishing pad. In this manner, the slurry occupies an interface between the pad and the surface of the wafer.  
         [0033]    The slurry initiates the polishing process by chemically reacting with the surface material being polished. The rotational movement of the polishing pad relative to the wafer causes abrasive particles entrained within the slurry to physically strip the reacted surface material from the wafer. The polishing slurry includes abrasives, for example, fumed silica, deionizedwater, alkalinesolvent, and buffer solution. The abrasives are typically composed of silica, alumina, or ceria. Sometimes, surfactants may be added according to desired purposes. A normal pH for this composition is usually between 7 and 13, preferably between 10.5 and 11.5. To achieve this pH, bases such KOH, NaOH or NH 4 OH are used.  
         [0034]    After t1 seconds from the start of the CMP process, a predetermined quantity of alcohol is added into the polishing slurry, which is fixed at a pH between 10.5 and 11.5 (step  54 ). In another case according to the present invention, a predetermined quantity of phenol is added into the polishing slurry. In still another case according to the present invention, both alcohol and phenol are added into the slurry. The time period t1 depends on the thickness of the polished silicate film and the composition of the film. For example, for a silicon dioxide formed by a plasma-enhanced chemical vapor deposition (PECVD) method having a thickness of about 1000 angstroms, t1 ranges from 55 seconds to 65 seconds. After the addition of the alcohol or phenol, a hydrophobic layer is formed over the polished silicate film. The concentration of the alcohol in the slurry may be within a range of about 0.01% to 10% by weight.  
         [0035]    The alcohol used is selected from a group consisting of methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, butyl alcohol, pentyl alcohol, n-hexanol, cyclohexyl alcohol, heptyl alcohol, octyl alcohol and the like. The phenol is selected from a group consisting of phenol, benzyl alcohol, diphenylcarbinol, and the like.  
         [0036]    Still referring to FIG. 2, after the addition of the alcohol or phenol (step  54 ), the CMP process goes on (step  58 ). A polishing end point is then detected (step  58 ) at time point t2 second. An over-polish step (step  60 ) is then performed after the end point for achieving a substantially global planar surface. Suggestion for the time period of the over-polish step is about 15 to 25 seconds, or 30% of t1. Finally, the hydrophobic layer formed in step  54  is removed by using ozone-containing water (step  64 ).  
         [0037]    Please refer to FIG. 3. FIG. 3 is a cross-sectional schematic diagram showing the status of the wafer  100  positioned against a polishing pad  204  after polishing for a time period t1 as set forth in the flow chart (FIG. 2). The polishing pad  204  may be made of various substances, depending on the material being polished. A popular polishing pad medium comprises polyurethane or polyurethane-impregnated polyester felts. An example of a somewhat hard polishing pad is the IC-1000 type pad commercially available from Rodel Products Corporation. A relatively soft pad is the SUBA 500 type pad, also manufactured by Rodel Products Corporation. The slurry sprayed onto the polishing pad  204  creates a slurry layer  202  at the interface between the polishing pad  204  and the wafer  100 . Gradually, the trench-fill dielectric  108  is polished to an even surface after t1 seconds. At t1, the polishing stop layer  106  is not yet exposed.  
         [0038]    Please refer to FIG. 4. FIG. 4 is a cross-sectional schematic diagram showing the wafer  100  positioned against a polishing pad  204  after the addition of ROH compound into the slurry layer  202  as set forth in the flow chart (FIG. 2). As shown in FIG. 4, when alcohol, such as isopropyl alcohol (IPA), is added into the slurry layer  202 , a hydrophobic layer  110  is formed over the trench-fill dielectric  108 . The hydrophobic layer  110  blocks the path of H2O and stops further diffusion of H2O into the trench-fill dielectric  108 . As a result, the hydration rate or the formation rate of the hydrated silicon is significantly reduced. At this phase, the surface of the trench-fill dielectric  108  is polished via mechanical interaction provided by the polishing pad  204  and the abrasive particles in the slurry layer  202 .  
         [0039]    A structure of a hydrophobic layer formed over a silicate film is shown in FIG. 5. As shown in FIG. 5, the alcohol molecules (R-O-H), having an RO group  122 , react with the silicon surface of the trench-fill dielectric silicate film  108  according to the following equation:  
         Si-O-Si+2ROH→2SiOR+H 2 O  
         [0040]    Under an alkaline environment, the OR group reacts with silicon dioxide to form Si-OR bondings  124 . With the aid of the hydrophobic group, i.e. OR group  122 , bonded on the surface of the trench-fill dielectric  108 , H 2 O molecules are blocked in the slurry layer  202  and the hydration of the silicon is theoretically interrupted. After the addition of alcohol at t1 time point, the polishing rate of the trench-fill dielectric  108  reduces.  
         [0041]    It should be remembered that the added alcohol will be consumed and additional H2O will be produced. The newly formed H2O will dilute the concentration of the added alcohol after the addition of the alcohol. To solve this problem, after 10 to 15 seconds from the first addition, a second addition of the alcohol is suggested. It is found that two additions of the alcohol result in a better dishing-free wafer than one addition.  
         [0042]    Please refer to FIG. 6. FIG. 6 is a cross-sectional schematic diagram showing the wafer  100  positioned against the polishing pad  204  after performing an over-polish step as set forth in FIG. 2. As shown in FIG. 6, after performing an over-polish step, the polishing stop layer  106  with higher hardness than the trench-fill dielectric  108  is removed in a planar manner to a remaining thickness of about 1000 to 2000 angstroms. By the protection of the hydrophobic layer  110 , the trench-fill dielectric  108  is not further over-polished which might cause a dishing profile in the wide trench  104 .  
         [0043]    Please refer to FIG. 7. FIG. 7 is a cross-sectional schematic diagram showing the wafer  100  after performing an ozone-containing water cleaning step. As shown in FIG. 7, since the hydrophobic layer  110  contains plenty of organic groups, such as alkyl groups and phenyl groups, they therefore need to be removed so as not to contaminate the device fabrication in the subsequent processes. One proposed method to remove the hydrophobic layer  110  is using ozone-containing water.  
         [0044]    In contrast to the prior art method, the present invention uses a simplified process employing alcohol addition to the slurry layer  202  to form a hydrophobic layer  110  which can effectively block H2O molecules from further diffusing into the polished silicate film (i.e. trench-fill dielectric  108 ). The created organic hydrophobic layer  110  can be completely removed using ozone-containing water. In summary, the proposed method according to the present invention offers a better choice for reducing dishing effects during a CMP process.  
         [0045]    Those skilled in the art will readily observe that numerous modifications and alterations of the method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.