Abstract:
For improving planarization of urethane polishing pads, at least one layer of the pad has a base resin and an isocyanate with a concentration within a range of 6.5-11.0 weight percent to obtain a high planarization property.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to polishing pads, and methods of producing the same.  
         [0002]     The present invention generally relates to polishing pads, in particular to chemical-mechanical polishing (CMP) pads. CMP is a process step in the semiconductor fabrication sequence that has generally become an integral part of the manufacture of semiconductor wafers. The process is used in a variety of applications in the semiconductor fabrication sequence. Different applications are sometimes best optimized with different polishing pad types and configurations. The present invention generally concerns itself with a process application known as “oxide”. Other applications include copper, copper barrier and poly.  
         [0003]     In any of these CMP processes, the silicon substrate is forcibly placed in direct contact with a moving polishing pad. A wafer carrier applies pressure against the backside of the substrate, usually while simultaneously forcibly applying rotation. During this process a slurry is made available, and is generally carried between the wafer and the pad by the motion of the pad. The composition of the slurry is dictated by the specific application.  
         [0004]     The CMP polishing pad is required to perform a plurality of engineering functions. It is required to polish, planarize short (&lt;100 micrometer) distances, planarize long (&gt;100 micrometer) distances up to a certain Planarization Length (see below) determined by the quality of the silicon substrate, not planarize beyond that length, transport slurry, maintain the same friction with the wafer for wafers polished—sequentially and with interruptions—for hundreds of wafers, clean the wafer surface, not scratch the wafer surface, be replaceable in minimal time, and others. This invention addresses the planarization capability of the pad.  
         [0005]     This invention comtemplates both short and long range planarization. Generally, long range planarization is controlled by the bulk properties of the pad and short range planarization is controlled by the surface properties of the pad. A concept useful in describing long range planarization is the Planarization Length (L), defined as a lateral dimension characteristic of the pad&#39;s ability to planarize. Intrinsic to this concept is Preston&#39;s equation, which maintains that when polishing, the removal rate is proportional to force. There are significant deviations to this relationship, but it holds generally, and for our purpose, it is sufficient. With Preston in mind, one can consider a feature to be planarized consisting of an upraised element. ( FIG. 1 ) A polishing pad will try to planarize the feature, and will succeed in doing so when the pressure exerted by the pad at the top of the feature exceeds the pressure exerted adjacent to the feature. Ala Preston, the removal rate at the top of the feature will exceed the removal rate adjacent to the feature and over time the feature will decrease in height. One definition of planarization length is the distance from the feature that the pressure has increased to 1/e of the pressure infinitely far from the feature (e is ln(10)).  
         [0006]     Short range planarization has no analog to L. Since the short range planarization is affected by the surface properties, it can vary dramatically using the same polishing pad by varying the amount of diamond conditioning received by the pad, an action that affects the pad&#39;s surface roughness. However, generally the pad surface is conditioned for the purpose of maintaining the removal rate. Therefore with regard to short range planarization, the engineering constraint is placed on the pad rather results-oriented. If written, it would read something like, “ . . . when consistently using a standard conditioning process the pad should exhibit adequate removal rate as well as high short range planarization . . . ”. Because of this, the short range planarization requirement generally comes from empirical results. For the copper application, these results would typically be expressed in dishing, the amount of material removed from a narrow copper line. For oxide applications, the results would typically be expressed by the planarization performance of a test structure designed to measure short range planarization.  
         [0007]     An improvement to polishing pads designed improve planarization is contemplated in James (U.S. Pat. No. 6,454,634), and in follow-on patents by James, U.S. Pat. No. 6,582,283, No. 6,736,709 and No. 6,749,485. While James offers an example of an additive that achieves the desired high KEL value, the teachings and data support the desired property only for what has been described above as short range planarization, namely planarization which occurs over a distance of less than 100 micrometers. The present invention considers formulations which affect both short and long range planarization.  
         [0008]     The measurement of planarization is made using planarization test structures. These are topographical structures which contain regular periodic arrays arranged in regions of different pitch and width. Due to practical limitations related to metrology the smallest of these to yield an accurate measurement had a 100 micrometer pitch at 50% density (i.e. alternating high and low structures of equal width) and the largest had a 500 micrometer pitch at 50% density. Planarization capability is measured by a repeating sequential polish/measure action in which the remaining amplitude and the average amount of oxide removed is recorded for each step. The sequence is complete when the structure is planarized below the noise level. This sequence yields a curve which can be further reduced to single FIGURE of merit of planarization called the Planarization Efficiency. In order to capture both short and long range planarization together, we have developed a parameter we call the General Planarization Efficiency (GPE) which consists of the average of the short and long range efficiencies. The GPE ranges from 0% to 100%, in which 0% represents no planarization at all (i.e. perfect etching), while 100% represents perfect planarization (material removed only from high areas, none whatsoever from low areas). This invention addresses formulations which lead to an improved GPE.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, it is an object of the present invention to provide a method of improving planarization of urethane polishing pads which is a further improvement of the existing methods, and also to provide a polishing pad which is characterized by better planarization properties.  
         [0010]     In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a method of improving the planarization property of a polishing pad, in accordance with which isocyanate is introduced in a base polyol resin of the urethane polishing pad within a concentration range of 6.5% to 11.0%.  
         [0011]     When isocyanate is introduced into the basic resin of a urethane pad with the concentration selected in accordance with the present invention, a GPE within the range of 75%-95% is obtained, which can be characterized as a very good planarization property.  
         [0012]     The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  illustrates a planarization of urethane polishing pads as a function of NCO concentration produced in accordance with the present invention  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     In accordance with the present invention, urethane polishing pads are produced in a conventional manner by providing a basic polyol resin with a corresponding composition, and isocyanate is introduced into the basic resin. It is to be understood that the whole pad can be produced in this way or only its working layer, in the event when the pad is a multi-layer pad. In a new and special way, the isocyanate is introduced with a concentration of between 6.5% and 11.0% into the basic resin.  
         [0015]     Tests have been conducted to determine the planarization of the pad produced in accordance with the present invention. Four polishing pads have been tested, in particular a polishing pad L-100 with isocyanate concentration of 4.0%, a polishing pad L-200 with isocyanate concentration of 7.5% and L-325 with isocyanate concentration 9.5% from the Adiprene series, and resin  2505  with isocyanate concentration 11.6% from the Royalcast series, all from Crompton Uniroyal Chemical. As shown in  FIG. 1 , when the urethane polishing pads with isocyanate concentration in accordance with the present invention were tested, they achieved the planarization within the range of 72-95%.  
         [0016]     A breakdown of the data into groups normally expected to affect planarization does not alter the result. For example, grooving is known to generally negatively affect the GPE of the pad by structurally weakening the surface. However, the aforementioned planarization range was found to hold both for pads with and without grooving. It is also known that the use of a soft subpad can negatively affect the GPE (by virtue of its effect on the long range planarization). Again in this case the results were unaffected when considering solo pads and pad stacks separately. It is therefore understood that the NCO concentration controls physical properties of the pad which affect both the short and the long range planarization capability.  
         [0017]     In accordance with an advantageous feature of the present invention, the concentration of the isocyanate can be within a range of 6.5-8.5%. In this case the urethane pad or its working layer is softer and suitable for barrier buff.  
         [0018]     In accordance with another preferable feature of the present invention, the concentration of the isocyanate can be within a range of 8.5-11.0%. In this case the urethane pad or its working layer is harder and is suitable for interlayer dielectric.  
         [0019]     The urethane pad or its layer can be composed of polyester polyurethane or a polyether polyurethane.  
         [0020]     The urethane pad or its working layer can contain abrasive particles which can be composed of silica, alumina, ceria, titania, diamond, or silicon carbide.  
         [0021]     On the other hand, the urethane pad or its working layer can be absent of abrasive particles.  
         [0022]     The urethane pad or its working layer in accordance with the present invention can also include a filler.  
         [0023]     The above presented ranges of planarization are highly efficient for the urethane polishing pads. As can be seen if this isocyanate concentration is less than 6.5% and more than 11.0%, the planarization property of the urethane polishing pads worsens.  
         [0024]     It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of methods and constructions differing from the types described above.  
         [0025]     While the invention has been illustrated and described as embodied in a method of improving planarization of urethane polishing pads, and urethane polishing pad produced by the same, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.  
         [0026]     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.