Patent Publication Number: US-6336841-B1

Title: Method of CMP endpoint detection

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of determining the endpoint of a chemical mechanical polishing (CMP) process, and more particularly, to a method of CMP endpoint detection involving the use of infrared spectroscopy. 
     2. Description of the Prior Art 
     Chemical mechanical polishing is a common method used in the semiconductor industry to planarize the surface of a semiconductor wafer. For example, it can be used to remove a first layer of a dual layer surface. Several methods are available for determining the endpoint of the CMP process, with the most common being optically monitoring a target layer. However, the target layer is required to be of a sufficient thickness so that during the CMP process, data can be detected by the photo detector of the intensity of a reflected light beam to produce a trace curve of which is then used to determine the CMP endpoint. Generally, the thickness of the target layer is required to be greater than 3000 angstroms so that the data generated by the reflecting light beam produces a trace curve. 
     Please refer to FIG. 1 of the schematic diagram of the method used to determine the CMP endpoint according to the prior art. As shown in FIG. 1, an unpolished semiconductor wafer  11  is positioned within a holder  13  of a wafer head  15 . Beneath the wafer  11  is a polishing pad  12  supported by a polishing platen  16 , with a window (not shown) penetrating both the pad  12  and the platen  16  to the surface of the target layer of the semiconductor wafer  11 . A motor  19  drives both the wafer head  15  and the polishing platen  16 , while a controller  18  controls both their rotational speeds. A vertical motor  20  is positioned for the vertical contacts between the wafer head  15  and the polishing platen  16 . In addition, the equipment of the CMP process also includes a slurry supplier tube  14 , to transfer a flow of slurry between the semiconductor wafer  11  and the polishing pad  12 . 
     During the CMP process, the wafer head  15  and the polishing platen  16  both rotate, respectively, at a specified rate of speed to allow the slurry to smoothly spray the polishing pad  12 . With the proper parameter settings, the target layer of the semiconductor wafer  11  can be polished via the chemical reaction produced between the slurry and the mechanical polishing of the polishing pad  12 . The CMP endpoint detecting system of the prior art determines the polishing endpoint by a trace curve  22 , processed by a computer  21 , of the light beam reflected from the target layer. More specifically, the equipment of the CMP process of the prior art includes an optical detecting device  17  to generate a light beam of a specific wavenumber. The light beam passes through the hole of the polishing pad  12  and is directed onto the target layer of the semiconductor wafer  11  at a predetermined angle. The intensity of the reflected light beam can be continually detected by the optical detecting device  17 . Then, the data is transmitted to the controller  18  and the computer  21  where the result is shown as a trace curve  22  on the computer screen. From the trace curve  22 , the CMP endpoint is then determined by the use of predetermined window logics  51  and  52  during abrupt changes in the intensity I of the reflected light. 
     However, the prior art method of determining the endpoint of the CMP process requires a target layer of a thickness above 3000 angstroms in order to produce a computer-generated trace curve, which is then used to detect the CMP endpoint. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide a novel method of CMP endpoint detection without the need for a specific target layer thickness. 
     In a preferred embodiment, the present invention provides an infrared spectroscopic method of removing a first layer from a semiconductor wafer without overpolishing the underlying second layer. The first layer and the second layer of the semiconductor wafer are composed of silicon oxide or silicon nitride. An infrared (IR) light source is directed onto the semiconductor wafer, and data related to IR absorptivity of each layer is collected to produce a standard IR absorbance curve for each layer of the semiconductor wafer. Since each layer absorbs IR light at different wavenumbers, two defined IR absorbance curves are observed whereby once the CMP process is performed, a change in the IR absorptivity and thus the absorbance curve of each layer is detected. The IR absorptivity of the first layer progressively decreases for a length of time until significant change in the absorbance curve is no longer detected. The endpoint of the CMP process is determined at a point when significant change in the IR absorptivity of the first layer is no longer detected and change in the IR absorptivity of the second layer occurs. 
     It is an advantage of the present invention that the endpoint of the CMP process is easily and precisely determined via infrared spectroscopy, whereby removal of a first layer exposes, but does not overpolish, the underlying second layer of a semiconductor wafer. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a method of determining the CMP endpoint according to the prior art. 
     FIG. 2 is a schematic diagram of a method of determining the CMP endpoint according to the present invention. 
     FIG. 3 is a graph illustrating the IR absorptivity versus wavenumber of silicon oxide and silicon nitride throughout the course of the CMP process, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Please refer to FIG. 2 of a schematic diagram of a method of determining the CMP endpoint according to the present invention. As shown in FIG. 2, a semiconductor wafer  30  is placed in a holder  23  and fixed in position by a wafer head  25 . The semiconductor wafer  30  rests atop a polishing pad  24  positioned on a platen  26 . The semiconductor wafer  30  has a first layer  31  and a second layer  29 . The first layer  31  and second layer  29  are each composed of silicon oxide or silicon nitride, of which both are transparent to infrared (IR) light and absorb IR light at different wavenumbers. An infrared light source  28 , is directed onto a window  27  which allows IR light from the IR light source  28  to pass through the platen  26 , the polishing pad  24 , the semiconductor wafer  30 , and the wafer head  25  to a detector  32 . The detector  32  then produces a graph, per period of the polishing pad  24 , displaying the IR absorptivity of both the silicon oxide and the silicon nitride of the semiconductor wafer  30  during the course of the CMP process. Thus, a graph displaying changes in IR absorptivity of the first layer  31  and second layer  29  is produced at a constant interval of time with a per unit time of milliseconds. 
     Please refer to FIG. 3 of the graph illustrating the IR absorptivity versus wavenumber of silicon oxide and silicon nitride throughout the course of the CMP process, according to the present invention. As shown in FIG. 3, the beginning of the CMP process is shown by an absorbance curve A which is shown having two distinct peaks  1 , 2 . Each peak represents IR absorptivity of each layer. For instance, the absorbance peak  2  of the first layer, composed of silicon oxide, is detected at a wavenumber between 1100-1000cm −1  with an absorbance value at approximately 1.5. The absorbance peak  1  of the second layer, composed of silicon nitride, is detected at a wavenumber of approximately 850-750cm −1  and with an absorbance value at approximately 0.3. 
     During progression of the CMP process, the absorbance curve decreases as shown by curves B and C of FIG.  3 . At one point, peak  2  of the absorbance curve C does not show a significant decrease. At this point the CMP process is at its endpoint since significant change in peak  2  of the silicon oxide, which is signalled by a three-point decrease in the slope of peak  2 , is no longer observed and is followed by the subsequent decrease in the IR absorptivity and therefore the absorbance peak  1  of the silicon nitride of the second layer. A lack of significant change in the absorbance peak  2  of the silicon oxide in combination with a beginning decrease in the absorbance peak  1  of the silicon nitride, signifies the endpoint of the CMP process. Since peak  1  shows a marked decrease from curve B to curve C, overpolishing of the second layer has occurred. Therefore, the CMP endpoint is determined to be at a point between curves B and C. 
     However, instead of the first layer being composed of silicon oxide and the second layer being composed of silicon nitride, they can be reversed so that the first layer is composed of silicon nitride and the second layer is composed of silicon oxide. At this point the CMP process is at its endpoint when no significant change occurs in the IR absorbance peak of the silicon nitride layer in combination with a decrease in the IR absorbance peak of the silicon oxide layer. 
     As well, the method of the present invention can also be used in a shallow trench isolation (STI) CMP process to remove a dielectric layer composed of silicon oxide so as to expose a stop layer, composed of silicon nitride, directly underlying the dielectric layer. The endpoint of the STI CMP process is therefore determined when a lack of significant change in the IR absorbance peak of the dielectric layer occurs in combination with a decrease in the IR absorbance peak of the stop layer. 
     In contrast to the prior art, the present invention provides an effective and simplified method of determining the endpoint of the CMP process whereby IR absorptivity is used to detect the removal of a first layer of a semiconductor wafer without overpolishing the underlying second layer. As well, in the prior art, a first layer of a thickness greater than 3000 angstroms is required to determine the CMP endpoint since a thickness less than 3000 angstroms will not produce a trace curve of which is used to detect the CMP endpoint. However, the method of the present invention does not require a first layer of a specified thickness. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bound of the appended claims.