Patent Publication Number: US-6709312-B2

Title: Method and apparatus for monitoring a polishing condition of a surface of a wafer in a polishing process

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a method and apparatus for monitoring a polishing condition of a surface of a wafer in a polishing process. The present invention is particularly useful for determining an end-point in a chemical mechanical polishing (CMP) process. 
     BACKGROUND OF THE INVENTION 
     Chemical mechanical polishing (also referred to as chemical mechanical planarization) or CMP is a proven process in the manufacture of advanced integrated circuits. CMP is used in almost all stages of semiconductor device fabrication. Chemical mechanical planarization allows the creation of finer structures via local planarization and for global wafer planarization to produce high density structures. 
     During a CMP process, a substrate is mounted to a carrier or polishing head. The exposed surface of the substrate is moved against a rotating polishing pad on a polishing platen. A polishing slurry is distributed over the polishing pad. The slurry includes an abrasive and at least one chemically reactive agent. The abrasive chemical solution is provided at the interface between the polishing pad and the wafer in order to facilitate the polishing. 
     It is generally desirable to control the CMP process to find an endpoint for polishing or to determine the thickness of a polished layer. 
     One prior art attempt to control the CMP process uses pre and/or post measurements of wafers with either manual or automatic processing. Systems are available which allow measurement of the wafers immediately before and after polishing. If the film thickness before and after polishing is known, it is possible to adjust the polishing parameters and to optimize the polishing process within a production sequence. However, such a pre and/or post measurement method has the disadvantage that at least the first wafer or the first few wafers have to be polished with the default parameter settings, i.e. without optimized parameters. Typically, these first wafers are targeted to underpolish, such that subsequent repolishing can be done to achieve the specification range. 
     Several methods have been suggested to obtain a reliable endpoint for the polishing process. Current methods include measuring temperature, shaft friction, vibration, sonic level, or frequency. Unfortunately, these methods do not work for all substrates, particularly when an oxide is polished. A large number of CMP processes use timed polishing steps for specific films or wafers. These processes generally lead to a relatively wide range of results, as the variation of factors such as polish head condition, slurry refreshing, down force, or pressure cause the polishing rate to change during the processing of a large batch of wafers. 
     Since overpolishing of wafers is catastrophic and severe overpolish may result in destroyed wafers, wafers are typically targeted to underpolish, since an under-polish condition may be removed by reprocessing the wafers to bring them up to specification. However targeting for an underpolish often leads to a significant number of wafers that require repolishing, thereby lowering the throughput and increasing the overall processing costs. Further, the time for which the underpolished wafers need to be repolished is usually calculated manually, taking the removed film thickness, the target thickness and the wafer polish time into account. Repolishing thus requires significant human resources. 
     While for larger device dimensions the process target specifications tend to be rather relaxed, there are increasing requirements to tighten the film removal range as device technologies shrink. 
     In view of the above, the present invention seeks to solve the above mentioned problems and shortcomings of the prior art and intends to provide a method and an apparatus which allows for an improved determination of the endpoint in a polishing process. 
     It would further be advantageous to have a method for polishing wafers with increased throughput, improved process uniformity and reduced processing costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial schematic illustration of a chemical mechanical polishing apparatus according to an embodiment of the present invention; 
     FIG. 2 is a process flow diagram illustrating an embodiment of a method according to the invention; 
     FIG.  3 ( a ) shows a schematic illustration of an illuminated field of view of a wafer to be polished before the start of a CMP process; 
     FIG.  3 ( b ) shows an illustration of an optical contrast profile across the field of view of FIG.  3 ( a ) as output by the optical sensor; 
     FIGS.  4 ( a ) and ( b ) show illustrations as in FIGS.  3 ( a ) and ( b ) in a situation where the CMP process has advanced; 
     FIGS.  5 ( a ) and ( b ) show illustrations as in FIGS.  3 ( a ) and ( b ) at the desired endpoint of the CMP process; 
     FIGS.  6 ( a ) and ( b ) show illustrations as in FIGS.  3 ( a ) and ( b ) in a situation where the CMP process has missed the endpoint and the wafer is damaged. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     According to the present invention, a method for monitoring a polishing condition of a surface of a wafer in a polishing process is provided, the method comprising the steps of: providing a wafer  16  to be polished, the wafer  16  having at least one optically distinguishable feature  20  below a transparent or translucent layer  22  to be polished; selecting one or more of said features  20  for monitoring; measuring an optical contrast profile  62 ;  72 ;  82 ;  92  (FIGS. 3-6) across one or more of said selected features  20 ; determining the polishing condition of the surface of the wafer  16  on the basis of the measured contrast profile  62 ;  72 ;  82 ;  92 ; and repeating the steps of measuring the optical contrast profile  62 ;  72 ;  82 ;  92  and determining the polishing condition until a predetermined polishing condition is reached. 
     According to another aspect of the present invention, a method for polishing wafers by a chemical mechanical polishing tool is provided, the method comprising the steps of setting polishing parameters of a chemical mechanical polishing tool; polishing at least one wafer  16  and monitoring a polishing condition of a surface of the wafer  16  by providing the wafer  16  to the polishing tool, the wafer having at least one optically distinguishable feature  20  below a transparent or translucent layer  22  to be polished, selecting one or more of said features  20  for monitoring, measuring an optical contrast profile  62 ;  72 ;  82 ;  92  across one or more of said selected features  20 , determining the polishing condition of the surface of the wafer  16  on the basis of the measured contrast profile  62 ;  72 ;  82 ;  92 ; and repeating the steps of measuring the optical contrast profile  62 ;  72 ;  82 ;  92  and determining the polishing condition until a predetermined polishing condition is reached, and adjusting the polishing parameters of said polishing tool on the basis of the results of monitoring the polishing condition to improve process throughput and process uniformity. 
     According to a further aspect of the present invention, there is provided an apparatus for monitoring a polishing condition of a surface of a wafer  16  having at least one optically distinguishable feature  20  below a transparent or translucent layer  22  to be polished, the apparatus comprising: 
     means for providing a wafer  16  to be polished; 
     means for selecting one or more of said features  20  for monitoring; 
     means  24 ,  26 ,  28  for measuring an optical contrast profile  62 ;  72 ;  82 ;  92  across one or more of said selected features  20 ; 
     means  30  for determining the polishing condition of the surface on the basis of the measured contrast profile  62 ;  72 ;  82 ;  92 ; and 
     means  30 ,  32  for determining whether a predetermined polishing condition is reached. 
     These and other features and advantages will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. 
     With respect to FIG. 1, which shows a partial schematic illustration of a chemical mechanical polishing apparatus according to an embodiment of the present invention, a polishing platen  10  carries a polishing pad  12 . A window  14  is provided in the polishing platen  10  and the polishing pad  12 , which allows optical access to wafers located on the polishing pad  12 . Arranged underneath the window  14  is an x-y stage  24 , which carries a light source  26 , such as a light emitting diode (LED) and an optical sensor  28 . 
     When carrying out a CMP process, a wafer  16  is placed on the polishing pad  12 , a slurry is added, and one or both of the wafer  16  and the polishing platen  10  are rotated. 
     From previous processing steps in the integrated circuit manufacturing process, the wafer  16  may have a complicated topography built on the original silicon substrate  18 . In the context of the present invention, however, it is only relevant that the wafer has at least one optically distinguishable feature  20  with at least one sharp edge below the transparent or translucent layer  22  that needs to be polished. The feature  20  could be a device feature from within the die, or a feature in a test area such as the scribe grid. Layer  22  may, for example, be an oxide layer. 
     FIG. 1 also shows a control unit  30  and a control unit memory  32  whose function will become clear from the detailed explanation below. In the following, the method for monitoring a polishing condition of a surface of a wafer in a polishing process is described with reference to FIG. 2, and particularly to FIGS. 3 to  6 , which show schematic illustration of the illuminated field of view of the wafer  16  and optical contrast profiles obtained from the optical sensor  28  at various stages of the CMP process. 
     Starting at reference sign  40  in the process flow diagram of FIG. 2, a wafer  16  is provided to a CMP tool in step  42 . It is properly oriented and loaded on a polishing head such that one or more of the relevant features  20  are accessible to the light source  26 . Next, in step  44 , one or more of the features  20  are selected for monitoring. As mentioned, feature  20  may be a device feature from within the die, or a feature in a test area. It is not necessary that the same feature  20  is monitored throughout repeated measurements. If a plurality of identical or similar features  20  exist on the wafer  16 , it may be sufficient to measure a different subset of features  20  in each measuring step. For example, devices such as DRAMs with their regularly repeating structures work well with such a scheme. If only one or a few features  20  are available on the wafer  16 , the x-y stage  24  will generally have to be adjusted to bring the light source  26  and the sensor  28  in a suitable position. 
     The method then proceeds to step  46 , in which an optical contrast profile of the selected features  20  is measured. Sensor  28 , which may be of the kind typically used in lithography to detect alignment features, measures the contrast profile across a certain field of view, as illustrated in FIG.  3 . In this figure, ( a ) shows across an exemplary field of view containing three identical features  20 , covered by an oxide layer  22  to be polished. Prior to the polishing process the oxide layer  22  extends up to a height level  60 . 
     FIG.  3 ( b ) shows the optical contrast profile  62  obtained from the optical sensor  28  in this situation. Attention is directed particularly to the intensity level  64  at the top surface of the features  20 , which is relatively low, and the width or sharpness  66  at the dark edges of the features  20 . As the oxide layer  22  covering the features  20  is still rather thick, the width appears relatively large, corresponding to a low sharpness level. The control unit  30  obtains the intensity level  64  and the sharpness  66  from the optical contrast profile  62  and determines the polishing condition of the wafer surface based on these values in step  48 . 
     Proceeding to step  50 , the control unit  30  compares the determined intensity  64  and sharpness  66  to predetermined endpoint values, stored in a control unit memory  32 . If, as in the situation of FIG. 3, the result of the comparison indicates, that the endpoint for polishing has not been reached, the method returns to step  46 , where, after a predetermined polishing time has lapsed, another measurement of the optical contrast profile is carried out. 
     As the polishing process continues, an increasing part of the layer  22  is being removed. FIG.  4 ( a ) illustrates the situation after a certain polishing time showing a reduced height level  70 . As the oxide layer  22  becomes thinner, the intensity level  74  of the optical contrast profile  72  at the top surface of the features  20  increases. At the same time, the edge sharpness increases, i.e. the transitions at the edges become less wide and deepen in contrast (FIG.  4 ( b )). 
     FIGS.  5 ( a ) and ( b ) illustrate a situation corresponding to the desired endpoint of the polishing process, in which a layer  22  of certain thickness (height level  80 ) remains. Comparing the intensity  84  and the sharpness  86  of the optical contrast profile  82  at this to the predetermined values, the control unit  30  concludes that the endpoint has been reached and the method terminates at  52 . 
     The comparison of the determined intensity and sharpness values to the stored values may, for example, be carried out by adding the weighted difference between the stored and the determined intensity value, and the weighted difference between the stored and the determined width. Appropriate weight factors can be found experimentally. If the result of this calculation is zero or negative, the desired endpoint has been reached. Otherwise, the magnitude of the positive result indicates, how much the current polishing condition deviates from the desired polishing condition. 
     For the sake of illustration, FIG. 6 shows a situation, in which the endpoint of the polishing process has been missed and the wafer features  20  have been damaged. FIG.  6 ( a ) shows the height level  90  of the oxide layer  22  to be in part even lower than the top surface of the features  20 , which have themselves been partly removed. The corresponding optical contrast profile  92  shows an intensity level  94  and a sharpness  96  well beyond the predetermined endpoint values. The skilled person will appreciate that there is a sufficient margin around the exact endpoint of FIG. 5, in which the method can determine that the process endpoint has reached to prevent overpolishing to an extent that damages the wafer  16 . 
     Further, a correlation chart can be produced to continually calculate the rate at which the film is being removed in the current polishing period, and track the total film removal for the process. This can also be to estimate the additional time needed before the expected endpoint at the current polishing rate. 
     An analysis of this kind of data during processing of a batch of wafers may be used to provide information on the film removal rate variation from wafer to wafer. Also, a feedback loop may be advantageously established to make adjustments to the CMP equipment hardware settings to adjust the polish rate to maximize throughput, provide improved process uniformity, and reduce processing costs. The feedback loop can further monitor the effect of equipment factors on polish rate and provide information relating to equipment performance and slurry effectiveness. It may, for example, show that part of the equipment requires maintenance or detect a change in the composition of the slurry at a batch change. Additionally, statistical process control can be done using this kind of data, especially for the same film type and polish process, to provide equipment control and improve the overall wafer processing performance. 
     While the invention has been described in terms of particular structures, devices and methods, those of skill in the art will understand based on the description herein that it is not limited merely to such examples and that the full scope of the invention is properly determined by the claims that follow.