Abstract:
Disclosed is a method for improving HSS CMP performance. After performing a HSS CMP process for a predetermined time, DI water is introduced and the polishing process is continued, so that the CMP rate and performance can be improved.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for improving high selective slurry (HSS) chemical-mechanical polishing (CMP) performance, and more particularly, to a method of adding deionized water in a CMP process for improving the overall HSS CMP performance.  
         [0003]     2. Description of the Prior Art  
         [0004]     In the semiconductor industry, chemical mechanical polishing (CMP) is the most common and important planarization tool applied. For example, the CMP process can be used to remove a topographical target of a thin film layer on a semiconductor wafer and to produce a wafer with both a regular and planar surface. In a CMP process, slurry is provided in a surface subject to planarization, and a mechanical polishing process is performed on the surface of the wafer. The slurry includes chemical agents and abrasives. The chemical agents may be PH buffers, oxidants, surfactants or the like, and the abrasives may be silica, alumina, zirconium oxide, or the like. The chemical reactions evoked by the chemical agents and the abrasion between the wafer, the abrasives, and the polishing pad can planarize the surface of the wafer.  
         [0005]     In recent history, CMP processes have been widely adopted in shallow trench isolation (STI) processes. Please refer to  FIG. 1  to  FIG. 3 .  FIG. 1  to  FIG. 3  are perspective diagrams showing a shallow trench isolation fabrication according to the prior art. As shown in  FIG. 1 , a pad oxide layer  12  and silicon nitride (Si 3 N 4 ) layer  14  are disposed on a substrate  10 , in which the substrate  10  further comprises a shallow trench  16  and a silicon dioxide (SiO 2 ) layer  18 . The silicon dioxide layer  18 , being served as a dielectric layer, is formed by a process such as a chemical vapor deposition (CVD) process. Next, a CMP process is performed on the silicon dioxide layer  18  outside the shallow trench  16 , in which the CMP process is stopped at the silicon nitride layer  14 , as shown in  FIG. 2 . The silicon nitride layer  14  and the pad oxide layer  12  are then removed and under an ideal condition, a height differential ΔD can be observed between the silicon dioxide layer  18  inside the shallow trench  16  and the active region of the substrate surface in proximity. If the CMP process is able to planarize and remove the silicon dioxide layer  18  outside the shallow trench  16  evenly and stop the process at the silicon nitride layer  14 , a positive height differential ΔD can be obtained. Hence, the silicon dioxide layer  18  inside the shallow trench  18  is higher than the substrate surface of the active region and thereby effectively reduces electrical leakage.  
         [0006]     In order to obtain a positive height differential ΔD, the planarization and the polishing endpoint has always been a challenge in CMP processes. In general, the determination is based on various factors including the characteristics (compactness) of the silicon dioxide layer, the uniformity of the silicon dioxide surface, the composition and pH value of the slurry, the polishing pad composition, the platen rotation speed, and the head down force of the wafer head.  
         [0007]     In STI processes, in order to completely remove the silicon dioxide layer  18  outside the shallow trench  16  and prevent an over etching of the silicon nitride layer  14  thereby damaging the active region devices, the selectivity ratio between the silicon dioxide and the silicon nitride should be increased. In the past, a solution used by most industries is to replace the traditional silica abrasive alkaline solution with high selectivity slurry (HSS) for performing CMP processes. Recently, the HSS has been widely applied in STI CMP processes of 0.13 um technology node and beyond for fabricating devices with higher reliability.  
         [0008]     Despite the fact that traditional oxide slurry STI CMP processes can easily reach a polishing rate of over 3000 A/min, the polishing rate for STI CMP processes by using HSS however is significantly slower. Please refer to  FIG. 4 .  FIG. 4  is a curve diagram showing the relationship between the polishing rate and polishing time of a STI CMP process by using HSS according to the prior art. As shown in  FIG. 4 , the polishing rate of the process decreases as the polishing time increases and consequently, it becomes virtually impossible for the process to reach a polishing rate of 1500 A/min. Moreover, HSS STIP CMP processes may also cause numerous problems including slurry residues, SiO 2  residues on the Si 3 N 4  layer, microscratches on the surface of the wafer, and thickness limitation of the SiO 2  layer, and therefore the process window will be limited.  
         [0009]     According to U.S. Pat. No. 6,132,294, a method for improving CMP process by allowing the wafer to detach easily from the polishing pad thereby preventing damages or microscratches is disclosed. According to the method, the CMP process is first performed by using traditional slurry comprised of silicon dioxide or aluminum oxide. After the end of the CMP process, the injection of slurry is stopped and water is injected into the process instead. At the same time, the rotation speed of the polishing pad is also increased for allowing the wafer to successfully detach from the polishing pad. Nevertheless, information regarding to HSS CMP processes has not been stated according to this patent, and the influence of HSS on slow polishing rate in STI processes was also unaddressed. Hence, the improvement of HSS in the speed, performance of CMP processes, and process window has always been a challenge.  
       SUMMARY OF INVENTION  
       [0010]     It is therefore an objective of the present invention to provide a method for improving HSS CMP performance and solving the above-mentioned problems by introducing a deionized water polishing step in the later stage of each CMP process.  
         [0011]     According to the present invention, a method for improving HSS CMP performance comprises: providing a polishing pad and a wafer head, wherein the wafer head carries a wafer; applying a high selectivity slurry on the polishing pad and applying a head down force to the wafer head for performing a chemical mechanical polishing process by contacting the wafer to the polishing pad, wherein the polishing pad and the wafer head each include a polishing pad speed and a wafer head speed; and applying deionized water to the polishing pad for continuing with the CMP process.  
         [0012]     By introducing deionized water in the later stage of each CMP process, the method is able to effectively dilute the concentration of the HSS, increase the overall polishing rate of the CMP process, and at the same time, reduce microscratch damage on the wafer and improve defect control.  
         [0013]     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 that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0014]      FIG. 1  to  FIG. 3  are perspective diagrams showing a shallow trench isolation fabrication according to the prior art.  
         [0015]      FIG. 4  is a curve diagram showing the relationship between the polishing rate and polish time of a HSS STI CMP process by using a HSS according to the prior art.  
         [0016]      FIG. 5  to  FIG. 9  are perspective diagrams showing a method for improving HSS CMP performance according to the present invention.  
         [0017]      FIG. 10  is a curve diagram showing the relationship between the removal rate of the silicon dioxide layer and the polishing time after deionized water is added according to the present invention.  
         [0018]      FIG. 11  is a curve diagram showing the relationship between the removal rate of the silicon nitride and the polishing time after deionized water is added. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Please refer to  FIG. 5  to  FIG. 9 .  FIG. 5  to  FIG. 9  are perspective diagrams showing a method for improving HSS CMP performance according to the present invention. According to the preferred embodiment of the present invention, the CMP process is utilized in a STI fabrication process for removing the silicon dioxide layer outside the shallow trench. As shown in  FIG. 5 , a first polishing pad  50  is disposed on a first polishing platen  52  and a wafer head  54  is utilized for fixing a wafer  56  in place. Preferably, the wafer  56  is a semiconductor wafer comprising integrated circuits or other semiconductor devices. The wafer  56  is fixed in a detachable manner on the wafer head  54 .  
         [0020]     As shown in  FIG. 6 , a head down force F 1  is applied to the wafer head  54  for contacting the wafer  56  to the first polishing pad  50  disposed on the first polishing platen  52 . A first CMP process is then performed by injecting a first HSS  60  to the first polishing pad  50  via a slurry feed  58 . During the first CMP process, each of the wafer head  54  and the first polishing pad  50  generates a wafer head speed and a first polishing pad speed that rotates in separate directions according to an arrow A and an arrow B.  
         [0021]     As shown in  FIG. 7 , after the first CMP process is performed for a predetermined time, another water feed  62  is used for providing deionized water  64  to the first polishing pad  50 . Preferably, the first CMP process is continued for another 5-60 seconds before being stopped completely. The stopping of the first CMP process will then detach the wafer  56  from the first polishing pad  50 . Due to the fact that an increase in the wafer head speed and the first polishing pad speed will result in a decrease in the overall polishing rate, the wafer head speed and the first polishing pad speed of the present method are maintained at constant speeds during the first CMP process when the deionized water  64  is injected to ensure that the polishing rate of the first CMP process can be constantly increased.  
         [0022]     As shown in  FIG. 8 , a second polishing pad  66  is disposed on a second polishing platen  68  for starting a second CMP process. Alternatively, the second polishing pad  66  can be replaced by the first polishing pad  50  by first removing the wafer  56  from the surface of the first polishing pad  50  by using the wafer head  54 , cleaning the first polishing pad  50  by using a conditioner and deionized water, and replacing the clean first polishing pad  50  as the pad used in the second CMP process. In the second CMP process, a head down force F 2  is first applied to the wafer head  54  for contacting the wafer  56  to the second polishing pad  66  and at the same time, a second HSS  72  is injected to the second polishing pad  66  via a slurry feed  70 . Consequently, a polishing process is performed on the wafer  56  by using the wafer head  54  to generate a wafer head speed toward a direction A and using the second polishing pad  66  to generate a second polishing pad speed toward a direction C.  
         [0023]     Next, the injection of the second HSS  72  is stopped after the second CMP process has been performed for a predetermined time, such as 50-80 seconds. After the injection of the second HSS  72  is stopped, the second CMP process is continued for another 5-60 seconds while injecting the deionized water  64  via a water feed  74  before reaching a complete stop.  
         [0024]     According to the present invention, the first high selectivity slurry  60  and the second high selectivity slurry  72  is a ceric-base slurry or a zirconic-base slurry, in which the slurry may contain materials such as ceria (CeO 2 ) or zirconia (ZrO 2 ).  
         [0025]     In addition, the head down force F 1  or F 2  of the wafer head can be selectively decreased during the first or second CMP process according to fabrication demand. Moreover, a third or fourth polishing pad can also be provided to perform a third or fourth CMP process. In order to increase the polishing rate of the HSS, deionized water can be injected in the end stage of every CMP process to dilute the concentrate of the HSS for improving the overall CMP rate and performance.  
         [0026]     By injecting deionized water (despite whether HSS is continuously injected or not) in the end of each HSS STI CMP process, the present invention provides a method that is able to dilute and decrease the adhesiveness of HSS and reduce the amount of excess slurry remains, thereby increasing the overall polishing performance and preventing microscratches on the wafer surface. Please refer to  FIG. 10  and  FIG. 11 .  FIG. 10  is a curve diagram showing the relationship between the removal rate of the silicon dioxide layer and the polishing time after deionized water is added whereas  FIG. 11  is a curve diagram showing the relationship between the removal rate of the silicon nitride and the polishing time after deionized water is added. As shown in  FIG. 10 , the removal rate of the silicon dioxide layer increases rapidly after the addition of deionized water for approximately 10 seconds, indicating that the present invention is capable of effectively increasing the speed of the HSS STI CMP process. As shown in  FIG. 11 , the removal rate of the silicon nitride did not increase significantly after the addition of deionized water, hence the selectivity ratio between silicon dioxide and silicon nitride in the CMP process can be well maintained.  
         [0027]     In contrast to the prior art, the present invention discloses a method by adding deionized water in the later stage of each HSS CMP process. After the deionized water is added, the choice of adding additional HSS can be further decided according to the actual polishing requirement. By using the present method, the polishing rate of the CMP process to the oxide layer can be greatly increased, which will in turn increase the selectivity ratio between silicon dioxide and silicon nitride. Moreover, the present invention also provides a solution for improving slurry residues, microscratches on wafer surface, process window limitations, and the overall performance.  
         [0028]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and 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.