Patent Publication Number: US-6336846-B1

Title: Chemical-mechanical polishing apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for fabricating semiconductor devices, and more particularly, to a chemical-mechanical polishing (CMP) apparatus and method. 
     2. Background of the Related Art 
     As the integration of semiconductor devices increases, multi-level interconnection technology has been put into practical use. Accordingly, local and global planarization of interlayer dielectric films has become important. Currently, a widely used CMP method of polishing the surface of a semiconductor wafer employs chemical components contained in a slurry solution, mechanical components of a polishing pad, and a polishing agent. 
     A CMP apparatus is mainly used in polishing the front face of a semiconductor wafer while fabricating semiconductor devices on the wafer. In general, in order to make the surface of the wafer as flat as possible, the wafer is planarized or softened at least one time during the fabrication process. In order to polish the wafer, the wafer is placed on a carrier, put into contact with the polishing pad covered with slurry and then pressed. While polishing is carried out, both the polishing pad and the wafer-loaded carrier rotate. 
     After polishing is carried out, the carrier moves upward so that the wafer is completely separated from the polishing pad. In this case, deionized water remains between the wafer and the surface of the polishing pad. Due to the deionized water, a strong adsorptive force is produced on the contact surface between the wafer and the polishing pad. If the carrier is raised in the presence of the adsorptive force, the wafer may separate from the carrier and remain fixed on the polishing pad. In such an event, subsequent processes would not be performed, and damage to the wafer may be caused. 
     SUMMARY OF THE INVENTION 
     To solve the above problems, it is an object of the present invention to provide a chemical-mechanical polishing (CMP) apparatus and method, by which a wafer stays on the carrier when the wafer-loaded carrier separates from the polishing pad after polishing is completed. 
     Accordingly, the present invention provides a CMP apparatus having a polishing pad covered with slurry, and a polishing head fixed on a semiconductor wafer for holding the surface of the semiconductor wafer in contact with the surface of the polishing pad, wherein the polishing head includes a wafer carrier on which the semiconductor wafer is fixed, and a retainer ring formed along the wafer carrier so as to guide the edges of the semiconductor wafer. The retainer ring has an opening through which air is supplied to the lower portion thereof, so that air is injected between the semiconductor wafer and the polishing pad through the opening before separating the semiconductor wafer from the polishing pad after the polishing process. 
     It is preferred that the opening is close to the semiconductor wafer and the opening is connected to an air injection opening formed in a shaft for supporting the polishing head and the rotary shaft of the polishing head. Also, the opening may vertically penetrate the inside of the retainer ring. 
     Also, the CMP apparatus may further include a sensor for sensing whether the semiconductor wafer is adhered to the wafer carrier after separating the wafer carrier from the polishing pad. 
     The present invention further provides a CMP method in which the polishing head moves upward so that the semiconductor wafer is separated from the polishing pad after polishing is complete, while the adsorptive force between the semiconductor wafer and the polishing pad is reduced by injecting air therebetween through the opening. 
     Here, the CMP method may further include sensing whether or not the semiconductor wafer is adhered to the polishing head after the polishing head is separated from the polishing pad, and displaying the sensing result to a user. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is a cross-sectional view illustrating a polishing head of a CMP apparatus according to the present invention; 
     FIG. 2A is a top view of a retainer ring of the CMP apparatus of FIG. 1; 
     FIG. 2B is a cross-sectional view taken along the line  2 B— 2 B in FIG. 2A; 
     FIG. 3 is a bottom view of the retainer ring of FIG. 2A; and 
     FIG. 4 is a flow chart illustrating a CMP method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described below in detail with reference to the attached drawings. However, the present invention may be embodied in may different forms and should not be construed as limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     Referring to FIG. 1, the CMP apparatus includes a polishing pad  100  and a polishing head  200 . The polishing pad  100  is mounted on a polishing platen  110  which rotates about a shaft and is driven by a driving motor (not shown). The polishing pad  100  rotates with the polishing platen  110 . While the polishing process is carried out, a slurry comprising a chemical solution and polishing particles is supplied onto the surface of the polishing pad  100 . 
     The polishing head  200  allows the surface of a semiconductor wafer  400  to contact the surface of the polishing pad  100  while the polishing process is carried out. During the polishing process, the polishing head  200  rotates about a rotary shaft. A motor  201  is fixed on a shaft  202  and the polishing head  200  rotates around the shaft  202  driven by the motor  201 . 
     A plurality of air pressure lines  203  are inserted into the shaft  202 . Air is supplied or exhausted through the air pressure lines  203 . The shaft  202  is connected to an external air supply duct  204 . In the air supply duct  204 , a valve  205  controls the supply or exhaustion of air and a gauge  206  measures the amount of air supplied to or exhausted from the air supply duct  204 . Since the shaft  202  rotates, the air supply duct  204  is not directly connected to the shaft  202  but is connected thereto through a rotary unit  207 . The rotary unit  207  surrounds the shaft  202  and includes a rotary portion which rotates with the shaft  202  and a stationary portion which is fixed even when the shaft  202  rotates. An opening used as passage of air is formed between the stationary portion and the rotary portion. The external air supply duct  204  is inserted into the stationary portion so that the air supplied through the air supply duct  204  moves from the stationary portion to the rotary portion through the opening to then be supplied to the air pressure lines  203  in the shaft  202 . The rotary portion of the rotary unit  207  is fixed with the side wall of the shaft  202  by O-rings  208 . 
     The shaft  202  is connected to a manifold  209  made of steel. Connection lines  210  corresponding to the plurality of air pressure lines  203  in the shaft  202  are inserted into the manifold  209 . The respective air pressure lines  203  in the shaft  202  are connected to the respective connection lines  210  in the manifold  209  one by one so that the air supplied through the respective air pressure lines  203  is transmitted through the corresponding connection lines  210 . Since the manifold  209  is fixed on the shaft  202 , it rotates as the shaft  202  rotates. 
     A wafer carrier  211  is disposed under the manifold  209  with first and second clamps  212  and  213  disposed therebetween. The first clamp  212  is fixed on the manifold  209  at its cental part, and the second clamp  213  is fixed on the manifold  209  at its edge. A rolling diaphragm  214  made of an elastic material is arranged in the space defined by the manifold  209 , the wafer carrier  211 , the first clamp  212  and the second clamp  213 . The rolling diaphragm  214  downwardly moves various devices disposed thereunder or restores them into their original locations by its expansion or contraction due to air supply or exhaustion. 
     The wafer carrier  211  is covered with a thin elastic membrane  215  which contacts the semiconductor wafer  400 , and a perforated plate  216 , having a plurality of through holes, is disposed thereon. The perforated plate  216  is fixed by third clamps  217  disposed at its edges. A flexure  218  fixed on a fourth clamp  219  at its one end is disposed above each third clamp  217 . The other end of the flexure  218  is fixed on the wafer carrier  211 . A ceramic plate  220  is disposed above and spaced a predetermined distance apart from the perforated plate  216 . A first pipe  221  is inserted into and penetrating the ceramic plate  220 . One end of the first pipe  221  is inserted into the connection line  210  of the manifold  209  and is movable vertically. 
     There is provided a chamber  222  defined by the perforated plate  216 , the third clamp  217  and the ceramic plate  220 . The pressure in the chamber  222  can be adjusted by supplying or exhausting air passing through the first pipe  221  penetrating the ceramic plate  220 . The portion of the wafer carrier  211  facing the fourth clamp  219  has a round groove, and an extensible/contractible cylindrical tube  223  is fixed in a hermetical space between the wafer carrier  211  and the fourth clamp  219  along the groove. The expansion/contraction due to supplying air into or exhausting air from the cylindrical tube  223  downwardly moves the devices disposed thereunder or restores them into their original locations. 
     A retainer ring  300  is fixed along the edges of the lower portion of the wafer carrier  211  in an annulate shape so as to guide the circumferential edges of the semiconductor wafer  400 . During the polishing process, an appropriate pressure is applied to the polishing pad  100  to improve the polishing profile. An opening  321  through which air is supplied to the lower portion of the retainer ring  300  is formed to penetrate the inside of the retainer ring  300 . The air is injected between the semiconductor wafer  400  and the polishing pad  100  through the opening  321  before the semiconductor wafer  400  is separated from the polishing pad  100  after the polishing process. 
     Referring to FIGS. 2A and 2B, when viewed from above, the retainer ring  300  is divided into two parts having different heights. A higher part  310  has a thread groove  311  into which a thread can be inserted, and a lower part  320  has an opening  321  through which air can pass. When viewed from below as in FIG. 3, a rectangular groove  330  is formed around the opening  321  of the retainer ring  300 . 
     Referring back to FIG. 1, the opening  321  formed in the retainer ring  300  is for injecting air between the semiconductor wafer  400  and the polishing pad  100  before separating the semiconductor wafer  400  from the polishing pad  100  after the polishing process. To this end, the opening  321  is connected to an external air injection unit. For example, as shown in the drawing, the opening  321  penetrates the wafer carrier  211 , the manifold  209  and the shaft  202 , to then be connected to the air injection duct  204 . As stated above, the air exhausted through the opening  321  is injected between the semiconductor wafer  400  and the polishing pad  100  before separating the semiconductor wafer  400  from the polishing pad  100  after the polishing process. Since the opening  321  is formed adjacent to the circumferential edge of the semiconductor wafer  400 , most of the air exhausted through the opening  321  is injected between the semiconductor wafer  400  and the polishing pad  100 . As described above, injecting air between the semiconductor wafer  400  and the polishing pad  100  weakens the adsorptive force produced between the semiconductor wafer  400  and the polishing pad  100 , thereby easily separating the semiconductor wafer  400  from the polishing pad  100  when the wafer carrier  211  moves away from the polishing pad  100 . 
     A sensor  500  for sensing whether or not the semiconductor wafer  400  has been separated from the polishing pad  100  after separating the semiconductor wafer  400  may be provided, preferably in close proximity to the wafer  400  and polishing pad  100 . The sensor senses the state of the surface of the polishing pad  100  or the surface of the thin elastic membrane  215 . 
     FIG. 4 is a flow chart illustrating a CMP method employing the above-described CMP apparatus. Referring to FIGS. 1 and 4, the CMP method according to the present invention will now be described. 
     First, in order to perform the polishing process, the semiconductor wafer  400  to be polished is seated on a wafer loader (not shown in FIGS. 1-3) positioned at a predetermined location. The wafer loader is disposed under the elastic membrane  215  of the polishing head  200  and spaced a predetermined distance therefrom. A robot (not shown in FIGS. 1-3) is typically used as means for carrying the semiconductor wafer  400  to the wafer loader. If the semiconductor wafer  400  is carried to the wafer loader, air is injected into the rolling diaphragm  214  of the polishing head  200 , so that the rolling diaphragm  214  expands. Accordingly, the retainer ring  300  descends toward the wafer loader such that the retainer ring surrounds the semiconductor wafer  400  and the semiconductor wafer  400  contacts the elastic membrane  215 . 
     Subsequently, in order to adsorb the semiconductor wafer  400  into the elastic membrane  215 , a vacuum state is created in the chamber  222 . Then, the elastic membrane  215  is adsorbed into the through hole of the perforated plate  216 . Accordingly, the semiconductor wafer  400  is adsorbed into the portion of the adsorbed elastic membrane  215 . In such a state, if the wafer loader is moved downwardly, the semiconductor wafer  400  is retained in a fixed position on the surface of the elastic membrane  215 . Next, the rolling diaphragm  214  is contracted to raise the retainer ring  300 . 
     Subsequently, the polishing head  200  on which the semiconductor wafer  400  is fixed is moved onto the polishing pad  100 . While the retainer ring  300  is moved downwardly, the motor  201  is operated to rotate the polishing head  200 . The polishing head  100  is also rotated by a motor connected to itself. At the almost same time as the rotation of the polishing head  200 , the cylindrical tube  223  is dilated to press the semiconductor wafer  400  down into contact with the surface of the polishing pad  100 . Then, the vacuum state of the chamber  222  is canceled to remove the adsorption state between the semiconductor wafer  400  and the elastic membrane  215  so that the semiconductor wafer  400  is made to contact the surface of the polishing pad  100  and the elastic membrane  215 . In this state, slurry is supplied onto the surface of the polishing pad  100  to perform the polishing process (step  410 ). 
     After the polishing process is completed, air is exhausted through the opening  321  in the retainer ring  300  (step  420 ). The air exhausted through the opening  321  is supplied to the polishing pad  100 . In particular, most of the air is supplied between the semiconductor wafer  400  and the polishing pad  100  so as to weaken the adsorptive force between the semiconductor wafer  400  and the polishing pad  100  which was created during the polishing process. Subsequently, a vacuum state is created in the chamber  222  so that the semiconductor wafer  300  is adsorbed into the elastic membrane  215 . Here, since the adsorptive force between the semiconductor wafer  400  and the polishing pad  100  is considerably weakened, the semiconductor wafer  400  is easily adsorbed into the elastic membrane  215 . 
     Then, the retainer ring  300  is raised until the semiconductor wafer  400  is spaced apart a predetermined distance from the surface of the polishing pad  100  (step  430 ). Next, the sensor is operated to sense the separation state of the semiconductor wafer  400  from the polishing pad  100  (step  440 ). If the semiconductor wafer  400  is separated from the polishing pad  100  and adhered to the elastic membrane  215 , subsequent processing steps are performed. However, if the semiconductor wafer  400  is still adhered to the polishing pad  100  without being separated therefrom, a warning message is displayed to a user and the user&#39;s further instruction is awaited (step  450 ). 
     As described above, according to the present invention, a semiconductor wafer is successfully separated from a polishing pad in order to transfer the semiconductor wafer to another processing stage after the polishing process is completed, since the separation is made after the adsorptive force therebetween has been considerably reduced. Therefore, the semiconductor wafer can be easily separated from the polishing pad, together with a polishing head.