Abstract:
A carrier device is adapted for use in a polishing apparatus with a turntable having a polishing surface. The carrier device includes a carrier for carrying an article to be polished and a control device operatively associated with the carrier. The control device includes an actuator operable to cause the carrier to urge the article against the polishing surace of the turntable to polish the article, a sensor operatively associated with the actuator and operable to sense a pressure as applied to the article when the article is urged against the polishing surface of the turntable and a control unit operatively associated with the actuator and the sensor so as to monitor the pressure during a polishing operation. The control unit is operable to control operation of the actuator in response to the pressure as monitored so as to keep the pressure at a target level and halt the polishing operation when the pressure is deviated from a predetermined range over a predetermined period of time.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to polishing apparatus for polishing a semiconductor wafer or other articles, and in particular, to a carrier device adapted for use in such polishing apparatus and designed to controllably carry an object with the object held in engagement with the polishing surface of a turntable during a polishing process. 
     FIG. 5 shows a typical semiconductor wafer polishing apparatus wherein a polishing process, commonly known as “chemical-mechanical planarization” (CMP), is carried out to finely polish a semiconductor wafer. The polishing apparatus generally includes a turntable  200 , a wafer carrier device  210  and a polishing surface dressing device  220 . 
     The turntable  200  is provided on its upper surface with a polishing pad made of polyurethane foam, artificial leather or the like or a fixed abrasive plate and is connected to a drive shaft  201 . The drive shaft is in turn connected to a motor (not shown) so that the turntable  200  is turned around its center axis. The wafer carrier device  210  includes a wafer carrier  211  in the shape of a disc and a motor  212  for ratating the wafer carrier  211  around its center axis through a transmission with a drive shaft  215  connected to the wafer carrier. The wafer carrier  211  is fluidly connected to a vacuum source (not shown) through a pipe  217 . The pipe is connected to the drive shaft to positively hold a semiconductor wafer against the bottom surface of the wafer carrier  211  by vacuum. The polishing surface dressing device  220  includes a dressing tool  221  and a motor  222  for rotating the dressing tool  221  around its center axis. The wafer carrier device  210  and the dressing device  220  are mounted on pivot shafts  231  and  241 , respectively, so that the wafer carrier  211  and the dressing tool  221  are brought into and out of engagement with the polishing pad of the turntable  200 . Further, the wafer carrier  211  and the dressing tool  221  are both moved in a vertical direction by actuators or air cylinders  252  and  272 . 
     During operation, the wafer carrier holds a semiconductor wafer on its bottom surface and brings it into engagement with the polishing pad on the rotating turntable  200  while being rotated around its center axis. After a desired numberof wafers are successively subjected to the polishing operation, the wafer carrier  211  is pivoted and brought out of engagement with the turntable and the dressing device  220  is pivoted to bring the dressing tool  221  into contact with the polishing pad on the turntable for conducting a dressing operation with respect to the polishing pad. 
     In such a polishing apparatus, there is a possibility that a crack may be created in a wafer which is being polished. If the polishing operation is continued in such a condition, the crack may become bigger so that the wafer carrier and/or the turntable may be eventually damaged. Further, a pressure controller may malfunction, whereby an inappropriate pressure such as an excessive pressure or insufficient pressure may be imposed on a wafer. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a carrier device with a control device for positively detecting such an anomalous situation in a polishing operation to make it posible to take a necessary action to cope with the anomalous situation. 
     In accordance with this invention, there is provided a carrier device adapted for use in a polishing apparatus with a turntable having a polishign surface. The carrier device comprises a carrier for carrying an article to be polished and a control device operatively associated with the carrier. The control device includes an actuator operable to cause the carrier to urge the article against the polishing surface of the turntable to polish the article, a sensor operatively associated with the actuator and operable to sense a pressure as applied to the article when the article is urged against the polishing surface of the turntable and a control unit operatively associated with the actuator and the sensor to monitor the pressure during a polishing operation. The control uhit is operable to control operation of the actuator in response to the pressure as monitored so as to keep the pressure at a target level and halt the polishing operation when the pressure deviated from a predetermined range over a predetermined period of the time. 
     Specifically, the carrier device further comprises a casing for supporting the carrier, the sensor being disposed between the casing and the acutator so as to sense the pressure as a reactive force developed when the actuator causes the carrier to urge the article against the polishing surface of the turntable. 
     The control unit may be adapted to halt the polishing operation after the current polishing operation for the article is completed if the pressure exceeds a first threshold pressure level over a predetermined period of time, and to immediately halt the polishing operation if the pressure exceeds a second threshold pressure level over a predetermined period of time, the second threshold pressure level being greater than the first threshold pressure level. 
     Further, The control unit includes a memory for storing the target value, the first threshold pressure level, and the second threshold presure level. 
     Furthermore, the control unit may be adapted to halt the polishing operation if the pressure drops below a first threshold pressure level and thereafter exceeds a second threshold presure level, the target level being less than the second threshold pressure level and greater than the first threshold pressure level. 
     The present invention further provides a method for controlling a carrier deice in a polishing apparatus which has a turntable having a polishing surface, the carrier device comprising a carrier for carrying an article to be polished and an actuator operable to cause the carrier to press the article against the polishing surface of the turn table. The method includes monitoring a pressure as applied to the article while the article is urged against the polishing surface of the turntable, regulating the actuator in response to the pressure as monitored so as to keep the pressure at a target level, and halting a polishing operation when the pressure deviates from the predetermined range over a predetermined period of time. 
     In another method, if the pressure exceeds a first threshold pressure level over a predetermined period of time, a polishing operation is halted after the current polishing operation for the article is completed, and if the pressure exceeds a second threshold pressure level greater than the first threshold pressure level over a predetermined period of time, the current polishing operation is immediately halted. 
     In another method, if the pressure drops below a first threshold pressure level and thereafter exceeds a second threshold pressure level greater than the first threshold level, a polishing operation is immediately halted. 
     The above and other features and advantages of the present invention will become apparent from the following description and the appended claims taken in conjunction with the accompaning drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a polishing apparatus employing a control device in accordance with this invention; 
     FIG. 2 is a schematic diagram showing a control unit employed in the control device shown in FIG. 1; 
     FIG. 3 is a flow diagram representative of a computer program executed for performing functions of the control unit; 
     FIG. 4 is a diagram showing various types of change in a presure under which an article to be polished is urged against a polishing surface in the polishing apparatus; and 
     FIG. 5 is a perspective view of a typical polishing apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a wafer carrier device  10  in accordance with an embodiment of the present invention which is employed in a semiconductor wafer polishing apparatus. The wafer carrier device  10  includes a wafer carrier  11  in the shape of a disk, a carrier drive shaft  30  connected centrally to the upper side of the wafer carrier, a motor  20  mounted on casing  50  and drivingly connected to the carrier drive shaft  30 , and an actuator or air cylinder  52  for moving vertically the carrier drive shaft  30  through the casing  50 . The actuator  52  is mounted on the casing  50  and has a piston rod  53 . A load cell or pressure sensor is disposed between the casing and the lower end of the piston rod. The bottom surface of thewafer carrier is adapted to hold a wafer. The actuator imposes a pressing force on the wafer carrier through the drive shaft to press the wafer against the polishing pad. As an important feature, the wafer carrier device includes a controller for controlling the pressing force. 
     The motor  20  is drivingly connected to a speed reducer  22  having an output shaft on which a pulley  23  is provided. 
     The carrier drive shaft  30  is hollow. The shaft is provided on its outer surface with a spline bushing  31  and a liner bushing  32  around which a sleeve  33  is mounted. The sleeve  33  is supported in the casing  50  by upper and lower bearings  37  and  38  so that the carrier drive shaft  30  is rotatable about its axis. An endless belt  25  is trained around the pulley  23  and the sleeve  33 . 
     The spline bushing  31  is engaged with a spline groove  34  formed on the outer surface of the shaft  30  through a bearing (not shown) so that the spline bushing  31  and the carrier drive shaft  30  are rotated together by means of the endless belt  25  driven by the motor  20  while they are axially movable relative to each other. The linear bushing  32  rotatably supports the carrier drive shaft  30  with the drive shaft allowed to axially move relative to the linear bushing. 
     The wafer carrier  11  is fluidly communicated with a vacuum source (not shown) through the hollow carrier shaft  30  and a pipe  55  connected to the upper end of the hollow carrier drive shaft  30 . This enables the wafer carrier to hold a semiconductor wafer against the bottom surface of the carrier by vacuum. 
     The carrier drive shaft  30  is provided at its upper end with a bracket  51  having a cylindrical support sleeve  51 ′ which is rotatably mounted on the upper end of the shaft. The bracket  51  is acially movable with the shaft. The bracket  51  supports the actuator or air cylinder device  52 . The casing  50  is fixedly mounted on a pivotal shaft  60  corresponding to the pivotal shaft  231  shwon in FIG.  5 . 
     The air cylinder  52  is fluidly connected to an air supply system through air conduits  83 - 1  and  83 - 2  which are fluidly communicated with lower and upper ends of the air cylinder  52 . The air supply system is controlled by control  80 . 
     The interior of the air cylinder  52  is divided into upper and lower chambers by a piston (not shown) provided at a top end of the piston rod  53 . The air cylinder  52  is moved down when a pressurized air is introduced into the lower chamber through the conduit  83 - 1  and moved up when a pressurized air is introduced into the upper chamber in the air cylinder through the conduit  83 - 2 . 
     The conduit  83 - 1  is fluidly connected to an air source (not shown) through is solenoid valve A having two positions, i.e., an air supply position and a vent position, a pressure controller  81  for controlling pressurized air supplied to the air cylinder  52 , and a pressure relief valve B. The conduit  83 - 2  is fluidly connected to an air source (not shwon) through 
     a solenoid valve  84 - 1  having two positions, i.e., an air supply position and a vent position and a pressure reduction valve  84 - 2 . 
     The control  80  includes a control unit  85  which, as shown in FIG. 2, comprises a CPU  85 - 1 , a memory  85 - 2  including a ROM and a RAM and an input/output ports  85 - 3 . The control unit  85  is adapted to receive signals representatie of loads sensed by load cell  75  and signals from an external input device  87  such as a key board and to output control signals to the pressure controller  81 , the solenoid valves A and  84 - 1 , an alarm  89  and other elements which are to be controlled by the control unit. 
     In a polishing operation, the wafer carrier device is first pivoted about the pivot shaft  60  to move the wafer carrier  11  to a position outside the truntable, and a first wafer is placed on the bottom surface of the carrier and held thereon by means of a vacuum force applied to the carrier. Then, the wafer carrier device is pivoted about pivot shaft  60  to bring the wafer carrier  11  to a predetermined position above the turntable. Thereafter, the solenoid valve  84 - 1  is moved to its vent position to open the conduit  83 - 2  to the atmosphere and the solenoid valve A is moved to its air supply position to supply a pressurized air, the pressure of which is controlled by the pressure controller  81 , into the lower chamber in the air cylinder  52 , whereby the cylinder  52  is lowered relative to the piston-rod  53  (thus, the casing  50 ) so that the carrier drive shaft  30  and the wafer carrier  11  are lowered thereby urging the wafer against the polishing surace of the turning turntable for polishing the first wafer. During the polishing operation, the turntable and the wafer carrier are rotated about their own center axes. When the first wafer has been completely polished, the wafer carrier device  10  is moved up and pivoted outside the turntable while halting the rotation of the wafer carrier. Thereafter, the polished first wafer is replaced by a second or new wafer. The second and following wafers are successively polished in the same way. 
     During the polishing, a pressure under which the wafer is urged against the polishing pad on the turntable is kept substantially constant by controlling the pressure controller  81  by means of the control unit  85 . This control is herein referred to as “pressure control”. 
     There is a possibility that a crack may occur in a wafer during the polishing operation, or the pressure controller  81  may malfunction. In this embodiment, the control unit  85  determines whether such an anomalous or critical situation has occurred on the basis of a change in a pressure for urging a wafer aainst the polishing pad so that an appropriate action is taken as is necessary. This is herein referred as “failure detection/control”. 
     FIG. 3 shows a flow diagram representative of a computer program executed by the control unit  85  for carrying out the “pressure control” and the “failure detection/control” in this embodiment during the above-stated polishing operation. 
     Before starting the polishing operation and thus the control operation by the control unit  85 , as operator inputs through the outside input device  87  various instructions or the information on various conditions with respect to the “pressure control” and the “failure detection/control” and stores them into the RAM of the memory  85 - 2 . The instructions include a desired value L 1  of magnitude of a pressure L at which a wafer carried by the wafer carrier  11  should be urged against the polishing pad on the turntable, first and second upper threshold values P 2  and P 1  and a lower threshold value P 3  of the pressure L. The relationship between these values if P 1 &gt;P 2 &gt;L 1 &gt;P 3  and it is set that when a pressure or load L sensed by the load cell  75  falls within a range of from P 3  to P 2 , the sensed pressure L is in a normal condition. Those values may be set in accordance with conditions of a polishing operation to be performed. Further, the instructions include C 1  which is a reference number to be compared with CA, CB, CC which will be explained herein below. 
     Referring to FIGS. 3 and 4, the control unit  85  will be explained. FIG. 4 shows various types of change (a)-(i) in magnitude of value of the force or pressure L sensed by the load cell  75 . As shown, in the flow chart of FIG. 3, before entering into the main routine of the program, CA, CB and CC are initialized to be “1”, steps  121 ,  122  and  123 , in addition to initialization for setting timers, registers and so on which are not designated in the drawings. 
     Normal Polishing Operation: 
     Change (a): 
     This change shows that a polishing operation performed by the polishing apparatus is in a normal condition so that a magnitude of the value of the sensed load L is kept within a range between P 2  and P 3 . 
     Accordingly, in FIG. 3, a process consisting of steps  124 ,  127 ,  130  and  140  is repeated, whereby the “pressure control” is executed all the time during the normal polishing operation to keep the force L to be L 1 . 
     Changes (b), (d) and (g): 
     Even if the force L becomes, as shown in (b), (d) and (g) in FIG. 4, larger than P 1  or P 2  or smaller than P 3  for a very short time, if the force thereafter returns to a normal level in the range P 2 -P 3 , such a change may be an error in measurement caused by, for instance, some minor noise and, thus, in this embodiment, the polishing operation is continued under the “pressure control”. 
     In the case of (b) in FIG. 4, when the sensed load L is first determined to be larger than P 1  &lt;step  124 &gt;, CA (i.e., a number of affirmative (i.e., Yes) determinations in step  124  which has been successively performed) is compared with a predetermined number C 1 . Since CA is initially set to be “1”, CA is incremented by “1” &lt; 126 &gt;. Thereafter, the sensed load L is determined to be larger than the second upper threshold P 2  &lt;step  127 &gt; and, then, CB (i.e., a number of affirmative (i.e., Yes) determinations in step  127  which has been successively performed) is compared with C 1  &lt; 128 &gt;. Since CB is initially set to be “1”, CB is incremented by “1” &lt; 129 &gt;. The process consequently goes to step  140  where the “pressure control” is continued and then returns to step  124  to repeatedly execute the same process, whereby CA and CB are both incremented for each repeated execution of the program. In the case of the change (b), since it is assumed that the magnitude value ofthe sensed load L returns to a normal level between P 2  and P 3  before either CA or CB becomes equal to C 1 , the program returns to the main loop which was explained above in connection with the change (a) in FIG.  4 . 
     In the change (d), a process of the successive steps  124 ,  127 ,  128 ,  129  and  140  is repeated whereby the polishing operation by the polishing apparatus is continued under the “pressure control” executed in step  140 . Before CB is incremented to be equal to C 1 , the sensed load L returns in a normal condition in which the value of the load is within a range of P 2 -P 3 , whereby the process executed by the control unit  85  returns to the main loop which was explained above in connection with the change (a) in FIG.  4 . 
     In the change (g), when the value of magnitude of the sensed load decreased below P 3  is sensed, the process consisting of the successive steps  124 ,  127 ,  130 ,  131 ,  132  and  140  is repeated, whereby the polishing operation is continued under the “pressure control”. Before CC (i.e., a number of affirmative (i.e., Yes) determinations in step  130  which have been successively performed) eventually becomes equal to C 1 , the sensed load L falls with the range between P 2  and P 3  and then the process returns to the main loop as stated above. 
     Polishing Operation under Anomalous Situation: 
     Changes (c), (e), (f) and (h) show that anomalous situations, for instance, a situation wherein a crack is formed in a wafer or the air cylinder deivce  52  malfunctions, have occurred, so that the value of magnitude of a pressure sensed by the load cell  75  goes outside of the range P 2 -P 3  and is maintained over a predetermined period. 
     Change (c): 
     In this change, the sensed load L increases over P 1  and continues over a predetermined time period and, consequently, it is necessary to stop the polishing operation immediately. 
     With reference to FIG. 3, in this case, a process consisting of a series of the steps  124 ,  125 ,  126 ,  127 ,  128 ,  129  and  140  is repeated a number of times. Although the process goes through step  140 , the value of magnitude of the sensed load remains above P 1  for a predetermined time so that CA is finally incremented to be equal to C 1 , and then the process proceeds to the step  133  and step  134  whereby an alarm is given and simultaneously the wafer carrier  11  is lifted up and the rotation of the wafer carrier is stopped. 
     Change (e): 
     The change shows that an anomalous situation which is not so critical such as, for instance, one in which [a creation of ] a crack in a wafer has occurred so that a sensed load L has increased to have a value of magnitude within the range P 2 -P 1  and been maintained at such a value over a predetermined time period. In this embodiment, the polishing operation is not stopped instantly. Instead, after finalization of the polishing operation with respect to a wafer being subjected to the poishing operation, the wafer carrier is lifted away from the turntable and then the rotation of the wafer carrier is halted for a while to enable an operator to check the apparatus and, if necessary, make some repairs before apolishing operation for a next new wafer is started. 
     In this case, a process consisting of a series of the steps  124 ,  125 ,  126 ,  127 ,  128 ,  129  and  140  is repeated a number of times and step  128  finally determines CB=C 1 ; then the process proceeds to the step  135  which executes a program for lifting the wafer carrier and, then, stopping the rotation of the wafer after completion of the polishing of the wafer which has been subjected to the polishing operation. 
     Change (f): 
     In this change, although the value of magnitude of a sensed load L increases over P 1  for very short time periods, it generally remains within the range P 2 -P 1 . In this embodiment, this change is supposed to show a situation which is anomalous but not so critical as, for instance, the formation of a crack in a wafer and, thus, the current poishing operation is continued until it is finalized and, thereafter, the operation of the wafer carrier device is halted. 
     In this case, a process consisting of a series of the steps  124 ,  125 ,  126 ,  127 ,  128 ,  129  and  140  and a process consisting of a series of the steps  124 ,  127 ,  128 ,  129  and  140  are alternatively executed, whereby CB is incremented for each execution of those process and eventually, in the step  128 , is determined to be equal to C 1  so that the process proceeds to the step  135  which executes a program for halting the operation of the wafer carrier device after the current polishing operation is completed. 
     Change (h): 
     This change shows that some trouble has occurred in the pressure controller  81  which causes an anomalous decrease in the force for urging a wafer supported by the wafer carrier  11  against the polishing pad on the turntable so that the value of magnitude of a load sensed by the load cell  75  decreases below P 3  for a long time over a predetermined time. In this embodiment, when such a change occurs, the current polishing operation for a wafer is continued until it is finalized and then the operation of the wafer carrier device is halted to enable an operator to check the apparatus for example by removing the polished wafer from the wafer carrier and, if necessary, repairing the apparatus. 
     With reference to FIG. 3, the process consisting of the steps  124 ,  127 ,  130 - 132  and  140  is repeated so that CC is incremented by “1” for each repeated processes and finally CC becomes equal to C 1  whereby the process eventually proceeds to the step  135 . 
     Change (i): 
     In this change, the value of magnitude of a load sensed by theload cell  75  first decreases below P 3  and then increases above P 2  and finally returns to normal. This change demonstrates that a wafer which was subjected to a polishing operation has separated from the wafer carrier  11 . Namely, the decrease in the value L to a value below P 3  demonstrates that the wafer has been thrown away from the wafer carrier  11  so that the wafer carrier has lost an object to press against the polishing pad on the turntable thereby causing a rapid decrease in the value L. The following increase in the value L to a value above P 2  demonstrates that the wafer carrier  11  which has lost the wafer was abruptly pressed against the polishing pad on the turntable by the air cylinder  52 . The following return of the value L to a normal level, i.e., P 2 &gt;L&gt;P 3  shows that the “pressure control” is continued in step  140  thereby lowering the force for pressing the wafer carrier toward the polishing pad by means of the air cylinder  52 . 
     In this embodiment, a wave showing a change in the value L such as that designated by (i) is pre-stored in the memory  85 - 2  of the control unit  85  and, when the control unit  85  detects an occurrenceof such a wave-like change in the value L, it determines that a wafer which was held by the wafer carrier has separated from the carrier and the wafer carrier should be lifted up immediately. 
     Although in FIG. 1 there is shown load cell  75  as a sensor for sensing a pressure under which the wafer carrier is urged against he polishing pad on the turntable, such a sensor is not limited to the load cell  75 . However, it is preferable to position such a sensor between the actuator or air cylinder  52  and the casing  50  so as to enable the sensor to sense a force which is actually imposed on the wafer carrier. This arrangement also makes it possible for the sensor to sense the force precisely without any delay so that the control unit can appropraitely control the wafer carrier in response to any anomalous situations which, as stated above, may occur during a polishing operation. Further, since in the embodiment described above the load cell  75  is fastened between the tip end of the piston-rod  53  and the casing  50 , the fastening of the load cell can be effected easily. 
     Further, an actuator for actuating the wafer carrier  11  is not limited to theair cylinder device  52  as shown in FIG.  1  and may be of various types. Furthermore, although, in the flow chart shown in FIG. 3, the reference number for CA, CB, and CC is set to be the same, i.e., C 1 , different reerence numbers may be assigned for CA, CB and CC. 
     In the embodiment of this invention, an anomalous situation which may occur in a wafer carrier device during a polishing operation is detected by means of the load cell  75  which is provided for sensing a pressure under which a wafer carrier by the wafer carrier should be urged against the polishing suface of the turntable mainly in order to control the actuator  52  thereby maintaining the pressure at a desired value. This means that it is not necessary to add another particular sensor for detecting such an anomalous situation and further makes it possible for the control unit to appropriately control the wafer carrier in response to any anomalous situations without delay. Further, in this embodiment, the control unit  85  is adapted to be able to take actions suitable for various types of change in magnitude of values of a pressure imposed on a wafer so that appropriate control of the wafer carrier device can be attained. 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example and were herein described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims.