Patent Publication Number: US-6659848-B1

Title: Slurry dispenser that outputs a filtered slurry to a chemical-mechanical polisher at a constant flow rate over the lifetime of the filter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to slurry dispensers that dispense slurry to a chemical-mechanical polisher used in semiconductor fabrication and, more particularly, to a slurry dispenser that outputs a filtered slurry to a polisher at a constant flow rate over the lifetime of the filter. 
     2. Description of the Related Art 
     A chemical-mechanical polisher is a device that removes excess material from the top surface of a semiconductor wafer. Chemical-mechanical polishers are commonly used to planarize the topography of a wafer, and to form damascene structures that are embedded in an insulation layer on a wafer. 
     FIG. 1 shows a block diagram that illustrates a conventional chemical-mechanical polisher  100 . As shown in FIG. 1, polisher  100  includes a round polish platen  110 , a round pad  112  that is connected to platen  110 , and a slurry dispenser  114  that dispenses a slurry  116  onto pad  112 . Slurry  116  includes water, a number of chemicals, and an abrasive material that has a large number of particles. 
     In addition, polisher  100  also includes a wafer carrier  120  that holds a wafer  122  so that the top surface of wafer  122  is parallel to the top surface of pad  112 . Polisher  100  further includes a vertical carrier  124  that moves wafer carrier  120  up and down so that the semiconductor materials formed on the top surface of wafer  122  are brought into contact with pad  112 . 
     In operation, pad  112  is rotated via platen  110  at a high rate of speed, slurry  116  is dispensed to pad  112 , and wafer  122  is rotated via carrier  120  at a high rate of speed and lowered until wafer  122  makes contact with pad  112 . Pad  112  and slurry  116  then remove the materials formed on the top surface of wafer  122 , beginning with the peaks, for as long as pad  112  and wafer  122  remain in contact. 
     One of the problems with chemical-mechanical polisher  100  is that when oversized particles are present in the abrasive material in slurry  116 , the surface of wafer  122  can become scratched and may affect the polishing removal rate and non-uniformity. With very small line widths, these scratches and degraded process characteristics can destroy or degrade the devices being fabricated on wafer  122 . 
     One approach to preventing scratches from oversized particles is to add a filter to slurry dispenser  114  that removes the oversized particles from slurry  116 . One problem with filters, however, is that filters increasingly restrict the flow of slurry  116  over time as the filters catch more and more oversized material. Eventually, the filters clog up and the flow of slurry stops. 
     Changes in the slurry flow rate effect the removal rate of the wafer material that is in contact with the pad which, in turn, makes it difficult to calculate how long the wafer material should remain in contact with the pad. In addition, polishers typically require a minimum slurry flow rate to remove material from a wafer, and prevent damage to the wafer. 
     As a result, to avoid damaging the wafer, the filter must be replaced before the decreasing slurry flow rate drops below the minimum slurry flow rate. Thus, there is a need for a slurry dispenser that outputs a filtered slurry, indicates when the filter needs to be replaced, and outputs the filtered slurry at a constant flow rate throughout the lifetime of the filter. 
     SUMMARY OF THE INVENTION 
     The present invention provides a slurry dispenser that utilizes a filter to remove oversized particles from a slurry to reduce the effects of scratches. In addition, the slurry dispenser of the present invention indicates when the filter needs to be replaced, and maintains a constant flow of slurry through the filter throughout the lifetime of the filter. 
     A slurry dispenser in accordance with the present invention includes a pump that receives a slurry from a slurry supply, and outputs a pumped slurry. The pump outputs the pumped slurry with a flow rate at a pump speed. The pump speed is controlled by a pump speed control signal. 
     The slurry dispenser also includes a slurry filter that removes oversized particles from the pumped slurry. The slurry filter has an input and an output. The dispenser additionally includes a flow meter that measures a flow rate of the pumped slurry, and outputs a measured flow signal that indicates a measured flow rate. 
     Further, the slurry dispenser includes a flow controller that receives the measured flow signal, compares the measured flow rate to a reference set point rate, and controls a value of the pump speed control signal in response to the difference between the measured flow rate and the reference set point rate. 
     The present invention also includes a method of dispensing slurry onto a pad of a chemical-mechanical polisher that includes the step of pumping a slurry from a slurry supply to output a pumped slurry. The pumped slurry has a flow rate at a pump speed. The pump speed is controlled by a pump speed control signal. 
     The method also includes the step of filtering the pumped slurry with a filter to remove oversized particles from the pumped slurry. The method additionally includes the steps of measuring a flow rate of the pumped slurry, and outputting a measured flow signal that indicates a measured flow rate. 
     The method further includes the steps of receiving the measured flow signal, comparing the measured flow rate to a reference set point rate, and controlling a value of the pump speed control signal in response to the difference between the measured flow rate and the reference set point rate. 
    
    
     A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a conventional chemical-mechanical polisher  100 . 
     FIG. 2 is a block diagram illustrating an example of a semiconductor polisher  200  in accordance with the present invention. 
     FIG. 3 is a graph illustrating the results of tests conducted on a chemical-mechanical polisher that illustrate the operation of the present invention. 
     FIG. 4 is a block diagram illustrating an example of a slurry dispenser  400  in accordance with an alternate embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows a block diagram that illustrates an example of a slurry dispenser  200  in accordance with the present invention. As described in greater detail below, the slurry used in a chemical-mechanical polisher is passed through a filter, and output at a constant flow rate throughout the lifetime of the filter. 
     As shown in FIG. 2, slurry dispenser  200  includes a peristaltic slurry pump  210  that receives a slurry  212  from a slurry supply  214 , and outputs a pumped slurry  216 . In operation, pumped slurry  216  has a flow rate that is controlled by the speed of pump  210  (number of revolutions per minute) which, in turn, is set by a pump control voltage VPC. 
     Further, slurry dispenser  200  also includes a slurry filter  218  that removes oversized particles from pumped slurry  216 . One of the advantages of filter  218 , as shown in FIG. 2, is that filter  218  is positioned very close to the point where slurry is dispensed onto the pad. As a result, slurry filter  218  is able to capture not only oversized particles that enter dispenser  200 , but also oversized particles that form within the slurry path of dispenser  200 . 
     Slurry filter  218  can be implemented with, for example, a depth filtration media, and should be generally capable of handling a high particle loading with solids concentrations in the range of 0.5% to 30%, including gels and agglomerates. (Oversized particles are particles that are larger than a predefined size.) In one embodiment, a one-micron slurry filter Model SLR015CE1 from Mykrolis is used. 
     In addition, dispenser  200  includes a flow control unit  220  that controls the flow rate of pumped slurry  216 . Flow control unit  220  includes a flow meter  222  that measures a flow rate of pumped slurry  216 , and outputs a measured flow rate signal MFRS that indicates the measured flow rate. 
     In this example, flow meter  222  is implemented with an electronic flow meter, such as the Model 4400 Flow Meter from NT International. Electronic flow meter  222 , which includes an input pressure transducer  224 -I and an output pressure transducer  224 -O, outputs a first measured pressure signal FMPS from output transducer  224 -O that indicates the slurry pressure. Although the Model  4400  only outputs a pressure signal FMPS from output transducer  224 -O, a pressure signal FMPS can be output from either transducer  224 -I or  224 -O. 
     Flow control unit  220  also includes a flow controller  226  that receives the measured flow rate signal MFRS and determines if the flow rate is within a predefined control band around a reference set point flow rate as represented by a set point signal SPS from a polisher control system  228 . For example, a Model CM-100 flow controller from NT International can be configured to have minimum and maximum limits of the measured flow rate signal MFRS around the reference set point flow rate. 
     When the measured flow rate is less than a minimum predefined flow rate for a predetermined period of time, controller  226  enables a loss of flow alarm signal AS to polisher control system  228  and aborts the polishing process. As a result, any interruption to the flow supply (such as leaking tubing, plugged tubing, or loss of bulk slurry supply) can be detected. Thus, when the flow drops below the minimum predefined flow rate, the loss of flow alarm signal AS is enabled. 
     Similarly, when the measured flow rate is greater than a maximum predefined flow rate for a predefined period of time, controller  226  enables the flow alarm signal AS to polisher control system  228  and aborts the polishing process. 
     To avoid aborting the polish process during changes to the reference set point flow rate (the reference set point flow rate is changed in accordance with the polish recipe), the alarm signal AS is ignored by polisher control system  228  for a predefined period of time, such as ten seconds. 
     If the alarm signal AS is not ignored for a period of time, the alarm signal AS will cause the polish process to stop every time the reference set point flow rate is changed by polish control system  228  to accommodate changes in the recipe because slurry dispenser  200  can not immediately change the flow rate. 
     Controller  226  constantly compares the measured flow rate to the reference set point flow rate. In response to the comparison, controller  226  varies the pump control voltage VPC to increase or decrease the pump speed until the measured flow rate is equal to the reference set point flow rate. 
     For example, when the measured flow rate signal MFRS indicates that the flow rate is less than the reference set point flow rate, controller  226  can increase the pump control voltage VPC to increase the speed of pump  210 . The increased pump speed increases the flow rate until the flow rate from the measured flow rate signal MFRS is equal to the reference set point flow rate. 
     Thus, one of the advantages of the slurry dispenser of the present invention is that the slurry dispenser provides a closed-loop slurry delivery system that allows the slurry flow rate to remain constant throughout the lifetime of the filter. When filter  218  begins to restrict the flow, the present invention detects this condition and increases the pump speed. As a result, the present invention insures that pumped slurry  216  is output to a pad PD on a polishing platen PP at a constant flow rate even as filter  218  catches more and more material and begins to clog. 
     As further shown in FIG. 2, dispenser  200  includes a pressure transducer  230  that measures the pressure of pumped slurry  216 , and outputs a second measured pressure signal SMPS that represents the measured pressure. Pressure transducer  230  can be implemented with, for example, the Model 4000 transducer from NT International. 
     Pressure transducer  224  and pressure transducer  230  are located on opposite sides of filter  218 , but can be in either order. In the example shown in FIG. 2, pressure transducer  224  measures the pressure on the input side of filter  218 , while pressure transducer  230  measures the pressure on the output side of filter  218 . 
     Slurry dispenser  200  further includes a differential pressure unit  232  that receives pressure signals FMPS and SMPS from pressure transducer  224  and pressure transducer  230 , respectively. When the difference in pressure across filter  218  reaches a predetermined value, differential pressure unit  232  enables a filter clogged signal FCS to polish controller  228 . For example, a Model D80 Display Module from NT International can be used to generate the filter clogged signal FCS. 
     The filter clogged signal FCS indicates that filter  218  needs to be replaced, and can cause polish controller  228  to illuminate a warning light, sound an audible alarm, and disallow additional product to be loaded into the polisher. 
     In addition, dispenser  200  can optionally include a three-way valve  234  that can pass either pumped slurry  216  or de-ionized (DI) water from a DI water supply  236  to a polish pad PD on platen PP. Valve  234  can be controlled by a select signal SSS output by polish controller  228  that indicates whether slurry is to pass or DI water is to pass. Dispenser  200  can also optionally include a manual valve  242  connected between filter  218  and transducer  230 . Dispenser  200  can further optionally include a manual value  244  that is located between slurry supply  214  and pump  210 . Valve  244  can be used for safety lock out and tag out procedures during maintenance activities. 
     In addition to the above advantages, another advantage of the present invention is that valve  234  allows the dispense section of the slurry line to be rinsed with DI water and kept clean without subjecting the media of filter  218  to the DI water, and pH shocking the slurry. Valve  234  closes the slurry line after pressure transducer  230 , and switchably opens the remaining portion of the slurry line to be rinsed with DI water from DI water supply  236 . 
     FIG. 3 shows a graph that illustrates the results of tests conducted on a chemical-mechanical polisher that illustrates the operation of the present invention. The tests measured the change in differential pressure across the filters, such as filter  218 , over a number of days. During the test, slurry flowed through the filters at a constant rate of 600 mL/minute. 
     As shown in FIG. 3, the average pressure differential, as shown by line PD, begins at about 0.5626 Kg/cmd (8 p/sid) when a new filter is installed, and increases to about 2.1097 Kg/cmd (30 p/sid) before the next filter is installed. The life span of a filter varies for a number of reasons, including the type of material that is removed, and typically lasts 5-7 days. 
     The removal rates of three polishers used were recorded for 113 days, and produced an average removal rate of 5764 angstroms per minute with a standard deviation of 298. The three polishers were then modified to incorporate the present invention. 
     The modified polishers were recorded for 198 days, and produced an average removal rate of 5904 angstroms per minute with a standard deviation of 236. Thus, the present invention produced a 141 angstrom per minute increase in the removal rate with a drop of 62 points in the standard deviation rate. 
     FIG. 4 shows a block diagram that illustrates an example of a slurry dispenser  400  in accordance with an alternate embodiment of the present invention. Dispenser  400  is similar to dispenser  200  and, as a result, utilizes the same reference numerals to designate the structures which are common to both dispensers. 
     As shown in FIG. 4, dispenser  400  differs from dispenser  200  in that dispenser  400  includes a fluid pressure surge dampener  410  that is connected between pump  210  and flow meter  222 . Dampener  410  includes a T-shaped fitting  412  that has a vertical extension  414  with a top end, and a cap  416  that covers the top end of vertical extension  414 . In addition, the region in vertical extension  414  that is adjacent to cap  416  is partially filled with air  420 . 
     In operation, pressure transients in the slurry lines are absorbed by surge dampener  410  so that flow meter  222  is not fooled by high pressure transients which occur when the various valves and pumps in the system turn on and off. Without surge dampener  410 , the alarm signal AS is prone to falsely detecting low flow conditions and abort the polish process. 
     It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.