Abstract:
A polishing apparatus can produce a uniform quality of polished products by supplying a polishing solution consistently without being affected by any disturbances in the solution supply source. The polishing apparatus comprises: a polishing section for polishing a workpiece by pressing the same against a polishing tool; a solution piping assembly to be connected to an external solution supply device for transferring a polishing solution therefrom to the polishing section; and a solution suction device provided in the solution piping assembly for introducing the polishing solution from the solution supply device to the polishing section at a desired flow rate.

Description:
TECHNICAL FIELD 
     The present invention relates to polishing apparatuses, and relates in particular to a polishing apparatus to provide consistent polishing by supplying polishing solution consistently regardless of disturbances in a polishing solution supply facility. 
     BACKGROUND ART 
     Advances in integrated circuit devices in recent years have been made possible by ultra fine wiring patterns and interline spacing. The trend towards high density of circuit integration leads to a requirement of extreme flatness of a substrate surface to satisfy the shallow depth of focus of stepper printer in photolithographic reproduction of microcircuit patterns. 
     One method of producing such a flat surface on a semiconductor wafer surface is to use a polishing apparatus having a polishing tool (for example, a polishing table having a polishing cloth) and a wafer holding section for holding the wafer and pressing and sliding the wafer against the polishing tool while supplying a polishing solution to the polishing surface. Such an apparatus can perform not only mechanical polishing but also chemical polishing using an alkaline or acidic polishing solution. 
     Polishing solution is normally prepared by mixing a stock solution and a dilution liquid in a mixing tank, which is used to supply a mixed solution through a delivery pipe to the solution nozzle of the polishing apparatus. The polishing facility may have associated cleaning mechanisms. When a plurality of polishing apparatuses are arranged in parallel to perform production of substrates, one solution supply device is normally provided for several polishing apparatuses. Also, in a production plant based on an even greater number of polishing apparatuses, consideration is given to a polishing solution delivering system having a stem pipe (circulating pipe) extending from one mixing tank and circulating around the plant and branch pipes branching from the stem pipe for delivering solution to each polishing apparatus, in an effort to reduce the operating and facility costs. 
     However, in such a conventional technology to supply the solution to each polishing apparatus by using a circulation pump, it is necessary to select a delivery capacity for the circulation pump so that the capacity would be sufficient to deliver a necessary quantity of solution at full plant operation, thus resulting in a high facility cost. Furthermore, because of fluctuations in solution supply rate caused by pressure changes in the delivery pipes due to changes in the number of operating polishing apparatuses, it is difficult to maintain a stable flow of solution of a given concentration. This affects the quality of polished products produced at various polishing apparatuses within the plant. It will be necessary to provide expensive flow control devices and complex process control methodology to overcome such flow rate fluctuation to provide a steady flow of polishing solution of a consistent quality to each polishing apparatus. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a polishing apparatus that enables production of a uniform quality of polished products by supplying a polishing solution consistently without being affected by any disturbances in the solution supply source. 
     The object has been achieved by providing a polishing apparatus comprising: a polishing section for polishing a workpiece by pressing the same against a polishing tool; a solution piping assembly to be connected to an external solution supply device for transferring a polishing solution therefrom to the polishing section; and a solution suction device provided in the solution piping assembly for introducing the polishing solution from the solution supply device to the polishing section at a desired flow rate. 
     Accordingly, an independent polishing unit with high flexibility is presented that can operate regardless of the presence or absence of fluid transport means for the polishing solution in the external polishing solution source. Even if a fluid transport means is to be provided in the external source of polishing solution, it is not necessary to choose an excessive capacity for the fluid transport means, and it is only necessary to operate a fluid transport means to suit the polishing load of polishing apparatuses. Thus, wasteful facility and operating costs can be eliminated. 
     The solution piping assembly may be provided with a flow control device for adjusting flow rates of polishing solution through the solution piping assembly. Accordingly, polishing solution can be supplied at a flow rate to suit the needs of individual polishing apparatuses, thereby providing stable and accurate flow control. 
     The solution piping assembly may be provided with a smoother for smoothing out pulsation of the polishing solution flowing therethrough. Accordingly, even when flow pulsation is produced in the fluid transport means over small time intervals, such as in peristaltic pumps, stable flow of polishing solution can be supplied. 
     The present polishing apparatus can supply polishing solution consistently regardless of any disturbances in the solution supply device so that a polishing operation can be carried out correctly at individual polishing apparatuses. When an external fluid transport device is utilized in the external solution supply source, it is not necessary to design an excessive capacity so that wasteful facilities and operating costs can be eliminated, thus resulting in integration of a solution supply facility even in a relatively large production plant having a large number of polishing apparatuses operating therein. Capital cost and space allowances can thus be lowered to bring genuine benefits to industries concerned with advanced semiconductor device production. 
     The polishing apparatus may be assembled in a polishing unit together with a storing section for storing a workpiece, and a transporting device for transporting the workpiece between the polishing apparatus and the storing section. 
     The polishing apparatus may be assembled in a polishing system together with a solution supply device, and a solution distribution pipe for distributing a polishing solution from the solution supply device to the polishing apparatus, in which the solution piping assembly is connected to the solution distribution pipe. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a plan view of the arrangement of a polishing unit in the polishing apparatus of the present invention; 
     FIG. 2 is an enlarged plan view of the polishing apparatus shown in FIG. 1; 
     FIG. 3 is a front view of the key section of the apparatus shown in FIG. 2; and 
     FIG. 4 is a schematic diagram showing the flow paths of the polishing solution through the polishing system of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, preferred embodiments will be presented with reference to the drawings. FIG. 1 shows an arrangement of the polishing apparatus P incorporated into a polishing unit  10 . Polishing unit  10  includes: a load/unload unit  12  disposed on the opposite-side of the polishing apparatus P; a transport device  16  having a robotic device  14  movable between the polishing apparatus P and the load/unload unit  12 ; and a plurality of inverters  18  and cleaners (or a dryer)  20  disposed on both sides of the transport device  16 . 
     As shown in an enlarged view in FIG. 2, polishing apparatus P includes: a turntable  30  having a polishing cloth bonded to a top thereof; a top ring unit  32  having a top ring  31  for holding a wafer and pressing the wafer towards the turntable surface; a dresser unit  34  having a dressing member  33  for conditioning the polishing cloth; and a polishing solution supply section  40  for steadily supplying a polishing solution from a solution supply device  36  by way of a solution nozzle  38 . Auxiliary devices include: a wafer pusher  42  for exchanging a wafer between the top ring  31  and the robotic device  14 ; and a cleaning section  44  for washing the dressing member  33  when it is not being used. The polishing apparatus P is constructed so that it may be placed in an isolated environment to prevent other devices in polishing unit  10  from being contaminated with splattered polishing solution. 
     Polishing solution supply section  40  is provided with a solution delivery pipe  46  connected to solution nozzle  38 , a solution suction pump  48  provided in the path of the delivery pipe  46 , a flow adjusting valve  50 ; and an accumulator  52  (refer  25  to FIG.  4 ). Solution delivery pipe  46  has a coupling  56  at one end, which is connected to a supply pipe  54  of the solution supply device  36 . As shown in more detail in FIG. 3, there are three suction pumps  48 , each a well known peristaltic pump operating through a flexible tube, housed vertically inside a casing  58 . 
     Obviously, the type, number and arrangement of suction pumps  48  are not limited to this example. Also, such pumps may be connected in series or parallel, and some may act as reserves. Further, the pumps may be switched through switching valves, and the manner of connecting the coupling to the pipe may be altered. 
     Flow rates through the peristaltic pumps  48  can be adjusted by adjusting the revolution speed, but in this embodiment, flow control valve  50  is used to provide a more precise adjustment of flow rates and suppress flow rate pulsation (a problem inherent in peristaltic pumps) to some extent. Accumulator  52  helps to further control pulsation to provide a stable supply of polishing solution to the solution nozzle  38 . 
     FIG. 4 shows an overall arrangement of the polishing system provided by connecting the polishing apparatus P to the solution supply device  36 . This polishing system is provided with the solution supply device  36 , a plurality of polishing apparatuses P in this embodiment, and circulation pipes  60  to deliver the solution to the vicinity of the polishing apparatuses P. 
     Solution supply device  36  includes: a stock solution tank  62  for storing a stock: solution; a dilution liquid tank  64  for storing a liquid to dilute the stock solution to a specific concentration; and a mixing tank  70  for merging the stock solution and dilution liquid supplied from the tanks  62 ,  64  through flow pipes  66 ,  68  to produce a polishing solution of a specific concentration. Flow pipes  66 ,  68  are respectively provided with pumps  72 ,  74  to transport the fluids under pressure, and flow control valves  76 ,  78 . Dilution liquid may also be obtained from a plant source in a form of deionized water supplied at a controlled flow rate. Stock solution may include an acidic, alkaline or neutral solution containing abrasive particles such as silica-gel, depending on the nature of the workpiece, and dilution liquid is normally deionized water containing no harmful impurities. 
     Circulation pipe  60  is provided with a circulation pump  80 , for circulating the polishing solution, and a pressure gage  82 . Circulation prevents precipitation of abrasive particles due to flow stagnation, so that the delivery pipe can be lengthened to deliver a consistent quality of polishing solution to a plurality of polishing apparatuses P from one solution supply source (mixing tank)  70 , thereby lowering the overall cost of the polishing system. 
     Mixing tank  70  has a liquid level detector to check the level of the stored solution, and it can be arranged to detect an upper limit, lower limit and bottom limit, for example, and output a signal to a controller  100 . Based on such signals, the controller  100  controls the liquid level such that, when the liquid surface is at the lower limit, pumps  72 ,  74  and flow control valves  76 ,  78  are operated to raise the level or stop filling when the upper limit is reached. Also, when the bottom limit is reached, an alarm is sounded and signals to stop polishing are outputted. 
     Operation of the polishing apparatus P of such a construction will be explained. Circulation pump  80  is activated and controlled so that the internal pressure of the fluid detected by the pressure sensor  82  remains above a threshold value to overcome internal resistance in the piping, and keep the solution circulating inside the pipes constantly. Therefore, when the polishing system is in operation, polishing solution is constantly circulated within the pipe  60 , thereby preventing changes in solution concentration caused by stagnation and blocking of the circulation pipe  60  due to precipitation of solid particles. 
     When the polishing apparatuses P are activated, each control device outputs flow rate command signals to the drive section for the peristaltic pump  48  and flow control valve  50  to operate at a pre-determined flow rate, thereby permitting polishing solution to flow at a given rate from the circulation pipe  60  to delivery pipe  46  to deliver polishing solution to the solution nozzle  38 . Regardless of the operating or nonoperating state of individual polishing apparatuses P, or changes in the tank solution level that can cause fluctuations in solution flow rate, the solution supply device  36  controls the internal pressure in the circulation pipe  60  within a certain range. In each of the polishing apparatus P, because each apparatus P is provided with its dedicated suction pump  48 , even if the internal pressure in the circulation pipe  60  varies widely, the solution flow rate can be kept constant by overcoming the effects of fluctuations. Flow pulsation caused by peristaltic pump  48  is smoothed out by the actions of the flow control valve  50  and accumulator  52 . Thus, the flow control valve  50  and the accumulator  52  function as a smoother for smoothing out pulsations of the polishing solution flowing through solution delivery pipe  46 . 
     Accordingly, in the present polishing system, by providing suction pumps  48  for each polishing apparatus, the number of polishing apparatuses that can be supplied by one solution supply device  36  is increased significantly, thereby enabling reduction in equipment and space costs by reducing the required number of solution supply devices  36 . Also, conditions of the polishing solution delivered are made more uniform in different polishing apparatuses P, thereby reducing quality variation of polished wafers from lot to lot by increasing the uniformity of polishing conditions in individual apparatuses P. 
     It is not necessary to have a high capacity circulation pump  80 , and therefore, it is possible to prevent inefficiencies of operating a high capacity circulation pump  80  at low flow rates when only a small number of polishing apparatuses P are in operation. 
     Based on the arrangement presented in this embodiment, flow sensors may be provided in the path of solution delivery pipe  46  so that the peristaltic pump  48  and flow control valve  50  may be controlled by feedback signals. This type of arrangement will enable control of each polishing apparatus P individually to suit different polishing requirements of workpieces. This will enable more precise polishing to be provided by improving responsiveness of the polishing system. 
     In the above embodiment, the circulation pump  80  is provided in the solution supply device  36 , but, because suction pumps  48  are provided for each polishing apparatus P, solution may be delivered directly from the mixing tank  70  to each apparatus P, depending on the number of operating apparatuses P and their locations without using the circulation pump  80 . The polishing system may be simplified by not providing a liquid transport pump for the solution supply device  36 . 
     Also, in the above explanation, the relative large polishing system is provided by arranging a large number of polishing apparatuses in parallel, but it is obvious that the present polishing apparatus is equally suitable for a small-scale operation having a few polishing apparatuses. In other words, this invention provides a polishing apparatus of high adaptability usable if accompanied by a suitable polishing solution source. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful as a polishing apparatus for providing a mirror polished surface on a substrate in a manufacturing process of a semiconductor wafer or liquid crystal display.