Patent Publication Number: US-6338671-B1

Title: Apparatus for supplying polishing liquid

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for supplying a polishing liquid used for polishing a surface of a semiconductor substrate, and more particularly to an apparatus for supplying a polishing liquid containing uniformly dispersed abrasive particles therein to a polishing surface of a polishing unit in a stable condition. 
     2. Description of the Related Art 
     Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 μm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. 
     It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surfaces of semiconductor wafers is to polish them with a polishing apparatus having a polishing unit (or section). 
     Conventionally, a polishing unit has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth constituting a polishing surface is attached to the upper surface of the turntable. A semiconductor substrate to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive particles is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor substrate held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated. 
     FIG. 7 is a schematic view showing the essential parts in an example of a polishing unit. The polishing unit comprises a turntable  142  having an upper surface to which a polishing cloth  140  is attached, a top ring  144  for holding a semiconductor wafer W as a polishing object and pressing the semiconductor wafer W against the polishing cloth  140  while rotating the semiconductor wafer W, and a polishing liquid supply nozzle  146  for supplying a polishing liquid Q to the polishing cloth  140 . The polishing cloth  140  constitutes a polishing surface. The top ring  144  is coupled to a top ring shaft  148 , and is vertically movably supported by an air cylinder (not shown). 
     The top ring  144  has an elastic pad  150  made of polyurethane or the like on its lower surface, and the semiconductor wafer W is held in such a manner that the semiconductor wafer W is closely brought in contact with the elastic pad  150 . The top ring  144  is further provided with a cylindrical guide ring  152  on its outer periphery so that the semiconductor wafer W is not dislodged from the lower surface of the top ring  144 . The guide ring  152  is fixed to the top ring  144 , and the lower end of the guide ring  152  projects from the holding surface of the top ring  144  so that the semiconductor wafer W is retained in a recess defined by the holding surface of the top ring  144  and the guide ring  152 . 
     In the polishing unit having the above structure, the semiconductor wafer W is held by the lower surface of the elastic pad  150  of the top ring  144 , and pressed against the polishing cloth  140  on the turntable  142  by the top ring  144 , and the turntable  142  and the top ring  144  are rotated so as to cause a relative sliding motion between the polishing cloth  140  and the semiconductor wafer W. At this time, a polishing liquid Q is supplied from the polishing liquid supply nozzle  146  to the polishing cloth  140 . The polishing liquid comprises abrasive particles such as silica particles suspended in a chemical solution such as an alkali solution, and the semiconductor wafer W is polished by a combination of chemical polishing with alkali and mechanical polishing with the abrasive particles. 
     In order to perform a high quality polishing in the above polishing unit, it is necessary to supply a polishing liquid having a constant concentration at a constant flow rate to the polishing surface of the polishing unit. A polishing liquid supply system may comprise a storage tank for storing a condensate comprising a mixture of, for example, alkali such as KOH or NH 4 OH and silica particles, an adjusting tank for adjusting the condensate to a desired concentration by diluting the condensate with a liquid such as pure water or chemical solution, a supply tank for storing temporarily a polishing liquid adjusted in the adjusting tank, and a polishing liquid supply pipe for delivering the polishing liquid to a nozzle from the supply tank. The polishing liquid supply system further comprises pipes for connecting the tanks. 
     It is known that a polishing rate and a polishing quality in a polishing process depend on the concentration of the polishing liquid used for polishing semiconductor wafers. On the other hand, for reducing equipment cost and operating cost, a common polishing liquid supply source is required to be used for a plurality of polishing units. For this reason, the polishing liquid having a given concentration is temporarily stored in the adjusting tank or the supply tank, and is then delivered to the respective polishing units. As a result, the quality of the polishing liquid stored in the adjusting tank or the supply tank is degraded with an elapse of time, and hence the abrasive particles tend to aggregate for thereby making the effective size of particles larger. Thus, an undesirable polishing liquid containing excessively large abrasive particles may be delivered to the polishing units to cause a polished surface of the semiconductor substrate to be scratched or to decrease a polishing rate. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an apparatus for supplying a polishing liquid which can continuously supply a polishing liquid having stable properties to a polishing section to perform a high quality polishing at all times. 
     In order to achieve the above object, according to a first aspect of the present invention, there is provided an apparatus for supplying a polishing liquid to a polishing section, comprising: a supply tank for storing a polishing liquid having given properties; a supply pipe for supplying the polishing liquid to the polishing section; a sensing device for detecting properties of the polishing liquid flowing through the supply pipe; and a stabilization device for maintaining properties of the polishing liquid stored in the supply tank or flowing through the supply pipe within an allowable range on the basis of an output signal from the sensing device. 
     According to the present invention, the properties of the polishing liquid supplied to the polishing section are continuously monitored, and when the polishing liquid has been degraded up to a condition that scratches are likely to be formed on a polished surface of the substrate by aggregated abrasive particles, various remedial procedures can be taken to stabilize the properties of the polishing liquid. Thus, the polishing liquid having stable properties is continuously supplied to the polishing section to provide a high quality polishing at all times. If it is found that the polishing liquid has been degraded to a large degree, then the operation of the polishing apparatus may be stopped to prevent inferior products from being produced. 
     In a preferred embodiment, the sensing device may measure at least one of particle size distribution, the number of coarse particles, oxidation-reduction potential, and solid material concentration in the polishing liquid. 
     According to the present invention, degradation of abrasive particles in the polishing liquid may be detected in real time directly or indirectly. 
     In a preferred embodiment, the stabilization device may uniformize particle size distribution. 
     The stabilization device for stabilizing the properties of polishing liquid may comprise an application device of ultrasonic energy to the polishing liquid, a filter for removing coarse particles, or an adding device of chemicals for adjusting the oxidation-reduction potential. The polishing liquid having a poor quality may be discarded, if necessary, to stabilize the properties of the polishing liquid. 
     In a preferred embodiment, the stabilization device comprises at least one of a filter for removing coarse particles, an ultrasonic generating device for breaking up coarse particles by ultrasonic energy, an adding device for supplying at least one of additives and abrasive particles to maintain a volume ratio of additives to abrasive particles in the polishing liquid at a constant value. 
     According to a second aspect of the present invention, there is provided a polishing apparatus for polishing a surface of a substrate, comprising: a turntable having a polishing surface; a top ring for holding a substrate and pressing the substrate against the polishing surface; and a polishing liquid supply unit for supplying a polishing liquid to the polishing surface, comprising: a supply tank for storing a polishing liquid having given properties; a supply pipe for supplying the polishing liquid to the polishing surface; a sensing device for detecting properties of the polishing liquid flowing through the supply pipe; and a stabilization device for maintaining properties of the polishing liquid stored in the supply tank or flowing through the supply pipe within an allowable range on the basis of an output signal from the sensing device. 
    
    
     The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an overall polishing apparatus incorporating a polishing liquid supply apparatus according to a first embodiment of the present invention; 
     FIG. 2 is a schematic diagram of a particle size distribution measuring device and a coarse particle size measuring device; 
     FIG. 3 is a schematic diagram of an oxidation-reduction electrometer; 
     FIG. 4 is a cross-sectional view of a solid material concentration measuring device; 
     FIG. 5 is a graph showing the relationship between the propagation velocity of ultrasonic waves and concentration of a polishing liquid at various temperatures of the polishing liquid; 
     FIG. 6 is a schematic diagram of an overall polishing apparatus incorporating a polishing liquid supply apparatus according to a second embodiment of the present invention; and 
     FIG. 7 is a schematic cross-sectional view of a conventional polishing unit. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, a polishing apparatus incorporating a polishing liquid supply apparatus (or unit) will be described below with reference to drawings. 
     As shown in FIG. 1, the polishing apparatus comprises a polishing liquid supply unit  10  and a polishing unit (or section)  12 . The polishing unit  12  comprises a turntable  142  and a polishing liquid supply nozzle  146 , but the polishing unit  12  may have the same structure as the conventional polishing unit shown in FIG.  7 . The turntable  142  has a polishing surface on an upper surface thereof. 
     The polishing liquid supply unit  10  comprises a plurality of storage tanks  14  for storing a condensate, an adjusting tank  16  for adjusting a concentration of condensate by diluting the condensate with pure water or chemical solution, and a supply tank  18  for temporarily storing a polishing liquid adjusted in the adjusting tank  16  and supplying the polishing liquid to the polishing unit  12 . A stirrer  22  having mixing blades is provided in each of the tanks  14 ,  16  and  18  to mix the liquid and particles by rotating the mixing blades with a motor  20 . A pure water supply line  24  is connected to the storage tanks  14  and the adjusting tank  16 , and the storage tanks  14  and the adjusting tank  16  are connected by a delivery pipe  28  having a pump  26 . 
     The adjusting tank  16  is connected to the supply tank  18  by a delivery pipe  32  having a pump  30  and a shutoff valve  32   a . The delivery pipe  32  has a return pipe  33  branched therefrom so that the polishing liquid is returned through a shutoff valve  33   a  to the adjusting tank  16 . The supply tank  18  is connected to a polishing liquid supply pipe  46  for the polishing unit  12  through a supply pipe  36  having a supply pump  34 . The supply pipe  36  has a return pipe  37  branched therefrom, and the return pipe  37  is connected to the supply tank  18  through a shutoff valve (circulation valve)  50 . 
     The delivery pipe  32  and the supply pipe  36  are respectively branched in the upstream sides of the pumps  30  and  34 , and the branched lines are connected to a discharge line  38  through respective shutoff valves  38   a  and  38   b . The discharge line  38  extending from the supply pipe  36  has a branched discharge line  44  having a discharge pump  40  and a discharge valve  42 . The supply pipe  36  has the polishing liquid supply pipe  46  for supplying the polishing liquid to the turntable  142  in the polishing unit  12 . The polishing liquid supply pipe  46  has a supply valve  48 , and the return pipe  37  branched from the supply pipe  36  has the shutoff valve  50  in the downstream side of the branch point of the polishing liquid supply pipe  46  branched from the supply pipe  36 . 
     The supply pipe  36  has sampling pipes  62   a ,  62   a  and  62   b  branched therefrom in the downstream side of the pump  34  and the upstream side of the branch point of the return pipe  37  branched from the polishing liquid supply pipe  46 , and the sampling pipes  62   a ,  62   a  and  62   b  have a particle size distribution measuring device  52 , a coarse particle measuring device  54  and an oxidation-reduction electrometer  56 , respectively. These pipes  62   a ,  62   a  and  62   b  are merged together in the downstream side of the measuring devices  52 ,  54  and  56  into a discharge pipe which is then connected to the discharge line  38 . The supply pipe  36  has a solid material concentration measuring device  58  in the downstream side of the branch points of the sampling pipes  62   a ,  62   a  and  62   b . The measurements of the respective devices  52 ,  54 ,  56  and  58  are inputted into a controller  60 . In this embodiment, each of the particle size distribution measuring device  52 , the coarse particle measuring device  54 , the oxidation-reduction electrometer  56 , and the solid material concentration measuring device  58  constitutes a sensing device. The particle size distribution measuring device  52  and the coarse particle measuring device  54  may use devices sold by Particle Sizing Systems. The coarse particle measuring device  54  may use a device sold under the trade name of Accusizer 780 OL. 
     As shown in FIG. 2, the particle size distribution measuring device  52  and the coarse particle measuring device  54  are connected to the supply pipe  36  through the sampling pipes  62   a , respectively. Each of the sampling pipes  62   a  has a sampling valve  64   a , a flow rate regulator  66   a , a line-mixer  68 , and a particle size sensor  70  in sequence. A diluting liquid pipe  76  extending from a diluting liquid supply source  72  is connected to the sampling pipe  62   a  through a diluting valve  74  and a flow rate regulator  66   b  at a location between the flow rate regulator  66   a  and the line-mixer  68 . 
     Accordingly, a portion of the polishing liquid flowing through the supply pipe  36  flows into the sampling pipe  62   a  at a controlled flow rate through the flow rate regulator  66   a , and the polishing liquid is mixed in the line-mixer  68  with the diluting liquid flowing through the flow rate regulator  66   b  by which a flow rate of the diluting liquid is controlled. Thus, after the polishing liquid having a given concentration is produced, it is supplied to the particle size sensor  70  as a sample liquid to be checked. The particle size sensor  70  measures the particle size distribution or the number of coarse particles in the sample liquid. The particle size distribution measuring device  52  and the coarse particle measuring device  54  have the respective particle size sensors  70  by which the particle size ranges to be measured are different from each other, but the sensors  70  have the same structure. By measuring the particle sizes different from each other, individually, the accuracy of measurement is improved. 
     In the polishing liquid supply system shown in FIG. 1, by using a bellows pump as the supply pump  34 , and a slurry of commercially available colloidal silica as a polishing liquid, the particle size distribution in the polishing liquid was measured by the particle size distribution measuring device  52 , and the number of coarse particles in the polishing liquid was measured by the coarse particle measuring device  54 . It was confirmed that the center of particle size distribution in the polishing liquid shifted towards the larger particle size side with an elapse of time after producing the polishing liquid. 
     As shown in FIG. 3, the oxidation-reduction electrometer  56  has the sampling pipe  62   b  connected to the supply pipe  36 . In the sampling pipe  62   b , there are provided a sampling valve  64   b  and a measuring vessel  80  having a sensor electrode  78  in sequence, and the measuring vessel  80  is connected to the discharge line  38 . A chemical supply source  82  is connected to the measuring vessel  80  through a chemical delivery pipe  86  having a metering pump  84 . In this arrangement, a small amount of chemicals such as hydrogen peroxide or potassium permanganate is added to the polishing liquid introduced into the measuring vessel  80 , and oxidation-reduction potential of the liquid may be measured by the electrode  78 . 
     In this example, the solid material concentration measuring device  58  utilizes ultrasonic waves, and comprises an ultrasonic transducer  90  and a reflection surface  92  housed in a casing  88  connected to the supply pipe  36  as shown in FIG.  4 . The ultrasonic transducer  90  and the reflection surface  92  are confronted with each other and disposed at a right angle to the flow direction of the polishing liquid. In this arrangement, ultrasonic waves are applied from the ultrasonic transducer  90  to the reflection surface  92 , and the propagation velocity of ultrasonic waves in the polishing liquid is measured, thereby measuring concentration of the polishing liquid. A temperature sensor is provided in the casing to correct any effect caused by temperature variations. 
     Next, the results measured by the solid material concentration measuring device  58  will be described. 
     FIG. 5 shows an example of the results measured by the device  58  on a polishing liquid containing colloidal silica particles. As shown in FIG. 5, the propagation velocity of ultrasonic waves differs depending on the concentration of the polishing liquid in each of temperatures of the polishing liquid, i.e., there is a certain correlation between the propagation velocity of ultrasonic waves and the concentration of the polishing liquid. Therefore, when the temperature of the polishing liquid and the propagation velocity of ultrasonic waves are known, the concentration of the polishing liquid can be measured. 
     As shown in FIG. 1, ultrasonic transducers  94   a ,  94   b  are provided on the bottoms of the adjusting tank  16  and the supply tank  18  in the polishing liquid supply unit  10  as a first stabilization device for stabilizing the properties of the polishing liquid. These ultrasonic transducers  94   a ,  94   b  allow the aggregated particles distributed over a wide range of particle sizes to be broken up by the energy of ultrasonic vibration. 
     As a result, the sizes of abrasive particles contained in the polishing liquid in the adjusting tank  16  and the supply tank  18  are uniformized. 
     In the downstream side of the supply pump  34  in the supply pipe  36 , a bypass line  98  capable of changing the flow direction of the polishing liquid by three-way valves  96   a ,  96   b  is disposed parallel to the supply pipe  36 , and the bypass line  98  is provided with a filter  100  as a second stabilization device for the polishing liquid. The filter  100  serves to remove coarse particles contained in the polishing liquid. 
     The adjustment tank  16  has a chemicals adding device  102  as a third stabilization device for the polishing liquid. The chemicals adding device  102  comprises a chemicals supply source  104 , and a chemicals supply line  106  connected to the adjusting tank  16  through a flow rate control valve  108 . By the chemicals adding device  102 , the volume ratio of the additives and abrasive particles contained in the polishing liquid in the adjusting tank  16  can be maintained at a constant value and the distribution of the particle sizes in the polishing liquid can be uniformized by adding acidic additives such as H 2 O 2  or nitric acid, or alkali additives such as KOH or NH 4 OH, or neutral chemicals such as surfactant to the polishing liquid in the adjusting tank  16 . 
     The operation of these stabilization devices is controlled by signals from the controller  60 . That is, the ultrasonic transducers  94   a ,  94   b , the three-way valves  96   a ,  96   b , and the flow rate control valve  108  of the chemicals adding device  102  are controlled by signals from the controller  60  on the basis of measurements obtained by the measuring devices  52 ,  54 ,  56  and  58 . 
     Next, the operation of the polishing apparatus having the above structure will be described below. The condensate stored in the storage tanks  14  is supplied to the adjusting tank  16  by operating the pump  26 , and is diluted to a certain concentration with pure water supplied from the pure water supply line  24 . After being adjusted to a desired concentration, the polishing liquid is fed to the supply tank  18  by operating the delivery pump  30 , and is stored therein. 
     The polishing liquid stored in the supply tank  18  flows through the supply pipe  36  by operating the supply pump  34 , and while polishing of the semiconductor wafer is conducted, the polishing liquid is supplied through the polishing liquid supply pipe  46  and the polishing liquid supply nozzle  146  to the polishing surface of the turntable  142  in the polishing unit  12  by opening the supply valve  48 . After polishing of the semiconductor wafer is completed, the supply valve  48  is closed, and the circulation valve  50  is opened to circulate the polishing liquid through the circulation path comprising the supply tank  18 , the supply pipe  36  and the return pipe  37 . Thus, even when the polishing liquid is not being supplied to the polishing unit, the polishing liquid is prevented from being stagnated in the pipes and the abrasive particles in the polishing liquid are prevented from being deposited in the pipes. 
     At this time, the polishing liquid flowing through the supply pipe  36  is monitored by the particle size distribution measuring device  52 , the coarse particle measuring device  54 , the oxidation-reduction electrometer  56 , and the solid material concentration measuring device  58  to determine particle size distribution, the number of coarse particles, oxidation-reduction potential, and the solid material concentration, and the measurements are inputted into the controller  60  for continuing monitoring. 
     The controller  60  judges whether there has been any change in the particle size distribution, and any coarse particles have been produced on the basis of the inputted measurements. When it is judged that a change has taken place in the particle size distribution, one or both of the ultrasonic transducers  94   a ,  94   b are operated to disperse the particles in the polishing liquid stored in one or both of the adjusting tank  16  and the supply tank  18  by the application of ultrasonic energy. When it is judged that there has been an increase in the concentration of coarse particles, the three-way valves  96   a ,  96   b  are switched to allow the polishing liquid to pass through the bypass line  98 , thereby removing the coarse particles in the polishing liquid by the filter  100 . Further, when it is judged that there has been a change in either the oxidation-reduction potential or the solid material concentration, the flow rate control valve  108  of the chemicals adding device  102  is opened, and by adding chemicals to the polishing liquid in the adjusting tank  16 , the oxidation-reduction potential or the solid material concentration of the polishing liquid is adjusted to keep the volume ratio of additives to abrasive particles at a constant value and to uniformize the particle size distribution in the polishing liquid. 
     When measurements of the polishing liquid flowing through the supply pipe  36  exceed predetermined limits set in the respective devices, it is judged that the polishing liquid has been degraded. Then, the discharge valve  42  is opened and the discharge pump  40  is operated to cause the polishing liquid in the supply tank  18  to be discharged through the discharge line  38 , and a new polishing liquid is prepared in the adjusting tank  16 . The polishing liquid supply unit  10  and the polishing unit  12  communicate with each other by communication lines so that the polishing unit  12  is controlled so as not to commence a new polishing operation, if the above undesirable state occurs. 
     A newly prepared polishing liquid is supplied to the supply tank  18  from which it is circulated within the supply pipe  36 , and the properties of the polishing liquid are monitored by the respective measuring devices. After it is confirmed that the measurements do not exceed the predetermined limits by the controller  60 , an instruction to commence a polishing operation is sent from the controller  60  to the polishing unit  12 . The polishing unit  12  resumes a polishing operation after the operator confirms that the abnormal condition has been removed. It is, of course, permissible to resume a polishing operation after the controller confirms given operating conditions so that the abnormal condition is automatically removed. 
     In the polishing apparatus, the limits predetermined in the respective measuring devices are different from one another depending on the kind of a polishing liquid. 
     Typical limits are as follows: for a colloidal silica type polishing liquid, the particles having a diameter of 1 μm or larger should be contained by less than 0.1% of the total amount of the polishing liquid; the number of coarse particles should be such that the particles equal to or larger than 5 μm is equal to or less than 100 particles/ml; the oxidation-reduction potential should be equal to or less than 1.0% of the initial value; and the solid material concentration should be equal to or less than ±5% of the setting. 
     In the above embodiment, although the measuring devices are positioned on or near the supply pipe  36 , they may be positioned at any places including the delivery pipe  32 , the adjusting tank  16  and the supply tank  18 , as long as the polishing liquid can contact the measuring devices. Especially, it is preferable that the oxidation-reduction electrometer  56  and the solid material concentration measuring device  58  are provided in the adjusting tank  16  or the delivery pipe  32 . 
     Further, as measuring devices for measuring degradation of the polishing liquid, a pH measuring device, a ζ potential measuring device, a turbidity measuring device, and a viscosity measuring device are included, and they may be provided in the polishing liquid supply system. Further, a chemicals concentration measuring device utilizing near infrared radiation may be employed to measure chemical components added to the polishing liquid. 
     FIG. 6 is a schematic diagram of an overall polishing apparatus incorporating a polishing liquid supply apparatus according to a second embodiment of the present invention. As shown in FIG. 6, a common supply tank is used to supply a polishing liquid to a plurality of polishing units. In FIG. 6, although two polishing units  12  are illustrated, three or more polishing units connected to the common supply tank may be provided. The polishing liquid supply unit  10  comprises a cylindrical buffer tube  110  serving as a container, a circulation pipe  112  extending from a bottom of the buffer tube  110 , passing through the locations near the polishing units  12 , and returning to the top end of the buffer tube  110 , and polishing liquid supply pipes  114  branched from the circulation pipe  112  and extending to the respective polishing units  12  to deliver the polishing liquid to the respective polishing units  12 . 
     The circulation pipe  112  has a circulation pump  116  for constantly circulating a given flow rate of the polishing liquid, and a back pressure regulating valve  118  and a pressure sensor  120  for maintaining the internal pressure of the pipe at a constant value or higher. Each of the polishing liquid supply pipes  114  has a polishing liquid supply valve  122  and a pump  124  for withdrawing the polishing liquid individually from the circulation pipe  112 . 
     The buffer tube  110  serves both as the adjusting tank  16  and the supply tank  18  in the first embodiment, and has a top end to which a delivery pipe  28 , a pure water supply line  24 , and a chemicals supply line  106  are connected. The buffer tube  110  has a first stabilization device comprising an ultrasonic transducer  94 , liquid level sensors  126   a ,  126   b ,  126   c  for detecting liquid levels, and an airbag  128  made of an extendable material. The airbag  128  serves to suppress fluctuations in the internal pressure caused by the changes in the liquid level of the buffer tube  110  while a space in the buffer tube  110  is hermetically sealed. 
     In this embodiment, the particle size distribution measuring device  52 , the coarse particle measuring device  54 , the oxidation-reduction electrometer  56 , and the solid material concentration measuring device  58  are provided at certain positions of the circulation pipe  112 , i.e., in the downstream side of the circulation pump  116 . A bypass line  98  branched from the circulation pipe  112  and having a filter  100  is provided in parallel to the circulation pipe  112 . A discharge line  38  is provided to remove the degraded polishing liquid from the buffer tube  110 . 
     The method of operating the polishing apparatus in this embodiment is basically the same as that in the first embodiment, and hence the explanation thereof will not be made. According to the polishing liquid supply unit of this embodiment, change of the concentration of polishing liquid caused by stagnation of the polishing liquid in the pipe can be prevented and clogging of the pipe caused by deposition of solid material can be prevented by constantly circulating the polishing liquid to be supplied to the portions near the polishing units  12 . As a result, since the overall length of the piping system can be lengthened, the polishing liquid can be stably supplied to many polishing units  12  from one buffer tube  110  serving as a supply source, and therefore the apparatus cost can be lowered. It may be possible to provide measuring devices for measuring the properties of the polishing liquid in each of the polishing liquid supply pipes  114 . 
     As described above, according to the present invention, the change of the properties of the polishing liquid is measured, and the properties of the polishing liquid is improved on the basis of the measurements. Therefore, since the effective size of particles does not become large, a polished surface of a semiconductor substrate is prevented from being damaged and a polishing rate of the semiconductor substrate is prevented from being decreased. Thus, in the polishing apparatus, semiconductor substrates can be stably and continuously polished in a good polishing condition. 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.