Abstract:
Disclosed herein is a method and an apparatus for applying a coating liquid to an object from a liquid-applying member at a first prescribed position, thereby forming a film on the object. Before the coating liquid at the first position, the coating liquid is applied at a second predetermined position. An impurity-detecting device detects the impurities contained in the coating liquid applied at the second position. A particle-counting device is provided, and a switching device is provided on a liquid-supplying pipe extending from a source of the coating liquid to the liquid-applying member. The switching device switches the supply of the coating liquid between the liquid-applying member and the impurity-detecting device. The impurities in the coating liquid can thereby monitored.

Description:
This application is a Division of application Ser. No. 08/915,737 filed on Aug. 21, 1997, now U.S. Pat. No. 5,938,847. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of coating a film on an object such as a semiconductor wafer or an LCD substrate, an apparatus for coating a film on such an object, and an apparatus for counting particles existing in the coating liquid. 
     In the manufacture of a semiconductor device, a circuit pattern is formed by means of so-called photolithography. The photolithography comprises the steps of: coating a photoresist on a semiconductor wafer, exposing the photoresist to light by using a photomask, and developed the photoresist thus exposed to light. 
     In the photoresist-coating step, the resist liquid is applied on to the center part of the semiconductor wafer from a nozzle located above the wafer, while the wafer held on a spin chuck is spinning at high speed. The resist liquid thus applied spread by virtue of the centrifugal force the wafer exerts while spinning. As a result, a resist film having an uniform thickness is formed on the entire surface of the semiconductor wafer. 
     The semiconductor wafer with the resist film coated on it is subjected to heat treatment, light-exposure, development and etching. A circuit pattern is thereby formed on the semiconductor wafer. The circuit pattern may not be a desired one if the resist film contains particles. 
     To form a resist film containing as few particle as possible, a filter is interposed between the nozzle and the resist liquid source to filter out particles from the resist liquid. The efficiency of the filter gradually decrease with time. Hence, the filter may fail to filter out particles after a long use. Unless the filter is replaced with a new one, many particles will remain in the resist liquid. 
     To decide whether or not the filter should be replaced with a new one, it is necessary to determine how much the efficiency of the filter has decreased. To this end it is required that the particles in the resist liquid be counted before the liquid is applied to semiconductor wafers. It is proposed that the particle counters commercially available be used to count the particles in the resist liquid. 
     Here arises a problem. The conventional particle counters are designed to count particles existing in low-viscosity liquids such as pure water and hydrofluoric acid, not to count particles in a high-viscosity liquid such as resist liquid which has viscosity of several cP to several hundred cP. If a conventional particle counter is placed between the nozzle and the semiconductor wafer and used for a long time to count particles in the resist liquid applied from the nozzle, the resist liquid sticks to the inner wall of the optical cell of the counter. Much time and labor are required to wash the particle counter. In view of this, the conventional particle counter cannot be used in an in-line fashion as is employed to count particles existing in pure water or hydrofluoric acid. 
     The counter must therefore be located outside the line of manufacturing semiconductor devices. In this case, the resist liquid must be sampled, and samples must be supplied to the particle counter. This also requires much time and labor. 
     The conventional particle counter cannot be used to count particles in the resist liquid, for another reason. It applies a light beam, such as a laser beam, to a liquid to count particles existing in the liquid. When the conventional particle counter applies a light beam to the resist liquid, the resist liquid emits light. This makes it difficult for the counter to count particles in the resist liquid with a sufficiently high accuracy. 
     BRIEF SUMMARY OF THE INVENTION 
     The first object of the invention is to provide a method of coating a film on a substrate, in which before a coating liquid (e.g., a resist liquid) is applied to the substrate from a liquid-applying member such as nozzle, it is determined whether the coating liquid contains impurities (e.g., particles) in an amount so large as to lower the yield of products to be made by using the film. 
     The second object of the present invention is to provide an apparatus which performs the film-coating method described above. 
     The third object of this invention is to provide an apparatus for coating a film on a substrate, in which the particles in the coating liquid used can be counted in in-line fashion. 
     The fourth object of the present invention is to provide an apparatus for counting the particle in such a coating liquid, in in-line fashion. 
     A first coating method designed to attain the first object is a method of coating a film on a substrate by applying a coating liquid to the substrate located at a first position. The method comprises the steps of: applying the coating liquid at a second position (generally known as “dummy dispensing position”) before applying the coating liquid at the first position; and detecting impurities contained in the coating liquid applied at the second position. 
     In most cases, the first position is above the center of the substrate. It suffices to set the second position away from the first position. Preferably, the second position should be set in an area not above the substrate, so that the coating liquid applied at the second position may not be applied to the substrate. To detect impurities, if any, contained in the liquid applied at the second position, the liquid may be collected, and a device such as a particle counter may be used to detect the impurities in the collected liquid. 
     As described above, the coating liquid is applied at the second position before it is applied at the first position, and the impurities contained in the liquid applied in the second position are detected. Hence, before applying the coating liquid to the substrate it can be determined whether too many particles exist in the coating liquid. The impurities may be detected immediately before the liquid is applied in the first position, at regular intervals, or every time the liquid is applied a prescribed number of substrates. 
     A second coating method designed to attain the first object is a method of coating a film on a substrate by applying a coating liquid to the substrate located at a first position, from one of a plurality of liquid-applying members. This method comprises the steps of: selecting one of the liquid-applying members; applying the coating liquid from the selected liquid-applying member at a second position before applying the coating liquid at the first position; and detecting impurities contained in the coating liquid applied at the second position; and moving the selected liquid-applying member to the first position and applying the coating liquid from the selected liquid-applying member to the substrate, only when the impurities are contained in the liquid in an amount less than a reference value. 
     Since a plurality of liquid-applying members are used in the second method, any desired one can be selected and used to apply the coating liquid to the substrate. 
     In the second method, the selected liquid-applying member is moved to the first position and applies the coating liquid to the substrate, only when the impurities are contained in the liquid in an amount less than a reference value. Thus, it can be determined whether or not too many particles exist in the coating liquid, before the coating liquid is applied to the substrate, as in the first method. If the impurities are contained in the liquid in an amount equal to or greater than the reference value, the selected liquid-applying member is not moved to the first position and the coating liquid is not applied to the substrate at all. 
     A first coating apparatus designed to achieve the second object is an apparatus for coating a film on a substrate by applying a coating liquid from a liquid-applying member to the substrate located at a first position. The apparatus comprises: a receptacle located at a second position, for receiving the coating liquid applied from the liquid-applying member; and a detecting device for detecting impurities contained in the coating liquid applied into the receptacle. 
     Preferably, the receptacle is located not above the substrate. It may be one which flares at its top. The receptacle may be connected to the detecting device by a tube, a pipe or the like. The detecting device is, for example, a particle counter which uses a laser beam to detect the impurities contained in the coating liquid. 
     The first apparatus can efficiently perform the fist coating method described above. 
     Even if a plurality of liquid-applying members, such as nozzles, are used, the first apparatus need not have a plurality of receptacles of the type described above need not be used. Only one receptacle is sufficient, in which case the apparatus is more simple, occupies a smaller space, and can be manufactured at a lower cost than otherwise. 
     The first apparatus may have a cleaning unit for cleaning a passage extending from at least the receptacle to the detecting device, through which the coating liquid is supplied. Once the passage is cleaned, no coating liquid examined previously remains in the passage. This ensures accurate detection of the impurities contained in the coating liquid now held in the receptacle. If the coating liquid is resist liquid, it suffices to supply solvent into the receptacle through the passage. 
     A second coating apparatus designed to achieve the second object is an apparatus for coating a film on a substrate by applying a coating liquid from a liquid-applying member to the substrate located at a first position, comprising: a liquid-applying member for applying the coating liquid; a detecting device for detecting impurities contained in the coating liquid; a first pipe connecting a source of the coating liquid to the liquid-applying member; a second pipe connecting the source of the coating liquid to the detecting device; and a switching device provided on the first pipe, for switching supply of the coating liquid between the liquid-applying member and the detecting device. 
     Unlike the first apparatus, the second apparatus is designed to detect impurities in the coating liquid in so-called in-line fashion. The liquid need not be applied from the liquid-applying member, for the purpose of detecting impurities in it. Without a receptacle, the impurities contained in the liquid can be detected. The switching device may be a switching valve such as a three-way valve. The second apparatus may have a plurality of liquid-applying members and a plurality of coating liquid sources. Even in this case, one pipe suffices to connect the coating liquid sources to the detecting device, and detecting device can examine different coating liquids which are to be applied from the liquid-applying members. 
     In the second apparatus, too, a cleaning unit may be used to clean a passage extending from at least the receptacle to the detecting device, through which the coating liquid is supplied. Once the passage is cleaned, no coating liquid examined previously remains in the passage. This ensures accurate detection of the impurities contained in the coating liquid now held in the receptacle. If the coating liquid is resist liquid, it suffices to supply solvent into the receptacle through the passage. 
     A first coating apparatus designed to achieve the third object is an apparatus which comprises: a coating section for coating a resist liquid on an object; a resist liquid source for supplying the resist liquid to the coating section; resist-supplying pipe for supplying the resist liquid from the resist liquid source to the coating section; a sampling pipe branched from the resist-supplying pipe; a valve provided at a node of the sampling pipe and the resist-supplying pipe, for switching supply of the coating liquid between the coating section and the sampling pipe; a particle-counting device for counting particles existing in the resist liquid supplied from the sampling pipe; and means for supplying the cleaning solution to the particle-counting device. 
     The sampling pipe, the valve, and the solution-supplying means cooperate, supplying the cleaning solution to the particle-counting device to clean the same. This prevents the resist liquid from sticking to the inner wall of the optical cell incorporated in the particle-counting device. Thus cleaned, the particle-counting device can be operated in in-line fashion with high efficiency. 
     A second coating apparatus designed to achieve the third object is an apparatus comprising: a coating section for coating a resist liquid on an object; a resist liquid source for supplying the resist liquid to the coating section; a plurality of resist-supplying pipes for supplying the resist liquid from the resist liquid source to the coating section; a plurality of sampling pipes branched from the resist-supplying pipes, respectively; a plurality of valves provided at nodes of the sampling pipes on the one hand and the resist-supplying pipes on the other, each for switching supply of the coating liquid between the coating section and one sampling pipe; a measuring pipe to which the sampling pipes are connected; a particle-counting device connected to the measuring pipe, for counting particles existing in the resist liquid supplied from each of the sampling pipes; and means for supplying the cleaning solution to the particle-counting device through the measuring pipe. 
     In this apparatus, various resist liquids can be supplied into the particle-counting device through the sampling pipes and the measuring pipe. Hence, one particle-counting device suffices to counting the particles existing in various resist liquids flowing through the resist-supplying pipes. Since the cleaning solution supplying means supplies the cleaning solution to the particle-counting device through the measuring pipe, the resist liquid is prevented from sticking to the inner wall of the optical cell incorporated in the particle-counting device. Thus cleaned, the particle-counting device can be operated in in-line fashion with high efficiency. 
     A third apparatus designed to achieve the third object is an apparatus of the same structure as the first and second apparatuses described above. The particle-counting device incorporated in the third apparatus has a particle-detecting section and a particle-counting section. The particle-counting section is designed to count only particles other than those which the particle-detecting section has detected from light emitted from resist liquid. Hence, the particle-counting device can count particles with high precision, because the particle-detecting section is not influenced by the light emitting from the resist liquid. 
     A fourth apparatus designed to achieve the third object is an apparatus comprising: a coating section for coating a resist liquid on an object; a resist liquid source for supplying the resist liquid to the coating section; resist-supplying pipe for supplying the resist liquid from the resist liquid source to the coating section; a sampling pipe branched from the resist-supplying pipe; a valve provided at a node of the sampling pipe and the resist-supplying pipe, for switching supply of the coating liquid between the coating section and the sampling pipe; and a particle-counting device for counting particles existing in the resist liquid supplied from the sampling pipe. The particle-counting device has a particle-detecting section and a particle-counting section for counting only particles other than those which the particle-detecting section has detected from light emitted from resist liquid. 
     The fourth apparatus is advantageous in the same respect as the third apparatus described above. 
     A fifth apparatus designed to achieve the third object of the present invention is an apparatus of the same structure as the second apparatus described above. The fifth apparatus is characterized in that the particle-detecting section has a relatively low sensitivity and is not influenced by the light emitting from the resin component of the resist liquid. Not influenced by such light, the particle-detecting section can detect particles having sizes over a broad range, serving to count particles in any resist liquid with high accuracy. 
     A sixth apparatus which is designed to achieve the third object of the invention is an apparatus identical in structure to any one of the first to fifth apparatuses described above. The sixth apparatus is characterized in that a filter is provided on the resist-supplying pipe and located upstream of the node of the sampling pipe and the resist-supplying pipe. Located upstream of that node, the filter can be found to have become less efficient when particles increases in number in the resist liquid. 
     A first particle-counting apparatus designed to achieve the fourth object of the invention comprises: a particle-detecting section; and a particle-counting section. The particle-counting section counts only particles other than those which the particle-detecting section has detected from light emitted from resist liquid. In other words, the particle-counting section is not influenced by the light emitted from the resist liquid. Hence, the particle-counting device can count particles with high precision. 
     A second particle-counting apparatus designed to achieve the fourth object of the invention is identical in structure to the first particle-counting apparatus. The second particle-counting apparatus is characterized in that the particle-detecting section has a relatively low sensitivity and is not influenced by the light emitting from the resin component of the resist liquid. The particle-detecting section has such a sensitivity as to detect particles having a size equal to or greater than 0.16 μm. Not influenced by such light, the particle-detecting section can detect particles having sizes over a broad range, serving to count particles in any resist liquid with high accuracy. 
     Additional object and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The object and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a perspective view of a resist-coating and -developing system incorporating a resist liquid coating apparatus which is the first embodiment of the invention; 
     FIG. 2 is a schematic view showing the resist liquid coating apparatus according to the first embodiment of the invention; 
     FIG. 3 is a plan view of the resist liquid coating apparatus shown in FIG. 2; 
     FIG. 4 is a schematic view showing a resist liquid coating apparatus which is the second embodiment of the present invention; 
     FIG. 5 is a plan view of the resist liquid coating apparatus shown in FIG. 4; 
     FIG. 6 is a diagram illustrating the resist liquid supplying system, pipes used to count particles in the resist liquid and washing liquid supplying system, all incorporated in the apparatus shown in FIG. 4; 
     FIG. 7 is a flow chart explaining how the particles in the liquid are counted and how the particle-detecting section of a particle counter is cleaned; 
     FIG. 8 is a schematic diagram showing the particle-detecting section of the particle-counting apparatus; 
     FIG. 9 is a graph representing the influence the resist liquid imposes on the light emission from the resist liquid when the particle-counting apparatus counts the particles existing in the resist liquid; 
     FIG. 10 is a graph depicting the influence the resist liquid imposes on the light emission from the resist liquid when the sensor used in the particle-counting apparatus is set at a low sensitivity; and 
     FIGS. 11A and 11B are graphs showing how the particles increased in numbers with time, as counted by the particle-counting apparatus. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The first embodiment of the present invention will be described, with reference to FIGS. 1 to  3 . 
     FIG. 1 shows a resist-coating and -developing system  1 . The system  1  is designed to wash semiconductor wafers, adhere resists to the wafers, heat the semiconductor wafers, cool the wafers to a prescribed temperature, expose the wafers to light, develop the resists on the wafers, and heat the wafers after developing the resists. 
     As shown in FIG. 1, the system  1  comprises a cassette station  2 , a first transport arm  3 , a transport mechanism  4 , a transport path  5 , a second transport arm  6 , a first arm track  7 , and a second arm track  8 . Cassettes C, each containing a plurality of wafers W, are aligned on the cassette station  2 , along the transport path  5 . The transport mechanism  4  can moved along the transport path  5 . It is designed to remove the wafers W from the cassettes C and transport them to the first main transport arm  3 . The transport arms  3  and  6  can move along the arm tracks  7  and  8 , respectively. 
     The resist-coating and -developing system  1  further comprises various wafer-processing apparatuses. The apparatuses are a brush washing apparatus  9 , a water-washing apparatus  10 , an adhesion apparatus  11 , a cooling apparatus  12 , two resist-coating apparatuses  13 , a heating apparatus  14 , and two developing apparatuses  15 . The apparatuses  9 ,  10 ,  11 ,  12  and  14  are arranged on one side of the arm tracks  7  and  8 , while the apparatuses  13  and  15  are arranged on the other side of the arm tracks  7  and  8 . 
     The brush washing apparatus  9  rotates wafers W removed from the cassettes C and washes the wafers W. The water-washing apparatus  10  applies water in the form of a high-pressure jet, to the surfaces of the wafers W, thereby washing the wafers W. The adhesion apparatus  11  renders each wafer W hydrophobic at a surface, making a resist firmly adhere to the surface. The cooling apparatus  12  cools the wafers W to a predetermined temperature. The resist-coating apparatuses  13  apply a resist liquid to the surfaces of the wafers W, coating a resist film on each wafer W. The heating apparatus  14  heats the wafers W coated with resist and also the wafers W exposed to light. The developing apparatuses  15  rotate the light-exposed wafers W and apply developing liquid to the wafers W, thereby developing the resist film on each wafer W. 
     The wafer-processing apparatuses  9  to  15  are arranged close to one another, at appropriate positions, so that they occupy but a relatively small space and operate with high efficiency. Wafers W are brought into and out of the apparatuses  9  to  14  by means of the transport arms  3  and  6 . 
     As shown in FIG. 1, the system  1  further comprises a casing  16 . The casing  16  contains the cassette station  2 , transport arms  3  and  6 , transport mechanism  4 , transport path  5 , arm tracks  7  and  8 , and wafer-processing apparatuses  9  to  15 . 
     The resist-coating apparatuses  13  are identical, each being the first embodiment of the present invention. One of the apparatuses  13  will be described, with reference to FIGS. 1,  2  and  3 . 
     As FIG. 1 shows, the resist-coating apparatus  13  comprises a casing  13   a.  As shown in FIGS. 2 and 3, the apparatus  13  has a processing chamber  21 , a spin chuck  22 , a chuck drive  23 , drain pipes  24 , and a drain tank  25 , which are provided in the casing  13   a.  The spin chuck  22  is provided in the chamber  21 . The chuck drive  23  is located below the chamber  21 . The drain pipes  24  are provided in the bottom of the chamber  21 . The drain tank  25  is located outside the chamber  21 . 
     The spin chuck  22  is designed to hold a wafer W in a horizontal position by vacuum suction. The chuck  22  can be rotated by the chuck drive  23 . The chuck drive  23  is, for example, a pulse motor. The drive  23  can rotate the spin chuck  22  at various controlled speeds. Gases can be exhausted from the center part of the bottom of the processing chamber  21  by a gas-exhaust means (not shown) such as a vacuum pump, which is provided outside the processing chamber  21 . Resist liquid and solvent can be drained through the drain pipes  24  into the drain tank  25 . 
     As seen from FIG. 3, the resist-coating apparatus  13  further comprises a holder  13   b,  four resist-applying nozzles N 1  to N 4 , four solvent-applying nozzles S 1  to S 4 , and four nozzle holders  31  to  34 , all provided in the casing  13   a.  The resist-applying nozzles N 1  to N 4  are paired with the solvent-applying nozzles S 1  to S 4 , respectively, constituting four nozzles units. The nozzle units are held by the nozzle holders  31  to  34 , respectively. The nozzle holders  31  to  34  are held by the holder  13   b.  The holder  13   b  has through holes (not shown). The nozzles N 1  to N 4  and S 1  to S 4  are held in these through holes, each with its open end exposed to the solvent atmosphere in the housing  13   a.  As shown in FIG. 3, the nozzle holders  31  to  34  have pins  31   a,    32   a,    33   a  and  34   a,  respectively. The pins  31   a  to  34   a  can be held by a scan arm  37   a,  which will be described later. 
     The nozzle holders  31  to  34  are identical in basic structure. Only the nozzle holder  31  shown in FIG. 2 will be described. As shown in FIG. 2, a resist-supplying tube  41  is connected at one end to the resist-applying nozzle N 1  and at the other end to a resist liquid source R 1  which is located outside the casing  13   a.  A resist liquid is therefore supplied from the source R 1  to the resist-applying nozzle N 1  through the resist-supplying tube  41 . A filter  42  is provided in the tube  41 , for filtering out impurities such as particles from the resist liquid. Mounted on the tube  41  is a resist-supplying mechanism  43 , such as a bellows pump, for supplying the resist liquid to the nozzle N 1  at a predetermined flow rate. 
     Since the three other nozzle holders  32 ,  33  and  34  are identical to the first nozzle holder  31  in basic structure, three resist liquids can be applied independently to the wafer W from the nozzles N 2 , N 3  and N 4 . Thus, the resist-coating apparatus  13  can coat the wafer W with four different resist liquids. 
     Two tubes  35   a  and  25   b  are connected to the nozzle holder  31 . A temperature-controlling fluid is supplied to the holder  31  through the tube  35   a  and therefrom through the tube  35   b.  The fluid maintains at a desired temperature the resist liquid which flows through the resist-supplying tube  41  and is eventually applied to the wafer W from the resist-applying nozzle N 1 . 
     As illustrated in FIG. 2, a solvent-supplying tube  45  is connected at one end to the solvent-applying nozzle S 1  and at the other end to a solvent source T which is located outside the casing  13   a.  A solvent is therefore supplied from the source T to the solvent-applying nozzle S 1  through the solvent-supplying tube  45 . Mounted on the tube  45  is a solvent-supplying mechanism  44 , such as a pump, for supplying the solvent to the solvent-applying nozzle S 1 . Two tubes  36   a  and  26   b  are connected to the nozzle holder  31 . A temperature-controlling fluid is supplied to the holder  31  through the tube  36   a  and therefrom through the tube  36   b.  The fluid maintains at a desired temperature the solvent which flows through the solvent-supplying tube  45  and is eventually applied to the wafer W from the solvent-applying nozzle S 1 . 
     The nozzle holder  31  holding the resist-applying nozzle N 1  and the solvent-applying nozzle S 1  can be moved from the holder  13   b  to a desired position above the wafer W by the scan arm  37   a  of a scan unit  37 . The scan unit  37  is so designed that the scan arm  37   a  can move in three-dimensional fashion, namely in X axis, Y axis and Z axis. 
     As mentioned above, the resist-applying nozzles N 1  to N 4  are paired with the solvent-applying nozzles S 1  to S 4 , respectively, constituting four nozzles units. Instead, only the resist-applying nozzles N 1  to N 4  may be held by the nozzle holders  31  to  34 , respectively, and the solvent-applying nozzles S 1  to S 4  may be replaced by a single solvent-applying nozzle which is secured to a certain part of the scan arm  37   a.    
     As shown in FIGS. 2 and 3, the resist-coating apparatus  13  has a resist receptacle  51  which is located outside the processing chamber  21  and below the scan unit  37 . The receptacle  51  is a pipe having a flaring open top. A probe  51   a  is connected to the lower end of the receptacle  51 , for examining the resist liquid supplied to the resist receptacle  51 . Connected to the probe  51   a  is a particle counter  52 . The counter  52  is designed to apply, for example, a laser beam to the resist liquid in the probe  51   a,  thereby to count the particles existing in the resist liquid. The resist liquid can be drained from the probe  51   a  through a drain pipe  53 , along with the resist liquid and solvent discharged from the drain tank  25 . 
     In operation, a wafer W is placed on the spin chuck  22  located in the processing chamber  21 . The spin chuck  22  automatically holds the wafer W by vacuum suction. The chuck drive  23  rotates the spin chuck  22 , whereby the wafer W is rotated. Of the resist-applying nozzles N 1  to N 4 , a nozzle Nx is selected to apply the desired resist liquid. The scan arm  37   a  is moved to the nozzle holder holding the nozzle Nx. The nozzle Nx selected may be, for example, the resist-applying nozzle N 1 . In this case, the arm  37   a  is moved to the nozzle holder  31 , grasps the holder S 1  and moves the same to a desired position above the wafer W. The solvent is first applied from the nozzle S 1  and the desired resist liquid is then applied from the nozzle N 1 . 
     With the resist-coating apparatus  13  it is possible to count the number of particles existing in an unit amount of the desired resist liquid, before the wafer W is mounted and held on the spin chuck  22 . More precisely, the scan arm  37   a  is moved to, for example, the nozzle holder  31  holding the resist-applying nozzle N 1  (i.e., the selected nozzle Nx). The scan arm  37   a  grasps the holder  31  and moves it to a position right above the resist receptacle  51 . The nozzle N 1  applies the resist liquid into the receptacle  51  in a predetermined amount. The particle counter  52  counts the particles existing in the resist liquid in the probe  51   a.  After the counter  52  finishes counting the particles, the nozzle S 1  applies the solvent into the receptacle  51 , washing the receptacle  51  and removing the residual resist liquid therefrom. 
     If the number of the particles the counter has counted is equal to or smaller than a reference value, a wafer W is placed on the spin chuck  22 , and the scan arm  37   a  moves the holder  31  to a position above the wafer W. The nozzle N 1  applies the resist liquid to the wafer W which is rotating. If the number of the particles the counter has counted is greater than the reference value, an alarm device (not shown) provided outside the resist-coating apparatus  13  generates an alarm, and the scan gram  37   a  moves the holder  31  back to the holder  13   b.  In this case, the apparatus  13  performs no further operation until measures are taken to reduce the number of particles existing in the resist liquid. 
     Any one of the other resist-applying nozzles N 2  to N 4 , for example the nozzle N 2  held by the nozzle holder  32 , may be connected to a source R 1  of the desired resist liquid. If this is the case, the scan arm  37   a  moves the nozzle holder  31  back to the holder  13   b  at the same time the alarm device generates an alarm, and grasps the nozzle holder  32  and moves the same to the position right above the resist receptacle  51 . Then, the nozzle N 2  applies the same resist liquid into the receptacle  51  in the prescribed amount. The counter  52  counts the particles existing in the resist liquid in the probe  51   a,  to determine whether the resist liquid should be applied to a wafer W or not. While these steps are being carried out in sequence, the operator may repair the resist-supplying system connected to the nozzle N 1  and including the filter  42  and the resist-supplying mechanism  43 , thereby reducing the number of particles existing in the unit amount of the resist liquid supplied to the nozzle N 1 . Hence, the resist-coating apparatus  13  need not be stopped and can continuously apply the desired resist liquid to wafers W. 
     The components of the resist-coating apparatus  13  are automatically controlled by a controller (not shown) provided in the resist-coating and -developing system  1 . 
     If it takes the counter  52  a considerably long time to count the particles existing in the resist liquid in the probe  51   a,  the counter  52  need not be operated every time the apparatus  13  coats the resist liquid on a wafer W. Rather, the counter  52  may count particles every time the apparatus  13  finishes coating of the liquid on a prescribed number of wafers W, or may count particles at regular intervals of several hours or several days. 
     As can be understood from the above, the amount of impurities (e.g., particles) contained in any resist liquid can be detected before the resist liquid is coated on wafers W. This ensures to form a high-quality resist film on a wafer W, which helps to provide a flawless circuit pattern on the wafer W. 
     Since the resist receptacle  51  is located outside the processing chamber  21 , it is always away from the wafer W placed in the chamber  21 . The resist liquid would not contaminate the spin chuck  22  provided in the chamber  21 , while the liquid is being supplied from any resist-applying nozzle into the receptacle  51 . To prevent the liquid from dripping down to the spin chuck  22 , it is desirable to locate the receptacle  51  at a level below the top of the processing chamber  21 . The receptacle  51  may be coupled to the holder  13   b.  In this case, the space in the casing  13   a  of the apparatus  13  can be smaller, and the liquid will have far less chance of dripping down to the chamber  21  or the spin chuck  22 , because the holder  13   b  is remote from the processing chamber  21 . 
     If the case where the resist receptacle  51  is coupled to the holder  13   b,  the scan arm  27   a  need not move the nozzle holders  31  to  34  from the holder  13   b  to a position above the resist receptacle  51 . Thus, one of the nozzles N 1  to N 4  can apply an amount of the resist liquid into the receptacle  51  while any other resist-applying nozzle is applying the resist liquid onto the wafer W held on the spin chuck  22 . While the resist liquid is being applied to several wafers W, one after another, an amount of the resist liquid to be applied to other wafers thereafter may be supplied into the receptacle  51  and the counter  52  counts the particles in the liquid in the probe  51   a.  The resist-coating can then be effected without a break. 
     As described above, the receptacle  51  is remote from the holder  13   b  in the resist-coating apparatus  13  illustrated in FIGS. 2 and 3. Even in the apparatus  13 , the scan arm  37   a  may move any nozzle holder holding the resist-applying nozzle not applying the resist liquid to the wafer W mounted on the chuck  22  from the holder  13   b  to the position above the receptacle  51 . The particles in the resist liquid can then be counted at any time desired. 
     In order to maintain the resist receptacle  51  clean enough for more accurate counting of particles, the open top of the receptacle  51  may be kept closed all time, but when the resist liquid is supplied into the receptacle  51  in the predetermined amount. For the same purpose, a cleaning unit may be connected to the receptacle  51 , for applying a solvent into the receptacle  51  to remove the residual resist liquid therefrom. Furthermore, a pump may be provided on the drain pipe  53  to drain the resist liquid and the solvent from the probe  51   a.    
     A resist-coating apparatus  140  according to the second embodiment of the present invention will be described, with reference to FIGS. 4 to  6 . 
     As shown in FIGS. 4 and 5, the resist-coating apparatus  140  comprises a casing  125   a,  a processing chamber  141 , a spin chuck  142  a chuck drive  143 , drain pipes  144 , and a drain tank  146 . The chamber  141 , the chuck  142 , drive  143 , pipes  144  and tank  146  are provided in the casing  125   a.  The spin chuck  142  is provided in the chamber  141 . The chuck drive  143  is located below the chamber  141 . The drain pipes  144  are provided in the bottom of the chamber  141  and connected to the drain tank  146 . The tank  146  is located outside the chamber  21 . 
     The spin chuck  142  is designed to hold a wafer W in a horizontal position by vacuum suction. The chuck  142  can be rotated by the chuck drive  143 . The chuck drive  143  is, for example, a pulse motor. The drive  143  can rotate the spin chuck  142  at various controlled speeds. Gases can be exhausted from the center part of the bottom of the processing chamber  141  by a gas-exhaust means (not shown) such as a vacuum pump, which is provided outside the processing chamber  141 . Resist liquid and solvent, which have been scattered from the wafer W being coated with the resist liquid, can be drained through the drain pipes  144  into the drain tank  145 . A drain pipe  147  is connected to the drain tank  146 . The resist liquid and the solvent can be drained through the drain pipe  147  from the tank  147 , and ultimately from the resist-coating apparatus  140 . 
     As seen from FIG. 15, the resist-coating apparatus  140  further comprises a holder  125   b,  four resist-applying nozzles N 1  to N 4 , four solvent-applying nozzles S 1  to S 4 , and four nozzle holders  151  to  154 , all provided in the casing  125   a.  The resist-applying nozzles N 1  to N 4  are paired with the solvent-applying nozzles S 1  to S 4 , respectively, constituting four nozzles units. The nozzle units are held by the nozzle holders  151  to  154 , respectively. The nozzle holders  151  to  154  are held by the holder  125   b.  The holder  125   b  has through holes (not shown). The nozzles N 1  to N 4  and S 1  to S 4  are held in these through holes, each with its open end exposed to the solvent atmosphere in the housing  125   a.  As will be described later, four resist-supplying pipes are connected to the nozzles N 1  to N 4 , respectively. Four different resist liquids can therefore be applied to the wafer W held on the spin chuck  142 . 
     As mentioned above, the resist-applying nozzles N 1  to N 4  are paired with the solvent-applying nozzles S 1  to S 4 , respectively, constituting four nozzles units. Instead, only the resist-applying nozzles N 1  to N 4  may be held by the nozzle holders  151  to  154 , respectively, and the solvent-applying nozzles S 1  to S 4  may be replaced by a single solvent-applying nozzle which is secured to a certain part of a scan arm  157   a  (later described). 
     As shown in FIG. 5, the nozzle holders  151  to  154  have pins  151   a,    152   a,    153   a  and  154   a,  respectively. The pins  151   a  to  154   a  can be held by the scan arm  157   a.  The scan arm  157   a  can be driven by a scan unit  157 , in three-dimensional fashion, namely in X axis, Y axis and Z axis. In operation, the scan unit  157  moves the scan arm  157   a  to any selected one of the nozzle holders  151  to  154 . Thus moved, the scan arm  157  grasps, for example, the pin  151   a  of the nozzle holder  151 . The scan unit  157  further moves the scan arm  157   a  to a position above the wafer W mounted on the spin chuck  142 . The selected nozzle holder  151  is thereby positioned above the wafer W. 
     As shown in FIG. 4, four tubes  155   a,    155   b,    156   a  and  156   b  are connected to each nozzle holder. A temperature-controlling fluid is supplied to the nozzle holder through the tube  155   a  and therefrom through the tube  155   b.  The fluid maintains at a desired temperature the resist liquid to be applied through the resist-applying nozzle to the wafer W. Similarly, a temperature-controlling fluid is supplied to the nozzle holder through the tube  156   a  and therefrom through the tube  156   b.  This fluid maintains at a desired temperature the solvent to be applied through the solvent-applying nozzle to the wafer W. 
     A system for supplying resist liquids to the resist-applying nozzles N 1  to N 4  of the resist-coating apparatus  140  will be described with reference to FIG.  6 . FIG. 6 shows not only the resist-supplying system, but also a counter for counting particles existing in the resist liquid and a system for supplying a cleaning solution. 
     As seen from FIG. 6, four resist-supplying pipes  161 ,  162 ,  163  and  164  are connected at one end to the resist-applying nozzles N 1  to N 4 , and at the other end to resist reservoirs R 1 , R 2 , R 3  and R 4 , respectively. 
     The resist-supplying pipes  161  to  164  extend parallel to one another. Meters L 1  to L 4  are mounted on the pipes  161  to  164 , respectively, for detecting the amounts of the resist liquids remaining in the reservoirs R 1  to R 4 . Pumps P 1  to P 4  are provided on the pipes  161  to  164 , respectively. Further, filters F 1  to F 4  are provided on the pipes  161  to  164 , respectively. Still further, air-operated valves V 1  to V 4  are provided on the resist-applying pipes  161  to  164 , respectively. Connected to the air-operated valves V 1  to V 4  are sampling pipes  171  to  174  which branch from the resist-applying pipes  161  to  164 , respectively. Each air-operated valve has two outlet ports D and M. The first outlet port D is connected to the resist-supplying pipe, while the second outlet port M is connected to the sampling pipe. 
     Usually, the first outlet ports D 1  to D 4  of the air-operated valves V 1  to V 4  are open and the second outlet ports M 1  to M 4  are closed, whereby the resist liquids flow through the pipes  161  to  164  to the resist-applying nozzles N 1  to N 4 , respectively. To count the particles in the resist liquids, whenever necessary, the second outlet ports M 1  to M 4  are opened, whereby the resist liquids flow through into the sampling pipes  171  to  174 , respectively. Since the valves V 1  to V 4  are located downstream of the filters F 1  to F 4 , it can be readily determined how much particles have increased in numbers in the resist liquid due to the decrease in the efficiency of the filter. 
     The sampling pipes  171  to  174  are connected to one pipe  175 . The pipe  175  is connected at the downstream end to a particle counter  180 . The particle counter  180  comprises a particle-detecting section  181  and a particle-counting section  182 . The particle-detecting section  181  has a light source and a sensor. The resist liquids can be supplied to the particle-detecting section  181  from the resist-supplying pipes  161  to  164  through the sampling pipes  171  to  174  and then through the pipe  175 . The section  181  detects the particles in any resist liquid supplied to it. The section  182  counts the particles the section  181  has detected. The resist liquid is then drained from the particle-detecting section  182  through the drain pipe  147 . 
     As shown in FIG. 6, a system for supplying a cleaning solution is provided, opposing the particle counter  180 . This system comprises a solution-supplying pipe  160 , a solution reservoir R 0 , a meter L 0 , a filter F 0 , and an air-operated valve V 0 . The solution-supplying pipe  160  extends parallel to the resist-supplying pipe  161 . The solution reservoir R 0  is connected to the upstream end of the pipe  160 . The tank  160  contains a cleaning solution, which is supplied through the pipe  160  under the pressure of N 2  gas. The solution may be forced through the pipe  160  by means of a pump, not by the pressure of N 2  gas. The meter L 0 , the filter F 0  and the valve V 0  are provided on the solution-supplying pipe  160 , in the order mentioned from the upstream end of the pipe  160 . 
     The meter L 0  is provided to detect the amount of the cleaning solution remaining in the reservoirs R 0 . The filter F 0  is designed to filter out particles from the cleaning solution. 
     The air-operated valve V 0  has two outlet ports D 0  and M 0 . The second outlet port M 0  is connected to the pipe  175 . Usually, the first outlet ports D 0  is opened and the second outlet port M 0  is closed. To supply the cleaning solution through the pipe  175 , the first outlet ports D 1  to D 4  are closed and the second outlet ports M 1  to M 4  are opened, whereby the cleaning solution flows to the particle-detecting section  181  of the particle counter  180  through the pipe  175 , passing the nodes of the pipe  175  and the sampling pipes  171  to  174 . The optical cell and the like incorporated in the particle-detecting section  181  is cleaned with the cleaning solution. The cleaning solution is discharged after use, from the section  181  through the drain pipe  174 . 
     To apply the resist liquid to the wafer W on the spin chuck  142  from the nozzle N 1 , for example, the pump P 1  draws the resist liquid from the reservoir R 1  via the meter L 1 . When a resist-applying signal is supplied to the valve V 1 , the pump P 1  supplies the resist liquid to the valve V 1  through the filter F 1 . At the same time the first outlet port D 1  of the valve V 1  is opened, whereby the resist liquid is supplied from the first outlet port D 1  to the nozzle N 1  via the resist-supplying pipe  161 . The nozzle N 1  applies the resist liquid to the wafer W held on the spin chuck  142 . After the resist liquid has been applied to the wafer W in the prescribed amount, the first outlet port D 1  of the valve V 1  is closed. The pump P 1  draws the resist liquid from the reservoir R 1  so that the resist liquid may be supplied to the nozzle N 1  and may be applied to the next wafer W. 
     The resist liquid is applied from the other resist-applying nozzles N 2 , N 3  and N 4 , exactly in the same way as from the resist-applying nozzle N 1 . 
     How the particles in the resist liquid are counted in the resist-coating apparatus  140  will be explained, with reference to the flow chart of FIG.  7 . 
     To count the particles existing in the resist liquid flowing through resist-supplying pipe  161 , a count-starting signal is supplied to the valve V 1  after the pump P 1  has drawn the resist liquid from the reservoir R 1  via the meter L 1 , and the second outlet port M 1  of the valve V 1  is opened (Step ST 1 ). The resist liquid is thereby supplied from the second outlet port M 1  to the particle-detecting section  181  of the particle counter  180  through the sampling pipe  171  and the pipe  175 . The section  181  detects the particles existing in the resist liquid (Step ST 2 ). Thereafter, the second port M 1  of the valve V 1  is closed, and the pump P 1  draws the resist liquid from the reservoir R 1  (Step ST 3 ). In order to achieve accurate counting of particles, the pipe  175  must be filled up with the resist liquid supplied from the reservoir R 1 . It is therefore desired that the sequence of Steps ST 1  to ST 3  be carried out several times. 
     Upon completion of the counting of particles, the resist liquid is drained from the pipe  175 . Then, the particle-detecting section  181  (particularly, the optical cell) of the counter  180  is cleaned. More precisely, the second outlet port M 0  of the air-operated valve V 0  provided on the solution-supplying pipe  160  is opened (Step ST 4 ). The cleaning solution is thereby supplied to the particle-detecting section  181  under the N 2  gas pressure, through the filter F 0 , the second outlet port M 0  of the valve V 0  and the pipe  175 . The pipe  157  and the section  181  are cleaned with the cleaning solution (Step ST 5 ). Upon completion of the cleaning, the second port M 0  of the air-operated valve V 0  is closed (Step ST 6 ). 
     To count the particles existing in the resist liquid flowing through resist-supplying pipe  162 , a count-starting signal is supplied to the valve V 2  after the pump P 2  has drawn the resist liquid from the reservoir R 2  via the meter L 2 , and the second outlet port M 2  of the valve V 2  is opened (Step ST 7 ). The resist liquid is thereby supplied from the second outlet port M 2  to the particle-detecting section  181  of the particle counter  180  through the sampling pipe  172  and the pipe  175 . The section  181  detects the particles existing in the resist liquid (Step ST 8 ). 
     Upon completion of the counting of particles, the second outlet port M 2  of the air-operated valve V 2  is closed (Step ST 9 ). Next, the first outlet port M 0  of the air-operated valve V 0  mounted on the pipe  160  is opened (Step ST 10 ). Further, the particle-detecting section  181  (particularly, the optical cell) of the counter  180  is cleaned (Step ST 11 ). Upon completion of the cleaning, the second port M 0  of the air-operated valve V 0  is closed (Step ST 12 ). 
     The particles in the resist liquid flowing through resist-supplying pipe  163  are then detected and counted (Steps ST 13  to ST 15 ), in the same way as those existing in the resist liquid flowing through the pipe  161 . 
     Thereafter, the particles in the resist liquid flowing through resist-supplying pipe  164  are detected and counted, in the same way as those existing in the resist liquid flowing through the pipe  161 . 
     The resists liquids flowing through the resist-supplying pipes  161 ,  162 ,  163  and  164  need not be subjected to the particle-counting process in the order specified above. Rather, they can automatically be subjected to the process in any other order and at any desired intervals, in accordance with an operation-sequence program stored in a memory. Whenever the number of the particles counted is greater than a reference value, an alarm device (not shown) generates an alarm. 
     The solution-supplying system including the solution-supplying pipe  160  can clean both the pipe  175  and the particle-detecting section  181  whenever necessary. The resist hardly remains in the optical cell of the section  181  or contaminates the section  181 . The particle counter  180  can therefore accurately count the particles which exist in the resist liquid flowing through each of the resist-supplying pipes  161  to  164 . The counter  180  is an efficient device since it can count the particles existing in the resist liquid flowing through a plurality of resist-supplying pipe, i.e., the pipes  161  to  164 . 
     The resist liquids may have different viscosities. Even in this case, each resist liquid can be supplied to the particle-detecting section  181  in an appropriate amount, provided that the pumps P 1  to P 4  are of the type which can supply resist liquid at a different flow rates. Needless to say, the pump provided on each resist-supplying pipe can supply resist liquid to the associated resist-applying nozzle in such a flow rate that the nozzle applies the liquid in a desired amount to the wafer W. 
     The solution-supplying pipe  160  is located farther from the particle counter  180  than the resist-supplying pipes  161  to  164 . The cleaning solution supplied from the line  160  to the pipe  175  can therefore clean the nodes of the pipe  175  and the sampling pipes  171  to  174 . 
     Two or more solution-supplying pipes may be used, not one pipe only, for supplying different types of cleaning solutions to the particle-detecting section  181  through the pipe  157 . If so, one of the clearing solutions can be selected in accordance with which type of a resist liquid has been supplied to the section  181 , so that the pipe  175  and the section  181  may be cleaned efficiently and thoroughly. 
     The particle counter  180  will be described in detail, with reference to FIG.  8 . 
     As described above, the particle counter  180  comprises the particle-detecting section  181  and the particle-counting section  182 . As shown in FIG. 8, the section  181  has a laser  183 , a sensor  184 , and an optical cell  185 . The laser  183  and the sensor  184  oppose each other. The cell  185  is located between the laser  183  and the sensor  184 . To count the particles existing in the resist liquid supplied into the optical cell  185 , the laser  183  emits a laser beam to the cell  185 , illuminating the particles in the resist liquid. The sensor  184  detects the particles thus illuminated and generates signals which correspond to the particles detected. The signals are input to the particle-counting section  182 . The section  182  processes the signals, generating data representing the number of the particles the sensor  184  has detected. 
     When the laser beam is applied to the resist liquid in the optical cell  185 , the resin component of the resist liquid emits light. The light emitted from the resin component lowers the accuracy of counting particles which have a size less than, for example, 0.25 μm. Influenced by this light, the sensor  184  makes counts a, b and c of particles having sizes less than size L, which are greater than the numbers of the particles of these sizes actually existing in the resist liquid, as is seen from FIG.  9 . (The shaded region in FIG. 9 indicates the counts the sensor  184  provides of non-existent particles, due to the light from the resin component.) Thus, the sensor  184  cannot accurately count particles having a size less than size L. 
     As can be understood from FIG. 9, the sensor  184  can make accurate counts d, e and f of the particles having sizes greater than size L, not influenced by the light emitted from the resin component of the resist liquid. Therefore, the counts the sensor  184  makes of only those particles which have a size equal to or greater than L may be used to determine whether or not the resist liquid contain an excessive number of particles, not using the counts of the particles having a size less than L. The threshold particle size L depends on the type of the resist liquid examined. Thus, the parameters set in the particle-counting section  182  for processing the signals generated by the sensor  184  should be changed in accordance with the type of the resist liquid. 
     Generally, particles assume so-called “logarithmic normal distribution” in terms of their sizes. The smaller the particles, the greater number of them. It is therefore necessary to count small particles in resist liquid, as well as large ones, in order to determine accurately whether or not the liquid contains too many particles. If the counts a, b and c the sensor  184  makes of particles having sizes less than size L and which are influenced by the light emitted from the resin component of the resist liquid are not considered, as mentioned above, it will be impossible to correctly determine whether the liquid contain an excessive number of particles. 
     In the present embodiment, the sensitivity of the sensor  184  is reduced. As a result, the counts it makes of relatively small particles are less influenced by the light emitted from the resin component of the resist liquid, as is illustrated in FIG.  10 . It is only the particles having a size less than size L′ which is less than size L. The size L′ is, for example, 0.16 μm. The sensitivity of the sensor  184  may be reduced to the sensitivity to detect particles having a size equal to or greater than 0.16 μm. Although the counts a′ to f′ the sensor  184  makes of the particles over the entire range of size are relatively reduced, they can be corrected on the basis of the counts the sensor  184  makes of particles existing in a liquid filled in the optical cell  185 , such as pure water, which contains nothing which emits light when the laser beam is applied to the liquid. 
     It is known that particles increases with time in the resist liquid in two distinctive manners, as is illustrated in FIGS. 11A and 11B. If the particles increases abruptly as shown in FIG. 11A, this is perhaps because the resist liquid in the reservoir has been contaminated or because any devices provided on the resist-supplying pipe malfunctions. In this event, the resist-coating apparatus  140  must be stopped immediately, and appropriate measures must be taken to reduce the number of particles. If the particles increases gradually as shown in FIG. 11B, this is probably because the filter or the pump provided on the resist-supplying pipe, or both have become less efficient over a long use. In this case, the either the filter or the pump, or both, must be replaced by new ones. In whichever manner the particles increase, the alarm device (not shown) gives an alarm to the operator or the host computer which controls the resist-coating and -developing system  1  (FIG. 1) when the number of the particles counted by the sensor  184  exceeds the reference value. 
     The present invention is not limited to the embodiments described above. Rather, various changes and modifications can be made. For instance, the resist-coating apparatus according to the invention may have only one resist-applying nozzle. Further, the piping system for supplying resist liquids and cleaning solution is not limited to the one shown in FIG.  6 . Still further, the resin liquid may be applied to LCD glass substrates, instead of semiconductor wafers W. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalent.