Method for cleaning a workpiece

Wafers (W) is immersed in a cleaning liquid (L) contained in a cleaning tank (20). The cleaning liquid (L) is supplied into the cleaning tank (20) so that the cleaning liquid (L) overflows the cleaning tank (20). The cleaning liquid (L) overflowed the cleaning tank (20) is filtered, circulated and returned into the cleaning tank (20). A motor-operated bellows pump (30) is connected by a suction pipe (51) to the cleaning tank (20). A particle counter (5) for counting particles contained in a sample of the cleaning liquid (L) sampled by the motor-operated bellows pump (30) is placed on the suction pipe (51) and connected to the suction side of the motor-operated bellows pump (30).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a cleaning apparatus for cleaning
 workpieces, such as semiconductor wafers or glass substrate for LCDs.
 2. Description of the Related Art
 Generally, a cleaning apparatus is employed prevalently in a semiconductor
 device fabricating process. The cleaning apparatus carries workpieces,
 such as semiconductor wafers or glass substrate for LCDs, (hereinafter
 referred to as "wafers") sequentially to cleaning tanks respectively
 containing chemical liquids and rinsing liquids for cleaning and the like.
 A known cleaning apparatus shown in FIG. 13 has a cleaning tank c having
 an inner tank a containing a cleaning liquid L in which wafers W are
 immersed and an outer tank b surrounding an upper end part of the inner
 tank a. A circulation line f connecting cleaning liquid supply nozzles d
 disposed in a lower part of the inner tank a and a drain port e formed in
 a bottom wall of the outer tank b is provided, for example, with an air
 bellows circulating pump g, a damper h and a filter i. Wafers W held on a
 wafer boat j are immersed in the cleaning liquid L contained in the inner
 tank a of the cleaning apparatus, the cleaning liquid L is supplied from a
 cleaning liquid source into the inner tank a so that the cleaning liquid L
 overflows the inner tank a into the outer tank b. The cleaning liquid L
 overflowed into the outer tank b is filtered and circulated. The wafers W
 are thus cleaned.
 As the cleaning liquid is used repeatedly, contaminative particles, such as
 particles removed from the wafers W, are accumulated in the cleaning
 liquid. The wafers subjected to cleaning in the cleaning liquid are
 contaminated, the yield of the cleaning process is reduced and the
 cleaning performance of the cleaning apparatus is reduced if the particle
 concentration of the cleaning liquid exceeds a predetermined level.
 As shown in FIG. 13, a branch line k for quality testing is connected to a
 part of the circulation line f on the discharge side of the circulating
 pump g, a testing means, for example, a particle counter m, is connected
 to the branch line k, and the discharge side of the particle counter m is
 connected to the outer tank b. A portion of the cleaning liquid L
 contained in the inner tank a is sampled and the number of particles
 contained in the sample cleaning liquid is measured to monitor the number
 of particles contained in the predetermined quantity of cleaning liquid L.
 In FIG. 13, indicated at n is a shutoff valve placed in the circulation
 line f, at p is a drain pipe connected to a drain port q formed in the
 bottom wall of the inner tank a, and at r is a drain valve placed in the
 drain pipe q.
 Since the circulating pump g is of an air bellows type, the flow rate of
 the sample cleaning liquid is unstable and, consequently, accurate
 measurement of particles cannot be achieved. Since the circulating pump g
 serves also as means for supplying the cleaning liquid to the particle
 counter m, the cleaning liquid is supplied at a flow rate exceeding the
 ability of the particle counter m. Furthermore, since the measurement of
 particles uses the sample cleaning liquid sampled at a part of the
 circulation line f on the discharge side of the circulating pump g, the
 particle counter m adds the number of particles produced by the
 circulating pump g to the number of particles originally contained in the
 cleaning liquid and, consequently, the number of particles originally
 contained in the cleaning liquid cannot accurately be measured.
 SUMMARY OF THE INVENTION
 The present invention has been made in view of the foregoing problems and
 it is therefore an object of the present invention to provide a cleaning
 apparatus capable of accurately measuring fine contaminative particles,
 such as particles contained in a cleaning liquid, of operating at an
 improved yield and of exercising improved cleaning performance, and to
 provide a cleaning method to be carried out by the cleaning apparatus.
 The foregoing object can be achieved by the followings.
 According to one aspect of the present invention, a cleaning apparatus
 comprises a cleaning tank for containing a cleaning liquid in which
 workpieces are immersed for processing, a circulation line connected to
 the cleaning tank and provided with a filtering device, and a measuring
 line separate from the circulation line, provided with a fixed-quantity
 delivery means and a measuring means for measuring fine contaminated
 particles, such as particles, contained in the cleaning liquid and having
 a suction end connected to the cleaning tank.
 In the cleaning apparatus of the present invention, the measuring means may
 be connected to the suction side of the fixed-quantity delivery means.
 In the cleaning apparatus of the present invention, the measuring means may
 be connected to the discharge side of the fixed-quantity delivery means.
 The measuring means is able to measure fine contaminative particles, such
 as particles, contained in the cleaning liquid accurately because a fixed
 quantity of the cleaning liquid can be taken out from the cleaning tank.
 The cleaning apparatus of the present invention may be provided with a
 control means for synchronously operating the fixed-quantity delivery
 means and the measuring means.
 The control means makes the measuring means carry out a measuring operation
 while the fixed-quantity delivery means is in suction operation.
 A fixed quantity of the cleaning liquid can be sampled from the cleaning
 tank and the measuring means is able to achieve the accurate measurement
 of the fine contaminative particles, such as particles, contained in the
 cleaning liquid. The synchronous operation of the fixed-quantity delivery
 means and the measuring means improves measuring accuracy.
 In the cleaning apparatus, the discharge end of the measuring line may be
 connected to the cleaning tank.
 When the discharge end of the measuring line is connected to the cleaning
 tank, the sample cleaning liquid can be returned to the cleaning tank for
 the effective use of the cleaning liquid.
 In the cleaning apparatus of the present invention, the cleaning tank may
 have an inner tank in which workpieces are immersed in the cleaning
 liquid, and an outer tank for containing the cleaning liquid overflowed
 the inner tank, and the suction side of the measuring line may be
 connected to the inner tank.
 When the cleaning apparatus is thus constructed, a fixed quantity of the
 cleaning liquid contained in the inner tank in which workpieces are
 immersed in the cleaning liquid can be sampled and fine contaminative
 particles, such as particles, contained in the cleaning liquid can
 accurately be measured by the measuring means.
 In the cleaning apparatus of the present invention, the discharge side of
 the measuring line may be connected to the outer tank.
 When the discharge side of the measuring line is connected to the outer
 tank, the sample cleaning liquid subjected to measurement can be
 discharged into the outer tank instead of directly returning the same into
 the inner tank in which the workpieces are immersed in the cleaning
 liquid. Accordingly, the cleaning ability of the cleaning liquid is not
 reduced and the cleaning liquid can be circulated for the effective use of
 the same.
 In the cleaning apparatus the present invention, the fixed-quantity
 delivery means may be a motor-operated bellows pump comprising a
 corrosion-resistant and chemical-resistant bellows, and a ball screw
 mechanism for driving the bellows.
 The use of the chemical-resistant, durable motor-operated bellows pump
 capable of pumping a fixed quantity of the cleaning liquid extends the
 life of the apparatus, and improves measuring accuracy and the reliability
 of the apparatus.
 In the cleaning apparatus of the present invention, the fixed-quantity
 delivery means may comprise a plurality of motor-operated bellows pumps
 arranged in parallel, and the bellows pumps may be driven so that the
 bellows pumps operate in different phases, respectively.
 When the motor-operated bellows pumps excellent in ability to pump a fixed
 quantity of fluid, chemical resistance and durability are employed the
 fixed-quantity delivery means, the life of the apparatus can be extended,
 measuring accuracy can be improved, the reliability of the apparatus can
 be enhanced, and fine contaminative particles, such as particles,
 contained in the cleaning liquid can accurately and continuously be
 measured.
 In the cleaning apparatus of the present invention, the control means may
 provide a detection signal when the contaminated particle number of the
 cleaning liquid determined on the basis of measured data provided by the
 measuring means exceeds a predetermined upper limit value.
 According to another aspect of the present invention, a cleaning method
 which immerses workpieces in a cleaning liquid contained in a cleaning
 tank and circulates the cleaning liquid contained in the cleaning tank
 through a circulation line provided with a filtering device for filtering
 the cleaning liquid comprises a step of sampling a fixed quantity of the
 cleaning liquid from the cleaning tank by a fixed-quantity delivery means,
 a step of measuring fine contaminative particles contained in the cleaning
 liquid sampled by the fixed-quantity delivery means by a measuring means,
 and a step of providing a detection signal representing the contaminative
 particle number of the cleaning liquid determined on the basis of measured
 data measured by the measuring means by a control means.
 The control means of the present invention may provide a cleaning liquid
 change request signal when the contaminative particle number of the
 cleaning liquid on the basis of measured data provided by the measuring
 means exceeds a predetermined upper limit value.
 The control means of the present invention may compare the contaminative
 particle number of the cleaning liquid sampled before a start of a first
 cleaning cycle and that of the cleaning liquid sampled after the
 completion of a cleaning cycle, and may provide the detection signal when
 a differential particle number between the respective contaminative
 particle numbers of the cleaning liquids, exceeds a predetermined upper
 limit value.
 The control means of the present invention may provide an abnormal
 workpiece signal when the contaminative particle number on the basis of
 the measured data provided by the measuring means exceeds a predetermined
 upper limit value.
 According to the present invention, a fixed quantity of the cleaning liquid
 can be sampled from the cleaning liquid contained in the cleaning tank at
 a sampling position other than the circulation line before starting a
 cleaning process or during a cleaning process, and the quantity of
 contaminative particles contained in the cleaning liquid is measured.
 Therefore, contaminative particles contained in the cleaning liquid can
 accurately be measured, and it is possible to inform the operator of an
 inappropriate condition of the cleaning liquid by a detection signal
 indicating the quantity of contaminative particles exceeding a
 predetermined upper limit value. It is also possible to inform the
 operator that the quantity of contaminative particles contained in the
 cleaning liquid is not greater than the predetermined upper limit value
 and the normal cleaning process can be achieved.
 The cleaning method of the present invention may further comprise a step of
 cleaning workpieces by immersing the same in the cleaning liquid contained
 in the cleaning tank after it is decided on the basis of a detection
 signal provided by the control means that the cleaning liquid is
 appropriate to cleaning.
 According to the present invention, the ability and the yield of the
 cleaning process can be improved by cleaning the workpiece after it is
 decided that the measured quantity of contaminative particles is not
 greater than the predetermined upper limit value indicating the upper
 limit of the quantity of contaminative particles for the cleaning liquid
 appropriate to cleaning.
 The cleaning method of the present invention may further comprise a step of
 changing the cleaning liquid when the cleaning liquid change request
 signal is provided continuously by the control means for a time exceeding
 a predetermined time.
 According to the present invention, the cleaning liquid is changed if the
 quantity of contaminative particles does not decrease below the
 predetermined upper limit value in the predetermined time. Therefore, the
 ability and the yield of the cleaning process can be improved.
 The cleaning method of the present invention may further comprise a step of
 changing the cleaning liquid when a differential particle number between
 the measured contaminative particle number of the cleaning liquid sampled
 before a start of a first cleaning cycle and that of the cleaning liquid
 sampled after the completion of a cleaning cycle exceeds a predetermined
 upper limit value.
 According to the present invention, the cleaning ability of the cleaning
 liquid is improved and the yield can be improved because the cleaning
 liquid is changed when the differential particle number, i.e., the
 difference between the measured contaminative particle number of the
 cleaning liquid sampled before the start of the first cleaning cycle and
 that of the cleaning liquid sampled after the completion of the cleaning
 cycle, exceeds the predetermined upper limit value.
 The cleaning method of the present invention may further comprise a step of
 comparing the measured contaminative particle concentration of the
 cleaning liquid sampled before a start of a cleaning cycle and that of the
 cleaning liquid sampled after the completion of the cleaning cycle, and
 providing the detection signal when a differential particle number between
 the respective contaminative particle numbers of the cleaning liquid,
 exceeds a predetermined upper limit value by the control means, and
 changing the cleaning liquid when the detection signal is provided by the
 control means.
 According to the present invention, the cleaning ability of the cleaning
 liquid is improved and the yield can be improved because the cleaning
 liquid is changed when the differential particle number, i.e., the
 difference between the measured contaminative particle number of the
 cleaning liquid sampled before the start of the cleaning cycle and that of
 the cleaning liquid sampled when the completion of a cleaning cycle,
 exceeds the predetermined upper limit value.
 The cleaning method of the present invention may change the cleaning liquid
 after the cleaning liquid has been used for a predetermined number of
 cleaning cycles or for a predetermined time.
 According to the present invention, the cleaning ability of the cleaning
 liquid is improved and the yield can be improved because the cleaning
 liquid is changed when the same has been used for the predetermined number
 of cleaning cycles on the basis of experimental data or for the
 predetermined time determined on the basis of experimental data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 First Embodiment
 A cleaning apparatus in a first embodiment according to the present
 invention will be described as applied to a semiconductor wafer cleaning
 system. Referring to FIG. 1, a cleaning system comprises, as principal
 components, a conveying section 2 for conveying a carrier 1 holding
 semiconductor wafers (workpieces) (hereinafter referred to simply as
 "wafers") in ahorizontal position, i.e., a wafer container, a wafer
 processing section 3 for processing the wafers W in a chemical liquid and
 a cleaning liquid and drying the same, and a wafer handling section 4
 interposed between the conveying section 2 and the processing section 3
 for transferring wafers W from the conveying section 2 to the processing
 section 3 and vice versa, adjusting the position of wafers W and changing
 the position of wafers W.
 The conveying section 2 has a carrier receiving unit 5a, a carrier
 delivering unit 5b and a wafer transfer unit 6, which are disposed at one
 end of the cleaning system. A conveying mechanism, not shown, is disposed
 between the carrier receiving unit 5a and the wafer transfer unit 6 to
 convey the carrier 1 from the carrier receiving unit 5a to the wafer
 transfer unit 6.
 Carrier lifters, not shown, are installed in the carrier receiving unit 5a
 and the wafer transfer unit 6, respectively. The lifters deliver empty
 carriers 1 to and receive the same from a carrier storage unit, not shown,
 disposed above the conveying section 2. A transfer robot, not shown,
 capable of moving in horizontal directions, i.e., directions along an X-
 and a Y-axis, and vertical directions, i.e., directions along a Z-axis is
 installed in the carrier storage unit. The transfer robot arranges empty
 carriers 1 received from the wafer transfer unit 6 and carries empty
 carriers 1 to the wafer transfer unit 6. Loaded carriers 1 containing
 wafers W also can be stored in the carrier storage unit.
 The wafer transfer unit 6 has an opening opening into the wafer handling
 section 4, and a lid operating device 8 is disposed in the opening of the
 wafer transfer unit 6. The lid operating device 8 removes a lid, not
 shown, from a carrier 1 and put the same on the carrier 1. The lid of a
 loaded carrier 1 loaded with unprocessed wafers W can be removed by the
 lid operating device 8 to carry the wafers W out of the carrier 1, and the
 lid can be put on the empty carrier 1 after all the wafers W have been
 carried out of the carrier 1. The lid of an empty carrier 1 transferred
 from the carrier storage unit to the wafer transfer unit 6 can be removed
 by the lid operating device 8, and the lid can be put on the carrier 1
 after the carrier 1 has been loaded with wafers W. A mapping sensor 9 is
 disposed near the opening of the wafer transfer unit 6 to count the number
 of wafers W contained in a carrier 1.
 Installed in the wafer handling section 4 are a wafer transfer arm 10,
 i.e., a horizontal conveying means, for receiving wafers W from the wafer
 transfer unit 6 and returning wafers W to the wafer transfer unit 6, a
 pitch changing mechanism, not shown, for holding a plurality of wafers W,
 such as fifth wafers W, in a horizontal position at predetermined
 intervals, a position changing mechanism 11 (position changing means)
 disposed between the wafer transfer arm 10 and the pitch changing
 mechanism to change a plurality of wafers W, such as twenty-five wafers W,
 from a horizontal position to a vertical position and vice versa, and a
 notch aligner (notch detecting means), not shown, for detecting notches,
 not shown, formed in wafers W held in a vertical position. The wafer
 handling section 4 has a conveying path 12 extended along the processing
 section 3. Wafer conveying devices 13 (wafer conveying means) travel along
 the conveying path 12.
 The processing section 3 comprises a first processing unit 14 for removing
 particles and, organic contaminants from wafers W, a second processing
 unit 15 for removing metallic contaminants from wafers W, a cleaning unit
 16 for removing a chemical oxide film formed on wafers W and drying wafers
 W, and a cleaning unit 17 for cleaning the wafer conveying device 13,
 which are arranged in a straight row. A cleaning apparatus in accordance
 with the present invention is applied to each of the first processing unit
 14, the second processing unit 15 and the cleaning unit 17. The wafer
 conveying devices 13 are disposed in sections of the conveying path 12
 corresponding to the units 14, 15, 16 and 17, respectively. Each wafer
 conveying device 13 is capable of moving in capable of moving in
 horizontal directions, i.e., directions along an X- and a Y-axis, and
 vertical directions, i.e., directions along a Z-axis, and of turning about
 a .theta.-axis.
 The cleaning apparatus in the first embodiment according to the present
 invention will be described hereinafter. Referring to FIG. 2, the cleaning
 apparatus comprises a cleaning tank 20 having an inner tank 21 containing
 a cleaning liquid, such as diluted hydrofluoric acid solution (DHF)
 prepared by diluting hydrofluoric acid (HF) or a rinsing liquid, such as
 pure water, and an outer tank 22 surrounding an upper open part of the
 inner tank 21 to contain the cleaning liquid L overllowing from the inner
 tank 21, cleaning liquid supply nozzles 23 disposed in a lower part of the
 inner tank 21, a circulation pipe 24 connecting the cleaning liquid supply
 nozzles 23 and a drain port 22a formed in the bottom wall of the outer
 tank 22, and a shutoff valve 25, an air bellows circulating pump 26, a
 damper 27 and a filter 28 arranged in that order from the side of the
 drain port 22a toward the cleaning liquid supply nozzles 23 on the
 circulation pipe 24. A wafer boat 29 capable of holding, for example,
 fifth wafers W is disposed in the inner tank 20 of the cleaning tank 20. A
 drain pipe 21c provided with a drain valve 21b is connected to a drain
 port 21a formed in the bottom wall of the inner tank 21. The circulation
 pipe 24, and the shutoff valve 25, the circulating pump 26, the damper 27
 and the filter 28 placed on the circulation pipe 24 constitute a
 circulation line.
 The cleaning apparatus is provided with, in addition to the circulating
 pump 26 placed on the circulation pipe 24, a motor-operated bellows pump
 (hereinafter referred to as "fixed-quantity delivery pump") 30. The
 fixed-quantity delivery pump 30 has a suction port 31 connected by a
 suction pipe 51 to the inner tank 21, and a discharge port 32 connected by
 a discharge pipe 52 to the outer tank 22. A particle counter (measuring
 means) 50 for measuring fine contaminative particle contained in the
 cleaning liquid L sucked from the inner tank 21 by the fixed-quantity
 delivery pump 30 is placed on the suction pipe 51. A central processing
 unit (abbreviated to "CPU") (control means) 60 controls the particle
 counter 50 so as to operate in synchronism with the sucking operation of
 the fixed-quantity delivery pump 30. Upon the detection of the actuation
 of the fixed-quantity delivery pump 30, the CPU 60 actuates the particle
 counter 50 to measure the quantity (particle number per milliliter) of
 particles contained in the cleaning liquid L while a sample cleaning
 liquid is being sampled from the cleaning liquid L contained in the inner
 tank 21. The CPU 60 provides a detection signal on the basis of the output
 signal of the particle counter 50. For example, the CPU 60 provides an
 alarm as a detection signal and displays an alarm or the like when the
 quantity (particle number) of the particles is greater than a
 predetermined thresh value.
 The suction pipe 51, the discharge pipe 52, the fixed-quantity delivery
 pump 30 and the particle counter 50 form a measuring line.
 As shown in FIG. 3, the fixed-quantity delivery pump 30 has a stationary
 end member 33 provided with a suction port 31 and a discharge port 32, a
 movable end member 34 disposed opposite to the stationary end member 33, a
 bellows 35 made of a corrosion-resistant, chemical-resistant synthetic
 rubber and extended between the stationary end member 33 and the movable
 end member 34, and a ball screw mechanism 36 for moving the movable end
 member 34 toward and away from the stationary end member 33. A threaded
 rod 38 is linked through a plurality of steel balls to a nut 39, and the
 nut 39 is connected to the movable end member 34 by a connecting member
 40. The threaded rod 38 is driven for rotation by a reversible stepping
 motor 37 to suck a fixed quantity (for example, 40 ml) of the cleaning
 liquid L from the inner tank 21 by making the bellows 35 expand and to
 discharge the sucked cleaning liquid L into the outer tank 22 by making
 the bellows 35 contract. Check valves 41 and 42 are placed in an end part
 of the suction pipe 51 connected to the suction port and in an end part of
 the discharge pipe 52 connected to the discharge port, respectively.
 As shown in FIG. 3, the particle counter 50 has a wholly or partly
 transparent measuring pipe 53 placed in the suction pipe 51, a laser light
 source 54 capable of emitting a laser beam and disposed on one side of the
 measuring pipe 53, and a photodetector 55 disposed on the other side of
 the measuring pipe 53 opposite to the laser light source 54.
 The fixed-quantity delivery pump 30 is driven to suck a quantity of the
 cleaning liquid L from the inner tank 21 and, at the same time, the
 particle counter 50 is operated. The laser light source 54 emits a laser
 beam to irradiate the cleaning liquid L flowing through the measuring pipe
 53 at a flow rate of 40 ml/min at the maximum. Deflection and interception
 of the laser beam by fine contaminative particles contained in the
 cleaning liquid L are detected by the photodetector 55 to count the number
 of particles contained in a predetermined quantity of the cleaning liquid
 L to determine the number of particles per milliliter (particles/ml). If
 the output shaft of the stepping motor 37 is operated in the normal and
 the reverse direction at the same rotating speed, a suction period, i.e.,
 a time necessary for a suction stroke, and a discharge period, i.e., a
 time necessary for a discharge stroke, are equal to each other and the
 measuring operation of the particle counter 50 is interrupted while the
 bellows 35 is in a discharge stroke as shown in FIG. 4(a). However, since
 the suction period and the discharge period are only a few seconds, the
 measurement of the particles is not affected by the interruption of the
 measuring operation of the particle counter 50. The time of the measuring
 operation of the particle counter 50 can be increased and the time of
 interruption of the measuring operation of the particle counter 50 can be
 reduced by driving the output shaft of the stepping motor 37 at a low
 rotating speed for the suction stroke of the bellows and at a high
 rotating speed for the discharge stroke of the bellows 35 as shown in FIG.
 4(b).
 The particle counter 50 can continuously be operated by providing the
 cleaning apparatus with a measuring line comprising the particle counter
 50 and two fixed-quantity delivery pumps 30A and 30B connected in a
 parallel to the particle counter 50 as shown in FIG. 5. The two
 fixed-quantity delivery pumps 30A and 30B are connected in parallel to the
 suction pipe 51 and the discharge pipe 52 and are operated in different
 phases, respectively; that is, the fixed-quantity delivery pump 30A is
 driven for a discharge stroke while the other fixed-quantity delivery pump
 30B is driven for a suction stroke, whereby the particle counter 50 is
 able to operate for the continuous measurement of particles contained in a
 predetermined quantity of the cleaning liquid L flowing through the
 measuring pipe 53.
 The configuration of the measuring line shown in FIG. 5 is the same as that
 of the measuring line shown in FIG. 3, except that the fixed-quantity
 delivery pump 30A is connected to the suction pipe 51 and the discharge
 pipe 52, and the other fixed-quantity delivery pump 30B is connected to a
 branch suction pipe 51A connected to the suction pipe 51, and a branch
 discharge pipe 52A connected to the discharge pipe 52 in the measuring
 line shown in FIG. 5, and hence parts shown in FIG. 5 and like or
 corresponding to those shown in FIG. 3 are designated by the same
 reference characters and the further description thereof will be omitted.
 The cleaning apparatus according to the present invention thus constructed
 cleans wafers W by immersing the wafers W in the cleaning liquid L
 contained in the inner tank 21 of the cleaning tank 20, supplying the
 cleaning liquid L from a cleaning liquid source, not shown, into the inner
 tank 21 so that the cleaning liquid L overflows the inner tank 21 into the
 outer tank 22, and filtering and circulating the cleaning liquid L
 collected in the outer tank 22. The fixed-quantity delivery pump 30 (or
 the fixed-quantity delivery pumps 30A and 30B) and the particle counter 50
 are operated during or before starting a cleaning process to sample a
 fixed quantity of the cleaning liquid L from the inner tank 21 through the
 measuring line separate from the circulation line and to measure particles
 contained in the sample cleaning liquid L. The sample cleaning liquid L
 sucked through the particle counter 50 by the fixed-quantity delivery pump
 30 is discharged through the discharge port 32 of the fixed-quantity
 delivery pump 30 and is returned into the outer tank 22 of the cleaning
 tank 20. Since the sample cleaning liquid L is not discharged into the
 inner tank 21 and is discharged into the outer tank 22, the cleaning
 ability of the cleaning liquid L contained in the inner tank 21 is not
 reduced and the cleaning liquid L can effectively used. The sample
 cleaning liquid may be discharged into a waste tank or the like instead of
 returning the same into the outer tank 22.
 The particle concentration (particle number) of the cleaning liquid L
 contained in the inner tank 21 is thus monitored. For example, the
 particle concentration of the cleaning liquid L is determined before
 starting a cleaning cycle, wafers W are carried into the inner tank 21 and
 are subjected to the cleaning process if a detection signal provided by
 the CPU 60 indicates a particle concentration not greater than a
 predetermined upper limit particle concentration, such as 10 particles/ml
 as shown in FIG. 6. Thus, wafers W can efficiently be cleaned. If the
 detection signal provided by the CPU 60 indicates a particle concentration
 (particle number) exceeding the upper limit particle concentration of 20
 particles/ml, the CPU 60 display an alarm to inform the operator of an
 inappropriate cleaning state.
 Although the cleaning apparatus in the first embodiment has been described
 on an assumption that the cleaning liquid L is DHF, the particle
 concentration (particle number) of the cleaning liquid can be monitored
 even if the cleaning liquid L is a mixed liquid of ammonia and hydrogen
 peroxide (APM) or a mixed liquid of sulfuric acid and hydrogen peroxide
 (SPM).
 Although the cleaning apparatus in the first embodiment has been described
 as applied to the semiconductor wafer cleaning system, needless to say,
 the cleaning apparatus is applicable to cleaning articles other than
 semiconductor wafers, such as glass substrates for forming LCDs and such.
 As is apparent from the foregoing description, the cleaning apparatus in
 accordance with the present invention thus constructed has the following
 excellent effects.
 A fixed quantity of the cleaning liquid can be sampled from the cleaning
 liquid contained in the cleaning tank, and particles contained in the
 sample cleaning liquid can accurately be counted by the measuring means.
 Therefore, the quality of the cleaning liquid can be monitored, the
 cleaning liquid of an optimum condition can be used for cleaning, so that
 the cleaning apparatus is able to operate at an improved yield and to
 exercise an improved cleaning ability.
 A fixed quantity of the cleaning liquid can be sampled from the cleaning
 liquid contained in the cleaning tank, particles contained in the sample
 cleaning liquid can accurately be counted by the measuring means and
 measuring accuracy can be improved by synchronously operating the
 fixed-quantity delivering means and the measuring means.
 A fixed quantity of the cleaning liquid can be sampled from the cleaning
 liquid contained in the inner tank in which workpieces are immersed in the
 cleaning liquid, and particles contained in the sample cleaning liquid can
 accurately be measured by the measuring means.
 Since the sample cleaning liquid can be discharged into the outer tank
 instead of discharging the same into the inner tank in which workpieces
 are immersed in the cleaning liquid, the cleaning liquid can be circulated
 without deteriorating the cleaning ability of the cleaning liquid
 contained in the inner tank and the cleaning liquid can effectively used.
 Since the durable, chemical-resistant motor-operated bellows pump capable
 of metering the cleaning liquid can be employed as the fixed-quantity
 delivering means, the life of the cleaning apparatus can be extended, and
 the measuring accuracy of the measuring means and the reliability of the
 cleaning apparatus can be improved.
 Since the durable, chemical-resistant motor-operated bellows pump capable
 of metering the cleaning liquid can be employed as the fixed-quantity
 delivering means, the life of the cleaning apparatus can be extended,
 measuring accuracy of the measuring means and the reliability of the
 cleaning apparatus can be improved, and particles contained in a large
 quantity of the cleaning liquid can continuously and accurately be
 measured.
 Second Embodiment
 A cleaning apparatus in a second embodiment according to the present
 invention is substantially identical with the cleaning apparatus in the
 first embodiment shown in FIGS. 1 to 6, except that the former carries out
 a cleaning method different from that carried out by the latter. Parts of
 the second embodiment like or corresponding to those of the first
 embodiment shown in FIGS. 1 to 6 are designated by the same reference
 characters and the description thereof will be omitted.
 In FIG. 2, the particle counter 50 is connected to the suction port 31 of
 the fixed-quantity delivery pump 30. However, the particle counter 50 need
 not necessarily be placed on the suction pipe 51 connected to the suction
 port 31 of the fixed-quantity delivery pump 30, but may be placed on a
 discharge pipe 52 connected to the discharge port 32 of a fixed-quantity
 delivery pump 30 as shown in FIG. 7. When the particle counter 50 is
 placed in the discharge pipe 52 connected to the discharge side of the
 fixed-quantity delivery pump 30, the sample cleaning liquid L sampled from
 the cleaning liquid L contained in an inner tank 21 is prevented from
 bubbling.
 The cleaning apparatus shown in FIG. 7 is the same in construction and
 function as the cleaning apparatus shown in FIG. 2, except that the
 particle counter 50 is connected to discharge side of the fixed-quantity
 delivery pump 30 in the cleaning apparatus shown in FIG. 7 and hence the
 fumer description of the cleaning apparatus shown in FIG. 7 will be
 omitted.
 In either the cleaning apparatus shown in FIG. 2 or the cleaning apparatus
 shown in FIG. 7, wafers W are immersed in the cleaning liquid contained in
 the inner tank 21 of the cleaning tank 20, the cleaning liquid L is
 supplied from the cleaning liquid source, not shown, into the inner tank
 21 so that the cleaning liquid L overflows the inner tank 21 into the
 outer tank 22, and the cleaning liquid collected in the outer tank 22 can
 be filtered and circulated while the wafers W are being cleaned. The
 fixed-quantity delivery pump 30 and the particle counter 50 are operated
 during or before starting a cleaning process to sample a fixed quantity of
 the cleaning liquid L from the inner tank 21 through the measuring line
 separate from the circulation line and to measure particles contained in
 the sample cleaning liquid L. The sample cleaning liquid L sucked through
 the particle counter 50 and discharged through the discharge port 32 of
 the fixed-quantity delivery pump 30 or the sample cleaning liquid
 discharged through the discharge port 32 of the fixed-quantity delivery
 pump 30 into the particle counter 50 is returned into the outer tank 22 of
 the cleaning tank 20 to use the sample cleaning liquid again for cleaning.
 Thus, the cleaning liquid can effectively used. The sample cleaning liquid
 may be discharged into a waste tank or the like instead of returning the
 same into the outer tank 22.
 A cleaning method in accordance with the present invention will be
 described hereinafter with reference to FIGS. 8 to 12. In step A (FIG.
 12), the fixed-quantity delivery pump 30 and the particle counter 50 are
 operated during or before starting a cleaning process to sample a fixed
 quantity of the cleaning liquid L from the inner tank 21 through the
 measuring line separate from the circulation line and to measure particles
 contained in the sample cleaning liquid L. If a particle concentration
 (particles/ml) determined on the basis of the number of particles counted
 by the particle counter 50 is greater than the upper limit particle
 concentration of, for example, 20 particles/ml, as shown in FIG. 8, the
 CPU 60 provides an alarm signal to inform the operator of an inappropriate
 cleaning condition (steps B and C). When the CPU 60 provides an alarm
 signal, the operator replaces the old cleaning liquid L with the new
 cleaning liquid L by discharges the old cleaning liquid L from the
 cleaning tank 20 and supplying the new cleaning liquid L into the cleaning
 tank 20 before starting the next cleaning cycle. The cleaning liquid L may
 be changed when the alarm signal provided by the CPU continues longer than
 a predetermined time. The cleaning liquid L is changed if the particle
 concentration of the cleaning liquid L does not decrease below the upper
 limit particle concentration before starting a cleaning cycle.
 The alarm signal provided by the CPU may be interpreted as a cleaning
 liquid change request signal or an abnormal wafer indication signal.
 If the measured particle concentration (particle number) is not greater
 than the upper limit particle concentration (particle number) and is on an
 acceptable level as shown in FIG. 8, wafers W are carried into the
 cleaning tank 20, more specifically, into the inner tank 21, the cleaning
 liquid L is supplied from the cleaning liquid source, not shown, into the
 inner tank 21 so that the cleaning liquid L overflows the inner tank 21,
 the cleaning liquid L overflowing from the inner tank 21 is filtered and
 circulated while the wafers W are subjected to a cleaning process for a
 predetermined cleaning time, such as 10 min (steps D and E). The wafers W
 are carried out of the cleaning tank 20 after the elapse of the cleaning
 time. After the wafers W have been taken out of the cleaning tank 20, the
 cleaning liquid L contained in the cleaning tank 20 is circulated through
 the circulation line while particles contained in the cleaning liquid L
 are filtered out. A plurality of wafers W, such as fifty wafers W, are
 carried into the cleaning tank 20 for the next cleaning cycle after the
 particle concentration of the cleaning liquid L has been decreased to an
 acceptable level as shown in FIG. 9. Then, the next cleaning cycle is
 executed. During the cleaning process, the cleaning liquid L contained in
 the inner tank 21 is sampled, and the number of particles contained in the
 sample cleaning liquid L is measured by the particle counter 50. If the
 particle concentration increases extraordinarily beyond the upper limit
 particle concentration during the cleaning process, the CPU 60 provides an
 alarm signal as shown in FIG. 6 to inform the operator of the abnormal
 condition, and the cleaning process is interrupted.
 The CPU 60 may compare a particle concentration (particle number) measured
 before starting the first cleaning cycle and a particle concentration
 (particle number) measured after the completion of every cleaning cycle,
 and may provide an alarm signal if the difference between the particle
 concentration measured before starting the first cleaning cycle and the
 particle concentration measured after the completion of every cleaning
 cycle exceeds a predetermined upper limit value. The operator is able to
 perceive that the wafers W being cleaned are abnormal from the alarm
 signal provided by the CPU 60, and the abnormal wafers W can be
 discriminated and separated from normal wafers.
 A plurality of cycles of the cleaning process are carried out to clean a
 plurality of lots of wafers W. Particles contained in the cleaning liquid
 L are counted by the particle counter 50 during the plurality of cycles of
 the cleaning process, and the differences S1, S2 and S3 (FIG. 11) between
 the particle concentrations measured in the successive cleaning cycles may
 be calculated. If the difference is greater than a predetermined upper
 limit value, such as the difference S3 (FIG. 11), the cleaning liquid L is
 changed (steps F and G). The cleaning liquid L may be changed when the
 difference between a particle concentration of the cleaning liquid L at
 the start of the nth cleaning cycle and a minimum particle concentration
 of the cleaning liquid L reached after the completion of the nth cleaning
 cycle is greater than a predetermined upper limit value. A number of
 cleaning cycles to be carried out or a period for which the cleaning
 process can be carried out before changing the cleaning liquid L may be
 determined beforehand on the basis of experimental data, and it is
 possible to change the cleaning liquid L at optimum time by referring to
 the predetermined number of cleaning cycles or the predetermined time.
 Although the cleaning method in accordance with the present invention has
 been described on an assumption that the cleaning liquid L is DHF, the
 particle concentration of the cleaning liquid can be determined and the
 quality of the cleaning liquid L can be monitored even if the cleaning
 liquid L is a mixed liquid of ammonia and hydrogen peroxide (APM) or a
 mixed liquid of sulfuric acid and hydrogen peroxide (SPM).
 Although the cleaning method in accordance with the present invention has
 been described as applied to the semiconductor wafer cleaning system,
 needless to say, the cleaning method is applicable to cleaning articles
 other than semiconductor wafers, such as glass substrates for forming LCDs
 and such.
 According to the present invention, a fixed quantity of the cleaning liquid
 contained in the cleaning tank is sampled from a part other than the
 circulation line before starting a cleaning process or during the cleaning
 process, and the quantity (particle number) of contaminative particles
 contained in the cleaning liquid is measured. Therefore, contaminative
 particles contained in the cleaning liquid can accurately be measured, and
 it is possible to inform the operator of an inappropriate condition of the
 cleaning liquid by a detection signal indicating a quantity of
 contaminative particles exceeding a predetermined upper limit value.
 Consequently, the cleaning liquid can be maintained in a quality suitable
 for cleaning, and cleaning ability and yield can be improved. It is also
 possible to inform the operator that the quantity of contaminative
 particles contained in the cleaning liquid is not greater than the
 predetermined upper limit value and the normal cleaning process can be
 achieved.
 Since a fixed quantity of the cleaning liquid contained in the cleaning
 tank is sampled from a part other than the circulation line and the
 quantity of contaminative particles contained in the sample cleaning
 liquid is measured, the quantity of contaminative particles contained in
 the cleaning liquid can accurately be measured, and the cleaning liquid is
 changed if particle concentration of the cleaning liquid decreases below
 the predetermined upper limit value to improve the ability and the yield
 of the cleaning process.
 Since a fixed quantity of the cleaning liquid contained in the cleaning
 tank is sampled from a part other than the circulation line and the
 quantity of contaminative particles contained in the sample cleaning
 liquid is measured, the quantity of contaminative particles contained in
 the cleaning liquid can accurately be measured, and the cleaning ability
 of the cleaning liquid and yield can be improved by changing the cleaning
 liquid if the difference between the particle concentration of the
 cleaning liquid before starting the first cleaning cycle and that of the
 cleaning liquid after the completion of a cleaning cycle is greater than a
 predetermined upper limit value.
 Since a fixed quantity of the cleaning liquid contained in the cleaning
 tank is sampled from a part other than the circulation line and the
 quantity of contaminative particles contained in the sample cleaning
 liquid is measured, the quantity of contaminative particles contained in
 the cleaning liquid can accurately be measured, and the cleaning ability
 of the cleaning liquid and yield can be improved by changing the cleaning
 liquid if the difference between the particle concentration of the
 cleaning liquid before starting a cleaning cycle and that of the cleaning
 liquid after the completion of the same cleaning cycle is greater than a
 predetermined upper limit value.
 The cleaning liquid can properly be changed and the cleaning ability of the
 cleaning liquid and yield can be improved by determining time for changing
 the cleaning liquid on the basis of a predetermined number of cleaning
 cycles to be carried out or a predetermined period for which the cleaning
 process may be carried out before changing the cleaning liquid, and the
 measured particle concentration of the cleaning liquid.