Abstract:
A flowmeter can measure a small flow rate of fluid with high measurement precision, can be made small-sized, can deal with a variety of types of chemicals, and can be produced easily at a low cost. The flowmeter includes: a casing having an enlarged portion and being disposed vertically; and a float enclosed in the enlarged portion of the casing and at least partly having a detection surface. The float is to be pushed up by a fluid flowing from below into the casing and flowing upwardly in the casing. The flowmeter also includes at least one displacement sensor, disposed outside the enlarged portion of the casing, for detecting an axial displacement of the float by magnetizing the detection surface of the float.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a flowmeter suited for measuring a small flow rate of fluid, and more particularly to a flowmeter suited for measuring a small flow rate of fluid in a semiconductor manufacturing apparatus for manufacturing, for example, a semiconductor device having a fine interconnect structure. 
   2. Description of the Related Art 
   For measuring the flow rate of a fluid, such as a gas or liquid, a flowmeter can be used which has a heating element disposed in a casing and measures the flow rate of a fluid by measuring the temperature of the heating element whose temperature changes with the flow rate of the fluid flowing in the casing. 
   A flowmeter is known which includes a plurality of light emitting diodes (LEDs) disposed on one side of a casing formed of an optically-transparent material, a plurality of light-receiving photodiodes disposed on the opposite side of the casing and facing the LEDs, and a float formed of a light-shielding material disposed in the casing. The flowmeter detects the position of the float by detecting the position of a light-receiving photodiode which has come to receive no light from an LED due to blocking of light by the float (see, for example, Japanese Patent laid-Open Publication No. H2-388162). 
   Another flowmeter is known which includes a float formed of a light-shielding material disposed in a casing formed of an optically-transparent material, and a plurality of imaging sensor units, each comprised of an optical lens and an image sensor, disposed along the casing to recognize an effective imaging range for the casing by a combination of the imaging ranges of the imaging sensor units. This flowmeter detects the position of the float based on electrical signal outputs from the imaging sensor units (see, for example, Japanese Patent Laid-Open Publication No. 2001-221666). 
   A similar flowmeter is known which uses lenses and CCD line sensors to project a one-dimensional image of a float on the line sensors and detects the position of the float from the position of the image (see, for example, Japanese Patent Laid-Open publication No. 2001-221666). 
   Further, another flowmeter is known which includes a float provided with a permanent magnet, a casing, and a plurality of magnetic sensors disposed outside the casing, and which detects the position of the float by detecting the magnetism of the permanent magnet of the float with one of the plurality of magnetic sensors (see, for example, Japanese Patent laid-open Publication No. H11-190644). In the conventional flowmeter using a heating element manufactured, for example, with a micromachine, the heating element is exposed to a fluid. When handling a corrosive fluid, it is therefore necessary to cover and protect the heating element with a protective material. In this case, because of the heat conductivity of the protective film, the measurement precision cannot be made high in measurement of a small flow rate of fluid. In addition, the response speed undesirably becomes slow, leading to difficult measurement of a small flow rate of fluid. 
   In the case of the conventional flowmeter having optical or magnetic sensors provided outside a casing for detecting the position of a float, the number of parts must be increased in order to enhance the measurement precision. This leads to difficult assembly, a larger-sized construction, and a higher cost. Further, a flowmeter having optical sensors needs the use of an optically-transparent casing, and therefore materials for the casing are restricted. In addition, such a flowmeter deals with some types of fluids with difficulty. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above situation in the related art. It is therefore an object of the present invention to provide a highly-reliable flowmeter which can measure a small flow rate of fluid with high measurement precision, can be made small-sized, can deal with a variety of types of chemicals, and can be produced easily at a low cost. 
   In order to achieve the object, the present invention provides a flowmeter comprising: a casing having an enlarged portion and which is disposed vertically; and a float enclosed in the enlarged portion of the casing and at least partly having a detection surface. The float is to be pushed up by a fluid flowing from below into the casing and flowing upwardly in the casing. The flowmeter also comprises at least one displacement sensor disposed outside the enlarged portion of the casing for detecting an axial displacement of the float by magnetizing the detection surface of the float. 
   According to the flowmeter of the present invention, an upward flow of a fluid in the vertical casing exerts an upward pressing force on the float enclosed in the enlarged portion of the casing. The float, in the balance between the pressing force and its own weight, floats and stands still at a position corresponding to the flow rate of the fluid flowing in the flowmeter. Since the float at least partly has a magnetic detection surface, the axial floating position (displacement) of the float can be detected precisely with high resolution by a displacement sensor, such as an inductance-type displacement sensor or eddy current-type displacement sensor, disposed outside the enlarged portion of the casing. 
   A preferable example of the displacement sensor is a sensor of very small size, comprised of a ferrite magnetic core and a coil wound on it and having such a very high resolution that it can detect displacement of the order of about 1 μm. The use of this displacement sensor can afford a sufficiently high measurement precision even when the size of the flowmeter is made small such that the stroke (movable range in the vertical direction) of the float is e.g. about 2 mm. Furthermore, this replacement sensor has high impact resistance, is highly reliable, and can be produced at a low cost. 
   The present invention also provides a casing for a flowmeter, having an enlarged portion provided with at least one displacement sensor outside and disposed vertically so that a fluid is allowed to flow from below into the casing and flow upwardly in the casing. 
   The present invention also provides a float at least partly having a detection surface and enclosed in an enlarged portion of a casing. The float is to be pushed up in the enlarged portion by a fluid flowing from below into the casing and flowing upwardly in the casing. 
   The flowmeter of the present invention detects an axial displacement of the float with a displacement sensor, such as an inductance-type displacement sensor or an eddy current-type displacement sensor. This enables high-precision measurement of a small flow rate of fluid, downsizing of the flowmeter and low-cost production of the flowmeter. Further, the casing and the float of the flowmeter can be made of a metal material, so that a small flow rate of fluid can be measured stably with high precision even in a corrosive environment. In addition, the flowmeter can be produced not by fine processing technology using, for example, a micromachine, but by ordinary technology. Accordingly, the flowmeter of solid construction can be produced at a low cost. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a vertical sectional view of a flowmeter according to a first embodiment of the present invention, 
       FIG. 1B  is a cross-sectional view taken along line B-B of  FIG. 1A , 
       FIG. 1C  is a vertical sectional view of a displacement sensor, and 
       FIG. 1D  is a plan view of the displacement sensor; 
       FIG. 2A  is a cross-sectional diagram showing the state of a float in a casing when there is no flow of fluid in the casing, 
       FIG. 2B  is a cross-sectional diagram showing the state of the float when a fluid is flowing into the casing, and 
       FIG. 2C  is a cross-sectional diagram showing the state of the float when the flow rate of the fluid has increased; 
       FIG. 3  is a graph showing the relationship between the flow rate of fluid and the axial displacement of the float as measured for floats having different specific gravities; 
       FIG. 4  is a vertical sectional view of a flowmeter according to a second embodiment of the present invention; 
       FIG. 5  is a vertical sectional view of a flowmeter according to a third embodiment of the present invention; and 
       FIG. 6  is a vertical sectional view of a variation of the flowmeter according to the third embodiment shown in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described with reference to the drawings. In the drawings, members or elements having the same operation or function are designated with the same reference numeral and a duplicate description thereof is omitted. 
     FIGS. 1A and 1B  show a flowmeter  10  according to a first embodiment of the present invention, and  FIGS. 1C and 1D  show a displacement sensor  15  used in the flowmeter  10 . The flowmeter  10  includes a casing  11  having an enlarged portion  12  and disposed vertically, and a float  13  enclosed in the enlarged portion  12 . Inductance-type displacement sensors  15 , for measuring an axial displacement of a float  13 , are provided outside the enlarged portion  12  of the casing  11 . The casing  11  is formed of, for example, a stainless steel, and the float  13  has a detection surface  13   a  of high-permeability magnetic material (permalloy). The displacement sensor  15  magnetizes the detection surface (magnetic surface)  13   a  through the casing  11 , and measures an axial displacement of the detection surface (magnetic surface)  13   a  from a change in the inductance. 
   In particular, the float  13  has at its upper and lower ends truncated conical portions of permalloy, high-permeability magnetic material, with the respective surfaces serving as detection surfaces  13   a . In addition, float  13  has a cylindrical portion  13   b  of non-magnetic aluminum material between the truncated conical portions. The diameter of the cylindrical portion  13   b  is, for example, about 20 mm and the height is, for example, about 30 mm. The specific gravity of the float  13  of this embodiment is set to about 2 so that it sinks in a stationary fluid (e.g. water) as a measuring object. The specific gravity of the float  13  can be set to an appropriated value, depending on the type of the fluid to be measured, the flow rate, etc., by adjusting the ratio between the heavy permalloy and the light aluminum, and the proportion of the hollow center portion of the float  13 . 
   As shown in  FIGS. 1C and 1D , the displacement sensor  15  is comprised of, for example, a ferrite core  15   a  and a coil  15   b  wound on the ferrite core  15   a . The ends of the coil  15   b  are connected to a not-shown sensor amplifier and a sensor output is taken out. The size of the ferrite core  15   a  is very small, for example, no more than several mm in diameter and no more than 2 to 3 mm in height. Accordingly, the size of the flowmeter  10 , i.e. the sum of the size of the enlarged portion  12  of the casing  11  and the size of the displacement sensor  15 , can be nearly the same as the size of the enlarged portion  12  of the casing  11 . Further, the inductance-type displacement sensor  15  can measure a displacement of the detection surface, the measuring object, with a resolution of the order of 1 μm in a contactless manner through the casing  11  of stainless steel. 
   In this embodiment, three inductance-type displacement sensors  15  on the upper end side of the enlarged section  12  and three inductance-type displacement sensors  15  on the lower end side are disposed at regular intervals in the circumferential direction. The respective three displacement sensors  15  are connected in series so that the total output can be taken out. Thus, the average output of the three displacement sensors  15  can be taken out to enhance the measurement precision. A circuit is designed so that a differential motion output is taken out from the total output of the upper end side sensors and the total output of the lower end side sensors. Thus, when the float  13  moves vertically (axially) in the enlarged portion  12 , the axial displacement can be measured by taking a differential motion output between the upper sensors and the lower sensors. 
     FIGS. 2A through 2C  illustrate the principle of flow rate measurement by the flowmeter  10 . As shown in  FIG. 2A , when there is no flow of fluid through the casing  11 , the float  13 , by its own weight, is in contact with the inner surface of the lower inverted truncated conical portion of the enlarged portion  12  of the casing  11 . When a fluid begins flowing into the casing  11 , the float  13  is pushed up by the flow of the fluid and stands still at a position where the upward pressing force of the flow of the fluid and the weight of the float  13  are balanced, as shown in  FIG. 2B . As the flow rate of the fluid flowing in the casing  11  increases, the float  13  moves up and stands still at a higher position, as shown in  FIG. 2C . 
   Thus, the floating position (axial displacement) of the float  13  changes approximately linearly with changes in the flow rate of the fluid flowing in the casing  11 . Accordingly, the flow rate of the fluid flowing in the casing  11  can be determined by detecting the axial displacement of the float  13  with the displacement sensors  15 . The displacement sensors  15  measure the distances Lu and Ld between the float  13  and the upper/lower truncated conical portions of the casing  11 , shown in  FIGS. 2A through 2C . The axial displacement component of the float  13  can be calculated from the distances Lu and Ld. In the flowmeter  10  of this embodiment, the float  13  has a diameter of 20 mmØ and a height of 30 mm and the stroke (movable distance in the axis direction) of the float  13  in the enlarged portion  12  is set to about 2 mm. The use of the displacement sensor  15  with a resolution of about 1 μm with respect to the displacement of the float  13  can measure an axial displacement of the float  13  with remarkably high precision. 
   In the flowmeter  10 , the clearance “C” (see  FIG. 1B ) between the inner diameter of the enlarged portion  12  of the casing  11  and the outer diameter of the float  13  is set to about 0.2 mm. Such clearance corresponds to the cross-sectional area of the casing (flow passage) of about 5 mmØ, which makes it possible to measure with high precision such a small flow rate as about 10-60 cc/min in terms of water, as will be described later. 
   A measurable flow rate range to the measuring object can be changed by changing the clearance “C” between the inner diameter of the enlarged portion  12  of the casing  11  and the outer diameter of the float  13 . For example, in a semiconductor manufacturing apparatus for manufacturing e.g. a semiconductor device having a fine interconnect structure, supply of a viscous fluid, such as a resist solution, is required to be controlled to a small flow rate, for example, about 10 cc/min. The requirement can be met and high-precision measurement of such a small flow rate becomes possible by providing a clearance corresponding to the cross-sectional area of the casing (flow passage) of e.g. about 2-3 mmØ. 
   Similarly, for a rough cleaning liquid, for example, control of the flow rate at about 2000-3000 cc/min is required. The requirement can be met and high-precision measurement of such a flow rate becomes possible by providing a clearance corresponding to the cross-sectional area of the casing (flow passage) of e.g. about 10 mmØ. Though the flowmeter of the present invention with a small clearance “C” is suitable for measurement of a small flow rate, the flowmeter, of course, can be used to measure a larger flow rate by making the clearance “C” larger. 
   In a semiconductor manufacturing apparatus, there are cases where various fluids, such as a fluorine-containing fluid, an acid, an alkali, a liquid chemical, a resist solution and a polishing slurry, are supplied under precise control of flow rate, for example, upon supply of a cleaning liquid to a cleaning apparatus, supply of a resist solution to a resist coating apparatus, and supply of a polishing slurry to a polishing apparatus. The flowmeter of the present invention can be advantageously used for flow rate measurement in such cases for the following reasons. Firstly, high-precision measurement of a small flow rate is possible. Secondly, the casing  11  and the float  13  can be made of a metal material, so that they can be highly stable to various chemicals. Thirdly, since a small-sized displacement sensor  15 , which requires no high assembling precision, can be used, the flowmeter can be made small-sized and can be easily incorporated in a semiconductor manufacturing apparatus, etc. Fourthly, the flowmeter is highly reliable. Fifthly, since an electrical signal corresponding to a flow rate can be taken out (produced), the flowmeter can be easily incorporated in a control system. 
   A description will now be made of actual measurement data on flow rate measuring characteristics of the flowmeter  10 .  FIG. 3  shows the relationships between the flow rate of a fluid flowing in the casing  11  and the axial displacements of the float  13  as measured for two different specific gravities of the float. The abscissa denotes the flow rate of fluid (water), and the ordinate denotes the axial displacement of the float  13 . The data for the float  13  having a specific gravity of 2 is shown in comparison with the data for the float  13  having a specific gravity of 3. As apparent from  FIG. 3 , the displacement of the float  13  having a specific gravity of 3 is smaller than the displacement of the float  13  having a specific gravity of 2, and the range of flow rate, in which the flow rate can be measured from the displacement in an approximately linear relationship therebetween, broadens to 10-90 cc/min. 
   Though the axial displacement of the float  13  is as small as about 0-0.5 mm, the axial displacement of the float  13  can be measured with sufficiently high precision by using a displacement sensor  15  with a resolution of the order of about 1 μm. 
   A flowmeter according to another embodiment of the present invention will now be described with reference to  FIGS. 4 through 6 . 
     FIG. 4  shows a flowmeter according to a second embodiment of the present invention. This flowmeter includes, in addition to the construction of the above-described flowmeter of the first embodiment, a radial magnetic bearing for controlling the radial position of the float  13 . In particular, the flowmeter of this embodiment includes radial displacement sensors  17  for detecting the radial position of the float  13 , a not-shown control device for controlling the position of the float  13  to maintain float  13  at a target radial position based on the position detected by the displacement sensors  17 , and electromagnets  18 . When the float  13  is radially displaced from the center of the casing  11 , (which is the target position), the displacement sensors  17  detect the eccentric displacement of the float  13  and the control device exerts a radial magnetic force on the float  13  by the electromagnets  18 , thereby returning the float  13  to the center of the casing  11  (i.e., the target position). 
   Four displacement sensors  17  and four electromagnets  18  are disposed respectively at regular intervals about float  13  in the circumferential direction, so that detection of displacement of the float  13  in the X, Y directions and control of the position of the float  13  can be conducted. The float  13  includes a cylindrical portion  13   c  of magnetic material, serving as a target of the electromagnets  18 , below the upper truncated conical detection surface  13   a , and a cylindrical portion  13   d  of magnetic material, serving as a target of the displacement sensors  17 , above the lower truncated conical detection surface  13   a.    
   Instead of using the radial displacement sensors  17 , it is also possible to use a sensorless radial magnetic bearing which comprises the electromagnets  18  which, by utilizing the winding, are provided with a displacement sensor function of detecting a radial displacement of the float  13 . 
   The flowmeter of this embodiment, whose construction is the same as the flowmeter of the first embodiment except for the provision of the radial magnetic bearing, has the same high-precision detection characteristics for small flow rates as the flowmeter of the first embodiment. In addition to this, with the provision of the radial magnetic bearing, it becomes possible with the flowmeter of this embodiment to always hold the float  13  in the center of the enlarged portion  12  of the casing  11 . This can prevent the float  13  from contacting the inner wall surface of the casing  11 , thereby preventing contamination due to contact between the float  13  and the casing  11 . 
   Furthermore, the electromagnets  18  of the radial magnetic bearing constantly exert a radial electromagnetic force on the float  13 . The radial magnetic force generates an axial shear force in the float  13 . Specifically, when the float  13  moves in the axial direction, the radial shear force counteracts the axial movement of the float  13 . Thus, the axial electromagnetic force, constantly applied from the electromagnets  18  on the float  13 , produces the same effect as produced by an increase in the specific gravity of the float  13 . Accordingly, it becomes possible to broaden the flow rate measurement range from 10-60 cc/min to e.g. 10-600 cc/min by adjusting the constant magnetic force of the electromagnets  18 . 
   Though in this embodiment an axial displacement of the float  13  due to a change in the flow rate of a fluid flowing in the casing  11  is detected with the displacement sensors  15 , it is also possible to detect a change in the flow rate of a fluid flowing in the casing  11  from a change in the electric current of the electromagnets  18  by utilizing the above-described magnetic shear force generated by the electromagnets  18 . 
   In particular, a minimum electric current is supplied to the electromagnets  18  when the flow rate of a fluid flowing in the casing  11  is zero. By using the magnetic shear force of the electromagnets  18 , the float  13  is held in a certain axial position detected with the displacement sensors  15 . When the flow rate of the fluid flowing in the casing  11  has increased, the upward pressing force applied from the fluid on the float  13  increases. At that moment, the electric current supplied to the electromagnets  18  is increased to increase the magnetic shear force in order to hold the float  13  in the certain axial position. In carrying out such control of the axial position of the float  13 , the larger the flow rate of the fluid flowing in the casing  11  is, the larger the magnetic shear force and thus the higher the electric current supplied to the electromagnets  18  that are needed to hold the float  13  in the certain axial position. Thus, there is a correlation between the flow rate of the fluid flowing in the casing  11  and the electric current supplied to the electromagnets  18 , and the correlation makes it possible to measure the flow rate of the fluid flowing in the casing  11  from the electric current supplied to the electromagnets  18 . 
     FIG. 5  shows a flowmeter according to a third embodiment of the present invention. The flowmeter of this embodiment differs from the flowmeter of the second embodiment in that the truncated conical portions at the upper and lower ends of the float  13  are eliminated and the detection surfaces  13   a  at the upper and lower ends of the float  13  are made flat surfaces (circular surfaces). Consequently, the enlarged portion  12  of the casing  11  is made rectangular (in cross-section), and the displacement sensors  15  are set on the surfaces perpendicular to the axial (longitudinal) direction of the enlarged section  12 . The remaining construction is the same as the flowmeters of the first and second embodiments. 
   With such distinctive features of this embodiment, the flowmeter can be made smaller-sized. It is, of course, possible to eliminate the radial displacement sensors  17  and the electromagnets  18 , together constituting the radial magnetic bearing. Even such flowmeter without a radial magnetic bearing has the same flow rate detection characteristics as the flowmeter of the first embodiment and can detect with sufficiently high precision a small flow rate even in a corrosive environment. 
   However, the truncated conical portions at the upper and lower ends of the float  13  of the flowmeter according to the first or second embodiment, together with the inverted truncated conical surfaces, facing the detection surfaces  13   a , of the enlarged portion  12  of the casing  11 , allows a fluid to flow smoothly and stably into the enlarged portion  12  of the casing  11 . In view of this, it is possible to leave the truncated conical portion at the lower end of the float  13  and the inversed truncated conical surface, facing the lower detection surface  13   a , of the enlarged portion  12  of the casing  11 , as shown by the variation in  FIG. 6 . This allows a fluid to flow smoothly and stably into the enlarged portion  12  of the casing  11 . In addition, the modifications to the flat detection surface  13   a  at the upper end of the float  13  and to the flat surface, facing the flat detection surface  13   a  and perpendicular to the axial direction, of the enlarged portion  12  of the casing  11 , can contribute to downsizing of the flowmeter. 
   It is, of course, possible also with this flowmeter to eliminate the radial displacement sensors  17  and the electromagnets  18 , together constituting the radial magnetic bearing. Even such flowmeter without a radial magnetic bearing has the same flow rate detection characteristics as the flowmeter of the first embodiment and can detect with sufficiently high precision a small flow rate even in a corrosive environment. 
   Though in the above-described first to third embodiments an inductance-type displacement sensor is used as the displacement sensor  15  to detect a displacement of the detection surfaces  13   a  of the permalloy float  13  from outside the casing  11  of non-magnetic steel, it is also possible to use an eddy current-type displacement sensor as the displacement sensor  15 . In this case, the presence of a steel casing  11 , which has a high electric conductivity, between the sensor  15  and the detection surface  13   a  incurs a considerable loss of eddy current. It is therefore preferred to use a different material for the casing. For example, a ceramic or resin material is preferably used for the casing in front of the eddy current-type displacement sensor so as to reduce eddy current loss. An eddy current-type sensor detects a displacement of an object based on a change in the impedance with a change in eddy current due to the displacement of the object. Accordingly, it is not necessary to use a high-permeability magnetic material for the detection surfaces  13   a  of the float  13 . A conductive material, which can generate an eddy current, will suffice. 
   In the case of detecting the position of a float through a metal casing by using an eddy current-type displacement sensor, the S/N ratio of a displacement sensor signal is low due to the influence of eddy current loss, etc caused by the casing. In order to reduce the influence and improve the S/N ratio, it is preferable to make the sensor driving current power-driven and provide a filter section for adequately removing noise from a sensor signal. Furthermore, in order to compensate phase shifting of sensor signal caused e.g. by a filter, it is preferable to provide a phase compensation section in a sensor signal section, a reference signal section and a synchronous detection signal section and optimize the sensor sensitivity. 
   Though the casing  11  and the float  13 , both made of a metal material, are used in the above-described embodiments, some liquid chemical as a measuring object can cause corrosion in a metal material. It is preferred in that case to use for the casing  11   a  resin or ceramic material having resistance to the chemical. Further, the float  13  is preferably coated and protected with a resin material having resistance to the chemical. 
   While the present invention has been described with reference to the preferred embodiments thereof, it will be appreciated by those skilled in the art that the present invention is not limited to the embodiments, but changes and modifications can be made therein within the spirit and scope of the present invention.