Apparatus for storing liquid, system for processing substrates, and method for determining liquid level - Patents.com

Abstract

To provide an art that identifies a liquid level of liquid in a tank on the basis of vibration applied to the liquid.SOLUTION: In a device pooling liquid to be used in processing a substrate, the device comprises: a fin that is inserted into a tank pooling the liquid, and configured so that a width dimension in a direction crossing a liquid surface of the liquid varies in accordance with a position in a liquid depth direction of the liquid; a vibration application unit that applies vibration to the fin; a vibration detection unit that detects the vibration when a wave formed on the surface of the liquid arrives at an inner wall surface of the tank by the fin to which the vibration is applied; and a liquid level identification unit that identifies a liquid level of the liquid in the tank on the basis of a correspondence relation between a size of amplitude of the detected vibration and the width dimension of the fin.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to an apparatus for storing a liquid, a system for processing a substrate, and a method for determining a liquid level. [Background technology]

[0002] In some semiconductor device manufacturing processes, a processing gas formed by vaporizing a processing liquid as a raw material is supplied to a semiconductor wafer (hereinafter referred to as "wafer") as a substrate for processing. In other cases, the processing liquid is supplied to the wafer in a liquid state for processing. In these cases, a mechanism for detecting the liquid level stored in a tank may be required.

[0003] Patent Document 1 describes a liquid level sensor that can be attached to the outer surface of bottles of various sizes by attaching electrodes to a knitted fabric structure. This liquid level sensor is configured to detect the level of the liquid contained in the bottle based on changes in capacitance detected by the electrodes. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-179973 Summary of the Invention [Problem to be solved by the invention]

[0005] The present disclosure provides a technique for determining the level of a liquid in a tank based on vibrations applied to the liquid. [Means for solving the problem]

[0006] The present disclosure provides an apparatus for storing a liquid used in processing a substrate, the apparatus comprising: a tank for storing the liquid; a fin inserted into the tank and configured so that its width dimension in a direction intersecting with the liquid surface changes depending on the position of the liquid in the liquid depth direction; a vibration applying unit that applies vibration to the fin; a vibration detection unit that detects vibrations when waves formed on the surface of the liquid by the fins to which vibrations are applied from the vibration application unit reach an inner wall surface of the tank; and a liquid level determination unit that determines the liquid level in the tank based on the correspondence between the amplitude of the vibration detected by the vibration detection unit and the width dimension of the fin. [Effects of the Invention]

[0007] According to the present disclosure, the liquid level in a tank can be determined based on vibrations applied to the liquid. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a configuration diagram of a wafer processing system according to the present disclosure. [Figure 2] FIG. 2 is a vertical cross-sectional side view showing an example of the configuration of a raw material liquid storage device. [Figure 3] 3A and 3B are a perspective view and a plan view showing an example of the configuration of a fin. [Figure 4] This is an example of a fin configuration having four fin members. [Figure 5] This is an example of a fin configuration having six fin members. [Figure 6] FIG. 2 is a vertical cross-sectional side view showing an example of the configuration of a vibration applying unit. [Figure 7] FIG. 10 is a first explanatory diagram relating to the operation of identifying the liquid level. [Figure 8] FIG. 10 is a second explanatory diagram relating to the operation of identifying the liquid level. [Figure 9] FIG. 10 is a third explanatory diagram relating to the operation of identifying the liquid level. [Figure 10] 10A and 10B are schematic diagrams showing the relationship between the convex portion of the fin and the magnitude of the amplitude. [Figure 11]10 is a graph showing an example of the correspondence relationship between the liquid level and the amplitude level of the detected vibration. [Figure 12] 10 is a first explanation showing another configuration example of the fins. [Figure 13] 10 is a second explanation showing another example of the configuration of the fins. DETAILED DESCRIPTION OF THE INVENTION

[0009] <Wafer processing system> An example configuration of a system (wafer processing system 1) for processing wafers W according to the present disclosure will be described with reference to Fig. 1. The wafer processing system 1 is configured to process wafers W by supplying a processing gas obtained by vaporizing a liquid L, which is a raw material liquid, in a raw material liquid storage device 11 to a wafer processing unit 12. Examples of the processing performed in the wafer processing unit 12 include a film formation process and an etching process.

[0010] The raw material liquid storage device 11 includes, for example, a tank 4 for storing the liquid L, a heater 112 for heating the liquid L, and a cabinet 111 for accommodating the tank 4, a carrier gas supply line 101 for supplying a carrier gas to the tank 4, and a raw material gas supply line 102 for supplying a processing gas obtained by vaporizing the liquid L together with the carrier gas to the wafer processing unit 12. The raw material liquid storage device 11 corresponds to the "device for storing liquid" in this example, and includes a mechanism for determining the liquid level of the liquid L stored in the tank 4, the detailed configuration of which will be described later.

[0011] The wafer processing unit 12 also includes a processing vessel 121 in which processing of the wafer W is performed, a mounting table 122 that holds the wafer W in the processing vessel 121, a gas shower head 123 that introduces processing gas into the mounting table 122, and a vacuum exhaust unit 124 that evacuates the processing vessel 121.

[0012] In the wafer processing system 1 configured as described above, a wafer W to be processed is loaded into the processing vessel 121 and placed on the mounting table 122, and the carrier gas is introduced into the tank 4 by opening the valves V1 to V3 on the carrier gas supply line 101 and the source gas supply line 102. Then, the liquid L is vaporized in the tank 4, and the resulting processing gas is supplied from the source gas supply line 102 to the processing vessel 121 via the gas shower head 123. The processing gas supplied to the processing vessel 121 performs a desired process on the wafer W, and then the processing gas is exhausted from the processing vessel 121 by the vacuum exhaust unit 124.

[0013] The raw material liquid storage device 11 constituting the wafer processing system 1 of this example is provided with a mechanism for identifying the level of the liquid L stored in the tank 4. By identifying the level of the liquid L in the tank 4, it becomes possible to know in advance the timing for replacing the tank 4 and to control the heating temperature of the liquid L and the supply flow rate of the carrier gas according to the level of the liquid L.

[0014] Here, a common method for identifying the liquid level of the liquid L in the tank 4 is to insert a float-type sensor into the tank 4 and detect the liquid level corresponding to the height position of the float. However, since the float-type sensor is configured to output the detected height position of the float as an electrical signal, when handling flammable liquid L, it may be difficult to install the float-type sensor in an area that comes into direct contact with the flammable liquid L or its vapor. Furthermore, in the method of detecting the liquid level from the outer wall surface of the tank 4 using a capacitance sensor, if there is other metal near the location where the sensor is installed, there is a risk that the results of the liquid level detection will be affected.

[0015] Therefore, the raw material liquid storage device 11 of this example can specify the height position of the liquid surface by applying vibrations to the liquid L in the tank 4 and detecting the vibrations when waves formed on the surface of the liquid L reach the inner wall surface of the tank 4. Hereinafter, with reference to Figs. 2 and 3, a configuration example of a mechanism for specifying the liquid level of the liquid L will be described.

[0016] <Finn 2> 2 shows a longitudinal side view of the tank 4 housed in the cabinet 111. Fins 2 are arranged in the tank 4 so as to extend along the depth direction (vertical direction) of the liquid L. The fins 2 are made of a rigid member such as metal or resin, and are configured so that their width dimension in the direction intersecting the liquid surface of the liquid L (horizontal direction) changes depending on the position of the liquid L in the depth direction.

[0017] 2, 3, and 7 to 10, the fin 2 has a plurality of protrusions 201, which are regions whose width dimension is larger than that of other regions. The plurality of protrusions 201 are arranged at intervals from one another in the direction of the depth of the liquid L.

[0018] 2 and 7 to 10, the protrusions 201 are assigned identification symbols (a) to (g) from top to bottom. When viewed from the top, the protrusions 201 (a) to (g) are configured so that their widths gradually decrease in the order of (a) → (b) → (c) → (d), and then gradually increase in the order of (d) → (e) → (f) → (g). According to this configuration, the protrusions 201 (a) and (g) with the largest widths are located at the upper and lower limits of the depth of the liquid L stored in the tank 4. The protrusion 201 (d) with the smallest width is located at the center of the depth of the liquid L.

[0019] Furthermore, looking at the individual convex portions 201, (b) and (c) have a configuration in which the width gradually decreases downward in the liquid depth direction, while (c) and (f) have a configuration in which the width gradually increases in the same direction. Furthermore, the convex portion 201 at the bottom (g) has a configuration in which the width gradually increases downward in the liquid depth direction and then remains constant.

[0020] Additionally, the components arranged to protrude laterally from the central axis C of the fin 2, indicated by the dashed line, are referred to as "fin members 20." In FIG. 2, the arrangement area of the fin members 20 is shown surrounded by a dashed line. In this case, the fin 2 of this example can be said to be configured such that two fin members 20, each having a congruent shape, are arranged 180° apart around the central axis C (FIGS. 3(a) and 3(b)).

[0021] The number of fin members 20 constituting the fin 2 is not limited to two. As shown in Figures 4(a) and (b), a fin 2a having four fin members 20 arranged at 90° intervals may be used. Alternatively, as shown in Figures 5(a) and (b), a fin 2b having six fin members 20 arranged at 60° intervals may be used.

[0022] The fin 2 having the above-described configuration is supported by being suspended from the lower end of a support rod 21 extending along the central axis C toward the liquid L. The support rod 21 is inserted into the tank 4 by penetrating a top plate portion 401 provided on the upper surface side of the tank 4. The support rod 21 is fixed, for example, by welding at a position (fixing portion 41) where it penetrates the top plate portion 401. Therefore, the inside of the tank 4 is kept airtight, and the support rod 21 cannot move up and down or rotate around the central axis C.

[0023] <Vibration applying unit 3> As shown in FIG. 2, a vibration applying unit 3 for applying vibration to the fins 2 via the support rod 21 is provided at the upper end of the support rod 21 that protrudes above the tank 4 from the top plate portion 401. 6 shows an example of the internal configuration of the vibration applying unit 3. The vibration applying unit 3 in this example is configured such that a rotating shaft 33 that rotates back and forth within a preset angle by a motor (not shown), a vibrator 32 that is provided so as to protrude from this rotating shaft 33 toward the upper end side of the support rod 21, and a connecting part 34 that connects the lower end of the vibrator 32 and the upper end of the support rod 21 are all provided within a common casing 31.

[0024] When the vibration applying unit 3 having the above configuration is used to apply reciprocating vibrations to the upper ends of the support rods 21 in a direction intersecting the plate surfaces of the fins 2 (the direction indicated by the dashed arrows in FIGS. 3(a) and 3(b)), the vibrations are transmitted to the fins 2 inside the tank 4 via the support rods 21. As a result, the fins 2 can be moved back and forth inside the tank 4 in a direction intersecting the plate surfaces of the fins 2.

[0025] The vibration applied to the fin 2 by the vibration applying unit 3 is not limited to the reciprocating vibration. For example, as in the fins 2a and 2b illustrated in Figures 4 and 5, the fin members 20 may be provided so as to radiate from a central axis C along the support rod 21. In this case, reciprocating vibration may be applied so that the support rod 21 precesses, and a moment in a direction that rotates the fin members 20 may be applied, as shown in Figures 4(b) and 5(b).

[0026] <Vibration detection unit> When vibrations are applied to the fins 2 by the vibration application unit 3, waves are formed on the surface of the liquid L stored in the tank 4. The tank 4 is provided with a vibration detection unit that detects vibrations when the waves propagate and reach the inner wall surface of the tank 4. As shown in FIG. 2, the vibration detection unit of this example is attached along the outer wall surface of the side wall portion 402 of the tank 4 and includes a linear piezoelectric element 51 that is a sensor unit that detects vibrations of the tank, and a signal output unit 52 that outputs a digital signal indicating the magnitude of the voltage output from the linear piezoelectric element 51.

[0027] The linear piezoelectric element 51 serves to detect vibrations of the side wall portion 402 and output the detected vibrations as a change in voltage. Known elements such as piezoelectric wires and piezoelectric cables can be used as the linear piezoelectric element 51. The linear piezoelectric element 51 is provided so as to extend in the vertical direction in correspondence with the arrangement area of the fins 2 on the tank 4.

[0028] In this case, the linear piezoelectric element 51 may be arranged vertically in a straight line along the direction of the depth of the liquid L in the tank 4. Furthermore, when providing the linear piezoelectric element 51 corresponding to the arrangement area of the fins 2 in the cylindrical tank 4, for example, the linear piezoelectric element 51 may be wound in a coil shape along the outer wall surface of the tank 4. FIG. 2 shows an example of the former arrangement. The linear piezoelectric element 51 thus attached along the outer wall surface of the linear piezoelectric element 51 of the tank 4 is connected to the signal output unit 52 via an electric cable.

[0029] <Control unit 10> As shown in FIGS. 1 and 2, the wafer processing system 1 includes a control unit 10. The control unit 10 is configured with a computer including a storage unit, memory, and a CPU that stores a program. The program contains instructions (steps) for outputting control signals from the control unit 10 to each unit of the wafer processing system 1 and for loading / unloading and processing the wafer W. The program is stored in a storage unit of the computer, such as a flexible disk, compact disk, hard disk, MO (magneto-optical disk), or non-volatile memory, and is read from the storage unit and installed in the control unit 10.

[0030] Here, the control unit 10 functions as a liquid level determining unit that determines the liquid level of the liquid L in the tank 4 based on the correspondence between the magnitude of the amplitude of vibration detected by the vibration detection unit (linear piezoelectric element 51, signal output unit 52) and the width dimension of the fin 2. A specific method for determining the liquid level will be described below together with the functions of the fin 2 and linear piezoelectric element 51 with reference to FIGS.

[0031] <effect> 7 to 9 schematically show how the liquid level of the liquid L stored in the tank 4 of the raw material liquid storage device 11 gradually decreases. In these figures, the dashed dotted line indicates the stationary liquid level of the liquid L when no vibration is applied to the fins 2.

[0032] 7, for example, the liquid level of the liquid L is at a height corresponding to the convex portion 201 of the fin 2 (a). At this time, when vibration is applied to the fin 2 via the support rod 21, the convex portion 201 of (a) stirs the surface of the liquid L, and a wave 601, shown schematically by a solid line, is formed on the surface of the liquid L. When this wave 601 propagates along the surface of the liquid L and reaches the inner wall surface of the side wall portion 402 of the tank 4, the wave 601 applies vibration to the side wall portion 402. This vibration is converted into a voltage by the linear piezoelectric element 51 and output to the signal output unit 52.

[0033] The signal output unit 52 outputs a digital signal 602 corresponding to the change in voltage input from the linear piezoelectric element 51. At this time, the change in voltage input from the linear piezoelectric element 51 is generated when the wave 601 formed on the surface of the liquid L vibrates the side wall portion 402, and therefore the digital signal 602 output from the signal output unit 52 is a signal having a frequency and amplitude corresponding to the wave 601. Therefore, by analyzing the digital signal 602 output from the signal output unit 52, the state of the wave 601 formed on the surface of the liquid L in the tank 4 can be known.

[0034] On the other hand, as described above, the fins 2 are configured so that the width dimension in the direction intersecting the liquid surface of the liquid L changes depending on the position in the depth direction of the liquid L. When vibration is applied to the fins 2 configured in this manner via the support rods 21, a stirring force is applied to the surface of the liquid L based on a moment whose magnitude depends on the width dimension of the fins 2 (fin members 20) (in this example, this coincides with the distance R from the support rod 21 (central axis C)).

[0035] For example, at the liquid level shown in Figure 7, the width dimension R a The largest stirring force is applied by the convex portion 201 of (a). Thereafter, for example, when the liquid L is consumed and the liquid level drops to the midpoint between the convex portion 201 of (a) and the convex portion 201 of (b), as shown in FIG. 8, the width dimension R0 of the fin 2 becomes the smallest, and the force stirring the surface of the liquid L also becomes the smallest. When the liquid level further drops to the position shown in FIG. 9, the width dimension R0 of the fin 2 becomes smaller than that of the convex portion 201 of (b) but larger than that of the convex portion 201 of (d). cThe stirring is carried out by the convex portion 201 having (c).

[0036] In this way, when different stirring forces are applied depending on the depth of the liquid L, the amplitude of the waves 601 formed on the surface of the liquid L changes. That is, at height positions where the convex portions 201 with large width dimensions R are arranged, waves 601 with large amplitudes are formed. On the other hand, at height positions where the convex portions 201 with small width dimensions R are arranged, waves 601 with small amplitudes are formed. Figure 10 schematically shows changes in the amplitude of the waves 601 formed when the liquid level of the liquid L is at height positions corresponding to each of the convex portions 201 (a) to (g).

[0037] Then, the signal output unit 52 outputs a digital signal 602 having an amplitude corresponding to the amplitude of each wave 601. Therefore, the liquid level of the liquid L in the tank 4 can be determined based on the correspondence between the magnitude of the amplitude of the digital signal 602 output from the signal output unit 52, i.e., the magnitude of the amplitude of the vibration of the side wall 402, and the width dimension R of the fin 2.

[0038] In reality, within the tank 4, the wave 601 has a complex waveform due to the position where the wave 601 is formed by the fin 2, the propagation direction of the wave 601, the distance between the position where the wave 601 is formed and the side wall 402, and the formation of a reflected wave of the wave 601 on the inner wall surface of the side wall 402. On the other hand, the frequency of the vibration applied from the vibration applying unit 3 can be known in advance, and in the configuration shown in Fig. 2, only the fin 2 applies the greatest force to the surface of the liquid L.

[0039] Therefore, for the digital signal 602 output from the signal output unit 52, it is possible to use, for example, a filter to extract the wave component having the largest amplitude and having the same frequency as the vibration applied from the vibration applying unit 3. By such signal processing, it is possible to obtain the digital signal 602 corresponding to the wave 601 formed by the fin 2, as shown schematically in Figures 7 to 9.

[0040] For the sake of convenience, the frequency of the vibration applied from the vibration applying unit 3 to the fins 2 is assumed to be constant, but from the perspective of detecting the magnitude of the amplitude, the frequency of the vibration applied to the fins 2 may be changed. Furthermore, for example, when inserting the tip of the carrier gas supply line 101 into the liquid L and performing bubbling to obtain processing gas, other forces that form waves 601 on the surface of the liquid L are applied in addition to the vibration of the fins 2. In this case, for example, the bubbling may be stopped, and the fins 2 may be vibrated during a period when processing gas is not being supplied, to determine the liquid level of the liquid L.

[0041] 11 schematically shows changes in the signal level of the digital signal 602 when the liquid level of the liquid L is gradually lowered in the raw material liquid storage device 11 shown in FIG. 2. As described above, the signal level of the digital signal 602 corresponds to the amplitude of the waves 601 formed by the vibration of the fins 2. At this time, according to FIG. 11, the amplitude of the waves 601 formed on the surface of the liquid L by the fins 2 changes in response to the width dimension R of the fins 2 as viewed along the liquid depth direction of the liquid L.

[0042] Therefore, for example, the control unit 10 acquires changes in the signal level of the digital signal 602 over time from the signal output unit 52, and determines the liquid level of the liquid L based on a correspondence with the previously determined shape of the fin 2. In this respect, the control unit 10 constitutes a liquid level determining unit of the present disclosure.

[0043] The change in the signal level of the digital signal 602 that is actually detected may have a lower resolution than that exemplified in Fig. 11. Even in such a case, by arranging the convex portions 201 at intervals in the direction of the depth of the liquid L, it is possible to digitally distinguish between the liquid levels corresponding to (a) to (g) and other liquid levels.

[0044] Based on the level of the liquid L in the tank 4 thus determined, it is possible to know in advance the timing for replacing the tank 4, and to control the operation of the wafer processing system 1 according to the level of the liquid L. 2 and the like is configured so that the amplitude of the wave 601 is maximized at the convex portions 201 at (a) and (g), which are the upper and lower limit positions of the depth of the liquid L. With this configuration, when the liquid level of the liquid L approaches the upper / lower limit positions, the signal level of the digital signal 602 also becomes maximum, so that it can also function as an alarm.

[0045] The raw material liquid storage device 11 of the present disclosure has the following advantages: The liquid level of the liquid L in the tank 4 is determined based on the vibrations applied to the liquid L from the fins 2, and therefore the vibrations can be detected from the outer wall surface side of the side wall portion 402. As a result, unlike a float-type sensor or the like, the liquid level can be measured without inserting a device that inputs and outputs electrical signals into the liquid L.

[0046] <Variations> 2 to 5, the fins 2, 2a to 2b may be configured as a fin 2c consisting of only one fin member 20, as shown in Fig. 12. Furthermore, it is not essential that multiple protrusions 201 are provided at intervals from each other, and the fins may be configured such that the width dimension changes continuously, as in the fin 2d shown in Fig. 13. Fig. 13 shows an example of a fin 2d configured so that the amplitude of the wave 601 increases as the level of the liquid L decreases.

[0047] 12 also shows a configuration in which the support rod 21 holds the fin 2c from the side. Alternatively, the support rod 21 can be inserted from the bottom side of the tank 4 to support the fins 2, 2a to 2d from their lower end sides.

[0048] In the example shown in FIG. 2, linear piezoelectric elements 51 are provided corresponding to the arrangement area of the fins 2, but the configuration of the sensor unit that detects vibrations of the side wall portion 402 is not limited to this. For example, a plurality of small patch-shaped piezoelectric elements or MEMS (Micro Electro Mechanical Systems) acceleration sensors may be provided corresponding to the height positions of the respective convex portions 201. These piezoelectric elements and acceleration sensors are connected in parallel to a common signal output unit 52. As described with reference to FIGS. 7 to 11, it is possible to identify the convex portions 201 that are forming waves according to the magnitude of the amplitude of the detected vibrations without having to identify the plurality of piezoelectric elements or acceleration sensors.

[0049] Furthermore, the sensor units such as the linear piezoelectric elements 51 are not limited to being attached to the outer wall surface of the side wall portion 402. For example, the linear piezoelectric elements 51, the patch-shaped piezoelectric elements described above, and the acceleration sensor may be embedded in the side wall portion 402.

[0050] Furthermore, the source liquid storage device 11 is not limited to being applied to a wafer processing system 1 configured to vaporize the liquid L, which is a source of a processing gas, and supply it to the wafer processing unit 12. For example, the source liquid storage device 11 of the present disclosure can also be applied to a wafer processing system in which the liquid L stored in the tank 4 is directly supplied to the wafer W to perform liquid processing.

[0051] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive, and the above-described embodiments may be omitted, substituted, or modified in various ways without departing from the scope and spirit of the appended claims. [Explanation of symbols]

[0052] L liquid W wafer 11 Raw material liquid storage device 2, 2a~2d fin 51 Linear piezoelectric element

Claims

1. An apparatus for storing a liquid used in processing a substrate, comprising: a tank for storing the liquid; a fin inserted into the tank and configured so that its width dimension in a direction intersecting with the liquid surface changes depending on the position of the liquid in the liquid depth direction; a vibration applying unit that applies vibration to the fin; a vibration detection unit that detects vibrations when waves formed on the surface of the liquid by the fins to which vibrations are applied from the vibration application unit reach an inner wall surface of the tank; and a liquid level determining unit that determines the liquid level in the tank based on the correspondence between the amplitude of the vibration detected by the vibration detection unit and the width dimension of the fin.

2. The device according to claim 1, wherein the fin has a plurality of convex portions, which are areas where the width dimension is larger than other areas, and the plurality of convex portions are arranged at intervals from each other in the liquid depth direction.

3. 3. The device described in claim 1 or 2, wherein the fins are supported by support rods that penetrate a top plate portion provided on the upper surface side of the tank and are inserted into the tank while being fixed to the top plate portion, and the vibration application unit is configured to apply vibrations to the fins via the support rods that extend outside the tank.

4. The apparatus of claim 3 , wherein the fins include a plurality of sets of congruent fin members spaced apart from one another around the central axis of the support rod.

5. 5. The device according to claim 1, wherein the vibration detection unit includes a sensor unit provided on an outer wall surface of the tank.

6. The device according to claim 5 , wherein the sensor portion is a linear piezoelectric element provided so as to extend along the liquid depth direction.

7. 1. A system for processing a substrate, comprising: A device for storing a liquid according to any one of claims 1 to 6; a substrate processing unit that processes substrates using the liquid stored in the tank.

8. The system according to claim 7 , wherein the substrate processing unit processes the substrate with a processing gas obtained by vaporizing the liquid in the tank.

9. 1. A method for determining a liquid level stored in a tank for use in processing a substrate, comprising: applying vibration to a fin inserted into the tank, the fin configured to change its width in a direction intersecting with the liquid surface depending on the position of the liquid in the depth direction; detecting vibrations generated when waves formed on the surface of the liquid by the fin to which the vibrations are applied reach the inner wall surface of the tank; and determining the level of the liquid in the tank based on a correspondence between the magnitude of the amplitude of the detected vibration and a width dimension of the fin.

10. 10. The method according to claim 9, wherein the fin has a plurality of protrusions, each of which is an area where the width dimension is larger than that of other areas, and the plurality of protrusions are arranged at intervals in the liquid depth direction.

11. the fins are supported by support rods that penetrate a top plate portion provided on an upper surface side of the tank and are inserted into the tank while being fixed to the top plate portion, The method according to claim 9 or 10, wherein in the step of applying vibration to the fins, the vibration is applied to the fins via the support rods that extend outside the tank.

12. 12. The method according to claim 9, wherein in the step of detecting the vibration, the vibration is detected from an outer wall surface side of the tank.

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