Abstract:
Apparatus and methods for determining the liquid level of a canister are described. A magnetic field generated by a floating magnet within the canister is measured by a plurality of magnetic sensors outside of the canister. Methods of calibrating the magnetic sensors and measuring the location of the floating magnet are also described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/263,737, filed Dec. 6, 2015, the entire disclosure of which is hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to methods and apparatus for measuring the level of the liquid in the container. More specifically, embodiments of the disclosure relate to the measurement of liquid levels in temperature controlled chemicals using magnetic sensors. 
       BACKGROUND 
       [0003]    Many processes, for example, semiconductor manufacturing, use methods and apparatus to measure the amount of a liquid or a fluid within a container. Conventional systems and methods of detecting the level of liquid in an ampoule are invasive and only provide discrete points (ultra-sonic and reed switch). A failure of a level sensor would require the ampoule to be returned to the chemical supplier. 
         [0004]    Conventional apparatus suffer from several issues that affect the accuracy of the measurements. Variations in the magnetic field which affect the results can occur from, for example, variations of the horizontal position of the floating magnet, orientation of the poles of the floating magnet, and variation of the magnetic field due to variation of magnetic properties of the materials and/or chemical ampule or temperature. 
         [0005]    Therefore, there is a need in the art for apparatus and methods to accurately and repeatedly measuring the level of a fluid within a container. 
       SUMMARY 
       [0006]    One or more embodiments of the disclosure are directed to level measurement systems comprising a canister comprising a sidewall, a top and a bottom defining an interior volume to contain a fluid. A floating magnet is within the interior volume. At least two magnetic sensors are outside of the canister and adjacent the sidewall. 
         [0007]    Additional embodiments of the disclosure are directed to level measurement systems comprising a precursor ampoule comprising a canister with a sidewall, a top and a bottom defining an interior volume to contain a precursor. A floating magnet is within the interior volume of the ampoule. The floating magnet has a density lower than a density of the precursor. A heater is outside of the canister and adjacent the sidewall. At least four magnetic sensors are outside of the canister and adjacent the sidewall. A microprocessor is connected to each of the magnetic sensors. 
         [0008]    Further embodiments of the disclosure are directed to methods of measuring a fluid level in a canister. A plurality of magnetic sensors are calibrated. The calibrated sensors are positioned adjacent the canister. The canister has a fluid and a floating magnet within an interior volume thereof. A magnetic field from the floating magnet is measured using the plurality of calibrated magnetic sensors. A position of the floating magnet is determined within the canister based on an output of the calibrated magnetic sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0010]      FIG. 1  shows a cross-sectional schematic view of a fluid level measuring device in accordance with one or more embodiment of the disclosure; 
           [0011]      FIG. 2  shows a cross-sectional schematic view of a fluid level measuring device in accordance with one or more embodiment of the disclosure; 
           [0012]      FIG. 3  shows a cross-sectional schematic view of a fluid level measuring device in accordance with one or more embodiment of the disclosure; 
           [0013]      FIG. 4  shows a cross-sectional schematic view of a fluid level measuring device in accordance with one or more embodiment of the disclosure; 
           [0014]      FIG. 5  shows a cross-sectional schematic view of a fluid level measuring device in accordance with one or more embodiment of the disclosure; and 
           [0015]      FIG. 6  shows a graph of the voltage output of magnetic sensors according to one or more embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Before describing several exemplary embodiments of the disclosure, it is to be understood that the disclosure is not limited to the details of construction or process steps set forth in the following description. The disclosure is capable of other embodiments and of being practiced or being carried out in various ways. 
         [0017]      FIG. 1  shows an apparatus  100  for generating a chemical precursor comprising a precursor ampoule  101  and resistive heater  102 . The ampule  101  has an inlet  103  and an outlet  104 . The level of a liquid  105  is measured by sensor  106 . Level sensors  106  like the variety shown are invasive and, in the case of failure, the sensor  106  cannot be replaced without sending the ampoule back to the chemical supplier. 
         [0018]    Some embodiments incorporate an ampoule magnet which floats inside the ampoule with magnetic sensors outside the ampoule. Systems of this sort have low accuracy because replacement of an empty ampoule with a new one does not allow for calibration of the level measurement because parameters of the floating magnet (e.g., position of magnet with respect to the sensors) will vary from ampoule to ampoule. Other variables include, but are not limited to, magnetic permeability of the stainless steel used in ampoule which may change from ampoule to ampoule. Therefore, the level measured by the sensor magnetic field will vary even at the same level of liquid in ampoule. 
         [0019]      FIG. 2  shows a system  200  in accordance with one or more embodiment of the disclosure. The system  200  comprises a canister  201 , which may be a chemical ampoule. The canister  201  includes a sidewall  202 , a top portion  203  and a bottom portion  204 . The canister  201  defines an interior volume  205  having an upper region  206  and a lower region  207 . 
         [0020]    A heater  210  surrounds the canister  201 . The heater  210  creates a temperature gradient between the upper region  206  and the lower region  207  in the interior volume  205  of the canister  201 . The heater  210  elevates the temperature of a precursor  220  to generate a precursor gas by sublimating or vaporizing the precursor  220  so that gaseous precursor accumulates in the upper region  206  of the canister  201 . The gaseous precursor can be swept out of the canister  201  by an inert carrier gas entering through inlet port  230  and exiting outlet port  240 . While the inlet port  230  is shown extending into the upper region  206  and the outlet port  240  is shown extending into the lower region  207 , those skilled in the art will understand that the inlet port  230  and outlet port  240  can extend to any suitable depth within the interior volume  205  of the canister  201 . For example, in some embodiments, the inlet port  230  extends into the lower region  207  of the canister  201  to allow the inert gas to bubble through or pass through the precursor  220 . In one or more embodiments, the inlet port  230  extends into the lower region  207  of the canister  201  and may contact the precursor  220 . In some embodiments, the outlet port  240  extends into the upper region  206  of the canister  201  and does not contact the precursor  220 . In one or more embodiments, the outlet port  240  extends into the lower region  207  of the canister  201  and may contact the precursor  220 . 
         [0021]    The heater  210  can be any suitable heater including, but not limited to, resistive heaters. In some embodiments, the precursor  220  is heated to a predefined temperature by a heater  210  disposed proximate to the sidewall  202 . In some embodiments, the heater  210  is configured to create a temperature gradient between a lower region  207  of the canister  201  and the upper region  206  of the canister  201 . The lower region  207  can be colder than or warmer than the upper region  206 . The temperature gradient may range from about 5° C. to about 15° C. 
         [0022]    A floating magnet  260  is within the canister  201 . The floating magnet  260  can be made of any suitable material. In some embodiments, the floating magnet  260  has a density less than the precursor  220 . The size of the floating magnet  260  can be any suitable size. For example, the length of the floating magnet  260  is less than the distance between the side walls  202  of the canister  201 . The strength of the magnetic field  262  of the floating magnet  260  is sufficient to be measured outside of the canister  201 . 
         [0023]    The floating magnet  260  can be free floating within the canister  201  or can be partially fixed in place. In some embodiments, the magnet is allowed to float on the precursor without additional structural support. In some embodiments, as shown in  FIG. 3 , the floating magnet  260  is connected to a guide  268  that extends along the z-axis of the canister  201  to allow the floating magnet  260  to move up and down with the precursor level unhindered. The guide  268  prevents the magnet from moving along the x- or y-axes to maintain a substantially uniform distance  269  between the floating magnet  260  and the sidewall  202 . As used in this manner, the term “substantially uniform distance” means that the distance between the floating magnet  260  and the sidewall  202  is within ±10% of the average distance  269 . 
         [0024]    Magnetic sensors  270   a ,  270   b ,  270   c  are located outside the canister  201 . The number of magnetic sensors can vary depending on, for example, the magnetic field  262  strength of the floating magnet  260  and the height of the canister  201  (i.e., the length of the sidewall  202  of the canister  201 ). In some embodiments, there are at least three magnetic sensors. In some embodiments, there are at least four magnetic sensors. In various embodiments, there are in the range of about 2 to about 10 magnetic sensors, or in the range of about 3 to about 9 magnetic sensors, or in the range of about 4 to about 8 magnetic sensors. 
         [0025]    The magnetic sensors  270  can be any suitable magnetic sensor. Suitable sensors include, but are not limited to, MEMS-based magnetic field sensors. An exemplary magnetic sensor for use with embodiments of the disclosure is a Hall effect sensor. A Hall effect sensor is a transducer which provides a variable voltage output as a function of or in response to a magnetic field. 
         [0026]    The spacing between the magnetic sensors  270  can be varied. The distance between adjacent sensors should be great enough to show a measurable difference between the magnetic field when the floating magnet is asymmetrically positioned. Therefore, the distance between the magnetic sensors may be a function of, at least, the dynamic operating range of the sensors. The spacing and number of magnetic sensors may also be based on the magnetic field strength of the floating magnet. The higher the magnetic field strength, the less magnetic sensors may be used. A floating magnet with a lower magnetic field strength may use a larger number of magnetic sensors than a higher field magnet. In some embodiments, the number of magnetic sensors is based on the height of the ampoule so that there are a fixed number of sensors per unit height. In one or more embodiments, magnetic sensors are positioned in the range of 0.5 inch to 2.5 inches apart, or in the range of about 1 inch to about 2 inches apart, or in the range of about 1 cm to about 7.5 cm apart, or in the range of about 2 cm to about 5 cm apart. 
         [0027]    A microprocessor  280  is connected to the magnetic sensors  270   a ,  270   b ,  270   c . The microprocessor  280  can be any suitable microprocessor that can obtain measurements from the magnetic sensors. The microprocessor  280  can be a stand-alone component or part of a larger processing system. 
         [0028]    Referring to  FIG. 4 , the magnetic sensors  270   a ,  270   b ,  270   c  of some embodiments are mounted in cavities  310   a ,  310   b ,  310   c , respectively, located in heater  210 . While only three cavities are shown, those skilled in the art will understand that there can be any suitable number of cavities. For example, in an embodiment having four magnetic sensors, there might be four cavities in the heater allowing a magnetic sensor to be positioned within each cavity. The cavities  310   a ,  310   b ,  310   c  can be precisely positioned to allow precise positioning of the magnetic sensors  270   a ,  270   b ,  270   c . In some embodiments, mounting the magnetic sensors in cavities allows the ampoule to be replaced without disturbing the position of the magnetic sensors. 
         [0029]    In use, the floating magnet  260  creates magnetic field  262  which can be measured by at least two of magnetic sensors  270   a ,  270   b ,  270   c . In the embodiment shown in  FIG. 4 , magnetic sensors  270   a  and  270   b  can measure magnetic field  260 . Magnetic sensor  270   c  may be too far from the floating magnet  260  to enable any appreciable measurement of magnetic field  262 . 
         [0030]    With reference to  FIG. 5 , calibration of the system is performed at least once without a container  201  (e.g., a chemical ampoule). During the calibration, a permanent magnet  401  is moved along the z-axis relative to the magnetic sensors  270   a ,  270   b ,  270   c . Calibration data is measured at several positions along the axis of movement of the permanent magnet. The readings of  403   a ,  403   b ,  403   c  outputs of the sensors is acquired by microprocessor  280 . The microprocessor  280  calculates ratios of signals of two sensors (e.g.,  403   a / 403   b ,  403   b / 403   c , and  403   a / 403   c ) and stores that data in memory which is part of the microprocessor  280  or a peripheral associated with the microprocessor. In some embodiments, more than two sensors to measure the position of the permanent magnet  401  and the microprocessor  280  calculates or record the values of the magnetic sensors  270   a ,  270   b ,  270   c.    
         [0031]    In one or more embodiments, the variation of the X-Y position of the permanent magnet  401  within the canister  201  has substantially no effect on the determination of the z-position of the magnet. In some embodiments, the variation of the X-Y magnet position equally affects the adjacent sensors. For example, in some embodiments, the accuracy of the calculation of the magnet position is independent of the absolute value of the magnetic field measurements. 
         [0032]    Referring to  FIG. 6 , in use, the level of liquid in the canister  201  is determined based on readings from the sensors  270  as determined by the microprocessor  280  based on the calibrated data. The microprocessor  280  defines the region that the floating magnet  260  is located within. For example, ( 403   a &lt; 403   b ) AND ( 403   b &gt; 403   c ) AND ( 403   a &gt; 403   c ) defines region C. After defining the region, the position of the floating magnet  260  can be calculated based on the ratio of the magnetic field readings for the region. The location of the floating magnet within region C is based on the voltage output contributions from magnetic sensor  270   a  and  270   b . There may also be some contribution from magnetic sensor  270   c  in addition depending on the proximity of the magnetic sensors to each other and the strength of the magnetic field of the floating magnet. In some embodiments, the location of the floating magnet is determined based on the output from less than all of the magnetic sensors. In some embodiments, the location of the floating magnet is determined based on the output from all of the magnetic sensors. In some embodiments, the location of the floating magnet is determined based on the two magnetic sensors closest to the region in which the magnet occupies. 
         [0033]    Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0034]    Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.