Apparatus for measuring the mass and / or the change in mass of an object

Abstract

An apparatus for measuring the mass and / or the change in mass of an object, comprising: a load cell; and a magnetic shield that is configured to at least partially magnetically shield at least part of the load cell.

Description

[0001] APPARATUS FOR MEASURING THE MASS AND / OR THE CHANGE IN MASS OF AN OBJECT

[0002] Field of the Invention

[0003] The present invention relates to an apparatus for measuring the mass and / or the change in mass of an object and particularly, although not exclusively, to an apparatus for measuring the mass and / or the change in mass of a wafer such as a semiconductor wafer.

[0004] Background

[0005] Microelectronic devices are fabricated on semiconductor (e.g. silicon) wafers using a variety of techniques, e.g. including deposition techniques and removal techniques. Semiconductor wafers may be further treated in ways that alter their mass, e.g. by cleaning, ion implantation, lithography and the like.

[0006] Wafer treatment techniques typically cause a change in mass at or on the surface of the semiconductor wafer. The configuration of the changes to the surface are often vital to the functioning of the device, so it is desirable for quality control purposes to assess wafers during production in order to determine whether they have the correct configuration.

[0007] Specialist metrology tools may be used within the production flow so that monitoring is conducted soon after the relevant process of interest and usually before any subsequent processing, i.e. between processing steps.

[0008] Measuring the change in mass of a wafer either side of a processing step is an attractive method for implementing product wafer metrology. It is relatively low cost, high speed and can accommodate different wafer circuitry patterns automatically. In addition, it can often provide results of higher accuracy than alternative techniques. The wafer in question is weighed before and after the processing step of interest. The change in mass is then correlated to the performance of the production equipment and / or the desired properties of the wafer.

[0009] Typically, a measure of the weight or mass of a wafer is performed before the processing step on a wafer mass metrology system and then the same system is used to measure the weight or mass of the wafer after the processing step. The measurement results are then used to calculate a change in mass of the wafer caused by the processing step.

[0010] One key component of a wafer mass metrology system is the load cell. One known type of load cell that can be used to perform such measurements is an electromagnetic force compensation load cell. At its most general, an electromagnetic force compensation load cell uses an electromagnetic force to balance out the weight force of the object. Typically, the weight force of the object is applied to a beam on a first side of a pivot about which the beam can pivot, and the electromagnetic force is applied to the beam on a second side of the pivot so that a moment of force due to the electromagnetic force cancels out a moment of force due to the weight force of the object. In such a situation, the beam does not move, because there is no net moment of force on the beam. The electromagnetic force that is required to prevent movement of the beam when the weight force is applied to the beam therefore corresponds to the weight force of the object. Movement (or lack of movement) of the beam can be detected using a detector, typically an optical detector.

[0011] An electromagnetic force compensation load cell includes an electromagnetic force compensation mechanism for generating the electromagnetic force. The electromagnetic force compensation mechanism typically comprises an electromagnetic coil that is attached to the beam, and a magnet that is appropriately positioned close to the electromagnetic coil. The electromagnetic force is generated by applying a current to the electromagnetic coil so that there is a magnetic force between the electromagnetic coil and the magnet. Typically, the magnet is a permanent magnet, which is typically positioned beneath the electromagnetic coil.

[0012] The electromagnetic force that is generated by such an electromagnetic force compensation mechanism depends on the current applied to the electromagnetic coil. Therefore, the current that is required to be supplied to the electromagnetic coil to prevent movement of the beam when the weight force is applied to the beam corresponds to the weight force of the object. The weight force of the object can therefore be determined by determining the current that is required to be supplied to the electromagnetic coil to prevent movement of the beam when the weight force is applied to the beam.

[0013] The load cell may comprise a controller or processor that is configured to multiply a measurement result of the load cell by a predetermined calibration factor, so that the load cell outputs a mass of the object. For example, the calibration factor may be determined by performing a measurement on a reference mass having a known mass with the load cell, or based on a known gravity “g” at the location of the load cell. The load cell may comprise an internal calibration mass for determining the calibration factor. Alternatively, the load cell may output a weight of the object.

[0014] The present inventors have realised that electromagnetic force compensation load cells, and indeed other types of electronic load cell, can be sensitive to external or local magnetic fields. In particular, an external or local magnetic field can impact the electromagnetic force compensation mechanism, for example such that a larger or smaller current than would otherwise be required needs to be supplied to the electromagnetic coil to prevent movement of the beam when the weight force is applied to the beam.

[0015] This will lead to an error in the measurement output of the load cell, and therefore an error in the measured mass and / or change in mass of the wafer.

[0016] For example, a uniform external or local magnetic field may lead to a uniform error in the measurement output of the load cell, which will negatively affect the accuracy of the measurement. In contrast, a fluctuating external or local magnetic field may lead to fluctuations in the measurement output of the load cell, thereby negatively affecting the measurement repeatability and the accuracy of the measurement. In general, the presence of such external or local magnetic fields may reduce an overall sensitivity of the apparatus to detect small differences in mass. For a uniform magnetic field it may be possible to calibrate the load cell to compensate for the magnetic field. However, such calibration may not be possible or appropriate for short term or transient fluctuations in the local magnetic field. Short term or transient fluctuations in the local magnetic field may therefore be particularly problematic in terms of their effect on the measurement accuracy. Such measurement errors may be caused by the magnetic field fluctuations directly, and / or by the local or external field magnetising / demagnetising a case of the load cell, depending on the magnetic remanence of the case of the load cell.

[0017] Such external or local magnetic fields may be caused by neighbouring process or metrology equipment in a wafer fabrication environment, and / or communication devices, and / or wafer / material handling systems such as hoists, or other sources inside or outside of a mass metrology apparatus comprising the load cell, for example.

[0018] In addition to electromagnetic force compensation load cells, this issue may also be relevant for magnetostriction load cells, which are a class of loadcell which work based on change in physical shape or dimensions of certain ferromagnetic materials do to an applied magnetic field (or vice versa). In addition, any load cell that uses an electromagnet may be affected by external or local magnetic fields in a manner similar to that described above.

[0019] Currently, to attempt to reduce an effect of such external magnetics fields on the measurement accuracy, it is known to prepare a survey to detect and quantify magnetic field strengths in different areas within a possible installation site of a mass metrology apparatus comprising the load cell. The mass metrology apparatus can then be installed in an area having a minimum magnetic field strength and / or a minimum variation in magnetic field strength. However, it can be difficult to find a suitable site within a fabrication environment with no or minimal external or local magnetic fields, or such a site may not be in an optimal position for installation of the mass metrology apparatus. Furthermore, such a survey only captures such magnetic fields once for a short period of time before the mass metrology apparatus is installed. It is therefore possible that the situation could change after the survey has been prepared, for example due to the installation of an additional process or metrology equipment in close vicinity at a later date, or that there may be short term or transient changes in the magnetic field.

[0020] The present invention has been devised in light of the above considerations.

[0021] Summary of the Invention

[0022] At its most general, the present invention relates to magnetically shielding the load cell, so that the measurement output of the load cell is less negatively affected by external or local magnetic fields.

[0023] According to a first aspect of the present invention, there is provided an apparatus for measuring the mass and / or the change in mass of an object, comprising: a load cell; and a magnetic shield that is configured to at least partially magnetically shield at least part of the load cell.

[0024] According to the first aspect of the present invention, at least part of the load cell is at least partially magnetically shielded by the magnetic shield. Therefore, an influence of an external or local magnetic field on a measurement output of the load cell can be removed or reduced. An accuracy and / or a repeatability of the measurement output of the load cell may therefore be improved. As a result, the apparatus may be able to more accurately and / or more repeatably measure the mass and / or the change in mass of an object.

[0025] The apparatus according to the first aspect of the present invention may have any one, or, where compatible, any combination of the following optional features.

[0026] The apparatus may be for measuring the mass and / or the change in mass of a wafer, for example a semiconductor wafer.

[0027] The apparatus may be for measuring the mass and / or the change in mass of a wafer having a predetermined diameter, for example 300 mm.

[0028] The apparatus may be configured or adapted to measure the mass and / or the change in mass of the object.

[0029] The apparatus may be a metrology apparatus, or a mass metrology apparatus, or a semiconductor wafer mass metrology apparatus, for example.

[0030] The load cell may be for measuring the weight or mass, and / or the change in weight or mass, of an object.

[0031] The load cell may be configured or adapted to measure the weight or mass, and / or the change in weight or mass, of the object

[0032] The load cell may be for weighing the object.

[0033] The load cell may be for performing a weight measurement on the object.

[0034] The load cell may be for generating measurement output indicative of the weight or mass, and / or the change in weight or mass, of the object.

[0035] The load cell may be for measuring a weight force of the object.

[0036] The load cell may comprise or be a weight force transducer.

[0037] The load cell may comprise or be a weight force sensor.

[0038] The load cell may generate measurement output based on a measurement of an amount of electromagnetic force compensation required to compensate for (or counterbalance or counteract) a weight force of an object.

[0039] The load cell may generate measurement output based on a measurement of an amount of current that needs to be supplied to an electromagnetic force compensation mechanism of the load cell to compensate for (or counterbalance or counteract) a weight force of an object.

[0040] The load cell may be configured to balance the weight force of the object with a force experienced when an electromagnetic coil of the load cell is energised in a magnetic field of the load cell. Specifically, the load cell may be configured to provide a current to the electromagnetic coil that is sufficient to cause the force experienced by the electromagnetic coil to balance the weight force of the object. The measurement output of the load cell may be generated based on the required current. The load cell may therefore comprise an electromagnetic coil arranged in a magnetic field and a controller for controlling a current supplied to the electromagnetic coil.

[0041] The load cell may comprise a permanent magnet that generates the magnetic field.

[0042] The permanent magnet may be positioned beneath or below the electromagnetic coil. Of course, the permanent magnet may be positioned differently relative to the electromagnetic coil, for example above the electromagnetic coil, and / or at the side of the electromagnetic coil, and / or inside the electromagnetic coil, and / or around the electromagnetic coil.

[0043] The electromagnetic coil and the permanent magnet may be configured to experience an attractive magnetic force therebetween when an appropriate current is supplied to the electromagnetic coil.

[0044] The load cell may comprise or be an electromagnetic force compensation or electromagnetic force restoration force sensor or load cell.

[0045] The load cell may comprise or be an electromagnetic force compensation or electromagnetic force restoration force sensor or load cell comprising an electromagnetic coil.

[0046] The apparatus, for example the load cell or a controller or processor of the apparatus, may be configured to calculate or determine the mass and / or change in mass of the object based on at least a measurement performed on the object by the load cell.

[0047] The load cell, or the apparatus, may comprise a controller or processor that is configured to multiply a measurement result of the load cell (for example a measurement of a weight force of the object by the load cell) by a predetermined calibration factor, to determine a mass and / or a change in mass of the object. For example, the calibration factor may be determined by performing a measurement on a reference mass having a known mass with the load cell, or based on a known gravity “g” at the location of the load cell. Specifically, the load cell may measure a weight force or a change in weight force of the object, and the weight force or change in weight force may then be converted into a mass or change in mass of the object. The load cell may comprise an internal calibration mass for determining the calibration factor.

[0048] The load cell may comprise an internal calibration mechanism that is configured to calibrate the load cell so that the load cell outputs a mass and / or a change in mass of the object based on a measurement of the weight force of the object.

[0049] The apparatus and / or the load cell may comprise a support for supporting the object during a measurement performed by the load cell.

[0050] The support may be configured or adapted to support the object during the measurement.

[0051] The support may be or comprise a pan. The term support may therefore be replaced with the term pan throughout, unless incompatible. The pan may be a weighing pan or a balance pan.

[0052] The support may be a weighing support. The support may comprise a support part for supporting the object and a mounting part for mounting the support on the load cell. For example, the mounting part may comprise a shaft such as a longitudinal shaft.

[0053] The support may comprise a pan part and a shaft connected to the pan part. The shaft may be a longitudinal shaft.

[0054] The support may be mounted on the load cell, or coupled to the load cell, or connected to the load cell.

[0055] Supporting an object may mean supporting the weight of the object.

[0056] Supporting an object may mean holding or carrying the object.

[0057] A magnetic shield may mean a part or component or device that is for, configured to, or adapted to, provide magnetic shielding and / or shield magnetic fields, and / or block magnetic fields, and / or reduce the magnitude of magnetic fields.

[0058] For example, the magnetic shield may comprise one or more layers or sheets or plates or films or coatings of one or more materials.

[0059] Magnetic shielding of the load cell may comprise protecting the load cell from the influence of an external or local magnetic field, and / or reducing the influence of the external or local magnetic field.

[0060] Magnetic shielding of the load cell may comprise reducing a magnitude of an external or local magnetic field experienced or felt by the load cell.

[0061] Magnetic shielding may comprise reducing or blocking an external or local magnetic field, or reducing a magnitude of an external or local magnetic field.

[0062] For example, the magnetic shield may be configured to reduce a magnitude of an external or local magnetic field to less than or equal to 50% of the original magnitude, or less than or equal to 25% of the original magnitude, or less than or equal to 10% of the original magnitude.

[0063] Magnetically shielding at least part of the load cell may comprise guiding or redirecting magnetic field lines along a path that doesn’t influence the at least part of the load cell, for example guiding magnetic field lines along a path around the at least part of the load cell.

[0064] The magnetic shield may at least partially magnetically shield the load cell.

[0065] The magnetic shield may magnetically shield at least part of the load cell.

[0066] The magnetic shield may magnetically shield the load cell.

[0067] The magnetic shield may magnetically shield only part of the load cell, for example only a part of the load cell that is particularly sensitive to the influence of an external or local magnetic field, for example an electromagnetic coil of the load cell.

[0068] The magnetic shield may magnetically shield at least an electromagnetic force compensation mechanism of the load cell, or at least part of an electromagnetic force compensation mechanism of the load cell. For example, the magnetic shield may magnetically shield at least an electromagnetic coil and a magnet of the electromagnetic force compensation mechanism.

[0069] At least part of the load cell means the load cell or part of the load cell.

[0070] At least partially magnetically shielding means magnetically shielding or partially magnetically shielding.

[0071] The magnetic shield may partially shield the load cell or part of the load cell. For example, the magnetic shield may not form a continuous or complete shield around the load cell or part of the load cell. For example, the magnetic shield may comprise one or more gaps or openings or holes or slots, for example for allowing a wire or wires to pass through the magnetic shield and / or for allowing a support or pan of the load cell to pass through the magnetic shield. The size of such gaps or openings may be minimised in order to maximise the magnetic shielding provided by the magnetic shield.

[0072] The magnetic shield may be configured to magnetically shield the load cell or part of the load cell, or to substantially magnetically shield the load cell or part of the load cell.

[0073] The magnetic shield may be configured to magnetically shield all of the load cell, or to substantially magnetically shield all of the load cell.

[0074] The magnetic shield may be arranged or adapted to at least partially magnetically shield the load cell.

[0075] The magnetic shield may be configured to shield one, a plurality, or all directional components of an external or local magnetic field. The magnetic shield may not shield all directional components of the external or local magnetic field. For example, the magnetic shield may be configured to block only one or two of three directional components of the external or local magnetic field (i.e. two or three of Bx, By, Bz). In particular, the load cell may be more sensitive to magnetic fields in a certain direction, and therefore it may be particularly beneficial to shield a directional component of the magnetic field having this direction, and / or not as important to shield directional components of the magnetic field having a different direction.

[0076] Shielding of only one or two of the directional components of an external or local magnetic field, or shielding of one or more directional components to a greater extent than another directional component, may be achieved in various ways, including providing a different thickness of the magnetic shield in different areas or sides of the magnetic shield, or only partly enclosing the load cell on certain sides or surfaces of the load cell (when the load cell is more sensitive to magnetic fields in a certain direction), or adding additional or multiple layers of magnetic shielding in the magnetic shield in certain areas or sides of the magnetic shield, for example.

[0077] The magnetic shield may be arranged to cover at least part of the load cell.

[0078] The magnetic shield may be arranged to cover part of the load cell.

[0079] The magnetic shield may be arranged to cover, or substantially cover, the load cell.

[0080] The magnetic shield may be arranged to cover, or substantially cover, all of the load cell.

[0081] Preferably the magnetic shield covers (or substantially covers or partially covers) all of the surfaces of the load cell or part of the load cell. The magnetic shield may enclose, or substantially enclose, or partially enclose, the load cell or part of the load cell. However, the magnetic shield may comprise one or more openings or gaps or holes or slots, for examples for allowing a wire or wires to pass through the magnetic shield and / or for allowing a support or pan of the load cell to pass through the magnetic shield.

[0082] The magnetic shield may be arranged around, or at least partially around, the load cell or part of the load cell.

[0083] The magnetic shield may surround, or substantially surround, or partially surround, the load cell or part of the load cell. The magnetic shield may completely or fully surround the load cell or part of the load cell.

[0084] The magnetic shield may house the load cell or part of the load cell.

[0085] The magnetic shield may comprise or form or be a housing or case around the load cell or part of the load cell.

[0086] The magnetic shield may comprise or form or be a hollow structure around the load cell or part of the load cell.

[0087] The term magnetic shield may be replaced with the term magnetic shielding throughout, unless incompatible.

[0088] Typically the magnetic shield comprises a material having a high magnetic permeability.

[0089] The magnetic shield may comprise a material having: a relative magnetic permeability, pr, greater than or equal to 1 ,000; or a relative magnetic permeability, pr, greater than or equal to 10,000; or a magnetic permeability, p, greater than or equal to 0.001 H / m; or a magnetic permeability, p, greater than or equal to 0.01 H / m.

[0090] The relative magnetic permeability is the magnetic permeability of the material divided by the magnetic permeability of free space po.

[0091] The magnetic shield may comprise a material having a high magnetic saturation. This may be in addition to, or instead of, the material having a high magnetic permeability. For example, a material that does not have a high magnetic permeability but that has a high magnetic saturation may be used because it can cope with a stronger magnetic field. For example, stainless steel or mild steel has a high magnetic saturation and can be used. Preferably such a material is used in addition to a material having a high magnetic permeability.

[0092] The magnetic shield may comprise a material having a magnetic saturation of > 0.1 T, for example.

[0093] The magnetic field may comprise a first material having a high magnetic permeability and a second material having a high magnetic saturation, for example arranged as different overlapping layers.

[0094] The magnetic shield may comprise a material having a low magnetic remanence.

[0095] The magnetic shield may comprise a material having a high magnetic permeability and / or a high magnetic saturation and / or a low magnetic remanence. For example, the magnetic shield may comprise one or more of: a ferromagnetic material, a Nickel-iron ferromagnetic alloy, Mu-metal, Supermalloy or Permalloy.

[0096] Ni-Fe alloys are advantageous materials to use for the magnetic shield because they have a high permeability. However, they do not have a high saturation, and therefore an additional material may also be provided that has a high saturation, for example mild steel or stainless steel.

[0097] The magnetic shield may comprise one or more layers.

[0098] The magnetic shield may comprise a single layer.

[0099] The magnetic shield may comprise a plurality of layers.

[0100] The plurality of layers may be arranged one or top of the other, and / or in a laminate, and / or in a layered or stacked arrangement, for example.

[0101] The plurality of layers may comprise a first layer having a first thickness and a second layer having a different second thickness.

[0102] Alternatively, the plurality of layers of material may have the same, or substantially the same, thickness.

[0103] There may be an air gap between two of the plurality of layers of material.

[0104] The plurality of layers may comprise first and second layers made of different materials. For example, each of the plurality of layers may be made of a different material. However, more than one layer of the same material may be included. the magnetic shield may have a thickness: greater than or equal to 0.2 mm; or greater than or equal to 1 mm.

[0105] The magnetic shield may comprise a plurality of different parts, which may be discrete and / or discontinuous. Preferably the different parts are combined together to form a continuous or substantially continuous magnetic shield.

[0106] The magnetic shield may be in addition to a case or housing of the load cell that houses an internal mechanism of the load cell. The load cell may therefore comprise a case or housing that houses an internal mechanism of the load cell. This case of housing may be made of aluminium, for example, which provides little or no magnetic shielding.

[0107] The magnetic shield may be positioned around or outside of the additional case or housing of the load cell.

[0108] The magnetic shield may have one or more holes or openings, for example for allowing a wire or other electrical connection to pass through the load cell.

[0109] The apparatus may comprise a support or pan having a mounting portion for mounting the support or pan on the load cell, wherein the mounting portion extends through an opening or hole or gap in the magnetic shield. For example, the mounting portion may be a shaft.. The support or pan may be for supporting the object during the measurement of the mass and / or the change in mass of the object.

[0110] The magnetic shield may comprise a housing or case or shell or enclosure around the load cell. The magnetic shield may be separable into at least two parts to allow the load cell or part of the load cell to be positioned inside the magnetic shield. For example, the magnetic shield may comprise a top portion and a bottom portion that fit together around the load cell or part of the load cell to form a housing or case or shell or enclosure around the loadcell.

[0111] The apparatus may further comprise a measurement chamber in which the load cell is housed. The measurement chamber may provide a controlled environment around the load cell, for example to minimise measurement errors caused by air currents.

[0112] The measurement chamber may comprise the magnetic shield. For example, the magnetic shield may be integral with the measurement chamber.

[0113] The measurement chamber may comprise, or be made of or substantially made of, aluminium, for example, which has a low magnetic permeability and therefore does not provide significant magnetic shielding of the load cell.

[0114] The magnetic shield may be on, or embedded into, an inner surface or inner wall or walls of the measurement chamber.

[0115] The magnetic shield may comprise a sheet or film or layer on, or embedded into, an inner surface or inner wall or walls of the measurement chamber. Preferably the magnetic shield is provided over all of the inner walls of the measurement chamber so as to surround the load cell positioned in the measurement chamber.

[0116] The load cell may be an electromagnetic force compensation load cell.

[0117] The load cell may comprise an electromagnetic force compensation mechanism.

[0118] The apparatus may further comprise a controller or processor that is configured to calculate the mass and / or the change in mass of the object based at least on a measurement performed by the load cell, and / or an output of the load cell.

[0119] The controller or processor may be integral to the load cell, or part of the load cell, or external to the load cell.

[0120] The apparatus may further comprise one or more sensors configured to sense one or more atmospheric conditions. For example, the one or more sensors may be arranged to sense one or more atmospheric conditions inside the measurement chamber.

[0121] For example, the one or more sensors may be configured to sense one or more of a pressure or temperature or humidity of the air surrounding the load cell and / or the air in the measurement chamber. The apparatus may further comprise a controller or processor that is configured to calculate the mass and / or the change in mass of the object based at least on a measurement performed by the load cell, or an output of the load cell, and an output of the one or more sensors.

[0122] For example, the controller or processor may be configured to calculate a buoyancy force acting on the object based on the output of the one or more sensors and to correct a weight or mass measurement for the object based on the calculated buoyance force.

[0123] The controller may be configured to correct the measurement of the mass and / or the change in mass for the effects of buoyancy acting on the object based on the output of the one or more sensors.

[0124] The magnetic shield may be electrically connected to ground. This may prevent electric charge, for example static charge, building up on the magnetic shield.

[0125] The magnetic shield may be electrically grounded.

[0126] The magnetic shield may be configured not to directly contact the load cell, for example an internal mechanism of the load cell, so as to avoid the generation of particles, which may subsequently contaminate a wafer being weights.

[0127] The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided.

[0128] Summary of the Figures

[0129] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0130] Figure 1 is a schematic illustration of part of a load cell that can used in an embodiment of the present invention.

[0131] Figure 2 is a schematic illustration of a load cell that can be used in an embodiment of the present invention.

[0132] Figure 3 is a schematic illustration of an apparatus according to an embodiment of the present invention.

[0133] Figure 4 is a schematic illustration of an apparatus according to a further embodiment of the present invention. Figure 5 is a schematic illustration of an apparatus according to a further embodiment of the present invention.

[0134] Detailed Description of the Invention

[0135] Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.

[0136] Figure 1 is a schematic illustration of part of a load cell that can be used in an embodiment of the present invention. Of course, the part of the load cell illustrated in Figure 1 is merely an example and other types or configurations of load cells may alternatively be used in embodiments of the present invention.

[0137] The load cell 1 is an electromagnetic force compensation load cell, or electromagnetic force restoration load cell.

[0138] Figure 1 illustrates the load cell 1 with a case or housing of the load cell 1 removed. Figure 1 therefore schematically illustrates an internal mechanism of the load cell 1.

[0139] As shown in Figure 1 , the load cell 1 comprises a pan 3 that is mounted on a shaft 5. The pan 3 is for supporting a wafer 2 while the weight or mass of the wafer 2 is measured using the load cell 1. The pan 3 may be substantially circular in shape when viewed from above, for example. As shown in Figure 1 , the pan 3 has a plurality of pins (or other protrusions such as bumps or balls) that extend from the upper surface of the pan 3 so as to contact a lower surface of the wafer 2 and support the wafer 2 spaced apart from the upper surface of the pan 3. For example, there may be three such pins.

[0140] The shaft 5 is connected to a beam 7 (or balance lever) on a first side of a pivot 9 around which the beam 7 can pivot. The connection is such that vertical movement of the shaft 5 acts to cause pivoting of the beam 7 around the pivot 9.

[0141] The load cell 1 further comprises an upper linkage 11 and a lower linkage 13. The upper linkage 11 and lower linkage 13 are each rotatably connected at a respective first end to the shaft 5 and at a respective second end to a fixed surface 15 by rotatable connections 17. The upper linkage 11 is connected to an upper portion of the shaft 5 and is located above the beam 7. The lower linkage 13 is connected to a lower portion of the shaft 5 and is located below the beam 7. The upper linkage 11 and lower linkage 13 may be rods or beams, for example. The upper linkage 11 and lower linkage 13 define or constrain possible motion of the shaft 5.

[0142] The shaft 5 is connected to the beam 7 via a third linkage 19 that is rotatably connected at its first end to the shaft 5 and that is rotatably connected at its second end to the beam 7.

[0143] When an object is loaded onto the pan 3, the weight force of the object acts on the pan 3 and the shaft 5 to move the pan 3 and the shaft 5 downwards. The connection between the shaft 5 and the beam 7 means that when an object is loaded on the pan 3, a downwards force is applied to the beam 7 on the first side of the pivot 9. This downwards force acts to rotate the beam 7 in an anti-clockwise (counterclockwise) direction around the pivot 9.

[0144] The load cell 1 further comprises an electromagnetic force compensation mechanism for generating a force on the beam 7 on the second side of the pivot 9, so as to balance out the force on the beam on the first side of the pivot 9 due to the weight of the object loaded on the pan 3.

[0145] Specifically, the electromagnetic force compensation mechanism comprises an electromagnetic coil 21 that is positioned on the beam 7 (or attached to the beam 7) on the second side of the pivot 9. Furthermore, the electromagnetic force compensation mechanism further comprises a magnet 23 that is positioned close to the electromagnetic coil 21 , for example beneath the electromagnetic coil 21 . The magnet 23 may be a permanent magnet or an electromagnet.

[0146] Rotation of the beam 7 in the anti-clockwise direction as described above would cause the electromagnetic coil 21 to move upwards, away from the magnet 23.

[0147] When an electrical current is applied to the electromagnetic coil 21 , the electromagnetic coil 21 produces a magnetic field. The interaction between the magnetic field produced by the electromagnetic coil 21 and the magnetic field produced by the magnet 23 causes the electromagnetic coil 21 to experience a force, the magnitude and direction of which is determined by the magnitude and direction of the current applied to the electromagnetic coil 21 .

[0148] By applying a current with a suitable magnitude and direction to the electromagnetic coil 21 , the electromagnetic coil 21 can be made to experience a downwards force towards the magnet 23 that causes a moment of feree on the beam 7 that is equal and opposite to the moment of feree on the beam 7 due to the weight of the object loaded on the pan 3.

[0149] In particular, a suitable current applied to the electromagnetic coil 21 can cause an attractive magnetic force between the electromagnetic coil 21 and the magnet 23, which acts in a downwards direction on the electromagnetic coil 21 , since the magnet 23 is positioned beneath the electromagnetic coil 21 , for example directly below the electromagnetic coil 21 .

[0150] The electromagnetic coil 21 therefore experiences an attractive force with the magnet 23, which acts on the electromagnetic coil 21 in a downwards direction towards the magnet 23, when a suitable current is supplied to the electromagnetic coil 21 .

[0151] The downwards force on the electromagnetic coil 21 , and therefore the moment of feree caused by the downwards force, depends on the magnitude of the current applied to the electromagnetic coil 21 . Therefore, the moment of feree caused by the weight of the object on the pan 3 can be determined by measuring the magnitude of the current that needs to be supplied to the electromagnetic coil 21 in order to keep the electromagnetic coil 21 in the same position when the object is placed on the pan 3. The current that needs to be supplied to the electromagnetic coil 21 in order to keep the electromagnetic coil 21 in the same position when the object is placed on the pan 3 is therefore directly related to the weight of the object, and can be used to calculate the weight and / or mass of the object. The load cell 1 further comprises a position sensor 25 for detecting a position of part of the beam 7, for example the position of an end of the beam 7. Specifically, the position sensor 25 is for detecting when the part of the beam 7 is in a predetermined position. The part of the beam 7 may be in the predetermined position when the beam 7 is horizontal, which may be a default or rest orientation of the beam 7 when no object is loaded on the pan 3. Therefore, the part of the beam 7 being in the predetermined position may indicate that the beam 7 is in the default or rest orientation of the beam 7.

[0152] The position sensor may be an optical sensor that comprises a light source and light sensor, wherein a light path between the light source and the light sensor is blocked when the part of the beam 7 is not in the predetermined position. For example, the part of the beam 7 may comprise an opening or aperture through which the light can pass and that is aligned with the light source and the light sensor when the part of the beam 7 is in the predetermined position.

[0153] The load cell 1 further comprises a controller 27 that is configured to control a current supplied to the electromagnetic coil 21 based on an output of the position sensor 25. Specifically, the controller 27 controls a magnitude of the current supplied to the electromagnetic coil 21 such that the position sensor 25 detects that the part of the beam 7 is in the predetermined position when the object is loaded on the pan 3. This corresponds to the force on the electromagnetic coil balancing out the force on the beam on the first side of the pivot 9 due to the weight of the object loaded on the pan 3.

[0154] In other words, the controller 27 is configured to control the supply of current to the electromagnetic coil 21 and to determine the current that needs to be supplied to the electromagnetic coil 21 to keep the electromagnetic coil 21 and therefore the pan 3 in the same positions when the object is loaded on to the pan 3.

[0155] The load cell 1 (for example the controller 27) may determine and output the weight of an object loaded on the load cell 1 , based on the current that needs to be supplied to the electromagnetic coil 21 to maintain the electromagnetic coil 21 and therefore the pan 3 in position when the object is loaded on the pan 3. Alternatively, the load cell 1 may determine and output the mass of an object loaded on the load cell 1 , for example using a calibration factor that is determined by performing a measurement on a reference mass having a known mass with the load cell 1 , or based on a known gravity “g” at the location of the load cell 1 . For example, the calculated weight of the object may be multiplied by the calibration factor to covert the weight into a mass. The load cell 1 may comprise an internal calibration mass for determining the calibration factor. As is well known, the weight and mass of the object are related together by the well-known equation W=mg.

[0156] Of course, the load cell 1 described above and illustrated in Figure 1 is only an example of a suitable load cell, and other types or configurations of load cell can be used in the present invention.

[0157] In particular, other types of electromagnetic force compensation load cells are known, for example where both the weight force of the object and the electromagnetic force act on a beam on the same side of the pivot in opposite directions. In addition, or alternatively, various changes can be made to the load cell 1 illustrated in Figure 1 . For example, the position sensor may be arranged to detect a position of the pan 3, shaft 5 or electromagnet 21 instead of part of the beam, or another component connected directly or indirectly to any of these components.

[0158] Furthermore, a configuration and / or position of the electromagnetic coil 21 and magnet 23 may be different to those illustrated in Figure 1 .

[0159] Furthermore, the connection between the shaft 5 and the beam 7 may be different to that illustrated in Figure 1 .

[0160] Furthermore, the upper and lower linkages 11 and 13 may be omitted in some embodiments.

[0161] Figure 2 shows the load cell 1 including a case or housing 29 of the load cell, which houses the internal mechanism of the load cell 1.

[0162] The case or housing 29 is made of a material with a relatively low magnetic permeability, and therefore provides no or minimal magnetic shielding of the load cell 1 . For example, the case or housing 29 may comprise aluminium.

[0163] As mentioned above, the present inventors have realised that electromagnetic force compensation load cells such as the load cell 1 illustrated in Figure 1 , and indeed other types of electronic load cell, can be sensitive to external or local magnetic fields. In particular, an external or local magnetic field can impact the electromagnetic force compensation mechanism, for example such that a larger or smaller current than would otherwise be required needs to be supplied to the electromagnetic coil to prevent movement of the beam. This will lead to an error in the measurement output of the load cell, and therefore an error in the measured mass or change in mass of the wafer.

[0164] In order to prevent or reduce such problems, in embodiments of the present invention magnetic shielding is provided that is arranged to magnetically shield the load cell.

[0165] Three alternative arrangements of such magnetic shielding are illustrated in Figures 3 to 5.

[0166] As shown in Figure 3, in a first embodiment of the present invention a magnetic shield 31 is provided that encloses and / or surrounds the load cell 1 . In particular, the magnetic shield 31 is provided around the whole load cell 1 to magnetically shield the whole load cell 1 .

[0167] The magnetic shield 31 may comprise or form or be a case or housing or container or enclosure around the load cell 1 .

[0168] The magnetic shield 31 may comprise a single material or a plurality of materials. For example, the magnetic shield may comprise one or more materials selected from a ferromagnetic material, Nickel-iron ferromagnetic alloy, Mu-metal, Supermalloy or Permalloy, for example.

[0169] The magnetic shield 31 may comprise a single layer of material, or a plurality of layers of either the same or different materials arranged in a laminate or layered or stacked arrangement.

[0170] The magnetic shield 31 may have a thickness of approximately 1 mm, for example. As shown in Figure 3, the pan 3 is connected to the load cell 1 by a shaft. The shaft extends though an opening in the magnetic shield 31 . For example, the opening may be a suitably sized hole in the magnetic shield 31 .

[0171] The shaft may be attached or connected to the load cell 1 , or mounted on the load cell 1 .

[0172] The magnetic shield may have other holes or openings, for example through which a wire can pass through the magnetic shield for supplying power to the load cell, and / or through which an electrical connection to ground can pass through the magnetic shield.

[0173] The apparatus may further comprise a measurement enclosure that houses the load cell 1 and magnetic shield 31 .

[0174] Figure 4 shows an alternative arrangement for the magnetic shield, wherein the magnetic shield 33 comprises a plurality of layers of one or more materials arranged in a laminate or layered or stacked structure or arrangement. The plurality of layers may comprise the same or different materials to each other. In addition, or alternatively, the plurality of layers may have the same thickness, or may have two or more different thicknesses.

[0175] Suitable materials for each of the plurality of layers may include a ferromagnetic material, Nickel-iron ferromagnetic alloy, Mu-metal, Supermalloy or Permalloy, for example.

[0176] The magnetic shield 33 may have any of the features of the magnetic shield 31 described above, unless incompatible.

[0177] The apparatus may further comprise a measurement enclosure that houses the load cell 1 and magnetic shield 33.

[0178] Figure 5 shows an alternative arrangement for the magnetic shield, wherein the magnetic shield 35 is provided on an inner surface or inner wall of a measurement chamber 37 that houses the load cell 1 .

[0179] The measurement chamber 37 is made of aluminium, which provides no or minimal magnetic shielding. The measurement chamber 37 provides a controlled atmosphere around the load cell, to reduce errors in the measurement of the mass and / or the change in mass that may otherwise be caused by convection currents and changes in temperature, pressure and humidity, for example.

[0180] In this embodiment, the magnetic shield 35 comprises a sheet or film or layer of material on, or embedded into, the inner surfaces or inner walls of the measurement chamber.

[0181] For example, the magnetic shield 35 may be provided on all of the inner top surface, inner side surfaces, and inner bottom surface of the measurement chamber 37, so as to surround the load cell 1 located in the measurement chamber 37..

[0182] The measurement chamber 37 may comprise a removable or openable lid, for inserting a wafer into the measurement chamber. The magnetic shield 35 may be provided on an inner surface of the lid.

[0183] Alternatively, the measurement chamber may instead comprise an opening or slot through which the wafer can be inserted to introduce the wafer into the measurement chamber 37. There may be a movable shutter or seal for selectively closing the opening or slot. In such an arrangement the magnetic shield may be provided on all the inner surfaces or walls of the measurement chamber 37 except for the opening or slot.

[0184] Suitable materials for the magnetic shield 35 may include one of more of a ferromagnetic material, Nickeliron ferromagnetic alloy, Mu-metal, Supermalloy or Permalloy, for example.

[0185] As discussed above, the magnetic shield 35 may comprise a single layer or a plurality of layers of the same or different materials.

[0186] The magnetic shield 35 may have any of the features of the magnetic shields 31 and 33 described above, unless incompatible.

[0187] Either of the embodiments illustrated in Figures 3 and 4 may also comprise the measurement chamber 37 (with or without the magnetic shield 35).

[0188] The apparatus may further comprise one or more sensors configured to sense one or more atmospheric conditions. For example, the one or more sensors may be arranged to sense one or more atmospheric conditions inside the measurement chamber 37.

[0189] For example, the one or more sensors may be configured to sense one or more of a pressure or temperature or humidity of the air the load cell 1 and / or the air in the measurement chamber 37.

[0190] The apparatus may further comprise a controller or processor that is configured to calculate the mass and / or the change in mass of the object based at least on a measurement performed by the load cell, or an output of the load cell, and an output of the one or more sensors.

[0191] For example, the controller or processor may be configured to calculate a buoyancy force acting on the object based on the output of the one or more sensors and to correct a weight or mass measurement for the object based on the calculated buoyance force.

[0192] The controller may be configured to correct the measurement of the mass and / or the change in mass for the effects of buoyancy acting on the object based on the output of the one or more sensors.

[0193] In an alternative arrangement, the magnetic shield may be provided around or outside of the measurement chamber 37, so that the whole measurement chamber 37 is shielded by the magnetic shield.

[0194] The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0195] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0196] For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.

[0197] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0198] Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0199] It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / - 10%.

Claims

Claims:1 . An apparatus for measuring the mass and / or the change in mass of an object, comprising: a load cell; and a magnetic shield that is configured to at least partially magnetically shield at least part of the load cell.

2. The apparatus according to claim 1 , wherein the magnetic shield is configured to magnetically shield at least part of the load cell.

3. The apparatus according to claim 1 or claim 2, wherein the magnetic shield is configured to shield one, a plurality, or all directional components of a local or external magnetic field.

4. The apparatus according to any one of the preceding claims, wherein the magnetic shield is arranged to cover at least part of the load cell.

5. The apparatus according to any one of the preceding claims, wherein the magnetic shield encloses, or substantially encloses, at least part of the load cell.

6. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises a case or housing around at least part of the load cell.

7. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises a material having a high magnetic permeability.

8. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises a material having: a relative magnetic permeability, pr, greater than or equal to 1 ,000; or a relative magnetic permeability, pr, greater than or equal to 10,000; or a magnetic permeability, p, greater than or equal to 0.001 H / m; or a magnetic permeability, p, greater than or equal to 0.01 H / m.

9. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises a material having a high magnetic saturation.

10. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises a material having a low magnetic remanence.

11. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises one or more of: a ferromagnetic material, a Nickel-iron ferromagnetic alloy, Mu-metal, Supermalloy or Permalloy.

12. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises one or more layers of material.

13. The apparatus according to any one of the preceding claims, wherein the magnetic shield comprises a plurality of layers of material.

14. The apparatus according to claim 13, wherein the plurality of layers of material comprise a first layer having a first thickness and a second layer having a different second thickness.

15. The apparatus according to claim 13 or claim 14, wherein there is an air gap between two of the plurality of layers of material.

16. The apparatus according to any one of the preceding claims, wherein the magnetic shield has a thickness: greater than or equal to 0.2 mm; or greater than or equal to 1 mm.

17. The apparatus according to any one of the preceding claims, wherein the apparatus comprises a support having a mounting portion for mounting the support on the load cell, and wherein the mounting portion extends through an opening in the magnetic shield.

18. The apparatus according to any one of the preceding claims, wherein the apparatus further comprises a measurement chamber in which the load cell is housed.

19. The apparatus according to claim 18, wherein the measurement chamber comprises the magnetic shield.

20. The apparatus according to claim 18 or claim 19, wherein the magnetic shield is on an inner surface and / or and inner wall of the measurement chamber.

21. The apparatus according to any one of the preceding claims, wherein the load cell is an electromagnetic force compensation load cell.

22. The apparatus according to any one of the preceding claims, wherein the load cell comprises an electromagnetic force compensation mechanism.

23. The apparatus according to any one of the preceding claims, wherein the apparatus further comprises a controller that is configured to calculate the mass and / or the change in mass of the object based at least on a measurement performed by the load cell.

24. The apparatus according to any one of the preceding claims, wherein the apparatus further comprises one or more sensors configured to sense one or more atmospheric conditions.

25. The apparatus according to claim 24, wherein the apparatus further comprises a controller that is configured to calculate the mass and / or the change in mass of the object based at least on a measurement performed by the load cell and an output of the one or more sensors.

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