Patent Publication Number: US-11396695-B2

Title: Electromagnetic module for physical vapor deposition

Description:
PRIORITY DATA 
     The present application is a continuation application of U.S. patent application Ser. No. 15/806,729, filed Nov. 8, 2017, the entirety of which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit industry has experienced rapid growth in the past several decades. Technological advances in semiconductor materials and design have produced increasingly smaller and more complex circuits. These material and design advances have been made possible as the technologies related to processing and manufacturing have also undergone technical advances. In the course of semiconductor evolution, the number of interconnected devices per unit of area has increased as the size of the smallest component that can be reliably created has decreased. 
     Physical vapor deposition (PVD) or sputtering has been widely used to deposit material layers to form gate stacks, barrier layers, and interconnect structures of semiconductor devices. In commonly seen sputtering processes, a target and a substrate is placed face-to-face in close proximity within a sputtering chamber. A gas is introduced into the sputtering chamber, where the gas is ignited to form a plasma. The gas ions in the plasma are drawn to the target by an electric field across the target, the back plate and the substrate. With sufficient energies, gas ions can dislodge atoms off of the target, allowing the dislodged target atoms to be deposited on the substrate. In reactive sputtering, a reactive gas is also introduced into the sputtering chamber and the target atom reacts with the reactive gas before depositing on the substrate. 
     Conventionally, to promote uniform erosion over the target surface, a rotating magnet module is used to apply a sweeping magnetic field over the target. The rotating magnet module may include an arrangement of permanent magnets that is rotated by a rotational mechanism around a center of the target. However, this conventional technique often results in uneven circular erosion patterns on the target, commonly known as the “racetracks.” Moreover, the rotational mechanism may displace magnets at or near the center of the rotating magnet module, resulting in under-erosion at or near the center of the target. 
     Thus, existing techniques have not proved entirely satisfactory in all respects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features, whether on the devices or the wafers and semiconductor features described herein, may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a schematic diagram of a sputtering system, according to various embodiments of the present disclosure. 
         FIG. 2  is a schematic top view of an electromagnet module, according to various embodiments of the present disclosure. 
         FIG. 3  is a schematic diagram of an electromagnet of an electromagnet module, according to various embodiments of the present disclosure. 
         FIG. 4  is a schematic diagram of an electromagnet coil that generates an eddy current in a target, according to various embodiments of the present disclosure. 
         FIG. 5  is a schematic diagram of two electromagnet coil over different target erosion profiles, according to various embodiments of the present disclosure. 
         FIGS. 6A and 6B  illustrate a flow chart of a method of depositing a material on a substrate, according to various embodiments of the present disclosure. 
     
    
    
     These figures will be better understood by reference to the following detailed description. 
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     It is further understood that the present disclosure relates generally to improving target erosion uniformity and deposition uniformity by use of a controllable electromagnet module over the target. More particularly, the present disclosure is related to an electromagnet array of individually controllable electromagnet. The electromagnet array not only can apply a static or sweeping magnetic field that promotes uniform target erosion but also can be dynamically tuned to address localized over- and under-erosion on the target. A controller connected to the electromagnet array can transmit an alternating current (AC) signal to each of the electromagnet such that an eddy current is generated in an area of a target adjacent to the electromagnet. By comparing impedance variations among the electromagnets, the location and depth of an erosion feature can be determined. The wafers and substrates described herein may take various forms including but not limited to wafers (or portions thereof) or substrates of individual devices such as chips (e.g., fabricated on a wafer). Various features may be formed on the substrate by the addition, subtraction, and alteration of material layers formed on the substrate to produce integrated circuits including those formed by CMOS-based processes, fin-like field effect transistor (FinFET) devices, MEMS devices, image sensors, and the like. Furthermore, as described above, specific embodiments described herein are exemplary only and not intended to be limiting. 
     Referring now to  FIG. 1 , shown therein is a schematic diagram of a sputtering system  100 , according to various embodiments of the present disclosure. The sputtering system  100  includes a vacuum chamber  110 . The vacuum chamber  110  is in fluid communication with a vacuum source  134  via a passage  130 . The vacuum source  134  can include one or more vacuum pumps. The passage  130  further includes a valve  132 , which is operable to shut off the fluid communication between the vacuum chamber  110  and the vacuum source  134 . The vacuum chamber  110  is also in fluid communication with a gas source  124  via a passage  120 . The gas source  124  is a source of gas used to generate a plasma within the vacuum chamber  110 . In some embodiments, the gas is an inert gas, such as argon. In some other embodiments, the gas can be oxygen or nitrogen. In some instances, there are more than one gas source and at least one of the gas sources is a source of an inert gas, such as argon. The passage  120  to the gas source  124  is controlled by a valve  122 , which is operable to modulate gas flow or completely shut off the fluid communication between the vacuum chamber  110  and the gas source  124 . The sputtering system  100  further includes a pedestal  310  for holding a substrate  300 . The substrate  300  can be a wafer, part of a wafer, or a substrate with fabricated features thereon. 
     The sputtering system  100  is configured to receive a target  140  (or sputtering target  140 ) within the vacuum chamber  110 . The target  140  can be formed of a metal, a conductive metallic compound, an alloy, or a metal solid solution. In some embodiments, the target  140  can be mounted on a back plate  150 . Non-exhaustive examples of materials for the target  140  and back plate  150  include aluminum (Al), copper (Cu), gold (Au), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), tantalum (Ta), graphite (C), tungsten (W), ruthenium (Ru), molybdenum (Mo), niobium (Nb), palladium (Pd), indium (In), gallium (Ga), boron (B), antimony (Sb), vanadium (V), tin (Sn), ytterbium (Yb), yttrium (Y), zirconium (Zr), chromium (Cr) and alloys and solid solutions thereof. The back plate  150  and the target  140  can be formed of the same material or different materials. In some implementations, the back plate  150  is formed of materials with properties that compensate for the material properties of the target  140 . For example, in cases where the material of the target  140  has low thermal conductivity, the material of the back plate  150  can be those with high thermal conductivity. Besides thermal conductivity, material properties to be considered can include electrical and magnetic conductivities. 
     As shown in  FIG. 1 , the substrate  300  is mounted on the pedestal  310 , which is electrically coupled to a power supply  320  via a transmission line  322 , and the target  140  is coupled to a power supply  160  via the back plate  150  and a transmission line  152 . That way, an electric field can be applied across the target  140  and the pedestal  310 . Both the pedestal  310  and the back plate  150  are made of conductive materials. In some embodiments, the power supply  320  is a radio frequency (RF) power supply having the transmission line  322  connecting to the pedestal  310  and a grounding line  321  connected to the ground. The conductive pedestal  310  that is connected to the power supply  320  is insulated from the vacuum chamber  110  by an insulation member  162 . In some instances, the power supply  160  is a direct current (DC) power supply having the transmission line  152  connecting to the back plate  150  and a grounding line  151  connected to the ground. In some embodiments represented by  FIG. 1 , the back plate  150  and/or the structure that supports it is insulated from the vacuum chamber  110  by insulation members  112  and  142 . The vacuum chamber  110  is therefore insulated by the insulation members  112 ,  142 , and  162  from the power supply  320  and the power supply  160 . The vacuum chamber can be grounded, as shown in  FIG. 1 . 
     In operation, the back plate  150  and the target  140  serve as a cathode and the pedestal  310  serves as an anode. The power supplies  160  and  320  can apply a static or a dynamically changing voltage across the cathode and the anode and thereby cause an electric field to be developed between the cathode and anode. The strength of the electric field, which is generated by the power supply  160 , the power supply  320 , or the combination of the two, is selected such that the gas from the gas source  124  can be ionized and ignited into a plasma  170 . In instances where argon (Ar) is fed from the gas source  124 , Ar atoms will give up an electron and exist in the plasma  170  as positively charged argon ions (Ar + ). As illustrated in  FIG. 1 , the positively charged argon ion in the plasma  170  will be accelerated by the electric field and bombard the surface of the target  140 . If the positively charged argon ion carries sufficient energy, it can dislodge atoms of the target, which are then deposited on the substrate  300 . 
     In some embodiments, the sputtering system  100  further includes an electromagnet module  200  over the target  140 . The electromagnet module  200  comprises of an electromagnet array of a plurality of electromagnets  202 . In this regard, because the electromagnet module  200  essentially refers to the electromagnet array, the electromagnet module  200  can be referred to the electromagnet array  200  from time to time. The electromagnet array  200  can be used to generate a magnetic field  180  near the target  140 . The magnetic field  180  can be designed to trap electrons near the target, thereby increasing the density of the plasma  170 , increasing ionization rate, and facilitating the sputtering process. In some embodiments, the electromagnet module  200  is connected to a controller  240  via a transmission line  220 . The controller  240  can control the magnitude and polarity of the magnetic flux of each of the electromagnets  202  in the electromagnet module  200 . That is, the magnetic field that is generated near the target  140  can be customized by changing the parameters or programs of the controller  240  for different geometries of the vacuum chamber  110 , different materials of the target  140 , different thicknesses of the target  140 , different plasma gas species, and different electric field strengths. In some embodiments, for a given vacuum chamber  110  and a given type of target  140 , experiments can be conducted to determine a default setting (or default pattern, default magnetization pattern) for the electromagnet array  200 . The default setting is determined based on uniformity of deposition rate on the substrate  300  and the uniformity of the consumption rate of the target  140  for a given set of target material and process parameters. Each of the given set of target material and process parameters can be referred to as a standard setting. In some embodiments, each standard setting can correspond to a unique default pattern of the electromagnet array  200  to achieve uniform deposition rate and uniform target consumption. In some other embodiments, the default pattern of the electromagnet array  200  can be determined based on a standard setting that includes a target material with mid-range target properties and mid-range process parameters. In those embodiments, the default pattern is a general one for all target materials and process parameters. In some implementations, unless the controller  240  is commanded to use a specific pattern, the default pattern is loaded at the initiation of a sputtering process using the sputtering system  100 . 
     Referring now to  FIG. 2 , shown therein is a schematic top view of the electromagnet array  200 , according to embodiments of the present disclosure. In some embodiments where the substrate  300  in  FIG. 1  is circular in shape, the pedestal  310  and the back plate  150  can also be circular and of the same size of the substrate  300 . In those embodiments, the electromagnet module  200  also has a circular shape that generally tracks the shape of the substrate  300 . However, as the electromagnet module  200  is formed of a plurality of electromagnets  202 , the shape of the electromagnet module  200  may only approximate a circle but is not perfectly circular, as shown in  FIG. 2 . In some instances, the electromagnet module  200  is larger than the target  140  such that the target  140  is subject to a more uniform magnetic field. The electromagnet array  200  can have a center point  206 , which can be either a geometric center or a center of gravity of the electromagnet array  200 . In some embodiments, the controller  240  can selectively energize a group of electromagnets  202  that form a energization pattern  204  (or pattern  204 ). When in use, electromagnets  202  within the energization pattern  204  is energized at a predetermined level to have magnetic fluxes with different polarities and magnitudes, while the electromagnets  202  outside the pattern  204  is energized at a constant level or baseline level. For illustration purposes, electromagnets  202  in the pattern  204  are shown in different colors to illustrate that they can have different polarities, such as N pole and S pole, or different magnitudes. In these embodiments, the controller  240  can sequentially and sweepingly, in a clockwise or counter-clockwise direction, energize a group of electromagnets  202  have a pattern substantially similar to pattern  204 , around the center point  206 . The sweeping energization of patterns  204  allows the electromagnet array  200  to mimic rotation of a fixed magnet arrangement around the center point  206 . For example, after the controller  240  energizes the pattern  204 , it can continue to sweep in counter-clockwise direction and subsequently energize a pattern  204 ′, which is similar but not identical to the pattern  204 . The sweeping pattern  204  at different positions on the electromagnet array  200  may not be identical because the packing of the electromagnets  200  may not allow the same pattern  204  to be reproduced at all angles around the center point  206 . 
     As shown in  FIGS. 1 and 2 , the individual electromagnets  202  in the electromagnet array  200  can be generally cylindrical in shape with a circular cross section. In some implementations, to increase the packing density of the electromagnets in the electromagnet array  200 , the electromagnets  202  can be made into other shapes. For example, to maximize the packing density of the electromagnet array  200 , the electromagnets  202  can be in the shape of a hexagonal prism (a cylinder with a hexagonal cross section). In some other instances, the electromagnets  202  can be in the shape of a rectangular column. In still other embodiments, the electromagnets  202  can have low profiles and are considered disk-like (circular, hexagonal, or circular) in shape. An individual electromagnet  202  is shown in  FIG. 3 . The electromagnet  202  includes a coil  221  and a core  222 . The coil  221  is made of highly conductive materials, such as copper or copper aluminum alloy. The core can be made of iron, iron alloys, nickel alloys, or iron-nickel alloys. The coil  221  includes two leads  221 A and  221 B. Both leads  221 A and  221 B are connected to the controller  240 . In some embodiments, the leads of each of the electromagnets  202  are individually connected to the controller  240 . In those embodiments, the smallest controllable unit in the electromagnet array  200  is an electromagnet  202  and the controller  240  can individually energize each electromagnet  202 . In some other embodiments, electromagnets  202  are first grouped according to the magnitudes and polarities of their magnetic fluxes. The leads of electromagnets  202  of the same group are first merged before they are connected to the controller  240  as a group. In those embodiments, the smallest controllable unit in the electromagnet array  200  is a group of electromagnets  202  and the controller  240  can energize a group individually. 
     Referring now to  FIG. 4 , shown therein a coil  232  that is in proximity of a conductive material  400 . In some embodiments, each of the electromagnets  202  in the electromagnet array  200  includes one coil  232  in addition to the coil  221 . In some other embodiments, the coil  221  includes the coil  232 . As shown in  FIG. 4 , the coil  232  is connected to an alternating current (AC) power supply  242 . When the AC power supply  242  feeds an AC signal with a frequency “F” to the coil  232 , an eddy current  410  is induced within the conductive material  400 . The eddy current  410  circulates in planes perpendicular to the magnetic field generated by the coil  232  near the surface of the conductive material  400 . The eddy current  410  would generate a magnetic field that opposes the magnetic field generated by the coil  232 , causing a change in impedance in the coil  232 . The impedance can be measured. The eddy current  410  has a standard depth of penetration “D,” which equals the depth at which the eddy current density decreases to 1/e or 37% (wherein e is the Euler&#39;s Constant) of the eddy current density at the surface of the conductive material  400 . At the depth of two Ds, the eddy current density would decrease to 1/e 2  (or about 13.5%) of the surface eddy current density. Further, at the depth of three Ds, the eddy current density would drop to 1/e 3  (or about 5%) of the surface eddy current density. At the depth of 5 Ds, the eddy current density would be below 1% of the surface eddy current density. The frequency F and the standard depth of penetration D are correlated by the following formula: 
             D   ≈     1       π   ⁢           ⁢   F   ⁢           ⁢   μσ               
where π is the mathematical constant that is approximately 3.14, μ is the magnetic permeability (H/mm), and σ is the electrical conductivity (in % IACS (International Annealed Copper Standard). By varying the frequency F of the AC signal from the AC power supply  242 , the eddy current  410  would have different standard depth of penetration D. According to the formula presented above, the standard depth of penetration D is inversely proportional to the square root of the frequency F. That is, the standard depth of penetration D becomes smaller when the AC signal from the power supply  242  has a higher frequency F.
 
     The eddy current  410  would decrease if a defect or a feature in the conductive material disrupts or reduces the eddy current. For example, given that the eddy current  410  mainly flows within about 5 standard depths of penetration (D), if the conductive material  400  becomes much less than 5 standard depth of penetration, the total eddy current would drop because the depth does not support the same thickness of the eddy current. The reduced eddy current would result in reduced magnetic field to counteract the magnetic field generated by the coil  232 , this reducing the impedance. Variation of impedance in the coil  232  can be measured to determine presence of erosion features in the target  140 . Referring now to  FIG. 5 , shown there are two coils  232 A and  232 B used to measure presence of an erosion feature  143 . The coil  232 A is over a portion  141  of the target  140  where a thickness D1 is uniform. The coil  232 B, however, is over the erosion feature  143  that includes a concave erosion profile that reduces the local thickness of the target  140  to D2 or otherwise disrupt the flow of an eddy current inducible at the location of the concave erosion profile. Although shown in  FIG. 5  as a concave profile, the erosion feature  143  can take many different shapes. The concave erosion profile of the erosion feature  143  can indicate a localized consumption, or localized over-consumption of the sputtering target  140 . In instances where the AC power supply  242  transmits an AC signal with a frequency F, the eddy current induced within the target would have standard depth of penetration D. For example, if D1 is greater than or equal to 5 standard depths of penetration (5 Ds) or more and D2 is smaller than 5 standard depths of penetration (5 Ds), the coils  232 A and  232 B would experience different impedances. This is so because the erosion feature  143  takes conductive target away or disrupt the flow of the eddy current such that the eddy current density around the erosion feature  143  is weaker or at least different from that around the portion  141 . In some embodiments, by varying the frequency F of the AC signals fed to the coils  232  and comparing impedance variations among different coils  232 , the location and depth of the erosion feature  143  can be determined. In some other embodiments, a table of standard impedance measurements for a target at a range of frequencies can be populated by measuring impedances in the coil  232  over a think target. Any abnormal impedance values can then detected by comparing the measured impedance to the impedance values in the table of standard impedance measurements. 
     Referring now to  FIGS. 6A and 6B , a method  10  for depositing a material on a substrate is provided. The method  10  is described below in conjunction with  FIGS. 1-4 . At operation  12  ( FIG. 6A ) of the method  10 , the substrate  300  is placed in the vacuum chamber  110  of the sputtering system  100 . The sputtering system  100  includes the vacuum chamber  110 , the sputtering target  140 , and the electromagnet array  200  over the sputtering target  140 . The electromagnet array  200  includes a plurality of electromagnets  202 . The target  140  can be formed of a metal, a conductive metallic compound, an alloy, or a metal solid solution. Examples of target materials include aluminum (Al), copper (Cu), gold (Au), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti), tantalum (Ta), graphite (C), tungsten (W), ruthenium (Ru), molybdenum (Mo), niobium (Nb), palladium (Pd), indium (In), gallium (Ga), boron (B), antimony (Sb), vanadium (V), tin (Sn), ytterbium (Yb), yttrium (Y), zirconium (Zr), chromium (Cr) and alloys and solid solutions thereof. The target  140  can be mounted on the back plate  150 . The back plate  150  and the target  140  can be formed of the same material or different materials. In some implementations, the material of the back plate  150  is selected to compensate for undesirable properties of the material of the target  140 , including, for example, low thermal conductivity, low electrical conductivity, and low magnetic conductivity. 
     At operation  14  ( FIG. 6A ) of the method  10 , an electric field is applied between the sputtering target  140  and the pedestal  310 . As shown in  FIG. 1 , the pedestal  310  that holds the substrate  300  is coupled to a power supply  320  via the transmission line  322 , and the target  140  is coupled to a power supply  160  via the back plate  150  and a transmission line  152 . That way, an electric field can be applied across the target  140  and the substrate  300 . Both the pedestal  310  and the back plate  150  are made of conductive materials. In some embodiments, the power supply  320  is a radio frequency (RF) power supply having the transmission line  322  connecting to the pedestal  310  and a grounding line  321  connected to the ground. At the operation  14 , the back plate  150  and the target  140  serve as a cathode and the pedestal  310  and the substrate  300  serve as an anode. The power supplies  160  and  320  can apply a static or a dynamically changing voltage across the cathode and the anode and thereby cause an electric field to be developed between the cathode and anode. 
     At operation  16  ( FIG. 6B ) of the method  10 , the plasma  170  is generated. The vacuum chamber  110  is in fluid communication with the gas source  124  via the passage  120 , which is controlled by the valve  122 . By opening the valve  122 , the gas from the gas source  124  enters into the vacuum chamber  110 . In some embodiments, the gas is an inert gas, such as argon. In some other embodiments, the gas can be oxygen, nitrogen or other suitable gases. Oxygen and nitrogen plasmas are often used in reactive sputtering processes to form oxides and nitrides. In those embodiments, the sputtering system  100  can include a second gas source for an inert gas, such as argon such that the plasma in the vacuum chamber  110  includes plasma of the inert gas along with plasma of oxygen or nitrogen. The strength of the electric field, which is generated by the power supply  160 , the power supply  320 , or the combination of the two, is selected such that the gas from the gas source  124  can be ionized and ignited into a plasma  170 . In instances where argon (Ar) is fed from the gas source  124 , Ar atoms will give up an electron and exist in the plasma  170  as positively charged argon ions (Art). While the operation  14  is depicted as preceding the operation  16 , the operation  16  can take place concurrently or after the operation  14  as well. The plasma forming gas can be present in the vacuum chamber  110  when the electric field is applied. Alternatively, the plasma forming gas can be fed to the vacuum chamber  110  after the electric field is applied. 
     While not described as a separate operation in the method  10 , the vacuum chamber  110  is allowed to be connected to the vacuum source  134  before the plasma-forming gas is fed into the vacuum chamber  110 . The vacuum chamber  110  is in fluid communication to the vacuum source  134  via the passage  130 , which is controlled by the valve  132 . In some embodiments, after the substrate  300  is mounted on the pedestal  310  and the target  140  is placed within the vacuum chamber  110 , the valve  132  is turned to an open position to allow fluid communication between the vacuum chamber  110  and the vacuum source  134 . Once the pressure with the vacuum chamber  110  reaches a steady low pressure, the valve  132  is turned to a closed position to shut off the fluid communication between the vacuum source  134  and the vacuum chamber  110 . This operation ensures that no unwanted gaseous impurity be present in the vacuum chamber  110  before the deposition process. In some embodiments where the substrate  300  is transported to the sputtering system  100  by a pressurized or vacuum wafer carrier, the valve  132  can be used to reduce or minimize the pressure differential between the wafer carrier and the vacuum chamber  110 . 
     At operation  18  ( FIG. 6A ) of the method  10 , the plurality of electromagnets  202  in the electromagnet array  200  is energized to generate the magnetic field  180  near the target  140 . The electromagnet module  200  is connected to the controller  240 . The controller  240  can control the magnitude and polarity of the magnetic flux of each of the electromagnets  202  in the electromagnet module  200 . By changing the magnitudes and polarities of the magnetic fluxes the electromagnets  202 , the magnetic field  180  can be configured to trap secondary electrons near the target  140 , thereby increasing the density of the plasma  170 , facilitating the sputtering process. In some embodiments, the magnetic field  180  can be customized by changing the parameters or programs of the controller  240  for different geometries of the vacuum chamber  110 , different shapes of the target  140 , different materials of the target  140 , different thicknesses of the target  140 , different plasma gas species, and different electric field strengths. In some embodiments, for a given vacuum chamber  110  and a given type of target  140 , experiments can be conducted to determine a default setting for the electromagnet array  200 . The default setting is determined based on uniformity of deposition rate on the substrate  300  and the uniformity of the consumption rate of the target  140  for a given set of target material and process parameters. Each of the given set of target material and process parameters can be referred to as a standard setting. In some embodiments, each standard setting can correspond to a unique default pattern of the electromagnet array  200  to achieve uniform deposition rate and uniform target consumption. In some other embodiments, the default pattern of the electromagnet array  200  can be determined based on a standard setting that includes a target material with mid-range target properties and mid-range process parameters. In those embodiments, the default pattern is a general one for all target materials and process parameters. In some implementations, unless the controller  240  is commanded to use a specific pattern, the default pattern is loaded at the initiation of a sputtering process using the sputtering system  100 . 
     After the operation  18  ( FIG. 6A ), the method  10  bifurcates into two branches. One branch includes operations  20  and  22  and the other includes operations  21 ,  23  and  25 . Both branches include monitoring the thickness of the target  140 . 
     In one embodiment, the method  10  determines an end of lifetime of the sputtering target  140  by determining that at least a portion of the target  140  is equal to or less than a predetermined thickness, which is referred to as a minimum target thickness. In the operation  20  ( FIG. 6B ), an alternating current (AC) signal with a frequency is transmitted to each of the plurality of electromagnets  202 . In some embodiments, the AC signal is generated by the controller  240 . As described above in conjunction with  FIGS. 4 and 5 , in some embodiments, each of the electromagnets  202  includes a coil  232 , which can be included in the coil  221  shown in  FIG. 3  or separate from the coil  221 . As shown in  FIG. 4 , the AC signal in the coil  232  can induce the eddy current  410  within a conductive material  400 , such as the target  140 . The eddy current  410  has the highest current density at the surface of the conductive material  400  (or the target  140 ) and the eddy current density decreases by 1/e per standard depth of penetration. As shown in  FIG. 5 , the coil  232  can experience an impedance variation if the portion of the target  140  underneath the coil  232  includes an erosion feature  143 . The frequency can be selected based on a correlation with the minimum target thickness such that the impedance variation indicates the target  140  is thinner than the minimum target thickness at least at one location. For example, the frequency can be selected such that the standard depth of penetration of that frequency is ⅕ of the minimum target thickness. In another instance, the frequency can be selected such that the standard depth of penetration of that frequency substantially equal to the minimum target thickness. 
     At the operation  22  ( FIG. 6B ) of the method  10 , an end of lifetime of the sputtering target  140  is determined by determining if an abnormal impedance variation in one of the plurality of electromagnets is present. In some embodiments, the coil  221  of each of the plurality of electromagnets  202  in the electromagnet array  200  includes a coil  232 . The controller  240  connected to the electromagnets  202  can transmit an AC signal to each of the coils  232  in the electromagnet array  200  to survey the target  140 . The frequency of the AC signal bears a correlation with the minimum target thickness such that the presence of an erosion feature, such as the erosion feature  143 , can be detected by anomaly in impedance of the coil  232 . For example, if the frequency is associated with a standard depth of penetration (D) equal to ⅕ of the minimum target thickness, the impedance of each of the coil  232  should be substantially the same if no erosion feature reduces the thickness of the target  140  to less than 5 times of the standard depth of penetration (5D). If any erosion feature reduces the thickness of the target  140  to less than 5 times of the standard depth of penetration (5D) at a location, the coil  232  over that location would experience a lower impedance due to weaker counteractive magnetic flux generated by a weaker or less organized eddy current. In another example, a standard impedance of a coil  232  over a target with the minimum target thickness is measured at a frequency. Subsequently, during a survey of the target  140  using the coils  232  in the electromagnet array at the same frequency, the impedances of each of the coil  232  is compared to the standard impedance. The end of lifetime of the target  140  can then be determined if the impedance of any coil  232  falls below the standard impedance or falls below the standard impedance by a certain percentage. As demonstrated by these examples, an abnormal impedance variation is an impedance that is either an outlier as compared to impedances measured under the same frequency or different from a standard impedance measurement. 
     In another embodiment of the method  10 , a location and a depth of an erosion feature is identified and the magnetization pattern of the electromagnet array  200  is modified to achieve more uniform target consumption. At the operation  21 , AC signals with a range of frequencies are transmitted to each of the plurality of electromagnets  202 . In some embodiments, the AC signals are generated by the controller  240 . As different frequencies of the AC signals correspond to different standard depths of penetration, at the operation  21 , the coils  232  of electromagnets  202  performs a depth-wise scan at locations under the coils  232  over the entire target  140 . The coils  232  scan the target  140  with one frequency at a time and step through the range of frequencies. 
     At the operation  23 , the depth and the location of an erosion feature within the sputtering target  140  is determined by identifying abnormal impedance variations at each frequency out of the range of frequencies. The range of frequencies corresponds to a range of standard depths of penetration. Similar to operations  20  and  22 , the impedance measured at each coil  232  at a frequency out of the range of frequencies is compared among the coils  232  or against a standard impedance. The same process is repeated for each of the frequency out of the range of frequencies. As the location of each coil  232  provides location information, and the depth-scan at each frequency provides depth information, the depth and the location of an erosion feature can be determined. 
     At the operation  25 , a magnetization pattern of the electromagnet array is modified to reduce the consumption rate of the location of the erosion feature on the target  140 , or to increase the consumption rate of the locations other than the location of the erosion feature on the target  140 , or both. One of the advantages provided by the electromagnet array  200  is that it is fully tunable in terms of the magnitude and polarity of the magnetic flux of each electromagnet  202  therein. Once the location and depth of the erosion feature is determined, the controller  240  can slow down the erosion at that location or speed up the erosion at other location by changing the magnetization pattern of the electromagnet array  200 . For example, by increasing the strength of the magnetic field  180  at a given location, more secondary electrons can be trapped at that location and the density of the plasma  170  at that location can be increased. The higher density of the plasma  170  can speed up the erosion/consumption of the target  140  at that location. In some embodiments, the determination of the strength of the magnetization pattern at given location can also take into consideration the depth of the erosion feature. If the erosion feature is located deep in the target  140  (i.e. farther away from the target or closer to the back plate  150 ), the increase and reduction of consumption rate have to be more drastic. If the erosion feature is located near the surface of the target  140  (i.e. closer to the target or farther away from the back plate  150 ), the increase and reduction of the consumption rate can be mild. In some embodiments where magnetization pattern of the electromagnet array  200  is set at a default pattern (or default setting, default magnetization pattern), the modification at the operation  25  is done to the default pattern. In some implementations, the depths and locations of the erosion features can be compiled as a thickness distribution of the sputtering target  140  and the default pattern can be modified to achieve a more uniform thickness distribution. 
     Thus, the present disclosure provides embodiments of sputtering systems and methods. In one embodiment, a sputtering system is provided. The sputtering system includes a chamber configured to receive a substrate, a sputtering target positioned within the chamber, and an electromagnet array over the sputtering target. The electromagnet array includes a plurality of electromagnets. In some embodiments, the electromagnet array of the sputtering system is connected to a controller and the controller is operable to control a magnitude and a polarity of a magnetic flux of each of the plurality of electromagnets. In some implementations of the present disclosure, the electromagnet array is generally circular in shape and the electromagnet array has a center point. In some implementations, the controller is operable to sweepingly energize a predetermined pattern of electromagnets out of the plurality of electromagnets to a predetermined level around the center point. In some instances, the electromagnets other than the predetermined pattern of electromagnets are energized at a baseline level different from the predetermined level. In some embodiments, the controller is further operable to transmit an alternating current (AC) signal with a frequency to each of the plurality of electromagnets to create an eddy current within a depth of the sputtering target. The depth bears a correlation with the frequency. In those embodiments, the controller is operable to detect an impedance variation if the eddy current flows through an erosion feature in the sputtering target. 
     In another embodiment, a method of depositing a material on a substrate is provided. The method includes placing the substrate in a chamber of a sputtering system. The sputtering system includes the chamber, a sputtering target positioned within the chamber, and an electromagnet array over the sputtering target. The electromagnet array includes a plurality of electromagnets; The method further includes applying an electric field between the sputtering target and the substrate; generating a plasma within the chamber; and energizing the plurality of electromagnets in the electromagnet array. In some embodiments, the energizing of the plurality of electromagnets includes energizing the plurality of electromagnets according to a default pattern that results in a uniform deposition thickness on the substrate and a uniform consumption rate across the sputtering target in a standard setting. In some embodiments, the method further includes modifying the default pattern based on a thickness distribution of the sputtering target. 
     In some implementations, the method according to the present disclosure further includes transmitting an alternating current (AC) signal with a frequency to each of the plurality of electromagnets, wherein the frequency correlates to a minimum target thickness. In some implementations, the method further includes determining an end of lifetime of the sputtering target by determining if an abnormal impedance variation in one of the plurality of electromagnets is present. In some embodiments, the method further includes transmitting alternating current (AC) signals with a range of frequencies to each of the plurality of electromagnets, wherein the range of frequencies correlate to a range of standard depths of penetration. In some implementations, the method further includes determining a depth and a location of an erosion feature within the sputtering target by identifying abnormal impedance variations at each frequency out of the range of frequencies. In some instances, the method further includes modifying the default pattern to reduce a consumption rate of the location of the erosion feature. In some embodiments, the method further includes modifying the default pattern to increase a consumption rate of the sputtering target in locations other than the location of the erosion feature. In some implementations, the erosion feature is a concave erosion profile resulted from localized consumption of the sputtering target. 
     In yet another embodiment, a method of determining an end of lifetime of a sputtering target is provided. The method includes placing a substrate in a chamber of a sputtering system. The sputtering system includes the chamber, the sputtering target positioned within the chamber, and an electromagnet array over the sputtering target. The electromagnet array includes a plurality of electromagnets. The method further includes transmitting an alternating current (AC) signal with a frequency to each of the plurality of electromagnets, wherein a standard depth of penetration of the frequency correlates to a minimum target thickness; and determining an end of lifetime of the sputtering target by determining if an abnormal impedance variation is present in one of the plurality of electromagnets is present. In some embodiments, determining if an abnormal impedance variation is present in one of the plurality of electromagnets includes comparing impedance variations in the plurality of electromagnets. In some embodiments, the standard depth of penetration is inversely proportional to a square root of the frequency and a square root of an electrical conductivity of the sputtering target. 
     The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.