Electrostatic wafer clamping device and method

Abstract

The invention relates to an electrostatic wafer clamping device (400) comprising: - at least two electrodes (405, 410), connected to at least two phases (406, 411) of a control unit, configured to generate an electrostatic attraction force on the wafer; - the phases; and - a control unit (415) for dynamically controlling the electrical potential of each phase, configured to command, for each phase and for each electrode connected to that phase, the establishment of a stable electrical state selected from among: - a positive-polarity electrical potential state; - a negative-polarity electrical potential state; and - a neutral-polarity electrical potential state, the control unit being configured such that, during at least one stable state, at least one phase from among the set of phases is in the neutral-polarity electrical potential state while at least one other phase is in a positive- or negative-polarity electrical potential state.

Description

[0001] DESCRIPTION

[0002] TITLE OF THE INVENTION: DEVICE AND METHOD FOR ELECTROSTATIC CLAMPING OF A WRAP

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present invention relates to a device and method for electrostatic wafer clamping. It is applicable, in particular, to the field of semiconductor circuit manufacturing.

[0005] STATE OF THE ART

[0006] The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise stated, it should not be assumed that any of the approaches described in this section constitutes prior art simply because of its inclusion in this section.

[0007] In the semiconductor industry, support wafers are used on which microelectronic circuits are manufactured. Given the very small size of these circuits, the stability of the support wafers during the manufacturing process is critical.

[0008] Beyond this stability requirement, the wafers must be maintained at a temperature defined by the manufacturing process. Given that the fabrication process is carried out under vacuum, to dissipate the heat generated by, or conversely to supply heat for, the fabrication of the semiconductor circuit, a heat transfer gas is used to thermally couple the wafer to hot or cold sources, thus regulating its temperature. Since this heat transfer gas must not break the vacuum, and the wafer is by definition removable, it is essential that the wafer does not leak gas.

[0009] For this reason, several approaches have been implemented to clamp these support plates. Such clamping is achieved using a chuck.

[0010] Such a chuck has several benefits:

[0011] - to allow the introduction of a thermal coupling gas between the chuck and the insert,

[0012] - orient the wafer (for example, to tilt the wafer in the case of an ion beam implantation process),

[0013] - flatten the wafer (when the wafer is convex or twisted) to obtain better thermal coupling, better deposition or better accuracy for high aspect ratio profiles and

[0014] - to support the inertial force applied to the insert due to acceleration and deceleration during insert transfer, insert scanning, or any other movement performed on the insert. A certain category of chucks is called electrostatic chucks, as opposed to mechanical chucks (of the collet type, for example). Such electrostatic chucks offer the following advantages:

[0015] - No mechanical contact is made on the treated face of the wafer, thus avoiding the generation of scratches, particles or contamination.

[0016] - no masking or edge effect is induced by the presence of mechanical grippers near the treated face of the insert and

[0017] - the insert used is flatter than in the case of inserts for mechanical chucks which require the insert to be convex in order to function.

[0018] An electrostatic mandrel comprises an electrically insulating body (typically made of ceramic such as Al₂O₃, A₂N, or BN) and at least one electrode made of an electrically conductive material (e.g., W, Mo, Pt, Pd, Ag, or graphite) covered by a dielectric material. The upper surface of this dielectric material can be arranged to correspond to a particular topography (called "dimple," "mesa," "embossing," etc.) in order to limit the contact areas between the mandrel and the insert, and to improve the gas pressure distribution, as referenced by 303 in Figure 3, for thermal coupling. Such an electrostatic mandrel is shown in Figure 1. Such a mandrel 100 comprises:

[0019] - an electrically insulating layer 105,

[0020] - at least one 110 electrode and

[0021] - a dielectric material 115.

[0022] When an insert is placed on the chuck, the corresponding electrical system can be understood as an electrical circuit made up of capacitors. When a voltage is applied to the electrodes, an electrostatic field appears between the electrodes and the insert, leading to a displacement of charges within the insert and the appearance of an electrostatic force according to Coulomb's law. Such a correspondence is shown in Figure 2, which corresponds to a so-called "bipolar" chuck, that is, one with two electrodes. The case of chucks with a single electrode, also called "monopolar" chucks, is slightly different but is not described in further detail because monopolar chucks are not relevant to the present invention.

[0023] This electrostatic force clamps the insert against the chuck. Such a device is shown in Figure 3. This device comprises:

[0024] - a chamber at vacuum level 301,

[0025] - a 305 plate,

[0026] - a thermal coupling gas layer 310,

[0027] - at least one electrode, 110 and 111,

[0028] - a dielectric material 115,

[0029] - an electrically insulating layer 105 and

[0030] - a pedestal 310. Such electrostatic chucks have a number of weaknesses which the present invention remedies.

[0031] Firstly, the clamping force "F", referenced 302, generated by the electrostatic field is relatively low.

[0032] The clamping pressure can generally be modeled by the following equation:

[0033] Where E corresponds to the relative permissiveness of the dielectric, it being understood that this dielectric layer also includes the thermal coupling gas layer, V to the clamping voltage (electrical potential difference between the wafer and the electrode) and d to the distance between the wafer and the electrode.

[0034] To increase clamping pressure, it is possible to increase the clamping tension or reduce the thickness of the dielectric material. However, increasing the clamping tension increases the risk of sticking, dielectric collapse, damage to the back face of the insert, and a reduction in the mean time before failure (MTBF), which is a measure of industrial performance.

[0035] In the remainder of this document, it is assumed that the clamping pressure is already optimized for the use case considered according to the type of electrostatic chuck (dimensions, material, manufacturing process used) with the exception of the electrode design.

[0036] Therefore, we consider that EJ e C r I ra I o LJ e is a constant and that V = V serr I ao LJ e = V m 11 ax .

[0037] Thus, firstly, in current systems, one of the challenges is to maximize the clamping force for a given clamping pressure (and, therefore, for a given clamping tension).

[0038] Secondly, because dielectric materials are not perfect, phenomena of sticking or "necking" occur. Such phenomena indicate that residual clamping pressure remains even though the clamping system has returned to a physical state in which the clamping pressure is theoretically zero.

[0039] One consequence of this bonding phenomenon can be insert shifting, which can lead to insert loss during transfer operations. These bonding phenomena can also lead to insert breakage when the bond is too strong to lift the insert during unloading, or because the insert shift causes it to fall.

[0040] The phenomenon of sticking is a major problem that can lead to a decrease in productivity, loss of inserts and the shutdown of production tools.

[0041] Secondly, in current systems, one of the challenges is to reduce the phenomenon of sticking without losing clamping force.

[0042] Thirdly, a constraint related to electrostatic clamping is the electrical charge of a wafer resulting from the clamping process. Consider a wafer that is not grounded or connected to a reference voltage, regardless of the reference or the connection method (grounding pin, conductive layer surrounding the wafer, plasma resulting from the manufacturing process). In such a case, the wafer's potential is said to be floating.

[0043] Since the wafer is coupled to the mandrel electrodes, the wafer's potential floats at a value V pia qU and with:

[0044] = MJi v plaque

[0045] Where Vi is the voltage of electrode i and Ci is the capacitance formed by electrode i and the portion of the plate that faces electrode i.

[0046] For the sake of simplification, we consider the case in which all the capacitances Ci are identical (for example, if all the electrodes have the same surface area and if the dielectric layers between the electrodes and the plate are identical), but the principle remains the same if the capacitances are not equal.

[0047] In such a case, if Ci is a constant for all i, then:

[0048] In which n is the number

[0049] If i V, is not constant, then the potential of the plate is not constant.

[0050] If the wafer is truly floating, then it floats at this voltage, which can impact the repeatability of the manufacturing process (an accumulation of charges in the wafer is undesirable).

[0051] If the wafer is connected to an external reference voltage, this induces a transfer of charges from the wafer to this reference, and such a transfer of charges is likely to damage the devices already manufactured on the wafer.

[0052] Thirdly, in current systems, one of the challenges is to reduce fluctuations in the "theoretical floating voltage" of the wafer resulting from electrostatic coupling.

[0053] In current systems, to solve the problem of the occurrence of the sticking phenomenon, different approaches have been proposed to promote the removal of residual charges after the insert has been loosened and before the removal of the insert.

[0054] Some methods involve electrically connecting the plate to neutral, either through mechanical contact with an electrically conductive object or via a plasma gun. Such approaches are illustrated, in particular, in US patent application US2011036990.

[0055] Some of these methods involve applying a specific loosening sequence in which the current, voltage amplitude, frequency, and / or clamping energy delivery cycle vary over time. Such approaches are illustrated, in particular, in US patent US6236555. All of these approaches are based on the assumption that there is sufficient charge accumulation in the insert, the dielectric, and / or the interface between the insert and the mandrel to generate adhesion.

[0056] To prevent, reduce or delay the onset of the sticking phenomenon, a solution most commonly implemented today is the reversal of the polarity of the electrodes.

[0057] Typically, as soon as the clamping method is monopolar or multipolar, the most common practice is to reverse the polarity of the clamping tension with each new pad. Such an approach is described, for example, in European patent application EP0294556.

[0058] This practice is not an ideal solution: while it is beneficial to limit the slow, long-term shift in the electrical state of the mandrel-dielectric, insert by insert, from the perspective of a single insert, the electrical state during clamping is static. Therefore, there is no reduction in accumulated charges for an individual insert. This can be problematic, particularly for long processes or for low-resistivity dielectrics, such as those used in high-temperature applications.

[0059] Another approach, rather than aiming for the removal of residual charges, consists of preventing this accumulation of charge.

[0060] Since this charge accumulation is linked to the V / d ratio (as previously discussed) and to time, and given that the V / d value is fixed to achieve the maximum clamping force, the concept behind these solutions is to reduce the clamping time for a given electrical state. This is achieved by replacing the constant clamping voltage Vi with an alternating signal. This approach is described, in particular, in US patent application US2005052817 and international patent application WO9411944.

[0061] The benefits of this approach are to stress the dielectric layer and the interface between the wafer and the dielectric for a short period by a voltage difference of a given polarity and then, for a similar period of time, by a voltage difference of the reversed polarity.

[0062] Nevertheless, using a "direct" polarity reversal sequence has a major weakness with regard to the minimum average clamping force during the entire clamping sequence and / or with regard to the flow of charges to the insert.

[0063] The benefits of this approach are exemplified below for five different cases, corresponding to electrostatic mandrels with one, two, three, four, and six electrodes. These examples are considered for a square-type alternating profile (i.e., one whose polarity reverses abruptly, as instantaneously as possible), as shown in Figure 3, as described in international patent application WO9411944.

[0064] The plate is also considered to be either floating or connected to ground.

[0065] For the sake of simplicity, the capacitance formed by each electrode with the opposite portion of the insert is assumed to be identical for all electrodes. Each electrode is assumed to be supplied with the same absolute voltage. To measure the impact of different scenarios on the minimum clamping force, the clamping voltage is assumed to be already optimized / maximized for the specific electrostatic chuck and process. For each scenario, the minimum clamping force is compared to the maximum clamping force F calculated when all electrodes are energized and the insert is referenced to OV, providing an optimal clamping pressure "Pserrage".

[0066] In the first case, corresponding to a monopolar chuck:

[0067] - if the plate is not connected to a reference voltage, the plate floats at the same voltage as the electrode, leading to a lack of clamping and significant charge transfer, and

[0068] - if the circuit board is connected to a reference voltage:

[0069] - if this reference voltage "Vref" is lower than the maximum voltage of the alternating current "Vac" used for clamping, there is a point in which the electrode has the same voltage as the reference voltage during polarity reversal, leading to a lack of clamping and loss of the insert,

[0070] - If this reference voltage is higher than the Vac voltage (or lower than the minimum voltage), the average voltage difference is not zero, leading to charge migration; - moreover, the wafer undergoes significant fluctuations in clamping force, which can generate significant defects on the underside of the wafer as well as the emission of particles; and finally, the maximum voltage difference between the wafer and the electrode sometimes exceeds the previously determined Vmax voltage, resulting in a major risk of irreparable damage to the device and / or the wafer.

[0071] - moreover, the clamping tension is very low: because Vserrage / d is already optimized such that Vserrage = Vref + Vac (for Vref > Vac), then the minimum clamping force is Vmin = Vref-Vac, corresponding to a value of little interest to competent people in the field.

[0072] In the second case, corresponding to a bipolar chuck:

[0073] - If the polarity of electrodes 1 and 2 is reversed simultaneously, there is a point where both electrodes are simultaneously at zero voltage, leading to a lack of clamping force.

[0074] - if the polarity of electrodes 1 and 2 is not reversed simultaneously, there is a moment during the tightening cycle in which the two electrodes have the same voltage Vserrage:

[0075] - If the plaque is not referenced, the plaque is lost.

[0076] - If the plate is referenced, it is not lost, but the theoretical floating tension of the plate fluctuates from +Vserrage to -Vserrage, which leads to a significant transfer of charge towards the plate.

[0077] - moreover, when an electrode is at OV, the actual clamping force corresponds to half of the maximum desired clamping force: Fmin = Fserrage / 2.

[0078] In the third case, corresponding to a three-pole mandrel: - in the case where the electrodes are reversed at different times:

[0079] - when one of the three electrodes is at OV, the clamping force is at two-thirds of the maximum clamping force,

[0080] - the theoretical floating voltage of the plate varies from +1 / 3 of Vserrage to -1 / 3 of Vserrage, leading to a significant charge transfer to the plate if this plate is referenced,

[0081] - if the plaque is not referenced:

[0082] - the platen tension varies from +1 / 3 of Vserrage to -1 / 3 of Vserrage, which can be a significant problem (for ion implementation for example),

[0083] - Furthermore, the voltage difference between the electrode and the plate is 4 / 3 of Vserrage, that is, one-third more than the optimized voltage Vserrage, leading to the need to implement an adapted voltage Vserrage corresponding to 75% of the preferred voltage Vserrage and

[0084] - the maximum clamping force then corresponds to Fmin = 2 / 3 x % Fserrage = 1 / 2 Tightening when an electrode is at OV.

[0085] - In the event that two electrodes are reversed simultaneously, in addition to the problems identified above, the clamping force Fmin corresponds to one-third of the theoretical clamping force Fserrage if the insert is referenced. If the insert's potential is not referenced, the insert is lost.

[0086] In the fourth case, corresponding to a four-pole chuck:

[0087] - when the polarity of the electrodes is reversed in pairs:

[0088] - the theoretical float voltage of the wafer is stable at 0V, such that there is no difference between referenced and unreferenced wafer voltages,

[0089] - when a pair of electrodes is at OV during the inversion phase, the actual clamping force Fmin is half the theoretical clamping force Fserrage,

[0090] - when the polarity of the electrodes is not reversed in pairs:

[0091] - there is a moment in which three electrodes share the same polarity, leading to an evolution of the theoretical float voltage of the plate between +1 / 2 and -1 / 2 Vserrage,

[0092] - if the plate is referenced, the minimum clamping force during polarity reversal corresponds to Fmin = 0.75 x Fserrage, and the fluctuation of the theoretical floating potential leads to a significant charge transfer towards the plate,

[0093] - if the plaque is not referenced:

[0094] - the minimum clamping force when three electrodes share the same polarity corresponds to Fmin = 0.75 x Fserrage,

[0095] - moreover, the voltage difference between the electrode and the plate will be 1.5 Vserrage, i.e. 50% more than the optimized Vserrage value, which could lead to damage to the system and therefore to an adaptation of the clamping voltage to 2 / 3 of Vserrage, leading to obtaining a minimum clamping force of Fmin = 0.75 x 2 / 3 Fserrage = 0.5 Fserrage.

[0096] In the fifth case, corresponding to a hexapolar chuck:

[0097] - when the polarity of the electrodes is reversed in pairs (case described in international patent application WO9411944):

[0098] - the theoretical floating voltage of the wafer remains stable at 0V and

[0099] - when a pair of electrodes is at 0V, the clamping force corresponds to 2 / 3 of the maximum clamping force,

[0100] - when the polarity of the electrodes is not reversed in pairs:

[0101] - there is a moment in which four electrodes share the same polarity, leading to an evolution of the theoretical float voltage of the plate between +1 / 3 and -1 / 3 Vserrage,

[0102] - if the plate is referenced, the minimum clamping force during polarity reversal corresponds to Fmin = 0.83 x Fserrage, and the plate is traversed by a significant current,

[0103] - if the plaque is not referenced:

[0104] - the minimum clamping force when three electrodes share the same polarity corresponds to Fmin = 0.81 x Fserrage,

[0105] - moreover, the voltage difference between the electrode and the plate will be 1.5 Vserrage, i.e. 50% more than the optimized Vserrage value, which could lead to damage to the system and therefore to an adaptation of the clamping voltage to 2 / 3 of Vserrage, leading to obtaining a minimum clamping force of Fmin = 0.81 x 0.75 Fserrage = 0.61 Fserrage.

[0106] Thus, there is no solution that satisfies the previously outlined constraints in a satisfactory manner.

[0107] PRESENTATION OF THE INVENTION

[0108] The present invention aims to remedy all or part of these drawbacks.

[0109] To this end, according to a first aspect, the present invention relates to an electrostatic plate clamping device, which comprises:

[0110] - at least two electrodes, connected to at least two phases of a control device, configured to generate an electrostatic attraction force on the wafer,

[0111] - said phases and

[0112] - said dynamic control device for the electrical potential of each electrode, configured to command, for each phase, and for each electrode connected to that phase, the restoration of a stable electrical state among:

[0113] - a state of electrical potential with positive polarity,

[0114] - a state of negative polarity electric potential and

[0115] - a state of electrical potential of neutral polarity, said control device being configured so that at least one phase among all the phases is of electrical potential of neutral polarity during at least one stable state while at least one other phase is in a state of electrical potential of positive or negative polarity.

[0116] These provisions offer several advantages to the clamping device:

[0117] - the clamping force is increased compared to devices with a similar number of electrodes used,

[0118] - the transfer of charges to the plate is minimized,

[0119] - the theoretical floating voltage of the plate is also limited and

[0120] - The electrical stress on the dielectric is limited, reducing the aging of this dielectric. Conceptually, the present invention aims to, instead of implementing sinusoidal or square electrical potential reversal sequences between the energized electrodes, integrate a stable electrical potential state corresponding to zero voltage (and therefore neutral polarity) into a reversal sequence.

[0121] Several use cases can therefore be implemented from this general inventive concept.

[0122] Thus, in some variants, a single electrode can be fixed at a voltage of zero volts at a given instant, rather than a pair (in a hexagonal configuration for example).

[0123] Polarity change profiles can vary in form, aiming in some cases to reduce or eliminate the theoretical floating voltage excursion of the package or to reduce or eliminate the clamping force excursion.

[0124] In certain embodiments, the control element is configured to command, for each electrode, the successive establishment of a stable electrical state among:

[0125] - a first state of electrical potential of positive or negative polarity,

[0126] - a second state of electric potential of neutral polarity, then

[0127] - a third state of electrical potential with polarity respectively negative or positive depending on the first state of electrical potential with polarity controlled.

[0128] In particular embodiments, the control element is configured to control the simultaneous change of state of a number of electrodes less than the total number of electrodes.

[0129] In particular embodiments, the control element is configured to command the establishment, for at least one electrode, of a change of state corresponding to a direct succession of transient electrical potential states selected from the transient states between:

[0130] - a state of electric potential of positive polarity and a state of electric potential of neutral polarity,

[0131] - a state of electrical potential of neutral polarity and a state of electrical potential of positive polarity, - a state of electrical potential of neutral polarity and a state of electrical potential of negative polarity and / or

[0132] - a state of electrical potential of negative polarity and a state of electrical potential of neutral polarity.

[0133] In particular embodiments, the control element is configured to command a simultaneous change of electrical potential states between at least one pair of electrodes, corresponding to an exchange of the voltage values ​​of these electrodes.

[0134] In particular embodiments, the control element is configured to control the simultaneous change of electrical potential state of two electrodes.

[0135] In particular embodiments, the device of the present invention comprises an odd number of phases and electrodes greater than 1.

[0136] In particular embodiments, the device which is the subject of the present invention comprises three phases and three electrodes.

[0137] In particular embodiments, the device which is the subject of the present invention comprises five phases and five electrodes.

[0138] In particular embodiments, the control element is configured to, during a transient state of change of electrical potential of polarity, control the voltage of each electrode in this transient state so that the sum of the products of the voltage difference between an electrode and the plate and the capacitance formed locally by this electrode-plate pair is equal to zero during the duration of the transient state.

[0139] In particular embodiments, the control element is configured to, during a transient state of change of electrical potential of polarity, control the voltage of each electrode in this transient state so that the sum of the products of the capacitance formed locally by the electrode-plate pair and the square of the voltage difference between this electrode and the plate is equal to a constant during the duration of the transient state, said constant being equal to this same sum during the stable states upstream and downstream of the transient state.

[0140] In particular embodiments, the rate of voltage variation and / or the maximum value of the charging current of the phases and / or electrodes during a transient state is limited to a predetermined limit value.

[0141] In particular embodiments, the control unit is configured to command, for at least two phases and / or electrodes, the execution of a polarization sequence according to a symmetrical sequencing pattern.

[0142] In certain embodiments, the control element is configured to command, for at least one phase and / or electrode, the execution of a periodic polarization sequence. In certain embodiments, the control element is configured to command, for at least one phase and / or electrode, the execution of a polarization sequence in which the duration of at least two stable states is identical.

[0143] In particular embodiments, the control element is configured to command, for at least one phase and / or electrode, the execution of a polarization sequence in which the duration of each stable electrical state is identical.

[0144] In particular embodiments, the control element is configured to command, successively at different phases and / or neighboring electrodes, the establishment of a stable electrical state of electrical potential of neutral polarity.

[0145] In particular embodiments, the control element is configured to command, successively to different phases and / or non-neighboring electrodes, the establishment of a stable electrical state of electrical potential of neutral polarity.

[0146] These embodiments can be achieved by changing the control of the different phases or by changing the connection of the different phases to the electrodes.

[0147] According to a second aspect, the present invention relates to a control element for an electrostatic insert clamping device, comprising at least two electrodes configured to generate an electrostatic attractive force on the insert, which includes:

[0148] - at least two phases configured to be connected to the electrodes,

[0149] - a means for dynamically controlling the electrical potential of each phase, configured to command, for each phase and for each electrode connected to that phase, the restoration of a stable electrical state among:

[0150] - a state of electrical potential with positive polarity,

[0151] - a state of negative polarity electric potential and

[0152] - a state of electrical potential of neutral polarity, said control means being configured so that at least one phase among the set of phases is of neutral polarity during at least one stable state while at least one other electrode is in a state of electrical potential of positive or negative polarity.

[0153] According to a third aspect, the present invention relates to a wafer processing equipment, characterized in that it comprises an electrostatic wafer clamping device which is the subject of the present invention.

[0154] According to a fourth aspect, the present invention relates to an electrostatic clamping method for inserts, which comprises:

[0155] - a dynamic control step of the electrical potential of each electrode, configured to control, for each phase, and for each electrode connected to that phase, the restoration of a stable electrical state among:

[0156] - a state of electrical potential with positive polarity,

[0157] - a state of negative polarity electric potential and

[0158] - a state of electrical potential of neutral polarity, at least one electrode among the set of electrodes being established in a state of neutral polarity during at least one stable state while at least one other electrode is in a state of electrical potential of positive or negative polarity during the step of dynamic control of the electrical potential.

[0159] BRIEF DESCRIPTION OF THE FIGURES

[0160] Other advantages, purposes and particular features of the invention will become apparent from the following non-limiting description of at least one particular embodiment of the device and method of the present invention, with reference to the accompanying drawings, in which:

[0161] Figure 1 schematically represents a particular embodiment of an electrostatic chuck as implemented by the present invention,

[0162] Figure 2 schematically represents an electrical circuit equivalence of an assembly comprising an electrostatic chuck and a plate.

[0163] Figure 3 schematically illustrates how a clamping force is generated from the electrostatic forces of each electrode in an electrostatic chuck.

[0164] Figure 4 schematically represents a particular embodiment of the device that is the subject of the present invention,

[0165] Figure 5 schematically represents a sequence of stable state variations for an electrode,

[0166] Figure 6 schematically represents a first sequence of stable state variations for three electrodes,

[0167] Figure 7 schematically represents a second sequence of stable state variations for three electrodes,

[0168] Figure 8 schematically represents three sequences of stable state variation for five electrodes,

[0169] Figure 9 schematically represents various patterns of transient polarity variation states,

[0170] Figure 10 schematically represents three electrode patterns for electrostatic mandrels and

[0171] Figure 11 schematically represents two clamping cycles by selective activation of electrostatic chuck electrodes.

[0172] DESCRIPTION OF IMPLEMENTATION METHODS

[0173] The present description is given by way of non-limiting attribution, each feature of an embodiment being able to be advantageously combined with any other feature of any other embodiment.

[0174] It should be noted from the outset that the figures are not to scale. As will be understood from this description, various inventive concepts can be implemented by one or more of the methods or devices described below, several examples of which are provided here. The actions or steps performed in the implementation of the method or device can be ordered in any appropriate manner. Consequently, it is possible to construct embodiments in which the actions or steps are performed in a different order than that illustrated, which may include performing certain acts simultaneously, even if they are presented as sequential acts in the illustrated embodiments.

[0175] The expression "and / or," as used in this document, should be understood as meaning "one or the other or both" of the elements thus joined, that is, elements that are present conjunctively in some cases and disjunctively in others. Multiple elements listed with "and / or" should be interpreted in the same way, that is, "one or more" of the elements thus joined. Other elements may also be present, other than those specifically identified by the "and / or" clause, whether or not they are related to those specifically identified elements.Thus, by way of non-limiting example, a reference to "A and / or B", when used in conjunction with an open language such as "including", may refer, in one embodiment, to A only (possibly including elements other than B); in another embodiment, to B only (possibly including elements other than A); in yet another embodiment, to A and B (possibly including other elements); etc.

[0176] As used here in the description, "or" should be understood inclusively.

[0177] As used in this description, the expression "at least one," when referring to a list of one or more items, should be understood as meaning at least one item chosen from one or more items in the list of items, but not necessarily including at least one of each item specifically listed in the list of items and not excluding any combination of items in the list of items. This definition also allows for the optional presence of items other than those specifically identified in the list of items to which the expression "at least one" refers, whether or not they are related to those specifically identified items.Thus, by way of non-limiting example, "at least one of A and B" (or, equivalently, "at least one of A or B", or, equivalently, "at least one of A and / or B") may refer, in one embodiment, to at least one, possibly including more than one, A, without B present (and possibly including elements other than B); in another embodiment, to at least one, possibly including more than one, B, without A present (and possibly including elements other than A); in yet another embodiment, to at least one, possibly including more than one, A, and at least one, possibly including more than one, B (and possibly including other elements); etc.

[0178] In the description below, all transitive expressions such as "comprising", "including", "carrying", "having", "containing", "implying", "holding", "composed of", and others, should be understood as open, that is, as meaning including but not limited to. Only the transitive expressions "consisting of" and "consisting essentially of" should be understood as closed or semi-closed transitive expressions, respectively.

[0179] In this description, it is noted that the technical objective of the present invention is to control different electrodes at different electrical potentials. Polarity indicates the sign of this electrical potential. Formally, to describe all these states, a "reference potential" is required. This reference potential can be arbitrarily set to 0.

[0180] Thus, we have a 0, so we can have both positive and negative values, and we have an amplitude relative to this reference potential. Therefore, here, we use "potential" or "voltage" interchangeably to refer to the potential difference between a given element and a reference electrical potential, commonly called "neutral" by the scientific community. This reference potential can be ground, but not necessarily. Similarly, the terms "neutral polarity" used in this document refer to an electrical potential equal to the reference potential, and the terms "positive polarity" and "negative polarity" refer to electrical potential differences of equal value with respect to the reference potential, but of opposite signs.

[0181] In this description, a "stable state of electrical potential of polarity" is defined as an electrical state corresponding to a predetermined target state and maintained for a comparatively long time compared to a "transient state of electrical potential of polarity," which corresponds to the necessary change in electrical potential between two target states. In a transient state of electrical potential, a voltage value is maintained for a short time compared to the voltage value maintained in a stable electrical state. It should be noted that, in some variations, the duration of a stable electrical state can approach zero, as the electrode polarities are then permanently in a reversal cycle.In such variants, the stable electrical state considered corresponds to a particular point on a polarity evolution curve, the "stable" state becoming a "target" and transient state, the stable electrical state of positive polarity corresponding to the maximum point, the stable electrical state of negative polarity corresponding to the minimum point and the stable electrical state of neutral electrical potential corresponding to an electrical potential equal to the reference potential (typically 0 V).

[0182] In the context of the present invention, a voltage source is called a "phase". Note that a phase can be connected to a plurality of electrodes.

[0183] Figure 4, which is not to scale, shows a schematic view of one embodiment of the device 400, the subject of the present invention. This electrostatic clamping device 400 comprises:

[0184] - at least two electrodes, 405 and 410, connected to at least two phases, 406 and 411, of a control element 415, configured to generate an electrostatic attractive force on the wafer,

[0185] - said phases, 406 and 411, and - said dynamic control device 415 for the electrical potential of each phase, configured to command, for each phase, and for each electrode connected to that phase, the restoration of a stable electrical state among:

[0186] - a state of electrical potential with positive polarity,

[0187] - a state of negative polarity electric potential and

[0188] - a state of electrical potential of neutral polarity, said control device being configured so that at least one phase among the set of phases is of electrical potential of neutral polarity during at least one stable state while at least one other phase is in a state of electrical potential polarity of positive or negative.

[0189] The phases, 406, 411, 421, 426 and 431, can be two or more in number, and preferably in an odd number which may be three or five.

[0190] The electrodes are made from any conductive material commonly used for electrostatic chuck electrodes. Such materials are widely known in the field of electrical chucks and are not listed here. Similarly, variations in electrode structure and dimensions are well-known in the field of electrostatic chucks.

[0191] For the present invention, any electrode structure and size can be implemented.

[0192] The purpose of the electrodes is to create a clamping force on the plate, as illustrated opposite Figure 3.

[0193] Although the other ordinary components of an electrostatic chuck are not shown in Figure 4, their presence can be implicitly deduced from the content of Figures 1, 2, and 3. Figure 4 shows a top view of an electrostatic chuck, with the relative positioning of the various electrodes clearly defined. Figure 10 shows different surface arrangements of the clamping electrodes. Reference A corresponds to a tripolar arrangement, while references B and C correspond to hexapolar arrangements. These different arrangements illustrate different geometries for the electrodes, compared to Figure 4. The electrodes can thus exhibit radial symmetry. In the arrangements shown in Figure 10, there are always at least two electrodes, 1105 and 1110, between the thermal coupling gas injection holes 1115 and the periphery of the chuck.

[0194] With regard to Figure 4, the electrostatic mandrel shown has five electrodes: 405, 410, 420, 425, and 430. All the electrodes may be identical. In some variations, at least one electrode has a surface area with dimensions different from at least one other electrode. In other variations, at least one electrode is made of a different material than at least one other electrode.

[0195] Similarly, a dielectric layer separating the wafer from the electrodes can be homogeneous with respect to each electrode or have distinct local characteristics. Likewise, each electrode can be supplied with the same absolute voltage, or at least one absolute voltage applied to at least one electrode can be different from at least one other absolute voltage applied to at least one other electrode.

[0196] The control unit 415 corresponds, for example, to an automaton equipped with a control means 416, such as a microcontroller or microprocessor configured to execute a set of computer instructions corresponding to a software, said software including control instructions for varying the polarity of the voltage of at least one electrode.

[0197] The exact nature of the control organ 415 is widely known in the field of control organ manufacturing and this nature is not limiting for the implementation of the present invention.

[0198] The control unit 415 is defined by its functional role, this role consisting at a minimum of organizing the variation of electrical potential of each electrode so as to respond to the different constraints mentioned in the discussion of the prior art.

[0199] Thus, the control unit 415 is configured to command, for each phase, and consequently for each electrode connected to that phase, a voltage application corresponding to +Vserrage, OV or -Vserrage.

[0200] The techniques for generating and managing the tensions of each phase are well known to those skilled in the art and are not detailed here.

[0201] Preferably, the control element 415 is configured to control a process according to a predetermined tightening sequence 600 corresponding to a succession of variations in the voltage of an electrode, as shown in Figure 5. Such a repeatable sequence corresponds, for example, to:

[0202] - a 605 state of positive polarity electrical potential,

[0203] - a 610 state of neutral polarity electric potential,

[0204] - a 615 state of negative polarity electric potential then

[0205] - a 610 state of electric potential of neutral polarity.

[0206] Such a sequence can be performed periodically, at a frequency, for example of 50Hz, 1 Hz, 0.1 Hz or 0.016Hz (corresponding to a period of one minute).

[0207] Finally, we observe two types of transient states, 620 and 625, between a stable electrical state of positive polarity electrical potential and a stable electrical state of neutral polarity electrical potential on the one hand, and between a stable electrical state of neutral polarity electrical potential and a stable electrical state of negative polarity electrical potential on the other hand.

[0208] Figure 6 shows the stacking of sequences 705, 710, and 715, similar to that illustrated in Figure 5, applied out of phase to three electrodes. In this configuration, at any given instant, two electrodes are powered with reversed polarity while one electrode is not powered. The specific phase shift used here corresponds to the duration of a stable electrical state, with all stable states for all electrodes having identical durations.

[0209] Figure 7 shows the stacking of variant sequences 805, 810, and 815, such as that illustrated in Figure 5, applied out of phase to three electrodes. In this configuration, the neutral polarity electrical potential state is shorter in duration for sequences 805 and 815 than for sequence 810.

[0210] Thus, in certain embodiments, the control element 415 is configured to command, for each electrode, the successive establishment of a stable electrical state among:

[0211] - a first state of electrical potential of positive or negative polarity,

[0212] - a second state of electric potential of neutral polarity, then

[0213] - a third state of electrical potential with polarity respectively negative or positive depending on the first state of electrical potential with polarity controlled.

[0214] In particular embodiments, such as that shown in Figure 4, the device 400 comprises five electrodes sequentially brought to a neutral electrical potential state. Such a bringing to a neutral electrical potential state can be arranged step by step, i.e. in the order of electrodes 405, 410, 420, 425 then 430, or in a star configuration, i.e. by skipping one electrode at a time, corresponding to the order of electrodes 405, 420, 430, 410 and 425.

[0215] Such organizations are represented in Figure 11, in which:

[0216] - a first organization A corresponds to a star-shaped tightening cycle, the white electrode being in a stable electrical potential state of neutral polarity, and

[0217] - a second organization B corresponds to a tightening cycle from one point to another, or circular.

[0218] Figure 8 illustrates the implementation of different sequences, 905, 910, and 915, of variation for a device comprising five electrodes. For each such sequence, a central line illustrates the neutral polarity electrical potential state, while a black rectangle positioned above this line illustrates a positive polarity electrical potential state, and a black rectangle positioned below this line illustrates a negative polarity electrical potential state. These different sequences illustrate the possibility of maintaining an electrode in a specific polarity electrical potential state during a cycle, as shown for sequences 910 and 915. The object of the present invention is therefore not limited to a particular sequence.

[0219] In particular embodiments, the control element 415 is configured to control the simultaneous change of state of a number of electrodes less than the total number of electrodes.

[0220] The number of electrodes changing state is, for example, an even number. This number of electrodes can correspond to all electrodes except one. In particular embodiments, the control element 415 is configured to command the establishment, for at least one electrode, of a state change corresponding to a direct succession of transient states selected from among the transient states between:

[0221] - a state of electric potential of positive polarity and a state of electric potential of neutral polarity,

[0222] - a state of electrical potential of neutral polarity and a state of electrical potential of positive polarity,

[0223] - a state of electric potential of neutral polarity and a state of electric potential of negative polarity and / or.

[0224] - a state of electrical potential of negative polarity and a state of electrical potential of neutral polarity.

[0225] In particular embodiments, the control element 415 is configured to control a simultaneous change of states between at least one pair of electrodes, corresponding to an exchange of the voltage values ​​of these electrodes.

[0226] In particular embodiments, the control element 415 is configured to, during a transient state of polarity change, control the voltage of each electrode in this transient state so that the sum of the products of the voltage difference between electrode and plate and the capacitance formed locally by the electrode-plate couple is equal to zero during the duration of the transient state.

[0227] This particular embodiment is represented by the following equation:

[0228] In certain embodiments, the following equation is verified only for two electrodes changing polarity simultaneously:

[0229] In variants of these particular embodiments, the other electrodes remain at the same voltage during the change of polarity of the two electrodes.

[0230] In variants of these particular embodiments, the voltage of the others is optimized to maximize the minimum clamping force during the change of polarity of the two electrodes.

[0231] In particular embodiments, the control element 415 is configured to, during a transient state of polarity change, control the voltage of each electrode in this transient state so that the sum of the products of the capacitance formed locally by the electrode-plate couple and the square of the voltage difference between electrode and plate is equal to a constant during the duration of the transient state, a constant equal to this same value in the upstream and downstream stable states.

[0232] This particular embodiment is represented by the following equation:

[0233] In certain embodiments, the following equation is verified only for two electrodes changing polarity simultaneously:

[0234] In variants of these particular embodiments, the other electrodes remain at a constant voltage during the change of polarity of the two electrodes.

[0235] In variants of these particular embodiments, the voltage of the others is optimized to reduce the theoretical floating voltage excursion of the plate during the change of polarity of the two electrodes.

[0236] In particular embodiments, the rate of voltage variation and / or the maximum value of the electrode charging current during a transient state is limited to a predetermined limit value.

[0237] Thus, as can be understood, in variants, the voltage change profile of a transient state can be linear or non-linear and over a more or less long determined duration.

[0238] Figure 9 shows different profiles, A, B and C, with corresponding variations:

[0239] - to a linear variation A, in which the voltage, 1001 and 1003, of two electrodes varies linearly while the voltage 1002 of one electrode is maintained constant in a negative polarity electrical potential state,

[0240] - to a linear variation B for which the voltage 1004 of the electrode, which does not change state, is adapted so as to maximize the clamping pressure of the insert, and

[0241] - to a limited variation C in maximum voltage variation speed, to avoid damaging the system, according to a predetermined variation profile, corresponding for example to:

[0242] - for the first part 1005 of the profile, the voltage of an electrode A drives the sequence of state changes,

[0243] - For this first part of the profile, the voltage of electrode B is a variable determined as a function of the voltage of electrode A to satisfy equation XC. V 2= constant, and

[0244] - For a first part 1005 of the profile, the voltage of an electrode A evolves according to the following equation 1007: V A (t) = J or linearly 1008 constraint if the rate of voltage change is greater than a predetermined limit rate of change, - for a second part 1006 of the subsequent profile, the voltage of electrode B is controlled and the voltage of electrode A becomes variable to comply with equation XC. V 2 = constant.

[0245] It should be noted that the present invention also relates to a series of specific steps for implementing the method that is the subject of the present invention. This electrostatic clamping method for inserts comprises: - a step 510 for dynamically controlling the electrical potential of each electrode, in order to control, for each electrode, the establishment of a stable electrical state among:

[0246] - a state of electrical potential with positive polarity,

[0247] - a state of negative polarity electrical potential and - a state of neutral polarity electrical potential, at least one electrode among all the electrodes being established in a state of neutral polarity electrical potential while at least one other electrode is in a state of positive or negative polarity electrical potential during the dynamic electrical potential control step. Preferably, during the control step 510, a transition profile from one stable electrical state to the other is controlled for each electrode.

[0248] Below, we present a first comparison, between a pentapolar mode (i.e. relating to a device subject to the present invention comprising five electrodes) corresponding to a particular embodiment of the present invention and a hexapolar mode (i.e. relating to a device subject to the prior art comprising six electrodes) corresponding to the prior art, of the powers consumed and the duration of stress of an electrode to clamp a wafer.

[0249] [Table 1] In this first table, we illustrate the comparison of performance between a hexapolar version of the prior art as described in patent US2005052817 and a pentapolar version which is the subject of the present invention.

[0250] The duration of the process is assumed to be identical. It is also assumed that the clamping voltages are identical in amplitude (therefore, the clamping force is slightly lower in the hexapolar version). Finally, it is assumed that the various stable states have the same duration for all electrodes.

[0251] Three cases are studied:

[0252] - Case 1: same duration of the stable electrical state between pentapolar and hexapolar,

[0253] - Case 2: same duration of the total polarity switching cycle (time taken for the various electrodes to regain the polarity they had at the very beginning of the cycle) and

[0254] - Case 3: same maximum duration of a stable electrical state = duration during which a given electrode remains at the same polarity.

[0255] The main objective of the present invention being to reduce the risk of sticking, which is considered by those skilled in the art to be proportional to the tightening time at a given polarity, case number 3 is undoubtedly the most relevant. However, the other cases are studied for all practical purposes.

[0256] For each of these cases, we compare:

[0257] - The energy consumed to process a wafer (to apply the process): this gives direct information on the energy that must be supplied to the system to ensure the clamping of the wafer, and therefore its thermalization and the proper execution of the recipe.

[0258] - Energy consumed by an electrode: provides information on the electrical stress borne by each power line supplying the electrodes, including sealed passages and high-voltage contacts inside and outside the device, which those skilled in the art know to be particularly fragile. This therefore provides information on the aging rate of these power lines, and consequently, on the risk of failure.

[0259] - The time during which each electrode is subjected to electrostatic stress: this provides direct information on the level of risk of dielectric charging, and therefore on the level of risk of bonding. It also provides direct information on the aging rate of the dielectric layer(s), and therefore on the risk of device failure.

[0260] Thus, for case number 3:

[0261] - the power consumed to clamp the plate during the manufacturing process is 29% lower with a pentapolar device of the present invention compared to a hexapolar device of the prior art,

[0262] - The aging of high-voltage power lines is 15% lower, and there are only 5 lines instead of 6, resulting in an overall risk of failure at the clamping connector level reduced by 29%.

[0263] - the aging of the dielectric layer is 15% lower and - all for a higher minimum clamping force.

[0264] Below, a second comparison is presented, between two pentapolar modes corresponding to particular embodiments of the present invention and a hexapolar mode corresponding to the prior art, of the powers consumed and the stress duration of an electrode to clamp a wafer.

[0265] [Table 2]

[0266] In this second table, we illustrate the comparison of clamping performance between a device subject to the present invention in pentapolar version, and a hexapolar device according to US patent 2005052817.

[0267] For the pentapolar version, cases are studied where the polarity switching profile respects EC.V = constant (called "V"). wa constant iron") and the case where the tilting profile complies with EC.V 2 = constant (called “F wafer constant”).

[0268] In both cases, we observe that the minimum available clamping force is greater, by 6% for the "V" case. wa constant iron" and 20% for the case "F wa f er constant”, and that the variation of this clamping force is much less than the case described in patent US2005052817.

[0269] As can be understood, the present invention also relates to a control element (415), as shown in Figure 4, for an electrostatic clamping device for a plate comprising at least two electrodes, connected to at least two phases, configured to generate an electrostatic attractive force on the plate, which comprises: - a means 416 for dynamically controlling the electrical potential of each electrode, configured to control, for each electrode, the establishment of a stable electrical state among:

[0270] - a state of electrical potential with positive polarity,

[0271] - a state of negative polarity electric potential and

[0272] - a state of electrical potential of neutral polarity, said control means 416 being configured so that, outside transient phases, at least one electrode among the set of electrodes is of electrical potential of neutral polarity during at least one stable state while at least one other electrode is in a state of electrical potential of positive or negative polarity.

[0273] One such means of control 416 is, for example, an electrical circuit controlled by an electronic circuit, implementing a microcontroller for example, configured to act as an automaton or to execute instructions representative of a computer program.

[0274] This control device can be associated with internal manufacturing recipes (operating autonomously) or receive manufacturing recipes from an external source and manage only the polarity switches. For example, the control device 416 autonomously generates the frequencies and the switching command, but receives from an external source the setpoint relating to the amplitude (= value of the electrical potential) of the polarity potential states.

[0275] Such a control means 416 can be operated autonomously or dynamically, by implementing a means of communication (such as an antenna or a communication cable) connected to an information network, such as the internet or a local network for example, so that instructions can be received by the control means 416.

[0276] Such instructions can be issued by software, logically, via an application programming interface (API) or via a graphical user interface.

Claims

25 DEMANDS 1. Electrostatic plate clamping device (400), characterized in that it comprises: - at least two electrodes (405, 410), connected to at least two phases of a control element, configured to generate an electrostatic attractive force on the wafer, - the said phases (406, 411) and - said dynamic control device (415) for the electrical potential of each phase, configured to command, for each phase and for each electrode connected to one of the phases, the restoration of a stable electrical state among: - a state of electrical potential with positive polarity, - -a state of negative polarity electric potential and - a state of electrical potential of neutral polarity, said control device being configured so that at least one phase among all the phases is of electrical potential of neutral polarity during at least one stable state while at least one other phase is in a state of electrical potential of positive or negative polarity.

2. Device (400) according to claim 1, wherein the control member (415) is configured to, for each electrode, control the successive establishment of a stable electrical state among: - a first state of electrical potential of positive or negative polarity, - a second state of electric potential of neutral polarity, then - a third state of electrical potential with polarity respectively negative or positive depending on the first state of electrical potential with polarity controlled.

3. Device (400) according to claim 1 or 2, wherein the control member is configured to control the simultaneous change of state of a number of electrodes less than the total number of electrodes.

4. Device (400) according to claim 3, wherein the control member (415) is configured to control the establishment, for at least one electrode, of a change of state corresponding to a direct succession of transient states selected from the transient states between: - a state of electric potential of positive polarity and a state of electric potential of neutral polarity, - a state of electrical potential of neutral polarity and a state of electrical potential of positive polarity, - a state of electrical potential of neutral polarity and a state of electrical potential of negative polarity, and / or - a state of electrical potential of negative polarity and a state of electrical potential of neutral polarity.

5. Device (400) according to any one of claims 3 or 4, wherein the control member (415) is configured to control a simultaneous change of states between at least one pair of electrodes, corresponding to an exchange of the voltage values ​​of these electrodes.

6. Device (400) according to any one of claims 1 to 5, which comprises an odd number of phases greater than 1.

7. Device according to claim 6, which comprises three phases.

8. Device (400) according to claim 6, comprising five phases (406, 411, 421, 426, 431) 9. Device (400) according to any one of claims 1 to 8, wherein the control member (415) is configured to, during a transient state of polarity change, control the voltage of each electrode in this transient state so that the sum of the products of the voltage difference between an electrode and the wafer and the capacitance formed locally by this electrode-wafer pair is equal to zero during the duration of the transient state.

10. Device (400) according to any one of claims 1 to 8, wherein the control member (415) is configured to, during a transient state of polarity change, control the voltage of each electrode in this transient state such that the sum of the products of the capacitance formed locally by the electrode-plate pair and the square of the voltage difference between this electrode and the plate is equal to a constant during the duration of the transient state, said constant being equal to this same sum during the stable states upstream and downstream of the transient state.

11. Device (400) according to claim 10, wherein the rate of voltage variation and / or the maximum value of the charging current of the phases and / or electrodes during a transient state is limited to a predetermined limit value.

12. Device (400) according to any one of claims 1 to 11, wherein the control member (415) is configured to control, for at least two phases and / or electrodes, the execution of a polarization sequence according to a symmetric sequencing pattern.

13. Device (400) according to any one of claims 1 to 12, wherein the control member (415) is configured to control, for at least one phase and / or electrode, the realization of a periodic polarization sequence.

14. Device (400) according to any one of claims 1 to 13, wherein the control member (415) is configured to control, for at least one phase and / or electrode, the execution of a polarization sequence in which the duration of each stable electrical state is identical.

15. Device (400) according to any one of claims 1 to 14, wherein the control member (415) is configured to control, for at least one phase and / or electrode, the execution of a polarization sequence in which the duration of at least two stable states is identical.

16. Device (400) according to any one of claims 1 to 15, wherein the control member (415) is configured to command, successively to different phases and / or neighboring electrodes, the restoration of a stable electrical state of electrical potential of neutral polarity. Tl 17. Device (400) according to any one of claims 1 to 15, wherein the control member (415) is configured to command, successively to different phases and / or non-neighboring electrodes, the restoration of a stable electrical state of electrical potential of neutral polarity.

18. Control element (415) for an electrostatic insert clamping device (400) comprising at least two electrodes (405, 410) configured to generate an electrostatic attractive force on the insert, characterized in that it comprises: - at least two phases (406, 411) configured to be connected to the electrodes, - a means (416) for dynamically controlling the electrical potential of each electrode, configured to command, for each phase, and for each electrode connected to that phase, the restoration of a stable electrical state among: - a state of electrical potential with positive polarity, - a state of negative polarity electric potential and - a state of electrical potential of neutral polarity, said control means being configured so that, in stable states, at least one phase among the set of phases is of electrical potential of neutral polarity while at least one other phase is in a state of electrical potential of positive or negative polarity.

19. Insert processing equipment, characterized in that it comprises an electrostatic insert clamping device according to any one of claims 1 to 17.

20. Electrostatic clamping method for inserts, characterized in that it comprises: - a step for dynamically controlling the electrical potential of each electrode, configured to control, for each phase, and for each electrode connected to that phase, the restoration of a stable electrical state among: - a state of electrical potential with positive polarity, - a state of negative polarity electric potential and - a state of electric potential of neutral polarity, at least one phase among the set of phases being established at an electric potential of neutral polarity during at least one stable state, while at least one other phase is in a state of electric potential of positive or negative polarity during the stage of dynamic control of the electric potential.

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