Lens, ion implantation system and method

Abstract

A lens, an ion implantation system, and a method are disclosed. An approach for reducing particle generation in an electrostatic lens is provided herein. In some embodiments, an ion implantation system may include: an electrostatic lens including an inlet for receiving an ion beam and an outlet for delivering the ion beam to a target, the electrostatic lens including a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of the ion beam line, wherein the first ground electrode is grounded and positioned adjacent to the outlet. The electrostatic lens may include a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and positioned adjacent to the outlet. The implantation system may also include a power supply operable to supply voltage and current to the electrostatic lens for controlling the ion beam.

Description

Technical Field

[0001] The present disclosure relates generally to semiconductor processing, and more particularly to an electrostatic filter, a lens, an ion implantation system, and a method that provide reduced particle generation. Background Art

[0002] Ion implantation is the process of introducing dopants or impurities into a substrate by bombardment. In semiconductor manufacturing, dopants are introduced to change electrical, optical or mechanical properties. For example, dopants can be introduced into an intrinsic semiconductor substrate to change the conductivity type and conductivity level of the substrate. When manufacturing integrated circuits (ICs), a precise doping profile improves the performance of the integrated circuit. In order to achieve the desired doping profile, one or more dopants can be implanted in the form of ions at various doses and various energy levels.

[0003] An ion implantation system may include an ion source and a series of beamline components. The ion source may include a chamber that produces the desired ions. The ion source may also include a power source and an extraction electrode assembly disposed near the chamber. The beamline components may include, for example, a mass analyzer, a first acceleration or deceleration stage, a collimator, and a second acceleration or deceleration stage. Much like a series of optical lenses used to manipulate a light beam, the beamline components may filter, focus, and manipulate ions or ion beams of the desired material type, shape, energy, and other characteristics. The ion beam passes through the beamline components and may be directed toward a substrate or wafer mounted on a platen or fixture. The substrate may be moved in one or more dimensions (e.g., translated, rotated, and tilted) by a device sometimes referred to as a multi-axis rotary arm (roplat).

[0004] Ion implanters produce stable and well-defined ion beams for a variety of ion species and extraction voltages. After several hours of operation using source gases such as AsH3, PH3, BF3, and other species, beam constituents eventually form deposits on the beam optics. The beam optics in the line-of-sight of the wafer can also be coated with residues from the wafer, including Si and photoresist compounds. These residues accumulate on beamline components, causing spikes in direct current (DC) potential during operation (e.g., in the case of electrically biased components). Eventually the residues flake off, causing an increased likelihood of particle contamination of the wafer.

[0005] One way to prevent material accumulation is to intermittently replace the beamline assembly of the ion implanter system. Alternatively, the beamline assembly can be manually cleaned, including de-energizing the ion source and releasing the vacuum in the system. After the beamline assembly is replaced or cleaned, the system is then evacuated and powered up to reach an operational state. Therefore, these maintenance processes can be time-consuming. In addition, the beamline assembly is unavailable during the maintenance process. Therefore, frequent maintenance processes can reduce throughput, thereby increasing overall manufacturing costs. Summary of the invention

[0006] In one approach, an ion implantation system may include an electrostatic lens including an inlet for receiving an ion beam and an outlet for delivering the ion beam to a target, the electrostatic lens including a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of an ion beam line, wherein the first ground electrode is grounded and positioned adjacent to the outlet. The electrostatic lens may also include a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and positioned adjacent to the outlet, and wherein the second suppression electrode is positioned further downstream along the ion beam line than the first suppression electrode. The implantation system may also include a power supply operable to supply voltage and current to the electrostatic lens for controlling the ion beam.

[0007] In another approach, a lens may include: a chamber having an entrance for receiving an ion beam and an exit for delivering the ion beam to a target; a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of an ion beam line, wherein the first ground electrode is grounded and located proximate to the exit. The lens may also include a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and located proximate to the exit.

[0008] In another approach, a method may include providing an electrostatic lens including an inlet for receiving an ion beam and an outlet for delivering the ion beam to a target, the electrostatic lens including a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of an ion beam line, wherein the first ground electrode is grounded and positioned adjacent to the outlet. The electrostatic lens may also include a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and positioned adjacent to the outlet. The method may also include supplying voltage and current to the electrostatic lens for controlling the ion beam. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The accompanying drawings illustrate exemplary approaches to the present disclosure hitherto conceived for practical application of its principles, and in which:

[0010] Figure 1 is a schematic diagram showing an ion implantation system according to an embodiment of the present disclosure.

[0011] Figure 2 According to the embodiment of the present disclosure Figure 1 A side cross-sectional view of an electrostatic filter of an ion implantation system is shown.

[0012] Figure 3 is a flow chart of a method according to an embodiment of the present disclosure.

[0013] The drawings are not necessarily drawn to scale. The drawings are merely representations and are not intended to depict specific parameters of the present disclosure. The drawings are intended to illustrate exemplary embodiments of the present disclosure and are therefore not to be considered as limiting the scope. In the drawings, the same reference numerals represent the same elements. DETAILED DESCRIPTION

[0014] Ion implantation systems, electrostatic filters or lenses, and methods according to the present disclosure will now be described more fully below with reference to the accompanying drawings, in which embodiments of the present disclosure are shown. The ion implantation systems, electrostatic filters, and methods may be embodied in many different forms and are not to be construed as limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the systems and methods to those skilled in the art.

[0015] Provided herein is a method for reducing particles in an ion implanter. An electrostatic filter may include a housing and a plurality of conductive beam optical devices (e.g., electrodes) located within the housing. The conductive beam optical device is arranged around an ion beam line directed toward a wafer and may include an inlet or terminal electrode near the inlet of the housing. The conductive beam optical device may also include a suppression electrode located downstream of the terminal electrode along the ion beam line, a powered electrode located downstream of the suppression electrode, and a ground electrode located downstream of the powered electrode. In some embodiments, those electrodes located below the ion beam are further away from the ion beam line than those electrodes located above the ion beam, so that the suppression electrode located at the bottom and the powered electrode are physically blocked or shielded from being coated by the envelope of back-sputter material returned from the wafer. In addition, in some embodiments, the suppression electrode located at the bottom may be positioned further downstream along the ion beam line than the suppression electrode located at the top. The electrostatic filter may also include an electrical system for delivering voltage and current to each of the conductive beam optical devices.

[0016] Reference now Figure 1 , an exemplary system according to the present disclosure is shown in the figure. An ion implantation system (hereinafter referred to as the "system") 10 represents a process chamber, which contains, among other components: an ion source 14 for generating an ion beam 18, an ion implanter, and a series of beamline components 16. The ion source 14 may include a chamber for receiving a gas stream 24 and generating ions therein. The ion source 14 may also include a power supply and an extraction electrode assembly disposed near the chamber. The beamline component 16 may include, for example, a mass analyzer 34, a first acceleration or deceleration stage 36, a collimator 38, and an electrostatic lens or electrostatic filter (EF) 40 that may correspond to the acceleration and deceleration stage. Although not shown, the beamline component 16 may also include a plasma flood gun (PFG) located downstream of the EF40.

[0017] In an exemplary embodiment, the beamline assembly 16 may filter, focus, and manipulate ions or an ion beam 18 having a desired species, shape, energy, and other characteristics. The ion beam 18 passing through the beamline assembly 16 may be directed toward a substrate mounted on a platen or fixture within a process chamber 46. As will be appreciated, the substrate may be moved in one or more dimensions (e.g., translation, rotation, and tilt).

[0018] As shown, there may be one or more feed sources 28 operable with the chamber of the ion source 14. In some embodiments, the material provided from the feed source 28 may include source material and / or other materials. The source material may contain a dopant substance introduced into the substrate in the form of ions. Meanwhile, the other materials may include a diluent, which is introduced into the ion source chamber of the ion source 14 with the source material to dilute the concentration of the source material in the chamber of the ion source 14. The other materials may also include a cleaning agent (e.g., an etchant gas), which is introduced into the chamber of the ion source 14 and transported within the system 10 to clean one or more beamline components 16.

[0019] In various embodiments, different substances may be used as source materials and / or other materials. Examples of source materials and / or other materials may include atomic or molecular substances containing boron (B), carbon (C), oxygen (O), germanium (Ge), phosphorus (P), arsenic (As), silicon (Si), helium (He), neon (Ne), argon (Ar), krypton (Kr), nitrogen (N), hydrogen (H), fluorine (F) and chlorine (Cl). Those of ordinary skill in the art will recognize that the substances listed above are non-limiting and other atomic or molecular substances may also be used. Depending on the application, the substance may be used as a dopant or the other material. Specifically, a substance used as a dopant in one application may be used as other materials in another application, and vice versa.

[0020] In an exemplary embodiment, source materials and / or other materials are provided in gaseous or vapor form to the ion source chamber of ion source 14. If the source materials and / or other materials are in non-gaseous or non-vapor form, an evaporator (not shown) may be provided near feed source 28 to convert the materials into gaseous or vapor form. In order to control the amount and rate at which source materials and / or other materials are provided to system 10, a flow rate controller 30 may be provided.

[0021] EF 40 may be configured to independently control the deflection, acceleration, deceleration, and focusing of ion beam 18. In one embodiment, EF 40 is a vertical electrostatic energy filter (VEEF). As will be described in more detail below, EF 40 may include an electrode configuration including a set of upper electrodes disposed above ion beam 18 and a set of lower electrodes disposed below ion beam 18. The set of upper electrodes and the set of lower electrodes may be stationary and have fixed positions. The potential difference between the set of upper electrodes and the set of lower electrodes may also vary along the central ion beam trajectory to reflect the energy of ion beam 18 at each point along the central ion beam trajectory, thereby independently controlling the deflection, acceleration, deceleration, and / or focusing of ion beam 18.

[0022] Although not limiting, the ion source 14 may include a power generator, a plasma exciter, a plasma chamber, and the plasma itself. The plasma source may be an inductively-coupled plasma (ICP) source, a toroidal coupled plasma source (TCP) source, a capacitively coupled plasma (CCP) source, a helicon source, an electron cyclotron resonance (ECR) source, an indirectly heated cathode (IHC) source, a glow discharge source, an electron beam generated ion source, or other plasma sources known to those skilled in the art.

[0023] The ion source 14 may generate an ion beam 18 for processing a substrate. In various embodiments, the ion beam (in cross section) may have a target shape, such as a spot beam or a ribbon beam as known in the art. To process the substrate, the ion beam 18 may be accelerated or decelerated to obtain a target energy by establishing a voltage (potential) difference between the ion source 14 and the wafer.

[0024] Reference now Figure 2, the EF 40 according to an exemplary embodiment will be described in more detail. As shown, the EF 40 includes an EF chamber 50 defined by a chamber housing 52. The EF 40 may also operate with one or more vacuum pumps (not shown) to adjust the pressure of the EF chamber 50. The EF 40 may be bounded along one end by a PFG 32 having an opening 37 to allow the ion beam 18 to pass through the opening 37 to reach the wafer 35. As shown, the PFG 32 is located between the EF 40 and the wafer 35, and the PFG 32 and the wafer 35 are oriented at an angle β relative to the ion beam line / trajectory 72. Although not limiting, the angle β may be between 5° and 30°. Due to the arrangement of multiple conductive beam optical devices 70A to 70J within the EF chamber 50, and due to the orientation of the EF 40 relative to the PFG 32 and the wafer 35, the EF 40 is considered to be "bent" or asymmetric.

[0025] As shown, the EF 40 may include one or more conductive beam optics 70A to 70J, which may be a plurality of graphite electrode rods disposed along an ion beam line / trajectory 72. In this embodiment, the conductive beam optics 70A to 70J are arranged in an asymmetric configuration relative to the ion beam line / trajectory 72. Although not limiting, the plurality of conductive beam optics 70A to 70J may include a set of entrance or terminal electrodes 70A, 70B proximate an entrance 75 of the EF 40 and a set of exit or ground electrodes 70I, 70J proximate an exit 76 of the EF 40. In some embodiments, the terminal electrodes 70A, 70B are held at a terminal voltage, while the ground electrodes 70I, 70J are grounded.

[0026] As further shown, EF 40 may include a set of suppression electrodes 70C, 70D disposed along opposite sides of ion beam line 72, downstream of terminal electrodes 70A and 70B, and a plurality of powered electrodes 70E to 70H disposed between suppression electrodes 70C, 70D and ground electrodes 70I, 70J. In some embodiments, the voltage of suppression electrodes 70C, 70D is less than or equal to the terminal voltage of terminal electrodes 70A to 70B. Although not limiting, in the case where ion beam 18 is a decelerating ion beam, powered electrodes 70E and 70G may be more positive than suppression electrode 70C, and powered electrodes 70F and 70H may similarly be more positive than suppression electrode 70D. In the case where ion beam 18 is an accelerating ion beam, powered electrodes 70E and 70G may be more negative than suppression electrode 70C, and powered electrodes 70F and 70H may be more negative than suppression electrode 70D.

[0027] As shown, each set of electrode pairs provides a space / opening to allow the ion beam 18 (e.g., a ribbon beam) to pass therethrough. Although not limiting, a first distance 'D1' between the suppression electrode 70C and the ion beam line 72 may be less than a second distance 'D2' between the suppression electrode 70D and the ion beam line 72. Additionally, in some embodiments, the suppression electrode 70D may be positioned further downstream along the ion beam line than the suppression electrode 70C. In other words, the distance 'D3' between the suppression electrode 70C and the ground electrode 70I is greater than the distance 'D4' between the suppression electrode 70D and the ground electrode 70J.

[0028] In an exemplary embodiment, the conductive beam optics 70A to 70J include multiple pairs of conductive pieces that are electrically coupled to each other. Alternatively, the conductive beam optics 70A to 70J may be a series of unitary structures that each include a hole for the ion beam to pass therethrough. In the illustrated embodiment, the upper portion and the lower portion of each electrode pair may have different electrical potentials (e.g., located in separate conductive pieces) to deflect the ion beam passing therethrough. Although the conductive beam optics 70A to 70J are shown as five (5) electrode pairs, a different number of elements (or electrodes) may be utilized. For example, the configuration of the conductive beam optics 70A to 70J may utilize a range of three (3) to ten (10) electrode sets.

[0029] In some embodiments, the ion beam 18 passing through the electrodes along the ion beam line 72 may contain boron or other elements. Electrostatic focusing of the ion beam may be achieved by using several thin electrodes (e.g., suppression / focusing electrodes 70C, 70D) to control the potential grading along the ion beam line 72. In the configuration of the conductive beam optics 70A to 70J shown, the ion beam 18 may be decelerated and deflected by the powered electrodes 70E to 70G.

[0030] In some embodiments, a power supply 78 (e.g., a direct current (DC) power supply) supplies voltage and current to the EF 40. The voltage / current may be supplied to the conductive beam optical devices 70A to 70J to generate plasma within the EF chamber 50. In various embodiments, the voltage and current provided by the power supply 78 may be constant or varied. In one embodiment, the conductive beam optical devices 70A to 70J are maintained at a range of DC potentials from 0.1 keV to 100 keV. The conductive beam optical devices 70A to 70J may be electrically driven in parallel (e.g., individually) or in series to enable each of the conductive beam optical devices 70A to 70J to operate uniformly and / or independently.

[0031] In some embodiments, as further shown, the terminal electrode 70B, the suppression electrode 70D, and the powered electrodes 70F, 70H can be shielded by the ground electrode 70J below the line of sight 67 to prevent deposition of sputtered material from the wafer 35, thereby keeping these electrodes below the line of sight 67 cleaner. As shown, the line of sight 67 can be defined by the inner surface 68 of the ground electrode 70J. This configuration will eliminate or at least reduce particle sources that may be generated at the electrodes 70B, 70D, 70F, 70H located at the bottom, and can eliminate or at least reduce the transport mechanism for material peeling off the electrodes 70A, 70C, 70E, 70G located at the top.

[0032] Now go to Figure 3 , a method 100 according to an embodiment of the present disclosure will be described. In box 101, the method 100 may include providing an electrostatic lens, the electrostatic lens including an entrance for receiving an ion beam and an exit for delivering the ion beam to a target. The electrostatic lens may include a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of the ion beam line, wherein the first ground electrode is grounded and positioned adjacent to the exit. The electrostatic lens may also include a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and positioned adjacent to the exit.

[0033] In some embodiments, the first terminal electrode and the second terminal electrode may be maintained at a terminal voltage. In some embodiments, the voltage of the first control electrode and the second inhibition electrode may be maintained at a level less than or equal to the terminal voltage. In some embodiments, the first inhibition electrode may be positioned at a first distance from the first ground electrode, and the second inhibition electrode may be positioned at a second distance from the second ground electrode, wherein the second distance is less than the first distance.

[0034] In block 103, method 100 may include supplying voltage and current to an electrostatic lens for controlling the ion beam. In some embodiments, voltage and current may be provided to a first set of powered electrodes along a first side of the ion beam line and a second set of powered electrodes along a second side of the ion beam line, wherein the first set of powered electrodes and the second set of powered electrodes are operable to decelerate and deflect the ion beam toward a wafer through the electrostatic lens.

[0035] For convenience and clarity, terms such as "top," "bottom," "upper," "lower," "vertical," "horizontal," "lateral," and "longitudinal" will be used herein to describe the relative placement and orientation of these components and their constituent parts relative to the geometry and orientation of the components of the semiconductor manufacturing apparatus as they appear in the figures. The terminology will include the words specifically mentioned, derivatives thereof, and words of similar meaning.

[0036] As used herein, an element or operation described in the singular and beginning with the word "a or an" is understood to also include multiple elements or operations. In addition, reference to "one embodiment" of the present disclosure is not intended to be interpreted as excluding the existence of other embodiments that also incorporate the features.

[0037] According to embodiments of the present disclosure, the terms "more positive" or "less positive", "greater" or "smaller" used herein with respect to voltage (electric potential) may refer to the relative voltages of two different entities. Thus, for example, 0V is "greater" or "more positive" than -5kV, while +10kV is "greater" or more positive than 0V. In addition, -10kV is "less positive" than -5kV. The terms "less negative", negative or "more negative" may also refer to relative voltages. For example, 0kV may be said to be more negative than +5kV, while +10kV is less negative than +5kV.

[0038] In view of the foregoing, embodiments of the present invention advantageously eliminate indirect substrate contamination caused by material re-sputtered from the substrate accumulating on the electrodes of the electrostatic filter, thereby forming an additional source of contamination due to subsequent sputtering or spalling from the electrodes.

[0039] Although certain embodiments of the present disclosure have been described herein, the present disclosure is not limited thereto, as the scope of the present disclosure is as broad as the art will allow and as this specification may indicate. Therefore, the foregoing description should not be construed as limiting. Other modifications within the scope and spirit of the appended claims will occur to those skilled in the art.

Claims

1. An ion implantation system comprising: An electrostatic lens comprising an inlet for receiving an ion beam and an outlet for delivering the ion beam to a target, the electrostatic lens comprising: a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of the ion beamline, wherein the first ground electrode is grounded and positioned adjacent the exit; and a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and positioned adjacent to the exit, wherein a first distance between the first suppression electrode and the ion beam line is less than a second distance between the second suppression electrode and the ion beam line, and wherein a third distance between the first ground electrode and the ion beam line is greater than a fourth distance between the second ground electrode and the ion beam line to position the second suppression electrode below a line of sight extending between the portal and the target; and a power supply operable to supply voltage and current to the electrostatic lens for controlling the ion beam, Wherein the second suppression electrode is positioned further downstream along the ion beam line than the first suppression electrode.

2. The ion implantation system of claim 1 , further comprising a first set of powered electrodes along the first side of the ion beam line and a second set of powered electrodes along the second side of the ion beam line, wherein the first set of powered electrodes and the second set of powered electrodes are operable to decelerate and deflect the ion beam.

3. The ion implantation system of claim 1, wherein the first terminal electrode and the second terminal electrode are maintained at a terminal voltage.

4. The ion implantation system of claim 3, wherein voltages of the first suppression electrode and the second suppression electrode are less than or equal to the terminal voltage.

5. The ion implantation system of claim 1 further comprising a plasma flood gun positioned between the electrostatic lens and a wafer, wherein the plasma flood gun and the wafer are oriented at an angle relative to the electrostatic lens.

6. The ion implantation system of claim 1, wherein a first distance between the first suppression electrode and the first ground electrode is greater than a second distance between the second suppression electrode and the second ground electrode.

7. A lens comprising: a chamber having an inlet for receiving an ion beam and an outlet for delivering the ion beam to a target; a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of the ion beamline, wherein the first ground electrode is grounded and positioned proximate to the exit; and a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beamline, wherein the second ground electrode is grounded and positioned proximate the exit, and wherein the second suppression electrode is positioned further downstream along the ion beamline than the first suppression electrode, wherein a first distance between the first suppression electrode and the ion beam line is less than a second distance between the second suppression electrode and the ion beam line, wherein a third distance between the first ground electrode and the ion beam line is greater than a fourth distance between the second ground electrode and the ion beam line to position the second suppression electrode below a line of sight extending between the entrance and the target, and wherein the line of sight is defined by an inner surface of the second ground electrode.

8. The lens of claim 7, further comprising a first set of powered electrodes along the first side of the ion beam line and a second set of powered electrodes along the second side of the ion beam line, wherein the second set of powered electrodes are positioned below the line of sight.

9. The lens of claim 8, wherein the first set of powered electrodes and the second set of powered electrodes are operable to decelerate and deflect the ion beam. 10 . The lens according to claim 7 , wherein the first terminal electrode and the second terminal electrode are maintained at a terminal voltage. 11 . The lens according to claim 10 , wherein voltages of the first suppression electrode and the second suppression electrode are less than or equal to the terminal voltage.

12. The lens of claim 7, wherein a first distance between the first inhibition electrode and the first ground electrode is greater than a second distance between the second inhibition electrode and the second ground electrode.

13. An ion implantation method comprising: An electrostatic lens is provided, the electrostatic lens comprising an inlet for receiving an ion beam and an outlet for delivering the ion beam to a target, the electrostatic lens comprising: a first terminal electrode, a first suppression electrode, and a first ground electrode disposed along a first side of the ion beamline, wherein the first ground electrode is grounded and positioned adjacent the exit; and a second terminal electrode, a second suppression electrode, and a second ground electrode disposed along a second side of the ion beam line, wherein the second ground electrode is grounded and positioned adjacent to the exit, wherein a first distance between the first suppression electrode and the ion beam line is less than a second distance between the second suppression electrode and the ion beam line, and wherein a third distance between the first ground electrode and the ion beam line is greater than a fourth distance between the second ground electrode and the ion beam line to position the second suppression electrode below a line of sight extending between the portal and the target; and supplying voltage and current to the electrostatic lens for controlling the ion beam, Wherein the second suppression electrode is positioned further downstream along the ion beam line than the first suppression electrode.

14. The method according to claim 13 also includes providing the voltage and the current to a first group of powered electrodes along the first side of the ion beam line and a second group of powered electrodes along the second side of the ion beam line, wherein the first group of powered electrodes and the second group of powered electrodes are capable of operating to decelerate and deflect the ion beam. 15 . The method according to claim 13 , further comprising maintaining the first terminal electrode and the second terminal electrode at a terminal voltage. 16 . The method of claim 15 , further comprising maintaining a voltage of the first inhibition electrode and the second inhibition electrode at a level less than or equal to the terminal voltage.

17. The method according to claim 13, further comprising: positioning the first suppression electrode at a fifth distance from the first ground electrode; and The second suppression electrode is positioned a sixth distance from the second ground electrode, wherein the sixth distance is less than the fifth distance.

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