Linear accelerator, ion implanter, and method of operating a linear accelerator

Abstract

A linear accelerator, an ion implanter, and a method of operating a linear accelerator are provided. The linear accelerator includes a radio frequency power supply assembly arranged to output a radio frequency signal and a drift tube assembly arranged to emit an ion beam and coupled to the radio frequency power supply assembly. The drift tube assembly can include a first ground electrode, an AC drift tube assembly disposed downstream of the first ground electrode, and a second ground electrode disposed downstream of the AC drift tube assembly, wherein the AC drift tube assembly includes at least one variable length AC drift tube.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to ion implantation apparatuses, and more particularly to a linear accelerator, an ion implanter, and a method of operating a linear accelerator. BACKGROUND

[0002] Ion implantation is a process by which dopants or impurities are introduced into a substrate by bombardment. Ion implantation systems can include an ion source and a series of beamline components. The ion source can include a chamber in which ions are generated. The ion source can also include a power supply and an extraction electrode assembly disposed proximate the chamber. The beamline components can 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 beam of light, the beamline components can filter, focus, and manipulate ions or ion beams having a particular species, shape, energy, and / or other qualities. The ion beam passes through the beamline components and can be directed toward a substrate mounted on a platen or chuck.

[0003] Implantation apparatuses capable of generating ion energies of approximately 1 MeV or greater are often referred to as high-energy ion implanters or high-energy ion implantation systems. One type of high-energy ion implanter is referred to as a linear accelerator (or LINAC), in which a series of electrodes arranged in a tube conduct an ion beam along a succession of tubes and accelerate the ion beam to increasingly higher energies, with the electrodes receiving an alternating current (AC) voltage signal. Known (radio frequency, RF) LINACs are driven by a radio frequency voltage supplied at 13.56 MHz to 120 MHz.

[0004] One issue with the operation of RF LINAC ion implanters is that the acceleration stages are arranged to accelerate ions having a particular mass / charge (m / q) ratio such that a maximum amount of ions can be conducted through the acceleration stage. Specifically, to maximize the acceleration of ions through a given acceleration stage, the length of a given AC drift tube can be optimized according to the speed at which ions enter the AC drift tube. In addition to the factor of ion energy entering the drift tube, this length will be determined by the m / q ratio. Thus, an RF LINAC having an AC drift tube sized to optimally accelerate a relatively lower mass species will not optimally accelerate a relatively higher mass species. The present disclosure is provided for these and other considerations. SUMMARY

[0005] In one embodiment, an apparatus is provided having a radio frequency power supply assembly arranged to output a radio frequency signal and a drift tube assembly arranged to emit an ion beam and coupled to the radio frequency power supply assembly. The drift tube assembly can include a first ground electrode, an AC drift tube assembly disposed downstream of the first ground electrode, and a second ground electrode disposed downstream of the AC drift tube assembly, where the AC drift tube assembly includes at least one variable length AC drift tube.

[0006] In another embodiment, an ion implanter can include an ion source to generate an ion beam, a buncher to output the ion beam in a bunched ion beam form, and a linear accelerator to transport and accelerate the bunched ion beam, the linear accelerator including a plurality of acceleration stages. In this regard, a given acceleration stage of the plurality of acceleration stages can include a radio frequency power supply assembly arranged to output a radio frequency signal and a drift tube assembly arranged to emit an ion beam and coupled to the radio frequency power supply assembly. The drift tube assembly can include a first ground electrode, an AC drift tube assembly disposed downstream of the first ground electrode, and a second ground electrode disposed downstream of the AC drift tube assembly, where the AC drift tube assembly includes at least one variable length AC drift tube.

[0007] In yet another embodiment, a method of operating a linear accelerator can include generating a first pulsed ion beam including a first m / q ratio and conducting the first pulsed ion beam through a variable length AC drift tube assembly arranged in a first configuration having a first drift tube coupler length. The method can include generating a second pulsed ion beam including a second m / q ratio different than the first m / q ratio and conducting the second pulsed ion beam through the variable length AC drift tube assembly arranged in a second configuration having a second drift tube coupler length different than the first drift tube coupler length. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1A An exemplary apparatus in a first configuration is shown in accordance with an embodiment of the present disclosure.

[0009] Figure 1B An exemplary apparatus in a second configuration is shown in accordance with an embodiment of the present disclosure. Figure 1A The exemplary apparatus shown.

[0010] Figure 2A Another exemplary apparatus in a first configuration is shown in accordance with an embodiment of the present disclosure.

[0011] Figure 2B An exemplary apparatus in a second configuration is shown in accordance with an embodiment of the present disclosure. Figure 2AAn exemplary apparatus is shown.

[0012] Figure 3 An exemplary ion implantation system is shown in accordance with embodiments of the present disclosure.

[0013] Figure 4 Electrical characteristics are shown as a function of position of different drift tube assemblies.

[0014] Figure 5A Another exemplary apparatus is shown in accordance with embodiments of the present disclosure.

[0015] Figure 5B Variations of the apparatus are shown. Figure 5A Drift tube details of variations of the apparatus are shown.

[0016] Figure 5C A table is presented showing ideal drift tube lengths for three different ion species (amu) and two different radio frequency voltage frequencies as a function of ion energy.

[0017] Figure 6 An exemplary process flow is presented.

[0018] The drawings are not necessarily drawn to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the disclosure. The drawings are intended to depict only typical embodiments of the disclosure, and therefore should not be considered as limiting the scope. In the drawings, like numbering represents like elements. DETAILED DESCRIPTION

[0019] In the following description, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration embodiments of the systems and methods in accordance with the present disclosure. The systems and methods can be practiced in many different forms and should not be construed as limited to the embodiments set forth 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.

[0020] In this document, terms such as "top," "bottom," "upper," "lower," "vertical," "horizontal," "lateral," and "longitudinal" can be used to describe the relative placement and orientation of these components and their constituent parts appearing in the figures with respect to the geometry and orientation of components of a semiconductor manufacturing device. The terms can include the specifically mentioned word, its derivatives, and words of similar import.

[0021] Elements or operations described herein in the singular tense and preceded with the word "a" or "an" are understood to potentially include a plurality of elements or operations. Furthermore, reference to "one embodiment" of the disclosure is not intended to be interpreted as excluding the presence of additional embodiments that also incorporate the described feature.

[0022] Methods for improved high-energy ion implantation systems and components based on beamline architectures, and specifically ion implanters based on linear accelerators, are provided herein. For brevity, ion implantation systems can also be referred to herein as "ion implanters." Various embodiments require novel methods that provide the ability to flexibly adjust the effective drift length within the acceleration stages of a linear accelerator.

[0023] Figure 1A An exemplary apparatus in a first configuration is shown in accordance with embodiments of the disclosure. Apparatus 100 represents a drift tube assembly 102 and associated components for accelerating an ion beam in an acceleration stage of a linear accelerator. Specifically, apparatus 100 defines a dual gap acceleration stage arrangement in which an ion beam is conducted through two acceleration gaps within apparatus 100. Apparatus 100 can include a radio frequency power supply assembly 110 that drives various components of drift tube assembly 102 using radio frequency signals. Radio frequency power supply assembly 110 is shown schematically without detail and can include power supplies, resonators, networks, and other known components. Drift tube assembly 102 is arranged as a series of hollow tubes through which an ion beam is conducted. The ion beam is accelerated as it passes through gaps G1 and G2.

[0024] As Figure 1A As shown in FIG. 1, drift tube assembly 102 can include a first grounded electrode 104, an AC drift tube assembly 108 disposed downstream of first grounded electrode 104, and a second grounded electrode 106 disposed downstream of AC drift tube assembly 108. As discussed in detail below, the AC drift tube assembly includes a variable length AC drift tube coupler formed by a first component 108A and a second component 108B. As such, in this and other embodiments that follow, such a configuration can be referred to as an AC drift tube coupler, where the configuration is formed by two separate components that are biased using the same radio frequency signal of the same phase. In other words, these components are electrically coupled to radio frequency power supply assembly 110 to simultaneously receive the radio frequency signal of the same phase, so the electrostatic potential at first component 108A and second component 108B is always the same. As a result, there is no electrostatic potential and no electric field between first component 108A and second component 108B.

[0025] According to various embodiments, the first component 108A and the second component 108B can be moved relative to each other along the direction of propagation of the ion beam (meaning along the drift tube axis (shown in dashed line)). Thus, since the value of the length Dl can be varied, the first component 108A and the second component 108B can be considered to form a variable length AC drift tube coupler. Figure 1B An AC drift tube assembly 108 is shown in a second configuration, where the value of Dl has been increased relative to the Figure 1A

[0026] Varying the length of the AC drift tube coupler changes the distance that the ion packets spend in the field-free region. Thus, although formed of two separate components, such an AC drift tube coupler forms a virtual AC drift tube having a variable length. Since, for a given ion energy, ions having different m / q ratios can enter the upstream side of the AC drift tube assembly 108 at different velocities, the ability to vary the length of the AC drift tube assembly enables the length Dl to be customized in such a way that the phase of the radio frequency signal seen by the ion beam 101 as it exits the AC drift tube assembly 108 is optimized for the best acceleration of the ion beam 101. In other words, for a given radio frequency frequency, by varying the length of the AC drift tube assembly 108 to maintain the flight time of the ion packets at a relatively constant value, the phase of the radio frequency signal at the exit (downstream side) of the AC drift tube assembly 108 can remain relatively constant, independent of the m / q ratio. Such a situation is shown in Figure 1A where the ion beam 101 enters the drift tube assembly 102 as a bunched ion beam in a series of packets at a given velocity. The ion beam 101 can have a first mass, while Figure 1B The ion beam 101 in the scenario shown has a second mass, such that the ion beam 101 has a higher velocity. Thus, the AC drift tube coupler (meaning the AC drift tube assembly 108) can be lengthened to account for the higher velocity of the ion beam 101, such that the individual ion packets of the ion beam 101 exit the AC drift tube assembly 108 at the appropriate time for optimal acceleration according to the applied radio frequency signal.

[0027] In the particular example shown in Figure 1A the AC drift tube assembly 108 includes a single AC drift tube coupler, where the first gap (gap G1) is between the first grounded electrode 104 and the AC drift tube assembly 108, and where the second gap (gap G2) is between the AC drift tube assembly 108 and the second grounded electrode 106.

[0028] In the particular example shown in Figure 1A and Figure 1B ​In different variations of the illustrated embodiment, the drift tube assembly 102 can be arranged to fix the gap G1 and the gap G2 at constant values. In other words, the first ground electrode 104 and the first component 108A can be moved in unison with respect to each other relative to the second component 108B such that the gap G1 does not change when D1 changes. Likewise, the second ground electrode 106 and the second component 108B can be moved in unison with respect to each other relative to the first component 108A such that the gap G2 does not change when D1 changes. In some embodiments, only the gap G1 can remain fixed, while in other embodiments only the gap G2 can remain fixed, depending on the required flexibility of ion processing and design complexity issues.

[0029] In other embodiments of the disclosure, the AC drift tube assembly can be formed by a first AC drift tube coupler and a second AC drift tube coupler. Such an arrangement is illustrated in Figure 2A The device 200 defines a triple gap configuration including a first gap (shown as gap G1) between the first ground electrode 104 and the first AC drift tube coupler 202, a second gap (shown as gap G2) between the first AC drift tube coupler 202 and the second AC drift tube coupler 204, and a third gap (shown as gap G3) between the second AC drift tube coupler 204 and the second ground electrode 106.

[0030] In known triple gap accelerator configurations, two different AC drift tubes can be coupled to opposite ends of a resonator that supplies a radio frequency voltage signal at a given frequency. In this way, a first voltage signal delivered to a first AC drift tube can be 180 degrees out of phase with a second voltage signal simultaneously delivered to a second AC drift tube. Figure 2A and Figure 2B The illustrated device 200 can operate under similar principles, where the assembly 210A delivers a voltage signal (shown as +V) that is 180 degrees out of phase with a voltage signal (shown as -V) delivered by the second assembly 210B to the second AC drift tube coupler 204. For example, the first AC drift tube coupler 202 can be coupled to one end of a resonator, while the second AC drift tube coupler 204 is coupled to an opposite end of the resonator in a manner that delivers a received radio frequency voltage signal that is 180 degrees out of phase between the two AC drift tube couplers.

[0031] According to various embodiments, the first component 202A and the second component 202B of the first AC drift tube coupler 202 can be moved relative to each other along a direction of propagation of the ion beam (i.e., along a drift tube axis (shown in dashed line)). Thus, the first component 202A and the second component 202B can be considered to form a variable length AC drift tube coupler since the value of the length D1 can vary. Figure 2BThe AC drift tube assembly 208 is shown in a second configuration in which the value of Dl has increased.

[0032] Instead of, or in addition to, the first and second components 202A, 202B being movable relative to each other, according to various further embodiments, the first and second components 204A, 204B of the second AC drift tube coupler 204 can be movable relative to each other along the drift tube axis. Figure 2B The AC drift tube assembly 208 is also shown in a second configuration in which the value of D2 has also increased.

[0033] In Figure 2A And Figure 2B In various variations of the embodiments shown, the drift tube assembly 208 can be arranged to fix the gap Gl, the gap G2, the gap G3, or any combination of these gaps at a constant value. In other words, the first ground electrode 104 and the first component 202A can be moved in unison relative to each other and relative to any other components of the drift tube assembly 208 such that the gap Gl does not change when Dl, D2, or both Dl and D2 change. Likewise, the second ground electrode 106 and the second component 204B can be moved in unison relative to each other and relative to the first component 108A such that the gap G3 does not change when D2 changes. In some embodiments, the second component 202B and the first component 204A can be mechanically coupled to each other in a manner such that the gap G2 does not change when Dl and / or D2 change. In some embodiments, only the gap Gl can remain fixed, while in other embodiments only the gap G2 can remain fixed, or only the gap G3 can remain fixed, or only the gap Gl and the gap G2 can remain fixed, or only the gap Gl and the gap G3 can remain fixed, or only the gap G2 and the gap G3 can remain fixed, depending on the required flexibility of ion processing and design complexity issues.

[0034] Figure 3 A schematic diagram of an apparatus according to embodiments of the present disclosure is depicted. Ion implanter 300 includes acceleration stages 314-A, 314-B of a LINAC (shown as linear accelerator 314). Ion implanter 300 can represent a beamline ion implanter, some elements of which are not shown for clarity of illustration. As is known in the art, ion implanter 300 can include an ion source 302 and a gas box 307. Ion source 302 can include an extraction system including an extraction assembly and a filter (not shown) to generate an ion beam 306 at a first energy. An example of a suitable ion energy for the first ion energy is in the range from 5 keV to 100 keV, although embodiments are not limited in this context. To form a high energy ion beam, ion implanter 300 includes various additional components for accelerating ion beam 306.

[0035] As shown, ion implanter 300 can include an analyzer 310 for analyzing ion beam 306 as in known devices by changing the trajectory of ion beam 306. Ion implanter 300 can also include a buncher 312 and a linear accelerator 314 (shown in dashed lines) disposed downstream of buncher 312, where linear accelerator 314 is arranged to accelerate ion beam 306 to form a high energy ion beam 315 of greater ion energy than ion beam 306 prior to ion beam 306 entering linear accelerator 314. Buncher 312 can receive ion beam 306 as a continuous ion beam and output ion beam 306 as a bunched ion beam to linear accelerator 314. As shown, linear accelerator 314 can include a plurality of acceleration stages (314-A to 314-N (not shown)) arranged in series. In various embodiments, the ion energy of high energy ion beam 315 can represent a final ion energy or an approximate final ion energy of ion beam 306. In various embodiments, ion implanter 300 can include additional components, such as a filter magnet 316, a scanner 318, a collimator 320, where the general functions of scanner 318 and collimator 320 are well known and will not be further elaborated upon herein. In this way, high energy ion beam represented by high energy ion beam 315 can be delivered to a terminal station 322 for processing a substrate 324. Non-limiting energy ranges for high energy ion beam 315 include 500 keV to 10 MeV, where the ion energy of ion beam 306 is increased stepwise by each acceleration stage of linear accelerator 314. According to various embodiments of the present disclosure, one or more of the acceleration stages of linear accelerator 314 can include a drift tube assembly having at least one variable length AC drift tube coupler, as elaborated upon with respect to the embodiment shown in FIG. 3. An advantage provided by ion implanter 300 is that the length of the AC drift tube coupler within linear accelerator 314 can be adjusted according to different ion species having different m / q ratios, and thus have different velocities when transmitted through the acceleration stages of linear accelerator 314. Figures 1A to 2B An advantage provided by ion implanter 300 is that the length of the AC drift tube coupler within linear accelerator 314 can be adjusted according to different ion species having different m / q ratios, and thus have different velocities when transmitted through the acceleration stages of linear accelerator 314.

[0036] Figure 4 Simulated electrical characteristics are shown as a function of position of different drift tube assemblies. In Figure 4In the diagram, the parameter δ(delta)t / q is plotted as a function of m / q, where δt / q represents the increase in ion energy generated by a given AC drift tube assembly within a given acceleration stage. The simulated energy increase is based on the application of an RF signal with an amplitude of 80 kV. The inner curve (the thicker line) represents the behavior of the fixed triple-gap drift tube assembly, while the middle curve represents the characteristics of the fixed double-gap drift tube assembly. The fixed triple-gap drift tube assembly produces a relatively constant peak value between 15 m / q and 20 m / q, representing an acceleration of 250 keV. The fixed double-gap configuration produces a relatively constant peak value over a wider range (between 12 m / q and 17 m / q), with less drop-off at higher or lower values. However, the peak acceleration is only 125 keV. The variable-length triple-gap drift tube configuration according to this embodiment is represented by the outer curve, where the length can be varied. In the latter configuration, the value of δt / q reaches a peak of 250 keV and remains constant between 9 amu / q and 29 amu / q, thus demonstrating a wide range of masses that can be maximized for acceleration.

[0037] Figure 5A Another exemplary apparatus according to an embodiment of the present disclosure is shown. The apparatus 500 is provided with a drift tube assembly 502 having a triple-gap drift tube configuration (shown as gap G1, gap G2, and gap G3), as described above for... Figure 2A and Figure 2B A general explanation. Figure 5B Show Figure 5A Details of a variant drift tube in the illustrated device. Device 500 includes support structures 504, 506, and 508, which are disposed along a common support member 501. Support structure 504 includes a first arm 504-A and a second arm 504-B. The first arm 504-A is mechanically coupled to a first ground drift tube 512, while the second arm 504-B is mechanically coupled to a first component 516-A of a first AC drift tube coupler 516. In this manner, the first ground drift tube 512 is mechanically rigidly coupled to the first component 516-A.

[0038] The support structure 506 includes a first arm 506-A and a second arm 506-B. The first arm 506-A is mechanically coupled to the second component 518-B of the second AC drift tube coupler 518, while the second arm 506-B is mechanically coupled to the second ground drift tube 514. In this manner, the second ground drift tube 514 is mechanically and rigidly coupled to the second component 518-B.

[0039] The support structure 506 includes a first arm 506-A and a second arm 506-B. The first arm 506-A is mechanically coupled to the first component 518-A of the second AC drift tube coupler 518, while the second arm 506-B is mechanically coupled to the second grounded drift tube 514. In this manner, the second grounded drift tube 514 is mechanically rigidly coupled to the first component 518-A.

[0040] The support structure 508 includes a first arm 508-A and a second arm 508-B. The first arm 508-A is mechanically coupled to the second component 516-B of the first AC drift tube coupler 516, while the second arm 508-B is mechanically coupled to the second component 518-B of the second AC drift tube coupler 518. In this manner, the second component 516-B of the first AC drift tube coupler 516 is mechanically coupled to the second component 518-B of the second AC drift tube coupler 518.

[0041] According to embodiments of the present disclosure, the support structure 504 and / or the support structure 506 can be moved in a sliding manner (in the direction of the arrows) relative to the support structure 508. Thus, moving the support structure 504 relative to the support structure 508 will adjust the value of the length of the first AC drift tube coupler 516 (shown as Dl). Likewise, moving the support structure 506 relative to the support structure 508 will adjust the value of the length of the second AC drift tube coupler 518 (shown as D2). At the same time, the aforementioned mechanical couplings of the different components of the drift tube assembly to the support structures will ensure that the value of the gap Gl, the value of the gap G2, and the value of the gap G3 do not change when either or both of the support structure 504 and the support structure 506 are moved relative to the support structure 508.

[0042] To illustrate the advantages of forming a variable length effective drift tube, Figure 5C A list of ideal tube lengths for different ion species (hydrogen, boron, and phosphorus) is shown as a function of ion energy up to 10 MeV. This length is the distance that a given ion travels in the time corresponding to an AC voltage of 180° or π radians. The tube length is also a function of the signal frequency, and typically 13.56 MHz is used for linear accelerators and a frequency of 40 MHz. It should be noted that for each ion energy, and at the two frequencies shown, a switch from phosphorus ions to boron ions corresponds to an increase in the ideal drift tube length of more than a factor of 2. In addition, the ideal drift tube length increases as the ion energy within a given species increases. For example, to accelerate boron ions using a 40 MHz radio frequency resonator, increasing the ion energy from 500 keV to 2 MeV increases the ideal drift tube length from 3.7 cm to 7.4 cm.

[0043] In view of the above considerations, the variable length drift tube assembly disclosed herein provides a useful method to handle a variety of ions with ideal drift tube lengths.

[0044] Figure 6 An exemplary process flow 600 is illustrated. At block 602, a first ion beam having a first m / q ratio is generated as a pulsed ion beam. The pulsed ion beam can be generated by a buncher disposed at the beginning of a linear accelerator to receive a continuous ion beam and output a bunched ion beam in a series of packets or pulses.

[0045] At block 604, the first ion beam is conducted through the variable length AC drift tube assembly when arranged in a first configuration having a first drift tube coupler length. The variable length drift tube assembly can be a double gap configuration with one variable length drift tube coupler or a triple gap configuration with one or two variable length drift tube couplers.

[0046] At block 606, a second ion beam having a second m / q ratio is generated as a pulsed ion beam. The pulsed ion beam can be generated by a buncher disposed at the beginning of a linear accelerator to receive a continuous ion beam and output a bunched ion beam in a series of packets or pulses.

[0047] At block 608, the second ion beam is conducted through the variable length AC drift tube assembly in a second configuration having a second drift tube coupler length. The variable length drift tube assembly can be a double gap configuration with one variable length drift tube coupler or a triple gap configuration with one or two variable length drift tube couplers. In the case of a double gap configuration, the one variable length drift tube coupler is adjusted to define the second drift tube coupler length. Also, in the case of a triple gap configuration, one variable length drift tube coupler is adjusted to define the second drift tube coupler length. Additionally, the second variable length drift tube coupler can or can not be adjusted to define a fourth length that is different than the third length in block 604.

[0048] In view of the above, the present disclosure provides at least the following advantages. As a first advantage, the variable length drift tube assembly of the present embodiments provides the ability to match the optimal drift tube length for a particular m / q for any given ion setting. A second advantage is that ion implanters with the variable length drift tube assembly do not need to be dedicated to only one ion species with a particular m / q ratio, and can accordingly reduce hardware costs and provide increased flexibility for implantation.

[0049] While certain embodiments of the present disclosure have been set forth in detail above, other embodiments are also within the scope of the present disclosure. The scope of the present disclosure is to be limited only by the appended claims, and the full scope of equivalents thereof. Therefore, the foregoing description is not to be considered as limiting the scope of the present disclosure. Other modifications can occur to those skilled in the art upon reading the description of the preferred embodiments. Thus, the scope of the present disclosure should be determined by the appended claims and equivalents thereof.

Claims

1. A linear accelerator comprising: a radio frequency power supply assembly arranged to output a radio frequency signal; and a drift tube assembly arranged to emit an ion beam and coupled to the radio frequency power supply assembly, the drift tube assembly comprising: a first ground electrode; an alternating current drift tube assembly disposed downstream of the first ground electrode; and a second ground electrode disposed downstream of the alternating current drift tube assembly, wherein the alternating current drift tube assembly comprises at least one variable length alternating current drift tube.

2. The linear accelerator of claim 1, wherein the alternating current drift tube assembly comprises a single alternating current drift tube coupler and defines a double gap configuration comprising a first gap between the first ground electrode and the alternating current drift tube assembly and a second gap between the alternating current drift tube assembly and the second ground electrode.

3. The linear accelerator of claim 2, wherein the single alternating current drift tube coupler comprises a first part and a second part, the second part being slidably movable relative to the first part along a drift tube axis.

4. The linear accelerator of claim 3, wherein the single alternating current drift tube coupler and the first ground electrode are co-movably with each other, wherein a first value of the first gap and a second value of the second gap are invariant when the first part is slidably moved relative to the second part along the drift tube axis.

5. The linear accelerator of claim 1, wherein the alternating current drift tube assembly comprises a first alternating current drift tube coupler and a second alternating current drift tube coupler and defines a triple gap configuration comprising a first gap between the first ground electrode and the first alternating current drift tube coupler, a second gap between the first alternating current drift tube coupler and the second alternating current drift tube coupler, and a third gap between the second alternating current drift tube coupler and the second ground electrode.

6. The linear accelerator of claim 5, wherein the first alternating current drift tube coupler comprises a first part and a second part, the second part being slidably movable relative to the first part along a drift tube axis.

7. The linear accelerator of claim 6, wherein the first part of the first alternating current drift tube coupler and the first ground electrode are co-movably with each other, wherein a first value of the first gap is invariant when the first part is slidably moved relative to the second part along the drift tube axis.

8. The linear accelerator of claim 7, wherein the second alternating current drift tube coupler comprises a third part and a fourth part, the fourth part being slidably movable relative to the third part along the drift tube axis, wherein a third value of the third gap is invariant when the third part is slidably moved relative to the fourth part along the drift tube axis. ​ 9. The linear accelerator of claim 8, wherein a second value of the second gap is invariant when the first component is moved in a sliding manner relative to the second component of the first RF drift tube coupler or when the third component is moved in a sliding manner relative to the fourth component of the second RF drift tube coupler.

10. An ion implanter, comprising: an ion source to generate an ion beam; a buncher to output the ion beam in a bunched ion beam form; and a linear accelerator to transport and accelerate the bunched ion beam, the linear accelerator comprising a plurality of acceleration stages, wherein a given acceleration stage of the plurality of acceleration stages comprises: a radio frequency power supply assembly arranged to output a radio frequency signal; and a drift tube assembly arranged to emit an ion beam and coupled to the radio frequency power supply assembly, the drift tube assembly comprising: a first ground electrode; an RF drift tube assembly disposed downstream of the first ground electrode; and a second ground electrode disposed downstream of the RF drift tube assembly, wherein the RF drift tube assembly comprises at least one variable length RF drift tube.

11. The ion implanter of claim 10, wherein the RF drift tube assembly comprises a single RF drift tube coupler and defines a double gap configuration, the double gap configuration comprising a first gap between the first ground electrode and the RF drift tube assembly and a second gap between the RF drift tube assembly and the second ground electrode.

12. The ion implanter of claim 11, wherein the single RF drift tube coupler comprises a first component and a second component, the second component being moveable in a sliding manner relative to the first component along a drift tube axis.

13. The ion implanter of claim 12, wherein the single RF drift tube coupler and the first ground electrode are moveable in unison with one another, wherein a first value of the first gap and a second value of the second gap are invariant when the first component is moved in a sliding manner relative to the second component along the drift tube axis.

14. The ion implanter of claim 10, wherein the RF drift tube assembly comprises a first RF drift tube coupler and a second RF drift tube coupler and defines a triple gap configuration, the triple gap configuration comprising a first gap between the first ground electrode and the first RF drift tube coupler, a second gap between the first RF drift tube coupler and the second RF drift tube coupler, and a third gap between the second RF drift tube coupler and the second ground electrode.

15. The ion implanter of claim 14, wherein the first RF drift tube coupler comprises a first component and a second component, the second component being moveable in a sliding manner relative to the first component along a drift tube axis. ​ 16. The ion implanter of claim 15, wherein the first component of the first RF drift tube coupler and the first ground electrode are movable in unison with one another, wherein a first value of the first gap is invariant as the first component is slidably moved along the drift tube axis relative to the second component.

17. A method of operating a linear accelerator, comprising: generating a first pulsed ion beam, the first pulsed ion beam comprising a first mass-to-charge ratio; conducting the first pulsed ion beam through a variable length RF drift tube assembly arranged in a first configuration, the first configuration having a first drift tube coupler length; generating a second pulsed ion beam, the second pulsed ion beam comprising a second mass-to-charge ratio different from the first mass-to-charge ratio; and conducting the second pulsed ion beam through the variable length RF drift tube assembly arranged in a second configuration, the second configuration having a second drift tube coupler length different from the first drift tube coupler length.

18. The method of claim 17, wherein the variable length RF drift tube assembly comprises only one variable length RF drift tube coupler, defining a double gap configuration.

19. The method of claim 17, wherein the variable length RF drift tube assembly comprises two variable length RF drift tube couplers, defining a triple gap configuration, wherein a length of at least one of the two variable length RF drift tube couplers is adjusted between the first configuration and the second configuration. ​

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