The disclosure relates generally to ion implantation apparatus and more particularly to high energy beamline ion implanters.
Ion implantation is a process of introducing dopants or impurities into a substrate via ion bombardment. Ion implantation systems may comprise an ion source and a series of beam-line components. The ion source may comprise a chamber where ions are generated. The ion source may also comprise a power source and an extraction electrode assembly disposed adjacent the chamber. The beam-line 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 for manipulating a light beam, the beam-line components can filter, focus, and manipulate ions or ion beam having particular species, shape, energy, and/or other qualities. The ion beam passes through the beam-line components and may be directed toward a substrate mounted on a platen or clamp.
Implantation apparatus 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 uses as ion acceleration stage a linear accelerator, or LINAC, where a series of electrodes arranged as tubes conduct and accelerate the ion beam to increasingly higher energy along the succession of tubes, where the electrodes receive an RF voltage signal. Known (RF) LINACs are driven by an RF voltage applied at frequencies between 13.56 MHz-120 MHz.
In known LINACs (for the purposes of brevity, the term LINAC as used herein may refer to an RF LINAC using RF signals to accelerate an ion beam) in order to reach a targeted final energy, such as one MeV, several MeV, or greater, the ion beam may be accelerated in multiple acceleration stages. Each successive stage of the LINAC may receive the ion beam at increasingly higher energy, and accelerate the ion beam to still higher energy. A given acceleration stage of a LINAC may employ a so-called double gap configuration with one RF powered electrode, or a so-called triple gap configuration, with two RF powered electrodes.
A given acceleration stage also may include a resonator, to drive the RF electrodes with an RF voltage at the chosen RF frequency. Examples of known configurations for resonators include solenoidal resonators having a solenoidal coil generally defining a circular cylindrical shape which coil is surrounded by an electrically grounded cylindrical resonator can (RF enclosure). From an electromagnetic point of view, the resonator is an RLC oscillating circuit comprising of the coil as inductive element and the resonator can as capacitive element. At resonance, the energy is transformed periodically from magnetic energy stored in the coil into electrostatic energy as a voltage difference between the powered RF electrodes. In these solenoidal configurations, an exciter coil is provided inside the resonator can but outside the resonator coil to generate an RF signal that is magnetically coupled to the resonator coil. In particular, in the resonant RF cavity, RF energy is transferred from an RF generator to the RLC oscillating circuit. For a given input RF power, the higher the shunt impedance (Zsh) of the resonator the higher the available acceleration voltage. The necessary RF energy is transferred from the RF generator to the RLC circuit by an RF exciter (exciter). In operation of the resonant cavity the exciter plays a double role: i) to match an output impedance of the RF generator (which impedance may be 50 Ω) and, ii) to maximize the power transfer from the RF generator to the RLC circuit.
Recently, so-called toroidal resonators have been proposed for use in acceleration stages, where a resonator coil defines a toroid shape and the surrounding can (cavity) has a cylindrical shape. This configuration may generate a closed magnetic field topology within the resonator. In this configuration, the placement of the exciter may need adjustment compared to known solenoidal designs, since the magnetic field is generally enclosed within the loops of the resonator coil.
With respect to these and other considerations the present disclosure is provided.
an exciter for a high frequency resonator is provided. The exciter may include an exciter coil inner portion, extending along an exciter axis, an exciter coil loop, disposed at a distal end of the exciter coil inner portion. The exciter may also include a drive mechanism, including at least a rotation component to rotate the exciter coil loop around the exciter axis.
In another embodiment, a resonator for a linear accelerator is provided. The resonator may include a toroidal resonator coil that defines a toroidal shape and an exciter, disposed at least partially within the toroidal resonator coil. The exciter may include an exciter coil inner portion, extending along an exciter axis, an exciter coil loop, disposed at a distal end of the exciter coil inner portion. The exciter may also have a drive mechanism, including at least a rotation component to rotate the exciter coil loop around the exciter axis.
In a further embodiment, a method of operating a linear accelerator is provided. The method may include sending RF power to an exciter of an RF resonator in the linear accelerator, where the RF resonator comprises a toroidal resonator coil and a resonator can, and where the exciter comprises an exciter loop that is disposed within the toroidal resonator coil. The method may further include conducting an ion beam through the linear accelerator, and rotating the exciter loop while the ion beam is conducted through the linear accelerator, wherein a power coupling between the exciter and the toroidal resonator coil is adjusted.
The drawings are not necessarily to scale. The drawings are merely representations, not intended to portray specific parameters of the disclosure. The drawings are intended to depict exemplary embodiments of the disclosure, and therefore are not be considered as limiting in scope. In the drawings, like numbering represents like elements.
An apparatus, system and method in accordance with the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, where embodiments of the system and method are shown. The system and method may be embodied in many different forms and are not be construed as being limited to the embodiments set forth herein. Instead, these embodiments are provided so this disclosure will be thorough and complete, and will fully convey the scope of the system and method to those skilled in the art.
Terms such as “top,” “bottom,” “upper,” “lower,” “vertical,” “horizontal,” “lateral,” and “longitudinal” may be used herein to describe the relative placement and orientation of these components and their constituent parts, with respect to the geometry and orientation of a component of a semiconductor manufacturing device as appearing in the figures. The terminology may include the words specifically mentioned, derivatives thereof, and words of similar import.
As used herein, an element or operation recited in the singular and proceeded with the word “a” or “an” are understood as potentially including plural elements or operations as well. Furthermore, references to “one embodiment” of the present disclosure are not intended to be interpreted as precluding the existence of additional embodiments also incorporating the recited features.
Provided herein are approaches for RF resonators, and in particular for improved high energy ion implantation systems and components, based upon a beamline architecture using linear accelerators. For brevity, an ion implantation system may also be referred to herein as an “ion implanter.” Various embodiments entail novel approaches that provide the capability of flexibly adjusting the effective drift length within acceleration stages of a linear accelerator.
The exciter shaft 17 may extend along an exciter axis, in this case defined as parallel to the Y-axis of the Cartesian coordinate system shown. The exciter coil 12 may further include an exciter loop 16, disposed at a distal end of the exciter coil inner portion 14. Thus, part of the exciter coil 12 is formed in the shaft 17, including the exciter coil inner portion 14 and conductive sleeve 20, while part of the exciter coil (exciter coil loop 16) extends beyond the exciter shaft 17.
The exciter coil loop 16 may define a circular shape that lies within a given plane, such as the X-Y plane. On a first end of the exciter coil loop 16 is connected to the distal end of the exciter coil inner portion 14, as shown, while on a second end, the exciter coil loop 16 is connected to conductive sleeve 20. This configuration allows the insulating sleeve 18 and exciter coil inner portion to be passed through a chamber wall 22 that may house the exciter coil 12, as well as associated hardware of a resonator.
As further shown in
In the embodiment of
The RF drift tube electrodes 102A are driven by a resonator 110. The resonator 110 includes an RF enclosure 112 to house a toroidal resonator coil, referred to as toroidal coil 114. The toroidal coil 114 and similar resonator coils are described in detail in the embodiments to follow. In brief, the exciter coil 12 may be arranged to receive RF power as part of an RF power delivery assembly, shown as the rf circuitry 124, including an RF generator 120 and impedance element 122. While not shown in the figures, the resonator 110 or similar resonators, as described below, may include a capacitive tuner, located externally to the toroidal coil 114 but inside of a resonator can (rf enclosure 112). In various non-limiting embodiments, the capacitive tuner may be movable, in a manner to adjust gross capacitance of the RLC circuit formed by the toroidal coil 114 and the resonator can (RF enclosure 112).
Moreover, as shown in
The exciter coil 12 and toroidal coil 114 in conjunction with the RF enclosure 112 act to generate RF voltages at the RF drift tube electrodes 102A. To obtain the relationships between the input RF power and the voltage generated on the accelerating electrodes (RF drift tube electrodes 102A), the resonant cavity including the exciter coil 12, toroidal coil 114, and the RF enclosure 112 is modelled as a lumped element circuit. Using Thevenin theorem, the RF generator circuit and resonator circuit can be transformed into a single circuit. The equivalent mutual impedance ZM can be written as
Z
M
=iωL
coil+ω2M/(Z0+iωLexcit) (1)
and similarly, the equivalent RF voltage VM is given as
V
M
=V
0
ωM/(Z0+iωLexcit) (2)
where i2=−1, ω=2πf is angular frequency, V0 and Z0 are the output voltage and impedance of the rf generator, M is mutual inductance of the exciter coil and resonator coil. As can be seen in eq. (2) the power transfer efficiency (which efficiency scales with the square of the voltage) depends on the coupling between the coils, which coupling is a function of the size, structure, physical spacing, relative location, and the properties of the environment surrounding the coils. In a simplest form, the mutual inductance for two concentric coils is given by the Maxwell formula.
M=4π√{square root over (Aa)}[(2/k−k)F−2E/k] (3)
with
k=2√{square root over (Aa)}/√{square root over ((A+a)2+s2)} (4)
where A and a are the radii of the circular coils, s the distance between their centers, and F, and E are the complete elliptic integrals of the first kind and second kind, respectively.
As coupling between an exciter coil and resonator coil depends on the amount of magnetic flux linkage between the exciter coil and resonator coil, for a given size of the resonator coil an optimum dimension of the exciter coil exists where the effect of coupling is maximum. In order to cover a wide range of frequencies, the exciter coil will have a high bandwidth of operation. Therefore, according to embodiments of the disclosure, the exciter coil 12 is designed as a low Q factor coil, meaning a low inductance coil. Thus, as shown in
where μ0 to and ϵ0 stand for magnetic permeability and dielectric permittivity of free space and ϵr for relative dielectric permittivity of the insulating sleeve material. Depending on the geometrical characteristics of the exciter a matching material can be chosen as insulator: air (ϵr=1), PTFE (ϵr=2), quartz (ϵr=3.7) , alumina (ϵr=9.8) or other ceramics. In general, in RF electronics the efficiency of power transmission from the generator to the load is characterized by a Voltage Standing Wave Ratio (VSWR), which parameter is the ratio between the amplitudes of the reflected voltage wave and forward voltage wave. As shown in
P
r
/P
f=((VSWR−1)/(VSWR+1))2 (6)
where Pr, Pf stand for the reflected and forward power, respectively. In one embodiment, by proper design of the exciter coil, the VSWR may be minimized to approach a value of 1. For the case depicted in
While the exciter coil 12 may be used to drive a resonator coil of any shape, in the present embodiments, such as in
To illustrate examples of this insertion geometry of exciter coil within a resonator coil,
Turning first to
A more advantageous insertion geometry for exciter coil 12 is depicted in
For an ideal case (no losses) the magnetic energy has been shown to convert entirely into electrostatic energy resulting in 1:1 energy conversion from the toroidal coil 114 (magnetic energy) to the accelerating ion (kinetic energy). However, in real systems there are losses limiting this energy conversion. In this case, the energy transfer is quantified by the shunt impedance (Zsh) of the resonator. For the same amount of input power, as higher Zsh as higher the voltage generated on the accelerating electrodes. Theoretical analysis shows Zsh scales with inductance of the coil as ˜L3/2 which relationship means the larger L becomes the larger is Zsh. On other hand, because the cavity forms an RLC circuit the circuit will oscillate with a certain frequency, which frequency at resonance is
f
0=1/2π√{square root over (LC)} (7)
where L is the inductance of the coil and C the capacitance of the system.
Therefore, the coil-can (enclosure) resonator system is designed to have an as-high-as-possible shunt impedance (Zsh), and simultaneously a natural resonance frequency (f0) as close as possible to the desired operating RF frequency (e.g., 13.56 MHz and 27.12 MHz). As noted above, the small departures of the resonant frequency from the operating frequency may be corrected with a capacitive tuning component (here, one possible location of a capacitive tuning component 140 is shown in the dashed lines),
As shown by Eqs (3)-(4), the mutual coupling depends on the sizes and relative location of the coils. Therefore, for a given resonator coil geometry there will be an optimal size of the inductive RF exciter, meaning the exciter coil loop 16 diameter. The electrical behavior induced by a set of exciter coils having identical characteristic impedance (Zch) but different loop radii was modelled against the same resonator coil. As can be seen in
In accordance with embodiments of the disclosure, the resonator may be firstly tuned for resonance in the absence of an ion beam. During operation due to thermal effects, the resonator frequency may drift from a designed value, requiring an operation to bring the resonator back to a resonance value. This return to resonance may be accomplished using a tuning system, including, for example, an adjustable capacitor. However, in the presence of beam, the resonator load impedance is changed as well, due to electrical resistance introduced by the beam. This impedance change will affect the power coupling, leading to a circumstance where the coupling of exciter loop to a resonator coil is not optimum. According to the present embodiments, the coupling of exciter 10 to the toroidal coil 114 may be adjusted with the provision of a movement mechanism for exciter 10, such as the drive mechanism of stage 24, discussed above. In other words, by rotating the exciter coil loop 16 around Oy axis, the coupling is readily changed, thus exposing more or less “effective” surface area, which change will maximize magnetic flux linkage between the exciter 10 and the toroidal coil 114.
As depicted in the VSWR and voltage behavior of
Ideally the center of the exciter coil loop 16 should align concentrically with the azimuthal axis of the toroid formed by a toroidal coil. However, small departures from symmetry of the two halves of the toroidal coil may a induce slight voltage imbalance on the powered drift tubes. In accordance with embodiments of the disclosure, this imbalance may be corrected by adjusting the insertion depth of the exciter coil 12. This adjusting may practically be achieved by moving the exciter coil loop 16 into the toroidal coil or withdrawing the exciter coil loop 16 from the toroidal coil to a new position, and subsequently fixing in the new position.
The ion implanter 300 may include an analyzer 310, functioning to analyze the ion beam 306 as in known apparatus, by changing the trajectory of the ion beam 306, as shown. The ion implanter 300 may also include a buncher 312, and a linear accelerator 314 (shown in the dashed line), disposed downstream of the buncher 312, where the linear accelerator 314 is arranged to accelerate the ion beam 306 to form a high energy ion beam 315, greater than the ion energy of the ion beam 306, before entering the linear accelerator 314. The buncher 312 may receive the ion beam 306 as a continuous ion beam and output the ion beam 306 as a bunched ion beam to the linear accelerator 314. The linear accelerator 314 may include a plurality of acceleration stages, represented by the resonators 110, arranged in series, as shown. In various embodiments, the ion energy of the high energy ion beam 315 may represent the final ion energy for the ion beam 306, or approximately the final ion energy. In various embodiments, the ion implanter 300 may include additional components, such as filter magnet 316, a scanner 318, collimator 320, where the general functions of the scanner 318 and collimator 320 are well known and will not be described herein in further detail. As such, a high energy ion beam, represented by the high energy ion beam 315, may be delivered to an end station 322 for processing a substrate 324. Depending on the ionization state (single, double, triple, . . . ionization) of the ionic species, non-limiting energy ranges for the high energy ion beam 315 include 500 keV-10 MeV, where the ion energy of the ion beam 306 is increased in steps through the various acceleration stages of the linear accelerator 314. In accordance with various embodiments of the disclosure, the acceleration stages of the linear accelerator 314 are powered by the resonators 110, where the design of resonators 110 may be in accordance with the embodiments of
At block 904, resonator conditions may be adjusted or set to tune the resonant frequency of the circuit formed by the RF power supply and resonator. The resonator conditions may be set by minimizing VSWR in one example. In particular, tuning may be accomplished by moving an adjustable capacitance component, such as a capacitor disposed in a resonance chamber housing the resonator coil and exciter loop.
At block 906, an ion beam is generated in a beamline ion implanter including a linear accelerator, using the current resonator circuit conditions, established at block 904.
At decision block 908, a determination is made as to whether the resonator is out of tune. For example, a relevant parameter such as the reflected power or VSWR may be monitored to see if the relevant parameter remains below a threshold. If so, the flow proceeds to block 910, where power coupling to the RF resonator is adjusted by rotating the exciter loop of the exciter. The flow then proceeds to block 912.
At block 912, beam processing is continued using the current resonator circuit conditions, where the current resonator circuit conditions may or may not represent conditions that are updated based upon the operation at block 910.
If at decision block 908, the resonator is not out of tune, the flow proceeds directly to block 912. After block 912, the flow may return to decision block 908 as beam processing continues. The flow loop between decision block 908 and block 912 may proceed while beam processing continues.
In view of the above, the present disclosure provides at least the following advantages. For one advantage, the configuration of exciter and resonator according to the present embodiments provides higher magnetic coupling efficiency and implicitly high power transfer compared with known resonators. At the same time the rotatable exciter configuration provides the advantage of another accessible tuning “knob” for adjusting the power transfer efficiency into a resonator.
While certain embodiments of the disclosure have been described herein, the disclosure is not limited thereto, as the disclosure is as broad in scope as the art will allow and the specification may be read likewise. Therefore, the above description is not to be construed as limiting. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.