Wave transmission lines and networks – Coupling networks – Frequency domain filters utilizing only lumped parameters
Reexamination Certificate
1998-09-28
2001-07-17
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Coupling networks
Frequency domain filters utilizing only lumped parameters
C333S175000, C315S505000, C250S492210
Reexamination Certificate
active
06262638
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to high-energy ion implantation systems and more particularly to a method and device for tuning and matching a resonator coil assembly for use in such systems.
BACKGROUND OF THE INVENTION
Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. High-energy ion implanters are used for deep implants into a substrate. Such deep implants are required to create, for example, retrograde wells. Eaton GSD/HE and GSD/VHE ion implanters are examples of such high-energy implanters. These implanters can provide ion beams at energy levels up to 5 MeV (million electron volts). U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, Eaton Corporation, and describing such an high-energy ion implanter, is incorporated by reference herein as if fully set forth.
A block diagram of a typical high-energy ion implanter
10
is shown in FIG.
1
. The implanter
10
comprises three sections or subsystems: a terminal
12
including an ion source
14
powered by a high-voltage supply
16
to produce an ion beam
17
of desired current and energy; an end station
18
which contains a rotating disc
20
carrying wafers W to be implanted by the ion beam; and a beamline assembly
22
, located between the terminal
12
and the end station
18
, which contains a mass analysis magnet
24
and a radio frequency (RF) linear accelerator (linac)
26
. The beamline assembly
22
conditions the ion beam output by the terminal
12
and directs the conditioned beam toward the target wafer W. A final energy magnet (not shown in
FIG. 1
) may be positioned between the linac
26
and the rotating disc.
The mass analysis magnet
24
functions to pass only ions of an appropriate charge-to-mass ratio to the linac. The mass analysis magnet is required because the ion source
14
, in addition to generating ions of appropriate charge-to-mass ratio, also generates ions of greater or lesser charge-to-mass ratio than that desired. Ions having inappropriate charge-to-mass ratios are not suitable for implantation into the wafers W.
The ion beam
17
passes through the mass analysis magnet
24
and enters the RF linac
26
which imparts additional energy to the ion beam passing therethrough. The RF linac produces particle accelerating fields which vary periodically with time, the phase of which may be adjusted to accommodate different atomic number particles as well as particles having different speeds. The RF linac
26
comprises a series of resonator modules
30
a
through
30
n
, each of which functions to further accelerate ions beyond the energies they achieve from a previous module.
FIG. 2
shows a known type of resonator module
30
, comprising a large inductive coil L contained within a resonator cavity housing
31
(i.e., a “tank” circuit). A radio frequency (RF) signal is capacitively coupled to a high-voltage end of the inductor L via capacitor C
C
. An accelerating electrode
32
is directly coupled to the high-voltage end of the inductor L. Each accelerating electrode
32
is mounted between two grounded electrodes
34
and
36
, and separated by gaps
38
and
40
, respectively. C
S
represents the stray capacitance of the high-voltage acceleration electrode
32
to ground. R
L
represents the losses associated with the resonant circuit comprising L and C
S
in a series loop (see FIG.
3
).
Values for C
S
and L are chosen for the circuit to achieve a state of resonance so that a sinusoidal voltage of large magnitude may be achieved at the location of the accelerating electrode
32
. The accelerating electrode
32
and the ground electrodes operate in a known “push-pull” manner to accelerate the ion beam passing therethrough, which has been “bunched” into “packets”. During the negative half cycle of the RF sinusoidal electrode voltage, a positively charged ion packet is accelerated (pulled by the accelerating electrode
32
) from the first grounded electrode
34
across gap
38
. At the transition point in the sinusoidal cycle, wherein the electrode
32
is neutral, the packet drifts through the electrode
32
(also referred to as a “drift tube”) and is not accelerated.
During the positive half cycle of the RF sinusoidal electrode voltage, positively charged ion packets are further accelerated (pushed by the accelerating electrode
32
) toward the second grounded electrode
36
across gap
40
. This push-pull acceleration mechanism is repeated at subsequent resonator modules having accelerating electrodes that also oscillate at a high-voltage radio frequency, thereby further accelerating the ion beam packets by adding energy thereto. The RF phase of successive accelerating electrodes in the modules is independently adjusted to insure that each packet of ions arrives at the appropriate gap at a time in the RF cycle that will achieve maximum acceleration.
FIG. 3
shows the equivalent circuit of the resonator module
30
of FIG.
2
. The time dependent input/output variables are voltage v(t) and current i(t). By taking advantage of the duality of time and frequency domain representation (the Fourier transform), time may be eliminated as a variable and replaced with &ohgr;, the radian frequency. In the harmonic steady state of resonance, v(t) and i(t) at frequency f are linearly related by the complex impedance Z(&ohgr;), such that V=Z(&ohgr;)I, where v(t)=V sin &ohgr;t and &ohgr;=2&pgr;f.
In the circuit of
FIG. 3
, the complex impedance Z of capacitor C
S
is proportional to 1/f, with I leading V by 90°; the complex impedance Z of inductor L is proportional to f, with I lagging V by 90°; and the resistive losses R
L
are generally independent of frequency, with I and V in-phase with each other. At resonance, maximum voltage is achieved at the accelerating electrode
32
for a given input RF signal, the currents in C
S
and L cancel because they are 180° out of phase, and all power in the circuit is dissipated through resistor R
L
. To attain a resonant state, &ohgr;=2&pgr;f=(LC)
−½
. For example, in the Eaton GSD series, &ohgr;=13.56 megahertz (MHz).
To maintain a state of resonance, the product of L×C
S
must remain constant. The quality factor Q of the resonant circuit also depends upon the ratio of R
L
/X, where X=&ohgr;L, or the ratio of stored energy per cycle over dissipated energy per cycle. Accordingly, drifts in C
S
and changes in L during operation may be accommodated by altering only one of these factors, in this case L, to “tune” the resonator circuit. Also, in order to obtain maximum power out of the resonator module
30
, the impedance of the resonator circuit must “match” that of the RF input source to minimize reflection of the input signal from the circuit back into the source.
FIG. 4
shows a prior art resonator module and the mechanisms provided for matching and tuning of the resonator circuit. The tuning mechanism comprises a servomotor (not shown) which moves a stem
44
of inductor L in and out of resonator cavity housing
31
in the directions shown by arrow
46
. By moving (stretching or compressing) the inductive coil L along axis
47
, the inductance value of the inductor can be altered. A collar
48
is provided at the high-current (up to 200 amps), low-voltage end of the inductor, through which the inductor stem slides in and out. However, this tuning mechanism provided in
FIG. 4
(i) requires significant power to stretch/compress the relatively stiff inductor; (ii) causes work hardening of the inductor which results in non-uniform inductance along the length of the coil; and (iii) requires a low-impedance, high-current collar which is subject to wear and potential breakdown over time.
The prior art matching mechanism shown in
FIG. 4
is provided by the capacitor C
C
which provides the capacitive coupling of the RF signal input from connector
50
to the inductor L. As shown more clearly in
FIG. 5
, the capacitor C
C
comprises a C-shaped
Axcelis Technologies Inc.
Bettendorf Justin P.
Kastelic John A.
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