Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2003-10-08
2004-08-10
Wells, Nikita (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C315S505000, C313S359100, C313S361100
Reexamination Certificate
active
06774378
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an ion beam implanter having a plurality of electrostatic quadrupoles for controlling ion beam divergence and, more particularly, to a method of tuning the plurality of electrostatic quadrupoles of such an ion beam implanter.
BACKGROUND ART
Ion beam implanters are widely used in the process of doping semiconductor wafers. An ion beam implanter generates an ion beam comprised of desired species of positively charged ions. The ion beam impinges upon an exposed surface of a workpiece such as a semiconductor wafer, substrate or flat panel, positioned in an implantation chamber, thereby “doping” or implanting the workpiece surface with desired ions.
One type of ion beam implanter suitable for deep implantation of ions into a semiconductor wafer workpiece utilizes an radio frequency (RF) accelerator (linac) to accelerate ions to high energy levels on the order of 1 million electron volts (MeV) per charge state. Such an accelerator typically utilizes multiple resonator modules, with each module including an accelerating electrode. The RF accelerator is controlled to take into account the mass, charge and initial velocity of the ions forming the ion beam. After traversing the RF accelerator resonator modules, a focused, high energy ion beam is directed to the workpiece to be implanted. A high energy ion beam implanter having an RF accelerator is disclosed in U.S. Pat. No. 4,667,111, issued on May 19, 1987 to Glavish et al. and assigned to the assignee of the present invention. The '111 patent is hereby incorporated herein in its entirety by reference.
Both the amplitude (in kilovolts (kV)) and the frequency (in Hertz (Hz)) of the accelerating electrode output signal must be determined as operating parameters for each resonator module. Moreover, when a multiple-stage RF accelerator is utilized, the phase difference (&PHgr;) (in degrees (°)) of each accelerating electrode output signal is a third operating parameter that must be determined. The resonator modules operational parameters of amplitude, frequency and phase must be determined and implemented by the control circuitry and electronics of the ion implanter (in conjunction with a human operator of the ion implanter). This process is referred to as “tuning” the ion beam.
A method and system for determining operating parameters of the resonator modules for a multi-stage RF accelerator is disclosed in U.S. Pat. No. 6,242,747, issued on Jun. 5, 2001 to Sugitani et al. and assigned to the assignee of the present invention. The '747 patent is incorporated herein in its entirety by reference.
In a multi-stage RF accelerator or linac, the ion beam passes through a central opening of the accelerating electrodes of each of the resonator modules. Positioned on either side of an accelerating electrode and axially spaced apart from the accelerating electrode are grounded electrodes. In the two gaps between an accelerating electrode and its flanking grounded electrodes appropriate electrical fields are generated within the gaps by the accelerating electrode to accelerate the ions as they pass through the gaps. For example, as a group of positive ions pass through a gap approaching an accelerating electrode, the accelerating electrode is energized to a negative voltage to generate an axial negative electric field in the gap approaching the accelerating electrode. This negative electrical field causes the positive ions in the particle bunch to accelerate through the negative electric field toward the accelerating electrode.
As the particle bunch of positive ions pass through the accelerating electrode, the voltage of the accelerating electrode is reversed to a positive voltage thereby generating an axial positive electric field in the gap through which the ions travel as they move away from the accelerating electrode. This positive field in the second gap further accelerates the particle bunch. By appropriate choice of module dimension and frequency of electrode energization, alternate ion sources that produce light or heavy ions can be successfully accelerated along the ion beam beam path between an ion source and the implantation chamber so that sufficient energy of the ions is achieved for proper implantation depth of the ions into the workpiece.
One issue that arises in a high energy implanter is that of beam divergence or diffusion. Within each electrode gap, the axial electric field created to accelerate ions within the gap causes radial focusing (that is, narrowing) of the beam in the first half of the gap and radial defocusing (that is, widening) of the beam in the second half of the gap. Unfortunately, because the electric radial defocusing forces in the second half of the gap are stronger than the radial focusing forces in the first half of the gap, the net result is overall radial defocusing as the beam passes through each gap. One method of compensating for radial defocusing is to provide electrostatic lenses, such as electrostatic quadrupoles (“electrostatic quadrupoles”), along the beam line to provide for convergence effect on the beam. As many as twelve or more electrostatic quadrupoles may be used along the beam line and may be advantageously positioned within the RF accelerator, in front of the RF accelerator (that is, upstream of the RF accelerator resonator modules), and/or behind the RF accelerator (that is, downstream of the resonator modules).
The basic function of the electrostatic quadrupoles is to focus the beam and to transport the beam from the ion source to the workpiece with a high transmission rate. The transmission rate is defined as the ratio of the final beam current to the injection beam current. The addition of electrostatic quadrupoles, needed for ion beam convergence, complicates the tuning process, because in addition to determining the operating parameters (amplitude, frequency and phase) for the resonator modules, the ion implanter control circuitry (in conjunction with the operator) must also determine operating parameters for the electrostatic quadrupoles. An electrostatic quadrupole is energized by applying a DC voltage to the electrodes of the quadrupole so as to create a DC voltage differential across oppositely positioned electrodes of the quadrupole. Typically, in a unipolar quadrupole there are two electrodes positioned 180 degrees apart, a DC voltage is applied the one electrode while the other electrode is held at ground potential or a reference voltage thereby resulting in an applied DC voltage across the electrode pair. Thus, each quadrupole must be “tuned” by determining a magnitude of the DC voltage applied across the quadrupole electrodes such that, in combination with all of the other electrostatic quadrupoles, transmission rate is optimized, that is, the highest transmission rate is achieved while still maintaining suitable beam quality, that is, a suitable beam energy with minimum energy spread. Because of the number of electrostatic quadrupoles in a typical high energy implanter (typically 12), tuning the quadrupoles to achieve a maximum or near maximum transmission rate is problematic.
The resonator modules and electrostatic quadrupoles of present high energy ion beam implanters are typically tuned by an automatic tuning program or software that is part of the ion implanter control electronics. Such an automatic tuning program (“autotune program”) utilizes a method of tuning that comprising sequential single parameter tuning, that is, a combination of single parameter tuning steps with each tuning step optimizing or setting a single control variable, that is, determining the amplitude, frequency and phase for each of the resonator modules and determining the magnitude of applied DC voltage for a single electrostatic quadrupole. Using this sequential tuning procedure, the autotune program tunes each parameter, that is, each resonator and each quadrupole individually until a satisfactory or acceptable beam is achieved. An example of a prior art sequential tuning program is depicted in the flow chart of shown in FIG.
3
.
E
Huang Yongzhang
Rutishauser Hans J.
Wu Xiangyang
Axcelis Technologies Inc.
Watts Hoffmann Co. LPA
Wells Nikita
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