Patent Number: 062427474
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 shows a block diagram representative of an ion implantation apparatus 1 utilizing an RF linac 23. A user input device 10 sends signals to a control calculation device 11 that receives (and stores) calculation codes from a storage device 18. The control calculation device sends signals to an operator display device 17. In addition, the control calculation device 11 sends signals to an amplitude control device 12, a phase control device 13, a frequency control device 14. Still further, the control calculation device sends signals to a convergence/divergence lens power supply 16 that powers a convergence/divergence lens 28. The amplitude control device 12 and the phase control device 13 send signals to the RF power supply 15 that powers the RF linac 23. The frequency control device 14 sends signals to the RF power supply 15 that powers the RF linac 23, and sends signals to the RF resonator portion 23-1 of the linac 23. In FIG. 2, a plan view of the ion implantation apparatus 1 of FIG. 1 is shown. An ion beam, represented by line 29, is extracted out of an ion source 21 and then passes through a mass analysis electromagnet 22 and is directed to an RF linac 23, which applies RF acceleration only on desired ions that pass through the mass analysis electromagnet 22. RF linac 23 can accelerate or decelerate an ion beam using the effect of RF fields, in a known manner. The accelerated or decelerated ion beam is deflected by an energy analysis electromagnet 24 and then undergoes energy analysis using a separation slit 25. Ions that pass through separation slit 25 are implanted into a wafer 27 in an implantation process chamber 26. A number of convergence/divergence lenses 28 for efficiently transporting the ion beam are placed in, in front of, or behind RF linac 23. Referring back to FIG. 1, the control system of RF linac 23 and convergence/divergence lens 28 is explained. Constituting the elements necessary for controlling RF linac 23 and lens (or lenses) 28 are: an input device 10 used for entering necessary conditions by an operator, a control calculation device 11 used for calculating values of various parameters from the entered conditions and for further controlling each constituting element, an amplitude control device 12 used for adjusting the RF amplitude, a phase control device 13 used for adjusting the RF phase, a frequency control device 14 used for adjusting the RF frequency, an RF power supply 15, a convergence/divergence lens power supply 16 used for convergence/divergence lens 28, a display device 17 used for displaying operation parameters, and a storage device 18 used for storing determined parameters. Moreover, numeric value calculation codes (programs) for calculating values of various parameters are stored in storage device 18 in advance. As previously discussed, RF linac 23 includes one or more RF resonators 23-1. Next, the operation of the ion implantation apparatus 1 is explained. An operator or a higher level computer enters into input device 10 the desired type of ions, the ionic valence value of ions, the extraction voltage of ion source 21, and the ion or ion beam energy value which is needed at the process chamber end of the machine. Using the internally stored numeric value calculation codes in parameter storage device 18, logic in the control calculation device 11 simulates the ion beam acceleration or deceleration, and the diversion/dispersion of the ion beam and calculates the RF linac operational parameters (amplitude, frequency and phase) for obtaining an optimum transport efficiency. At the same time, the control calculation device 11 calculates operational parameters (at least the electrical current or electrical voltage) of convergence/divergence lenses 28 for efficiently transporting an ion beam. The calculated various parameters are displayed on display device 17. As for the acceleration or deceleration conditions which are beyond the capability of RF linac 23, a message indicating that there are no solutions is displayed on display device 17. Among the parameters, the parameter related to the amplitude is sent from control calculation device 11 to amplitude control device 12, which adjusts the amplitude of the output of RF power supply 15. The parameter related to the phase is sent to phase control device 13, which adjusts the phase of the output of RF power supply 15. The parameter related to the frequency is sent to frequency control device 14. Frequency control device 14 controls the output frequency of RF power supply 15 while it also controls the resonance frequency of RF resonator 23-1 of RF linac 23. Control calculation device 11 also controls convergence/divergence lens power supply 16 using the calculated parameters for the convergence/divergence lenses 28. Ions which enter RF linac 23 and convergence/divergence lenses 28, whose operations are controlled as described above, are accelerated or decelerated to the desired energy and deflected by energy analysis electromagnet 24. Then, the ions undergo energy analysis using separation slit 25. The ions that pass through separation slit 25 are implanted into wafer 27 in implantation process chamber 26. The various parameters that are calculated using the numeric value calculation codes are stored in parameter storage device 18, after the calculation or the actual operation to obtain a beam. The control calculation device 11 simulates the acceleration or deceleration of an ion beam based on the numeric value calculation codes which are stored in advance, and automatically calculates at least one of the RF parameters of amplitude, frequency and phase. The control calculation device 11 can then operate the ion implantation apparatus by reading the stored parameters. Thus, thereafter, the desired ion beam can be obtained merely by reference to the stored parameters and without numeric calculations. Specific conditions (such as the geometrical dimensions, number of acceleration stages, a utilized frequency band, the maximum value of the amplitude, the number of convergence/divergence lenses, the maximum values thereof and so forth) of the RF linac and the convergence/divergence lens system of the ion implantation apparatus, can be incorporated into the numeric value calculation codes which are stored by control calculation device 11 in storage device 18. In this manner, a set of the codes can be switched for various types of RF linac systems and convergence/divergence lens systems. Next, with reference to FIG. 3, the calculation procedure based on the numeric value calculation codes is explained. Here, the explanation is performed for a case in which RF resonators 23-1 consist of the first through fourth RF resonators. The process includes nine steps, referenced herein as S1-S9. In Step S1, an operator or a higher level computer enters the calculation conditions into input device 10. Here, an ion source extraction voltage, an ion mass, and an ionic valence value of ions are entered as incoming beam conditions, and the final energy value EF of the ions or ion beam is entered as an outgoing beam condition. In Step S2, the initialization calculation is performed. In other words, a plurality of outgoing beam energy values (E1 through E8) are calculated using the predetermined eight combinations of phase and voltage for the given incoming beam conditions. Here, E1 is the theoretically the lowest energy and E8 the largest energy. The combinations of phase and voltage are determined so that the outgoing energy levels E1 through E8 are separated by approximately the same energy incremental values. In Step S3, the final energy value EF and each of the calculated outgoing beam energy values (E1 through E8) are compared. In Step S4, conversion calculation is performed. In the conversion calculation, if for example, E4&lt;EF&lt;E5, then the value of voltage or phase is altered between the conditions of E4 and E5 until an outgoing beam energy becomes equal to the desired final energy value EF. In Step S5, temporary operational parameters for the RF linac are obtained as a result of repeated calculations of Step S4. In Step S6, the optimization of the bunching phase (first resonator) of the linac is performed. In other words, using the temporary parameters as the initial set, the phases of the resonance frequencies of the second through fourth RF resonators are varied until a phase combination which maximizes the transport efficiency of RF linac 23 is found. In Step S7, RF linac operational parameters are obtained as the result of Step S6. In Step S8, optimization for convergence/divergence lenses 28 is performed. In other words, simulation for the ion beam is performed by varying the parameters of convergence/divergence lenses 28 against the RF parameters of RF linac 23 which are obtained in the above step. The simulation includes the lateral spread of the ion beam. Thus, the strength of convergence/divergence lenses 28 for the maximum transport efficiency is obtained. In the final step S9, the final parameters are obtained. This is done by combining the RF parameters with the parameters for the convergence divergence lenses 28. As previously discussed, in the prior art, parameters are determined within an ion implantation apparatus and the determination provides analytical solutions (in other words, the solution of equations). Conversely, the most prominent feature of the present invention lies in the improvement by which numeric value calculation codes have been developed so that they can be applied to an RF acceleration system or convergence lens system for which analytical solutions cannot be obtained. Simulation utilizing numerical calculation is performed within an ion implantation apparatus and thereby parameters can be automatically determined. Acceleration parameters of an RF system or parameters of convergence/divergence lenses for totally new acceleration conditions were conventionally obtained by expending a very large amount of effort and time through a procedure such as the one illustrated in FIG. 6. The present inventions allow such parameters to be automatically determined by merely an operator or a higher level computer entering acceleration conditions (e.g., ionic valence value of ions, a desired energy value, etc.). FIG. 4 briefly illustrates the entire procedure. An operator enters the acceleration conditions and a final energy value into the input device 10, and the control calculation device 11 determines the optimum solution (by numeric simulation), and determines the linac and convergence/divergence lens operational parameters. In other words, the operation can now be performed with the same ease as the operation performed for a prior art ion implantation apparatus that accelerates ions utilizing an electrostatic field. Thus, regarding the process to determine a new set of linac operational parameters, there are advantages. The time required to determine the parameters is drastically reduced (approximately one minute according to the experiment results.) Effort by an operator to determine parameters is almost eliminated. Optimum parameters can be determined without iteration by trial-and-error. The quality of determined parameters does not depend on the skill of an operator and hence, is reproducible. Even when a higher level computer enters acceleration conditions or a final energy value, the apparatus of the present invention can automatically determine the parameters. Hence, it is possible to achieve completely automatic operation of the apparatus. As explained hereinabove, according to the present invention, operating conditions of an ion implantation apparatus that utilizes an RF acceleration method can be determined with ease in a short period of time. Moreover, an ion beam having any energy value can be obtained in a short period of time. Accordingly, a preferred embodiment has been described for a method and system for optimizing linac operational parameters in an ion implantation apparatus. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented with respect to the foregoing description without departing from the scope of the invention as defined by the following claims and their equivalents.