Abstract:
A method of monitoring particle beam current in an ion implanter in which the ion beam is analyzed to separate it into a separate sub-beam for each ion charge state. At least one sub-beam, having a charge state different from the desired charge state, is intercepted, and the current of the intercepted sub-beam is measured. This current is useful as an estimate of the current of the desired sub-beam which is used for the implantation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to implantation of ions into silicon wafers and to the measurement of ion dose during an ion implantation process. 
     2. Description of the Related Art 
     In a conventional ion implanter dose control system, the main ion beam is sampled either periodically or continuously during wafer implantation to provide a measure of the ion particle current (particles/sec.). 
     The ion beam produced by an ion implanter is subject to both long and short term variations in intensity. Since the beam is generally much smaller than the wafer target, providing a spatially uniform dose on the wafer requires that either the beam is translated across the wafer or that the wafer is translated through the beam. It is common practice to modulate the translational velocity by a factor derived from the beam particle current to compensate for variations in beam current during an implant. 
     Before the translational velocity correction can be determined, the main (or implant) beam current must be known. There are many ways of satisfying this requirement. Two of the commonest are: 
     1) Multiplexing the beam (either spatially or temporally) to provide a sample of its intensity during the implant. 
     2) Collecting and measuring the current striking the wafer directly. 
     Both of these techniques suffer from the disadvantage that they measure the beam current in the proximity of the wafer. The wafer may induce local environmental perturbations which adversely affect the accuracy of the beam current measurement, resulting in dose errors. For example, if the wafer outgasses under the impact of the beam, the resultant vacuum degradation will cause undesired charge exchange of the beam, leading to an error in the beam current measurement. 
     SUMMARY OF THE INVENTION 
     The invention provides a novel means of measuring and ensuring that silicon wafers receive the correct ion dose (particles/cm 2 ) during an ion implantation process. 
     In a conventional ion implanter dose control system, the main ion beam is sampled periodically during wafer implantation to provide an estimate of the ion particle current (particles/sec.). 
     In the present invention, the particle current of the main ion beam is estimated by measuring the ion current of an alternate (unused, mass resolved) ion beam which possesses a charge state which is undesirable for wafer implantation. Since the alternate (or monitor) beam is not used for wafer implantation, it can be monitored continuously and in a region of stable vacuum quality where ion currents can be measured most accurately. The ion current in the monitor beam is proportional to the ion current in the main beam and their ratio can be measured between wafer implantation batches. Therefore, by measuring the ion current of the monitor beam, the ion current of the main beam can be inferred. This method provides greater accuracy, especially in the presence of hydrogen outgassing from photoresist coated wafers, since the measurement is performed upstream from the poor vacuum of the wafer process chamber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawings, in which: 
     FIG. 1 is a top view of an ion implanter; 
     FIG. 2 is a top view, partly in section, of a portion of the ion implanter of FIG. 1 which embodies the present invention; 
     FIG. 3 is a diagrammatic view of the path of a multicomponent ion beam through a magnetic field; 
     FIG. 4 is an alternative embodiment of the invention which uses a smaller, moveable Faraday cup; 
     FIG. 5 is an alternative embodiment of the invention which uses a multiplicity of smaller, fixed Faraday cups, and 
     FIG. 6 is an alternative embodiment of the invention which uses a single cup to monitor the intensity of the neutral beam. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, and first to FIG. 1 thereof, therein is shown an overall plan view of an ion implanter  1  suitable for use with the invention. Although the invention is applicable to ion implanters generally without limiting the invention, one suitable implanter might be, for example, the MeV Ion Implanter System manufactured by Genus, Inc., Ion Technology Division, 4 Stanley Tucker Drive, Newburyport, Mass. as its model G1520 under the trademark “Tandetron”. The G1520 is typically used to implant boron, phosphorous or arsenic in the energy range 25 KeV-2.65 MeV into silicon wafers. Typical values of particle current range from 1×10 11  to 1×10 16  particles/sec. Typical dose values range from 5×10 10  to 1×10 16  particles/square cm. Depending on the energy of the implant, the ions striking the wafer may be singly, doubly or triply charged. When performing an 800 keV singly charged phosphorous implant, the G1520 uses a 10° magnet operating at 4667 gauss to select the desired charge state. 
     The system shown in FIG. 1 is divided into five major modules: the injector  2 , the tandem acceleration region  3 , the beam filter module  4 , the process chamber module  5 , and the wafer handling module  6 . The present invention is located in the beam filter region  4 , which includes a 10° uniform field magnet  7 , a drift region, and the final charge state selection aperture. This unit selects the desired ion beam by separation of the charge states (1+, 2+, 3+ and 4+) formed in the center of the accelerator module  3 . No mass analysis is performed at this stage, only energy analysis by selection of different charge states of the same species. The positive ion beam currents are optimized in a setup Faraday cup prior to entry into the process chamber  5 . 
     The beam filter module  4  is shown in greater detail in FIG.  2 . The ion beam from the accelerator module  3  enters the beam filter module  4  along the accelerated ion beam axis  8  at the vacuum waveguide entrance  9 . Passing through the magnetic field produced by the 10° analyzer magnet  7 , the ion beam is split into separate beams for each of the charge states contained in the main beam. A filter slit  10  in the path of the accelerated ion beam is so positioned in relation to the magnetic field strength that the desired charge state is selected. In the example of FIG. 2, the selected charge state is charge state ( 1 ). The other charge states follow different trajectories  11 , 12 , 13 , while the neutral beam remains along beam axis  8 . In accordance with the present invention, a Faraday cup  14  is located to read the current of one or more of the unselected charge states. In the example of FIG. 2, the unselected charge states which are read are charge states  2 , and  3 . 
     The Faraday cup  14  is supported by an insulator  15  which also serves to conduct cooling water to and from the cup. The cup is surrounded by front  16  and rear  17  bias electrodes, which are maintained at a negative potential. The bias electrodes ensure that no externally generated electrons reach the cup, and also that any electrons generated in the cup do not escape, but are returned to the cup. The acceptance area of the assembly is defined by a water cooled grounded aperture  18 . 
     The action of the magnetic field of FIG. 2 may be understood with reference to FIG.  3 . Referring thereto, when an ion beam  21  passes through a magnetic field  22 , the various constituent ions experience angular deflections determined by their mass, velocity and charge. This technique is commonly used to filter ions of a particular mass/velocity/charge combination from a multicomponent beam. The different angular deflections result in the various components of the beam being separated spatially. An “analyzing slit”  24  is positioned to pass only the desired main beam  23 ; alternate, unwanted components  25  experience different angular deflections and do not pass through the analyzing slit  24  and do not contribute to the implantation of the wafer. 
     During the time of an implant, the intensity of one or more of these alternate, unwanted components  25  is proportional to the intensity of the desired main beam  23 . For example, if the input to a tandem accelerator is composed of ions of a single mass, velocity and charge state, the output beam will contain ions of a single mass but a variety of velocities and charges. The physics of the acceleration process allow the relative proportions of the various components of the output beam to be constant over a wide range of output beam intensity. 
     Thus the intensity of the main beam  23  may be monitored by measuring the intensity of one or more of the alternate, unwanted component beams  25 , at any convenient point downstream of the 10° magnet. It may be found convenient to measure the unwanted component&#39;s intensity in the vicinity of the analyzing slit, making use of the pre-existing spatial separation of the beam components in this area; however, other implementations are possible without departing from the spirit and scope of the present invention. The intensity of the unwanted component is preferably measured at a location  26  well separated from the point of beam impact on the wafer; thus environmental perturbations caused by beam/wafer interaction will have minimal effect on the beam intensity measurement. 
     To permit an absolute measurement of the main beam current, it is determine the ratio of the main beam current to the monitor beam current to the monitor beam current. This determination can be made by measuring both currents at a convenient time when the main beam is not performing an implant. 
     There are a number of alternative embodiments of the invention which may be preferred under specific circumstances. FIG. 4 illustrates an arrangement in which the single fixed Faraday cup is replaced by a smaller, moveable cup  30 . The smaller cup may be positioned to monitor a single unselected charge state (or possibly more than one at a time), thus rejecting the other unselected beams. This may be desirable if the intensity of one of the unselected beams is not proportional to the intensity of the selected beam. Under these circumstances it will be necessary to reject the unselected beam to obtain a reliable measure of the beam current. 
     Another embodiment is shown in FIG. 5, in which the single fixed Faraday cup has been replaced by a plurality of smaller, fixed cups  31 , 32 , 33 , positioned to intercept the beams generated by specific unselected charge states. If the charge state  1  beam is selected (i.e. is being passed by the analyzing slit  10 ), Faraday cup  31  will intercept the neutral (charge state  0 ) beam, Faraday cup  32  will intercept the charge state  2  beam, and Faraday cup  33  will intercept the charge state  3  beam. If the field generated by the magnet  7  is reduced, a higher charge state beam will be selected, in which case Faraday cup  31  will collect beams with a charge state lower then the charge state of the selected beam. For example, if charge state  3  is passing through the analyzer slit  10 , charge states  1  and  2  will be collected by Faraday cup  31 . 
     Another embodiment is shown in FIG.  6 . Collector  34  is positioned to intercept the neutral (charge state  0 ) beam, which is always passed undeviated by the magnetic field. The impact of the beam will stimulate the emission of secondary electrons from the collector. Some of these electrons will escape, causing a current to flow in an ammeter connected to the collector; the magnitude of the current will provide an estimate of the intensity of the neutral beam. Ground shield  35  surrounds the collector, shielding it from other charged particles that may be present. In this embodiment, unselected charge state beams  11 - 13  impact grounded aperture  18 . 
     Having thus described the principles of the invention, together with several illustrative embodiments thereof, it is to be understood that, although specific terms are employed, they are used in a generic and descriptive sense, and not for purposes of limitation, the scope of the invention being set forth in the following claims.