Generating particle beams and measuring and regulating their characteristics

Abstract

A workpiece is treated with particles where an apparatus, including a particle source, produces a particle beam and directs the particle beam toward a surface of a work piece. A faraday cup assembly is positioned to intercept the particle beam for measuring a characteristic of the particle beam. The faraday cup assembly includes a faraday cup for gathering a charge and an aperture assembly having an aperture through which charged particles travel to the faraday cup. The faraday cup is located relative to the aperture assembly to intercept the charged particles. The aperture assembly is constructed to vary the size of the aperture.

Description

BACKGROUND

This invention relates to generating particle beams and measuring and regulating their characteristics, specifically using a Faraday cup or a plurality of Faraday cups.

As is well known, particle beams can be used for a variety of applications. One such application is ion implantation where beams of ion are generated and directed toward workpieces to implant ions into the workpieces. The workpieces can be wafers or other target substrates. The beams of ions can have large cross-section, that is, larger than or equal to about 1 cm², preferably larger than or equal to about 7 cm². Such beams of ion can be employed in conjunction with scanning arrangements to produce deposit of ions over wide areas. The scanning arrangements may be implemented to perform magnetic scanning of the beams in one direction and mechanical scanning of the workpieces in another direction.

During an ion implantation process, it is generally desirable to implant the ions uniformly across the workpiece. This uniformity of the implantation process depends on the ion beam having the same characteristics in its entire scan region and during the entire implantation process. However, an ion beam can have different characteristics in different points of the scan and at different times during the ion implantation process. Therefore, to ensure uniform implantation, the characteristics of the ion beam need to be monitored and the parameters affecting its characteristics need to be adjusted accordingly. Additionally, in some applications, it is desirable to change the ion beam characteristics to implant ions according to a new set of requirements and characteristics. For example, it may be desirable to increase or decrease the ion beam current for a new set of workpieces. In that case, it can be more efficient and less expensive to change the ion beam characteristics "on the fly," that is without stopping the implantation process to, for example, reconfigure the measuring instruments for the new ion beam.

The uniformity of the ion implantation process can be characterized by the value of an implantation dose delivered to the entire workpiece and also by the uniformity of the dose across the wafer. Implantation dose (D) may be defined as the number of ions delivered to a workpiece per unit of area. Since the value of the delivered dose depends on the value of the ion beam current, one method of monitoring and regulating the dose is to monitor the ion beam current by using Faraday cups. Once the dose is determined, the parameters governing the implantation process can be adjusted to compensate for variations in beam uniformity. Such parameters can include the speed of mechanical scanning of the workpiece and the parameters governing the ion beam current.

SUMMARY

In one general aspect, the invention features treating a workpiece with particles by an apparatus, including a particle source, for producing a particle beam and directing the particle beam toward a surface of a work piece. A Faraday cup assembly is positioned to intercept the particle beam for measuring a characteristic of the particle beam. The Faraday cup assembly includes a Faraday cup for gathering a charge and an aperture assembly having an aperture through which charged particles travel to the Faraday cup. The Faraday cup is located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture.

In another aspect, the invention features the Faraday cup assembly.

Embodiments of the invention can include one or more of the following features.

The variable aperture assembly comprises a first plate having a second aperture and a second plate having a third aperture. The first plate is disposed adjacent to the second plate such that the second aperture is positioned relative to the third aperture to define the first-mentioned aperture and the first and second plates are capable of sliding relative to one another, where sliding one of the plates relative to the other one of the plates varies the position of the second aperture relative to the third aperture and thereby varies the size of the first-mentioned apertures.

At least some of the devices further comprise a driver connected to the one of the plates to cause the one of the plates to slide relative to the other one of the plates.

The driver is further connected to the other one of the plates to cause the other one of the plates to slide relative to the one of the plates. The driver causes the one of the plates and the other one of the plates to slide a substantially equal distance.

The Faraday cup is elongated and is characterized by a first elongated axis and the aperture is elongated and is characterized by a second elongated axis, the cup and the opening being arranged relative to one another such that the first and second elongated axes are substantially parallel to one another.

In some embodiments, the particle beam is an ion beam and the apparatus further includes a plurality of magnets directing the ion beam toward a surface of a work piece and scanning the ion beam. The ion beam has a large cross-section, for example, larger than or equal to about 1 cm² and preferably larger than or equal to about 7 cm². The beam has a perveance in the order of or greater than 0.02 (mA) (AMU)^1/2 (KeV)-3/2. The ions in the beam may be argon, nitrogen, boron, arcin, phosphine, phosphorus, arsenic, and antimony. The beam may be a ribbon-shaped beam.

The Faraday assembly and the apparatus for producing and directing the particle beam form parts of a device which further includes a processor connected to receive input signals representing the charge gathered by the Faraday cup assembly and to output device control signals to control the operation of the device. A computer readable memory stores a software program comprising instructions for processing the input signals to supply the output device control signals to control the operation of the device. According to the instructions in the software, the processor receives signals representing the charge gathered by the Faraday cup assembly, processes the signals to determine whether to adjust the signals to control the operation of the device, and, if determined to adjust the device control signals, adjusts the ion implanter control signals. The device control signals include signals to the apparatus which control characteristics of the particle beam such as the current and uniformity of the particle beam.

The processor can also be connected to output aperture control signals to vary the size of the apertures and the software contains further instructions for processing the input signals to output aperture control signals for determining the size of the apertures. According to the instructions in the software, the processor processes the input signals, determines whether to vary the size of the aperture control signals, and if determined to vary the apertures, adjusts to aperture control signals to vary the size of the aperture.

A suppressor electrode for connection to a power supply is disposed between the Faraday cup and the variable opening aperture assembly, and having an aperture for the charged particles to travel through to the Faraday cup.

A cooling plate is located adjacent to and in contact with the first or second plate, the cooling plate being grounded.

The Faraday cup is constructed with graphite.

A Faraday cup array is positioned to intercept the particle beam for measuring a characteristic of the particle beam. The Faraday cup array comprises a plurality of Faraday cups for gathering charge and a variable aperture assembly having a plurality of apertures though which charged particles from the particle beam travel to the plurality Faraday cups. Each one of the plurality of Faraday cups is located relative to the variable aperture assembly to intercept the charged particles traveling through at least one of the plurality of apertures. The aperture assembly constructed to vary the size of at least one of the apertures. The first-mentioned Faraday cup assembly is a part of the Faraday cup array.

In another aspect, the invention features the Faraday cup array.

Embodiments of the invention can include one or more of the following features.

The variable aperture assembly comprises a first plate having a second plurality of apertures and a second plate having a third plurality of apertures, where the first plate is disposed adjacent to the second plate such that the second plurality of apertures are positioned relative to the third plurality of apertures to define the first-mentioned plurality of apertures. The first and second plates are capable of sliding relative to one another, where sliding one of the plates relative to the other one of the plates varies the position of the second plurality of apertures relative to the third plurality of apertures and thereby varies the size of the first-mentioned plurality of apertures.

A driver is provided for causing one of the plates to slide relative to the other plate. The driver includes a first arm connected to the one of the plates and the driver moves the arm to cause the sliding; a second arm connected to the other one of the plates and the driver moves the second arm to cause the sliding; and a rotating wheel having a axis of rotation, where the first and second arms are pivotally connected at one end to the wheel at a first and second connection points located at a distance from the axis of rotation and pivotally connected at another end to the one of the plates.

A housing is provided to house the plurality of Faraday cups in an arrangement to form an array of Faraday cups in at least one row. The Faraday cups are elongated and are characterized by elongated axes and the apertures are elongated and are characterized by elongated axes, the Faraday cups and apertures being positioned relative to one another such that the elongated axes of the Faraday cups are substantially parallel to at least one of the elongated axes of the apertures.

In another aspect, the invention features providing a Faraday cup for gathering and measuring charge of a flow of charged particles, and varying a size of an aperture, where the aperture controls the flow of the charged particles to the Faraday cup.

Embodiments of the invention include one or more of the following features.

A particle beam is generated and directing toward a work piece according to selected characteristics. The Faraday cup is positioned in the path of the particle beam, at least a portion of the particle beam traveling through the aperture to supply the flow of particles. The charge gathered by the Faraday cup is then measured.

The size of the aperture is varied to vary size of a cross section of the portion of the particle beam reaching the Faraday cup.

The size of aperture is varied in response to the value of the measured charge.

The size of the aperture is varied to cause the measured charge value to tend toward a preselected value representing a predetermined signal to noise ratio.

Measuring the charge further comprises integrating the value of the charge gathered by the Faraday cup.

In some embodiments, the step of measuring the charge and the step of varying the size of the aperture is performed in response to instructions from the computer.

The measured charge value is integrated where the integrated value represents the current of the particle beam. The integrated value is compared to a pre-selected value to determine a difference between the values. The selected characteristics is then varied based on the determined difference.

In some embodiments, the steps of integrating the measured charge value, measuring the charge and comparing the measured charge value to the pre-selected value are performed by a computer. The steps of varying the size of the aperture and varying the selected characteristics are performed in response to instructions from the computer.

The measured charge value is compared to a preselected value to determine a difference between the values, the measured charge value representing a profile of the beam. The selected characteristics is varied based on the determined difference.

In some embodiments, the steps of measuring the charge and comparing the measured charge value to the preselected value are performed by a computer. The steps of varying the size of the aperture and varying the selected characteristics are performed in response to instructions from the computer.

Other features and advantages of the invention will become apparent from the following description of preferred embodiments, including the drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic plan view of an ion implanter and its beam path.

FIGS. 2A-2D are side view illustrations of the position of a Faraday cup array relative to a semiconductor wafer in two embodiments of an end station of ion implanter of FIG. 1.

FIG. 3 is a perspective view of a Faraday cup array.

FIG. 4 is a cross-sectional view of a portion of the Faraday cup array of FIGS. 2A and 2B.

FIGS. 5A and 5B are cross-sectional views of single Faraday cups in the Faraday cup array of FIGS. 2A and 2B.

FIGS. 6A-6B and 7A-7B show the relationship between differently sized ion beams and size of apertures in embodiments of a Faraday cup assembly aperture.

FIGS. 8A and 8B are illustrative graphs of the integrated signals from a Faraday cup assembly for two different aperture widths.

FIGS. 9A and 9B are illustrative graphs of the beam profile readings from a Faraday cup assembly for two different aperture widths.

FIG. 10 is an illustrative graph of the relationship between the signal to noise ratio and the width of apertures 72.

FIG. 11 is a schematic diagram of an ion implanter control system.

FIG. 12 is a flow chart of the operation of the ion implanter control system of FIG. 11.

DESCRIPTION

FIG. 1 shows an example of an ion implanter 10, employed to produce a highly uniform deposit of ions over the entire surface of a workpiece, such as a semiconductor wafer 24. General features of the beam line of such an ion implanter are disclosed in e.g. U.S. Pat. No. 5,393,984 (hereinafter, the "'984 patent"), and in the commonly assigned applications "Ion source", filed on May 22, 1998, Ser. No. 09/083,814, "Ion Implantation with Charge Neutralization", filed on May 22, 1998, Ser. No. 09/083,707, and "Transmitting a Signal Using Duty Cycle Modulation", filed on Dec. 1, 1997, Ser. No. 08/982,210, "Space Charge Neutralization of an Ion Beam", filed on May 22, 1998, Ser. No. 09/083,706. The '984 patent and above referenced patent applications are hereby incorporated by reference in their entirety.

Ion implanter 10 has an ion source 12, an extractor electrode 14, an analyzer magnet 16, a scanner magnet 18, a collimator magnet 20, a plasma charge neutralizer 22 and wafer 24. Ion implanter 10 of FIG. 1 produces a ribbon-shaped beam which in some embodiments has a range of energies from 1 keV to 100 keV. The beam is a high current, high perveance beam (in some embodiments the beam has a perveance in the order of or greater than 0.02 (mA)(amu)^1/2 (KeV)-3/2), as explained in the '984 patent. The beam current in certain embodiments ranges from about 10 micro-amps to about 40 milli-amps. Implanter 10 magnetically scans ion beam 40 over wafer 24 in one direction. In other embodiments, the beam may be electrostatically scanned.

Ion source 12 of the implanter may be any type of ion generating source, including a Bernas type source, a Freeman source or a microwave ECR source. Ion source 12 generates positively charged ions for implantation, including gases such as argon, nitrogen, disassociated boron (as in BF3), arcin, and phosphine. Ions of solids may also be implanted after vaporization. Such solids include phosphorus, arsenic, and antimony. Other material may also be implanted. The ions emerge from an orifice, extracted by extraction electrode 14, which has a negative potential compared to the source. The shape and position of extractor electrode 14 is such that a well-defined ion beam emerges from the electrode.

Analyzer magnet 16 then analyzes the ion beam by removing undesired impurities according to the ion momentum to charge ratio (Mv/Q, where v is the velocity of the ion, Q is its charge, and M is its mass).

Scanner magnet 18 then scans the ion beam in a direction perpendicular to the path of the beam. The scanning range is approximately 400 mm. Following scanning, collimator magnet 20 reorients the ion beam such that paths of ions within the beam are generally parallel over the entire range of the scan.

In the embodiment shown, the ion beam is a ribbon-shaped broad beam having a beam height (that is, the length of the beam along the major axis of the cross section of the beam) of 90 mm at the source and 60 mm at the wafer, while the dimension along the orthogonal, minor axis is 2.5 mm at the source and 10 mm at the wafer. Other beams, however, can have various dimensions which would provide a large cross-sectional beam area at the wafer, e.g. a cross-sectional area larger than or equal to about 1 cm², preferably larger than or equal to about 7 cm². Such beams may be ribbon-shaped. Such beams may also have cross-sections which are circular, oval, or other shapes.

Ion implanter 10 is sized to enable implantation on wafers that have a diameter of up to 300 millimeters. An end station 26 holds wafer 24, at a selected angle within a range of angles of incidence of the beam to the wafer, preferably from normal incidence to the ion beam to less than 10°. End station 26 also moves the wafer mechanically to scan the wafer in a second direction, normal to the direction of magnetic scan and in the plane in which the wafer rests. The direction of the mechanical scan of the wafer and the magnetic scan of the ion beam may be substantially orthogonal to one another.

End station 26 further includes a Faraday cup array 50 which we will now describe. As shown schematically in FIGS. 2A and 2B, Faraday cup 50 is positioned downstream from wafer 24. FIG. 2A shows wafer 24 positioned for implantation by ion beam 40. FIG. 2B shows wafer 24 after having been implanted and removed from the path of ion beam 40. An outline 24' of wafer 24 (in a tilted position) illustrates a possible position of wafer 24 during implantation relative to the position of Faraday cup array 50 during ion beam current measurement. Removal of wafer 24 results in Faraday cup array 50 being positioned in the path of ion beam 40 to intercept the particles and gather charge for measurement readings. Note that Faraday cup array 50 can be retractable, that is, it can be moved into position for measuring the beam current, and after measurement, be retracted from its measuring position. In its retracted position, Faraday cup array 50 would be situated away from the path of the beam.

FIGS. 2C and 2D show the position of Faraday cup array 50 relative to wafer 24 in an alternative embodiment of end station 26. In this embodiment, Faraday cup array 50 is situated in approximately the same plane as wafer 24. In some embodiments, Faraday cup array 50 can tilt and be mechanically scanned in the same manner as wafer 24 so as to intercept the ion beam in the same manner as wafer 24 would. Faraday cup array 50 in this embodiment is retractable and is put in position for measurement when wafer 24 is removed. Since Faraday cup array 50 is in the same plane as wafer 24 during operation, measurements by Faraday cup array 50 accurately reflect the implantation dose applied to wafer 24.

Referring to FIGS. 3, 4, 5A, and 5B, Faraday cup array 50 has an array of Faraday cup assemblies 60 located in a row. Each Faraday cup assembly has a Faraday cup 62 located in a housing 64 and a number of components shared with the other Faraday cup assemblies 60. The shared components are a suppressor electrode 66, a cooling plate 68, and a variable aperture assembly 70 defining a plurality of variable apertures 72. Variable aperture assembly 70 further includes an upper plate 74A, a lower plate 74B and a driver or driving mechanism 75. Driver 75 causes an upper plate 74A and a lower plate 74B to slide relative to one another and thereby vary the size of apertures 72 as will be described in detail below. Driver 75 includes arms 76A and 76B, rotating surfaces 78, and a motor 82. Upper plate 74A may be constructed out of graphite or glassy graphite which reduces flaking and secondary emission/contamination.

As stated above, each Faraday cup assembly 60 includes a Faraday cup 62 and a corresponding aperture 72. Each Faraday cup 62 is essentially an electrode for gathering charge and is typically connected to ground in series with a current measuring device. The shape, geometry, and dimensions of Faraday cups 62 are not limited to ones disclosed here and can vary depending on the application. Here, Faraday cups 62 are made out of graphite, disposed in front of an array of apertures 72. Each one of Faraday cups 62 is positioned relative to one of the apertures 72 such that charges which travel through the aperture are intercepted by that cup. Variable aperture assembly 70 defines a plurality of apertures 72 allowing charged particles of ion beam 40 to travel through and reach Faraday cup 62. As will be described in detail below, the width and therefore size of apertures 72 are variable and may be varied on demand during the implantation process.

As shown particularly in FIG. 3, Faraday cup array 50 has a number of Faraday cup assemblies 60 arranged as a linear array. Generally, Faraday cup array 50 is placed so as to span the entire scan path of ion beam 40 and be able to obtain readings from ion beam 40 along its entire path. Apertures 72 and Faraday cups 62 are generally longer than the largest expected dimension of the beam along an axis parallel to apertures 72. This ensures that charged particles from the entire beam will enter Faraday cups 62.

Faraday cups 62 and apertures 72 are generally evenly spaced except for pairs of Faraday cups 62A, 62B and corresponding pairs of apertures 73A, 73B at either end. Faraday cup pairs 62A, 62B and corresponding aperture pairs 73A, 73B are positioned more closely to one another than is the case with other Faraday cups 62 and apertures 72. This positioning assists with reducing the effect of obtaining a false reading when ion beam 40 does not clear the aperture 72 located at the end of the scan path of ion beam 40. In that case, ion beam 40 would continually bombard the corresponding Faraday cup 62 in both scan directions without leaving the cup. In other words, ion beam 40 would bombard that Faraday cup for a period up to twice as long as other Faraday cups 62. Since, as will be described below, in some cases, the measured charge value is an integral of the gathered charge, the reading from Faraday cup 62 at the end of the scan can therefore be up to twice as high as that of the other Faraday cups 62 in Faraday cup array 50. Hence, that reading would have to be discarded. In that case, because in each pair of Faraday cups at the end of the scan, apertures 72 and Faraday cups 62 are positioned close to one another, the reading from the other Faraday cup 62 in the pair can be used as an accurate estimate of the beam characteristic at the end of its scan path.

Generally, each aperture 72 is an opening through which particles of an ion beam can travel through to reach a corresponding Faraday cup 62. Variable aperture assembly 70 is designed and constructed such that the size of apertures 72 can be varied by varying the width of the opening area apertures 72. Each aperture 72 is defined by one of apertures 72A of upper plate 74A and a corresponding one of apertures 72B of lower plate 74B. As mentioned above, plates 74A and 74B are constructed and arranged so as to be able to slide with respect to one another. To understand how the size of apertures 72 is varied consider one of apertures 72A of upper plate 74A and a corresponding one of apertures 74A of lower plate 74B. As these apertures 72A and 72B become more or less aligned, the width and therefore size of the opening areas through which ion beam 40 can travel become respectively larger or smaller. Also, note that as apertures 72A and 72B become more or less aligned, the central axes of apertures 72 remain relatively centered on Faraday cups 62.

For example, FIG. 5A shows plates 74A and 74B having moved such that width w1 is the width of the effective opening area for aperture 72. FIG. 5B in turn shows plates 74A and 74B having slid a distance such that apertures 72A and 72B are substantially aligned and the effective width of aperture 72 is w2. Therefore, a beam current measurement at any point in time by Faraday cup 62 in FIG. 5A corresponds to a narrower portion or slice of ion beam 40 than a beam current measurement by Faraday cup 62 in FIG. 5B.

It should be noted that although Faraday cup array 50 is positioned with respect to ion beam 40 such that ion beam 40 travels substantially along axis 84, at points in the scan, path of ion beam 40 may diverge from this axis. Therefore, aperture 72 may appear as larger or smaller to ion beam 40. However, the divergence is typically not significant. It should also be noted that in other embodiments, the structure defining aperture 72 may be implemented such that the size of the aperture does not vary at all as the angle of incidence of the beam changes.

We will now describe the mechanism for causing plates 74A and 74B to slide with respect to one another in order to vary the width and therefore the size of apertures 72. Each one of plates 74A and 74B is attached to one end of arms 76A and 76B. The other end of arms 76A and 76B are pivotally connected to a rotating surface 78, here a disk, at a distance from the center of rotation 80 of rotating surface 78. A motor 82 can rotate rotating surface 78 causing the arms 76A and 76B to move in opposite directions. Their movement in turn results in plates 76A and 76B to slide relative to one another in opposite directions. When plates 76A and 76B slide relative to one another, apertures 72A and their corresponding apertures 72B become more or less aligned and the width and size of apertures 72 vary accordingly. Note that, since both plates 74A and 74B slide substantially the same distance with respect to one another, apertures 72 remain substantially centered (that is, centered on axes 84 shown in FIGS. 5A and 5B) as their size changes.

In an alternative embodiment of the driving mechanism for causing plates 74A, 74B to slide with respect to one another, arms 76A, 76B may be connected to a conventional driving mechanism for moving the arms linearly in the same plane as plates 74A, 74B. Such conventional mechanisms may include conventional pneumatic, hydraulic, gear, or belt driven mechanisms.

As mentioned above, Faraday cup assemblies 60 share cooling plate 68 and suppressor electrode 66. Cooling plate 68 has an array of openings which are aligned with apertures 72 so as to allow particles traveling through apertures 72 to travel past cooling plate 68 without obstruction. Each opening in cooling plate 68 preferably has dimensions larger than those of maximum opening area of apertures 72.

Cooling plate 68 is positioned adjacent to and in contact with lower plate 74B. Each of upper and lower plates 74A, 74B are both fabricated from conductive material such as graphite. During operation, upper plate 74A is bombarded with ions in the ion beam. This bombardment has two effects. First, a positive charge is continually applied to plate 74A. Second, the ions stopped by plate 74A release their energy to plate 74A and heat plate 74A. Cooling plate 68 assists with countering these two effects. During operation, cooling plate 68 is grounded which results in plates 74A and 74B being also grounded. Therefore, cooling plate 68 allows the charge delivered to plate 74A to flow to ground. Additionally, cooling plate 68 acts as a heat sink and assist with cooling plates 74A and 74B.

Suppressor electrode 66 is disposed between Faraday cups 62 and variable aperture assembly 70. Suppressor electrode 66 has an array of openings located so as to allow beam traveling through apertures 72 to reach Faraday cups 62. During operation, suppressor electrode 66 is kept at a negative potential. Because of its charge, suppressor electrode 66 prevents secondary electrons from leaving Faraday cup 62. It should be noted that although an electrostatic suppression electrode is used in this embodiment, other embodiments may use magnetic or electromagnetic fields for suppression of secondary electrons.

Referring to FIG. 4, Faraday cups 62 can be connected for various types of measurements in various circuit configurations. Here, Faraday cups 62 are connected for two types of measurements (beam current and beam profile) in two respective circuit configurations. In the first circuit configuration, the signal from each Faraday cup 62 is connected to an integrator 82, implemented using conventional circuitry. The signal from the integrator is supplied to a measurement/display device 84 such as an oscilloscope or an ammeter connected in series to ground. Measurement/display device 84 may also be a computer or a component of a computer. This circuit configuration allows obtaining an ion beam current measurement from each Faraday cup 62, as will be described below in detail. In the second circuit configuration, the signal from each Faraday cup 62 is supplied to measurement/display device 84 directly and without being integrated. This circuit configuration allows measuring a profile of the ion beam, as will be described below in detail.

Having described the structure of an embodiment of a variable aperture Faraday cup array, we will now describe methods of using Faraday cup array 50 to measure and regulate characteristics of ion beams during ion implantation. More specifically, according to these methods of using Faraday cup array 50, Faraday cup array 50 can be used to measure characteristics of the ion beam to determine both the value of total dose delivered to a workpiece and the uniformity of that dose. These values can then be used to adjust the characteristics of the ion beam to ensure a selected dose is delivered to the workpiece and that the delivered dose is uniformly applied across the workpiece. Additionally, these methods will be described in the context of measuring and regulating ion beam characteristic as those characteristics are changed during the implantation process, without stopping the implantation process, so as to implant different batches of workpieces according to different implantation requirements.

As mentioned, Faraday cup array 50 can be used to determine both the value of the total dose delivered to the workpiece and the uniformity of that dose. Determining the total dose value allows for adjusting the ion beam current to obtain a desired dose value. As will be described in detail below, the total dose value is determined by measuring the beam current. This measurement then can be used to determine the dose delivered to the workpiece.

Determining the uniformity of the dose in turn allows for modifying the beam characteristics so as to increase the uniformity of the dose across the workpiece. The uniformity of a dose delivered to the workpiece depends on various factors including the uniformity of the ion beam current across the scan path of the beam and the uniformity of the beam profile across a cross-section of the beam and across the entire scan path. Therefore, it is desirable to measure these values to determine uniformity of the dose and to determine how to adjust the parameters governing the operation of ion implanter 10 to achieve the desired uniformity. However, each of these two types of measurements requires different characteristics in the Faraday cups used for measuring the charge.

We will now discuss each of these measurements in turn. First, consider measuring the ion beam current. Referring to FIGS. 6A and 6B, a Faraday cup and an ion beam can be sized so that the entire beam fits through the opening and the Faraday cup can measure the entire beam current. However, referring to FIGS. 7A and 7B, the Faraday cup and the ion beam can also be sized (a more typical situation) so that the cross-sectional area of the beam at the plan of incidence with the Faraday cup is larger than the opening to the Faraday cup. Therefore, the cup measures the charge from particular portions or slices of the ion beam as opposed to the charge from the entire beam as in FIGS. 6A and 6B. Therefore, to obtain the current of the beam in the case of FIGS. 7A and 7B, these measured charge values are integrated.

The measured charge value has a noise component and a signal component. FIGS. 8A and 8B are illustrative graphs of the integrated signals from a Faraday cup assembly 60, where the signal to noise ratio SNR1 for the graph in FIG. 8A is less than the signal to noise ratio SNR2 for the graph in FIG. 8B. As is apparent from FIGS. 8A and 8B, when the signal from a Faraday cup 62 is integrated, the final integrated value is made up of two components: an integrated signal component illustrated by areas 82A and 82B in graphs in FIGS. 8A and 8B; and an integrated noise component illustrated by areas 84A and 84B. As signal to noise ratio increases, the ratio of their integrated values also increases. Hence, the higher the signal to noise ratio is, the more accurately the final integrated measurement value represents the signal component and the ion beam current. Therefore, when measuring the ion beam current, it is desirable to optimally increase the signal to noise ratio and preferably not allow the signal to noise ratio fall below a threshold previously determined to provide accurate ion beam current measurements.

Second, consider measuring a beam profile. In contrast to the accuracy of beam current measurements, the accuracy of beam profile measurements depends less on having a high signal to noise ratio. This is the case because the signal from a Faraday cup measuring the profile is not integrated.

In measuring the beam profile, the width of the Faraday cup aperture determines the resolution of the measurement. FIGS. 9A and 9B illustrate this. FIGS. 9A and 9B are illustrative graphs of the beam profile readings from a single Faraday cup, where a width W1 of the aperture in FIG. 9A is greater than a width W2 of the aperture in FIG. 9B. Because of the wider width W1, the beam profile readings in FIG. 9A do not have as fine a resolution as those in FIG. 9B since at each point in time a larger cross-section of the beam is measured. Therefore, in the case of measuring a beam profile, a lower aperture opening is more desirable.

Hence, performing these two types of measurements for the same ion beam can require two sets of instruments, each of which is designed for taking one of the above measurements. Additionally, during operation, where it is desirable to change the ion beam characteristics during the implantation process without stopping the operation of the implanter, an additional issue arises: instruments which are designed for accurately taking the above measurements for an ion beam with a particular set of characteristics are not necessarily suitable for taking measurements from an ion beam with different set of characteristics.

Faraday cup 50 can be used for accurately taking both types of measurements for ion beams having different characteristics. Faraday cup array 50 can be used to measure the current of a beam at a high signal to noise ratio and can subsequently be used to measure the beam profile of that beam using a narrow aperture opening. Faraday cup array 50 can further be used for making accurate ion beam current and profile measurements as the ion beam characteristics are changed "on the fly," that is without stopping generating and directing the beam. We will now describe methods of using Faraday cup array 50 for making such measurements and for modifying the ion beam to achieve uniform beam current and beam profile.

As stated above, to obtain an accurate ion beam current measurement, the signal to noise ratio should not fall below a predetermined threshold value for obtaining accurate measurements. However, the signal to noise ratio generally decreases as the ion beam current decreases. For beams having low currents (for example, beams having a current less than approximately 10 microamps and, in some cases, more than approximately 10 nanoamps), the signal to noise ratio can fall below desirable levels and can indeed become so low that the signal may be indiscernible.

To increase the signal to noise ratio, the size of apertures 72 in Faraday cup array 50 are varied according to a method which will be now described. Generally, a decrease in ion beam current decreases signal to noise ratio because the charge Q gathered by Faraday cups 62 and Faraday cups array 50 decreases as the current decreases. The charge Q in an exemplary one of Faraday cups 62 can be approximately estimated by the following formula (note that this equation is a simplified equation and other factors such as edge effects, etc. may also be taken into account in determining the value of Q): ##EQU1##

where v is the scan speed which is equivalent to the distance ion beam 40 travels during each scan cycle divided by the period of the scan cycle; Q is the integrated value of the charge gathered by the Faraday cup; and w is the width of the opening area aperture 72 opening assuming a normal incidence of the beam.

Therefore, the gathered charge value can be increased by increasing the width of apertures 72, that is, by increasing w. The relationship between the gathered charge value Q and the aperture width can be shown by the following sensitivity equation: ##EQU2##

This equation shows the relative sensitivity of the value of gathered charge to the value of a change in the width of apertures 72. Note that the relationship between these two variables is usually nonlinear.

As the width of apertures 72 varies, however, the value of the noise which appears in the measurements can also vary. The value of the noise includes at least two components: a constant noise value and a variable noise value. The constant noise value does not vary significantly as the width of apertures 72 is varied. Sources for the constant noise value include instrument noise. Therefore, to increase the ratio of the signal to the constant noise, the aperture width can be increased to increase the gathered charge value Q and therefore increase the ratio of the signal to the constant noise.

The variable noise value, however, varies as the width of apertures 72 varies because of various factors. One such factor is the width of apertures 72. Sources of the variable noise value include sources outside the Faraday cup, such as secondary particle emissions in ion implanter, which increase as the width of apertures 72 increases. Therefore, increasing the width of apertures 72 can increase the value of this noise component. If this increase is proportionally greater than the increase in the gathered charge value Q, the ratio of the signal to the variable noise value and the over all signal to noise ratio drop.

FIG. 10 is an illustrative graph 90 of the relationship between the signal to noise ratio and the width of apertures 72. As is readily apparent, graph 90 shows that the relationship between the signal to noise ratio and the width of apertures 72 is a nonlinear relationship. (It should be noted that the shape of graph 90 depends on various factors, including, the ion beam which is being measured and the particular configuration of ion implanter). Based on graph 90, depending on the width of apertures 72, increasing the width of the aperture can increase or decrease the signal to noise ratio.

One technique for determining how far to increase the width of the aperture for accurate ion beam current reading is to determine a measured charge threshold or a range of such values which based on experience or calculation have been found to provide appropriate signal to noise ratio. Then, during operation of implantation, when the measured charge value falls below the measured charge threshold or outside the range, the width of apertures 72 is increased to compensate. In this manner, the width of apertures 72 is widened only as much as is necessary. This reduces the possibility of decreasing the signal to noise ratio by widening apertures 72.

After obtaining ion beam current measurements, the beam current values can be used for determining the over all dose delivered by the ion beam to the workpiece and the distribution dose and its uniformity in the scan path. The ion beam current is first estimated by the following equation (based on Eq. 1): ##EQU3##

This estimated value then can be used to determine the estimated dose based on the following equation: ##EQU4##

where D_Estimated is the estimated dose; I_Estimated is the estimated current; T is the scan period; n is the number of scans; e is the average charge per implanted ions; and A is the wafer area. The estimated dose value can then be used to determine the total dose delivered to the wafer. The estimated dose value can also be used to determine the dose at each point in the scan and therefore the dose uniformity across the scan path.

We will now describe a method of using Faraday cup array 50 to measure an accurate beam current profile for the entire scan path and for a particular spot in the scan. First, the width of apertures 72 is narrowed to obtain the narrowest width possible given a predetermined acceptable signal to noise ratio. In order to determine how narrow apertures 72 can be made, a current value for an acceptable signal to noise ratio is determined and the width of the aperture is varied until that current value is reached. As previously stated, because the signal is not integrated for beam profile measurement, the signal to noise ratio in this case need not be as high as that required for ion beam current reading.

The narrow width then results in measuring the beam profile at a high resolution. The measured beam profile then can be used to determine the beam current variance across the ion beam and therefore the variance in the density of ions across the work piece.

Having described methods of using Faraday cup array 50 measuring characteristics of a beam, we will now describe a method for measuring ion beam current and profile using Faraday cup array 50 to determine the ion beam uniformity and the implantation dose where the beam characteristics are changed during the implantation process. According to the method, as the ion beam characteristics are charged for a new set of implantation criteria, apertures 72 are adjusted to obtain a current measurement at a high signal to noise ratio. This measurement is used to compute the total dose delivered to the workpiece and the dose uniformity across the scan path. Next, apertures 72 are narrowed to measure the beam profile at a minimum acceptable signal to noise ratio. The measured values are then used to compute the uniformity of the dose across the beam profile and the scan path. Based on these measurement results, the various parameters governing the operation of the implanter and affecting the beam uniformity are adjusted to increase uniformity while obtaining the desired value for the dose delivered to the workpiece. The parameters which can be adjusted include the current controlling the various magnets, the ion source electrode potentials, and the acceleration and deceleration potentials. Therefore, when operating ion implanter 10 according to this method, ion implanter 10 can be used to implant workpieces using two different beam characteristics changing from one beam to another without having to shutting down implanter 10 in order to reconfigure ion implanter 10 or its measuring devices.

Other embodiments are within the scope of the following claims.

For example, the above described method of operating implanter 10 and specifically operating Faraday cup array 50 for monitoring and controlling the beam can be automated using a computer. Referring to FIG. 11, charge measurement signals 102 from Faraday cups 62 are supplied to an input/output (I/O) interface and signal convertor 104 which pre-processes the signals and provides converted signals 102' to a processor 106. Processor 106, executing a software program 112 stored in a memory 108, processes converted signals 102' from Faraday cup array 50. In accordance with the results of that processing, processor 106 then provides appropriate control signals 110' to I/O interface and signal convertor 104. I/O interface and signal convertor 104 converts those signals to control signals 110 for adjusting apertures 72 of Faraday cup array 50. Additionally, in accordance with the results of processing signals 102', processor 106 also provides control signal 112' to I/O interface and signal convertor 104. I/O interface and signal convertor 104 converts those signals to control signals 112 which are transmitted to the various components of ion implanter 10 to adjust the characteristics of the ion beam.

FIG. 12 is a flow chart of steps taken by processor 106 when executing program 112. The user first selects the desired dose to be delivered to a workpiece. After receiving signals 102, processor 106 determines the ion beam current (step 202). If the ion beam current is not within the range of current values which provide acceptable signal to noise ratio for ion beam current measurement (step 202), processor 106 determines whether the ion beam current is above the range (step 204). If the ion beam current is above the range (step 206), processor 106 causes control signals 110 be supplied to Faraday cup array 50 to decrease the size of apertures 72 (step 208). If the ion beam current is not above the range, that is, the ion beam current is below the range (step 206), processor 106 causes control signals 110 to be supplied to Faraday cup array 50 to increase the size of apertures 72 (step 210). Steps 202 and 210 are repeated until the measured current value falls within the range of value producing acceptable signal to noise ratio (step 204).

When the current measured current value falls within the range, processor 106 uses the measured ion beam current values to compute the estimated value of the total dose delivered to the workpiece and the dose at each Faraday cup which signifies dose uniformity across the scan path (step 212). Processor 106 compares the value of the total delivered dose to the operator input value. If they are not substantially the same (step 214), processor 106 causes control signals 112 to be supplied to implanter 10 based on the difference between the value of the total delivered dose and the operator input value to cause the total dose value tend toward the operator input value (step 216). Processor 106 then proceeds to step 202.

However, if processor 106 determines that the value of the total delivered dose is substantially the same as that inputted by the operator (step 214), processor 106 then proceeds to measure and analyze the beam profile. To do so, processor 106 causes control signals 110 to be sent to Faraday cup array 50 to reduce the size of apertures 72 for the lowest acceptable signal to noise ratio for obtaining the beam profile readings (step 218). Processor 106 then measures and analyzes the beam profile across the scan path and at each Faraday cup location (step 220). If the beam profile is not acceptable in that it is not sufficiently uniform at each Faraday cup location and across the scan path or if the dose uniformity across the scan path is not acceptable (step 222), processor 106 causes control signals 112 to be supplied to the implanter based on the measured values and the difference between the measured values and values previously determined to provide acceptable results (step 216). However, if the beam profile and dose uniformity are acceptable in that they are sufficiently uniform (step 222), processor 106 determines to proceed with implanting ions in the workpiece at the new dose level.

Additionally, embodiments of Faraday cup assemblies and Faraday cup arrays disclosed here may be used in machines other than implanters, such as broad electron beam and neutral beam machines. They can also be used at other stages along the path of the ion beam or with other components. For example, the Faraday cup assemblies and arrays may be used after the analyzer or collimator magnets. The Faraday cup assemblies and arrays may also be used in end stations commonly known as "batch systems" in which the mechanical scanning is implemented by having multiple wafers placed on an outer perimeter of a spinning disc

It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and embodiments are within the scope of the following claims.

It should be noted that size and shape of apertures 72 are not limited to those disclosed here. As to size, FIGS. 6A-B and 7A-B show two exemplary sizes of aperture 72. In FIGS. 6A-B, the size of apertures 72 is larger than the size of the beam cross-section. In contrast, in FIGS. 7A-B, the size of apertures 72 is smaller than the beam cross-section. As to shape, apertures 72 can for example be circular or elliptical. Moreover, various different mechanisms and designs may be used to implement the apertures and the manner in which their size is varied.

Claims

1. A device for treating a workpiece with particles, comprising:

apparatus, including a particle source, for producing a particle beam and directing the particle beam toward a surface of a work piece, and

a faraday cup assembly positioned to intercept the particle beam for measuring a characteristic of the particle beam, the faraday cup assembly comprising:

a faraday cup for gathering a charge,

an aperture assembly having an aperture through which charged particles travel to the faraday cup, the faraday cup being located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture, wherein the variable aperture assembly comprises:

a first plate having a second aperture,

a second plate having a third aperture,

wherein the first plate being disposed adjacent to the second plate such that the second aperture is positioned relative to the third aperture to define the first mentioned aperture, and

wherein the first and second plates are capable of sliding relative to one another, wherein sliding one of the plates relative to the other one of the plates varies the position of the second aperture relative to the third aperture and thereby varies the size of the first-mentioned apertures.

2. The device of claim 1 further comprising:

a driver connected to said one of the plates to cause said one of the plates to slide relative to said other one of the plates.

3. The device of claim 2 wherein the driver is further connected to said other one of the plates to cause said other one of the plates to slide relative to said one of the plates.

4. The device of claim 3 wherein the driver causes said one of the plates and said other one of the plates to slide a substantially equal distance.

5. The device of claim 1 further comprising a faraday cup array positioned to intercept the particle beam for measuring a characteristic of the particle beam, the faraday cup array comprising:

a plurality of faraday cups for gathering charge,

a variable aperture assembly having a plurality of apertures though which charged particles from the particle beam travel to the plurality faraday cups, each one of the plurality of faraday cups being located relative to the variable aperture assembly to intercept the charged particles traveling through at least one of the plurality of apertures, the aperture assembly constructed to vary the size of at least one of the apertures,

wherein the first-mentioned faraday cup assembly is a part of the faraday cup array.

6. The device of claim 1 wherein the faraday cup is elongated and is characterized by a first elongated axis, and wherein the aperture is elongated and is characterized by a second elongated axis, the cup and the opening being arranged relative to one another such that the first and second elongated axes are substantially parallel to one another.

7. The device of claim 1 further including a processor connected to receive input signals representing the charge gathered by the faraday cup assembly and to output device control signals to control the operation of the device, and

a computer readable memory storing a software program comprising instructions for processing the input signals to output the device control signals to control the operation of the device,

wherein, according to the instructions in the software, the processor receives signals representing the charge gathered by the faraday cup assembly, processes the signals to determine whether to adjust the signals to control the operation of the device, and, if determined to adjust the device control signals, adjusts the device control signals.

8. The device of claim 1, further comprising:

a suppressor electrode for connection to a power supply, the suppressor electrode being disposed between the faraday cup and the variable opening aperture assembly, and having an aperture for the charged particles to travel through to the faraday cup.

9. A device for treating a workpiece with particles, comprising:

apparatus, including a particle source, for producing a particle beam and directing the particle beam toward a surface of a work piece, and

a faraday cup assembly positioned to intercept the particle beam for measuring a characteristic of the particle beam, the faraday cup assembly comprising:

a faraday cup for gathering a charge,

an aperture assembly having an aperture through which charged particles travel to the faraday cup, the faraday cup being located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture,

wherein the particle beam is an ion beam and the apparatus further includes a plurality of magnets directing the ion beam toward a surface of a work piece and scanning the ion beam.

10. The device of claim 9 wherein the ion beam has a large cross-section.

11. The device of claim 10 wherein the large cross-section larger than or equal to about 1 cm².

12. A device for treating a workpiece with particles, comprising:

apparatus, including a particle source for, producing a particle beam and directing the particle beam toward a surface of a work piece, and

a faraday cup assembly positioned to intercept the particle beam for measuring a characteristic of the particle beam, the faraday cup assembly comprising:

a faraday cup for gathering a charge,

an aperture assembly having an aperture through which charged particles travel to the faraday cup, the faraday cup being located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture,

the device further including a processor connected to receive input signals representing the charge gathered by the faraday cup assembly and to output device control signals to control the operation of the device, and

a computer readable memory storing a software program comprising instructions for processing the input signals to output the device control signals to control the operation of the device,

wherein, according to the instructions in the software, the processor receives signals representing the charge gathered by the faraday cup assembly, processes the signals to determine whether to adjust the signals to control the operation of the device, and, if determined to adjust the device control signals, adjusts the device control signals,

wherein the processor is further connected to output aperture control signals to vary the size of said apertures and the software contains further instructions for processing the input signals to output aperture control signals for determining the size of said apertures,

wherein, according to the instructions in the software, the processor processes the input signals, determines whether to vary the size of the aperture control signals, and if determined to vary the apertures, adjusts to aperture control signals to vary the size of the aperture.

13. A device for treating a workpiece with particles, comprising:

apparatus, including a particle source, for producing a particle beam and directing the particle beam toward a surface of a work piece, and

a faraday cup assembly positioned to intercept the particle beam for measuring a characteristic of the particle beam, the faraday cup assembly comprising:

a faraday cup for gathering a charge,

an aperture assembly having an aperture through which charged particles travel to the faraday cup, the faraday cup being located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture,

the device further comprising:

a suppressor electrode for connection to a power supply, the suppressor electrode being disposed between the faraday cup and the variable opening aperture assembly, and having an aperture for the charged particles to travel through to the faraday cup,

the device further comprising a cooling plate adjacent to and in contact with the first or second plate, the cooling plate being grounded.

14. The faraday cup assembly comprising:

a faraday cup for gathering a charge,

an aperture assembly having an aperture through which charged particles travel to the faraday cup, the faraday cup being located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture,

wherein the variable aperture assembly comprises:

a first plate having a second aperture

a second plate having a third aperture,

wherein the first plate being disposed adjacent to the second plate such that the second aperture is positioned relative to the third aperture to define the first-mentioned aperture, and

wherein the first and second plates are capable of sliding relative to one another, wherein sliding one of the plates relative to the other one of the plates varies the position of the second aperture relative to the third aperture and thereby varies the size of the first mentioned apertures,

the faraday cup further comprising:

a driver connected to said one of the plates to cause said one of the plates to slide relative to said other one of the plates,

the faraday cup assembly further comprising a cooling plate adjacent to and in contact with the first or second plate, the cooling plate being grounded.

15. The faraday cup assembly of claim 14 wherein the driver is further connected to said other one of the plates to cause said other one of the plates to slide relative to said one of the plates.

16. The faraday cup assembly of claim 14 wherein the driver causes said one of the plates and said other one of the plates to slide a substantially equal distance.

17. The faraday cup assembly of claim 14 wherein the faraday cup is elongated and is characterized by a first elongated axis, and wherein the aperture is elongated and is characterized by a second elongated axis, the cup and the opening being arranged relative to one such that the first and second elongated axis are substantially parallel to one another.

18. The faraday cup assembly of claim 14, further comprising:

a suppressor electrode for connection to a power supply, the suppressor electrode being disposed between the faraday cup and the variable opening aperture assembly, and having an aperture for the particles to travel through to the faraday cup.

19. The faraday cup assembly comprising:

a faraday cup for gathering a charge,

an aperture assembly having an aperture through which charged particles travel to the faraday cup, the faraday cup being located relative to the aperture assembly to intercept the charged particles, the aperture assembly constructed to vary the size of the aperture,

wherein the faraday cup is constructed with graphite.

20. A faraday cup array comprising:

a plurality of faraday cups for gathering charge,

a variable aperture assembly having a plurality of apertures through which charged particles from the particle beam travel to the plurality faraday cups, each one of the plurality of faraday cups being located relative to the variable aperture assembly to intercept the charged particles traveling through at least one of the plurality of apertures, the aperture assembly constructed to vary the size of at least one of the apertures,

a driver connected to said one of the plates to cause said one of the plates to slide relative to said other one of the plates, and

a cooling plate adjacent to and in contact with the first or second plate, the cooling plate being grounded.

21. The faraday cup array of claim 20 wherein the variable aperture assembly comprises:

a first plate having a second plurality of apertures,

a second plate having a third plurality of apertures,

wherein the first plate being disposed adjacent to the second plate such that the second plurality of apertures are positioned relative to the third plurality of apertures to define the first-mentioned plurality of apertures, and

wherein the first and second plates are capable of sliding relative to one another, wherein sliding one of the plates relative to the other one of the plates varies the position of the second plurality of apertures relative to the third plurality of apertures and thereby varies the size of the first-mentioned plurality of apertures.

22. The faraday cup array of claim 20 wherein the driver is further connected to said other one of the plates to cause said other one of the plates to slide relative to said one of the plates.

23. The faraday cup array of claim 22 wherein the driver causes said one of the plates and said other one of the plates to slide a substantially equal distance.

24. The faraday cup array of claim 20 wherein the faraday cups are elongated and are characterized by elongated axes, and wherein the apertures are elongated and are characterized by elongated axes, the faraday cups and apertures being positioned relative to one another such that the elongated axes of the faraday cups are substantially parallel to at least one of the elongated axes of the apertures.

25. The faraday cup array of claim 20, further comprising:

a housing to house the plurality of faraday cups in an arrangement to form an array faraday cups in at least one row.

26. The faraday cup array of claim 20, further comprising:

a suppressor electrode for connection to a power supply, the suppressor electrode being disposed between one of the faraday cups and the variable aperture assembly, and having an aperture for the charged particles to travel through to said one of the faraday cups.

27. A faraday cup array comprising:

a plurality of faraday cups for gathering charge,

a variable aperture assembly having a plurality of apertures through which charged particles from the particle beam travel to the plurality faraday cups, each one of the plurality of faraday cups being located relative to the variable aperture assembly to intercept the charged particles traveling through at least one of the plurality of apertures, the aperture assembly constructed to vary the size of at least one of the apertures,

wherein the variable aperture assembly comprises:

a first plate having a second plurality of apertures,

a second plate having a third plurality of apertures,

wherein the first plate being disposed adjacent to the second plate such that the second plurality of apertures are positioned relative to the third plurality of apertures to define the first-mentioned plurality of apertures, and

wherein the first and second plates are capable of sliding relative to one another, wherein sliding one of the plates relative to the other one of the plates varies the position of the second plurality of apertures relative to the third plurality of apertures and thereby varies the size of the first-mentioned plurality of apertures,

the faraday cup array further comprising:

a driver connected to said one of the plates to cause said one of the plates to slide relative to said other one of the plates,

wherein the driver includes:

a first arm connected to said one of the plates and the driver moves said arm to cause said sliding;

a second arm connected to said other one of the plates and the driver moves the second arm to cause said sliding; and

a rotating wheel having a axis of rotation, wherein the first and second arms are pivotally connected at one end to the wheel at a first and second connection points located at a distance from the axis of rotation and pivotally connected at another end to said one of the plates.

28. A faraday cup array comprising:

a plurality of faraday cups for gathering charge,

a variable aperture assembly having a plurality of apertures through which charged particles from the particle beam travel to the plurality faraday cups, each one of the plurality of faraday cups being located relative to the variable aperture assembly to intercept the charged particles traveling through at least one of the plurality of apertures, the aperture assembly constructed to vary the size of at least one of the apertures,

wherein at least one of the faraday cups is constructed with graphite.

29. A method comprising:

providing a faraday cup for gathering and measuring charge of a flow of charged particles, and

varying a size of an aperture, wherein the aperture controls the flow of the charged particles to the faraday cup,

wherein the size of aperture is varied in response to the value of the measured charge.

30. The method of claim 29 further comprising:

generating and directing a particle beam toward a work piece according to selected characteristics;

positioning the faraday cup in the path of the particle beam, at least a portion of the particle beam traveling through the aperture to supply the flow of particles; and

measuring the charge gathered by the faraday cup.

31. The method of claim 30 further comprising varying the size of the aperture to vary size of a cross section of the portion of the particle beam reaching the faraday cup.

32. The method of claim 30 further comprising:

integrating the measured charge value, the integrated value representing the current of the particle beam;

comparing the integrated value to a pre-selected value to determine a difference between the values; and

varying the selected characteristics based on the determined difference.

33. The method of claim 32 wherein the steps of integrating the measured charge value, measuring the charge and comparing the measured charge value to the pre-selected value are performed by a computer; and

the steps of varying the size of the aperture and varying the selected characteristics are performed in response to instructions from the computer.

34. The method of claim 30 further comprising:

comparing the measured charge value to a pre-selected value to determine a difference between the values, the measured charge value representing a profile of the beam; and

varying the selected characteristics based on the determined difference.

35. The method of claim 34 wherein the steps of measuring the charge and comparing the measured charge value to the pre-selected value are performed by a computer; and

the steps of varying the size of the aperture and varying the selected characteristics are performed in response to instructions from the computer.

36. The method of claim 29 wherein varying the size of the aperture further comprises varying the size of the aperture to cause the measured charge value to tend toward a preselected value representing a predetermined signal to noise ratio.

37. The method of claim 36 wherein measuring the charge further comprises integrating the value of the charge gathered by the faraday cup.

38. The method of claim 36 wherein the step of measuring the charge and the step of varying the size of the aperture is performed in response to instructions from the computer.