WO2002084713A2

WO2002084713A2 - Occluding beamline ion implanter

Info

Publication number: WO2002084713A2
Application number: PCT/US2002/011292
Authority: WO
Inventors: David S. Holbrook
Original assignee: Varian Semiconductor Equipment Associates, Inc.
Priority date: 2001-04-11
Filing date: 2002-04-10
Publication date: 2002-10-24
Also published as: WO2002084713A3; TW552629B

Abstract

Systems and methods for ion implantation of dopant materials into semiconductor wafers are provided. An ion implanter includes an ion source for generating an ion beam, a support platen for supporting a workpiece during ion implantation, a scan mechanism for scanning the workpiece relative to the ion beam, an occluder for blocking at least a portion of the ion beam from reaching the workpiece during at least a portion of workpiece scanning, and an occluder translator for translating the occluder during workpiece scanning. The scan mechanism produces translation of the workpiece and spinning of the workpiece about an axis of rotation that is located at or near the center of the workpiece. The ion implanter may include a uniformity control system for controlling the dose distribution of ions implanted into the workpiece.

Description

OCCLUDING BEAMLINE ION IMPLANTER

Cross Reference to Related Application

This application claims the benefit of provisional application Serial No. 60/283,044, filed April 11, 2001, which is hereby incorporated by reference in its entirety.

Field of the Invention This invention relates to systems and methods for ion implantation of dopant materials into semiconductor wafers and, more particularly, to systems and methods for implanting dopant materials into semiconductor wafers at low energies to form ultrashallow junctions.

Background of the Invention

Ion implantation has become a standard technique for introducing conductivity- altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.

A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1000 Angstroms and may eventually require junction depths on the order of 200 Angstroms or less.

The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. Ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 50 keV to 400 keV, and may not function efficiently at the energies required for a shallow junction implantation. At low implant energies, such as energies of 2 keV and lower, the ion current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication cost and is unacceptable to semiconductor device manufacturers.

Another trend in ion implanters is toward single wafer implanters, wherein one wafer at a time is implanted. Batch ion implanters have been utilized to achieve high throughput, but are large and expensive and place multiple, highly expensive wafers at risk.

One approach to single wafer ion implantation employs a so-called ribbon ion beam. The ribbon ion beam has a width that is at least as great as the diameter of the wafer, and the wafer is mechanically scanned in a direction perpendicular to the long dimension of the ribbon beam to distribute the ions over the wafer surface. This approach provides highly satisfactory performance but suffers from certain drawbacks. In particular, the ribbon beam is required to be highly uniform across its width. It is more expensive to create a uniform ion beam, as compared to not imposing a uniformity constraint.

Another well-known approach to single wafer ion implantation employs two- dimensional scanning of the ion beam over the wafer surface. The scanning can be electrostatic, magnetic, or a combination thereof. This approach also provides highly satisfactory performance. However, in order to permit scanning of a large wafer, a relatively long beamline is required. The long beamline is detrimental at ultralow energies, because the beam expands as a result of space charge expansion.

Accordingly, there is a need for new and improved methods and apparatus for ion implantation of a semiconductor wafer, particularly at ultra low energies.

Summary of the Invention

Systems and methods for ion implantation of dopant materials into workpieces, such as semiconductor wafers, are provided. An ion implanter includes an ion source for generating an ion beam, a wafer support platen for supporting a semiconductor wafer during ion implantation, a drive mechanism for scanning the semiconductor wafer relative to the ion beam, and an occluder for blocking at least a portion of the ion beam from reaching the semiconductor wafer during at least a portion of the wafer scan. The occluder affects the spatial ion current distribution incident on the wafer. The drive mechanism produces two components of wafer scanning, including spinning of the wafer about an axis of rotation, which may be located at or near the center of the wafer, and linear translation of the wafer relative to the ion beam. Preferably, the linear translation causes the ion beam to pass through the axis of rotation of the spinning wafer. The drive mechanism may include a spin motor and a linear translator. An occluder translator may be provided for translating the occluder during wafer scanning.

According to a first aspect of the invention, an ion implanter is provided. The ion implanter comprises an ion source for generating an ion beam, a support platen for supporting a workpiece during ion implantation, a scan mechanism for scanning the workpiece relative to the ion beam, an occluder for blocking at least a portion of the ion beam from reaching the workpiece during at least a portion of workpiece scanning, and an occluder translator for translating the occluder during workpiece scanning. The occluder is positioned between the ion source and the wafer and is spaced from the wafer. Preferably, the occluder is positioned well upstream of the wafer to limit wafer contamination through sputtering of the occluder by the ion beam. An image of the occluder is projected by the ion beam into the wafer (i.e., an occluder shadow is ion imaged onto the wafer). The occluder is translated relative to the ion beam during ion implantation. Preferably, the occluder is translated in synchronism with wafer translation such that the occluder image remains fixed on the wafer surface, excluding the effect of wafer spinning. It will be understood that occluder translation includes linear translation but not spinning.

The occluder may be a plate that is positioned between the ion source and the wafer so as to block some, all or none of the ion beam from reaching the wafer, depending on the position of the wafer and the occluder relative to the ion beam. A purpose of the occluder is to control dose distribution of the implanted dopant material over the surface of the semiconductor wafer. More particularly, a purpose of the occluder is to control dose uniformity near the center of rotation of the spinning wafer. In one embodiment, the occluder comprises a plate having a straight edge. The straight edge is oriented perpendicular to the direction of wafer translation and may pass through the axis of rotation of the spinning wafer. In another embodiment, the occluder comprises a plate having two edges which meet at a vertex. In one example, the edges form an angle of 90°. The one or more edges of the occluder may be straight, curved or arbitrarily shaped. In one embodiment, the vertex defined by the occluder edges may be located on the axis of rotation of the spinning wafer. In another embodiment, the vertex and/or an edge of the occluder may be displaced relative to the axis of rotation of the spinning wafer to control uniformity. In another embodiment, the occluder has a hole which passes all or part of the ion beam.

The ion implanter preferably includes a uniformity control system. The uniformity control system includes a uniformity monitor coupled to a uniformity controller, which may be incorporated into a system controller. The uniformity monitor measures dose as a function of radial position on the wafer and provides the measured values to the uniformity controller. The uniformity controller determines uniformity errors based on the measured values and computes translation velocity corrections to produce a desired uniformity. The uniformity controller provides velocity control signals to the linear translator in the wafer scan mechanism to achieve the desired uniformity. The translation velocity may be increased at a given radial position to reduce the implanted dose at that radial position or may be decreased in order to increase the implanted dose at that radial position.

In one embodiment, the translation velocity is near 1/r, where r is the distance between the ion beam and the axis of rotation of the spinning wafer. In another embodiment, the occluder configuration can be selected to reduce the translation velocity variation over the scan range and in particular to permit a constant or nearly constant translation velocity. The translational velocity of the wafer with respect to the occluded image of the ion beam controls the spatial uniformity of the dose delivered to the wafer. Moderating the uniformity-dependent translational velocity profile may be beneficial, especially near the center of the wafer where 1/r effects dominate. There are several factors which moderate the required translational velocity profile near the center of the wafer: 1) the size of the ion beam spot on the wafer surface; 2) the shape of the occluder; and 3) blur (prenumbral or other) of the image of the occluder at the surface of the wafer.

According to another aspect of the invention, a method for ion implantation of a workpiece is provided. The method comprises the steps of positioning a workpiece in the path of an ion beam, spinning the workpiece about an axis of rotation, translating the spinning workpiece relative to the ion beam, and occluding at least a part of the ion beam from reaching the workpiece during at least a portion of the step of translating the workpiece relative to the ion beam.

Brief Description of the Drawings For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

FIG. 1 is a schematic block diagram of an ion implanter in accordance with an embodiment of the invention;

FIG. 2 is a schematic diagram that illustrates the spinning wafer and a first embodiment of the occluder, as viewed along the ion beam axis;

FIG. 3 is a schematic diagram that illustrates the spinning wafer and a second embodiment of the occluder, as viewed along the ion beam axis; FIG. 4 is a schematic diagram that illustrates the spinning wafer and a third embodiment of the occluder, as viewed along the ion beam axis;

FIG. 5 is a schematic diagram that illustrates the spinning wafer and a fourth embodiment of the occluder, as viewed along the ion beam axis;

FIG. 6 is a schematic diagram that illustrates the spinning wafer and an occluder that is displaced relative to the axis of rotation of the spinning wafer, as viewed along the ion beam axis;

FIG. 7 is a schematic ray diagram that illustrates blurring of the ion beam edge for a first spacing between the occluder and the wafer plane;

FIG. 8 is a schematic diagram that illustrates blurring of the ion beam edge for a second spacing between the occluder and the wafer plane;

FIG. 9 is a top view of one embodiment of a uniformity monitor suitable for use in the ion implanter of FIG. 1;

FIG. 10A is a graph of translation speed and dose as a function of radial position, for the case of non-uniform dose; and FIG. 1 OB is a graph of translation speed and dose as a function of radial position, wherein the translation speed is corrected to provide uniform dose.

Detailed Description A schematic block diagram of an ion implanter in accordance with an embodiment of the invention is shown in FIG. 1. An ion beam generator 10 includes an ion source 12 for generating ions of a desired dopant material, an extraction electrode 14 positioned in proximity to an aperture in ion source 12, an extraction power supply 16 for biasing extraction electrode 14 negatively with respect to ion source 12 and a gas source 18 for supplying a gas to be ionized to ion source 12. Ions are extracted from ion source 12 by extraction electrode 14 to form an ion beam 20. A mass analyzer 30, which may include an analyzer magnet 32 and a mask 34 having a resolving aperture 36, selects a desired ion species from the particles generated by the ion beam generator 12.

The ion implanter further includes an end station 50 having a platen 52 for supporting a semiconductor wafer 54 or other workpiece in the path of ion beam 20, such that ions of the desired species are implanted into the semiconductor wafer 54. A drive system 60 produces wafer scanning as described in detail below.

The ion implanter further includes an occluder 64 for blocking all or a portion of the ion beam from reaching wafer 54 during at least a portion of the wafer scan. Occluder 64 is preferably spaced from wafer 54 to limit wafer contamination caused by sputtering of occluder 64. The occluder 64 may be positioned along the ion beam 20 between wafer 54 and ion source 12 in an ion optical arrangement which permits an image of occluder 64 to be projected onto wafer 54. An occluder translator 66 produces linear translation of occluder 64 perpendicular to the path of ion beam 20 as described below. A uniformity monitor 70 is used for measuring and adjusting the dose distribution of the dopant material over the wafer surface. A system controller 72 controls the components of the ion implanter to achieve a desired operation.

The ion implanter may include additional components known to those skilled in the art. For example, end station 50 typically includes automated wafer handling equipment for introducing wafers into the ion implanter and for removing wafers after ion implantation. It will be understood that the entire path traversed by the ion beam is evacuated during ion implantation. Different ion implanter configurations may be utilized within the scope of the invention, and the configuration of FIG. 1 is given by way of example only.

The drive system 60 includes a spin motor 80 for spinning platen 52 and wafer 54 about an axis of rotation 82. The axis of rotation 82 is parallel to ion beam 20 and is normal to the surface of wafer 54. Drive system 60 further includes a linear translator 84, which may be coupled by a shaft 86 to spin motor 80. The linear translator 84 produces linear translation of spinning wafer 54 such that ion beam 20 passes through the axis of rotation 82 of spinning wafer 54. The drive system 60 thereby produces radial translation of the spinning wafer 54 relative to ion beam 20.

As indicated above, drive system 60 produces wafer scanning, including wafer spinning and linear translation, relative to ion beam 20. It will be understood that scanning may be produced by movement of wafer 54, by movement of ion beam 20 or by a combination of wafer movement and beam movement. In a preferred embodiment, the ion beam 20 is stationary, and scanning is produced by movement of wafer 54. In this configuration, beam scanning is not required and a relatively short beamline may be utilized. As indicated above, wafer scanning includes a spinning component and a translation component. In order to achieve a desired dose uniformity, the translation velocity should be slow in comparison with the spin velocity. Preferably, the translation velocity and the spin velocity are controlled such that the ion beam is distributed relatively uniformly around annular or nearly annular regions of the spinning wafer as the spinning wafer is translated relative to the ion beam. Preferably, the wafer spinning velocity is in a range of about 100 to 10,000 rpm, and the wafer translation velocity is in a range of about 50 to 500 millimeters per second. Preferably, the axis of rotation 82 of spinning wafer 54 is located at or near the center of wafer 54 so as to balance the forces exerted on wafer 54 during spinning.

The occluder translator 66 produces linear ranslation of occluder 64 during wafer scanning to achieve a desired dose distribution. The ion beam 20 projects an image of occluder 64 onto spinning wafer 54. Preferably, system controller 72 synchronizes the operation of linear translator 84 and occluder translator 66, such that the image of occluder 64 remains fixed in position on spinning wafer 54 during wafer scanning. This is achieved by translating occluder 64 and wafer 54 along parallel paths in the same direction. The translation velocities of occluder 64 and wafer 54 may be different so as to ensure that the image of occluder 64 remains fixed on wafer 54, at least during the time that the occluder blocks all or part of the ion beam. In another embodiment, the translation velocities of the occluder 64 and the wafer 54 are adjusted such that the image of occluder 64 moves on wafer 54 in order to achieve a desired dose distribution.

For an ion beam of zero diameter, it can be shown that scanning the ion beam from the edge of the wafer to the center of the wafer at a rate that is proportional to 1/r, where r is the distance between the ion beam and the axis of rotation of the spinning wafer, produces uniform dose distribution. However, ion beams with zero diameter do not exist. Real ion beams have finite cross-sectional dimensions and are somewhat difficult to control as to size and shape. For a finite sized ion beam, with a scan rate near 1/r, a uniformity producing solution does not exist for arbitrary beam spot distributions. In general, a scan rate near 1/r creates a dose error in an annular region near the center of the wafer. The center of the wafer can be dosed correctly, but in doing so an annular region of different dose is formed near the center of the wafer.

In accordance with an aspect of the invention, the occluder 64 is provided to control the dose distribution of implanted dopant material over the wafer surface. More particularly, occluder 64 permits control of dose uniformity in a region near the axis of rotation 82 of spinning wafer 54.

Referring to FIG. 2, a schematic diagram of spinning wafer 54 and a first embodiment of the occluder 64 is shown. In the embodiment of FIG. 2, occluder 64 is configured as a plate having an edge 90. Edge 90 is a straight line which is perpendicular to the direction of wafer translation and which passes through axis of rotation 82 of spinning wafer 54. Edge 90 may be configured as a knife edge, as known in the art of ion beam apertures.

Operation of occluder 64 in controlling dose uniformity is described with reference to FIG. 2. As shown, spinning wafer 54 and occluder 64 are translated relative to ion beam 20. Thus, spinning wafer 54 moves to produce radial translation of ion beam 20 relative to wafer 54. In FIG. 2, ion beam 20 is illustrated at positions 92, 94 and 96 relative to spinning wafer 54 and occluder 64. As shown, the full cross-section of ion beam 20 is incident on spinning wafer 54 for a major portion of the wafer translation, as indicated at position 94. As wafer 54 is translated such that the ion beam 20 is near the axis of rotation 82, as indicated at position 96, a portion of the ion beam is blocked by occluder 64. Thus, the ion dose implanted in wafer 54 is reduced in comparison with the dose that would be implanted in the absence of occluder 64. The occluder 64 thereby has the effect of reducing ion dose in areas of wafer 54 where ion beam 20 is incident, at least in part, on occluder 64. By controlling the degree to which occluder 64 blocks ion beam 20 over the wafer surface, dose distribution can be controlled. A number of factors affect dose uniformity in the ion implanter shown in FIG. 1 and described above. One factor that affects dose uniformity is the translation velocity of spinning wafer 54 with respect to ion beam 20 as a function of radial position of ion beam 20 on wafer 54. As a first approximation, the translation velocity may be near 1/r, where r represents the radial position of ion beam 20 on spinning wafer 54 relative to axis of rotation 82. Adjustments to the translation velocity may be utilized to adjust dose uniformity, as described below. Also, the occluder configuration may be selected to provide a desired translation velocity profile and dose uniformity, as described below.

Additional factors that affect dose uniformity are the size and shape of the ion beam. It will be understood that the size and shape of the ion beam are likely to be different for different ion species and different ion energies. It is believed that the configuration described herein, which utilizes an occluder in combination with wafer spinning and translation, is relatively insensitive to beam size and shape.

The parameters associated with the occluder also affect dose uniformity. As described below, the occluder may have different edge configurations, including a single edge, two edges which meet at a vertex, or more than two edges. The edge or edges may be straight or curved. The edge configuration affects dose uniformity and may be utilized to achieve a desired result. In another embodiment, the occluder has at least one hole, or aperture, which passes all or part of the ion beam to control dose distribution over the wafer surface. In this embodiment, the size and shape of the hole affects dose uniformity. In addition, dose uniformity is affected by the position of the occluder relative to the wafer during wafer scanning, including the position of the occluder in a plane parallel to the plane of the wafer and the spacing between the occluder and the wafer. Furthermore, dose uniformity is affected by any movement of the occluder image relative to the wafer during wafer scanning. The occluder configuration is selected to produce a desired dose distribution and preferably is selected to limit beam blockage. Typically, a uniform dose distribution is desired. As a general proposition, those occluder configurations which achieve the desired uniformity with limited beam blockage should be utilized. This approach maximizes the ion current reaching the wafer and thereby maximizes throughput. By way of example, an occluder having a small aperture may enhance uniformity, but at the expense of decreased throughput.

In the ion implanter of FIG. 1, the ion beam 20 is preferably normal to the surface of wafer 54 so as to avoid a variable incidence angle with respect to the crystal structure of the semiconductor wafer during wafer spinning. An occluder configuration in the form of a plate having straight edge 90 is shown in

FIG. 2. It is believed that the occluder configuration of FIG. 2 may be utilized in a variety of applications, with different beam sizes and shapes, to achieve dose uniformity over the surface of wafer 54. Other occluder configurations are illustrated in FIGS. 3-6. Like elements in FIGS. 2-6 have the same reference numerals. A second embodiment of the occluder is shown in FIG. 3. An occluder 100 is configured as a plate having edges 102 and 104 which meet at a vertex 106. In the embodiment of FIG. 3, vertex 106 is located on axis of rotation 82, and edges 102 and 104 form an angle of 90°. Edges 102 and 104 and are preferably oriented angles of +45° and - 45° with respect to the direction of wafer translation. A third embodiment of the occluder is shown in FIG. 4. An occluder 110 is configured as a plate having curved edges 112 and 114, and a straight edge 116 joining curved edges 112 and 114. Preferably, the center of edge 116 is located on axis of rotation 82. Curved edges 112 and 114 provide variable blockage of ion beam 20 as a function of radial position during wafer translation. It will be understood that different curvatures may be utilized and that one or both of edges 112 and 114 may be straight or curved. Furthermore, edge 116 may be straight or curved and may have any suitable length. A fourth embodiment of the occluder is shown in FIG. 5. An occluder 120 is configured as a plate having edges 122 and 124, which meet at a vertex 126. Preferably, each of edges 122 and 124 is oriented at an angle of with respect to a direction 128 of wafer translation. The angle may be less than 90° or more than 90°, depending on the desired beam blockage as a function of ion beam position on spinning wafer 54. Furthermore, edges 122 and 124 may be straight or curved.

In the embodiment of FIG. 2, edge 90 of occluder 64 is positioned on axis of rotation 82. In the embodiments of FIGS. 3 and 5, the vertices 106 and 126 are positioned on the axis of rotation 82 of the spinning wafer. In the embodiment of FIG. 4, the center of edge 116 is located on axis of rotation 82. Referring now to FIG. 6, a configuration wherein edge 90 of occluder 64 is displaced by a distance d relative to axis of rotation 82 is shown. The displacement of edge 90 may be used to achieve a desired beam blockage during wafer translation to thereby achieve a desired dose uniformity. The displacement of edge 90 relative to axis of rotation 82 may be utilized to compensate for occluder image blurring and or to adjust the dose uniformity profile in a particular situation. The displacement d may be positive or negative with respect to axis of rotation 82. Furthermore, for occluders having edges which meet at a vertex, the vertex may be displaced in a positive or negative direction with respect to axis of rotation 82 to achieve a desired dose uniformity.

Preferably, the occluder edge configuration is symmetrical with respect to the path of wafer translation relative to the ion beam. Thus, for example, in FIG. 5 the edges 122 and 124 of occluder 120 are symmetrically located with respect to direction 128 of wafer translation. The occluders shown in FIGS. 2-4 also have symmetrical configurations. Blurring of the occluder image projected on wafer 54 is illustrated in FIGS. 7 and 8 for different spacings between occluder 64 and wafer 54. In FIG. 7, occluder 64 is spaced from wafer 54 by a distance Si . An image of the occluder 64 is projected onto wafer 54. The image of occluder 64 on wafer 54 has a blurred region 152 with a width bi. A region to the left of blurred region 152 is entirely blocked by occluder 64, and a region to the right of blurred region 152 is entirely unblocked. Referring now to FIG. 8, occluder 64 is spaced from wafer 54 by a distance s₂ which is greater than the distance S_t shown in FIG. 7. A blurred region 154 has a width b₂ that is greater than the width bi of blurred region 152 in FIG. 7. Thus, it can be seen that image blurring is a function of the spacing between occluder 64 and wafer 54. The ion beam 20 typically varies in size, shape and intensity for different ion species and different ion energies. The different ion beam parameters are likely to produce different dose uniformity profiles in implanted wafers. Accordingly, the ion implanter preferably includes a uniformity control system. As shown in FIG. 1, the uniformity control system includes a uniformity monitor 70 coupled to system controller 72, which includes a uniformity controller. The uniformity monitor 70 measures dose as a function of radial position on the wafer for a given ion beam and provides the measured values to the uniformity controller. The uniformity controller determines uniformity errors based on the measured values and computes translation speed corrections to produce a desired uniformity. The uniformity controller provides speed control signals to the linear translator 84 to achieve the desired uniformity. The translation speed may be increased at a given radial position to reduce the implanted dose at that radial position or may be decreased in order to increase the implanted dose at that radial position.

An embodiment of uniformity monitor 70 is shown in FIG. 9, as viewed along the direction of ion beam 20. Uniformity monitor 70 is mounted on shaft 86 for translation by linear translator 84 (FIG. 1). Uniformity monitor 70 includes a plurality of dosimetry cups, or Faraday cups, for measuring ion beam current. As shown in FIG. 9, uniformity monitor 70 includes arc-shaped Faraday cups 200, 202, 204 and 206 and a semi-circular Faraday cup 210. Faraday cups 200, 202, 264, 206, and 210 all have a common center of curvature 212. In a preferred embodiment, Faraday cups 200, 202, 204, 206 and 210 have entrance apertures with equal areas. The width of each Faraday cup in a direction perpendicular to the direction of wafer translation is at least as great as the ion beam dimension. As the uniformity monitor 70 is translated by linear translator 84 relative to ion beam 20, the Faraday cups 200, 202, 204, 206 and 210 receive ion beam current that represents the ion beam current implanted in wafer 54 during ion implantation. A practical implementation of dose monitor 70 may have more arc-shaped Faraday cups than shown in FIG. 9 to increase the spatial resolution of the uniformity measurement. An output current from each Faraday cup is supplied to the uniformity controller in system controller 72 (FIG. 1). The measured currents from each of the Faraday cups provide a representation of the dose uniformity of implanted dopant material in wafer 54. As indicated above, Faraday cups 200, 202, 204, 206 and 210 preferably have entrance apertures with equal areas. This configuration is unique because the geometry performs the math to convert from physical measurement of the ion beam to scan rate determination. The output of each Faraday cup is proportional to the required scan rate at that point in the scan. The Faraday outputs a one dimensional signal whose values are proportional to the wafer dose in the spinning wafer system. In another embodiment, the entrance apertures of the Faraday cups have different areas, and the ratios of the areas are taken into account in calculating dose uniformity.

Adjustment of dose uniformity using the uniformity control system is described with reference to FIGS. 10A and 10B. In FIG. 10 A, the uniformity monitor 70 is scanned with a translation speed profile 230 that increases with decreasing radius and produces a dose uniformity profile 232 having a region 234 where the dose is greater than the desired value. The uniformity control system detects the region 234 and corresponding radius values and determines a translation speed correction to provide uniform dose. As shown in FIG. 10B, a corrected translation speed profile 240 provides increased translation speed for the radius values in region 234 where the dose is above the desired value, so as to decrease the implanted dose and provide a uniform dose uniformity profile 242. Similarly, the implanted dose can be increased for selected values of radius by decreasing the translation speed at that radius. After the translation speed has been adjusted to provide the desired uniformity profile, ion implantation of wafer can proceed. As noted above, the wafer translation velocity to achieve uniform dose on the spinning wafer may be approximately 1/r, for an ion beam having a small cross-sectional dimension. According to another feature of the invention, the occluder configuration can be selected to provide a specified translation velocity profile, while achieving the desired dose distribution. For example, the occluder configuration can be selected to reduce the translation velocity variation over the scan range and in particular to permit a constant or nearly constant translation velocity. This relaxes the requirements on the linear translator 84 (FIG. 1) and the occluder translator 66, and permits a tighter control loop to be utilized. By way of example, an occluder having a 1/r edge configuration can be utilized with a spinning wafer having a constant translation velocity relative to a uniform transverse ribbon ion beam.

The translational velocity of the wafer with respect to the occluded image of the ion beam controls the spatial uniformity of the dose delivered to the wafer. Moderating the uniformity dependent translational velocity profile is important, especially near the center of the wafer where 1/r effects dominate. There are several factors in this invention which moderate the required translational velocity profile near the center of the wafer: 1) the size of the ion beam spot on the wafer surface; 2) the shape of the occluder; and 3) blur (prenumbral or other) of the image of the occluder at the surface of the wafer.

It should be understood that various changes and modifications of the embodiments shown in the drawings described in the specification may be made within the spirit and scope of the present invention. Accordingly, it is intended that all matter contained in the above description and shown in the accompanying drawings be interpreted in an illustrative and not in a limiting sense. The invention is limited only as defined in the following claims and the equivalents thereto. What is claimed is:

Claims

1. An ion implanter comprising: an ion source for generating an ion beam; a support platen for supporting a workpiece during ion implantation; a scan mechanism for scanning the workpiece relative to the ion beam; an occluder for blocking at least a portion of the ion beam from reaching the workpiece during at least a portion of workpiece scanning; and an occluder translator for translating the occluder during workpiece scanning.

2. An ion implanter as defined in claim 1, wherein said occluder is positioned between said ion source and the support platen.

3. An ion implanter as defined in claim 1, wherein said scan mechanism produces translation of the workpiece and spinning of the workpiece about an axis of rotation.

4. An ion implanter as defined in claim 3, wherein the axis of rotation is located at or near the center of the workpiece.

5. An ion implanter as defined in claim 3, wherein the workpiece is translated such that the ion beam passes through the axis of rotation.

6. An ion implanter as defined in claim 1, wherein said scan mechanism comprises a spin motor for spinning the wafer about an axis of rotation and a linear translator for producing linear translation of the workpiece and the spin motor relative to the ion beam.

7. An ion implanter as defined in claim 3, wherein said occluder is positioned to block at least a portion of the ion beam near the axis of rotation.

8. An ion implanter as defined in claim 1, wherein an occluder image produced by said occluder remains fixed on the workpiece during workpiece scanning.

9. An ion implanter as defined in claim 1, wherein an occluder image produced by said occluder moves on the workpiece during workpiece scanning.

10. An ion implanter as defined in claim 1 , wherein said occluder comprises a plate having a straight beam-occluding edge.

11. An ion implanter as defined in claim 10, wherein the beam-occluding edge passes tlirough the axis of rotation of the workpiece.

12. An ion implanter as defined in claim 10, wherein the beam-occluding edge is displaced from the axis of rotation of the workpiece.

13. An ion implanter as defined in claim 1, wherein said occluder comprises a plate having two beam-occluding edges that meet at a vertex.

14. An ion implanter as defined in claim 13, wherein the beam-occluding edges are straight.

15. An ion implanter as defined in claim 13, wherein the beam-occluding edges are curved.

16. An ion implanter as defined in claim 13, wherein the beam-occluding edges meet at an angle of 90°.

17. An ion implanter as defined in claim 13, wherein the beam-occluding edges are disposed at an angle with respect to a direction of translation of the workpiece.

18. An ion implanter as defined in claim 1 , wherein said occluder comprises a plate having two or more beam-occluding edges.

19. An ion implanter as defined in claim 18, wherein the beam-occluding edges are symmetrically disposed with respect to a path of workpiece translation.

20. An ion implanter as defined in claim 13, wherein the vertex of the beam-occluding edges is located on the axis of rotation of the workpiece.

21. An ion implanter as defined in claim 13, wherein the vertex of the beam-occluding edges is displaced from the axis of rotation of the workpiece.

22. An ion implanter as defined in claim 1 , further comprising a uniformity control system for controlling the dose distribution of ions implanted into the workpiece.

23. An ion implanter as defined in claim 22, wherein the uniformity control system comprises a uniformity monitor for measuring ion dose as a function of position on the workpiece and a uniformity controller for controlling workpiece translation in response to the measured dose to produce a desired dose distribution.

24. An ion implanter as defined in claim 23, wherein said uniformity controller determines workpiece translation velocity corrections in response to the measured dose.

25. An ion implanter as defined in claim 23, wherein said uniformity monitor comprises a plurality of Faraday cups coupled to said scan mechanism.

26. An ion implanter as defined in claim 25, wherein said Faraday cups include a semicircular Faraday cup and one or more arc-shaped Faraday cups.

27. An ion implanter as defined in claim 25, wherein said Faraday cups have equal area entrance apertures.

28. An ion implanter as defined in claim 22, wherein said uniformity control system measures ion dose as a function of radial position with respect to the axis of rotation.

29. An ion implanter as defined in claim 1, wherein the scan mechanism translates the workpiece at a velocity that is an approximate function of 1/r, where r represents the distance between the ion beam and the axis of rotation.

30. An ion implanter as defined in claim 1, wherein the scan mechanism translates the workpiece at approximately constant velocity.

31. An ion implanter as defined in claim 1 , further comprising a system controller for controlling said scan mechanism and said occluder translator.

32. An ion implanter as defined in claim 1, wherein the workpiece is translated at a translation velocity and is rotated at a spin velocity, wherein the translation velocity and the spin velocity are controlled such that the ion beam is distributed around annular or nearly annular regions of the spinning workpiece as the spinning workpiece is translated relative to the ion beam.

33. An ion implanter as defined in claim 1, wherein the workpiece and the occluder are translated in planes perpendicular to the ion beam.

34. A method for ion implantation of a workpiece, comprising the steps of: positioning a workpiece in the path of an ion beam; spinning the workpiece about an axis of rotation; translating the spinning workpiece relative to the ion beam; and occluding at least a part of the ion beam from reaching the workpiece during at least a portion of the step of translating the spinning workpiece.

35. A method as defined in claim 34, wherein the step of occluding at least a part of the ion beam comprises positioning an occluder between an ion source and the workpiece, and translating the occluder during at least a portion of the step of translating the spinning workpiece.

36. A method as defined in claim 34, wherein the axis of rotation is located at or near the center of the workpiece.

37. A method as defined in claim 34, wherein the workpiece is translated such that the ion beam passes through the axis of rotation.

38. A method as defined in claim 34, wherein the step of occluding at least a part of the ion beam comprises blocking at least a portion of the ion beam near the axis of rotation.

39. A method as defined in claim 35, wherein the occluder is translated such that an occluder image produced by the occluder remains fixed on the workpiece during at least a portion of the step of translating the spinning workpiece.

40. A method as defined in claim 35, wherein the occluder is translated such that an occluder image produced by the occluder moves on the workpiece during at least a portion of the step of translating the spinning workpiece.

41. A method as defined in claim 34, wherein the step of occluding at least a part of the ion beam comprises positioning in the ion beam an occluder comprising a plate having a beam-occluding edge.

42. A method as defined in claim 34, further comprising the steps of measuring ion dose as a function of position on the workpiece and controlling translation of the spinning workpiece in response to the measured dose to produce a desired dose distribution.

43. A method as defined in claim 34, wherein the step of translating the spinning workpiece comprises translating the workpiece at a velocity that is an approximate function of 1/r, where r represents the distance between the ion beam and the axis of rotation.

44. A method as defined in claim 33, wherein the step of translating the spinning workpiece comprises translating the workpiece at approximately constant velocity.

45. A method as defined in claim 34, wherein the workpiece is translated at a translation velocity and is rotated at a spin velocity, further comprising the step of controlling the translation velocity and the spin velocity such that the ion beam is distributed around annular or nearly annular regions of the spinning workpiece as the spinning workpiece is translated relative to the ion beam.

46. Apparatus for ion implantation of a workpiece, comprising: means for positioning a workpiece in the path of an ion beam; means for spinning the workpiece about an axis of rotation; means for translating the spinning workpiece relative to the ion beam; and means for occluding at least a part of the ion beam from reaching the workpiece during at least a portion of the step of translating the spinning workpiece.

47. Apparatus as defined in claim 46, wherein said means for occluding comprises an occluder for blocking at least a portion of the ion beam from reaching the workpiece and an occluder translator for translating the occluder during workpiece translation.

48. Apparatus as defined in claim 46, wherein the axis of rotation is located at or near the center of the workpiece.

49. Apparatus as defined in claim 46, wherein the workpiece is translated such that the ion beam passes through the axis of rotation.

50. Apparatus as defined in claim 47, wherein an occluder image produced by said occluder remains fixed on the workpiece during translation of the spinning workpiece.

51. Apparatus as defined in claim 47, wherein an occluder image produced by said occluder moves on the workpiece during translation of the spinning workpiece.

52. Apparatus as defined in claim 46, further comprising means for measuring ion dose as a function of position on the workpiece and means for controlling workpiece translation in response to the measured dose to produce a desired dose distribution.

53. Apparatus as defined in claim 46, wherein the means for translating the spinning workpiece translates the workpiece at a velocity that is an approximate function of 1/r, where r represents the distance between the ion beam and the axis of rotation.

54. Apparatus as defined in claim 46, wherein the means for translating the spinning workpiece translates the workpiece at approximately constant velocity.