METHOD AND APPARATUS FOR CONSTANT ANGLE OF INCIDENCE SCANNING IN ION BEAM SYSTEMS

Abstract: An ion beam (18) is scanned over a semiconductor wafer (30) in an ion implanter by deforming the wafer to have a concave contour selected to provide a constant angle of incidence of the scanned beam on the wafer surface. In one embodiment, the beam is scanned along one axis with a constant angle of incidence and is scanned about a center of deflection (46) along a second orthogonal axis. The wafer is deformed to have a concave cylindrical contour. The wafer can be clamped to a concave cylindrical platen surface by electrostatic or peripheral clamping. In a second embodiment, the beam is scanned in two dimensions about a center of deflection and the wafer is deformed to have a concave spherical contour. The wafer can be clamped to a concave spherical platen surface by electrostatic clamping.

Description: METHOD AND APPARATUS FOR CONSTANT ANGLE OF INCIDENCE SCANNING IN ICN BEAM SYSTEMS Field of the Invention This invention relates to the field of ion optics and, more particularly, to methods and apparatus for scanning an ion beam over a workpiece surface while maintaining a substantially constant angle of incidence between the beam and the workpiece surface. The workpiece is typically a semiconductor wafer. Eackground of the Invention Ion implantation has become a standard technique for introducing impurity dopants into semiconductor wafers. A beam of ions is generated in a source and is directed with varying degrees of acceleration toward a target wafer. Ion implantation systems typically include an ion source, ion optics for removing undesired ion species and for focusing the beam, means for deflecting the ion beam over the " target area, and an end station for mounting and exchanging wafers. In most cases, the cross-sectional area of the ion beam is substantially smaller than the area of the target wafer. Therefore, relative motion between the wafer and'the ion beam is necessary in order to distribute the implanted dose over the wafer surface. In serial ion implantation systems, wafers are implanted one at a time and the ion beam is scanned in two dimensions over the workpiece surface by electrostatic deflection plates. As used herein, "angle of incidence" means the instantaneous angle between the ion beam and the wafer surface.In serial systems, the angle of incidence of the beam on the wafer varies with beam deflection angle when the wafer is mounted flat. When the wafer is clamped to a convex or domed surface for enhanced cooling, variations in angle of incidence are increased. As a result of variations in the angle of incidence, the implanted ion dose per unit area of the wafer varies with the deflection angle. The trend in semiconductor wafers has been toward larger diameters (up to eight inches) to achieve economy of scale. As wafer dimensions increase, deflection angles and dose variations are further increased. To limit dose variations, the length of the . scanning apparatus can be increased so as to maintain a more nearly constant deflection angle. Neither increased dose variation nor additional system length is desirable, since dose uniformity requirements have become more stringent, while clean room floor space is at a premium. Special scanning techniques can be used to compensate for dose variations due to scan angle, as described in U.S. Patent Nos. 4,283,631 issued August 11, 1981 and 4,449,051 issued May 15, 1984, both assigned to the assignee of the present application. However, such techniques are complex and do not compensate for the effects described below. It is known that incident ions of a given energy penetrate into the crystal lattice of the target wafer by different distances, depending on their angle of incidence with respect to various crystal planes. For certain angles of incidence, an effect „W,O Λ „8„8/ , n 0_2 n 92„ n 0 -4- known as channeling results in relatively large penetration depths. The variations in incident angle for a scanned ion beam result in different penetration depths, and a corresponding variation in 05 device characteristics, over the surface area of the wafer. It is customary in ion implantation to tilt the target wafer by a few degrees with respect to the incident ion beam to avoid certain angles of incidence and to thereby minimize channeling.. 10 A further undesirable effect of incidence angle variations is shadowing. In the manufacture of integrated circuits, the wafer surface is frequently nonplanar after various etching and deposition steps. When it is desired to implant impurities 15 into wells with ' vertical walls, any variations in incidence angle cause a shadowing of part of the well by surrounding higher levels and produce non-uniform implantation of the wells. Constant angular incidence has been acheived in 20 the prior art with electron beams using magnetic and electrostatic double deflection scanning, as shown in V.S. Patent Nos. 4,101,813 issued July 18, 19 . 78 and 4,117,339 issued September 26, 1978 * Double deflection systems are practical in the case . of 25 electron beams due to the typically small electron currents and the small electron mass. One drawback of double deflection systems is that they require a careful synchronization of time-varying electrical inputs to the deflection elements. Although double deflection systems can, in theory, be applied to ion beam systems, these systems have severe practical limitations. Ion implantation systems are required to work with beams in the range up to 100 milliamps and to implant high mass ions (such as arsenic). High current ion beams must be space charge neutralized by electrons traveling with the beam to avoid beam expansion, or blowup, due to charge repulsion. The use of electrostatic deflection elements results in the removal of the neutralizing electrons and an unacceptable beam expansion due to space charge repulsion. Magnetic elements do not remove electrons f om the neutralized beam, but for high atomic mass ions, magnetic deflection elements are large, heavy and power consuming. A magnetic double deflection system for use in an ion implanter is disclosed in U.S. Patent No. 4,367,411 issued January 4, 1983 and assigned to the assignee of the present application. The disclosed system produces normal beam scanning in one dimension. Two dimensional double deflection scanning of heavy ions would require extremely large and impractical elements. Ion beam lithography utilizes a finely focused ion beam which is deflected over the surface of a workpiece. Scanning with a constant angle of incidence is desirable in the case of ion beam lithography for the same reasons as set forth above in connection with ion implantation. It is a general object of the present invention to provide improved methods and apparatus for ion beam scanning. It is another object of the present invention to provide an ion beam system with a constant angle of incidence between the ion beam and the workpiece. It is another object of the present invention to provide ion beam treatment methods and apparatus wherein a thin, flexible workpiece is deformed to match a predetermined scanning pattern such that a constant angle of incidence is achieved ^ - Summary of the Invention According to the present invention, these and other objects and advantages are achieved in apparatus for charged particle beam scanning of a thin, flexible workpiece with a constant angle of incidence. The apparatus comprises means for forming a charged particle beam, first scanning means for deflecting the beam over the workpiece along a first axis with a substantially constant -1- angle cf incidence, second scanning means for deflecting the beam over the workpiece along a second axis orthogonal to the first axis about a center of deflection at a distance R from the workpiece, and mounting means for positioning the workpiece in the path of the ion beam. The mounting means includes means for deforming the workpiece to have a concave contour coincident with the surface of an imaginary cylinder having an axis passing through the center of deflection parallel to the - first axis, and having a radius R. In a preferred embodiment, the first scanning means includes a first electrostatic deflector for deflecting the beam and an angle correction magnet positioned downstream of the electrostatic deflector for steering the scanned ion beam into parallel paths. The mounting means includes a support platen with a surface having a concave contour and a clamping ring for clamping the workpiece against the concave contour. According to another aspect of the present invention, there is provided apparatus for charged particle beam scanning of a thin, flexible workpiece with a substantially constant angle of incidence comprising means for forming a charged particle beam, scanning means for deflecting the beam over the workpiece surface in a two-dimensional pattern about.a center of deflection at a distance R from the workpiece and mounting means for positioning the workpiece in the path of the beam. The mounting means includes means for deforming the workpiece to have a concave contour coincident with the surface of an imaginary sphere having a center at the center of deflection, and having a radius R. According to yet another aspect of the present invention, there is provided a method for charged particle beam scanning of a thin, flexible workpiece with a substantially constant angle of incidence. The method comprises the steps of forming a charged particle beam, positioning the workpiece in the path of the ion beam, scanning the beam in a predetermined two-dimensional pattern, and deforming the workpiece to have a concave contour selected to provide a constant angle of incidence between the deflected beam and the workpiece. Brief Description of the Drawings For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference may be had to the accompanying drawings, which are incorporated herein by reference and in which: FIG. 1 is a plan view of an ion implantation system in accordance with the present invention; FIG. 2 is a side elevation of the ion implantation system of FIG. 1; FIGS. 3A and 3B illustrate a workpiece mounting arrangement for the system of FIGS. 1 and 2; FIG. 4 illustrates workpiece mounting by electrostatic clamping; and FIG. 5 illustrates an alternate embodiment of the present invention. Detailed Description of the Invention An .ion implantation system in accordance with the present invention is shown in FIGS. 1 and 2. A high voltage terminal 10 is held at high voltage potential relative to ground by a high power supply 12. Terminal 10 contains ion source apparatus required to form a beam of ions of a desired species. In common practice, an ion source 14 is provided to ionize either a gas derived from a gas handling system or a vapor formed by vaporizing a solid material. A typical ion source 14 requires power supplies to sustain an ionizing discharge, to impose an axial magnetic field across the discharge region, and to shape the electric field at the aperture of the ion source, thus achieving effective extraction of a well-de ined high current ion beam from the ion source 14. A variety of ion sources are known in the art. See, for example, Aitken, "Ion Sources', Ion Implantation Techniques, Springer-Verlag, 1982. An ion beam 18 diverging from the source 14 is momentum analyzed in an analyzer magnet 20, which is energized from an analyzer power supply (not shown) . The analyzed beam passes through an analyzer exit slit 24 to an accelerator tube 26 where it encounters a carefully designed field gradient from the high voltage terminal 10 to ground potential. Optical elements such as a ^ quadrupole lens 28, which may take the form of a quadrupdle triplet or other focusing elements, operate to produce a spatial and energy focus at a target 30. The target 30 is typically a semiconductor wafer. A deflection system comprising x-axiε electrostatic deflection plates 36, y-axiε electrostatic defleciton plates 38 and an angle correction magnet 40 scans the beam over the desired area of the target 30. The waveforms applied to the deflection plates 36, 38 and their synchronization to form the desired scanning pattern are generated by a scan control system 42. The operation of the deflection system is described in detail hereinafter. The beam 18 is deflected by a fixed angle from beam axis 44 sufficient to completely separate the beam from a neutral component arising principally from charge exchange collisions betwen residual gases and the charged beam. In typical ion implantation systems, target wafer 30 is positioned in a target chamber containing beam defining apertures, beam monitoring and integrating apparatus and equipment for introducing the semiconductor wafer into the vacuum system and positioning and cooling the wafer during ion implantation. A unique mounting arrangement for the target 30 is described in detail hereinafter. The entire region traversed by the ion beam 18 between the source 14 and the target 30 is maintained at high vacuum by a vacuum pumping system (not shown). Except for" the deflection system and the arrangement for mounting of the target 30, the above-described system is exemplified by a Model 350D Ion Implanter, manufactured and sold by Varian Extrion Division, Gloucester, Massachusetts. In accordance with the present invention, the deflection system provides a constant angle of incidence scan -in the x direction and a conventional scan about a center of deflection 46 in the y direction. The x direction scanning is produced by the combined action of x-axis deflection plates 36 and angle correction magnet 40. The beam 18 is deflected in a conventional manner by deflection plates 36, which are energized by using a ramp voltage to provide scanning and a DC voltage to provide a fixed offset relative to the beam axis 44. Angle correction magnet 40 is positioned downstream of the deflection plates 36 so as to intercept the scanned beam. The angle correction magnet 40 comprises approximately sector-shaped pole pieces 40a and 40b, separated by gap 40c and energized by windings 40d and 40e. The magnetic elements produce in the gap 40c a non-time-varying magnetic field parallel to the y-axis. The bending or steering of the beam produced by the correction magnet 40 varies with beam deflection such that the - beam exiting therefrom follows parallel paths as the beam is scanned. As a result, the beam intercepts the target 30 with constant angle of incidence as it is scanned by the x-axis deflection plates 36. The operation of sector-shaped magnets to provide parallel scanning is known in the art. See, for example, U.S. Patent No. 4,367,441. Since a constant angle of incidence scan is provided in the x direction, target 30 is maintained flat along the x direction. After exiting from the angle correction magnet 40 the beam passes between y-axis deflection plates 38. Typically, a ramp scanning voltage of lower frequency than the x direction scanning voltage is applied to the deflection plates 38 to produce y direction scanning about the center of deflection 46. The distance from the center of deflection 46 to the target 30 is denoted as R. Usually, the composite scan pattern produced by the deflection system is a raster type scan pattern which can be square-, rectangular or circular and covers the surface of the target wafer and overlaps the edges to the extent necessary for uniformity and for beam- sensing. The scanning ' produced by the deflection plates 38 results in a variation of angle of incidence for a flat target " 30. In accordance with the present invention, the target 30, which is presumed to be thin and somewhat flexible, is deformed to have a concave contour. The concave contour is chosen to produce a constant angle of incidence between the - ion beam and the target 30. In the embodiment of FIGS. 1 and 2, the target .30 is shaped to coincide with the surface of an imaginary cylinder having an axis passing through the center of deflection 46 parallel to the x-axis and a radius equal to the distance R .between ' the center of deflection 46 and the surface of the workpiece 30. As a result, the target 30 takes on a shape which is a portion of a cylindrical surface parallel to the x-axis. A mounting arrangement for providing the above-described cylindrical contour is shown in FIGS. 3A and 3E. The mounting arrangement comprises a wafer support platen 50 having a surface with the desired concave contour and a clamping ring 52 for peripheral clamping of the target wafer 30 to the. platen surface. FIG. 3A is a cross-section taken, along the y-axis while FIG. 3B is a cross-section taken along the x-axis. The clamping ring 52 presses the wafer 30 against the contoured platen surface and holds it in position. It will be understood that the concave contour of the platen 50 has a relatively slight curvature to avoid damage to the wafer caused by excessive bending. In one example of the present invention, the distance between the center of deflection 46 and the wafer surface is about 64 inches. Thus, the deflection at the center of a 200mm diameter wafer is only about 0.125 inches. In an alternate embodiment of the mounting arrangement, peripheral wafer clamping is replaced by electrostatic clamping as shown in FIG. 4. Electrostatic clamping of wafers to flat surfaces is known in the art. A voltage source 60 is connected between a wafer 62 and an electrode, which can be a conductive platen 64, spaced from the wafer 62 by a dielectric layer 66 to produce electrostatic attraction of the wafer 62 to the platen surface. Electrostatic clamping has the advantages of exposing one entire wafer surface for treatment with no surface area lost to clamping, and of operating in vacuum. Both peripheral clamping and electrostatic clamping can be assisted by an optional vacuum chuck 70, located in the platen surface and connected to a vacuum pump 72, as shown in FIG. 4. When the vacuum pump 72 is in operation, the vacuum chuck 70 draws the wafer 62 against the platen surface. It will be understood that the vacuum chuck 70 operates only while the treatment chamber or vacuum lock is at relatively high pressure during wafer exchange and is shut off when the system is vacuum pumped. However, the vacuum chuck can be used to fix the wafer in position against the concave contour of the platen surface, after which peripheral clamping or electrostatic clamping is used to hold the wafer in position during ion beam treatment. Any of the well known techniques for wafer cooling can be applied to the mounting arrangement of the present invention. For example, a gas at a pressure in the range of about one to fifty Torr can be introduced from a gas source 74 through a passage 76 in the platen 50 (see FIGS. 3A and 3B) into a thermal transfer region between the wafer 30 and the platen surface, as shown and described in U.S. Patent No. 4,457,359 issued July 3, 1984 and assigned to the assignee of the present application. The thermal transfer region can be sealed by a peripheral O-ring 7S in the platen surface. Alternatively, a thermally conductive rubber pad (not shown) can be positioned between the wafer and the platen surface to enhance thermal conductivity. Furthermore, the platen 50 can be cooled by circulation of cooling water through passages 80 in the platen 50 to remove heat transferred from the wafer. The cylindrical concave contour required by the preferred embodiment of the present invention is advantageous for several reasons. The wafer is required to be deformed only in one dimension, thereby limiting the risk of damage to the wafer. In addition, the cylindrical contour makes clamping with a peripheral clamping ring relatively easy. Finally, known cooling techniques can be utilized with this arrangement. A variety of optical elements can be utilized in place of the angle of correction masnet 40 for steering the scanned beam into parallel paths. Examples of such other optical elements are a quadrupole lens, a space charge lens and a solenoid lens. Space charge lens have been described by Mobley et al in IEEE Trans. Nucl. Sci., Vol. NS-26, No. 3 (June 1979) pp. 3112-3114 and by Booth et al in Nucl. Instru . Methods, Vol. 151, (1978) pp. 143-147. Generally, the optical element is positioned with its focal point at the center of deflection of the x-axis deflection plates 36 to obtain a scanned beam with parallel paths. . An alternate embodiment of the present invention is shown in FIG. 5. The ion beam 18 is scanned in two dimensions by x-axis deflection plates 84 and y-axis deflection plates 86 and 88. As in the previous embodiment, a wafer 90 is positioned in the path of the scanned beam by a mounting arrangement 92. The y-axis deflection plates 86 and 88 are divided into Flates 86 upstream of the x-axis plates 84 and plates 88 downstream of the x-axis plates 84. The divided configuration provides a center of deflection 94 at a single point for scanning in two dimensions. The mounting arrangement 92 deforms the wafer 90 to have a concave contour which lies on the surface of an imaginary sphere having a center at the center of deflection 94 and a radius equal to the distance from the center of deflection 94.to the wafer surface. In one example of this configuration. the distance between the center of deflection 94 and the wafer surface is about 64 inches. When the wafer 90 is deformed to have a concave spherical contour as described, the scanned beam is incident upon the wafer surface at a constant angle over the wafer surface area. In the mounting arrangement 92 shown in FIG. 5, a wafer support platen 96 is provided with a concave spherical surface as described above. With a spherical contour, peripheral clamping is relatively ineffective, since an edge bending moment is required. A clamping ring must be inward of the wafer edge to provide the necessary bending moment and in such position blocks part of the useful area of the wafer. Electrostatic clamping can be utilized effectively in the spherical situation, as shown in FIG. 5, since the clamping force is applied over the entire wafer surface area. In addition, a vacuum chuck can be utilized in the platen in conjunction with electrostatic clamping as described hereinabove. While there has been shown and described what is at present considered the preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: CLAIMS What is claimed is: 1. Apparatus for charged particle beam scanning of a thin, flexible workpiece with a substantially constant angle of incidence comprising: means for forming a charged particle beam of -predetermined parameters; first scanning means for deflecting the beam over the workpiece along a first axis with a substantially constant angle of incidence; second scanning means for deflecting the beam over the workpiece along a -second axis orthogonal to the first axis about a center of deflection at a distance R from the workpiece; and mounting means for positioning the workpiece in the path of the beam, said mounting means including means for deforming the workpiece to have a concave contour coincident with the surface of an imaginary cylinder having an axis passing through said center of deflection parallel to said first axis, and having a radius R. 2. Apparatus for charged particle beam scanning as defined in Claim 1 wherein said first scanning means comprises a first beam deflector for deflecting the beam along the first axis and an angle correction magnet positioned downstream from the first beam deflector, said angle correction magnet having a field gradient and pole shape for steering the beam from the first beam deflector into parallel paths as the beam is scanned. 3. Charged particle beam apparatus as defined in Claim 2 wherein the first beam deflector includes electrostatic scanning plates. 4. Charged particle beam apparatus as defined in Claim 3 wherein the second scanning means comprises electrostatic deflection plates positioned downstream of said angle correction magnet. 5. Charged particle beam apparatus as defined in Claim 1 wherein said mounting means includes a workpiece support platen having a surface with said concave contour, and a clamping ring for clamping a peripheral edge of the workpiece against the platen surface. 6. Charged particle beam apparatus as defined in Claim 1 wherein said mounting means includes a workpiece support platen having a surface with said concave contour, and means for electrostatic clamping of said workpiece to said platen surface. 20 ' ' -22- 7. Charged particle beam apparatus as defined in Claim .6 wherein said mounting means further includes vacuum chuck means positioned in said platen surface for drawing said workpiece against said platen 5 surface prior to vacuum pumping of the apparatus. 8. Charged particle beam apparatus as defined in Claim 1 wherein said mounting means includes means for cooling of said workpiece during treatment with 0 said charged particle beam. 9. Charged particle beam apparatus as defined in Claim 1 wherein ' said first scanning means includes a first deflector for scanning the beam along said 15 fir-st axis and a quadrupole lens positioned downstream of said first deflector for steering the beam into parallel paths as it is scanned by said first deflector. 20 10. Charged particle beam apparatus as defined in Claim 1 wherein said first scanning means includes a firβt deflector for scanning the beam along said first axis and a space charge lens positioned downstream of said first deflector for steering the 25 beam into parallel paths as it is scanned by said first deflector. 11. Charged particle beam apparatus as defined in Claim 1 wherein said first scanning means includes a first deflector for scanning the beam along said first axis and a solenoid lens positioned downstream of said first deflector for steering the beam into parallel paths as it is scanned by said first deflector. 12. Charged particle beam apparatus as defined in Claim 1 adapted for ion implantation of a semiconductor wafer. 13. Apparatus for charged particle beam scanning of a thin, flexible workpiece with a substantially constant angle of incidence comprising; means for forming a charged particle beam of - ' predetermined parameters; scanning means for deflecting the beam over the workpiece surface in a two dimensional pattern about a center of deflection at a distance R from the workpiece; and mounting means for positioning the workpiece in the path of the beam, said mounting means including means for deforming the workpiece to have a concave contour coincident with a surface of an imaginary sphere having a center at said center of deflection, and a radius R. 14. Charged particle beam apparatus as defined in Claim 13 wherein said scanning means includes x-axis electrostatic scanning plates and y-axis electrostatic scanning plates and means for applying scanning voltages to said scanning plates. 15. Charged particle beam apparatus as defined in Claim 13 wherein said mounting means .includes a workpiece support platen having a surface with said concave contour, and means for electrostatic clamping of said workpiece to said platen surface. 16. Charged particle beam apparatus as defined in Claim 15 wherein said mounting means further includes a vacuum chuck positioned in said platen surface for drawing said workpiece thereagainst prior to vacuum pumping of said charged particle beam apparatus. 17. Charged particle beam apparatus as defined in Claim 16 wherein said mounting means further includes means for cooling said workpiece during scanning by said charged particle beam. 18. Charged particle beam apparatus as defined in Claim 13 adapted for ion implantation of a semiconductor wafer. 19. A method for charged particle beam scanning of a thin, flexible workpiece with a substantially constant angle of incidence comprising the steps of: forming a charged particle beam of predetermined parameters; positioning the workpiece in the path of the beam; scanning the beam over the workpiece surface; and deforming the workpiece to have a concave ■ contour selected to provide constant angle of incidence of the scanned ion beam upon the workpiece surface. 20. The method as defined in Claim 19 wherein said step of scanning the beam includes deflecting the beam over the workpiece along a first axis with a - substantially constant angle of incidence and deflecting the beam over the workpiece along a second axis orthogonal to the first axis about a center of deflection at a distance R from the workpiece and wherein the workpiece is deformed to have a concave contour coincident with the surface of an imaginary cylinder having an axis passing through the center of deflection parallel to the first axis, and having a radius R. 21. A method as defined in Claim 19 wherein the beam is deflected over the workpiece surface in a two dimensional pattern about a center of deflection at a distance R from the workpiece and the workpiece is deformed to have a concave contour coincident with the surface of an imaginary sphere having a center at the center of deflection, and having a radius R.

Inventors: MOBLEY RICHARD M, PEDERSEN BJORN O, POLLOCK JOHN D, MOBLEY, RICHARD, M., PEDERSEN, BJORN, O., POLLOCK, JOHN, D.
Applicants: VARIAN ASSOCIATES, VARIAN ASSOCIATES, INC.
Application year: 1987
Main classification: G21K5
IPC codes: G21K1, G21K5, H01J37