Ion beam scanning system

Abstract: An ion implantation system ( 10 ) including structure ( 28 ) to create a dipole field through which the beam passes. The strength and direction of the dipole field is controlled to adjust an angle of impact between the workpiece, typically a semiconductor wafer and the ion beam. A side-to-side scanning motion is used to provide controlled doping of an entire semiconductor wafer ( 60 ).

Description: 1. Field of the Invention The present invention concerns an ion beam implant system that includes structure for deflecting ions to a surface of a silicon wafer so that the ions impact the silicon wafer at controlled impact angles as they scan across the wafer surface. 2. Background Art To enhance the yield in fabricating integrated circuits from semi-conductor wafers by ion beam doping, it is important to achieve doping uniformity across the wafer surface. One factor that affects doping uniformity is the uniformity of impact angle between the ion beam and the silicon wafer during ion implantation. In prior art medium current implanters, an ion beam is electrostatically deflected from an original trajectory by several degrees to sweep across the silicon wafer and dope the silicon wafer in a controlled, uniform manner. When the diameter of the silicon wafer is on the order of 10 centimeters, such deflection produces small angular deviations of impact angle. As the size of the silicon wafer increases, however, the deviation in impact angle from the center to the outer edge of the wafer also increases. To sweep across larger diameter wafers, the medium current ion beam trajectory has increased, causing problems in beam transmission. Increasingly, even modest angle variations have become unacceptable for these implanters. Various techniques have been tried to avoid angle variation caused by beam deflection. The most common procedure is to deflect the beam back after its original deflection before it reaches the region of impact with the silicon wafer. Examples of prior art patents addressing the variation in impact angle are U.S. Patent No. 4,794,305 to Matsukawa; U.S. Patent No. 4,276,477 to Enge; U.S. Patent No. 4,687,936 to McIntyre et al .; and U.S. Patent No. 4,922,106 to Berriam et al . U.S. Patent No 5,091,655 to Dykstra et al. discloses an ion beam implantation system where an ion beam is controllably deflected from an initial trajectory as it passes through spaced parallel plates. Once deflected, the ion beam enters an accelerator that both redeflects the once-deflected ion beam and accelerates the ions within the beam to a final desired energy. Ions within the beam exiting the accelerator follow trajectories that impact the workpiece at controlled impact angles. As a beam is deflected electrostatically, it is also focused in the plane of deflection. The focusing strength is proportional to the square of the angle of deflection. As a result, the manner in which the beam is imaged varies through the angles of deflection. As the angle of deflection becomes larger, the focusing effect also becomes larger. The consequence of this focusing effect is that the beam size and shape at the target varies as a function of position, making doping uniformity difficult to achieve even though the impact angle of ions with the workpiece is well controlled. European patent application EP-A-0 445 964 discloses an electrostatic scanning system for an ion beam. The beam is passed through a quadrupole lens to focus the beam, a Y-scanner to angularly scan the beam across the target, an X-scanner to give the beam a constant deflection, and a converter to convert the angularly scanned beam into a parallel scanned beam. Disclosure of the Invention The method for ion beam treating a workpiece according to the present invention is defined by claim 1. The ion beam implant system according to the present invention is defined by claim 4. Preferred embodiments are set out in the dependent claims. An embodiment concerns an ion beam implant system for controllably treating a workpiece and including a source for providing ions that treat the workpiece and a workpiece support for orienting the workpiece relative to the source. Ions from the source are emitted along a first trajectory until they reach an electrode structure that sets up a dipole field which deflects the ions from the first trajectory. By adjusting electrode voltages applied to the electrode structure, the ions exit the electrode structure at different orientations within a cone of possible trajectories. A second electrode redeflects the ions to a target plane so that the ions intersect the plane at a controlled angle. The target or workpiece is mounted for relative movement through the ion beam so that an entire workpiece is treated. Fixing the angle of deflection through which the ions are bent allows the length of the beam line from the source to the target to be fixed. The beam merely rotates about the initial axis of propagation. This assures good beam integrity and assures that each point on the target is impacted by a controlled distribution of incident angles. These and other objects, advantages and features of the invention will become better understood from the detailed description of a preferred embodiment of the invention which is described in conjunction with the accompanying drawings. Brief Description of the Drawings Figure 1 is a schematic of an ion implantation system for treating a workpiece at an implantation station; Figure 2 is a perspective view showing ion beam deflection about an azimuthal direction; Figure 2A is a schematic showing the ion beam impacting a workpiece; and Figure 3 is a schematic side view showing deflection of an ion beam. Detailed Description of the Embodiments Figure 1 depicts an ion implantation system 10 having an ion source 12 for generating an ion beam 14. An extraction power supply 15 biases an extraction electrode of the source to a potential of approximately 20 kilovolts to accelerate ions from the source 12 along a trajectory leading to an ion mass analyzing magnet 16. The magnet 16 bends the beam 14 at approximately a right angle and directs ions having an appropriate mass along a travel path through a shutter 20. The shutter 20 rejects ions having an inappropriate mass from the ion beam 14. The ion beam 14 next passes through a multi-pole deflection electrode array 28. Control voltages applied by a power supply 29 energize elements of the electrode array 28 to create an electric field having a dipole component. As the ion beam 14 passes through a region bounded by the electrode array 28, ions are deflected at a controlled angle and by a controlled amount based upon the electric potential applied to the electrodes of the array. In a preferred and disclosed embodiment, the array includes eight electrically conductive elongated rods 40-47 (Fig. 2) aligned parallel to each other along the initial trajectory of the ion beam as the beam exits the analyzing magnet. The angular spacing between adjacent electrodes is most preferably 360/8=-45°. Ions deflected by the electrode array 28 pass through a pair of curved, constant potential electrodes 48, 49 which redeflect the ions. After this double deflection of the ions, the ions impact a workpiece at a controlled angle of impact. Figure 1 depicts an implantation station 50 having a wafer support 51 that orients a semiconductor wafer 60 at a position to intercept ions that have been twice deflected. Ion beam collisions with other particles degrade beam integrity so the entire beam path from the source 12 to the implantation station 50 is evacuated. At the ion implantation station 50, a chamber 52 is evacuated and wafers are inserted into and withdrawn from load locks to avoid repeated pressurization and depressurization of the chamber 52. A mechanized arm 54 located outside the chamber 52, grasps a wafer supported on a shuttle 62 that has obtained the wafer from a cassette 64. The arm 54 first positions each undoped wafer on a wafer orienter 66. The orienter 66 rotates the undoped wafer to a particular orientation before ion implantation so that as ions strike the wafer, they encounter a specific orientation of the wafer crystal lattice structure. The wafer is moved into a load lock 68 so that a second arm 70 can move the wafers to an implant position within the chamber. At the ion implantation site, a wafer support 51 orients the wafer with respect to the beam and moves the wafer back and forth through the ion beam. Once the wafers have been treated, they are moved by an arm 72 into a second load lock 74. From this second load lock, the wafers are moved by an arm 76 to a shuttle 78 which stores the treated wafers in a cassette 80. Returning to Figure 2, the ion beam 14 that enters a region bounded by the multiple electrodes of the array 28 is a well defined, collimated beam that has passed through the shutter 20. Through appropriate control of the electrode voltages by the power supply 29, this beam can be made to deflect away from its initial path by a controlled amount and at a controlled angle. In accordance with the present invention, the amount of deflection is held constant and the azimuthal amount of deflection controlled. A time-varying voltage applied by the power supply can, as pictured in Figure 2, cause the ion beam 14 to sweep out an arcuate range of trajectories as the ion beam leaves the array 28. At any time, the beam follows only one of the paths depicted in Figure 2 so that Figure 2 represents a time lapse ion beam depiction. Turning to Figure 2A, a portion of an arc 82 that is swept out by the ion beam 14 is shown intersecting a semi-conductor wafer 60. Controlled power supply energization of electrodes that make up the array 28 sweeps back and forth along the arc 82. The support 51 is coupled to an actuator (not shown) that moves the wafer 60 back and forth as designated by the arrow in Figure 2A to achieve a complete ion implantation of the wafer 60. By appropriate tilting of the wafer 60, the angle of impact between the ion beam and wafer is controlled and relatively uniform ion impact angles are achieved. The power supply 29 has a control output for each electrode in the array 28. By switching the voltages applied to the array in a regular periodic manner, the dipole field component can be made to sweep back and forth to sweep the ion beam in a controlled arc across the wafer 60. The deflection plates 48, 49 redeflect the ion beam after the initial deflection by the electrode array 28. As seen in Figure 2, the electrode arrays 48, 49 also form an arc which eliminates the focusing aspect of the prior art double deflection schemes noted above. The angles of deflection are fixed so that the length of the beam line from the electrode array 28 to the workpiece support 51 is fixed. In fact, the shape of the ion beam 14 can be made invariant, it simply rotates about the initial axis of propagation. This guarantees stable angle integrity. Large angles of deflection may be achieved by the multi-pole electrode array 28. The consequent strong focusing of the beam is fixed and, therefore, can be properly addressed by use of the arcuate electrode arrays 48, 49. Short beam line transmission is achieved while maintaining large angles of scan for large-diameter wafers. Figure 3 depicts a modification in the Figure 2 arrangement to achieve large scanning angles without large initial deflection angles. This arrangement utilizes a similar multi-pole array of electrodes, but limits the amount of deflection produced by such an array to a small amount, for example, six degrees. A post deflector adds the remaining deflection. This additional deflection is achieved with a cylindrically symmetric electrode 90 positioned downstream from the first multi-pole element array. From that point on, the Figure 3 and Figure 2 embodiments are the same. Figure 3 shows a cross section through the beam based upon this alternate ion beam scanning arrangement. The present invention has been described with a degree of particularity, but it is the intent that the invention encompass all modifications and alterations from the disclosed design falling within the scope of the appended claims.

Claims: A method for ion beam treating a workpiece (60) comprising the steps of: a) causing an ion beam (14) to move along an initial trajectory; b) orienting the workpiece at a target location; c) causing the trajectory of the ions to diverge away from the initial trajectory by a controlled deflection amount by a dipole electric field at a region intercepted by the ion beam; d) redeflecting said ions by generating a static electric field that redeflects ions subsequent to a divergence of the beam; and e) moving the workpiece in a back and forth manner to cause ions passing through the static electric field to treat an entire workpiece surface, characterised in that the trajectory of the ions is varied by adjusting the direction of said dipole electric field to vary the azimuthal angle of the ion beam deflection by scanning the deflected ion beam about an axis of rotation without changing the angle by which the beam is deflected from the axis of rotation. The method of Claim 1 wherein the trajectory of the ions is varied by cyclically adjusting the orientation of the dipole electric field causing the beam to sweep back and forth across the workpiece. The method of claim 1 or 2, comprising generating a time-varying multipole electric field at said region intercepted by the ion beam such that the deflection angle is fixed and scans in an arcuate path. An ion beam implant system (10) for controllably treating a workpiece (60) comprising: a) source means (12) for providing ions to treat the workpiece; b) workpiece support means (51) for orienting the workpiece at a target plane relative the source means ; c) beam forming means (16) for causing ions emitted by the source means to form an ion beam (14) moving in a first trajectory; d) dipole deflection means (28) for deflecting ions in said ion beam from said first trajectory; e) electrode deflection means (48,49) for redeflecting ions deflected by the dipole deflection means to the target plane; f) scan means for moving the workpiece support means to cause ions to strike the workpiece at controlled regions on the workpiece surface; and g) control means (29) for the dipole deflection means (28) , characterised in that said dipole deflection means comprise a multipole array of electrodes for deflecting the ion beam by a fixed amount, and said control means (29) is for applying time-varying control signals to the array of electrodes to vary the azimuthal direction of the deflection and cause the beam to sweep across an arc at a workpiece plane as the azimuthal direction at which the ion beam is deflected changes. The ion beam implant system of Claim 4 wherein the dipole deflection means (28) comprises elongated metallic electrodes (40-47) extending parallel to each other and generally parallel to an entrance trajectory of the beam as the beam enters a region bounded by the metallic electrodes. The ion beam implant system of Claim 5 wherein the control means individually controls the electric potential on each elongated metallic electrode. The ion beam implant system of Claim 4, 5 or 6 wherein the electrode deflection means comprises spaced arcuate electrode plates (48,49) biased at control voltages to redeflect ions to the workpiece. The ion beam implant system of Claim 4, 5 or 6 where the electrode deflection means comprises two spaced apart curved electrodes (48,49) and a power supply (102) for applying a fixed electric potential between the two spaced apart curved electrodes.

Inventors: BENVENISTE VICTOR MAURICE, BENVENISTE, VICTOR MAURICE
Applicants: AXCELIS TECH INC, AXCELIS TECHNOLOGIES, INC.
Application year: 1993
Main classification: H01J37
IPC codes: H01L21, H01J37, C23C14