Patent Number: 062918281
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION A cross sectional plan view of a high-energy ion implanter 10 is shown in FIG. 1. Although the invention is described herein as being incorporated into an accelerating electrode assembly in the RF linac portion of a high-energy ion implanter, it is understood that the invention may be incorporated into ion implanter components other than electrode assemblies, and in ion implanter types other than high-energy. The implanter 10 comprises five sections or subsystems: an injector 12 including an ion source 14 for producing an ion beam 15 and a mass analysis magnet 16 for mass analyzing the ion beam; a radio frequency (RF) linear accelerator (linac) 18 including a plurality of resonator modules 20a-20n for accelerating the ion beam to a higher energy; a final energy magnet (FEM) 22 for performing final mass analysis of the accelerated ion beam; a resolver housing 23 for final resolution and conditioning of the ion beam; and an end station 24 which contains a rotating disc 26 carrying wafers W to be implanted by the ion beam. Positioned after the mass analysis magnet 16 at the entrance to the linac 18 is a variable aperture 30 for controlling the amount of beam current passing into the linac. Immediately downstream of the aperture 30 is a first flag Faraday 31 for measuring the beam current out of the aperture 30 and into the linac 18. A second flag Faraday 35 is located in the resolver housing 23 for measuring the current of the ion beam prior to implanting into the wafer W. The mass analysis magnet 16 functions to pass to the RF linac 18 only the ions generated by the ion source 14 having an appropriate charge-to-mass ratio. The mass analysis magnet is required because the ion source 14, in addition to generating ions of appropriate charge-to-mass ratio, also generates ions of greater or lesser charge-to-mass ratio than that desired. Ions having inappropriate charge-to-mass ratios are not suitable for implantation into the wafer. The ion beam that passes through the mass analysis magnet 16 is typically comprised of a single isotope and enters the RF linac 18 which imparts additional energy to the ion beam passing therethrough. The RF linac produces particle accelerating fields which vary periodically with time, the phase of which may be adjusted to accommodate different atomic number particles as well as particles having different speeds. Each of the plurality of resonator modules 20 in the RF linac 18 functions to further accelerate ions beyond the energies they achieve from a previous module. Intermediate each of the resonator modules 20 is an electrostatic quadrupole lens 60. The quadrupole lens 60 refocuses the ion beam passing therethrough, to counter the effect of net radial defocusing as the ion beam passes through a particular resonator module 20. Although not shown in FIG. 1, quadrupole lenses may also be positioned immediately before and after the RF linac 18. FIG. 2 shows an electrostatic quadrupole lens 60 in more detail. The lens 60 comprises a housing 62 including a mounting flange 64 provided with bolt holes 66 for mounting the lens 60 to the linac block. The housing 62 may be constructed of aluminum and may be provided with a groove 68 into which may be partially installed an RF shield device. Contained within the housing 62 are the operational components of the electrostatic quadrupole lens, as better shown in the exploded view of FIG. 3. As shown in FIG. 3, the electrostatic quadrupole lens housing 62 comprises a central housing portion 70 surrounded on either side by outer portions 72a and 72b. Annular graphite plugs 74a and 74b surround openings 76a and 76b, respectively, in the housing outer portions that permit the ion beam to pass therethrough. Endcaps 78a and 78b maintain the position of the graphite plugs 74a and 74b and complete the lens housing 62. Fasteners such as bolts or screws (not shown) are used to attach the endcaps to the housing outer portions at locations 80, and to attach the housing outer portions to the central housing portion at locations 82. The central housing portion 70 is secured to the mounting flange 64 using bolts 83 (see also FIG. 4). The lens housing 62 encloses the electrostatic quadrupole electrodes 84a through 84d which are oriented radially outward from an ion beam axis 86, approximately 90.degree. apart from each other (see also FIG. 4). The electrodes 84 are comprised of graphite and are positioned within the lens housing by standoff rods 92. A first pair of electrodes 84a and 84c oppose each other approximately 180.degree. apart, and a second pair of electrodes 84b and 84d also oppose each other approximately 180.degree. apart. A pair of standoff rods attaches to the back of each electrode 84 by means of a clamp 94 and a screw 96. As such, eight standoff rods 92 are used to position the four electrodes 84a through 84d within the lens housing 62. The ends of standoff rods 92 seat into recesses in the outer portions 72a and 72b of the lens housing. The fasteners that attach the endcaps 78 to these outer portions screw directly into the standoff rods to fix their position within the housing 62. Electrical power is delivered to the electrodes 84 via slots 100 and 102 provided in the mounting flange 64 and central housing portion 70, respectively. Electrical lead 104 passes through these slots and attaches to electrode 84a. Jumper wire 106 connects electrode 84a to electrode 84c, such that both of these electrodes operate at the same voltage. Similarly, electrical lead 108 passes through these slots and attaches to electrode 84b. Jumper wire 110 connects electrode 84b to electrode 84d, such that both of these electrodes operate at the same voltage. Screws 112 secure the electrical leads 104 and 108 and the jumper wires 106 and 110 to the same clamps 94 that secure the standoff rods 92 to the electrodes 84. The electrical leads which pass through the slot 100 in the mounting flange 64 are each fixedly mounted to the back of the flange by means of a terminal assembly 114. Each terminal assembly 114 comprises a lower terminal portion 116 which abuts the electrically grounded flange 64, an upper terminal portion 118 to which the electrical lead is fixedly attached, and an insulator 120 separating the upper and lower terminal portions. The insulator 120 is required because, while the flange 64 is electrically grounded, the electrical leads, and hence the electrodes to which they are attached, operate at high voltage (e.g., +20 kilovolts (KV) and -20 KV for leads 104 and 108, respectively). The electrical leads 104 and 108 are fixedly attached to the upper terminal portions 118 by washers 122 and terminal screws 124 (see also FIG. 4). Because the electrical leads and the electrodes to which they are attached operate at high voltage, the standoff rods 92 are made of an insulating material. In the preferred embodiment the standoff rods 92 are made of quartz (Si0.sub.2). Alternatively, other quartz-like insulating materials may be used, e.g., Pyrexg.RTM., which is a trademarked name for a heat resistant and chemical resistant glass material. As used herein, glass-like materials shall mean either Pyrex.RTM. or quartz (SiO.sub.2). As shown in FIG. 3, the standoff rods 92 attach at either end to the outer portions 72 of the electrically grounded lens housing 62, and support the high voltage electrodes 84a through 84d. In one example of operation, the four electrodes 84a through 84d are energized, so that electrodes 84a and 84c are operated at a potential of +20 KV and electrodes 84b and 84d are operated at a potential of -20 KV. The four energized electrodes form a quadrupole electrical field, having quadrupole components in the region between the electrodes, to radially focus the ion beam passing therethrough. Although the invention has been described in terms of quartz (SiO2) or Pyrex.RTM., it is contemplated that other materials may be substituted for the standoff rods 92. Quartz, as a non-metallic oxide of silicon, is an inexpensive and abundantly available choice for the standoffs. Standoff rods 92 made of quartz function well as electrical insulators, and have outer surfaces which have been found to resist accumulation of graphite sputtered off of the electrodes 84 as the ion beam passes through the quadrupole lens 60. In this manner, the quartz rods 92 prevent a conductive graphite coating from accumulating on the surface thereof. Accordingly, a preferred embodiment of electrode standoff rods for use in an ion implanter has been described. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented with respect to the foregoing description without departing from the scope of the invention as defined by the following claims and their equivalents.