Patent Number: 
Section: description

The present invention will now be described with reference to the drawings wherein like reference numerals are used to refer to like elements throughout. The present invention provides for beam containment in a low energy, high current ion implantation system at low pressures without requiring the introduction of externally generated plasma by enhancing the beam plasma using a multi-cusp magnetic field in combination with RF or microwave energy to create an ECR condition in a mass analyzer. However, it will be appreciated that the invention may be advantageously employed in applications other than those illustrated and described herein. Referring now to the drawings, in FIG. 1A, a low energy ion implanter 10 is illustrated, having a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes an ion source 20 powered by a high voltage power supply 22. The ion source 20 produces an ion beam 24 which is provided to the beamline assembly 14. The ion beam 24 is conditioned by a mass analysis magnet 26. The mass analysis magnet 26 passes only ions of appropriate charge-to-mass ratio to a wafer 30. The conditioned ion beam 24 is then directed toward the target wafer 30 in the end station 16. Referring also to FIG. 1B, an ion implanter 100 is illustrated in greater detail in accordance with an exemplary aspect of the present invention, and has an ion source 112, a mass analysis magnet 114, a beamline assembly 115, and a target or end station 116. An expansible stainless steel bellows assembly 118, which permits movement of the end station 116 with respect to the beamline assembly 115, connects the end station 116 and the beamline assembly 115. Although FIG. 1B illustrates an ultra low energy (ULE) ion implanter, the present invention has applications in other types of implanters as well. The ion source 112 comprises a plasma chamber 120 and an ion extractor assembly 122. Energy is imparted to an ionizable dopant gas to generate ions within the plasma chamber 120. Generally, positive ions are generated, although the present invention is applicable to systems wherein negative ions are generated by the source 112. The positive ions are extracted through a slit in the plasma chamber 120 by the ion extractor assembly 122, which comprises a plurality of electrodes 127. Accordingly, the ion extractor assembly 122 functions to extract a beam 128 of positive ions from the plasma chamber 120 and to accelerate the extracted ions into the mass analysis magnet 114. The mass analysis magnet 114 functions to pass only ions of an appropriate charge-to-mass ratio to the beamline assembly 115, which comprises a resolver housing 123 and a beam neutralizer 124. The mass analysis magnet 114 includes a curved beam path 129 within a passageway 139 defined by an aluminum beam guide having side walls 130, evacuation of which is provided by a vacuum pump 131. The ion beam 128 that propagates along this path 129 is affected by the magnetic field generated by the mass analysis magnet 114, to reject ions of an inappropriate charge-to-mass ratio. The strength and orientation of this dipole magnetic field is controlled by control electronics 132 which adjust the electrical current through the field windings of the magnet 114 through a magnet connector 133. The dipole magnetic field causes the ion beam 128 to move along the curved beam path 129 from a first or entrance trajectory 134 near the ion source 112 to a second or exit trajectory 135 near the resolving housing 123. Portions 128xe2x80x2 and 128xe2x80x3 of the beam 128, comprised of ions having an inappropriate charge-to-mass ratio, are deflected away from the curved trajectory and into the walls of an aluminum beam guide 130. In this manner, the magnet 114 passes to the resolving housing 123 only those ions in the beam 128 which have the desired charge-to-mass ratio. The passageway 139 further comprises a magnetic device including one or more magnets 170 disposed laterally along the beam path 129. The magnets 170 are mounted above and below the beam path 129 to create a multi-cusped magnetic field (not shown in FIG. 1B) in the passageway 139. A high frequency electric field (not shown in FIG. 1B) is also provided in the passageway 139 via a microwave injection port 172 which couples a power source 174 with the passageway 139. The multi-cusped magnetic field and the high frequency electric field in the passageway 139 cooperatively interact to create an electron cyclotron resonance condition in at least one region (not shown in FIG. 1B) of the passageway in order to provide beam containment of the ion beam 128, as described in greater detail infra. The resolver housing 123 includes a terminal electrode 137, an electrostatic lens 138 for focusing the ion beam 128, and a dosimetry indicator such as a Faraday flag 142. The beam neutralizer 124 includes a plasma shower 145 for neutralizing the positive charge that would otherwise accumulate on the target wafer as a result of being implanted by the positively charged ion beam 128. The beam neutralizer and resolver housings are evacuated by a vacuum pump 143. Downstream of the beam neutralizer 124 is the end station 116, which includes a disk-shaped wafer support 144 upon which wafers to be treated are mounted. The wafer support 144 resides in a target plane which is generally perpendicularly oriented to the direction of the implant beam. The disc shaped wafer support 144 at the end station 116 is rotated by a motor 146. The ion beam thus strikes wafers mounted to the support as they move in a circular path. The end station 116 pivots about point 162, which is the intersection of the path 164 of the ion beam and the wafer W, so that the target plane is adjustable about this point. FIG. 2 illustrates an exemplary mass analyzer beam guide 200 for use in a low energy ion implantation system (e.g., low energy ion implanter 10 of FIG. 1B), having an arcuate longitudinal passageway 202 defined by inner and outer arcuate side walls 204 and 206, respectively, along an ion beam path 208. The beam guide 200 extends longitudinally along the path 208 from an entrance end 210 to an exit end 212 through an arc angle xcex8 which may be approximately 135 degrees, for example. Beam guide 200 further comprises a microwave injection port 214 which provides coupling of RF or microwave energy from a power source 216 with the passageway 202 via a cable 218. The beam guide further includes a mass analysis magnet comprising two arcuate magnet poles (not shown in FIG. 2) to provide a dipole magnetic field in the passageway 202 which allows ions of a selected charge-to-mass ratio to reach the exit end 212 along the path 208. FIGS. 3A and 3B illustrate an end elevation view and a sectional plan view, respectively, of the exemplary mass analyzer beam guide 200 of FIG. 2, having a plurality of magnets 220 associated therewith for generating a multi-cusped magnetic field in accordance with an aspect of the invention. Magnets 220 extend laterally between an inner radius R1 and an outer radius R2 in the passageway 202 in a longitudinally spaced relationship along the path 208, with an angular spacing xcex82, which may be, for example, 5.326 degrees. In one exemplary implementation of the invention, the inner radius R1 may be about 300 mm and the outer radius R2 may be about 500 mm. The passageway 202 is further defined by top and bottom walls 222 and 224, respectively. The dipole field may be generated externally to the beamguide 200 by an electromagnet (not shown). In another implementation of the invention the magnets 220 are embedded into one or both of the beamguide walls 222 and 224 in slots machined from the outside thereof, such that the magnets 220 remain outside of the vacuum chamber. In addition, it will be recognized that magnets 220 may be provided in one or both of the top and bottom walls 222, and 224, respectively, or on one or both of the side walls 204 and 206, respectively, or any combination thereof. FIGS. 4 and 5 respectively illustrate the mass analyzer beam guide 200 in longitudinal and lateral section along section lines 4xe2x80x944 and 5xe2x80x945 of FIG. 2. As seen in FIG. 5, magnets 220 are magnetized longitudinally along the propagation direction of the ion beam path 208, and are staggered such that adjacent magnets have like polarity poles facing each other. For clarity, the magnets 220 having south poles facing toward the entrance end 210 of the beam guide 200 are indicated as 220A and magnets 220 having south poles facing toward the exit end 212 of the guide 200 are indicated as 220B. In order to facilitate the mass analysis function, a dipole magnetic field is established in the passageway 206, for example, via an external electromagnet (not shown) having vertical field lines 230 as illustrated in FIG. 4. Referring also to FIG. 6, the exemplary bipolar magnets 220A and 220B create individual magnetic fields, illustrated for simplicity with exemplary field lines 232A and 232B which cooperate to form multi-cusped magnetic fields near and spaced from the top and bottom walls 222 and 224, respectively, in the passageway 206. The exemplary placement of magnets 220A and 220B illustrated in the various figures illustrates similarly oriented magnets 220 vertically aligned (e.g., magnet 220A directly above magnet 220A, magnet 220B directly above magnet 220B). However, it will be appreciated that orientations other than those specifically illustrated and described herein are possible and are contemplated as falling within the scope of the present invention. The orientation of magnets 220A and 220B illustrated in FIGS. 5 and 6, for example, advantageously provides additive magnetic field lines in the areas between adjacent magnets 220, although this is not required for the invention. Where RF or microwave energy is provided in the passageway 206 (e.g., via power source 216 and microwave injection port 214 of FIG. 2), the cooperative interaction between the magnetic and electric fields results in the creation of an electron cyclotron resonance (ECR) condition in regions 234 spaced a distance 236A and 236B from the magnets 220. The ECR condition in regions 234 advantageously provides enhancement of the beam plasma associated with an ion beam traveling through the passageway 206 along the path 208, whereby beam integrity is improved along the longitudinal length of the mass analyzer beam guide 200. The creation of an ECR condition in one or more regions 234 around an ion beam prevents beam xe2x80x9cblow-upxe2x80x9d by facilitating the transfer of energy to the plasma surrounding the beam, thereby enhancing the plasma. An electron cyclotron resonance condition occurs when an alternating electric field is applied to a charged particle in a static magnetic field, such that the frequency of the electric field matches the natural frequency of rotation of the charged particle around the static magnetic field lines. Where this resonance condition is attained (e.g., in regions 234), a single frequency electromagnetic wave can accelerate a charged particle very efficiently. It will be appreciated that the sizing, orientation, and spacing of the magnets 220 within the passageway 206 allow the location of the ECR regions 234 to be generated in accordance with desired ion beam containment goals. For example, the strength of the magnets 220 may be varied in order to change the distance 236A and/or 236B between the inner surfaces of the magnets 220 and the ECR regions 234. In this manner, the distances 236A and 236B may be adjusted according to the passageway size and/or the desired ion beam size. In addition, the spacing between adjacent magnets 220 may be changed in order to vary the spacing between adjacent ECR regions 234. Furthermore, the relative orientations of the magnetic pole faces of adjacent magnets may be varied in order to provide additive magnetic field lines between adjacent magnets 220. Many different magnet sizes, orientations, and spacings are possible and contemplated as falling within the scope of the present invention. In accordance with the present invention, the multi-cusped magnetic field employed to obtain the ECR condition may be successfully superimposed near the edges of the dipole field. The plasma produced at the resonance surface where the correct magnetic field strength value is obtained expands toward the center of the ion beam along the dipole field lines, in a direction opposite to the field gradient. The introduction of the electric field into the beam guide passageway 202 may further be aided by the use of a waveguide in the passageway as illustrated and described in greater detail infra. Referring now to FIGS. 7A and 7B, another aspect of the invention is illustrated in reference to mass analyzer beam guide 200, wherein a sectional side elevation view is provided. The beam guide 200 comprises top and bottom walls 222 and 224, respectively, an outer sidewall 206, and an inner side wall (not shown) defining a passageway 202 through which an ion beam (not shown) propagates along a path 208. A plurality of magnets 220A and 220B (collectively designated as 220) are provided in similar fashion to the magnets 220 of FIGS. 3A-6 which extend laterally between the inner side wall and the outer side wall 206, in a spaced relationship to each other such that the longitudinally opposite magnet poles of adjacent magnets 220 face one another. Oriented in this fashion, the magnets 220 provide a multi-cusped magnetic field in the passageway 202 near the top and bottom walls 222 and 224, which field is illustrated by exemplary field lines 232A and 232B. A mass analysis electromagnet (not shown) outside of the beamguide may provide a dipole magnetic field (not shown) adapted to provide the mass analysis functionality discussed supra. Unlike the mass analyzer implementations in the previous figures, the beam guide 200 of FIGS. 7A and 7B further comprises one or more waveguides 250. The waveguide comprises a suitable propagation medium such as quartz, that is metalized on all sides by a thin coating (e.g., aluminum). Since the skin depth at 2.54 GHz is less than one micrometer, a metalization layer coating thickness of a few microns is adequate. Laterally extending ports or slots 254 are provided in the inwardly facing metalization layers of the waveguides 250 between adjacent magnets 220 for coupling RF or microwave energy from the waveguide 250 into the passageway 202 of the beam guide 200 as described in greater detail infra. The waveguides 250 may be coupled to an RF or microwave power source (e.g., source 216 of FIG. 2) through any known method (e.g., windows, antennas, and the like), whereby standing wave resonance may be established in the waveguides 250 along the longitudinal length thereof. It will be appreciated that although two waveguides (e.g., upper and lower) 250 are illustrated in the figures, that other configurations, including a single waveguide 250, may be employed according to the invention. The RF or microwave energy provides electric fields in the passageway 202 illustrated by exemplary electric field lines 256A and 256B in FIG. 7B which cooperatively interact with the multi-cusped magnetic fields generated by the magnets 220 to provide ECR regions 234 spaced from the top and bottom walls 222 and 224. As discussed supra, the ECR condition promotes the enhancement of the beam plasma associated with an ion beam (not shown) propagating through the passageway 202 of the beam guide 200 along the path 208, whereby the integrity of the beam is maintained by the reduction or elimination of beam xe2x80x9cblow-upxe2x80x9d. The ports or slots 254 in the waveguide 250 extend laterally between the inner side wall (not shown in FIGS. 7A and 7B) and the outer side wall 206 having a width 260 and adjacent ports or slots 254 are longitudinally spaced by an angular pitch distance 262 which is the pitch spacing of the magnets 220. Referring also to FIGS. 8A and 8B, another exemplary waveguide 250 is illustrated in section, mounted between wall 222 and the multi-cusped field magnets 220. According to another aspect of the invention, the waveguide 250 comprises upper and lower metalized layers 280 and 282, respectively above and below a base layer 284 adapted to propagate RF or microwave energy for introduction into the passageway 202 of the beam guide 200. Laterally extending ports or slots 254 are provided in the lower support layer 282 exposing the base layer 284 to the interior of the passageway 202. In addition, O-rings 286 may be provided encircling the slots 254 in order to seal the magnets from the vacuum region. According to still another exemplary aspect of the invention, the base layer 284 may be made from quartz, the upper and lower metalized layers 280 and 282, respectively, may be made from aluminum, the O-rings 286 may be made from a suitable elastomer, and the beam guide cover 288 may be made from aluminum. Alternatively, however, other materials may be employed and are contemplated as falling within the scope of the present invention. Referring now to FIGS. 8C and 8D, side section views of the exemplary beamguide 200 and waveguide 250 are illustrated. In accordance with the invention, the top wall 222 may include a recess for supporting the waveguide 250, as well as a seating surface for compressing the o-ring 286 around the slot 254. The beamguide 200 may further include a top cover 290 allowing removable mounting of the waveguide 250 in the top wall 222. Referring also to FIG. 8D, the top wall 222 may also include a recess or pocket in which the magnets 220 are seated. The o-rings 286 around the slots 254 thus providing for isolation of the magnets from the vacuum of the inner passageway 202. Referring now to FIG. 9, the waveguide 250 is shown installed in a beam guide 200, where the waveguide 250 extends along the path 208 of the ion beam propagation. The pitch spacing of the magnets 220 is the same as that of the waveguide ports or slots 254, having an angular value of xcex82, for example 5.326 degrees, providing for 25 equally spaced magnets 220 along an angular beam guide length of xcex81, for example, approximately 135 degrees. In operation, RF or microwave energy (e.g., provided by power source 216 via cable 218 and microwave injection port 214) is propagated in the waveguide 250 located behind the multi-cusped magnetic field generating magnets 220. The energy is coupled to the beam plasma (not shown) via the periodically distributed ports or slots 254 for creation of the ECR condition (e.g., in regions 234 of FIGS. 7A and 7B) conducive to plasma enhancement employed for beam containment. As illustrated further in FIG. 10, the waveguide 250 furthers the generation of RF or microwave electric fields of sufficient magnitude orthogonal to the fixed magnetic fields at many locations (e.g., regions 234 of FIGS. 7A and 7B) along the beam propagation path 208. Toward that end, the length of the waveguide 250 may be set at a multiple of xc2xd wavelengths (e.g., nxcex/2, where n is an integer) corresponding to the RF or microwave power source frequency (e.g., 2.45 GHz), with the coupling ports or slots 254 located at xc2xd wavelength locations. The waveguide 250 may therefore constitute a resonant structure where standing waves can be produced therein with the ports or slots 254 located where the E field is minimum and the H field is maximum (e.g., xe2x80x9cHxe2x80x9d coupling). The length of the ports or slots 254 in the waveguide 250 may be maximized (e.g., slots 254 are nearly as long laterally as the width of the waveguide 250) and the width may be optimized for nominal impedance matching. For example, in the exemplary waveguide 250, the angular slot spacing (an hence the spacing of the magnets 220) is approximately 5.326 degrees, the inner radius R1 is approximately 370 mm, and the outer radius R2 is approximately 430 mm. The length of the ports or slots 254 in this example is approximately 50 mm, and the width is approximately 5 mm. In order to obtain consistent electric field patterns in the beam guide 200, it is desirable to excite a single dominant propagation mode. For example, the TE10 propagation mode for rectangular cross-section waveguides provides an electric field that is normal to the broadwall of the guide with a (1) peak in the center of the broadwall. The field magnitude is constant along the direction parallel to the narrow wall (e.g., xe2x80x9c0xe2x80x9d peaks). This TE10 has the lowest cut-off frequency. The cut-off frequencies for the TEx0 modes depend only on the broadwall dimension. Higher order modes TEn0 have progressively higher cut-off frequencies. According to one aspect of the invention, by choosing the size of the broadwall such that the cut-off frequency for the TE20 mode is slightly larger than the operating frequency (e.g., 2.45 GHZ), the widest possible waveguide 250 is selected which will only propagate the single TE10 mode. Once the waveguide dimensions are so chosen, the propagation wavelength is determined. An electric field develops across the ports or slots 254 outside the waveguide 250 in the interior of the beam guide passageway 202, which is oriented along the ion beam propagation direction (e.g., path 208). A magnetic field (e.g., multi-cusped field) is generated that is perpendicular to the electric field, with the proper magnitude for creating the ECR resonance condition in the regions 234 of the passageway 202. For example, a BF2+ion beam at an energy of 1.19 keV requires a magnetic field strength of 873 Gauss to follow the proper trajectory in a mass analyzer of a nominal 400 mm bending radius for creating the ECR condition. The ECR regions 234 may be located advantageously close enough to the slots 254 in the waveguide 250 to benefit from the high electric field intensity, yet sufficiently spaced from any surface (e.g., magnets 220, waveguide 250, etc.) to minimize plasma losses. For example, the ECR regions 234 of FIGS. 7A and 7B may be located a distance 236 away from the magnets 220, which may be in the range of about 4 to 6 mm, with a nominal distance of about 5 mm providing proper operation. Referring now to FIG. 11, a method 300 of providing ion beam containment in a low energy ion implantation system is illustrated. The method begins at step 302 wherein an ion beam is produced along a longitudinal path using an ion source. A mass analyzer is provided at step 304 having an inner passageway, a high frequency power source, a mass analysis magnet mounted in the inner passageway, and a magnetic device mounted in the inner passageway. The ion beam is received in the mass analyzer from the ion source at step 306, and ions of appropriate charge-to-mass ratio are directed at step 308 from the mass analyzer along the path toward a wafer or other target to be implanted with ions. At step 310, an electric field is generated in the passageway using a high frequency power source. A multi-cusped magnetic field is generated at step 312 using a magnetic device mounted in the passageway, which may create advantageously an ECR condition therein. Although the invention has been shown and described with respect to a certain applications and implementations, it will be appreciated that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a xe2x80x9cmeansxe2x80x9d) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms xe2x80x9cincludesxe2x80x9d, xe2x80x9cincludingxe2x80x9d, xe2x80x9chasxe2x80x9d, xe2x80x9chavingxe2x80x9d, and variants thereof are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term xe2x80x9ccomprisingxe2x80x9d.