Elastomeric sliding seal for vacuum bellows

An improved bellows assembly (18) is provided for use in, for example, an ion implanter (10). The bellows assembly comprises a first mounting portion (56) located at one end of the bellows assembly for fixedly mounting the bellows assembly to a first vacuum chamber (16); a second mounting portion (54) located at an opposite end of the bellows assembly for slidably mounting the bellows assembly to a second vacuum chamber (15); and a steel bellows (60) located between the first and second mounting portions. The bellows extends generally along a longitudinal axis (64) and is expansible and contractible along this axis. The second mounting portion permits radial slidable movement of the bellows assembly with respect to the second chamber in a first plane substantially perpendicular to this axis. The second mounting portion comprises a first sliding plate (74) having a first wear plate (78), a second sliding plate (76) having a second wear plate (80), and an elastomeric seal (82) for maintaining a vacuum condition at the slidable mating surface provided between the first and second wear plates.

FIELD OF THE INVENTION
 The present invention relates generally to ion implantation equipment, and
 more specifically to an elastomeric sliding seal for a vacuum bellows in
 such equipment.
 BACKGROUND OF THE INVENTION
 Ion implantation has become a standard accepted technology of industry to
 dope workpieces such as silicon wafers or glass substrates with impurities
 in the large scale manufacture of items such as integrated circuits and
 flat panel displays. Conventional ion implantation systems include an ion
 source that ionizes a desired dopant element which is then accelerated to
 form an ion beam of prescribed energy. The ion beam is directed at the
 surface of the workpiece to implant the workpiece with the dopant element.
 The energetic ions of the ion beam penetrate the surface of the workpiece
 so that they are embedded into the crystalline lattice of the workpiece
 material to form a region of desired conductivity.
 Ion energy is used to control junction depth in semiconductor devices. The
 energy levels of the ions that make up the ion beam determine the degree
 of depth of the implanted ions. High energy processes such as those used
 to form retrograde wells in semiconductor devices require implants of up
 to a few million electron volts (MeV), while shallow junctions may only
 demand energies below 1 thousand electron volts (1 keV).
 A typical ion implanter comprises three sections or subsystems: (i) a
 terminal for outputting an ion beam, (ii) a beamline for mass resolving
 and adjusting the focus and energy level of the ion beam, and (iii) a
 target chamber which contains the semiconductor wafer to be implanted by
 the ion beam. The continuing trend to smaller and smaller semiconductor
 devices requires a beamline construction which serves to deliver high beam
 currents at low energies. The high beam current provides the necessary
 dosage levels, while the low energy levels permit shallow implants.
 Source/drain junctions in semiconductor devices, for example, require such
 a high current, low energy application.
 Low energy ion beams which propagate through a given beamline construction
 suffer from a condition known as beam "blow-up", which refers to the
 tendency for like-charged ions within the ion beam to mutually repel each
 other. Such mutual repulsion causes a beam of otherwise desired shape to
 diverge away from an intended beamline path. Because the problem of beam
 blow-up increases with increasing beamline lengths, a design objective of
 preferred beamline constructions is to minimize or shorten the length of
 the beamline.
 Typically, the target chamber is oriented generally perpendicularly with
 respect to the axis of the shortened beamline so that the ion beam strikes
 normal to the plane of the substrate. However, certain implants require
 the ion beam to strike the substrate at an orientation several degrees
 from normal. In order to permit such implants, the target chamber is made
 pivotable about the axis of the beam path. For example, a tilt-twist
 mechanism may be provided to allow pivoting in each of two perpendicular
 axes that generally lie in the plane of a substrate in the target chamber.
 An expansible bellows provides the interface between the beamline and the
 movable target chamber.
 For applications where the bellows is required to move in simple axial
 compression or extension, no lateral forces are present, and the bellows
 corrugations can adequately handle the extensive or compressive forces in
 the axial direction. However, when the target chamber pivots with respect
 to the beamline path, the bellows typically experience shear forces in the
 plane perpendicular to the beam path. The bellows mounting is urged
 laterally within this plane (ie., the bellows mounting tends to undergo a
 lateral offset). Even small lateral movements in metal welded bellows may
 cause large shear stresses at mounting locations.
 Fixedly mounting the bellows on both ends focuses these shear stresses in
 the plane perpendicular to the beam path (and parallel planes) at the
 locations of the fixed mountings. This shear stress may result in
 premature failure of the bellows by reducing the number of cycles in its
 lifetime. Because the implantation process is typically performed in a
 high vacuum (e.g., down to 1.times.10.sup.-7 torr) process chamber to
 prevent dispersion of the ion beam and minimize the risk of contamination
 of the substrate by airborne particulates, any breach in the integrity of
 the bellows will result in loss of this vacuum condition. The loss of
 vacuum and the resulting contamination of the interior of the bellows will
 compromise the implantation process being performed.
 It is an object of the present invention, then, to provide a means for
 alleviating the shear stress in a vacuum bellows. It is a further object
 of the present invention to provide an improved bellows for connecting two
 portions of an ion implanter that move with respect to each other. It is
 yet a further object of the invention to provide an elastomeric sliding
 seal for a bellows, including a bellows for use in an ion implanter.
 SUMMARY OF THE INVENTION
 An improved bellows is provided for use in, for example, an ion implanter.
 The bellows assembly comprises a first mounting portion located at one end
 of the bellows assembly for fixedly mounting the bellows assembly to a
 first vacuum chamber; a second mounting portion located at an opposite end
 of the bellows assembly for slidably mounting the bellows assembly to a
 second vacuum chamber; and a steel bellows located between the first and
 second mounting portions. The bellows extends generally along a
 longitudinal axis and is expansible and contractible along this axis. The
 second mounting portion permits radial slidable movement of the bellows
 assembly with respect to the second chamber in a first plane substantially
 perpendicular to this axis. The second mounting portion comprises a first
 sliding plate having a first wear plate, a second sliding plate having a
 second wear plate, and an elastomeric seal for maintaining a vacuum
 condition at the slidable mating surface provided between the first and
 second wear plates.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
 Referring now to FIG. 1 of the drawings, an ion implanter, generally
 designated 10, is shown as comprising an ion source 12, a mass analysis
 magnet 14, a beamline assembly 15, and a target or end station 16. An
 expansible stainless steel bellows 18 assembly, which permits movement of
 the end station 16 with respect to the beamline assembly 15, connects the
 end station 16 and the beamline assembly. Although FIG. 1 shows an ultra
 low energy (ULE) ion implanter, the present invention has applications in
 other types of implanters as well. The invention also has applications in
 any vacuum environment wherein a bellows that couples two components must
 undergo more than simple axial compression or extension, without
 compromising the integrity of the vacuum.
 The ion source 12 comprises a plasma chamber 20 and an ion extractor
 assembly 22. Energy is imparted to an ionizable dopant gas to generate
 ions within the plasma chamber 20. Generally, positive ions are generated,
 although the present invention is applicable to systems wherein negative
 ions are generated by the source. The positive ions are extracted through
 a slit in the plasma chamber 20 by the ion extractor assembly 22 which
 comprises a plurality of electrodes 27. The electrodes are charged with
 negative potential voltages, increasing in magnitude as the distance from
 the plasma chamber slit increases. Accordingly, the ion extractor assembly
 functions to extract a beam 28 of positive ions from the plasma chamber
 and accelerate the extracted ions into the mass analysis magnet 14.
 The mass analysis magnet 14 functions to pass only ions of an appropriate
 charge-to-mass ratio to the beamline assembly 15, which comprises a
 resolver housing 23 and a beam neutralizer 24. The mass analysis magnet 14
 includes a curved beam path 29 which is defined by an aluminum beam guide
 30, evacuation of which is provided by a vacuum pump 31. The ion beam 28
 that propagates along this path is affected by the magnetic field
 generated by the mass analysis magnet 14, to reject ions of inappropriate
 charge-to-mass ratio. The strength and orientation of this magnetic field
 is controlled by control electronics 32 which adjust the electrical
 current through the field windings of the magnet 14 through magnet
 connector 33.
 The magnetic field causes the ion beam 28 to move along the curved beam
 path 29, from a first or entrance trajectory 34 near the ion source 12 to
 a second or exit trajectory 35 near the resolving housing 23. Portions 28'
 and 28" of the beam 28 comprised of ions having an inappropriate
 charge-to-mass ratio are deflected away from the curved trajectory and
 into the walls of aluminum beam guide 30. In this manner, the magnet 14
 passes to the resolving housing 23 only those ions in the beam 28 which
 have the desired charge-to-mass ratio.
 The resolver housing 23 includes a terminal electrode 37, an electrostatic
 lens 38 for focusing the ion beam, and a dosimetry indicator such as a
 Faraday flag 42. The beam neutralizer 24 includes a plasma shower 45 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 28. The beam neutralizer and resolver housings are evacuated by
 vacuum pump 43.
 Downstream of the beam neutralizer 24 is the end station 16, which includes
 a disk-shaped wafer support 44 upon which wafers to be treated are
 mounted. The wafer support 44 resides in a target plane which is
 (generally) perpendicularly orientated to the direction of the implant
 beam. The disc shaped wafer support 44 at the end station 16 is rotated by
 motor 46. The ion beam thus strikes wafers mounted to the support as they
 move in a circular path. The end station 16 pivots about point 62 which is
 the intersection of the path 64 of the ion beam and the wafer W so that
 the target plane is adjustable about this point. In this manner, the angle
 of ion implantation may be slightly modified from the normal. The
 expansible bellows, shown in more detail in FIG. 2, permits this relative
 movement of the end station 16 and the beamline assembly 15. Bellows in
 the UIE environment must be compact to minimize the length of the
 beamline. As such, the number of expansible bellows corrugations is
 limited, as is its ability to absorb lateral shear stresses.
 As shown in FIG. 2, the expansible bellows assembly 18 comprises a fixed
 mounting bracket 50, a bellows subassembly 52, and a sliding seal assembly
 54. The bellows main assembly comprises an end station mounting bracket
 56, a sliding seal mounting bracket 58, and a metal bellows 60 disposed
 therebetween. Although shown in cross section, the expansible bellows
 assembly 18 must be capable of maintaining a vacuum, and as such comprises
 an enclosed assembly, that is, the metal bellows 60 is generally
 cylindrical in shape, and the sliding seal assembly 54 and brackets 50, 56
 and 58 are generally annular in shape.
 Although the stiffness of the metal bellows is generally about 1200
 lb./in., the bellows can pivot about pivot point 62 in the plane that is
 perpendicular to axis 64 along which the ion beam 28 travels. As shown in
 FIG. 1, point 62 is the intersection of the axis 64 and the plane of a
 wafer supported by the disc shaped wafer support 44. The disc shaped wafer
 support 44 is provided with tilt and twist mechanisms (not shown) which
 pivot the wafer support, respectively, about the two linear axes that
 define the plane normal to the axis 64. Pivoting the wafer support 44
 about the point 62 causes lateral forces and resulting shear stresses to
 be exerted in this plane and parallel planes. The pivoting causes an arc
 to be swept equal to the pivot angle and the distance from point 62.
 The fixed mounting bracket 50 may be provided with threaded bores (not
 shown) through which the bracket 50 may be bolted to the beam neutralizer
 housing. Alternatively, the mounting bracket may be integral with the beam
 neutralizer housing. Groove 68 is provided for a sealing element such as
 an elastomeric O-ring (not shown) to maintain a vacuum seal between the
 bellows assembly 18 and the beam neutralizer housing.
 The end station mounting bracket 56 is provided with threaded bores 70
 through which the bracket 56 may be bolted to the end station 16.
 Alternatively, the mounting bracket may be made integral with the end
 station. Groove 72 is provided for a sealing element such as an
 elastomeric O-ring (not shown) to maintain a vacuum seal between the
 bellows assembly 18 and the end station.
 Located between the fixed mounting bracket 50 and the bellows subassembly
 52 is the sliding seal assembly 54. The sliding seal assembly absorbs the
 lateral forces and resulting shear stresses exerted by the pivoting wafer
 support 44, which would otherwise need to be absorbed by the body of the
 bellows. The sliding seal assembly 54 comprises a first sliding plate 74,
 a second sliding plate 76, a first wear plate 78, a second wear plate 80,
 and an elastomeric seal 82. The wear plates are attached to their
 respective sliding plates or made integral therewith, and the first
 sliding plate/wear plate 74, 78 is secured to the fixed mounting bracket
 50 by means of bolts 79. Similarly, the second sliding plate/wear plate
 76, 80 is secured to the sliding seal mounting bracket 58 by means of
 bolts 81.
 The sliding plates 74, 76, like the brackets 50, 56 and 58 are made of
 aluminum or steel that is ground flat. The wear plates 78, 80 are made of
 a material having a low coefficient of friction such as DLC (diamond-like
 carbon) which is applied to the ground steel or aluminum sliding plates
 74, 76 by a vacuum plasma deposition process. The DLC provides a nearly
 frictionless, mirror-like hard and smooth slidable surface.
 The elastomeric seal 82 is generally annular in shape, having a C-shaped
 cross section. The seal may be formed of a fluoroelastomer dipolymer, such
 as Viton.RTM. (type 9711), which is a registered trademark of the E. I.
 DuPont de Nemours and Company, Wilmington, Del. Portions of the seal 82
 are disposed and captured between the sliding plates 74, 76 and their
 respective brackets 50, 58 by bolts 79, 81. Ridges or end portions 84, 86
 of the seal 82 reside in grooves in their respective sliding plates 74, 76
 to provide a vacuum tight seal and secure the position of the seal 82.
 In operation, the end station 16 pivots about point 62 in the plane that is
 perpendicular to the beam axis 64. This plane is parallel to that formed
 by the interface of the wear plates 78, 80. The relative movement of the
 wear plates minimizes any lateral shear forces in the plane of the wear
 plate interface. Small lateral movements of up to 2 centimeters (cm)
 thereby prevent these lateral shear forces from being transmitted to the
 bellows subassembly 52. As such, the sliding seal assembly 54 permits true
 radial movement of the bracket 58 with respect to beam axis 64.
 The use of an elastomeric seal such as Viton.RTM. between the fixed
 mounting bracket 50 and the bellows subassembly 52 permits such lateral
 movement without compromising the vacuum condition within the bellows. The
 Viton.RTM. can be easily replaced by removing bolts 79 and 81 when the
 interior of the bellows is brought up to atmospheric pressure.
 The Viton.RTM. also minimizes particle generation within the vacuum. By
 fixing the elastomeric end of the bellows, and by making the bellows
 reentrant, the potential for an ion beam or other corrosive environment
 from attacking the elastomer is minimized. Specifically, because the
 elastomeric seal 82 is not within the direct line of sight of the ion
 beam, instead being obstructed by flange 92 in the sliding seal mounting
 bracket 58, the ion beam cannot degrade the seal. Further, the flange 92
 prevents particles resulting from any other seal degradation from being
 transferred to the interior of the bellows.
 Accordingly, a preferred embodiment of an improved bellows for 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.