Charged particle beam extraction and formation apparatus

Details Charged particle beam extraction and formation apparatus Charged particle beam extraction and formation apparatus

1999-09-07

2003-07-08

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

With positive or negative ion acceleration

Plural apertured electrodes

C313S299000, C313S306000, C313S348000

Reexamination Certificate

active

06590324

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to the field of charged particle sources and more specifically to charged particle sources with grid electrode optics forming apertures for the charged particles.
BACKGROUND OF THE INVENTION
Gridded, broad-beam ion sources, first developed for ion propulsion engines for spacecraft, are used in a variety of applications, such as ion beam etching (IBE), ion beam sputter deposition (IBSD), materials modification, and nuclear fusion technology. Ions are usually extracted from a discharge plasma by multi-grid ion optics. The plasma generator and the ion optics assembly are the two major components of the broad-beam ion source.
The plasma is usually generated by a type of high voltage glow discharge, hot-cathode discharge, vacuum arc discharge, or RF discharge. Ions extracted from the plasma are accelerated and focused into an ion beam by applying relevant potentials to an electrode in contact with the plasma and other grid electrodes (ion optics). The optimum number of grid electrodes is defined by application requirements, such as cost, weight, sensitivity to contamination of exposed surfaces by grid material, and beam collimation.
For many ion beam etch and ion beam sputter deposition applications, grid assemblies which provide low ion beam divergence are needed. Grid assemblies using three or more grid electrodes are preferable for this purpose. Such grid assemblies are able to provide low beam divergence over a wide range of beam current and beam voltage (ion energy). In addition, when operated under proper conditions, grid assemblies with three or more grid electrodes are not subject to grid erosion from charge exchange ions generated in the ion beam. For comparison, one and two-grid systems are mechanically simpler but have a limited range of operation at low beam divergence and are subject to grid erosion. Consequently, three-grid ion optics, with longer grid life, are more compatible with high purity materials processing requirements.
In a three-grid assembly, the grid in contact with the plasma is conventionally called the screen grid, and has a positive potential close to the plasma potential that defines the ion energy. The next grid downstream in the beam usually is set at a negative potential, and is called an accelerator grid. For low beam divergence operation, the absolute value of accelerator potential should not be greater than 0.3 times the value of the screen grid potential. The third grid is most commonly connected to ground potential, as are the target and chamber components. The third grid is called the decelerator grid.
ION OPTICS DESIGN AND OPERATIONAL CONSIDERATIONS
In a majority of broad ion source applications for high throughput production processes (or high thrust ion engines), the plasma generator and the multi-grid optics assembly must provide high beam current density at the ion optics and beam target. In turn, the maximum ion beam current (Ib) is very sensitive to the total extraction voltage Vt, which is sum of absolute values of screen potential (Vs) and accelerator potential (Va), and the spacing between these grids (d). To a good degree of accuracy this dependence can be expressed by the Child-Langmuir equation, Ib~(Vt)
3/2
/d
2
. As illustrated, ion beam current is inversely proportional to the square of the grid spacing; smaller grid spacing produces significantly higher ion beam current.
In the technologies mentioned above, (IBE, IBSD, and ion thruster applications), the ion energy is relatively low and usually does not exceed approximately a few kilovolts. To achieve high ion beam densities and low beam divergence, the inter-electrode spacing in the grid assembly must be on the order of 0.5-2.0 mm. This small spacing must be maintained over large beam diameters, up to 50 cm and more. Furthermore, technological requirements for ion beam uniformity (1% or less) and beamlet divergence (less than 3-5 degrees half-angle) dictate tight tolerances for grid inter-spacing and hole alignment. Grid inter-spacing tolerance is typically ±0.050 to 0.10 mm over the entire grid assembly. Grid hole misalignment is maintained at less than 0.05 mm with a 2 mm grid hole diameter. Maintaining these tight tolerances requires strict manufacturing control coupled with exceptionally stable grid structures and mounting configurations. Providing and maintaining these demanding tolerances is substantially complicated by thermal gradients which can exist between the center and periphery of the grids and also between the grid electrodes and grid support components.
In addition to the need for accurate grid inter-spacing during manufacture, the ion optics are also subject to repeated operational thermal cycling between “hot” (plasma on) and “cold” (plasma off) states. In a design where the mounting portion of the grid assembly is placed outside of the plasma generator, the temperature gradients are great. It has been observed in this configuration for the temperature of the center portion of the grids as much as 200 degrees Celsius higher than the temperature of the outer diameter of the grids.
Different techniques have been proposed to improve the thermal and mechanical stability of grids. These techniques include holding the grids in tension, supporting the screen grid in its center by a post contained in the plasma generator, adding stiffening ribs and using inter-electrode support spacers. However, at present, a common technique to provide stability is by forming the grid electrodes in a dished hemispherical configuration. As a result of the three dimensional shape, a dished grid has different mechanical stability when compared to a flat grid. A dished grid also has different thermal characteristics when compared to a flat grid. One difference is that thermal deformation of a dished grid is more predictable in magnitude and direction.
In addition to different thermal and mechanical characteristics when compared to flat grids, dished grid assemblies are more appropriate for special applications where highly focused or defocused ion beams are required. Concave grids (where the dishing is toward the plasma source) produce a focused ion beam that can be used in ion beam sputter deposition systems with relatively small target areas and high density ion beams. On the other hand, convex grids (where the dishing is away from the plasma source) produce defocused beams used in ion beam processing systems, such as in substrate surface cleaning, when a relatively large substrate or target area is exposed to a low density ion beam.
With a flat peripheral area on the grid (either flat or dished grid with an outer flange), it is known to use relatively massive stiffening ring arrangements to support and stiffen the grid. These stiffening rings are also usually fabricated from the same material as the grids, and are fastened to the flat peripheral area of the grid. In turn, the grid stiffening rings are fastened to each other and/or to the grid mounting base with some form of fastener. The fasteners are varied and include rigid posts, screws, nuts, washers, insulating bushings, and “sputter cups.” “Sputter cups” protect insulator surfaces from shorting out due to deposition of conductive materials. Because these designs have multiple parts and tend to be somewhat complex, they usually require some manual grid alignment, at least for initial set-up.
However, grid stiffening rings are exposed to rapid thermal transitions. It is commonly known that the relatively massive rings can introduce larger temperature gradients in the radial direction. In addition, if there is poor thermal contact between the edge of the grid electrode and the stiffening ring, a transient azimuthal temperature variation will occur. Nonuniformity in the temperature distribution can lead to grid distortion with consequent aperture misalignment and beamlet vectoring, which can cause ion impingement on the accelerator and decelerator grids. Finally, utilization of molybdenum stiffening rings appreciably increases the construction weight and cost.
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