Patent Application
20120168622
The present disclosure relates to ion beams. More particularly, the present disclosure relates to producing a mass analyzed ion beam within ion implantation systems.
For many applications, such as formation of solar cells using ion implantation, the ability to implant at high current in an efficient manner is needed to reduce production costs. Large area sources may have various configurations.
Known beamline implanters may include an ion source, extraction electrodes, a mass analyzer magnet, corrector magnets, and deceleration stages, among other components. The beamline architecture provides a mass analyzed beam such that ions of a desired species are conducted to the substrate (workpiece). However, one disadvantage of the beamline implanter architecture is that the implantation current and therefore the throughput may be insufficient for economical production in applications such as implantation of solar cells.
Plasma doping tools (PLAD) may provide a more compact design that is capable of producing higher beam currents at a substrate. In a PLAD tool, a substrate may be immersed in a plasma and provided with a bias with respect to the substrate to define the ion implantation energy. However, PLAD system designs suffer from the fact that a mass analysis capability does not exist, thereby preventing the screening of ions of undesirable mass from impinging on the substrate.
It will therefore be apparent that a need exist to improve ion implanter architecture, especially in the case of high throughput large ion beams.
Embodiments of the present disclosure are directed to implanters that include a large area ion extraction system and a single-magnet configuration that produce a mass resolution for ion beams incident on a workpiece. In accordance with one embodiment, a system for producing a mass analyzed ion beam for implanting into a workpiece includes an ion extraction plate having a set of apertures configured to extract ions from an ion source to form a plurality of beamlets. The system also includes a magnetic analyzer configured to provide a magnetic field to deflect ions in the beamlets in a first direction that is generally perpendicular to a principle axis of the beamlets and a mass analysis plate having a set of apertures wherein first ion species having a first mass/charge ratio are transmitted through the mass analysis plate and second ion species having a second mass/charge ratio are blocked by the mass analysis plate. A workpiece holder is configured to move with respect to the mass analysis plate in a second direction perpendicular to the first direction, wherein a pattern of ions transmitted through the mass analysis plate forms a continuous ion beam current along the first direction at the substrate.
In another embodiment, a method of providing a large area mass analyzed ion beam to a substrate includes forming unanalyzed beamlets that define a beam footprint having a long axis, said beamlets formed by extracting ions from an ion source through a plurality of slots in an extraction plate. The method further includes deflecting a first and second group of ions in the unanalyzed beamlets over respective first and a second deflection distances in a first direction generally parallel to the long axis of the beam footprint with a magnetic field, and blocking the second group of ions with an analysis plate. The method also includes translating the substrate with respect to the analysis plate in a second direction perpendicular to the first direction, wherein a pattern of ions transmitted through the analysis plate forms a continuous ion beam current along the first direction at the substrate.
For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:
a and 6b present a side plan view and top cross-sectional view, respectively, of an exemplary arrangement of extraction and mass analysis plates;
c presents a top cross-sectional view of another exemplary arrangement of extraction and mass analysis plates.
a and 7b depict side plan views of alternative exemplary magnetic analyzers;
a depicts a side plan view of exemplary mass analyzed beamlets;
b and 9c depict respective ion current profiles from beamlets transmitted to a workpiece in the absence of a dithering magnet under overlap and underlap conditions, respectively; and
d depicts an ion current profile from beamlets transmitted to a workpiece in the presence of a dithering magnet.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, like numbers refer to like elements throughout.
In the description and figures to follow a set of Cartesian coordinate system is consistently used to define and describe the operation of embodiments.
Magnet assembly 114 may include magnets that are arranged to produce a moderate dipole magnetic field that is configured to produce an orthogonal force on a passing charged particle. When beamlets 112 pass through the magnet assembly, ions within the beamlets may experience a deflecting force that acts to deflect lighter ions 116 a greater lateral distance from their initial trajectories than the deflection imparted to heavier ions 118, which may travel in a substantially straighter trajectory as shown. As used herein, the terms “lighter ions” and “heavier ions” generally refer to ions having relatively smaller mass/charge ratios and those ions having relatively larger mass/charge ratios, respectively.
System 100 also includes a screening plate 120 that include apertures 122, which may be configured to pass ions 118. Apertures 122 may also be configured to block ions 116, whose trajectories are more curved, resulting in a displacement that causes their trajectories to intercept the screening plate 120. Accordingly, screening plate 120 may produce a series of beamlets 112a that are mass analyzed beamlets, wherein the beamlets 112a have a larger fraction of the straighter-trajectory ions (which may be heavier ions). As depicted in
As viewed in
As depicted in
Embodiments of the present invention may also provide a diffuser to diffuse mass analyzed beamlets together. In the example of
Accordingly, ion implanter 100 provides a compact ion beam architecture than provides high current of a desired species over large areas, such as large workpieces, while still providing a mass analyzed beam to the workpiece in which unwanted ion species are screened out before impacting the workpiece. In some particular embodiments, the ion implanter operates to screen lighter ions from heavier ions that are transmitted to a workpiece.
With reference also to
Referring to both
In operation, aperture plates 306, 406 may be used as an electrode to extract ions from ion source 102 and form a plurality of beamlets (not shown) as described above with respect to
In some embodiments, the beam footprint length L1, L2 may range from a several millimeters to about 20 centimeters and the width W may range from a few centimeters to about 1 meter or so. In one example, the width W may be increased by providing a longer aperture plate containing more apertures having the same width d and length LA. In operation, therefore, aperture plates 306, 406 may be used to produce ribbon beams whose width (corresponding to the dimension W) is on the order of one meter. Such beams may be scanned with respect to a substrate platen, for example, in the y-direction to provide implantation over a large area.
The arrangement of aperture plates 306, 406 provides the further advantage in that a continuous beam current may be provided to a workpiece even when separate beamlets impact a workpiece. This may be accomplished by scanning the workpiece in the y-direction with respect to the aperture plates. For example, the spacing S of apertures 310 of aperture plate 306, their length LA, and angle with respect to the y-direction are sufficient to define an overlap region O in the x-direction. Thus, when scanned in the y-direction, the pattern of beamlets (ions) formed from the angled apertures may form a continuous overlapping beam current at a workpiece. The angle formed by apertures 310 in aperture plate 406 is less than that in aperture plate 306, such that a slight beamlet underlap U is defined. However, when a workpiece is scanned in the y-direction with respect to aperture plate 406, the beamlet divergence after exiting the apertures 310, as well as the use of a dithering magnet (described further below with respect to
Another advantage of the ion beam implanter arrangements of the present disclosure is that the compact, high ion current geometry of PLAD-style systems is provided together with a mass analysis capability. In particular, the present embodiments provide ion beams having a width up to about one meter that may be conveniently mass analyzed by providing ion deflections on the order of as little as a few millimeters. As detailed further below with respect to
Embodiments may specifically provide a mass analyzed beam for implanting dopant species into a workpiece, such as a solar cell or an integrated circuit substrate. The ion species may be derived from a plasma source that may contain, in addition to the dopant species, unwanted ion species, such as hydrogen ions (Hx+).
Continuing with the example of
In operation, extraction aperture plate 606 of system 600 may extract ions as unanalyzed ion beamlets 612 that pass through apertures 610 substantially parallel to the direction z, as illustrated in
The differential deflection Δdef may be defined as the difference in deflection in the x-direction between that experienced by the lighter ions and that experienced by the heavier ions while the ions traverse between extraction plate 606a and mass analysis plate 606b. As shown in
In the example shown, an offset e-m between extraction and mass analysis apertures is provided in the x-direction, which may help ensure that the entire width wb of sub-beams of light ions 616 is blocked, while the entire width wb of sub-beams of heavy ions 618 passes through the lower apertures 610. In some embodiments, the spacing S between apertures may be greater than or equal to the aperture width d, to help ensure that the beamlet width wb of deflected ions 616 is not greater than the spacing between apertures, which might permit at least some deflected ions 616 to pass through at least one of a pair of adjacent apertures.
Advantageously, as further depicted in
In embodiments, the total distance traversed by ion beamlets between aperture plates 606a and 606b may be on the order of 15 cm, for example, between about 5 cm to about 50 cm, depending on the required mass resolution. In one particular example, for a mass analyzed 10 keV phosphorous ion beam that is stripped of Hx+ (x=1, 2, 3) ions, a differential deflection Δdef of about 6 mm may be produced for a 200 Gauss, 15 cm long orthogonal B field (along the z-direction). This, in turn, requires a separation between plates 606a and 606b of at least 15 cm to allow room for a magnet assembly to be placed therebetween to provide the required 15 cm long B field. Accordingly, an aperture arrangement whose extraction assembly plate is separated from the mass analysis plate by at least 15 cm and whose apertures 610 produce beamlets 612 having widths wb less than about 6 mm may be effective in producing a 10 keV phosphorous beam in which a large fraction of Hx+ (x=1, 2, 3) contamination is removed using a 200 Gauss magnetic field. This, in turn may require arranging the width d of slots 610 to be about 10 mm or smaller.
In other embodiments, extraction aperture plate 606a and mass analysis plate 606b may be configured to selectively block higher mass ions, as depicted in arrangement 650 of
a and 7b are side plan views that present details of respective magnetic assemblies 700 and 720, which may act as magnetic analyzers according to alternative embodiments of the disclosure. Magnetic assembly 700 presents a housing 702 that contains two separated sets of permanent magnets 704 whose poles are aligned generally in a common direction parallel to the y-axis, so as to produce a magnetic (B) field aligned in the y-direction. This field may produce a deflection force on charged particles that pass through gap 710 between magnets 704. For example, beamlets 708 may be deflected in the x-direction while traveling through gap 710 in the z-direction (out of page). In order to screen the magnetic field from ion source and other components, housing 702 may comprise a low carbon steel, or similar material.
In the embodiment of
In one example, the dithering magnet may generate an oscillating magnetic field with a triangular sawtooth waveform that can smooth out the beamlets 900. In order to facilitate improved beam uniformity, a dither magnet may be disposed immediately adjacent to a mass analysis plate, as depicted in
b and 9c depict respective cross-sectional profiles 904 and 906, respectively, of ion current at a workpiece surface as a function of position along the x-direction for beamlets in which no dithering magnet is used and respective beamlet overlap or underlap is present in the x-direction. Thus, the ion current profiles of
d presents a beam current profile 908 for beamlets in which a dithering magnet is employed to smooth the beamlets. The smoothed beamlets exhibit a uniform current density, as compared to the fluctuation in current density apparent in the unsmoothed beamlets of
In other embodiments, the spacing, length and angle of slots in an analysis plate may be such that a uniform beam current is produced when a workpiece is scanned in the y-direction without the use of a dithering magnet.
In summary, the inventive ion implantation system of the present disclosure provides a mass analyzed ion beam in a compact geometry that facilitates the ability to produce high ion currents at the workpiece due to the proximity of ion source and workpiece. Moreover, the extraction plate architecture that provides an analyzed beam is scalable to larger beam dimensions without the need to scale features such as magnetic field strength. In other words, the local deflection distance required to provide a mass analyzed beam is independent of the overall beam dimensions. Exemplary ion implantation systems of the present disclosure may be used, for example, where high throughput, high current implantation is required using a single ion species and where only a single ion energy is employed. In such a case, a permanent magnet configuration that produces an optimized and unchanging magnetic field strength may be used in conjunction with a fixed configuration of extraction and mass analysis plates. Moreover, even if beam energy is to be varied to some extent, a permanent magnet configuration may be used, by accommodating variations in energy by adjusting the relative positions in x-direction of extraction aperture slots with respect to analysis slots (see
The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. For example, embodiments disclosed hereinabove have generally depicted the scenario in which one species of heavier ions is transmitted by a mass analysis plate, while one species of lighter ions is blocked. However, more than one ion species may be blocked in other embodiments by the appropriate choice of parameters including aperture width, aperture separation, magnetic field strength, ion energy, and the like. In addition, embodiments of this disclosure include arrangements in which only partial screening of unwanted ion species may occur. In other words, exemplary operating parameters such as ion energy, magnetic field strength and aperture arrangements may permit a fraction of a total species of unwanted ions to propagate to a workpiece (as well as a fraction of desired species to be blocked) in cases where exposure of the workpiece to that fraction of unwanted species is tolerable. Moreover, in further embodiments, the individual apertures of the extraction plates and mass analysis plates need not be elongated nor have any particular shape.
Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.
