Patent Grant
8698401
This invention relates generally to ion and plasma sources, and more particularly it pertains to those sources in which ions are generated with an inductively coupled radio-frequency discharge.
A plasma can be defined as an electrically conducting gas that satisfies quasi-neutrality. For singly charged ions, the type most often generated in ion and plasma sources, this means that the density of electrons and ions is approximately equal (ne≈ni). An ion or plasma source typically has a discharge region in which ions are generated by the collisions of energetic electrons with molecules of the working gas, a region of ion acceleration, and a region through which the beam of energetic ions travels after it leaves the source. Beams from industrial ion or plasma sources are used for etching, deposition and property modification. These sources operate in vacuum chambers, which are continually pumped while the source is operating to maintain a background pressure of approximately 10−3 Torr (0.13 Pascals) or less for ion sources and up to several times that high for some plasma sources. Ion or plasma sources are also used for space propulsion, in which case the beam provides propulsion for a spacecraft and the background pressure is much less than 10−3 Torr.
Both gridded and gridless ion and plasma sources are used in industrial applications and space propulsion. For a gridless ion source, a quasi-neutral plasma extends from the discharge region, through the acceleration region, into the beam. (An exception exists for a short distance of the acceleration region of an anode-layer source.) There may also be some overlap of the ion generation, ion acceleration, and beam regions in a gridless source. Such sources have been called both ion and plasma sources. For consistency herein, they are called “plasma sources.” In a gridless plasma source the acceleration can be electromagnetic—caused by the interaction of an electron current with a magnetic field, which establishes an electric field in a quasi-neutral plasma. The electron current that interacts with the magnetic field is supplied by a source of electrons at the exit of the source. This acceleration process is described in more detail in an article by Zhurin, et al., in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1.
The ion acceleration in a plasma source can also take place as the result of the expansion from a high plasma density to a low plasma density as it leaves the source. At the low background pressures assumed herein, the plasma potential and the density are related by the Boltzmann relation,
ne=ne,oexp(Vp/Te), (1)
where ne,o is the reference plasma density where the plasma potential is defined as zero, Vp is the plasma potential at a density ne, and Te is the electron temperature in electron-volts. From Equation (1), the decrease in plasma density as the plasma leaves the plasma source results in a decrease in plasma potential that serves to accelerate the ions. The electrons in the beam are again supplied by the continuous plasma from the discharge region.
Yet another means of accelerating ions in a quasi-neutral plasma is described in U.S. Pat. No. 4,862,032—Kaufman, et al. As described therein, a gradient in magnetic field can interact with electrons to generate an electric field in a plasma, and the electric field will accelerate ions.
In a gridded source, electrons are present in the plasma of the discharge region, but they are excluded from the acceleration region between grids. The ion acceleration in such a source is electrostatic, i.e., caused by the voltage difference between the grids. The beam from a gridded ion source must be a quasi-neutral plasma (to avoid the mutual repulsion of a beam consisting only of positively charged ions), so electrons are added after electrostatic acceleration by an electron-emitting neutralizer. Gridded sources have been almost always been called “ion sources,” and that nomenclature is used herein. The means of extracting ions from a discharge plasma, accelerating them between electrically charged grids, and adding electrons to form a beam of quasi-neutral plasma are well understood by those skilled in the art and are described by Kaufman, et al., in the AIAA Journal, Vol. 20 (1982), beginning on page 745. It is also understood by those skilled in the art that, in the event of only grounded surfaces for the beam to impinge on, it is sometimes possible for the electrons added to the beam to come only from the secondary emission of ions striking grounded surfaces.
Beam nomenclature: If a source is called an “ion source,” the beam from it is usually called an “ion beam,” even though that beam satisfies quasi-neutrality and is a plasma. If a source is called a “plasma source,” the beam is usually called a “plasma” or “plasma beam,” although it has also sometimes been called an “ion beam.” Herein it is called simply a “beam,” which is defined as being comprised of energetic ions accompanied by sufficient electrons to make it a quasi-neutral plasma, regardless of whether the source is a plasma source or an ion source.
The particular sources described in the aforesaid article by Kaufman, et al., in the AIAA Journal use a direct-current discharge to generate ions. It is also possible to use electrostatic ion acceleration with a radio-frequency discharge, as described in U.S. Pat. No. 5,274,306—Kaufman, et al. for a capacitively coupled discharge, and U.S. Pat. No. 5,198,718—Davis, et al. for an inductively coupled discharge. These publications are incorporated herein by reference.
Plasma sources are described in the aforementioned U.S. Pat. No. 4,862,032—Kaufman, et al., and in the aforementioned article by Zhurin, et al., in Plasma Sources Science & Technology. The particular sources described in these publications use a direct-current discharge to generate ions. It is also possible for a gridless source to use a radio-frequency discharge, as described in U.S. Pat. No. 5,304,282—Flamm. These publications are also incorporated herein by reference. It should be noted that the aforesaid patent by Flamm uses the free expansion of a plasma for ion acceleration that was described previously.
The most common geometric configuration for either an ion (gridded) or plasma (gridless) source is one that generates a beam with a circular cross section. However, linear configurations, in which the cross section of the beam is greatly extended in one direction, have also been used. One such linear source is described by Wykoff, et al., in an article in Proceedings of the Eighth International Conference on Vacuum Web Coating, Las Vegas, Nev., Nov. 6-8, 1994, beginning on page 81. This publication is also incorporated herein by reference. In addition, beams with an annular cross section are described in the aforementioned article by Zhurin.
This patent is concerned with the generation of ions for a source, either ion or plasma, using an inductively coupled radio-frequency discharge. The beams from such sources have presented problems in that the distribution of energetic ions departed substantially from what was expected and/or needed. An ion source with a circular beam can be assumed to illustrate these problems. Such a source has a general axial symmetry and that symmetry would be expected to be reproduced in the beam. That is, while radial variations in ion current density might be expected, the beam would be expected to have symmetry about the axis of source symmetry. It is true that asymmetry can be introduced by such things as an asymmetric variation in spacing between ion-optics grids, but it is assumed that the design and construction of the ion source is carried out by those skilled in the art and the sources do not incorporate such obvious shortcomings.
To be more specific, the primary concern here is with those perturbations or departures from expectations associated with the inductor, comprised of multiple turns of high conductivity wire, that couples radio-frequency energy to the ion-generating discharge. There have been increasingly difficult requirements for precision in the control of beams from ion and plasma sources. At present, it is difficult to use the beams from these sources in many applications if the distributions of ion current density are not controlled to give reproducibility or beam symmetry within several percent. In some cases, that control results in a several-percent requirement for uniformity over most of the cross section of that beam.
In light of the foregoing, it is a general object of the invention to mitigate the variations of ion current density in the beam from an inductively coupled radio-frequency ion or plasma source that result from the terminations of the multiple-turn inductor that is used to generate ions in that source.
Another general object of the invention is to provide a modified radio-frequency inductor for an ion or plasma source that is simple to fabricate and use, while giving improved uniformity in the azimuthal direction (the angle around the axis) for a circular beam or in the long direction for a linear beam.
Yet another general object of the invention is to provide a modified radio-frequency inductor for an ion or plasma source that provides such improved uniformity, while requiring energy from only a single radio-frequency power supply.
Still another general object of the invention is to provide a modified radio-frequency inductor for an ion or plasma source that minimizes the radio-frequency power required to obtain such improved uniformity.
A specific object of the invention is to provide a modified radio-frequency inductor for an ion or plasma source that does not require a complicated and expensive discharge-chamber shape to obtain such uniformity.
Another specific object of the invention is to provide a modified radio-frequency inductor for an ion or plasma source that does not require the presence of an additional magnetic field in the discharge region to obtain such uniformity, said magnetic field being generated by either a stationary or moving permanent magnet.
Still another specific object of the invention is to provide a modified radio-frequency inductor for an ion or plasma source that does not require the presence of an additional magnetic field in the discharge region to obtain such uniformity, said magnetic field being generated by either a stationary or moving electromagnet.
A still further specific object of the invention is to mitigate the variations of ion current density in the beam from an inductively coupled radio-frequency ion or plasma source that result from the terminations of the inductor that is used to generate ions in that source without a variety of ad hoc modifications to that source.
In accordance with one embodiment of the present invention, the dielectric discharge chamber of a generally axially symmetric ion source has a hollow cylindrical shape. One end of the discharge chamber is closed with a dielectric wall. The working gas is introduced through an aperture in the center of this wall. The ion-optics grids are at the other end of the discharge chamber, which is left open. The inductor is a helical coil of copper conductor that surrounds the cylindrical portion of the dielectric discharge chamber. The modification that produces uniformity about the axis of symmetry is a shorted turn of the helical-coil inductor at the end of the inductor closest to the ion-optics grids.
Features of the present invention which are believed to be patentable are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objectives and advantages thereof, may be understood by reference to the following descriptions of specific embodiments thereof taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements and in which:
Referring to
The usual material choices are quartz or alumina for dielectric discharge chamber 11, copper wire or wire plated with copper or silver to at least the radio-frequency “skin depth” for inductor 15, and graphite or molybdenum for grids 18A and 18B.
In operation, a source of radio-frequency (rf) energy (not shown in
The mixture of electrons and ions forms a quasi-neutral, electrically-conductive gas called a plasma within region 14. This plasma is in contact with electrically conductive grid 18A and assumes a potential close to that of the grid, which is connected to the positive terminal of a first direct-current (dc) power supply (not shown in
The ions that reach ion-optics grid 18A (usually called the screen grid) are formed into beamlets by the apertures in that grid. (A beamlet is the portion of an ion beam that passes through a single aperture of electrostatic ion optics.) These ions are accelerated by the electric field between grids 18A and 18B and, in normal operation, continue on to form a beam in external volume 19 to the right of grids 18A and 18B in
Inductor 15 is part of a resonant inductive-capacitive circuit. The resonant condition is necessary for the current in the conductor to be large enough to sustain a discharge that generates ions. To have a high “Q” (approximately the ratio of rf inductive or capacitive impedance to circuit resistance at resonance), the inductor must be made of a high conductivity material, usually copper. Other possibilities include, but are not limited to, silver and gold. As indicated previously, the high-conductivity material may be limited to a thin layer or plating, equal to or greater than the “skin thickness” at the frequency used.
It should be noted that, while the source shown in
Still referring to
Also in
Referring to
It is also necessary to consider different types of symmetry for ion source 10 shown in
Referring to
Referring to
There are several features that are of interest in ion source 30. The first of these features is re-entrant dielectric discharge chamber 31A, 31B, and 31C, with extensions 31D and 31E. Back wall 31 of the discharge chamber is not necessarily made of a dielectric material. It is stated in the aforesaid patent that the relative dimensions of the re-entrant discharge chamber and the sizes and locations of extensions 31D and 31E can be optimized for beam uniformity. It is recognized in the aforesaid patent that the ion current density, ji, in a beam from a nominally axially symmetric source is not axially symmetric, but is a function of both radius, r, from the axis of that source and the azimuthal angle, φ, about that axis,
ji=f(r,φ). (2)
The approach used therein is to treat radial and azimuthal features in no particular order or priority. For example, re-entrant cavity 31B and 31C addresses radial variations, and extensions 31D and 31E on that cavity address both radial and azimuthal variations, but no relative priority is given in their use.
Other features described in the aforesaid patent include additional inductor 35 with ends 36 and 37, and, in ion source 40 and
To summarize the prior art, nominally axially symmetric ion and plasma sources that use inductively coupled radio-frequency energy have variations of ion current density in their beams. These variations include both radial and azimuthal components. A variety of techniques has been used to make these beams more uniform. As mentioned previously, ion and plasma sources with shapes other than axially symmetric have also been used, and similar techniques could be used to produce uniform beams from such sources. For example, a primary concern for a linear source is usually the generation of a beam that does not vary significantly in ion current density along the length of the plasma source. An elongated re-entrant chamber could be used to this end, together with extensions on the re-entrant chamber contoured to produce the desired uniformity.
Referring to
Referring to
The beam was surveyed with a screened probe at a distance of about 2 cm from the ion optics. (A screened probe is described by Kahn, et al., in an article in the 48th Annual Technical Conference Proceedings of the Society of Vacuum Coaters, 2005, beginning on page 17, 2005.) Surveys were made through the axis from different directions to find the maximum departure from axial symmetry. For this maximum-departure direction and 3 and 4 cm radii, approximately midway between the axis and the maximum 7 cm radius of the ion optics, the ion current density varied ±2.6% and ±2.5% from the mean values at these radii when no shorted turn was used at the ion-optics end of the inductor. With a shorted turn at the end of the inductor, the variation at 3 and 4 cm radii dropped to ±0.1% and ±0.2% from the mean values at these radii.
The effectiveness of the shorted turn in reducing departures from axial symmetry is striking and the explanation of this effectiveness is not obvious from prior art. The aforesaid U.S. Patents by Kanarov, et al., and Yevtukhov, et al. indicate a variety of techniques can be used for achieving a uniform ion current density, but they give no priority in mitigating the radial and azimuthal variations and tend to treat the two at the same time. (See for example, the use of extensions 112a and 112b in FIG. 1 of Kanarov, et al.)
From a more fundamental technical viewpoint, if the cause of particular problem is understood, it can often be compensated for, or corrected, or mitigated at or near the source of the problem. This usually results in a more global solution than using a variety of compensations or corrections at a distance from the source of the problem which, in turn, can often require further compensations or corrections at still further locations.
Asymmetric operation of an ion source is normally the result of an asymmetry in the apparatus. If a source that is nominally axially symmetric is examined closely, it is apparent that there is very little departure from axial symmetry in that source. Departures from axial symmetry in the ion optics were mentioned previously, but it was also mentioned that such departures are understood by those skilled in the art and need not be a cause of asymmetry in operation. If the ion optics are ruled out, the most significant departure from axial symmetry is in the rf inductor, because it has a finite number of turns and the beginning and ending of the inductor constitute asymmetries.
The number of turns used in the inductors of rf ion and plasma sources typically ranges from several up to perhaps a dozen. A departure from symmetry would therefore be expected for the rf magnetic field near the end of an inductor. The local departure of that field, compared to the circumferentially averaged value near that location, would be expected to have a magnitude of the order of 1/N, where N is the number of turns in the inductor. The use of a shorted turn proximate to the end of an inductor and approximately following the contour of a turn near the end of that inductor appears from
The mechanism for the suppression of the departure from axial symmetry is Lenz's law. The induced voltage around a closed path (equal to the integral of the electric field over the path length) is proportional to the variation with time of the magnetic flux Φ passing through that closed path,
∫E·d1∝dΦ/dt. (3)
When the closed path follows a closed circuit of a material with a high electrical conductivity, the induced voltage around this closed path is approximately zero and,
dΦ/dt≈0. (4)
Note that Equation (4) does not imply that
dφ/dA≈0 (5)
everywhere within the shorted circuit. It is still possible for a positive value of flux density, dΦ/dA, at one location within the shorted circuit to be balanced by a negative value elsewhere. Nevertheless, the experimental effect of a shorted circuit of inductor as shown in
It may be noted that there are usually closed circuits of metallic conductors in or near the ion optics of an ion source. The most common metallic material used for plasma or ion sources, however, is nonmagnetic stainless steel, with a resistivity approximately 50 times that of copper. (The resistivity of copper is about 1.7 micro-ohm-cm, while that of 304 stainless steel is about 90 micro-ohm-cm.) The effectiveness of stainless steel for forming closed circuits of conductor near an inductor is thus negligible compared to copper or another high-conductivity material.
The preceding discussion has focused on the generation of an axially symmetric beam, i.e., one with an axially symmetric distribution of ion current density. Here the focus is on generating a beam that is also uniform over a significant area. The ion source configuration used to produce the profile shown by triangular symbols in
Mathematically, this is equivalent to assuming that the variables in the function on the right side of Equation (2) can be separated,
f(r,φ)=f(r)·f(φ). (6)
In
The radial correction in the apparatus was made by varying the diameters of the holes in screen grid 18A. The same 14-cm ion source used to generate the symmetric profile (triangles) in
A procedure gives the desired variation in screen hole diameters. Several screens are made with different screen hole diameters. Ion-beam profiles are then obtained using those screens, while operating the ion source at the same beam voltage, accelerator voltage, rf power, and working-gas flow rate. The desired screen-hole diameter at each radius can then be found by interpolating between the profiles to obtain the desired current density. In this manner, different hole diameters are obtained at different radii, and are plotted as the “empirical variation” in
Using the method of varying screen hole diameters described above in connection with
Another conclusion that can be drawn from the profile in
The screen-hole diameter was selected as the variable to offset the radial variation in ion current density after the asymmetry in the beam was corrected with a shorted turn at the end of the inductor. The aforesaid patent by Speiser teaches that screen-hole diameter, grid spacing, and hole locations may all be varied. The aforesaid patent by Kanarov teaches that screen grid thickness may also be varied. Although the screen grid parameters would be expected to have more effect on the extraction of ions in the discharge region, accelerator grid parameters would also be expected to have some effect. The shape of the grids (e.g., dished as described by Kaufman, et al., in an article in the Journal of Vacuum Science and Technology, Vol. 16, beginning on page 899) could also be used to correct a radial variation in ion current density. These examples should show that a wide range of ion-optics parameters may be used to offset a variation in the radial direction of an ion source.
Referring to
Prior art was presented that showed source and inductor configurations other than approximately axially symmetric are well known. A linear beam shape is described by Wykoff, et al., in the aforesaid article in the Proceedings of the Eighth International Conference on Vacuum Web Coating. An annular beam shape is described by Zhurin, et al., in the aforesaid article in Plasma Sources Science & Technology. Irregular beam shapes for specific applications are a further possibility.
As example of a plasma inductor with a non-cylindrical shape that uses a closed circuit mitigation of the inductor termination, see
The operation of plasma source 70 is similar to ion source 60 in
Referring to
For a more general approach, the present invention should be presented in terminology that does not depend on the geometric configuration of the apparatus. To this end, the localized effect of an inductor termination or end should be offset, remedied, or mitigated by a closed circuit of high-conductivity material (copper, silver, gold, etc.) that follows the shape of the inductor of interest and is spatially located close to the last turn of the inductor having that end. Source 80 in
Referring to
The introduction of working gas in
As described in connection with
The rf transmission lines from the sources of radio-frequency (rf) energy to the inductors used in the generation of ions should also be mentioned. Depending on the frequency, the transmission line may consist of a coaxial cable or a closely spaced parallel pair of conductors. Properly designed, the transmission lines have little effect on the rf magnetic fields in the discharge regions of ion or plasma sources. For example, parallel conductors of a transmission line can frequently be spaced close enough to minimize the rf fields near the inductor while, at the same time, being far enough apart that negligible rf current is conducted through the capacitive coupling between the two conductors. However, the connections between the end of the transmission line and the ends of the inductor can contribute to the termination effects of an inductor. In the examples given herein, the connections from the transmission line to the inductor were assumed to be part of the inductor terminations and were not considered further.
While particular embodiments of the present invention have been shown and described, and various alternatives have been suggested, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broadest aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of that which is patentable.
This application is based upon, and claims priority from, our Provisional Application No. 61/335,302, filed Jan. 5, 2010.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3311772 | Speiser et al. | Mar 1967 | A |
| 3958883 | Turner | May 1976 | A |
| 4862032 | Kaufman et al. | Aug 1989 | A |
| 5003225 | Dandl | Mar 1991 | A |
| 5198718 | Davis et al. | Mar 1993 | A |
| 5274306 | Kaufman et al. | Dec 1993 | A |
| 5304282 | Flamm | Apr 1994 | A |
| 6127275 | Flamm | Oct 2000 | A |
| 6777699 | Miley et al. | Aug 2004 | B1 |
| 7183716 | Kanarov et al. | Feb 2007 | B2 |
| 7309961 | Park et al. | Dec 2007 | B2 |
| 7557362 | Yevtukhov et al. | Jul 2009 | B2 |
| 20080122367 | Vinogradov et al. | May 2008 | A1 |
| Entry |
|---|
| Wykoff et al., 50-CM Linear Gridless Source,,pp. 81-88, Eigth Annual International Conference On Vacuum Web Coating, 1994. |
| Kaufman, et al., Focused ion beam designs for sputter deposition, J.Vac.Sci. Technol. 16(3) May/Jun. 1979, pp. 899-905. |
| Kaufman et al., Technology and applications of broad-beam ion sources used in sputtering, J.Vac.Sci. Technol. 21(3) Sep./Oct. 1982, pp. 725-736. |
| Zhurin, et al., Physics of closed drift thrusters, Plasma Sources Sci.Technol. 8 (1999), pp. R1-R20. |
| Kaufman et al., Ion Source Design for Industrial Applications, AIAA Journal. vol. 20, No. 6, Jun. 1982, pp. 745-760. |
| Number | Date | Country | |
|---|---|---|---|
| 20110163674 A1 | Jul 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61335302 | Jan 2010 | US |
