Plasma source for a focused ion beam system

Abstract

The present invention provides an inductively coupled, magnetically enhanced ion beam source, suitable to be used in conjunction with probe-forming optics to produce an ion beam without kinetic energy oscillations induced by the source.

Claims

1. A dual beam system for milling and deposition, comprising: a plasma tube containing an ionizable, non-metallic plasma gas and a source aperture at an end of the plasma tube; an antenna disposed around the exterior of the plasma tube, the antenna including an adjustable circuit to adjust the phase shift across the antenna; an extractor that enables extraction of the plasma from the source aperture into an ion beam; a focusing mechanism to focus the ion beam; a sample stage for supporting and moving a sample; and a scanning electron microscope.

2. The system of claim 1, further comprising a magnet placed in proximity of the source aperture.

3. The system of claim 1, wherein the focusing mechanism causes an ion beam of high brightness exceeding 2000 A/cm.sup.2/sr with an extracted beam energy of 10 keV.

4. The system of claim 1, wherein the system exhibits an energy spread that is less than 3 eV.

5. The system of claim 1, wherein the system exhibits an energy spread that is less than 4 eV.

6. The system of claim 1 in which the scanning electron microscope includes means for forming and focusing an electron beam.

7. The system of claim 6 in which the scanning electron microscope is disposed such that the ion beam and the electron beam can each be directed to the sample.

8. The system of claim 1 in which the ionizable, non-metallic plasma gas comprises a reactive species.

9. The system of claim 1 in which the ionizable, non-metallic plasma gas is selected from a group comprising: oxygen (O.sub.2), nitrogen (N.sub.2), and sulfur hexafluoride (SF.sub.6).

10. The system of claim 1 further comprising a turbo-molecular pump for evacuating the plasma source and maintaining high vacuum in an upper column optics region of the system.

11. The system of claim 1, in which the extractor includes low-emittance growth extraction optics.

12. The system of claim 11, in which the low-emittance growth extraction optics generate a collimated beam.

13. The system of claim 11, in which the low-emittance growth extraction optics generate a divergent beam.

14. The system of claim 1, in which the adjustable circuit is adjusted so that the phase shift across the antenna produces negligible modulation of the plasma.

15. The system of claim 1, in which the adjustable circuit is adjusted so that the phase shift across the antenna is substantially 180 degrees.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 shows a preferred embodiment of an ion plasma source according to the method of the present invention.

(3) FIG. 2 shows a circuit for adjustment of power transfer to the plasma.

(4) FIG. 3 shows the axial energy spread data for this invention.

(5) FIG. 4 shows a preferred embodiment of the present invention.

(6) FIG. 5 shows an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) FIG. 1 shows a diagram of an embodiment of an ion plasma source 10 according to the method of the present invention. A coil 1000 is capacitively coupled by impedance 210 to an RF source 200. Note that the capacitances shown in FIG. 1 are nominal values that readily can be selected by one of skill in the art according to the frequency of operation of the coil, as will be described further below. Coil 1000 is preferably a multi-turn coil that wraps around a dielectric plasma tube 2000 so that the axis of the coil substantially coincides with the axis of chamber 2000 and the beam axis.

(8) When driven by source 200, coil 1000 forms a helical RF antenna. Driving the coil with an RF source can impart a time-varying potential to the plasma, due to capacitive coupling. That is, the coil can produce a radial electric field that modulates the plasma. This is undesirable because it creates a spread in the beam energy, resulting in chromatic aberration. However, in a preferred embodiment of the present invention, the antenna is driven at one end by a signal that is out of phase with the signal at the opposite end by as much as 180. This creates a region interior to the coil where the potential fluctuations are substantially zero at all times. In this region there is substantially no time-varying modulation of the plasma arising from the time-varying voltage across the coil 1000. Thus, the phase of the antenna can be adjusted to minimize modulation of the ionization potential of the plasma in the region where ions are extracted in response to an applied acceleration field. The energy of the ions extracted from the plasma, according to this method, is substantially un-modulated by the RF voltage across the antenna.

(9) However, source 200 does indeed cause electrons to move. Because of the orientation of the coil, free electrons in the plasma circulate around the plasma skin, causing them to collide with atoms to produce ions. This can produce plasma of very high ion density with relatively low thermal ion energy. A fixed-strength annular magnet about 5 to 10 milli-meters thick, or a variable-strength electromagnet 3000, that produces an axial field strength of nominally 200 to 1000 Gauss is placed between an end of the coil and a region 3500 of extraction, and is provided to increase plasma density. The magnet reduces electron diffusion and loss to the walls of the plasma chamber. Thus, the RF source is inductively coupled to the plasma and the annular magnet increases the plasma density in the extraction region.

(10) A split Faraday shield 6000 can be used to screen out the capacitive field of the coil, but this is less desirable for two main reasons. First, a degree of capacitive coupling is required to ignite the plasma. Using a split Faraday shield usually requires another external power source (e.g., a Tesla coil) to ignite the plasma. Second, split Faraday shields typically result in some energy loss, due to Eddy currents induced in the shield. Without the Faraday shield, the balanced antenna approach still results in a sufficient time varying electric field in areas of the plasma chamber to cause the initial field ionization required to initiate the plasma.

(11) A beam voltage 400 is electrically connected to a beam energy cap 420, which has an additional low pass filter 410 to ensure negligible RF pick-up to the beam voltage. An extractor voltage source 600, that is negative with respect to the potential applied to the source electrode 4000, is applied to the extraction electrode 4500. Skimmer electrode 5000 is at ground potential and provides an aperture through which the dense ion beam passes to produce an ion beam that can be focused with appropriate optics. Thus, the beam is extracted from the extraction region 3500, with a beam waist formed in the skimmer aperture 5000, and thus propagates along the beam axis in response to an applied acceleration. Alternatively, beam voltage 400 can be electrically connected directly to the source electrode 4000 instead of to the beam energy cap 420.

(12) Note that the configuration of electrodes in the extraction region shown in FIG. 1 is representative, but other electrode configurations could be implemented instead. Indeed, the invention contemplates that extraction optics generating a collimated or divergent beam will be optimum, as opposed to forming a waist in the beam. Essentially, extraction optics that result in the least emittance growth and minimal Coulomb interactions are the most favorable for a high brightness ion source.

(13) FIG. 2 shows a circuit of the present invention, including a plasma impedance, Zp, 2010 in parallel with an unknown coil inductance characteristic 1010. In series with the parallel combination of the plasma impedance 2010 and inductance 1010 is capacitance 8000. This series-parallel combination is in parallel with a second capacitance 7000. This parallel-series-parallel combination is in series with a third capacitance 210. This entire network is in parallel with the source 200. Clearly, the phase shift across the coil and plasma impedance can be controlled by the selection of capacitance values 210, 7000 and 8000. One can therefore select capacitance values 210, 7000, and 8000 to obtain a phase shift across the coil and plasma of 180 degrees.

(14) Suppose for example that the source is at 13.56 Mega-Hertz (MHz) with a 50 ohm output impedance. Then choosing the following capacitances results in a phase shift across the coil and plasma of about 180 degrees: capacitance 210=50 pico-Farads (pF), capacitance 7000=330 pF, capacitance 8000=340 pF. The combined componentscapacitances 210, 7000, and 8000, and inductance 1010may be viewed as an impedance matching network that matches the 50 ohm source impedance to the plasma impedance 2010 load. When the network is matched, maximum power from the source is transferred to the plasma impedance load. These values were implemented for an Argon plasma tube with an inner diameter of 20 milli-meters (mm), an outer diameter of 26 mm, and a length of 100 mm. The wall of the plasma tube was 3 mm-thick quartz. The coil was 30 mm long with three turns and a diameter of about 50 mm.

(15) Thus, the present invention provides a circuit adjustment to achieve a maximum transfer of power to the plasma, coupled with negligible modulation of the plasma potential, resulting in negligible axial energy spread of the extracted ions. FIG. 3 shows a graph of the axial ion energy distribution resulting from the antenna in a balanced state and an unbalanced state. When the antenna is in the balanced state, that is, when the phase shift across the antenna is about 180 degrees, then the axial ion energy is very narrowly distributed about zero electron-Volts (eV). This is shown by the solid curve in FIG. 3, which exhibits a full width at half magnitude of less than 3 eV. In contrast, when the antenna is unbalanced, the axial ion energy is distributed over a broad energy range. This is shown by the dashed curve in FIG. 3, which exhibits a full width at half maximum of greater than 70 eV.

(16) The embodiment described above minimizes the effects of capacitive coupling on the ions, leaving only the influence of the pre-sheath potential gradient. The potential gradient of the pre-sheath region is finite, but small, and is generally about half the mean electron energy (T.sub.e), where T.sub.e is only 3 eV for the type of source described above, giving an inescapable lower limit to the axial energy spread (E) of 1.5 eV.

(17) The present invention may be conveniently operated at low RF power, nominally imparting 25 Watts to the plasma. At this power level a brightness of 200 Acm.sup.2 sr.sup.1 (Amperes/centimeter-squared/steradian) can be generated at only 5 keV with an ion current density of 19.6 mA cm.sup.2. This implies a thermal energy of 0.15 eV and a plasma density of 810.sup.11 cm.sup.3. Pulse plasma densities of 110.sup.14 ions cm.sup.3 have been attained with this source, implying that a source brightness of >110.sup.5 A cm.sup.2 sr.sup.1 is obtainable at a beam energy of 50 keV, with current density of: J.sub.i=0.6n.sub.iq{square root over (k.sub.BT.sub.eM.sub.i)}2.4 Acm.sup.2, where E.sub.0=50 keV, and E.sub.=0.15 eV. This yields

(18) ${}_{\max }=\frac{J_i{E}_0}{E_{}}>1{10}^5{\mathrm{Acm}}^{-2}{\mathrm{sr}}^{-1}.$

(19) FIG. 4 shows a focused ion beam system 100 that includes an evacuated envelope 10 upon which is located a plasma source 11 with an RF antenna, implemented as described above, to provide a dense plasma for ion beam focusing column 16. Ion beam 18 passes from source 11 through column optics 16 and between electrostatic deflection mechanism 20 toward specimen 22, which comprises, for example, a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26. A turbo-molecular pump 8 is employed for evacuating the source and maintaining high vacuum in the upper column optics region. The vacuum system provides within chamber 26 a vacuum of typically between approximately 1107 Torr and 5104 Torr, with nominally 10 mTorr in the plasma source and <1106 Torr in the column optics chamber.

(20) High voltage power 34 is connected to ion source 11 as well as to appropriate electrodes in focusing column 16 for forming an approximately 0.1 keV to 50 keV ion beam 18 and directing the same downward. RF power supply 33 and impedance matching circuit 27 is also provided to energize a coil of source 11, as described above. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20 whereby beam 18 may be controlled to trace out a corresponding pattern on the upper surface of specimen 22. In some systems, the deflection plates are placed before the final lens, as is well known in the art.

(21) The ion beam source 11 is brought to a focus at specimen 22 for either modifying the surface 22 by ion milling, material deposition, or for the purpose of imaging the surface 22. A charged particle multiplier 40 used for detecting secondary ion or electron emission for imaging is connected to video circuit and amplifier 42, the latter supplying drive for video monitor 44 also receiving deflection signals from controller 36. The location of charged particle multiplier 40 within chamber 26 can vary in different embodiments. For example, a preferred charged particle multiplier 40 can be coaxial with the ion beam and include a hole for allowing the ion beam to pass. A scanning electron microscope 41, along with its power supply and controls 45, are optionally provided with the FIB system 8.

(22) Signals applied to deflection controller and amplifier 36, cause the focused ion beam to move within a target area to be imaged or milled according to a pattern controlled by pattern generator 38. Emissions from each sample point are collected by charged particle multiplier 40 to create an image that is displayed on video monitor 44 by way of video circuit 42. An operator viewing the image may adjust the voltages applied to various optical elements in column 16 to focus the beam and adjust the beam for various aberrations.

(23) Focusing optics in column 16 may comprise mechanisms known in the art for focusing or methods to be developed in the future. For example, two cylindrically symmetric electrostatic lenses can be implemented to produce a demagnified image of the round virtual source. Because of the low axial energy spread in the extracted beam, chromatic blur is minimal and efficient focusing of the beam can be achieved even at low acceleration voltages (ie low beam energies). These properties in conjunction with appropriate focusing optics can be used to generate nanometer, to micrometer scale spot sizes with a range of kinetic energies (0.1 keV-50 keV) and beam currents from a few pico-amperes to several micro-amperes.

(24) A large transfer of power from the coil to the plasma is desirable to achieve high ionization density. Hence, efficient power dissipation is required to limit the operating temperature of the plasma source. Desirably, heat is conducted away from the plasma tube efficiently. This can be achieved by forming a dielectric and metallic shell around the plasma, inside the coil, with high thermal conductivity to efficiently conduct heat away from the plasma.

(25) The source can be expected to have a predictable and relatively long life span (ie >>1000 hours). This type of source differs from a DC plasma source, because ions leave the plasma and strike the surrounding walls at energies that are nominally 15-20 eV (5.2 Te for an argon plasma), resulting in negligible sputtering of source chamber material. A typical DC source has a plasma potential that is nominally 50-500 V higher than a cathodic electrode, resulting in significant sputtering of the cathode that ultimately results in the end of life for these sources. Substantial elimination of plasma potential modulation, according to the methods described herein, also substantially reduces the probability of ions striking the source electrode with energy above the sputter threshold. Ions transit the plasma sheath in a time that is substantially less than the period of the RF signal. Hence, ions leave the plasma with kinetic energies that are determined largely by the temporal plasma potential induced by the capacitive field from the antenna.

(26) The realization of very high plasma densities (up to 1014/cm.sup.3), low thermal ion energies (down to 0.1 eV), low axial energy spread (1.5-3 eV), the ability to operate with either inert or reactive gases, and the potential for very long life due to minimal erosion of source materials, makes a magnetically enhanced, inductively coupled plasma source ideal to be used in conjunction with probe forming FIB optics.

(27) The present invention can provide beam currents from a few pico-amperes to current greater than 10.sup.11, greater than 10.sup.10 amps, greater than 10.sup.9 amps, greater than 10.sup.8 amps, greater than 10.sup.7 or current of several micro-amperes. A source brightness of at least 10.sup.4 A/cm.sup.2/sr, at least 10.sup.5 A/cm.sup.2/sr, and up to 10.sup.6 A/cm.sup.2/sr or more at 50 keV can be achieved. The axial energy spread is less than 3 eV, less than 2.5 and could be as low as 1.5 eV. This contrasts sharply with present day Liquid Metal Ion Sources (LMIS), which can provide a beam brightness on the order of 10.sup.6 A/cm.sup.2/sr, but with an energy spread on the order of 5 eV. Also, LMI sources are generally only suitable for generation of beam currents in the pico- to nano-ampere range. A further advantage of the present invention is the ability to operate with any inert gas as well as many reactive gases, (e.g., O.sub.2, N.sub.2, SF.sub.6, etc. . . . ). The ion beam from the present invention is capable of being focused into a beam diameter of a few nanometers, up to several tens of micrometers. Inert gas beams can readily be generated making the invention suitable for applications where gallium or other metallic ion beams might be problematic.

(28) In fact, the axial energy spread is dictated only by the static potential gradient of the pre-sheath region of the plasma. Ions can be generated at any point on the pre-sheath potential gradient, with ions created at the top of the gradient ultimately acquiring more kinetic energy than those created at the bottom. The energy distribution is determined by the height of the potential hill, which is determined by the mean electron energy in the plasma (T.sub.e), according to:
Vk.sub.BT.sub.e2q(4)
With a mean electron temperature of 3.4810.sup.4 K (3 eV), the resulting potential drop in the pre-sheath is 1.5V.

(29) As a point of reference, a 100 nm diameter, 100 pA argon FIB, with a nominal landing energy of 20 keV requires an image side brightness (B) of 7200 A cm.sup.2 sr.sup.1 according to:

(30) $\begin{array}{cc}B=\frac{4I}{{}^2{}_i^2{d}^2}& \left(5\right)\end{array}$
with .sub.i being the convergence angle, d the spot size, and I the beam current. If an axial energy spread (E) of nominally 2 eV is assumed, along with a demagnifying two lens optical system having a chromatic aberration coefficient, C.sub.c=86 mm and a spherical aberration coefficient, C.sub.s=120 mm, the beam is chromatically dominated under the optimum conditions. Furthermore, we can assume that our beam will have equal contributions from chromatic blur and the demagnified geometric source size. Hence, the contributions from chromatic aberration disk (d.sub.c) and the geometric spot size will each be 100{square root over (2)}=71 nm. Hence, for the values indicated above, the optimum .sub.i is determined to be 7.5 mrads, resulting in an image side brightness of 710.sup.3 A cm.sup.2 sr.sup.1 and a source brightness of 1.510.sup.4 A cm.sup.2 sr.sup.1 at 20 keV.

(31) In order to generate a source brightness of 1.510.sup.4 Acm.sup.2 sr.sup.1 in an argon plasma, a magnetically enhanced Inductively Couple Plasma (ICP) source can be used. In order for this plasma source to provide a brightness of greater than 1.510.sup.4 A cm.sup.2 sr.sup.1, at a nominal energy of 20 keV, a current density from the source of 225 mA/cm.sup.2 is required assuming a thermal energy of 0.1 eV, according to:

(32) $\begin{array}{cc}{}_{\max }=\frac{J_i{E}_0}{E_{}}& \left(6\right)\end{array}$
Finally, in order to achieve J.sub.i=225 mA/cm.sup.2 we require a plasma density of at least 910.sup.12 ions/cm.sup.3, from:

(33) $\begin{array}{cc}{n}_i=\frac{J_i}{0.6q\sqrt{k_B{T}_e{M}_i}}& \left(7\right)\end{array}$

(34) where n.sub.i=plasma ion density (m.sup.3), J.sub.i=2250 A m.sup.2, T.sub.e=3.4810.sup.4 K, k.sub.B=1.3810.sup.23 J K.sup.1, q=1.610.sup.19 C, M.sub.i=39.9481.6610.sup.27 kg. The plasma source invention described herein provides all the necessary plasma attributes (ion density, mean thermal ion energy and axial energy spread) to result in a high brightness ion source suitable for nanometer scale FIB applications. Thus, the present invention provides a low energy spread (<3 eV) and low mean thermal ion energy (<0.15 eV) at low RF power, on the order of 25 Watts imparted to the plasma. Also, the source exhibits very high beam current stability (<0.1% drift per hour). At higher RF powers the beam brightness increases while still maintaining low thermal ion energies within the plasma to enable realization of a targeted brightness with a plasma density below that achieved in a pulsed mode.

(35) An alternative embodiment of the present invention is shown schematically in FIG. 5. An RF generator 950 is presented with a source resistance 951. This is coupled to shunt capacitance 952. A primary winding 955 wrapped around a ferrite toroidal core 957 with a Faraday shield 956 is coupled to the RF source through inductance 954 and capacitance 953. Capacitance 953 compensates for leakage inductance 954. Ferrite core 957 may be formed from two commercially available cemented-together cores wrapped with Teflon tape, each core rated at 1 kilo-Watt (kW). In a preferred embodiment, the primary winding is distributed around the toroid as copper tape. Wrapped over this is more Teflon insulation tape, followed by a Faraday shield, followed by another layer of Teflon.

(36) There are two balanced secondary windings 958 and 959 that may be implemented as single passes of copper tape traversing through the interior of core 957. These may be held in place by the hardware of the capacitors and stand-off feed-through connectors for the antenna wires. The secondary windings are coupled together through adjustable capacitance 960 and are coupled to ground through the parallel combination of like capacitances 961 and 962 and like resistances 963 and 964. The opposite ends of the windings 958 and 959 are coupled across the parallel combination of the antenna inductance 965 and the plasma impedance 966. Values for the circuit components may be obtained from a SPICE program assuming a value for the plasma impedance.

(37) The transformer formed by the primary and secondary windings provides an impedance transformation of one to (T1/T2)**2, where T1 is the number of turns in the primary and T2 is the number of turns in the secondary. This aids in transforming the low plasma impedance 966 to the 50 ohm impedance of the generator. For example, with a turns ratio of 7 to 2, the impedance transformation is one to 12.25.

(38) The secondary is center-tapped and coupled to ground to provide a balanced circuit independent of the impedance match. The secondary windings and antenna, in conjunction with capacitances 960, 961, and 962, form a series resonant circuit. Variable capacitance 960 enables tuning of the circuit that is relatively insensitive to changes in plasma impedance. The balun leakage inductance 954 is compensated by capacitance 953, and the parallel capacitance 952 completes the match to 50 ohms. The matching provided by this embodiment is relatively insensitive to component tolerances. Moreover, the balanced circuit produces an RF field that is substantially symmetrical across the plasma length.

(39) Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Because the invention can be used in different applications for different purposes, not every embodiment falling within the scope of the attached claims will achieve every objective. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.