Patent Publication Number: US-4549082-A

Title: Synthetic plasma ion source

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to ion sources and especially to an ion source utilizing a charge exchange process to change a molecular beam to an ion beam. 
     Ion beam sources have found various applications, one being in the fabrication of integrated circuits. Integrated circuit fabrication is now beginning a new phase. A few years ago, the goal was the fabrication of very large scale integrated circuits (VLSI) in which as many functions as possible were crowded on one chip. Now there is a drive to develop very high speed integrated circuits (VHSIC), in which the objective is to increase speed by shrinking components to submicron dimensions. For the initial stages of VHSIC work, electron beam microlithography is being used. 
     The objective of particle beam lithography is to write a pattern on a semiconductor surface with a tiny focused spot of charged particles. This pattern can then be treated to form an integrated circuit. Essential to this process is the source of the charged particles; the closer it is to being an ideal monochromatic point source, the smaller the final spot size can be. Also, the more current the source produces, the faster the pattern can be written. A means is described herein which constitutes such a nearly ideal ion source, superior to any previous charged particle source intended for microlithography. 
     Ions have an advantage over electrons in being approximately one hundred times more effective at exposing resist material used in chip fabrication. This would reduce the cost/wafer/hour for an ion beam of the same current and spot size as an electron beam. 
     Another advantage of ions over electrons is that much greater resolution is possible, due to the absence of backscattering by the substrate (the proximity effect). 
     Ions also permit entirely new techniques to be used. SiO 2  can be exposed directly, without first exposing a layer of resist. Etching of SiO 2  by sputtering could be done directly and this step could be followed immediately by implantation within the same vacuum chamber. 
     OBJECTS OF THE INVENTION 
     An object of this invention is to provide a charged particle source having higher current, smaller spot size and more efficiency than other charged particle sources. 
     Another object is to provide a charged particle source which avoids the backscattering effect exhibited by electron beam sources. 
     A further object is to provide intense ion beams. 
     SUMMARY OF THE INVENTION 
     The objects and advantages of the present invention are achieved in a crossed-beam charge-exchange ion gun by utilizing an electron beam to form a synthetic plasma by projecting electrons into the region where a positively charged ion beam is projected through a molecular beam of the same gas species, e.g., Argon, as the ion beam. The charged ions ionize the molecules and the ions in the newly formed ion beam are kept from flying apart by the neutralizing effect on their charges of the intermingled electrons. 
     A second aspect of the invention is the use of an aperture both as a beam collimator and an aperture stop (also known as a lens stop) to improve the optical qualities of an immersion lens. The immersion lens includes the lens formed by the curvature at the ion-emitting edge of the plasma plus the field in the volume of the sheath extending from the curved edge of the plasma to the cathode. 
     With respect to conventional plasma ion sources, e.g., Duoplasmatrons, the present invention provides a cold, collimated ion beam in the plasma before extraction. The invention provides improvements over liquid metal and field ionization sources in that it effects ionization in a field-free region, leading to less spherical and chromatic aberration, and also makes the attainment of greater currents possible. Relative to previously reported molecular-beam ion sources, the invention provides means for extracting intense beams, rather than just individual ions, by converting the beam source to a hybrid beam/plasma source. 
     Some fields to which the present invention has application are the following: 
     (a) Lithography: direct and indirect (mask-making); 
     (b) Micromachining and etching by sputtering; 
     (c) Materials analysis (secondary ion mass spectroscopy) (SIMS), induced X-rays, Auger spectrometry); 
     (d) Microscopy (scanning and transmission); 
     (e) Ion implantation. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic diagram showing an ion beam source in accordance with the invention. 
     FIG. 2a is a simplified schematic diagram of a trochoidal monochromator used in the invention. 
     FIG. 2b is a schematic diagram showing a blow-up of a small area of FIG. 2a showing the charge-exchange area, the aperture tube in the anode and the secondary ion beam. 
     FIG. 3 is a partial cross-sectional view of an ion gun for use in the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows in schematic form the essential features of a molecular-beam charge-exchange ion source in accordance with the present invention. A gas reservoir 1 contains a pressurized gas, preferably a noble gas such as argon (Ar) or helium (He). The high-pressure gas (assume Ar) is allowed to exit from the reservoir through a small hole in a nozzle 12 (see FIG. 3) into an evacuated region. Since the pressure at first is high, the gas undergoes hydrodynamic flow as it emerges from the nozzle 12, and then makes a transition to molecular flow. This process produces a very intense beam. Following the nozzle 12, are an aperture 13, called a skimmer, and a subsequent aperture 14 called a collimator. These apertures produce a cylindrically symmetric collimated molecular beam 2. 
     A gun that can be used to form the molecular beam is shown and described in U.S. Pat. No. 3,616,596, issued to R. Campargue on Feb. 17, 1969, and in a section by R. Campargue in the book entitled &#34;Rarefield Gas Dynamics, Supp. 3, Vol. II, published in 1966 by Academic Press, N.Y., editor J. H. Leeuw, the section being called &#34;High Intensity Supersonic Molecular Beam Apparatus&#34;, Pgs. 279-298 (see, especially, FIG. 1 which corresponds in general to FIG. 3 herein). 
     An electron beam 3, preferably of low-energy electrons, e.g., a few tenths of an eV, is formed by an electron emitting filament 17 and projected through a drift region 11 where it is subjected to an EXB field which bends its path to make the path coincide with that of the molecular beam 2 so that electrons intermingle with Ar molecules in the combined beam. A primary ion beam 5 of positively charged argon ions (Ar + ) is orthogonally projected through the intermingled electron-molecule beam in an ionization region where the beams cross. The ions in the primary ion beam may be of any gaseous species having an ionization potential fairly close in value to that of the gaseous species forming the molecular beam. A charge exchange by means of a electron tunneling process occurs which transfers positive charge from the primary beam Ar +  ions to the neutral Ar molecules, resulting in a secondary ion beam moving substantially undeflected through a plasma region 6. The plasma extends through a small aperture 10 in a tube 10&#39; formed in an anode 8. Spaced downstream of the anode 8 is a cathode 9. The electrons are reflected backwards by the field set up between the anode and cathode and the substantially electron-free region 18 of the secondary ion beam between the plasma and the cathode 9 is called the sheath. The sheath edge 7 abuts the edge of the plasma 6 which projects through the aperture stop 10 into the aperture tube 10&#39;. 
     The plasma produced by projection of electrons into a region containing ions is called a synthetic plasma. The synthetic plasma region 6 includes the region of intersection of the molecular and ionic Ar beams. If the plasma is produced sufficiently close to the edge of the sheath, only a small percentage of the secondary ions will be neutralized by recombination with electrons. The edge of the sheath which abuts the plasma projecting through the aperture 10 forms a meniscus, which is part of an immersion lens ion-optical properties of the lens and the curvature of the meniscus being controllable by varying the ion density or the cathode-anode (i.e., the extractor) potential. The meniscus alters the secondary ion beam so that it acts as though it emanates from a point source, i.e., the meniscus provides the beam with a virtual or real focal point (point source) depending on the concavity or convexity of the meniscus respectively. The beam, as it continues on its course, is then bent by the field in the sheath so that the beam still forms a very small point of focus (small in comparison to the focus area of conventional ion beam guns). 
     The part of the plasma which lies to the right of the lens, or apertures, stop 10 is called the plasma column herein. The plasma column exists because the ions therein are collimated by coming through the lens stop 10 and are confined radially by their directed motion, not by walls or the magnetic field. Therefore, the column is called free-standing. The shape and position of the meniscus 7 formed at the junction between the downstream edge of the plasma and the upstream edge of the sheath is shown in FIG. 2b. The meniscus may be converging or diverging although the latter is preferable. The immersion lens (immersion objective) comprises the meniscus and the sheath field, the optical properties of the immersion lens (and the position and shape of the meniscus) depending in a calculable way on the internal plasma conditions, such as electron temperature, ion kinetic energy and ion density, and the shapes of, and the voltage difference between, the anode and cathode. 
     A molecular beam gun structure which can be employed is that shown in FIG. 3. This structure is known in the art and is modified in accordance with the present invention by introducing immediately after the collimator 14 a trochoidal monochromator, shown in FIG. 2a, an anode 8 and a cathode 9, which are both coaxial and cylindrically symmetric. The monochromator has a series of spaced plates 21 before and after a drift region 11 in which crossed electric (E) and magnetic (B) fields bend the electron beam 3 into coincidence with the molecular beam 2. FIG. 1 shows the axial length over which the B-field must extend. In practice, however, it will extend over a more extensive length since it is difficult to confine the magnetic field. The anode 8, with its aperture 10&#39;, is formed from the final plate of the monochromator, the various elements of which are separated by sapphire balls 22. 
     The electrons travelling in the secondary ion beam before they are reflected at the plasma/sheath edge prevent the slow-moving Ar +  ions in the secondary ion beam from flying apart due to like-charge repulsion. Thus, adding the electrons maintains the collimation of the ion beam originally established in the molecular beam. Also, the optics of the immersion lens beginning at the plasma/sheath edge is improved by using the aperture 10 both as a collimator and a lens stop. The collimated molecular beam emerging from the collimating hole 14 still has some divergence and the tube aperture 10 acts to collimate the secondary ion beam still further. In addition, the area of the aperture 10 is made smaller than the cross-sectional area of the anode aperture 8&#39; so that the aperture 10 acts as a lens stop, thereby avoiding the spherical aberration in the immersion lens that would result if the plasma made contact with the anode aperture 8&#39;. 
     Best-mode parameters for the elements of the ion gun are as follows: 
     (a) distance from nozzle to interaction (charge exchange) region: 5 cm. 
     (b) gas density at ionization region: ≃1×10 13  cm -3   
     (c) primary ion current: 10-100MA. 
     (d) secondary ion kinetic energy within the plasma: 0.063 eV 
     (e) plasma electron temperature: 0.1 eV 
     (f) aperture tube length≃aperture tube radius 
     (g) anode filling factor (plasma column diameter/anode tube diameter): ˜0.6 
     (h) extraction field strength: ˜0.6 V/micron 
     (i) B ˜100 Gauss 
     (j) trochoidal monochromator parameters transverse drift distance, 1.6 mm; distance between the drift electrodes, or &#34;dees&#34;, 1.6 mm; voltage between &#34;dees&#34;, a few tenths of a volt. For a discussion of the monochromator, see, for example, &#34;Optimization of the trochoidal electron monochromator,&#34; McMillan and Moore, Rev. Sci. Instrum. 51(7), Jul. 1980, and reference cited therein. 
     For a given set of ion source parameters, the extraction voltage (between anode and cathode) can be &#34;tuned&#34; to give a beam with the least spherical aberration for those parameters. 
     Table 1 gives best results obtained for ion beam performance when tuning is done in a computer simulation of ion extraction with an immersion lens of the type shown in FIG. 1. 
     
                       TABLE 1                                                     
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                        Final Beam                                        
Plasma Column                                                             
            Final Beam  Diameter  Beam                                    
Diameter    Energy      (Spot Size)                                       
                                  Current                                 
(microns)   (keV)       (microns) (μ amps)                             
______________________________________                                    
24           10         0.1       0.4                                     
24          100         0.03      0.4                                     
80           10         0.3       4.4                                     
80          100         0.1       4.4                                     
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     A variation of the invention consists in eliminating the tube 10&#39; in favor of an aperture in each of a pair of spaced plates, the second, or downstream, plate acting as the anode and having the larger aperture size, and the first, or upstream, plate having the same potential as the anode and having a smaller aperture size. As can be seen, this arrangement is the equivalent of the tube arrangement since the area of the tube frontal aperture 10 is smaller than the area of the aperture 8&#39; in the anode 8. The area of the anodic aperture 8&#39; is, of course, the same as the cross-sectional area of the aperture tube 10&#39;. 
     In another variation of the invention, the electrons could be projected into the molecular beam in the region in which the primary ion beam is being injected; in this case, no B-field is present. 
     In a further variation of the invention, ionization of the molecular beam could be effected by subjecting the beam to electromagnetic radiation. 
     Other variations in structure are possible. For example, the anode and cathode do not have to be in the form of straight plates but could be formed with surfaces of revolution. For example, to obtain less spherical aberration, the anode could be arcuate, either convexly or concavely. A grid (Wehnelt) electrode, which is common in the art, could be inserted between the anode and the cathode to further shape the field of the immersion lens. Also, although in crossed-beam ion guns, the preference is to make the beams cross orthogonally, this does not necessarily have to be the geometrical configuration that is employed. Moreover, while the electron beam has been shown as being intermingled with the collimated molecular beam prior to its ionization, it is equally possible to inject the electrons at the point of ionization or thereafter. 
     What has been described herein is an improved ion beam source which employs a charge-exchange process and a low-energy-electron beam to form a synthetic plasma of ions and electrons. The low-energy electrons shield the ionization region from the extraction region, prevent secondary ions from being unduly accelerated and dispersed before reaching the extraction region, and provide a controllable plasma meniscus at the sheath edge. High intensity, collimation and monochromaticity (low temperature) is achieved while the beam is still in the molecular, uncharged state. A B-field is provided which, in addition to its use in the trochoidal monochromator, confines low-energy electrons to the molecular beam and plasma region, suppresses plasma instabilities driven by the primary beam and removes hot electrons. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.