Abstract:
In one embodiment of this invention, the apparatus for sputter deposition within an evacuated volume comprises a compact gridless ion source into which an ionizable gas is introduced and from which ions leave with directed energies at or near the sputtering threshold and a sputter target near that source, biased negative relative to the surrounding vacuum enclosure, and located within the beam of ions leaving that source. Particles sputtered from the target are deposited on a deposition substrate spaced from both the ion source and the sputter target. An energetic beam of electrons can be generated by the incident ions striking the negatively biased sputter target and the deposition substrate is located either within or outside of this beam, depending on whether the net effect of bombardment by energetic electrons is beneficial or detrimental to that particular deposition process.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation-in-part of our copending application Ser. No. 09/471,662, filed Dec. 24, 1999, which was a continuation-in-part of our earlier application Ser. No. 09/078,727, filed May 14, 1998, now abandoned, and the present application also claims benefit of our Provisional Application Ser. No. 60/177,894, filed Jan. 24, 2000.  
     
    
     
       FIELD OF INVENTION  
         [0002]    This invention relates generally to the deposition of thin films, and more particularly to the deposition of thin films with controlled bombardment, or lack thereof, by energetic electrons.  
           [0003]    This invention can find application in a variety of thin film applications such as the deposition of decorative or protective films, the deposition of conducting films for solid state electronics, the deposition of insulating or dielectric films for either solid state electronics or optical devices, or the deposition of magnetic films for recording heads or recording media.  
         BACKGROUND ART  
         [0004]    Low-pressure sputter deposition is widely used for the deposition of thin films. The low background pressure of approximately one millitorr (0.13 Pascal) or less is important to minimize the collisions of sputtered particles with molecules of the background gas as they travel from the sputter target to the deposition substrate. Minimizing these collisions is important both for the cleanliness of the deposited film and avoiding degradation of the sputtered-particle energy, which is important for obtaining good adhesion and high density for the deposited film.  
           [0005]    A closely related prior art is the sputter deposition from a grounded target that results from an energetic beam of ions being directed against that target. The most widely used ion source for such an application is the gridded ion source described in an article by Kaufman, et al., in the  AIAA Journal,  Vol. 20 (1982), beginning on page 745, incorporated herein by reference. Although it would be less likely, a gridless ion source could also be used. The end-Hall type of gridless ion source is described in U.S. Pat. No. 4,862,032—Kaufman, et al., while the closed-drift type of gridless ion source is described in U.S. Pat. No. 5,359,258—Arkhipov, et al., both of which are incorporated herein by reference. The primary advantage of such sputter deposition apparatus is the low pressure that is possible at the deposition substrate. The high pressure required for the generation of ions is confined to the inside of the ion source. The total gas flow is thereby reduced, compared to having the entire volume within the vacuum enclosure at high pressure, and moderate pumping permits the deposition substrate to be maintained at a low background pressure.  
           [0006]    There are problems with the prior art of sputter deposition from a target using an energetic beam of ions against that target.  
           [0007]    One problem is that the ion beam generated by the ion source must be directed only at the sputter target. Even with carefully machined and expensive ion optics grids, it is common for some energetic ions to strike other hardware besides the target and thereby introduce contamination directly into the deposited film, or through contamination sputtered onto the target, introduce contamination indirectly into the deposited film.  
           [0008]    Another problem is the reflection of energetic neutrals from the sputter target. Energetic ions become neutralized in striking the target, and are reflected diffusely to strike the thin film being deposited on the substrate. These collisions with the substrate introduce damage sites in the deposited film.  
           [0009]    Yet another problem is the reduced ion current capacity of ion optics for gridded ion sources at energies low enough to minimize the above problem of energetic neutral reflection. As described in the above article by Kaufman, et al., in the  AIAA Journal,  the ion current capacity of these ion optics varies approximately as the three-halves power of the voltages. Operation at low ion voltages—and energies—therefore severely restricts the ion beam current and thus the process rate.  
           [0010]    A related problem is the large gas flow required to operate a gridded ion source when the source must be large to offset the reduction in ion current capacity due to operating the source at low voltages.  
           [0011]    In summary, complicated and expensive apparatus is required for sputter deposition with energetic ion beams. Attempts to reduce the damage due to energetic neutrals by reducing the ion energy can result in an increase in the size of the ion source used which, in turn, can result in the increase of the gas flow and a need for larger, more expensive vacuum pumps.  
           [0012]    A more recent technology is described in copending application Ser. No. 09/471,662 filed on Dec. 24, 1999, which is expressly incorporated herein by reference. The contamination in the deposited film is reduced in this more recent technology by using an ion source that generates a low-energy ion beam, wherein the energies of ions in this beam are sufficiently low to minimize or eliminate any sputtering due to collisions of these ions with ground-potential surfaces. The ion energy required to sputter the target is obtained, not by acceleration to a high energy in the ion source, but by biasing the target negative relative to ground potential—defined as the potential of the surrounding vacuum enclosure. This approach has a significant advantage over other ion-beam deposition processes in that it is not necessary to confine the ion beam to the sputter target to achieve a high degree of purity for the deposited films.  
           [0013]    Because of the negative bias of the target, the secondary electrons emitted from the target are accelerated to a high energy in the plasma sheath adjacent to the sputter target. In this manner, secondary electrons emitted from the target become energetic electrons after being accelerated through the plasma sheath. Under the proper operating conditions, these energetic electrons form a directed beam away from the target. Depending on the locations of the deposition substrate, this beam of energetic electrons could strike, or not strike, the surface of the deposition substrate upon which the film was being deposited.  
           [0014]    Energetic electrons are known to cause a variety of effects, both adverse and beneficial, on films deposited on deposition substrates, as described by Ball in  J. of Applied Physics,  Vol. 43 (1972), beginning on page 3047, and by Chapman, et al., in  J. of Applied Physics,  Vol. 45 (1974), beginning on page 2115, both of which are incorporated herein by reference. There has been, however, no recognition that specific operating parameters could control the formation, or lack of formation, of a well-defined electron beam by the acceleration of secondary electrons from a sputter target.  
           [0015]    Electron-beam sources have been studied separately, as described by Kaufman, et al., in  J. of Vacuum Science and Technology,  Vol. A3 (1985), beginning on page 1774. An electron beam in a plasma background has an upper limit on electron current beyond which the beam becomes unstable and is scattered diffusely. This maximum electron current for a well-defined beam depends both on the operating parameters and on the specific electron beam geometry.  
           [0016]    Using stability limits from electron source technology, the stability of an electron beam formed by secondary electrons can be determined, permitting the directing of the electron beam formed by a biased target either toward or away from deposition substrates. Conversely, it is possible to operate at conditions resulting in electron beam instability, so that the energetic electron beam is scattered broadly, thereby reducing both the total energy and current density of electrons arriving at a particular substrate.  
           [0017]    Depending on the substrate and the film being deposited upon it, the beam of energetic electrons could have beneficial or detrimental effects. Examples of the beneficial effects are: the energetic electrons can be used to enhance chemical reactions at the substrate surface, to increase nucleation of a deposited film, as well as to drive an electrically floating substrate surface to a negative potential and thereby enhance the bombardment of the substrate surface by low-energy background ions, thereby modifying film stress or producing increased film adhesion, density, or refractive index. Examples of detrimental effects are: the energetic electrons can damage sensitive film and substrate materials, either directly by collision damage or overheating, or indirectly by electrostatic charging. Knowledge of the presence and location of this energetic electron beam, which depends in turn on being able to determine its stability or instability, thus permits the use of this electron beam in deposition when the effects are beneficial and the avoidance of this electron beam when the effects are detrimental.  
         SUMMARY OF INVENTION  
         [0018]    In light of the foregoing, it is an overall general object of the invention to provide an improved apparatus for sputter deposition that confines the high pressure required for ion generation to an ion source and deposits thin films in a low background pressure, in which the energetic electrons generated from a sputter target biased negative relative to ground can be used in a controlled manner.  
           [0019]    Another object of the present invention is to provide an apparatus in which these energetic electrons are used to enhance chemical reactions at substrate surfaces.  
           [0020]    A further object of the present invention is to provide an apparatus in which these energetic electrons are used to bias floating-potential substrate surfaces to a negative potential, thereby enhancing bombardment of substrate surfaces by low-energy background ions.  
           [0021]    Yet another object of the present invention is to provide an apparatus in which the energetic electrons that are generated from a sputter target biased negative relative to ground are suppressed or directed away from the deposition substrates by a magnetic field located near the sputter target.  
           [0022]    In accordance with one specific embodiment of the present invention, the apparatus for sputter deposition within an evacuated volume comprises a compact gridless ion source to generate a beam of ions into which an ionizable gas is introduced and from which ions leave with directed energies at or near the sputtering threshold, a sputter target near that source and located within the beam of ions leaving that source, a grounded shield surrounding the sputter target and defining that portion of the target which is to be exposed to sputtering, a power supply to bias the target negative relative to ground so that ions are attracted to and sputter only the target. Particles sputtered from the target are deposited on a deposition substrate separate from both the ion source and the sputter target. The deposition substrate is located either within or outside of the beam of energetic electrons generated by the negatively biased sputter target, depending on whether the net effect of bombardment by energetic electrons is beneficial or detrimental to the deposition process. Ground is defined as the potential of the surrounding vacuum enclosure, which is typically at earth ground.  
       
    
    
     DESCRIPTION OF FIGURES  
       [0023]    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 which:  
         [0024]    [0024]FIG. 1 is a schematic cross-sectional view of an apparatus for sputter deposition constructed in accordance with a specific embodiment of the present invention;  
         [0025]    [0025]FIG. 2 is an enlarged schematic cross-sectional view of a portion of FIG. 1;  
         [0026]    [0026]FIG. 3 is a schematic cross-sectional view of an apparatus for sputter deposition constructed in accordance with an alternate embodiment of the present invention;  
         [0027]    [0027]FIG. 3 a  is a schematic cross-sectional view of an alternate assembly of target, target support, and grounded target enclosure constructed in accordance with an another alternate embodiment of the present invention;  
         [0028]    [0028]FIG. 4 is a schematic cross-sectional view of an apparatus for sputter deposition constructed in accordance with another alternate embodiment of the present invention; and  
         [0029]    [0029]FIG. 5 is a schematic cross-sectional view of a portion of FIG. 4 showing a magnetic field region located near the target.  
         [0030]    It may be noted that the aforesaid schematic cross-sectional views represent the surfaces in the plane of the section while avoiding the clutter which would result were there also a showing of the background edges and surfaces of the overall assemblies.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Referring to FIG. 1, there is shown a schematic cross-sectional view of an apparatus  12  for sputter deposition constructed in accordance with a specific embodiment of the present invention. As is customary with apparatus for sputter deposition, a grounded vacuum enclosure  14  surrounds an evacuated volume  16 , which is maintained at a rarefied pressure by pumping through a port  18 . In the apparatus for sputter deposition there is an ion source  20  which is shown in simplified block form because a variety of ion source designs could be used, many of which are commercially available. An ionizable gas  22  is introduced into ion source  20 . Energetic ions are generated from ionizable gas  22  by ion source  20  and flow outward in an ion efflux which has a central portion  24  and a fringe  26 , with all ions having energies of about 50 eV or less. Central portion  24  of the ion efflux is directed generally at sputtering target  28  and more particularly at exposed surface  29   a  of target  28 . Target  28  is in physical contact with target support  30  and partially enclosed by grounded target enclosure  32 . Target support  30  may be cooled, using a technology that is well-known to those skilled in the art. Energy is supplied to the ions that reach target  28 , by biasing target support  30  and target  28 , which is in electrical and physical contact with the target support, to a negative potential relative to ground. Ground is defined as the potential of the surrounding vacuum enclosure, which is typically at earth ground. Edge  34  of target enclosure  32  defines the perimeter of the exposed surface  29   a  of target  28 . To keep the ions from the ion efflux from penetrating behind edge  34  of target enclosure  32 , spacing  36  between target  28  and target enclosure  32  should be approximately equal to or less than the plasma sheath thickness at the target. Sputtered particles  38  leave exposed surface  29   a  in a range of directions, typically approximated with a cosine distribution. Some of sputtered particles  38  arrive at deposition substrates  40   a  and  40   b  and are deposited thereon to form thin films on the exposed surfaces  42   a  and  42   b  of said deposition substrates. A reactive gas  44  may be introduced through auxiliary port  46 . Sputtered particles  38 , which have energies in the range of several eV and which are sufficient to cause chemical reactions, can combine with the reactive gas at the exposed surfaces  42   a  and  42   b  of deposition substrates  40   a  and  40   b  to form deposited films that are compounds of the sputtered material and the reactive gas.  
         [0032]    Continuing to refer to FIG. 1, an energetic electron beam  48  is emitted from exposed surface  29   a  of target  28  and accelerated therefrom along approximately straight-line trajectories that depart from target  28  in a normal direction from the exposed surface  29   a . This electron beam can have important effects on the properties of the deposited film. To explain the presence of this electron beam, it is necessary to examine the exposed surface of the target and the region near thereto wherein the electrons originate and are accelerated.  
         [0033]    Referring now to FIG. 2, a portion of target support  30 , a portion of target  28  in contact therewith, and a portion of exposed surface  29   a  of target  28  are shown. Also shown is boundary  50  of plasma sheath  54  spaced a sheath thickness L from the exposed surface  29   a . In volume  52  outside of the plasma sheath there are both electrons and ions in approximately equal numbers (to satisfy the plasma requirement of quasineutrality). Electrons are excluded from sheath  54  between the target and boundary  50  of the plasma sheath by the negative field from the target  28 . The sheath thickness L can be determined from Child&#39;s law,  
           j   i =(4ε o /9)(2 e/m   i ) ½   V   {fraction (3/2)}   /L   2 ,  (1) 
         [0034]    where j i  is the ion current density, ε o  is the permittivity of free space, e/m i  is the charge-to-mass ratio of the ions, and V is the voltage across the sheath—all in SI (rationalized mks) units. For a typical value of L, one can assume argon ions at 1 mA/cm 2  with a negative target bias of 300 V, which give L=2.1 mm. Sheath thickness L is thus seen to be much larger than any microscopic texturing of the target surface, but also much smaller than the typical target size of at least several centimeters. The electrons that form electron beam  48  are emitted from exposed surface  29   a  of target  28  due to collisions of ions from ion efflux  24  thereon. Due to the relative dimensions of the exposed surface  29   a , the sheath thickness L, and any microscopic roughness on the target, these secondary electrons are accelerated by the electric field within the sheath on trajectories that follow straight lines oriented normal to the local target surface  29   a  where they are emitted. The mean-free-path of energetic electrons is normally long compared to the target-substrate distance; for example, several meters in an argon background pressure of one millitorr (0.13 Pascal) with an electron energy of 300 eV. Because of the long mean-free-path, the electron beam  48 , if stable, can maintain its direction without significant scattering over the target-substrate distance. The energy gained by the electrons being accelerated through sheath  54  will equal, in eV, the negative bias voltage relative to ground, in volts. The current of these secondary electrons is significant, typically corresponding to 10 percent or more of the ion current arriving at the target. This current may be measured with probes or estimated from secondary electron coefficients.  
         [0035]    There is a stability limit on the current that can be carried by an electron beam in a plasma background. The ions can neutralize the space charge of the electrons, and thereby offset most steady-state space charge effects in such an electron beam. When a disturbance occurs, though, the much more massive ions cannot move rapidly enough to offset the growth of any electron-beam instability. The stability limit for an electron beam is thus approximately the same with or without the presence of charge-neutralizing ions.  
         [0036]    The maximum electron current for a stable, well-defined beam varies as the electron energy to the 3/2 power. For a cylindrical beam of radius r flowing coaxially through a grounded enclosure of radius R, the maximum stable electron current, I e,max , is given by Kaufman, et al., in Chapter XVI of  Operation of Broad - Beam Sources , Commonwealth Scientific Corporation, Alexandria, Va., 1984 as  
           I   e,max =1.963πε o (2 e/m   e ) ½   V   {fraction (3/2)} /[1+2 ln ( R/r )]  (2) 
         [0037]    where ε o  is the permittivity of free space, e is the electronic charge, m e  is the electron mass, V is the voltage difference through which the electrons have been accelerated (and corresponding to the electron energy in eV). In the aforesaid article by Kaufman, et al., in the  J. of Vacuum Science and Technology,  the presence of a background plasma, which is typical of ion-beam and plasma environments, results in an effective value of R/r≈3, even though the actual wall distance corresponded to a much larger ratio. This value of R/r can be used in Equation (2) to obtain an approximate expression for maximum electron current.  
           I   e,max ≈1,963πε o (2 e/m   e ) ½   V   {fraction (3/2)} /3.2  (3) 
         [0038]    Equations (2) and (3) are recommended for calculations of maximum stable electron currents for circular beams formed by circular targets. Equations are given for other beam, and target, shapes by Kaufman, et al., in the aforesaid Chapter XVI in the book,  Operation of Broad - Beam Sources.    
         [0039]    Using the limits of Equations (2) or (3), or the equivalent equations for other beam configurations, the stability of an electron beam formed by secondary electrons can be determined, permitting the directing of the electron beam toward or away from deposition substrates. Conversely, it is possible to operate at conditions resulting in electron beam instability, so that the energetic electron beam is scattered broadly, thereby reducing the energy and current density arriving at a particular substrate.  
         [0040]    In general, an increase in bias voltage will increase electron energy and therefore shift operation toward stability, while an increase in ion current directed toward the target will increase the generation of secondary electrons and therefore shift operation toward instability. As described in the aforesaid article by Kaufman, et al., in the  J. of Vacuum Science and Technology , the region of intermediate stability is moderate, with operation tending to be either stable or unstable. The onset of instability is measured with a sudden decrease in electron current arriving at a probe, indicating that the beam is being scattered by instability.  
         [0041]    In summary, depending on operating parameters understood by those skilled in the plasma-physics art, a stable and well-defined beam of energetic secondary electrons  48  may be directed out from exposed surface  29   a  of target  28 , in addition to the much more diffuse distribution of sputtered particles  38  used to deposit films on the substrate surfaces  42   a  and  42   b . In the embodiment of this invention shown in FIGS. 1 and 2, the directionality of such an electron beam  48  can be used to assure arrival of the energetic electrons at the deposited film (the location of substrate  40   a ) or the avoidance of the energetic electrons (the location of substrate  40   b ). Alternatively, the operating conditions may be selected so that an unstable beam of electrons is generated, resulting in a diffuse electron beam with much lower density at any one location.  
         [0042]    The presence or absence of this electron beam can produce important effects on the properties of the deposited film. For an undesirable effect, the energetic electrons can damage sensitive film and substrate materials, either directly by collisions or indirectly by charging. For a desirable effect, the energetic electrons can be used to enhance chemical reactions at the substrate surface, as well as to drive an electrically floating substrate surface to a negative potential and thereby enhance the bombardment of the substrate surface by low-energy background ions. Knowledge of the presence and location of electron beam  48  thus permits the use of its energetic electrons in deposition when the effects are beneficial and the avoidance or substantial reduction of these energetic electrons when the effects are detrimental.  
       ALTERNATE EMBODIMENTS  
       [0043]    The embodiment of this invention shown in FIGS. 1 and 2 shows sputter target  28  with a flat exposed surface  29   a , which is the shape most often used in sputtering applications. In FIG. 3, there is shown a similar deposition apparatus wherein the only difference is that exposed surface  29   b  has a concave spherical shape in which the radius of curvature is large compared to the sheath thickness. Because the secondary electrons follow straight-line trajectories oriented normal to the local target surface  29   b  where the electrons are emitted, the concave shape of exposed surface  29   b  results in a focused shape for electron beam  48  with increased electron current density at substrate surface  42   a.    
         [0044]    In FIG. 3 a  an alternate assembly of target  28 , target support  30 , and grounded target enclosure  32  is shown, which may be used in an apparatus similar to that shown in FIG. 3. The only difference from the similar assembly shown in FIG. 3 is that the exposed surface  29   c  of target  28  is convex, resulting in an expanding or defocused shape for the electron beam that is generated.  
         [0045]    It should be evident to those skilled in the art that other shapes may be used for the exposed surface of the target: multiple flat planes, convex spherical surfaces, or multiple combinations of shapes, with the objective of enhancing or reducing the arrival of energetic electrons at different locations on a substrate or with different substrates.  
         [0046]    Another alternate embodiment of the present invention is shown in FIG. 4, where the grounded target enclosure  56  is also a permeable path for a magnetic circuit and is magnetically energized by electromagnet winding  58 .  
         [0047]    To avoid the excessive clutter of showing both sputtered particles  38  and the magnetic field lines in the same drawing, the magnetic field lines are omitted in FIG. 4, while the sputtered particles are omitted in FIG. 5, which otherwise duplicates the assembly of target  28 , target support  30 , target enclosure  56 , and electromagnet winding  58  from FIG. 4. For the magnetic field shown in FIG. 5 to contain energetic secondary electrons emitted by the exposed surface  29   a  of target  28 , the thickness T must equal or exceed the cyclotron radius for these electrons at their maximum energy. The cyclotron radius, r e , for an electron is  
         
       r 
       e 
       =m 
       e 
       v 
       e 
       /eB, 
     
         [0048]    where m e  and v e  are the electron mass and velocity, e is the electronic charge, and B is the magnetic field strength, with all again in SI units. For a 300 eV electron, corresponding to a negative target bias of 300 V, the cyclotron radius is about 2 cm at a mean magnetic field strength of 30 Gauss (30×10 −4  Tesla). In practice, the thickness of the magnetic field should be 50-100 percent larger than the cyclotron radius, resulting in a 3-4 cm thickness for a 30 Gauss mean field strength.  
         [0049]    Although a magnetically permeable path energized with an electromagnet winding is indicated in FIGS. 4 and 5, other magnetic configurations should be readily evident. As one example, a permanent magnet could be used for magnetic energization. As another example, electromagnet windings without any magnetically permeable path could also be used. As a last example, multiple electromagnet windings could be used simultaneously with out-of-phase alternating currents to generate a rotating magnetic field.  
         [0050]    Moderate magnetic field strengths over thicknesses of several centimeters can thus be used to contain the energetic secondary electrons until they collide with grounded surfaces or dissipate their energy in collisions with background particles. With such containment, a variety of substrate locations,  40   a  or  40   b  for example, may be used without exposure of the substrate or the deposited film thereon to energetic electrons.  
         [0051]    While particular embodiments of the present invention have been shown and described, and various alternatives have been suggested, it will be obvious to those of ordinary skill 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.