Abstract:
A method is described that involves applying a first voltage to a first mesh located above a wafer. The wafer has a charge sensitive material exposed thereon. The method also involves applying a second voltage to a second mesh located above the wafer. The method also involves depositing a layer of material by ion beam deposition onto the charge sensitive material while the voltages are applied to their respective meshes.

Description:
FIELD OF INVENTION  
       [0001]     The field of invention relates generally to the semiconductor arts, and, more specifically, deposition on charge sensitive materials with ion beam deposition.  
       BACKGROUND  
       [0002]     Ion beam deposition is a deposition technique that directs a high energy ion beam toward a target made of material to be sputter deposited onto a wafer (e.g., a semiconductor wafer having features pattered thereon that help form a plurality of electronic semiconductor chips). A simplistic depiction of an ion beam deposition system is presented in  FIG. 1 .  
         [0003]     According to the depiction of  FIG. 1 , a deposition chamber  100  is coupled to an ion source component  101 . The deposition chamber  100  also includes a target  102  and a wafer  103 . A plasma is formed within the ion source component  101 . The plasma&#39;s constituent atoms (e.g., Argon (Ar) atoms or Xenon (Xe) atoms) and electrons collide with one another causing at least some of these atoms to lose one or more electrons such that they become positively charged ions. The positively charged ions are extracted from the ion source component  101 , formed into a beam  104  and directed to a target  102 . The target  102  is made of material which is to be deposited onto the wafer  103 . When the ion beam&#39;s ions collide with the target  102 , the target&#39;s constituent atoms are knocked off the target  102 . These atoms then deposit on the wafer  103  such that a film of the target material is formed on the wafer  103 .  
         [0004]     Unfortunately, if ion beam deposition is used to deposit target material onto a “charge sensitive” material (such as a ferroelectric polymer exposed on the surface of the wafer), the charge sensitive material is observed to be “degraded” after the ion beam deposition process is performed. Ion beam deposition has therefore not gained acceptance as a legitimate deposition technique for deposition onto charge sensitive materials. Alternative deposition techniques, such as thermal evaporation, are therefore used to deposit onto charge-sensitive materials even though ion bean deposition is capable of providing higher quality deposited films (e.g., in terms of defects in film microstructure) than these alternative deposition techniques.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:  
         [0006]      FIG. 1  (prior art) shows an ion beam deposition system;  
         [0007]      FIG. 2  shows a wafer and a region just above the wafer;  
         [0008]      FIGS. 3   a  and  3   b  show a sub-chamber assembly for use within an ion beam deposition system;  
         [0009]      FIG. 4  shows detected charge in a region just above a wafer as a function of voltage applied to the upper and lower meshes depicted on  FIGS. 3   a  and  3   b;    
         [0010]      FIG. 5  shows an ion beam deposition system that includes the sub-chamber assembly depicted in  FIGS. 3   a  and  3   b.    
     
    
     DETAILED DESCRIPTION  
       [0011]     In order to address the issues associated with ion beam deposition onto “charge sensitive” materials, the dynamics of ion beam deposition and its effects on a charge sensitive material should be better understood.  FIG. 2  shows a wafer  203  that may be assumed to have an exposed layer of charge sensitive material on its surface. Here, a charge sensitive material can be assumed to be a material that structurally decomposes in the presence of electrically charged particles (e.g., positively charged ions or negatively charged electrons). For instance, a layer or trench of ferroelectric polymer can “unravel” in the presence of electrons and/or positively charged ions. Examples of charge sensitive materials other than ferroelectric polymers include various plastics or other materials containing organic bonds that are sensitive to plasma damage (e.g., including low-K dielectrics now that incorporate organic material into SiO2 to lower the dielectric constant).  
         [0012]     Note that plasma exposure is routinely used to promote adhesion on plastics such as polyimide, polypropylene, etc.—adhesion improves because the plastic is ripped apart at the surface and therefore better able to form new chemical bonds with a material. With respect to the deposition of insulating materials (tantalum oxide, silicon oxide, calcium fluoride, etc.), if the growing film is bombarded with charged particles it builds up a charge and therefore a potential. Once the dielectric strength of the film is exceeded a breakdown occurs, which destroys an area of the film. Usually in these sorts of applications particles of the opposite polarity are purposely introduced to the system to cancel the charge so that no potential develops.  
         [0013]     The most basic dynamic process of ion beam deposition involves the deposition of “charge neutral” target atoms onto the surface of the wafer  203 . That is, impingement of the ion beam with the target creates a number of “intact” atoms that have neither lost electron(s) nor gained electron(s) and are therefore electrically neutral as they deposit onto the surface of the wafer  203 . Deposition of these charge-neutral atoms is encouraged in the case of deposition onto charge sensitive materials because charge-neutral atoms are not believed to promote any electrical reaction with the charge sensitive material, and, as a consequence, no structural decomposition of the charge sensitive material should result.  
         [0014]     The problematic correlation between the structural quality of the charge sensitive material being deposited upon and the ion beam deposition process is believed to be related to the abundance of charged particles (most notably, positively charged ions and negatively charged electrons) that exist just above the surface of the wafer  203  (e.g., in region  209 ) during deposition. Essentially, the ion beam deposition process naturally lends itself to the creation of not only charge-neutral target atoms within the deposition chamber as described just above, but also, positively charged ions and negatively charged electrons. These charged particles are capable of being present just above the wafer during the deposition process such that a charge sensitive material that is being deposited upon electrically reacts with these charged particles thereby causing its structural decomposition.  
         [0015]     Referring back to  FIG. 1 , some ion beam deposition dynamics believed to cause the creation of these undesirable charged particles include: 1) emission of electrons from the plasma  105  into the deposition chamber  102  (e.g., through the ion source component&#39;s “porous” interface  106  with the deposition chamber  100 ); 2) emission of secondary electrons from the chamber&#39;s background gas atoms and/or target atoms and/or ion beam ions (e.g., resulting from high energy atomic collisions); 3) charge transfer the ion beam&#39;s  104  ions and the deposition chamber&#39;s background gas atoms (specifically, if a background gas atom passes near the ion beam  104 , an electron from the background gas atom may transfer to a positively charged ion in the ion beam leaving the background gas atom as a low energy positively charged ion); 4) positive ions of target material that are knocked off the target by the ion beam  104  (note that these ions may range from high to low energy depending on the specific collision dynamics of each); and, 5) electrons generated from item 4) just above.  
         [0016]     Of the various ion beam deposition dynamics described above, note that categories 1), 2) and 5) may be deemed to create “low energy” charged particles because they create electrons. Here, electrons may be regarded as possessing low kinetic energy. In the case of 1) the electron energy is low because the accelerating field the electron sees is the sheath of the ion source plasma (about 40 V or so), the ions on the other hand are purposely accelerated through 100s of volts (typically ˜1000-1500 V in ion beam deposition). 2) and 5) are essentially the same thing (secondaries) which are low-energy by nature (usually &lt;50 eV). Also, note that category 3) above may be deemed to create a low energy charged particle because background gas atoms are not accelerated like the ions in the ion beam  104 .  
         [0017]     Thus, background gas atoms tend to drift in the deposition chamber  100  at much lower speeds than the ions in the ion beam  104  and therefore may also be deemed to posses low kinetic energy. Background gas atoms are typically inert atoms such as Ar or Xe. They may be separately added to the deposition chamber  100  and/or may diffuse into the chamber  100  from the plasma  105 . Also, as indicated in the parenthetical comment following category 4), some percentage of the ionized target material atoms that exist in the chamber  100  may be low energy particles also.  
         [0018]     Thus, of the ion beam deposition process dynamics categories listed above, particles created according to categories 1) through 3) and 5) and some percentage of category 4) correspond to the creation of low energy particles within the chamber  100 . It therefore follows that a “not insignificant” percentage of the charged particles that reside within the deposition chamber  101 , including those residing in region  209  of  FIG. 2 , are low energy particles rather than high energy particles.  
         [0019]     Because a “not insignificant” percentage of the charged particles within region  209  are believed to be low energy particles, there exists some opportunity that they can be removed from the region  209  just above the surface of the wafer  203 . If so, the result will be a less electrically reactive cloud just above the surface of the wafer  203  that, by its nature, will induce less electrical reaction with the charge sensitive material on the wafer  203  than otherwise would occur if no attempt to remove the low energy charged particles existed (as in prior art approaches). Because less electrical reaction is induced with the charge sensitive material, the removal of the low energy charged particles should result in the charge sensitive material suffering less structural degradation from the ion deposition process.  
         [0020]      FIGS. 3   a  and  3   b  depict an apparatus for preventing low energy charged particles from migrating to the region  309  that exists just above the surface of the wafer  303 .  FIG. 3   a  shows a top view (looking down onto the wafer from above the wafer).  FIG. 3   b  shows a cross sectional view. According to the approach depicted in  FIGS. 3   a  and  3   b , a base plate  310  and a stacked structure that includes mesh frames  314 ,  315  essentially forms a “sub-chamber”  311  that contains the region  309  just above the wafer  303  where the removal of charged particles is desired. Ideally, only particles that enter the sub chamber  311  through chamber opening  310  are capable of being deposited onto the wafer  303 .  
         [0021]     A first, upper mesh  312  is fixed approximately at the opening to the sub-chamber  311  such that a particle can enter the sub-chamber  311  (and deposit on the wafer  303 ) only if it flows through the upper mesh  312 . An electrostatic potential (in one implementation, a DC voltage) is applied to the upper mesh  312 . The electrostatic potential induces electric field lines that emanate from (or terminate on) the mesh  312  (depending on the polarity of the potential).  
         [0022]     The electric field lines “affect” the motion of low energy charged particles that are moving toward the sub chamber  311  such that they are prevented from entering the sub-chamber  311 . However, the electric field lines do not affect the motion of charge neutral particles that are moving toward the sub-chamber  311 . Recalling the above discussion that it is desirable that charge neutral and not charged particles reside in the region  309  just above the wafer  303 , note that the structure of  FIGS. 3   a  and  3   b  has the desired affect of thwarting the flow of at least low energy charged particles toward region  309  but not thwarting the flow of charge neutral target atoms toward region  309 .  
         [0023]     A mesh structure is essentially any structure having an arrangement of openings (typically in a repetitive pattern). The specific embodiment of  FIGS. 3   a  and  3   b  shows two mesh structures  312 ,  313 , where, each mesh structure  312 ,  313  is made of a first plurality of electrically conductive wires that span across an opening to the sub chamber and a second plurality of electrically conductive wires that span across the same sub-chamber opening, where, the first plurality of wires and the second plurality of wires run perpendicular to one another (e.g., so that each mesh corresponds to a grid of openings formed with its respective wires). The meshes are each made of electrically conductive material such as a metal or metal alloy (e.g., stainless steel).  
         [0024]     A first of the mesh structures is given a negative potential and a second of the mesh structures is given a positive potential (relative to the baseplate  310  and wafer  303  which are electrically grounded (i.e., 0 volts)). In a preferred implementation the higher mesh  312  is given a negative potential and the lower mesh  313  is given a positive potential.  
         [0025]     In this case, the negative potential mesh  312  sinks (i.e., terminates) electric field lines which has the corresponding effect of repelling negatively charged electrons. The positive potential mesh  313  sources (i.e., emanates) electric field lines which has the corresponding effect of repelling positively charged particles (such as positively charged ions). In a further feature of the preferred implementation, the potential of the lower, positively charged mesh  313  has a higher absolute value than the higher, negatively charged mesh  312  (e.g., the positively charged mesh  313  has a positive potential in the hundreds of volts but the negatively charged mesh  312  has a negative potential in only the tens of volts). According to this design, electrons should be repelled from reaching region  309  before reaching the higher mesh  312  and positively charged particles should be repelled from reaching region  309  before reaching the lower mesh  313 .  
         [0026]     In an implementation where the sub-chamber  311  opening is circular, the meshes  312 ,  313  are secured in a circular frame (e.g., “rings”  314  and  315 ). Of course, other frame shapes are possible such as square or rectangular. The lower mesh  313  is electrically isolated from the baseplate  310  and the upper mesh  312  with beads or rings  319  made of electrically insulating material (e.g., a ceramic, quartz (glass), sapphire (ion source grid assemblies are typically insulated/spaced by sapphire balls), or a polymer such as polyimide.)  
         [0027]      FIGS. 3   a  and  3   b  also show a third, lowest ring  316  that approximately encircles the region  309  just above the wafer  303  where only charge neutral particles are desired. In an implementation the lowest ring  316  does not support a mesh nor is set to a fixed potential by the deposition equipment. Rather, the lowest ring  316  is used as a device that measures the charge in region  309 . Here, if a net positive charge is present in region  309 , a voltmeter  317  that is coupled across ring  316  to ground should measure a positive voltage and an ammeter  318  should detect a current flow into ground. Contrariwise, if a net negative charge is present in region  309 , the voltmeter  317  should measure a negative voltage and the ammeter  318  should detect a current flow from ground.  
         [0028]     Ideally, no voltage is detected by the voltmeter  317  and no current is detected by the ammeter  318  signifying that the region  309  just above is free of charge.  FIG. 4  plots the voltage detected by the voltmeter  317  across a range of applied voltage magnitudes and polarities to both the upper (V upper ) and lower (V lower ) meshes for the following, exemplary, ion beam deposition conditions. For the plot shown, the conditions were: beam voltage =1200 V (thus the beam ions have ˜1200 eV of energy); suppressor voltage=150 V (this voltage keeps electrons from flowing back into the source and also controls the spread (focus) of the ion beam); rf power=600 W (ion source power); Ar gas flow=10 sccm through the ion source; target material=A1 (different materials will emit more or fewer secondary electrons and reflect/backscatter the beam at different energies and probabilities).  
         [0029]      FIG. 4  shows a large operating region  401  where negligible charge is detected just above the wafer when a positive potential is applied to the upper mesh and a negative potential is applied to the lower mesh. Region  401  “extends” far to the right off the depicted scale in  FIG. 4  (e.g., V upper &gt;100 volts and 0&gt;V lower &gt;−50 volts). According to one perspective, a large positive potential is applied to the lower mesh (e.g., in hundreds of volts such as 400 volts) and a smaller negative potential is applied to the upper mesh (e.g., in hundreds of volts such as −40 volts).  
         [0030]     Referring back to  FIG. 3   b , the arrangement of potential in this fashion is believed to support the objective of keeping the region  309  just above the wafer  303  free of charged particles for the following reasons. Firstly, electrons, being essentially massless, are more easily kept from the sub chamber  311  with a “weaker” electric field such as an electric field produced from application of a potential to a mesh that is only in the tens of volts. As such, the application of a negative potential in the tens of volts to the upper mesh  312  forms a weak electric field above the upper mesh  312  that strongly repels electrons from the sub-chamber  311  and only weakly accelerates heavier positively charged ions toward the sub-chamber  311 . Thus, it is believed that, according to this setup, for the most part, positive ions pass through the upper mesh  312  but electrons do not.  
         [0031]     The placement of a positive voltage in the hundreds of volts to the lower mesh  312  in combination with keeping the distance between the upper and lower meshes relatively short (e.g., less than 1.0 centimeters such as 0.5 cm) provides for a much stronger electric field between the two meshes  312 ,  313  that repels positive ions from the region  309  just above the wafer. For instance, in an application where V lower =+400 volts, V upper =−40 volts and the distance between the two meshes is 0.5 cm, the electric field strength between the two meshes  312 ,  313  is on the order of 440V/0.5 cm=880 V/cm. The much stronger field between the two meshes  312 ,  313  (as compared to the field strength above the upper mesh  312 ) is believed to be better able to prevent the penetration of heavier positive ions to the region just above the wafer  309 . Additionally, in this application, the lower mesh  313  is  30 cm above the wafer surface.  
         [0032]     Referring back to  FIG. 4 , note that negligible charge is detected in regions other than those corresponding to V lower &lt;0 and V upper &gt;0. As such, even though it appears that V lower &lt;0 and V upper &gt;0 yield a wide operating range where little or no charge is observed above the wafer, nevertheless, it appears that other operating regions may be used outside the V lower &lt;0 and V upper &gt;0 boundary. Note that the third, lower ring  316  need not be installed in an application where the physics within the chamber are well enough understood that detection of the charge within the region just above the wafer is not necessary.  
         [0033]     Note that the use of the mesh apparatus should be better than neutralization techniques because charged particles are prevented from reaching the film in the first place (by contrast, with respect to neutralization techniques, film degradation occurs before charge can be neutralized). In applications where trapped charges in the growing film might be important (e.g., electrical insulators) it may be best to avoid introducing charges as much as possible rather than trying to neutralize them, the mesh apparatus is a potentially better solution.  
         [0034]      FIG. 5  shows an ion beam deposition system that includes a sub chamber designed according to the principles outlined above. According to the depiction of  FIG. 5 , a deposition chamber  500  is coupled to an ion source component  501 . The deposition chamber  500  also includes a target  502  and a wafer  503 . A plasma is formed within the ion source component  501 . The plasma&#39;s constituent atoms (e.g., Argon (Ar) atoms or Xenon (Xe) atoms) and electrons collide within one another causing at least some of these atoms to lose one or more electrons such that they become positively charged ions. The positively charged ions are extracted from the ion source component  501 , formed into a beam  504  and directed to a target  502 . The target  502  is made of material which is to be deposited onto the wafer  503 .  
         [0035]     The wafer  503  has exposed on its surface a charge sensitive material. When the ion beam&#39;s ions collide with the target  502 , the target&#39;s constituent atoms are knocked off the target  502 . Some of these atoms are electrically neutral while others are positively ionized. Electrically neutral atoms flow largely uninhibited through the mesh structure into the sub chamber assembly  511  and deposit on the wafer  503  such that a film of the target material is formed on the wafer  503 . The positively ionized atoms (as well as electrons) are repelled from the sub chamber assembly because voltages are applied to the meshes (with wiring) while the ion source is energized to produce an ion beam during sputter deposition.  
         [0036]     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.