Patent Publication Number: US-2015079796-A1

Title: Charged-Particle-Beam Processing Using a Cluster Source

Description:
The present application is a divisional application of U.S. patent application Ser. No. 11/590,570 filed Oct. 31, 2006, which is incorporated by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to charged-particle-beam processing. 
     BACKGROUND OF THE INVENTION 
     Charged-particle beams, such as ion beams and electron beams, are used for processing work pieces in nanotechnology because charged-particle beams can form very small spots. For example, focused ion beam systems are able to image, mill, deposit, and analyze with sub-micron precision. Focused ion beam systems are commercially available, for example, from FEI Company, Hillsboro, Oreg., the assignee of the present application. The ions can be used to sputter, that is, physically eject, material from a work piece to produce features, such as trenches, in the work piece. An ion beam can also be used to activate an etchant gas to enhance sputtering, or to decompose a precursor gas to deposit material near the beam impact point. An ion beam can also be used to form an image of the work piece, by collecting secondary particles ejected by the impact of the ion beam. The number of secondary particles ejected from each point on the surface is used to determine the brightness of the image at a corresponding point on the image. Focused ion beams are often used in the semiconductor industry. In one application, for example, a focused ion beam is used to cut a small trench into an integrated circuit to expose a cross section of a vertical structure for observation or measurement using an ion beam or an electron beam. 
     Electron beams can also be used to process a work piece. Electron beam processing is described, for example in U.S. Pat. No. 6,753,538 to Mucil et al. for “Electron Beam Processing.” Electron beams are more commonly used for forming images in a process called electron microscopy. Electron microscopy provides significantly higher resolution and greater depth of focus than optical microscopy. In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding point on the surface. 
     In a transmission electron microscope (TEM), a broad electron beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically thinned to a thickness of less than 100 nm. One method of preparing samples includes using a focused ion beam to cut a thin sample from a work piece, and then using the ion beam to thin the sample. 
     In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface. 
     When a charged-particle beam impacts a surface, there is the potential for damage or alteration of the surface. Focused ion beam systems typically use gallium ions from liquid metal gallium ion source. Gallium ions are relatively heavy, and a gallium ion accelerated through a typical 30,000 volts will inevitably alter the work piece surface. Plasma ion systems, such as the one described in WO20050081940 of Keller et al. for a “Magnetically Enhanced, Inductively Coupled, Plasma Source for a Focused Ion Beam System,” which is hereby incorporated by reference, can use lighter ions, which cause less damage, but the ions will still typically alter the work piece surface. Electrons, while much lighter than ions, can also alter a work surface. When a user desires to measure a work piece with an accuracy of nanometers, changes in the work piece caused by the impact of charged particles can be significant, especially in softer materials, such as photoresist and low-k and ultra-low-k dielectric materials, such as polyphenylene materials. 
     Currently, technicians quantify the dimensional change that is caused by the charged-particle beam deposition of the protective layer, and then apply a correction factor to subsequent measurements to obtain an estimate of the true dimension. Such estimates are not always accurate because of the variation in the alteration by the charged-particle beam. 
     When a user desires to use an ion beam to extract a sample viewing with a TEM, as described for example, in U.S. Pat. No. 5,270,552 to Ohnishi, et al. “Method for Separating Specimen and Method for Analyzing the Specimen Separated by the Specimen Separating Method,” the user typically scans the focused ion beam in an imaging mode to locate the region of interest. The scanning causes damage to the surface. When the region of interest is located and the beam begins to mill a trench, there is additional damage to the work piece because the edges of the beam are not perfectly sharp. That is, the beam is typically Gaussian shaped, and the ions in the tail of the Gaussian distribution will damage the work piece at the edge of the trench. Damage has been found not just on fragile materials, but also on relatively hard materials. 
     To protect the work piece surface, it is common to apply a protective layer before charged-particle-beam processing. One method of applying a protective layer is charged-particle-beam deposition, that is, using a charged-particle beam to provide energy to decompose a gas to deposit a material on the surface. The protective layer shields the area around the cut and preserves the characteristics of the features that are to be imaged and measured. Commonly used deposition gasses include precursor compounds that decompose to deposit tungsten, platinum, gold, and carbon. For example, tungsten hexacarbonyl can be used to deposit tungsten, methylcyclopentadienyl trimethyl platinum can be used to deposit platinum, and styrene can be used to deposit carbon. Precursor gases to deposit many different materials are known in the art. The preferred material to be deposited as a protective layer depends on the application, including the composition of the underlying target surface, and the interaction between the protective layer material and the target surface. 
     Although charged-particle-beam-assisted deposition can locally apply a layer at the precise location where the layer is needed, applying a protective layer using charged-particle beam deposition has several disadvantages. Charged-particle-beam-assisted deposition is relatively slow and, in some processes, up to sixty percent of the total processing time is consumed in deposition of the protective layer. When an ion beam is initially scanned onto the target surface to deposit material, the beam sputters material away from the surface for an initial period of time until a sufficient amount of deposition material accumulates to shield the surface from the ion beam. Even though that period of time may be small, it can be large enough to allow a significant amount of material to be removed, which causes the accuracy of the cross-sectional analysis to be compromised. 
     Electron and laser beams can be used to generate secondary electrons to decompose a precursor gas to deposit a protective layer, but those beams may also damage the underlying surface—especially when they are at sufficient energy and/or current density levels for achieving favorable processing time. It is generally not practical to use such beams because deposition will be too slow if the beams are sufficiently “weak” to avoid harm to the underlying surface. Physical vapor deposition (“PVD”) sputter methods could be used to deposit protective layers in some applications, but they normally cannot be utilized for production control applications in wafer fabrication facilities because such methods cannot be used to locally apply a deposition layer onto a targeted part of the wafer surface. U.S. Pat. App. No. 60/773,396, which is assigned to the assignees of the present invention, describes a method of PVD that can provide a localized layer. A charged-particle beam is used to sputter material from a target onto the surface. The charged-particle beam is not directed to the surface itself and damage is avoided. This method, however, is time consuming. 
     Another method of applying a protective coating is described is U.S. Pat. No. 6,926,935 to Arjavec et al. for “Proximity Deposition.” In this method, the charged-particle beam is not directed at the area of interest, but to a region outside the area of interest. Secondary electrons decompose the precursor gas over the area of interest to provide a protective layer. As the protective layer is being created around the edge of the region of interest, the charged-particle beam can be moved inward. This method is also time consuming. 
     Colloidal silver applied with a brush has long been used to produce a conductive protective layer in scanning electron microscopy. The silver particles used are relatively large. Using a brush to apply the layer can damage the substrate and cannot provide a localized layer. 
     Another method of applying a protective coating is to use a felt tip pen, such as a Sharpie brand pen from the Sanford division of Rubbermaid corporation. The ink from a Sharpie pen is suitable for use in a vacuum chamber, because it dries thoroughly, and there is little outgassing in the vacuum chamber. Touching the pen to the region of interest would alter the surface, so the ink is applied near the region of interest, and the ink then wicks onto the region of interest. Compounds in the ink protect some surfaces. The area affected by the felt tip is very large compared to the sub-micron features of modern integrated circuits, and the positioning accuracy of the ink is insufficient. 
     Applying a protective layer of fullerene molecules for computer disk drive components is described, for example, in U.S. Pat. No. 6,743,481 to Hoehn et al. for “Process for Production of Ultrathin Protective Overcoats” and U.S. Pat. Pub. No. 20020031615 of Dykes et al. for “Process for Production of Ultrathin Protective Overcoats.” Fullerenes are ejected from a source by the impact of an ion beam or an electron beam, and some of the fullerenes are ejected in the direction of the target and coat it. 
     The industry needs a method of rapidly and accurately applying a localized protective layer without damaging a work piece surface. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to deposit onto or etch a work piece surface using a cluster beam to reduce surface damage. 
     This invention includes using a cluster ion source to etch a surface or to deposit material. Using clusters typically reduces substrate damage compared to using individual ions. The invention is particularly useful for depositing a protective layer for charged-particle beam processing. In some embodiments, a precursor gas is decomposed by a cluster beam to deposit a protective layer. In other embodiments, the components of the cluster deposit onto the work piece surface to provide a protective layer for charged particle beam processing. Embodiments of the invention also include systems that allow for deposition of a protective layer using a cluster source and for additional charged particle beam processing. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more through understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a preferred embodiment of the invention system including a plasma source that is capable of generating charged clusters of atoms and directing the clusters toward a sample. 
         FIG. 2  shows a preferred method in accordance with the invention. 
         FIG. 3  shows a preferred plasma cluster source. 
         FIG. 4  shows an alternative cluster source. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The following is a detailed description of example embodiments of the invention depicted in the accompanying drawings. The example embodiments are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. The detailed descriptions below are designed to make such embodiments obvious to a person of ordinary skill in the art. 
     In some embodiments of the invention, a cluster beam provides energy to decompose a precursor gas to deposit a layer, while minimizing damage to the work piece surface. The cluster beam can also be used to etch the work piece surface, optionally with an etch-enhancing gas. In other embodiments, a cluster beam directly deposits a protective layer for charged particle beam processing, preferably the protective layer deposition and the charged particle beam processing occurring in the same vacuum chamber. 
       FIG. 1  shows a simplified illustration of an embodiment of the invention that comprises a charged particle beam system  100  including a cluster ion source  102 , a gas injection system  104 , and preferably an electron beam column  106 . A gas injection system is described, for example, in U.S. Pat. No. 5,851,413 to Casella et al. for “Gas Delivery Systems for Particle Beam Processing,” assigned to the assignee of the present invention. Another gas delivery system is described in U.S. Pat. No. 5,435,850 to Rasmussen for a “Gas Injection System,” also assigned to the assignee of the present invention. Cluster source  102  provides a beam  108  of clusters of atoms or molecules directed toward a region of interest  110  on a work piece  112 , which is positioned within a vacuum chamber  114  on a stage  116  (not shown), preferably capable of moving in at least two directions. 
       FIG. 2  shows a preferred process in accordance with the invention. In step  210 , a work piece is inserted into vacuum chamber  114 . In step  212 , a portion of the work piece  112  is imaged using the electron beam column  106  to locate markings on the work piece surface to register the coordinate system of the stage  116  with the coordinate system of the work piece  112  so that the stage  116  can be moved to position region of interest  110  within an area toward to which the cluster beam can be directed. In step  214 , a jet of a precursor gas is directed toward the work piece  112  in the vicinity of the region of interest  110 . Many deposition precursor gases are known, including tetramethylorthosilane (TMOS), tetraethylorthosilane (TEOS), tetrabutoxysilane Si(OC 4  H 9 ), dimethyl acetylacetonate gold, organometallic compounds, such as tungsten hexacarbonyl (W(CO) 6 ), and methylcyclopentadienyl trimethyl platinum (C 9 H 16 Pt). 
     In step  216 , the beam of clusters is directed to the work piece surface to provide energy to decompose the precursor gas and deposit a protective layer. Clusters of different types of atoms or molecules can be used with the invention. The term “cluster” includes groups of multiple molecules or atoms. For example carbon in the form of fullerenes (e.g., C 60 , C 70 , C 80  , or C 84 ), gold (Au 3 ), bismuth (Bi 3 ), and xenon (Xe 40 ) and other clusters are useful for deposition or etching. The term “cluster” includes not only groups of multiple fullerenes, but also single fullerene molecules which are considered as clusters of carbon atoms. Because each cluster has only one or a few unbalanced electric charges, the charge-to-mass ratio of the clusters can be significantly less than the charge-to-mass ratio of individual atoms or molecules used in charged-particle beams. Although the energy of the entire cluster can be relatively large, for example, several hundred to several thousand electron volts, the energy per component is low. 
     A typical protective layer is preferably between 0.05 μm and 1 μm thick, more preferably between 0.1 μm and 0.8 μm thick, and most preferably about 0.2 μm thick. A preferred protective layer is sufficiently conductive to dissipate any electrical charge produced by the impact of the charged-particle beam onto the work piece. A preferred protective layer is “vacuum friendly,” that is, it does not “outgas” or continue to evaporate in a vacuum chamber to interfere with the charged-particle beam or contaminate the work piece. A preferred protective layer stabilizes the structures on the work piece. The preferred protective layer does not interact with or alter the structures on the work piece and provides mechanical strength so that the dimensions of structures change little or not at all under the impact of the charged-particle beam. The deposition is limited to areas near the impact point of the cluster beam. 
     Because the clusters have masses that are greater than that of the gallium ions in a typical prior art method, damage to the work piece is confined to surface layer that is thinner than the damage layer caused by a beam of gallium ions. Depending on composition, morphology, and size of clusters used, and the accelerating voltage, damage to the substrate can be confined within the top 20 nm, the top 10 nm, the top 5 nm or the top 2 nm. For example, using a 15 kV accelerating voltage for clusters of C 60 , Au, or Bi clusters would typically limit damage to the top 5 nm of the work piece. By comparison, a typical gallium ion beam damages the surface to a depth of about 30 nm to 50 nm. The preferred accelerating voltage can vary with the type of clusters and the material of the work piece. 
     In some embodiments, the protective layer can be composed of the material from the clusters, such as a carbon layer deposited from fullerenes, and in such embodiments a deposition precursor gas is unnecessary. That is, the clusters directly deposit onto the work piece. In other embodiments, the protective layer is composed of decomposition products from the precursor gas. In other embodiments, a combination of the material in the cluster beam and decomposition products of a precursor gas are deposited. 
     In step  218 , the region of interest, protected by the protection layer, is processed by a charged particle beam. For example, the region of interest can be viewed using scanning electron microscopy, or the region can be etched, or an additional material can be deposited using an ion beam. If a multi-source system is used as the source of the cluster beam, the plasma material that was the source of the clusters can be removed from the plasma chamber, and the plasma chamber can be filled with a different gas for performing a material removal process or a material deposition process, with or without an etch-enhancing gas or a precursor gas being directed to the region of interest. An example of a multi-source system is described in U.S. Prov. Pat. App. 60/830,978 filed Jul. 14, 2006 for “A Multi-Source Plasma Focused Ion Beam System,” which is hereby incorporated by reference and which is assigned to the assignee of the present invention. 
     In other embodiments, an etch-enhancing gas can be used, and the beam of clusters provides energy to initiate a chemical reaction to etch the work piece with reduced damage outside of the etched area. Etch enghacing gases include XeF2, F2, C12, Br2, I2, fluorocarbons, such as trifluoro acetamide and trifluoroacetic acid and trichloroacetic acid, water, ammonia, and oxygen. 
     A preferred implementation of the invention uses a plasma source as described in U.S. Pat. App. Pub. No. 2005/0183667 for a “Magnetically Enhanced, Inductively Coupled Plasma Source for a Focused Ion Beam System,” which is hereby incorporated by reference. Such an ion source provides a beam having very low chromatic aberration and can be focused to a relatively small spot at a relatively high beam current, making it suitable for precise micromachining and deposition. Such a source can provide beams of clusters, as well as beams of individual atoms and molecules for charged particle beam processing. For example, a cluster beam can be used to deposit a protective layer, and then an argon beam can be used with xenon difuoride to micromachine a feature in the work piece. 
       FIG. 3  shows a simplified schematic diagram of a preferred RF excited, plasma ion chamber. A ceramic plasma ion chamber  300  is wrapped by a coil  302 . The coil is excited by an RF source(not shown). Ceramic plasma ion chamber  300  is a cylinder with aperture electrodes  304  at one end. The aperture electrodes exhibit an aperture centered on the cylinder axis of ceramic plasma ion chamber  300 . An ion beam leaves ceramic plasma ion chamber  300  through the aperture of the electrodes  304  and passes through an ion beam focusing column  306  to produce a deflectable focused ion beam  308 . 
     Ceramic plasma ion chamber  300  receives, through a valve  309 , gas from one or more of a plurality of sources  310 ,  312 ,  314 . Sources may comprise cluster gases, such as those described above, inert gases such as xenon (Xe) or helium (He), reactive gases such as oxygen (O 2 ), or precursor or etch-enhancing gases as described above. Valve  309  may be provided to select in sequence each of a plurality of different gases from the sources. Thus, one may choose one ion species for milling or etching and choose a second different ion species for deposition. 
     The source is convenient in that one material can be provided to the plasma source to produce a beam of clusters to provide a protective layer, and then a second material can be provided to the plasma source for further processing of the work piece. For example, one may introduce a gas containing C 60  to decompose a precursor gas to deposit a protective layer, and then introduce a gas such as Xe for sputtering. 
       FIG. 4  shows a schematic of an alternative embodiment of a cluster source  400 . The cluster source  400  can be in its own vacuum chamber, or can be in a vacuum chamber that includes one or more additional charged particle beam columns, such as focused ion beam columns, e.g., liquid metal or plasma source columns, or electron beam columns. The non-cluster sources can be used, for example, to process a work piece after a protective layer is applied using the cluster source. The vacuum chamber can include a gas injection system to direct a deposition precursor gas or an etch-enhancing gas toward the work piece. 
     Cluster source  400  includes a crucible  402  containing a source material  404 . A power source  408  heats a heating coil  410  to provide energy to evaporate source material  404 . Evaporated atoms or molecules of source material  404 , referred to generally as components, expand through a nozzle  420 , which causes the atoms or molecules to condense into clusters  422 . Clusters are preferably loosely bound groups of atoms or molecules. The components preferably do not form a crystalline structure in the clusters, but are amorphous, resembling a liquid state. Components can also be evaporated by a laser or a beam of electrons or ions. 
     The clusters  422  are charged in an ionizer  430 , which may, for example, include an electron source  432  having an electrical potential to accelerate electrons  434  toward an electrode  436 . The electrons  434  collide with clusters  422 , ionizing some of the components. Typically only a small number of components, such as one or two, are ionized, so that the electrical charge on each cluster is small. The clusters are optionally passed though a mass filter  440 , which may use a combination of electrostatic fields, magnetic fields, and apertures so that a beam of cluster  442  having masses in a desired range exit the mass filter  440  and are directed toward the work piece  450  Skilled persons will recognize that the cluster source described in  FIG. 4  is simplified, and that many variations and improvements are known and used. 
     Such cluster sources have been used, for example, to provide coating for optical components. A typical cluster from the source of  FIG. 4  includes between 2 and 10,000 atoms or molecules. The components of a cluster may be loosely bound to each other, although components in some clusters, such as the carbon atoms in fullerenes, are more strongly bound. An efficient cluster source produces a relatively large number of clusters of the desired size, compared to the number of unattached components or clusters of undesirable sizes that are produced. Clusters are charged and typically directed toward the target by electrostatic forces. Because the components can be loosely bound in the cluster, the energy of the cluster can cause it to disintegrate upon impact and allow the components to spread out into a relatively thin, uniform layer on the surface. 
     Cluster ion sources, including those using C 60 , have been used in secondary mass ion spectroscopy to eject material from the surface of a work piece. The low energy per component insures that little momentum is transferred to individual atoms and molecules in the work piece, resulting in very little mixing of the work piece material. 
     The term charged-particle-beam “processing” as used herein includes imaging, as well as sputtering, etching and depositing. 
     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. 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. We claim as follows: