Abstract:
A method of fabricating a film of magnetic nanocomposite particles including depositing isolated clusters of magnetic nanoparticles onto a substrate surface and coating the isolated clusters of magnetic nanoparticles with an insulator coating. The isolated clusters of magnetic nanoparticles have a dimension in the range between 1 and 300 nanometers and are separated from each other by a distance in the range between 1 and 50 nanometers. By employing PVD, ablation, and CVD techniques the range of useful film thicknesses is extended to 10-1000 nm, suitable for use in wafer based processing. The described methods for depositing the magnetic nanocomposite thin films are compatible with conventional IC wafer and Integrated Passive Device fabrication.

Description:
CROSS REFERENCE TO RELATED CO-PENDING APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/763,327 filed on Jan. 30th, 2006 and entitled “SYSTEMS AND METHODS FOR FORMING MAGNETIC NANOCOMPOSITE MATERIALS”, the contents of which are expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The present invention relates to systems and method for forming magnetic nanocomposite materials via thin film deposition techniques. 
       BACKGROUND OF THE INVENTION  
       [0003]    Magnetic nanocomposite materials are composites of magnetic and insulator materials and have ultrafine grains or particles with dimensions of less than 300 nanometers. When small enough particles of magnetic materials, of order 2-300 nm in size, are sheathed in thin layers of insulators, of order 1-50 nm in thickness, and formed in such a way that the exchange coupling between the particles of nearest neighboring particles is realized, the resulting materials have both excellent magnetic and insulating properties. Materials of this sort have been made into useful forms for electronic devices such as inductors by techniques such as squeegee application of epoxies loaded with such particles or by electroplating. Such techniques are useful for films of 100-1000 microns and 5-100 microns, respectively that are useful in forming thick film passive devices such as inductors. However, for devices integrated within a semiconductor wafer or thin film passive device wafer, film thicknesses of less than a few microns are desired. Accordingly, there is a need for a thin film deposition technique for magnetic nanocomposite materials that is appropriate for integration with conventional IC wafer and Integrated Passive Device fabrication. 
       SUMMARY OF THE INVENTION 
       [0004]    In general, in one aspect, the invention features a method of fabricating a film of magnetic nanocomposite particles including depositing isolated clusters of magnetic nanoparticles onto a substrate surface and then coating the isolated clusters of magnetic nanoparticles with an insulator coating. The isolated clusters of magnetic nanoparticles have a dimension in the range between 1 and 300 nanometers and are separated from each other by a distance in the range between 1 and 50 nanometers. 
         [0005]    Implementations of this aspect of the invention may include one or more of the following features. The depositing and coating are repeated until a desired film thickness is achieved. The method also includes measuring the film thickness. The film thickness is in the range between 10 and 1000 nanometers. The isolated clusters of magnetic nanoparticles are deposited via a physical vapor deposition (PVD) process. The isolated clusters of magnetic nanoparticles are coated with an insulator via chemical vapor deposition (CVD) process or via PVD process. The insulator coating thickness is in the range between 1 and 30 nanometers. The method may further include aggregating the isolated clusters of magnetic nanoparticles before the coating. The aggregating may include thermally annealing the deposited isolated clusters of magnetic nanoparticles, or irradiating the deposited isolated clusters of magnetic nanoparticles with a light source such as lasers or UV light sources. The magnetic nanoparticles may be Fe, Ni, Co, NiCo, FeZn, borides of these materials, ferrites, rare-earth metals, or alloy combinations thereof. The substrate may be fused silica, oxidized silicon, quartz, or silicon, GaAs, GaN, high temperature glass, alumina, silicon nitride, silicon carbide, semiconductor materials, refractive insulators, or organic printed circuit board materials. The insulator coating may be SiO 2 , Si 3 N 4 , Al 2 O 3 , oxides, ceramics, polymers, organic material or ferrites, epoxies, Teflon®, and silicones or combinations thereof. The depositing and the coating may occur simultaneously and in the same reactor. The isolated clusters of magnetic nanoparticles may be deposited via sputtering a target comprising the magnetic material. The isolated clusters of magnetic nanoparticles may be deposited via CVD. In the CVD process the magnetic nanoparticles may be formed by decomposing carbonyl precursors of the magnetic material via electromagnetic radiation. Alternatively, the isolated clusters of magnetic nanoparticles may be deposited via an ion cluster beam (ICB) deposition process. The deposition of isolated clusters of magnetic nanoparticles may include ablating the magnetic nanoparticles from a target comprising the magnetic material and condensing the magnetic nanoparticles onto the substrate surface. The magnetic nanoparticles may be ablated from the target by electromagnetic radiation from a source such as lasers, UV light, Radio Frequency (RF) waves or microwaves. The ablated magnetic nanoparticles may be further ionized by a particle beam such as electron beam, ion beam, or molecular beam. The target may be rotated and/or rocked during the ablation process. The substrate may be rotated and/or rocked during the deposition process. The coating of the isolated clusters of magnetic nanoparticles with the insulator coating may include ablating particles of the insulator from a target comprising the insulator and condensing the ablated insulator particles onto the magnetic nanoparticles and the substrate surface. The ablating of the magnetic nanoparticles and the ablating of the insulator particles may occur simultaneously in the same reactor. The deposition may be enhanced by a magnetic field or electric field. The coating may be enhanced by an electric field or magnetic field. 
         [0006]    In general, in another aspect, the invention features an apparatus for fabricating a film of magnetic nanocomposite particles including equipment for depositing isolated clusters of magnetic nanoparticles onto a substrate surface and equipment for coating the isolated clusters of magnetic nanoparticles with an insulator coating. The isolated clusters of magnetic nanoparticles have a dimension in the range between 1 and 300 nanometers and are separated from each other by a distance in the range between 1 and 50 nanometers. 
         [0007]    Implementations of this aspect of the invention may include one or more of the following features. The apparatus may further include equipment for measuring the thickness of the film. The deposition equipment may be a physical vapor deposition (PVD) reactor. The coating equipment may be a chemical vapor deposition (CVD) reactor or a PVD reactor. The apparatus may further include equipment for aggregating the isolated clusters of magnetic nanoparticles before the coating. The aggregating equipment may be equipment for thermally annealing the deposited isolated clusters of magnetic nanoparticles, or equipment for irradiating the deposited isolated clusters of magnetic nanoparticles with a light source such as lasers or UV light sources. The deposition equipment and the coating equipments are comprised in the same reactor. The deposition equipment comprises a sputtering reactor or a CVD reactor. In the CVD reactor the magnetic nanoparticles may be formed by decomposing carbonyl precursors of the magnetic material via electromagnetic radiation. The deposition equipment comprises an ion cluster beam (ICB) deposition reactor. The deposition equipment comprises equipment for ablating the magnetic nanoparticles from a target comprising the magnetic material and equipment for condensing the magnetic nanoparticles onto the substrate surface. The ablating equipment comprises an electromagnetic radiation source such as lasers, UV light, Radio Frequency (RF) waves or microwaves. The deposition equipment may further comprise equipment for ionizing the ablated magnetic nanoparticles and the ionizing equipment may be a particle beam source such as an electron beam, an ion beam, or a molecular beam. The target may be rotated and/or rocked during the ablation. The substrate may be rotated and/or rocked during the deposition. The coating equipment comprises equipment for ablating particles of the insulator from a target comprising the insulator and equipment for condensing the insulator particles onto the magnetic nanoparticles and the substrate surface. The ablating of the magnetic nanoparticles and the ablating of the insulator particles may occur simultaneously in the same reactor. The deposition equipment may further comprise a source for a magnetic field or electric field. The coating equipment may further comprise a source for an electric field or magnetic field. 
         [0008]    Among the advantages of this invention may be one or more of the following. By employing PVD, ablation, and CVD techniques the range of useful film thicknesses of magnetic nanocomposite particles is extended to 10-1000 nm (0.01-1 micron), so called thin films, for use in wafer based processing. The distribution of core particle sizes is maintained from deposition to deposition so as to assure repeatability in the film properties. Depending on the properties desired, this may be a narrow or broad distribution of particles. Oxidation of the core materials is prevented as this is known to have a deleterious effect on the magnetic properties of such materials caused by the antiferromagnetic effect in these oxides. Maximum performance of the magnetic nanocomposite films is achieved by optimizing the core particle size, their size distribution, particle isolation distance and inter-particle isolation distances to achieve the desired tradeoffs in magnetic permeability, frequency response and dielectric permittivity. An insulating layer of controlled thickness is deposited on the conducting magnetic nanoparticles to eliminate potential eddy current losses when a device based on these magnetic nanoparticles is operated at frequencies in the range between 1 MHz to 100 GHz. The magnetic and dielectric properties of the film are simultaneously adjusted to achieve optimal tunable device characteristics. Chemical reaction or alloying of the magnetic metal nanoparticles and their coated insulating layers is avoided. The described methods for depositing the magnetic nanocomposite thin films are appropriate for integration with conventional IC wafer and Integrated Passive Device production. Patterning of these thin films can be realized via inorganic liftoff, sputter etching, or ion milling techniques 
         [0009]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and description below. Other features, objects and advantages of the invention will be apparent from the following description of the preferred embodiments, the drawings and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]    Referring to the figures, wherein like numerals represent like parts throughout the several views: 
           [0011]      FIG. 1A  is a cross-sectional side view of a PVD magnetic nanocomposite material; 
           [0012]      FIG. 1B  is a top view of the PVD magnetic nanocomposite material of  FIG. 1A ; 
           [0013]      FIG. 1C  is a cross-sectional side view of an aggregated PVD magnetic nanocomposite material; 
           [0014]      FIG. 1D  is a top view of the aggregate PVD magnetic nanocomposite material of  FIG. 1C ; 
           [0015]      FIG. 2  is a cross-sectional side view of an aggregate PVD magnetic nanocomposite material after four cycles of the sequence shown in  FIGS. 1A-1D ; 
           [0016]      FIG. 3  is a schematic diagram of the apparatus for the combined PVD-CVD deposition of a magnetic nanocomposite material; 
           [0017]      FIG. 4  is a schematic diagram of the apparatus for the deposition of a magnetic nanocomposite material from carbonyl precursors; 
           [0018]      FIG. 5  is a schematic diagram of the apparatus for the deposition of a magnetic nanocomposite material from ion cluster beam; 
           [0019]      FIG. 6  is a schematic diagram of the apparatus for deposition of a magnetic nanocomposite material by ablation; and 
           [0020]      FIG. 7  is a block diagram of the method for fabricating a film of magnetic nanocomposite particles, according to this invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    The invention describes several methods for depositing magnetic nanocomposite thin films appropriate for integration with conventional IC wafer and Integrated Passive Device fabrication. 
         [0022]    Referring to  FIG. 7 , a method  100  for fabricating a film of magnetic nanocomposite particles includes the steps of depositing isolated clusters of magnetic nanoparticles ( 102 ), aggregating the deposited isolated clusters of magnetic nanoparticles ( 104 ) and then coating the isolated clusters of magnetic nanoparticles with an insulator coating ( 106 ). Next, the thickness of the deposited film is measured ( 107 ) and the process repeats itself until a desired thickness of the film is achieved ( 108 ). The film thickness is in the range between 10 and 1000 nanometers. Examples of magnetic nanoparticles include Fe, Ni, Co, NiCo, FeZn, borides of these materials, ferrites, rare-earth metals, or alloy combinations thereof. Examples of substrates include fused silica, oxidized silicon, quartz, or silicon, GaAs, GaN, alumina, silicon nitride, silicon carbide, semiconductor materials, refractive insulators, or organic printed circuit board materials. Examples of insulator coating include SiO 2 , Si 3 N 4 , Al 2 O 3 , oxides, ceramics, polymers, organic material or ferrites, epoxies, Teflon®, and silicones or combinations thereof. 
         [0023]    In one embodiment, the magnetic nanoparticles are deposited onto the substrate surface via physical vapor deposition (PVD) technique, so as to form isolated islands of magnetic material. This deposition method takes place in a PVD reactor. PVD is a physical deposition process that does not involve chemical reactions. Examples of PVD techniques that are applicable include evaporative deposition, electron beam PVD (EB-PVD), sputter deposition and pulsed laser deposition, among others. Commercial PVD reactors are offered by Applied Materials (Santa Clara, Calif.), Novellus Systems (San Jose, Calif.) and Balzers (Liechtenstein). In other examples, custom made PVD reactors are used. Referring to  FIG. 1A , clusters (islands) of magnetic nanoparticles (core material)  110  are deposited onto a surface  113  of substrate  114 . The surface  113  of substrate  114  includes a nucleating layer  112 . Examples of nucleating layer  112  include plasma activated SiO 2  using techniques such as SUSS MicroTec&#39;s nanoPrep technology. The first deposition cycle is truncated at a point where islands formed around nucleation sites are stopped before the islands coalesce in cross section. Next, the deposited core material  110  is aggregated by heating the substrate. In other examples aggregation of the core material occurs by irradiating the deposited core material  110  with a laser beam or a high intensity UV light. Aggregation causes the core material particles to become more spherical in shape. Next, the aggregated core material particles  110  are coated with an insulating coating  116  via chemical vapor deposition (CVD) process. CVD is a chemical process used to produce high-purity solid materials. In a typical CVD reactor a substrate is exposed to volatile precursors of a certain material, which then react and/or decompose on the substrate surface to form a film of the material. CVD techniques that are applicable include low pressure CVD (LPCVD), Metalorganic CVD (MOCVD), Plasma enhanced CVD (PECVD), Rapid thermal CVD (RTCVD) and Vapor phase epitaxy (VPE), among others. Commercial CVD reactors are offered by Applied Materials (Santa Clara, Calif.), Novellus Systems (San Jose, Calif.) and Balzers (Liechtenstein). In one example, core material  110  is Fe particles, nucleating layer  112  is SiO 2 , substrate  114  is fused silica and coating  116  is SiO 2 . The thickness  90  of the as deposited core material is 10 nanometers, the thickness of the insulating layer  92  is 5 nanometers and the overall thickness  91  of the coated core material is 30 nanometers, shown in  FIG. 1C . The average diameter  93  of the coated particle is 30 nanometers, shown in  FIG. 1D . Referring to  FIG. 2 , the process repeats itself for four cycles and the resulting film  120  includes particles  1 , 2 , 3 , 4 , from the corresponding four cycles. Particles  1 , 2 , 3 , 4  and surface  113  are coated with the insulator coating  116 . In one example, the overall thickness  95  of the film  120  is 150 nanometers. In other examples, particles  1 , 2 , 3 , 4  are coated with the insulator coating  116  via a PVD process. The sizes, shapes, and densities of the core material islands  110  are suggestive of a variation that is tuned to achieve desired material properties, i.e. the process conditions are altered to provide variation in the size and density of the core material particles and the inter particle spacing and dielectric matrix thickness. These variations are known to affect the final magnetic, dielectric, and smoothness of the final film. If the smoothness or flatness of the final surface is inadequate, a chemical mechanical polishing (CMP) process can be interposed in the sequence or after the final deposition to provide the desired topology. In this case, it may be desirable to coat the surface with an insulating layer to isolate exposed nanoparticles the environment or subsequent conductive layers. The inter-core spacing distance  96  and the inter-core isolation distance  97 , shown in  FIG. 2 , are important parameters in determining the degree of exchange coupling, low frequency and RF eddy current losses realized in the final film. In one example, the inter-core spacing distance  96  is between 25 and 150 nanometers and the inter-core isolation distance  97  is between 1 and 30 nanometers. 
         [0024]    In another embodiment, a combined PVD and CVD process is applied to fabricate the magnetic nanocomposite films. Referring to  FIG. 3 , PVD deposition of the magnetic core material takes place in the presence or a reactive gas  134  to form the insulating coating on the magnetic nanoparticles  132 . In one example, the reactive gas is reactive silane and oxygen or silane and ammonia in the presence of a background pressure of Argon to form an insulating coating of SiO 2  or Si 3 N 4 , or Al 2 O 3 . The Argon pressure is adjusted to effect changes in the size of the particles and the silane/oxygen ratio is adjusted to effect changes in the thickness of the coating insulator. As shown in  FIG. 3 , a target  130  of core material is sputtered in a back pressure of Argon that has been adjusted to achieve the desired particle size. These particles  132  are then coated with silica, as they are transported across the reactor  140  by the reaction of silane and oxygen  134  and continuously deposited on the substrate  114 . The sputtering and CVD processes may proceed simultaneously or sequentially including in an overlapping manner so as to effect changes in the core particle size, coating thickness, and deposition rates. Particles  132  may also be biased with an electric field  142  or a magnetic field (not shown). 
         [0025]    In another embodiment a multistage CVD process takes place in a cold wall flow reactor  150 , shown in  FIG. 4 . In this method, aggregates of Ni, Co, NiCo, Fe, FeZn, Borides of these materials, alloys of these materials, or the like ferromagnetic materials are formed from their carbonyl precursors and then are coated with an SiO 2  overcoat in a subsequent downstream step in the same reactor. The SiO 2  overcoat is formed using a continuous or a pulsed formation process from a silane—oxygen reaction. Referring to  FIG. 4 , the cold wall flow reactor  150  includes two stages  152 ,  154  in which the core particles are first formed  152  and then coated with an insulator  154 . Carbonyls  151  introduced in the upper part of the chamber  152  are decomposed with the assist of incident light  153  to form particles of the core material  155  that are subsequently coated with SiO 2  in the lower part of the chamber  154  where silane  156  and oxygen  157  are introduced. The coated particles  158  are subsequently deposited on substrate  114  suitable for electronic device fabrication at the bottom of the reactor  150 . The carbonyl flow rate, light intensity, silane flow rate and oxygen flow rate are varied to effect changes in the characteristics of the film deposited on the product substrate. 
         [0026]    Another method for forming the magnetic nanoparticles is by applying the Ion Cluster Beam (ICB) deposition process. Referring to  FIG. 5 , aggregates  162  of Ni, Co, NiCo, Fe, FeZn, Borides of these materials, alloys of these materials, or the like, ferromagnetic materials such as ferrites are formed in an Ion Cluster Beam (ICB) Deposition reactor  160  in the presence of silane  166  and oxygen  164 , or in the downstream presence of silane or oxygen. The continuously formed aggregates  162  are then to coated with a film of SiO 2    166  and then are deposited on a substrate  114 , suitable for the formation of electronic devices. As shown in  FIG. 5 , the ion cluster beam source  161  introduces a cloud of particles of the core material  162 . The clusters typically contain a few 10s of atoms of the core material. These clusters  162  then pass through the reactor zone  165  where silane  166  and oxygen  164  react to form SiO 2  which then coats the core material particles. The resulting coated particles  166  as well as SiO 2  itself are deposited on the substrate  114  and form a continuous film comprising of particles of core material dispersed in a silica matrix. The entry rate of core ion clusters and the silane-oxygen flows are controlled to vary the properties of the resulting nanocomposite film deposited on the product substrate. 
         [0027]    In another embodiment, the nanocomposite film is deposited by ablating a target of nanocomposite material. According to this method nanocomposite particles, such as silica coated aggregates Ni, Co, NiCo, Fe, FeZn, Borides of these materials, alloys of these materials, or ferromagnetic materials, are first formed into a target. In one example, the target is a disk having a thickness of 1-10 mm in thickness and 25-1000 mm in diameter and is formed by pressure sintering. This target  171  is then affixed to one plate  174  of a parallel plate vacuum chamber  170  with the other plate  176  holding the substrate  114  suitable for electronic device formation, shown in  FIG. 6 . Referring again to  FIG. 6 , the nanocomposite particles  175  are ablated from the target  171  by incident illumination  178  by one or several laser beams  179  that scan the surface of the target. Increasing the number of lasers used enhances the deposition rate and uniformity of the deposited film. 
         [0028]    In one example, the lasers are arrayed around the periphery of the chamber  170  in one or more rows such that the beams  178  are aligned to impinge on the target  171  below the critical angle (to the normal) of reflection but so as to strike the target at several nominal radii from its central axis and each beam is mechanically or electro-optically scanned across a range of radii. The target  171  may be rotate about the central axis  181  so that the impinging laser beams  178  will uniformly ablate the target material. The substrate  114  may also be rotated  183  in like manner as the target  171 , but asynchronously so as to improve the uniformity of the film deposition. Further, the target may also be optionally washed with one or more electron or ion beams  180  to assist in charging the ablated particles  175 . The plates  174 ,  176  are biased by a variable AD/DC potential  182  in such a way as to induce the ablated particles to be preferentially transported to the substrate  114  where the nanocomposite particles are deposited as a film. 
         [0029]    In yet another embodiment, the nanocomposite film is deposited by ablating a target of the magnetic material and a target of the insulating material. In this method magnetic nanoparticles, such as silica coated aggregates of Ni, Co, NiCo, Fe, FeZn, Borides of these materials, alloys of these materials, or ferromagnetic materials such as ferrites, are first formed into a target. In one example, the target is a disk having a thickness of 1-10 mm and a diameter of 25-1000 mm and is formed by pressure sintering. A second target is formed from the insulator ceramic or polymeric material. Both the magnetic particle target and the insulator target are positioned on plate  174  of reactor  170  and substrate  114  is placed on plate  176  so as to face both the magnetic particle target and the insulator target. The magnetic nanoparticles are ablated from the magnetic particle target by incident illumination by one or several laser beams that scan the surface of this target. The insulator target is also ablated simultaneously with the magnetic particle target. Co-evaporation of the two targets, followed by condensation onto the substrate  114  forms films comprising of insulator coated on the metal particles. Increasing the number of lasers used enhances the deposition rate and uniformity of the deposited film. 
         [0030]    Other embodiments are within the scope of the following claims. For example, a magnetic or an electric field is employed in the deposition zones of  FIG. 4-FIG .  6  between the target/inlet area and the deposition substrate area. These fields increase the deposition rates and control the size and location of the deposited particles. The magnetic or electric field may be constant or pulsed. Furthermore, microwaves of RF waves are used for the ablation process instead of or in addition to laser or other light source. In each of the cases above where an inorganic insulator is employed, an organic insulator may be substituted. In general, the use of an organic insulator changes the resulting dielectric properties, mechanical properties, water absorption capacity, and tolerance to temperature extremes in such a way as to make films produced in such a fashion substantially different from those employing inorganic insulators. The materials prepared with organic insulators have the advantage of not exposing the core materials to oxidation which has been noted to have deleterious effects on the magnetic properties of the films. In one example, the thickness of the deposited film is measured by Rutherford Back Scattering. 
         [0031]    Several embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.