Abstract:
Methods and apparatuses for implementing magnetic field to assist PECVD to locally or globally coat the internal surface of the work piece are presented. Several permanent magnet assembly designs have been presented to provide efficient and effective magnetic field inside the work piece, which acts partially as the working chamber. The magnet assembly generates magnetic flux inside the working chamber, which increases the efficiency of PECVD process, enable PECVD process under higher gas pressure and to improve the uniformity, deposition rate, better adhesion and reduce the process temperature.

Description:
REFERENCE CITED 
     Us Patent Documents 
       [0000]    
       
         1) K. Baba et.al, Surface and Coating Technology, Vol. 74-75, 1995, P292. 
         2) Hitomi Yamaguchi, et.al, J. Manufacturing Science and Engineering, Vol. 129, 2007, P885. 
         3) Hiroyuki Yoshiki, et.al, J. Vac. Sci. A 26(3), May/June 2008, P338; 
         4) Hiroyuki Yoshiki, et.al, Vacuum 84 (2010)559; 
         5) Shamim M. Malik, et.al, J. Vac. Sci. A 15(6), November/December 1997, P2875; 
         6) R. Hytry, et.al, Surface and Coating Technology, 74-75 (1995)43; 
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         18) U.S. Pat. No. 5,224,441. 
       
     
       FIELD OF INVENTION 
       [0019]    The invention is related to methods and apparatus facilitating functional material deposition on the internal surface of work pieces, more specifically to designs of adding permanent magnet array assembly to generate magnetic field inside the work pieces in order to improve the uniformity, deposition rate, better adhesion and reduce the process temperature for the plasma enhanced chemical vapor deposition (PECVD) on the internal surface of tubular work piece. 
       BACKGROUND ART 
       [0020]    Many techniques have been developed in order to improve the surface mechanical properties, such as, wear, erosion, corrosion, friction, and biocompatible properties, of a work piece. These include typically two techniques:
       1. Surface treatment with plasma only, such as Beam Ion Implantation (BII) and Plasma Immersion Ion Implantation (PIII). No material is deposited onto the work piece. This method can only affect the top surface of the work piece, and the treated depth is generally believed to be too thin for many applications.   2. Functional coating the surface of a work piece. The widely used methods includes:
           1) Physical Vapor Deposition (PVD);   2) Chemical Vapor Deposition (CVD);   3) Plasma Enhanced Chemical Vapor Deposition (PECVD);   
               
 
         [0026]    Coating the internal surface of a work piece is more important as there are many applications that require the properties of the internal surface of the work piece to be modified and improved. For example, in the case, such as aircraft landing gear hydraulic cylinders, automotive engine cylinder liners, military gun barrels, and pipes for transmitting petroleum and chemical products, better wear, erosion, corrosion, and friction properties can generally be achieved via hard internal-surface coating, such as CrN, TiN, or DLC. In other cases, functional SiO2, TiO2, or DLC thin film need to be coated onto the internal wall of a fine tube, or implanted medical devices to enhance antivirus, antibacterial, and biocompatible properties. 
         [0027]    PECVD process has relatively large deposition rate, lower process temperature (&lt;200 C, or at room temperature), large scale, and most of all, all the surface exposed to the reactive gas can be coated evenly. Therefore it is generally considered as a 3D coating process, and widely used in industry applications. It is especially useful for coating the internal surface of a work piece. 
         [0028]    Malik et al. developed a PECVD technique to deposit DLC films onto internal surface of a tube. In order to maintain the discharge in a small tube, they inserted a ground electrode into the tube center so that a hollow glow discharge can be generated and sustained. However, when the tube diameter becomes even smaller and the length becomes even longer, it is getting more difficult to sustain the hollow glow discharge. 
         [0029]    A microwave antenna was also applied into the tube to enhance the plasma by Baba and Hatada et.al in another attempt to coating the internal surface of a tube using PECVD technology. They also placed an electromagnetic coil outside the tube with high voltage applied onto the tube. By moving the coil location, they were able to generate plasma inside tubes and control the deposition rate locally. However, the tube can only be coated one section at a time due to the limitation of localized plasma generated only at the coil location. 
         [0030]    In this invention, to take the advantage of both PECVD process and the magnetic field impact on plasma, magnet array assembly is employed outside/inside a tube to further enhance the plasma density for PECVD process. With the help of the magnet array assembly, higher deposition rate, lower process temperature, better adhesion and better uniform deposition rate, can be obtained for coating the internal surface of a work piece. 
       SUMMARY OF THE INVENTION 
       [0031]    The present invention includes, at least, a permanent magnet array assembly embodied on a PECVD system for internal wall coating of work piece with high aspect ratio and/or complicated internal structures such as pipe and tube. The method includes: isolate the work piece and use it as effective vacuum chamber; input gas precursor and/or gas mixture for designed internal coating layer structure; apply either pulse DC or RF energy and use the permanent magnet array assembly to enhance the glow discharge in the whole or only certain portion of the work piece; to efficiently deposition functional layer(s) on the internal wall of the work piece. 
         [0032]    The design of adding the permanent magnet arrays assembly in the present apparatus is of importance. Firstly, the magnetic field restricts the path of free electron within the glow discharge and effectively increase the efficiency of ionization within the glow discharge and allow the sustain of the glow discharge under high gas pressure of precursor (or process gas mixture) with the possibility of sustain glow discharge even under atmosphere pressure to allow high rate deposition and lower process temperature. The sustaining of the glow discharge at lower vacuum or even at atmosphere pressure reduce overall cost of ownership of the apparatus while still retaining the quality of the internal coating with higher deposition rate. Secondly, the magnet array assembly can be rotated or oscillate around the longer axial of the work piece with dwell time optimized for special purpose. For example, the dwell time can be optimized for uniformity coating thickness along the work piece in one case, while in the other case, the deposition thickness can be specified at particular location for the purpose of enhance the functionality and performance of the work piece. 
         [0033]    In this invention, there are two types of designs for the permanent magnet array assembly: one is implemented outside of the work piece; the other is put inside of the work piece. The type of permanent magnet array assembly for use outside of the work piece is particularly useful for the work piece with small internal wall diameter such as the capillary tube used within the medical instruments or the microfluidic devices. For the work piece with large enough internal wall diameter, the magnet array assembly design for the internal use could be better. Of course, when the situation is allowed, both types of the permanent magnet array assemblies can be implemented at the same time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  illustrates one of the embodiments of the coating apparatus set up for current invention. 
           [0035]      FIG. 2  illustrates a second embodiment of the coating apparatus set up for the current invention. 
           [0036]      FIG. 3  illustrates one embodiment of the principle of permanent magnet array design for current invention. 
           [0037]      FIG. 4A  and  FIG. 4B  illustrates the principle of the Halbach permanent magnet array arrangement in the prior art. 
           [0038]      FIG. 5  illustrates the embodiment of implementing of the principles of the magnet array design into the apparatus of current invention. 
           [0039]      FIG. 6  illustrates another embodiment of implementing of the principles of the magnet array design into the apparatus of current invention. 
           [0040]      FIG. 7  illustrates another embodiment of implementing of the principles of the magnet array designs into the apparatus of current invention. 
           [0041]      FIG. 8  illustrates an embodiment of the coating apparatus with the magnet assembly inserted into the work piece. 
           [0042]      FIG. 9  illustrates another embodiment of the coating apparatus with the magnet assembly inserted into the work piece. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    The following description is provided in the context of particular applications and the details, to enable any person skilled in the art to make and use the invention. However, for those skilled in the art, it is apparent that various modifications to the embodiments shown can be practiced with the generic principles defined here, and without departing the spirit and scope of this invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles, features and teachings disclosed here. 
         [0044]    The present invention relates to method and apparatus design to facilitate the disposition of the functional coating on the internal wall of the work piece with particularly emphasis on implementing the permanent magnet array assembly to enhance the glow discharge during PECVD coating. Depending on the size of the permanent magnet array assembly relative to the work piece, the glow discharge can be generated either through the whole work piece or localized only in a portion of the work piece at any given time. 
         [0045]    With reference of the  FIG. 1 , a conductive and non- or low-magnetic work piece  100 , with an internal wall  101  is connected from both ends by end components  110 , which are electrically insulated from the work piece  100 . The deposition working chamber consists the work piece  100  and end components  110 . On one of the end component  110 , there is a gas inlet  120  connecting to the external gas supply with several gas control devices  160  such as mass flow controller (MFC) for accurate control of the gas input into the working chamber. On the other end component  110 , there is a gas outlet  130 , which could connect to vacuum pump. When the process requires the pressure of the working chamber below the atmosphere pressure, the end components  110  can be made of the vacuum tight sealing components. The conductive work piece  100  is connected to external DC pulse power supply  140  to allow negative bias to add onto the work piece  100  to assist ignition and sustain the glow discharge while allowing the ion bombardment or/and implantation of the internal wall  101  of the work piece whenever the process is needed. The end components  110  are grounded to form an electrical close loop for the DC pulse power supply. The permanent magnet array assembly  150 , which will be illustrated further in the following context, is placed outside of the work piece  100  to assist the glow discharge igniting and sustaining. The permanent magnet array assembly  150  can fully or partially circularly rotate around the central axial of the work piece for improving coating efficiency, uniformity and/or tuning local coating thickness. When the thickness of the magnet array assembly is smaller than the length of the work piece, the permanent magnet array assembly  150  can also move along the center axial of the work piece. The presence of the permanent magnet assembly  150  enhances the glow discharge, which could increase the efficient of PECVD process, enable PECVD at higher process pressure, and/or improve the coating quality. 
         [0046]    With reference of the  FIG. 2 , a non-conductive work piece  200 , with an internal wall  201  is connected from both ends by end components  210 . The deposition working chamber consists the work piece  200  and end components  210 . On one of the end component  210 , there is a gas inlet  220  connecting to the external gas supply with several gas control devices  260  such as mass flow controller (MFC) to allow accurate control of the gas input into the working chamber. On the other end component  210 , there is a gas outlet  230 , which could connect to vacuum pump if needed. When the process requires maintaining the pressure of the working chamber below the atmosphere pressure, the end components  210  can be made of vacuum tight sealing components. There are two electrodes  242  and  243  locating outside of the work piece, which acts as radio frequency (RF) electrodes/antennas to allow the energy coupled into the glow discharge inside the working chamber. One piece of the electrode  242  is connected to RF matching box  241  and external RF power  240 , while the other piece of the electrode  243  is grounded. The length of the electrodes could be the same as or slight shorter than the length of the work piece  200 . The permanent magnet array assembly  250 , which will be illustrated further in the following context, is placed outside of the work piece  200  to assist the glow discharge igniting and sustaining. The permanent magnet array assembly  250  can fully or partially circularly rotate around the central axial of the work piece for enhancing the PECVD process. When the thickness of the magnet array assembly is smaller than the length of the work piece, the permanent magnet array assembly  250  can also move along the center axial of the work piece. The presence of the permanent magnet assembly  150  enhances the glow discharge, which could increase the efficient of PECVD process, enable PECVD at higher process pressure, and/or improve the coating quality. 
         [0047]    With reference of the  FIG. 3 , a cross section view (perpendicular to the long axial of the work piece) of portion of the permanent magnet array segment, which may be embodied in the present apparatus for internal wall coating, is shown. In this illustration, the permanent magnet array segment  300  is placed adjacent and outside the work piece  310  with an internal wall  311 . The magnet segment  300  consists of adjacent permanent magnet bars  301  and  302 , support insulator  304  and soft magnetic rail  303 . The magnetization directions of the permanent magnet bars  301  and  302  aim toward the work piece with northern and southern poles of the adjacent magnet bars pointing towards the opposite directions so that the magnetic fringe field  305  can penetrate through the wall of the work piece to assist the PECVD process. The distance between the adjacent magnet elements can be optimized based on magnetic strength of bar permanent magnets as well as the wall thickness and material properties of the work piece  310 . 
         [0048]    As shown in  FIG. 4A , a typical Halbach array is illustrated. In Halbach array, adjacent magnets are arranged with their magnetization directions alternating in directions perpendicular with each other. As shown in  FIG. 4B , the field above the plane is in the same direction for both structures, but the field below the plane is in opposite directions. Such arrangement of permanent magnets will reinforce the magnetic field on one side of the array while cancel the field to near zero on the other side, which is called “a one-sided flux”. The advantages of one sided flux distribution are twofold:
       1. The field is twice as large on the side on which the flux is confined;   2. No stray field is produced on the opposite side. This helps with field confinement.       
 
         [0051]      FIG. 5  illustrates an embodiment of a permanent magnet ring assembly  500  that may be implemented in PECVD apparatus of present invention. Although the exact details of the magnet ring assembly can vary, there are at least three basic components in this assembly, namely outer soft magnetic ring  501 , the non-magnetic support part(s)  502 , the field generating magnet-assembled ring  504 , which is sandwiched between  501  and  502 . Within magnet assembled ring  504 , all magnets are made of permanent magnetic materials, such as NdFeB, SmCo, AlNiCo. Each magnet element is engineered in a particular shape, eg. fan shape, to match the local contour of external wall of the work piece  510 . All the magnets are closely packed and arranged according to the principle of the Halbach array in this embodiment so that big enough magnetic flux  503  can be generated and penetrate through the wall of the work piece  510  and help to enhance the glow discharge during PECVD process. All the magnets are assembled into the pre-design location with the help of the non-magnetic supporting element  502 . The exact materials, arrangement and sharp of the supporting element  502  can vary. A soft magnetic shield in circular shape  501  is used to cover the outmost surface of the ring assembly to form a magnetic close loop to eliminate the entire magnetic fringe field outside the soft magnetic ring  501 . Although the magnet assembly  500  in  FIG. 5  is configured via the principle of the Halbach as shown in  FIG. 4 , it can also be assembled using the arrangement and magnet design principle shown in  FIG. 3 , which serves the same purpose of generating magnetic field  503  seen in  FIG. 5 . Between the work piece  510  and magnet assembly  500 , there is a gap  520 , which enables the magnet assembly  500  can move, free of major friction, around the center of the work piece  511  as well as along the long axle of the work piece. 
         [0052]      FIG. 6  illustrates an alternative embodiment of a permanent magnet ring assembly  600  that may be implemented in PECVD apparatus of the present invention. The schematic drawing in  FIG. 6  is self explanatory. Basically, the magnet assembly is a sector of the magnet assembly shown in  FIG. 5 , with similar functional non-magnetic supporting part(s)  602  and soft magnetic ring  601  for flux close. Since the magnetic assembly  600  is only cover portion of the surface of the work piece, in order to coat uniform deposit around the whole internal wall of the work piece  610 , the magnet assembly has to rotate round the center axle of the work piece  611 . Although there is only one sector is shown in  FIG. 6 , multiple sectors can be used to give the balance between the mechanical movement and the number of magnet assemblies for cost saving purpose. 
         [0053]      FIG. 7  illustrates an embodiment of a permanent magnet ring assembly  700  that may be implemented in PECVD apparatus of the present invention. Noticeably, the magnet ring assembly  700  is placed inside the work piece  710 . This arrangement is of importance when the work piece is made by materials with high magnetic moment or the wall thickness of the work piece is too thick for the externally arranged magnet assembly as shown in  FIG. 5  and  FIG. 6  to easily generate large enough magnetic field to penetrate through the wall of the work piece and assist PECVD process. The magnet assembly  700  consists of a center support rod  704  covered with a surface ring  701  made by soft magnetic materials, such as soft Fe, magnet-assembled ring  705  and non-magnetic supporting structure(s)  702 . The permanent magnet elements within magnet-assembled ring  705  are arranged based on principle shown in either  FIG. 3  or  FIG. 4 . The purpose of the magnet assembly  700  is to generate the magnetic flux  703  around it and into the space of the internal work piece  710 . 
         [0054]    With reference of the  FIG. 8 , a conductive and non- or low-magnetic work piece  800 , with an internal wall  801  is connected from both ends by end components  810 , which is electrically insulated from the work piece  800 . The deposition working chamber consist the work piece  800  and end components  810 . On one of the end component  810 , there is a gas inlet  820  connecting to the external gas supply with several gas control devices  860  such as mass flow controller (MFC) to allow accurate control of the gas input into the working chamber. On the other end component  810 , there is a gas outlet  830 , which can connect to vacuum pump if needed. When the process requires maintaining the pressure of the working chamber below the atmosphere pressure, the end components  810  can be made of vacuum tight sealing components. The conductive work piece  800  is connected to external DC pulse power supply  840  to allow negative bias to add onto the work piece  800  to assist ignition and sustain the glow discharge while allowing the ion bombardment or/and implantation of the internal wall  801  of the work piece whenever the process is needed. The end components  810  are grounded to form an electrical close loop for the DC pulse power supply. The permanent magnet assembly  850 , which has been illustrated in details in  FIG. 7 , is placed inside of the work piece  800  to assist the glow discharge igniting and sustaining. The permanent magnet assembly  850  can fully or partially circularly rotate around the central supporting rod  851  for improving coating uniformity. The central supporting rod  851  can move smoothly through the end component  810  via mechanical bearing component  856 , which is am existing know-how from prior art. To maintain the vacuum seal, the mechanical bearing component  856  can be bearings with magnetic fluidic seal. When the length of the magnet array is smaller than the length of the work piece, the permanent magnet array  850  can also move along the center axial of the work piece. 
         [0055]    With reference of the  FIG. 9 , a non-conductive work piece  900 , with an internal wall  901  is connected from both ends by end components  910 . The deposition working chamber consist the work piece  900  and end components  910 . On one of the end component  910 , there is a gas inlet  920  connecting to the external gas supply with several gas control devices  960  such as mass flow controller (MFC) to allow accurate control of the gas input into the working chamber. On the other end component  910 , there is a gas outlet  930 , which can connect to vacuum pump. When the process requires the pressure of the working chamber below the atmosphere pressure, the end components  910  can be vacuum tight sealing components. There are two electrodes  942  and  941  locating outside of the work piece, which acts as radio frequency (RF) electrodes to allow the energy coupled into the glow discharge inside the working chamber. One piece of the electrode  942  is connected to RF matching box  941  and external RF power  940 , while the other piece of the electrode  941  is grounded. The length of the electrodes could be the same as or slight shorter than the length of the work piece. The permanent magnet-array assembly  950 , which has been illustrated in details in  FIG. 7 , is placed inside of the work piece  900  to assist the glow discharge igniting and sustaining. The permanent magnet array assembly  950  can fully or partially circularly rotate around the central supporting rod for improving coating uniformity. The central supporting rod  951  can move smoothly through the end component  910  via mechanical bearing component  956 , which is an existing know-how from prior art. To maintain the vacuum seal, the mechanical bearing component  956  can be made of bearings with magnetic fluidic seal. When the length of the magnet array is smaller than the length of the work piece, the permanent magnet array  950  can also move along the center axial of the work piece.