Abstract:
An ion source and a polishing system using the ion source are disclosed. The ion source includes a discharge chamber, an electron emitter, a cathode, a screen grid, an accelerator grid, and a screen electrode. The discharge chamber is configured for accommodating discharge gas. The electron emitter is disposed in the discharge chamber. The cathode, the screen grid, the accelerator grid, and the accelerator grid are separately aligned in the discharge chamber in an ascending order with respect to the respective distance thereof from the electron emitter. The electron emitter, the cathode, the screen grid, the accelerator grid, and the accelerator grid are powered in order of descending voltages. The screen electrode defines an adjustable orifice to permit adjustment of an ion-beam ejecting area associated with the orifice. The polishing system further employs a movable stage and control and monitor components, in addition to the ion source.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to ion sources and, more particularly, to an ion source and a polishing system using the ion source.  
       DESCRIPTION OF RELATED ART  
       [0002]     Ion sources are typically devices that ionize gas molecules and then focus, accelerate, and emit them as narrow ion beams. The ion beams can be used for various technical and technological purposes such as cleaning, activation, polishing, thin-film coating, aligning, or etching.  
         [0003]     Many ion sources need to emit focused ion beams for some technological purposes, for example, for selectively etching and/or polishing of a small area. With advances in technology, there is increasingly a demand for precision polishing of surfaces for a variety of purposes. For example, highly polished precision surfaces are needed in molds, optical elements (e.g., optical lenses), and in the fabrication of semiconductor elements (e.g., silicon wafers).  
         [0004]     A typical focused ion source can be used to alter selected regions on a surface of a workpiece by polishing thereof. This ion source generally includes a discharge chamber configured for accommodating a discharge gas and an ion-optical system disposed in the discharge chamber. The ion-optical system is configured for generating ion beams derived from the discharge gas and accelerating the ion beams to an appropriate level of energy. The areas of polishing can be controlled to within a surface roughness in the nanometer range. However, in a traditional electrical discharge machining (EDM), the areas of machining become relatively rough, within surface roughness in the micrometer range. That is, this focused ion source has a relatively higher degree of roughness precision than the EDM.  
         [0005]     However, many, if not most, workpieces have some areas that are difficult to reach/treat and/or small areas, such as, for example, a narrow area, slot, slope surface, sharp angle surface, concave portion, and/or convex portion. During polishing, the aforementioned ion source usually has a problem with the difficult and/or small areas, making polishing of such areas difficult to accurately control or at least making adjustment between different areas challenging. In other words, this ion source has generally not proven suitable for treating surfaces with difficult and/or small areas.  
         [0006]     What is needed, therefore, is an ion source that can adjustably and controllably eject ion beams.  
         [0007]     What is also needed, therefore, is a polishing system using the above-described ion source.  
       SUMMARY OF INVENTION  
       [0008]     In accordance with a preferred embodiment, an ion source includes a discharge chamber, an electron emitter, a cathode, a screen grid, an accelerator grid, and a screen electrode. The discharge chamber is configured for accommodating a discharge gas. The electron emitter is disposed in the discharge chamber. The cathode, the screen grid, the accelerator grid, and the accelerator grid are separately aligned in the discharge chamber in an ascending order with respect to a distance from the electron emitter. The electron emitter, the cathode, the screen grid, and the accelerator grid are powered in a descending order of voltages. The screen electrode defines an adjustable orifice to adjust an ejecting area through which the ion beams can travel, thereby permitting control of the resulting beam diameter.  
         [0009]     A polishing system includes a platform, a control device connected with the platform, a monitor device operatively linked with the control device, and an ion source. The monitor device is configured (i.e., structured and arranged) for measuring and monitoring surface characteristics of a workpiece to be polished and for transmitting information about the surface characteristics to the control device. The control device regulates the movement and rotation of the platform according to the information received from the monitor device. The ion source is mounted on the platform and is configured for ejecting ion beams. The ion source includes a discharge chamber, an electron emitter, a cathode, a screen grid, an accelerator grid, and a screen electrode. The discharge chamber is configured for accommodating a discharge gas. The electron emitter is disposed in the discharge chamber. The cathode, the screen grid, the accelerator grid, and the accelerator grid are separately aligned in the discharge chamber in an ascending order with respect to a distance thereof from the electron emitter. The electron emitter, the cathode, the screen grid, and the accelerator grid are powered in order of descending voltages. The screen electrode defines an adjustable orifice to selectably vary an ejecting area through which the ion beams can be transmitted.  
         [0010]     Other advantages and novel features will be drawn from the following detailed description of preferred embodiments when conjunction with the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Many aspects of the present ion source and polishing system can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present ion source and the related polishing system. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
         [0012]      FIG. 1  is a schematic, cross-sectional view of an ion source, according to a preferred embodiment;  
         [0013]      FIG. 2  is schematic view of a plasma bridge neutralizer, applied in the ion source of  FIG. 1 ;  
         [0014]      FIG. 3  is a schematic view of a polishing system, using the ion source of  FIG. 1 ;  
         [0015]      FIG. 4  is a schematic view of an alternative polishing system, using the ion source of  FIG. 1 ; and  
         [0016]      FIG. 5  is a flow chart of a method for polishing a workpiece by using the polishing system of FIGS.  3  or  4 . 
     
    
     DETAILED DESCRIPTION  
       [0017]     Embodiments of the present ion source and polishing system will now be described in detail below and with reference to the drawings.  
         [0018]      FIG. 1  illustrates an ion source  100 , in accordance with a preferred embodiment. The source  100  includes a discharge chamber  10 , an electron emitter  11 , a magnetic coil  12 , a cathode  13 , a screen grid  15 , an accelerator grid  16 , a screen electrode  17 , and a neutralizer  18 , advantageously in that general, uninterrupted order. The discharge chamber  10  includes an inlet  101  and an outlet  102  in two opposite walls thereof. The inlet  101  is configured for supplying discharge gas (e.g., Argon)  112  into the discharge chamber  10 . The outlet  102 , meanwhile, is configured for allowing ion beams to eject/exit therefrom. The electron emitter  11 , the magnetic coil  12 , the cathode  13 , the screen grid  15 , the accelerator grid  16 , the screen electrode  17 , and the neutralizer  18  are separately aligned in the discharge chamber  10 , in an ascending order with respect to their respective distance from the inlet  101 .  
         [0019]     The electron emitter  11  is surrounded by the magnetic coil  12  and disposed along an axis thereof . The electron emitter  11  may be a cathode filament made of, for example, tungsten or tantalum. The electron emitter  11  is configured (i.e., structured and arranged) for emitting electrons  111  while electric current is applied thereto.  
         [0020]     The cathode  13  can, for example, be shaped as a conic dome. The cathode  13  is advantageously coaxial and adjacent to the magnetic coil. The cathode  13  has a top portion at a distance from the electron emitter. The top portion defines an opening. The cathode  13  has a lower voltage than the electron emitter  11 . The cathode  13  can be connected to a negative terminal of a discharge power supply. For example, to provide an ion beam of singly charged argon (Ar) ions with a desired energy of about 1000 eV, the negative terminal of the beam supply is connected to a discharge anode and set to 1000 V. Due to such negative voltage, the electrons  111  are thus extracted from the electron emitter  11 . The electrons  111  sequentially move helically towards and through the cathode  13 , due to a magnetic field generated by the magnetic coil  12 . During such movement, most of the electrons  111  collide with the discharge gas  112 . These collisions ionize the discharge gas  112 , thereby generating cations  113 . Likewise, some cations  113  also collide the discharge gas  112  to further ionize the discharge gas  112 , thereby generating cations  113  yet again.  
         [0021]     In addition, some free electrons  112 , without having collided with the discharge gas, also pass through the opening  131  of the cathode  13 . After passing through the opening  131 , the cations  113  and the free electrons  112  again collide with the discharge gas  112  to obtain more cations  113 . Advantageously, a shield layer  14  is coated on the cathode  13 , the shield layer  14  being configured for protecting the cathode  13  from being bombarded by the more cations  113 . The shield layer  14  can be made of a material such as alumina, magnesium oxide, or silicon dioxide.  
         [0022]     The screen grid  15  is spaced from the cathode  13  and defines a plurality of clearances  151  between adjacent grids. The screen grid  15  can be a conductive electrode, which is connected to a positive high voltage ion beam power supply. Thus, the screen grid  15  is the electrode controlling the potential of the cations  113 , which is also effectively also the “beam voltage.” The beam voltage can be in the range from 100 V to 900 V, which is lower than potential of the cathode  13  so that the cations  113  can be attracted thereto and thus move towards the screen grid  15 . Some cations  113  bombard/impact the screen grid  15 , while other cations  113  pass through the clearances  151  and sequentially move towards the accelerator grid  16 .  
         [0023]     The accelerator grid  16  is spaced from the screen grid  15  and defines a plurality of gaps  161  between adjacent grids. The gaps  161  of the accelerator grid  16  are substantially aligned or coaxial with the clearances  151  of the screen grid  15 . The accelerator grid  16  has a lower voltage than the screen grid  15  so that the cations  113  can be accelerated to move towards the accelerator grid  16 . For example, the accerelator voltage could be about −400 V. Similarly, a minority of the cations  113  may impact the accelerator grid  16 , while a majority of the cations  113  avoid such a collision and pass through the gaps  161  and sequentially move towards the screen electrode  17 . After passing through the screen grid  15 , some cations  113  are lack of enough energy to continue later operation. In this circumstance, the accelerator grid  16  can provide these cations  113  further energy (hence the name acceleration grid) so as to these cations  113  have enough energy/momentum to continue later operation (i.e., to proceed to an ultimate use destination, e.g., a polishing site). Other still sufficiently energetic cations  113 , accelerated by the accelerator grid  16 , will, in turn, have a relatively better polishing capacity. As a result, the accelerator grid  16  can improve polishing efficiency of the ion source  100 .  
         [0024]     The screen electrode  17  is spaced from the accelerator grid  16 . The screen electrode  17  can be at electrical ground potential. The screen electrode  17  defines an orifice  171  structured and arranged for allowing the cations  113  to pass through the screen electrode  17  and for thereby facilitating the ejection of the cations  113  out of the discharge chamber  10 .  
         [0025]     An adjusting member  172  is slideably mounted on the screen electrode  17 . The sildeable member  172  includes two lids  172   a  and  172   b . The two lids  172  and  172   b  can slide relative to each other on the screen electrode  17 , for example, along a corresponding slideway (not shown) defined in the screen electrode  17 . The two lids  172   a  and  172   b  cooperatively cover part or all area of the orifice  171  and can be selectably, slidably moved in order to facilitate an adjustment of a space size and/and shape of the orifice  171 . Accordingly, an ejecting area of the cations  113  from the orifice  171  can be adjusted by sliding one or both of the lids  172   a  and  172   b  of the screen electrode  17 .  
         [0026]     The neutralizer  18  is positioned in the vicinity of the outlet  102 , through which the cations  11  can exit, and is adjacent to the orifice  171 . The neutralizer  18  is used to provide electrons for current and space charge neutralization of the cations  113 , for example, to reduce inter-ion repulsion within the stream of cations  113 . If not neutralized, the cations  113  will eject from the outlet  102  and then bombard on a small area of a workpiece (not shown) to be polished, which generally faces towards the outlet  102 . As a result, an excess positive charge can be locally formed on/at that small area of the workpiece. The positive charge would yield an electric field around the polished area. The electric field thus formed has a disadvantageous influence on the incoming cations  133 , thereby disturbing the sequential polishing process (i.e., polishing beyond the initial bombardment could tend to be impaired or at least not as controllable). With the advent of the neutralizer  18 , the neutralizer  18  emits numerous electrons. The electrons emitted interact with the cations  113  before the cations  113  bombard the workpiece, thereby preventing the formation of the electric field and ensuring the continuous and effective polishing of the workpiece. Alternatively, the neutralizer  18  could be configured for emitting electrons toward the workpiece so as to neutralize the excess positive charge formed on the workpiece.  
         [0027]     The neutralizer  18  may be a hot filament neutralizer or a plasma bridge neutralizer. The hot filament neutralizer includes a filament, for example, a tungsten filament or a tantalum filament. The filament is configured for emitting electrons upon being heated.  
         [0028]      FIG. 2  illustrates a plasma bridge neutralizer (PBN). The plasma bridge neutralizer includes an RF (radio frequency) coil  181 , a discharge house  182 , a cup-shaped collector  183 , an electron extraction electrode  184 , and a barrier  185 . The RF coil  181  is coiled (i.e., wound) on the discharge house  182 . The discharge house  182  may be made of a ceramic material. The collector  183  is accommodated in the discharge house  182  and is spaced therefrom. The barrier  185  has a passage  185   a  communicating with the collector  183 . The passage  185   a  is configured for supplying a discharge gas to the collector  183 . The brarrer  185  is configured for preventing leakage of electrical power. The collector  183  has a negative bias voltage to attract the cations  113 . The extraction electrode  184  has a positive bias voltage to extract the electrons.  
         [0029]     During neutralizing, an RF energy is applied to the RF coil  181 , and synchronously an electromagnetic wave is coupled to the discharge house  182  to form a plasma plume. This plasma plume acts as a conductive path or plasma bridge between the extraction electrode  184  and the cations  113 . The cations  113  are neutralized at the conductive path or plasma bridge with the electrons extracted from the extraction electrode  184 .  
         [0030]      FIG. 3  illustrates a polishing system  200 , using the ion source  100  above described, for polishing a workpiece  27  having a surface  270  to be polished. In addition to the ion source  100 , the polishing system  200  includes a platform  21 , a monitor device  23 , and a control device  24 . The ion source  100  is mounted on the platform  21 . The platform  21  can be a X-Y-Z three-dimensional stage and cause the ion source  100  move to any direction and rotate or tilt to reach any difficult area to be polished. The monitor device  23  is electrically connected with the control device  24 . The control device  24 , in turn, is electrically connected with the platform  21  and, further advantageously, with the ion source  100 .  
         [0031]     The monitor device  23  is configured for monitoring surface characteristics (e.g., roughness, contour) of the surface  270  in real time during a polishing process and then transmitting the surface characteristic information to the control device  24 . The monitor device  23  may, beneficially, employ a Fizeau interferometer (precision about micrometer scale), a Nomarshi microscope (precision about 1.22 times wavelength of light wave), and/or fringes of equal chromatic order (precision below 1 nanometer).  
         [0032]     The control device  24  receives the information about the surface characteristics from the monitor device  23  and then adjusts and controls the ion source  100  to polish the workpiece  27  according to the information. The information indicates whether area of the surface  270  being currently polished needs to be further polished and/or whether other adjustments (e.g., ion beam power or position) need to be made. The control device  24  drives the platform  21  to move/rotate so as to adjust and direct the ion source  100  towards the area of the surface  270  being currently polished.  
         [0033]      FIG. 4  illustrates an alternative polishing system  300 , using the ion source  100  above described. In addition to the ion source  100 , the polishing system  300  includes a platform  31 , a monitor device  33 , a control device  34 , a memory component  35 , and an input device  36 . The platform  31 , the monitor device  33 , and the control device  34  are essentially similar to the platform  21 , the monitor device  23 , and the control device  24  of the polishing system  200 , respectively.  
         [0034]     The input device  36  electrically connects with the memory component  35  and is configured for inputting initial information/data regarding the surface characteristics of the workpiece  27  into the memory component  35 . The initial surface characteristic data is original information about the surface  270  before polishing, for example, length, width, geometry, microstructure, contour, hardness, etc. The memory component  35  is electrically connected with the control device  34  and is configured for storing the original surface characteristic information and then transmitting this original information to the control device  34 . The control device  34  controls the ion source  100  to perform the primary polishing process according to, at least in part, to the original information transmitted thereto. Upon beginning the polishing process, the monitor device  33  is then able act in a manner similar to the monitor device  23  to facilitate adjustments to the operation of the control device  34  during processing.  
         [0035]     Referring to  FIGS. 4 and 5 , a method for polishing a workpiece using the polishing system  300  includes the steps of: providing a workpiece having a surface to be polished; monitoring at least one surface characteristic of the surface of the workpiece via a monitor device and then transmitting information about the at least one surface characteristic to a control device; and polishing the surface via an ion source under the control of the control device according to the information.  
         [0036]     The workpiece to be polished may be, e.g., a mold. The mold can be made, for example, of a material selected from the group consisting of: stainless steel, stainless steel with a nickel phosphide (NiP) coating, metal alloys, ceramic (such as tungsten carbide (WC) or silicon carbide (SiC)), glass, glass-ceramics, and combinations thereof. The mold typically has a surface to be polished, for example, a mold surface.  
         [0037]     Preferably, the original information about surface characteristics of the surface  270  of the workpiece is obtained via the monitor device  33  or, alternatively, via a peripheral surface profilmeter. Additionally, certain information (e.g., dimensions, shape, hardness, etc.) may potentially be input by the user or via a data bank. The original information, however obtained, is inputted into the memory component  35  via the input device  36 . The memory component  35  then supplies the original information to the control device  34  so that the control device  34  can control the ion source  100  to perform a primary polishing process according to the original information.  
         [0038]     During the polishing process, the surface characteristics of any area of the surface  270  is monitored in real time via the monitor device  33  and a corresponding resultant information about surface characteristics of the surface  270  is then transmitted to the control device  34 . According to this information, the control device  34  timely controls the positioning of the platform  31  and/or adjusts the ion source  100  to perform both precise and accurate polishing. At the same time, the two lids  172   a  and  172   b  can slide on the screen electrode  17  so as to adjust the ejecting area/shape available for the cations  113  (See  FIG. 1  ) to satisfy different requirements of polishing properties on different areas. For example, in polishing some difficult and/or small areas, the two lids  172   a  and  172   b  can slide closely to minimize the ejecting area of the cations  113  (See  FIG. 1 ) thereby facilitating the fine and accurate polishing of these areas. Thus, even if the mold surface, i.e., surface  270 , has difficult surface geometries to be polished, such as for example, a narrow area, slot, slope surface, sharp angle surface, concave portion, or convex portion, the polishing system  300  can accurately polish the surface  270 . The surface roughness Ra can be controlled in the nanometer or subnanometer range, for example, from about 0.2 nanometers to about 1.0 nanometer.  
         [0039]     Furthermore, the ion source  100  may employ other alternative structures and configurations in other embodiments. For example, the adjusting member  172  can include four or more lids symmetrically surrounding the orifice  171 . The lids can be made of a transformable material that can elongate while heated. Moreover, the screen electrode  17  can be a retractable electrode along a radial direction of the discharge chamber  10 , thereby being able to self-adjust the space size and shape of the orifice  171 . The electron emitter  11  can be, e.g., a hot filament. In addition to use in a polishing process, the ion source  100  can be applied in other technical and technological purposes, such as cleaning, activation, thin-film coating, aligning, and/or etching.  
         [0040]     It will be understood that the above particular embodiments and methods are shown and described by way of illustration only. The principles and features of the present invention may be employed in various and numerous embodiments thereof without departing from the scope of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.