Abstract:
The invention describes how contaminant particles may be removed from a surface without in any way damaging that surface. First, the positional co-ordinates of all particles on the surface are recorded. Optionally, only particles that can be expected to cause current or future damage to the surface are included. Then, using optical tweezers, each particle is individually removed and then disposed of. Six different ways to remove and dispose of particles are described.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to the general field of integrated circuit manufacture with particular reference to the removal of contaminant particles.  
       BACKGROUND OF THE INVENTION  
       [0002]     Contaminant particles are among the most common problems associated with IC (integrated circuit) manufacturing processes. They may cause device yield loss because of photo defocus in successive layers, pattern bridging, contact/via opens, CMP (chemical mechanical polishing) scratching, etc. Current in-line inspection tools, such as KLA (an on-line defect inspection tool produced by KLA-Tensor) make it easy to accurately determine the location and size of contaminant particles on wafers but there is no tool for removing these particles except scrubber cleaners. Unfortunately, scrubber cleaning is not a suitable method for dielectric layers, patterned layers, water-absorbent layers, metal layers (corrosion concern), etc and the device yield loss remains the same even though we know the position of these contaminant particles.  
         [0003]     Optical traps, sometimes referred to as optical tweezers, utilize a light source to produce radiation pressure. Radiation pressure is a property of light that creates small forces by absorption, reflection, or refraction of light by a dielectric material. Relative to other types of forces, the forces generated by radiation pressure are almost negligible-only a few picoNewtons. However, a force of only a few picoNewtons is sufficient to allow attachment to particles of the sizes just discussed.  
         [0004]     Optical tweezers utilize the force that exists when a transparent material with a refractive index greater than the surrounding medium is placed in a light gradient. As light passes through polarizable material, it induces a dipole moment. This dipole interacts with the electromagnetic field gradient, resulting in a force directed towards the brightest region of the light, normally the focal region. Conversely, if an object has a refractive index less than the surrounding medium, such as an air bubble in water, the object experiences a force drawing it toward the darkest region.  
         [0005]     As long as the frequency of the laser is below the natural resonances of the particle being trapped, the dipole moment will be in phase with the driving electric field. A schematic view of a light tweezer setup is illustrated in  FIG. 1 . Light  11 , typically laser light, enters a high numerical aperture objective lens  12  of an optical system and is focused  16  to a diffraction limited region (spot)  13  on a particle  14 . Because the intensity profile of the laser light is not uniform, an imbalance in the reaction forces generates a three-dimensional gradient force  15  with the brightest light in the center. The gradient force  15  pulls the object toward the brightest point. Thus, the forces generated by the optical system “traps” the object. Such gradient forces are formed near any light focal region.  
         [0006]     The sharper or smaller the focal region  13 , the steeper the gradient. To overcome scattering forces near the focal region and hence prevent the object from being ejected along the direction of the light beam, the optical system must produce the steepest possible gradient forces. Sufficiently steep gradient forces can be achieved by focusing laser light to a diffraction-limited spot through a microscope objective of high numerical aperture (N.A.).  
         [0007]     A routine search of the prior art was performed with the following references of interest being found:  
         [0008]     In U.S. Pat. No. 6,055,106, Grier et al. describe an apparatus for manipulating small dielectric particles. In U.S. Pat. No. 5,953,166, Shikano discloses a laser trapping apparatus while in U.S. Pat. No. 5,689,109 Schutze discloses an apparatus and method for the manipulation, processing and observation of small particles. Weetall et al., in U.S. Pat. No. 5,620,857 use tightly focused laser beams as optical tweezers while Burns et al., in U.S. Pat. No. 5,245,466, create arrays using light beams coupled to microscopic polarizable matter. U.S. Pat. No. 5,079,169 is a method for optically manipulating polymer filaments. Ashkin et al. describe a nondestructive optical trap for biological particles in U.S. Pat. No. 4,893,886. Finally, in U.S. Pat. No. 5,512,745, Finer et al. shows an optical trap system while Ashkin (U.S. Pat. No. 3,808,550), and Shivashankar et al. (U.S. Pat. No. 6,139,831) show optical trap related patents.  
       SUMMARY OF THE INVENTION  
       [0009]     It has been an object of the invention to remove contaminant particles from a surface.  
         [0010]     Another object of the invention has been that the act of removing said particles not damage the surface in any way.  
         [0011]     A further object has been that the invention allow a choice to be made between removing all particles on the surface and removing only particles that have the potential to damage the surface.  
         [0012]     These objects have been achieved by first recording the positional co-ordinates of all particles on the surface. Optionally, only particles that can be expected to cause current or future damage to the surface are included. Then, using optical tweezers, each particle is individually removed and then disposed of. Six different ways to remove particles are described. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  illustrates the general principle of optical tweezers.  
         [0014]      FIG. 2  shows a first embodiment of the invention in which a contaminant particle is removed from a downward facing surface.  
         [0015]      FIGS. 3-7  show five variations of a second embodiment of the invention in which a contaminant particle is removed from an upward facing surface. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Since optical tweezers have proved to be an effective tool for three dimensional trapping and manipulation of particles, the present invention uses them as its means for removing contaminant particles from the surface of an integrated circuit wafer. The process of the present invention can be divided into three main steps:  
         [0017]     (1) Detect and record the location and size of all contaminant particles or, optionally, only selected particles that are predicted to reduce product yield. Typically, particles have a mean diameter between about 0.12 and 0.5 microns, with about 1 micron being an upper limit to what can be handled by optical tweezers at the current state of the art. In the 0.15 micron process, any particle whose size exceeds about 0.25 microns is likely to cause a yield loss. This step is accomplished by using an in-line inspection tool. An example of this is KLA which was mentioned earlier. Optionally, the same lens system used to generate the optical tweezers may also be used for obtaining said positional co-ordinates.  
         [0018]     (2) Using the optical tweezers, access each location separately and remove all, or only selected, particles from the semiconductor wafer surface, depending which takes least time (including the time to execute step  1 ). As a practical matter, the optical system needs to have a NA of at least 0.8 and the light source needs to be a laser (such as Ar or he-Cd) operating with an intensity of at least 10 W.  
         [0019]     (3) Dispose of the removed particles, usually, though not necessarily, by reducing the power level (including full turnoff) until the particle is drawn out of the trap by gravitational forces.  
       1 st  Embodiment  
       [0020]     Referring now to  FIG. 2 , we show there the first of two embodiments of the invention. Schematically represented is silicon wafer  21  whose lower surface  22  contains one or more integrated circuits. Contaminant particle  14  is seen after it has been drawn away from surface  22  by the light trap  13  at the focal point of focused laser beam  16 . This was accomplished by first positioning the lens ( 12  in  FIG. 1 ) so as to bring the focal point of converging beam  16  directly below where the particle  14  had been sitting on the surface  22  at a distance from the particle of between about 200 and 500 nanometers.  
         [0021]     Positioning of the beam  16  was, in turn, achieved by controlling motion of the lens from a data base that contained the coordinates (on the wafer surface) of all particles that were to be removed. Depending on the relative times to populate the data base and to remove particles, all particles on surface  22  could be removed or removal could be limited to particles that, because of their position on the surface, were expected to damage the integrated circuit, either immediately, during later processing, or on life.  
         [0022]     A key feature of this embodiment is that a stream of gas (symbolized by arrow  25 ) is caused to flow past and around wafer  21  in a downward direction. As a consequence, when the power level of the laser is reduced (or made zero), there is no longer sufficient force within the light trap to hold the particle and it gets swept away from the wafer through a combination of gravitational forces and the carrying power of the gas stream.  
       2 nd  Embodiment  
       [0023]     Referring now to  FIG. 3 , the first of several variations of the second embodiment is shown. Schematically represented is silicon wafer  21  whose upper surface  22  contains one or more integrated circuits. Contaminant particle  14   a  is seen after it has been drawn away from surface  22  by the light trap  13 a at the focal point of focused laser beam  16   a . This was accomplished by first positioning the lens ( 12  in  FIG. 1 ) so as to bring the focal point of converging beam  16  directly above where the particle  14  had been sitting on the surface  22  at a distance from the particle of between about 200 and 500 nanometers.  
         [0024]     Positioning of the beam  16  was, in turn, achieved by controlling motion of the lens from a data base that contained the co-ordinates (on the wafer surface) of all particles that were to be removed. Depending on the relative times to populate the data base and to remove particles, all particles on surface  22  could be removed or removal could be limited to particles that, because of their position on the surface, were expected to damage the integrated circuit, either immediately, during later processing, or on life.  
         [0025]     Variation 1: Continuing our reference to  FIG. 3 , it is seen that disposal of the removed particle  14   a  is achieved by moving the focal point  13   a  of the laser to position  13   b  where particle  14   b  is no longer above the wafer surface. The laser power level is then reduced or terminated causing particle  14   b  to be removed by gravity.  
         [0026]     Variation 2: Referring now to  FIG. 4 , as in variation 1, disposal of the removed particle  14   a  also begins by a lateral movement of focal point  13   a  to a new position  13   c . Unlike variation  1 , however,  14   c  is higher than  14   a  and continues to be above the wafer. A key feature of variation  2  is that catcher plate  41  is then inserted between focal point  13   c  and the surface so that when the power level is reduced or terminated, the particle falls onto the catcher plate which is then removed.  
         [0027]     Variation 3: Referring now to  FIG. 5 . This resembles variation 1 in that the light beam and particle do not move. Instead, wafer  21   a  is moved to position  21   b  which is sufficiently removed from  21   a  so that particle  14  is no longer above the wafer surface. The laser power level is then reduced or terminated causing particle  14  to be removed by gravity.  
         [0028]     Variation 4: This is illustrated in  FIG. 6 . Unlike the previous three variations, neither the focal point nor the wafer get moved. Instead, tube  61  is provided. This has one open end with the other end being connected to low pressure chamber  62 , low pressure meaning a pressure lower than that at focal point  13 . Tube  61  is brought into position so that its open end is close to particle  14  at which point the particle gets pulled out of the light trap, moving along direction  63  and getting sucked into tube  61  and thence into low pressure container  62 . Typically the pressure at focal point  13  would be about atmospheric or low pressure chamber  62  could be used.  
         [0029]     Variation 5: This is illustrated in  FIG. 7 . Here too neither the focal point nor the wafer get moved. Instead, tube  71  is provided. This has one open end with the other end being connected to pressure source  72 . Tube  71  is brought into position so that its open end is close to particle  14  at which point the particle gets pushed out of the light trap, moving along direction/trajectory  73  and not falling below the level of surface  22  until it is clear of the wafer. Typically the pressure at focal point  13  would be about atmospheric.  
         [0030]     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.