Patent Publication Number: US-2005126590-A1

Title: Cleaning sheet for a probe

Description:
BACKGROUND OF THE INVENTION  
      This invention relates to a cleaning sheet for and a method of removing foreign substances attached to the tip and side surfaces of a probe used for inspecting a planar target object such as a semiconductor device having integrated circuits installed thereon.  
      Chips are produced by installing semiconductor elements and integrated circuits on a semiconductor wafer through various wafer-producing processes. The chips thus produced on a semiconductor wafer are cut off from the wafer after a current-passing test and are packaged. Tests with a current are carried out also before and after the packaging to separate faulty products stringently from qualified products.  
      Such current-passing tests are carried out by means of a test apparatus of a known kind such as a wafer prober. A position-matching process by moving a prober needle (“probe”) serving as an electrode of a tester and the electrodes on the chip (pads or lead lines) and a contacting process between the probe and the chip electrodes are repeated for carrying out electrical measurements of different kinds.  
      While such position-matching and contacting processes are repeated by moving the probe and the electrodes on the chips, the tip of the probe slides over the electrodes on the chips, scraping off portions of the electrodes. Such portions of the electrodes that have been scraped off become attached to the tip and the side surfaces of the probe as foreign substances.  
      The foreign substances, that thus become attached to the probe, are metals such as aluminum. If such a metal is oxidized, the electrical contact resistance between the probe and the electrodes on the chip becomes larger, making it impossible to carry out accurate electrical measurements. Thus, it is necessary to clean the tip portion of the probe regularly after each time a specified number of contact processes have been carried out in order to remove such foreign substances from the probe.  
      Removal of such foreign substances from the tip portion of a probe is carried out by using the same apparatus used for the current-passing tests such as the aforementioned wafer prober, except the target object to be tested such as a semiconductor wafer attached to the apparatus is replaced with a cleaning device having a similar shape. Examples of a prior art cleaning device of this kind include hard polishing plates of a grinding stone, glass and a ceramic material (as disclosed, for example, in Japanese Patent Publications Tokkai 7-199141, 5-209896, 5-166893, 4-96342 and 3-105940) and sheets with a polishing layer formed on an uneven surface of an elastic member having protrusions and indentations (as disclosed, for example, in Japanese Patent Publication Tokkai 2000-332069). The probe is pressed against the surface of such a cleaning device, as done in the aforementioned current-passing test.  
      If a polishing plate of a hard material is used as the cleaning device and the probe is pressed too hard against it, its tip portion will become deformed and hence it is necessary to reduce the pressure with which the probe can be pressed against the cleaning device. As a result, only the portions very close to the tip can be cleaned. If a cleaning device of the type with a polishing layer formed on an elastic member is used, on the other hand, the tip of the probe penetrates the polishing layer and cannot be cleaned sufficiently. Thus, the common practice has been to provide separately a device for cleaning the tip and another device for cleaning the side surfaces, attaching them to a current-passing test apparatus consecutively one at a time. It was therefore both cumbersome and time-consuming to clean a probe.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of this invention in view of the above to provide a single cleaning sheet capable of simultaneously cleaning both the tip and the side surfaces of a probe.  
      A cleaning sheet embodying this invention, with which the above and other objects can be accomplished, may characterized as comprising a base sheet, a first polishing layer formed on the base sheet, a porous foamed layer having air bubbles inside formed on the base sheet, and a second polishing layer formed on the foamed layer. The surface of the foamed layer may be flat or preferably porous with the second polishing layer having openings corresponding to those on the porous surface of the foamed layer.  
      For cleaning a probe by using such a cleaning sheet, the tip of the probe is caused to penetrate the surface of the second polishing layer, to pass through the foamed layer and to be pressed against the first polishing layer.  
      With the present invention, a single cleaning sheet can clean both the tip and the side surfaces of a probe. If an adhesive with a high heat resistance is used to paste the cleaning sheet of this invention to a dummy wafer, furthermore, the tip portion of a probe can be cleaned without stopping the apparatus during a current-passing inspection process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIGS. 1A and 1B  are each a sectional view of a cleaning sheet embodying this invention.  
       FIG. 2  is a schematic sectional view of a probe being cleaned with a cleaning sheet embodying this invention.  
       FIG. 3  is a schematic side view of a wafer prober. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 3  shows a wafer prober  20  of a known kind. When a cleaning process according to this invention is effected, a cleaning sheet  10  embodying this invention to be described below and shown in  FIG. 1  is attached to such a wafer prober  20  instead of a target object such as a semiconductor wafer which is normally attached thereto in order to be inspected thereby. As shown in  FIGS. 1A and 1B , the cleaning sheet  10  is comprised of a base sheet  11 , a first polishing layer  12  formed on the surface of the base sheet  11 , a porous foamed layer  13  having air bubbles  15  inside formed on the surface of the first polishing layer  12 , and a second polishing layer  14  formed on the surface of this foamed layer  13 .  
      A sheet with thickness 50 μm-200 μm and a flat surface of a plastic material such as polyester and polyethylene terephthalate (PET) may be used as the base sheet  11 .  
      The first polishing layer  12  is formed by using particles of a material such as silica, aluminum oxide and diamond with average diameter of 0.001 μm-10 μm as abrading particles and fixing them by means of a polyester or urethane resin binder. Such a layer may be formed on the surface of the base sheet  11  by dispersing abrading particles inside a resin solution obtained by dissolving a resin binder with a solvent such as methylethyl ketone to prepare a polishing paint, applying this polishing paint on the surface of the base sheet  11  a known coating method such as the reverse coating method and drying the polishing paint thus applied. The thickness of the first polishing layer  12  is within the range of 10 μm-50 μm.  
      The foamed layer  13  is a sheet of a foamed material such as foamed urethane having a thickness within the rage of 50 μm-300 μm. This foamed sheet is pasted to the surface of the first polishing layer  12  formed on the base sheet  11 . The surface of this foamed sheet may be flat as shown in  FIG. 1A  or may have portions removed by a buffing process to form a porous surface by causing openings  16  made of air bubbles to appear exposed on this porous surface as shown in  FIG. 1B .  
      A foaming sheet, to be used as the foaming layer  13  with a flat surface as shown in  FIG. 1A , may be formed as follows. First, a plastic sheet serving as a form-providing film with a flat and smooth surface is coated with a foaming resin solution obtained by dissolving a foaming resin material in a solvent. Foaming polyurethane resin and an organic solvent such as dimethyl formaldehyde may be used. If necessary, water, a foaming aid such as fleon and a foam stabilizer such as silicone oil may also added. After the solvent in the foaming resin solution is replaced with water inside water, this foaming resin solution is dried so as to remove the water component. Air bubbles are thereby formed where the solvent was replaced by water. The foamed layer formed on the plastic sheet is thereafter peeled off from the plastic sheet to obtain a foamed sheet. The surface at which the foamed layer was peeled off from the plastic sheet is smooth and flat, like the surface of the plastic sheet on which the foaming resin solution was applied.  
      The foamed sheet (forming the foamed layer  13 ) thus prepared is pasted to the surface of the first polishing layer  12  formed on the base sheet  11  by using a polyester or urethane resin adhesive.  
      The second polishing layer  14  is formed by coating the surface of this foamed layer  13  with the polishing paint described above by a known coating method such as the reverse coating method and then drying it. The second polishing layer  14  is preferably formed to be thin enough to allow the probe  22  to penetrate it, as shown in  FIG. 2 . The thickness of the second polishing layer  14  is in the range of 10 μm-50 μm.  
      A probe for inspecting a planar object such as a semiconductor device with integrated circuits installed thereon may be cleaned to remove foreign substances attached to its tip and side surfaces by using a test apparatus of a known kind such as shown at  20  in  FIG. 3  for carrying out a current-passing test. As shown in  FIG. 3 , the test apparatus  20  has a table  23  on which a target object for inspection such as a semiconductor device is intended to be attached. After such a target object for inspection is attached, the table  23  is moved horizontally and vertically for positioning and the target object is pressed against a probe  22  of a probe card  21  in order to carry out electrical measurements of different kinds on the target object. When the probe  22  is to be cleaned, the cleaning sheet  10  as described above is attached to the table  23 , instead of the target object for inspection and the table  23  is similarly moved similarly to press the probe  22  against the surface of the cleaning sheet  10  attached to the table  23 .  
      The cleaning sheet  10  may be cut into a circular, quadrangular or any other shape and pasted on the table  23  by means of a double-side adhesive sheet. Alternatively the cleaning sheet  10  may be pasted on a flat plate of an appropriate shape and this plate may then be attached to the table  23 . In summary, the manner of attaching the cleaning sheet  10  onto the table  23  does not limit the scope of the invention. The cleaning sheet  10  may be attached to the table  23  or to such a flat plate by preliminarily coating the back surface of the base sheet  11  with an adhesive agent (not shown), say, to a thickness in the range of 20 μm-30 μm.  
      Chips on a semiconductor wafer are tested usually by varying the temperature in the range of −40° C.-150° C. and in the manner of an assembly line. The cleaning of the tip of the probe  22  is carried out according to this invention by inserting a dummy wafer of the same shape as semiconductor wafers but having a cleaning sheet of this invention pasted thereon at a regular interval in this assembly line. This method is advantageous because the tests can carried out continuously and hence more efficiently without stopping the test apparatus but when the temperature exceeds 100° C., the adhesive force of the cleaning sheet to the dummy wafer becomes adversely affected and the cleaning sheet may become displaced or peeled off. In view of such occurrences, an adhesive agent with resistance against heat such as acryl resin should preferably be used in order to stabilize the adhesive force of the cleaning sheet  10  to the dummy wafer especially under higher-temperature environment.  
      When the tip of the probe  22  is pressed against the surface of the cleaning sheet  10  on the table  23 , it penetrates the second polishing layer  14 , passes through the foamed layer  13  as shown in  FIG. 2  and is pressed against the surface of the first polishing layer  12 . The side surfaces of the probe  22  are cleaned while its tip is penetrating the second polishing layer  14 , passing the foamed layer  13  and being pressed against the first polishing layer  12 . The tip of the probe  22  is cleaned while being pressed against the surface of the first polishing layer  12 .  
      The cleaning method explained above with reference to  FIG. 2  is carried out after the test apparatus is stopped by attaching the cleaning sheet  10  at a proper place on the test apparatus. As explained above, however, this is not intended to limit the scope of the invention. The probe  22  may be cleaned during a current-passing test by pasting the cleaning sheet on a dummy wafer by using a heat-resistant adhesive agent.  
      As a test example, a cleaning sheet embodying this invention was produced first by coating a surface of a plastic sheet of polyethylene terephthalate (PET) with thickness 75 μm with a polishing paint having silica particles with average particle diameter of 0.3 μm dispersed in a resin solution obtained by dissolving a polyester resin in a solvent such as methylethyl ketone by the reverse coating method, forming a first polishing layer with thickness of 20 μm on the surface of the plastic sheet by drying it and aging it for three days at 40° C. Next, a foamed sheet of foamed urethane resin with thickness 100 μm was attached to the surface of this first polishing layer to form a foamed layer of thickness 50 μm on the surface of the first polishing layer and it was then aged for three days at 40° C. Next, the aforementioned polishing paint was applied on the surface of this foamed layer by the reverse coating method, dried to form a second polishing layer with thickness 10 μm and aged for 3 days at 40° C. The second polishing layer had a porous surface with openings corresponding to those on the surface of the foamed layer.