Patent Publication Number: US-6991520-B2

Title: Abrasive machine and method of abrading work piece

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an abrasive machine and a method of abrading a work piece, e.g., silicon wafer. 
     A technique of Chemical-Mechanical Polishing (CMP) is considered as an important technique for high density multi-layered wiring. 
     There are many related factors in the CMP, such as kinds of slurry, rotational speed of an abrasive plate, kinds of abrasive pads, temperature, etc. Therefore it is difficult to select optimum abrading conditions under which desired abrasion rate and flatness can be gained. 
     SUMMARY OF THE INVENTION 
     The present invention has been invented so as to overcome the disadvantage of the CMP. 
     An object of the present invention is to provide an abrasive machine and a method of abrading a work piece capable of changing pressure applied to the work piece, which seldom relates to the CMP factors, and easily defining the optimum abrading conditions. 
     To achieve the object, the present invention has following structures. 
     Namely, the abrasive machine of the present invention comprises: 
     a pressure vessel having a lid which opens or closes the pressure vessel, the pressure vessel being capable of increasing and reducing inner pressure; 
     an abrasive plate being provided in the pressure vessel; 
     a pressing plate being provided on the abrasive plate, the pressing plate pressing a work piece, which has been set between the abrasive plate and the pressing plate, onto the abrasive plate; 
     a driving unit relatively moving the abrasive plate with respect to the pressing plate so as to abrade the work piece; and 
     a pressure source being connected to the pressure vessel, the pressure source increasing or reducing the inner pressure of the pressure vessel. 
     On the other hand, the method of the present invention is a method of abrading a work piece in an abrasive machine comprising: a pressure vessel having a lid which opens or closes the pressure vessel, the pressure vessel being capable of increasing and reducing inner pressure; an abrasive plate being provided in the pressure vessel; a pressing plate being provided on the abrasive plate, the pressing plate pressing a work piece, which has been set between the abrasive plate and the pressing plate, onto the abrasive plate; a driving unit relatively moving the abrasive plate with respect to the pressing plate so as to abrade the work piece; and a pressure source being connected to the pressure vessel, the pressure source increasing or reducing the inner pressure of the pressure vessel, 
     the method comprises the steps of: 
     setting the work piece between the abrasive plate and the pressing plate; 
     introducing a gas into the pressure vessel; and 
     relatively moving the abrasive plate with respect to the pressing plate by the driving unit so as to abrade the work piece. 
     In the present invention, the abrasive plate and the pressing plate are provided in the pressure vessel, and the work piece can be abraded in a state-of increasing or reducing the inner pressure of the pressure vessel, so that the abrading conditions can be easily controlled by adjusting the inner pressure of the pressure vessel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of examples and with reference to the accompanying drawings, in which: 
         FIG. 1  is a front sectional view of an abrasive machine of an embodiment of the present invention; 
         FIG. 2  is a plan view of the abrasive machine, in which a lid is opened; 
         FIG. 3  is a plan view of a bell jar; 
         FIG. 4  is an explanation view a pressure source connected to the bell jar; 
         FIG. 5  is an explanation view of a driving unit of another example; 
         FIG. 6  is an explanation view of the driving unit of other example; 
         FIG. 7  is an explanation view of a mechanism for moving an abrasive plate; 
         FIG. 8  is a sectional view of a press-type pressing plate; 
         FIG. 9  is a graph showing a relationship between air pressure and abrasion rate; 
         FIG. 10  is a graph showing a relationship between oxygen gas pressure and abrasion rate; 
         FIG. 11  is a graph showing a relationship between nitrogen gas pressure and abrasion rate; 
         FIG. 12  is a graph showing a relationship between argon gas pressure and abrasion rate; 
         FIG. 13  is a graph of rate of abrading a copper layer in various gas atmospheres; 
         FIG. 14  is a graph of rate of abrading an Si substrate in the various gas atmospheres; 
         FIG. 15  is a graph of rate of abrading an SiO 2  layer in the various gas atmospheres; 
         FIG. 16  is a sectional view of copper wires implanted in a barrier metal layer; and 
         FIG. 17  is a sectional view of the implanted copper wires exposed. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a front sectional view of an abrasive machine of an embodiment of the present invention; 
       FIG. 2  is a plan view of the abrasive machine, in which a lid is opened; and  FIG. 3  is a plan view of a bell jar. 
     The bell jar  12  has a lid  14  and acts as a pressure vessel capable of bearing increase and reduce of pressure therein. The lid  14  is pivotably attached to a body proper  16  of the bell jar  14  by a shaft  15  so as to open and close the body proper  16 . 
     A lower end of a clamping bolt  18  is pivotably attached to the body proper  16  by a shaft  21 ; an upper end of the bolt  18  is capable of entering a gap between U-shaped forks of a fixed arm  19 . By turning a nut  20 , the lid  14  can air-tightly close the body proper  16 . In the present embodiment, six clamping bolts  18  are provided with angular separations of 60 degrees. 
     The body proper  16  is made of steel having a prescribed thickness and formed into a bottomed cylindrical shape. A top plate of the lid  14  is curved upward. With this pressure-resisting structure, the bell jar  12  can act as the pressure vessel. A bottom section  16   a  of the body proper  16  is a flat plate, and its thickness is much thicker than that of a cylindrical section so as to bear inner pressure. 
     Note that, the shape of the bell jar  12  is not limited to the cylindrical shape. Other pressure vessels which have enough pressure-resisting structure can be employed in the present invention. 
     An abrasive plate  23  is provided in the bell jar  12 . 
     An abrasive cloth or an abrasive pad (not shown), which is made of a known material, is adhered on an upper face of the abrasive plate  23 . 
     A connecting member  24 , which is formed into a cylindrical shape, is fixed on a bottom face of the abrasive plate  23 . The connecting member  24  is connected to a rotary shaft  26 , which is rotatably held by a bearing  25  of the bottom section  16   a,  by a key  27 . With this structure, the abrasive plate  23  is rotated together with the rotary shaft  26 . A symbol  28  stands for a sealing member. 
     A lower end part of the abrasive plate  23  is supported by a thrust bearing  29 . A supporting member  30  is provided on the bottom section  16   a,  and the thrust bearing  29  is provided on the supporting member  30 . 
     A cover  31  encloses an outer circumferential face of the abrasive plate  23  so as to remain prescribed amount of slurry on the abrasive plate  23 . Note that, the cover  31  may be omitted. 
     A supporting base  32  supports the bell jar  12  and has four legs  32   a.  An adjustable bolt  33  is provided to a lower end of each leg  32   a  so as to adjust height of the supporting base  32  and levelness of the bell jar  12 . 
     A motor  35 , which acts as a driving unit, is attached to the supporting base  32 . A motor shaft of the motor  35  is connected to the rotary shaft  26 , so that the motor  35  can rotate the abrasive plate  23 . In the present embodiment, the motor  35  is provided outside of the bell jar  12 , but the motor  35  may be provided in the bell jar  12 . 
     A pressing plate  36  for pressing a work piece (not shown) is provided on the abrasive plate  23 . The pressing plate  36  applies own weight to the abrasive plate  23  as a pressing force. The work piece to be abraded is set or sandwiched between the abrasive plate  23  and the pressing plate  36 . 
     A plurality of weights  37  are mounted on the pressing plate  36  so as to adjust the pressing force. The weights  37  act as a press unit for applying pressure to the work piece. 
     Note that, number of the weights  37  is optionally determined on the basis of abrading conditions. 
     A roller  38 , which is coaxial with the abrasive plate  23 , and a roller, which is provided above an outer edge of the abrasive plate  23 , contact an outer edge of the pressing plate  36 , so that the pressing plate  36  can be held at a prescribed position on the abrasive plate  23 . The rollers  38  and  39  are rotatably held by an arc-shaped arm  40  provided in the bell jar  12 . 
     In  FIG. 2 , the abrasive plate  23  is rotated in a direction “A”. By rotating the abrasive plate  23 , the pressing plate  36  too is rotated, about its own axis, in the same direction. 
     Note that, the roller  38  may be rotated by a motor (not shown) so as to rotate the pressing plate  36 , which contacts the roller  38 , in a prescribed direction. 
     A proper amount of slurry is stored in the body proper  16 . In the present embodiment, a lower part of the body proper  16  acts as a slurry storing section  16   b  (see  FIG. 4 ). 
     As shown in  FIG. 4 , the slurry stored in the body proper  16  is circulated by a circulation pump  43 . 
     The circulation pump  43  is connected to a pipe  44 , which is connected to the slurry storing section  16   b,  and a pipe  45 , which is connected to an upper part of the body proper  16 . The slurry stored in the slurry storing section  16   b  is drawn by the pump  43  and supplied onto the abrasive plate  23  via the pipe  45 . The slurry, which has been used to abrade the work piece, is collected in the slurry storing section  16   b.    
     The slurry storing section  16   b,  the circulation pump  43  and the pipes  44  and  45 , etc. constitute a slurry supplying unit. Note that, a symbol  44   a  shown in  FIG. 1  stands for a connecting section of the pipe  44 . 
     The slurry storing section  16   b,  of course, may be provided outside of the bell jar  12 . 
     In  FIG. 4 , a pressurizing unit  47  and a pressure reduction unit  48  constitute a pressure source. 
     The pressurizing unit  47  is connected to the body proper  16  via a pipe  49  so as to introduce a pressurized fluid into the bell jar  12 . In the present embodiment, air, oxygen, nitrogen and argon gas are employed as the fluid. Other gasses may be optionally employed. The fluids are selected and supplied into the bell jar  12  by a switching valve (not shown). A pressure reduction valve  51  is provided so as to supply the fluid into the bell jar  12  with predetermined pressure. Symbols  52  and  53  are valves, and a symbol  54  is a flow control valve capable of controlling amount of flow of the fluid. 
     Note that, a mixed gas may be employed as the fluid. 
     The pressure reduction unit  48  is connected to a part of the pipe  49 , which is located between the valves  52  and  53 . A symbol  56  stands for a valve. 
     The pressure reduction unit  48  includes a vacuum pump. 
     Note that, a symbol  49   a  shown in  FIG. 1  stands for a connecting section of the pipe  49 . 
     By closing the valve  56  and opening the valves  52  and  53 , the pressurized fluid can be introduced into the bell jar  12 , so that inner pressure of the bell jar  12  can be increased. On the other hand, by closing the valve  52  and opening the valves  53  and  56 , the pressure reduction unit  48  sucks the fluid in the bell jar  12 , so that the inner pressure of the bell jar  12  can be reduced. 
     A pressure gauge  57 , which acts as a measuring equipment, measures the inner pressure of the bell jar  12 . Other equipments for measuring temperature, humidity, etc. may be provided if required. 
     A safety valve  58  releases the pressurized fluid outside when the inner pressure of the bell jar  12  exceeds a prescribed value. A symbol  60   a  stands for a viewing window (see  FIG. 3 ). 
     Another example of the driving unit is shown in  FIG. 5 . 
     A motor  64  including a stator  59  and a rotor  63  is provided in the bell jar  12 , and the abrasive plate  23  is fixed on the rotor  63 . A motor driver  65  is provided outside of the bell jar  12 , and electric power is supplied to stator coils via wires  66 . Note that, the motor  64  is a known electric motor. 
     In this driving unit, only the wires  66  should be sealed, therefore the sealing mechanism can be simplified. 
     Further, another example of the driving unit is shown in  FIG. 6 . 
     In this example, the abrasive plate  23  is rotated by magnetic coupler means. Namely, a first magnet rotor  67 , in which North magnetic poles and South magnetic poles are alternately formed on an outer circumferential face, is rotated by a motor  68 . By rotating the first magnet rotor  67 , a second magnet rotor  69  is rotated. The abrasive plate  23  is fixed on the second magnet rotor  69 . 
     With this structure, the abrasive plate  23  can be rotated without contacting any members located outside, therefore an inner space of the bell jar  12  can be clean. 
     In the present embodiment, the abrasive plate  23  is rotated about its own axis. In another embodiment, the abrasive plate  23  may be moved in a plane parallel to an abrasive face (the upper face) of the abrasive plate  23 . This embodiment is shown in  FIG. 7 . 
     In  FIG. 7 , a plurality of crank shafts  70  are attached to the abrasive plate  23 , and the crank shafts  70  are synchronously rotated by a driving unit (not shown), which is provided outside of the bell jar  12 . With this structure, the abrasive plate  23  can be moved along a circular orbit with fixed heading. Namely, all points in the abrasive plate  23  equally rotate in a direction “B”. 
     In the above described embodiment, the work piece is merely pressed onto the abrasive plate  23  by the pressing plate  36 . The work piece may be adhered on a bottom face of the pressing plate  36 . In this case, the abraded work piece is peeled from the pressing plate  36  when the abrasion is completed. 
     The pressing member  36  may have sucking means for holding the work piece by producing negative pressure. In this case, the sucking means may suck and hold the work piece directly or with an elastic bucking member. 
     In the above described embodiment, the weights  37  are employed as the press unit. A cylinder unit (not shown) provided on the arm  40  may be employed to apply pressure to the work piece. 
     Further, a pressure head-type pressing plate may be employed. An example of the pressure head-type pressing plate  36  is shown in  FIG. 8 . 
     A holding member  73  is suspended in a head proper  72  by an elastic ring member  74 , e.g., diaphragm. With this structure, a pressure chamber  75  is formed. The pressurized fluid is introduced into the pressure chamber  75 , so that the work piece held on a bottom face of the holding member  73  is pressed onto the abrasive plate  23 . Preferably, the pressing plate  36  is rotated about a rotary shaft  76  by a motor (not shown). A driving mechanism including the motor may be provided on the arm  40 . 
     Further, the pressing plate  36  may be vertically moved by a cylinder unit (not shown) so as to move to and away from the abrasive face (the abrasive cloth) of the abrasive plate  23 . In this case, the rotary shaft  76  may be rotatably held by a holding arm (not shown), and the holding arm may be vertically moved by a cylinder unit (not shown) provided on the arm  40 . 
     The driving mechanism allows the rotary shaft  76  to vertically move in a prescribed range and transmits torque of the motor. 
     The pressurized fluid is introduced into the pressure chamber  75  via a fluid path  77  formed in the rotary shaft  76 . The fluid is introduced into the fluid path  77  via a rotary joint (not shown). 
     A restraining ring  78  prevents the holding member  73  from coming out from the head proper  72  and guides the vertical movement of the holding member  73 . 
     An O-ring  79  is provided between an inner circumferential face of the head proper  72  and an outer circumferential face of the holding member  73 . The O-ring  79  absorbs horizontal movement of the holding member  73  and prohibits the slurry to enter the head proper  72 . 
     Experiments were executed in the abrasive machine  10  under the following conditions. Note that, the inner air pressure of the bell jar  12  was varied; and the copper layer, the SiO 2  layer and the Si substrate of the work piece were abraded. 
     The conditions were,
         Abrasive cloth: IC1000/SUBA400 (trade name), diameter 200 mm;   Slurry: silica slurry “SS-25” for SiO 2  
           colloidal silica “Compol-80” for Si   alumina slurry for Copper;   
           Pressing force of the pressing plate  36 : 100–500 g/cm 2 ;   Rotational speed of the abrasive plate  23 : 15–90 rpm; and   Abrasion time: 2–4 min.       

     The work piece were abraded with the fixed pressing force, the fixed rotational speed and the fixed abrasion time under above conditions. The results are shown in  FIG. 9 . 
     In  FIG. 9 , the inner pressure of zero is the atmospheric pressure. Namely, the horizontal axis or the inner pressure of the bell jar  12  indicates the pressure added to and reduced from the atmospheric pressure. 
     As clearly shown in  FIG. 9 , abrasion rate under the atmospheric pressure was minimum; the abrasion rate was increased in nearly proportion to increasing and reducing the inner pressure. 
     Especially, in the case of abrading the SiO 2  layer and the Si substrate, the abrasion rate of 200 KPa was about twice as great as that of the atmospheric pressure; and the abrasion rate of 500 KPa was about 2.5 times as great as that of the atmospheric pressure. 
     In the case of abrading the copper layer, the minimum abrasion rate appeared on the negative pressure side (about −50 KPa). Namely, the minimum abrasion rate was slightly shifted toward the negative pressure side, but the abrasion rate was increased on the both sides of the minimum as well as the SiO 2  layer and the Si substrate. 
     The inventor thinks that the reasons of increasing the abrasion rate under the positive pressure are: the fluid pressure is applied to the pressing plate  36 ; and the slurry is permeated into the abrasive cloth by the fluid pressure. 
     The reason of increasing the abrasion rate under the negative pressure is not clearly found. The inventor thinks that frictional heat between the work piece and the abrasive cloth is hardly radiated due to pressure reduction so that temperature rises and reaction rate is increased. By increasing the reaction rate, the abrasion rate is increased under the negative pressure. 
       FIG. 10  is a graph showing a relationship between oxygen gas pressure and the abrasion rate. Oxygen was used as the fluid instead of the air. 
     Tendency of the case of employing oxygen is nearly equal to that of the case employing the air. Especially, in the case of abrading the copper layer, the abrasion rate was much increased under high pressure. 
     According to the results, the abrasion rate can be controlled by adjusting the inner pressure of the bell jar  12  without changing other conditions. 
     For example, when the abrasion is started and the work piece is roughly abraded, the inner pressure of the bell jar  12  is increased or reduced so as to abrade the work piece with high abrasion rate; when the work piece is finished and the work piece, the inner pressure of the bell jar  12  is returned to zero or the atmospheric pressure so as to abrade the work piece with low abrasion rate. 
     Of course, the abrasion rate may be controlled by combining other factors, e.g., the rotational speed of the abrasive plate  23 . 
     In the case of using a plurality of kinds of slurry or abrasive cloth, a plurality of abrading stations are provided in one abrasive machine, so that the abrasive machine must be large. However, the inner pressure of the bell jar  12  and the rotational speed of the abrasive plate  23  can be changed at one abrading station, so number of the abrading stations can be reduced, the abrading conditions can be easily determined, a size of the abrasive machine can be smaller and manufacturing cost of the machine can be reduced. 
     The slurry accommodated in the bell jar  12  is pressurized and circulated, so load of the circulation pump  43  is not so great. 
     If the slurry storing section is provided outside of the bell jar  12 , the slurry is introduced into the bell jar  12  whose inner pressure has been increased, so that a high power circulation pump is required. 
     The slurry may stay in the bell jar  12 . In this case, the abrasive plate  23  is inclined with respect to the horizontal plane, by adjusting the adjustable bolts  33 , so as to dip a lower part of the surface of the abrasive plate  23  in the slurry. With this structure, the slurry can be always permeated into the abrasive cloth for abrading the work piece. 
       FIG. 11  is a graph showing a relationship between nitrogen gas pressure and the abrasion rate. An inert gas, e.g., nitrogen, was used as the fluid instead of the air. The air in the bell jar  12  was purged by nitrogen, then the inner pressure was increased and reduced. 
     Under the negative pressure, the abrasion rate was increased as well as the case of employing air and oxygen (see  FIGS. 9 and 10 ). 
     On the other hand, under the positive pressure, especially in the case of abrading the copper layer, the abrasion rate was reduced until 400 KPa. 
     The inventor thinks that the copper layer is easily oxidize, therefore a mechanism of the abrasion under the non-oxygen atmosphere (see  FIG. 11 ) is different from that under the oxygen atmosphere (see  FIGS. 9 and 10 ). Namely, under the oxygen atmosphere, the copper layer is etched by the slurry and the oxidation, so that the abrasion rate is high; under the non-oxygen atmosphere, the copper layer is etched by the slurry only, so that the abrasion rate is low. 
       FIG. 12  is a graph showing a relationship between argon gas pressure and the abrasion rate. Argon was used as the fluid. 
     As clearly shown in the drawing, tendency of the case of employing the argon gas is nearly equal to that of the case employing the nitrogen gas. 
       FIG. 13  is a graph of the rate of abrading the copper layer in various gas atmospheres. 
       FIG. 14  is a graph of the rate of abrading the Si substrate in the various gas atmospheres. 
       FIG. 15  is a graph of the rate of abrading the SiO 2  layer in the various gas atmospheres. 
     By properly selecting the pressurized fluid, the abrasion rate can be controlled by adjusting the fluid pressure only. In the case of selectively employing the fluids (gasses), a plurality of gas supplying units which respectively supply different gasses are provided in one abrasive machine  10 , and the gas supplying units are selected by a switching valve. 
     A method of abrading implanted copper wires, which are insulated by the SiO 2  layer, will be explained with reference to  FIG. 16  as an example. 
     A barrier metal layer  61  prevents the copper from diffusing into the SiO 2  layer  60 . The barrier metal layer  61  is made of tantalum nitride (TaN) or made by spattering tantalum (Ta). The copper layer  62  is made by electrolytic plating, etc. 
     The copper layer  62  is abraded, for example, in the pressurized air, with high abrasion rate until the barrier metal layer  61  is exposed. 
     A metal constituting the barrier metal layer  61  is harder than copper, if the abrasion is further continued, the copper layer  62  is abraded more, so that of the implanted wires will be too thin. 
     Thus, for example, the copper layer  62  is abraded in the pressurized nitrogen (see  FIG. 11 ) with low abrasion rate; the barrier metal layer  61  is abraded with high abrasion rate. With this manner, the barrier metal layer  61  and the copper layer  62  can be abraded at the same abrasion rate, so that proper implanted wires  62   a  can be formed as shown in  FIG. 17 . 
     The fluid in the bell jar  12  can be changed in one abrasion cycle so as to change the abrasion rate, so that the abrading conditions can be easily changed. The rotational speed of the abrasive plate  23 , of course, may be controlled simultaneously. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by he foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.