Patent Publication Number: US-2003230323-A1

Title: Apparatus and method for improving scrubber cleaning

Description:
FIELD OF THE INVENTION  
       [0001] The present invention generally relates to a method for scrubber cleaning semiconductor wafers having a surface coating and more particularly, to an apparatus and method for ensuring thorough cleaning of the center of a semiconductor wafer during a scrubber cleaning process to prevent excess oxide growth on the center of the wafer.  
       BACKGROUND OF THE INVENTION  
       [0002] In the fabrication process for semiconductor devices, numerous fabrication steps, as many as several hundred, must be executed on a silicon wafer in order to complete integrated circuits on the wafer. Since the processing of silicon wafers requires extreme cleanliness in the processing environment to minimize the presence of contaminating particles or films, the surface of the silicon wafer is frequently cleaned after each processing step. For instance, the wafer surface is cleaned after the deposition of a surface coating layer such as oxide or after the formation of a circuit by a processing step such as etching. A frequently-used method for cleaning the wafer surface is a wet scrubbing method.  
       [0003] In cleaning a wafer surface by a wet scrubbing method, a wafer is rotated at a high speed, i.e., at least about 200 RPM and preferably, about 1,000 RPM, simultaneously with a jet of high-pressure deionized water sprayed on top. The water jet is normally sprayed at a pressure of about 2,000-3,000 psi. The water movement on top of the wafer surface displaces any contaminating particles that are lodged on the wafer surface. One limitation of the wafer jet scrubbing method is that the process only moves particles from side to side in openings, such as oxide windows, without removing the particle. Furthermore, as the image size decreases, it becomes more difficult for water to reach the particles in openings because of increased surface tension.  
       [0004] It has also been noted that in a water jet scrubbing process conducted on a silicon wafer that is coated with an insulating material, i.e., an oxide layer as an inter-metal dielectric layer, some regions of the film are damaged at the wafer center by the cumulated stress from the water jet when the aperture size of the jet nozzle is too large or is distorted. The damaged film can be identified by a KLA scan, even though a large number of wafers must be tested since the probability of such damage is only about 10-30%.  
       [0005]FIG. 1 illustrates a silicon wafer  10  the upper surface of which is scanned in a waterjet scrubbing method using a conventional wafer scrubbing apparatus  8 . The wafer  10  is normally positioned on a wafer platform  17  which is typically rotatably mounted on a wafer stage  16 . The wafer platform  17  rotates the wafer  10  at a predetermined rotational speed, which may be between about 200 RPM and about 2,000 RPM. A water jet  22  of deionized water is ejected from a water jet nozzle  26  typically mounted on a nozzle rack  28  above the surface of the wafer  10 . The water jet  22  has a water pressure of typically about 50 kg/cm 2 . As it strikes the surface of the wafer  10  at an angle of typically about 45°, the water jet  22  is scanned along a top of the wafer surface by a lateral sweeping motion of the water jet nozzle  26  to define a generally curved or arcuate trace  12  which normally traverses the center  14  of the wafer  10 , as illustrated in FIG. 2. The surface of the wafer  10  is scanned by the water jet  22  at least once, and preferably, several times. Centrifugal force acting on the water flow on the surface of the wafer  10  due to the rotating wafer platform  17  and wafer  10  removes contaminating particles or films from the surface of the wafer  10 .  
       [0006] When the coating on the surface of the wafer is fused silicate glass (FSG) or other low-density film, stress defects tend to occur at the wafer center due to the cumulated stress from the water jet striking the wafer. Furthermore, organic particles tend to remain at the wafer center due to the reduced centrifugal force acting on the particles at the center of the wafer during the scrubbing process. These factors tend to increase the quantity of native oxide growth at the center region compared to other regions of the wafer, as measured by ellipsometer measurements. Moreover, the increased oxide thickness at the wafer center causes a reduced electrical charge at the wafer center as compared to the electrical charge at the wafer edge.  
       [0007] It has been found that horizontal movement of the wafer stage beneath the water jet nozzle during the scrubbing process provides a more uniform dispersement of the sprayed water along the entire surface of the disc. This has been found to substantially improve removal of organic particles from the wafer which would otherwise tend to remain at the wafer center due to reduced centrifugal force at the center, as well as reduce the water spray-induced damage to low-density film coatings at the wafer center by spreading the impact energy of the spray across a larger surface area on the wafer.  
       [0008] Accordingly, an object of the present invention is to provide an apparatus and method for substantially reducing the presence of particles remaining at the center of a semiconductor wafer after a wafer scrubbing process.  
       [0009] Another object of the present invention is to provide an apparatus and method for substantially reducing the possibility of wafer damage during a wafer scrubbing process.  
       [0010] Still another object of the present invention is to provide an apparatus and method for improving the efficacy of a scrubber cleaning process in the fabrication of integrated circuits on semiconductor wafers.  
       [0011] Yet another object of the present invention is to provide an apparatus and method which spreads impact energy of water or other fluid sprayed onto a wafer surface over a large surface area on the wafer to prevent or minimize the likelihood of impact damage to the wafer center during a wafer scrubbing operation.  
       [0012] A still further object of the present invention is to provide an apparatus and method for reducing excessive oxide growth at the center of a semiconductor wafer due to defects in the wafer scrubbing operation.  
       [0013] Yet another object of the present invention is to provide a method and apparatus which utilizes horizontal linear movement combined with a spinning motion of a semiconductor wafer to both reduce the presence of particles remaining at the center of a semiconductor wafer after a wafer scrubbing process and prevent or minimize the likelihood of cleaning water or fluid spray impact damage to the wafer center during the wafer scrubbing process.  
       [0014] Still another object of the present invention is to provide a method and apparatus which utilizes linear motion of a spinning wafer or a jet nozzle with respect to the other to disperse a pressurized fluid jet sprayed onto the surface of the wafer over substantially the entire surface of the wafer and thereby reduce the presence of particles remaining at the center of the wafer after a wafer scrubbing operation as well as reduce the possibility of spray-induced damage to the wafer center during the operation.  
       SUMMARY OF THE INVENTION  
       [0015] These and other objects and advantages are provided in a method and apparatus comprising a wafer platform which rotates a semiconductor wafer at a predetermined speed while being moved in a horizontal linear motion with respect to a stationary jet nozzle spraying a water or fluid jet onto the wafer during a wafer scrubbing process. The coupled rotary and linear motions of the wafer facilitates through washing or rinsing of the wafer surface and spreads impact energy of water or fluid sprayed onto a wafer surface over a large surface area on the wafer, resulting in a substantial reduction of particles remaining at the center of the wafer after the wafer scrubbing operation and preventing or minimizing the likelihood of impact damage to the wafer center during the wafer scrubbing process. In another embodiment, the water or fluid jet nozzle moves along a horizontal axis while the spinning wafer remains stationary.  
       [0016] A method of the present invention may be carried out by the operating steps of providing a semiconductor wafer having a film layer coated on top thereof, positioning the wafer on a wafer holder or platform in a scrubbing chamber, and scanning a water jet in multiple passes across the film on the wafer while both rotating the wafer and moving the wafer holder or platform in a horizontal linear motion with respect to the water jet.  
       [0017] Another method of the present invention may be carried out by the operating steps of providing a semiconductor wafer having a film layer coated on top thereof, positioning the wafer on a wafer holder or platform in a scrubbing chamber, and scanning a water jet in multiple passes across the film on the wafer while both rotating the wafer and moving the water or fluid jet in a horizontal linear motion with respect to the spinning wafer.  
       [0018] The method of improving wafer scrubbing of the present invention may further include the steps of providing a jet nozzle and ejecting a jet of water or other scrubbing fluid from the jet nozzle onto the surface of the film coating the upper surface of the wafer. The semiconducting wafer may be a silicon wafer, and the film coated on the upper surface of the wafer may be fused silicate glass (FSG) or any other lower density film. The jet may be formed of deionized water and may have a water pressure of at least about 50 kg/cm 2 .  
       [0019] The method of the present invention may further include the step of scanning a water or fluid jet in multiple passes across the surface of the disc as the disc is rotated at speeds of typically between about 200 RPM and about 2,000 RPM. The disc is simultaneously moved in a horizontal linear manner with respect to the water or fluid jet at speeds typically of from about 1 cm/min. to about 10 cm/min. The multiple passes may be made across a surface area corresponding to at least about one half of the disc upper surface area or across the entire disc upper surface area. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020] The invention will now be described, by way of example, with reference to the accompanying drawings, wherein:  
     [0021]FIG. 1 is an illustration of a conventional apparatus and method for cleaning a wafer positioned in a wet scrubber by a water jet traversing the top surface and through the center of the wafer;  
     [0022]FIG. 2 illustrates a trace made by a water jet traversing the top surface and through the center of a wafer, according to the conventional apparatus and method of FIG. 1;  
     [0023]FIG. 3 illustrates use of an apparatus and method of the present invention for cleaning a wafer positioned in a wet scrubber by a water jet traversing the top surface of the wafer;  
     [0024]FIG. 4 illustrates multiple traces made by a water jet traversing half of the top surface of a wafer, according to the apparatus and method of the present invention;  
     [0025]FIG. 5 illustrates multiple traces made by a water jet traversing the entire top surface of a wafer, according to the apparatus and method of the present invention; and  
     [0026]FIG. 6 illustrates another embodiment of the wafer scrubbing apparatus of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0027] The present invention comprises an apparatus and method for substantially improving the scrubber cleaning process of semiconductor wafers typically having a thin insulating film deposited on the upper surface thereof. The present invention facilitates more uniform scrubbing of all regions on the surface of the film, to prevent or minimize the possibility of film damage and remove organic film particles which otherwise have a tendency to remain on the film at the center region of the disc after the scrubbing operation and contribute to excessive oxide film growth at the wafer center.  
     [0028] Referring initially to FIG. 3 of the drawings, a wafer scrubbing apparatus of the present invention is generally indicated by reference numeral  32  and typically includes a wafer stage  34 , fitted with a rotatable wafer turntable or platform  36 . An elongated rack  38 , provided with multiple rack teeth  40 , is mounted on the bottom surface of the wafer stage  34 . The rack teeth  40  mesh with multiple pinion teeth  44  provided around the circumference of a circular pinion  42 . The pinion  42  may be rotatably mounted on any suitable support  48  and is engaged by a synchronized motor  46  or other suitable powering mechanism for the pinion  42 . The wafer stage  34  is further typically slidably or rollably mounted on horizontal track or tracks  58 . Accordingly, the synchronized motor  46  can be operated to rotate the pinion  42  which, in turn, drives the wafer stage  34  via the rack  38  in a selected direction and at a selected speed along a substantially horizontal plane or axis, as indicated by the double-headed arrow  60 . A process controller  62  with enabling software may be operably connected to the synchronized motor  46  typically by means of suitable wiring  64  for controlling the speed and direction of horizontal linear travel of the wafer stage  34  on the track or tracks  58 . A jet nozzle  26  is mounted typically on a nozzle rack  28  above the surface  56  of a wafer  52  resting on the wafer platform  36 , and remains stationary with respect to the wafer  52 , wafer platform  36  and wafer stage  34  throughout a wafer-scrubbing process as hereinafter described. Alternatively, the jet nozzle  26  may be adapted for linear motion along a horizontal axis while the spinning wafer platform  36  and wafer  52  remain stationary. While the rack  38  and pinion  42  heretofore described are capable of driving the wafer stage  34  horizontally, it is understood that any other suitable mechanism known by those skilled in the art may be used to advance the wafer stage  34  horizontally along the track or tracks  58  as heretofore described.  
     [0029] Referring next to FIGS. 3 and 4, according to a typical method of using the wafer scrubbing apparatus  32  of the present invention, a wafer  52  having a film  56  coated on the upper surface thereof is initially positioned on the wafer platform  36 . The film  56  may be fused silicate glass (FSG) or any other low-density insulative film coated on the wafer  52 . As the wafer platform  36  is rotated on the wafer stage  34  at a speed of from about 200 rpm to about 2,000 rpm, and preferably, about 1,000 rpm, the synchronized motor  46  is operated to drive the wafer stage  34  along the tracks  58  at a speed of from about 1 cm/min. to about 10 cm/min., and preferably, at about 2.5 cm/min. Simultaneously, a fluid jet  22  which is typically but not necessarily deionized water is ejected from the jet nozzle  26  onto the film  56  on the surface of the wafer  52 . The water jet  22  ejected from the jet nozzle  26  may have a pressure of at least about 50 kg/cm 2 , and the jet nozzle  26  is typically moved in a lateral, sweeping motion to eject the water jet  22  onto the film  56  in such a manner as to define a curved trace  50  initially across the center  54  of the wafer  52 . After that, multiple traces  50  are successively defined by the sweeping water jet  22  across at least one half of the surface area of the film  56  on the wafer  52  as illustrated in FIG. 4, as the wafer stage  34  is advanced horizontally along the tracks  58  for a distance which corresponds to the radius of the wafer  52 . Accordingly, in the case of a 100 mm diameter wafer  52 , the wafer stage  34  is advanced a total distance of 5 cm, which is the diameter of the wafer  52 . Alternatively, the wafer stage  34  may be advanced along the tracks  58  for a distance which corresponds to the diameter of the wafer  52 , typically 10 cm, in which case the sweeping water jet  22  defines multiple traces  50  across the entire surface area of the film  56  on the rotating wafer  52 , as illustrated in FIG. 5. The multiple traces  50  may be made on the film  56  across the various sections of the wafer  52  either once or multiple times, as needed. It has been found that defining the multiple traces  50  across the film  56  on the wafer  52 , facilitated by horizontal movement of the wafer stage  34 , the laterally-sweeping motion of the jet nozzle  26  and the rotating motion of the wafer  52 , provides a more uniform dispersement of the sprayed water or other scrubbing fluid along the entire surface of the film  56 . This has been found to substantially improve removal of organic particles from the wafer  52  which would otherwise tend to remain at the wafer center  54  due to reduced centrifugal force at the wafer center  54 , as well as reduce water or fluid jet-induced damage to the low-density film  56  particularly at the wafer center  54  by spreading the impact energy of the water jet  22  across a relatively large surface area on the film  56 .  
     [0030] Referring next to FIG. 6, another embodiment of the wafer scrubbing apparatus of the present invention is generally indicated by reference numeral  68  and includes a rotatable multi-wafer turntable or platform  70  which is drivingly engaged by a drive motor or other drive mechanism  74  through a drive belt or chain  76 . A jet nozzle  72  is mounted above the platform  70  for directing a water or fluid jet  82  sequentially onto each of multiple wafers  78  supported on the platform  70  as the platform  70  is rotated by operation of the drive motor  74 . Furthermore, the jet nozzle  72  is mounted for bidirectional horizontal axial movement above and along the radius of the platform  70 , as indicated by the arrow  80 . This axial motion of the jet nozzle  72  may be accomplished by operation of a rack and pinion arrangement (not shown), as heretofore described with respect to the wafer scrubbing apparatus  32  of FIG. 3, or any other suitable method known by those skilled in the art. It is understood that the wafer platform  70  instead of the jet nozzle  72  may be adapted for horizontal axial movement in an alternative embodiment of the invention. Accordingly, defining multiple traces  84  of the water jet  82  across a film  79  on the upper surface of the wafer  78  is facilitated by horizontal axial movement of the water jet nozzle  72 , combined with the rotating motion of the platform  70 . This provides a substantially uniform dispersement of the sprayed water along the entire surface of the film  79  on the wafer  78 . Accordingly, removal of organic particles from the wafer  78  which would otherwise tend to remain at the wafer center  86  due to reduced centrifugal force at the wafer center  86 , is significantly enhanced. Moreover, water jet-induced damage to the low-density film  79  on each wafer  78 , particularly at the wafer center  86 , is substantially minimized or eliminated by spreading the impact energy of the water jet  82  across a relatively large surface area on the film  79 .  
     [0031] While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications can be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.  
     [0032] Having described our invention with the particularity set forth above, we claim: