Patent Publication Number: US-2006000109-A1

Title: Method and apparatus for reducing spin-induced wafer charging

Description:
FIELD OF THE INVENTION  
      The present invention relates to methods for drying wafers during the fabrication of semiconductor integrated circuits on the wafers. More particularly, the present invention relates to a method and apparatus for reducing wafer charging during spin-drying of a wafer.  
     BACKGROUND OF THE INVENTION  
      The fabrication of various solid state devices requires the use of planar substrates, or semiconductor wafers, on which integrated circuits are fabricated. The final number, or yield, of functional integrated circuits on a wafer at the end of the IC fabrication process is of utmost importance to semiconductor manufacturers, and increasing the yield of circuits on the wafer is the main goal of semiconductor fabrication. After packaging, the circuits on the wafers are tested, wherein non-functional dies are marked using an inking process and the functional dies on the wafer are separated and sold. IC fabricators increase the yield of dies on a wafer by exploiting economies of scale. Over 1000 dies may be formed on a single wafer which measures from six to twelve inches in diameter.  
      Various processing steps are used to fabricate integrated circuits on a semiconductor wafer. These steps include deposition of a conducting layer on the silicon wafer substrate; formation of a photoresist or other mask such as titanium oxide or silicon oxide, in the form of the desired metal interconnection pattern, using standard lithographic or photolithographic techniques; subjecting the wafer substrate to a dry etching process to remove the conducting layer from the areas not covered by the mask, thereby etching the conducting layer in the form of the masked pattern on the substrate; removing or stripping the mask layer from the substrate typically using reactive plasma and chlorine gas, thereby exposing the top surface of the conductive interconnect layer; and cooling and drying the wafer substrate by applying water and nitrogen gas to the wafer substrate.  
      The numerous processing steps outlined above are used to cumulatively apply multiple electrically conductive and insulative layers on the wafer and pattern the layers to form the circuits. The final yield of functional circuits on the wafer depends on proper application of each layer during the process steps. Proper application of those layers depends, in turn, on coating the material in a uniform spread over the surface of the wafer in an economical and efficient manner.  
      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.  
      In an SRD cleaning process, a wafer is normally positioned on a wafer platform which is typically rotatably mounted on a wafer stage. The wafer platform rotates the wafer at a predetermined rotational speed, which may be between typically about 30 RPM and about 5,000 RPM. Simultaneously, a water jet of de-ionized water is ejected onto the upper surface of the rotating wafer from a nozzle opening in the nozzle.  
      As it strikes the surface of the wafer, the water jet is typically scanned along a top of the wafer surface by a lateral sweeping motion of the water jet nozzle to define a generally curved or arcuate trace which normally traverses the center of the wafer. The surface of the wafer is scanned by the water jet at least once, and preferably, several times. Centrifugal force acting on the water flow on the surface of the wafer due to the rotating wafer platform and wafer removes contaminating particles or films from the surface of the wafer. 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. After completion of the jet-scrubbing process, the wafer is subjected to a spin-drying step in which the wafer is rotated and nitrogen or clean dry air (CDA) is blown against the wafer surface.  
       FIG. 1  illustrates a silicon wafer  10  the upper surface of which is sprayed in an SRD (spin-rinse-dry) waterjet scrubbing method using a conventional wafer scrubbing apparatus  8 . The wafer  10  is normally positioned on a wafer platform  12  which is supported by a shaft  14 . The shaft  14  is engaged by a motor  16  for rotation. The wafer platform  12  rotates the wafer  10  at a predetermined rotational speed, which may be between about 30 RPM and about 5,000 RPM. A water jet  18  of deionized water is ejected from a water jet nozzle  20  positioned above the surface of the wafer  10 . The water jet  18  has a water pressure of typically about 50 kg/cm 2 . As it strikes the surface of the wafer  10 , the water jet  18  may be scanned along a top of the wafer surface by a lateral sweeping motion of the water jet nozzle  20  to define a generally curved or arcuate trace which normally traverses the center of the wafer  10 . The surface of the wafer  10  is scanned by the water jet  18  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  12  and wafer  10  removes contaminating particles or films from the surface of the wafer  10 .  
      After the rinsing step is completed, the wafer  10  is subjected an SRD drying step. Accordingly, the wafer  10  remains on the wafer platform  12 . The motor  16  rotates the shaft  14  and wafer platform  12  in one direction, as indicated by the curved arrows, at rotational speeds of up to 4,000 rpm. This evaporates the residual rinsing water from the surface of the wafer  10 .  
      One of the limitations of the SRD drying step is that rotation of the wafer frequently results in electrostatic charging of the wafer surface. The presence of electrostatic charges on the surface of the wafer increases particle contamination of the wafer surface. This ultimately leads to defect densities in the finished chips or die fabricated on the wafer, as revealed by chip testing. Furthermore, electrostatic charges on the surface of the wafer frequently interfere with the operation of production equipment, thus decreasing up-time, interrupting process flow or requiring re-processing of the semiconductor products. The problems caused by electrostatic charges tend to be more of a problem in photolithography areas than in other areas of semiconductor production. Accordingly, a method and apparatus for reducing or eliminating wafer charging during spin-rinse-drying of wafers is needed.  
      An object of the present invention is to provide a novel method for reducing electrostatic charging of wafers during a spin-drying process.  
      Another object of the present invention is to provide a novel wafer charging reduction method which is effective in reducing defects in integrated circuit devices formed on a wafer.  
      Still another object of the present invention is to provide a novel method for reducing electrostatic charging of a wafer during a spin-drying process by sequentially rotating the wafer in the clockwise and counterclockwise directions, respectively, or in the counterclockwise and clockwise directions, respectively.  
      A still further object of the present invention is to provide a novel method for reducing electrostatic charging of a wafer during a spin-drying process, which method is effective in removing a rinsing liquid from high aspect ratio trenches and other openings in devices fabricated on the wafer.  
      Yet another object of the present invention is to provide a novel apparatus for reducing electrostatic charging of wafers, which apparatus includes an SRD (spin-rinse-dry) apparatus that is capable of sequentially rotating a wafer in opposite directions during spin-drying of the wafer.  
     SUMMARY OF THE INVENTION  
      In accordance with these and other objects and advantages, the present invention is generally directed to a novel method for reducing or eliminating electrostatic charging of wafers during a spin-dry step of wafer cleaning. The method includes rinsing a wafer, typically by dispensing a cleaning liquid such as deionized water on the wafer while spinning the wafer; and spin-drying the wafer by sequentially rotating the wafer in opposite directions. This reduces the accumulation of electrostatic charges on the wafer as compared to unidirectional rotation of the wafer during the spin-drying step. Furthermore, the method is effective in removing the deionized water or other rinsing liquid from high aspect ratio trenches and other openings in devices fabricated on the wafer.  
      The present invention is further directed to a novel apparatus for reducing electrostatic charging of wafers during a spin-drying process. The apparatus includes an SRD (spin-rinse-dry) apparatus that is capable of sequentially rotating a wafer in opposite directions during spin-drying of the wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  is a schematic of a typical conventional spin-rinse dryer, illustrating spin-rinsing of a wafer, followed by spin-drying of the wafer;  
       FIG. 2  is a schematic of an illustrative embodiment of a spin-rinse dryer according to the present invention, illustrating spin-rinsing of a wafer, followed by spin-drying of the wafer according to the method of the present invention; and  
       FIG. 3  is a flow diagram which summarizes sequential process steps carried out according to the method of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is generally directed to a novel method which is effective in eliminating or at least reducing spin-induced electrostatic charging of semiconductor wafers during a spin-drying step of wafer cleaning carried out at various points during the fabrication of integrated circuits (ICs) on the wafers. According to the method, a wafer is rinsed following an IC device fabrication step, typically by dispensing deionized water or other cleaning liquid on the wafer as the wafer is rotated. The wafer is then subjected to a multi-stage spin-drying step in which the wafer is sequentially rotated in opposite directions. This bi-directional rotation of the wafer reduces the accumulation of electrostatic charges on the wafer as the wafer is dried. Furthermore, bi-directional rotation of the wafer during the spin-drying step is effective in removing the deionized water or other cleaning liquid from high aspect ratio trenches and other openings in devices fabricated on the wafer.  
      The present invention is further directed to a novel apparatus for reducing electrostatic charging of wafers during a spin-drying process. The apparatus includes an SRD (spin-rinse-dry) apparatus that is capable of sequentially rotating a wafer in opposite directions during spin-drying of the wafer. The SRD apparatus includes an electric motor and a controller which is operably connected to the motor for selective rotation of the motor in a clockwise or counterclockwise direction at a selected rotational speed.  
      Referring to  FIG. 2 , an illustrative embodiment of an SRD (spin-rinse-dry) wafer scrubbing apparatus according to the present invention is generally indicated by reference numeral  28 . The SRD wafer scrubbing apparatus  28  includes an electric motor  36 . The lower end of a shaft  34  is engaged by the electric motor  36  for rotation thereby. A wafer platform  32 , which is adapted for supporting a wafer  30 , is supported on the upper end of the shaft  34  for rotation by the shaft  34 . A water jet nozzle  40  is positioned above the wafer platform  32  for ejecting a jet  38  of pressurized cleaning liquid, such as deionized water, onto the wafer  30  during a spin-rinsing step of wafer cleaning, which will be hereinafter described.  
      By use of techniques known to those skilled in the art, the controller  42  is operably connected to the motor  36 , such as through suitable controller wiring  43 , in such a manner as to facilitate rotation of the shaft  34  and wafer platform  32  in a selected counterclockwise or clockwise direction, as indicated by the curved counterclockwise arrow  44  and clockwise arrow  46 , respectively. A speed controller dial or switch (not shown) may be provided on the controller  42  to facilitate manual selection between various rotational speeds of the wafer platform  32  as the motor  36  rotates the wafer platform  32  in the selected clockwise or counterclockwise direction. Alternatively, the controller  42  may include a microprocessor with supporting software which enables programmed operation of the motor  36 . In that case, the controller  42  can be programmed to operate the motor  36  in such a manner that the wafer platform  32  is rotated at a selected rotational speed in one direction and then at a selected rotational speed in the opposite direction during the spin-drying process of wafer cleaning.  
      Referring again to  FIG. 2 , in conjunction with the flow diagram of  FIG. 3 , the method of the present invention is carried out typically as follows. First, the wafer  30  is placed on the wafer platform  32  for initial spin-rinsing of the wafer  30  in an SRD wafer cleaning process, as indicated in step  1  of  FIG. 3 . In the IC fabrication processes preceding the SRD cleaning process, various IC fabrication steps are carried out on the wafer  30 . These may include a photolithography process, for example, in which a photoresist layer (not shown) is deposited on a conductive layer (not shown) on the wafer  30 ; the photoresist is exposed to ultraviolet light through a mask or reticle, wherein exposed portions of the photoresist are rendered either soluble or insoluble in a developing chemical; and the photoresist is developed, wherein the photoresist is exposed to the developing chemical to remove the soluble portions of the photoresist from the wafer. After the development step of photolithography, the SRD spin-rinsing step  1  may be carried out to remove residual photoresist particles from the wafer prior to further processing. However, it is understood that the SRD spin-rinsing step  1  may be carried out at any point during processing, as deemed necessary for the removal of particulate contaminants from the wafer.  
      During the SRD spin-rinsing step  1 , a jet of pressurized cleaning liquid  38 , such as deionized (DI) water, is ejected from the dispensing nozzle  40  and onto the wafer  30 . Simultaneously, the wafer platform  32  rotates the wafer  30  at a predetermined rotational speed, which may be between typically about 30 RPM and about 5,000 RPM. The cleaning liquid  38  typically strikes the center of the rotating wafer  30  and is drawn outwardly by centrifugal force toward the edge of the wafer  30 , washing particles from the wafer  30 . The SRD spin-rinsing step  1  may be carried out using process parameters which are known to those skilled in the art.  
      Upon completion of the SRD spin-rinsing step  1 , the wafer  30  is subjected to a spin-drying process according to the method of the present invention. Accordingly, as indicated in step  2  of  FIG. 3 , the wafer  30  is initially rotated in a first direction, such as a counterclockwise direction, as indicated by the counterclockwise arrow  44  of  FIG. 2 . Preferably, the wafer  30  is rotated in the first direction at a gradually-increasing rotational speed until the wafer  30  reaches a target rotational speed of typically about 20˜5,000 rpm for a period of typically about 1˜60 sec.  
      Next, as indicated in step  3 , rotation of the wafer  30  is gradually stopped. As indicated in step  4 , the wafer  30  is then rotated in the second direction. The second direction of wafer rotation is the clockwise direction as indicated by the clockwise arrow  46  (in the event that the first direction was the counterclockwise direction) or the counterclockwise direction as indicated by the counterclockwise arrow  44  (in the event that the first direction was the clockwise direction). Preferably, the wafer  30  is rotated in the second direction at a gradually-increasing rotational speed until the wafer  30  reaches a target rotational speed of typically about 300˜5,000 rpm for a period of typically about 1˜60 sec.  
      As indicated in step  5 , rotation of the wafer  30  is then stopped. As indicated in step  6 , steps  1 - 5  or  2 - 5  may then be repeated, as necessary, to complete drying of the wafer  30 . It will be appreciated by those skilled in the art that sequential rotation of the wafer  30  in the clockwise and counterclockwise directions in alternating order prevents or minimizes the accumulation of electrostatic charges on the wafer  30  which otherwise tends to attract potential device-contaminating particles to the wafer  30  during spin-drying. This reduces the number of defects and enhances the yield of devices on the wafer  30 .  
      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.