Abstract:
A method and apparatus for cleaning a wafer in a dual brush cleaning system is disclosed. Two brushes, preferably made of PVA and wetted by cleaning fluids, are positioned opposite one another and spaced apart enough to allow a portion of a wafer to be inserted between their working surfaces and make frictional engagement with them. The top brush is rotated at a first speed and the bottom brush is rotated at a second faster speed sufficient for the freely rotating wafer to rotate at the same speed and in the same direction as the top brush. The bottom brush may have raised areas on its surface to assist in efficiently gripping and rotating the wafer. A common rotation speed and direction causes a uniform relative velocity between the top brush and the wafer that results in an improved cleaning operation.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the art of cleaning workpieces such as semiconductor wafers during various stages in the manufacturing process of integrated circuits, and more particularly, relates to an improved method and apparatus whereby a freely rotating wafer is cleaned between two adjacent motor driven brushes wherein there is a substantially uniform relative velocity distribution between the top brush and the wafer&#39;s surface. 
     BACKGROUND OF THE INVENTION 
     A flat disk or “wafer” of single crystal silicon is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Semiconductor wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. 
     Several of the processes used to manufacture semiconductor wafers introduce particles or contaminates to the surfaces of the wafer. For example, chemical-mechanical polishing (CMP) involves placing the wafer on a polishing pad in the presence of slurry. Slurry typically contains chemicals that etch away material from the wafer&#39;s surface and abrasive particles that assist in mechanically removing material from the wafer&#39;s surface. Slurry may contain, for example, KOH and colloidal or fumed silica abrasive particles. The wafer is then pressed against the polishing pad and relative motion is created between the wafer&#39;s surface and the polishing pad to remove material from the wafer&#39;s surface. The wafer&#39;s surface is thereby exposed not only to the chemicals and particles contained in the slurry, but also to material removed from the wafer&#39;s surface. The process of pressing and causing relative motion between the wafer&#39;s surface and these contaminates undesirably adheres the contaminates to the wafer&#39;s surface. 
     The wafer&#39;s surface on which integrated circuitry is to be constructed must be extremely clean in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. The material layers (deposited thin film layers, usually made of metals for conductors or oxides for insulators) applied to the wafer while building interconnects for the integrated circuitry must also be made extremely clean to avoid contamination of the circuitry. 
     Conventional post-CMP wafer cleaning commonly uses a combination of buffing, double-sided brush scrubbing, megasonic cleaning and spin-rinse drying to remove contaminates from the wafer&#39;s surface. The present invention relates to double-sided brush scrubbing so buffing, megasonic cleaning and spin-rinse drying, all presently known cleaning techniques, will not be explained in detail so as to not obscure the present invention. 
     One conventional style of double-sided brush scrubbing is accomplished by placing a freely rotating wafer between the working surfaces of a top and a bottom brush. In conventional cleaning, the top and bottom brushes are rotated by a motor in the same direction and at the same speeds and have the same pattern of raised features on their working surfaces. The motorized rotating top and bottom brushes grip the edge of the freely rotating wafer and cause the wafer to also rotate in the same direction as the brushes. 
     Applicant has discovered that in conventional cleaners the top brush commonly rotates at about twice the speed of the wafer resulting in a nonuniform relative velocity between the top brush and the wafer across the wafer&#39;s surface. The consequences of applicant&#39;s discovery are graphically illustrated in FIGS. 2 a  and  2   b . The velocities at various points across the rotating wafer  100  and rotating top brush  101  are represented by the arrows between lines C 2  and C 3  and lines C 1  and C 3  respectively. The line C 1  representing the speed of top brush  101  is steeper than the line C 2  representing the speed of the wafer  100  since, as previously mentioned, the top brush  101  commonly rotates at about twice the speed of the wafer  100 . The relative velocity between the top brush  101  and the wafer  100  may be found by subtracting their individual velocities at various points. The relative velocity between the top brush  101  and the wafer  100  is illustrated in FIG. 2 b . Specifically, applicant has discovered that as the wafer  100  rotates in a conventional dual brush cleaning system, the relative velocity between the top brush  101  and the wafer  100  is higher near the center and lower near the edge of the wafer  100 . 
     Applicant has further discovered that conventional dual brush cleaning systems typically remove particles satisfactorily from the center of the wafer  100 , but often leave a band of contaminates near the edge of the wafer  100 . Applicant has thus discovered a need for a dual brush cleaning system with a high uniform (across the entire width of the wafer  100 ) relative velocity between the top brush  101  and the wafer  100 . 
     What is therefore needed is an apparatus and method of cleaning wafers that produce a high uniform relative velocity between the top brush and the wafer&#39;s surface. 
     SUMMARY OF THE INVENTION 
     Therefore it is an object of the present invention to provide an apparatus and method for cleaning workpieces that addresses and resolves the shortcomings of the prior art described above. Another object of the present invention is to provide a dual brush cleaning system where all points on the workpiece&#39;s surface experience substantially similar top brush velocities during the cleaning process as the workpiece rotates between the brushes. 
     The apparatus portion of the present invention relates to a dual brush cleaning system for cleaning a workpiece&#39;s surface. The cleaning system has two brushes, a top and a bottom, each with a working surface on one end and is advantageously connected to a shaft on the other end. The working surfaces are positioned opposite of each other and spaced apart enough for a workpiece to be placed, gripped and cleaned between them. The shafts for both brushes may be connected to means, such as one or more motors, for rotating the brushes at separate speeds. 
     The apparatus is preferably able to create the condition of providing a uniform relative velocity between the top brush and the workpiece across the width of the workpiece as the workpiece rotates between the brushes. Sensors may be inserted to monitor in real-time the rotational speeds of the top brush and the workpiece and to then adjust the speed of the bottom brush as needed. However, the cleaning system may be simplified by determining through empirical means the bottom brush speed necessary to obtain a uniform relative velocity between the top brush and the workpiece. Once the desired bottom brush speed has been determined that produces the desired results for a particular application, the bottom brush speed may simply be set at this level. 
     While the top brush is used primarily to clean the workpiece&#39;s top surface, the bottom brush is used primarily (although it also cleans the workpiece&#39;s bottom surface) to rotate the workpiece. It is thus important for the working surface of the bottom brush to have an efficient grip on the workpiece to more efficiently rotate the workpiece. Applicant has discovered that a bottom brush with a plurality of nubbs (raised areas) on its working surface may be used to efficiently grip the workpiece and provide the necessary rotational motion to the workpiece. 
     The method portion of the present invention may be practiced in a dual brush cleaning system by inserting a workpiece, preferably a semiconductor wafer, between the working surfaces of a top and bottom brush. The brushes may be rotated after the workpiece has been inserted, but are preferably already rotating at the speeds necessary to produce a substantially uniform relative velocity between the top brush&#39;s working surface and the workpiece&#39;s surface. 
     These and other aspects of the present invention are described in full detail in the following description, claims and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the appended drawing figures, wherein like numerals denote like elements, and: 
     FIG. 1 a  is a top view of a dual brush cleaning system; 
     FIG. 1 b  is a side view of the dual brush cleaning system shown in FIG. 1 a;    
     FIG. 2 a  is a top view of a dual brush cleaning system illustrating the velocity of the workpiece and the velocity of the top brush when the top and bottom brushes rotate at the same speed; 
     FIG. 2 b  is a top view of a dual brush cleaning system illustrating the relative velocity of the workpiece and the top brush when the top and bottom brushes rotate at the same speed as in FIG. 2 a;    
     FIG. 3 a  is a top view of a dual brush cleaning system illustrating the condition of the bottom brush rotating at a speed faster than the speed of the top brush such that the top brush and workpiece rotate at the same speed; 
     FIG. 3 b  is a top view of a dual brush cleaning system illustrating the relative velocity of the workpiece and the top brush when the top brush and workpiece rotate at the same speed as in FIG. 1 a;    
     FIG. 4 a  is a bottom view of a preferred working surface for the top brush; 
     FIG. 4 b  is a top view of a preferred working surface for the bottom brush; and 
     FIG. 5 is a flowchart of a preferred method for implementing the present invention. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Wafers are often exposed to contaminates during the manufacturing process of integrated circuits. For example, during CMP a wafer is typically pressed against a polishing surface in the presence of chemicals for etching the wafer&#39;s surface and abrasive particles for mechanical removing material from the wafer&#39;s surface. CMP is thus a particularly dirty process requiring wafers to be cleaned prior to being sent to the next manufacturing step. 
     A buffing step may be used after CMP to remove the gross contaminates as well as to remove any microscratches present on the wafer&#39;s surface. Buffing may be accomplished by lightly pressing, about 1.5 psi, a wafer&#39;s surface against a politex pad, made commercially available by Rodel Incorporated from Newark Delaware, in the presence of KOH, mild slurry and/or DI water. The present invention does not require any particular buffing process, but superior cleaning results may be obtained if an effective buffing step is used to remove the gross contaminates prior to the cleaning step. 
     Referring to FIGS. 1 a  and  1   b , the components of an exemplary dual brush cleaning system necessary to practice the present invention will now be discussed in greater detail. The cleaning system has at least two brushes, a top  101  and a bottom  103 , each with a working surface on one end and, preferably, a shaft,  102  and  104 , connected to the other end. The brushes  101  and  103  may be advantageously made of compressible, porous polyvinyl alcohol (PVA) foam that becomes soft when wet. The brushes  101  and  103  may be disk-shaped with a diameter slightly larger than the radius of the wafer  100 . For example, the brushes  101  and  103  preferably have a diameter of about 128 mm for a wafer  100  having a diameter of 200 mm. Syntak Corporation and Rippey Corporation make PVA brushes commercially available that are suitable for practicing the present invention with semiconductor wafers. 
     The working surfaces of the brushes  101  and  103  are positioned facing each other and close enough so that a wafer  100  placed between the brushes  101  and  103  slightly compresses and makes strong friction engagement with the brushes  101  and  103 . The brushes during operation are preferably kept wet to keep the brushes soft and pliant to prevent them from damaging the wafer  100  and to suspend and carry away contaminates that have been liberated from the surface of the wafer  100 . For semiconductor wafers, the fluids should primarily comprise DI water, but may also have about 2% NH 4 OH by volume. 
     The shafts  102  and  104  for both brushes  101  and  103  may be connected to means, such as one or more motors  105  and  106 , for rotating the brushes  101  and  103  at separate speeds. Another alternative is to connect the motor(s)  105  directly to the brushes, for example, by a belt encircling the brushes  101  and  103 . A single motor with gearing mechanisms (not shown) for rotating the brushes  101  and  103  at separate speeds may be used, but preferably both brushes  101  and  103  have an independently controlled motor. The type of motor(s)  105  and  106  is not critical for the present invention, but it is desirable that the motor(s) produce few particles or contaminates. The motor(s)  105  and  106  must also be able to rotate the brushes  101  and  103  at the necessary speeds (typically between 50 and 500 rpm or more) to practice the present invention. 
     The dual brush cleaning system is advantageously able to create the condition of providing a uniform relative velocity between the top brush  101  and the wafer  100  across the width of the wafer  100  as the wafer  100  rotates between the brushes  101  and  103 . This desirable uniform velocity condition is illustrated in FIGS. 3 a  and  3   b . The velocities at various points across the rotating wafer  100  and rotating top brush  101  are represented by arrows between lines A 2  and A 3  and lines A 1  and A 3  respectively. The line A 1  representing the speed of the top brush  101  has the same slope as line A 2  representing the speed of the wafer  100  since a uniform velocity condition may be created when the top brush  101  rotates at the same speed as the wafer  100 . The relative velocity between the top brush  101  and the wafer  100  may be found by subtracting their individual velocities at various points. The relative velocity between the top brush  101  and the wafer  100  when both are rotated at the same rpm is illustrated in FIG. 3 b . Specifically, Applicant has discovered that when the wafer  100  rotates at the same velocity as the top brush  101 , a desirable uniform relative velocity condition is created between the top brush  101  and the wafer  100  across the entire surface of the wafer  100 . 
     The cleaning step of removing contaminates from the wafer&#39;s surface relies heavily on mechanical forces, such as those resulting from brush  101  (and brush  103  for the back wafer&#39;s surface) contact with contaminates. These forces are determined by the number of brush-particle collisions and the brush-wafer relative speed at the time of the collision. As the number of collisions and the relative speed increases, the opportunities for particle removal also increase. Thus, the length of time the wafer  100  spends between the rotating brushes  101  and  103 , the brush pressure against the wafer  100  and the brush rotating speed are all critical in obtaining the best possible cleaning result. In general, the longer the brushing time, the greater the brush pressure and the faster the brush rotating speed, the better the cleaning result. Specifically, a wafer cleaning time of about 80 seconds between a top brush  101  rotating at about 110 rpm and a bottom brush rotating at about 300 rpm with a brush pressure of 3.5 psi has been found to produce acceptable results. These process parameters for PVA brushes  101  and  103  having a diameter of 128 mm, in combination with the brush contours discussed below, have been found to rotate a 200 mm wafer  100  at the same speed as the top brush  101 , i.e., 110 rpm. 
     Sensors may be inserted to monitor in real-time the rotational speeds of the top brush  101  and the wafer  100  and to then adjust the speed of the bottom brush  103  as needed. However, the cleaning system may be simplified by determining through empirical means the bottom brush speed necessary to obtain a uniform relative velocity between the top brush  101  and the wafer  100 . Once the desired bottom brush speed has been determined that produces the desired results for a particular application, the bottom brush speed may simply be set to this level. 
     Referring to FIGS. 4 a  and  4   b , while the top brush  101  is used primarily to clean the wafer&#39;s top surface, the bottom brush  103  is used primarily (although it also cleans the wafer&#39;s bottom surface) to rotate the wafer  100 . It is thus important for the working surface of the bottom brush  103  to have an efficient grip on the wafer  100  to properly rotate the wafer  100 . A bottom brush  103  with a plurality of round-shaped nodules (nubbs)  108  separated by open spaces on its working surface have been found to efficiently grip the wafer  100  and provide the necessary rotational motion to the wafer  100 . A top brush  101  with a plurality of raised wipers  107  separated by open spaces has been found to efficiently remove contaminates on the wafer&#39;s surface. 
     An exemplary method of the present invention for cleaning a wafer  100  will now be discussed with reference to the apparatus in FIGS. 1 a  and  1   b  and the process flowchart in FIG.  5 . After CMP and a buffing step, or other process steps that leave contaminates on the wafer&#39;s surface, a wafer  100  may be cleaned in a dual brush cleaning system such as that described above. The PVA brushes  101  and  103  may be kept compliant and contaminates transported away by continually rinsing the brushes  101  and  103  with DI water and, optionally, 2% NH 4 OH by volume (step  501 ). The brushes  101  and  103  preferably have been previously positioned opposite of each other such that a wafer  100  inserted between their working surfaces would be under about 1.0 psi. The top brush  101  may be rotated at a first speed (step  502 ) while the bottom brush  103  may be rotated in the same direction at a second faster speed (step  503 ) that results in the wafer  100  rotating at the same speed as the top brush  101 . Specifically, a top brush rotation speed of about 110 rpm and a bottom brush rotation speed of about 300 rpm have been found to produce the desired condition of a uniform relative velocity between the top brush  101  and the wafer  100 . A wafer  100  may be inserted between the top brush  101  and the bottom brush  103  for a period of time, up to 80 seconds or even longer, to produce the desired level of cleanliness (step  505 ). The wafer may then be withdrawn from between the top brush  101  and the bottom brush  103  (step  506 ). 
     The wafer  100  is preferably then dried with remaining loose contaminates removed from the surface. This may be accomplished by rinsing the wafer with DI water and then spinning the wafer (preferably at a speed faster than 1000 rpm) in a spin-rinse dryer (not shown) to remove any remaining fluids and contaminates on the surface by centrifugal force. 
     Although the foregoing description sets forth a preferred exemplary embodiment and method of operation of the invention, the scope of the invention is not limited to this specific embodiment or described method of operation. Modification may be made to the specific form and design of the invention without departing from its spirit and scope as expressed in the following claims. For example, although the present invention was described using a wafer  100  as the described workpiece, any number of workpieces may also be cleaned using the present invention.