Patent Publication Number: US-6334229-B1

Title: Apparatus for cleaning edges of contaminated substrates

Description:
This application is a division of application Ser. No. 08/640,459, filed May 1, 1996, now U.S. Pat. No. 5,861,066. 
    
    
     FIELD OF INVENTION 
     The present invention relates to the field of semiconductor wafer processing; more particularly, the present invention relates to cleaning semiconductor substrates (wafers). 
     DESCRIPTION OF RELATED ART 
     Semiconductor manufacturers use semiconductor wafers as the base for manufacturing integrated circuits. In one step of the manufacturing process, the wafers are put through chemical mechanical polishing (CMP). CMP is becoming the main planarization technology for both dielectric and metal layers. For the CMP of dielectric layers, such as BPSG, BPTEOS, and PECVD Oxides (often referred to as the ILD 0 , ILD 1 , ILD 2  . . . layers, respectively), a fumed silica-based slurry is normally used. Other slurries, such as dispersed silica, fumed or dispersed allumina, are also being used for CMP of both oxides and metals (such as tungsten (W) and titanium (Ti)). When the CMP process is completed, the wafers&#39; surfaces are covered in particles, referred to as a slurry residue. At later steps in the process flow, some of this slurry residue is redistributed across the front of the wafer, thereby resulting in a loss in die yield and/or device performance. To prevent the slurry redistribution, all surfaces of a wafer must be free of contamination. 
     Different post CMP cleaning methods have been introduced in the last few years. These include cleaning the wafers in wet stations using conventional wet cleaning methods, such as SC 1 , HF and megasonic cleaning. Other cleaning methods in use are based on scrubbing wafers with brushes of various kinds and configurations using DI water or a combination of DI with chemicals such as Ammonia and Citric acid. 
     One system used to remove wafer contaminants is a double sided scrubber. In a double sided scrubber, a semiconductor wafer is scrubbed simultaneously on both sides by brushes. Since the wafer is being scrubbed simultaneously on both sides by the brushes, there must be a way of holding the wafer in place and rotating the wafer so the entire surface of the wafer is cleaned. A mechanism used for this purpose is commonly referred to as a roller. 
     Today, double sided scrubbers are usually automated and comprise a conveyor type mechanism, rollers, and brushes. In general, the wafer lies flat on the conveyor mechanism and the conveyor mechanism moves the wafer into the brushes. While being scrubbed, the wafer is supported (or held horizontally) by the conveyor mechanism, brushes, rollers, or a combination thereof. FIG. 1 illustrates a conventional double sided wafer scrubber. Referring to FIG. 1, a wafer  101  is being scrubbed by brushes, one of which is shown as brush  102  and the other being beneath wafer  101  and directly below brush  102 . Rollers  103  rotate wafer  101  so the entire wafer surface may be cleaned. Each of brushes  102  is rotated about its central axis by a motor  104 . The rotary motion of rollers  103  is then transferred to wafer  101  when the edge of each of rollers  103  comes into contact with the outer edge of wafer  101 . 
     Brush cleaning systems can effectively clean the front and backs of semiconductor substrates. However, brushes do not provide a sufficient amount of mechanical energy at the edge/bevel to remove contamination. In other words, although the double sided scrubbers are extremely effective at cleaning the front and back side of a wafer, they can leave a slurry residue on the bevel. Likewise, all of the above methods fail to clean the very edge and bevel area of the substrate (wafer) as well. In other words, all current scrubbing methods and apparatus are unable to clean the very edge and bevel area of the substrate (wafer). 
     The present invention provides a method and apparatus that cleans the edge of substrates, including the bevel area when present. 
     SUMMARY OF THE INVENTION 
     A method and apparatus for cleaning edges of substrates is described. The present invention provides a cleaning mechanism that cleans particles off the edge of the wafer based on friction and/or a difference in tangential velocity at a point of contact between the wafer and the cleaning mechanism. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only. 
     FIG. 1 illustrates a conventional double sided wafer scrubber. 
     FIG. 2A illustrates the top view of the side brush mechanism of the present invention. 
     FIG. 2B illustrates a side view of the side brush mechanism of the present invention. 
     FIG. 3 illustrates one embodiment of the side brush mechanism of the present invention incorporated into one of the rollers. 
     FIG. 4 illustrates a side section view of a pad and roller combination used for scrubbing the edge of a substrate. 
     FIG. 5 illustrates one embodiment of the double-sided scrubber system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A method and apparatus for cleaning edges of contaminated substrates is described. The cleaning process may be used in double sided scrubber systems or other systems, such as, for instance, chemical mechanical polishing systems or flat panel display manufacturing systems. In the following description, numerous specific details are set forth such as rotation speeds, chemicals, pressures, etc., in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known components, structures and techniques have not been shown in detail in order to avoid obscuring the present invention. 
     Overview of the Present Invention 
     The present invention provides a method and apparatus that cleans the edge of substrates, including the bevel area when present. In the present invention, particles are removed from the edge and/or bevel area (or any other surface sloping from the edge to the top or bottom of the substrate) using a side scrubbing mechanism incorporated into a scrubber tool. In one embodiment of the present invention, the side scrubbing mechanism comprises a brush added to a double-sided scrubber, which is described below. 
     Although the present invention is described in conjunction with the scrubbing of a wafer, it will be appreciated that any similarly shaped, i.e. generally flat, substrate, may be processed by the methods and apparatuses of the present invention. Further, it will be appreciated that reference to a wafer or substrate may include a bare or pure semiconductor substrate, with or without doping, a semiconductor substrate with epitaxial layers, a semiconductor substrate incorporating one or more device layers at any stage of processing, other types of substrates incorporating one or more semiconductor layers such as substrates having semiconductor on insulator (SOI) devices, two or multiple substrates bonded to each other, or substrates for processing other apparatuses and devices such as flat panel displays, multichip modules, etc. 
     FIGS. 2A and 2B illustrate a simplified top view and a simplified side view of the side scrubbing mechanism of the present invention, respectively. The side scrubber may be added to a double-sided scrubber. Note that FIGS. 2A and 2B illustrate only an exemplary configuration and other configurations are possible. 
     Referring to FIGS. 2A and 2B, a wafer  200  is shown being cleaned by top-side brush  201  as it moves through the scrubber (from left to right as depicted). Along either or both of two side locations,  203  and  204 , are side brush scrubbing mechanisms. Referring to FIG. 2B, top-side brush  201  is again shown with wafer  200 . Bottom-side brush  202  is shown below wafer  200 , along with a single side brush  203 . In one embodiment, the side brush is made of Poly Vinyl Alcohol (PVA), nylon, polyurethane, or other abrasive materials. As wafer  200  moves through the scrubber system, brushes  201  and  202  clean the top and bottom (e.g., the front and back) of wafer  200 , while side brush(es)  203  removes particles along the edge and bevel areas of wafer  200  (due to, e.g., friction, tangential velocity difference, etc.). 
     One benefit of the present invention is that through the combination of the side scrubbing mechanism and the top and bottom brushes all of the exposed areas of the wafer which may be contaminated with slurry particles are cleaned. This includes the top surface, bottom surface and the edge/bevel area. 
     The side scrubbing mechanism of the present invention may be shaped in a variety of shapes. For instance, in one embodiment, the side scrubbing mechanism may be shaped like a roller, while in an alternative embodiment, the side scrubbing mechanism may be shaped like a “dog bone.” Furthermore, the side scrubbing mechanism may be flat or with nibs (like that typically found on the top and bottom brushes). 
     FIG. 3 illustrates one embodiment of the side brush of the present invention incorporated into a roller. Referring to FIG. 3, wafer  310  is placed between brushes  320  of the double sided scrubber. Motor  340  rotates roller  330  of the present invention. When roller  330  is in contact with wafer  310  friction is created between their edges. Thus, the rotating motion of rollers  330  and  331  and the friction that is created causes wafer  310  to rotate. The rotation of wafer  310  between brushes  320  allows the entire surface of the wafer to be cleaned. The two rollers  330  and  331  contact the wafer at two locations to rotate the wafer and to hold it in place (i.e., prevent forward motion) as it is scrubbed. The edge/bevel area is cleaned due to a difference in tangential velocity at the point of contact between the substrate and the side scrubbing mechanism. 
     In one embodiment, roller  330  rotates at 40 revolutions per minute (rpm), while roller  331  rotates at 60 rpm. In this manner, the ratio of rotational speeds between rollers  330  and  331  is 1.5, or a ratio that is approximate thereto. In alternate embodiments, the rotational speed of roller  330  may be 10-15% less than the rotational speed of roller  331 . Note that any difference between rotation speeds of the wafer and the roller may be used to clean the edge/bevel areas. 
     To enhance particle removal, roller  330  includes a pad made of an abrasive (e.g., nylon, PVA, polyurethane, etc.). FIG. 4 illustrates one embodiment of roller  330  with pad  401 . An exemplary wafer  402  is also shown. In one embodiment, pad  401  comprises a SubaIV pad manufactured by Rodel of Newark, Del. Other abrasive pads such as IC1000, suba500, politex (all manufactured by Rodel) can also be used. 
     Note that the pad can be of different thickness and surface texture to increase and/or even maximize the cleaning action. The pad may also be shaped to remove particles only from an edge, where for instance, the wafer is without bevel areas that cannot be cleaned by the top and bottom brushes. 
     To further facilitate particle removal, water jets may be used to propel water into or near the point of contact between rollers  330  and  331  and the wafer, such as shown in FIG.  3 . Such water jets may be positioned such that the direction of water flows from a plane aligned with the rotational axis of the wafer and contact points between the wafer and the side scrubbing mechanism. In such a case, the water may simply carry the particles away that are removed from the wafer by the side scrubbing mechanism or may, if at sufficient pressure, cause removal of particles by itself. The water jet can also act as pad conditioning when it is directed toward the side scrubbing mechanism. Although two jets are shown, only one jet may be used to help facilitate particle removal. Note that the water jets are held in place by support structures which are well-known in the art. In one embodiment, the water jets are held in place above the wafer. Such a jet may be as simple as a barbed coupling with reducing barb to increase the velocity of the created stream. In one embodiment, the barbed coupling is ⅛″ to {fraction (1/16)}″ in diameter. In another embodiment, the jet may include a nozzle that produces a fanned, knife edge pattern. Water jets are well-known in the art. Note also that jets that spray other chemicals may be used, instead of water, to facilitate particle removal. 
     The pad may be cleaned occasionally to remove build-up of particles. The side scrubbing mechanism of the present invention may be self-cleaning. In one embodiment, the side scrubbing mechanism may flow DI water or a combination of DI water and a chemical such as NH 4 OH or NH 4 OH/H 2 O 2  mixture through itself. In an alternate embodiment, the side scrubbing mechanism may be self-cleaning by spraying DI or a combination of DI and a chemical such as NH 4 OH or NH 4 OH/H 2 O 2  onto it during substrate cleaning to reduce build-up. In cases where water is used, manual or self-cleaning is generally not required, but may be used. 
     The present invention may be used for post CMP of both oxide and metal, including, but not limited to, layers of W, Ti, Al, or Cu. However, the present invention is not limited to use in CMP applications. Also, the present invention may be extended to accommodate all substrate sizes in the future. 
     Furthermore, the present invention is easily integrated into existing post-CMP scrubber technologies. That is, wafers (e.g., dielectric, metal) may undergo CMP using, for instance, a slurry (e.g., fumed silica-based, dispursed silica, fumed or dispersed alumina, etc.). Such CMP is well-known to those skilled in the art. As a result of the CMP process, the surface of the wafers is covered with particles (e.g., slurry residue). 
     After the CMP process, the wafers are cleaned. In one embodiment, the wafers are cleaned using a scrubber, which scrubs the top and bottom sides of the wafer using a single-sided scrubber or a double-sided scrubber. 
     The edge (and bevel areas) of the wafer are also cleaned in accordance with the present invention. In one embodiment, the top and bottom (the two sides) of the wafers are cleaned simultaneously with the edge (and bevel areas) of the wafer. 
     An Exemplary Scrubber 
     FIG. 5 illustrates a conceptual view of a double sided wafer scrubber (scrubber) as may be used by one embodiment of the present invention. The scrubber includes a number of stations. Each of theses stations logically represent one or more steps in the wafer cleaning process. These stations can also include the hardware and software that completes one of the steps in the cleaning process. The cleaning process includes the steps executed by the scrubber on the wafers. In one embodiment, the scrubber can process multiple wafers simultaneously; one or more wafers are being processed in each of the stations at a point in time. 
     Dirty wafers are loaded at one end of the scrubber; clean wafers are unloaded from the other end of the scrubber. 
     In load station  510  (also known as the input station), the operator loads a cassette  580  into the scrubber. The cassette  580  contains a number of dirty wafers. Wafers are automatically moved from load station  510  to brush  1  station  520  on transport belt  1   515 . Transport belt  1   515  is moved by DC motor  593 . Wafer  501  represents a dirty wafer being automatically removed from cassette  580  and placed on transport belt  1   515 . 
     In brush  1  station  520 , a dirty wafer  502 , is brushed and sprayed (water jets not shown), to remove some of the particles from the dirty wafer  502 . Brushes  521  scrub both sides of the dirty wafer  502 . The height of the top brush is controlled by a stepper motor (not shown). Side brush  590  scrubs the edge and bevel areas of dirty wafer  502 . The once brushed wafers are then automatically moved to brush  2  station  530 . This is done by transport belt  2   516 , controlled by a second DC motor (not shown). 
     In brush  2  station  530 , a once brushed wafer  503  is brushed and sprayed (water jets not shown), to remove more of the particles from the once brushed wafer  503 . Brushes  531  scrub both sides of the once brushed wafer  503 . The height of the top brush of brushes  531  are controlled by stepper motor  591 . Although not shown, brush  2  station  530  may also include a side brush, like side brush  590 , to clean the edge and bevel area of once brushed wafer  503 . The twice brushed wafers are then automatically moved to spin &amp; dry station  540 , via transport belt  3   517 . 
     Spin &amp; dry station  540  rinses the wafers, spins them, and dries them. Wafer  504  represents a wafer being processed in the spin &amp; dry station  540 . At this point, the wafer has been cleaned. Note, for one particular type of wafer, the wafer must have been kept wet during the load station  510 , brush  1  station  520 , and brush  2  station  530 . Only after being brushed and rinsed can this type of wafer then be spun and dried. The spun and dried wafer is then moved to the output station  550 . 
     In output station  550 , the clean wafer is put into a cassette  581 . Wafer  505  represents a clean wafer being put into cassette  581 . The cassette  581 , when full of clean wafers, can then be removed by the operator. This completes the cleaning process. 
     Control system housing  570  houses a number of components that comprise the heart of the control system for the scrubber. Control system housing  570  includes a host cage  571  having a host board  572 . The host board  572  provides the overall control for the scrubber. The host board  572  typically includes one or more host processors implemented in one or more physical packages. The host board  572  can include a board from Gespac, Inc., of Scottsdale, Ariz. (a Motorola 68030 based processor board, part number MPU-3OH8). The host cage  571  provides support for the host board  572  and other boards in the host cage (e.g. sensor input boards, a video card for operator display  560 , a board for communicating signals from the host board  572  to the rest of the control system). 
     The host board can communicate to the rest of the control boards through another board in the host cage  571 , (communication board  578 ) or through a connector directly to the host board  572 . A control board is typically a modular circuit formed on a printed circuit board, that controls motors or other devices within a scrubber. Typically, the communications from the host cage pass through a communications board  578 . The communications board, in turn, communicates with other devices through a bus  577 . 
     Bus  577  supports an easily extensible and modular control system. In the scrubber of FIG. 5, the bus  577  links the host board  572 , the communications board  578 , the stepper motor backplane  575  and the DC motor backplane  573 . Messages between the various devices attached to the bus  577  can be communicated according to a protocol described below. A message is a packet of information to be communicated from one point to another point. 
     The stepper motor backplane  575  supports a stepper motor control board  576 . This stepper motor control board  576  controls the movement of stepper motor  591  via stepper motor bus  592 . Similarly, the DC motor backplane  573  supports a DC motor control board  574 . The DC motor control board  574  controls the movement of the DC motor  593  and DC motor  595  via DC motor bus  594 . 
     In one embodiment of the present invention, each of these backplanes support up to four motor control boards. However, one of ordinary skill in the art would understand that the present invention is not limited to backplanes that support only four motor control boards. 
     Operator display  560  typically includes a monitor like a cathode ray tube, or flat panel display. In one embodiment, operator display  560  also includes a touch sensitive screen allowing the operator to interact with the scrubber control system. 
     Note that FIG. 5 is a conceptual drawing. Some components are represented by one symbol so as to not overly obscure the present invention. For example, it is possible to have transport belt  3   517  be made of two or more physical transport belts, each belt being moved by a different DC motor. 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the various embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, references to details of various embodiments are not intended to limit the scope of the claims which in themselves recite only those features regarded as essential to the invention. 
     Thus, a method and apparatus for cleaning edges of substrates has been disclosed.