Abstract:
The present invention provides a method and system for blowing dust and other foreign particles off semiconductor substrate surfaces. The system may blow ionized air, which neutralizes electrostatic charges on the substrate surface thereby facilitating the release of particles from the substrate and reducing the risk of damage to semiconductor substrates from electrostatic discharge. The system also provides a vacuum for capturing dust particles blown off the surface, thus preventing the particles from contaminating clean room environments and redepositing on the cleaned substrate or other substrates.

Description:
TECHNICAL FIELD 
     The present invention generally relates to semiconductor processing, and in particular to an apparatus and method for removing small dust particles from substrates prior to assembly of Fine-pitch Ball Grid Array (FBGA) packages. 
     BACKGROUND OF THE INVENTION 
     In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down device dimensions (e.g., to submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. These may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry such as corners and edges of various features. 
     Assembly of devices with small feature size has necessitated the introduction of new assembly methodologies. One such methodology, which is now widely used, is Ball Grid Array (BGA) packaging. BGA packages are formed by placing an array of pads on a semiconductor substrate and solder balls on each pad. 
     A problem that arises, particularly when the BGA has a fine pitch, involves small dust particles interfering with the attachment of solder balls to pads. This problem is illustrated in FIG. 1, where a small dust particle  100  prevents solder ball  110  from properly contacting pad  120 . As a consequence, ball  110  may be missing from the final array or insecurely attached to pad  120 , resulting in defects. 
     High suction vacuums have been used to clean substrates. However, such vacuums are one or more of bulky, noisy, prone to mechanical breakdown, and expensive to maintain. Furthermore, high suction vacuums are not entirely effective in removing dust particles and create problems keeping the substrates in place. 
     One reason contaminants appear on substrates is that they are attracted by electrostatic charges that develop on the substrates during processing. High suction vacuum systems do not effectively mitigate development of these charges. Aside from attracting contaminants to the substrate surface, electrostatic charges create a risk electrostatic discharge, which can damage the semiconductor package. 
     There has been an unsatisfied need for better apparatus and methods that can be employed in FBGA packaging to effectively remove dust particles from substrates. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and system for blowing dust and other foreign particles off semiconductor substrate surfaces. The system may blow ionized air, which neutralizes electrostatic charges on the substrate surface thereby facilitating the release of particles from the substrate and reducing the risk of damage to semiconductor substrates from electrostatic discharge. The system also provides a vacuum for capturing dust particles blown off the surface, thus preventing the particles from contaminating clean room environments and redepositing on the cleaned substrate or other substrates. 
     One aspect of the present invention provides a cleaning system including a body, a blower outlet mechanically attached to the body, and a vacuum inlet mechanically attached to the body, wherein the shape of the body and the positioning of the blower outlet and vacuum inlet permit the blower outlet and vacuum inlet to be simultaneously placed in proximity to a surface to be cleaned whereby air blown from the blower outlet blows dust particles off the surface to be cleaned and blown particles are taken up through the vacuum inlet. 
     Another aspect of the invention provides a cleaning system including means for blowing ionized air onto a surface and means for capturing dust particles blown off the surface. 
     A further aspect of the invention provides a method of packaging semiconductor units including the steps of placing the units on a conveyor, conveying the units past a device that blows dust particles off the units with ionized air and captures blown dust particles with a vacuum, and subsequently conveying the units to an area where solder balls are attached to the units. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a dust particle blocking a solder ball from properly contacting a pad. 
     FIG. 2 is an illustration of a device according to the present invention, viewed from an oblique angle. 
     FIG. 3 is an illustration of a device according to the present invention, viewing the device in cross-section from one side. 
     FIG. 4 is a schematic of a BGA process according to on aspect of the present invention. 
     FIG. 5 is a schematic of the BGA process of FIG. 4, where the substrate has advanced to the ball attach area. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One aspect of the present invention involves cleaning substrates by blowing air across their surfaces and capturing blown particles using a vacuum. The blown air emanates from a blower and is preferably ionized. Ionized air helps prevent static discharge, which could damage semi-conductor parts, and also facilitates release of particles from the substrate surface. The vacuum system is positioned to capture dust particles blown off the surfaces. Blown particles are immediately captured, thereby preventing them from migrating into the environment around the cleaning device and re-contaminating the substrate being cleaned, or other substrates. The invention is useful in integrated circuit packaging systems, such as a Fine-pitch Ball Grid Array packaging system. 
     The cleaning system removes particles having any size that can be captured by the system. In one embodiment, the cleaning system is particularly suited for removing particles having sizes from about 10 to about 500 μm. In another embodiment, the system is particularly suited for removing particles having sizes from about 50 to about 300 μm. 
     The present invention will now be described with reference to the drawings, wherein like reference numerals refer to like elements throughout. 
     FIGS. 2 an  3  are illustrations showing main parts of a device and system  202  for removing particles in accordance with the invention. Apparatus  202  includes body  220 , blower nozzle  200 , vacuum inlet  250 , air curtain  240 , intake manifold  270  formed by body  220 , a vacuum system, which is not shown, vanes  260  within intake manifold  270 , and wheels  230 . Substrate  210  is not part of the device, but is illustrated to put the device in context. Likewise, the illustrations include transportation rail  280  with rollers  290  that carry substrate  210  past device  202 . 
     Nozzle  200  is connected to an air blower that pressurizes air and expels it through nozzle  200 , which is the blower outlet. In one embodiment, the air blower compresses air to a pressure from about 20 to about 120 psi. In another embodiment, the blower compresses air to a pressure from about 25 to about 50 psi. The blown air is preferably ionized air. The air may be ionized, before, during, or after compression. A commercially available ionized air blower may be used, a SemiTone Model AC7200 Air Knife for example. Ionized air is formed by splitting some of the air molecules into positively and negatively charged ions. Oppositely charged ions are attracted to and neutralize charges on substrate surfaces. 
     Nozzle  200  is preferably broad and flat, having a width that is slightly less than or greater than or equal to the width of the substrates the device is to clean. Nozzle  200  is mounted on body  220  so that nozzle  200  directs air at an angle against the substrate to be cleaned  210 . Preferably, the nozzle directs air against the substrate at an angle that is from about 5 to about 45 degrees from a plane parallel to the substrate surface. 
     Wheels  230 , which are attached to body  220 , are positioned to roll across the surface to be cleaned  210 . Wheels  230  maintain a gap between nozzle  200  and substrate  210 . In one embodiment, this gap is from about 0.1 mm to about 2.5 cm. In another embodiment, the gap is from about 0.1 mm to about 1 cm. In yet another embodiment, the gap is from about 0.1 mm to about 2 mm. Optionally, one pair of wheels  230  is attached to body  220  through nozzle  200 . 
     Air curtain  240  forms part of the inlet to the vacuum system. Only a portion of the vacuum system is shown. For device  202 , the forward edge  242  of the air curtain  240  marks the beginning of the vacuum inlet. The vacuum inlet is relatively close to the blower outlet, which is at the end of nozzle  200 . In one embodiment, the vacuum inlet is from about 0.01 to about 10 cm from the blower outlet. In another embodiment, the vacuum inlet is from about 0.1 to about 5 cm from the blower outlet. In a further embodiment, the vacuum inlet is from about 0.5 to about 2 cm. On device  202 , this distance is 0.5 cm or greater, measured from nozzle  200  to the forward edge  242  of air curtain  240 . 
     Air curtain  242  is designed to slide across the surface of the substrate to be cleaned  210  and direct particles blown off the surface into vacuum manifold  270 . A major portion (at least about 50%) of the air expelled by the blower is taken up through the vacuum intake. In one embodiment, at least about 90% of the air is taken up. In another embodiment, at least about 95% of the air is taken up. Likewise, a majority of the particles (at least about 50% by weight) blown off the surface of the substrate is taken up by the vacuum system. In one embodiment, at least about 90% of the blown dust and other foreign particles is captured by the vacuum system. In a further embodiment, at least about 95% of the blown dust and other foreign particles is captured by the vacuum system. 
     The vacuum inlet opens into inlet manifold  270 . Manifold  270  is an integral part of body  220 . Inlet manifold  270  has a shape that facilitates keeping particles within the vacuum system once they are blown through the vacuum inlet. Inlet manifold  270  narrows along its length. Preferably, from its broadest point, the manifold cross section narrows and reduces by a factor from about 2 to about 20. 
     Manifold  270  contains a plurality of vanes  250 , which are attached to manifold  270 . Vanes  250  channel air flowing within the manifold toward the narrow end of the manifold, which is the manifold exit. As air flows through the narrowing channels formed by vanes  250 , the air accelerates, further contributing to the ability of the system  202  to trap particles within the vacuum system. 
     The volume of air flow into the vacuum system is usually equal to or greater than the flow rate from the blower. In one embodiment, the volumetric flow rate into the vacuum system is from about 10 to about 1000 cfm (cubic feet per minute). In another embodiment, the volumetric flow rate is from about 30 to about 400 cfm. In a further embodiment, the volumetric flow rate is from about 75 to about 200 cfm. 
     The vacuum system, of which only the inlet side is shown, is generally designed to convect air and particles out of a clean room environment. Therefore the vacuum system of the apparatus according to the invention may be no more than a coupling adapted to attach to the main exhaust of a clean room. The vacuum system may include a vacuum pump or a connection to a vacuum pump or similar apparatus. 
     FIGS. 4 and 5 are schematics of a Ball Attach system using a cleaning device according to the invention. Cleaning device  400  is positioned above transportation rail  410 . In FIG. 4, substrate  420  is cleaned by device  400 . FIG. 5 shows substrate  420  carried by transportation rail  410  to Ball Attach site  430 . In one embodiment, cleaning device  400  is physically attached to transportation rail  410  from an area below cleaning device  400 . In another embodiment, cleaning device  400  is not attached to transportation rail  410 , but is positioned above it. Orientation and level control systems on the transportation rail in the vicinity of the cleaning device may be used to help avoid scratching the substrates. 
     Although the invention has been described as being used in a Ball Attach system, it has other applications. Devices of the invention may be used advantageously in other bonding processes where contaminants and/or electrostatic charges are a concern, e.g., die attaching and wire bonding. 
     What has been described above is the present invention and several of its specific embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.