Abstract:
A method that includes, obtaining a substrate, placing a reinforcing layer over a first side of the substrate; and thinning the substrate by removing material from an opposite side of the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention pertains in general to microchip processing and in particular to a thin wafer backgrinding operation. 
     BACKGROUND OF THE INVENTION 
     The microelectronics industry is consistently achieving an ever shrinking size of devices and ever greater levels of device integration, resulting in higher interconnect densities. A steady succession of interconnect developments and strategies has evolved in the packaging industry to meet the demand of these high interconnect densities. Nearly all have been aimed at one goal, reduced size and with few exceptions, reduced size also translates into lower cost. Thus in state of the art packaging, dual-in-line packages have been largely replaced by surface mount packages, and newer developments, such as chip-on-chip (COC) and multichip modules (MCM), are meeting the high density interconnect demand. These and similar developments are directed toward reducing the package area, i.e. the x-y dimension of the package. 
     The issue of package thickness has been addressed by techniques for thinning the wafers from which the chips are singulated. The thinning operation is performed on fully processed wafers by mounting the wafer, processed side down, on a temporary carrier such as an adhesive tape and grinding the backside of the wafer. A variety of wafer thinning techniques have been proposed and used, ranging from machines providing simple mechanical abrasion using, e.g., an abrasive grinding wheel, to chemical etching and polishing techniques, and combinations of these, e.g. chemical mechanical polishing (CMP). In a typical wafer thinning process, a 200 mm diameter wafer of completed ICs may be reduced from an initial thickness of 26 to 30 mils to a final thickness of only 12 mils before it is remounted and diced. 
     The individual IC chips, or dies, are then packaged, which may involve assembly into an MCM or COC tile. The term tile as used herein refers to a sub-assembly of at least two components, a substrate and at least one active chip flip-chip bonded to the substrate. The substrate of the tile may or may not be an active chip. In a common arrangement, the tile comprises two or more components, a substrate, and one or more chips that can be alone, side-by-side, or chip-on-chip, and where the substrate may be active or passive. The chip-on-chip may comprise two stacked chips, or two or more chips stacked on one, usually larger, chip. The term substrate refers in this context to a support element, either active or passive, and the term chip typically refers to a fully processed, i.e. finished, semiconductor IC device. In the preferred case, all of the elements in the tile are semiconductor, typically silicon although the substrate may also be ceramic. 
     In the assembly operation, the singulated die are handled through a die mounting and bonding tool, and additional interconnections made as needed. To withstand this additional processing without fracture, a die thickness of 10 mils or greater is generally adequate. However, a die thickness of less than 8 mils, which would otherwise be desirable for many applications, is prohibited by the exposure to handling after thinning and by wafer warpage. 
     Wafer physical warpage is created by stresses that are inherent in the metal layers of the wafer. In addition, tapes that are placed onto the wafer to allow for processing can contribute to wafer warpage. This can be a result of unbalanced tape tension, tape material shrinkage, etc. in which these stresses are transferred to the wafer creating a distortion of the wafer such as bowing. 
     The limitation on thickness of the die applies also to a support wafer or substrate. This limitation, 10 mils or greater, is generally accepted in the industry as a norm, and tiles of less than 20 mils have not been attainable. This constraint rules out the use of stacked chips or tiles in several important applications, such as so-called smart cards, i.e. credit cards with imbedded chips. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which: 
     FIG. 1 is a flow-diagram of one embodiment for a method of using a reinforcing layer as a stiffener to support a patterned wafer during a backgrinding operation to thin the wafer. 
     FIG. 2A is an illustration of the wafer with the device side facing up at the start of a process for backgrinding the wafer. 
     FIG. 2B is an illustration of a backgrind tape placed over the device side of the wafer. 
     FIG. 2C is an illustration of a reinforcing layer placed onto the backgrind tape. 
     FIG. 2D is an illustration of the wafer thinning operation. 
     FIG. 2E is an illustration of a wafer mounting operation. 
     FIG. 2F is an illustration of the removal of the stiff reinforcing film. 
     FIG. 2G is an illustration of the removal of the backgrind tape. 
     FIG. 2H is an illustration of the operation to separate the individual dies from the wafer using a wafer saw. 
    
    
     DETAILED DESCRIPTION 
     A method for reducing warpage of a substrate during a substrate thinning process by adding a stiff reinforcing layer is described. In one embodiment, the substrate is a wafer and the reinforcing layer is applied over a protective coating such as a backgrind tape, where the backgrind tape can be placed onto the wafer in preparation for the wafer thinning process. The wafer structural reinforcement can be applied as a thin adhesive backed metal alloy taping material. Alternatively, the reinforcement can be an adhesive backed polymer laminated with reinforcing fibers or the reinforcement can be a flexible polymer film, which can be heat or UV-cured and where the stiffness level can change with the cure. 
     For purposes of discussing the invention, it is to be understood that various terms are used by those knowledgeable in the art to describe apparatus, techniques, and approaches. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in gross form rather than in detail in order to avoid obscuring the present invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention. 
     In one embodiment, the reinforcing layer is an aluminum foil tape where the aluminum foil (backing) can be a soft, non-heat treated aluminum with an acrylic adhesive coating on at least one side. The aluminum foil tape can have a total tape thickness of approximately 5 mils where the aluminum backing can be approximately 3 mil (0.003″) thick with a tensile strength of approximately 30 lbs./in. width and with an elongation at break of approximately 8%. The aluminum foil tape can perform in a temperature use range of approximately −64°-300° F. 
     FIG. 1 is a flow-diagram of one embodiment for a method of using a reinforcing layer as a stiffener to support a patterned wafer during a grinding, i.e. a backgrinding operation to thin the wafer. The patterned wafer is obtained  102  and the device side is placed facing up on a vacuum chuck and where the bottom side contacting the vacuum chuck is the non-device side  104 . A protective coating of a pressure sensitive backgrind tape can applied to the wafer device side  106  where the backgrind tape can be a single pre-cut piece that has been die cut from sheet stock. The backgrind tape may be a low-warpage tape such as the ICROS® tape brands manufactured by Mitsui Chemicals America, Inc., located in Purchase, N.Y. The backgrind tape can be placed onto the wafer by a roller or alternatively by a vacuum or through the use of a press, such that minimum stresses are placed into the tape that could contribute to wafer warpage. Next, the reinforcing layer is placed over the backgrind tape  108 . The reinforcing layer can be added in strips from a tape or pre-cut from sheet stock to the shape of the wafer and placed onto the wafer as a single circular piece. A backgrinding process can then be performed where the wafer non-device side is ground until the wafer thickness is reduced to a dimension  110 . After wafer thinning, a chemical etch can be performed for planarization of the ground surface  112 . After wafer planarization, the wafer can be placed into a ring frame where the center can be a nylon fabric  114 . Next, tabs placed in the reinforced tape and the backgrind tape can allow for automated tape removal  116 . A dicing tape, such as Nitto SPV 224B manufactured by the Nitto Denko Corporation, Osaka, Japan, can next be applied to the wafer non-device side  118  to adhere the wafer to a base structure and after which a wafer saw can dice the individual dies from the wafer  120 . 
     FIGS. 2A-2H are illustrations of one embodiment of a method for using a reinforcing film to provide structural support to a wafer during the wafer thinning process. FIG. 2A is an illustration of the wafer  202  with the device side  206  facing up at the start of a process for backgrinding the wafer  202 . The wafer  202  can be placed onto a vacuum base (not shown) that can hold the wafer  202  in a position. 
     FIG. 2B is an illustration of a backgrind tape  204  placed over the device side of the wafer. The wafer  202  can be any diameter such as, for example, 200 mm or 300 mm and the tape  204  can be applied over the wafer  202  to protect the device side  206  during the later backgrind operation to reduce or thin the wafer thickness. The backgrind tape  204  can be pre-cut to the approximate wafer diameter and placed onto the wafer  202  as a single piece film. The backgrind tape  204  can be placed onto the wafer  202  without stretching to minimize stresses from being placed into the tape  204  that can transfer to the wafer  202 . Reducing induced stresses in the tape  204  as applied can be accomplished in a variety of ways, such as, for example, by contacting the tape  204  to the wafer  202  at a wafer edge and using a roller (not shown) to lay the film  204  down onto the wafer  202  in a linear fashion as the roller travels across the wafer  202 . 
     FIG. 2C is an illustration of a reinforcing layer  208  placed onto the backgrind tape  204 . The reinforcing layer  208 , such as, for example, an aluminum foil backing with adhesive, can be pre-cut to a shape to approximately cover the previously applied backgrind tape  204 . The reinforcing layer  208  can be applied by any mechanical means (again such as by roller) that places only minimal distortion forces into the wafer  202 , i.e. maintains the wafer  202  as flat after placement of the reinforcing layer  208 . 
     FIG. 2D is an illustration of the wafer thinning operation. Wafer thinning can be accomplished by a mechanical abrasion process where the wafer non-device side  210  can be ground down and after such grinding, a chemical etch process can further polish the ground surface  210 . Alternatively, the wafer thinning can be accomplished by a chemical etch, a dry plasma etch, or a Chemical Mechanical Polish (CMP) process. Dry etch processing systems remove material from the surface of a silicon wafer, or from films deposited on the wafer, through exposure to plasma, a highly reactive chemical species created in an etch reactor. The reactor not only produces the plasma but also controls the chemical and physical reactions that occur on the wafer surface at the atomic level. Through the etch process, selected materials are removed from the wafer or film, which shapes the profile and critical dimensions of the remaining materials. CMP can use a wet formula comprising chemicals that attack the material to be removed. This wet formula can be a slurry that also contains abrasive compounds that can act to mechanically remove material. During the wafer thinning operation, the wafer  202  should remain flat while contacting the abrading medium (not shown) such that after the grinding operation, the wafer thickness t remains uniform. After wafer thinning, the wafer  202  and reinforcing layer  208  can be rinsed. 
     FIG. 2E is an illustration of a wafer mounting operation. After the wafer thinning operation, a ring mount  212  can be obtained that has a wire mesh center  214 . The wire mesh center  214  can have a pressure sensitive adhesive (not shown) applied to contact the wafer non-device side  216  previously ground. The wafer  202  can be positioned to contact the center of the wire mesh  214  and become affixed. 
     FIG. 2F is an illustration of the wafer during reinforcing film removal. Reinforcing film removal occurs after the wafer  202  has been mounted within the ring mount  212 . A tab (not shown) can be added to the shape of the reinforcing film  208  such that the reinforcing film  208  removal process can be automated. A machine (not shown) for automatically removing the reinforcing film  208  can be used such as an ATRM-2100, ATRM-2200, or ATRM4000 de-taper manufactured by the Takatori Corporation located in Kashihara City, Japan. 
     FIG. 2G is an illustration of the removal of the backgrind tape. Again, a tab (not shown) can be placed in the backgrind tape  204  or a tap secondarily attached to the backgrind tape  204  such that the de-taper machine can be used for automatically removing the backgrind tape  204 . 
     FIG. 2H is an illustration of the operation to singulate or separate the individual die  218  from the wafer  202  using a wafer cutting tool  220 . The wafer  202  can be sectioned using an automated process that employs the wafer cutting tool  220  such as, for example, a circular rotating diamond blade  220 . After the wafer dies are separated, the individual die  218  can be run through an automated washing machine to clean (not shown). 
     In another embodiment of the reinforcing layer used as a stiffener for a patterned wafer, biaxial oriented fibers can be placed within a resin. The fibers can have a tensile strength to provide stiffness to the reinforcing layer, such as, for example, fiberglass and the biaxial direction can place the fibers at an angle relative to each other and where, in one embodiment, the angle is approximately 90 degrees. The fibers may be placed such that fibers in one direction are stacked over the fibers in the other direction or, alternatively, the fibers in the two directions may be woven into each other. The fibers can be held with resin that is a polymer binder and where the binder can have no adhesive applied, or an adhesive such as a pressure sensitive adhesive can be applied to at least one side. If no adhesive is placed onto the reinforcing layer, the backgrind coating can be a double back tape, i.e. having adhesive on both sides, such that the reinforcing layer can be attached to the backgrind coating by this backgrind adhesive. The reinforcing film can be cut in a single piece to fit onto the wafer with a stamping operation such as a metal cutter formed in the shape of the reinforcing film to fit onto the wafer. Alternatively, the reinforcing film can be applied from a roll of tape, where, in one embodiment, the two fiber orientations relative to the direction of the tape can be in a +45°,−45° fiber direction. However, other embodiments can vary the orientation such as, for example, to have one fiber orientation in the direction of the tape length and the other fiber orientation in the direction with the width of the tape. The weight of the fiberglass fabric can be in the range of approximately 2-10 oz/yrd2. 
     In yet another embodiment for the reinforcing layer used as a stiffener for a patterned wafer, a thermoset polymer is used. The thermoset polymer may or may not have reinforcing fibers and can be applied as an uncured or partially cured polymer and where further cross-linking is accomplished after the polymer has been applied to the wafer. After full cross-linking, i.e. cure, the stiffness of the plastic film can be increased as a result of the cross-linking and where the cross-linking can be accomplished such as, by example, through exposure to UV or to heat. The thermoset polymer can be a material such as epoxy, urethane, phenolic, or blends of thermoset and thermoplastic materials. The requirement for the thermoset or thermoset blended material can be that the material stiffness should increase after cure to improve wafer anti-warpage properties. 
     The application of the thermoset polymer can be accomplished in a variety of ways, such as, for example, spray-on of an uncured thermoset or the placing onto the wafer of a partially-cured film, either of which would require a subsequent cure. The application of any of the reinforcing layer materials should not create stresses within the reinforcing layer that are high enough to overcome the reinforcing layer stiffness that would translate into wafer distortion. 
     Thus a method for reinforcing a substrate to improve anti-warping properties, and in particular, to reinforcing a wafer to maintain wafer flatness during wafer thinning processes has been described. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.