Abstract:
A method for forming device packages includes forming a perimeter comprising a reactive foil and a bonding material interposed between a first wafer and a second wafer, pressing the first and the second wafers against the reactive foil and the bonding material, initiating the reactive foil, wherein the reactive foil heating the bonding material to create a bond between the first and the second wafers, and singulating the first and the second wafers into the device packages.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates to the use of reactive foils for wafer bonding and for forming device packages.  
         DESCRIPTION OF RELATED ART  
         [0002]    Wafer bonding techniques are used in IC (integrated circuit) and MEMS (micro-electromechanical systems) manufacturing. By achieving package function at the wafer level, it is possible to realize cost savings via massive parallel assembly. While MEMS packaging has been incorporated at the device fabrication stage of the micromachining process, there is a need for a more uniform packaging process to produce higher yields and to lower costs. Hermeticity and low-temperature sealing are two key elements that present formidable challenges to the goal of process uniformity.  
           [0003]    MEMS devices and IC&#39;s are generally fragile devices that are sensitive to high temperatures and high voltages required for conventional wafer bonding techniques. Conventional wafer bonding techniques include anodic bonding, intermediate-layer bonding, and direct bonding. Anodic bonding typically takes place at 300 to 450° C. and requires the application of high voltages. Direct bonding typically takes place at 1000° C. and requires extremely good surface flatness and cleanliness. Intermediate-layer bonds are typically formed with brazes or solders such as AuSi (gold silicon), AuGe (gold germanium), and AuSn (gold tin). All of these brazes and solders have melting temperatures that can degrade temperature sensitive materials and devices.  
           [0004]    Thus, what is needed is a method that bonds wafers without exposing MEMS devices and IC&#39;s to high temperatures and high voltages.  
         SUMMARY  
         [0005]    In one embodiment of the invention, a method for forming device packages includes forming a perimeter comprising a reactive foil and a bonding material interposed between a first wafer and a second wafer, pressing the first and the second wafers against the reactive foil and the bonding material, initiating the reactive foil whereby the reactive foil heats the bonding material to create a bond between the first and the second wafers, and singulating the first and the second wafers into the device packages. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIGS. 1, 2, and  3  illustrate cross-sections of wafer structures for forming device packages in various embodiments of the invention.  
         [0007]    [0007]FIG. 1A illustrates a top view of the pattern formed by the reactive foil and the bonding material around devices on a wafer in one embodiment of the invention.  
         [0008]    [0008]FIGS. 4A, 4B,  4 C, and  4 D illustrate the resulting cross-sections of a wafer structure formed by a method in one embodiment of the invention.  
         [0009]    [0009]FIGS. 5A and 5B illustrate the resulting cross-sections of a wafer structure formed by a method in another embodiment of the invention.  
         [0010]    [0010]FIG. 6 illustrates the resulting cross-section of a wafer structure formed by a method in another embodiment of the invention.  
         [0011]    [0011]FIGS. 7A and 7B illustrate the resulting cross-sections of a wafer structure formed by a method in another embodiment of the invention.  
         [0012]    [0012]FIG. 8 illustrates the resulting cross-section of a wafer structure formed by a method in another embodiment of the invention.  
         [0013]    [0013]FIGS. 9A and 9B illustrate alternative layer geometries for the reactive foil and the bonding material in embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]    In accordance with one aspect of the invention, devices packages are formed with reactive foils at both the die and wafer levels. In one embodiment of the invention, a reactive foil and a bonding material are patterned to form one or more perimeters around devices atop a first wafer. A second wafer is aligned with the first wafer and the reactive foil is initiated to start an exothermic reaction that releases heat. The heat from the reactive foil is localized away from the devices. The heat melts the bonding material to form a bond between the first and the second wafers. The bonded wafer structure is singulated to form individual packages containing the devices.  
         [0015]    [0015]FIG. 1 illustrates a cross-section of a structure  10  that forms one or more device packages in one embodiment of the invention. Structure  10  includes a base wafer  12  and a cover wafer  14 . Depending on the application, one or both of wafers  12  and  14  can have bulk or surface micro-machined features. Micro-machined features include electronic devices, electromechanical sensors, micro-actuators, optical components, and mechanical alignment marks.  
         [0016]    A reactive foil  16  is formed atop wafer  12 . Reactive foil  16  includes alternating layers of reactive materials (e.g., aluminum and nickel) that produces an exothermic reaction when initiated. Reactive foil  16  can be formed atop wafer  12  by magnetron sputtering and patterned by methods described later in the present disclosure. For more details on reactive foils, please refer to (1) International Application No. PCT/US01/14052, International Publication No. WO 01/83205, published Nov. 8, 2001, (2) International Application No. PCT/US01/14053, International Publication No. WO 01/83182, (3) U.S. patent application Ser. No. 09/846,422, US Patent Application Publication No. US 2001/0038029, published Nov. 8, 2001, (4) U.S. patent application Ser. No. 09/846,447, US Patent Application Publication No. US 2001/0046597, published Nov. 29, 2001.  
         [0017]    A bonding material  18  is next formed atop reactive foil  16 . Bonding material  18  can be a solder, a braze, or any other bonding agent requiring heat to transform the bonding material into its final state. Bonding material  18  can be formed atop reactive foil  16  by evaporation or sputtering. Reactive foil  16  and bonding material  18  are then patterned to form part of one or more perimeters  17  (FIG. 1A) around device areas  19  (FIG. 1A) on wafer  12 . Wafers  12  and  14  are then aligned to the required accuracy. A nominal force is applied to press wafers  12  and  14  against reactive foil  16  and bonding material  18 . This prevents movement of wafers  12  and  14  when reactive foil  16  is initiated. Reactive foil  16  is initiated to provide a localized heat source for bonding material  18 . As a result, bonding material  18  bonds wafers  12  and  14 . After the exothermic reaction, reactive foil  16  leaves behind an intermetallic mixture consisting of the materials from the reactive foil (e.g., aluminum and nickel) and the bonding material.  
         [0018]    Alternatively, bonding material  18  (e.g., solder) can be deposited atop reactive foil  16  by plating or screen printing, instead of evaporation or sputtering, after reactive foil  16  has been patterned. A photoresist, such as a plating or screen printing mask, is first applied to areas that do not need solder and then the entire device or wafer is plated or screen printed with bonding material  18  to form the desired bond lines. The photoresist mask is then cleaned off. Plating or screen printing offers a cost advantage over evaporation and sputtering of the bonding material.  
         [0019]    Bonding wafers  12  and  14  form a partial package for an electronic device such as an IC laser, a MEMS device such as an electromechanical sensor, or an optoelectronic device such as a semiconductor laser (e.g., Fabry-Perot, distributed feedback, vertical cavity surface emitting lasers), light emitting diodes, and photodetectors (e.g., positive-intrinsic-negative photodetectors and monitor diodes). After the reactive foil bonding, a device  20  is placed, aligned, and bonded to base wafer  12  through a hole  22  in cover wafer  14 . Although not shown in FIG. 1, one skilled in the art understands there are multiple devices  20  in structure  10 . Structure  10  is singulated to form individual device packages.  
         [0020]    [0020]FIG. 2 illustrates a cross-section of a structure  50  that forms one or more device packages in another embodiment of the invention. Structure  50  includes base wafer  12  and a cover wafer  52 . In this embodiment, base wafer  12  has device  20  constructed thereon or placed, aligned, and bonded prior to reactive foil bonding. Furthermore, cover wafer  52  has a cavity  54  to accommodate device  20 . Depending on the application, wafers  12  and  14  can also have other bulk or surface micro-machined features.  
         [0021]    As similarly described above, wafers  12  and  52  are bonded using reactive foil  16  and bonding material  18 . Bonding wafers  12  and  52  may form a complete hermetic package for electronic and MEMS devices. As one skilled in the art understands, structure  50  may include multiple devices  20  and may be singulated to form individual device packages.  
         [0022]    [0022]FIG. 3 illustrates a cross-section of a structure  80  that forms one or more device packages in another embodiment of the invention. Structure  80  includes base wafer  12 , an intermediate ring wafer  82 , and a cover wafer  84 . Depending on the application, wafers  12 ,  82 , and  84  can have bulk or surface micro-machined features.  
         [0023]    Wafers  12  and  82  are bonded using reactive foil  16  and bonding material  18 . Reactive foil  16  and bonding material  18  are formed atop wafer  12  and then patterned to form perimeters  17  (FIG. 1A) on wafer  12 . Wafers  12  and  82  are then aligned to the required accuracy. A nominal force is applied to press wafers  12  and  82  against reactive foil  16  and bonding material  18 . This prevents movement of wafers  12  and  82  when reactive foil  16  is initiated. Reactive foil  16  is then initiated and, as a result, bonding material  18  creates a bond between wafers  12  and  82 .  
         [0024]    Device  20  is next placed, aligned, and bonded to base wafer  12  through a hole  86  in ring wafer  82 . Wafers  82  and  84  are then bonded using a reactive foil  16 A and a bonding material  18 A. Reactive foil  16 A and bonding material  18 A are formed atop cover wafer  84  and then patterned to form perimeters on cover wafer  84 . These perimeters on cover wafer  84  correspond to and are located opposite of perimeters  17  (FIG. 1A) on wafer  12 . Wafers  82  and  84  are then aligned to the required accuracy. A nominal force is applied to press wafers  82  and  84  against reactive foil  16 A and bonding material  18 A. This prevents movement of wafers  82  and  84  when reactive foil  16 A is initiated. Reactive foil  16 A is then initiated and, as a result, bonding material  18 A creates a bond between wafers  82  and  84 .  
         [0025]    Bonding wafers  12 ,  82 , and  84  may form a complete hermetic package for electronic and MEMS devices. As one skilled in the art understands, structure  80  may include multiple devices  20  and may be singulated to form individual device packages.  
         [0026]    In accordance with another aspect of the invention, a method is provided to pattern the reactive foil. Patterning the reactive foil is difficult because it is reactively thick (e.g., 20 to 100 microns) compared to conventional metal layers (e.g., 1 micron) in semiconductor processing. In one embodiment of the invention, two layers of photoresist form a lift-off mask used to pattern the reactive foil. In another embodiment of the invention, a mechanical lift-off mask is used to pattern the reactive foil. In yet another embodiment of the invention, a lithographic etch is used to pattern the reactive foil.  
         [0027]    [0027]FIGS. 4A to  4 D illustrate a method to pattern a reactive foil in one embodiment of the invention. Referring to FIG. 4A, a first photoresist layer  112  is formed atop a wafer  114 . A second photoresist layer  116  is then formed atop of photoresist  112 . The material of photoresist  116  is selected to develop at a slower rate than photoresist  112 . Regions  122  of photoresist  112  and regions  124  of photoresist  116  are exposed to light  118  through a mask or reticle  120 . Photoresists  112  and  116  are then developed to form windows  126  through photoresists  116  and  112  as shown in FIG. 4B. As a result of the different development rate, photoresist  116  forms overhangs  127  above photoresist  112 .  
         [0028]    Referring to FIG. 4C, a reactive foil  128  is deposited atop photoresist  116  and through windows  126  onto wafer  114 . Reactive foil  128  can be deposited by sputtering. Bonding material  130  is next deposited atop reactive foil  128 . Bonding material  120  can be deposited by evaporation or sputtering. Overhangs  127  (FIG. 4B) prevents a continuous layer of reactive foil  128  and bonding material  130  to form over wafer  114 .  
         [0029]    Referring to FIG. 4D, photoresists  116  and  112  are stripped, and reactive foil  128  and bonding material  130  thereon are lifted off. The remaining reactive foil  128  and bonding material  130  on wafer  114  form the desired bond pattern (e.g., perimeters  17  in FIG. 1A) between wafer  114  and another wafer (e.g., a ring or a cover wafer).  
         [0030]    In another embodiment, instead of using two photoresist layers with different development rates, the top photoresist layer can be treated with chlorobenzene to reduce its development rate to achieve the undercut profile. Alternatively, a single thick photoresist layer can have its top surface treated with chlorobenzene to achieve the undercut profile.  
         [0031]    In yet another embodiment, bonding material  130  is plated or screen printed on top of reactive foil  128  after reactive foil  128  alone has been deposited and patterned with the steps shown in FIGS. 4A to  4 D.  
         [0032]    [0032]FIGS. 5A and 5B illustrate another method to pattern a reactive foil in one embodiment of the invention. Referring to FIG. 5A, photoresist layer  112  is formed atop wafer  114 . Photoresist  112  is patterned by exposing regions  122  to light  118  through a mask or reticle  164 .  
         [0033]    Referring to FIG. 5B, photoresist layer  116  is formed atop photoresist  112 . In this embodiment, photoresists  112  and  116  can have the same development rate. Photoresist  116  is patterned by exposing regions  124  to light  118  through a mask or reticle  168 . Reticle  168  has smaller windows than reticle  164 . Thus, region  124  is smaller than region  122 .  
         [0034]    Regions  122  and  124  are then developed to form windows  126  with overhangs  127  as shown in FIG. 4B. Reactive foil  128  and bonding material  130  are next formed and then patterned with the remaining photoresists  112  and  116  as described above in reference to FIGS. 4C and 4D. Alternatively, bonding material  130  can be plated or screen printed on top of reactive foil  128  after reactive foil  128  alone has been deposited and patterned with the steps shown in FIGS. 5A and 5B.  
         [0035]    [0035]FIG. 6 illustrates another method to pattern a reactive foil in one embodiment of the invention. In this embodiment, a mechanical mask or stencil  192  is used to pattern reactive foil  128  and bonding material  130 . Mask  192  can be made of stainless steel, glass, or silicon wafer into which undercut windows  194  are machined or etched. Reactive foil  128  and bonding material  130  are deposited through windows  194  to form the desired bond pattern. The excess reactive foil  128  and bonding material deposited on mask  192  can be stripped so mask  192  can be reused. Alternatively, bonding material  130  can be plated or screen printed on top of reactive foil  128  after reactive foil  128  alone has been deposited and patterned with the steps shown in FIG. 6.  
         [0036]    [0036]FIGS. 7A and 7B illustrate another method to pattern a reactive foil in one embodiment of the invention. Referring to FIG. 7A, reactive foil  128  is formed over wafer  114 . Bonding material  130  is formed atop reactive foil  128 . Bonding material  130  can be deposited on top of reactive foil  128  by evaporation or sputtering. Alternatively bonding material  130  can be plated or screen printed on top of reactive foil  128 . A photoresist layer  220  is formed atop bonding material  130 . Photoresist  220  is then patterned by exposing regions  222  to light  224  through a mask or reticle  226 .  
         [0037]    Regions  222  are then developed to form etching windows  228  in photoresist  220 . Regions  128 A and  130 A left unprotected by the remaining photoresist  220  are etched away, and the remaining photoresist  220  is stripped, to form the structure shown in FIG. 4D.  
         [0038]    In accordance with another aspect of the invention, a reactive foil is used to bond a large number of devices, such as IC lasers, in a parallel fashion. Referring to FIG. 8, wafer  114  is bulk or surface micro-machined according to the specific application. A metal layer  250  is formed and patterned to form metal lines atop wafer  114 . A reactive foil  252  and a conductive bonding material  254  are formed and patterned atop metal layer  250 . Reactive foil  252  and bonding material  254  can be patterned by any of the methods described above.  
         [0039]    Devices  256  are aligned and placed atop bonding material  254 . A nominal force is applied to press device  256  and wafer  114  against reactive foil  252  and bonding material  254 . This prevents movement of devices  256  and wafer  114  when reactive foil  252  is initiated. Reactive foil  252  is then initiated to heat bonding material  254 . As a result, bonding material  254  forms a bond between devices  256  and their corresponding metal  250 . After the exothermic reaction, reactive foil  252  leaves behind an intermetallic mixture consisting of the materials from the reactive foil (e.g., aluminum and nickel) and the bonding material. The steps described above can be used to bond the devices in FIGS. 1, 2, and  3  to their base wafers.  
         [0040]    In the embodiments described above, the layer geometry consists of a bonding material (e.g., solder) on top of a reactive foil. However, different layer geometries can be used to bond the wafers and to form device packages. FIG. 9A illustrates a layer geometry where reactive foil  128  is formed on top of bonding material  130  interposed between wafers  114 A and  114 B to bond the wafers. FIG. 9B illustrates a layer geometry where a sandwich of bonding material  130 A, reactive foil  128 , and bonding material  130 B is interposed between wafers  114 A and  114 B to bond the wafers. Furthermore, this layer geometry may be repeated.  
         [0041]    Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.