Abstract:
More complete bonding of wafers may be achieved out to the edge regions of the wafer by constrained bond strengthening of the wafers in a pressure bonding apparatus after direct wafer bonding. The pressure bonding process may be accompanied by the application of not above room temperature.

Description:
BACKGROUND  
         [0001]    This invention relates generally to wafer bonding.  
           [0002]    In wafer bonding, two semiconductor wafers may be placed in a face-to-face configuration. A layer on one semiconductor wafer may be transferred to the other semiconductor wafer in a process called wafer bonding. A wide variety of layers may be transferred between semiconductor wafers. One application for wafer bonding is in connection with forming silicon on insulator (SOI) devices.  
           [0003]    Generally, a pair of opposed flat silicon wafers are contacted to one another so that they physically and chemically bond. A layer is transferred from a donor wafer to a handle wafer.  
           [0004]    One problem with existing wafer bonding processes is that a peripheral region of the handle wafer, generally about 3 to 5 millimeters, may remain unbonded. This unbonded peripheral region is a region on the outer periphery of the wafer extending radially inwardly from the edge of the wafer to a distance of about 3 to 5 millimeters.  
           [0005]    As a result of this unbonded region, islands of material, debris, particles, and flakes may collect in the unbonded region created by the resulting edge. These particles may ultimately release, resulting in problematic defects. In addition, the wafers may only have a useable surface area up to 3 to 5 millimeters inwardly of the outermost edge. The unbonded area may result in some loss of useable wafer area.  
           [0006]    Thus, there is a need for better ways to wafer bond wafers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a cross-sectional view of one embodiment of the present invention;  
         [0008]    [0008]FIG. 2 is a cross-sectional view of wafers in accordance with one embodiment of the present invention; and  
         [0009]    [0009]FIG. 3 is a partial cross-sectional view of the results of wafer bonding in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]    Referring to FIG. 1, a pair of wafers  12   a  and  12   b  may be located in a free state condition on a bonding plate  20 . Bonding may be initiated at the edge or center of the wafers  12 .  
         [0011]    During bonding, elastic deformation of donor wafer  12   b  and handle wafer  12   a  may occur microscopically, compensating for surface roughness, topography, flatness, and profile in the center and edge regions of the wafers. The strength of the surface Van der Waals forces may not be sufficient to elastically deform the areas at the edges of the wafers  12 .  
         [0012]    Even where bonding occurs at the edges of the wafers  12 , the bonding forces may not be strong enough to overcome the natural tendency for the wafers to pull apart due to the surface characteristics. This leads to non-layer transfer areas resulting in loss of transferred device film of up to 5 millimeters unbonded inboard of the circular area at the handle wafer edge.  
         [0013]    Thus, as shown in FIG. 1, the wafers  12  may be pressed together to direct bond and transfer the film  14  from the donor wafer  12   b  to the handle wafer  12   a . The donor wafer  12   b  may be mounted on a mounting plate  20 . The wafers  12   a  and  12   b  may be pressed together centrally or peripherally as indicated by the pressure element  16  and the associated arrow. At such time, the wafers  12  may be held in alignment by the jig  18 .  
         [0014]    After direct bonding, the bonded pair may be put into a pressure bonding apparatus, shown in FIG. 2, to flatten and bring into contact unbonded areas that split from the initial bonding of the handle and donor wafers  12 . In this case, a pair of rigid, flat, parallel plates  22   a  and  22   b  may be positioned on either opposed surface of the bonded wafers  12  and pressure may be applied substantially uniformly across at least one plate  22  while the other plate  12  is supported. In one embodiment, the applied pressure may be from 0.01 pounds per square inch to 0.35 pounds per square inch. The pressure may be applied for 10 to 30 minutes in some embodiments.  
         [0015]    Bond strengthening may be achieved by heat treatment of the bonded pair in the pressure bonding apparatus shown in FIG. 2. The heat treatment may convert Van der Waals surface interactions into stronger covalent bonds between donor and handle wafers  12  over the entire wafer contact area.  
         [0016]    Then subsequent layer exfoliation results in more complete device layer transfer as shown in FIG. 3. As indicated in FIG. 3, the film  14  from the donor wafer  12   b  may be transferred close to the peripheral edge  24  of the wafer  12   a . In one embodiment, the wafer  12   a  may be a silicon on insulator wafer having bulk silicon  28  covered by an insulator  30  over which is bonded the film  14 .  
         [0017]    The heat processing may involve temperatures of 100 to 600° C. for times from 1 to 30 minutes in some embodiments of the present invention.  
         [0018]    As a result, in some embodiments, even where-wafer non-uniformities occur, direct wafer bonding of donor and handle wafers accompanied by constrained annealing of the bonded pair facilitate complete wafer bonding. As a result, the 3 to 5 millimeter region of non-bonding with conventional processes may be reduced, facilitating complete wafer surface bonding. In some embodiments, less than 3 millimeters of edge exclusion  26  may occur with complete surface area contact and film  14  bonding across the wafer  12   a.    
         [0019]    This more complete bonding may reduce the edge region that tends to collect particles and flakes. This may reduce the ensuing defects caused by such particles in some embodiments.  
         [0020]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.