Abstract:
A silicon direct bonding method including preparing two silicon substrates having corresponding bonding surfaces, forming a trench in at least one bonding surface of the two silicon substrates, and thermally bonding the two silicon substrates to one another. The trench may be along a dicing line. The trench may communicate with an outer edge of the bonded substrates.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a silicon direct bonding method. More particularly, the present invention relates to a silicon direct bonding method by which void formation caused by gases is suppressed.  
         [0003]     2. Description of the Related Art  
         [0004]     Typically, a silicon substrate called a ‘wafer’ is used to manufacture semiconductor devices. For example, various semiconductor devices may be formed through micromachining, processing, etc., including, e.g., forming a predetermined material layer on the silicon substrate, etching a surface of the silicon substrate, etc.  
         [0005]     In manufacturing a semiconductor device, two silicon substrates may be bonded to each other. One method of bonding is silicon direct bonding (SDB), which has been generally applied to bonding silicon substrates. Generally, the SDB method may include the following operations. After two silicon substrates are prepared, the substrates are cleaned and a thin film of ions and/or molecules, e.g., OH − , H + , H 2 O, H 2 , O 2 , etc., is formed on the bonding surfaces of the two substrates. The two substrates are then put in close contact with one another, which results in the substrates becoming attached to each other. In detail, the two substrates are attached due to the power of the Van der Waals force existing between the ions/molecules on the opposing substrates. This Van der Waals force serves to maintain the substrates in position, i.e., they are pre-bonded by it. If the two pre-bonded substrates are then subjected to a thermal bonding process, e.g., by being put into a thermal treatment furnace and heated up to approximately 1000° C., the two substrates may be strongly bonded due to interdiffusion between atoms of the two opposing substrates.  
         [0006]     In the SDB process just described, gases may be generated during the thermal bonding process by the ions/molecules that exist between the two substrates. The gases may not be completely discharged and may remain to cause voids at the junction of the two substrates. The voids may decrease bond strength between two silicon substrates and may elevate the defect rate of the resultant bonded semiconductor devices, detrimentally affecting yield. Moreover, the void problem may become more significant as substrate sizes increases and the bonding areas increase accordingly.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention is therefore directed to a SDB method by which void formation caused by gases is suppressed, which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.  
         [0008]     It is therefore a feature of an embodiment of the present invention to provide a SDB method in which a trench is formed on one or more bonding surfaces of the opposing silicon substrates, so that gases generated during a thermal treatment process may be discharged, thereby reducing or eliminating void formation caused by the gases.  
         [0009]     It is therefore another feature of an embodiment of the present invention to provide a SDB method in which a trench is formed along a dicing line used to singulate the bonded substrates.  
         [0010]     At least one of the above and other features and advantages of the present invention may be realized by providing a silicon direct bonding method including preparing two silicon substrates having corresponding bonding surfaces, forming a trench in at least one bonding surface of the two silicon substrates, and thermally bonding the two silicon substrates to one another.  
         [0011]     The method may further include cleaning the two silicon substrates after forming the trench. A silicon oxide film may be formed on at least one surface of the two silicon substrates, and the trench may be formed in the silicon oxide film. The trench may be formed along at least a part of a plurality of dicing lines.  
         [0012]     The dicing lines may include a first plurality of lines that extend in a first direction and a second plurality of lines that extend in a second direction perpendicular to the first direction. The method may further include forming a plurality of trenches along one or both of the first and second plurality of lines.  
         [0013]     The trench may extend to the outer edge of the substrate. The trench may be formed to a predetermined depth. Forming the trench may include etching. Forming the trench may further include depositing a photoresist layer on one of the bonding surfaces, forming a pattern in the photoresist layer, and using the patterned photoresist layer as an etching mask.  
         [0014]     At least one of the above and other features and advantages of the present invention may also be realized by providing a method of forming a bonded semiconductor structure including providing two silicon substrates, at least one of the substrates having a plurality of active devices formed thereon, forming a plurality of trenches in a bonding surface of at least one of the two silicon substrates, thermally bonding the two silicon substrates together, and singulating the bonded substrates into a plurality of bonded semiconductor structures, wherein the bonded substrates are singulated along dicing lines, and the plurality of trenches corresponds to the dicing lines.  
         [0015]     Thermally bonding the two silicon substrates together may form a bonded substrate structure, the bonded substrate structure including a plurality of channels at an interface of the two silicon substrates, the plurality of channels corresponding to the plurality of trenches. The plurality of channels may communicate to a circumferential edge of the bonded substrate structure. The plurality of trenches may include first trenches formed in a first direction and second trenches formed in a second direction perpendicular to the first direction, the second trenches intersecting the first trenches. The method may further include, before thermally bonding, applying a thin film to bonding surfaces of the two silicon substrates, the thin film including one or more of OH −  ions, H +  ions, H 2 O molecules, H 2  molecules and O 2  molecules. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0017]      FIGS. 1A-1E  illustrate cross-sectional views of stages in a method of bonding two substrates according to a first embodiment of the present invention;  
         [0018]      FIG. 2  illustrates a perspective view of a trench formed on a bonding surface of a substrate according to the first embodiment of the present invention;  
         [0019]      FIGS. 3A-3C  illustrate cross-sectional views of stages in a method of bonding two substrates according to a second embodiment of the present invention; and  
         [0020]      FIG. 4  illustrates a perspective view of a trench formed on a bonding surface of a substrate according to the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     Korean Patent Application No. 10-2005-0000831, filed on Jan. 5, 2005, in the Korean Intellectual Property Office, and entitled: “Silicon Direct Bonding Method,” is incorporated by reference herein in its entirety.  
         [0022]     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the figures, the dimensions of layers and regions are exaggerated for clarity of illustration. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being “under” another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout.  
         [0023]     In the SDB method according to the present invention, gases generated during a thermal treatment process may be discharged through one or more trenches existing at the interface of opposing silicon substrates, so that void formation at the interface may be prevented or minimized.  
         [0024]      FIGS. 1A-1E  illustrate cross-sectional views of stages in a method of bonding two substrates according to a first embodiment of the present invention, and  FIG. 2  illustrates a perspective view of a trench formed on a bonding surface of a substrate according to the first embodiment of the present invention. Referring to  FIG. 1A , a SDB method according to the first embodiment of the present invention may be used to bond first and second substrates  110 ,  120 . The first and second substrates  110 ,  120  may be, e.g., silicon substrates. The first and second silicon substrates  110 ,  120  may be a type of silicon wafer commonly used in the manufacture of semiconductor device. The first silicon substrate  110  may include a first bonding surface  111  and the second silicon substrate  120  may include a second bonding surface  121  corresponding to the first bonding surface  111 .  
         [0025]     Referring to  FIG. 1B , a photoresist PR may be coated on one of the first and second silicon substrates  110  and  120 . As illustrated, the PR is coated on the first bonding surface  111  of the first silicon substrate  110 . Subsequently, the PR may be patterned in a predetermined pattern through an exposure and development process to expose a part of the first bonding surface  111 .  
         [0026]     Referring to  FIG. 1C , the first bonding surface  111  may be etched to a predetermined depth, using the PR as an etching mask, thereby forming one or more trenches  114 . Etching of the first bonding surface  111  may be performed by, e.g., dry etching using a method such as reactive ion etching (RIE), a wet etching method, etc. After etching, the PR may be stripped, leaving the trench  114  formed in the first bonding surface  111 , as shown in  FIG. 1D .  
         [0027]     In the above-described operations, it will be appreciated that the trench may be formed in the second substrate  120 , or in both the first and second substrates  110 ,  120 , and that the above-described operations are merely exemplary. Further, a cross-sectional shape of the trenches, while shown as being rectangular, may be any shape, particularly in accordance with a formation process of the trenches.  
         [0028]     Referring to  FIG. 2 , the trench  114  may be formed along a plurality of dicing lines L D1  and L D2 , in order not to affect semiconductor devices formed on the silicon substrates  110 ,  120 . As used herein, dicing refers to singulation of the substrates, wherein a plurality of semiconductor devices formed on the two silicon substrates  110 ,  120  are separated into individual dies by, e.g., cutting. The dicing lines L D1  and L D2  may include first lines L D1  that extend in a first direction and second lines L D2  that extend in a second direction perpendicular to the first direction. A plurality of trenches  114  may be formed along the first lines L D1  and the second lines L D2 , as shown in  FIG. 2 . Alternatively, trenches  114  may be formed along only one of the first lines L D1  and the second lines L D2 .  
         [0029]     The trenches  114  may be formed to extend to the circumference of the two silicon substrates  110 ,  120  and may communicate to the outside of the silicon substrates  110 ,  120 . Thus, gases generated between the silicon substrates  110 ,  120  may be discharged to the outside of the silicon substrates  110 ,  120 , as will be described in further detail below.  
         [0030]     The first silicon substrate  110  and the second silicon substrate  120  may be cleaned (not shown) after forming the trenches  114 . The cleaning operation may include, e.g., a cleaning process and a drying process.  
         [0031]     A thin film (not shown) may be formed on the first bonding surface  111  of the first silicon substrate  110  and on the second bonding surface  121  of the second silicon substrate  120 . The thin film may include, e.g., ions and/or molecules such as OH − , H + , H 2 O, H 2  and O 2 , etc.  
         [0032]     Referring to  FIG. 1E , bonding surfaces  111 ,  121  of the first and second silicon substrates  110 ,  120  may be brought into close contact with one another, such that the two silicon substrates  110 ,  120  are pre-bonded by Van der Waals forces between the above-described ions/molecules. The two silicon substrates  110 ,  120  in the pre-bonded state may then be thermally bonded. Thermal bonding may include, e.g., putting the pre-bonded substrates  110 ,  120  into a thermal treatment furnace and thermally heating to approximately 1000° C. Thus, the two silicon substrates  110  and  120  may strongly bonded due to interdiffusion between atoms of the two silicon substrates  110 ,  120 .  
         [0033]     During thermal bonding, gases may be generated by ions/molecules existing at the interface between the two silicon substrates  110 ,  120 . According to the present invention, the gases may flow into the trench  114 , and may flow through the trench  114  to be smoothly discharged to the outside of the silicon substrates  110 ,  120 . The gases may exit the trench  114  at the circumferential edge of the silicon substrates  110 ,  120 .  
         [0034]      FIGS. 3A-3C  illustrate cross-sectional views of stages in a method of bonding two substrates according to a second embodiment of the present invention, and  FIG. 4  illustrates a perspective view of a trench formed on a bonding surface of a substrate according to the second embodiment of the present invention. Referring to  FIG. 3A , in the SDB method according to the second embodiment of the present invention, a silicon oxide film  112  may be formed on one of two silicon substrates  110 ,  120 . As illustrated, the silicon oxide film  112  is formed on the surface of the first silicon substrate  110 . The surface of the silicon oxide film  112  may serve as a first bonding surface  111 ′. It will be appreciated that the silicon oxide film  112  may also be formed on the second silicon substrate  120 , or on both the first and second silicon substrates  110 ,  120 . If the silicon oxide film  112  is formed on one or both of the silicon substrates  110 ,  120 , the bond strength between the two silicon substrates  110 ,  120  may be enhanced.  
         [0035]     Referring to  FIG. 3B , the silicon oxide film  112  formed on the first silicon substrate  110  may be etched to a predetermined depth, thereby forming a trench  114 ′. The formation of the trench  114 ′ may be performed as described above with respect to  FIGS. 1B and 1C . The trench  114 ′ may be formed to penetrate the entire thickness of the silicon oxide film  112 , as shown in  FIG. 3B , or may be formed to a depth that is less than the thickness of the silicon oxide film  112  (not shown).  
         [0036]     Referring to  FIG. 4 , the trench  114 ′ can be formed along all or part of dicing lines L D1 , L D2 . The trench  114 ′ may also be formed in various other configurations suitable to allow gases to be smoothly discharged from the bonding area of the silicon substrates  110 ,  120 . The trench  114 ′ may be formed to extend to the circumference of the two silicon substrates  110 ,  120  and may communicate with the outside of the silicon substrates  110 ,  120  via a circumferential edge thereof.  
         [0037]     The first and second silicon substrates  110 ,  120  may be cleaned, a film of ions/molecules may be applied, and, as shown in  FIG. 3C , the first and second silicon substrates  110 ,  120  may be brought into close contact with one another. The first and second silicon substrates  110 ,  120 , in the closely-contacted state, may then be subjected to a thermal bonding process by, e.g., being put into a thermal treatment furnace and thermally heated to approximately 1000° C. Thus, the first and second silicon substrates  110 ,  120  may be bonded by interdiffusion of atoms between the first and second silicon substrates  110 ,  120 .  
         [0038]     During thermal treatment, gases generated by ions/molecules that exist at the interface between the first and second silicon substrates  110 ,  120  may be discharged through the trench  114 ′ to the outside of the silicon substrates  110 ,  120  in a similar fashion to that described above with respect to the first embodiment.  
         [0039]     As described above, in the SDB method according to the present invention, a trench may be formed on one or more bonding surfaces of the two substrates to be bonded, such that gases generated during thermal treatment may be smoothly discharged. Thus, void formation may be reduced or eliminated at the junctions of the two bonded substrates. Accordingly, the bond strength of the two substrates may be enhanced, defect rates lowered, and yields improved.  
         [0040]     Exemplary embodiments of the present invention have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.