Patent Publication Number: US-10763292-B2

Title: Interconnect apparatus and method for a stacked semiconductor device

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This is a continuation application of U.S. application Ser. No. 15/395,360, entitled “Interconnect Apparatus and Method” which was filed on Dec. 30, 2016 which is a divisional application of U.S. application Ser. No. 13/890,841, entitled “Interconnect Apparatus and Method” which was filed on May 9, 2013 and issued as U.S. Pat. No. 9,536,777 on Jan. 3, 2017, which claims priority to U.S. Provisional Application Ser. No. 61/780,465, entitled “Interconnect Apparatus and Method” which was filed on Mar. 13, 2013, all of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The semiconductor industry has experienced rapid growth due to continuous improvements in the integration density of a variety of electronic components (e.g., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size (e.g., shrink the semiconductor process node towards the sub-20 nm node), which allows more components to be integrated into a given area. As the demand for miniaturization, higher speed and greater bandwidth, as well as lower power consumption and latency has grown recently, there has grown a need for smaller and more creative packaging techniques of semiconductor dies. 
     As semiconductor technologies further advance, stacked semiconductor devices have emerged as an effective alternative to further reduce the physical size of a semiconductor device. In a stacked semiconductor device, active circuits such as logic, memory, processor circuits and the like are fabricated on different semiconductor wafers. Two or more semiconductor wafers may be installed on top of one another to further reduce the form factor of the semiconductor device. 
     Two semiconductor wafers may be bonded together through suitable bonding techniques. The commonly used bonding techniques include direct bonding, chemically activated bonding, plasma activated bonding, anodic bonding, eutectic bonding, glass frit bonding, adhesive bonding, thermo-compressive bonding, reactive bonding and/or the like. Once two semiconductor wafers are bonded together, the interface between two semiconductor wafers may provide an electrically conductive path between the stacked semiconductor wafers. 
     One advantageous feature of stacked semiconductor devices is much higher density can be achieved by employing stacked semiconductor devices. Furthermore, stacked semiconductor devices can achieve smaller form factors, cost-effectiveness, increased performance and lower power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a cross sectional view of a stacked semiconductor device prior to a bonding process in accordance with various embodiments of the present disclosure; 
         FIG. 2  illustrates a cross sectional view of the semiconductor device shown in  FIG. 1  after a bottom anti-reflection coating (BARC) layer and a plurality of hard mask layers are formed over the first semiconductor wafer in accordance with various embodiments of the present disclosure; 
         FIG. 3  illustrates a cross sectional view of the semiconductor device shown in  FIG. 2  after a patterning process is applied to the hard mask layers and the BARC layer of the first semiconductor wafer in accordance with various embodiments of the present disclosure; 
         FIG. 4  illustrates a cross sectional view of the semiconductor device shown in  FIG. 3  after an etching process is applied to the substrate of the first semiconductor wafer in accordance with various embodiments of the present disclosure; 
         FIG. 5  illustrates a cross section view of the semiconductor device shown in  FIG. 4  after another etching process is applied to the semiconductor device in accordance with various embodiments of the present disclosure; 
         FIG. 6  illustrates a cross sectional view of the semiconductor device shown in  FIG. 5  after the remaining photoresist layer has been removed in accordance with various embodiments of the present disclosure; 
         FIG. 7  illustrates a cross section view of the semiconductor device shown in  FIG. 6  after a dielectric layer is deposited over the semiconductor device in accordance with various embodiments of the present disclosure; 
         FIG. 8  illustrates a cross sectional view of the semiconductor device shown in  FIG. 7  after an etching process is applied to some portions of the dielectric layer in accordance with in accordance with various embodiments of the present disclosure; 
         FIG. 9  illustrates a cross sectional view of the semiconductor device shown in  FIG. 6  after a conductive material has been filled in the openings in accordance with various embodiments of the present disclosure; 
         FIG. 10  illustrates a cross section view of the semiconductor device shown in  FIG. 9  after a chemical mechanical polish (CMP) process is applied to the top surface of the semiconductor device in accordance with various embodiments of the present disclosure; 
         FIG. 11  illustrates a cross sectional view of the semiconductor device shown in  FIG. 10  after a dielectric layer is formed on the semiconductor device in accordance with various embodiments of the present disclosure; 
         FIG. 12  illustrates a cross sectional view of another stacked semiconductor device in accordance with various embodiments of the present disclosure; 
         FIG. 13  illustrates a cross sectional view of yet another stacked semiconductor device in accordance with various embodiments of the present disclosure; 
         FIG. 14  illustrates a cross sectional view of a backside illuminated imager sensor including a stacked wafer structure in accordance with various embodiments of the present disclosure; 
         FIG. 15  illustrates a top view of the hard mask in accordance with various embodiments of the present disclosure; and 
         FIG. 16  illustrates another top view of the hard mask in accordance with various embodiments of the present disclosure. 
     
    
    
     Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to preferred embodiments in a specific context, a method for forming interconnect structures for a stacked semiconductor device. The invention may also be applied, however, to a variety of semiconductor devices. Hereinafter, various embodiments will be explained in detail with reference to the accompanying drawings. 
       FIG. 1  illustrates a cross sectional view of a stacked semiconductor device prior to a bonding process in accordance with various embodiments of the present disclosure. Both the first semiconductor wafer  110  and the second semiconductor wafer  210  include a semiconductor substrate (e.g., first substrate  102  and second substrate  202 ) and a plurality of interconnect structures (e.g., metal lines  106 ,  108 ,  206  and  208 ) formed over the semiconductor substrate. The first semiconductor wafer  110  is used as an example to illustrate the detailed structure of the semiconductor wafers prior to a bonding process. 
     As shown in  FIG. 1 , the first semiconductor wafer  110  may comprises a first substrate  102  and a plurality of inter-metal dielectric layers  104  formed over the first substrate  102 . In addition, a plurality of metal lines such as metal lines  106  and  108  are formed in the inter-metal dielectric layers  104 . 
     The first substrate  102  may be formed of silicon, although it may also be formed of other group III, group IV, and/or group V elements, such as silicon, germanium, gallium, arsenic, and combinations thereof. The first substrate  102  may also be in the form of silicon-on-insulator (SOI). The SOI substrate may comprise a layer of a semiconductor material (e.g., silicon, germanium and/or the like) formed over an insulator layer (e.g., buried oxide and/or the like), which is formed in a silicon substrate. In addition, other substrates that may be used include multi-layered substrates, gradient substrates, hybrid orientation substrates, any combinations thereof and/or the like. 
     The first substrate  102  may further comprise a variety of electrical circuits (not shown). The electrical circuits formed on the first substrate  102  may be any type of circuitry suitable for a particular application. In accordance with some embodiments, the electrical circuits may include various n-type metal-oxide semiconductor (NMOS) and/or p-type metal-oxide semiconductor (PMOS) devices such as transistors, capacitors, resistors, diodes, photo-diodes, fuses and/or the like. 
     The electrical circuits may be interconnected to perform one or more functions. The functions may include memory structures, processing structures, sensors, amplifiers, power distribution, input/output circuitry and/or the like. One of ordinary skill in the art will appreciate that the above examples are provided for illustrative purposes only and are not intended to limit the various embodiments to any particular applications. 
     The inter-metal dielectric layers  104  are formed over the first substrate  102 . As shown in  FIG. 1 , the inter-metal dielectric layers  104  may comprise a plurality of metal lines such as metal lines  106  and  108 . 
     The metal lines  106  and  108  may be made through any suitable formation process (e.g., lithography with etching, damascene, dual damascene, or the like) and may be formed using suitable conductive materials such as copper, aluminum, aluminum alloys, copper alloys or the like. 
     As shown in  FIG. 1 , the first semiconductor wafer  110  will be stacked on top of the second semiconductor wafer  210 . In some embodiments, a plurality of bonding pads are formed in the first semiconductor wafer  110  and the second semiconductor wafer  210  respectively. Furthermore, the bonding pads located at the second semiconductor wafer  210  are aligned face-to-face with their corresponding bonding pads located at the first semiconductor wafer  110 . The first semiconductor wafer  110  and the second semiconductor wafer  210  are bonded together through suitable bonding techniques such as direct bonding. 
     In accordance with some embodiments, in a direct bonding process, the connection between the first semiconductor wafer  110  and the second semiconductor wafer  210  can be implemented through metal-to-metal bonding (e.g., copper-to-copper bonding), dielectric-to-dielectric bonding (e.g., oxide-to-oxide bonding), metal-to-dielectric bonding (e.g., oxide-to-copper bonding), any combinations thereof and/or the like. 
     It should be noted that the bonding show in  FIG. 1  may be at wafer level. In the wafer-level bonding, wafers  110  and  210  are bonded together, and are then sawed into dies. Alternatively, the bonding may be performed at the chip level. 
     It should further be noted that the first semiconductor wafer  110  may be a backside illumination sensor and the second semiconductor wafer  210  may be a logic circuit. The backside illuminated image sensor may be formed in an epitaxial layer over a silicon substrate. According to the fabrication process of backside illuminated image sensors, the silicon substrate has been removed in a backside thinning process. A portion of epitaxial layer remains. A photo active region is formed in the remaining epitaxial layer. 
     The photo active regions may comprise, for example, photo-diodes formed by implanting impurity ions into the epitaxial layer. Furthermore, the photo active regions may be a PN junction photo-diode, a PNP photo-transistor, an NPN photo-transistor or the like. In accordance with an embodiment, the photo active regions may comprise a p-type layer formed on an n-type region, wherein the n-type region is formed on an epitaxial layer grown from a p-type semiconductor substrate. 
     The second semiconductor wafer  210  may comprise a logic circuit. The logic circuit may be an analog-to-digital converter. In addition, the logic circuit may be a data processing circuit, various embodiments may also include other circuits connected to a backside illuminated image sensor, such as a memory circuit, a bias circuit, a reference circuit and the like. 
     After the first semiconductor wafer  110  is bonded on the second semiconductor wafer  210 , a thinning process may be applied to the backside of the first semiconductor wafer in accordance with an embodiment. According to the fabrication processes of backside illuminated image sensors, the substrate is thinned until the epitaxial layer is exposed. More particularly, the backside the substrate may be thinned to a thickness in a range from about 2 um to about 2.15 um. Such a thin substrate layer allows light to pass through the substrate and hit photo diodes embedded in the substrate without being absorbed by the substrate. 
     The thinning process may be implemented by using suitable techniques such as grinding, polishing and/or chemical etching. In accordance with an embodiment, the thinning process may be implemented by using a chemical mechanical polishing (CMP) process. In a CMP process, a combination of etching materials and abrading materials are put into contact with the back side of the substrate and a grinding pad (not shown) is used to grind away the back side of the substrate until a desired thickness is achieved 
       FIG. 2  illustrates a cross sectional view of the semiconductor device shown in  FIG. 1  after a bottom anti-reflection coating (BARC) layer and a plurality of hard mask layers are formed over the first semiconductor wafer in accordance with various embodiments of the present disclosure. The BARC layer  112  is formed on a backside of the first substrate  102 . Throughout the description, the side of the first substrate  102  adjacent to the BARC layer  112  is referred to the backside of the first substrate  102 . 
     The BARC layer  112  may be formed of a nitride material, an organic material, an oxide material and the like. The BARC layer  112  may be formed using suitable techniques such as chemical vapor deposition (CVD) and/or the like. 
     A first hard mask layer  113  is formed over the BARC layer  112 . A second hard mask layer  115  is formed over the first hard mask layer  113 . In some embodiments, the first hard mask layer  113  may be formed of polysilicon. The second hard mask layer  115  is formed of oxide. Throughout the description, the first hard mask layer  113  is alternatively referred to as a poly hard mask layer  113 . The second hard mask layer  115  is alternatively referred to as an oxide hard mask layer  115 . The poly and oxide hard mask layers may be formed using suitable techniques such as CVD and/or the like. 
       FIG. 3  illustrates a cross sectional view of the semiconductor device shown in  FIG. 2  after a patterning process is applied to the hard mask layers and the BARC layer of the first semiconductor wafer in accordance with various embodiments of the present disclosure. A patterned mask  302  such as a photoresist mask and/or the like may be formed over the oxide hard mask layer  115  using suitable deposition and photolithography techniques. A suitable etching process, such as a reactive ion etch (RIE) or other dry etch, an anisotropic wet etch, or any other suitable anisotropic etch or patterning process may be applied to the hard mask layers and the BARC layer. As a result, a plurality of openings  301  and  303  are formed in the hard mask layers and the BARC layer. 
     After the openings  301  and  303  have been formed, the remaining photoresist layer (e.g., mask  302 ) may be removed by using suitable photoresist stripping techniques such as chemical solvent cleaning, plasma ashing, dry stripping and/or the like. The photoresist stripping techniques are well known and hence are not discussed in further detail herein to avoid repetition. 
       FIG. 4  illustrates a cross sectional view of the semiconductor device shown in  FIG. 3  after an etching process is applied to the substrate of the first semiconductor wafer in accordance with various embodiments of the present disclosure. After the photoresist mask has been removed by a suitable removal process, a suitable etching process, such as a dry etching, a wet etching or any other suitable patterning process may be applied to the first substrate  102  of the first semiconductor wafer  110 . During the etching process, the oxide layer  115  may function as a hard mask layer. As shown in  FIG. 4 , a plurality of openings  114  and  116  are formed in the first substrate  102 . The etching process may be performed on the first substrate  102  until the first inter-metal dielectric layer  104  is exposed. Subsequently, the oxide hard mask layer  115  may be removed by a suitable removal process. 
       FIG. 5  illustrates a cross section view of the semiconductor device shown in  FIG. 4  after another etching process is applied to the semiconductor device in accordance with various embodiments of the present disclosure. A suitable etching process, such as a dry etch, an anisotropic wet etch, or any other suitable anisotropic etch or patterning process, may be performed on the semiconductor device to form openings  504  and  506 . During the etching process, the poly layer  113  and the metal lines  106 ,  108  and  206  may function as hard mask layers. 
     The openings  504  and  506  are respective extensions of the openings  114  and  116  shown in  FIG. 4 . In particular, the openings  504  and  506  extend through the inter-metal dielectric layers  104  and  204  as well as the bonding interface of two stacked wafers. As shown in  FIG. 5 , the metal lines  106 ,  108 ,  206  and  208  are exposed after the openings  504  and  506  have been formed. 
     It should further be noted that the metal lines  106  and  108  are formed of suitable metal materials such as copper, which is of a different etching rate (selectivity) from the inter-metal dielectric layers (e.g., the inter-metal dielectric layers  104  and  204 ). As such, the metal lines  106  and  108  may function as a hard mask layer for the etching process of the inter-metal dielectric layers  104  and  204 . A selective etching process may be employed to etch the inter-metal dielectric layers  104  and  204  rapidly while etching only a portion of the metal lines  106  and  108 . As shown in  FIG. 5 , the exposed portion of the hard mask layer (e.g., metal lines  106  and  108 ) may be partially etched away, thereby forming a recess such as the recess  502  as shown in  FIG. 5 . The depth of the recess  502  may vary depending on a variety of applications and design needs. 
       FIG. 6  illustrates a cross sectional view of the semiconductor device shown in  FIG. 5  after the remaining poly layer has been removed in accordance with various embodiments of the present disclosure. During the etching process shown in  FIG. 5 , the poly layer  113  may be partially etched away or fully etched away. After the etching process, a suitable removal process may be employed to remove the remaining portion of the poly layer  113 . As shown in  FIG. 6 , the top surface of the BARC layer  112  is exposed after the poly layer  113  has been removed. 
       FIG. 7  illustrates a cross section view of the semiconductor device shown in  FIG. 6  after a dielectric layer is deposited over the semiconductor device in accordance with various embodiments of the present disclosure. As shown in  FIG. 7 , a dielectric layer  702  is formed over the bottoms and sidewalls of the openings  701  and  703 . Furthermore, the dielectric layer  702  is formed over the top surface of the semiconductor device as shown in  FIG. 7 . 
     The dielectric layer  702  may be formed of various dielectric materials commonly used in integrated circuit fabrication. For example, the dielectric layer  702  may be formed of silicon dioxide, silicon nitride or a doped glass layer such as boron silicate glass and the like. Alternatively, dielectric layer may be a layer of silicon nitride, a silicon oxynitride layer, a polyamide layer, a low dielectric constant insulator or the like. In addition, a combination of the foregoing dielectric materials may also be used to form the dielectric layer  702 . In accordance with some embodiments, the dielectric layer  702  may be formed using suitable techniques such as sputtering, oxidation, CVD and/or the like. 
       FIG. 8  illustrates a cross sectional view of the semiconductor device shown in  FIG. 7  after an etching process is applied to some portions of the dielectric layer in accordance with in accordance with various embodiments of the present disclosure. The dielectric layer  702  may be patterned and portions of the dielectric layer  702  may be removed. As shown in  FIG. 8 , the remaining dielectric layer may include two portions. The first portion is formed along the sidewalls of the bottom trench. The second portion is formed along the sidewalls of the upper trench. Throughout the description, the first portion is alternatively referred to as a first dielectric layer  801 . The second portion is alternatively referred to as a second dielectric layer  803 . 
     The removal process of some portions of the dielectric layer  702  may be a suitable etching process such as wet-etching, dry-etching and/or the like. The detailed operations of either the dry etching process or the wet etching process are well known in the art, and hence are not discussed herein to avoid repetition. 
       FIG. 9  illustrates a cross sectional view of the semiconductor device shown in  FIG. 8  after a conductive material has been filled in the openings in accordance with various embodiments of the present disclosure. In some embodiments, a plurality of auxiliary layers such as a seed layer may be deposited prior to a plating process, through which the conductive material is filled into the openings. 
     The seed layer (not shown) may be may be formed of copper, nickel, gold, any combination thereof and/or the like. The seed layer may be formed by suitable deposition techniques such as PVD, CVD and/or the like. 
     Once the seed layer has been deposited in the openings, a conductive material, which includes tungsten, titanium, aluminum, copper, any combinations thereof and/or the like, is filled into the openings, forming conductive plugs  902  and  904 . In some embodiments, the conductive material may be filled in the openings through an electroplating process. 
       FIG. 10  illustrates a cross section view of the semiconductor device shown in  FIG. 9  after a CMP process is applied to the top surface of the semiconductor device in accordance with various embodiments of the present disclosure. A planarization process, such as CMP, etch back step and the like, may be performed to planarize the top surface of the semiconductor device. As shown in  FIG. 10 , a portion of the conductive material has been removed as a result. As shown in  FIG. 10 , there may be two conductive plugs  902  and  904  formed in the semiconductor device after the CMP process is performed on the semiconductor device. 
     As shown in  FIG. 10 , each conductive plug (e.g., conductive plugs  902  and  904 ) may comprise two portions. A first portion is from the metal line  206  to the hard mask layer formed by the metal lines  106  and  108 . The first portion is of a width W 1  as shown in  FIG. 10 . A second portion is from the hard mask layer to the backside of the first substrate  102 . The second portion is of a width W 2  as shown in  FIG. 10 . In some embodiments, W 2  is greater than or equal to W 1 . 
       FIG. 11  illustrates a cross sectional view of the semiconductor device shown in  FIG. 10  after a dielectric layer is formed on the semiconductor device in accordance with various embodiments of the present disclosure. The dielectric layer  1102  may comprise commonly used dielectric materials, such as silicon nitride, silicon oxynitride, silicon oxycarbide, silicon carbide, combinations thereof, and multi-layers thereof. The dielectric layer  1102  may be deposited over the semiconductor device through suitable deposition techniques such as sputtering, CVD and the like. 
     The conductive plugs (e.g., conductive plug  902 ) include two portions as described above with respect to  FIG. 10 . The portion from the hard mask layer (e.g., metal line  106 ) to the metal line  206  may be alternatively referred to as a three-dimensional structure  1115  throughout the description. 
     One advantageous feature of the stacked wafer having the conductive plugs  902  and  904  shown in  FIG. 11  is that the active circuits of both semiconductor wafers are connected to each other through a single conductive plug (e.g., conductive plug  902 ). Such a single conductive plug helps to further reduce form factor. Furthermore, in comparison to stacked semiconductor devices connected by multiple conductive plugs, the single conductive plug coupled between two semiconductor wafers shown in  FIG. 11  helps to cut power consumption and prevent parasitic interference. 
     It should be noted while  FIG. 11  illustrates two semiconductor wafers stacked together, one skilled in the art will recognize that the stacked semiconductor device shown in  FIG. 11  is merely an example. There may be many alternatives, variations and modifications. For example, the stacked semiconductor device may accommodate more than two semiconductor wafers. 
       FIG. 12  illustrates a cross sectional view of another stacked semiconductor device in accordance with various embodiments of the present disclosure. The stacked semiconductor device  1200  is similar to the stacked semiconductor device  100  shown in  FIG. 11  except that the hard mask layer is formed by contacts, which is located adjacent to the interface between the first substrate  102  and the inter-metal dielectric layers  104 . 
     The contacts may be formed in an inter-layer dielectric layer (not shown). The inter-layer dielectric layer may comprise a material such as boron phosphorous silicate glass (BPSG), although any suitable dielectrics may be used for either layer. The inter-layer dielectric layer may be formed using a process such as PECVD, although other processes may alternatively be used. 
     The contacts  1006  and  1008  may be formed through the inter-layer dielectric layer with suitable photolithography and etching techniques. Generally, these photolithography techniques involve depositing a photoresist material, which is masked, exposed, and developed to expose portions of the inter-layer dielectric layer that are to be removed. The remaining photoresist material protects the underlying material from subsequent processing steps, such as etching. 
     The contacts  1006  and  1008  may comprise a barrier/adhesion layer (not shown) to prevent diffusion and provide better adhesion for the contacts  1006  and  1008 . In some embodiments, the contacts  1006  and  1008  may be formed of any suitable conductive material, such as a highly-conductive, low-resistive metal, elemental metal, transition metal, or the like. In accordance with an embodiment, the contacts  1006  and  1008  are formed of tungsten, although other materials, such as copper, aluminum and/or the like, could alternatively be utilized. In an embodiment in which the contacts  1006  and  1008  are formed of tungsten, the contacts  1006  and  1008  may be deposited by CVD techniques known in the art, although any method of formation could alternatively be used. 
     As shown in  FIG. 12 , the conductive plugs (e.g., conductive plugs  1202  and  1204 ) include two portions. The portion from the hard mask layer (e.g., contact  1006 ) to the metal line  206  may be alternatively referred to as a three-dimensional structure  1214  throughout the description. 
       FIG. 13  illustrates a cross sectional view of yet another stacked semiconductor device in accordance with various embodiments of the present disclosure. The stacked semiconductor device  1300  is similar to the stacked semiconductor device  100  shown in  FIG. 11  except that the etching hard mask is formed by redistribution lines, which are located adjacent to the interface of two semiconductor wafers. 
     The redistribution lines  1306  and  1308  may be a single material layer, or a multi-layered structure and may be made of metals such as titanium, titanium nitride, aluminum, tantalum, copper and combinations thereof. The redistribution lines  1306  and  1308  may be made by any suitable method known in the art such as physical vapor deposition (PVD), sputter, CVD, electroplating and/or the like. 
     The conductive plugs (e.g., conductive plugs  1302  and  1304 ) include two portions. The portion from the hard mask layer (e.g., redistribution lines  1306  and  1308 ) to the metal line  206  may be alternatively referred to as a three-dimensional structure  1314  throughout the description. 
     It should be noted that the first semiconductor wafer  110  may be bonded on the second wafer  210  through a suitable metal-dielectric bonding technique such as a copper-silicon oxide nitride (Cu—SiON) bonding process. 
     It should further be noted while  FIG. 11 ,  FIG. 12  and  FIG. 13  illustrate hard mask layers formed by metal lines, contacts and redistribution lines respectively, one skilled in the art will recognize that hard mask layers shown in  FIGS. 11-13  are merely examples. There may be many alternatives, variations and modifications. For example, the hard mask layer may be formed by a plurality of isolation regions, poly-silicon regions, any combinations thereof and/or the like. 
       FIG. 14  illustrates a cross sectional view of a backside illuminated imager sensor including a stacked wafer structure in accordance with various embodiments of the present disclosure. The backside illuminated image sensor  1400  comprises two semiconductor wafers, namely a sensor wafer  1201  and an application-specific integrated circuit (ASIC) wafer  1203 . As shown in  FIG. 14 , the sensor wafer  1201  is stacked on top of the ASIC  1203 . In some embodiments, the sensor wafer  1201  and the ASIC wafer  1203  are connected to each other through suitable three-dimensional structures such as the three-dimensional structure  1115  shown in  FIG. 11 , the three-dimensional structure  1214  shown in  FIG. 12 , the three-dimensional structure  1314  shown in  FIG. 13  and any combinations thereof. 
     The ASIC wafer  1203  may comprise a plurality of logic circuits such as logic circuits  1206  and  1208 . In some embodiments, the logic circuits may be an analog-to-digital converter. However, the logic circuits may be other functional circuits that may be utilized within a backside illuminated image sensor. For example, the logic circuits  1206  and  1208  may be a data processing circuit, a memory circuit, a bias circuit, a reference circuit, any combinations thereof and/or the like. 
     The ASIC wafer  1203  may further comprise a plurality of interconnection layers and a plurality of metal lines  1220 ,  1222 ,  1224  and  1226  embedded in the interconnection layers. The metal lines  1220 ,  1222 ,  1224  and  1226  may function as interconnection structures. As indicated by the arrows shown in  FIG. 14 , the metal lines  1220 ,  1222 ,  1224  and  1226  provide signal paths between logic circuits  1206  and  1208 , and the sensor wafer  1201 . 
     The metal lines  1220 ,  1222 ,  1224  and  1226  may be made through any suitable formation process (e.g., lithography with etching, damascene, dual damascene, or the like) and may be formed using suitable conductive materials such as copper, aluminum, aluminum alloys, copper alloys or the like. 
     The sensor wafer  1201  is fabricated by CMOS process techniques known in the art. In particular, the sensor wafer  1201  comprises an epitaxial layer over a silicon substrate. According to the fabrication process of backside illuminated image sensors, the silicon substrate has been removed in a backside thinning process until the epitaxial layer is exposed. A portion of epitaxial layer may remain. A p-type photo active region and an n-type photo active region (not shown respectively) are formed in the remaining epitaxial layer. 
     The photo active regions such as the p-type photo active region and the n-type photo active region may form a PN junction, which functions as a photodiode. As shown in  FIG. 14 , the imager sensor  1110  may comprise a plurality of photodiodes. 
     The sensor wafer  1201  may comprise a transistor (not shown). In particular, the transistor may generate a signal related to the intensity or brightness of light that impinges on the photo active regions. In accordance with an embodiment, the transistor may be a transfer transistor. However, the transistor may be an example of the many types of functional transistors that may be utilized within a backside illuminated image sensor. For example, the transistor may include other transistors located within a backside illuminated image sensor, such as a reset transistor, a source follower transistor or a select transistor. All suitable transistors and configurations that may be utilized in an image sensor are fully intended to be included within the scope of the embodiments. 
     The sensor wafer  1201  may comprise a plurality of interconnection layers and metal lines embedded in the interconnection layers. The metal lines  1120 ,  1122 ,  1124  and  1126  may provide signal paths between the sensor wafer  1201  and the ASIC wafer  1203 . In particular, as indicated by the arrows shown in  FIG. 14 , an external signal may enter the backside illuminated image sensor  1400  through the aluminum copper pad  1112 , and then reach the metal routing (e.g., metal line  1120 ) through interconnect structures such through vias (not shown). The external signal may further pass through a three-dimensional structure  1210 . The three-dimensional structure  1210  may be the three-dimensional structure  1115  shown in  FIG. 11 , the three-dimensional structure  1214  shown in  FIG. 11 , the three-dimensional structure  1314  shown in  FIG. 13  and/or any combinations thereof. 
     After the external signal passes the three-dimensional structure  1210 , the external signal may reach the logic circuit  1206  through the metal routing (e.g., metal line  1220 ) of the ASIC wafer  1203 . 
     When a signal leaves the logic circuit  1206 , it reaches the image sensor  1110  through a conductive path formed by the metal routing (e.g., metal line  1222 ) of the ASIC wafer  1203 , the three-dimensional structure  1210 , the metal routing (e.g., metal line  1122 ) of the sensor wafer  1201 . 
     After the image sensor  1110  generates a signal, the signal is sent to the logic circuit  1208  through a path formed by the metal routing (e.g., metal line  1124 ) of the sensor wafer  1201 , the three-dimensional structure  1210 , the metal routing (e.g., metal line  1224 ) of the ASIC wafer  1203 . Furthermore, the signal may be sent outside of the backside illuminated image sensor  1400  from the logic circuit  1208  through a path formed by the metal routing (e.g., metal line  1226 ) of the ASIC wafer  1203 , the three-dimensional structure  1210 , the metal routing (e.g., metal line  1126 ) of the sensor wafer  1201  and the aluminum copper pad  1114 . 
     The logic circuit  1206  and  1208  may be coupled to aluminum copper pads  1112  and  1114 . As shown in  FIG. 14 , the aluminum copper pads  1112  and  1114  may be formed on the backside of the sensor wafer  1201 . 
     It should be noted that the location of the aluminum copper pads  1112  and  1114  shown in  FIG. 14  is merely an example. A person skilled in the art will recognize that there may be many alternatives, modifications and variations. For example, the aluminum copper pads  1112  and  1114  may be formed on the non-bonding side of the ASIC wafer  1203 . The form factor of a backside illuminated image sensor can be reduced by forming the aluminum copper pads  1112  and  1114  on the non-bonding side of the ASIC wafer  1203 . 
     One advantageous feature of having input/output terminals formed on the non-bonding side of the ASIC wafer  1203  is that the density as well as quantum efficiency of the backside illuminated image sensor  1400  can be improved as a result. 
       FIG. 15  illustrates a top view of the hard mask in accordance with various embodiments of the present disclosure. As described above with respect to  FIG. 11 ,  FIG. 12  and  FIG. 13 , the hard mask layers may be formed by metal lines, contacts and redistribution lines respectively. While the cross sectional view  1501  shows the hard mask layer includes two portions (e.g., metal lines  106  and  108 ), these two portions may be from a continuous ring shaped region as illustrated by the top view  1502 . The top view  1502  of the hard mask layer shows the hard mask layer is of a ring shape. The inside diameter of the ring shaped hard mask layer is denoted as W 1 . 
     It should be noted that the internal circle of the ring shaped hard mask layer can be replaced by other suitable shapes such as a square as shown by the top view  1504 . It is within the scope and spirit of various embodiments of the present disclosure that the top view of the hard mask layer may comprise other shapes, such as, but no limited to oval, triangular, polygonal and/or the like. 
       FIG. 16  illustrates another top view of the hard mask in accordance with various embodiments of the present disclosure. The top views of  FIG. 16  are similar to those shown in  FIG. 15  except that the ring shape is replaced by a square with an opening. The top view  1602  shows a square with a square shaped opening. The top view  1604  shows a square with a circular shaped opening. 
     In accordance with an embodiment, a method comprises bonding a first semiconductor chip on a second semiconductor chip, applying an etching process to the first semiconductor chip and the second semiconductor chip until a metal surface of the second semiconductor chip is exposed, wherein as a result of applying the etching process, an opening is formed in the first semiconductor chip and the second semiconductor chip and plating a conductive material in the opening to from a conductive plug. 
     In accordance with an embodiment, a method comprises bonding a first semiconductor chip on a second semiconductor chip, applying an etching process to the first semiconductor chip and the second semiconductor chip until a metal surface of the second semiconductor chip is exposed, forming an opening extending through the first semiconductor chip and partially through the second semiconductor chip, wherein a portion of the opening is surrounded by a conductive element of the first semiconductor chip, the portion comprising two different widths and plating a conductive material in the opening to from a conductive plug. 
     In accordance with an embodiment, a method comprises bonding a first semiconductor chip on a second semiconductor chip, wherein an interconnect structure of the first semiconductor chip is in contact with an interconnect structure of the second semiconductor chip, applying an etching process to the first semiconductor chip and the second semiconductor chip until an interconnect element of the second semiconductor chip is exposed, forming an opening extending through the first semiconductor chip and partially through the second semiconductor chip, wherein the opening extends into an interconnect element of the first semiconductor chip and plating a conductive material in the opening to from a conductive plug. 
     Although embodiments of the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.