Abstract:
The invention provides a chip package and fabrication method thereof. In one embodiment, the chip package includes: a semiconductor substrate having opposite first and second surfaces, at least one bond pad region and at least one device region; a plurality of conductive pad structures disposed on the bond pad region at the first surface of the semiconductor substrate; a plurality of heavily doped regions isolated from one another, underlying and electrically connected to the conductive pad structures; and a plurality of conductive bumps underlying the heavily doped regions and electrically connected to the conductive pad structures through the heavily-doped regions.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/788,091 filed on May 26, 2010, now U.S. Pat. No. 8,497,534 which claims priority of Provisional U.S. Patent Application Nos. 61/235,146 and 61/235,153, both filed on Aug. 19, 2009, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a chip package, and in particular, to a wafer-level chip package and fabrication method thereof. 
     2. Description of the Related Art 
     A wafer level packaging technique for chip packaging has been developed. A wafer level package is first completed and then a dicing step is performed to form separated chip packages. A redistribution pattern in a chip package is mainly designed to be in direct contact with metal pads. Thus, the process for forming the redistribution pattern must correspond with the design of the metal pads. 
     It is desired to have a novel chip package and a fabrication method thereof to address the above issues. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a chip package is provided. An exemplary chip package comprises: a semiconductor substrate having opposite first and second surfaces, at least one bond pad region, and at least one device region; a plurality of conductive pad structures disposed on the bond pad region at the first surface of the semiconductor substrate; a plurality of heavily doped regions isolated from one another, underlying and electrically connected to the conductive pad structures; and a plurality of conductive bumps underlying the heavily doped regions and electrically connected to the conductive pad structures through the heavily doped regions. 
     According to another aspect of the invention, a method for fabricating a chip package is provided. An exemplary method comprises: providing a semiconductor wafer having opposite first and second surfaces, wherein the semiconductor wafer comprises at least one bond pad region, at least one device region, and a plurality of conductive pad structures on the first surface and disposed on the bond pad region; forming a plurality of heavily doped regions underlying the conductive pad structures, wherein the heavily-doped regions are isolated from one another and electrically connected to the conductive pad structures; and forming a plurality of conductive bumps underlying the heavily-doped regions, wherein the conductive bumps are electrically connected to the conductive pad structures through the heavily doped regions. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS. 1-2  are cross sections showing a method for fabricating a semiconductor chip according to an embodiment of the invention; 
         FIGS. 3A-3G  are cross sections showing a method for fabricating a carrier wafer according to another embodiment of the invention; 
         FIGS. 4-5  are cross sections showing a method for fabricating a semiconductor chip according to another embodiment of the invention; 
         FIGS. 6A-6B  are cross sections showing a method for fabricating a semiconductor chip according to yet another embodiment of the invention; 
         FIGS. 7A-7D  are cross sections showing a method for fabricating a semiconductor chip according to a further embodiment of the invention; and 
         FIGS. 8A-8D  are cross sections showing a method for fabricating a semiconductor chip according to a still further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     In the drawings or the description, similar or same reference numerals are used to designate similar or same elements. In addition, shapes or thickness of elements shown in the drawings may be exaggerated for clarity or simplicity. Further, each element shown in the drawings will be described. It should be understood that any element not shown or described may be any kind of conventional element as known by those skilled in the art. In addition, the disclosed embodiment is merely a specific example for practicing the invention, without acting as a limitation upon its scope. 
     A CMOS image sensor device package is used as an example. However, a micro-electromechanical system (MEMS) chip package or other semiconductor chips may also be suitable for use. That is, it should be appreciated that the chip package of the embodiments of the invention may be applied to electronic components with active or passive devices, or digital or analog circuits, such as opto electronic devices, micro-electromechanical systems (MEMS), micro fluidic systems, and physical sensors for detecting heat, light, or pressure. Particularly, a wafer scale package (WSP) process may be applied to package semiconductor chips, such as image sensor devices, light-emitting diodes (LEDs), solar cells, RF circuits, accelerators, gyroscopes, micro actuators, surface acoustic wave devices, pressure sensors, or ink printer heads. 
     The wafer scale package process mentioned above mainly means that after the package process is accomplished during the wafer stage, the wafer with chips is cut to obtain separate independent packages. However, in a specific embodiment, separate independent chips may be redistributed overlying a supporting wafer and then be packaged, which may also be referred to as a wafer scale package process. In addition, the above mentioned wafer scale package process may also be adapted to form chip packages of multi-layer integrated circuit devices by stacking a plurality of wafers having integrated circuits. 
     According to a feature of the invention, the electrical connections between conductive pad structures and conductive bumps are achieved by the use of heavily doped regions. As such, it is not necessary for a redistribution pattern to be in direct contact with conductive pad structures. In one embodiment, the heavily doped regions are disposed in the semiconductor substrate underlying the conductive pad structures. In another embodiment, the heavily doped regions are disposed in a carrier substrate bonded to the semiconductor substrate. 
     Referring to  FIGS. 1-2 , cross-sectional views illustrating the steps for forming a chip package on a semiconductor wafer according to an embodiment of the invention are shown. In this embodiment, the heavily doped regions are disposed in the semiconductor substrate underlying the conductive pad structures. As shown in  FIGS. 1-2 , a semiconductor wafer  300  is first provided, which is typically a silicon wafer. The semiconductor wafer includes an insulating layer  301 , which may be formed by semiconductor processing steps such as a thermal oxidation or chemical vapor deposition step. In one embodiment, a silicon-on-insulator (SOI) substrate may be used. Alternatively, the semiconductor wafer may be formed by combing two wafers together, using a wafer bonding process, wherein one of the wafers is provided with an insulating layer. The semiconductor wafer are defined with a plurality of device regions  100 A surrounded by peripheral bonding pad regions  100 B. Thereafter, insulating walls  305  connecting to the insulating layer  301  are formed in the semiconductor wafer  300  to isolate a plurality of regions as heavily doped regions  300 B. A semiconductor device  302  such as an image sensor device or MEMS is fabricated in the device region  100 A. Overlying the semiconductor wafer  300  and the semiconductor device  302  is an intermetal dielectric (IMD) layer  303 , which is typically a low-k dielectric such as porous oxide. A plurality of conductive pad structures  304  are fabricated in the IMD layer  303  on the peripheral bonding pad regions  100 B. The insulating walls and the insulating layer may be formed of an insulating material such as silicon oxide, or an insulation space such as air gap or vacuum. The conductive pad structures  304  are preferably made of materials such as copper (Cu), aluminum (Al), or other suitable metals. It should be noted that the semiconductor wafer comprises a plurality of heavily doped region  300 B in the peripheral bonding pad regions  100 B, wherein the heavily doped regions  300 B are isolated by insulating walls  305  and electrically connected to the conducive pad structures  304 . The heavily doped regions  300 B may be formed by doping ions (e.g., phosphors or arsenic ions) of a high concentration (e.g., 1E14-6E15 atoms/cm 2 ) by, for example, diffusion or ion implantation processes, to form a conductive path. In an embodiment, one heavily doped region corresponds to one conductive pad structure. However, when a plurality of conductive pad structures are used as a common output, one heavily doped region may correspond to a plurality of conductive pad structures at the same time. 
     In addition, the semiconductor wafer  300 , produced by wafer foundries, may be covered with a chip passivation layer  306 . Meanwhile, in order to electrically connect the devices in the chip to external circuits, the chip passivation layer  306  may be defined in advance by wafer foundries to form a plurality of openings  306   h  exposing the conductive pad structures  304 . 
     Next, as shown in  FIG. 3A , a packaging layer  500  is bonded to the semiconductor wafer. For simplicity, only the conductive pad structures  304 , the insulating walls  305 , and the insulating layer  301  are shown in the semiconductor wafer  300 . The packaging layer  500  may be, for example, a transparent substrate such as glass, another blank silicon wafer, or another wafer having integrated circuits. In one embodiment, a spacer layer  310  is used to separate the packaging layer  500  and the semiconductor substrate such that a cavity  316  surrounded by the spacer layer  310  is formed. The spacer layer  310  may be a sealant resin or a photosensitive insulating material, such as epoxy, solder mask, and so on. In addition, the spacer layer  310  may be formed on the semiconductor wafer  300 , and then bonded with the opposing packaging layer  500  using an adhesion layer. On the other hand, the spacer layer  310  may also be formed on the packaging layer  500 , and then bonded with an opposing semiconductor substrate  300  using an adhesion layer. 
     Referring to  FIG. 3B , using the packaging layer  500  as a supporting substrate, the backside  300   a  of the semiconductor wafer is etched by, for example, an anisotropic etch process, to remove portions of the semiconductor wafer  300  and the insulating layer  301  to form openings  300   h  therethrough to expose the heavily doped regions  300 B. It should be noted that each of the openings  300   h  corresponds to the heavily doped regions  300 B in the peripheral bonding pad regions  100 B isolated by the insulating walls  305 . 
     As shown in  FIG. 3C , an insulating layer  320  which exposes the heavily doped regions  300 B is formed in the openings  300   h . The insulating layer  320  may be a silicon oxide layer formed by thermal oxidation or plasma chemical vapor deposition processes. For example, the insulating layer  320  may be formed in the openings  300   h  and extend to the backside  300   a  of the semiconductor wafer  300 , and then the portion of the insulating layer at the bottom of the openings  300   h  would be removed by conventional photolithography and etching processes to expose the heavily doped regions  300 B. 
     Next, as shown in  FIG. 3D , a conductive pattern  330  is formed in the openings  300   h . In this embodiment, the conductive pattern serves as a redistribution pattern and therefore, the conductive pattern is formed on the sidewalls of the openings  300   h  and further extended to the bottom surface  300   a  of the semiconductor wafer  300   a  and the heavily doped regions  300 B. The redistribution pattern  330  may be formed by physical vapor deposition, chemical vapor deposition, electroplating, and eletroless plating processes, and so on. The redistribution pattern  330  may be formed of metals such as copper, aluminum, gold, or combinations thereof. Alternatively, the redistribution pattern  330  may be formed of conductive oxides such as indium tin oxide (ITO), indium zinc oxide (IZO), or a combination thereof. In one embodiment, a conductive layer is conformally formed on the entire semiconductor wafer, and then patterned to form the redistribution pattern as shown in  FIG. 3D . 
     Thereafter, referring to  FIG. 3E , the formation of a passivation layer  340  is shown. In an embodiment of the invention, the passivation layer  340  may, for example, be a solder mask. A solder mask material may be applied overlying the backside  300   a  of the semiconductor wafer to form the passivation layer  340 . Then, the passivation layer  340  is patterned to form a plurality of terminal contact openings, exposing portions of the redistribution pattern  330 . Then, an under bump metallurgy (UBM) and a conductive bump  350  are formed at the terminal contact openings. For example, the UBM may be formed of a conductive material such as a metal or metal alloy, and may be nickel, silver, aluminum, cooper, or alloys thereof. Alternatively, the UBM may be a doped polysilicon, single crystalline silicon, or conducting glass layer. In addition, a refractory metal material such as titanium, molybdenum, chromium, or titanium-tungsten layer may be used alone or in combination with other metal layers. In a specific embodiment, a nickel/gold layer may be partially or entirely formed overlying a surface of the metal layer. Through the redistribution pattern  330 , the conductive bumps  350  may be electrically connected to the heavily doped regions  300 B instead of the conductive pad structures  304 . In an embodiment of the invention, the conductive bump  350  is used to transmit input/output (I/O), ground, or power signals of the device  302 . Subsequently, the semiconductor wafer is diced along the scribe line SC on the peripheral bonding pad region, to thereby form a plurality of chip packages. 
     The heavily doped regions  300 B in the peripheral bonding pad regions are isolated by the insulating walls  305 . Therefore, the redistribution pattern  330  can electrically connect to the heavily doped regions  300 B, and it is not necessary for the redistribution pattern to be in direct contact with the conductive pad structures  304 . In addition, the heavily doped regions  300 B in the peripheral bonding pad regions may have an area that is wider than that of the conductive pad structures  304  such that the contact openings  300   h  have a larger process window for alignment. 
     Furthermore, as shown in  FIG. 3F , the depth of the opening  300   h  may penetrate beyond the insulating layer  301  such that the redistribution pattern  330  may extend into the heavily doped regions  300 B, or even reach the conductive pad structures  304  to thereby increase the contact area (as shown in  FIG. 3G . In other words, the insulating layer  301  may be at the bottom of the openings  300   h  or below the openings. 
     Referring to  FIGS. 4-5 , cross-sectional views illustrating the steps for forming a chip package on a semiconductor wafer according to another embodiment of the invention are shown. In this embodiment, the heavily doped regions are disposed in a carrier substrate. As shown in  FIGS. 4-5 , a semiconductor wafer  300  is first provided, which is typically a silicon wafer. The semiconductor wafer includes an upper surface  300   a  and a bottom surface  300   b . In addition, a plurality of scribe line regions and substrates corresponding to chips are defined in the semiconductor wafer, wherein each of the chips includes at least one device region  100 A surrounded by a peripheral bonding pad region  100 B. Thereafter, a semiconductor device  302  such as an image sensor device or MEMS is fabricated on the upper surface  300   a  in the device region  100 A. Overlying the semiconductor wafer  300  and the semiconductor device  302  is an intermetal dielectric (IMD) layer  303 , which is typically a low-k dielectric such as porous oxide. A plurality of conductive pad structures  304  are fabricated in the IMD layer  303  on the peripheral bonding pad region  100 B. The conductive pad structures  304  are preferably made of materials such as copper (Cu), aluminum (Al), or other suitable metals. 
     In addition, the semiconductor wafer  300 , produced by wafer foundries, may be covered with a chip passivation layer  306 . Meanwhile, in order to electrically connect the devices in the chip to external circuits, the chip passivation layer  306  may be defined in advance by wafer foundries to form a plurality of openings  306   h  exposing the conductive pad structures  304 . 
     Next, as shown in  FIG. 6A , a semiconductor wafer  600  such as a blank silicon wafer or a silicon wafer with integrated circuits is provided as a carrier substrate, which includes an upper surface  600   a  and a bottom surface  600   b . A plurality of openings  600   h  are formed by removing portions of the semiconductor wafer  600  from the upper surface  600   a . The openings  600   h  are then filled with insulating layers  610 , for example, formed of polymer materials such as polyimide. Alternatively, an insulating layer such as silicon oxide may be formed by semiconductor processing steps. For example, a silicon oxide layer is blanketly formed by thermal oxidation or plasma chemical vapor deposition processes, and thereafter, the oxide layer on the upper surface  600   a  and/or bottom surface  600   b  of the silicon wafer  600  may be removed. It should be noted that the silicon wafer  600  is a heavily doped substrate, which may be formed by doping ions (e.g., phosphors or arsenic ions) of a high concentration (e.g., 1E14-6E15 atoms/cm2) by, for example, diffusion or ion implantation processes, to form a conductive path. In an embodiment, one heavily doped region corresponds to one conductive pad structure. However, when a plurality of conductive pad structures are used as a common output, one heavily doped region may correspond to a plurality of conductive pad structures at the same time. 
     Referring to  FIG. 7A , the semiconductor substrate  300  with a semiconductor device is bonded to the carrier substrate  600 . For example, the semiconductor substrate  300  is flipped upside down with its upper surface  300   a  bonded to the upper surface  600   a  of the carrier substrate  600  such that the semiconductor device  302  is away from the carrier substrate  600 , while the conductive pad structures  304  are facing and bonded to the upper surface  600   a  of the carrier substrate  600 . For simplicity, only the conductive pad structures  304 , the semiconductor device  302 , and the IMD layer  303  are shown in the semiconductor substrate  300 . 
     Thereafter, as shown in  FIG. 7B , the semiconductor substrate  300  is thinned from the bottom surface  300   b  thereof (as indicated by the dash lines) to a suitable thickness by, for example, etching, milling, grinding, or polishing processes. For example, when the semiconductor device is an image sensor, the thinned silicon substrate  300  should be thin enough to permit a sufficient amount of light to pass therethrough for the image sensor  302  to sense incident light and generate signals. In this embodiment, the bottom surface  300   b  of the semiconductor substrate  300  is used as a light incident surface. 
     After completion of the thinning process, a packaging layer  500  is bonded to the bottom surface  300   b  of the semiconductor wafer  300 , as shown in  FIG. 7C . The packaging layer may be for example, a transparent substrate such as glass, another blank silicon wafer, or another wafer having integrated circuits. In one embodiment, a spacer layer  310  is used to separate the packaging layer  500  and the semiconductor substrate such that a cavity  316  surrounded by the spacer layer  310  is formed. The spacer layer  310  may be a sealant resin or a photosensitive insulating material, such as epoxy, solder mask, and so on. In addition, the spacer layer  310  may be formed on the bottom surface  300   b  of the silicon substrate  300 , and then bonded with the opposing packaging layer  500  using an adhesion layer. On the other hand, the spacer layer  310  may also be formed on the packaging layer  500 , and then bonded with an opposing bottom surface  300   b  of the silicon substrate  300  using an adhesion layer. 
       FIG. 7D  illustrates an optional process, wherein the carrier substrate is thinned from the bottom surface  600   b  thereof, using the packaging layer  500  as a supporting substrate. For example, the backside  600   b  of the carrier substrate is polished by a chemical mechanical polishing process to expose surfaces of the insulating layers  610  such that the insulating layers constitute an insulating wall  610  to isolate the heavily doped regions  600 B in the carrier substrate  600  which correspond to the peripheral bonding pad regions  100 B. 
     Thereafter, a passivation layer  640  is formed. In an embodiment of the invention, the passivation layer  640  may, for example, be a solder mask. A solder mask material may be applied overlying the bottom surface  600   b  of the carrier substrate  600   b  to form the passivation layer  640 . Then, the passivation layer  640  is patterned to form a plurality of contact openings, exposing portions of the bottom surface  600   b  of the carrier substrate. Then, an under bump metallurgy (UBM) and a conductive bump  350  are formed at the contact openings. For example, the UBM may be formed of a conductive material such as a metal or metal alloy, and may be nickel, silver, aluminum, cooper, or alloys thereof. Alternatively, the UBM may be a doped polysilicon, single crystalline silicon, or conducting glass layer. In addition, a refractory metal material such as titanium, molybdenum, chromium, or titanium-tungsten layer may be used alone or in combination with other metal layers. In a specific embodiment, a redistribution pattern can be used to redistribute the position of the conductive bump  650 . 
     In an embodiment of the invention, the conductive bump  650  is used to transmit input/output (I/O), ground, or power signals of the device  302 . Subsequently, the semiconductor wafer is diced along the scribe line SC on the peripheral bonding pad region, to thereby form a plurality of chip packages. 
     In addition, the heavily doped regions  600 B in the carrier substrate  600  which correspond to the peripheral bonding pad region are isolated by the insulating wall  610 . Therefore, the conductive bumps  650  can electrically connect to the heavily doped regions  600 B by direct contact or by the redistribution pattern. It is not necessary for the conductive bumps  650  to be in direct contact with the conductive pad structures  304 . In addition, the heavily doped regions  600 B in the carrier substrate  600  which correspond to the peripheral bonding pad regions may have an area that is wider than that of the conductive pad structures such that the contact openings have a larger process window for alignment. 
     Referring to  FIGS. 8A-8D , cross-sectional views showing the steps for forming a chip package according to another embodiment of the invention are shown, wherein a primary difference with the previous embodiment is that the carrier substrate  600  is a silicon-on-insulator (SOI) substrate which includes an insulating layer  630 . Insulating walls  610  extending to the insulating layer  630  are formed in the carrier substrate  600  to isolate the heavily doped regions  600 B which correspond to the peripheral bonding pad regions  100 B. The insulating walls and the insulating layer may be formed of silicon oxide. The heavily doped regions  600 B may be formed by an ion implantation process, which may be performed before or after the formation of the insulating walls  610 . Next, as shown in  FIG. 8B , a portion of thickness of the semiconductor wafer  300  is removed from the backside  300   b  thereof. Referring to  FIGS. 8C-8D , the carrier substrate  600  is thinned after a packaging layer  500  is disposed thereon, and then a passivation layer  640  and conductive bumps  650  are formed in sequence. However, in another embodiment, the insulating layer  630  is not removed but left intact during the thinning process of the carrier substrate  600 . In other embodiments, the insulating layer  630  is formed by semiconductor processing steps such as a thermal oxidation and chemical vapor deposition step. Alternatively, the insulating layer is formed by combing two wafers together, using a wafer bonding process, wherein one of the wafers is provided with an insulating layer. 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.