Abstract:
An image-sensor device includes a substrate including a pixel region and a logic region. A logic transistor is disposed in the logic region and is surrounded by a logic isolation feature. A radiation-sensing region is disposed in the pixel region of the substrate. An epitaxial pixel isolation feature is disposed in the pixel region and surrounds the radiation-sensing region. A doped region with a same doping polarity as the radiation-sensing region is located between a bottom of the radiation-sensing region and the back surface of the substrate. The epitaxial pixel isolation feature is in direct contact with the doped region. The doped region extends continuously under the pixel region and the logic region. The epitaxial pixel isolation feature is in direct contact with the doped region, and the logic isolation feature is spaced apart from the doped region.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This Application is a Continuation of U.S. application Ser. No. 14/089,263 filed on Nov. 25, 2013, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    An image-sensor device is one of the building blocks in a digital imaging system such as a digital still or a video camera. An image-sensor device includes a pixel array (or grid) for detecting light and recording intensity (brightness) of the detected light. The pixel array responds to the light by accumulating a charge—for example, the more light, the higher the charge. The accumulated charge is then used (for example, by other circuitry) to provide a color and brightness signal for use in a suitable application, such as a digital camera. One type of image sensor is a backside illuminated (BSI) image sensor device. BSI image sensor devices are used for sensing a volume of light projected towards a backside surface of a substrate (which supports the image sensor circuitry of the BSI image sensor device). The pixel grid is located at a front side of the substrate, and the substrate is thin enough so that light projected towards the backside of the substrate can reach the pixel grid. BSI image sensor devices provide a reduced destructive interference, as compared to front-side illuminated (FSI) image sensor device. 
         [0003]    Integrated circuit (IC) technologies are constantly being improved. Such improvements frequently involve scaling down device geometries to achieve lower fabrication costs, higher device integration density, higher speeds, and better performance. Along with the advantages realized from reducing geometry size, improvements are being directly to the IC devices. One such IC device is an image sensor device. 
         [0004]    Due to device scaling, improvements to BSI technology are continually being made to further improve the quality of BSI image sensor devices. Although existing BSI image-sensor devices and methods of fabricating BSI image sensor devices have been generally adequate for their intended purposes, as device scaling-down continues, they have not been entirely satisfactory in all respects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0005]    For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings. 
           [0006]      FIG. 1  shows an enlarged top view of a pixel region of an image-sensor device, in accordance with some embodiments. 
           [0007]      FIG. 2  shows a cross-sectional view of a pixel region shown in  FIG. 1  and a periphery region of the image sensor device, in accordance with some embodiments. 
           [0008]      FIG. 3  shows an enlarged top view of a pixel region of an image-sensor device, in accordance with some embodiments. 
           [0009]      FIG. 4  shows a flow chart illustrating a method for manufacturing an image sensor device, in accordance with some embodiments. 
           [0010]      FIGS. 5-9  are cross-sectional views of a pixel region shown in  FIG. 3  and a peripheral region of an image-sensor device at various stages of the manufacturing process, in accordance with some embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Moreover, the performance of a first process before a second process in the description that follows may include embodiments in which the second process is performed immediately after the first process, and may also include embodiments in which additional processes may be performed between the first and second processes. Various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. Furthermore, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the like elements in various figures and embodiments are identified by the same or similar reference numerals. 
         [0012]      FIG. 1  shows an enlarged top view of a pixel region  101  of an image-sensor device  100 , in accordance with some embodiments. The image-sensor device  100  includes an array of pixels shown in  FIG. 1 . Each pixel region  101  is arranged into a column and a row. The pixel region  101  refers to a unit cell containing one photodetector  106  and various circuitry for converting electromagnetic radiation to an electrical signal. In some embodiments, the photodetector  106  includes a photodiode for recording an intensity or brightness of radiation (light). The pixel region  101  may contain various transistors including a transfer transistor  110 , a reset transistor  112 , a source-follower transistor  114 , a select transistor  116 , other suitable transistors, or combinations thereof. The pixel region  101  may also include various doped regions in the substrate, for example doped regions  118 A,  118 B and  120 . The doped regions ( 118 A,  118 B and  120 ) are configured as source/drain regions of previous mentioned transistors. The doped region  120  is also referred to as a floating diffusion region, which is between the transfer transistor  110  and the reset transistor  112 . A conductive feature  132  overlaps a portion of a gate stack of the source-follower transistor  114  and connects to the floating diffusion region. The image-sensor device  100  also includes various isolation features formed in a substrate to isolate various regions of the substrate. In some embodiments, a dielectric isolation feature  108  is formed in the pixel region to isolate the photodetector  106 , the transfer transistor  110 , the reset transistor  112 , the source-follower transistor  114  and the select transistor  116 . Additional circuitry, input, and/or output in a periphery region may be coupled to the pixel array to provide an operation environment for the pixel region  101  and support external communications with the pixel region  101 . For example, the pixel array may be coupled with readout circuitry and/or control circuitry in the periphery region. 
         [0013]      FIG. 2  shows a cross-sectional view of the pixel region  101  along line A-A in  FIG. 1  and a periphery region  102  of the image-sensor device  100 , in accordance with some embodiments. The image-sensor device  100  includes a substrate  104  having a front surface  104 A and a back surface  104 B. In some embodiments, the substrate  104  is a semiconductor substrate including silicon. Alternatively or additionally, the substrate  104  includes another elementary semiconductor, such as germanium and/or diamond, a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium arsenide and/or indium antimonide, an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP, or combinations thereof. The substrate  104  may be doped with a p-type or n-type dopant. The P-type dopant includes boron, BF 2 , gallium, indium, other suitable p-type dopants, or combinations thereof. The N-type dopant includes phosphorous, arsenic, other suitable n-type dopants, or combinations thereof. In the depicted embodiments, the substrate  104  is a p-type substrate. The substrate  104  may have a doping concentration ranging from about 1E15/cm 3  to about 5E16/cm 3 . The substrate  104  may be implanted using a process such as in-situ doping, ion implantation or diffusion in various steps and techniques. 
         [0014]    The pixel region  101  includes one or more photodetectors  106 , such as a photodiode. As shown in  FIG. 2 , the photodetector  106  includes a radiation-sensing region  106 A and a pinned layer  106 B. The radiation-sensing region  106 A is a doped region having a doping polarity which is different from that of the substrate  104 , along the front surface  104 A of the substrate  104 . In the depicted embodiments, the radiation-sensing region  106 A is an n-type doped region. The pinned layer  106 B is a doped region layer disposed overlapping the radiation-sensing region  106 A at the front surface  104 A of the substrate  104 . In the depicted embodiments, the pinned layer  106 B is a p-type doped layer. 
         [0015]    The pixel region  101  further includes various transistors, such as the transfer transistor  110 , the reset transistor  112 , the source-follower transistor  114  (shown in  FIG. 1 ) and the select transistor  116  (shown in  FIG. 1 ). Each transistor has a corresponding gate stack disposed over the front surface  104 A of the substrate  104 . The gate stack of the transfer transistor  110  overlies a portion of the radiation-sensing region  106 A. The pixel region  101  also includes various doped regions in the substrate  104 . The doped regions correspond to gate stacks of previous mentioned transistors as source/drain regions. For example, doped regions  120  and  118 A are source/drain regions of the reset transistor  112 . In some embodiments, the doped regions  118 A and  120  are heavily doped regions with a doping polarity which is different from that of the substrate  104 . In the depicted embodiments, the doped regions  118 A and  120  are n-type regions. 
         [0016]    The gate stack of each transistor includes a gate dielectric layer and a gate electrode layer. The gate dielectric layer may include a dielectric material, such as silicon oxide, a high-k material, other dielectric materials, or combinations thereof. Examples of the high-k dielectric material may include HfO 2 , HfSiO, HfSiO, HfTaO, HfTiO, HfZrO, zirconium oxide, aluminum oxide, hafnium oxide, hafnium dioxide-alumina (HfO 2 —Al 2 O 3 ) alloy or combinations thereof. The gate electrode layer may include polysilicon and/or a metal including Al, Cu, Ti, Ta, W, Mo, TaN, NiSi, CoSi, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN or combinations thereof. 
         [0017]    The periphery region  102  may include readout and/or control circuitry coupled to the pixel region  101  to provide an operational environment for the pixel region  101 . In the depicted embodiments, a PMOS transistor  122  and an NMOS transistor  124  are shown. The PMOS transistor  122  includes a gate stack  122 A and source/drain regions  122 B having p-type doping polarity formed in an n-type well  122 C. The NMOS transistor  124  includes a gate stack  124 A and source/drain regions  124 B having n-type doping polarity formed in a p-type well  124 C. 
         [0018]    The image-sensor device  100  further includes various isolation features in the pixel region  101  and the peripheral region  102 . For example, the image-sensor device  100  may include a number of dielectric isolation features  108  and a number of doped isolation features  128  formed in substrate  104  of the pixel region  101 . The dielectric isolation features  108  include silicon oxide, silicon nitride, silicon oxynitride, other insulating material, or combination thereof. Each of the dielectric isolation features  108  has a depth D 1  ranging from about 500 Å to about 3000 Å. The formation of the dielectric isolation features  108  may include an etching process to etch trenches from the front surface  104 A of the substrate  104  and a deposition process to fill the trenches with a dielectric material. In some embodiments, the dielectric isolation features  108  may be surrounded by a doped liner layer  126 . The doped liner layer  126  may have a doping polarity which is different from that of the radiation-sensing region  106 A. The doped liner layer  126  is able to remedy surface defects of the dielectric isolation features  108  after annealing. 
         [0019]    In some embodiments, the doped isolation regions  128  extend from the front surface  104 A into the substrate  104 . The doped isolation features  128  have a doping polarity which is different from the radiation-sensing region  106 A for acting as isolation wells. In the depicted embodiments, the doped isolation features  128  are p-type doped regions. The dielectric isolation features  108  and the doped isolation features  128  surround the radiation-sensing region  106 A of the photodetector  106  to prevent horizontal leakage paths between the photodetector  106  and other regions. 
         [0020]    The image-sensor device  100  further includes a number of dielectric isolation features  130  formed in the peripheral region  102 . Each of the dielectric isolation features  130  extends from the front surface  104 A into the substrate  104 . The dielectric isolation features  130  may isolate the PMOS transistor  122  and the NMOS transistor  124  in the peripheral region  102 . The dielectric isolation features  130  may include silicon oxide, silicon nitride, silicon oxynitride, other insulating material, or combinations thereof. 
         [0021]    In some embodiments, the image-sensor device  100  includes a doped layer  132  and a doped layer  134  formed near the back surface  104 B of the substrate  104 . The doped layer  132  is formed near the back surface  104 B between the doped isolation features  128 . The doped layer  134  has a doping polarity which is the same as the radiation-sensing region  106 A and typically extends to the radiation-sensing region  106 A. In the depicted embodiments, the doped layer  132  is an n-type layer. The doped layer  132  may have a doping concentration that is less than the doping concentration of the radiation-sensing region  106 A. The doped layer  132  may create an electric field to help separate the electron-hole pairs and drive electrons to the radiation-sensing region  106 A. In some embodiments, the doped layer  134  has a doping polarity which is different from the radiation-sensing region  106 A to isolate the photodetector  106 . In some embodiments, the doped layer  134  is formed by one or more implantation processes from the back surface  104 B of the substrate  104 . 
         [0022]    The image-sensor device  100  further includes a multilayer interconnect (MLI)  136  over the front surface  104 A of the substrate  104 , including over the photodetector  106 . The MLI  136  is coupled to various components of the image-sensor device  100 , for example the photodetector  106 , such that the various components of the image-sensor device  100  are operable to properly respond to illuminated light (imaging radiation). The MLI  136  includes various conductive features  138  and  140 . The conductive features  138  may be vertical interconnects  138 , such as contacts and/or vias, and the conductive features  140  may be horizontal interconnects, such as lines. The various conductive features  138  and  140  include conductive materials, such as aluminum, aluminum/silicon/copper alloy, titanium, titanium nitride, tungsten, polysilicon, metal silicide, or combinations thereof. 
         [0023]    The various conductive features  138  and  140  of the MLI  136  are interposed in a dielectric layer  142 . The ILD layer  142  may include silicon dioxide, silicon nitride, silicon oxynitride, TEOS oxide, phosphosilicate glass (PSG), borophosphosilicate glass nitride (BPSG), fluorinated silica glass (FSG), carbon doped silicon oxide, Black diamond® (Applied materials of Santa Clara, Calif.), amorphous fluorinated carbon, low-k dielectric material, polyimide, or combinations thereof. The ILD layer  142  may have a multilayer structure. 
         [0024]    The image-sensor device  100  may further include a carrier wafer  144  disposed over the front surface  104 A of the substrate  104 . In some embodiments, the carrier wafer  144  is bonded to the MLI  136 . The carrier wafer  144  includes silicon or glass. The carrier wafer  144  can provide protection for the various features (such as the photodetector) formed on the front surface  104 A of the substrate  104 , and can also provide mechanical strength and support for processing the back surface  104 B of the substrate  104 . 
         [0025]    The image-sensor device  100  may further include an antireflective layer  146 , a color filter  148  and a lens  150  disposed over the back surface  104  of the substrate  104 . The antireflective layer  146  includes a dielectric material, such as silicon nitride or silicon oxynitride. 
         [0026]    The color filter  148  is disposed over the antireflective layer  146 , and is aligned with the radiation-sensing region  106 A of the photodetector  106 . The color filter  148  is designed so that it filters through light of a predetermined wavelength. For example, the color filter  148  may filter though visible light of a red wavelength, a green wavelength, or a blue wavelength to the photodetector  106 . In an example, the color filter  148  includes a dye-based (or pigment-based) polymer for filtering out a specific frequency band (for example, a desired wavelength of light). 
         [0027]    The lens  150  is disposed over the color filter  148  and is also aligned with the radiation-sensing region  106 A of the photodetector  106 . The lens  150  may be in various positional arrangements with the photodetector  106  and the color filter  148 , such that the lens  150  focuses an incident radiation  152  on the radiation-sensing region  106 A of the photodetector  106 . Alternatively, the position of the color filter  148  and the lens  150  may be reversed, such that the lens  150  is disposed between the antireflective layer  146  and the color filter  148 . 
         [0028]    In an operation according to one or more embodiments, the image-sensor device  100  is designed to receive an incident radiation  152  traveling towards the back surface  104 B of the substrate  104 . The lens  150  directs the incident radiation  152  to the color filter  148 . The incident radiation  152  then passes from the color filter  148  through the antireflective layer  146  to the substrate  104  and the corresponding photodetector  106 , specifically to the radiation-sensing region  106 A. When exposed to the incident radiation  146 , the photodetector  106  responds to the incident radiation  152  by accumulating charges. When the gate of transfer transistor  110  is turned on, the charges are transferred from the photodetector  106  to the floating diffusion region  120 . Through the connection of the conductive feature  132  (shown in  FIG. 1 ), the source-follower transistor  114  may convert the charges from the floating diffusion region  120  to voltage signals. The select transistor  116  may allow a single row of the pixel array to be read by read-out electronics. The reset transistor  112  acts as a switch to reset the floating diffusion region  120 . When the reset transistor  112  is turned on, the floating diffusion region  120  is effectively connected to a power supply, clearing all integrated charges. In the depicted embodiments, each of the transistors in the pixel region  101  keeps off when a bias voltage is not applied, and is turned on by applying a positive bias voltage. 
         [0029]    As shown in  FIG. 2 , the photodetector  106  is surrounded by various doped regions, such as the doped liner layer  126 , the doped isolation regions  128  and the doped layer  132 . The doped layer  126  and  132  and the doped isolation regions  132  can prevent horizontal current leakage and enhance performance of the photodetector  106 . However, the formation of the doped layers  126  and  132  and the doped isolation regions  128  may require high implant energies to implant dopants therein. Such high implant energies could cause damage in the image-sensor device  100 , which may make dark current and white pixel issues worse. In addition, the doped layers  126  and  132  and the doped isolation regions  128  may require one or more annealing processes to activate the implanted dopants. A high temperature of each of the annealing processes could further cause diffusion of the doped regions and other doped regions in the substrate  104 . Further diffusion of other doped regions could be detrimental to the performance and reliability of the image-sensor device  100 . 
         [0030]      FIG. 3  shows an enlarged top view of a pixel region  301  of an image-sensor device  300  (illustrated in  FIGS. 5-9 ), in accordance with some embodiments. The image-sensor device  300  includes an array of pixels as shown in  FIG. 3 . Each pixel region  301  is arranged in a column and a row. In some embodiments, from the top view, the connections and arrangements of each feature in the image-sensor device  300  is the same as or similar with the corresponding features shown in  FIG. 1 . Accordingly, the connection and arrangement of the each feature in the image-sensor device  300  are not repeatedly discussed herein. 
         [0031]      FIG. 4  is a flow chart of a method  200  of forming an image-sensor device  300  according to some embodiments of this disclosure. The method  200  starts with operation  201 , in which a substrate having a pixel region and a periphery region is provided. Afterwards, the method  200  continues with operation  202  in which a number of first trenches are etched in the pixel region. The method  200  continues with operation  203  in which a number of epitaxial isolation features are formed in the first trenches. Afterwards, the method continues  200  with operation  204  in which a number of second trenches are formed in the peripheral region. Afterwards, the method  200  continues with operation  205  in which a number of dielectric isolation features are formed in the second trenches. 
         [0032]      FIGS. 5 to 9  are cross-sectional views of the pixel region  301  along line A-A in  FIG. 3  and a peripheral region  302  of the image-sensor device  300  at various stages of the manufacturing process, in accordance with some embodiments. In some embodiments, the image-sensor device  300  is fabricated according to the method  200 . Various figures have been simplified for a better understanding of the concepts of the present disclosure. 
         [0033]    The method  200  proceeds from operation  201  and continues to operation  202 .  FIG. 5  is a cross-sectional view of the image-sensor device  300  after performing operations  201  and  202 , in accordance with some embodiments. A substrate  304  is provided. The substrate  304  has a front surface  304 A and a back surface  304 B. The substrate  304  is similar with the substrate  104  as shown in  FIG. 2  but has a doping polarity which is different from that of the substrate  104 . For example, the substrate  304  has a first doping polarity that is n-type or p-type. The substrate  304  is an n-type substrate when the substrate  104  is a p-type substrate, or vice versa. In the depicted embodiments, the substrate  304  is an n-type substrate. In some embodiments, the substrate  304  may have a doping concentration ranging from about 1E15/cm 3  to about 1E16/cm 3 . 
         [0034]    In some embodiments, a hard mask layer  362  is formed over the front surface  304 A of the substrate  304 . The hard mask layer  362  may have a multilayer structure. In some embodiments, the hard mask layer  362  includes a pad layer (not shown), a dielectric layer (not shown) over the pad layer, and an imaging enhancement layer (not shown) over the dielectric layer. The pad layer, such as an oxide layer, act as a stress buffer layer between the substrate  304  and the overlying dielectric layer. The dielectric layer may include a nitrogen-containing material, such as silicon nitride or silicon oxynitride. Alternatively, the dielectric layer includes an amorphous carbon material, silicon carbide or tetraethylorthosilicate (TEOS). The imaging enhancement layer may include an organic layer, or a polymer layer or silicon-rich oxide (SRO). The imaging enhancement layer can enhance the accuracy of an image being transferred from an overlying photoresist layer. The hard mask layer  362  may be formed through a process such as a chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD). Then, the hard mask layer  362  is patterned through suitable photolithographic and etching processes to form a number of holes  362 A and expose a portion of the front surface  304 A of the substrate  304  in the pixel region  301 . 
         [0035]    The exposed portion of the substrate  304  are removed by a suitable etching process through the holes  362 A to form a number of first trenches  364  in the pixel region  301 . The etching process may be reactive ion etching (REI) or other dry etching processes. A depth D 2  of each of the first trenches  364  may be in a range from about 2000 Å to about 5000 Å. 
         [0036]    The method  200  continues with operation  203  in which epitaxial isolation features are formed in the first trenches.  FIG. 6  is a cross-sectional view of the image-sensor device  300  after performing operation  203 , in accordance with some embodiments. A number of epitaxial isolation features  308  are formed in the first trenches  362 . Each of the epitaxial isolation features  308  may have the depth D 2 . In some embodiments, an epitaxy process is performed to deposit an epitaxy material. The epitaxial material is formed by epitaxially growing a semiconductor material. The semiconductor material may include single elements such as germanium (Ge) or silicon (Si), or compound semiconductor materials, such as gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), or semiconductor alloy, such as silicon germanium (SiGe), gallium arsenide phosphide (GaAsP). The epitaxial growing process may include selective epitaxy growth (SEG), cyclic deposition and etching (CDE), chemical vapor deposition (CVD) techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy (MBE), other suitable epitaxy processes, or combinations thereof. 
         [0037]    Each of the epitaxial isolation features  308  has a second doping polarity which is different from the first doping polarity. In the depicted embodiments, the epitaxial isolation features  308  are p-type epitaxial isolation features. In some embodiments, the doping concentration of the epitaxial isolation features  308  is in a range from about 1E17 per cm 3  to about 1E18 per cm 3 . The epitaxial isolation features  308  may be doped through in-situ doping as the material is grown such that there is no need to further perform another implant process and annealing process to activate the implanted dopant. In some embodiments, each of the epitaxial isolation features  308  has a gradient doping concentration. For example, the doping concentration of the epitaxial isolation features  308  increases toward the front surface  304 A of the substrate  304 . Such gradient doping concentration may effectively isolate the heavily doped regions of the transistors (shown in  FIG. 3 ) formed near the front surface  304 A. 
         [0038]    The epitaxy material may overfill the first trenches  362 . A planarization process, such as a chemical mechanical polishing (CMP) process and/or an etching process, is applied to the epitaxy material to reduce the thickness of the epitaxy material to expose the top surface of the hard mask layer  362 . In some embodiments, the hard mask layer  362  is removed after the planarization process. The epitaxial isolation features  308  are further planarized until they are substantially planar to the front surface  304 A of the substrate  304 . 
         [0039]    The method  200  continues with operation  204  in which a number of second trenches are etched into the substrate  304  of the peripheral region  302 .  FIG. 7  shows a cross-sectional view of the image-sensor device  300  after performing operation  204 , in accordance with some embodiments. A hard mask layer  366  is formed over the front surface  304 A of the substrate  304 . 
         [0040]    The hard mask layer  366  may include a multilayer structure and material which are similar with the hard mask layer  362 . Afterwards, the hard mask layer  366  is patterned through suitable photolithographic and etching processes to form a number of holes  366 A. The holes  366 A expose a portion of the front surface  304 A of the substrate  304  in the peripheral region  302 . 
         [0041]    The exposed portion of the substrate  304  is removed by a suitable etching process through the holes  366 A to form a number of second trenches  368  in the peripheral region  302 . The etching process may include reactive ion etching (RIE) or other dry etching techniques. Each of the second trenches has a depth D 3  extending from the front surface  304 A into the substrate  304 . The depth D 3  may be substantially the same with the depth D 1 . 
         [0042]    The method  200  continues with operation  205  in which a number of dielectric isolation features are formed in the second trenches.  FIG. 8  shows a cross-sectional view of the image-sensor device  300  after performing operation  205 . As shown in  FIG. 8 , a number of dielectric isolation features  330  are formed in the second trenches  368 . In some embodiments, a dielectric material is deposited into the second trenches  368  by a suitable deposition process. The deposition process may include chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma enhanced chemical vapor deposition (PECVD) or other suitable deposition methods. The dielectric material may include silicon oxide, silicon nitride, silicon oxynitride, other insulating material, or combinations thereof. 
         [0043]    The dielectric material may overfill the second trenches  368 . A planarization process, such as a chemical mechanical polishing (CMP) process and/or an etching process, is applied to the dielectric material to expose the top surface of the hard mask layer  366 . In some embodiments, the hard mask layer  366  is removed after the planarization process. The dielectric isolation features  330  are further planarized until they are substantially planar to the front surface  304 A of the substrate  304 . 
         [0044]    It is understood that additional operations can be provided before, during and after operation  205  of the method  200 .  FIG. 9  is a cross-sectional view of the image-sensor device  300  after operation  205 . The photodetector  106  is formed in the pixel region  301 . The photodetector  106  includes the radiation-sensing region  106 A and the pinned layer  106 B. The radiation-sensing region  106 A is surrounded by the epitaxial isolation features  308 . The depth of the epitaxial isolation features is greater than that of the radiation-sensing region  106 A. In some embodiments, the radiation-sensing region  106 A also has a doping polarity, such as the first doping polarity, which is the same as the substrate  304  and different from the epitaxial isolation features  308 . Accordingly, the epitaxial isolation features  308  may form a P-N junction with the radiation-sensing region  106 A to prevent lateral current leakage. In the depicted embodiments, the radiation-sensing region  106 A is doped with n-type species along the front surface  304 A of the substrate  304 . The pinned layer  106 B is doped with p-type species overlapping in the radiation-sensing region  106 A at the front surface  304 A of the substrate  304 . In some embodiments, the radiation-sensing region  106 A has a doping concentration greater than that of the substrate  304 . For example, the radiation-sensing region  106 A may have a doping concentration ranging from about 1E15 per cm 3  to about 1E16 per cm 3 . 
         [0045]    As shown in  FIG. 9 , by forming the epitaxial isolation features  308 , there is no need to form the dielectric features  108  (shown in  FIG. 2 ), the doped liner layer  126  (shown in  FIG. 2 ) and the doped isolation feature  128  (shown in  FIG. 2 ). In addition, there is also no need to form the doped layer  132  (as shown in  FIG. 2 ). The substrate  304  can provide the same function as the doped layer  132  since it has the same doping polarity (e.g., the first doping polarity) as the radiation-sensing region  106 A. Therefore, high implant energies and a number of annealing processes can be eliminated in the manufacturing process of the image-sensor device  300 . 
         [0046]    The floating diffusion region  120  and the doped regions  118 A and  118 B (shown in  FIG. 3 ) are formed in the pixel region  301 . The floating diffusion region  120  and the doped regions  118 A and  118 B may be heavily doped regions of the first doping polarity. For example, the floating diffusion region  120  and the doped regions  118 A and  118 B may have a doping concentration ranging from about 1E15 per cm 3  to about 8E15 per cm 3 . In the depicted embodiments, the floating diffusion region  120  and the doped regions  118 A and  118 B are n-type doped regions. 
         [0047]    In the periphery region  102 , an n-type well  122 C and a p-well  124 C are formed in the substrate  104  by implantation. Source/drain regions  122 B and source/drain regions  124 B are formed in the corresponding n-type well  122 C and p-well  124  by implantation. 
         [0048]    Gate stacks are formed on the front surface  304 A of the substrate  304  and in the pixel region  301  and peripheral region  302 . Each of the gate stacks includes a gate dielectric layer and a gate electrode layer. The gate stacks and the doped regions  120 ,  118 A and  118 B (shown in  FIG. 3 ) construct a transfer transistor  310 , a reset transistor  312 , a source-follower transistor  314  (shown in  FIG. 3 ) and a select transistor  316  (shown in  FIG. 3 ) in the pixel region  102 . As described above, in some embodiments, each of the substrate  304  and the doped regions  120 ,  118 A and  118 B has the first doping polarity. Accordingly, the channel portions of each of the transistors  310 ,  312 ,  314  and  316  in the pixel region  301  have the same doping polarity as the doped regions  120 ,  118 A and  118 B with less doping concentration. In some embodiments, each of the transistors  310 ,  312 ,  314  and  316  keeps off by applying a negative bias voltage, and is turned on by applying a positive bias voltage or stopping to apply the negative bias voltage. For example, the negative bias voltage is in a range from about 3.3 to about −3.3 V. 
         [0049]    The gate stacks  122 A and  124 A corresponding to the n-type well  122 C and the p-well  124 C are formed in the periphery region  102 . The gate stacks  122 A and source/drain regions  122 B in the n-type well  122 C construct a PMOS transistor. Likewise, the gate stacks  124 A and source/drain regions  124 B in the p-type well  124 C construct a NMOS transistor. The gate stacks in the pixel region  310  and the peripheral region  302  are formed by suitable processes, including deposition, lithography patterning, and etching processes. 
         [0050]    The image-sensor device  300  further includes a multilayer interconnect (MLI)  136  disposed over the front surface  304 A of the substrate  304 . The MLI  136  is coupled to various components of the image-sensor device  300 , such as the photodetector  106 , such that the various components of the image sensor device  100  are operable to properly respond to illuminated light (imaging radiation). The MLI  136  includes various conductive features  138  and  140 , which may be vertical interconnects, such as contacts and/or vias and horizontal interconnects, such as lines. The conductive features  138  and  140  are formed by suitable processes, including deposition, lithography patterning, and etching processes to form vertical and horizontal interconnects. 
         [0051]    The various conductive features  138  and  140  of the MLI  136  are disposed in an interlayer dielectric (ILD) layer  142 . The ILD layer  142  may include silicon dioxide, silicon nitride, silicon oxynitride, TEOS oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), fluorinated silica glass (FSG), carbon doped silicon oxide, low-k dielectric material, or combinations thereof. The ILD layer  142  may have a multilayer structure. The ILD layer  142  may be formed by suitable processes, including spin-on coating, chemical vapor deposition (CVD) or plasma enhanced chemical vapor deposition (PECVD). In one example, the MLI  136  and the ILD layer  142  may be formed in an integrated process including a damascene process. In some embodiments, further process steps are included after the MLI  136  formation. As illustrated in  FIG. 9 , the carrier wafer  144  is bonded to the MLI  136 . The carrier wafer  146  provides mechanical strength and support for processing the back surface  304 B of the substrate  304 . A planarization process, such as a chemical mechanical polishing (CMP) process, is applied to the back surface  304 B of the substrate  304  to reduce a thickness of the substrate  304 . The doped layer  134  is formed by an implantation process, diffusion process, annealing process or combinations thereof through the back surface  304 B. The antireflective layer  146 , the color filter  148  and the lens  150  disposed over the back surface  304   b  of the substrate  304  are also formed, in some embodiments. The color filter  348  and the lens  350  are aligned with the radiation-sensing region  106 A of the photodetector  106 . 
         [0052]    Although the impurity type of the substrate (such as  104 ,  304 ) and the doped regions (such as regions  106 A,  106 B,  126 ,  118 A,  118 B,  120 ,  122 B,  122 C,  124 B,  124 C,  126 ,  128 ,  132 ,  134 ,  308 , etc.) are specified in the illustrated embodiments, the teaching of the embodiments is readily available for the formation of a device with doping polarities of the substrate and these doped regions inverted. In addition, the sequences of the operations  201  to  205  may be reasonably rearranged. For example, the operations  204  and  205  can be performed prior to the operations  202  and  203 . 
         [0053]    Embodiments of mechanisms for forming an image-sensor device are described above. The image-sensor device including epitaxial isolation features are formed to prevent horizontal current leakage. The epitaxial isolation features may be formed without an annealing process. In addition, the image-sensor device includes a substrate that is doped with the same doping polarity as a radiation-sensing region. The substrate itself can provide a function to create an electric field to help separate the electron-hole pairs and drive electrons to the radiation-sensing region. There is no need to form another doped layer near the back surface of the substrate. Accordingly, high energy implantation and a number of annealing processes can be avoided in the manufacturing process of the image-sensor device. The performance and reliability of the image-sensor device can be enhanced. 
         [0054]    In accordance with some embodiments, an image-sensor device is provided. The image-sensor device includes a substrate having a front surface and a back surface. The image-sensor device also includes a radiation-sensing region, and the radiation-sensing region is operable to detect incident radiation that enters the substrate through the back surface. The image-sensor device further includes a doped isolation region formed in the substrate and adjacent to the radiation-sensing region. In addition, the image-sensor device includes a deep-trench isolation structure formed in the doped isolation region. The deep-trench isolation structure includes a trench extending from the back surface and a negatively charged film covering the trench. 
         [0055]    In accordance with some embodiments, an image-sensor device is provided. The image-sensor device includes a substrate having a front surface and a back surface. The image-sensor device also includes a radiation-sensing region formed in the substrate. The radiation-sensing region is operable to detect incident radiation that enters the substrate through the back surface. The radiation-sensing region further includes an epitaxial isolation feature formed in the substrate and adjacent to the radiation-sensing region. The radiation-sensing region and the epitaxial isolation feature have different doping polarities. 
         [0056]    In accordance with some embodiments, an image-sensor device is provided. The image-sensor device includes a substrate having a front surface and a back surface, and the substrate has a first doping polarity. The image-sensor device also includes a radiation-sensing region formed in the substrate. The radiation-sensing region is operable to detect incident radiation that enters the substrate through the back surface. The radiation-sensing region has the first doping polarity and a doping concentration which is higher than that of the substrate. The image-sensor device further includes a heavily doped region formed in the substrate and adjacent to the radiation-sensing region. The heavily doped region has the first polarity and a doping concentration which is higher than that of the substrate. The heavily doped region and the radiation-sensing region define a channel region of the first polarity therebetween. 
         [0057]    In accordance with some embodiments, an image-sensor device is provided. The method includes providing a substrate having a front surface and a back surface. The method also includes forming a radiation-sensing region of a first doping polarity in the substrate. The method further includes forming an epitaxial isolation structure in the substrate and adjacent to the radiation-sensing region. The epitaxial isolation structure has a second doping polarity which is different from the first doping polarity. 
         [0058]    Although embodiments of the present disclosure and their 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 disclosure as defined by the appended claims. For example, it will be readily understood by those skilled in the art that many of the features, functions, processes, and materials described herein may be varied while remaining within the scope of the present disclosure. 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 operations described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or operations, 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 disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or operations.