Abstract:
Backside illuminated sensors and methods of manufacture are described. Specifically, a backside illuminated sensor with a dipole modulating layer near the photodiode is described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No.  61 / 875 , 676 , filed Sep.  9 ,  2013 . 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention generally relate to devices and processes for forming the same. More specifically, embodiments of the invention are directed to backside illuminated image sensors with a tunable electrostatic effect and methods of forming the same. 
         [0003]    In the formation of a backside illuminated (BSI) CMOS image sensors (CIS), a dielectric film is deposited above the photodiode. This dielectric film, frequently made of hafnium oxide, often has fixed negative charges to create a hole accumulation layer to suppress dark current caused by the interface between the photodiode and the dielectric layer. However, the electrostatic effect of the interface is not eliminated. 
         [0004]    There is a need to control the electrostatic charge for many applications, including charges that result in dark current for optical applications. Typically, this is done by using hafnium oxide only films, and changing the thickness or implant conditions of the fixed negative charge inducing species. 
         [0005]    However, there remains a need in the art for methods to provide control the electrostatic effect for a given film thickness. 
       SUMMARY 
       [0006]    One or more embodiments of the invention are directed to methods of forming a device. A photodiode is formed on a surface adjacent to and in electrical communication with a gate electrode. An oxide layer is formed on the surface. An insulator having a thickness is deposited on the oxide layer. The insulator comprises a dipole modulating layer and a fixed charge layer. 
         [0007]    Another embodiment of the invention is directed to a backside illuminated sensor comprising a substrate with a gate electrode thereon. A photodiode is adjacent to and in electrical communication with the gate electrode. An oxide layer is at a surface of the substrate. An insulator is above the oxide layer, the insulator comprising a dipole modulating layer and a fixed charge layer. 
         [0008]    A further embodiment of the invention is directed to a backside illuminated sensor comprising a substrate with a gate electrode thereon. A photodiode is in electrical communication with the gate electrode. An oxide layer is at a surface of the substrate. An insulator having a thickness less than about 150 Å is above the oxide layer. The insulator comprises a dipole modulating layer and a fixed charge layer. The fixed charge layer comprises a first fixed charge sub-layer and a second fixed charge sub-layer and the dipole modulating layer is between the first fixed charge sub-layer and the second fixed charge sub-layer. An antireflective coating is over the insulating layer. A color filter and a lens are positioned over the antireflective coating to direct light toward the photodiode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0010]      FIG. 1  shows a partial schematic cross-section of an optical device in accordance with one or more embodiments of the invention; 
           [0011]      FIG. 2  shows a partial schematic cross-section of an optical device in accordance with one or more embodiments of the invention; 
           [0012]      FIG. 3  shows a partial schematic cross-section of an optical device in accordance with one or more embodiments of the invention; and 
           [0013]      FIG. 4  shows a partial schematic cross-section of an optical device in accordance with one or more embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Embodiments of the invention generally relate to new methods to reduce the dark current source of optical filters that are induced by the defects created by the grinding/polishing process. In one embodiment, the surface may or may not be treated by using decoupled plasma oxidation or thermally to create an oxide layer, followed by depositing a dipole layer, and then depositing a hafnium oxide layer. In another embodiment, the surface may or may not be treated by decoupled plasma oxidation or thermally to create an oxide layer, followed by a hafnium oxide layer, followed by a dipole layer, and topped with a hafnium oxide layer. In another embodiment, the surface may or may not be treated by decoupled plasma oxidation or thermally to create an oxide layer, followed by a hafnium oxide layer, followed by a dipole layer. 
         [0015]    As used in this specification and the appended claims, the term “substrate”, “substrate surface”, “surface”, “wafer”, “wafer surface” and the like are used interchangeably, referring to a surface, or portion of a surface, upon which a process acts. It will also be understood by those skilled in the art that reference to a substrate can also refer to only a portion of the substrate, unless the context clearly indicates otherwise. Additionally, reference to depositing on a substrate can mean both a bare substrate and a substrate with one or more films or features deposited or formed thereon. 
         [0016]    As used in this specification and the appended claims, the terms “reactive gas”, “precursor”, “reactant”, and the like, are used interchangeably to mean a gas that includes a species which is reactive with a substrate surface. For example, a first “reactive gas” may simply adsorb onto the surface of a substrate and be available for further chemical reaction with a second reactive gas. 
         [0017]    It may be beneficial to control the electrostatic charge for many applications, including charges that result in dark current for optical applications. Typically, this is done by using hafnium oxide only films or silicon oxide-hafnium oxide films, and changing the thickness or implant conditions. Embodiments of the invention provide a method that can design or control the placement and amount of the electrostatic effect for a given film thickness, therefore enhancing the injection of photoelectrons into CIS photodiode array 
         [0018]    Embodiments of the invention add aluminum or titanium oxides, amongst other oxides, which are able to shift the flatband voltage and total charge. This, in turn, is able to reduce or eliminate the dark current for the optical sensor. The degree of that shift can be engineered into the film by modulating surface treatment and the placement and ratio in the film. 
         [0019]      FIGS. 1 and 2  show a partial cross-sectional view of a device  100  in accordance with one or more embodiments of the invention. The device  100  shows a portion of a single pixel  102  of an optical device. The device  100  includes a photodiode  110  formed in a silicon insulating layer  120  and on a surface  112 . The photodiode can be any suitable photodiode. There is typically a thickness of ˜3 micron that separates the photodiode from the fixed charge area. 
         [0020]    The fixed charge layer  140  can be made of any suitable dielectric material. For example high-k dielectrics (those with dielectric constants greater than silicon dioxide) can be used including, but not limited to, hafnium oxide. 
         [0021]    The dipole modulating layer  130  can be any suitable compound including, but not limited to, a silicon dioxide surface treatment layer and titanium oxides, aluminum oxides, lanthanum oxides, zirconium oxides, tantalum oxides, cerium oxide, and yttrium oxides. Without being bound by any particular theory of operation, it is believed that the dipole modulating compound induces an increased negative charge that cancels the positive charge due to the surface defects created by the polishing process on the silicon surface to expose the photodiode  110 , reducing the dark current for the optical sensor. In one or more embodiments, the dipole modulating layer  130  comprises titanium oxide. 
         [0022]    The device  100  shown in  FIGS. 1 and 2  further comprises an optional antireflective coating  150  on the insulating layer  120 . The antireflective coating  150  can be any suitable material known to those skilled in the art that can provide higher transmission/lower reflection. 
         [0023]    A color filter  160  is positioned over the fixed charge layer  140 . If an antireflective coating  150  is present, then the color filter  160  may be positioned over the antireflective coating  150 . The purpose of the color filter  160  is to filter wavelengths of light that are not being evaluated by the individual pixel  102 . For example, if the pixel  102  is a green pixel, the majority of extraneous light may be filtered out by the color filter  160  so that any resulting electrical signal generated by the photodiode  110  is the product of the particular wavelengths of light. 
         [0024]    A lens  170  is positioned over the color filter  160 . The lens  170  collimates or focuses incoming light onto the photodiode  110 . 
         [0025]    The photodiode  110  is positioned on pixel circuitry  180 . The pixel circuitry  180  can comprise a plurality of interconnects (not shown), insulators (not shown) and gate electrodes  185 , as will be understood by those skilled in the art. In some embodiments, a gate electrode  185  is formed adjacent to and in electrical communication with the photodiode  110 . 
         [0026]    The position and thickness of the dipole modulating layer  130  can have an impact on the charge. In some embodiments, an oxide layer  125 , for example, a silicon oxide layer for a silicon substrate is added. The oxide layer  125  can be formed by any suitable method including, but not limited to, chemically deposited oxide, thermal oxidation, or decoupled plasma oxidation. This oxide layer may be, for example a surface treatment on the substrate. The thickness of the oxide layer  125  can vary and is typically up to about 30 Å thick, but can be thicker or thinner. As shown in  FIGS. 1 through 3 , the dipole modulating layer  130  is positioned immediately under the fixed charge layer  140  and is in contact with the oxide layer  125  on one side and the fixed charge layer  140  on the other side. This configuration may be referred to as a bottom placement of the dipole modulating layer  130  as viewed with the lens  170  on top. As used in this specification and the appended claims, the terms “bottom”, “middle” and “top” used in this context refer to the orientation of layers when the lens  170  side is position at the top, as shown in  FIG. 2 . 
         [0027]    In some embodiments, the fixed charge layer  140  is formed between and contacts both the photodiode  110  and the dipole modulating layer  130 . The dipole modulating layer  130  of the embodiment can contact the antireflective coating  150  on the other side. This is the reverse of the configuration shown in  FIG. 3 . Stated differently, in some embodiments, the fixed charge layer  140  is formed between and contacts both the oxide layer and the dipole modulating layer 
         [0028]      FIG. 4  shows another embodiment in which the insulator  125  comprises two dielectric sub-layers (i.e., two fixed charge sub-layers), a first fixed charge sub-layer  140   a  and a second fixed charge sub-layer  140   b . The first fixed charge sub-layer  140   a  is shown contacting the photodiode  110  and the dipole modulating layer  130  and the dipole modulating layer  130  is positioned between the first fixed charge sub-layer  140   a  and the second fixed charge sub-layer  140   b . The thickness of the first fixed charge sub-layer  140   a  will directly affect the distance of the dipole modulating layer  130  from the photodiode  110 . This difference may also impact the reduction in dark current. 
         [0029]    The thickness T of the insulator  125  can be any suitable thickness. In some embodiments, the insulator  125  is less than about 150 Å thick measured from the top of the photodiode  110  to the top of the insulator  125  layer. The dipole modulating layer  130  can be formed with any suitable thickness. In some embodiments, the thickness of the dipole modulating layer is in the range of about 1 Å to about 100 Å, or in the range of about 2 Å to about 90 Å, or in the range of about 3 Å to about 80 Å, or in the range of about 4 Å to about 70 Å, or in the range of about 5 Å to about 60 Å, or in the range of about 10 Å to about 50 Å. In one or more embodiments, the thickness of the dipole modulating layer  130  is greater than about 1 Å, 2 Å, 3 Å, 4 Å, 5 Å, 6 Å, 7 Å, 8 Å, 9 Å, 10 Å, 15 Å, 20 Å, 25 Å, 30 Å, 35 Å, 40 Å, 45 Å or 50 Å. 
         [0030]    The ratio of the dipole modulating layer  130  to the fixed charge layer  140  in the insulating layer  120  can also have an impact on the charge. In some embodiments, the dipole modulating layer  130  and the fixed charge layer  140  are present in a ratio in the range of about 1:100 to about 100:1, or in the range of about 1:10 to about 10:1, in the range of about 1:5 to about 5:1, or in the range of about 1:2 to about 2:1. 
         [0031]    When the dipole modulating layer  130  is spaced from the photodiode, the thickness of the first fixed charge sub-layer  140   a  can impact the charge as this thickness affects the distance that the dipole modulating layer  130  is positioned from the photodiode  110 . In some embodiments, the first fixed charge sub-layer  140   a  has a thickness in the range of about 1 Å to about 100 Å, or in the range of about 2 Å to about 90 Å, or in the range of about 3 Å to about 80 Å, or in the range of about 4 Å to about 70 Å, or in the range of about 5 Å to about 60 Å, or in the range of about 10 Å to about 50 Å. In one or more embodiments, the thickness of the first fixed charge sub-layer  140   a  is greater than about 1 Å, 2 Å, 3 Å, 4 Å, 5 Å, 6 Å, 7 Å, 8 Å, 9 Å, 10 Å, 15 Å, 20 Å, 25 Å, 30 Å, 35 Å, 40 Å, 45 Å or 50 Å. In one or more embodiments, the first fixed charge sub-layer  140   a  has a thickness in the range about 1% to about 90% of the total thickness T of the insulator  125 , or in the range of about 2% to about 80% or in the range of about 3% to about 70%, or in the range of about 4% to about 60% or in the range of about 5% to about 50%. 
         [0032]    In some embodiments, one or more of the dipole modulating layer  130  and the fixed charge layer  140  are deposited by atomic layer deposition (ALD). Atomic layer deposition may allow for precise control of the thickness of the individual layers as ALD processes are theoretically self-limiting. 
       EXAMPLES 
       [0033]    Each example has similar insulator thickness of about 50 Å to about 100 Å. The position of the dipole modulating film was changed from the bottom (immediately adjacent the oxide layer), to the middle, to the top (without additional insulator above). The results are summarized in Table 1. The units for the numbers are total charge (E 10 [#/cm 2 ]). The baseline is the HfO x  only. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Tunability by Dopant Placement 
               
             
          
           
               
                   
                 Aluminum 
                 Titanium 
               
             
          
           
               
                 HfO x  only 
                 Bottom 
                 Middle 
                 Top 
                 Bottom 
                 Middle 
                 Top 
               
               
                   
               
               
                 31.18 
                 −1.35 
                 33.08 
                 32.70 
                 −776.29 
                 −226.15 
                 9.06 
               
               
                   
               
             
          
         
       
     
         [0034]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.