Patent Document

PRIORITY DATA 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/626,664, filed Jan. 24, 2007, now U.S. Pat. No. ______, issued ______, which claims benefit of U.S. Provisional Patent Application Ser. No. 60/798,876, filed May 9, 2006, both of which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor. 
         [0003]    Backside illuminated sensors are used for sensing a volume of exposed light projected towards the backside surface of a substrate. The pixels are located on 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 pixels. Backside illuminated sensors provide a high fill factor and reduced destructive interference, as compared to front-side illuminated sensors. 
         [0004]    A problem with backside illuminated sensors is that different wavelengths of radiation to be sensed experience different effective absorption depths in the substrate. For example, blue light experiences a more shallow effective absorption depth, as compared to red light. Improvements in backside illuminated sensors and/or the corresponding substrate are desired to accommodate different wavelengths of light. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
           [0006]      FIG. 1  is a top view of a sensor including a plurality of pixels, according to one or more embodiments of the present invention. 
           [0007]      FIGS. 2-5  are sectional views of a sensor having a plurality of backside illuminated pixels, constructed according to aspects of the present disclosure. 
           [0008]      FIG. 6  is a graph of light sensitivity vs. wavelength for a sensor having backside substrate thicknesses of uniform size. 
           [0009]      FIG. 7  is a graph of light sensitivity vs. wavelength for a sensor having backside substrate thicknesses of varying size. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. 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. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, 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 interposing the first and second features, such that the first and second features may not be in direct contact. 
         [0011]    Referring to  FIG. 1 , an image sensor  50  provides a grid of backside illuminated (or back-illuminated) pixels  100 . In the present embodiment, the pixels  100  are photosensitive diodes or photodiodes, for recording an intensity or brightness of light on the diode. Alternatively, the pixels  100  may also include reset transistors, source follower transistors, pinned layer photodiodes, and transfer transistors. The image sensor  50  can be of various different types, including a charge-coupled device (CCD), a complimentary metal oxide semiconductor (CMOS) image sensor (CIS), an active-pixel sensor (ACP), or a passive-pixel sensor. Additional circuitry and input/outputs are typically provided adjacent to the grid of pixels  100  for providing an operation environment for the pixels and for supporting external communications with the pixels. 
         [0012]    Referring now to  FIG. 2 , the sensor  50  includes a silicon-on-insulator (SOI) substrate  110  including silicon and carbon dioxide. Alternatively, the substrate  110  may comprise an epitaxial layer or other combination of layers. In other embodiments, the substrate  110  may comprise an elementary semiconductor such as silicon, germanium, and diamond. The substrate  110  may also comprise a compound semiconductor such as silicon carbide, gallium arsenic, indium arsenide, and indium phosphide. The substrate  110  may comprise an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, and gallium indium phosphide. 
         [0013]    In the present embodiment, the substrate  110  comprises P-type silicon formed over a silicon dioxide base. Silicon doping may be implemented using a process such as ion implantation or diffusion in various steps. The substrate  110  may comprise lateral isolation features to separate different devices formed on the substrate. The thickness of the substrate  110  has been thinned to allow for etching of the backside of the substrate. This reduction in thickness may be accomplished by back grinding, diamond scrubbing, chemical mechanical planarization (CMP), or other similar techniques. 
         [0014]    The sensor  50  includes a plurality of pixels  100  formed on the front surface of the semiconductor substrate  110 . For the sake of example, the pixels are further labeled  100 R,  100 G, and  100 B to correspond with example light wavelengths of red, green, and blue, respectively. As noted above, the pixels  100  (also referred to as radiation-sensing elements) sense different wavelengths of radiation (light) and record an intensity or brightness of the radiation (light). The pixels  100  each comprise a light-sensing region (or photo-sensing region) which in the present embodiment is an N-type doped region having dopants formed in the semiconductor substrate  110  by a method such as diffusion or ion implantation. In continuance of the present example, the doped regions are further labeled  112 R,  112 G, and  112 B to correspond with the pixels  100 R,  100 G, and  100 B, respectively. In some embodiments, the doped regions  112  can be varied one from another, such as by having different material types, thicknesses, and so forth. 
         [0015]    The sensor  50  further includes additional layers, including first and second metal layers  120 ,  122  and inter-level dielectric  124 . The dielectric layer  124  comprises a low-k material, as compared to a dielectric constant of silicon dioxide. Alternatively, the dielectric layer  124  may comprise carbon-doped silicon oxide, fluorine-doped silicon oxide, silicon oxide, silicon nitride, and/or organic low-k material. The metal layers  120 ,  122  may include aluminum, copper, tungsten, titanium, titanium nitride, tantalum, tantalum nitride, metal silicide, or any combinations thereof. 
         [0016]    Additional circuitry also exists to provide an appropriate functionality to handle the type of pixels  100  being used and the type of light being sensed. It is understood that the wavelengths red, green, and blue are provided for the sake of example, and that the pixels  100  are generally illustrated as being photodiodes for the sake of example. 
         [0017]    Referring now to  FIG. 3 , the substrate  110  includes a plurality of absorption depths  114 R,  114 G, and  114 B located beneath the corresponding pixels  100 R,  100 G, and  100 B, respectively. Each wavelength (e.g., red, green, and blue light) has a different effective absorption depth when it passes through the substrate  110 . For example, blue light experiences a more shallow effective absorption depth, as compared to red light. Thus, the absorption depth  114 R,  114 G, and  114 B for each color pixel  100 R,  100 G, and  100 B varies accordingly. As an example, the absorption depth  114 R beneath the pixel  100 R for red light is between 0.35 μm to 8.0 μm. The absorption depth  114 G beneath the pixel  100 G for green light is between 0.15 μm to 3.5 μm. The absorption depth  114 B beneath the pixel  100 B for blue light is between 0.10 μm to 2.5 μm. 
         [0018]    The absorption depths  114  may be formed by a variety of different techniques. One technique is to apply a photosensitive layer to the backside of the substrate  110 , pattern the photosensitive layer, and etch the substrate according to the pattern. For example, a wet etch process may be used to remove the unwanted silicon substrate. This process can be repeated to create different absorption depths. 
         [0019]    Referring now to  FIG. 4 , the sensor  50  includes a planarization layer  130  located between the pixels  100 R,  100 G, and  100 B and the color filters  160 R,  160 G, and  160 B (shown in  FIG. 5 ). The planarization layer  130  is made up of an organic or polymeric material that has a high transmittance rate for visible light. This allows light to pass through the planarization layer  130  with very little distortion so that it can be detected at the light-sensing regions in the substrate  110 . The planarization layer  130  may be formed by a spin coating method which provides for a uniform and even layer. 
         [0020]    Referring now to  FIG. 5 , the sensor  50  is designed to receive light  150  directed towards the back surface of the semiconductor substrate  110  during applications, eliminating any obstructions to the optical paths by other objects such as gate features and metal lines, and maximizing the exposure of the light-sensing region to the illuminated light. The illuminated light  150  may not be limited to visual light beam, but can be infrared (IR), ultraviolet (UV), and other radiation. 
         [0021]    The sensor  50  further comprises a color filter layer  160 . The color filter layer  160  can support several different color filters (e.g., red, green, and blue), and may be positioned such that the incident light is directed thereon and there through. In one embodiment, such color-transparent layers may comprise a polymeric material (e.g., negative photoresist based on an acrylic polymer) or resin. The color filter layer  160  may comprise negative photoresist based on an acrylic polymer including color pigments. In continuance of the present example, color filters  160 R,  160 G, and  160 B correspond to pixels  100 R,  100 G, and  100 B, respectively. 
         [0022]    The sensor  50  may comprise a plurality of lenses  170 , such as microlenses, in various positional arrangements with the pixels  100  and the color filters  160 , such that the backside-illuminated light  150  can be focused on the light-sensing regions. 
         [0023]    Referring to  FIG. 6 , a graph  200  shows a comparison of the sensitivities for the various pixels when responding to red, green, or blue light. The vertical axis of the graph  200  shows light or radiation sensitivity, and the horizontal axis shows light or radiation wavelength. As can be seen from the graph  200 , if the absorption depths are uniform, the light sensitivity  205  between the different pixels in response to red, green, and blue radiation wavelengths would be different. The blue light has a shorter wavelength than the green and red light and thus, the blue light has a shorter effective absorption depth in the substrate. In the present example, the pixel for receiving blue light would have a reduced level of light sensitivity, as compared to the pixels for receiving green and red light. 
         [0024]    Referring now to  FIG. 7 , a graph  210  shows a comparison of the sensitivities for the pixels  100 R,  100 G, and  100 B, when responding to red, green, or blue light, respectively. Since the sensor  50  has absorption depths  114 R,  114 G, and  114 B with varying thicknesses, then a more even distribution of light sensitivity  215  can be obtained between the different pixels  100 R,  100 G, and  100 B in response to different wavelengths of radiation. In the present example, the wavelengths are red, green, and blue, and the pixels  100 R,  100 G, and  100 B have corresponding color filters  160 R,  160 G, and  160 B. It is understood that variations in junction depths and dopant concentrations may be combined with aspects of the present disclosure to achieve a more uniform spectral response and to improve performance of the sensor  50 . 
         [0025]    Thus, provided is an improved sensor device and method for manufacturing same. In one embodiment, a backside illuminated sensor includes a semiconductor substrate having a front surface and a back surface and a plurality of pixels formed on the front surface of the semiconductor substrate. The sensor further includes a plurality of absorption depths formed within the back surface of the semiconductor substrate. Each of the plurality of absorption depths is arranged according to each of the plurality of pixels. 
         [0026]    In some embodiments, the plurality of pixels are of a type to form a CMOS image sensor. In other embodiments, the plurality of pixels are of a type to form a charge-coupled device. In other embodiments, the plurality of pixels are of a type to form an active-pixel sensor. In still other embodiments, the plurality of pixels are of a type to form a passive-pixel sensor. 
         [0027]    In some other embodiments, the sensor includes red, green, and blue color filters aligned with corresponding red, green, and blue pixels and a planarization layer that lies between the color filters and the pixels. The sensor further includes microlenses over the color filters, a dielectric layer disposed above the front surface of the semiconductor substrate, and a plurality of metal layers over the semiconductor substrate. 
         [0028]    In another embodiment, a method is provided for forming a backside illuminated sensor. The method includes providing a semiconductor substrate having a front surface and a back surface and forming a first, second, and third pixel on the front surface of the semiconductor substrate. The method further includes forming a first, second, and third thickness within the back surface of the semiconductor substrate, wherein the first, second, and third thickness lies beneath the first, second, and third pixel, respectively. In some embodiments, the method includes forming color filters aligned with the plurality of pixels and forming a planarization layer between the color filters and pixels. The method further includes providing a dielectric layer and a plurality of metal layers above the front surface of the semiconductor substrate. 
         [0029]    The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure.

Technology Category: 5