Abstract:
An image sensor the image sensor comprising an absorption layer disposed on a silicon substrate, the absorption layer having at least one of SiGe or Ge, and an antireflection layer disposed directly thereon.

Description:
BACKGROUND 
       [0001]    The disclosure relates generally to an image sensor and, more particularly, to an image sensor having enhanced absorption coefficient for improved pixel performance and a method of manufacturing the same. 
         [0002]    Complementary metal oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charge-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, CMOS image sensors typically require lower voltages, consume less power, enable random access to image data, may be fabricated with compatible CMOS processes, and enable integrated single-chip cameras. 
         [0003]    CMOS image sensors utilize light-sensitive CMOS circuitry to convert light energy into electrical energy and can be designed to be illuminated from a front surface or from a back surface. The light-sensitive CMOS circuitry typically comprises a photodiode formed in a silicon substrate. As the photodiode is exposed to light, an electrical charge is induced in the photodiode. The photodiode is typically coupled to a MOS switching transistor, which is used to sample the charge of the photodiode. Colors may be determined by placing filters over the light-sensitive CMOS circuitry. 
         [0004]    The light received by pixels of the CMOS image sensor is often based on the three primary colors: red, green, and blue (R, G, B), and additional colors can be identified and/or created with various combinations and intensities (e.g., when red and green overlap they form yellow).  FIG. 1  is a graph showing the relationships between the wavelengths of blue, green, and red light and their respective quantum efficiencies. Incident light with a long wavelength such as red light (wavelength of about 600 nm) has a decreased absorption coefficient. As pixel performance is dependent on the absorption characteristics of the photodiode, this decreased absorption coefficient results in decreased red signals and an imbalance in color ratio. 
         [0005]    Furthermore, pixel sensitivity for receiving the incident light is lowered with the trend of size reduction of pixels of the CMOS image sensor and cross-talk is caused between different pixels with incident light, especially for incident light having a long wavelength, thereby degrading the overall performance of pixels of the CMOS image sensor. 
         [0006]    Hence, what is needed is an improved image sensor that provides for increased absorption coefficient and therefore improved pixel performance, especially for light having a long wavelength. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0007]    The features, aspects, and advantages of the disclosure will become more fully apparent from the following detailed description, appended claims, and accompanying drawings in which: 
           [0008]      FIG. 1  is a graph showing the quantum efficiency (%) of the wavelength, λ (nm) of blue, green, and red light. 
           [0009]      FIG. 2  illustrates an image pixel array region and surrounding logic region of an image sensor in accordance with an embodiment of the present disclosure. 
           [0010]      FIG. 3  is a cross-sectional view of a portion of an image sensor in accordance with an embodiment of the present disclosure. 
           [0011]      FIG. 4  is a graph showing the relationships of the light absorption coefficient (cm −1 ) of a SiGe layer having various Ge content at different wavelengths (μm). 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    In the following description, numerous specific details are set forth to provide a thorough understanding of embodiments of the present disclosure. However, one having an ordinary skill in the art will recognize that embodiments of the disclosure can be practiced without these specific details. In some instances, well-known structures and processes have not been described in detail to avoid unnecessarily obscuring embodiments of the present disclosure. 
         [0013]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. It should be appreciated that the following figures are not drawn to scale; rather, these figures are merely intended for illustration. 
         [0014]    With reference now to  FIG. 2 , there is shown an image sensor  100 , which comprises a grid or array of backside illuminated pixel regions  102 . Each pixel region  102  may comprise a photosensitive diode  300  (not shown in  FIG. 2  but illustrated in  FIG. 3 ), which generates a signal related to the intensity or brightness of light that impinges on the photosensitive diode  300 . 
         [0015]    The image sensor  100  may also comprise a logic region  104  located adjacent to the array of pixel regions  102 . The logic region  104  may have additional circuitry and contacts for input and output connections to and from the array of pixel regions  102 . The logic region  104  is utilized to provide an operating environment for the pixel regions  102  and to moderate communications between the array of pixel regions  102  and outside devices (not shown). 
         [0016]      FIG. 3  illustrates a simplified cross-sectional view of an individual pixel region  102  of image sensor  100 . While pixel regions  102  and logic region  104  ( FIG. 2 ) may be manufactured adjacent to each other, they may also be formed in separate sections of the image sensor  100 . Both the pixel region  102  and the logic region  104  may be formed on a single substrate  302 . The substrate  302  may be formed above a carrier wafer  304  having an insulating layer  306  therebetween. The insulating layer  306  may include a plurality of metal lines  308  for communication between the pixel region  102 , logic region  104 , and/or other devices. The insulation layer  306  may comprise of inter-level dielectric and/or inter-metal dielectric layers. 
         [0017]    The substrate  302  may have a thickness between about 0.1 μm and 10 μm and may be a semiconductor material such as silicon, germanium, diamond, or the like. Alternatively, compound materials such as silicon germanium, silicon carbide, gallium arsenic, indium arsenide, indium phosphide, silicon germanium carbide, gallium arsenic phosphide, gallium indium phosphide, combinations of these, and the like, may also be used. Additionally, the substrate  302  may comprise a silicon-on-insulator (SOI) substrate. The substrate  302  may be doped with a p-type dopant, such as boron, aluminum, gallium, or the like, although the substrate  302  may alternatively be doped with an n-type dopant, as is known in the art. 
         [0018]    The substrate  302  additionally comprises a plurality of isolation structures  310  designed to separate and isolate various devices formed on or in the substrate  302 , and also to separate the pixel region  102  from the logic region  104 . The isolation structures  310  may be shallow trench isolations generally formed by etching the substrate  302  to form a trench and filling the trench with dielectric material as is known in the art. Also formed in the substrate  302  within the pixel region  102  are transistors, source/drain regions, gate structures, metal layers, and/or passivation layers, the details of which are not described so as not to obscure embodiments of the present disclosure. In an embodiment in which the substrate  302  is a p-type substrate, the source/drain region may be formed by implanting appropriate n-type dopants such as phosphorous, arsenic, antimony, or the like. It should be noted that one of ordinary skill in the art will realize that all suitable transistors, source/drain regions, gate structures, metal layers, passivation layers, and/or configurations and their processes and steps that may be utilized in an image sensor are fully intended to be included within the scope of the present disclosure. 
         [0019]    The pixel region  102  may comprise a photosensitive diode  300  that senses the amount of light that impinges the photosensitive diode  300  from a backside of the substrate  302 , and may include a pinned layer photodiode. As one skilled in the art will recognize other photosensitive diodes may also be used. For example, a non-pinned layer photodiode may alternatively be used. Any suitable photodiode may be utilized with the present disclosure, and all of these photodiodes are intended to be included within the scope of the present disclosure. 
         [0020]    Formed above the substrate  302  is a layer of a semiconductor material, silicon germanium (SiGe) layer  312 . Having a lower energy bandgap, the SiGe layer  312  enhances the image sensors incident light absorption capability and improves the image sensor&#39;s quantum efficiency, especially for longer wavelengths of light, such as red light. SiGe layer  312  may comprise from about 0.1% Ge to about 100% Ge.  FIG. 4  is a graph showing the relationships of the light absorption coefficient (cm −1 )of SiGe layer with various Ge content at various wavelengths (μm) by S. J. Koester et al., “IEEE J. of Quantum Electronics,” vol. 12, No. 16, p. 1489, 2006. Note an increase in the absorption coefficient of light where there is an increase of the Ge content. 
         [0021]    SiGe layer  312  may have a thickness from about 5 angstroms to about 5,000 angstroms and may be selectively grown on the substrate  302  of the image sensor using an epitaxial chemical vapor deposition (CVD) method in-situ. In one embodiment, the epitaxial layer  312  may have a thickness from about 0.5 nm and about 500 nm. In some embodiments, epitaxial layer  312  may have a thickness from about 1 nm to about 100 nm. Ge concentrations in the SiGe layer  312  should be determined based on the thermal budget of the manufacturing process, and in one embodiment the Ge concentration is from about 0.1% to about 100% for a temperature from about 500° C. to about 1,000° C. If the fabrication process will reach temperatures of up to 1,000° C., Ge concentration in the SiGe layer  312  should be kept between 10% and 40%. If the process temperature will remain below 500° C., pure Ge can be used instead. 
         [0022]    In addition to epitaxial growth, SiGe layer  312  may also be formed on silicon substrate  302  by implanting Ge species into silicon substrate  302 . According to one embodiment of the present disclosure, the Ge implant is performed at room temperature, in a vacuum pressure chamber with a pressure from about 10 −8  torr to about 10 −7  torr, and for a process time from about 10 minutes to 180 minutes. The higher the Ge dosage desired, the longer the process time required. However, it is not easy to obtain Ge content in the SiGe layer  312  to be higher than about 10% using a Ge implant process. To form the SiGe layer  312  with a higher Ge content for better light absorption capability, in some embodiments it is preferable to fabricate the SiGe layer  312  using the epitaxial method as high Ge content of over 10%, such as 20%, 50%, or even up to 100% may be obtained. Other methods as are known to those skilled in the art may also be used. 
         [0023]    After growing the SiGe layer  312 , the image sensor  100  may pass through a furnace or rapid thermal anneal (RTA) process. The furnace annealing process is performed in an atmosphere of nitrogen (N 2 ), Hydrogen (H 2 ) or a combination of N 2  and H 2 . 
         [0024]    To prevent incoming photons from reflecting off the surface of the photodiode  300 , and thereby failing to register a charge, an anti-reflection layer  314  may be formed above the SiGe layer  312 . Anti-reflection layer  314  may comprise silicon nitride (SiN). 
         [0025]    Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, charge coupled devices (CCDs) may be utilized in place of the CMOS devices within the image sensor, different materials may be utilized for the photodiode, or different methods of formation may be utilized for the various layers and devices. These devices, steps, and materials may be varied while remaining within the scope of the present disclosure.