Abstract:
A method of manufacturing an image sensor using a microlens mold is provided. The method includes: forming an interlayer dielectric layer on a semiconductor substrate having photodiodes; forming color filter layers on the interlayer dielectric layer; forming a planarization layer on the color filter layers; coating photoresist on the planarization layer; aligning a mold having a lens shaped pattern on the semiconductor substrate with the photoresist applied thereon; pressing the mold and the semiconductor substrate closely to each other such that a pattern formed in the mold is transferred onto the photoresist; and separating the mold from the semiconductor substrate, thereby forming micro-lenses.

Description:
RELATED APPLICATION(S) 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of Korean Patent Application Nos. 10-2005-0132256 filed Dec. 28, 2005 and 10-2005-0131289 filed Dec. 28, 2005, both of which are incorporated herein by reference in their entireties. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method of manufacturing an image sensor. 
       BACKGROUND OF THE INVENTION 
       [0003]    In general, image sensors are semiconductor devices for converting optical images into electric signals, and are generally classified as charge coupled devices (CCDs) or CMOS (Complementary Metal Oxide Semiconductor) image sensors. 
         [0004]    The CMOS image sensor includes a photodiode for detecting light and a logic circuit for converting detected light into electric signals representing image data. As the quantity of light received in the photodiode increases, the photo sensitivity of the image sensor is improved. 
         [0005]    To improve the photo sensitivity, either a fill factor, which is the ratio of a photodiode area to the whole area of the image sensor, must be increased, or a photo-gathering technology is used to change the path of light incident onto an area other than the photodiode area such that the light can be gathered in the photodiode. 
         [0006]    A representative example of the photo-gathering technology is to make a micro-lens. That is, a convex micro-lens is formed on a top surface of a photodiode region using a material having superior light transmittance, thereby refracting the path of incident light in such a manner that a greater amount of light can be transmitted into the photo-diode. 
         [0007]    In this case, light parallel with an optical axis of the micro-lens is refracted by the micro-lens so that the light is focused on a certain position of the optical axis. 
         [0008]    Meanwhile, since resolution is determined depending on the number of photodiodes receiving images, the current tendency has tended toward increasing the number of pixels (high-pixels) and miniaturizing the size of the pixels (micro-sized pixels), when manufacturing elements for an image sensor. 
         [0009]    Because of the development of the micro-sized pixels and the high-pixels, external images are input into an image plane through a micro-lens. 
         [0010]    Color filters are formed with primary color filter layers or complementary color filter layers for the purpose of color reproduction. That is, the color filters are formed through an on-chip scheme in such a manner that the primary color filter layers can reproduce red, green and blue colors, and the complementary color filter layers reproduce cyan, yellow and magenta colors through color separation. 
         [0011]    Meanwhile, in order to effectively and appropriately utilize incident light, a micro-lens is provided to improve light condensing efficiency. The micro-lens is formed by thermally reflowing photoresist. 
         [0012]    However, when reflowing the photoresist to maximize the size of the micro-lens such that a greater amount of light can be condensed, a bridge is produced between neighboring micro-lenses. For this reason, a CD (Critical Dimension) is maintained in a certain degree to enhance uniformity. 
         [0013]    Among the image sensors having the aforementioned feature, the CMOS image sensor is classified into 3T-type, 4T-type, 5T-type or the like depending on the number of transistors in each unit pixel. The 3T-type CMOS image sensor includes one photodiode and three transistors, and the 4T-type CMOS image sensor includes one photodiode and four transistors. An equivalent circuit and a layout for a unit pixel of the 3T-type CMOS image sensor is described below with reference to  FIGS. 1 and 2 . 
         [0014]      FIG. 1  is an equivalent circuit diagram of a general 3T-type CMOS image sensor, and  FIG. 2  is a layout showing a unit pixel of a general 3T-type CMOS image sensor. 
         [0015]    As shown in  FIG. 1 , the unit pixel of a conventional 3T-type CMOS image sensor includes one photodiode PD and three nMOS transistors T 1 , T 2  and T 3 . The cathode of the photodiode PD is connected to the drain of the first NMOS transistor T 1  and the gate of the second nMOS transistor T 2 . 
         [0016]    The sources of the first and second nMOS transistors T 1  and T 2  are connected to a power source line through which a reference voltage VR is supplied, and the gate of the first NMOS transistor T 1  is connected to a reset line through which a reset signal RST is supplied. 
         [0017]    The source of the third nMOS transistor T 3  is connected to the drain of the second nMOS transistor T 2 , the drain of the third nMOS transistor T 3  is connected to a reading circuit (not shown) through a signal line, and the gate of the third nMOS transistor T 3  is connected to a row selection line through which a selection signal SLCT is supplied. 
         [0018]    Thus, the first, second and third nMOS transistors T 1 , T 2  and T 3  are referred to as reset, drive and selection transistors Rx, Dx and Sx, respectively. 
         [0019]    Referring to  FIG. 2 , in the unit pixel of the conventional CMOS image sensor, an active region  10  is defined having a wide portion and a narrow portion. A photodiode  20  is formed on the portion having a broad width in the active region  10 , and gate electrodes  30 ,  40  and  50  of three transistors are formed overlapping the narrow portion of the active region  10 . 
         [0020]    That is, reset, drive and selection transistors Rx, Dx and Sx are formed by the gate electrodes  30 ,  40  and  50 , respectively. 
         [0021]    Here, impurity ions are implanted into portions of the active region  10  except below each of the gates  30 ,  40  and  50  of each of the transistors so as to form a source/drain region for each of the transistors. Thus, a power source voltage Vdd is supplied into the source/drain region between the reset and drive transistors Rx and Dx, and a reading circuit (not shown) is connected to the source/drain region at one side of the select transistor Sx. 
         [0022]    Although not shown in the figures, the gate electrodes  30 ,  40  and  50  described above are respectively connected to signal lines, and each of the signal lines are provided with a pad at one end thereof to be connected to an external driving circuit. 
         [0023]    Hereinafter, a method of manufacturing a CMOS image sensor according to the related art will be described with reference to the accompanying drawings. 
         [0024]      FIGS. 3 to 6  are sectional views illustrating a method of manufacturing a CMOS image sensor according to the related art. 
         [0025]    Referring to  FIG. 3 , an interlayer dielectric layer  13  is formed on a semiconductor substrate  11  having a plurality of light sensing elements, e.g., photodiodes  12 . 
         [0026]    Here, the interlayer dielectric layer  13  may be formed as multiple layers. 
         [0027]    Then, a dyeable resist is coated on the interlayer dielectric layer  13 , and color filter layers  14  for filtering light for each wavelength band are then formed by performing an exposure and development process. 
         [0028]    Subsequently, a planarization layer  15  is formed on the color filter layers in order to adjust a focus distance and secure planarity for forming a lens layer. 
         [0029]    Referring to  FIG. 4 , a resist layer  16   a  for forming micro-lenses is applied on the planarization layer  15 , and a reticle  17  with openings is aligned above the resist layer  16   a.    
         [0030]    Subsequently, the resist layer  16   a  is selectively exposed to correspond to the openings of the reticle  17  by radiating light such as laser onto the semiconductor substrate  11  through the reticle  17 . 
         [0031]    Referring to  FIG. 5 , the exposed resist layer  16   a  is developed to form micro-lens patterns  16   b.    
         [0032]    Referring to  FIG. 6 , hemispherical micro-lenses  16  are formed by reflowing the micro-lens patterns  16   b  at a predetermined temperature. 
         [0033]    However, since micro-lenses are formed through a reflowing method in the aforementioned CMOS image sensor according to the related art, there are some problems as follows: 
         [0034]    First, it is very difficult to control bleaching and reflowing processes after exposing and developing processes due to the sensitivity of the photoresist for micro-lenses. 
         [0035]    That is, the intervals between micro-lenses may form unequally. For example, as shown in  FIG. 6 , interval width A is narrower than interval width B. 
         [0036]    Second, since micro-lenses are very sensitive to the thermal process, high-priced equipment capable of fine temperature adjustment is required. When the temperature is not adjusted properly, it is difficult to form perfect spherical lenses. 
         [0037]    That is, where a temperature is not adjusted properly, lenses may connect to each other or may be separated by a long distance so that precise images cannot be obtained. The formation of perfectly spherical lenses has become an important point for determining the quality of a complimentary oxide semiconductor image sensor. 
         [0038]    The latest technology trend focuses on techniques for reducing the path of light to enhance the quality of the image sensors. One such technique incorporates aligning the micro-lenses to as low a portion as possible after the process for forming complimentary metal oxide has been finished. 
         [0039]    In addition, there is a case where the micro-lens is formed after reducing the thickness of a passivation layer before the lens of the image sensor is formed. In this case, if the conventional semiconductor process is used, uniformity of photoresist becomes bad due to the step difference, and the aperture or shape of a lens adjacent to the stepped portion becomes deteriorated when forming the lenses. 
       SUMMARY OF THE INVENTION 
       [0040]    Accordingly, an object of embodiments of the present invention is to provide a method of manufacturing an image sensor, wherein micro-lenses are formed using an imprint method so that the micro-lenses can be uniformly formed, and a characteristic of the image sensor can be enhanced. 
         [0041]    In accordance with a preferred embodiment of the present invention, there is provided a method of manufacturing an image sensor, including: forming an interlayer dielectric layer on a semiconductor substrate having photodiodes; forming color filter layers on the interlayer dielectric layer; forming a planarization layer on the color filter layers; coating photoresist on the planarization layer; aligning a mold with the shape of a lens patterned therein on the semiconductor substrate with the photoresist applied thereon; pressing the mold and the semiconductor substrate closely to each other such that a pattern formed in the mold is transferred onto the photoresist; and separating the mold from the semiconductor substrate to form micro-lenses. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]      FIG. 1  is an equivalent circuit diagram of a general 3T-type CMOS image sensor; 
           [0043]      FIG. 2  is a layout showing a unit pixel of a general 3T-type CMOS image sensor; 
           [0044]      FIGS. 3 to 6  are sectional views illustrating a method of manufacturing a CMOS image sensor according to the related art; 
           [0045]      FIGS. 7 to 10  are sectional views illustrating a process of forming a pattern with a desired shape using an imprint lithography method; 
           [0046]      FIGS. 11 to 18  are sectional views illustrating a method of manufacturing an image sensor according to a first embodiment of the present invention; and 
           [0047]      FIGS. 19 to 23  are sectional illustrating a method of manufacturing an image sensor according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0048]    Hereinafter, a method of manufacturing an image sensor according to embodiments of the present invention will be described with reference to the accompanying drawings. 
         [0049]      FIGS. 7 to 10  are sectional views illustrating a process of forming a pattern with a desired shape using an imprint lithography method according to an embodiment of the present invention. 
         [0050]    Referring to  FIG. 7 , a solid mold  31  can be prepared with a desired pattern formed thereon. In one embodiment, the solid mold  31  can be formed of silicon or the like. 
         [0051]    A thermoplastic polymer thin film  33  can be formed by being coated on a semiconductor substrate  32 . 
         [0052]    The mold  31  with the pattern formed thereon can be aligned above the semiconductor substrate  32  with the polymer thin film  33  coated thereon. 
         [0053]    Referring to  FIG. 8 , the mold  31  with the pattern formed thereon and the semiconductor substrate  32  with the polymer thin film  33  formed thereon can be aligned such that the formed pattern faces the polymer thin film  33 . 
         [0054]    Referring to  FIG. 9 , the semiconductor substrate  32  and the mold  31  can be introduced between press plates so as to be treated at high temperature and high pressure in a state in which they are in close contact with each other. The press plates can cause the formed pattern to press into the polymer thin film  33 . 
         [0055]    Referring to  FIG. 10 , when the semiconductor substrate  32  and the mold  31  are separated from each other, it can be seen that the pattern formed on the mold  31  is transferred onto the polymer thin film  33  formed on the semiconductor substrate  32  such that a polymer thin film pattern  33   a  is formed. 
         [0056]    Because a solid mold of Si or the like can be used in the aforementioned imprint lithography method, there is an advantage in that a pattern can be easily implemented up to about 6 nm. 
         [0057]      FIGS. 11 to 18  are sectional views illustrating a method of manufacturing an image sensor according to a first embodiment of the present invention. 
         [0058]    Referring to  FIG. 11 , an interlayer dielectric layer  103  can be formed on a semiconductor substrate  101  having a plurality of light sensing elements, e.g., photodiodes  102 , and various transistors (such as shown in  FIG. 1 ). 
         [0059]    In one embodiment, the interlayer dielectric layer  103  can be formed as multiple layers. Although not shown in the figures, after one interlayer dielectric layer is formed, a light shielding layer can be formed to prevent light from being incident onto a portion of the substrate between the regions of the photodiodes  102 , and then another interlayer dielectric layer can be formed on the light shielding layer. 
         [0060]    Referring to  FIG. 12 , a dyeable resist can be applied on the interlayer dielectric layer  103 , and color filter layers (R, G, B)  104  for filtering light for each wavelength band can then be formed by performing an exposing and developing process. 
         [0061]    In a specific embodiment, the color filter layers  104  for filtering light for each wavelength band can be formed as a single layer by applying a corresponding photoresist material, and patterning the photoresist material through a photo-etching process using an additional mask. In one embodiment, the color filter layers  104  can have a thickness of 1 to 5 μm. 
         [0062]    Referring to  FIG. 13 , in order to obtain reliability and EMC in packaging and to prevent the penetration of moisture or a heavy metal from the outside, a planarization layer  105  can be formed by, for example, depositing a silicon nitride film on the entire surface of the semiconductor substrate  101  including the color filter layers  104 . 
         [0063]    Because optical transmission is very important in an image sensor, the planarization layer  105  can be formed to have a thickness of 1000 Å to 6000 Å so as to eliminate an interference phenomenon of thin films due to the thickness thereof. 
         [0064]    Here, a desired bonding pad (not shown) can be formed for interconnection by opening pad and scribe line sections of the planarization layer  105  and then performing a dry or wet etching using photoresist as an etch mask. 
         [0065]    Referring to  FIG. 14 , in order to increase an amount of light incident onto the photodiodes  102 , a trench  106  with a predetermined depth from a surface of the planarization layer  105  can be formed by selectively removing a portion of the planarization layer  105  at which micro-lenses will be formed later. 
         [0066]    Referring to  FIG. 15 , a photoresist  107  for micro-lenses can be coated on the entire surface of the semiconductor substrate  101  including in the trench  106 . 
         [0067]    Referring to  FIG. 16 , a mold  108  in which a pattern is formed in the shape of a desired lens (e.g., a hemispheric shape) can be aligned above the semiconductor substrate  101  having the photoresist  107  applied thereon. 
         [0068]    In one embodiment, PDMS (poly dimethylsiloxane) can be used as a material for the mold  108 . 
         [0069]    Referring to  FIG. 17 , the mold  108  and the semiconductor substrate  101  having the photoresist  107  applied thereon can be pressed into close contact with each other, and a thermal process is then performed while applying pressure thereto. 
         [0070]    The thermal process can be performed such that the photoresist  107  can maintain the shape imprinted by the pattern of the mold  108 . 
         [0071]    Referring to  FIG. 18 , the mold  108  can be separated from the semiconductor substrate  101  to form micro-lenses  109  having the same shape as the pattern of the mold  108  within the trench  106  of the planarization layer  105 . 
         [0072]    Accordingly, in embodiments of the present invention, the distance between the micro-lens  109  and the photodiode  102  can be reduced by forming the trench  106  on the planarization layer  105  and then forming the micro-lenses  109  so that loss of light incident onto the photodiode  102  through the micro-lens  109  can be minimized, thereby enhancing the sensitivity of the image sensor. 
         [0073]      FIGS. 19 to 23  are sectional views illustrating a method of manufacturing an image sensor according to a second embodiment of the present invention. 
         [0074]    Referring to  FIG. 19 , photodiodes  202  can be formed within a semiconductor substrate  201 , and an interlayer dielectric layer  203  can be formed on the semiconductor substrate  201 . 
         [0075]    Then, color filter layers (R, G, B)  204  for filtering light can be formed on the interlayer dielectric layer  203  using a dyeable resist. 
         [0076]    Thereafter, a planarization layer  205  can be formed on the color filter layers  204 , and a photoresist  206  for micro-lenses can be applied on the planarization layer  205 . 
         [0077]    Such processes can be the same as described above in reference to the first embodiment. 
         [0078]    Referring to  FIG. 20 , the photoresist  206  can be selectively patterned through an exposing and developing process to form micro-lens patterns  206   a  having a predetermined interval. 
         [0079]    Referring to  FIG. 21 , a mold  207  in which a pattern is formed in the shape of a desired lens (e.g., a hemisphere) can be aligned above the semiconductor substrate  201  having the micro-lens patterns  206   a  formed thereon. 
         [0080]    In one embodiment, PDMS (poly dimethylsiloxane) can be used as a material of the mold  207 . 
         [0081]    Referring to  FIG. 22 , the mold  207  and the semiconductor substrate  201  having the micro-lens patterns  206   a  formed thereon can be pressed into close contact with each other, and a thermal process is then performed while applying pressure thereto. 
         [0082]    The thermal process can be performed such that the micro-lens pattern  206   a  can maintain an exact shape after the mold  207  has imprinted the semiconductor substrate  201  with the micro-lens patterns  206   a  formed thereon. 
         [0083]    Referring to  FIG. 23 , the mold  207  can be separated from the semiconductor substrate  201  to reveal micro-lenses  208  having the same shape as the pattern of the mold  207  on the planarization layer  205 . 
         [0084]    As described above, a method of manufacturing a CMOS image sensor according to embodiments of the present invention has advantages as follows. 
         [0085]    That is, micro-lenses with a desired shape can be formed using an imprint method in the present invention so that advantages can be expected as follows: 
         [0086]    First, the exact shape of a micro-lens can be formed without requiring a reflow process. In particular, embodiments of the subject method can form the micro-lenses while maintaining proper interval spacing. 
         [0087]    Second, although the conventional method of forming a lens has a disadvantage in that the shape of the lens may change depending on a condition of a heating and exposing process, a precisely designed mold can be imprinted into the photoresist so that factors adversely effecting the shape of a lens can be reduced. 
         [0088]    Third, although a heating and exposing process should be performed again after a general photo process in the conventional method of forming a lens, such a process is shortened in the present invention so that productivity can be enhanced. 
         [0089]    It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application.