Abstract:
Disclosed is a CMOS image sensor and a method for manufacturing a CMOS image sensor. The method includes: (a) forming a resist film on a semiconductor substrate comprising a light sensing part, a protecting layer over the light sensing part, and an exposed bonding pad; (b) forming a color filter array on the thin resist film; (c) forming a plurality of microlenses over the color filter array; and (d) etching the resultant structure until the bonding pad is exposed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for manufacturing a semiconductor device, and more specifically, to a method for manufacturing a complementary metal oxide semiconductor (CMOS) image sensor.  
         [0003]     2. Description of the Related Art  
         [0004]     An image sensor, as a kind of semiconductor device, transforms optical images into electrical signals. Image sensors can be generally classified into charge coupled devices (CCDs) and CMOS image sensors. Among these image sensors, a CMOS image sensor comprises a photo diode for detecting incident light and transforming it into electrical signals, and logic circuits for transmitting and processing the electrical signals.  
         [0005]     In processes for manufacturing a CMOS image sensor, it is desired to increase a so-called fill factor, defined as a ratio of light sensing area to total image sensor area, for the purpose of improving light sensitivity. However, since the light sensing part is formed only in an area other than the area where logic circuits are formed, there are limits to improving the fill factor of the device. For such reason, forming a plurality of microlenses on or over color filters of a CMOS image sensor has been widely employed as one alternative light condensing technique for changing a path of light incident on regions other than the light sensing part and concentrating light to the light sensing part.  
         [0006]     A conventional method for manufacturing a CMOS image sensor including microlenses is hereinafter described referring to  FIGS. 1   a  to  1   d.    
         [0007]     A conventional CMOS image sensor comprises: a light sensing part  13  including a photo diode  11  for accepting incident light, and for generating and accumulating electric charges; a protecting layer  21  formed on a structure of the light sensing part; color filter arrays  23 ; a planarization layer  25 ; and a plurality of microlenses  27 .  
         [0008]     In a conventional method of manufacturing such structured CMOS image sensor, as shown in  FIG. 1   a , the protecting layer  21  with a silicon nitride base is formed on a semiconductor substrate  10  that comprises the light sensing part  13  including the photo diodes  11 , and on a wiring bonding pad  15 . Then, as shown in  FIG. 1B , a portion of the protecting layer  21  on the wiring bonding pad  15  is removed, exposing an upper surface of the wiring bonding pad  15 . This opening process generally involves a photolithography process. More specifically, a photoresist material is applied and patterned on the protecting layer  21 , then a portion of the protecting layer  21  is etched and removed to expose the wiring bonding pad  15 . Afterwards, a remaining photoresist material is removed using a reactive ion etch.  
         [0009]     Next, as shown in  FIG. 1   b , the color filter array  23  is formed on the protecting layer  21 . Here, the color filter array  23  is formed in a primary color system, i.e., including a red filter (R), a green filter (G), and a blue filter (B), using photoresist materials containing a red, green, or blue pigment, respectively. Formation of each color filter involves a series of coating, exposure and development processes according to the photolithography technique. Alternatively, the color filter array can be formed in a complementary color system including cyan, yellow, and magenta filters.  
         [0010]     Then, as shown in  FIG. 1   c , the planarization layer  25  is formed on the color filter array  23 . The planarization layer  25  removes steps (uneven horizontal surfaces) in the topography between the color filters  23 , thus enabling uniform formation of microlenses. In addition, a thickness of the planarization layer  25  is controlled so that a focal length is adjusted appropriately. The planarization layer  25  can comprise a photoresist, oxide, or nitride base material.  
         [0011]     Next, a photoresist layer is applied, exposed, and developed on the planarization layer  25 , thus forming a plurality of photoresist patterns. These photoresist patterns are thermally reflowed and cured to form a lens, thus resulting in a plurality of microlenses  27  shown in  FIG. 1   d.    
         [0012]     According to the above-described conventional method, the remaining photoresist material in the bond pad opening process is ashed by a reactive ion etch. Thus, surface properties of the protecting layer  21  can be locally changed according to the conditions (ambient and other) in the ashing process. As a result, adhesion of color photoresist on the protecting layer  21  may deteriorate so that some of the patterned color filters peel off. In general, the color filter array  23  is formed of organic materials. Especially, in the case where the color photoresist contains a large amount of pigment, the peeling phenomenon may occur more frequently because of relatively large-sized pigment particles affecting adhesion to the protecting layer  21 . There is a need in the art to solve the peeling problems of the color filters because it can induce deterioration and/or failure of certain characteristics of the device (e.g., discoloration).  
         [0013]     In the case where the peeling phenomenon occurs, the color filter array  23  and microlenses  27  may be reworked and reproduced by stripping one or more of the color filter array  23 , the planarization layer  25 , and the microlenses  27 , and repeating the photolithography process(es) for the color filter array  23 , the planarization layer  25 , and the microlenses  27 , up to several times. However, a developing solution used in the photolithography process generally comprises TMAH ((Tetramethylammonium hydroxide) which erodes the exposed wiring bonding pad  15 . Therefore, the number of repetitions of the stripping and photolithography process is restricted. Moreover, several repetitions of the photolithography process can lead to contamination of wiring bonding pad  15 , thus resulting in a wiring failure.  
         [0014]     Meanwhile, according to the conventional method, the microlenses  27  are formed distant from each other by about 0.21 μm˜0.5 μm, for the purpose of preventing formation of bridges between the microlenses  27  during the curing and reflowing processes of the corresponding photoresist pattern. However, the gap between microlenses  27  results in at least some loss of the light incident between microlenses  27 , and especially a problem that the resolution of color signals may be less than optimal due to oblique light incident to adjacent pixels.  
       SUMMARY OF THE INVENTION  
       [0015]     It is, therefore, an object of the present invention to provide a method for manufacturing a CMOS image sensor, wherein a bonding pad is protected by a thin resist film before formation of a color filter array, and the thin resist film covering the bonding pad is removed after formation of microlenses, thus inhibiting, suppressing or preventing the color filter array from peeling off.  
         [0016]     Another object of the present invention is to provide a method for manufacturing a CMOS image sensor, in which unnecessary gaps between microlenses are reduced, minimized or eliminated, thereby improving the light condensing efficiency of the image sensor, and reducing or preventing adverse effects of oblique light incident to adjacent pixels, and ultimately enabling realization of more vivid colors.  
         [0017]     It is still another object of the present invention to provide a method for manufacturing a CMOS image sensor having improved light sensitivity by improving the shape uniformity of the microlenses.  
         [0018]     To achieve the above objects, an embodiment of a method for manufacturing a CMOS image sensor according to the present invention comprises the steps of: (a) forming a resist film on a semiconductor substrate comprising a light sensing part, a protecting layer over the light sensing part, and an exposed bonding pad; (b) forming a color filter array on the resist film; (c) forming a plurality of microlenses over the color filter array; and (d) etching the resultant structure until the bonding pad is exposed again.  
         [0019]     Alternatively, a method for manufacturing a CMOS image sensor according to the present, can comprise the steps of: forming an organic material layer over an entire surface of the substrate, including a plurality of microlenses and a bonding pad; and etching the resultant structure until the bonding pad is exposed.  
         [0020]     These and other aspects of the invention will become evident by reference to the following description of the invention, often referring to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]      FIGS. 1   a  to  1   d  are cross-sectional views illustrating a conventional method for manufacturing a CMOS image sensor.  
         [0022]      FIGS. 2   a  to  2   f  are cross-sectional views illustrating one embodiment of a method for manufacturing a CMOS image sensor according to the present invention.  
         [0023]      FIGS. 3   a  to  3   c  are cross-sectional views illustrating another embodiment of a method for manufacturing a CMOS image sensor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Hereinafter, one embodiment of a manufacturing method for a CMOS image sensor according to the present invention will be described with reference to  FIGS. 2   a  to  2   f.    
         [0025]     Referring to  FIG. 2   a , a protecting layer  201  with a silicon nitride base (e.g., which may comprise silicon nitride) is formed on a semiconductor substrate  100  that comprises light sensing parts  103  including photo diodes  101 , one or more metallization or wiring layers (not shown), and a bonding pad  105  (e.g., for attachment of a wire adapted for transferring electrical signals to and/or from the image sensor). Then, a portion of the protecting layer  201  on or over the bonding pad  105  is removed, thus exposing an upper surface of the bonding pad  105 . This opening process of the bonding pad  105  is generally performed using a photolithography process. More specifically, a photoresist material is applied on the entire protecting layer  201  (e.g., spin-coated onto the entire substrate) and patterned (e.g., by conventional irradiation and development), then a portion of the protecting layer  201  (e.g., at least the portion over bonding pad  105 ) is etched and removed. The remaining photoresist pattern is then removed (e.g., using a reactive ion etch and/or ashing process).  
         [0026]     Next, as shown in  FIG. 2   b , a thin resist film  202  is formed on entire surfaces of the exposed bonding pad  105  and the protecting layer  201 , thus covering the bonding pad  105 . For example, the resist film  202 , which can comprise an organic material, may have a thickness of not more than 50 nm (e.g., 10 to 50 nm). Thereafter, it is hard-cured. The resist film  202  functions as a protector for the bonding pad  105 , and (optionally) as a planarization layer for improving the profile (e.g., the flatness or horizontal nature of the topography) and the uniformity of a color filter array  203  to be formed thereon in a subsequent process. It is preferable that the resist film  202  comprise an organic material base having a superior (or predetermined) transparency to visible light (e.g., at least 80%, 90%, or more). More preferably, the resist film  202  comprises a thermosetting resin, such as an acrylic resin, etc., that has little reactivity with the color filter array  203 .  
         [0027]     Thereafter, as shown in  FIG. 2   c , the color filter array  203  is formed on the protecting layer  202 . Here, the color filter array  203  comprises a primary color system, i.e., including a red filter (R), a green filter (G), and a blue filter (B), using a photoresist material containing a red, green, and blue pigment, respectively. Formation of each color filter involves performing at least three times the photolithography process including coating, exposure and development of each individual photoresist material. The present embodiment exemplifies the primary color filter system, however, the color filter array can be alternatively formed in a complementary color system including cyan, yellow, and magenta filters.  
         [0028]     As shown in  FIG. 2D , a planarization layer  205  is formed on the color filter array  203 . The planarization layer  205  removes steps (e.g., differences in height across the substrate topography) between the color filters  203 , thus enabling uniform formation of microlenses  207 . In addition, the planarization layer  205  may have a thickness of about 0.5 μm ˜1.5 μm and may be selected so that a focal length of light passing through an overlying microlens is adjusted appropriately (e.g., to focus the light onto an underlying photodiode  101 ). The planarization layer  25  can comprise a photoresist, oxide, or nitride base material. The planarization layer  205  is an optional component and it may be omitted.  
         [0029]     Next, a photoresist layer is applied, exposed, and developed on the planarization layer  205 , thus forming a photoresist pattern (e.g., a plurality of microlens bodies). The photoresist pattern is bleached, thermally reflowed and cured to form lenses having a desired curvature, thus resulting in a plurality of microlenses  207  as shown in  FIG. 2E .  
         [0030]     Here, the number of microlenses  207  preferably relates to the number of pixels in the image sensor, and they have a size (e.g., area) as large as possible, in order to improve light sensitivity and condensing efficiency to incident light. However, it is difficult to uniformly fabricate so many (relatively) large microlenses because of morphology effects due to the underlying layer and photo effects in the microlenses. Therefore, gaps (e.g.,  303 ) between the microlenses  207  should be formed to the extent that they can be controlled appropriately (e.g., reproduced within manufacturing tolerances or limits), as shown in  FIGS. 2E and 2F .  
         [0031]     Next, the substrate is etched (e.g., it undergoes a blank etch) using an oxygen (O 2 ) plasma or other plasma comprising an oxygen atom source (e.g., O 3 , N 2 O, NO, CO 2 , etc., which may further contain a carrier gas or noble gas such as N 2 , Ar, He, etc.) so that the portion of the resist film  202  covering the bonding pad  105  is removed, and simultaneously, the microlenses  207  may be etched. In this process, irregular tails (or “bridges”) between microlenses  207  may also be removed, and gaps  303  between microlenses  207  may be shallow etched. Especially, regions  303  between microlenses  207  may be etched to in the form of a concave lens, thus resulting in “gapless” microlenses, as shown in  FIG. 2   f . Such plasma etching consequently enables maximizing the size of the microlenses, thereby enabling condensation of incident light to the photodiode by an amount approaching 100%. Furthermore, since the light sensitivity can be improved and an oblique light incident to adjacent pixels can be reduced or prevented, it is possible to realize clearer images using a CMOS image sensor manufactured according to the present invention.  
         [0032]     Meanwhile, referring to  FIG. 2   f , the shallow etching of gaps  303  between microlenses  207  utilizes the difference of etching selectivity between the photoresist ingredients of microlenses  207  and the planarization layer  205 . In the case where these two materials have a similar etching selectivity, an ideal concave form between microlenses may not form as readily as may be desired. In such case, as shown in  FIG. 3   a , it is preferable that an organic material layer  305  is further formed over the entire surface of the substrate before the etch process of  FIG. 2F . The organic material layer  305  can comprise a photoresist or thermosetting resin, and it can have a thickness of about 30 nm ˜100 nm. Preferably, when using the organic material layer  305 , the microlenses  207  may be irradiated with ultraviolet radiation to prevent an undesired reaction between the organic material layer  305  and microlenses  207 .  
         [0033]     As shown in  FIG. 3   a , forming organic material layer  305  uniformly over the entire surface of the substrate, especially microlenses  207 , can facilitate formation of the concave profiles between microlenses  207 . More specifically, when the substrate is dry etched by an oxygen-containing plasma (e.g., a plasma comprising O 2 ) after forming the organic material  305 , the concave profiles of the organic material layer  305  between microlenses can be maintained. Although the resist film  202  and the organic material layer  305  are both present on the bonding pad  105 , the etch should proceed until the bonding pad  105  is exposed. For this reason, the amount of the organic material layer  305  remaining after the etch depends on the total thickness of the resist film  202 .  FIGS. 3   b  and  3   c  show the cases where the organic material layer remaining after the etch is present in different amounts. However, the operation of the CMOS image sensor is not affected in either case. For example, in the case where a relatively considerable amount of the organic material remains, as shown in  FIG. 3   b , the remaining organic material layer  305   a  still has a concave profile  303   a  between microlenses. In the other case where the organic material remains locally in a relatively small amount (and, e.g., where the planarization layer  205  is partially etched during the etching process), as shown in  FIG. 3   c , the region between microlenses still has a concave profile  303   b.    
         [0034]     According to the present invention, a resist film for protecting the bonding pad is formed over the bonding pads before forming the color filters and removed from the bonding pads after forming the microlenses, which enables a reduction and/or prevention of erosion of the bonding pad, as well as a change in surface properties of the protecting layer. As a result, the peeling phenomenon of the color filter array can be prevented. In addition, the present invention enables (a) improvement in shape uniformity of the microlenses and (b) formation of gapless microlenses. Moreover, the present invention has such advantages that the size of microlens can be maximized, thus the light sensitivity can be improved. Especially, using the organic material layer facilitates formation of a concave profile between microlenses.  
         [0035]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be therein without departing from the spirit and scope of the invention as defined by the appended claims.