Abstract:
The present invention discloses a method for manufacturing an image sensor which makes the boundaries between microlenses clear by forming a guide layer in advance and can increase the focal distance of light and the quantity of light by forming the spheres of the microlenses to have a constant height.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for manufacturing an image sensor, and more particularly, to a method for manufacturing a CMOS image sensor which can maximize the color filter array effect of a CMOS image sensor and achieve simplification and stabilization of the process.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, a microlens is formed in order to maximize the effect of the color filter array (CFA) of a CMOS image sensor. Such a microlens is shaped by coating a photosensitive photoresist of silicon oxides based having high light transmittance by a spin-on coating machine, patterning the same by use of a mask, and curing the same by thermally flowing it.  
         [0005]      FIG. 1  is a sectional view for showing a CMOS image sensor formed according to one of conventional techniques.  
         [0006]     As shown in  FIG. 1 , a semiconductor device with a color filter and a microlens includes a semiconductor substrate  10  which has underwent shallow trench isolation (STI) and a passivation processes, a first overcoating layer (OCL)  12  formed on the semiconductor substrate  10 , a color filter array (CFA) layer  14  formed on the overcoating layer  12 , a second overcoating layer  16  and a microlens (ML) layer  18  formed on the second overcoating layer  16 .  
         [0007]     In case of forming a microlens with a size of less than 4 μm by the method as shown in  FIG. 1 , it is problematic in that the uniformity of size and height is lowered due to the inaccuracy of the boundary portions of the edges.  
         [0008]     Moreover, it is difficult to measure a critical dimension (CD) due to the inaccuracy of a pattern, and the quantity of light approaching the photo-diode region is lowered due to the instability of the process.  
         [0009]     Furthermore, it is difficult to adjust the thickness of a CFA and the CD size of the CFA due to the non-uniformity of the sub-topology upon forming of the CFA layer  14 , thereby making the process unstable.  
         [0010]     In a typical image sensor, the image of an object is stored in photo-electrons focused on the above-mentioned PD and then converted into an electric signal. Accordingly, the quantity of the image converted into an electric signal is decreased or becomes non-uniform, thus the image properties of the image sensor device are deteriorated and the sensor function is also deteriorated.  
         [0011]     Further, although an etching process has been often employed in order to overcome the circumstance similar to the above-mentioned problem, this is such a process against the reduction of defects and this increases the number of process steps and becomes disadvantageous in terms of cost saving.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention is designed in consideration of the problems of the prior art, and therefore it is a primary object of the present invention to provide a method for manufacturing an image sensor which makes the boundaries between the microlenses clear by forming a guide layer in advance and can increase the focal distance of light and the quantity of light by forming the spheres of the microlenses to maintain a constant height.  
         [0013]     It is another object of the present invention to provide a method for manufacturing a CMOS image sensor which makes a color filter array free of the topology of passivation by preserving a planarization process (an overcoating layer) as it is simply by an overcoating layer resist patterning process without accompanying etching and removal processes and which makes it easier to control the size of the color filter array.  
         [0014]     To achieve the above object, there is provided a method for manufacturing a CMOS image sensor according to the present invention, comprising the steps of: a) forming an overcoating layer photoresist on a semiconductor substrate on which a shallow trench isolation and a passivation are formed; b) patterning the overcoating layer photoresist by using an overcoating layer mask; c) forming a blue layer on the patterned overcoating layer photoresist; d) forming a blue pixel by exposing the blue layer by the use of a blue layer exposure light mask; e) forming a red pixel and a green pixel by repetitively performing steps c) and d) to the red and green pixel thereby forming a pattern of a color filter array layer; and f) forming a microlens structure by coating of a microlens layer and then patterning and thermal flowing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Other objects and aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which:  
         [0016]      FIG. 1  is a sectional view for showing a CMOS image sensor formed according to one of conventional techniques; and  
         [0017]      FIGS. 2   a  to  2   i  are sectional views showing a method for manufacturing a CMOS image sensor according to a preferred embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Hereinafter, a preferred embodiment of the present invention will be described in more detail referring to the drawings. In addition, the following embodiment is for illustration only, not intended to limit the scope of the invention.  
         [0019]      FIGS. 2   a  to  2   i  are sectional views for showing a method for manufacturing a CMOS image sensor according to a preferred embodiment of the present invention.  
         [0020]     Firstly, as shown in  FIG. 2   a , an overcoating layer photoresist  102  is formed on a semiconductor substrate  100  on which a shallow trench isolation and a passivation are formed.  
         [0021]     According to the preferred embodiment of the present invention, a color filter array device composed of blue, red and green pixels representing colors on the device is formed with a 3.2 μm×3.2 μm size. In order to maximize the function of a CFA device by increasing the quantity of external light, a microlens is formed of a photosensitive photoresist of a silicon oxide based having a high transmittance with a 2.5 μm×2.5 μm size.  
         [0022]     Additionally, there is a large topology gap of a passivation layer, which is the uppermost layer of the semiconductor substrate  100 , in a CFA forming region. Thus, a first overcoating layer, which is a photosensitive overcoating layer photoresist  102  having a high transmittance is formed at a thickness of about 6,000 to 12,000 Å and planarized.  
         [0023]     Next, as shown in  FIG. 2   b , the positive overcoating layer photoresist  102  is patterned by exposure equipment by the use of an overcoating layer mask  104 . In this process, only the pixel on which a CFA is to be formed, a pad terminal and some regions are opened by using the overcoating layer photoresist  102  as a positive, and the remaining input/output regions, peri regions, etc. undergoes masking so as to leave the overcoating layer photoresist  102 .  
         [0024]     According to the preferred embodiment of the present invention, the overcoating layer photoresist  102  is left at about 500 to 1,000 Å as far as the passivation upon the exposure of an overcoating layer to be used as a planarization layer, or completely removed to reduce the distance between the color filter array and the photo-diode existing in the semiconductor substrate  100  so as to reduce the loss of light.  
         [0025]     Next, as shown in  FIG. 2   c , a blue layer  108  patterned by the above mentioned exposure and development process is coated at 9,000 Å. At this time, the overcoating layer photoresist  102  serves as a guide  107  between microlenses (ML) to be formed in the subsequent process and prevents bridging caused by the overflow of the microlenses.  
         [0026]     Next, as shown in  FIG. 2   d , the blue layer  108  is exposed by using a blue layer exposure light mask  109 .  
         [0027]     Continually, as shown in  FIG. 2   e , an over-exposure is carried out in a blank state without a mask in order to stably embody the microlenses to be formed on the color filter array, thereby enabling a blue layer  108 ′ to maintain a thickness of 7,000 to 8,000 Å which is lower than the thickness of the overcoating layer guide  107 . This is for the formation of the overcoating layer guide  107  preventing bridging between the microlenses to be formed on the color filter array layer.  
         [0028]     Next, the steps explained with reference to  FIGS. 2   c  to  2   f  described above are carried out to a red pixel  110  and a green pixel  111  in the same manner, thereby completing the patterning of the color filter array layers  110 ,  111  and  114  as shown in  FIG. 2   g.    
         [0029]     Continually, as shown in  FIG. 2   h , a microlens  112  is coated at a thickness of about 5500 to 7500 Å, and then patterned and thermally flown to form a microlens  112 ′ as shown in  FIG. 2   i.    
         [0030]     According to the preferred embodiment of the present invention, even if there is no overcoating layer planarization layer (second overcoating layer) that is conventionally used before forming a microlens layer, only a very fine topology exists, thereby making it possible to coat a microlens of 6,000 to 7,000 Å. And, since a negative overcoating layer resist is employed, it is easy to pattern a color filter array region. Besides, an overcoating layer guide  107  is formed between the color filter arrays  110 ,  111  and  114 , thus there is no bridging occuring between microlenses even in the case of overflowing.  
         [0031]     Accordingly, the progress of the microlens process is more stabilize than a conventional one and the height of the color filter array and microlenses are lower by about 12,000 Å than those of conventional ones, thereby minimizing the loss of light through the microlenses and improving the optical integrity in photo-diode regions owing to the microlenses.  
         [0032]     As seen from above, the present invention can simplify and stabilize the process and has no drawbacks in comparison with the conventional art in which a double overcoating layer is employed for an etching process and planarization. By this, the properties of a CMOS image sensor device as well as the color filter array properties are improved and process enhancement is accomplished through simplification of the process, thereby saving time and cost.