Abstract:
The present invention relates to a method for fabricating an inorganic microlens. The method includes the steps of: depositing an inorganic layer on a substrate; forming a hemispherical photoresist pattern on the inorganic layer; and performing a blanket etch-back process to thereby form a hemispherical inorganic microlens.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for fabricating an image sensor; and, more particularly, to a method for fabricating an image sensor with an inorganic microlens for improving a light concentration degree of the image sensor.  
       DESCRIPTION OF RELATED ARTS  
       [0002]     An image sensor is a device that converts the one dimensional or above the two dimensional optical information into an electric signal. The image sensor is classified into two types; they are, an image pickup tube and a solid image pickup device. The image pickup tube is widely used in fields related to measurement, control and recognition with adaptation of an image processing technology focused on television, and various applied technologies on the image pickup tube have been developed. A currently commercialized solid image pickup device of the image sensor is classified into a metal oxide semiconductor (MOS) type and a charge coupled device (CCD) type.  
         [0003]     A complementary metal oxide semiconductor (CMOS) image sensor is a device that converts an optical image into an electric signal. Particularly, the CMOS image sensor adopts a switching mode that sequentially detects outputs with use of MOS transistors fabricated as the same number of pixels. Compared to a CCD image sensor, the CMOS image sensor is more convenient in its driving mode and is capable of realizing various scanning types. Also, the realization of a signal processing circuit into a single chip makes it possible to minimize the CMOS image sensor. Furthermore, the CMOS image sensor is advantageous in lowering power consumption and reducing manufacturing costs because of the use of a compatible CMOS technology.  
         [0004]     In the image sensor, it is important to convert incident lights into electric signals without any loss. It is a photodiode that performs the above conversion function. However, in a unit pixel of a conventional image sensor, the photodiode is formed in a limited area since the unit pixel is complexly formed not only of the photodiode but also of circuits for processing signals generated within the unit pixel. To overcome this problem, a microlens is formed within an upper part of the unit pixel to condense lights incident to regions other than a photodiode region into the photodiode region. The use of the microlens improves a light concentration degree of the image sensor.  
         [0005]      FIGS. 1 and 2  are diagrams showing a conventional method for fabricating a microlens.  
         [0006]     Referring to  FIG. 1 , in a conventional CMOS image sensor, field oxide layers  102 , photodiodes  103 , an inter-metal insulation layer  104 , metal wires  105  and a passivation layer  106  are sequentially formed on a silicon substrate  101 . On top of this resulting structure, a first planarization layer  107 , color filters  108  and a second planarization layer  109  are formed.  
         [0007]     Next, an array of microlenses is formed on the second planarization layer  109 . More specifically, an organic photoresist pattern  110  is formed on a region corresponding to each photodiode  103 . Thereafter, a thermal process is performed.  
         [0008]     Referring to  FIG. 2 , edges of the organic photoresist pattern  110  are susceptible to heat exerted by the thermal process, and thus, being flowed down and forming a plurality of hemispherical microlenses  110 A.  
         [0009]     However, the organic photoresist used for the microlens  110 A is made of a fragile material. Thus, the microlens  110 A may be prone to physical shocks causing damages to the microlens  110 A, e.g., a crack denoted as ‘A’ in  FIG. 2 . Also, a defect might be generated by a contamination ‘B’ when the microlens  110 A is exposed to a chemical which melts the organic material used in the microlens  110 A. Furthermore, it is difficult to remove particles ‘C’ adsorbed onto the organic photoresist  110  due to a highly viscous material used in the organic photoresist  110 . These adsorbed particles may block lights from transmitting through the array of microlenses, thereby generating black spots on a display device with the image sensor having the microlens.  
       SUMMARY OF THE INVENTION  
       [0010]     It is, therefore, an object of the present invention to provide a method for fabricating an image sensor with an inorganic microlens capable of easily suppressing incidences of crack and contamination and of removing particles.  
         [0011]     In accordance with an aspect of the present invention, there is provided a method for fabricating an inorganic microlens of an image sensor, including the steps of: depositing an inorganic layer on a substrate; forming a hemispherical photoresist pattern on the inorganic layer; and performing a blanket etch-back process to thereby form a hemispherical inorganic microlens. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The above and other objects and features of the present invention will become better understood with regard to the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:  
         [0013]      FIGS. 1 and 2  are diagrams illustrating a conventional method for fabricating a microlens; and  
         [0014]     FIGS.  3  to  5  are cross-sectional views illustrating a method for fabricating an inorganic microlens in accordance with a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.  
         [0016]     FIGS.  3  to  5  are cross-sectional views showing a method for fabricating an inorganic microlens in accordance with a preferred embodiment of the present invention.  
         [0017]     Referring to  FIG. 3 , field oxide layers  202 , photodiodes  203 , an inter-metal insulation layer  204 , metal wires  205  and a passivation layer  206  are sequentially formed on a silicon substrate  201 . Then, a first planarization layer  207 , color filters  208  and a second planarization layer  209  are sequentially formed on top of the above resulting structure.  
         [0018]     Next, an array of inorganic microlenses is formed on the second planarization layer  209 . More specifically, an inorganic oxide layer  210  is formed on the second planarization layer  209 . An organic photoresist pattern  211  is then formed on a region of the inorganic oxide layer  210  corresponding to each photodiode  203 . A thermal process is performed thereafter.  
         [0019]     Herein, the organic photoresist pattern  211  is not used for forming the array of microlenses but for the purpose of an etch mask when the inorganic oxide layer  210  is subjected to an etching process. The inorganic oxide layer  210  is formed with a thickness ranging from about 5000 Å to about 30000 Å depending on a focal length of the microlenses and a size of the photodiode  203 . Also, the inorganic oxide layer  210  is deposited at a low temperature ranging from about 100° C. to about 200° C. in order to prevent degradation of the color filters  208 .  
         [0020]     Referring to  FIG. 4 , after the thermal process, the organic photoresist pattern  211  becomes hemispherical. Hereinafter, the hemispherical organic photoresist pattern is denoted with a reference number  211 A. Then, a blanket etch-back process is performed to the inorganic oxide layer  210  by using the hemispherical organic photoresist pattern  211 A as an etch mask. At this time, the blanket etch-back process uses an etch gas of CHF 3  and CF 4  mixed in a ratio of about 2 to 1 and proceeds under condition that an etch selectivity between the organic photoresist  211  and the inorganic oxide layer  210  is almost about 1 to 1. However, the composition ratio of the above etch gas is variable depending on an employed equipment and material characteristics. Generally, an amount of the CHF 3  gas is higher than that of the CF 4  gas.  
         [0021]     Referring to  FIG. 5 , the hemispherical organic photoresist pattern  211 A is removed. After the blanket etch-back process, the inorganic oxide layer  210  is patterned as the same shape of the hemispherical organic photoresist pattern  211 A, thereby obtaining an array of hemispherically patterned inorganic oxides  210 A. Each of the hemispherically patterned inorganic oxides  210 A serves as an inorganic microlens.  
         [0022]     Meanwhile, if the organic photoresist pattern  211  is formed by performing a photolithography process along with the use of a conventional microlens mask, adjacent hemispherically patterned inorganic oxides  210 A are not completely separated since each microlens should be maintained with an intended size. Also, this non-separated shape of the inorganic microlens  210 A does not result in a functional defect in the microlens.  
         [0023]     If these hemispherically patterned inorganic oxides  210 A are needed to be separated, each of the hemispheres in the hemispherical organic photoresist pattern  211 A is formed with a size bigger than an intended size of the microlens. Then, the inorganic oxide layer  210  is over-etched with use of the hemispheric organic photoresist pattern  211 A as an etch mask to thereby form the completely separated hemispheric inorganic microlenses  210 A each with the intended size.  
         [0024]     Since the microlens fabricated in accordance with the preferred embodiment of the present invention is made of an inorganic material, it is possible to solve the problems of a crack generated by a physical shock, a contamination-related defect and a focusing difficulty created by particles remaining on the viscous organic microlens. As a result, it is further possible to improve yields of image sensors with enhanced competitiveness in current markets.  
         [0025]     Although the above preferred embodiment exemplifies the use of oxide as the inorganic material for forming the microlens, such a nitride layer and an oxynitride layer can be still used as the inorganic material. Also, a thickness and a composition ratio of the nitride layer and oxynitride layer can be varied to control an index of refraction.  
         [0026]     While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope and spirit of the invention as defined in the following claims.