Patent Publication Number: US-7595216-B2

Title: Method for manufacturing CMOS image sensor

Description:
RELATED APPLICATION(S) 
   This application claims the benefit under 35 USC §119(e) of Korean Patent Application No. 10-2005-0134453 filed Dec. 29, 2005, which is incorporated herein by reference in its entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to a method for manufacturing an image sensor, and more particularly, to a method for manufacturing a complementary metal oxide silicon (CMOS) image sensor. 
   BACKGROUND OF THE INVENTION 
   In general, an image sensor is a semiconductor device that transforms an optical image to electrical signals. The image sensor is generally classified as a charge coupled device (CCD) or a CMOS image sensor. 
   The CCD includes a matrix of photodiodes (PD). Each photodiode converts an optical signal into an electric signal. The CCD also includes a plurality of vertical charge coupled devices (VCCDs). The VCCDs are formed between vertical lines of the photodiodes in the matrix for transmission of charges generated at the photodiodes in a vertical direction. The CCD further includes horizontal charge coupled devices (HCCDs) for transmission of the charges transmitted through the VCCDs in a horizontal direction. In addition, the CCD includes a sense amplifier for sensing the charges transmitted in the horizontal direction and outputting an electric signal. 
   However, the CCD is disadvantageous in that it has a complicated driving method, exhibits high power consumption, and is produced via a complicated fabrication process involving multiple photo process stages. 
   Moreover, the CCD has another disadvantage in that it is difficult to include a CCD in a small product due to the difficulty in integrating a control circuit, a signal processing circuit, and an A/D converter, and the like on a CCD chip. 
   Recently, the CMOS sensor has been heralded as the next generation image sensor that can overcome the disadvantages of CCDs. 
   The CMOS image sensor is a device that employs CMOS technology to capture an image. Specifically, a control circuit, a signal processing circuit, and the like are used as peripheral circuits for successively detecting outputs from pixels using MOS transistors. A MOS transistor is formed on the semiconductor substrate for each pixel. 
   That is, the CMOS image sensor has a photodiode and a MOS transistor formed within each unit pixel. By monitoring the switching of the MOS transistors, the CMOS image sensor successively detects electric signals from the photodiodes of the unit pixels to reproduce an image. 
   The CMOS image sensor exhibits low power consumption and has a simple fabrication process as a result of fewer photo process stages. 
   The following is a description of a method for fabricating a CMOS image sensor according to the related art with reference to the accompanying drawings. 
     FIGS. 1A and 1B  are cross-sectional views of a CMOS image sensor for describing a method of fabricating a CMOS image sensor according to the related art. 
   Referring to  FIG. 1A , an interlayer dielectric  12  is formed on a semiconductor substrate (not shown). Here, the substrate includes a plurality of photo sensing devices, for example, photodiodes  11 . 
   The interlayer dielectric  12  may be formed with multi layers. Although not shown, after one interlayer dielectric layer has been formed, a light shielding layer may be formed and another interlayer dielectric layer can then be formed thereon. Here, the light shielding layer functions to block light incident on parts of the substrate other than photodiode regions. 
   Next, a planarized passivation layer  13  is formed on the interlayer dielectric  12  in order to protect a device from moisture and to prevent the device from being scratched. 
   Then, color filters  14 , which filter light by wavelengths, are formed by coating the passivation layer  13  with a dyable resist, and then performing exposure and developing processes. 
   Here, a photolithography process is selectively performed in the red (R), green (G), and blue (B) color filter layers  14  three times to form a color separating layer. 
   Next, a planarization layer  15  is formed on the color filter layers  14  in order to adjust a focusing distance and secure the flatness for forming a lens layer. 
   Further, a thermal treatment is performed at a temperature greater than 200° C. so as to cure the planarization layer  15 . 
   Then, a resist layer for a microlens is coated on the planarization layer  35 , and a reticle (not shown) having an opening portion is aligned above the resist layer. Next, light such as a laser beam is irradiated to an entire surface of the semiconductor substrate using the reticle as a mask to selectively expose the resist layer through the openings of the reticle. 
   Then, the exposed resist layer  16   a  is developed to form a microlens pattern  20 . 
   Thereafter, as shown in  FIG. 2B , the microlens patterns  20  are reflown at a predetermined temperature to form microlenses  22 . 
   However, during the forming process of the microlens pattern  20  it is difficult to precisely define intervals between microlens patterns because of the limit of the resolution of photolithographic equipment. Moreover, after completion of the reflow process, adjacent microlenses will not have a zero-gap. 
   Consequently, some light may not be focused to a light receiving section of the CMOS image sensor, which results in the deterioration in the sensitivity of the CMOS image sensor. 
   BRIEF SUMMARY 
   Accordingly, embodiments of the present invention are directed to a method for manufacturing a CMOS image sensor that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of embodiments of the present invention is to provide a method for manufacturing a CMOS image sensor, which improves sensitivity by maintaining a zero-gap between microlenses. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a method of manufacturing a CMOS (complementary metal oxide silicon) image sensor comprising: forming an interlayer dielectric on a semiconductor substrate on which a plurality of photodiodes are formed; forming a plurality of color filters at regular intervals on the interlayer dielectric; forming a planarization layer on an entire surface of the semiconductor substrate including the color filters; forming sacrificial resist patterns on the planarization layer that are spaced apart from each other; forming spacers at sidewalls of the sacrificial resist patterns; removing the sacrificial resist patterns while remaining the spacers; forming a resist layer on the planarization layer on which the spacers remain; removing the spacers; and reflowing the resist layer at a predetermined temperature to form a microlens. 
   In a specific embodiment, the spacers can be formed by depositing and dry-etching a plasma enhanced-tetraethoxysilane (PE-TEOS) system liner oxide layer on the sacrificial resist pattern. 
   The sacrificial resist patterns can be formed by coating and patterning the resist layer by exposure and developing processes. 
   In embodiments, the reflowing of the resist layer can be performed at a temperature ranging from 300 to 700° C. 
   The method can further comprise recessing a part of the resist layer within the spacers by a dry etching process in order to form the resist layer of a uniform thickness between the spacers. 
   In another aspect of the present invention, there is provided a method of manufacturing a CMOS (complementary metal oxide silicon) image sensor comprising: providing a semiconductor substrate on which a planarization layer is formed for forming a microlens; forming sacrificial resist patterns on the planarization layer that are spaced apart from each other; forming spacers at sidewalls of the sacrificial resist patterns; removing the sacrificial resist patterns; forming a resist layer on the planarization layer on which only the spacers remain; removing the spacers; and reflowing the resist layer at a predetermined temperature to form a microlens. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings: 
       FIGS. 1A and 1B  are cross-sectional views of a CMOS image sensor for describing a method of fabricating a CMOS image sensor according to the related art; and 
       FIGS. 2A through 2F  are cross-sectional views of a CMOS image sensor for describing a method of manufacturing a CMOS image sensor according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   Hereinafter, a method for manufacturing a CMOS image sensor according to embodiments of the present invention will be explained in detail referring to the accompanying drawings. 
     FIGS. 2A through 2F  are cross-sectional views of a CMOS image sensor for describing a method of manufacturing a CMOS image sensor according to an embodiment of the present invention. 
   Referring to  FIG. 2A , an interlayer dielectric  32  can be formed on an entire surface of a semiconductor substrate (not shown). The semiconductor substrate can have at least one photodiode  31  formed thereon for producing charges according to an amount of incident light. 
   In one embodiment, the interlayer dielectric  32  may be formed with multi layers. Although not shown, after one interlayer dielectric layer has been formed, a light shielding layer may be formed and then another interlayer dielectric layer can be formed thereon. Here, the light shielding layer functions to block light incident on parts other than regions having photodiodes  31 . 
   Next, a planarized passivation layer  33  can be formed on the interlayer dielectric  32  in order to protect a device from moisture and to prevent the device from being scratched. 
   The passivation layer  33  can be coated with a dyable resist and patterned to form red (R), green (G), and blue (B) color filter layers  34 , which filter light by wavelengths. 
   In an embodiment, a photolithography process can be selectively performed in the red (R), green (G), and blue (B) color filter layers  34  three times to form a color separating layer. 
   Then, an UV exposure process can be carried out in the respective color filter layers  34  to reform any unstable state of surfaces thereof. 
   Next, a planarization layer  35  can be formed on the color filter layers  34  in order to adjust a focusing distance and secure the flatness for forming a lens layer. 
   In one embodiment, a thermal treatment can be performed at a temperature ranging from 150 to 300° C. so as to cure the planarization layer  35 . 
   Then, a first resist layer can be coated on the planarization layer  35 , and a reticle (not shown) having an opening portion can be aligned above the first resist layer. Next, light such as a laser beam can be irradiated to an entire surface of the semiconductor substrate using the reticle as a mask to selectively expose the first resist layer through the openings of the reticle. The exposed resist layer can be developed to form a sacrificial resist pattern  36 . 
   Subsequently, as shown in  FIG. 2B , a plasma enhanced-tetraethoxysilane (PE-TEOS) system liner oxide layer can be deposited on the resulting structure and a dry etching can be performed on the liner oxide layer to form a spacer  38  at sidewalls of the sacrificial resist pattern  36 . 
   Next, as shown in  FIG. 2C , the sacrificial resist pattern  36  can be removed by a strip process while leaving the spacers  38  to remain on the planarization layer  35 . 
   Subsequently, as shown in  FIG. 2D , a second resist layer  40  can be coated on the planarization layer  35  on which the spacers  38  are formed. 
   A microlens pattern can be defined by the second deposited resist layer  40  through the spacers  38 . 
   In a further embodiment, when it is difficult to form the second resist layer of a uniform thickness between the spacers, after the deposition of the second resist layer, a dry etching process can be performed on the entire surface of the wafer to recess a non-uniform region of the second resist layer between the spacers  38 . 
   Next, as shown in  FIG. 2E , a wet etching process can be performed on the entire surface of the resulting structure to remove the spacers  38 . Accordingly, the second resist layers, which have uniform spacing due to the spacers  38 , remain, thereby forming the microlens patterns  40 . 
   Referring to  FIG. 2F , the microlens patterns  40  can be reflown at a predetermined temperature to form a plurality of microlenses  42 . 
   In one embodiment, the reflow temperature ranges from approximately 300 to 700° C. in order to form the microlenses  42 . 
   Accordingly, because the microlens patterns have regular intervals due to the spacers, a zero-gap may be also maintained between adjacent microlenses after the completion of the reflow process. This allows light to be focused to a light receiving section without losing light that travels without passing through a microlens. Therefore, the sensitivity of the CMOS image sensor can be improved. 
   In accordance with embodiments of the present invention, since the microlens patterns have regular intervals due to removal of the spacers, a zero-gap may be also maintained between adjacent microlenses after a completion of the reflow process. This allows light to be focused to a light receiving section without the loss of light traveling straight without passing through the microlens, with the result that the sensitivity of the CMOS image sensor can be improved. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.