Abstract:
A method for manufacturing a flash memory cell with a floating gate and a control gate having an increased coupling ratio due to an increase in gate capacitance. The gate size is increased by reducing a groove width in a photoresist pattern used to define the gate region. The groove width is reduced by employing a slope-etching process to form the photoresist pattern.

Description:
[0001]    The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean Patent Application No. 10-2005-0135335 (filed on Dec. 30, 2005), which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    A flash memory cell has advantages similar to an erasable programmable read only memory (EPROM) and an electrically erasable and programmable read only memory (EEPROM). A flash memory cell includes a floating gate, an insulating layer, a control gate, a tunnel oxide layer, and an isolation layer formed over a silicon substrate. The flash memory cell also includes a source and drain region formed on an exposed portion of a silicon substrate. The flash memory cell can electrically store and erase 1-bit using one transistor. 
         [0003]    The flash memory cell stores charges in the floating gate and the floating gate is insulated from the control gate. Data is stored and erased by applying power to the control gate which is coupled to the floating gate through the insulating material. The ratio of power transferred from the control gate to the floating gate through the insulating material is referred to as a coupling ratio. The value of the coupling ratio is proportional to the capacitance generated by an overlap between the floating gate and the control gate. 
         [0004]      FIGS. 1A to 1D  are flow diagrams showing a process of manufacturing a flash memory cell. 
         [0005]    Referring to  FIG. 1A , after a trench is formed by etching a predetermined region of a semiconductor substrate  100 , a device isolation layer  102  is formed by a series of processes for burying an insulator into the trench. A first oxide layer  104 , a first polycrystalline silicon layer  106 , an insulating layer  108  and a photoresist pattern  110  for defining a floating gate are sequentially formed over the substrate  100  on which the device isolation layer  102  is formed. The first oxide layer  104  is deposited with a thickness ranging from approximately 90 Å to 100 Å and the first polycrystalline silicon layer  106  is deposited with a thickness ranging from approximately 950 Å to 1050 Å. The insulating layer  108  is deposited by using e.g., a nitride layer, a tetra ethyl ortho silicate (TEOS) or the like, at a thickness ranging from 2200 Å to 2300 Å. 
         [0006]    After the insulating layer  108  is anisotropically etched along the photoresist pattern  110 , the photoresist pattern  110  is removed by an ashing process using, e.g., Ar, O 2  or the like, so that a floating gate region is defined through the patterned insulating layer  108 , as shown in  FIG. 1B . 
         [0007]    After an insulator, e.g., TEOS or a nitride layer, is deposited on the semiconductor substrate  100  where a floating gate region is defined with a thickness ranging from approximately 740 Å to 760 Å, a spacer insulating layer  112  is formed on side surfaces of a patterned slot of the insulating layer  108  by performing reactive ion etching (RIE) on the deposited insulator, as shown in  FIG. 1C . 
         [0008]    Thereafter, as shown in  FIG. 1D , a floating gate  106   a  is formed by etching the first polycrystalline silicon layer  106  and the first oxide layer  104  by using the insulating layer  108  and the spacer insulating layer  112  as a mask so that the device isolation layer  102  is exposed. Thereafter, a control gate  116  is formed by removing the spacer insulating layer  112  and the insulating layer  108  and by sequentially depositing a second insulating layer  114  and a second polycrystalline silicon layer on an upper surface of the semiconductor substrate  100 . 
         [0009]    Accordingly, for increasing a capacitance of a floating gate and a control gate, after an insulator is patterned through a photolithography process, a patterning process is repeatedly performed using an insulator to form a fine pattern. Accordingly, it takes long time to perform the manufacturing process and production yield is decreased due to a difficulty of forming the fine pattern. 
       SUMMARY 
       [0010]    Embodiments relate to a method for manufacturing a flash memory cell; and, more particularly, to a method for manufacturing a flash memory cell by simplifying processes of forming a floating gate and a control gate. 
         [0011]    Embodiments relate to a method for manufacturing a flash memory cell for increasing a capacitance of a gate by forming a fine pattern through a reflow process after a photoresist pattern is formed. 
         [0012]    Embodiments relate to a method for manufacturing a flash memory cell with a gate line of a floating gate and a control gate, the method including the steps of depositing a first oxide layer over a semiconductor substrate having a device isolation layer; depositing a first polycrystalline silicon layer over the first oxide layer; forming a floating gate photoresist pattern over the first polycrystalline silicon layer; slope etching the photoresist pattern through a photoresist reflow process; forming a floating gate by etching the first polycrystalline silicon layer and the first oxide layer along the slope etched photoresist pattern to expose the isolation layer; removing the photoresist pattern; and forming the control gate by sequentially depositing a second oxide layer and a second polycrystalline silicon layer over the semiconductor substrate and the floating gate. 
         [0013]    The slope etching may be performed through a reactive ion etching (RIE) process. 
         [0014]    The reactive ion etching may be performed under a pressure condition of approximately 60 mTorr to 80 mTorr. 
         [0015]    The reactive ion etching may be performed with a power input of approximately 50 W to 100 W. 
         [0000]    The slope etching creates a sloped sidewall in a groove in the etched photoresist pattern which causes a decrease in the width of a groove in the photoresist pattern. The process creates a groove width in the photoresist pattern smaller than a critical dimension of the semiconductor device fabrication process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIGS. 1A to 1D  are process flow diagrams showing a process of manufacturing a flash memory cell according to a prior art; 
           [0017]    Example  FIG. 2  is a diagram schematically depicting a reactive ion etching apparatus for manufacturing a flash memory cell in accordance with embodiments; 
           [0018]    Example  FIGS. 3A to 3D  are process flow diagrams showing a process of fabricating the flash memory cell in accordance with embodiments; and 
           [0019]    Example  FIG. 4  is a layout of the flash memory cell fabricated in accordance with embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A first oxide layer and a first polycrystalline silicon layer are formed sequentially over a semiconductor substrate where a device isolation layer is formed; and a photoresist pattern which defines a floating gate forming region is formed over the semiconductor substrate; and the photoresist pattern is slope etched through a photoresist reflow process; and a floating gate is formed by etching along the slope etched photoresist pattern; and a control gate is formed by sequentially depositing a second oxide layer and a second polycrystalline silicon layer over a top of the semiconductor substrate where the floating gate is formed. Using this method, the object of the embodiments can be easily achieved. 
         [0021]      FIG. 2  is a diagram schematically depicting a reactive ion etching apparatus for manufacturing a flash memory cell in accordance with embodiments, wherein the reactive ion etching apparatus includes a chamber  200 , a reactive gas storing unit  200   a  and a power supply unit  200   b . A wafer  202 , a wafer chuck  204  for lifting and supporting the wafer  202  and a wafer lift  206  for moving the wafer  202  up and down are included in the chamber  200 . In addition, a reactive gas injection pipe  202   a  is installed on a top portion of the chamber  200  to supply an etchant gas from the reactive gas storing unit  200   a . The chamber  200  is connected to the power supply unit  200   b  for supplying power. 
         [0022]    By using the above-mentioned reactive ion etching device, a first oxide layer, a polycrystalline silicon layer and an insulating layer are sequentially formed over a semiconductor substrate where a device isolation layer is formed; after the insulating layer is slope etched, a floating gate is formed by etching the polycrystalline silicon layer and the first oxide layer by using the slope etched insulating layer as a mask; a second oxide layer is formed over the floating gate; and a control gate is formed over the second oxide layer. 
         [0023]      FIGS. 3A to 3D  are process flow diagrams depicting a process of fabricating the flash memory cell in accordance with embodiments.  FIG. 4  is a layout of the flash memory cell fabricated in accordance with embodiments. The method of fabricating the flash memory cell in accordance with embodiments will be described with reference to the accompanying drawings. 
         [0024]    Referring to  FIG. 3A , after a trench is formed by etching a predetermined region of a semiconductor substrate  300 , a device isolation layer  302  is formed by performing a series of processes for burying an insulating material into the trench. A first oxide layer  304 , a first polycrystalline silicon layer  306  and a photoresist pattern  308  for defining a floating gate are sequentially formed over the device isolation layer  302 . The first oxide layer  304  is deposited with a thickness ranging from approximately 90 Å to 100 Å and the first polycrystalline silicon layer  306  can be deposited with a thickness ranging from approximately 950 Å to 1050 Å. The formed trench corresponds to a trench line  402  shown in  FIG. 4  and a plurality of trench lines  402  are formed in parallel to a direction of a bit line BL. 
         [0025]    A top portion of the semiconductor substrate  300  is slope etched through a photoresist reflow process, whereby the already patterned and etched photoresist layer shown in  FIG. 3A  is reetched and shaped into a pattern with rounded transitions from the top surfaces to the sidewalls, and narrower grooves or holes exposing polycrystalline silicon layer  306  as shown in  FIG. 3B . The grooves or holes in the photoresist pattern  308  with the inclining sidewalls will be used to etch a floating gate pattern. 
         [0026]    Herein, the slope etching through the photoresist reflow process is performed by reactive ion etching (RIE), under a pressure of approximately 60 mTorr to 80 mTorr in the chamber and at a power of approximately 50 W to 100 W supplied through the power supply unit  200   b  using the reactive ion etching device shown in  FIG. 2 . 
         [0027]    Since the photoresist pattern  308  obtained through the above-mentioned photoresist reflow process is patterned into wider area, the width of a groove in the insulating layer is reduced relative to a groove patterned exclusively through an anisotropic etching process. This is because sides of the groove are formed to be inclined with a predetermined angle θ through the photoresist reflow process. Therefore, since a smaller groove is formed, a semiconductor device which has features smaller than a standard critical dimension (CD), for example, features smaller than 0.18 μm in a 0.18 μm process which otherwise has 0.18 μm design rules, can be fabricated. 
         [0028]    Thereafter, the first polycrystalline silicon layer  306  and the first oxide layer  304  are etched along the photoresist pattern  308  until the device isolation layer  302  is exposed. In this way, a floating gate  306   a  is formed, as shown in  FIG. 3C . 
         [0029]    Thereafter, after removing the photoresist pattern  308 , a control gate  312  is formed by sequentially depositing a second oxide layer  310  and a second polycrystalline silicon layer over the whole top surface of the semiconductor substrate  300  where the floating gate  306   a  is formed, as shown in  FIG. 3D . Through this process, as shown in  FIG. 4 , a plurality of gate lines  404  are formed parallel to a direction of a word line WL, and the floating gate  306   a  is formed between neighboring device isolation areas  302  (see  FIG. 3D ). The control gate  312  is formed over the floating gate  306   a  and the device isolation layer  302 . 
         [0030]    Next, as shown in  FIG. 4 , a source region  406  and a drain region  408  are formed by injecting impurity ions on the semiconductor substrate  300 , using the gate line  404  including the floating gate  306   a  and the control gate  312  as a mask. 
         [0031]    Therefore, in the accordance with embodiments, a slope etching process is performed through a reflow process for a photoresist pattern to sequentially form a floating gate and a control gate during the fabrication of a flash memory cell. 
         [0032]    As above-mentioned, in accordance with embodiments, a photoresist pattern is slope etched through a reactive ion etching process under a pressure of approximately 60 mTorr to 80 mTorr in the chamber, at a power of approximately 50 W to 100 W. The manufacturing process can be simplified and the manufacturing cost reduced, while production yield is increased by forming a floating gate along the photoresist pattern patterned through this process. 
         [0033]    By using the photoresist pattern, which has a slope formed through a slope etching process in a photoresist reflow process, as a floating gate mask, a fine pattern may be formed for a semiconductor device having a feature smaller than a regular critical dimension, for example 0.18 μm. Therefore, errors in a pattern generated due to a complicated fabricating process can be prevented and, thus, the reliability of a semiconductor unit can be improved. 
         [0034]    It will be obvious and apparent to those skilled in the art that various modifications and variations can be made in the embodiments disclosed. Thus, it is intended that the disclosed embodiments cover the obvious and apparent modifications and variations, provided that they are within the scope of the appended claims and their equivalents.