Abstract:
A method of manufacturing a flash memory device wherein a stacked structure of an oxide and nitride or the reverse is applied to insulation spacers provided on sidewalls of gates for forming source/drain regions. After completing the source/drain regions, spacers are formed on sidewalls of the gates by using an oxide film as a contacting buffer, thus minimizing the interference between gates and reducing the stress to cells, overcoming the disturbance of threshold voltage.

Description:
BACKGROUND  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to methods of manufacturing flash memory devices and, more particularly, to a method of manufacturing a flash memory device minimizing interference between gate lines, reducing stress to memory cells, and improving disturbance of threshold voltages.  
         [0003]     2. Discussion of Related Art  
         [0004]     In manufacturing transistors or flash memory devices, insulation spacers are formed on sidewalls of gate lines after constructing the gate lines. The insulation spacers are provided to prevent the gate lines from being connected to contact plugs that are subsequently formed. In addition, the insulation spacers are provided to ensure sufficient space between gate lines and source/drain regions in an ion implantation process for forming the source and drain regions in LVPMOS (low voltage PMOS) and LVNMOS (low voltage NMOS) fields. Unless the spaces between the gates lines and the source/drain regions are sufficiently defined, a short channel effect will be generated, which degrades electrical characteristics of the device.  
         [0005]     The insulation spacer is usually made of an oxide film and a nitride film. When the spacer is formed of a nitride film, the nitride film is removed after forming source/regions through an ion implantation process. In this case, while there is an advantage of obtaining wider regions for source and drain contacts within the same area than in the case of using an oxide film alone, the nitride film remains at narrow spaces between the gate lines (specifically, wordlines of a NAND flash memory device). The dielectric constant of the nitride film is typically 6 to 8, which is larger than the dielectric constant of the oxide film (e.g., 3.6 to 3.9). Thus, an increase of interference effects between the gate lines and disturbance of threshold voltages results, which will be described in detail as follows.  
         [0006]      FIGS. 1A and 1B  are diagrams illustrating variations of threshold voltages in memory cells according to bias conditions of peripheral cells.  
         [0007]     Referring to  FIG. 1A , a string of a NAND flash memory device includes a drain selection transistor (not shown) connected in series between a bitline and a common source line, pluralities of memory cells (nine cells C 13  through C 21  are shown), and a source selection transistor (not shown). Here, 16, 32, or 64 memory cells are serially connected. This structure of the string is well known, so it will not be described in further detail.  
         [0008]     In programming the 16th memory cell C 16  in the structure of the string, a wordline assigned to the memory cell C 16  is supplied with a pass voltage 10V. Also, a power source voltage or the pass voltage is applied to a selection line of the drain selection transistor and a source selection transistor, and 0V is applied to a bitline (not shown).  
         [0009]     When the spacers are formed of the oxide film, the threshold voltage of the programmed memory cell C 16  varies by about 0.144V due to interference from the pass voltage applied to the adjacent memory cells C 15  and C 17 . Otherwise, when the spacers are formed of oxide and nitride films, the threshold voltage of the programmed memory cell C 16  varies by 0.212V.  
         [0010]     Referring to  FIG. 1B , it can be seen that when the first memory cell C 1  is programmed under the same condition, the interference by its adjacent memory cell C 2  relatively decreases to lessen the variation of threshold voltage.  
         [0011]     From the foregoing, it can be seen that the first memory cell C 1  is less affected from the interference than is cell C 16 . Also, if the spacer includes a nitride film with a high dielectric constant, the effect of interference is more pronounced, to increase the variation in threshold voltage of the memory cell.  
       SUMMARY OF THE INVENTION  
       [0012]     The invention is directed to a method of manufacturing a flash memory device capable of minimizing interference between gate lines, reducing stress to memory cells, and improving disturbance of threshold voltages.  
         [0013]     In one aspect, the invention provides a method of manufacturing a flash memory device, including the steps of: forming first junction regions in active regions of a semiconductor substrate where source selection lines, wordlines, and drain selection lines are formed; forming first spacers with an insulation film on sidewalls of the source and drain selection lines, the insulation film filling spaces among the wordlines, between the wordline and the source selection line, and the wordline and the drain selection line; forming second spacers on the first spacers on the sidewalls of the source and drain selection lines; forming second junction regions in the first junction regions among the second spacers; and removing the second spacers.  
         [0014]     In a preferred embodiment, the insulation film is formed of an oxide film.  
         [0015]     In a preferred embodiment, the insulation film is formed with the least thickness necessary to fill the spaces among the wordlines, between the wordline and the source selection line, and the wordline and the drain selection line.  
         [0016]     In a preferred embodiment, the second spacers are formed of a nitride film.  
         [0017]     In a preferred embodiment, the respective thicknesses of the first and second spacers are established on a target distance between the second junction region and one of the source and drain selection lines.  
         [0018]     In a preferred embodiment, the second spacers are removed by a wet etching process using phosphoric acid.  
         [0019]     In another aspect the invention provides a method of manufacturing a flash memory device, including the steps of forming a low-concentration impurity region in a semiconductor substrate where pluralities of wordlines and selection lines are formed; forming a buffering nitride film entirely over the semiconductor substrate including the wordlines and the selection lines; forming oxide-film spacers on sidewalls of the selection lines by an etching process after forming an oxide film on the buffering nitride film; forming source/drain regions in predetermined areas of the semiconductor substrate by way of an ion implantation process with using the oxide-film spacers; removing the oxide-film spacers and the buffering nitride film; and forming contact-buffering oxide-film spacers on the sidewalls of selection lines, filling up spaces among the wordlines.  
         [0020]     In a preferred embodiment, the oxide-film spacers are removed by a wet etching process with using an HF solution of H 2 O and HF mixed in a ratio of 50:1 through 110:1.  
         [0021]     In a preferred embodiment, the oxide-film spacers are removed by a wet etching process with using an HF solution of H 2 O and BOE mixed in a ratio of 20:1 through 300:1.  
         [0022]     In a preferred embodiment, the buffering nitride film is removed by a wet etching process with using phosphoric acid. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate example embodiments of the present invention and, together with the description, serve to explain principles of the present invention. In the drawings:  
         [0024]      FIGS. 1A and 1B  are diagrams illustrating variations of threshold voltages in memory cells according to bias conditions of peripheral cells;  
         [0025]      FIGS. 2A through 2F  are sectional diagrams illustrating processing steps by a method of manufacturing a flash memory device in accordance with a first embodiment of the invention; and  
         [0026]      FIGS. 3A through 3F  are sectional diagrams illustrating processing steps by a method of manufacturing a flash memory device in accordance with a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]     Preferred embodiments of the invention are described below in more detail with reference to the accompanying drawings. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout the specification.  
         [0028]     Hereinafter, it will be described about an exemplary embodiment of the invention in conjunction with the accompanying drawings.  
         [0029]      FIGS. 2A through 2F  are sectional diagrams illustrating processing steps by a method of manufacturing a flash memory device in accordance with a first embodiment of the invention.  
         [0030]     Referring to  FIG. 2A , a source selection line SSL, wordlines WL 0 ˜WLn, and a drain selection line DSL are disposed in predetermined intervals to each other on a semiconductor substrate  201 . Methods of forming the lines are well known in this technical field, and will not be described in detail.  
         [0031]     Referring to  FIG. 2B , first junction regions  202 ,  202   s , and  202   d  are formed in active regions of the semiconductor substrate  201 . In detail, the first junction regions  202  are formed in the semiconductor substrate  201  between the wordlines WL 0 ˜WLn, between the wordline WL 0  and the source selection line SSL, and between the wordline WLn and the drain selection lines DSL.  
         [0032]     The first junction region  202   s  formed between the source selection lines SSL acts as a common source region while the first junction region  202   d  formed between the drain selection lines DSL acts as a source region connected to the bitline.  
         [0033]     Referring to  FIG. 2C , first spacers  203  are formed on sidewalls of the wordlines WL 0 ˜WLn, the source selection lines SSL, and the drain selection lines DSL.  
         [0034]     In detail, an oxide film is formed in the minimum thickness capable of filling up spaces between the wordlines WL 0 ˜WLn. For example, the thickness of the oxide film is variable according to an integration density of the device, which may be formed in thickness of 100 Å through 1000 Å. A continuous blanket etch-back process is carried out to complete the structure of the first spacers  203  on the sidewalls of the source selection lines SSL and drains selection lines DSL.  
         [0035]     At this time, since intervals between the wordlines WL 0 ˜WLn, between the wordline WL 0  and the source selection line SSL, and between the wordline WLn and the drain selection line DSL are relatively narrow, the oxide film tends to remain therein without being clearly etched away. While the first spacers  203  are also formed even between the lines, they are settled, filling the spaces among the wordlines, and before the source and drain selection lines because the spaces among the lines are narrow. Thus, the first junction regions  202  are filled while the common source line  202   s  between the source selection lines SSL and the drain region  202   d  between the drain selection line DSL are partially filled.  
         [0036]     Next, referring to  FIG. 2D , second spacers  204  are formed on sidewalls of the source selection line SSL over the common source region  202   s  and sidewalls of the drain selection line DSL over the drain region  202   d . In detail, after a nitride (Si 3 N 4 ) film is formed on the overall structure of the substrate, a blanket etch-back process is conducted to remain the nitride film just on sidewalls of the source selection line SSL over the common source region  202   s  and sidewalls of the drain selection line DSL over the drain region  202   d . The thickness of the nitride film is variable in accordance with an integration density of the device, preferably in the range of 100 Å through 1000 Å. During this, as the first spacer  203  located over the first junction region  202  keeps the almost same height with the wordlines WL 0 ˜WLn, the first spacer  203  over the first junction region  202  are entirely removed therefrom.  
         [0037]     Meanwhile, as the distance between a second junction region  205  ( FIG. 2E ) and a selection line which will be formed by the subsequent processing steps is dependent upon the thicknesses of the first and second spacers  203  and  204 , it is preferred to optimize the thicknesses of the first and second spacers  203  and  204 .  
         [0038]     Referring to  FIG. 2E , second junction regions  205  are formed in the semiconductor substrate  201  between the source selection lines SSL and between the drain selection lines DSL. The impurity concentration of the second junction region  205  is higher than that of the first junction region  202 . And, the second junction region  205  is settled deeper than the first junction region  202 .  
         [0039]     Subsequently, referring to  FIG. 2F , plugs are formed over the common source region  202   s  and the drain region  202   d . During this, the second spacers  204  are removed in order to ensure a wider space for the plugs. The second spacers  204  may be removed by means of a wet etching process utilizing phosphoric acid (H 3 PO 4 ). Here, the wet etching process may be conducted for one minute through thirty minutes to completely remove the second spacers  204 .  
         [0040]     As a result, the spaces between the wordlines WL 0 ˜WLn are filled only with the oxide films for the first spacers  203 . And, the first spacers  203 , which are made of an oxide film with a lower dielectric constant than a nitride film, remain on the sidewalls of the source and drain selection lines SSL and DSL. The first spacers  203  remaining on the sidewalls of the source and drain selection lines SSL and DSL are used as contact-buffering spacers, minimizing interference during a programming operation and ensuring the spaces for the plugs at most.  
         [0041]      FIGS. 3A through 3F  are sectional diagrams illustrating processing steps by a method of manufacturing a flash memory device in accordance with a second embodiment of the present invention.  
         [0042]     Referring to  FIG. 3A , source and drain selection lines, DSL and SSL, and wordlines (“Cell”) are formed with predetermined patterns in a semiconductor substrate  300 . Thereafter, ionic impurities with low concentration are implanted into the substrate to form low-concentration impurity regions  301 . The low-concentration impurity regions  301  are disposed between the wordlines and between the source and drain selection lines SSL and DSL.  
         [0043]     Referring to  FIG. 3B , a buffering nitride film  302  is deposited on the entire face of the semiconductor substrate  300  where the source and drain selection lines SSL and DSL and the wordlines (Cell) are formed. The buffering nitride film  302  is provided to prevent abnormal oxidation of the selection lines SSL and DSL and the wordlines (Cell), and also provided to prevent etching damage to the selection lines SSL and DSL and the wordlines (Cell) while removing an oxide film to be formed in the subsequent processing step. Thus, the buffering nitride film  302  need not be thick. The buffering nitride film  302  is preferably deposited by a chemical vapor deposition process (hereinafter, referred to as CVD). During this, it is preferred for the CVD process to be carried out at a temperature of 700° C. to 800° C. under a pressure of 0.3 mTorr through 0.4 mTorr. Subsequently, an oxide film  303  is formed on the buffering nitride film  302 . It is preferable for the oxide film  303  to be deposited by a CVD process at a temperature of 800° C. to 900° C.  
         [0044]     Next, referring to  FIG. 3C , the oxide film  303  is partially etched away, remaining on sidewalls of the source and drain selection lines SSL and DSL, resulting in spacers  304  on the sidewalls the source and drain selection lines SSL and DSL.  
         [0045]     Subsequently, referring to  FIG. 3D , ionic impurities with high concentration are injected into the substrate to form source/drain regions  305 . During this step, for an NMOS transistor, arsenic (As) is injected thereinto with a dose of 4.0 through 6.0E15 atoms/cm 2  at an energy of 20 KeV through 40 KeV. For a PMOS transistor, BF 2  is injected thereinto with a dose of 2.0 through 4.0E15 atoms/cm 2  at an energy of 30 KeV through 50 KeV.  
         [0046]     Next, referring to  FIG. 3E , the spacers  304  are removed by an etching process. During this step, the spacers  304  are removed, preferably by a wet etching process with using a solution of deionized (DI) water and HF in a ratio of 50:1 through 100:1. It is also possible to dilute the DI water with BOE in preparing the solution for the wet etchant. Here, it is preferable to set the ratio between the DI water and the BOE in 20:1 through 300:1. Thereafter, the buffering nitride film  302  is removed. The buffer nitride film  302  is removed by way of a wet etching process with using phosphoric acid (H 3 PO 4 ).  
         [0047]     Referring to  FIG. 3F , after depositing an oxide film entirely over the semiconductor substrate including the drain and source selection lines DSL and SSL and the wordlines (Cell), contact-buffering oxide spacers  306  are formed on the sidewalls of the drain and source selection lines DSL and SSL by an etching process. During this step, spaces between the wordlines (Cell) are filled up with the oxide film.  
         [0048]     As above, the spacers are formed with the stacked structure of the oxide and nitride films on sidewalls of the gate lines, and the oxide film remaining after removing the nitride film, after forming the spacers and the source/drain regions, is used as the contact-buffering spacers on the sidewalls of the gate lines. Thus, the invention minimizes interference arising from the remaining nitride film among the gate lines. And, as spaces for the source/drain regions are obtained by removing the nitride film, the inventive method improves electrical characteristics and integration density in the device. Moreover, as the spacers are formed with the stacked structure of the buffering nitride film and oxide films for the source/drain regions, and the spacers of contact-buffering oxide film are formed on the sidewalls of the gate lines after removing the nitride film, after forming the spacers and the source/drain regions. Thus, the invention minimizes the interference arising from the remaining nitride film among the gate lines and lessens the disturbance of threshold voltages.  
         [0049]     Although the invention has been described in connection with the embodiment of the invention illustrated in the accompanying drawings, it is not limited thereto. It will be apparent to those skilled in the art that various substitution, modifications and changes may be thereto without departing from the scope and spirit of the invention.