Abstract:
An EEPROM device manufacturing method is disclosed. The method includes the steps of oxidation, polysilicon deposition, and etching to form first polysilicon layers of a select transistor and a floating gate electrode. The method also includes a second polysilicon deposition step followed by an etching step to form a logic gate electrode and a control gate electrode at the same time. This method prevents damage to the silicon substrate and reduces the number of process steps compared to conventional manufacturing methods.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for manufacturing a nonvolatile memory device, and more particularly, to a method for manufacturing a nonvolatile memory device which can prevent damage to a silicon substrate and reduce a number of process steps by forming a logic gate electrode and cell control gate electrode at the same time instead of an SAE etching process. 
     2. Description of the Related Art 
     Nonvolatile memory devices are a type of semiconductor memory that can preserve stored data even if its power supply is interrupted. Of these nonvolatile memory devices, an EEPROM (electrically erasable and programmable read only memory is a nonvolatile memory device capable of stable storage even if there is no power supply, and also is a modified erasable and programmable read only memory (EPROM) having the characteristic of electrically erasing stored data and rewriting the same. Thus, the EEPROM can be used conveniently in various applications requiring program rewriting. 
     The EEPROM can write and erase data by electrically changing the charge of a device constituting a chip, and can reprogram data in a state stored in the system since it is capable of electrical reading and writing. In the EEPROM, a programming operation is enabled by generating channel hot electrons at a drain side, accumulating the electrons in a floating gate electrode and increasing the threshold voltage of a cell transistor, and an erasing operation is enabled by generating a high voltage between a source/substrate and the floating gate electrode, emitting the electrons accumulated in the floating gate electrode and decreasing the threshold voltage of the cell transistor. 
     In order to have the EEPROM cell to do the write/erase operations of byte (8 bits) unit, it is necessary that a unit cell have one select transistor. The select transistor used is defined simultaneously with the floating gate electrode of an EEPROM cell utilizing a first polysilicon. 
     Of the EEPROM, a device having a small chip size and having excellent write and erase characteristics includes a flash memory. In case of a flash cell, the flash cell has no select gate, thus it is capable of only block or chip unit erasing. A floating gate electrode defined as a first polysilicon is etched on a field oxide film. Thus, after the deposition of a second polysilicon, the loss of the field oxide film occurs even if the first polysilicon and the second polysilicon are stacked and etched, thereby causing no damage on the silicon substrate. 
     On the other hand, in case of the EEPROM cell, the select transistor using the first polysilicon and the floating gate electrode of the EEPROM cell are formed in an active region unlike a flash memory. Thus, the active region is opened where no first polysilicon exists in the self align etch (SAE) process of the first polysilicon and the second polysilicon. Due to this, the damage of the active open region by the over-etching of silicon occurs as much as the first polysilicon etching target. 
     Hereinafter, the problems of the method for manufacturing a nonvolatile memory device according to the conventional art will be described with reference to the accompanying drawings. 
       FIG. 1  is a sectional view showing an EEPROM device and a logic device formed according to the conventional art. 
     As shown in  FIG. 1 , a select transistor  110  formed on the silicon substrate  100 , a first polysilicon  121  acting as a floating gate electrode and a second polysilicon  122  acting as a control gate electrode are formed in an EEPROM device region A. Further, a second polysilicon  122 ′ acting as a logic gate electrode  130  is formed in a logic device region B that is to be formed on the same wafer as the EEPROM device region, thus the logic gate electrode has the same transistor characteristics as a general analog logic device. 
       FIG. 2  is a plane view showing an EEPROM device and a logic device formed according to the conventional art. An active damage occurs in the process of stacking and etching an EEPROM cell in a C region, and the damage occurred in the active region during the etching process leads to a leakage current. Due to this, a stand-by current is increased, thereby generating a junction leakage current in the drain active side where a contact is to be formed. 
       FIGS. 3   a  to  3   j  are sectional views showing sequential processes for manufacturing a nonvolatile memory device according to the conventional art. 
     Firstly, as shown in  FIG. 3   a , a field oxide film  310  is formed on a silicon substrate  300  by a LOCOS method for device isolation, to thus isolate a logic region and an EEPROM cell region. And, a deep N-well and P-well ion implantation are carried out. 
     Continually, as shown in  FIG. 3   b , a channel ion implantation of the select transistor and an ion  320  implantation for controlling the threshold voltage of the EEPROM cell are carried out. 
     Then, as shown in  FIG. 3   c , a dual gate oxidization process for forming a gate oxide film  330  of the select transistor and a tunnel oxide film  330 ′ of the EEPROM cell is performed. 
     Then, a first polysilicon is deposited and a selective photograph and etching process is performed to thus form a gate electrode  340  and a floating gate electrode  340 ′ of EEPROM cell as shown in  FIG. 3   d . The first polysilicon is formed in the active region except the field oxide film  310 . 
     Afterwards, as shown in  FIG. 3   e , in order to insulate the floating gate electrode  340 ′ of the EEPROM cell and a second polysilicon to be deposited hereinafter, an Oxide-Nitride-Oxide (“ONO”) film  350  is formed on the entire surface of a wafer, and a selective photograph and etching process is performed to completely cover the floating gate electrode  340 ′. 
     Then, as shown in  FIG. 3   f , an ion implantation process for controlling the threshold voltage is performed to a logic device region B and a logic gate oxide film  360  is formed. 
     Continually, as shown in  FIG. 3   g , a second polysilicon  370  that is to be used as a control gate electrode of the EEPROM cell and as a gate of the logic device is deposited. Then, as shown in  FIG. 3   h , the logic device region B is blocked, and the second the polysilicon  370 ′ of the EEPROM cell region undergoes an SAE (self align etch) process. 
     Next, as shown in  FIG. 3   i , the SAE process is, completed to make the first polysilicon/second polysilicon of the EEPROM cell region have a stacked shape. At this time, although there is no problem in the portion where the first polysilicon  340 ′ exists at the lower end of the second polysilicon  370 ′, the silicon substrate is exposed at the portion where the first polysilicon  340 ′ does not exist at the lower end of the second polysilicon  370 . Thus, in the SAE process, the silicon substrate is damaged as much as an etching target for etching the ONO film  350  and the first polysilicon  340 ′ after the second polysilicon etching process. Also, more serious damage occurs in the active region of a D portion where no ONO film  350  exists, thereby giving rise to a leakage current of the EEPROM cell. 
     Thereafter, as shown in  FIG. 3   j , the EEPROM cell region A is blocked by photoresist and then the second polysilicon  370  of the logic region is selectively etched to form a logic gate electrode  370 ″. 
     According to the method for manufacturing a nonvolatile memory device according to the conventional art, upon the SAE process for the process of forming a gate of a stacked shape of the first polysilicon/second polysilicon in the EEPROM cell, the silicon substrate of the active region at the portion having no first polysilicon, particularly, at the portion having no ONO film, thus giving rise to a leakage current. As a result, there occurs a problem that the standby current is increased to give rise to a leakage current in the drain active side. 
     SUMMARY OF THE INVENTION 
     The present invention is designed in consideration of the problems of the prior art, and therefore it is an object of the present invention to provide a method for manufacturing a nonvolatile memory device, and more particularly, to a method for manufacturing a nonvolatile memory device which can prevent damage to a silicon substrate and reduce the number of process steps by etching a floating gate electrode at the same size as a conventionally finally formed floating gate electrode, covering the overall floating gate electrode with an ONO film and etching a control gate electrode of an EEPROM cell and a logic gate electrode under the same conditions as a logic gate etching. 
     To achieve the above object, there is provided a method for manufacturing a nonvolatile memory device according to the present invention, comprising the steps of: forming a gate oxide film of a select transistor and a tunnel oxide film of an EEPROM cell on a semiconductor substrate where a cell region and a logic region are isolated and a predetermined substructure is formed; depositing a first polysilicon on the resultant material where the gate oxide film of the select transistor and the tunnel oxide film are formed; forming a gate electrode of the select transistor and a floating gate electrode of the EEPROM cell by selectively etching the first polysilicon; forming an ONO film so as to cover the floating gate electrode; carrying out a threshold voltage control ion implantation to the logic region and forming a logic gate oxide film; and forming a logic gate electrode and a control gate electrode of the EEPROM cell at the same time by depositing a second polysilicon on the entire surface of the resultant material and etching the second polysilicon. 
     In the method for manufacturing a nonvolatile memory device according to the present invention, it is possible to prevent damage to an active region by forming a floating gate electrode having the same length as the channel length upon the process of selectively etching the first polysilicon and additionally etching the first polysilicon upon a subsequent control gate electrode etching process. 
     In the method for manufacturing a nonvolatile memory device according to the present invention, it is possible to improve the insulating properties by overlapping 0.2 to 0.3 μm of the floating gate electrode with an ONO film and completely covering the same. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a sectional view showing an EEPROM device and a logic device formed according to the conventional art; 
         FIG. 2  is a plane view showing an EEPROM device and a logic device formed according to the conventional art; 
         FIGS. 3   a  to  3   j  are sectional views showing processes for manufacturing a nonvolatile memory device according to the conventional art; and 
         FIGS. 4   a  to  4   i  are sectional views showing processes for manufacturing a nonvolatile memory device according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, a preferred embodiment of the present invention will be described in more detail referring to the drawings. In addition, the following embodiment is just for illustration only, not intended to limit the scope of the invention, and the identical component to the conventional art uses the identical reference numeral and name. 
       FIGS. 4   a  to  4   i  are sectional views showing processes for manufacturing a nonvolatile memory device according to the present invention sequentially. 
     Firstly, as shown in  FIG. 4   a , a field oxide film  410  is formed on a silicon substrate  400  by a LOCOS method for device isolation, to thus isolate a logic region and an EEPROM cell region. And, a deep N-well and P-well ion implantation is carried out. 
     Continually, as shown in  FIG. 4   b , a channel ion implantation of the select transistor and an ion  420  implantation for controlling the threshold voltage of the EEPROM cell are carried out. 
     Then, as shown in  FIG. 4   c , a dual gate oxidization process for forming a gate oxide film  430  of the select transistor and a tunnel oxide film  430 ′ of the EEPROM cell is performed. At this time, the gate oxide film  430  of the select transistor is oxidized to have a thickness of 150 to 250 Å in consideration of the transistor margin for a high voltage, and then only a tunnel oxide film forming region is sheeted off in order to form a tunnel oxide film of the EEPROM cell to thus form a tunnel oxide film at a thickness of 85 to 95 Å by a reoxidization process. Then, a first polysilicon is deposited and a selective photograph and etching process is performed to thus form a gate electrode  440  and a floating gate electrode  440 ′ of an EEPROM cell. At this time, the gate electrode  440  serves to transfer a high voltage of the lower end of a drain to the EEPROM cell, and the floating gate electrode  440 ′ serves to implant or emit electrons upon a FN tunneling to thus make a cell threshold voltage in a desired write/erase state. 
     Additionally, the channel length of the floating gate electrode  440 ′ is made the same as the channel length of the first polysilicon of a stack cell conventionally finally formed by a SAE process. That is, the channel length is etched so as to be 0.2 to 0.6 μm smaller than the length of a conventional first polysilicon after the etching of the first polysilicon. 
     Afterwards, as shown in  FIG. 4   e , in order to insulate the floating gate electrode  440 ′ of the EEPROM cell and a second polysilicon to be deposited hereinafter, an ONO film  450  is formed on the entire surfaces of a wafer, and a selective photograph and etching process is performed to overlap 0.2 to 0.3 μm of the floating gate electrode  440 ′ and completely cover it. 
     Then, as shown in  FIG. 4   f , an ion implantation process for controlling the threshold voltage is performed to a logic device region B and a logic gate oxide film  460  is formed. 
     Continually, as shown in  FIG. 4   g , a second polysilicon  470  that is to be used as a control gate electrode of the EEPROM cell and a gate of the logic device is deposited. Then, as shown in  FIG. 4   h , the logic device region B and the second polysilicon  470  of the EEPROM cell region are etched. The second polysilicon etching process for both of the logic region and the EEPROM cell region, is carried out in the same manner as the logic gate etching method so that the logic gate electrode  470 ′ and the control gate electrode  470 ″ of the EEPROM cell can be formed at the same time. At this time, the EEPROM cell control gate electrode  470 ″ having the same thickness as the logic gate electrode  470 ′ is formed. Thereafter, no additional etching process is performed, thus the number of process steps can be reduced. Besides, no damage is applied to the silicon substrate, thus preventing generation of a leakage current. 
     Then, as shown in  FIG. 4   i , a spacer  480  is formed on the side walls of the gate electrode  440 , the EEPROM cell stack gate (first polysilicon/second polysilicon) and the logic gate electrode, and then a selective photograph and ion implantation process is performed to form a source/drain junction region  490 . 
     In this way, according to the method for manufacturing a nonvolatile memory device according to the present invention, upon a floating gate electrode etching process, the first polysilicon is etched so as to be the same as a conventionally formed floating gate electrode, that is, so as to be 0.2 to 0.6 μm smaller as compared to a conventional floating gate electrode etching process, then the floating gate electrode is covered with an ONO film and then the control gate electrode of the EEPROM cell and the logic gate electrode are etched at the same time under the same condition as a logic gate etching. Therefore, it is possible to overcome the problem that, in a conventional SAE process, the silicon substrate at the portion where the first polysilicon does not exist is damaged. Moreover, a number of photograph and etching process steps can be reduced. 
     As seen from above, the present invention is advantageous in that, in the process of selectively etching the second polysilicon, the logic gate electrode and the control gate electrode of the EEPROM cell are etched at the same time under the same condition as a logic gate etching, thus requiring no additional etching process and reducing the number of process steps. 
     Furthermore, upon etching the floating gate electrode of the EEPROM cell, the floating gate electrode is etched to have the same length as the channel length, and then the etching process for the first polysilicon is not performed upon the second polysilicon etching process. Thus, the active region of the silicon substrate is not damaged to thus prevent generation of an initial current and a leakage current.