Abstract:
A method of operating a memory cell for 3D array of this invention is described as follows. Carriers of a first type are injected into a charge storage layer of the memory cell by applying a double-side biased (DSB) voltage to double sides of the memory cell. Carriers of a second type are injected into the charge storage layer by applying FN voltages.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation application of and claims the priority benefit of U.S. application Ser. No. 12/326,283, filed on Dec. 2, 2008, now allowed. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to semiconductor devices, and more particularly relates to a method of operating a non-volatile memory (NVM). 
     2. Description of the Related Art 
     Nonvolatile memory has been widely used in many electronic products, wherein the most populated non-volatile memory devices are those having a charge storage layer and being written and erased electrically, such as EEPROM and flash memory. 
     Such a non-volatile memory is formed based on a bulk semiconductor substrate conventionally, with the charge storage layer disposed between a control gate and the substrate. Recently, nonvolatile memory formed based on a thin semiconductor film with the thin- film transistor (TFT) technology has been provided, wherein each cell is a thin-film transistor. By utilizing the TFT technology, it is possible to repeatedly form a semiconductor film and a layer of TFT cells based thereon and thereby fabricate a 3D non-volatile memory array. 
     A non-volatile memory cell of TFT type is conventionally programmed through positive Fowler-Nordheim (+FN) electron tunneling into the charge storage layer and erased by ejecting electrons out of the charge storage layer. For the efficiency of +FN programming is lower, the programming time required is longer generating more heat. This makes the operating method unsuitable to a 3D non-volatile memory array because a 3D memory array particularly suffers from difficult heat dissipation in the prior art. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, this invention provides a method of operating a non-volatile memory cell, which has a higher programming efficiency reducing the program time and heat generation and is therefore suitable for a 3D non-volatile memory. 
     The method of operating a memory cell for 3D array of this invention is described as follows. Carriers of a first type are injected into a charge storage layer of the memory cell by applying a double-side biased (DSB) voltage to double sides of the memory cell. Carriers of a second type are injected into the charge storage layer by applying FN voltages. 
     In an embodiment, the DSB voltage includes a DSB band-to-band tunneling hot carrier injection voltage. 
     In an embodiment, the FN voltage includes a positive FN (+FN) tunneling voltage. 
     In an embodiment, the carriers of the first type are electric holes and the carriers of the second type are electrons. 
     In an embodiment, the step of injecting the second type of carrier into the charge storage layer includes applying a first voltage to a control gate of the memory cell and applying a second voltage to a source and a drain of the memory cell. The first voltage is different from the second voltage such that FN tunneling of the second type of carrier into the charge storage layer is applied. 
     In an embodiment, the first voltage is higher than 0V and the second voltage is 0V. 
     In an embodiment, the first voltage ranges from 15 V to 20 V. 
     In an embodiment, the memory cell is a thin-film transistor (TFT), and the thin-film transistor includes a semiconductor layer, the charge storage layer and a control gate, where the semiconductor layer and the control gate both comprise doped silicon, and the charge storage layer includes two oxide layers and a nitride layer between the two oxide layers, so that the memory cell is a TFT SONOS cell. 
     A method of operating a memory cell for 3D array is provided. The method includes lowering a threshold voltage value of the memory cell by applying a DSB voltage to double sides of the memory cell and increasing the threshold voltage of the memory cell by applying FN voltage. 
     In an embodiment, the DSB voltage includes a DSB band-to-band tunneling hot carrier injection voltage. 
     In an embodiment, the FN voltage includes a positive FN (+FN) tunneling voltage. 
     In an embodiment, the step of increasing the threshold voltage of the memory cell includes applying a first voltage to a control gate of the memory cell and applying a second voltage to a source and a drain of the memory cell, wherein the first voltage is different from the second voltage such that the FN voltage is applied. 
     In an embodiment, the first voltage is higher than 0V and the second voltage is 0V. 
     In an embodiment, the first voltage ranges from 15 V to 20 V. 
     In an embodiment, the memory cell is a thin-film transistor (TFT), and the thin-film transistor includes a semiconductor layer, a charge storage layer and a control gate. The semiconductor layer and the control gate both comprise doped silicon, and the charge storage layer includes two oxide layers and a nitride layer between the two oxide layers, so that the memory cell is a TFT SONOS cell. 
     Since in this invention the memory cell is programmed with FN tunneling after being pre-erased with DSB injection, the programming efficiency is raised due to the charges of the opposite sign in the charge storage layer so that the time required for the programming is reduced. Moreover, because less heat is produced due to the shorter programming time, the operating method of this invention is particularly suitable for a 3D-memory array that suffers from the heat dissipation issue in the prior art. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the pre-erasing step in a method of operating a non-volatile memory cell according to an embodiment of this invention. 
         FIG. 2  illustrates the programming step in a method of operating a non-volatile memory cell according to an embodiment of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  respectively illustrate the pre-erasing step and programming step in a method of operating a non-volatile memory cell according to an embodiment of this invention. 
     Referring to  FIG. 1 , the non-volatile memory cell includes a semiconductor layer  120  as a floating body disposed on an insulator  110  on a substrate  100 , a charge storage layer  130  over the semiconductor layer  120 , a control gate  140  over the charge storage layer  130 , and a source region  150  and a drain region  160  in the semiconductor layer  120  beside the control gate  140 . 
     The substrate  100  may be a silicon substrate. The insulator  110  may be a silicon oxide layer that is usually formed by CVD. The semiconductor layer  120  may be a doped polysilicon film formed by LPCVD. The charge storage layer  130  may be a charge trapping layer, which is usually a silicon nitride (SiN) layer between a bottom oxide layer  132  and a top oxide layer  134 . The control gate  140  may include doped polysilicon. When the semiconductor layer  120  and control gate  140  include doped polysilicon and the charge storage layer  130  is a SiN trapping layer two oxide layers  132  and  134 , the memory cell is a TFT SONOS cell. 
     In addition, it is possible that the semiconductor layer  120  under the control gate  140  is P-doped and the source region  150  and the drain region  160  are N-doped so that the cell is an N-type transistor, which is taken as an example in the descriptions below. 
     Referring to  FIG. 1  again, in the pre-erasing step, a first voltage Vg e  is applied to the control gate  140  and a second voltage Vs e (=Vd e ) to the source region  150  and the drain region  160  (double-side biased, DSB), wherein Vg e  is sufficiently lower than Vs e  (Vd e ) such that band-to-band tunneling hot hole (BTBTHH) injection into the charge storage layer  130  is caused. For example, the first voltage is lower than 0V and the second voltage higher than 0V. In such a case, it is possible that Vg e  ranges from −10 V to −20 V and Vs e  (=Vd e ) ranges from 8 V to 12 V. In a specific embodiment, Vg e  is about −15V and Vs e  (=Vd e ) about 10V. 
     Referring to  FIG. 2 , in the programming step, a third voltage Vg p  is applied to the control gate  140  and a fourth voltage Vs p  (=Vd p ) is applied to the source region  150  and the drain region  160 , wherein Vg p  is sufficiently higher than Vs p  (Vd p ) such that +FN tunneling of electrons into the charge storage layer is caused. For example, Vs p  (=Vd p ) is 0V and the third voltage is higher than 0V. In such a case, Vg p  may range from 15 V to 20 V. In a specific embodiment, Vg p  is about 20V. 
     On the other hand, the memory device according to this embodiment includes a memory cell as shown in FIG.  1 / 2 , a first logic for pre-erasing the memory cell through DSB BTBTHH injection as in  FIG. 1 , and a second logic for programming the memory cell through +FN tunneling of electrons as in  FIG. 2 . The first logic might apply the above bias configuration for pre-erasing. The second logic might apply the above bias configuration for programming. 
     It is particularly noted that though this invention is exemplified by the operation of a TFT-type non-volatile memory cell in the embodiment, the operating method of this invention is also applicable to many other types of non-volatile memory cells having a charge storage layer, a control gate and source/drain regions, in consideration of the mechanisms of the DSB programming and FN-tunneling erasing. That is, the memory cell in the memory device of this invention is not limited to be a TFT-type NVM cell, but may alternatively be one of many other types of NVM cells. 
     Since in this embodiment the memory cell is programmed with electron FN tunneling after being pre-erased with DSB hole injection, the programming efficiency is raised due to the positive charge in the charge storage layer so that the time required for programming is reduced. Moreover, because less heat is produced due to the shorter programming time, the operating method of this invention is particularly suitable for a 3D-memory array that suffers from the heat dissipation issue in the prior art. 
     Furthermore, though the first type of carrier is electric hole, the second type of carrier is electron, the DSB injection includes DSB BTBTHH injection and the FN tunneling includes +FN tunneling of electrons in the above embodiment, this invention is not limited to the combination. In another embodiment, for example, the 1 st  type of carrier is electron, the 2 nd  type of carrier is electric hole, the DSB injection includes DSB electron injection and the FN tunneling includes FN tunneling of electric holes. 
     This invention has been disclosed above in the embodiments, but is not limited thereto. It is known to those of ordinary skill in the art that some modifications and innovations may be made without departing from the spirit and scope of this invention. Hence, the scope of this invention should be defined by the following claims.