Abstract:
A new structure of a non-volatile semiconductor memory cell array and a method of fabricating the memory arrays. The circuit layout of the memory array not only comprises of the conventional floating gates, control gates, cell sources and cell drains, but also adds the local source regions to increase the coupling ratio. Besides, the new design can reduce the number of metal contact windows, further increase the packing density of the memory array. Furthermore, an additional isolation region is formed between two bit lines so as to increase the distance between two bit lines, which can minimize the possibility of cross talk due to shirking spacing.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method of fabricating non-volatile semiconductor memory devices, and more particularly to a memory device employing local sources having improved coupling ratio. 
     2. Description of the Prior Art 
     The trend of recent semiconductor device developing is that high density non-volatile memory has been expected to replace some part of the huge external storage device market of computers, because of its easy to access and low power dissipation. 
     Please refer now to FIG. 1, there is shown a cross sectional view of the conventional stack non-volatile memory array cell. A tunnel oxide 2 and a first polysilicon 3A layers are grown on a semiconductor substrate 1. The tunnel oxide layer and the first polysilicon layers are patterned by the conventional lithography and plasma-etching techniques to form the floating gates. A first dielectric 4 and a second polysilicon 5A layers are then deposited overlaying the floating gates. The first dielectric layer is typically made of oxide/nitride/oxide (ONO) sandwich structure. Thereafter, the control gates are patterned. 
     Next, source 6 and drain 7 regions are formed by ion implanting dopants into the semiconductor substrate which is not covered by the gates. A second dielectric layer 8 (usually made of doped oxide) is deposited on the entire substrate surface. Finally, contact windows 9 are opened by lithography and plasma-etching techniques again. The basic structure of a non-volatile memory cell is completed. Referring now to FIG. 2, there is shown the cell layout corresponding to FIG. 1. The areas marked 10 are the isolation regions. 
     According to the description above, there must be a contact window for every two cells that limits the packing density of the memory array. Besides, the silicon substrate which is not covered by the first polysilicon (S area of FIG. 2) has great possibilities to be undercut during the self-aligned etching process, which increases the resistance of the source lines. Furthermore, the distance between two adjacent metal lines is too close when cell size continuously shirking that creates cross talk problem. All these drawbacks degrade the performance of the non-volatile memory arrays. Therefore, the present invention provides a new design approach which reduces the cell area compared with the conventional cell without suffering the above problems. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a method of fabricating the non-volatile semiconductor memory array having higher coupling ratio. 
     It is another object of the present invention is to provide a novel design of the non-volatile semiconductor memory cell which can reduce the cell area and increase the packing density. 
     It is yet another object of the present invention to provide an easy and manufacturable non-volatile memory cell structure which can cut down the production cost. 
     It is a further object of the present invention to provide a low operation voltage non-volatile memory array which is portable. 
     It is still a further object of the present invention to provide an additional isolation region between two adjacent bit lines of the non-volatile memory cell which can improve the reliability of the device. 
     In accordance to these objects of this invention, a new method of fabricating non-volatile semiconductor memory array with high coupling ratio is achieved. First, a field oxide layer for isolation and a tunnel oxide layer are formed on the surface of the silicon substrate. Then, the active device area and isolation region are defined by the conventional lithography and etching techniques. Next, a first polysilicon layer is deposited, followed by the conventional lithography and plasma etching techniques to form the floating gate pattern. Then, a first dielectric, a metal polycide and a second dielectric layers are sequentially deposited overlaying the floating gate. Thereafter, the control gate pattern is formed by the conventional lithography and plasma etching techniques again. Next, the ion implantation technique is employed to form the cell source and cell drain regions of the non-volatile memory array. A cell source/drain oxide layer is then grown on the silicon substrate surface, followed by the spacer oxide layer deposition. The oxide spacers are formed on the sidewalls of the floating gates by the vertically unisotropically plasma-etching technique. Finally, the local bitline and local source are formed simultaneously that is one of the key points of the present invention. The non-volatile semiconductor memory cell array of the present invention is accomplished. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings forming a material part of this description, there is shown: 
     FIG. 1 shows a cross-sectional view of the non-volatile memory cell according to the prior art. 
     FIG. 2 shows the prior art non-volatile memory cell circuit layout corresponding to FIG. 1. 
     FIG. 3A shows the circuit layout of the non-volatile of the non-volatile memory cell according to the first preferred embodiment of the present invention. 
     FIG. 3B shows the equivalent circuit diagram corresponding to FIG. 3A. 
     FIG. 3C shows the circuit layout of the non-volatile memory cell according to the alternating embodiment of the present invention. 
     FIG. 3D shows the equivalent circuit diagram corresponding to FIG. 3C. 
     FIG. 3E through 3K schematically show the cross sectional view of the non-volatile memory cell according to the preferred embodiments of the present invention. 
     FIG. 3L shows the examples of the cell operation conditions obtainable with the preferred embodiments of the present invention. 
     FIG. 3M shows the Program Erase diagrams and their bias conditions corresponding to FIG. 3L. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now more particularly to FIG. 3A, there is shown the non-volatile memory cell circuit layout of the first preferred embodiment of the present invention. The memory cell comprises of: top select lines 31, selecting the cell address of the memory array; common source line 32; a plural columns and rows of floating gates 33, locating on the surface of a silicon substrate; a plural rows of control gate 34, continuously locating above the floating gates 33; a plural columns of local bitlines 35, locating on the surface of the silicon substrate; a plural columns of cell sources 36 and cell drains 37, alternatively arranging under the local bitlines 35, and the cell sources 36 and cell drains 37 are separated by the control gates 34; a plural rows of local sources 38, alternatively arranging with the cell drains 37 in each row; a plural columns of isolation region 39, isolating the adjacent cell drain 37 and local source 38. 
     Referring now to FIG. 3B, there is shown the equivalent circuit diagram corresponding to FIG. 3A. The local source lines are connected to the common source line 32 which is controlled by one bottom select 30 only. The symbols of WL(0)iB WL(1)iB . . . iBWL(n-2)iBWL(n-1) are representing the wordlines, while the symbols of Data(0)iB . . . iBData(m-1) are representing the datalines. The horizontal wordlines and the vertical datalines cross each other to form the n by m array structure. 
     Referring now to FIG. 3C, there is shown the non-volatile memory cell circuit layout of the alternating embodiment of the present invention. The layout is basically the same with the first preferred embodiment of FIG. 3A. The only difference is that each local source line 38 is independently controlled by its own control line. 
     Referring now to FIG. 3D, there is shown the equivalent circuit diagram corresponding to FIG. 3D. The local source lines are individually controlled by independent control lines of BS(0)iB . . . iBBS(m-1). 
     Referring to FIGS. 3E through 3L, the preferred embodiments of the present invention will be described in details. In the following description, numerous details are set forth in order to provide a thorough understanding of the present invention. It will be appreciated by one skilled in the art that variations of these specific details are while still achieving the results of the present invention. In other instance, well-known processing steps are not described in detail in order not unnecessarily to obscure the present invention. 
     Referring now to FIG. 3E, there is shown a cross sectional view of a partially completed silicon substrate which is cut from the word line direction BB of FIG. 3A. The starting material of the present invention is a monocrystalline P-type silicon substrate 41. First, a field oxide layer 42 for isolation and a tunnel oxide layer 46 are formed on the surface of the silicon substrate 41. The field oxide layer 42 may be formed by forming a layer of Silicon Oxide and Silicon Nitride thereon. Thereafter, an active device area and an isolation region are defined by the conventional lithography and etching techniques. The field oxide layer may serve as the isolation region. Because the silicon oxide layer on the silicon nitride grows slowly, a thin silicon oxide layer may be formed after the silicon nitride layer is removed. The active device area comprises of: cell channel; and cell source/drain regions. 
     The field oxide layer 42 is grown by thermal oxidation method, using steam at a temperature between about 900 to 1200° C., to a thickness of about 3000 to 6500 Angstroms. The tunnel oxide layer 46 is also formed by thermal oxidation, to a thickness of about 50 to 120 Angstroms. 
     Referring now to FIG. 3F, there is shown a cross sectional view of the silicon substrate also from the word line direction BB. First, a first polysilicon layer 47 is deposited, followed by the conventional lithography and plasma etching techniques to form the floating gate pattern. Then, a first dielectric 49, a metal polycide 51 and a second dielectric 53 layers are sequentially deposited overlaying the floating gate. Thereafter, the control gate pattern is formed by the conventional lithography and plasma etching techniques again. 
     The first polysilicon layer 47 is usually formed by in-situ phosphorus doped LPCVD method under a mixture of (15%PH 3  +85%SiH 4 ) and (5%PH 3  +95%N 2 ), the total pressure is about 1 torr, and at the reaction temperature about 550° C. to obtain a thickness of between 500 to 2000 Angstroms. The first polysilicon plasma etching process can use magnetic enhanced reactive ion etching (MERIE), electron cyclotron etching (ECR) or the conventional reactive ion etching (RIE) methods with reactant gases such as Cl 2 , SF 6  and HBr. The first dielectric layer 49 is usually using oxide/nitride/oxide (ONO) triple layer with the equivalent oxide thickness of about 100 to 300 Angstroms. The polycide layer 51 is usually made of metal polycide materials such as tungsten silicide (WSi) or titanium silicide (TiSi) by low pressure chemical vapor deposition (LPCVD) technique to a thickness of about 1000 to 3000 Angstroms. The second dielectric layer is usually made of oxide or nitride by LPCVD technique also to a thickness of about 1000 to 3000 Angstroms. 
     Next, FIGS. 3G through 3K are the cross sectional views of the silicon substrate from the local bitline direction CC. Referring now to FIG. 3G, the ion implantation technique is employed to form the cell source and cell drain regions of the non-volatile memory array such as flash memory or the electrical programmable read only memory (EPROM) arrays. This is accomplished via ion implantation of arsenic (As 75 ) ions, at an energy of 20 to 80 keV, and a dose between about 1E15 to 5E15 cm -2 . 
     Referring now to FIG. 3H, first, a cell source/drain oxide layer 57 is grown on the silicon substrate surface, followed by the spacer oxide layer 59 deposition. The cell source/drain oxidation step is also severed as dopant drive in step, which is performed at a temperature range of about 900 to 1000° C. for about 30 minutes in a nitrogen ambient environment with less oxygen for the purpose of obtaining the desired junction depth. The spacer oxide layer 59 is usually made of silicon dioxide such as TEOS by the LPCVD or PECVD technique to a thickness of about 2000 to 4000 Angstroms. 
     Referring now to FIG. 3I, the photoresist pattern 61 is formed by the conventional lithography technique to protect the oxide layer above the cell source 36 region, but unprotect the drain area 37 and source area 38. The oxide spacers 59 are then formed on the sidewalls of the floating gates by the vertically unisotropically plasma-etching technique. The oxide plasma etching process is used the magnetic enhanced reactive ion etching (MERIE) method with reactant gases such as CF 4 , CHF 3  and Ar. 
     Referring now to FIGS. 3J and 3K, the local bitline and local source line are formed simultaneously that is one of the key points of the present invention. First, a conducting layer 63 which has a thickness of about 2000 to 4000 Angstroms is deposited over the entire silicon substrate. Then, the local bitline and local source line patterns are formed by the conventional lithography and plasma etching techniques. The conducting layer is usually made of low resistivity materials such as polysilicon, tungsten, tungsten silicide (WSi) or titanium silicide (TiSi). FIG. 3J is the cross sectional view of the silicon substrate from the local bitline 35 of FIG. 3A, while FIG. 3K is the cross sectional view of the silicon substrate from the local source line 32 of FIG. 3A. The non-volatile semiconductor memory cell array of the present invention is completed. 
     Finally, referring now to FIG. 3L, an example of the cell operation conditions is shown. To program the cell, a pulse of low positive voltage (LV) is applied to the selected local bitline (drain) 37, with the selected wordline (control gate) 34 tied to a high negative voltage (-HV), the electrons are injected from floating gate 33 to local bitline 37 as shown in FIG. 3M(a). On the other hand, to erase the cell, a pulse of high positive voltage (HV) is applied to the selected wordline (control gate) 34, with the selected bitline (drain) 37 and local source 38 both tied to ground, the electrons are moving from channel to floating gate 33 as shown in FIG. 3M(b). To read the data stored in the cell, a pulse of low positive voltage (LV) is applied to both the selected wordline (control gate) 34 and bitline (drain) 37. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention.