Abstract:
A method for fabricating a one time programmable read only memory (OPTROM) device. A first conductive layer, a first semiconductor layer, an anti-fuse layer, a second semiconductor layer are sequentially formed on a substrate. The second semiconductor layer, the anti-fuse layer, the first semiconductor layer, and the first conductive layer are then patterned along the first direction into a first conductive line. The second semiconductor layer, the anti-fuse layer, and the first semiconductor layer are patterned into a memory cell. A dielectric layer is deposited over the substrate, wherein oxygen plasma sputtering is performed to clean the substrate before deposition. A second conductive line is formed over the second dielectric layer, running generally orthogonal to the first conductive line.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for fabricating a semiconductor memory device, and more particularly, to a method of fabricating a one-time programmable read only memory (OTPROM) device.  
         [0003]     2. Description of the Related Art  
         [0004]     An anti-fuse memory device is a three dimensional (3D) memory device with a memory cell comprising an anti-fuse layer interposed between a PN diode. When the anti-fuse layer is intact, the cell is an open electrical circuit. When the anti-fuse layer is breached, the cell is a diode. The anode material and the cathode material are continuous orthogonally extending strips. Compared to conventional 2D memories, 3D anti-fuse memory is better suited to integration, meaning more memory devices can be built on a single wafer, thereby reducing cost.  
         [0005]     U.S. Pat. No. 6,420,215 discloses a memory cell with low leakage. The disclosed memory cell places an anti-fuse layer between the anode and the cathode. When the anti-fuse layer is intact, the cell is electrically an open circuit. When the anti-fuse layer is breached, the anode material and cathode material converge in a small-diameter filament, and a diode is formed. The small filament gives the diode a very small area and perimeter. Thus the diode&#39;s leakage is relatively small.  
         [0006]     U.S. Pat. No. 6,525,953 discloses an exemplary vertically-stacked, field-programmable, nonvolatile memory comprising multiple layers of first and second crossing conductors. Pillars are self-aligned at the intersection of adjacent first and second crossing conductors, and each pillar comprises at least an anti-fuse layer. The pillars form memory cells with the adjacent conductors, and each memory cell includes first and second diode components separated by the anti-fuse layer. The diode components form a diode only after the anti-fuse layer is breached.  
         [0007]      FIG. 1  is a schematic layout of a conventional anti-fuse OPTROM device comprising a word line (WL), a bit line (BL), and a memory cell electrically connecting the word line to the bit line. FIGS.  2  to  3  are cross sections illustrating the fabrication procedure.  
         [0008]     Referring to  FIG. 2 , a semiconductor substrate  10 , such as monocrystalline silicon, is provided. A layer of p + -doped polysilicon  20  is formed on the substrate  10 . A titanium layer  30  is deposited on the p + -doped polysilicon layer  20 . Titanium nitride is formed on the titanium layer to serve as adhesion layer. A rapid thermal process (RTP) is performed reacting p + -doped polysilicon layer and titanium layer into a titanium silicide (TiSi 2 ) layer  30 . The titanium silicide (TiSi 2 ) layer  30  possesses characteristics of low resistivity and excellent thermal stability. A layer of titanium nitride (not shown) is formed over the titanium silicide layer  30 . Next, a p + -doped polysilicon layer  40  is formed over the titanium nitride layer.  
         [0009]     A rapid thermal oxidation (RTO) process is subsequently performed to form an anti-fuse layer  50 , such as silicon oxide, on the P + -polysilicon layer  40 . An n-doped polysilicon layer  60  is formed over the anti-fuse layer  50 .  
         [0010]      FIG. 3  is cross section illustrating the procedure of defining word lines and memory cells. The n-doped polysilicon layer  60 , the anti-fuse layer  50 , the p + -doped polysilicon layer  40 , the titanium silicide layer  30  and the p + -doped polysilicon layer  20  are sequentially lithographically etched generally along the first direction to form a word line. The n-doped polysilicon layer  60 , the anti-fuse layer  50 , the p + -doped polysilicon layer  40  are then lithographically etched to form a memory cell stack.  
         [0011]     In accordance with the above mentioned processes, however, silicon residue  70  will remain on the surface of the titanium silicide layer  30  during lithographical etching the p + -doped polysilicon layer  40  causing short between memory cells and lowering production yield.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, an object of the present invention is to provide a method for fabricating an OTPROM device overcoming the shortcomings associated with the related art.  
         [0013]     Another object of the present invention is to provide an oxygen pre-sputtering process prior to dielectric deposition for removing the silicon residue.  
         [0014]     To obtain the above objects, the present invention provides a method for fabricating a semiconductor memory device, comprising providing a substrate, sequentially forming a first conductive layer, a first type doped semiconductor layer, a first dielectric layer, a second type doped semiconductor layer on the substrate, patterning the second type doped semiconductor layer, the first dielectric layer, the first type doped semiconductor layer, and the conductive layer along the first direction, thereby turning the conductive layer into a first conductive line, patterning the second type doped semiconductor layer, the first dielectric layer, and the first type doped semiconductor layer into a memory cell, depositing a second dielectric layer overlying the substrate, wherein oxygen plasma sputtering is employed to clean the substrate before deposition, planarizing the second dielectric layer to expose the memory cell, and forming a second conductive line overlying the second dielectric layer, running generally orthogonal to the first conductive line.  
         [0015]     To obtain the above objects, the present invention provides a method for fabricating a one time programmable read only memory (OTPROM) device, comprising providing a substrate, sequentially forming a stack of p + -doped silicon/TiSi 2 /TiN/p + -doped silicon/first dielectric/n-type doped silicon layers on the substrate, patterning the stack of p + -doped silicon/TiSi 2 /TiN/p + -doped silicon/first dielectric/n-type doped silicon layers along the first direction, thereby turning the stack of p + -doped silicon/TiSi 2 /TiN layers into a word line, patterning the stack of p + -doped silicon/first dielectric/n-type doped silicon layers into a memory cell, depositing a second dielectric layer overlying the substrate, wherein oxygen plasma sputtering is employed to clean the substrate before deposition, planarizing the second dielectric layer to expose the memory cell, and forming a stack of n + -type doped silicon/TiN/TiSi 2 /n + -type doped silicon/n-type doped silicon layers over the second dielectric layer and patterning the same into a bit line, running generally perpendicular to the word line.  
         [0016]     To obtain the above objects, the present invention provides a semiconductor memory device, comprising a first conductive line disposed on a semiconductor substrate, the surface of the first conductive line being substantially silicon residue free, a second conductive line running generally perpendicular to the first conductive line, a memory cell between the first line and the second line, and a dielectric layer, surrounding the memory cell, wherein the surface of the first conductive line being oxygen plasma sputtered preventing silicon residue.  
         [0017]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The present invention will become more fully understood from the detailed description given hereinafter and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0019]      FIG. 1  is a schematic layout of a conventional anti-fuse OPTROM device;  
         [0020]     FIGS.  2  to  3  are cross sections illustrating the fabrication procedure; and  
         [0021]     FIGS.  4  to  8  are cross sections illustrating the fabrication procedures of an embodiment according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     The present invention will hereinafter be described with reference to the accompanying drawings.  
         [0023]     Compared with the related art, the key feature of the present invention is performing oxygen pre-sputtering on the surface of the titanium silicide layer to remove silicon residue, thereby improving production yield.  
         [0024]     FIGS.  4  to  8  are cross sections illustrating the fabrication procedures of an embodiment according to the present invention.  
         [0025]     Referring to  FIG. 4 , a semiconductor substrate  100 , such as monocrystalline silicon, is provided. A conductive layer comprising a polysilicon layer  200  and a composite layer  220  of TiN/TiSi 2  is deposited over the substrate  100 . The polysilicon layer  200  is heavily doped polysilicon, such as p + -doped polysilicon, using chemical vapor deposition (CVD) to achieve a thickness between about 1500 Å to 2500 Å, more preferably 2000 Å. Dopant, such as boron (B) or boron fluoride (BF 2 ), is added to the polysilicon layer  200  with a dosage of exceeding 10 19  atoms/cm 3 .  
         [0026]     Next, a metal layer  220 , such as titanium, is deposited over the p + -doped polysilicon layer  200  to achieve a thickness between about 200 Å to 800 Å, more preferably 500 Å. A titanium nitride with a thickness between about 100 Å is formed over the titanium layer  200  to serve as adhesion layer. After rapid thermal processing (RTP), the p + -doped polysilicon layer  200  reacts the titanium layer to a titanium silicide (TiSi 2 ) layer  220 . The titanium silicide (TiSi 2 ) layer  220  possesses characteristics of low resistance and excellent thermal stability. The rapid thermal processing (RTP) is performed at a temperature of about 400 to 1200° C., more preferably 675° C. with inert gases. The resistivity of the titanium silicide (TiSi 2 ) layer  220  is approximately 10 to 200 μΩ-cm.  
         [0027]     A heavily p + -doped polysilicon layer  240  is formed over the TiN/TiSi 2  layer  220  as top polysilicon layer  240 . The p + -doped polysilicon layer  240  is heavily doped polysilicon, such as p + -doped polysilicon, using chemical vapor deposition (CVD) to achieve a thickness between about 400 Å to 600 Å, more preferably 500 Å. Dopant, such as boron (B) or boron fluoride (BF 2 ), is added to the polysilicon layer  240  with a dosage of exceeding 10 19  atoms/cm 3 .  
         [0028]     A rapid thermal oxidation (RTP) process is performed at a temperature of about 400° C. to 650° C. for 30 to 60 seconds using N 2  and O 2  gases. The rapid thermal oxidation (RTO) process is subsequently performed to form the p + -doped polysilicon layer  240 . The surface of the p + -doped polysilicon layer  240  is oxidized to form a thin silicon oxide layer as an anti-fuse layer  260  with a thickness of about 5 Å to 20 Å, more preferably 14.5 Å.  
         [0029]     An n-doped polysilicon layer  280  is formed over the anti-fuse layer  260 . The n-doped polysilicon layer  280  is doped polysilicon, such as n-doped polysilicon, using chemical vapor deposition (CVD) to achieve a thickness of between about 3000 Å to 4000 Å, more preferably 3500 Å. Dopant, such as phosphorus (P) or arsenic (As), is added to the polysilicon layer  280  with a dosage of about 10 15  to 10 17  atoms/cm 3 .  
         [0030]      FIG. 5  is cross section of  FIG. 1  along the line A-A′illustrating the procedure of defining word lines (WL). The n-doped polysilicon layer  280 , the anti-fuse layer  260 , the top p + -doped polysilicon layer  240 , the titanium silicide layer  220  and the bottom p + -doped polysilicon layer  200  are sequentially lithographically etched generally along the first direction (east-to-west) to form long straight strips serving as word lines (WL).  
         [0031]      FIG. 6  is cross section of  FIG. 1  along the line B-B′ illustrating the procedure of defining a memory pillar. The n-doped polysilicon layer  280 , the anti-fuse layer  260 , the p + -doped polysilicon layer  240  are then lithographically etched to form a memory cell  270 . During the above mentioned etching processes, particulate silicon residue  300  will remain on the surface of the titanium silicide layer  220  causing a BL bridge problem. To remove the silicon residue  300 , pre-sputtering  400  is performed before dielectric deposition using O 2  with a flow rate of 300-400 sccm and Ar gas with a flow rate of 200-250 sccm, at a temperature of about 225 to 275° C., and a power of about 1000-1500 W. More preferably, the pre-sputtering  400  is performed using O 2  with a flow rate of 340 sccm and Ar gas with a flow rate of 240 sccm, at a temperature of about 250° C., and a power of about 1300 W.  
         [0032]     Oxidization may alternatively be performed during the above pre-sputtering  400  to transform the silicon residue  300  into silicon oxide which is an insulator.  
         [0033]     Referring to  FIG. 7 , the spaces between each memory cell  270  and each first conductive line  230  are then filled with dielectric  500  such as silicon dioxide, using high density plasma chemical vapor deposition (HDPCVD). During the HDPCVD process, the density of the active ions exceeds that of the conventional CVD process. As a result, the HDPCVD process is cable of accomplishing both deposition and etching simultaneously such that substantially void-free filling is achieved. The dielectric  500  is then planarized by chemical mechanical polishing (CMP) exposing the surface of the memory cell  270 .  
         [0034]     Referring  FIG. 8 , a second conductive line  650  is formed on the second dielectric  500 , substantially orthogonal to the first conductive line  230 . The second conductive line  650  comprises a stack of an n + -doped polysilicon layer  600 , a metal silicide layer  620 , an n + -doped polysilicon layer  640  and an n-doped polysilicon layer  660 .  
         [0035]     The N + -doped polysilicon layer  600  is heavily doped polysilicon, such as n + -doped polysilicon, using chemical vapor deposition (CVD) to achieve a thickness between about 1500 Å to 2500 Å, more preferably 2000 Å. Dopant, such as phosphor (P) or arsenic (As), is added to the polysilicon layer  600  with a dosage exceeding 10 19  atoms/cm 3 .  
         [0036]     A metal layer  620 , such as titanium, with a thickness of between about 200 Å to 800 Å, more preferably 500 Å is deposited over the n + -doped polysilicon layer  600 . A titanium nitride layer with a thickness of about 100 Å (not shown) is formed over the titanium layer  620  to serve as an adhesion layer to a thickness between about. After rapid thermal processing (RTP), the n + -doped polysilicon layer  600  reacts the titanium layer  620  to a titanium silicide (TiSi 2 ) layer  620 . The titanium silicide (TiSi 2 ) layer  620  possesses characteristics of low resistance and excellent thermal stability. The rapid thermal processing (RTP) is performed at a temperature of about 400 to 1200° C., more preferably 675° C. with inert gases. The resistivity of the titanium silicide (TiSi 2 ) layer  620  is approximately 10-200 μΩ-cm.  
         [0037]     A second type doped polysilicon layer  640  is heavily doped polysilicon, such as n + -doped polysilicon, using chemical vapor deposition (CVD) to achieve a thickness between about 400 Å to 600 Å, more preferably 500 Å. Dopant, such as phosphor (P) or arsenic (As), is added to the polysilicon layer  640  with a dosage exceeding 10 19  atoms/cm 3 .  
         [0038]     The n-doped polysilicon layer  660  is deposited on the n + -doped polysilicon layer  640 , using chemical vapor deposition (CVD) to achieve a thickness between about 3000 Å to 4000 Å, more preferably 3500 Å. Dopant, such as phosphor (P) or arsenic (As), is added to the polysilicon layer  660  with a dosage of between about 10 15  and 10 17  atoms/cm 3 .  
         [0039]     The n-doped polysilicon layer  660 , n + -doped polysilicon layer  640 , metal silicide layer  620 , and an n + -doped polysilicon layer  660  are sequentially lithographically etched creating a second conductive line  650  running generally perpendicular to the first conductive line as a bit line (BL).  
         [0040]     Again referring to  FIG. 8 , the completed anti-fuse semiconductor memory device  800  is described as follows. The semiconductor memory device  800  comprises a first conductive line  120  overlying a semiconductor substrate  100  running through a first direction (e.g., east-to-west). The surface of the first conductive line  120  is substantially silicon residue free. A memory cell  140  is disposed over the first conductive line  120 . A second conductive line  160  electrically connecting the memory cell  140  is formed, running orthogonal (e.g., north-to-south) to the first conductive line  120 . The spaces between each conductive line  230  and each memory cell  270  are filled with dielectric layer  500 .  
         [0041]     The invention been thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modification as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.