Abstract:
A buried bit line ROM is disclosed having orthogonal sets of buried bit lines and polysilicon word lines. Polysilicon spacers are disposed on either side of each of the bit lines. The polysilicon spacers are slightly doped. The bit lines have a doping profile so that the edges of each bit line is doped less and the center of each bit line is doped more.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for making self-aligned buried N +   bit lines in a semi-conductor device. Specifically, it relates to a high performance buried-bit line (&#34;B/L&#34;) mask ROM process which is suitable for very high density ROM product. 
     RELATED APPLICATION 
     A patent application entitled &#34;Buried Bit Line ROM with Low Bit Line Resistance&#34; bearing Ser. No. 08/092,189 was filed on Jul. 14, 1993, now U.S. Pat. No. 5,430,673 and is assigned to the assignee hereof. This application contains subject matter related to the subject matter of the present application and is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     A ROM is an array of ROM cells. A top view of a conventional array of ROM cells is shown in FIG. 2. A cross-section of an individual ROM cell is shown in FIG. 1. A ROM array has some cells which are conductive and some cells which are not conductive. A ROM mask process allows selective implantation of the ROM code so that some cells are conductive and some are not. 
     Referring to FIG. 2, the ROM array is formed on a substrate 11. Illustratively, a p-type substrate is used. A plurality of N +   buried bit lines 12 are formed in the substrate 11. A plurality of word lines 14 are also formed on the surface of the substrate orthogonal to the bit lines. 
     The cross-section of the individual ROM cell of FIG. 1 is taken along the AA&#39; line of FIG. 2. Each cell of the ROM has two parallel buried bit lines 12 which form source and drain regions for the cell. A channel 3 is formed between the two bit lines in the cell. A gate oxide layer 4 is formed on top of the substrate 1. The oxide layer 4 is thick at portions designated 5 which are located above the bit lines 2 and thin at portions designated 6 located over the channel 3. A polysilicon word line 7 is formed over the oxide gate layer 4. A photo resist layer 8 is formed and patterned on the polysilicon layer as a mask. The photo-resist layer is patterned so that the polysilicon layer across the source/drain region makes the channel conducting when a voltage is applied on the polysilicon layer. The boron 9 is then implanted in the selected cells to turn off the conducting state for the &#34;off&#34; cell (this programs the cell). 
     Each bit line has a width W. The spacing between adjacent bit lines is S. Illustratively, W is 0.6 microns and S is 0.6 microns. When the cell dimension shrinks, i.e., a smaller device size, the bit line width also shrinks and the bit line sheet resistance increases. In addition, the punch-through voltage between adjacent bit lines becomes unacceptably low. 
     It is an object of the present invention to provide a buried bit line structure and a method for making a buried bit line structure which eliminates the sheet resistance and punch through voltage problems in high density buried bit line ROMs with small bit line width and small bit line spacing. 
     SUMMARY OF THE INVENTION 
     The present invention uses an extra polysilicon layer in self-aligned direct contact with an N +   buried bit line and without any trenches in the device area. Additionally, the process incorporates a lightly doped (&#34;LDD&#34;) effect in the N +   bit line that improves the ROM coded cell breakdown voltage and leakage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a cross section of a conventional buried bit line ROM cell, 
     FIG. 2 illustrates a top view of a buried bit line mask ROM array, 
     FIG. 3(a) and (b) illustrate two embodiments of the device of the present invention, 
     FIGS. 4(a) to (h) illustrate the process of forming one embodiment of the present invention, and 
     FIGS. 5(a) to (h) illustrate the process of forming another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 3(a) and 3(b) illustrate buried bit line ROM cells of the present invention. The ROM cell comprises a substrate 30 of a first dopant type having buried bit lines 31 of a second dopant type. Illustratively, the substrate is p-type and the bit lines are N + . The bit lines 31 are disposed parallel to each other. Polysilicon spacers 33 are deposited on either side of each bit line 31. The bit lines 31 have a doping profile such that the portion 34 of the bit lines 31 under the spacers 33 are doped less then the center of the bit lines 31. This doping profile is a result of the polysilicon spacers which are doped N - . Because the bit lines are doped less at their edges than at their centers the punch-through voltage is increased. A gate oxide layer 32 is deposited over the spacers 33. A word line 35 is deposited over the gate oxide 32. The word line 35 is oriented orthogonally to the bit lines 31. A channel region 37 can be made non-conducting via implantation. 
     In an alternative embodiment shown in FIG. 3(b), an additional oxide layer 36 is deposited between the spacers 33 and the bit lines 31. 
     The process for forming the ROM cell of the present invention is illustrated in FIGS. 4(a)-4(h) and 5(a) to 5(h). As shown in FIGS. 4(a) and 5(a), the starting point for this process is a p-type silicon substrate 30. An oxide layer (SiO 2 ) 40 is deposited on the surface of the substrate 30. The oxide layer is between 100 and 200 Å (Angstroms). This oxide layer is the gate oxide. Next, a silicon nitride layer 41 is deposited on top of the oxide layer 40. The nitride layer is between 2000 and 3000 Å. 
     A layer of photo-resist is then deposited on the nitride layer 41. The photo-resist is then patterned to form the photo-resist stripes 42 which are separated by openings 43. Using the photo-resist stripes as a mask, the nitride layer 41 is etched, e.g., by dry etch, and, then, the photo-resist stripes 42 are removed. The resulting trenches 44 in the layer 41 are shown in FIG. 4(b). In an alternative embodiment, the nitride layer 41 and oxide layer 40 are etched, thereby forming trenches 60 into the substrate 30, as shown in FIG. 5(b). 
     Referring to FIGS. 4(c) and 5(c), a polysilicon layer 45 is deposited in trenches 44 and 60, respectively, to a thickness between 2000 and 3000Å. A polysilicon spacer 46 (&#34;poly 1 spacer 46&#34;) is formed by anisotropic plasma etch or overetch of the polysilicon layer, as shown in FIGS. 4(d) and 5(d). The area 34 of the substrate 30 is lightly doped because of dopants in the polysilicon spacer 46. These polysilicon dopants are N - . Phosphorous is implanted through the spacers 46 to form bit lines 31, as shown in FIGS. 4(e) and 5(e), by ion implantation, as indicated by arrows 47, at an energy of approximately 50 to 100 keV. The implantation creates N buried bit lines 31 in the substrate. The dose of the N +   dopant is 1×10 15  /cm 2 . The poly spacers block some of the implanted N +   dopant, therefore, the region 34 becomes N - . Since the poly spacer has a sloped shape, the N -   region 34 has the shape as indicated in FIG. 4(e) and 5(e). This doping of the area in the channel beneath the poly 1 spacer 46 creates a N +  /N -  doping profile. This light doping increases the punch-through voltage from bit line to bit line. 
     The nitride layer stripes 41 are etched or stripped away and, then the oxide layer 40 is etched, as shown in FIGS. 4(f) and 5(f). A gate oxide layer 48, shown as 32 in FIGS. 3(a) and (b), is regrown by oxidation over the poly 1 spacer 46. This gate oxide 48 has a thickness between 90 and 150Å. The original oxide layer 40 may remain between the spacers 46 and the buried bit lines 31. In the embodiment of FIG. 4(f), a portion of the oxide layer 40 remains between the spacers 46 and the bit lines 31. This portion is shown as 36 in FIG. 3(b). 
     Next, referring to FIGS. 4(g) and 5(g), a polysilicon word line 49 (&#34;poly 2 word line 49&#34;) is deposited or formed over the gate oxide 48. The poly word fine is patterned by conventional masking processes and etching processes. This process is not shown in the figures. 
     As shown in FIGS. 4(h) and 5(h), the poly 2 word line 49 is now masked and etched. A layer of photo-resist 50 is deposited on the poly 2 word line 49. The photo-resist layer 50 is formed into stripes for ROM code masking. A boron ROM code 51 is then implanted, as indicated by the arrows 52, between the spacers 46 and the buried bit lines 31, i.e, in the channel region 51. The boron implantation occurs by using an energy of about 150 kev and a dose of about 1×10 14  /cm2. The selected boron ion may be B 11 . The ROM code implant is an implanted Boron ion which is self-aligned. 
     The rest of the process steps for completing the ROM are conventional back-end processes including BPSG, contacts, metallization and passivation. 
     The process and product of the present invention have many advantages, including: 
     1. The bit line N +   implant using phosphorous (P 31 ) through the sloped poly 1 spacer creates a N +  /N -   doping profile, which has the LDD effect. This doping profile and LDD effect is advantageous for better punch-through resistance in a smaller device size. 
     2. For the cell shown in FIG. 3(b), when the poly 2 word line is applied with a voltage, some fraction of this voltage is coupled to part of the poly 1 spacer above the N -   area, therefore, the read or cell current of the memory transistor will increase due to an effect similar to a fully overlapped LDD structure. 
     3. The ROM code implant will be self-aligned into the center area of the channel resulting in a higher breakdown voltage and lower bit line resistance. 
     Finally, the above-described embodiment of the invention are intended to be illustrated only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the spirit and scope of the following claims.