Abstract:
A ROM array comprises orthogal sets of buried bit lines and polysilicon wordlines. The buried bit lines comprise trenches with insulating material on the side walls, the trenches then being filled with polysilicon. Theis reduces bit line sheet resistance and increases the punch through voltage between adjacent bit lines.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a buried bit line ROM with a low bit line sheet resistance and a high punch through voltage. 
     BACKGROUND OF THE INVENTION 
     High sheet resistance (R S ) of buried bit lines in a ROM becomes more severe when the cell dimension shrinks as memory density increases. When the cell dimension shrinks, 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. 
     FIG. 1 illustrates a buried bit line ROM 10. The ROM 10 comprises a P-type silicon substrate 11. A plurality of N +  -type buried bit lines 12 are formed in the ROM 10. Each bit line has a width W and the spacing between adjacent bit lines is S. A plurality of polysilicon word lines 14 are also formed on the surface of the substrate 11. The ROM 10 comprises a plurality of cells. One such cell 30 is delineated in FIG. 1 and shown in a cross-section view taken along the line AA&#39; in FIG. 2. 
     As shown in FIG. 2, the cell 30 comprises two adjacent buried bit lines 12 which form source and drain regions for the cell. A channel 22 of length S is formed between the two bit lines in the cell 30. A gate oxide layer 16 is formed on top of the substrate 11. The oxide layer is thick at portions 18 which are located above the bit lines 12 and thin at portion 20 located over the channel 22. A polysilicon word line 24 is formed over the gate oxide layer 16. 
     As indicated above, when the bit line width W is too small, the bit line sheet resistance is unacceptably high. In addition, when the bit line spacing S is too small, the punch through voltage between adjacent bit lines is 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 a trench filled with polysilicon to form the buried bit lines in a ROM or EPROM. This reduces bit line resistance and increases punch through voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a conventional buried bit line ROM array. 
     FIG. 2 illustrates a cross-section of one ROM cell in the array of FIG. 1. 
     FIGS. 3, 4, 5, 6 and 7 illustrate the formation of buried bit lines by the formation of polysilicon filled trenches. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 3-7 illustrate the formation of buried bit lines in accordance with the present invention. 
     As shown in FIG. 3, the starting point is a silicon substrate 11 which illustratively is of P-type. The substrate is doped with boron atoms. An oxide (SiO 2 ) layer 50 with a thickness of about 200 Angstroms is deposited on the surface of the substrate. A silicon nitride layer 52 with a thickness of 1000 Angstroms is then deposited on top of the oxide 50. 
     A layer of photo-resist (PR) is then deposited on top of the nitride 52. The layer of photo-resist is then patterned to form the photo-resist stripe portions 54, which stripes are separated by the openings 55. 
     Using the photo-resist stripes 54 as a mask, the substrate 11 is etched using a dry etch to a depth of 1 micron. The photo-resist portions 54 are then removed. The resulting trenches 56 are shown in FIG. 4. 
     SiO 2  is then deposited or thermally oxidized on the side walls and bottom of the trenches 56 to a thickness of 1000˜2000 Angstroms. 
     The SiO 2  is then etched and over-etched to remove the SiO 2  from the bottom of the trenches 56 to form the oxide spacers 57 and the oxide spacer over-etch region 58. The depth of the oxide spacer over-etch region 58 is about 0.1 micron. 
     As shown is FIG. 5, the nitride layer 52 is then stripped and the trenches are filled with polysilicon material 59 to form the bit lines 12. 
     The polysilicon material is doped to be N +  -type. The doping can be accomplished using POCl 3 . Alternatively As can be implanted using ion implantation at 50 KeV. The dose of the N +  dopant in the polysilicon 59 is 5E15/cm 2  -2E16/cm 2 . Then, the oxide layer 50 is stripped. 
     As shown in FIG. 6, N +  side diffusion is utilized in the regions 61, 62, 63 of each of the two bit lines in the cell 30 to form the source and drain of the memory cell 30. Thus, the source is formed by one set of regions 61, 62, 63 of one bit line 12 and the drain is formed by another set of regions 61, 62, 63 of an adjacent bit line 12. 
     As shown in FIG. 7, a gate oxide layer 16 is then formed on the substrate 11. The gate oxide has a thickness of about 500 Angstroms on top of the bit lines 12 and a thickness of about 150 Angstroms on top of the channel between the bit lines 12. The polysilicon wordline 24 is then formed on top of the oxide layer 16. 
     The rest of the processes for completing the ROM array are entirely conventional back-end processes including BPSG, contacts, metalization and ROM coding. 
     The buried bit lines formed using the above described poly trench method result in low bit line sheet resistance and good bit line-to-bit line punch through voltage. Therefore, the bit line pitch (bit line width plus spacing between adjacent bit lines) can be reduced for a high density ROM array. 
     The above described process can also be used for bit lines in an EPROM (Erasable Programmable Read Only Memory). 
     Finally, the above described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the spirit and scope of the following claims.