Abstract:
A process of fabricating a non-volatile read only memory device (ROM) wherein the conductive word lines have desirable very narrow widths and are closely spaced. The invention provides a process for forming word lines with a smaller width and line pitch than is possible with conventional processes. A first set of word lines is formed. Next, a second set of word lines is formed in between the first word lines using oxide spacers to define the second word lines.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     This invention relates to a read only memory (ROM) type semiconductor device and in particular, to an improved process for forming high density self aligned conductive lines (word lines). 
     (2) Description of the Prior Art 
     In the quest to achieve microminiaturization of integrated circuit devices, individual elements have been made very small and the elements have been closely packed. As ROM devices are scaled down in dimension, there is a continuous challenge to produce high density conductive lines (i.e., word lines) using a minimum number of process steps. In conventional methods for forming conductive lines, the minimum line pitch is limited by the photolithography precision. The line pitch is the distance between on side of a line and the same side of an adjacent line. This shown in FIG. 1 where pitch 20 illustrates the word line pitch. This minimum line pitch 20 limits the miniaturization of the ROM. 
     In the conventional prior art process, a thin insulating layer 12, typically oxide is deposited on a monocrystalline silicon substrate 10 as shown in FIG. 1. Subsequently, a conductive layer, often composed of polycrystalline silicon, is deposited on the insulating layer. Next, a photoresist layer is formed on the conductive layer. The photoresist layer is exposed and developed to define a pattern of elongated spaced parallel lines. The exposed conductive layer is etched to form conducting lines 14 (i.e., word lines) and to expose portions of the first insulating layer. Finally, conventional semiconductor techniques are used to form arm complete the non-volatile memory device. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a process of forming a memory device with high density conductive lines. 
     Another object of the present invention is no provide a process for fabricating high density conductive lines with line pitches and line widths smaller than possible using a conventional process. 
     In accordance with the above objects, a process for fabricating high density conductive lines on a semiconductor substrate is provided. A thin insulating layer is formed on the surface of a monocrystalline semiconductor substrate. Next, a first blanket conductive layer, preferably polycrystalline silicon, is deposited over the first insulating layer. First conductive lines are formed from the conductive layer by using conventional photolithographic and etching processes. 
     Subsequently, a conformal polycrystalline silicon layer is formed over the first conductive lines and the exposed portions of the insulation layer. The conformal polycrystalline silicon layer is completely oxidized to form a conformal silicon oxide layer. Then, the conformal silicon oxide layer is anisotropically etched to form silicon oxide spacers on the vertical sidewalls of the conductive lines and to expose the tops of the first conductive lines. Next, a thin oxide layer and a second polycrystalline silicon layer are grown on the exposed surfaces. The second polycrystalline silicon layer is anisotropically etched to form second conductive lines in the areas between the spacers. As a result of this process, the smallest line pitch achievable for the first and second conductive lines is approximately half of the line pitch for conductive lines formed using the prior art process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings show the following: 
     FIG. 1 is cross-sectional view in broken section in greatly enlarged scale that illustrates a device structure in a stage of fabrication in accordance with the prior art processes 
     FIGS. 2 through 6 are a sequence of cross-sectional views in broken section in greatly enlarged scale that illustrate a device structure in various stages of fabrication in accordance with the process of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present inventions will be described in detail with reference to the accompanying drawings. It should be noted that the drawings are in greatly simplified form. In practice the memory device structure will be one of many supported on a common substrate connected with suitable metallurgy in various electronic electric circuit configurations. The substrate shall be a monocrystalline silicon semiconductor body with many devices fabricated therein, as is well known in the art. The substrate 10 is preferably formed of monocrystalline silicon having a crystalline orientation of &lt;1 0 0&gt;. The background substrate dopant is preferably P-type, with a concentration in the range of 5E15 to 5E17 atoms/cm 3 . As shown in FIG. 1, first thin insulating layer 12, preferably oxide, is formed on substrate 10. Layer 12 typically has a thickness is in the range of 100 to 200 Angstroms, more preferably a thickness of 130 Angstroms. Next, a first conductive blanket layer is formed over the first insulating layer 12. 
     The conductive blanket layer has a thickness in the range of 2000 to 4000 Angstroms, and more preferably a thickness of 3000 Angstroms. The conductive layer is preferably formed of polycrystalline silicon or a polycide. The first conductive layer can be doped using insitu doping or by ion implantation. Typically, Arsenic or Phosphorous ion impurities are used. Conductive layer impurity concentrations are in the range of 5E18 to 5E21 Atoms/cm 3 , and more preferably 1E20 Atoms/cm 3 . 
     Over the first conductive layer, a photoresist layer is formed, exposed and developed to define a pattern of elongated spaced parallel resist lines. This pattern is used as a etch mask and the exposed conductive layer is anisotropically etched to form first conductive lines 14, (i.e., word lines) with substantially vertical sidewalls. Also, the etching process exposes portions of the first insulating layer between the conductive lines. Preferably, the conductive layer is etched by a commercially available plasma dry etcher with significantly high polysilicon to silicon oxide selectivity and preferably higher than 20 to 1. The first conductive lines 14 have a line pitch in range of 0.8 to 1.6 microns, and more typically a line pitch of 1.2 microns for a 0.6 micron feature size process. 
     Referring to FIG. 2, The photoresist is then removed. Subsequently, a conformal polycrystalline silicon layer 22 is formed over the conductive lines 14 and the exposed portion of the first insulating layer 12. The conformal layer 22 has a thickness in the range of 500 to 2000 Angstroms, and a thickness more preferably of 1000 Angstroms. 
     Subsequently, polysilicon conformal layer 22 is completely oxidized to form a conformal silicon oxide layer 24 as illustrated in FIG. 3. Layer 24 has a thickness in the range of 2000 to 4000 Angstroms, and more preferably a thickness of 3000 Angstroms. Portions of polycrystalline silicon conductive line 14 can be consumed in the oxidation process. Also, portions of the substrate between the first conductive lines 14 can also be consumed in the oxidation process. Normally, the ratio of the thickness of polycrystalline silicon consumed to oxide thickness grown is approximately 45 percent. 
     As shown in FIG. 4, the oxide layer 24 is anisotropically etched to expose the top surfaces of the conductive lines 14 forming silicon oxide spacers 26 having vertical sidewalls. Preferably the oxide layer 24 is removed by anisotropic reactive ion etching with CHF 3  and oxygen as the etching ambient. The spacers 26 have a width in the range of 1150 to 4000 Angstroms. Also, areas 27 of the substrate between the spacers 26 are exposed by the etch. Next, the exposed substrate areas 27 and conductive lines 14 are oxidized to form a second thin insulating layer 32 as shown in FIG. 5. The thin insulating layer 32 has a thickness in the range of 100 to 200 Angstroms, and more preferably with a thickness of 130 Angstroms. 
     Next, as illustrated in FIG. 5, a second polycrystalline silicon layer 30 having a substantially planar top surface is deposited on the substrate so that the lines 14 and oxide layer 32 are covered. The second polycrystalline silicon layer 30 has a thickness in the range of 2000 to 5000 Angstroms (measured from the top of oxide layer 32 to the top of polysilicon layer 30), and more typically has a thickness of 3000 Angstroms. The second polycrystalline layer 30 can be doped using insitu doping, diffusion or by ion implantation. Typically ion impurities that can be used are Arsenic or Phosphorous. Moreover, layer 30 is has an impurity concentration in the range of 5E18 to 5E21 atoms/cm 3 . 
     Now, the second polycrystalline silicon layer 30 is anisotropically etched to a depth below the surface of layer 32 above the first conductive lines 14. This etch also forms the second conductive lines 34 in the areas 27 between the spacers 26. 
     The invention provides a method for forming conductive lines that are nearly twice as density as compared to lines formed using the conventional process as shown FIG. 1, using the same photolithography ground rules. For example, for a 0.6 micron ground rule photolithography technology, the smallest feature definable is a 0.6 micron width or length. Therefore, as shown in FIG. 1, the minimum line pitch 20 would be 1.2 microns (i.e., 0.6 microns for line width 18 and 0.6 microns 16 for space). Using the invention and the same 0.6 micron photolithographic ground rule, as shown in FIG. 6, the minimum line pitch 36 is 0.6 microns, (i.e., 0.3 microns for the line width 38 and 0.3 microns for the space 40). First conductive line 14 and therefore, line width 38 are reduced by the oxidation that formed the spacers 26 which can consume a portion of the first conductive line 14. Also, the space width 40 is controlled by the oxidation process as opposed to the conventional process where space width 26 is controlled by less precise photolithographic processes. In the invention, oxidation processes which are used to form conductive lines and spacers, are much more controllable than photolithographic processes thus allowing smaller device dimensions. Overall, the invention achieves line pitches approximately half of that possible using the conventional process. 
     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the are that various changes in form and details may be made without departing from the spirit and scope of the invention.