FIG. 1 shows a schematic plan view of a conventional MOSFET-based ROM device 10 which is specially designed for permanent storage data. This ROM device includes a plurality of memory cells as exemplarily indicated by the dash boxes labeled with the reference numerals 12, 14, which can be accessed via an array of intercrossed world lines WL1, WL2, WL3 and an array of buried bit lines BL1, BL2, BL3. World lines passe over the tops of the buried bit lines. Each of the memory cells is associated with one segment of the word line between each neighboring pair of the bit lines under which one channel region is formed labeled with the reference numeral 16. Whether the memory cell 12 stores the data 0 or 1 is dependent on whether its associated channel region 16 is doped with impurity or not.
FIG. 2 shows a cross section of the device 10 taken along line 1B--1B in FIG. 1. Device 10 includes a P-doped silicon substrate 20 with buried N+ bit lines 26. Over the substrate is formed a gate oxide layer 22 upon which is formed a polysilcon world lines 24. Photoresist mask with a pattern 25 is formed over the polysilicon word line 24 for the process of code ion implantation. The section in FIG. 2 shows code implantation of implanted boron ion region 28.
To achieve microminiaturization of integrated circuit devices, individual elements have been made very small and the elements have been closely packed. As read only memory devices scaled down in dimensions, there is a continuous challenge to maintain close spacing between the buried bit lines to obtain density cell structures. Also, the spacing between the bit lines and code implant areas is critical to maintaining consistent bit line resistance that is a important factor to ROM read speed. Code implants and bit lines are composed of an opposite type impurities such that N+ bit lines and P+ code implants or P+ bit lines and N+ code implants. A large bit line resistance due to a code impurity overlap will degrade the data reading speed.
As shown in FIG. 2, conventional photolithography processes are used to form photoresist pattern 25 on the surface, leaving opening area where the code implants are desired. The remaining photoresist pattern 25 is used as a code implant mask. When the bit lines are N+ type regions, a boron ion is normally used for the code implant. The surface is implanted with boron forming code implant region 28.
Referring to FIG. 3, a ROM code implant region is located below an opening area in the photoresist mask 25 at the intersection of the word line and the space between two adjacent bit lines. The section in FIG. 3 shows a code implantation 30 of implanted boron ions 29. The code implantation 30 overlap the buried bit lines 26 due to the mask shift indicated by dash line thereby increasing the bit line resistance due to the opposite type impurity. On the other hand, the most advanced ULSI has a minimum feature size of 0.15 um, which is very close to the resolution limit of optical photolithography because of the light source. The code implantation area is about 0.15 um.sup.2 in a 0.15 um process. The light refraction is often introduced to the resolution difference, which may cause the overlap. The photo masks manufacturing therefore need optic proximity correction process. Hence, in a conventional photolithography process, overlay control is very time-consuming to carry out. In addition, to reduce manufacturing cost, the process of masks manufacturing must be relatively simple.