Contactless mask progammable rom

Abstract

A contactless mask programmable read-only memory (Mask ROM) is described, comprising a plurality of word lines extending in row direction and a plurality of diffusions arranged in rows and columns in a substrate. In the Mask ROM, two rows of diffusions are separated by a word line. Two adjacent diffusions in the same column, the word line between the two diffusions, and the substrate between the two diffusions together constitute a memory cell. The memory cells include a plurality of first memory cells and a plurality of second memory cells. The channel length, the gate oxide width or the channel dopant concentration of the first memory cells is different from that of the second memory cells, such that the threshold voltage of the first memory cells is substantially different from that of the second memory cells.

What is claimed is:  
     
         1 . A contactless mask programmable read-only memory (Mask ROM), comprising a plurality of word lines extending in a row direction and a plurality of MOS-type memory cells, wherein 
 two terminal memory cells in one column are coupled to a first voltage source of a first level and a second voltage source of a second level, respectively, wherein the second level is different from the first level; and    the memory cells include a plurality of first memory cells and a plurality of second memory cells, wherein a first threshold voltage of the first memory cells is substantially different from a second threshold voltage of the second memory cells, such that a type (first or second) of a selected memory cell can be identified during a reading operation.    
     
     
         2 . The contactless Mask ROM of  claim 1 , wherein a second channel length of the second memory cells is smaller than a first channel length of the first memory cells and is small enough to cause short channel effect, such that the second threshold voltage of the second memory cells is substantially lower than the first threshold voltage of the first memory cells.  
     
     
         3 . The contactless Mask ROM of  claim 1 , wherein a second channel dopant concentration of the second memory cells is lower than a first channel dopant concentration of the first memory cells, such that the second threshold voltage of the second memory cells is substantially lower than the first threshold voltage of the first memory cells.  
     
     
         4 . The contactless Mask ROM of  claim 1 , wherein a second gate dielectric thickness of the second memory cell is smaller than a first gate dielectric thickness of the first memory cells, such that the second threshold voltage of the second memory cells is substantially lower than the first threshold voltage of the first memory cells.  
     
     
         5 . The contactless Mask ROM of  claim 1 , wherein the first voltage source comprises a bit line.  
     
     
         6 . The contactless Mask ROM of  claim 5 , wherein the second voltage source comprises ground.  
     
     
         7 . The contactless Mask ROM of  claim 1 , wherein two columns of memory cells are separated by an isolation layer.  
     
     
         8 . The contactless Mask ROM of  claim 1 , wherein the word lines comprises doped polysilicon.  
     
     
         9 . The contactless Mask ROM of  claim 1 , wherein the substrate is a P-type substrate and diffusions of the MOS-type memory cells are of N-type.  
     
     
         10 . A contactless mask programmable read-only memory (Mask ROM), comprising: 
 a plurality of word lines extending in a row direction; and    a plurality of diffusions arranged in rows and columns in a substrate, wherein    two rows of diffusions are separated by a word line;    two adjacent diffusions in the same column, the word line between the two diffusions, and the substrate between the two diffusions together constitute a memory cell, and a plurality of memory cells are thus defined; and    the memory cells include a plurality of first memory cells and a plurality of second memory cells, wherein a second channel length of the second memory cells is smaller than a first channel length of the first memory cells and is small enough to cause short channel effect, such that a second threshold voltage of the second memory cells is substantially lower than a first threshold voltage of the first memory cells and a type (first or second) of a selected memory cell can be identified during a reading operation.    
     
     
         11 . The contactless Mask ROM of  claim 10 , wherein one terminal diffusion in a column of diffusions is coupled to a bit line.  
     
     
         12 . The contactless Mask ROM of  claim 11 , wherein the other terminal diffusion in the column of diffusions is coupled to ground.  
     
     
         13 . The contactless Mask ROM of  claim 10 , wherein two columns of diffusions are separated by an isolation layer.  
     
     
         14 . A contactless mask programmable read-only memory (Mask ROM), comprising: 
 a plurality of word lines extending in a row direction; and    a plurality of diffusions arranged in rows and columns in a substrate, wherein    two rows of diffusions are separated by a word line;    two terminal diffusions in each column of diffusions are coupled to a voltage and ground, respectively;    two adjacent diffusions in the same column, the word line between the two diffusions, and the substrate between the two diffusions together constitute a memory cell, so that a plurality of memory cells are defined; and    the memory cells include a plurality of first memory cells and a plurality of second memory cells, wherein a second channel dopant concentration of the second memory cells is lower than a first channel dopant concentration of the first memory cells, such that a second threshold voltage of the second memory cells is substantially lower than a first threshold voltage of the first memory cells and a type (first or second) of a selected memory cell can be identified during a reading operation.    
     
     
         15 . The contactless Mask ROM of  claim 14 , wherein one terminal diffusion in a column of diffusions is coupled to a bit line.  
     
     
         16 . The contactless Mask ROM of  claim 15 , wherein the other terminal diffusion in the column of diffusions is coupled to ground.  
     
     
         17 . The contactless Mask ROM of  claim 14 , wherein two columns of diffusions are separated by an isolation layer.  
     
     
         18 . A contactless mask programmable read-only memory (Mask ROM), comprising: 
 a plurality of word lines extending in a row direction; and    a plurality of diffusions arranged in rows and columns in a substrate, wherein    two rows of diffusions are separated by a word line;    two adjacent diffusions in the same column, the word line between the two diffusions, and the substrate between the two diffusions together constitute a memory cell, so that a plurality of memory cells are defined; and    the memory cells include a plurality of first memory cells and a plurality of second memory cells, wherein a second gate dielectric thickness of the second memory cells is smaller than a first gate dielectric thickness of the first memory cells, such that a second threshold voltage of the second memory cells is substantially lower than a first threshold voltage of the first memory cells and a type (first or second) of a selected memory cell can be identified during a reading operation.    
     
     
         19 . The contactless Mask ROM of  claim 18 , wherein one terminal diffusion in a column of diffusions is coupled to a bit line.  
     
     
         20 . The contactless Mask ROM of  claim 19 , wherein the other terminal diffusion in the column of diffusions is coupled to ground.