Electrically erasable programmable read only memory (eeprom) cell and forming method thereof

Abstract

An electrically erasable programmable read only memory (EEPROM) cell includes a first gate, a second gate and an erasing gate. The first gate and the second gate are disposed on a substrate, wherein the first gate includes a first floating gate and a first control gate stacked from bottom to top, and the second gate includes a second floating gate and a second control gate stacked from bottom to top. The erasing gate is sandwiched by the first gate and the second gate, wherein a side part of the first floating gate and a side part of the second floating gate right below the erasing gate both have multiple tips. The present invention also provides a method of forming said electrically erasable programmable read only memory (EEPROM) cell.

What is claimed is: 
     
         1 . An electrically erasable programmable read only memory (EEPROM) cell, comprising:
 a first gate and a second gate arranged on a substrate, wherein the first gate comprises a first floating gate and a first control gate stacked from bottom to top, and the second gate comprises a second floating gate and a second control gate stacked from bottom to top; and   an erasing gate sandwiched between the first gate and the second gate, wherein one side part of the first floating gate directly under the erasing gate and one side part of the second floating gate directly under the erasing gate have multiple tips.   
     
     
         2 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 1 , further comprising:
 a source line located in the substrate directly under the erasing gate.   
     
     
         3 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 1 , wherein the side part of the first floating gate directly under the erasing gate and the side part of the second floating gate directly under the erasing gate are both two-step side parts. 
     
     
         4 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 3 , wherein each two-step side part comprises two steps. 
     
     
         5 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 1 , further comprising:
 a first word line disposed a the side of the first gate opposite to the erasing gate, and a second word line disposed on a side of the second gate opposite to the erasing gate.   
     
     
         6 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 5 , further comprising:
 a plurality of bit lines disposed in the substrate, wherein one of the bit line is disposed at a side of the first word line opposite to the erasing gate, and another one of the bit line is disposed at a side of the second word line opposite to the erasing gate.   
     
     
         7 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 6 , further comprising a plurality of bit line contact plugs disposed directly above the bit lines. 
     
     
         8 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 1 , wherein the other side part of the first floating gate opposite to the erasing gate and the other side part of the second floating gate opposite to the erasing gate have vertical sidewalls. 
     
     
         9 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 1 , further comprising:
 a first hard mask disposed on the first control gate, and a second hard mask disposed on the second control gate.   
     
     
         10 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 9 , wherein a top surface of the first hard mask and a top surface of the second hard mask are higher than a top surface of the erasing gate. 
     
     
         11 . The electrically erasable programmable read only memory (EEPROM) cell according to  claim 10 , further comprising:
 a plurality of spacers disposed on the erasing gates, and disposed on the sides of the first hard mask and the second hard mask.   
     
     
         12 . A method of forming an electrically erasable programmable read only memory (EEPROM) cell, comprising:
 sequentially forming a floating gate layer and a control gate stacked on a substrate;   forming a first spacer on the floating gate layer and at a first side of the control gate;   removing an exposed top of the floating gate layer to form a pre-floating gate layer, wherein the pre-floating gate layer has a stepped side part;   forming a second spacer on the pre-floating gate layer and on the first side of the control gate; and   removing the exposed part of the pre-floating gate layer, so as to form a floating gate, wherein the floating gate has a two-step side part.   
     
     
         13 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 12 , wherein the step of sequentially forming the floating gate layer and the control gate stacked on the substrate comprises:
 sequentially forming the floating gate layer, a control gate layer and a hard mask layer to cover the substrate;   patterning the hard mask layer and the control gate layer to form the control gate and a hard mask.   
     
     
         14 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 12 , wherein the step of forming the first spacer on the floating gate layer and on the first side of the control gate comprises:
 forming the first spacer on the floating gate layer, and on the first side and a second side of the control gate respectively; and   removing the first spacer on the second side of the control gate, but the first spacer on the first side of the control gate is remained.   
     
     
         15 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 14 , wherein
 before removing the first spacer on the second side of the control gate, further comprising:   forming a first doped region in the substrate at a side of the first spacer on the second side of the control gate.   
     
     
         16 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 12 , after forming the pre-floating gate layer with the stepped side part, further comprising:
 removing the first spacer on the first side of the control gate.   
     
     
         17 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 12 , wherein the step of forming the second spacer on the pre-floating gate layer and on the first side of the control gate comprises:
 forming the second spacer on the pre-floating gate layer and on the first side and the second side of the control gate respectively; and   removing the second spacer on the second side of the control gate, but remaining the second spacer on the first side of the control gate.   
     
     
         18 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 17 , before removing the second spacer on the second side of the control gate, further comprising:
 forming a second doped region in the substrate on the side of the second spacer on the second side of the control gate.   
     
     
         19 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 12 , wherein a width of the first spacer is half a width of the second spacer. 
     
     
         20 . The method for forming an electrically erasable programmable read only memory (EEPROM) cell according to  claim 12 , after forming the floating gate, further comprising:
 forming a polysilicon layer on the substrate on the side of the floating gate, wherein a part of the polysilicon layer on the side of the two-step side part serves as an erasing gate, and a part of the polysilicon layer opposite to the two-step side part serves as a word line.