Non-volatile memory cell having a floating gate and a coupling gate with improved coupling ratio therebetween
View Patent ↗A non-volatile memory cell having a split gate, wherein the floating gate and the coupling/control gate have complimentary non-planar shapes. The shape may be a step shape. An array of such cells and a method of manufacturing the cells are also disclosed.
1. A non-volatile memory cell comprising:
a semiconductor substrate of a first conductivity type having a top surface;
a first region of a second conductivity type in said substrate along the top surface;
a second region of the second conductivity type, in said substrate along the top surface, spaced apart from the first region;
a channel region between the first region and the second region;
a word line gate positioned over a first portion of the channel region, immediately adjacent to the first region, said word line gate spaced apart from the channel region by a first insulating layer;
a floating gate positioned over another portion of the channel region, said floating gate having a lower surface separated from the channel region by a second insulating layer, and an upper surface opposite the lower surface; said floating gate having a first side wall adjacent to but separated from the word line gate; and a second side wall opposite the first side wall, wherein said upper surface having a non-planar contour from said first side wall to said second side wall;
a coupling gate positioned over the upper surface of the floating gate and insulated therefrom by a third insulating layer, said coupling gate having a lower surface that has a contour that follows the contour of said upper surface of said floating gate; and
an erase gate positioned adjacent to the second side wall of the floating gate; said erase gate positioned over the second region and insulated therefrom;
wherein said upper surface of said floating gate has a single step like non-planar contour such that the second side wall is taller than said first side wall.
2. The memory cell of claim 1 wherein said erase gate overhangs a portion of said floating gate.
3. An array of non-volatile memory cells comprising:
a semiconductor substrate of a first conductivity type having a top surface;
a plurality of memory cells arranged in an array with a plurality of rows and a plurality of columns; each of said memory cells comprising a first region of a second conductivity type in said substrate along the top surface; a second region of the second conductivity type, in said substrate along the top surface, spaced apart from the first region in a column direction, with a channel region between the first region and the second region, each of said channel region having a first portion and a second portion, with the first portion immediately adjacent to the first region;
a word line gate, extending in a row direction perpendicular to the column direction, positioned over a plurality of the first portion of channel regions, said word line gate spaced apart from each channel region by a first insulating layer;
a floating gate positioned over the second portion of each channel region, said floating gate having a lower surface separated from the channel region by a second insulating layer, and an upper surface opposite the lower surface; said floating gate having a first side wall adjacent to but separated from the word line gate; and a second side wall opposite the first side wall, wherein said upper surface having a non-planar contour from said first side wall to said second side wall;
a coupling gate, extending in the row direction, positioned over the upper surface of a plurality of floating gates and insulated therefrom by a third insulating layer, said coupling gate having a lower surface that has a contour that follows the contour of said upper surface of said floating gate; and
an erase gate, extending in the row direction across a plurality of columns and positioned adjacent to the second side wall of a plurality of floating gates; said erase gate positioned over the second region and insulated therefrom;
wherein said upper surface of each of said floating gates has a single step like non-planar contour such that said second side wall of each floating gate is taller than said first side wall of each floating gate.
4. The array of claim 3 wherein said second region extends in the row direction across a plurality of columns.
5. The array of claim 4 wherein said erase gate overhangs a portion of each of said plurality of floating gates.
6. A non-volatile memory cell comprising:
a semiconductor substrate of a first conductivity type having a top surface;
a first region of a second conductivity type in said substrate along the top surface;
a second region of the second conductivity type, in said substrate along the top surface, spaced apart from the first region;
a channel region between the first region and the second region;
a word line gate positioned over a first portion of the channel region, immediately adjacent to the first region, said word line gate spaced apart from the channel region by a first insulating layer;
a floating gate positioned over another portion of the channel region, said floating gate having a lower surface separated from the channel region by a second insulating layer, and an upper surface opposite the lower surface; said floating gate having a first side wall adjacent to but separated from the word line gate; and a second side wall opposite the first side wall, wherein said upper surface having a non-planar contour from said first side wall to said second side wall;
a third insulating layer on said upper surface of said floating gate, said third insulating layer having a uniform thickness extending from said first side wall to said second side wall;
a coupling gate positioned over said third insulating layer; and
an erase gate positioned adjacent to the second side wall of the floating gate; said erase gate positioned over the second region and insulated therefrom;
wherein said upper surface of said floating gate has a single step like non-planar contour such that said second side wall is taller than said first side wall.
7. The memory cell of claim 6 wherein said erase gate overhangs a portion of said floating gate.
8. A method of fabricating a non-volatile memory cell, said method comprising:
forming a first polysilicon layer on a first insulating layer on a semiconductor substrate, said first polysilicon layer having a top surface with a planar contour;
etching the top surface of said first polysilicon layer to produce a non-planar contour;
forming a second insulating layer on said top surface of said first polysilicon layer, with said second insulating layer substantially uniform in thickness over said top surface;
forming a second polysilicon layer on said second insulating layer, said second polysilicon layer having a bottom surface with a contour that substantially follows the non-planar contour of said top surface of said first polysilicon layer;
masking and cutting said second polysilicon layer, said second insulating layer, and said first polysilicon layer, to form a coupling gate and a floating gate, respectively;
forming a word line gate and an erase gate, on adjacent but respective opposite sides of said coupling gate and said floating gate;
forming source and drain regions in said substrate;
wherein said non-planar contour of said top surface of said first polysilicon layer has a single step such that a first side wall of the floating gate adjacent but separated from the erase gate is shorter than a second side wall of the floating gate opposite the first side wall and adjacent but separated from the word line gate.
9. The method of claim 8 wherein said source is formed underneath the erase gate.
10. The method of claim 8 , wherein said drain is formed adjacent to the word line gate.