Method of forming split gate memory cells with thinned tunnel oxide
A method of forming a memory device includes forming a floating gate on a memory cell area of a semiconductor substrate, having an upper surface terminating in an edge. An oxide layer is formed having first and second portions extending along the logic and memory cell regions of the substrate surface, respectively, and a third portion extending along the floating gate edge. A non-conformal layer is formed having a first, second and third portions covering the oxide layer first, second and third portions, respectively. An etch removes the non-conformal layer third portion, and thins but does not entirely remove the non-conformal layer first and second portions. An etch reduces the thickness of the oxide layer third portion. After removing the non-conformal layer first and second portions, a control gate is formed on the oxide layer second portion and a logic gate is formed on the oxide layer first portion.
1. A method of forming a memory device, comprising:
providing a semiconductor substrate with a substrate upper surface having a memory cell region and a logic region;
forming a floating gate disposed vertically over and insulated from the memory cell region of the substrate upper surface, wherein the floating gate includes an upper surface that terminates in an edge;
forming an oxide layer having a first portion that extends along the logic region of the substrate upper surface, a second portion that extends along the memory cell region of the substrate upper surface, and a third portion that extends along the edge of the floating gate;
forming a non-conformal layer having a first portion that covers the oxide layer first portion, a second portion that covers the oxide layer second portion, and a third portion that covers the oxide layer third portion, wherein the third portion of the non-conformal layer has a thickness that is less than a thickness of the first and second portions of the non-conformal layer;
performing an etch that removes the third portion of the non-conformal layer, and thins but does not entirely remove the first and second portions of the non-conformal layer;
performing an oxide etch that reduces a thickness of the third portion of the oxide layer, wherein the first and second portions of the oxide layer are protected from the oxide etch by the first and second portions of the non-conformal layer;
removing the first and second portions of the non-conformal layer;
forming a control gate having a first portion on the second portion of the oxide layer and a second portion that extends up and over the floating gate, wherein the control gate is insulated from the edge of the floating gate by the third portion of the oxide layer having the reduced thickness; and
forming a logic gate on the first portion of the oxide layer.
2. The method of claim 1 , further comprising:
oxidizing the upper surface of the floating gate so that the upper surface of the floating gate is concave and the edge is a sharp edge.
3. The method of claim 1 , wherein the forming of the control gate and the forming of the logic gate comprise:
forming a polysilicon layer on the first, second and third portions of the oxide layer;
selectively removing portions of the polysilicon layer leaving a first portion of the polysilicon layer as the formed control gate and leaving a second portion of the polysilicon layer as the formed logic gate.
4. The method of claim 1 , further comprising:
forming a first source region in the substrate adjacent an end of the floating gate;
forming a first drain region in the substrate adjacent an end of the control gate;
forming a second source region in the substrate adjacent a first end of the logic gate; and
forming a second drain region in the substrate adjacent a second end of the logic gate.
5. The method of claim 4 , wherein the forming of the first drain region, the second source region and the second drain region are performed simultaneously by an implantation process.
6. The method of claim 1 , wherein the non-conformal layer is formed of a flowable material.
7. The method of claim 1 , wherein the non-conformal layer is formed of a BARC material.
8. The method of claim 1 , wherein the non-conformal layer is formed of a photoresist material.
9. The method of claim 1 , wherein the non-conformal layer is formed of a silicon-on-glass material.
10. A method of forming a memory device, comprising:
providing a semiconductor substrate with a substrate upper surface having a memory cell region and a logic region;
forming a floating gate disposed vertically over and insulated from the memory cell region of the substrate upper surface, wherein the floating gate includes an upper surface that terminates in an edge;
forming a first oxide layer having a first portion that extends along the logic region of the substrate upper surface, a second portion that extends along the memory cell region of the substrate upper surface, and a third portion that extends along the edge of the floating gate;
forming a non-conformal layer having a first portion that covers the first portion of the first oxide layer, a second portion that covers the second portion of the first oxide layer, and a third portion that covers the third portion of the first oxide layer, wherein the third portion of the non-conformal layer has a thickness that is less than a thickness of the first and second portions of the non-conformal layer;
performing an etch that removes the third portion of the non-conformal layer, and thins but does not entirely remove the first and second portions of the non-conformal layer;
performing an oxide etch that removes the third portion of the first oxide layer, wherein the first and second portions of the first oxide layer are protected from the oxide etch by the first and second portions of the non-conformal layer;
forming a second oxide layer that extends along the edge of the floating gate, wherein the second oxide layer has a thickness that is less than a thickness of the first oxide layer;
removing the first and second portions of the non-conformal layer;
forming a control gate having a first portion on the second portion of the first oxide layer and a second portion that extends up and over the floating gate, wherein the control gate is insulated from the edge of the floating gate by the second oxide layer; and
forming a logic gate on the first portion of the first oxide layer.
11. The method of claim 10 , further comprising:
oxidizing the upper surface of the floating gate so that the upper surface of the floating gate is concave and the edge is a sharp edge.
12. The method of claim 10 , wherein the forming of the control gate and the forming of the logic gate comprise:
forming a polysilicon layer on the first, second and third portions of the first oxide layer;
selectively removing portions of the polysilicon layer leaving a first portion of the polysilicon layer as the formed control gate and leaving a second portion of the polysilicon layer as the formed logic gate.
13. The method of claim 10 , further comprising:
forming a first source region in the substrate adjacent an end of the floating gate;
forming a first drain region in the substrate adjacent an end of the control gate;
forming a second source region in the substrate adjacent a first end of the logic gate; and
forming a second drain region in the substrate adjacent a second end of the logic gate.
14. The method of claim 13 , wherein the forming of the first drain region, the second source region and the second drain region are performed simultaneously by an implantation process.
15. The method of claim 10 , wherein the non-conformal layer is formed of a flowable material.
16. The method of claim 10 , wherein the non-conformal layer is formed of a BARC material.
17. The method of claim 10 , wherein the non-conformal layer is formed of a photoresist material.
18. The method of claim 10 , wherein the non-conformal layer is formed of a silicon-on-glass material.