Select transistors with tight threshold voltage in 3D memory
Disclosed herein is a 3D memory with a select transistor, and method for fabricating the same. The select transistor may have a conductive floating gate, a conductive control gate, a first dielectric between the conductive floating gate and the conductive control gate, and a second dielectric between a body and the conductive floating gate. In one aspect, a uniform gate dielectric is formed using lateral epitaxial growth in a recess adjacent a crystalline semiconductor select transistor body, followed by forming the gate dielectric from the epitaxial growth. Techniques help to prevent, or at least reduce, a leakage current between the select transistor control gate and the select transistor body and/or the semiconductor substrate below the select transistor. Therefore, select transistors having a substantially uniform threshold voltage, on current, and S-factor are achieved. Also, select transistors have a high on-current and a steep sub-threshold slope.
1. An apparatus, comprising:
a semiconductor substrate having a major surface;
a select transistor comprising:
a body formed from a solid pillar of semiconductor in contact with the semiconductor substrate;
a conductive floating gate that is completely surrounded by insulators in direct contact with the conductive floating gate;
a conductive control gate;
a first dielectric between the conductive floating gate and the conductive control gate; and
a second dielectric between the body and the conductive floating gate; and
a string of memory cells that extends vertically with respect to the major surface of the semiconductor substrate, the string having a string channel, wherein the solid pillar of semiconductor of the select transistor is in contact with the string channel.
2. The apparatus of claim 1 , wherein the conductive floating gate surrounds the solid pillar of semiconductor.
3. The apparatus of claim 1 , wherein the conductive floating gate comprises polysilicon.
4. The apparatus of claim 1 , wherein the conductive floating gate comprises metal.
5. The apparatus of claim 1 , wherein the solid pillar of semiconductor is single crystalline silicon.
6. The apparatus of claim 1 , wherein the memory cells each comprise a dielectric charge storage region.
7. The apparatus of claim 1 , further comprising a common source line, wherein the string of memory cells is a NAND string, the select transistor is a source side select transistor configured to connect one end of the NAND string to the common source line.
8. A method of fabricating a 3D memory array, comprising:
forming a select transistor adjacent to a semiconductor substrate having a major surface, forming the select transistor comprising:
forming a solid pillar of semiconductor in contact with the semiconductor substrate, the solid pillar being a body of the select transistor;
forming a gate dielectric adjacent the solid pillar of semiconductor;
forming a conductive floating gate adjacent the gate dielectric, wherein the gate dielectric is between the between the body and the conductive floating gate;
forming an inter-gate dielectric adjacent the conductive floating gate;
forming a conductive control gate adjacent the inter-gate dielectric, wherein the inter-gate dielectric is between the conductive floating gate and the conductive control gate, wherein the conductive floating gate is completely surrounded by insulators in direct contact with the conductive floating gate; and
forming a string of memory cells that extends vertically with respect to the major surface of the semiconductor substrate, comprising forming a string channel in contact with the solid pillar of semiconductor of the select transistor.
9. The method of claim 8 , wherein forming the conductive floating gate comprises forming conductive floating surrounding the solid pillar of semiconductor.
10. The method of claim 9 , wherein forming the solid pillar of semiconductor in contact with the semiconductor substrate and forming the gate dielectric adjacent the solid pillar of semiconductor comprises:
epitaxially growing a crystalline semiconductor in contact with the semiconductor substrate; and
oxidizing the epitaxially grown crystalline semiconductor to form the gate dielectric from crystalline semiconductor, the solid pillar of semiconductor is formed from remaining oxidized epitaxially grown crystalline semiconductor.
11. The method of claim 9 , further comprising:
forming a plurality of alternating layers of insulator material and sacrificial material above the semiconductor substrate;
forming an opening through the alternating layers of the insulator material and the sacrificial material to the semiconductor substrate, wherein forming the solid pillar of semiconductor in contact with the semiconductor substrate comprises epitaxially growing crystalline semiconductor on the semiconductor substrate;
removing a layer of the sacrificial material to expose a sidewall of the epitaxially grown crystalline semiconductor, removing the layer leaves a recess; and
forming the conductive floating gate in the recess.
12. The method of claim 11 , wherein forming the string of memory cells comprises forming a dielectric film in the opening, the dielectric film is configured as charge storage regions of the memory cells.
13. The method of claim 8 , wherein the solid pillar of semiconductor is single crystal silicon.
14. The method of claim 8 , wherein forming the conductive floating gate comprises forming the conductive floating gate from polysilicon.
15. The method of claim 8 , wherein forming the conductive floating gate comprises forming the conductive floating gate from metal.
16. The method of claim 8 , wherein forming the string of memory cells comprises forming a NAND string.
17. A non-volatile memory system, comprising:
a semiconductor substrate having a major surface;
a string select transistor comprising:
a body formed from a solid pillar of semiconductor in contact with the semiconductor substrate;
a conductive control gate;
a conductive floating gate;
an inter-gate dielectric that provides electrical isolation between the conductive floating gate and the conductive control gate; and
a gate dielectric that provides electrical isolation between the body and the conductive floating gate; and
a NAND string of memory cells that extends vertically with respect to the major surface of the semiconductor substrate, the NAND string having a NAND string channel, wherein the solid pillar of semiconductor of the string select transistor is in contact with the NAND string channel.
18. The non-volatile memory system of claim 17 , wherein the memory cells of the NAND string each comprise a dielectric charge storage region.
19. The non-volatile memory system of claim 17 , further comprising a common source line, wherein the string select transistor is a source side select transistor configured to connect one end of the NAND string to the common source line.