Methods for forming multilayer horizontal NOR-type thin-film memory strings
Various methods overcome the limitations and achieve superior scaling by (i) replacing a single highly challenging high aspect ratio etch step with two or more etch steps of less challenging aspect ratios and which involve wider and more mechanically stable active strips, (ii) using dielectric pillars for support and to maintain structural stability during a high aspect ratio etch step and subsequent processing steps, or (iii) using multiple masking steps to provide two or more etch steps of less challenging aspect ratios and which involve wider and more mechanically stable active strips.
1. A method for high aspect ratio etching, comprising:
preparing above a surface of a semiconductor substrate a plurality of material multi-layers, stacked one on top of another along a first direction substantially orthogonal to the surface of the semiconductor substrate, wherein each material multi-layer comprises first and second semiconductor layers;
patterning and etching the material multilayers along the first direction using a first mask to form a first set of trenches that divide the material layers into a first group of multi-layer stacks, wherein each of the first set of trenches extends along a second direction substantially parallel to the surface of the semiconductor substrate;
filling the first set of trenches with a dielectric material; and
patterning and etching the first group of multi-layer stacks along the first direction using a second mask to form a second set of trenches that divide the first group of multi-layer stacks into a second group of multi-layer stacks, wherein each of the second set of trenches extends along the second direction parallel the first set of trenches.
2. The method of claim 1 , wherein etching the material multi-layer and etching the first group of multi-layers each involve an etch of an aspect ratio less than 40.
3. The method of claim 1 , wherein the second mask provides mask structures each overlapping (i) at least two neighboring ones of the first group of multi-layer stacks and (ii) a dielectric material in one of the first set of trenches between the neighboring multi-layer stacks.
4. The method of claim 1 , further comprising (i) providing an intermediate mask on the material multi-layers, the intermediate mask providing masking features for defining both the first set of trenches and the second set of trenches, and wherein the first mask is provided on top of the intermediate mask and provides masking structures that protect the second set of trenches during etching the material multi-layers.
5. The method of claim 4 , wherein the second mask is formed by the dielectric material filling between masking structures of the first mask.
6. The method of claim 1 , further comprising replacing a portion of the dielectric material in each of the first set of trenches with a conductive column that extends along the first direction.
7. The method of claim 6 , wherein replacing a portion of the dielectric material in the first set of trenches comprises:
etching the dielectric material in the first set of trenches to provide a plurality of shafts that extends along the first direction;
depositing a charge storage material conformally in each shaft; and
filling each shaft with a conductive material to form the conductive columns.
8. The method of claim 6 , further comprising, prior to preparing the material multi-layers, providing a plurality of conductors above the surface of the semiconductor substrate, each conductor extending along a third direction substantially orthogonal to each of the first and second directions.
9. The method of claim 8 , further comprising providing a conductive path to enable each conductive column to be electrically connected to one of the conductors.
10. The method of claim 6 , further comprising providing a plurality of conductors above the second group of multi-layer stacks, each conductor extending along a third direction substantially orthogonal to each of the first and second directions.
11. The method of claim 10 , further comprising providing conductive paths to enable each conductive column to be electrically connected to one of the conductors.
12. The method of claim 10 , further comprising:
providing circuit elements in the semiconductor substrate; and
electrically connecting selected ones of the conductive columns to corresponding ones of the circuit elements, each of the selected conductive columns being situated substantially above its corresponding circuit element.
13. The method of claim 12 , further comprising providing an isolation layer between the circuit elements and the conductive columns, wherein electrically connecting selected ones of the conductive columns to their corresponding circuit elements comprises creating conductive paths through the isolation layer.
14. The method of claim 1 , further comprising creating a stair case structure at one or more ends of each multi-layer stack in the second group of multilayer stacks, wherein each staircase structure has steps that successively extends the layers of the multi-layer stack along the second direction, and wherein each step exposes the first semiconductor layer or the second semiconductor layer, or another conductive layer, of a multi-layer in the multi-layer stack.
15. The method of claim 14 , further comprising providing an electrically insulating material over the staircase structure and providing via connections along the first direction to electrically connect to the exposed semiconductor layer, or the conductive layer, of a step of the staircase structures.
16. The method of claim 1 , wherein each material multi-layer further comprises a sacrificial layer between the first and second semiconductor layers, the method further comprising replacing a portion of the sacrificial layer by a third semiconductor layer.
17. The method of claim 1 , wherein each material multi-layer further comprises a sacrificial layer adjacent one or both of the first semiconductor layer and the second semiconductor layer, the method further comprising replacing at least a portion of the sacrificial layer by a conductive material.