3-dimensional NOR strings with segmented shared source regions
A NOR string includes a number of individually addressable thin-film storage transistors sharing a bit line, with the individually addressable thin-film transistors further grouped into a predetermined number of segments. In each segment, the thin-film storage transistors of the segment share a source line segment, which is electrically isolated from other source line segments in the other segments within the NOR string. The NOR string may be formed along an active strip of semiconductor layers provided above and parallel a surface of a semiconductor substrate, with each active strip including first and second semiconductor sublayers of a first conductivity and a third semiconductor sublayer of a second conductivity, wherein the shared bit line and each source line segment are formed in the first and second semiconductor sublayers, respectively.
1. A process for forming a memory structure comprising:
forming circuitry in a semiconductor substrate, the semiconductor substrate having a planar surface;
forming a plurality of active layers, with successive active layers being isolated from each other by isolation layers, each active layer comprising first and second semiconductor sublayers of a first conductivity type, a third semiconductor layer of a second conductivity type opposite the first conductivity type;
patterning and etching the active layers anisotropically to form a first set of trenches from the top of the active layers along a first direction substantially perpendicular to the planar surface, such that each trench extends lengthwise along a second direction substantially parallel to the planar surface;
filling the first set of trenches with a sacrificial material;
patterning and etching the sacrificial material anisotropically along the first direction to form a second set of trenches running lengthwise along a third direction substantially parallel the planar surface and substantially orthogonal to the second direction, thereby exposing a portion of each of the plurality of active layers; and
isotropically etching the exposed portions of the active layers to remove exposed portions of the first, second and third semiconductor sublayers of each active layer.
2. The process of claim 1 , wherein the active layers each further comprise a conductive layer adjacent the first semiconductor sublayer, the conductive layer being resistant to the isotropically etching step.
3. The process of claim 1 , subsequent to the isotropically etching step, further comprising:
selectively removing the sacrificial material from the first set of trenches to expose the active layers;
providing a charge-trapping layer in trenches over the exposed active layers;
filling the first of trenches with a conductive material; and
patterning and anisotropically etching the conductive material to provide pillars of conductive material.
4. The process of claim 3 , wherein (i) the first semiconductor sublayer and the second semiconductor sublayer of each active layer provides a shared bit line and shared source line segments for thin-film storage transistors in a NOR string; (ii) the third semiconductor sublayer provides channel regions for the thin-film storage transistors; and (iii) pillars of conductive material provide word lines for the thin-film storage transistors.