Semiconductor device with localized charge storage dielectric and method of making same
View Patent ↗An array of transistors includes a plurality of transistors, a plurality of word lines extending in a first direction and a plurality of bit lines extending in a second direction. Each transistor includes a source, a drain, a channel and a localized charge storage dielectric. A first transistor of the plurality of transistors and a second transistor of the plurality of transistors share a common source/drain. A first localized charge storage dielectric of the first transistor does not overlap the common source/drain and a second localized charge storage dielectric of the second transistor overlaps the common source/drain.
1. A method of making an array of transistors, comprising:
first forming an active region or a word line film;
second forming a charge storage dielectric layer over the active region or the word line film;
third forming a first mask over a first plurality of regions on the charge storage dielectric layer;
fourth patterning the charge storage dielectric layer using the first mask to form a plurality of localized charge storage dielectrics; and
fifth forming the word line film over the plurality of localized charge storage dielectrics if the active region was formed first, or forming the active region over the plurality of localized charge dielectrics if the word line film was formed first;
wherein each transistor of the array of transistors comprises:
a first lateral transistor portion comprising (i) a source; (ii) a first channel portion located adjacent to the source; and (iii) one of the localized charge storage dielectrics; and
a second lateral transistor portion comprising (i) a drain and (ii) a second channel portion located adjacent to the drain, wherein the second transistor portion excludes a localized charge storage dielectric.
2. The method of claim 1 , further comprising forming a tunneling dielectric and a blocking dielectric which are located in the first lateral transistor portion and in the second lateral transistor portion of each transistor.
3. The method of claim 2 , wherein:
the step of forming the active region occurs before the step of forming the word line film;
forming the tunneling dielectric comprises forming the tunneling dielectric on the active region;
forming the charge storage dielectric layer comprises forming the charge storage dielectric layer on the tunneling dielectric;
patterning the charge storage dielectric layer comprises patterning the charge storage dielectric layer without patterning the tunneling dielectric;
forming the blocking dielectric comprises forming the blocking dielectric on the localized charge storage dielectrics such that the blocking dielectric contacts the tunneling dielectric in the second lateral transistor portions where the charge storage dielectric layer has been removed; and
forming the word line film comprises forming the word line film on the blocking dielectric.
4. The method of claim 3 , further comprising selectively implanting threshold voltage adjusting dopants into the active region using the first mask as a mask.
5. The method of claim 3 , further comprising selectively forming a silicon oxide dielectric using the first mask as a mask in the second lateral transistor portions where the charge storage dielectric layer has been removed.
6. The method of claim 3 , wherein the active region comprises a portion of a single crystal semiconductor substrate or an epitaxial layer located on the single crystal semiconductor substrate.
7. The method of claim 3 , wherein the active region comprises a polysilicon active layer.
8. The method of claim 7 , further comprising:
forming a plurality of bit lines extending in a first direction;
forming an insulating fill between the bit lines;
depositing the polysilicon active layer on the bit lines and the insulating fill to perform the step of forming the active region;
outdiffusing source and drain dopants from the bit lines into the polysilicon active layer to form at least portions of sources and drains in the polysilicon active layer separated by channels;
forming a second mask over the word line film located on the blocking dielectric; and
patterning the word line film, the blocking dielectric layer, the localized charge storage dielectrics, the tunneling dielectric and the polysilicon active layer using the second mask to form a plurality of first rails extending in a second direction different than the first direction.
9. The method of claim 2 , wherein:
the step of forming the active region occurs after the step of forming the word line film;
forming the blocking dielectric comprises forming the blocking dielectric on the word line film;
forming the charge storage dielectric layer comprises forming the charge storage dielectric layer on the blocking dielectric;
patterning the charge storage dielectric layer comprises patterning the charge storage dielectric layer without patterning the blocking dielectric;
forming the tunneling dielectric comprises forming the tunneling dielectric on the localized charge storage dielectrics such that the tunneling dielectric contacts the blocking dielectric in the second lateral transistor portions where the charge storage dielectric layer has been removed; and
forming the active region comprises forming a polysilicon active layer on the tunneling dielectric.
10. The method of claim 9 , further comprising:
forming a second mask over the polysilicon active layer;
patterning the polysilicon active layer, the blocking dielectric, the localized charge storage dielectric, the tunneling dielectric and the word line film using the second mask to form a plurality of first rails extending in a first direction;
forming a first insulating fill between the first rails;
forming at least portions of sources and drains in the polysilicon active layer separated by channels;
forming a plurality of bit lines extending in a first direction on the polysilicon active layer and on the first insulating fill; and
forming a second insulating fill between the bit lines.
11. The method of claim 1 , further comprising monolithically forming a plurality of thin film transistors in a plurality of vertically stacked device levels, wherein each of the thin film transistors comprises a localized charge storage dielectric located over a source but not over a drain.
12. The method of claim 11 , further comprising:
forming a plurality of first top gate thin film transistors, wherein each of the first top gate transistors comprises a first localized charge storage dielectric located over a first source but not over a first drain;
monolithically forming a plurality of second bottom gate thin film transistors over the plurality of first top gate thin film transistors, wherein each of the second bottom gate transistors comprises a second localized charge storage dielectric located under a second source but not under a second drain;
monolithically forming a plurality of third top gate thin film transistors over the plurality of second bottom gate thin film transistors, wherein each of the third top gate transistors comprises a third localized charge storage dielectric located over a third source but not over a third drain; and
monolithically forming a plurality of fourth bottom gate thin film transistors over the plurality of third top gate thin film transistors, wherein each of the fourth bottom gate transistors comprises a fourth localized charge storage dielectric located under a fourth source but not under a fourth drain.