Semiconductor device having shallow trench isolation structure comprising an upper trench and a lower trench including a void
In one embodiment, a semiconductor device has an active region defined by an isolation layer formed inside an STI trench that includes an upper trench and a lower trench having a substantially curved cross-sectional profile under the upper trench so that the lower trench is in communication with the upper trench. Since the upper trench has a sidewall tapered with a positive slope, a good gap filling property can be obtained when filling the upper trench with an insulating layer. By forming a void in the lower trench, a dielectric constant at the bottom of the isolation layer is lower than a dielectric constant at an oxide layer, thereby improving the isolation property. The isolation layer includes a first insulating layer formed inside only the upper trench and covering an inner wall of the upper trench in the form of a spacer.
1. A semiconductor device comprising:
a semiconductor substrate having an upper trench and a lower trench to collectively define an active region, the lower trench having a substantially curved cross-sectional profile, the lower trench being in communication with the upper trench; and
an isolation layer formed inside the upper trench and the lower trench of the semiconductor substrate, the isolation layer comprising:
a first insulating layer formed inside only the upper trench and covering an inner wall of the upper trench;
a second insulating layer overlying the first insulating layer and covering an inner wall of the lower trench; and
a third insulating layer overlying the second insulating layer and extending from the upper trench to the lower trench to define a closed void inside the lower trench,
wherein the upper trench has a first width at an upper portion and a second width at a lower portion, the first width being greater than the second width, and the lower trench has a third width corresponding to a maximum width of the lower trench, the third width being greater than the second width and less than the first width.
2. The semiconductor device of claim 1 , wherein the isolation layer is formed on the semiconductor substrate to define a plurality of active regions each having an island shape with a major axis extending in a first direction and a minor axis extending in a second direction perpendicular to the first direction on a main surface of the semiconductor substrate, and the third width is less than the first width in a minor axis direction of the active region.
3. The semiconductor device of claim 2 , wherein the third width is greater than the first width in a major axis direction of the active region.
4. The semiconductor device of claim 2 , wherein the major axis is longer than the minor axis.
5. The semiconductor device of claim 4 , wherein the major axis and minor axis define outer boundaries of each of the island shape active regions.
6. The semiconductor device of claim 1 , wherein the first insulating layer is formed of a medium temperature deposition of oxide (MTO).
7. The semiconductor device of claim 1 , wherein the second insulating layer comprises a nitride liner covering the first insulating layer.
8. The semiconductor device of claim 7 , wherein the third insulating layer comprises an oxide layer covering the nitride liner around the void within the lower trench.
9. The semiconductor device of claim 1 , wherein the substantially curved cross-sectional profile represents a generally circular arc.
10. The semiconductor device of claim 1 , wherein the upper trench has a tapered inner wall with a positive slope.
11. The semiconductor device of claim 1 , wherein the void has a substantially curved cross-sectional profile having a generally circular shape.
12. A semiconductor device comprising:
a semiconductor substrate having an upper trench and a lower trench to collectively define an active region, the lower trench having a substantially curved cross-sectional profile, the lower trench being in communication with the upper trench; and
an isolation layer formed inside the upper trench and the lower trench of the semiconductor substrate, the isolation layer comprising:
a first insulating layer formed inside only the upper trench and covering an inner wall of the upper trench;
a second insulating layer overlying the first insulating layer and covering an inner wall of the lower trench;
a third insulating layer overlying the second insulating layer and extending from the upper trench to the lower trench to define a closed void inside the lower trench; and
a fourth insulating layer interposed between the first insulating layer and the second insulating layer in the upper trench, the fourth insulating layer being interposed directly between the substrate and the second insulating layer in the lower trench.
13. The semiconductor device of claim 12 , wherein the fourth insulating layer comprises an oxide layer.
14. The semiconductor device of claim 13 , wherein the fourth insulating layer is a grown oxide layer.
15. The semiconductor device of claim 12 , wherein:
the first insulating layer is formed of a medium temperature deposition of oxide (MTO);
the second insulating layer comprises a nitride liner covering the first insulating layer;
the third insulating layer comprises an oxide layer covering the nitride liner around the void within the lower trench; and
the fourth insulating layer comprises an oxide layer.