Method of making a semiconductor device having a strained semiconductor active region using edge relaxation, a buried stressor layer and a sacrificial stressor layer
The present invention relates to creating an active layer of strained semiconductor using a combination of buried and sacrificial stressors. That is, a process can strain an active semiconductor layer by transferring strain from a stressor layer buried below the active semiconductor layer and by transferring strain from a sacrificial stressor layer formed above the active semiconductor layer. As an example, the substrate may be silicon, the buried stressor layer may be silicon germanium, the active semiconductor layer may be silicon and the sacrificial stressor layer may be silicon germanium. Elastic edge relaxation is preferably used to efficiently transfer strain to the active layer.
1. A method of forming a semiconductor device, comprising:
providing a substrate;
forming a buried stressor layer over the substrate and forming an active layer on the buried stressor layer, the buried stressor layer between the substrate and the active layer and formed to have lateral in-plane stress at a surface of the buried stressor layer adjacent the active layer when the active layer is formed;
forming trenches in the substrate and filling the trenches with trench fill material;
forming a sacrificial stressor layer over the active layer, the sacrificial stressor layer formed to have lateral in-plane stress when the sacrificial stressor layer is formed;
causing the trench fill material to go from a compliant state to a comparatively stiffer state, such that strain is subsequently retained in the active layer by the trench fill material on either side of the active layer; and
removing the sacrificial stressor layer.
2. The method of claim 1 , wherein the causing includes heating the trench fill material to become more fluid and subsequently cooling the trench fill material to become more solid.
3. The method of claim 1 , wherein the trench fill material filling one of the trenches comprises silicon dioxide comprising boron or phosphorus impurities.
4. The method of claim 1 , wherein the causing anneals the trench fill material in an annealing gas ambient so that the trench fill material becomes stiffer.
5. The method of claim 1 , wherein the trench fill material filling one of the trenches comprises silicon dioxide formed from a chemical reaction.
6. The method of claim 1 , wherein the trench fill material filling one of the trenches is deposited using a TEOS source, and wherein the causing anneals the trench fill material.
7. The method of claim 1 , wherein the buried stressor layer is formed after forming trenches.
8. The method of claim 1 , wherein the buried stressor layer comprises a silicon germanium layer and the active layer comprises silicon.
9. The method of claim 8 , wherein the sacrificial stressor layer comprises a silicon germanium layer.
10. The method of claim 9 , the method further comprising determining a maximum combined thickness for the buried stressor layer and the sacrificial stressor layer.
11. The method of claim 8 , wherein the sacrificial stressor layer comprises silicon nitride.
12. The method of claim 1 , wherein the sacrificial stressor layer comprises silicon nitride.
13. The method claim 1 , wherein the strain retained in the active layer is tensile strain.