Method for making source and drain regions of a MOSFET with embedded germanium-containing layers having different germanium concentration

A method comprising:

• forming a gate stack over a substrate;
• forming a source/drain region adjacent to the gate stack, the source/drain region extending from a top surface of the substrate into the substrate, the forming the source/drain region comprising;
• performing a first epitaxy to grow a first silicon germanium layer, the first silicon germanium layer having a first germanium percentage;
• increasing a flow rate ratio of a flow rate of germanium-containing precursors to a flow rate of silicon-containing precursors;
• performing a second epitaxy with the increased flow rate ratio to grow a second silicon germanium layer over the first silicon germanium layer, the second silicon germanium layer having a second germanium percentage higher than the first germanium percentage;
• reducing the flow rate ratio of a flow rate of germanium-containing precursors to a flow rate of silicon-containing precursors;
• and
• performing a third epitaxy with the reduced flow rate ratio to grow a third silicon germanium layer over the second silicon germanium layer, the third silicon germanium layer having a third germanium percentage lower than the second germanium percentage, the third germanium percentage continuously increasing along a bottom-to-top direction.

A method comprising:

• forming a gate stack over a substrate;
• forming an opening extending into the substrate, the opening being adjacent a side of the gate stack;
• epitaxially growing a first silicon germanium layer in the opening, the first silicon germanium layer having a first germanium percentage;
• epitaxially growing a second silicon germanium layer over the first silicon germanium layer, at an interface of the first silicon germanium layer and the second silicon germanium layer, there is an abrupt increase from the first germanium percentage to a second germanium percentage;
• epitaxially growing a third silicon germanium layer over the second silicon germanium layer, at an interface of the second silicon germanium layer and the third silicon germanium layer, there is an abrupt decrease from the second germanium percentage to a third germanium percentage, wherein a higher portion of the third silicon germanium layer has a germanium percentage higher than a germanium percentage in a lower portion of the third silicon germanium layer;
• forming a silicon cap over and contacting the third silicon germanium layer;
• and
• performing a silicidation to silicide the silicon cap.