Semiconductor device and method for forming the same

A method, comprising:

• forming first semiconductor layers vertically stacked over a first region of a substrate and second semiconductor layers vertically stacked over a second region of the substrate, wherein a bottommost one of the first semiconductor layers is higher than a bottommost one of the second semiconductor layers;
• forming a first gate structure over the first semiconductor layers and a second gate structure over the second semiconductor layers;
• etching, by using the first and second gate structures as etch masks, the first semiconductor layers and the second semiconductor layers to form a first recess within the first semiconductor layers and a second recess within the second semiconductor layers;
• forming a first epitaxy layer in the first recess and a second epitaxy layer in the—second—recess, wherein the first epitaxy layer is thicker than the second epitaxy layer, comprising:
• performing a first deposition process to form a first layer of the first epitaxy layer and the second epitaxy layer;
• and
• performing a second deposition process to form a second layer of the first epitaxy layer over the first layer of the first epitaxy layer, and
• forming a third epitaxy layer over the first epitaxy layer and a fourth epitaxy layer over the second epitaxy layer.

A method, comprising:

• forming a semiconductor material over a first region and a second region of a substrate;
• removing a portion of the semiconductor material from the second region of the substrate;
• after removing the portion of the semiconductor material, forming first semiconductor layers vertically stacked over the first region of the substrate and second semiconductor layers vertically stacked over the second region of the substrate, wherein a number of the first semiconductor layers is different from a number of the second semiconductor layers;
• forming a first gate structure over the first semiconductor layers and a second gate structure over the second semiconductor layers;
• etching, by using the first and second gate structures as etch masks, the first semiconductor layers and the second semiconductor layers to form a first recess within the first semiconductor layers and a second recess within the second semiconductor layers;
• forming a first epitaxy layer in the first recess and a second epitaxy layer in the—second—recess, wherein the first epitaxy layer is thicker than the second epitaxy layer;
• and
• forming a third epitaxy layer over the first epitaxy layer and a fourth epitaxy layer over the second epitaxy layer, wherein an interface between the first epitaxy layer and the third epitaxy layer and an interface between the second epitaxy layer and the fourth epitaxy layer are at different levels.