Method of tuning threshold voltages of transistors

A structure comprising:

• a semiconductor substrate comprising a first semiconductor region and a second semiconductor region;
• a first transistor comprising:
• a first gate dielectric over the first semiconductor region;
• a first work function layer over and contacting the first gate dielectric;
• a second work function layer over and contacting the first work function layer;
• and
• a first conductive region over the second work function layer;
• and
• a second transistor comprising:
• a second gate dielectric over the second semiconductor region;
• a third work function layer over and contacting the second gate dielectric, wherein the first work function layer and the third work function layer have a same work function and
• a fourth work function layer over and contacting the third work function layer, wherein the third work function layer and the fourth work function layer are formed of different materials, and have a same conductivity type as the first work function layer and the third work function layer;
• and
• a second conductive region over the fourth work function layer.

A structure comprising:

• a bulk semiconductor substrate;
• a first Fin Field-Effect Transistor (FinFET) comprising:
• a first semiconductor fin protruding higher than the bulk semiconductor substrate;
• a first high-k dielectric layer on first sidewalls and a first top surface of the first semiconductor fin;
• a first work function layer over and contacting the first high-k dielectric layer;
• a second work function layer over and contacting the first work function layer;
• and
• a first glue layer over the second work function layer;
• and
• a second FinFET comprising:
• a second semiconductor fin protruding higher than the bulk semiconductor substrate;
• a second high-k dielectric layer on second sidewalls and a second top surface of the second semiconductor fin;
• a third work function layer over and contacting the second high-k dielectric layer, wherein the first work function layer and the third work function layer are formed of a same material and have a first conductivity type;
• a fourth work function layer over and contacting the third work function layer, wherein the second work function layer and the fourth work function layer are formed of materials different from each other, and are of the first conductivity type;
• and
• a second glue layer over the fourth work function layer.