1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor device, and more particularly, to a manufacturing method of a semiconductor device including transistors having different threshold voltages.
2. Description of the Prior Art
The development of semiconductor integrated circuit technology progresses continuously and circuit designs in products of the new generation become smaller and more complicated than those of the former generation. The amount and the density of the functional devices in each chip region are increased constantly according to the requirements of innovated products, and the size of each device has to become smaller accordingly.
Poly-silicon is conventionally used as a gate electrode in semiconductor devices, such as the metal-oxide-semiconductor (MOS). With the trend towards scaling down the size of semiconductor devices, however, conventional poly-silicon gates face problems such as inferior performance due to boron penetration and unavoidable depletion effects. This increases equivalent thickness of the gate dielectric layer, reduces gate capacitance and worsens a driving force of the devices. Therefore, work function metals that are suitable for use as the high-k gate dielectric layer are used to replace the conventional poly-silicon gate to be the control electrode. However, transistors having different conductivity types and/or different threshold voltages are required in integrated circuits, and gate stacks of these transistors may be different from one another for presenting different conductivity types and/or different threshold voltages. Accordingly, for the related industries, it is important to integrate manufacturing processes of these gate stacks efficiently.