1. Field of the Invention
The present invention relates to a semiconductor process, and more particularly, to a semiconductor process, which forms a cap layer with a material different from a hard mask layer, covers two gates with a material layer, and back etches the material and the gates to make the two gates have the same height.
2. Description of the Prior Art
For decades, chip manufacturers have developed more and more small metal-oxide-semiconductor (MOS) transistors to make them faster. As the semiconductor processes advance to very deep sub micron era, such as 65-nm node or beyond, increasing the driving current for MOS transistors has become a critical issue. In order to improve device performance, crystal strain technology has been developed. Crystal strain technology has become more and more attractive as a mean for obtaining better performances in the field of CMOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strain makes CMOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel.
An epitaxial layer is often formed in a substrate beside a gate for putting a strain on a semiconductor crystal. In a first step for forming the epitaxial layer, a first spacer is formed to define the position of the epitaxial layer, then the substrate is etched to form a recess, and the epitaxial layer is formed in the recess. After the epitaxial layer is formed, the first spacer needs to be removed to form a second spacer, which defines the position of a source/drain in the substrate beside the gate.
For a complementary metal-oxide semiconductor (CMOS) or a static random access memory (SRAM), an NMOS transistor and a PMOS transistor are located on both sides, and the materials and the forming methods of the epitaxial layers of the NMOS transistor and the PMOS transistor are different. For example, a silicon germanium is suited for forming in the substrate beside a gate of the PMOS transistor but the silicon germanium is not suited for forming in the substrate beside a gate of the NMOS transistor. Furthermore, a silicon carbide is suited for forming in the substrate beside a gate of the NMOS transistor. As the silicon germanium is formed in the substrate beside the gate of the PMOS transistor, a lithography process is performed on the PMOS transistor to form a first spacer and a recess, and the first spacer is removed after the epitaxial layer is formed. The cap layer of the PMOS transistor is etched during the step of forming the silicon germanium epitaxial layer, without the cap layer of the NMOS transistor being etched. Therefore, thicknesses of the gates of the NMOS transistor and the PMOS transistor are different. Moreover, if the cap layer of the PMOS transistor is over-etched, the gate layer below the cap layer may be exposed, or the first spacer may not be removed completely.
The modern methods of solving the problem of the thickness difference between the gate of the NMOS transistor and the gate of the PMOS transistor in a static random access memory (SRAM) may be described as following. A lithography process is additionally performed to thin the cap layer of the NMOS transistor, but the thinning process is complex and the photoresists formed during the two lithography processes (respectively performed on the PMOS transistor and the NMOS transistor) would induce misalignment at the boundary between the NMOS transistor and the PMOS transistor, resulting in the cap layer at the boundary being over-etched or not enough etched, hence degrading the performance of the static random access memory (SRAM).
Therefore, a semiconductor process is especially needed in modern industries to solve problems such as thickness difference between gates of two transistors, spacer residues, and exposed gate layers.