Epitaxial process

1. Field of the Invention

The present invention relates generally to an epitaxial process, and more specifically to an epitaxial process forming spacers at a same time, which are further paired with hard masks to form epitaxial structures.

2. Description of the Prior Art

For decades, chip manufacturers have developed faster metal-oxide-semiconductor (MOS) transistors by making them smaller. As the semiconductor processes advance to very deep sub micron technologies, such as 65-nm node or beyond, how to increase the driving current for MOS transistors has become a critical issue. In order to improve the device performances, crystal strain technology has been developed. Crystal strain technology is becoming more and more attractive as a means for getting better performances in the field of MOS transistor fabrication. Putting a strain on a semiconductor crystal alters the speed at which charges move through that crystal. Strains make the MOS transistors work better by enabling electrical charges, such as electrons, to pass more easily through the silicon lattice of the gate channel. In the known arts, attempts have been made to use a strained silicon layer, which has been grown epitaxially on a silicon substrate with a silicon germanium (SiGe) structure or a silicon carbide (SiC) structure disposed in between. In this type of MOS transistor, a biaxial tensile strain is induced in the epitaxy silicon layer due to the difference of size between the lattice constants of silicon germanium or silicon carbide and silicon. As a result, the band structure is altered, and the carrier mobility is increased. This enhances the speed performances of the MOS transistors. Furthermore, forming epitaxial structures having good electrical performances in a plurality of transistors respectively has become an important issue, especially when forming epitaxial structures with different electrical types.

The present invention provides an epitaxial process that simultaneously forms spacers on a substrate beside two gates, which are then paired with hard masks to form epitaxial structures having good performances beside the two spacers respectively.

The present invention provides an epitaxial process including the following steps. A substrate including a first area and a second area is provided. A first gate and a second gate are formed on the substrate of the first area and the second area respectively. A first spacer and a second spacer are respectively formed on the substrate beside the first gate and the second gate at the same time. A first epitaxial structure is formed beside the first spacer and then a second epitaxial structure is formed beside the second spacer by the first spacer and the second spacer defining the locations of the first epitaxial structure and the second epitaxial structure respectively.

The present invention provides an epitaxial process including the following steps. A first hard mask is deposited on a second area of a substrate but exposing a first area of the substrate completely. A first epitaxial structure is formed in the exposed first area. A second hard mask is deposited on the substrate of the first area and on the first hard mask of the second area. The second hard mask and the first hard mask in the second area are removed. A second epitaxial structure is formed in the second area. The second hard mask located in the first area is removed.

According to the above, the present invention provides an epitaxial process, which forms a first spacer and a second spacer on a substrate beside a first gate and a second gate respectively, and then sequentially forms a first hard mask and a second hard mask to expose a first area and a second area respectively, so that a first epitaxial structure and a second epitaxial structure can be formed in the first area and the second area respectively. This way, the present invention just needs to perform a wet etching process only once to form the first epitaxial structure and the second epitaxial structure. Compared to performing wet etching processes many times, the first epitaxial structure and the second epitaxial structure formed by the present invention can have improved structures, show less damages of the already formed parts, such as cap layers, simplifications the process and increases the processing throughput.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

FIGS. 1-11 schematically depict cross-sectional views of an epitaxial process according to an embodiment of the present invention.

FIGS. 1-11 schematically depict cross-sectional views of an epitaxial process according to an embodiment of the present invention. A gate-last for high-K last process is applied in this embodiment, but the present invention can also use a gate-last for high-K first process, a gate-first process or a polysilicon gate process etc.

As shown in FIG. 1 , a substrate 110 is provided. The substrate 110 may be a semiconductor substrate such as a silicon substrate, a silicon containing substrate, a III-V group-on-silicon (such as GaN-on-silicon) substrate, a graphene-on-silicon substrate or a silicon-on-insulator (SOI) substrate. The substrate 110 includes a first area A and a second area B. In this embodiment, the first area A is a PMOS transistor area and the second area B is an NMOS transistor area, but not limited thereto. In this embodiment, two first gates G 1 and two second gates G 2 are respectively formed in the first area A and the second area B, wherein each of the first gates G 1 and the second gates G 2 includes a dielectric layer 122 , an electrode layer 124 and a cap layer 126 from bottom to top. The dielectric layer 122 may be an oxide layer, which may be formed through a thermal oxide process or a chemical oxide process etc; the electrode layer 124 may be a silicon layer, and the electrode layer 124 is a sacrificial electrode layer, which will be replaced by a metal gate through a metal gate replacement process due to a gate-last for high-K last process being applied in this embodiment; the cap layer 126 may be a single layer or a dual layer, which may be formed through a nitride layer or/and an oxide layer, but it is not limited thereto. A liner (not shown) may be selectively formed on the substrate 110 beside each of the first gates G 1 and the second gates G 2 , wherein the liner (not shown) may be a nitride layer, but not limited thereto. In this embodiment, two first gates G 1 and two second gates G 2 are respectively formed in the first area A and the second area B for simplifying and clarifying the present invention, so that later formed epitaxial structures can be depicted completely. In another embodiment, the number of first gates G 1 and second gates G 2 may range from one to more than two.

Please refer to FIG. 1 ; two spacers 132 are formed on the substrate 110 beside the first gates G 1 and the second gates G 2 respectively to define the positions of lightly doped source/drain regions located in the substrate 110 beside the spacers 132 . Two lightly doped source/drain regions 134 are respectively formed in the substrate 110 beside the two spacers 132 , wherein the dopants of the lightly doped source/drain regions 134 depend upon the electrical types. For example, the first area A may be doped with trivalent ions such as boron, and the second area B may be doped with pentavalent ions such as phosphorous.

As shown in FIGS. 2-3 , a first spacer 142 a and a second spacer 142 b are formed on the substrate 110 beside the first gates G 1 and the second gates G 2 respectively. More precisely, a spacer material 142 is entirely deposited on the first gates G 1 , the second gates G 2 and the substrate 110 . The spacer material 142 may be a single layer or a dual layer, which may be composed of a nitride layer or/and an oxide layer etc. In this embodiment, the spacer material 142 may be a nitride layer, which may be formed from a precursor of hexachlorosilane (HCD). An etching process P 1 is performed to pattern the spacer material 142 to form the first spacer 142 a and the second spacer 142 b on the substrate 110 beside the first gates G 1 and the second gates G 2 respectively at the same time. In a preferred embodiment, the etching process P 1 is a dry etching process.

As shown in FIG. 4-5 , a patterned first hard mask 150 a is formed on the substrate 110 in the second area B but the substrate 110 in the first area A is exposed completely. More precisely, as shown in FIG. 4 , a first hard mask layer 150 is entirely deposited on the substrate 110 , the first gates G 1 and the second gates G 2 in the first area A and the second area B. As shown in FIG. 5 , a patterned photoresist K 1 is formed to cover the second area B. The patterned photoresist K 1 may be formed by patterning a blanketly formed photoresist (not shown), but it is not limited thereto. An etching process P 2 is performed to remove the first hard mask layer 150 in the first area A but keeping the patterned first hard mask 150 a in the second area B, wherein the material of the first hard mask 150 may be a thin oxide layer or a thin nitride layer, and its thickness may be merely 30 angstroms, so as to be removed more easily later, but still suited for being a hard mask while rendering a part of the substrate 110 to be removed to form recesses. The etching process P 2 may be a dry etching process or a wet etching process etc. In a preferred embodiment, the etching process P 2 is a dry etching process, so as to have a better etched profile.

As shown in FIGS. 6-7 , a first epitaxial structure 160 is formed beside the first spacer 142 a . In this embodiment, the first epitaxial structure 160 is formed in the substrate 110 beside the first spacer 142 a . More precisely, as shown in FIG. 6 , an etching process P 3 is performed to etch the substrate 110 beside the first spacer 142 a to form a recess R 1 in the substrate 110 of the first area A. Since the first hard mask 150 a covers the second area B, the substrate 110 in the second area B will not be etched. Then, the patterned photoresist K 1 is removed in the second area B while the first hard mask 150 a in the second area B is maintained. As shown in FIG. 7 , the first epitaxial structure 160 is formed in the substrate 110 beside the first spacer 142 a . In this embodiment, since the first area A is a PMOS transistor area, the first epitaxial structure 160 is an epitaxial structure suited for being used in a PMOS transistor, such as a silicon germanium epitaxial structure. In another embodiment, the first epitaxial structure 160 may be directly formed on the substrate 110 beside the first spacer 142 a without etching the substrate 110 . Moreover, the high temperature during the formation of the first epitaxial structure 160 can repair the lattice structure damaged while performing the etching process P 2 or while etching the recess R 1 etc.

As shown in FIG. 8 , a second hard mask layer 170 is deposited on the first epitaxial structure 160 , the substrate 110 and the first gates G 1 in the first area A, and on the patterned first hard mask 150 a in the second area B. The second hard mask layer 170 may be a nitride layer or an oxide layer, but it is not limited thereto. In a preferred embodiment, the second hard mask 170 is a nitride layer, so as to be removed easily and to reduce the damages in the first epitaxial structure 160 below due to the removing of the second hard mask layer 170 .

As shown in FIG. 9 , a patterned photoresist K 2 is formed to cover the first area A, which may be obtained by patterning a blanketly formed photoresist (not shown), but it is not limited thereto. An etching process P 4 is performed to remove a part of the second hard mask layer 170 and the first hard mask 150 a in the second area B but still maintaining a patterned second hard mask 170 a in the first area A. In a preferred embodiment, the etching process P 4 is a dry etching process for having better etched profile. Then, the patterned photoresist K 2 is removed. Thus, as shown in FIG. 10 , a second epitaxial structure 180 is formed on the substrate 110 in the second area B. In this embodiment, the second epitaxial structure 180 is directly formed on the substrate 110 beside the second spacer 142 b without etching the substrate 110 . Moreover, since the second area B is an NMOS transistor area, the second epitaxial structure 180 is a silicon phosphorous epitaxial structure, especially suited for being used in semiconductor processes lower than 20 nanometers. In another embodiment, the second epitaxial structure 180 may be a silicon carbide epitaxial structure, suited for being used in an NMOS transistor, but it is not limited thereto. In this embodiment, the silicon phosphorous epitaxial structure is directly formed on the substrate 110 ; in another embodiment, other epitaxial structures may be formed in the substrate 110 beside the second spacer 142 b . In other words, an etching process may be performed to form a recess in the substrate 110 beside the second spacer 142 b , and then the second epitaxial structure is formed into the recess. This depends upon the practical needs. Furthermore, the high temperature during the formation of the second epitaxial structure 180 can repair the lattice structure damaged while performing the etching process P 4 or while etching the recess etc.

The patterned second hard mask 170 a of the first area A is removed, and the first epitaxial structure 160 in the first area A is therefore exposed as shown in FIG. 11 . The patterned second hard mask 170 a in this embodiment is a nitride layer, so that it can be removed by a hot phosphoric acid containing wet etching process, but it is not limited thereto. This means that the process for forming the first epitaxial structure 160 in the first area A and forming the second epitaxial structure 180 in the second area B is completed.

According to the above, the present invention just needs the hot phosphoric acid containing wet etching process to be performed only once to form the first epitaxial structure 160 and the second epitaxial structure 180 with different electrical types in the first area A and the second area B respectively. Nowadays epitaxial processes still need to perform the hot phosphoric acid containing wet etching processes many times to define and form the first epitaxial structure 160 and the second epitaxial structure 180 respectively. Compared to the previous processes, the hot phosphoric acid containing wet etching process only needs to be performed once in the present invention, thereby reducing the damages of the first epitaxial structure 160 and the second epitaxial structure 180 caused by these etching processes, the structural integrity of the first epitaxial structure 160 and the second epitaxial structure 180 can be maintained, and good electrical performance are achieved. Besides, the damages in the formed parts, such as the cap layer 126 , the first spacer 142 a and the second spacer 142 b , caused by performing these etching processes many times, would expose the electrode layer 124 , leading to degrading the qualities of the metal gates formed in a later performed metal gate replacement process, and potential circuit leakages can be reduced in the present invention. Furthermore, the first spacer 142 a and the second spacer 142 b formed simultaneously in the present invention can simplify the process and increase the processing throughput.

Thereafter, main spacers (not shown) may be selectively formed on the substrate 110 beside the first spacer 142 a and the second spacer 142 b . An ion implantation process may be performed to form two source/drain regions 190 in the substrate 110 beside the main spacers (not shown) in the first area A and the second area B respectively. The dopants of the source/drain regions 190 may be trivalent ions or pentavalent ions such as boron or phosphorous etc, depending upon the electrical types of the formed semiconductor components and the practical needs. A salicide process may be performed to form a metal silicide (not shown) on the source/drain regions 190 now or after contact hole formation. An interdielectric layer is formed and planarized to entirely cover the substrate 110 and remove the cap layer 126 , and the electrode layer 124 is therefore exposed. A metal gate replacement process is performed to remove the electrode layer 124 and the dielectric layer 122 , and then a dielectric layer having a high dielectric constant, a barrier layer, a work function metal layer and a low resistivity material etc are filled to replace the first gates G 1 and the second gates G 2 by metal gates. Contact plugs may be formed in the interdielectric layer and electrically connect the source/drain regions 190 . An upper interconnect structures may be formed. And, other semiconductor processes may be performed so on.

To summarize, the present invention provides an epitaxial process, which forms a first spacer and a second spacer on the substrate beside a first gate and a second gate respectively, and then sequentially forms a first hard mask and a second hard mask to expose a first area and a second area respectively, so that a first epitaxial structure and a second epitaxial structure can be formed in the first area and the second area respectively. This way, the present invention just needs a wet etching process to be performed only once to form the first epitaxial structure and the second epitaxial structure. Compared to performing many wet etching processes, the first epitaxial structure and the second epitaxial structure formed by the present invention can have improved structures (especially for silicon germanium epitaxial structures with high concentration, which are easily etched by wet etching processes with etchants of hot phosphoric acid). Besides, the damages in the formed parts, such as the cap layer, the first spacer and the second spacer, caused by performing theses etching processes, that would expose the electrode layer 124 , leading to degrading the qualities of the metal gates formed in a later performed metal gate replacement process and circuit leakages may occur, can be reduced in the present invention. Furthermore, the first spacer and the second spacer formed simultaneously in the present invention can simplify the process and increase the processing throughput.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.