Manufacturing method for semiconductor structure

[0001] 1. Field of the Invention

[0002] The invention relates to a manufacturing method for a semiconductor structure, and more particularly, to a manufacturing method for a semiconductor structure applied with strained-silicon technique.

[0003] 2. Description of the Prior Art

[0004] With semiconductor processes entering the era of the deep submicron meter below 65 nanometer (nm), it has been more and more important to increase the metal-oxide semiconductor (MOS) drive current. To improve device performance, strained-silicon technique such as selective epitaxial growth (SEG) method is developed to form epitaxial layers serving as the source/drain of the MOS. Because a lattice constant of the epitaxial layer is different from that of silicon, such characteristic is employed to cause alteration to the band structure of the silicon in the channel region. Accordingly, carrier mobility of the channel region is enhanced and thus device performance is improved.

[0005] Please refer to FIG. 1, which is a schematic drawing illustrating a semiconductor structure applied with the conventional SEG method. As shown in FIG. 1, a semiconductor device 150 is positioned on a substrate 100. The semiconductor structure 150 includes a gate 110 and a gate dielectric layer 112. A spacer 114 is formed on the sidewalls of the gate 110 and the gate dielectric layer 112, lightly-doped drains (LDDs) (not shown) are formed in the substrate 100 at two sides of the gate 110, and recesses 120 are respectively formed in the substrate 100 at two sides of the spacer 114. The recess 120 includes an epitaxial layer 112 formed therein. The epitaxial layer 112 is formed by the SEG method and serves as the source/drain by performing ion implantation before or after the SEG method. As shown in FIG. 1, a channel region 130 is formed between the source/drain and underneath the gate 110. The epitaxial layers 122 formed in the source/drain region render compressive or tensile stress to the channel region 130 and thus the carrier mobility in the channel region 130 is enhanced.

[0006] However, as size of the semiconductor structure keeps shrinking, the stress provided by the epitaxial layer 122 is more and more susceptible to shapes, configuration, and material choice of the epitaxial layer 122. Furthermore, it is well-known that the epitaxial layer 122 is formed along the surface of the recess 120 during the SEG method. Therefore shapes and crystalline orientation of each surface of the recess 120 also render impacts to the epitaxial layer 122. For example, the recess 120 of the conventional semiconductor device 150 typically includes a V shape, therefore the epitaxial layer 122 formed along the surfaces of the recesses 120 obtains a V-shaped pointed end (as emphasized by circle A). Moreover, it is found that device leakage always occurs at the pointed end.

[0007] Therefore, there is still a need for a manufacturing method for a semiconductor structure that is able to form the recesses having the ideal shape, which is beneficial to form the epitaxial layer having a proper shape that improves effective stress to the channel region.

[0008] According to an aspect of the present invention, there is provided a manufacturing method for a semiconductor structure. The manufacturing method includes providing a substrate having at least a gate structure formed thereon, performing a first wet etching process to etch the substrate at two sides of the gate structure, performing a second wet etching process to etch the substrate to form a recess respectively at the two sides of the gate structure, and performing a SEG method to form an epitaxial layer having a diamond shape with a flat bottom respectively in the recesses.

[0009] According to another aspect of the present invention, there is also provided a manufacturing method for a semiconductor structure. The manufacturing method includes providing a substrate having at least a gate structure formed thereon, performing a tetra methyl ammonium hydroxide (TMAH) wet etching process to etch the substrate at two sides of the gate structure with a TMAH solution, and performing a base wet etching process to etch the substrate to form a recess respectively at two sides of the gate structure. The base wet etching process includes an etching ratio of (100) plane orientation to (111) orientation of the substrate, and the etching ratio is between 1 and 2.

[0010] According to the manufacturing method for a semiconductor structure provided by the present invention, two-stepped etching process is performed to form the recesses at two sides of the gate structure: First, a first wet etching process including TMAH solution is performed to form pre-recesses in the substrate at the two sides of the gate structure. Then, a second wet etching process including a bas solution is performed to etch the bottom and sidewalls of the pre-recesses to form the recesses. Because the base solution used in the second wet etching process has the etching ratio of (100) plane orientation to (111) orientation of the substrate, and the etching ratio is between 1 and 2, the obtained recess is formed to have a diamond shape. Furthermore, the diamond-shaped recess includes a flat bottom. It is noteworthy that the diamond-shaped recess has at least a pointed end directing toward the channel region, therefore the epitaxial layer formed along the surface of the recess obtains a pointed end toward the channel region. Accordingly, effective stress provided by the epitaxial layer to the channel region is enhanced. On the other hand, the epitaxial layer formed along the flat bottom of the recess obtains a flat bottom, consequently. Therefore, device leakage that used to occur at the pointed end is avoided.

[0011] 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.

[0014] Please refer to FIGS. 2-5, which are drawings illustrating a manufacturing method for a semiconductor structure provided by a preferred embodiment of the present invention. As shown in FIG. 2, the preferred embodiment first provides a substrate 200. The substrate 200 includes a gate structure 220 formed thereon, and the gate structure 220 includes a gate dielectric layer 212, a gate 210, and a cap layer 214 sequentially and upwardly stacked on the substrate 200. It is well-known to those skilled in the art that the cap layer 214 is formed to cover the gate 210 to protect the gate 210 from damage that may be caused in any process such as photolithograph process, ion implantation, etching process, or any needed cleaning process in the semiconductor fabricating process. LDDs 216 are formed in the substrate 200 at two sides of the gate 210 and the gate dielectric layer 212 of the gate structure 220. A spacer 218 is formed on sidewalls of the gate 210 and the gate dielectric layer 212. As shown in FIG. 2, the spacer 218 preferably is a multi-layered structure including an L-shaped seal layer 218a and an insulating layer 218b covering the seal layer 218a. The spacer 218 formed on the sidewalls of the gate 210 and the gate dielectric layer 212 after forming the LDDs 216 is used to protect the sidewalls of the gate 210 and the gate dielectric layer 212 and to define positions for forming the source/drain.

[0015] Please refer to FIG. 3. Next, a first wet etching process 230 is performed to etch the substrate 200 at two sides of the gate structure 220 with a tetra methyl ammonium hydroxide ((CH₃)₄NOH, TMAH) solution. In other words, the first wet etching process 230 is a TMAH wet etching process. In the preferred embodiment, a concentration of TMAH in the TMAH solution is lower than 5%, and a concentration of water (H₂O) is higher than 95%. The first wet etching process 230 is performed at a temperature of about 70° C., but not limited to this. More important, a performance duration of the first wet etching process 230 is between 20 seconds and 30 seconds according to the preferred embodiment. During forming the LDDs 216, the cap layer 214 and the gate 210 serve as an implantation mask, and p-type or n-type dopants, such as phosphorus (P), boron (B), or arsenic (As), required by p-type or n-type semiconductor structure are implanted into the substrate 200 to form the LDDs 216. It is noteworthy that dopants implanted into the substrate 200 change a wet etching rate of the substrate 200. Therefore the first wet etching process 230 is performed with the TMAH solution to etch LDDs 216 at the two sides of the gate structure 220 to breakthrough the LDDs 216 to its bottom, according to the preferred embodiment. As shown in FIG. 3, the first wet etching process 230 is performed to form a pre-recess 232 in the substrate 200 respectively at the two sides of the gate structure 220. Furthermore, the pre-recess 232 is formed at where the source/drain is to be formed. It is also noteworthy that a sidewall of the pre-recess 232 formed by the first wet etching process 230 has an included angle B with the surface of the substrate 200, and the included angle B is substantially 54.7 degrees)(°).

[0016] Please refer to FIG. 4. Then, a second wet etching process 240 is performed to etch the substrate 200 at the two sides of the gate structure 220 with a base solution. The second wet etching process 240 includes a base solution, the base solution includes an etching ratio of (100) plane orientation to (111) orientation of the substrate 200, and the etching ratio is between 1 and 2. For example, the base solution includes ammonia (NH₄OH), hydrogen peroxide (H₂O₂), and H₂O, and a proportion of NH₄OH, H₂O₂, and H₂O is about 10:1:1200. Furthermore, the second wet etching process 240 is performed at room temperature. It is noteworthy that a performance duration of the second wet etching process 240 is longer than the performance duration of the first wet etching process 230. For example, the performance duration of the second wet etching process 240 is about 180 seconds. Since the second wet etching process 240 has the etching ratio of (100) plane orientation to (111) orientation of the substrate 200, which is between 1 and 2, the second wet etching process 240 vertically etches the bottom of the pre-recess 232 and laterally etches the sidewalls of the recess 232 to form a recess 242 in the substrate 200 at where used to be the pre-recess 232.

[0017] As shown in FIG. 4, the recess 242 formed by the second wet etching process 240 has a bottom surface 242a, an opening 242b, a first slanted sidewall 242c connecting to the opening 242b, and a second slanted sidewall 242d connecting to the first slanted sidewall 242c and the bottom surface 242a. Since the base solution used in the second wet etching process 240 has the etching ratio of (100) plane orientation to (111) orientation of the substrate 200, which is between 1 and 2, the recess 242 formed by the second wet etching process 240 obtains a flat bottom surface 242a, not a V-shaped bottom surface having the pointed end as the prior art. In addition, the included angle B formed by the first slanted sidewall 242c and the substrate 200 is still 54.7°. It is also noteworthy that the first slanted sidewall 242c and the second slanted sidewall 242d of the recess 242 formed by the second wet etching process 240 respectively has a (111) plane orientation while the bottom surface 242a of the recess 242 has a (100) plane orientation.

[0018] Please refer to FIG. 5. Next, a SEG method 250 is performed to form an epitaxial layer 252 in each recess 242, and the epitaxial layer 252 fills up the recess 242. It is well-known to those skilled in the art that in the SEG method 250, the epitaxial layer 252 is to grow along each surface of the recess 242. Therefore the epitaxial layer 252 is formed along the bottom surface 242a, the first slanted sidewall 242c, and the second slanted sidewall 242d of the recess 242 in the preferred embodiment. Consequently, a diamond-shaped epitaxial layer 252 is obtained as shown in FIG. 5. It is noteworthy that since the recess 242 includes the flat bottom surface 242a, the epitaxial layer 252 obtains a flat bottom accordingly. Furthermore, the epitaxial layer 252 can include a silicon germanium (SiGe) epitaxial layer or a silicon carbide (SiC) epitaxial layer required by p-type or n-type semiconductor structure. Additionally, the cap layer 214 is selectively removed and followed by performing a silicide process to form silicide (not shown) at least on the surface of the epitaxial layers 252.

[0019] In the prior art, a dry etching process has been used to form the recess, particularly, the dry etching process is used to breakthrough the LDDs. However, the dry etching process causes server lattice damage to the substrate 200. Therefore the epitaxial layer formed along the damaged lattice of the substrate 200 by the SEG method always has dislocation defects, which has adverse impact to the provided stress. Different from the prior art, the preferred embodiment provides twp-stepped etching process (the first wet etching process 230 and the second wet etching process 240) with introducing the first wet etching process 230 to breakthrough the LDDs 216. It is noticeable that the first wet etching process 230 renders no damages to the lattice of the substrate 200, therefore the dislocation defect is eliminated from the epitaxial layer 252, and thus the epitaxial layer 252 is able to provide effective stress. In the preferred embodiment, the feature that the second wet etching process 240 has the etching ratio of (100) plane orientation to (111) orientation of the substrate 200, which is between 1 and 2, is employed to form the recess 242 having the flat bottom surface 242a in the substrate 200. Accordingly, the epitaxial layer 252 formed along the flat bottom surface 242a obtains a diamond shape with a flat bottom. It is noteworthy that the epitaxial layer 252 obtains a pointed end as designated by the circle C along the first slanted sidewall 242c and the second slanted sidewall 242d of the recess 242, and the pointed end of epitaxial layer 252 provides tensile or compressive stress more effectively. Consequently, the carrier mobility in the channel region is improved. More important, the flat bottom of the epitaxial layer 252 avoids the device leakage that occurs when the epitaxial layer has the V-shaped pointed end. Accordingly, reliability of the semiconductor structure is improved.

[0020] According to the manufacturing method for a semiconductor structure provided by the present invention, two-stepped etching process is performed to form the recesses at two sides of the gate structure: First, a first wet etching process including TMAH solution is performed to form pre-recesses in the substrate at the two sides of the gate structure. Then, a second wet etching process including a bas solution is performed to etch the bottom and sidewalls of the pre-recesses to form the recesses. Because the base solution used in the second wet etching process has the etching ratio of (100) plane orientation to (111) orientation of the substrate, and the etching ratio is between 1 and 2, the obtained recess is formed to have a diamond shape. Furthermore, the diamond-shaped recess includes a flat bottom. It is noteworthy that the diamond-shaped recess has at least a pointed end directing toward the channel region, therefore the epitaxial layer formed along the surface of the recess obtains a pointed end toward the channel region. Accordingly, effective stress provided by the epitaxial layer to the channel region is enhanced. On the other hand, the epitaxial layer formed along the flat bottom of the recess obtains a flat bottom, consequently. Therefore, device leakage that used to occur at the pointed end is avoided. Briefly speaking, the manufacturing method provided by the present invention improves not only enhances the device performance but also the device reliability.

[0021] 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.