CN115188665A - Semiconductor structure and forming method thereof - Google Patents

Abstract

A semiconductor structure and a method of forming the same, wherein the method comprises: providing a substrate; forming a dielectric layer on the surface of the substrate; forming a first opening in the dielectric layer, a second opening in the dielectric layer, and an initial third opening in the substrate, wherein the second opening is located at the bottom of the first opening and is communicated with the first opening, the side wall of the second opening is recessed relative to the side wall of the initial third opening, and the bottom of the second opening exposes a part of the top surface of the substrate; etching the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening, and forming a third opening in the substrate; and forming epitaxial layers in the third opening, the second opening and the first opening, so that the defect of undercut caused by etching the substrate on the side wall of the initial third opening is avoided, and the performance of the formed device is improved.

Description

Semiconductor structure and method of forming the same

technical field

The present invention relates to the technical field of semiconductor manufacturing, and in particular, to a semiconductor structure and a method for forming the same.

Background technique

Silicon-based optoelectronic integration integrates microelectronics technology and photonics technology. It is the integration and development of microelectronics technology and an important frontier research field in the development of information technology. His research contents include silicon-based high-efficiency light sources, silicon-based high-speed photodetectors, silicon-based high-speed optical modulators, and low-loss optical waveguide devices. The epitaxial growth of germanium on silicon substrate is the preferred material for silicon-based high-speed long-wavelength photodetectors.

At present, the main processes for growing germanium materials on silicon substrates include processes for growing germanium on silicon patterned substrates. The method for growing germanium on a silicon pattern substrate comprises: preparing a silicon oxide film on the silicon substrate, then photolithographically etching the silicon oxide to expose a window for growing germanium, and germanium will selectively grow at the position of the silicon exposed by the window, and Lateral overgrowth merges on the silicon oxide surface to form a complete germanium epitaxial layer.

However, the process of growing germanium on the existing silicon pattern substrate needs to be further improved.

SUMMARY OF THE INVENTION

The technical problem solved by the present invention is to provide a semiconductor structure and a method for forming the same, so as to improve the performance of the formed semiconductor structure.

In order to solve the above technical problems, the technical solution of the present invention provides a semiconductor structure, comprising: a substrate; a dielectric layer located on the surface of the substrate; an epitaxial layer located in the dielectric layer and the substrate, the epitaxial layer comprising a first area, a second area adjacent to and on the first area, and a third area adjacent to and on the second area, the The first region is located in the substrate, the second region and the third region are located in the dielectric layer, the sidewall of the second region is protruded from the sidewall of the third region, the first region The sidewalls of the second region are flush with or protrude from the sidewalls of the first region.

Optionally, the material of the epitaxial layer includes germanium.

Optionally, the material of the dielectric layer includes silicon oxide.

The technical solution of the present invention also provides a method for forming a semiconductor structure, including: providing a substrate; forming a dielectric layer on the surface of the substrate; forming a first opening in the dielectric layer and a second opening in the dielectric layer , and an initial third opening located in the substrate, the second opening is located at the bottom of the first opening and communicates with the first opening, the sidewall of the second opening is recessed relative to the sidewall of the first opening, and the second opening The sidewall is recessed relative to the sidewall of the initial third opening, and the bottom of the second opening exposes the top surface of the substrate portion; the substrate top exposed to the inner wall of the initial third opening and the bottom of the second opening The surface is etched to form a third opening in the substrate; an epitaxial layer is formed in the third opening, the second opening and the first opening.

Optionally, before forming the dielectric layer, a sacrificial layer is formed on a surface of a part of the substrate, and the dielectric layer is located on the sidewalls and the top surface of the sacrificial layer.

Optionally, the method for forming the first opening, the second opening and the initial third opening includes: forming a mask layer on a partial surface of the dielectric layer; etching using the mask layer as a mask the dielectric layer until part of the sacrificial layer is exposed to form the first opening; the sacrificial layer and the substrate exposed by the first opening are etched using the mask layer as a mask, The initial third opening is formed in the substrate, the initial second opening is formed in the dielectric layer, and the sidewall of the initial second opening exposes the sacrificial layer; after the initial third opening is formed, The remaining sacrificial layer is removed to form the second opening.

Optionally, the formation process of the first opening includes one or a combination of a dry etching process and a wet etching process; the formation process of the initial third opening and the initial second opening It includes one or a combination of a dry etching process and a wet etching process.

Optionally, the process of removing the remaining sacrificial layer is an isotropic etching process.

Optionally, the material of the sacrificial layer is different from the material of the dielectric layer; the material of the sacrificial layer is different from the material of the substrate.

Optionally, the method for forming the first opening, the second opening and the initial third opening includes: etching the dielectric layer until part of the sacrificial layer is exposed, and forming in the dielectric layer a first opening; after the first opening is formed, the sacrificial layer is removed to form the second opening; after the second opening is formed, the substrate exposed at the bottom of the first opening is etched to The initial third opening is formed.

Optionally, the formation process of the first opening includes one or a combination of a dry etching process and a wet etching process.

Optionally, the process of removing the sacrificial layer is an isotropic etching process.

Optionally, a protective layer is located between the substrate and the dielectric layer.

Optionally, the material of the protective layer includes silicon oxide; the material of the sacrificial layer includes polysilicon.

Optionally, the method for forming the protective layer and the sacrificial layer includes: forming a gate oxide material layer on the substrate; forming a dummy gate material layer on the gate oxide material layer; forming a dummy gate material layer on the dummy gate A patterned layer is formed on the electrode material layer; using the patterned layer as a mask, the dummy gate material layer and the gate oxide material layer are etched to form the sacrificial layer and the protection layer.

Optionally, while forming the sacrificial layer and the protective layer, the gate oxide material layer is also etched to form a gate oxide layer, and the dummy gate material layer is also etched to form a dummy gate for MOS devices are formed in other regions.

Optionally, the process of etching the inner wall of the initial third opening and the top surface of the substrate exposed at the bottom of the second opening includes one or both of a dry etching process and a wet etching process. combination.

Optionally, the process of etching the inner wall of the initial third opening and the top surface of the substrate exposed at the bottom of the second opening is a dry etching process, and the dry etching process adopts an etching gas. Including hydrogen chloride.

Compared with the prior art, the technical solutions of the embodiments of the present invention have the following beneficial effects:

In the method for forming a semiconductor structure provided by the technical solution of the present invention, the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening are etched, and a third opening is formed in the substrate. In the etching process, because the second opening exposes the top surface of the substrate adjacent to the sidewall of the initial third opening to an etching solution or an etching gas, the initial third opening is also exposed to the etching solution or gas. When the substrate exposed by the sidewalls of the three openings is etched, the top surface of the substrate exposed at the bottom of the second opening is also etched, so as to avoid only the substrate with the sidewalls of the initial third opening being etched The "undercut" defect caused by etching, thereby improving the performance of the formed device.

Description of drawings

1 to 3 are schematic cross-sectional views of a semiconductor structure formation process;

4 to 9 are schematic structural diagrams of each step of a method for forming a semiconductor structure according to an embodiment of the present invention;

10 to 15 are schematic structural diagrams of each step of a method for forming a semiconductor structure in another embodiment of the present invention.

Detailed ways

It should be noted that "surface" and "upper" in this specification are used to describe the relative positional relationship in space, and are not limited to whether they are in direct contact.

As described in the background art, the performance of a semiconductor structure formed by using an existing process of growing germanium on a silicon pattern substrate needs to be improved. The description and analysis are now combined with the formation process of a semiconductor structure.

1 to 3 are schematic cross-sectional views of a process of forming a semiconductor structure.

Referring to FIG. 1 , a substrate 100 is provided; an oxide layer 101 is formed on the substrate 100 .

Referring to FIG. 2, the oxide layer 101 and the substrate 100 are etched to form openings 103 in the oxide layer 101 and the substrate 100; the surface of the substrate 100 on the bottom surface of the opening 103 is removed damage layer.

Referring to FIG. 3 , after removing the damaged layer, a germanium material layer 104 is formed in the opening 103 .

In the above method, the material of the substrate 100 is silicon, and the germanium material layer 104 is epitaxially grown on the surface of the substrate 100 at the bottom of the opening 103 . To prepare for the epitaxial growth of germanium on the silicon surface, it is necessary to remove the surface damage layer of the substrate 100 on the bottom surface of the opening 103. Usually, a HCl vapor phase etching and polishing process is used, which is conducive to the reaction of HCl gas and silicon at high temperature, and the Damaged layer removed. During the process, since the sidewalls of the openings 103 are exposed to the etching gas, the sidewalls of the openings 103 located in the substrate 100 will be etched laterally to form the sidewalls of the openings 103 . "Undercut" defect A of the wall (shown in Figure 3). The defect A affects the performance of the formed germanium photodetector, and the size and depth of the defect A are positively correlated with the dark current of the device. To eliminate the effects of "undercut" defects.

In an embodiment to solve the above problem, in the HCl vapor phase etching and polishing process, dichlorosilane (DCS) precursor gas is introduced to remove the damaged layer, and at the same time, crystalline silicon is deposited on the surface of the opening 103, so that silicon is deposited on the surface of the opening 103 The surface of the substrate 100 exposed by the opening 103 is epitaxially grown, so that the surface of the substrate 100 exposed by the opening 103 is repaired and the size of the "undercut" defect A is reduced. However, the method of introducing the precursor gas cannot completely eliminate the "undercut" defect A, and the epitaxial growth process has strict requirements on temperature, and excessive temperature will affect the selection of the substrate 100 surface for crystalline silicon growth If the temperature is too low, the quality of crystalline silicon will be affected.

In order to solve the above problem, in a method for forming a semiconductor structure provided by the present invention, the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening are etched, and the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening are etched, A third opening is formed. In the etching process, because the second opening exposes the top surface of the substrate adjacent to the sidewall of the initial third opening to an etching solution or an etching gas, the initial third opening is also exposed to the etching solution or gas. When the substrate exposed by the sidewalls of the three openings is etched, the top surface of the substrate exposed at the bottom of the second opening is also etched, so as to avoid only the substrate with the sidewalls of the initial third opening being etched The "undercut" defect caused by etching, thereby improving the performance of the formed device.

In order to make the above objects, features and beneficial effects of the present invention more clearly understood, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

4 to 7 are schematic structural diagrams of each step of a method for forming a semiconductor structure according to an embodiment of the present invention.

Referring to FIG. 4 , a substrate 200 is provided; a dielectric layer 201 is formed on the surface of the substrate 200 .

The material of the substrate 200 includes silicon.

The material of the dielectric layer 201 includes silicon oxide.

In this embodiment, a protective layer 203 is located between the substrate 200 and the dielectric layer 201 .

The material of the protective layer 203 includes silicon oxide.

In this embodiment, before forming the dielectric layer 201 , a sacrificial layer 202 is formed on a part of the surface of the substrate 200 , and the dielectric layer 201 is located on the sidewalls and the top surface of the sacrificial layer 202 .

The material of the sacrificial layer 202 includes polysilicon.

The sacrificial layer 202 is used to occupy space for the subsequent formation of the second opening, so the material of the sacrificial layer 202 can be other materials different from that of the dielectric layer 201, and in the etching process for forming the second opening , the sacrificial layer 202 has a larger etching selectivity ratio relative to the dielectric layer 201 to reduce the etching damage to the dielectric layer 201 caused by the etching process. In this embodiment, the material of the sacrificial layer 202 is polysilicon.

The method for forming the protective layer 203 and the sacrificial layer 202 includes: forming a gate oxide material layer (not shown in the figure) on the substrate 200; forming a dummy gate material layer on the gate oxide material layer (not marked in the figure); forming a patterned layer on the dummy gate material layer (not marked in the figure); using the patterned layer as a mask, etching the dummy gate material layer and all The gate oxide material layer is formed to form the sacrificial layer 202 and the protective layer 203 .

In another embodiment, when the sacrificial layer 202 and the protective layer 203 are formed, the gate oxide material layer is also etched to form a gate oxide layer (not shown in the figure), and the dummy gate material layer is formed. It is also etched to form dummy gates (not shown in the figure) for forming MOS devices in other regions. The sacrificial layer 202 and the dummy gate are formed at the same time, and the protective layer 203 and the gate oxide layer are formed at the same time, which saves the process and reduces the production cost.

Subsequently, a first opening in the dielectric layer, a second opening in the dielectric layer, and an initial third opening in the substrate are formed, and the second opening is located at the bottom of the first opening and communicated with the first opening , the sidewall of the second opening is recessed relative to the sidewall of the first opening, the sidewall of the second opening is recessed relative to the sidewall of the initial third opening, and the bottom of the second opening exposes the top surface of the substrate portion .

In this embodiment, for the formation method of the first opening, the second opening and the initial third opening, please refer to FIG. 5 to FIG. 7 . In other embodiments, the method for forming the first opening, the second opening and the initial third opening includes: forming a mask layer on a partial surface of the dielectric layer; using the mask layer as a mask to etch etching the dielectric layer until part of the sacrificial layer is exposed to form the first opening; etching the sacrificial layer and the substrate exposed by the first opening using the mask layer as a mask , the initial third opening is formed in the substrate, the initial second opening is formed in the dielectric layer, and the sidewall of the initial second opening exposes the sacrificial layer; after the initial third opening is formed , removing the remaining sacrificial layer to form the second opening.

Referring to FIG. 5 , the dielectric layer 201 is etched until a portion of the sacrificial layer 202 is exposed, and a first opening 205 is formed in the dielectric layer 201 .

In this embodiment, the method for forming the first opening 205 further includes: forming a mask layer 204 on a part of the surface of the dielectric layer 201 ; and etching the dielectric layer 201 by using the mask layer 204 as a mask until the Part of the sacrificial layer 202 is exposed to form the first opening 205 .

The material of the mask layer 204 is photoresist.

Referring to FIG. 6 , after the first opening 205 is formed, the sacrificial layer 202 is removed to form the second opening 206 .

The process of removing the sacrificial layer 202 is an isotropic etching process.

The process of removing the sacrificial layer 202 includes one or a combination of a dry etching process and a wet etching process. The process of removing the sacrificial layer 202 has a larger etching selectivity ratio of the sacrificial layer 202 to the dielectric layer 201, which is beneficial to protect the dielectric layer 201 from etching during the process of removing the sacrificial layer 202 damage.

In this embodiment, the process of removing the sacrificial layer 202 is a wet etching process. The wet etching process adopts potassium hydroxide solution. Since the potassium hydroxide solution has a larger selection ratio for polysilicon material and silicon oxide material, it is beneficial to remove the sacrificial layer 202 and make the dielectric layer 201 and the dielectric layer 201. The protection layer 203 is not damaged by etching, and the protection layer 203 further protects the substrate 200 from damage by etching.

Referring to FIG. 7 , after the second opening 206 is formed, the substrate 200 exposed at the bottom of the first opening 205 is etched to form the initial third opening 207 .

Referring to FIG. 8 , the inner wall of the initial third opening 207 (as shown in FIG. 7 ) and the top surface of the substrate 200 exposed from the bottom of the second opening 206 are etched to form a first Three openings 208 .

The process of etching the inner wall of the initial third opening 207 and the top surface of the substrate 200 exposed at the bottom of the second opening 206 includes one or both of a dry etching process and a wet etching process. combine.

The etching process is used to remove the surface damage layer of the substrate 200 exposed from the inner wall of the initial third opening 207 to prepare for the subsequent formation of an epitaxial layer in the third opening. Meanwhile, in the etching process, since the second opening 206 exposes the top surface of the substrate 200 adjacent to the sidewall of the initial third opening 207 to the etching solution or the etching gas, While the substrate exposed by the sidewalls of the initial third opening 207 is etched, the top surface of the substrate exposed at the bottom of the second opening 206 is also etched, so as to avoid only the initial first opening 206 being etched. The "undercut" defect caused by the etching of the substrate on the sidewalls of the three openings 207 further improves the performance of the formed device. In this embodiment, the top surface of the substrate exposed from the bottom of the second opening 206 is etched, and the protective layer 203 is also etched.

In this embodiment, the sidewalls of the formed third openings 208 are flush with the sidewalls of the second openings 206 . In other embodiments, the sidewall of the second opening 206 is recessed relative to the sidewall of the third opening 208 , and the shape of the third opening 208 is determined by etching conditions.

In this embodiment, the process of etching the inner wall of the initial third opening 207 and the top surface of the substrate 200 exposed at the bottom of the second opening 206 is a dry etching process, and the dry etching process adopts The etching gas includes hydrogen chloride.

Subsequently, an epitaxial layer is formed in the third opening 208 , the second opening 206 and the first opening 205 .

In this embodiment, before forming the epitaxial layer, the mask layer 204 is also removed.

Referring to FIG. 9 , an epitaxial layer 209 is formed in the third opening 208 , the second opening 206 and the first opening 205 .

The material of the epitaxial layer 209 includes germanium.

The substrate 200 exposed by the third opening 208 is used as a seed crystal, and the epitaxial layer 209 is formed by an epitaxial growth technique.

The epitaxial layer 209 includes a first region (not shown in the figure) located in the third opening 208 , a second region (not shown in the figure) located in the second opening 206 , and a first region located in the first opening The third zone within 205 (not shown in the figure). The topography of the epitaxial layer 209 is determined by the topography of the first opening 205 , the second opening 206 and the third opening 208 .

Correspondingly, an embodiment of the present invention further provides a semiconductor structure, please continue to refer to FIG. 9 , including: a substrate 200 ; a dielectric layer 201 located on the surface of the substrate 200 ; a dielectric layer 201 and the substrate 200 The inner epitaxial layer 209, the epitaxial layer 209 includes a first region (not marked in the figure), a second region adjacent to the first region and located on the first region (not marked in the figure) , and a third region (not marked in the figure) adjacent to the second region and located on the second region, the first region is located in the substrate 200, the second region and the second region are The third region is located in the dielectric layer 201 , the sidewall of the second region is protruding from the sidewall of the third region, and the sidewall of the second region is flush with or protrudes from the first region side wall.

The material of the epitaxial layer 209 includes germanium.

The material of the dielectric layer 201 includes silicon oxide.

In this embodiment, a protective layer 203 is located between the substrate 200 and the dielectric layer 201 .

10 to 15 are schematic structural diagrams of each step of a method for forming a semiconductor structure in another embodiment of the present invention.

In this embodiment, for the formation methods of the first opening, the second opening and the initial third opening, please refer to FIG. 10 to FIG. 13 .

Referring to FIG. 10 , a substrate 300 is provided; a dielectric layer 301 is formed on the surface of the substrate 300 .

The material of the substrate 300 includes silicon.

The material of the dielectric layer 301 includes silicon oxide.

In this embodiment, before forming the dielectric layer 301 , a sacrificial layer 302 is formed on a part of the surface of the substrate 300 , and the dielectric layer 301 is located on the sidewalls and the top surface of the sacrificial layer 302 . In other embodiments, before the sacrificial layer 302 is formed, a protective layer is also formed on the surface of the dielectric layer 301 .

The material of the sacrificial layer 302 is different from the material of the dielectric layer 301 ; the material of the sacrificial layer 302 is different from the material of the substrate 300 . In this embodiment, the material of the sacrificial layer 302 is silicon nitride.

The sacrificial layer 302 is used to occupy space for the subsequent formation of the second opening. In the process of forming the second opening, an etching process with a larger selectivity ratio for the sacrificial layer 302 and the dielectric layer 301 may be selected, so as to reduce the effect of the etching process on the dielectric layer 301 . damage. In the process of forming the second opening, an etching process with a larger selectivity ratio between the sacrificial layer 302 and the substrate 300 may be selected, so as to reduce the impact of the etching process on the substrate 300 . damage.

The method for forming the sacrificial layer 302 includes: forming a sacrificial material layer (not marked in the figure) on the substrate 200; forming a patterned layer (not marked in the figure) on the sacrificial material layer; The patterned layer is a mask, and the sacrificial material layer is etched to form the sacrificial layer 302 .

Referring to FIG. 11, a mask layer 303 is formed on a part of the surface of the dielectric layer 301; the dielectric layer 301 is etched using the mask layer 303 as a mask until part of the sacrificial layer 302 is exposed, forming the The first opening 304 .

The material of the mask layer 303 includes photoresist.

The formation process of the first opening 304 includes one or a combination of a dry etching process and a wet etching process. In this embodiment, the formation process of the first opening 304 is a dry etching process, and the dry etching process is conducive to forming openings with better shapes.

Referring to FIG. 12 , the sacrificial layer 302 and the substrate 300 exposed by the first opening 304 are etched using the mask layer 303 as a mask to form the initial first Three openings 306 , an initial second opening 305 is formed in the dielectric layer 301 , and the sidewall of the initial second opening 305 exposes the sacrificial layer 302 .

The formation process of the initial third opening 306 and the initial second opening 305 includes one or a combination of a dry etching process and a wet etching process. In this embodiment, the formation process of the initial third opening and the initial second opening 305 is a dry etching process. The third opening 306 and the initial second opening 305 are formed in the same process, which saves process steps and is beneficial to reducing production costs.

Referring to FIG. 13 , after the initial third opening 306 is formed, the remaining sacrificial layer 302 is removed to form the second opening 307 .

The process of removing the remaining sacrificial layer 302 is an isotropic etching process.

The process of removing the sacrificial layer 302 includes one or a combination of a dry etching process and a wet etching process. The process of removing the sacrificial layer 302 has a larger etching selectivity ratio of the sacrificial layer 302 to the dielectric layer 301, which is beneficial to protect the dielectric layer 301 from etching during the process of removing the sacrificial layer 302 damage.

In this embodiment, the process of removing the sacrificial layer 302 is a wet etching process. The wet etching process adopts phosphoric acid solution, because phosphoric acid solution has a larger selection ratio of silicon nitride material and silicon oxide material, which is beneficial to the process of removing the sacrificial layer 302, so that the dielectric layer 301 is not etched At the same time, since the phosphoric acid solution has a large selection ratio of silicon oxide material and silicon material, it is beneficial to protect the substrate 300 from etching damage during the process of removing the sacrificial layer 302 .

Referring to FIG. 14 , etching is performed on the inner wall of the initial third opening 306 (as shown in FIG. 13 ) and the top surface of the substrate 300 exposed at the bottom of the second opening 307 (as shown in FIG. 13 ). A third opening 308 is formed in the substrate 300 .

The process of etching the inner wall of the initial third opening 207 and the top surface of the substrate 200 exposed at the bottom of the second opening 206 includes one or both of a dry etching process and a wet etching process. combine.

The etching process is used to remove the surface damage layer of the substrate 200 exposed from the inner wall of the initial third opening 207 to prepare for the subsequent formation of an epitaxial layer in the third opening. Meanwhile, in the etching process, since the second opening 206 exposes the top surface of the substrate 200 adjacent to the sidewall of the initial third opening 207 to the etching solution or the etching gas, While the substrate exposed by the sidewalls of the initial third opening 207 is etched, the top surface of the substrate exposed at the bottom of the second opening 206 is also etched, so as to avoid only the initial first opening 206 being etched. The "undercut" defect caused by the etching of the substrate on the sidewalls of the three openings 207 further improves the performance of the formed device.

In this embodiment, the process of etching the inner wall of the initial third opening 207 and the top surface of the substrate 200 exposed at the bottom of the second opening 206 is a dry etching process, and the dry etching process adopts The etching gas includes hydrogen chloride.

Subsequently, an epitaxial layer is formed in the third opening 308 , the second opening 307 and the first opening 304 .

In this embodiment, before forming the epitaxial layer, the mask layer 204 is also removed.

Referring to FIG. 15 , an epitaxial layer 309 is formed in the third opening 308 , the second opening 307 and the first opening 304 .

The material of the epitaxial layer 309 includes germanium. The substrate 200 exposed by the third opening 308 is used as a seed crystal, and the epitaxial layer 309 is formed by an epitaxial growth technique.

Although the present invention is disclosed above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be based on the scope defined by the claims.

Claims (18)

1. a semiconductor structure, is characterized in that, comprises: substrate; a dielectric layer on the surface of the substrate; An epitaxial layer within the dielectric layer and the substrate, the epitaxial layer including a first region, a second region adjacent to the first region and on the first region, and a second region adjacent to the first region The second area is adjacent to the third area located on the second area, the first area is located in the substrate, the second area and the third area are located in the dielectric layer, and the first area is located in the dielectric layer. The sidewalls of the second region are protruded relative to the sidewalls of the third region, and the sidewalls of the second region are flush with or protrude from the sidewalls of the first region. 2. The semiconductor structure of claim 1, wherein the material of the epitaxial layer comprises germanium. 3. The semiconductor structure of claim 1, wherein the material of the dielectric layer comprises silicon oxide. 4. A method for forming a semiconductor structure, comprising: provide a substrate; forming a dielectric layer on the surface of the substrate; A first opening located in the dielectric layer, a second opening located in the dielectric layer, and an initial third opening located in the substrate are formed, the second opening is located at the bottom of the first opening and communicated with the first opening, so the second opening sidewall is recessed relative to the first opening sidewall, the second opening sidewall is recessed relative to the initial third opening sidewall, and the second opening bottom exposes the top surface of the substrate portion; etching the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening to form a third opening in the substrate; An epitaxial layer is formed within the third opening, the second opening and the first opening. 5 . The method for forming a semiconductor structure according to claim 4 , wherein, before forming the dielectric layer, a sacrificial layer is formed on the surface of the substrate part, and the dielectric layer is located on the sidewall of the sacrificial layer and the sidewall of the sacrificial layer. 6 . top surface. 6 . The method for forming a semiconductor structure according to claim 5 , wherein the method for forming the first opening, the second opening and the initial third opening comprises: forming part of the surface of the dielectric layer. 7 . mask layer; using the mask layer as a mask to etch the dielectric layer until part of the sacrificial layer is exposed to form the first opening; using the mask layer as a mask to etch the first opening An opening exposes the sacrificial layer and the substrate, the initial third opening is formed in the substrate, the initial second opening is formed in the dielectric layer, and the sidewalls of the initial second opening are exposed The sacrificial layer is removed; after the initial third opening is formed, the remaining sacrificial layer is removed to form the second opening. 7. The method for forming a semiconductor structure according to claim 6, wherein the forming process of the first opening comprises one or a combination of a dry etching process and a wet etching process; The formation process of the initial third opening and the initial second opening includes one or a combination of a dry etching process and a wet etching process. 8 . The method for forming a semiconductor structure according to claim 6 , wherein the process of removing the remaining sacrificial layer is an isotropic etching process. 9 . 9 . The method for forming a semiconductor structure according to claim 5 , wherein the material of the sacrificial layer is different from the material of the dielectric layer; the material of the sacrificial layer is different from the material of the substrate. 10 . 10 . The method for forming a semiconductor structure according to claim 5 , wherein the method for forming the first opening, the second opening and the initial third opening comprises: etching the dielectric layer until the dielectric layer is etched. 11 . Part of the sacrificial layer is exposed, and a first opening is formed in the dielectric layer; after the first opening is formed, the sacrificial layer is removed to form the second opening; after the second opening is formed, etching is performed the substrate exposed at the bottom of the first opening to form the initial third opening. 11 . The method for forming a semiconductor structure according to claim 10 , wherein the forming process of the first opening comprises one or a combination of a dry etching process and a wet etching process. 12 . 12. The method for forming a semiconductor structure according to claim 10, wherein the process of removing the sacrificial layer is an isotropic etching process. 13 . The method for forming a semiconductor structure according to claim 10 , wherein a protective layer is located between the substrate and the dielectric layer. 14 . 14. The method for forming a semiconductor structure according to claim 13, wherein a material of the protective layer comprises silicon oxide; and a material of the sacrificial layer comprises polysilicon. 15. The method for forming a semiconductor structure according to claim 13, wherein the method for forming the protective layer and the sacrificial layer comprises: forming a gate oxide material layer on the substrate; forming a dummy gate material layer on the material layer; forming a patterned layer on the dummy gate material layer; using the patterned layer as a mask, etching the dummy gate material layer and the gate oxide material layer , forming the sacrificial layer and the protective layer. 16 . The method for forming a semiconductor structure according to claim 15 , wherein when the sacrificial layer and the protective layer are formed, the gate oxide material layer is also etched to form a gate oxide layer, and the dummy layer is formed. 17 . The gate material layer is also etched to form dummy gates for forming MOS devices in other regions. 17 . The method for forming a semiconductor structure according to claim 4 , wherein the process of etching the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening comprises dry etching. 18 . One or a combination of an etching process and a wet etching process. 18 . The method for forming a semiconductor structure according to claim 17 , wherein the process of etching the inner wall of the initial third opening and the top surface of the substrate exposed from the bottom of the second opening is dry etching. 19 . etching process, and the etching gas used in the dry etching process includes hydrogen chloride.

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