143. The sixth epitaxial layer 156 may have lower surface 156B having a substantially U-like rounded shape. Each of the first, second, third, fourth and fifth upper surfaces 151T, 152T, 153T, 154T and 155T may be curved toward the substrate 101, as illustrated in FIGS. 2 and 3.

Each of the second to fifth epitaxial layers 152, 153, 154, and 155 may have a maximum thickness in the vertical direction Z, in a central portion in the first direction X. Accordingly, the sixth epitaxial layer 156 filling the gap of the fifth epitaxial layer 155 may have an aspect ratio, smaller than that an aspect ratio of the source/drain hole 150H. In example embodiments, each of the second to fifth epitaxial layers 152, 153, 154, and 155 may have a vertical thickness increased in a direction toward the central portion in the first direction X. In example embodiments, thicknesses of the second to fifth epitaxial layers 152, 153, 154, and 155 in the central portions in the first direction X may be the same or different from each other. Each of the second to fifth epitaxial layers 152, 153, 154, and 155 may include a center portion in the first direction X, and that center portion of each of the second to fifth epitaxial layers 152, 153, 154, and 155 may have a thickest thickness of the respective second to fifth epitaxial layers 152, 153, 154, and 155, as illustrated in FIG. 3.

The sixth epitaxial layer 156 has an aspect ratio, lower than an aspect ratio of the source/drain hole 150H, so that occurrence of dislocation or formation of voids in the source/drain region 150 may be suppressed. Accordingly, electrical characteristics and reliability of the semiconductor device 100 may be improved.

An epitaxial layer, formed within the source/drain region 150, may have a crystal growth rate varying depending on a crystal direction. For example, the epitaxial layer constituting the source/drain region 150 may have a relatively low growth rate in the vertical direction Z, as compared to that in the horizontal direction X. When the epitaxial layer is formed in the source/drain hole 150H, growth thereof in the vertical direction may not be completed until growth thereof in the horizontal direction is completed. Accordingly, dislocation may occur or voids may be formed in the source/drain region 150. As a contacted polysilicon pitch (CPP) between adjacent gate structures is decreased, and the aspect ratio of the source/drain hole 150H is increased to increase the degree of integration of the semiconductor device, probability of occurrence of defects in the source/drain regions 150 may be increased.

The defects, occurring in the source/drain region 150, may deteriorate electrical performance of the semiconductor device. For example, when the semiconductor device 100 is a PMOS, the epitaxial layers having a high germanium (Ge) concentration may act as a stressor applying compressive force to the channel layers 141, 142, and 143 to increase charge mobility in the channel layers 141, 142, and 143. However, when dislocation occurs in the source/drain region 150, strain relaxation may occur in a region in which an interatomic bond is broken. Accordingly, since the source/drain region 150 is incapable of applying sufficient compressive stress to the channel layers 141, 142, and 143, the performance of the semiconductor device may be deteriorated due to an increase in resistance of the channel layers 141, 142, and 143.

The source/drain region 150 according to example embodiments may include second to fifth epitaxial layers 152 to 155 having a maximum thickness in a central portion in the first direction X. Accordingly, the aspect ratio of the sixth epitaxial layer 156 filling the gap of the fifth epitaxial layers 155 may be reduced. Accordingly, defects caused by the dislocation or the voids in the sixth epitaxial layer 156 may be suppressed. As a result, defects occurring in the source/drain region 150 may be suppressed without limiting a device CPP to improve the performance of the semiconductor device.

In example embodiments, the first to fifth epitaxial layers 151, 152, 153, 154, 155 may meet at a first point P. For example, uppermost ends of the first to fifth epitaxial layers 151, 152, 153, 154, and 155 may meet at the first point P. The first point P may be disposed on the same level as the upper surface 156T of the sixth epitaxial layer 156.

The first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 may have a discontinuous germanium (Ge) concentration in a boundary between continuously disposed layers. The first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 may include first to sixth germanium (Ge) concentrations, respectively. The first Ge concentration of the first epitaxial layer 151 may be lower than the second Ge concentration of the second epitaxial layer 152. The second Ge concentration of the second epitaxial layer 152 may be higher than the third Ge concentration of the third epitaxial layer 153. The third Ge concentration of the third epitaxial layer 153 may be lower than the fourth Ge concentration of the fourth epitaxial layer 154. The fourth Ge concentration of the fourth epitaxial layer 154 may be higher than the fifth Ge concentration of the fifth epitaxial layer 155. The fifth Ge concentration of the fifth epitaxial layer 155 may be lower than the sixth Ge concentration of the sixth epitaxial layer 156. The germanium (Ge) concentrations of the first to sixth epitaxial layers 151 to 156 will be described later with reference to FIGS. 4A and 4B.

The gate structure 160 may be disposed to extend in one direction, for example, the Y-direction and to extend on (e.g., traverse or intersect) the active region 105 and the channel structures 140 on the active region 105 and the channel structures 140. Channel regions of transistors may be formed in the active region 105 and the channel structures 140 intersecting the gate structure 160. The gate structure 160 may include a gate electrode layer 163, a gate dielectric layer 162 between the gate electrode layer 163 and the plurality of channel layers 141, 142, and 143, spacer layers 161 on side surfaces of the gate electrode layer 163, and a gate capping layer 164 on an upper surface of the gate electrode layer 163.

The gate dielectric layer 162 may be disposed between the active region 105 and the gate electrode 163 and between the channel structure 140 and the gate electrode 163, and may be disposed to cover at least a portion of surfaces of the gate electrode 165. For example, the gate dielectric layer 162 may be disposed to surround all surfaces of the gate electrode 165, other than the upper surface of the gate electrode 163. The gate dielectric layer 162 may extend between the gate electrode 163 and the spacer layers 161, but the extension of the gate dielectric layer 162 is not limited thereto. The gate dielectric layer 162 may include an oxide, a nitride, and/or a high-κ dielectric material. The high-κ dielectric material may refer to a dielectric material having a dielectric constant, higher than a dielectric constant of a silicon oxide (SiO₂). The high-κ dielectric material may include or may be one of, for example, an aluminum oxide (Al₂O₃), a tantalum oxide (Ta₂O₃), a titanium oxide (TiO₂), an yttrium oxide (Y₂O₃), a zirconium oxide (ZrO₂), a zirconium silicon oxide (ZrSi_xO_y), a hafnium oxide (HfO₂), a hafnium silicon oxide (HfSi_xO_y), a lanthanum oxide (La₂O₃), a lanthanum aluminum oxide (LaAl_xO_y), a lanthanum hafnium oxide (LaHf_xO_y), a hafnium aluminum oxide (HfAl_xO_y), and/or a praseodymium oxide (Pr₂O₃).

The gate electrode 163 may be disposed to extend upwardly of the channel structure 140 while filling spaces between the plurality of channel layers 141, 142, and 143 on the active region 105. The gate electrode 163 may be spaced apart from the plurality of channel layers 141, 142, and 143 by the gate dielectric layer 162. The gate electrode 163 may include a conductive material. For example, the gate electrode 163 may include a metal nitride (for example, a titanium nitride (TiN), a tantalum nitride (TaN), and/or a tungsten nitride (WN)), a metal material (for example, aluminum (Al), tungsten (W), and/or molybdenum (Mo)), and/or a semiconductor material (for example, doped polysilicon).

The gate electrode layer 163 may have a multilayer structure including two or more layers. The spacer layers 161 may be disposed on opposite side surfaces of the gate electrode 163. The spacer layers 161 may insulate the source/drain regions 150 and the gate electrode layer 163 from each other. In example embodiments, the spacer layers 161 may have a multilayer structure. The spacer layers 161 may include, for example, an oxide, a nitride, an oxynitride, and/or a low-x dielectric material.

The gate capping layer 164 may be disposed on the gate electrode layer 163, and a lower surface of the gate capping layer 164 may be surrounded by the gate electrode layer 163 and the spacer layers 161.

The interlayer insulating layer 190 may be disposed to cover the source/drain region 150, the gate structure 160, and the isolation layer 110. The interlayer insulating layer 190 may include, for example, an oxide, a nitride, an oxynitride, and/or a low-x dielectric material.

The contact plug 180 may penetrate through the interlayer insulating layer 190 to be connected to the source/drain region 150, and may apply an electrical signal to the source/drain region 150. The contact plug 180 may be disposed on the source/drain region 150. In an example embodiment, the contact plug 180 may be disposed to have a length, greater than a length of the source/drain region 150 in the Y-direction. The contact plug 180 may have an inclined side surface in which a width of a lower portion is narrower than a width of an upper portion depending on an aspect ratio, but a shape of the side surface of the contact plug 180 is not limited thereto. The contact plug 180 may be disposed to recess the source/drain region 150 to a predetermined depth. In example embodiments, the contact plug 180 may be disposed to penetrate through the fourth epitaxial layer 154 and to penetrate through at least a portion of the third epitaxial layer 153. The contact plug 180 may include, for example, a metal nitride (for example, a titanium nitride (TiN), a tantalum nitride (TaN), and/or a tungsten nitride (WN)) and a metal material (for example, aluminum (Al), tungsten (W), and/or molybdenum (Mo)).

Next, the germanium (Ge) concentrations of the first to sixth epitaxial layers 151 to 156 of the source/drain region 150 will be described with reference to FIGS. 4A and 4B.

FIG. 4A is an enlarged view of a portion of a semiconductor device according to example embodiments. For ease of description, the contact plug 180 in FIG. 3 is not shown in FIG. 4A.

FIG. 4B is a diagram illustrating a distribution of germanium (Ge) concentration in a source/drain region in a semiconductor device according to example embodiments. FIG. 4B is a schematic diagram illustrating a concentration profile of germanium (Ge) in the source/drain region 150 along I-I′ of FIG. 4A.

Referring to FIGS. 4A and 4B, in FIG. 4A, I-I′ is a center line of a source/drain region 150 in a first direction X. In FIG. 4B, a period X1-X2 corresponds to the first epitaxial layer 151, a period X2-X3 corresponds to the second epitaxial layer 152, a period X3-X4 corresponds to the third epitaxial layer 153, a period X4-X5 corresponds to the fourth epitaxial layer 154, a period X5-X6 corresponds to the fifth epitaxial layer 155, and a period X6-X7 corresponds to the sixth epitaxial layer 156.

Each of the first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 may be a semiconductor layer including silicon (Si) and germanium (Ge). The first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 may have first to sixth germanium (Ge) concentrations C1, C2, C3, C4, C5, and C6, respectively. The germanium (Ge) concentration may be discontinuous in boundaries between continuous epitaxial layers.

The first Ge concentration C1 of the first epitaxial layer 151 may be lower than the second Ge concentration C2 of the neighboring second epitaxial layer 152. In example embodiments, the first Ge concentration C1 of the first epitaxial layer 151 may be about 5 atomic percent (at %) to about 8 at %. When the first epitaxial layer 151 has the first Ge concentration within the above-mentioned range, stress caused by a difference in lattice constants between silicon (Si) and germanium (Ge) may be reduced.

The second Ge epitaxial layer 152 may have the second Ge concentration C2 that is higher than the first Ge concentration C1 of the neighboring first epitaxial layer 151 and the third Ge concentration C3 of the neighboring third epitaxial layer 153. The fourth epitaxial layer 154 may have the fourth Ge concentration G4 that is higher than the third Ge concentration C3 of the neighboring third epitaxial layer 153 and the fifth Ge concentration C5 of the neighboring fifth epitaxial layer 155. The sixth epitaxial layer 156 may have the sixth Ge concentration C6 that is higher than the fifth Ge concentration C5 of the neighboring fifth epitaxial layer 155. In example embodiments, each of the second Ge concentration C2 of the second epitaxial layer 152, the fourth Ge concentration C4 of the fourth epitaxial layer 154, and the sixth Ge concentration G6 of the sixth epitaxial layer 156 may be about 30 at % to about 70 at %.

The third epitaxial layer 153 may have the third Ge concentration C3 that is lower than the second Ge concentration C2 of the neighboring second epitaxial layer 152 and the fourth Ge concentration C4 of the neighboring fourth epitaxial layer 154. The fifth epitaxial layer 155 may have the fifth Ge concentration G5 that is lower than the fourth Ge concentration C4 of the neighboring fourth epitaxial layer 154 and the sixth Ge concentration C6 of the neighboring sixth epitaxial layer 156.

The second epitaxial layer 152, the fourth epitaxial layer 154, and the sixth epitaxial layer 156, respectively containing a relatively high concentration of germanium (Ge) as compared with neighboring layers, may constitute a first region of the source/drain region 150. The third epitaxial layer 153 and the fifth epitaxial layer 155, interposed between e epitaxial layers of the first region and containing a relatively low concentration of germanium (Ge) as compared with neighboring layers, may constitute a second region of the source/drain region 150. The first region may include first silicon-germanium (SiGe), and the second region may include second silicon-germanium (SiGe). A concentration of germanium (Ge) included in the first silicon-germanium (SiGe) may be higher than a concentration of germanium (Ge) included in the second silicon-germanium (SiGe). In the specification, the epitaxial layer included in the second region may be referred to as an “insertion layer.”

In FIGS. 1 to 3, 4A and 4B, the first region of the source/drain region 150 is illustrated as including three epitaxial layers (second, fourth and sixth epitaxial layers 152, 154, and 156), and the second region is illustrated as including two epitaxial layers (third and fifth epitaxial layers 153 and 155), but example embodiments are not limited thereto. Each of the first region and the second region of the source/drain region 150 may include a plurality of epitaxial layers. For example, the second region of the source/drain region 150 may include two or more to 50 or less epitaxial layers.

The third epitaxial layer 153 and the fifth epitaxial layer 155 may be formed in a reflow process of the second epitaxial layer 152 and the fourth epitaxial layer 154. For example, the second epitaxial layer 152 may be conformally formed on the first epitaxial layer 151 to have a uniform thickness. Then, the second epitaxial layer 152 and the third epitaxial layer 153 may be reflowed by a process of applying heat, or the like, at the same time as the third epitaxial layer 153 may be formed on the second epitaxial layer 152. Accordingly, the second epitaxial layer 152 and the third epitaxial layer 153 may have a maximum thickness in a central portion in the first direction X.

Similarly, the fourth epitaxial layer 154 may be conformally formed on the third epitaxial layer 153 to have a uniform thickness. Then, the fourth epitaxial layer 154 and the fifth epitaxial layer 155 may be reflowed by a process of applying heat, or the like, at the same time as the fifth epitaxial layer 155 may be formed on the fourth epitaxial layer 154. Accordingly, the fourth epitaxial layer 154 and the fifth epitaxial layer 155 may have a maximum thickness in the central portion in the first direction X.

Germanium (Ge) source gas may not be supplied in the process of forming the third and fifth epitaxial layers 153 and 155 performed together with the reflow process. The other process conditions may be maintained to be the same as or similar to process conditions for forming the second and fourth epitaxial layers 152 and 154. Accordingly, the third and fifth epitaxial layers 153 and 155 may have a germanium (Ge) concentration that is lower than a Ge concentration of the neighboring epitaxial layers, and may have a discontinuous germanium (Ge) concentration in boundaries with the neighboring epitaxial layers.

Each of the first to sixth Ge concentrations C1, C2, C3, C4, C5, and C6 of the first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 may have a profile increased in a direction toward an upper surface of the source/drain region 150. However, the concentration profiles of the first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 are not limited thereto. In other embodiments, the first to sixth Ge concentrations C1, C2, C3, C4, C5, and C6 may have constant values in the first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156, respectively.

As described above, the first to sixth epitaxial layers 151, 152, 153, 154, 155, and 156 have different material compositions (for example, Ge concentrations), and thus may be substantially distinguished from each other through analysis such as transmission electron microscopy energy-dispersive X-ray spectroscopy (TEM-EDX), or the like.

FIGS. 5 to 7 are cross-sectional views of semiconductor devices according to example embodiments, respectively.

In the example embodiment of FIGS. 5 to 7, in the case in which reference numerals are the same as those of FIGS. 1 to 3 but are different in alphabets, it intends to describe an example embodiment different from the example embodiment of FIGS. 1 to 3 and features described with the same reference numerals may be the same or similar.

The semiconductor device 100a of FIG. 5 is different from the semiconductor device 100 of FIGS. 1 to 3 in terms of a shape of a source/drain region 150a.

Referring to FIG. 5, the source/drain region 150a may include first to sixth epitaxial layers 151, 152a, 153a, 154a, 155a, and 156a. The second to fifth epitaxial layers 152a, 153a, 154a, 155a, and 156a may meet at a first point Pa. For example, uppermost ends of the second to fifth epitaxial layers 152a, 153a, 154a, and 155a may meet at the first point Pa. The first point Pa may be disposed on a level, lower than a level of an uppermost end of the first epitaxial layer 151 and a level of an uppermost surface of the sixth epitaxial layer 156a. The sixth epitaxial layer 156a may have a lower surface 156Ba contacting an upper surface of the fifth epitaxial layer 155a, a side surface 156S contacting an upper surface of the first epitaxial layer 151, and an upper surface 156Ta contacting an interlayer insulating layer 190.

The semiconductor device 100b of FIG. 6 is different from the semiconductor device 100 of FIGS. 1 to 3 in terms of a shape of a source/drain region 150b.

Referring to FIG. 6, the source/drain region 150b may include first to sixth epitaxial layers 151, 152b, 153b, 154b, 155b and 156b. The second and third epitaxial layers 152b and 153b may meet at a first point Pb1, and the fourth and fifth epitaxial layers 154b and 155b may meet at a second point Pb2. For example, uppermost ends of the second and third epitaxial layers 152b and 153b meet at the first point Pb1, and uppermost ends of the fourth and fifth epitaxial layers 154b and 155b meet at the second point Pb2. The first point Pb1 may be disposed on a level, lower than a level of the second point Pb2.

The fourth epitaxial layer 154b may have a lower surface 154Bb contacting an upper surface of the third epitaxial layer 153b, and a side surface 154S contacting an upper surface of the first epitaxial layer 151, and an upper surface 154Tb contacting a lower surface of the fifth epitaxial layer 155b.

The sixth epitaxial layer 156b may have a lower surface 156Bb contacting an upper surface of the fifth epitaxial layer 155b, a side surface 156S contacting an upper surface of the first epitaxial layer 151, and an upper surface 156Tb contacting an interlayer insulating layer 190.

FIG. 7 shows cross-sectional views of a semiconductor device according to example embodiments. Specifically, FIG. 7 illustrates regions, respectively corresponding to cross-sections taken along lines I-I′, II-II′ and III-III′ of FIG. 1. For ease of description, only a group of components (e.g., major components) of the semiconductor device are illustrated in FIG. 7.

Referring to FIG. 7, a semiconductor device 100c may include an active region 105, an isolation layer 110, a source/drain region 150c, a gate structure 160, a contact plug 180, and an interlayer insulating layer 190. The semiconductor device 100c may include a FinFET device, a transistor in which an active region 105 has a fin structure. The FinFET device may include a transistor disposed around the active region 105 and the gate structure 160, intersecting each other. For example, the FinFET device may be a PMOS transistor. Hereinafter, the same reference numerals as those of FIGS. 1 to 3 denote corresponding components, and descriptions overlapping the above descriptions will be omitted.

The source/drain regions 150c may be disposed within a recess region, recessed from an upper surface of the active region 105, on at least one side of the gate structure 160. The source/drain regions 150c may include a plurality of epitaxial layers, for example, first to sixth epitaxial layers 151c, 152c, 153c, 154c, 155c, and 156c. The first to sixth epitaxial layers 151c, 152c, 153c, 154c, 155c, and 156c may be sequentially disposed in the recess region. The second to fourth epitaxial layers 152c, 153c, 154c, and 155c may be disposed below the sixth epitaxial layer 156c to reduce an aspect ratio of the sixth epitaxial layer 156c. Accordingly, dislocation or formation of voids in the source/drain region 150c may be suppressed to improve performance of the semiconductor device 100c.

FIGS. 8A to 8J are cross-sectional views illustrating a method of manufacturing a semiconductor device according to example embodiments. In some embodiments, processes illustrated in FIGS. 8A and 8J may be sequentially performed. FIGS. 8A to 8J illustrate an example embodiment of a method of manufacturing the semiconductor device of FIGS. 1 to 3, and illustrates cross-sections corresponding to FIG. 2.

Referring to FIG. 8A, sacrificial layers 120 and channel layers 141, 142, and 143 may be alternately stacked on a substrate 101.

The sacrificial layers 120 may be replaced with a gate dielectric layer 162 and a gate electrode layer 163 through a subsequent process, as illustrated in FIG. 2. The sacrificial layers 120 may be formed of a material having etching selectivity with respect to the channel layers 141, 142, and 143. The channel layers 141, 142, and 143 may include a material, different from a material of the sacrificial layers 120. In an example embodiment, the channel layers 141, 142, and 142 may include silicon (Si), and the sacrificial layers 120 may include silicon-germanium (SiGe).

The sacrificial layers 120 and the channel layers 141, 142, and 143 may be formed by performing an epitaxial growth process using the substrate 101 as a seed. Each of the sacrificial layers 120 and the channel layers 141, 142, and 143 may have a thickness within a range of about 1 angstrom (Å) to 100 nanometers (nm). The number of the channel layers 141, 142, and 143, stacked alternately with the sacrificial layer 120, may vary according to example embodiments.

Referring to FIG. 8B, a portion of a stack structure of the sacrificial layers 120 and the channel layers 141, 142, and 143 and a portion of the substrate 101 may be removed to form active structures.

The active structure may include sacrificial layers 120 and channel layers 141, 142, and 143, stacked alternately with each other, and may further include an active region 105 formed to protrude upwardly beyond an upper surface of the substrate 101 by removing a portion of the substrate 101. The active structures may be formed to have a line shape extending in one direction, for example, an X-direction, and may be disposed to be spaced apart from each other in a Y-direction.

In a region in which a portion of the substrate 101 is removed, isolation layers 110 may be formed by filling the region with an insulating material and then recessing a resultant structure such that the active region 105 protrudes. An upper surface of the isolation layers 110 may be formed to be lower than an upper surface of the active region 105.

Referring to FIG. 8C, sacrificial gate structures 170 and spacer layers 161 may be formed on the active structures.

The sacrificial gate structures 170 may be sacrificial structures, formed in a region in which the gate dielectric layer 162 and the gate electrode layer 163 are disposed, on the channel structure 140 through a subsequent process as illustrated in FIG. 2. The sacrificial gate structure 170 may include first and second sacrificial gate layers 172 and 175 and a mask pattern layer 176, sequentially stacked. The first and second sacrificial gate layers 172 and 175 may be patterned using a mask pattern layer 176. The first and second sacrificial gate layers 172 and 175 may be an insulating layer and a conductive layer, respectively. For example, the first sacrificial gate layer 172 may include a silicon oxide, and the second sacrificial gate layer 175 may include polysilicon. The mask pattern layer 176 may include a silicon nitride. The sacrificial gate structures 170 may have a line shape extending in one direction and intersecting the active structures. For example, the sacrificial gate structures 170 may extend in the Y-direction, and may be disposed to be spaced apart from each other in the X-direction.

Spacer layers 161 may be formed on opposite sidewalls of the sacrificial gate structures 170. The spacer layers 161 may be formed by forming a layer having a uniform thickness along upper and side surfaces of the sacrificial gate structures 170 and the active structures and then anisotropic etching the layer. The spacer layers 161 may be formed of a low-x dielectric material or may include, for example, SiO, SiN, SiCN, SiOC, SiON, and/or SiOCN.

Referring to FIG. 8D, a recess region RC may be formed by removing the exposed sacrificial layers 120 and the channel layers 141, 142, and 143 between the sacrificial gate structures 170, and thus, a channel structures 140 may be formed.

The exposed sacrificial layers 120 and the channel layers 141, 142, and 143 may be removed using the sacrificial gate structures 170 and the spacer layers 161 as masks. The remaining sacrificial layers 120 may be removed to a predetermined depth from a side surface in the X-direction to have an inwardly concave side surfaces. Side surfaces of the remaining channel layers 141, 142, and 143 in the X-direction may be etched to have outwardly convex side surfaces. However, the shapes of the side surfaces of the sacrificial layers 120 and the channel layers 141, 142, and 143 are not limited to those illustrated in the drawing. Side surfaces of the sacrificial layers 120 and the channel layers 141, 142, and 143 may be formed to be coplanar with each other in a direction, perpendicular to an upper surface of the substrate 101.

Referring to FIG. 8E, a first epitaxial layer 151 may be formed in the recess region RC.

The first epitaxial layer 151 may extend to contact the channel layers 141, 142, and 143 and the sacrificial layers 120 within the recess region RC. Accordingly, an upper surface of the first epitaxial layer 151 may be formed in a recessed shape, and may be formed to have a substantially U-shape. A surface of the first epitaxial layer 151, contacting the channel layers 141, 142, 143 and the sacrificial layers 120, may have a wavy shape. A lowermost end of the upper surface 151T of the first epitaxial layer 151 may be disposed on a level, higher than a level of a lower surface of a lowermost sacrificial layer 120.

The first epitaxial layer 151 may include silicon-germanium (SiGe). The first epitaxial layer 151 may be formed by supplying silicon (Si) and germanium (Ge) source gases while supplying a carrier gas. In an example embodiment, the carrier gas may be, for example, hydrogen (H₂) gas, the silicon (Si) source gas may be, for example, silane (SiH₄), dichlorosilane (SiH₂Cl₂; DCS), and/or chlorosilane (SiH₃Cl; MCS), and the germanium (Ge) source gas may be, for example, germanium tetrahydride (GeH₄). However, the carrier gas, the silicon (Si) source gas, and the germanium (Ge) source gas are not limited thereto.

The first epitaxial layer 151 may include a first germanium (Ge) concentration, lower than a Ge concentration of the sacrificial layers 120. In an example embodiment, the first epitaxial layer 151 may include Ge at a concentration of about 5 at % to about 8 at %. The first epitaxial layer 151 may include a Ge concentration, lower than the Ge concentration of the sacrificial layers 120, and may have etching selectivity, lower than etching selectivity of the sacrificial layers 120. Accordingly, in the process of FIG. 8I to be described later, the sacrificial layers 120 may be selectively removed and the source/drain regions 150 protected by the first epitaxial layer 151 may remain.

Referring to FIG. 8F, a second preliminary epitaxial layer 152P may be formed on the first epitaxial layer 151.

The second preliminary epitaxial layer 152P may be conformally formed on an upper surface of the first epitaxial layer 151 to have a substantially uniform thickness. Accordingly, an upper surface of the second preliminary epitaxial layer 152P may have a recessed shape, similar to the upper surface of the first epitaxial layer 151. The second preliminary epitaxial layer 152P may be formed to cover an entire upper surface of the first epitaxial layer 151.

The second preliminary epitaxial layer 152P may include silicon-germanium (SiGe). The second preliminary epitaxial layer 152P may be formed by supplying silicon (Si) and germanium (Ge) source gases while supplying a carrier gas. The second preliminary epitaxial layer 152P may have a second Ge concentration, higher than the first Ge concentration of the first epitaxial layer 151. In an example embodiment, the second preliminary epitaxial layer 152P may include Ge at a concentration of about 30 at % to about 70 at %.

Referring to FIG. 8G, a third epitaxial layer 153 may be formed while forming a second epitaxial layer 152 by reflowing the second preliminary epitaxial layer (152P of FIG. 8F).

The second preliminary epitaxial layer (152P of FIG. 8F) may be reflowed by heat, applied in a process of forming the third epitaxial layer 153, or the like, to form the second epitaxial layer 152. The second epitaxial layer 152 may have a maximum thickness in a central portion in a first direction X.

The third epitaxial layer 153 may include silicon-germanium (SiGe). The third epitaxial layer 153 may be formed by supplying a silicon (Si) source gas while supplying a carrier gas. However, the germanium (Ge) source gas may not be supplied in a process of forming the third epitaxial layer