Semiconductor substrate, manufacturing method therefor, and semiconductor device

0001The present invention relates to a semiconductor substrate, manufacturing method therefor, and semiconductor device and, more particularly, to a semiconductor substrate including, e.g., regions with different strained states in a surface layer, i.e., a strained semiconductor region and unstrained semiconductor region, a manufacturing method therefor, and a semiconductor device using the semiconductor substrate.

0002In a strained silicon under tensile stress, it is known that mobility of electrons in Si increases due to the effect of internal stress in the strained Si layer. The strained Si layer is defined as an Si layer whose lattice constant deviates from an original Si lattice constant owing to the stress. An Si layer without stress having an original Si lattice constant will be called as a relaxed Si layer, compared to the strained Si layer. Also for silicon germanium (SiGe) to be described later, an SiGe layer with stress will be called a strained SiGe layer, and an SiGe layer without stress will be called as a relaxed SiGe layer.

0003The operation speed of a semiconductor device can be increased when the device is formed using, e.g., a strained Si layer with tensile stress for a channel of an n-channel metal-oxide semiconductor field effect transistor (MOSFET: to be referred to as an nMOS hereinafter). A high-performance, advanced-function semiconductor device can be manufactured by embedding semiconductor devices with various functions on a strained Si substrate having a strained Si layer.

0004Although the electron mobility increases in an Si layer under tensile stress, but the hole mobility becomes equal to or lower than that in the bulk Si. Strained Si is therefore suitable for an nMOS in terms of semiconductor device characteristics. A pMOS favors strained SiGe which is higher in hole mobility than Si. In this situation, a semiconductor substrate having two semiconductor regions with different strained states has been proposed. An example of the substrate is shown in FIGS. 18A and 18B.

0005This example employs a strained silicon on insulator (SOI) substrate 3 (FIG. 18A) having a thin strained Si layer 21 formed above a substrate 1 and a buried oxide film (BOX) layer 11 and relaxed SiGe layer 12 without stress which are sandwiched between the strained Si layer 21 and the Si substrate 1. In this semiconductor substrate, a relaxed Si region B having a relaxed Si layer 22 without stress is formed on part of the substrate surface while a strained Si region A having the strained Si layer 21 remains in the rest of the surface.

0006The relaxed Si region B can be formed as follows. First, the entire surface is thermally oxidized to form an oxide film (SiO₂ film), and the strained Si layer 21 in the region B is exposed by patterning. In the region B, the Si substrate 1 is exposed by selectively removing the exposed strained Si layer 21, relaxed SiGe layer 12, and BOX layer 11. A relaxed Si layer 22 without stress is formed on the Si substrate 1 by selective epitaxial growth, then the SiO₂ film on the strained Si layer 21 surface is removed. In this manner, a semiconductor substrate having the strained Si region A whose surface is the strained Si layer 21 and the relaxed Si region B whose surface is the relaxed Si layer 22 without stress is formed, as shown in FIG. 18B.

0007In a process of manufacturing a semiconductor substrate having the strained Si region A and relaxed Si region B by selective epitaxial growth, for example, with the structure shown in FIG. 18B, annealing at a relatively high temperature such as thermal oxidization or hydrogen annealing is generally performed before selective epitaxial growth. The annealing generates misfit dislocations near an interface between the strained Si layer 21 and the relaxed SiGe layer 12 due to the difference between their lattice constants, thereby relaxing the strain of the strained Si layer 21. Thermal oxidization of the thin strained Si layer 21 makes the layer 21 thinner, and also makes it difficult to control the thickness of the strained Si layer 21, exactly.

0008During the annealing, Ge diffuses from the relaxed SiGe layer with high Ge concentration into the strained Si layer 21 formed on the relaxed SiGe layer. As a result, the strain of the strained Si layer 21 decreases, failing to sufficiently increase the electron mobility.

0009Another example is a semiconductor substrate having a strained Si region A and relaxed Si region B by using a strained Si substrate 2, as a starting material, in which a strained Si layer 21 is formed on an SiGe buffer layer 31 on an Si substrate 1, as shown in FIG. 19A. In the SiGe buffer layer 31, its Ge concentration is low in a portion close to the Si substrate 1 in order to suppress the generation of above-mentioned misfit dislocations, and gradually increases toward the surface.

0010In this example, the relaxed Si region B is formed as follows. First, part of the strained Si layer 21 is covered with a mask, and the relaxed Si region B being formed an Si layer without stress is exposed. Then, the strained Si layer 21 and SiGe buffer layer 31 in the region B are selectively etched away to expose the surface of the unstrained Si substrate 1. The exposed surface is defined as the relaxed Si region B, as shown in FIG. 19B. In this structure, a step is formed between the strained Si region A on the SiGe buffer layer 31 and the relaxed Si region B from which the SiGe buffer layer 31 is removed.

0011As described above, the SiGe buffer layer 31 is a so-called graded SiGe layer in which the Ge concentration in Si is not uniform in the direction of thickness, lower near the Si substrate 1, and gradually higher toward the surface. In manufacturing a semiconductor substrate with the structure shown in FIG. 19B, the SiGe buffer layer 31 is etched away, as described above. However, since the Ge concentration in the SiGe buffer layer 31 near the Si substrate 1 is low, the selectivity in the etching between the SiGe buffer layer 31 and Si substrate 1 becomes low, failing to control the etching amount. Consequently, the etching uniformity in a wafer, between wafers and among lots becomes worse, and the level of the relaxed Si region B becomes nonuniform.

0012When a semiconductor device is manufactured using a semiconductor substrate suffering the above problems, for example, planarization becomes difficult in the manufacturing process due to the step of the substrate, and desired patterning is hardly achieved. The characteristics of manufactured semiconductor devices vary owing to, e.g., variations in the thickness of the strained Si layer 21 in the surface or variations in the Ge concentration of the SiGe layer 12 in contact with the strained Si layer 21, failing to obtain desired characteristics.

0013The SiGe buffer layer 31 contains many misfit dislocations in order to relax the difference in lattice constant between the Si substrate 1 and the SiGe buffer layer 31. When a deep semiconductor element, e.g., a trench memory cell, is formed in the strained Si region A having misfit dislocations, the trench may cross the misfit dislocation. The dislocation acts as a current leakage path, and the element characteristics is degraded by an increased leakage current.

0014In order to manufacture a semiconductor device with excellent desired characteristics, a semiconductor substrate must be obtained in which (1) the entire substrate is flat without any step between the strained Si region and the relaxed Si region, (2) the strained Si layer and relaxed Si layer, in which semiconductor elements are to be formed, are uniform in, for example, the thicknesses and impurity concentrations, and (3) lattice defects such as misfit dislocations do not exist in a trench formation region in the substrate.

0015In an aspect of the invention, a semiconductor substrate comprises a support substrate, a first semiconductor region including a first silicon layer formed above the support substrate, a second semiconductor region including a strained second silicon layer formed above the support substrate, a surface of the second silicon layer being formed at substantially the same level as a surface of the first silicon layer, and an insulating film at an interface between the first semiconductor region and the second semiconductor region.

0016In another aspect, a semiconductor device comprises a support substrate, a first semiconductor region including a second silicon layer formed on a first silicon layer above the support substrate, a second semiconductor region including a strained third silicon layer formed on a silicon germanium layer above the support substrate, a surface of the third silicon layer being formed at substantially the same level as a surface of the second silicon layer, an insulating film which is formed on an interface between the first semiconductor region and the second semiconductor region below an isolation insulator formed between the first semiconductor region and the second semiconductor region, a trench memory cell which is formed in the first semiconductor region, and a field effect transistor which is formed in the second semiconductor region.

0017In another aspect, a semiconductor substrate manufacturing method is comprised for forming a first insulating film on a semiconductor substrate including a first semiconductor layer, patterning the first insulating film, removing part of the semiconductor substrate to form a recess, forming a second insulating film on an entire surface, removing the second insulating film from a bottom surface in the recess, forming a second semiconductor layer in the recess from a material different from a material of the first semiconductor layer, removing the first and second insulating films from a surface of the first semiconductor layer, and simultaneously forming a first silicon layer on the first semiconductor layer and a second silicon layer on the second semiconductor layer, a surface of the second silicon layer being substantially flush with a surface of the first silicon layer.

0018This summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.

0019The invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a sectional view showing an example of a semiconductor substrate according to the first embodiment;</li><li>FIGS. 2A to 2F are sectional views for explaining an example of a semiconductor substrate manufacturing process according to the first embodiment;</li><li>FIG. 3 is a sectional view showing an example of a semiconductor device using the semiconductor substrate according to the first embodiment;</li><li>FIG. 4 is a sectional view showing an example of a semiconductor substrate according to the second embodiment;</li><li>FIGS. 5A to 5C are sectional views for explaining an example of a semiconductor substrate manufacturing process according to the second embodiment;</li><li>FIG. 6 is a sectional view showing an example of a semiconductor device using the semiconductor substrate according to the second embodiment;</li><li>FIG. 7 is a sectional view showing an example of a semiconductor substrate according to the third embodiment;</li><li>FIGS. 8A to 8F are sectional views for explaining an example of a semiconductor substrate manufacturing process according to the third embodiment;</li><li>FIG. 9 is a sectional view showing an example of a semiconductor device using the semiconductor substrate according to the third embodiment;</li><li>FIG. 10 is a sectional view showing an example of a semiconductor substrate according to the fourth embodiment;</li><li>FIGS. 11A to 11F are sectional views for explaining an example of a semiconductor substrate manufacturing process according to the fourth embodiment;</li><li>FIG. 12 is a sectional view showing an example of a semiconductor device using the semiconductor substrate according to the fourth embodiment;</li><li>FIG. 13 is a sectional view showing an example of a semiconductor substrate according to the fifth embodiment;</li><li>FIGS. 14A to 14F are sectional views for explaining an example of a semiconductor substrate manufacturing process according to the fifth embodiment;</li><li>FIG. 15 is a sectional view showing an example of a semiconductor device using the semiconductor substrate according to the fifth embodiment;</li><li>FIGS. 16A to 16C are sectional views showing an example of a semiconductor substrate according to the first modification to the fifth embodiment;</li><li>FIGS. 17A to 17C are sectional views showing an example of a semiconductor substrate according to the second modification to the fifth embodiment;</li><li>FIGS. 18A and 18B are sectional views showing an example of a semiconductor substrate manufacturing process according to the prior art; and</li><li>FIGS. 19A and 19B are sectional views showing another example of a semiconductor substrate manufacturing process according to the prior art.</li></ul>

0020Embodiments of the present invention will be described in detail below with reference to the several views of the accompanying drawing.

(First Embodiment)

0021As shown in FIG. 1, the first embodiment uses, as a starting material, a substrate 4 (to be referred to as a silicon germanium on insulator (SGOI) substrate hereinafter) prepared by forming a relaxed SiGe layer 12 on a BOX layer 11 serving as an insulating layer on an Si substrate 1 serving as a support substrate. A semiconductor substrate according to the first embodiment has a strained Si region A1 including a strained Si layer 21 formed on the relaxed SiGe layer 12 by an epitaxial growth of Si, and a relaxed Si region B1 including a relaxed Si layer 22 without stress that is formed on a selectively epitaxially grown Si layer 15 at almost the same level as the strained Si layer 21. According to the feature of the semiconductor substrate, control of the film thickness of the strained Si layer 21 is facilitated at high quality.

0022The manufacturing process will be explained with reference to FIGS. 2A to 2F. <ul id="ul0002" list-style="none" compact="compact"><li>(1) As shown in FIG. 2A, a silicon oxide film (SiO₂ film) 16 is formed on the entire surface of an SGOI substrate 4. The SGOI substrate 4 is prepared by forming, for example, a 100-nm thick relaxed SiGe layer 12 having a Ge concentration of 15% on a 100-nm thick BOX layer 11 on an Si substrate 1. The SiO₂ film 16 has a thickness of, e.g., 100 nm and may be formed by low pressure chemical vapor deposition (LPCVD).</li><li>(2) As shown in FIG. 2B, the SiO₂ film 16 in the region B1 where a relaxed Si layer is to be formed is removed. The relaxed SiGe layer 12 and a part of the BOX layer 11 in the region B1 are etched away. Etching can be anisotropic or isotropic etching, but anisotropic etching such as reactive ion etching (RIE) is preferable for micropatterning.</li><li>(3) As shown in FIG. 2C, a second SiO₂ film 17 is formed by thermal oxidization. Subsequently, a silicon nitride film (SiN film) 18 is deposited on the entire surface by, e.g., LPCVD. Although the first embodiment adopts the SiN film 18, another film such as an SiO₂ film may be used.</li><li>(4) As shown in FIG. 2D, the SiN film 18 deposited on the BOX layer 11 is etched by, e.g., RIE. The exposed BOX layer 11 is removed by wet etching to expose the Si substrate 1. Etching of the BOX layer 11 can be reactive etching such as RIE, but, for example, wet etching is preferable so as not to damage the surface of the Si substrate 1.</li><li>(5) As shown in FIG. 2E, an Si layer 15 is selectively epitaxially grown on the surface of the exposed Si substrate 1. As selective epitaxial growth conditions, for example, 0.25 slm of dichlorsilane (SiH₂Cl₂) and 0.1 slm of hydrogen chloride (HCl) are supplied using a hydrogen (H₂) carrier gas at a pressure of 10 Torr and a substrate temperature of 800°C. The growth thickness of the epitaxial Si layer 15 can be set equal to the total thickness of the relaxed SiGe layer 12 and BOX layer 11. If any damage exists in the surface of the Si substrate 1 in step (4), the crystal quality of the selectively epitaxially grown Si layer 15 degrades. The epitaxial Si layer 15 can be grown by solid phase epitaxial growth instead of selective epitaxial growth in a vapor phase. Solid phase epitaxial growth is performed as follows. An amorphous Si film is deposited thicker than the SiGe layer 12 thickness on the entire surface. The Si film is annealed at a high temperature to change the amorphous Si film on the Si substrate 1 in the region B1 into a single-crystal epitaxial Si layer 15 by solid phase epitaxial growth. At this time, amorphous Si on the SiN film 18 in the region A1 changes into poly-crystal Si. In order to planarize the surface and to remove the poly-Si at the same time, chemical-mechanical polishing (CMP) is executed to form the structure shown in FIG. 2E. </li><li>(6) After that, the SiN film 18 and the SiO₂ films 16 and 17, which cover the relaxed SiGe layer 12, are sequentially removed by wet etching using a phosphoric acid-containing solution and dilute hydrofluoric acid. An Si layer is epitaxially grown on the entire surface to form Si layers 21 and 22. Epitaxial growth is done, e.g., to a thickness of 20 nm by using 300 slm of an SiH₂Cl₂ source gas at a pressure of 10 Torr and a temperature of 800°C. Accordingly, as shown in FIG. 2F, the strained Si layer 21 and the relaxed Si layer 22 without stress can be simultaneously formed at almost the same level on the relaxed SiGe layer 12 and the epitaxial Si layer 15, respectively.</li></ul>

0023The strained state of the surface of the semiconductor substrate manufactured according to the first embodiment was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layer 12 and the relaxed Si layer 22 without stress was formed on the epitaxial Si layer 15.

0024As shown in FIG. 3, after forming an isolation IS in the semiconductor substrate, in the strained Si region A1, for example, an nMOS having a source/drain SDa, gate insulator GIa, and gate electrode GEa is formed. In the relaxed Si region B1, for example, a trench type dynamic random access memory (DRAM) cell having a source/drain SDb, gate insulator GIb, gate electrode GEb, and trench capacitor TC is formed. In the semiconductor device, part of the SiN film 18 remains below the isolation IS at the interface between the strained Si region A1 and the relaxed Si region B1. The SiN film 18 has an effect of completely isolating elements. The operation of the semiconductor device was evaluated to confirm that the operation speed was higher and the leakage current of the memory cell was lower than those of a semiconductor device manufactured by a conventional technique.

0025In this way, a semiconductor substrate can be manufactured in which both the strained Si region A1 having the strained Si layer 21 at the surface and the relaxed Si region B1 having the relaxed Si layer 22 without stress are formed at almost the same level. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.

(Second Embodiment)

0026The second embodiment is related to a semiconductor substrate preferable to manufacturing a complementary metal-oxide semiconductor (CMOS), based on the first embodiment. Similar to the first embodiment, as shown in FIG. 4, the second embodiment uses an SGOI 4, as a starting material, prepared by forming a relaxed SiGe layer 12 on a BOX layer 11 on an Si substrate 1. A semiconductor substrate according to the second embodiment has a strained Si region A1 including a strained Si layer 21 formed on the relaxed SiGe layer 12 on the BOX layer 11 by epitaxial growth of Si, and a relaxed Si region B2 including a relaxed Si layer 22 without stress that is formed on a strained SiGe layer 13 on a selectively epitaxially grown Si layer 15 at almost the same level as the strained Si layer 21. According to the feature of the semiconductor substrate, control of the film thicknesses of the strained Si layer 21 and relaxed Si layer 22 is facilitated at high quality.

0027The manufacturing process will be explained with reference to FIGS. 5A to 5C.

0028FIG. 5A is a sectional view identical to FIG. 2D. Manufacturing steps up to the step of FIG. 5A are the same as steps (1) to (4) in the first embodiment, and will be briefly explained. <ul id="ul0003" list-style="none" compact="compact"><li>(1) For example, a 100-nm thick SiO₂ film 16 is formed on an SGOI substrate 4.</li><li>(2) The SiO₂ film 16, relaxed SiGe layer 12, and a part of BOX layer 11 are removed from the region B2 where the relaxed Si region B2 is being formed.</li><li>(3) A thermal oxide film 17 is formed, then an SiN film 18 is deposited on the entire surface.</li><li>(4) The SiN film 18 on the BOX layer 11 and the remaining BOX layer 11 are so removed as not to leave any damage in the Si substrate 1, thereby forming a structure shown in FIG. 5A.</li><li>(5) As shown in FIG. 5B, an Si layer 15 and strained SiGe layer 13 are successively formed by selective epitaxial growth on only the Si substrate 1 exposed in the region B2. As selective epitaxial growth conditions of Si, 0.25 slm of SiH₂Cl₂ and 0.1 slm of HCl are supplied using an H₂ carrier gas at a pressure of 10 Torr and a temperature of 800°C. Subsequently, the strained SiGe layer 13 is epitaxially grown in the same apparatus using 0.4 slm of SiH₂Cl₂ and 0.2 slm of GeH₄ at a pressure of 10 Torr and a temperature of 800°C. The thickness of the strained SiGe layer 13 is preferably 10 to 30 nm. The total thickness of the epitaxial Si layer 15 and strained SiGe layer 13 can be set equal to the total thickness of the relaxed SiGe layer 12 and BOX layer 11.</li><li>(6) Thereafter, the SiN film 18 and the SiO₂ films 16 and 17, which cover the relaxed SiGe layer 12, are sequentially removed by wet etching using a phosphoric acid-containing solution and dilute hydrofluoric acid. An Si layer is epitaxially grown on the entire surface to form Si layers 21 and 22. Epitaxial growth is done to a thickness of 20 nm by using 300 slm of an SiH₂Cl₂ source gas at a pressure of 10 Torr and a temperature of 800°C. As shown in FIG. 5C, the strained Si layer 21 and the relaxed Si layer 22 without stress can be simultaneously formed at almost the same level on the relaxed SiGe layer 12 and on the strained SiGe layer 13 formed on the epitaxial Si layer 15, respectively.</li></ul>

0029Formation of the epitaxial Si layer 22 on the strained SiGe layer 13 can also be prevented by performing epitaxial growth while covering the strained SiGe layer 13 with, e.g., an SiO₂ film.

0030The strained state of the surface of the semiconductor substrate manufactured according to the second embodiment was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layer 12 and the relaxed Si layer 22 without stress was formed on the strained SiGe layer 13 on the epitaxial Si layer 15.

0031As shown in FIG. 6, after forming an isolation IS in the semiconductor substrate, in the strained Si region A1, for example, an nMOS having a source/drain SDa, gate insulator GIa, and gate electrode GEa is formed. In the relaxed Si region B2, for example, a pMOS having a source/drain SDb, gate insulator GIb, and gate electrode GEb is formed. The pMOS is so manufactured as to form a channel in the strained SiGe layer 13. In the semiconductor device, the SiO₂ film 17 and SiN film 18 partially remain below the isolation IS at the interface between the strained Si region A1 and the relaxed Si region B2. The SiO₂ film 17 and SiN film 18 have an effect of completely isolating elements. As a result of evaluating the operation of the semiconductor device, the switching speeds of both the nMOS and pMOS increased in comparison with a semiconductor device manufactured by a conventional technique.

0032As described above, a semiconductor substrate can be manufactured in which both the strained Si region A1 having the strained Si layer 21 on the surface and the relaxed Si region B2 having the relaxed Si layer 22 without stress are formed at almost the same level. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.

(Third Embodiment)

0033As shown in FIG. 7, the third embodiment adopts a substrate (to be referred to as a silicon-germanium on nothing (SGON) hereinafter) prepared by forming a relaxed SiGe layer above a cavity, instead of the SGOI substrate 4. The third embodiment forms an SGON structure by using an Si substrate 1 as a starting material. As shown in FIG. 7, a semiconductor substrate according to the third embodiment has a strained Si region A2 including a strained Si layer 21 formed on a relaxed SiGe layer 12 having a cavity 33 by epitaxial growth of Si, and a relaxed Si region B3 including a relaxed Si layer 22 without stress that is formed on the Si substrate 1 at almost the same level as the strained Si layer 21. According to the feature of the semiconductor substrate, control of the film thickness of the strained Si layer 21 is facilitated at high quality. In addition, the third embodiment can reduce the number of manufacturing steps and the manufacturing cost in comparison with the first and second embodiments.

0034The manufacturing process of the third embodiment will be explained with reference to FIGS. 8A to 8F. <ul id="ul0004" list-style="none" compact="compact"><li>(1) As shown in FIG. 8A, an SiO₂ film 16 is formed on an Si substrate 1 by, e.g., thermal oxidization or CVD. The SiO₂ film 16 in a region A2 where a strained Si layer is to be formed is patterned and removed.</li><li>(2) As shown in FIG. 8B, the Si substrate 1 exposed in the region A2 is removed by, e.g., 2 µm. The entire surface is thermally oxidized to form a second SiO₂ film 17, and an SiN film 18 is formed on the entire surface by, e.g., CVD. Although the third embodiment employs the SiN film 18, another film such as an SiO₂ film can be used.</li><li>(3) As shown in FIG. 8C, the SiN film 18 in the region A2 is removed by anisotropic etching such as RIE. The exposed second SiO₂ film 17 is removed by, e.g., wet etching to expose the Si substrate 1. A strained SiGe layer 13 is formed on the exposed Si substrate 1 by selective epitaxial growth to a thickness of, e.g., 2 <i>µ</i>m by which the Si substrate 1 has been removed in step (2). As selective epitaxial growth conditions of the SiGe layer 13, for example, 0.4 slm of SiH₂Cl₂ and 0.2 slm of GeH₄ are supplied using an H₂ carrier gas at a pressure of 10 Torr and a temperature of 800°C.</li><li>(4) As shown in FIG. 8D, a plurality of trenches 32 are formed in the strained SiGe layer 13. The trench 32 has, e.g., a diameter of 0.2 µm and a depth of 2 µm.</li><li>(5) The substrate having the trenches 32 is annealed in hydrogen. The annealing condition is, e.g., a temperature of 850°C, a pressure of 300 Torr, and a time of 10 min. In annealing, Si and Ge atoms in the strained SiGe layer 13 are migrated, and the trenches 32 couple with each other to form a cavity 33. At the same time, the strain of the SiGe layer is relaxed to form a relaxed SiGe layer 12 above the cavity, thus forming a structure shown in FIG. 8E.</li><li>(6) The SiN film 18 and the SiO₂ films 16 and 17, which cover the surface of the Si substrate 1, are sequentially removed by wet etching using a phosphoric acid-containing solution and dilute hydrofluoric acid. An Si layer is epitaxially grown on the entire surface to form Si layers 21 and 22. Epitaxial growth is done to a thickness of 20 nm by using 300 slm of an SiH₂Cl₂ source gas at a pressure of 10 Torr and a temperature of 800°C. Consequently, as shown in FIG. 8F, the strained Si layer 21 and the relaxed Si layer 22 without stress can be formed at almost the same level on the relaxed SiGe layer 12 and the Si substrate 1, respectively.</li></ul>

0035The strained state of the surface of the semiconductor substrate manufactured according to the third embodiment was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layer 12 and the relaxed Si layer 22 without stress was formed on the Si substrate 1.

0036As shown in FIG. 9, after forming an isolation IS in the semiconductor substrate, in the strained Si region A2, for example, an nMOS having a source/drain SDa, gate insulator GIa, and gate electrode GEa is formed. In the relaxed Si region B3, for example, a trench DRAM cell having a source/drain SDb, gate insulator GIb, gate electrode GEb, and trench capacitor TC is formed. In the semiconductor device, the SiO₂ film 17 and SiN film 18 partially remain below the isolation IS at the interface between the strained Si region A2 and the relaxed Si region B3. The SiO₂ film 17 and SiN film 18 have an effect of completely isolating elements. The operation of the semiconductor device was evaluated to confirm that the operation speed was higher and the leakage current of the memory cell was lower than those of a semiconductor device manufactured by a conventional technique.

0037In this fashion, a semiconductor substrate can be manufactured in which both the strained Si region A2 having the strained Si layer 21 on the surface and the relaxed Si region B3 having the relaxed Si layer 22 without stress are formed at almost the same level. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.

(Fourth Embodiment)

0038The fourth embodiment concerns a substrate applied to a CMOS semiconductor device, similar to the second embodiment. As shown in FIG. 10, the semiconductor substrate according to the fourth embodiment has a strained Si region A3 including a strained Si layer 21 that is epitaxially grown on a relaxed SiGe layer 12-2 on an Si substrate 1, and a relaxed Si region B2 including a relaxed Si layer 22 without stress that is epitaxially grown on a strained SiGe layer 13 on an Si layer 15 formed on the Si substrate 1. This semiconductor substrate, therefore, has the strained Si region A3 and relaxed Si region B2 at almost the same level. According to the feature of the semiconductor substrate, control of the film thicknesses of the strained Si layer 21 and relaxed Si layer 22 is facilitated at high quality.

0039The manufacturing process of the fourth embodiment will be explained with reference to FIGS. 11A to 11F. <ul id="ul0005" list-style="none" compact="compact"><li>(1) As shown in FIG. 11A, an SiO₂ film 16 is formed to a thickness of 100 nm by, e.g., thermal oxidization or CVD on an Si substrate 1. The SiO₂ film 16 in the prospective formation region of the strained Si region A3 is patterned and removed to expose the Si substrate 1.</li><li>(2) As shown in FIG. 11B, a relaxed SiGe layer 12-1 is epitaxially grown to a thickness of, e.g., 2 <i>µ</i>m on the Si substrate 1 in the region A3. At this time, a poly-SiGe layer 35 is formed on the SiO₂ film 16 in the region B2. The relaxed SiGe layer 12 can be formed by solid phase epitaxial growth instead of vapor phase epitaxial growth. The relaxed SiGe layer 12-1 can be formed as a so-called graded SiGe layer in which the Ge concentration is low near the Si substrate 1 and increases along with growth. The strain of the SiGe layer 12-1 was evaluated by X-ray diffraction analysis to confirm that 98% of the entire structure was the relaxed SiGe layer 12-1 whose lattices were relaxed. After the SiGe layer 12-1 with a predetermined thickness is formed, the surface is planarized by CMP. </li><li>(3) As shown in FIG. 11C, for example, an SiO₂ film 36, serving as a mask, is formed on the entire surface, and patterned to expose the poly-SiGe layer 35. The mask can be formed by another material such as an SiN film other than SiO₂. The poly-SiGe layer 35 is removed by, e.g., RIE using the SiO₂ film 36 as a mask. The poly-SiGe layer 35 can be removed by wet etching as far as the selectivity between poly-SiGe and single-SiGe is high. </li><li>(4) The mask SiO₂ film 36 and the SiO₂ film 16 on the surface of the Si substrate 1 in the region B2 are removed. As shown in FIG. 11D, a second SiO₂ film 17 and SiN film 18 are formed on the entire surface by, e.g., thermal oxidization and CVD, respectively.</li><li>(5) As shown in FIG. 11E, the SiN film 18 above the Si substrate 1 in the region B2 is removed by, e.g., RIE, and the second SiO₂ film 17 is removed with, e.g., a dilute hydrofluoric acid solution, thus exposing the Si substrate 1. Subsequently, selective epitaxial growth is performed to form an epitaxial Si layer 15 on the exposed Si substrate 1. As selective epitaxial growth conditions of Si, for example, 0.25 slm of SiH₂Cl₂ and 0.1 slm of HCl are supplied using an H₂ carrier gas at a pressure of 10 Torr and a temperature of 800°C. The thickness of the epitaxial Si layer 15 can be set equal to that of the relaxed SiGe layer 12-1.</li><li>(6) Thereafter, the SiN film 18 and second SiO₂ film 17 which cover the surface of the relaxed SiGe layer 12-1 are sequentially removed by wet etching using a phosphoric acid-containing solution and dilute hydrofluoric acid.</li></ul>

0040An SiGe layer is epitaxially grown to a thickness of, e.g., 200 nm on the entire surface to form SiGe layers 12-2 and 13. Epitaxial growth conditions of the SiGe layer are, e.g., 0.4 slm of SiH₂Cl₂, 0.2 slm of GeH₄, a pressure of 10 Torr, and a temperature of 800°C. The SiGe layer 12-2 grown on the relaxed SiGe layer 12-1 is a relaxed SiGe layer, and the SiGe layer 13 grown on the epitaxial Si layer 15 is a strained SiGe layer.

0041An Si layer is epitaxially grown to a thickness of, e.g., 20 nm on the entire surface to form Si layers 21 and 22. Epitaxial growth conditions are, e.g., 0.3 slm of SiH₂Cl₂ in an H₂ carrier gas, a pressure of 10 Torr, and a temperature of 800°C. Accordingly, as shown in FIG. 11F, the strained Si layer 21 and the relaxed Si layer 22 without stress can be formed at almost the same level on the relaxed SiGe layers 12-2 and 12-1 over the strained SiGe layer 13 on the epitaxial Si layer 15, respectively.

0042The strained state of the surface of the semiconductor substrate manufactured according to the fourth embodiment was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layers 12-1 and 12-2 and the relaxed Si layer 22 without stress was formed on the strained SiGe layer 13 on the Si substrate 1.

0043As shown in FIG. 12, after forming an isolation IS in the semiconductor substrate, in the strained Si region A3, for example, an nMOS having a source/drain SDa, gate insulator GIa, and gate electrode GEa is formed. In the relaxed Si region B2, for example, a pMOS having a source/drain SDb, gate insulator GIb, and gate electrode GEb is formed. The pMOS is so manufactured as to form a channel in the strained SiGe layer 13. In the semiconductor device, the SiO₂ film 17 and SiN film 18 partially remain below the isolation IS at the interface between the strained Si region A3 and the relaxed Si region B2. The SiO₂ film 17 and SiN film 18 have an effect of completely isolating elements. As a result of evaluating the operation of the semiconductor device, the switching speeds of both the nMOS and pMOS increased in comparison with a semiconductor device manufactured by a conventional technique.

0044Accordingly, a semiconductor substrate can be manufactured in which both the strained Si region A3 having the strained Si layer 21 on the surface and the relaxed Si region B2 having the relaxed Si layer 22 without stress are formed at almost the same level. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.

(Fifth Embodiment)

0045The fifth embodiment uses, as a starting material, a bulk strained Si substrate 2 prepared by forming a strained Si layer 14 on, e.g., a 2 µm thick relaxed SiGe layer 12 on an Si substrate 1. A semiconductor substrate according to the fifth embodiment has a strained Si region A3 including a strained Si layer 21 on the relaxed SiGe layer 12, and a relaxed Si region B1 including a relaxed Si layer 22 without stress on a selective epitaxial Si layer 15, by epitaxially growing Si on the entire surface, as shown in FIG. 13. That is, this semiconductor substrate has the strained Si region A3 and relaxed Si region B1 at almost the same level. As the features of the semiconductor substrate, the film thickness of the strained Si layer 21 can be easily controlled, and a high-quality strained Si region A3 and relaxed Si region B1 can be formed.

0046The manufacturing process will be explained with reference to FIGS. 14A to 14F. <ul id="ul0006" list-style="none" compact="compact"><li>(1) As shown in FIG. 14A, an entire strained Si layer 14 of a bulk strained Si substrate 2 is thermally oxidized. A formed oxide film is removed with a dilute hydrofluoric acid-containing solution to expose a relaxed SiGe layer 12. The strained Si layer 14 can also be etched away using a solution containing a mixture of fluoric acid and nitric acid. Note that a relaxed SiGe substrate having no strained Si layer 14 can also be adopted. In this case, a step of removing the strained Si layer 14 can be omitted. An SiO₂ film 16 and SiN film 18 are sequentially deposited on the entire surface of the SiGe layer 12 by, e.g., CVD.</li><li>(2) As shown in FIG. 14B, the SiO₂ film 16 and SiN film 18 in the region B1 where a relaxed Si layer is to be formed are patterned, then are removed. The relaxed SiGe layer 12 in the region B1 is etched away. Etching is preferably isotropic etching using a solution. The SiGe layer 12 can be completely removed by simultaneously etching part of the Si substrate 1.</li><li>(3) As shown in FIG. 14C, a second SiO₂ film 17 is formed by thermal oxidization. Subsequently, a second SiN film 19 is deposited on the entire surface by, e.g., CVD.</li><li>(4) As shown in FIG. 14D, the second SiN film 19 deposited above the Si substrate 1 is etched by, e.g., RIE. The exposed second SiO₂ film 17 is removed by wet etching to expose the Si substrate 1. Etching of the second SiO₂ film 17 can be ion etching such as RIE, but wet etching is preferable so as not to damage the surface of the Si substrate 1.</li><li>(5) As shown in FIG. 14E, an Si layer 15 is selectively epitaxially grown on the surface of the exposed Si substrate 1. As selective epitaxial growth conditions of Si, for example, 0.25 slm of SiH₂Cl₂ and 0.1 slm of HCl are supplied using an H₂ carrier gas at a pressure of 10 Torr and a temperature of 800°C. The thickness of the epitaxial Si layer 15 can be set equal to the thickness of the relaxed SiGe layer 12. If any damage exists on the surface of the Si substrate 1 in step (4), the crystallinity of the selectively epitaxially grown Si layer 15 degrades. In place of selective epitaxial growth, the epitaxial Si layer 15 can be grown by solid phase epitaxial growth.</li><li>(6) The SiN films 18 and 19 and the SiO₂ films 16 and 17 which cover the relaxed SiGe layer 12 are sequentially removed by wet etching using a phosphoric acid-containing solution and dilute hydrofluoric acid. An Si layer is epitaxially grown on the entire surface to form Si layers 21 and 22. Epitaxial growth is done to a thickness of 20 nm by using 0.3 slm of an SiH₂Cl₂ source gas at a pressure of 10 Torr and a temperature of 800°C. The growth rate can be controlled by the temperature and pressure. The growth rate was 3 to 450 Å/min at a temperature of 700°C to 800°C and a pressure of 10 to 60 Torr. By the epitaxial growth, as shown in FIG. 14F, the strained Si layer 21 and the relaxed Si layer 22 without stress can be formed at almost the same level on the relaxed SiGe layer 12 and the epitaxial Si layer 15, respectively.</li></ul>

0047The strained state of the surface of the semiconductor substrate manufactured according to the fifth embodiment was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layer 12 and the relaxed Si layer 22 without stress was formed on the epitaxial Si layer 15.

0048As shown in FIG. 15, in the strained Si region A3, for example, an nMOS having, a source/drain SDa, gate insulator GIa, and gate electrode GEa is formed. In the relaxed Si region B1, for example, a trench DRAM cell having a source/drain SDb, gate insulator GIb, gate electrode GEb, and trench capacitor TC is formed. In the semiconductor device, the SiO₂ film 17 and SiN film 19 partially remain below the isolation IS at the interface between the strained Si region A3 and the relaxed Si region B1. The SiO₂ film 17 and SiN film 19 have an effect of completely isolating elements. The operation of the semiconductor device was evaluated to confirm that the operation speed was higher and the leakage current of the memory cell was lower than those of a semiconductor device manufactured by a conventional technique.

0049Hence, a semiconductor substrate can be manufactured in which both the strained Si region A3 having the strained Si layer 21 on the surface and the relaxed Si region B1 having the relaxed Si layer 22 without stress are formed at almost the same level. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.

(First Modification to Fifth Embodiment)

0050In the first modification, a number of manufacturing steps in the fifth embodiment is reduced and so modified as to be suited to high integration. The first modification employs, as a starting material, a relaxed SiGe substrate 5 prepared by forming, e.g., a 2 <i>µ</i>m thick relaxed SiGe layer 12 on an Si substrate 1. This can omit a step of removing a strained Si layer by, e.g., thermal oxidation. In addition, the first modification can prevent to concentrate Ge in the surface of the relaxed SiGe layer 12 due to unexpected thermal oxidation of the relaxed SiGe layer 12 during the removal step of the strained Si layer. The relaxed SiGe layer 12 in the region A3, where strained Si is to be formed, is removed by anisotropic etching. Therefore, this modification can also prevent undercut below the mask SiN film 18 and SiO₂ film 16 by lateral etching, and is suitable for high integration.

0051A process modified from the fifth embodiment will be explained with reference to FIGS. 16A to 16C. <ul id="ul0007" list-style="none" compact="compact"><li>(1) An SiO₂ film 16 and SiN film 18 are formed on the entire surface of a relaxed SiGe substrate 5. As shown in FIG. 16A, the SiO₂ film 16 and SiN film 18 in the relaxed Si region B1, where an unstrained Si layer is to be formed, are patterned and removed.</li><li>(2) Subsequently, as shown in FIG. 16B, the relaxed SiGe layer 12 and part of Si substrate 1 are removed by anisotropic etching such as RIE using the SiO₂ film 16 and SiN film 18 as a mask. At this time, the anisotropic etching prevents lateral etching of relaxed SiGe layer 12 below the mask.</li></ul>

0052Thereafter, steps (3) to (6) in the fifth embodiment are executed. More specifically, a second SiO₂ film 17 and second SiN film 19 are formed on the entire surface. The second SiN film 19 and second SiO₂ film 17 on the Si substrate 1 are removed, and an Si layer 15 is formed on the exposed Si substrate 1 by selective epitaxial growth. The SiN films 18 and 19 and the SiO₂ films 16 and 17 on the surface of the relaxed SiGe layer 12 are removed, and an Si layer is epitaxially grown on the entire surface to form Si layers 21 and 22. Consequently, as shown in FIG. 16C, a semiconductor substrate can be formed in which the strained Si layer 21 and the relaxed Si layer 22 without stress can be formed at almost the same level on the relaxed SiGe layer 12 and the epitaxial Si layer 15, respectively. In the semiconductor substrate of the first modification, the width of the boundary region between the strained Si layer 21 and the relaxed Si layer 22 without stress can be narrowed as compared with the semiconductor substrate of the fifth embodiment.

0053The strained state of the surface of the semiconductor substrate manufactured according to the first modification was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layer 12 and the relaxed Si layer 22 without stress was formed on the epitaxial Si layer 15.

0054On the semiconductor substrate, similar to FIG. 15, for example, an nMOS is formed in the strained Si region A3, and a trench DRAM cell is formed in the relaxed Si region B1. The operation of the semiconductor device was evaluated to confirm that the operation speed was higher and the leakage current of the memory cell was lower than those of a semiconductor device manufactured by a conventional technique.

0055A semiconductor substrate can be manufactured in which both the strained Si region A3 having the strained Si layer 21 on the surface and the relaxed Si region B1 having the relaxed Si layer 22 without stress are formed at almost the same level. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.

(Second Modification to Fifth Embodiment)

0056In the second modification, a number of manufacturing steps of the fifth embodiment is reduced and so modified as to be suited to high integration. At the same time, the second modification avoids the possibility of damaging the Si substrate 1 by RIE in the forming process of the relaxed Si region B1 in the first modification. Similar to the first modification, the second modification uses, as a starting material, a relaxed SiGe substrate 5 prepared by forming, e.g., a 2 <i>µ</i>m thick relaxed SiGe layer 12 on an Si substrate 1. The second modification can simplify the process, similar to the first modification to the fifth embodiment. Further, the relaxed SiGe layer 12 in the region A3 where strained Si is to be formed is removed by anisotropic etching, then, the Si substrate 1 is removed by wet etching. Undercut below the mask SiN film 18 and SiO₂ film 16 by lateral etching can be suppressed, and any damage to the Si substrate 1 by RIE can also be prevented.

0057A process according to the second modification to the fifth embodiment will be described with reference to FIGS. 17A to 17C. <ul id="ul0008" list-style="none" compact="compact"><li>(1) Similar to the first modification, an SiO₂ film 16 and SiN film 18 are formed on the entire surface of a relaxed SiGe substrate 5. As shown in FIG. 17A, the SiO₂ film 16 and SiN film 18 in the relaxed Si region B1 where an unstrained Si layer is to be formed are patterned and removed.</li><li>(2) Subsequently, as shown in FIG. 17B, a relaxed SiGe layer 12 in the region B1 is removed by anisotropic etching such as RIE using the SiO₂ film 16 and SiN film 18 as a mask. At this time, anisotropic etching suppresses lateral etching below the mask. Part of the Si substrate 1 is removed by wet etching. This hardly damages the surface of the Si substrate 1 and can suppress the undercut amount below the mask to very small. For the wet etching of the Si substrate 1, it can be used, e.g., a mixture of an alkaline solution and a hydrogen peroxide.</li></ul>

0058Thereafter, steps (3) to (6) in the fifth embodiment are executed. More specifically, a second SiO₂ film 17 and second SiN film 19 are formed on the entire surface. The second SiN film 19 and second SiO₂ film 17 on the Si substrate 1 are removed, and an Si layer 15 is formed on the exposed Si substrate 1 by selective epitaxial growth. The SiN films 18 and 19 and the SiO₂ film 16 on the surface of the relaxed SiGe layer 12 are removed, and an Si layer is epitaxially grown on the entire surface to form Si layers 21 and 22. As shown in FIG. 17C, a semiconductor substrate can be manufactured in which the strained Si layer 21 and the relaxed Si layer 22 without stress can be formed at almost the same level on the relaxed SiGe layer 12 and the epitaxial Si layer 15, respectively. In the semiconductor substrate of the second modification, the width of the boundary region between the strained Si layer 21 and the relaxed Si layer 22 without stress can be narrowed. At the same time, the second modification avoids the possibility of damaging the surface of the Si substrate 1 by RIE. As a result, the crystallinity of the epitaxial Si layer 15 can improve.

0059The strained state of the surface of the semiconductor substrate manufactured according to the second modification was evaluated by Raman spectroscopy analysis. It was confirmed that the strained Si layer 21 was formed on the relaxed SiGe layer 12 and the relaxed Si layer 22 without stress was formed on the epitaxial Si layer 15.

0060On the semiconductor substrate, similar to FIG. 15, for example, an nMOS is formed in the strained Si region A3, and a trench DRAM cell is formed in the relaxed Si region B1. The operation of the semiconductor device was evaluated to confirm that the operation speed was higher and the leakage current of the memory cell was lower than those of a semiconductor device manufactured by a conventional technique.

0061A semiconductor substrate can, therefore, be manufactured in which both the strained Si region A3 having the strained Si layer 21 on the surface and the relaxed Si region B1 having the relaxed Si layer 22 without stress are formed at almost the same level with the strained Si layer 21. By using this semiconductor substrate, a higher-speed, higher-performance semiconductor device than a semiconductor device manufactured by a conventional method can be manufactured.