Abstract:
A method of fabricating a semiconductor structure. According to one aspect of the invention, on a first semiconductor substrate, a first compositionally graded Si 1-x Ge x  buffer is deposited where the Ge composition x is increasing from about zero to a value less than about 20%. Then a first etch-stop Si 1-y Ge y  layer is deposited where the Ge composition y is larger than about 20% so that the layer is an effective etch-stop. A second etch-stop layer of strained Si is then grown. The deposited layer is bonded to a second substrate. After that the first substrate is removed to release said first etch-stop Si 1-y Ge y  layer. The remaining structure is then removed in another step to release the second etch-stop layer. According to another aspect of the invention, a semiconductor structure is provided. The structure has a layer in which semiconductor devices are to be formed. The semiconductor structure includes a substrate, an insulating layer, a relaxed SiGe layer where the Ge composition is larger than approximately 15%, and a device layer selected from a group consisting of, but not limited to, strained-Si, relaxed Si 1-y Ge y  layer, strained S 1-z Ge z  layer, Ge, GaAs, III-V materials, and II-VI materials, where Ge compositions y and z are values between 0 and 1.

Description:
PRIORITY INFORMATION 
   This application claims priority from provisional application Ser. No. 60/281,502 filed Apr. 4, 2001. 

   BACKGROUND OF THE INVENTION 
   The invention relates to the production of a general semiconductor substrate of relaxed Si 1-x Ge x -on-insulator (SGOI) for various electronics or optoelectronics applications, the production of strained Si or strained SiGe field effect transistor (FET) devices on SGOI, and the production of monocrystalline III-V or II-VI material-on-insulator substrates. 
   Relaxed SGOI is a very promising technology as it combines the benefits of two advanced technologies: the conventional SOI technology and the disruptive SiGe technology. The SOI configuration offers various advantages associated with the insulating substrate, namely reduced parasitic capacitances, improved isolation, reduced short-channel-effect, etc. The SiGe technology also has various advantages, such as mobility enhancement and integration with III-V devices. 
   One significant advantage of the relaxed SGOI substrate, is to fabricate high mobility strained-Si, strained-Si 1-x Ge x  or strained-Ge FET devices. For example, strained-Si MOSFETs can be made on the SGOI substrate. The strained-Si MOSFETs on the SGOI has attracted attention because it promises very high electron and hole mobilities, which increase the speed of the electronic circuit. Other III-V optoelectronic devices can also be integrated into the SGOI substrate by matching the lattice constants of III-V materials and the relaxed Si 1-x Ge x . For example, a GaAs layer can be grown on Si 1-x Ge x -on-insulator where x is equal or close to 1. SGOI may serve as an ultimate platform for high speed, low power electronic and optoelectronic applications. 
   There are several methods for fabricating SGOI substrates and SGOI FET devices. In one method, the separation by implantation of oxygen (SIMOX) technology is used to produce SGOI. SIMOX uses a high dose oxygen implant to bury a high concentration of oxygen in a Si 1-x Ge x  layer, which will then be converted into a buried oxide (BOX) layer upon annealing at a high temperature. One of the main drawbacks is the quality of the resulting Si 1-x Ge x  film and the BOX layer. In addition, the Ge segregation during the high temperature anneal also limits the amount of Ge composition to a value that is low, such as 10%. Due to the low Ge composition, the device fabricated on those SGOI substrates has limited performance. For example the strained-Si MOSFETs fabricated on the SGOI by the SIMOX process have limited electron or hole mobility enhancement due to the low Ge composition, since the mobility enhancement is dependent on Ge composition through the degree of the strain in the strained-Si layer. 
   In a second method, a conventional silicon-on-insulator (SOI) substrate is used as a compliant substrate. In this process, an initially strained Si 1-x Ge x  layer is deposited on a thin SOI substrate. Upon an anneal treatment, the strain in the Si 1-x Ge x  layer is transferred to the thin silicon film underneath, resulting in relaxation of the top Si 1-x Ge x  film. The final structure is a relaxed-SiGe/strained-Si/insulator. The silicon layer in the structure is unnecessary for an ideal SGOI structure, and may complicate or undermine the performance of devices built on it. For example, it may form a parasitic back channel on the strained-Si, or may confine unwanted electrons due to the band gap offset between the strained-Si and SiGe layer. 
   In a third method, a similar SGOI substrate is produced using a p ++  layer as an etch stop. On a first Si substrate, a compositionally graded SiGe buffer is deposited, followed by deposition of multiple material layers including a relaxed SiGe layer, a p ++  etch stop layer, and a Si layer. After bonding to a second substrate, the first substrate is removed. In an etching process, the compositionally graded SiGe buffer is etched away and etching stops at P ++  etch stop layer, resulting in a relaxed-SiGe/Si/insulator structure. The presence of the silicon layer in the structure may be for the purpose of facilitating the wafer bonding process, but is unnecessary for ideal SGOI substrates. Again, the silicon layer may also complicate or undermine the performance of devices built on it. For example, it may form a parasitic back channel on this strained-Si, or may confine unwanted electrons due to the band gap offset between the Si and SiGe layer. Moreover, the etch stop of p ++  in the above structure is not practical when a first graded Si 1-y Ge y  layer has a final y value larger than 0.2. This is because the etch rate of KOH will slow down dramatically when KOH reaches the Si 1-y Ge y  layer with a Ge composition larger than 0.2, and that layer is itself a very good etch stop for KOH. Therefore, KOH will not be able to remove practically all of the first compositionally graded Si 1-y Ge y  layer (when y is larger than 0.2) and the second relaxed SiGe layer, thus using a p ++  layer as an etch-stop for KOH is not practical. 
   Other attempts include re-crystallization of an amorphous Si 1-x Ge x  layer deposited on the top of a SOI (silicon-on-insulator) substrate. Again, such a structure is not an ideal SGOI substrate and the silicon layer is unnecessary, and may complicate or undermine the performance of devices built on it. The relaxation of the resultant SiGe film and quality of the resulting structure are main concerns. 
   In a recent method, relaxed Si 1-x Ge x -on-insulator is produced by using 20% SiGe layer as an etch-stop. First a compositionally graded Si 1-x Ge x  buffer (where x is less than about 0.2) and then a uniform Si 1-y Ge y  etch-stop layer (where y is larger than about 0.2) are deposited on the first substrate. Then the deposited layer is bonded to a second insulating substrate. After removing the first substrate and graded buffer layer utilizing the Si 1-y Ge y  as an etch-stop, a Si 1-y Ge y -on-insulator (SGOI) results. The method makes use of an experimental discovery that Si 1-y Ge y  with Ge composition larger than about 20% is a good etch-stop for all three conventional Si etchant systems, KOH, TMAH and EDP, and the selectivity is better than the conventional p ++  etch stop. In this method the etch-stop Si 1-y Ge y  layer is part of the final SGOI structure. However, as the Ge composition in the final SGOI structure is fixed by the etch-stop Si 1-y Ge y , if the desired Ge composition in the final SGOI structure is much higher or lower than 0.2, the above method is not practical. If it is much lower than 0.2, for example 0.1, Si 0.9 Ge 0.1  is not a good etch stop at all. If it is much larger than 0.2, the Ge composition difference between the etch-stop layer and surface layer in the grade buffer is too big and there is large lattice constant difference between the two layers, which prevents the growth of a relaxed etch-stop Si 1-y Ge y  layer with good quality. 
   From above, clearly an improved method is needed to fabricate a relaxed SGOI substrate with high Ge composition and wide range of Ge composition. An improved method is needed to fabricate strained-Si or strained-SiGe FET devices on SGOI substrate with high Ge composition. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, the invention provides a method of semiconductor device fabrication, and more specifically, a method of production of a general semiconductor substrate of relaxed SGOI for various electronics or optoelectronics applications, a method of production of strained Si or strained SiGe FET devices on SGOI, and the production of monocrystalline III-V or II-VI material-on-insulator substrates. The invention provides a method of producing a relaxed Si 1-x Ge x -on-insulator substrate with high Ge composition and wide range of Ge composition, and the Ge composition may be much less or much higher than 20%. The invention provides an improved method to fabricate strained-Si or strained-SiGe MOSFET devices on SGOI substrate with high Ge composition. When strained-Si n-MOSFETs are fabricated on relaxed Si 1-x Ge x -on-insulators substrates with a high Ge composition, 25% for example, there is significant enhancement on electron mobility as compared to the co-processed bulk-Si MOSFETs on conventional bulk Si substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 ( a )- 1 ( d ) are flow process diagrams of a SGOI substrate fabrication process; 
     FIG.  1 ( e ) is a block diagram of relaxed SiGe and strained-Si regrowth on a relaxed SGOI substrate for strained-Si MOSFET application; 
       FIG. 2  is a block diagram of a compositionally graded Si 1-x Ge x  buffer deposited epitaxially on a Si substrate; 
       FIG. 3  is a micro-photograph of an exemplary strained-Si surface channel MOSFET device fabricated on relaxed Si 0.75 Ge 0.25 -on-insulator; 
       FIG. 4  is a graph showing the measured experimental effective electron mobility as a function of effective vertical electric field from the exemplary strained-Si MOSFET device shown in  FIG. 3 ; 
       FIG. 5  is a block diagram of another embodiment of a of SGOI structure with 10% Ge; 
       FIG. 6  is a block diagram of another embodiment of a SGOI structure with 80% Ge using two etch-stops; 
       FIG. 7  is a block diagram showing the production of an III-V on insulator structures; and 
       FIG. 8  is a block diagram of another embodiment of a SGOI structure with improved SiGe layer thickness uniformity. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   FIGS.  1 ( a )- 1 ( d ) are flow process diagrams of an experimental fabrication process of a SGOI substrate with Ge composition of 25% in accordance with one embodiment of the invention. Starting with a 4-inch Si (100) substrate  2 , high quality relaxed Si 0.75 Ge 0.25  layer  4  is grown at 900° C. by UHVCVD using a compositionally graded Si 1-x Ge x  buffer  6  technique as described in U.S. Pat. No. 5,221,413 issued to Brasen et al., which is incorporated herein by reference in its entirety. Using this technique, a compositionally graded Si 1-x Ge x  buffer  6  can be grown epitaxially on Si substrate, which allows a relaxed SiGe layer to be grown on the top of the buffer with low threading dislocation density. 
   FIG.  2 . is a block diagram of a compositionally graded Si 1-x Ge x  buffer  30 . The compositionally graded Si 1-x Ge x  buffer  30  is a multi-layer structure where the Ge composition in each layer is changing gradually from a beginning value to a final value. For example, the compositionally graded Si 1-x Ge x  buffer  30  shown in  FIG. 2  has 16 layers, and the Ge composition x in the first layer is 0% and is increasing gradually to 2%, 4%, 6% until 30% in the last layer (layer  16 ). Such a compositionally graded Si 1-x Ge x  buffer  30  allows a high quality relaxed Si 0.7 Ge 0.3  layer to be grown on the top of the buffer with low threading dislocation density. 
   Referring to FIGS.  1 ( a )- 1 ( d ), a compositionally graded Si 1-x Ge x  buffer  6  is epitaxially grown on a 4-inch Si (100) substrate  2 , where the Ge composition x is increasing gradually from zero to 25% with a grading rate of 10% Ge/μm. Within the compositionally graded Si 1-x Ge x  buffer  6 , a portion of the buffer  6  with Ge composition larger than about 20% forms a natural etch stop. A 2.5 μm-thick undoped, relaxed Si 0.75 Ge 0.25  cap layer  4  is then deposited, as shown in FIG.  1 ( a ). The slow grading rate and high growth temperature result in a completely relaxed cap layer  4  with threading dislocation densities of ˜10 5  cm −2 . As shown in FIG.  1 ( b ), the wafer  2  is then flipped over and bonded to a second Si substrate  10 , which is thermally oxidized. The oxide  12  in the second substrate will become the insulator layer in the final SiGe-on-insulator substrate. The bonded pair is then annealed at 850° C. for 1.5 hrs. The bonded pair is grounded to remove the donor wafer substrate  8 , as shown in FIG.  1 ( c ). The wafer  8  is then subjected to a TMAH solution to etch away a portion of the compositionally graded Si 1-x Ge x  buffer  6  with Ge composition less than 20%. The etching process stops approximately at a 20% SiGe layer  14  within the compositionally graded Si 1-x Ge x  buffer  6  and the 20% SiGe layer  14  is used as a natural etch stop. 
   After performing the etching process, the remaining portion of the compositionally graded Si 1-x Ge x  buffer  14  with a Ge composition between 20% to 25% and part of the relaxed Si 0.75 Ge 0.25  layer  4  are removed by chemical-mechanical polishing (CMP), resulting in a relaxed Si 0.75 Ge 0.25 -on-insulator substrate, as shown in FIG.  1 ( d ). The CMP process is also essential in planarizing the SGOI surface for epitaxial regrowth in the next step. As shown in FIG.  1 ( e ), in order to make a strained-Si device  22 , a 100 nm p-type (doping 10 16  cm −3 ) relaxed Si 1-x Ge x  layer  18  is grown at 850° C. with a Ge composition of 25%, followed by 8.5 nm-thick undoped strained-Si layer  20  grown at 650° C. Electronic devices may be fabricated on the above semiconductor structure. In particular, a large size strained-Si n-MOSFETs can be fabricated on the above structure and significant electron mobility enhancement is observed from the strained-Si MOSFETs. 
     FIG. 3  is a micro-photograph of a strained-Si, surface channel n-MOSFETs on the relaxed SGOI substrate. The n-MOSFET includes gate stack  24  that has a 300 nm low temperature oxide (LTO)  26  deposited via LPCVD at 400° C., and a 50 nm of poly-Si  28  deposited at 560° C. The large thickness of the LTO gate  24  dielectric facilitates the process, as described below. Capacitors fabricated with LTO have demonstrated interface state densities on par with thermal oxides (˜5×10 10  cm −2  eV −1 ). The measured fixed oxide charge density is about 2.4×10 11  cm −2 . 
   The gate stack  24  is then patterned and etched into MOSFET structures. A key step is the use of a buffered oxide etchant (BOE) to undercut the gate polysilicon, forming a large “T-gate” geometry. Arsenic ion implants (35 keV, total dose 1×10 15  cm −2 ) are performed to dope both the source/drain  30  and gate  24  regions at 4 perpendicular directions with a 7° tilt to extend the source/drain regions under the T-gate structure. The dopant is activated via RTA at 1000° C. for 1 s. Since the strained-Si layer  32  is in equilibrium, no relaxation via misfit dislocation introduction occurred. Blanket Ti/Al metallization is performed via e-beam deposition at a perpendicular incidence. Due to the extreme geometry of the “T-gate” FET structure and large gate LTO  26  thickness, breaks occur in the metal which isolate the source, gate, and drain regions  24  and  30  without further lithography. 
   Long channel n-MOSFETs (effective channel length L eff =200 μm) are used to evaluate the electron mobility as a function of the vertical field. The effective electron mobility μ eff  is extracted from the linear regime device current that is defined as:
 
μ eff =( L   eff   /W   eff ) I   DS   /[C   ox ( V   GS   −V   T ) V   DS ],  Eq. 1
 
where L eff  is effective channel length, W eff  is effective channel width, I DS  is current from the drain to source, C ox  is the oxide capacitance, V GS  is gate to source voltage, V DS  is the drain to source voltage, wherein in this embodiment, V DS =0.1 V. The oxide capacitance is defined as
 
 C   ox =ε ox   /t   ox   Eq. 2
 
where ε ox  is the dielectric constant of oxide, and t ox  is the oxide thickness. The oxide capacitance is obtained from C-V measurements on the device, and the oxide thickness t ox =326 nm is also extracted from the C-V measurements. The effective vertical field E eff  is given by
 
 E   eff =( Q   b   +Q   inv /2)/ε s .  Eq. 3
 
where Q b  is the bulk depletion charge, Q inv  is the inversion charge, and ε s  is the dielectric constant of Si. Because of uncertainties in the strained-Si/Si 0.75 Ge 0.25  doping, the bulk depletion charge Q b  is not computed from the usual N A x d,max  approximation. Instead, Q b  is extracted from
 
 E   ox ε ox   =Q   inv   +Q   b ,  Eq. 4
 
where E ox  is the electric field in the gate oxide. As a result, the effective field can be approximated by
 
 E   eff   =[E   ox ε ox   −Q   inv /2]/ε s .  Eq. 5
 
The inversion charge Q inv  is taken to be
 
 C   ox ( V   GS   −V   T ). E   ox   Eq. 6
 
and is assumed to be equal to V GS /t ox , which holds under the conditions of strong inversion and V GS &gt;&gt;V DS , such that the potential difference between the strongly-inverted Si surface and the S/D regions is negligibly small compared with the large potential drop across the thick gate oxide.
 
     FIG. 4  is a graph demonstrating the measured effective electron mobility as a function of the effective vertical electric field on a strained-Si on SGOI. The graph also demonstrates the mobilities of two other controls, such as conventional bulk Si MOSFETs  34  and strained-Si MOSFETs  38  on relaxed bulk SiGe substrate, for comparison. Since all three devices have the same geometry and are processed simultaneously, possible errors due to factors such as the extraction of the ring geometry factor, and approximations in E eff  evaluation do not impact the relative comparison of the electron mobility characteristics. As shown in  FIG. 4 , the measured mobility for the CZ Si control device  34  is close to the universal mobility curve  40 .  FIG. 4  also shows that the measured electron mobility enhancement for strained Si MOSFETs  36  fabricated on SGOI as compared to the mobility of co-processed bulk Si MOSFETs  38  is significant (˜1.7 times). In addition, the electron mobilities are comparable for devices fabricated on SGOI  36  and bulk relaxed SiGe layers  38 , thus demonstrating the superior mobility performance introduced by the strained-Si channel is retained in this SGOI structure. This enhancement factor of 1.7 is consistent with previously reported experimental and theoretical values for strained-Si n-MOSFETs on bulk relaxed SiGe films. 
   This demonstrates that the fabrication of relaxed SGOI structures and strained-Si FET devices on SGOI with high Ge composition of 25% is practical. This also demonstrates that strained-Si MOSFETs fabricated on a SGOI substrate can significantly improve electron mobility. In contrast to the method of fabrication of SGOI by SIMOX process where the high annealing temperature limits the Ge composition to a low value, the process of forming a SGOI in accordance with the invention has a low thermal budget and thus is compatible with a wide range of Ge composition in the SGOI substrate. This embodiment of invention allows fabrication of a SGOI substrate and a strained-Si FET device with high Ge composition, and the Ge composition can be much higher than the Ge composition in the relaxed Si 1-y Ge y  etch-stop layer where y has a value close to 20%. 
   In a variation of the above process, before the step of bonding, various of material layers like strained-Si, strained-SiGe, relaxed SiGe may also grown on the relaxed Si 0.75 Ge 0.25  cap layer  4 . For example, a three layer system, a strained-Si, a strained-SiGe and a relaxed SiGe layer, may be deposited before bonding. Therefore, after bonding and layer removal steps, the strained-Si and strained-SiGe layers are on the SGOI structure and can be used to fabricate both n-MOSFET and p-MOSFET devices immediately without a regrowth step. 
     FIG. 5  is a block diagram of a low Ge composition SGOI substrate. The Ge composition in the SGOI substrate can be considerably less than the Ge composition in a relaxed Si 1-y Ge y  etch-stop layer where y has a value close to 20%. For example, a SGOI substrate with Ge composition of 10% can be fabricated. As shown in  FIG. 5 , a compositionally graded Si 1-x Ge x  buffer  46  is epitaxially grown on a silicon substrate  44 , where the Ge composition x is increasing gradually from about zero to about 20%. A uniform etch-stop layer  48  of relaxed Si 1-y Ge y  is deposited where Ge composition y is larger than or close to about 20%. Then a second compositionally graded Si 1-z Ge z  buffer  50  is grown on the etch-stop layer  48  where Ge composition z is decreasing gradually from a value close to 20% to a smaller value, in this embodiment 10%. Finally a uniform relaxed Si 0.9 e 0.1  layer  52  is grown. 
   After flipping over and bonding to a second substrate, the first substrate is removed. A wet etch of KOH or TMAH removes the first graded buffer and stops at the etch-stop layer  48 . After the etch-stop layer  48  and second compositionally graded Si 1-z Ge z  buffer  50  are removed, the relaxed Si 0.9 Ge 0.1  layer  52  is released, resulting in a Si 0.9 Ge 0.1 -on-insulator substrate. In summary, this process allows the production of SGOI with Ge composition much less than 20%. 
   The embodiment outlined in  FIG. 1  is also applicable to the fabrication of SGOI structures with very high Ge composition, for example 80%. However, the Si 0.2 Ge 0.8  layer in the final SGOI structure may not have good thickness uniformity for such high Ge composition. The SiGe layer thickness uniformity is important. For example, to fabricate strained-Si MOSFET devices on a SGOI structure, the performance of the devices strongly depends on the thickness of the Si 0.2 Ge 0.8  layer. A uniform SiGe layer is highly desired. To fabricate SGOI with Ge composition of 80% using the method described in  FIG. 1 , it necessitates the deposition of a relative thick compositionally graded Si 1-x Ge x  buffer where the Ge composition is increasing gradually from zero to 80%. A TMAH or KOH etch step etches away the portion of the compositionally graded Si 1-x Ge x  buffer where Ge composition is less than 20% and stops at 20% SiGe layer within the compositionally graded Si 1-x Ge x  buffer. The remaining portion of the compositionally graded Si 1-x Ge x  buffer is still considerably thick, where Ge composition varies from about 20% to 80%. For example, the remaining portion of the compositionally graded Si 1-x  Ge x  buffer with Ge composition from 20% to 80% has a thickness of 6 μm if the buffer is grown with a grading rate of 10% Ge/μm. 
   This 6 μm thick buffer needs to be removed in order to explore the Si 0.2 Ge 0.8  layer, for example by means of CMP. This removing step may induce significant non-uniformity. There are two possible sources of non-uniformity. First, the growth of the SiGe film itself may be not uniform across the whole substrate. For example, it is observed that the SiGe buffer can vary more than 10% in thickness if the surface of the Si substrate is placed in parallel to the direction of reactant gas flow in the CVD reactor during growth. In this orientation, one part of the substrate is in contact with higher concentration of gas than the other part since the gas concentration is decreasing along its flow pass as gas gets consumed. Therefore, the growth rate is different, resulting in differences of layer thickness. To avoid this non-uniformity, it is preferred that the surface of the Si substrate be placed normal to the direction of reactant gas flow in the reactor during the growth. 
   The second source comes from the removing process of the buffer layer. For example, if the buffer layer is removed by a polishing technique such as CMP, the CMP process may induce some uniformity. Although the CMP can improve the local uniformity, it may induce some global non-uniformity across the wafer. For example, the CMP process may polish the edge of the wafer faster than the center. As a result, the final SGOI structure has a non-uniform SiGe layer. Using two or more etch-stops, the system can improve the uniformity as described in the embodiment below. 
     FIG. 6  is block diagram of a SGOI substrate with improved SiGe layer uniformity using two etch stop layers, which is especially suitable for SGOI substrates with high Ge composition. As shown in  FIG. 6 , a compositionally graded Si 1-x Ge x  buffer  56  is grown on a silicon substrate  54 , where Ge composition x is increasing gradually from zero to about 0.2. A uniform etch-stop layer  60  of Si 0.8 Ge 0.2  is deposited, and then a continuing compositionally graded Si 1-y Ge y  buffer  62  is provided where Ge composition y is increasing gradually from about 0.2 to a higher value, for example 0.8. A second etch-stop layer  64  of strained-Si is then grown. A uniform Si 0.2 Ge 0.8  layer  66  is deposited with a Ge composition of 80%. After flipping over and bonding to a second insulating substrate, the first substrate is removed. During a first etching step, the first compositionally graded Si 1-x Ge x  buffer  56  is removed and the etching stops at the first etch-stop layer  60  of Si 0.8 Ge 0.2 . With another etching step, the second compositionally graded Si 1-y Ge y  buffer  62  is removed and the etching stops at the second etch-stop layer  64  of strained-Si. Removing the second etch-stop layer  64 , the final relaxed Si 0.2 Ge 0.8  layer  66  is released, resulting in a Si 0.2 Ge 0.8 -on-insulator substrate. {In the above process, the surface of the deposited layers may be very rough due to the crosshatch in the SiGe buffer. A smoother strained-Si and relaxed SiGe layer may be wanted. A CMP step can be used for this purpose to smooth for example the compositionally graded Si 1-y Ge y  buffer  62 , before depositing the second etch-stop layer  64 . 
     FIG. 7  is a block diagram of a GaAs-on-insulator substrate. As shown in  FIG. 7 , a compositionally graded Si 1-x Ge x  buffer  74  is grown on a silicon substrate  72 , where Ge composition x is increasing gradually from zero to about 1, i.e., to pure Ge composition. Within the compositionally graded Si 1-x Ge x  buffer, a portion of the buffer with Ge composition larger than about 20% forms a natural SiGe etch stop. Then a second etch-stop layer  76  of strained-Si is grown, followed by a relaxed Ge layer  78 . A uniform GaAs layer  80  is then deposited. After flipping over and bonding to a second insulating substrate, the first substrate is removed. During the first etching step, the portion of the compositionally graded Si 1-x Ge x  buffer  74  with Ge composition smaller than 20% is removed and the etching stops at the first etch-stop layer. With the second etching step, the remaining compositionally graded Si 1-x Ge x  buffer  74  is removed and the etching stops at the second etch-stop layer  76  of strained-Si. Removing the second etch-stop layer  76  of strained-Si and the Ge layer  78  results in a GaAs-on-insulator structure. 
   In all of the above-mentioned SGOI or GaAs-on-insulator fabrication processes, wafer bonding is used. In order to bond two surfaces, the surfaces should be smooth enough, with a very small surface roughness. However, the as-grown SiGe layer, strained Si layer, Ge layer or GaAs layer can be rough. Typically, the compositionally graded SiGe buffer shows a very rough surface due to the cross-hatch (a dislocation-induced phenomenon). The CMP process is conventionally used to smooth the surface before bonding. However, as described above, CMP may induce global non-uniformity across the wafer. Moreover, in some cases, there may not be enough thickness for a surface to be polished. For example, if a layer is a strained Si etch-stop layer, its thickness is very small in order to keep it strained without relaxation, for example 10 nm. 
   Two approaches may be used to solve this issue. The first approach is before depositing the last thin material layer (e.g., the last layer is a strained Si layer), polish the SiGe buffer layer to achieve enough surface smoothness. Then grow the last strained Si etch-stop layer, which results in a smoother final surface. If the surface is smooth enough, the structure can be bonded directly. Even if polishing is still needed, it will reduce the thickness to be polished. 
   The second approach requires before bonding to deposit an additional insulating material layer like an oxide layer on the first structure. Afterward, polish this additional insulating layer to achieve enough surface smoothness, and then bond the polished insulating layer to a second substrate. 
     FIG. 8  is a block diagram of a Si 0.8 Ge 0.2 -on-insulator substrate with improved Si 0.8 Ge 0.2  layer uniformity. As shown in  FIG. 8 , a compositionally graded Si 1-x Ge x  buffer  84  is grown on a silicon substrate  82 , where Ge composition x is increasing gradually from zero to about 20%. Then a Si 0.8 Ge 0.2  etch-stop layer  86  with selected thickness is deposited. The Si 0.8 Ge 0.2  etch-stop layer  86  will also contribute to the SiGe layer in the final Si 0.8 Ge 0.2 on-insulator substrate. The thickness of the Si 0.8 Ge 0.2  etch-stop layer  86  is thick enough to sustain the selective etch process. This thickness is also chosen deliberately such that the resulting final Si 0.8 Ge 0.2 -on-insulator substrate has a desired Si 0.8 Ge 0.2  layer thickness. For example, for the purpose of fabricating high mobility strained-Si MOSFET on Si 0.8 Ge 0.2 -on-insulator substrate, a final Si 0.8 Ge 0.2  layer  86  thickness of 100 nm or less may be desired. After the deposition of Si 0.8 Ge 0.2  etch-stop layer  86 , an additional insulating layer is deposited, for example an oxide layer  88 . The oxide layer  88  is polished by CMP to achieve surface smoothness required by wafer bonding. By doing this, the polishing of Si 0.8 Ge 0.2  etch-stop layer  86  is avoided. Without the polishing step, the Si 0.8 Ge 0.2  etch-stop layer  86  can maintain its good uniformity. After flipping over and bonding to a second substrate, the first substrate is removed. After a selective etching process with TMAH or KOH, which removes the compositionally graded Si 1-x Ge x  buffer and stops at the Si 0.8 Ge 0.2  etch-stop layer  86 , a final Si 0.8 Ge 0.2 -on-insulator substrate results. The structure has good SiGe layer uniformity. Polishing may be used to smooth the Si 0.8 Ge 0.2  surface after etching without removing too much material. Then strained-Si is grown on the SGOI structure and strained-Si MOSFET may be fabricated on the SGOI with Ge composition of 20%. 
   Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.