A method of fabrication of a substrate for an epitaxial growth

0001The present invention relates to a method of fabrication of a substrate for an epitaxial growth, comprising: obtaining a relaxed epitaxial base layer on an auxiliary substrate.

0002A method of the above-mentioned type is described in US 6,039,803 where a GeSi layer is grown epitaxially on a silicon substrate having a lattice constant different from the GeSi layer. First, a graded buffer GeSi layer is formed on the silicon substrate, serving as lattice adaptation between the silicon substrate and a relaxed GeSi layer formed on the graded buffer GeSi layer. This method results in a high quality epitaxial GeSi layer but it is difficult and very time consuming, leading to high costs of the produced GeSi layers, so that these layers are often only applied in very specific instances despite their excellent electronic properties.

0003It is the object of the present invention to fabricate substrates which allow for more efficient epitaxial growth of a material with a desired lattice parameter on another material with a different lattice parameter, wherein the material can be grown with a high thermodynamic and crystallographic stability.

0004The object is solved by a method of the above-mentioned type wherein the method further comprises: transferring at least a part of the epitaxial base layer onto a carrier substrate forming a base substrate; and further growing of the material of the epitaxial base layer on the carrier substrate.

0005In the transferring step, at least a part of the epitaxial layer is detached from the auxiliary substrate and is taken and put onto the carrier substrate. Surprisingly, during and after the transfer, the epitaxial base layer maintains a good crystal quality and thermodynamic stability as on the auxiliary substrate. Usually difficult and time-consuming material growth on the auxiliary substrate must be performed until a transferable material thickness of the epitaxial base layer is provided, whereas the further growth of the epitaxial base layer can be simply performed on the carrier substrate. The carrier substrate can have a simpler or more common structure compared to the complicated and cost-intensive auxiliary substrate. It is therefore possible to grow a thicker epitaxial base layer on the carrier substrate with a quality comparable to the high quality achieved when the epitaxial layer is grown on the auxiliary substrate, but with increased efficiency.

0006According to a variant of the invention, the relaxed epitaxial base layer can be lattice-mismatched with the auxiliary substrate.

0007In a favourable embodiment of the invention, the epitaxial base layer is grown with a thickness of about 0,1 µm to 5 µm on the auxiliary substrate. These lesser thicknesses are surprisingly sufficient for a comfortable and thermodynamically stable transfer to the carrier substrate.

0008According to a further example of the invention, the epitaxial base layer is transferred onto the carrier substrate using the following steps: implanting atomic species in the epitaxial base layer, forming a pre-weakened area; bonding the implanted epitaxial base layer with the carrier substrate; and detaching the bonded structure at the pre-weakened area. The implanting step makes it possible for the pre-weakened area to be formed in an accurate but efficient way. The bonding step allows good adhesion between the epitaxial base layer and the carrier substrate so that at least part of the epitaxial base layer adheres to the carrier substrate when detaching occurs.

0009It is furthermore advantageous to select the carrier substrate from at least one of the group consisting of silicon, silicon dioxide, fused silica, oxidised silicon, germanium, gallium nitride, indium phosphide and gallium arsenide. This way, the properties of these materials can be combined with the very specific electronic properties of the epitaxial base layer in a crystallographically and thermodynamically stable heteroepitaxial structure.

0010According to a preferable embodiment of the invention, the base substrate is thermally treated after the transferring step. This method can stabilise the material compound of the epitaxial base layer and the carrier substrate and/or reduce defects which may be caused during the transferring step. The thermal treatment can be applied before and/or after the further growing of the epitaxial base layer on the carrier substrate.

0011In a further example of the invention, a surface of the epitaxial base layer is prepared for epitaxy after the transferring step. This can help to facilitate and to improve a subsequent epitaxial growth on the transferred epitaxial base layer. A preparation for epitaxy can be performed by, for example, chemical-mechanical polishing (CMP), chemically or by planarisation.

0012It is further advantageous if the further-grown material of the epitaxial layer has less dislocation density than the epitaxial base layer obtained on the auxiliary substrate. This makes the further grown material more suitable for subsequent applications.

0013According to a further aspect of the present invention, the part of the epitaxial base layer transferred has a smaller dislocation density than the epitaxial base layer grown on the auxiliary substrate. In this way, the quality of the transferred epitaxial base layer is improved and very well suited for further processing.

0014According to another advantageous embodiment of the invention, the epitaxial base layer is further grown on the carrier substrate up to a thickness of about 0,1 µm to 5 µm. The increased thickness achieved by this method is especially favourable for a detachment of the further-grown epitaxial base layer, making it possible to transfer at least a part of the further-grown epitaxial base layer to another substrate.

0015A further example of the invention suggests that the further grown epitaxial base layer is of silicon germanium. A good quality can be obtained by the growth of this material.

0016According to a specific embodiment of the present invention, at least a part of the material of the epitaxial base layer which is further grown on the carrier substrate is transferred to another substrate, forming a further combined structure. Thus, a part of said epitaxial base layer can be detached and applied to another substrate. By applying this step, epitaxial base layers of good quality can be obtained and they can be applied to a desired substrate, forming the further combined structure.

0017According to a variant of the invention, the transfer of at least a part of the further-grown layer to another substrate can be repeated with the remainder of said further-grown layer on the carrier substrate. With repetition, a good multiplication of an epitaxial base layer of high quality can be attained. It is possible to use the further-grown layer subsequently for detaching parts thereof and applying them to other structures. If desired, the detachment can be carried out by implanting species in the base epitaxial layer, forming a pre-weakened area, a subsequent bonding of the implanted epitaxial base layer, and finally detaching the bonded structure at the pre-weakened area.

0018A variant of the invention suggests that the transfer can be repeated after re-claiming and/or planarisation of the remaining further-grown layer. This provides a good surface for the later application of the further-grown layer to another substrate.

0019In a beneficial variant of the invention, at least one second epitaxial layer is grown on the further-grown epitaxial base layer on the carrier substrate. This method can be used to grow a second epitaxial layer on the same carrier substrate with material properties other than those of the base epitaxial layer but with a comparably high crystal quality.

0020In a yet further preferable embodiment of the invention, the at least one second epitaxial layer is of silicon. Thus it is possible to produce a high quality strained silicon layer on the epitaxial base layer which can be favourably used in certain electronic devices due to the excellent electronic properties of high crystal quality strained silicon.

0021In an especially favourable embodiment of the invention, the at least one second epitaxial layer is formed with a thickness of about 10 nanometres to about 20 nanometres. Thereby a very high degree of strain in the second epitaxial layer can be produced which can lead to an increased carrier mobility in the second epitaxial layer.

0022In another advantageous example of the invention, a further epitaxial layer of the material of the base epitaxial layer is grown on the second epitaxial layer. This way, the second epitaxial layer can be sandwiched between two epitaxial layers with the same material properties. The second epitaxial layer therefore defines an end of a base epitaxial layer which makes it possible to use the second epitaxial layer in particular as an etch stop for a transfer of at least part of the epitaxial base layer from the carrier substrate to another substrate.

0023In another beneficial variant of the invention, a periodical structure with alternate epitaxial base layers and at least one second epitaxial layer is formed. This method results in a structure which is especially favourable for a repeated transfer of at least a part of the base epitaxial layer to another substrate, whereas the second epitaxial layer can be helpful for a defined transfer of at least a part of the base epitaxial layer or it can be transferred together with at least a part of the base epitaxial layer to the other substrate.

0024It is furthermore advantageous when at least a part of the material of the epitaxial base layer is transferred together with at least a part of the second epitaxial layer to the other substrate. This way, not only can the good material characteristics of the epitaxial base layer be transferred to the other substrate, but also the excellent material and electronic properties of the second epitaxial layer can be used at the other substrate.

0025In another preferable variant of the invention, a surface of the further combined structure is finished. By this method, the surface properties of the resulting structure can be advanced so that the combined structure is very well-prepared for a high variety of further process steps.

0026Specific embodiments of the present invention will become more apparent from the following detailed description with reference to the accompanying drawings, in which: <dl id="dl0001"><dt>Fig. 1</dt><dd>schematically shows a source substrate;</dd><dt>Fig. 2</dt><dd>schematically shows a formation of a graded buffer layer on the source substrate of Fig. 1;</dd><dt>Fig. 3</dt><dd>schematically shows a formation of a relaxed layer of the structure shown in Fig. 2;</dd><dt>Fig. 4</dt><dd>schematically shows an implantation step performed on the structure of Fig. 3, being a partial stage of the transferring step according to the present invention;</dd><dt>Fig. 5</dt><dd>schematically shows a bonding step of the structure of Fig. 4 with a carrier substrate, being a partial stage of the transferring step according to the present invention;</dd><dt>Fig. 6</dt><dd>schematically shows a splitting step of the structure of Fig. 5, being a partial stage of the transferring step according to the present invention;</dd><dt>Fig. 7</dt><dd>schematically shows a further growth on a split base substrate of Fig. 6 according to the present invention;</dd><dt>Fig. 8</dt><dd>schematically shows a growth of a second layer on the structure of Fig. 7;</dd><dt>Fig. 9</dt><dd>schematically shows a formation of a periodical structure based on the structure of Fig. 8;</dd><dt>Fig. 10</dt><dd>schematically shows an implantation step performed on the structure of Fig. 9;</dd><dt>Fig. 11</dt><dd>schematically shows a split part of the structure of Fig. 10 transferred to another substrate;</dd><dt>Fig. 12</dt><dd>schematically shows the structure of Fig. 11 after an etching step;</dd><dt>Fig. 13</dt><dd>schematically shows another implantation step performed on the structure of Fig. 9;</dd><dt>Fig. 14</dt><dd>schematically shows a split part of the structure of Fig. 13 transferred to another substrate; and</dd><dt>Fig. 15</dt><dd>schematically shows the structure of Fig. 14 after an etching step.</dd></dl>

0027Fig. 1 schematically shows a source substrate 1 having a surface 2, used as a starter material for the present invention. The source substrate 1 is a wafer in the embodiment shown, but in other embodiments can be any kind of substrate available in the semiconductor industry, such as a chip or material compound. The first substrate 1 can be already structured, or can be unstructured as shown in Fig. 1. The source substrate 1 is of a single crystalline material such as single crystal silicon, germanium, GeSi or an A_III-B_V semiconductor. In the embodiment shown, the source substrate 1 is of single crystalline silicon.

0028Fig. 2 schematically shows a formation of a graded buffer layer 3 on the surface 2 of the source substrate 1 shown in Fig. 1. The graded buffer layer 3 is epitaxially grown on the source substrate 1 and consists of a material with a lattice constant different from a lattice constant of the source substrate 1.

0029In the embodiment shown the graded buffer layer 3 consists of GeSi, the germanium content of which is gradually increased, beginning from the source substrate 1 as shown by the arrow A. The graded buffer GeSi layer 3 is grown with a certain thickness t₀ and has a surface 4. In another embodiment, the source substrate 1 consists of germanium and the graded buffer layer 3 consists of GeSi, the germanium content of which is gradually decreased beginning from the source substrate 1.

0030In other examples of the present invention, the material of the buffer layer 3 can be of Si_1-xGe_x, Si_1-x-yGe_xC_y, Si_1-yC_y, GaAs, InP, GaN, GaP or other A_III-B_V-materials.

0031The buffer layer 3 serves as a lattice adaptation layer between the source substrate 1 and an epitaxial layer 5 which will be formed on the buffer layer 3.

0032It is also possible that a so-called "compliant" substrate can be used as a starter material for the present invention, instead of the source substrate 1 with the buffer layer 3. Compliant substrates have an implanted region at a certain depth, allowing a slippage effect of a substrate layer above the implanted region in relation to the bulk substrate. This way, a difference between lattice constants of a layer grown on the implanted substrate and the substrate can be absorbed.

0033In a further example of the present invention, a relaxed layer can be obtained by implantation followed by a heat treatment. This is described in US Patent No. 6,464,780 B1, the disclosure of which is hereby incorporated in its entirety by reference. It suggests to produce a mono-crystalline layer on a substrate with a non-adapted lattice. A mono-crystalline layer is deposited upon a surface of a lattice-mismatched mono-crystalline substrate and then a buried defect-rich layer is created in said mono-crystalline substrate. This prevents the development of threading dislocations in said mono-crystalline layer by ion implantation, e.g. by hydrogen. The defect-rich layer can be found in said mono-crystalline substrate as close as possible to said surface without disturbing said surface. Said mono-crystalline layer is then relaxed and the formation of threading deformations is restricted. Its lattice parameters come close to the original lattice structure.

0034The structure shown in Fig. 2 or a compliant substrate is used as an auxiliary substrate 7 for the following process steps.

0035Fig. 3 schematically shows a formation of a relaxed epitaxial GeSi layer 5 on the surface 4 of the graded buffer layer 3 of the structure shown in Fig. 2. The epitaxial layer 5 is grown with a high crystal quality up to a thickness t₀ which is large enough to allow a detachment of at least part thereof in a later step.

0036In another possible embodiment (not shown), the epitaxial layer 5 is grown on a compliant substrate serving as a lattice adapter.

0037The process shown in Figs. 1 to 3 results in a heteroepitaxial structure having on top the relaxed layer 5 with a high thermodynamic and crystallographic stability. To achieve such a result, the above steps have to be made with great care, entailing a relatively long process time to produce the relaxed layer 5.

0038The steps shown in Figs. 1 to 3 or the steps in which the epitaxial layer 5 is grown on a compliant substrate comprise an initial epitaxial growth of the epitaxial layer 5. Although for fabrication of the epitaxial layer 5 a growth on a buffer layer 3 or on a compliant substrate is described, the formation of the epitaxial is not limited to these fabrication methods but can be performed with any other available method. The epitaxial layer 5 functions as epitaxial base layer 5 in the following process steps.

0039Fig. 4 schematically shows an implantation step performed on the auxiliary substrate 7 shown in Fig. 3. The implantation step represents a partial stage of a transferring step of the present invention used to transfer at least a part of the high quality grown epitaxial base layer 5 from the complicated auxiliary substrate 7 on a common carrier substrate 10.

0040Atomic species 8, such as hydrogen and/or helium ions, are implanted through the surface 6 of the epitaxial base layer 5, forming at a certain depth <i>d</i> of the epitaxial base layer 5 a pre-weakened area 9. The depth <i>d</i> can be in the epitaxial base layer 5 or can be at the interface between the epitaxial base layer 5 and the buffer layer 3.

0041In a variant of the invention, an oxide layer may be formed before the ion implantation.

0042Fig. 5 schematically shows a bonding step according to a partial stage of the transferring step. The structure shown in Fig. 4 is bonded in this step with a carrier substrate 10 at the implanted surface 6. The carrier substrate 10 can be of a material such as silicon, silicon dioxide, fused silica, oxidised silicon, germanium, gallium nitride, indium phosphide or gallium arsenide. Thereby, the pre-weakened area 9 is brought near to the bonded interface between the carrier substrate 10 and the implanted auxiliary substrate 7.

0043Fig. 6 schematically shows the beginning of a detachment step in which the structure shown in Fig. 5 is urged into two parts at the pre-weakened area 9. One of the parts forms a base substrate 17 consisting of the carrier substrate 10 and at least a part 52 of the epitaxial base layer 5. Fig. 6 shows the structure in two parts.

0044In another embodiment of the invention where the pre-weakened area is formed at the interface between the epitaxial base layer 5 and the buffer layer 3, the structure is urged into two parts at this interface. This way the whole epitaxial base layer can be transferred to the carrier substrate 10.

0045It should be understood for a person skilled in the art that any available technique other than that shown in Figs. 3 to 5 can be used to transfer at least a part of the epitaxial base layer 5 onto the carrier substrate 10.

0046After detachment, a surface 13 of the base substrate 17 has an increased roughness. Therefore, the base substrate 17 can be thermally treated to improve the roughness of the surface 13 and/or to increase the bonding force between the epitaxial base layer part 52 and the carrier substrate 10, in a so-called stabilisation process. The thermal treatment can furthermore help to prevent any extension of misfit dislocations or even reduce misfit dislocation density, or other crystalline defects, since it does not contain an unstable structure such as a graded buffer layer.

0047It also aims at obtaining a surface characteristic which is suitable for a subsequent epitaxial growth. The surface treatment of the surface 13 can be made by Chemical-Mechanical-Polishing (CMP) processes, CMP touch polishing, dry or wet chemical etching and/or annealing steps including rapid thermal annealing (RTA) or rapid thermal oxidation (RTO). The surface 13 can furthermore be treated physically, for example by ion bombardment or by a combination of the above methods.

0048The finishing of the surface 13 is preferably carried out after the layer transfer shown in Fig. 6, but can likewise be applied after the stabilisation process such as thermal treatment or can be combined with such a stabilisation process. The appropriate step order for finishing depends on the technology and materials used. In a further embodiment, the surface finishing step of the surface 13 can be omitted.

0049Fig. 7 schematically shows a further epitaxial growth on the structure shown in Fig. 6. In this step, the epitaxial base layer part 52 of Fig. 6, having a thickness t₁ after the splitting step, is epitaxially grown up to the thickness t₂ of about 0,1 µm to 5 µm, resulting in a thick epitaxial layer 520 on the carrier substrate 10, as shown in Fig. 7.

0050Surprisingly, the further-grown epitaxial base layer part 520 with the increased thickness t₂ has a good high crystal quality and thermodynamic stability like the original epitaxial base layer 5 grown on the auxiliary substrate 7. The further grown epitaxial base layer part 520 has less dislocation density than the epitaxial base layer 5 grown on the auxiliary substrate 7. Furthermore, the part of the epitaxial base layer transferred has a smaller dislocation density than the epitaxial base layer 5 grown on the auxiliary substrate 7.

0051In the embodiment shown, the further-grown epitaxial base layer part 520 has the same Ge concentration of the GeSi layer 5. A greater thickness t₂ is preferable for a further repeated layer transfer, starting from the further-grown epitaxial base layer part 520.

0052The structure shown in Fig. 7 can be used for a further detachment of a part of the further grown layer 520 and transferring same to another substrate for forming a further combined structure. In this way, and by subsequent transferrals of at least a part of the further grown epitaxial base layer 520, a large number of structures shown in Fig. 7 can be created. Thus, it is possible to repeat the transfer of at least a part of the further-grown layer 520 to another substrate. The remainder of said further-grown layer 520 can be used for transferring at least parts thereof to one or another of the substrates 12. For example, the method described with regard to Fig. 4 for the epitaxial base layer 5, and transferring a part 52 thereof by bonding same to further carrier substrate 10 and detaching same with the method described in Figs. 4 to 6, can be applied.

0053If a first part of the further-grown layer 520 is transferred, the surface can be re-claimed and/or planarised before remainder of the further-grown layer 520 is transferred.

0054Fig. 8 schematically shows a growth of a second epitaxial layer 11 on the further-grown layer 520. In the embodiment shown, the second epitaxial layer 11 consists of silicon and has a thickness t_s of about 10 nanometres to 20 nanometres. Due to different lattice constants between the second epitaxial layer 11 and the further-grown epitaxial base layer part 520, the second epitaxial layer 11 can be grown on layer 520 with a certain strain. Therefore, in the embodiment shown, a strained silicon layer 11 is formed on the high quality GeSi layer 520.

0055Fig. 9 schematically shows a continued growth of a further epitaxial layer 521 on the second epitaxial layer 11. The further epitaxial layer 521 consists of the same material as the further-grown epitaxial base layer part 520 below the second epitaxial layer 11. Thereby, a periodical structure 16 is formed on the carrier substrate 10, in which the second epitaxial layer 11 is sandwiched between the further-grown epitaxial base layer part 520 and the further epitaxial layer 521. In the embodiment shown, the thin strained silicon layer 11 is sandwiched between the relaxed GeSi layers 520, 521, wherein the strained layer 11 is elastically strained with a very low defect density.

0056Although not shown, the periodical structure 16 can comprise much more than the layers shown in Fig. 9 with a repeated arrangement of base epitaxial layers 520, 521 and second epitaxial layers 11. Using such a construction, each one or two or more periodicities may be transferred to another substrate. This way, many further combined structures can be produced, all having very good crystallinity comparable with the quality of the originally grown base epitaxial layer 5 grown on the auxiliary substrate 7. The repeated transfer allows therefore a cloning of the epitaxial base layers and/or the second epitaxial layer 11 on a variety of substrates.

0057Fig. 10 schematically shows an implantation step performed on the structure shown in Fig. 9. Species 18, such as hydrogen or helium ions, are implanted through a surface 22 of the further epitaxial layer 521, through the layer 521, through the second epitaxial layer 11 and to a certain depth d of the further-grown epitaxial base layer part 520. The implanted species form there a pre-weakened area 19 defining a later detachment line of the structure.

0058The next steps are similar to those shown in Figs. 5 and 6. First, the implanted structure shown in Fig. 10 is bonded at its implanted surface 22 with another substrate 12. The other substrate 12 consists of, for instance, silicon, silicon dioxide or gallium arsenide, but can be of any other material available in the semiconductor industry.

0059Then the bonded structure consisting of the structure shown in Fig. 10 and the other substrate 12 is detached along the pre-weakened area 19, resulting in two parts.

0060Fig. 11 shows one of the parts, consisting of the other substrate 12, the further epitaxial layer 521, the second epitaxial layer 11 and at least a part of the further-grown epitaxial base layer part 5201.

0061Fig. 12 schematically shows a combined structure 20 which corresponds to the structure shown in Fig. 11 after an etching step in which the residual part 5201 of the epitaxial base layer part 520 is etched away. In the etching step, the second epitaxial layer 11 serves as an etch stop layer causing stopping of etching at the surface 14 of the second epitaxial layer 11. The etching step results in a structure consisting of the other substrate 12, the further epitaxial layer 521 which has a very high degree of crystal quality and thermodynamic stability and the second epitaxial layer 11 which is, in the embodiment shown, a thin, strained silicon layer with a high degree of crystal quality.

0062Fig. 13 schematically shows a further variant of an implantation step performed on the structure shown in Fig. 9. Species 28, such as hydrogen and/or helium ions, are implanted through the surface 22 of the further epitaxial layer 521, through the further epitaxial layer 521, to a certain depth <i>d</i> of the second epitaxial layer 11, forming there a pre-weakened area 29. The next steps for obtaining a detachment at said pre-weakened area 29 are similar to those shown in Figs. 5 and 6.

0063First, the structure shown in Fig. 13 is bonded with another substrate 12, consisting of, for example, silicon, silicon dioxide or gallium arsenide, but which can be of any other material available in the semiconductor industry. The bonding interface is at the former implanted surface 22. Then, the bonded structure is detached along the pre-weakened area 29, resulting in two parts.

0064Fig. 14 shows one of these parts, consisting of the other substrate 12, the further-grown epitaxial base layer part 520 and a residual part 111 of the former second layer 11.

0065Fig. 15 shows the structure of Fig. 14 after an etching step in which the residual part 111 of the former second epitaxial layer 11 is removed, wherein the interface between the further-grown epitaxial base layer part 520 and the layer 111 is used as an etch stop. The structure shown in Fig. 15 is a combined structure 21 having a surface 15 with an increased roughness.

0066The roughness of surface 15 can be reduced in a final finishing step which is not shown. For final surface treatment, the surface 15 can be planarised by CMP processes or CMP touch polishing, or by other chemical or physical treatments as mentioned above with reference to the finishing of the surface 13.