Abstract:
A stress absorbing microstructure assembly including a support substrate having an accommodation layer that has plurality of motifs engraved or etched in a surface, a buffer layer and a nucleation layer. The stress absorbing microstructure assembly may also include an insulating layer between the buffer layer and the nucleation layer. This assembly can receive thick epitaxial layers thereon with concern of causing cracking of such layers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 11/287,379 filed Nov. 28, 2005 now U.S. Pat. No. 7,163,873 which is a divisional of application Ser. No. 10/755,007 filed Jan. 8, 2004, now U.S. Pat. No. 7,009,270. The entire content of each earlier application is expressly incorporated herein by reference thereto. 
    
    
     FIELD OF INVENTION 
     The invention relates to a novel substrate having a mechanical stress absorption system. In particular the invention is directed to a support substrate for the deposition of a nucleation layer thereon. 
     BACKGROUND OF INVENTION 
     Attempts have been made to produce epitaxial layers from materials such as GaN, GaAs, InP, GaAlAs, InGaAs, AlN, AlGaN, and even SiGe. It is known that relatively thick layers of these materials, for example, layers having a thickness greater than 1 or 2 μm, are required for good crystalline qualities. The advantages of an epitaxial layer not being stressed (or only slightly stressed), and having a low defect density, i.e., having a dislocation density of less than 10 6 /cm 2  are also known. 
     Techniques such as MOCVD have been utilized to obtain epitaxial growth of thick layers of GaN, for example, a layer having a thickness greater than 12 μm on a substrate for epitaxy. The epitaxial growth of such GaN layers has been essentially on the bulk substrates of sapphire, SiC, or Si materials. These three substrates are the most frequently used since they are the most readily available substrates, although a few tests have been carried out on substrates such as ZnO or LiGaO 2 . 
     Currently, epitaxial GaN layers homogeneously deposited on a substrate surface have a dislocation density in the range 10 8  and 10 10 /cm 2  regardless of the nucleation surface used. Additionally, the stresses in thick GaN layers obtained by MOCVD (growth temperature 1000-1100° C.) clearly depend on the coefficient of thermal expansion of the epitaxial substrate, which determines the stresses of thermoelastic origin that are imposed on the system. 
     For example, GaN layers produced on sapphire are in compression, while those obtained on SiC are under slight tension, and those on silicon are under high tension. Tension stresses produce a strong tendency for cracks to form in the epitaxial film, thereby destroying it. The layers subjected to the compression stresses, however, are also problematic. 
     These problems are particularly true for epitaxial growth on silicon substrates. For silicon epitaxy support, the limit beyond which cracks appear is about 1 μm to 2 μm, which is a limiting factor in producing thick, good quality epitaxial layers. 
     Growth tests on SOI (silicon on insulator) substrates have shown that the use of that type of substrate can reduce the crystal defect density in the epitaxially grown layer because of the compliant nature of the very thin film of silicon present on the oxide. However, that system is limited in its capacity to absorb stresses, in particular for thick GaN layers (like silicon, at best a film thickness of 1 μm to 2 μm). 
     It appears that the crystal quality improves by growth on substrates having motifs. The dislocation densities obtained are of the order of 10 6 /cm 2 . Epitaxial lateral overgrowth (ELO) techniques exist, along with techniques known as pendeoepitaxy (PE), lateral overgrowth from trenches (LOFT), and cantilever epitaxy (CE). All of these techniques are based on lateral overgrowth and coalescence of the epitaxially grown layer to ultimately form a continuous film. The continuous films obtained have precise zones with improved crystal quality (the epitaxial lateral overgrowth (ELOG) technique), or have a homogeneous film of crystal quality (LOFT technique). Those solutions have been demonstrated for sapphire, SiC, and Si (111). 
     Although these solutions improve the crystal quality of the epitaxially grown film, they cannot effectively solve the problem of stress in the epitaxially grown films. Thus, a need exists for a substrate or a support that can absorb high levels of stress during crystal growth, and in particular during thick epitaxial growth of a material. In particular there is a need for a support that absorb stresses when the coefficient of thermal expansion of the epitaxial growth material is different from that of the substrate. The present invention now satisfies that need 
     SUMMARY OF INVENTION 
     In one aspect of the invention, a support substrate is provided which has a mechanical stress absorbing system that is capable of absorbing stresses such as those produced by heating or cooling of the substrate. The present invention also relates to a microstructure comprising the novel support substrate, a nucleation or growth layer; and a buffer layer or intermediate layer. Also provided is a method of forming the substrate assembly. 
     The mechanical stress absorption system of the support substrate is capable of absorbing thermoelastic stresses, such as those generated at the surface of a substrate during temperature changes. The mechanical stress absorbing system comprises an array of stress absorbing elements, which can be obtained by machining the support substrate, e.g., by ion etching. The absorbing element include motifs such as spaced studs, trenches, or saw cuts or any other geometrical motif that has a flexibility or elasticity in a plane that is parallel to the surface of the substrate. During times of stress, the absorbing elements compensate or accommodate such stressed due to its flexibility or elasticity properties. 
     The microstructure of the invention comprises the support substrate having stress absorbing elements, a nucleation layer, and a buffer or an intermediate layer. The microstructure may further include an oxide layer to form an SOI structure. 
     Advantageously, the buffer layer of the assembly is also capable of absorbing or accommodating stresses that arise during epitaxial growth of the nucleation or growth layer on the support substrate. Further, the microstructure of the present invention is capable of accommodating epitaxial layers having thicknesses in the order of a few μm, for example as much as 4 μm, and in particular thick GaN layers. 
     The nucleation layer may include by way of example and not limitation, monocrystalline material, such as Si, SiC, GaN, sapphire, AlN or diamond. In one embodiment, the nucleation layer is obtained by transfer from a transfer-substrate. 
     The support substrate, for example and not limitation, includes Si, or SiC, and the buffer layer includes amorphous silicon, porous silicon, polysilicon, amorphous silicon dioxide SiO 2 , amorphous silicon nitride Si 3 N 4 , silicon carbide (SiC), gallium nitride (GaN), sapphire or aluminum nitride (AlN). 
     In a preferred embodiment, the microstructure comprises nucleation layer formed from silicon, a buffer layer that is either polycrystal or porous, a support substrate formed from silicon, and an electrically insulating layer between the nucleation layer and the buffer layer. For example and not limitation, the insulating layer may be an oxide, e.g., silicon oxide, or a boro-phospho-silicate glass layer. Thus, the structure of the invention is compatible with SOI (silicon on insulator) type structures. 
     In another embodiment, the intermediate layer is formed between the support substrate and the nucleation layer. If the structure is an SOI type structure, the intermediate layer may be formed between the superficial silicon layer and the insulating layer, for example silicon oxide. 
     In a further embodiment, the nucleation layer and the support substrate is formed from silicon, and an oxide or an electrically insulating layer is located between the nucleation layer and the substrate. This embodiment is therefore also compatible with an SOI type structure. 
     The buffer layer or intermediate layer may be formed between the nucleation layer or the oxide layer and the support substrate, said buffer layer being porous or polycrystal, for example, for example formed from Si, amorphous silicon, porous silicon, polysilicon, SiC, GaN, sapphire or AlN. 
     The present invention also provides a method of fabricating a stress absorbing microstructure comprising the steps of implanting atomic species on a transfer substrate to define a plane of weakness so that a portion of the substrate can be easily detached from the transfer substrate. A mechanical stress absorbing system that preferably includes a plurality of motifs are formed in the surface of a support substrate. The transfer substrate having an implanted layer of species is assembled to the support substrate in a face to face orientation. Thereafter, the plane of weakness of the transfer substrate is treated to further weaken the plane of weakness without generating cracking of the transfer substrate. A portion of the transfer substrate is detached from the transfer substrate so that a portion of the transfer substrate remains on the support substrate to form microstructure assembly of the invention. If desired, the assembled transfer substrate and support substrate can be heated to about 1000° C. to reinforce bonding between the transfer substrate and the support substrate and without causing cracking in the transfer or support substrates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a support substrate with a deposited buffer layer and a nucleation layer; 
         FIG. 2  shows a support substrate with a deposited buffer layer, oxide layer and nucleation layer; 
         FIG. 3A  illustrates a substrate of the invention with a plurality of motifs thereon; 
         FIG. 3B  illustrates a substrate of the invention with a plurality of motifs; 
         FIG. 3C  illustrates a substrate of the invention with notches; 
         FIG. 4  illustrates a two dimensional pattern of motifs on a substrate; 
         FIG. 5A  illustrate a transfer substrate and a support substrate with motifs; and 
         FIG. 5B  illustrates the substrate of  5 A with deposited transfer substrate from  5 A. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides a support substrate having a mechanical stress absorption system. In one aspect of the present invention, and as shown in  FIG. 3A , a support substrate  20  or rigid support, is provided with this system as an accommodation layer  22 . The accommodation layer is elastic or has a certain degree of elasticity at least in a plane xy, parallel to the plane of layer  24 , and layer  26 . The accommodation layer  22  comprises at least one motif, such as notches, and/or trenches, both of which may be etched into the substrate layer  20 . Alternatively, any other geometric motif that has a stress absorbing effect may be used. Preferably, the at least one motif has an elasticity or flexibility in a plane parallel to the plane of layers  24 ,  26 . As known in the art, the resulting elasticity can be calculated by applying the conventional beam theory. 
     In one embodiment, as shown in  FIG. 3B , the accommodation layer  23 , is formed at the rear face of substrate  20 . Thus, potential difficulties associated with adhering layer  26  and the substrate  20  is minimized. 
     Advantageously, the embodiments of the present invention are capable of absorbing stresses. Additionally, the two mechanical stress accommodation systems as shown in  FIGS. 3A and 3B  can be present in the same substrate. 
     In another embodiment, and as shown in  FIG. 3C , motifs in the form of notches  25  such as “saw-cuts,” are made in the substrate  20 . 
     In accordance with the invention, the motifs as shown in  FIGS. 3A ,  3 B, and  3 C are on at least one side of the substrate of the invention. However, it is also within the present invention to have the motifs such as the notches or trenches illustrated herein on both the front and rear faces of the substrate. 
     The etched or hollowed out motifs preferably repeat themselves in a two-dimensional periodic pattern or in one dimension as shown in  FIG. 4 . 
     For example and not limitation the trenches have a depth p equal to about 10 μm, a width l=1 μm and are spaced apart by an amount of about e=1 μm. The trenches are hollowed into the substrate  20  to generate a mechanical stress absorption system. 
     Nucleation layer  24  for example and not limitation may be a layer of monocrystalline material obtained by transferring a thin layer from a first substrate, for example using the “SMART-CUT” method or by fracturing the substrate. Alternatively, the nucleation layer includes silicon, silicon carbide, gallium nitride, sapphire, aluminum nitride or diamond. 
     Buffer or intermediate layer  26  and the for example, be a polycrystal or porous layer or amorphous layer. For example and not limitation, the buffer layer includes of Si, SiC, GaN, sapphire or AlN or silicon nitride. 
     Substrate  20  may be for example comprised of silicon, silicon carbide, sapphire, aluminum nitride or diamond. 
     In another aspect of the present invention, the structure of  FIGS. 3A ,  3 B, or  3 C can also be a SOI type structure, wherein layer  26  is an oxide layer or insulator layer and layer  24  is a layer of silicon. For example, with reference to  FIG. 2 , substrate  16  may be etched so as to form motifs on at least one of the faces parallel to the plane of the layers  10 ,  12 ,  14  to form an elastic accommodation layer as described above with reference to  FIGS. 3A to 3C . Thereafter the structure may comprise a buffer layer  14 , an oxide layer  12 , and a thin semiconductor material layer  10 . 
     Support substrate of the present invention that have motifs such as hollowed out trenches or etched notches and the like also have substantially reduced surface areas. Thus, the contact surface area is reduced. Therefore, molecular bonding the substrate to deposited layer may be modified to overcome the reduced contact surface areas. For example, the distribution of the trenches or of the notches could be optimized to allow spontaneous bonding. To this end, the geometric parameters of the patterns could be adjusted, e.g. the width and/or the periodicity of said patterns. 
     Further, in order to obtain an etched substrate and to be able to preserve a flat bonding surface, it is possible to obturate the surface of the substrate in part or completely prior to bonding. Even complete obturation over the entire depth of the trenches or of the etched patterns enables an absorption effect of the stresses to be conserved. 
     In one example, if the surface is formed from silicon, a step for smoothing the surface of the substrate  20  in a stream of hydrogen can be carried out to close the etching pits in part or completely by migration of silicon atoms, as illustrated in  FIG. 4 , in which reference number  28  indicates filling of a trench with silicon over a certain depth h. 
     In a further example, a non-conforming material (for example an oxide) is deposited to obturate the trenches at the surface. The deposit can be carried out by a non-optimized shallow trench isolation (STI) filling method. Such a method is, for example, described in C. P. Chang et al. “A Highly Manufacturable Corner Rounding Solution for 0.18 μm Shallow Trench Insulation”, IEMD 97-661. 
     Advantageously, the assembled support forms an element that can mechanically absorb stresses by movement and/or deformation of the bars or notches or the walls of the trenches under the effect of the thermoelastic stress. 
     As mentioned above, a buffer or intermediate layer is interposed between a nucleation or growth layer and a substrate. The buffer layer can absorb a quantity of stresses, for example by generating crystalline defects in said layer or by mechanical displacement of material in said layer.  FIG. 1  illustrates a nucleation layer  2 , buffer layer  4  and a support substrate  6  of the present invention. 
     The substrate  6  includes Si or SiC or sapphire (Al 2 O 3 ) or aluminum nitride (AlN). The buffer layer  4  is a polycrystal, porous, or amorphous layer. It can be formed by CVD techniques and can be formed from silicon (Si), silicon carbide (SiC), gallium nitride (GaN), sapphire or aluminum nitride (AlN), silicon dioxide (SiO 2 ), or silicon nitride Si 3 N 4 . The buffer layer can be a thin layer of amorphous silicon, polysilicon or porous silicon (obtained by intentional porosification or by porous deposit). 
     The nucleation layer  2  is, for example, a layer of monocrystalline material, obtained by transferring a thin layer from a first substrate, for example using the fracture method known as “SMART-CUT” (see  FIGS. 5A and 5B  relating to this subject, or even the article by A. J. Auberton-Hervé cited below in this description). 
     Typically, the thickness of the nucleation layer is of the order of about 0.1 μm to 2 μm thick, for example 0.5 μm; the thickness of the buffer layer is of the order of a few tenths of μm, for example about 0.1 μm to about 1 μm or 2 μm, and the substrate can be of the order of several hundred μm, or in the range 100 μm to 700 μm, for example about 500 μm or 525 μm. 
     The coefficients of thermal expansion C 1  and C 2  of the nucleation layer  2  and of the substrate  6  can be different. For example, SiC has a coefficient of thermal expansion of 4.5×10 −6 K −1 , Si has a coefficient of 2.5×10 −6 K −1 , alumina (Al 2 O 3 ) has a coefficient of 7×10 −6 K −1 . 
     This difference in the coefficients of the layer  2  and of the substrate  6  can generate stresses during phases of temperature rise or fall, in particular once the relative difference |C 1 -C 2 |/C 1  or |C 1 -C 2 |/C 2  is at least 10% or 20% or 30% at ambient temperature, i.e. about 20° C. or 25° C. 
     Stresses generated during an excursion in temperature are absorbed by the buffer layer  4 . In the case of a polycrystal layer, the stresses are absorbed therein by defect generation. In the case of a porous layer, the pores allow local displacement of material which mechanically absorb the tensions or stresses. In the case of an amorphous layer, the privileged relaxation mode of the stresses occurs by creep of the layers present. 
     Also in accordance with the invention is an SOI type structure in which the oxide becomes viscous at a lower temperature, for example a boro-phospho-silicate glass (BPSG). The viscous layer absorbs the tensions and stresses by creep. 
     The buffer layer as described above may be interposed in an SOI structure between the oxide or insulating layer and the substrate, as shown in  FIG. 2 , where  10  designates a thin layer of semiconductive material, preferably monocrystalline, e.g. formed from silicon, silicon carbon SiC, gallium nitride GaN, sapphire, or AlN. Reference number  12  designates a layer of SiO 2  oxide, layer  14  represents the buffer layer and reference number  16  represents a substrate formed from a semiconductive material, e.g. thick silicon. 
     The oxide layer of the SOI structure acts as a stress accommodation layer because the crystal growth methods are carried out at temperatures of the order of several hundred degrees (for example: 1000° C.). At those temperatures, the oxide becomes viscous and absorbs some of the stresses. The buffer layer  14  will also absorb some of said stresses, but in a different manner as it does not become viscous. 
     The relative difference in the coefficient of thermal expansion between the nucleation layer  10  and the substrate  16  can therefore, likewise, be greater than 10% or 20% or 30% at ambient temperature (20° C. or 25° C.). 
     For an SOI structure, the buffer layer  14  can, for example, result from a deposit of amorphous or polycrystal silicon that can box in and absorb stresses and is, for example, in the range 10 nm to 1 μm or 0.1 μm to 2 μm thick. 
     Typically, the thickness of the layer  10 , which can be formed by transfer, is about 10 nm to 300 nm, or is even in the range 0.1 μm to 2 μm. The thickness of the layer  12 , which can be formed by deposit, is of the order of a few hundred nm, for example in the range 100 nm to 700 nm, for example 400 nm. 
     The substrate  10  can be of substantially the same thickness as the substrate  6  in  FIG. 1 . 
       FIGS. 5A and 5B  illustrate a method of preparing the structure of the present invention. 
     In a first step ( FIG. 5A ), substrate  40  is implanted with ionic or atomic species to define a thin layer  52  of implanted species which extends substantially parallel to the surface  41  of the substrate  40 . A layer or plane of weakness or fracture is formed, defining in the volume of the substrate  40  a lower region  45  intended to constitute a thin film and an upper region  44  constituting the bulk of the substrate  40 . Hydrogen is generally implanted, but other species can also be implanted, including co-implantation of hydrogen and helium. 
     The substrate  42  is provided with motifs such as engraved patterns, for example, as described above. Engraving is performed from the surface  43  and/or from the surface  47 . 
     The two substrates  40  and  42  are then assembled with face  43  against face  41  using a wafer bonding technique (assembling wafers using any technique that is known in the microelectronics field) or by adhesive contact (e.g. molecular adhesion) or by bonding. With regard to these techniques, reference could be made to the work by Q. Y. Tong and U. Gösele, “Semiconductor Wafer Bonding”, (Science and Technology), Wiley Interscience Publications. 
     A portion  44  of the substrate  40  is then removed by a thermal or mechanical treatment that causes a fracture along the plane of weakness  52 . An example of that technique is described in the above-mentioned article by A. J. Auberton-Hervé et al. The structure obtained is that shown in  FIG. 5B . 
     In order to reinforce the bonding interface or the join between the substrate  42  (or its face  43 ) and the thin layer  45  (or the contact face  41 ), it may be desirable to raise the temperature to about 1000° C. 
     During the different temperature rise phases, the structure of the motifs etched in the substrate  42 , in particular their flexibility or elasticity, compensates for or absorbs the stresses and the varied differences due to differences between the coefficients of the thermal expansion of the two substrates  40 ,  42 . The relative difference between said coefficients can, as already mentioned above, be at least 10% or at least 20% or at least 30% at ambient temperature. 
     The film  45  can also be a nucleation or growth layer such as the layer  2 ,  10 , or  24  in  FIGS. 1 to 3C  (the substrate  42  being similar to the substrate  6 ,  16 ,  20  in  FIGS. 1 to 4 ). However, unlike those structures, the structure in  FIG. 5B  does not present a buffer layer. 
     The film  45  can also be replaced by an assembly of superimposed films. In other words, this aspect of the invention not only concerns a monolayer on substrate system, but any multilayer system that employs layers deposited on a substrate. It is, for example, the association of the nucleation layer and the buffer layer in  FIGS. 1 to 3C . 
     The formation of a plane of weakness can be obtained by methods other than ion implantation. Thus, it is also to make a layer of porous silicon, as described in the article by K. Sakaguchi et al. “ELTRAN® by Splitting Porous Si layers”, Proceedings of the 9th International Symposium on Silicon-on-Insulator Tech. and Device, 99-3, the Electrochemical Society, Seattle, p. 117-121 (1999). 
     Other techniques also enable the substrates to be thinned without implementing ion implantation and without creating a plane of weakness: such techniques include polishing and etching.