A method of producing a substrate by transferring a donor wafer comprising foreign species, and an associated donor wafer

0001The present invention generally relates to methods of fabricating substrates by stacking thin layers of semiconductor materials; it is preferably applicable to monocrystalline thin layers.

0002The method known as "Smart-Cut"®, based on implanting hydrogen and/or other rare gases and molecular bonding, allows thin films to be thinned and assembled on supports. More precisely, implanting atomic species creates embrittlement in the layer at the depth at which it is to be detached from a donor wafer. It is then assembled with a support or stiffener by molecular bonding. The implanted layer is then transferred by carrying out treatment, in particular a heat or mechanical treatment, to produce a cleavage at the embrittlement zone. The thickness of the thinned film depends on the application, but it is in general of the order of a few hundred or tens of nanometers. The surface obtained can then be polished using a chemical or a mechanical-chemical method.

0003Such a method can produce heterostructures which are impossible to obtain by epitaxy. When the heat treatments carried out to fracture and/or strengthen the bonding interface are carried out at a lower temperature than during epitaxy, it also has the advantage of reducing inter-diffusion phenomena. From an economic viewpoint, said method also allows the layer left behind after fracturing, known as the negative, to be recycled.

0004One industrial application of that method is the silicon on insulator (SOI) substrate composed of a thin film of monocrystalline silicon electrically insulated from a bulk substrate. In general, the bulk substrate is silicon, and the insulating silicon layer is amorphous silica.

0005That method is also applicable to a wide range of materials, whether they form the implanted layer (SiC, GaAs, InP, LiNbO₃, ...), the support or stiffener (monocrystalline or polycrystalline silicon, gallium arsenide, polycrystalline indium phosphide, quartz, etc.), or any bonding layer (SiO₂, Si₃N₄, Pd, etc.).

0006It is also possible to use that method to produce "pseudo-substrates" intended to receive an additional layer by epitaxial growth on the transferred thin layer.

0007That can provide several advantages: <ul id="ul0001" list-style="bullet" compact="compact"><li>size: since some substrates are not available in standard industrial sizes, it is thus possible to carry out a method of transferring a thin layer onto a support or stiffener with a larger diameter. In particular, a 4 inch diameter InP film can be transferred onto a 6 inch diameter support so as to remain compatible with 6 inch standard micro-electronics fabrication facilities;</li><li>brittleness: the brittleness of certain bulk substrates (for example InP) can cause the substrates and components to break during fabrication and manipulation and may thereby significantly increase production costs. The layer transfer method can advantageously be employed if a stiffener can provide strength to the structure (for example, a thin InP layer on an Si or GaAs support);</li><li>cost: the high cost of certain substrates may justify using a layer transfer method to transfer a very thin layer (a few tens of nanometers thick) onto a cheap stiffening substrate; the operation being repeated after recycling the donor wafer (negative);</li><li>compliant effect: this term represents a certain adaptability of the thin layer, particularly as regards dimensions. In this respect, epitaxial growth is known to require a good match between the lattice parameters and thermal expansion coefficients of the substrate assembly and the epitaxial layer. By way of example, on bulk GaAs substrate, the maximum lattice mismatch must not exceed about 1%, otherwise stacking defects occur in the epitaxial layer. Of course, techniques have recently been developed that allow higher mismatches between the lattice parameters, while proposing a multilayer structure with an epitaxial seed layer that is sufficiently thin to be able to match itself to the characteristics of the epitaxially grown material by deformation.</li></ul>

0008It has also to be observed that InP as a substrate for the micro-electronics industry is rapidly gaining popularity. Because of its intrinsic properties, InP and its alloys (InGaAs, AlInAs, InGaP, InGaAsP, InGaAsN, etc.) that can be epitaxially grown thereon with a lattice match, allow in particular transistors to be produced with excellent cutoff and transition frequencies. InP technology is thus the most favorable for producing very high speed optical transmission networks.

0009In optoelectronics, emitters and receivers produced using InP technology can function within wavelength ranges that are used in optical telecommunications.

0010That combination of characteristics means that this group of materials is the only group that can currently wholly integrate the associated photonic functions and electronic functions of control and amplification in the optoelectronics field.

0011Finally, in the field of microwave amplification, the high power or low noise levels developed by high energy mobility (HEMT) field effect transistors produced using InP technology also contributes to the great success of InP technology.

0012Currently available substrates formed from InP substrates and the like are bulk substrates obtained by ingot preparation techniques. There are two principal techniques for growth by pulling: liquid encapsulated Czochralski, LEC, and vertical gradient freezing, VGF, as well as a variety of variations and improvements.

0013However, the production of large, high quality InP crystals is fraught with difficulties that involve the crystallization properties of the material. Low twin crystal creation energy and low stacking fault energy facilitate the appearance of defects in the crystalline structure produced, and the density of said defects increases with ingot size.

0014Incorporating certain impurities into the melt mixture is also known, either to provide N or P type doping or to render the material semi-insulating, which is accomplished by compensation, preferably with iron.

0015Substrates are sliced from said ingots along the desired crystallographic direction, generally (100) or (111). Subsequent mechanical-chemical polishing produces a finished substrate on which epitaxial growth, which is known per se, can be carried out.

0016However, growing iron-compensated InP by pulling impinges against a physical property, namely the extremely low segregation coefficient of iron in InP [K(Fe) = 10^-3]. That causes excessive iron incorporation close to the seed as growth commences, followed by depletion of iron in the melt. The iron concentration gradient from the head to the tail of the ingot results in a variation in iron concentration, which can be as high as one order of magnitude, for example 10¹⁶.cm^-3 at one end of the ingot axis to 10¹⁷.cm^-3 at the other end. Compensation of the substrate, and thus its resistivity, will vary substantially depending on its original position in the ingot.

0017To overcome that problem it is possible, of course, to proceed to an a posteriori bulk substrates compensation. A technique for incorporating iron by ion implantation can be cited in this respect, however it would irreversibly damage the InP material.

0018It is known that InP can be compensated using a diffusion technique. Generally, sealed quartz tube diffusion is employed, at a high temperature (about 900°C) with a compound that is rich in iron and phosphorus, providing a vapor pressure of several atmospheres. The presence of phosphorus prevents desorption of the phosphorus component of the InP from the substrate surface.

0019However, the thickness of bulk substrates impose very long diffusion times (typically at least 80 hours (h) for a 600 micrometer (µm) substrate), and thus, that technique is not readily compatible with mass production using bulk substrates.

0020Because of the size, brittleness, or cost considerations mentioned above, or to provide the substrate with a compliant nature for epitaxy, the skilled person would wish to use a Smart-Cut® type technique to transfer a thin layer of InP onto a support. That technique has been carried out for unintentionally doped InP layers, or those doped with the usual dopants (S, Sn and Zn), or compensated by the presence of iron (see the article by E. Jalaguier et al in Proc 11^th Int Conf InP and Related Materials, pp 26-7 (1999)).

0021However, in the case of semi-insulating InP compensated with iron or another compensating material, undesirable interactions are observed between the implanted species (typically hydrogen) and the complexes present in the material and involving iron atoms.

0022More precisely, although studies have demonstrated the favorable role played by the presence of dopants or impurities in the material on the coalescence kinetics of implanted species prior to cleavage, the Applicant has discovered that iron acts differently, probably by modifying the migration kinetics of hydrogen during implantation then during annealing. This means firstly, that the selection of implantation conditions (dose, energy, temperature) and annealing conditions (duration, temperature) becomes much more difficult. Further, the roughness of the free face of the transferred thin layer following fracture is increased, thus increasing the polishing work and producing a material loss which harms the industrial efficiency and the economics of the method.

0023The present invention aims to overcome this disadvantage.

0024To this end, in a first aspect, the invention provides a method of producing a substrate comprising a thin crystalline layer transferred from a donor wafer onto a support, said thin layer including one or more foreign species intended to modify its properties, the method being characterized in that it comprises the following steps in sequence: <ul id="ul0002" list-style="bullet" compact="compact"><li>implanting atomic species into a zone of the donor wafer that is substantially free of the foreign species, to form an embrittlement zone below a bonding face, the embrittlement zone and the bonding face delimiting a thin layer to be transferred;</li><li>bonding the donor wafer at the level of its bonding face to a support;</li><li>applying stresses in order to produce a cleavage in the region of the embrittlement zone and to obtain a substrate comprising the support and the thin layer;</li></ul> and in that it further comprises a step of diffusing foreign species into the thickness of the thin layer prior to implantation or after fracture, suited to modify the properties of the thin layer, in particular its electrical or optical properties.

0025Certain preferred but non-limiting aspects of this method are as follows: <ul id="ul0003" list-style="bullet" compact="compact"><li>the step of diffusing foreign species is carried out after cleavage;</li><li>the step of diffusing foreign species is carried out prior to implantation;</li><li>the step of diffusing foreign species is carried out to a depth that is smaller than the implantation depth;</li><li>the method includes, after cleavage, a thinning step suited to remove the portion of the transferred layer which is depleted in foreign species;</li><li>the method further includes, prior to bonding, a step of producing a bonding layer on the donor wafer and/or on the support;</li><li>the bonding layer forms a buried insulator in the final substrate;</li><li>the material of the donor wafer is a III-V semiconductor compound;</li><li>the foreign species include a species that can render the compound semi-insulating by diffusion;</li><li>the compound is indium phosphide;</li><li>the foreign species is/are selected from the group constituted by iron and rhodium;</li><li>the foreign species include a combination of a shallow acceptor such as mercury or cadmium and a shallow donor such as titanium or chromium;</li><li>the implanted species comprise at least one species selected from hydrogen ions and rare gas ions;</li><li>the support material is selected to be mechanically stronger than the material of the thin layer;</li><li>the method includes a subsequent epitaxial growth step carried out on the thin layer of the substrate;</li><li>the lattice of the epitaxially grown material is mismatched with the material of the thin layer.</li></ul>

0026In a second aspect, the invention provides a donor wafer comprising at least one layer of a crystalline material for implementing a method of transferring thin layers of semiconductor material of a predetermined thickness removed from said wafer onto a support for fabricating substrates for microelectronics, optoelectronics or optics, the wafer being characterized in that it comprises, on the removal side and over a depth that is smaller than said predetermined thickness, at least one diffused foreign species that can modify the properties of the material of said donor wafer.

0027Preferred but non-limiting aspects of said wafer are as follows: <ul id="ul0004" list-style="bullet" compact="compact"><li>the material of the wafer is a III-V semiconductor compound, and said foreign species is capable of rendering the material of the wafer semi-insulating;</li><li>the III-V semiconductor compound is indium phosphide;</li><li>the foreign species is/are selected from the group comprising iron and rhodium.</li></ul>

0028Other aspects, objects, and advantages of the present invention will become apparent from the following detailed description of preferred embodiments of the invention, given by way of non-limiting example and made with reference to the accompanying drawings in which: <ul id="ul0005" list-style="none" compact="compact"><li>Figures 1A to 1E show different steps of a method in accordance with a first embodiment of the invention; and</li><li>Figures 2A to 2F show different steps of a method in accordance with a second embodiment of the invention.</li></ul>

0029The steps of a first method of the invention are described below using as an example a thin layer from an InP donor wafer 20, transferred to a support or stiffener 10 formed from silicon.

0030Firstly, the faces to be bonded to the support 10 and the donor wafer 20 are provided with a bonding layer (typically oxide or nitride) respectively 11, 21, to form a hydrophilic surface for molecular bonding (Figure 1A).

0031To this end, the donor wafer 20 and the support 10 firstly undergo chemical attack based on hydrofluoric acid to remove the natural oxide layer. The support 10 is oxidized by thermal oxidation. That technique, which is particularly suitable for silicon, cannot be envisaged for InP and thus plasma vapor phase deposition is employed. The bonding layers 11, 21 are generally about a few hundred nanometers thick.

0032Atomic species implantation is then carried out into the donor wafer 20 at its face provided with the bonding layer 21, to form a general plane embrittlement zone 22 which defines a thin layer to be transferred 23 between it and the layer 21 (Figure 1B).

0033The term "atomic species implantation" as used in the present text means any introduction by bombardment of atoms or molecules, which may or may not be grouped, and which may or may not be ionized. Said implantation can be carried out using an ion beam implanter, a plasma immersion implanter, etc. Different types of species can be implanted, for example rare gas ions (for example H⁺, H₂⁺, He⁺). It is also possible to carry out co-implantation with an element such as boron.

0034Preferably, ionic implantation into wafer 20 is carried out after heating said wafer. The range of temperatures used differs depending on the materials. In the case of InP, the temperature is preferably in the range 150°C to 250°C. The dose used to implant hydrogen ions into that material is preferably in the range 10¹⁶ to 5 × 10¹⁷ H⁺/cm².

0035The next step is molecular bonding of the implanted donor wafer and the support, at the bonding layers 21, 11 (Figure 1C). This bonding requires flat, smooth surfaces. The surfaces are polished using conventional mechanical-chemical polishing techniques. In the present case of hydrophilic type bonding, it is also preferable to increase the surface concentration of molecules terminating in OH, that allow bonding. To this end, the donor wafer and support can be immersed in a RCA or SC1 solution (H₂O: H₂O₂: NH₄OH = 5:1:0.2-1). It is then dried at a temperature below 90°C. The donor wafer and support are then assembled at ambient temperature under slight pressure, and the assembly is annealed, typically between 250°C and 400°C. Annealing acts firstly to reinforce bonding at the bonding interface, and secondly to cause micro-cracks to appear which, on coalescing, start to fracture the InP film along the embrittlement plane created by the implanted zone (Figure 1D). In that figure, reference number 30 designates the insulating layer substantially formed by initial bonding layers 21, 11.

0036Preferably, the surface of the thin layer that remains exposed is then thinned to remove the superficial implanted zone that is rich in hydrogen atoms. Different techniques can be used: wet/dry etching and/or mechanical-chemical polishing. A dry etching thinning technique which may prove to be particularly suitable is a spray thinning and smoothing technique.

0037To obtain a semi-insulating layer 25 by diffusion of iron 24 therein (Figure 1E) from the thin layer of InP 23, the assembled structure is placed in a sealed quartz tube at a high temperature (about 900°C), in a gas mixture composed of iron and phosphorus (FeP₂). The pressure is typically several atmospheres. The diffusion period, which is essentially proportional to the thickness of the InP layer to be diffused, is estimated to be about ten minutes for a thickness of the order of one micrometer. The FeP₂ gas is preferably obtained from high purity iron powder and from red phosphorus in a molar ratio of 1:2.

0038Exposure to the high temperature employed during this diffusion also has an annealing function, which endows the InP material with a semi-insulating nature with an iron concentration that is much lower than in the case of a non-annealed material (typically from 10¹⁵ atoms.cm² instead of 10¹⁷ atoms.cm², in known manner and as described by R. Fornari et al in "Conductivity Conversion of Lightly Fe-doped InP Induced by Thermal Annealing: A Method for Semi-Insulating Material Production", J Appl Phys 81 (11) 1997, pp 7604-11).

0039It is considered that this effect derives from a large reduction in shallow donors (concentration about 4 × 10¹⁵ atoms.cm^-3).

0040The reduction of the necessary concentration of iron allows in this case a very high mobility of residual free electrons. Further, said annealing treatment produces a thermal stability that improves the aptitude of the substrate to implantation for the fabrication of devices.

0041Because the atomic species implanted into the material of the InP donor layer are there even though the material is still depleted in iron, the implantation, coalescence and cleavage phases can be carried out under good conditions, without the perturbation described above caused by the iron.

0042In a variation, iron can be diffused from a diffusion source constituted by a solid thin film, rich in iron, assembled onto the free face of the transferred thin layer 23. The assembly is exposed to a heat treatment that encourages diffusion. This method protects the surface of the thin layer and prevents any phosphorus desorption.

0043Treatment of the final surface of the structure comprising the support 10, the intermediate insulating layer 30 and the thin layer 25 is preferably carried out by spray projection of neutral atoms (argon) in clusters to obtain an initial planarization and smoothing of the surface by mechanical and/or mechanical-chemical polishing. The final roughness value, measured in terms of standard deviation, is of the order of a few Angstroms.

0044The substrate can either be delivered as is to the component industry destination, for epitaxial growth over the thin layer 25, thus forming a growth seed, or it can be provided with an epitaxially grown layer following the sequence of steps described above.

0045The preparation required for epitaxy can include a step of stabilizing the surface oxide and using tensioactives to provide the surface with a hydrophilic nature.

0046We shall now describe a further implementation of the invention, referring to Figures 2A to 2F.

0047We start by diffusing iron onto the donor wafer 20 (Figure 2A), but in a shallow depth layer 25. More precisely, said depth is smaller than the depth to which the atoms will subsequently be implanted to form the embrittlement zone (Figure 2C). Figure 2B shows the intermediate production of the bonding layer 21.

0048Figure 2D illustrates the bonding step.

0049Migration of the implanted species can result in fracture from the donor wafer 20 of a thin layer 23 comprising the layer 25 that has received iron diffusion and a layer 26 in which iron is substantially absent (Figure 2E). Since said migration occurs in a zone in which iron atoms are absent, or in any case sufficiently rare not to perturb said migration and the resulting coalescence, cleavage can occur under good conditions.

0050This second implementation is advantageous in that it avoids the high temperature annealing required to diffuse iron into a multi-layer structure. Differences between the thermal expansion coefficients are susceptible of generating very high stresses. By exposing a bulk (non composite) substrate to a high temperature for diffusion, expansion of the substrate does not involve shear stresses.

0051Hence, prior to the surface treatment, the structure illustrated in Figure 2E of the drawings is obtained, with a zone 26 of the thin layer that is substantially free of iron, and a zone 25 containing diffused iron, adjacent to the buried insulating zone 30.

0052The depth of the implanted zone 22 with respect to the thickness of the zone 25 is selected so that subsequent thinning can completely remove zone 26, and if necessary a small fraction of the zone 25, to obtain a final thin layer 25 that is homogeneously iron compensated throughout its thickness.

0053A number of variations of the invention are possible.

0054The diffused material can be any material incorporated into a thin layer before or after transfer, in particular to modify the physical, chemical or electrical properties thereof, and which somehow counters migration of the implanted species during the implantation step.

0055In this respect, although iron is currently the only element used on an industrial scale to render InP semi-insulating, compensation elements with a lower diffusivity than iron - to limit contamination of other parts of the structure by iron and the risk of iron depletion - could also be used.

0056In this regard, as indicated in the article by A. Näser et al, "Thermal Stability of the Mid-gap Acceptor Rhodium in Indium Phosphide", Appl Phys Lett, 67, 479-481 (1995), rhodium is known to be a deep acceptor in InP (dual activation energy of 620 milli electron-volts (meV) and 710 meV) which is thermally very stable to diffusion in InP. While that element may prove to be problematic in use for bulk InP substrates as it cannot compensate all of their volume, it becomes practical with a thin layer of transferred InP in accordance with the present invention. Typically, according to the study by A. Näser et al cited above, the dose of rhodium would be about 1 × 10¹⁷ atoms.cm^-3 to a depth of 250 nm.

0057InP compensation can also be achieved using a combination of elements, for example a shallow acceptor such as Hg or Cd combined with a deep acceptor such as Ti or Cr. Resistivities of 10⁴ to 10⁵ ohm.cm are obtained. Mercury and titanium have the advantage of having a much lower diffusivity than iron and could limit contamination of neighboring parts of the structure that is produced.

0058In conclusion, the present invention can produce a structure in which we have: <ul id="ul0006" list-style="bullet" compact="compact"><li>a thin film of InP with diffused Fe;</li><li>an optional amorphous bonding layer (SiO₂, Si₃N₄, etc.);</li><li>a support, for example monocrystalline, polycrystalline Si, etc.</li></ul> using a thin film transfer method without perturbation by iron.

0059More precisely, by selecting non-doped InP, transfer of a thin InP film using a layer transfer method such as Smart-Cut® is facilitated.

0060Further, iron diffusion of InP in a gas source, since it is carried out at a very high temperature (about 900°C or more) produces results as regards homogeneity of iron distribution, compensation efficiency (resistivity/iron concentration ratio improved) and consequently mobility of residual free electrons.

0061Since said diffusion is carried out in a thin film of low thickness (typically &lt; 1 µm), its duration is typically a few minutes, i.e., about two orders of magnitude shorter than diffusion into a bulk substrate, which takes a prohibitive length of time, and becomes compatible with industrial application.

0062As regards size, since the maximum diameter of InP wafers is currently 100 mm, they can be transferred to a larger diameter support and thus these pseudo-substrates can subsequently be used in fabrication facilities for larger standards. As an example, it becomes possible to use certain GaAs technology equipment (standard substrate size: 150 mm) with said pseudo-substrates.

0063Furthermore, compared with a bulk substrate, the support, for example Si, of the structure results in increased strength. This advantage leads to a reduction in losses during transport, manipulation and fabrication of components and circuits.

0064Finally, depending on the transition layer employed, the invention can produce a pseudo substrate with a compliant character for epitaxy because of the very thin InP layer. This means that epitaxial growth of a material with a lattice mismatch of 1% or more and/or with a thermal expansion coefficient that differs from that of InP can be facilitated.

0065Clearly, the present invention is applicable to all materials that may contain foreign elements perturbing coalescence of the implanted species necessary for a layer transfer method such as Smart-Cut®. It is not limited to the implementations described and shown; the skilled person will be capable of supplying many variations and modifications.