The use of steps of bonding and layer transfer are among the various fabrication methods employed in the field of microelectronic and optoelectronics. According to this method, a first so-called “donor substrate”, optionally covered with a layer of insulator which encapsulates it, is subjected to implantation of atomic and/or ionic species in order to create a weakened zone within it. This substrate is subsequently bonded by molecular adhesion to a second so-called “handle” substrate, then the donor substrate is separated into two parts along this weakened zone so that a desired thickness of the material of the donor substrate, and optionally the layer of insulator if it is present, is transferred onto the handle substrate.
After this transfer, the donor substrate becomes a so-called “negative” remainder, whereas the handle substrate becomes a so-called “positive” multilayer substrate. Such a layer transfer method makes it possible in particular to fabricate substrates known by the abbreviation “SOI”, standing for “Silicon-On-Insulator”, and to do so at a competitive cost, albeit so long as the donor substrate can be recycled.
FIG. 1 schematically represents in cross section the shape of a donor wafer (here the right side) of an example of a negative obtained after a first layer transfer. This is the negative remainder that is recycled in order to form a new donor substrate, which can be used in a fabrication method as described above. This figure shows a negative referenced overall by 1, which comprises a donor substrate 10 having a so-called “front” face 101 because it is the one which was in contact with the handle substrate, and an opposite so-called “rear” face 102. The donor substrate 10 is chamfered in an annular peripheral zone both on its front face and on its rear face. Initially, it was fully covered with a layer of insulator 2, but after separation of the layer of material extending into the non-chamfered region of the substrate, between the front face 101 and the weakened zone 103, it is found that the negative comprises a non-transferred annular ring 104.
This non-transferred ring 104 extends between the boundary 105 of the separation and the outer edge of the substrate or, in other words, is next to the implanted chamfered zone. This non-transferred ring 104 comprises a portion of insulator 2a and a portion of silicon 106. This ring may, for example, have a thickness of several hundreds of nanometers and a width on the order of from 1 to 3 mm. Its presence is associated with the fact that the chamfered portion of the donor substrate does not adhere sufficiently to the handle substrate and is therefore not transferred.
The recycling carried out according to the techniques of the prior art involves eliminating all of the insulator 2, that is to say both the insulator 2a, as well as the insulator 2 present on the rear face and on the edge; eliminating the residual ring 106 of silicon in order to get rid of the step-shaped profile; removing a thickness of silicon corresponding at least to the thickness damaged by the implantation, over the entire front side of the substrate; and polishing the front face of the negative, so as to recover a surface condition referred to as “mirror polished”, for example carrying out chemical mechanical polishing known to the person skilled in the art by the abbreviation “CMP”.
Such recycling may, for example, be carried out according to the method described in document EP 1 156 531, which consists in carrying out a step of deoxidizing the negative, for example by etching in a bath of acid, a step of grinding the extreme edge of the wafer and finally a step of polishing the implanted surface.
In practice, it is found that equipment which only removes material on the front face of the substrate does not make it possible to remove a regular thickness. For this reason, it is preferable to use equipment that carries out double-sided polishing of the negative, which leads to removal of close to 5 μm of material on each face of the donor substrate.
The aforementioned recycling methods have numerous drawbacks, namely in particular:                a high cost owing to the presence of at least two, or even three polishing steps that require expensive and bulky equipment which is difficult to maintain and a high consumption of consumable products such as polishing slurries and polishing pads.        the complexity of the method.        the large removal of material each time recycling is carried out, on the order of 10 μm, which rapidly leads to substrates being obtained which are too thin and thereby too fragile, in particular after several recycling cycles. Such substrates no longer comply with the SEMI specifications, and therefore can no longer be used again, in particular as handle substrates.        
Lastly, another drawback resides in the fact that the implanted but non-bonded annular ring is at least partially degraded during the annealing treatment carried out in order to separate the layer coming from the donor substrate. This results in a bubbling phenomenon which produces very many particles that can contaminate not only the positive but also the equipment subsequently used for various treatments of these wafers, and in particular the equipment for cleaning after separation.
Document WO 2005/038903 describes the transfer of an active layer by assembling two chamfered wafers. In order to prevent a non-bonded wafer edge from breaking in an uncontrolled fashion and inducing the presence of particles on the other surfaces, it is suggested to eliminate an edge zone of the active layer. Since this step is very intricate, document WO 2005/038903 proposes to carry out routing in a peripheral portion of one of the wafers before bonding the two wafers together.
FIG. 2 schematically represents in cross section the shape which a part (here the right side) of a donor substrate of the SOI type would theoretically have after having undergone the treatment of routing before bonding described in document WO2005/038903. However, the solutions proposed in the aforementioned document WO2005/038903 for carrying out the routing do not make it possible to obtain such an abrupt side edge, that is to say one which is as perpendicular to the front face as represented in FIG. 2.
As can be seen for comparison, the layer of insulator 2 and a part of the layer of material of the donor substrate 10 in a peripheral annular zone extending over the front face have been removed, where this removal is carried out for example by etching, over a width corresponding at least to that of the chamfered zone and over a thickness at least equal to the depth of the implantation zone. The active layer delimited by the weakened zone has the reference 107.
Specifically, tests carried out by the Applicant, by using an approach that involves successively etching the layers of silicon oxide then of silicon with the aid of a plasma, have shown that the routing obtained has a shallow slope as represented in appended FIG. 3A. Inter alia, etching of the silicon oxide continues during the step of etching the silicon.
Furthermore, and as can be seen in FIG. 3B, the resulting negative retains a non-transferred ring including a layer of oxide and a layer of implanted silicon, which substantially impairs the advantages expected of the routing method for future recycling of the negative.
Lastly, the routing solution proposed in document WO2005/038903 involves protecting the treated positive substrate by depositing a protective layer (for example of oxide), then removing an annular peripheral ring of it by lithography, carrying out the routing by etching the unprotected layer of the positive, and subsequently removing the protective mask. The use of an additional step of forming a protective mask, however, increases the costs and, above all, is a source of contamination. Moreover, it is preferable to avoid introducing impurities before the bonding since the intention is to reduce the defectiveness of the substrates obtained.
The Bosch method or process is also a known method that involves covering the front face of a substrate with the aid of a mask then, by anisotropic etching of this mask, in forming patterns on the order of one micrometer on the front face with a view to subsequently forming electronic or electromechanical components, but in no case in routing the edges of a full wafer over a width of 2 or 3 mm.
The aim of the invention is to resolve the aforementioned drawbacks of the prior art.