fig2 shows a wafer 20 which includes a layer 22 having an uneven surface 220 , on a layer of a substrate 21 . it is noted that a layer 25 covers the uneven surface 220 . the layer 22 can be made from , for example , diamond cvd , si 3 n 4 , aln or a polycrystalline material such as polycrystalline silicon . such materials are rugged and can , for example , be employed as a layer of a soi ( silicon on insulator ) wafer , to improve the properties of thermal conductivity of the soi wafer . it may therefore be preferable to form the insulation layer of such a wafer not from sio 2 ( wherein the thermal conductivity properties are not good ), but by one or several materials which have high thermal conductivity , such as diamond or si 3 n 4 . layers of such materials are generally obtained by epitaxy , but the surface of such epitaxial layers is uneven . the layer 22 typically is comprised of material having a substantial hardness ( diamond , for example ). but the layer may be difficult to polish evenly because it is a heterogeneous material ( such as a polycrystalline material ) which is delicate . for these reasons , the layer 22 may be difficult to polish , or in general , be difficult to plane . the layer 22 may be comprised of a material with a high retail price , in which case material losses should be minimized as much as possible . the layer 22 may have been obtained by depositing material onto the flat surface of the substrate 21 . a chemical vapor deposition ( cvd ) technique may be used to deposit the material to form the layer 22 , which is a chemical deposit of a material in a vacuum . it will be noted that the surface 220 of the layer 22 is uneven , but the other face 221 which interfaces with the substrate 21 is flat . the face of the substrate 21 is in fact itself flat . in the embodiment of the invention shown in this figure , the wafer 20 is a multilayer wafer , which means that it is composed of several layers ( in this case , substrate layer 22 plus layer 21 ). however , it is possible to utilize the present method to produce uniform implantation in a monolayer wafer with an uneven surface , or to produce uniform implantation inside a layer having an uneven surface and situated on or in a multilayer wafer . referring to fig2 , the wafer 20 comprises a substrate layer 21 under the layer 22 . species are implanted and pass through the uneven surface of layer 22 to create a zone 23 in the thickness of the substrate 21 that corresponds to the maximum concentration of the implanted samples . in this embodiment , the covering layer 25 was applied to the wafer 20 before implantation . for example , the covering layer 25 may have been deposited upon the surface layer 22 . it is noted that the surface 250 of the covering layer 25 is planar . such a planar surface may be obtained by processing surface 250 by using a planing process that includes a polishing technique , for example . it is noted that the zone 23 of fig2 is more even than the zone 13 of fig1 . covering the uneven surface 220 of the layer 22 with a layer having a flat surface prior to the implantation stage results in increasing the implantation of species to a substantially uniform depth . in the case where the invention is used to conduct a smart - cut ® type process , an even weakened zone 23 can be created by implanting species through the covering layer and the uneven surface of the layer 22 . the invention can thus be implemented within the scope of a smart - cut ® or other transfer layer type process . in this case , following an implantation step the surface 250 is adhered to a stiffener , the substrate 21 is detached at the weakened zone 23 , and optionally , the free surface created at the zone 23 is processed . it is also possible to insert a fine , electrically insulating layer between the surface 250 and the stiffener , before adhering the stiffener to the surface 250 . for example , an electrically insulating layer made of sio 2 could be deposited on the surface 250 . in this case , the layer 25 and a supplementary layer ( the insulating layer ) together form the covering layer . the fine insulating layer can be deposited on the covering material , which has already been applied to the surface 220 , prior to the implantation step , so that the fine layer is part of the covering layer 25 ( see the cases illustrated in fig3 ). generally , the covering layer 25 can be made of several layers of different materials , as described later in this text . when the present method is used with a smart - cut ® type process , the substrate 21 can be made of monocrystalline silicon . monocrystalline silicon can also be used for other applications . hydrogen and / or helium may be used to implant species and when conducting a smart - cut ® type process . fig3 illustrates the effect caused by the thickness of the covering layer on the local depth of implantation , and the effect caused by planing the covering layer . the four cases represent four local conditions of implantation through a covering layer 25 and a layer with an uneven surface 22 , to a depth within the thickness of a substrate 21 ( wherein reference numbers 21 , 22 , 25 correspond to the different layers illustrated in fig2 ). the results shown in fig3 are based on simulations wherein the layer 21 had the characteristics of monocrystalline silicon , the layer 22 had an uneven surface and the characteristics of carbon ( carbon corresponding to diamond ), and the layer 25 was composed of two layers . the two layers were a layer of silicon deposited on the uneven surface of the layer 22 of carbon ( in fig3 the irregularities of this layer correspond to the different thicknesses of carbon in the different simulations ), and a layer of sio 2 covering the layer of silicon . the four simulations shown in fig3 correspond to four conditions wherein hydrogen was implanted . the first case corresponds to a carbon layer having a thickness of 2000 angstroms , covered by a 2000 angstroms thick silicon layer and a 1000 angstroms thick sio 2 layer . in this case , the depth of implantation of atomic species into the silicon substrate 21 is 1170 angstroms . the second case corresponds to a carbon layer thickness of 1000 angstroms , covered by a 2000 angstroms thick silicon layer and a 1000 angstroms thick sio 2 layer . in this second case , the depth of implantation into the silicon substrate 21 is 2793 angstroms . the third case corresponds to a carbon thickness layer of 2000 angstroms , covered by a 1000 angstroms thick silicon layer and 1000 angstroms thick sio 2 layer . in this third case , the depth of implantation into the silicon substrate 21 is 2383 angstroms . the fourth case corresponds to a carbon thickness layer of 1000 angstroms , covered by a 1000 angstroms thick silicon layer and 1000 angstroms thick layer of sio 2 . in this fourth case , the depth of implantation into the silicon substrate 21 is 3865 angstroms . the implantation conditions ( dose , energy , etc .) were the same in all four cases . these results illustrate several phenomena . first , when implanting occurs through a covering layer and through the carbon layer 22 having an uneven surface , the effect of the irregular surface of the layer 22 on the implantation depth is not only reproduced but is amplified . this is illustrated by comparing cases 1 and 2 which shows that there is a difference of 1623 angstroms in the depth of implantation , even though the difference in the thickness of the carbon layers is only 1000 angstroms . the same result is illustrated by comparing cases 3 and 4 which shows a difference of 1482 angstroms in the depth of implantation , even though the difference in thickness of the carbon layers is only 1000 angstroms . second , when there is an uneven layer 22 and the surface of the covering layer 25 is planed , the difference in the depth of implantation for cases having different carbon layer thicknesses is substantially reduced . a comparison of cases 2 and 3 illustrates that there is a difference of only 410 angstroms in the depth of implantation , even though the carbon layers differ in thickness by 1000 angstroms . the comparison of cases 2 and 3 corresponds to a situation in which the layer of silicon that contributes to forming the covering layer 25 has been planed prior to the deposit of the sio 2 layer . the comparison of these two cases thus illustrates the benefit of planing the silicon layer before depositing the sio 2 layer . a planed covering layer is particularly advantageous to minimize the differences in implantation depth generated by the differences in thickness of the carbon layer 22 which has an uneven surface . it is also possible , according to a variant of the invention , to locally adapt or modify the thickness of the covering layer 25 in different places on the wafer . consequently , the depth of implantation may be selectively controlled with the goal of further increasing evenness . a local modification of the thickness of the covering layer 25 can be accomplished at the time the layer is deposited , or by subsequent processing ( such as by planing or using a selective chemical etching process , and the like ). fig3 illustrates an implementation according to the invention concerning a covering layer 25 formed from si and sio 2 layers . as described below , the nature of the material or materials utilized for the covering layer can also have an influence on the differences in implantation depth . fig4 is a graph that illustrates the average depth of implantation ( sequenced rp ( a ), in angstroms ) as a function of the energy of implantation ( along the abscissa , wherein the energy of implantation is in kev ), for implanting hydrogen in different materials . fig4 shows that , for the same conditions , the average depth of implantation differs according to the nature of the material into which the hydrogen is implanted . each material can thus be characterized by a particular coefficient of implantation depth . fig4 illustrates that certain materials can be classified in decreasing order of this coefficient ( that is , for the same implantation conditions the material having a higher coefficient will have greater implantation depths ). as shown , the materials having greatest implantation depths in decreasing order are sio 2 , si , germanium , carbon ( diamond ), and aluminum oxide . the differences concerning the coefficient of implantation depth can be advantageously exploited within the scope of the invention . it is possible to improve the uniformity of implantation by using a covering layer 25 made of one or more materials , in which each material has a coefficient of implantation depth that is as close as possible to the coefficient of implantation depth associated with the material of the uneven surface layer 22 . by using such a covering layer 25 , and by planing ( preferably including polishing ) the surface of the covering layer , the best conditions are created for minimizing the differences in depth of the implanted species that could occur due to the differences in thickness of the layer 22 . in this case an overall layer that includes the combination of the layers 22 and 25 is created . this overall layer has a constant thickness ( due to the planing process ), and a coefficient of depth of implantation that is as homogeneous as possible . it is also possible to combine several layers of different materials inside the covering layer 25 so that the average of the coefficients of implantation depth of the different materials corresponds to the value of the coefficient of implantation depth of the uneven layer 22 . the average of coefficients can be weighted by the respective thicknesses of the different layers that form the covering layer 25 .