Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a division of application Ser. No. 10/840,599 filed May 5, 2004, the entire content of which is expressly incorporated herein by reference. 
     
    
     BACKGROUND ART  
       [0002]     The present invention relates to processing semiconductor wafers, especially for use in applications such as electronics, microelectronics, and optoelectronics. In particular, the invention pertains to an implantation process for a wafer that includes at least a layer having an uneven surface, wherein implanting occurs through the uneven surface.  
         [0003]     It is known to employ implantation processes for semiconductor wafers to obtain various effects. For example, implantation can be used to create a weakened zone within the thickness of the wafer. The SMART-CUT® process is an example of an implantation method that produces a weakened zone. A general description of the SMART-CUT® process can be found, for example, in “Silicon-On-Insulator Technology: Materials to VLSI”, 2 nd  edition by Jean-Pierre Colinge, from Kluwer Academic Publishers, pages 50 and 51.  
         [0004]     When a weakened zone is to be created in the thickness of the wafer, for example by using a layer transfer method such as a SMART-CUT® type process, it is generally preferred for the weakened zone to have an even surface that extends substantially in a plane. In general, for numerous implantation applications, it is desirable to implant species so that the implanted samples are evenly distributed in the thickness of the wafer at a particular implantation depth under the surface of the wafer. Implantation that occurs through an uneven surface adversely affects the uniformity of the depth of implantation of the samples.  FIG. 1  illustrates this phenomenon. Referring to  FIG. 1 , samples have been implanted in a wafer  10  through a layer  12  of material having an uneven surface  120 , and through a portion of the substrate  11  upon which the layer  12  is located. Due to the uneven character of the surface  120 , the depth of implantation (shown by the line  13 ) is also uneven. This is a disadvantage.  
         [0005]     In this text “uneven surface” means a surface having a roughness value that is greater than a value on the order of several angstroms in root mean square (rms) value. It should also be noted that in the present text an even surface is one that has a roughness value less than several angstroms rms.  
       SUMMARY OF THE INVENTION  
       [0006]     Provided is a method for implanting atomic species through an uneven surface of a semiconductor layer. The method includes applying a covering layer upon the uneven surface in an amount sufficient and in a manner to increase surface uniformity, and then implanting the atomic species through the covering layer and uneven surface to obtain a more uniform depth of implantation of the atomic species in the layer.  
         [0007]     In a preferred embodiment, the semiconductor layer is a multilayer wafer comprising at least a substrate and a layer having an uneven surface. The covering layer is preferably deposited onto the uneven surface layer to form a substantially flat upper surface through which the atomic species can be implanted. Advantageously, the material of the covering layer is selected to increase the uniformity of the depth of implantation. In a preferred embodiment, the material of the covering layer is chosen to minimize differences between a coefficient of implantation depth of the semiconductor material of the uneven surface layer and a coefficient of implantation depth of the covering layer material. In one embodiment, the uneven surface layer comprises at least one of diamond, Si 3 N 4 , AlN, or polycrystalline silicon.  
         [0008]     In another embodiment, the covering layer may include at least two layers of different material. The materials selected for each layer of the covering layer are advantageously selected to increase the uniformity of the depth of implantation. In an beneficial implementation, the materials selected for each layer of the covering layer are chosen to minimize the difference between a coefficient of implantation depth of the semiconductor material of the uneven surface layer and the overall coefficient of implantation depth of the covering layer. In an advantageous embodiment, the covering layer is applied such that it includes a plane surface. It also is advantageous to process the surface of the covering layer to form the pane surface prior to implanting atomic species. Such processing may include a planning process or polishing. In a preferred embodiment, implantation is conducted using at least one of hydrogen or helium. The method may also include conducting a layer transfer method.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Other aspects, purposes and advantages of the invention will become clear after reading the following detailed description with reference to the attached drawings, in which:  
         [0010]      FIG. 1  illustrates implantation of samples in a wafer through a semiconductor layer having an uneven surface, and through a part of the substrate on which the semiconductor layer is located;  
         [0011]      FIG. 2  illustrates the effect on the uniformity of the implantation depth when a covering layer is used prior to implantation;  
         [0012]      FIG. 3  shows simulation results that illustrate the effect of the thickness of the covering layer on the local depth of implantation, as well as the effect of planing the covering layer; and  
         [0013]      FIG. 4  is a graph showing the differences in coefficients of implantation depth for different materials when implanting hydrogen. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]      FIG. 2  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.  
         [0015]     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.  
         [0016]     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.  
         [0017]     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.  
         [0018]     Referring to  FIG. 2 , 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  FIG. 2  is more even than the zone  13  of  FIG. 1 .  
         [0019]     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.  
         [0020]     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  FIG. 3 ). Generally, the covering layer  25  can be made of several layers of different materials, as described later in this text.  
         [0021]     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.  
         [0022]      FIG. 3  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  FIG. 2 ). The results shown in  FIG. 3  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  FIG. 3  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.  
         [0023]     The four simulations shown in  FIG. 3  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.  
         [0024]     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.  
         [0025]     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).  
         [0026]      FIG. 3  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.  
         [0027]      FIG. 4  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.  FIG. 4  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.  FIG. 4  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.  
         [0028]     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.  
         [0029]     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 .

Technology Category: 5