Abstract:
A method for manufacturing an integrated circuit containing fully and partially depleted MOS transistors, including the steps of forming similar MOS transistors on a thin silicon layer formed on a silicon-germanium layer resting on a silicon substrate; attaching the upper surface of the structure to a support wafer; eliminating the substrate; depositing a mask and opening this mask at the locations of the fully-depleted transistors; oxidizing the silicon-germanium at the locations of the fully-depleted transistors in conditions such that a condensation phenomenon occurs; and eliminating the oxidized portion and the silicon-germanium portion, whereby there remain transistors with a thinned silicon layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the field of integrated circuits and, more specifically, of integrated circuits of very small dimensions, on the order of some ten nanometers.  
         [0003]     2. Discussion of the Related Art  
         [0004]     In the field of integrated circuits containing transistors having dimensions on the order of some ten nanometers, MOS transistors formed on a thin silicon layer are here considered. It has been acknowledged that the behavior of such transistors is different according to whether, when the transistor is on, the channel region takes up the entire thickness of a very thin silicon layer or takes up but a portion of the thickness of a slightly thicker silicon layer. In the first case, it is spoken of fully depleted transistors and, in the second case, of partially depleted transistors. For example, for a MOS transistor having a gate length on the order of 10 nm, a fully depleted transistor will be formed in a silicon layer of a thickness from 10 to 20 nm and the source and drain regions will take up this entire thickness. However, a partially depleted transistor will be, for example, formed in a silicon layer having a thickness of approximately 70 nm, the source and drain regions penetrating down to a depth ranging from approximately 10 to 20 nanometers.  
         [0005]     It may be desired to form, in a same integrated circuit, fully depleted transistors which will have the advantage of a great switching speed and partially depleted transistors which will have the advantage of being able to stand slightly higher voltages than fully depleted transistors.  
       SUMMARY OF THE INVENTION  
       [0006]     An aspect of the present invention provides an integrated circuit containing these two types of transistors and a manufacturing method enabling obtaining with accuracy the two transistor types.  
         [0007]     An embodiment of the present invention provides a method for manufacturing an integrated circuit containing fully depleted MOS transistors and partially depleted MOS transistors, comprising the steps of:  
         [0008]     a) forming similar MOS transistors on a thin silicon layer formed on a silicon-germanium layer resting on a silicon substrate;  
         [0009]     b) attaching the upper surface of the formed structure to a support wafer;  
         [0010]     c) eliminating said substrate until the silicon-germanium layer is apparent;  
         [0011]     d) depositing a mask and opening this mask at the locations of transistors which are desired to be fully depleted;  
         [0012]     e) oxidizing the silicon-germanium at the locations of transistors which are desired to be fully depleted in conditions such that a condensation phenomenon occurs and that the front between the silicon and the silicon-germanium moves in the silicon layer; and  
         [0013]     f) eliminating the oxidized portion and the silicon-germanium portion, whereby there remain transistors with a thinned silicon layer.  
         [0014]     According to an embodiment of the present invention, step a) comprises the step of coating the structure with layers, the last one of which is a polished planar layer.  
         [0015]     According to an embodiment of the present invention, said polished planar layer is a silicon oxide layer.  
         [0016]     According to an embodiment of the present invention, step b) comprises the step of applying the structure on a silicon wafer by molecular bonding.  
         [0017]     According to an embodiment of the present invention, said thin silicon layer has an initial thickness on the order of from 50 to 70 nm and a thickness after thinning on the order of from 10 to 20 nm.  
         [0018]     According to an embodiment of the present invention, the method further comprises steps of contacting from the side from which the substrate has been eliminated.  
         [0019]     According to an embodiment of the present invention, the method further comprises steps of placing of a new wafer on the side from which the substrate has been eliminated.  
         [0020]     The foregoing and other objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     FIGS.  1  to  4  are simplified cross-section views illustrating successive steps of manufacturing of a fully depleted transistor and of a partially depleted transistor in a same integrated circuit according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0022]     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not drawn to scale.  
         [0023]     FIGS.  1  to  4  are simplified cross-section views of a portion of a semiconductor device illustrating successive steps of the manufacturing of a fully-depleted transistor and of a partially depleted transistor in the same integrated circuit according to an embodiment of the present invention.  
         [0024]     First, as illustrated in  FIG. 1 , a thin single-crystal silicon-germanium layer  2  is grown on a semiconductor substrate  1 , for example, silicon. On layer  2 , a thin single-crystal silicon layer  3  is grown, which is divided into wells or active areas in which elementary components are formed. The separation into wells is ensured by insulating regions  5 , currently designated in the art as STIs (for shallow trench insulation), extending across the entire thickness of silicon layer  3 . Wells containing transistors will here be considered. Two MOS transistors comprising a gate  11  formed on an insulator  12  and surrounded with spacers  13  so that a channel is capable of forming between source and drain regions  14  and  15  in well  3  have been very schematically shown in the drawing. Each of the transistors, as shown, is a partially depleted transistor, that is, the depth of the source and drain regions is much smaller than the thickness of the well in which the transistor is formed. To give an example of an order of magnitude, silicon-germanium layer  2  may have a thickness on the order of from 50 to 100 nm, thin silicon layer  3  may also have a thickness on the order of from 50 to 100 nm, and the source and drain regions are capable of penetrating into the well down to a depth from 10 to 30 nm.  
         [0025]     It should be understood by those skilled in the art that the representation of the transistors is very simplified. In particular, more complex structures of spacers as well as contact recovery silicide regions and metallizations are usually provided, although not shown. The contacts and the metallizations may be formed at this stage or subsequently, after the full manufacturing of the fully and partially depleted devices. The structure is coated with an assembly of layers designated with reference numeral  17  alternately containing insulating layers and portions of conductive layers to ensure interconnects between components. The upper portion of this assembly of layers  17  is, in the shown example, a silicon oxide layer planarized to be planar and polished.  
         [0026]     At the step illustrated in  FIG. 2 , the upper surface of layer  17  is stuck on a new substrate  21 , preferably, a silicon wafer. The sticking may be performed by molecular bonding or by any adequate method.  
         [0027]     Then, initial substrate  1 , which should be at the top in the representation of  FIG. 2 , is eliminated, for example, in a lapping step followed by an etching which is extremely selective between the silicon of substrate  1  and the silicon-germanium of layer  2 . Then, silicon-germanium layer  2  is coated with a mask, for example, a silicon nitride mask  23  which is open at the locations where fully depleted MOS transistors are desired to be formed.  
         [0028]     At the step illustrated in  FIG. 3 , the apparent silicon-germanium in the opening formed in silicon nitride layer  23  is oxidized. A known condensation phenomenon then occurs, according to which the germanium of the silicon-germanium layer migrates into silicon well  3 - 2 . Thus, after a given time, the front between the silicon and the silicon-germanium has moved down (in the representation of  FIG. 3 ) to reach the immediate neighborhood of the drain/source junctions. This front line is designated in  FIG. 3  with reference numeral  31 . However, the external portion of the initial silicon-germanium layer, on the side of the opening in silicon nitride layer  23 , oxidizes. The oxidized region is designated with reference numeral  32 .  
         [0029]     An advantage of this condensation phenomenon is that it is very well controlled and that, if an oxidation is performed for a determined time, the depth by which the front between the silicon-germanium and the silicon moves can be accurately determined, as described in T. Tezuka et al., IEEE Proceedings of 2004 Symposium on VLSI Technology, pp. 198-199 and in T. Tezuka et al., IEEE Proceedings of 2005 Symposium on VLSI Technology, pp. 80-81.  
         [0030]      FIG. 4  shows a next step of the method according to an embodiment of the present invention in which, after having carried out the previously-described oxidation-condensation steps, the silicon oxide formed in the opening in layer  23  is selectively etched. Indeed, a silicon nitride layer  23  had been selected to be able to selectively etch the silicon oxide with respect to this layer. Of course, those skilled in the art may select other appropriate materials, the important thing being to obtain this etch selectivity with respect to silicon oxide  32  formed at the step illustrated in  FIG. 3 . In a second step, the silicon-germanium is selectively etched with respect to silicon to keep a silicon well  3 - 2  of decreased thickness with respect to its initial thickness. For example, the oxidation-condensation step will have been carried on until the front between the silicon-germanium and the silicon is at a distance on the order of from 10 to 20 nm from the surface (smaller in the drawing) of the gate insulator.  
         [0031]     First partially depleted transistors in wells  3 - 1  of a much greater thickness than the depth of the source-drain regions and second fully depleted transistors for which the thickness of the silicon layer is on the order of from 10 to 15 nanometers will thus have been obtained, as desired.  
         [0032]     Various ways to complete the device may be envisaged.  
         [0033]     A first way to complete the device comprises, after having possibly removed nitride and SiGe layers  23  and  2 , respectively, depositing a protection layer, for example, an oxide layer. Vias are then pierced through the structure from the upper surface in  FIG. 4 , at locations where it will have been avoided to form active devices, to form contact recoveries with metallizations provided in layer assembly  17 . Some direct contacts having their surfaces usually designated as rear surfaces which are now apparent may besides be taken from the upper surface. Dielectrics may also be deposited, before depositing the protection layer, on the rear surfaces of the transistors to place the transistor wells under compression or tension which, in a known fashion, modifies the mobility of the carriers in these wells and thus the features of the transistors. Advantage may also be taken of the fact that access is had to the usually rear surface of the well of a transistor to deposit, with an interposed insulating layer, a conductive layer that can be used as a “rear gate” enabling giving the transistors additional control capacities, the front and rear gates of a transistor being simultaneously or independently controlled.  
         [0034]     A second way to complete the device comprises filling the openings illustrated in  FIG. 4  above the fully depleted transistors, eliminating layer  23 , eliminating or not SiGe layer  2 , and depositing a silicon oxide layer (or oxidizing the apparent surface of the SiGe layer) to attach the oxidized surface of the structure to a new substrate which would take up the position of previously-eliminated substrate  1 . In this case, substrate  21  will then be eliminated and an integrated circuit structure having the usual aspect (the gates being on the top side, on the apparent surface side), but on which fully depleted transistors and partially depleted transistors will have been formed, will have been obtained.  
         [0035]     It should be understood by those skilled in the art that the present invention is likely to have many variations, as concerns the indicated dimensions as well as the nature of the various materials. Such materials must indeed be selected according to their features of being semiconductors, insulators, or conductors, and according to their mutually selective etch features.  
         [0036]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.