Abstract:
A method for forming a empty area under a layer of a given material, including forming on a substrate a stacking of a photosensitive layer and of a layer of the given material; insolating a portion of the photosensitive layer or its complement according to whether the photosensitive layer is positive or negative with an electron beam crossing the layer of the given material; and removing the portion of the photosensitive layer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to integrated circuit manufacturing methods. 
   The method of the present invention more specifically aims at forming, under a layer of a first material, empty areas and/or portions of another material. 
   2. Discussion of the Related Art 
   Known ion implantation techniques enable forming buried layers in a substrate by having implanted ions react with the substrate. An example of an ion implantation is the implantation of oxygen O 2  in a silicon substrate to form buried silicon oxide layers SiO 2 . 
   This implantation technique is however relatively destructive for the upper part of the substrate. Further, it does not enable forming a buried layer of any type of material. Further, the thickness of the buried layer is difficult to control. 
   Another known buried layer forming technique is that implemented for example upon forming of devices of SON (silicon on nothing) type. After the epitaxial growth of a silicon/germanium layer followed by a silicon layer, one or several portions of the silicon layer are etched to be able to remove, by etching, the underlying silicon/germanium layer. A dielectric or conductive layer is then deposited under the silicon layer by filling the space previously taken up by the silicon/germanium layer. 
   Once the layer has been deposited, no known technique enables removing chosen portions of the dielectric or conductive layer. A disadvantage of this technique thus is that the deposited layer necessarily has a shape identical to that of the silicon/germanium layer. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide such a method for forming, under a layer of a given material, portions of another material and/or empty areas having any shape. 
   Another object of the present invention is to provide such a method which is non-destructive for the layer of a given material. 
   Another object of the present invention is to provide such a method that enables forming portions of any other material. 
   To achieve these and other objects, the present invention provides a method for forming an empty area under a layer of a given non photosensitive material comprising; forming on a substrate a stacking of a photosensitive layer and of a layer of the given material; insolating a portion of the photosensitive layer or its complement according to whether the photosensitive layer is positive or negative with an electron beam crossing the layer of the given material; and removing said portion of the photosensitive layer. 
   According to an embodiment of the above-mentioned method, at least one opening of said layer of the given material emerging into said portion is formed prior to the removal of said portion of the photosensitive layer. 
   According to an embodiment of the above-mentioned method, the method further comprises filling with a second material the space previously taken up by the removed portion of the photosensitive layer. 
   According to an embodiment of the above-mentioned method, the method further comprises removal of the remaining portions of the photosensitive layer. 
   According to an embodiment of the above-mentioned method, the method comprises, prior to the removal of said remaining portions of the photosensitive layer, forming of at least one opening of said layer of the given material emerging into one of said remaining portions and further comprising filling with a third material different from the second material the space previously taken up by the remaining portions of the photosensitive layer. 
   According to an embodiment of the above-mentioned method, the removal of said portion of the photosensitive layer is performed by means of a plasma, said portion volatilizing after transformation in crossing said layer of the given material. 
   According to an embodiment of the above-mentioned method, the removal of the remaining portions of the photosensitive layers is performed by means of an oxygen plasma, the remaining portions volatilizing after transformation in crossing said layer of the given material. 
   According to an embodiment of the above-mentioned method, the method further comprises, at forming a stacking of a photosensitive layer and of a layer of the given material, the forming of a second photosensitive layer on the stacking, and the insolation step is performed to insolate the two photosensitive layers simultaneously, whereby the insolated portions of the two layers are superposed. 
   The present invention also provides a method for forming an interconnect network comprising: covering a substrate with a stacking of a first insulating layer, of a first photosensitive layer, and of a second insulating layer; forming openings in the stacking; filling the openings with a conductive material to form contacts; covering the second insulating layer and the contacts with a second photosensitive layer; insolating several pairs of portions of the photosensitive layer or their complements according to whether the photosensitive layer is positive or negative, the portions of a same pair being superposed and placed above and under the second insulating layer; removing the pairs of portions of the photosensitive layer; filling with a conductive material the space previously taken up by the portions of each of the pairs located above the second insulating layer. 
   The present invention also provides a method for forming a gate-all-around transistor comprising: forming a single-crystal silicon bridge running above an active area of a semiconductor substrate, the bridge being laid on an insulation area surrounding the active area; forming a thin silicon oxide layer around the bridge and on the active area; covering the structure and the empty space located between the active area and the bridge with a photosensitive layer; insolating two first portions of the photosensitive layer, or their complements according to whether the photosensitive layer is positive or negative, the first two portions of the photosensitive layer being superposed and placed above and under the central portion of the bridge; removing the two first and second portions of the photosensitive layer; filling with a conductive material the space previously taken up by the two first and second portions to form a gate; removing the remaining portions of the photosensitive layer; covering the structure and the empty spaces located under the bridge on either side of the gate with a silicon nitride; and performing an anisotropic etch of the nitride to form spacers on the sides of the gate portion located above the bridge. 
   The foregoing 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 
       FIGS. 1A to 1D  are cross-section views of structures obtained after successive steps of the method of the present invention; 
       FIG. 2  is a perspective view of an intermediary structure obtained in a conventional method for forming SON-type devices; 
       FIGS. 3A to 3F  are cross-section views of structures obtained after successive steps of an example of implementation of the method of the present invention; and 
       FIGS. 4A to 4F  are cross-section views of structures obtained after successive steps of another example of implementation of the method of the present invention. 
   

   DETAILED DESCRIPTION 
   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 drawings are not to scale. 
   The general aspects of the method of the present invention are described in relation with  FIGS. 1A to 1D . Examples of implementation of the method are described hereafter for the forming of a transistor of SON type and for the forming of an interconnect network enabling connecting the components of a circuit to one another. 
   In an initial step, illustrated in  FIG. 1A , a stacking of a photosensitive layer  2  and of a thin layer  3  of a given material is formed on a substrate  1 . Substrate  1  may have any structure obtained after a step of a standard integrated circuit forming method. The stacking of photosensitive layer  2  and of thin layer  3  may be obtained according to various methods as will appear in the examples of implementation of the method of the present invention described hereafter. Thin layer  3  may be formed of a dielectric, semiconductor, or conductor material. 
   According to the present invention, an insolation of one or several portions of photosensitive layer  2  with an electron beam crossing thin layer  3  is performed. The penetration depth of the electron beam through thin layer  3 , photosensitive layer  2  and possibly substrate  1  depends on the electron acceleration power, on the respective thickness of the layers, and on their nature. It will be within the abilities of those skilled in the art to define the thicknesses of layers  2  and  3  as well as the acceleration power of the electron beam enabling insolation of photosensitive layer  2  while ensuring that the electron beam remains coherent in crossing thin layer  3  to have a good definition of the insolated portions. As an example, with a thin silicon layer  3  of a 100-nm thickness and a 50-keV acceleration power, the insolated portions of the photosensitive layer can be defined with an accuracy under 10 nm. 
   In  FIG. 1A , five portions  5 ,  6 ,  7 ,  8 , and  9  have been shown from left to right. In the case where the photosensitive layer is positive, portions  6  and  8  are insolated. In the case where the photosensitive layer is negative, portions  5 ,  7 , and  9  are insolated. 
   After insolation, the insolated (or non-insolated, according to cases) portions, here portions  6  and  8 , are removed. Various methods may be implemented. 
   In the case where the chosen type of photosensitive layer enables it, portions  6  and  8  may be removed by an “evaporation” method. This method comprises placing the structure in a plasma which modifies the structure of portions  6  and  8  without modifying that of portions  5 ,  7  and  9 . The used plasma may be an oxygen (O 2 ), hydrogen (H 2 ), or nitrogen (N 2 ) plasma. After modification of the structure of the portions of photosensitive layer  6  and  8 , the photosensitive layer is removed by passing through thin layer  3 . 
   Another method for removing portions of photosensitive layer  6  and  8  is illustrated in  FIG. 1B . Openings O 1  and O 2  are formed in thin layer  3  above portions  6  and  8 . In this example, opening O 1  is formed above the left-hand part of portion  6  and extends above the right-hand part of portion  5 . Opening O 2  is formed above the central part of portion  8 . Portions  6  and  8  of the photosensitive layer are then removed by selective development. In the example shown in  FIG. 1B , openings of thin layer  3  have been shown above each of the portions of photosensitive layer  2  which are desired to be removed. However, in certain structures such as that described hereafter in relation with  FIGS. 3A to 3F , it is possible for the portions of the photosensitive layer which are desired to be suppressed to be accessible “sideways”. In this case, it is not indispensable to form openings prior to the removal of certain portions of the photosensitive layer. 
   In the case where openings O 1  and O 2  have been formed, an additional step, illustrated in  FIG. 1C , comprising filling the space previously taken up by portions  6  and  8  with a second material different from that of thin layer  3 , may be provided Portions  10  and  11  of the second material have thus been formed under thin layer  3 . 
   In the case where the recesses under thin layer  3  have been formed by selective chemical development and where these recesses have been filled with a second material, an additional step, illustrated in  FIG. 1D , comprising removing the remaining portions of photosensitive layer  6 ,  7 , and  9 , may be provided. To remove the remaining portions of the photosensitive layer, openings, in this example O 3  and O 4 , may be formed as previously in thin layer  3  above the portions to be removed, portions  5  and  9  in this example. In the case where the chosen type of photosensitive layer allows it, it may also be provided to place the structure in a plasma (O 2 , H 2 , N 2 ) likely to modify the structure of portions  5 ,  7  and  9  of the photosensitive layer so that the photosensitive layer is removed by passing through thin layer  3 . In this example, portion  7  is removed across thin layer  3 . 
   In the case where openings O 3  and O 4  have been formed, an additional step, comprising filling the space previously taken up by portions  5 ,  7 , and  9  with a third material different from the second material, may be provided. 
   An advantage of the method of the present invention is that it enables forming, under a layer of a given material empty areas and/or portions of another material having any desired shape. Further, since the insolation by an electron beam is a very accurate technique, the shapes of the portions can be defined with great accuracy. 
   Another advantage of the method of the present invention is that it does not damage the definitive portions or layers of the formed structures. 
   A specific embodiment of the method of the present invention is described hereafter in an example of the forming of a gate all around (GAA) transistor. 
     FIG. 2  is a perspective view of an intermediary structure obtained after a step of a conventional gate-all-around transistor forming method. An active area  100  of a semiconductor substrate  101  is defined by an insulating region  102  formed at the surface of substrate  101 . Active area  100  has in this example a substantially rectangular shape in top view. A silicon layer forms a bridge  103  running over active area  100  and bearing against insulating region  102  on either side of active area  100 . The portion of bridge  103  located above active area  100  is a single crystal region, the portion located above insulating region  102  is polycrystalline. Active area  100  and bridge  103  are separated by a empty area Active area  100  is visible in top view on each side of bridge  103 . 
   The structure shown in  FIG. 2  is conventionally obtained by epitaxial growth of a silicon/germanium layer above active area  100  and epitaxial growth of a single-crystal silicon layer above the silicon germanium layer. The bilayer is then etched to form a strip transverse to active area  100 . The remaining portion of the silicon/germanium layer is removed by etching and silicon bridge  103  is thus obtained. 
     FIGS. 3A to 3F  are cross-section views of the structures obtained after successive steps of an example of implementation of the method of the present invention performed based on the structure shown in  FIG. 2 . The cross-section views are shown along a plane longitudinally cutting bridge  103 . 
   In a first step, illustrated in  FIG. 3A , a thin silicon oxide layer SiO 2  is grown by thermal oxidation all around silicon bridge  103  and on active area  100 . A photosensitive layer  111  is then deposited above the structure and in the empty space located between bridge  103  and active area  100 . 
   At the next step, illustrated in  FIG. 3B , photosensitive layer  111  is insolated by using an electron beam capable of crossing the photosensitive layer located above bridge  103 , bridge  103  being covered with a thin silicon oxide layer and the photosensitive layer placed under bridge  103 . In the case where the photosensitive layer is positive, a portion  120  of photosensitive layer  111  located above the central portion of bridge  103  and a portion  121  of photosensitive layer  111  located above portion  120  under bridge  103  are insolated. Portions, not shown, of the photosensitive layer located in prolongation of portions  120  and  121  on each side of bridge  103  are further insolated. In the case where the photosensitive layer is negative, portions  122  and  123  of the photosensitive layer respectively located above the left-hand and right-hand portions of bridge  103  as well as portions  124  and  125  respectively located under the left-hand and right-hand portions of bridge  103  are insolated. Portions of the photosensitive layer, not shown, located in prolongation of portions  122  to  125  on each side of bridge  123 , are also insolated. 
   At the next step, illustrated in  FIG. 3C , portions  120  and  121  as well as portions located in prolongation of portions  120  and  121  on each side of bridge  103  are removed. 
   At the next step, illustrated in  FIG. 3D , a conductive material, for example, polysilicon, is deposited in the space previously taken up by portions  120  and  121  and by the portions placed in prolongation of portions  120  and  121  on each side of bridge  103 . Gate portions  130  and  131  respectively above and under bridge  103  and gate portions, not shown, on each side of bridge  103  in prolongation of portions  130  and  131 , are then obtained. These portions altogether form one and the same gate surrounding bridge  103 . 
   At the next step, illustrated in  FIG. 3E , the remaining portions of the photosensitive layer, that is, portions  122  to  125 , as well as the portions placed in prolongation of portions  122  to  125  on each side of bridge  103 , are removed. 
   At the next step, illustrated in  FIG. 3F , a nitride deposition is performed to entirely cover the previously-obtained structure and to fill the empty spaces located under bridge  103  on either side of portion  131 . An anisotropic etching of the nitride is then performed to expose the upper surface of gate portion  130 , of the thin oxide layer  110  placed on bridge  103 , and of active area  100  on either side of bridge  103 . After this nitride etching, spacers  140  and  141  have been formed on the sides of portion  130  and nitride portions  142  and  143  under bridge  103  on either side of gate portion  131 . The portion of bridge  103  located under polysilicon portions  130  and  131  forms the transistor channel. The portions of bridge  103  located on either side of channel  144  form source/drain areas  145  and  146 . 
   An advantage of the previously-described method is that it enables forming a gate-all-around transistor such that the gate portions placed above and under the silicon bridge are aligned. 
   Another embodiment of the method of the present invention is described hereafter in an example of realization of an integrated circuit interconnect network. 
   In an initial step, illustrated in  FIG. 4A , a stacking of an insulating layer  201 , of a photosensitive layer  202 , and of an insulating layer  203 , are formed on a substrate  200 . Substrate  200  may have any structure obtained after a step of a standard integrated circuit forming method. As an example, three conductive areas  205 ,  206 , and  207  are shown at the surface of substrate  200 . Conductive areas  205 ,  206 , and  207  are for example terminals of access to components of the integrated circuit formed in substrate  200 . Insulating layers  201  and  203  may be thin silicon oxide layers or layers of an insulator preferably exhibiting a small dielectric constant. 
   At the next step, illustrated in  FIG. 4B , openings O 10 , O 11 , and O 12  are formed in the stacking of layers  201  to  203  above conductive areas  205 ,  206 , and  207 . Openings O 10 , O 11 , and O 12  may be formed according to a method comprising the following steps. A nitride layer followed by a photosensitive layer is deposited on insulating layer  203 . Portions of the photosensitive layer above conductive areas  205 ,  206 , and  207  are insolated and removed. The unprotected portions of the nitride layer are etched. Insulating layer  203 , photosensitive layer  202 , and insulating layer  201  are successively etched according to an anisotropic etch method enabling formation of openings having substantially vertical walls. The remaining portions of the nitride layer are finally removed. 
   At the next step, illustrated in  FIG. 4C , openings O 10 , O 11 , and O 12  are filled with a conductive material such as copper to form contacts C 1 , C 2 , and C 3  above conductive areas  205 ,  206 , and  207 . The filling of openings O 10 , O 11 , and O 12  may consist of performing a copper deposition over the entire structure, then performing a chem.-mech polishing to expose insulating layer  203 . Insulating layer  203  and contacts C 1 , C 2 , and C 3  are then covered with a photosensitive layer  210 . 
   At the next step, illustrated in  FIG. 4D , the two layers of photosensitive layer  202  and  210  are insolated with an electron beam passing through photosensitive layer  210 , insulating layer  203 , and photosensitive layer  202 . In this example, five portions  220 ,  221 ,  222 ,  223 , and  224  are defined from left to right of photosensitive layer  210  and five portions  230 ,  231 ,  232 ,  233 , and  234  respectively above portions  220  to  224  are also defined in photosensitive layer  202 . Portion  221  extends above contacts C 1  and C 2 , portion  223  is placed above contact C 3 . In the case where the photosensitive layer is positive, portions  221 ,  223 ,  231 , and  233  are insolated. In the case where the photosensitive layer is negative, portions  220 ,  222 ,  224 ,  230 ,  232 , and  234  are insolated. 
   At the next step, illustrated in  FIG. 4E , portions  221  and  223  of photosensitive layer  210  are removed according to a conventional development method. Portions  231  and  233  are then removed by placing the structure in a plasma so that the photosensitive layer volatilizes after transformation in passing through insulating layer  203 . The removed portions of the photosensitive layer are then replaced with empty areas  240  and  241 . Another way to remove portions  231  and  233  comprising forming small openings in insulating layer  203  above these portions, then of removing them by wet etch. 
   At the next step, illustrated in  FIG. 4F , the space previously taken up by portions  221  and  223  of the photosensitive layer is filled with a conductive material such as copper to form conductive lines  250  and  251 . The forming of the conductive layers may be performed by depositing copper over the entire structure and by performing a chem.-mech polishing to expose the upper surface of portions  220 ,  222 , and  224  of the photosensitive layer. Conductive areas  205  and  206  are then connected via contacts C 1  and C 2  and conductive line  250 . 
   The previously-described step sequence comprising forming contacts, then conductive lines, may be repeated several times to form several conductive line levels. 
   An advantage of the specific embodiment of the present invention is that it enables forming an interconnect network such that each conductive line is insulated from the other conductive lines by two very thin portions of an insulating layer and by an empty area forming the best possible insulator. 
   Further, the insulation between two lines placed on two successive levels being very good, it is possible to decrease the height of the contacts, which enables significantly reducing their resistance. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, it may be provided to form on a substrate a stacking of several photosensitive layers and of layers of one or several given materials before performing an insolation of all the photosensitive layers by means of an electron beam crossing the layer assembly. Further, various methods may be implemented to form a stacking of photosensitive layers and of layers of one or several materials. 
   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.