Abstract:
A 3-D structure formed in a recess of a substrate delimited by walls, including a large number of rectangle parallelepipedic blades extending from the bottom of the recess to the substrate surface while being oriented perpendicularly to one another and formed in a pattern covering the whole surface of the recess, some blades being non-secant to one of the walls, each non-secant blade being connected to one of the walls by at least another perpendicular blade.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 08/55906, filed on Sep. 3, 2008, entitled “HIGH-DENSITY 3-DIMENSIONAL STRUCTURE,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to 3-dimensional structures and, more specifically, to supports for such structures. The present invention relates to structures having at least one dimension on the order of one micrometer, or even of a few tenths of a micrometer or less. 
         [0004]    2. Discussion of the Related Art 
         [0005]    To form 3-D structures, a method comprises using a substrate, for example, made of silicon, in which a great number of very narrow trenches, close to one another, for example at a step on the order of one micrometer, separated by thin silicon blades, are formed. Micro-electro-mechanical devices (MEMS) are thus for example formed. Various layers may also be arranged on the walls and bottom of these trenches to form electronic components. Fuel cell cores or cells forming micro-batteries may also be formed on the walls and the bottom of these trenches. 
         [0006]    In the case of electronic components, capacitors or diodes having their active areas extending on the walls and bottom of the trenches are for example formed. Thus, capacitors having very high capacitances per surface unit are obtained. 
         [0007]      FIG. 1  illustrates an example of a support for 3-D components. The support is formed of a substrate  1 , for example, made of silicon, in which trenches  3  are formed. Trenches  3  have a significantly elongated rectangular shape, at the substrate surface. The trenches extend parallel to one another. Thus, each trench  3  is separated from another trench by a silicon blade  5 . 
         [0008]      FIG. 2  illustrates a partial cross-section view of a capacitor formed on a support such as that in  FIG. 1 . In  FIG. 2 , only two trenches  3  are shown. 
         [0009]    A stack of a first conductive layer  5  and of an insulating layer  7  extends on the surface of substrate  1  and on the walls and bottom of trenches  3 . A second conductive layer  9  fills the space remaining in trenches  3  and extends, above the substrate, on insulating layer  7 . Conductive layers  5  and  9  form the two capacitor electrodes. Contacts, not shown, are taken in appropriate fashion on each of these electrodes. 
         [0010]    The capacitance of the capacitor of  FIG. 2  is linked to the facing surface between each of electrodes  5  and  9  and to the thickness of insulating layer  7 . A recurrent object in the forming of 3-D capacitors is to increase their capacitances without increasing the surface area that they take up at the substrate surface. The facing surface between electrodes is thus desired to be increased. For this purpose, it is desirable to increase height h and to decrease thickness e of the blades between trenches to form more trenches per area unit. 
         [0011]    However, when it is dug too deeply or when very close trenches are formed, mechanical stress that may deform the blades and the trenches appears on each of the blades separating the trenches. This limits the increase of the ratio of the trench depth to the blade thickness. 
       SUMMARY OF THE INVENTION 
       [0012]    At least one embodiment of the present invention aims at supports for 3-D structures having a significant integration density, likely to comprise high and thin blades. 
         [0013]    At least one embodiment of the present invention also aims at any type of mechanical, electronic or other component, formed on such a support. 
         [0014]    Thus, to achieve all or part of these objects as well as others, an embodiment of the present invention provides a 3-D structure formed in a recess of a substrate delimited by walls, comprising a large number of parallelepipedic blades extending from the bottom of the recess to the substrate surface while being oriented perpendicularly to one another and formed in a pattern covering the whole surface of said recess, some blades being non-secant to one of the walls, each non-secant blade being connected to one of the walls by at least another perpendicular blade. 
         [0015]    According to an embodiment of the present invention, the trenches separating the blades altogether form, at the substrate surface, a pattern following a Moore fractal curve. 
         [0016]    According to an embodiment of the present invention, the recess has a depth, in the substrate, which is greater than 100 μm. 
         [0017]    According to an embodiment of the present invention, the blades have a thickness smaller than 0.8 μm and are separated from one another by a distance shorter than 0.8 μm. 
         [0018]    According to an embodiment of the present invention, the substrate and the blades are made of silicon. 
         [0019]    An embodiment of the present invention further provides a device comprising a structure such as described hereabove on the relief of which extends a capacitor, a diode, a fuel cell, or a micro-battery. 
         [0020]    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  
         [0021]      FIG. 1 , previously described, illustrates an example of a known 3-D component support; 
           [0022]      FIG. 2 , previously described, illustrates an example of a capacitor formed on a support such as that in  FIG. 1 ; 
           [0023]      FIGS. 3A and 3B  illustrate the effect of buckling phenomena on a silicon blade; 
           [0024]      FIGS. 4A and 4B  illustrate the effect of the electrostatic forces between two silicon blades; 
           [0025]      FIGS. 5A and 5B  illustrate, in top view, two patterns likely to be submitted to mechanical deflections; 
           [0026]      FIG. 6  is a top view of a support according to an embodiment of the present invention; and 
           [0027]      FIG. 7  is a top view of another support according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0028]    For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. 
         [0029]      FIGS. 3A ,  3 B,  4 A, and  4 B illustrate the effects of two mechanical stresses which appear on blades when the latter have a significant height or a low thickness. More specifically,  FIGS. 3A and 3B  illustrate the deformations due to buckling and  FIGS. 4A and 4B  illustrate the deformations due to electrostatic forces between blades. 
         [0030]      FIG. 3A  illustrates an isolated blade  5  extending vertically from a plane  11 . Blade  5  has a height h, a thickness e, and a length l. 
         [0031]    When height h of the blade is large and its thickness e is small, a buckling phenomenon may occur due to the weight of the blade. This results in the deformation of the blade, as illustrated in  FIG. 3B : the blade settles on itself, its height h decreases, and it substantially forms an S or a C in cross-section. 
         [0032]      FIG. 4A  illustrates two parallel blades  5 , of the same type as that in  FIG. 3A .  FIG. 4B  illustrates the deformation of the blades of  FIG. 4A  due to the electrostatic forces between the blades. The base of blades  5  being attached to support  11 , the electrostatic forces bring the facing walls of the two blades closer to each other, or even against each other, as shown in  FIG. 4B . 
         [0033]    The forming of blades of significant height h and of small thickness e increases the risks of deformation of such blades by the effects mentioned hereabove. Thus, parasitic mechanical and electrostatic phenomena limit dimensional selection possibilities. 
         [0034]    It is provided to form a support comprising blades having a shape such that buckling phenomena and deformations due to electrostatic forces are attenuated. For this purpose, the inventor provides forming, in the support, sets of blades. Each set comprises an assembly of orthogonal blades, with at least one of them that joins a wall of a recess in which the blade sets are formed. Thus, portions which are substantially I-shaped, in top view, can be defined in each assembly. 
         [0035]    In an I-shaped portion comprising a center bar and two parallel end branches, blades forming the parallel branches of the I enable the holding of the blade forming the central bar thereof. The stiffness of a blade assembly, like any mechanical structure, is characterized by its moment of area. Said moment is particularly high in the case of I-shaped portions. 
         [0036]      FIG. 5A  shows an example of a blade formed of seven elementary squares of side length a, extending along an axis x. The perimeter of this blade thus is  16   a.    FIG. 5B  illustrates a blade assembly forming an I, also comprising seven elementary squares of side length a. This assembly also has a perimeter equal to  16   a.  Thus, the surface of layers formed on the blade walls will be the same in the case of  FIGS. 5A and 5B . In  FIG. 5B , an axis y follows the direction of the central bar of the I and an axis x is perpendicular to axis y. Theory shows that the moment of area along axis x of the blade of  FIG. 5A  is approximately 11 times smaller than the moment of area along axis x of the I shape of  FIG. 5B  and approximately 10 times smaller than the moment of area along axis y of this last pattern. This difference in moment of areas explains the difference in stiffness between the patterns of  FIGS. 5A and 5B . 
         [0037]    To further increase the resistance of the blade assemblies against mechanical and electrostatic phenomena, the present invention provides for each blade assembly to comprise at least one blade which is directly connected to a wall of a recess in which the different blade assemblies are formed. It will be within the abilities of those skilled in the art to form such a structure comprising assemblies of blades perpendicular to one another, each assembly comprising at least one blade connected to a wall of the recess. 
         [0038]    Among the different possible configurations, the inventor provides forming trenches having a shape, in top view, substantially following a Moore fractal curve. Such a fractal curve has the advantage of naturally fitting into a square, which is particularly appropriate for the geometry of electronic chips, and of being able to be easily computer-modeled. 
         [0039]      FIG. 6  is a top view of a support according to such an embodiment, the support being for example made of silicon. Support  1  comprises a recess delimited by walls  13 ,  15 ,  17 , and  19 . Blades  21  extending from the bottom of the recess surround trenches  23 , trenches  23  altogether forming a pattern following a Moore curve of order  4 . Each blade  21  is either directly connected to one of walls  13 ,  15 ,  17 , or  19 , or indirectly connected to one of these walls via one or several blades  21  perpendicular to the first blade. A stiff structure is thus obtained, each blade enabling an efficient holding of the surrounding blades. 
         [0040]      FIG. 7  illustrates a variation of  FIG. 6  in which trenches  23  follow a Moore curve of order  5 . 
         [0041]    Advantageously, for a same substrate surface and blades of same thicknesses, a structure such as that in  FIG. 6  has a mechanical resistance from 5 to 10 times greater than that of a structure such as in  FIG. 1 . Thus, trenches having a height which is at least 5 times greater than before can be formed, without risking to adversely affect the structure stiffness. The blades may have a thickness much smaller than one micrometer, for example, 0.8 μm, or even smaller than one tenth of a micrometer, for a height greater than 100 μm, and be separated by a distance smaller than one micrometer, for example 0.8 μm, or even smaller than one tenth of a micrometer. In the case of capacitors, capacitors having capacitances at least 5 times as large as previously are obtained. The inventor expects to obtain capacitors having a capacitance per area unit greater than approximately 250 nF/mm 2 . 
         [0042]    It should be noted that the embodiments of  FIGS. 6 and 7  are examples only, and that any pattern comprising blades, which by their assembly substantially form I portions and where each assembly comprises at least one blade directly connected to a wall of the support, will also be suitable. 
         [0043]    Rounded versions of the Moore curves with no sharp angle may also be used to ease the implementation. This is similar to IPN-type shapes (normal girder profile), which have the specificity of a good resistance to out-of-line efforts. Such structures are formed based on an I, where the end branches of the I are widened at the level of its central bar. Such structures enable to further increase the mechanical resistance of patterns formed on a silicon substrate. 
         [0044]    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.