Abstract:
The use of a conductive bidimensional perovskite as an interface between a silicon, metal, or amorphous oxide substrate and an insulating perovskite deposited by epitaxy, as well as an integrated circuit and its manufacturing process comprising a layer of an insulating perovskite deposited by epitaxy to form the dielectric of capacitive elements having at least an electrode formed of a conductive bidimensional perovskite forming an interface between said dielectric and an underlying silicon, metal, or amorphous oxide substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the forming of capacitive elements in an integrated circuit. The present invention more specifically relates to the forming of capacitive elements based on perovskite oxides.  
         [0003]     The invention applies, for example, to the forming of high-density memory integrated circuits in which the memory point capacitors are formed in a perovskite layer to form the dielectric.  
         [0004]     2. Discussion of the Related Art  
         [0005]     A difficulty of the use of perovskite oxides in the forming of semiconductor circuits is linked to the poor crystal growth of perovskites on silicon substrate. The thin perovskite layer or film deposited on a silicon substrate does not maintain the properties of the single crystal or of the ceramic. An interface must thus be interposed between the silicon substrate and this perovskite. The same problems are posed for the crystal growth of perovskites on a metal layer or on an amorphous oxide, which form the other types of layers above which it is generally desired to form integrated capacitors.  
         [0006]     To maintain the single-crystal structure of the perovskite forming the dielectric of the capacitive elements, the interface on which this perovskite is deposited must enable deposition by epitaxy. Further, this interface must adhere in crystallized form on the underlying substrate and form a diffusion and reaction barrier between the silicon (oxidized or not) and the perovskite.  
         [0007]     Among the perovskites generally used to form the dielectric layers deposited by epitaxy on silicon substrates, one can find:  
         [0008]     barium strontium titanates (BST) having formula (Ba,Sr)TiO 3 ;  
         [0009]     lead zirconate titanates (PZT), of formula Pb(Zr,Ti)O 3  and their derivatives such as, for example, lead lanthanum zirconate titanates (PLZT); and  
         [0010]     double layer perovskites such as strontium bismuth tantalate (SBT) of formula SrBi 2 Ta 2 O 9 , or strontium bismuth niobate (SBN) of formula SrBi 2 Nb 2 O 9 .  
         [0011]     To enable deposition of such perovskites on a silicon oxide substrate (SiO 2 ), an interface formed of a BiT perovskite (Bi 4 Ti 3 O 12 ) is generally used.  
         [0012]     A disadvantage of using BiT as an interface is that it is an insulating perovskite. Accordingly, to form a capacitive element from an SiO 2  substrate, the BiT must be covered with another conductive perovskite, for example, LSCO ((La,Sr)CoO 3 ), before depositing the dielectric layer.  
         [0013]     A disadvantage then is a complexity of the forming process and of the obtained structure.  
         [0014]     An example of use of BiT as an interface for the epitaxy of a perovskite is described in the article: “Template Approaches to Growth of Oriented Oxide Heterostructures on SiO 2 /Si” by R. Ramesh et al. published in 1994 in “Journal of Electronic Materials”, Vol. 23, no 1.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention aims at providing a novel interface between a silicon, metal, or amorphous oxide substrate and a perovskite deposited by epitaxy and forming the dielectric of an integrated capacitive element.  
         [0016]     The present invention especially aims at providing an interface preserving the single-crystal properties of the perovskite forming the dielectric.  
         [0017]     The present invention also aims at providing an interface of simple structure.  
         [0018]     The present invention further aims at providing a method for forming such an interface which is compatible with technologies currently used in microelectronics and which is easy to form.  
         [0019]     To achieve these and other objects, the present invention provides the use of a conductive bidimensional perovskite as an interface between a silicon, metal, or amorphous oxide substrate and an insulating perovskite deposited by epitaxy.  
         [0020]     According to an embodiment of the present invention, the conductive bidimensional perovskite is used to form one of the electrodes of capacitive elements having their dielectric formed by the perovskite deposited by epitaxy.  
         [0021]     According to an embodiment of the present invention, the conductive bidimensional perovskite is strontium lanthanum nickelate of formula La (2−x) Sr x NiO (4±y) .  
         [0022]     The present invention also aims at a method for forming capacitive elements in an integrated circuit on a silicon, metal, or amorphous oxide substrate, comprising the deposition by epitaxy of an insulating perovskite to form the dielectric of the capacitive elements, and the use as an interface between the insulating perovskite and the substrate of a conductive bidimensional perovskite.  
         [0023]     According to an embodiment of the present invention, the interface is exclusively formed of said conductive bidimensional perovskite.  
         [0024]     According to an embodiment of the present invention, the interface perovskite is selected to exhibit a quadratic crystal structure.  
         [0025]     According to an embodiment of the present invention, the interface perovskite is of formula:  
         [0026]     La (2−x) A x BO (4±y)  where A is the dopant, and where B is a metal.  
         [0027]     According to an embodiment of the present invention, the dopant rate and the oxygen content are selected according to the desired conductivity.  
         [0028]     The present invention further provides an integrated circuit comprising a layer of an insulating perovskite deposited by epitaxy to form the dielectric of capacitive elements, at least an electrode of the capacitive elements being formed of a conductive bidimensional perovskite forming an interface between said dielectric and an underlying silicon, metal, or amorphous oxide substrate.  
         [0029]     According to an embodiment of the present invention, the interface is exclusively formed of said conductive bidimensional perovskite.  
         [0030]     According to an embodiment of the present invention, said conductive bidimensional perovskite is strontium lanthanum nickelate.  
         [0031]     The foregoing objects, features, and advantages of the present invention, as well as others, will be discussed in detail in the following non-limiting description of specific embodiments and implementation modes. 
     
    
     DETAILED DESCRIPTION  
       [0032]     A feature of the present invention is to provide the use of a conductive bidimensional perovskite as an interface between the substrate and the dielectric perovskite.  
         [0033]     A bidimensional perovskite is defined as having a crystal axis along which mesh parameter “c” is very different from the mesh parameters (“a” and “b”) of the other axes which are, as for them, of the same order of magnitude. The perovskite of the present invention is selected to have mesh parameters “a” and “b” close to most of the dielectric perovskites to be deposited by epitaxy.  
         [0034]     Preferably, this perovskite exhibits a quadratic crystal structure, which gives it a capacity to grow along the axis of parameter “c”. Its growth on a silicon substrate, be it oxidized or not, is thus favored.  
         [0035]     Another feature of the present invention is that the interface is formed of a single conductive material. It can then form the electrode of the capacitive element and directly receive the conventional dielectric perovskite.  
         [0036]     According to a preferred embodiment of the present invention, the selected perovskite is a nickel-based perovskite. For example, strontium lanthanum nickelate of formula La (2−x)Sr   x NiO (4±y) .  
         [0037]     The deposition of such a perovskite directly on a silicon oxide substrate is performed according to an embodiment of the present invention by laser ablation under the following conditions. The substrate is brought to a temperature ranging between 650 and 750° C., for example, 700° C. The assembly is processed in a residual oxygen atmosphere between 0.05 and 0.5 millibars, for example, 0.1 millibar. Strontium lanthanum nickelate further has the feature of being a product existing in solid form, which enables using a single target for the laser ablation.  
         [0038]     The thickness of the deposition ranges, preferably, between 200 and 700 nanometers. The minimum limit aims at avoiding interactions with the underlying substrate. The maximum limit preserves the stability of the obtained layer. Preferably, the thickness will be on the order of from 200 to 300 nanometers.  
         [0039]     The other features of the laser ablation deposition are within the abilities of those skilled in the art according to the application. As a specific example, a cooling under oxygen at a pressure of approximately 100 millibars or more may be provided. The wavelength of the used laser will preferably be smaller than 300 nanometers. Its frequency will range between 1 and 20 Hertz and the duty cycle will be selected for a pulse duration ranging between 10 and 20 nanoseconds. Such ablation conditions are a preferred, though non-limiting, embodiment.  
         [0040]     It should be noted that the present invention enables deposition of the two perovskites (conductive interface perovskite to form the electrode and insulating perovskite of the dielectric) without opening the laser ablation chamber. This contributes to optimizing the manufacturing process.  
         [0041]     An advantage of the present invention is that the perovskite can be deposited indifferently on silicon, metal, or an amorphous oxide (for example, SiO 2 ).  
         [0042]     Another advantage of the present invention is that the perovskite which has been grown on the above-mentioned substrate enables epitaxial deposition of an insulating perovskite to form the dielectric of the capacitive element. All the previously-mentioned perovskite families can be deposited by epitaxy on an interface according to the present invention.  
         [0043]     The conductivity of the bidimensional perovskite used as an interface according to the present invention can be adjusted by the dopant rate (in the above example, strontium) and/or the oxygen content.  
         [0044]     Considering the above example of ablation deposition, it has been possible to deposit an La 0,9 Sr 1,1 NiO 4  perovskite with a crystal orientation quality greater than 95% on an SiO 2  or silicon substrate in a plane 100 with a conductivity on the order of one milliohm.centimeter. An insulating Ba (1−x) Sr x TiO 3  perovskite deposition by the same method on the above-mentioned interface has exhibited a very good orientation (mismatch of 4% in mesh parameters).  
         [0045]     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, other deposition conditions than those indicated hereabove may be provided, provided to respect a deposition by laser ablation of the conductive perovskite interface for a deposition immediately overlying a dielectric perovskite by epitaxy.  
         [0046]     Further, other materials than those indicated as an example hereabove may be used. It should however be noted that, to obtain the advantages of the present invention, the conductive perovskite interface must be deposited directly on a silicon, metal, or amorphous oxide substrate and directly receive the dielectric perovskite of the capacitive element. This last perovskite deposited by epitaxy may however be formed of several layers according to the desired thickness.  
         [0047]     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.