Abstract:
A microelectronic structure is described which contains a first conductive layer for preventing oxygen diffusion at the structure. The first conductive layer contains a base material and at least one oxygen-binding admixture that is provided with at least one element from the fourth subgroup or the lanthane group. In a preferred embodiment, the microelectronic structure is used in semiconductor storage components with a metal oxide dielectric as a condenser dielectric.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of copending International Application No. PCT/DE99/03832, filed Dec. 1, 1999, which designated the United States. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Field of the Invention  
           [0003]    The invention is in the field of semiconductor technology and relates to a microelectronic structure which has at least one substrate and one first conductive layer. Microelectronic structures such as these are used in particular in semiconductor memories.  
           [0004]    Materials having a high dielectric constant (Epsilon&gt;20) or having ferroelectric characteristics are being increasingly used in order to further increase the integration density in semiconductor memories. Those materials which are of major interest at the moment are metal oxide dielectrics, which are deposited at relatively high temperatures in the presence of oxygen. Prominent representatives are, for example, barium-strontium-titanate ((Ba,Sr)TiO 3 , BST), lead-zirconate-titanate (PbZrTiO 3 , PZT), strontium-bismuth-tantalate (SrBi 2 Ta 2 O 9 , SBT) and derivatives of the above mentioned materials. The high deposition temperatures required and the oxygen atmosphere that is present pose stringent requirements on the already formed structures on the semiconductor substrates, in particular on the lower electrode of the storage capacitor and on any barrier layer located under the electrode. Oxygen-resistant noble metals, in particular, have been proposed as electrode materials. Since noble metals such as these, in particular the preferred platinum, form interfering metal suicides with silicon, a barrier layer which is normally disposed between the electrode and the silicon substrate or polysilicon layer is intended to prevent the silicon from diffusing into the platinum electrode. The barrier layer is formed of titanium or a titanium-titanium nitride.  
           [0005]    However, this has the disadvantage that the titanium oxidizes relatively quickly at relatively high deposition temperatures (above 500° C.) and in consequence prevents a conductive connection from being formed between the electrode and the silicon. A range of measures have thus been proposed in order to protect the barrier layer against oxidation during the deposition of the metal oxides.  
           [0006]    One option is, for example, to bury the barrier in an oxygen-resistant nitride layer, and this is proposed, for example, in U.S. Pat. No. 5,619,393. In this solution, the barrier layer is surrounded in the form of a collar by the nitride layer, and its upper face is completely covered by the electrode, which extends to beyond the collar. However, production of such a structure involves a relatively large number of process steps. A further option of avoiding the problem of oxidation of the barrier layer is to use a structure in which the upper electrode, rather than the lower electrode, is connected via a conductive layer to the associated selection transistor. This makes it possible to dispense with a conductive barrier layer underneath the lower electrode. However, the structure, which is described, for example, in U.S. Pat. No. 5,122,477, has the disadvantage that it occupies a relatively large amount of space and is thus unsuitable for very large scale integrated memory components.  
         SUMMARY OF THE INVENTION  
         [0007]    It is accordingly an object of the invention to provide a microelectronic structure which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which allows simple and reliable protection of an oxygen-sensitive layer, and to specify a method for producing such a structure.  
           [0008]    With the foregoing and other objects in view there is provided, in accordance with the invention, a microelectronic structure. The microelectronic structure contains at least one substrate and a first conductive layer disposed on the substrate. The first conductive layer is composed of at least one basic material having at least one oxygen-bonding additive containing at least one element selected from the group consisting of Group IVb elements and lanthanum group elements. A second conductive layer is disposed on the first conductive layer and contains a noble metal. A metal oxide dielectric at least partially covers the second conductive layer.  
           [0009]    The object is achieved according to the invention in the case of a microelectronic structure of the type mentioned initially by the first conductive layer being composed of at least one basic material having at least one oxygen-bonding additive, which contains at least one element from Group IVb or from the lanthanum group.  
           [0010]    The basic idea of the invention is to provide a conductive layer with suitable oxygen-bonding additives. These are intended to prevent diffusion of oxygen and/or of oxides which assist diffusion, and thus to protect those structures which are located under the conductive layer against oxidation. To this end, the first conductive layer is composed of at least one basic material which is electrically conductive and, is very largely oxygen-resistant, and in which the oxygen-bonding additive is distributed as uniformly as possible. The important feature is that the oxygen-bonding additive is present even before the action of the oxygen on the structures to be protected in the basic material and thus prevents oxygen diffusion through the first conductive layer.  
           [0011]    Normally, the at least one oxygen-bonding additive forms an alloy or a mixed layer with the basic material, which may be formed of one or more components, in which case the oxygen-bonding additive may also at least partially be present in the form of a finely distributed deposit in the basic material. The advantages of a uniform distribution of the oxygen-bonding additive are, in particular, the uniform oxygen resorption capability of the first conductive layer, the adaptation of the resorption capability by variation of the layer thickness of the first conductive layer and a uniform and very largely stress-free increase in volume due to the oxygen bonding.  
           [0012]    Elements from Group IVb and from the lanthanum group have been found to be particularly advantageous as oxygen-bonding additives, with zirconium, hafnium, cerium or a combination of the elements being particularly preferred. It is also advantageous for the oxygen-bonding additive to be added to the basic material with a proportion by weight of between 0.5% and 20%, preferably of between 1% and 10%.  
           [0013]    Suitable basic materials for the first conductive layer are noble metals, in particular platinum, palladium, rhodium, iridium, ruthenium, osmium, rhenium, conductive oxides of the abovementioned metals or a mixture of the abovementioned compounds and elements.  
           [0014]    It is also preferable for the microelectronic structure to have a metal oxide dielectric that at least partially covers the first conductive layer. The metal oxide dielectric is used, in particular in semiconductor memories, as a capacitive dielectric, with the first conductive layer being at least part of one electrode of the storage capacitor. Since the metal oxide dielectric is normally applied directly to the first conductive layer, any barrier layer which is preferably located underneath the first conductive layer must be protected against attack by oxygen while it is being deposited in an atmosphere containing oxygen.  
           [0015]    The metal oxide dielectric preferably contains a compound of the general nature ABO, where O represents oxygen, and A and B each represent at least one element in the group containing barium, strontium, tantalum, titanium, lead, zirconium, niobium, lanthanum, calcium and potassium. The general compound ABO often has a crystal structure similar to perovskite, which is a critical factor for the desired dielectric (high dielectric constant) characteristics or for the ferroelectric characteristics. One example of such a compound is SrBi 2 Ta 2 O 9 .  
           [0016]    A second conductive layer, which preferably contains a noble metal, in particular platinum, is preferably disposed between the first conductive layer and the metal oxide dielectric in order to improve the electrical characteristics of the metal oxide dielectric. The additional conductive layer first represents an inert and smooth boundary surface for the growth of the metal oxide dielectric, and second assists the crystal growth of the metal oxide dielectric during its deposition and during subsequent heat treatment and, furthermore, provides additional oxidation protection.  
           [0017]    The bonding capacity of the first conductive layer with regard to oxygen should be set as appropriate by choice of the amount of additive, so that no further additional layers to prevent oxygen diffusion are required. An additive level of between 8 and 10%, for example, is sufficient to prevent the oxygen diffusion, which occurs during deposition and heat treatment of the metal oxide dielectrics, through the first conductive layer, whose thickness is about 100 nm, virtually completely. The first conductive layer may thus be thinner, in order to save costs.  
           [0018]    In accordance with an added feature of the invention, a barrier layer is disposed between the first conductive layer and the substrate.  
           [0019]    In accordance with an additional feature of the invention, the barrier layer contains titanium.  
           [0020]    With the foregoing and other objects in view there is further provided, in accordance with the invention, a method for producing the microelectronic structure. The method includes the steps of preparing a substrate, and simultaneously applying a basic material and a oxygen-bonding additive to the substrate to form a first conductive layer. The oxygen-bonding additive contains at least one element selected from the group consisting of Group IVb elements and lanthanum group elements. A second conductive layer is deposited onto the first conductive layer, the second conductive layer contains a noble metal such as platinum. Finally, a metal oxide dielectric is applied to the second conductive layer.  
           [0021]    The second part of the object is achieved by the method for producing a microelectronic structure which has at least one substrate and a first conductive layer, with the first conductive layer being composed of at least one basic material having at least one oxygen-bonding additive which contains at least one element from Group IVb or from the lanthanum group. The method includes the steps of preparing a substrate, the simultaneous application of the basic material and of the oxygen-bonding additive to the substrate in order to form the first conductive layer.  
           [0022]    In the method, the basic material and the oxygen-bonding additive are preferably applied to the substrate at the same time, so that the first conductive layer is formed there as a mixture of the basic material and of the oxygen-bonding additive. If the deposition temperatures and the additive level of the oxygen-bonding additive are selected appropriately, the latter can at least partially be deposited from the basic material, or can form a mixed crystal together with the basic material.  
           [0023]    It is advantageous to apply the basic material and the oxygen-bonding additive to the substrate by a physical sputtering method. This is preferably done using a common source for the basic material and the oxygen-bonding additive, with this being achieved in a simple manner by a sputtering target which consists of the basic material and has wafers which contain the oxygen-bonding additive applied to it. Therefore, there is no need to provide a mixed source. In fact, it is easy to vary the nature of the oxygen-bonding additive and its additive level.  
           [0024]    By way of example, a pressure of about 0.02 mbar and a substrate temperature of about 200° C. are preferably used for producing an iridium layer with an oxygen-bonding hafnium additive.  
           [0025]    In accordance with an added mode of the invention, there is the step of applying the basic material and the oxygen-bonding additive to the substrate by a physical sputtering method using a common source.  
           [0026]    Once the first conductive layer has been applied, the metal oxide dielectric is applied by metal organic chemical vapor deposition (MOCVD) methods or spin-on methods.  
           [0027]    The microelectronic structure is preferably used in a memory apparatus, with the first conductive layer representing a first electrode which, together with a further electrode and the metal oxide dielectric that is disposed between these electrodes, forms a storage capacitor. A large number of such storage capacitors are preferably disposed on one substrate.  
           [0028]    Furthermore, the microelectronic structure is generally suitable for use as an oxygen diffusion barrier, in order to protect oxygen-sensitive areas of the microelectronic structure, in particular a semiconductor structure, against attack by oxygen.  
           [0029]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0030]    Although the invention is illustrated and described herein as embodied in a microelectronic structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0031]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 is a diagrammatic, partial, sectional view of a storage capacitor using a microelectronic structure according to the invention;  
         [0033]    [0033]FIG. 2 is a partial, sectional view of a second embodiment of the storage capacitor;  
         [0034]    [0034]FIG. 3 is a partial, sectional view of a third embodiment of the storage capacitor; and  
         [0035]    [0035]FIG. 4 is an illustration of a sputtering reactor for producing the microelectronic structure. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown a storage capacitor  5  which is disposed on a substrate  10 . The storage capacitor  5  contains a lower electrode  15 , which is formed in layers from an iridium oxide layer  20 , an iridium layer  25  and a platinum layer  30 . Optionally, it is also possible to use ruthenium oxide and ruthenium instead of iridium oxide and iridium. Together, the iridium oxide layer  20  and the iridium layer  25  represent a first conductive layer. At least one of the iridium oxide  20  or iridium layers  25  contains an oxygen-bonding additive, which is preferably formed by hafnium. Depending on its additive level of between 1% and 10%, this can form a mixed crystal with the respective layer or may be partially present in the form of a deposit.  
         [0037]    The platinum layer  30  represents a second conductive layer in the present embodiment. The lower electrode  15 , which is formed in layers, was preferably structured by jointly etching the three layers  20 ,  25  and  30 . This is done, for example, by an anisotropic etching process with a high physical component that is achieved, for example, in an argon sputtering process. Chlorine or hydrogen bromide (HBr) can be added to the argon plasma to assist the process.  
         [0038]    A barrier layer  35  containing titanium is located underneath the lower electrode  15 . It is used to improve the adhesion characteristics of the lower electrode  15  on the substrate  10 , and to prevent silicon diffusion. This is particularly necessary since the lower electrode  15  is connected through a contact hole  40  (which is filled with polysilicon) in the substrate  10  to a selection transistor, which is not illustrated here in any more detail. A barrier layer  35 , which is formed of titanium-titanium nitride, is preferably structured jointly together with the lower electrode  15 . Therefore, only a single etching step is required for the structure containing the lower electrode  15  and the barrier layer  35 .  
         [0039]    The lower electrode  15  is completely covered by a SBT layer  45 , with the latter representing a metal oxide dielectric. The SBT layer  45  thus also makes direct contact with the edge areas of the barrier layer  35 . Therefore, these areas are not protected during the deposition of the SBT layer  45 . However, since the penetration depth of the oxygen diffusion into the barrier layer  35  is limited, the entire barrier layer  35  is not oxidized, but only those areas that are directly adjacent to the SBT layer  45 . The central area of the barrier layer  35 , which is located in particular in the area of the contact hole  40 , is protected against oxidation by the lower electrode disposed above it and in particular by the hafnium additive contained in the iridium oxide layer  20  or the iridium layer  25 . Furthermore, the iridium layer  25  itself acts as a protective layer, since iridium is at least partially oxidized in the SBT process conditions (about 800° C., atmosphere containing oxygen), and thus impedes oxygen diffusion.  
         [0040]    After the application of the SBT layer, a further electrode  50  is deposited over the entire area onto the SBT layer  45 . Together with the lower electrode  15  and the SBT layer  45 , the further electrode  50  forms the ferroelectric storage capacitor  5 .  
         [0041]    A second embodiment of the structure is illustrated in FIG. 2 and allows the barrier layer  35  to be protected better. In this structure, the platinum layer  30  also covers the side areas of the layer stack containing the barrier layer  35 , the iridium oxide layer  20  and the iridium layer  25 , so that the SBT layer  45  does not make direct contact with the barrier layer  35 . Another advantageous feature of the structure is that the entire boundary surface between the lower electrode  15  and the SBT layer  45  is formed by the platinum layer  30 , and the boundary surface characteristics and the storage characteristics of the SBT layer  45  are thus improved.  
         [0042]    [0042]FIG. 3 illustrates a third embodiment of the structure. In the structure, the barrier layer  35  is formed only in the area of the contact hole  40 , so that the barrier layer  35  is completely covered by the iridium oxide layer  20 . The barrier layer  35  is thus completely protected against oxidation during the SBT deposition. Optionally, in the structure, the platinum layer  30  can also be continued over the side areas of the iridium oxide layer  20  and of the iridium layer  25 , in order to improve the capacitor characteristics.  
         [0043]    It has been found that the oxygen absorption when using hafnium leads only to a relatively minor increase in the volume of the iridium oxide layer  20  and of the iridium layer  25 , so that any mechanical stresses which occur in consequence do not lead to damage.  
         [0044]    Reference is made to FIG. 4 in order to illustrate the method according to the invention for producing a microelectronic structure in which the first conductive layer is formed of a basic material and an oxygen-bonding additive. A sputtering reactor  55  is illustrated schematically here, having a substrate mount  60  and a target holder  65 , which at the same time act as the cathode and anode, respectively. A silicon wafer  70 , which subsequently forms the substrate  10 , is located on the substrate mount  60 . An iridium wafer  75  with hafnium wafers  80  placed on it is attached to the target holder  65 , which is disposed opposite the silicon wafer  70 . Together, the wafers represent the common source during the sputtering process. The proportion of hafnium that is deposited can be set by choice of the wafer size of the hafnium wafer. Hafnium and iridium are precipitated jointly from the respective sources by the argon plasma produced in the sputtering reactor  55 , and are applied as a mixture to the silicon wafer  70 . It is also possible to replace the iridium wafer  75  by an iridium oxide wafer.  
         [0045]    In order to improve the adhesion strength of the sputtered layers on the silicon wafer  70 , the wafer can be heated by heating applied from underneath the wafer. Advantageous temperatures are in the range 200° to 500° C.