Abstract:
A semiconductor device which exhibits reduced electromigration which includes an electrically conductive support member having a thin film of chromium overlaying the conductive support and a layer of semiconductor material overlaying said chromium film. The invention also includes a method of reducing electromigration and resistance and resistance in conductive supporting layers of semiconductor devices by forming a chromium layer over at least one surface of the conductive support layer of the device. The conductive support may be copper or aluminum with a semiconductive layer of germanium or silicon.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to chromium films, and more specifically to the use of chromium films to reduce electromigration in semiconductor and related devices. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits get smaller and smaller. This makes it possible to place more and more transistors on a given size of an integrated circuit. This requires that connecting lines between transistor and other devices on the silicon chip get smaller and smaller. High current densities are used in such conductors. 
     Currents in metals are usually carried by electrons. However, if current densities are in the 10 6  A/cm 2  range, and the temperature is high, metal ions are also moving. This phenomena is called electromigration and changes the geometry of the conductors. Hillocs and cracks form and lead to the degradation and failure of electrical conductors. Ions move usually in the same direction as electrons, because the momentum transfer from electrons to ions is responsible for the movement of the ions. 
     Surface diffusion and surface coatings influence electromigration. Surface coating as protection against failure is a subject of controversy. Opinions range from “no-effect” to “cure-all” according to a review by F. M. D&#39;Heurly and R. Rosenberg (1). These authors suggest that the surface must be considered mainly for the effect of vacancy movement on the surface. Clearly, overlayers produce specific vacancy concentrations at the interface. 
     Schwarz (2) in his review of electromigration in interconnects and contacts gives a summary of results of such studies. Aluminum strips covered with silicon dioxide showed an increase by a factor of 3 in the Mean Time to Failure (MTF) (3), or by a factor of 23 by first anodizing films and then adding a porous layer (4). Sputter conditions of the silicon dioxide influenced the MTF (5). Al-Cu conductors (6) with 1.5 μm of glass had a longer lifetime than those without glass. The glass overlayer produced changes in the failure mechanism. 
     A brief review included in a recent report on electron microscopy studies of electromigration defects (7) shows that there is now an increasing interest in electromigration on very clean surfaces. Most of these studies involve semiconductor surfaces or adatoms on semiconductor surfaces. A few studies were concerned with clean copper surfaces (8, 9), and only one paper was listed in which systems with Ni-adatoms on polycrystalline W- and Mo-surfaces were studied. 
     As stated above, one of the major results of the study of electromigration is, that the movement of atoms is strongly influenced by the momentum transfer from electrons to ions. Only non-elastic collision can produce such a momentum transfers. Systems, in which collisions between electrons and ions are mostly elastic, should show low electromigration. This would be found in systems, in which the surface resistance is very low. 
     It seems that this concept of low surface resistance as failure inhibitor has not been considered. This may be due to the fact, that the scattering process of electrons at the surface is difficult to modify, so it did not seem to be an adjustable parameter. 
     The surface resistance in metals can be studied in resistance measurements of thin films. Films less than 2 nm thick are usually discontinuous, 2 to 6 nm thick films are semicontinuous (10). The resistivity of thin continuous films is in first approximation inversely proportional to the thickness t. For instance, Fuad et al. (11) found for chromium prepared at base pressures in the 10 −6  Torr range: 
     
       
           p= 2·10 −8   /t (Ω m   2 ) 
       
     
     with t the thickness of the film measured in meter. If t≧80 nm, the resistivity is usually constant. It is larger than the bulk resistivity, because the thin film has a high defect density. 
     Surface scattering can be modified, if one adds overlayers on a thin film. This leads usually to a resistance increase. For instance, if one adds to an annealed Au-film a fractional monolayer of gold atoms (12), the resistance R of this film increases, in spite of the fact that the average film thickness increases. The resistance increase can be explained with an increase in non-elastic electron scattering. The annealed Au-film has in most sections rather flat surfaces. Add-atoms, even if they are from the same material, destroy this smoothness, and produce an increase in the resistivity. If one adds an overlayer of different atoms to a film, the resistance of the film will usually increase, even if the overlayer atoms are metallic. This resistance increase would not be expected in simple theories, where 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Thickness of 
                   
                 ΔR/ due to a 0.7 nm 
               
               
                   
                 Substrate 
                 Substrate 
                 Overlayer 
                 thick overlayer 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Silver 
                 15 
                 nm 
                 Ge 
                 +20% 
               
               
                   
                 Gold 
                 22.3 
                 nm 
                 Permalloy 
                 +12% 
               
               
                   
                 Aluminum 
                 10 
                 nm 
                 Germanium 
                  0% 
               
               
                   
                 Copper 
                 12.9 
                 nm 
                 SiO 2   
                  0% 
               
               
                   
                 Copper 
                 12.9 
                 nm 
                 Chromium 
                     −4% 
               
               
                   
                   
               
             
          
         
       
     
     one would assume that one has two parallel conductors, each with its given conductance, and the total conductance should be the sum of the conductances of both films. Naturally, the conductance of the overlayer should be very small, since it is very thin, but it should give a positive contribution. 
     The resistance increase due to overlayers is due to increased non-elastic scattering in the bilayer structure. The scattering of electrons in a sample with a thin overlayer takes place not only on the (“new”) surface, but also on the interface between the original film and the overlayer (the “old” surface). 
     One exception to the rule that thin overlayers increase the resistance is the case Were Cr-atoms are deposition on Cu-films. Table 1 show that a Cr-overlayer 0.7 nm thick reduces the resistance of the film by 4%. This is about the value expected from the film thickness change. 
     We measured the influence of a chromium overlayer of 0.3 nm on a 4.3 nm thick copper film and found a strong resistance decrease. The relative resistance change was −46%. The resistance decreased in a 6 μm thick Cu-films by 60% if a 1 μm thick Cr-overlayer was added. These changes are much stronger than the 4% decrease found by Chopra and Randlett (13). Chopra and Randlett&#39;s result can be explained with the thickness increase in their sample. Our resistance decrease is much stronger and represent a new discovery. 
     That chromium overlayers can influence electromigration was found in the case of silver conductors by Rosenberg et al. (14). These authors studied grain boundary contributions to transport. They studied damage mechanisms, effects of diffusivity, vacancies and grain boundaries on electromigration. They described one test, in which they covered the top of an Ag-film with a 10 nm thick diffusion resistant chromium overlayer to avoid grooving on the silver film. The side of the Ag-film was bare. After subjecting the film to a high current, the top surface covered with chromium showed no defects due to electromigration, whereas the bare edge of the silver film showed a growth pattern. However, it is possible that the edge of an uncoated film shows more defect growth than an (uncoated) top surface because the sharp curvature of the edge surface changes the surface energy (higher vapor pressure, different vacancy concentration than other sections of the film). The typical studies for failure, namely the deteriation of the silver film with time as indicated by resistance changes was not studied, nor was the time to failure measured. No suggestion was made that chromium was superior to other (metallic) overlayers to reduce electromigration defects. 
     We studied magnetic properties, the Hall effect, the Seebeck effect, and the electrical resistivities of Cr-films on different substrates and with different overlayers over many years (15-23). Some magnetic properties were unusual (15-18). The magnetic diamagnetism was much larger than found in any other materials, except superconductors (15-18). This indicates that the films are most likely partly superconducting. The Seebeck coefficient (19) and the Hall coefficient (20) were close to zero for very thin Cr-films on Ge-substrates, characteristic for superconducting materials. 
     We measured the electrical resistance of ultra thin Cr-films (21-22). The resistance per square in Ohms, R (square), at room temperature is given between 0.11 nm and 10 nm by: 
     
       
           R (square)=2000 /t   1−3   
       
     
     if t is given in nm. These films have at room temperature resistance per square values which are by an order of magnitude smaller than the resistance per square values of silver films with the same thickness, tested at 20 K. The low resistance per square value of the Cr-film shows, that inelastic scattering on Cr-surfaces is very low. The resistance decreased further if Ge- or Si-overlayers were deposited on top of the Cr-film (21-23). 
     The resistance decreases in our Cu-film with overlayers of chromium show, that Cr-overlayers decrease inelastic electron surface scattering. Also, interfacial scattering at the “Cr-overlayer”—“Cu-substrate” are low. Ge- or Si-layers deposited on top of the Cr-overlayers lead to a further reduction in surface resistance. This again leads to an increase in the reliability of these conductors, and will also help to protect the films against corrosion. 
     SUMMARY OF THE INVENTION 
     It is the objective of this invention to increase the lifetime of interconnects in integrated circuits by adding overlayers which decrease the electrical surface resistance. This objective is accomplished by coating these conductors such as gold, copper or aluminum with an overlayer of chromium. Additionally, a layer of germanium or silicon may be used to cover the chromium overlayer. Aside from reducing the electrical surface resistance, these chromium films will reduce surface diffusion since the diffusion coefficient of chromium is much lower than the diffusion coefficient of copper at the same testing temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred mode of practicing the invention, read in connection with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a device illustrating one embodiment of the invention. 
     FIG. 2 is a perspective view of a device illustrating a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Conductors in large scale integrated devices have to carry very high current densities. This leads to electromigration. That is induced by energy and momentum transfer from electrons to ions. This requires that the electron is scattered in an inelastic process by the metallic ions. Very thin chromium films can be produced in such a way that they have very low surface resistance values. This demonstrates that inelastic surface scattering is very low in this material. Chromium overlayers on copper show also low surface resistance values. This proves that a chromium film on copper has also low inelastic surface scattering. Therefore, if chromium overlayers are prepared in such a way that they have low resistance values, then they will produce systems with low electromigration and long service life. 
     Such films have been prepared by evaporation of materials and their subsequent adsorption on substrates, but they can also be prepared in a molecular beam apparatus, in which first the conducting, then the chromium and finally the germanium (or silicon) films are deposited in a ultra high vacuum system. Under these conditions, the chromium films show the low surface resistance values required to produce the low electromigration. Chemical vapor deposition techniques may also be employed if they produce low resistance films. 
     FIG. 1 illustrates one embodiment of the present invention which consists of a conductive substrate ( 10 ), a chromium conductor ( 12 ), and a semiconducting overlayer ( 14 ). 
     FIG. 2 illustrates a second embodiment of a proposed device of the present invention. On a substrate of silicon, doped silicon or oxidized silicon ( 20 ), an interfacial layer ( 22 ) is deposited. This layer may be metallic, a compound, or an intermediate phase. It is inserted to improve adhesion between ( 20 ) and ( 24 ), or to prevent undesired diffusion. It should also be selected to give a low electrical interface resistance. This can be achieved by using low resistance chromium as interfacial layer. On top of this interfacial layer, the metallic conductor ( 24 ) is deposited. This conductor could be copper. A chromium film ( 26 ) is deposited on top of the metallic conductor, which in turn may be coated with a semiconductor like germanium or silicon ( 28 ). 
     Using a chromium or chromium alloy film as interfacial layer ( 22 , FIG. 2) between the substrate ( 20 , FIG. 2) and the conductor ( 24 , FIG. 2) will also improve the performance of the device, because this system can be prepared in such a way in a high vacuum system, that the interfacial resistance is very low. This reduces heating in the conductor, which in turn reduces electromigration on the interface between the conducting film ( 24 , FIG. 2) and the chromium film on top of it ( 26 , FIG.  2 ). 
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     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.