Abstract:
A new method is provided for the creation of copper interconnects. An opening is created in a layer of dielectric, a layer of barrier material is deposited. The layer of barrier material extends over the surface of the layer of dielectric. A film of copper is deposited over the surface of the layer of barrier material. The copper film is polished down to the surface of the layer of barrier material, creating a first copper interconnect. The created first copper interconnect is subjected to a thermal anneal, inducing copper hillocks in the surface of the first copper interconnect by releasing copper film stress in the first copper interconnect. The copper hillocks are then removed by polishing the surface of the created first copper interconnect down to the surface of the surrounding layer of dielectric, creating a second and final copper interconnect.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    (1) Field of the Invention  
           [0002]    The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method for the creation of layers of copper having improved surface characteristics.  
           [0003]    (2) Description of the Prior Art  
           [0004]    After or as part of the creation of semiconductor devices, these devices or parts thereof must be interconnected. For these interconnections, metal such as aluminum or their alloys or copper are the preferred materials. Copper has of late been the material of choice in view of the more attractive performance characteristics of copper such as a low cost and low resistivity. Copper however has a relatively large diffusion coefficient into surrounding dielectrics such as silicon dioxide and silicon. Copper from an interconnect may diffuse into the silicon dioxide layer causing the dielectric to become conductive and decreasing the dielectric strength of the silicon dioxide layer. Copper interconnects should therefore be encapsulated by at least one diffusion barrier to prevent diffusion into the silicon dioxide layer. Silicon nitride is a diffusion barrier to copper, but the prior art teaches that the interconnects should not lie on a silicon nitride layer because it has a high dielectric constant compared with silicon dioxide. The high dielectric constant causes an undesired increase in capacitance between the interconnect and the substrate. Copper further has low adhesive strength to various insulating layers, while it has been proven inherently difficult to mask and etch a blanket copper layer into intricate circuit structures.  
           [0005]    The deposition and processing of a layer of semiconductor material typically creates a thin film of material in which molecular stress in introduced due to the thermal processing of the deposited layer. This thermal stress results in the accumulation of sub-layers of the material, which show themselves as hillocks over the surface of the created thin film. This occurrence of surface hillocks is particularly troublesome where multiple overlying layers of copper are used as part of the structure since lower layer hillocks will have a magnifying effect on overlying layers of copper. The film stress is typically attributed to a mismatch of the Thermal Coefficient of Expansion (CTE) over adjacent layers of material, a mismatch that is more pronounced where one of the overlying layers of material comprises metal.  
           [0006]    In view of the negative effects that can be introduced by surface hillocks, where these hillocks occur on the surface of patterned copper, a method of reducing or eliminating such hillocks is of benefit in processing semiconductor devices that use copper as an interconnect medium. The invention addresses concerns of hillock formation and provides a method whereby such a formation of hillocks of avoided.  
           [0007]    U.S. Pat. No. 6,368,948 B (Ngo et al.) shows a process to reduce copper hillocks.  
           [0008]    U.S. Pat. No. 4,1704,367 (Alvis et al.) reveals a process to suppress hillocks using an Ar ion implant and thermal cycles.  
           [0009]    U.S. Pat. No. 6,348,410 B1 (Ngo et al.) shows a low temperature copper hillock suppression process.  
           [0010]    U.S. Pat. No. 5,447,887 (Fliplak et al.) shows a capping copper process.  
         SUMMARY OF THE INVENTION  
         [0011]    A principle objective of the invention is to create copper interconnects that are free of surface hillocks.  
           [0012]    Another objective of the invention is to create a layer of copper interconnect whereby the layer has reduced stress therein.  
           [0013]    Yet another objective of the invention is to create copper interconnect metal of increased reliability.  
           [0014]    In accordance with the objectives of the invention a new method is provided for the creation of copper interconnects. An opening is created in a layer of dielectric, a layer of barrier material is deposited. The layer of barrier material extends over the surface of the layer of dielectric. A film of copper is deposited over the surface of the layer of barrier material. The copper film is polished down to the surface of the layer of barrier material, creating a first copper interconnect. The created first copper interconnect is subjected to a thermal anneal, inducing copper hillocks in the surface of the first copper interconnect by releasing copper film stress in the first copper interconnect. The copper hillocks are then removed by polishing the surface of the created first copper interconnect down to the surface of the surrounding layer of dielectric, creating a second and final copper interconnect.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIGS. 1 through 3 shows the conventional method of creating a copper interconnect, as follows:  
         [0016]    [0016]FIG. 1 shows the cross section of a semiconductor surface over the surface of which a copper interconnect has been formed in a first layer of dielectric and surrounded by a barrier layer.  
         [0017]    [0017]FIG. 2 shows hillock formation in the surface of the created copper interconnect.  
         [0018]    [0018]FIG. 3 shows a cross section after a second layer of dielectric has been deposited over the surface of the first layer of dielectric, including the surface of the created copper interconnect.  
         [0019]    [0019]FIGS. 4 through 6 show the invention, as follows:  
         [0020]    [0020]FIG. 4 shows the cross section of a semiconductor surface over the surface of which a copper interconnect has been formed in a first layer of dielectric and surrounded by a barrier layer, the barrier layer is shown as extending over the surface of the first layer of dielectric.  
         [0021]    [0021]FIG. 5 shows hillock formation in the surface of the created copper interconnect, thermal anneal is applied to the created copper interconnect to release stress and further stimulate hillock formation.  
         [0022]    [0022]FIG. 6 shows a cross section after a surface of the created copper interconnect has been polished, the layer of TaN has been removed from the surface of the first layer of dielectric. The structure is now ready for the deposition of additional layers of dielectric (not shown) over the surface of the first layer of dielectric, including the surface of the created copper interconnect. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    The conventional method of creating a copper interconnect is first highlighted using FIGS. 1 through 3.  
         [0024]    In FIG. 1 is shown a cross section of the surface of a substrate  10 , a first layer  12  of dielectric has been deposited over the surface of the substrate  10 . The first layer of dielectric has been patterned and etched, creating an opening in the surface of the first layer  12  of dielectric for the creation of copper interconnect therein. Before the deposition of the layer of copper, a layer  14  of barrier material is over the surface of the first layer  12  of dielectric for reasons of avoiding diffusion of the copper into the first layer  12  of dielectric. Then the layer  16  of copper is deposited, using conventional metal deposition methods, after which the deposited layer  16  of copper is polished down to essentially the surface of the first layer  12  of dielectric. This results in the cross section that is shown in FIG. 1. The created layer of copper as shown in the cross section of FIG. 1 is now ready for deposition of a second layer  20  of dielectric thereover as shown in the cross section of FIG. 3. Since elevated temperatures are applied for the deposition of the second layer  20  of dielectric, this temperature stimulates the relocation of copper molecules, a stimulation that is caused by internal stress that is created in a deposited film of copper at the time that a layer of copper is deposited.  
         [0025]    This temperature stimulated molecular copper relocation results in the copper hillocks  18 , FIG. 2, forming over the surface of the created layer  16  of copper. The copper hillocks  18  in other words form during and as a result of the deposition of the second layer  20  of dielectric and is further stimulated by the elevated temperature that is required for the deposition of the second layer  20  of dielectric. The formed copper hillocks  18  are in profile further propagated through the second layer  20  of dielectric and show up over the surface thereof as surface irregularities  19 , irregularities that are highly undesirable from the point of view of creating a high-performance semiconductor device.  
         [0026]    It is the objective of the invention to prevent surface irregularities  19  during and as a result of forming copper interconnects. This is highlighted and explained in detail using FIGS. 4 through 6.  
         [0027]    Shown in the cross section of FIG. 4 is the surface of a substrate  10 , over the surface of which a first layer  22  of dielectric has been deposited. An opening, which may be an opening created as part of a damascene process and may therefore have the cross section of a single or a double damascene opening, is etched into the surface of the first layer  22  of dielectric for the creation of a copper interconnect therein. A single damascene opening comprises an interconnect via or an interconnect trench, a double damascene opening comprises an interconnect via and an interconnect trench. A layer  24  of barrier material is deposited over the surface of the first layer  22  of dielectric after the opening has been created in the surface thereof, this layer  24  of barrier material covers inside surfaces of the opening created in the surface of layer  22 . After the deposition of the layer  24  of barrier material, a layer  26  of copper is blanket deposited over the surface of the layer  24  of barrier material, this layer  26  of copper is polished down to the surface of the layer  24  of dielectric, resulting in the cross section that is shown in FIG. 4.  
         [0028]    Prior to the deposition of a second layer of dielectric over the surface of the first layer  22  of dielectric, thereby including the surface of the created layer  26  of copper interconnect, the invention stimulates stress release and therewith stimulates the formation of copper hillocks  28 , FIG. 5, over the surface of the copper layer  26  by applying a thermal anneal to the structure shown in cross section in FIG. 5. After the formation of the hillocks  28 , FIG. 5, stress relieve has been achieved in the created layer  26  of copper. Additional high temperature processing, as is required for the deposition of a second layer of dielectric, will not again result in the formation of hillocks over the surface of copper layer  26 .  
         [0029]    By therefore removing the copper hillocks  28 , a removal that is accomplished by polishing the surface of the copper layer  26  that is shown in the cross section of FIG. 5, the created surface of the copper interconnect  26 ′, FIG. 6, becomes and will remain free of copper hillocks, even if the layer  26 ′ is additionally exposed to high temperature processing conditions. The polishing of the surface of copper layer  26  can be accomplished applying methods of Chemical Mechanical Polishing (CMP).  
         [0030]    It must be noticed in the cross sections of FIGS. 5 and 6 that the surface of layer  26 , FIG. 5, is higher than the surface of the finally polished layer of copper  26 ′ shown in the cross section of FIG. 6. This difference in elevation of the surfaces of layers  26  and  26 ′ of copper is caused by the presence of layer  24  over the surface of the first layer  22  of dielectric. By therefore polishing the layer  26 , FIG. 5, of copper down to the surface of the first layer of dielectric, copper that is located in layer  26 , FIG. 5, above the plane of the surface of the first layer  22  of dielectric is also removed. It is to be expected that this additionally removed copper contains molecular irregularities since this layer of copper is close to the copper hillocks  28 . The additional removal of this layer of copper therefore assures that irregularities that are close to the surface of copper layer  26 , FIG. 5, are also removed after the copper hillocks  28  have been removed. This further improves the planarity of the polished layer  26 ′, FIG. 6, and further removes the likelihood of copper hillocks forming after the layer  26 ′, FIG. 6, of copper has been polished.  
         [0031]    A typical diffusion barrier layer  24 , deposited by for instance rf. sputtering, may contain silicon nitride, phosphosilicate glass (PSG), silicon oxynitride, aluminum, aluminum oxide (AlxOy), tantalum, tungsten, titanium nitride, niobium, or molybdenum. The preferred material for the barrier layer  24  of the invention is titanium nitride (TaN). A barrier layer is preferably about 100 and 500 Angstrom thick and more preferably about 300 Angstrom thick.  
         [0032]    Further can be cited the deposition of a barrier layer of SiN using PECVD under a temperature between about 200 and 500 degrees C., a pressure between about 1 mTORR and  100  TORR, a time between about 2 and 100 seconds, an environment of SiH 4 +NH 3 +N 2  or Si 2 H 6 +NH 3 +N 2  or SiH 4 +N 2 +Ar using a plasma or thermal process.  
         [0033]    The deposition of a first and second layer of dielectric may comprise the deposition of a layer of silicon oxide, deposited using either LPCVD, PECVD, or APCVD processing, at a temperature between about 200 to 800 degrees C. to a thickness between about 1,000 to 10,000 Angstrom. It must thereby be pointed out that the second layer of dielectric, FIG. 3, must be created using a thermal process for the hillocks  19 , FIG. 2 to appear on the surface of the second layer of dielectric.  
         [0034]    The etching of a layer of dielectric can be accomplished by applying an anisotropic, RIE processing, using CHF 3  as an etchant.  
         [0035]    Conductors are typically isolated from each other and from underlying conductive elements by a suitable dielectric, a suitable dielectric being, for instance silicon dioxide (“oxide”, doped or undoped) or silicon nitride (“nitride”), silicon oxynitride, fluoropolymer, parylene, polyimide, tetra-ethylortho-silicate (TEOS) based oxides, boro-phosphate-silicate-glass (BPSG), phospho-silicate-glass (PSG), boro-silicate-glass (BSG), oxide-nitride-oxide (ONO), plasma enhanced silicon nitride (PSiNx), oxynitride. A low dielectric constant material, such as hydrogen silsesquioxane. HDP-FSG (high-density-plasma fluorine-doped silicate glass) is a dielectric that has a lower dielectric constant than regular oxide.  
         [0036]    The most commonly used and therefore the preferred dielectrics are silicon dioxide (doped or undoped), silicon oxynitride, parylene or polyimide, spin-on-glass, plasma oxide or LPCVD oxide.  
         [0037]    The step of anneal of the invention can be applied using a rapid thermal anneal in a temperature range between about 300 and 400 degrees C. for a time between about 20 and 40 seconds. The anneal temperature may be varied dependent on the thickness of the created copper interconnect and the thickness of the deposited first layer of dielectric.  
         [0038]    If a thermal deposition process is used for the deposition of a second layer of dielectric (not shown) over the surface of layer  22 , the hillocks  19  as shown in the cross section of FIG. 3 will be visible. The invention therefore allows for the second layer of dielectric to be deposited over the surface of the first layer  22  of dielectric while using a thermal process and, since the source of the hillocks has been removed, these hillocks will no longer appear over the surface of the second layer of dielectric.  
         [0039]    The invention can be summarized as follows:  
         [0040]    Provide a substrate  
         [0041]    Deposit a first layer of dielectric over the surface of the substrate  
         [0042]    Create an opening in the surface of the first layer of dielectric  
         [0043]    Deposit a layer of barrier material over the surface of the first layer of dielectric, including inside surfaces of the created opening  
         [0044]    Deposit a layer of copper of the surface of the layer of barrier material  
         [0045]    Polish the deposited layer of copper down to the surface of the layer of barrier material, creating a copper interconnect  
         [0046]    Apply a thermal anneal to the created structure, most notably to the deposited and polished layer of copper  
         [0047]    Polish the layer of barrier material down to the surface of the first layer of dielectric, thereby polishing the created copper interconnect, and  
         [0048]    Deposit a second layer of dielectric over the surface of the first layer of dielectric, including the surface of the polished layer of copper.  
         [0049]    Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.