Abstract:
A new method is provided for the creation of a copper seed interface capability. A first seed layer of copper alloy and a second seed layer of copper is provided over an opening in a layer of dielectric. The opening is filled with copper, the first and second seed layers are annealed.

Description:
BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   The invention relates to the fabrication of integrated circuit device, and more particularly, to a method of creating a copper interconnect without the need for a separate barrier layer. 
   (2) Description of the Prior Art 
   In the creation of semiconductor devices, the creation of conductive interconnects has become increasingly more important due to the continuing reduction in device parameters, a reduction that is driven by requirements of improved device performance. Metal is typically used for the creation of conducting interconnects comprising such materials as aluminum, tungsten, titanium copper polysilicon, polycide or alloys of these metal. For the creation of metal interconnects a Ti/TiN/AlCu/TiN process is the preferred method. Electrically conductive materials that can be used for the metal lines include but are not limited to Al, Ti, Ta, W, Mo, Cu, their alloys or a combination of these materials. 
   Due to increased requirements of low resistance of interconnect metal, copper has become more attractive as a material for the creation of interconnect metal. The invention relates to the fabrication of copper conductive lines and vias that provide the conductive interconnections of integrated circuits in semiconductor devices or the interconnections in a multilayer substrate over the surface of which semiconductor devices are mounted. More particularly, the fabrication of conductive lines and vias using damascene and dual damascene processes. 
   In fabricating Very and Ultra Large Scale Integration (VLSI and ULSI) circuits with the dual damascene process, an insulating or dielectric material, such as silicon oxide, of a semiconductor device is patterned with several thousand openings for the conductive lines and vias. These openings are filled at the same time with metal and serve to interconnect the active and passive elements of the integrated circuit. The dual damascene process also is used for forming multilevel conductive lines of metal, such as copper, in layers of insulating material, such as polyimide, of multi-layer substrates over which semiconductor devices are mounted. 
   Damascene is an interconnection fabrication process in which grooves are formed in an insulating layer and filled with metal to form the conductive lines. Dual damascene is a multi-level interconnection process in which, in addition to forming the grooves of single damascene, conductive via openings also are formed. In the standard dual damascene process, the insulating layer is coated with a layer of photoresist, which is exposed through a first mask with an image pattern of via openings, the via pattern is anisotropically etched in the upper half of the insulating layer. The photoresist now is exposed through a second mask with an image pattern of the conductive line openings or trenches, after being aligned with the first mask of the via pattern to encompass the via openings. By anisotropically etching the openings for the conductive lines in the upper half of the insulating material, the via openings already present in the upper half of insulating material are simultaneously etched and replicated in the lower half of the insulating material. After the etching is complete, both the vias and line openings are filled with metal. Dual damascene is an improvement over single damascene because it permits the filling of both the conductive grooves and vias with metal at the same time, thereby eliminating process steps. 
   Copper is being increasingly used as an interconnect metal due to its low cost and low resistivity. Copper however has a relatively large diffusion coefficient into surrounding dielectrics such as silicon dioxide and into silicon. Copper from an interconnect may diffuse into the silicon dioxide layer causing the dielectric to become conductive while decreasing the dielectric strength of the silicon dioxide layer. Copper interconnects are therefore typically encapsulated by at least one diffusion barrier, comprising for instance silicon nitride, to prevent diffusion into the silicon dioxide layer. Copper is known to have low adhesive strength to various insulating layers, masking and etching a blanket layer of copper layer presents a challenge. 
   To provide a starting material for electroplating of a copper interconnect line to the surrounding layer of dielectric or insulation, a seed layer is typically deposited over the barrier layer. The invention addresses this aspect of the creation of copper interconnects and provides a method that allows the creation of such copper interconnects without the need for a separate barrier layer. 
   U.S. Pat. No. 6,333,560 B1 (Uzoh) shows a barrier-less copper interconnect process. 
   U.S. Pat. No. 6,124,198 (Moslehi) shows a barrier-less interconnect process. 
   U.S. Pat. No. 6,358,848 B1 (Lopatin) shows a Ca doped Cu seed layer. 
   U.S. Pat. No. 6,181,012 B1 (Edelstien et al.) shows a barrier-less copper process. 
   U.S. Pat. No. 6,090,710 (Andricacos et al.) and U.S. Pat. No. 5,969,422 (Ting et al.) are related copper interconnect patents. 
   SUMMARY OF THE INVENTION 
   A principle objective of the invention is to provide a method of forming an underlying layer for copper interconnects. 
   A new method is provided for the creation of a copper interface. A first seed layer of copper alloy and a second seed layer of copper is provided over an opening in a layer of dielectric. The opening is filled with copper, the first and second seed layers are annealed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a cross section of a semiconductor substrate over the surface of which a first level of metal is available, layers of dielectric have been deposited, an opening for a copper interconnect has been created through the multiple payers of dielectric. The created opening has a cross section of a dual damascene structure. 
       FIG. 2  shows the cross section of the semiconductor substrate after layer of copper alloy has been deposited as a first seed layer. 
       FIG. 3  shows a cross section after a thin seed layer of copper has been deposited. 
       FIG. 4  shows a cross section after a layer of copper has been deposited over the second seed layer, filling the opening created through the layers of dielectric. 
       FIG. 5  shows a cross section after polishing of the deposited layer of copper, including the first and second seed layers over the surface of the layers of dielectric. 
       FIG. 6  shows a cross section after a cap layer has been deposited over the surface of the layers of dielectric, including the surface of the therein created copper interconnect. 
       FIG. 7  shows a cross-section after the step of anneal has been completed. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Copper, as previously stated, suffers from high diffusivity in common insulating materials such as silicon oxide and oxygen-containing polymers. For instance, copper tends to diffuse into polyimide during high temperature processing of the polyimide. 
   This causes severe corrosion of the copper and the polyimide due to the copper combining with oxygen in the polyimide. This corrosion may result in loss of adhesion, delamination, voids, and ultimately a catastrophic failure of the component. Copper interconnects are therefore conventionally encapsulated in at least one diffusion barrier to prevent diffusion into the surrounding layer of dielectric such as a layer of silicon dioxide. A typical barrier layer is formed using rf. sputtering of titanium nitride, tantalum, tungsten, niobium, molybdenum, Ti/TiN or Ti/W and is more preferably formed using TiN. The barrier layer can also be used to improve the adhesion of the subsequent overlying tungsten layer. A barrier layer is preferably about 100 and 500 Angstrom thick and more preferably about 300 angstrom thick. The addition of a barrier layer has a negative impact on device performance by increasing contact resistance and series resistance of the in this manner created copper interconnect. These negative effects are to be avoided, more so in the era of sub-micron and deep sub-micron device feature sizes since this negative impact has a proportionally larger negative impact on these devices. 
   To further enhance the adhesion of a copper interconnect line to the surrounding layer of dielectric or insulation, a seed layer is typically deposited over the barrier layer. A conventional seed layer can be deposited using a sputter chamber or an Ion Metal Plasma (IMP) chamber at a temperature of between about 0 and 300 degrees C. and a pressure of between about 1 and 100 mTorr, using copper or a copper alloy as the source at a flow rate of between about 10 and 400 sccm and using argon as an ambient gas. The minimum thickness of a seed layer is about 50 Angstrom, this thickness is required to achieve a reliable gap fill. 
   The invention, shown using  FIGS. 1 through 7 , starts with,  FIG. 1 , a blanket unprocessed semiconductor substrate  10  over the surface of which is created a interconnect structure through successive overlying layers of dielectric. 
   Shown in the cross section of  FIG. 1  are a layer  11  which represents the layer of semiconductor devices that is created in or over the surface of substrate  10 . The electrical point of first level copper contact  20  is representative of the points of electrical contact provided in the surface of substrate  10  that provide access to the semiconductor devices created in or over the surface of substrate  10  as represented by layer  11 . 
   Further shown in the cross section of  FIG. 1  are a first layer  12  of dielectric, a second layer  14  of dielectric and a third layer  16  of dielectric. Overlying the three layers of dielectric are a first layer  13  of etch stop material, a second layer  15  of etch stop material and a third layer  17  of etch stop material. Opening  25 , created through the layers of dielectric and etch stop material as shown in the cross section of  FIG. 1 , will be recognized as having the cross section of a dual damascene structure. Conventional methods of photolithographic exposure and development are used to create opening  25 , opening  25  exposes the surface of copper contact point  20 . 
   Layers  12 ,  14  and  16  of dielectric are preferably formed using a low-k dielectric material. 
   As a next step,  FIG. 2 , a layer  18  is deposited over inside surface of opening  25  and over the surface of the patterned and etched layers of dielectric. Layer  18 , of critical importance to the invention, is one of the two layers that replace and serve as the conventional seed layer between a there-over to be created copper interconnect and the surrounding layers of dielectric. Layer  18  is therefore referred to as a first seed layer. Layer  18  of the invention comprises a copper alloy layer, deposited using methods of CVD, to a preferred thickness of between about 50 and 300 Angstrom. 
   The copper alloy layer  18  may, herewith highlighting doping materials as examples without thereby being limited to these materials, be doped by Cr, Pd, Sn, Ti, Zr, Mg, Al, whereby all of these materials have as common characteristic that these materials prevent oxidation of the surface of a thereover deposited copper seed layer. The copper alloy layer  18  therefor may comprise CuCr, CuPd, CuSn, etc., in accordance with the listed doping elements. 
   As a next step, shown in cross section in  FIG. 3 , a layer  19  of copper seed material is deposited over the surface of the copper alloy layer  18 , this deposition is performed to a preferred thickness of between about 300 and 800 Angstrom. Layer  19  is referred to as a second seed layer for a thereover to be created copper interconnect. The first seed layer  18  combined with the second seed layer  19  form a copper seed layer, this combined copper seed layer is preferred to be created to a thickness between about 800 and 1,200 Angstrom. 
   The structure that is shown in cross section in  FIG. 3  is now ready for the deposition of a layer  22 ,  FIG. 4 , of copper thereover, using conventional methods of metal deposition such as ECP, filling opening  25  in addition to depositing copper over the surface of layer  19  of copper seed. 
   After the structure that is shown in cross section in  FIG. 4  has been created, a Rapid Thermal Anneal is applied to the structure that is shown in cross section in  FIG. 4 , resulting in copper stabilization and the stimulation of an interaction between the doped elements in layer  18  and the sidewalls of the dielectrics  14  and  16 . This interaction forms, in the interface between layer  18  and the surrounding dielectrics  14  and  16  of low-k dielectric material, oxide compounds such as MgO x , AlO x , HfO x , TiO x , ZrO x  and the like, dependent on the doping element that has been provided as a dopant in layer  18 . 
   The Rapid Thermal Anneal can be performed applying a temperature of no less than about 350 degrees C. for a time of no less than about 10 minutes. 
   By now removing the deposited layer  22  of copper and the layers  18 / 19  of doped copper and copper seed from above the surface of layer  16  of dielectric, applying for this purpose preferably methods of Chemical Mechanical Polishing (CMP), the structure that is shown in cross section in  FIG. 5  is obtained. 
   The layer that interfaces between the created copper interconnect  22 ,  FIG. 5 , and the surrounding layers of dielectrics  14  and  16  replaces the conventional barrier/seed layer, and performs the conventional function of barrier layer over which a seed layer is deposited. The need for a conventional barrier layer surrounding the created copper interconnect  22 ,  FIG. 5 , is therefore removed. 
   As final steps of the invention,  FIG. 6 , a cap layer  24  can be created over the surface of the created copper interconnect  22  and the top layer  16  of low-k dielectric. 
   The cap layer  24  may further interact with the underlying layer  22  of copper and form a layer  26 ,  FIG. 7 , between the cap layer  24  and the underlying copper interconnect  22 , further protecting the surface of copper layer  22 . Layer  26  forms only if SiC is used for cap layer  24 , no cap layer  26  will form if for instance SiN is used for the cap layer  24 , this due to enablement of chemical interaction between the cap layer  24  and the underlying layer  22  of copper, which is dependent on the chemical composition of the interfacing materials. 
   It must be pointed out that the doping element and the concentration of the doping element in the first seed layer  18  must be carefully controlled. If the level of doping in the first seed layer  18  is too high, severe leakage may be introduced by the doping element and originating in the created interconnect  22  of copper. 
   It must further be pointed out the whereas the cross sections htat are shown in  FIGS. 1-7  address the deposition of one copper alloy layer over which one copper layer is deposited, the invention is not limited to depositing only two layers for the creation of a barrier-free layer of material surrounding a copper interconnect. Multiple layers of copper alloy may be used for this purpose in combination with multiple layers of copper seed. 
   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.