Abstract:
A method of filling features in a dielectric layer is provided. A pure Co or pure Ru adhesion layer is deposited against surfaces of the features, wherein the adhesion layer is separated from some of the surfaces of the features of the low-k dielectric layer by no more than 10 Å. The features are filled with Cu or a Cu alloy.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method of forming semiconductor devices on a semiconductor wafer. More specifically, the invention relates to forming copper interconnects in low-k dielectric layers. 
     In forming semiconductor devices, dual damascene copper interconnects are placed in low-k dielectric layers. This may be done by depositing copper or a copper alloy into features etched into the low-k dielectric layer. The deposited copper may be deposited by electrodeposition or by electroless deposition. 
     SUMMARY OF THE INVENTION 
     To achieve the foregoing and in accordance with the purpose of the present invention, a method of filling features in a dielectric layer is provided. A pure Co or pure Ru adhesion layer is deposited against surfaces of the features, wherein the adhesion layer is separated from some of the surfaces of the features of the low-k dielectric layer by no more than 10 Å. The features are filled with Cu or a Cu alloy. 
     These and other features of the present invention will be described in more details below in the detailed description of the invention and in conjunction with the following figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a high level flow chart of an embodiment of the invention. 
         FIGS. 2A-F  are schematic views of the formation of structures using the inventive process. 
         FIG. 3  is a schematic view of the formation of structures using another embodiment. 
         FIG. 4  is a schematic view of the formation of structures using another embodiment. 
         FIG. 5  is a schematic view of the formation of structures using another embodiment. 
         FIG. 6  is a schematic view of the formation of structures using another embodiment. 
         FIG. 7  is a schematic view of the formation of structures using another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. 
     In the formation of copper interconnects, dual damascene features of trenches and vias are etched into a dielectric layer. Normally, a barrier layer containing tantalum or some other barrier material is used to fill the features, before the features are filled with copper. The barrier layer is used to protect the dielectric from copper diffusion into the dielectric and for other purposes. The tantalum based barrier in the copper interconnects significantly reduces the cross-section area of the copper, as the interconnect dimensions get smaller and smaller, which leads to an exponential increase of the copper line resistance. 
       FIG. 1  is a high level flow chart of an embodiment of the invention. In this embodiment, a substrate with a dielectric layer with dual damascene features of vias and trenches is provided (step  104 ). A barrier layer is selectively deposited on the bottoms of the vias (step  104 ). Copper contacts are electrolessly deposited (ELD) on bottoms of the vias over the barrier layer (step  112 ). A barrier layer is formed over the copper contacts (step  116 ). An adhesion layer is formed in the features (step  120 ). The remaining features are filled with copper (step  124 ). 
     In a preferred embodiment of the invention, a substrate with a dielectric layer with dual damascene features of vias and trenches is provided (step  104 ).  FIG. 2A  is a schematic cross-sectional view of a stack  200  with a substrate  204  with a metal layer  208  under a dielectric layer  212 . There may be one or more layers between the substrate  204  and the metal layer  208 . The metal layer  208  may be a continuous layer or a patterned layer in a dielectric layer. There may be one or more layers between the metal layer  208  and the dielectric layer  212 . Dual damascene features  216  have been etched into the dielectric layer  212 . The dual damascene features  216  comprise vias  224 , which are etched completely through the dielectric layer  212 , and vias  220 , which are only partially etched trough the dielectric layer  212 . In this embodiment, the dielectric layer is a low-k dielectric layer of either a carbon doped silicon oxide or an organic polymer layer. In addition, the bottoms of the vias are in electrical contact with the metal layer  208 . Preferably, the vias are etched completely through to the metal layer  208 . 
     A barrier layer is selectively deposited on bottoms of the vias (step  108 ). In this embodiment, the selective deposition is provided by electroless deposition, which is possible, since the deposition is on a conductive layer. In this embodiment, the barrier layer comprises cobalt and tungsten. The barrier layer may further comprise boron, nickel, molybdenum, or phosphorous.  FIG. 2B  is a schematic cross-sectional view of the stack  200  after the barrier layer  228  has been selectively deposited on only bottoms of the vias  224 . 
     A copper pre-fill is provided to form contacts on the bottoms of the vias (step  112 ). In this embodiment the copper pre-fill is pure copper. In other embodiments, the copper pre-fill may be copper with small amounts of other alloy materials. In this embodiment, the selective deposition is accomplished by electroless deposition.  FIG. 2C  is a schematic cross-sectional view of the stack  200  after the copper contacts  232  has been selectively deposited on only bottoms of the vias  224 . In another embodiment, a cobalt pre-fill may be used instead of the copper pre-fill. 
     A top contact barrier layer is selectively deposited over the copper contacts (step  112 ). In this embodiment, the selective deposition is provided by electroless deposition. In this embodiment, the barrier layer comprises cobalt and tungsten. The barrier layer may further comprise boron or phosphorous.  FIG. 2D  is a schematic cross-sectional view of the stack  200  after the top contact barrier layer  236  has been selectively deposited on the copper contacts  232 . 
     An adhesion layer is deposited within the features (step  120 ). Preferably, the adhesion layer is cobalt or ruthenium. More preferably, the adhesion layer is pure cobalt or pure ruthenium. Preferably, the adhesion layer has a thickness in the range of 10 to 30 Å. Preferably, the adhesion layer is formed by chemical vapor deposition (CVD) or atomic layer deposition (ALD). Preferably, the adhesion layer is no more than 10 Å away from the low-k dielectric layer of the walls of the trenches. More preferably, the adhesion layer is deposited directly against the low-k dielectric layer on the walls and bottoms of the trenches.  FIG. 2E  is a schematic cross-sectional view of the stack  200  after the adhesion layer  240  has been deposited (step  120 ). 
     The thickness of the pure cobalt or ruthenium adhesion layer being in the range of 10 to 30 Å results in a continuous layer, which prevents copper electromigration. If the pure cobalt or ruthenium adhesion layer does not have sufficient adhesion to the dielectric layer  212 , because of the particular dielectric material, then an additional adhesion layer of another material may be formed on the dielectric layer  212  before forming the pure cobalt or ruthenium adhesion layer. The additional adhesion layer does not need to be continuous and therefore is less than 10 Å in thickness. This results in the pure copper or ruthenium adhesion layer being less than 10 Å from the dielectric layer  212 . 
     The features are copper filled (step  124 ). The copper filling may be by ELD or electroplating or by some other method. In this embodiment the copper fill is pure copper. In other embodiments, the copper fill may be copper with small amounts of other alloy materials.  FIG. 2F  is a schematic cross-sectional view of the stack  200  after the features have been filled with copper fill  244  (step  124 ). 
     The barrier layers help prevent electromigration of the copper. The barrier layers may also improve copper adhesion. This embodiment fills the vias with the copper contacts  232 , so that the copper contacts are in direct contact with the low-k dielectric  212 , without a barrier layer. This allows an increased copper contact cross-section, while preventing copper diffusion, because the low-k dielectric is either a carbon doped silicon oxide or an organic polymer. 
     In another embodiment of the invention, the step of forming the barrier on top of the copper contact pre-fill (step  116 ) is omitted. In such a process, the providing the substrate with dual damascene features (step  104 ), forming a barrier layer on the bottoms of the vias (step  108 ), the depositing the copper contact pre-fill (step  112 ), the depositing the adhesion layer (step  120 ), and the copper filling the features (step  124 ) are the only steps performed.  FIG. 3  is a cross-sectional view of a stack  300 , with copper filling formed by this process. The stack  300  comprises a substrate  304  with a metal layer  308  under a dielectric layer  312 , a barrier layer  316  on the bottom of the vias, a copper contact pre-fill  316 , with an adhesion layer  324  directly deposited on the dielectric layer  312  within the entire surface of the trenches, and a copper fill  328  filling the remainder of the features. 
     In another embodiment of the invention, the steps of forming the barrier on bottoms of the vias (step  108 ), the copper pre-fill (step  112 ), and the forming the barrier over the contacts (step  116 ) are omitted. In such a process, the providing the substrate with dual damascene features (step  104 ), the depositing the adhesion layer (step  120 ), and the copper filling the features (step  124 ) are the only steps performed.  FIG. 4  is a cross-sectional view of a stack  400 , with copper filling formed by this process. The stack  400  comprises a substrate  404  with a metal layer  408  under a dielectric layer  412 , with an adhesion layer  416  directly deposited on the dielectric layer  412  within the entire surface of the features, and a copper fill  420  filling the remainder of the features. 
     In this embodiment, by omitting various steps, the resulting process is simpler and quicker. However, the pre-filling the via in the previous embodiment makes the copper fill of the trenches easier. In addition, the placement of the adhesion layer within the vias reduces the cross-sectional area of the copper within the vias. 
     In another embodiment of the invention, the copper contact pre-fill (step  112 ) and the forming the barrier layer over the contacts (step  116 ) are omitted. In such a process, the providing the substrate with dual damascene features (step  104 ), the forming the barrier layer only on the bottom of the vias (step  108 ), the depositing the adhesion layer (step  120 ), and the copper filling the features (step  124 ) are the only steps performed.  FIG. 5  is a cross-sectional view of a stack  500 , with copper filling formed by this process. The stack  500  comprises a substrate  504  with a metal layer  508  under a dielectric layer  512 , with a barrier layer  516  and an adhesion layer  520  directly deposited on the dielectric layer  512  within the entire surface of the features except at the bottom of the vias, and a copper fill  524  filling the remainder of the features. 
     In another embodiment of the invention, the steps of forming the barrier on bottoms of the vias (step  108 ) and the forming the barrier over the contacts (step  116 ) are omitted. In such a process, the providing the substrate with dual damascene features (step  104 ), the depositing the copper contact pre-fill (step  112 ), the depositing the adhesion layer (step  120 ), and the copper filling the features (step  124 ) are the only steps performed.  FIG. 6  is a cross-sectional view of a stack  600 , with copper filling formed by this process. The stack  600  comprises a substrate  604  with a metal layer  608  under a dielectric layer  612 , a copper contact pre-fill  616 , with an adhesion layer  620  directly deposited on the dielectric layer  612  within the entire surface of the trenches, and a copper fill  620  filling the remainder of the features. 
     In another embodiment of the invention, the steps of forming the barrier on bottoms of the vias (step  108 ) and the forming the barrier over the contacts (step  116 ) are omitted. In such a process, the providing the substrate with dual damascene features (step  104 ), the depositing the copper contact pre-fill (step  112 ), the depositing the adhesion layer (step  120 ), and the copper filling the features (step  124 ) are the only steps performed.  FIG. 6  is a cross-sectional view of a stack  600 , with copper filling formed by this process. The stack  600  comprises a substrate  604  with a metal layer  608  under a dielectric layer  612 , a copper contact pre-fill  616 , with an adhesion layer  620  directly deposited on the dielectric layer  612  within the entire surface of the trenches, and a copper fill  620  filling the remainder of the features. 
     In another embodiment of the invention, the step of forming the barrier on bottoms of the vias (step  108 ) is omitted. In such a process, the providing the substrate with dual damascene features (step  104 ), the depositing the copper contact pre-fill (step  112 ), forming a barrier layer on top of the copper contacts (step  116 ), the depositing the adhesion layer (step  120 ), and the copper filling the features (step  124 ) are the only steps performed.  FIG. 7  is a cross-sectional view of a stack  700 , with copper filling formed by this process. The stack  700  comprises a substrate  704  with a metal layer  708  under a dielectric layer  712 , a copper contact pre-fill  716 , a barrier layer  720  on top of the contact  716 , with an adhesion layer  724  directly deposited on the dielectric layer  712  within the entire surface of the trenches, and a copper fill  728  filling the remainder of the features. 
     In some embodiments the low-k dielectric is porous. In other embodiments, the low-k dielectric is not porous. In various embodiments the low-k dielectric is defined as a dielectric with a k value of less than 3.0. Some embodiments allow the copper to be deposited against the dielectric layer without a barrier layer in between. By eliminating the barrier layer between the copper and the low-k dielectric these embodiments allow sufficient room for the copper contacts at small feature widths. Generally, barrier layers between the copper and the dielectric have a high resistance, which reduces the cross-section of highly conductive materials. In some embodiments an additional adhesion layer of less than 10 Å is placed between the adhesion layer of cobalt or ruthenium. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and various substitute equivalents, which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and various substitute equivalents as fall within the true spirit and scope of the present invention.