Abstract:
A method for sealing a porous dielectric layer atop a substrate, wherein the dielectric layer is patterned to form at least a trench and at least a via, comprises applying a first plasma to a surface of the dielectric layer to silanolize the surface, treating the surface of the dielectric layer with a silazane to form a monolayer of silane molecules on the surface, and applying a second plasma to the surface of the dielectric layer to induce a polymerization of at least a portion of the silane molecules. The polymerized silane molecules form a cross-linked matrix that builds over a substantial portion of the surface of the dielectric layer and seals at least some of the exposed pores.

Description:
BACKGROUND  
       [0001]     Modern integrated circuits generally contain several layers of interconnect structures fabricated above a substrate. The substrate may have active devices and/or conductors that are connected by the interconnect structure.  
         [0002]     Interconnect structures, typically comprising trenches and vias, are usually fabricated in, or on, an interlayer dielectric (ILD). It is generally accepted that, the dielectric material in each ILD should have a low dielectric constant (k) to obtain low capacitance between conductors. Decreasing this capacitance between conductors, by using a low dielectric constant, results in several advantages. For instance, it provides reduced RC delay, reduced power dissipation, and reduced cross-talk between the metal lines.  
         [0003]     To obtain the desired low dielectric constant, porosity is often introduced into the dielectric material. These pores typically increase problems that inherently exist when further processing is done on dielectric material. For example copper formed in the trenches and vias, without a barrier, may diffuse into an underlying layer causing the shorting of adjacent copper lines or line-to-line leakage. Moreover, when vias and trenches are etched in the porous dielectric material, pores are often exposed on the surface of the dielectric. Therefore, interconnect structures employ a barrier layer over the surface of the dielectric to protect from copper diffusing into the dielectric material. Common materials used for this barrier layer are tantalum, tantalum nitride, tantalum carbide, and titanium nitride.  
         [0004]     Yet, any discontinuity, like the discontinuities  130  in  FIG. 1A , in the barrier film  120  will result in the diffusion of copper atoms or penetration of plating solution into the dielectric layer  110 . This diffusion can also cause copper lines to short or induce electrical leakage from line-to-line to occur, as well as cause destruction of the dielectric layer  110 . As shown in  FIG. 1B , the prior art requires the deposition of a thicker barrier layer  140 , typically greater than 30 nm, to physically cover the exposed pores and adequately protect the dielectric layer  110 . Nevertheless, this thicker barrier layer  140  takes up additional volume in a via or a trench, which increases the line resistance by reducing the volume available for copper and adding series resistance to an underlying copper connection.  
         [0005]     These integration challenges are also present in related art methods of sealing the exposed pores on the surface of dielectric materials. One technique requires the use of thin films of material, such as SiC, to seal the pores; yet, these thin films often take too much volume and increase the dielectric constant of the interconnect structure. Another technique requires the use of plasma gas, such as nitrogen, argon, or helium, to increase the density of the dielectric surface. Nevertheless, plasma is very directional, which makes deposition of a continuous film extremely difficult. This also results in a higher dielectric constant of film stack.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1A  is a prior art cross-sectional elevation view of a trench and via defined by a porous interlayer dielectric after a thin film has been deposited over the surface of the dielectric.  
         [0007]      FIG. 1B  is a prior art cross-sectional elevation view of a trench and via defined by a porous interlayer dielectric, after a thick barrier layer has been deposited over the surface of the dielectric.  
         [0008]      FIG. 2  is a semiconductor substrate with a patterned, porous dielectric layer.  
         [0009]      FIG. 3  is a close-up view of the surface of a dielectric layer that includes three exposed pores.  
         [0010]      FIG. 4  is a method for sealing the exposed pores on a dielectric layer.  
         [0011]      FIGS. 5A  to  5 D illustrate examples of unsaturated silazane molecules.  
         [0012]      FIG. 6  is a close-up view of the surface of a dielectric layer that includes a monolayer of silane molecules.  
         [0013]      FIG. 7  is a close-up view of the surface of a dielectric layer that includes a polymerized layer of silane molecules that seals exposed pores of the surface.  
         [0014]      FIG. 8  is a top view of an exposed pore being sealed using a method of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0015]     Described herein are systems and methods of sealing a porous dielectric layer using a plasma induced surface polymerization process. In the following description, various aspects of the illustrative implementations will be described using terms commonly employed by those skilled in the art to convey the substance of their work to others skilled in the art. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some of the described aspects. For purposes of explanation, specific numbers, materials and configurations are set forth in order to provide a thorough understanding of the illustrative implementations. However, it will be apparent to one skilled in the art that the present invention may be practiced without the specific details. In other instances, well-known features are omitted or simplified in order not to obscure the illustrative implementations.  
         [0016]     Various operations will be described as multiple discrete operations, in turn, in a manner that is most helpful in understanding the present invention, however, the order of description should not be construed to imply that these operations are necessarily order dependent. In particular, these operations need not be performed in the order of presentation.  
         [0017]     Turning to  FIG. 2 , a semiconductor substrate  200  is illustrated which generally includes several active device regions (not shown), including other structures such as gates, local interconnects, metal layers, and other active or passive device structures or layers. An etch stop  210  is also illustrated in  FIG. 2 , which may be comprised of numerous materials. For example, etch stop  210  may be comprised of silicon nitride (Si 3 N 4 ) or silicon carbide (SiC).  
         [0018]     A porous dielectric layer  220  may be deposited on the etch stop  210  layer and patterned to produce an interconnect structure  230 , for instance, a trench  230 A and a via  230 B. For clarity, only some of the surface pores are shown in  FIG. 2 . Although not illustrated, it should be noted that the entire porous dielectric layer  220  generally consists of continuous and open pores. The porous dielectric layer  220  may be deposited using conventional processes such as chemical vapor deposition (CVD), atomic layer deposition (ALD), or plasma enhanced chemical vapor deposition (PECVD). The porous dielectric layer  220  may be formed using any known suitable dielectric materials, including but not limited to oxides such as silicon dioxide (SiO 2 ) and carbon doped oxide (CDO), organic polymers such as perfluorocyclobutane (PFCB), or fluorosilicate glass (FSG). Some examples of oxide dielectrics include: LKD-5109, which may be obtained from JSR; Nanoglass-E, which may be obtained from Honeywell; and Zirkon, which may be obtained from Shipley. Some examples of polymer dielectrics are porous silk, which may be obtained from Dow Chemical and GX-3P, which may be obtained from Honeyvell. Some examples of CDOs are the BDII family from Applied Materials, and Coral from Novellus. Ordinary masking and etching processes, such as dual damascene techniques, may be used to form the interconnect structure  230 , as well as any other trenches or vias needed within the dielectric layer  220 .  
         [0019]     The surface of the etched dielectric layer  220  tends to have a number of exposed pores  250 , as shown in  FIG. 2 . This is caused in part by the porous nature of the dielectric layer  220  and tends to be exacerbated by the interconnect etching process which exposes many of the pores  250 . As stated above, copper that is later formed in the interconnect structure  230 , without a barrier layer, may diffuse into the dielectric layer  220  and cause the shorting of adjacent copper lines or line-to-line leakage.  
         [0020]      FIG. 3  is a close-up view of the surface of the dielectric layer  220  that includes three of the exposed pores  250 . Silicon based porous dielectrics invariably have hydrophilic surfaces after the dual damascene patterning process, and the surface of the dielectric layer  220  contains silanol molecules (Si—OH) that typically result from the etching and cleaning processes used to form the trenches  230 A and vias  230 B of the interconnect structure  230 . These silanol bonds are highly reactive towards silazanes, some of which are described below.  
         [0021]      FIG. 4  illustrates one implementation of a pore-sealing process  400  that may be used to treat the surface of the dielectric layer  220  and seal the exposed pores  250 . This reduces the likelihood of copper diffusing into the dielectric layer  220  and enables a thinner barrier layer to be used within the interconnect structure. In some implementations, the sealing of the exposed pores  250  may enable the barrier layer to be eliminated altogether.  
         [0022]     In some implementations, the pore-sealing process  400  of the invention may begin by exposing the dielectric layer  220  to a carbon dioxide (CO 2 ) or nitrogen oxide (N 2 O) based plasma (see  402  of  FIG. 4 ). This process is used to silanolize the surface of the dielectric layer  220 . In other words, the CO 2  or N 2 O based plasma is used to form Si—OH molecules along at least a portion of the surface of the dielectric layer  220 . This step is optional and depends on whether the surface of the dielectric layer  220  was substantially silanolized by the etching and cleaning processes used to form the interconnect structure. In alternate implementations, chemicals may be used to silanolize the dielectric layer  220 , such as hydrogen peroxide or other alkaline materials. Generally, the process conditions for the CO 2  or N 2 O based plasma include a pH of 11 for the alkaline chemicals and temperatures around 50° C. or higher for the peroxide based chemicals.  
         [0023]     In an alternate implementation, a wet chemical may be used to silanolize the surface of the dielectric layer  220 . In such an implementation, an oxidizing wet chemical or an ozone-based chemical may be used.  
         [0024]     Next, the pore-sealing process  400  of the invention exposes the silanolized surface of the dielectric layer  220  to a silazane (see  404  of  FIG. 4 ). In implementations of the invention, the silazane may be in a vapor or a liquid form either at room temperature or an elevated temperature, for instance, around 100° C. to 200° C. It is preferred that silazanes having attached polymerizable groups, such as vinyl, phenyl, or benzyl groups, be used. For instance, any of a number of unsaturated silazanes may be used, including but not limited to vinyl silazanes such as 1,3-Divinyl-1,3-Diphenyl-1,3-Dimethyldisilazane (shown in  FIG. 5A ), 1,3-Diphenyl-1,1,3,3,-Tetramethyldisilazane (shown in  FIG. 5B ), 1,3-Bis-(4-Biphenyl)-Tetramethyldisilazane (shown in  FIG. 5C ), and 1,3-Divinyltetramethyldisilazane (shown in  FIG. 5D ). Other unsaturated silazanes, including but not limited to hexamethyldisilazane (HMDS) and benzyl silazane, may be used.  
         [0025]     When the silanolized surface of the dielectric layer  220  is exposed to a silazane, a physical and/or chemical adsorption of the silazane occurs at the surface of the dielectric layer  220 . The adsorption process is generally highly exothermic and tends to be spontaneous and have a quick reaction rate. The physical and/or chemical adsorption of the silazane tends to be self limiting at conventional process conditions and is generally confined to a first monolayer due to the nature of the reaction. As shown in  FIG. 6 , the silanol molecules are replaced at the surface of the dielectric layer  220  with a monolayer of silane molecules (once the silazane molecules are adsorbed, they become silane molecules). In the implementation of  FIG. 6 , the pore-sealing process  400  uses vinyl silazane molecules having phenyl (Ph) side groups. It should be noted that in other implementations of the invention, the silazane used may be purely vinyl with no phenyl side groups or may be alternate silazanes as described above.  
         [0026]     As can be seen in  FIG. 6 , the monolayer of silane molecules provides a dangling polymerizable species that now covers the surface of the dielectric layer  220 . Accordingly, the pore-sealing process  400  of the invention induces a polymerization of the unsaturated silanes by exposing the monolayer of silane molecules to a plasma (see  406  of  FIG. 4 ). A plasma generates radicals and positively charged ions that are ideal sources for addition to the unsaturation groups in the silane molecules. The plasma activates the unsaturated groups on the silane molecules to link among themselves and seal one or more exposed pores  250  on the surface of the dielectric layer  220 . The pore sealing is surface limited and not pore-filling. As such, the dielectric constant of the porous dielectric layer  220  remains substantially unaffected.  
         [0027]     In one implementation of the invention, the plasma used to initiate the polymerization is a diluted ethylene-based plasma that is chemically akin to the groups dangling from the surface. This cross linking of the side-groups generally results in a polyethylene type material. Furthermore, the ethylene plasma can be used along with the silane to manipulate the final dielectric wall characteristics for adhesion needs. In some implementations of the invention, the general plasma process conditions may fall into the following ranges: radio-frequency (RF) power level of 500-2000 Watts @ 13.56 MHz, ethylene (C 2 H 4 ) and helium (He) flows of 200-2000 sccm, and a showerhead-electrode gap of 0.3-1.0″.  
         [0028]     For the vinyl silane implementation of  FIG. 6 , the ethylene plasma induces the unsaturated C═C bonds of the vinyl silane molecules to polymerize into a cross-linked matrix, as shown in  FIG. 7 . The phenyl side groups, if present, may become saturated and form polymer chains, denoted in  FIG. 7  as (CH 2 ) x . The cross-linked matrix may build over a substantial portion of the surface of the dielectric layer  220  and seal the exposed pores  250 . The end result is the formation of a continuous, polymerized layer of carbon rich film that is cross-linked due to the unsaturation of the silane molecules prior to the plasma treatment. As stated above, the pore sealing is surface limited and not pore-filling, therefore the dielectric constant of the porous dielectric material remains substantially unaffected. The polymerized carbon rich film is generally impervious to precursors and chemicals used in subsequent barrier layer deposition processes and may increase the barrier layer&#39;s adhesion to the dielectric layer  220 .  
         [0029]     The polymerization of the monolayer may be inititated by various means, including but not limited to physical means such as thermal initiation or electron beam initiation, or chemical/radio-frequency means such as plasma initiation. In implementations of the invention, plasmas other than ethylene plasmas may be used to induce the polymerization of the unsaturated silane molecules. For instance, in other implementations, alternate plasmas that may be used include, such as a combination of argon and helium with the ethylene plasma.  
         [0030]      FIG. 8  shows a top view of one exposed pore  250  during the plasma induced surface polymerization method of the invention.  FIG. 8  begins with an exposed pore  250 . Next, the dielectric layer  220  in and around the exposed pore  250  adsorbs the silazane molecules, such as unsaturated vinyl, phenyl, or benzyl silazane molecules. Then a plasma, such as ethylene plasma, is used to initiate a polymerization of the unsaturated silane molecules to form a cross-linked matrix. The polymerization process is surface driven and surface limited, and continues until it results in the formation of a solid, carbon rich layer over the surface of the dielectric layer  220 .  
         [0031]     Accordingly, a process for sealing porous dielectrics using plasma-induced surface polymerization of unsaturated silazanes has been described. The methods of the invention enable efficient pore sealing of the dielectric materials where the pore sealing is surface limited and not pore-filling. As such, the dielectric constant of the porous dielectric material remains substantially unaffected. The final surface tends to be a carbon rich layer that tends to have greater adhesion than the starting surface for subsequently deposited barrier layers.  
         [0032]     The above description of illustrated implementations of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific implementations of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
         [0033]     These modifications may be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific implementations disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.