Abstract:
Methods and apparatuses for using a semi-permeable membrane to deliver a reagent to a surface in a topographically selective manner are provided. The methods and apparatuses are particularly useful for removing sulfur-containing electrocatalysts from copper surfaces using a semi-permeable membrane to deliver an oxidizing agent to a catalyst-coated surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 from U.S. Provisional Application Ser. No. 60/894,487, filed Mar. 13, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention is directed to methods and apparatuses for using a semi-permeable membrane to deliver a reagent to a surface in a topographically selective manner. The methods and apparatuses are particularly useful for removing sulfur-containing electrocatalysts from copper surfaces using a semi-permeable membrane to deliver an oxidizing agent to a catalyst-coated surface. 
       BACKGROUND 
       [0003]    Interconnections on integrated circuits are fabricated by the Cu damascene process in which the interconnect circuit pattern is lithographically etched into the surface of a dielectric layer on the surface of the wafer. The etching creates “recessed areas” in the dielectric layer that can be several nanometers to several microns in depth. The remaining surface of the dielectric layer forms “non-recessed areas” that surround the “recessed areas.” The pattern is then coated with thin conformal layers of a barrier metal, such as Ta, followed by Cu. Additional Cu is then electroplated over the entire surface of a wafer to fill completely the recessed areas with Cu. 
         [0004]    In order to assure complete filling of the smallest the recessed areas, the electroplating chemistry incorporates an electrocatalyst, most commonly 3-mercapto-1-propane sulfonic acid (MPS), a salt of MPS, or a corresponding disulfide. These electrocatalysts adsorb to the Cu surface and increase the rate of Cu electrodeposition relative to areas of bare Cu lacking an adsorbed electrocatalyst. This effect is amplified in very small recessed areas because the surface concentration increases during filling. Because the electrocatalyst is present on all surfaces of the wafer, in the course of filling larger recessed areas, excess Cu is deposited everywhere and must be removed. This is typically achieved by chemical mechanical polishing, but may also be accomplished by membrane-mediated electropolishing (MMEP) methods disclosed by S. Mazur et al., (co-pending applications U.S. Ser. No. 10/976,897; U.S. Ser. No. 10/986,048; and U.S. Ser. No. 11/291,697). 
         [0005]    Removing excess Cu introduces significant expense, yield loss and waste disposal problems for the fabrication of integrated circuits. It is therefore desirable to minimize the amount of excess Cu required to fill the circuit features. 
         [0006]    One way to achieve this objective would be to interrupt electrodeposition immediately after filling the small recessed areas with Cu, and to then remove the electrocatalysts in a topographically selective manner from the non-recessed areas, while leaving the electrocatalysts in the recessed areas. In this way, subsequent plating becomes concentrated in the recessed areas and can be stopped with minimal accumulation of Cu on the non-recessed areas. 
         [0007]    MMEP has been shown to be highly effective for topographically selective removal of MPS, but it is accompanied by a significant amount of Cu removal. Typically, in order to completely remove MPS from the surface of the non-recessed areas, several nm of Cu must also be removed. It is therefore desirable to improve the efficiency of MPS removal relative to Cu removal. 
       SUMMARY OF THE INVENTION 
       [0008]    One aspect of this invention is an apparatus comprising:
       a) a fully or partially enclosed container;   b) a semi-permeable membrane having an internal surface and an external surface, wherein the membrane forms a surface of the fully or partially enclosed container; and   c) a reagent-containing fluid which at least partially fills the fully or partially enclosed container, wherein the reagent-containing fluid contacts at least a portion of the internal surface of the membrane.       
 
         [0012]    Another aspect of this invention is a process comprising:
       a) providing an apparatus comprising:
           i) a fully or partially enclosed container;   ii) a semi-permeable membrane having an internal surface and an external surface, wherein the membrane forms a surface of the fully or partially enclosed container; and   iii) a reagent-containing fluid which at least partially fills the fully or partially enclosed container, wherein the reagent-containing fluid contacts at least a portion of the internal surface of the membrane;   
           b) providing a workpiece having a surface and optionally an oxidizable or reducible compound adsorbed onto the surface of the workpiece;   c) contacting the surface of the workpiece with the external surface of the membrane; and   d) allowing at least a portion of the reagent to diffuse through the membrane to react with the workpiece surface or an oxidizable or reducible compound adsorbed onto the surface of the workpiece.       
 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]      FIG. 1  is a schematic cross-section of an apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    One embodiment of the process of this invention can be used to deliver reagents to the surface of a workpiece. The reagents can react either with the surface of the workpiece, or with an oxidizable or reducible compound that is adsorbed onto the surface of the workpiece. If the surface of the workpiece has topographical features, i.e., recessed areas and non-recessed areas, one embodiment of the process of this invention can be used to deliver reagents selectively to the non-recessed areas. In this way, material can be removed from the non-recessed areas of the workpiece without also removing material from the recessed areas. 
         [0022]    One aspect of this invention is an apparatus comprising:
       a) a fully or partially enclosed container;   b) a semi-permeable membrane having an internal surface and an external surface, wherein the membrane forms a surface of the fully or partially enclosed container; and   c) a reagent-containing fluid which at least partially fills the fully or partially enclosed container (which can also be referred to as a “vessel”), wherein the reagent-containing fluid contacts at least a portion of the internal surface of the membrane.       
 
         [0026]    In some embodiments, the apparatus can be used in the membrane-mediated delivery of a reagent to the non-recessed areas of a workpiece having non-recessed areas. For example, in integrated circuit (IC) interconnects, the recessed areas can be about 0.5 micron below the surrounding non-recessed areas. In printed wiring boards, the recessed areas may be 10 to 50 microns below the surrounding areas. 
         [0027]    Suitable reagents include oxidizing agents and reducing agents. Suitable oxidizing agents include ozone, hydrogen peroxide, peracids, and salts of high valent transition metal ions (e.g., Fe(NO 3 ) 3  or Ce(NH 4 ) 2 (NO 3 ) 6 ). Some reagents, for example, hydrogen peroxide, can be used at concentrations as high as 70%. The transition metal salts are more typically used at concentrations of 0.01 M-1.0 M. Peracids can be made in situ by combining a carboxylic acid with hydrogen peroxide. 
         [0028]    Preferably, the reagent-containing fluid is maintained at a hydrostatic pressure greater than ambient atmospheric pressure, and the membrane is sufficiently flexible to expand under the influence of this pressure to establish a convex external surface (a “bulge” or “blister”) to contact the workpiece. 
         [0029]    Suitable semi-permeable membranes for use with oxidizing agents are those which are stable in the presence of the oxidizing agent(s) and which are permeable to the oxidizing agent(s). Suitable membranes include copolymers of fluorinated and/or perfluorinated olefins and monomers containing strong acid groups. Perfluorosulfonate ionomer membranes and perfluorocarboxylate ionomer membranes are suitable. Other semi-permeable membranes can also be used. 
         [0030]      FIG. 1  shows a schematic of an apparatus in which A represents a reagent-containing fluid, B represents a semi-permeable membrane, C represents a workpiece, and D represents recessed areas in a workpiece. 
         [0031]    In the membrane-mediated processes of this invention, a membrane is interposed between the reagent and the workpiece. In some embodiments, the workpiece has small topographic features such as recessed and non-recessed areas. By providing a membrane that is thick and/or stiff enough that it does not conform to the small topographic features of the workpiece, the membrane will not contact the surfaces of the recessed areas. In this way, the reagent is delivered selectively to the non-recessed areas, and the process can selectively remove material that is adsorbed onto the non-recessed areas of the workpiece, without removing material that is in the recessed areas. In one embodiment, the workpiece is a metal-coated substrate, e.g., a damascene wafer. 
         [0032]    One aspect of this invention is a process comprising:
       a) providing an apparatus comprising:
           i) a fully or partially enclosed container;   ii) a semi-permeable membrane having an internal surface and an external surface, wherein the membrane forms a surface of the fully or partially enclosed container; and   iii) a reagent-containing fluid which at least partially fills the enclosed container, wherein the reagent-containing fluid contacts at least a portion of the internal surface of the membrane;   
           b) providing a workpiece having a surface and optionally an oxidizable or reducible compound adsorbed onto the surface of the workpiece;   c) contacting the surface of the workpiece with the external surface of the membrane; and   d) allowing at least a portion of the reagent to diffuse through the membrane to react with the workpiece surface or an oxidizable or reducible compound adsorbed onto the surface of the workpiece.       
 
         [0040]    Suitable reaction temperatures are from 10-80° C. In one embodiment, the reaction temperature is within the range of 15-50° C. Reaction times are from 0.1 sec to several minutes, depending on the concentration and composition of the reagent. 
         [0041]    In one embodiment, a compound is adsorbed onto the workpiece prior to contacting the surface of the workpiece with the external surface of the membrane. In one embodiment, the workpiece has adsorbed onto it a sulfur-containing electrocatalyst (e.g., MPS, 3-mercapto-1-propane sulfonic acid) and the reagent in the reagent-containing fluid is an active oxidizing agent. 
         [0042]    Removal of an adsorbed oxidizable compound is accomplished by oxidizing it to form a soluble species. After the oxidizing agent diffuses through the membrane, it reacts with the adsorbed oxidizable compound, converting it to a form that has less affinity for the workpiece surface and can be washed or rinsed or dissolved away. The soluble species is then removed from the workpiece surface by rinsing with a suitable or solution, i.e., a solvent or solution that will dissolve the soluble species. For example, the oxidized adsorbed oxidizable compound can be removed from the workpiece surface by immersing the workpiece in a suitable solvent (e.g., water) or by periodically rinsing the surface with a suitable solvent. 
         [0043]    Oxidation of the adsorbed oxidizable compound is accomplished when a portion of the external surface of the membrane contacts a portion of the non-recessed areas of the workpiece. As used herein, “contact” (of the membrane and the workpiece) means that preferably the workpiece and the membrane are within close proximity, e.g., between 1 nm and 1 micron. Typically, the apparatus is moved across the surface of workpiece, especially if the area of the surface to be treated is larger than the contact area of the membrane with the surface. 
         [0044]    In one embodiment, the workpiece is coated with a thin layer (less than 1 micron thick) of water-immiscible hydrocarbon or halocarbon before being contacted with the membrane. Suitable hydrocarbons include heptane and toluene. While it is not intended that the invention be bound by any particular mechanism or theory, it is believed the hydrocarbon or halocarbon lubricates the surface and also improves the selectivity of the adsorbed oxidizable compound removal by slowing or stopping diffusion of the oxidizing agent into the recessed areas of the workpiece. In this way, adsorbed oxidizable compound is removed preferentially, preferably only, from those areas in direct contact with the membrane, leaving the electrocatalyst in the recesses. This leads to more copper plating out in the recesses than on the non-recessed areas of the workpiece in a subsequent plating process. 
         [0045]    Removal of an adsorbed reducible compound is carried out in an analogous process, except that the reagent is a reducing agent, for example sodium borohydride. 
         [0046]    Selective oxidation of the adsorbed oxidizable compound can also be accomplished by a membrane-mediated electrochemical process. In such a process, the half-cell is configured as described in U.S. Ser. No. 10/976,897. 
         [0047]    The amount of adsorbed compound removed can be determined by XPS and/or cyclic voltammetry. 
       EXAMPLES 
     Example 1 
     Preparation of MPS-Activated and MPS-Free Cu Surfaces 
       [0048]    A 12″ silicon wafer pre-plated with approximately 150 nm of Cu (Novellus Systems, Inc., Tualatin, Oreg.) was mounted on a spin-coater (Headway Research, Inc., Garland, Tex., Model PWM32) and treated at 300 rpm with reagents in the following sequence: DI (de-ionized) water; 10 ml of 5% H 2 SO 4 ; DI water; 10 ml 0.1% MPS (3-mercapto-1-propane sulfonic acid) in 5% H 2 SO 4 ; and DI water. The MPS-coated wafer was then dried at 1000 rpm. By skipping the treatment with the 0.1% MPS solution, the same procedure was used to prepare wafers free of MPS. 
       Example 2 
     Analysis of Cu Surfaces by Cyclic Voltammetry and XPS 
       [0049]    Cyclic voltammetry measurements (EG&amp;G PARC, Princeton, N.J., Model 173 potentiostat and Model 175 programmer) were made at a scan rate of 10 mV/sec, operating at potentials between −0.40V and −1.00 V versus Hg/HgSO 4 . For this purpose, selected areas of the wafer were masked by applications of a 2.5 cm square piece of Teflon® tape with a round opening 1 cm in diameter (0.785 cm 2 ). A pyrex flange joint 7 cm long with a 2 cm O-ring was centered over the hole in the tape mask and clamped onto the wafer to form a cylindrical cell with liquid-tight seal. 10 ml of electrolyte solution (medium acid Cu plating solution, Novellus Systems, Inc., Tualatin, Oreg.) was added to the cell. A Hg/HgSO 4  reference electrode (Radiometer Analytical SAS, Villeurbanne, France, Model Ref 601) was inserted into the cell along with a Cu foil counter electrode. An electrical connection was made to the surface of the wafer outside the area of the cell and connected to the potentiostat as the working electrode. Measurements on wafers freshly prepared as in Example 1 exhibited currents 20 to 30 mA higher than on MPS-free wafers at potentials from −0.7 to −1.0 V versus Hg/HgSO 4  ( FIG. 1 ). This demonstrates the catalytic effect of MPS on cathodic reaction of the Cu surface and provides a means to detect the presence of MPS on a Cu surface. 
         [0050]    X-ray photoelectron spectroscopy (XPS) was used to analyze the surfaces. On an MPS-activated Cu surface, the signal from S (2p electrons) represented 4% of all elements detected. In contrast, on MPS-free wafers prepared as in Example 1, the signal from S represented only 0.2% of all elements detected. 
       Example 3 
     Oxidation of MPS with Iron and Cerium Nitrates 
       [0051]    A wafer fragment activated with MPS as in Example 1 was immersed in a solution of 0.5M Fe(NO 3 ) 3  for approximately 5 sec and immediately rinsed with DI water. Visual inspection showed that all of the Cu had been removed exposing, the silver-colored Ta sub-layer. One area of a second MPS-activated wafer fragment was exposed to a solution of 0.05M Fe(NO 3 ) 6  for 15 sec and immediately rinsed with DI water. Cyclic voltammetry indicated no detectable loss of MPS relative to an un-treacted area of the same wafer fragment. 
         [0052]    A third MPS-activated wafer fragment was exposed to a solution of 0.05M Ce(NH 4 ) 2 (NO 3 ) 6  for 10 sec. Cyclic voltammetry indicated little if any MPS was removed. When this experiment was repeated with 30 sec exposure the thickness of Cu layer was reduced from 147 to 110 nm and cyclic voltammetry showed more than 50% of the MPS had been removed. 
       Example 4 
     Oxidation with Aqueous Hydrogen Peroxide 
       [0053]    A wafer fragment activated with MPS as in Example 1 was immersed in a 50% solution of hydrogen peroxide in water (Aldrich) for 15 sec and then rinsed with DI water. Discoloration on the surface indicated the formation of copper oxides, but the thickness of Cu was only reduced by 3±2 nm. Cyclic voltammetry showed that more than 50% of the MPS had been removed. 
       Example 5 
     Membrane-Mediated Oxidation with Aqueous Hydrogen Peroxide 
       [0054]    A membrane cell with a window 1′ in diameter was fitted with a Nafion® PFSA membrane (N1110-H, E. I. du Pont de Nemours and Company, Wilmington, Del.) and filled with a 50% solution of hydrogen peroxide in water (Aldrich) adjusted to a hydrostatic pressure of approximately 1 psi. An MPS-activated wafer was rinsed with DI water, then brought into contact and gently stroked with the external surface of the membrane for 30 sec, and then rinsed in DI water. Cyclic voltammetry showed approximately 50% of the MPS had been removed. 
       Example 6 
     Oxidation with Ozone 
       [0055]    A wafer fragment activated with MPS as in Example 1 was immersed in water saturated with ozone for 30 sec and then rinsed with DI water. Cyclic voltammetry showed that virtually all of the MPS had been removed. As second wafer fragment activated with MPS as in Example 1 was exposed to a stream of gaseous ozone for 30 sec and then rinsed with DI water. Cyclic voltammetry indicated approximately 20% of the MPS had been removed. 
       Example 7 
     Membrane-Mediated Oxidation with Adipic Acid in H 2 O 2    
       [0056]    A wafer with 600 nm deep recessed features was activated with MPS as in Example 1. A drop of heptane was placed on the wafer surface. A membrane cell with a window 3″ in diameter was fitted with a Nafion® PFSA membrane (N1110-H) and filled with a 20% solution of hydrogen peroxide in water (Aldrich) and 0.1 wt % adipic acid and adjusted to a hydrostatic pressure of approximately 1 psi. The membrane was brought down dry onto the surface of the wafer and moved back and forth (2 cm/sec, 2 rpm) on the heptane-wetted area for 60 sec. The heptane was then evaporated and the wafer rinsed with 5% sulfuric acid in de-ionized water. 
         [0057]    After rinsing with dilute sulfuric acid, the adipic acid/H 2 O 2 -treated wafer was plated with an additional layer of Cu (constant potential of −0.6 V vs. Hg/HgSO 4 ; area=12.5 cm 2 ; 17 coulombs). Profilometry of the surface indicated that the step height had been reduced from about 600 nm to 200-400 nm. 
         [0058]    This demonstrates that the membrane-mediated oxidation of MPS is topographically selective, removing MPS selectively from the non-recessed area, rather than from the recessed area.