Abstract:
The present invention provides plating solutions, particularly metal plating solutions, designed to provide uniform coatings on substrates and to provide substantially defect free filling of small features, e.g., micron scale features and smaller, formed on substrates with none or low supporting electrolyte, i.e., which include no acid, low acid, no base, or no conducting salts, and/or high metal ion, e.g., copper, concentration. Additionally, the plating solutions may contain small amounts of additives which enhance the plated film quality and performance by serving as brighteners, levelers, surfactants, grain refiners, stress reducers, etc.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of co-pending U.S. patent application Ser. No. 09/992,117, filed Nov. 13, 2001, which is a continuation of U.S. patent application Ser. No. 09/484,616, filed Jan. 18, 2000, now issued as U.S. Pat. No. 6,350,366, which is a continuation of U.S. patent application Ser. No. 09/114,865, filed Jul. 13, 1998, now issued as U.S. Pat. No. 6,113,771, which claims benefit of U.S. provisional patent application Serial No. 60/082,521, filed Apr. 21, 1998. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to new formulations of metal plating solutions designed to provide uniform coatings on substrates and to provide defect free filling of small features, e.g., micron scale features and smaller, formed on substrates.  
           [0004]    2. Background of the Related Art  
           [0005]    Electrodeposition of metals has recently been identified as a promising deposition technique in the manufacture of integrated circuits and flat panel displays. As a result, much effort is being focused in this area to design hardware and chemistry to achieve high quality films on substrates which are uniform across the area of the substrate and which can fill or conform to very small features.  
           [0006]    Typically, the chemistry, i.e., the chemical formulations and conditions, used in conventional plating cells is designed to provide acceptable plating results when used in many different cell designs, on different plated parts and in numerous different applications. Cells which are not specifically designed to provide highly uniform current density (and the deposit thickness distribution) on specific plated parts require high conductivity solutions to be utilized to provide high “throwing power” (also referred to as high Wagner number) so that good coverage is achieved on all surfaces of the plated object. Typically, a supporting electrolyte, such as an acid or a base, or occasionally a conducting salt, is added to the plating solution to provide the high ionic conductivity to the plating solution necessary to achieve high “throwing power”. The supporting electrolyte does not participate in the electrode reactions, but is required in order to provide conformal coverage of the plating material over the surface of the object because it reduces the resistivity within the electrolyte, the higher resistivity that otherwise occurs being the cause of the non-uniformity in the current density. Even the addition of a small amount, e.g., 0.2 Molar, of an acid or a base will typically increase the electrolyte conductivity quite significantly (e.g., double the conductivity).  
           [0007]    However, on objects such as semiconductor substrates that are resistive, e.g., metal seeded wafers, high conductivity of the plating solution negatively affects the uniformity of the deposited film. This is commonly referred to as the terminal effect and is described in a paper by Oscar Lanzi and Uziel Landau, “Terminal Effect at a Resistive Electrode Under Tafel Kinetics”, J. Electrochem. Soc. Vol. 137, No. 4 pp. 1139-1143, April 1990, which is incorporated herein by reference. This effect is due to the fact that the current is fed from contacts along the circumference of the part and must distribute itself across a resistive substrate. If the electrolyte conductivity is high, such as in the case where excess supporting electrolyte is present, it will be preferential for the current to pass into the solution within a narrow region close to the contact points rather than distribute itself evenly across the resistive surface, i.e., it will follow the most conductive path from terminal to solution. As a result, the deposit will be thicker close to the contact points. Therefore, a uniform deposition profile over the surface area of a resistive substrate is difficult to achieve.  
           [0008]    Another problem encountered with conventional plating solutions is that the deposition process on small features is controlled by mass transport (diffusion) of the reactants to the feature and by the kinetics of the electrolytic reaction instead of by the magnitude of the electric field as is common on large features. In other words, the replenishment rate at which plating ions are provided to the surface of the object can limit the plating rate, irrespective of current. Essentially, if the current density dictates a plating rate that exceeds the local ion replenishment rate, the replenishment rate dictates the plating rate. Hence, highly conductive electrolyte solutions that provide conventional “throwing power” have little significance in obtaining good coverage and fill within very small features. In order to obtain good quality deposition, one must have high mass-transport rates and low depletion of the reactant concentration near or within the small features. However, in the presence of excess acid or base supporting electrolyte, (even a relatively small excess) the transport rates are diminished by approximately one half (or the concentration depletion is about doubled for the same current density). This will cause a reduction in the quality of the deposit and may lead to fill defects, particularly on small features.  
           [0009]    It has been learned that diffusion is of significant importance in conformal plating and filling of small features. Diffusion of the metal ion to be plated is directly related to the concentration of the plated metal ion in the solution. A higher metal ion concentration results in a higher rate of diffusion of the metal into small features and in a higher metal ion concentration within the depletion layer (boundary layer) at the cathode surface, hence faster and better quality deposition may be achieved. In conventional plating applications, the maximum concentration of the metal ion achievable is typically limited by the solubility of its salt. If the supporting electrolyte, e.g., acid, base, or salt, contain a co-ion which provides a limited solubility product with the plated metal ion, the addition of a supporting electrolyte will limit the maximum achievable concentration of the metal ion. This phenomenon is called the common ion effect. For example, in copper plating applications, when it is desired to keep the concentration of copper ions very high, the addition of sulfuric acid will actually diminish the maximum possible concentration of copper ions. The common ion effect essentially requires that in a concentrated copper sulfate electrolyte, as the sulfuric acid (H 2 SO 4 ) concentration increases (which gives rise to H +  cations and HSO 4   −  and SO 4   −  anions), the concentration of the copper (II) cations decreases due to the greater concentration of the other anions. Consequently, conventional plating solutions, which typically contain excess sulfuric acid, are limited in their maximal copper concentration and, hence, their ability to fill small features at high rates and without defects is limited.  
           [0010]    Therefore, there is a need for new formulations of metal plating solutions designed particularly to provide good quality plating of small features, e.g., micron scale and smaller features, on substrates and to provide uniform coating and defect-free fill of such small features.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention provides plating solutions with none or low supporting electrolyte, i.e., which include no acid, low acid, no base, or no conducting salts, and/or high metal ion, e.g., copper, concentration. Additionally, the plating solutions may contain small amounts of additives which enhance the plated film quality and performance by serving as brighteners, levelers, surfactants, grain refiners, stress reducers, etc.  
         DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
         [0012]    The present invention generally relates to electroplating solutions having low conductivity, particularly those solutions containing no supporting electrolyte or low concentration of supporting electrolyte, i.e., essentially no acid or low acid (and where applicable, no or low base) concentration, essentially no or low conducting salts and high metal concentration to achieve good deposit uniformity across a resistive substrate and to provide good fill within very small features such as micron and sub-micron sized features and smaller. Additionally, additives are proposed which improve leveling, brightening and other properties of the resultant metal plated on substrates when used in electroplating solutions with no or low supporting electrolyte, e.g., no or low acid. The invention is described below in reference to plating of copper on substrates in the electronic industry. However, it is to be understood that low conductivity electroplating solutions, particularly those having low or complete absence of supporting electrolyte, can be used to deposit other metals on resistive substrates and has application in any field where plating can be used to advantage.  
           [0013]    In one embodiment of the invention, aqueous copper plating solutions are employed which are comprised of copper sulfate, preferably from about 200 to about 350 grams per liter (g/l) of copper sulfate pentahydrate in water (H 2 O), and essentially no added sulfuric acid. The copper concentration is preferably greater than about 0.8 Molar.  
           [0014]    In addition to copper sulfate, the invention contemplates copper salts other than copper sulfate, such as copper fluoborate, copper gluconate, copper sulfamate, copper sulfonate, copper pyrophosphate, copper chloride, copper cyanide and the like, all without (or with little) supporting electrolyte. Some of these copper salts offer higher solubility than copper sulfate and therefore may be advantageous.  
           [0015]    The conventional copper plating electrolyte includes a relatively high sulfuric acid concentration (from about 45 g of H 2 SO 4  per L of H 2 O (0.45M) to about 110 g/L (1.12M)) which is provided to the solution to provide high conductivity to the electrolyte. The high conductivity is necessary to reduce the non-uniformity in the deposit thickness caused by the cell configuration and the differently shaped parts encountered in conventional electroplating cells. However, the present invention is directed primarily towards applications where the cell configuration has been specifically designed to provide a relatively uniform deposit thickness distribution on given parts. However, the substrate is resistive and imparts thickness non-uniformity to the deposited layer. Thus, among the causes of non-uniform plating, the resistive substrate effect may dominate and a highly conductive electrolyte, containing, e.g., high H 2 SO 4  concentrations, is unnecessary. In fact, a highly conductive electrolyte (e.g., generated by a high sulfuric acid concentration) is detrimental to uniform plating because the resistive substrate effects are amplified by a highly conductive electrolyte. This is the consequence of the fact that the degree of uniformity of the current distribution, and the corresponding deposit thickness, is dependent on the ratio of the resistance to current flow within the electrolyte to the resistance of the substrate. The higher this ratio is, the lesser is the terminal effect and the more uniform is the deposit thickness distribution. Therefore, when uniformity is a primary concern, it is desirable to have a high resistance within the electrolyte. Since the electrolyte resistance is given by l/κπr 2 , it is advantageous to have as low a conductivity,κ, as possible, and also a large gap, l, between the anode and the cathode. Also, clearly, as the substrate radius, r, becomes larger, such as when scaling up from 200 mm wafers to 300 mm wafers, the terminal effect will be much more severe (e.g., by a factor of 2.25). By eliminating the acid, the conductivity of the copper plating electrolyte typically drops from about 0.5 S/cm (0.5 ohm −1 cm −1 ) to about {fraction (1/10)} of this value, i.e., to about 0.05 S/cm, making the electrolyte ten times more resistive.  
           [0016]    Also, a lower supporting electrolyte concentration (e.g., sulfuric acid concentration in copper plating) often permits the use of a higher metal ion (e.g., copper sulfate) concentration due to elimination of the common ion effect as explained above. Furthermore, in systems where a soluble copper anode is used, a lower added acid concentration (or preferably no acid added at all) minimizes harmful corrosion and material stability problems. Additionally, a pure or relatively pure copper anode can be used in this arrangement. Because some copper dissolution typically occurs in an acidic environment, copper anodes that are being used in conventional copper plating typically contain phosphorous. The phosphorous forms a film on the anode that protects it from excessive dissolution, but phosphorous traces will be found in the plating solution and also may be incorporated as a contaminant in the deposit. In applications using plating solutions with no acidic supporting electrolytes as described herein, the phosphorous content in the anode may, if needed, be reduced or eliminated. Also, for environmental considerations and ease of handling the solution, a non acidic electrolyte is preferred.  
           [0017]    Another method for enhancing thickness uniformity includes applying a periodic current reversal. For this reversal process, it may be advantageous to have a more resistive solution (i.e., no supporting electrolyte) since this serves to focus the dissolution current at the extended features that one would want to preferentially dissolve.  
           [0018]    In some specific applications, it may be beneficial to introduce small amounts of acid, base or salts into the plating solution. Examples of such benefits may be some specific adsorption of ions that may improve specific deposits, complexation, pH adjustment, solubility enhancement or reduction and the like. The invention also contemplates the addition of such acids, bases or salts into the electrolyte in amounts up to about 0.4M.  
           [0019]    A plating solution having a high copper concentration (i.e., &gt;0.8M) is beneficial to overcome mass transport limitations that are encountered when plating small features. In particular, because micron scale features with high aspect ratios typically allow only minimal or no electrolyte flow therein, the ionic transport relies solely on diffusion to deposit metal into these small features. A high copper concentration, preferably about 0.85 molar (M) or greater, in the electrolyte enhances the diffusion process and reduces or eliminates the mass transport limitations. The metal concentration required for the plating process depends on factors such as temperature and the acid concentration of the electrolyte. A preferred metal concentration is from about 0.8 to about 1.2M.  
           [0020]    The plating solutions of the present invention are typically used at current densities ranging from about 10 mA/cm 2  to about 60 mA/cm 2 . Current densities as high as 100 mA/cm 2  and as low as 5 mA/cm 2  can also be employed under appropriate conditions. In plating conditions where a pulsed current or periodic reverse current is used, current densities in the range of about 5 mA/cm 2  to about 400 mA/cm 2  can be used periodically.  
           [0021]    The operating temperatures of the plating solutions may range from about 0° C. to about 95° C. Preferably, the solutions range in temperature from about 20° C. to about 50° C.  
           [0022]    The plating solutions of the invention also preferably contain halide ions, such as chloride ions, bromide, fluoride, iodide, chlorate or perchlorate ions typically in amounts less than about 0.5 g/l. However, this invention also contemplates the use of copper plating solutions without chloride or other halide ions.  
           [0023]    In addition to the constituents described above, the plating solutions may contain various additives that-are introduced typically in small (ppm range) amounts. The additives typically improve the thickness distribution (levelers), the reflectivity of the plated film (brighteners), its grain size (grain refiners), stress (stress reducers), adhesion and wetting of the part by the plating solution (wetting agents) and other process and film properties. The invention also contemplates the use of additives to produce asymmetrical anodic transfer coefficient (α a ) and cathodic transfer coefficient (α c ) to enhance filling of the high aspect ratio features during a periodic reverse plating cycle.  
           [0024]    The additives practiced in most of our formulations constitute small amounts (ppm level) from one or more of the following groups of chemicals:  
           [0025]    1. Ethers and polyethers including polyalkylene glycols  
           [0026]    2. Organic sulfur compounds and their corresponding salts and polyelectrolyte derivatives thereof.  
           [0027]    3. Organic nitrogen compounds and their corresponding salts and polyelectrolyte derivatives thereof.  
           [0028]    4. Polar heterocycles  
           [0029]    5. A halide ion, e.g., Cl −   
           [0030]    Further understanding of the present invention will be had with reference to the following examples which are set forth herein for purposes of illustration but not limitation. 
       
    
    
     EXAMPLE I  
       [0031]    An electroplating bath consisting of 210 g/L of copper sulfate pentahydrate was prepared. A flat tab of metallized wafer was then plated in this solution at an average current density of 40 mA/cm 2  and without agitation. The resulting deposit was dull and pink.  
       EXAMPLE II  
       [0032]    To the bath in example I was then added 50 mg/L of chloride ion in the form of HCl. Another tab was then plated using the same conditions. The resulting deposit was shinier and showed slight grain refinement under microscopy.  
       EXAMPLE III  
       [0033]    To the bath of Example II was added the following:  
                                             Compound   Approximate Amount (mg/L)                                Safranine O   4.3       JANUS GREEN B   5.1       2-HYDROXYETHYL DISULFIDE   25       UCON ® 75-H-1400 (Polyalkylene glycol   641       with an average molecular weight of       1400 commercially available from       Union carbide)                  
 
         [0034]    Another tab was plated at an average current density of 10 mA/cm 2  without agitation. The resulting deposit had an edge effect but was shinier and showed grain refinement.  
       EXAMPLE IV  
       [0035]    To the bath of Example II was added the following:  
                                                                 Compound   Approximate Amount (mg/L)                                        2-Hydroxy-Benzotriazole   14           EVAN BLUE   3.5           Propylene Glycol   600                      
 
         [0036]    Another tab was plated at an average current density of 40 mA/cm 2  with slight agitation. The resulting deposit had an edge effect but was shinier and showed grain refinement.  
       EXAMPLE V  
       [0037]    To the bath of Example II was added the following:  
                                             Compound   Approximate Amount (mg/L)                                BENZYLATED POLYETHYLENIMINE   3.6       ALCIAN BLUE   15       2-HYDROXYETHYL DISULFIDE   25       UCON 75-H-1400 (Polyalkylene glycol   357       with an average molecular weight of       1400 commercially available from       Union carbide)                  
 
         [0038]    Another tab was plated at an average current density of 20 mA/cm 2  without agitation. The resulting deposit had and edge effect but was shinier and showed grain refinement.  
       EXAMPLE VI  
       [0039]    A copper plating solution was made by dissolving 77.7 g/liter of copper sulfate pentahydrate (0.3 Molar CuSO 4 ×5H 2 O), and 100 g/liter of concentrated sulfuric acid and 15.5 cm 3 /liter of a commercial additive mix in distilled water to make sufficient electrolyte to fill a plating cell employing moderate flow rates and designed to plate 200 mm wafers. Wafers seeded with a seed copper layer, about 1500Å thick and applied by physical vapor deposition (PVD), were placed in the cell, face down, and cathodic contacts were made at their circumference. A soluble copper anode was placed about 4″ below, and parallel to, the plated wafer. The maximal current density that could be applied, without ‘burning’ the deposit and getting a discolored dark brown deposit, was limited to 6 mA/cm 2 . Under these conditions (6 mA/cm 2 ), the copper seeded wafer was plated for about 12 minutes to produce a deposit thickness of about 1.5 μm. The copper thickness distribution as determined from electrical sheet resistivity measurements was worse than 10% at 1 sigma. Also noted was the terminal effect which caused the deposit thickness to be higher next to the current feed contacts on the wafer circumference.  
       EXAMPLE VII  
       [0040]    The procedure of example VI was repeated except that no acid was added to the solution. Also the copper concentration was brought up to about 0.8M. Using the same hardware (plating cell) of example VI, same flow, etc. it was now possible to raise the current density to about 40 mA/cm 2  without generating a discolored deposit. Seeded wafers were plated at 25 mA/cm 2  for about 3 min to produce the same thickness (about 1.5 μm) of bright, shiny copper. The thickness distribution was measured again (using electrical resistivity as in example VI) and was found to be 2-3% at 1 sigma. The terminal effect was no longer noticeable.