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  Pulse plating process for deposition of gold-tin alloyUSPTO Application #: 20060237324Title: Pulse plating process for deposition of gold-tin alloy Abstract: The invention relates to a solution for use in connection with the deposition of a gold-tin alloy on an electroplatable substrate. This solution generally includes water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions. Optionally, a deposit conditioning agent can be added to the solution to assist in providing a uniform and bright deposit. The solution has a pH of between about 2 and about 10 and the deposit has a gold content of between about 65% to about 90% by weight and a tin content between about 10% and about 35% by weight. An advantageous way for providing a stable gold-tin deposit is by a pulse plating technique. (end of abstract) Agent: Winston & Strawn LLP - Washington, DC, US Inventors: USPTO Applicaton #: 20060237324 - Class: 205247000 (USPTO) Related Patent Categories: Electrolysis: Processes, Compositions Used Therein, And Methods Of Preparing The Compositions, Electrolytic Coating (process, Composition And Method Of Preparing Composition), Depositing Predominantly Alloy Coating, Gold Is Predominant Constituent The Patent Description & Claims data below is from USPTO Patent Application 20060237324. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation-in-part of U.S. application Ser. No. 10/847,468 filed May 18, 2004 which claims the benefit of U.S. provisional application 60/472,826 filed May 21, 2003, and a continuation-in-part of U.S. application Ser. No. 11/337,246 filed Jan. 19, 2006 which claims the benefit of provisional application 60/645,949 filed Jan. 21, 2005. The entire content of each prior application is expressly incorporated herein by reference thereto. BACKGROUND OF THE INVENTION [0002] Alloys of gold and tin, particularly in the compositional range of about 75 to 80 percent gold by weight, are useful as solders for the interconnection of electronic components. Gold-tin alloys are also useful in many microelectronic applications including chip bonding and wafer bump plating. The 80-20 wt % (70-30 at %) gold-tin eutectic alloy is particularly desirable as a solder. The alloy may be applied by vacuum deposition or as a solid preform, however, electrodeposition, due to its low cost, is a preferred method of application. It is generally acknowledged that the ability to electroplate such materials allows the flexibility to deposit overall or in selected areas at will, and to adjust deposit thicknesses as required. For these reasons, much effort has been made to develop workable electroplating systems for these alloys, and numerous references are provided in the literature about such systems. [0003] Prior art electroplating baths for the deposition of gold-tin alloy have been found to be incapable of depositing the eutectic alloy over a usable current density range. This was clearly demonstrated in "Film growth characterization of pulse electro deposited Au/Sn tin films" by Djurfors and Ivey (GaAs MANTECH, 2001), where in FIG. 1 they show a step transition from 16 at % Sn to 50 at % at a current density of around 1.5 mA/cm.sup.2. According to the authors this is a result of the deposition of two distinct phases: Au.sub.5Sn (16 at % Sn) at low current density and AuSn (50 at % Sn) at high current density. This has been further confirmed by test work which has shown that prior art electrolytes will not typically yield the desired eutectic alloy. [0004] The prior art electrolytes, using complexing agents such as citric acid, pyrophosphate, gluconic acid, ethylene diamine tetra acetic acid ("EDTA"), and the like, typically yield alloys which are either tin rich (<50% Au) or gold rich (95% Au), or have tin rich or gold rich regions at different current densities. An 80/20 wt % eutectic gold-tin alloy cannot be deposited over a usable current density range. Moreover, many prior art baths suffer from poor stability making them of little practical interest. [0005] U.S. Pat. No. 4,634,505 by Kuhn, et al. describes a bath using trivalent cyanide gold complex and a tin IV oxalate complex, which operates at pH below 3, but this bath gives deposits with less than 1% Sn, and is not useful for depositing a eutectic alloy. [0006] U.S. Pat. No. 4,013,523 by Stevens et al. describes a bath using trivalent gold complex and tin as stannic halide complex. The pH is less than 3 and the bath allegedly is capable of depositing an 80-20 wt % gold alloy. [0007] U.S. Patent Application No. 2002063063-A1 by Uchida et al. describes a non-cyanide formulation where the gold complexes used include gold chloride, gold sulfite, gold thiosulfite among others. The electrolyte includes stannic and stannous salts of sulfonic acids, sulfosuccinates, chlorides, sulfates, oxides and oxalates. The tin is complexed with EDTA, DTPA, NTA, IDA, IDP, HEDTA, citric acid, tartaric acid, gluconic acid, and glucoheptonic acid among others. The deposit is brightened by a cationic macromolecular surfactant. [0008] Japanese patent application 56136994 describes a solution which uses sulfite gold complex in combination with stannous tin pyrophosphate complex at a pH of 7 to 13. [0009] German patent DE 4406434 describes a solution using the trivalent cyanide gold complex in conjunction with stannic tin complexes. The pH is 3-14 and an 80-20 eutectic alloy deposit allegedly may be provided. [0010] U.S. Pat. No. 6,245,208 by Ivey et al. discloses a non-cyanide formulation which uses gold chloride in combination with sodium sulfite, stannous tin, a complexing agent (ammonium citrate), and uses ascorbic acid as a stabilizer. Eutectic alloy deposits are claimed and bath stability on the order of weeks is reported. [0011] As noted above these baths are not always stable and have been found to be insufficient in providing eutectic gold tin alloys on electroplatable substrates. [0012] Djurfors and Ivey (GaAs MANTECH, 2001) describe a method in which on-off pulsing current is used and alternating plating period of higher and lower on-off pulsing current are applied during electroplating. The said time periods are on the order of minutes and a deposit consisting of distinct layers of different alloy composition gold tin alloy are formed. This layered deposit may then be annealed to form a homogeneous deposit of the desire composition. [0013] Accordingly, there is a need for a process for electroplating deposits of a eutectic gold-tin alloy on various substrates, and this is now provided by the present invention. SUMMARY OF THE INVENTION [0014] The invention relates to a method for electroplating gold-tin alloy deposits on a substrate where the gold content is between about 65% to less than 90% by weight and a tin content of above 10% to about 35% by weight. The method comprises contacting the substrate with one of the solutions described herein, such solutions generally comprising water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions, with the solution having a pH of between about 2 and about 10 so that the deposit will have the gold and tin contents mentioned above. Preferably, the alloy content is near the eutectic compositions of 80% by weight gold and 20% by weight tin, but can be in the range of 75% to less than 85% gold and 15% to 25% tin. [0015] The gold-tin deposits are generally matte and of the desired alloy content. If desired, the solution can further comprise a deposit conditioning agent in an amount sufficient to enhance the appearance or uniformity of the gold-tin deposit. Preferred deposit conditioning agents include 2,2'-dipyridyl, polyethylene imines, and many others and are present in an amount effective to enhance the smoothness or brightness of the gold-tin deposit. [0016] A pulsed current is applied though the solution to provide the gold-tin alloy electrodeposit upon the substrate. This is used to achieve stable deposits of the desired gold-tin alloy. The pulsed current preferably comprises an uninterrupted, sequential, off-on, continuously repeating pulsing sequence over a base current applied across plating cell electrodes that applies high and low current densities in the solution for predetermined millisecond time periods. The pulsed current is generally off from about 1 to 25 milliseconds and then is turned on for about 1 to 25 milliseconds to provide the pulsed current. A typical base current density is between about 1 ASF and about 20 ASF with the pulsed current density ranging from 0.1 to 8 ASF. A preferred base current density is between about 2 ASF and about 10 ASF with the pulsed current density ranging from 0.2 to 5 ASF. The pulsed current is preferably on for a shorter time than when it is off. For example, the pulsed current can be off for between about 5 milliseconds and about 10 milliseconds followed by being on for about 1 millisecond to about 4 milliseconds. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0017] It has now been found that an alloy with a significant tin content, exemplified by the eutectic 80/20 wt % gold-tin alloy, can be deposited. Thus, while alloys such as about 70% gold--about 30% tin and about 90% gold--about 10% tin are obtainable, the eutectic alloy, or as close to the eutectic alloy as possible, is preferred due to the well known advantages of such an alloy. [0018] As used herein, the term "about," when modifying a numerical value, is used to refer to a variance ranging from 0% to 20% of the value of the number being modified. For example, the term "about 20" refers to a numerical value of 20 (0% variance) or a numerical range of 18-22 (10% variance) or a maximal numerical range of 16 to 24 (20% variance). As will be clear to others skilled in the art, other numerical ranges are contemplated by the invention. [0019] In one embodiment, the electrolyte solution comprises water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, and complexed gold ions. If desired, a deposit conditioning agent can be added to the solution in an amount sufficient to enhance the appearance or uniformity of the gold-tin deposit. As noted above, any one of a wide variety of such agents can be used, including 2,2'-dipyridyl or polyethylene imines, and are present in an amount effective to enhance the smoothness or brightness of the gold-tin deposit. The skilled artisan can conduct routine tests to determine the optimum agents for any particular electrolyte. [0020] The tin ions can be added in any solution soluble form that provides stannous ions. Any two-valent tin salt, including sulfate, chloride, methane sulfonate, oxalate, or any other suitable stannous tin salt, can be used to provide these stannous ions, and the specific tin salt is not critical. Stannic tin may also be added to the solution; however, some stannous tin must be present in the electrolyte for the invention to function properly. This is because a non-alkaline electrolyte containing only stannic tin ions will not provide any appreciable amounts of tin in the deposit and will instead result in a deposit that is almost pure gold. The stannous tin ion concentration in the inventive solution is between about 1 g/l and about 20 g/l and more preferably between about 2 g/l and about 10 g/l. Continue reading... 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