Patent Application
20060163080
Gold-tin alloys are 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.
Prior art electroplating baths for the deposition of gold-tin alloy have been found by the current inventor 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/cm2. According to the authors this is a result of the deposition of two distinct phases: Au5Sn (16 at % Sn) at low current density and AuSn (50 at % Sn) at high current density. This has been further confirmed by our work which has shown that prior art electrolytes will not typically yield the desired eutectic alloy.
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.
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. The formulation also uses oxalic acid as a conducting salt. However, this bath gives deposits with less than 1% Sn, and therefore is not useful for depositing a eutectic alloy.
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.
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. Oxalate is listed among the possible buffer compounds.
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.
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.
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.
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, particularly when small parts for electronic components or composite substrates are to be plated.
U.S. Pat. Nos. 6,071,398 and 6,402,924 disclose methods of electrodepositing a metal such as copper onto a substrate. The methods comprise applying a pulsed periodic reverse current across the electrodes of a plating cell utilizing a peak reverse current density and peak forward current density; and varying the ratio of peak reverse current density to peak forward current density in periodic cycles to provide metal deposits of uniform thickness and appearance upon the substrate. The deposition of metal alloys is not mentioned in these patents.
Polyalkylene imines are generally known in the art as useful additives to gold electroplating baths. U.S. Pat. No. 3,864,222 discloses that polyalkylene imines can be incorporated into gold and gold alloy plating baths as agents for the general improvement of the brightness and other properties of the resulting electrodeposit. While certain alloys of gold-silver and gold-nickel are disclosed, gold tin is not mentioned, possibly due to the difficulties in obtaining a stable bath of such alloys.
Accordingly, there is a need for a stable electroplating bath for the deposition of a eutectic gold-tin alloy on various substrates, and this is now provided by the present invention.
The invention relates to a solution for use in connection with the deposition of a gold-tin alloy on an electroplatable substrate. Thus solution generally comprises water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, complexed gold ions, and an alloy stabilization agent that includes an imine functional group. The alloy stabilization agent is present in an amount sufficient to stabilize the deposited alloy and enable a eutectic gold-tin deposit to be provided over a usable current density range. The solution has a pH of between about 2 and about 10 and the deposit having a gold content of less than 90% by weight and a tin content greater 10% by weight.
Polyalkylene imines are advantageously used as alloy stabilization agents, with polyethylene imines having a molecular weight of between about 600 and about 2000 being most preferred. The deposit preferably has a gold content of between about 70% and about 80% by weight and a tin content of between about 20% and about 30% by weight.
The complexing agent for the stannous tin ions generally is an organic acid or a salt thereof, with oxalic acid, citric acid, gluconic acid, malonic acid, ascorbic acid, iminodiacetic acid or a solution soluble salt thereof being preferred. The complexed gold ions are advantageously gold cyanide or gold sulfite complexes.
The invention also relates to a method for electroplating of a gold-tin alloy deposit on a substrate. The method comprises contacting the substrate with a solution comprising water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, complexed gold ions, and an alloy stabilization agent, with the solution having a pH of between about 2 and about 10 so that the deposit will have a gold content less than 90% by weight and a tin content greater than 10% by weight. A pulsed current is applied though the solution to provide the gold-tin alloy electrodeposit upon the substrate.
The pulsed current preferably comprises an uninterrupted, sequential, off-on, continuously repeating pulsing sequence 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.
Alternatively, a base current can be provided with the pulsed current being applied upon the base current. A typical base current is between about 1 ASF and about 20 ASF with the pulsed current ranging from 0.1 to 8 ASF. A preferred base current is between about 2 ASF and about 10 ASF with the pulsed current 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.
This method is preferably used for electroplating a gold-tin alloy deposit on composite articles that include electroplatable and non-electroplatable portions in order to provide a gold-tin alloy metal electrodeposit on the electroplatable portions of the articles without deleteriously affecting the non-electroplatable portions of the articles.
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 over a usable range of current densities from the electrolytes disclosed herein. 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.
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.
The electrolyte solution comprises water, stannous tin ions, a complexing agent to render the stannous tin ions soluble, complexes gold ions, and an alloy stabilization agent.
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.
Also, the concentration of stannous ions may be adjusted in relation to the gold ion concentration to provide the desired alloy. FIG. 2 illustrates the relationship between metal ion concentration ratio in the solution and the composition of the deposited alloy. One of ordinary skill in the art can optimize the metal concentrations in any particular solution to obtain the desired gold-tin alloy.
An antioxidant or reducing agent is preferably included to help maintain the tin ions as stannous tin. Catechol, hydroquinone, or phenolsulfonic acid, or other agents known it the art to prevent tin oxidation can be used with catechol being preferred. The amount of this agent is between about 0.1 g/l and about 5 g/l, and preferably between about 0.5 g/l and 2 g/l.
A complexing agent is present in the solution to assist in rendering and maintaining the stannous tin ions soluble in the solution at the operational pH. Any suitable organic acid can be used for this purpose. Examples of complexing agents useful in the present invention include but are not limited to oxalic acid, citric acid, ascorbic acid, gluconic acid, malonic acid, and iminodiacetic acid. Generally, carboxylic acids are preferred, but iminodiacetic acid and ascorbic acid, which are not carboxylic acids, are also preferred complexors. Moreover, any other complexing agent that can complex the stannous tin in the solution can be used. The most preferred complexing agents are oxalic, citric, gluconic and malonic acids. Salts of these acids can also be used.
The complexing agent is present in the solution in at least a sufficient concentration to maintain the stannous tin soluble at the solution pH. Additionally, it is desirable to maintain an excess of complexing agent beyond the minimum concentration to improve solution conductivity and to provide pH buffering. The complexer concentration is typically between about 10 g/l and about 300 g/l and is most preferably between about 40 g/l and about 150 g/l.
The gold ions are preferably provided in the solution as a gold cyanide complex, most preferably monovalent gold cyanide, although, trivalent gold cyanide may also be used. Non-cyanide sulfite gold complex can also be used in the present invention when short solution lives are acceptable; otherwise this complex would not be preferred as the stability of this complex is inferior to the others. The most preferred gold ion complex is potassium gold cyanide. The preferred concentration of gold ion complex in the present invention is between about 2 g/l and about 20 g/l and most preferably between about 3 g/l and about 10 g/l.
In the present invention it has been found that the addition of an alloy stabilization agent having an imine functional group will produce an electrolyte, which will deposit the desired eutectic or similar alloys over an acceptable range of current densities. In the absence of such an additive the deposit may be either tin or gold rich or may have tin or gold rich regions in different areas caused by different current densities.
The alloy stabilization agent is important to the proper operation of the present invention. The most effective alloy stabilization agents are those which have imine functional groups. The preferred alloy stabilization agents are polyalkylene imine compounds, with polyethylene imine polymers being most preferred. These polymers are formed by the polymerization of ethylene imines or substituted ethylene imines or are derived from the addition of ethylene imines to organic or inorganic molecules. Typical polymers include polyethylene imines, polypropylene imines, polyhydroxyethylene imines, polyethylene imine adducts, and ethylene imine adducts. Of these, the polymers having molecular weights of 600 to 2000 have found to be most effective. The concentration of the alloy stabilization agent is in the range of 0.1 to 10 ml/l and is most preferably in the range of 0.5 to 2 ml/l.
Other additives can be added to the solution to modify the grain structure of the deposit. These include metallic additives such as nickel, cobalt, arsenic, lead, thallium, or selenium. Organic additives such as those described in U.S. Patent Application No. 2002063063 may also be used, if desired. Additionally, other salts or buffers may be optionally added to the electrolyte to improve conductivity or pH stability. Examples, of such additives include simple salts such as potassium MSA, potassium sulfate, as well as others well known in the art.
The pH of the electrolyte is between about 2 and about 10, is generally less than 8 and is most preferably between about 3 and about 5.5. The preferred pH of the solution is generally dependent upon the gold complex that is used. For instance, potassium gold cyanide is not stable below a pH of 3.5, but a trivalent cyanide gold complex is stable at lower pH values. Sulfite gold complexes are generally not stable below pH 6 and are most stable at pH 8 and higher. Since the solution of the present invention is useful in microelectronics applications, it is desirable to have a pH of less than 8 and preferably less than 7 to prevent solution attack on photoresist masks that are often applied to the electrodeposition substrates. Additionally, it has been found that deposit appearance of tin containing alloys begins to degrade at pH values greater than about 4.7. The pH can be adjusted to the desired ranges using an acid or base, as necessary.
The solution temperature is typically between about 20° C. and about 70° C. and is most preferably between about 38° C. and about 50° C. Temperature has a direct effect on the composition of the deposited alloy, with higher temperature resulting in higher gold concentrations. FIG. 1 shows the relationship between electrolyte temperature and deposit alloy composition. As is clearly shown the alloy composition can be varied very effectively by controlling temperature.
The electrolyte of the present invention may be operated using insoluble anodes including platinized titanium, platinized niobium, or iridium oxide electrode. It is also possible to use soluble anodes, however, this is not typically practiced in precious metals plating.
An advantageous way for providing the desired deposit is by a pulse plating technique. Gold/Tin deposits can be controlled to a large extent by bath chemistry, but plating at varying current densities can change the proportions of the metals in the deposit. For example, at a given gold to tin ratio in the bath, the alloy of the plated deposit will very greatly by increasing or decreasing the current density. At higher current densities, the deposit tends to be higher in gold content, while at lower current densities, the deposit tends to be higher in tin. Thus, variations from the desired eutectic 80 gold-20 tin deposit can be encountered depending upon applied current density. Furthermore, it is not always possible to plate at optimum current densities to obtain the desired alloy. i.e., burning can occur under high current density conditions, while dullness of the deposit is obtained under low current density conditions. Gold to tin ratios, pH, additives and temperature of the solution also have an effect on the plated alloy, and are controllable to a large degree, but often a more precise control is desired.
This additional control is now provided by the present invention. By applying a pulse plating technique, several advantages are obtained. The content of the alloy deposit can be more accurately controlled by varying the applied current density. Pulse plating also decreases plating time when thick deposits (5-25 microns) are to be provided. In addition, the possibility of burning in the high current density portions on parts with sharp geometries is reduced or eliminated.
The process is primarily suited for plating typical substrates such as metallized silicon wafers with photo resist masking or a metallic substrate that has a photo resist masking. The metallic or metallized portions are readily platable while the masking is not easily platable, thus enabling the deposited metal to be applied in a circuit or other metal pattern.
As noted, the pulsed current preferably comprises an uninterrupted, sequential, off-on, continuously repeating pulsing sequence across plating cell electrodes that applies high and low current densities in the solution. One way to do this is to apply a pulsed current that is at a low value from about 1 to 25 milliseconds and then is increased to a higher current density for a second period of about 1 to 25 milliseconds to provide the pulsed current.
A preferred way to do this is to provide a constant base current and then apply a smaller, pulsed current upon the base current. A typical base current is between about 1 ASF and about 20 ASF with the pulsed current ranging from 0.1 to 8 ASF. A preferred base current is between about 2 ASF and about 10 ASF with the pulsed current ranging from about 0.2 ASF to about 5 ASF. The pulsed current is preferably on for a shorter time than when it is off.
The most preferred embodiment includes pulse plating using a combination of a constant current with a superimposed pulsed current. The use of a constant D.C. current density at low current densities of about 4 ASF to about 5 ASF produces a gold rich alloy at good deposition rate. By adding a pulsed current, such as about 1 ASF to about 1.5 ASF at about 2 seconds on and about 8 seconds off using a separate rectifier wired in parallel, the electrolyte is refreshed at the plated interface and then is spiked at the higher current density, thus reducing the possibility of burning the deposit. An additional benefit is the ability to control the tin accurately in the deposit by simply increasing the current on the pulse rectifier. For example, the pulsed current can be off for between about 5 milliseconds and about 10 milliseconds followed by being on for 1 to 4 milliseconds to achieve optimum results for the bath chemistries disclosed herein.
With the present invention, the interval of the direct current pulses is on the order of milliseconds to seconds. The alloy composition may be adjusted by varying the relative lengths of the intervals. The desired pulsing direct current may be obtain by connecting a constant current DC rectifier in parallel with a pulsing (on/off) constant current power supply. This configuration produces a current source which fluctuates between high and low current densities. By way of non-limiting example, values which have been successfully used are a low current density of 5 ASF for 8 milliseconds and a high current density of 6.25 ASF for 2 milliseconds. Other values can be used and the previous conditions are given as one preferred example only. A skilled artisan can determine by routine testing the most preferred pulse plating conditions for any particular electroplating solution to achieve the desired alloy content of the deposit.
Other electroplating processes of the invention may include two constant voltage power supplies connected in series to produce two alternating voltages, or a programmable power supply such as a galvanostat or potentiostat used to supply the current for the plating process. Moreover, other wave forms, such as a sinusoidal wave form, can be superimposed on a direct current and act in the same fashion as the pulsed current processing technique described herein.
The following examples illustrate useful embodiments of the invention.
A eutectic gold-tin alloy electrodeposit is obtained from the following solution and under the following electroplating conditions.
The above electrolyte will deposit a matte to semibright 80-20 wt % gold-tin alloy at current densities up to 10 ASF at temperatures between 105° F (40.5° C.) and 120° F. (48.8° C.).
A eutectic gold-tin alloy electrodeposit is obtained from the following solution and under the following electroplating conditions.
The above electrolyte will deposit a matte to semibright 80-20 wt % gold-tin alloy at current densities up to about 10 ASF at temperatures between 100° F. (37.7° C.) to 120° F. (48.8° C.).
A eutectic gold-tin alloy electrodeposit is obtained from the following solution and under the following electroplating conditions.
The above electrolyte will deposit a matte to semibright 80-20 wt % gold-tin alloy at current densities up to 10 ASF at temperatures between 105° F. (40.5° C.) and 130° F. (54.4° C.).
A eutectic gold-tin alloy electrodeposit is obtained from the following solution and under the following electroplating conditions.
This electrolyte solution was utilized to demonstrate the use of pulses of direct current of different current densities to electrodeposit gold-tin alloys. Briefly, copper coupons (1.25″×1.25″) were plated in the electrolyte at a temperature of about 110° F. (45° C.).
Coupon #1 was plated using a conventional 5 ASF DC current and yielded a deposit of 86 wt % gold as measured by EDAX; the melting temperature was measured as 590° F. (310° C.) to 610° F. (320° C.).
Coupon #2 was plated alternately at 5 ASF for 8 ms and 2 ms at 6.25 ASF. This yielded a deposit of 76 wt % gold as measured by EDAX; the melting temperature was measured as about 535° F. (280° C.).
The results demonstrate that the pulsed plating technique of the present invention can help provide a more uniform alloy deposit, one that is closer to the eutectic composition so that a lower melting (or reflow) temperature can be used.
This experiment was designed to test the effect of pulse rectification modification on increasing tin in the deposit. As a first step, the following one liter solution was prepared with a Gold:Tin ratio of 1:1.25.
The bath was maintained at a temperature of 110° F. (43.33° C.) and agitated with a stir bar during electroplating. A 1.25″×1.25″ copper lid was utilized for the electroplating of all test samples. The results are as follows:
Sample 1A was electroplated at 5 ASF (No Pulse). The resultant gold:tin alloy had a thickness of 5 microns, a composition of 82.1% gold, an SEM-measured gold value of 86.2% and a melting temperature of 590-608° F. (310-320° C.).
Sample 1B was electroplated at 5 ASF with pulsing at 2.5 ASF on for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 5 microns, a composition of 72.7% gold, an SEM-measured gold value of 80.06% and a melting temperature of 572° F. (300° C.).
Sample 1C was electroplated at 5 ASF with pulsing at 5 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 5 microns, a composition of 70.6% gold, an SEM-measured gold value of 83.48% and did not remelt to liquid.
Sample 1D was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 5 microns, a composition of 75.9% gold, an SEM-measured gold value of 83.24% and a melting temperature of 617° F. (325° F.).
Sample 1E was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 10 microns, a composition of 75.5% gold, an SEM-measured gold value of 75.94% and a melting temperature of 527° C. (275° F.).
Sample 1F was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 15 microns, a composition of 76.7% gold, an SEM-measured gold value of 74.96% and a melting temperature of 572° F. (300° F.).
Sample 1G was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 20 microns, a composition of 77.2% gold, an SEM-measured gold value of 76.1 and a melting temperature of 536° F. (280° C.).
Sample 1H was electroplated at 5 ASF with pulsing at 1.25 ASF for 2 milliseconds and an 8 millisecond off interval. The resultant gold:tin alloy had a thickness of 25 microns, a composition of 77.4% gold, an SEM-measured gold value of 73.8% and a melting temperature of 527° F. (275° F.).
Alloy thickness and measurement of percent Au was determined by X-ray diffraction and represent an average from 5 measurements. (Numerical values with sample C were obtained with 3 measurements) Scanning Electron Microscopy was also used to separately measure the gold percentage of the alloy.
These results of the experiment also indicate that the pulsed plating technique of the present invention can help provide a more uniform alloy deposit, one that is closer to the eutectic composition so that a lower melting (or reflow) temperature can be used.
This application claims the benefit of provisional application 60/645,949 filed Jan. 21, 2005, the entire content of which is expressly incorporated herein by reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| 60645949 | Jan 2005 | US |
