Method for preparing gold-silver alloy plating layer, corresponding plating layer and product

Abstract

Disclosed are a method for preparing a gold-silver alloy plating layer, a corresponding plating layer, and a product, belonging to the technical field of electroplating. The method for preparing a gold-silver alloy plating layer comprises the following steps: pretreating a workpiece to be electroplated, preparing a gold-silver electroplating solution, and setting an electroplating temperature; measuring and determining a low current density range and a high current density range, placing the workpiece to be electroplated into the electroplating solution, and controlling a current density to periodically switch between the low current density range and the high current density range; after obtaining a gold-silver plating layer having a predetermined gold-silver deposition amount, ending the electroplating; and annealing the gold-silver plating layer. The present invention eliminates the influence of slight current density fluctuations on gold content in the gold-silver alloy plating layer, thereby solving the technical problem, caused by photoresist openings, of random differences in gold content being present in a gold-silver alloy plating layer at different positions. By means of adjusting the duration of the high current density range and the low current density range time cycles to control the thickness of each plating layer, the gold content in the final plating layer can be accurately controlled.

Description

A method for preparing gold-silver alloy coating and corresponding coating and product Technical Field

[0001] The present invention belongs to the field of electroplating technology and relates to a method for preparing a gold-silver alloy coating and corresponding coatings and products, and in particular to a method for preparing a gold-silver alloy coating for packaging lead frames, liquid crystal driver chips, optoelectronic chips, and the like. Background Art

[0002] In the semiconductor industry, due to gold's excellent properties, pure gold electroplating is often used in chip packaging structures to achieve electrical interconnections between the chip and the substrate. This is widely used in liquid crystal displays, image sensors, memory, microprocessors, and microwave radio frequency chips. The corresponding end-use electronic products include mobile phones, televisions, computers, tablets, and cameras. Gold is very expensive. Silver, another precious metal, also has good electrical conductivity and oxidation resistance, but its price is much lower than gold. If silver could partially replace gold—that is, electroplating a gold-silver alloy instead of pure gold—the cost of chip packaging structures could be greatly reduced (pure silver packaging is subject to issues such as silver pins, silver migration, and susceptibility to oxidation and sulfidation).

[0003] Alloy electroplating involves the co-deposition of two or more metal ions. The fundamental requirement for metal co-deposition is that the deposition potentials of the two metals must be very close or equal, so few metals can achieve co-deposition. Examples of metal pairs that can achieve co-deposition include Pb (-0.126 V) and Sn (-0.136 V), Ni (-0.25 V) and Co (-0.277 V), and Cu (+0.34 V) and Bi (+0.32 V), whose standard potentials are very close. For metals with significantly different equilibrium potentials, co-deposition can be achieved by adding a suitable complexing agent to the plating solution to bring the deposition potentials of the two metal ions closer together. For example, in the co-deposition of zinc and silver, although the equilibrium potential of silver is 1.5 V more positive than that of zinc, in a cyanide system, the deposition potential of silver is even more negative than that of zinc.

[0004] For electroplating gold-silver alloy, since gold and silver are precious metals, the currently commercially available system is still mainly based on the cyanide system. Although there are reports on electroplating gold-silver alloy using non-cyanide systems (such as US11255021B2, US7938948B2 and JP3300519B2), these systems are generally less stable than cyanide systems. This is because the reaction between cyanide ions and gold ions (Au + / Au(CN)2 - Stability constant ~10 38 ) and silver ions (Au + / Au(CN)2 - Stability constant ~10 22) has a high complexation coefficient, and the amount of free gold and silver ions in the solution is very small, so the cyanide system plating solution has high stability. The equilibrium potential of gold and silver deposition in the cyanide system can be calculated based on the stability coefficient:

[0005]

[0006] Therefore, in theory, it is difficult to obtain a gold-silver alloy coating with a stable gold content within a wide current density operating range using a cyanide system for gold-silver alloy electroplating.

[0007] Regarding the existing technology for preparing gold-silver alloys by electroplating, in addition to the aforementioned patents for cyanide-free gold-silver alloy electroplating, patents such as EP2879169B1, TWI495766B, CN101225536B, DE4406419C1, JP1987164890A, GB2046794A, CH629260A5, US4121982A, SU665027A1, JP1985014115B2, GB1070683A, and GB1023339A disclose technical solutions for cyanide-containing gold-silver alloy electroplating. The common feature of these technical solutions is the use of constant current direct current technology to electroplating the gold-silver alloy.

[0008] In semiconductor manufacturing, horizontal cup or vertical rack plating machines are typically used to form a metal layer on the wafer surface through electroplating. The different electric field distributions (i.e., current densities) in the center and edge regions of the wafer, or in areas with dense and sparse photoresist openings, result in different electroplating heights. While baffles can be used to adjust the electric field distribution between the center and edge regions of the wafer, the differences in current density caused by the different photoresist openings remain a currently unresolved technical issue. Given that each customer's chip pattern design is unique, these technical issues are common.

[0009] As previously analyzed, due to the significant difference in the precipitation potentials of gold and silver ions in the plating solution, it is difficult to produce a gold-silver alloy coating with a specified gold content within a wide current density range. Furthermore, since varying photoresist openings are common in semiconductor manufacturing, this inevitably leads to variations in current density across different areas during electroplating, resulting in random variations in gold content across different locations within the gold-silver alloy coating. Technical issues

[0010] The present invention aims to provide a method for preparing a gold-silver alloy coating, the corresponding coating, and the product. By periodically switching between experimentally determined low and high current densities, stable, uniform low-gold and high-gold coatings can be obtained within these low and high current density ranges, respectively. After electroplating, annealing is performed to form a gold-silver intersolvent, ensuring a consistent gold content throughout the coating. Because the gold content of each coating layer formed during the electroplating process is stable and uniform, the thickness of each coating layer can be controlled by adjusting the time period between the low and high current density intervals, thereby precisely controlling the gold content of the final coating. This objective of the present invention is achieved through the following technical solutions. Technical Solutions

[0011] One aspect of the present invention is to provide a method for preparing a gold-silver alloy coating, comprising the following steps:

[0012] S1 pre-treats the workpiece to be electroplated.

[0013] S2 prepares gold and silver electroplating solution and sets the electroplating temperature.

[0014] S3: Test the gold-silver electroplating solution of step S2 at the electroplating temperature set in step S2 to obtain a relationship curve between the gold content in the gold-silver alloy coating and the current density; based on the relationship curve, determine the change rate of the gold content in the coating relative to the current density at 15wt% / ASD (ASD is A / dm 2 ) or less. Within the low and high current density ranges, current density fluctuations have little effect on the gold content in the coating, and a coating with a stable and uniform gold content can be obtained.

[0015] S4: Place the pretreated workpiece to be electroplated into the gold-silver electroplating solution of step S2, control the current density to be in the low current density range or the high current density range determined in step S3, maintain it for a time period, then switch the current density to the high current density range or the low current density range, maintain it for another time period, and repeat this periodic switching; after obtaining the gold-silver electroplating layer with the predetermined gold and silver deposition amount, the electroplating is terminated.

[0016] In the present invention, the definition of the rate of change R of the gold content in the coating relative to the current density is expressed by the following formula:

[0017]

[0018] Where R is the rate of change of the gold content in the coating with respect to the current density, and the unit is wt% / ASD; J1 is a certain current density value, and J2 is a current density value greater than J1, and the unit is ASD; C1 is the gold content in the coating when the current density is J1, and C2 is the gold content in the coating when the current density is J2, and the unit is wt%.

[0019] After careful research, the inventors discovered that the gold-silver alloy electroplating process exists in low and high current density ranges. Within these ranges, current density fluctuations have little effect on the gold content in the coating, resulting in a coating with a stable and uniform gold content. Outside these ranges, current density fluctuations either significantly affect the gold content in the coating, or gold and silver are difficult to co-deposit. The presence of these two current density ranges is universal in the gold-silver alloy electroplating process, a phenomenon determined by the electrochemical properties of gold and silver. The specific values ​​of these two current density ranges vary with the composition of the gold-silver electroplating solution and the electroplating temperature, and require experimental determination.

[0020] The inventors utilized this discovery to control the current density during the gold-silver alloy electroplating process, switching it periodically between low and high current density ranges determined through experimental testing. This eliminated the impact of small current density fluctuations on the gold content in the gold-silver alloy coating, thereby resolving the technical issue of random variations in gold content in the gold-silver alloy coating at different locations due to photoresist openings. By adjusting the time periods of the high and low current density ranges, and thereby controlling the thickness of each coating layer, the gold content in the final coating can be accurately controlled. A detailed analysis is provided below.

[0021] Assume that the gold contents in the gold-silver alloy coatings obtained in the high current density range and the low current density range are c high and c low The target gold content in the final gold-silver alloy coating product is c, and the unit is wt%. The relationship between these three values ​​is c low <c<c high Assume that the target thickness of the final gold-silver alloy coating product is T, and correspondingly, the mass of the final gold-silver alloy coating product is m, and there is a corresponding relationship between the two. Assume that the total thickness of the gold-silver alloy coating obtained in the high current density range and the low current density range is T respectively. high and T low , correspondingly, their respective masses are m high and m low .

[0022] m=m high +m low

[0023] m×c=m high ×c high+m low ×c low

[0024] As can be seen from the above formula, the target gold content can be achieved by controlling the mass ratio of the gold-silver alloy coatings obtained in the high current density range and the low current density range, respectively. The mass and thickness of the gold-silver alloy coating are correlated, and the thickness is correlated with the duration and number of electroplating cycles. Therefore, by controlling the ratio of the duration of the high current density range to the duration of the low current density range, the target gold content can be achieved. Controlling the total electroplating duration (i.e., the number of time cycles) can achieve the desired coating thickness.

[0025] Furthermore, the method further includes step S5: annealing the gold-silver electroplated layer to make the gold content in the entire plating layer consistent.

[0026] Furthermore, the duration of each time period in step S4 is 50-1000ms (milliseconds). If the time period is too short, the switching is too frequent, making the operation difficult; if the time period is too long, the annealing time required to ensure a consistent gold content in the final coating is too long, affecting production efficiency.

[0027] Furthermore, the thickness of the gold and silver electroplated layer obtained in each time period in step S4 is 10-500 nm.

[0028] Furthermore, in step S5, the annealing temperature is 250-300°C, and the annealing time is 5-120 minutes. The annealing temperature and annealing time can be determined experimentally within the above parameter ranges based on the area and thickness of the gold-silver alloy electroplated layer. Generally, the larger the area and the thicker the thickness, the higher the annealing temperature and the longer the annealing time required.

[0029] Another aspect of the present invention is to provide a gold-silver alloy coating obtained by the above method, wherein the gold content in the gold-silver alloy coating can be accurately controlled by adjusting the time period of the high current density interval and the low current density interval.

[0030] Another aspect of the present invention is to provide a semiconductor chip comprising the above-mentioned gold-silver alloy plating layer.

[0031] Furthermore, the semiconductor chip includes a liquid crystal display, an image sensor, a memory, a microprocessor and a microwave radio frequency chip.

[0032] Another aspect of the present invention is to provide an electronic product including the semiconductor chip.

[0033] Furthermore, the electronic products include mobile phones, televisions, computers, tablet computers and cameras. Beneficial effects

[0034] The present invention has the following beneficial technical effects: it eliminates the influence of small fluctuations in current density on the gold content in the gold-silver alloy plating layer, thereby solving the technical problem of random differences in the gold content in the gold-silver alloy plating layer at different positions due to photoresist openings, and by adjusting the time period length of the high current density interval and the low current density interval, thereby controlling the thickness of each layer of the plating layer, the gold content in the final plating layer can be accurately controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 shows the electrochemical polarization curves of gold and silver electroplating, respectively.

[0036] Figure 2 is a curve showing the relationship between the gold content and current density in the gold-silver alloy coating using DC electroplating for different electroplating solutions and electroplating temperatures.

[0037] FIG3 is a schematic diagram of electroplating a gold-silver alloy using the electroplating method provided by the present invention. Modes for Carrying Out the Invention

[0038] The following is a clear and complete description of the technical solution of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0039] Figure 1 shows the electrochemical polarization curves for gold and silver electroplating alone. The gold electroplating solution used contained potassium aurous cyanide (molecular formula KAu(CN)2) with a gold ion concentration of 8 g / L, 60 g / L potassium pyrophosphate as a conductive salt, 20 g / L 5,5-dimethylhydantoin, and water as the solvent. The pH of the plating solution was 9. The silver electroplating solution used contained potassium silver cyanide (molecular formula KAg(CN)2) with a silver ion concentration of 4 g / L, 60 g / L potassium pyrophosphate as a conductive salt, 20 g / L 5,5-dimethylhydantoin, and water as the solvent. The pH of the plating solution was 9. The above compositions of the gold and silver electroplating solutions are exemplary and do not constitute limitations of the present invention. Those skilled in the art can prepare gold and silver electroplating solutions with other compositions as needed.

[0040] As shown in Figure 1, the deposition potentials for electroplating gold and silver differ significantly. The initial deposition potential for silver is -0.3 V, while the initial deposition potential for gold is -0.6 V. This indicates that electroplating within the range of 0 to -0.6 V only produces silver deposits. From -0.6 V to -0.75 V, region C in Figure 1, the formation of an electroplated gold-silver alloy begins. Within this range, the deposition current densities for both gold and silver do not vary significantly. Due to the high deposition current density of silver, the gold content in the deposited layer is relatively low. From -0.75 V to -1.23 V, region B in Figure 1, the silver deposition current initially fluctuates significantly, while the gold deposition current remains relatively stable. Then, as silver deposition enters the limiting current diffusion region, the silver deposition current remains relatively stable, while the gold deposition current begins to decline rapidly. This indicates that within this range (region B in Figure 1), due to the large variations in either the silver or gold deposition rates, it is difficult to obtain an alloy coating with a stable gold-silver content. Starting at -1.23V, entering region A in Figure 1, gold deposition enters the current density diffusion zone, and silver also remains in this region, resulting in a gold-silver alloy coating with a stable gold and silver content. It should be noted that starting at -1.38V, the current density for silver electroplating decreases further. This is because hydrogen begins to evolve on the silver surface, which reduces the electroplating efficiency and may increase stress in the coating. Therefore, operation in higher current density regions is not recommended.

[0041] Figure 2 shows the relationship between the gold content and current density in a gold-silver alloy coating using direct current electroplating. Three gold-silver electroplating solutions containing both gold and silver ions were prepared, including: potassium aurous cyanide with a gold ion concentration of 10 g / L, potassium silver cyanide with silver ion concentrations of 5 g / L, 8 g / L, and 10 g / L, respectively; 60 g / L potassium pyrophosphate as a conductive salt; 20 g / L 5,5-dimethylhydantoin; water as the solvent; a plating solution pH of 9; and electroplating at 35°C or 25°C. The above gold-silver electroplating solution compositions are exemplary and do not constitute limitations of the present invention. Those skilled in the art can prepare gold-silver electroplating solutions with other compositions as needed.

[0042] As can be seen from the top curve in Figure 2, when the current density is operated from 0.3ASD to 0.7ASD, the gold content of the coating increases from 22.6wt% to 27.1wt%. The gold content in the coating changes slightly with current density, with a rate of change R of 11.3wt% / ASD. When the current density changes from 0.7ASD to 1.3ASD, the gold content of the coating increases from 27.1wt% to 55.7wt% with increasing current density. The change in current density has a significant impact on the gold and silver content in the coating, with a rate of change R of 47.7wt% / ASD. When the current density changes from 1.3ASD to 1.7ASD, the gold content of the coating increases from 55.7wt% to 60.0wt%. The gold content in the coating changes slightly with current density, with a rate of change R of 10.8wt% / ASD. These results are consistent with the electrochemical polarization curves in Figure 1. In the low current density range or the high current density range, the gold or silver content in the coating is relatively stable because the deposition current of gold or silver changes very little. However, in the intermediate current density range, or because the current density of silver or gold changes greatly, it is difficult to obtain a coating with a stable gold and silver content.

[0043] Figure 2 also shows the relationship between the gold content and current density in the gold-silver alloy coating using DC electroplating, for different gold-silver plating solutions and plating temperatures. When the gold-silver content in the plating solution and the plating temperature vary, the gold-silver alloy electroplating process generally exhibits a high current density range and a low current density range. Within these ranges, current density fluctuations have minimal impact on the gold content in the coating, resulting in a coating with a stable and uniform gold content. The specific values ​​within these two current density ranges vary with the composition of the gold-silver plating solution and the plating temperature, and require experimental determination.

[0044] Based on the above research results, the inventors have obtained a method for preparing a gold-silver alloy coating provided by the present invention, comprising the following steps:

[0045] S1 pre-treats the workpiece to be electroplated.

[0046] S2 prepares gold and silver electroplating solution and sets the electroplating temperature.

[0047] S3. Testing the gold-silver electroplating solution of step S1 at the electroplating temperature set in step S2 to obtain a relationship curve between the gold content in the gold-silver alloy coating and the current density; based on the relationship curve, determining a low current density range and a high current density range in which the rate of change of the gold content in the coating relative to the current density is less than 30%.

[0048] S4: Place the pretreated workpiece to be electroplated into the gold-silver electroplating solution from step S2, and control the current density to periodically switch between the low current density range and the high current density range determined in step S3. The duration of each low current density range and high current density range is 50-1000 ms. Electroplating is terminated after a gold-silver electroplated layer with a predetermined gold and silver deposition amount is obtained.

[0049] S5: Annealing the gold and silver electroplated layers to achieve a uniform gold content throughout the entire layer. The annealing temperature is 250-300°C and the annealing time is 5-120 minutes.

[0050] The electroplating method of step S4 is shown in Figure 3, where the horizontal axis represents the time period and the vertical axis represents the current density. In the high current density range, a coating with a high and stable gold content can be obtained. Then, switching to the low current density range, a coating with a low and stable gold content can be obtained. Repeating this operation can produce a structure with many layers of alternating gold and silver content. high and t low Indicates the time period length of the high current density interval and the low current density interval. By adjusting these two parameters, the thickness of the plating with high gold content and low gold content can be controlled, thereby adjusting the gold content required for the final plating.

[0051] It should be noted that different plating solutions contain different concentrations of gold and silver ions, and the plating temperature varies. The gold content of the plated layer will vary between high and low current density ranges, but this does not affect the uniform gold and silver content of the plated layer obtained in the high and low current density ranges. In practical applications, for plating solutions and plating temperatures with different gold and silver compositions, the gold content of the plated layer obtained in the high and low current density ranges can be tested first. Then, by adjusting the time cycle, the gold-silver alloy plated product with the desired gold content can be obtained.

[0052] The present invention is described in detail below through some specific examples. Example 1

[0053] A gold-silver electroplating bath containing both gold and silver ions was prepared. The bath contained: potassium aurous cyanide with a gold ion concentration of 10 g / L, potassium silver cyanide with a silver ion concentration of 5 g / L, 60 g / L potassium pyrophosphate as a conductive salt, and 20 g / L 5,5-dimethylhydantoin. The solvent was water, the pH of the bath was 9, and electroplating was performed at 35°C. Electroplating was performed at 0.3 ASD for 100 ms, followed by 1.4 ASD for 100 ms, and this cycle was repeated. The total electroplating time was 30 minutes. After plating, the samples were annealed at 290°C for 60 minutes. The gold content of the deposited samples was determined by XRF, and sectioning was performed to determine the uniformity of the gold-silver alloy. Example 2

[0054] The difference from Example 1 is that the electroplating is performed at 0.4 ASD for 200 ms, then switched to 1.3 ASD for 400 ms, and the switching is repeated periodically. The other conditions are the same. Example 3

[0055] The difference from Example 1 is that the electroplating is performed at 0.5 ASD for 100 ms, then switched to 1.5 ASD for 50 ms, and the switching is repeated periodically. The other conditions are the same. Example 4

[0056] The difference from Example 1 is that the electroplating is performed at 0.4 ASD for 300 ms, then switched to 1.6 ASD for 100 ms, and the switching is repeated periodically. The other conditions are the same. Example 5

[0057] The difference from Example 1 is that the electroplating is performed at 0.6 ASD for 800 ms, then switched to 1.3 ASD for 200 ms, and the switching is repeated periodically. The other conditions are the same. Example 6

[0058] The difference from Example 1 is that the plating is performed at 0.7 ASD for 400 ms, then switched to 1.7 ASD for 50 ms, and the switching is repeated periodically. The other conditions are the same. Example 7

[0059] The difference from Example 1 is that the electroplating is performed at 0.5 ASD for 1200 ms, then switched to 1.4 ASD for 300 ms, and the switching is repeated periodically. The other conditions are the same.

[0060] The electroplating conditions, the gold content of the plated layer after annealing, and the degree of gold-silver mixing after annealing of Examples 1-7 are shown in the following table.

[0061]

[0062] As can be seen from the table above, by adjusting the time cycle length, Examples 1-6 can produce gold-silver alloy coatings with gold contents ranging from 24.2 to 49.7 wt%, and section observation shows that the gold and silver coatings are uniformly mixed. In Example 7, however, the gold content varies in different regions, and section observation shows that the gold and silver in the coating are not uniformly mixed. This is because when the time cycle length is too long, the coating thickness is relatively large, and diffusion takes a long time. Although increasing the annealing temperature or extending the annealing time may eventually achieve uniform mixing, this is difficult to accept in actual production because increasing the temperature may affect the quality of the chip, and extending the annealing time will reduce production efficiency.

[0063] Although the embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art may make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention. The scope of protection of the present invention is defined by the claims and their equivalents.

Claims

1. A method for preparing a gold-silver alloy coating, characterized in that: The following steps are involved: S1 pre-treats the workpiece to be electroplated; S2 prepares gold and silver electroplating solution and sets the electroplating temperature; S3. Testing the gold-silver electroplating solution of step S2 at the electroplating temperature set in step S2 to obtain a relationship curve between the gold content and the current density in the gold-silver alloy coating. Based on the relationship curve, determining a low current density range and a high current density range in which the rate of change R of the gold content in the coating relative to the current density is less than 15 wt% / ASD. The rate of change R of the gold content in the coating relative to the current density is defined by the following formula: ， Where, R is the rate of change of the gold content in the coating relative to the current density, in wt% / ASD; J1 is a certain current density value, J2 is a current density value greater than J1, both in ASD; C1 is the gold content in the coating when the current density is J1, and C2 is the gold content in the coating when the current density is J2, both in wt%; S4: placing the pretreated workpiece to be electroplated into the gold-silver electroplating solution of step S2, controlling the current density to be within the low current density range determined in step S3 for a time period, and then switching the current density to the high current density range for another time period; or controlling the current density to be within the high current density range determined in step S3 for a time period, and then switching the current density to the low current density range for another time period; such periodic switching is repeated, with each time period lasting 50-800 ms; after obtaining a gold-silver electroplated layer with a predetermined gold-silver deposition amount, the electroplating is terminated; S5 Annealing treatment of gold and silver electroplating layer.

2. The method according to claim 1, characterized in that The thickness of the gold and silver electroplated layer obtained in each time period in step S4 is 10-500 nm.

3. The method according to claim 1, characterized in that In step S5, the annealing temperature is 250-300° C., and the annealing time is 5-120 min.

4. A gold-silver alloy coating obtained by the method according to any one of claims 1 to 3.

5. A semiconductor chip, characterized in that: The invention comprises the gold-silver alloy plating layer as claimed in claim 4.

6. The semiconductor chip according to claim 5, wherein: The semiconductor chip includes a liquid crystal display, an image sensor, a memory, a microprocessor and a microwave radio frequency chip.

7. An electronic product, characterized in that: Comprising the semiconductor chip according to claim 5 or 6.

8. The electronic product according to claim 7, characterized in that: The electronic products include mobile phones, televisions, computers and cameras.

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