System and method for solder bump plating

- M/A-COM, Inc.

The present invention relates to a system single-metal plating, a system for binary-metal plating, and a system for solder-bump plating. The solder-bump plating system comprises a single-metal plating system for plating a preconditioned substrate to form a single-metal plated substrate; and a binary-metal plating system for plating the single-metal plated substrate. In a preferred implementation, the solder-bump plating system of the present invention is configured to single-metal plate a preconditioned substrate using a first plating solution to provide a single-plated substrate; to single-metal plate the single-plated substrate using a second plating solution to provide a double-plated substrate; and to binary-metal plate the double-plated substrate using a third plating solution to provide a solder-bump plated substrate. A novel apparatus for use in plating a substrate comprises an electrically conductive holder comprising a handle portion having at least one electrical contact and a holder portion having at least one flexible electrical contact.

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Description
FIELD OF THE INVENTION

The invention relates generally to solder bump plating, and more particularly to a system and method for single-metal plating, a system and method for binary-metal plating, and a system and method for solder bump plating.

BACKGROUND OF THE INVENTION

The microelectronic industry has been making a tremendous improvement toward miniaturization of circuitry with greater performance. Similarly the packaging and assembly of integrated circuits is aggressively making the effort to follow the microelectronic trend. As it is commonly known to one of ordinary skill in the art, microelectronic production substrates are commonly fabricated through solder bump plating. In fact, the semiconductor industry as a whole is aggressively adapting and employing microelectronic substrates using solder bumps. Generally, the process of producing microelectronic substrates using solder bump plating typically involves a substrate being preconditioned and coated with layers of various alloys and metals. Appropriate metals, chosen for their respective art recognized purposes, are then plated in layers onto the substrate. Conductive solder bumps are then fixedly attached to the substrate. Thus, a plurality of electronic components can then later be connected to the substrate by means of the conductive bumps.

One conventional process of solder bump plating is ‘electroless plating’. In this process, plating tanks are filled with solutions containing metal ions. A reducing chemical agent is added to the solution to act as an electro-chemical reaction catalyst. Through diffusion of the metal ions, a layer of metal is plated onto a substrate. There are, however, significant disadvantages to this approach. Due to the addition of a reducing chemical agent to the plating solution, equilibrium no longer exists, rendering the solution unstable. Moreover, as it is well recognized in the art, an unstable solution can readily decompose in the likely event of a contaminant being introduced to the solution. Thus, due to such clear disadvantages, electroless plating is highly unreliable.

Another conventional process of solder bump plating is ‘fountain plating’. However, as it is recognized by those of ordinary skill in the art, the conventional fountain plating process is extremely complex and requires specialized professionals, production systems, tools, and equipment. Generally, the fountain plating process requires an exact positioning of a substrate, such that the substrate is positioned both above and perpendicular to a source of a pressurized stream of a plating solution. In a generally fountain-type fashion, the plating solution flows upward, contacting the bottom portion of the substrate, in turn, plating the substrate. As it can be appreciated, fountain plating requires highly sophisticated and complex tools and equipment further requiring a plurality of customized parts. Hence, specialized training is required simply to maintain the system. Furthermore, the sophistication and complexity alone necessitates the requirement for highly specialized professionals just for operation.

In order to meet future demand, the microelectronic industry is constantly searching for both viable techniques and more importantly, cost effective ways to achieve its goal of producing microelectronic substrates using solder bumps. Accordingly, it is desirable to have a reliable, inexpensive, easily operated, and maintainable system and method for providing solder bump plated microelectronic substrates.

SUMMARY OF THE INVENTION

The present invention relates to a method and system for single-metal plating a substrate. The single-metal plating system comprises a single-metal plating solution comprising a material source of metal ions of a single metal, an anode, a cathode, a preconditioned substrate electrically connected to the cathode, and a current source connected to the anode and cathode. The anode, cathode, and substrate are immersed in the single-metal plating solution, thereby forming a circuit. The current source is then used to apply a low current density to the circuit, thereby causing metal ions in the solution to become uniformly deposited onto the preconditioned substrate.

In an alternate embodiment, the present invention relates to a method and system for binary-metal plating a substrate. The binary-metal plating system comprises a binary-metal plating solution comprising a material source of metal ions of two or more metals, an anode, a cathode, a preconditioned substrate electrically connected to the cathode, a current source connected to the anode and cathode, and an agitation means. The anode, cathode, and substrate are immersed in the binary-metal plating solution, thereby forming a circuit. The current source is then used to apply a high current density to the circuit while the agitation means agitates the solution, thereby causing metal ions in the solution to become uniformly deposited onto the preconditioned substrate.

In an alternate embodiment, the present invention relates to a system and method for solder-bump plating a substrate, the system comprising a single-metal plating system for plating a preconditioned substrate, thereby forming a single-metal plated substrate; and a binary-metal plating system for plating the single-metal plated substrate. In a preferred implementation, the solder-bump plating system of the present invention is configured to single-metal plate a preconditioned substrate using a first plating solution to provide a single-plated substrate; to single-metal plate the single-plated substrate using a second plating solution to provide a double-plated substrate; and to binary-metal plate the double-plated substrate to provide a solder-bump plated substrate.

The present invention also relates to a novel apparatus for use in plating a substrate. The apparatus comprises an electrically conductive holder comprising a handle portion having at least one electrical contact for coupling to a current source and a holder portion having at least one flexible electrical contact for supporting a substrate and for electrically contacting the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary preconditioned substrate used in accordance with the present invention.

FIG. 2 illustrates a single-metal plating system in accordance with an exemplary embodiment of the present invention.

FIG. 3 illustrates a binary-metal plating system in accordance with an exemplary embodiment of the present invention.

FIGS. 4A and 4B illustrates an apparatus for use in plating a substrate in accordance with an exemplary embodiment of the present invention.

FIG. 5 illustrates a single-metal plating method in accordance with an exemplary embodiment of the present invention.

FIG. 6 illustrates a binary-metal plating method in accordance with an exemplary embodiment of the present invention.

FIG. 7 illustrates a solder-bump plating method in accordance with an exemplary embodiment of the present invention.

FIGS. 8 illustrates a solder-bump plating system in accordance with an exemplary embodiment of the present invention.

FIG. 9A illustrates an exemplary preconditioned substrate for use in accordance with an exemplary embodiment of the present invention.

FIG. 9B illustrates an exemplary copper-plated substrate in accordance with the present invention.

FIG. 9C illustrates an exemplary copper and nickel plated substrate in accordance with the present invention.

FIG. 9D illustrates an exemplary solder-bump plated substrate in accordance with the present invention.

FIG. 9E illustrates an exemplary stripped solder-bump plated substrate in accordance with the present invention.

FIG. 9F illustrates a reflowed solder bump in accordance with the present invention.

FIG. 10 illustrates a solder-bump plating method in accordance with an exemplary embodiment of the present invention.

 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Described herein are novel methods and systems for use in solder-bump plating. In general, solder-bump plating involves single-metal plating a substrate, followed by binary-metal plating the substrate. To this end, the present invention provides novel plating methods and systems for both single-metal plating and binary-metal plating. In addition, the present invention provides a novel cathode/handle for use in securing a substrate during the plating processes. Preferably, the methods and systems of the present invention are utilized in connection with suitable, ‘pre-conditioned’ substrates. A ‘pre-conditioned’ substrate, as the term is used herein, refers to any substrate that is conditioned for use in single-metal plating, binary-metal plating, and/or solder-bump plating in accordance with the present invention. To illustrate, reference is made to FIG. 1, wherein an exemplary pre-conditioned substrate 101 is shown. This exemplary substrate 101, however, is for illustrative purposes only and should not be deemed as limiting upon the present invention. Indeed, the present invention may be utilized in connection with any suitable substrate having any suitable configuration.

Referring now to FIG. 1, an exemplary pre-conditioned substrate 101 for use in accordance with the present invention is shown. The pre-conditioned substrate 101 comprises a substrate layer 111, a first titanium tungsten (TiW) layer 112, a copper (Cu) layer 113, a second TiW 114 layer, and a resist layer 115. Portions of the second TiW layer 114 and the resist layer 115 have been removed to expose a portion 113A of the copper layer 113. Ultra-violet (UV) exposure, dry etching, and/or any other known technique may be utilized for removing portions of the resist layer 115 and TiW layer (114). As is further discussed below, the exposed copper portion 113A provides a suitable foundation for metal and/or solder bump plating in accordance with the present invention.

The substrate layer 111 may comprise any substrate desired to be plated. For example, the substrate layer 111 may comprise an integrated circuit, a semiconductor chip, a wafer, or any other substrate known to those skilled in the art. To illustrate, in an exemplary implementation, the substrate layer 111 may comprise an AlGaAs PIN diode microelectronic wafer. In addition, although the substrate layer 111 is shown having TiW 112, 114 and copper 113 layers, any suitable conditioning layers appropriate for the particular implementation may be utilized in accordance with the present invention. Similarly, any appropriate preconditioning process, such as metal plating, dry etching, wet etching, photo-resist exposure, may be utilized to dispose and/or remove preconditioning layers from the substrate layer 111.

Referring now to FIG. 2, an exemplary single-metal plating system 200 in accordance with the present invention is shown. The single-metal plating system 200 comprises a plating container 211, plating solution 212 disposed therein, a cathode 231, an anode 232, and a current source 241. The plating container 211 may be any suitable container composed of any inert, non-plating material large enough to adequately space a cathode and anode therein. For example, a tank, beaker, dish, or any other appropriate container may be used as a plating container 211. Container 211 may further comprise, for example, a cover, handle(s), inlets, outlets, valves, hoses, tubes, meters, gauges, measuring devices, and/or any other appropriate components. In an exemplary embodiment of the present invention, the container 211 may comprise a plastic tank such as, for example, a polypropylene tank.

Disposed within the container 211 is a plating solution 212. The plating solution 212 may be any conductive solution comprising a source of metal ions of a single metal. To illustrate, the single metal may comprise, for example, one of Co, Pd, Au, Ag, Cr, Cd, Al, In, Fe, Pb, and Pt. Example single-metal plating solutions may comprise, for example, CuSO4. CuCN, Ni(SO3NH2)2, or Ni SO4, or any other single-metal solution. In one exemplary implementation of the single-metal plating system 200, the solution 212 comprises a copper solution, such as CuSO4. In another exemplary implementation, the solution 212 comprises a nickel solution, such as Ni(SO3NH2)2.

The single-metal plating system 200 further comprises an anode 232 and cathode 231. The anode 232 may be any electrically conductive apparatus of an inert material comprising at least one electrical connection point, also referred to as a contact point. In an exemplary embodiment, the anode 232 comprises a bar of metal wherein the metal is the same metal as that of the single-metal ions contained in the plating solution 212. To illustrate, if the plating solution 212 were a copper based solution, then preferably, the anode 232 would comprise a bar of copper. Similarly, if the plating solution 212 were a nickel based solution, then preferably, the anode 232 would comprise a bar of nickel. Alternatively, an inert anode such as a platinized titanium anode may be used as anode 232.

The cathode 231 of the present system 200 preferably comprises an electrically conductive material and one or more electrically conductive connection points. As further detailed in FIGS. 4A-4B, the cathode 231 of the present system 200 preferably comprises an electrically conductive holder component 433 and a cover component 434. The holder component 433 preferably comprises the one or more electrically conductive connection points, or prongs 435a-d. In the configuration illustrated in FIGS. 4A-4B, the holder 433 comprises four prongs 435a, 435b, 435c, and 435d. It should be understood, however, that any number of connection points or prongs may be utilized. Preferably, the prongs 435a-d are affixed to the holder 433 at equal distances from one another, protruding towards a center of the holder 433. Additionally, it is preferred that the cover 434 compliment the shape of the holder 433. It should be noted that the cover 434 also functions to support both the substrate 101 and the holder 433 in the plating solution 212.

The cathode 231 of the system 200 is also preferably coated with a non-reactive material such as polyurethane, Teflon, or other appropriate material in order to prevent a reaction from taking place when immersed in a reactive solution. As is known to those skilled in the art, the non-reactive coating may also function as an electrical barrier, preventing the flow of current where desired. Therefore, in order to allow current to pass through the cathode 231, while preventing current from passing directly into the solution 212, connection point 436 and prongs 435a-d of the holder 433 are left uncoated so as to provide an electrical current path from a current source, through to the prongs 435a-d.

Referring again to FIG. 2, the anode 232 and cathode 231 are immersed into the plating solution 212, thereby forming an open circuit. In a preferred embodiment, the anode 232 and cathode 231 are positioned in the solution 212 at a distance of between three (3) and ten (10) inches from each other. As further discussed below, this range of distances is useful in obtaining uniform plating of the substrate 101. As is well known to those skilled in the art, a solution 212 comprising a material source of metal ions may function as an electrically conductive medium. Thus, in the present system 200, the metal plating solution 212 is functioning as a conductive medium electrically coupling the anode 232 and cathode 231. As further discussed below, this open circuit is closed by connecting the anode 232 and cathode 231 to a current source 241.

Coupled to the cathode 231 is the preconditioned substrate 101 discussed above. As is illustrated in FIG. 4B, the preconditioned substrate 101 is positioned onto the cathode holder 433 such that the substrate 101 effectively contacts all of prongs 435a-d, yet remains within the perimeter of the holder 433 and cover 434. In this manner, when the preconditioned substrate 101 is positioned onto holder 433, the prongs 435a-d function as electrical contact points, thus creating an electrical connection between the preconditioned substrate 101 and the cathode 231. In addition, once the substrate 101 is fastened to the holder 433, an area of the substrate 101 desired to be plated will remain exposed to the holder's 433 opening.

As is further illustrated in FIG. 4B, once the preconditioned substrate 101 is positioned atop the cathode holder 433, the cover 434 is positioned atop the preconditioned substrate 101. Alternatively, the substrate 101 may first be positioned onto the cover 434, with the cathode holder 433 positioned thereon such that all of the prongs 435a-d effectively contact the substrate 101, yet remains within the perimeter of the holder 433 and cover 434.

In either event, the cover 434 is securely fastened to holder 433 via any appropriate fastening means, such as for example, screws, bolts, dowels, adhesive, epoxy, or any other fastening means known in the art. Once the substrate 101 is secured to the holder 433, an area of the substrate 101 desired to be plated will remain exposed via the holder's 433 opening.

The prongs 435a-d of the cathode 231 are preferably formed of a flexible material and oriented at an angle between 1° and 179° above a horizontal axis of the cathode holder 433. In this manner, the prongs 435a-d may provide a flexible cushioning to minimize any damage to the preconditioned substrate 101 that may occur from fastening the cover 434 to holder 433.

Connected to the anode 232 and cathode 231 is a current source 241. This current source 241 is preferably connected to cathode 231 at connection point 436, and to anode 232 at a connection point 261, thereby forming a closed circuit. The current source 241 may be any appropriate device known to supplying current, wherein the current may be set in the mAmp range. In a preferred embodiment, the current source 241 comprises a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC.

Once the current source 241 is connected to the anode 232 and cathode 231, the current source 241 is utilized to provide an optimal current density to the closed circuit formed in the container 211, thereby causing the metal ions in the solution 212 to be reduced and become uniformly deposited onto the substrate 101. In use, the current source 241 applies a low current density to cathode 231 via connection point 436, causing current to flow through the cathode 231 to preconditioned substrate 101 via prongs 435a-d. Low current density is preferably in the range of 10 mA/cm2 or less, but greater than 0 mA/cm2. In one particular implementation, the current density applied for optimized plating is approximately 7.3 mA/cm2.

As will be recognized by those skilled in the art, applying a current density to the system 200 while the anode 232, cathode 231, and preconditioned substrate 101 are immersed in the plating solution 212 causes metal ions in the solution 212 to diffuse at an accelerated rate. As a result, metal cations and anions of the plating solution 212 will separate. The metal cations migrate toward the cathode 231, as denoted by arrow 221 of FIG. 2, while the metal anions of the plating solution 212 migrate toward the anode 232, as denoted by arrow 222 of FIG. 2. In a preferred implementation, the metal cations of the plating solution 212 form deposits of metal on the preconditioned substrate 101, while anions of the plating solution 212 react with the metal of anode 232 and form chemical bonds or by-product.

As noted above, applying a current density to the system 200 increases the rate of diffusion of ions, thus increasing the plating rate of metal ions onto the preconditioned substrate 101. It should be understood, however, that there is a trade off between the amount of current density employed and plating uniformity. For instance, as current density is increased, uniformity of metal ion deposits may decrease, resulting in a non-uniform layer of plated metal on the preconditioned substrate 101. Thus, it is preferable to apply a current density that optimizes a plating rate, without sacrificing plating uniformity. An instrument useful in achieving this optimal current density may include, for example, an UltraGage® 9500 benchtop wafer metrology system manufactured by ADE® Corporation, hereinafter a ‘measurement instrument’.

As known to those skilled in the art, a measurement instrument is a tool that can measure over six hundred (600) points on a substrate to determine its plating thickness and plating uniformity. As such, a user may utilizing a measurement instrument in conjunction with the present system 200 to adjust the applied current density until a uniform thickness is achieved. In addition, a measurement instrument may be utilized to ensure that the optimal current density is only applied until a desired plating thickness is achieved. Once the desired plating thickness is achieved, the current source 241 may be disabled, and the now single-metal plated substrate 101 may be removed from plating solution 212, rinsed, and utilized in any appropriate manner.

Referring now to FIG. 5, a flow diagram illustrating a single-metal plating method 500 in accordance with the present invention is shown. The method 500 begins at step 511, wherein a single-metal plating solution comprising a material source of metal ions is provided. Preferably, the plating solution is a conductive solution comprising a source of metal ions of a single metal. For example, the plating solution may comprise CuSO4, CuCN, Ni(SO3NH2)2, Ni SO4, or any other single-metal solution.

Next, in step 512, a preconditioned substrate is electrically connected to a cathode. In a preferred implementation, the preconditioned substrate may comprise a substrate layer, a layer of titanium tungsten, a layer of copper, a second layer of titanium tungsten, and layer of resist. The substrate layer may comprise, for example, an integrated circuit, semiconductor chip, wafer or any other substrate known in the art. The preconditioning of the substrate may include a process such as UV exposure, development, wet etching, and/or dry etching. Other preconditioning processes such as metal plating, dry etching, wet etching, photo-resist exposure, development and the like may be utilized in accordance with the present invention.

The cathode may be made of any non-reactive material capable of conductance and of providing at least one electrical connection point. In a preferred implementation, the cathode preferably comprises an electrically conductive holder component and a cover component. The holder component may further comprise one or more electrically conductive prongs. The one or more prongs are preferably affixed to the holder uniformly, protruding towards a center of the holder. The cover preferably compliments the shape of the holder. The cathode holder is preferably coated with a non-reactive material such as polyurethane, Teflon, or other appropriate material in order to prevent a reaction from occurring when the cathode is immersed in the single-metal plating solution. The contact points and/or prongs and a portion of the holder are preferably left uncoated, thus enabling electrical current to pass through the cathode to the substrate coupled thereto and into the solution.

The electrical connection of the preconditioned substrate to the cathode (step 512) may be by way of any appropriate means known to those skilled in the art. In a preferred implementation, the preconditioned substrate is positioned on the cathode such that the preconditioned substrate effectively contacts all electrical contact points of the cathode. If a cathode holder having a cover is used, for example, the substrate may be positioned onto the cathode holder such that the substrate effectively contacts the cathode holder's contact prong(s). Once the substrate is properly positioned, the cover may be securely fastened to holder portion of the cathode so as to securely hold the substrate in place. Fastening means such as screws of a non-reactive material be utilized to secure the cover to the holder, although any known fastening means may be utilized in accordance with the present invention.

Alternatively, the substrate may first be positioned onto the cover, with the cathode holder positioned thereon such that all of the prongs effectively contact the substrate, yet remains within the perimeter of the holder 433 and cover 434. In either event, once the substrate is secured to the holder, an area of the substrate desired to be plated will preferably remain exposed via an opening in the holder.

Next, in step 513, an anode and the cathode are immersed into the solution, thereby coupling the two and creating an open circuit. Preferably, the anode and cathode are separated by a distance of between 3 and 10 inches in the solution. The anode may be made of any non-reactive material capable of conductance. In a preferred implementation, the anode is a bar of metal, wherein the metal is the same metal as that of the metal ions in the single-metal plating solution. For example, if the single-metal solution comprises copper, then preferably, the anode comprises a bar of copper. Alternatively, the anode may comprise any inert material such as, for example, platinized titanium.

Since the substrate has previously been coupled to the cathode (step 512), once the cathode is immersed (step 513), the substrate will also be immersed. It should be understood, however, that the preconditioned substrate may be immersed before, concurrently, or after the immersion of the cathode (step 513). If immersed before or after the cathode, the electrical coupling of the substrate to the cathode (step 512) would occur in the solution.

As is well known to those skilled in the art, a solution comprising a material source of metal ions may function as an electrically conductive medium. Thus, in the present configuration, the single-metal plating solution functions as a conductive medium electrically coupling the anode and cathode, thereby forming an open circuit. As further discussed below, a closed circuit is later formed by connecting the anode and cathode to a current source.

The immersion of the anode, cathode, and substrate into the single-metal plating solution results in the diffusion of ions of the single-metal plating solution, causing metal ions of the single-metal plating solution to be deposited onto the preconditioned substrate to form a layer of plated metal on the substrate. The diffusion of ions is inherently a slow process. In order to increase the rate of diffusion and hence, the rate at which ions are reduced and plated onto the substrate, a current density is employed at step 514. Here, a low current density is applied to the circuit formed by the anode, the cathode connected to the preconditioned substrate, and the single-metal plating solution. Applying current density in this manner causes metal cations and anions of the plating solution to separate, wherein the metal cations are reduced and form deposits of metal onto the preconditioned substrate. The current density is applied until a desired plating thickness is achieved.

In a preferred implementation, an instrument such as a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC, is used to apply the low current density (step 514) to the cathode. The low current density may be, for example, in the range of 10 mA/cm2 or less, but greater than 0 mA/cm2. In a specific implementation, the current density applied for optimized plating is 7.3 mA/cm2. In addition, a measurement may be utilized both to determine the desired low current density, and to achieve the desired plating thickness. Alternatively, an instrument such as a profilometer may be utilized to measure the plating thickness to confirm that the desired thickness is achieved. Once the desired thickness is achieved, the current source is disabled, and the single-metal plated substrate may be removed from the plating solution and utilized as desired.

Referring now to FIG. 3, an exemplary binary-metal plating system 300 in accordance with the present invention is shown. The binary-metal plating system 300 comprises a plating container 311, plating solution 312 disposed therein, a cathode 331, an anode 332, a current source 341, and an agitator 351. The plating container 311 may be any suitable container composed of any inert, non-plating material large enough to adequately space a cathode and anode therein. For example, a tank, beaker, dish, or any other appropriate container may be used as a plating container 311. Container 311 may further comprise, for example, a cover, handle(s), inlets, outlets, valves, hoses, tubes, meters, gauges, measuring devices, and/or any other appropriate components. In an exemplary embodiment of the present invention, the container 311 is a plastic tank, such as for example, a polypropylene tank.

Disposed within the container 311 is a plating solution 312. The plating solution 312 may be any conductive solution comprising a source of metal ions of two or more metals. For example, plating solution 312 may comprise SnPb, SnBi, SnCu, SnAg, or any other binary-metal solution or a solution of three metals such as SnAgCu. In one exemplary implementation of the binary-metal plating system 300, the solution 312 comprises SnPb substantially in a composition ratio of 63% Sn, 37% Pb. As used herein, the phrase “substantially in a composition ratio” indicates that the stated ratio may be adjusted by plus or minus 10%. Thus, a SnPb “substantially in a composition ratio” of 63% Sn and 37% Pb, may include, for example, a SnPb solution with a composition ration of 60% Sn and 40% Pb, or 58% Sn and 42% Pb, etc.

The binary-metal plating system 300 further comprises an anode 332 and cathode 331. The anode 332 may be any electrically conductive apparatus of an inert material comprising at least one electrical connection point. In an exemplary embodiment, the anode 332 comprises a bar of metal comprising the same metal as that of the binary-metal ions contained in the plating solution 312. To illustrate, if the plating solution 312 were a tin-lead based solution, then preferably, the anode 332 would comprise a bar of tin and lead. Alternatively, the anode 332 may comprise-inert materials such as, for example, platinized titanium.

The anode 332 may be any electrically conductive apparatus of an inert material comprising at least two electrical connection points. The cathode 331 of the present system 300 preferably comprises an electrically conductive material and one or more electrically conductive connection points. As further detailed in FIGS. 4A-4B, the cathode 331 of the present system 300 preferably comprises an electrically conductive holder component 433 and a cover component 434. The holder component 433 preferably comprises the one or more electrically conductive connection points, or prongs 435a-d. In the configuration illustrated in FIGS. 4A-4B, the holder 433 comprises four prongs 435a, 435b, 435c, and 435d. It should be understood, however, that any number of connection points or prongs may be utilized. Preferably, the prongs 435a-d are affixed to a generally ring-shaped holder 433 at equal distances from one another, protruding towards a center of the holder 433. Additionally, it is preferred that the cover 434 compliment the shape of the holder 433.

The cathode 331 of the system 300 is also preferably coated with a non-reactive material such as polyurethane, Teflon, or other appropriate material in order to prevent a reaction from taking place when immersed in a reactive solution. As is known to those skilled in the art, the non-reactive coating may also function as an electrical barrier, preventing the flow of current where desired. Therefore, in order to allow current to pass through the cathode 331, while preventing current from passing directly into the solution 312, connection point 436 and prongs 435a-d of the holder 433 are left uncoated so as to provide an electrical current path from a current source, through to the prongs 435a-d.

Referring again to FIG. 3, the anode 332 and cathode 331 are immersed into the plating solution 312, thereby forming an open circuit. In a preferred embodiment, the anode 332 and cathode 331 are positioned in the solution 312 at a distance of between three (3) and ten (10) inches from each other. As further discussed below, this range of distances is useful in obtaining uniform plating of the substrate 101. As is well known to those skilled in the art, a solution 312 comprising a material source of metal ions may function as an electrically conductive medium. Thus, in the present system 300, the metal plating solution 312 is functioning as a conductive medium electrically coupling the anode 332 and cathode 331. As further discussed below, this open circuit is closed by connecting the anode 332 and cathode 331 to a current source 341.

Coupled to the cathode 331 is the preconditioned substrate 101 discussed above. As is illustrated in FIG. 4B, the preconditioned substrate 101 is positioned onto the cathode holder 433 such that the substrate 101 effectively contacts all of prongs 435a-d, yet remains within the perimeter of the holder 433 and cover 434. In this manner, when the preconditioned substrate 101 is positioned onto holder 433, the prongs 435a-d function as electrical contact points, thus creating an electrical connection between the preconditioned substrate 101 and the cathode 331. As is further illustrated in FIG. 4B, once the preconditioned substrate 101 is positioned atop the cathode holder 433, the cover 434 is positioned atop the preconditioned substrate 101. Alternatively, the substrate 101 may first be positioned onto the cover 434, with the cathode holder 433 positioned thereon such that all of the prongs 435a-d effectively contact the substrate 101, yet remains within the perimeter of the holder 433 and cover 434.

In either event, the cover 434 is securely fastened to holder 433 via any appropriate fastening means, such as for example, screws, bolts, dowels, adhesive, epoxy, or any other fastening means known in the art. Once the substrate 101 is secured to the holder 433, an area of the substrate 101 desired to be plated will remain exposed via the holder's 433 opening.

The prongs 435a-d of the cathode 331 are preferably formed of a flexible material and oriented at an angle between 1° and 179° above a horizontal axis of the cathode holder 433. In this manner, the prongs 435a-d may provide a flexible cushioning to minimize any damage to the preconditioned substrate 101 that may occur from fastening the cover 434 to holder 433.

Connected to the anode 332 and cathode 331 is a current source 341. This current source 341 is preferably connected to cathode 331 at connection point 436, and to anode 332 at a connection point 361, thereby forming a closed circuit. The current source 341 may be any appropriate device known to supply current, wherein the current may be set in the mAmp range. In a preferred embodiment, the current source 341 is a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC.

Once the current source 341 is connected to the anode 332 and cathode 331, the current source 341 is utilized to provide an optimal current density to the closed circuit formed in the container 311, thereby causing the metal ions in the solution 312 to be reduced and become uniformly deposited onto the substrate 101. In use, the current source 341 applies a high current density to cathode 331 via connection point 436, causing current to flow through the cathode 331 to preconditioned substrate 101 via prongs 435a-d. “High” current density is preferably in the range of greater than greater than 10 mA/cm2 to 3,000 mA/cm2. In one particular implementation, the current density applied for optimized plating is approximately 43 mA/cm2.

In addition to applying a high current density to the system 300, the agitator 351 provides agitation to the system to improve plating uniformity. In an exemplary embodiment, the agitator 351 may comprise a magnetic stirrer positioned within the container 311, although any type of stirrer may be utilized in accordance with the present invention. In fact, agitation may be applied to solution 312 by any appropriate means known to those skilled in the art. For example, heating, stirring, shaking, etc. may all be utilized, alone or in combination, to agitate the solution 312. In implementations utilizing the preferred magnetic stirrer, it is preferred that the stirrer 351 be controlled electronically to achieve a desired agitation rate. In one exemplary implementation, for example, the magnetic stirrer 351 may be set to stir the solution 312 at a rate of 400 r.p.m.

As will be recognized by those skilled in the art, applying a current density to the system 300 while the anode 332, cathode 331, and preconditioned substrate 101 are immersed in the plating solution 312, and while the agitator 351 is agitating the solution 312, causes metal ions in the solution 312 to diffuse at an accelerated rate. As a result, metal cations and anions of the plating solution 312 separate. The metal cations migrate toward the cathode 331, as denoted by arrow 321 of FIG. 3, while the metal anions of the plating solution 312 migrate toward the anode 332, as denoted by arrow 322 of FIG. 3. In a preferred implementation, the metal cations of the plating solution 312 form deposits of plated metal on the preconditioned substrate 101, while anions of the plating solution 312 react with the metal of anode 332 and form chemical bonds or by-product. Furthermore, preferably the applied current density and agitation are adjusted, such that the resulting layer of plated binary-metal is of a similar composition to that of the binary-metal of the plating solution 312 used.

As noted above, applying a current density to the system 300, while agitating the solution 351 of the system 300 increases the rate of diffusion and mass transfer of ions, thus increasing the plating rate of metal ions onto the preconditioned substrate 101. Accordingly, it is preferable to apply a current density that optimizes a plating rate, without sacrificing plating uniformity. An instrument useful in achieving this optimal current density may include, for example, a measurement instrument.

Once a desired plating thickness is achieved, the current source 341 may be disabled, and the now single-metal plated substrate 101 may be removed from plating solution 312, rinsed, and utilized in any appropriate manner.

Referring now to FIG. 6, a flow diagram illustrating a binary-metal plating method 600 in accordance with the present invention is shown. The method 600 begins at step 611, wherein a binary-metal plating solution comprising a material source of metal ions is provided. Preferably, the plating solution is a conductive solution comprising a source of metal ions of two metals. For example, the plating solution may comprise tin and lead, SnBi, SnAg, and other solutions comprising two metals.

Next, in step 612, a preconditioned substrate is electrically connected to a cathode. In a preferred implementation, the preconditioned substrate may comprise a substrate layer, a layer of titanium tungsten, a layer of copper, a second layer of titanium tungsten, and layer of resist. The substrate layer may comprise, for example, an integrated circuit, semiconductor chip, wafer or any other substrates known in the art. The preconditioning of the substrate may include a process such as UV exposure, development, wet etching, and/or dry etching. Other preconditioning processes such as metal plating, dry etching, wet etching, photo-resist exposure, development and the like may be utilized in accordance with the present invention.

The cathode may be made of any non-reactive material capable of conductance and of providing at least one electrical connection point. In a preferred implementation, the cathode preferably comprises an electrically conductive holder component and a cover component. The holder component may further comprise one or more electrically conductive prongs. The one or more prongs are preferably affixed to the holder uniformly, protruding towards a center of the holder. The cover preferably compliments the shape of the holder. The cathode holder is preferably coated with a non-reactive material such as polyurethane, Teflon, or other appropriate material in order to prevent a reaction from occurring when the cathode is immersed in the single-metal plating solution. The contact points and/or prongs and a portion of the holder are preferably left uncoated, thus enabling electrical current to pass through the cathode to the substrate coupled thereto and into the solution.

The electrical connection of the preconditioned substrate to the cathode (step 612) may be by way of any appropriate means known to those skilled in the art. In a preferred implementation, the preconditioned substrate is positioned on the cathode such that the preconditioned substrate effectively contacts all electrical contact points of the cathode. If a cathode holder having a cover is used, for example, the substrate may be positioned onto the cathode holder such that the substrate effectively contacts the cathode holder's contact prong(s). Once the substrate is properly positioned, the cover may be securely fastened to holder portion of the cathode so as to securely hold the substrate in place. Alternatively, the substrate may first be positioned onto the cathode cover, with the cathode holder positioned thereon such that all of the prongs effectively contact the substrate, yet remains within the perimeter of the holder and cover.

Fastening means such as screws of a non-reactive material be utilized to secure the cover to the holder, although any known fastening means may be utilized in accordance with the present invention. Once the substrate is secured to the holder, an area of the substrate desired to be plated will remain exposed via the holder's opening.

Next, in step 613, an anode and the cathode are immersed into the solution, thereby coupling the two and creating an open circuit. Preferably, the anode and cathode are separated by a distance of between 3 and 10 inches in the solution. The anode may be made of any non-reactive material capable of conductance. In a preferred implementation, the anode is a bar of metal, wherein the metal is the same metal as that of the metal ions in the binary-metal plating solution. For example, if the binary-metal solution comprises tin and lead, then preferably, the anode comprises a bar of tin and lead.

Since the substrate has previously been coupled to the cathode (step 612), once the cathode is immersed (step 613), the substrate will also be immersed. It should be understood, however, that the preconditioned substrate may be immersed before, concurrently, or after the immersion of the cathode (step 613). If immersed before or after the cathode, the electrical coupling of the substrate to the cathode (step 612) would occur in the solution.

As is well known to those skilled in the art, a solution comprising a material source of metal ions may function as an electrically conductive medium. Thus, in the present configuration, the binary-metal plating solution functions as a conductive medium electrically coupling the anode and cathode, thereby forming an open circuit. As further discussed below, a closed circuit is later formed by connecting the anode and cathode to a current source.

The immersion of the anode, cathode, and substrate into the binary-metal plating solution results in the diffusion of ions of the binary-metal plating solution, causing metal ions of the binary-metal plating solution to be reduced and deposited onto the preconditioned substrate to form a layer of plated metal on the substrate. The diffusion of ions is inherently a slow process. In order to increase the rate of diffusion and mass transfer, hence, the rate at which ions are reduced and plated onto the substrate, a current density, together with agitation, is employed at step 614. Here, a high current density is applied to the circuit formed by the anode, the cathode connected to the preconditioned substrate, and the binary-metal plating solution. At the same time, the solution is agitated using any desired agitation means. In an exemplary embodiment, the agitation means comprises a magnetic stirrer. Applying current density and agitating in this manner causes metal cations and anions of the plating solution to separate, wherein the metal cations are reduced and form deposits of binary-metal onto the preconditioned substrate. Furthermore, preferably the applied current density and agitation are adjusted, such that the resulting layer of plated binary-metal is of a similar composition to that of the binary-metal of the plating solution used. The current density and agitation is applied until a desired plating thickness is achieved.

In a preferred implementation, an instrument, such as a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC, is used to apply the low current density (step 614) to the cathode. The high current density may be, for example, in the range of greater than 10 mA/cm2 to 3,000 mA/cm2. In a specific implementation, the current density applied for optimized plating is 43 mA/cm2. In a preferred embodiment, a measurement instrument is utilized to determine the optimal current density and to determine a substrate plating thickness. Optionally, a instrument such as a profilometer may be used to measure a plating thickness on the substrate.

Once the desired thickness is achieved, the now binary-metal plated substrate may be removed from the plating solution and utilized as desired.

Referring now to FIG. 7, a flow diagram illustrating a solder bump plating method 700 in accordance with the present invention is shown. This solder-bump plating method 700 simply comprises single-metal plating a substrate (step 701) and binary metal plating the substrate (step 703). In an exemplary embodiment, the single-metal plating step (701) may be performed in accordance with the single-metal plating method 500 of the present invention. Additionally or optionally, the binary-metal plating step (703) may be performed according to the binary-metal plating method 600 of the present invention. In use, a preconditioned substrate is first single-metal plated (step 701). Next, the now single-metal plated substrate is binary-metal plated (step 703).

In an exemplary implementation of the solder-bump plating method 700, a substrate is single-metal plated (step 701) at least twice, before being binary-metal plated (step 703), wherein the single-metal plating step (703) may be performed any number of times as appropriate for the particular implementation. In such an implementation, each time the substrate is single-metal plated (step 701), the same or different single-metal may be utilized. To illustrate, a preconditioned substrate may first be single-metal plated (step 701) using a copper solution to form a layer of copper on the substrate. Next, the substrate may be single-metal plated (step 701) using a nickel-based solution, thereby providing a substrate layered in copper and nickel. The copper and nickel plated substrate may then be binary-metal plated using a tin-lead solution, thereby providing a solder-bump plated substrate.

Referring now to FIG. 8, an exemplary system 800 for solder-bump plating in accordance with the present invention is shown. The system 800 comprises three plating containers 811A-C, each comprising a plating solution 812A-C, a cathode 831, an anode 832A-C, and a current source 841, respectively. One of the plating containers 81 IC further comprises an agitator 851. The plating containers 811A-C may each be any suitable container 811A-C composed of any inert, non-plating material. For example, a tank, beaker, dish, or any other appropriate container may be used as a plating container 811A-C. The containers 811A-C may further comprise, for example, covers, handles, inlets, outlets, valves, hoses, tubes, meters, gauges, measuring devices, and/or any other appropriate components. In an exemplary embodiment of the present invention, the containers 811A-C are plastic tanks.

Disposed within each container 811A-C is a plating solution 812A-C, respectively. The first and second plating solutions 812A-B may each comprise any conductive solution comprising a source of metal ions of a single metal. For example, the plating solution 812A in the first container 811A may comprise a copper-based solution, while the plating solution 812B in the second container 811B may comprise a nickel-based solution. The third plating solution 812C comprises any binary-metal solution, such as for example, a tin-lead based solution substantially in a composition ratio of 63% tin and 37% lead, ±10%.

The solder-bump plating system 800 further comprises an anode 832A-C and a cathode 831, respectively, for use with each of the containers 811A-C. The anodes 832A-C may be any electrically conductive apparatus of an inert material comprising at least one electrical connection point. In an exemplary embodiment, the anodes 832A-C each comprise a bar of metal wherein the metal is the same metal as that of the metal ions contained in their respective plating solutions 812A-C. To illustrate, if the first plating solution 812A were copper-based, then preferably, the first anode 832A would comprise a bar of copper. Similarly, if the second plating solution 812B were nickel-based, then preferably, the second anode 832B would comprise a bar of nickel. Likewise, if the third plating solution 812C were tin-lead based, then the third anode 832C would preferably comprise a bar of tin and lead. Alternatively, the anodes 832A-C may comprise inert metals such as, for example, screens of platinized titanium and/or any other inert metal.

The cathode 831 of the present system 800 preferably comprises an electrically conductive material and one or more electrically conductive connection points for coupling to a substrate 801. Additionally, the cathode 831 is preferably coated with a non-reactive material such as polyurethane, Teflon, or other appropriate material in order to prevent a reaction from taking place when immersed in any of the plating solutions 812A-C. This cathode 831 also preferably comprises an electrically conductive holder component and a cover component (not shown), for use in securing and coupling to a substrate 801 desired to be solder-bump plated. An exemplary cathode is illustrated in FIGS. 4A-B.

Each anode 832A-C is shown immersed in the plating solution 812A-C of a respective plating container 811A-C. The cathode 831, on the other hand, may be used successively with each of the plating containers 811A-C. As further discussed below, the cathode 831 is first immersed in the first plating container 81 IA for use in single-metal plating a substrate 801. Then, the cathode 831 is immersed into the second container 811B for use in single-metal plating the substrate 801 again. The cathode 831 is then immersed into the third container 811C for binary-metal plating the substrate 801. By immersing the cathode 831 into a plating container 811A-C (which comprises a metal solution and an anode, respectively) an open circuit is formed. In a preferred embodiment, once immersed in a plating solution 812A-C, the anode 832A-C and cathode 831 are positioned in the respective solutions 812A-C at a distance of between three (3) and ten (10) inches from each other. This range of distances is useful in obtaining uniform plating of the substrate 801.

Connected to the anode 832A-C and cathode 831 in each respective container 811A-C is a current source 841. By connecting the current source 831 to the anode 832A-C and cathode 831 in this manner, a closed circuit may be formed in each container 811A-C. The current source 841 may be any appropriate device known to supply current, wherein the current may be set in the mAmp range. In a preferred embodiment, the current source 841 is a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC.

In use, the current source 841 is connected to the first anode 832A and to the cathode 831 (to which the substrate 801 is coupled). The current source 841 is then utilized to provide an optimal low current density to the closed circuit formed in the first container 811A, thereby causing the metal ions in the solution 812A to be reduced and become uniformly deposited onto the substrate 801. The low current density applied to the first solution 812A is preferably in the range of 10 mA/cm2 or less, but greater than 0 mA/cm2. In one particular implementation, the current density applied for optimized plating is approximately 7.3 mA/cm2.

Once the substrate 801 is plated to a desired thickness, the cathode 831 and substrate 801 are removed from the first container 81 IA and immersed in the second container 811B. As noted above, this second container 811B may contain the same type of solution as in the first container 811 A, or preferably, a different type of metal solution. In an exemplary implementation, the first plating solution 812A is a copper-based solution and the second plating solution 812B is a nickel-based solution.

Once immersed in the second container 811B, the cathode 831 and anode 832B are connected to the current source 841. The current source 841 is then utilized to provide an optimal low current density to the closed circuit formed in the second container 811B, thereby causing the metal ions in the second solution 812B to be reduced and become uniformly deposited onto the substrate 801. The low current density applied is preferably in the range of 10 mA/cm2 or less, but greater than 0 mA/cm2. In one particular implementation, the current density applied for optimized plating is approximately 7.3 mA/cm2.

Upon achieving a desired plating thickness onto the substrate 801, the cathode 831 and substrate 801 are removed from the second container 811B and immersed into the third container 811C. At this point, the substrate 801 has two single metal plating layers. The first layer is comprised of the metal contained in the first solution 812A (e.g., copper), and the second layer is comprised of metal contained in the second solution 812B (e.g., nickel).

Once immersed in the third container 811C, the cathode 831 and anode 832C are connected to the current source 841. The current source 841 is then utilized to provide an optimal high current density to the closed circuit formed in the third container 811C. At the same time, the agitator 851 is utilized to agitate the binary-metal solution 812C, thereby causing the metal ions in the binary-metal solution 812C to be reduced and become uniformly deposited onto the substrate 801. Furthermore, preferably the applied current density and agitation are adjusted, such that the resulting layer of plated binary-metal is of a similar composition to that of the binary-metal of the plating solution 812C used. The high current density applied to the circuit formed in the third container 811C is preferably in the range of greater than 10 mA/cm2 to 3,000 mA/cm2. In one particular implementation, the current density applied for optimized plating is approximately 43mA/cm2, while the agitation rate is approximately 400 r.p.m. Preferably, the agitator comprises an electronically controlled magnetic stirrer positioned within the container 811C.

Upon achieving a desired plating thickness on the substrate 801, the substrate 801 may be removed and utilized as desired.

Referring now to FIGS. 9A-9E, an exemplary substrate 901A for plating using a solder-bump plating system (e.g., system 800 of FIG. 8) according to the present invention is shown. For illustrative purposes, the exemplary substrate 901A will be described as being plated using copper-based, nickel-based, and tin/lead-based solutions. It should be understood, however, that any desired plating solutions may be utilized in accordance with the present invention.

Shown in FIG. 9A is an exemplary preconditioned substrate 901A desired to be solder-bump plated. The substrate 901A comprises a substrate layer 911, a first titanium tungsten (TiW) layer 912, a copper (Cu) layer 913, a second TiW 914 layer, and a resist layer 915. Portions of the second TiW layer 914 and the resist layer 915 have been removed to expose a portion of the copper layer 913A. The exposed copper area 913A provides a suitable foundation for solder-bump plating in accordance with the present invention.

The preconditioned substrate 901A is first single-metal plated using a copper-based solution, thereby providing a first copper plating layer 921 on the substrate 901B. The plated substrate 901B is then single-metal plated using a nickel-based solution, thereby providing a nickel plating layer 931, as shown in FIG. 9C. The now copper and nickel plated substrate 901C is then plated using a binary-metal solution comprising tin and lead, thereby forming a solder bump 941, as shown in FIG. 9D.

Once the solder bump 941 has been formed, the substrate 901D, may be stripped and the solder bump(s) 941 reflowed to improve the shape and appearance of the solder bump(s) 941. As known to those in the art, the processes of stripping and reflowing may be utilized to produce spherical and shiny solder bumps.

Therefore, in accordance with the present invention, the solder bump plated substrate 901D may first be stripped of one or more layers of metal through any appropriate conventional stripping means, including wet chemistries or dry chemistries or combination of both wet and dry chemistries. To illustrate, titanium tungsten layers 912, 914 may be stripped using wet chemistries such as H2O2, a mixture of EDTA and NH4OH, a mixture of EDTA and H2O2, any other wet chemistry solutions, or dry chemistries such as mixtures of CH4+CHF3+He, SF6+He, CF4+CHF3+He or SF6, and/or any other appropriate means appropriate for removal of titanium tungsten. The copper layer 913 is then preferably stripped using wet chemistries such as a cyanide solution CS-130 produced by Technic, Inc., a potassium cyanide solution KCN, a sodium solution NaCN, or any other wet chemistry solutions appropriate for removing copper.

Once all layers of metal have been stripped off, as shown in FIG. 9E, the tin-lead layer 941 of the solder-bump plated substrate 901E may be reflowed, using any conventional reflow techniques. Preferably, the process of reflowing involves heating solder-bump plated substrate 901E to a temperature slightly higher than its melting point, commonly referred to as “Eutectic”. In the exemplary substrate 901E, the tin-lead layer is plated from a tin-lead binary-metal substantially in a composition ratio of, for example, 63% tin and 37% lead, +10%. Accordingly, the substrate 901E may be reflowed at, for example, a temperature of 183° C. or higher. At this temperature, the tin and lead will melt together forming a tin/lead bump 951, as shown in FIG. 9F. Because of a surface tension of the tin/lead bump 941, and because of the bump's 941 propensity to migrate toward a lower energy state, the bump 941 is transformed into a spherical and shiny solder bump 951. In FIG. 9F, the existing structures of Ni, Cu and TiW layers underneath the solder bump 951 are not shown.

Referring now to FIG. 10, a flow diagram is shown illustrating an exemplary solder bump plating method 1000 in accordance with the present invention is shown. For illustrative purposes, the plating method 1000 will be described as using copper, nickel, and tin-lead plating solutions. It should be understood, however, that the plating method 1000 is not limited thereto. Indeed, any desired plating solution may be utilized without departing from the spirit of the present invention.

The method 1000 begins at step 1011, wherein a first single-metal plating solution comprising a material source of metal ions is provided. For purposes of discussion, this first single-metal plating solution is a conductive solution comprising a source of copper ions. To illustrate, the first plating solution may comprise CuSO4, CuCN, or any other copper-based solution.

Next, in step 1012, a preconditioned substrate is electrically connected to a cathode. In a preferred implementation, the preconditioned substrate may comprise a substrate layer, a layer of titanium tungsten, a layer of copper, a second layer of titanium tungsten, and layer of resist. The substrate layer may comprise, for example, an integrated circuit, semiconductor chip, wafer or any other substrates known in the art. The preconditioning of the substrate may include a process such as UV exposure, development, wet etching, and/or dry etching. Other preconditioning processes such as metal plating, dry etching, wet etching, photo-resist exposure, development and the like may be utilized in accordance with the present invention.

The cathode may be made of any non-reactive material capable of conductance and of providing at least one electrical connection point. In a preferred implementation, the cathode preferably comprises an electrically conductive holder component and a cover component. The holder component may further comprise one or more electrically conductive prongs. The one or more prongs are preferably affixed to the holder uniformly, protruding towards a center of the holder. The cover preferably compliments the shape of the holder. The cathode holder is preferably coated with a non-reactive material such as polyurethane, Teflon, or other appropriate material in order to prevent a reaction from occurring when the cathode is immersed in the single-metal plating solution. The contact points and/or prongs and a portion of the holder are preferably left uncoated, thus enabling electrical current to pass through the cathode to the substrate coupled thereto and into the solution.

The electrical connection of the preconditioned substrate to the cathode (step 1012) may be by way of any appropriate means known to those skilled in the art. In a preferred implementation, the preconditioned substrate is positioned on the cathode such that the preconditioned substrate effectively contacts all electrical contact points of the cathode. If a cathode holder having a cover is used, for example, the substrate may be positioned onto the cathode holder such that the substrate effectively contacts the cathode holder's contact prong(s). Alternatively, the substrate may first be positioned onto the cathode cover, with the cathode holder positioned thereon such that all of the prongs effectively contact the substrate, yet remains within the perimeter of the holder and cover.

In either event, once the substrate is properly positioned, the cover may be securely fastened to holder portion of the cathode so as to securely hold the substrate in place. Fastening means such as screws of a non-reactive material be utilized to secure the cover to the holder, although any known fastening means may be utilized in accordance with the present invention. Once the substrate is secured to the holder, an area of the substrate desired to be plated will remain exposed via the holder's opening.

Next, in step 1013, an anode and the cathode are immersed into the solution, thereby coupling the two and creating an open circuit. Preferably, the anode and cathode are separated by a distance of between 3 and 10 inches in the first plating solution. The anode may be made of any non-reactive material capable of conductance. For purposes of the present illustration, the anode comprises a copper bar. Alternatively, the anode may comprise an inert metal such as, for example, a screen of platinized titanium.

Since the substrate has previously been coupled to the cathode (step 1012), once the cathode is immersed (step 1013), the substrate will also be immersed. It should be understood, however, that the preconditioned substrate may be immersed before, concurrently, or after the immersion of the cathode (step 1013). If immersed before or after the cathode, the electrical coupling of the substrate to the cathode (step 1 012) would occur in the solution.

As is well known to those skilled in the art, a solution comprising a material source of metal ions may function as an electrically conductive medium. Thus, in the present configuration, the single-metal plating solution functions as a conductive medium electrically coupling the anode and cathode, thereby forming an open circuit. As further discussed below, a closed circuit is later formed by connecting the anode and cathode to a current source.

The immersion of the anode, cathode, and substrate into the copper-plating solution results in the diffusion of copper ions, resulting in copper ions being deposited onto the preconditioned substrate to form a layer of copper thereon. The diffusion of ions, however, is inherently a slow process. Thus, in order to expedite this process, a current density is employed at step 1014. Here, a low current density is applied to the circuit formed by the anode, the cathode (connected to the preconditioned substrate), and the copper plating solution. The current density is applied until a desired plating thickness is achieved.

In a preferred implementation, an instrument, such as a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC, is used to apply the low current density (step 1014) to the cathode. The low current density may be, for example, in the range of 10 mA/cm2 or less, but greater than 0 mA/cm2. In a specific implementation, the current density applied for optimized plating is 7.3 mA/cm2. In a preferred embodiment, a measurement instrument is utilized to determine the optimal current density and/or to determine when a desired plating thickness is achieved.

Once a desired plating thickness of copper is achieved, the current source and cathode are removed from the first (copper) plating solution. Then, at step 1021, a second single-metal plating solution comprising a material source of metal ions is provided. For purposes of this illustration, this second single-metal plating solution is a conductive solution comprising a source of nickel ions. To illustrate, the second plating solution may comprise nickel sulfomate, Ni(SO3NH2)2, or NiSO4, or any other nickel-based solution.

The cathode (used in plating the substrate with copper) and the copper-plated substrate are then immersed into the nickel-based solution (step 1022), together with a second anode, thereby creating an open circuit in the second (nickel) plating solution. Alternatively, rather then reusing the cathode used to copper-plate the substrate, a second cathode may be utilized. The anode is made of any non-reactive material capable of conductance. For purposes of the present illustration, the second anode comprises a nickel bar. Alternatively, the second anode may comprise an inert metal such as, for example, a screen of platinized titanium.

Once immersed, the anode and cathode are preferably separated by a distance of between 3 and 10 inches in the second (nickel) plating solution. As will be understood by those skilled in the art, immersing the anode and cathode into the nickel-based solution forms an open circuit, with the solution itself functioning as an electrically conductive medium.

The immersion of the anode, cathode, and substrate into the nickel-plating solution results in the diffusion of ions of the nickel plating solution, causing nickel ions to be deposited onto the preconditioned substrate to form a layer of nickel on top of the previously formed layer of copper. In order to expedite this diffusion process, a current density is employed at step 1023. Here, a low current density is applied to the circuit formed by the anode, the cathode (connected to the preconditioned substrate), and the nickel plating solution. The current density is applied until a desired plating thickness is achieved. As in the copper-plating step 1014, an instrument such as a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC, is preferably used to apply the low current density (step 1023). The low current density applied may be, for example, in the range of 10 mA/cm2 or less, but greater than 0 mA/cm2. In a specific implementation, the current density applied for optimized nickel plating is 7.3mA/cm2. In a preferred embodiment, a measurement instrument is utilized to determine the optimal current density and/or to determine when a desired plating thickness is achieved.

Once a desired plating thickness of nickel is achieved, the current source and cathode are removed from the second (nickel) plating solution. Then, at step 1031, a binary-metal plating solution comprising a material source of metal ions is provided. For purposes of this illustration, this binary-metal plating solution is a conductive solution comprising a source of tin and lead ions.

The cathode (used in plating the substrate with copper and nickel) and the copper/nickel-plated substrate connected thereto, are then immersed into the tin-lead based solution (step 1032), together with a third anode, thereby creating an open circuit in the binary (tin-lead) plating solution. Alternatively, rather then reusing the cathode used to copper- and nickel-plate the substrate, a different cathode may be utilized. The anode used for binary plating is preferably made of any non-reactive material capable of conductance. For purposes of the present illustration, this third anode comprises a tin-lead bar. Alternatively, the third anode may comprise an inert metal such as, for example, a screen of platinized titanium or any inert metal.

Once immersed, the anode and cathode are preferably separated by a distance of between 3 and 10 inches in the binary (tin-lead) plating solution. As will be understood by those skilled in the art, immersing the anode and cathode into the tin-lead based solution forms an open circuit, with the solution itself functioning as an electrically conductive medium.

The immersion of the anode, cathode, and substrate into the tin-lead plating solution results in the diffusion of ions of the tin-lead plating solution, causing tin and lead ions to be deposited onto the preconditioned substrate. In order to expedite the diffusion and mass transfer processes, a current density and agitation is employed at step 1033. Here, a high current density is applied to the circuit formed by the anode, the cathode connected to the preconditioned substrate, and the binary-metal plating solution. At the same time, the solution is agitated using any desired agitation means. In an exemplary embodiment, the agitation means comprises a magnetic stirrer. Applying a high current density and agitating in this manner causes metal cations and anions of the plating solution to separate and augment mass transfer, wherein the metal cations form deposits of metal ions onto the preconditioned substrate. Furthermore, preferably the applied current density and agitation are adjusted, such that the resulting layer of plated binary-metal is of a similar composition to that of the binary-metal of the plating solution used. The current density and agitation are applied (step 1033) until desired plating thickness is achieved. As in the copper and nickel-plating steps 1014 and 1023, an instrument such as a power supply that can deliver direct current (DC), pulsed-DC, and/or reversed-pulsed DC, is preferably used to apply the high current density (step 1033). The high current density applied may be, for example, in the range of greater than 10 mA/cm2 to 3,000 mA/cm2. In a specific implementation, the current density applied for optimized tin-lead plating is 43 mA/cm2, while an agitation rate of 400 r.p.m. is used to agitate the tin-lead solution. In a preferred embodiment, a measurement instrument is utilized to determine the optimal current density and/or to determine when a desired plating thickness is achieved.

Once the desired plating thickness is achieved, the now binary-metal plated substrate may be removed from the plating solution and utilized as desired.

Optionally, in order to improve the texture and shape of the tin-lead solder bumps, the plated substrate may be striped and the solder bumps are reflowed to produce spherical and shiny solder bumps (step 1034). To this end, plated substrate may first be stripped of one or more layers of metal through any appropriate conventional stripping means, including via the use of either wet chemistries, dry chemistries and/or a combination of wet and dry chemistries. Once the unwanted layers of metal has been stripped from the substrate, the solder bumps may be reflowed using any conventional reflow techniques.

Unlike conventional single-metal plating, binary-metal plating, and solder-bump plating processes, the plating methods of the present invention may be performed at room temperature.

Claims

1. A system of binary-metal plating comprising:

a solution comprising a material source of metal ions of two or more metals;
an anode and a cathode immersed in the solution, thereby forming a circuit;
a preconditioned substrate electrically connected to the cathode;
a current source connected to the anode and cathode for applying a high current density to the circuit; and
a means for agitating the solution, wherein application of the high current density and the agitating means to the solution causes metal ions in the solution to be reduced and to become uniformly deposited onto the substrate.

2. The system of claim 1, wherein the material source of metal ions comprises tin and lead ions.

3. The system of claim 2, wherein the material source of metal ions are substantially in a composition ratio of 63% tin ions and 37% lead ions.

4. The system of claim 1, wherein the preconditioned substrate comprises one of an integrated circuit, a semiconductor chip, and a wafer.

5. The system of claim 1, wherein high current density is within a range of greater than 10 mA/cm2 to 3,000 mA/cm2.

6. The system of claim 1, wherein the cathode comprises:

an electrically conductive holder having at least one electrical contact; and
a cover.

7. The system of claim 1, wherein the current source comprises a power supply configured to deliver at least one of direct current (DC), pulsed-DC, and reversed-pulsed DC.

8. The system of claim 1, wherein the agitation means comprises an electrically controlled magnetic stirrer.

9. The system of claim 1, further comprising an instrument configured to measure at lease one of plating thickness and plating uniformity.

10. A method of binary-metal plating comprising:

providing a solution comprising a material source of metal ions of two or more metals;
immersing an anode and a cathode into the solution, thereby forming a circuit;
electrically connecting a preconditioned substrate to the cathode;
applying a high current density to the circuit while agitating the solution, thereby causing metal ions in the solution to be reduced and to become uniformly deposited onto the substrate.

11. The method of claim 10, wherein the material source of metal ions comprises tin and lead ions.

12. The method of claim 10, wherein the cathode comprises:

an electrically conductive holder having at least one electrical contact; and
a cover.

13. The method of claim 10, wherein high current density is within a range of greater than 10 mA/cm2 to 3,000 mA/cm2.

14. The method of claim 10, wherein the agitating step is performed via an electrically controlled magnetic stirrer.

15. The method of claim 10, wherein the steps of the method are performed at room temperature.

16. A method of solder bump plating comprising:

single-metal plating a preconditioned substrate to form a single-metal plated substrate;
and binary-metal plating the single-metal plated substrate.

17. The method of claim 16, wherein the single-metal plating step comprises:

providing a single-metal plating solution comprising a material source of metal ions of a single metal;
immersing a first anode and a cathode into the single-metal plating solution, thereby forming a first circuit;
electrically connecting a preconditioned substrate to the cathode; and
applying a low current density to the first circuit, thereby causing metal ions in the solution to be reduced and to become uniformly deposited onto the substrate; and wherein the binary-metal plating step comprises:
providing a binary-metal solution comprising a material source of metal ions of two or more metals;
immersing a second anode and the cathode into the binary-metal plating solution, thereby forming a second circuit;
applying a high current density to the second circuit while agitating the binary-metal plating solution, thereby causing metal ions in the binary-metal plating solution to be reduced and to become uniformly deposited onto the substrate.

18. The method of claim 17, further comprising:

single-metal plating the preconditioned substrate one or more times using one or more single-metal plating solutions prior to the binary-metal plating step.

19. The method of claim 18, further comprising:

single-metal plating the preconditioned substrate using a first single-metal plating solution, thereby forming a single-metal plated substrate;
single-metal plating the single-metal plated substrate using a second single-metal plating solution, thereby forming a double-metal plated substrate; and
binary-metal plating the double-metal plated substrate.

20. The method of claim 19 further comprising:

removing one or more preconditioning layers from the binary-metal plated substrate.

21. The method of claim 20 further comprising:

reflowing the binary-metal plated substrate.

22. The method of claim 21, wherein the first single-metal plating solution comprises copper ions.

23. The method of claim 21, wherein second single-metal plating solution comprises nickel ions.

24. The method of claim 21, wherein high current density is within a range of greater than 10 mA/cm2 to 3,000 mA/cm2.

25. The method of claim 21, wherein low current density is within a range of greater than 9 mA/cm2 to 10 mA/cm2.

26. The method of claim 21, wherein the cathode comprises:

an electrically conductive holder having at least one electrical contact; and
a cover.

27. The method of claims 21, wherein binary-metal plating solution is agitated via an electrically controlled magnetic stirrer.

28. The method of claim 21, wherein the steps of the method are performed at room temperature.

29. A system for solder bump plating comprising:

a first plating solution comprising a material source of metal ions of a first single metal;
a first anode immersed in the first solution;
a preconditioned substrate electrically connected to a cathode;
a second plating solution comprising a material source of metal ions of a second single metal;
a second anode immersed in the second solution;
a third plating solution comprising a material source of metal ions of two or more metals;
a third anode immersed in the third solution;
a means for applying a current density to the first, second, and third plating solutions; and
a means for agitating the third solution,
wherein the cathode and substrate are immersed in the first plating solution, thereby forming a first circuit to which a low current density is applied for causing metal ions in the first plating solution to be reduced and to become uniformly deposited onto the substrate forming a first plating layer thereon;
wherein the cathode and substrate are immersed in the second plating solution, thereby forming a second circuit to which a low current density is applied for causing metal ions of the second plating solution to be reduced and to become uniformly deposited onto the first plating layer thereby forming a second plating layer thereon; and
wherein the cathode and substrate are immersed in the third plating solution, thereby forming a third circuit to which a high current is applied, together with agitation from the agitating means, thereby causing metal ions in the third solution to be reduced and to become uniformly deposited onto the second plating layer thereby forming a third plating layer thereon.

30. The system of claim 29, wherein the cathode comprises:

an electrically conductive holder having at least one electrical contact; and
a cover.

31. The system of claims 29, wherein means for agitating the third plating solution comprises an electrically controlled magnetic stirrer.

Patent History
Publication number: 20080116077
Type: Application
Filed: Nov 21, 2006
Publication Date: May 22, 2008
Applicant: M/A-COM, Inc. (Lowell, MA)
Inventor: Prasit Sricharoenchaikit (Westborough, MA)
Application Number: 11/602,736
Classifications
Current U.S. Class: Agitating Or Moving Electrolyte During Coating (205/148); With Agitator (204/273); With Means For Measuring, Testing, Or Sensing Current, Voltage, Or Power (204/229.8); Forming Multiple Superposed Electrolytic Coatings (205/170)
International Classification: C25D 5/10 (20060101); C25D 17/00 (20060101); C25D 3/02 (20060101); C25D 21/10 (20060101);