Field of the invention

The present invention relates to a method and apparatus for pulse-plating. A novel pulse sequence is also disclosed.

Background

Galvanic plating of electrically-conducting materials and objects is well-known and has been used for many years to provide conductive or protective layers of metal on an object. In the simplest form of galvanic plating, the object to be plated is connected to an electric circuit such that it acts as an anode, and is immersed in a bath containing a solution of metal ions. A counter electrode is also connected to the circuit and is also immersed in the bath. An electric current is supplied to the circuit, and the object becomes plated with metal.

More recent developments in the field of galvanic plating allow the thickness, conductivity and brightness of the layer to be varied. Multiple layers or layers comprising a mixture of metals may also be introduced. Modern galvanic plating can further introduce visually attractive features to the object.

US patent application US2004/0140219 discloses a system and method for pulse current plating. The current is varied from a first level to a second level to provide for differing rates of deposition of a conductive material on the substrate. A relaxation period is included in the pulse sequence to allow the conductive material to reach equilibrium.

US patent number 6,750,144 discloses a method for electrochemical metallization and planarization of semiconductor substrates. A series of plating steps is used, each one comprising a train of pulses. The method is used for filling recesses of different sizes on semiconductor substrates.

The article "Plating with Pulsed and periodic-Reverse Current" By Tai-Ping Sun, CC. Wan & Y. M. Shy, in Metal Finishing from May 1979 discusses the effects of current wave forms on film structure, surface morphology and microhardness.

A particular challenge for practitioners of galvanic plating is to obtain a layer of even thickness, especially when the object to be coated has a complicated or delicate structure. Electric field variations across the surface of an object give rise to variations in the thickness of the plating, particularly where sharp angles on the object's surface concentrate the electric field. At these points, the thickness of the plating is also found to increase. With increasing miniaturisation, problems arise as plating extends to areas where it is not desired, often causing short-circuits.

One solution to the problem of uneven plating is to add chemicals known as "levellers" to the bath. These promote even plating, but may be expensive, increase the need for waste stream purification or adversely affect the quality of the plating.

Severe problems arise when the object to be plated contains cavities, particularly deep cavities. It is particularly difficult to provide a layer of even thickness inside such cavities. Galvanic plating using DC tends to deposit a thick layer of metal around the opening of the cavity and little or no plating occurs at the deeper surfaces of the cavity.

Summary of the invention The present invention provides a method for pulse-plating which addresses the disadvantages of the prior art. Particularly, it allows even, uniform coatings to be applied, even deep inside cavities in the object.

The invention relates to a method for plating a object with a metal coating, said method comprising the steps of: a. bringing the object into contact with a solution comprising metal ions; b. bringing a counter electrode into contact with said solution comprising metal ions; c. connecting an electrical circuit between the object and the counter electrode; d. applying a constant DC current I ₁ to the electrical circuit for a time period ti so as to provide a current density A-i on the surface of the object so that the object becomes plated with a metal coating;

e. applying a second constant DC current I ₂ in the opposite direction as I ₁ to the electrical circuit for a time period t ₂ so as to provide a current density A ₂ on the surface of the object; f. applying a third constant DC current I ₃ in the same direction as Ii to the electrical circuit for a time period t ₃ so as to provide a current density A ₃ on the surface of the object; wherein steps e. and f. may take place in either order; g. repeating steps e. and f., and optionally step d, until the object is plated with the required profile of the metal. wherein steps e and f take place in immediate succession, such that time periods t ₂ and t ₃ are sequential.

The invention further relates to an object plated according to the described method, and to an apparatus for carrying out the method. The invention also relates to the pulse sequence of the described method, and a set of instructions for carrying out the pulse sequence of the described method.

Brief description of the Figures

Figure 1 is a graphical illustration of the variation of current according to the present invention.

Figure 2 is a graphical illustration of the variation of current according to the present invention when steps d, e and f are repeated sequentially.

Figure 3 is a plan view of a base station filter.

Detailed description

The term "cavity" is used in the present context to mean a part of the object in which material been drilled, moulded, gouged or otherwise removed from the object to leave an empty space. It includes those cases in which holes are made completely through an object from one surface to another.

As mentioned above, the invention relates to a method for plating a object with a metal coating, said method comprising the steps of: a) bringing the object into contact with a solution comprising metal ions; b) bringing a counter electrode into contact with said

solution comprising metal ions; c) connecting an electrical circuit between the object and the counter electrode; d) applying a constant DC current I ₁ to the electrical circuit for a time period t-, so as to provide a current density A ₁ on the surface of the object so that the object becomes plated with a metal coating; e) applying a second constant DC current I ₂ in the opposite direction as I ₁ to the electrical circuit for a time period t ₂ so as to provide a current density A ₂ on the surface of the object; f) applying a third constant DC current I ₃ in the same direction as I ₁ to the electrical circuit for a time period t ₃ so as to provide a current density A ₃ on the surface of the object; wherein steps e. and f. may take place in either order; g) repeating steps e. and f., and optionally step d, until the object is plated with the required profile of the metal, wherein steps e and f take place in immediate succession, such that time periods t ₂ and t ₃ are sequential.

Figure 1 illustrates an embodiment of the invention in its most simple form, and shows a graph of how the applied current varies with time. A DC current Ii is applied for a time t-i (step d, above), followed by a short reversal of current to I ₂ for a time t ₂ (step e, above). The current is reversed again to a value I ₃ for a time t ₃ (step f, above). As can be seen from Fig. 1 , currents I ₂ and I ₃ (steps e and f, above) are repeated until the required profile of the metal is obtained. It is not necessary to begin the pulse-plating sequence with the reverse current (I ₂ ); rather I ₃ can be applied first (i.e. steps e. and f. may take place in either order).

Figure 2 shows how the applied current varies with time in an embodiment of the invention in which, after steps e and f have been repeated a number of times, step d is repeated (application of a current I ₁ ).

In the above method, A ₁ is between 0.1 and 5.0 amperes/dm ² , preferably between 0.2 and 2.0 amperes/dm ² and more preferably between 0.25 and 0.75 amperes/dm ² . A ₂ is generally greater than A ₃ . For example, A ₂ may be at least 1.5 times greater, at least 1.7 times greater, at least 2 times greater or at least 3 times greater than A ₃ . A ₂ is between 0.5 and 5.0 amperes/dm ² , preferably between 0.5 and 2.0 amperes/dm ² and more preferably between 0.5 and 1.75 amperes/dm ² . Furthermore, A ₃ is between 0.5 and 5.0 amperes/dm ² , preferably between 0.5 and 2.0 amperes/dm ² and more preferably between 0.5 and 1.5 amperes/dm ² .

The length of the time period ti is adjusted so that the initial coating of metal reaches the desired thickness. For example, I ₁ may be between 10 minutes and 2 hours, preferably between 20 minutes and 1.5 hours, more preferably around 50 minutes.

It is preferred that t ₃ is less than t ₂ . For example, t ₃ may be less than 80% of t ₂ , preferably less than 70% of X ₂ , most preferably less than 55% of X ₂ . The time period t ₂ is between 0.2ms and 10ms, preferably between 1 and 5 ms and more preferably between 1.5 and 2.5ms. Time period t ₃ is between 0.1ms and 5 ms, preferably between 0.5ms and 2.5ms and more preferably between 0.75ms and 1.25ms.

If a time delay is introduced between I ₂ and I ₃ , the method will not work. The process is very sensitive to changes in pulse lengths and time durations between pulses. In a particular embodiment, it is important that the time periods t ₂ and t ₃ are the same length when steps e. and f. are repeated.

The current (I) is applied such that the desired current density (A) on the surface of the object can be obtained. The surface area is always important for platers and if no computer-calculated values exist, one has to calculate the surface manually from the actual geometry of the object. The surface area is defined as the part of the component to be plated. The current density is defined as the total current divided by the total surface area to be plated.

The method functions without the use of chemical additives known as "levellers". The method is best suited in those cases where the object contains one or more cavities. It is particularly effective for those cavities which are deeper than they are wide, i.e. wherein the aspect ratio for the cavity, as defined by the ratio between the diameter (D) and the depth (L) of the largest cylinder which fits in the cavity, is between 0.25 and 3 inclusive, preferably between 0.5 and 2.5 inclusive, more preferably 0.75 and 2 inclusive.

As illustrated in Figure 3, for a cavity with an aspect ratio of 1 , application of a 3 micron layer of silver or copper to the bottom of a cavity through conventional DC-plating will result in at least 3-4 times this depth of metal on the top sides of the cavity. This can be avoided with the method of the present invention.

The value for I ₁ is selected in order to reach the best possible coverage and metal build up in the bottom of the cavity. A typical value is around 0.5 A/dm ² . The best coverage is achieved with DC-plating but the best levelling is achieved with pulse plating. The values of I ₂ and I ₃ are chosen depending on what one would like to achieve. By combining these two features, in subsequent steps, it's possible to have a thicker layer in the bottom then on the edges of the cavity.

The method according to the present invention allows the cavity to be selectively etched a predetermined distance from the plane defining the opening of the cavity, without the use of a mask. Alternatively, appropriate use of the method allows the cavity to be selectively etched a predetermined distance from the deepest point of the cavity, without the use of a mask. Examples of predetermined distances which might be desired are 50 % or 75 % of the depth of the cavity. By changing the current densities and ratios for I ₂ and I ₃ , it is possible to control how deep in the cavity the etching takes place, how fast the etching proceeds and the resulting thickness profile of the etched metal layer.

Through choice of the current and time it is possible for a cavity to be plated in e.g. only the lower half, or alternatively only in the upper half. By "etching" is meant the removal of metal which has been plated.

For plating deep cavities, the method involves the consecutive steps of:

1. Plating the cavities until the requested thickness is reached in the bottom of the cavity, with DC current at a low current density. For plating of copper, to obtain a typical base station filter with a minimum thickness of 3 micron in the bottom, this DC-plating will be performed at a current density (A ₁ ) of 0.5 A/dm ² under 35 minutes (ti).

2. Applying PRC, (Pulse Reverse Current), with a direction opposite to that of the initial current, i.e. etching for levelling the metal layer. For a typical filter described above, the method typically involves a forward pulse length (t ₃ ) of 1.0 ms and a current density (A ₃ ) of 1.0 A/dm ² and a reverse pulse length (t ₂ ) of 2.0 ms and a current density (A ₂ ) of 1.7 A/dm ² . For copper plating of a filter as under point 1 , this pulse plating (repeating steps e. and f. of the method) will go on for approx. 8-10 min.

Under the inventive current/time profile used for etching, the pulse plating step will only affect the metal layer in the bottom of the cavity very little.

Suitable metals for use in the present method are Ag, Au, Cu, Ni, Cr and Zn. Typically, the metal ion is present in a concentration of between 0.1 g/l and 500 g/l. The metals are present in ionic form, having a one or more counter ions which may be selected from the group comprising sulphate, nitrate, chloride, fluoride, bromide, iodide, phosphate, hydroxide, cyanide and/or pyrophosphate.

In one embodiment, the solution comprising metal ions additionally contains one or more surfactants. Preferably, these are selected from the group consisting of non-ionic surfactants, and are more preferably substituted alkylene oxide compounds. In a most preferred embodiment, the surfactant is nonylphenol ethoxylate. Commonly, the surfactant is present in a concentration of between 0.01 g/l and 10g/l, preferably between 0.1g/l and 1 g/l and more preferably between 0.1 g/l and 0.3g/l. The use of polyoxyalkylene compounds will keep the brightness close to what is achieved after DC- plating.

The solution comprising metal ions may additionally contain a polymer. The polymers to be used in the bath are preferably oxyethylene-based (homo, graft and block copolymers), and more preferably polyethyleneglycol with an average molecular weight between 100 and 4000. The polymers are usually present in a concentration ranging from 0.01 g/l to 10.0g/l inclusive, preferably from 0.01 g/l to 1.0g/l inclusive, more preferably from 0.10g/l to 1.0g/l.

The invention further relates to an object plated by the method described herein. Objects which are of particular interest are microwave components and base station filters. The objects which can be plated by the method of the present invention can be made of aluminium, zinc, steel including stainless steel and polymers.

The invention further relates to apparatus for carrying out the method described herein, said apparatus comprising: a. a container suitable for containing a solution comprising metal ions,

b. a solution comprising metal ions, c. a current source comprising means for varying the current I under a time t as set out herein, d. electrical connections, and e. a counter electrode.

The invention also concerns a pulse sequence for carrying out the above-described method, said pulse sequence comprising a. applying a constant DC current I ₁ to the electrical circuit for a time period t-i so as to provide a current density A ₁ on the surface of the object so that the object becomes plated with a metal coating; b. applying a second constant DC current I ₂ in the opposite direction as Ii to the electrical circuit for a time period X ₂ so as to provide a current density A ₂ on the surface of the object; c. applying a third constant DC current I ₃ in the same direction as l-i to the electrical circuit for a time period t ₃ so as to provide a current density A ₃ on the surface of the object; d. repeating steps e. and f., and optionally step d, until the object is plated with the required profile of the metal wherein Ai, A ₂ , A ₃ , t ₂ and t ₃ are as defined above. Particularly, it is of interest that the time periods t,, t ₂ and t ₃ in the above-mentioned pulse sequence are sequential.

The present invention also concerns a list of computer-readable instructions, said instructions when loaded into the memory of a processor connected to a DC power supply cause the processor to vary the current I of the power supply according to the above-mentioned pulse sequence.

A computer-readable instruction for carrying out the pulse-plating sequences of the method may comprise the following directions: a. applying a constant DC current I ₁ to the electrical circuit for a time period U so as to provide a current density A ₁ on the surface of the object so that the object becomes plated with a metal coating; b. applying a second constant DC current I ₂ in the opposite direction as l-i to the electrical circuit for a time period t ₂ so as to provide a current density A ₂ on the surface of the object;

c. applying a third constant DC current I ₃ in the same direction as I ₁ to the electrical circuit for a time period t ₃ so as to provide a current density A ₃ on the surface of the object; d. repeating steps e. and f., and optionally step d, until the object is plated with the required profile of the metal

The invention is not limited to the description and examples given herein, but may be varied within the scope of the claims.

Example 1

Figure 3 is a plan view of a base station filter made of aluminium (approximately 1 :1 scale). The filter was first plated with a layer of nickel, using an electroless Ni plating bath as commonly used in the art to a thickness of 2.5 μm. The Ni-plated filter was then subjected to Cu plating, first with a DC current, then under alternating reverse and forward pulses and then with a second DC current:

First DC phase ca. 0.5 A/dm ² for 60 min

Reverse pulse ca. 1.7 A/dm ² for 2ms

Forward pulse ca. 1.0 A/dm ² for 1 ms Total length of time for which the pulse-plating was applied 30 min

Second DC phase ca. 0.5 A/dm ² for 4 min

The thickness of the copper layer was measured by X-ray. The measurement points were as follows. 1 , 2: On the edge of the filter

3: At a depth of 26 mm in a horizontal plane

4,5: On the bottom of the cavity at a depth of ca. 48 mm, (aspect ratio, (depth/diameter) ca. 1.1)

6,7: In the bottom of a cavity in a resonator rod at a depth of ca. 8 mm, (aspect ratio = 1.1)

The following thicknesses were achieved:

(-) = not measured

As can be seen, the first DC plating step alone gives a coat of Cu which is very unevenly distributed between the edges (1 and 2) and the bottom (4-7) of the base station filter. Pulse-plating can remove Cu which is deposited on the edges 1 and 2. Application of a further DC step creates a Cu coating which is more evenly distributed between the edges (1 and 2) and the bottom (4-7) than previously known methods.

Example 2

A typical base station with open cavities was pulse-plated with silver. The platable area of the filter was 48 dm ² , and filters were racked back to back in order to reduce silver outplating on the backside. A minimum 3.0 μm layer of silver in the bottom of the cavity was required. This requirement was achieved by the pulse-plating process according the present invention in less total process time and with a 28 % reduction in silver consumption compared to conventional DC-plating.

The maximum silver thickness, on the edges was 7 μm for pulse-plating and 15 μm for DC-plating. For the pulse-plating process, 16.2 Ah was applied in the forward direction and 10.9 Ah in the backward direction were consumed i. e. the resulting forward consumption is 5.3 Ah. For DC-plating 7.4 Ah were consumed.

Example 3 The pulse-plating process according to the invention works also for very narrow dimensions. As a result, the invented pulse-plating process can also be used for applications on Printed Circuit Boards. The technology can be used in the same manner for via holes as for cavities for microwave filters. Using the procedure of sequential DC

and pulse reverse plating, it's possible to receive even thicker metal inside the hole, than on the flat board.