WO2016091718A1

WO2016091718A1 - Improved coated copper wire for bonding applications

Info

Publication number: WO2016091718A1
Application number: PCT/EP2015/078555
Authority: WO
Inventors: Murali Sarangapani; Jürgen Scharf; Rajkumar THIRUNARAYANAN; Ahmad Abdillah HARON
Original assignee: Heraeus Deutschland Gmbh & Co Kg; Heraeus Materials Singapore Pte., Ltd.
Priority date: 2014-12-11
Filing date: 2015-12-03
Publication date: 2016-06-16
Also published as: TW201633482A; SG10201408304UA

Abstract

The present invention relates to a wire comprising a core with a surface, a first coating layer with a layer surface and a further coating layer, wherein A) the core comprises a) at least 99.95 wt.% of copper, b) an amount X of at least one element selected from silver and gold, c) an amount Y of at least one element selected from phosphorus, magnesium and cerium, wherein the ratio of X and Y is in the range of from 0.03 to 50; B) the first coating layer is composed of at least one element selected from the group comprising of palladium, platinum and silver, wherein the first coating layer is superimposed over the surface of the core, C) the further coating layer is superimposed over the layer surface of the first coating layer, wherein the further coating layer is composed of gold. The present invention further relates to a method for manufacturing a wire as aforementioned and to an electric device comprising the wire of the invention.

Description

Improved Coated Copper Wire for Bonding Applications

Bonding wires are used in the manufacture of semiconductor devices for electrically intercon- necting an integrated circuit and a printed circuit board during semiconductor device fabrication. Further, bonding wires are used in power electronic applications to electrically connect transistors, diodes and the like with pads or pins of the housing. While bonding wires were made from gold in the beginning, nowadays less expensive materials are used such as copper. While copper wire provides very good electrical and thermal conductivity, ball-bonding as well as wedge-bonding of copper wire has its challenges. Moreover, copper wires are susceptible to oxidation.

With respect to wire geometry, most common are bonding wires of circular cross-section and bonding ribbons which have a more or less rectangular cross-section. Both types of wire ge- ometries have their advantages making them useful for specific applications. Thus, both types of geometry have their share in the market. For example, bonding ribbons have a larger contact area for a given cross-sectional area. However, bending of the ribbons is limited and orientation of the ribbon must be observed when bonding in order to arrive at acceptable electrical contact between the ribbon and the element to which it is bonded. Turning to bonding wires, these are more flexible to bending. However, bonding involves either welding and/or larger deformation of the wire in the bonding process, which can cause harm or even destroy the bonding pad and underlying electric structures of the element, which is bonded thereto.

Some recent developments were directed to bonding wires having a copper core and a protec- tive coating layer. As core material, copper is chosen because of high electric conductivity. With regard to the coating layer, palladium is a possible choice. These coated bonding wires combine the advantages of the copper wire with less sensitivity to oxidation.

Nevertheless, there is an ongoing need for further improving bonding wire technology with regard to the bonding wire itself and the bonding processes.

Accordingly, it is an object of the invention to provide improved bonding wires.

Another object of the invention is to provide a bonding wire which has good processing prop- erties and which has no specific needs when interconnecting.

Another object of the invention is to provide a bonding wire which has excellent electrical and thermal conductivity. Another object of the invention is to provide a bonding wire which exhibits an improved reliability.

Another object of the invention is to provide a bonding wire which exhibits excellent bondabil- ity. Another object of the invention is to provide a bonding wire which shows an improved bonda- bility with respect to a stitch bonding.

Another object of the invention is to provide a bonding wire which shows an improved bonda- bility with respect to a second bonding which is a stitch bonding, whilst the bonding performance for a first bonding which was a ball bonding is at least sufficient.

Another object of the invention is to provide a bonding wire which has improved resistance to corrosion and/or oxidation.

Another object of the invention is to provide a system for bonding an electronic device, which can be used with standard chip and bonding technology, which system shows reduced failure rate at least with respect to a second bonding. Another object of the invention is to provide a method for manufacturing an inventive bonding wire, whereby the method basically shows no increase in manufacturing costs compared with known methods.

The wires of the present invention have been found to solve at least one of the objects men- tioned above. Further, a process for manufacturing these wires has been found which overcomes at least one of the challenges of manufacturing wires. Further, electric devices comprising the wires of the invention were found to be more reliable, both at the interface between the wire according to the invention and other electrical elements, e.g., the printed circuit board, the pad/pin etc., as well at interfaces within the electric devices, where bonding wires are connect- ed to other electric or electronic parts thus constituting the electric device.

A contribution to the solution of at least one of the above objects is provided by the subject- matter of the category-forming claims. The dependent sub-claims of the category-forming claims represent preferred embodiments of the invention, the subject-matter of which also makes a contribution to solving at least one of the objects mentioned above. A first aspect of the invention is a wire comprising a core comprising with a surface, a first coating layer with a layer surface and a further coating layer, wherein

A) the core comprises

a) at least 99.95 wt.% of copper, preferably at least 99.98 wt.% of copper; b) an amount X of at least one element selected from silver and gold;

c) an amount Y of at least one element selected from phosphorus, magnesium and cerium,

wherein the ratio of X and Y is in the range of from 0.03 to 50, preferably in the range of from 2 to 8, or from 4 to 5;

B) the first coating layer is composed of at least one element selected from the group comprising of palladium, platinum and silver,

wherein the first coating layer is superimposed over the surface of the core;

C) the further coating layer is superimposed over the layer surface of the first coating layer,

wherein the further coating layer is composed of gold;

wherein the wire has an average diameter in the range of from 8 to 80 μιη, preferably in the range of from 12 to 55 μιη.

The wire is preferably a bonding wire for bonding in microelectronics. The wire is preferably a one-piece object. Numerous shapes are known and appear useful for wires of the present invention. Preferred shapes are - in cross-sectional view - round, ellipsoid and rectangular shapes.

Preferably, the copper content of the core is at least 99.95 wt.% and not more than 99.995 wt.%. This allows for using common supplies of copper material for wire bonding without exceedingly high costs due to high purity specifications.

In a preferred embodiment, the core of the wire of the invention comprises in total in the range of from 0 to 100 ppm, yet more preferred less than 30 ppm, of further components. The low amount of these further components ensures a good reproducibility of the wire properties. In the present context, the further components, often also referred as "inevitable impurities", are minor amounts of chemical elements and/or compounds which originate from impurities present in the raw materials used or from the manufacturing process which produced the wire. Examples of such further components are: Ni, Mn, Pt, Cr, Ca, La, Al, B, Zr, Ti, S, Fe. Further components present in the core are usually not added separately. The presence of the further components originates from impurities present in one or more of components a), b) and c).

In a preferred embodiment, the core of the wire of the invention comprises less than the following amounts of further components:

a) Any one of Ni, Mn in <15 ppm each;

b) Any one of: Pt, Cr, Ca, La, Al, B, Zr, Ti in <2 ppm each;

c) Any one of S, Fe in <10 ppm each; Yet more preferred at least two of the aforementioned limits are met by the material forming the core, most preferred all limits are met by the material forming the core.

The core of the wire in the present context is defined as a homogenous region of bulk material. Since any bulk material always has a surface region which might exhibit different properties to some extent, the properties of the core of the wire are understood as properties of the homogeneous region of bulk material. The surface of the bulk material region can differ in terms of morphology, composition (e.g. oxygen content) and other features. The surface can be an outer surface of the inventive wire in preferred embodiments. In further embodiments, the surface of the wire core can be an interface region between the wire core and a coating layer superim- posed on the wire core.

The term "superimposed" in the context of the present invention is used to describe the relative position of a first item, e.g. a copper core, with respect to a second item, e.g. a coating layer. "Superimposed" characterizes, that further items, such as an intermediate layer, can - but no need to be arranged between the first and the second item. Preferably, the second item is at least partially superimposed over the first item, e.g. for at least 30 %, 50 %, 70 % or for at least 90 %, each with respect to the total surface of the first item. Most preferably, the second item is completely superimposed over the first item. The term "intermediate layer" in the context of this invention refers to a region of the wire between the copper core and the coating layer. In this region, a combination of materials of both, the core and the coating layer, is present.

The term "thickness" in the context of this invention is used to define the size of a layer in per- pendicular direction to the longitudinal axis of the copper core, which layer is at least partially superimposed over the surface of copper core.

The average diameter is obtained by the "sizing method". According to this method the physical weight of the wire for a defined length is determined. Based on this weight, the diameter of the wire is calculated using the density of the wire material (density of copper: p_Cu = 8.92 g/cm³). The average diameter is calculated as arithmetic mean of five measurements on five cuts of a particular copper wire.

For the present invention, the term bonding wire comprises all shapes of cross-sections and all usual wire diameters, though bonding wires with circular cross-section and thin diameters are preferred.

The line intercept method for determining the average size of crystal grains is a standard metallo graphic practice. There, a wire is cut parallel to the direction of the wire and the cross- section generated thereby is etched. The size of a crystal grain in the present context is defined as the longest of all sections of straight lines which can be passed through the grain. The average size of crystal grains is the arithmetic mean of at least seven measurements of crystal grains in the core / bulk material. The testing is performed according to ASTM El 12-96 standard, section 16.3, page 13. Such thin wires mostly, but not necessarily have a cross-sectional view essentially in the shape of a circle. The term "a cross-sectional view" in the present context refers to a view of a cut through the wire, wherein the plane of the cut is perpendicular to the longitudinal extension of the wire. The cross-sectional view can be found at any position on the longitudinal extension of the wire. A "longest path" through the wire in a cross-section is the longest chord which can be laid through the cross-section of the wire within the plane of the cross-sectional view. A "shortest path" through the wire in a cross-section is the longest chord perpendicular to the longest path within the plane of the cross-sectional view defined above. If the wire has a perfect circular cross-section, then the longest path and the shortest path become indistinguishable and share the same value. The term "diameter" is the arithmetic mean of all geometric diameters of any plane and in any direction, wherein all planes are perpendicular to the longitudinal extension of the wire.

A preferred embodiment of the invention is a wire, wherein the total amount Y of the at least one element selected from phosphorus, magnesium and cerium is in the range of from 15 to 300 ppm, preferably in the range of from 30 - 200 ppm, or in the range of from 40 to 80 ppm, each value in ppm based on the total weight of the core.

A further preferred embodiment of the invention is a wire, wherein the at least one element forming the amount Y is selected from phosphorous. It has been found that such an amount of phosphorous in the core improves the core's resistance against oxidation while the Vickers micro hardness of the core is not altered.

A further preferred embodiment of the invention is a wire, wherein the total amount X of at least one element selected from silver and gold is in the range of from 5 to 10000 ppm, preferably in the range of from 20 to 5000 ppm, or from 50 ppm to 2000 ppm, or from 100 ppm to 800 ppm, and yet more preferred in the range of from 100 - 350 ppm, each value in ppm based on the total weight of the core. A further preferred embodiment of the invention is a wire, wherein the first coating layer has a thickness in the range of from 40 nm to less than 0.5 μιη, preferably in the range of from 40nm to 200nm, or in the range of from 40 nm to 80 nm. In the event, the first coating layer is composed of palladium, a further embodiment of the invention has first a coating layer which has a thickness of less than 0.5 μιη. Yet more preferred, further coating layer has a thickness of less than 0.05 μιη. A sufficiently thin further coating layer causes only little changes to most properties of the overall wire. However, some properties are remarkably improved, in particular regarding the bonding process.

Concerning the composition of the first coating layer, the palladium content of the layer is at least 50 wt.%, more preferably at least 95wt.%, each wt.% based on the total weight of the first coating layer. Particularly preferred, the coating layer consists of pure palladium. Pure palladium usually has less than 1 wt.% of further components, with regard to the total amount of pal- ladium in the first coating layer. In a further preferred embodiment, the further components present in the first coating layer are noble metals.

A further preferred embodiment of the invention is a wire, wherein the further coating layer has a thickness in the range of from 1.0 nm to less than 500 nm, preferably in the range of from lnm to lOOnm, or from lnm to 50nm.

A further preferred embodiment of the invention is a wire, wherein the core has a content of silver or gold in the range of 5 ppm to 10000 ppm, preferably in the range of 5 ppm to 1000 ppm, yet more preferred in the range of 200 ppm to 250 ppm. It was observed that the presence of at least small amounts of silver improves the mechanical properties, e.g. awards some softness to the wire.

A further preferred embodiment of the invention is a wire, which has a further coating layer, in which the gold content is at least 50 wt.%>, more preferably at least 95 wt.%>, each based on the total amount wt.% of the further coating layer. Particularly preferred, the further coating layer consists of pure gold. Pure gold usually has less than 1 wt.%> of further components, with re- gard to the total amount of gold in the further coating layer. In a further preferred embodiment, the further components present in the first coating layer are noble metals.

A further preferred embodiment of the invention is a wire, wherein the core of the wire has an average size of crystal grains in the range of from 3 to 5.0 μιη, the average size determined according to line-intercept method (see definition above).

A further preferred embodiment of the invention is a wire, wherein in the range of from 18 to 42 % of the grains of the wire are oriented in <100> direction; and in the range of from 27 to 38 % of the grains of the wire are oriented in < 11 1 > direction; each % with respect to the total number of crystals with orientation parallel to the drawing direction of the wire. Grains of the wire are oriented in the specified direction if the direction of the crystal grains deviates less than from -15° to +15°, whereby drawing direction of the wire was used as reference orientation The <100> and <11 1> texture percentages were calculated by counting the number of crystals with <100> and the number of crystals with <111> orientation. These numbers were divided by the sum of both <100> and <111>, since usually no crystal grains with orientation <010> were identified.

A further preferred embodiment of the invention is a wire which is characterized by at least one, preferably two or more, or all of the following features: a) The ratio of elongation value AL of the wire and the average diameter d of the wire (1) is in the range of from 0.05 %/μηΐ ΐο 1.5 %/μιη; preferably in the range of from 0.25 %/μι ίο 0.75 %/μιη;

The elongation AL of the wire is given as a percentage value. If an initial length L of the wire is given and the wire is stretched to a length L' until it breaks, the elongation is defined by AL = (L'-L)/L. β) the wire meets the equation 0.000025 < AL /(d*CX) < 0.3, wherein

AL = elongation of the wire in %; d = average diameter of the wire in μιη;

CX = content of silver or gold, measured in ppm;

Such wire has an optimized silver or gold content in some aspects, e.g. lOppm or 225ppm of Ag. Preferably, the wire meets the relation 0.0002 < AL/(d*CX) < 0.15, and most preferably the wire meets the relation 0.001 < AL/(d*CX) < 0.08. the wire meets the equation 0.0008 < AL /(d*CY) < 0.15, wherein

AL = elongation of the wire in %;

d = average diameter of the wire in μιη;

CY = content of phosphorus, magnesium or cerium, measured in ppm;

Such wire has an optimized phosphorous or magnesium or cerium content dependent on the respective demands for its elongation and diameter. Preferably, the wire meets the relation 0.001 < AL/(d*CY) < 0.08, and most preferably the wire meets the relation 0.003 < AL/(d*CY) < 0.03. the micro hardness of the wire core is not more than 125 HV, determined using a Fischer scope HI IOC tester with Vickers diamond indenter applying lOmN force for 5s dwell time. The micro hardness of the wire core limits the maximum forces which can be applied on the bond pad during the bonding procedure. Such limitation can be of advantage if mechanically sensitive structures are arranged under the bond pad. Examples of such are bond pads which have a soft coating material like aluminum or gold. For example, such a sensitive structure can comprise one or several layers of porous silicon dioxide, in particular of silicon dioxide with a dielectric constant of less than 2.5. Such porous soft materials become increasingly common since they contribute to improve a device's performance. In line with this, the mechanical properties of a wire of a preferred embodiment of the invention are optimized in the aforementioned way which avoids cracking or other damaging of the weak layers. ε) the electrical resistivity of the wire is in the range of from 1.69 to 1.90 μΩ*αιι, preferably in the range of from 1.75 to 1.86 μΩ*αη. γ) the nano-hardness of at least one, preferable both, the first coating layer and the further coating layer, is in the range of from 340 to 430 HV.

A second aspect of the invention is a method for manufacturing a wire, preferably a wire as described for the first aspect of the invention, which method comprises at least the steps of i. Providing a precursor item comprising:

a) at least 99.95 wt.% of copper as the main component of the core, preferably at least 99.98 wt.% of copper;

b) an amount X of at least one element selected from silver and gold, c) an amount Y of at least one element selected from phosphorus, magnesium and cerium,

ii. drawing the precursor item to a core precursor, until the core precursor has an average diameter in the range of from 50 to 250 μιη, preferably in the range of from 80 to 200 μιη;

iii. coating the core precursor with at least one element selected from the group consisting of palladium, platinum or silver, whereby a first coating layer is formed on the core precursor;

iv. further coating gold on the first coating layer of the core precursor obtained in step iii., whereby a further coating layer is formed;

v. drawing the coated core precursor obtained from step iv. to a final diameter in the range of from 8 to 80 μιη;

vi. annealing the product prepared in step v.;

whereby the wire according to the second aspect of the invention is obtained, wherein this wire has an average diameter in the range of from 8 to 80 μιη. Preferred embodiments of the second aspect of the invention are those, which have been described for the first aspect of the invention in the above. A precursor item as in step i. can be obtained by doping copper with an amount of phosphorus, magnesium and/or cerium, silver and/or gold, optionally by doping with further elements. Doping can be realized by producing a melt of said components and copper and cooling the melt to form a homogeneous piece of copper based precursor item.

A first embodiment to the second aspect of the invention is a method, wherein the annealing of the product in step vi. is performed at a temperature of at least 400 °C, preferably of at least 430 °C, or of at least 540 °C. Higher annealing temperatures can provide for higher values for the elongation of the wire.

Concerning further parameters for annealing, in particular thin wires need not be exposed to the annealing temperature for long. In most cases annealing is done by pulling the wire through an annealing oven of a given length and with a defined temperature profile at a given speed. An exposure time of a thin wire to the annealing temperature is typically in the range of 0.1 second to 10 seconds.

A third aspect of the invention is a method for connecting an electrical device, comprising the steps of

I. providing a wire as described for the first aspect of the invention or one of the embodiments thereto, or a wire obtained by a method according the second aspect of the invention or one of the embodiments thereto;

II. bonding the wire provided in step I. to a first bond pad of the device by either ball bonding or wedge bonding; and

III. bonding the wire of step I. which is bonded to a first bond pad to a second bond pad of the device by wedge bonding;

wherein step III. is performed without the use of a forming gas; and

wherein step II. is performed in presence of an inert gas or forming gas. In a first embodiment to the third aspect of the invention, the wire is bonded in step II. by ball bonding and in step III. by wedge bonding. Wires according to the third aspect of the invention or to the embodiment thereto have excellent properties with respect to oxidation effects. Even better protection against oxidation of the copper core is achieved by complete encapsulation of the core with thin gold over palladium coating layers are present in combination with a certain amount of silver and phosphorous in the core material. The resulting properties allow for processing the wire by purging forming gas and hence lead to clean, axi- symmetrical free air ball form. Forming gas is known in the art as a mixture of an inert gas like nitrogen with hydrogen, wherein the hydrogen content may provide for reduction reactions of oxidized wire material. In the sense of the invention, omitting of forming gas means that no reactive compounds like hydrogen is used. Nevertheless, use of an inert gas like nitrogen can still be advantageous.

A fourth aspect of the invention is a method for manufacturing a wire according to the invention, comprising the steps;

a. providing a copper core precursor item as in the second aspect of the invention;

b. drawing the precursor until a final diameter of the wire core is reached;

c. coating the copper core with the material of the first and further coating layer, either before or after step b - drawing the precursor;

d. annealing the coated and drawn wire at a defined temperature for a minimum time.

Concerning steps b to c, the manufacturing method is generally known in the art. It is pointed out that the coating layer might be applied by any known or suitable method like mechanical coating, electrolytic plating, electroless plating, physical vapor deposition (PVD), chemical vapor deposition (CVD) and more. The coating can be done before or after the drawing of the wire, which may be dependent on properties of the respective coating and coating method. In particular, the coating might be performed at an intermediate step, with a drawing of the wire or precursor occurring before as well as after the coating step. As to step d, annealing is performed in a controlled way as known in the art in order to achieve a softening of the wire and/or optimizing the crystal structure of the wire according to the respective demands. Preferably, the annealing is done dynamically while the wire is moved through an annealing oven and wound onto a spool after having left the oven.

A fifth aspect of the invention is an electrical device comprising a first and a second bonding pad, and a wire according to the first aspect of the invention or an embodiment thereto, or a wire obtained by a method according the second aspect of the invention or one of the embodi- ments thereto, wherein the wire is connected to at least one of the bonding pads using ball- wedge bonding. An electric device in the present context includes electronic devices. Electronic devices are those which comprise semiconducting elements.

An embodiment of the fifth aspect is an electrical device, wherein the wire is connected to both pads by ball-wedge bonding.

A sixth aspect of the invention is a system for bonding an electronic device, comprising a first bond pad, a second bond pad and a wire according to the invention, wherein the wire is connected to at least one of the bonding pads by means of stitch-bonding. This combination of an inventive wire in a system is preferred due to the fact that the wire has especially beneficial properties with respect to stitch bonding.

In an embodiment of the fifth and sixth aspect of the invention is a method, wherein the pro- cess window for the at least one stitch bond to a gold bond pad has a value of at least 11550 mA*g for a wire which has a diameter of 18 μιη.

The definition of a process window area for bonding wires is known in the art and is widely used to compare different wires. In principle, it is the product of a bonding window of an ultra- sonic energy used in the bonding and a bonding window of a force used in the bonding, wherein the resulting bond has to meet certain pull test specifications, e.g. a pull force of 2.5 grams, no non-stick on lead etc.. The actual value of the process window area of a given wire further depends on the wire diameter as well as the bond pad material. In order to give a specific definition of the properties of an inventive wire, the claimed process window value is based on a wire diameter of 18 μιη = 0.7 mil, wherein the bond pad consists of gold. The scope an in- ventive system is not limited to wires of this diameter and bond pads made of gold, but names this data only for definition purpose.

Description of the figures The subject matter of the invention is exemplified in the figures. The figures, however, are not intended to limit the scope of the invention or the claims in any way.

In Figure 1 , a wire 1 is depicted. Figure 2 shows a cross sectional view of wire 1. In the cross sectional view, a copper core 2 is in the middle of the cross sectional view. The copper core 2 is encompassed by a palladium coating layer 3. The palladium coating layer 3 is encompassed by a thin gold coating layer 41. On the limit of copper wire 2, a surface 15 of the copper core is located. On the limit of palladium coating 3, a surface 42 of the palladium coating is located. On a line L through the cen- tre 23 of wire 1 the diameter of copper core 2 is shown as the end to end distance between the intersections of line L with the surface 15. The diameter of wire 1 is the end-to-end distance between the intersections of line L through the centre 23 and the outer limit of wire 1. In addition, thickness of coating layer 3 and 41 are depicted. Figure 3 shows a process for manufacturing a wire according to the invention.

Figure 4 depicts an electric device 10 comprising two elements 11 and a wire 1. The wire 1 electrically connects the two elements 11. The dashed lines mean further connections or circuitry which connects the elements 11 with external wiring of a packaging device surrounding the elements 11. The elements 11 can comprise bond pads, lead fingers, integrated circuits, LEDs or the like.

Figure 5 shows a sketch of a wire pull test. To a substrate 20, a wire 1 is bonded in bonds 21 at an angle 19 of 45°. A pull hook 17 pulls wire 1. The angle 32, which is formed when the pull hook 17 pulls the wire 1, is 90°.

Figure 6: A graph and picture were obtained by a typical scratch test on gold and palladium plated wire surface. The plated layers on the wire of this example show good adhesion. Brief discussion of the graph and observations are provided in test methods and in the section Results.

The graph in Figure 7 demonstrates a second bond process window (ultrasonic energy (USG) versus force) of 0.7mil gold flash palladium coated copper wire (samples 1 & 3) in comparison with gold flash palladium coated copper wire of 0.7mil diameter (sample "Comp 1").

The graph in Figure 8 demonstrates a second bond process window (scrub amplitude versus force) of 0.7mil gold flash palladium coated copper wire (samples 1 & 3) in comparison with bare copper wire of 0.7mil diameter (sample "uncoated bare copper"). Figure 9 illustrates grain size measurements carried out in the present invention as per ASTM standard El 12-96, section 16.3, page 13. GFAB is the grain size of the FAB, GW is the grain size of the Wire, LFAB is the length of the intercept line marked in a FAB, LW is the length of the intercept line marked in the wire, NFAB is the number of grains intersecting the line, and NW is the number of grains intersecting the line.

Examples

The invention is further exemplified by examples. These examples serve for exemplary elucidation of the invention and are not intended to limit the scope of the invention or the claims in any way. Example 1

A quantity of copper material of at least 99.99 % purity ("4N-copper") was molten in a crucible. Small amounts of master alloy were added to the copper melt and an uniform distribution of the added components was ascertained by induction chunking. The amounts of added silver and phosphorous were calculated to contribute to a share as shown in Table.1. Then a wire core precursor was cast from the melt.

Table 1 : Wire core composition

The cross section of the wire core was of circular shape. The wire had an average diameter of 201 μιη. The average diameter was determined by individual measurements of diameter at different spots of the wire which led to measurement results in the range of from 200.5 to 201.5 μιη.

The wire core was then coated with a layer consisting of palladium (Pd) of at least 99% purity. The thickness of the palladium coating layer was 702nm. Accordingly, the coating layer did not change the wire diameter significantly. Then, the palladium plated wire was coated with a layer of gold of at least 99% purity. The thickness of the coating layer was 31nm. Accordingly, the coating layer did not change the wire diameter significantly. After these coating steps , the wire plated with palladium and gold was then drawn in 3 major drawing stages with 22 steps in each stage to form a wire core with an average diameter of 18 μιη, wherein an elongation of the precursor item of from 6 to 18 % in length was performed in each step. In the examples presented herein, 11% elongation was practised for each of stage 1 to stage 3. A slipping agent was employed during drawing.

In an alternative way of manufacture, the wire core was coated with a Pd layer and a gold layer, both of at least 99% purity, at slightly thicker diameter than final wire size. In this case, the average wire diameter was in the range 75 to 250 micron. It is preferred to coat at 200μιη with a coating thickness in the range of 700 - 800 nm or less for palladium and in the range of 30 - 35 nm or less for gold. The wires were degreased in-line, palladium electroplated, followed by gold electroplating, washed and spooled for final wire drawing. During electro-plating the wire speed was maintained at 5 to 25m/min., voltage applied was between IV and 8V, current applied was from 0.05 to 5 A. The pH of palladium plating bath was maintained between 7 and 10 and gold plating was maintained in the range from 4 to 6. The plating was processed at 40 to 60°C.

The plated wire core precursor was then drawn in 3 major drawing stages with 22 steps in each stage to form a wire core with an average diameter of 18 μιη, wherein an elongation of the precursor item of from 6 to 18 % in length was performed in each step. In the examples pre- sented herein, 11% elongation was practised form 1 to stage 2 and 8% elongation was practised for stage3. A slipping agent was employed during drawing.

The cross section of the wire core was of circular shape. The wire had an average diameter of 18 μιη. The average diameter was determined by individual measurements of diameter at dif- ferent spots of the wire which led to measurement results in the range of from 17.5 to 18.5 μιη. Results

The scratch test using diamond indenter and nano indentation revealed excellent adhesion of gold plated layer to palladium plated layer and palladium plated layer to copper core surface without any sign of plated layer peeling (see e.g., Fig.6). The diamond indenter penetrated to a depth of 1600nm from plated wire surface to copper core and running for ΙΟΟμιη length, where peeling was absent when tested at about 10 locations. The hardness of gold plating tested using nano hardness indentation approach are provided in Table 2. The depth of indentation illustrates indenter has penetrated mainly in gold plated layer and the reported value should reflect the gold plating hardness of about 400HV.

Table 2: Hardness measurement of the gold and palladium plated wire surface

The coated wire was annealed in an annealing step in order to further adjust parameters like elongation, hardness, crystal structures and the like. The annealing is performed dynamically by running the wire through an annealing oven of a length of 30 cm and a temperature of 430 °C. In the present examples, the annealing time, which is the time during which a given piece of the moving wire remains in the heated oven, was about 0.76 s. After leaving the oven, the wire is spooled on reels for packaging.

Table 3 and Table 4 show data of examples sample 1 to sample 8 of wires according to the invention. The exemplary inventive wires have been produced identically, only the annealing temperatures in the annealing step was varied. None of the further annealing parameters apart from the temperature has been changed. Further, Table 5 shows data of eight samples and commercially purchased bonding wire Compl . As the exact production process of these wires (Compl) is not known, data of annealing temperatures is not available. For comparison data of uncoated bare copper wire is included. All of the evaluated wires (inventive wires and comparative wires) have a diameter of 0.7 mil = 18 μιη.

Table3: Wire core composition, diameter 18 μιη, values in ppm, gold and palladium plated.

Table 4: Calculating the ratio of elongation to diameter and dopants of gold and palladium plated 18 μιη diameter wire. Values in ppm.

With: AL Elongation (in %)

BL Break load (in grams)

T Annealing temperature (in degrees Celsius)

d diameter (= 18 μιη for all wires) Solid- solution concentration in ppm

Deoxidizer concentration in ppm

Table 5: Second bond performance of 18μιη diameter gold and palladium plated wire with different core compositions

Remarks: The FAB ("free air ball shape") was evaluated visually and categorized into four classes (decreasing quality): best - good - fair - bad. The FAB is the shape of the ball of a ball bonding performed under standard conditions which are the same for all of the evaluated wires. The FAB is excellent when the molten drop of the wire solidifies with spherical and axis- symmetrical ball like shape. Fair describes a molten drop of wire solidify, but the ball size is smaller than the specification and/or tilted.

The second bond process window areas were defined either as the product of the respective differences between upper and lower borders of the scrub amplitude and the applied force, or as product of the respective differences between upper and lower borders of the ultrasonic en- ergy (USG) and the applied force. The second bond window value is generally preferred to have a lower value, means lower the energy required to bond. The data of Table 3 to Table 4 show that wires according to the invention have some outstanding properties whilst maintaining at least good quality throughout all of the evaluated properties. In particular, the inventive wires generally show a wide the second bond window area, i.e. 10450 mA.g or more, e.g. as wide as 11550mA. g. For the comparative examples, the wire Compl shows also a high second bond window area, but it has high bond force. The high bond force indicates more energy is required to supply to the bond, which is disadvantageous for the process stability (Fig.7). In addition, the 2^nd bond process windows were plotted for other pa- rameters versatile in industrial measurement, here: force versus scrub amplitude. Based on Fig.8, it becomes evident that samplel and sample3 revealed a wider bond process window than uncoated bare copper wire. Furthermore, the Compl sample has a significantly higher ball hardness of 105HV. As a result, the wire Compl would be less suited at least for ball bonding applications when compared with the inventive wires. To the contrary, the inventive wires are particularly useful for first and second bond applications. Inventive wires can be a preferred choice for wedge-wedge-bonding applications, but also show excellent results for ball-wedge- bonding applications.

Test Methods

All tests and measurements were conducted at T = 20 °C and a relative humidity RH = 50 %. a. Average size of crystal grains by line intercept method The size of crystal grains was determined using a standard metallographic technique,

ASTM El 12-96, section 16.3, page 13. A sample of the wires core was cut parallel to the direction of the wire and the cross-section obtained thereby was etched. In the present case, a solution of 2g FeCl₃ and 6 ml concentrated HC1 in 200 ml deionized water was used for the etching. The crystal grain size was determined according to the line in- tercept principle. The size of a crystal grain in the present context was defined as the longest of all sections of straight lines which passed through the grain. The measured average size of crystal grains was the arithmetic mean of at least seven measurements of crystal grains in the core material. The schematic in Figure 9 illustrates grain size measurement carried out in the present invention as per above mentioned ASTM standard, where G_FAB is the grain size of the FAB, Gw is the grain size of the Wire, L_FAB is the length of the intercept line marked in a FAB, Lw is the length of the intercept line marked in the wire, N_FAB is the number of grains intersecting the line, and Nw is the number of grains intersecting the line. b. Elongation (AL)

The tensile properties of the wires were tested using an Instron-5300 instrument. The wires were tested at 1 (one) inch/min speed, for 10 inch gauge length. The load and elongation on fracture (break) were acquired as per ASTM standard F219-96. The elongation was difference in the length of the wire before and after tensile testing, calculated from the recorded load versus extension tensile plot. c. Vickers Hardness (micro hardness)

The hardness was measured using a Fischer scope HI IOC testing equipment with a Vickers indenter. A force of 10 mN was applied to a test specimen of wire for a dwell time of 5 s. The testing was performed in a cross-sectional cut on the center of the coated and annealed wire core along the longitudinal axis which is also the wire axis. d. Coating Layer thickness

For determining the thickness of a coating layer and the average diameter of the core, the wire was cut perpendicular to the maximum elongation of the wire. The cut wire was carefully grinded and polished to avoid smearing of soft materials. A picture was recorded through an optical microscope or a scanning electron microscope (SEM), wherein the magnification was chosen so that the full cross-section of the wire was observed. This procedure was repeated at least 5 times. All values were provided as arithmetic mean of the 5 measurements. e. Process window area

Measurements of ball-bonding process window area were done by standard procedure. The test wires were bonded using a KNS-iConn bonder tool (Kulicke & Soffa Industries Inc, Fort Washington, PA, USA). The definition of a 2^nd bond process window area for bonding wires was known in the art and was widely used to compare different wires. In principle, it is the product of scrub amplitude and force used in the bonding, wherein the resulting bond has to meet certain pull test specifications, e.g. a pull force of 2.5 grams, no non-stick on lead etc.. The actual value of the 2^nd bond process window area of a given wire further depends on the wire diameter as well as the lead finger plated material. In order to give a specific definition of the properties of an inventive wire, the process window value were based on a wire diameter of 18 μιη = 0.7 mil, wherein the lead finger consists of silver.

The four corners of the process window were derived by overcoming the two main failure modes:

(1) supply of too low force and scrub amplitude lead to non-stick on lead (NSOL) of the wire, and

(2) supply of too high force and scrub amplitude lead to short tail (SHTL).

The scope of the inventive system was not limited to wires of this diameter and lead fingers made of silver, but names this data only for definition purpose. f. Free air ball The electric flame off (EFO) current and time defines the specification of the FAB. On EFO firing, the tip of the fractured Cu wire melts and form axi- symmetrical spherical FAB, further stitch the wire on lead frame such that FAB stand on air. This mode of bonding was referred to as cherry pits. The procedures are described in the K S Pro- cess User Guide for Free Air Ball (Kulicke & Soffa Industries Inc, Fort Washington,

PA, USA, 2002, 31 May 2009). The FAB diameter was measured using optical microscope at 200X to 500X magnification in micron scale. Morphology of the FAB was observed using scanning electron microscope (SEM). g. Electrical Conductivity

Both ends of a test specimen, i.e. a wire of 0.5 m in length, were connected to a power source providing a constant current/voltage. The resistance was recorded with a device for the supplied voltage. The unit used for measuring was Resistomat model 2316, and the test was repeated with at least 10 test specimens. The arithmetic mean of the ten measurements was used for the calculations given below.

The resistance R was calculated according to R = V / I. The specific resistivity p was calculated according to p = (R x A) / 1, wherein A is the mean cross-sectional area of the wire and 1 the length of the wire between the two measuring points of the device for measuring voltage.

The specific conductivity σ was calculated according to σ = 1 / p.

h. Nano -hardness

The gold and palladium plating adhesion was characterized by measuring the hardness and scratch tests using a diamond indenter and a nano-indentation testing unit. The plated wire surface was scratched with 50mN load, to a depth of 800nm and length of ΙΟΟμηι. The test parameters used for nano hardness were;

• Indentation maximum load is 50mN

• loading rate is 0.05mN/s

• Peak hold time is 5s

• Unloading rate is 0.05mN/s

• Calculation model is Oliver and Pharr method. Electron Back-Scattered Diffraction (EBSD)

The main steps adopted to measure wire texture were sample preparation, getting good Kikuchi pattern and component calculation;

(a) The bonding wires were first potted using epoxy and polished as per standard metallographic technique. Ion milling was applied in the final sample preparation step to remove any mechanical deformation of the wire surface, contamination and oxidation layer. The ion-milled cross-sectioned sample surface was sputtered with gold. Then ion milling and gold sputtering were carried out for two further rounds.

(b) The sample was loaded in a FESEM (field emission scanning electron microscope) equipped with an electron back-scattering diffraction (EBSD) detector. The electron back-scattering patterns (EBSP) which contain the wire crystallo- graphic information were obtained.

(c) These patterns were further analyzed for grain orientation fraction, grain size, etc. Points of similar orientation were grouped together to form texture component. To distinguish different texture component, maximum tolerance angle of 15° was used. The wire drawing direction was set as a reference orientation. The <100> and <111> texture percentages were calculated by measurement of the percentage of crystals with <100> and <111> orientation parallel to the reference orientation. Commonly <010> component was absent. j. Tensile Test

Tensile test on the wires were performed using an Instron-5300 for 0.25m gage length and 1 inch/mm test speed.

Embodiments of the invention

I A wire (1), comprising a core (2) with a surface (21), a first coating layer (3) with a layer surface (31) and a further coating layer (4), wherein

A) the core (2) comprises

a) at least 99.95 wt.% of copper;

b) an amount X of at least one element selected from silver and gold;

wherein the ratio of X and Y is in the range of from 0.03 to 50;

B) the first coating layer (3) is composed of at least one element selected from the group comprising of palladium, platinum and silver,

wherein the first coating layer (3) is superimposed over the surface (21) of the core (2);

C) the further coating layer (4) is superimposed over the layer surface (31) of the first coating layer (3),

wherein the further coating layer (4) is composed of gold;

wherein the wire (1) has an average diameter in the range of from 8 to 80 μιη.

II The wire (1) according to embodiment I, wherein the amount Y is in the range of from 20 to 300 ppm, based on the total weight of the core (2).

III The wire (1) according to embodiment I or II, wherein the amount X is in the range of from 5 to 10000 ppm, based on the total weight of the core (2).

IV The wire (1) according to any one of the precedent embodiments, wherein the first coating layer (3) has a thickness in the range of from 40 nm to less than 0.5 μιη.

V The wire (1) according to any one of the precedent embodiments, wherein the further coating layer (4) has a thickness in the range of from 1.0 nm to less than 50 nm. VI The wire (1) according to any one of the precedent embodiments, wherein the core (2) has an average size of crystal grains in the range of from 3 to 5.0 μιη, the average size determined according to line-intercept method.

VIIThe wire (1) according to any one of the precedent embodiments, wherein in the range of from 18 to 42 % of the grains of the wire are oriented in <100> direction; and in the range of from 27 to 38 % of the grains of the wire are oriented in < 111 > direction; each % with respect to the total number of crystals with orientation parallel to the drawing direction of the wire (1).

VIII The wire (1) according to any one of the precedent embodiments, characterized by at least one of the following features:

a) The ratio of elongation value AL of the wire (1) and the average diameter d of the wire (1) is in the range of from 0.05 %/μιη to 1.5 %/μιη;

β) the wire (1) meets the equation 0.000025 < AL /(d*CX) < 0.3, wherein

AL = elongation of the wire in %;

d = average diameter of the wire in μιη;

CX = content of silver or gold, measured in ppm;

γ) the wire (1) meets the equation 0.0008 < AL /(d*CY) < 0.15, wherein

AL = elongation of the wire in %;

d = average diameter of the wire in μιη;

CY = content of phosphorus, magnesium or cerium, measured in ppm; δ) the micro hardness of the wire core (2) is not more than 125 HV; ε) the electrical resistivity of the wire (1) is in the range of from 1.69 to

1.90 μΩ*αη. IX A method for manufacturing a wire (1), comprising at least the steps of i. Providing a precursor item (5) comprising:

a) at least 99.95 wt.% of copper as the main component of the core (2), b) an amount X of at least one element selected from silver and gold, c) an amount Y of at least one element selected from phosphorus, magnesium and cerium,

wherein the ratio of X and Y is in the range of from 0.03 to 50;

ii. drawing the precursor item (5) to a core precursor (2a);

iii. coating the core precursor (2a) with at least one element selected from the group consisting of palladium, platinum or silver whereby a first coating layer (3) on the core precursor (2a) is formed;

iv. further coating gold on the first coating layer (3) of the core precursor (2a) obtained in step iii., whereby a further coating layer (4) is formed;

v. drawing the coated core precursor obtained from step iv. to a final diameter of 8-80 μιη;

vi. annealing the product prepared in step v.;

whereby the wire (1) is obtained,

X The method of embodiments IX, wherein the annealing of the product in step vi. is performed at a temperature of at least 400 °C.

XI A method for connecting an electrical device (10), comprising the steps of

I. providing a wire (1) according to any one of embodiment I to VIII, or a wire obtained by a method according to any one of embodiments IX to X,

II. bonding the wire (1) to a first bond pad (11) of the device (10) by either ball bonding or wedge bonding; and III. bonding the wire (1) to a second bond pad (11) of the device (10) by wedge bonding;

wherein step III. is performed without the use of a forming gas; and wherein step II. is performed in presence of an inert gas or forming gas.

XII An electrical device (6) comprising a first and a second bonding pad (11, 11), and a wire (1) according to any one of embodiments I to VIII, or a wire obtained by a process according to any one of embodiments IX to X, wherein the wire (1) is connected to at least one of the bonding pads (11, 11) using ball- wedge bonding.

XIII The electrical device (6) according to embodiment XII, wherein the process window for the at least one stitch bond to a gold bond pad has a value of at least 11550 mA*g provided the wire (1) has a diameter of 18 μιη.

Reference Numerals

Claims

1. A wire (1), comprising a core (2) with a surface (21), a first coating layer (3) with a layer surface (31) and a further coating layer (4), wherein

A) the core (2) comprises

a) at least 99.95 wt.% of copper;

b) an amount X of at least one element selected from silver and gold;

wherein the ratio of X and Y is in the range of from 0.03 to 50;

wherein the further coating layer (4) is composed of gold;

2. The wire (1) according to claim 1, wherein the amount Y is in the range of from 20 to 300 ppm, based on the total weight of the core (2).

3. The wire (1) according to claim 1 or 2, wherein the amount X is in the range of from 5 to 10000 ppm, based on the total weight of the core (2).

4. The wire (1) according to any one of the precedent claims, wherein the first coating layer (3) has a thickness in the range of from 40 nm to less than 0.5 μιη.

5. The wire (1) according to any one of the precedent claims, wherein the further coating layer (4) has a thickness in the range of from 1.0 nm to less than 50 nm.

6. The wire (1) according to any one of the precedent claims, wherein the core (2) has an average size of crystal grains in the range of from 3 to 5.0 μιη, the average size determined according to line-intercept method.

7. The wire (1) according to any one of the precedent claims, wherein in the range of from 18 to 42 % of the grains of the wire are oriented in <100> direction; and in the range of from 27 to 38 % of the grains of the wire are oriented in < 11 1 > direction; each % with respect to the total number of crystals with orientation parallel to the drawing direction of the wire (1).

8. The wire (1) according to any one of the precedent claims, characterized by at least one of the following features:

β) the wire (1) meets the equation 0.000025 < AL /(d*CX) < 0.3, wherein

AL = elongation of the wire in %;

d = average diameter of the wire in μιη;

CX = content of silver or gold, measured in ppm;

γ) the wire (1) meets the equation 0.0008 < AL /(d*CY) < 0.15,

wherein

AL = elongation of the wire in %;

d = average diameter of the wire in μιη;

1.90 μΩ*αη.

9. A method for manufacturing a wire (1), comprising at least the steps of i. Providing a precursor item (5) comprising:

wherein the ratio of X and Y is in the range of from 0.03 to 50;

ii. drawing the precursor item (5) to a core precursor (2a);

vi. annealing the product prepared in step v.;

whereby the wire (1) is obtained,

10. The method of claim 9, wherein the annealing of the product in step vi. is performed at a temperature of at least 400 °C.

11. A method for connecting an electrical device (6), comprising the steps of

I. providing a wire (1) according to any one of claims 1 to 8, or a wire obtained by a method according to any one of claims 9 to 10,

II. bonding the wire (1) to a first bond pad (61) of the device by either ball bonding or wedge bonding; and

III. bonding the wire (1) to a second bond pad (62) of the device by wedge bonding; wherein step III. is performed without the use of a forming gas; and wherein step II. is performed in presence of an inert gas or forming gas.

12. An electrical device (10) comprising a first and a second bonding pad (11, 11), and a wire (1) according to any one of claims 1 to 8, or a wire obtained by a process according to any one of claims 9 to 10, wherein the wire (1) is connected to at least one of the bonding pads (11, 11) using ball- wedge bonding.

13. The electrical device (10) according to claim 12, wherein the process window for the at least one stitch bond to a gold bond pad has a value of at least 11550 mA*g provided the wire (1) has a diameter of 18 μιη.