Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B32—LAYERED PRODUCTS
  • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
  • B32B15/00—Layered products comprising a layer of metal
  • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
  • B32B15/018—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of a noble metal or a noble metal alloy
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/02—Making non-ferrous alloys by melting
  • C22C1/03—Making non-ferrous alloys by melting using master alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
  • C22C9/06—Alloys based on copper with nickel or cobalt as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/08—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of copper or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/1601—Process or apparatus
  • C23C18/1633—Process of electroless plating
  • C23C18/1646—Characteristics of the product obtained
  • C23C18/165—Multilayered product
  • C23C18/1651—Two or more layers only obtained by electroless plating
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/1601—Process or apparatus
  • C23C18/1633—Process of electroless plating
  • C23C18/1646—Characteristics of the product obtained
  • C23C18/165—Multilayered product
  • C23C18/1653—Two or more layers with at least one layer obtained by electroless plating and one layer obtained by electroplating
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
  • C23C28/02—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material
  • C23C28/023—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings only including layers of metallic material only coatings of metal elements only
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
  • C23C28/04—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings of inorganic non-metallic material
  • C23C28/042—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D only coatings of inorganic non-metallic material including a refractory ceramic layer, e.g. refractory metal oxides, ZrO2, rare earth oxides
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D7/00—Electroplating characterised by the article coated
  • C25D7/06—Wires; Strips; Foils
  • C25D7/0607—Wires
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  • H01L2224/45147—Copper (Cu) as principal constituent
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Definitions

• the invention relates to an 8 to 80 pm thick wire comprising a copper core comprising: copper, nickel in an amount in the range of from 0.005 to 5 wt.-% (weight-%, % by weight), and, optionally, silver in an amount in the range of from 0.005 to 1 wt.-%, wherein all amounts in wt.-% are based on the total weight of the core; wherein the copper core has an average size of crystal grains in the range of from 1.5 to 30 pm.
• the invention further relates to a process for manufacturing such wire.
• the use of bonding wires in electronics and microelectronics applications is well-known state of the art. 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 electric and thermal conductivity, bonding of copper wire has its challenges.
• copper wires are susceptible to corrosion, and oxidation.
• the wires of the invention have been found to solve at least the object of improved corrosion and moisture resistance. Further, a process for manufacturing these wires has been found.
• a first aspect of the invention is a wire comprising a core comprising or consisting of (a) nickel in an amount in the range of from 0.005 to 5 wt.-%, preferably 0.1 to 0.6 wt.-% or even 0.45 to 0.55 wt.-%,
• (b) optionally, silver in an amount in the range of from 0.005 to 1 wt.-%, preferably 0.1 to 0.6 wt.-% or even 0.45 to 0.55 wt.-%,
• wt.-% and wt.-ppm are based on the total weight of the core, wherein the,core has an average size of crystal grains in the range of from 1.5 to 30 ⁇ , the average size determined according to the line intercept method, wherein the wire has an average diameter in the range of from 8 to 80 ⁇ or even 12 to 55 ⁇ .
• 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 invention. Preferred shapes are - in cross-sectional view - round, ellipsoid and rectangular shapes.
• 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 metallographic practice. There, a wire is cut perpendicular 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 E1 12-96 standard, section 16.3, page 13.
• the core of the wire comprises (a) 0.005 to 5 wt.-% of nickel and (c) 94 to 99.98 wt.-% of copper.
• the core of the wire may or may not comprise component (b), i.e. silver. If the core of the wire comprises silver, the silver amount is in the range of from 0.005 to 1 wt.-%.
• the core of the wire of the invention comprises (d) 0 to 100 wt.-ppm of further components. The low amount of these further components ensures a good
• the further components are minor amounts of chemical elements and/or compounds which originate from impurities present in the raw materials used or from the wire manufacturing process.
• Such further components are: Mn, Pt, Cr, Ca, Ce, Mg, La, Al, B, Zr, Ti, S, Fe.
• the core comprises or consists of
• nickel in an amount in the range of from 0.005 to 5 wt.-%, preferably 0.1 to 0.6 wt.-% or even 0.45 to 0.55 wt.-%,
• nickel in an amount in the range of from 0.005 to 5 wt.-%, preferably 0.1 to 0.6 wt.-% or even 0.45 to 0.55 wt.-%,
• an embodiment of the invention is a wire comprising (a) nickel, (b) optionally, silver, and (c) copper, as disclosed above. 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 the nickel, silver and copper. In an embodiment, the core of the wire of the invention comprises less than the following amounts of further components:
• the wire core comprises elemental phosphorus as a dopant in an amount in the range of from 40 to 80 wt.-ppm, based on the total weight of 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 wire core. In an alternative, it can be an interface region between the wire core and a coating layer superimposed on the wire core.
• first item e.g. a copper core
• 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.
• 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.
• intermediate layer in the context of this invention refers to a region of the wire between the core and the coating layer. In this region, a combination of materials of both, the core and the coating layer, is present.
• thickness in the context of this invention is used to define the size of a layer in perpendicular direction to the longitudinal axis of the core, which layer is at least partially superimposed over the surface of the core.
• the ratio between the diameter of the core and the average size of crystal grains in the core is in the range of from 2 to 14 or even 2 to 7.
• the average diameter of the wire is in the range of from 15 to 28 ⁇ .
• the average size of crystal grains in the core is preferably in the range of from 1 .5 to 6 pm.
• the average diameter of the wire is in the range of from > 28 to 38 pm.
• the average size of crystal grains in the core is preferably in the range of from 2 to 10 pm.
• the average diameter of the wire is in the range of from > 38 to 50 pm.
• the average size of crystal grains in the core is preferably in the range of from 5 to 15 pm.
• the average diameter of the wire is in the range of from > 50 to 80 pm.
• the average size of crystal grains in the core is preferably in the range of from 7 to 30 pm.
• the core has a surface, wherein a coating layer is superimposed over the surface of the core.
• the mass of the coating layer is not more than 2.5 wt.-%, preferably 2 wt.-% or less, each with respect to the total mass of the core.
• a coating layer When a coating layer is present, it often has a minimum mass of 0.1 wt.-% or more or 0.5 wt.-% or more, each with respect to the total mass of the core.
• Applying a low amount of material as coating layer preserves the characteristics which are defined by the material of the core of the wire.
• the coating layer awards particular characteristics to the wire surface such as being inert against environment, resistance to corrosion, improved bondability, etc.
• the thickness of the coating layer is in the range of from 60 to 70 nm for a wire of an average diameter of 18 pm.
• the coating layer may have a thickness in the range of from 90 to 100 nm, for example.
• the coating layer is made of an element selected from the group consisting of palladium, platinum and silver.
• the coating layer can be a monolayer of one of said elements.
• the coating layer can be a multilayer of a number of super-positioned adjacent layers, wherein each layer is made of one element selected from the group consisting of palladium, platinum and silver.
• Each layer which is part of the coating layer is deposited individually from one of the aforementioned pure metal elements.
• plating such as electroplating and electroless plating, deposition of the material from the gas phase such as sputtering, ion plating, vacuum evaporation and physical vapor deposition, and deposition of the material from the melt.
• a further coating layer is superimposed over the coating layer.
• the mass of the further coating layer is not more than 0.2 wt.-%, preferably not more than 0.1 wt.-%, each with respect to the total mass of the core.
• the thickness of the further coating layer lies in the range of from 2 to 4 nm for a wire having an average diameter of 18 pm.
• the further coating layer may have a thickness of 3.5 to 5.5 nm, for example.
• the further coating layer is a gold layer.
• the wire of the invention is characterized at least by one of the following features: a) The corrosion resistance has a value of at most 0% bonded ball lift; see “Test method G" as described below.
• the moisture resistance has a value of at most 0% bonded ball lift; see "Test method I" as described below.
• the hardness of the wire core is not more than 120 HV, preferably not more than 115 HV, or not more than 110 HV; see “Test method J” as described below. 6)
• the process window area for stitch bonding has a value of at least 40 pm g, or of at least 90 pm-g, or of at least 120 prrvg, each provided the wire has an average diameter of 18 pm; see section "Test method C” as described below.
• the resistivity of the wire is not more than 1.80 ⁇ -cm.
• the average bonded ball roundness has a value of at most 0.0025 pm, or of at most 0.002 pm, or of at most 0.0018 pm each provided the wire has an average diameter of 18 pm; see section "Test method E" as described below,
• the average bonded ball concentricity has a value of at most 0.00056 pm, or of at most 0.0005 pm, or of at most 0.0004 pm each provided the wire has an average diameter of 18 pm; see section "Test method E" as described below.
• a second aspect of the invention is a process for manufacturing a wire in any of its embodiments disclosed above.
• the process comprises at least the process steps of
• nickel in an amount in the range of from 0.005 to 5 wt.-%, preferably 0.1 to 0.6 wt.-% or even 0.45 to 0.55 wt.-%,
• (b) optionally, silver in an amount in the range of from 0.005 to 1 wt.-%, preferably 0.1 to 0.6 wt.-% or even 0.45 to 0.55 wt.-%,
• a precursor item as provided in process step (1 ) can be obtained by alloying and/or doping copper with the desired amount of nickel and, optionally, silver. Alloying and doping is realized by producing a melt of said components and copper and cooling the melt to form a homogeneous piece of copper based precursor item.
• such precursor item is in the form of a rod having a diameter of, for example, 2 to 25 mm and a length of, for example, 5 to 100 m.
• Such rod can be made by casting a copper alloy melt comprising or consisting of (a) nickel in an amount in the range of from 0.005 to 5 wt.-%, (b) optionally, silver in an amount in the range of from 0.005 to 1 wt.-%, (c) copper in an amount in the range of from 94 to 99.98 wt.-%, and (d) 0 to 100 wt.-ppm of further components in an appropriate mold of room temperature, followed by cooling and solidifying, wherein all amounts in wt.-% and wt.-ppm are based on the total weight of the copper alloy melt.
• the copper alloy itself can be prepared by conventional processes known to the person skilled in the art of metal alloys, for example, by melting together the copper, the nickel and, optionally, the silver in the desired ratio. In doing so, it is possible to make use of conventional copper-nickel and copper-silver master alloys.
• the melting process can for example be performed making use of an induction furnace and it is expedient to work under vacuum or under an inert gas atmosphere.
• the materials used can have a purity grade of, for example, 99.99 wt.-% and above.
• these coating layers are preferably applied to the wire precursor.
• the skilled person knows how to calculate the thickness of such coating layers on a precursor item to obtain the coating layers in the thickness disclosed for the embodiments of the wire, i.e. after elongating the precursor item with the one or more coating layers to the wire precursor.
• numerous techniques for forming a coating layer of a material according to the embodiments on a copper or copper alloy surface are known. Preferred techniques are plating, such as electroplating and electroless plating, deposition of the material from the gas phase such as sputtering, ion plating, vacuum evaporation and physical vapor deposition, and deposition of the material from the melt.
• process step (2) In order to superimpose a single- or multi-layer metal coating to the wire core as disclosed for some of the embodiments of the first aspect of the invention, it is expedient to interrupt process step (2) once a certain precursor diameter in the range of, for example, 80 to 200 pm is reached. Then the single- or multi-layer metal coating may be applied, for example, by one or more electroplating process steps. Thereafter process step (2) is continued until the desired and final diameter of the wire core is obtained.
• the precursor item is elongated to form a wire precursor, until the desired diameter of the wire core is obtained.
• Numerous techniques are known to elongate a precursor item to form a wire precursor are known and appear useful in the context of the invention. Preferred techniques are rolling, swaging, die drawing or the like, of which die drawing is particularly preferred. In the latter case the precursor item is drawn in several process steps until the desired and final diameter of the wire core is reached.
• the desired and final diameter of the wire core may be in the range of from 8 to 80 pm or, preferably, in the range of from 12 to 55 pm.
• Such wire die drawing process is well known to the person skilled in the art.
• the precursor item may be drawn in 8 major stages and each stage with 15 to 25 process steps, wherein in each drawing process step an elongation of the precursor item in the range of from 6 to 18 % irv length is performed.
• the % of elongation may be same or different for each drawing process step.
• the elongated wire precursor obtained after completion of process step (2) is annealed, preferably in a tubular oven.
• the annealing is strand annealing (final annealing), which is a continuous process allowing for a fast production of the wire with high reproducibility.
• Strand annealing means that the annealing is done dynamically while the wire precursor is moved through a preferably tubular annealing oven and spooled onto a reel after having left the oven.
• the annealing is performed at an object temperature in the range of, for example, 440 to 700 °C for 0.1 to 0.4 seconds, preferably 470 to 650 °C for 0.1 to 0.3 seconds; these temperature/time conditions allow for achieving or adjusting the desired average crystal grain size of the wire core.
• the annealing is typically performed by pulling the elongated wire precursor through a conventional annealing oven, typically in the form of a cylindrical tube of a given length and with a defined temperature profile at a given annealing speed which may be chosen in the range of, for example, from 4 to 30 meters/minute, or from 14 to 16
• annealing time/object temperature parameters can be defined and set.
• annealing at a temperature below the temperature of maximum elongation can result in beneficial wire properties as the wire morphology can be influenced in a positive way.
• selecting the annealing temperature to be higher than the temperature of maximum elongation while maintaining the annealing time constant is of further advantage.
• this manufacturing principle can be used to adjust the average grain size of the wire, e.g. to larger average grain sizes. By this adjustment, other properties like e.g. wire hardness, ball-bonding behavior etc. can be influenced in a positive manner.
• annealing is performed at a temperature above the temperature of maximum elongation wherein the elongation value of the wire after annealing is not more than 98% of the maximum elongation value.
• process step (3) may be performed at a temperature which is at least 10 °C, preferably at least 50 °C, or at least 80 °C higher than the temperature of maximum elongation TAL(ma )- Often, the temperature in process step (3) is not more than 200 °C higher than TAL(ma )-
• the temperature of maximum elongation TAL(ma ) is determined by testing the elongation at break of a specimen (wire) at different temperatures.
• the data points are collected in a graph, showing the elongation (in %) as a function of temperature (°C).
• the resulting graph is often referred to as an "annealing curve".
• a temperature is observed where the elongation (in %) reaches a maximum. This is the temperature of maximum elongation TAL(ma )-
• Figure 1 shows an exemplary annealing curve of a nickel-alloyed 18 pm copper wire according to sample 3 (Table 1 ).
• the annealing temperature is the variable parameter of the x-axis.
• the graph shows the measured values of the break load (BL, in grams) and the elongation (EL, in %) of the wire.
• the elongation was determined by tensile testing. Elongation measurements exhibited a typical local maximum value of about 10 % in the displayed example, which was achieved at an annealing temperature of around 470 °C. If the wire according to sample 3 was not annealed at this temperature of maximum elongation, but at 560 °C, which was 90 °C above the temperature of the maximum elongation the result is an elongation value of about 9.8%, which is more than 2% below the maximum elongation value.
• the annealing may be performed in an inert or reducing atmosphere.
• inert atmospheres as well as reducing atmospheres are known in the art and are used for purging the annealing oven which is typically a tubular annealing oven.
• nitrogen is preferred.
• reducing atmospheres hydrogen is preferred.
• Another preferred reducing atmosphere is a mixture of hydrogen and nitrogen. Preferred mixtures of hydrogen and nitrogen are 90 to 98 vol.-% nitrogen and, accordingly, 2 to 10 vol.-% hydrogen, wherein the vol.-% total 100 vol.-%.
• Preferred mixtures of nitrogen/hydrogen are equal to 93/7, 95/5 and 97/3 vol.-%/vol.-%, each based on the total volume of the mixture.
• Applying reducing atmospheres in annealing is particularly preferred, if some parts of the surface of the wire are sensitive to oxidation by oxygen of the air, e.g. if copper of the wire is exposed to its surface.
• a preferred combination of said process parameters is: object temperature of 500 to 650 °C for 0.1 to 0.3 seconds, preferably in further combination with the use of 90-98 vol.-% nitrogen/2-10 vol.-% hydrogen mixtures as purging gas at a gas exchange rate in the range of 50 to 63 min "1 .
• the wire of the invention is finished.
• the finished wire is typically spooled and vacuum sealed immediately after completion of process step (3), i.e. without delay, for example, within ⁇ 1 to 5 hours after completion of process step (3) and then stored for further use as bonding wire. Storage in vacuum sealed condition should not exceed 6 months. After opening the vacuum seal the wire should be used for wire bonding within no longer than 7 days. It is preferred that all process steps (1 ) to (3) as well as spooling and vacuum sealing are carried out under clean room conditions (US FED STD 209E cleanroom standards, 1 k standard).
• a third aspect of the invention is a wire obtainable by a process according to the second aspect of the invention or of an embodiment thereof. It has been found that said wire is well suited for use as a bonding wire in wire bonding applications.
• Wire bonding technique is well known to the skilled person. In the course of wire bonding it is typical that a ball bond (1 st bond) and a stitch bond (2 nd bond, wedge bond) are formed. During bond forming a certain force (typically measured in grams) is applied, supported by application of scrub amplitude (typically measured in pm). The mathematical product of the difference between the upper and the lower limits of the applied force and the difference between the upper and the lower limits of the applied scrub amplitude in a wire bonding process defines the wire bonding process window:
• the wire bonding process window defines the area of force/scrub amplitude
• the wire of the invention exhibits a considerably wide wire bonding process window.
• a wire of the invention 18 pm in diameter exhibits a wire bonding process window in the range of, for example, 40 to 120 pm g for stitch bonding.
• the size of crystal grains was determined using a standard metallographic technique, ASTM E112-96, section 16.3, page 13. A sample of the wire core was cross-sectioned and then etched. In the present case, a solution of 2 g FeC and 6 ml concentrated HCI in 200 ml deionized water was used for the etching. The crystal grain size was determined according to the line intercept 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 tensile properties of the wires were tested using an lnstron-5300 instrument. The wires were tested at 1 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 the difference in the length of the wire before and after tensile testing (AL/L), calculated from the recorded load versus extension tensile plot.
• the 2 nd bond (stitch bond) process window area is the product of the difference between the upper and the lower limits of the force used in the bonding and the difference between the upper and the lower limits of the applied scrub amplitude, 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 process window values were based on a wire having an average diameter of 18 pm, wherein the lead finger to which the wire was bonded consisted of silver.
• Bonded ball (1 st bond) morphologies were quantified by measuring the average roundness and concentricity.
• the wires were bonded using standard or advanced firing mode.
• the 4N bare copper and nickel alloyed copper wires were bonded using standard firing mode, while silver-nickel alloyed copper wires were bonded using advanced firing mode.
• Bonded ball was observed in high power Nikon microscope MM40 at 500X magnification.
• a well rounded ball showed 0.001 pm and worst ball revealed 0.003 pm average roundness.
• 12 edges were marked along the outer circle.
• a well concentric bonded ball showed 0.0001 ⁇ and worst ball revealed of 0.0009 pm.
• the wires were ball bonded to AI-0.5wt.-%Cu bond pads.
• the test devices with the so- bonded wires were soaked in salt-solution at 25 °C for 2, 4, 6, 8 and 10 minutes, washed with Dl water and later with acetone.
• the salt-solution contained 30 wt.-ppm NaCI in de-ionised (Dl) water.
• the number of lifted balls were examined under lower power scope (Stereoscope - SZX16) at 10 to 100X magnification. Observation of a higher number of lifted balls indicated severe interfacial galvanic corrosion.
• the wires were ball bonded to AI-0.5wt.-%Cu bond pads.
• the test devices with the so- bonded wires were stored at 130 °C temperature, 85 % relative humidity (RH) for 20 hours in highly accelerated stress test (HAST) chamber and later examined for number of lifted balls under lower scope (Stereoscope - SZX 6) at 10 to 100X magnification. Observation of a higher number of lifted balls indicated severe interfacial galvanic corrosion.
• the hardness was measured using a Fischer scope H1 10C 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 on the center of the annealed wire core.
• a quantity of copper material of at least 99.99 % purity (“4N-copper") was melt in a crucible.
• a small amount of master alloy was added to the copper melt and uniform distribution of the added component was ascertained by stirring. The following master alloys were used.
• the chemical composition of the copper, copper-nickel and copper-nickel-silver alloy wires was controlled by ICP (inductively coupled plasma) analysis using a Perkin Elmer ICP-OES 7100DV model.
• ICP inductively coupled plasma
• the wires were dissolved in concentrated nitric acid and the solution was used for the ICP analysis.
• Table 1 shows the composition of samples 1 to 6 of inventive copper-nickel alloy wires of 18 pm average diameter.
• the nickel content of the wires varied as indicated as did the amount of phosphorus.
• a comparative wire (Ref) consisted of 4N-copper.
• Table 1 Chemical composition of copper and copper-nickel alloy wires, average diameter 18 pm, values in wt.-ppm
• Table 2 shows the composition of sample wires 7 to 10 each of which having 18 pm average diameter. The silver, nickel and phosphorus content of the wires varied as indicated. A comparative wire (Ref) consisted of 4N-copper. Table 2: Chemical composition of copper and copper-nickel-silver alloy wires, average diameter 18 pm, values in wt.-ppm
• the wires were annealed in a final annealing process step.
• the annealing was performed dynamically as strand annealing, by running the wires through a tubular annealing oven of 30 cm length and an annealing temperature of 560°C (wires 1 -6) or 650 °C (wires 7-10) with a speed of 1 m/s. After leaving the oven the wires were spooled on a reel for packaging.
• the annealing time was the exposure time of a given piece of the moving wire within the heated oven, which was 0.3 s. Within the oven zone, a constant temperature was adjusted.
• the average grain sizes of wire samples 1 to 10 were measured. The result was in the range of 3 pm to 6 pm for samples 1 to 6 and in the range of 1.5 pm to 6 pm for samples 7 to 10.
• Table 3 shows test results obtained with the 8 mm rods and the bonded wires of samples 1 to 6.
• Table 4 shows test results obtained with the 8 mm rods and the bonded wires of samples 7 to 10.
• Wires 1-6 and 7-10 resulted in process windows which were well suited for industrial application.
• a significant improvement of the corrosion and moisture resistance of the cast rods and bonded balls was observed at least with a nickel content in a range of 0.5 wt.-% to 5 wt.-% or with a combination of silver in a range of 0.1 to 0.5 wt.-% and nickel in a range of 0.1 to 0.5 wt.-%.

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