as is known in the art , the maximum current capability ( imax ) and reliability of a connection formed between a copper bump on a die substrate and its package by a soldering reflow process is affected by the formation of a layer of copper - tin ( cu — sn ) intermetallic compounds ( imcs ) at the interface between the copper bump and the package terminal , typically a solder ball or bump . the cu — sn imc has a high growth rate , and thus the imc layer at the joint becomes thick and porous due to the known kirkendall effect and high current density . increased thickness of the intermetallic compound and kirkendall voids formed within the compound cause high contact resistance and reduce connection reliability because of the brittle nature of the imc . iron ( fe ) has a much slower reaction rate with tin ( sn ) than copper . in one embodiment , an iron or iron alloy cap layer on a copper bump acts as a barrier to the reaction between copper from the bump and tin from the solder . a fe — sn imc layer that is formed in this embodiment generally has a lower thickness , fewer kirkendall voids , and lower contact resistance than the cu — sn imc that would be formed in the absence of the cap layer . in one embodiment with an iron alloy cap layer , the reaction rate between iron and tin is very slow after an initial formation of a 0 . 1 to 0 . 2 micron think layer of fe — sn imc , probably due to further reaction being inhibited by the initially formed layer . in other embodiments , a nickel or iron - nickel alloy cap may be used . nickel has properties similar to those of iron in terms of how imc layers are formed between nickel and tin , and thus a ni — sn imc layer formed between a copper bump with a nickel or iron - nickel alloy cap and a package solder ball or bump is also of lower thickness and contact resistance than a cu — sn imc layer formed between an un - capped copper bump and a package solder ball or bump . fig1 ( prior art ) shows process phases of a fabrication process in which a copper bump 120 is formed on the substrate or inter - layer dielectric of a fabricated integrated circuit die 140 . the copper bump is formed by electroplating an aperture in a photo - resistive mask 110 over a copper pad 130 formed in the substrate in process phase 101 . once the copper bump is formed by an electroplating process , the photoresist may be stripped away , as known in the art , in a process phase 102 leaving the exposed copper bump . the die may then be attached to a package 180 as in the process stage depicted at 103 by a solder reflow process . as is known the copper bump and the tin in the reflowed solder 170 react to form a thick cu — sn imc layer 160 . in one embodiment depicted in fig2 , an electroplating process is used in stage 201 to create a copper bump as previously described with reference to fig1 . another cap or barrier layer 210 is then plated over the copper bump . this barrier layer is formed of a ferrous or nickel or a combination ferro - nickel ( fe — ni ) alloy . finally , in the depicted embodiment , a pre - tin layer is formed over the barrier layer , 220 . the pre - tin layer may be formed by electroplating an alloy of tin , indium , or a noble metal such as silver , gold or palladium over the copper bump . in a next phase of the process , 202 , the photoresist 110 is stripped away to expose the capped copper bump with a pre - tin layer . this bump may then be coupled to a package 180 by a reflow soldering process as depicted at 203 . in this embodiment , though a fe — sn or ni — sn imc 240 may be formed by reaction of the barrier layer with the tin in the reflowed solder 170 , the thickness of the layer 240 is smaller than the thickness of the layer formed when no barrier is present as depicted in fig1 at 160 . a variation on the process of fig2 similar to that depicted in fig2 in which the formation of a pre - tin layer is omitted would still have the feature of a thinner imc and may be used in some embodiments . fig3 depicts a process and structure embodiment in which the first stage is similar to that described earlier with reference to fig1 , starting with the formation of a copper bump by electroplating in 301 . after the photoresist is stripped away , a layer of barrier metal 310 made of either a ferrous or nickel alloy or a combination fe — ni alloy is formed over substantially the entire surface of the copper bump by an electroless plating process as depicted in stage 302 . in stage 303 , a depiction of the formation of a connection with a package shows the copper bump with the electroless barrier and the formation of a thin intermetallic layer . because fe or ni has a slower reaction rate with sn than cu , a thinner intermetallic layer 160 is formed than those formed in the previously discussed versions in this embodiment . variations of the embodiment depicted in fig3 include the addition of a pre - tin formed over the barrier 310 layer by either electroplating or electroless plating in order to improve the reflow process of connection to a package with solder bumps or balls . in fig4 , another embodiment is depicted . in this embodiment , a copper bump is formed as previously discussed , at 401 . after the formation of the bump , the photoresist 110 is retained and a physical vapor deposition process is used to form a layer of barrier metal 420 such as a fe , ni , or fe — ni alloy over the copper bump and resist layer as depicted at process phase 402 . in some embodiments , a pre - tin layer 410 may be further deposited over the barrier layer as well . other embodiments may omit this layer . the excess pre - tin layer , barrier layer metal and photo - resist are then removed by a lithography , etch , and photoresist stripping process as is known in the art . this leaves a copper bump with a barrier layer and pre - tin layer as depicted at 403 . when connected to a package by a reflow process as depicted at 404 , a thin imc is formed between the barrier alloy and the tin in the reflowed solder . many other embodiments of the claimed subject matter are possible . many modifications may be made to adapt the teachings of the description and the drawings to a particular situation without departing from their scope . therefore , it is intended that the claimed subject matter not be limited to the various exemplary embodiments disclosed in the description and the drawings , but rather to all embodiments falling within the scope of the claims below .