Patent Application
20150115442
1. Field
The present invention relates to a redistribution layer. Moreover, the present invention relates to a method of forming a redistribution layer.
2. Description of the Related Art
In the art so-called ball grid arrays (BGA) are used for a plurality of applications in the field of packaged semiconductor chips. Often these BGAs comprises a so called redistribution layers (RDLs) used to connect contact pads or contact points of the chip to a substrate or package having a different contact layout or contact arrangement than the chip. However since the redistribution layer itself and the connected layers or elements comprise different materials, e.g. a supporting layer like polyimide and copper paths used for the redistribution of contact points, mechanical or thermo-mechanical stress occurs due to temperature change from assembly temperature and different thermal expansion coefficients of the different materials in the redistribution layer, the chip and optionally packages.
The repeated thermo-mechanical stress leads to repeated plastic deformation and then to breakages or failures of electrical contacts, e.g. due to breakages of the copper paths or connector lines. The copper is typically weakened by impurities of Cl and S origin from pattern plating. In order to decrease the probability of breakages of the conductor paths of the redistribution layer specific forms or shapes of the (solder) ball of the BGA and the connections of the balls to the conductor paths are used. For example, a so called teardrop design is used which is schematically shown in
This teardrop design reduces the stress induced by the different temperature expansion coefficients by compensating the relative movement from the different components of the components of the RDL, the connected chip and the supporting layer. The teardrop shape of the solder balls may be sufficient to reduce the probability of breakage in BGAs having relative great balls (and thus a relative great amount of solder material for compensating) and a relative large pitch between the different balls of the grid. However, the known techniques may not be suitable anymore for a reduced ball size, e.g. less than 300 micrometer, and a reduced ball pitch, e.g. 0.5 millimeter or below, since the solder balls may not provide sufficient compensation material anymore due to a reduced volume and to increased shearing angles due to a low vertical distance between the board comprising the RDL and a used package. Solders with higher stiffness may be required but then will lead to more stress in the RDL.
Furthermore, US 2005127527 A1 discloses a general electronic component having metal-coated elevations formed of a rubber-elastic, silicone-based elastomere by a printing process. The rubber-elastic elevations are used for compensating for mechanical stress occurring and to compensate for unevenness between the electronic component and a carrier.
Thus, the probability of breakage of used solder contacts or connections in the RDL may increase in case of reduced pitch and/or in case the boards are reduced in size. In addition the problems may become more severe in case the chips shall be used in security relevant applications, e.g. in the automotive technology, having high reliability needs, while at the same time demanding relatively large sized chips due to the necessary functionality to be implemented in the chip or package.
In general, there may be a need to provide a redistribution layer and a method of forming a redistribution layer which withstands a high number of thermocycles and/or having a low probability of breakages.
According to an exemplary aspect a redistribution layer for a chip is provided, wherein the redistribution layer comprises at least one electrical conductor path connecting two connection points with each other, wherein the at least one electrical conductor path is arranged on a planar supporting layer and wherein the electrical conductor path comprises copper and at least one other further electrical conductive material in an amount of more than 0.04 mass percent.
According to an exemplary aspect a redistribution layer for a chip is provided, wherein the redistribution layer comprises at least one electrical conductor path connecting two connection points with each other, wherein the at least one electrical conductor path is arranged on a planar supporting layer and wherein the conductor path comprises a highly conductive base material and a tensile strength enhancement compound.
According to an exemplary aspect a method of forming a redistribution layer for a chip is provided, wherein the method comprises providing a planar supporting layer, forming at least one electrical conductor path on the planar supporting layer, wherein the conductor path comprises a highly conductive base material and a tensile strength enhancement compound.
The provision of a compound or material in addition to a highly conductive base material, e.g. copper, may enable that the conductor path exhibits an increased tensile strength when compared to a conductor path which is formed out of substantially pure copper as used in the art. Thus, it may be possible that the conductor paths may resist or withstand a greater number of thermocycles even when the package carrying the redistribution layer (RDL) has a size which is large, e.g. more than 8 mm×8 mm. In other words the additional compound may strengthen the conductor path by providing an increased flexibility, e.g. provides an elastic conductor path. Thus, the additional compound may increase the number of thermocycles for the RDL, and the respective chip for which the RDL is used, before a breakage of a conductor path occurs.
The accompanying drawings, which are included to provide a further understanding of exemplary embodiments and constitute a part of the specification, illustrate exemplary embodiments.
In the drawings:
In the following, further exemplary embodiments of the transducer arrangement and the method of manufacturing the same will be explained. It should be noted that embodiments described in the context of the redistribution layer may also be combined with embodiments of the method of forming the redistribution layer and vice versa.
According to an exemplary aspect a packaged chip is provided, wherein the packaged chip comprises a chip bonded to a redistribution layer according to an exemplary aspect and a ball grid array attached to the redistribution layer.
In particular, the amount of the at least one other electrical conductive material may be more than 0.05 mass percent, preferably more than 0.1% mass percent. The contact points may be formed by contact pads and/or vias.
In particular, the tensile strength enhancement compound may be an electrical conductive material, e.g. an alloy or mixture of an electrical conductive material and a further compound either conductive or non-conductive. In particular, the tensile strength enhancement compound may be formed or may be improved in a cold working or strain hardening process, e.g. when the conductor path including the tensile strength enhancement compound is exposed to stress or strain a strain hardening process may be induced, e.g. during an actual usage of an device comprising the redistribution layer.
The term “compound” may particularly denote an admixing or additive which is willingly incorporated and may have to be distinguished from an impureness or impurity which is typically present in every material. For example, the tensile strength enhancement compound may be added willingly in a sufficient amount in order to provide a tensile strength of the electrical conductor path which is above a given threshold.
The term “electrical conductive” may particularly denote any material having an electrical resistivity which is lower than a given threshold, e.g. lower than 2*106 Ω*m (ohm*meter) at 20° C. Thus, according to such definition of the term “electrical conductive” some elements or compounds typically denoted as non-conductive, e.g. boron, may be considered to be “electrical conductive”. On contrast elements, having a lower electrical conductivity, like sulphur and chlorine, may not considered to be “electrical conductive” in this sense.
The term “conductor path” may particularly denote a continuous path formed by an electrical conductive material, e.g. copper. In particular, also a seed layer of copper, e.g. having a thickness of less than 1 micrometer like 0.5 micrometer, for example, may already count as a conductor path.
The term “redistribution layer” may be interpreted in a broad sense as a layer, in particular a structured metal containing layer, on a chip that makes contact points, e.g. contact pads or vias, of a chip, e.g. an integrated chip, available in another location. Thus, in this sense a layer, which is restricted in size to the size or area of the chip, e.g. a layer which is formed at the so called back end of line, may fall under this definition. However, also a layer which extends beyond the size of the chip, e.g. a layer which is formed in a packaged chip e.g. an embedded wafer level ball grid array, may fall under this definition as well.
According to an exemplary embodiment of the redistribution layer the supporting layer comprises polyimide.
Alternatively or additionally, the supporting layer may comprise polybenzoxazole (PBO). The provision of a supporting layer or an intermediate layer comprising or consisting substantially of polyimide or PBO may have the advantage that the supporting layer already exhibits some amount of flexibility or bendability improving the durability of the RDL or the RDL formed on or in the supporting layer.
According to an exemplary embodiment of the redistribution layer the package carrying the redistribution layer has a size of at least 4 mm×4 mm.
In particular, the redistribution layer may have a size of more than 8 mm×8 mm, more preferably the redistribution layer may have a size of more than 15 mm×15 mm. An upper limit for the size may be about 150 mm×150 mm or even more.
According to an exemplary embodiment of the redistribution layer the conductor paths have a tensile strength of more than 100 MPa. In particular, the tensile strength may be more than 150 MPa, 250 MPa, more particular more than 300 MPa and preferably more than 400 MPa, wherein these strengths are measured in the bulk at room temperature.
According to an exemplary embodiment of the redistribution layer the conductor path is formed by a multilayer stack having an uppermost layer.
In particular, the term “multilayer stack” may include any stack having at least two layers. A total thickness of the multilayer stack may be in the range 2 micrometer to 50 micrometer, more particular in the range of 5 micrometer to 25 micrometer, preferably in the range of 5 micrometer to 10 micrometer, e.g. about 8 micrometer.
In particular, the at least one other further electrical conductive material is formed as an additional layer on a sub conductor path of copper.
The at least one other further electrical conductive material may form an additional layer which is arranged on top of a copper layer or a sub conductor path of copper. Thus, the conductor path may be formed by a sub conductor path of copper and a sub conductor path formed by the further electrical conductive material (optionally comprising copper as well). Alternatively, the at least one other further electrical conductive material may be formed underneath the copper conductor paths or may be mixed with the copper compound and forming an alloy with the same.
The use of an additional layer for the at least one other further electrical conductive material may be an efficient way to provide a strengthening element or function for the copper compound or copper conductor paths. In particular, the provision of an extra layer may provide an efficient and flexible way to provide a strengthening element or compound, since two steps are used to form the conductor path each of which steps may be adapted to the specific needs of processing the respective materials.
According to an exemplary embodiment of the redistribution layer the at least one other further electrical conductive material is selected out of the group consisting of: tin, silver, iron, nickel, aluminium, carbon, silicon, phosphorus, titanium, vanadium, boron and any combination thereof.
In particular, all materials which are suitable for an elastic or flexible conductor path or line may be used as materials for the electrical conductive material. The at least one further electrical conductive material may be arranged in an additional layer over the copper conductor paths. Useful layer comprise In, Al, Zn, Ni. In particular, indium and tin may be a suitable material for strengthening the conductor path since already a small amount of tin within copper may increase the tensile strength of the conductor path to a sufficient amount.
According to an exemplary embodiment of the redistribution layer the at least one other further electrical conductive material forms an alloy with the copper.
In particular, in case that the alloy has a sufficient electrical conductivity it may not necessary to form conductor paths of pure or substantially pure copper. Thus, when using an alloy or mixture of copper and the further electrical conductive material for forming the conductor path it may be possible that only a single forming step for forming the conductor path is necessary instead of two or multiple sub-steps forming a highly conductive sub conductor path (comprising the copper, for example) and an additional sub conductor path (comprising the further electrical conductive material).
According to an exemplary embodiment of the redistribution layer the multilayer stack is an annealed multilayer stack.
The annealing may improve the durability and/or tensile strength of the at least one conductive path. For example, only one annealing step is performed after forming the uppermost layer of the multilayer conductor path.
According to an exemplary embodiment of the redistribution layer the uppermost layer comprises an electrical conductive material with an electrical resistivity below a given threshold.
In particular, the given threshold may be in the range of the electrical resistivity of copper. For example, the given threshold may be 3*10−8 Ω*m, in particular 2.65*10−8 Ω*m, at 20° C. For example, the uppermost layer may comprise or may (essentially) consist of copper and/or silver. The provision of an uppermost or top layer having a high electrical conductivity may allow for the provision of a redistribution layer which is suitable for high frequency applications, e.g. radio frequency, since despite the skin effect a sufficient conductivity may be ensured when using a highly conductive material for the uppermost layer.
According to an exemplary embodiment of the redistribution layer the amount of the tensile strength enhancement compound is chosen so that a tensile strength of the electrical conductor path of more than 100 MPa is achieved.
In particular, the tensile strength may be more than 200 MPa, more particular more than 250 MPa and preferably more than 300 MPa or 400 MPa, wherein the tensile strengths are measured in the bulk at room temperature.
In particular, the tensile strength enhancement compound may have a percentage of more than 0.04 mass percent.
According to an exemplary embodiment the redistribution layer is connected to a ball grid array. In particular, the ball grid array may be a wafer level ball grid array (WLB) or an embedded wafer level ball grid array (eWLB).
According to an exemplary embodiment of the redistribution layer the further electrical conductive material has a grain size of less than 2 micrometer.
In particular, the grain size may be 1 micrometer or below, e.g. in the range between 10 nanometer and 1 micrometer. The provision of a specific grain size in particular a small grain size may enable that a growing of grains of the base material of the conductor path, e.g. copper, may be avoided or at least reduced such leading to the fact that the resistance to breakage is not reduced over time, e.g. during a number of heat cycles.
According to an exemplary embodiment of the redistribution layer the tensile strength enhancement compound is selected out of the group consisting of: tin, silver, iron, nickel, aluminium, carbon, silicon, phosphorus, titanium, vanadium, boron and any combination thereof.
In particular, the tensile strength enhancement compound may be an electrical conductive material, e.g. an alloy of an electrical conductive material and a further compound either conductive or non-conductive. In particular, the highly conductive material may be copper, aluminium or silver, for example. It should be mentioned that the boron may function as a kind of adhesive or chemical binding agent binding small amounts of impurities, e.g. sulphur and/or chlorine, typically present in base materials, e.g. plating copper. Thus, in case boron is used as the tensile strength enhancement compound the electrical conductor path may comprise the base material, e.g. copper (including a small amount of impurities), and only a small amount of boron for compensating for the weakening effects of the impurities. The total amount of materials other than the base material may thus be rather low, e.g. it may be below 0.04 mass percent or even below 0.03 mass percent of the material of the electrical conductor path.
According to an exemplary embodiment the method of forming a redistribution layer for a chip further comprises forming a first sub conductor path of copper, and forming a second sub conductor path on top of the first sub conductor path which second sub conductor path comprises the strength enhancement compound.
According to an exemplary embodiment of the method of forming a redistribution layer the forming of the at least one electrical conductor path is performed by pattern plating.
In particular, a first sub-step of forming and/or the second sub-step of forming may be performed by plating. Platting may be a simple and efficient method for forming a patterned layer, i.e. conductor paths.
According to an exemplary embodiment of the method of forming a redistribution layer the at least one electrical conductor path is performed by printing, in particular by printing in a pattern.
The annealing step may increase the durability and/or flexibility of the conductor path.
According to an exemplary embodiment of the method of forming a redistribution layer for a chip the forming of the at least one electrical conductor path comprises a strain hardening step after plating the conductor path.
According to an exemplary embodiment the method of forming a redistribution layer for a chip further comprise fixing a chip to the redistribution layer.
In particular, the chip may be bonded, soldered, plated or attached in any other suitable way to the redistribution layer.
Summarizing a gist of an exemplary embodiment may be seen in providing a redistribution layer for a chip, in particular for a packaged chip, for a wafer level ball grid array or for an embedded wafer level ball grid array, having an improved strength due to the addition of an additive or an additional strengthening compound. In particular, an additional compound may be used which modifies a stiff copper contact path or conductor path leading to a conductor path exhibiting a slightly elastic or flexible characteristic.
The strengthening compound may be arranged as an additional layer or may be mixed into a base compound (preferably highly conductive, like copper or wolfram) of the conductor path. The conductor paths may connect two or more contact points, e.g. contact points and/or contact pads, with each other. Due to the strengthening compound the probability of a breakage of the conductor paths may be reduced. In particular, the number of thermocycles before a stress induced breakage occurs may be increased, so that high performance specifications may be fulfilled as demanded in specific technologies, e.g. in the automotive technology.
The above and other needs, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings, in which like parts or elements are denoted by like reference numbers.
The illustration in the drawing is schematically and not necessarily to scale.
After the plating, in particular in case of Sn or In, an annealing step may be performed finally after building the layer stack. In case of NiP also a galvanic process may be used to form the additional layer. Preferably, pattern plating is used but the additional layer may also be formed after a lack stripping and seed-etching. When forming the additional layer of NiP in a galvanic process the forming of the NiP layer may be formed by an electro-less galvanic process. However, it should be noted that the above described tensile strength enhancement compounds may be mixed with a highly conductive material (like copper) and the conductor paths may be formed in a single step and single layer comprising the mixture or alloy.
With these steps the redistribution layer is in principle formed. However, after the forming of the at least one conductor path additional optional steps may be performed, e.g. an annealing step 303, which may be performed in case tin or indium is used in a layer over a copper layer, and/or a step connecting the RDL to a BGA 304 and/or a step connecting the RDL to a chip. In addition or alternatively to the optional annealing step a cold working process or strain hardening process may be performed on the conductor path during the manufacturing of the RDL or even during the use of the RDL. In such a cold working process the strength of the conductor path will be increased during the first stress or strain cycles of the RDL, e.g. in the case that nickel or boron are used as the tensile strength enhancement compound. During such a process a mixed crystal may be formed having an improved strength and thus may enable an improved lifetime of an IC-RDL connection. In particular, boron may function as a kind of adhesive compensating for the small impurities within plated copper (typically sulphur and chlorine) which typically reduces the strength of the copper.
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It should be noted that in case that the strengthening compound itself has a sufficiently high conductivity the base conductor path of copper may be omitted, e.g. the conductor paths comprising a copper base material and the strengthening compound may be formed only when forming the RDL. That is in this case a base conductor RDL layer may be omitted and the conductor may be formed by a semi-additive galvanic process and/or etching process from a single layer comprising copper as a base material and the tensile strength enhancement compound. Further the outermost layer of thickness due to skin depths shall be pure copper or an element or alloy close to its conductivity for better RF signal transmission.
It should be noted that the term “comprising” does not exclude other elements or features and the “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs shall not be construed as limiting the scope of the claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
