Patent Publication Number: US-2018053709-A1

Title: Method for manufacturing semiconductor devices, and corresponding device

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to the manufacture of semiconductor devices such as integrated circuits. 
     One or more embodiments may apply to the formation of electrically conductive contacts, for example copper contacts, in such devices. 
     Description of the Related Art 
     The market of semiconductor circuits, in particular integrated circuits, imposes on manufacturers increasingly stringent specifications, for example in sectors such as the automotive sector. 
     For instance, there is felt the desire to provide packages of semiconductor circuits based upon the use of so-called metal lead frames (LF) that are able to withstand thermomechanical stresses, such as the ones induced by assembly processes or by repeated activation/deactivation (on/off) cycles. 
     For instance, it is desirable to achieve a good level of intrinsic robustness in regard to phenomena such as delamination so as to be able to arrive at solutions that can be defined as delamination-free, namely with a package structure that is able to withstand phenomena of delamination between different materials such as, for example, the electrically conductive material (copper) of the lead frame and the resin or compound of the package. 
     Documents such as US 2009/0315159 A1 are representative of the known art. 
     BRIEF SUMMARY 
     According to one or more embodiments, a method manufacturesg a semiconductor device including at least one electrically-conductive metal member in a non-conductive package material. The manufacturing includes: 
     providing a first metal layer covering said electrically-conductive metal member, said first metal layer including a flat morphology, and 
     providing a second metal layer covering partly said first metal layer and leaving at least one surface portion of said first metal layer uncovered, said second metal layer including a rough morphology. 
     One or more embodiments may also regard a corresponding semiconductor device. 
     The claims form an integral part of the description of examples of embodiment provided herein. 
     One or more embodiments may envisage resorting to a material (e.g., copper) of a rough type that is able to improve adhesion to the resin/compound of the package so that the overall structure will be practically insensitive to phenomena of delamination (e.g., between the copper material and the resin) that might be induced by thermal cycles that are set up, for example, following upon switching-on and switching-off of the device during its service life. 
     In one or more embodiments, adhesion between the two different materials may be due to mechanisms of a mechanical and chemical nature, i.e., to the presence of asperities (peaks or troughs) to which the resin can adhere and to the increase in the contact surface that is such as to enable a higher chemical interaction per unit surface. 
     One or more embodiments may be based upon the observation of the fact that, as compared to a surface made, for example, of copper of a standard type, by resorting to a rough version it is possible to obtain an increase in the effective surface, for example, in the region of 100-150%. 
     A rough surface may, on the other hand, lead to onset of undesired phenomena such as a modification in the characteristics of wettability, with increase of the effect known as “epoxy bleed-out” (EBO). 
     One or more embodiments may consequently envisage providing a stack of two layers of material, for example, two copper layers, the first layer being made of copper of the bright (brite) type, for example electroplated over the entire surface of the lead frame, for instance for a minimum thickness of 1 μm, which is such as to enable also a stable attachment, for example, of the semiconductor die, and the second layer being made of rough copper, formed, for example, by electroplating also in this case for a minimum thickness of approximately 1 μm over dedicated areas of the lead frame, for example to enable a stable connection of the wire-bonding type, using for example copper wires. 
     One or more embodiments may be based upon the realization of the fact that the techniques that envisage a spot deposition of silver (e.g., of a semi-bright type) may not offer a sufficient degree of adhesion for the resin/compound of the package. Deposition of the layers is, moreover, problematical and the deposited layers have a rough morphology, which may give rise to delamination between the silver and the resin, with the bonding wires (e.g., copper wires) that are susceptible to failure in the presence of thermal cycles. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       One or more embodiments will now be described, purely by way of non-limiting example, with reference to the annexed drawings, wherein: 
         FIG. 1  is a schematic cross-sectional view of a semiconductor device; 
         FIG. 2  is a view approximately corresponding to the arrow II of  FIG. 1 , which illustrates characteristics of embodiments; and 
         FIG. 3  is a synthetic representation of modalities of production of embodiments. 
     
    
    
     It will be appreciated that, for clarity and simplicity of illustration, the various figures and the parts visible therein may not be represented all at the same scale. 
     DETAILED DESCRIPTION 
     In the ensuing description, various specific details are illustrated, in order to provide an in-depth understanding of various examples of embodiments. The embodiments may be obtained without one or more of the specific details, or with other methods, components, materials, etc. In other cases, known structures, materials, or operations are not illustrated or described in detail so that the various aspects of the embodiments will not be obscured. 
     Reference to “an embodiment” or “one embodiment” in the framework of the present description is intended to indicate that a particular configuration, structure, or characteristic described in relation to the embodiment is comprised in at least one embodiment. Consequently, phrases such as “in an embodiment”, “in one embodiment”, and the like that may be present in various points of the present description do not necessarily refer exactly to one and the same embodiment. Moreover, particular conformations, structures, or characteristics may be combined in any adequate way in one or more embodiments. 
     The references used herein are provided simply for convenience and hence do not define the sphere of protection or the scope of the embodiments. 
       FIG. 1  is a schematic illustration of a semiconductor device  10 , such as an integrated circuit. 
     In one or more embodiments, the device  10  may comprise a semiconductor chip or die (constituting the integrated circuit proper)  12  mounted on a so-called die pad  14  that can be connected to the outside through electrically conductive contacts (lead tips)  16 . The die pad  14  and lead tips  16  may be parts of a so-called “lead frame” (LF), to which there may be bonded electrical wires  18  to constitute a so-called wire-bonding configuration, which extends between the lead frame (lead tip)  16  and electrical contact pads, or bonding pads, (not visible in the figures) provided on the surface of the die  12 . 
     The ensemble of parts described can then be located in a casing or package  20  comprising a resin or compound that is such as to enable molding of the package of the device  10 . 
     What has been described schematically above corresponds to solutions known in the art, which renders a more detailed description herein superfluous. 
     As already discussed previously, one or more embodiments tackle the problem of possible delamination phenomena that may arise, for example, between the structures made of metal material (e.g., copper) of the leads  16  and the resin of the package  20 . 
     One or more embodiments tackle the above aspect by improving adhesion between the aforesaid materials, this being obtained by increasing interpenetration of the above materials (mechanical approach) and by creating bonds that are able to form bridges between the two materials (chemical approach). 
     In one or more embodiments, to increase interpenetration between the materials, it is possible to roughen the surface of the metal material (e.g., copper) in such a way that, when the material of the package (resin)  20  is, for example, spread on the lead frame comprising the leads  16 , this material can encounter a greater number of bonding sites. 
     In one or more embodiments, it is possible to envisage growth of a number of layers of metal material. 
     In what follows, one or more embodiments will be exemplified with reference, as metal material, to copper, envisaging that the growth of layers of material is applied to the lead frame, for instance, to the leads  16 , with particular attention paid, for example, to the sites for bonding the wires  18 . 
     The above is provided merely by way of example and hence is not to be understood as in any way limiting the scope of the embodiments, for example as regards the choice of the metal material. 
     For instance, in one or more embodiments, the copper may be replaced by silver or nickel. 
     For instance, in one or more embodiments, it is possible to envisage that on the base layer, made, for example, of copper, and designated by  100  (in practice, the copper material that would otherwise constitute the base structure both of the die pad  14  and of the leads  16  of the lead frame), there will be formed, for instance, with the growth techniques exemplified in what follows:
         a first layer  102  with a smooth morphology (e.g., of the type defined, in the case of copper, as bright (brite) copper); and   a second (localized) layer  104  with a rough morphology.       

     In one or more embodiments (see, for example,  FIG. 2 , for simplicity of representation) the first flat layer  102  may comprise copper that is deposited, on the base raw-copper structure  100 , for example electrolytically with a fine-grain structure, for example with a thickness of 1 to 2 μm and a roughness (rugosity), expressed as surface ratio (SR), i.e., the ratio f r =A r /A g  between the real or effective surface area A r  and the geometrical surface area A g , of approximately 1. 
     The second layer  104  (which is also, in one or more embodiments, electrodeposited) may have a granular (nodular) structure with a surface roughness SR (once again defined as surface ratio, i.e., as the ratio A r /A g ) comprised, for example, in the range between 1.2 and 3.0 with a thickness of, for example, between 1 and 3 μm. 
     Of course, reference to electrolytic deposition is not to be understood as in any way limiting the embodiments. 
     In one or more embodiments, it is in fact possible to resort, both for the layer  102  and for the layer  104 , to the use of other methods such as chemical-vapor deposition (CVD), sputtering, electroless plating, spray-coating, etc. 
     In one or more embodiments (see, for example,  FIG. 1 ), the first layer  102  (i.e., the smooth layer) can be deposited over the entire surface of the base structure  100 , i.e., over the surface of the lead frames, as well as, optionally, over the surface of the die pad  14 . 
     In one or more embodiments, the second layer  104  (i.e., the rough layer) may, instead, be deposited selectively (e.g., using a mechanical mask or some other masking technique) so as to extend selectively only over certain areas, such as those of the leads  16 . 
     Such selective deposition can be carried out either on one side or on both sides of the lead frame, with selective deposition on one or more areas of interest. 
     For instance,  FIG. 1  refers to the possibility of depositing the rough layer  104  at the “proximal” tips of the leads  16  of the lead frame, where bonding of the wires  18  may be carried out. 
     In one or more embodiments, the area of interest on which (also) the rough layer  104  is deposited may involve other areas of the leads  16  and/or areas of the die pad  14 , for example, with a morphology of deposition of a ring type in the case of the lead frame  16  and a localized deposition, for example in the case of the die pad  14 . 
     One or more embodiments as exemplified may afford advantages of various nature. 
     For instance, in one or more embodiments, it is possible to perform a die attach (DA) of the chip or die  12  on the copper die pad  14 , without running the risk of giving rise to creeping effects. 
     In particular, in one or more embodiments, it is possible to avoid resorting, for attaching the die  12  to the die pad  14 , to die-attach (DA) techniques using high-melting-point alloys, for example alloys with a lead (Pb) base, which can give rise to creeping phenomena, with migration of the molten lead beyond the area of the die pad, with consequent possible need to reject the device. 
     In one or more embodiments, it is possible to carry out a die attach adhesively, without giving rise to appreciable risks of triggering EBO phenomena. In particular, in the case of die attach of an adhesive type, for example with epoxy glues, it is possible to prevent onset of phenomena of bleed-out of the solvents, which are likely to induce delamination, a phenomenon that may arise in a particularly accentuated form on very rough surfaces. 
     One or more embodiments may then facilitate wire bonding of the wires  18  (e.g., copper wires) to the leads  16 . In one or more embodiments, soldering of the bonding wires  18  to the leads of the lead frame  16  can be carried out without any appreciable risks of giving rise to the phenomenon known as “outer lead-tip de-wetting”, i.e., the phenomenon that may arise mainly in the case of pre-plated lead frames with high roughness that is to be reduced through rather severe pre-treatments prior to soldering, with the corresponding need to increase the thickness of the soldering mass to enhance levelling of the asperities. 
     In addition, the rough area  104  is able to improve adhesion to the compound (resin) of the package  20 , so reducing (and virtually eliminating) any risks of delamination. 
     In one or more embodiments, a multilayered structure  102 ,  104 , as exemplified in the figures, affords a solution to various problems that are liable to emerge in the case where just a structure of a rough type were to be used, for example considering that the increased roughness may at times favor undesired phenomena of capillarity. 
     It is likewise possible to prevent phenomena that are linked to the high roughness of copper as a whole and are likely to increase adhesion to the resin of the package  20 , which, however, is unfavorable to removal of the so-called runners that are to be removed. 
     Similar considerations apply also to deflashing, i.e., the mechanism linked to the fact that the residue of flash are difficult to remove especially on slugs of power packages. 
     One or more embodiments avoid these drawbacks thanks to the possibility of providing roughness only in certain areas that would benefit from the roughness (e.g., the tips where bonding of the wires  18  is carried out). It is thus possible to achieve an ideal synthesis between the increase of adhesion and prevention of delamination and of the drawbacks outlined previously. 
     The possible recourse, for the layer  102 , to smooth (bright) copper likewise enables various advantages linked to its intrinsic purity to be achieved. 
     For instance, it facilitates die-attach bonding of the die  12  to the die pad  14  using a soft-solder die-attach (SSDA) technique, with the possibility of operating with a wider operating window. 
     As regards the wire-bonding operation, one or more embodiments may enable bonding with copper wire  18 , also in this case with wider operating windows thanks to the greater ductility and lower hardness as compared to the copper of the base structure. 
     Furthermore, since the layer  102  is, for example, an electrodeposited layer, it is possible to attain a good level of absence of defects and porosities. 
     One or more embodiments may comprise the operating steps represented schematically in the diagram of  FIG. 3 . 
     After an initial degreasing step  1000 , it is possible to proceed to an activation step  1002  followed by deposition (in a step  1004 ) of the layer  102 , for instance of bright copper (with a thickness of, for example, 2 μm), and then—in a step  1006 —to deposition (e.g., spot deposition) of the rough copper  104  (also here, for example, with a thickness of around 2 μm). 
     In practice, processes such as electrodeposition processes are well-suited for application of copper layers either in full mode, i.e., over the entire surface (e.g., the layer  102 ) or selectively (e.g., the rough layer  104 ). Chemical-deposition technologies likewise constitute technically valid solutions. 
     With current techniques, plating of the lead frames may be carried out using machines of the reel-to-reel (R2R) type, along with mechanical masks that are able to produce localized deposits (spots) of silver. 
     This technology may be adapted, with minor variations at a system level, to a localized deposition of the rough material  104 . 
     One or more embodiments may consequently envisage a method for manufacturing semiconductor devices (e.g.,  10 ) comprising at least one electrically conductive metal element (e.g., the leads  16  of the lead frame) in a non-conductive package material (e.g.,  20 ), the method comprising:
         providing a first metal layer (e.g.,  102 ), which covers said at least one metal element, said first layer having a smooth (flat) morphology; and   providing a second metal layer (e.g.,  104 ), which in part covers said first layer, leaving at least one portion of the surface of the first layer exposed, said second layer having a rough morphology.       

     One or more embodiments may comprise bonding, preferably by soldering, electrically conductive wires (e.g.,  18 ) to said second layer. 
     In one or more embodiments:
         said first layer may have a thickness of 1-2 μm; and/or   said second layer may have a thickness of 1-3 μm.       

     In one or more embodiments:
         said first layer may have a roughness (rugosity) with a surface ratio (SR) of approximately 1; and/or   said second layer may have a roughness with a surface ratio (SR) of 1.2 to 3.0.       

     In one or more embodiments, at least one, and preferably all, from among said metal element, said first layer, and said second layer may comprise copper. 
     In one or more embodiments said first layer may comprise bright (brite) copper. 
     One or more embodiments may comprise obtaining said first layer and said second layer using one of the following techniques: electrolytic deposition, chemical-vapor deposition, sputtering, electroless plating, and spray-coating. 
     In one or more embodiments, said metal element may comprise at least one contact lead (e.g.,  16 ) provided with a lead tip, where the method comprises providing said second layer (e.g., only) on said tip. 
     One or more embodiments may comprise providing a die pad (e.g.,  14 ) for mounting a semiconductor die (e.g.,  12 ) in said package, and the method may comprise:
         providing said first layer for covering said die pad; and   attaching a semiconductor die on said first layer on said die pad.       

     One or more embodiments may regard a semiconductor device comprising:
         at least one electrically conductive metal element in a non-conductive package material;   a first metal layer for covering said metal element, said first layer having a smooth (flat) morphology; and   a second metal layer for covering partially said first layer, leaving at least one portion of the surface of said first layer ( 102 ) exposed, said second layer having a rough morphology.       

     One or more embodiments may comprise:
         a die pad (e.g.,  14 ) for mounting a semiconductor die in said package;   said first layer for covering said die pad; and   a semiconductor die attached on said die pad on said first layer.       

     In one or more embodiments, said metal element may comprise at least one lead provided with a tip, with said second layer ( 104 ) provided (e.g., only) on said tip. 
     Without prejudice to the underlying principles, the details of construction and the embodiments may vary, even significantly, with respect to what has been illustrated herein purely by way of non-limiting example, without thereby departing from the sphere of protection. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.