Abstract:
There is provided a method of forming a plating layer, the method including: forming a seed layer on a substrate; forming a pattern layer on the seed layer, the pattern layer formed of a thermoplastic resin and including openings; forming a plating layer on portions of the seed layer corresponding to the openings; and removing the pattern layer. This method ensures that the plating layer is formed with a sufficient thickness and the substrate, particularly, a ceramic substrate suffers minimal chemical damage during a plating process. Moreover, the plating layer is formed with a more uniform thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 2008-0051807 filed on Jun. 2, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method of forming a plating layer, and more particularly, to a method of forming a plating layer which ensures a sufficient plating thickness of the plating layer and minimum chemical damage to a substrate, notably, a ceramic substrate, during a plating process. 
         [0004]    2. Description of the Related Art 
         [0005]    In general, a multilayer ceramic substrate is utilized as a part incorporating an active device such as a semiconductor integrated circuit (IC) chip and a passive device such as a capacitor, an inductor and a resistor, or a simple semiconductor IC package. More specifically, the multilayer ceramic substrate is widely used to implement various electronic parts such as a power amplifier (PA) module substrate, a radio frequency (RF) diode switch, a filter, a chip antenna and diverse package parts and a converged device. 
         [0006]    Conventionally, to form external electrodes of this multilayer ceramic substrate, an Ni plating layer and an Au plating layer are formed on a metal pattern printed on a surface of a ceramic sintered body by electroless plating and electroplating, respectively. However, in a case where the external electrodes are formed by this method, the Ni/Au plating layer is not sufficiently thick. Besides, the Ni/Au plating layer is not uniform in thickness since current is hardly supplied to an entire area of the substrate uniformly. Accordingly, the external electrodes when bonded to a probe tip are degraded in bonding force while experiencing higher electrical resistance. Moreover, in a case where a plating solution permeates into the ceramic substrate during the plating process, the ceramic substrate may be decolored or eroded, which subsequently leads to reduction in strength. 
         [0007]    These problems undermine reliability of the multilayer ceramic substrate. Thus, there has been a demand in the art for a method of ensuring the plating layer is formed with a uniform and sufficient thickness. 
       SUMMARY OF THE INVENTION 
       [0008]    An aspect of the present invention provides a method of forming a plating layer in which a plating layer is formed with a sufficient thickness while a substrate, particularly a ceramic substrate is minimized in chemical damage during the plating process. 
         [0009]    According to an aspect of the present invention, there is a method of forming a plating layer, the method including: forming a seed layer on a substrate; forming a pattern layer on the seed layer, the pattern layer formed of a thermoplastic resin and including openings; forming a plating layer on portions of the seed layer corresponding to the openings; and removing the pattern layer. 
         [0010]    The pattern layer may be formed of one material selected from a group consisting of polyethylene, polyvinylidene fluoride, liquid crystal polymer and a combination thereof. The pattern layer has a thickness of 20 to 30 μm to ensure a sufficient thickness of the plating layer. 
         [0011]    The removing the pattern layer may include heating the pattern layer. The removing the pattern layer may include heating the pattern layer at a temperature of 200 to 300° C. for 2 to 3 hours. 
         [0012]    The seed layer may include first and second layers, the first layer formed of one material selected from a group consisting of Ti, Cr, ZnO and a combination thereof, and the second layer formed on the first layer and containing Cu. Here, the first layer may have a thickness of 0.05 to 0.3 μm. The second layer may have a thickness of 0.3 to 1 μm. 
         [0013]    The forming a seed layer may include performing one of sputtering and E-beam evaporation. 
         [0014]    The forming a plating layer may include performing electroplating, but the present invention is not specifically limited thereto. 
         [0015]    The forming a seed layer on a substrate may include forming the seed layer on an entire top surface of the substrate. 
         [0016]    The forming a plating layer may include forming a Cu layer, an Ni layer and an Au layer sequentially. 
         [0017]    The substrate may be a ceramic substrate having an internal electrode and a conductive via therein, the internal electrode and the conductive via electrically connected to the plating layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0019]      FIGS. 1A to 1D  are cross-sectional views illustrating a method of forming a plating layer according to an exemplary embodiment of the invention; 
           [0020]      FIG. 2  is a detailed view illustrating a seed layer shown in  FIG. 1 ; and 
           [0021]      FIG. 3  illustrates a process which may be added to the embodiment shown in  FIG. 1  according to an exemplary embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the shapes and dimensions may be exaggerated for clarity, and the same reference signs are used to designate the same or similar components throughout. 
         [0023]      FIGS. 1A to 1D  are cross-sectional views illustrating a method of forming a plating layer according to an exemplary embodiment of the invention. 
         [0024]    First, as shown in  FIG. 1A , a substrate  101  is provided and a seed layer  102  is formed on a top surface of the substrate  101 . The substrate  101  may include a conductive via and an internal electrode formed therein. Particularly, the substrate  101  may adopt a ceramic substrate such as a low co-fired or high co-fired ceramic. However, the present invention is not limited thereto, and any kind of substrate may be utilized as long as the substrate requires a plating layer as an external electrode. The seed layer  102  serves as a seed for a plating layer which will be formed in a later process. In the present embodiment, the seed layer  102  may be formed on an entire area of the top surface of the sintered substrate  101  not by a screen printing but by sputtering or E-beam deposition. As described above, the seed layer  102  is formed as a thin film on the entire top surface of the substrate  101 . Accordingly, as will be described later, the plating layer can be easily formed by electroplating. 
         [0025]      FIG. 2  is a detailed cross-sectional view illustrating a seed layer shown in  FIG. 1 . Referring to  FIG. 2 , the seed layer  102  is configured as a two-layer structure including first and second layers. The first layer is a Ti layer  102   a  and the second layer is a Cu layer  102   b . Here, the Ti layer  102   a  serves to enhance adherence between the substrate  101 , e.g., made of ceramic and the plating layer. The Ti layer  102   a  may have a thickness ta ranging from 0.05 to 0.3 μm. However, alternatively, the first layer may be formed of Cr or ZnO in addition to Ti, or these materials may be used in combination. The Cu layer  102   b  functions as a substantial seed and considering this seed function, the Cu layer  102   b  may have a thickness tb of about 0.3 to 1.0 μm. Meanwhile, although not illustrated, a metal pad layer made of e.g., Ag may be additionally formed between the seed layer  102  and the substrate  101 . 
         [0026]    Afterwards, as shown in  FIG. 1B , a pattern layer  103  is formed on the seed layer  102 . Here, the pattern layer  103  has openings O provided therein to serves as an area for forming the plating layer. Particularly, in the present embodiment, the pattern layer  103  is formed of a thermoplastic resin to be thermally removed. Accordingly, as will be described later, after forming the plating layer, the pattern layer  103  can be easily removed, with minimum damage to the substrate  101  and plating layer. The pattern layer  103  may be formed of polyethylene, polyvinylidene fluoride (PVDF), and liquid crystal polymer (LCP). 
         [0027]    The pattern layer  103  has a thickness t 1  determined by considering a thickness of a desired plating layer. The present embodiment aims to form a thick plating layer by electroplating. Given this, the pattern layer  103  may have a thickness t 1  of 20 to 30 μm. Meanwhile, the pattern layer  103  may be formed by various methods for forming thermoplastic resin patterns, for example, by spin coating after a mask process. 
         [0028]    Thereafter, as shown in  FIG. 1C , a plating layer  104  is formed on portions of the seed layer  102  corresponding to the openings O. Although not described in detail, to perform this plating process, the substrate having the seed layer  102  and the pattern layer  103  formed thereon is immersed in a plating bath containing a plating solution, and then electroplating is preformed to induce electrical chemical reaction. As described above, it is construed that the electroplating can be carried out since the seed layer  102  is provided as a thin film on the entire top surface of the substrate  101 . In the present embodiment, the plating layer  104  can be formed on the portions of the pattern layer  103  corresponding to the openings by electroplating to have a great thickness. This allows for superior bonding between the substrate  101  and the plating layer  104 . Here, the plating layer  104  may be formed of a three-layer structure of Cu/Ni/Au even though configured differently according to a material for the seed layer  102 . 
         [0029]    Next, as shown in  FIG. 1D , the pattern layer  103  is removed from the substrate  101 . As described above, the pattern layer  103  is formed of a thermoplastic resin such as polyethylene, which can be easily removed by adequate heating. Here, the pattern layer  103  may be heated at 300 to 400° C. and for 2 to 3 hours to be removed. Also, the plating layer  104  may be heated while being covered by the ceramic substrate to undergo minimum damage. 
         [0030]    As described above, the pattern layer  103  can be easily removed by heat, not by a chemical method. This allows the plating layer  104  and the substrate  101  to be chemically undamaged. The pattern layer  103 , if formed of a photosensitive material, needs to be removed using a strong acid or a strong base. This may chemically impair the plating layer  104  and the substrate  101 . However, in the present embodiment, the pattern layer  103  is substantially free from such damage. Accordingly, adherence force between the plating layer  104  and the substrate  101  is enhanced. Also, another electrical device may be bonded to the plating layer  104  more strongly. 
         [0031]    Meanwhile according to another exemplary embodiment of the invention, as shown in  FIG. 3 , the seed layer  102  may be partially removed to have a shape identical to a shape of the plating layer  104  to obtain a desired electrode structure.  FIG. 3  illustrates a process which may be added to the embodiment of  FIG. 1  according to an exemplary embodiment of the invention. Here, the seed layer  102  may be removed using an adequate mask by a known process in the art. 
         [0032]    The inventors of the present invention conducted experiments for demonstrating superior effects of the present invention. Hereinafter, the plating layers formed by the conventional method and the method of the present invention will be compared. 
         [0033]    First, by the conventional method, a plating layer having a three-layer structure of Cu/Ni/Au was formed without employing a thermoplastic pattern. Meanwhile, a plating layer having a three-layer structure of Cu/Ni/Au was formed by the method of the present invention. 
         [0034]    Here, in the conventional method, the Ni layer was formed by electroless plating and the Au layer was formed by electroplating. In the present invention, both Ni and Au were formed by electroplating. As a result of comparing the thickness between the plating layers formed according to the conventional method and the method of the present invention, for the conventional plating layer, the Cu layer, Ni layer, and Au layer had an average thickness of 3.2 μm, 6.4 μm, and 0.69 μM, respectively. On the other hand, for the plating layer of the present invention, the Cu layer, Ni layer, Au layer had an average thickness of 8.2 μm, 4.1 μm, and 2.1 μm, respectively. As described above, the plating layer of the present invention can be formed with a greater thickness than the conventional plating layer and in addition is more uniform in thickness. 
         [0035]    Then, adherence force of the conventional plating layer and the plating layer of the present invention was compared. The plating layer formed according to the present invention, when bonded to a probe tip, is significantly increased in bonding force. That is, a shear stress required for separating the plating layer from the probe tip bonded thereto was averaged about 36N/mm 2  in the conventional method, and was 82N/mm 2  in the present invention, which is at least twice higher than the shear stress of the present invention. 
         [0036]    As set forth above, according to exemplary embodiments of the invention, a plating layer can be formed with a sufficient thickness and with minimum chemical damage to a substrate, particularly, a ceramic substrate, during a plating process. Moreover, the plating layer formed by the method of forming the plating layer according to the invention can be more uniform in thickness. 
         [0037]    While the present invention has been shown and described in connection with the exemplary embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.