Abstract:
There is provided a multilayered ceramic substrate where a groove is formed in a intermediate stack having a relatively big thermal expansion coefficient or a step is formed at an edge portion of the intermediate stack so that cracks occurring due to differences in the thermal expansion coefficient among stacks is prevented from spreading to the edge portion, thereby inhibiting occurrence of edge cracks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of Korean Patent Application No. 2008-0034848 filed on Apr. 15, 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 multilayered ceramic substrate including ceramic green sheets stacked, and more particularly, to a low-temperature co-fired multilayered ceramic substrate. 
         [0004]    2. Description of the Related Art 
         [0005]    With greater efforts made to achieve smaller and more cost-effective portable electronic devices, studies for integrating passive devices constituting the electric devices have been conducted actively with ardent interest. 
         [0006]    Active devices are mostly high-density integrated circuits based on the silicon technology and have been incorporated into only several chip parts. Meanwhile, passive devices such as a resistor, a capacitor and an inductor have been hardly integrated and individually attached onto a circuit board by soldering. 
         [0007]    Therefore, a demand for integrating the passive devices has been increased to reduce the size of the passive devices and enhance performance and reliability thereof. As a method for solving this problem, a low temperature co-fired ceramics (LTCC)-based integration technology has been vigorously studied. 
         [0008]    Generally, in the LTCC technology, a metal is applied on a glass-mixed ceramic substrate, and a plurality of ceramic green sheets each having a metal electrode formed thereon are stacked and pressurized. Then, the ceramic green sheets are subjected to co-firing at a low temperature of 800° C. to 1000° C. to form a multilayered substrate. 
         [0009]      FIG. 1  is a schematic view illustrating a conventional low-temperature co-fired ceramic substrate. 
         [0010]    As shown, an organic binder and a plasticizer are added to a powder having a ceramic power and a sintered agent mixed therein to prepare a slurry. Then, the slurry is formed using tape casting and then cut into a predetermined size to manufacture green sheets S. 
         [0011]    The green sheets S are provided in plural numbers to manufacture a multilayered substrate. Each of the green sheets S may be provided thereon with an internal connection terminal. The internal connection terminal is formed by filling a conductive paste in a via hole perforated in the green sheet to electrically connect upper and lower ones of the green sheets. Also, the each green sheet may be provided with inner electrodes by screen-printing a conductive paste which is a high melting point metal. 
         [0012]    Moreover, the green sheets S prepared as above are stacked in a necessary number, and heated and pressurized to form stacks. 
         [0013]    Meanwhile, the green sheets S are different in physical properties such as permittivity, permeability, or thermal expansion coefficient due to differences in mixed materials added in preparing the slurry. Therefore, as shown in  FIG. 2 , in order to form a multilayered ceramic substrate, stacks  20  and  30  with low thermal expansion coefficients are disposed above and below a stack with a relatively bigger thermal expansion coefficient, respectively to be horizontally symmetrical with each other. 
         [0014]    However, in the multilayered ceramic substrate  1  with this multilayered structure, an intermediate stack  10  with a big thermal expansion coefficient suffers tensile stress due to rapid temperature change during a sintering process. This tensile stress arising from differences in the thermal expansion coefficient as described above may disadvantageously cause cracks c to the intermediate stack  10 . 
         [0015]    Furthermore, cracks occurring in the intermediate stack  10  may spread to an edge portion of the substrate to cause edge cracks. This induces moisture to be infiltrated into the substrate to lead to defects in the product and undermine reliability thereof. 
       SUMMARY OF THE INVENTION 
       [0016]    An aspect of the present invention provides a multilayered ceramic substrate in which a groove is formed in a intermediate stack having a relatively big thermal expansion coefficient or a step is formed at an edge portion of the intermediate stack to block cracks caused by differences in the thermal expansion coefficient among stacks from spreading to the edge portion, thereby inhibiting occurrence of edge cracks. 
         [0017]    According to an aspect of the present invention, there is provided a multilayered ceramic substrate including: a first stack formed of ceramic green sheets having a first thermal expansion coefficient; a second stack formed of ceramic green sheets having a second thermal expansion coefficient different from the first thermal expansion coefficient, the second stack stacked on one of upper and lower surfaces of the first stack; and a buffer part defined by a machined portion in at least one of the upper and lower surfaces of the first stack so as to prevent a crack occurring inside the first stack from spreading to an edge portion of the first stack to cause an edge crack. 
         [0018]    The multilayered ceramic substrate may further include a third stack formed of ceramic green sheets having a third thermal expansion coefficient different from the first thermal expansion coefficient, the third stack stacked on the other one of the upper and lower surfaces of the first stack. 
         [0019]    The first stack may have a thermal expansion coefficient greater than a thermal expansion coefficient of the second stack. 
         [0020]    The first stack may have a thermal expansion coefficient greater than a thermal expansion coefficient of the third stack. 
         [0021]    The second thermal expansion coefficient of the second stack may be substantially identical to the third thermal expansion coefficient of the third stack. 
         [0022]    The buffer part may include a groove provided in the at least one of the upper and lower surfaces of the first stack to induce crack occurrence. 
         [0023]    The groove may be formed inside an outer periphery of the first stack. 
         [0024]    The buffer part may have a thermal expansion coefficient identical to the first thermal expansion coefficient, and includes an auxiliary layer stacked on the at least one of the upper and lower surfaces of the first stack. 
         [0025]    The buffer part may include a groove defined by a stack of the auxiliary layer to prevent crack occurrence. 
         [0026]    The auxiliary layer may include at least one of the ceramic green sheets of the first stack. 
         [0027]    The buffer part may include a step formed such that the edge portion of the first stack has a thickness greater than an inner portion thereof. 
         [0028]    The buffer part may include a groove formed in the at least one of the upper and lower surfaces of the first stack to induce crack occurrence, and a step formed such that the edge portion of the first stack has a thickness greater than an inner portion thereof. 
         [0029]    The groove may be formed inside the edge portion of the first stack where the step is formed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    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: 
           [0031]      FIG. 1  is a schematic view illustrating general low-temperature co-fired ceramic substrates; 
           [0032]      FIG. 2A  is a cross-sectional view illustrating cracks generated in a first stack in a multilayered ceramic substrate where the ceramic substrates of  FIG. 1  are stacked; 
           [0033]      FIG. 2B  is a plan view illustrating the first stack of  FIG. 2A ; 
           [0034]      FIG. 3A  is a cross-sectional view illustrating grooves formed in a first stack in a multilayered ceramic substrate according to an exemplary embodiment of the invention; 
           [0035]      FIG. 3B  is a cross-sectional view illustrating another substrate where grooves are formed in a first stack; 
           [0036]      FIG. 3C  is a cross-sectional view illustrating grooves formed in a first stack; 
           [0037]      FIG. 3D  is a plan view illustrating a first stack having grooves formed therein; 
           [0038]      FIG. 4A  is a cross-sectional view illustrating a step formed on a first stack in a multilayered ceramic substrate according to an exemplary embodiment of the invention; 
           [0039]      FIG. 4B  is a plan view illustrating the first stack of  FIG. 4A ; 
           [0040]      FIG. 5A  is a cross-sectional view illustrating grooves and a step formed in a first stack in a multilayered ceramic substrate according to an exemplary embodiment of the invention; and 
           [0041]      FIG. 5B  is a plan view illustrating the first stack of  FIG. 5A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0042]    Exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
         [0043]    First, a multilayered ceramic substrate will be described with reference to  FIG. 3 . 
         [0044]      FIG. 3  is a schematic view illustrating a multilayered ceramic substrate la according to an exemplary embodiment of the invention, in which  FIGS. 3  A and B are cross-sectional views illustrating grooves formed in a first stack,  FIG. 3C  is a cross-sectional view illustrating grooves formed in a first stack, and  FIG. 3D  is a plan view illustrating a first stack having grooves formed therein. 
         [0045]    As shown in  FIG. 3A , the multilayered ceramic substrate  1   a  of the present embodiment includes a first stack  10   a,  a second stack  20  and a buffer part  40 . The second stack  20  is disposed on one of an upper surface and lower surface of the first stack  10   a.    
         [0046]    Also, as shown in  FIG. 3B , the multilayered ceramic substrate  1   a  includes a first stack  10   a,  a second stack  20 , a third stack  30  and a buffer part  40 . The second stack  20  may be formed on the upper surface of the first stack  10   a  and the third stack  30  may be formed on the lower surface of the first stack  10   a.    
         [0047]    The stacks  10   a,    20 , and  30  are formed by stacking a plurality of ceramic green sheets S. The stacks may be identical to or different from one another in physical properties according to physical properties of the stacked green sheets S. 
         [0048]    That is, the green sheets S are classified by the thermal expansion coefficient (CTE) and corresponding ones of the green sheets S with identical expansion coefficients are stacked to form the respective stacks  10   a,    20 , and  30  with different thermal expansion coefficients. 
         [0049]    The first stack  10   a  may have a thermal expansion coefficient different from thermal expansion coefficients of the second stack  20  and third stack  30 , respectively. However, particularly, the second stack  20  and the third stack  30  may have thermal expansion coefficients substantially identical to each other. 
         [0050]    Therefore, the first stack  10   a  and the second stack  20  have a different thermal expansion coefficient from each other and the first stack  10   a  and the third stack  30  have a different thermal expansion coefficient from each other. Also, the second stack  20  and the third stack  30  may have a thermal expansion coefficient identical to or different from each other. 
         [0051]    Furthermore, the first stack  10   a  has a first thermal expansion coefficient greater than a second thermal expansion coefficient of the second stack  20  and a third thermal expansion coefficient of the third stack  30 , respectively. 
         [0052]    Meanwhile, as in  FIG. 3A  or  FIG. 3B , the buffer part  40  is formed by machining the upper surface of the first stack  10   a  so as to prevent cracks generated inside the first stack  10   a  from spreading to an edge portion of the first stack to cause occurrence of edge cracks. 
         [0053]    In the present embodiment, the buffer part  40  includes grooves  11  formed in the upper surface of the first stack  10   a  to induce occurrence of cracks c. Alternatively, the grooves  11  may be formed in the lower surface of the first stack  10   a.    
         [0054]    Also, to form the grooves  11 , the surface of the first stack  10   a  may be machined, for example, by irradiating a laser beam onto the surface of the first stack  10   a,  but not limited thereto. 
         [0055]    The grooves  11  may be formed in consideration of circuit patterns (e). The grooves  11  are formed inside an outer periphery of the first stack  10   a  to be spaced apart from the outer periphery at a predetermined distance. 
         [0056]    Moreover, as shown in  FIG. 3C , the buffer part  40  is formed of a material identical to the first stack  10   a,  and also has a thermal expansion coefficient identical to the first stack  10   a.  The buffer part  40  may include an auxiliary layer  13  formed on at least one of the upper and lower surfaces of the first stack. 
         [0057]    Also, the buffer part  40  may include grooves formed by stacking the auxiliary layer  13  to induce occurrence of cracks c. Here, the grooves  12  may be formed by machining a surface of the buffer part  40  inside the outer periphery of the first stack  10   a.    
         [0058]    That is, the auxiliary layer  13  formed of a material identical to the first stack  10   a  and also having a thermal expansion coefficient identical to the first stack  10   a  is additionally stacked on the first stack  10   a  and the grooves  12  are formed therein. Alternatively, the auxiliary layer  13  having the grooves  12  formed therein may be additionally stacked. 
         [0059]    Here, the auxiliary layer  13  may be formed by stacking at least one of the green sheets S constituting the first stack  10   a.    
         [0060]    As described above, the grooves  11  or  12  are formed to arbitrarily design such that cracks c occur regularly along the grooves  11  or  12 . Also, as shown in  FIG. 3D , the grooves  11  or  12  allow the cracks to occur only inside the stacks  10  while blocking the cracks from spreading to edge portions of the stacks  10 . 
         [0061]    A multilayered ceramic substrate according to an exemplary embodiment of the invention will be described with reference to  FIGS. 4A and 4B . 
         [0062]    In the embodiment of  FIGS. 4A and 4B , the multilayered ceramic substrate  1   b  is configured in a substantially identical manner to the embodiment of  FIG. 3 . 
         [0063]    However, the embodiment of  FIG. 4  is different from the embodiment of  FIG. 3  in terms of a detailed construction of the first stack. Thus, hereinafter, overlapping parts with the previous embodiment will be omitted and only construction of the first stack will be mainly described. 
         [0064]    As shown in  FIG. 4 , the multilayered ceramic substrate  1   b  of the present embodiment includes a first stack lob, a second stack  20 , a third stack  30  and a buffer part  50 . Also, a second stack  20  is stacked on an upper surface of the first stack  10   b  and a third stack  30  is stacked on a lower surface of the first stack  10   b.    
         [0065]    Although not illustrated, alternatively, the multilayered ceramic substrate  1   b  of the present embodiment includes a first stack lob, a second stack  20  and a buffer part  50 . The second stack  20  may be formed on one of the upper and lower surfaces of the first stack  10   b.    
         [0066]    The stacks  10   b,    20  and  30  have respective thermal expansion coefficients identical to the previous embodiment and thus will not be described further. 
         [0067]    In the present embodiment, the buffer part  50  includes a step  14  formed such that an edge portion of the first stack  10   b  has a thickness greater than a thickness of an inner portion thereof. 
         [0068]    The step  14  may be formed by stacking at least one of green sheets S constituting the first stack  10   b  along the edge portion of the first stack  10   b.    
         [0069]    This allows cracks c generated inside the first stack  10   b  from spreading to the edge portion of the first stack  10   b.    
         [0070]    Meanwhile, a multilayered ceramic substrate according to another exemplary embodiment of the invention will be described with reference to  FIGS. 5A and 5B . 
         [0071]    In the embodiment of  FIGS. 5A and 5B , the multilayered ceramic substrate  1   c  is configured in a similar manner to the embodiment of  FIG. 3  and construction of a first stack will be mainly described while the overlapping parts are omitted. 
         [0072]    As shown in  FIG. 5 , the multilayer ceramic substrate  1   c  of the present embodiment includes a first stack  10   c,  a second stack  20 , a third stack  30  and a buffer part  60 . A second stack  20  is stacked on an upper surface of the first stack  10   c  and a third stack  30  is stacked on a lower surface of the first stack  10   c.    
         [0073]    In the present embodiment, the buffer part  60  includes grooves  11  formed in the upper surface of the first stack  10   c  to induce occurrence of cracks c and a step  14  formed with a predetermined thickness along an edge portion of the first stack  10   c  such that the edge portion of the first stack has a thickness greater than an inner portion thereof. 
         [0074]    Here, the grooves  11  may be formed inside the edge portion of the first stack  10   c  where the step  14  is formed. 
         [0075]    As described above, the grooves are provided in at least one of the upper and lower surfaces of the first stack disposed between the second stack and the third stack. Also, the step is formed with a predetermined thickness along the edge portion of the first stack to prevent inner cracks from spreading to the edge portion of the first stack and thus reduce defects by preventing infiltration of moisture. 
         [0076]    As set forth above, according to exemplary embodiments of the invention, cracks generated by differences in the thermal expansion coefficient when the temperature changes during sintering are prevented from spreading to an edge portion of a substrate, thereby inhibiting infiltration of moisture. Accordingly, this reduces defects in the product and enhances reliability thereof. 
         [0077]    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.