Abstract:
This invention provides a method for forming a micro blind via on a copper clad laminate (CCL) substrate. A CCL substrate having a dielectric layer sandwiched by a first copper layer and a second copper layer is prepared. A laser absorbing layer is formed on the first copper layer. The laser absorbing layer is subjected to laser drilling. A micro blind via is drilled into the first copper layer and the dielectric layer in one step.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a process for manufacturing a circuit board and, more particularly, to a method for forming a micro blind via on a one side of a double-side copper clad laminate substrate utilizing laser drilling technique. According to this invention, it is not necessary to form a copper window during the fabrication of the micro blind via. 
         [0003]    2. Description of the Prior Art 
         [0004]    In recent history, printed circuit boards (PCBs) have been widely used for carrying various electronic components and devices. As there is demand for electronic products to be lighter, smaller, and portable, research and development of printed circuit boards is unavoidably moving toward a direction of miniaturization, higher integration, lower thickness, and the use of multiple layers. 
         [0005]    In general, printed circuit boards having circuit layouts on both sides of the circuit boards have been widely used in numerous electrical apparatuses such as air-conditioners, telephones, and fax machines. Nevertheless, it is essential to have a “bridge”, or otherwise referred to as through holes, for communicating between the circuits and electrical wires on both sides of the board. The useable area of a typical double-sided printed circuit board is often twice that of a typical single-sided printed circuit board, and in contrast to a single-sided printed circuit board, the double-sided printed circuit board is more suitable to be used for products with complex circuits. 
         [0006]    Please refer to  FIGS. 1-5 .  FIGS. 1-5  are schematic, cross-sectional diagrams illustrating a prior art process of forming a plated micro blind via on a double-side copper clad substrate utilizing laser drilling technique. First, as shown in  FIG. 1 , a copper clad substrate (CCL)  10  is provided. The CCL  10  comprises an intermediate base layer  12  and copper films  14   a  and  14   b  sandwiching about the intermediate base layer  12 . The copper film  14   a  covers the upper side of the CCL  10 , while the copper film  14   b  covers the lower side of the CCL  10 . 
         [0007]    As shown in  FIG. 2 , a photoresist layer  16   a  and a photoresist layer  16   b  are formed on the copper films  14   a  and  14   b , respectively. A photolithographic process including exposure, development and baking steps is carried out to form an opening  26  in the photoresist layer  16   a . Thereafter, using the photoresist layer  16   a  and the photoresist layer  16   b  as a hard mask, a copper wet etching process is performed to etch the exposed copper film  14   a  through the opening  26 , thereby forming a copper window  24  in the copper film  14   a.    
         [0008]    As shown in  FIG. 3 , a laser drilling process is performed to drill a micro blind via  30  into the intermediate base layer  12  through the copper window  14 . The micro blind via  30  exposes a portion of the copper film  14   b . Typically, the laser drilling process employs CO2 laser having a wavelength, which the copper film  14   b  does not absorb. Therefore, the laser drilling process does not etch through the copper film  14   b  and stops on the copper film  14   b.    
         [0009]    Subsequently, a conventional de-smear process is performed to remove the residuals on the surface of the CCL  10 . After the de-smear process, an electro-less copper plating process is carried out to plate a thin copper layer  32  onto the surface of the CCL  10  and on the interior sidewall of the micro blind via  30 . 
         [0010]    As shown in  FIG. 4 , another lithographic process including exposure and development is performed to form a photoresist layer pattern  40   a  and a photoresist layer pattern  40   b  on the copper films  14   a  and  14   b , respectively. The photoresist layer pattern  40   a  and photoresist layer pattern  40   b  define the circuit layout to be formed on the upper side and lower side of the CCL  10 . 
         [0011]    As shown in  FIG. 5 , a plating process is then carried out. Copper wiring patterns  50   a  and  50   b  are formed in the area that is not masked by the photoresist layer pattern  40   a  and photoresist layer pattern  40   b . Finally, protective tin layers  52   a  and  52   b  is plated on the copper wiring patterns  50   a  and  50   b.    
         [0012]    The above-described prior art method has a drawback in that the copper window  24  is required before performing the laser drilling process in order to define the laser burning area in advance. Therefore, it requires one additional exposure/development step and one more copper etching step, thus increases the process time and reduces the throughput. The additional exposure/development step and copper etching step also increases the production cost. 
       SUMMARY OF THE INVENTION  
       [0013]    It is one object of the present invention to provide an improved method for directly forming a micro blind via on a one side of a double-side copper clad laminate substrate utilizing laser drilling technique without the need of forming a copper window in advance. 
         [0014]    According to the claimed invention, a method for directly laser drilling a micro blind via on a copper clad laminate (CCL) substrate is provided. A double-side CCL substrate is prepared. The CCL substrate comprises a first copper film and a second copper film sandwiching about an intermediate base layer. A laser absorbing layer is then formed on the first copper film. A laser drilling process is performed to impinge a laser beam directly onto the laser absorbing layer to burn away a portion of the laser absorbing layer, the first copper film and the intermediate base layer, thereby exposing a portion of the second copper film and forming a micro blind via in the CCL substrate. 
         [0015]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0016]      FIGS. 1-5  are schematic, cross-sectional diagrams illustrating a prior art process of forming a plated micro blind via on a double-side copper clad substrate utilizing laser drilling technique. 
           [0017]      FIGS. 6-12  are schematic, cross-sectional diagrams illustrating a preferred exemplary process of directly forming a micro blind via on a double-side copper clad substrate utilizing laser drilling technique in accordance with one preferred embodiment of this invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0018]    Please refer to  FIGS. 6-12 .  FIGS. 6-12  are schematic, cross-sectional diagrams illustrating a preferred exemplary process of directly forming a micro blind via on a double-side copper clad substrate utilizing laser drilling technique in accordance with one preferred embodiment of this invention. As shown in  FIG. 6 , a double-side copper clad laminate (CCL) substrate  100  is provided. The CCL substrate  100  comprises an intermediate base layer  102  and copper films  104   a  and  104   b  sandwiching about the intermediate base layer  102 . The intermediate base layer  102  may be made of prepreg or any other suitable dielectric insulating materials. 
         [0019]    As shown in  FIG. 7 , a laser absorbing layer  106   a  and laser absorbing layer  106   b  are formed on the copper films  104   a  and  104   b , respectively. The laser absorbing layer  106   a  and laser absorbing layer  106   b  is capable of absorbing a laser radiation with a pre-determined laser wavelength. According to the preferred embodiment of this invention, the laser absorbing layer  106   a  and laser absorbing layer  106   b  is comprised of copper oxide. The method for forming the laser absorbing layer  106   a  and laser absorbing layer  106   b  may include the step of contacting the copper films  104   a  and  104   b  with solution containing NaClO/NaOH. However, other materials that are capable of absorbing laser radiation may be used. This invention should not be limited to copper oxide and it formation method. Preferably, the laser absorbing layer  106   a  has a thickness of about 0.5-1.5 micrometers. 
         [0020]    As shown in  FIG. 8 , after the formation of the laser absorbing layer  106   a  and laser absorbing layer  106   b , a laser drilling process is carried out. The laser beam directly impinges on the laser absorbing layer  106   a , and etches the laser absorbing layer  106   a , the copper film  104   a  and the intermediate layer  102 , thereby forming a micro blind via  130 . According to the preferred embodiment, the laser is CO2 laser. The laser absorbing layer  106   a  is capable of absorbing the wavelength of the CO2 laser and has a laser absorbility of 50% or higher. The laser drilling process stops on the copper film  104   b . 
         [0021]    It is one distinct feature of the present invention that the copper window is not required before the laser drilling process. The laser directly impinges on the laser absorbing layer  106   a , which absorbs the laser energy in a very short period of time and concurrently releases heat to burn away and vaporizes the underlying copper film  104   a  and the intermediate base layer  102  around the area on which the laser beam impinges. A photo mask for defining the copper window is thus omitted. The method provided according to this invention is therefore much simpler and more cost-effective. 
         [0022]    Furthermore, other metal layers or material films that are capable of absorbing the laser radiation as described above may be used to cover the laser absorbing layer  106   a.    
         [0023]    It is another distinct feature of the present invention that overhang structure  124  is formed around the rim of the micro blind via  130  as specifically indicated in  FIG. 8 . 
         [0024]    As shown in  FIG. 9 , after the laser drilling process, the remaining laser absorbing layers  106   a  and  106   b  are removed to expose the copper films  104   a  and  104   b.    
         [0025]    As shown in  FIG. 10 , a conventional de-smear process is carried out to remove the residuals on the surface of the CCL substrate  100 . The residuals may be sputtered substances during the laser drilling. After the de-smear process, an electro-less copper plating process is carried out to plate a thin copper layer  132  onto the surface of the CCL substrate  100  and on the interior sidewall of the micro blind via  130 . 
         [0026]    As shown in  FIG. 11 , a lithographic process including exposure and development is performed to form a photoresist layer pattern  140   a  and a photoresist layer pattern  140   b  on the copper films  104   a  and  104   b , respectively. The photoresist layer pattern  140   a  and photoresist layer pattern  140   b  define the circuit layout to be formed on the upper side and lower side of the CCL substrate  100 . 
         [0027]    As shown in  FIG. 12 , a plating process is then carried out. Copper wiring patterns  150   a  and  150   b  are formed in the area that is not masked by the photoresist layer pattern  140   a  and photoresist layer pattern  140   b . According to this preferred embodiment, the micro blind via  130  is not completely filled with plated copper  150   a . Preferably, ⅓ of the depth of the micro blind via  130  is covered with the copper. In another case, the micro blind via  130  is completely filled with plated copper. 
         [0028]    Finally, protective tin layers  152   a  and  152   b  is plated on the copper wiring patterns  150   a  and  150   b.    
         [0029]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.