Abstract:
A high frequency substrate includes a first metal layer, a first dielectric layer, a second metal layer, a second dielectric layer and a high-frequency signal transmission line. The first dielectric layer is formed on the first metal layer, and the second metal layer is formed on the first dielectric layer. The first and second metal layers are maintained in stable voltage status due to the high dielectric coefficient of the first dielectric layer. Besides, the second dielectric layer is formed on the second metal layer. High speed and high frequency transmission are achieved when signals transmitting in the high-frequency transmission line formed on the second dielectric layer due to the low dielectric coefficient of the second dielectric layer.

Description:
[0001]    This application claims the benefit of Taiwan application Serial No. 91138177, filed Dec. 31, 2002.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a high frequency substrate, and more specifically to a high frequency substrate capable of increasing the signal transmission speed of the high-frequency signal transmission line and maintaining the power plane and the ground plane in stable voltage status.  
           [0004]    2. Description of the Related Art  
           [0005]    While wireless network and satellite communication are gaining rapid growth, electronic products tend to have the design of high speed, high frequency and high capacity. Consequently, the present electronic products need to have the support of a high frequency substrate in order that a high frequency and high speed performance can be achieved.  
           [0006]    Please refer to FIG. 1, the cross-sectional view of a conventional high frequency substrate. In FIG. 1, the high frequency substrate  100  comprises a power plane  102 , a ground plane  106 , a high-frequency signal transmission line  110  and dielectric layers  104  and  108 . The dielectric layer  104  is formed on the power plane  102 ; the ground plane  106  is formed on the dielectric layer  104 . The dielectric layer  108  is formed on the ground plane  106  while the high-frequency signal transmission line  110  is deposited on the dielectric layer  108 . Of which, the dielectric constant of the air outside the high-frequency signal transmission line  110  is 1 while the dielectric constant of the dielectric layers  104  and  108  is ε r  wherein the value of ε r  being greater than 1, say 4 for instance. Besides, the dielectric layers  104  and  108  are normally dense structures.  
           [0007]    Suppose that the signal transmission speed in the high-frequency signal transmission line  110  is Vp, then Vp□C/(ε eff ) 1/2 , wherein C is the speed of the light while ε eff  is the effective dielectric constant. That is to say, the signal transmission speed Vp is inversely proportional to the square root of the effective dielectric constant ε eff . It is noteworthy that ε eff  varies with the intensity of signal transmission frequency. In other words, an electromagnetic field will be generated during signal transmission. Of which, the dielectric constant for the medium of the high electromagnetic area can be regarded as ε eff .  
           [0008]    Unlike in the case of low-frequency operation where the electromagnetic field generated during signal transmission concentrates in the air outside the high-frequency signal transmission line  110 , the electromagnetic field generated during signal transmission almost entirely concentrates in the dielectric layer  108  because the high-frequency signal transmission line  110  normally has a high frequency of signal transmission. So the effective dielectric constant ε eff  approximately equals to the dielectric constant ε r  of the dielectric layer  108 . For instance, the value of ε eff  equals 4. Hence, the signal transmission speed Vp will slow down, which increases the energy loss of signals and in turn affects signal transmission quality greatly.  
           [0009]    Moreover, the voltages of the power plane  102  and the ground plane  106  are interfered by one another due to the low dielectric constant of the dielectric layer  104 ; for example, ε r  is 4. The values of voltage in the power plane  102  and the ground plane  106  vary; the power plane  102  and the ground plane  106  are unable to be maintained in stable voltage status so that the high frequency substrate  100  is subjected to huge impacts and unable to keep in well progress. All of this begs for improvement.  
         SUMMARY OF THE INVENTION  
         [0010]    In views of the aforesaid difficulties of the conventional techniques, the present invention aims to provide a high frequency substrate with a high frequency transmission line formed on a low-dielectric-coefficient dielectric layer, so that high speed and high frequency transmission are thus achieved. Further more, the high frequency substrate of the invention has a high-dielectric-coefficient dielectric layer formed between the power plane and the ground plane, so that the voltage of the power plane and the ground plane are maintained stable with this particular design.  
           [0011]    It is therefore an object of the present invention to provide an improved high frequency substrate at least having a first metal layer, a first dielectric layer, a second metal layer, a second dielectric layer and a high-frequency signal transmission line. The first dielectric layer is formed on the first metal layer, and the first dielectric layer is made of a high dielectric coefficient material. The second metal layer is formed on the first dielectric layer and the second dielectric layer is formed on the second metal layer; the second dielectric layer is made of a low dielectric coefficient material. The high-frequency signal transmission line is formed on the second dielectric layer.  
           [0012]    It is another object of the present invention to provide an improved high frequency substrate at least having a first metal layer, a first dielectric layer, a second metal layer, a second dielectric layer, a third dielectric layer and a high-frequency signal transmission line. The first dielectric layer is formed on the first metal layer, and the first dielectric layer is made of a high dielectric coefficient material. The second metal layer is formed on the first dielectric layer and the second dielectric layer is formed on the second metal layer. The second metal layer has an opening; the third dielectric layer is filled within the opening and on the second metal layer. The third dielectric layer is made of a low dielectric coefficient material and the high-frequency signal transmission line is formed on the third dielectric layer.  
           [0013]    It is a further object of the present invention to provide a substrate at least having a first dielectric layer, a second dielectric layer and a signal transmission line. The first dielectric layer has an opening and the second dielectric layer is filled within the opening; the signal transmission line is formed on the second dielectric layer. While the second dielectric layer is made of a low dielectric coefficient material, the signal transmission line thereon is a high-frequency signal transmission line. Relatively, while the second dielectric layer is made of a high dielectric coefficient material, the signal transmission line thereon is a low-frequency signal transmission line.  
           [0014]    It is further another object of the present invention to provide a method of forming a substrate. The method includes the steps described as follow: first, a first dielectric layer is performed. Then, portions of the first dielectric layer are etched so that an opening is formed on the first dielectric layer. Further, a second dielectric layer is filled within the space; a signal transmission line is finally formed on the second dielectric layer. While the second dielectric layer is made of a low dielectric coefficient material, the signal transmission line thereon is a high-frequency signal transmission line. Relatively, while the second dielectric layer is made of a high dielectric coefficient material, the signal transmission line thereon is a low-frequency signal transmission line.  
           [0015]    Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 (Prior Art) is a cross-sectional view of a conventional high frequency substrate;  
         [0017]    [0017]FIG. 2 is a cross-sectional view of a high frequency substrate in accordance with the first preferred embodiment of the invention;  
         [0018]    [0018]FIG. 3 is a cross-sectional view of a high frequency substrate in accordance with the second preferred embodiment of the invention; and  
         [0019]    [0019]FIG. 4 is a cross-sectional view of a high frequency substrate in accordance with the third preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The present invention provides a high frequency substrate with a high frequency transmission line on a low-dielectric-coefficient dielectric layer. Thus, high speed and high frequency transmission are achieved and also the disadvantages and limitations of existing high frequency substrate are overcome. Further more, the high frequency substrate of the invention has a high dielectric coefficient dielectric layer formed between the power plane and the ground plane. With this particular design, the voltage of the power plane and the ground plane are maintained stable. The implementation of the high frequency substrate is described as follows, taking three examples, with reference to the accompanying drawings.  
       EXAMPLE 1  
       [0021]    [0021]FIG. 2 is a cross-sectional view of a high frequency substrate in accordance with the first preferred embodiment of the invention. Referring to FIG. 2, the high frequency substrate  200  has two metal layers  202  and  206 , two dielectric layers  212  and  214  and a high-frequency signal transmission line  210 . The dielectric layer  212  is formed on the metal layer  202  and the dielectric layer  212  is made of a high dielectric coefficient material; the value of the dielectric coefficient thereof is more than 4. The metal layer  206  is formed on the dielectric layer  212  and the dielectric layer  214  is formed on the metal layer  206 . The dielectric layer  214  is made of a low dielectric coefficient material; the value of the dielectric coefficient thereof is less than 4. The high-frequency signal transmission line  210  is on the dielectric layer  214 .  
         [0022]    The dielectric coefficient of the dielectric layer  214  is less than the dielectric coefficient of the traditional dielectric layer  108  shown in FIG. 1. It will be appreciated that the signal transmission speed in the high-frequency signal transmission line  210  will be enhanced in accordance with the relationship that the signal transmission speed is inversely proportional to the square root of the effective dielectric constant. The energy loss of the signal transmission is decreased so that high speed and high frequency transmission can be achieved.  
         [0023]    Further, the dielectric coefficient of the dielectric layer  212  is greater than the dielectric coefficient of the traditional dielectric layer  104  shown in FIG. 1. It will be appreciated that the metal layers  202  and  206  (such as a power plane and a ground plane) can be maintained in stable voltage status and the high frequency substrate  200  can be kept in well progress.  
       EXAMPLE 2  
       [0024]    [0024]FIG. 3 is a cross-sectional view of a high frequency substrate in accordance with the second preferred embodiment of the invention. Referring to FIG. 3, the high frequency substrate  300  has two metal layers  302  and  306 , a high-frequency signal transmission line  310 , a low-frequency signal transmission line  318  and three dielectric layers  312 ,  314  and  316 . The dielectric layer  312  is on the metal layer  302  and the dielectric layer  312  is made of a high dielectric coefficient material; the value of the dielectric coefficient thereof is more than 4. The metal layer  306  is on the dielectric layer  312  and the dielectric layer  316  is formed on the metal layer  306 . The dielectric layer  316  is possessed of an opening  322  so that portions of the metal layer  306  are exposed via the opening  322 .  
         [0025]    Further, the dielectric layers  312  and  316  are made of different materials and the dielectric coefficients thereof are different. The dielectric layer  314  is within the opening  322  and on the metal layer  306 , so that the dielectric layer  314  and the dielectric layer  316  are both on the metal layer  306 . The dielectric layer  314  is made of a low dielectric coefficient material; the value of the dielectric coefficient thereof is less than 4. The high-frequency signal transmission line  310  is on the dielectric layer  314  and a low-frequency signal transmission line  318  is relatively on the dielectric layer  316 .  
         [0026]    The dielectric coefficient of the dielectric layer  314  is less than the dielectric coefficient of the traditional dielectric layer  108  shown in FIG. 1. It will be appreciated that the signal transmission speed in the high-frequency signal transmission line  310  will be enhanced in accordance with the relationship that the signal transmission speed is inversely proportional to the square root of the effective dielectric constant. The energy loss of the signal transmission is decreased, so that high speed and high frequency transmission can be achieved.  
         [0027]    Consideration should be given here to the dielectric layer  314 . The top superficial measure of the dielectric layer  314  is equal to or larger than the bottom superficial measure of the high-frequency signal transmission line  310 . The low-frequency signal transmission line  318  is disposed on the dielectric layer  316  so that the low-frequency signal transmission line  318  can be maintained in stable voltage status.  
         [0028]    Moreover, the dielectric coefficient of the dielectric layer  312  is greater than the dielectric coefficient of the traditional dielectric layer  104  shown in FIG. 1. It will be appreciated that the metal layers  302  and  306  (such as a power plane and a ground plane) can be maintained in stable voltage status and the high frequency substrate  300  can be kept in well progress.  
       EXAMPLE 3  
       [0029]    [0029]FIG. 4 is a cross-sectional view of a high frequency substrate in accordance with the third preferred embodiment of the invention. Referring to FIG. 4, the high frequency substrate  400  has two metal layers  402  and  406 , a high-frequency signal transmission line  410 , a low-frequency signal transmission line  418  and four dielectric layers  412 ,  414 ,  416  and  420 . The dielectric layer  412  is on the metal layer  402  and the dielectric layer  412  is made of a high dielectric coefficient material; the value of the dielectric coefficient thereof is more than 4. The metal layer  406  is formed on the dielectric layer  412  and the dielectric layer  416  is formed on the metal layer  406 . The dielectric layer  616  is possessed of openings  422  and  424 , so that portions of the metal layer  406  are exposed via the openings  422  and  424 . Further, the dielectric layers  414  and  416  are respectively filled within the opening  422  and within the opening  424  but are both on the metal layer  406 , so that the dielectric layers  414 ,  416  and  420  are all on the metal layer  406 . The dielectric layer  414  is made of a low dielectric coefficient material; the value of the dielectric coefficient thereof is less than 4. The dielectric layers  412  and  416  are both made of same high dielectric coefficient materials. The high-frequency signal transmission line  410  is on the dielectric layer  414  and a low-frequency signal transmission line  418  is on the dielectric layer  416 .  
         [0030]    The dielectric coefficient of the dielectric layer  414  is less than the dielectric coefficient of the traditional dielectric layer  108  shown in FIG. 1. It will be appreciated that the signal transmission speed in the high-frequency signal transmission line  410  will be enhanced in accordance with the relationship that the signal transmission speed is inversely proportional to the square root of the effective dielectric constant. The energy loss of the signal transmission is decreased so that high speed and high frequency transmission can be achieved.  
         [0031]    Consideration should be given here to the dielectric layer  414 . The top superficial measure of the dielectric layer  414  is equal to or larger than the bottom superficial measure of the high-frequency signal transmission line  410 . The low-frequency signal transmission line  418  is disposed on the dielectric layer  416  so that the low-frequency signal transmission line  418  can be maintained in stable voltage status. Further, the dielectric coefficient of the dielectric layer  412  is more than the dielectric coefficient of the traditional dielectric layer  104  shown in FIG. 1. It will be appreciated that the metal layers  402  and  406  (such as a power plane and a ground plane) can be maintained in stable voltage status and the high frequency substrate  400  can be kept in well progress.  
         [0032]    Moreover, the high frequency substrates of foregoing preferred embodiment of the invention can be made by build-up or lamination. The following description will present the forming a high-frequency substrate by build-up, taking the high-frequency substrate  400  in the third preferred embodiments of the invention as an example.  
         [0033]    Referring to FIG. 4, a metal layer  402  is first formed. A dielectric layer  412  with a high dielectric coefficient material is formed on the metal layer  402 . Next, covering a metal layer  406  on the dielectric layer  412  and covering a dielectric layer  420  on the metal layer  406 . Further, removing portions of the dielectric layer  420  by etching, mechanical drilling, laser drilling and the like, so that openings  422  and  424  are formed on the metal layer  406 . Then, depositing a low dielectric coefficient material to fill the opening  422  and forming the dielectric layer  414  by spin coating or by printing. Depositing a high dielectric coefficient material to fill the opening  424  and forming the dielectric layer  416  by spin coating or by printing. Next, applying a high-frequency signal transmission line  410  and a low-frequency signal transmission line  418  on the dielectric layers  414  and  416  by patterning and electroplating respectively.  
         [0034]    The above steps are not just limited in what are described above. For example, the two metal layers of three preferred embodiments of the invention,  202  and  206 ,  302  and  306 ,  402  and  406  can be a power plane and a ground plane, respectively. The low dielectric coefficient material is polytetrafluroethylene (PTFE). Also, the high dielectric coefficient material of which the value of the dielectric coefficient is more than 4 could be bismaleimide triazine (BT), FR-4 epoxy resin, or polyimide (PI).  
         [0035]    In summary, this high-frequency substrate of the present invention at least offers the following advantages:  
         [0036]    a. With the particular design that high-frequency substrate with a high-frequency transmission line on a low-dielectric-coefficient dielectric layer, high speed and high frequency transmission are achieved and also the disadvantages and limitations of existing high-frequency substrate are overcome.  
         [0037]    b. The high-frequency substrate of the invention has a high dielectric coefficient dielectric layer formed between the power plane and the ground plane. With this particular design, the voltage of the power plane and the ground plane are maintained stable.  
         [0038]    While the invention has been described by three preferred embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.