Abstract:
A technique for providing a multi-layer substrate which is capable of signal transmission at multiple propagation speeds is disclosed. In one embodiment, the technique is realized by constructing a multi-layer substrate by creating air channels in dielectric layers adjacent to a conductor. The air channels may also be filled with an alternative dielectric material. At least three types of multi-layer substrates may be produced through this technique. Furthermore, signal tracks of varying lengths can be provided to accommodate differing delays.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This patent application is related to U.S. patent application Ser. No. 09/749,411, filed concurrently herewith and entitled “Suspended Stripline Structures to Reduce Skin Effect and Provide Low Loss Transmission of Signals with High Data Rates or High Frequencies,” which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a technique for varying signal transmission delay times and increasing signal reach within a substrate and, more particularly, to a technique for using air channels within a multi-layer substrate for varying signal transmission delay times and increasing signal reach. 
     BACKGROUND OF THE INVENTION 
     The present state of the art is shown in FIG.  12  and is a typical multi-layer substrate. The typical substrate includes multiple dielectric layers  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 , and  9 . Between the dielectric layers are multiple signal tracks  10 ,  11 ,  12  and  13 . Metal reference layers, including ground layers  20 ,  21  and  22  are also formed between the dielectric layers. The metal reference layers additionally include a power layer  25  provided between the dielectric layers  5  and  6 , and a primary layer  28  and a secondary layer  29 , which form the outermost layers of the substrate. 
     Signal loss over the distance of the tracks of the signal layers  20 - 22  frequently occurs with high bit rates or high signal frequencies. Furthermore, the signals with high bit rates or high frequencies often become skewed over the track distance. 
     When signals with high bit rates or high frequencies are used, such as digital signaling at 10 Gb/s or higher, long signal tracks can result in significant delays. 
     Signal velocity (v) is calculated as follows: 
     
       
           v=c /∈  (2) 
       
     
     where c is equal to the speed of light and ∈ is the dielectric constant. When a typical dielectric having a dielectric constant ∈=3.9 is used, signal velocity is reduced to approximately half the speed of light. 
     With regard to signal reach, the wavelength (λ) of one bit of information is: 
     
       
         λ= c /( f ∈)  (1) 
       
     
     where c is equal to a speed of 3.0×10 10  cm/s, f is equal to a frequency of 10×10 9  and ∈ is equal to the dielectric constant of a commonly used substrate material, which is commonly between 3.0 and 4.7. Accordingly, the larger the dielectric constant, the shorter the signal reach. 
     Suspended substrate striplines have been used to minimize the above-identified problem of losses in striplines. Prior U.S. Pat. Nos. 4,521,755, 4,614,922, and 5,712,607 disclose the use of suspended substrate striplines. In all of the aforementioned patents, which are hereby incorporated by reference herein, the striplines are attached to a substrate which is mounted so as to be surrounded by air on both sides. 
     In U.S. Pat. No. 4,521,755, the disclosed structure is intended to promote uniform current density and lower losses. In the structure disclosed in U.S. Pat. No. 4,521,755, a channel is formed inside of a conductor. A substrate having striplines is mounted inside the channel. 
     In U.S. Pat. No. 4,614,922 an upper housing having an upper channel and a lower housing having a lower channel are provided. A center board is positioned between the conductive housings. A transmission strip and a conductive surface are formed on the center board. The structure is intended for use in a microwave circuit. 
     U.S. Pat. No. 5,712,607 discloses the use of an air-dielectric stripline which includes a dielectric layer sandwiched between two spacer layers. Conductive traces are attached to the dielectric layers. Channels are formed in the spacer layer. 
     None of the aforementioned patents discloses the use of air channels in multi-layer substrates in order to synchronize signals or more generally, the use of channels for the adjustment of signal transmission times through different signal tracks within the substrate. The previous suspended striplines have employed a mechanical construction, which is too bulky for high density circuit packages. 
     In view of the foregoing, it would be desirable to provide a technique for synchronizing signals and adjusting signal transmission times within a multi-layer substrate which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for providing air channels for synchronizing signals within a multi-layer substrate in an efficient and cost effective manner. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, a multi-layer substrate is provided. The multi-layer substrate comprises a first dielectric layer having a first dielectric constant and a first stripline disposed adjacent the first dielectric layer. The multi-layer substrate further comprises a second dielectric layer having a channel therein, the channel filled with a substance having a second dielectric constant different from the first dielectric constant and a second stripline adjacent the channel of the second dielectric layer. 
     According to another aspect of the present invention, a technique for providing a substrate in which signals are transmitted at more than one propagation speed is provided. In one embodiment, the technique is realized by a substrate having striplines with differing signal propagation speeds, the substrate comprising multiple dielectric layers. A first stripline is disposed adjacent a first dielectric layer having a first dielectric constant and has a first signal propagation speed. A second stripline is disposed adjacent a second dielectric layer having an air channel therein, the air channel having a second dielectric constant different from the first dielectric constant. The second stripline has a second signal propagation speed different from the first signal propagation speed. 
     In accordance with an additional aspect of the invention, a method for forming suspended striplines within a multi-layer substrate is provided. The method comprises the steps of forming a first substrate layer having conductive material on one side, etching the conductive material into a set of striplines, and applying a second substrate layer over the conductive material. The method further comprises forming a channel in one of the substrate layers and attaching a third substrate layer to the substrate layer having the channel. 
     The present invention will now be described in more detail with reference to exemplary embodiments thereof as shown in the appended drawings. While the present invention is described below with reference to preferred embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as disclosed and claimed herein, and with respect to which the present invention could be of significant utility. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings. These drawings should not be construed as limiting the present invention, but are intended to be exemplary only. 
     FIG. 1 is a cross-sectional view of a multi-layered substrate in accordance with an embodiment of the invention; 
     FIG. 2 is a cross-sectional view of a multi-layered substrate in accordance with an alternative embodiment of the invention; 
     FIG. 3 is a cross-sectional view of a multi-layered substrate in accordance with another alternative embodiment of the invention; 
     FIG. 3A is a perspective cross-sectional view of the multi-layered substrate of FIG. 3 having an additional signal track in accordance with another alternative embodiment of the invention; 
     FIG. 4 is a flow chart illustrating a method of the invention; 
     FIG. 5 illustrates a cross-section of an embodiment of the invention after a first method step; 
     FIG. 6 illustrates a cross-section of an embodiment of the invention in a further stage of construction; 
     FIG. 7 illustrates a cross-section of an embodiment of the invention in yet a further stage of construction; 
     FIG. 8 illustrates a cross-section of an embodiment of the invention in yet a further stage of construction; 
     FIG. 9 is a cross-sectional view of an embodiment of the invention in a further stage of construction; 
     FIG. 10 is a cross-sectional view of an embodiment of the invention in a further stage of construction; 
     FIG. 11 is a cross sectional view of an embodiment of the invention in its final stage of construction; and 
     FIG. 12 illustrates a prior art substrate. 
     Throughout the drawing figures, like elements &amp; features are designated by the same reference numeral and may not be described in detail for all drawing figures. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring to FIG. 1, there is shown a substrate  100  of a first embodiment of the invention. The substrate  100  includes metal reference layers including a primary layer  101  and a secondary layer  110  forming opposing boundaries. The primary layer  101  and the secondary layer  110  are typically formed from copper, but they may be formed from any other suitable equivalent material. Between the primary and secondary layers  101 ,  110 , multiple dielectric layers  120 ,  121 ,  122 ,  123 ,  124 ,  125 ,  126 ,  127 , and  128  are disposed. Multiple signal tracks  102 ,  104 ,  107 , and  109  are formed between the dielectric layers. Additional metal reference layers including ground planes  103 ,  105 , and  108  are also formed between adjacent dielectric layers and a power layer  106  is formed between dielectric layers. Finally, an air channel  130  is formed in the dielectric layer  121  through which the signal tracks  102  extend. 
     The aforementioned dielectric layers  120 - 128  may be formed from any suitable material and may in fact be formed from differing dielectric materials having different dielectric constants, so as to vary signal propagation speed. In an exemplary embodiment, dielectric layers  125 - 128  are formed from 10 mils of dielectric material Getek™, which has a dielectric constant of 3.9. The dielectric layer  124  is formed from 5 mils of Getek™. The dielectric layers  121  and  122  are formed from 10 mils of dielectric material Rogers™ RT5880, which has a dielectric constant of 2.2. The dielectric layer  123  is formed from 10 mils of a pressed dielectric which may have a single a dielectric constant or may be a laminated dielectric slab having multiple dielectric constants, for example 4.0/8.0/16. If the latter approach is chosen, different dielectric materials can be implemented to create differing delay times in one dielectric layer. 
     The signal tracks  102 ,  104 ,  107 , and  109 , ground planes  103 ,  105 , and  108 , power layer  106 , primary layer  101 , and secondary layer  110  are preferably formed of copper. In an embodiment of the invention, the signal tracks  102 ,  103 ,  107 , and  109 , and the primary  101 , secondary  110 , and ground planes  103  and  108  are formed from 0.65 mils of copper. The power layer  106  and ground layer  105  are formed from 1.3 mils of copper. 
     The air channel  130  is preferably laser ablated into the dielectric layer  121 . The air channel  130  may be formed by other methods including but not limited to the manufacturing of trenches using microvia technology, mechanical pressing, or mechanical milling. The dielectric constant of the air channel is 1.0 and the thickness dimension of the air channel preferably corresponds to the thickness of the dielectric layer  121 , which is 10 mils in the provided embodiment. 
     In one possible configuration, the signal tracks  102  are bordered on one side by the dielectric layer  120 , which may have a dielectric constant of approximately 2.2 and on an opposite side by the air channel  130 , which has a dielectric constant of 1.0. Regardless of the material chosen for the dielectric layers, the air channel  130  will always have a dielectric constant of approximately 1.0, thereby enhancing signal propagation speed. 
     FIG. 2 illustrates a second embodiment of the substrate of the invention. As in the first embodiment, the substrate  200  includes multiple dielectric layers and signal tracks. A primary layer  201  and a secondary layer  210  form opposite peripheries of the substrate. Dielectric layers  220 ,  221 ,  222 ,  223 ,  224 ,  225 ,  226 ,  227 ,  228 , and  229  are interspersed throughout the substrate. Ground planes  203 ,  205 , and  208  are formed between dielectric layers. A power layer  206  is also provided between the dielectric layers  225  and  226 . 
     The aforementioned dielectric layers  220 - 229  may be formed from any suitable material and may in fact be formed from differing dielectric materials having different dielectric constants, so as to vary signal propagation speed. In an exemplary embodiment, the dielectric layers  225 - 229  are formed from Getek™, which as set forth above, has a dielectric constant of 3.9. The dielectric layers  226 - 229  may have a thickness of approximately 10 mils and the dielectric layer  225  may have a thickness of approximately 5 mils. The dielectric layer  221  can be formed from approximately 5 mils of Rogers™ RT5880 having a dielectric constant of approximately 2.2. The dielectric layer  223  can be formed from approximately 5 mils of pressed dielectric having a dielectric constant of 4.0/8.0/16.0 and 5 mils of Rogers™ RT5880. The dielectric layer  224  may be formed from approximately 10 mils of the aforementioned pressed dielectric. 
     The signal tracks  202 ,  204 ,  207 , and  209  are preferably formed of copper having a thickness of approximately 0.65 mils. The primary layer  201 , secondary layer  210 , and two ground plane layers  203  and  208  are formed from copper having a thickness of approximately 0.65 mils. In the illustrated embodiment, the power layer  206  and ground plane layer  205  are formed of approximately 1.3 mils of copper. 
     The air channel  230  is preferably laser ablated into the dielectric layer  222  and the air channel  231  is preferably laser ablated into the dielectric layer  220 . The channels  230  and  231  create a suspended substrate  221  because the substrate  221  is bordered on two sides by the air channels  230  and  231 . 
     A third embodiment of a substrate  300  of the invention is illustrated in FIG.  3 . Dielectric layers  310 ,  311 ,  312 ,  313 ,  314 ,  315 , and  316  and air channels  320 ,  321 ,  323 , and  323  surround signal tracks  301  and  302 . This structure represents merely a portion of a multi-layered substrate and would likely be incorporated into a complete substrate structure having the primary, secondary, ground, and power layers as shown in FIGS. 1 and 2. 
     The signal tracks  301  and  302  are surrounded by the air channels  320 - 323 , thereby creating suspended striplines. To form the air channel  320 , the dielectric layer  311  is preferably laser ablated. The dielectric layer  312  is preferably laser ablated to form the air channel  321 . The dielectric layer  314  is preferably laser ablated to form the air channel  322  and the dielectric layer  315  is preferably laser ablated to form the air channel  323 . 
     In any of the three above-described embodiments, the signal tracks may be formed of different lengths, and the air channels may be filled with alternative dielectric materials. For instance, referring to FIG. 3A, in substrate  300 A signal track  301  is shown having a length that is longer than the length of signal track  324 , and channels  322  and  323  are shown filled with an alternative dielectric material (e.g., a non-air dielectric material). In order to synchronize a signal which sees a dielectric constant of 1.0 with a signal that sees a dielectric constant of 3.0, a signal track (e.g., signal track  301 ) adjacent an air channel with a 1.0 dielectric constant may be shorter than a signal track (e.g., signal track  324 ) adjacent a dielectric having a dielectric constant of 3.0. Furthermore, it may be desirable for various applications to have signals within the substrate reach their destinations at different times. 
     FIG. 4 illustrates a method by which the three aforementioned substrate embodiments can be constructed. FIGS. 5-11 show each method step individually. 
     In step S 1  of FIG. 4, a substrate layer having copper foil on one side is produced. Step S 1  is shown in FIG. 5 in which a substrate  2  is covered with a copper foil  1 . 
     In step S 2  of FIG. 4, the copper foil is etched into striplines. This step is shown in FIG. 6 in which the substrate  2  is covered with etched copper foil  1 . 
     In step S 3  of FIG. 4, a second substrate is applied over the copper foil. This step is shown in FIG. 7 in which a substrate  3  is placed upon the etched copper foil  1 . 
     In step S 4  of FIG. 4, a channel is laser ablated into one of the substrates. Step S 4  is shown in FIG. 8 in which a channel  4  is created in the substrate  3 , thereby exposing the copper track  1 . 
     In step S 5  of FIG. 4, a third substrate is applied to the substrate having the laser ablated channel. Step S 5  is shown in FIG. 9 in which a substrate  5  is placed over the substrate  3 . 
     In step S 6 , a channel is laser ablated into the substrate on the opposite side. Step S 6  is shown in FIG. 10 in which a channel  6  is laser ablated into the substrate layer  2 , thereby exposing the copper track  1 . 
     In step S 7 , a fourth substrate is applied on the side milled in step S 6 . This step is shown in FIG. 11 in which a substrate  7  is placed adjacent the substrate  2  overlaying the air channel  6 . 
     The final structure shown in FIG. 11 is a suspended stripline, which corresponds to the embodiment of the invention as shown in FIG.  3 . Similar procedures can be used to produce the non-suspended stripline using air channels of FIG.  1  and the suspended stripline substrate of FIG.  2 . 
     The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the present invention, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such modifications are intended to fall within the scope of the following appended claims. Further, although the present invention has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present invention as disclosed herein.