Patent Publication Number: US-6985055-B2

Title: Transmission line comprised of interconnected parallel line segments

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application P2002-324021 filed on Nov. 7, 2002; the entire contents of which are incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a transmission line for high-speed signals in semiconductor integrated circuits, and a semiconductor device using the same. 
   2. Description of the Related Art 
   In recent years, speeds of semiconductor integrated circuits have been increasing, whereby microprocessors that operate at a clock speed of several GHz and high frequency circuits operating at several GHz have been achieved. The prevention of a cross talk between signal lines and attention for a high frequency signal response including line inductance are necessary for transmitting signals in a semiconductor integrated circuit at a GHz level. A coplanar waveguide or a microstrip line is a representative transmission line for transmitting high frequency signals. 
   An example of applying the coplanar waveguide to a clock line in a semiconductor integrated circuit has been reported (see F. Ishihara, Proceedings of ASP-DAC2000, pp. 647–652, 2000). 
   Furthermore, the microstrip line is generally applied to a printed board and the like. There is also an example of a structure of the microstrip line for reducing parasitic capacitance in a semiconductor integrated circuit (see M. Mizuno, IEEE ISSCC 2000, pp. 366–367, 2002). 
   A line width may be increased in order to decrease the line impedance for ultra high-speed signal transmission. However, by increasing the line width, a coupling effect between signal lines in the upper and lower layers relatively increases, and an effect of cross talk noise becomes a problem. Furthermore, an error may increase in the line impedance of additional signal lines predicted prior to wiring processing. 
   In the microstrip line, a ground plane that forms a return path requires a large amount of wiring resources, and may be a large obstacle for general signal lines. 
   SUMMARY OF THE INVENTION 
   A first aspect of the present invention lies in a transmission line, and includes: a first input electrode located in a first level; a plurality of parallel stripe-shaped signal lines in the first level, one end of the signal lines connected to the first input electrode; a first output electrode connected to another end of the signal lines, facing to the first input electrode; a second input electrode adjacent to the first input electrode in a second level facing the first level; a plurality of stripe-shaped ground lines positioned alternately in between and at outer sides of each of the signal lines in the first level, one end of the ground lines connected to the second input electrode; and a second output electrode adjacent to the first output electrode in the second level and connected to another end of the ground lines. 
   A second aspect of the present invention lies in a transmission line, and includes: a first signal line located in a first level; a first ground line located in a second level facing the first level, the first ground line parallel to and facing the first signal line, and having a separated section at least at one portion; and bridge ground lines located in the first level on both sides of the first signal line overlaying the separated section, and connected respectively to one separated end and another separated end of the first ground line. 
   A third aspect of the present invention lies in a semiconductor device, and includes: a route line having a first microstrip line configuration, being connecting a route driver cell to a route relay buffer cell, the route driver cell receiving signals to be supplied to a terminal circuit; a route branch line having a second microstrip line configuration, being branched from the route relay buffer cell and connected to a first relay buffer cell; and a relay branch line having a coplanar waveguide configuration, being branched from the first relay buffer cell and connected to a second relay buffer cell for supplying the signals to the circuit. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram of a coplanar waveguide according to a first embodiment of the present invention; 
       FIGS. 2A and 2B  are cross-sections of the coplanar waveguide according to the first embodiment of the present invention; 
       FIG. 3  is a schematic diagram of a microstrip line according to a second embodiment of the present invention; 
       FIG. 4  is a cross-section of the microstrip line according to the second embodiment of the present invention; 
       FIG. 5  is a schematic diagram describing a transmission line in a semiconductor device according to a third embodiment of the present invention; and 
       FIG. 6  is a schematic diagram illustrating an example of the transmission line in the semiconductor device according to the third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified. 
   First Embodiment 
   A coplanar waveguide according to a first embodiment of the present invention, as shown in  FIG. 1 , has a plurality of stripe-shaped signal lines  21   a ,  21   b ,  21   c ,  21   d , located in parallel between a plurality of stripe-shaped ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  in a first level. The signal lines  21   a ,  21   b ,  21   c ,  21   d  are, for example, connected at one end to a first input electrode  23  on the left-hand side facing the page of  FIG. 1  and connected at another end to a first output electrode  25  on the right-hand side facing the page in the first level. The ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  are connected at one end to a second input electrode  24 , which is positioned adjacent to the first input electrode  23 , and at another end to a second output electrode  26 , which is positioned adjacent to the first output electrode  25 , and in contact with first plugs  32   a ,  32   b ,  32   c ,  32   d ,  32   e  and second plugs  34   a ,  34   b ,  34   c ,  34   d    34   e , respectively, in a second level that faces the first level where the signal lines  21   a ,  21   b ,  21   c ,  21   d  and ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  are positioned. Furthermore, first and second input terminals  27  and  28  are provided to the first and second input electrodes  23  and  24 , respectively; and first and second output terminals  29  and  30  are provided to the first and second output electrodes  25  and  26 , respectively. The first and second input terminals  27  and  28  are connected to a circuit on a signal transmission side; and the first and second output terminals  29  and  30  are connected to a circuit on a signal reception side. 
   The structure of the coplanar waveguide according to the first embodiment of the present invention is described using  FIGS. 2A and 2B , which are cross-sections cut along the lines IIA—IIA and IIB—IIB in  FIG. 1 , respectively. For convenience of description, a second interlayer insulating film  15  is shown as the uppermost layer of multi-level interlayer insulating films provided on a semiconductor substrate  11  as shown in  FIGS. 2A and 2B , however, interlayer insulating films may be further provided on the second interlayer insulating film  15 . Furthermore, the second input electrode  24  and second output electrode  26  are provided on a surface of a first interlayer insulating film  13 , which is an underlying layer adjacent to the second interlayer insulating film  15 . Needless to say, however, the second input electrode  24  ( FIG. 2A ) and second output electrode  26  (not shown in  FIG. 2A ) may be provided on a surface of another interlayer insulating film further underlying the first interlayer insulating film  13 , or the second input electrode  24  and second output electrode  26  may be provided on a surface of an interlayer insulating film provided on an upper layer of the second interlayer insulating film  15 . 
   As shown in  FIG. 2B , the plurality of signal lines  21   a ,  21   b ,  21   c ,  21   d  of a line width wc and a line thickness t are alternately disposed with the ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  at intervals of a gap width s on a surface of the second interlayer insulating film  15  that is deposited on the first interlayer insulating film  13 . The insulating films may be a silicon oxide (SiO 2 ) film and the like on the semiconductor substrate  11 . The signal lines  21   a ,  21   b ,  21   c ,  21   d  are connected to the first input electrode  23  and first output electrode  25  at the surface of the second interlayer insulating film  15 . On the other hand, the ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  are connected to the second input electrode  24  and second output electrode  26 , which are provided on another surface that faces the surface of the second interlayer insulating film  15  such as the first interlayer insulating film  13 , so as to avoid a short circuit with the signal lines  21   a ,  21   b ,  21   c ,  21   d . For example, as shown in  FIG. 2A , the ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  are connected to the second input electrode  24 , which is disposed on the surface of the first interlayer insulating film  13 , via the first plugs  32   a ,  32   b ,  32   c ,  32   d ,  32   e  that are buried in contact holes provided in the second interlayer insulating film  15 . 
   In the coplanar waveguide according to the first embodiment of the present invention, the electromagnetic coupling between the parallel signal lines  21   a ,  21   b ,  21   c ,  21   d  and ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  becomes relatively stronger. Accordingly, it is possible to provide a coplanar waveguide which is substantially unaffected by noise due to cross talk from wiring layers underlying the second interlayer insulating film  15  or wiring layers deposited on the second interlayer insulating film  15 . Furthermore, in a coplanar waveguide having three coplanar conductors, a current flows mostly near a surface in each of the coplanar conductors due to a skin effect in a high frequency region and the line resistance exponentially may increase. However, in the coplanar waveguide according to the first embodiment of the present invention, since the signal lines  21   a ,  21   b ,  21   c ,  21   d  and ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  are divided into a plurality of parallel transmission lines, increases in line resistance may be avoided. Furthermore, in order to make the most use of the advantages of the coplanar waveguide according to the first embodiment of the present invention and to save as much wiring resources as possible, for the line thickness t of the signal lines  21   a ,  21   b ,  21   c ,  21   d , the line width wc of the signal lines  21   a ,  21   b ,  21   c ,  21   d  and the ground lines  22   a ,  22   b ,  22   c ,  22   d ,  22   e  may be less than twice the line thickness t, and the gap width s less than the line thickness t. 
   Second Embodiment 
   As shown in  FIG. 3 , a microstrip line according to a second embodiment of the present invention has a first signal line  41  located in a first level and separated first ground lines  42   a  and  42   b  located in a second level which faces the first level. In the first level, bridge ground lines  43   a  and  43   b , which are provided on both sides of the first signal line  41 , are located such that portions of the bridge ground lines  43   a  and  43   b  face and overlap the first ground lines  42   a  and  42   b . The bridge ground line  43   a  is connected to the first ground lines  42   a  and  42   b  via contact with plugs  44   a ,  44   b  and plugs  44   e ,  44   f  respectively, which are provided at portions of the integrated circuit that face and overlap the first ground lines  42   a  and  42   b . The bridge ground line  43   b  is connected to the first ground lines  42   a  and  42   b  via contact with plugs  44   c ,  44   d  and plugs  44   g ,  44   h  respectively, which are provided at portions of the integrated circuit that face and overlap the first ground lines  42   a  and  42   b . Accordingly, a first signal line  47 , a second signal line  48  and the like can be located between the first ground lines  42   a  and  42   b  lying under the bridge ground lines  43   a  and  43   b.    
   The structure of the microstrip line according to the second embodiment of the present invention is described using  FIG. 4 , which is a cross-section cut along the line of IV—IV in  FIG. 3 . For convenience of description, a second interlayer insulating film  15  is shown as the uppermost layer of a plurality of interlayer insulating films provided on a semiconductor substrate  11  as shown in  FIG. 4 . Needless to say, however, interlayer insulating films may be further provided on the second interlayer insulating film  15 . 
   As shown in  FIG. 4 , the first signal line  41  of a line width w and the bridge ground lines  43   a  and  43   b  on both sides thereof are located on a surface of the second interlayer insulating film  15 , which is deposited on the first interlayer insulating film  13  above the semiconductor substrate  11 . The bridge ground lines  43   a  and  43   b  are connected to the first ground line  42   a , which is provided on the first interlayer insulating film  13 , in contact with the plugs  44   a ,  44   b  and the plugs  44   c ,  44   d  respectively, which are buried in contact holes provided in the second interlayer insulating film  15 . The first signal line  41  is separated from the first ground line  42   a  by an interlayer distance h. Furthermore, the first and second signal lines  47  and  48  are provided on the surface of the first interlayer insulating film  13 . 
   With the microstrip line according to the second embodiment of the present invention, the separated first ground lines  42   a  and  42   b , which face the first signal line  41  extending along the surface of the second interlayer insulating film  15 , are connected by the bridge ground lines  43   a  and  43   b , which are located on the same surface of the second interlayer insulating film  15  as the first signal line  41 . Accordingly, it is possible to connect circuits, which are disposed on both sides of the first ground lines  42   a  and  42   b , by the first and second signal lines  47  and  48  with a shorter distance across the first ground lines  42   a  and  42   b  without detouring. It should be noted that the first signal line  41  and the bridge ground lines  43   a  and  43   b  provide a coplanar waveguide having a 3-strip structure. 
   The first ground lines  42   a  and  42   b  are used as grounds for the microstrip line according to the second embodiment of the present invention instead of the ground planes generally used for microstrip lines. The line width of the first ground lines  42   a  and  42   b  is greater than or equal to the line width w of the first signal line  41 , and less than a value of (w+20*h). The line width of the first ground lines  42   a  and  42   b  is set so as to assure a return current of approximately 75% to 95% for the microstrip line. Thus, fluctuations in electrical characteristics such as impedance of a transmission line are kept within 10% of the characteristic impedance, allowing for the saving of wiring resources. Furthermore, when a total length of connecting sections with the bridge ground lines  43   a  and  43   b  is less than 20% of the length of the first ground lines  42   a  and  42   b , it is possible to control fluctuations in the characteristic impedance along the length of the transmission line within ±5%. 
   An example where the first ground lines  42   a  and  42   b  underlie the first signal line  41  has been illustrated in the second embodiment of the present invention. However, a first ground line may be in an upper layer, or a microstrip line may alternatively be sandwiched by the first ground lines in both the upper and lower layers. 
   Third Embodiment 
   A transmission line used in a semiconductor device according to a third embodiment of the present invention is characterized by using in combination a coplanar waveguide and a microstrip line described in the first and second embodiments. Since other elements are the same as in the first and second embodiments, duplicate descriptions are omitted. 
   As shown in  FIG. 5 , the transmission line used in the semiconductor device according to the third embodiment of the present invention is, for example, applied for clock signal lines in a logic circuit and the like. To begin with, a clock signal is provided to a route driver cell  60  from a clock signal source, and relayed to a route relay buffer cell  61  from a route line  71 , which is a first microstrip line. The clock signal is further propagated from the route relay buffer cell  61  to first relay buffer cells  62   a ,  62   b ,  62   c ,  62   d  from route branch lines  72   a ,  72   b ,  72   c ,  72   d , which are second microstrip lines. The branched clock signals are then further propagated from each of the first relay buffer cells  62   a ,  62   b ,  62   c ,  62   d  to each of second relay buffer cells  64   a ,  64   b ,  64   c ,  64   d ,  64   e ,  64   f ,  64   g ,  64   h ,  64   i ,  64   j ,  64   k ,  64   l ,  64   m ,  64   n ,  64   o ,  64   p  from relay buffer branch lines  74   a ,  74   b ,  74   c ,  74   d ,  74   e ,  74   f ,  74   g ,  74   h ,  74   i ,  74   j ,  74   k ,  74   l ,  74   m ,  74   n ,  74   o ,  74   p , which are coplanar waveguides. 
   When supplying a signal to many destinations, such as the clock signal lines in a logic circuit and the like, buffer cells for relaying are inserted along the clock signal line so as to distribute the clock signal. The coplanar waveguide according to the first embodiment of the present invention employs a structure of the signal lines and ground lines that are separated in thin lines and positioned in parallel, thus a parasitic capacitance increases and a line resistance also easily increases. Accordingly, the microstrip line according to the second embodiment of the present invention is advantageous to send high-speed clock signals on a GHz level. However, the wiring resources cannot be sufficiently acquired when the microstrip lines are heavily used. 
   In the semiconductor device according to the third embodiment of the present invention, as shown in  FIG. 6  for example, a microstrip line  104  is used in the region near the clock signal source where a signal line  107   a  is sparsely located. On the other hand, a coplanar waveguide  102  is used in the region where signal lines  107   b ,  107   c ,  107   d ,  107   e ,  107   f  are highly dense at the clock signal supply destination. The micro strip line  104  has a clock input buffer cell  100 , which provides the clock signal to one end of a first signal line  141  in a first level. First ground lines  142   a  and  142   b , which are located in a second level that faces the first level, in which the first signal line  141  is located, are separated, and bridge ground lines  143   a  and  143   b  are provided on the first level. The signal line  107   a  is located under the bridge ground lines  143   a  and  143   b.    
   Another end of the first signal line  141  is connected to a first input electrode  123  of the coplanar waveguide  102  through contact with a relay buffer cell  103 . Furthermore, the first ground line  142   b  is connected to a second input electrode  124 . Second signal lines  121   a ,  121   a ,  121   b ,  121   c ,  121   d  and second ground lines  122   a ,  122   b ,  122   c ,  122   d ,  122   e  of the coplanar waveguide  102  are formed on the first level. Accordingly, multiple signal interconnects  107   b ,  107   c ,  107   d ,  107   e ,  107   f  may be positioned under the coplanar waveguide  102 . The second signal lines  121   a ,  121   b ,  121   c ,  121   d  and second ground lines  122   a ,  122   b ,  122   c ,  122   d ,  122   e  are connected at another end on the opposite side of the microstrip line  104  to a first output electrode  125  and a second output electrode  126 . The first output electrode  125  is further connected to a clock output buffer cell  101 , supplying the clock signal to other, nearby circuits. 
   According to the transmission lines of the semiconductor device of the third embodiment of the present invention, deterioration of signal propagation characteristics may be reduced and the wiring resource consumption may be reduced. Here, when the characteristic impedance and the line resistance per unit length of the coplanar waveguide according to the first embodiment are respectively designated as Zc and Ro, the length of the coplanar waveguide may be determined so as to be less than a value of (2*Zc/Ro). The signals to be propagated may drastically attenuate, when the length of the coplanar waveguide become longer than the value of (2*Zc/Ro). 
   Other Embodiments 
   The present invention has been described as discussed above, however the descriptions and drawings that constitute a portion of this disclosure should not be perceived as limiting this invention. Various alternative embodiments and operational techniques will become clear to persons skilled in the art from this disclosure 
   In the first through third embodiments of the present invention, an insulating film such as SiO 2  and the like has been illustrated as the first and the second interlayer insulating films  13 ,  15 . However, an insulating film such as a silicon nitride (Si 3 N 4 ) film or an alumina (Al 2 O 3 ) film, for example, may be used. Additionally, even with interlayer insulating films having an wiring structure including electrical conducting layers such as polysilicon films, amorphous silicon films, metal films or the like, the same results may be achieved. 
   Furthermore, the coplanar waveguide according to the first embodiment of the present invention may be applied to a differential signal line. Namely, a pulse signal is provided to one end of the coplanar waveguide, while a differential pulse signal is provided to another end. Since the pulse signals of opposite polarity are transmitted among adjacent lines, electromagnetic coupling is relatively strengthened, making it more difficult to be affected by a noise due to cross talk from other wiring layers.