Patent Publication Number: US-6909346-B1

Title: Switching arrangement using HDI interconnects and MEMS switches

Description:
GOVERNMENTAL INTEREST 
   This invention was made with government support under Contract/Grant MDA972-00-C-0043 (DARPA). The United States Government has a non-exclusive, non-transferable, paid-up license in this invention. 

   FIELD OF THE INVENTION 
   This invention relates to switch arrangements which may be used for making andor breaking electrical connections, and more particularly to such switches using microelectromechanical (MEMS) devices in conjunction with high density interconnects (HDI). 
   BACKGROUND OF THE INVENTION 
     FIG. 1  is a simplified perspective or isometric view of a portion of a conventional “microstrip” transmission line  10 . In  FIG. 1 , the structure  10  includes a planar dielectric plate  12 . An elongated “strip” electrical conductor  14  extends over the upper surface  12   us  of the dielectric plate  12 , and an electrically conductive “ground plane”  16  extends over the entirety of the lower surface  12   ls , at least in the region generally under the strip conductor  14 . Structure  10 , and other generally similar structures such as “stripline,” tend to constrain the electrical fields associated with propagating electromagnetic waves to lie principally in a portion of the dielectric plate  12  lying between the strip conductor  14  and the ground plane  16 , all as is well known in the art. In order to prevent excessive transmission perturbations or “losses” attributable to reflections of propagating electromagnetic energy, the “surge” or “characteristic” impedance of a transmission line, such as transmission line  10  of  FIG. 1 , must be maintained along its length, or at the very least must change “slowly” along its length, where the rate of change of characteristic impedance is in part dependent upon the wavelength. The type of transmission line illustrated in  FIG. 1  is one of those commonly used in High Density Interconnect (HDI) technology, which is useful when making very compact, complex or repairable electronic systems. 
     FIG. 2   a  is a simplified cross-sectional representation of a prior-art arrangement using a microelectromechanical (MEMS) switch in conjunction with high density interconnect (HDI) structures. MEMS structures are mechanical structures made, in general, by processes which are akin to those used to fabricate solid-state integrated circuits, including photolithography and resist, etching, multiple layers of material. In  FIG. 2   a , a transmission line  10  includes a layer of dielectric  12 , which has a strip conductor  14  on its upper surface, extending front a left end LE to near a transverse plane T 6 . A ground plane or conductor  16  extends from the left edge LE to near a transverse plane T 2 . At the right end RE of  FIG. 2   a , a similar transmission line  210  includes a dielectric slab  212  defining an upper surface  212   us  and a lower surface  212   ls , and a strip conductor  214  overlying upper surface  212   us  from near a transverse plane T 14  to right end RE. A ground plane  216  extends below, and in contact with, lower surface  212   ls  from the right end RE to transverse plane T 18 . 
   A MEMS switch structure designated generally as  220  lies under HDI interconnect transmission-line structures  10  and  210  in  FIG. 2   a . MEMS switch structure  220  includes a MEMS dielectric substrate  222  defining an upper surface  222   us  and a lower surface  222   ls . The movable mechanical element in MEMS structure  220  is illustrated as an electrically conductive switch contact  224 , which is fabricated so that a drive structure (not illustrated in  FIG. 2   a ) can cause it to move upward and downward (relative to the orientation of the FIGURE) in the direction of double-headed arrow  250 . In order to incorporate the movable element  224  into a transmission line, a further strip conductor  234  is deposited on or otherwise supported by the upper surface  222   us  of dielectric plate  222 , extending partially under movable switch element  224 , with a break  235  in the continuity of strip conductor  234  generally at the location of the movable element  224 . When the movable element  224  is in its uppermost state or condition, which is the position illustrated in  FIG. 2   a , there is no continuity between the left and right portions of strip conductor  234 , and the switch is therefore OPEN or nonconductive. Conversely, when the movable conductor element  224  is in its lowermost state or condition, it is in contact with both left and right halves or portions of strip conductor  234 , and provides electrical continuity therebetween. In this state, the switch is said to be CLOSED. It should be noted in passing that European usage looks on a switch as one might a gate, and a nonconductive state is known as CLOSED, while the conductive state is known as OPEN. Movable switch element  224  is controlled to the UP or DOWN state by MEMS controllers, not illustrated. 
   In order to avoid transmission-line discontinuities which might perturb proper transmission, it is desirable to have strip conductor  234  of  FIG. 2   a  in the form of a transmission line. The transmission line of MEMS structure  220  includes a further ground plane  226  lying below lower surface  222   ls  of MEMS substrate  222 , at least in the region lying below strip conductor  234  and movable element  224 . 
   In order to provide a space or “room” for the desired movement of movable conductive element  224  of the MEMS structure  220 , a layer  240  of dielectric is placed between the transmission line structure  210  and the MEMS structure  220 , with a gap or opening  242  at the location of movable element  224 . Finally, the connections are completed by a plurality of through vias and metallizations. More particularly, a through via  250  extends at transverse plane T 2  from ground plane  16  to a metallization  251 , and a further through via  252  extends at a transverse plane T 4  from metallization  251  to ground plane  226 . Thus, the combination of through vias  250  and  252 , in conjunction with metallization  251 , provides contact between the right-most end of ground plane  16  and the left-most end of ground plane  226 . In addition, a through via  256  extends at transverse plane T 18  from ground plane  216  to a metallization  257 , and a further through via  254  extends at a transverse plane T 16  from metallization  257  to ground plane  226 . Thus, the combination of through vias  254  and  256 , in conjunction with metallization  257 , provides contact between the left-most end of ground plane  216  and the right-most end of ground plane  226 . Some strip conductor connections are made by means of a through via  260  extending at a plane T 6  through dielectric plate  12  to a metallization  261 , and a further through via  262  extending through dielectric plate  240  at plane T 8  from metallization  261  to the left-most end of strip conductor  234 . The strip conductor connections are completed by means of a through via  266  extending at a plane T 14  through dielectric plate  212  to a metallization  267  lying between dielectric plates  212  and  240 , and a further through via  254  extending at a plane T 12  through dielectric plate  240  to the right-most end of strip conductor  234 . Thus, through vias  264  and  266 , in conjunction with metallization  267 , provides electrical continuity from strip conductor  214  to the right end of strip conductor  234 . In general, it may be said that the fields associated with a propagating electromagnetic wave are constrained to lie between the strip conductor/ground plane sets  14 , 16 ;  234 ,  226 ;  214 ,  216 . 
     FIG. 2   b  illustrates the electric field resulting at transverse plane T 1  of  FIG. 2   a  from application of a direct voltage to strip conductor  14  relative to ground  16  of  FIG. 1   a . In  FIG. 2   b , the dielectric  12  is not hatched, in order to make the electric field lines  290  more visible. As illustrated, the electric field lines  290  extend from the strip conductor  14 , principally through the dielectric material  12 , and terminate on ground conductor or plane  16 .  FIG. 2   c  illustrates the electric field resulting at transverse plane T 9  of  FIG. 2   a  from application of a direct voltage to strip conductor  14  relative to ground  16  of  FIG. 1   a . As illustrated, the electric field structure  292  of  FIG. 2   c  is virtually identical to that of  FIG. 2   b , with the field lines extending principally through the dielectric material  222  from the strip conductor  234  to the ground plane  226 . The similarity of the field structure is an indication that the surge impedance of this section of transmission line is similar to that of the section illustrated in  FIG. 2   b.    
   SUMMARY OF THE INVENTION 
   In general, the invention relates to a transmission line structure including first and second mutually separated strip conductors lying on an upper side of an upper dielectric sheet, and a ground conductor juxtaposed with the lower side of the upper dielectric sheet. A further strip conductor lies on a lower side of a lower dielectric sheet, with its ends registered with the ends of the first and second strip conductors. In one embodiment, a gap in the further strip conductor is controllably bridged by a MEMS switch element, which may lie below the second dielectric sheet or in a cavity defined in the second dielectric sheet. 
   A transmission-line structure according to an aspect of the invention comprises a first dielectric sheet defining first and second broad sides. The first broad side of the first dielectric sheet bears first and second separate electrically conductive planar strips. Each of the separate electrically conductive planar strips defines at least a first end. The first end of the first planar strip and the first end of the second planar strip are spaced apart by a distance. A second dielectric sheet defines first and second broad sides. The first broad side of the second dielectric sheet defines a single continuous electrical conductor which defines first and second nonconductive regions. The first and second nonconductive regions are spaced apart by about the distance. The first broad side of the second dielectric sheet is juxtaposed with the second broad side of the first dielectric sheet, with at least portions of the first and second nonconductive regions of the continuous electrical conductor registered with the first ends of the first and second planar strips, respectively. The transmission-line structure also includes a nonconductive planar surface bearing a third electrically conductive planar strip defining first and second ends. The first and second ends of the third planar strip are separated by about the distance. The nonconductive planar surface is associated with the second side of the second dielectric sheet, with the first and second ends of the third planar strip registered with the first ends of the first and second planar strips, respectively. A first electrically conductive through via arrangement connects the first end of the first planar strip to the first end of the third strip through the first nonconductive region. A second electrically conductive through via arrangement connects the first end of the second planar strip to the second end of the third strip through the second nonconductive region, to thereby form the first, second and third planar strips into a continuous strip conductor in which at least a portion of each of the first, second and third planar strips overlies a side of the continuous electrical conductor to thereby form a strip transmission line including at least portions of the first, second and third planar strips. 
   A preferred embodiment of the transmission-line structure further includes a gap in the third planar strip, and mechanically operated switch means making controllable electrical and mechanical contact with a portion of the third planar strip on each side of the gap. In one version of this preferred embodiment, the mechanically operated switch means lies on a side of the gap which is remote from the first dielectric sheet, and moves toward and away from the second dielectric sheet in order to make and break connection. In another version of this preferred embodiment, the mechanically operated switch means lies within a cavity defined in the second dielectric sheet. 
   Another embodiment of the transmissionline structure includes a gap in the third planar strip, and a planar signal processing module with at least first and second signal ports. The first and second signal ports are mechanically and electrically connected to portions of the third planar strip on each side of the gap. In a preferred version of this embodiment, the signal processing module performs amplification, and the first and second signal ports are signal input and output ports, respectively. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a simplified perspective or isometric view of a portion of a prior-art transmission line; 
       FIG. 2   a  is a simplified cross-sectional view of a prior-art transmission-line switch including a MEMS switch in an HDI structure,  FIG. 2   b  is a representation of the electric field structure at a first location along the structure of  FIG. 2   a , and  FIG. 2   c  is a representation of the electric field structure at a second location along the structure of  FIG. 2   a;    
       FIG. 3   a  is a simplified cross-sectional view of a transmission line structure according to an aspect of the invention,  FIGS. 3   b ,  3   c , and  3   d  are plan views of various layers of the structure of  FIG. 3   a , and  FIGS. 3   e  and  3   f  are representations of the electric field structure at different locations along the structure of  FIGS. 3   a ,  3   b ,  3   c , and  3   d;    
       FIG. 4   a  is a simplified representation of a transmission-line structure similar to that of  FIGS. 3   a ,  3   b ,  3   c , and  3   d , with the inclusion of a movable MEMS switch element in the open or OFF state, and  FIG. 4   b  is similar to  FIG. 4   a  but shows the switch element in the closed or ON state; 
       FIG. 5   a  is a simplified representation of a transmission-line structure similar to  FIG. 4   a , but has the movable MEMS switch element lying in a cavity defined in a dielectric layer, and  FIG. 5   b  is a plan view of the structure of  FIG. 5   a , showing a particular layer of conductors; and 
       FIG. 6  is a simplified cross-sectional representation of a switch structure similar to that of  FIG. 5   a , with the addition of a further transmission line structure defining a gap and a MMIC electronic device making contact with at least side of the gap. 
   

   DESCRIPTION OF THE INVENTION 
     FIG. 3   a  is a simplified cross-sectional illustration of a transmission-line arrangement according to an aspect of the invention, including upper and lower dielectric layers  312  and  392 , respectively. The lower surface of dielectric layer  312  is designated  312   ls  The upper surface of dielectric layer  312  is designated  312   us , and bears a pattern MT 2  of metallization which is illustrated in plan view in  FIG. 3   b . The metallization layer MT 2  includes a left-most top microstrip conductor  314   l  and a corresponding right-most microstrip conductor  314   r . In  FIG. 3   b , the metallization portion is hatched, to aid in visualizing the metallization portion separated from the upper surface  312   us  of dielectric sheet  312 . As illustrated in  FIG. 3   b , strip conductor or top microstripline  314   l  terminates at a transverse plane Tb in an enlarged pad  350 , provided to aid in registering the various layers together, and possibly to provide some excess capacitance to aid in impedance matching. Similarly, right strip conductor  314   r  terminates at a transverse plane Tf in an enlarged pad  360 . The distance between transverse planes Tb and Tf is designated S 1 . 
     FIG. 3   c  illustrates the conductor pattern of metallization layer MT 1 , which lies between dielectric layers  312  and  316  of  FIG. 3   a . As illustrated in  FIG. 3   c , almost the entire surface or plane MT 1  is occupied by a conductive ground plane  316 . Near transverse planes Tb and Tc, an opening  370  provides clearance for a pad  351 , which extends at least from transverse plane Tb to transverse plane Tc, in a manner which is isolated from ground conductor  316 . Similarly, at transverse planes Te and Tf, an opening  380  provides clearance for a pad  381 , which extends at least from transverse plane Te to transverse plane Tf, also isolated from ground conductor  316 . Pads  351  and  381  provide terminals for plated-through vias which make connections among the layers of metallization. More particularly, a through via  352  extends from upper-layer pad  350  through dielectric layer  312  to central-layer pad  351  at transverse plane Tb, and a through via  362  extends at transverse plane Tf through dielectric layer  312  to make electrical connection between upper-layer pad  360  and middle-layer pad  381 . 
     FIG. 3   d  illustrates in plan view the conductive or metallization pattern of bottom layer MTO of  FIG. 3   a . In  FIG. 3   d , a strip conductor  326  extends from a pad  326   l  at transverse plane Tc to a corresponding pad  326   r  at plane Te. Thus, pad  326   l  lies under a portion of pad  351  of  FIG. 3   c , and pad  326   r  lies under a portion of pad  381 . As illustrated in  FIG. 3   a , a plated-through or conductive via  372  extends at transverse plane Tc from upper surface  392   us  through dielectric layer  392  to lower surface  392   ls , to electrically interconnect middle-layer metallization  351  to lower-level metallization pad  326   l . Similarly, a plated-through or conductive via  374  extends at transverse plane Te through dielectric layer  392  to electrically interconnect middle-layer metallization  381  to lower-level metallization pad  326   r . Thus, a voltage applied to upper or top level strip conductor  314   l  of  FIG. 3   a  relative to ground  316  creates a field pattern at a transverse plane Ta which is illustrated in  FIG. 3   e . In  FIG. 3   e , the electric field lines  390  extend from the strip conductor  314   l , principally through the dielectric material  312 , and terminate on ground conductor or plane  316 . Comparison of  FIGS. 2   b  with  3   e  shows that the field patterns are similar, so that the structure of  FIGS. 3   a ,  3   b ,  3   c , and  3   d  at transverse plane Ta corresponds to the structure of  FIG. 2   a  at transverse plane T 1 . Also, a voltage applied to upper or top level strip conductor  314   l  of  FIG. 3   a  relative to ground  316  creates a field pattern at a transverse plane Td of  FIG. 3   d  which is illustrated in  FIG. 3   f . In  FIG. 3   f , the electric field lines  394  extend from the strip conductor  326 , principally through the dielectric material  392 , and terminate on ground conductor or plane  316 . Comparison of  FIGS. 3   e  with  3   f  shows that the field patterns  390 ,  394  are similar, except for the physical positions of the strip conductors  314   l ,  326 , respectively, relative to the ground plane  316 . since the physical position of components has no effect on electrical systems other than as it affects the field structure (in other words, gravity has no effect on the electrical performance), the structure of  FIGS. 3   a ,  3   b ,  3   c , and  3   d  at transverse plane Td corresponds to that at transverse plane Ta. 
     FIGS. 4   a  and  4   b  are similar to  FIG. 3   a , and corresponding parts or elements are designated by like reference designations or alphanumerics. The arrangement of  FIG. 4   a  includes a break, opening or nonconductive portion  412  of conductor strip  326  at or near a transverse plane T 40 , which divides conductor strip  326  into a left portion  326   L  and a right portion  326   R . A MEMS switch element in the form of a conductive strip  410  is positioned below opening  412 , and arranged by a MEMS actuator  460  for motion in the direction of double-headed arrow  450  between the illustrated position with conductive element  410  not in electrical contact with conductor  326  and a second position, illustrated in  FIG. 4   b , in which conductive element  410  is in contact with strip conductor  326   L ,  326   R  on both sides of break  412 . 
   Microelectromechanical actuators for accomplishing such motion are known in the art. The length of break  412  is a distance S, which is less than the length of movable element  410 . In the “making contact” position of conductive element  410  illustrated in  FIG. 4   b , the opening or break  412  is bridged by conductive element  410 , thereby providing a path for the flow of electric current along the strip  326   L ,  326   R . The state of the switch element represented by  FIG. 4   a  is nonconductive, OPEN or OFF, and the state of the switch element represented by  FIG. 4   b  is conductive, CLOSED or ON. Thus, motion of a conductive element driven by a MEMS actuating device relative to a gap in a conductor can cause the transmission-line structure of  FIG. 3   a  to act effectively as a switch having ON and OFF states. 
     FIG. 5   a  is a simplified cross-sectional view of a switch  500  generally similar to switch  400  of  FIG. 4   a , and in which like reference designations refer to the same elements. The arrangement of  FIG. 5   a  differs from that of  FIG. 4   a  in that the movable MEMS switch element  410  lies above strip conductor portions  326   L  and  326   R , rather than below. The motion of movable MEMS element  410  continues to be in the direction indicated by arrow  450 . In order to provide space for movable MEMS switch element  410 , a cavity designated generally as  510  is defined in dielectric sheet or layer  392  in the region around movable MEMS switch element  410 . 
   Those skilled in the art of transmission lines know that the removal of dielectric material from a location adjacent the strip conductor tends to reduce the capacitance per unit length of the transmission line including the strip conductor, thereby tending to make the transmission line “inductive” or higher impedance in the affected region. In order to compensate for the effects of removing dielectric from dielectric sheet or plate  392  in the region around movable MEMS switch element  410 , the strip conductor is made wider than it would otherwise be.  FIG. 5   b  is a plan view of layer MT 0  of  FIG. 5   a , showing the left and right strip conductors, and also showing the location of cavity  510 . In the region of cavity  510 , the wider portion of strip conductor  326   L  is designated  526   L , and the wider portion of strip conductor  326   R  is designated  526   R . In order to maintain the impedance of the transmission line structure in the region of the movable MEMS switch element  410 , the element itself is made to a width about equal to that of the wider portions  526   L  and  526   R . Also illustrated in  FIG. 5   b  are the electrostatic MEMS switch drive pads  570   a  and  570   b , to which voltage is applied to cause motion of the movable MEMS switch element  410 . 
     FIG. 6  is a simplified cross-sectional view of a switch  500  as described in conjunction with  FIGS. 5   a  and  5   b , with the addition of a monolithic microwave integrated circuit (MMIC) electronic device, thereby forming a structure  600  including a MEMS switch connected to a MMIC device by means of HDI connections. In  FIG. 6 , device  500  corresponds to the like element of  FIG. 5 , and a switched version of the signal applied to top microstripline  314   l  appears at microstripline  314   r . In  FIG. 6 , MMIC element  620  is designated as being an amplifier, and is mounted below the lower surface  392   ls  of dielectric sheet or layer  392  with its input port  626   l  connected to pad  650  at the right end of strip conductor  314   r  by way of a combination of through via  652 , pad  651 , and through via  672 . Similarly, the output port  626   r  of MMIC amplifier  620  is connected to a pad  660  on the upper surface  312   us  of dielectric layer  312  by way of a through via  662 , a pad  681 , and a further through via  674 . 
   A salient advantage of at least some arrangements according to the invention lies in reduced electromagnetic reflections attributable to ground discontinuities or ground current reflections, which is particularly important in microwave applications. 
   Other embodiments of the invention will be apparent to those skilled in the art. For example, the “MMIC amplifier  620 ” could be, or include, a phase shifter, a low-noise amplifier, a power amplifier, filter components, or a further MEMS switch. The structure could include plural items corresponding to “MMIC amplifier  620 ,” or more than one MEMS switch, or both. Adhesives may be used to join the various surfaces of the dielectric sheets and MEMS or other substrates. 
   Thus, a transmission-line structure according to an aspect of the invention comprises a first dielectric sheet ( 312 ) defining first ( 312   us ) and second broad ( 312   ls ) sides. The first broad side ( 312   us ) of the first dielectric sheet ( 312 ) bears first ( 314   l ) and second ( 314   r ) separate electrically conductive planar strips. Each of the separate electrically conductive planar strips defines at least a first end ( 350 ,  360 , respectively). The first end ( 350 ) of the first planar strip ( 314   l ) and the first end ( 360 ) of the second planar strip ( 314   r ) are spaced apart by a distance (S 1 ). A second dielectric sheet ( 392 ) defines first ( 392   us ) and second ( 392   ls ) broad sides. The first broad side ( 392   us ) of the second dielectric sheet ( 392 ) defines a single continuous electrical conductor (ground  316 ) which defines first ( 370 ) and second ( 380 ) nonconductive regions. The first and second nonconductive regions are spaced apart by about the distance (S 1 ). The first broad side ( 392   us ) of the second dielectric sheet ( 392 ) is juxtaposed with the second broad side ( 312   ls ) of the first dielectric sheet ( 312 ), with at least portions of the first ( 370 ) and second ( 380 ) nonconductive regions of the continuous electrical conductor ( 316 ) registered with the first ends ( 350 ,  360 ) of the first ( 314   l ) and second ( 314   r ) planar strips, respectively. The transmission-line structure also includes a nonconductive planar surface ( 392   ls ;  460   us ) bearing a third electrically conductive planar strip ( 326 ) defining first ( 326   l ) and second ( 326   r ) ends. The first ( 326   l ) and second ( 326   r ) ends of the third planar strip ( 326 ) are separated by about the distance (S 1 ). The nonconductive planar surface ( 392   ls ;  460   us ) is associated with the second side ( 392   ls ) of the second dielectric sheet ( 392 ), with the first ( 326   l ) and second ( 326   r ) ends of the third planar strip ( 326 ) registered with the first ends ( 350 ,  360 ) of the first ( 314   l ), and second ( 314   r ) planar strips, respectively. A first electrically conductive through via ( 352 ,  372 ) arrangement connects the first end ( 350 ) of the first planar strip ( 314   l ) to the first end ( 326   l ) of the third strip ( 326   L ) through the first nonconductive region ( 370 ). A second electrically conductive through via arrangement ( 362 ,  374 ) connects the first end ( 360 ) of the second planar strip ( 314   r ) to the second end ( 326   r ) of the third strip ( 326   R ) through the second nonconductive region ( 380 ), to thereby form the first ( 314   l ), second ( 314   r ) and third ( 326 ) planar strips into a continuous strip conductor in which at least a portion of each of the first ( 314   l ), second ( 314   r ) and third ( 326 ) planar strips overlies a side of the continuous electrical conductor ( 316 ) to thereby form a strip transmission line including at least portions of the first, second and third planar strips. 
   A preferred embodiment of the transmission-line structure further includes a gap ( 412 ) in the third planar strip ( 326 ), and mechanically operated switch means ( 410 ) making controllable electrical and mechanical contact with a portion ( 326   l ) ( 326   r ) of the third planar strip ( 326 ) on each side of the gap ( 412 ). In one version of this preferred embodiment, the mechanically operated switch means lies on a side of the gap ( 412 ) which is remote from the first dielectric sheet ( 312 ), and moves toward and away from the second dielectric sheet ( 392 ) in order to make and break connection. In another version of this preferred embodiment, the mechanically operated switch means lies within a cavity ( 510 ) defined in the second dielectric sheet ( 392 ). 
   Another embodiment of the transmission-line structure includes a gap ( 626   g ) in the third planar strip ( 626 ), and a planar signal processing module ( 620 ) with at least first ( 626   l ) and second ( 626   r ) signal ports. The first ( 626   l ) and second ( 626   r ) signal ports are mechanically and electrically connected to portions of the third planar strip ( 626   l ,  626   r ) on each side of the gap ( 626   g ). In a preferred version of this embodiment, the signal processing module ( 620 ) performs amplification, and the first and second signal ports are signal input and output ports, respectively.