Patent Publication Number: US-6710689-B2

Title: Floating contactor relay

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to relays and in particular to a relay having a floating contactor. 
     2. Description of Related Art 
     FIG. 1 is a block diagram of a portion of a typical prior art integrated circuit (IC) tester  10  including a set of channels  12 , one for each of several terminals of an IC device under test (DUT)  14 . Each channel  12  includes a channel control and data acquisition circuit  16 , a comparator  18  and a tristate driver  20 . A relay  24  links an input of comparator  18  and an output of driver  20  to a DUT terminal  26 . Another relay  25  connects a parametric measurement unit (PMU)  28  within channel  12  to DUT terminal  26 . A host computer  30  communicates with the channel circuits  16  of each channel  12  via a parallel bus  32 . 
     Tester  10  can carry out both digital logic and parametric tests on DUT  14 . Before starting a digital logic test, the control and data acquisition circuit  16  of each channel  12  closes relay  24  and opens relay  25  to connect comparator  18  and driver  20  to DUT terminal  26  and to disconnect PMU  28  from terminal  26 . Thereafter, during the digital logic test, the channel control signal may turn on driver  20  and signal it to send a logic test pattern to DUT terminal  26  when the DUT terminal  26  is acting as a DUT input. When terminal  26  is a DUT output, circuit  16  turns off driver  20  and supplies an “expect” bit sequence to an input of comparator  18 . Comparator  18  produces an output FAIL signal indicating whether successive states of the DUT output signal matches successive bits of the expect bit sequence. Circuit  16  either stores the FAIL data acquired during the test for later access by host computer  30  or immediately notifies host computer  30  when comparator  18  asserts the FAIL signal. 
     PMU  28  includes circuits for measuring analog characteristics of the DUT  14  at terminal  26  such as, for example, the DUT&#39;s quiescent current. Before starting a parametric test, the channel control circuit  16  opens relay  24  and closes relay  25  to connect the channel&#39;s PMU  28  to DUT terminal  26  and to disconnect comparator  18  and driver  20  from terminal  26 . Host computer  30  then programs PMU  28  to carry out the parametric test and obtains test results from the PMU. 
     Relays  24  and  25  are normally preferred over solid state switches for routing signals between DUT  14 , PMU  28 , driver  20  and comparator  18  because a relay, having very low loss, does not substantially influence test results. We would like to position comparator  18 , driver  20 , relays  24  and  25 , and circuit  16  as close as possible to DUT terminal  26  to minimize the signal path lengths between terminal  26 , comparator  18  and driver  20 . When the signal paths are too long, the signal delays they cause can make it difficult or impossible to provide the signal timing needed to properly test DUT  14 , particularly when the DUT operates at a high speed. Thus to minimize signal path distances we want to use relays  24  and  25  that are as short as possible and which can be reached via short signal paths. 
     In some prior art testers, one or more channels  12  are implemented on each of a set of printed circuit boards (“pin cards”) that are mounted in a cylindrical chassis to form a test head. FIG. 2 illustrates a simplified plan view of a typical test head  34 . FIG. 3 is a partial sectional elevation view of the test head  34  of FIG.  2 . FIGS. 4 and 5 are expanded front and side elevation views of a lower portion of one of a set of pin cards  36  mounted within test head  34 . Pin cards  36  are radially distributed about a central axis  38  of test head  34  and positioned above an integrated circuit device under test (DUT)  14  mounted on a printed circuit board, “load board”  39 . A set of pogo pins  41  provide signal paths between relays  24  and  25  mounted on pin cards  36  and contact points on the surface of load board  39 . Microstrip traces on load board  39  connect the contact points to terminals of DUT  14 . 
     Relays  24 ,  25  are mounted near the lower edges of each pin card  36  as close as possible to central axis  38  to minimize the signal path distance to DUT  14 . However from FIG.  2  we can see that the space between pin cards  36  is relatively limited near axis  38 . Thus in order to position relays  24 ,  25  close to axis  38  we want to use relays that are relatively thin but which are fast and reliable. 
     FIG. 6 is a simplified sectional elevation view of a conventional reed relay  40  including a glass tube  42  containing a pair of conductive reeds  44  and  45  serving as the relay&#39;s contacts  47 . A wire  46  wraps many turns around tube  42  to form a coil  48 . Reeds  44 ,  45  are normally spaced apart, but when a voltage is applied across opposite leads  50 ,  52  of coil  48 , magnetic flux produced by the coil flexes reeds  44 ,  45  causing them to contact one another so that a current may flow through the relay contacts  47 . A conductive sheath  43  partially surrounds tube  42  to provide a ground surface. The spacing between reeds  44 ,  45  and shield  43  influences the characteristic impedance of the transmission line formed by reeds  44  and  45  when they are in contact. 
     The magnetic force produced by coil  48  on reeds  44 , 45  is proportional to the product of the magnitude of the current passing through coil  48  and the number of turns of coil about tube  42 . A large number of coil turns is provided to minimize the amount of current needed to operate relay  40 . However the large number of turns contributes to the thickness of relays; a relay&#39;s coil typically contributes more than half the thickness of the relay. 
     FIG. 7 is a schematic diagram of a typical circuit for driving coils of a set of N relays  40 . One end of each relay&#39;s coil  48  is connected to a voltage source  54  while the other end of the relay&#39;s coil is connected to ground through one of a set of N switches  49  controlled by one of control signals C 1 -CN. For example when a control signal Cl turns on one of switches  49 , the current passes through relay coil  48  thereby causing the relay&#39;s contacts  47  to close. When control signal C 1  turns off switch  49 , current stops passing though coil  48  and allows contacts  47  to open. 
     When switch  49  opens, the magnetic field produced by coil  48  collapses producing a transient voltage spike across coil  48  that is limited by a diode  56  connected across the coil. Without diode  56  the voltage spike would pass though voltage source  54  and appear as undesirable noise in other circuits receiving power from voltage source  54 . Reeds  44  and  45  are also subject to contact bounce, wear, sticking and stress failure. 
     The opposing faces of reeds  44  and  45  have capacitance when relay  48  is open and that “stub” capacitance can influence high frequency signals. Referring to FIG. 1, for example, when the relay  24  linking unit  16  is closed and the other relay  24  is open during high frequency tests, the stub capacitance of the open relay can distort signals passing between the DUT and driver and receiver  20  and  18 . 
     Since reeds  44  and  45  large enough to carry large currents have substantial inertia, and since reed inertia slows relay operation, relay reed size represents a trade-off between relay speed and current carrying capacity. Reeds  44  and  45 , tube  42 , shield  43 , coils  46  and diode  56  all contribute to the size of relay  40  and the bulk of that relay makes it difficult to concentrate several such relays into a small volume. Since relay bulk can limit the number relays  24  (FIG. 1) that can be placed in a small area near a DUT terminal, only a such few relays can be used in each channel  12 . The limitation of number of relays  24  in turn limits the number of test components such as devices  16  and  28  that can alternatively access 
     What is needed is a compact, low-noise, low-stub capacitance, long-life relay for use as relays  24  and  25  of the integrated circuit tester of FIG.  1  and other applications which can switch relatively quickly for the amount of current it must carry and with little contact bounce. 
     BRIEF SUMMARY OF THE INVENTION 
     A relay in accordance with the invention includes one or more conductive coils embedded in an insulating substrate having multiple horizontally disposed layers. The relay also includes at least one set of contacts bordering a space containing a contactor in which at least a portion of its surface is conductive and shaped to mate with the contacts. The contactor is “free-floating” (i.e., unattached to any other object) and free to move within the space adjacent to the contacts. The contactor includes material such as iron or nickel so that a magnetic field can apply a motive force on the contactor. Current passing through the coil or coils produces magnetic fields which can selectively either position the contactor within the space so that its conductive surface mates with the contacts to provide a signal path therebetween, or so that its conductive surface does not mate with the contacts and does not provide a signal path therebetween. 
     A relay in accordance with a first embodiment of the invention includes first and second coils. When a current passes through the first coil it produces a first magnetic field pulling the contactor onto the contacts. When current alternatively passes through the second coil it produces a second magnetic field pulling the contactor away from the contacts. Thus the switching state of the relay is determined by whether current passes through the first or second coil. 
     A relay in accordance with a second embodiment of the invention employs a spherical contactor having first and second hemispheres of opposite magnetic polarity. The first hemisphere has a conductive surface while the second hemisphere has a non-conductive surface. When current passes through the coil in a first direction it creates a first magnetic field forcing the conductive surface of the contactor&#39;s first hemisphere onto the contacts thereby creating a signal path between the contacts. When current passes through the coil in a second direction it creates a second magnetic field forcing the non-conductive surface of the contactor&#39;s second hemisphere onto the contacts thereby breaking the signal path between the contacts. 
     A multiple pole relay in accordance with at third embodiment of the invention includes a spherical contactor free to roll around a torroidal channel formed in the substrate. Several contacts are distributed around an output periphery of the channel while a common contact covers an inner surface of the channel. A separate coil is embedded in the substrate proximate to each contact. Whenever a current is applied to one of the coils, it creates a magnetic field attracting the contactor so that the contactor positions itself to provide a conductive path between the contact proximate to that coil and the central contact. 
     When a relay in accordance with the invention employs a very small contactor which can be moved by relatively small magnetic fields, the relay&#39;s coils and cores can be relatively small. Thus many such relays can be concentrated into a relatively small volume. Since the relay&#39;s coils, cores, and contacts, and in some embodiments the contactor, are embedded in a substrate such as a printed circuit board, the relay requires little or no space on the surface of the substrate. Since it does not include any springs, reeds or other parts that substantially deform wherein making when breaking a signal path, a relay in accordance with the invention is less subject to contact bounce and material stress failures than conventional relays. 
     It is accordingly an object of the invention to provide a very compact, high-speed, low stub capacitance, long-lived relay that is relatively unaffected by contact bounce. 
     The claims portion of this specification particularly points out and distinctly claims the subject matter of the present invention. However those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING(S) 
     FIG. 1 is a block diagram of a portion of a typical prior art integrated circuit (IC) tester, 
     FIG. 2 illustrates a simplified plan view of the test head of the tester of FIG. 1, 
     FIG. 3 is a partial sectional elevation view of the test head FIG.  2 . 
     FIG. 4 is an expanded front elevation view of a lower portion of one of a set of pin cards of the test head of FIG. 2, 
     FIG. 5 is an expanded side elevation view of a lower portion of one of a set of pin cards of the test head of FIG. 2, 
     FIG. 6 is a simplified sectional elevation view of a prior art reed relay, 
     FIG. 7 is a schematic diagram of a prior art circuit for driving coils of a set of reed relays, 
     FIG. 8 is a sectional elevation view of a relay in accordance with the invention, 
     FIGS. 9-11 are partial plan views of the relay of FIG. 8, 
     FIG. 12 is a schematic diagram of the relay of FIG. 8 along with a current source and a switch for controlling the relay, 
     FIG. 13 illustrates a relay in accordance with a first alternative embodiment of the invention, 
     FIG. 14 is a schematic diagram illustrating the relay of FIG. 13 along with a switch and a current source for controlling the relay, 
     FIG. 15 is a plan view of a relay in accordance with a second embodiment of the invention. 
     FIG. 16 is a sectional elevation view of the relay of FIG. 14, 
     FIG. 17 is a schematic diagram illustrating the relay of FIG. 14 along with a multiplexer and a current source for controlling the relay, 
     FIG. 18 is a sectional elevation view of a relay in accordance with a third embodiment of the invention, 
     FIG. 19 is a schematic diagram the relay of FIG. 18 along with a multiplexer and a current source for controlling the relay, 
     FIG. 20 is a plan view of a relay in accordance with a fourth embodiment of the invention, 
     FIG. 21 is a sectional elevation view of the relay of FIG. 20, 
     FIG. 22 is a schematic diagram the relay of FIG. 20 along with a multiplexer and a current source for controlling the relay, 
     FIG. 23 is a sectional elevation view of a relay in accordance with a fifth embodiment of the invention, 
     FIG. 24 is a sectional elevation view of a relay in accordance with a sixth embodiment of the invention, 
     FIG. 25 is a sectional elevation view of a relay in accordance with a seventh embodiment of the invention, 
     FIG. 26 is a sectional elevation view of a hybrid circuit employing relays in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 8 is a sectional elevation view of a relay  60  in accordance with the invention formed within the insulating substrate  62  having multiple substrate layers  64 A- 64 Q. Substrate layers  64 A- 64 Q may be formed of any of a wide variety of insulating substrate materials such as for example, silicon dioxide, other semiconductor oxides, silicon nitride, silicon oxynitride, ceramics, phosphor-silicate glass and other glasses, and conventional printed circuit board substrate materials. FIGS. 9,  10  and  11  are partial plan views of relay  60  along section lines  9 — 9 ,  10 — 10  and  11 — 11  of FIG.  8 . Relay  60  includes a pair of multiple-turn coils  66  and  68  formed by conductive traces  67  residing on the various substrate layers  64  and interconnected by vias  69 . Traces  67  may be, for example conductive metals or conductive semiconductor materials such as polysilicon. Although for simplicity each coil  66  and  68  is illustrated in FIG. 8 as having 18 turns, coils  66  and  68  can have a much larger number of turns. Each coil  66  and  68  surrounds a separate core  70  or  72 , each extending vertically partially through substrate  60  and formed of iron or other suitable magnetic core material. Relay  60  also includes a small spherical contactor  74  residing between cores  70  and  72  in a cavity of layer  64 H. Contactor  74  is “free-floating” in that it is not attached to any other object and is free to move anywhere within the cavity in layer  64 H. 
     Conductive layers  76  and  78  on the upper and lower surfaces of substrate layer  64 H are formed to provide one pair of conductive contacts  80  directly above contactor  74  and another pair of conductive contacts  82  directly below contactor  74 . Layers  76  and  78  may, for example, be made of metal such as copper, silver or gold, or of semiconductor material. Contactor  74 , suitably including iron or other material attracted by magnetic fields, has a conductive surface for providing a signal path between contacts  80  or between contacts  82 . Conductive layer  79 A below coil  66  and conductive layer  79 B above coil  68  act as electro-static shields. 
     FIG. 12 is a schematic diagram illustrating relay  60  along with a current source  84  and a solid-state multiplexer  86  for controlling the relay. Multiplexer  86 , in turn controlled by an externally generated control signal CONT, routes current from current source  84  either through coil  66  or through coil  68 . Multiplexer  86  normally routes current through coil  68  to produce a magnetic flux in core  72  pulling contactor  74  down onto contacts  82 . The conductive surface of contactor  74  provides a signal path between those contacts so that contacts  82  are normally closed. Contacts  80  are normally open because contactor  82  normally does not provide a signal path between them. However when the CONT signal tells multiplexer  86  to route the current from current source  84  through coil  66 , the coil induces magnetic flux in core  70  pulling contactor  74  upward onto contacts  80  thereby closing those contacts. Contacts  82  open because contactor  82  no longer provides a signal path between them. 
     As may be apparent on close inspection of FIG. 8, one side of contact  80  is slightly lower than the other side of contact  80  so that when contactor  74  rises it strikes one side of the contact before it strikes the other side, thereby causing contactor  82  to rotate slightly about a first horizontal axis. Similarly one side of contacts  82  is slightly higher than the other side so that when coil  68  pulls contactor  74  downward, the contactor strikes one side of contact  80  first and then rotate slightly about a second horizontal axis perpendicular to the first axis. Thus as relay  60  repeated opens and closes contacts  80  and  82  contactor  74  rotates about two perpendicular horizontal axes. The contactor&#39;s rotating action helps to wipe contacts  80  and  82  to keep them free of contaminants and to prevent the contactor from deforming. 
     Another control signal CONTX controls the amount of current source  84  generates. Normally the current is only large enough to produce sufficient magnetic fields move contactor  74  up and down. However should any contaminants eventually cause contactor  74  to become stuck on either of contacts  80  or  82 , the CONTX signal can signal current source  84  to temporarily provide larger currents producing stronger magnetic fields in coils  66  and  68 . By alternately switching the large current between coils  66  and  68 , vibrations produced on contactor  82  can free it. The ability to free a stuck contactor helps to prolong the life of the relay. 
     Unlike prior art reed relays, relay  60  does not rely on parts that flex and therefore and is therefore less subject to stress failures. When contactor  74  is very small, relay  60  can be very small, and since relay  60  is wholly embedded in substrate  62 , it takes up no space above the substrate. Note that since contacts  82  are spaced apart and have relatively little opposed surface area, they have very little stub capacitance in the open state. The low contact capacitance makes relay  62  particularly suitable for high frequency applications. 
     FIG. 13 illustrates a relay  90  embedded in a substrate  92  in accordance with an alternative embodiment of the invention. Relay  90  includes a spherical contactor  94 , a single core  96  embedded in substrate  92  below contactor  94 , a single coil  98  formed by traces  100  surrounding core  96 , and a pair of contacts  104  formed in a conductive layer  106  on the upper surface of substrate  92 . A cover  110  mounted on substrate  92  covers contactor  94 . The contactor  94  suitably has a core magnetized iron, nickel or other magnetic material so that contactor  94  has a north and south pole. The surface of the contactor&#39;s southern hemisphere is coated with conductive material such as, for example gold or silver, while the surface of the contactor&#39;s northern hemisphere is coated with an insulator such as glass or ceramic material. A conductive layer  107  above coil  98  acts as an electrostatic shield. 
     FIG. 14 is a schematic diagram illustrating relay  90  along with a pair of multiplexers  112  and  113  and a current source for controlling the relay. Multiplexers  112  and  113 , controlled by externally generated control signals CONT 1  and CONT 2  may route current from current source  114  in either direction through coil  98 . When the current passes through coil  98  in one direction, the upper end of core  96  becomes a northern magnetic pole and pulls the southern pole of contactor  94  onto contacts  104 . Since the surface of the contactor&#39;s southern hemisphere is conductive it provides a signal path between contacts  104 . When switch  112  thereafter routes current from current source  114  in the opposite direction through coil  98 , the upper end of core  96  becomes a southern magnetic pole repelling the contactor&#39;s southern pole and attracting the contactor&#39;s northern pole. Contactor  94  thus rotates so that its northern pole now points downward. Since the surface of the contactor&#39;s northern hemisphere is non-conductive, the signal path between contacts  104  is broken. 
     FIG. 15 is a plan view and FIG. 16 is a sectional elevation view of an eight-pole, single-throw relay  120  in accordance with the invention. A spherical contactor  122  having a conductive surface  123  rolls in a torroidal channel  124  formed in the upper surface of a circuit board  126 . A set of eight contacts  128  formed in a conductive layer on the surface of substrate  126  are distributed about the circular periphery of channel  124 . A single common contact  130  covers the inner circumference of channel  124 . A set of eight iron cores  132  are embedded in substrate  26  under channel  124 , each surrounded by a separate coil  134  formed by traces and vias embedded within substrate  126 . Contactor  122  suitably includes a ceramic core  136  coated by iron or nickel  138  and a conductive gold outer layer  123 . A cover (not shown) residing on the surface of circuit board  126  suitably encloses contactor  122  and channel  124 . 
     FIG. 17 is a schematic diagram illustrating relay  120  and a multiplexer  137  and current source  139  for controlling the relay. Multiplexer  137  responds to externally generated control data (CONT) by directing the current output of current source  139  to one of coils  134 . The coil  134  receiving the current magnetizes the core  132  it surrounds. The magnetic field from that core attracts contactor  122  so that it rolls around channel  124  and positions itself over that particular coil. The conductive surface  123  of contactor  122  provides a signal path between the adjacent contact  124  and central contact  130 . and as a signal path to central contact  130 . On system startup, the CONT signal suitably cycles the current from current source  138  to each of coils  134  in turn so as to place contactor  122  in a known position. 
     FIG. 18 is a sectional elevation view of a sixteen-pole, double-throw relay  140  including a contactor  142  similar to contactor  122  of FIG. 16 residing in a torroidal channel  144  embedded wholly within a substrate  145 . Relay  140  is similar to relay  120  of FIGS. 15 and 16 except that in addition to eight cores  146  and coils  148  below channel  144 , it has another eight cores  150  and coils  152  above the channel. It also has a separate set of eight upper contacts  154  distributed about the circular periphery of the channel and an upper common contact  156  in addition to eight lower contacts  158  and lower common contact  157 . 
     FIG. 19 is a schematic diagram illustrating relay  140  and a multiplexer  160  and current source  159  for controlling the relay. Multiplexer  160  responds to externally generated control data (CONT) by directing the current output of current source  159  to one of coils  148  or  152 . When one of lower coils  148  receives the current, it magnetizes the core  146  it surrounds. The magnetic field from that core attracts contactor  142  so that it positions itself over that particular coil with the conductive surface of contactor  142  providing a signal path between the adjacent lower contact  158  and lower common contact  157 . When one of upper lower coils  152  receives the current from current source  159 , it magnetizes its corresponding core  150  and current magnetic field from that core attracts contactor  142  so that it positions itself to provide a signal path between the adjacent upper contact  154  and upper common contact  156 . 
     FIG. 20 is a plan view and FIG. 21 is a sectional elevation view of a relay  161  in accordance with the invention having eight terminals A-H. FIG. 22 is a schematic diagram of relay  161  along with a multiplexer  168  and current source  169  for controlling it. A spherical contactor  162  resides in a circular, dish-shaped channel  163  on the upper surface of a substrate  164 . A set of eight contacts  165  formed in a conductive layer on the surface of substrate  164  are distributed about the circular periphery of channel  163 . A set of relay coils  166  and cores  167  embedded within substrate  164  under channel  163  are positioned so that when any one coil  166  receives current from source  169 , its related core  167  produces a magnetic field pulling contactor  162  over two adjacent contacts  165 . Contractor  162  then completes a signal path between the two adjacent contacts. Thus relay  161  can interconnect any pair of adjacent relay terminals A-H. 
     A version of relay  61  having three terminals A, B and C instead of eight may replace prior art relays  24  and  25  of FIG.  1 . In addition to providing alternative signal paths from DUT terminal  26  to receiver/drier  18 , 20  or to parametric measurement unit  28 , such a relay could also provide a signal path between driver, receiver  18 , 20  and parametric measurement unit  28  while isolating DUT terminal  26 . This would, for example, permit the use of parametric measurement unit  28  for calibrating driver  20  and receiver  18  without being affected by the input impedance of DUT terminal  26 . 
     FIG. 23 illustrates a relay  170  generally similar to relay  60  of FIG. 8 except that it has a bullet-shaped contact element  171  instead of a spherical contact element  82 . It should be apparent that other contactor shapes, such as for example polyhedrons, could be employed in various versions of the relay described above when suitable adjustments are made to the shape of the relay contacts the contactor contacts. 
     FIG. 24 illustrates a relay  172  wherein a magnetic field created by current passing though a coil  173  embedded in a substrate  174  moves a magnetized core  175  upward to push a conductive spherical contactor  176  onto contacts  177  formed in a conductive layer above the contactor. When the direction of current through coil  173  is reversed, core  175  moves downward permitting contactor  176  to fall onto contacts  178  formed on a conductive layer below the contactor. An upper tip  179  of core  175  is slanted so that contactor  176  rotates slightly each time core  175  pushes the contactor upward. 
     FIG. 25 illustrates a relay  180  in accordance with the invention in which an elongate conductive contactor  182 , a permanent magnet having north and south magnetic poles, resides in a space  183  within a substrate  184  surrounded by an embedded coil  185 . When current passes through coil  185  in a first direction, coil  185  generates a magnetic field driving contactor  182  upward and to that it makes contact with a pair of upper conductive contacts  186 . When a current passes though coil  185  in a second direction, coil  185  generates a magnetic field driving contactor  182  downward onto a pair of lower conductive contacts  187 . 
     Embedded relays in accordance with the invention may be used, for example, to provide relay contacts at the input/output terminals of a hybrid circuit. FIG. 26 illustrates a hybrid circuit  190  including two “flip-chip” integrated circuit chips  192  mounted on a substrate  194  residing within an integrated circuit package  196 . Solder balls  198  link input/output pads on the surfaces of chips  192  to vias  200  extending downward to contact and coil terminals of relays  205  embedded in substrate  194 . Additional vias  204  extend downward from contacts of embedded relays  205  to solder balls  206  connecting hybrid circuit  190  to traces  207  on the surface of a larger substrate  208 . 
     While the embodiments of the relay are described herein above as being implemented within conventional multiple-layer printed circuit boards, other embodiments of the relay could be implemented on other types of multiple layer substrates including, for example, substrates formed of ceramic and semiconductor materials. 
     While the forgoing specification has described preferred embodiment(s) of the present invention, one skilled in the art may make many modifications to the preferred embodiment without departing from the invention in its broader aspects. The appended claims therefore are intended to cover all such modifications as fall within the true scope and spirit of the invention.