Abstract:
A component is provided for an impedance change in a coplanar waveguide which includes two grounding conductors and a signal line lying between the grounding conductors, as well as a conducting connecting element, which has a covering surface for the two grounding conductors and the signal line, and is electrically insulated, so that in each case a capacitor is formed. The connecting element and the lines are situated and arranged so that the respective capacitor between the grounding conductors and the connecting element has an invariable capacitance, but the capacitor between the connecting element and the signal line has a variable capacitance. A structure is also provided in which in an exactly opposite way, the respective capacitor between the grounding conductors and the connecting element has a variable capacitance, but the capacitor between the connecting element and the signal line has an invariable capacitance. Furthermore, a method for producing such a component is also provided.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a component as well as a method for producing a component for impedance change in a coplanar waveguide.  
       BACKGROUND INFORMATION  
       [0002]     Components and production methods for components for impedance change in coplanar waveguides have become known in various specific embodiments.  
         [0003]     In one specific embodiment of a micromechanically produced high-frequency short-circuiting switch, a thin metal bridge is stretched between grounding conductors of a coplanar waveguide. This bridge is electrostatically drawn to a thin dielectric disposed on a signal line lying between the ground elements, thereby increasing the capacitance of a “plate-type capacitor” formed by the bridge and the signal line. This capacitance change influences the propagation properties of the electromagnetic waves guided on the waveguide. In the “off” state (the metal bridge is pulled downwards to the signal line) a major part of the power is to be reflected. In the “on” state, however, (the metal bridge is above), a large part of the power is transmitted.  
         [0004]     In German Laid-Open Document DE 100 37 785 A1, a device for impedance change on a coplanar waveguide is discussed, in which the grounding conductors are connected by a connecting piece and the signal line has a bridge, at the location of the connecting piece, which, in turn, may be operated electrostatically. The advantage of this specific embodiment is that the length of the metal bridge, that is, the length of the bridge over the element connecting the grounding conductors, is not a function of the clearance between the grounding conductors of the coplanar waveguide. Accordingly, the clearance between the grounding conductors of the waveguide may be selected independently of the length of the bridge, and vice versa,  
         [0005]     A disadvantage of both the mentioned specific embodiments is that, for the electrostatic operation of the respective bridge, the grounding conductors and the signal line have to have a direct current control voltage applied to them.  
         [0006]     One structure from the related art that does not have this disadvantage is shown in  FIGS. 5   a  and  5   b  of the attached drawings.  FIG. 6  shows, in addition, a greatly schematized equivalent circuit diagram for this structure. Component  101 , illustrated in  FIGS. 5   a  and  5   b , for impedance change of a section of a waveguide  102  includes two external grounding conductors  103 ,  104  and a signal line  105  lying between them. A bridge arrangement  106  having a self-supporting bridge  107  is constructed over grounding conductors  103  and  104 , as well as signal line  105 . A section along section line V-V, having a nondeflected bridge  107  and a deflected bridge  107  (drawn in broken lines) is illustrated in  FIG. 5   b . Bridge  107  is stretched between vertical member elements  108  that are electroplated on at each end.  
         [0007]     In order to obtain a compact structure, each grounding conductor  103  and  104  has a recess  103   a  or  104   a  in the vicinity of bridge  107 . 0   
         [0008]     The bridge, via a connection  109 , may have applied to it a control dc voltage with respect to lines  103 ,  104 ,  105 , in order to draw the bridge against lines  103 ,  104 ,  105  via electrostatic forces. To avoid a short circuit, in the vicinity below the bridge, an insulating layer  110  is laid over lines  103 ,  104 ,  105  (for this, see especially the sectional arrangement).  
         [0009]     Component  101  may be described by an equivalent circuit diagram according to  FIG. 6 , with a view to its high frequency properties. Symmetrical to two line sections  111 ,  112  with symbolically shown characteristic wave resistance  113 , a grounded branch  114  branches off, which has the following components: A first mutual capacitance  115 , an inductance  116  and an ohmic resistance  117 , followed by a second mutual capacitance  118 . Before the second mutual capacitance, a voltage source  119  is symbolically connected.  
         [0010]     First mutual capacitance  115  is defined by the intersection of signal line  105  and  107 , and may assume, in particular, two capacitance values according to the two positions shown in  FIG. 5   b . Inductance  116  comes about from the bridge sections between signal line  105  and respective grounding conductor  103 ,  104 . The same sections define ohmic resistance  117 . Mutual capacitance  118  is established by the intersection of bridge  107  with the respective narrow region of grounding conductors  103  or  104 , and may also, just as first mutual capacitance  114 , in particular, assume two values corresponding to the positions of bridge  107  shown in  FIG. 5   b . Using such a construction, one may achieve, for example, a capacitance change by approximately a factor of 100, whereby component  101  may be used in a prespecified frequency range as a high frequency switch.  
         [0011]     In principle, using this construction, a decoupling of the control signal of the switchable capacitances of lines  103 ,  104 ,  105  is implemented, and thereby the possibility is given of using such switching elements in change-over switches, distribution networks or phase shifters.  
         [0012]     However, it has turned out that such a bridge, having uniformly reproducible switching properties, is not simple to implement, if it can be done at all.  
       SUMMARY OF THE INVENTION  
       [0013]     The exemplary embodiment and/or exemplary method of the present invention is based on the object of making available a component described above, having mutual capacitances decoupled with regard to the control signal, which has improved switching parameters.  
         [0014]     This object is attained by the features described herein.  
         [0015]     Advantageous and expedient embodiments for implementing the present invention are specified in the dependent claims.  
         [0016]     First of all, the exemplary embodiment and/or exemplary method of the present invention relates to a component for impedance change in a coplanar waveguide, which includes two grounding conductors and a signal line that lies between the grounding conductors, as well as a conducting connecting element which has a covering surface (area) for the two grounding conductors and the signal line, and is insulated, so that, in each case, a capacitor is formed. Now, an aspect of the exemplary embodiment and/or exemplary method of the present invention is that the connecting element and the conductors are positioned and designed in such a way that the respective capacitor between the grounding conductors and the connecting element has an invariable capacitance, but the capacitor between the connecting element and the signal line has a variable capacitance. This procedure is based on the knowledge that it is very difficult, in the case of the exemplary embodiment and/or exemplary method that was last mentioned above, to get the switchable bridge to lie reproducibly upon the grounding conductors in a planar manner on the outside, that is, in  FIG. 5   b , in the area of vertical member elements  108  in the activated state. This disadvantage may be avoided by executing the mutual capacitance to the grounding conductors as an invariable capacitor. This may be achieved especially in that vertical member elements  108  shown in  FIG. 5   b  are shifted into an area above insulated grounding conductors  103 ,  104 . Because of this procedure, one only has to make certain that bridge  107  lies reproducibly in a planar manner on the insulating layer above signal line  105 .  
         [0017]     However, in an alternative specific embodiment, it is conceivable with the same advantages that the mutual capacitance is designed to be invariable to the signal line but the mutual capacitances to the respective grounding conductors is designed to be variable.  
         [0018]     In both cases, the mutual capacitances lie in series with an inductance and form a resonant circuit, whose resonant frequency, because of the variable capacitance or capacitances, is able to reflect (mirror) two working points, e.g. transmission and reflection of a signal having a prespecified frequency. For the desired function of the resonant circuit, it is thus sufficient if a mutual capacitance is switchable.  
         [0019]     In one embodiment of the present invention, the connecting element is able to be deformed mechanically, which may be elastically, in such a way that the distance between the connecting element and the line that, together with the connecting element, forms the variable capacitance, is variable in the area of the covering surface, for instance, via electrostatic forces.  
         [0020]     However, it is also possible that the signal line or the grounding conductors, in a subsection, in which it(they) cover(s) the connecting element, is(are) at a distance mechanically deformable in such a way that the distance may be adjusted in the area of the respective cover surface. Consequently, in this specific embodiment, the grounding conductors are not connected by a bridge, but a bridge is provided, for instance, in the signal line, under which the connecting element runs, the connecting element being coupled by covering surfaces with the grounding conductors and at least one insulating layer intercalated in between, in a fixed capacitive manner, to the grounding conductors. Consequently, this variant has the advantage that the bridge may be designed independently of the clearance from the grounding conductors, and, at the same time, the capacitive coupling between grounding conductors and signal lines may be switched at a comparatively greater reproducibility.  
         [0021]     For the switching of the variable capacitor, the connecting element may have a voltage applied to it. With that, electrostatic forces on the capacitor may, for example, be used between the connecting element and the signal line, in order to be able to switch over its capacitance, for instance, between two values.  
         [0022]     In a method for producing the components just described, for impedance change in a coplanar waveguide, which includes two grounding conductors and a signal line that lies between the grounding conductors, as well as a conducting connecting element which has an overlapping surface to the two grounding conductors and the signal line, and is insulated, so that, in each case, a capacitor is formed, the major aspect is in the following method steps:  
         [0023]     One or more conductive layers are deposited on the substrate for developing the connecting element, and are subsequently patterned, which may be by photolithographic methods. Then an insulating layer is deposited, and, on the insulating layer, the grounding conductors as well as the signal lines are built up with a bridge over the connecting element. By this method one obtains a component in which the connecting element is coupled via capacitors having a fixed capacitance to the grounding conductors and to the signal line via a capacitor that is variable in its capacitance, which, however, may be switched at comparatively good reproducibility.  
         [0024]     If a not highly insulated substrate is used, it is additionally advantageous if an insulating layer is first generated on the substrate, before building up the structure. This may be done, for instance by thermal oxidation or by applying a PECVD layer (PECVD stands for plasma-enhanced chemical vapor deposition). Thermal oxide is advantageous with regard to a low damping of a high frequency signal.  
         [0025]     Furthermore, the insulating layer deposited on the connecting element may be patterned. In this way, not only a terminal for the connection of the connecting element is able to be exposed, but possibly also areas on connecting bars which are used for later electroplating for the electrical connection of sections on which structures are to be “electroplated on”.  
         [0026]     The grounding conductors and at least a part of the signal lines may be generated via an electroplating step. For this it is advantageous if a starting layer is deposited first. This starting layer is favorably patterned via a lift-off process. Thereby it is avoided that damage occurs in the dielectric that has already been applied to the connecting element. In addition, one does not have to notice whether the starting layer is able to be patterned selectively from the material of which the connecting element is made.  
         [0027]     For the further construction of the lines with a bridge over the connecting element in the area of the signal line, it is advantageous if a sacrificial layer is applied and patterned. In this context, the area of the subsequent bridge is also covered by the sacrificial layer. Now, in one electroplating step, each exposed area of the sacrificial layer may be reinforced with electroplating if, in addition, a starting layer is present in this area. One may permit the galvanic layer to grow to the extent that it overlaps over the sacrificial layer, and, in cross section, so to speak, a mushroom structure is created.  
         [0028]     Now, in one additional step, an additional metallization is laid down and patterned, over the sacrificial layer, using electroplating reinforcement. Hereby is created, in the first place, the bridge of the signal line, the remaining areas, may be formed, in a top view, corresponding to the contour of the signal line and the grounding conductors. The sacrificial layer is subsequently removed thereafter, which may be anisotropically, down to the area under the bridge.  
         [0029]     Subsequently to these measures, in a subsequent step, for instance, connecting crosspieces for the electroplating step may be removed, without running the risk of damaging the bridge. Such connecting crosspieces are required in order, in the electroplating step, to connect electrically to one another all areas in which an electroplating structure is to grow on a starting layer.  
         [0030]     Finally, the sacrificial layer is also removed under the bridge metallization, whereby a component is created that is made up of essentially one coplanar waveguide, in which the grounding conductors are capacitively coupled in each case via a continuous connecting element, and the signal line is also capacitively coupled to the connecting element via a flexible bridge, that is, a switchable bridge. Thereby the impedance may be changed at this place, by applying a control voltage to the insulated connecting element, which results in electrostatic forced on the bridge at a corresponding shift in the position of the bridge.  
         [0031]     The effect is a capacitance change, which will be explained further below, in connection with the exemplary embodiments, with reference to an equivalent circuit diagram of such a structure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]      FIG. 1   a  shows a schematic representation, in which a first HF switching element having integrated control voltage decoupling in a top view ( FIG. 1   a ).  
         [0033]      FIG. 1   b  shows a schematic representation, in which a first HF switching element having integrated control voltage decoupling in a section along cutting line I-I in  FIG. 1   a.    
         [0034]      FIG. 2   a  shows an additional high frequency switch in a corresponding view.  
         [0035]      FIG. 2   b  shows an additional high frequency switch in another corresponding view.  
         [0036]      FIG. 3  an equivalent circuit diagram which applies to both high frequency switches as in  FIGS. 1   a  and  1   b , or  FIGS. 2   a  and  2   b.    
         [0037]      FIG. 4   a  to  4   l  show different processing states in the production of a high frequency switch according to  FIGS. 1   a  and  1   b , in each case in a schematic, perspective representation.  
         [0038]      FIG. 5   a  shows a high frequency switch in a top view, which is available from the related art.  
         [0039]      FIG. 5   b  shows a high frequency switch in a section along cutting line V-V of  FIG. 5   a , which is available from the related art.  
         [0040]      FIG. 6  shows an electrical equivalent circuit diagram for the high frequency switch, according to  FIGS. 5   a  and  5   b.   
     
    
     DETAILED DESCRIPTION  
       [0041]      FIG. 1   a  shows a high frequency switch  1 , which includes a piece of a coplanar waveguide  2 . Waveguide  2  has two grounding conductors  3 ,  4  as well as a signal line  5 . Signal line  5  is designed in a region above a connecting element  6  in the form of a bridge  7  (see especially sectional view according to  FIG. 1   a ). High frequency switch  1  is built up on a substrate  8 , on which an insulating layer  9  was first deposited. Thereupon follows connecting element  6  having a connecting pad  10 . Except for one contact place for connecting pad  10 , connecting element  6  is covered by an additional insulating layer  11 . Then there follows, in the structure of coplanar wave guide  2 , a starting layer  12  for respective grounding conductor  3 ,  4  and signal line  5  (not to be seen in the section of  FIG. 1   b ), a comparatively thick layer  13 , which was reinforced galvanically and a cover layer  14 , of which bridge  7  is also formed.  
         [0042]     Now, if, via terminal pad  10 , a voltage is applied to connecting element  6 , electrostatic forces act on bridge  7 , which, in terms of direct current, is at ground potential, which draw bridge  7  to connecting element  6  until bridge  7  lies on insulating layer  11  in the area over connecting element  6 .  
         [0043]     The appertaining electrical equivalent circuit diagram is explained in the light of  FIG. 3 . In this context, the same reference numerals were used as in  FIG. 6 , except for the second mutual capacitance, which in  FIG. 6  has the reference numeral  118 , since in this respect the electrical equivalent circuit diagram does not differ. In  FIG. 3 , the second mutual capacitance is given reference numeral  15 .  
         [0044]     By contrast to the exemplary embodiment and/or exemplary method according to  FIG. 6  or  FIGS. 5   a  and  5   b , second mutual capacitance  15  is fixed in its capacitance. In  FIGS. 1   a  and  1   b  this corresponds to the intersection of connecting element  6  with grounding conductors  3 ,  4 . Inductors  116  and ohmic resistors  115 , for the range of the connecting element, are between signal line  5  and respective grounding conductors  3 ,  4 . The variable mutual capacitance is established by the intersection of bridge  7  with connecting element  6 . During activation via pad  10  in  FIGS. 1   a  and  1   b , one may set, for example, two values, one maximum value and one minimum value of the capacitance. In the equivalent circuit diagram, voltage source  119  is responsible for the electrostatic activation of bridge  7 .  
         [0045]     The equivalent circuit diagram as in  FIG. 3  comes about also for a high frequency switch according to  FIGS. 2   a  and  2   b . However, the high frequency switch according to  FIGS. 2   a  and  2   b  differs quite substantially from the high frequency switch according to  FIGS. 1   a  and  1   b  in that, instead of a longitudinal bridge along signal line  5 , in  FIG. 2 a  lateral bridge  21  is implemented between grounding conductors  3 ,  4 .  
         [0046]     In order to make this possible, high frequency switch  20  has the following construction: On substrate  8  having insulating layer  9 , at first there is not a connecting element, but the line structures of coplanar waveguide  22  having grounding conductors  3 ,  4  and signal line  5 . In the vicinity of bridge  21 , in each case an insulating layer  23 ,  24 ,  25  is provided. This is followed by a vertical member element  26 , in each case on outside grounding conductor  3 ,  4 . Vertical member elements  26  have three layers, as seen in section: First a starting layer  27 , followed by a galvanically grown layer  28 , and covered by a cover layer  29  which, from an electrical point of view, corresponds to connecting element  6 , and from which bridge  21  is formed. Vertical member structure  26  along with bridge  21  may be acted upon with a control voltage via a contact pad  30 .  
         [0047]     What applies for both principles according to  FIGS. 1 and 2  is that mutual capacitance  15  (formed from the respective mutual capacitances of connecting element  6  and vertical member element  26 ) is connected in series with the actual switching capacity  115 , inductance  117  and ohmic resistance  116 , and therewith forms a resonant circuit. If mutual capacitance  15  is selected to be large, compared to switching capacity  115  in the controlled, that is, lower state of respective bridge  7 ,  21 , then, with respect to a resonant frequency of the resonant circuit, the switch behaves like a corresponding switch without integrated control dc voltage decoupling. But if one reduces mutual capacitance  15 , one then obtains an additional degree of freedom for the purpose of shifting the resonant frequency of the resonant circuit towards higher frequencies. Furthermore, because of that, in the non-controlled state, one may reduce the capacitance effective for the high frequency, and thereby particularly also the insertion attenuation, without this going along with an increase in the switching voltage. The attracting force for the bridge comes about from the derivation of the energy stored in the capacitor, with which constant mutual capacitances  15  have nothing to do in this regard.  
         [0048]     For a high frequency switch according to  FIGS. 2   a  and  2   b  there is further the advantage that the length of bridge  21  may be changed independently of the coplanar line geometry by the position of vertical member  26 . This is very important, since in that way a mechanical switching voltage and an inductance may be simply varied. In addition, specific embodiments according to  FIGS. 2   a  and  2   b  also become possible for higher frequencies which, in order to avoid parasitic modes, require very small signal line widths.  
         [0049]      FIGS. 4   a  to  4   l  are intended to clarify the production of a high frequency switch  1  according to  FIGS. 1   a  and  1   b . According to  FIG. 4   a , the starting point is, for example, a high ohmic, p-doped silicon substrate  8  having a thickness of 300 μm. For the insulation of a high frequency component built up onto this, substrate  8  may be thermally oxidized to generate an insulating layer  9 . Up to now, a PECVD layer has a higher damping.  
         [0050]     Subsequently, (see  FIG. 4   c ) a layer of molybdenum-tantalum (MoTa) is applied in a thickness which may be 100 to 400 nm, using a sputtering process. Other metallizing processes are possible too, however, a refractory metal, such as molybdenum-tantalum, should be used. In addition to which, molybdenum-tantalum is comparatively non-noble, and is able to be etched wet-chemically, at the end of the process sequence, selectively with respect to all other metals used. This is especially important for connecting bar  40  for carrying out the electroplating.  
         [0051]     In order to lower the comparatively high resistance of molybdenum-tantalum, especially for the area of the connection between the mutual capacitances, aluminum or a multi-layer system made of aluminum and molybdenum-tantalum may also be used instead.  
         [0052]     In any case, the applied layer is patterned, in order to generate from it connecting element  6 . In the area of later grounding conductors  3 ,  4 , this is made of an area  41  having a predefined size, in order to specify fixed mutual capacitance  15 , of narrow connecting crosspieces  42  to a middle electrode area, by which the coupling to the later signal line is established.  
         [0053]     After that, an insulating layer, e.g. PECVD SiO x  is depositied, for example at 300° (C). Instead of PECVD SiO x , silicon oxynitride (SiON), silicon nitride (Si 3 N 4 ) or another insulator may be used. The insulating layer is also patterned, especially in the area of the connecting bars as well as at a connecting place  43  for a later connecting pad  10  for having a control voltage act upon the high frequency component (see  FIG. 4   d ).  
         [0054]     According to  FIG. 4   e , onto this layer sequence, a starting metallization layer  12  may be sputtered on, for instance, at a thickness of 300 nm (titanium-tungsten, gold or chromium-copper coming into consideration as the metals) and patterned in the shape of the intended waveguide structure, with regard to the grounding conductor and the signal line, which may be by a lift-off process. By the use of the lift-off process, previously applied insulating layer  11  is not affected. With regard to the structure of the signal line, one should note that this is interrupted in the area of electrode  43  (at this location there will follow the later connection by bridge  7  situated above it).  
         [0055]     Besides that, using the starting metallization, supply line  44  to connecting pad  10  is generated.  
         [0056]     Next, there follows the generation of a sacrificial layer  45  and the corresponding patterning according to the structure of the intended grounding connectors  3 ,  4  and control line  5 , the area above electrode  43  for forming the bridge also being covered. Photo-resist, for example, is suitable as sacrificial layer  45 , at a thickness of 3.5 to 4 μm ( FIG. 4   f ).  
         [0057]     Layer  13  is then generated in an electroplating process. As the material for the electroplating process, copper, for instance, is suitable. This process step may be seen in  FIG. 4   g.    
         [0058]     In a further process step (see  FIG. 4   h ), cover layer  14  is generated, together with bridge  7 . For this, for example, aluminum or aluminum-silicon-copper is applied at a thickness of 300 to 800 nm, and is patterned corresponding to the structures of grounding conductors  3 ,  4  and signal line  5 . This means that bridge  7  continues as cover layer in the electroplated area of signal line  5 .  
         [0059]      FIG. 4   i  shows that now sacrificial layer  45  is removed in an anisotropic etching step, such as by RIE O 2  plasma etching, except for the area under bridge  7 .  
         [0060]      FIG. 4   k  shows the process state after which, selective with respect to all other metals, such as in hydrogen peroxide (H 2 O 2 ), the molybdenum-tantalum of connecting bars  40  has been removed. By sacrificial layer  45  being still present under bridge  7 , it is avoided that bridge  7  is affected by this process step.  
         [0061]     As the next to last process step, sacrificial layer  45  is also removed from under bridge  7 , whereby a structure according to  FIG. 4   l  remains, which corresponds to the structure according to  FIGS. 1   a  and  1   b . Removing the sacrificial layer under bridge  7  requires an isotropic etching step which, for example, may be carried out in a plasma barrel etcher in O 2  plasma.  
         [0062]     In comparison to other methods, critical planarization steps or differential etching steps are avoided by the method just described. In particular, the method described represents a solution of the “island problem”:  
         [0063]     In the production of phase shifters, surfaces are to be reinforced by electroplating, which, however, at the end of the production process are electrically insulated from other surfaces. But, for electroplating deposition, all surfaces must be connected to one another in a conductive manner. Therefore, it becomes necessary to remove again these connections in a step after the electroplating deposition. The present technological sequence permits the wet-chemical removal of these connection lines without destroying the micromechanical bridge.