Abstract:
The present invention relates to an arrangement in a multilayered electronic circuit. In a transition between two planar transmission lines, a compensating element is used to keep the average capacitance per length unit more constant during the transition. A via conductor the passes near an edge of a planar conductor pattern, the via conductor and the planar conductor having a mutual capacitive coupling within a predetermined range. A compensating conductor is formed between the planar conductor and the via conductor, which conductor is connected to the planar conductor by a compensating via. If the segment of the via conductor which belongs to the same via hole pattern as the compensating via is displaced, the compensating via is also displaced. The compensating planar pattern is then disconnected from the planar conductor. This improves yield in a given multilayer process.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to an arrangement in a multilayered electronic circuit for radio frequency applications, as well as a method of manufacturing such a circuit and a RF-circuit comprising such an arrangement.  
         DESCRIPTION OF RELATED ART  
         [0002]    Multilayered RF-circuits are used more and more in communications technology and elsewhere. The reason for this is that they allow smaller circuits to be built. One process resulting in multilayered RF-circuits is the LTCC process, an abbreviation for Low Temperature Cofired Ceramics. This process generally involves steps as follows. First holes are punched in predetermined patterns in a number of substrates. The holes are filled with a conductive material. When the circuit is finished, this material forms vias, which provide for vertical transitions within the circuit. Then planar conductive patterns are printed on at least some of the substrates. The pattern may also include different passive and active components that are formed on the substrate. The planar conductive patterns provide horizontal transitions within the circuit. Then the substrates are stacked in a predetermined order and with a predetermined orientation. In a final step, they are fired at a relative low temperature (for instance 850° C.) to form a solid circuit.  
           [0003]    An LTCC circuit normally contains a number of transmission lines. These may be devised as microstrip lines, striplines or even coaxial lines. A microstrip line, for instance, then consists of a planar ground conductor extending in a first plane and a planar signal conductor extending parallel to the ground conductor in a second plane adjacent to the first plane. The ground conductor normally has a wider lateral extension in its plane than the signal conductor has.  
           [0004]    When two microstrip transmission lines, extending in different sets of layers are to be interconnected, this is arranged by means of via conductors. Then a first via interconnects the respective planar ground conductors, while another via interconnects the respective planar signal conductors.  
           [0005]    A problem with this kind of transition is that the capacitance per length unit between the signal conductor and the ground conductor often is lower in the transition region as compared to the horizontally extending transmission line parts. This results in reflections when a signal having a predetermined frequency is propagated in the transmission line towards the transition region. The transition, thus, is not matched and this limits the RF performance, e.g. bandwidth, of the circuit.  
           [0006]    A solution to this problem is to add a compensating element in the transition. This may be arranged in the following manner. If, for instance, the respective ground conductors of two interconnected planar transmission lines are placed above their respective signal conductors, the via interconnecting the signal conductors will pass in the vicinity of one edge of the lower planar ground conductor. If a projection is arranged in this planar conductor, protruding towards the via, this will result in a higher capacitance between the ground conductor and the signal via. Thus, the average capacitance per length unit between the signal conductor and the ground conductor is increased. This results in a circuit with improved RF performance.  
           [0007]    It should be noted, however, in this context that the tolerances of the relative positions of adjacent layers as well as the relative positions of a via hole pattern and a planar conductor pattern within one layer in an LTCC circuit are relatively large.  
           [0008]    This causes a problem with the above-described compensation arrangement. If a small change from the ideal distance between the ground conductor and parts of the signal via exists, the capacitance between them will differ from the intended one. The relation between distance and capacitance is a non-linear one. A relatively small displacement, positioning a segment part of the via conductor a bit closer to the planar ground conductor may render the compensating capacitance far too large. This is likely to reduce the bandwidth of the circuit to such an extent that it is useless for the intended purposes. A transition designed to be fully compensated in the ideal position will therefore have a relatively low yield when produced, i.e. only a relatively small number of circuits in a batch of a given size will function properly. This, of course, makes the circuits very expensive.  
           [0009]    A procedure to deal with this problem is to under-compensate the transition, i.e. to design a ground conductor which, in an ideal relative position between the conductor and the via, has a capacitance somewhat less than the optimal capacitance. This results in higher yield but smaller bandwidth. Thus, a trade-off between yield and RF performance, e.g. bandwidth, exists.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention seeks to diminish the aforementioned problems.  
           [0011]    One object of the present invention is to provide a low cost multilayered RF-circuit.  
           [0012]    Another object is to achieve a multilayered RF-circuit with improved bandwidth.  
           [0013]    Yet another object of the present invention is to achieve a method for producing a multilayered RF-circuit with improved yield.  
           [0014]    The above-mentioned objects are achieved by means of an arrangement in a multilayered circuit, a method for manufacturing such a circuit in accordance with the invention as described below and multilayered RF-circuit comprising a similar compensating arrangement.  
           [0015]    It has been observed by the inventor, that in a circuit manufactured in an LTCC process, the tolerances of the relative positions of different planar conductors within one layer are very small (typically around 5-10 μm) compared to the tolerances of the relative positions of adjacent layers (which are typically around 50-100 μm). The same applies for the relative positions of individual via holes within the via hole pattern in one layer, which also have small tolerances (typically around 5-10 μm). The manufacturing tolerances as regards position of the hole pattern vis-a-vis the position of the planar conductor pattern within one layer, however, is comparable to the tolerances of the relative positions of adjacent layers.  
           [0016]    In accordance with the invention, the compensation in a transition between two planar transmission lines is made more process tolerant by utilising this observation. In an arrangement in accordance with the invention a first planar conductor, which may for instance constitute ground conductor in a planar transmission line, is formed on a first substrate layer among a plurality of substrate layers forming a multilayered circuit. The circuit further comprises a first via hole, formed in the first substrate layer or in a substrate layer adjacent to the first substrate layer on the side of the first planar conductor. The first via hole is filled with a conductive material and may form a segment part of a signal via providing a transition between two transmission lines. The capacitive coupling between the conductive material in the first via hole and the first planar conductor is intended to be within a predetermined range. In accordance with the invention a second planar conductor, which may be called a compensating planar conductor, is formed between the first via hole and the first planar conductor, on the same side of the same substrate layer as the first planar conductor. The surfaces of the first and the compensating planar conductors are formed to be non-intersecting. The first and the compensating planar conductors are interconnected by means of a conductive material disposed in a second via hole, which is formed in the same substrate layer as the first via hole. The second via hole may be called a compensating via hole.  
           [0017]    Assume that the first planar conductor constitutes a ground conductor in a planar transmission line and that the conductive material with which the first via hole is filled forms a segment part of a signal via conductor, which intersects the plane in which the first planar conductor extends at a distance from its nearest edge. In an ideal case, in an arrangement in a circuit as defined above, a certain capacitance, within an intended range, is achieved between the first and second planar conductors on one hand and the intersecting via on the other. The equivalent model in this situation may schematically be described as a single capacitance C. If during the manufacturing of the circuit the hole pattern in question is displaced from its ideal position so that a part of the intersecting via approaches the first conductor, the compensating structure changes as well. This is due to the fact that the compensating via hole also is displaced. At a certain displacement of the hole pattern, the conductive material of the compensating via hole ceases to be in contact with both the first and the compensating conductor. This changes the schematic equivalent model of the compensating structure into two capacitances connected in series. The resulting capacitance of two capacitances connected in series may be written as 1/(1/C 1 +1/C 2 ) . This serves to compensate for the displacement of the segment of the intersecting via. The capacitance between the intersecting via and the planar ground conductor may therefore still be within the intended predetermined range.  
           [0018]    This results in a less expensive circuit, since its fault tolerant qualities provide higher yield in a given manufacturing process.  
           [0019]    It also allows multilayered RF-circuits with improved bandwidth properties to be produced at a relatively low cost.  
           [0020]    A corresponding method may be defined where, in a step when via holes are formed in a layer, a via hole being positioned as said compensating via hole is formed. In another step in accordance with the invention, wherein a conductor pattern is disposed on a layer, a conductor corresponding to the compensating planar conductor as mentioned above is formed.  
           [0021]    This results in a manufacturing process producing multilayered RF-circuits with improved yield.  
           [0022]    An arrangement in a multilayered electronic circuit for radio frequency applications in accordance with the invention is then characterised as it appears from claims  1  or  10 .  
           [0023]    A method of manufacturing such a circuit in accordance with the invention is then characterised as it appears from claim  7 . A multilayered RF-circuit comprising a compensating arrangement is further characterised in claim  10 .  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0024]    [0024]FIG. 1 illustrates some steps in an LTCC process.  
         [0025]    [0025]FIGS. 2 a  and  2   b  show a vertical transition between two transmission lines of the stripline type according to known art.  
         [0026]    [0026]FIGS. 3 a ,  3   b ,  3   c  and  3   d  illustrate schematically compensating arrangements for a transition between two transmission lines in a multilayer structure.  
         [0027]    [0027]FIGS. 4 a  and  4   b  illustrate a compensating arrangement in accordance with the invention for a transition between two transmission lines.  
         [0028]    [0028]FIG. 5 a  illustrates schematically another compensating arrangement in accordance with the invention for a transition between two transmission lines. FIG. 5 b  illustrates a via conductor.  
         [0029]    [0029]FIG. 6 illustrates another compensating arrangement in accordance with the invention for a transition between two transmission lines.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]    [0030]FIG. 1 illustrates some steps in an LTCC process. The substrate sheets  101  used in the process are preferably made of a thin dielectric ceramic material. In a first step, holes  102  are punched in predetermined patterns in these substrates  101 . These holes  102  are filled with a suitable conductive material, such as for instance gold. Then conductors  103  are printed on one or in some cases two sides of the substrate by using a screen printing process. Components  104  such as resistors may also be disposed on the substrate  101 . In circuits operating at microwave frequencies, components such as capacitances and filters of different kinds may often be formed by different configurations of planar conductors.  
         [0031]    When all substrate sheets are prepared they are stacked into a pile  105 . The pile  105  may include up to more than 40 layers, but 10 is considered a more normal number. The pile  105  may be laminated and preheated before it is fired in an oven at around 850° C., typically for about 2 hours. After the firing the vias formed by the conductive material in the via holes interconnect different layers in the circuit. Vias may also be formed which extend through more than one layer, i.e. including plural segments.  
         [0032]    [0032]FIGS. 2 a  and  2   b  show a vertical transition between two transmission lines of the stripline type according to known art. A first view is shown in FIG. 2 a . A first transmission line  201 ,  202 ,  203  consists of an upper ground conductor  201 , a signal conductor  202  and a lower ground conductor  203 . These conductors are each printed on a respective substrate layer  207 . A second transmission line  204 ,  205 ,  206  including an upper ground conductor  204 , a signal conductor  205  and a lower ground conductor  206  extends vertically offset from the first transmission line. In the depicted case, the first  201 ,  202 ,  203  and second  204 ,  205 ,  206  transmission lines are printed on non-intersecting sets of layers. This is however not necessary, the signal conductor  202  of the first transmission line and the upper ground conductor  204  of the second transmission line could for instance be printed on the same substrate layer.  
         [0033]    A first number of vias  208  interconnect the upper ground conductors  201 ,  204  of the first and second transmission lines. A second via  210  interconnects the signal conductors  202 ,  205  of the first and second transmission lines. A third number of vias  209  interconnect the lower ground conductors  203 ,  206  of the first and second transmission lines. This is seen more clearly in FIG. 2 b . For clarity reasons, the proportion of the vias are slightly changed in FIG. 2 b . The thickness of a substrate layer is often less than twice as large as the diameter of a via hole.  
         [0034]    The transmission lines shown in FIG. 2 b  are stripline transmission lines, each having upper  201  and lower  203  planar ground conductors with an interposed planar signal conductor  202 . The lateral extension  211  of the signal conductor  202  is normally less than the lateral extensions  212  of the respective ground conductors  201 ,  203 .  
         [0035]    A microstrip transmission line (not shown) could be obtained by taking away the upper or the lower ground conductor of one of the transmission lines shown in FIG. 2 b . A quasi-coaxial transmission line (not shown) could be obtained by interconnecting the upper and lower ground conductors of a transmission line, by means of vias. The vias are then placed at regular distances at the lateral sides of the ground conductor. The invention described herein works with stripline, microstrip and quasi-coaxial transmission lines.  
         [0036]    [0036]FIGS. 3 a ,  3   b ,  3   c  and  3   d  illustrate compensating arrangements for a transition between two transmission lines in a multilayer structure. The arrangement depicted in FIG. 3 a  includes a signal conductor  301 . A signal via  305  connects the signal conductor  301  to another planar signal conductor (not shown), situated in another layer. A planar ground conductor  302  is printed on a lower layer in the structure compared to the layer of the signal conductor  301 . Ground vias  306  connect the ground conductor  302  to another ground conductor (not shown) higher up in the structure. The signal via  305  intersects the plane in which the planar ground conductor  302  extends. A compensating element  303  is formed as a projection in the ground conductor. The compensating element  303  projects towards the area where the signal via  305  intersects the plane in which the planar ground conductor  302  extends. In an ideal case, for instance as depicted in FIG. 3 a , the compensating element serves to raise the capacitance between the planar ground conductor and the signal via  305  to a suitable level which improves the transmission properties of the transition.  
         [0037]    Due to the manufacturing tolerances of an LTCC process, however, the corresponding transitions in a number of circuits of a batch with a given size are likely to look similar to the transition shown in FIG. 3 b . In this transition the via hole pattern in the layer immediately above the layer on which the planar ground conductor  302  is printed has been moved to the right. At least this segment part of the signal via  305  has thus left its intended position  307 . Other parts of the via, i.e. parts which are built from material disposed in corresponding via holes in other layers, may still be situated near the ideal position. The same applies to the ground vias  306 .  
         [0038]    At least a part of the signal via  305  is thus situated closer to the planar ground conductor  302  and its compensating element  303  than was intended. This results in a higher capacitance between the signal via and the planar ground conductor than was intended. Since the relation between distance and capacitance is a non-linear one, a small move of the via hole pattern in question may render the capacitance far too large. This may be very detrimental to the transmission properties of the transition. Therefore in most cases the transitions have been under-compensated in order to increase the yield of the LTCC process. This, of course, also reduces the RF performance, e.g. bandwidth, of the circuit.  
         [0039]    In FIGS. 3 c  and  3   d , the identical problem as shown in FIGS. 3 a  and  3   b  is illustrated with a different compensating structure  303 .  
         [0040]    [0040]FIGS. 4 a  and  4   b  illustrate a compensating arrangement in accordance with the invention for a transition between two transmission lines. The transmission line involves a planar signal conductor  401 , to which a signal via conductor  405  is connected. As in FIGS. 3 a  and  3   c , ground via conductors  406  are connected to a planar ground conductor  402 . In accordance with the invention, a compensating planar conductor  404  is printed on the same substrate as and close to the planar ground conductor  402 . The compensating planar conductor  404  is printed in the vicinity of the area where the signal via  405  intersects the plane in which the planar ground conductor  402  and the compensating planar conductor  404  extend. The planar ground conductor  402  and the compensating planar conductor  404  are non-intersecting, there is a minimum distance between their edges which is larger than zero, but smaller than the diameter of a via hole.  
         [0041]    In an ideal case the planar ground conductor  402  and the compensating conductor  404  are interconnected by a compensating via  409  consisting of the conductive material, with which a compensating via hole is filled. This via is formed in the layer in which the planar ground conductor  402  is printed or in an adjacent layer on the side of the planar ground conductor  402 . This compensating via  409  makes the compensating arrangement more tolerant of displacement of the segment of the signal via  405  which is situated in the same layer as the compensating via  409 . The compensating planar conductor  404  may be formed to surround the signal via  405  in a circular segment manner. In this position the schematic equivalent model of the compensating structure may be written as a single capacitance C.  
         [0042]    If during manufacturing of the circuit a displacement of a via hole pattern takes place, similar to the ones which take place between FIGS. 3 a  and  3   b  or  3   c  and  3   d , the relevant via hole pattern is displaced as is shown in FIG. 4 b . A part of the signal via  405  is then situated closer to the planar ground conductor  402 . In an arrangement according to the invention however, the topology of the compensating arrangement has also changed. The compensating via  409  belongs to the same via hole pattern as the displaced segment of the signal via  405 . Therefore, the compensating via  409  is also displaced, which disconnects the compensating planar conductor  404  from the planar ground conductor  402 . The schematic equivalent model of the compensation can therefore be described as two capacitances connected in series, which compensates for the displacement of the signal via segment towards the planar ground conductor. The capacitance of the compensating arrangement may thus still be within the intended range even though a certain displacement of a signal via segment has taken place.  
         [0043]    [0043]FIG. 5 a  is a three-dimensional view of a transmission line transition in accordance with the invention. A first transmission line is shown, including upper  501  and lower  503  planar ground conductors as well as a planar signal conductor  502 . The first transmission line extends in a first set of layers in a multilayered RF-circuit. A second transmission line includes upper  504  and lower  506  planar ground conductors as well as a planar signal conductor  505 . The second transmission line extends in a second set of layers in a multilayer RF-structure. The respective upper and lower planar ground conductors, as well as the planar signal conductors of the first and second transmission lines are interconnected by means of via conductors  508 ,  509 ,  510 , which extend in a direction substantially perpendicular to the planes in which the planar conductors extend. The via conductor  510 , interconnecting the respective planar signal conductors  502 ,  505 , intersects the plane in which the upper ground conductor  504  of the second transmission lines extends. The via conductor  510  is not in contact with the upper ground conductor  504 .  
         [0044]    A compensating arrangement  511 ,  512  is arranged between the upper ground conductor  504  of the second transmission line and the signal via  510 . The compensating arrangement consists of a compensating planar conductor  511  and a compensating via  512 . If the part  513  of the signal via  510  which belongs to the same via hole pattern as the compensating via  512  is moved during manufacturing, the compensating via is moved as well. This serves to disconnect the compensating planar conductor  511  from the relevant planar ground conductor  504 . As is shown in FIG. 5 the compensating planar conductor  511  may be formed, at least partly in a recess in the relevant planar ground conductor  504 .  
         [0045]    In FIG. 5 a , for clarity reasons, a compensating conductor in accordance with the invention has been placed between the upper ground conductor  504  of the lower transmission line and the signal via only. Such arrangements may however also be used, for instance, between the signal via  510  and the lower ground conductor  503  of the first transmission line or between the planar signal conductor  502  of the first transmission line and the signal vias  508 , interconnecting the upper planar ground conductors  501 ,  504  of the first and second transmission lines.  
         [0046]    As in FIG. 2 b  the proportion of the vias are slightly changed in FIG. 5 a . A via conductor with more appropriate proportions is shown in FIG. 5 b . The via is then built up from four via segments  515 ,  516 ,  517 ,  518 . The segments are slightly displaced vis-à-vis one another during manufacturing.  
         [0047]    [0047]FIG. 6 illustrates another compensating arrangement in accordance with the invention for a transition between two transmission lines. A planar signal conductor  601  is connected to a corresponding planar signal conductor (not shown), which is situated in another layer, by means of a signal via conductor  603 . The signal via conductor  603  intersects the plane in which a planar ground conductor  602  extends, in the vicinity of one of its edges. A variable compensating arrangement  604   a - 604   g ,  605   a - 605   g  is arranged between the planar ground conductor  602  and the via  603 . This is used to compensate for a displacement from the ideal position of a substrate segment of the via conductor  603  vis-a-vis the planar ground conductor  602 . The compensating arrangement consists of a number of planar compensating conductors  605   a - 605   g , which are non-intersecting with the planar ground conductor  602 , but which extend in the same plane. The compensating conductors  605   a - 605   g  are placed between the planar ground conductor  602  and the signal via conductor  603  and form together a half-circular pattern. Each of the compensating conductors  605   a - 605   g  are, in an ideal relative position of the relevant segment of the via  603  and the planar ground conductor  602 , connected to the ground conductor by means of a corresponding compensating via  604   a - 604   g . The compensating vias  604   a - 604   g  consist of conductive material disposed in via holes in the substrate in which the relevant segment of the signal conductor via  603  for which process tolerance is to be achieved extends.  
         [0048]    The compensating arrangement as depicted in FIG. 6 is capable of compensating for movements of the via hole pattern in relation to the relevant planar conductor pattern in two dimensions. If the via hole pattern is moved upward during manufacturing, the segment of the signal conductor via  603  approaches the upper planar compensating conductors  605   a ,  605   b . In this case one or both of these conductors may be released by their respective compensating vias  604   a ,  604   b , while other compensating conductors are still connected to the planar ground conductor  602 . A movement to the right in the figure of the via hole pattern during manufacturing may cause the centre compensating conductors  605   c ,  605   d ,  605   e  to be released by their respective compensating vias  604   c ,  604   d ,  604   e . The lower planar compensating conductors  605   f ,  605   g , for instance, on the other hand, may still be connected to the planar ground connector  602  by the compensating via  604   g.    
         [0049]    It should be noted that the scope of the invention is not limited to the embodiments described above. Various changes may be made without departing from the scope of the appended claims. For instance, process tolerance may be achieved separately for a via segment constituted by a filled via hole in the substrate on which a planar conductor is printed and a via segment constituted by a filled via hole in the substrate adjacent to the substrate on which the planar conductor is printed on the side of this conductor. These via segments may both belong to the same via conductor, but their tolerances could be independent.  
         [0050]    It is also possible to make the compensation stepwise variable by providing multiple compensating conductors. Then, if a first movement of a via hole pattern in certain direction takes place during manufacturing only a first compensating conductor is released by its corresponding via, while a second compensating conductor is still connected to a planar ground conductor. If a second movement takes place, which is larger than the first movement, then also the second conductor is released.  
         [0051]    The invention is useful also in transitions in multilayered RF circuits, where the transition in itself is not undercompensated. One such an example is where limitations in size render the transition itself over-compensated.  
         [0052]    The compensating via of a compensating arrangement in accordance with the invention may in the same time be used to connect a planar conductor on a first layer to a planar conductor on another layer.  
         [0053]    The invention is also applicable to other processes than the LTCC process, such as the LTTT process, where LTTT is an abbreviation for Low Temperature Transfer Tape.