DE102005037950B3 - Simplified construction of microwave circuits in LTCC technology with fewer through holes - Google Patents

Abstract

The method involves providing electromagnetic fields or dielectric displacement currents by special circuit techniques such as the use of redundant leads or laterally overlapping planar conductive structures to build up ceramic layers to form the LTCC circuit structure. The planar conductive structures lie in different planes of the structure. A fine mesh earth plane with no more than 50 per cent metal is provided between the planar structures to be coupled together.

Description

One Method for simplified construction of microwave circuits in LTCC technology with reduced number of through holes.

Multilayer ceramic circuits are increasingly being used for Microwave applications in LTCC technology (low temperature cofired ceramic) realized. The LTCC technology is a ceramic multilayer printed circuit board technology and stands for a ceramic substrate system that is considered inexpensive in electrical engineering Substrate technology is used, with ten to fifty layers on top of each other can be stacked. The tracks are usually from highly conductive, burned silver or gold conductive pastes. The Metallizations are first as metallic conductive pastes screen-printed on layer by layer the unfired ("green") ceramics printed and after stacking and pressing the multi-layered construction together burned in the process furnace (cofired). The sintering temperature of the LTCC Glass ceramic is at approx. 850 ° C. This relatively low temperature only allows the use of good conductive gold and silver conductors instead of bad conductive molybdenum or tungsten conductor tracks, which in connection with the HTCC technology (high temperature cofired ceramic) must be used. These procedures are already known for some time.

In a patent document from the year 1997 ( DE 197 07 253 C2 ), the technological details of the conductor structures made of burned silver or gold conductive pastes are already being discussed extensively.

in the Compared to conventional Ceramic products whose production requires sintering temperatures of about 1300 ° C (HTCC), the LTCC technology comes out with about 850 ° C, which in the production incurred energy consumption is significantly reduced.

Not most recently because of the materials used is in the LTCC technology a big Potential for Microwave applications. Many suitable for the microwave range Circuits are already known. So are simple circulator circuits (US 2003/0090335 A1), field effect transistor mixer (US 2003/0151136 A1) or complex tunable filter structures (US 2004/0178867 A1) in LTCC technology. Both the ceramic substrates as well the gold and silver conductors have correspondingly good physical properties and electrical properties, which for the required high grades for low-loss Circuits in the microwave range are required. Furthermore the material and manufacturing costs are competitive with others Substrate systems such as HTCC and conventional printed circuit board technology (PCB: printed circuit board) z. B. on FR4 or PTFE (polytetrafluoroethylene) substrates.

A method for simplified construction of microwave circuits is known from the device for transmitting broadband high-frequency signals ( DE 10 2004 038 574 A1 ) known. There coplanar lines are used, wherein the conductor structure is arranged on two opposite sides of at least one dielectric substrate layer of a predetermined thickness such that the conductor structure overlaps in predetermined coupling regions, whereby the coupling regions of the conductor structure transmit the high-frequency signals by an electromagnetic coupling. The device is characterized in that the thickness of the substrate layer is smaller than a quarter of the wavelength and that a plurality of electromagnetic couplings are arranged serially one behind the other.

Further Characteristic features are that a stepless characteristic impedance transition in a transition area the symmetrical coplanar line on a microstrip line is designed and that between selected sections of the conductor track structure a via is attached, which has a conductive signal transition between the selected sections the conductor structure causes.

In This device will therefore already the electrical coupling the respective circuit components serving electromagnetic fields or dielectric displacement currents used. Serve with special circuit measures like the use of at the end of empty running lines connected to produce an electric stray field at the end, the use of in different levels of laterally overlapping planar conductor structures, which form mutually coupling capacities and the use of resonant coupler structures and resonant ones Coupling lines at localized places directly and predominantly perpendicular through the substrate layers used for the construction, wherein the respective planar conductor structures to be coupled in different Layers of the circuit structure lie. The whole setup is like that also possible in LTCC technology.

Because of the predominant structure of the device in Koplanartechnik the respective ground planes (called reference paths here) are arranged laterally symmetrical to the signal path. As a result, there is no closed ground plane between the signal paths in the overlap area. Also, the different ground planes are without defined reference potential and only capacitively coupled. This is easily possible because no sensitive and complex electronic microwave circuit is integrated, but it is only a purely passive coupling connection between z. B. a coaxial line and a conductor track structures in microstrip technology (microstrip) is, the respective ground planes are to be isolated by this device so straight from each other.

The Galvanic isolation with good transmission properties at the same time Frequencies greater than 6 GHz is the task of this described device.

in the Range of single-ended microstrip technology (microstrip) the device does not see any overlap in front. At the transition between the coplanar technique and the unbalanced microstrip technology Rather, the device sees an ordinary galvanic connection in the form of a metallized through hole (via) in front of a to ensure broadband connection.

1 shows an example (Sample Production Line) for the typical manufacturing process of an LTCC circuit. The starting material for the ceramic layers before firing is still a flexible film (Greenscheet), which is supplied on rolls with different layer thicknesses of 250 microns down to 50 microns. The pieces suitable for the respective application are cut to size (preprocessing), cleaned, dried (blanking, drying) and prepared. For galvanic contacting of the individual layers are then in the slides z. B. by means of a laser beam device through holes (vias) drilled (via forming), which are filled in an additional process step with an electrically conductive paste (via filling). In a further process step, thick film pastes for the conductor tracks are applied to the ceramic films (conductor printing, fine line process). With the aid of resistor pastes, dielectric or ferrimagnetic pastes, further components such as resistors, capacitors and coils ( EP 0 581 206 B1 ) will be realized. The individual films are then inspected again under the microscope (inspection) and then stacked very precisely, laminated (laminated) together and finally fired together (cofiring) to give a hermetically sealed rigid block.

After that find further final processing steps (post-firing, dicing), the assembly (SMT) and contacting with hybrid components (wire bonding) as well as a visual inspection (inspection) before the finished Measured the LTCC circuit with regard to its electrical properties (RF characterization) becomes.

These short explanation The LTCC technique shows that the essential and complex process steps So in connection with the galvanic contacting of the tracks in the different levels through the filled with conductive paste and thereby metallized through holes (vias) occur. A Reducing the number of these through holes would thus simplify manufacture.

It shows 2 a cross-section through an exemplary selected LTCC conductor track structure with four layers. The conductors shown in black are interconnected by means of metallized through-holes (vias). These tracks can be located both on the always accessible upper and lower outer levels and in the inaccessible after firing intermediate levels. Through holes that are inaccessible after firing are referred to as buried vias.

Next the production engineering effort for the numerous processing steps for the targeted, accurate production and production of the with conductive Paste filled and thereby metallized through holes (vias) there is one more another serious disadvantage. The through holes (vias) require a relatively large Volume. For one thing must the through holes even enough be great. This is how the smallest diameter becomes typical for the standard process with 150μm specified. On the other hand, the contact surface of the through holes (via cover pad) enough be great. Here is for the standard process requires about twice the hole diameter, So about 300 microns, at least (Special process) but 250 microns. The minimum distance of the centers of two through holes from each other (via pitch) is with 400 μm to take into account.

This, for the Realization of the necessary for galvanic contacting with filled with conductive paste and thereby metallized through holes (vias), necessary space can not for be used for other functional purposes and therefore leads to a regular drastic enlargement of the electronic circuit.

The object of the invention is to reduce the number of through holes (vias) filled with conductive paste and thereby metallized, despite the use of a coherent ground plane (ground) as in the case of predominant realization of printed conductor structures in single-ended microstrip technology (microstrip) or for electrical screening tion of the electronic microwave circuit (ring mixer, microwave power amplifier, microwave oscillator circuit) from the radiation field of the antenna array is required to significantly reduce and thereby reduce the one hand, the manufacturing cost and on the other hand, the spatial dimensions of the realized microwave circuits.

These Task is achieved by the features listed in claim 1 solved.

The particular advantages of the invention are that on a big one Number of through holes (vias) can be dispensed with. In terms of production technology, this means that the process steps for the creation of the through holes (via forming) and for filling the through holes (via filling). Furthermore, this means that the Process step to control the through-holes (inspection) is shortened, which saves time and money. Another, whole significant advantage is that the dimensions of the LTCC circuit with the same functionality can be significantly reduced because that did not consume from the through holes Volume occupied by circuitry relevant components can be. An embodiment of the The invention is illustrated in the drawing and will be described below described in more detail.

It demonstrate

3 1 shows the cross section through an LTCC circuit structure as an exemplary embodiment for the coupling of resonant strip conductor structures using the example of a microwave strip filter structure;

4 a scale top view of the coupling region of a microwave trace filter structure with the typical dimensions of a cover plate metallization for a through hole,

5 the cross section through an LTCC circuit structure as an embodiment of the layer structure for the coupling of non-resonant interconnect structures using the example of the coupling of a microwave power amplifier to a microwave oscillator circuit,

6 the plan view of some expedient designs of coupled non-resonant interconnect structures,

7 the cross-section through an LTCC circuit structure as an embodiment for the coupling of a non-resonant interconnect structure with a resonant track structure using the example of a connected to a microwave power amplifier (PA) antenna arrangement and

8th the top view of the coupling region of a non-resonant track structure with a resonant planar antenna array.

to Realization of the interconnect structures in unbalanced microstrip technology (microstrip) or for electrical shielding of the electronic Microwave circuit (ring mixer, microwave power amplifier, microwave oscillator circuit) from the radiation field of the antenna arrangement becomes a ground plane (ground) needed, to achieve a defined potential with the help of metallized Through holes (vias) with the ground line of the electronic microwave circuit is galvanically connected. The key to the invention presented here lies in the very low shielding attenuation of a ground plane in LTCC technology. In contrast to shielding housings whose Wall thickness metallization in the millimeter range, are already the continuous copper surfaces only 35 μm thick in PCB technology. In the LTCC technique, however, the metalizations of the Grounding levels typically reached only layer thicknesses of about 10 microns. In addition, the grounding levels in the LTCC technology by no means Completely are closed surfaces, but rather formed from a fine-meshed network of tracks whose metallization total less than 50% of the covered area is. A large Part of the ground plane thus consists of non-conductive ceramic. This is required to detachments the respective ceramic layers to avoid each other, because a Completely continuous metallization would as intermediate layer separate the ceramic layers. These could then not merge together and it could be in a continuous Metallization form lateral cracks, which replace the lead to individual levels would. While in multi-layer PCB technology, the layer thickness of the individual layers about 500 microns is, can the layer thicknesses of the individual layers in LTCC technology already in the Standard Process up to 40 μm (after firing) are reduced. Due to the thin metallization, the low coverage the metallization and the small distances between the individual levels is the screen attenuation a ground plane in LTCC technology very low.

Also is the value for the permittivity The ceramic with a value of 7.8 almost twice as high as at a FR4 substrate material the PCB technology.

It So it makes sense, whenever in a microwave circuit dielectric displacement currents flow have to, to let them flow through the ground plane, d. H. the special ones Conditions of the ground plane in LTCC technology for the Realization of circuits to use to avoid avoidable through holes.

The layer structure for a typical microwave filter structure, constructed from planar resonant lines in unsymmetrical microstrip technology (microstrip), shows 3 as an exemplary embodiment of the realization of an inventive LTCC microwave circuit with a reduced number of through holes. The microwave filter exemplified here consists of a metal feeding line (filter 1) located on top of the LTCC circuit, a buried metal resonant line (filter 2) located in the middle of the ceramic and a discharging metallic line (filter 3) located on the bottom of the LTCC circuit. These lines are created by firing the applied with the help of a screen printing conductive paste and completely solid. Between these lines are the two ground planes (Ground 1 and Ground 2). These ground planes are not completely solid, but they have fine meshes to ensure the secure adhesion of the respective ceramic layers (Layer 1 with Layer 2 or Layer 3 with Layer 4) after firing. The on the upper ceramic level (layer 1) idling feeding line (filter 1) has at its open end an electrical stray field. In conventional microwave filter structures, which are constructed from planar resonant lines in asymmetrical microstrip technology in a plane, the resonant line to be coupled would likewise be arranged on the upper ceramic level (layer 1) at some distance from the feeding line (filter 1). wherein the distance provides for the necessary decoupling, so that the resonance line is not damped too much. This distance is in the inventive structure of the filter according to 3 saved in lateral terms, which reduces the dimensions of the filter. In this filter structure described by way of example, the resonance line (filter 2) is located between the second ceramic plane (layer 2) and the third ceramic plane (layer 3). In this case, the respective interconnects can even be arranged overlapping, because they are indeed in different levels. Between the first ceramic level (Layer 1) and the second ceramic level (Layer 2) is the first ground plane (Ground 1). This ensures the necessary damping of the stray field of the on the upper ceramic level (Layer 1) idling feeding line (Filter 1). Also arranged between the second ceramic layer (Layer 2) and the third ceramic layer (Layer 3) resonant line (Filter 2) is idle at both ends and has there stray fields. The stray fields of the respective tracks couple through the ground planes, while experiencing some attenuation, which is desirable. Analogously, as already described in connection with the feeding line (filter 1), there is also a coupling of the stray fields of the resonance line (filter 2) and the discharging line (filter 3).

The in the afferent Line (Filter 1) supplied Electromagnetic energy thus passes through the stray fields of the respective at the end idle lines through the ground plane (Ground 1) across the resonance line (filter 2) again through a ground plane (Ground 2) on the laxative Line (filter 3). But only those frequency components of the Electromagnetic energy transmitted to a simple or Multiples of the resonant frequency of the resonant line, d. H. the electrical or physical length of the resonant line (Filter 2) determines the transmitted Frequency spectrum, because the stray field at the end of the resonant line in case of resonance by the voltage overshoot on the line especially pronounced is.

The microwave filter structure described here by way of example can be expanded as desired. It usually makes sense to use multiple resonant lines. These can either be in ever new ceramic levels, or be returned to the top ceramic layer. Thus, for example, a microwave filter structure is conceivable in which three resonant lines are used. Accordingly, the in 3 shown laxative line (filter 3) may be another resonant line, which is coupled to a further, third resonant line between the second ceramic level (Layer 2) and the third ceramic level (Layer 3). This would in turn be connected to a laxative line located on the upper ceramic layer (Layer 1). The resonant cables would not have to be stretched, but could be circular or U-shaped. As a result, the feed line and the drain line at the top ceramic level (layer 1) could be only a few millimeters apart, and the entire areal and spatially extended microwave filter structure would be buried and invisible inside the LTCC circuit structure.

3 shows that not a single connection hole is needed to build the previously described microwave filter structure. Which area can be saved thereby is illustrated 4 , This illustration shows the plan view of two end-to-end planar resonant conductors gene.

It is in 4 To scale, on the one hand the width of a resonant line (planar line) with a characteristic impedance of about 50 ohms and on the other hand, the dimensions of a circular cover plate metallization (via cover pad) for a through hole (via) shown. At a ceramic level height of 40 μm after firing, a permittivity of 7.8 and a track thickness of approximately 10 μm, a track width of about a resonant line (planar-tuned line) with a characteristic impedance of about 50 ohms results 40 μm. The diameter of a circular cover plate metallization (via cover pad) for a through hole (via) is at least about 250 μm as standard. It is easy to see that a galvanic connection or - when using a dielectric paste - a capacitive connection of the upper resonant line (planar-tuned line) with the buried planar (line) through the ceramic layer through Relation to the resonant lines would take a lot of volume.

The Coupling of interconnect structures is also useful for non-resonant circuit structures. In particular, if repercussions the individual active circuit components with each other low to be held, a low coupling is desired. So may, for example, the retroactivity a modulated transmitter output stage or one for modulation (direct sequence spread spectrum) used with digital signals ring mixer to the microwave oscillator circuit in no case lead to power or frequency dips. Often, therefore, one is forced the oscillator signal first to reinforce and then with the help of attenuators to weaken again about the repercussions to keep the microwave oscillator circuit low.

In such cases, the in 5 illustrated inventive construction of the coupling of a microwave power amplifier to a microwave oscillator circuit. The exemplary layer structure shows a planar transmission line (line 1) connected to the oscillator, which is located on top of the ceramic layer (layer 1) of an LTCC circuit. The planar transmission line (line 2) connected to the microwave power amplifier is located on the underside of the ceramic layer (layer 2). Between both ceramic levels, the ground plane (ground) is arranged. So that the non-resonant planar transmission lines can be coupled sufficiently intensively with one another, an overlapping of the two lines is generally necessary here.

This overlap is in 6 clearly recognizable. 6 shows the top view of some expedient designs of coupled non-resonant interconnect structures. The outlines of the tracks on the top of the ceramic level are drawn through and the outlines of the tracks on the bottom of the ceramic level shown in phantom. Conceivable and appropriate forms are described below. A simple overlap of the ends of two planar lines requires the smallest area. The end of the planar transmission line (line 1) located on the upper side of the ceramic layer (layer 1) of an LTCC circuit, and the end of the planar transmission line located on the underside of the ceramic layer (layer 2) of an LTCC circuit (FIG. line 2) cover the respective other line. As a result, the conductor tracks act like capacitor plates. The ground plane (ground) forms with respect to the signals to be transmitted only one equipotential plane in the middle of the coupling capacitor thus formed. The dielectric of the LTCC ceramic additionally acts capacitively. Coupling capacities in the femtofarad range can be achieved, which is completely sufficient for many applications in the microwave range. An increase in the capacitance value can be achieved by increasing the overlapping area. Thus, the ends of the two exemplified planar transmission lines (line 3) and (line 4) are designed circular. This increases the overlapping area. Alternatively, the ends can also be designed rectangular. Such an embodiment show the planar transmission lines (line 5) and (line 6). Here, the achievable values for the coupling capacity are directly proportional to the overlap area.

An exemplary embodiment for the coupling of a non-resonant interconnect structure with a resonant interconnect structure is disclosed in US Pat 7 shown by the example of a connected to a microwave power amplifier (PA) antenna arrangement. For example, the non-resonant feed line connected to the microwave power amplifier (PA) is routed to the bottom of the ceramic layer (layer 2) of an LTCC circuit. The resonant planar antenna array is then located on top of the ceramic layer (layer 1) of the LTCC circuit. In between, according to the standard process, a mesh-like closed ground plane (ground), which has no additional openings or apertures. The feeder line running at its end extends over a certain length parallel to the antenna line, which in turn is connected in an electrically conductive manner to the actual planar radiator elements and forms an overall resonant unit with them.

The 8th shows the top view of the coupling region of the located on the bottom (contour shown in phantom) non-resonant feed line (feed line) with the located on the top (contour shown in solid) resonant planar antenna array (antenna). The output of the microwave power amplifier (PA) is shown in this drawing as a schematic source only. The electromagnetic coupling mechanism is similar to that used in directional couplers or standing wave bridges. Also in this case, the ground plane (ground) again forms an equipotential plane, which is arranged substantially symmetrically in the electromagnetic coupling field of the planar lines and therefore causes relatively low attenuation.

Claims (1)

Method for simplified construction of microwave circuits in LTCC technology with reduced number of through holes, where for the electrical coupling of the respective circuit components serving electromagnetic fields or dielectric displacement currents by special circuit measures like the use of end-to-end pipes attached to her End create an electric stray field, the use of in different levels of laterally overlapping planar conductor structures, the mutually coupling capacities form and the use of resonant coupler structures and resonant coupling lines at localized places directly and mostly vertically the ceramic layers used to construct the LTCC circuit structure guided with the respective planar conductor structures to be coupled lie in different levels of the LTCC circuit structure and a finely meshed ground plane, the maximum 50% metal covering, between these respective zuppelnden planar conductor structures is arranged.