WO1998009341A1 - Improved isolation in multi-layer structures - Google Patents

Abstract

A multi-layer circuit structure with selectively disposed ground planes, selectively disposed signal lines for transmitting signals of a given wavelength at a first level and a second level, dielectric material disposed between each of said ground planes and said signal lines, selectively disposed transition regions for electrically connecting one of the signal lines at said first level to one of said signal lines at said second level, said transition regions having selectively located vias for interconnecting said ground planes, has said vias of a first transmission line being spaced at a distance of other than multiple half wavelengths of said signal relative to said vias of a second transmission line.

Description

IMPROVED ISOLATION IN MULTI-LAYER STRUCTURES

The present invention relates to methods of improving isolation between signal lines in multi-layer boards .

Multi-layer structures are used quite frequently in microwave, rf and other high frequency applications. The structures have a great deal of advantage because of their ability to reduce the overall area of circuit board structure which has clear benefits in the wireless industry where size considerations are of the utmost importance. Additionally, multi- layer boards have been accepted in the industry because of relatively low cost and good performance. However, isolation of neighboring transmission lines remains a great problem in the high frequency multi-layer structure.

A typical multilayer board structure in cross - section is shown in Figure 1. In this particular structure, a lower and upper ground plane 101 enclose selectively located signal lines 102, 103 and ground layers 104, 105 and 106. This structure has improved isolation characteristics which are desired provided the ground layers 104, 105 and 106 are maintained at a good ground level. Clearly, the isolation between the rf signal lines can be reduced significantly when the ground lines 104, 105 and 106 are floating. To this end, the reduction in isolation can be as great as 65 dB if the grounds float. The lines 104, 105, and 106 are therefore connected to ground by using metal coated via holes. One approach suggests placement of numerous vias that are small in diameter and are placed as close as possible. .Another approach suggests placement of vias with separation less than a quarter wavelength at the frequency of interest. However, this is insufficient to provide adequate isolation and is sometimes counter productive. Additionally the importance of via inductance to isolation degradation is not understood. Additionally, it is often necessary to have signal line transitions between layers of the multi- layer board which can adversely affect the isolation between the signal lines. For example, a typical cross-sectional view of a signal line in a transition between levels is shown in Figure 2. The uppermost and lowermost ground planes for the transition in Figure 2 are shown as 201 and 202. Additionally, an intermediate ground plane within the board for purposes of isolation is shown in Figure 2 as 203 and 204. The signal line 205 makes a transition from one level to another for various and sundry purposes. This transition can be problematic from the standpoint of isolation. In this transition, a neighboring signal line to 205 could readily couple signal line 205 if the grounding transition between the layers 201, 202, 203 and 204 proves inadequate. Accordingly, it is desired to have a better ground capability at levels 203, 204 in order to effect good isolation between signal line 205 and a neighboring signal line not shown. To be clear, the discontinuities at transitions result in the creation of other modes at the transition which can couple to neighboring signal lines.

As the overall performance of the system is hindered by poor isolation of signals at a transition, it is imperative to have a board which has improved isolation. While vias are used for grounding purposes, there is a need to effect proper grounding. Accordingly, what is desired in an improved isolation scheme in multi-layer rf and other high frequency transmission line circuit boards in which the isolation is maintained at an acceptable level.

The present invention relates to an improved structure for isolation of transmission lines in a microwave or rf multi-layer structure. The present invention is drawn to a method of the placement of vias as well as their dimensions and geometric shape in order to most effectively reduce the forward and reverse coupling between transmission lines in multilayer structures. As stated above, at the transition points of transmission lines in multi-layer structures from one level to another, there is the potential for significant coupling between transmission lines. Accordingly, there is a marked decrease in isolation in most systems. Particularly at transition points, because signal line transitions occur at points of transition between the levels, it is essential that a good ground be transitioned with the signal line at transitions from one level in a multilayer board to another. Otherwise, undesired modes are radiated from the transitioning transmission line to neighboring transmission lines, adversely effecting the isolation of the neighboring line from the transition line. This undesired effect is reduced dramatically with a reduction of both forward and reverse coupling by proper placement of ground vias as well as the size and geometric shape. To this end, at a transition, where ground planes for a transmission line are also transitioned, the inductance between the two ground planes at a point of transition should be reduced as greatly as is possible. The present invention is drawn to placing a number of vias about a point of transition in order to effect the transition of the ground planes. It has been found that the placement of a number of vias is effective in this role, as the effective inductance is approximately inversely proportional to the number of vias and directly proportional to the inductance for each individual via. The present invention is also drawn to the placement of vias as far apart as possible but less than integral half wavelengths apart at the frequency of operation at non-transition regions. To this end, placing the vias as far apart as possible reduces the likelihood of phase or partial phase induced coupling of signals with signal vectors of nearby signal lines. By placing the vias less than half wavelength apart natural resonance conditions are avoided.

The dimensions of the vias are chosen to reduce the inductance of the individual vias. One approach is to make the vias as large as possible to reduce as greatly as possible the inductance of each individual via. This is found to have a rather dramatic effect and results in improved isolation by reducing the inductance induced coupling between the signal lines associated with the vias. This desired result can be effected by a variety of techniques, however. To this end, the cross - sectional area as well as the orientation of the via with respect to the transmission line are important parameters to be considered in designing the vias. The greater the cross-sectional area the lower the individual inductance of the via. The orientation also has a dramatic effect on the inductance and inductive coupling. The end result is a system having an isolation on the order of at least 60-70 dB at frequencies of 1-2 GHz.

It is an object of the present invention to improve the isolation between transmission lines in a single or a multi-layer high frequency structure.

It is a further object of the present invention to improve isolation between transmission lines at points of interlevel transition.

It is a feature of the present invention to have ground vias selectively located at points of transition in order to properly transition the ground plane along with the transition of the signal line.

It is a further feature of the present invention to locate the vias as far apart as possible in the transition region, while avoiding placement of vias at half wavelengths apart. It is a further feature of the present invention to have vias with an area great enough to reduce effectively the natural inductance of the via. It is an advantage of the present invention to have a simple structure for effecting via placement readily adaptable to high volume manufacturing techniques .

Figure 1 is a cross-sectional view of a typical transmission line structure having center grounds and rf signal lines.

Figure 2 is a cross-sectional view of a signal line in transition from one level to another with three ground planes . Figure 3 is a top view of a particular level of transmission line structure of a multi -layer board.

Figure 4 is an enlarged view of a via on a board at a given level .

Figures 5 and 6 are graphical representations of the effect of line segment length on forward and reverse isolation.

Figures 7 and 8 are graphical representations of the effect of via density on forward and reverse coupling. Figures 9 and 10 are graphical representations of forward and reverse isolation as a function of via inductance .

Figures 11a - lie show a transition region in 3 levels . Figure 12 is a view of a layer of a multilayer board having a wide ground shield and vias therein for improved isolation.

Figure 13 is a view of a layer of a multilayer board having narrow ground rails and vias therein for improved isolation.

Figure 14 is a view of a layer of a multilayer board where both narrow shields with a wide center shield and views therein for improved isolation.

Figures 15 and 16 are some of the various shapes of vias of the present invention.

Turning to Figures 3 and 4, a top view of a section or level of multi -layer transmission line structure is shown. To this end, signal ^'lines 301 and ground lines 302 having vias 303 effect the isolation of the signal line. The vias shown in Figures 3 and 4 at 303, 403 respectively, are holes drilled into the substrates to required depth having metallization therein to effect the proper electrical connection to ground. The material of choice for transmission medium depends on frequency; FR4 is good to 1 GHz and PTFE is suitable for higher frequency application. The via holes in these materials are drilled either "mechanically" or by laser ablation, and then plated with 0.7 - 1.4 mil metal. While the preferred embodiments are drawn to single or multilayer circuit boards, it is entirely possible that the teachings of the present disclosure can be applied to integrated circuits generally to include LSI and VLSI technology. In that event, standard LSI and VLSI processing can be used to effect vias, transmission lines and ground planes. As is discussed herein below, the preferred structure for the via is a cylindrical shape. However, the objects of the present invention can be achieved by fabricating vias having a variety of shapes and orientations to effect the desired inductance level .

As is well known, adjacent transmission line pairs, having an electromagnetic wave traveling on one transmission line in a given direction, couple electromagnetically to the neighboring transmission line. This coupling induces a signal in the direction opposite the transmission direction of the first signal line in a TEM environment, one with a homogeneous dielectric medium. In a medium in which vias are located, as well as other nonhomgeneous dielectric media, forward coupling can be effected, inducing electromagnetic waves in the same direction as the initial signal line. In terms of S parameters, forward coupling would be S₃₁ and reverse coupling would be S₄₁ in a four port system shown in Figure 3 with the ports as are shown. With these basic ^'principles in mind, the following discoveries have been made with the respect to the proper placement of vias to effect improved isolation in multi-level transmission structures. The effects of the increasing the number segments, with the distance between vias kept the same, is shown in Figures 5 and 6. Figure 5 shows the simulated forward coupling while Figure 6 shows the simulated reverse coupling for 1, 4, 8 and 16 segments of quarter wavelength at 1 GHz with a via placed every quarter wavelength at 1 GHz apart. Longer lines show marginally more forward coupling: a quarter wavelength line has 4 dB more isolation than four wavelength long lined at 1 GHz. An aspect of the present invention is the proper placement of vias with respect to wavelength. For a transmission line of a given wavelength, it has been found that placement of vias at integral half wavelengths apart results in a high Q resonator structure that stores a great deal of energy therein. This energy readily couples to a neighboring transmission line, and reduces the isolation. Accordingly, it is very important to make sure that any and all vias are not located at integral half wavelengths apart. It had been suggested previously that less than a quarter wavelength separation of ground vias is required for improved isolation. The present invention however reveals that a placement of the vias should in fact be as far apart as possible from one another, however not reaching the one half wavelength or multiples thereof in spacing. The only basic exception to this general rule for via placement is discussed herein with regard to the number of vias at a point of transition of a ground plane from one level to another. The placement of the vias plays a fundamental role in the isolation between signal lines. As stated above, the placement of vias relative to one another with respect to transmission line^'s is generally that they should be placed as far apart as possible without being placed at multiple half wavelengths from one another for the reasons as stated above. Another very important consideration is the placement of vias relative to points of transition. To this end, a point of transition of a ground plane from one level to the next is more properly effected by reducing the inductance of the vias as much as is possible. As stated above, the placement of one via at the point of a transition of a ground plane from one level to the next results in an undesired inductive effect. This translates into an impedance (reactance) which can be reduced by the placement of multiple vias. The present invention envisions the use of multiple vias near a point of transition so that the effective inductance of the vias is reduced. As stated above, the effective inductance is approximately inversely proportional to the number of vias and directly proportional to the individual inductance of the vias. So, the two general observations above with regard to placement of vias relative to a point of transition as well as relative to vias in an neighboring transmission line is that the vias should be placed in multiple locations about a transition point, as close to the transition point as possible, and of equal importance as far apart from the vias of neighboring transmission lines, however not at multiple half wavelengths at the wavelength of the signal in the transmission lines. Another affect which contributes to a reduction in isolation is the number of vias in a given wavelength of transmission line. The effect of the density of vias on forward isolation is shown at Figure 7. One segment translates to two vias while four segments have five vias and sixteen segments have seventeen vias. The effect of density of vias on forward coupling is readily apparent; there is no improvement in forward isolation. In fact this simulation shows a degradation in forward isolation as the number of via per wavelength is increased. However, as is noted in Figure 8, reverse coupling is much less dependent on via density than forward coupling.

Finally, returning to Figures 9 and 10, the forward and reverse coupling respectively with regard to via inductance for the individual via is disclosed. Another discovery of the present invention is that the particular individual inductance of a given via directly effects the forward coupling. To reduce forward coupling and thereby improve forward isolation, it is extremely important to reduce the individual inductances of the vias. This is accomplished by making the via area greater (such as by increasing the radii of circular vias) , as it is well known that inductance reduces rapidly with increasing area. This has beneficial input on manufacturing, as it is easier to fabricate holes for the vias in a circuit board having a greater diameter than those with smaller diameter.

Conventional boards use many 8 mil diameter holes. While the preferred embodiment of the present invention envisions a cylindrical via, or a via with a circular cross section, other vias are clearly within the purview of the disclosure of the present invention. For example, the vias could be made of any regular or irregularly shaped polygon as is shown in Figure 15. If the via is milled using a circular drill bit, an attempt to produce a regular polygon will result in something close to that shape but not identical thereto.

Additionally, the shapes of vias as is shown in Figure 16 are possible as well. In any event, the main thrust of the present disclosure with regard to the shape of the vias is that an increase in area of the individual via and therefore a decrease in inductance and subsequent coupling is desired. This can be achieved by combinations of small vias which can be regular or irregular polygons or the variations of the vias shapes shown in Figures 15 and 16 as would be readily apparent to one of ordinary skill in the art. Finally, it is important to note that the orientation of the vias with respect to the transmission line can influence the isolation as well. The via needs to be oriented in such a matter that it can most effectively ground the rf current. Typically for cross-section in Figure 1, the EM simulated inductance is 0.006 nH while a 14 mil diameter via is about 0.002 nH and 22 mil diameter is less than 0.0002 nH . For 1 GHz application inductance below 0.002 nH i.e. diameter more than 14 mil may be acceptable .

From the last three observations with respect to vias of neighboring transmission lines, it is imperative to have the vias located as far apart from one another as possible having the cross-sectional area being as large as possible to reduce inductance and yet have the distance between the vias not equal to a half wavelength. Accordingly, it is clear that the vias should be placed as far apart as possible without being at multiple of a half wavelength. As per simulation results in Figure 7 for the structure in Figure 1 forward isolation can be at least 1 dB better when vias every quarter wavelength are used compared to every 1/8 of a wavelength. Thus the size, position and density of vias are critical factors to be considered in order to effectively improve isolation.

Turning to Figure 11, three levels of transmission line are shown, showing the transition zone. The transition zone refers to the area where rf signals are passed from one level to the next level on a multi -layer board. Shown in Figure 11 is a typical transition of an exemplary 50 ohm transmission line in a stripline medium to different ground references through the interlayer via. In this region considerable coupling between the lines can occur. To reduce coupling, vias close to the transitions have to be included for the reasons stated above. In this case, with a single via connecting all grounds together the isolation is improved by 10 dB and by having three vias connecting all grounds the isolation improves by 15dB.

Figures 12 through 14 show additional embodiments of the teaching of the present invention. Each shows a particular level of a multilayer board. Figure 12 shows a typical top view of a given level on the multi-layer board having two widely separated signal lines 1201 with a ground shield 1202 and vias 1203. The dielectric material is shown at 1204. The ground shield serves as a ground plane for elements or transmission lines at another level not shown. The ground shield 1202 has vias 1203 close to the lines to force ground at the shield edges. However, there are no ground vias in the center of the shield. The section shown in Figure 12 has a reduced level of isolation in the instance where the width of the ground shield 1202 is on the order of a half wavelength resonance condition is achieved as described above . The drawbacks with respect to forward isolation are identical to those discussed above with regard to resonance conditions. Additionally, in the system shown in Figure 12, the vias 1203 must remain as far apart as possible but not at multiple half wavelengths for the reasons discussed above.

Turning to Figure 13, the ground shield or guard rail 1304 is shown to have a much smaller width as is compared to the relatively wide ground shield shown in Figure 12. It is of interest to note that with regard to terminology, the use of the words guard rail and ground shield for purposes of distinction, and that in either case the physical structure is a ground plane within the meaning of transmission line engineering. The structure shown in Figure 13 has vias 1303, transmission lines 1301 and dielectric 1302. It has been found that the narrower ground shields provide much better isolation than the wider ground shields and it is postulated that that in the rf signal layer the metallization should be restricted to avoid a standing wave condition as well as to suppress leaky modes from propagating. This embodiment is found to provide better isolation than that shown in Figure 12 by the avoidance of a resonance condition and the suppression of leaky modes.

In another embodiment, shown in Figure 14, a wide shield 1401 with vias 1402 is shown. In addition, the structure has narrow guard shields 1403 with vias 1402. The signal lines are shown at 1404 with dielectric 1405. This structure has a higher degree of isolation than that of the structures of Figures 12 and 13. The arrangement of having a signal line, a narrow ground shield 1403, a wide ground shield 1401 and the separating dielectric results in the separation of signal and ground levels that suppresses the coupling and improves isolation. The separation of signal lines by the placement of the two ground shields 1403, 1401 along with the placement of the vias 1402 aids in the effectiveness of this structure. In addition, the center ground layer 1401 can be used for other purposes in a transmission line structure.

The invention having been described in detail, it is clear that the application of the present invention to other structures is certainly possible. To the extent that the placement of ground vias at other than multiple half wavelengths and in a reduced density as well as in segment lengths to avoid phase coupling are utilized in order to reduce isolation in multi-level rf and other high frequency structures, such are deemed within the purview of the present invention.

Claims

Claims :

1. A multilayer circuit structure having selectively disposed ground planes; selectively disposed signal lines for transmitting signals of a given wavelength at a first level and a second level; dielectric material disposed between each of said ground planes and said signal lines; selectively disposed transition regions for electrically connecting one of signal lines at said first level to one of said signal lines at said second level, said transition regions having selectively located vias for interconnecting said ground planes, characterized in that: said vias of a first transmission line being spaced at a distance of other than multiple half- wavelengths of said wavelength of said signals relative to said vias of a second transmission line.

2. A multilayer structure as recited in claim 1 wherein said distance is less than integer half wavelengths .

3. A multilayer circuit structure as recited in any of the preceding claims wherein said signal lines of said first and said second levels have electrical isolation from one another on the order of 60 - 70 dB at a frequency of 1 - 2 GHz .

4. A multilayer structure as recited in any of the preceding claims wherein said vias are substantially cylindrical and have a diameter greater than 10 mils.

5. A multilayer circuit structure as recited in any of the preceding claims wherein said vias have a diameter in the range 14 mils to 22 mils.

6. A multilayer circuit structure as recited in any of the preceding claims wherein said vias have a diameter on the order of 10 mils - 22 mils.

7. A multilayer structure as recited in any of the preceding claims wherein said vias have an inductance on the order of 0.05nH or less at 1 Ghz .

8. A multilayer structure as recited in any of the preceding claims wherein said transition regions have at least three of said vias.

9. A multilayer structure as recited in any of the preceding claims wherein said vias are regularly shaped polygons .

10. A multilayer structure as recited in any of the preceding claims wherein said vias are irregularly shaped polygons .

11. A transmission line structure for transmitting electromagnetic waves at a given frequency, the structure having at least one selectively disposed ground plane and selectively disposed signal lines for transmitting signals of a given wavelength said at least one ground plane and said signal lines disposed on a dielectric substrate; and selectively disposed vias for effecting electrical ground connections, characterized in that said vias being separated by a distance of other than multiple half wavelengths of said waves at said frequency.

12. A transmission line structure as recited in claim 11 wherein one of said signal lines is isolated from another of said signal lines by 60 - 70 dB and said given frequency is 1-2 GHz.

13. A transmission line structure as recited in any of the preceding claims wherein said vias are substantially cylindrical and have a diameter greater than 10 mils.

14. A transmission line structure as recited in any of the preceding claims wherein said vias have a diameter in the range 14 mils to 22 mils.

15. A transmission line structure as recited in any of the preceding claims wherein said vias have a diameter on the order of 10 mils - 22 mils.

16. A transmission line structure as recited in any of the preceding claims wherein said vias are regularly shaped polygons.

17. A transmission line structure as recited in any of the preceding claims wherein said vias are irregularly shaped polygons .