JP2015088642A - Differential transmission lines, multilayer circuit board, and optical module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical module that allows preventing phase shift and mismatch of characteristic impedance of differential transmission lines, and being downsized.SOLUTION: Differential transmission lines are provided on a multilayer circuit board 6 including a stack of a plurality of insulating layers 6a to 6f in such a manner as to have a first signal line and a second signal line that transmit differential signals. The first and second signal lines respectively include interlayer wiring portions 22a and 22b disposed between the plurality of insulating layers, input-side via connection portions 21a and 21b connecting from input terminals 10 provided on a surface of the multilayer circuit board to one end portions of the interlayer wiring portions, and output-side via connection portions 23a and 23b connecting from output terminals 11 provided on the surface of the multilayer circuit board to the other end portions of the interlayer wiring portions. The interlayer wiring portion of the first signal line and the interlayer wiring portion of the second signal line are disposed between different insulating layers, and a ground layer 24b is disposed between the interlayer wiring portion of the first signal line and the interlayer wiring portion of the second signal line.

Description

  The present invention relates to a differential transmission line for transmitting a differential signal, a multilayer circuit board on which the differential transmission line is disposed, and an optical module on which the multilayer circuit board is mounted.

An optical module for transmitting and receiving an optical signal includes an optical semiconductor element that converts an electrical signal into an optical signal, an electronic cooling element that operates to maintain the optical semiconductor element at a predetermined temperature based on a supplied control signal, and And a package including the optical semiconductor element and the electronic cooling element.
A differential signal is used as an electrical signal supplied to the optical semiconductor element. In the differential signal, since the positive phase signal flowing through one of the two signal lines and the reverse phase signal flowing through the other have the same magnitude and opposite directions, the magnetic fields generated by the signals cancel each other. In addition, since the signal level is small, generation of unnecessary radiation noise and crosstalk noise can be suppressed as compared with the single-ended transmission method. In addition, since the influence of external noise is canceled out by one signal and the other signal, the noise resistance is superior to that of the single-ended transmission method.
Accordingly, optical modules that employ a differential transmission method have been developed so far. For example, Patent Document 1 discloses a technology called “optical module”, which can suppress deterioration of an electrical signal transmitted between a printed circuit board and an optical semiconductor element by suppressing heat inflow from the printed circuit board. It is disclosed. Hereinafter, the prior art will be described with reference to FIG. 2 of Patent Document 1 and FIG. 11 of the present application. When explaining using FIG. 2 of patent document 1, the code | symbol in a figure is quoted as it is.

In FIG. 2 of Patent Document 1, an optical module 110 includes an optical semiconductor element 112 that converts an electrical signal into an optical signal, a ceramic package 114 that hermetically seals and holds the optical semiconductor element 112, and an interior of the ceramic package 114. It has the lead | read | reed 116 which electrically connects with the exterior, and the printed circuit board 118 which outputs the electrical signal transmitted to the optical semiconductor element 112.
In addition, the optical module 110 includes a first flexible substrate 124 on which a plurality of first transmission lines 122 that electrically connect the printed circuit board 118 and the package external side portion 120 of the leads 116 are patterned, and an optical semiconductor. A second flexible substrate 130 on which a plurality of second transmission lines 128 that electrically connect the element 112 and the package inner side portion 126 of the lead 116 are patterned is provided.
The optical semiconductor element 112 is driven by a differential signal output from the differential driver 152. The differential signal is a line that forms a differential transmission line with a set of two (first transmission lines 122-1 and 122-2, leads 116-1 and 116-2, and second transmission lines 128-1 and 128-2. -2) is transmitted to the optical semiconductor element 112.
Here, in the optical module 110, in addition to suppressing noise generated by the electric signal supplied to the optical semiconductor element 112 and improving noise resistance of the electric signal supplied to the optical semiconductor element 112, the optical module 110 can There is another demand for module miniaturization.

JP 2006-13202 A JP-A-1-286493

  However, the second transmission lines 128-1 and 128-2 forming a differential transmission line by a set of two disclosed in Patent Document 1 are formed on the same plane on the second flexible substrate 130 in the package 114. Therefore, a space for installing the flexible substrate 130 in the package 114 is required. For this reason, there is a possibility that the recent demand for miniaturization of optical modules cannot be satisfied satisfactorily.

A method for solving this problem will be described with reference to FIG. FIGS. 11A and 11B are configuration diagrams for explaining the prior art in which a differential transmission line is built in the side wall of the package of the optical module, and FIG. It is arrow sectional drawing. In this figure, the same components as those disclosed in FIG. 2 of Patent Document 1 are denoted by the same reference numerals, and description thereof is omitted.
As the above-described solution, as shown in FIG. 11, the second flexible substrate 130 (FIG. 11) which is originally required in the package 114 by incorporating differential transmission lines 128-1 and 128-2 in the side wall S of the package 114. It is conceivable to omit the space (not shown in FIG. 11) and to narrow the entire width of the package 114.
However, when the second transmission lines 128-1 and 128-2 formed on the same plane on the second flexible substrate 130 are built in the side wall S of the ceramic package 114, the following problem occurs.
That is, the impedances of the second transmission lines 128-1 and 128-2, which are differential transmission lines arranged substantially parallel to each other, must be kept within a specified value. Relative dielectric constant = 3.5, relative dielectric constant of air = 1), the second transmission lines 128-1 and 128-2 are made of alumina having a dielectric constant higher than that of a flexible substrate or air (ratio of alumina). If an attempt is made to incorporate in the side wall S of the ceramic package 114 having a dielectric constant of 9.5) or the like, the capacitance due to capacitive coupling between the second transmission lines 128-1 and 128-2 becomes too large, so that the second transmission line 128- The characteristic impedance of 1,128-2 becomes too lower than specified.
In order to increase the characteristic impedance of the second transmission lines 128-1 and 128-2, a method of reducing the line width of each line or increasing the distance between the lines can be considered, but the former increases the loss, The latter has a problem that the width of the package 114 cannot be reduced.
As another means for reducing the size of the optical module 110, the second transmission lines 128-1 and 128-2 are arranged in parallel in the vertical direction inside the side wall S of the package 114, thereby omitting the space described above. It is conceivable to narrow the width.
As an example in which the differential transmission lines are arranged in parallel vertically, the “multilayer wiring board” disclosed in Patent Document 2 is already known.

  However, when the differential transmission lines 128-1 and 128-2 are arranged in parallel in the vertical direction inside the side wall S of the package 114, between the input terminal In and the differential transmission lines 128-1 and 128-2, and It is necessary to connect the output terminal Out and the differential transmission lines 128-1 and 128-2 with vias penetrating in the depth direction. Since the via line length (via depth) differs between the two lines, the entire isometric property is lost. In the differential transmission line, there is a problem that when the overall lengths of the two lines are different, the phase on the output end side is shifted and the phase difference appears as common mode noise.

  The present invention has been made in response to such a conventional situation, and provides an optical module that can be miniaturized by preventing a phase shift of a differential transmission line and also preventing a mismatch of characteristic impedance. It is another object of the present invention to provide a differential transmission line capable of providing a miniaturized optical module and a multilayer circuit board on which the differential transmission line is disposed.

In order to achieve the above object, a differential transmission line according to a first aspect of the present invention includes a first signal line and a second signal that transmit a differential signal to a multilayer circuit board in which a plurality of insulating layers are laminated. A differential transmission line provided with a line, wherein the first and second signal lines are each provided with an interlayer wiring portion disposed between the plurality of insulating layers and an input provided on the surface of the multilayer circuit board An input-side via connecting portion connecting from the terminal to one end of the interlayer wiring portion; an output-side via connecting portion connecting from the output terminal provided on the multilayer circuit board surface to the other end of the interlayer wiring portion; The interlayer wiring portion of the first signal line and the interlayer wiring portion of the second signal line are arranged between different insulating layers, and the interlayer wiring portion of the first signal line and the second signal Characterized in that a ground layer is disposed between the interlayer wiring portions of the wire A.
In the differential transmission line configured as described above, the interlayer wiring portions of the first signal line and the second signal line are disposed between different insulating layers, and the ground layer is disposed between these interlayer wiring portions. The impedance of the first signal line and the second signal line in each interlayer wiring portion acts so as to depend on the electric capacitance (capacitance) between each signal line and the ground layer.
In addition, since the interlayer wiring portion of the first signal line and the second signal line is arranged between different insulating layers, it is generated due to the difference in via depth between the input side via connection portion and the output side via connection portion of each signal line. There is a phase difference between the two signals, and this appears as degradation of impedance matching. However, since the interlayer wiring portion of the first signal line and the second signal line is shielded by the ground layer, the signal causing the phase difference Acts so as not to cause mode conversion due to direct coupling.
In the present specification and claims, a terminal that is one end of a differential transmission line and receives an external signal is referred to as an input terminal, and a terminal that is the other end of the differential transmission line and externally receives a signal. A terminal that outputs is called an output terminal.

A differential transmission line according to a second aspect of the present invention is the differential transmission line according to the first aspect, wherein an interlayer wiring portion of the first signal line and an interlayer wiring portion of the second signal line. Of these, one of the interlayer wiring portions is provided with a meander wiring portion for canceling out the difference in line length between the first signal line and the second signal line.
In the differential transmission line having the above configuration, the meander wiring portion provided in the interlayer wiring portion has an effect of canceling out the difference in the line length between the first signal line and the second signal line.
Furthermore, in the invention according to the second aspect, even with respect to the phase difference of the signal between the first signal line and the second signal line caused by inserting the meander wiring part in one of the interlayer wiring parts, Since each interlayer wiring part is shielded by the ground layer, it acts so as not to cause mode conversion due to the direct coupling of the signals having the phase difference.

A multilayer circuit board according to a third aspect of the present invention is a multilayer circuit board in which a plurality of insulating layers are laminated, and includes an input terminal and an output terminal. The differential transmission line according to claim 1 or 2 is provided.
The multilayer circuit board having the above configuration has the same operation as that of the differential transmission line according to claim 1 or claim 2.

Furthermore, an optical module according to claim 4 includes a package capable of hermetically sealing an internal space, an optical semiconductor element disposed in the internal space of the package, and an electron that suppresses overheating of the optical semiconductor element. The multilayer circuit board according to claim 3, further comprising: a cooling element; and an input terminal provided outside the package; and an output terminal provided in the internal space. A terminal is connected to the optical semiconductor element, and a differential signal input from the outside via the input terminal is transmitted to the optical semiconductor element.
Since the optical module having the above configuration includes the multilayer circuit board according to the third aspect, the optical module has the same function as that of the third aspect. That is, it has the same action as the invention according to claim 1 or claim 2.

In the differential transmission line according to claim 1 of the present invention, since the interlayer wiring portion of the first signal line and the second signal line is disposed between different insulating layers, the width of the signal line can be reduced even in a narrow insulating layer. The loss of the transmission signal due to the thin signal line can be suppressed.
In addition, since each impedance in the interlayer wiring portion of the first signal line and the second signal line depends on the electric capacity between each signal line and the ground layer, the first signal line and the second signal line By controlling the width of the signal line to a range corresponding to the thickness and dielectric constant of the insulating layer, the impedance in the interlayer wiring portion of the first signal line and the second signal line can be controlled to a desired range. It is.
Therefore, even when the first signal line and the second signal line are not equally spaced, there is no characteristic impedance mismatch. In addition, since the two signal lines are shielded by the ground layer, mode conversion is also possible due to the direct coupling of signals that have phase differences due to via length differences at the input and output via connections. Does not occur. Therefore, the transmission characteristics of the differential signal are not deteriorated.

In the differential transmission line according to claim 2 of the present invention, in addition to the effect of the invention according to claim 1, the meander wiring portion provided in the interlayer wiring portion includes the first signal line and the second signal line. In order to cancel out the difference in line length, the phase shift on the output terminal side due to the difference in overall length between the first signal line and the second signal line, and the occurrence of common mode noise due to the phase difference are suppressed. Is possible.
Moreover, the characteristic impedance mismatch, which is the effect of the invention according to claim 1, is also suppressed against the non-uniform spacing between the first signal line and the second signal line caused by providing the meander wiring portion. Therefore, the characteristic impedance mismatch does not occur even in the invention described in the claims.
More specifically, since the meander wiring portion is provided, the inter-wiring distance between the interlayer wiring portion of the first signal line and the interlayer wiring portion of the second signal line arranged between different insulating layers varies. Fluctuates and reflection occurs, resulting in deterioration of transmission characteristics of differential signals.
However, if a ground layer is provided between the interlayer wiring portions, the width of the first signal line and the second signal line can be controlled within a range corresponding to the thickness and dielectric constant of the insulating layer. The impedance in the interlayer wiring portion of the first signal line and the second signal line can be controlled within a desired range. Therefore, the meander wiring portion can be provided to eliminate the difference in the overall length between the first signal line and the second signal line, and to suppress the occurrence of common mode noise due to the phase shift and phase difference.
Further, the insertion of the meander wiring portion in one of the interlayer wiring portions causes a signal phase difference between the first signal line and the second signal line, but each interlayer wiring portion is shielded by the ground layer. Therefore, mode conversion due to direct coupling of signals having a phase difference does not occur, and the transmission characteristics of differential signals are not deteriorated from this point.

  In the multilayer circuit board according to claim 3 of the present invention, in addition to the effect of the invention according to claim 1 or 2, the impedance in the interlayer wiring portion of the first signal line and the second signal line is set to a desired value. It is possible to control the range, and there is no mismatch in characteristic impedance. Therefore, the multilayer circuit board can be miniaturized without degrading the transmission characteristics of differential signals.

  In the optical module according to claim 4 of the present invention, the multilayer circuit board according to claim 3 is provided, so that the invention according to claim 3 is added to the effect of the invention according to claim 1 or 2. The effect of can be demonstrated. That is, the optical module can be reduced in size without degrading the transmission characteristics of the differential signal.

It is a block diagram of the optical module using the differential transmission line which concerns on embodiment of this invention, and the multilayer circuit board which has arrange | positioned it. FIG. 2 is a cross-sectional structural view taken along line AA in FIG. 1. 1 is an exploded perspective view of a multilayer circuit board according to an embodiment of the present invention. It is an arrow directional cross-section figure of the code | symbol BB in FIG. It is an arrow directional cross-section figure of code | symbol CC in FIG. (A) is a cross-sectional structure diagram showing the first embodiment of the multilayer circuit board according to the present invention, (b) is a cross-sectional structure diagram showing the first prior art, (c) is a cross-sectional structure diagram showing the second prior art. It is. It is a graph which shows the transmission signal frequency dependence of the transmission loss of the transmission signal about Example 1, the 1st prior art, and the 2nd prior art of the multilayer circuit board concerning the present invention. (A) is a perspective conceptual diagram of Example 2 of the multilayer circuit board based on this invention, (b) is the arrow sectional structure figure of the code | symbol DD line in (a). (A) is a perspective conceptual diagram of the multilayer circuit board according to the third prior art, and (b) is a cross-sectional structural view taken along line EE in (a). (A) is a graph of Sdd11 for the differential transmission line in the multilayer circuit board according to the third prior art, and (b) is a graph of Sdd11 for the differential transmission line in Example 2 of the multilayer circuit board according to the present invention. It is. (A), (b) is a block diagram for demonstrating the prior art which incorporates a differential transmission line in the side wall of the package of an optical module, (b) is an arrow XX line of (a). It is sectional drawing.

Hereinafter, a differential transmission line according to an embodiment of the present invention, a multilayer circuit board on which the differential transmission line is arranged, and an optical module on which the multilayer circuit board is mounted will be described with reference to FIGS. .
FIG. 1 is a configuration diagram of an optical module using a differential transmission line according to an embodiment of the present invention and a multilayer circuit board on which the differential transmission line is arranged, and FIG. 2 is a cross-sectional view taken along line AA in FIG. FIG.
1 and 2, an optical module 1 includes a base 2 having a rectangular parallelepiped plate shape, a multilayer circuit board 6 having a shape substantially the same as that of the base 2 and having a through hole in the center, and the multilayer circuit board. 6 includes a frame body 7 having a through hole communicating with the through hole 6, and the optical semiconductor element 4 and the electronic cooling element 13 are formed by the upper surface of the base 2, the through hole of the multilayer circuit board 6, and the through hole of the frame body 7. A cavity 5 for housing is formed.
The opening area of the through hole of the frame body 7 is larger than the opening area of the through hole of the multilayer circuit board 6, and an internally exposed surface 8 exposed in the cavity 5 is formed on the upper surface of the multilayer circuit board 6. Further, the long side dimension of the frame body 7 is shorter than the long side dimension of the multilayer circuit board 6, and an externally exposed surface 9 that is exposed to the outside of the optical module 1 is formed on the upper surface of the multilayer circuit board 6.
A seal ring 14 is joined to the upper surface of the frame body 7. The seal ring 14 is a member for hermetically sealing the inside of the cavity 5 by welding the lid 15. The seal ring 14 and the lid 15 are made of Kovar having good weldability in which nickel and cobalt are mixed into iron.
The base 2 is made of a material having high thermal conductivity such as copper tungsten. The multilayer circuit board 6 and the frame body 7 are both formed by laminating a plurality of ceramic insulating layers such as alumina, and constitute the multilayer ceramic structure 3 by integrating them.

  A hole 16 is formed on one side surface of the multilayer ceramic structure 3. A pipe 18 containing a translucent window member 17 made of sapphire, glass or the like is fitted into the hole 16, and an optical fiber connection made of Kovar or the like into which an optical fiber 19 is inserted and fixed is inserted into the pipe 18. A tube 20 is connected. The pipe 18 is made of Kovar or the like for welding the optical fiber connection pipe 20, and is brazed to the multilayer ceramic structure 3 in order to make the cavity 5 airtight.

An input terminal 10 for transmitting an externally supplied electric signal to the optical semiconductor element 4 is formed on the externally exposed surface 9 of the multilayer circuit board 6. The electrical signal is supplied in the form of a differential signal composed of a positive phase signal and a negative phase signal that is opposite in phase to the positive phase signal, and the input terminal 10 is connected to a first input terminal 10a to which a positive phase signal is supplied. And a second input terminal 10b to which a reverse phase signal is supplied.
The multilayer circuit board 6 includes a differential transmission line having two signal lines for transmitting a differential signal supplied to the input terminal 10 to an output terminal 11 formed on the internal exposed surface 8 (see FIG. 3). Is formed.
Specifically, the positive phase signal supplied to the first input terminal 10 a is transmitted to the first output terminal 11 a via the signal lines 21 a, 22 a, 23 a, and then the optical semiconductor element 4 via the bonding wire 12. Is input. The signal lines 21a, 22a, and 23a are first signal lines.
The other signal line is a signal line for transmitting a reverse phase signal supplied to the second input terminal 10b. After being transmitted to the second output terminal 11b via the signal lines 21b, 22b, and 23b, the bonding wire is used. 12 is input to the optical semiconductor element 4. These signal lines 21b, 22b, and 23b are second signal lines. The differential transmission line is composed of a pair of these first signal line and second signal line.
The optical semiconductor element 4 is driven by the differential signal supplied by the first signal lines 21a, 22a, and 23a and the second signal lines 21b, 22b, and 23b, and transmits the laser light signal to the transparent window member 17 side. Output to. The optical signal output from the optical semiconductor element 4 is output to the optical fiber 19.

Next, the differential transmission line according to the embodiment of the present invention will be described with reference to FIGS. 3 is an exploded perspective view of the multilayer circuit board according to the embodiment of the present invention, FIG. 4 is a cross-sectional view taken along the line BB in FIG. 3, and FIG. FIG. Note that FIG. 3 shows the multilayer circuit board broken down into six boards for easy understanding of the structure, but FIGS. 4 and 5 show cross sections without being broken down.
In FIG. 3, the multilayer circuit board 6 includes first signal lines 21a, 22a, and 23a for transmitting differential signals supplied from the outside to the input terminal 10 to the output terminal 11, and second signal lines 21b, 22b, A differential transmission line having two signal lines 23b is formed.
The first signal lines 21a, 22a, and 23a connect the first input terminal 10a and the first output terminal 11a, the interlayer wiring portion 22a disposed between the insulating layers of the multilayer circuit board 6, and the first input terminal 10a and the interlayer An input-side via connection portion 21a that connects between the wiring portions 22a, and an output-side via connection portion 23a that connects the first output terminal 11a and the interlayer wiring portion 22a are provided.
Similarly, the second signal lines 21b, 22b, and 23b connect the second input terminal 10b and the second output terminal 11b, the interlayer wiring part 22b disposed between the insulating layers of the multilayer circuit board 6, and the first input. An input-side via connection portion 21b that connects between the terminal 10b and the interlayer wiring portion 22b, and an output-side via connection portion 23b that connects the first output terminal 11b and the interlayer wiring portion 22b are provided.
The interlayer wiring portion 22 a of the first signal line and the interlayer wiring portion 22 b of the second signal line are provided in different layers of the multilayer circuit board 6. Specifically, the interlayer wiring part 22a is provided on the circuit board 6b, and the interlayer wiring part 22b is provided on the circuit board 6d.

As shown in FIG. 4, the input side via connection portion 21a of the first signal lines 21a, 22a, and 23a is a via conductor penetrating from the sixth circuit board 6f to the third circuit board 6c of the multilayer circuit board. The first input terminal 10a and the interlayer wiring part 22a are connected.
On the other hand, the input side via connection portion 21b of the first signal lines 21b, 22b, and 23b is configured as a via conductor that penetrates from the sixth layer circuit board 6f to the fifth layer circuit board 6e of the multilayer circuit board. The two input terminals 10b and the interlayer wiring part 22b are connected.

As shown in FIG. 5, the output side via connection portion 23a of the first signal lines 21a, 22a, and 23a is a via that penetrates from the sixth layer circuit board 6f to the third layer circuit board 6c of the multilayer circuit board 6. It is comprised as a conductor and connects between the 1st output terminal 11a and the interlayer wiring part 22a. The output-side via connection portion 23b of the second signal lines 21b, 22b, and 23b is configured as a via conductor that penetrates from the sixth-layer circuit board 6f to the fifth-layer circuit board 6e of the multilayer circuit board 6, The output terminal 11b and the interlayer wiring part 22b are connected.
A ground layer 24b is disposed between the two interlayer wiring portions 22a and 22b, and the ground layer 24a and the interlayer wiring portion are provided on the circuit board 6a below the circuit board 6b on which the interlayer wiring portion 22a is provided. A ground layer 24c is provided on the upper circuit board 6e of the circuit board 6d provided with 22b.
The ground layers 24a to 24c constitute conductive metal electrodes, and are connected to ground terminals 26a, 26b, and 26c on the externally exposed surface 9 of the multilayer circuit board 6 through ground vias 25a, 25b, and 25c. The metal electrodes constituting the ground layers 24a to 24c do not need to be so-called solid electrodes having the entire surface as electrodes, but are desirably as wide as possible while having at least a width equal to or larger than the signal line. However, since it is only a guideline, it is desirable to determine the area of the electrode in consideration of the entire multilayer circuit board 6 and characteristics.
By providing such a ground layer 24b, the interlayer wiring portion 22a of the first signal line and the interlayer wiring portion 22b of the second signal line are shielded. The impedance of the interlayer wiring portion 22a of the first signal line depends on the capacitance between the interlayer wiring portion 22a and the ground layer 24a and between the interlayer wiring portion 22a and the ground layer 24b. The impedance of the wiring part 22b depends on the capacitance between the interlayer wiring part 22b and the ground layer 24b and between the interlayer wiring part 22b and the ground layer 24c.
Therefore, by controlling the width of the signal lines in the interlayer wiring portions 22a and 22b to a range corresponding to the thickness and dielectric constant of the circuit boards 6b, 6c, 6d and 6e which are insulating layers, the first signal lines and the first signal lines It is possible to control the impedance in the interlayer wiring portions 22a and 22b of the two signal lines within a desired range.
Therefore, even when the first signal line and the second signal line are not equally spaced, there is no characteristic impedance mismatch. This also increases the degree of design freedom with respect to the arrangement of the first signal line and the second signal line.
With respect to the effect of increasing the degree of freedom in design, the width of the signal line can be achieved even in a narrow insulating layer that is possible because the interlayer wiring portions of the first signal line and the second signal line are arranged between different insulating layers. This is also possible because the transmission signal loss can be easily suppressed by thinning the signal lines.
In addition, since the two signal lines are shielded by the ground layer 24b, a signal having a phase difference due to a difference in via length in the input side via connection portions 21a and 21b and the output side via connection portions 23a and 23b is directly displayed. There is no mode conversion by combining. Therefore, the transmission characteristics of the differential signal are not deteriorated.
Further, in the multilayer circuit board according to the present embodiment, a meander wiring portion 27 for canceling out the difference in line length with the first signal line is formed in the interlayer wiring portion 22b of the second signal line. .

Specifically, as is apparent from FIG. 3, vias are formed in the first signal lines 21a, 22a, and 23a in multiple layers as compared with the second signal lines 21b, 22b, and 23b. As a result, the entire track length becomes longer. Therefore, the meander wiring portion 27 is provided in the interlayer wiring portion 22b of the second signal line so as to be longer than the interlayer wiring portion 22a of the first signal line so that the entire line length is the same. .
By providing such a meander wiring part 27, the difference between the line lengths of the first and second signal lines including the input side via connection parts 21a and 21b and the output side via connection parts 23a and 23b is canceled. The phase shift of the differential transmission line can be eliminated. Generation of common mode noise can be suppressed by eliminating the phase shift.
Moreover, as in the present embodiment, even if the meandering wiring portion 27 for canceling the difference in line length is provided in one signal line in the interlayer wiring portion 22b and the equal spacing between the two signal lines is lost, As described above, the first signal line is controlled by controlling the width of the signal line in the interlayer wiring portions 22a and 22b within a range corresponding to the thickness and dielectric constant of the circuit boards 6b, 6c, 6d, and 6e that are insulating layers. In addition, since it is possible to control the impedance in the interlayer wiring portions 22a and 22b of the second signal line within a desired range, there is no characteristic impedance mismatch. Therefore, the multilayer circuit board 6 according to the present embodiment can eliminate the phase shift of the differential transmission line including the via connection portion and can prevent the mismatch of the characteristic impedance.
Further, the provision of the interlayer wiring portions 22a and 22b causes a signal phase difference between the first signal line and the second signal line, but the interlayer wiring portions 22a and 22b are shielded by the ground layer 24b. Therefore, mode conversion due to the direct coupling of signals having a phase difference does not occur. Therefore, by providing the ground layer 24 between the interlayer wiring portions 22a and 22b and shielding it, even if the meander wiring portion 27 is provided and the line length of the entire signal line is made the same, the interlayer wiring It is possible to suppress the direct coupling of signals that cause a phase difference due to the provision of the portions 22a and 22b, and thus it is possible to exhibit an excellent effect that transmission characteristics of differential signals are not deteriorated. .
Therefore, the optical module 1 on which the multilayer circuit board 6 having such a differential transmission line is mounted can be miniaturized while preventing the phase shift of the differential transmission line and preventing the mismatch of characteristic impedance. it can.

Example 1
Hereinafter, for the multilayer circuit board according to the present embodiment, a test is made as an example so that the characteristic impedance is 50Ω, and further, a differential transmission line of a conventional example is created, and a line loss and a space saving property are compared. Will be described with reference to FIGS.
6A is a cross-sectional structure diagram showing a first embodiment of the multilayer circuit board according to the present invention, FIG. 6B is a cross-sectional structure diagram showing the first prior art, and FIG. 6C is a cross section showing the second prior art. FIG.
The multilayer circuit board of the first embodiment shown in FIG. 6A is composed of four insulating layers 28a to 28d, and two differential transmission lines between the insulating layers 28a and 28b and between the insulating layers 28c and 28d. 29a and 29b are arranged. Further, ground layers 30a and 30c are formed on the lower surface of the insulating layer 28a and the upper surface of the insulating layer 28d, respectively, and a ground layer 30b is formed between the insulating layers 28b and 28c. These ground layers 30a to 30c are ground vias. 31 is connected. Accordingly, the differential transmission line 29a is sandwiched between the insulating layers 28a and 28b disposed between the ground layers 30a and 30b, and the differential transmission line 29b is disposed between the ground layers 30b and 30c. , 28d.
In the multilayer circuit board according to Example 1 configured as described above, the width w1 of the sidewalls of the insulating layers 28a to 28d made of alumina ceramic is 1.2 mm, and the thickness t1 of each of the insulating layers 28a to 28d is 0.3 mm. Thus, a differential transmission line was prepared in which the dimensions of each part were adjusted so that the characteristic impedance was 50Ω within the prototype package.
In the present embodiment, the line width w2 of the differential transmission lines 29a and 29b is set to be as wide as 105 μm in order to suppress the signal passing loss in the line, but between the differential transmission lines 29a and 29b arranged above and below. Since the ground layer 30b is further disposed and the ground layers 30a and 30c are further disposed so as to sandwich the differential transmission lines 29a and 29b, the capacitance component of the characteristic impedance does not increase, and the characteristic impedance is defined. Value (50Ω). The thickness t2 of the differential transmission lines 29a and 29b is 35 μm, and the material is tungsten.

On the other hand, in the circuit board according to the first prior art shown in FIG. 6B, the differential transmission lines 33a and 33b are arranged in parallel on the flexible board 32 as disclosed in FIG. A circuit board in which the ground layer 34 is provided on the lower surface of the flexible board 32 is assumed.
In the conventional technology configured as described above, the line width w4 of the differential transmission lines 33a and 33b is 80 μm, the distance between lines g is 100 μm, and the characteristic impedance is adjusted to 50Ω as in the first embodiment. Prototype. Further, the thickness t4 of the differential transmission lines 33a and 33b is 35 μm and the material is copper, the width W of the flexible substrate 32 is 1 mm, the thickness T is 50 μm, and the material is polyimide.
Since the differential transmission lines 33a and 33b according to the first prior art are composed of the flexible substrate 32 and air having a low dielectric constant, the capacitance component of the characteristic impedance is originally kept low. For this reason, it is possible to increase the line width w4 while keeping the characteristic impedance within the specified value to suppress the passage loss, or to reduce the distance g between lines. However, a package that employs the differential transmission lines 33a and 33b according to the prior art requires a space for accommodating the flexible substrate 32 in the cavity of the package, and cannot be reduced in size.

Further, in the circuit board according to the second prior art shown in FIG. 6C, the differential transmission line of the first prior art is built in the side wall of the package. In FIG. 6C, the second prior art includes two layers of insulating layers 35a and 35b made of alumina ceramic and differential transmission lines 36a and 36b disposed therebetween. Further, a ground layer 37a is formed on the lower surface of the insulating layer 35a, and a ground layer 37b is formed on the upper surface of the insulating layer 35b. These ground layers 37a and 37b are connected by a ground via 38.
In the circuit board according to the second prior art configured as above, the side wall width w5 of the insulating layers 35a and 35b made of alumina ceramic is 1.2 mm, and the thickness t5 of each of the insulating layers 35a and 35b is 0. A differential transmission line having a dimension of 3 mm and adjusted to have a characteristic impedance of 50Ω in a prototype package was prepared. The differential transmission lines 36a and 36b have a line width w6 of 50 μm, a thickness t6 of 35 μm, and a material of tungsten.
In the second prior art, since the space between the two differential transmission lines 36a and 36b is made of alumina ceramic having a high dielectric constant, the capacitance component of the characteristic impedance increases. It will be lower than. For this reason, in order to keep the characteristic impedance within a specified value, it is necessary to increase the distance g between the differential transmission lines 36a and 36b to 200 μm and the line width w6 to 50 μm.
The multilayer circuit board according to the first embodiment shown in FIGS. 6A, 6B, and 6C, the circuit board according to the first prior art, and the circuit board according to the second prior art are used in the line. Since the transmission loss of the transmission signal was measured, the result will be described below with reference to FIG.

FIG. 7 is a graph showing the transmission signal frequency dependence of the transmission loss of the transmission signal in Example 1 of the multilayer circuit board according to the present invention, the first conventional technique, and the second conventional technique.
In FIG. 7, (a) is a measurement result regarding the passage loss of the differential transmission lines 33a and 33b according to the first prior art, and (b) is a passage loss of the differential transmission lines 36a and 36b according to the second prior art. (C) is a graph which shows the measurement result regarding the passage loss of the differential transmission lines 29a and 29b which concern on Example 1, respectively.
As shown in the graph, it can be seen that in the first conventional technique (a), the transmission signal has a low transmission loss in the entire measured frequency band. On the other hand, in the second prior art (b), as a result of narrowing the line width w6 of the differential transmission lines 36a and 36b, the transmission loss has increased considerably.
On the other hand, in Example 1 (c), the line width w2 can be widened, so that it is possible to suppress the loss slightly to the extent of the first prior art (a).

(Example 2)
A differential line as shown in the following figure was created as Example 2, and a differential line as shown in the following figure was created as a comparative example. And Sdd11 was measured about each differential line, and the result of the following graph was obtained. As shown in the graph below, it was confirmed that the Sdd11 decreased in Example 2 compared to the comparative example.
Hereinafter, for the multilayer circuit board according to the present embodiment, a multilayer circuit board as shown in FIG. 8 is prototyped as Example 2, and the multilayer circuit board according to the third prior art is shown in FIG. A circuit board is prototyped and a test for comparing Sdd11 is performed for each differential transmission line, which will be described with reference to FIGS.
FIG. 8A is a conceptual perspective view of a multilayer circuit board according to a second embodiment of the present invention, FIG. 8B is a cross-sectional structural view taken along line DD in FIG. 9A, and FIG. The perspective conceptual diagram of the multilayer circuit board which concerns on a 3rd prior art, (b) is an arrow sectional structure figure of the code | symbol EE in (a).
The multilayer circuit board of Example 2 shown in FIGS. 8A and 8B is composed of four insulating layers 40a to 40d as in Example 1, and the first input terminal is formed on the upper surface of the insulating layer 40d. An input terminal 42 including 42a and a second input terminal 42b is disposed, and an output terminal 43 including a first output terminal 43a and a second output terminal 43b is disposed on the lower surface of the insulating layer 40a.
The first input terminal 42a and the second input terminal 42b are respectively connected via differential transmission lines including input side via connection portions 44a and 44b, interlayer wiring portions 45a and 45b, and output side via connection portions 46a and 46b. The first output terminal 43a and the second output terminal 43b are connected. The interlayer wiring portion 45a is disposed between the insulating layers 40a and 40b, and the interlayer wiring portion 45b is disposed between the insulating layers 40c and 40d.
Furthermore, ground layers 41c, 41b, and 41a are formed on the upper surface of the insulating layer 40d, between the insulating layers 40b and 40c, and on the lower surface of the insulating layer 40a, respectively. Therefore, the interlayer wiring portion 45a includes the ground layers 41a and 41b. The interlayer wiring part 45b is sandwiched between the ground layers 41b and 41c, and the interlayer wiring parts 45a and 45b are shielded from each other.
As shown in FIG. 8B, the length in which the interlayer wiring portions 45a and 45b are arranged in parallel on the plane shown in cross section is 10 mm. The material is the same as in Example 1.

The multilayer circuit board of the third prior art shown in FIGS. 9A and 9B is composed of three layers of insulating layers 50a to 50c, and a first input terminal 52a and a second input layer are formed on the upper surface of the insulating layer 50c. An input terminal 52 including an input terminal 52b is disposed, and an output terminal 53 including a first output terminal 53a and a second output terminal 53b is disposed on the lower surface of the insulating layer 50a.
The first input terminal 52a and the second input terminal 52b are connected via differential transmission lines including input side via connection portions 54a and 54b, interlayer wiring portions 55a and 55b, and output side via connection portions 56a and 56b, respectively. The first output terminal 53a and the second output terminal 53b are connected. The interlayer wiring portion 55a is disposed between the insulating layers 50a and 50b, and the interlayer wiring portion 55b is disposed between the insulating layers 50b and 50c.
Further, ground layers 51b and 51a are formed on the upper surface of the insulating layer 50c and the lower surface of the insulating layer 50a, respectively. Therefore, the interlayer wiring portion 45a is sandwiched between the ground layers 41a and 41b, and the interlayer wiring portion 45b is configured to be sandwiched between the ground layers 41b and 41c, but unlike the second embodiment, the interlayer wiring portions 55a and 55b are not shielded from each other.
As in the second embodiment, as shown in FIG. 9B, the length in which the interlayer wiring portions 55a and 55b are arranged in parallel on the plane shown in the cross section is 10 mm. The material is the same as in Examples 1 and 2.
Thus, in order to compare the case where the two interlayer wiring portions of the differential transmission line are each shielded by the ground layer, the differential transmission line is a so-called mixed mode S parameter. By measuring the response in the differential mode and comparing the results, it is explained that the characteristics of Example 2 of the present invention are excellent. Hereinafter, the measurement results will be described with reference to FIG.

FIG. 10A is a graph of Sdd11 for the differential transmission line in the multilayer circuit board according to the third prior art, and FIG. 10B is Sdd11 for the differential transmission line in Example 2 of the multilayer circuit board according to the present invention. It is a graph of. Sdd11 is a kind of S parameter, and this S parameter is generally used as a parameter for evaluating the characteristics of the electronic circuit when designing the electronic circuit. Specifically, Sdd11 inputs an AC signal from one of the ports (port 1) in the circuit to be measured out of the differential mode response in the mixed mode S parameter. The degree of attenuation or amplification transmitted is expressed in a logarithm, where a negative number means attenuation and a positive number means amplification.
As is clear from comparison between FIGS. 10A and 10B, it can be seen that the Sdd11 of Example 2 has a higher attenuation in all the measurement frequency bands, that is, shows a good characteristic with less reflection.
As described above with reference to the first and second embodiments, the differential transmission line according to the present invention, the multilayer circuit board including the differential transmission line, and the optical module including the multilayer circuit board are configured to transmit differential signals. It is possible to greatly reduce the size of the apparatus while suppressing the deterioration of the performance.

  As described above, the present invention provides a differential transmission line for transmitting a differential signal supplied to an optical semiconductor element used for transmitting and receiving an optical signal, a multilayer circuit board including the differential transmission line, and a multilayer The present invention can be used in the field of manufacturing optical modules on which circuit boards are mounted.

  DESCRIPTION OF SYMBOLS 1 ... Optical module 2 ... Base 3 ... Multilayer ceramic structure 4 ... Optical semiconductor element 5 ... Cavity 6 ... Multilayer circuit board 6a-6f ... Circuit board 7 ... Frame body 8 ... Internal exposure surface 9 ... External exposure surface 10 ... Input Terminal 10a ... First input terminal 10b ... Second input terminal 11 ... Output terminal 11a ... First output terminal 11b ... Second output terminal 12 ... Bonding wire 13 ... Electronic cooling element 14 ... Seal ring 15 ... Lid 16 ... Hole 17 ... Translucent window member 18 ... Pipe 19 ... Optical fiber 20 ... Optical fiber connection pipe 21a, 21b ... Input side via connection part 22a, 22b ... Interlayer wiring part 23a, 23b ... Output side via connection part 24a, 24b, 24c ... Ground Layers 25a, 25b, 25c... Ground vias 26a, 26b, 26c... Ground terminals 27. 28b, 28c ... Insulating layers 29a, 29b ... Differential transmission lines 30a, 30b, 30c ... Ground layers 31 ... Ground vias 32 ... Flexible substrates 33a, 33b ... Differential transmission lines 34 ... Ground layers 35a, 35b ... Insulating layers 36a , 36b ... differential transmission lines 37a, 37b ... ground layer 38 ... ground vias 40a, 40b, 40c, 40d ... insulating layers 41a, 41b, 41c ... ground layer 42 ... input terminal 42a ... first input terminal 42b ... second input Terminal 43 ... Output terminal 43a ... First output terminal 43b ... Second output terminal 44a, 44b ... Input side via connection part 45a, 45b ... Interlayer wiring part 46a, 46b ... Output side via connection part 50a, 50b, 50c ... Insulating layer 51a, 51b ... ground layer 52 ... input terminal 52a ... first input terminal 52b ... Second input terminal 53 ... Output terminal 53a ... First output terminal 53b ... Second output terminal 54a, 54b ... Input side via connection 55a, 55b ... Interlayer wiring 56a, 56b ... Output via connection 110 ... Optical module 112 ... Optical semiconductor element 114 ... Package 116 ... Lead 128-1, 128-2 ... Differential transmission line 154 ... Electrode 156 ... Bonding wire g ... Distance between lines w ... Width t ... Thickness S ... Side wall

Claims (4)

A differential transmission line provided with a first signal line and a second signal line for transmitting a differential signal on a multilayer circuit board in which a plurality of insulating layers are laminated,
The first and second signal lines are respectively connected to an interlayer wiring portion disposed between the plurality of insulating layers and an input terminal provided on the multilayer circuit board surface to one end portion of the interlayer wiring portion. A side via connection portion, and an output side via connection portion for connecting from an output terminal provided on the multilayer circuit board surface to the other end portion of the interlayer wiring portion,
The interlayer wiring portion of the first signal line and the interlayer wiring portion of the second signal line are arranged between different insulating layers, and the interlayer wiring portion of the first signal line and the interlayer wiring of the second signal line A differential transmission line, wherein a ground layer is disposed between the sections.   Of the interlayer wiring portion of the first signal line and the interlayer wiring portion of the second signal line, one of the interlayer wiring portions has a line length difference between the first signal line and the second signal line. The differential transmission line according to claim 1, wherein a meander wiring part for canceling out is provided. A multilayer circuit board in which a plurality of insulating layers are laminated,
A multilayer circuit board comprising an input terminal and an output terminal, and the differential transmission line according to claim 1 or 2 between the input terminal and the output terminal. A package capable of hermetically sealing the internal space;
An optical semiconductor element disposed in an internal space of the package;
An electronic cooling element for suppressing overheating of the optical semiconductor element;
The multilayer circuit board according to claim 3, comprising an input terminal provided outside the package and an output terminal provided in the internal space.
An optical module, wherein the multilayer circuit board connects the output terminal to the optical semiconductor element and transmits a differential signal input from the outside via the input terminal to the optical semiconductor element.