Differential wiring board and semiconductor light emitting device

Abstract

To reduce the device size in a differential drive scheme.SOLUTION: A differential wiring board comprises: a dielectric layer 10 having a first surface 12 and a second surface 14, including a first portion 16 having a first thickness h1 between the first surface 12 and the second surface 14, and including a second portion 18 having a second thickness h2 thinner than the first thickness h1 between a first surface 12 and a second surface 14; a first conductor line 22 having a first wiring width W1 and arranged on the first surface 12 of the first portion 16; a second conductor line 24 having a second wiring width W2 narrower than the first wiring width W1 and arranged on the first surface 12 of the second portion 18; and a ground conductor 26 arranged on the second surface 14 of the first portion 16 and the second surface 14 of the second portion 18 so as to overlap the first conductor line 22 and second conductor line 24. The first conductor line 22 and the second conductor line 24 constitute a differential transmission track.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a differential wiring board and a semiconductor light emitting device. [Background technology]

[0002] A differential drive method is known as a means for ensuring the quality of electrical signals in optical modules for communication. Patent Document 1 discloses differential driving of an electroabsorption modulated laser (EML). Non-Patent Document 1 is also known as a general calculation formula for differential transmission lines. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-29649 A [Non-Patent Document 1] IEEE Transactions on Electronic Computers, 1967, Vol. EC-16, Issue 2 Summary of the Invention [Problem to be solved by the invention]

[0004] In the differential driving method, the wiring width of a pair of differential transmission lines is the same in order to balance the phase-inverted signal pair. Also, as disclosed in Reference 1, in order to mount an EML on one of the pair of differential transmission lines, a wide wiring width must be ensured. For these reasons, it is difficult to miniaturize the differential driving method compared to the single-ended driving method.

[0005] The present invention aims to achieve miniaturization using a differential drive system. [Means for solving the problem]

[0006] A differential wiring board according to the present invention comprises: a dielectric layer having a first surface and a second surface, including a first portion having a first thickness between the first surface and the second surface and including a second portion having a second thickness between the first surface and the second surface that is thinner than the first thickness; a first conductor line having a first wiring width and arranged on the first surface of the first portion; a second conductor line having a second wiring width narrower than the first wiring width and arranged on the first surface of the second portion; and a ground conductor arranged on the second surface of the first portion and on the second surface of the second portion so as to overlap the first conductor line and the second conductor line, wherein the first conductor line and the second conductor line constitute a differential transmission line.

[0007] According to the present invention, the second wiring width is narrower than the first wiring width, which enables miniaturization. Although the second wiring width is narrower than the first wiring width, the second thickness is thinner than the first thickness, which allows a balance to be achieved between the signal pairs that are differentially transmitted.

[0008] A semiconductor light emitting device according to the present invention comprises the differential wiring board and a semiconductor optical element mounted on the differential wiring board. [Brief description of the drawings]

[0009] [Figure 1] 1 is a plan view of a differential wiring board according to a first embodiment. [Diagram 2] 2 is a cross-sectional view taken along line II-II of the differential wiring board shown in FIG. [Diagram 3] FIG. 2 is a diagram showing a simulation model of the differential wiring board according to the first embodiment. [Figure 4] FIG. 13 is a diagram showing a simulation model of a differential wiring board according to a comparative example. [Diagram 5] FIG. 4 is a diagram showing frequency characteristics of the first embodiment and a comparative example obtained by a simulation using a three-dimensional electric field analysis tool. [Figure 6] FIG. 11 is a cross-sectional view of a differential wiring board according to a first modified example. [Figure 7] FIG. 11 is a cross-sectional view of a differential wiring board according to Modification 2. [Figure 8] FIG. 11 is a cross-sectional view of a differential wiring board according to Modification 3. [Figure 9] FIG. 11 is a cross-sectional view of a differential wiring board according to a fourth modified example. [Figure 10] FIG. 11 is a plan view of a differential wiring board according to a second embodiment. [Figure 11] 11 is a cross-sectional view taken along the line XI-XI of the differential wiring board shown in FIG. [Figure 12] FIG. 11 is a cross-sectional view of a differential wiring board according to a third embodiment. [Figure 13] FIG. 11 is a plan view of a semiconductor light emitting device according to a fourth embodiment. [Figure 14] 14 is a cross-sectional view taken along line XIV-XIV of the semiconductor light-emitting device shown in FIG. 13. [Figure 15] FIG. 2 is a cross-sectional view of a semiconductor optical element. [Figure 16] FIG. 13 is a plan view of a semiconductor light emitting device according to a fifth embodiment. [Figure 17] FIG. 13 is a plan view of a semiconductor light emitting device according to a sixth embodiment. [Figure 18] 18 is a cross-sectional view taken along line XVIII-XVIII of the semiconductor light-emitting device shown in FIG. 17. [Figure 19] FIG. 13 is a plan view of a semiconductor light emitting device according to a seventh embodiment. [Figure 20] 20 is a cross-sectional view taken along the line XX-XX of the semiconductor light-emitting device shown in FIG. 19. [Figure 21] FIG. 13 is a cross-sectional view of a semiconductor light emitting device according to an eighth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In all the drawings, the same reference numerals denote the same or equivalent functions, and the repeated description thereof will be omitted. Note that the size of the figures does not necessarily correspond to the magnification.

[0011] [First embodiment] FIG. 1 is a plan view of a differential wiring board according to a first embodiment. FIG. 2 is a cross-sectional view of the differential wiring board shown in FIG. 1 taken along line II-II. The differential wiring board has a dielectric layer 10. The dielectric layer 10 is made of a dielectric material such as aluminum nitride. The dielectric layer 10 is made of a single layer. The dielectric layer 10 has a first surface 12 and a second surface 14. The dielectric layer 10 includes a first portion 16. The first portion 16 has a first thickness h1 between the first surface 12 and the second surface 14. The dielectric layer 10 includes a second portion 18. The second portion 18 has a second thickness h2 between the first surface 12 and the second surface 14 that is smaller than the first thickness h1. The first portion 16 and the second portion 18 are flush with each other on the first surface 12. The second surface 14 has a step (height conversion portion) 20 between the first portion 16 and the second portion 18. That is, the difference in thickness between the first portion 16 and the second portion 18 appears on the second surface 14 .

[0012] The differential wiring board has a first conductor line 22. The first conductor line 22 has a first wiring width W1. The first conductor line 22 is disposed on the first surface 12 of the first portion 16. The differential wiring board has a second conductor line 24. The second conductor line 24 has a second wiring width W2 narrower than the first wiring width W1. The second conductor line 24 is disposed on the first surface 12 of the second portion 18. The second conductor line 24 is located at the center of the second portion 18 in the direction in which the first conductor line 22 and the second conductor line 24 are arranged. The step 20 of the second surface 14 is located directly below the center of the space between the first conductor line 22 and the second conductor line 24. In other words, the distance d1 from the first conductor line 22 to the center and the distance d2 from the second conductor line 24 to the center are equal.

[0013] The first conductor line 22 and the second conductor line 24 are made of a material such as gold, and may be made of a laminated material. The first conductor line 22 and the second conductor line 24 form a differential transmission line. The first wiring width W1 and the second wiring width W2, and the first thickness h1 and the second thickness h2 are set so that the characteristic impedance of the differential transmission line is 100Ω.

[0014] The differential wiring board has a ground conductor 26. The ground conductor 26 is made of a material such as gold, and a laminated material may also be used. The ground conductor 26 is disposed on the second surface 14 of the first portion 16 and the second surface 14 of the second portion 18. The ground conductor 26 is continuous with the second surface 14 of the first portion 16 and the second surface 14 of the second portion 18. The ground conductor 26 is also formed on a side surface of the first portion 16 adjacent to the second portion 18. Since the ground conductor 26 is integrally formed, patterning is not required, and the manufacturing process can be simplified.

[0015] The ground conductor 26 overlaps the first conductor line 22 and the second conductor line 24. As a result, the first conductor line 22 and the ground conductor 26 form a high-frequency line of a microstrip line with a GND. The second conductor line 24 and the ground conductor 26 also form a high-frequency line of a microstrip line with a GND.

[0016] According to this embodiment, the second wiring width W2 is narrower than the first wiring width W1, which enables miniaturization. Although the second wiring width W2 is narrower than the first wiring width W1, the second thickness h2 is thinner than the first thickness h1, so that the signal pair that is differentially transmitted can be balanced and there is no deterioration in the characteristic impedance.

[0017] 3 is a diagram showing a simulation model of the differential wiring board according to the first embodiment. The first conductor line 22 and the second conductor line 24, as well as the first portion 16 and the second portion 18, have the characteristics described above. Specific dimensions are as shown in the figure. The differential impedance was set to 100Ω.

[0018] 4 is a diagram showing a simulation model of a differential wiring board according to a comparative example. The width of a pair of conductor lines is the same, and the thickness of the dielectric layer is also uniform. The specific dimensions are as shown in the figure. The differential impedance was set to 100Ω.

[0019] 5 is a diagram showing frequency characteristics of the first embodiment and the comparative example obtained by simulation using a three-dimensional electric field analysis tool. As is clear from this result, the first embodiment, like the comparative example, obtains sufficient transmission characteristics (S21) in the range from 0 GHz to 60 GHz. According to this embodiment, in applications up to at least 50 GHz, it is possible to obtain characteristics equivalent to those of a general differential wiring board while reducing the size of the differential wiring board by 20%. Note that the dimensions are merely examples and may be appropriately selected according to conditions such as the desired differential impedance, the required frequency band, and the dielectric constant of the dielectric layer 10.

[0020] [Variation 1] 6 is a cross-sectional view of a differential wiring board according to Modification 1. The differential wiring board differs from the first embodiment in that the dielectric layer is multi-layered (e.g., layers 10A and 10B having different dielectric constants). The rest of the details are the same as those described in the first embodiment.

[0021] [Variation 2] 7 is a cross-sectional view of a differential wiring board according to Modification 2. The differential wiring board differs from the first embodiment in that the ground conductor is divided. The ground conductor includes a first ground conductor 26A disposed on the second surface 14 of the first portion 16. The ground conductor includes a second ground conductor 26B disposed on the second surface 14 of the second portion 18 and separated from the first ground conductor 26A. The ground conductor 26 is not formed on the side of the first portion 16 adjacent to the second portion 18. The rest of the details described in the first embodiment are applicable.

[0022] [Variation 3] 8 is a cross-sectional view of a differential wiring board according to Modification 3. The step 20B on the second surface 14 is not located directly below the center of the space between the first conductor line 22 and the second conductor line 24. Thus, even if the position of the step 20B varies, this poses little practical problem.

[0023] [Variation 4] FIG. 9 is a cross-sectional view of a differential wiring board according to the fourth modification. The dielectric layer 10 has a third portion 28, the thickness of which changes, between the first portion 16 and the second portion 18. The third portion 28 does not overlap either the first conductor line 22 or the second conductor line 24. The second conductor line 24 is located at the center of the second portion 18 in the direction in which the first conductor line 22 and the second conductor line 24 are arranged. The third portion 28 is integrated with the first portion 16 at a first thickness h1, and is integrated with the second portion 18 at a second thickness h2. The second surface 14 of the first portion 16 and the second surface 14 of the second portion 18 are connected by an inclined surface of the third portion 28. Even with this shape, no difference in high-frequency characteristics is observed that is problematic in practical use.

[0024] [Second embodiment] Fig. 10 is a plan view of a differential wiring board according to a second embodiment of the present invention, and Fig. 11 is a cross-sectional view of the differential wiring board shown in Fig. 10 taken along line XI-XI.

[0025] The dielectric layer 210 includes a plurality of first portions 216. The dielectric layer 210 includes a plurality of second portions 218. The plurality of first portions 216 and the plurality of second portions 218 are arranged alternately. The first conductor line 222 is disposed on the first surface 212 of each of the plurality of first portions 216. The second conductor line 224 is disposed on the first surface 212 of each of the plurality of second portions 218. The first conductor line 222 and the second conductor line 224 configure a differential transmission line for each of the multi-channels CH.

[0026] Each of the multiple first portions 216 includes, on the first surface 212, a wiring region 230 in which the first conductor line 222 is formed. Each of the multiple first portions 216 includes, on the first surface 212, a margin region 232 adjacent to the wiring region 230 with a margin width Wm in a direction opposite to the second conductor line 224. The multiple first portions 216 are each equal in margin width Wm.

[0027] The ground conductor 226 is continuous with the second surface 214 of the first portion 216 and the second surface 214 of the second portion 218. The second surface 214 has a step 220 between the first portion 216 and the second portion 218. The step 220 of the second surface 214 is located directly below the center of the space between the first conductor line 222 and the second conductor line 224. The second conductor line 224 is located in the center of the second portion 218 in the direction in which the first conductor line 222 and the second conductor line 224 are arranged. The rest of the details are the same as those described in the first embodiment.

[0028] According to this embodiment, since the second wiring width W2 is smaller than the first wiring width W1 in each channel CH, a small array type differential wiring board can be realized. Alternatively, as a modified example, the size of the array type differential wiring board may be maintained and the distance between adjacent channels CH may be increased. This makes it possible to reduce crosstalk. Alternatively, the differential wiring board may be made smaller and the distance between adjacent channels CH may be increased.

[0029] [Third embodiment] 12 is a cross-sectional view of a differential wiring board according to the third embodiment. Each of the multiple first portions 316 has a wiring region 330 on the first surface 312 in which a first conductor line 322 is formed. Each of the multiple first portions 316 has no margin on the first surface 312 adjacent to the wiring region 330 in the direction opposite to the second conductor line 324. The step 320 of the second surface 314 is located directly below the center of the space between the first conductor line 322 and the second conductor line 324. The rest of the contents are the same as those described in the second embodiment.

[0030] [Fourth embodiment] Fig. 13 is a plan view of a semiconductor light emitting device according to a fourth embodiment. Fig. 14 is a cross-sectional view of the semiconductor light emitting device shown in Fig. 13 taken along line XIV-XIV. The semiconductor light emitting device has a differential wiring board 400. The contents described in the first embodiment are applicable to the differential wiring board 400.

[0031] A semiconductor optical element 434 is mounted on the differential wiring board 400. The semiconductor optical element 434 has a semiconductor laser 436 for emitting light. The semiconductor optical element 434 has an electroabsorption modulator 438 for modulating the light by the electroabsorption effect. The semiconductor optical element 434 is an electroabsorption modulator integrated laser in which the semiconductor laser 436 and the electroabsorption modulator 438 are integrated together.

[0032] 15 is a cross-sectional view of a semiconductor optical device 434. A semiconductor laser 436 and an electroabsorption modulator 438 are fabricated in a semiconductor substrate 440. The semiconductor substrate 440 is an n-type InP substrate. The semiconductor laser 436 has a multiple quantum well 446A sandwiched between a lower SCH (Separated Confinement Heterostructure) layer 442A and an upper SCH layer 444A on the semiconductor substrate 440. A diffraction grating layer 448 is provided on the upper SCH layer 444A and is covered with a cladding layer 450A. The semiconductor laser 436 may be a distributed Bragg reflector (DBR) or a distributed feedback (DFB).

[0033] The electroabsorption modulator 438 has a multiple quantum well 446B sandwiched between a lower SCH layer 442B and an upper SCH layer 444B on a semiconductor substrate 440. The upper SCH layer 444B is covered with a cladding layer 450B. The cladding layers 450A and 450B may be the same or different materials. The electroabsorption modulator 438 and the semiconductor laser 436 may not be integrated on the same semiconductor substrate 440, but may be formed on different semiconductor substrates 440.

[0034] The semiconductor optical element 434 has a lower electrode 452 (e.g., a cathode) common to the electroabsorption modulator 438 and the semiconductor laser 436. The lower electrode 452 may be formed separately for each of the electroabsorption modulator 438 and the semiconductor laser 436. The semiconductor laser 436 has an upper electrode 454 (e.g., an anode) for applying a DC voltage. A DC voltage is applied between the upper electrode 454 and the lower electrode 452. The semiconductor optical element 434 has an upper electrode 456 (e.g., an anode) of the electroabsorption modulator 438. An AC voltage is applied between the upper electrode 456 and the lower electrode 452.

[0035] The semiconductor optical element 434 (lower electrode 452) faces the first conductor line 422 so as not to protrude from the first conductor line 422. A first wiring width W1 of the first conductor line 422 is set to a sufficient size for mounting the semiconductor optical element 434, and is constant in the extending direction. An upper electrode 456 of the electroabsorption modulator 438 is connected to the second conductor line 424 by a wire 468. Although not shown, an upper electrode 454 of the semiconductor laser 436 is electrically connected to a DC current source by a connecting means such as a wire.

[0036] The differential wiring board 400 has a pad 458 arranged on the first surface 412 so as to be adjacent to the first conductor line 422 in the opposite direction to the second conductor line 424. The pad 458 is arranged in the first portion 416. The differential wiring board 400 has a matching resistor 460 connected in series between the pad 458 and the first conductor line 422. The matching resistor 460 is connected for impedance matching. The semiconductor light emitting device has a wire 470 that connects the upper electrode 456 of the electroabsorption modulator 438 and the pad 458. As a result, the matching resistor 460 is connected in parallel to the electroabsorption modulator 438.

[0037] A high frequency signal for differential signal transmission is input to the first conductor line 422 and the second conductor line 424. The high frequency signal is applied to the lower electrode 452 and the upper electrode 456 of the electroabsorption modulator 438, respectively, and the electroabsorption modulator 438 is driven by the differential signal.

[0038] [Fifth embodiment] 16 is a plan view of the semiconductor light emitting device according to the fifth embodiment. The second conductor line 524 has a stub 562 extending in the opposite direction to the first conductor line 522. The stub 562 is made of the same material or the same laminate material as the second conductor line 524. The portion of the first conductor line 522 adjacent to the pad 558 and the portion of the second conductor line 524 from which the stub 562 extends are adjacent to each other.

[0039] The second conductor line 524 has a stub 562, and thus its electrical characteristics become closer to those of the first conductor line 522 connected to the matching resistor 560 and the pad 558. As a result, the high-frequency characteristics of the high-frequency lines of the first conductor line 522 and the second conductor line 524 have closer performance, making it possible to improve the signal quality when driven by a differential signal. The other details are the same as those described in the fourth embodiment.

[0040] [Sixth embodiment] Fig. 17 is a plan view of the semiconductor light emitting device according to the sixth embodiment Fig. 18 is a cross-sectional view of the semiconductor light emitting device shown in Fig. 17 taken along line XVIII-XVIII.

[0041] The semiconductor light emitting device has a differential wiring board 600. The contents described in the second embodiment (FIG. 11) and the fourth embodiment (FIG. 13) are applicable to the differential wiring board 600. The semiconductor light emitting device has a semiconductor optical element 634. The contents described in the fourth embodiment (FIG. 13) are applicable to the semiconductor optical element 634.

[0042] Each of the multiple first portions 616 includes, on the first surface 612, a wiring region 630 in which a first conductor line 622 is formed. Each of the multiple first portions 616 includes, on the first surface 612, a margin region 632 adjacent to the wiring region 630 with a margin width Wm (see FIG. 11 ) in a direction opposite to the second conductor line 624. The multiple first portions 616 are equal in the margin width Wm. A pad 658 is disposed in the first portion 616.

[0043] In each channel CH, a high-frequency signal for differential signal transmission is input to the first conductor line 622 and the second conductor line 624. The high-frequency signal is applied to a lower electrode (not shown) and an upper electrode 656 of the electro-absorption modulator 638, respectively, and the electro-absorption modulator 638 is driven by the differential signal.

[0044] [Seventh embodiment] Fig. 19 is a plan view of the semiconductor light emitting device according to the seventh embodiment Fig. 20 is a cross-sectional view of the semiconductor light emitting device shown in Fig. 19 taken along the line XX-XX.

[0045] The differential wiring board 700 has inter-channel conductors 764 (e.g., vias such as filled vias and through-hole vias) arranged on the first surface 712 between adjacent channels CH. The differential wiring board 700 has connecting conductors 766 that penetrate the dielectric layer 710 and connect the ground conductors 726 and the inter-channel conductors 764. The connecting conductors 766 penetrate the first portion 716, avoiding the second portion 718. The pads 758 are arranged in the first portion 716.

[0046] In this embodiment, the inter-channel conductor 764 connected to the ground conductor 726 is disposed between adjacent channels CH, so that electrical crosstalk between adjacent channels CH can be reduced.

[0047] [Eighth embodiment] 21 is a cross-sectional view of a semiconductor light emitting device according to the eighth embodiment. The differential wiring board 800 has an inter-channel conductor 864 arranged on the first surface 812 between adjacent channels CH. The differential wiring board 800 has a connection conductor 866 that penetrates the dielectric layer 810 and connects the ground conductor 826 and the inter-channel conductor 864. The connection conductor 866 penetrates the second portion 818, avoiding the first portion 816. The pad 858 is arranged in the second portion 818.

[0048] Since the inter-channel conductor 864 is formed in the thin second portion 818, the through hole can be made small. For example, a small diameter via can be formed. This makes it possible to provide a more compact semiconductor light emitting device.

[0049] The present invention is not limited to the above-described embodiment, and various modifications are possible. For example, the configurations described in the embodiments can be replaced with substantially the same configurations, configurations that provide the same effects, or configurations that can achieve the same purpose. [Explanation of symbols]

[0050] 10 dielectric layer, 10A layer, 10B layer, 12 first surface, 14 second surface, 16 first portion, 18 second portion, 20 step, 20B step, 22 first conductor line, 24 second conductor line, 26 ground conductor, 26A first ground conductor, 26B second ground conductor, 28 third portion, 210 dielectric layer, 212 first surface, 214 second surface, 216 first portion, 218 second portion, 220 step, 222 first conductor line, 224 second conductor line, 226 ground conductor, 230 wiring area, 232 blank area, 312 first surface, 314 second surface, 316 first portion, 320 step, 322 first conductor line, 324 second conductor line, 330 wiring area, 400 differential wiring board, 412 First surface, 416 First portion, 422 First conductor line, 424 Second conductor line, 434 Semiconductor optical element, 436 Semiconductor laser, 438 Electroabsorption modulator, 440 Semiconductor substrate, 442A Lower SCH layer, 442B Lower SCH layer, 444A Upper SCH layer, 444B Upper SCH layer, 446A Multiple quantum well, 446B Multiple quantum well, 448 Diffraction grating layer, 450A Cladding layer, 450B Cladding layer, 452 Lower electrode, 454 Upper electrode, 456 Upper electrode, 458 Pad, 460 Matching resistor, 468 Wire, 470 Wire, 522 First conductor line, 524 Second conductor line, 558 Pad, 560 Matching resistor, 562 Stub, 600 Differential wiring board, 612 First surface, 616, first portion, 622, first conductor line, 624, second conductor line, 630, wiring area, 632, margin area, 634, semiconductor optical element, 638, electroabsorption modulator, 656, upper electrode, 658, pad, 700, differential wiring board, 710, dielectric layer, 712, first surface, 716, first portion, 718, second portion, 726, ground conductor, 758, pad, 764, inter-channel conductor, 766, connecting conductor, 800, differential wiring board, 810, dielectric layer, 812, first surface, 816, first portion, 818, second portion, 826, ground conductor, 858, pad, 864, inter-channel conductor, 866, connecting conductor, CH, channel, h1, first thickness, h2, second thickness, W1, first wiring width, W2, second wiring width, Wm, margin width, d1, distance, d2, distance.

Claims

1. a dielectric layer having a first surface and a second surface, including a first portion having a first thickness between the first surface and the second surface, and a second portion having a second thickness between the first surface and the second surface that is smaller than the first thickness; a first conductor line having a first wiring width and disposed on the first surface of the first portion; a second conductor line having a second wiring width narrower than the first wiring width and disposed on the first surface of the second portion; a ground conductor arranged on the second surface of the first portion and the second surface of the second portion so as to overlap the first conductor line and the second conductor line; having the first conductor line and the second conductor line constitute a differential transmission line, a portion of the first conductor line having the first wiring width and a portion of the second conductor line having the second wiring width are adjacent to each other in a direction perpendicular to a direction in which the first conductor line extends, the dielectric layer has a third portion between the first portion and the second portion, the third portion having a varying thickness; The third portion overlaps neither the first conductor line nor the second conductor line.

2. 2. The differential wiring board according to claim 1, The ground conductor is continuous with the second surface of the first portion and the second surface of the second portion.

3. 2. The differential wiring board according to claim 1, the ground conductor includes a first ground conductor arranged on the second surface of the first portion, and a second ground conductor arranged on the second surface of the second portion and separated from the first ground conductor.

4. 4. The differential wiring board according to claim 1, the second surface has a step between the first portion and the second portion, The step on the second surface is located directly below the center of the space between the first conductor line and the second conductor line.

5. 5. The differential wiring board according to claim 1, The second conductor line is located at the center of the second portion in the direction in which the first conductor line and the second conductor line are arranged.

6. 6. A differential wiring board according to claim 1, the first portion is a plurality of first portions, the second portion is a plurality of second portions; the first conductor line is disposed on the first surface of each of the plurality of first portions; the second conductor line is disposed on the first surface of each of the plurality of second portions; The first conductor line and the second conductor line are each a part of a multi-channel differential wiring board that constitutes the differential transmission line.

7. 7. The differential wiring board according to claim 6, each of the plurality of first portions includes, on the first surface, a wiring region in which the first conductor line is formed, and a margin region adjacent to the wiring region with a margin width in a direction opposite to the second conductor line; The plurality of first portions each have the same margin width.

8. 7. The differential wiring board according to claim 6, Each of the multiple first portions has a wiring area on the first surface in which the first conductor line is formed, and has no margin adjacent to the wiring area in the direction opposite to the second conductor line.

9. 9. A differential wiring board according to claim 6, an inter-channel conductor disposed on the first surface between adjacent channels of the multiple channels; a connection conductor penetrating the dielectric layer and connecting the ground conductor and the inter-channel conductor; The differential wiring board further comprises:

10. 10. The differential wiring board according to claim 9, The connection conductor passes through the first portion while avoiding the second portion.

11. 10. The differential wiring board according to claim 9, The connection conductor passes through the second portion while avoiding the first portion.

12. The differential wiring board according to claim 1 , a semiconductor optical element mounted on the differential wiring substrate; A semiconductor light emitting device comprising:

13. 13. The semiconductor light emitting device according to claim 12, the semiconductor optical device is an electroabsorption modulator integrated laser in which an electroabsorption modulator and a semiconductor laser are integrally integrated, the electroabsorption modulator integrated laser has a lower electrode common to the electroabsorption modulator and the semiconductor laser, and an upper electrode of the electroabsorption modulator; the lower electrode faces the first conductor line without protruding therefrom; The upper electrode is connected to the second conductor line by a wire.

14. 14. The semiconductor light emitting device according to claim 13, a pad disposed on the first surface so as to be adjacent to the first conductor line in a direction opposite to the second conductor line; a matching resistor connected in series between the pad and the first conductor line; a wire connecting the upper electrode and the pad; The semiconductor light emitting device further comprises:

15. 15. The semiconductor light emitting device according to claim 14, The pad is disposed on the first portion of the semiconductor light emitting device.

16. 15. The semiconductor light emitting device according to claim 14, The pad is disposed on the second portion of the semiconductor light emitting device.

17. 17. The semiconductor light emitting device according to claim 14, the second conductor line has a stub extending in a direction opposite to that of the first conductor line, A semiconductor light emitting device, wherein a portion of the first conductor line adjacent to the pad and a portion of the second conductor line from which a stub extends are adjacent to each other.

18. a dielectric layer having a first surface and a second surface, including a first portion having a first thickness between the first surface and the second surface, and a second portion having a second thickness between the first surface and the second surface that is smaller than the first thickness; a first conductor line having a first wiring width and disposed on the first surface of the first portion; a second conductor line having a second wiring width narrower than the first wiring width and disposed on the first surface of the second portion; a ground conductor arranged on the second surface of the first portion and the second surface of the second portion so as to overlap the first conductor line and the second conductor line; having the first portion and the second portion are adjacent to each other in a direction perpendicular to a direction in which the first conductor line extends, the first conductor line and the second conductor line constitute a differential transmission line, the dielectric layer has a third portion between the first portion and the second portion, the third portion having a varying thickness; The third portion overlaps neither the first conductor line nor the second conductor line.

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