JP5366210B2 - Optical waveguide - Google Patents

Abstract

The bulk average refractive index of at least a region of an upper clad (24) overlapping with an optical waveguide core (23) is set larger than the bulk average refractive index of at least a region of a lower clad (22) between the optical waveguide core (23) and the substrate (21).

Description

  The present invention relates to a structure of an optical waveguide in a minute optical circuit. In particular, the present invention relates to a structure of a thin optical waveguide formed on a substrate.

  Intra-LSI optical wiring technology is being studied as a means for realizing high-capacity transmission and high functionality while realizing low cost and high mass productivity by Si photonics. If an optical device using an optical waveguide using a material having a large refractive index difference is realized, an optical communication device having a smaller size and lower power consumption than the conventional type can be realized.

  As an optical waveguide using a material with a high refractive index difference, a structure using Si on an Si substrate as an optical waveguide core (L. Pavesi and DJ Lockwood, "Silicon Photonics" Springer, 2003, pp.269-271. ( Reference 1)) is known. As typical optical waveguides, ridge waveguides, rib waveguides, thin wire waveguides, and photonic crystal line defect waveguides are known. Many technologies related to optical waveguides using compound semiconductors such as GaAs have also been proposed (S. Noda and T. Baba, “Roadmap on Photonic Crystals”, Kluwer Academic Publishers, 2003, pp. 45-49. ( Reference 2)).

  Such an optical waveguide is used when the waveguide itself has a function as a device or various functional elements (light emitting element, light receiving element, optical branching element, optical coupling element, optical demultiplexing element) integrated on a micro optical circuit. In many cases, it is used as an optical wiring element that connects between an element, an optical multiplexing element, an optical modulation element, an optical switching element, an optical memory element, an optical buffer element, and the like. From such a viewpoint, in an optical waveguide connecting these various functional elements, or in a waveguide device in which the waveguide itself functions as a functional device, the electromagnetic field mode propagating through the optical waveguide propagates smoothly, and the optical waveguide itself The low propagation loss is extremely important in any optical device having an optical waveguide.

  From this point of view, the technology to reduce propagation loss by absorbing the material itself in waveguides of various materials (G.-L. Bona, R. Germann and BJOffrein, “SiON high-refractive- index waveguide and planar lightwavecircuits ”, IBM J. Res. & Dev. 47 (28) 239-249 (2003). (Ref. 3), Chung-Yen Chao and L. JayGuo,“ Thermal-flow technique for reducing surface roughness and controllinggap size in polymer microring resonators ", Applied Physics Letters, 84, 2479 (2004). (Reference 4), JP 09-033742 A, JP 2004-144935 A, JP 2005-062538 A), Technology to reduce structural fluctuations contained in the waveguide manufacturing process (Kevin K. Lee, Desmond R. Lim, Lionel C. Kimerling, Jangho Shin, and Franco Cerrina, “Fabrication ofultralow-loss Si / SiO2 waveguides by roughnes s reduction ”, Optics Letters, 26, 1888 (2001). (Reference 5), Jun-ichi Takahashi, TaiTsuchizawa, Toshifumi Watanabe, and Sei-ichi Itabashi,“ Oxidation-inducedimprovement in the sidewall morphology and cross-sectional profile of siliconwire waveguides ”, Journal of Vacuum Science & Technology B, 22, 2522 (2004). (Reference 6), JP 2005-140815 A) and the like have been proposed.

  However, the low-loss optical waveguides described in the above-mentioned documents 3 to 6, JP-A 09-033742, JP-A 2004-144935, JP-A 2005-062538, and JP-A 2005-140815. The technology for realizing the above only deals with the case where the optical waveguide is not affected by the substrate on which it is normally manufactured. Therefore, when the electromagnetic field mode of the optical waveguide is affected on the substrate and a large propagation loss occurs, it cannot be used to prevent an increase in the propagation loss.

  On the other hand, considering the integration of the LSI circuit and the optical waveguide, if the embedded insulating film is thinned for the LSI circuit, the influence of the absorption of the substrate material and the refractive index of the substrate are different from the cladding. There is a possibility that the influence of such as cannot be ignored.

  The object of the present invention is to solve such a problem and to reduce the propagation loss due to the influence of the substrate even when the thickness of the clad located between the substrate and the optical waveguide core is thinned. It is to provide an optical waveguide that can be used.

  In the optical waveguide of the present invention, a first clad (lower clad), an optical waveguide core, and a second clad (upper clad) are laminated in this order on a substrate, and at least the second clad overlaps with the optical waveguide core. The region is formed of a material having a volume average refractive index larger than that of at least the region between the optical waveguide core and the substrate of the first cladding.

  In this specification, “upper” or “upper” refers to a direction away from the substrate when the substrate is horizontally disposed as a lowermost layer. “Lower” or “lower” refers to a direction approaching the substrate.

  The optical waveguide core is formed on the surface of the first cladding, and the second cladding is made of a material whose volume average refractive index is larger than the volume average refractive index of the first cladding. It is desirable to be formed over.

  It is desirable that the composition of the material constituting the optical waveguide core and the second cladding is SiOxNy, (x, y = 0 to 1), and the material constituting the first cladding is SiO2.

  Further, it is desirable that the value of the volume average refractive index of the second clad with respect to the volume average refractive index of the first clad is 1.5 times or more.

  The optical waveguide core has a length (width) in a direction parallel to the surface of the substrate and crossing the optical waveguide direction, which is 1.5 times or more and 2.5 times a length (height) in the direction perpendicular to the surface of the substrate. It is desirable to be formed as follows.

  The refractive index of the optical waveguide core is nC, the refractive index of the first cladding is nL, the refractive index of the second cladding is nU, and the refractive index difference between the optical waveguide core and the first cladding is ΔnL = (nC2−nL2). / (2 × nC2), where the refractive index difference between the optical waveguide core and the second cladding is ΔnU = (nC2−nU2) / (2 × nC2), the refractive index between the optical waveguide core and the second cladding. The ratio ΔnU / ΔnL × 100 between the difference ΔnU and the refractive index difference ΔnL between the optical waveguide core and the first cladding is preferably 40% or more and 60% or less.

  The optical waveguide of the present invention can be used in at least a part of an optical circuit in an optical communication device for optical interconnection between LSI chips in which LSI chips are connected via an optical circuit. Further, it can be used for at least a part of an optical circuit in an optical communication device for LSI on-chip optical interconnection in which an electronic circuit and an optical circuit are integrated in one LSI chip.

  According to the present invention, in the technology related to the present invention, the propagation loss of the electromagnetic field mode propagating through the optical waveguide increases due to the influence of the substrate such as the influence of the absorption of the material of the substrate and the refractive index of the substrate being different from the cladding. Even if the thickness of the lower clad located between the substrate and the optical waveguide core is reduced to such an extent that the leakage mode is caused by the influence of the substrate, it is positioned on the opposite side of the optical waveguide core. Propagation loss can be sufficiently reduced by setting the refractive index of the upper cladding large.

  Since the optical waveguide of the present invention can reduce propagation loss and reduce the required light source output, the power consumption required for optical interconnection between LSI chips and between LSI on-chip can be reduced. Further, when it is intended to realize a desired propagation loss specification, the thickness of the lower clad positioned between the substrate and the optical waveguide core can be further reduced. This means that in the integration of the LSI circuit and the optical waveguide, it is possible to reduce the thickness of the embedded insulating film related to the LSI.

It is sectional drawing which shows an example of Si wafer used in order to produce an optical waveguide. It is sectional drawing which shows the structure of an optical waveguide. It is a figure which shows the dispersion | distribution relationship of an optical waveguide. It is a figure which shows optical power density distribution of the waveguide mode with respect to a perpendicular | vertical direction to a board | substrate when the refractive index of a lower clad and an upper clad is equal. It is a figure which shows the optical power density distribution of the waveguide mode with respect to the orthogonal | vertical direction to a board | substrate in case a refractive index difference exists between a lower clad and an upper clad. It is a figure which shows the distance dependence (TE-like mode) between the board | substrate-waveguide core regarding a propagation loss. It is a figure which shows the distance dependence (TM-like mode) between the board | substrate-waveguide core regarding a propagation loss. It is a figure which shows the refractive index difference dependence (TE-like mode) between the waveguide core-cladding regarding a propagation loss. It is a figure which shows the refractive index difference dependence (TM-like mode) between the waveguide core-cladding regarding a propagation loss. It is a figure which shows the relationship between material composition and material refractive index. It is sectional drawing which shows the structural example of the optical communication device for the optical interconnection between LSI chips. It is sectional drawing which shows the structural example of the optical communication device for LSI on-chip optical interconnection.

  Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a cross-sectional view showing an example of a Si wafer used for producing an optical waveguide. This Si wafer has a configuration in which an Si layer 13 is formed on an Si substrate 11 in such an arrangement as to sandwich an SiO 2 layer 12 of an oxide insulating film. The surface of the Si layer 13 is an air layer 14. Such a Si wafer is known as a technology booster capable of maintaining the performance improvement due to semiconductor scaling and realizing the performance improvement without depending on the scaling. This configuration is of high quality with respect to film thickness and composition distribution, and it is possible to obtain a relatively large substrate, so it is most often used as a basic structure for manufacturing various optical waveguide devices in Si photonics. It has been.

  FIG. 2 is a sectional view showing the structure of the optical waveguide formed on the substrate. Using the Si substrate 11 and the SiO2 layer 12 of the Si wafer shown in FIG. 1 as the substrate 21 and the lower cladding (first cladding) 22 as they are, the Si layer 13 is processed to form the optical waveguide core 23, and the upper cladding By forming the (second cladding) 24 as a laminated film having a finite thickness, an optical waveguide is manufactured. That is, the optical waveguide has a structure in which the optical waveguide core 23 is formed on the surface of the lower cladding 22 and the upper cladding 24 is formed so as to cover the side surface and the surface of the optical waveguide core 23. Here, the width of the optical waveguide core 23, that is, the length in the direction parallel to the surface of the substrate 21 and crossing the optical waveguide direction is wcore, and the height of the optical waveguide core 23, that is, the length in the direction perpendicular to the surface of the substrate 21. Hcore, and the height of the lower cladding 22, that is, the distance between the substrate 21 and the optical waveguide core 23 is h. As the material of the upper clad 24, a material having a volume average refractive index larger than that of the lower clad 22 is used.

  FIG. 3 shows the dispersion relationship of the optical waveguide. The vertical axis represents the normalized waveguide refractive index, and the horizontal axis represents the wavelength. Here, in the optical waveguide structure shown in FIG. 2, the width wcore of the optical waveguide core 23 = 2.2 μm, the height hcore = 1.1 μm, the refractive index difference Δn between the optical waveguide core 23 and the upper and lower claddings 22 and 24. Was set to 2.7%. The film thickness of the lower clad 22 and the upper clad 24 was made sufficiently large (a waveguide structure that is vertically and horizontally symmetrical). When the 0th order mode, the first order mode, the second order mode,... Are assigned from the long wavelength side, the 0th order to the second order mode are shown for both the TE-like mode and the TM-like mode. Note that the TE-like mode refers to a mode having even parity in a symmetric operation related to a mirror surface having a normal line in a direction perpendicular to the substrate. The TM-like mode is a mode having an odd parity. Further, regarding only the 0th to 2nd modes, the TE-like mode is an electromagnetic field mode in which an electric field component in a direction parallel to the substrate and a magnetic field component in a direction perpendicular to the substrate are large. An electromagnetic field mode having a large electric field component in a direction perpendicular to the substrate and a magnetic field component in a direction parallel to the substrate is the TM-like mode. There is almost no difference between the TE-like mode and the TM-like mode, and the dispersion relations are overlapped.

  FIG. 4 is a diagram showing the optical power density distribution of the waveguide mode in the direction perpendicular to the substrate 21 when the lower cladding 22 and the upper cladding 24 have the same refractive index. The wavelength is 850 nm, and the 0th-order mode of both the TE-like mode and the TM-like mode is shown. The horizontal axis in FIG. 4 indicates the position in the height direction of the optical waveguide (direction perpendicular to the substrate 21), and the position of “zero” indicates the center of the optical waveguide core 23. It can be seen that the optical power density distribution in the waveguide mode is symmetrical in the upper and lower cladding regions of the optical waveguide and exhibits exponential attenuation.

  FIG. 5 is a diagram showing an optical power density distribution similar to that in FIG. 4 when the volume average refractive index of the upper cladding 24 is larger than the volume average refractive index of the lower cladding 22. Here, the refractive index of the optical waveguide core 23 is nC, the refractive index of the lower cladding 22 is nL, the refractive index of the upper cladding 24 is nU, and the refractive index difference between the optical waveguide core 23 and the lower cladding 22 is ΔnL = (nC2 −nL2) / (2 × nC2), the refractive index difference between the optical waveguide core 23 and the upper cladding 24 is ΔnU = (nC2−nU2) / (2 × nC2), and the refraction between the optical waveguide core 23 and the upper cladding 24 is performed. An example in which the ratio ΔnU / ΔnL × 100 between the rate difference ΔnU and the refractive index difference ΔnL between the optical waveguide core 23 and the lower cladding 22 is 37% is indicated by a solid line. Further, the distribution in the case where ΔnU / ΔnL × 100 is 100%, that is, the same distribution as that in FIG.

  FIG. 6 is a diagram showing the dependence of propagation loss on the distance between the substrate and the optical waveguide core. When the height h of the lower cladding 22 in FIG. 2 is changed, the TE- in the optical waveguide structure shown in FIG. The propagation loss of the 0th-order guided mode of the like mode is shown. Here, the ratio ΔnU / ΔnL × 100 between the refractive index difference ΔnU between the optical waveguide core 23 and the upper cladding 24 and the refractive index difference ΔnL between the optical waveguide core 23 and the lower cladding 22 is 100%, 75%, and Each case is 50%. The wavelength is 850 nm.

  When the wavelength is 850 nm, Si forming the substrate 21 absorbs the electromagnetic field, and the refractive index of Si is larger than the refractive index of the lower cladding 22, so the distance between the substrate 21 and the bottom surface of the optical waveguide core 23 (= When the height h) of the lower cladding 22 decreases, the propagation loss increases rapidly. On the other hand, when the refractive index of the upper clad 24 is increased, ΔnU / ΔnL decreases, and the propagation loss decreases as ΔnU / ΔnL decreases for the same value of h.

  Thus, by making the refractive index of the upper clad 24 larger than the refractive index of the lower clad 22, the loss due to the substrate 21 is reduced, and an optical waveguide with a lower waveguide loss can be realized.

  FIG. 7 is a diagram showing a change in propagation loss with respect to the height h of the lower cladding. FIG. 7 shows the propagation loss of the 0th-order waveguide mode in the TM-like mode in the optical waveguide structure shown in FIG. 2 when the height h of the lower cladding 22 in FIG. 2 is changed. Here, similarly to FIG. 6, the case where the ratio ΔnU / ΔnL × 100 of the refractive index differences ΔnU and ΔnL is 100%, 75% and 50%, respectively, is shown. The wavelength is 850 nm.

  Also in the TM-like mode, similarly to the TE-like mode, when the Si of the substrate 21 absorbs the electromagnetic field and the height h of the lower cladding 22 is reduced, the propagation loss increases rapidly. On the other hand, when the refractive index of the upper clad 24 is increased, ΔnU / ΔnL decreases, and the propagation loss decreases as ΔnU / ΔnL decreases for the same value of h.

  Thus, by making the refractive index of the upper clad 24 larger than the refractive index of the lower clad 22, the loss due to the substrate 21 is reduced, and an optical waveguide with a lower waveguide loss can be realized.

  FIG. 8 is a graph showing a change in propagation loss with respect to ΔnU / ΔnL. The height of the lower cladding 22 is set to h = 2 μm, and the optical waveguide structure shown in FIG. 2 has a zero-order waveguide mode in the TE-like mode. Indicates propagation loss. Here, the thickness of the upper clad 24 shown in FIG. 2 was set to 3.5 μm immediately above the waveguide core 22 and the wavelength was set to 850 nm.

  When the refractive index of the upper clad 24 is increased, ΔnU / ΔnL decreases, and in a region where ΔnU / ΔnL is 50% or more and 100% or less, propagation loss decreases as ΔnU / ΔnL decreases. You can see the effect. In the region where ΔnU / ΔnL is further reduced, as shown in FIG. 5, the optical power density distribution of the electromagnetic field mode to the upper clad 24 becomes larger, so the influence of the upper clad 24 appears remarkably. As the error bar (width of fluctuation of propagation loss) shown at the data point increases, the propagation loss starts to increase. Therefore, there is an optimum value for ΔnU / ΔnL, and it is considered that ΔnU / ΔnL × 100 = 40% or more and 60% or less is optimal.

  Thus, by making the refractive index of the upper clad 24 higher than that of the lower clad 22 and making the value an optimum value (ΔnU / ΔnL × 100 = 40% or more, 60% or less), the substrate Therefore, an optical waveguide having a lower waveguide loss can be realized.

  FIG. 9 is a diagram showing a change in propagation loss with respect to ΔnU / ΔnL, where the height of the lower cladding 22 is h = 2 μm, and propagation of the 0th-order waveguide mode of the TM-like mode in the optical waveguide structure shown in FIG. Indicates loss. Here, the thickness of the upper clad 23 in FIG. 2 was set to 4 μm immediately above the waveguide core 22 and the wavelength was set to 850 nm.

  When the refractive index of the upper clad 24 is increased, ΔnU / ΔnL decreases, and in a region where ΔnU / ΔnL is 50% or more and 100% or less, propagation loss decreases as ΔnU / ΔnL decreases. You can see the effect. In a region where ΔnU / ΔnL is further reduced, the optical power density distribution of the electromagnetic field mode to the upper clad 24 becomes larger as shown in FIG. 5, so that the influence of the upper clad 24 appears remarkably, and the data in FIG. The error bar (width of fluctuation of propagation loss) indicated by the dot becomes large. Therefore, there is an optimum value for ΔnU / ΔnL, and it is considered that ΔnU / ΔnL × 100 = 40% or less and 60% or less are optimal.

  Thus, by making the refractive index of the upper clad 24 higher than that of the lower clad 22 and making the value an optimum value (ΔnU / ΔnL × 100 = 40% or more, 60% or less), the substrate Therefore, an optical waveguide having a lower waveguide loss can be realized.

  FIG. 10 is a diagram showing the relationship between material composition and material refractive index (H. Baumann and RESah, Journal of Vacuum Science & Technology A, vol. 23, no. 3, pp. 545-550 (2005), FIG. 8 (Ref. 7)). FIG. 10 shows that the change in refractive index can be realized by changing the composition of the material. That is, the refractive index can be changed in accordance with changes in the composition of Si, O, H, and N of the SiON film.

  The composition of the material constituting the optical waveguide core 23 and the upper cladding 24 is preferably SiOxNy, (x, y = 0 to 1), and the material constituting the lower cladding 22 is preferably SiO2.

  FIG. 11 is a cross-sectional view showing a configuration example of an optical communication device for optical interconnection between LSI chips using the optical waveguide of the present invention. This optical communication device has a configuration in which LSI chips are connected via an optical circuit, and includes an LSI mounting board 1101 and a photodiode / light source mounting board 1102. The LSI mounting board 1101 includes a concave mirror 1103, an optical signal output optical waveguide 1104, and an optical signal input optical waveguide 1105 in order to input / output optical signals to / from the photodiode / light source mounting board 1102. The photodiode / light source mounting board 1102 includes a light source electrical wiring layer 1106, a photodiode electrical wiring layer 1107, a VCSEL (surface emitting laser) light source 1108, and a photodiode 1109 that are monolithically formed. The power supplied to the VCSEL light source 1108 and the photodiode 1109 is supplied from the circuit in the LSI package 1110 through the electrical wiring vias 1111 and 1112. The optical waveguide of the present invention is used for at least a part of the optical signal output optical waveguide 1104 and the optical signal input optical waveguide 1105.

  By applying the present invention to the optical waveguide portion, the required light source output is reduced, so that the power consumption required for the optical interconnection between chips on the LSI mounting board is reduced.

  FIG. 12 is a cross-sectional view showing a configuration example of an optical communication device for LSI on-chip optical interconnection using the optical waveguide of the present invention. In this optical communication device, an optical wiring layer 1201 configured by providing a photodetector in a part of an optical waveguide in the structure shown in FIG. 2 is attached to an LSI chip (LSI layer 1203) via a filling layer 1202. It is configured together. Here, an MSM (metal / semiconductor / metal) photodetector having a structure in which a semiconductor absorption layer 1204 and an electrode 1205 are provided along a part of the optical waveguide core 23 is shown as the photodetector. The electrode 1205 is an electrical connection pad provided on the surface of the upper clad 24 via an electrode pad 1206 provided between the lower clad 22 and the upper clad 24 and an electrical connection via 1207 provided in the upper clad 24. 1208 is electrically connected. The LSI layer 1203 includes a transistor 1212, a Cu via wiring 1213, and an LSI electrode pad 1214 on a Si substrate 1211.

  In this structure, an optical signal propagating through the optical waveguide core 23 is absorbed by the semiconductor absorption layer 1204 and converted into an electrical signal. The electrode 1205, the electrode pad 1206, the electrical connection via 1207, the LSI electrode pad 1214, and the Cu via wiring 1213. Is input to the transistor 1212.

  Wirings on the LSI chip including the multilayer wiring are mutually connected to the electrode pads of the optical wiring layer via LSI electrode pads, electrical connection pads, and electrical connection vias.

  By applying the present invention to the optical waveguide portion of the optical wiring layer 1201, the required light source output is reduced, so that the power consumption required for the interconnection in the LSI chip is reduced.

  In each of the above embodiments, for the sake of simplicity, the configuration in which the side surface and the upper surface of the optical waveguide core are covered with the upper clad and the refractive index distribution does not exist in the upper clad has been described. The present invention is not limited to such an embodiment, and the volume average refractive index of at least the region overlapping the optical waveguide core in the upper cladding is such that the volume average refractive index of at least the region between the optical waveguide core and the substrate in the lower cladding. The present invention can be similarly applied to any structure as long as it is configured to be larger than the refractive index.

  While the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

  This application claims the priority on the basis of Japanese application No. 2007-297650 for which it applied on November 16, 2007, and takes in those the indications of all here.

Claims (3)

The first clad, the optical waveguide core and the second clad are laminated in this order on the substrate,
The optical waveguide core is formed on the surface of the first clad, and has a length in a direction parallel to the surface of the substrate and across the optical waveguide direction in a direction perpendicular to the surface of the substrate. 1.5 times or more and 2.5 times or less,
The second clad is formed by covering a side surface and a surface of the optical waveguide core with a material whose volume average refractive index is 1.5 times or more larger than the volume average refractive index of the first clad,
The refractive index of the optical waveguide core is nC, the refractive index of the first cladding is nL, the refractive index of the second cladding is nU, and the refractive index difference between the optical waveguide core and the first cladding is ΔnL = ( NC ² −nL ² ) / (2 × nC ² ), when the refractive index difference between the optical waveguide core and the second cladding is ΔnU = (nC ² −nU ² ) / (2 × nC ² ) The ratio ΔnU / ΔnL × 100 between the refractive index difference ΔnU between the optical waveguide core and the second cladding and the refractive index difference ΔnL between the optical waveguide core and the first cladding is 40% or more and 60%. The following is an optical waveguide. An optical communication device for optical interconnection between LSI chips, in which LSI chips are connected via an optical circuit, wherein the optical waveguide according to claim 1 is used in at least a part of the optical circuit. optical communication devices for optical interconnection between LSI chips. An optical communication device for LSI on-chip optical interconnection in which an electronic circuit and an optical circuit are integrated in one LSI chip, wherein the optical waveguide according to claim 1 is used for at least a part of the optical circuit. was, optical communication devices for optical interconnection of the LSI chip.

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