JP4820917B2 - Manufacturing method of substrate type optical waveguide device having grating structure - Google Patents

Description

  The present invention relates to a method for manufacturing a substrate-type optical waveguide device having a grating structure.

  In recent years, with the development of optical fiber communication systems, particularly the invention of erbium-doped optical fiber amplifiers (EDFA) and dense wavelength division multiplexing communication systems (DWDM), the amount of information transmitted in optical fiber communication networks has increased rapidly. In preparation for further increase in data capacity, research and development are underway on modulation schemes that increase the number of wavelengths to be multiplexed and have high frequency utilization efficiency. In the DWDM system, for example, optical components having more advanced functions are required, such as an optical dispersion compensator that compensates the chromatic dispersion and dispersion slope of each channel more precisely than conventionally used dispersion compensating optical fiber modules. The In addition, research and development has been conducted on variable optical dispersion compensators that can cope with temporal changes and path changes in the dispersion characteristics of optical transmission lines, and polarization mode dispersion compensators that dynamically compensate for polarization mode dispersion. Yes.

  On the other hand, with the rapid expansion of the size and number of installed information communication systems, the enormous amount of power consumed by computer systems and high-end routers is becoming a problem not only from an economic perspective but also from the perspective of environmental impact. There is a need for Green ICT (Information and Communication Technology) that saves power and reduces environmental impact. If various transmission devices such as routers can be miniaturized, the efficiency of housing the equipment in the data center or communication carrier station will be improved and the space utilization efficiency will be improved, and the air conditioning power of the data center or station will be improved. It can be greatly reduced, contributing to energy saving. Therefore, power saving and downsizing are also required for optical components used in various optical transmission apparatuses.

As a technique for manufacturing a small and high-performance optical component, silicon photonics technology for manufacturing an optical waveguide device using a CMOS manufacturing process has been spotlighted and research and development are being promoted. By constructing an optical waveguide using a high refractive index material such as silicon (Si) or silicon nitride (SiN), an optical waveguide device using various conventional silica ( SiO ₂ ) -based glasses as main constituent materials of a core and a clad It becomes possible to reduce the size. Further, by doping Si with an impurity element to obtain a semiconductor material, it is possible to apply a voltage from the outside to adjust the refractive index, and to realize an optical property variable device. Since the manufacturing process is suitable for large-scale mass production, the price of optical components can be expected to be reduced in the future.

Conventionally, a planar optical waveguide device having a Bragg grating pattern, as shown in FIG. 46, and an equal pitch grating structure pitch P _G is a constant of the protrusion 201 and the recess 202 provided in the side wall of the optical waveguide 200, FIG. 47, the pitch of the convex portion 301 and the concave portion 302 provided on the side wall of the optical waveguide 300 is such that P _G ⁱ > P _G ^j > P _G ^k > P _G ^l > P _G ^m > P _G ^n. A gradually changing chirped pitch type grating structure is known.

  In Patent Document 1, a Bragg grating having a certain period is formed in an optical waveguide such as an optical fiber or a substrate-type optical waveguide, and a sampling structure is formed in the optical waveguide so as to overlap the Bragg grating. A chromatic dispersion compensation element that performs chromatic dispersion compensation in a plurality of wavelength channels is disclosed. The sampling structure includes a pattern that is phase-sampled at a certain period longer than the period of the Bragg grating. Each period of phase sampling is divided into a plurality of spatial regions in a direction along the optical axis of the optical waveguide, and the phase of the Bragg grating changes discontinuously at the boundary where adjacent spatial regions are in contact with each other. FIG. As shown in 1A to 1D, there is no discontinuous change in phase within one spatial region.

  Non-Patent Document 1 is an academic paper by the inventors of Patent Document 1, and technical information that complements Patent Document 1 is disclosed. First, a Bragg grating pattern of a single channel at the center wavelength is designed using the knowledge of Patent Document 1. The grating pattern is derived from the desired reflection and chromatic dispersion spectral characteristics by the inverse scattering method. However, in optical fiber Bragg gratings, there is a limit to the range in which the refractive index can be changed in order to produce a grating pattern, so the above spectral characteristics are inverse Fourier transformed and apodized so as not to exceed the limit. Add As described above, a pattern in which the pitch of the Bragg grating continuously changes with the position is obtained. Thereafter, a Bragg grating pattern of a plurality of channels is designed by phase sampling. In optical fiber Bragg gratings, phase sampling is effective because the range of change in refractive index is limited.

  Patent Document 2 discloses that a device having complicated optical characteristics such as an optical dispersion compensator can be realized by designing and manufacturing a substrate type optical waveguide device by solving the inverse scattering problem.

The resolution of the photolithographic process in each technology node of the CMOS device manufacturing technology is not only determined by shortening the wavelength of the light source of the exposure apparatus, but can also be improved by introducing a resolution enhancement technique such as a phase shift mask. A technology node of 400 nm or more uses an i-line light source having a wavelength of 365 nm. A KrF excimer laser with a wavelength of 248 nm was used in each of the technology nodes of 250 nm, 180 nm, and 130 nm. At present, an ArF excimer laser with a wavelength of 193 nm has been introduced, and further, immersion exposure technology has been developed, and technology nodes of 90 nm, 65 nm, and 45 nm have been put into practical use.
The phase shift method is conventionally known as a method for improving the resolution limit in the reduction projection exposure method using a stepper exposure apparatus. According to Non-Patent Document 2, the resolution limit of the phase shift method is improved about twice as compared with the exposure method using a normal transmission mask.

  Conventionally, various optical passive components such as a photonic crystal optical waveguide, a silicon fine wire optical waveguide, and an AWG have been studied as an optical component for an optical fiber communication system using silicon photonics technology, in addition to a modulator and a light receiving and emitting element. Although there is a trend toward commercialization of transceiver modules, research on silicon photonics technology is still in its infancy. Much of the research so far has been carried out using a direct drawing process using an electron beam (EB) apparatus, and knowledge about the photolithography process using a photomask has not yet been accumulated. In the manufacture of a silica glass-based optical waveguide with an initial relative refractive index difference (commonly known as Δ) of about 0.3%, the core width of the optical waveguide is sufficiently thick at about 7 μm, and a photomask with an equal magnification projection is used. It was done. On the other hand, in a high relative refractive index difference optical waveguide used in silicon photonics technology, the effective refractive index perceived by signal light is high, so the core size of a single mode optical waveguide is one-several to several tens of minutes In addition, the interval between the periodic structures of the photonic crystal optical waveguide and the grating optical waveguide is very small. Therefore, a finer process technology is required.

  On the other hand, unlike LSIs in which electronic circuit elements such as DRAMs and CPUs are integrated, optical waveguide devices require a sufficient thickness or depth to form the peripheral structure such as the thickness of the optical waveguide core or cladding. Therefore, it is not always possible to apply the most advanced fine process, and there are many cases where it is necessary to use an older generation technology node, such as the need for thick film resist coating. In addition, for optical components for optical fiber communication systems, which have orders of magnitude less demand than established integrated circuits such as DRAMs and CPUs, the use of industrial 12-inch wafer manufacturing processes for mass production is not necessarily directly linked to cost reduction. However, the production of an appropriate amount using a 6-inch wafer or an 8-inch wafer by an old generation process often results in cost reduction. For example, Non-Patent Document 3 discloses a silicon photonics optical waveguide device for an optical fiber communication system manufactured using a 130 nm technology node. The 130 nm technology node is a process in which, for example, a stepper exposure apparatus having a wavelength of 248 nm is used and resolution is improved using a phase shift mask.

US Pat. No. 6,707,967 Japanese Patent Application Laid-Open No. 2004-077665

H. Li, Y. Sheng, Y. Li, and JE Rothenberg, "Phased-Only Sampled Fiber Bragg Gratings for High-Channel-Count Chromatic Dispersion Compensation," Journal of Lightwave Technology, Vol. 21, No. 9, pp. 2074-2083 (2003) Marc D. Levenson, NS Viswanathan, Robert A. Simpson, "Improving Resolution in Photolithography with a Phase-Shifting Mask," IEEE Transactions on Electron Devices, Vol.ED-29, No.12, pp. 1828-1836 (DECEMBER 1982 ) T. Pinguet, V. Sadagopan, A. Mekis, B. Analui, D. Kucharski, S. Gloeckner, "A 1550 nm, 10 Gbps optical modulator with integrated driver in 130 nm CMOS," 2007 4th IEEE International Conference on Group IV Photonics, (19-21 Sept. 2007)

  Conventionally known equi-pitch grating structures and chirped pitch grating structures provide advanced functionality such as optical dispersion compensation characteristics that simultaneously compensate for chromatic dispersion and dispersion slope of multiple channels as optical characteristics of substrate-type optical waveguide devices. I can't do it. In addition, when manufacturing the device using silicon photonics technology, it is not easy to manage the processing accuracy of each dimension in a structure with gradually changing dimensions such as a chirped pitch type grating structure, and more process management is possible. An easy structure is required.

  The grating structure based on the phase sampling pattern described in Patent Document 1 and Non-Patent Document 1 realizes a multi-channel type optical dispersion compensator even in an optical waveguide having a relatively small effective refractive index amplitude such as a fiber Bragg grating (FBG). I can do it. However, when portions having a high refractive index are arranged according to a predetermined rule along the optical axis of the optical waveguide, the length of the optical waveguide increases as the functionality increases. For this reason, it is not suitable for the purpose of reducing the length and length of the high-functional device.

  In order to realize an optical waveguide device having advanced functions such as an optical dispersion compensator, a grating optical waveguide based on a change in the core width of the optical waveguide is used by using an inverse scattering method as described in Patent Document 2. It is preferable to design and realize a substrate-type optical waveguide device using silicon photonics technology based on the design. However, Patent Document 2 suggests that a special process such as a LIGA process using X-ray lithography may be required when the change in the width of the core is extremely fine. .

  Therefore, the present invention is a substrate type optical waveguide device having a grating structure that can achieve a high degree of functionality, can be reduced in length and reduced in size, and can easily manage processing accuracy in the manufacturing process. It is an object to provide a manufacturing method.

  In order to solve the above problems and achieve the above object, the present invention employs the following means. That is, according to one aspect of the present invention, the core of the optical waveguide has convex portions that are wide portions of the core and concave portions that are narrow portions of the core along the longitudinal direction of the core. A substrate-type optical waveguide device having a non-uniformity in the width of the core in the convex portion and the width of the core in the concave portion, which is made of a high refractive index material. A high refractive index material layer forming step of forming a high refractive index material layer constituting at least a part of the core having a recess, and a photoresist layer forming step of forming a photoresist layer on the high refractive index material layer And using the first photomask which is a phase shift type photomask, the width of the light shielding region is substantially equal to the width of the core in the concave portion at the position corresponding to the concave portion, and corresponds to the convex portion. A first exposure step of forming a light-shielding region on the photoresist layer, the light-shielding region having a width greater than the core width of the convex portion, and exposing the photoresist layer outside the light-shielding region; Using a second photomask that is a mold type photomask, the width of the light-shielding region is larger than the width of the core in the concave portion at the position corresponding to the concave portion, and the width of the light-shielding region at the position corresponding to the convex portion. A second exposure step of forming on the photoresist layer a light shielding region substantially equal to a core width of the convex portion, and exposing the photoresist layer outside the light shielding region; and developing the photoresist layer And developing the high refractive index material layer using the photoresist pattern obtained by the developing step to form the convex portion and the concave portion. And etching step for forming, characterized in that it has at least.

The pitch defined as the total value of the length in the longitudinal direction of the convex portion adjacent to the longitudinal direction of the core and the length in the longitudinal direction of the concave portion may be a non-uniform pitch and a non-chirp pitch. .
Throughout the grating structure, the pitch P _G may also be met _{(P G -P) / ΔP =} N. Here, P is a predetermined pitch reference value, M is an integer value greater than a predetermined value 1, ΔP is a value obtained by dividing P by M, and N is an integer.
In the main pitch in the grating structure, the N may be +1, −1, or 0.

The pitch defined as the total value of the width of the core in the convex part, the width of the core in the concave part, and the length in the longitudinal direction of the convex part adjacent to the longitudinal direction of the core and the length in the longitudinal direction of the concave part is Alternatively, it may be designed by solving an inverse scattering problem using desired optical characteristics.
The inverse scattering problem may be solved using the Zakharov-Shabat equation.

According to the above-mentioned substrate type optical waveguide device manufacturing method, it is possible to reduce the length and reduce the size while achieving high functionality as compared with the conventional chirped grating whose pitch changes gradually. Yes, the processing accuracy in the manufacturing process can be easily managed.
Since the grating optical waveguide was designed by solving the inverse scattering problem using the Zakharov-Shabat equation, an optical dispersion compensator that simultaneously compensates the group delay dispersion and dispersion slope of an optical fiber transmission line for many DWDM channels at once. A substrate-type optical waveguide device having such complicated optical characteristics can be configured in a small size with a short waveguide length.
Since this can be manufactured by silicon photonics technology using CMOS manufacturing process, large-scale mass production is possible, and future price reduction can be expected. Further, a small device can be obtained by adopting a high relative refractive index difference optical waveguide structure.
As a result of designing a grating optical waveguide by solving the inverse scattering problem using the Zakharov-Shabat equation, the grating optical waveguide has a plurality of discrete values with a pitch that is uneven in the core width of the optical waveguide and in the lateral width of the groove structure. Would be something like By taking a plurality of discrete values with a certain grating pitch, the process management becomes easy unlike the chirp type.
A first light shielding region is formed on the photoresist layer using a phase shift photomask so that the lateral width of the light shielding region corresponding to the convex portion is extended to be wider than the design dimension of each core width of the convex portion. A light-shielding region is formed on the photoresist layer using a binary photomask so that the exposure step and the width of the exposure region corresponding to the convex portion are substantially equal to the design dimension of each core width of the convex portion. The grating structure is formed by the exposure step 2. Therefore, a grating optical waveguide having a complicated shape can be manufactured with a structure as designed even if an old-generation exposure device having a wavelength of 248 nm is used.

It is a fragmentary perspective view of the core which shows 1st Embodiment of a board | substrate type optical waveguide device. It is a partial top view of the core which concerns on 1st Embodiment of this invention. 1 is a cross-sectional view of a substrate-type optical waveguide device according to a first embodiment of the present invention. It is explanatory drawing which shows an example of the form which connected the board | substrate type | mold optical waveguide device and the optical transmission line. It is a graph which shows an example of change of effective refractive index n _eff to core width w in a 1st embodiment. It is a graph which shows an example of a reflectance spectrum. It is a graph which expands and shows a part of FIG. It is a graph which shows an example of a group delay spectrum. It is a graph which expands and shows a part of FIG. It is a graph which shows an example of potential distribution. It is a graph which expands and shows a part of FIG. It is a fragmentary perspective view which shows the manufacturing process of 1st Embodiment. It is a fragmentary perspective view which shows the manufacturing process of 1st Embodiment. It is a fragmentary perspective view which shows the manufacturing process of 1st Embodiment. It is a top view which shows a part of chromium pattern of the phase shift type photomask for side wall grating structures. It is a top view which shows a part of phase (pi) shift pattern of the phase shift type photomask for side wall grating structures. It is a top view which shows a part of phase zero shift pattern of the phase shift type photomask for side wall grating structures. It is a top view which shows a part of structure of the phase shift type photomask for side wall grating structures. It is a top view which shows a part of reverse pattern of the binary type photomask for side wall grating structures. It is a top view explaining the relationship between the light-shielding area | region and core by the binary type photomask for side wall grating structures. It is a top view explaining the relationship between the light shielding area | region by the binary type photomask of the modification 1, and a core. It is a top view explaining the relationship between the light shielding area | region by the binary type photomask of the modification 2, and a core. It is a top view which shows a part of photoresist pattern for side wall grating structures. It is a top view which shows a part of pattern of the phase shift type photomask for side wall grating structures concerning the 2nd or 3rd comparative example. These are the fragmentary perspective views of the core which show 2nd Embodiment of a board | substrate type optical waveguide device. These are the fragmentary top views of the core which concerns on 2nd Embodiment of this invention. These are sectional drawings of the board | substrate type optical waveguide device which concerns on 2nd Embodiment of this invention. It is a partial top view of the core for demonstrating _win and wout _in 2nd Embodiment. Is a graph showing an example of a change in n _eff for w _in the second embodiment. Is a graph showing an example of a change in w _out due to the change of w _in the second embodiment. Is a graph showing changes in w _in and w _out for n _eff in the second embodiment. It is a fragmentary perspective view which shows the manufacturing process of 2nd Embodiment. It is a fragmentary perspective view which shows the manufacturing process of 2nd Embodiment. It is a fragmentary perspective view which shows the manufacturing process of 2nd Embodiment. It is a fragmentary perspective view which shows the manufacturing process of 2nd Embodiment. It is a fragmentary perspective view which shows the manufacturing process of 2nd Embodiment. It is a top view which shows a part of chromium pattern of the phase shift type photomask for upper grating structures. It is a top view which shows a part of phase (pi) shift pattern of the phase shift type photomask for upper grating structures. It is a top view which shows a part of phase zero shift pattern of the phase shift type photomask for upper grating structures. It is a top view which shows a part of structure of the phase shift type photomask for upper grating structures. It is a top view which shows a part of reverse pattern of the binary type photomask for upper grating structures. It is a top view which shows a part of photoresist pattern for upper grating structures. It is sectional drawing which shows 3rd Embodiment of a board | substrate type optical waveguide device. Is a graph showing an example of a change in n _eff for w _in the third embodiment. Is a graph showing an example of a change in w _out due to the change of w _in the third embodiment. Is a graph showing an example of a change in w _in and w _out for n _eff in the third embodiment. It is a graph which shows an example of the effective refractive index distribution of 3rd Embodiment. It is a graph which expands and shows a part of FIG. It is a graph which shows an example of distribution of the grating pitch of a 3rd embodiment. It is a graph which expands and shows a part of FIG. 4 is a scanning electron microscope (SEM) photograph obtained by photographing a part of a groove filling body formed in Example 3 from above. It is a scanning electron microscope (SEM) photograph which expands and shows a part of FIG. 4 is a scanning electron microscope (SEM) photograph obtained by photographing a part of the side wall grating structure formed in Example 3 from an obliquely upper side. 4 is a scanning electron microscope (SEM) photograph obtained by photographing a part of the side wall grating structure formed in Example 3 from above. It is a top view which shows an example of the conventional single pitch type | mold grating structure. It is a top view which shows an example of the conventional chirp type | mold grating structure.

Hereinafter, the present invention will be described based on preferred embodiments.
<First Embodiment of Substrate Type Optical Waveguide Device>
1A to 1C schematically show a first embodiment of a substrate-type optical waveguide device of the present invention. 1A is a perspective view of a part of a core 1 of an optical waveguide, FIG. 1B is a top view of the same part of the core 1, and FIG. 1C is a cross-sectional view of a substrate type optical waveguide device. A perspective view of the substrate type optical waveguide device is shown in FIG. In FIG. 1C, reference numeral 2 is used for the side wall of the core 1 without distinguishing the concave portion 2 a and the convex portion 2 b of FIGS. 1A and 1B.
This substrate type optical waveguide device is a substrate type optical waveguide device in which an optical waveguide is formed on a substrate 5. The optical waveguide has a lower clad 6 formed on the substrate 5, a core 1 formed on the lower clad 6, and an upper clad 7 formed on the core 1 and the lower clad 6. Moreover, the grating structure 2 is comprised from the recessed part 2a and the convex part 2b which were formed in the both-sides wall of the core 1 as a periodic change of the width | variety w of the core 1. As shown in FIG.
Here, the core width w refers to the width of the core 1 in the direction perpendicular to the longitudinal direction of the optical waveguide, that is, the direction in which the signal light is guided, and parallel to the substrate. The concave portion 2a has a narrow core width, and the convex portion 2b has a wide core width.
The upper surface 3 and the bottom surface 4 of the core 1 are flat.

The distance that the recess 2a continues in the longitudinal direction of the optical waveguide (left and right direction in FIG. 1B) is referred to as the length of the recess in the longitudinal direction. Moreover, the distance which the convex part 2b continues in the longitudinal direction of an optical waveguide is called the length of the longitudinal direction of a convex part. The grating pitch at that position is a set of adjacent convex portions and concave portions, and the sum of the length in the longitudinal direction of the convex portions and the length in the longitudinal direction of the concave portions.
As will be described later in detail, the substrate type optical waveguide device of the present embodiment takes any value of the discretized pitch obtained as a result of solving the inverse scattering problem. That is, the substrate type optical waveguide device of the present embodiment is different from any of the conventionally known equal pitch grating structure, chirped pitch grating structure, and sampled grating structure.
FIG. 1B shows that the grating pitch takes different values such as P, P + ΔP, and P−ΔP depending on the position in the longitudinal direction of the optical waveguide. Regarding the core width w, FIG. 1B shows a portion where the core width w tends to increase from left to right. As will be described later, the same optical waveguide includes a portion where the core width w tends to decrease from left to right in other portions (not shown).
As described above, since the grating pitch and the core width change in a complicated manner obtained as a result of solving the inverse scattering problem, desired functionality can be imparted to the optical waveguide.

(Device usage example)
FIG. 2 shows an example of a form 100 in which the substrate type optical waveguide device 101 and the optical transmission lines 103 and 105 are connected. Since the device 101 is a reflective device having a grating structure, the start end is the light input incident end and at the same time the light output end. As shown in FIG. 2, an input / output optical fiber is usually connected through a circulator 102 for use. The circulator 102 includes an incident optical fiber 103 that propagates incident signal light, a coupling optical fiber 104 that connects the substrate-type optical waveguide device 101 and the optical circulator 102, and an outgoing optical fiber 105 that propagates outgoing signal light. Is connected.
Further, when an input / output conversion unit called a normal mode field converter or a spot size converter is added at a position where the substrate type optical waveguide device 101 and the coupling optical fiber 104 are optically connected, the coupling optical fiber 104 and the device 101 are added. This is preferable because the connection loss can be reduced.

(Device manufacturing method)
In order to obtain a substrate type optical waveguide device having a grating structure capable of obtaining desired optical characteristics, in the present invention, a potential distribution over the light propagation direction of the optical waveguide is obtained, and this is converted into an equivalent refractive index distribution of the optical waveguide, Convert to the dimensions of the optical waveguide. The potential distribution is calculated from the wave equation that introduces a variable that is the power wave amplitude propagating forward and backward of the optical waveguide, for example, to the Zakharov-Shabat equation with the potential derived from the logarithmic derivative of the equivalent refractive index of the optical waveguide. A design method that estimates the potential distribution to achieve the desired reflection spectrum by reducing it as a backscattering problem that numerically derives the potential function from the complex reflection spectrum, which is the spectrum of the reflectance intensity and phase of the grating optical waveguide. Can be used to design.
This makes it possible to design and manufacture a Bragg grating element having complicated optical characteristics that cannot be realized by a conventionally known equal pitch grating element or chirped pitch grating element. For example, in a DWDM optical fiber communication system 40 A device having desired optical characteristics such as an optical chromatic dispersion compensator that simultaneously compensates for chromatic dispersion and dispersion slope of a transmission line optical fiber in a channel can be realized.

(Design method of potential distribution)
The technique for designing the potential distribution from the desired complex reflection spectrum using the inverse scattering problem is as follows.
In the numerical formula in the design procedure to be described later, the mathematical formula is shown with the longitudinal direction of the grating optical waveguide, that is, the light propagation direction as the z axis. The left-right direction in FIG. 1B is the z-axis direction. A grating region start end of the grating optical waveguide device is set to z = 0, and an end end is set to a z maximum value coordinate. The maximum z value is the region length of the grating optical waveguide portion.

  First, an electromagnetic field propagating in an optical waveguide is described by Sipe's paper (JE Sipe, L. Poladian, and C. Martijn de Sterke, “Propagation through nonuniform grating structures,” Journal of the Optical Society of America A, Vol. 11, Issue 4, pp. 1307-1320 (1994)) is formulated as follows.

Assuming that the time variation of the electromagnetic field is exp (−iωt), the complex amplitude E (z) of the electric field in the optical waveguide and the complex amplitude H (z) of the magnetic field are expressed as follows. The following equations (1) and (2) are obtained by Maxwell's Equations.

Where E (z) is the complex amplitude of the electric field, H (z) is the complex amplitude of the magnetic field, i is the imaginary unit, ω is the angular frequency, μ ₀ is the permeability of vacuum, ε ₀ is the permittivity of vacuum, n _eff Represents the effective refractive index of the optical waveguide.

In order to construct coupled-mode equations from equations (1) and (2), E (z) and H (z) are traveling waves as in the following equations (3) and (4). (Power wave propagating forward ) Converted to amplitude A ₊ (z) and backward wave (power wave propagating backward) amplitude A ₋ (z) . The device is a reflective device that realizes desired optical characteristics as a reflection spectrum. The reflected wave corresponds to the backward wave amplitude A ₋ (z) .

However, n _av is a reference refractive index (average effective refractive index) of the optical waveguide, and this n _av is a standard for n _eff (z). These variables A ₊ (z) and A ₋ (z) satisfy the following expressions (5) and (6), where c _light is the speed of _light in vacuum.

Here, the wave number k (z) is expressed by the following equation (7). Here, c _light is the speed of _light in vacuum.

Further, q (z) in the equation (8) is a potential distribution in the coupled mode equation.

When n (z) in Expression (5) and Expression (6) is substituted with the same expression as n _eff (z) in Expression (7) and Expression (8), Expression (5) and Expression (6) become (9) It is reduced to the Zakharov-Shabat equation shown in equation (10).

Solving the inverse scattering problem shown by the Zakharov-Shabat equation is to solve the Gel'fand-Levitan-Marchenko type integral equations, which will be described later. (PV Frangos and DL Jaggard, “A numerical solution to the Zakharov-Shabat inverse scattering problem,” IEEE Transactions on Antennas and Propagation, Vol. 39, Issue. 1, pp. 74-79 (1991)). .
Also, Xiao's paper (G. Xiao and K. Yashiro, “An Efficient Algorithm for Solving Zakharov-Shabat Inverse Scattering Problem,” IEEE Transaction on Antennas and Propagation, Vol. 50, Issue 6, pp. 807-811 (2002) ) Discloses an efficient solution of the Zakharov-Shabat equation.

The optical characteristic of the substrate type optical waveguide device having the grating structure of the present invention is defined by the following equation (11) as a complex reflection spectrum r (k) at the start end of the optical waveguide (the output light is also output from here). The

As shown in the following equation (12), the Fourier transform of r (k) is the impulse response R (z) of this system.

By giving a desired group delay characteristic and reflectance distribution with respect to the wavelength as the complex reflection spectrum r (k), the potential distribution function q (z) for realizing this can be numerically solved.
In the present invention, the design is performed using an amplitude modulation type grating in which the amplitude of the grating changes and the phase changes depending on the amplitude. Therefore, in the complex reflection spectrum used as design input data, a predetermined group delay time characteristic is obtained from the origin of the frequency (that is, 0 Hz) in order to improve the separation between the envelope of the amplitude of the grating and the phase of the vibration of the grating. Include all available frequency regions.

First, the solutions of the equations (3) and (4) are expressed as the following equations (13) and (14).

A ₊ (z) and A ₋ (z) propagate in the + z direction and the −z direction, respectively. The integral term in Equation (13) and Equation (14) represents the influence of reflection. From Equation (13) and Equation (14), the coupled mode equation is transformed into the following Gel'fand-Levitan-Marchenko type integral equations (15) and (16).

Here, the normalized time y is y _{= c light} t (t is time), a z> y. R (z) is an inverse Fourier transform of the complex reflection spectrum r (k) with the wave number as a variable, and corresponds to an impulse response. The potential distribution q (z) is obtained by solving the equations (15) and (16) by giving R (z), and is given by the equation (17).

By applying the obtained potential distribution q (z) to the following equation (18), an effective refractive index distribution n _eff (z) of the grating optical waveguide is obtained.

  In the present invention, the potential distribution q (z) in the equations (8) and (17) is a real number. As a result, the calculation for converting the complex reflection spectrum r (k) into the impulse response (time response) R (z) becomes a real number type, and the amplitude changes and the phase changes depending on the amplitude.

The effective refractive index distribution n _eff (z) thus obtained is such that the high refractive index value and the low refractive index value appear alternately at a short pitch (period), and shows a grating optical waveguide structure. It has become. This grating structure is a non-uniform one in which the refractive index difference between the adjacent high refractive index value and the low refractive index value corresponding to the core width w in the concave and convex portions on the side wall of the optical waveguide core is not constant but gradually changes. In addition, the pitch at which the refractive index changes takes a limited discrete value. Any of the conventionally known equal pitch grating optical waveguide, chirped pitch grating optical waveguide, and sampled grating optical waveguide is used. Has a new structure that does not match.

  The grating optical waveguide of the present invention forms a grating pattern by changing the amplitude of the Bragg grating, and is an amplitude modulation type in which the sign of the gradient of the envelope of the amplitude of the grating is inverted. The sampled grating optical waveguide is characterized in that an optical waveguide region in which the amplitude is continuously zero is interposed between two points where the sign is inverted. On the other hand, such a structure does not appear in the amplitude modulation type grating optical waveguide of the present application. The inversion of the sign indicates a steep steepness or discontinuity that occurs at an isolated single coordinate point. That is, the sign of the envelope gradient is inverted at a certain z coordinate. Since the amplitude becomes zero only at an isolated coordinate point where the sign of the envelope slope is inverted, there is virtually no region where the amplitude remains constant zero. This makes it possible to shorten the waveguide length as compared with the sampled Bragg grating.

There are a plurality of isolated coordinate points on the waveguide where the sign of the envelope gradient is reversed. Each coordinate point is accompanied by a phase discontinuous change. Since the local period (pitch) changes when the phase changes discontinuously, the pitch is different from half of the value obtained by dividing the center wavelength in the spectrum of interest at the coordinate point by the average value n _av of the effective refractive index of the optical waveguide. Takes a value. The accuracy of specifying the coordinate point where the sign of the envelope gradient is inverted depends on the discrete step of the waveguide coordinate z on the horizontal axis. If the step is ΔP, the accuracy of specifying the coordinate point is in the range of ± ΔP. As described above, in the amplitude modulation type grating optical waveguide of the present invention, the sign of the slope of the envelope of the amplitude of the grating is inverted, and as a result, there are coordinate points at which the pitch changes discretely.
The discretized grating pitch can be expressed as P ± NΔP, where N is an integer related to the discretization parameter when solving the inverse scattering problem.

The discrete change in pitch is a feature not found in chirped Bragg gratings. In the chirped Bragg grating, the pitch continuously changes along the optical waveguide direction. The chirped Bragg grating, which varies the amplitude at the same time of the Bragg grating, changes in the amplitude remains is used for realization of the secondary properties such as apodization, the main such as the number of channel phase characteristics of the reflection spectrum of the filter This characteristic is achieved by changing the frequency of the Bragg grating along the light guiding direction. With the procedure disclosed here, a chirped grating cannot be constructed. In order to construct a chirped grating, it is necessary to switch the conversion from the complex reflection spectrum r (k) to the time response (impulse response) R (z) to the complex type. As a result, the potential distribution q (z) obtained by Expression (17) is a complex number. If q (z) is a complex number, when determining the q (z) the effective refractive index distribution from n _eff (z), for _{n eff} (z) is a real number, to take only the real part of q (z) is required. Therefore, the amplitude modulation type grating structure of the present invention and the conventionally known chirped grating structure are classified into different categories by different design methods. Since it is opposed to the amplitude modulation type, the chirped grating structure is classified as a frequency modulation type.

In the present invention, including all the other embodiments, the calculation used for conversion from the complex reflection spectrum to the impulse response is a real number type, and is intended for an amplitude modulation type Bragg grating. The conditions for selecting the amplitude modulation type Bragg grating are summarized as follows.
(I) The frequency range of the specified spectrum characteristic is all included from the origin (frequency zero) to the region where the corresponding spectrum channel exists.
(II) The real type is selected in the conversion from the above complex reflection spectrum to the impulse response.

In the actual calculation procedure, first, the maximum value of z is specified by determining the total length of the grating optical waveguide device. For example, in the case of an optical dispersion compensator, the maximum value of the group delay time to be generated in the grating optical waveguide is determined from the group delay dispersion value to be compensated and the channel bandwidth. By multiplying by the speed c _light and further dividing by the average value n _av of the effective refractive index, the minimum required element length can be determined. The total length of the element is obtained by adding a certain extra length to this. Subsequently, the discretization step is determined. As an example, 18,000Ramuda element full length relative to the designed center wavelength lambda, by setting the discretization ticks z positioned lambda / 40, the potential distribution of the optical dispersion compensator for 720,000 points from _{z 0} to z ₇₂₀₀₀₀ q (z) will be calculated.

  As an example of the desired optical characteristic for the wavelength given as the complex reflection spectrum r (k), the distribution of reflectance is as shown in FIGS. 4 and 5, and the group delay characteristic is as shown in FIGS. FIG. 8 and FIG. 9 show the obtained potential distribution q (z).

Based on the optical waveguide cross-sectional structure obtained in advance, specifically, the relationship between the core dimension and the equivalent refractive index, the potential distribution q (z) obtained by solving the inverse scattering problem is converted into the effective refractive index distribution n _eff (z). Next, the core size distribution in the light propagation direction (longitudinal direction) of the optical waveguide is calculated.

FIG. 3 shows the correspondence between the effective refractive index n _eff and the core width w of the optical waveguide device according to the first embodiment shown in FIGS. 1A to 1C. In this case, the cladding material is silica ( SiO ₂ ) and the core material is silicon nitride (SiN). The thickness t of the core was 1.4 μm. Mode 1 and mode 2 correspond to a so-called TE mode and TM mode, respectively. In order to obtain the corresponding relationship between the effective refractive index n _eff and the core width w as shown in FIG. 3, the value of the core width w is changed, and the electromagnetic field of the eigen propagation mode is changed from the cross-sectional structure of each optical waveguide. The distribution is obtained by a mode solver program employing various methods such as a mode matching method, a finite element method, or a beam propagation method, and the effective refractive index n _eff is calculated.

In the case of the first embodiment, since the structure of the optical waveguide has polarization dependence, it is necessary to select in advance whether the device to be designed is for the TE mode or the TM mode. For example, when designing a TE mode device, the core width w at each z coordinate can be obtained from the effective refractive index distribution n _eff (z) and the graph of mode 1 in FIG. From FIG. 3, the reference refractive index (average effective refractive index) n _av is set to 1.95, for example, by taking about the center of the range in which the relationship between the effective refractive index and the structural dimension of the optical waveguide is examined.

(Optical waveguide manufacturing process)
Next, the manufacturing process of the optical waveguide device of the first embodiment will be described.
First, as shown in FIG. 10, a high refractive index material layer 1a that is a material of the core 1 is formed (a high refractive index material layer forming step). Here, after forming the lower clad 6 on the support substrate 5, the high refractive index material layer 1 a is formed on the lower clad 6. The support substrate 5 is, for example, a silicon wafer, and the lower clad 6 is an SiO ₂ film deposited with an appropriate thickness using a CVD apparatus or the like. In addition, the high refractive index material layer 1a is obtained by depositing a SiN film for forming an optical waveguide core with a desired thickness using a CVD apparatus or the like.

  Next, as shown by a two-dot chain line in FIG. 10, a photoresist pattern 60 is formed on the high refractive index material layer 1a. The photoresist pattern 60 corresponds to the designed grating structure 2 of the optical waveguide. 13 to 16 show a first photomask pattern used for forming the photoresist pattern 60, and FIG. 17A shows a second photomask pattern used for forming the photoresist pattern 60. Further, the photoresist pattern 60 obtained in FIG. 18 is shown in more detail. Note that only a very small portion of the optical waveguide in the longitudinal direction is shown in FIGS.

In order to produce the photoresist pattern 60 shown in FIG. 18, two photomasks, a first photomask that is a Levenson-type phase shift photomask and a second photomask that is a binary photomask, are used. use. Each photomask can be produced by drawing using CAD or the like. In the following description, a light shielding region formed by projecting the chromium pattern of the first photomask onto the photoresist layer is abbreviated as “first light shielding region”, and the chromium pattern of the second photomask is abbreviated as “first light shielding region”. May be abbreviated as a “second light-shielding region”. In this case, a portion (for example, a light shielding film) that blocks light in the photomask is exemplarily referred to as a “chrome pattern”. However, in the present invention, the material of the light shielding film is not limited to chromium (Cr), and MoSi or the like is used. It can also be used. Further, the phase shift type photomask is not limited to the Levenson type phase shift mask, and a halftone type phase shift mask or the like may be used. As the photomask substrate, a glass substrate such as silica glass is preferably used.
The first Levenson type phase shift photomask has a structure as shown in FIG. The pattern shown in black in FIG. 13 is a chromium pattern made of chromium (Cr), and the pattern shown in black as a reverse pattern in FIG. 14 is a transmission pattern (“phase π shift” corresponding to the phase shift amount π (180 °). The pattern shown in black as a reverse pattern in FIG. 15 is a transmission pattern corresponding to zero phase shift amount (abbreviated as “phase zero shift pattern”). In the first phase shift photomask, the chrome pattern is extended so that the light-shielding region corresponding to the convex portion 2b is sufficiently wider than the design dimension of the core width of the convex portion 2b. The exposure region by the first phase shift type photomask exists outside the first light shielding region.
The second binary type photomask is used to set the core width of the convex portion 2b to a dimension as designed. A pattern shown in black as a reverse pattern in FIG. 17A is a transmission pattern of the second binary photomask. The exposure area by the second binary photomask is present outside the second light shielding area.
By carrying out a two-step exposure process by applying a set of two photomasks, an area that is commonly included in the first light-shielding area and the second light-shielding area becomes an unexposed area. By combining the exposure areas, an exposure pattern shown in white in FIG. 18 is obtained. Further, a photoresist pattern shown in black in FIG. 18 is obtained by the development process.

As described above, in order to obtain the photoresist pattern 60 used for forming the grating structure including the concave portion 2a and the convex portion 2b, the core width at each of the position 60a corresponding to the concave portion 2a and the position 60b corresponding to the convex portion 2b. It is necessary to increase the solubility during development by exposing the photoresist outside the unexposed portion (the portion remaining by development) only in this range. As the photoresist, a photoresist having a property of increasing the solubility by exposure (that is, a positive type) is used.
When trying to obtain the photoresist pattern 60 of the present embodiment, the core width in the concave portion 2a and the core width in the convex portion 2b not only repeatedly increase and decrease along the longitudinal direction of the optical waveguide, but also increase and decrease The problem is that the pitch is extremely small.

For example, as illustrated in Comparative Examples 1 and 2 to be described later, a method using a single exposure process using only one of a binary photomask and a phase shift photomask is conceivable.
However, in the method using only a binary photomask, resolution is difficult unless the pitch of the core structure is sufficiently longer than the wavelength used for exposure.
Further, in the method using only the phase shift type photomask, the phase conflict is caused by overlapping (that is, canceling out) the light having the phase shift amount π and the light having the phase shift amount π outside the position corresponding to the convex portion 2b. As a result, there is a problem that underexposure occurs and a linear structure in which an underexposed portion remains outside the position 60b corresponding to the convex portion 2b of the developed photoresist pattern 60 occurs.

Furthermore, in order to remove the linear structure resulting from the use of the phase shift photomask, a method using both the phase shift photomask and the binary photomask can be considered. As a method at this time, as described in Comparative Example 3 to be described later, a method of using a phase shift type photomask to shield a region corresponding to both the core width in the concave portion 2a and the core width in the convex portion 2b, A method of using a phase shift photomask in an additional exposure process for removing unintended linear structures formed by phase competition can be considered. But in this case,
(I) the lateral width of the chromium pattern of the phase shift photomask at the position of the recess 2a,
(Ii) the lateral width of the chrome pattern of the phase shift type photomask at the position of the convex portion 2b, and (iii) the lateral width of the chrome pattern of the binary type photomask at the position of the convex portion 2b,
These three need to be produced with high accuracy as designed. Further, the width of the resist after development and the core width after etching at the position corresponding to the convex portion 2b are determined based on the width of the portion where the two light-shielding regions are overlapped by the chrome patterns of (ii) and (iii). Therefore, there is a problem that the core width of the convex portion 2b is shortened when the exposure position by the two photomasks is shifted laterally in the direction of the core width in the two exposure steps.

Therefore, in this embodiment, as will be described in detail later,
(I) the lateral width of the chrome pattern of the phase shift photomask at the position of the recess 2a, and (ii) the lateral width of the chrome pattern of the binary photomask at the position of the protrusion 2b,
The two are made as accurate as designed,
(Iii) the lateral width of the chromium pattern of the phase shift photomask at the position of the convex portion 2b, and (iv) the lateral width of the chromium pattern of the binary photomask at the position of the concave portion 2a,
These two are not tailored to the design dimensions. That is, in this embodiment, the core width in the recess 2a is adjusted to the design dimension using the phase shift photomask, and the core width in the protrusion 2b is adjusted to the design dimension using the binary photomask. Even if the exposure positions by the two photomasks are shifted laterally in the direction of the core width in the two exposure steps, the influence on the resist lateral width after development and the core width after etching is small. As a result, a highly accurate photoresist pattern 60 can be produced.
Furthermore, if the lateral width of the second light-shielding region is smaller than the core width in the recess 2a at the position of the recess 2a, the core width in the recess 2a actually formed after etching becomes smaller than the design dimension. Therefore, the lateral width of the second light shielding region is made wider than the core width of the concave portion 2a at the position of the concave portion 2a.

  In this specification, “the width of the light shielding region corresponds to the core width” means that the width of the light shielding region is adjusted so as to obtain the designed core width. If the lateral width of the light shielding area formed by reducing the chrome pattern of the photomask by a predetermined ratio and projecting it onto the photoresist layer is equal to the core width or within the allowable error range, This corresponds to the case where the area width corresponds to the core width. Therefore, the dimension of the chrome pattern is determined in consideration of the reduction ratio.

The method for forming the photoresist pattern 60 is, for example, as follows.
An unexposed photoresist layer is formed on the high refractive index material layer 1a (photoresist layer forming step). The photoresist layer forming step can be performed by coating, for example.
Next, the photoresist layer is exposed using a phase shift type photomask (first exposure step). As described above, the first light shielding region by the chrome pattern of the phase shift type photomask is such that the lateral width of the light shielding region corresponds to the core width in the concave portion 2a at the position of the concave portion 2a of the core 1, and the position of the convex portion 2b. Then, the horizontal width of the light shielding region is larger than the core width of the convex portion 2b. The exposure area exists outside the first light shielding area. Therefore, in the first exposure process, the designed core width is shielded from light at the position corresponding to the recess 2a, and the area wider than the design dimension is shielded from light at the position corresponding to the protrusion 2b.
Next, the photoresist layer is exposed using a binary photomask (second exposure step). As described above, the second light-shielding region by the chrome pattern of the binary type photomask is such that the width of the light-shielding region is larger than the core width of the concave portion 2a at the position of the concave portion 2a of the core 1, and the light-shielding region at the position of the convex portion 2b. Corresponds to the core width of the convex portion 2b. The exposure area exists outside the second light shielding area. Therefore, in the second exposure step, a range wider than the design dimension is shielded from light at the position corresponding to the recess 2a, and the designed core width is shielded from light at the position corresponding to the protrusion 2b.

In consideration of the characteristics of the binary photomask, it is preferable to reduce the change in the lateral width of the second light shielding region along the longitudinal direction of the optical waveguide.
For example, as in the binary photomask having the reverse pattern 62 in FIG. 17C (modified example 1), the horizontal width of the second light-shielding region at the position of the concave portion 2a is larger than the designed core width Wb at the position of the convex portion 2b. The design core when the width of the second light-shielding region at the position of the concave portion 2a is the position of the convex portion 2b, as in the case of a small size or a binary photomask having the reverse pattern 63 of FIG. 17D (Modification 2) A case where the width is larger than the width Wb is also conceivable.
However, in the case shown in FIG. 17C, when the exposure position by the binary photomask is shifted in the waveguide longitudinal direction (z direction), the core width of the convex portion 2b may be partially narrowed (thinned) stepwise. There is. Further, in the case shown in FIG. 17D, when the exposure position by the binary photomask is shifted in the waveguide longitudinal direction (z direction), there is a possibility that only a part of the extended portion of the convex portion 2b remains slender.
Therefore, it is preferable that the lateral width of the second light-shielding region at the position of the recess 2a takes an intermediate value between the lateral widths of the second light-shielding regions at the positions of the two adjacent convex portions 2b. In other words, the width of the second light-blocking region and s ₁ at the position of the convex portion 2b adjacent to the positive direction on the z-axis of a concave portion 2a, the in the position of the convex portion 2b adjacent to the negative direction on the z-axis 2 when the width of the light shielding region and s _2, the lateral width of the second shaded portion at the position of the concave portion 2a is, _{s 1} or s ₂ or less (in the case of _{s 1} ≦ s _2), or, _{s 2} or more s ₁ The following (when s ₂ ≦ s ₁ ) is preferable.

In particular, as shown in FIG. 17B, at a position where the widths of the second light-shielding regions at the positions of the adjacent two convex portions 2b are different from each other, a step 61a that changes the width of the second light-shielding region is provided between the recess 2a. Preferably, on both sides of the step 61a, the lateral width of the second light shielding region in the recess 2a is made equal to the lateral width of the second light shielding region in the adjacent projection 2b.
Thereby, even if the position shift of the binary photomask occurs in the longitudinal direction (z direction) of the optical waveguide, the accuracy of the core width Wb at the position of the convex portion 2b is not easily lowered. The change in the width of the second light-shielding area along the longitudinal direction of the optical waveguide, since only made once per one grating pitch P _G, mask fabrication is facilitated.

The exposure can be performed using a stepper exposure apparatus. The wavelength of the light used for exposure can be appropriately set according to the characteristics of the photoresist, and examples thereof include 248 nm.
After the two-step exposure process, a developing process for developing the photoresist layer, an etching process for etching the high refractive index material layer 1a using the photoresist pattern obtained by the developing process, and a process for removing the remaining photoresist To do.
Thereby, as shown in FIG. 11, the core 1 which has the grating structure 2 which consists of the convex part 2b and the recessed part 2a in a side wall can be formed.
Further, as shown in FIG. 12, an upper clad 7 (for example, SiO ₂ ) is deposited with an appropriate thickness using a CVD apparatus or the like. The thickness of the upper cladding 7 deposited on the core 1 may be different from the thickness of the upper cladding 7 deposited on the lower cladding 6. If necessary, a planarization step can be performed by chemical mechanical polishing (CMP) or the like so that the height from the substrate 5 is uniform.

  Through the above steps, a substrate type optical waveguide having a grating structure on the core side wall can be manufactured. Normally, when using a substrate type optical waveguide device having a large relative refractive index difference, a mode field diameter converter is required for optical connection with an optical fiber. In general, a step called a mode field conversion unit or a spot size conversion unit is formed before and after the above-described steps, and they are integrated on the same substrate so as to be optically connected to the optical waveguide. .

The phase shift type photomask used in the first exposure process has a phase π shift pattern (180 ° shift pattern) and a phase zero shift pattern as a transmission pattern through which light (for example, ultraviolet light) used for exposure of the photoresist in the exposure process passes. This is a type of Levenson type phase shift mask based on a structure in which (0 ° shift pattern) is alternately provided.
When this phase shift type photomask is used by a conventionally known technique, a chromium pattern corresponding to a desired grating shape is formed as shown in FIG. However, as will be described later in Comparative Example 2, since the phase π shift pattern and the phase zero shift pattern are adjacent to each other, there is a problem that the accuracy of the tip of the convex portion and the extension thereof is inferior.
Therefore, in the present invention, the problem is solved by combining the phase shift photomask and the binary photomask by a novel method as described above.

Example 1
The optical dispersion compensator of the substrate type optical waveguide having the structure shown in FIGS. 1A to C and having a grating structure on the side wall of the optical waveguide core with silicon nitride (SiN) as the core and silica glass ( SiO ₂ ) as the cladding is designed. Produced.
The cross-sectional structure of the optical waveguide was designed according to the structure of FIG. 1C, and the correspondence relationship between the effective refractive index n _eff of the waveguide and the waveguide width w shown in FIG. 3 was obtained. The TE mode device was designed and the mode 1 relationship was selected.
Subsequently, a grating pattern was designed. The design center frequency was 188.4 THz. That is, the design center wavelength is 1591.255 nm. ITU-T G.L over 100 channels with L-Band and 45 channels with channel bandwidth of 50 GHz. The dispersion delay single mode optical fiber (DSF) 100 km specified in 653 is compensated for the group delay dispersion and dispersion slope. As the optical characteristics of the optical fiber line to be compensated, the group delay dispersion is −295 ps / nm, the dispersion slope ( Relative Dispersion Slope (RDS) was assumed to be 0.018 / nm. The amplitude intensity reflectance in the channel band was set to 95%. The reflectance spectrum of the complex reflection spectrum r (λ) prepared based on these set values is shown in FIGS. 4 and 5, and the group delay spectrum is shown in FIGS. The total length of the element was 18,000λ, the discretization step of the z position was set to λ / 40, the inverse scattering problem was solved to obtain the prepared spectrum, and the potential distribution q (z) was obtained. The results are shown in FIGS.

Subsequently, the optical waveguide dimensions in FIG. 3 are selected from the vicinity of the center of the effective refractive index range, the reference refractive index (average effective refractive index) n _av is 1.95, and the frequency corresponding to the center wavelength is 188. The potential distribution q (z) was converted to an effective refractive index distribution n _eff (z) at 4 THz (that is, the center wavelength of 1591.255 nm). The optical waveguide dimensions were determined from the obtained effective refractive index distribution n _eff (z) and FIG.
A first phase shift photomask shown in FIG. 16 and a second binary photomask shown in FIG. 17A are manufactured based on the designed optical waveguide dimensions, and an optical waveguide is manufactured using these photomasks. . A stepper exposure apparatus having a wavelength of light used for exposure of 248 nm was used.
When the obtained optical waveguide was observed with a scanning electron microscope (SEM), it was confirmed that a grating structure as designed was formed on the core side wall.

(Comparative Example 1)
An attempt was made to produce the same grating structure as in Example 1 using a normal binary photomask. In this case, the chrome pattern of the photomask is a pattern obtained by similarly expanding the resist pattern shown in FIG. 18 (this is the same as the designed grating structure).
In this case, in a line and space pattern that can be produced using a stepper exposure apparatus having a wavelength of 248 nm, the minimum value of the line width is 190 nm and the minimum value of the space width is 200 nm. Considering sufficient manufacturing tolerance, the grating pitch needs to be 450 nm. At this time, both the line width and the space width are required to be wider than 220 nm.
However, the grating structure designed in Example 1 has a main grating pitch P of 339 nm, and normal binary photomasks cannot be exposed correctly. Note that the length in the longitudinal direction of the convex portion 2b of the grating structure 2 in FIG. 1A corresponds to the line width, and the length in the longitudinal direction of the concave portion 2a corresponds to the space width.
Therefore, as a result of studying the limitations of the stepper used, in this case, the DUV exposure apparatus with a wavelength of 248 nm whose numerical aperture is limited to 0.68, the design is sufficiently resolved in lithography without further resolution improvement. For this purpose, the grating pitch needs to be at least 400 nm.

(Comparative Example 2)
An attempt was made to produce a grating structure similar to that of Example 1 using a conventionally known Levenson type phase shift mask. In this case, the structure of the photomask is as shown in FIG. 19, and the chrome pattern shown in black in FIG. 19 is a pattern in which the designed grating structure is similarly enlarged.
The Cr-free transmission pattern has a structure with two types of thickness so that the phase π shift pattern and the phase zero shift pattern are alternately repeated.
In this case, in a line and space pattern that can be produced using a stepper exposure apparatus having a wavelength of 248 nm, the minimum value of the line width was 140 nm and the minimum value of the space width was 180 nm. At this time, if the grating pitch is 323 nm or more, resolution is possible.
However, in the photomask structure of FIG. 19, the phase π shift pattern and the phase zero shift pattern are adjacent to each other at the position corresponding to the tip of the convex portion of the core. Become. An unintended residual linear structure is formed on the extension of the convex portion of the core due to phase competition.
That is, it is difficult to appropriately manufacture the grating structure of the present invention with a conventionally known Levenson type phase shift mask.

(Comparative Example 3)
In order to remove the linear structure of Comparative Example 2 by trim exposure, a first stage using a Levenson type phase shift mask shown in FIG. 19 and a second stage exposure using a binary photomask shown in FIG. 17A, An attempt was made to produce a grating structure similar to that in Example 1.
Here, the longitudinal direction (that is, the signal propagation direction) of the optical waveguide is defined as the z direction, and the direction perpendicular to the substrate and parallel to the substrate (that is, the core width direction) is defined as the x direction. If the phase-shift photomask and the binary photomask are displaced in the x direction, one of the convex portions that are paired with each other across the central axis of the core is mistakenly exposed in the second stage exposure. . As a result, there arises a problem that the core width Wb of the convex portion 2b is shortened. On the other hand, a part of the linear structure to be removed remains.
That is, even if trim exposure is used in combination with a conventionally known Levenson type phase shift mask, it is difficult to appropriately manufacture the grating structure of the present invention.

In contrast, the grating structure according to the present invention can be appropriately manufactured according to the first embodiment. In the case of Example 1, it is the same as Comparative Example 3 in that the first stage using the Levenson type phase shift mask and the second stage using the binary photomask are the same, but the core width of the recess 2a is The phase shift type photomask is used to match the design dimension, and the core width of the convex portion 2b is adjusted to the design dimension using the binary type photomask. Therefore, even if two types of masks are exposed by being shifted in the core width direction (x direction) in the two exposure steps, the influence on the core width is small.
Further, in the case of Example 1, as shown in FIG. 17B, the horizontal width of the second light shielding region at the position of the concave portion 2a is equal to the horizontal width of the second light shielding region at the position of the two adjacent convex portions 2b. Further, at a location where the horizontal widths of the second light-shielding regions at the positions of the adjacent two convex portions 2b are different from each other, the position of the step 61a that changes the horizontal width of the second light-shielding region is provided in the middle of the concave portion 2a. . For this reason, even if the mask is exposed while being shifted in the longitudinal direction (z direction) of the optical waveguide, if the displacement is within half of the length of the concave portion 2a in the longitudinal direction, left and right in the signal light propagation direction of the optical waveguide. The length at which the tip of the convex part 2b located at the second stage is exposed in the second stage is not shifted. As a result, it is possible to prevent the linear structure from remaining.

<Second Embodiment of Substrate Type Optical Waveguide Device>
20A to 20C schematically show a second embodiment of the substrate-type optical waveguide device of the present invention. 20A is a perspective view of a part of the core 10 of the optical waveguide, FIG. 20B is a top view of the same part of the core 10, and FIG. 20C is a cross-sectional view of the substrate type optical waveguide device. A perspective view of the substrate type optical waveguide device is shown in FIG. In FIG. 20C, reference numerals 12 and 13 are used for the side wall and the groove-like structure of the core 10 without distinguishing the concave portions 12a and 13a and the convex portions 12b and 13b in FIGS. 20A and 20B.
This substrate-type optical waveguide device is a substrate-type optical waveguide device in which an optical waveguide is formed on a substrate 15. The optical waveguide has a lower clad 16 formed on the substrate 15, a core 10 formed on the lower clad 16, and an upper clad 17 formed on the core 10 and the lower clad 16.
Further, in order to eliminate the problem of polarization dependency of optical characteristics, the grating structure 12 is provided on the side wall of the optical waveguide core and the grooved grating structure 13 is provided on the upper part of the core. The bottom surface 14 of the core 10 is flat.

As shown in FIG. 21, the side wall grating structure 12 includes a concave portion 12 a and a convex portion 12 b formed on both side walls of the core 10 as a periodic change of the core width w _out . The core width w _out is the width of the core 10 in the direction perpendicular to the longitudinal direction of the optical waveguide, that is, the direction in which the signal light is guided, and parallel to the substrate. The concave portion 12a has a narrow core width, and the convex portion 12b has a wide core width.
The distance in which the concave portion 12a continues in the longitudinal direction of the optical waveguide (left and right direction in FIG. 20B) is referred to as the longitudinal length of the concave portion. Moreover, the distance which the convex part 12b continues in the longitudinal direction of an optical waveguide is called the length of the longitudinal direction of a convex part. The grating pitch ( P _{G in} FIG. 21) at that position is a set of adjacent convex portions and concave portions and the sum of the width of the convex portions and the width of the concave portions.

The upper surface 11 of the core 10 has a grooved grating structure 13.
The grooved grating structure 13 has a convex portion 13 b formed at a position corresponding to the convex portion 12 b of the sidewall grating structure 12. The convex portion 13 b is a part of the core 10 and protrudes toward the inside of the grooved grating structure 13. At the position where the convex portion 13b is formed, the lateral width of the groove-like structure 13 is narrowed. Further, the grooved grating structure 13 has a recess 13 a formed at a position corresponding to the recess 12 a of the sidewall grating structure 12. The concave portion 13a is a part of the core 10 and has a concave shape relative to the convex portion 13b. At the position where the recess 13a is formed, the lateral width of the groove-like structure 13 is wide. In other words, the width w _in the grooved structure 13 at the convex portion 13b is narrow, whereas, have a relationship that the width w _in the groove-like structure 13 is reversed as wide in the recess 13a.

(Device manufacturing method)
The manufacturing method of the substrate type optical waveguide device of the second embodiment is almost the same as the manufacturing method of the first embodiment. In this embodiment, the optical waveguide has two sets of grating structures, but the design of the grating pattern is the same as in the first embodiment until the design process for calculating the potential distribution q (z).
In the present embodiment, it shows the w _in dependence of the effective refractive index for the TE polarization (mode1) and TM polarization (mode2) in FIG. 22A, showing the relationship between w _in and w _out in Figure 22B. Also in Figure 23 shows the correspondence between the w _in and w _out to the effective refractive index n _eff of the optical waveguide.

In this case, the cladding material is silica ( SiO ₂ ) and the core material is silicon nitride (SiN). The thickness of the clad located at the lower part and the upper part of the core is 2 μm. t _in is 0.1 μm and t _out is 1.4 μm.
Given the relationship between w _in and w _out as shown in FIG. 22B, can be reduced polarization dependence of effective refractive index of the waveguide, as shown in FIG. 22A. The effective refractive index of the TE polarization is regarded as the effective refractive index of the optical waveguide, is plotted to calculate the correspondence between the effective refractive index and w _in and w _out, Figure 23 is obtained. That is, in this embodiment, the set of w _in and w _out are obtained corresponding to a n _eff, devices designed is polarization independent.
From the effective refractive index distribution n _eff (z) and FIG. 23, the width w _in and the core width w _out of the groove-like structure at each z coordinate can be obtained. From FIG. 23, the reference refractive index (average effective refractive index) n _av is set to 1.935, for example, by taking approximately the center of the range in which the relationship between the effective refractive index and the structural dimension of the optical waveguide is examined.

(Optical waveguide manufacturing process)
Next, the manufacturing process of the optical waveguide device of the second embodiment will be described.
First, as shown in FIG. 24, the high-refractive-index material layer 10a serving as the core material is formed up to a portion below the groove-like region (the height range where the groove-like structure 13 is formed) (first High refractive index material layer forming step).
Further, a low refractive index material layer 17a for forming the grooved grating structure 13 is deposited on the high refractive index material layer 10a with a desired thickness (low refractive index material layer forming step).

In the first high refractive index material layer forming step, the lower clad 16 is formed on the support substrate 15, and then the first high refractive index material layer 10 a is formed on the lower clad 16. The support substrate 15 is, for example, a silicon wafer, and the lower clad 16 is a SiO ₂ film deposited with an appropriate thickness using a CVD apparatus or the like. The first high refractive index material layer 10a is obtained by depositing a SiN film for forming the optical waveguide core 10 with a desired thickness using a CVD apparatus or the like. Here, the desired thickness of the first high-refractive-index material layer 10a is the depth of the groove of the grooved grating structure 13 ( t in FIG. 20C) from the final thickness of the SiN film ( t _{out in} FIG. 20C). _in ).
In the low refractive index material layer forming step, a SiO ₂ film is deposited on the SiN film as a low refractive index material layer 17a for forming the grooved grating structure 13 with a desired thickness. Here, the desired thickness of the low refractive index material layer 17a is a value equal to or greater than the depth of the groove of the grooved grating structure 13 ( t _{in in} FIG. 20C). The thickness of the low-refractive index material layer 17a has a margin for allowing a decrease in the thickness of the groove filling member 18 in a planarization step after a second high-refractive index material layer forming step, which will be described later, as necessary. it is preferably set to a value obtained by adding to the design value t _in.

  Next, as shown by a two-dot chain line in FIG. 24, a photoresist pattern 50 is formed on the low refractive index material layer 17a. This photoresist pattern 50 is for forming a groove filling 18 (see FIG. 25) corresponding to the designed grooved grating structure 13. 29 to 32 show a first photomask pattern used for forming the photoresist pattern 50, and FIG. 33 shows a second photomask pattern used for forming the photoresist pattern 50. FIG. 34 shows the photoresist pattern 50 obtained in more detail. 29 to 34 show only a small part in the longitudinal direction of the optical waveguide.

  In the optical waveguide device shown in FIG. 28, the groove filling body 18 fills the inside of the groove-like structure 13 and is integrated with the upper clad 17. The groove filling body 18 has a concave portion 18a that is a narrow groove width portion and a convex portion 18b that is a wide groove width portion. The groove filling body 18 has a shape complementary to the core 10 around the groove structure 13. That is, the concave portion 18 a of the groove filling body 18 corresponds to the convex portion 13 b of the groove-like structure 13, and the convex portion 18 b of the groove filling body 18 corresponds to the concave portion 13 a of the groove-like structure 13.

In order to produce the photoresist pattern 50 shown in FIG. 34, a first Levenson type phase shift type photomask and a second binary type photomask are used. Each photomask can be produced by drawing using CAD or the like.
The first Levenson type phase shift photomask has a structure as shown in FIG. The pattern shown in black in FIG. 29 is a chromium pattern made of chromium (Cr), and the pattern shown in black as a reverse pattern in FIG. 30 is a transmission pattern (“phase π shift” corresponding to the phase shift amount π (180 °). The pattern shown in black as the reverse pattern in FIG. 31 is a transmission pattern corresponding to zero phase shift (abbreviated as “phase zero shift pattern”). In the first phase shift photomask, the light-shielding region corresponding to the recess 13a is larger than the design dimension of the lateral width of the groove-like structure 13 in the recess 13a (projection 18b) (that is, the lateral width of the groove filling body 18 in the projection 18b). Extend the chrome pattern to be wide enough. The exposure region by the first phase shift type photomask exists outside the first light shielding region.
The second binary photomask is used for setting the lateral width of the groove-like structure 13 to the design value win _in the convex portion 13a. A pattern shown in black as a reverse pattern in FIG. 33 is a second binary photomask transmission pattern. The exposure area by the second binary photomask is present outside the second light shielding area.
By carrying out a two-step exposure process by applying a set of two photomasks, an area that is commonly included in the first light-shielding area and the second light-shielding area becomes an unexposed area. An exposure pattern shown in white in FIG. 34 is obtained by combining the exposure regions. Further, a photoresist pattern shown in black in FIG. 34 is obtained by the development process.
In particular, as shown in FIG. 33, the second binary photomask has a horizontal width of the second light-shielding region at the position of the convex portion 13b equal to the horizontal width of the second light-shielding region at the position of the two adjacent concave portions 13a. It is preferable to take an intermediate value. For example, at a location where the widths of the second light-shielding regions at the positions of the two adjacent recesses 13a are different from each other, a step position for changing the width of the second light-shielding region is provided in the middle of the convex portion 13b. On both sides, it is preferable that the lateral width of the second light shielding region in the convex portion 13b is equal to the lateral width of the second light shielding region in the adjacent concave portion 13a.

The method of forming the photoresist pattern 50 for the upper groove grating structure 13 is performed in the same manner as the photoresist pattern 60 for the sidewall grating structure 12 described above.
That is, an unexposed photoresist layer is formed on the low refractive index material layer 17a (photoresist layer forming step).
Next, the photoresist layer is exposed using the phase shift photomask shown in FIG. 32 (first exposure step).
Next, the photoresist layer is exposed using the binary photomask shown in FIG. 33 (second exposure step).
After the two-step exposure process, a developing process for developing the photoresist layer, an etching process for etching the low refractive index material layer 17a using the photoresist pattern obtained by the developing process, and a process for removing the remaining photoresist (Groove filling material forming step). Thereby, as shown in FIG. 25, the groove part filling body 18 which consists of the convex part 18b and the recessed part 18a can be formed in a side wall.

After the formation of the groove filling body 18, a high refractive index material (for example, SiN) constituting the core 10 is deposited with a desired thickness using a CVD apparatus or the like (second high refractive index material layer forming step).
The high refractive index material layer obtained in this manner is the sum of the thickness of the high refractive index material layer 10a in FIG. 25 and the thickness newly deposited after the formation of the groove filler 18 is the final thickness of the core 10 (FIG. 20 C t _out ) or more. Since the high refractive index material is deposited also on the groove filling body 18, the surface is flattened by chemical mechanical polishing (CMP) or the like as shown in FIG. It is made not to remain (flattening process). The thickness of the high refractive index material layer 10b after polishing corresponds to the final thickness of the core 10.
When the thickness of the low refractive index material layer 17a is made larger than the design value tin _in the low refractive index material layer forming step described above, the thickness of the groove filling body 18 is decreased by a predetermined amount in the planarization step, It is preferable because the high refractive index material remaining on the groove filling body 18 can be more reliably prevented. In this case, the thickness of the groove filling body 18 after polishing corresponds to the groove depth ( t _{in in} FIG. 20C) of the grooved grating structure 13.

  Next, as shown by a two-dot chain line in FIG. 26, a photoresist pattern 60 is formed on the high refractive index material layer 10b. This photoresist pattern 60 corresponds to the designed sidewall grating structure 12. The sidewall grating structure 12 can be formed in the same manner as in the first embodiment, except that this embodiment requires alignment of the sidewall grating structure 12 and the grooved grating structure 13.

An etching process for etching the high refractive index material layer 10b using the photoresist pattern 60 obtained by the development process, and then a process for removing the remaining photoresist are performed. As a result, as shown in FIG. 27, the core 10 having the grating structure 12 including the convex portions 12b and the concave portions 12a on the side wall and the groove-like grating structure 13 including the convex portions 13b and the concave portions 13a on the upper side can be formed.
Further, as shown in FIG. 28, an upper clad 17 (for example, SiO ₂ ) is deposited with an appropriate thickness using a CVD apparatus or the like. The thickness of the upper cladding 17 deposited on the core 10 may be different from the thickness of the upper cladding 17 deposited on the lower cladding 16. If necessary, a planarization step can be performed by chemical mechanical polishing (CMP) or the like so that the height from the substrate 15 is uniform.

(Example 2)
20A to 20C, a polarization-independent substrate having a grating structure on each of the core side wall and the core upper portion of the optical waveguide, using silicon nitride (SiN) as a core and silica glass ( SiO ₂ ) as a cladding. The optical dispersion compensator of the type optical waveguide was designed and manufactured.
The cross-sectional structure of a light waveguide designed according to the structure of FIG. 20C, was obtained correspondence relationship between w _in and w _out to the effective refractive index of the optical waveguide as shown in FIG. 23.

  Subsequently, a grating pattern was designed. The design center frequency was 188.4 THz. That is, the design center wavelength is 1591.255 nm. ITU-T G.L over 100 channels with L-Band and 45 channels with channel bandwidth of 50 GHz. It is assumed that the dispersion delay single mode optical fiber (DSF) 100 km specified in 653 is compensated for the group delay dispersion and the dispersion slope, and the optical characteristics of the optical fiber line to be compensated are a group delay dispersion of −295 ps / nm, a dispersion slope (Dispersion Slope, RDS) was assumed to be 0.018 / nm. The amplitude intensity reflectance in the channel band was set to 95%. The reflectance spectrum of the complex reflection spectrum r (λ) prepared based on these set values is shown in FIGS. 4 and 5, and the group delay spectrum is shown in FIGS. The total length of the element was 18,000λ, the discretization step at the z position was set to λ / 40, the inverse scattering problem was solved so as to obtain the prepared spectrum, and the potential distribution q (z) was obtained. The results are shown in FIGS.

Subsequently, the optical waveguide dimensions in FIG. 23 are selected from the vicinity of the center of the effective refractive index range, the reference refractive index (average effective refractive index) n _av is 1.935, and the frequency corresponding to the center wavelength is 188. The potential distribution q (z) was converted to an effective refractive index distribution n _eff (z) at 4 THz (that is, the center wavelength of 1591.255 nm).
The core width of the optical waveguide was determined from the obtained effective refractive index distribution n _eff (z) and the relationship between n _eff (z) and w _out shown in FIG. Further, the resultant effective refractive index distribution n _eff (z), to determine the dimensions of the grooved structure and a relationship n _eff (z) and w _in shown in Figure 23.

A first phase shift photomask shown in FIG. 32 and a second binary photomask shown in FIG. 33 are manufactured based on the dimensions of the designed groove-like structure, and the groove-like structure is produced using these photomasks. Formed. The formation of the groove-like structure was carried out by taking a method in which only a part of the upper clad to be filled was previously produced as the groove filling body 18 and a high refractive index material for the optical waveguide core was deposited later on both sides thereof. .
Therefore, the portion that becomes the convex portion of the grating structure when viewed from the core material corresponds to the concave portion of the groove portion filling body, and the portion that becomes the concave portion of the grating structure when viewed from the core material corresponds to the convex portion of the groove portion filling body. . That is, it should be noted that the relationship between the line width and the space width is reversed. A stepper exposure apparatus having a wavelength of 248 nm was used.
When the obtained groove filling body was observed with a scanning electron microscope (SEM) at the stage where the groove filling body was formed, it was confirmed that the groove filling body as designed was formed.

Further, based on the designed optical waveguide dimensions (core width), the first phase shift photomask shown in FIG. 16 and the second binary photomask shown in FIG. 17A are manufactured, and these photomasks are used. An optical waveguide having a side wall grating structure was manufactured. A stepper exposure apparatus having a wavelength of light used for exposure of 248 nm was used.
When the obtained optical waveguide was observed with a scanning electron microscope (SEM), it was confirmed that a grating structure as designed was formed on the core side wall.

<Third Embodiment of Substrate Type Optical Waveguide Device>
FIG. 35 shows a cross-sectional view of a third embodiment of the substrate-type optical waveguide device. This substrate-type optical waveguide device 20 has a double core structure including inner cores 21 and 22 for making optical characteristics variable and an outer core 24 for solving the problem of polarization dependence of optical characteristics. Adopted.
This double core structure exists on the lower cladding 26 formed on the substrate 25. The upper and both sides of the composite core are covered with an upper clad 27. The upper clad 27 and the lower clad 26 are made of a material lower than the average refractive index of the double core structure. The material of the upper clad 27 and the material of the lower clad 26 may be the same or different.

The inner cores 21 and 22 are divided into two parts through a central gap 23, and each has ribs 21b and 22b and slabs 21a and 22a.
The outer core 24 is disposed on the inner cores 21 and 22. The refractive index of the outer core 24 is lower than the average refractive index of the inner cores 21 and 22. Although not shown in FIG. 35, side wall grating structures and upper groove grating structures similar to the core 10 of FIGS. 20A to 20C are formed on the side wall 24b of the outer core 24 and the groove structure 24c on the upper surface 24a, respectively. ing. Specifically, the sidewall grating structure where the core width w _out periodically changing of the outer core 24, and the width w _in the grooved structure 24c formed on the upper surface 24a of the outer core 24 is periodically changed An upper grooved grating structure is provided.

The manufacturing method of the optical waveguide device 20 of the present embodiment is the same as that of the second embodiment except that the inner cores 21 and 22 and the central gap 23 are formed between the lower clad 26 and the outer core 24. It is the same.
First, an SiO ₂ film to be the lower clad 26 and a thin film silicon layer to be the inner cores 21 and 22 are formed on a silicon wafer to be the support substrate 25. This process is replaced by preparing a commercially available SOI (Silicon on Insulator) wafer having a SiO ₂ film such as a thermal oxide film called a BOX layer and a thin silicon layer formed thereon on the silicon wafer. It is possible.

The silicon of the SOI layer is appropriately patterned by a photolithography process and an etching process, and a P-type semiconductor region and an N-type semiconductor region are formed by implanting an impurity element (dopant). An impurity (dopant) that imparts conductivity to the high-refractive index core made of a semiconductor can be appropriately selected and used according to the base medium. For example, when the base medium is a group IV semiconductor material such as silicon, a group III element such as boron (B) is used as an additive that imparts P-type conductivity, and phosphorus ( V group elements such as P) and arsenic (As) are used.
Alternatively, a nano-gap insulating structure may be employed in which SiO ₂ is deposited by patterning fine grooves in the SOI layer silicon to reduce leakage current. The inner cores 21 and 22 can realize a variable optical property function by applying a voltage from the outside to cause a change in refractive index by the carrier plasma effect. If necessary, a central gap 23 having a nanogap insulating structure is formed, and then the shapes of the silicon ribs 21b and 22b and the silicon slabs 21a and 22a of the inner cores 21 and 22 are processed by a photolithography process and an etching process.

After the inner cores 21 and 22 are formed, the outer core 24 is formed. In the case of the second embodiment described above, the first high refractive index material layer 10a of FIG. 24 is formed on the lower clad 16, but in the case of this embodiment, the high refractive index material layer that is the material of the outer core 24. Is formed on the inner cores 21 and 22. Thereafter, the outer core 24 having the side wall grating structure and the upper groove grating structure can be manufactured by the same process as the method shown in FIGS. Further, SiO ₂ to be the upper clad 27 is deposited on the upper side and both sides of the outer core 24. Further, metal connections and electrode pads for applying a voltage to the inner cores 21 and 22 are formed as necessary.

Optical waveguide device produced by the present invention, throughout the grating structure, the pitch P _G has the characteristic that satisfies the _{(P G -P) / ΔP =} N. Here, P is a predetermined pitch reference value, ΔP is a value obtained by dividing P by M, M is an integer value greater than a predetermined 1, and N is an integer.

As one specific example, the result of converting the potential distribution q (z) shown in FIGS. 8 and 9 to the effective refractive index distribution n _eff (z) based on the structure of Example 3 described later is shown in FIGS. 39. FIG. 38 is an overall view corresponding to an optical waveguide device total length of about 12.2 mm, and FIG. 39 is an enlarged view of the vicinity of 3.308 mm.

A simplified grating in which the effective refractive index distribution n _eff (z) shown in FIGS. 38 and 39 is alternately repeated with a steep (stepwise) change between a convex portion having a constant amplitude and a concave portion having a constant amplitude. Integrated (averaged) to structure. The distribution of the grating pitch obtained in this way is shown in FIGS.
As shown in FIGS. 40 and 41, the grating structure of the present invention has non-uniform optical waveguide dimensions (core width, groove width) at the convex portions and concave portions, and the grating pitch takes a limited discrete value. Has characteristics.

40 and 41 show an example of the grating pitch distribution of the optical dispersion compensator having an element length of about 12.2 mm and a grating period of about 36,000. This is an example in which the optical waveguide dimensions are calculated by designing for L-Band with the reference refractive index (average effective refractive index) n _av being 2.348 and the center wavelength λc being about 1591 nm, and the main grating pitch P is P = Λc / ( n _av × 2) = 339 nm.

Since the inverse scattering problem is solved and the potential distribution q (z) is obtained, it is subdivided into λ / 40 as a discrete step of the z position, so M is 20 and ΔP = P / M = 17 nm. FIG. 40 shows P-10ΔP, P-6ΔP, P-5ΔP, P-4ΔP, P-3ΔP, P-2ΔP, P-ΔP, P, P + ΔP, P + 2ΔP, P + 3ΔP, P + 4ΔP, P + 5ΔP, P + 6ΔP, P + 7ΔP, P + 8ΔP. The presence of grating pitches corresponding to 169 nm, 237 nm, 254 nm, 271 nm, 288 nm, 305 nm, 322 nm, 339 nm, 356 nm, 373 nm, 390 nm, 407 nm, 424 nm, 441 nm, 457 nm, and 474 nm has been observed. There are no grating pitches of P-11ΔP or less and P + 9ΔP or more, and grating pitches of 186 nm, 203 nm, and 220 nm corresponding to P-9ΔP, P-8ΔP, and P-7ΔP.
In FIG. 41, the range of 3.308 to 3.318 mm in the total length of about 12.2 mm (FIG. 40) is shown enlarged. This range corresponds to the range in which a part of the patterns of the four photomasks shown in FIGS. 13 to 16, 17A, 29 to 32, and 33 are displayed. In this region, many pitches are 339 nm corresponding to P, and some pitches are 322 nm corresponding to P−ΔP.

  In general design cases, P is the largest, followed by P ± ΔP. These three types are the main pitches, and the frequency of appearance of the corresponding grating pitch tends to decrease as the integer value N of P ± NΔP increases thereafter. Although not shown in this specification, for example, in the design example of a single channel optical filter, almost all grating pitches are P, only a few P ± ΔP are observed, and P ± NΔP where N is 2 or more appears. There is also a case of not. In Example 3, pitches of P-9ΔP, P-8ΔP, and P-7ΔP did not appear. In addition, P was not observed at all in other optical dispersion compensator design examples, and two types of pitches of P ± ΔP were almost the same number and were the main pitches.

  In this way, taking discrete values of a limited number (small number) of pitches is effective in maintaining the processing accuracy in the CMOS manufacturing process. In the CMOS manufacturing process, it is a common process control technique to measure dimensions using a scanning electron microscope (SEM) such as DICD (Development Inspection Critical Dimension) and FICD (Final Inspection Critical Dimension). It is difficult to manage the pitch accuracy with a structure having a gradually changing pitch such as a grating, but the structure having a small number of discrete values as in the present invention or the only one such as an equal pitch type. Process management is easy in a structure having a pitch.

(Example 3)
An optical dispersion compensator for a substrate-type optical waveguide having the structure shown in FIG. 35, in which silicon (Si) is an inner core, silicon nitride (SiN) is an outer core, and silica glass ( SiO ₂ ) is a cladding was designed and manufactured. .
The cross-sectional structure of a light waveguide designed according to the structure of FIG. 35, the w _in dependence of the effective refractive index for the TE polarization (mode1) and TM polarization (mode2) as shown in FIG. 36A, as shown in FIG. 36B the relationship between w _in and w _out, to determine the correspondence between the w _in and w _out to the effective refractive index of the optical waveguide as shown in FIG. 37.
In designing the optical waveguide structure, the material and dimensions of each part adopted are as follows. Inner cores 21 and 22 are made of silicon (Si), central gap 23 is made of silica glass ( SiO ₂ ), outer core 24 is made of silicon nitride (SiN), substrate 25 is made of silicon (Si), and lower cladding 26 is made of silica glass ( SiO ₂ ). The upper clad 27 was made of silica glass ( SiO ₂ ). The dimensions of each part are as follows: t ₁ = 250 nm, t ₂ = 50 nm, w ₁ = 280 nm, w ₂ = 160 nm, t _out = 600 nm, t _in = 100 nm, the thickness of the lower cladding 26 is 2000 nm, and the maximum thickness of the upper cladding 27 (Thickness on the slabs 21a and 22a) was 2000 nm.

The design of the grating pattern is the same as in the first and second embodiments up to the design process for calculating the potential distribution q (z). Subsequently, the reference refractive index (average effective refractive index) n _av was set to 2.348 by selecting from the vicinity of the center of the effective refractive index range n _eff shown on the horizontal axis of FIG. Further, as a device for L-Band, the frequency corresponding to the center wavelength is set to 188.4 THz (that is, the center wavelength is 1591.255 nm), and the potential distribution q (z) shown in FIGS. It converted into effective refractive index distribution n _eff (z) shown in FIG.
The core width of the optical waveguide was determined from the obtained effective refractive index distribution n _eff (z) and the relationship between n _eff (z) and w _out shown in FIG. Further, the resultant effective refractive index distribution n _eff (z), to determine the dimensions of the groove-like structure from the n _eff (z) and w _in shown in Figure 37.

A first phase shift photomask shown in FIG. 32 and a second binary photomask shown in FIG. 33 are manufactured based on the dimensions of the designed groove-like structure, and the groove-like structure is produced using these photomasks. Formed. The formation of the groove-like structure was carried out by using a method in which only a part of the upper clad to be filled was previously produced as a groove filling body, and a high refractive index material for the optical waveguide core was subsequently deposited on both sides thereof.
When the groove filling body obtained by the groove filling body forming step was observed with a scanning electron microscope (SEM), as shown in FIGS. 42 and 43, the groove filling body made of SiO ₂ had a grooved grating structure as designed. It was confirmed that a complementary structure was formed.

Further, based on the designed optical waveguide dimensions (core width), the first phase shift photomask shown in FIG. 16 and the second binary photomask shown in FIG. 17A are manufactured, and these photomasks are used. An optical waveguide having a side wall grating structure was manufactured. A stepper exposure apparatus having a wavelength of light used for exposure of 248 nm was used.
When the obtained optical waveguide was observed with a scanning electron microscope (SEM), as shown in FIGS. 44 and 45, it was confirmed that a sidewall grating structure as designed was formed on the outer core made of SiN.

DESCRIPTION OF SYMBOLS 1,10 ... Core, 1a, 10a, 10b ... High refractive index material layer, 2, 12 ... Side wall grating structure, 2a, 12a ... Concave part, 2b, 12b ... Convex part, 13 ... Groove-like grating structure (groove-like structure) , 13a ... concave portion, 13b ... convex portion, 3, 11 ... upper surface, 4, 14 ... bottom surface, 5, 15, 25 ... substrate (support substrate), 6, 16, 26 ... lower cladding, 7, 17, 27 ... upper portion Clad, 17a ... Low refractive index material layer, 18 ... Groove filler, 18a ... concave, 18b ... convex, 20 ... Substrate type optical waveguide device, 21,22 ... Inner core, 21a, 22a ... Slab, 21b, 22b ... Rib, 23 ... center gap, 24 ... outer core, 24a ... upper surface, 24b ... sidewall, 24c ... groove-like structure, 50 ... photoresist pattern for groove filling, 60 ... photoresist pattern for sidewall.

Claims (8)

The core of the optical waveguide has a grating structure having a convex portion that is a wide portion of the core and a concave portion that is a narrow portion of the core along the longitudinal direction of the core, and A method of manufacturing a substrate-type optical waveguide device in which the width of the core in the convex portion and the width of the core in the concave portion are non-uniform,
A high refractive index material layer forming step of forming a high refractive index material layer comprising at least part of the core made of a high refractive index material and having the convex part and the concave part;
A photoresist layer forming step of forming a photoresist layer on the high refractive index material layer;
Using the first photomask which is a phase shift type photomask, the horizontal width of the light shielding region is substantially equal to the width of the core in the concave portion at the position corresponding to the concave portion, and the light shielding is performed at the position corresponding to the convex portion. A first exposure step of forming a light-shielding region on the photoresist layer, wherein a lateral width of the region is larger than a core width of the convex portion, and exposing the photoresist layer outside the light-shielding region;
Using a second photomask that is a binary photomask, the width of the light shielding region is larger than the width of the core in the recess at the position corresponding to the recess, and the width of the light shielding region at the position corresponding to the projection. Forming a light shielding region on the photoresist layer substantially equal to the width of the core in the convex portion, and exposing the photoresist layer outside the light shielding region;
A developing step of developing the photoresist layer;
An etching step of etching the high refractive index material layer using the photoresist pattern obtained by the developing step to form the convex portion and the concave portion;
A method for manufacturing a substrate-type optical waveguide device, comprising:   The pitch defined as the sum of the length in the longitudinal direction of the convex portions adjacent to the longitudinal direction of the core and the length in the longitudinal direction of the concave portions is an unequal interval pitch and a non-chirp pitch. A method for manufacturing a substrate-type optical waveguide device according to claim 1. Throughout the grating structure, the pitch P _{G is, (P G -P) / ΔP} = manufacturing method of the planar optical waveguide device according to claim 2, characterized in that satisfy N.
Here, P is a predetermined pitch reference value, ΔP is a value obtained by dividing P by M, M is an integer value greater than a predetermined 1, and N is an integer.   4. The method of manufacturing a substrate type optical waveguide device according to claim 3, wherein the N is one of +1, −1, and 0 at a main pitch in the grating structure. 5.   The width of the core in the convex part, the width of the core in the concave part, and the pitch are designed by solving an inverse scattering problem using desired optical characteristics as input. A method for producing a substrate-type optical waveguide device according to claim 1. 6. The method of manufacturing a substrate type optical waveguide device according to claim 5 , wherein the inverse scattering problem is solved using a Zakharov-Shabat equation.   The pitch defined as the total value of the width of the core in the convex part, the width of the core in the concave part, and the length in the longitudinal direction of the convex part adjacent to the longitudinal direction of the core and the length in the longitudinal direction of the concave part is 2. The method of manufacturing a substrate type optical waveguide device according to claim 1, wherein the substrate type optical waveguide device is designed by solving an inverse scattering problem to be used by inputting desired optical characteristics.   8. The method of manufacturing a substrate type optical waveguide device according to claim 7, wherein the inverse scattering problem is solved using a Zakharov-Shabat equation.