JP6475640B2 - Grating coupler and manufacturing method thereof - Google Patents

Description

  The present invention relates to a grating coupler and a manufacturing method thereof, and more specifically to a grating coupler having a special laminated structure as an optical waveguide structure and a manufacturing method thereof.

  Conventionally, an optical waveguide element in which an optical waveguide is formed on a substrate and light is propagated in a waveguide mode in this optical waveguide. In order to emit guided light to the outside of the optical waveguide or to transmit external light into the optical waveguide. Various grating couplers having a diffraction grating formed on the surface of an optical waveguide have been proposed.

  FIG. 5 is a cross-sectional view schematically showing a cross-sectional structure of an example of a grating coupler. Hereinafter, the structure and function of the grating coupler will be briefly described with reference to FIG.

  The grating coupler 500 has an action of receiving an optical signal through an optical waveguide and converting an optical axis by diffraction. 5A includes a BOX layer 512, a core layer 514 having a higher refractive index than that of the BOX layer 512, and an upper cladding layer 516 having the same refractive index as that of the BOX layer 512 in this order. And the core layer 514 is formed with a diffraction grating portion, and the optical path can be changed only by forming the diffraction grating on the waveguide. Note that the grating coupler 500 includes, for example, (1) uniform GC without focus, (2) uniform GC with focus, (3) non-uniform GC with focus, and the like, as shown in FIG. There are those having various structures.

  Hereinafter, an operation principle and the like for emitting guided light to the outside of the optical waveguide will be schematically described.

As shown in FIG. 5A, when a thin diffraction grating is formed in the thickness direction (x direction) of the waveguide of the grating coupler 500, the waveguide light and the emitted light are phase-matched in the propagation direction (z direction). It must meet, matching condition, the propagation constant of the guided light beta _0, the propagation constant of the radiation and beta _q, a β _{q =} β _{0 +} qK. Where Λ is the period of the diffraction grating, K = 2π / Λ, and q is a value corresponding to the order of the emitted light (0, ± 1, ± 2,...).

In this case, with respect to the normal of the diffraction grating, binding conditions between the emitted light and the waveguide for emitting at an angle θ, the wavelength used for lambda, the effective refractive index of the waveguide to N, refraction n _c of the upper cladding layer If the ratio, Λ is the period of the diffraction grating, then n _c sin θ = N + qλ / Λ.

Considering that the N> n _c, radiation becomes to be limited to the order of q ≦ -1, With a minimum power distribution ratio is high -1st order synchrotron radiation, high efficiency utilization of the radiation It will be illustrated. In addition to the above-described primary radiation light and the like, there is also return light P _{ref in the} waveguide direction, core layer transmitted light P _trans , and radiation light P _down to the substrate 510 side via the BOX layer 512.

Here, the emission angle θ can be arbitrarily designed according to the period Λ, width w, depth d, and thickness D of the optical waveguide shown in FIG. 5A. It is as follows.
Λ: 530 to 550 nm
FF (= 1-w / Λ): 0.3 to 0.6
d: 60 to 80 nm
D: 180-200 nm

  As described above, the case where guided light is emitted outside the optical waveguide by the grating coupler has been described. However, it has been widely performed that external light is incident on the optical waveguide by such a grating coupler.

  For example, in the configuration for emitting the guided light as shown in FIG. 5A to the outside of the optical waveguide, the FF is reduced for the diffraction grating of several periods on the light incident side, or the depth d is set to be d. It is known that the coupling strength of the radiated light is improved when the FF is reduced, and there is a manufacturing limit to reducing the FF. Therefore, the depth d is set for the diffraction grating of several periods on the light incident side by the double etching method. Making it small is proposed (refer nonpatent literature 1).

“CMOS-compatible high efficiency double-etched apodized waveguide grating coupler”, OPTICS EXPRESS Vol. 21, No. 7 (2013/4/8) 7868-7874

  In the configuration for emitting guided light to the outside of the optical waveguide, when the double etching method as described above is employed to reduce the depth d of the grating coupler on the light incident side, the depth accuracy of the diffraction grating portion is increased. Need to be high. However, since it is difficult to accurately control the etching depth even in a normal etching process, there is a problem that manufacturing tolerance is small and it is difficult to achieve sufficient depth accuracy. In double etching, it becomes more difficult to accurately control the etching depth.

  The basic configuration of the grating coupler of the present invention for solving the above problems is roughly as follows.

  An optical waveguide structure of a grating coupler is configured by a laminated structure in which an Si layer, a thin dielectric layer, and an upper Si layer having a refractive index substantially equal to the Si layer and having a diffraction grating portion are formed. The depth of the groove of the diffraction grating portion was made substantially constant so that the bottom surface of the groove was positioned on the surface of the thin dielectric layer. Thereby, even if the depth d of the grating coupler is reduced, the depth accuracy of the diffraction grating portion can be kept high.

  A multi-stage configuration in which the depth d of the grating coupler is reduced only in the vicinity of the end of the grating coupler forming region is as follows.

  On the Si layer, a laminated structure composed of a thin dielectric layer and an upper Si layer having a refractive index substantially equal to that of the Si layer is laminated in a required number of steps in the grating coupler forming region, and a diffraction grating portion near the end of the grating coupler forming region. The optical waveguide structure has a structure in which the depth of the groove is smaller (shallow) than the depth of the groove outside the vicinity of the end portion, and the diffraction grating is formed both near and outside the end portion of the grating coupler forming region. The groove depth of the diffraction grating portion is substantially constant so that the bottom surface of the groove of the portion is positioned on the surface of the thin dielectric layer.

Next, the basic configuration of the manufacturing method of the grating coupler of the present invention is roughly as follows.
(1) An SOI substrate for forming a grating coupler is prepared.
(2) In the grating coupler formation region, a thin dielectric layer is formed on the SOI substrate, and an upper Si layer is formed thereon.
(3) In the grating coupler formation region, the upper Si layer is etched to form a diffraction grating portion of the grating coupler. In the etching process of the upper Si layer in the grating coupler forming region, the thin dielectric layer in the lower layer is used as an etch stopper.
(4) The groove of the diffraction grating portion is filled with the oxide cladding, and the oxide cladding is planarized to form the upper cladding layer.

  Note that a polycrystalline Si layer can be used as the upper Si layer.

The basic configuration of the method for manufacturing a multi-stage grating coupler according to the present invention is roughly as follows.
(1 ′) An SOI substrate for forming a grating coupler is prepared.
(2 ′) In the grating coupler formation region, a thin dielectric layer is formed on the SOI substrate, and an upper Si layer is formed thereon, thereby forming a laminated structure including the thin dielectric layer and the upper Si layer. Such a laminated structure is formed in the required number of steps.
(3 ′) In the vicinity of the end of the grating coupler formation region, the upper Si layer is etched to form a diffraction grating portion of the grating coupler. When etching the upper Si layer in the vicinity of the edge of the grating coupler formation region, the thin dielectric layer in the lower layer is used as an etch stopper, and the depth is substantially constant in the vicinity of the edge of the grating coupler formation region. Shallow trenches are formed.
(4 ′) Outside the vicinity of the end of the grating coupler forming region, the multi-layer laminated structure is etched to form a diffraction grating portion of the grating coupler. When etching the laminated structure outside the vicinity of the edge of the grating coupler formation region, the thin dielectric layer in the lowermost layer is used as an etch stopper, and the depth is substantially outside the vicinity of the edge of the grating coupler formation region. A constant deep groove is formed.
(5 ′) Finally, the groove of the diffraction grating portion is filled with the oxide cladding, and the oxide cladding is planarized to form the upper cladding layer. This completes a multi-stage configuration in which the depth d of the grating coupler is reduced only in the vicinity of the end of the grating coupler formation region.

  Note that a polycrystalline Si layer can be used as the upper Si layer.

  According to the basic configuration of the grating coupler and the manufacturing method of the present invention, when the diffraction grating portion of the grating coupler is formed on the upper Si layer, the dielectric layer provided below the upper Si layer is used as an etch stopper. By using it, it is possible to eliminate variations in etching depth when forming the diffraction grating portion.

  Further, according to the multi-stage grating coupler of the present invention and the method of manufacturing the same, the dielectric layer provided below the upper Si layer has different depths both near and outside the edge of the grating coupler formation region. By using it as an etch stopper, variations in the etching depth when forming the diffraction grating portion can be eliminated.

FIG. 1 is a cross-sectional view schematically showing a cross-sectional structure of an embodiment of the grating coupler of the present invention. FIG. 2 is a cross-sectional view schematically showing a cross-sectional structure of an embodiment of a multi-stage grating coupler according to the present invention. FIG. 3 is a cross-sectional process diagram for explaining an embodiment of the method for manufacturing a grating coupler of the present invention. FIG. 4 is a cross-sectional process diagram for explaining an embodiment of a method for manufacturing a multi-stage grating coupler according to the present invention. FIG. 5 is a cross-sectional view schematically showing a cross-sectional structure of an example of a grating coupler.

  FIG. 1 is a cross-sectional view schematically showing a cross-sectional structure of an embodiment of the grating coupler of the present invention.

  The cross-sectional structure illustrated in FIG. 1 includes a thin dielectric layer 112 and a Si layer 120 on an SOI substrate formed by laminating a BOX layer 102 and a Si layer 120 having a higher refractive index than the BOX layer 102 on the substrate 101. A polycrystalline Si layer 130 having a refractive index substantially equal to that of the diffraction grating portion and a top clad layer 116 filling the groove 150 of the diffraction grating portion of the polycrystalline Si layer 130. The structure is shown.

  Compared with the cross-sectional structure shown in FIG. 5A described above, the Si layer 120, the thin dielectric layer 112, and the Si layer 120 have a refractive index substantially equal to that of the diffraction grating portion formed. The laminated structure composed of the crystalline Si layer 130 has a waveguide function corresponding to the core layer 314 in FIG. 3A, and the effective refractive index N of the waveguide in this case is the combined refractive index of the laminated structure. It is equivalent to.

  In the above-described example, the polycrystalline Si layer 130 is used. However, instead of the polycrystalline Si layer 130, a Si layer similar to the Si layer 120 may be used as the upper Si layer.

  As described above, the grating coupler includes, for example, (1) uniform GC without focus, (2) uniform GC with focus, and (3) non-uniform with focus, as shown in FIG. There are various types such as GC, and any type can be adopted in the present invention.

  FIG. 2 is a cross-sectional view schematically showing a cross-sectional structure of an embodiment of a multi-stage grating coupler according to the present invention. FIG. 2 illustrates a two-stage grating coupler.

  The cross-sectional structure illustrated in FIG. 2 includes an SOI substrate formed by laminating a BOX layer 202 and a Si layer 220 having a higher refractive index than the BOX layer 202 on the substrate 201, and a thin dielectric formed on the SOI substrate. A first layer in which a polycrystalline Si layer 230 having a refractive index substantially equal to that of the layer 212 and the Si layer 220 is laminated, and a polycrystalline Si layer 270 having a refractive index substantially equal to that of the thin dielectric layer 252 and the Si layer 220 are laminated. The second layer, the shallow groove 250 of the diffraction grating portion formed in the polycrystalline Si layer 270 of the second layer in the vicinity of the end of the grating coupler formation region, and the second outside of the vicinity of the end of the grating coupler formation region. The thin dielectric layer 252 and the polycrystalline Si layer 270, and the deep groove 290 in the diffraction grating portion formed in the first polycrystalline Si layer 230, and the shallow groove and the deep groove are buried. In which the phased upper cladding layer 216. illustrating a multi-stage structure composed.

  In contrast to the cross-sectional structure shown in FIG. 5A described above, the multistage structure including the first layer and the second layer has a waveguide function corresponding to the core layer 514 in FIG. 5A. In this case, the effective refractive index N of the waveguide corresponds to the combined refractive index of the multistage structure.

  In the above example, the polycrystalline Si layers 230 and 270 are used. However, one or both of the polycrystalline Si layers 230 and 270 may be replaced with a Si layer similar to the Si layer 120.

  As described above, the grating coupler includes, for example, (1) uniform GC without focus, (2) uniform GC with focus, and (3) non-uniform with focus, as shown in FIG. There are various types such as GC, and any type can be used for the multi-stage grating coupler of the present invention.

  FIG. 3 is a cross-sectional process diagram for explaining an embodiment of a method for manufacturing a grating coupler of the present invention, and this manufacturing method includes steps (a) to (e).

  First, as shown in FIG. 3A, an SOI substrate used for forming a grating coupler is prepared. In this SOI substrate, an Si layer 320 is further formed on the upper surface of a buried oxide layer (BOX layer) 302 formed on the upper surface of the support substrate 301.

  Next, as shown in FIG. 3B, in the grating coupler formation region, a relatively thin dielectric layer 312 is formed on the Si layer 320 in the grating coupler formation region, for example, by thermal oxidation. The dielectric layer 312 may be at least one layer selected from, for example, a silicon oxide layer, a silicon nitride layer, another insulating layer, and the like.

  Next, as shown in FIG. 3C, a polycrystalline silicon layer 330 is formed in the grating coupler formation region.

  Next, as shown in FIG. 3D, after forming a resist pattern in the grating coupler formation region, the polycrystalline silicon layer 330 is etched by a reactive etching method to form a number of grooves 350 in the diffraction grating portion. To do. The dielectric layer 312 functions as an etch stopper when the groove 350 is etched.

  Next, as shown in FIG. 3E, in the grating coupler formation region, a large number of grooves 350 in the diffraction grating portion are filled with an oxide cladding, and planarization is performed by CMP (chemical-mechanical polishing process) to form an upper cladding layer 316. Form.

  In the above example, the polycrystalline Si layer 230 is used. However, a Si layer similar to the Si layer 220 may be used instead of the polycrystalline Si layer 230.

  FIG. 4 is a cross-sectional process diagram for explaining an embodiment of a manufacturing method of a multi-stage grating coupler according to the present invention (a manufacturing method of a 2-stage grating coupler in the case of this embodiment). Includes steps (a ′) to (g ′).

  First, as shown in FIG. 4A ', an SOI substrate used for forming a grating coupler is prepared. In this SOI substrate, an Si layer 420 is further formed on the upper surface of a buried oxide layer (BOX layer) 402 formed on the upper surface of the support substrate 401.

  Next, as shown in FIG. 4B ', in the grating coupler formation region, a relatively thin dielectric layer 412 is formed on the Si layer 420 in the grating coupler formation region, for example, by thermal oxidation. The dielectric layer 412 may be at least one layer selected from, for example, a silicon oxide layer, a silicon nitride layer, another insulating layer, and the like.

  Next, as shown in FIG. 4C ', a polycrystalline silicon layer 430 is formed in the grating coupler formation region.

  Next, as shown in FIG. 4D ′, a relatively thin dielectric layer 452 is formed on the polycrystalline silicon layer 430 in the grating coupler formation region, and the polycrystalline silicon layer 470 is formed on the dielectric layer 452. Form.

  Next, as shown in FIG. 4 (e ′), after forming a resist pattern on the polycrystalline silicon layer 470, the polycrystalline silicon layer 470 is etched by a reactive etching method so as to be near the end of the grating coupler forming region. A shallow groove 450 of the diffraction grating portion is formed. In this case, the dielectric layer 452 functions as an etch stopper when the shallow groove 450 is etched. The number of shallow grooves is not limited to the illustrated example, and may be any number.

  Next, as shown in FIG. 4F ′, after a resist pattern is formed on the polycrystalline silicon layer 470, the polycrystalline silicon layer 470, the thin dielectric layer 452, and the polycrystalline silicon layer 430 are formed by reactive etching. Are sequentially etched to form a deep groove 490 in the diffraction grating portion outside the vicinity of the end of the grating coupler formation region. When the thin dielectric layer 452 is etched in the above etching process, the etching gas is switched to an etching gas having a high selectivity with respect to the dielectric layer 452, and after the etching of the dielectric layer 452, the selectivity with respect to the polycrystalline silicon layer 430 is changed. When the etching gas is switched to a high etching gas (for example, HBr), the lowermost dielectric layer 412 functions as an etch stopper when the deep groove 490 is etched.

In the above example, the etching gas is switched in order to etch the thin dielectric layer 452. However, such switching of the etching gas is only an example, and for example, if dilute hydrofluoric acid is used, the etching gas is switched. Both the crystalline silicon layer 470 and the thin dielectric layer 452 can be etched. However, in this case, instead of using a resist mask, the resist mask may be transferred to a hard mask pattern such as SiN _x to use the hard mask pattern. This is because the resist is easily peeled off during the dilute hydrofluoric acid treatment, whereas SiN _x is stable against the HBr etching and dilute hydrofluoric acid treatment.

  Next, as shown in FIG. 4G ′, in the grating coupler formation region, the shallow groove 450 and the deep groove 490 of the diffraction grating portion are filled with an oxide cladding, and planarized by CMP to form an upper cladding layer 416. Form.

  In the above example, the polycrystalline Si layers 430 and 470 are used. However, one or both of the polycrystalline Si layers 430 and 470 may be replaced with the same Si layer as the Si layer 420.

  In the above embodiment, the manufacturing method of the two-stage grating coupler has been described. However, in the case of manufacturing a three-stage or more grating coupler, the above step (d ′) may be repeated as many times as necessary.

  The embodiments of the present invention have been described above with reference to the drawings. However, those skilled in the art can use other similar embodiments and can appropriately form the embodiments without departing from the present invention. It should be noted that changes or additions can be made. In addition, this invention should not be limited to said embodiment, and should be interpreted based on description of a claim.

101, 201, 301, 401, 501: support substrates 102, 202, 302, 402, 502: BOX layers 112, 212, 252, 312, 412, 452, 512: dielectric layers 116, 216, 316, 416, 516 : Upper cladding layers 120, 220.320, 420: Si layers 130, 230, 270, 330, 430, 470: polycrystalline Si layers 140, 240, 340, 440: rib-type Si waveguides 150, 250, 290, 350 , 450, 490: groove 514: core layer

Claims (5)

  On the Si layer, a laminated structure composed of a thin dielectric layer and an upper Si layer having a refractive index substantially equal to that of the Si layer is laminated in a required number of steps in the grating coupler forming region, and a diffraction grating portion near the end of the grating coupler forming region. The groove of the diffraction grating portion has a structure in which the depth of the groove is smaller than the depth of the groove outside the vicinity of the end portion as the optical waveguide structure, both in the vicinity of the end portion of the grating coupler forming region and in the vicinity of the end portion. A grating coupler having a multistage configuration in which the depth of the grooves of the diffraction grating portion is substantially constant so that the bottom surface of the diffraction grating portion is positioned on the surface of the thin dielectric layer. 2. The grating coupler according to claim 1 , wherein the upper Si layer is a polycrystalline Si layer. 3. The grating coupler according to claim 1, wherein the Si layer is a Si layer of an SOI substrate composed of a support substrate, a BOX layer, and a Si layer. First step of preparing an SOI substrate comprising three layers of a support substrate, a BOX layer, and an Si layer, forming a thin dielectric layer on the SOI substrate in a grating coupler formation region, and forming an upper Si layer thereon When,
A second step of forming a thin dielectric layer on the upper Si layer, and further repeating the step of forming an upper Si layer thereon a required number of times to form a laminated structure comprising the thin dielectric layer and the upper Si layer; ,
Etching the uppermost upper Si layer in the vicinity of the end of the grating coupler forming region to form a shallow groove of the diffraction grating portion in the vicinity of the end of the grating coupler forming region, Using the thin dielectric layer under the upper Si layer as an etch stopper;
Etching the laminated structure outside the vicinity of the end of the grating coupler forming region to form a deep groove of the diffraction grating part outside the vicinity of the end of the grating coupler forming region, the bottom of the laminated structure A thin dielectric layer located at a first step is used as an etch stopper, and a shallow groove and a deep groove of the diffraction grating portion are filled with an oxide cladding, and the oxide cladding is planarized to form an upper cladding layer. Process,
Including
A method of manufacturing a grating coupler, comprising forming a multi-stage configuration in which the depth of the grating coupler is reduced only in the vicinity of the end of the grating coupler forming region. 5. The method of manufacturing a grating coupler according to claim 4 , wherein the upper Si layer is a polycrystalline Si layer.

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