Composite waveguide modulator and method of making the same

Abstract

This invention relates to the field of silicon photonics technology and discloses a composite waveguide modulator and its fabrication method. The fabrication method includes: providing a substrate structure; forming a modulation electrode layer having a first cladding layer and a modulation electrode on the substrate structure; forming a silicon nitride waveguide layer having a second cladding layer and a first silicon nitride waveguide on the modulation electrode layer, wherein the first silicon nitride waveguide and the modulation electrode are spaced apart in a direction perpendicular to the surface of the substrate structure and misaligned in a direction parallel to the surface of the substrate structure; forming a lithium niobate waveguide layer including a third cladding layer and a lithium niobate waveguide on the silicon nitride waveguide layer, wherein the lithium niobate waveguide and the first silicon nitride waveguide are spaced apart in a direction perpendicular to the surface of the substrate structure and at least partially overlap in a direction parallel to the surface of the substrate structure. In this invention, the design of first the modulation electrode, then the first silicon nitride waveguide, and finally the lithium niobate waveguide can improve the CMOS process compatibility of silicon photonic devices based on electro-optic materials such as lithium niobate, enrich device functionality, and increase integration density.

Description

Technical Field

[0001] This invention relates to the field of silicon photonics technology, specifically to composite waveguide modulators and their fabrication methods. Background Technology

[0002] In the field of silicon-based optoelectronics, electro-optic modulators are key components for realizing high-speed optical communication and optical computing. In recent years, electro-optic modulators based on lithium niobate thin films (LNOI) have attracted widespread attention due to their excellent electro-optic coefficient, high bandwidth, and low half-wave voltage. Compared with traditional silicon-based modulators, lithium niobate modulators exhibit significant advantages in terms of high speed, low power consumption, and high linearity.

[0003] In related technologies, LNOI wafers, typically composed of stacked silicon wafers, silicon dioxide cladding, and lithium niobate thin films, are used as the fabrication platform for electro-optic devices. This fully utilizes the high electro-optic coefficient of the lithium niobate thin film and the high refractive index difference between silicon dioxide and lithium niobate, providing an ideal platform for the fabrication of high-performance electro-optic modulators. To further improve modulation performance and achieve compatibility with CMOS processes, related technologies have also explored heterogeneous integration of lithium niobate thin films with silicon nitride waveguides to form composite waveguide structures, thereby reducing optical transmission loss and improving modulation efficiency. For example, a silicon waveguide is first fabricated on a silicon-on-insulator substrate, followed by the deposition of a silicon oxide cladding and chemical mechanical polishing. Then, a silicon nitride waveguide is grown using low-pressure chemical vapor deposition (LPCVD). Next, silicon oxide is deposited using plasma-enhanced chemical vapor deposition (PECVD) and planarized again using chemical mechanical polishing to form a bonding surface with the LNOI wafer. After bonding, the silicon substrate on the back of the LNOI wafer is removed, and metal electrodes are fabricated on a lithium niobate film using physical vapor deposition (PVD) and damascus processes.

[0004] However, in the above technical solutions, on the one hand, the metal electrode fabrication step is arranged after the bonding of lithium niobate, which makes the entire process unable to be completed on a standard CMOS production line, resulting in poor process compatibility, increased manufacturing costs, and limitations on the integration and mass production capabilities of silicon photonic devices. Summary of the Invention

[0005] This invention provides a composite waveguide modulator and its fabrication method to solve the problems of poor process compatibility and difficulty in further improving the optoelectronic modulation efficiency in existing devices.

[0006] In a first aspect, the present invention provides a method for fabricating a composite waveguide modulator, comprising: Provide the base structure; A modulation electrode layer is formed on one side surface of the substrate structure. The modulation electrode layer includes a first cladding layer and a modulation electrode located within the first cladding layer. A silicon nitride waveguide layer is formed on the side of the modulation electrode layer away from the substrate structure. The silicon nitride waveguide layer includes a second cladding and a first silicon nitride waveguide located within the second cladding. The first silicon nitride waveguide and the modulation electrode are spaced apart in a direction perpendicular to the surface of the substrate structure and staggered in a direction parallel to the surface of the substrate structure. A lithium niobate waveguide layer is formed on the side of the silicon nitride waveguide layer away from the substrate structure. The lithium niobate waveguide layer includes a third cladding and a lithium niobate waveguide located within the third cladding. The lithium niobate waveguide and the first silicon nitride waveguide are spaced apart in a direction perpendicular to the surface of the substrate structure and at least partially overlap in a direction parallel to the surface of the substrate structure. The lithium niobate waveguide and the first silicon nitride waveguide are configured to form a composite waveguide structure.

[0007] Beneficial Effects: In the fabrication method of the composite waveguide modulator of the present invention, after forming the substrate structure, the metal modulation electrode is first fabricated, and then a first silicon nitride waveguide and a lithium niobate waveguide are sequentially formed on top of it, ultimately forming a modulator with a buried modulation electrode and a modulation region lithium niobate-silicon nitride composite waveguide structure. This process, which proceeds with the modulation electrode first, then the first silicon nitride waveguide, and finally the lithium niobate waveguide, firstly, the arrangement of the lithium niobate waveguide after the modulation electrode and the first silicon nitride waveguide helps improve process compatibility. The final fabricated lithium niobate waveguide does not cause problems with the preceding process flow on the standard CMOS production line due to lithium niobate material, thus helping to reduce costs and improve mass production capabilities. Secondly, in the present invention, the modulation electrode is fabricated first, located below the subsequently fabricated first silicon nitride waveguide and lithium niobate waveguide. This buried modulation electrode ensures a small longitudinal optical field coupling distance between the first silicon nitride waveguide and the lithium niobate waveguide, improving optical field coupling efficiency and providing more flexible process control.

[0008] In one alternative embodiment, a modulation electrode layer is formed on one side surface of the substrate structure, including: A first sub-cladding layer is disposed on one side surface of the substrate structure; An electrode groove is formed on the side of the first sub-cladding that is away from the substrate structure. A metal material is filled into the electrode groove to form a modulation electrode; A second sub-cladding layer is formed on the side of the first sub-cladding layer that is away from the substrate structure. The second sub-cladding layer covers the modulation electrode and together with the first sub-cladding layer forms the first cladding layer.

[0009] Beneficial effects: By first forming an embedded modulation electrode through grooving, the metal modulation electrode is completely covered by the first cladding layer, which ensures that the performance of the modulation electrode is not affected by subsequent processes, and also ensures the efficient fabrication of subsequent composite waveguide structures.

[0010] In one alternative embodiment, a silicon nitride waveguide layer is formed on the side of the modulation electrode layer away from the substrate structure, comprising: A silicon nitride wafer is fabricated, comprising a second substrate layer, a second isolation layer, and a silicon nitride layer stacked together. The silicon nitride wafer is bonded to the surface of the modulation electrode layer away from the substrate structure, wherein the silicon nitride layer is connected to the first cladding layer of the modulation electrode layer; Remove the second substrate layer and at least a portion of the second isolation layer; The silicon nitride layer is patterned to obtain the first silicon nitride waveguide; A second cladding layer is disposed on the side of the first silicon nitride waveguide away from the modulation electrode layer, and the second cladding layer covers the first silicon nitride waveguide and the first cladding layer.

[0011] Beneficial effects: A first silicon nitride waveguide is formed above the modulation electrode in the longitudinal direction and misaligned with the modulation electrode in the transverse direction, reducing the optical field interference and optical loss of the modulation electrode on the first silicon nitride waveguide; a cladding material is disposed on the modulation electrode layer to form a second cladding covering the first silicon nitride waveguide, thereby forming a silicon nitride waveguide layer with the first silicon nitride waveguide and the second cladding.

[0012] In one alternative embodiment, the step of preparing a silicon nitride wafer includes: Provide a second substrate layer; A second isolation layer is formed on one side surface of the second substrate layer; A silicon nitride layer is formed on the side of the first isolation layer away from the second substrate layer using a low-pressure chemical vapor deposition process. The silicon nitride layer is annealed.

[0013] Beneficial effects: In this invention, since the fabrication of the entire silicon nitride wafer and the device body are spatially separated, when the modulation electrode is an embedded electrode, a low-pressure chemical vapor deposition process can also be used to form a silicon nitride layer, forming a silicon nitride thin film with low transmission loss. This not only forms a high-quality silicon nitride thin film, but also avoids the adverse effects of metal electrodes and lithium niobate waveguides on electric field, optical field, etc., thereby ensuring the modulation effect.

[0014] In one optional embodiment, after forming the silicon nitride waveguide layer on the side of the modulation electrode layer away from the substrate structure, and before forming the lithium niobate waveguide layer on the side of the silicon nitride waveguide layer away from the substrate structure, the method further includes: A stop layer is formed on the surface of the silicon nitride waveguide layer facing away from the substrate structure, and the stop layer at least covers the first silicon nitride waveguide. A fourth cladding layer is formed, covering the stop layer and the second cladding layer.

[0015] Beneficial effects: After the first silicon nitride waveguide is fabricated, a protective layer is first formed on the first silicon nitride waveguide, which is also the stop layer when the lithium niobate waveguide is fabricated on the first silicon nitride waveguide, ensuring that the performance of the first silicon nitride waveguide is not affected by subsequent process steps.

[0016] In one alternative embodiment, after forming a fourth cladding layer covering the stop layer and the second cladding layer, and before forming a lithium niobate waveguide layer on the side of the silicon nitride waveguide layer away from the substrate structure, the embodiment further includes: forming a contact electrode that extends inward from a portion of the surface of the fourth cladding layer to a connection modulation electrode.

[0017] Beneficial effects: The contact electrode is fabricated after the first silicon nitride waveguide is fabricated and before the lithium niobate waveguide is fabricated. The part of the contact electrode located next to the first silicon nitride waveguide is thinner than the modulation electrode and is relatively far away from the silicon nitride waveguide. This allows for the input of electrical signals without affecting the generation and transmission of optical signals.

[0018] In one alternative embodiment, a lithium niobate waveguide layer is formed on the side of the silicon nitride waveguide layer away from the substrate structure, comprising: A bonding groove is formed on the fourth cladding layer, exposing the stop layer; Remove the stop layer; A lithium niobate chip is disposed in a bonding trench. The lithium niobate chip includes a third substrate layer, a third isolation layer and a lithium niobate layer stacked together. The lithium niobate layer is bonded to the second cladding layer of the silicon nitride waveguide layer facing the bottom of the bonding trench to form a lithium niobate waveguide. Remove the second substrate layer; Filler adhesive is placed in the bonding groove to form a third cladding surrounding the lithium niobate waveguide.

[0019] Beneficial effects: The heterogeneous integration of lithium niobate waveguides on top of first silicon nitride waveguides through grooving and bonding processes helps to control the coupling distance between the lithium niobate waveguides and the first silicon nitride waveguides, and also helps to form higher quality lithium niobate waveguides and reduce pollution.

[0020] In one alternative embodiment, the substrate structure includes a stacked first substrate layer, a first isolation layer, and a silicon device layer, the silicon device layer including a variety of silicon elements, and a first cladding layer of the modulation electrode layer adapted to cover the surfaces of the various silicon elements and the first isolation layer.

[0021] Beneficial effects: Various silicon components can be used in conjunction with various silicon nitride components, which helps to reduce signal input and output losses.

[0022] In a second aspect, the present invention also provides a composite waveguide modulator, comprising: a substrate structure, a modulation electrode layer, a silicon nitride waveguide layer, and a lithium niobate waveguide layer. The modulation electrode layer is formed on one side surface of the substrate structure, including a first cladding and a modulation electrode located at least within the first cladding. The silicon nitride waveguide layer is formed on the side of the modulation electrode layer away from the substrate structure, including a second cladding and a first silicon nitride waveguide located within the second cladding. The first silicon nitride waveguide and the modulation electrode are spaced apart in a direction perpendicular to the surface of the substrate structure and staggered in a direction parallel to the surface of the substrate structure. The lithium niobate waveguide layer is formed on the side of the silicon nitride waveguide layer away from the substrate structure, including a third cladding and a lithium niobate waveguide located within the third cladding. The lithium niobate waveguide and the first silicon nitride waveguide are spaced apart in a direction perpendicular to the surface of the substrate structure and at least partially overlap in a direction parallel to the surface of the substrate structure. The lithium niobate waveguide and the first silicon nitride waveguide are configured to form a composite waveguide structure.

[0023] Beneficial Effects: The composite waveguide modulator of the present invention includes a buried modulation electrode and a composite waveguide structure of lithium niobate-silicon nitride in the modulation region. First, the lithium niobate waveguide is located at the top of the device, which helps improve process compatibility. The lithium niobate waveguide does not cause the problem of lithium niobate material affecting the preceding process flow on the standard CMOS production line, which helps to reduce costs and improve mass production capabilities. Second, in this embodiment, the modulation electrode is located at the bottom of the first silicon nitride waveguide and the lithium niobate waveguide. This buried modulation electrode also ensures a small longitudinal optical field coupling distance between the first silicon nitride waveguide and the lithium niobate waveguide, improving optical field coupling efficiency and making process control more flexible.

[0024] In one alternative embodiment, it further includes a contact electrode that extends inward from a portion of the surface of the cladding structure away from the substrate structure to connect to the modulation electrode.

[0025] Beneficial effects: The contact electrode is fabricated after the first silicon nitride waveguide is fabricated and before the lithium niobate waveguide is fabricated. The part of the contact electrode located next to the first silicon nitride waveguide is thinner than the modulation electrode and is relatively far away from the silicon nitride waveguide. This allows for the input of electrical signals without affecting the generation and transmission of optical signals. Attached Figure Description

[0026] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1This is a schematic flowchart illustrating the fabrication method of the composite waveguide modulator according to an embodiment of the present invention. Figure 2 This is a schematic diagram of the structure of a silicon-on-insulator substrate according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the base structure according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure after the first sub-cladding layer is provided on the substrate structure according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the structure after the electrode grooves are formed in an embodiment of the present invention; Figure 6 This is a schematic diagram of the structure after the modulation electrode is formed in the electrode groove according to an embodiment of the present invention; Figure 7 This is a schematic diagram of the structure after the second sub-cladding layer is formed on the modulation electrode according to an embodiment of the present invention; Figure 8 This is a schematic diagram of the structure of a silicon nitride wafer according to an embodiment of the present invention; Figure 9 This is a schematic diagram of the structure after the silicon nitride wafer is bonded to the first cladding layer according to an embodiment of the present invention; Figure 10 This is a schematic diagram of the structure after removing the second substrate layer in an embodiment of the present invention; Figure 11 This is a schematic diagram of the structure after the first silicon nitride waveguide is formed according to an embodiment of the present invention; Figure 12 This is a schematic diagram of the structure after the second cladding is formed on the first silicon nitride waveguide according to an embodiment of the present invention; Figure 13 This is a schematic diagram of the structure after the initial stop layer is formed on the second cladding layer according to an embodiment of the present invention; Figure 14 This is a schematic diagram of the structure after the stop layer is formed according to an embodiment of the present invention; Figure 15 This is a schematic diagram of the structure after the fourth cladding layer is formed on the stop layer according to an embodiment of the present invention; Figure 16 This is a schematic diagram of the structure after forming the electrode through-hole and contact window in an embodiment of the present invention; Figure 17 This is a schematic diagram of the structure after the contact electrode is formed in the electrode through hole and the contact window according to an embodiment of the present invention; Figure 18 This is a schematic diagram of the structure after forming a bonding groove in the fourth cladding layer according to an embodiment of the present invention; Figure 19 This is a schematic diagram of the structure after removing the bottom stop layer of the bonding groove in an embodiment of the present invention; Figure 20This is a schematic diagram of the structure after a lithium niobate chip is placed in the bonding groove according to an embodiment of the present invention; Figure 21 This is a schematic diagram of the structure of the composite waveguide modulator according to an embodiment of the present invention.

[0028] Explanation of reference numerals in the attached figures: 1. Substrate structure; 11. First substrate layer; 12. First isolation layer; 13. Silicon device layer; 13a. First silicon waveguide; 13b. Second silicon waveguide; 13c. Grating structure; 10. Silicon-on-insulator substrate; 101. Bottom silicon; 102. Intermediate layer; 103. Top silicon; 2. Modulation electrode layer; 21. First cladding layer; 211. First sub-cladding layer; 212. Second sub-cladding layer; 22. Modulation electrode; 23. Electrode groove; 3. Silicon nitride waveguide layer; 31. Second cladding layer; 32. First silicon nitride waveguide; 33. Second silicon nitride waveguide; 34. Third silicon nitride waveguide; 30. Silicon nitride wafer; 301. Second substrate layer; 302. Second isolation layer; 303. Silicon nitride layer; 4. Lithium niobate waveguide layer; 41. Third cladding layer; 42. Lithium niobate waveguide; 40. Lithium niobate chip; 401. Third substrate layer; 402. Third isolation layer; 403. Lithium niobate layer; 5. Stopping layer; 50. Initial stopping layer; 6. Fourth cladding layer; 601. Bonding groove; 7. Contact electrode; 701. Interconnect electrode; 702. Pad; 701a. Electrode via; 702a. Contact window. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] like Figures 1 to 21 As shown, the present invention provides a method for fabricating a composite waveguide modulator. Figure 1 The diagram below illustrates the process of this preparation method, which specifically includes the following steps: Step S100: Provide the substrate structure 1.

[0031] For example, the substrate structure 1 can be a silicon substrate, or it can be fabricated using a silicon-on-insulator substrate 10. (See reference...) Figure 2 and Figure 3The silicon-on-insulator substrate 10 includes a bottom silicon layer 101, an intermediate layer 102, and a top silicon layer 103 stacked together. The intermediate layer 102 is a silicon oxide layer, and the thickness of the top silicon layer 103 can be 220 nm. The top silicon layer 103 of the silicon-on-insulator substrate 10 is patterned to obtain various silicon devices, such as a grating structure 13c, a first silicon waveguide 13a, and a second silicon waveguide 13b obtained through photolithography and etching. The trench depth of the grating structure 13c is 70 nm, and the grating structure 13c is formed before the silicon waveguide. The bottom silicon layer 101 of the silicon-on-insulator substrate 10 serves as the first substrate layer 11 of the substrate structure 1, the intermediate layer 102 serves as the first isolation layer 12 of the substrate structure 1, and the various silicon devices serve as the silicon device layer 13 of the substrate structure 1.

[0032] In step S200, a modulation electrode layer 2 is formed on one side surface of the substrate structure 1. The modulation electrode layer 2 includes a first cladding layer 21 and a modulation electrode 22 located within the first cladding layer 21.

[0033] refer to Figures 4 to 7 An embedded modulation electrode 22 can be formed by first setting a portion of the first cladding 21 on the substrate structure 1, then slotting and filling it with metal on its upper side; then covering another portion of the first cladding 21 to form a first cladding 21 covering the modulation electrode 22, and finally forming a modulation electrode layer 2 with the modulation electrode 22 embedded in the first cladding 21.

[0034] In step S300, a silicon nitride waveguide layer 3 is formed on the side of the modulation electrode layer 2 away from the substrate structure 1. The silicon nitride waveguide layer 3 includes a second cladding layer 31 and a first silicon nitride waveguide 32 located within the second cladding layer 31. The first silicon nitride waveguide 32 and the modulation electrode 22 are spaced apart in a direction perpendicular to the surface of the substrate structure 1 and are offset in a direction parallel to the surface of the substrate structure 1.

[0035] refer to Figures 8 to 12 First, a plurality of silicon nitride elements are formed on the surface of the first cladding 21 on the side of the modulation electrode layer 2 that is relatively far from the substrate structure 1. For example, at least a first silicon nitride waveguide 32 is formed above the modulation electrode 22 in the longitudinal direction and offset from the modulation electrode 22 in the transverse direction. For example, the transverse offset here can be understood as, Figure 11 As shown, when three modulation electrodes 22 are formed, two first silicon nitride waveguides 32 are respectively disposed in the interval regions of the three modulation electrodes 22; finally, a cladding material is disposed on the modulation electrode layer 2 to form a second cladding 31 covering the first silicon nitride waveguides 32, thereby forming a silicon nitride waveguide layer 3 having the first silicon nitride waveguides 32 and the second cladding 31, as shown. Figure 12 As shown.

[0036] In step S400, a lithium niobate waveguide layer 4 is formed on the side of the silicon nitride waveguide layer 3 away from the substrate structure 1. The lithium niobate waveguide layer 4 includes a third cladding layer 41 and a lithium niobate waveguide 42 located within the third cladding layer 41. The lithium niobate waveguide 42 and the first silicon nitride waveguide 32 are spaced apart in a direction perpendicular to the surface of the substrate structure 1 and at least partially overlap in a direction parallel to the surface of the substrate structure 1. The lithium niobate waveguide 42 and the first silicon nitride waveguide 32 are configured to form a composite waveguide structure.

[0037] For example, a lithium niobate waveguide 42 is formed on the side of the silicon nitride waveguide layer 3 that is relatively far from the substrate structure 1. The outer side of the lithium niobate waveguide 42 is covered by a third cladding layer 41. The lithium niobate waveguide 42 can be a planar wide waveguide. The lithium niobate waveguide 42 and the silicon nitride waveguide correspond at least partially in the longitudinal direction. In this embodiment, it is preferable that the lithium niobate waveguide 42 completely covers the underlying silicon nitride waveguide to form a strongly coupled silicon nitride-lithium niobate composite waveguide structure.

[0038] In summary, in the fabrication method of the composite waveguide modulator in this embodiment, after forming the substrate structure 1, the metal modulation electrode 22 is first fabricated, and then the first silicon nitride waveguide 32 and the lithium niobate waveguide 42 are sequentially formed on top of it, ultimately forming a modulator with an embedded modulation electrode 22 and a modulation region lithium niobate-silicon nitride composite waveguide structure. This process, which involves first the modulation electrode 22, then the first silicon nitride waveguide 32, and finally the lithium niobate waveguide 42, has several advantages. First, the lithium niobate waveguide 42 is placed after the modulation electrode 22 and the first silicon nitride waveguide 32, which helps improve process compatibility. The final lithium niobate waveguide 42 does not cause problems with the lithium niobate material affecting the preceding processes on the standard CMOS production line, thus helping to reduce costs and improve mass production capabilities. Second, the first silicon nitride waveguide 32 and the lithium niobate waveguide 42 require optical field overlap to achieve efficient electro-optic modulation, and their vertical spacing typically needs to be strictly controlled within tens to two... Within a hundred nanometers, based on this, compared to the conventional approach where the metal modulation electrode 22 is fabricated between the first silicon nitride waveguide 32 and the lithium niobate waveguide 42, which increases the longitudinal distance between the first silicon nitride waveguide 32 and the lithium niobate waveguide 42, in this embodiment, the modulation electrode 22 is fabricated first and located below the subsequently fabricated first silicon nitride waveguide 32 and lithium niobate waveguide 42. This buried modulation electrode 22 can ensure a smaller longitudinal optical field coupling distance between the first silicon nitride waveguide 32 and the lithium niobate waveguide 42, improve the optical field coupling efficiency, and achieve flexible control of the process.

[0039] In some embodiments, reference Figures 4 to 7 The step S200 of forming the modulation electrode layer 2 on one side surface of the substrate structure 1 specifically includes the following steps: Step S201: A first sub-cladding layer 211 is provided on one side surface of the substrate structure 1.

[0040] Specifically, such as Figure 4 As shown, a first sub-cladding layer 211 is provided to cover one side of the silicon device layer 13 in the substrate structure 1, and the surface of the first sub-cladding layer 211 is planarized by chemical mechanical polishing (CMP).

[0041] In step S202, an electrode groove 23 is formed on the side of the first sub-cladding 211 that is away from the substrate structure 1.

[0042] Specifically, such as Figure 5 As shown, an electrode groove 23 with a depth of less than 1 μm, such as 300 nm, is opened on one side after the first sub-cladding 211 is planarized. Step S203: Fill the electrode groove 23 with metal material to form the modulation electrode 22; Specifically, such as Figure 6 As shown, copper metal is filled into the electrode tank 23 by electroplating, and an embedded modulation electrode 22 is formed by damascus process.

[0043] In step S204, a second sub-cladding layer 212 is formed on the side of the first sub-cladding layer 211 away from the substrate structure 1. The second sub-cladding layer 212 covers the modulation electrode 22 and together with the first sub-cladding layer 211 forms the first cladding layer 21.

[0044] Specifically, such as Figure 7 As shown, some cladding material is covered on top of the modulation electrode 22, that is, a second sub-cladding 212 is formed, and the surface of the second sub-cladding 212 is smoothed again by CMP process, thereby forming a first cladding 21 covering the modulation electrode 22, and finally forming a modulation electrode layer 2 having the modulation electrode 22 and the first cladding 21.

[0045] In summary, the above steps form an embedded modulation electrode 22, which is completely covered by the first cladding layer 21. This ensures that the performance of the modulation electrode 22 is not affected by subsequent processes and also ensures the efficient fabrication of the subsequent composite waveguide structure.

[0046] In some embodiments, step S300, which involves forming a silicon nitride waveguide layer 3 on the side of the modulation electrode layer 2 away from the substrate structure 1, specifically includes the following steps: Step S301: Prepare a silicon nitride wafer 30, which includes a second substrate layer 301, a second isolation layer 302 and a silicon nitride layer 303 stacked together.

[0047] Specifically, such as Figure 8As shown, the second substrate layer 301 in the silicon nitride wafer 30 can be an 8-inch silicon wafer with qualified parameters, and the second isolation layer 302 is a silicon oxide thin film with a thickness of 100 nm. Then, a full-surface silicon nitride layer 303 is formed on the second isolation layer 302. In this embodiment, the fabrication process of the silicon nitride wafer 30 is completely separated from the device structure for forming the modulation electrode layer 2 in both space and time, which helps to form a high-quality silicon nitride layer 303.

[0048] In step S302, a silicon nitride wafer 30 is bonded to the surface of the modulation electrode layer 2 away from the substrate structure 1, wherein the silicon nitride layer 303 is connected to the first cladding layer 21 of the modulation electrode layer 2.

[0049] Specifically, such as Figure 9 As shown, firstly, the surface of the silicon nitride wafer 30 and the device surface with the modulation electrode layer 2 are activated by plasma process; then, with the silicon nitride layer 303 facing the first cladding layer 21, the silicon nitride wafer 30 is pre-bonded and placed on the first cladding layer 21, and the bonding and fixing of the silicon nitride wafer 30 and the surface of the first cladding layer 21 is completed by pressure and low temperature annealing process.

[0050] Step S303: Remove the second substrate layer 301 and at least a portion of the second isolation layer 302.

[0051] Specifically, such as Figure 10 As shown, most of the thickness of the second substrate layer 301 can be removed by coarse grinding; then the remaining second substrate layer 301 can be removed by fine grinding, and in other embodiments, part of the second isolation layer 302 may also be removed.

[0052] Step S304: The silicon nitride layer 303 is patterned to obtain the first silicon nitride waveguide 32.

[0053] Specifically, such as Figure 11 As shown, the patterning of the silicon nitride layer 303 can be performed in two photolithography and etching steps. The second isolation layer 302 is etched first, and the silicon nitride layer 303 is etched second, thereby obtaining multiple silicon nitride elements, including at least the first silicon nitride waveguide 32. Other silicon nitride elements include the second silicon nitride waveguide 33 and the third silicon nitride waveguide 34, etc. The second silicon nitride waveguide 33 is coupled with the second silicon waveguide 13b to form a composite waveguide, and the third silicon nitride waveguide 34 can be coupled with the grating structure 13c. Retaining a portion of the second isolation layer 302 on the silicon nitride layer 303 for patterning helps to avoid damage to the silicon nitride material during the patterning process, forming a high-quality first silicon nitride waveguide 32. Of course, the second isolation layer 302 and the silicon nitride layer 303 can also be etched in one step.

[0054] In step S305, a second cladding layer 31 is provided on the side of the first silicon nitride waveguide 32 away from the modulation electrode layer 2, and the second cladding layer 31 covers the first silicon nitride waveguide 32 and the first cladding layer 21.

[0055] Specifically, such as Figure 12 As shown, a second cladding 31 is disposed on the first cladding 21 on which the first silicon nitride waveguide 32 is formed. The second cladding 31 covers the first silicon nitride waveguide 32. The second cladding 31 is also made of silicon oxide material, which is the same material as the second isolation layer 302.

[0056] In summary, the above steps form a first silicon nitride waveguide 32 located above the modulation electrode 22 in the longitudinal direction and offset from the modulation electrode 22 in the transverse direction, reducing the optical field interference and optical loss of the first silicon nitride waveguide 32 caused by the modulation electrode 22. A cladding material is then deposited on the modulation electrode layer 2 to form a second cladding 31 covering the first silicon nitride waveguide 32, thereby forming a silicon nitride waveguide layer 3 having the first silicon nitride waveguide 32 and the second cladding 31. For example, the transverse offset here can be understood as, as... Figure 11 As shown, when three modulation electrodes 22 are formed, two first silicon nitride waveguides 32 are respectively disposed in the interval region of the three modulation electrodes 22.

[0057] Furthermore, such as Figure 8 As shown, step S301 of preparing the silicon nitride wafer 30 specifically includes the following steps: Step S3011: Provide a second substrate layer 301.

[0058] Specifically, the second substrate 301 is an 8-inch silicon wafer with various parameters, adapted to the first substrate 11.

[0059] In step S3012, a second isolation layer 302 is formed on one side surface of the second substrate layer 301.

[0060] Specifically, a silicon oxide thin film with a thickness of 100nm~500nm can be grown through a thermal oxidation process to form a second isolation layer 302.

[0061] In step S3013, a silicon nitride layer 303 is formed on the side of the first isolation layer 12 away from the second substrate layer 301 using a low-pressure chemical vapor deposition process.

[0062] Specifically, since the modulation electrode 22 in this embodiment is a pre-prepared embedded electrode, when the first silicon nitride waveguide 32 is subsequently fabricated by directly forming a silicon nitride thin film on the first cladding layer 21, the silicon nitride thin film can usually only be deposited using plasma-enhanced chemical vapor deposition (PECVD). However, the silicon nitride thin film prepared by this process has high transmission loss. If low-pressure chemical vapor deposition (LPCVD) is used to deposit the silicon nitride thin film, the high temperature will cause the metal modulation electrode 22 to separate from the dielectric and fail. In this embodiment, the entire silicon nitride wafer 30 is spatially separated from the device body. Therefore, when the modulation electrode 22 is an embedded electrode, low-pressure chemical vapor deposition (LPCVD) can also be used to form the silicon nitride layer 303, forming a silicon nitride thin film with low transmission loss. This not only forms a high-quality silicon nitride thin film but also avoids the adverse effects of the metal electrode and the lithium niobate waveguide 42 on the electric field and optical field, thereby ensuring the modulation effect.

[0063] Step S3014: Anneal the silicon nitride layer 303.

[0064] Specifically, the silicon nitride layer 303 can be annealed at a high temperature of 1100℃~1300℃ for 1h~3h to remove H from the film. + Impurities are reduced, internal stress is decreased, and the quality of silicon nitride films is improved.

[0065] In some embodiments, after step S300 of forming the silicon nitride waveguide layer 3 on the side of the modulation electrode layer 2 away from the substrate structure 1, and before step S400 of forming the lithium niobate waveguide layer 4 on the side of the silicon nitride waveguide layer 3 away from the substrate structure 1, the method further includes: like Figure 13 and Figure 14 As shown, a stop layer 5 is formed on the side surface of the silicon nitride waveguide layer 3 facing away from the substrate structure 1, and the stop layer 5 at least covers the first silicon nitride waveguide 32.

[0066] Specifically, firstly, a silicon nitride thin film is grown on the entire surface of the silicon nitride waveguide layer 3 using a PECVD process. Alternatively, it can be amorphous silicon or a PVD-deposited TiN, Al, or other thin film to form an initial stop layer 50. The thickness of the initial stop layer 50 can be 50nm to 150nm. Then, the initial stop layer 50 is photolithographically and etched using a patterning process to form a stop layer 5 that at least covers the first silicon nitride waveguide 32.

[0067] like Figure 15 As shown, a fourth cladding layer 6 is formed that covers the stop layer 5 and the second cladding layer 31.

[0068] Specifically, a silicon oxide film with a thickness between 1 and 2 μm can be grown on the second cladding layer 31 using a PECVD process to form a fourth cladding layer 6 covering the stop layer 5. Then, the surface of the fourth cladding layer 6 is planarized by CMP.

[0069] In summary, after fabricating the first silicon nitride waveguide 32, a protective layer is first formed for the first silicon nitride waveguide 32, that is, the stop layer 5 is formed when fabricating the lithium niobate waveguide 42 on the first silicon nitride waveguide 32, to ensure that the performance of the first silicon nitride waveguide 32 is not affected by subsequent process steps.

[0070] In some embodiments, after the step of forming the fourth cladding 6 covering the stop layer 5 and the second cladding 31, and before forming the SS400 of the lithium niobate waveguide layer 4 on the side of the silicon nitride waveguide layer 3 away from the substrate structure 1, the method further includes: like Figure 16 and Figure 17 As shown, a contact electrode 7 is formed, which extends inward from a portion of the surface of the fourth cladding 6 to the connection modulation electrode 22.

[0071] Specifically, firstly, electrode vias 701a and contact windows 702a are etched within the cladding structure, including the fourth cladding layer 6, the third cladding layer 41, and the second cladding layer 31, using photolithography and etching processes. Then, copper metal is filled into the electrode vias 701a and contact windows 702a, followed by damascus etching to form interconnect electrodes 701 and pads 702, thus completing the fabrication of the contact electrode 7. The metal electrode within the electrode via 701a located beside the first silicon nitride waveguide 32 is extremely thin compared to the modulation electrode 22, and thus has virtually no impact on the optical mode field, and therefore does not affect the modulation efficiency.

[0072] In summary, the contact electrode 7 is fabricated after the first silicon nitride waveguide 32 is fabricated and before the lithium niobate waveguide 42 is fabricated. The portion of the contact electrode 7 located next to the first silicon nitride waveguide 32 is thinner than the modulation electrode 22 and is relatively far away from the silicon nitride waveguide. This ensures that the generation and transmission of optical signals are not affected while the electrical signal input is achieved.

[0073] In some embodiments, step S400, which involves forming a lithium niobate waveguide layer 4 on the side of the silicon nitride waveguide layer 3 away from the substrate structure 1, specifically includes the following steps: Step S401: A bonding groove 601 is formed on the fourth cladding layer 6, exposing the stop layer 5.

[0074] Specifically, such as Figure 18 As shown, bonding grooves 601 for setting lithium niobate waveguide 42 are formed in the fourth cladding layer 6 by photolithography and etching processes, and the etching stops on the stop layer 5.

[0075] In addition, before opening the bonding groove 601, it is necessary to ensure that the pad 702 is exposed. If necessary, dry etching of the cladding material above the pad is performed to ensure the exposure of the pad 702.

[0076] Step S402: Remove stop layer 5.

[0077] Specifically, such as Figure 19 As shown, the stop layer 5 prepared by the silicon nitride film can be removed by a wet process, such as by using a hot phosphoric acid wet process to remove the stop layer 5.

[0078] In step S403, a lithium niobate chip 40 is disposed in the bonding trench 601. The lithium niobate chip 40 includes a third substrate layer 401, a third isolation layer 402 and a lithium niobate layer 403 stacked together. The lithium niobate layer 403 is bonded to the second cladding layer 31 of the silicon nitride waveguide layer 3 facing the bottom of the bonding trench 601 to form a lithium niobate waveguide 42.

[0079] Specifically, the externally prepared initial lithium niobate wafer can be cut into rectangular lithium niobate chips 40 with a length and width greater than 1 cm, and the bonding surface can be activated by plasma. Then, with the lithium niobate layer 403 facing the bottom of the bonding groove 601, the lithium niobate chip 40 is directly pre-bonded to the bottom of the bonding groove 601 using the D2W method. Afterwards, the bonding of the lithium niobate chip 40 is completed by pressurized low-temperature annealing. Figure 20 As shown.

[0080] Step S404: Remove the second substrate layer 301.

[0081] Specifically, such as Figure 21 As shown, the third substrate layer 401 and part of the third isolation layer 402 on the back side of the lithium niobate layer 403 can be removed by grinding; then the surface is planarized by CMP.

[0082] In step S405, filler adhesive is placed in the bonding groove 601 to form a third cladding 41 surrounding the lithium niobate waveguide 42.

[0083] Specifically, such as Figure 21 As shown, since the bonding groove 601 is typically larger than the lithium niobate chip 40 for ease of installation, it needs to be filled with cladding material after bonding to ensure good optical characteristics and overall modulator stability. The filler is usually a refractive index-matching adhesive. Since lithium niobate has already formed on the chip, it will not be deposited with silicon dioxide via CVD on the manufacturing line (especially CMOS process lines) to avoid contaminating the cavity. Furthermore, the filler thickness can be flush with or not flush with the surface of the fourth cladding layer 6, as long as it meets the designed optical performance requirements.

[0084] In summary, the fabrication of the lithium niobate waveguide 42 through grooving and bonding processes helps to control the coupling distance between the lithium niobate waveguide 42 and the first silicon nitride waveguide 32, and also helps to form a higher quality lithium niobate waveguide 42 and reduce contamination.

[0085] In some embodiments, the first cladding layer 21, the second cladding layer 31, the third cladding layer 41, the fourth cladding layer 6, the first isolation layer 12, the second isolation layer 302, and the third isolation layer 402 are all made of silicon oxide material to ensure structural consistency and stability.

[0086] like Figure 21 As shown, this embodiment also provides a composite waveguide modulator, including: a substrate structure 1, a modulation electrode layer 2, a silicon nitride waveguide layer 3, and a lithium niobate waveguide layer 4. The modulation electrode layer 2 is formed on one side surface of the substrate structure 1, including a first cladding layer 21 and a modulation electrode 22 located at least within the first cladding layer 21. The silicon nitride waveguide layer 3 is formed on the side of the modulation electrode layer 2 away from the substrate structure 1, including a second cladding layer 31 and a first silicon nitride waveguide 32 located within the second cladding layer 31. The first silicon nitride waveguide 32 and the modulation electrode 22 are perpendicular to the substrate structure. The first silicon nitride waveguide 32 and the first niobate waveguide 4 are spaced apart in the direction perpendicular to the surface of the substrate structure 1 and staggered in the direction parallel to the surface of the substrate structure 1. The lithium niobate waveguide 4 is formed on the side of the silicon nitride waveguide 3 away from the substrate structure 1, including a third cladding 41 and a lithium niobate waveguide 42 located in the third cladding 41. The lithium niobate waveguide 42 and the first silicon nitride waveguide 32 are spaced apart in the direction perpendicular to the surface of the substrate structure 1 and at least partially overlap in the direction parallel to the surface of the substrate structure 1. The lithium niobate waveguide 42 and the first silicon nitride waveguide 32 are configured to form a composite waveguide structure.

[0087] The composite waveguide modulator of this embodiment includes a buried modulation electrode 22 and a lithium niobate-silicon nitride composite waveguide structure in the modulation region. First, the lithium niobate waveguide 42 is located at the top of the device, which helps improve process compatibility. The lithium niobate waveguide 42 does not cause problems with the preceding process flow on the standard CMOS production line due to the lithium niobate material, which helps reduce costs and improve mass production capabilities. Second, in this embodiment, the modulation electrode 22 is located at the bottom of the first silicon nitride waveguide 32 and the lithium niobate waveguide 42. This buried modulation electrode 22 can ensure a small longitudinal optical field coupling distance between the first silicon nitride waveguide 32 and the lithium niobate waveguide 42, improve optical field coupling efficiency, and enhance process control flexibility.

[0088] In some embodiments, the composite waveguide modulator described above further includes a contact electrode 7, which extends inward from a portion of the surface of the cladding structure away from the substrate structure 1 to connect to the modulation electrode 22.

[0089] The contact electrode 7 is fabricated after the first silicon nitride waveguide 32 is fabricated and before the lithium niobate waveguide 42 is fabricated. The part of the contact electrode 7 located next to the first silicon nitride waveguide 32 is thinner than the modulation electrode 22 and is relatively far away from the silicon nitride waveguide. This allows for the input of electrical signals without affecting the generation and transmission of optical signals.

[0090] Further functional descriptions of the above structures are the same as those of the corresponding embodiments described above, and will not be repeated here.

[0091] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for fabricating a composite waveguide modulator, characterized in that, include: Provide the base structure; A modulation electrode layer is formed on one side surface of the substrate structure, the modulation electrode layer including a first cladding layer and a modulation electrode located within the first cladding layer; A silicon nitride waveguide layer is formed on the side of the modulation electrode layer away from the substrate structure. The silicon nitride waveguide layer includes a second cladding layer and a first silicon nitride waveguide located within the second cladding layer. The first silicon nitride waveguide and the modulation electrode are spaced apart in a direction perpendicular to the surface of the substrate structure and staggered in a direction parallel to the surface of the substrate structure. A lithium niobate waveguide layer is formed on the side of the silicon nitride waveguide layer away from the substrate structure. The lithium niobate waveguide layer includes a third cladding layer and a lithium niobate waveguide located within the third cladding layer. The lithium niobate waveguide and the first silicon nitride waveguide are spaced apart in a direction perpendicular to the surface of the substrate structure and at least partially overlap in a direction parallel to the surface of the substrate structure. The lithium niobate waveguide and the first silicon nitride waveguide are configured to form a composite waveguide structure.

2. The method for fabricating the composite waveguide modulator according to claim 1, characterized in that, The formation of a modulation electrode layer on one side surface of the substrate structure includes: A first sub-cladding layer is disposed on one side surface of the substrate structure; An electrode groove is formed on the side of the first sub-cladding that is away from the substrate structure; A metal material is filled into the electrode groove to form a modulation electrode; A second sub-cladding layer is formed on the side of the first sub-cladding layer away from the substrate structure. The second sub-cladding layer covers the modulation electrode and together with the first sub-cladding layer forms the first cladding layer.

3. The method for fabricating the composite waveguide modulator according to claim 1, characterized in that, The formation of a silicon nitride waveguide layer on the side of the modulation electrode layer away from the substrate structure includes: A silicon nitride wafer is prepared, the silicon nitride wafer comprising a second substrate layer, a second isolation layer and a silicon nitride layer stacked together; The silicon nitride wafer is bonded to the side surface of the modulation electrode layer away from the substrate structure, wherein the silicon nitride layer is connected to the first cladding layer of the modulation electrode layer; Remove the second substrate layer and at least a portion of the second isolation layer; The silicon nitride layer is patterned to obtain the first silicon nitride waveguide; A second cladding layer is disposed on the side of the first silicon nitride waveguide away from the modulation electrode layer, and the second cladding layer covers the first silicon nitride waveguide and the first cladding layer.

4. The method for fabricating the composite waveguide modulator according to claim 3, characterized in that, The step of preparing silicon nitride wafers includes: Provide a second substrate layer; A second isolation layer is formed on one side surface of the second substrate layer; The silicon nitride layer is formed on the side of the first isolation layer away from the second substrate layer using a low-pressure chemical vapor deposition process. The silicon nitride layer is then annealed.

5. The method for fabricating the composite waveguide modulator according to claim 1, characterized in that, After forming a silicon nitride waveguide layer on the side of the modulation electrode layer away from the substrate structure, and before forming a lithium niobate waveguide layer on the side of the silicon nitride waveguide layer away from the substrate structure, the method further includes: A stop layer is formed on the surface of the silicon nitride waveguide layer facing away from the substrate structure, and the stop layer at least covers the first silicon nitride waveguide. A fourth cladding layer is formed that covers the stop layer and the second cladding layer.

6. The method for fabricating the composite waveguide modulator according to claim 5, characterized in that, After the formation of the fourth cladding layer covering the stop layer and the second cladding layer, and before the formation of the lithium niobate waveguide layer on the side of the silicon nitride waveguide layer away from the substrate structure, the method further includes: A contact electrode is formed, which extends inward from a portion of the surface of the fourth cladding to connect to the modulation electrode.

7. The method for fabricating the composite waveguide modulator according to claim 5, characterized in that, The formation of a lithium niobate waveguide layer on the side of the silicon nitride waveguide layer away from the substrate structure includes: A bonding groove is formed on the fourth cladding layer, and the bonding groove exposes the stop layer; Remove the stop layer; A lithium niobate chip is disposed in the bonding groove. The lithium niobate chip includes a third substrate layer, a third isolation layer and a lithium niobate layer stacked together. The lithium niobate layer is bonded to the second cladding layer of the silicon nitride waveguide layer facing the bottom of the bonding groove to form a lithium niobate waveguide. The lithium niobate waveguide and the silicon nitride waveguide constitute a lithium niobate-silicon nitride composite waveguide. Remove the second substrate layer; A filler adhesive is placed in the bonding groove to form a third cladding layer surrounding the lithium niobate waveguide.

8. The method for fabricating a composite waveguide modulator according to any one of claims 1-7, characterized in that, The substrate structure includes a first substrate layer, a first isolation layer, and a silicon device layer stacked together. The silicon device layer includes a variety of silicon elements, and the first cladding layer of the modulation electrode layer is adapted to cover the surfaces of the various silicon elements and the first isolation layer.

9. A composite waveguide modulator, characterized in that, include: Substrate structure; A modulation electrode layer is formed on one side surface of the substrate structure, the modulation electrode layer including a first cladding layer and at least a modulation electrode located within the first cladding layer; A silicon nitride waveguide layer is formed on the side of the modulation electrode layer away from the substrate structure. The silicon nitride waveguide layer includes a second cladding layer and a first silicon nitride waveguide located within the second cladding layer. The first silicon nitride waveguide and the modulation electrode are spaced apart in a direction perpendicular to the surface of the substrate structure and staggered in a direction parallel to the surface of the substrate structure. A lithium niobate waveguide layer is formed on the side of the silicon nitride waveguide layer away from the substrate structure. The lithium niobate waveguide layer includes a third cladding layer and a lithium niobate waveguide located within the third cladding layer. The lithium niobate waveguide and the first silicon nitride waveguide are spaced apart in a direction perpendicular to the surface of the substrate structure and at least partially overlap in a direction parallel to the surface of the substrate structure. The lithium niobate waveguide and the first silicon nitride waveguide are configured to form a composite waveguide structure.

10. The composite waveguide modulator according to claim 9, characterized in that, Also includes: A contact electrode is provided extending inward from a portion of the surface of the cladding structure away from the substrate structure to connect to the modulation electrode.

Images