Thin film lithium niobate device and method of making same

Abstract

The application discloses a thin-film lithium niobate device and a manufacturing method thereof. The thin-film lithium niobate device comprises, from bottom to top, a substrate, a buffer layer, a lithium niobate layer, an insulation layer and a hot electrode. The lithium niobate layer is a thin-film lithium niobate. The lithium niobate layer comprises a hot phase shift region corresponding to the hot electrode and lithium niobate waveguides located on both sides of the hot phase shift region along a first direction. The first direction is the direction of light conduction. A dielectric waveguide is arranged on the lithium niobate layer. The dielectric waveguide corresponds to the position of the hot electrode and is located between the lithium niobate layer and the hot electrode. The lithium niobate waveguide and the dielectric waveguide jointly constitute an optical waveguide. Compared with the prior art, the application has the advantages that by forming the dielectric waveguide in the hot phase shift region of the hot electrode, the change of the thermal-optical bias phase caused by the Z interface generated by the etching of the lithium niobate waveguide can be avoided, thereby providing a thermal-optical bias structure for eliminating the thermal-electric effect charge accumulation and avoiding the drift of the thermal-optical bias operating point.

Description

Technical Field

[0001] This invention relates to optical communication technology, and in particular to a thin-film lithium niobate device and a method for fabricating the device. Background Technology

[0002] Driven by new-generation information technologies such as 5G, the Internet of Things, virtual reality, and artificial intelligence, the broadband wave has swept the globe, and information capacity has grown exponentially over the past decade, putting enormous pressure on the bandwidth and energy consumption of optical communication networks.

[0003] Electro-optic modulators are core devices for realizing photoelectric conversion of information and a key link in overcoming the two major technical challenges of bandwidth and energy consumption. New electro-optic modulator chips with high bandwidth, low power consumption, low loss, and miniaturization are revolutionary technologies that will completely replace traditional lithium niobate crystal devices, and are one of the core technologies that countries around the world are focusing their efforts on developing.

[0004] Lithium niobate possesses photorefractive, nonlinear, electro-optic, acousto-optic, piezoelectric, and thermoelectric effects, and is widely used in various optoelectronic devices. Among these, lithium niobate modulators are currently the mainstream electro-optic modulator products. For normal operation, lithium niobate modulators require the operating point to be set at the quadrature bias point.

[0005] The DC bias operating point of lithium niobate modulators based on electro-optic bias drifts over time. Improving the fabrication process of lithium niobate modulators can reduce DC drift to some extent, but cannot eliminate it completely. Therefore, additional control circuitry is needed to lock the modulator's operating point, increasing the control complexity and cost. This is a major limiting factor for lithium niobate modulators. The optical waveguide of thin-film lithium niobate modulators is achieved through etching, resulting in a large refractive index difference and strong optical confinement. Therefore, thin-film lithium niobate modulators can achieve very low (less than 1V) modulation voltages while maintaining a large modulator bandwidth, attracting widespread attention and research.

[0006] Thin-film lithium niobate modulators commonly use X-cut thin-film lithium niobate. See also Figures 14-17To achieve the highest modulation efficiency, the optical waveguide is positioned along the Y-axis, and the electric field is positioned along the Z-axis. The thin-film lithium niobate modulator is also affected by DC bias drift. This thin-film lithium niobate modulator, from bottom to top, includes a substrate 1', a buffer layer 8', a lithium niobate waveguide 2', an insulating layer 3', and a thermoelectric electrode 4'. Using the thermo-optic effect to set the operating point of the thin-film lithium niobate modulator can effectively reduce bias drift. For etched thin-film lithium niobate waveguides, etching creates a Z-interface 100 on both sides of the lithium niobate waveguide 2'. The thermoelectric effect causes charge accumulation at the etched Z-interface 100. The charge accumulated at the etched Z-interface generates an electro-optic effect that alters the phase of the thermo-optic bias. At low temperatures, the thermo-optic coefficient of lithium niobate decreases, increasing the influence of the thermoelectric effect and thus increasing the difficulty of locking the thermo-optic bias operating point. In severe cases, the operating point may become unlockable. Summary of the Invention

[0007] The first technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a thin-film lithium niobate device that eliminates thermoelectric charge accumulation, avoids operating point drift, and improves stability.

[0008] The second technical problem to be solved by the present invention is to provide a method for fabricating the above-mentioned thin-film lithium niobate device.

[0009] The technical solution adopted by the present invention to solve the first technical problem mentioned above is as follows: a thin-film lithium niobate device, comprising, from bottom to top, a substrate, a buffer layer, a lithium niobate layer, an insulating layer, and a thermal electrode, wherein the lithium niobate layer is a thin-film lithium niobate, and the lithium niobate layer includes a thermally modulated phase shift region corresponding to the thermal electrode and lithium niobate waveguides located on both sides of the thermally modulated phase shift region along a first direction, wherein the first direction is the direction of light transmission; characterized in that: a dielectric waveguide is disposed on the lithium niobate layer, the dielectric waveguide being positioned corresponding to the thermal electrode and located between the lithium niobate layer and the thermal electrode, wherein the lithium niobate waveguide and the dielectric waveguide together constitute an optical waveguide.

[0010] Preferably, the dielectric waveguide is a waveguide made of silicon nitride.

[0011] To further eliminate charge accumulation caused by thermoelectric effect, the lithium niobate layer has etched Z-interfaces on both sides of the thermally tuned phase shift region along a second direction, which is a transverse direction perpendicular to the light transmission direction. The dielectric waveguide passes through the thermally tuned phase shift region, and the Z-interfaces on both sides are connected by a first metal electrode to form a short circuit.

[0012] To reduce the loss due to the mode field mismatch between the lithium niobate layer and the dielectric waveguide, the lithium niobate waveguide has a first transition section along the optical transmission direction. The size of the first transition section gradually decreases from the direction near the thermally tuned phase shift region to the direction away from the thermally tuned phase shift region. The dielectric waveguide has a second transition section located on both sides of the thermally tuned phase shift region along the first direction. The size of the second transition section gradually decreases from the direction near the thermally tuned phase shift region to the direction away from the thermally tuned phase shift region. The positions, shapes, and sizes of the first and second transition sections correspond to each other.

[0013] The technical solution adopted by the present invention to solve the second technical problem mentioned above is: a method for fabricating a thin-film lithium niobate device as described above, characterized by comprising the following steps:

[0014] S1) A base wafer is provided, the base wafer including a substrate, a buffer layer and an unetched lithium niobate layer, and a lithium niobate waveguide is etched on the lithium niobate layer of the base wafer, the lithium niobate waveguide being located on both sides of a thermally modulated phase shift region defined by the lithium niobate layer along a first direction;

[0015] S2) A dielectric film is grown on the lithium niobate layer, and then the dielectric waveguide is etched; then an insulating layer is grown on the lithium niobate layer and the dielectric waveguide.

[0016] S3) Fabricate a thermal electrode on a dielectric waveguide, wherein the position of the thermal electrode corresponds to the thermally modulated phase shift region of the lithium niobate layer;

[0017] S4) Fabricate a second metal electrode to connect the two thermal electrodes.

[0018] To reduce the loss due to the mode field mismatch between the lithium niobate layer and the dielectric waveguide, in step S1), a first transition section is etched on the lithium niobate waveguide, and the size of the first transition section gradually decreases from the direction close to the thermally tuned phase shift region to the direction away from the thermally tuned phase shift region; in step S2), along the first direction, the dielectric waveguide has a second transition section located on both sides of the thermally tuned phase shift region of the hot electrode, and the size of the second transition section gradually decreases from the direction close to the thermally tuned phase shift region to the direction away from the thermally tuned phase shift region.

[0019] To further eliminate charge accumulation caused by thermoelectric effect, in step S1), the lithium niobate layer has etched Z-interfaces on both sides of the thermally tuned phase shift region along a second direction, where the second direction is a transverse direction perpendicular to the light transmission direction; after step S3), the following steps are also included: removing the insulating layer portion outside the thermally tuned phase shift region to expose the Z-interface; in step S4), a first metal electrode is also fabricated to connect the lithium niobate layer to the Z-interfaces on both sides of the thermally tuned phase shift region.

[0020] To further reduce the influence of the Z-interface and eliminate the charge accumulation caused by the thermoelectric effect, during the process of removing the insulating layer outside the thermally modulated phase shift region, the portion of the lithium niobate layer located on the side of the two Z-interfaces that is far from each other is etched through, thereby directly exposing the top surface of the buffer layer on the side of the two Z-interfaces that is far from each other, so that the bottom of each Z-interface contacts the top surface of the buffer layer.

[0021] Compared with the prior art, the advantages of the present invention are: by forming a dielectric waveguide in the thermally modulated phase shift region of the thermoelectrode, the change in thermo-optical bias phase caused by the Z interface generated by the etching of the lithium niobate waveguide can be avoided, thereby providing a thermo-optical bias structure that eliminates the accumulation of thermoelectric effect charge and avoids the drift of the thermo-optical bias operating point. Attached Figure Description

[0022] Figure 1 This is a top view of the first embodiment of the present invention, showing the thermoelectric electrode fabricated on a dielectric waveguide.

[0023] Figure 2 This is a cross-sectional view of a waveguide in the first embodiment of the present invention, showing the thermal electrode fabricated on a dielectric waveguide.

[0024] Figure 3 This is a top view of step one of the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention.

[0025] Figure 4 This is a cross-sectional view of step one of the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention.

[0026] Figure 5 This is a top view of step two in the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention.

[0027] Figure 6 This is a cross-sectional view of step two in the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention.

[0028] Figure 7 This is a top view of step three in the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention;

[0029] Figure 8 This is a cross-sectional view of step three in the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention.

[0030] Figure 9 This is a top view of step four in the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention.

[0031] Figure 10 A cross-sectional view of step four in the fabrication process of the thin-film lithium niobate device according to the first embodiment of the present invention;

[0032] Figure 11This is a top view of step four in the fabrication process of the thin-film lithium niobate device according to the second embodiment of the present invention.

[0033] Figure 12 A cross-sectional view of step four in the fabrication process of the thin-film lithium niobate device according to the second embodiment of the present invention;

[0034] Figure 13 This is a top view of step five in the fabrication process of the thin-film lithium niobate device according to the second embodiment of the present invention.

[0035] Figure 14 A cross-sectional view of step five in the fabrication process of the thin-film lithium niobate device according to the second embodiment of the present invention; Figure 15 This is a schematic diagram of the etched waveguide cross-section along the Y direction for an X-cut thin-film lithium niobate device in the prior art;

[0036] Figure 16 A top view of the prior art method of fabricating thermal electrodes on an etched lithium niobate layer;

[0037] Figure 17 This is a cross-sectional view of a waveguide in the prior art where a thermal electrode is fabricated on an etched lithium niobate layer. Detailed Implementation

[0038] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions.

[0039] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential," etc., indicating orientation or positional relationships, are based on the orientation or positional relationships shown in the accompanying drawings and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Since the embodiments disclosed in this invention can be arranged in different directions, these terms indicating direction are only for illustration and should not be regarded as limitations. For example, "upper" and "lower" are not necessarily limited to directions opposite to or consistent with the direction of gravity. In addition, features defined with "first" and "second" may explicitly or implicitly include one or more of such features.

[0040] Example 1

[0041] See Figures 1-3 A thin-film lithium niobate device, in this embodiment, is a modulator. Alternatively, it can be any device with a thermally modulated structure.

[0042] The thin-film lithium niobate device comprises, from bottom to top, a substrate 1, a buffer layer 8, a lithium niobate layer 2, an insulating layer 3, and a thermal electrode 4. The substrate 1 can be a silicon substrate or a quartz substrate, and the buffer layer 8 and the insulating layer 3 can be made of silicon oxide.

[0043] The lithium niobate layer 2 is a thin-film lithium niobate, and the hot electrode 4 is located above the lithium niobate layer 2 at the corresponding position. The lithium niobate layer 2 forms a thermally modulated phase shift region 23 corresponding to the hot electrode 4 and lithium niobate waveguides 24 located on both sides of the thermally modulated phase shift region 23 in the first direction. The cross-section of the thermally modulated phase shift region 23 is approximately rectangular, and the first direction is the direction of light conduction. Figure 1 (The diagram shows the left-right direction). A dielectric waveguide 5 is etched on the lithium niobate layer 2, extending along the first direction and passing through the thermally modulated phase shift region 23. In this embodiment, when used as a modulator, there are two parallel dielectric waveguides 5; alternatively, when used in other devices, there may be only one dielectric waveguide 5, or N parallel waveguides, where N≥3. The thermal electrode 4 is located above the dielectric waveguide 5. Since the lithium niobate layer 2 is etched on the buffer layer 8, it extends along both sides in the second direction (along the transverse direction perpendicular to the light transmission direction). Figure 1 The image shows the top and bottom sides. Figure 2 The diagram shows etched Z-interfaces 22 formed on both sides. Since these Z-interfaces 22 are very far from the dielectric waveguide 5, the effect of the thermoelectric effect on the phase is reduced. To further eliminate the charge accumulation caused by the thermoelectric effect, the Z-interfaces 22 on both sides can be connected through the first metal electrode 7 to form a short circuit, thereby quickly neutralizing the charge generated by the thermoelectric effect. In this embodiment, the first metal electrode 7 is annular. Alternatively, it can be made into other deformed shapes with openings, as long as the Z-interfaces 22 on both sides are connected.

[0044] Therefore, with the device formed as described above, the thermally modulated phase shift region does not involve etching the lithium niobate waveguide; instead, a dielectric waveguide 5 is used, thus avoiding the Z-interface generated by etching the lithium niobate waveguide. The dielectric waveguide 5 provides lateral confinement of the light. Figure 1 (As shown in the up-down direction). The optical waveguide is composed of a lithium niobate waveguide 24 and a dielectric waveguide 5, with light distributed in the dielectric waveguide 5 and the lithium niobate waveguide 24.

[0045] To reduce the loss due to mode field mismatch between lithium niobate waveguide 24 and dielectric waveguide 5, a tapered waveguide structure was fabricated between lithium niobate waveguide 24 and dielectric waveguide 4 to achieve mode transition. For details, see [link to details]. Figure 1In the first direction, the lithium niobate waveguide 24 has a first transition section 21, the size of which gradually decreases from the direction near the thermally tuned phase shift region 23 to the direction away from the thermally tuned phase shift region 23. The dielectric waveguide 5 has second transition sections 51 corresponding to both sides of the thermally tuned phase shift region 23, the size of which gradually decreases from the direction near the thermally tuned phase shift region 23 to the direction away from the thermally tuned phase shift region 23. Here, the size refers to the width of the waveguide, i.e. Figure 1 The dimensions in the vertical direction are shown. The positions, shapes, and dimensions of the first transition segment 21 and the second transition segment 51 correspond. The transition segments on both sides can be symmetrical or asymmetrical.

[0046] The method for fabricating the thin-film lithium niobate device of the present invention includes the following steps:

[0047] S1) See also Figure 3 and Figure 4 A lithium niobate waveguide 24 is etched on the base wafer. The lithium niobate waveguide 24 is located on both sides of the thermally modulated phase shift region 23 defined by the lithium niobate layer 2, and a first transition section 21 is etched on the lithium niobate waveguide 24. The thermally modulated phase shift region 23 has etched Z-interfaces 22 on both sides. The base wafer includes a substrate 1, a buffer layer 8 and a lithium niobate layer 2. The buffer layer 8 is covered by the lithium niobate layer 2 before etching. The base wafer can be prepared in advance or purchased, or it can be prepared before this step.

[0048] S2) See also Figure 5 and Figure 6 A dielectric film, preferably silicon nitride, is grown on top of the lithium niobate layer 2 and then etched into two parallel dielectric waveguides 5, each having a second transition section 51 as described above; then a silicon oxide insulating layer 3 is grown on the lithium niobate layer 2 and the dielectric waveguides 5.

[0049] S3) See also Figure 7 and Figure 8 Thermistors 4 are fabricated on each dielectric waveguide 5, and thermistors 4 are made of metal.

[0050] S4) See also Figure 9 and Figure 10 Remove part of the insulating layer 3 outside the thermally tuned phase shift region 23 to expose the Z interface 22;

[0051] S5) See also Figure 1 and Figure 2 A second metal electrode 6 is fabricated, thereby connecting two hot electrodes 4 to the two ends of the hot electrode 4 respectively; a first metal electrode 7 is fabricated, connecting the lithium niobate layer 2 to the Z interface 22 on both sides of the thermally modulated phase shift region 23.

[0052] Example 2

[0053] In this embodiment, the difference from Embodiment 1 above lies in the following steps S4 and S5 of the manufacturing method:

[0054] S4) See also Figure 11 and Figure 12 The insulating layer 3 outside the thermally modulated phase shift region 23 is removed to expose the Z-interface 22. In this embodiment, the etching in this step etches through the portion of the lithium niobate layer 2 located outside the two Z-interfaces 22, thereby directly exposing the top surface of the buffer layer 8 outside the Z-interface 22, so that the bottom of the Z-interface 22 contacts the top surface of the buffer layer 8. In the first embodiment, a thinner layer of lithium niobate is still retained outside the Z-interface 22, and the bottom of the Z-interface 22 is higher than the top surface of the buffer layer 8. Here, the outside of the Z-interface 22 refers to the side of the two Z-interfaces 22 that are far apart from each other.

[0055] S5) See also Figure 13 and Figure 14 A second metal electrode 6 is fabricated, thereby connecting two hot electrodes 4 to the two ends of the hot electrode 4 respectively; a first metal electrode 7 is fabricated, connecting the lithium niobate layer 2 to the Z interface 22 on both sides of the thermally modulated phase shift region 23. Since the outer part of the Z interface 22 is etched through, the bottom of the first metal electrode 7 contacts the top surface of the buffer layer 8. In the embodiment, the bottom of the first metal electrode 7 contacts the lithium niobate layer 2.

[0056] The effects of Z-interface 22 can be completely eliminated by using the methods described above.

Claims

1. A thin-film lithium niobate device comprising, from bottom to top, a substrate (1), a buffer layer (8), a lithium niobate layer (2), an insulating layer (3) and a thermal electrode (4), the lithium niobate layer (2) being a thin-film lithium niobate, the lithium niobate layer (2) comprising a thermal phase shift region (23) corresponding to the thermal electrode (4) and lithium niobate waveguides (24) located on either side of the thermal phase shift region (23) along a first direction, the first direction being the direction of light propagation; characterized in that: A dielectric waveguide (5) is arranged on the lithium niobate layer (2), the dielectric waveguide (5) corresponds to the position of the thermal electrode (4) and is located between the lithium niobate layer (2) and the thermal electrode (4), the lithium niobate waveguide (24) and the dielectric waveguide (5) jointly constitute an optical waveguide, and the dielectric waveguide (5) is a waveguide made of silicon nitride.

2. The thin film lithium niobate device of claim 1, wherein: The lithium niobate layer (2) is formed with etched Z interfaces (22) on both sides of the thermal phase shift region (23) along a second direction, the second direction is a transverse direction perpendicular to the light transmission direction, the dielectric waveguide (5) passes through the thermal phase shift region (23), and the Z interfaces (22) on both sides are connected by the first metal electrode (7) to form a short circuit.

3. The thin film lithium niobate device of claim 2, wherein: The lithium niobate waveguide (24) has a first transition section (21), the size of the first transition section (21) gradually decreases from the direction close to the thermal phase shift region (23) to the direction away from the thermal phase shift region (23), the dielectric waveguide (5) has a second transition section (51) located on both sides of the thermal phase shift region (23) along a first direction, the size of the second transition section (51) gradually decreases from the direction close to the thermal phase shift region (23) to the direction away from the thermal phase shift region (23), and the position, shape and size of the first transition section (21) and the second transition section (51) correspond.

4. A method of fabricating a thin film lithium niobate device as claimed in claim 1, characterized by: The method comprises the following steps: S1) providing a base wafer, the base wafer comprising a substrate (1), a buffer layer (8) and an unetched lithium niobate layer (2), etching a lithium niobate waveguide (24) on the lithium niobate layer (2) of the base wafer, the lithium niobate waveguide (24) being located on both sides of a determined thermal phase shift region (23) of the lithium niobate layer (2) along a first direction; S2) growing a dielectric film on the lithium niobate layer (2), and then etching a dielectric waveguide (5); then growing an insulating layer (3) on the lithium niobate layer (2) and the dielectric waveguide (5); S3) manufacturing a thermal electrode (4) on the dielectric waveguide (5), the position of the thermal electrode (4) corresponding to the thermal phase shift region (23) of the lithium niobate layer (2); S4) manufacturing a second metal electrode (6) to connect the two thermal electrodes (4).

5. The method of claim 4, wherein: In step S1), a first transition section (21) is etched on the lithium niobate waveguide (24), the size of the first transition section (21) gradually decreases from the direction close to the thermal phase shift region (23) to the direction away from the thermal phase shift region (23); in step S2), along the first direction, the dielectric waveguide (5) has a second transition section (51) corresponding to both sides of the thermal phase shift region (23), the size of the second transition section (51) gradually decreases from the direction close to the thermal phase shift region (23) to the direction away from the thermal phase shift region (23).

6. The method of claim 4, wherein: In step S1), the lithium niobate layer (2) has etched Z interfaces (22) on both sides of the thermal phase modulation region (23) along a second direction which is a transverse direction perpendicular to the light transmission direction; after step S3), the following steps are further included: removing the insulating layer (3) outside the thermal phase modulation region (23) to expose the Z interfaces (22); in step S4), a first metal electrode (7) is further made to connect the Z interfaces (22) of the lithium niobate layer (2) on both sides of the thermal phase modulation region (23).

7. The method of claim 6, wherein: In the process of removing the insulating layer (3) outside the thermal phase modulation region (23), the part of the lithium niobate layer (2) on the side away from the two Z interfaces (22) is etched through, so that the top surface of the buffer layer (8) is directly exposed on the side away from the two Z interfaces (22), and the bottom of each Z interface (22) contacts the top surface of the buffer layer (8).

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