Integration of photonic optical gyroscopes with microelectromechanical sensors

Abstract

Aspects of the present disclosure are directed to the monolithic integration of an optical gyroscope fabricated on a planar silicon platform as a photonics integrated circuit with a MEMS accelerometer on the same die. The accelerometer can be controlled by the electronic circuitry that controls the optical gyroscope. Gaps may be introduced between adjacent waveguide turns to reduce crosstalk and improve the sensitivity and packing density of the optical gyroscope.

Description

[Technical Field]

[0001] The present disclosure relates to various structures and fabrication methods for integrating photonics-based optical gyroscopes with micro-electro-mechanical system (MEMS)-based sensors on the same chip. In other words, the present disclosure relates to a photonics integrated circuit (PIC) (also referred to in the claims as an integrated photonics chip) that is monolithically integrated with a MEMS-based sensor on the chip. [Background technology]

[0002] A gyroscope (sometimes called a "gyro") is a sensor capable of measuring angular velocity. Gyroscopes can be mechanical or optical and vary in accuracy, performance, cost, and size. Mechanical gyros based on the Coriolis effect are typically inexpensive but lack high performance and are prone to measurement errors caused by temperature, vibration, and electromagnetic interference (EMI). Optical gyros typically have the highest performance and rely on interferometric measurements based on the Sagnac effect (a rotation-induced phenomenon encountered in interferometry). Optical gyros have the advantage over mechanical gyros because they have no moving parts and are therefore less susceptible to shock, vibration, and temperature changes than mechanical gyros.

[0003] The most common optical gyroscope is the fiber optic gyroscope (FOG). FOG construction typically includes a long loop of polarization-maintaining (PM) fiber (the loop may comprise a coil or fiber spool with several turns). Laser light is launched into both ends of the PM fiber, traveling in different directions. When the fiber loop / coil is rotating, the light beams experience different optical path lengths relative to each other. By setting up an interferometer system, it is possible to measure small optical path length differences that are proportional to the area of ​​the enclosed loop and the angular velocity of the rotating coil.

[0004] While FOGs can have very high precision, they are also large, very expensive, and difficult to assemble because the devices are built on separate optical components that must be precisely aligned, often involving manual alignment and making them difficult to scale for mass production. The present disclosure provides a solution to this problem, as further described below.

[0005] Multiple gyroscopes and other sensors (such as accelerometers and possibly magnetometers) may be packaged together as an inertial measurement unit (IMU) within a moving object to sense various motion parameters along the X, Y, and Z axes. For example, a 6-axis IMU may package 3-axis accelerometers and 3-axis gyroscopes together to measure the absolute spatial displacement of a moving object. Applications of IMUs include, but are not limited to, military operations (e.g., fighter jets, submarines), commercial aircraft / drone navigation, robotics, autonomous vehicle navigation, virtual reality, augmented reality, gaming, etc.

[0006] We propose replacing fiber with waveguide-based integrated photonics components for cost-effective and easy integration into semiconductor platforms, which are more promising for mass production of gyroscopes. This application describes various structures, including silicon nitride (SiN) waveguide cores fabricated on silicon platforms, and the integration of MEMS accelerometers on the same silicon platform with gyroscopes, as detailed below. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] U.S. Provisional Application No. 62 / 923,234 [Patent Document 2] U.S. Nonprovisional Application No. 17 / 071,697 [Patent Document 3] U.S. Patent Application Publication No. 2021 / 0116246 [Patent Document 4] U.S. Provisional Application No. 62 / 858,588 [Patent Document 5] U.S. Provisional Application No. 62 / 896,365 [Patent Document 6] U.S. Provisional Application No. 62 / 986,379 [Patent Document 7] U.S. Nonprovisional Application No. 16 / 894,120 [Patent Document 8] U.S. Patent No. 10,969,548 [Patent Document 9] U.S. Provisional Application No. 62 / 872,640 [Patent Document 10] U.S. Provisional Application No. 62 / 904,443 [Patent Document 11] U.S. Nonprovisional Application No. 16 / 659,424 [Patent Document 12] U.S. Patent No. 10,731,988 Summary of the Invention

[0008] Aspects of the present disclosure are directed to the monolithic integration of an optical gyroscope fabricated on a planar silicon platform as a photonics integrated circuit with a MEMS accelerometer on the same die. The accelerometer can be controlled by the electronic circuitry that controls the optical gyroscope. Gaps may be introduced between adjacent waveguide turns to reduce crosstalk and improve the sensitivity and packing density of the optical gyroscope. [Means for solving the problem]

[0009] In particular, an integrated photonics chip is disclosed that includes a waveguide coil having multiple waveguide turns looping around a central region surrounded by the waveguide coil, each waveguide turn being parallel to adjacent waveguide turns, for use as a rotation sensing element of an optical gyroscope, and a microelectromechanical systems (MEMS)-based motion sensing device monolithically integrated in the central region surrounded by the waveguide coil, wherein the waveguide coil and the MEMS accelerometer are fabricated on a common platform.

[0010] The optical gyroscope and MEMS-based motion sensing device are packaged together as a modular, integrated inertial measurement unit (IMU), where the MEMS-based motion sensing device provides coarse rotational sensing values ​​for all axes of motion, and the optical gyroscope can provide finer rotational sensing values ​​for one or more selected axes of motion.

[0011] The MEMS-based motion sensing device may also include an accelerometer for one or more axes of motion, hi some embodiments, the MEMS device may be a six-axis gyroscope and accelerometer.

[0012] The common platform for the integrated photonics chip can be a silicon photonics platform, with each waveguide turn comprising a waveguide core sandwiched between an upper cladding and a lower cladding, hi one embodiment, the waveguide core comprises silicon nitride, and the upper and lower claddings comprise oxide.

[0013] Structural modifications can be introduced on either side of each waveguide turn to reduce crosstalk between adjacent waveguide turns, thereby increasing the spatial density of waveguide turns that can be fabricated within a given area of ​​an integrated photonics chip.

[0014] The predetermined area may depend on the exposure field of the reticle used to fabricate the waveguide coil and the MEMS-based motion sensing device. Increasing the spatial density of the waveguide turns increases the central area enclosed within the waveguide coil and the number of waveguide turns surrounding the central area, thereby increasing the sensitivity of the rotation sensing element.

[0015] The structural modifications may include voids, including voids, voids filled with metal, or voids filled with an inert gas or liquid.

[0016] In some embodiments, the gap is in the form of a high aspect ratio rectangular slit or groove in which the longitudinal dimension of the gap is substantially greater than the lateral dimension of the gap such that the gap extends substantially above and below the waveguide core along the longitudinal direction.

[0017] Also disclosed is a method for monolithically fabricating an integrated photonics chip comprising a waveguide coil and a MEMS-based motion sensing device on a common platform, the method including the steps of: designating a central region on the common platform for fabricating the MEMS-based motion sensing device, the central region being surrounded by a waveguide coil comprising a plurality of waveguide turns looping around the central region, each waveguide turn being parallel to adjacent waveguide turns, the waveguide coil being used as a rotation sensing element of an optical gyroscope; fabricating the waveguide coil on the common platform; protecting the fabricated waveguide coil by depositing an etch stop layer above the waveguide coil; and fabricating the MEMS-based motion sensing device in the designated central region.

[0018] The steps of fabricating the MEMS-based motion sensing device further include depositing and patterning an electrode on the etch stop layer in a designated central region; depositing and patterning a first sacrificial layer on the etch stop layer and the patterned electrode; depositing and patterning a structural layer on the patterned sacrificial layer and the patterned electrode; depositing and patterning a second sacrificial layer on the patterned structural layer; patterning the structural layer to generate pillars as part of the MEMS-based motion sensing device, wherein the second sacrificial layer is also patterned on the pillars; and removing the first sacrificial layer and the second sacrificial layer, thereby generating a suspending structure that functions as the motion sensing element of the MEMS-based device.

[0019] The method may further include forming gaps on either side of each waveguide turn to reduce crosstalk between adjacent waveguide turns, thereby increasing the spatial density of waveguide turns that can be fabricated within a given area of ​​the integrated photonics chip.

[0020] An integrated photonics chip can have multiple layers or planes, and portions of the photonics components can be distributed among the multiple planes. In this way, the total footprint of the photonics chip can remain small, but more functionality can be packed into the photonics chip, and longer waveguide coil lengths can be introduced without increasing the footprint of the device.

[0021] The present disclosure will become more fully understood from the following detailed description and accompanying drawings of various embodiments of the disclosure. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a schematic top view of an optical gyroscope coil (also called a waveguide coil) and a MEMS accelerometer having multiple turns on the same die, according to an embodiment of the present disclosure. [Figure 2A] 1 is a diagram of a process flow for monolithically integrating a MEMS accelerometer onto an optical gyroscope chip with silicon nitride waveguides according to an embodiment of the present disclosure, specifically a cross-sectional view at the start of the MEMS-SiPhOG integration process flow. [Figure 2B] FIG. 1 is a diagram of a process flow for monolithically integrating a MEMS accelerometer onto an optical gyroscope chip with silicon nitride waveguides, in accordance with an embodiment of the present disclosure, specifically showing an additional layer of SiN formed on top of an oxide layer that serves as a lower cladding. [Figure 2C] FIG. 1 is a diagram of a process flow for monolithically integrating a MEMS accelerometer onto an optical gyroscope chip with silicon nitride waveguides, in accordance with an embodiment of the present disclosure, showing in detail a sacrificial layer deposited and patterned over an etch stop layer and patterned electrodes. [Figure 2D] FIG. 1 is a diagram of a process flow for monolithically integrating a MEMS accelerometer into an optical gyroscope chip with silicon nitride waveguides, showing a patterned structural layer deposited over a patterned sacrificial layer and electrodes, according to an embodiment of the present disclosure. [Figure 2E] FIG. 1 is a diagram of a process flow for monolithically integrating a MEMS accelerometer onto an optical gyroscope chip with silicon nitride waveguides, showing a second sacrificial layer deposited and patterned over the structural layer, according to an embodiment of the present disclosure. [Figure 2F] FIG. 1 is a diagram of a process flow for monolithically integrating a MEMS accelerometer onto an optical gyroscope chip with a silicon nitride waveguide, according to an embodiment of the present disclosure, showing the patterning of a structural layer to create pillars in the area designated for the MEMS accelerometer. [Figure 3] 1 is a schematic cross-sectional view illustrating a fully processed integrated device including a MEMS accelerometer and an optical gyroscope according to an embodiment of the present disclosure. [Figure 4] 1A-1C are schematic cross-sectional views illustrating alternative configurations (cross-sections) of a gyroscope waveguide chip that can serve as a basis for monolithically integrating a MEMS accelerometer, according to embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0023] Aspects of the present disclosure are directed to the monolithic integration of compact, ultra-low-loss integrated photonics-based waveguides with microelectromechanical systems (MEMS)-based sensing devices. These waveguides can be used, for example, as optical elements on planar photonic integrated circuits (PICs) in photonic integrated optical gyroscopes. As discussed in the background section, the key to the high performance of fiber-based optical gyroscopes is the long lengths of high-quality, low-loss optical fiber used to measure the Sagnac effect. The inventors recognize that with the advent of integrated silicon photonics suitable for wafer-scale processing, there is an opportunity to replace FOGs with smaller integrated photonics chip solutions without sacrificing performance. Photonics-based optical gyros offer the potential to offer comparable performance to FOGs, while reducing size, weight, power, and cost, and, in addition, being mass-producible and immune to vibration and electromagnetic interference. When an integrated optical gyroscope is fabricated on a silicon platform, it is abbreviated as SiPhOG® (Silicon Photonics Optical Gyroscope). This disclosure takes a step towards combining sensing elements by monolithically integrating a MEMS accelerometer onto a photonics gyroscope chip.

[0024] One of the key elements of this integrated photonics solution is the fabrication of a very low-loss waveguide core out of silicon nitride (Si3N4) surrounded by a cladding of oxide or fused silica. The entire waveguide structure (including the core and cladding) is sometimes referred to as a SiN waveguide for simplicity. The propagation loss of a SiN waveguide can be well below 0.1 dB / meter. This is a significant improvement over current state-of-the-art SiN processes, which have propagation losses in the 0.1 dB / centimeter range.

[0025] FIG. 1 shows a SiPhOG®-MEMS composite system 100 fabricated on a gyroscope waveguide die 10. Light is launched at a first end 14 of a gyroscope waveguide coil 20 having several turns. While only four turns are shown here for clarity, a practical device may use many more turns (e.g., hundreds of turns) based on the required sensitivity of the gyroscope. After propagating through the waveguide coil, the light exits at a second end 16. Note that light can be launched from either end 14 or 16, so each end can function as an “input end” or “output end.” For simplicity, the first end 14 will be referred to as the “input end,” the second end 16 as the “output end,” and the portion 18 of the waveguide near the second end 16 will be referred to as the “output waveguide” 18. In some embodiments, light can be launched at both ends 14 and 16 to obtain a phase difference signal from counter-propagating light beams. The design of the waveguide coil takes into account phase interference between counter-propagating beams and / or cross-coupling between adjacent waveguides, such as 110 and 112. The accelerometer device 30 is integrated onto the gyroscope waveguide die 10. In one embodiment, the accelerometer die 12 including the MEMS accelerometer device 30 may be hybrid-integrated onto the gyroscope waveguide die 10. For example, a precursor to this disclosure was filed as U.S. Provisional Application No. 62 / 923,234 on October 18, 2019, entitled "INTEGRATED PHOTONICS OPTICAL GYROSCOPES OPTIMIZED FOR AUTONOMOUS TERRESTRIAL AND AERIAL VEHICLES," which describes hybrid integration of MEMS sensors with photonics gyroscopes at a module level. This application was amended on October 15, 2020 to U.S. Non-Provisional Application No. 17 / 071,697, which has been published as U.S. Patent Application Publication No. 2021 / 0116246, all of which are incorporated herein by reference. This disclosure focuses on in-plane integration of a MEMS accelerometer device 30 onto a gyroscope waveguide die 10.In other words, a self-contained inertial measurement unit (IMU) including a photonics optical gyroscope and a MEMS accelerometer can be fabricated as a single device on a single die, which may be referred to as SiPhOG-X.

[0026] FIG. 2A shows a cross-sectional view at the beginning of the MEMS-SiPhOG integration process flow. Specifically, FIG. 2A shows substrate 102, which may be a silicon substrate. The thickness of substrate 102 may be a standard wafer thickness, e.g., 725 μm. Note that the thicknesses of the various material layers are not drawn to scale. However, dashed line 101 is introduced in the middle of layer 102 for clarity only, to convey the idea that substrate 102 is much thicker than the rest of the material layers shown in FIGS. 2A-2F. The thicknesses of layers 106 and 116 may be in the range of 15 μm on either side of substrate 102, with a thickness "h1." Layer 106 serves as a lower cladding for waveguide cores 110 and 112. Note that adjacent waveguide cores 110 and 112 correspond to each turn of waveguide coil 20 shown in FIG. 1. The pitch p1 between the cores may be in the range of 20 μm, which can be significantly reduced by introducing structural modifications between adjacent waveguide cores, as described below with respect to FIG. 4. The waveguide cores 110 and 112 may have a thickness "h" and a width "w." A non-limiting exemplary dimension for "h" may be 60-100 nm, and "w" may be 2-3 μm. The waveguide cores 110 and 112 are made from silicon nitride (SiN). The upper cladding 104 is formed on the waveguide cores 110 and 112. The thickness "h2" of the upper cladding layer 104 may be in the range of 2-3 μm. If necessary for subsequent process flows to produce the MEMS accelerometer, the thickness may be reduced to approximately 1 μm. It should be noted that when layers 106, 110 (and 112) and 104 are formed on one side of substrate 102, corresponding layers 116, 118 and 114 are also formed on the other side of substrate 102, even though those layers are not used for waveguiding purposes, or those layers can create waveguides in different layers, if desired.While both the top cladding 104 and the bottom cladding 106 are shown to be of the same material 108, e.g., silicon oxide, in various embodiments, the bottom cladding can be a pre-grown oxide and the top cladding can be a deposited oxide such as TEOS (Tetra Ethyl Ortho Silicate) or other composition. Similarly, both layers 114 and 116 have the same material 120, which is identical to material 108. Dashed box 113 indicates the area allocated for the turns of the optical gyroscope coil 20 on die 10, and dashed box 111 indicates the area allocated for subsequent fabrication of a MEMS accelerometer on the same die 10.

[0027] 2B shows an additional layer 122 of SiN formed on oxide layer 108. This SiN layer 122 acts as an etch stop layer for MEMS accelerometer fabrication. Electrodes 124 and 126 can be deposited and patterned on layer 122 in the area 111 designated for the MEMS accelerometer.

[0028] 2C shows a sacrificial layer 128 deposited and patterned over the etch stop layer 122 and the patterned electrodes 126 and 124. The sacrificial layer 128 may be an oxide layer.

[0029] 2D shows a patterned structural layer 130 deposited over the patterned sacrificial layer 128 and electrodes 126, 124. The structural layer 130 can be made from polysilicon germanium (SiGe). This layer fills the gaps between the patterned sacrificial layer 128.

[0030] 2E shows a second sacrificial layer 131 deposited and patterned on structural layer 130. Layer 131 may be the same material as layer 128, for example, an oxide. Bond pads 132 and 134 are formed and patterned on second sacrificial layer 131.

[0031] 2F shows the step of patterning structural layer 130 to create pillars 130a, 130b, 130c, 130d, and 130e in designated area 111 for the MEMS accelerometer. A second sacrificial layer 131 is also patterned over the pillars (131a, 131b, 131c).

[0032] 3 shows MEMS device 30 after sacrificial layers 128 and 131 have been removed, thereby creating a free-floating structure 130c midway between the surrounding frame represented by posts 130b and 130d. This free-floating structure 130c is essential to the operation of the MEMS accelerometer.

[0033] Note that each waveguide core 110, 112 corresponds to a turn of the waveguide coil 20 shown in FIG. 1. To maintain single mode operation and avoid coupling between adjacent waveguides, a minimum pitch p1 must be maintained between adjacent waveguide cores. Non-limiting exemplary values ​​for p1 can be 14-16 μm, or 20 μm. This pitch limits the total length of the waveguides on the die 10 (see FIG. 1) and also the maximum area enclosed by the waveguide coil 20.

[0034] FIG. 4 illustrates one approach to reducing the pitch and thereby packing the turns of the waveguide coil 20 more closely while reducing crosstalk between adjacent waveguides. The embodiment of FIG. 4 automatically increases the overall length of the waveguide and the enclosed area, thereby increasing the sensitivity of the optical gyroscope. By introducing air gaps 450 on both sides of the waveguide core, the optical mode is largely confined within one turn of the waveguide, preventing leakage of the optical signal into adjacent turns of the waveguide. In other words, the air gaps 450 function as physical insulators between adjacent waveguide cores 410, 412, 414, and 416. Reducing the pitch p2 between two adjacent waveguide cores automatically accommodates more turns within the same reticle field, increasing both the overall length and the enclosed area in each layer or plane. The pitch p2 can easily be made smaller than 10 μm with the air gaps, i.e., much smaller than the pitch p1 shown in FIG. 2A. Note that instead of air, the gaps may be filled with other non-reactive fluids, such as inert gases. Also, instead of air gaps, subwavelength grating structures or metal barriers can be used between adjacent waveguides to reduce the pitch without increasing crosstalk. Note that MEMS accelerometers can be fabricated on modified SiN waveguide chips (e.g., with air gaps) in much the same way as described with respect to Figures 2A-2F.

[0035] Note that one option could be to distribute the entire length of the SiN waveguide coil with multiple turns (and / or the ring with a single turn) into different vertically separated layers (e.g., two or more layers), which would result in improved gyro sensitivity without increasing the form factor. Details of stacked multilayer gyro configurations are contained in U.S. Provisional Application No. 62 / 858,588, entitled "Integrated Silicon Photonics Optical Gyroscope on Fused Silica Platform," filed June 7, 2019. Sequel U.S. Provisional Application No. 62 / 896,365, entitled "Single-layer and Multi-layer Structures for Integrated Silicon Photonics Optical Gyroscopes," filed September 5, 2019, describes additional embodiments. A third U.S. Provisional Application No. 62 / 986,379, entitled "Process Flow for Fabricating Integrated Photonics Optical Gyroscopes," was filed March 6, 2020. These three applications are combined into U.S. Non-provisional Application No. 16 / 894,120, entitled "Single-layer and Multi-layer Structures for Integrated Silicon Photonics Optical Gyroscopes," filed June 5, 2020, and ultimately issued as U.S. Patent No. 10,969,548 on April 6, 2021. These applications are incorporated herein by reference.Additionally, system-level integration of silicon photonics-based front-end chips with SiN waveguide chips is encompassed in U.S. Provisional Application No. 62 / 872,640, entitled "System Architecture for Silicon Photonics Optical Gyroscopes," filed July 10, 2019, and U.S. Provisional Application No. 62 / 904,443, entitled "System Architecture for Silicon Photonics Optical Gyroscopes with Mode-Selective Waveguides," filed September 23, 2019. These two applications were combined into U.S. Nonprovisional Application No. 16 / 659,424, entitled "System Architecture for Silicon Photonics Optical Gyroscopes," filed October 21, 2019, and ultimately issued as U.S. Patent No. 10,731,988 on August 4, 2020. These applications are also incorporated herein by reference.

[0036] However, in the aforementioned applications, the need to space adjacent waveguides in a single plane with a pitch that prevents unwanted cross-coupling also created the need to fabricate a two-layer device. Therefore, to maintain approximately the same footprint, the entire length of the waveguide spiral was distributed among two or more planes. The present disclosure provides a solution that allows adjacent waveguides to be more densely packed within a single plane, i.e., shortening the pitch between adjacent waveguides in each plane. Note that the terms "layer" and "plane" are used interchangeably when describing the distribution of waveguide coils among multiple planes. Densely packing the waveguides in a single plane can completely eliminate the need to fabricate a multilayer device, or at least reduce the number of layers required to achieve an adequate total length of the waveguide coil, which is directly related to the sensitivity of the optical gyroscope.

[0037] In summary, incorporating a MEMS sensor on the same chip as a photonics optical gyroscope can utilize both the Coriolis force and the Sagnac effect to achieve precise inertial sensing, including rotation and acceleration sensing. Even a lower-precision mechanical gyroscope can be integrated on the same die for axes that do not require the high-precision optical values ​​of a Sagnac effect gyroscope. Monolithic integration of SiPhOGs with MEMS sensors expedites bringing all the electronic control circuitry for the various sensors onto the same chip.

[0038] It should be noted that some sensing applications may require a high-precision optical gyroscope on only one axis to supplement or replace low-precision measurements from a low-cost mechanical gyroscope (e.g., a MEMS-based gyroscope), while the other two axes may continue to use low-precision measurements from the low-cost mechanical gyroscope. One such example is the gyroscope in the safety sensor on which current and future generations of autonomous vehicles, particularly automatic driver assistance systems (ADAS) for the Level 2.5 / Level 3 (L2.5 / L3) market, rely. In ADAS, because the vehicle is on the XY plane of a hard road, high-precision angle measurement may be desired only on the Z axis (yaw axis) to determine heading. In this situation, angle measurement on the X and Y axes (pitch and roll axes) may not be safety-critical. The inventors have recognized that lowering the cost of high-precision optical gyroscopes for at least one axis will reduce the overall cost of the IMU and facilitate mass-market penetration. Additionally, if desired, other two-axis mechanical gyroscopes may also be replaced or supplemented by optical gyroscopes with properly designed system-level integration of all three axes (pitch, roll, and yaw axes), for example, in unmanned aerial vehicles (e.g., drones), construction, agriculture, industrial, maritime vehicles, L4 / L5 markets, and certain military applications.

[0039] In the foregoing specification, embodiments of the present disclosure have been described with reference to certain exemplary embodiments. It will be apparent that various modifications can be made without departing from the broader spirit and scope of the embodiments of the present disclosure as set forth in the following claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. In addition, directional terms such as "upper," "lower," etc., are not intended to limit the scope of the present disclosure to any fixed orientation, but rather encompass various permutations and combinations of orientations. [Explanation of symbols]

[0040] 10 Gyroscope waveguide die 12 accelerometer die 14 Input terminal 16 Output terminal 18 Output waveguide 20 Gyroscope waveguide coil 30 Accelerometer Device 100 SiPhOG®-MEMS composite system 101 dashed line 102 Circuit Board 104 Upper Cladding 106 Lower Cladding 108 Material 110 Waveguide core, layer 111 Area allocated for MEMS accelerometers 112 Waveguide core, layer 113 Area allocated for turns of optical gyroscope coil 114 layers 116 layers 118 layers 120 Material 122 Etch stop layer 124 electrodes 126 electrode 128 Sacrificial Layer 130 Structural Layer 130a Pillar 130b pillar 130c pillar 130d pillar 130e Pillar 131 Second Sacrificial Layer 131a Pillar 131b Pillar 131c pillar 132 Bond Pad 134 Bond Pad 410 Waveguide Core 412 Waveguide Core 414 Waveguide Core 416 Waveguide Core 450 void h thickness h1 thickness h2 thickness p1 pitch w width

Claims

1. a waveguide coil having a plurality of waveguide turns looping around a central region enclosed by the waveguide coil, each waveguide turn being parallel to an adjacent waveguide turn, the waveguide coil being used as a rotation sensing element of an optical gyroscope; a microelectromechanical systems (MEMS) based motion sensing device monolithically integrated in the central region surrounded by the waveguide coil, wherein the waveguide coil and the MEMS based motion sensing device are fabricated on a common platform; and An integrated photonics chip comprising: An integrated photonics chip, wherein the common platform is a silicon photonics platform, and each waveguide turn comprises a waveguide core sandwiched between an upper cladding and a lower cladding.

2. The integrated photonics chip of claim 1 , wherein the optical gyroscope and the MEMS-based motion sensing device are packaged together as a modular, integrated inertial measurement unit (IMU).

3. 3. The integrated photonics chip of claim 2, wherein the MEMS-based motion sensing device provides coarse rotational sensing values ​​for all axes of motion and the optical gyroscope provides finer rotational sensing values ​​for one or more selected axes of motion.

4. The integrated photonics chip of claim 2 , wherein the MEMS-based motion sensing device comprises an accelerometer for one or more axes of motion.

5. The integrated photonics chip of claim 1 , wherein the waveguide core comprises silicon nitride and the upper cladding and the lower cladding comprise oxide.

6. 10. The integrated photonics chip of claim 1, further comprising structural modifications introduced on either side of each waveguide turn to reduce crosstalk between adjacent waveguide turns, thereby increasing the spatial density of waveguide turns that can be fabricated within a given area of ​​the integrated photonics chip.

7. The integrated photonics chip of claim 6 , wherein the predetermined area depends on the exposure field of a reticle used to fabricate the waveguide coil and the MEMS-based motion sensing device.

8. 7. The integrated photonics chip of claim 6, wherein increasing the spatial density of waveguide turns increases the central area enclosed within the waveguide coil and increases the number of waveguide turns surrounding the central area, thereby increasing the sensitivity of the rotation sensing element.

9. The integrated photonics chip of claim 6 , wherein the structural modifications include gaps.

10. The integrated photonics chip of claim 9 , wherein the gap comprises one of an air gap, a metal-filled gap, or an inert gas or liquid-filled gap.

11. 10. The integrated photonics chip of claim 9, wherein the gap is in the form of a high aspect ratio rectangular slit or groove, the longitudinal dimensions of the gap being substantially greater than the lateral dimensions of the gap, such that the gap extends substantially longitudinally above and below the waveguide core.

12. 10. The integrated photonics chip of claim 1, wherein a first portion of the waveguide coil is on a first plane, a second portion of the waveguide coil is on a second plane, and the first plane and the second plane are stacked vertically relative to each other.

13. 13. The integrated photonics chip of claim 12, wherein light evanescently couples between the first portion of the waveguide coil on the first plane and the second portion of the waveguide coil on the second plane.

14. 1. A method for monolithically fabricating an integrated photonics chip comprising a waveguide coil and a MEMS-based motion sensing device on a common platform, comprising: designating a central region on the common platform for fabricating the MEMS-based motion sensing device, the central region being surrounded by the waveguide coil comprising a plurality of waveguide turns looping around the central region, each waveguide turn being parallel to adjacent waveguide turns, the waveguide coil being used as a rotation sensing element of an optical gyroscope; fabricating the waveguide coils on the common platform; protecting the fabricated waveguide coil by depositing an etch stop layer over the waveguide coil; fabricating the MEMS-based motion sensing device in the designated central area; A method comprising:

15. fabricating the MEMS-based motion sensing device comprises:

15. The method of claim 14, further comprising depositing and patterning an electrode over the etch stop layer in the designated central region.

16. depositing and patterning a first sacrificial layer over the etch stop layer and the patterned electrode; depositing and patterning a structural layer over the patterned sacrificial layer and the patterned electrodes; depositing and patterning a second sacrificial layer over the patterned structural layer; 16. The method of claim 15, further comprising:

17. patterning the structural layer to create pillars as part of the MEMS-based motion sensing device, the second sacrificial layer also being patterned onto the pillars; removing the first sacrificial layer and the second sacrificial layer, thereby creating a floating structure that functions as a motion sensing element of the MEMS-based device; 17. The method of claim 16, further comprising:

18. 15. The method of claim 14, further comprising forming a gap on either side of each waveguide turn to reduce crosstalk between adjacent waveguide turns, thereby increasing the spatial density of waveguide turns that can be fabricated in a given area of ​​the integrated photonics chip.

19. The step of fabricating the waveguide coil comprises: fabricating a first portion of the waveguide coil on a first plane; fabricating a second portion of the waveguide coil on a second plane, wherein light evanescently couples between the first portion of the waveguide coil on the first plane and the second portion of the waveguide coil on the second plane; 15. The method of claim 14, further comprising:

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