RU2739829C1 - Optomechanical vibration micro-sensor - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to micro-optoelectromechanical systems or optical MOEMS systems, more specifically to miniature inertial (vibration) optical sensors using MOEMS systems. Disclosed is an optomechanical vibration micro-sensor which is fully integrated into a silicon substrate with an optical circuit for reading interference type, with a planar structure based on silicon oxynitride waveguides, wherein micro sensor design consists of waveguide Mach-Zehnder interferometer, one of arms of which is located along cantilever perimeter, the other shoulder on substrate. Oscillations of the cantilever cause change in the length of the optical waveguide due to deformation of the layer in which the waveguide is located. Changing the length of one arm of the interferometer results in a change in the phase ratio of waves summed at the output communication element. Communication element in the interferometer at the input and output can be directional couplers (connected waveguides) or multimode communication devices. Difference in intensity of light flux between two waveguides in arms of interferometer is proportional to acceleration in wide range of amplitudes and allows interferometer to detect mechanical displacement.

EFFECT: simplified design, increased reliability, sensitivity and possibility of operation under severe environmental conditions (strong electromagnetic fields and high temperature).

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Description

The invention relates to micro-optoelectromechanical systems or optical MEMS systems, more precisely to the field of miniature inertial (vibration) optical sensors using MOEMS systems.

From the prior art, inertial sensors are known, the operation of which is based on the use of a change in capacitance of parallel plate electrodes or piezoresistors in a silicon crystal. However, the sensitivity of such sensors tends to decrease with decreasing sensor size. Therefore, for further miniaturization, other solution methods are needed for inertial sensors.

As a solution to the above problem, optical methods are very promising. Generally, the optical signal is immune to electromagnetic interference and parasitic capacitance. In addition, the signal strength of the optical device does not necessarily depend on the size of the device. Therefore, it is possible to manufacture an inertial sensor using optical signals with dimensions smaller than conventional inertial sensors. In terms of a number of indicators (sensitivity, noise immunity), the most promising option is with an inertial waveguide sensor using light interference.

Known optical sensor for measuring the parameters of ground vibrations based on a planar optical waveguide structure / 1 /. The sensor uses an optical interferometer to capture the phase shift in optical signals in response to external vibrations. This sensor provides the magnitude and frequency of vibration disturbance and its direction. The interferometer includes a first leg having a first optical path length and a second leg having a second optical path length. An optical signal generator is connected to the interferometer to supply it with a frequency modulated optical signal. A pair of electrodes is located along one of the branches of the interferometer. A wire coil (inductor) wound around a stationary magnetic core is connected between a pair of electrodes. The inductor has freedom of movement in the longitudinal direction along the rod and can oscillate in response to the movement of the ground, thereby inducing voltage. This voltage is applied to a pair of electrodes, which causes phase modulation of the optical signal directed by the interferometer arm to the output coupler of the interferometer. Next, the optical signals output from the interferometer are processed by a photodetector to determine the speed and acceleration associated with the movement of the earth.

This device is not a fully integrated optical-mechanical microstructure. In it, the sensitive element itself is external, which significantly increases the dimensions of the device. One of the problems of optical sensors of this type is the difficulty of obtaining high sensitivity, the complexity of coupling with optical fibers.

Known accelerometer based on MOEMS, which uses a Mach-Zehnder interferometer (IMC) - the type of optical modulator / 2 /.

This accelerometer is suitable for use in a system that uses optical fibers as signal lines, since no optoelectric conversion is required to connect the sensor to the optical fibers. In addition, the combination of the proposed accelerometer and optical wiring can be used in an environment with high electromagnetic noise.

In this sensor, the central part of one of the arms in the MZI has a floating structure (cantilever). When inertia is applied perpendicularly to the transducer, the floating MZI waveguide deflects and expands. As a result, the output intensity of the MZI is modulated. The MZI intensity modulation is:

where I is the intensity of the light wave at the output, I ₀ is the intensity of the initial light wave, n is the refractive index of the waveguide, and ΔL is the mechanically increased length of the floating waveguide under the action of the inertial force.

However, since the mass of the floating waveguide is small, the applied inertial force due to acceleration is also small, since the inertial force due to acceleration is proportional to the mass of the object. Thus, an additional corrective mass is needed, which is placed on the floating waveguide to enhance its deflection. However, the floating waveguide is too narrow to impose a corrective mass on it. Therefore, a console is added to support this mass. This cantilever crosses the floating waveguide in the same plane. When the correction mass moves under the applied force, the floating waveguide and the cantilever deflect simultaneously.

However, this accelerometer has a number of problems. One of the problems is the light loss at the intersection of the cantilever and the floating waveguide. The waveguide crossing loss tends to increase as the width of the cantilever increases.

In addition, this device uses waveguides based on a silicon-on-insulator (SOI) structure, absorption of the material in silicon leads to self-heating and bending of the waveguide, which leads to errors in displacement measurements. Silicon has a large thermo-optical effect (dn / dT ~ 2 × 10-4 ° C), therefore SOI waveguides are not optimal for high-resolution displacement measurements. Typically, silicon waveguides exhibit relatively high light losses (~ 5-10 dB / cm). Silicon waveguides made from SOI material are more expensive than standard silicon. Finally, the relatively high refractive index of silicon (n _silicon = 3.5 at λ = 1550 nm) requires relatively small sizes of waveguides (width w <500 nm) to maintain a single-mode condition (the condition for the correct operation of the interferometer) / 3 /. This increases the need for high precision manufacturing, which further increases cost.

The problem to be solved by the claimed invention is to implement a vibration sensor that meets the requirements for compactness, reliability and low cost, to meet the requirements of high sensitivity and the ability to operate in harsh environmental conditions (strong electromagnetic fields and high temperatures).

The problem is solved primarily by using the design of an optomechanical vibration microsensor, fully integrated into a silicon substrate with an optical readout circuit of the interference type, with a planar structure based on silicon oxynitride waveguides, characterized in that the microsensor design consists of a Mach-Zehnder waveguide interferometer, one of the arms of which is located along the perimeter of the cantilever (floating waveguide), the other arm is on the substrate.

Oscillations of the cantilever lead to a change in the length of the optical waveguide due to deformation of the layer in which the waveguide is located. A change in the length of one of the interferometer arms leads to a change in the phase relationship of the waves summed at the output coupling element. The difference in the intensity of the light flux between the two waveguides in the arms of the interferometer is proportional to the acceleration over a wide range of amplitudes, which allows the microsensor to detect mechanical displacement.

The location and geometry of the "floating" waveguide in one of the arms of the Mach-Zehnder interferometer along the perimeter of the cantilever are selected from the requirements for increasing the sensitivity, which can vary depending on the length of the arm of the cantilever (and, accordingly, the length of the waveguide), minimizing bending losses in the curved sections of the waveguides and compactness of the device.

From the literature / 4 / it is known that propagation losses (including bending losses) of about 0.1 dB / cm were obtained for the contrast of the waveguide structure Δn = 1.5-2% (the size of the oxynitride core is 2.5 × 2, 0 μm ² , R = 2 mm), which is significantly less than in the case of a SOI sensor. This geometry and structure of waveguides is the basic reference for the design of this microsensor.

Secondly, the problem to be solved by the claimed invention is solved by choosing a material for manufacturing an optical waveguide structure and an inertial mass. It is known that the sensitivity of the sensor is directly related to the level of total optical signal loss. Losses such as leakage to the substrate and absorption losses by hydrogen impurities can be reduced by using thermally grown silicon oxide with a thickness of 7-15 μm in the upper and lower buffer layers of waveguides. In this case, the central core layer (oxynitride) is applied directly to the lower buffer layer (thermal silicon oxide). The upper buffer layer (thermal silicon oxide) is transferred by bonding to a pre-oxidized silicon wafer. Excess topcoats can be removed by chemical or chemical-mechanical polishing. Thus, two problems are solved - the insertion loss is reduced, the structure has a lower mechanical stress than in the version with the traditional use of thick plasma silicon oxide as the upper buffer layer of the waveguide structure. The use of this method of manufacturing a waveguide structure is known / 5 /, but is innovative in the context of application for a "floating" waveguide.

Higher sensor sensitivity can be achieved by using multimode coupling elements at the input and output of the Mach-Zehnder interferometer. These coupling elements are easier to manufacture from a technological point of view, with less stringent aspect ratio requirements for photolithography and etching, and provide better process reproducibility. According to calculations, multimode coupling elements can be 15 microns long and 300 microns long.

The invention is illustrated by drawings and drawings:

FIG. 1 optomechanical vibration microsensor with an optical readout circuit of the interference type, with a planar structure based on SiON waveguides, with coupling elements in the form of directional couplers, where

1 - Mach-Zehnder interferometer

2,3 - coupling elements of the Mach-Zehnder interferometer

4 - cantilever

5,6 - input-outputs of the Mach-Zehnder interferometer

7 - the released area of the silicon substrate

8 - etching grooves SiON, SiO ₂

9 - arms of the Mach-Zehnder interferometer

FIG. 2 an optomechanical vibration microsensor with an interference-type optical readout circuit, with a planar structure based on silicon oxynitride waveguides, with coupling elements in the form of multimode modules, where

10.11 - communication elements of the Mach-Zehnder interferometer in the form of multimode modules.

Mechanical design parameters can be selected based on frequency response requirements. The thickness of the waveguide structure (15 µm) is selected to control the longitudinal sensitivity (along an axis parallel to the substrate). According to calculations, the length of the cantilever arm and the mass of the cantilever part of the waveguide (inertial mass) can be 600 μm and 2.5 mg, respectively, in order to ensure high-frequency linearity. It is known to use the linear portion of the displacement response, where acceleration and displacement are proportional, the resonant frequency must be higher than the upper limit of the operating range. To achieve 5% linearity in the 30-450 Hz range, it must be higher than 1.5KHz.

The mechanical sensitivity of the sensor is determined by the frequency response requirements: the optical circuit must be able to detect displacements up to 10 nm (expected resolution: 0.5 m / s). The optical design is optimized using the BPM method to improve sensitivity as well as provide linearity of the output signal for an offset range between -0.5 μm and 0.5 μm.

As an example, the microsensor structure has been designed and optimized to operate in accordance with the following requirements:

linearity in amplitude 5% (range: 0.5 m / s ² - 400 m / s ² ),

frequency range - from 30 to 2000 Hz,

operating temperature range - up to 100 ° С.

This design of the sensor provides for placement on a single substrate in addition to the sensor element of the transmitter (laser) and photodetector modules to increase the sensitivity and eliminate unwanted additional communication interference, as well as to further miniaturize the device. All components can be placed on a single substrate by hybrid assembly.

Thus, the expected technical result, namely, in terms of compactness, reliability, low cost, high sensitivity and ability to work in harsh field conditions, can be achieved using the original microsensor design, optimal materials and methods for its implementation and the possibility of integral performance for creating functionally complete device.

The fully integrated optical sensor of this design can be used to measure small displacements and low frequency vibrations, and control rotating machinery in harsh electromagnetic conditions.

Information sources.

1. US patent No. 5497233,

2. "A Study of Mach-Zehnder Interferometer Type Optical Modulator Applicable to an Accelerometer" Japanese Journal of Applied Physics 50 (2011) - prototype,

3. US patent No. 9395177,

4. "Low-Loss Fiber-Matched Low-Temperature PECVD Waveguides with Small-Core Dimensions for Optical Communication Systems" IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 9, NO. 9, SEPTEMBER 1997,

5. "Planar waveguides with less than 0.1 dB / m propagation loss fabricated with wafer bonding" Optics Express, November 2011.

Claims (1)

An optomechanical vibration microsensor, fully integrated into a silicon substrate with an interference-type optical readout circuit, with a planar structure based on silicon oxynitride waveguides, characterized in that it contains a Mach-Zehnder waveguide interferometer, one of the arms of which is located along the perimeter of the cantilever, the other arm on the substrate , the coupling elements in a Mach-Zehnder interferometer at the input and output are directional couplers or multimode coupling devices, and the optical part of the microsensor is bonding bonding, where thermal silicon oxide acts as the lower and upper buffer layers of the waveguide structure.

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