A preparation method and application of a piezoresistive angle sensor

Abstract

The application discloses a preparation method and application of a piezoresistive angle sensor, and comprises the following steps: performing thermal oxygen treatment on the surface of a silicon wafer to grow an oxide layer on the surface of the silicon wafer; performing photoetching and oxide layer etching on the region of the silicon wafer required to form a piezoresistive strip according to the product structure, and etching to the silicon substrate; performing ion implantation on the silicon wafer to form a piezoresistive strip with a piezoresistive effect in the region not protected by the oxidized silicon; removing the silicon oxide film and performing rapid annealing; performing metallization plating on the silicon wafer; performing photoetching and etching on the metal film to form a metal lead; the metal lead is connected with piezoresistive strips with the same resistance value to form a Wheatstone bridge, and a finished product is obtained. The application is that the piezoresistive angle sensor is installed in a vehicle-mounted laser radar micro mirror to read the angle change of the micro mirror. The piezoresistive angle sensor prepared by the application has the advantages of simple structure, low cost and high measurement precision.

Description

Technical Field

[0001] This invention relates to the field of microelectromechanical systems (MEMS) technology, and in particular to a method for fabricating and applying a piezoresistive angle sensor. Background Technology

[0002] LiDAR, millimeter-wave radar, and cameras are recognized as the three key sensing technologies for autonomous driving. Compared to millimeter-wave radar and cameras, LiDAR technology, with its superior spatial 3D resolution capabilities, has become the preferred technology for Level 3 / 4 autonomous driving.

[0003] LiDAR technology boasts high resolution, high precision, and strong resistance to active interference, making it the optimal technology for autonomous driving from a product and technology perspective. However, achieving breakthroughs in the mass production of automotive-grade LiDAR is a crucial milestone for the realization of autonomous driving technology. Currently, there are four main LiDAR technology routes: traditional mechanical LiDAR, MEMS, Flash, and OPA. Each has its own characteristics. Traditional mechanical LiDAR suffers from low reliability and high cost. Among the latter three solid-state LiDAR technologies, MEMS (Micro-Electro-Mechanical Systems) can directly integrate extremely compact micromirrors onto silicon-based chips. By flexibly controlling the rotation of these micromirrors through circuitry, the laser emitted by the laser emitter is deflected, enabling scanning of the external environment and generating high-density point cloud maps with ultra-high scanning speed. Under the premise of meeting automotive-grade LiDAR performance requirements, MEMS LiDAR, with its reasonable price and rapid mass production capabilities, is the most promising technology solution for practical application.

[0004] MEMS lidar currently faces two major technical challenges: ensuring angular accuracy and passing automotive-grade testing. The mainstream methods for directly detecting the angle of MEMS galvanometers are photoelectric detection, induced electromotive force method, and induced capacitance method.

[0005] The photoelectric detection method fixes the relative positions of a micromirror, a laser, and a photodetector. The photodetector is positioned within the scanning path of the micro-torsional mirror. When the laser beam emitted from the laser is reflected by the micro-torsional mirror and scans through the first photodetector, the first photodetector captures the position signal of the micro-torsional mirror and uses this signal to calculate the amplitude and phase of the micro-torsional mirror's vibration. This method is complex, expensive, and has high assembly and debugging costs. Furthermore, it has low integration, making large-scale mass production difficult, and it is even more difficult to pass automotive-grade testing.

[0006] The induced electromotive force method indirectly calculates the rotation angle of a micromirror by measuring the induced electromotive force generated by the induction coil cutting within a magnetic field as the mirror rotates. It is mostly applicable to electromagnetically driven MEMS micromirrors and is characterized by low precision and high delay.

[0007] Inductive capacitance method: This method involves forming a rotating comb structure on the micromirror. During rotation, the change in the relative area between the comb and the fixed inductive comb shape creates an inductive capacitance, which changes with the rotation angle of the comb. This method is commonly used in electrostatically driven MEMS micromirrors, but it has low compatibility with other driving methods and suffers from low signal-to-noise ratio and low measurement accuracy. Summary of the Invention

[0008] The purpose of this invention is to provide a method for fabricating a piezoresistive angle sensor and its application. The piezoresistive angle sensor fabricated by this invention has the advantages of simple structure, low cost, and high measurement accuracy.

[0009] The technical solution of this invention: A method for manufacturing a piezoresistive angle sensor, comprising the following steps:

[0010] Step 1: Perform thermal oxidation treatment on the silicon wafer surface to grow an oxide layer on the silicon wafer surface;

[0011] Step 2: Based on the product structure, perform photolithography and oxide layer etching on the area of ​​the silicon wafer where the piezoresistive strip needs to be formed, etching down to the silicon substrate;

[0012] Step 3: Ion implantation is performed on the silicon wafer to form piezoresistive strips with piezoresistive effect in the areas not protected by silicon oxide;

[0013] Step 4: Remove the silicon oxide film and perform rapid annealing;

[0014] Step 5: Metallize the silicon wafer;

[0015] Step 6: Perform photolithography and etching on the metal thin film to form metal leads;

[0016] Step 7: Connect the piezoresistive strips with the same resistance value using metal leads to form a Wheatstone bridge, thus obtaining the finished product.

[0017] In the above-mentioned method for preparing a piezoresistive angle sensor, in step 1, the thermal oxidation treatment involves placing a silicon wafer in a reaction tube made of quartz glass, heating the reaction tube to a temperature of 900-1200°C using a resistance wire heating furnace, and allowing oxygen or water vapor to pass through the reaction tube at a flow rate of 1 cm / s, causing a chemical reaction on the surface of the silicon wafer.

[0018] The aforementioned application of the piezoresistive angle sensor involves installing the piezoresistive angle sensor in the micromirror of an onboard LiDAR system to read the angle changes of the micromirror.

[0019] In the aforementioned application of the piezoresistive angle sensor, the angle is read by converting the piezoresistive effect of the piezoresistive strip into a rate of change of resistance using a Wheatstone bridge. The rate of change of resistance is then passed through a filter circuit and an amplifier circuit. The output signal of the amplifier circuit is then connected to an A / D converter, and the output signal of the A / D converter is then connected to an MCU or FPGA. Finally, the MCU or FPGA processes the signal to obtain the angle information.

[0020] In the aforementioned application of the piezoresistive angle sensor, the piezoresistive effect of the piezoresistive strip is expressed as follows:

[0021] ΔR / R=(1+2u+ΠE)×ε

[0022] In the formula: ΔR is the resistance change measured by the piezoresistive effect and the Wheatstone bridge, R is the initial impedance of the resistor; u is Poisson's ratio; ε is stress deformation, ε=ρ×θ, ρ is the torsional radius of the cross section, θ is the torsional angle, E is the elastic modulus, under the action of shear stress, E should be replaced by the shear modulus G;

[0023] θ=(AR / R) / (IIGρ)

[0024] Π is the piezoresistive coefficient matrix:

[0025]

[0026] The piezoresistive coefficient along each crystal direction in the crystal axis coordinate system is represented by π. ij Let i equal 1, 2 and 3 to represent the relative change in resistance along the x, y and z coordinate directions, and i equal 4, 5 and 6 to represent the relative change in resistance perpendicular to the x, y and z coordinate directions; j equal 1, 2 and 3 to represent the tensile stress along the x, z and y directions, and j equal 4, 5 and 6 to represent the shear stress perpendicular to the x, z and y directions.

[0027] Compared with existing technologies, the piezoresistive angle sensor fabrication method of this invention forms a doped piezoresistive strip on the surface of a movable silicon structure using ion implantation. Utilizing the piezoresistive effect of doped silicon, the strain of the piezoresistive strip caused by torsion, compression, and tension of the movable silicon structure is converted into a rate of change in resistance. This rate of change in resistance is then read using a Wheatstone bridge. This method simplifies the fabrication process, resulting in a simple piezoresistive angle sensor with low cost and high measurement accuracy. In application, this invention utilizes the piezoresistive effect region obtained by partial doping on a silicon-based material to convert the strain caused by micromirror torsion into an electrical readout of resistance change. This readout is further calibrated to reflect the angle change of the micromirror's torsion beam, enabling closed-loop control of the angle during micromirror vibration and further improving the scanning accuracy of MEMS lidar. This invention features a simple structure and low cost, eliminating the need for other detection devices or equipment such as lasers and photodetectors. It boasts a measurement accuracy of up to 0.01°, strong applicability, and can be integrated into MEMS galvanometers with different driving methods and many other micro-actuator chips. This provides a broad prospect for integrating angle sensors onto automotive-grade MEMS galvanometers and offers a suitable solution for the mass production of high-precision lidar products for automotive applications. Attached Figure Description

[0028] Figure 1 This is a flowchart of Embodiment 1 of the present invention.

[0029] Figure 2 This is a schematic diagram of the Wheatstone bridge structure of the piezoresistive angle sensor in Embodiment 1 of the present invention;

[0030] Figure 3 This is a flowchart of Embodiment 2 of the present invention. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this should not be construed as limiting the present invention.

[0032] Example 1: A method for fabricating a piezoresistive angle sensor, such as... Figure 1 As shown, it includes the following steps:

[0033] Step 1: Perform a thermal oxidation treatment on the silicon wafer surface to grow an oxide layer on the silicon wafer surface; the thermal oxidation treatment involves placing the silicon wafer in a reaction tube made of quartz glass, heating the reaction tube to a temperature of 900-1200°C using a resistance wire heating furnace, and allowing oxygen or water vapor to pass through the reaction tube at a flow rate of 1 cm / s, causing a chemical reaction on the silicon wafer surface.

[0034] Step 2: Based on the product structure (the product structure here depends on the specific design of the micromirror), perform photolithography and oxide layer etching on the area in the silicon wafer where the piezoresistive strip needs to be formed, etching down to the silicon substrate.

[0035] Step 3: Ion implantation is performed on the silicon wafer to form piezoresistive strips with piezoresistive effect in the areas not protected by silicon oxide;

[0036] Step 4: Remove the silicon oxide film and perform rapid annealing;

[0037] Step 5: Metallize the silicon wafer;

[0038] Step 6: Perform photolithography and etching on the metal thin film to form metal leads;

[0039] Step 7: Connect the piezoresistive strips with the same resistance value using metal leads to form a Wheatstone bridge, thus obtaining the finished product. For example... Figure 2 As shown, a Wheatstone bridge is a device consisting of four resistors used to accurately measure the resistance of one of the resistors (while the resistances of the other three are known). The Wheatstone bridge uses changes in resistance to measure changes in a physical quantity. A microcontroller collects and processes the voltage across the variable resistor to calculate the corresponding change in the physical quantity, making it a highly accurate measurement method.

[0040] This embodiment of the piezoresistive strip utilizes the piezoresistive effect of doped silicon (the resistivity of monocrystalline silicon, diffuse-doped silicon, and polycrystalline silicon materials changes significantly after being subjected to stress; this phenomenon is called the piezoresistive effect). It converts the strain of the piezoresistive strip caused by torsion, compression, and tension of the movable silicon structure into a rate of change in resistance (the strength of the piezoresistive effect can be represented by the piezoresistive coefficient π, which is defined as the relative change in resistivity under unit stress). The piezoresistive effect exhibits anisotropic characteristics; the resistivity change will differ depending on the direction of applied stress and the direction of current flow.

[0041] For example, the piezoresistive coefficient π of N-type silicon is measured at room temperature when stress is applied along the 100° direction and a current is passed through it. 100 =102.2×10 -11 m 2 / N, and when current flows along the 010 direction, its piezoresistive coefficient π 010 =53.7X10 -11 m 2 / N).

[0042] Example 2: Application of a piezoresistive angle sensor. The piezoresistive angle sensor prepared in Example 1 is installed in a micromirror of an automotive lidar system to read the angle changes of the micromirror. Figure 3 As shown, the angle is read by converting the piezoresistive effect of the piezoresistive strip into a rate of change of resistance value using a Wheatstone bridge. Then, the rate of change of resistance value is passed through a filter circuit and an amplifier circuit. The output signal of the amplifier circuit is then connected to an A / D converter, and the output signal of the A / D converter is then connected to an MCU or FPGA. Finally, the MCU or FPGA processes the signal to obtain the angle information.

[0043] Specifically, the piezoresistive effect of the piezoresistive strip is expressed as:

[0044] ΔR / R=(1+2u+ΠE)×ε

[0045] In the formula: ΔR is the resistance variation value measured according to the piezoresistive effect and the Wheatstone bridge, R is the initial impedance of the resistor; u is the Poisson's ratio, where (1+2u)<<ΠE and can be neglected; ε is the stress deformation, ε=ρ×θ, ρ is the cross-section torsion radius, θ is the torsion angle, E is the elastic modulus, and under the action of shear stress, E shall be replaced by the shear modulus G;

[0046] θ=(ΔR / R) / (ΠGρ)

[0047] Π is the piezoresistive coefficient matrix:

[0048]

[0049] Wherein, the piezoresistive coefficients in all crystal directions along the crystal axis coordinate system are represented by π ij Where i equal to 1, 2 and 3 represents the relative resistance change along the x, y and z coordinate directions, and i equal to 4, 5 and 6 represents the relative resistance change in the directions perpendicular to the x, y and z coordinate directions; j equal to 1, 2 and 3 represents the tensile stress along the x, z and y directions, and j equal to 4, 5 and 6 represents the shear stress in the directions perpendicular to the x, z and y directions.

[0050] In terms of application, the present invention utilizes the piezoresistive effect region obtained by doping partial regions on the silicon-based material, which can convert the strain caused by the torsion of the micromirror into electrical readout of resistance value change, and further calibrate it as the angle change of the micromirror torsion beam, so as to realize closed-loop control of the angle during the vibration process of the micromirror, and further improve the scanning accuracy of the MEMS LiDAR.

Claims

1. A method for fabricating a piezoresistive angle sensor, characterized in that: Includes the following steps: Step 1: Perform thermal oxidation treatment on the silicon wafer surface to grow an oxide layer on the silicon wafer surface; Step 2: Based on the product structure, perform photolithography and oxide layer etching on the area of ​​the silicon wafer where the piezoresistive strip needs to be formed, etching down to the silicon substrate; Step 3: Ion implantation is performed on the silicon wafer to form piezoresistive strips with piezoresistive effect in the areas not protected by silicon oxide; Step 4: Remove the silicon oxide film and perform rapid annealing; Step 5: Perform metallization coating on the silicon wafer; Step 6: Perform photolithography and etching on the metal thin film to form metal leads; Step 7: Connect the piezoresistive strips with the same resistance value using metal leads to form a Wheatstone bridge, thus obtaining the finished product; In step 1, the thermal oxidation treatment involves placing the silicon wafer in a reaction tube made of quartz glass. The reaction tube is heated to a temperature of 900–1200°C using a resistance wire heating furnace. Oxygen or water vapor passes through the reaction tube at a flow rate of 1 cm / s, causing a chemical reaction to occur on the surface of the silicon wafer.

2. The application of the piezoresistive angle sensor according to claim 1, characterized in that: The piezoresistive angle sensor is installed in the micromirror of the vehicle-mounted lidar to read the angle changes of the micromirror.

3. The application of the piezoresistive angle sensor according to claim 2, characterized in that: The angle is read by converting the piezoresistive effect of the piezoresistive strip into a rate of change of resistance using a Wheatstone bridge. The rate of change of resistance is then passed through a filter circuit and an amplifier circuit. The output signal of the amplifier circuit is then connected to an A / D converter, and the output signal of the A / D converter is then connected to an MCU or FPGA. Finally, the MCU or FPGA processes the signal to obtain the angle information.

4. The application of the piezoresistive angle sensor according to claim 3, characterized in that: The piezoresistive effect of the piezoresistive strip is expressed as follows: ； In the formula: It is based on the piezoresistive effect and the resistance change measured by the Wheatstone bridge. Let be the initial impedance of the resistor; Poisson's ratio; It is stress deformation. , It is the torsional radius of the cross section. It's about twisting the angle. Let be the elastic modulus, under shear stress. shear modulus replace; ； It is the piezoresistive coefficient matrix: ； Wherein, the piezoresistive coefficient along each crystal direction in the crystal axis coordinate system is used as Indicates, using The values ​​1, 2, and 3 represent the relative changes in resistance along the x, y, and z coordinate directions. The values ​​4, 5, and 6 represent the relative changes in resistance in the directions perpendicular to the x, y, and z coordinates; This is equivalent to using 1, 2, and 3 to represent tensile stresses along the x, z, and y directions. The values ​​4, 5, and 6 represent shear stresses perpendicular to the x, z, and y directions, respectively.

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