CN114911051B - Low-temperature-drift electrostatic MEMS (micro-electromechanical system) micro-mirror and implementation method thereof - Google Patents
Low-temperature-drift electrostatic MEMS (micro-electromechanical system) micro-mirror and implementation method thereof Download PDFInfo
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- CN114911051B CN114911051B CN202210493453.7A CN202210493453A CN114911051B CN 114911051 B CN114911051 B CN 114911051B CN 202210493453 A CN202210493453 A CN 202210493453A CN 114911051 B CN114911051 B CN 114911051B
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- G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
- G02B26/08—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light
- G02B26/0816—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements
- G02B26/0833—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD
- G02B26/0841—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the direction of light by means of one or more reflecting elements the reflecting element being a micromechanical device, e.g. a MEMS mirror, DMD the reflecting element being moved or deformed by electrostatic means
Abstract
The invention discloses a low-temperature-drift electrostatic MEMS (micro-electromechanical systems) micro-mirror and an implementation method thereof, belonging to the field of micro-opto-electro-mechanical systems (MOEMS). The micro-mirror structure comprises a mirror surface, a torsion beam, a comb tooth driver, a comb tooth supporting structure, an electric isolation channel, a bonding pad and a cavity, wherein the micro-mirror structure sequentially comprises a structural layer, an insulating layer and a substrate layer from top to bottom. The invention realizes the low-angle temperature drift compensation of the electrostatic MEMS micro-mirror by optimizing the micro-mirror structure to adjust the micro-mirror damping; compensating the temperature drift of the frequency introduced by the Young modulus of the structural layer material by using the thermal stress between the structural layer and the substrate layer to realize low-frequency temperature drift compensation; on the basis of carrying out low-angle temperature drift compensation and low-frequency temperature drift compensation on the electrostatic MEMS micro-mirror, the working stability and the accuracy of motion control of the electrostatic MEMS micro-mirror are improved. The invention does not need to additionally increase a position detection module and feedback control, and has the advantages of simple structure, small volume, low cost and easy realization. The low temperature drift comprises low frequency temperature drift and low angle temperature drift.
Description
Technical Field
The invention belongs to the field of micro-opto-electro-mechanical systems (MOEMS), and particularly relates to a low-temperature drift electrostatic MEMS (micro-electromechanical system) micro-mirror and an implementation method thereof.
Background
The electrostatic comb MEMS micro-mirror has the advantages of rapid scanning, low power consumption, large resonance angle, simple structure and mass manufacturing, and has wide application in the fields of 3D imaging, laser radar (LiDAR), projection display and the like. For each application, the MEMS micro-mirror is required to oscillate at a certain frequency and optical scanning angle. However, the temperature drift of the resonant frequency and the scanning angle of the MEMS micromirror causes instability of operation of the micromirror, which severely limits the application of the MEMS micromirror.
Typically, the electrostatic comb-drive MEMS micro-mirror is fabricated on a silicon-on-insulator (SOI) wafer, the device layers are used to form the mirror, the torsion beams and the comb-drive, and the substrate layer is used to form the cavity for mirror motion. Since the device layer and the structure layer are both silicon materials, and the young modulus of silicon is affected by temperature, the resonant frequency of the electrostatic MEMS micro-mirror generates temperature drift. Meanwhile, since the micromirror is air-damped to vary with temperature, the scanning angle of the micromirror also generates temperature drift. The main solution to the MEMS micro-mirror temperature drift is to add real-time mirror position detection, such as capacitance detection, piezoresistive detection, and optical detection, but this will increase the chip size, the production cost, and the system complexity.
Disclosure of Invention
The invention mainly aims to provide a low-temperature drift electrostatic MEMS (micro-electromechanical system) micromirror and an implementation method thereof. The invention does not need to additionally increase a position detection module and feedback control, and has the advantages of simple structure, small volume, low cost and easy realization.
The low temperature drift comprises low-frequency temperature drift and low-angle temperature drift, the low-frequency temperature drift means that the resonant frequency temperature drift of the electrostatic MEMS micro-mirror is lower than 1 ppm/DEG C, and the low-angle temperature drift means that the maximum scanning angle temperature drift of the electrostatic MEMS micro-mirror is lower than 10 ppm/DEG C.
The purpose of the invention is realized by the following technical scheme.
The invention discloses a low-temperature-drift electrostatic MEMS (micro-electromechanical system) micromirror, which comprises a mirror surface, a torsion beam, a comb tooth driver, a comb tooth supporting structure, an electric isolation channel, a bonding pad and a cavity.
The micro-mirror structure comprises a structural layer, an insulating layer and a substrate layer from top to bottom in sequence. Wherein the mirror surface, the torsion beam, the comb tooth driver, the comb tooth support structure and the electrical isolation channel are located on the structural layer and are formed by etching the structural layer. The two sides of the mirror surface are connected with the frame of the micro mirror through the comb tooth supporting structure and the torsion beam and rotate around the axis of the torsion beam. The comb tooth drivers are located on two sides of the comb tooth supporting structure, and the electric isolation of the movable comb teeth and the static comb teeth is realized through electric isolation channels. The bonding pad is formed by depositing a structural layer film and is used for electrically connecting the movable comb teeth and the static comb teeth. The movable comb teeth electrode is connected with the bonding pad through the torsion beam to form a positive electrode, and the fixed comb teeth electrode is connected with the bonding pad through the external frame to form a negative electrode. The cavity is formed by etching the substrate layer and is located below the mirror surface to provide space for rotation of the mirror surface.
The low-angle temperature drift compensation of the electrostatic MEMS micro-mirror is realized by optimizing the micro-mirror structure to adjust the micro-mirror damping.
And compensating the temperature drift of the frequency introduced by the Young modulus of the silicon of the structural layer material by utilizing the thermal stress between the structural layer and the substrate layer to realize low-frequency temperature drift compensation.
On the basis of carrying out low-angle temperature drift compensation and low-frequency temperature drift compensation on the electrostatic MEMS micro-mirror, the working stability and the accuracy of motion control of the electrostatic MEMS micro-mirror are improved.
Preferably, optimizing the micromirror structure to adjust micromirror damping includes two ways:
the first method is as follows: parameters of a comb tooth driver are fixed, the size of the mirror surface of the electrostatic MEMS micro-mirror is adjusted, damping adjustment of the electrostatic MEMS micro-mirror is achieved, and low-angle temperature drift compensation of the electrostatic MEMS micro-mirror is achieved.
The second method comprises the following steps: parameters of a comb tooth driver are fixed, the shape or the size of the mirror surface of the electrostatic MEMS micro-mirror is adjusted, damping adjustment of the electrostatic MEMS micro-mirror is achieved, and low-angle temperature drift compensation of the electrostatic MEMS micro-mirror is achieved.
The principle of adjusting the damping to control the temperature drift of the scanning angle is as follows:
when the electrostatic MEMS micro-mirror works in a resonance state, under the condition of a given driving voltage V, the scanning angle theta of the electrostatic MEMS micro-mirror can be amplified by the quality factor Q value of the micro-mirror, and the scanning angle theta is proportional to the Q value.
θ＝SQV 2 (1)
Where S is a shape factor and is related only to the shape of the mirror.
And the Q value of the micromirror is inversely proportional to the air damping b experienced by the micromirror.
Where I is the moment of inertia of the mirror and ω is the resonant angular frequency of the micromirror.
Therefore, the scanning angle of the micromirror is mainly affected by air damping, and is inversely proportional to the air damping coefficient b, which varies with temperature to cause temperature drift of the scanning angle.
Temperature coefficient of scan angle alpha θ Temperature coefficient alpha with air damping coefficient b Opposite sign, thus controlling the temperature coefficient α of air damping of the micromirror b Temperature drift alpha capable of controlling scanning angle of micromirror θ 。
α θ ＝-α b (4)
While the electrostatic comb-driven micromirror is damped by two kinds of air: resistance damping at the mirror surface and viscous damping between the moving and static comb teeth, the two types of damping being related to the structural parameters of the micromirror. Total damping coefficient b total Is the sum of the resistive damping and the viscous damping:
b total ＝b drag +b vis (5)
sliding film damping coefficient b between moving and static comb teeth vis Effective viscosity mu of mainly air eff In relation to this, the comb structure is controlled to adjust the size thereof by reference:
wherein N is the broach number, and d is the broach width, and lc is the length of broach, and lo is the width that the broach supported, and g is the broach clearance.
Damping coefficient of resistance b at mirror surface drag Mainly in relation to the air density ρ, the size of the mirror is adjusted by controlling its dimensions:
wherein c is drag In order for the coefficient of resistance to be related to the shape of the mirror,
where L is the length of the mirror surface and W is the width of the mirror surface.
Meanwhile, since the effective viscosity is in direct proportion to the temperature and the air density is in inverse proportion to the temperature, the temperature coefficient of the effective viscosity and the temperature coefficient of the air density are constant and have opposite signs
α ρ <0。
Wherein mu 0 Is the air viscosity at a temperature of 273K, ρ 0 Is the air density at a temperature of 273K.
At the same time, the temperature coefficient of the total damping α b Comprises the following steps:
wherein beta is vis The proportion of the viscous damping in the total damping is related to comb tooth parameters; beta is a drag The proportion of the resistance damping in the total damping is related to the shape and the size of the mirror surface.
The temperature coefficient of the scan angle is:
thus, β is adjusted by controlling the structure of the micromirror drag And beta vis So that the resistance damping and the viscous damping compensate each other, thereby allowing the temperature coefficient alpha of the scanning angle θ Close to 0, the purpose of reducing the angle drift of the MEMS micro-mirror is achieved, and further low angle temperature drift is achieved.
The first method is as follows: the shape and the size parameters of the mirror surface are fixed, the comb tooth parameters of the electrostatic MEMS micro-mirror are adjusted, the damping adjustment of the electrostatic MEMS micro-mirror is realized, and the low-angle temperature drift compensation of the electrostatic MEMS micro-mirror is realized. The specific implementation method comprises the following steps:
the low-angle temperature drift compensation step I of the fixed mirror surface:
the mirror shape and size parameters are fixed. The mirror surface shape comprises a circle, a rectangle, an ellipse and a diamond.
And (2) compensating the low-angle temperature drift of the fixed mirror surface:
from equation (11), the ratio of viscous damping to total damping, β, is known vis Related to the number N of the comb teeth, the width d of the comb teeth, the length lc of the comb teeth, the width lo of the comb teeth support and the gap g of the comb teeth. Fixing the size of the electrostatic MEMS micro-mirror surfaceThe number N of comb teeth of the section comb tooth driver, the width d of the comb teeth, the length lc of the comb teeth, the gap g of the comb teeth and the width lo supported by the comb teeth realize the damping adjustment of the electrostatic MEMS micro-mirror, so that the viscous damping and the resistance damping compensate each other, and the low-angle temperature drift of the electrostatic MEMS micro-mirror is realized.
Preferably, in order to further reduce the chip size, avoid the broach to support length too long, the broach is arranged on the mirror surface to realize the electric isolation of mirror surface broach through electric isolation groove, avoid leading to the broach actuation and inefficacy because of the vibration, the mirror surface broach only provides the effect that increases the damping, and does not provide drive power.
The second method comprises the following steps: parameters of a comb tooth driver are fixed, the shape or the size of the mirror surface of the electrostatic MEMS micro-mirror is adjusted, damping adjustment of the electrostatic MEMS micro-mirror is achieved, and low-angle temperature drift compensation of the electrostatic MEMS micro-mirror is achieved. The specific implementation method comprises the following steps:
the low-angle temperature drift compensation step of the fixed comb teeth comprises the following steps:
comb drive parameters are fixed. Comb tooth driver parameters include, broach number N, broach width d, the length lc of broach, the width lo that the broach supported, broach clearance g.
And (2) low-angle temperature drift compensation of the fixed comb teeth:
according to the formula (12), the ratio beta of the resistance damping in the total damping drag Form factor c of mirror surface drag The mirror length L is related to the mirror width W. Adjusting the ratio beta of the resistance damping to the total damping by changing the mirror shape or the mirror size of the MEMS micromirror drag And the damping adjustment of the electrostatic MEMS micro-mirror is realized, so that the resistance damping and the viscous damping are mutually compensated, and the low-angle temperature drift of the electrostatic MEMS micro-mirror is realized.
Compensating the temperature drift of the frequency introduced by the Young modulus of the silicon of the material of the structural layer by utilizing the thermal stress between the structural layer and the substrate layer to realize low-frequency temperature drift compensation, and preferably realizing the low-frequency temperature drift in the following two ways:
the method comprises the following steps: the thermal stress between the structural layer and the substrate layer is realized by changing the substrate material, the frequency temperature drift introduced by the silicon Young modulus of the structural layer material is compensated, and the low-frequency temperature drift compensation is realized.
The first step of substrate layer frequency temperature drift compensation: the device layer is made of silicon material, and the torsion beam is positioned on the structural layer and made of silicon.
The electrostatic comb-drive MEMS micro-mirror is a typical silicon resonator, and the resonant frequency of the electrostatic comb-drive MEMS micro-mirror is reduced along with the increase of the temperature due to the temperature dependence of the Young modulus of silicon, and the temperature coefficient of the resonant frequency is-30 ppm/DEG C.
The resonance frequency f of the electrostatic MEMS micro-mirror, without considering the thermal stress, is:
where k is the spring constant of the twist beam, w is the twist beam width, l is the twist beam length, t is the twist beam thickness, I is the moment of inertia of the mirror surface, and ν is the Poisson's ratio of silicon.
And a second substrate layer frequency temperature drift compensation step: and changing the substrate material, and compensating the frequency temperature drift of the electrostatic MEMS micro-mirror by using the thermal stress between the substrate material and the silicon material of the device layer to realize the low-frequency temperature drift of the micro-mirror.
The substrate layer is instead a material with a coefficient of thermal expansion greater than that of silicon and insulating, including glass, sapphire, ceramic, silicon carbide, gallium nitride. Since the substrate material is insulating, the electrostatic MEMS micro-mirror does not require an additional insulating layer. By utilizing the stress rigidization effect, the thermal stress between the substrate layer and the structural layer applies tensile stress to the torsion beam, so that the resonance frequency of the micromirror is increased, and the frequency temperature drift introduced by the Young modulus of silicon is compensated. The tensile thermal stress F between the device layer silicon and the substrate material is:
F＝F 2 -F 1 ＝(E 2 α 2 t 2 -E 1 α 1 t 1 )LΔT (15)
wherein is E 2 Is the Young's modulus of the substrate material, E 1 Is the Young's modulus, α, of silicon 2 Is the coefficient of thermal expansion, alpha, of the substrate material 1 Is the coefficient of thermal expansion of siliconL is the contact length between silicon and the substrate material, t 2 Is the thickness of the substrate material, t 1 Is the thickness of silicon.
Since the thermal expansion coefficient of the substrate material is larger than that of silicon, and the amount of strain of the device layer silicon in the axial direction is smaller than that of the substrate material substrate when the temperature rises, the device layer receives a force from the tension of the substrate, and the tensile stress acts on the torsion beam, resulting in an increase in the resonance frequency.
Resonant frequency f of the electrostatic MEMS micromirror in the case of thermal stress F Comprises the following steps:
wherein k is F Is the equivalent spring constant of the tensile stress F, which is proportional to the magnitude of the tensile stress F, and F is proportional to the thermal expansion coefficient alpha of the substrate material 2 Thickness t of the substrate layer 2 And is proportional to the contact length L between the device layer and the substrate layer.
By adjusting the coefficient of thermal expansion alpha of the structural layer 2 Thickness t of the substrate layer 2 And the contact length between the device layer and the substrate layer Lcontrol k F Is used for compensating the temperature coefficient of the Young modulus of silicon, thereby reducing the temperature drift of the resonant frequency.
The second method comprises the following steps: the stress regulating layer is added below the substrate layer, and the frequency temperature drift introduced by the Young modulus of the structural layer material silicon is compensated through the thermal stress between the substrate layer and the stress regulating layer, so that the low-frequency temperature drift compensation is realized.
Compensating the frequency temperature drift of the stress control layer by the first step: the device layer and the substrate layer are made of silicon materials, and the torsion beam is located on the structural layer and made of silicon.
And a second stress control layer frequency temperature drift compensation step: by adding the stress regulating layer on the substrate layer, the thermal expansion coefficient of the stress regulating layer material is obviously larger than that of silicon, including glass, sapphire, ceramic, silicon carbide, gallium nitride and the like. Through the thermal stress between the stress regulating layer and the substrate layer, the thermal stress borne by the substrate layer acts on the torsion beam in the structural layer to apply tensile stress on the torsion beam, so that the resonance frequency of the micromirror is increased, and the frequency temperature drift introduced by the Young modulus of silicon is compensated.
According to the formulas (15) and (16), the thermal stress between the substrate layer and the stress control layer is changed by adjusting the thermal expansion coefficient of the stress control layer, the thickness of the stress control layer and the contact area between the stress control layer and the substrate layer, so that the tensile stress applied to the torsion beam is adjusted to compensate the temperature coefficient of the Young modulus of silicon, and the temperature drift of the resonant frequency is reduced.
The invention also discloses a method for realizing the low-temperature-drift electrostatic MEMS micro-mirror based on different substrate materials, which is used for manufacturing the low-temperature-drift electrostatic MEMS micro-mirror, does not need an additional insulating layer and comprises the following steps:
step (a): and carrying out anodic bonding on the front surface of the SOI wafer and a substrate wafer, wherein the substrate wafer is made of an insulating material with a thermal expansion coefficient larger than that of silicon. The substrate wafer is used to form a substrate layer.
A step (b): the back side of the SOI wafer is thinned by chemical mechanical polishing. And isotropically etching the residual silicon, exposing the buried oxide layer of the SOI wafer, and removing the buried oxide layer of the silicon-on-insulator wafer through vapor phase hydrofluoric acid etching to form a device layer.
Step (c): and etching the back surface of the substrate wafer to form a cavity structure.
A step (d): and depositing a metal film on the front surface of the device layer and patterning to form a metal pad and a metal layer on the mirror surface.
A step (e): the front etching device layer forms a mirror surface, a torsion beam, a comb tooth supporting structure, a comb tooth driver and an electric isolation channel, namely the low-temperature drift electrostatic MEMS micro-mirror is realized based on different substrate materials.
Meanwhile, the invention provides a method for realizing a low-temperature-drift MEMS micro-mirror based on a stress control layer, which is used for manufacturing a low-temperature-drift electrostatic MEMS micro-mirror and comprises the following steps:
step (a): the upper silicon layer of the SOI wafer is a device layer, and the lower silicon layer is a substrate layer.
Step (b): and etching the back surface of the SOI wafer to the buried oxide layer to form a cavity structure, and removing the buried oxide layer through wet etching.
Step (c): the back surface of the SOI is bonded with the wafer for stress regulation and control through bonding, and the wafer for stress regulation and control is made of a material with a thermal expansion coefficient larger than that of silicon.
Step (d): and depositing a metal film on the front surface of the device layer and patterning to form a metal pad and a metal layer on the mirror surface.
A step (e): the front etching device layer forms a mirror surface, a torsion beam, a comb tooth supporting structure, a comb tooth driver and an electric isolation channel, namely the low-temperature drift electrostatic MEMS micro-mirror is realized based on different substrate materials.
Has the beneficial effects that:
1. according to the angle temperature drift control method of the electrostatic comb tooth drive MEMS micro-mirror based on damping, disclosed by the invention, the angle temperature drift is reduced by controlling the structural size of the micro-mirror, an additional angle position detection module is not needed, the chip volume and the cost are reduced, and the method has the advantages of simple structure, small volume, low cost and easiness in implementation.
2. The invention discloses a frequency temperature drift control method of an electrostatic comb driving MEMS (micro-electromechanical system) micromirror based on thermal stress control, which reduces the temperature drift of resonant frequency by controlling the thermal expansion coefficient and the size of a substrate layer, does not need phase detection and feedback control, and can reduce the cost of a chip and the complexity of a system.
3. The invention discloses a method for realizing a low-temperature drift micro-mirror based on different material substrates, which can manufacture a low-temperature drift electrostatic comb tooth driven MEMS micro-mirror, does not need an additional insulating layer, and improves the working stability and the accuracy of motion control of the electrostatic MEMS micro-mirror on the basis of carrying out low-angle temperature drift compensation and low-frequency temperature drift compensation on the electrostatic MEMS micro-mirror.
4. The invention discloses a method for realizing a low-temperature-drift micro-mirror based on a stress control layer, which can manufacture a low-temperature-drift electrostatic comb tooth driven MEMS micro-mirror.
Drawings
FIG. 1 is a schematic diagram of an electrostatic MEMS micro-mirror 3D structure;
FIG. 2 is a schematic diagram of structural parameters of an electrostatic MEMS micro-mirror;
fig. 3 is an optimized view of the structure of the electrostatic MEMS micro-mirror. (a) a schematic diagram of a rectangular mirror structure; (b) schematic diagram of circular mirror structure; (c) a schematic diagram of a rectangular mirror tooth structure; (d) a schematic diagram of a tooth structure of a circular mirror;
fig. 4 thermal stress analysis of the electrostatic MEMS micro-mirror. (a) a torsion beam force diagram; (b) base:Sub>A force diagram between the glass substrate and the structural layer of the A-A section; (c) base:Sub>A stress diagram between the stress control layer and the micromirror structure layer of the A-A section;
FIG. 5 is a process flow of a low temperature drift electrostatic MEMS micro-mirror based on a glass substrate;
fig. 6 shows a process flow of a low temperature drift electrostatic MEMS micro-mirror based on a stress control layer.
Wherein: 1-electrostatic MEMS micro-mirror integral structure, 2-structural layer, 3-insulating layer, 4-substrate layer, 5-torsion beam, 6-mirror surface, 7-comb driver, 7.1-mirror surface damping regulation comb, 8-electric isolation groove, 8.1-mirror tooth electric isolation groove, 9-bonding pad, 10-cavity, 11-stress regulation layer, 12-SOI wafer, 13-SOI wafer substrate silicon, 14-SOI wafer device layer, 15-frequency compensation substrate wafer, 16-SOI wafer substrate residual silicon, 17-SOI wafer oxygen embedding layer, 18-mirror surface metal film, 19-frequency compensation stress regulation wafer
Detailed Description
For a better understanding of the objects and advantages of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings and examples.
Example 1:
the embodiment discloses a design method and a manufacturing process of a low-temperature-drift electrostatic comb-drive MEMS (micro-electromechanical system) micromirror. Electrostatic MEMS micro-mirror 3D structure as shown in fig. 1. MEMS micro mirror major structure is located structural layer 2, including torsion beam 5, mirror surface 6, broach driver 7 and electrical isolation groove 8, and the electrical insulation of broach driver is guaranteed to insulating layer 3 and electrical isolation groove 8. A cavity 10 is formed in the substrate layer 4 to facilitate deflection of the mirror plate 6.
The low angle temperature drift can be achieved by optimizing the micromirror structure control so that the resistive damping and viscous damping compensate each other. The resistive damping is dependent on the shape and size of the mirror 6, such as the mirror length and mirror width in fig. 2. The viscous damping is related to the parameters of the comb drive, such as the number N of comb teeth, the width d of the comb teeth, the length lc of the comb teeth, the width lo of the comb support, the comb gap g in fig. 2. According to the damping adjustment principle, two ways are available for adjusting the damping of the micromirror so as to reduce the angle temperature drift of the electrostatic MEMS micromirror and realize low angle temperature drift.
In one embodiment, the structural parameters of the comb tooth driver are fixed, the shape and the size of the mirror surface are adjusted, and then the resistance damping is adjusted, so that the resistance damping and the viscous damping are mutually compensated, and the low-angle temperature drift is realized, as shown in fig. 3 (a) and 3 (b). The parameter of broach driver, broach number N, broach width D, the length lc of broach, the width lo that the broach supported, broach clearance g remain unchanged, and the shape of micro-mirror switches between rectangle and circular, changes the size of mirror surface simultaneously, like mirror surface length L, mirror surface width W or mirror surface diameter D to reach the angle drift that the damping compensation viscidity damping of adjusting micro-mirror resistance brought, realize low angle temperature and float.
In another embodiment, the shape and size of the mirror surface are fixed, the structural parameters of the comb teeth are adjusted, and then the viscous damping is adjusted, so that the viscous damping and the resistive damping are mutually compensated, and the low-angle temperature drift is realized, as shown in fig. 3 (c) and 3 (d). The shape of the mirror is rectangular or circular and the size remains unchanged. Adjust the parameter of broach driver, broach number N, broach width d, the length lc of broach, the width lo that the broach supported, broach clearance g to reach the angle drift that the damping compensation resistance damping of adjusting micro-mirror viscidity brought, realize low angle temperature drift. In particular, in order to increase the viscous damping adjustable range and further reduce the size of the chip, the damping control comb teeth 7.1 are arranged on the mirror surface and are electrically insulated from the comb teeth driver 7 on the comb teeth support structure through the electrical isolation grooves 8.1, and the damping control comb teeth 7.1 on the mirror surface only provide compensation of viscous damping and do not provide driving force.
The frequency temperature drift compensation principle is as shown in fig. 4, and the resultant tensile force F generated by the thermal stress acts on the torsion beam 5, as shown in fig. 4 (a), the frequency temperature drift introduced by the young modulus of silicon in the device layer 2 is compensated, so as to realize the low frequency temperature drift of the electrostatic MEMS micromirror.
In one embodiment, a substrate material having a thermal expansion coefficient greater than that of silicon and insulating may be used instead of the silicon substrate, the force diagram of which is shown in fig. 4 (b). When the temperature rises, the strain amount of the silicon material of the device layer along the axial direction is smaller than that of the substrate material, so that the device layer is subjected to a tensile force F from the substrate 2 And the device layer will generate compressive stress F on the torsion beam 1 By adjusting the coefficient of thermal expansion of the substrate material, the thickness t 2 And a length L such that F 2 >F 1 The resultant force is tensile stress F acting on the torsion beam 5, so as to compensate the frequency temperature drift introduced by the Young modulus of silicon in the device layer 2, and further realize the low frequency temperature drift of the electrostatic MEMS micro-mirror.
In another embodiment, a layer of stress modulating material having a coefficient of thermal expansion greater than that of silicon is bonded under a silicon substrate, the stress map of which is shown in fig. 4 (c). When the temperature rises, the strain quantity of the device layer and the substrate layer along the axial direction is smaller than that of the stress regulation layer, so that the device layer is subjected to tensile force F from the stress regulation layer 2 And the device layer will generate compressive stress F on the torsion beam 1 Regulating the thermal expansion coefficient and the thickness t of the stress regulating material 2 And a length L such that F 2 >F 1 The resultant force is tensile stress F acting on the torsion beam 5, so that the frequency temperature drift introduced by the Young modulus of silicon in the device layer 2 is compensated, and the low frequency temperature drift of the electrostatic MEMS micro-mirror is realized.
In order to realize the fabrication of the low temperature drift MEMS micro-mirror, in one embodiment, the present invention provides a fabrication process of a glass substrate low temperature drift MEMS micro-mirror, as shown in fig. 5.
(a) Preparing an SOI wafer 12;
(b) Device layer 14 of SOI wafer 12 is bonded to substrate wafer 15;
(c) The CMP thins the substrate layer 13 of the SOI wafer 12 to below 10 μm to form a residual silicon layer 16;
(d) Isotropically etching the residual silicon layer 16 to a buried oxide layer 17 of the SOI wafer;
(e) The buried oxide layer 17 of the SOI wafer is etched by Vapor HF, and an alignment mark is etched on the front surface;
(f) Etching the substrate wafer 15 by wet etching or sand blasting to form a cavity 10 of the electrostatic MEMS micro-mirror;
(g) Evaporating a metal film to form mirror metal 18 and a bonding pad 9;
(h) And etching the SOI device layer 14 on the front surface, releasing the micro-mirror structure, and forming the mirror surface 6, the comb tooth driver 7, the torsion beam and the electric isolation channel.
In another embodiment, the present invention provides a manufacturing process of a low temperature drift MEMS micro-mirror based on a stress control layer, as shown in fig. 6.
(a) Preparing an SOI wafer 12;
(b) Etching a substrate layer 13 of the SOI wafer 12 to form a cavity 10 of the electrostatic MEMS micro-mirror;
(c) BOE wet etching is carried out, and the buried oxide layer 17 of the SOI wafer is removed;
(d) Anodic bonding bonds the stress wafer 19 and the SOI substrate layer 13 together;
(e) The buried oxide layer 17 of the SOI wafer is etched by Vapor HF, and an alignment mark is etched on the front surface;
(f) Etching the substrate wafer 15 by wet etching or sand blasting to form a cavity 10 of the electrostatic MEMS micro-mirror;
(g) Evaporating a metal film to form mirror metal 18 and a bonding pad 9;
(h) And etching the SOI device layer 14 on the front surface, releasing the micro-mirror structure, and forming the mirror surface 6, the comb tooth driver 7, the torsion beam and the electric isolation channel.
The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (6)
1. A low temperature floats static MEMS micro-mirror which characterized in that: the comb-tooth-shaped mirror comprises a mirror surface, a torsion beam, a comb-tooth driver, a comb-tooth supporting structure, an electric isolation channel, a bonding pad and a cavity;
the micro-mirror structure comprises a structural layer, an insulating layer and a substrate layer from top to bottom in sequence; wherein the mirror surface, the torsion beam, the comb tooth driver, the comb tooth support structure and the electrical isolation channel are positioned on the structural layer and are formed by etching the structural layer; two sides of the mirror surface are connected with a frame of the micromirror through a comb tooth supporting structure and a torsion beam and rotate around the axis of the torsion beam; the comb tooth drivers are positioned on two sides of the comb tooth supporting structure, and the electric isolation of the movable comb teeth and the static comb teeth is realized through electric isolation channels; the bonding pad is formed by depositing a structural layer film and is used for electrically connecting the movable comb teeth and the static comb teeth; the movable comb tooth electrode is connected with the bonding pad through the torsion beam to form a positive electrode, and the fixed comb tooth electrode is connected with the bonding pad through the external frame to form a negative electrode; the cavity is formed by etching the substrate layer and is positioned below the mirror surface to provide a space for the rotation of the mirror surface;
the low-angle temperature drift compensation of the electrostatic MEMS micro-mirror is realized by optimizing the micro-mirror structure to adjust the micro-mirror damping;
compensating the temperature drift of the frequency introduced by the Young modulus of the structural layer material by using the thermal stress between the structural layer and the substrate layer to realize low-frequency temperature drift compensation;
on the basis of carrying out low-angle temperature drift compensation and low-frequency temperature drift compensation on the electrostatic MEMS micro-mirror, the working stability and the accuracy of motion control of the electrostatic MEMS micro-mirror are improved;
the optimization of the micromirror structure to adjust the micromirror damping includes two ways:
the method I comprises the following steps: fixing the shape and size parameters of the mirror surface, adjusting comb tooth parameters of the electrostatic MEMS micro-mirror, realizing damping adjustment of the electrostatic MEMS micro-mirror, and realizing low-angle temperature drift compensation of the electrostatic MEMS micro-mirror;
the second method comprises the following steps: fixing parameters of a comb tooth driver, adjusting the shape or size of the mirror surface of the electrostatic MEMS micro-mirror, realizing damping adjustment of the electrostatic MEMS micro-mirror, and realizing low-angle temperature drift compensation of the electrostatic MEMS micro-mirror;
the temperature drift of the scanning angle is controlled by adjusting the damping,
when the electrostatic MEMS micro-mirror works in a resonance state, under the condition of a given driving voltage V, the scanning angle theta of the electrostatic MEMS micro-mirror can be amplified by the quality factor Q value of the micro-mirror, and the scanning angle theta is in direct proportion to the Q value;
θ＝SQV 2 (1)
wherein S is a shape factor and is only related to the shape of the mirror surface;
the Q value of the micromirror is inversely proportional to the air damping b suffered by the micromirror;
wherein I is the rotational inertia of the mirror surface, and omega is the resonance angular frequency of the micromirror;
therefore, the scanning angle of the micromirror is mainly affected by air damping, and is inversely proportional to the air damping coefficient b, and the air damping coefficient b changes with the temperature to cause the temperature drift of the scanning angle;
temperature coefficient of scan angle alpha θ Temperature coefficient alpha with air damping coefficient b Opposite in sign, thus controlling the temperature coefficient α of the air damping of the micromirror b Temperature drift alpha for controlling scanning angle of micromirror θ ；
α θ ＝-α b (4)
While the electrostatic comb-driven micromirror is damped by two kinds of air: resistance damping at the mirror surface and viscous damping between the moving comb teeth and the static comb teeth, wherein the two types of damping are related to the structural parameters of the micro-mirror; total damping coefficient b total Is the sum of the resistive damping and the viscous damping:
sliding film damping coefficient b between moving and static comb teeth vis Effective viscosity mu of mainly air eff In relation to this, the comb structure is controlled to adjust the size thereof by reference:
wherein N is the number of comb teeth, d is the width of the comb teeth, lc is the length of the comb teeth, lo is the width supported by the comb teeth, and g is the gap between the comb teeth;
damping coefficient of resistance b at mirror surface drag Mainly related to the air density ρ, its size is adjusted by controlling the size of the mirror:
wherein c is drag In order for the coefficient of resistance to be related to the shape of the mirror,
is the peak angular velocity, L is the length of the mirror, W is the width of the mirror;
meanwhile, because the effective viscosity is in direct proportion to the temperature and the air density is in inverse proportion to the temperature, the temperature coefficient of the effective viscosity and the temperature coefficient of the air density are constant and have opposite signs
α ρ <0；
Wherein mu 0 Is the air viscosity at a temperature of 273K, ρ 0 Is the air density at 273K;
at the same time, the temperature coefficient of the total damping α b Comprises the following steps:
α b ＝α ρ β drag +α μeff β vis (10)
wherein beta is drag The proportion of the resistance damping in the total damping is related to the shape and the size of the mirror surface,β vis the proportion of viscous damping in the total damping is related to comb teeth parameters;
the temperature coefficient of the scan angle is:
α θ ＝-α b ＝-(α ρ β drag +α μeff β vis ) (13)
thus, β is adjusted by controlling the structure of the micromirror drag And beta vis So that the resistance damping and the viscous damping compensate each other, thereby allowing the temperature coefficient alpha of the scanning angle θ Close to 0, the purpose of reducing the angle drift of the MEMS micro-mirror is achieved, and further low angle temperature drift is realized.
2. The low temperature drift electrostatic MEMS micro-mirror of claim 1, wherein: the method I comprises the following steps: fixing the shape and size parameters of the mirror surface, adjusting comb tooth parameters of the electrostatic MEMS micro-mirror, realizing damping adjustment of the electrostatic MEMS micro-mirror, and realizing low-angle temperature drift compensation of the electrostatic MEMS micro-mirror; the specific implementation method comprises the following steps of,
the first compensation step of the low-angle temperature drift of the fixed mirror surface:
fixing the shape and size parameters of the mirror; the mirror surface shape comprises a circle, a rectangle, an ellipse and a rhombus;
and (2) compensating the low-angle temperature drift of the fixed mirror surface:
from equation (11), the ratio of viscous damping to total damping, β, is known vis The number N of the comb teeth, the width d of the comb teeth, the length lc of the comb teeth, the width lo of the comb tooth support and the gap g of the comb teeth are related; fixing the size of the mirror surface of the electrostatic MEMS micro-mirror, adjusting the number N of comb teeth of a comb tooth driver, the width d of the comb teeth, the length lc of the comb teeth, the gap g of the comb teeth and the width lo supported by the comb teeth to realize the damping adjustment of the electrostatic MEMS micro-mirror,the viscous damping and the resistance damping are mutually compensated, and the low-angle temperature drift of the electrostatic MEMS micro-mirror is realized;
the second method comprises the following steps: fixing parameters of a comb tooth driver, adjusting the shape or size of the mirror surface of the electrostatic MEMS micro-mirror, realizing damping adjustment of the electrostatic MEMS micro-mirror, and realizing low-angle temperature drift compensation of the electrostatic MEMS micro-mirror; the specific implementation method comprises the following steps:
the low-angle temperature drift compensation method for the fixed comb teeth comprises the following steps:
fixing comb drive parameters; the parameters of the comb tooth driver comprise the number N of comb teeth, the width d of the comb teeth, the length lc of the comb teeth, the width lo of the comb tooth support and the comb tooth gap g;
and (2) compensating the low-angle temperature drift of the fixed comb teeth:
according to the formula (12), the ratio beta of the resistance damping in the total damping drag Form factor c of mirror surface drag Mirror length L, relative to mirror width W; adjusting the ratio beta of the resistance damping to the total damping by changing the mirror shape or the mirror size of the MEMS micromirror drag And the damping adjustment of the electrostatic MEMS micro-mirror is realized, so that the resistance damping and the viscous damping are mutually compensated, and the low-angle temperature drift of the electrostatic MEMS micro-mirror is realized.
3. The low temperature drift electrostatic MEMS micro-mirror of claim 2, wherein: the thermal stress between the structural layer and the substrate layer is utilized to compensate the frequency temperature drift introduced by the Young modulus of the structural layer material silicon, so that the low-frequency temperature drift compensation is realized, and the low-frequency temperature drift is realized by the following two modes:
the method comprises the following steps: the thermal stress between the structural layer and the substrate layer is realized by changing the substrate material, the frequency temperature drift introduced by the Young modulus of the structural layer material silicon is compensated, and the low-frequency temperature drift compensation is realized;
first substrate layer frequency temperature drift compensation step: the material of the device layer is silicon material, the torsion beam is positioned on the structural layer, and the material of the torsion beam is silicon;
the electrostatic comb drive MEMS micro-mirror is a typical silicon resonator, and the resonant frequency of the electrostatic comb drive MEMS micro-mirror is reduced along with the increase of the temperature due to the temperature dependence of the Young modulus of silicon;
the resonance frequency f of the electrostatic MEMS micro-mirror without considering the thermal stress is:
wherein k is a spring constant of the torsion beam, w is a width of the torsion beam, l is a length of the torsion beam, t is a thickness of the torsion beam, I is a moment of inertia of the mirror surface, and ν is a poisson ratio of silicon;
and a second substrate layer frequency temperature drift compensation step: changing a substrate material, and compensating the frequency temperature drift of the electrostatic MEMS micro-mirror by utilizing the thermal stress between the substrate material and the silicon material of the device layer to realize the low-frequency temperature drift of the micro-mirror;
the substrate layer is changed into a material which has a thermal expansion coefficient larger than that of silicon and is insulated, wherein the material which has the thermal expansion coefficient larger than that of silicon and is insulated comprises glass, sapphire, ceramic, silicon carbide and gallium nitride; because the substrate material is insulating, the electrostatic MEMS micro-mirror does not need an additional insulating layer; by utilizing the stress rigidization effect, the thermal stress between the substrate layer and the structural layer applies tensile stress to the torsion beam, so that the resonance frequency of the micromirror is increased, and the frequency temperature drift introduced by the Young modulus of silicon is compensated; tensile thermal stress F between device layer silicon and substrate material
F＝F 2 -F 1 ＝(E 2 α 2 t 2 -E 1 α 1 t 1 )LΔT (15)
Wherein is E 2 Is the Young's modulus of the substrate material, E 1 Is the Young's modulus, α, of silicon 2 Is the coefficient of thermal expansion, alpha, of the substrate material 1 Is the coefficient of thermal expansion of silicon, L is the contact length between silicon and the substrate material, t 2 Is the thickness of the substrate material, t 1 Is the thickness of silicon;
since the thermal expansion coefficient of the substrate material is larger than that of silicon, the amount of strain of the device layer silicon in the axial direction is smaller than that of the substrate material substrate when the temperature rises, so that the device layer receives a force from the stretching of the substrate, and the tensile stress acts on the torsion beam, resulting in an increase in the resonance frequency;
resonant frequency f of the electrostatic MEMS micromirror under thermal stress F Comprises the following steps:
wherein k is F Is the equivalent spring constant of the tensile stress F, which is proportional to the magnitude of the tensile stress F, and F is proportional to the thermal expansion coefficient alpha of the substrate material 2 Thickness t of the substrate layer 2 Proportional to the contact length L between the device layer and the substrate layer;
by adjusting the coefficient of thermal expansion alpha of the structural layer 2 Thickness t of the substrate layer 2 And the contact length between the device layer and the substrate layer Lcontrol k F The temperature coefficient of (2) is used for compensating the temperature coefficient of the Young modulus of silicon, so that the temperature drift of the resonant frequency is reduced;
the second method comprises the following steps: compensating the frequency temperature drift introduced by the Young modulus of the structural layer material silicon by adding a stress regulating layer below the substrate layer and compensating the low frequency temperature drift through the thermal stress between the substrate layer and the stress regulating layer;
compensating the frequency temperature drift of the stress control layer by the first step: the device layer and the substrate layer are made of silicon materials, and the torsion beam is positioned on the structural layer and made of silicon;
and a second stress control layer frequency temperature drift compensation step: by adding the stress regulating layer on the substrate layer, the thermal expansion coefficient of the stress regulating layer material is obviously greater than that of silicon, including glass, sapphire, ceramic, silicon carbide and gallium nitride; the thermal stress applied to the substrate layer acts on the torsion beam in the structural layer through the thermal stress between the stress regulating layer and the substrate layer to apply tensile stress on the torsion beam, so that the resonance frequency of the micromirror is increased, and the frequency temperature drift introduced by the Young modulus of silicon is compensated;
according to the formulas (15) and (16), the thermal stress between the substrate layer and the stress control layer is changed by adjusting the thermal expansion coefficient of the stress control layer, the thickness of the stress control layer and the contact area between the stress control layer and the substrate layer, so that the tensile stress applied to the torsion beam is adjusted to compensate the temperature coefficient of the Young modulus of silicon, and the temperature drift of the resonant frequency is reduced.
4. The low temperature drift electrostatic MEMS micro-mirror of claim 3, wherein: in order to further reduce chip size, avoid the broach to support length too long, the broach is arranged on the mirror surface to realize the electrical isolation of mirror surface broach through the electrical isolation groove, avoid leading to the broach actuation and inefficacy because of the vibration, the mirror surface broach only provides the effect that increases damping, does not provide drive power.
5. A method for realizing a low temperature drift electrostatic MEMS micro-mirror based on different substrate materials, which is used for manufacturing the low temperature drift electrostatic MEMS micro-mirror as claimed in claim 1, 2, 3 or 4, and is characterized in that: no additional insulating layer is required, including the steps of,
a step (a): carrying out anodic bonding on the front surface of the SOI wafer and a substrate wafer, wherein the substrate wafer is made of an insulating material with a thermal expansion coefficient larger than that of silicon; the substrate wafer is used for forming a substrate layer;
step (b): thinning the back of the SOI wafer through chemical mechanical polishing; isotropically etching the residual silicon, exposing the oxygen burying layer of the SOI wafer, and removing the oxygen burying layer of the silicon wafer on the insulator by gas-phase hydrofluoric acid etching to form a device layer;
step (c): etching the back of the substrate wafer to form a cavity structure;
step (d): depositing a metal film on the front surface of the device layer and patterning to form a metal pad and a metal layer on the mirror surface;
a step (e): the front etching device layer forms a mirror surface, a torsion beam, a comb tooth supporting structure, a comb tooth driver and an electric isolation channel, namely the low-temperature drift electrostatic MEMS micro-mirror is realized based on different substrate materials.
6. A method for realizing a low temperature drift MEMS micro-mirror based on a stress control layer, which is used for manufacturing the low temperature drift electrostatic MEMS micro-mirror as claimed in claim 1, 2, 3 or 4, and is characterized in that: a stress regulating layer is bonded below the substrate, which comprises the following steps,
step (a): the upper silicon layer of the SOI wafer is a device layer, and the lower silicon layer is a substrate layer;
step (b): etching the back surface of the SOI wafer to the buried oxide layer to form a cavity structure, and removing the buried oxide layer through wet etching;
step (c): bonding the back surface of the SOI with a wafer for stress regulation and control by bonding, wherein the wafer for stress regulation and control is made of a material with a thermal expansion coefficient larger than that of silicon;
a step (d): depositing a metal film on the front surface of the device layer and patterning to form a metal pad and a metal layer on the mirror surface;
a step (e): the front etching device layer forms a mirror surface, a torsion beam, a comb tooth supporting structure, a comb tooth driver and an electric isolation channel, namely the low-temperature drift electrostatic MEMS micro-mirror is realized based on stress regulation and control layers made of different materials.
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