CN110927960A - Thermal drive deformable micromirror - Google Patents

Abstract

The invention discloses a thermally-driven deformable micromirror, and belongs to the technical field of adaptive optical systems. The micro mirror is composed of a lens, an electrode, a supporting beam and a silicon structure frame, wherein the supporting beam supports the lens in the center of the silicon structure frame in a suspension manner, the lens is divided into three layers from top to bottom, the three layers are sequentially a metal reflecting layer, a silicon dioxide layer and a silicon layer, the supporting beam and the silicon layer of the lens are integrally formed by the same material, the silicon structure frame and the supporting beam are insulated by the silicon dioxide layer, the electrode is arranged on the surface of the silicon structure frame and connected with the supporting beam, after the electrode is electrified, the supporting beam and the lens form a current loop and generate heat, and the lens is deformed by utilizing the difference of the thermal expansion coefficients of the metal and. The invention achieves the purpose of mirror surface deformation by controlling the heating temperature of two different materials of metal and silicon through electrification.

Description

Thermal drive deformable micromirror

Technical Field

The invention relates to a deformable micromirror structure, in particular to a micromirror deformed by thermal driving, and belongs to the technical field of adaptive optical systems.

Background

Since texas instruments have successfully produced commercially available digital micromirrors, various types of micromirrors have been developed as if they were used in spring rain. The development of the Micro-mirror, on the one hand, has benefited from the progress of Micro-electro mechanical Systems (MEMS) technology, including surface process, bulk silicon processing process, self-assembly process, etc. On the other hand, market demand is also driving the development of micromirror technology. Most of the micromirrors are micromirrors with actuating arms driving the lenses to move, such as electrostatic micromirrors, which use array-type comb teeth to drive the lenses to deflect; the piezoelectric micro-mirror drives the lens to deflect or displace by utilizing the reverse piezoelectric effect of the material; the electromagnetic micro-mirror controls the deflection of the lens by utilizing the electromagnetic effect of the magnetic field coil; the electrothermal micro-mirror utilizes the thermal expansion effect of the material to drive the lens to deflect and lift. Although the driving modes of the micro mirrors are different, the micro mirrors are essentially changed by driving the lens to move by the driving arm, which is widely applied to optical scanning systems, such as laser radar (LiDAR), optical switches, coherence tomography, face recognition systems, portable spectrometers, and the like.

However, in adaptive optics systems, a deformable mirror is required for wavefront correction purposes. Adaptive optics is proposed based on astronomical observations, and the purpose is to improve the quality of spatial optical communication, while atmospheric attenuation and turbulence will reduce the spatial optical communication quality to a great extent. If wavefront distortion can be detected by an effective means and compensated in real time to achieve the purpose of correcting wavefront, the anti-interference capability of spatial communication will be improved. Therefore, it is imperative to develop a micro-deformable mirror capable of realizing wavefront correction. However, the current micro-deformable mirror has the defects of high driving voltage, small deformation and complex preparation process.

Disclosure of Invention

In view of the above, the present invention provides a thermally actuated deformable micromirror, which achieves the objective of mirror deformation by controlling the heating temperatures of two different materials.

The utility model provides a thermal drive deformable micromirror, this micromirror comprises lens, electrode, corbel and silicon structural framework, the corbel is with the unsettled support of lens at silicon structural framework's central authorities, the lens divide into the three-layer from top to bottom, is metal reflection stratum, silica layer and silicon layer in proper order, the corbel is the integrated molding of homogeneous material with the silicon layer of lens, it is insulating by the silica layer between silicon structural framework and the corbel, the electrode sets up and links to each other at silicon structural framework's surface and corbel, and to the electrode circular telegram back, corbel and lens formation current return circuit and generate heat, utilize the thermal expansion coefficient difference of metal and silicon to make the lens produce the deformation.

Further, the lens is of a circular structure, and the radius of the silicon layer in the lens is the same as that of the aluminum layer.

Furthermore, the number of the support beams is four, and the four support beams are uniformly arranged along the circumferential direction of the lens.

Furthermore, the supporting beams adopt a fork type cantilever beam structure, the cantilever beams are arranged in a roundabout manner in a plane, the fork ends of the cantilever beams are positioned on the silicon structure frame, and the junction ends are positioned on the lens.

Further, a resistance wire is fixed on the surface of the lens, and a lead is arranged on the surface of the support beam and conducts the resistance wire and the electrode.

A processing method of a thermal drive deformable micromirror is realized by the following steps:

the method comprises the following steps: growing a layer of Silicon dioxide On the front surface of an SOI (Silicon-On-Insulator) substrate as an insulating layer, sputtering a layer of aluminum, and forming a reflector by utilizing a photoetching process;

step two: patterning the front surface of the SOI substrate by a photoetching process to form a support beam structure, and then etching and forming by using a DRIE (reactive Ion etching) process;

step three: patterning a back cavity structure to be etched on the back surface of the SOI substrate by photoetching, and etching the back cavity to the SOI middle silicon dioxide layer by using a DRIE (deep ion etching) process;

step four: and corroding the exposed silicon dioxide layer by using corrosive liquid, and releasing the micro mirror to finish the manufacture.

Has the advantages that:

1. according to the invention, the silicon is heated by using the branch beam of the silicon material instead of the heating wire, the silicon is used as an intrinsic semiconductor material, the resistivity of the silicon can be controlled by doping, and the additional manufacture of the heating wire is avoided, so that the preparation process can be simplified, the transmission loss can be reduced, and the heating efficiency is improved.

2. The support beam adopts a fork-type cantilever beam structure, the cantilever beams are arranged in a roundabout way in a plane, and the structure ensures that the lens and the silicon structure frame have a stable connection structure and have corresponding elastic deformation capacity.

3. The processing method of the invention utilizes the structural characteristics of the SOI substrate with a sandwich structure, processes the top layer silicon in the substrate into the support beam, etches the middle layer silicon dioxide and then uses the silicon as the insulating layer, forms the silicon structure frame with the bottom layer silicon, and has the advantages of simple process flow and low production difficulty.

4. The structure has the advantages that the resistance wire is fixed on the surface of the lens, the lead is arranged on the surface of the support beam, and the resistance wire is conducted, so that the temperature of different positions of the lens surface can be changed through the heating resistance wire, and the local deformation of the lens surface can be controlled.

Drawings

FIG. 1 is a schematic diagram of the overall structure of a thermally actuated deformable micromirror according to the present invention;

FIG. 2 is a cross-sectional view of a lens;

FIG. 3 is a schematic view of the present invention with the silicon structural frame removed;

FIG. 4 is a half sectional view of FIG. 3;

FIG. 5 is a schematic view showing a structure of a support beam with a heating resistor integrated on the mirror surface

FIG. 6 is a schematic diagram of a micromirror with a mirror surface covered by metal (such as aluminum, gold or other high reflectivity metal)

FIG. 7 is a schematic view of a micro-mirror structure with increased density of heater strips and increased number of support beams

FIG. 8 is a schematic diagram of the deformation of a Bimorph structure under the change of temperature;

FIG. 9 is a deformation diagram of a lens with temperature variation;

FIG. 10 is a graph of displacement at various positions of a lens as temperature increases;

FIG. 11 is a schematic flow chart of a process for fabricating a thermally actuated deformable micromirror according to the present invention.

The solar cell comprises a 1-silicon structural frame, 2-electrodes, 3-support beams, 4-lenses, 5-aluminum reflecting layers, 6-silicon dioxide layers, 7-silicon layers, 8-top silicon, 9-middle silicon dioxide, 10-bottom silicon, 11-photoresist, 12-ultraviolet rays, 13-aluminum layers, 14-heating wires and 15-metal leads.

Detailed Description

The invention is described in detail below by way of example with reference to the accompanying drawings.

As shown in figure 1, the invention provides a thermally-actuated deformable micromirror, which comprises a lens 4, an electrode 2, a supporting beam 3 and a silicon structure frame 1, wherein the circular lens 4 is suspended and supported in the center of the silicon structure frame 1 by the four supporting beams 3, as shown in figure 2, the lens 4 is divided into three layers from top to bottom, namely an aluminum reflecting layer 5, a silicon dioxide layer 6 and a silicon layer 7, as shown in figures 3 and 4, the supporting beam 3 and the silicon layer 7 of the lens 4 are integrally formed by the same material, the silicon structure frame 1 and the supporting beam 3 are insulated by the silicon dioxide layer 6, the electrode 2 is arranged on the surface of the silicon structure frame 1 and connected with the supporting beam 3, after the electrode 2 is electrified, the supporting beam 3 and the silicon layer 7 in the lens 4 form a current loop and generate heat, the lens 4 is deformed by utilizing the difference of thermal expansion coefficients of the two materials of aluminum and silicon, and is not limited to the two materials of silicon, such as silicon gold material, silicon aluminum material.

As shown in fig. 5, 6 and 7, the additional structure of the present invention is not limited to the shape and number of the beams, for example, the shape may be serpentine or spiral, and the number is evenly distributed around the lens. Heating resistance wires can be integrated on the silicon layer 7 through related processes, and the purpose of the heating resistance wires is to enable the temperature at different positions of the mirror surface to be controllable, so that the local deformation of the micro mirror is changed. Except for the lead wires 15 and the heating resistance wires 14, the number of the lens 4 is three, including the silicon layer 7, the silicon dioxide insulating layer 6 and the reflecting layer 5.

When the temperature of the material is changed, the stretching effect is generated due to the existence of thermal stressIf two materials are combined together, deformation will occur when heated, as shown in FIG. 8, the thermal expansion coefficients of material 1 and material 2 are α₁And α₂Wherein α₁>α₂. The curvature radius after bending deformation of the materials 1 and 2 under the temperature change Δ T is as follows:

wherein E is₁、E₂Young's moduli, t, of materials 1 and 2, respectively₁、t₂The thickness of material 1 and material 2, respectively.

The thermally driven deformable micromirror of the present invention is simulated by finite element method, in order to simplify the calculation, in the simulation process, the silicon structure frame 1 of the connected support beam 3 is removed, and the support beam 3 is fixedly supported by adopting fixed constraint, and simultaneously, the insulating layer in the middle is not considered, and the radius of the silicon layer 7 in the lens is the same as that of the aluminum reflecting layer 5.

Since the melting point of aluminum (933.15K) is low, the maximum temperature was set to 400K during the simulation in order to reduce the impact of creep on the device. Fig. 8 corresponds to the deformation results of the thermal deformation mirror at 300K, 310K, 320K, 330K, 340K, 350K, 360K, 370K, 380K, 390K, and 400K, respectively.

It can be seen from fig. 9 that the radius of curvature of the mirror surface gradually decreases and the position of the mirror surface changes as the temperature increases.

Fig. 10 is a curve of the change of the lens outer circle and the lens center with temperature, and it can be seen from the graph that the lens displacement changes obviously with the increase of temperature and has a good linear relationship. The slope of lens center displacement with temperature change is much larger than lens circumference displacement because the lens circumference is connected by the clamped beams and movement has limitations.

The invention also provides a preparation method of the thermal driving deformable micro-mirror, which comprises the following steps:

the method comprises the following steps: a double-side polished SOI (Silicon-On-Insulator) substrate having a sandwich structure is provided, including top Silicon 8, Silicon dioxide 9, and bottom Silicon 10, as shown in fig. 11 (a). Next, a layer of silicon dioxide 6 is grown on the front surface of the SOI by using a plasma enhanced chemical vapor deposition method, then photoresist 11 is applied, photolithography is performed by using ultraviolet light 12, development is performed to pattern an insulating layer to be formed, post-baking is performed on a hot plate for 30min, and then the silicon dioxide layer 6 is etched by using hydrofluoric acid and ammonium fluoride solution, as shown in fig. 11(b) (c) (d) (e).

Step two: the aluminum layer 13 is sputtered on the SOI using a magnetron sputtering apparatus, as shown in fig. 11 (f). The aluminum layer to be etched is patterned by development, post-baked on a hot plate for 30min, and then etched by a mixed solution of hydrochloric acid and phosphoric acid, as shown in fig. 11(g) (h) (i).

Step three: the photoresist is homogenized on the SOI wafer, the supporting beam structure to be formed is patterned by a photolithography process, the wafer is baked for 30min after being hot-mixed, and then the region without the mask on the top layer is etched clean by DRIE (deep Reactive Ion etching), as shown in FIG. 11(j) (k) (l).

Step four: and (3) photoresist is uniformly coated on the back surface of the silicon wafer, photoetching is carried out, a back cavity structure needing etching is patterned, and the postbaking time after photoetching is prolonged to 60min because the etching depth is thicker at this time so as to increase the etching selection ratio. The back cavity is then etched using DRIE until the intermediate silicon dioxide layer 9 is etched, as shown in fig. 11(m) (n) (o).

Step five: and (3) putting the micromirror obtained in the fourth step into BOE corrosive liquid, corroding the exposed silicon dioxide layer until the micromirror is released, then cleaning the chip, scribing to obtain a single micromirror, and finishing the manufacturing, wherein the manufacturing is shown in fig. 11 (p).

In summary, the above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (5)

1. The utility model provides a thermal drive deformable micromirror, its characterized in that, this micromirror comprises lens, electrode, corbel and silicon structural framework, the corbel is with the unsettled support of lens at silicon structural framework's central authorities, the lens divide into the three-layer from top to bottom, is metal reflection stratum, silica layer and silicon layer in proper order, the corbel is the integrated into one piece of homogeneous material with the silicon layer of lens, it is insulating by the silica layer between silicon structural framework and the corbel, the electrode sets up and links to each other with the corbel on silicon structural framework's surface, and to the electrode circular telegram back, corbel and lens form the current return circuit and generate heat, utilize the thermal expansion coefficient difference of aluminium and silicon to make the lens produce the deformation.

2. The thermally actuated deformable micromirror of claim 1 wherein the radius of the silicon layer in the mirror plate is the same as the radius of the aluminum layer.

3. A thermally actuated deformable micromirror as claimed in claim 1 or 2, wherein said supporting beam is in a bifurcated cantilever beam structure, the cantilever beams are in a circuitous arrangement in a plane, the bifurcated ends of the cantilever beams are located on the silicon structural frame, and the merged ends are located on the lens.

4. A thermally actuated deformable micromirror as claimed in claim 1, wherein the surface of said mirror plate is fixed with resistance wires, and conductive wires are arranged on the surface of the supporting beam and conduct the resistance wires and electrodes.

5. A method for processing a thermally actuated deformable micromirror, comprising the steps of:

the method comprises the following steps: growing a layer of silicon dioxide on the front surface of the SOI substrate as an insulating layer, sputtering a layer of aluminum, and forming a reflecting mirror surface by utilizing a photoetching process;

step two: patterning the front surface of the SOI substrate by a photoetching process to form a support beam structure, and then etching and forming by using a DRIE (deep etching) process;

step three: patterning a back cavity structure to be etched on the back surface of the SOI substrate by photoetching, and etching the back cavity to the SOI middle silicon dioxide layer by using a DRIE (deep ion etching) process;

step four: and corroding the exposed silicon dioxide layer by using corrosive liquid, and releasing the micro mirror to finish the manufacture.

Images