KR20080096090A - Resonantly scanning mems mirror operated by the electromagnetic force with low damping and high optical efficiency - Google Patents

Abstract

A MEMS mirror is provided to increase light efficiency of a driving mirror and reduce damping by solving a problem of resistance between a mirror and air in high frequency operation. A support frame(103) is supported by two torsion bars penetrating a silicon substrate up and down. A cantilever beam(101) has sharp both wings which is narrow compared to the width and is connected to two torsion bars symmetrically. A coil wire is placed at the backside of the cantilever beam pass a current.

Description

Resonantly scanning MEMS mirror operated by the electromagnetic force with low damping and high optical efficiency

1 is a perspective view of a conventional laser application electromagnetic force driving mirror,

2 is a cross-sectional view taken along the line AA ′ of FIG. 1;

3 is a photograph of a laser application galvanometer drive mirror according to the prior art,

4 is a perspective view of a general torsion bar actuator for explaining the driving principle;

5 is a perspective view of a driving mirror according to the present invention;

6 is a plan view and a rear view of the drive mirror according to the present invention;

7 is a cross-sectional view taken along line AA ′ of FIG. 5;

8 is an elliptical incident beam at the incidence of a driving mirror and a 45 degree angle according to the present invention;

9A to 9L are schematic views illustrating a manufacturing method of a driving mirror according to the present invention.

<Description of the symbols for the main parts of the drawings>

101: cantilever beam 102: torsion bar

103 frame 104 permanent magnet

105: magnetic field line induction frame 200: substrate

201: mirror thin film layer 202: coil electrode

203: first insulating layer 206: second insulating layer

208: connecting electrode 209: hard mask

The present invention relates to a MEMS resonant driving mirror using an electromagnetic field having a low damping and high fill factor. The driving mirror has a low residual stress, a damping coefficient, a low air resistance, and high light efficiency.

Conventional driving mirrors for laser scanners include single-axis or two-axis electromagnetic drive mirrors of Nippon Signal, Japan, electrostatic drive mirrors of 0.6 mm to 2 mm in size, ARI, USA, and electromagnetic drive mirrors for eyepiece-type SVG displays from Microvion, USA. . 1 to 3 show a MEMS electromagnetic force driving mirror for a conventional laser scanner. The cantilever beam 3 of the MEMS electromagnetic force driving mirror for the laser scanner includes a reflection mirror 1 having a reflective metal layer in the center, a coil lead portion 2 for driving a current around the driving mirror or at the rear of the mirror, and a driving mirror. It consists of a driving torsion bar 4 and the like on both sides, and a permanent magnet 5 is placed on both sides of the driving mirror chip at the wafer level. When a current is applied to the coil lead 2 outside the reflective mirror 1 of the cantilever beam 3 inside the permanent magnet 5 of the driving mirror, the cantilever beam is torsion bar 4 by the Lorentz force. Rotation is driven around the axis to change the path of the laser light. When an alternating current is applied to the coil of the driving mirror, the torsion bar is used as the axis of rotation, and the driving mirror is alternately driven. The laser light is scanned on the screen by scanning two driving mirrors or one driving mirror for driving the x and y axes.

1 to 3, the cantilever beam 3 of the drive mirror for the conventional laser display has a square or rectangular or circular structure. Friction between the cantilever beam and air occurs when driving at high frequencies near the resonant frequency. This damping effect causes the driving angle to be somewhat reduced. There is a need to devise a structure to reduce such damping.

5 is a photograph of a conventional galvanometer mechanical drive mirror. Referring to FIG. 5, when the laser beam is incident at a 45 degree angle to the mirror surface, the incident beam becomes an ellipse at the mirror surface. It becomes clear that a design of the structure of the drive mirror in consideration of the reflection efficiency for the elliptical laser beam at the mirror surface is required. Conventional galvanometer drive mirrors are characterized by their bulkiness and high price.

Therefore, the present invention is to improve the light efficiency of the MEMS electromagnetic force driving mirror for laser display and at the same time to reduce the damping effect by solving the resistance between the driving mirror and the air when driving at a high frequency.

The first task for enhancing the light efficiency of the drive mirror to be achieved in the present invention is to increase the fill factor by placing a coil conductor through which a current flows on the rear surface of the drive mirror. To do this, a coil lead is placed on the back of the reflecting mirror. The second task to improve the reflection efficiency is to design the shape of the reflecting mirror in consideration of the incident angle of the laser beam.

To reduce the damping effect of the drive mirror to be achieved in the present invention is to design the friction portion with the air of the cantilever beam in a streamlined form, considering the kinetic dynamics such as increasing the size of the air gap, the size, shape, etc. of the cantilever beam It is to devise efficiently.

Accordingly, the present invention relates to an electromagnetic force driving mirror having an electrode coil under the mirror and adjusting the shape of the mirror to increase reflection efficiency and reduce damping coefficient.

The driving mirror for the laser display according to the present invention is supported by two torsion bars 102 by penetrating vertically through a thin silicon substrate 200 which is stress-relieved by plasma treatment after double-sided chemical mechanical polishing. A frame 103; A frame 103 which penetrates the silicon substrate 200 up and down and is supported by two torsion bars 102; A cantilever beam 101 having a sharp tip and a narrower width than the length of the two wings in a driving direction connected to be symmetrical with the two torsion bars; A coil lead 202 positioned at a rear side of the cantilever beam and configured to flow a current; It is characterized in that it comprises two permanent magnets 104 having different polarities, respectively fixed to the magnetic field line induction frame 105 outside the frame 103 so as to be symmetrical with respect to the rotation axis of the torsion bar 102.

According to a preferred embodiment of the present invention as shown in Figs. 5 and 6, the driving mirror is electromagnetically driven and is reflected in a state in which a magnetic field by the permanent magnet 104 is hung in a direction perpendicular to the driving axis of the cantilever beam 101. When a current is applied to the coil 202 wound below the mirror, the cantilever beam is driven about the torsion bar 102 by the Lorentz force. On the contrary, when current is applied, it is driven in the opposite direction. The driving motion equation of the cantilever beam is represented by the following equation (1).

....... 식 (1)

....... Formula (1)

Θ of the left side term in equation (1) is the drive angle of the cantilever beam with the torsion bar as the drive shaft, J is the moment of inertia of the cantilever beam 101, c is the damping coefficient, K _t Is the spring constant for the torsion bar. The right side term is the rotational force term by Lorentz force, proportional to the applied current i _D , where A is a constant.

The smaller the damping coefficient, the larger the driving angle. In the present invention, the invention relates to a driving mirror capable of reducing the damping coefficient and increasing the driving angle. Referring to Equation (2), the damping becomes very small when the width D of the cantilever beam is smaller than the length L _m .

..............식 (2)

................... Equation (2)

    In equation (2)

................식 (3)

Equation (3)

Μ is the air viscosity, h _o is the air-gap of the cantilever beam, b = D / L _m to be.

   9, when the laser beam is incident on the reflective mirror surface of the cantilever beam, the circular beam becomes elliptical at the mirror surface. The sharp tip of the wing in the driving direction has a larger light effective area to reflect the elliptical laser beam.

   Accordingly, the driving mirror according to the present invention is an elliptical structure in which the shape of the cantilever beam is pointed at the tip of the wing as shown in FIGS. Since the width of the cantilever beam is short compared to the length, the damping coefficient is reduced. In addition, the tip of the wing in the driving direction has the effect of cutting the air and has a larger light effective area to reflect the elliptical laser beam.

6 is a diagram illustrating the front and rear surfaces of the driving mirror chip excluding the permanent magnet and the magnetic field line induction frame. FIG. 7 is a cross-sectional view taken along line AA ′ of FIG. 5. The thin silicon substrate 200, which has been stress-free by SF ₆ plasma treatment after chemical mechanical polishing, is a wafer provided by DISCO, Japan, for example. The front surface of the cantilever beam 101 is coated with a gold (Au) thin film using a red wavelength, and an aluminum thin film having high reflectance in visible light when the entire visible light is used is coated at 500 mW or more. The reflecting mirror has a size of 2 mm x 3 mm to 5 mm x 7 mm, and the shape of the cantilever beam connected to the two torsion bars to be symmetrical is sharp at both ends of the wings in the driving direction to reduce the resistance and to reduce the damping. To this end, the width in the driving direction is narrower than the length, and in order to increase the fill factor of the reflective mirror, coil coils are provided on the rear surface thereof as shown in FIG. The length of the torsion bar 102 is 1 ~ 2mm and its line width is 60 ~ 600μm, the thickness is 30 ~ 500μm. Two magnets 104 having different polarization directions are provided outside the frame 103 so as to be symmetrical with respect to the rotation axis of the torsion bar 102. Two magnets are fixed to the magnetic field line induction frame 105.

Hereinafter, with reference to FIGS. 9A-9L, the manufacturing method of the drive mirror chip by this invention is demonstrated.

In the method of manufacturing a driving mirror chip according to the present invention, the method may include manufacturing a thin silicon wafer 200 in which residual stress is eliminated, and etching the upper silicon layer on the wafer 200 to produce a frame 103 and a cantilever beam 101. And forming a torsion bar 102 and dicing a metal thin film for reflection on the surface of the cantilever beam.

The residual stress-releasing thin silicon wafer 200 of FIG. 9A is subjected to hydrofluoric acid (HF) and HNO ₃ after chemical mechanical polishing on both surfaces of a general silicon wafer. SF ₆ after rinsing in mixed solution The surface of the silicon wafer is etched with plasma to relieve the residual stress of the mechanical polishing surface. For example, it is manufactured using Disco Grinding equipment of DISCO.

9A to 9B, a description will be given of a step of fabricating a coil lead after applying an insulating layer to the lower portion of the thin silicon wafer 200 in which residual stress is eliminated.

FIG. 9B is a diagram illustrating a process of applying a silicon oxide film, which is a first insulating layer 203, to a rear surface of the thin silicon wafer. The silicon oxide film is applied using, for example, plasma enhanced chemical vapor deposition (PECVD) equipment. As shown in FIG. 9C, a photoresist 205 is coated on the insulating layer 203 to form a post-exposure pattern. The photoresist here uses a negative photoresist dedicated to lift-off for forming the electrode. Next, as shown in FIG. 10D, the electrode thin film is deposited by sputtering or vacuum evaporation, and then subjected to a lift-off process in which an ultrasonic solution is placed in an acetone solution and subjected to ultrasonic cleaning. Likewise an electrode coil is formed. Next, as shown in FIG. 9F, a silicon oxide film, which is the second insulating layer 206, is deposited by plasma enhanced chemical vapor deposition. Next, as shown in FIG. 9G, the via hole 207 connecting the coil electrode 202 is drilled, and the lift-up process is performed as described above to form the connecting electrode 208 as shown in FIG. 9H.

9I to 9K, the method of forming the frame 103, the driving mirror 101, and the torsion bar 102 by etching the silicon layer on the silicon wafer 200 and the silicon oxide layer on the bottom thereof is described.

First, referring to FIG. 9I, a silicon oxide film is deposited on the silicon wafer 200. The photoresist is applied and patterned by a photolithography method using a double-sided aligner, followed by etching with CHF ₃ and O ₂ mixed plasma to form a hard mask 209, and then removing the photoresist. . Next, SF ₆ as shown in Figure 9j. The silicon layer is etched by plasma, and then the hard mask 209 and the lower silicon oxide layer are removed by the CHF ₃ and O ₂ mixed plasma, and the frame 103, the driving mirror 101, and the torsion bar 102 are vertically penetrated. ) Is formed.

9K to 9L illustrate a step of performing a dicing saw process after forming the reflective metal thin film on the surface of the driving mirror on the silicon wafer.

Referring to FIG. 9K, a gold (Au) thin film or an aluminum (Al) or aluminum nitride (AlN) thin film is deposited on the surface of the cantilever beam 101, the torsion bar 102, and the frame 103 by 500 Å or more. Referring to FIG. 10L, a final driving mirror chip in a wafer state is completed on a wafer by a dicing saw or a laser cutting method for each driving mirror chip on a wafer.

The present invention relates to a MEMS resonance driving mirror using an electric field having low damping and high light efficiency. The resonance frequency is 2 kHz to 8 kHz, and the maximum driving angle is obtained using the resonance frequency. The driving mirror according to the present invention is an oval structure in which the tip of the cantilever beam has a sharp tip as shown in FIG. 5, which has a small damping coefficient, a large driving angle, and a large light efficiency. Since the width of the cantilever beam is shorter than the length, the damping coefficient is reduced. The sharp tip of the wing in the driving direction also allows the reflecting mirror to have a larger light effective area for reflecting the elliptical laser beam.

Claims (3)

    A frame 103 which penetrates the silicon substrate 200 up and down and is supported by two torsion bars 102; A cantilever beam 101 having a sharp tip and a narrower width than the length of the two wings in a driving direction connected to be symmetrical with the two torsion bars; And a coil conductor (202) positioned on a rear surface of the cantilever beam and configured to flow a current.     The two magnets of claim 1, wherein the magnetic field polarization directions are different from each other outside the frame 103 so as to be symmetrical with respect to the rotation axis of the torsion bar 102, and fixed to the magnetic field line induction frame 105.      2. The cantilever beam according to claim 1, wherein the cantilever beam is elliptical or diamond-shaped having a sharp tip of the driving blade and a long axis in the driving axial direction.

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