KR20070112927A - Resonant scanning mirror with mems technology for the laser display - Google Patents

Abstract

A resonant scanning mirror for a laser display is provided to increase a fill factor of the mirror by arranging a coil line behind the mirror. A resonant scanning mirror for a laser display includes a frame(103), a reflective mirror(101), a coil line and two magnets. The frame penetrates a silicon substrate and is supported by two torsion bars. The reflective mirror is arranged to be symmetrical with respect to the two torsion bars. The coil line is formed behind the reflective mirror. A current flows through the coil line. The magnets with different polarities are arranged to be symmetrical with respect to each other around a rotation axis of the torsion bar. The magnets are arranged outside the frame. A stress on the silicon substrate is relieved by performing a plasma treatment after a CMP(Chemical Mechanical Polishing) process on the silicon substrate.

Description

MEMS resonant drive mirror for laser display {Resonant scanning mirror with MEMS technology for the laser display}

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

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

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

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

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

6 is a perspective view of a cantilever portion of a driving mirror according to the present invention;

7 is a rear perspective view of a cantilever portion of a drive mirror according to the present invention;

8A to 8M are schematic views illustrating a manufacturing method of the electromagnetic force driving mirror of the present invention.

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

101: reflection mirror 102: torsion bar

103 frame 104 permanent magnet

200: substrate 201: mirror thin film layer

202: coil electrode 208: connecting electrode

203: first insulating layer 206: second insulating layer

209: hard mask

The present invention relates to an electromagnetic force driving mirror for a laser application display that improves the reflection efficiency in the visible light region and at the same time has excellent durability. In detail, the coil wafer 202 is positioned on the rear surface of the reflecting mirror 201 using the thin wafer 200 in which residual stress is eliminated, thereby increasing durability and the fill factor of the reflecting mirror 201, and the surface of the reflecting mirror. Has a maximum reflectance with respect to the center wavelength of visible light (550 nm), and the surface of the reflecting mirror 201 and the torsion bar 102 is coated with a diamond coating to protect the mirror surface and reduce friction of the driving part, thereby increasing durability. It is an invention for a drive mirror for excellent laser display.

Conventional driving mirrors for laser scanners include short axis or biaxial 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 SVG displays, Microvion, USA. 1 is an electromagnetic force driving mirror for a typical representative laser scanner. The electromagnetic force driving mirror for the laser scanner includes a reflection mirror (1) having a reflective metal layer at the center of the cantilever portion, a coil lead portion (2) through which current flows around the driving mirror, and a driving torsion bar (4) on both sides of the driving mirror. The permanent magnets 5 are placed on both sides of the drive mirror chip at the wafer level. When a current is applied to the coil lead 2 outside the reflective mirror 1 of the cantilever 3 inside the permanent magnet 5 of the driving mirror, the cantilever is axially driven by the Lorentz force. Rotation drive is used to change the path of the laser light. When the AC current is reduced at the resonant frequency of the drive mirror of several kHz, the drive is alternated at a large angle. 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 and 2, the conventional electromagnetic force driving mirror used an SOI wafer 10. The SOI wafer is bonded together with the silicon oxide 8 between the first silicon wafer 7 and the second silicon wafer 9 which are manufactured by chemical mechanical polishing. The SOI wafer has an increased residual stress because an etch mark is formed on the wafer surface following the mechanical rotation of the CMP. Therefore, there is a problem that the SOI wafer can be easily destroyed by external impact even during the process or after the final product is manufactured.

Referring to FIG. 2, in the conventional electromagnetic force driving mirror, a coil conducting wire 2 through which a current flows is positioned around the reflecting mirror 1 and the reflecting mirror 1. In the conventional electromagnetic force driving mirror, when the number of coils is increased to increase the first silicon wafer driving angle by using the SOI wafer, the fill factor of the driving mirror is reduced, thereby reducing the light efficiency. The cantilever 3 is too hard to drive by friction of the torsion bar 4 during several kHz driving, and the durability is poor.

Therefore, it is an object of the present invention to improve the durability of the driving mirror by improving the reflection efficiency of the driving mirror for the conventional laser scanner and at the same time eliminating friction between the torsion bar and the support anchor when driving at a high frequency.

The improvement of the reflection efficiency to be achieved in the present invention is to increase the fill factor by placing the coil conductors on the rear surface of the driving mirror. To this end, coil leads are fabricated on the rear surface of the reflective mirror using a double-side polished wafer instead of a conventional SOI wafer.

The improvement of the reliability of the drive mirror to be achieved in the present invention is to produce a drive mirror that is not easily destroyed during external impact. To this end, the surface of the polished surface of the double-side polished wafer is subjected to plasma treatment to use a stress-relieved wafer. In addition, in order to solve the frictional force of the torsion bar of the driving mirror is to coat a low friction protective film thin film on the connection portion between the driving portion and the support anchor of the torsion bar. Therefore, the present invention is to manufacture the driving mirror that can protect the mirror thin film by adjusting the dimensions of the torsion bar to increase the resonant frequency and at the same time to the diamond coating on the torsion bar to reduce the friction force and also to the diamond coating the surface of the mirror.

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 reflection mirror 101 connected to be symmetrical with the two torsion bars; A coil lead 202 positioned at a rear side of the reflective mirror and formed to flow a current; Two magnets having different polarities are provided outside the frame 103 so as to be symmetrical with respect to the rotation axis of the torsion bar 102.

3 is a perspective view illustrating a MEMS resonance driving mirror for a laser display according to an exemplary embodiment of the present invention. 4 is a cross-sectional view taken along the line AA ′ of FIG. 3. The thin silicon substrate 200 which is stress-free by SF6 Plasma treatment after double-sided chemical mechanical polishing is, for example, a wafer provided by DISCO of Japan. The front surface of the driving mirror 101 is coated with a thin film of gold (Au) when the red wavelength is used, and an aluminum thin film having high reflectance in the visible light when the entire visible light is used. The driving mirror has a size of 4 mm × 4 mm, which is suitable for a laser display for advertisement and entertainment, and a coil lead is provided on the rear side of the driving mirror to increase the fill factor of the driving mirror. The length of the torsion bar 102 is 1 ~ 1.5 mm and its line width is 60 ~ 600 μm, thickness is 30 ~ 50 μm. Two magnets 104 having different polarities are provided outside the frame 103 so as to be symmetrical with respect to the rotation axis of the torsion bar 102.

Figure 6 is a perspective view of the cantilever portion of the drive mirror according to the present invention, Figure 7 is a rear perspective view of the cantilever portion of the drive mirror according to the present invention. When a current is applied to the coil conductor located at the outermost part of the rear surface of the driving mirror according to the present invention, the torsion bar 102 is driven by the Lorentz force. On the contrary, when current is applied, it is driven in the opposite direction. The resonance frequency, which is the performance of the drive mirror, is calculated by the following equation (1).

...... 식 (1)

Expression (1)

이고

When a> b in formula (1),

ego

이다.

to be.

Where K _s is the spring constant of the torsion bar 103, I _m is the moment of inertia of the driving mirror 101, I _p is the moment of inertia of the torsion bar 103, and G is the torsion bar. Torsion modulus of (103), where a is 1/2 of the line width of the torsion bar 103 and b is 1/2 of the thickness of the torsion bar. ρ is the density of silicon, t _m is the thickness of the drive mirror, L _m is the length of the drive mirror 101 in a direction parallel to the torsion bar 103 of the drive mirror 101, D is parallel to the torsion bar 103 The length of the drive mirror 101 in one direction.

The following is a table showing the resonant frequency of the drive mirror of 4mm × 4mm according to the present invention.

The maximum driving angle (θ) of the driving mirror was obtained by modeling the relationship between the net torque (τ) and each dimension of the electromagnetic force driving mirror, as shown in the following table. The drive angle was modeled when the same torque was applied to the drive mirror.

.......................식 (2)

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

A large driving angle is obtained when the length of a is 250 μm, the length of b is 16 μm, and the thickness of the drive mirror is 30 μm when the resonant frequency of the 4 mm x 4 mm drive mirror obtained by the present invention is about 4 kHz.

Hereinafter, the manufacturing method of the drive mirror by this invention is demonstrated with reference to FIGS. 8A-8M.

The manufacturing method of the driving mirror according to the present invention comprises the steps of manufacturing a thin wafer 200, the residual stress is eliminated, and after applying an insulating layer to the lower portion of the wafer 200 to produce a coil lead (the step, and the wafer (200) etching the upper silicon layer to form a frame 103, a driving mirror 101, a torsion bar 102, and forming a reflective metal thin film on the driving mirror surface on the wafer and then ashing ( Dicing).

The residual stress-releasing thin wafer 200 of FIG. 8A is etched on the silicon wafer surface with SF3 plasma after chemical mechanical polishing on both sides of the normal silicon wafer, followed by cleaning in a hydrofluoric acid (HF) and HNO ₃ mixed solution. Treatment to relieve the residual stress of the mechanical polishing surface. For example, it is manufactured using DISCO Grinding Equipment of DISCO (Japan).

Referring to FIGS. 8B to 8H, after the insulating layer is applied to the lower portion of the thin wafer 200 where the residual stress is removed, a description is given of a step of manufacturing the coil lead.

FIG. 8B shows a coating of the silicon oxide film, which is the first insulating layer 203, on the back side of the thin wafer. The silicon oxide film is deposited using, for example, plasma enhanced chemical vapor deposition (PECVD) equipment. As shown in FIG. 8C, a pattern is formed by applying a photoresist 205 on the insulating layer 203. Here, the photoresist uses a negative photoresist dedicated to lift-off for forming an electrode. Next, as shown in FIG. 8D, the electrode thin film is deposited by a sputtering or evaporating method, and then subjected to a lift-off process of ultrasonic cleaning by putting in an acetone solution to form an electrode coil as shown in FIG. 8E. Next, a silicon oxide film, which is a second insulating layer 206, is deposited using PECVD (Plasma Enhanced Chemical Vapor Deposition) equipment, as shown in Fig. 8F, and then a via connecting the coil electrode 202 as shown in Fig. 8G. The hole 207 is drilled and the lift-up process is performed as described above to form the connection electrode 208 as shown in FIG. 8H.

8I to 8K, the method of forming the frame 103, the driving mirror 101, and the torsion bar 102 by etching the silicon layer on the wafer 200 is described. The photolithography method of the following process uses a double-sided aligner.

First, referring to FIG. 8I, a silicon oxide film is deposited on a surface of the wafer 200. CHF _{3 /} O ₂ after photoresist is applied and patterned by photolithography The photoresist is removed after the hard mask 209 is formed by etching with plasma such as the like. The hard mask serves as a protective layer of silicon under the hard mask during deep etch. Next, as shown in FIG. 8J, the silicon layer is etched in SF6 plasma, and then CHF _{3 /} O ₂ is etched. The hard mask 209 is removed by a plasma such as a pattern of the frame 103, the driving mirror 101, and the torsion bar 102. Next, after patterning by photolithography on the back of the wafer as shown in FIG. 8k, CHF _{3 /} O ₂ The first and second insulating layers are etched by plasma to form a frame 103, a driving mirror 101, and a torsion bar 102 that penetrate up and down.

8L to 8M illustrate a step of dicing after forming a reflective metal thin film on the driving mirror surface on the wafer.

Referring to FIG. 8L, at least 500 angstroms of a gold thin film or an aluminum thin film are deposited on the surfaces of the driving mirror 101, the torsion bar 102, and the frame 103. The protective film is a silicon oxide film or an enhanced coating. Alternatively, the protective film is deposited by sputtering a similar diamond of low friction and low stress, which is used in conventional Diamond MEMS (Micro Electro mechanical Systems) technology. Referring to FIG. 8M, each driving mirror chip of the wafer is diced with a laser or the like to separate the driving chip from the wafer to complete the final wafer-level driving mirror product.

Through the present invention, a large drive mirror having a size of 4 mm × 4 mm for the MEMS resonant drive mirror for laser display is an invention useful for laser displays for advertisement and entertainment. To this end, the drive mirror made of the gold or aluminum thin film has a resonant frequency of 2 kHz. ~ 8 kHz, the maximum driving angle is obtained at this resonance frequency. When two driving mirrors are driven in the x-direction and the other driving mirror in the y-direction, laser light is scanned on the screen to realize a large screen of a laser show in the visible region and an advertising billboard.

Coil wires are placed on the back of the drive mirror so that the fill factor of the drive mirror is excellent, making the manufacturing process simpler and more durable by using thin wafers with high fill factor of the mirror and low stress. Drive mirrors made of low-stress wafers are impact-resistant and efficient in reducing warpage. The protective film is coated on the surface of the driving mirror and the torsion bar to protect the mirror thin film. In particular, the low friction and low stress pseudo diamond thin film used in Diamond MEMS protects the mirror and reduces the friction of the driving torsion bar. Promoted.

Claims (4)

   A frame 103 which penetrates the silicon substrate vertically and is supported by two torsion bars; A reflection mirror connected to be symmetrical with the two torsion bars; A coil lead positioned in a rear portion of the reflective mirror to form a current; The laser application resonance drive mirror, characterized in that it comprises two magnets each having a different polarity outside the frame so as to be symmetrical with respect to the rotation axis of the torsion bar. The silicon substrate of claim 1, wherein the silicon substrate is subjected to plasma treatment after chemical mechanical polishing. 3. The chemical mechanical polishing of claim 2, wherein the chemical mechanical polishing is double side polishing. The low friction, low stress pseudo diamond coating according to claim 1, wherein the torsion bar surface and the mirror surface of the drive mirror are fabricated by the Diamond MEMS method.

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