CN1661413A - Digitally controlled self-positioning free-space micromachined optical switch - Google Patents

Abstract

The present invention relates to a numerically controlled self-positioning free space micromechanical optical switch. It is an optical switch device used for optical cross-connection in fiber-optic communication network. It has a biaxial micromirror main body structure formed from left supporting point, right supporting point, left supporting bean, right supporting beam, upper supporting beam, lower supporting beam, frame and reflecting mirror. Said invention also provides their connection mode.

Description

Digitally controlled self-positioning free-space micromachined optical switch

Technical field

The invention relates to an optical switch device, which is used for optical cross-connection in an optical fiber communication network, and belongs to the technical field of semiconductor micro-device manufacturing.

Background technique

With the continuous improvement of the informatization level of the whole society, the optical-electrical-optical switching mode in the current optical fiber communication network has become the "electronic bottleneck" to further expand the communication capacity. All-optical switching is the development direction of the optical fiber communication network, and optical cross-connection is The key to realizing all-optical switching, the optical switch is the core device in the optical cross-connect device.

In recent years, with the improvement of application requirements and the development of science and technology, people have researched and developed a variety of optical switches based on different materials and physical effects, such as solid-state waveguide optical switches, liquid crystal optical switches, holographic optical switches, thermo-optical switches, semiconductor Amplifier optical switch, micro-electro-mechanical system (MEMS, referred to as micro-mechanical) optical switch, etc. Among them, MEMS optical switches can be mass-produced with a process similar to integrated circuits, and can be manufactured together with control circuits, with small volume and low cost. The characteristics of MEMS optical switches have nothing to do with the format, wavelength, protocol, modulation method, polarization, transmission direction, etc. of optical signals, and are superior to other types of optical switches in terms of loss and scalability, so MEMS optical switches and their arrays are most likely It has become the mainstream of core optical switching devices and has become the most concerned development direction in the field of optical switch research.

MEMS optical switches use micro-mirrors obtained by micro-machining to realize the conversion of optical paths, and there are mainly two types: two-dimensional and three-dimensional. A two-dimensional MEMS optical switch means that the incident light beam and the outgoing light beam are in the same plane, and the optical switch can only change the propagation direction of light within this plane. Therefore, in the application of two-dimensional MEMS optical switches, the micro-mirror, the input fiber and the output fiber are located in the same plane, and the micro-mirror only has two states of entering and exiting the optical path, corresponding to the reflection and straight-through of the optical path, an N× N switch arrays can connect N input optical fibers and N output optical fibers. To control the two-dimensional MEMS optical switch, it is only necessary to apply sufficient driving voltage to make the micromirror move. It is mainly used in a switch matrix with a small number of ports.

The three-dimensional MEMS optical switch means that the micro-mirror used to convert the optical path can deflect around two orthogonal axes in the plane where it is located. By controlling the two deflection angles, the incident light can be reflected in the three-dimensional space according to the required angle. Therefore, this optical switch is also called a free space optical switch. In the application of three-dimensional MEMS optical switches, the input optical fiber and the output optical fiber are respectively arranged in a two-dimensional array, and the two groups of optical switches are also arranged in a two-dimensional array, and one group of optical switches corresponds to the input optical fiber one by one, which is called the input switch group. , and another group of optical switches corresponds to the output optical fiber one by one, which is called the output switch group. The incident light from the input fiber is irradiated on the micro mirror corresponding to the input fiber in the input switch group, and the deflection angle of the mirror is controlled so that the reflected light is irradiated on the micro mirror corresponding to the specified output fiber in the output switch group. on the reflector, and then control the deflection angle of the reflector so that the reflected light is irradiated on the corresponding output optical fiber, so as to realize the optical cross-connection between any input optical fiber and any output optical fiber.

The application of the three-dimensional MEMS optical switch shows that only 2N optical switches are needed to realize the optical cross-connection of N input optical fibers and N output optical fibers. Therefore, a three-dimensional, ie, free-space optical switch array is mainly used in a switch matrix with a large number of ports.

It can be seen from the above that whether the light reflected by the free-space optical switch can be irradiated on the output fiber determines whether the optical cross-connect function can be realized, and the accuracy of the irradiation determines the performance of the optical cross-connect, that is, insertion loss. In order to precisely control the direction of the reflected light, two deflection angles of the optical switch need to be precisely controlled. So far, people mainly solve this problem through the detection of deflection angle and closed-loop control, that is, two sets of detection and control electrodes are set under the micro-mirror, and each set of detection electrodes detects the rotation angle of the micro-mirror around an axis, and then through the corresponding The control electrode exerts a driving force to stabilize the micromirror at the desired corner. Due to the small size of the micromirror, the capacitance formed between the micromirror and the detection electrode is very small, and the capacitance change generated when the deflection angle of the micromirror changes is even smaller, making accurate detection more difficult. On the other hand, the response time is another important performance index of the optical switch, which depends to a large extent on the dynamic characteristics of the closed-loop loop, and the loop needs a certain correction network. All of these lead to quite complicated detection and feedback control circuits for the deflection angle of the micromirror. The complexity of the detection control circuit has exploded and the scale is huge.

Contents of Invention

Technical problem: Aiming at the small size of MEMS devices, weak capacitance changes, and difficult detection, so that the precise control of the reflection angle of free space MEMS optical switches is difficult, the present invention solves the problem from the structural design, providing a digitally controlled self-positioning free space micro The mechanical optical switch realizes the fixed angle deflection and automatic positioning of the mirror surface, removes the complicated angle detection and control circuit, and realizes the direct digital control of the free space MEMS optical switch.

Technical solution: A boss structure is set at the appropriate position on the back of the micromirror and the frame. As long as a relatively sufficient voltage is applied to the substrate electrode, the mirror surface and the frame will deflect until the boss contacts the substrate.

The present invention is made up of left supporting point, right supporting point, left supporting beam, right supporting beam, upper supporting beam, lower supporting beam, frame, reflecting mirror surface; wherein, reflecting mirror surface is positioned at the middle of frame, and the two ends of left supporting beam It is fixed to the outside of the frame and the left support point, and the two ends of the right support beam are respectively fixed to the outside of the frame and the right support point; the upper end of the upper support beam is fixed to the upper part of the frame, and the lower end of the upper support beam is fixed to the reflector surface The upper end of the lower support beam is fixed to the lower part of the frame, and the upper end of the lower support beam is fixed to the lower end of the mirror surface. An outer boss and an inner boss are respectively arranged on the back of the frame and the mirror surface.

1. Structure The current domestic two-layer polysilicon surface technology only provides one structural layer with a single thickness, which determines that the micro-mirror and its supporting structure have the same thickness. In order to adapt to this technological condition, the present invention adopts the overall structure of biaxial external support and built-in micromirrors.

In the overall structure of the present invention, the supporting point of the structure is also a fixed end, and the large plane in the middle is a mirror surface, which is connected to the frame through the inner beam, and the frame is connected to the fixed point through the outer beam. The micromirror can be twisted around the inner beam (Y axis) relative to the frame to change the propagation direction of the beam in the XOZ plane, and can also be twisted with the frame around the outer beam (X axis) relative to the base to change the beam in the YOZ plane direction of propagation. When the micromirror is twisted around the inner beam and the outer beam at the same time, the propagation direction of light can be changed in three-dimensional space, so that a single device can realize the distribution of one input light to eight output directions or the distribution of eight input lights to one output direction reuse.

In order to achieve a certain torsion angle of the mirror and require a small driving force, and reduce the area occupied by the entire device, the inner beam and the outer beam are designed as a square wave structure to reduce their torsional stiffness and compact structure.

2. Driving The present invention adopts an electrostatic driving method, which has a simple and reliable structure, and is especially suitable for batch processing and electromechanical integration.

When a voltage is applied between the electrodes arranged on the device substrate and the device's movable structure, an electrostatic attraction will be generated between the two, and the relative electrostatic attraction between different substrate electrodes and the device's movable structure can be controlled. The size can generate an appropriate torsional moment on the movable structure, so that it can be twisted around the corresponding axis.

On the substrate under the main structure of the biaxial micromirror, drive electrodes are arranged, including external electrodes for driving the frame and internal electrodes for directly driving the mirror surface. The difference between the electrostatic attractive forces generated by the two outer electrodes on the frame makes the frame and the mirror twist around the outer beam, and the difference between the electrostatic attractive forces generated by the two inner electrodes on the mirror makes the mirror twist around the inner beam relative to the frame. If the two outer electrodes If there is a certain difference between the attraction force to the frame and the attraction force to the mirror surface of the two internal electrodes, the mirror surface will twist around the inner beam and the outer beam at the same time.

3. Positioning In the optical path exchange based on the biaxial micromirror, a prominent problem is the need to precisely control the twist angle of the mirror in order to accurately reflect the incident light to the exiting fiber. In a biaxial micromirror with continuous deflection (that is, the deflection angle can be continuously changed), this purpose must be achieved through angle detection and closed-loop control, that is, the detection electrode and the control electrode are simultaneously set on the mirror surface and the substrate under the frame. The deflection angle of the mirror is detected by means of the detection electrodes, and then the angle is adjusted or stabilized at a desired value by means of the control electrodes.

Since the deflection angles of the reflectors are all discrete values (i.e. do not need to change continuously) in the actual light exchange application, the present invention uses the convenience brought by the planar micromachining process to set a kind of convex angle at the appropriate position of the micromirror and the back of the frame. In this way, as long as a relatively sufficient voltage is applied to the substrate electrode, the mirror and the frame will deflect until the boss contacts the substrate, so that the mirror deflects at a fixed angle and realizes automatic positioning. This self-positioning structure not only ensures the accuracy and repeatability of the corner of the mirror, but also eliminates the complicated detection and control circuit. In addition, it can also prevent the mirror or the frame from being bonded to the substrate due to accidental reasons, which is very important for surface processing. It is a common and troublesome problem for devices.

This kind of biaxial micromirror based on the boss structure can realize 1:8 or 8:1 optical path conversion under digital control, and a certain scale of digitally controllable optical cross-connect array can be realized by combining multiple similar devices. .

4. Reflective layer In order to improve the reflection ability of the mirror surface, a layer of metal is plated on the polysilicon structure layer. In view of the actual situation of the domestic surface processing technology, metal Al is easier to sputter and adhere to the polysilicon surface than Au. The present invention uses metal Al as the surface of the polysilicon. The metallic reflective layer of the lens. The reflectivity of the mirror surface increases with the increase of the thickness of the metal layer, but an excessively thick metal layer will have an adverse effect on the mechanical properties of the structure. The thickness of the aluminum layer is 50nm to reduce the influence of the mirror metal layer on the structure and mechanical properties of the entire device At the same time, it is easy to process and realize.

Beneficial effect: the present invention overcomes the problem that the detection and control of the deflection angle of the reflective mirror are difficult due to the small size of the MEMS device and the weak capacitance change. As long as a relatively sufficient voltage is applied to the substrate electrode, the mirror and the frame will deflect until the boss contacts the substrate to achieve automatic positioning. This self-positioning structure not only ensures the accuracy and repeatability of the mirror rotation angle, but also saves complex detection and control circuits, and realizes free-space optical cross-connection under digital control. On the other hand, the self-positioning bosses can also prevent the mirror or frame from bonding to the substrate by accident, thereby avoiding this common and troublesome problem in surface-processed devices.

Using this device to form an array of a certain scale can realize direct digital control of multi-channel optical cross-connections, avoiding the volume, weight, and power of large and complex analog circuits when using analog-controlled free-space optical switches to realize optical switching arrays. Consumption, cost and reliability issues.

Description of drawings

Fig. 1 is a front view of the device of the present invention. There are: left support point 1, right support point 6, left outer support beam 2, right outer support beam 5, inner upper support beam 3, inner lower support beam 8, frame 4, mirror surface 7.

Fig. 2 is a reverse view of the device of the present invention. Inner boss 71 and outer boss 41 are arranged therein.

Fig. 3 is a distribution diagram of electrodes on the device substrate of the present invention. There are: inner electrode 91 and outer electrode 92 .

Detailed ways

Structurally, the digitally controlled self-positioning free space micromechanical optical switch of the present invention consists of a left support point 1, a right support point 6, a left support beam 2, a right support beam 5, an upper support beam 3, a lower support beam 8, and a frame 4. Reflecting mirror surface 7 forms the main structure of the biaxial micromirror; wherein, reflecting mirror surface 7 is located in the middle of frame 4, and the two ends of left support beam 2 are respectively fixed with the outside of frame 4 and left support point 1, the right support beam 5 Both ends are respectively fixed to the outside of frame 4 and right support point 6; Fixed in the lower part of the frame 4, the upper end of the lower support beam 8 is fixed on the lower end of the mirror surface 7. An outer boss 41 and an inner boss 71 are provided on the backs of the frame 4 and the mirror surface 7 respectively. On the substrate under the main structure of the biaxial micromirror, the driving electrodes are arranged, including the outer electrode 92 for driving the frame and the inner electrode 91 for directly driving the mirror surface.

The digitally controlled self-positioning free space MEMS optical switch of the present invention has been realized by the polysilicon surface micromachining process, and the specific process steps are as follows:

The first step: clean the substrate silicon wafer, deposit silicon oxide (thickness 3000 angstroms), silicon nitride (thickness 2000 angstroms) and the first layer of polysilicon (thickness 3000 angstroms); apply photoresist, register photo Engraving the substrate electrode and lead pattern, reactive ion etching the first layer of polysilicon, degelling and cleaning.

Step 2: Deposit PSG sacrificial layer (thickness 2 microns), apply photoresist, register photolithographic boss pattern, reactive ion etching PSG sacrificial layer (3000 angstroms), remove glue and clean.

The third step: apply photoresist, register the pattern of photolithographic support pillars, reactive ion etching PSG sacrificial layer, remove glue and clean.

Step 4: Deposit the second layer of polysilicon (thickness 2 microns), deposit PSG thin film layer, high temperature annealing heat treatment.

Step 5: Apply photoresist, register the photolithographic structure pattern, reactive ion etch the second layer of polysilicon, remove the glue and clean.

Step 6: Apply photoresist, register photolithographic mirror pattern, sputter 50nm thick aluminum metal film, remove glue and clean.

Step 7: Corrode the PSG sacrificial layer in KOH solution to release the structure and perform anti-adhesion treatment.

According to the above, the present invention can be realized.

Claims (3)

1. A digitally controlled self-positioning free space micromechanical optical switch, characterized in that it consists of a left support point (1), a right support point (6), a left support beam (2), a right support beam (5), an upper support beam (3), lower support beam (8), frame (4), reflection mirror surface (7) form biaxial micromirror main structure; Wherein, reflection mirror surface (7) is positioned at the middle of frame (4), left support beam (2) The two ends of the frame (4) are respectively fixed to the outside of the frame (4) and the left support point (1), and the two ends of the right support beam (5) are respectively fixed to the outside of the frame (4) and the right support point (6); The upper end of the beam (3) is fixed on the upper part of the frame (4), the lower end of the upper support beam (3) is fixed on the upper end of the mirror surface (7), and the lower end of the lower support beam (8) is fixed on the frame (4). Bottom, the upper end of the lower support beam (8) is fixed on the lower end of the mirror surface (7). 2. The digitally controlled self-positioning free-space micromechanical optical switch according to claim 1, characterized in that an outer boss (41) and an inner boss ( 71). 3. The digitally controlled self-positioning free-space micromachined optical switch according to claim 1, characterized in that on the substrate below the main structure of the biaxial micromirror, drive electrodes are arranged, including external electrodes (92) for driving the frame ) and internal electrodes (91) for directly driving the mirror.

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