WO2006011692A1

WO2006011692A1 - Optical switching device using a micro piezoelectric actuator and method for fabricating same

Info

Publication number: WO2006011692A1
Application number: PCT/KR2004/001872
Authority: WO
Inventors: Young-Geun Park; Kyu-Ho Hwang
Original assignee: M2N Inc
Current assignee: M2N Inc
Priority date: 2004-07-26
Filing date: 2004-07-26
Publication date: 2006-02-02
Anticipated expiration: 2007-01-26

Abstract

An optical switching device includes a mirror, a first and the second actuators for adjusting a tilt angle of the mirror on the X-axis, a third and a fourth actuators on the Y-axis, gimbals for supporting the first and the second actuators and a driving substrate for applying a driving signal to the actuators. The gimbals include at least one groove therein in a lateral direction thereof to prevent the gimbals from being bent when the piezoelectric layers are constricted or expanded. Further, under the mirror, there is formed at least one groove to maintain the flatness of the mirror when the mirror is tilted by the actuators. By forming the groove under the mirror, an entire surface of the mirror can be used to reflect an optical signal.

Description

OPTICAL SWITCHING DEVICE USING A MICRO PIEZOELECTRIC ACTUATOR AND METHOD FOR FABRICATING SAME

Technical Field

The present invention relates to an optical switching device using a micro piezoelectric actuator and a method for fabricating same; and, more particularly, to a method for fabricating an optical switching device having a stable structure, in which a polishing process is carried out after polysilicon is deposited in a stepped structure, so that succeeding deposition process and etching process can be easily performed.

Background Art

A switching device used in an optical communication system is an important factor for determining a maximum transmission capacity of the communication system. Recently, an optical switch has been widely used in order to miniaturize the switching device and increase a switching capacity. Especially, a switching technology using a micro mirror, which is controlled by using a MEMS (micro electro¬ mechanical system) technology, is gaining popularity. The MEMS refers to a 3-D microstructure fabricating technology developed from a semiconductor manufacturing process and, especially, is used for fabricating a mechanical structure having a micron or nano size. The optical switch fabricated by using the MEMS technology is technically classified into two types in accordance with optical channel switching methods. To be specific, one is a type using a micro mirror and the other is a type using micro fluids having different refractive indices.

The optical switch using the micro mirror is divided further into a 2D planar switch having a two-dimensional array and a 3D free-spatial switch having a three- dimensional array. Although the 2D planar switch has many advantages in that it allow optical fibers to be readily arranged and has a simple structure due to its employment of an on-off operation type mirror, it is difficult to accommodate therein a large number of (e.g., 32x32 or more) ports. Therefore, the 3D switch having a greater expandability is more suitable for use in a backbone network requiring a Tbps level capacity. In general, actuators used in a micro device such as the aforementioned optical switch has been driven by an electrostatic force. However, the actuator using the electrostatic force has the disadvantage in that it accompanies an increase of a driving voltage and has non- linear characteristics. Further, it is not preferred due to the occurrence of a pull-in phenomenon of a micro mirror being adhered to a substrate. Furthermore, it is difficult to precisely control an optical channel or change a position of a mirror by revolving the micro mirror with respect to two driving axes, i.e., the X-axis and the Y-axis.

Accordingly, an optical switching device using a micro piezoelectric actuator and a method for fabricating same, which are disclosed in PCT application publication No.WO 03/089957, have been proposed to vary a position of the mirror. Fig. 1 shows an example of an optical switching device using a micro piezoelectric actuator.

As illustrated in Fig. 1, when a micro mirror 90 rotates pivoting on the X-axis by driving a piezoelectric layer 65 of a first and a second actuator 60 and 61, for example, a gimbal 160 supporting the first and the second actuator 60 and 61 can be bent due to a constriction or an expansion of the piezoelectric layer 65. In this case, a change in a shape of the gimbal 160 is transmitted to a third and a fourth actuator 260 and 261 connected to the gimbal 160, thereby causing a movement of a mirror 90 on the Y-axis. Therefore, it is difficult to individually control a movement of the mirror 90 on the X-axis and that on the Y- axis.

Thus, in order to prevent a movement of the micro mirror 90 on the X-axis and that on the Y-axis from being coupled, a groove 75 is formed on the gimbal 160 in a length direction thereof, or another groove 70 is formed on a portion where the gimbal 160 is connected to the first and the second actuator 60 an 61. In this way, the gimbal 160 can precisely transmit the movements of the first and the second actuator 60 and 61 on the X-axis to the mirror 90 without affecting, e.g., the movements of the third and the fourth actuator 260 and 261 on the Y-axis.

In order to form a groove or a step on the gimbal 160 or the like, a step is formed on a driving substrate formed on a semiconductor substrate by using a dry or a wet etching process during an initial state of a fabricating process of an optical switching device using a micro piezoelectric actuator. Then, a membrane layer, a bottom electrode layer, a piezoelectric material layer and a top electrode layer are deposited and patterned on the driving substrate on which the step is formed, thereby forming the piezoelectric actuator.

However, if the piezoelectric material layer is formed on the driving substrate having the step formed thereon by using a spin coating method in a state where a shape of the step is maintained even after a deposition of the membrane layer and the bottom electrode layer formed of, e.g., SiNx, the piezoelectric material layer may not be uniformly coated due to the stepped shape. Further, in case of SiNx, since an internal stress thereof is high and a film thickness thereof is comparative thin (generally, less than 1 μm) , a balance between a stress of the membrane layer of SiNx and that of a film to be deposited thereon is required in order to increase a flatness of a finally formed actuator, which makes an entire fabricating process complex.

Moreover, in the optical switching device, in case the mirror connected to the actuator is moved by the actuator, a step or a groove is formed on a surface of the mirror in order to maintain the flatness of the mirror. In this case, however, there is a drawback in that an area where a light is reflected on the surface of the mirror is restricted.

Disclosure of Invention

It is, therefore, an object of the present invention to provide an optical switch and a method for fabricating same, which forms a stable structure of the optical switch in a fabricating process thereof and facilitates a deposition process after a formation of a groove or a step which prevents two-axis movements of actuators from being coupled and maintains a flatness of a mirror.

In accordance with the present invention, there is provided a method for fabricating an optical switching device using a first, a second, a third and a fourth micro piezoelectric actuator, the method comprising the steps of: forming a driving substrate including a driving circuit for generating a driving signal; forming a groove for maintaining a flatness of the optical switching device and a mirror on the driving substrate; depositing a protection layer on an upper surface of the driving substrate; depositing a polysilicon layer to bury a stepped portion formed by the groove on the protection layer; planarizing a surface of the deposited polysilicon; depositing an insulating layer on the polysilicon layer; forming a piezoelectric device layer on the insulating layer; forming a piezoelectric layers of the respective actuators by etching the piezoelectric device layer and the insulating layer; forming, by patterning the insulating layer, a mirror supporting layer positioned between the first and the second actuator, a membrane and a connecting part of the respective actuators, a gimbal for supporting the first and the second actuator, a first and a second transmitting part for connecting the first and the second actuator to the mirror supporting region and a third and a fourth transmitting part for connecting the third and the fourth actuator to the gimbal; depositing a mirror on the mirror supporting region; forming a passivation layer on the optical switching device; removing a part of the driving substrate, where corresponds to lower surfaces of the mirror and the actuators; and removing the passivation layer and the protection layer.

Brief Description of Drawings

The above and other objects and features of the present invention will become apparent from the following description of preferred embodiments given in conjunction with accompanying drawings, in which:

Fig. IA is a perspective view of an optical switching device using a micro piezoelectric actuator in accordance with a prior art;

Figs. IB to ID show diagrams depicting enlarged principal parts 210, 220 and 230 of the micro piezoelectric actuator illustrated in Fig. IA; Fig. 2A illustrates a perspective view of an optical switching device using a micro piezoelectric actuator in accordance with a preferred embodiment of the present invention;

Figs. 2B to 2D provide diagrams describing enlarged principal parts 310, 320 and 330 of the micro piezoelectric actuator illustrated in Fig. 2A;

Fig. 3A presents a perspective view of an optical switching device using a micro piezoelectric actuator in accordance with another preferred embodiment of the present invention; Figs. 3B to 3D represent diagrams showing enlarged principal parts 410, 420 and 430 of the micro piezoelectric actuator illustrated in Fig. 3A;

Fig. 4 provides a plane view of the optical switching device using a micro piezoelectric actuator illustrated in Fig. 2A; and

Figs. 5A to 5K illustrate steps of a method for fabricating an optical switching device using a micro piezoelectric actuator in accordance with a preferred embodiment of the present invention.

Best Mode for Carrying Out the Invention

Fig. 2A illustrates a perspective view of an optical switching device using a micro piezoelectric actuator in accordance with a preferred embodiment of the present invention. Further, Figs. 2B to 2D provide diagrams describing enlarged principal parts 310, 320 and 330 of the micro piezoelectric actuator illustrated^' in Fig. 2A. Referring to Fig. 2A, there is illustrated an optical switching device including a mirror 90; a first and a second actuator 60 and 61 for controlling a tilting angle of the mirror 90 on the X-axis; and a third and a fourth actuator 260 and 261 for controlling the tilting angle of the mirror 90 on the Y-axis by controlling the tilting angle of the first and the second actuator 60 and 61. Each of the actuators 60, 61, 260 and 261 of the optical switching device is implemented in a linear shape.

Each of the first and the second actuator 60 and 61 includes a first membrane 60a, a second membrane 60b, a piezoelectric layer 65 formed on at least one of the first and the second membrane 60a and 60b and a connecting part 22 connected between the first and the second membrane 60a and 60b. Further, the connecting part 22 includes two elastic bodies 22b and a connecting member 22a coupled therebetween. Herein, the membrane 60a or 60b is connected to one terminal of the respective elastic bodies 22b.

Although each of the elastic bodies 22b has a zigzag shape in Figs. 2B and 2C, the shapes of the elastic bodies 22b may be configured differently. For example, as shown in Figs. 3B and 3C, elastic bodies 22b' of a connecting part 22' may be implemented by using a vertically jagged shaped spring.

As shown in Fig. 2A, each of the connecting parts 22 of the first and the second actuator 60 and 61 is preferably positioned at an opposite side with respect to each other across the mirror 90. Further, each of the connecting parts 22 is connected to the mirror 90 through a transmitting part 30, so that the first and the second actuator 60 and 61 can control a tilting angle of the mirror 90 on the X-axis.

Meanwhile, each of the third and the fourth actuator 260 and 261 includes a third membrane 260a, a fourth membrane 260b, a piezoelectric layer 265 formed on at least one of the third and the fourth membrane 260a and 260b and a connecting part 42 connected between the third and the fourth membrane 260a and 260b. Further, the connecting part 42 includes two elastic bodies 42b and a connecting member 42a coupled therebetween. Herein, the membrane 260a or 260b is connected to one terminal of the respective elastic bodies 42b.

Although each of the elastic bodies 42b has a zigzag shape in Figs. 2B and 2C, the shapes of the elastic bodies 42b may be configured differently. For example, as shown in Figs. 3B and 3C, elastic bodies 42b' of a connecting part 42' may be implemented by using a vertically jagged shaped spring.

As shown in Fig. 2A, each of the connecting parts 42 of the third and the fourth actuator 260 and 261 is preferably positioned at an opposite side with respect to each other across the mirror 90. For example, it is preferable that a virtual straight line drawn between the connecting parts 42 positioned at the opposite sides with respect to each other across the mirror 90 is perpendicular to a virtual line drawn between the connecting parts 22 positioned at the opposite sides with respect to each other across the mirror 90.

Each of the connecting parts 42 is connected to the gimbal 160 through a transmitting part 52, so that the third and the fourth actuator 260 and 261 can control tilting angles of the first and the second actuator 60 and 61. Accordingly, a tilting of the mirror 90 can be controlled.

As depicted in Fig. 2D, the gimbal 160 may be configured to have thereon a groove or a step 75 in its length direction. The groove 75 serves to prevent the gimbal 160 from being bent when the piezoelectric layer 65 is constricted or expanded in response to a driving signal. If the gimbal 160 is bent as the piezoelectric layer 65 is constricted or expanded, the tilting of the first and the second actuator 60 and 61 can be transmitted to the third and the fourth actuator 260 and 261. Accordingly, it is difficult to control the movement of the mirror on the X- axis independently the movement thereof on the X-axis. In the same manner, by forming another groove 70 on the gimbal 160, the movements of the third and the fourth actuator 260 and 261 on the Y-axis, i.e., the tilting, can be precisely transmitted to the mirror 90 without affecting the movements of the first and the second actuator 60 and 61 on the X-axis.

The grooves 70 and 75 formed on the gimbal 160 are formed inside the gimbal 160 so that they cannot be seen from an outside. In other words, the gimbal 160 has a structure in which a plurality of layers are deposited. Such multi-layered gimbal 160 can be formed by filling the grooves 70 and 75 formed on a multi-layered layer with, e.g., polysilicon; performing a chemical mechanical polishing (CMP) process on a surface of the polysilicon; and depositing at least one layer on the planarized polysilicon.

In the meantime, each of the piezoelectric layers 65 and 265 includes a top electrode, a bottom electrode and a piezoelectric material layer positioned between the top and the bottom electrode. The piezoelectric material layer contains, e.g., PZT, PbTiO₃, PLZT, PbZrO₃, PLT, PNZT, LiNbO₃ or LiTaO₃. Further, the top and the bottom electrode are formed of a conductive material. In other words, the top electrode contains, e.g., Al, Ru, Au, Ag or RuO₂, PT or the like, and the bottom electrode contains, e.g., Ru or Au, PT, Ta having a high conductivity. The top and the bottom electrode are connected to a driving circuit (not shown) through electrode bridges 270 and 271.

As shown in Fig. 2A, the mirror 90 is positioned between the first and the second actuator 60 and 61. The mirror 90 can be preferably formed of a metal having a high reflexibility, e.g., Au or Pt. A mirror supporting layer composed of the aforementioned insulating layer and/or piezoelectric layer can be formed in a lower portion of the mirror 90.

Referring to Figs. 2A and 3A, there is illustrated the mirror 90 having a circular shape. However, the mirror 90 may have a rectangular shape or another polygonal shape.

Meanwhile, at least one groove or step 91 may be formed for maintaining a flatness and a reflexibility of the mirror 90 itself in case the mirror 90 is tilted by the actuator. In this case, the groove 91 formed on the mirror 90 is formed in a lower portion of the mirror 90 so that it cannot be seen from an outside. In other words, the mirror 90 has a structure in which a plurality of layers are deposited. Such multi-layered mirror 90 can be formed by filling the groove 91 with polysilicon; performing a chemical mechanical polishing (CMP) process on a surface of the polysilicon; and depositing at least one layer among the multi-layers on the planarized polysilicon. Accordingly, it is possible to maintain the flatness of the mirror 90. Further, by forming a groove on the surface of the mirror 90, it can be solved the problem that a reflexibility or a reflecting region of the mirror 90 is restricted. Hereinafter, a method for fabricating an optical switching device including a micro piezoelectric actuator in accordance with a preferred embodiment of the present invention will be described with reference to Figs. 2A To 2D and 3A to 3D. Figs. 5A to 5K sequentially illustrate steps of the method for fabricating an optical switching device including a micro piezoelectric actuator in accordance with a preferred embodiment of the present invention. Cross- sectional views shown in Figs. 5A to 5K depict a cross section of the optical switching device, which is taken along a line A-A' illustrated in Fig. 4, in accordance with the preferred embodiment of the present invention. For the convenience of explanation, some components of the optical switching devices are illustrated out of scale in Figs. 5A to 5K.

As illustrated in Fig. 5A, a driving substrate 2 having a driving circuit for generating a driving signal is formed on a semiconductor substrate (not shown) . Further, a groove is formed on the driving substrate 2 by a dry or a wet etching. The groove formed on the driving substrate 2 becomes a groove or a step formed under or inside the mirror and a gimbal in the present optical switching device. The groove formed on the driving substrate 2 enables the optical switching device of the present invention to maintain a flatness without being affected by a constriction and an expansion of the actuator.

As shown in Fig. 5B, a protection layer 4 is deposited on the driving substrate 2. The protection layer 4 prevents the driving substrate 2 from being damaged in succeeding processes and also prevents a layer deposited on the protection layer 4 from being etched when the driving substrate 2 is etched. The protection layer 4 may be made of, e.g., Siθ₂ or SiNx.

Next, as depicted in Fig. 5C, a polysilicon layer 199 is deposited on the protection layer 4. In this case, the polysislicon layer 199 is formed as follows. That is, polysilicon is deposited on the protection layer 4 and, then, a heat treatment is performed thereon for about 2 to 3 hours at a temperature higher than about 1000°C to thereby remove a residual stress thereof. Such processed polysilicon has no residual stress and thus is not much affected by another layer or film deposited thereon. A surface of the heat- treated polysilicon layer 199 is planarized by a chemical mechanical polishing (CMP) process. With the planarization of the surface of the polysilicon layer 199, a flatness of the mirror and the piezoelectric layer deposited in a succeeding process can be maintained.

Further, sequentially deposited on the polysilicon layer 199 are an insulating layer 200 (e.g., Siθ₂ or SiNx), a bottom electrode 202 (e.g., a platinum layer) and a piezoelectric material layer 204. Herein, the piezoelectric layer 204 contains, e.g., PZT, PbTiO₃, PLZT, PbZrO₃, PLT, PNZT, LiNbO₃ or LiTaO₃. Furthermore, a top electrode layer 206 (e.g., a platinum layer) is deposited on the piezoelectric material layer 204. The top and the bottom electrode layer 206 and 202 and the piezoelectric material layer 204 comprises a piezoelectric device layer.

Thereafter, as illustrated in Figs. 5E to 5G, predetermined portions (corresponding to a piezoelectric actuator, a gimbal, a mirror or the like) in the device are formed by etching the piezoelectric device layer. That is, as shown in Fig. 5G, the piezoelectric layer 65 of the first and the second actuator 60 and 61 is formed on the insulating layer 200. Moreover, a ring pattern 92 surrounding a periphery of the mirror is formed on the insulating layer 200. Although it is not illustrated in Fig. 5G, the piezoelectric layer 265 of the third and the fourth actuator is formed on the insulating layer 200.

Referring to Figs. 5E to 5G, the top electrode layer 206, the piezoelectric material layer 204 and the bottom electrode layer 202 are sequentially formed by an etching.

However, the top electrode layer and the piezoelectric device layer can be formed at once by using a mask.

Then, as illustrated in Fig. 5H, the insulating layer 200 is etched, so that a part thereof is removed. In other words, the insulating layer 200 is patterned, thereby forming the membranes 60a and 60b of the first and the second actuator 60 and 61 and the membranes 260a and 260b of the third and the fourth actuator 260 and 261. In this case, elastic bodies 22b (not shown) coupled between the membranes 60a and 60b and the connecting parts 22a are also formed. Besides, elastic bodies 42b (not shown) coupled between the membranes 260a and 260b and the connecting part 42a are also formed. In addition, a gimbal 160 supporting the first and the second actuator 60 and 61 is formed and connected to the third and the fourth actuator 260 and 261 through a transmitting part. Further, another transmitting part is formed to connect the first and the second actuator 60 and 61 to the mirror. As depicted in Fig. 5H, a mirror supporting region 200a is formed by etching the insulating layer 200. Next, the mirror 90 is formed on the mirror supporting region 200a.

As described with reference to Fig. 5A, a step for maintaining a flatness of the mirror has already been formed in a lower portion of the mirror supporting region 200a, i.e., in the driving substrate 2. Therefore, even in case the mirror is tilted in the optical switching device, the flatness of the mirror itself can be maintained. Furthermore, since a step is formed on the lower portion of the mirror, not in the mirror itself, an entire surface of the mirror can be used as a reflecting region.

Although the mirror supporting region 200a includes a single layer, i.e., a part of the insulating layer 200, in Fig. 5H, the mirror supporting region 200a may include multilayers 200, 202, 204 and 206 without etching a portion where the mirror is deposited during the etching process illustrated in Figs. 5A to 5G. In this case, the mirror 90 may be formed in a different way from what has been described with reference to Fig. 5H. In other words, the mirror 90 may be formed by patterning a mirror layer deposited on an entire surface of the mirror supporting region 200a, or by depositing the mirror 90 after forming a mask on the mirror supporting region 200a except where the mirror 90 will be formed.

Thereafter, a photo resist (PR) layer (not shown) is formed on the structure illustrated in Fig. 5H. Further, a developing process and a metal deposition process are carried out in order to form an electrode bridge for connecting an actuator to a pad of a driving circuit.

Next, as shown in Fig. 51, a passivation layer 6 is formed on the structure illustrated in Fig. 5H. Further, a part of the driving substrate 2 is selectively removed such that a lower portion of the protection layer 4 is exposed through an opening 8 (see, Fig. 5J) . Finally, as depicted in Fig. 5K, the passivation layer 6 and the' protection layer 4 are etched. In thus formed optical switching device, lower portions of the mirror and the actuator are exposed and, accordingly, operations of the mirror and the actuator are not restricted.

In this embodiment, the optical switching device using the piezoelectric actuator has been described. However, the technology applied to this embodiment can be applied to an MEMS device such as an optical scanner, an optical attenuator, an optical array, an optical motor or the like. Further, the optical switching device in accordance with the present invention may be employed as one element of an M x N array of the optical switching device.

While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

1. A method for fabricating an optical switching device using a first, a second, a third and a fourth micro piezoelectric actuator, the method comprising the steps of: forming a driving substrate including a driving circuit for generating a driving signal; forming a groove for maintaining a flatness of the optical switching device and a mirror on the driving substrate; depositing a protection layer on an upper surface of the driving substrate; depositing a polysilicon layer to bury a stepped portion formed by the groove on the protection layer; planarizing a surface of the deposited polysilicon; depositing an insulating layer on the polysilicon layer; forming a piezoelectric device layer on the insulating layer; forming a piezoelectric layers of the respective actuators by etching the piezoelectric device layer and the insulating layer; forming, by patterning the insulating layer, a mirror supporting layer positioned between the first and the second actuator, a membrane and a connecting part of the respective actuators, a gimbal for supporting the first and the second actuator, a first and a second transmitting part for connecting the first and the second actuator to the mirror supporting region and a third and a fourth transmitting part for connecting the third and the fourth actuator to the gimbal; depositing a mirror on the mirror supporting region; forming a passivation layer on the optical switching device; removing a part of the driving substrate, where corresponds to lower surfaces of the mirror and the actuators; and removing the passivation layer and the protection layer.

2. The method of claim 1, further comprising the step of: performing a heat treatment, after depositing the polysilicon layer, on the deposited polysilicon for about 2 to 3 hours at a temperature higher than about 1000"C.

3. The method of claim 1, wherein the step of planarizing the surface of the polysilicon layer is carried out by a chemical mechanical polishing (CMP) process.

4. The method of claim 1, wherein the piezoelectric device layer has a top electrode, a bottom electrode and a piezoelectric material layer positioned between the top and the bottom electrode.

5. The method of claim 1, wherein the piezoelectric layers of the respective actuators have a top electrode, a bottom electrode and a piezoelectric material layer positioned between the top and the bottom electrode.

6. The method of claim 1, wherein the step of removing a part of the driving substrate is carried out by using one of a dry etching and a wet etching or a combination thereof.

7. The method of claim 1, wherein in the step of forming a groove on the driving substrate, the groove is formed a portion on the driving substrate where the gimbal and the mirror are formed.

8. The method of claim 1, further comprising, after the step of patterning the insulating layer, the steps of: depositing a photo resist (PR) layer on the optical switching device; developing a part of the PR layer, which corresponds to a portion where an electrode bridge for connecting the respective actuators to the driving circuit is formed; depositing a metal on the PR layer; forming the electrode bridge by patterning the metal; and removing the PR layer.