Patent Publication Number: US-2005139577-A1

Title: Microelectromechanical system comb actuator and manufacturing method thereof

Description:
TECHNICAL FIELD  
      The present invention relates to a microelectromechanical system (MEMS), and more particularly, to a MEMS comb actuator materialized in an insulating material and a manufacturing method thereof.  
     BACKGROUND ART  
      Recent rapid development of surface micro-machining technology leads to development of MEMS apparatuses having various functions. MEMS apparatuses have many advantages in terms of size, cost, and reliability and have thus been developed for comprehensive applications.  
      In particular, as an interest in optical communication systems increases, technology concerning optical communication apparatuses or devices widely used in a communication network has been actively developed. With such development of optical communication technology, MEMS apparatuses are increasingly used in order to endow functions to optical communication devices. More specifically, at present, many techniques of materializing a planar lightwave circuit (PLC), i.e., an optical circuit integrated on a substrate, have been developed. These techniques are forming various types of waveguides replacing existing optical fiber in a very small region of a silica or polymer layer formed on a silicon substrate. At an early stage, these techniques were usually used to manufacture an arrayed waveguide grating (AWG), which is an optical device dividing a wavelength and mixing wavelengths in a wavelength division multiplexing (WDM) system. Recently, techniques of manufacturing a combined device by combining an AWB device with functional devices, such as an optical attenuator and an optical switch, have been developed. A MEMS actuator is widely used to drive the optical attenuator and the optical switch.  
       FIG. 1  shows an example of a conventional MEMS comb actuator applied to an optical device. Referring to  FIG. 1 , an optical switch  10  includes a plurality of waveguides  12   a ,  12   b ,  12   c , and  12   d  and a reflective mirror  14 , which is disposed among the plurality of waveguides  12   a ,  12   b ,  12   c , and  12   d  to reflect light transmitted through the waveguides  12   a ,  12   b ,  12   c , and  12   d , thereby changing the traveling path of the light. When the reflective mirror  14  is moved in an arrow direction R and thus displaced from a position among the waveguides  12   a ,  12   b ,  12   c , and  12   d , light from the first waveguide  12   a  is directly incident on the fourth waveguide  12   d , and light from the second waveguide  12   b  is directly incident on the third waveguide  12   c . Conversely, when the reflective mirror  14  is moved in an arrow direction F, light from the first and second waveguides  12   a  and  12   b  is reflected from the reflective mirror  14 , and thus the traveling path of the light is changed toward the third and fourth waveguides  12   c  and  12   d.    
      The rectilinear motion of the reflective mirror  14  is carried out by a MEMS comb actuator  20  combined with the reflective mirror  14 . The MEMS comb actuator  20  includes two combs  22  and  24 , which are electrically is separated from each other. One of the two combs  22  and  24 , for example, the comb  22 , is a stationary comb fixed to a substrate. The other, for example, the comb  24 , is a movable comb separated from the substrate. The movable comb  24  is supported by a spring  28  connected to a post  26  fixed to the substrate.  
      When a voltage is applied to the two combs  22  and  24  structured as described above, the movable comb  24  supported by the spring  28  is pulled down to the fixed comb  22  due to static electricity. However, due to the elasticity of the spring  28 , the movable comb  24  does not closely contact the fixed comb  22  but is separated from the fixed comb  22  by a predetermined gap. When the voltage applied to the two combs  22  and  24  is cut off, the movable comb  24  returns to its original position due to the force of restitution of the spring  28 . With such rectilinear motion of the movable comb  24 , the reflective mirror  14  combined with the movable comb  24  rectilinearly moves in the arrow direction F or R. Here, the moving distance of the movable comb  24  and the reflective mirror  14  can be adjusted by adjusting the magnitude of the voltage applied to the two combs  22  and  24 .  
       FIGS. 2A through 2D  show processes of manufacturing the conventional MEMS comb actuator shown in  FIG. 1 . Referring to  FIG. 2A , the conventional MEMS comb actuator is usually manufactured using a Silicon On Insulator (SOI) wafer  30 , in which an insulating layer  33  is formed between two silicon substrates  31  and  32 . The SOI wafer  30  is manufactured by forming the insulating layer  33  made of silicon oxide on the first silicon substrate  31  and then bonding the second silicon substrate  32  to the insulating layer  33 . Thereafter, as shown in  FIG. 2B , photoresist is deposited on the second silicon substrate  32  and then patterned, thereby forming an etch mask  42 . Next, as shown in  FIG. 2C , the first silicon substrate  32  is etched through the etch mask  42 , thereby forming trenches  44 , and then the etch mask  42  is removed. Next, as shown in  FIG. 2D , the exposed insulating layer  33  made of silicon oxide is etched through the trenches, thereby forming a silicon structure  34  separated from the first silicon substrate  31 .  
      As described above, the conventional MEMS comb actuator is constituted by a conductive silicon structure because in order to apply a voltage to a stationary comb and a movable comb of the MEMS comb actuator, the materials of the stationary and movable combs must have conductivity. In the meantime, as described above, a waveguide is formed on an insulating material layer, such as a silica layer or polymer layer, formed on a silicon substrate. When the material of the MEMS comb actuator is different from that of the waveguide passing light therethrough, it is difficult to integrally construct the MEMS comb actuator and a waveguide portion on a single substrate. Conventionally, therefore, a hybrid technique of forming a functional optical device such as an optical switch driven by the MEMS comb actuator by separately manufacturing the MEMS comb actuator and the waveguide portion and then combining them.  
      However, according to the hybrid technique, manufacturing processes of the MEMS comb actuator and the waveguide portion must be separately carried out, and a process of combining them is additionally needed, so manufacturing cost increases. Moreover, an alignment error may occur when the MEMS comb actuator is combined with the waveguide portion, thereby degrading performance.  
      In the meantime, when optical fiber is used instead of a waveguide, the optical fiber is aligned and combined with the MEMS structure made of silicon. In this case, manufacturing cost also increases due to alignment of the optical fiber, and an alignment error also occurs. In addition, reliability can be decreased as time lapses and temperature changes.  
     DISCLOSURE OF THE INVENTION  
      The present invention provides a microelectromechanical system (MEMS) comb actuator materialized in an insulating material, such as silica or polymer, so that the MEMS comb actuator can be integrally formed with an optical device on a single substrate.  
      The present invention also provides a method of manufacturing a MEMS comb actuator using an insulating material such as silica or polymer.  
      According to an aspect of the present invention, there is provided a MEMS comb actuator including a stationary comb, which is fixed to a substrate; a movable comb, which is separated from the substrate; a post fixed to the substrate; and a spring, which is connected to the post to be separated from the substrate so as to movably support the movable comb. The stationary comb, the movable comb, the post, and the spring are formed in an insulating material layer formed on the substrate, and a metal coating layer having conductivity is formed at least on the surface of the stationary comb and the movable comb.  
      Preferably, the insulating material layer is made of silica or polymer, the metal coating layer is made of one of aluminum and gold, and the substrate is a silicon substrate.  
      The metal coating layer may be formed on the top and side surfaces of each of the stationary comb and the movable comb. Preferably, the metal coating layer formed on the surface of the movable comb extends across the surfaces of the spring and the post.  
      According to another aspect of the present invention, there is provided a method of manufacturing a MEMS comb actuator. The method includes (a) preparing a substrate; (b) forming an insulating material layer having a predetermined thickness on the substrate; and (c) selectively etching the insulating material layer and the substrate, thereby forming a stationary comb fixed to the substrate, a movable comb separated from the substrate, a post fixed to the substrate, and a spring connected to the post to be separated from the substrate so as to movably support the movable comb in the insulating material layer, and forming a metal coating layer having conductivity on the surfaces of the stationary comb and the movable comb.  
      Step (c) includes forming an etch mask on the top of the insulating material layer; etching the insulating material layer exposed through the etch mask, thereby forming trenches; etching the substrate through the trenches to a predetermined depth, thereby forming structures separated from the substrate in the insulating material layer; and forming the metal coating layer.  
      Alternatively, step (c) includes forming an etch mask on the top of the insulating material layer; etching the insulating material layer exposed through the etch mask, thereby forming trenches; forming a metal coating layer at least on the surfaces of portions, which constitute the stationary comb and the movable comb; etching the metal coating layer formed on the bottoms of the trenches to expose the substrate; and etching the substrate to a predetermined depth, thereby forming structures separated from the substrate in the insulating material layer.  
      The insulating material layer may be made of silica. In this case, the insulating material layer can be formed using flame hydroxide deposition (FHD) and can be etched using reactive ion etching (RIE).  
      The insulating material layer may be made of a polymer. In this case, the insulating material layer can be formed using at least one method selected from the group consisting of laminating, spray coating, and spin coating and can be etched using photolithography.  
      The substrate may be etched using wet etch.  
      Preferably, the metal coating layer is made of one of aluminum and gold. In this case, the metal coating layer can be formed using chemical vapor deposition (CVD) or a sputtering process.  
      According to the present invention, a MEMS comb actuator can be integrally formed with an optical device formed in an insulating material, such as silica or polymer, on a single substrate, so totals of manufacturing time and cost are reduced. In addition, an alignment error does not occur.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a plane view of an example of a conventional microelectromechanical system (MEMS) comb actuator applied to an optical device.  
       FIGS. 2A through 2D  are diagrams showing the stages in a method of manufacturing the conventional MEMS comb actuator shown in  FIG. 1 .  
       FIG. 3  is a plane view of a MEMS comb actuator according to a preferred embodiment of the present invention.  
       FIG. 4  is a partial perspective view of the MEMS comb actuator taken to along the line A-A′ shown in  FIG. 3 .  
       FIGS. 5A through 5E  are sectional views of the stages in a method of manufacturing a MEMS comb actuator according to a first preferred embodiment of the present invention, which are taken along the line B-B′ shown in  FIG. 3 .  
       FIGS. 6A and 6B  are sectional views of the stages in a method of manufacturing a MEMS comb actuator according to a second preferred embodiment of the present invention, which are taken along the line B-B′ shown in  FIG. 3 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.  
       FIG. 3  is a plane view of a microelectromechanical system (MEMS) comb actuator according to a preferred embodiment of the present invention.  FIG. 4  is a partial perspective view of the MEMS comb actuator taken along the line A-A′ shown in  FIG. 3 .  
      Referring to  FIGS. 3 and 4 , a MEMS comb actuator  200  according to the present invention is formed on and supported by a silicon substrate  100 . The silicon substrate  100  can be replaced with a substrate, for example, a glass substrate, which is made of an easily processible material. The MEMS comb actuator  200  includes a stationary comb  220 , a movable comb  240 , posts  260 , and springs  280 .  
      The stationary comb  220  is composed of a stationary stage  222  fixed to the silicon substrate  100  and a plurality of stationary fingers  224  protruding from one side of the stationary stage  222  in the shape of the teeth of a comb. The movable comb  240  is separated from the silicon substrate  100  by a predetermined gap to rectilinearly move. The movable comb  240  includes a movable stage  242  and a plurality of movable fingers  244  protruding from one side of the movable stage  242  in the shape of the teeth of a comb to face the stationary fingers  224 . The stationary comb  220  and the movable comb  240  are physically and electrically separated from each other. The stationary fingers  224  and the movable fingers  244  are interlaced with each other with a predetermined gap.  
      The posts  260  are separated from the movable comb  240  and disposed at both sides, respectively, of the movable comb  240 . The posts are fixed to the silicon substrate  100 .  
      A spring  280  is disposed between each of the two posts  260  and the movable comb  240  and separated from the silicon substrate  100 . In other words, the ends of the springs  280  are connected to the respective posts  260 , and the other ends thereof are connected to the respective ends of the movable comb  240 , so that the springs  280  elastically support the movable comb  240 .  
      The stationary comb  220 , the movable comb  240 , the posts  260 , and the springs  280  are formed on an insulating material layer  110  on the silicon substrate  100 . In other words, the MEMS comb actuator  200  of the present invention is made of an insulating material. Various kinds of insulating material can be used, but it is preferable to use silica or polymer usually used to manufacture optical devices.  
      As described above, since the MEMS comb actuator  200  of the present invention is made of an insulating material such as silica, conductive metal coating layers  150   a  and  150   b  are formed at least on the surfaces of the respective stationary and movable combs  220  and  240  in order to apply a voltage to the stationary comb  220  and the movable comb  240 . The metal coating layers  150   a  and  150   b  can be made of any conductive metal, but it is preferable to use aluminum or gold frequently used in semiconductor manufacturing processes. As shown in  FIG. 4 , the metal coating layers  150   a  and  150   b  can be formed on the top and side surfaces of the stationary comb  220  and the movable comb  240 . The metal coating layers  150   a  and  150   b  are electrically connected to a bonding pad (not shown).  
      The metal coating layers  150   a  and  150   b  can be formed only on the surfaces of the stationary comb  220  and the movable comb  240 . In this case, the metal coating layer  150   b  formed on the surface of the movable comb  240  is connected to the bonding pad through a wire (not shown), so the wire may snap due to the rectilinear movement of the movable comb  240 . Accordingly, as shown in  FIG. 3 , it is preferable that the metal coating layer  150   b  formed on the surface of the movable comb  240  extends across the surfaces of the springs  280  and the posts  260 . Here, the wire can be connected to a portion of the metal coating layer  150 , which is formed on the surface of the posts  260  and thus does not move. In addition, the stationary stage  222  of the stationary comb  220  fixed to the silicon substrate  100  and the posts  260  fixed to the silicon substrate  100  can be defined by the metal coating layers  150   a  and  150   b , respectively, formed on their surfaces.  
      In operation of the MEMS comb actuator  200  having the above-described structure according to the present invention, when a voltage is applied to the metal coating layers  150   a  and  150   b  formed on the surfaces of the stationary comb  220  and the movable comb  240 , electrostatic power is generated between the metal coating layers  150   a  and  150   b , and thus the movable comb  240  is drawn to the stationary comb  220 . Here, the moving distance of the movable comb  240  can be adjusted by controlling the elasticity of the springs  280  and the magnitude of the voltage applied to the metal coating layers  150   a  and  150   b . When the voltage applied to the metal coating layers  150   a  and  150   b  is cut off, the movable comb  240  returns to its original position due to the force of restitution of the springs  280 .  
      As described above, although the MEMS comb actuator  200  of the present invention is made of an insulating material, such as silica or polymer, it can satisfactorily perform its function due to the metal coating layers  150   a  and  150   b . Accordingly, the MEMS comb actuator  200  can be integrally formed with an optical device formed on an insulating material, such as a polymer or silica, on a single substrate.  
      The following description concerns preferred embodiments of a method of manufacturing a MEMS comb actuator having the above-described structure according to the present invention.  
       FIGS. 5A through 5E  are sectional views of the stages in a method of manufacturing a MEMS comb actuator according to a first preferred embodiment of the present invention, which are taken along the line B-B′ shown in  FIG. 3 .  
      Referring to  FIG. 5A , in the first embodiment, a silicon substrate  100  is prepared as a substrate supporting an MEMS comb actuator. Although a glass substrate instead of the silicon substrate  100  can be used, it is more effective for mass production to use the silicon substrate  100  since a silicon wafer widely used in manufacturing semiconductor devices can be used.  
      In the meantime,  FIG. 5A  shows only a part of a silicon wafer. Several tens through several hundreds of MEMS comb actuators according to the present invention can be formed on a single wafer in the form of chips.  
      Thereafter, an insulating material layer, for example, a silica layer  110 , is formed on the top of the prepared silicon substrate  100  to a predetermined thickness. As described above, the insulating material layer can be formed of other insulating material, for example, a polymer, than silica. Hereinafter, it is assumed that the insulating material layer is the silica layer  110  made of silicon oxide, for example, SiO 2 . More specifically, the silica layer  110  can be formed to have a thickness of about 40 μm using chemical vapor deposition (CVD) or flame hydrolysis deposition (FHD). It is preferable to use FHD, which is more advantageous in forming a relatively thick material layer.  
      In the meantime, when a polymer layer instead of the silica layer  110  is used as the insulating material layer, the polymer layer can be formed to a thickness of about 40 μm on the silicon substrate  100  using a method such as laminating, spray coating, or spin coating.  
      Next, referring to  FIG. 5B , an etch mask  120  is formed on the top of the silica layer  110 . The etch mask  120  can be formed by depositing photoresist on the top of the silica layer  110  and then patterning the photoresist.  
      Subsequently, the silica layer  110  exposed through the etch mask  120  is etched, thereby forming trenches  130 , as shown in  FIG. 5C . The silica layer  110  can be etched using dry etching such as reactive ion etching (RIE).  
      In the meantime, when the polymer layer instead of the silica layer  110  is used as the material layer, the structure shown in  FIG. 5C  can be formed using photolithography.  
      Next, referring to  FIG. 5D , the silicon substrate  100  exposed through the trenches  130  is etched to a predetermined depth. More specifically, the silicon substrate  100  is wet etched to a thickness of about 5-10 μm using a silicon etchant, for example, tetramethyl ammonium hydroxide (TMAH) or KOH. As a result, silica structures  112  separated from the silicon substrate  100  are formed, as shown in  FIG. 5D . Here, each silica structure  112  has a thickness of about 5 μm and a height of about 40 μm. The silica structures  112  are separated from one another by a distance of about 3-5 μm.  
      The silica structures  112  constitute the movable stage  242  and the movable fingers  244  of the movable comb  240  shown in  FIG. 3  and a part of the stationary comb  220 , i.e., the stationary fingers  224 , shown in  FIG. 3 . Although not shown in  FIG. 5D , the springs  280  shown in  FIG. 3  are formed using such silica structures described above.  
      In  FIG. 5D , silica layer portions  110 ′ remaining on the silicon substrate  100  form the posts  260  shown in  FIG. 3 . Although not shown in  FIG. 5D , the stationary stage  222  of the stationary comb  220  shown in  FIG. 3  is formed using such remaining portions of the silica layer  110  as described above.  
      Referring to  FIG. 5E , a metal coating layer  150  having conductivity is formed on the surface of the resultant structure shown in  FIG. 5D . More specifically, the metal coating layer  150  can be formed by depositing aluminum or gold on the surfaces of the remaining silica layer  110 ′ and the silica structures  112  to a thickness of about 0.5 μm using a CVD or sputtering process.  
      It is preferable to form the metal coating layer  150  only on the top and side surfaces of the remaining silica layer  1101  and the silica structures  112 . Although the metal coating layer  150  can be formed only on the surfaces of portions constituting the stationary comb  220  of  FIG. 3  and the movable comb  240 , it is preferable to additionally form the metal coating layer  150  on the surfaces of portions constitute the springs  280  and the posts  260 . As described above, this metal coating layer  150  can define the stationary stage  222  of the stationary comb  220  and the posts  260 .  
       FIGS. 6A and 6B  are sectional views of the stages in a method of manufacturing a MEMS comb actuator according to a second preferred embodiment of the present invention, which are taken along the line B-B′ shown in  FIG. 3 . In the second embodiment, the same stages as those of the first embodiment shown in  FIGS. 5A through 5C  are performed, and thus a description thereof will be omitted.  
      After forming the trenches  130  by etching the silica layer  110  on the silicon substrate  100  in the stage shown in  FIG. 5C , the metal coating layer  150  is formed on the surface of the resultant structure, as shown in  FIG. 6A . The metal coating layer  150  is formed on the same portions and in the same manner as in the first embodiment.  
      Thereafter, as shown in  FIG. 6B , the metal coating layer  150  formed on the bottom of the trenches  130  is etched, thereby exposing the silicon substrate  100 . Then, the silicon substrate  100  is etched to a predetermined depth, thereby forming the same structure as shown in  FIG. 5E . The silicon substrate  100  is etched using the same etching method as that used in the first embodiment.  
      As described above, the manufacturing method according to the second embodiment of the present invention is almost the same as that according to the first embodiment of the present invention, with the exception that the metal coating layer  150  is formed before the silicon substrate  100  is etched.  
      According to a manufacturing method of the present invention, a MEMS comb actuator can be materialized in an insulating material, such as silica or polymer. Consequently, the MEMS comb actuator can be integrally formed with an optical device on a single substrate.  
      While this invention has been particularly shown and described with reference to preferred embodiments thereof, the preferred embodiments should be considered in descriptive sense only, and it will be understood by those skilled in the art that various changes in form and details may be made therein. For example, a MEMS comb actuator of the present invention can be made using various insulating materials in addition to silica and polymer. Instead of silicon, other easily processible materials can be used to make a substrate. In addition, in depositing and etching each layer, various deposition and etching methods not mentioned in the above-described embodiments can be used. The specific numerical values suggested in the description of the manufacturing methods can be freely adjusted within a range allowing a manufactured MEMS comb actuator to normally operate. Moreover, a MEMS comb actuator according to the present invention can be various technological fields as well as the field of optical communication including an optical switch and optical attenuator. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims.  
      Industrial Applicability  
      As described above, according to the present invention, a MEMS comb actuator can be materialized in an insulating material, such as silica or polymer and thus can be integrally formed with an optical device formed in the insulating material on a single substrate. Therefore, a conventional process of separately manufacturing a MEMS comb actuator and an optical device part and combining them is not necessary, so totals of manufacturing time and cost are reduced. In addition, an alignment error does not occur. Consequently, high reliability of a functional optical device driven by a MEMS comb actuator can be achieved, and a competitive price can be secured.