Abstract:
An electrostatic movable micro mirror chip includes an upper mirror plate and a lower electrode plate positioned and jointed together via pairs of fitting solder and positioning grooves. It improves the optical quality of the mirror chip by a lower joining temperature. The fitting and jointing achieves easy positioning and interconnection. The fabrication time and cost is less. The mirror in the mirror chip is plated with metallic coating on both sides so as to balance the stress and improve its flatness.

Description:
FIELD OF THE INVENTION 
   The invention generally relates to an electrostatic movable micro mirror chip applicable to optical communication or optical display industry, and in particular relates to an electrostatic movable micro mirror chip in which solder is used for positioning and jointing. 
   BACKGROUND OF THE INVENTION 
   In the optical communication industry, requirements of optical components are getting more and more severe. The optical devices suppliers have to provide smaller while cheaper components to meet customer&#39;s requirements. By applying semiconductor manufacturing process and other machining processes, kinds of high-precision and high-quality micro mechanical elements can be made in a mass production scale. 
   Movable micro mirror chip is a kind of micro component applicable in optical communication or display modules. The micro mirror chip mainly includes a mirror layer and an actuation layer. The mirror layer is a suspended mirror actuated to swing or revolve by electrostatic force of the actuation layer. The micro mirror chip works as an attenuator in optical communication by steering the light beam direction. The biggest challenge in fabrication of a conventional movable micro mirror chip is the positioning alignment of the mirror layer and the actuation layer. The alignment process requires operations of hands or specific positioning machines that cause a high manufacturing cost very difficult to be reduced. 
   U.S. Pat. No. 6,442,307 discloses a micro mirror device, in which the mirror layer and the actuation layer are jointed together by solder joints. Though the construction solves conduction and jointing problem, the solder joint does not provide precise positioning. Therefore, a spacer is user for the positioning function. However, when the solder melts, it loses its form. In order to stabilize the jointing and to remove the spacer after soldering, special materials for the spacer is required that makes the material selection very difficult. Also, the height of the solder layers is hard to be controlled after the large temperature variation. Moreover, since the mirror layer and the actuation layer all require electrical circuit to operate, there are wire-bonding areas on the layers that are easy to be stained by re-flow solder because there is no protection manner. The melted solder may stain the bonding areas and make the afterward wire bonding impossible, or even ruin the whole chip and increase fabrication cost of the product. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide an electrostatic movable micro mirror chip that has good performance, easy packaging, good positioning during jointing, and well protection to the signal connection area for preventing it from solder stain. 
   An electrostatic movable micro mirror chip according to the invention includes an upper mirror plate and a lower electrode plate. The upper mirror plate carries a mirror that is supported by a cantilever beam for swinging relative to the upper mirror plate; or supported by two arms for revolutions relative to the upper mirror plate. The lower electrode plate is formed a cavity for the mirror freely moving therein. The cavity is arranged with electrodes for actuating the mirror through electrostatic force. 
   There are fitting solder and positioning grooves formed correspondingly on the upper mirror plate and the lower electrode plate for positioning with each other and being melted and fixed by heat. There is a shrinking pipe or a stopping wall located between the fitting solder and the signal connection area so as to prevent the signal connection area from solder stain during soldering. 
   Both sides of the mirror are coated with metallic layers for good reflection, interconnection and stress-balance for preventing it from deformation. The electrostatic movable micro mirror chip of the invention utilizes fitting solder and positioning grooves for both positioning and jointing. It reduces manufacturing cost and saves time. The signal connection area is prevented from solder stain by means of a shrinking pipe or a stopping wall located between the fitting solder and the signal connection area. The mirror is both-side coated for eliminating any stress generated by construction, material and temperature process and enhancing flatness of the mirror. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description given hereinbelow. However, this description is for purposes of illustration only, and thus is not limitative of the invention, wherein: 
       FIG. 1A  is an exploded view of an electrostatic movable micro mirror chip of the invention; 
       FIG. 1B  is a perspective view of an electrostatic movable micro mirror chip of the invention; 
       FIGS. 2A to 2F  are sequential process views of making an upper mirror plate in the invention; 
       FIG. 2G  is an addition process view of making an upper mirror plate in the invention; 
       FIGS. 3A to 3E  are sequential process views of making a lower electrode plate in the invention; 
       FIGS. 3F and 3G  are sequential process views of a second embodiment in making a lower electrode plate of the invention; 
       FIGS. 4A to 4C  are sequential process views of jointing a mirror plate and an electrode plate in the invention; 
       FIGS. 5A and 5B  are functional views of the fitting solder and the positioning grove in the invention; 
       FIG. 6  is a descriptive view of another embodiment of the invention; and 
       FIGS. 7A ,  7 B are descriptive views of protection for signal connection area in the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIGS. 1A and 1B , an electrostatic movable micro mirror chip according to the invention includes an upper mirror plate  10  and a lower electrode plate  20 . The middle portion of the upper mirror plate  10  carries a mirror  11  that is supported by two arms  12 ,  13  for revolutions relative to the upper mirror plate  10 . The upper mirror plate  10  has a top surface  102  and a bottom surface  101 . The top surface  102  is formed with a cavity  16 ,  21  corresponding to the mirror  11 . The mirror  11  has a reflective surface facing up. The top surface  102  is formed with two positioning grooves  14 ,  15  located preferably on both sides of the mirror  11 . 
   The lower electrode plate  20  has its top surface  201  jointed with the bottom surface  101  of the upper mirror plate  10 . The top surface  210 , corresponding to the mirror  11 , is formed with a cavity  16 ,  21  for the mirror  11  freely moving therein. Both sides of the cavity  16 ,  21 , corresponding to the positioning grooves  14 ,  15 , are formed with fitting solder  23 ,  22  having correspondent shapes of the positioning groove  14 ,  15 . There is a solder seed layer at the root of the fitting solder. The solder seed layer of the fitting solder has a little bit larger area than the fitting solder. Therefore, upon soldering, the fitting solder  22 ,  23  are fitted into the positioning grooves  14 ,  15  and then heated to joint the upper mirror plate  10  and the lower electrode plate  20  together. 
   The fabrication process of the upper mirror plate  10  is as follows. First, preparing a substrate  30 , as shown in  FIG. 2A . The substrate  30  is usually of silicon on insulator (SOI). Then, etching to form a cavity  16  (as shown in  FIGS. 2B and 2C ) from the bottom and etching to form the suspended mirror  11 , the arms  12 ,  13  and the positioning grooves  14 ,  15  from the top (as shown in  FIGS. 2D and 2E ). Finally, plate a metallic coating  111  on the mirror  11  from the bottom (as shown in  FIG. 2F ). The mirror  11  has a thickness less than the upper mirror plate  10 . 
   The fabrication process of the lower mirror plate  20  is as follows. First, preparing a substrate, as shown in  FIG. 3A . Then, forming the groove  21  and the signal connection area  25 , as shown in  FIGS. 3B and 3C . The signal connection area  25  and the actuation manner will be described later. Furthermore, arranging metallic electrodes  41  (as shown in  FIG. 3D ) and the solder  22 ,  23  (as shown in  FIG. 3E ). 
   When jointing, as shown in  FIG. 4A , turning over the upper mirror plate  10  to let the cavity  16  and the metallic coating  111  faces up. Then, positioning the positioning grooves  14 ,  15  to the fitting solder  23 ,  22 , and heating to solder and fix them, as shown in  FIG. 1B . The process simultaneously solves the problems of positioning and jointing. The actuation is achieved by applying electrical voltage to the signal connection area  25  and the cavity  21  through wire bonding (as shown in  FIG. 1A ). The upper mirror plate  10  is actuated by directly applying voltage to the metallic coating  111  of the mirror  11  (as shown in  FIG. 4A ). When the electrical voltage being applied to the mirror  11  and the cavity  21 , the mirror  11  is attracted to rotate by the arms  12 ,  13  (as shown in  FIG. 4B ). However, in this arrangement, the electrode of the upper mirror plate  10  is located on top of the mirror  11 , as shown in  FIG. 4C . If wiring or other method is applied to the mirror  11 , it is hard to process and will interfere the movement of the mirror  11 . Therefore, both sides of the mirror  11  are processed with metallic coatings  111 ,  112 , as shown in  FIG. 2G . After soldering, the signal connection area  24  of the lower electrode plate  20  and the metallic coating  112  of the mirror  11  are electrically connected to actuate the chip through the signal connection area  24  and no need of wiring to the mirror  11 . The both-side coating also gets an advantage of stress balance. Because the fitting solder  22 ,  23  require thermal welding, the chip also requires temperature variation tests; these temperature variations usually cause stress (such as bending) to the mirror  11  and influence the flatness of the mirror  11  due to the different material of the mirror substrate and the coating  111 . Therefore, when coating at both sides  111 ,  112 , not only a lower actuation voltage can operate the chip, the materials, such as gold, silver, titanium, chromium and aluminum, for both sides of the mirror is chosen with the same; or different materials while compensated by thickness, so as to keep the mirror flat. Moreover, because the upper mirror plate  10  is turned over for jointing, the metallic coating  112  of the mirror  11  is almost at the same height of the bottom surface  101  of the upper mirror plate  10 . Therefore, the actuation height is decided by the depth of the cavity  21 . In comparison to construction of prior arts, the depth is easier to be controlled. As for the solder material, through the invention can balance the stress, it is still preferred of using solder of lower melting temperature, such as the solder model In52/Sn48 melted at 118 centigrade degree, model In97/Ag3 at 146 centigrade degree, model In80/Pb15/Ag5 at 149 to 154 centigrade degree and so on. 
   Because of the both side metallic coating  111 ,  112  of the mirror  11 , the manner of actuation can also be adjusted. As shown in  FIGS. 1A and 1B , there is another signal connection area  24  formed near the fitting  22 . And, when plating the metallic coating  112 , the bottom surface  101  of the upper mirror plate  10  is also plated with metallic coating as shown in  FIG. 2E . Therefore, after soldering, the signal connection area  24  formed on the lower electrode plate  20  is used to actuate. Because the upper mirror plate  10  is also externally actuated via the lower electrode plate  20 , the problem of uneasy actuation of the metallic coating  111  of the mirror  11  on the upper mirror plate  10  is solved. 
   However, since both sides of the mirror  11  have metallic coating  111 ,  112  as shown in  FIG. 4C , when revolving the mirror  11 , electrical short by contact of the mirror  11  to the cavity  21  has to be prevented. Therefore, there is at least an insulation stud  211  (as shown in  FIG. 3F ) or insulation layer  26  (as shown in  FIG. 3G ) formed on bottom of the cavity  21  for preventing electrical short. It is preferable to have a plurality of insulation studs at a same side, for example, as shown in  FIG. 1A , three insulation studs  211 ,  212 ,  213  are formed. Of course, an array of studs (not shown in the drawing) can be used. 
   Since the aforesaid positioning grooves  14 ,  15  and fitting solder  22 ,  23  are used for positioning and jointing; their positions can be flexibly adjusted. For example, arranging a positioning groove  15  on the lower electrode plate  20  and arranging the fitting solder  22  on the upper mirror plate  10  (as shown in  FIG. 5A ); arranging both positioning grooves  14 ,  15  on the lower electrode plate  20  and arranging the fitting solder  22 ,  23  on the upper mirror plate  10  (as shown in  FIG. 5B ), or any other number and shape of the positioning grooves and fitting solder can be arranged to get the same effect. Further, the fitting solder can be an electrically conductive resin. 
   The invention is not only applicable to revolvable mirror with two arms  12 ,  13 , as shown in  FIG. 1A , but also applicable to swinging mirror with a cantilever  18  as shown in  FIG. 6 . On the other hand, the methods of prevention of solder stain from the fitting solder  22 ,  23  to the signal connection area  24 ,  25  are shown in  FIGS. 7A and 7B . The fitting solder  23  at the left side is located on a different level to the signal connection area  25  so that it is only required to keep the seed layer  231  of the solder  23  a suitable distance from the signal connection area  25 . The fitting solder  22  at the right side connects to the signal connection area  24  via a shrinking pipe  222 . A stopping wall  27  is further formed between them. The stopping wall  27  is made of material not adhesive to the solder. The shrinking pipe  222 , in accompany with the stopping wall  27 , restrains the flow of melted solder for preventing it from flowing into the signal connection area  24 . A correspondent groove  17  is formed for receiving and positioning the stopping wall  27 . 
   The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.