Abstract:
A large micro-mirror, e.g. 3 mm by 4 mm, in accordance with the present invention has sufficient rigidity to ensure a low mirror curvature, e.g. a radius of curvature greater than 5 meters, and a low mass in order to ensure a high oscillation frequency, e.g. greater than 1000 Hz. A method of making the micro-mirror utilizes bulk micro-machining technology, which enables the manufacture of a honeycomb structure sandwiched between two solid and smooth silicon layers without any indentations or holes. The honeycomb sandwich structure provides the rigidity and low mass needed to obtain a micro-mirror with a low mirror curvature and high resonant frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. Patent Application No. 60/807,658 filed Jul. 18, 2006, which is incorporated herein by reference. 
     
     TECHNICAL FIELD 
       [0002]    The present invention relates to a micro-electro-mechanical (MEMS) device, and in particular to a MEMS device including a tilting platform having a sandwiched structure with a closed cellular core. 
       BACKGROUND OF THE INVENTION 
       [0003]    Conventional MEMs mirrors for use in optical switches, such as the one disclosed in U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 to Buzzetta et al, redirect beams of light to one of a plurality of output ports, and include an electro-statically controlled mirror pivotable about a single axis. Tilting MEMs mirrors, such as the ones disclosed in U.S. Pat. No 6,491,404 issued Dec. 10, 2002 in the name of Edward Hill, and United States Patent Publication No. 2003/0052569, published Mar. 20, 2003 in the name of Dhuler et al, which are incorporated herein by reference, comprise a mirror pivotable about a central longitudinal axis. The MEMs mirror device  1 , disclosed in the aforementioned Hill patent, is illustrated in  FIG. 1 , and includes a rectangular planar surface  2  pivotally mounted by torsional hinges  4  and  5  to anchor posts  7  and  8 , respectively, above a substrate  9 . The torsional hinges may take the form of serpentine hinges, which are disclosed in U.S. Pat. No. 6,327,855 issued Dec. 11, 2001 in the name of Hill et al, and in United States Patent Publication No. 2002/0126455 published Sep. 12, 2002 in the name of Robert Wood, which are incorporated herein by reference. 
         [0004]    One of the main challenges facing MEMS designers of larger sized mirrored platforms, e.g. 2 mm to 3 mm in length, is the conflicting requirement of high mirror resonance frequency and low stress-induced mirror curvature. The former demands a relatively thin light mirror, while the latter requires a relatively thick structure. When the mirror is too thin, the reflective surfaces will have excessive curvature induced by the stresses in the reflective coatings or internal stresses in the mirror itself, which results in excessive optical coupling losses. However, making the mirror too thick makes it heavy, thereby lowering the resonant frequency for a given hinge stiffness. Moreover, increasing the hinge stiffness to compensate for a heavy mirror would require too high a voltage to drive the mirror electrostatically to the desired angle. 
         [0005]    U.S. Pat. No. 6,791,730 issued Sep. 14, 2004 to Sniegowski et al discloses a micro-mirror structure including stiffening ribs or rails between upper and lower layers. The structure disclosed in the Sniegowski et al reference is realized using surface micro-machining processes, which are generally limited to the manufacture of relatively small mirrors. Many optical switching applications require large area mirrors tilting to a relatively high angle, thereby requiring a large swing space underneath, which is difficult to achieve using surface micro-machining. Moreover, when closed cells are used as the stiffening members, access holes have to be etched on the optically active upper layer to allow for the removal of any sacrificial layers that are disposed therebetween, resulting in a plurality of dimples or the like formed in an upper surface of the upper layer, which have an adverse impact on the optical performance capabilities thereof, thereby making the mirror unacceptable in many applications. 
         [0006]    An object of the present invention is to overcome the shortcomings of the prior art by providing a sandwich structure including upper and lower smooth and solid skins, and a closed cellular core to minimize curvature and maximize resonance frequency. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the present invention relates to a micro-mirror device comprising: 
         [0008]    a mirrored platform including a sandwich structure having an upper uniform, smooth and uninterrupted layer; a core layer having a closed cellular structure; and a lower uniform, smooth and uninterrupted layer; 
         [0009]    a hinge structure enabling the mirrored platform to rotate about an axis of rotation above a substrate; and 
         [0010]    attracting means for rotating the mirrored platform about the axis of rotation. 
         [0011]    Another aspect of the present invention relates to a method of manufacturing the micro-mirror device, comprising the steps of: 
         [0012]    a) providing a first semiconductor on insulator structure including a first semiconductor layer, a first insulator layer, and a second semiconductor layer; 
         [0013]    b) etching the second semiconductor layer to form a core layer having closed cellular structures; 
         [0014]    c) providing a second semiconductor on insulator structure including a third semiconductor layer and a second insulator layer; 
         [0015]    d) bonding the second semiconductor on insulator structure to the first semiconductor on insulator structure forming a sandwich structure having an upper uniform, smooth and uninterrupted skin layer, and a lower uniform, smooth and uninterrupted skin layer, with the core layer therebetween; 
         [0016]    e) providing a substrate with an electrode mounted thereon; 
         [0017]    f) mounting the sandwich structure on the substrate above the electrode; and 
         [0018]    g) etching the sandwich structure to form hinges and a rotatable platform. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein: 
           [0020]      FIG. 1  is an isometric view of a conventional micro-mirror structure; 
           [0021]      FIG. 2   a  is an isometric view of the large micro-mirror structure in accordance with the present invention; 
           [0022]      FIG. 2   b  is a partly sectioned isometric view of the large micro-mirror structure of  FIG. 2   a;    
           [0023]      FIG. 3  is a cross-sectional view of the micro-mirror structure of  FIG. 2   a;    
           [0024]      FIG. 4  is an isometric view of a portion of the core layer of the micro-mirror structure of  FIG. 2   a;    
           [0025]      FIG. 5  is an isometric view of a portion of the core layer of the micro-mirror structure of  FIG. 2   a;    
           [0026]      FIGS. 6   a  to  6   g  illustrate a process of manufacturing the micro-mirror structure of  FIG. 2   a;    
           [0027]      FIGS. 7   a  to  7   g  illustrate an alternative embodiment of the process of  FIGS. 6   a  to  6   g  for manufacturing the micro-mirror structure of  FIG. 2   a ; and 
           [0028]      FIGS. 8   a  to  8   g  illustrate an alternative embodiment of the process of  FIGS. 6   a  to  6   g  for manufacturing the micro-mirror structure of  FIG. 2   a;    
       
    
    
     DETAILED DESCRIPTION 
       [0029]    With reference to  FIGS. 2   a  and  2   b , a micro-electro-mechanical (MEMS) mirror device  11  with a relatively large mirrored platform  12 , e.g. 3 mm×4 mm, includes symmetric torsional hinges  13  and  14  enabling the mirrored platform  12  to pivot about a horizontal axis X above a substrate  15 . The hinges  13  and  14  may be serpentine hinges, as hereinbefore discussed. One or more electro-static electrodes  16 , or some other means of attracting or repulsing one side of the mirrored platform  12  towards or away from the substrate  15 , are provided beneath the mirrored platform  12  for controlling the angular position thereof. The mirrored platform  12  tilts towards the substrate  15  when a voltage is applied to one of the electrodes  16  with respect to the grounded mirrored platform  12  due to the electrostatic force of attraction. Spring forces in the hinges  13  and  14  can restore the mirrored platform  12  to a horizontal position. A second electrode  16  can be provided for mirror actuation in the opposing direction, if bi-direction actuation is desired. The mirrored platform  12  is coated with a reflective metallic layer  17 , e.g. gold or aluminum, for redirecting beams of light incident thereon 
         [0030]    In accordance with the present invention, and with reference to  FIGS. 3 to 5 , the mirrored platforms  12  are comprised of a sandwich structure  71  including of a light core  72 , with closed cells  73 , which are preferably regular hexagonal as shown in  FIGS. 5 and 6 , and upper and lower stiff outer skins  74  and  75  symmetrically attached to both sides of the core  72 . The curvature stability is provided by the two solid skins  74  and  75 , separated by the relatively thick core  72 , which gives rise to a large second moment of area and hence high bending resistance. Practically negligible curvature, e.g. approximately 0.2 m −1  with a radius of curvature of approximately 5 m, can be achieved, while achieving a resonance frequency of 1000 Hz without making the mirror hinges  13  and  14  becoming too stiff to be actuated to the required angle at acceptable voltage levels, for a mirror size of 2 mm to 3 mm. 
         [0031]    Preferably each closed cell is hexagonal in shape (see  FIG. 5 ) with walls that are 4 to 6 um thick, 40 to 60 um long, and 40 um to 60 um high; however, other closed cellular shapes are also possible including square, rectangular, pentagonal etc. Ideally, each of the upper and lower outer skins  74  and  75  are 4 to 6 um thick, e.g. one tenth to one fifteenth the thickness of the core  72 . To ensure the proper optical performance the upper skin  74  has an upper surface that is solid and smooth without any indentations or holes. 
         [0032]    The first and second torsional hinges  13  and  14  may be made of any one or more of the upper and lower stiff outer skins  74  and  75  and the core layer  72 , but preferably is contiguous with the core layer  72 , as the other two layers  74  and  75  may be too thin and weak in bending. In a preferred embodiment, the bottom and top skins  74  and  75  are removed in the areas of the hinge  13  and  14 . The hinges  13  and  14  may also be made contiguous with a combination of the upper and lower stiff outer skins  74  and  75  and the core layer  72 . 
         [0033]    The mirrored platform  12  is made relatively thick and light by having a cellular structure as a thick core  72 . The cells  73  may be designed to have core density of &lt;10% of the bulk density, thereby reducing the mass of the sandwich structure  71  drastically, and hence the mass moment of inertia for torsional micro-mirror applications may be reduced drastically, which enables relatively high resonance frequency for a given hinge spring constant. The thick core  72  provides a relatively large second moment of area, therefore a high bending resistance and greater control over stress-induced curvature. Furthermore, both the upper and lower outer skins  74  and  75  form solid, smooth, flat, contiguous, uniform, uninterrupted and undisturbed surfaces, free from holes, dimples or other irregularities. 
         [0034]    A typical process is illustrated in  FIGS. 6   a  to  6   g , in which a first SOI structure  101  is provided, including an first silicon layer  102  providing a handle, a first oxide etch stop layer  103 , e.g. silicon oxide, forming the lower skin  75 , and an second silicon layer  104  forming the core  72 . In the second step ( FIG. 6   b ) the second silicon layer  104  is patterned and etched down to the first oxide layer  103  forming the core layer  72  with cells  73 . The second silicon layer  104  also includes wing sections on either side thereof for forming the hinges  13  and  14 , as well as a cap for mounting on the substrate  15 . In the next step, illustrated in  FIG. 6   c , a second SOI structure  105 , including a third silicon layer  106 , e.g. a silicon wafer, with a second oxide layer  107 , e.g. silicon oxide, thermally (or by some other method) grown thereon, is bonded, e.g. fusion bonded, to the second silicon layer  104  of the first SOI structure  101 . In the next step ( FIG. 6   d ) one of the handle layers, i.e. first silicon layer  102 , is removed, e.g. etched away, along with portions of the first oxide stop layer  103 , to define the mirrored platform structure  12  in which the first and second oxide layers  103  and  107  form the upper and lower skins  74  and  75 , respectively. The purpose of the remaining handle layer  106  is to provide ruggedness for handling the wafer during further processing of the sandwich mirror, e.g. bonding to a substrate during device construction. 
         [0035]    In finishing steps ( FIGS. 6   e  to  6   g ) a wafer  108  is provided defining the substrate  15  with a recess  109  surrounded by supporting walls  110 , providing the necessary clearance for the mirrored platform  12 . The electrodes  16  are patterned on the lower surface of the recess  109 . The substrate  15  may be comprised of glass (pyrex®), silicon or other suitable material. For silicon substrates an oxide layer (not shown) may be used to electrically isolate the mirror layer  17  from the substrate  15 . The cap sections surrounding the mirrored platforms  12  of the honeycomb sandwich, from  FIG. 6   d , is then bonded to the walls  110  of the substrate  15  ( FIG. 6   f ). The final steps are illustrated in  FIG. 6   g , wherein: i) the honeycomb handle wafer, e.g. third silicon layer  106 , is removed, ii) the reflective metal layer  17  is deposited on the top of the upper skin  74 , e.g. the second oxide layer  107 , and iii) deep reactive ion etching (DRIE) is performed to fabricate the hinges  13  and  14  and float the mirrored platform  12  above the substrate  15 . Preferably, the first and second torsional hinges  13  and  14  are etched into only the second silicon layer  104 , i.e. the core layer  72 , (as in  FIG. 3 ); however, the first and second hinges can be formed from the second silicon layer  104 , i.e. the core layer  72 , and the first or second oxide layers  103  and  107 , i.e. the lower and upper skin layers  75  and  74 , respectively, or into all three of the second silicon layer  104 , i.e. the core layer  72 , and the first and second oxide layers  103  and  107 , i.e. the upper and lower skin layers  74  and  75 . 
         [0036]      FIGS. 7   a  to  7   g  illustrate another process in which a first SOI structure  111  is provided, including an first silicon layer  112  providing a handle, a first oxide etch stop layer  113 , and a second silicon layer  114  forming the core  72  and the lower skin  74 . In the second step, illustrated in  FIG. 7   b , the second silicon layer  114  is etched down to form the core layer  72  with cells  73 , while leaving a thin layer, e.g. 3 to 6 um, of the second silicon layer  114  for the lower skin  75 . The second silicon layer  114  also includes wing sections on either side thereof for forming the hinges  13  and  14 , as well as a cap for mounting on the substrate  15 . A second SOI structure  115  is provided in the next step ( FIG. 7   c ) including a thin, e.g. 3 um to 6 um, third silicon layer  116  forming the upper skin  74 , a second etch stop oxide layer  117 , and a fourth silicon layer  118  providing a handle. The second SOI structure  115  is bonded, e.g. fusion bonded, onto the second silicon layer  114  of the first SOI structure  111 , whereby the third silicon layer  116  is adjacent to the second silicon layer  114 . In the final step ( FIG. 7   d ) one of the handle layers, i.e. the first silicon layer  112 , with the corresponding oxide layer, e.g. the first oxide layers  113 , is removed, e.g. etched away, to define the mirrored platform structure  12  in which the third silicon layer  116  and the thin layer of the second silicon layer  114  form the upper and lower skins  74  and  75 , respectively. 
         [0037]    As above, in the finishing steps ( FIGS. 7   e  to  7   g ) a wafer  108  is provided defining the substrate  15  with a recess  109  surrounded by supporting walls  110 , providing the necessary clearance for the mirrored platform  12 . The electrodes  16  are patterned on the lower surface of the recess  109 . The substrate  15  may be comprised of glass (pyrex®), silicon or other suitable material. The cap sections surrounding the mirrored platforms  12  of the honeycomb sandwich, from  FIG. 7   d , is then bonded to the walls  110  of the substrate  15  ( FIG. 7   f ). The final steps are illustrated in  FIG. 7   g , wherein: i) the honeycomb handle wafer, e.g. fourth silicon layer  118  and the second oxide layer  117 , is removed, ii) the reflective metal layer  17  is deposited on the top of the upper skin  74 , e.g. the third silicon layer  116 , and iii) deep reactive ion etching (DRIE) is performed to fabricate the hinges  13  and  14  and float the mirrored platform  12  above the substrate  15 . Preferably, the first and second torsional hinges  13  and  14  are etched into only the second silicon layer  114 , i.e. the core layer  72 , (as in  FIG. 3 ); however, the first and second hinges can be formed from the second silicon layer  114 , i.e. the core layer  72 , and the second or third silicon layers  114  or  116 , i.e. the upper or lower skin layers  74  and  75 , or into all three of the second silicon layer  114 , i.e. the core layer  72 , the thin portion of the second silicon layer  114 , i.e. the lower skin layer  75 , and the third silicon layer  116 , i.e. the upper skin layer  74 , as in  FIG. 7   g.    
         [0038]      FIGS. 8   a  to  8   d  illustrate another process in which a first double SOI structure  121  is provided, including an first silicon layer  122 , in between first and second oxide etch stop layers  123  and  124 , a second silicon layer  125  forming the core  72 , and a bottom silicon layer  126  forming a handle. The first silicon layer  122  and the first oxide layer  123  combine to form the lower skin  75 . In the second step, illustrated in  FIG. 8   b , the second silicon layer  125  is etched down to the first oxide stop layer  123  to form the core layer  72  with cells  73 . The second silicon layer  125  also includes wing sections on either side thereof for forming the hinges  13  and  14 , as well as a cap for mounting on the substrate  15 . A second SOI structure  127  is provided in the next step ( FIG. 8   c ) including an third thin silicon layer  128 , e.g. 2 um to 6 um, a third etch stop oxide layer  129 , a fourth silicon layer  130 , and a fourth etch stop oxide layer  131 . The third silicon layer  128 , along with the third oxide layer  129  forms the upper skin  74 . The second SOI structure  127  is bonded, e.g. fusion bonded, onto the first SOI structure  121 , whereby the third oxide layer  129  is adjacent to the second silicon layer  125 . In the final step ( FIG. 8   d ) one of the handle layers, e.g. the bottom silicon layer  126 , with the corresponding oxide layer, e.g. the second oxide layer  124 , is removed, e.g. etched away, along with the portions of the first silicon layers  122  and the first oxide layer  123  surrounding the mirrored platform  12 , to define the mirrored platform structure  12  in which the first silicon layer  122  and the first oxide layer  123  combine to form the lower skin  75 , and the third etch stop oxide layer  129  along with the third silicon layer  128  forms the upper skin  74 . 
         [0039]    As above, in finishing steps ( FIGS. 8   e  to  8   g ) a wafer  108  is provided defining the substrate  15  with a recess  109  surrounded by supporting walls  110 , providing the necessary clearance for the mirrored platform  12 . The electrodes  16  are patterned on the lower surface of the recess  109 . The substrate  15  may be comprised of glass (pyrex®), silicon or other suitable material. The cap sections surrounding the mirrored platforms  12  of the honeycomb sandwich, from  FIG. 8   d , is then bonded to the walls  110  of the substrate  15  ( FIG. 8   f ). The final steps are illustrated in  FIG. 8   g , wherein: i) the honeycomb handle wafer, e.g. fourth silicon layer  130  and fourth oxide layer  131 , is removed, ii) the reflective metal layer  17  is deposited on the top of the upper skin  74 , e.g. the third silicon layer  128 , and iii) deep reactive ion etching (DRIE) is performed to fabricate the hinges  13  and  14  and float the mirrored platform  12  above the substrate  15 . Preferably, the first and second torsional hinges  13  and  14  are etched into only the second silicon layer  125 , i.e. the core layer  72 , (as in  FIG. 3 ); however, the first and second hinges can be formed from the second silicon layer  125 , i.e. the core layer  72 , and the first or third silicon layers  122  or  128 , i.e. the upper or lower skin layers  74  and  75 , (as in  FIG. 8   g ) or into all three of the second silicon layer  125 , i.e. the core layer  72 , and the first and third silicon layers  122  and  128 , i.e. the lower and upper skin layers  74  and  75 , (as in  FIG. 7   g ). 
         [0040]    The aforementioned processes are preferably executed using SOI structures; however, other semiconductor structures can be used with suitable insulator, e.g. silicon on fused silica or quartz, silicon on glass such as pyrex, silicon carbide on oxidized silicon, and indium phosphide (inP) or gallium arsenide (GaAs) on oxidized silicon.