Abstract:
An arbitrary gap between the two chips of a MEMS device arranged in a flip-chip arrangement is achieved by etching into a first substrate to form mesas which act as spacers between which, or even on which, any required circuit elements are formed. Points of a layer at a first surface of the second substrate within which MEMS structures are made are bonded to the mesas of the first substrate. The second substrate is then removed, leaving the structures bonded to the mesas. The mesas may be formed by placing a hard mask, such as silicon oxide, which defines the desired pattern of mesas on the first substrate, and then etching the unmasked portion of the substrate using a mixture of potassium hydroxide (KOH) with isopropanol (IPA) or, tetramethyl ammonium hydroxide (TMAH) mixed with a surfactant, e.g., nonylphenol ethoxy ether or other equivalent compounds.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to the art of micro-electromechanical systems (MEMS) devices and methods of their making, and more particularly, to arrays of tiltable micro mirrors, e.g., small mirrors, which can reflect light.  
         BACKGROUND OF THE INVENTION  
         [0002]    One solution for all-optical switching employs two MEMS devices each containing an array of tiltable micro mirrors, e.g., small mirrors, which can reflect light, which herein refers to any radiation in the wavelength of interest, whether or not in the visible spectrum. An optical path is established for light supplied from an input source, e.g., an optical fiber, to an output, e.g., an output fiber, by steering the light using a first micro mirror on the first optical MEMS device, the first micro mirror being associated with the input fiber, onto a second micro mirror on the second optical MEMS device which is associated with the output fiber. The second micro mirror then steers the light into the output fiber. Each fiber connected to the system is considered a port of the system, the input fibers being the input ports and the output fibers being the output ports.  
           [0003]    There are various prior art methods of making such an array of tiltable micro mirrors. Typically the array is made in two parts. The first part includes the electrodes which control the tilt of the micro mirrors and some type of spacer which holds the second part offset from the electrodes. The second part includes the micro mirrors and their springs and any other supporting structure.  
           [0004]    The spacers of one prior art mirror array are made from polyimide, which is photo-patternable type of plastic that is deposited on the substrate. Disadvantageously, such spacers are a) not flat at the top, b) are relatively soft, e.g., compared to silicon, c) must be hard baked at high temperatures, and d) the height of the resulting spacers is not uniform from device to device even when the same processing is employed.  
           [0005]    In another prior art arrangement, the micro mirrors are manufactured from a silicon on insulator (SOI) wafer and then portions of the back of the wafer are etched to allow the mirror to move freely. The unetched portions serve as the spacers and keep the micro mirrors elevated with respect to the electrodes which are on a second wafer. See for example U.S. Pat. No. 6,201,631, which is incorporated by reference as if fully set forth herein. However, such micro mirrors are relatively fragile, and the height of the spacers is dictated by the thickness of the wafer on which the micro mirrors were formed, which is typically greater than 200 μm.  
           [0006]    Yet another prior art method creates micro mirrors as suspended structures by depositing a thick sacrificial layer on a substrate, with appropriate patterning to make holes therethrough via etching. Then, a material to form the micro mirrors is conformally deposited on top of the sacrificial layer. The layer for the micro mirrors is patterned, and the micro mirrors are formed. A portion of the conformally deposited mirror material is then etched away to allow access to the sacrificial layer. Finally, the sacrificial layer is removed via etching. This process suffers from the fact that it takes a long time to grow the thick sacrificial layer, and the height of the suspended micro mirrors is typically limited by thickness of the sacrificial layer, so the height is often limited to no more than 5 μm.  
           [0007]    A process similar to that used for making accelerometers as taught in “ISAAC:integrated silicon automotive accelerometer” by Leland ‘Chip’ Spangler and Christopher J. Kemp, published in Sensors and Actuators A 54 (1996), pages 523-529, which is incorporated by reference as if fully set forth herein, is unsuitable for making arrays of micro mirrors. This is because when using that process it is too hard to control the final thickness of the membrane that would be used for the mirror and other delicate structures, such as springs, which would also be formed from the same membrane.  
         SUMMARY OF THE INVENTION  
         [0008]    I have recognized that MEMS devices with an arbitrary gap between the two chips in a flip-chip arrangement can easily be achieved, in accordance with the principles of the invention, by etching into a first substrate to form mesas which act as the spacers and between which, or even on which, any required circuit elements are formed, e.g., after the mesas are formed. Thereafter, points of a layer at a first surface of the second substrate within which MEMS structures are made are bonded to the mesas of the first substrate. The second substrate is then removed, leaving the structures bonded to the mesas.  
           [0009]    In accordance with an aspect of the invention, the mesas may be formed by placing a hard mask, such as silicon oxide, which defines the desired pattern of mesas on the first substrate, and then etching the unmasked portion of the substrate using an anisotropic etchant, such as a mixture of potassium hydroxide (KOH) with isopropanol (IPA). In accordance with another aspect of the invention, since KOH and IPA are incompatible with complementary metal oxide semiconductor (CMOS) type processes, in lieu of using KOH and IPA, tetramethyl ammonium hydroxide (TMAH) mixed with a surfactant, e.g., nonylphenol ethoxy ether or other equivalent compounds, which is compatible with CMOS processing, can be used.  
           [0010]    In one embodiment of the invention, micro mirrors with an arbitrary gap between the micro mirrors and their respective electrodes can easily be achieved by etching into a first substrate to form mesas which act as spacers between which, or even on which, the electrodes are formed after the mesas are formed. Thereafter, the micro mirrors and their supporting structure, which are made on the surface of a second substrate, are bonded to the mesas of the first substrate. The second substrate is then removed, leaving the micro mirror array bonded to the mesas. The inventive method enables the mesa tops be flat to enable a good bond with the mirror supporting structure. The inventive method also enables the space between the mesas be flat, so that electrodes deposited in the space are flat and have a uniform distance to their respective mirror in its neutral position.  
           [0011]    In accordance with an aspect of the invention, various ones of the mesas may be of different sizes. In accordance with another aspect of the invention, various ones of the mesas may be used to implement functionality other than the supporting function of spacing. For example, a) a mesa with a hole in it may be used to seat a ball lens, b) a mesa with V-grooves may be used to seat optical fibers, c) long narrow mesas placed between micro mirrors may function as windbreakers, d) a mesa with a pattern etched into its top, e.g., a fiducial mark, can be used for alignment purposes, and e) the like.  
           [0012]    In accordance with another embodiment of the invention, vias may be made through the mesas or through any of the substrate to facilitate the making of electrical connections. In accordance with another aspect of the invention, mesas of different heights can be made by removing the mask which was covering any mesa for which it is desirable that it have less height at a point during the etching process and then resuming etching. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]    In the drawing:  
         [0014]    [0014]FIG. 1 shows a cross-section of substrate with hard mask forming thereon a pattern of locations at which the mesas will be formed, in accordance with the principles of the invention;  
         [0015]    [0015]FIG. 2 shows a cross-section of the substrate of FIG. 1 after it has been etched with a wet anisotropic etch;  
         [0016]    [0016]FIG. 3 shows a cross-section of the substrate of FIG. 2 after the hard mask is removed;  
         [0017]    [0017]FIG. 4 shows the substrate as it appears in FIG. 3 and also a second substrate which is facing upside down with respect to the first substrate;  
         [0018]    [0018]FIG. 5 shows a three-dimensional view of a portion of FIG. 4 in which the part of the micro-electromechanical system that is going to move is a paddle;  
         [0019]    [0019]FIG. 6 shows the two substrates of FIG. 4 bonded to each other before removal of the sacrificial layer and the “handle wafer”;  
         [0020]    [0020]FIG. 7 shows removal of the sacrificial layer;  
         [0021]    [0021]FIG. 8 shows removal of the “handle wafer”;  
         [0022]    [0022]FIG. 9 shows a mesa in which is seated a ball lens, in accordance with an aspect of the invention;  
         [0023]    [0023]FIG. 10 shows a mesa with V-grooves in respective ones of which are seated optical fibers, in accordance with an aspect of the invention;  
         [0024]    [0024]FIG. 11 shows micro mirrors between which have been formed long narrow mesas to function as windbreakers, in accordance with an aspect of the invention;  
         [0025]    [0025]FIG. 12 shows a mesa into which has been etched a pattern which may be used as a fiducial mark for alignment purposes, in accordance with an aspect of the invention;  
         [0026]    [0026]FIG. 13 shows crossed intersecting mesas which can function as a fiducial mark, in accordance with an aspect of the invention;  
         [0027]    [0027]FIG. 14 shows another embodiment of the invention which employs vias; and  
         [0028]    [0028]FIG. 15 shows mesas of different heights, in accordance with aspect of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0029]    The following merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.  
         [0030]    In the claims hereof any element expressed as a means for performing a specified function is intended to encompass any way of performing that function including, for example, a) a combination of circuit elements which performs that function or b) software in any form, including, therefore, firmware, microcode or the like, combined with appropriate circuitry for executing that software to perform the function. The invention as defined by such claims resides in the fact that the functionalities provided by the various recited means are combined and brought together in the manner which the claims call for. Applicant thus regards any means which can provide those functionalities as equivalent as those shown herein.  
         [0031]    Unless otherwise explicitly specified herein, the drawings are not drawn to scale.  
         [0032]    Additionally, unless otherwise explicitly specified herein, any lens shown and/or described herein is actually an optical system having the particular specified properties of that lens. Such an optical system may be implemented by a single lens element but is not necessarily limited thereto. Similarly, where a mirror is shown and/or described what is actually being shown and/or described is an optical system with the specified properties of such a mirror, which may be implemented by a single mirror element but is not necessarily limited to a single mirror element. This is because, as is well known in the art, various optical systems may provide the same functionality of a single lens element or mirror but in a superior way, e.g., with less distortion. Furthermore, as is well known in the art, the functionality of a curved mirror may be realized via a combination of lenses and micro mirrors and vice versa. Moreover, any arrangement of optical components that are performing a specified function, e.g., an imaging system, gratings, coated elements, and prisms, may be replaced by any other arrangement of optical components that perform the same specified function. Thus, unless otherwise explicitly specified here, all optical elements or systems that are capable of providing specific function within an overall embodiment disclosed herein are equivalent to one another for purposes of the present disclosure.  
         [0033]    The term micro-electromechanical systems (MEMS) device as used herein is intended to mean an entire MEMS device or any portion thereof. Thus, if a portion of a MEMS device is inoperative, or if a portion of a MEMS device is occluded, such a MEMS device is nonetheless considered to be a MEMS device for purposes of the present disclosure.  
         [0034]    In the description, identically numbered components within different ones of the FIGS. refer to the same components.  
         [0035]    [0035]FIG. 1 shows a cross-section of substrate  101 , typically &lt;100&gt;silicon, with hard mask  103 , e.g., silicon nitride or silicon oxide (SiO 2 ), forming thereon a pattern of locations at which the silicon will not be etched and consequently at which mesas will be formed, in accordance with the principles of the invention. FIG. 2 shows a cross-section after substrate  101  has been etched with a wet anisotropic etch, e.g., using, in accordance with aspects of the invention, a mixture of potassium hydroxide (KOH) with isopropanol (IPA) or tetramethyl ammonium hydroxide (TMAH) mixed with a surfactant, e.g., nonylphenol ethoxy ether or other equivalent compounds, which is compatible with CMOS processing, as the etchant. As a result of the etching process, mesas  105  are formed, each of which still has hard mask  103  on its upper surface. It is advantageous if the mesa tops are flat, to enable a good bond portion of the second substrate to be bonded thereto, e.g., the mirror support springs. It is also advantageous that the space between the mesas be flat so that any circuit elements, e.g., electrodes, which are deposited in the space are flat. For micro mirrors it is desirable to have a uniform distance between the micro mirrors in their neutral position and their respective electrodes below them. Use of the aforementioned etchants yields mesas of adequate and uniform height while leaving the spaces in between appropriately flat.  
         [0036]    [0036]FIG. 3 shows a cross-section of substrate  101  after hard mask  103  is removed via an etching step, e.g., using hydrofluoric acid as the etchant if silicon oxide was used as the hard mask. Also shown in FIG. 3 are circuit elements, e.g., electrodes,  107  formed in the valleys between mesas  105 . Note that the valleys are the open spaces that were formed as a result of the etching of substrate  101  to form mesas  105 . Circuit elements such as electrodes  107  may be formed using essentially conventional techniques for forming such circuit elements on flat substrates. However, due to the topography of the substrate given the presence of mesas  105 , when applying photoresist to pattern the electrodes, the photoresist is conformally coated on the wafer, e.g., using a spray coater, onto substrate  101  to form the required conformal coating. Alternatively, the photoresist could be electroplated to form a conformal coating. Other well known techniques for depositing circuit elements on non-flat substrate may be employed to deposit circuit elements on the surfaces of the mesas themselves as well.  
         [0037]    Note that for micro mirrors, in addition to electrodes, there is also formed the wiring that is necessary to provide control signals to the electrodes. However, such wiring is not shown in FIG. 3 due to the difficulty of showing such wires. Those of ordinary skill in the art will readily know how to form such wiring, which uses the same techniques as the making of the electrodes. Furthermore, various other active and passive circuit elements may also be formed on substrate  101 .  
         [0038]    [0038]FIG. 4 shows substrate  101  as it appears in FIG. 3 and also a second substrate  401  which is facing upside down with respect to substrate  101 , but is not yet bonded thereto. Thus, on the “upper” surface of substrate  401 , which is facing downward toward substrate  101  because it has been “flipped”, is a) sacrificial layer  403 , e.g., a deposited layer of silicon dioxide, which will be removed to release those parts which are to move and b) mechanical layer  405 , e.g., silicon deposited on sacrificial layer  403 , from which is formed those parts of the micro-electromechanical system that is going to move, e.g., the micro mirrors. Mechanical layer  405  could be made of, at least in part, or be coated with, a reflecting material, such as a metal or a reflective dielectric stack. Also shown in FIG. 4 is optional bonding agent  407 , e.g., solder, glass, or any other appropriate agent, which will help bond mechanical layer  405  to mesas  105  of substrate  101 . FIG. 5 shows a three-dimensional view of a portion of FIG. 4 in which the part of the micro-electromechanical system that is going to move is a paddle formed in mechanical layer  405 .  
         [0039]    [0039]FIG. 6 shows substrate  101  bonded to substrate  401  before removal of sacrificial layer  403  and unnecessary portion  601  of substrate  401 , which is referred to as the “handle wafer”.  
         [0040]    In one embodiment of the invention, sacrificial layer  403  is removed, e.g., via etching, which causes handle wafer  601  to become detached from the remaining structure thereby enabling the moveable parts of mechanical layer  405  to move. This is shown in FIG. 7. In another embodiment of the invention, the handle wafer is first removed via etching. This is shown in FIG. 8. Sacrificial layer  403  is then removed, also via etching, which allows the moveable parts to move. Typically, one etchant is employed to remove the handle wafer and a different etchant is employed to remove the sacrificial layer. A sacrificial layer need not be employed if it is possible to control the etching such that it stops when it reaches mechanical layer  405 .  
         [0041]    Advantageously, if mechanical layer  405  is not already reflective, well controlled metalization of the moving parts may now be achieved at the surface where the sacrificial layer formerly had been, e.g., using a shadow mask to prevent the metal from coating other parts. Alternatively, the moving part may have been fabricated so as to already have metalization or another reflective material which is exposed upon removal of the sacrificial layer. Further alternatively, a hole or cavity, e.g., the size of the moving parts and at the location of the moving parts, may be drilled through the handle wafer and sacrificial layer through to the moving part layer so that metalization of the moving parts may be achieved through the cavity.  
         [0042]    Further advantageously, high density arrays of circular micro mirrors may be formed since the mesas which hold the micro mirrors up are below the micro mirrors, and so the mesas can be located off center from the rows formed by the micro mirrors, i.e., in those gaps formed at the point where three or more neighboring micro mirrors meet if there are two complete adjacent rows or columns of micro mirrors meeting.  
         [0043]    In accordance with an aspect of the invention, various ones of the mesas may be different sizes. In accordance with another aspect of the invention, various ones of the mesas may be used to implement functionality other than the supporting function of spacing. FIG. 9 shows mesa  905  made in accordance with the principles of the invention in which is seated ball lens  919 , in accordance with an aspect of the invention. FIG. 10 shows mesa  1005  with V-grooves  1021  in respective ones of which are seated optical fibers  1023  in accordance with an aspect of the invention. FIG. 11 shows micro mirrors  1127  between which have been formed long narrow mesas  1125  to function as windbreakers in accordance with an aspect of the invention. This windbreaking function is useful since when one of micro mirrors  1127  is moved it may generate a wind which could affect the positioning of adjacent ones of micro mirrors  127 . The windbreaking mesa blocks the wind and prevents the position of the adjacent micro mirror from being disturbed. FIG. 12 shows mesa  1205  into which has been etched pattern  1229 , which may be used as a fiducial mark for alignment purposes.  
         [0044]    Note that although the mesas have been shown herein as substantially square in shape, this is for clarity and pedagogical purposes only. In practice, the mesas may have any shape. Thus, FIG. 13 shows crossed intersecting mesas  1305  which can function as a fiducial mark, in accordance with an aspect of the invention.  
         [0045]    [0045]FIG. 14 shows, in accordance with another embodiment of the invention, via  1431  connecting electrode  1433  through the mesa  105  and substrate  1435  to connection point  1437  as well as via  1439  connecting electrode  1441  though substrate  1435  to connection point  1443 .  
         [0046]    [0046]FIG. 15 shows, in accordance with aspect of the invention, mesas of different heights which are made by removing the mask which was covering any mesa to have less height at a point during the etching process and then resuming etching. Thus, mesas  105  are used as spacers while mesa  1550  is used for another function, such as those described hereinabove.