Abstract:
A method for manufacturing a mechanical grating device is presented. The device consists of a plurality of parallel-suspended ribbons that are deformed using, for example, an electrostatic force to actuate alternate ribbons. Actuation is a deformation of the ribbon resulting from an applied voltage to affect the height of the ribbons above a substrate. The method for manufacturing a mechanical gating device comprises the steps of: 
     providing a spacer layer on top of a protective layer which covers a substrate; 
     etching a channel entirely through the spacer layer; 
     depositing a sacrificial layer at least as thick as the spacer layer; 
     rendering the deposited sacrificial layer optically coplanar by chemical mechanical polishing; 
     providing a tensile ribbon layer completely covering the area of the channel; 
     providing a conductive layer patterned in the form of a grating; 
     transferring the conductive layer pattern to the ribbon layer and etching entirely through the ribbon layer; and 
     removing entirely the sacrificial layer from the channel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to U.S. Ser. No. 09/216,289 filed Dec. 18, 1998 entitled A Mechanical Grating Device, and further reference is made to U.S. Ser. No. 09/215,973 filed Dec. 18, 1998 entitled Method For Producing Coplanar Surface Structures. 
     FIELD OF THE INVENTION 
     This invention relates to the field of the fabrication of a device which modulates an incident light beam. The device is a mechanical grating device. More particularly, this invention discloses a method for manufacturing a mechanical grating device, wherein the device manufactured according to the inventive method possesses a significant improvement in the output of the diffracted light beam. 
     BACKGROUND OF THE INVENTION 
     Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360, issued May 10, 1994 to Bloom et al., entitled “Method and Apparatus for Modulating a Light Beam”. This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode, and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. The deformable elements are designed to have a thickness equal to λ/4 where λ is the wavelength of the incident light source. They are supported a distance of λ/4 above, and parallel to, the substrate. When the deformable elements are actuated (for example a sufficient switching voltage is applied), the deformable elements are pulled down and the incident light is diffracted. Optical systems can intercept the diffracted light. For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image. Furthermore, since gratings are inherently dispersive, this modulator can be used for color displays. 
     U.S. Pat. No. 5,677,783, issued Oct. 14, 1997 to Bloom et al., entitled “Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate” discloses a method of making a deformable grating apparatus for modulating a light beam and including means for obviating stiction between grating elements and underlying substrate. Referring to FIG. 1, a perspective cut-away view of a prior art light modulator  10  is shown. An insulating protective layer  24  of silicon nitride topped with a buffer layer of silicon dioxide  26  is deposited on a silicon substrate  22 . This is followed by the deposition of a sacrificial silicon dioxide layer  16 . A silicon nitride layer  30  is next deposited in which is defined the deformable elements  12 . Both the thickness of the sacrificial silicon dioxide layer  16  and the silicon nitride layer  30  are critical in determining the amplitude modulation and thus the efficiency of the grating device. In order to achieve freestanding beams the sacrificial silicon dioxide layer  16  is etched away in the active area. The remaining sacrificial silicon dioxide layer  16  not removed acts as a supporting frame  14  for the deformable elements  12 . The last fabrication step provides an aluminum film (not shown) in order to enhance the reflectance of the beams and provide an electrode for application of a voltage between the deformable elements  12  and the substrate  22 . 
     There are many problems with this prior art device. The thickness of both the sacrificial oxide layer  16  and silicon nitride layer  30  have to each be λ/4. Because these thicknesses determine the grating amplitude of the modulator, their dimensions are critical. Variations in either of these thicknesses will result in unwanted diffraction of light in the off state, as well as lower diffraction efficiency in the on state, thus lower contrast ratios. There is no freedom to adjust the thickness of the deformable element  12  for optimization of its mechanical properties. 
     There is no defined etch stop in the device structure during removal of the sacrificial oxide layer  16 . This requires a carefully controlled time-dependent etch to ensure that the remaining sacrificial oxide layer  16  is able act as the supporting frame  14 . The profile left by the wet etch openings between the beams leaves an uneven wall below the deformable elements  12  where they contact the supporting frame  14 . Such effects will cause variations in the electromechanical properties of the devices. 
     The etching process to remove the sacrificial oxide layer is also a wet process. During this wet processing step it has been seen that stiction tends to occur in that the deformable elements tend to adhere and remain adhered to the substrate. Special drying techniques can be used to overcome this problem but complicate the process. Removal of the sacrificial layer using a dry process is preferred. 
     U.S. Pat. No. 5,661,592, issued Aug. 26 ,1997 to Bornstein et al., entitled “Method of Making and an Apparatus for a Flat Diffraction Grating Light Valve” discloses a method for making a deformable grating apparatus which attempts to address the problems associated with this prior art device. An insulating layer is deposited on the substrate. A phosphosilicate glass (PSG) sacrificial layer is next deposited. The PSG sacrificial layer is selectively patterned removing the PSG sacrificial layer except in regions where the deformable grating elements are to be formed. The PSG is reflowed at high temperature to lower the angle of its sidewall. Silicon nitride is then deposited conformably over the PSG and patterned into deformable elements. The PSG sacrificial layer is then removed by wet etching. By selectively patterning the PSG sacrificial layer the region under the beams is more uniform relying now on the uniformity of the reflow of the PSG sacrificial layer. However the removal of the PSG sacrificial layer is still a wet process with the corresponding disadvantages as described above. 
     The conformal deposition of the silicon nitride over the step height formed by the patterned PSG sacrificial layer region also has topography determined by the step height. In patterning the deformable elements this topography will limit the minimum spacing between the deformable elements. Increased spacing between elements will cause increased light scattering decreasing the efficiency of the grating. The use of a PSG sacrificial layer also requires a high temperature reflow step that would complicate its integration with CMOS circuitry on the same substrate. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for manufacturing a mechanical grating device. The mechanical grating device has to have equal actuation conditions for the deformable elements throughout the whole device in order to improve the diffraction efficiency of the device. 
     The object is achieved with a method for manufacturing a mechanical grating device comprising the steps of: 
     providing a spacer layer on top of a protective layer which covers a substrate; 
     etching a channel entirely through the spacer layer; 
     depositing a sacrificial layer at least as thick as the spacer layer; 
     rendering the deposited sacrificial layer optically coplanar by chemical mechanical polishing; 
     providing a tensile ribbon layer completely covering the area of the channel; 
     providing a conductive layer patterned in the form of a grating; 
     transferring the conductive layer pattern to the ribbon layer and etching entirely through the ribbon layer; and 
     removing entirely the sacrificial layer from the channel. 
     Advantages 
     This method of fabrication produces a device with the desired performance and the specific steps of fabrication to allow manufacturing methods that are common to the microelectronics industry that are reproducible and uniform across the area of the device being produced. According to the used method the advantages of the mechanical or the electromechanical grating device produced with the inventive method are: an improved definition of the position of the channel walls beneath the deformable ribbon elements allow reproducible ribbon length. The reproducible length of the deformable ribbon elements affects resonance frequency, speed of actuation, damping affects due to resonance coupling, and air flow restriction beneath continuous areas of the ribbon layer, etc. 
     A further advantage is, that the actuation of the ribbon elements of the device may also be carried out for example by heat or mechanic force activation. Actuation is a deformation of the ribbon resulting from an applied force to affect the height of the ribbons above a substrate. Furthermore, the formation of a ground plane at the surface of the silicon wafer allows top side access and better charge confinement within the substrate. Additionally the device structure and materials selected are compatible with standard CMOS fabrication methods and allow a fabrication process sequence that make the fabrication of the electromechanical grating compatible with the integration of CMOS circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter of the invention is described with reference to the embodiments shown in the drawings. 
     FIG. 1 is a perspective, partially cut-away view of the prior art grating device; 
     FIG. 2 is a perspective, partially cut-away view of the mechanical grating device of the present invention; 
     FIG. 3 is a top view of the mechanical grating device of the present invention; 
     FIG. 4 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the layer built-up of one embodiment of the invention; 
     FIG. 5 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate etching of a channel; 
     FIG. 6 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the deposition of a sacrificial layer; 
     FIG. 7 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the removal of the sacrificial layer exceeding the channel; 
     FIG. 8 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate deposition of the ribbon layer; 
     FIG. 9 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the provision of an interconnection between the ribbon layer and the substrate; 
     FIG. 10 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the provision of a reflective layer; 
     FIG. 11 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the patterning of the reflective layer and the ribbon layer; 
     FIG. 12 is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the removal of a sacrificial layer within the channel; and 
     FIG. 13 is a cross-sectional view along plane B—B indicated in FIG. 3 to show one embodiment of the device fabricated according to the inventive method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is a perspective, partially cut-away view of a mechanical grating device  100  of the present invention. The mechanically deformable structures of the mechanical grating device  100  are formed on top of a base  50 . The present embodiment as shown in FIG. 2 discloses a mechanical grating device  100 , which can be operated by the application of an electrostatic force. According to the fact, that the actuation force of the mechanical grating device  100  is electrostatic the base  50  comprises the several layers of different materials. The base  50  comprises a substrate  52 . The material of the substrate  52  is chosen from the materials glass, plastic, metal and semiconductor material. The substrate  52  is covered by a bottom conductive layer  56 . In this embodiment the thin bottom conductive layer  56  is necessary since it acts as an electrode for applying the voltage to actuate the mechanical grating device  100 . The thin bottom conductive layer  56  is covered by a protective layer  58 . The bottom conductive layer  56  is selected from the group consisting of aluminum, titanium, gold, silver, tungsten, silicon alloys and indium tinoxide. Above the protective layer  58  a standoff layer  60  is formed which is followed by a spacer layer  65 . On top of the spacer layer  65 , a ribbon layer  70  is formed which is covered by a reflective layer  78 . In the present embodiment the reflective layer  78  has also to be conductive in order to provide electrodes for the actuation of the mechanical grating device  100 . The electrodes are patterned from the reflective and conductive layer  78 . 
     The spacer layer  65  has a longitudinal channel  67  formed therein. The longitudinal channel  67  comprises a first and second side wall  67   a  and  67   b  and a bottom  67   c . The channel  67  is open to the top and covered by a first and a second set of deformable ribbon elements  72   a  and  72   b . Each deformable ribbon element  72   a  and  72   b  spans the channel  67  and is secured to the surface of the spacer layer  65  on either side of the channel  67 . The bottom  67   c  of the channel  67  is covered by the protective layer  58 . As mentioned above, the ribbon layer  70  is covered by the reflective layer  78 . The reflective layer  78 , which is conductive as well, is patterned such that there is a first and a second conducting region  78   a  and  78   b . Both, the first and the second conductive region  78   a  and  78   b  have according to the patterning, a comb-like structure and are arranged at the surface of the mechanical grating device in an interdigitated manner. The first and second conductive region  78   a  and  78   b  are mechanically and electrically isolated from one another. According to the pattern of the reflective layer  78  the ribbon layer  70  is patterned in the same manner. As a result there are the first and the second set of deformable ribbon elements  72   a  and  72   b  spanning the channel  67  and in the direction of the channel  67  are arranged such that every other deformable ribbon element belongs to one set. 
     In the embodiment as shown in FIG. 2, a plurality of standoffs  61  are positioned on the bottom  67   c  of the channel  67 . The standoffs  61  are patterned from the standoff layer  60  (detailed description of the patterning process see below) such that a group of standoffs  61  is associated only with the deformable ribbon elements  72   a  and  72   b  of the first or the second set. In the embodiment shown here, the group of standoffs  61  is associated with the second set of deformable ribbon elements  72   b . The standoffs  61  may also be patterned in the form of a single bar. 
     Referring to FIG. 3, a top view of the mechanical grating device  100  of the present invention is shown. A first view plane A—A, perpendicular to the length of the mechanical grating device  100  provides a cross-sectional view of the mechanical grating device  100  as shown in FIGS. 4 to  12 . A second view plane B—B, perpendicular to the first view plane A—A of the mechanical grating device  100  provides a cross-sectional view of the mechanical grating device  100  as shown in FIG.  13 . The mechanical grating device  100 , as shown is FIG. 3, is a device which can be actuated by the application of an electrostatic force. It is clear that a person skilled in the art can imagine other ways for actuating the grating device, for example thermal actuation, piezoelectric actuation or any combination. In the embodiment shown in FIG. 3, a first and a second, electrically conducting region  78   a  and  78   b  are formed on the surface of the mechanical grating device  100 . The first and the second electrically conducting region  78   a  and  78   b  are electrically and mechanically isolated from each other to allow the application of voltage separately to a first and to a second set of deformable ribbon elements  72   a  and  72   b . The first conducting region  78   a  applies the voltage to the first set of deformable ribbon elements  72   a  and the second conducting region  78   b  provides the voltage to the second set of deformable ribbon elements  72   b . The first conducting region  78   a  is in contact with the bottom conductive layer  56  designated at the base  50 . The connection is established by an interconnect  75  (see for example FIG.  12 ). The thin bottom conductive layer  56  may be formed above any layer which is below the bottom  67   c  of the channel  67 . From the view of FIG. 3, regions of the spacer layer  65  and protective layer  58  are visible because of pattering of first and second conductive region  78   a  and  78   b  to achieve electrical and mechanical isolation of the deformable ribbon elements  72   a  and  72   b . For operation of the mechanical grating device  100  the electrostatic force is produced by a voltage difference between the thin bottom conducting layer  56  and the first or the second conducting layer  78   a  or  78   b  which are formed atop of each deformable ribbon element  72   a  and  72   b . According to the embodiment shown here, the voltage for actuating the ribbon elements is applied to the second set of deformable ribbon elements  72   b , those elements are drawn to the bottom  67   c  of the channel  67 . The resulting pattern is that every second deformable element (all deformable ribbon elements designated with the reference numeral  72   b ) are in contact with the base of the channel. It is clear that a switching of the second set of deformable elements is also possible. The same bi-level pattern can be achieved. 
     Referring now to FIG. 4, which is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the layer built-up of one embodiment of the invention which uses standoffs  61  formed at the bottom  67   c  of the channel  67 . The base  50  comprises the substrate  52 , covered by the bottom conductive layer  56 , and a protective layer  58  on top of the bottom conductive layer  56 . As mentioned above the substrate  52  can be glass, plastic, metal or a semiconductor material. In the case of silicon as the substrate  52  the bottom conductive layer  56  can also be generated by an ion implantation technique. The base  50  defines a surface  53 . The surface  53  of the base  50  is covered by a standoff layer  60 . The standoff layer  60  defining a top surface  54   a  of the standoffs  61 . The standoff layer  60  is then covered by the spacer layer  65 . The spacer layer  65  is selected from the group consisting of silicon oxide, silicon nitride and polyimide. The top surface  54   a  of the standoff layer  60  will be used to define an actuation height resulting from operation. The spacer layer  65 , which is for example of silicon oxide, is deposited by chemical vapor deposition. The total height of the actuation of the deformable ribbon elements  72   a  and  72   b  is defined by the thickness of the spacer layer  65  having an upper surface level  64   a.    
     Referring now to FIG. 5 which is a cross-sectional view along plane A—A, indicated in FIG. 3, to illustrate etching of a channel  67 . The patterning of the spacer layer  65  is carried out using standard photolithographic processing and etching methods to define the longitudinal channel  67 . The active region of the mechanical grating device  100  will be located in the area of the channel  67 . The standoff layer  60  is then patterned using photolithographic processing and etching methods to produce the standoffs  61 , as illustrated in FIG.  5 . Although not illustrated, these patterns can consist of pedestals or lines. The standoffs  61  act as mechanical stops for the actuation of the mechanical grating device and the upper surface of the standoffs  61  is defined by surface  54   a . The actuated deformable ribbon elements  72   a  or  72   b  came in contact with the surface  54   a  of the standoffs  61 . 
     Referring now to FIG. 6, which is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the deposition of a sacrificial layer  66 . To allow additional layers atop the existing structure, as shown in FIG. 5, a conformal sacrificial layer  66  is deposited to a thickness greater than the separation of the top surface  54   a  of the standoff layer  60  and the top surface  64   a  of the spacer layer  65  (see FIG.  6 ). The material for the sacrificial layer  66  is different from the spacer layer  65  and is selected from the group consisting of silicon oxide, silicon nitride, polysilicon, doped-polysilicon, silicon-germanium alloys and polyimide. 
     Referring now to FIG. 7, which is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the removal of the sacrificial layer  66  exceeding the channel  67 . The sacrificial layer  66  is planarized to a level substantially near the top surface  64   a  of the spacer layer  65 . Chemical mechanical polishing methods are used to achieve the polished structure. The polished surface of sacrificial layer  66  filling the channel is preferably polished to be optically coplanar with surface  64   a  such that a light beam reflected from the top surface of the next-to-be-deposited ribbon layer  70  would always be reflected specularly if it were scanned along ribbon layer  70  even in the region in which ribbon layer  70  contacts sacrificial layer  66  and top surface  64   a  of spacer layer  65 . As is well known in the practice of optical engineering, this requires a surface planarity of less than about  200  Angstrom units. In this case the surface of the to-be-deposited ribbon elements  72   a  and  72   b  remain optically coplanar on both their bottom and top surfaces after removal of the material  66  providing the material of ribbon elements  72   a  an d 72   b  is deposited with uniform thickness and uniform tensile stress. Also in this case, ribbon elements  72   a  and  72   b  have no mechanically irregularities at the points at which they contact spacer layer  65 , thereby ensuring the ribbons pull down uniformly and predictably during device operation. 
     Referring now to FIG. 8, which is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate deposition of the ribbon layer  70 . The ribbon layer  70  is provided above the top surface  64   a  of the spacer layer  65 , thereby also covering the area of the channel  67 . Silicon nitride is a well-suited material for the ribbon layer  70  and can be patterned to provide the needed mechanical structure. Its material properties are well suited for the application because of the intrinsic tensile stress easily controlled by the deposition process. 
     Referring now to FIG. 9, which is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the provision of an interconnection  76  between the ribbon layer  70  and the bottom conductive layer  56  which is part of the structure of the base  50 . Contact to the bottom conductive layer  56  is accomplished by etching at least one opening  74  using well-known etching methods. Then depositing of a thick conducting layer  76  is carried out (the thick conducting layer may be an aluminum alloy) which is followed by photolithographic processing and etching methods to limit the area coated by the thick conducting layer  76  to an area around at least one opening  74 . The thick conducting layer provides an interconnect  75  between the ribbon layer and the bottom conductive layer  56 . 
     Referring now to FIG. 10, which is a cross-sectional view along lane A—A indicated in FIG. 3 to illustrate the provision of a reflective layer  78 . According to the embodiment of an electromechanical grating device  100  the applied reflective layer  78  needs to have good electric conducting properties. The reflective layer  78  is deposited atop the ribbon layer  70 . The light reflecting properties of the reflective layer  78  improve the efficiency of diffraction when operating the electromechanical grating device  100 . It is important that the device has a maximized efficiency of diffraction. The material for the reflective layer  78  with conducting properties is selected from the group consisting of aluminum, titanium, gold, silver, tungsten, silicon alloys and indium tinoxide. 
     Referring now to FIG. 11, which is a cross-sectional view along plane A—A indicated in FIG. 3 to illustrate the patterning of the reflective layer  78  and the ribbon layer  70 . The reflective layer  78  (also conducting properties) and the ribbon layer  70  are patterned using photolithographic processing and etching. First the reflective layer  78  is patterned. Then the ribbon layer  70  is patterned using the remaining reflective layer  78  as a mask for etching. This etching process defines first and second conductive region  78   a  and  78   b  (see FIG. 2 and 3) of the reflective layer  78 . The first and second conductive region  78   a  and  78   b  provide an electrical and mechanical isolation of the deformable ribbon elements  72   a  and  72   b . The interconnect  75  connects the first conductive region  78   a  with the bottom conductive layer  56 . 
     Referring now to FIG. 12, which is a cross along plane A—A indicated in FIG. 3 to illustrate the removal of a sacrificial layer  66  material within the channel  67 . The sacrificial layer  66  filling the cavity  67  is removed by dry etching methods using xenon difluoride to yield the device cross-sectional view of the channel  67  as illustrated in FIG.  12 . The inventive method of production of the mechanical grating device  100  results in channel  67  wherein the first and second side walls  67   a  and  67   b  define a channel  67  which has a constant cross-section in the longitudinal dimension of the mechanical grating device  100 . Additionally the above described method provides a mechanical grating device  100  wherein each of the deformable ribbon elements has an exact defined top surface  70   a  and bottom surface  70   b . In the un-actuated state, no force is applied to the deformable ribbon elements  72   a  and  72   b , all the deformable ribbon elements  72   a  and  72   b  are coplanar with respect to their top and bottom surface  70   a  and  70   b . Preferably, the surface of the to-be-deposited ribbon elements  72   a  and  72   b  remain optically coplanar on both their bottom and top surfaces after removal of the material  66 . 
     FIG. 13, a cross-sectional view along plane B—B, illustrates the mechanical grating device  100  with no applied voltage to the second reflective region  78   b . In case there is no voltage applied between the thin bottom conducting layer  56  and the first or the second reflective layer  78   a  or  78   b  which are formed atop of each deformable ribbon element  72   a  and  72   b , all of the ribbon elements  72   a  and  72   b  are coplanar. In the embodiment shown in FIG. 13 the top layer on the deformable ribbon elements  72   a  and  72   b  is a reflective and conductive layer  78   a  and  78   b  which defines a top surface  70   a  of the coplanar ribbon elements  72   a  and  72   b . The surface of the ribbon elements  72   a  and  72   b  facing the base  50  of the mechanical grating device  100  is designated as a bottom surface  70   b . On the bottom  67   c  of the channel  67  a plurality of standoffs  61  are formed. Each standoff  61  defines a top surface  54   a  which faces the bottom surface  64   a  of the of the ribbon elements  72   a  and  72   b . The depth channel  67  defines the distance between the bottom surface  70   b  of the ribbon elements  72   a  and  72   b  and the bottom  67   c  of the channel  67  or the top surface  54   a  of the standoffs  61 . The plurality of standoffs  61  is distributed on the bottom  67   c  of the channel  67 , such that every second deformable ribbon element  72   a  or  72   b  is associated with a standoff  61 . In the embodiment shown here each of the deformable ribbon elements  72   b  of the second set are associated with standoffs  61 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST 
       10  diffraction grating 
       11  incident light beam 
       12  deformable elements 
       13  diffracted beam 
       14  frame 
       13  spacer layer 
       20  base 
       22  substrate 
       24  passivating layer 
       26  conducting layer 
       30  thin layer 
       50  base 
       50   a  top surface of base 
       52  substrate 
       53  surface of the base 
       54   a  top surface of standoffs 
       56  bottom conductive layer 
       58  protective layer 
       60  standoff layer 
       61  standoff 
       64   a  upper surface level of the spacer layer 
       65  spacer layer 
       66  sacrificial layer 
       67  channel 
       67   a  first side wall of the channel 
       67   b  second side wall of the channel 
       67   c  bottom of the channel 
       70  ribbon layer 
       70   a  top surface of the coplanar ribbon elements 
       70   b  bottom surface of the coplanar ribbon elements 
       72   a  first set of deformable ribbon elements 
       72   b  second set of deformable ribbon elements 
       74  opening 
       75  interconnect 
       76  thick conducting 
       78   a  first conducting region 
       78   b  second conducting region 
       100  mechanical grating device