Patent Publication Number: US-2009218312-A1

Title: Method and system for xenon fluoride etching with enhanced efficiency

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 11/083,030, filed Mar. 17, 2005, and claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Patent Application No. 60/613,423, filed on Sep. 27, 2004, the disclosure of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates generally to fabricating electronic devices. More particularly, the disclosure relates to an apparatus and method useful for fabricating a microelectromechanical systems device. 
     2. Description of the Related Art 
     Microelectromechanical systems (MEMS) include micromechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, and/or other micromachining processes that etch away parts of substrates and/or deposited material layers or that add layers to form electrical and electromechanical devices. Some of these processes are similar to those originally developed for use in semiconductor manufacturing. 
     A spatial light modulator is an example of a MEMS. A variety of different types of spatial light modulators can be used for imaging applications. One type of a spatial light modulator is an interferometric modulator. An interferometric modulator may comprise a pair of conductive plates, one or both of which may be partially transparent and capable of relative motion upon application of an appropriate electrical signal. One plate may comprise a stationary layer deposited on a substrate, the other plate may comprise a metallic membrane suspended over the stationary layer. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed. 
     SUMMARY OF CERTAIN EMBODIMENTS 
     The system, method, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled “Detailed Description of Certain Embodiments” one will understand how the features of this invention provide advantages that include, for example, improved throughput, control, and process flexibility. 
     Provided herein is an apparatus and a method useful for manufacturing MEMS devices. An aspect of the disclosed apparatus provides a substrate comprising an etchable material exposed to a solid-state etchant, wherein the substrate and the solid-state etchant are disposed in an etching chamber. In some embodiments, the solid state etchant is moved into close proximity to the substrate. In other embodiments, a configurable partition is between the substrate and the solid-state etchant is opened. The solid-state etchant forms a gas-phase etchant suitable for etching the etchable material. In some preferred embodiments, the solid-state etchant is solid xenon difluoride. The apparatus and method are advantageously used in performing a release etch in the fabrication of optical modulators. 
     Some embodiments provide an apparatus for etching comprising a chamber, a support for a substrate on which a microelectromechanical systems device is formed, and solid xenon difluoride, wherein the support and the solid xenon difluoride are disposed within the chamber. 
     Other embodiments disclosed herein provide an apparatus for etching comprising an etchant module and an etching chamber, wherein the etching chamber comprises an interior, an exterior, and a support for a substrate therein, wherein the apparatus has a first configuration, in which the etchant module is disposed in the interior of the etching chamber and is in fluid communication with a substrate disposed on the support, and a second configuration, in which the etchant module is not in fluid communication with the substrate disposed on the support. In some embodiments, the etchant module is movable between a retracted position and an extended position; in the retracted position, the etchant module is substantially outside the etching chamber; and in the extended position the etchant module is substantially within the etching chamber. 
     Other embodiments provide an apparatus for etching comprising: an etching chamber; a support for a substrate on which microelectromechanical device is formed; an etchant module; and a means for exposing a substrate on the support to the etchant module within the etching chamber. 
     Other embodiments provide an apparatus for etching comprising a support for a substrate on which a microelectromechanical systems device is formed and solid xenon difluoride, wherein the support and the solid xenon difluoride are proximate for a vapor formed from the solid xenon difluoride to etch a substrate comprising an etchable material. In some embodiments, the support and solid xenon difluoride are less than about 10 cm apart. 
     Other embodiments disclosed herein provide a method for fabricating a microelectromechanical systems device and a microelectromechanical systems device fabricated according to the method, wherein the method comprises: supporting a substrate in an etching chamber comprising an interior, an exterior, and a support for a substrate; and disposing an etchant module in the interior of the etchant chamber and in fluid communication with the substrate, wherein a solid-state etchant is supported in the etchant module. In some embodiments, the microelectromechanical systems device is an interferometric modulator. 
     Other embodiments provide a method for fabricating a microelectromechanical systems device and a microelectromechanical systems device fabricated according to the method, wherein the method comprises: disposing within an etching chamber a substrate comprising an etchable material, and disposing within the etching chamber a solid etchant, wherein the solid etchant forms a gas-phase etchant capable of etching the etchable material. 
     Other embodiments provide a method for fabricating a microelectromechanical systems device and a microelectromechanical systems device fabricated according to the method, wherein the method comprises: disposing a substrate within an etching chamber; extending an etchant module into the etching chamber; and allowing the gas-phase etchant to etch the material. A solid etchant is supported on the etchant module, and the solid etchant forms a gas-phase etchant capable of etching a material on the substrate. 
     Other embodiments provide a method for fabricating a microelectromechanical systems device and a microelectromechanical systems device fabricated according to the method, wherein the method comprises: providing solid xenon difluoride within an etch chamber; supporting a substrate comprising an etchable material within the etch chamber; and etching the etchable material from the substrate with a vapor generated by the solid xenon difluoride. 
     Other embodiments provide a method for fabricating a microelectromechanical systems device and a microelectromechanical systems device fabricated according to the method, wherein the method comprises: supporting a substrate comprising an etchable material within the etch chamber; and positioning solid xenon difluoride sufficiently proximate to the substrate such that a vapor formed by the solid xenon difluoride etches the etchable material. In some embodiments, the support and solid xenon difluoride are less than about 10 cm apart. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the invention will be readily apparent from the following description and from the appended drawings (not to scale), which are meant to illustrate and not to limit the invention. 
         FIG. 1  is an isometric view depicting a portion of one embodiment of an interferometric modulator display in which a movable mirror of a first interferometric modulator is in a reflective, or “on,” position at a predetermined distance from a fixed mirror and the movable mirror of a second interferometric modulator is in a non-reflective, or “off” position. 
         FIG. 2  is a system block diagram illustrating one embodiment of an electronic device incorporating a 3×3 interferometric modulator display. 
         FIG. 3  is a diagram of movable mirror position versus applied voltage for one exemplary embodiment of an interferometric modulator of  FIG. 1 . 
         FIG. 4  is an illustration of sets of row and column voltages that may be used to drive an interferometric modulator display. 
         FIG. 5A  and  FIG. 5B  illustrate one exemplary timing diagram for row and column signals that may be used to write a frame of display data to the 3×3 interferometric modulator display of  FIG. 2 . 
         FIG. 6A  is a cross section of the device of  FIG. 1 .  FIG. 6B  is a cross section of an alternative embodiment of an interferometric modulator.  FIG. 6C  is a cross section of an alternative embodiment of an interferometric modulator 
         FIG. 7A-FIG .  7 E illustrate in cross section certain intermediate structures in the fabrication of an embodiment of an interferometric modulator. 
         FIG. 8  illustrates an embodiment of an apparatus useful for performing a release etch in the fabrication of a MEMS device. 
         FIG. 9  is a flowchart illustrating an embodiment of a method for performing a release etch using the apparatus of  FIG. 8 . 
         FIG. 10A  is a perspective view of an embodiment of an apparatus suitable for performing a release etch in the fabrication of a MEMS device.  FIG. 10B  and  FIG. 10C  are detail views of a module for the apparatus illustrated in  FIG. 10A .  FIG. 10D  and  FIG. 10E  are top views and cross sections, respectively, of another embodiment of an etching chamber. 
         FIG. 11A-FIG .  11 D illustrate alternative embodiments for an etchant module. 
         FIG. 12A  and  FIG. 12B  illustrate alternative embodiments for etching chambers. 
         FIG. 13  is a flowchart illustrating an embodiment of a method for performing a release etch using the apparatus illustrated in  FIG. 10A  or  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS 
     As described in more detail below, preferred embodiments disclosed herein provide an etching chamber comprising a support for a MEMS substrate and a solid etchant disposed within the etching chamber. In some embodiments, the solid etchant is supported in a module that is movable between a position distal of the support for the MEMS substrate and a position proximal of the support. In other embodiments, a configurable partition between the MEMS substrate and the solid etchant is opened. In some preferred embodiments, the solid etchant is xenon difluoride. Also described herein are embodiments of methods of using the apparatus in the fabrication of a MEMS device, and in particular, an interferometric modulator. These and other embodiments are described in greater detail below. 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the invention may be implemented in any device that is configured to display an image, whether in motion (e.g. video) or stationary (e.g. still image), and whether textual or pictorial. More particularly, it is contemplated that the invention may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g. odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g. display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures (e.g. tile layouts), packaging, and aesthetic structures (e.g. display of images on a piece of jewelry). More generally, the invention may be implemented in electronic switching devices. 
     Spatial light modulators used for imaging applications come in many different forms. Transmissive liquid crystal display (LCD) modulators modulate light by controlling the twist and/or alignment of crystalline materials to block or pass light. Reflective spatial light modulators exploit various physical effects to control the amount of light reflected to the imaging surface. Examples of such reflective modulators include reflective LCDs, and digital micromirror devices. 
     Another example of a spatial light modulator is an interferometric modulator that modulates light by interference. One interferometric modulator display embodiment comprising a reflective MEMS display element is illustrated in Error! Reference source not found. In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, a bi-stable display element reflects incident light to a user. When in the dark (“off” or “closed”) state, a bi-stable display element reflects little visible light to the user. Depending on the embodiment, the display  110  may be configured to reflect more visible light in the “off” state than in the “on” state, i.e., the light reflectance properties of the “on” and “off” states are reversed. MEMS pixels can also be configured to reflect only selected colors, producing a color display rather than black and white. 
       FIG. 1  is an isometric perspective view depicting two adjacent pixels in a row of one embodiment of a visual display, comprising a MEMS interferometric modulator. An interferometric modulator display comprises a row/column array of these interferometric modulators. Each interferometric modulator includes a pair of mirrors positioned at a distance from each other to form a resonant optical cavity. In one embodiment, at least one of the mirrors in partially transmissive. In one embodiment, one of the mirrors may be moved between at least two positions. In the first position, the movable mirror is positioned at a first distance from the other mirror so that the interferometric modulator is predominantly reflective. In the second position, the movable mirror is positioned at a different distance, e.g. adjacent to the fixed mirror, such that the interferometric modulator is predominantly absorbing. 
     The depicted portion of the pixel array includes two adjacent interferometric modulators  12   a  and  12   b  in a row. In the depicted embodiment of the interferometric modulator, a movable mirror  14   a  is illustrated in the reflective (“relaxed”, “on”, or “open”) position at a predetermined distance from a fixed, partial mirror  16   a,    16   b . The movable mirror  14   b  of the interferometric modulator  12   b  is illustrated in the non-reflective (“actuated”, “off”, or “closed”) position adjacent to the partial mirror  16   b.    
     The fixed mirrors  16   a,    16   b  are electrically conductive, and may be fabricated, for example, by depositing layers of chromium and indium-tin-oxide onto a transparent substrate  20  that are patterned into parallel strips, and may form row electrodes. The movable mirrors  14   a,    14   b  along the row may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes  16   a,    16   b ) on the substrate  20 , with aluminum being one suitable material, and may form column electrodes. 
     With no applied voltage, a cavity  19  exists between the two layers  14 ,  16 . However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel charges, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable electrode is forced against the stationary electrode (a dielectric material may be deposited on the stationary electrode to prevent shorting and control the separation distance) as illustrated by the pixel on the right in  FIG. 1 . The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation can control the reflective vs. non-reflective state of each pixel. 
       FIG. 2  through  FIG. 5  illustrate one exemplary process and system for using an array of interferometric modulators in a display application.  FIG. 2  is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor  21  which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor  20  may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application. 
     In one embodiment, the processor  20  is also configured to communicate with an array controller  22 . In one embodiment, the array controller  22  includes a row driver circuit  24  and a column driver circuit  26  that provide signals to the array  30 . The cross section of the array illustrated in  FIG. 1  is shown by the lines  1 - 1  in  FIG. 2 . Portions of the array controller  22  as well as additional circuitry and functionality may be provided by a graphics controller which is typically connected between the actual display drivers and a general purpose microprocessor. Exemplary embodiments of the graphics controller include 69030 or 69455 controllers from Chips and Technology, Inc., the S1D1300 series from Seiko Epson, and the Solomon Systech 1906. 
     For MEMS interferometric modulators, the row/column actuation protocol may take advantage of a hysteresis property of these devices illustrated in  FIG. 3 . It may require, for example, a 10 volt potential difference to cause a pixel to deform from the relaxed state to the actuated state. However, when the voltage is reduced from that value, the pixel may not relax until the voltage drops below 2 volts. There is thus a range of voltage, about 3 V to about 7 V in the example illustrated in  FIG. 3 , where there exists a stability window within which the device will remain in whatever state it started in. The row/column actuation protocol is therefore designed such that during row strobing, pixels in the strobed row that are to be actuated are exposed to a voltage difference of about 10 volts, and pixels that are to be relaxed are exposed to a voltage difference of close to zero volts. After the strobe, the pixels are exposed to a steady state voltage difference of about 5 volts such that they remain in whatever state the row strobe put them in. After being written, each pixel sees a potential difference within the “stability window” of 3-7 volts in this example. This feature makes the pixel design illustrated in  FIG. 1  stable under the same applied voltage conditions in either an actuated or relaxed pre-existing state. Since each pixel of the interferometric modulator, whether in the actuated or relaxed state, is essentially a capacitor formed by the fixed and moving mirrors, this stable state can be held at a voltage within the hysteresis window with almost no power dissipation. Essentially no current flows into the pixel if the mirror is not moving and the applied potential is fixed. 
     In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row  1  electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row  2  electrode, asserting the appropriate pixels in row  2  in accordance with the asserted column electrodes. The row  1  pixels are unaffected by the row  2  pulse, and remain in the state they were set to during the row  1  pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of other protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention. 
       FIG. 4  and  FIG. 5  illustrate one possible actuation protocol for creating a display frame on the 3×3 array of  FIG. 2 .  FIG. 4  illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves of  FIG. 3 . In the  FIG. 4  embodiment, actuating a pixel involves setting the appropriate column to −V bias , and the appropriate row to +ΔV. Relaxing the pixel is accomplished by setting the appropriate column to +V bias , and the appropriate row to the same +ΔV. In those rows where the row voltage is held at zero volts, the pixels are stable in whatever state they were originally in, regardless of whether the column is at +V bias , or −V bias . 
       FIG. 5B  is a timing diagram showing a series of row and column signals applied to the 3×3 array of  FIG. 2  which will result in the display arrangement illustrated in  FIG. 5A , where actuated pixels are non-reflective. Prior to writing the frame illustrated in  FIG. 5A , the pixels can be in any state, and in this example, all the rows are at 0 volts, and all the columns are at +5 volts. In this state, all pixels are stable in their existing actuated or relaxed states. 
     In the  FIG. 5A  frame, pixels ( 1 , 1 ), ( 1 , 2 ), ( 2 , 2 ), ( 3 , 2 ) and ( 3 , 3 ) are actuated. To accomplish this, during a “line time” for row  1 , columns  1  and  2  are set to −5 volts, and column  3  is set to +5 volts. This does not change the state of any pixels, because all the pixels remain in the 3-7 volt stability window. Row  1  is then strobed with a pulse that goes from 0, up to 5 volts, and back to zero. This actuates the ( 1 , 1 ) and ( 1 , 2 ) pixels and relaxes the ( 1 , 3 ) pixel. No other pixels in the array are affected. To set row  2  as desired, column  2  is set to −5 volts, and columns  1  and  3  are set to +5 volts. The same strobe applied to row  2  will then actuate pixel ( 2 , 2 ) and relax pixels ( 2 , 1 ) and ( 2 , 3 ). Again, no other pixels of the array are affected. Row  3  is similarly set by setting columns  2  and  3  to −5 volts, and column  1  to +5 volts. The row  3  strobe sets the row  3  pixels as shown in  FIG. 5A . After writing the frame, the row potentials are zero, and the column potentials can remain at either +5 or −5 volts, and the display is then stable in the arrangement of  FIG. 5A . It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the present invention. 
     The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,  FIG. 6A-FIG .  6 C illustrate three different embodiments of the moving mirror structure.  FIG. 6A  is a cross section of the embodiment of  FIG. 1 , where a strip of metal material  14  is deposited on orthogonally extending supports  18 . In  FIG. 6B , the moveable mirror is attached to the supports at the corners only, on tethers  32 . In  FIG. 6C , the mirror  14  is suspended from a deformable film  34 . This embodiment has benefits because the structural design and materials used for the mirror  14  can be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer  34  can be optimized with respect to desired mechanical properties. The production of various types of interferometric devices is described in a variety of published documents, including, for example, U.S. Published Application 2004/0051929. A wide variety of well known techniques may be used to produce the above described structures involving a series of material deposition, patterning, and etching steps. 
     Interferometric modulators of the general designs described above and disclosed in U.S. Pat. No. 5,835,255, the disclosure of which is incorporated by reference, and those illustrated in  FIG. 6A-FIG .  6 C include a cavity  19  between the mirrors  14  and  16  through which the mirror  14  moves with respect to the mirror  16 . In some embodiments, the cavity  19  is created by forming a sacrificial layer that is removed in a latter stage in the processing, as described in greater detail below. 
     U.S. Provisional App. No. 60/613466 entitled “Device and Method for Interferometric Modulation Having Oxide-Stops” filed on Sep. 27, 2004, the disclosure of which is incorporated by reference, also discloses manufacturing techniques for the fabrication of an interferometric modulator. A sacrificial layer is formed and etched away to release the secondary mirror/conductor from the primary mirror/conductor, thereby forming a cavity and permitting movement therebetween. This etch is also referred to herein as a “release etch,” because the flexible membrane is released by the etch thereby permitting flexure of this membrane. 
     As discussed more fully below, in some preferred embodiments, solid XeF 2  is a source of a gas-phase etchant used in the release etch. As such, the following description refers to solid XeF 2  as the source of the gas-phase etchant, although those skilled in the art will understand that the disclosure is not so limited. Methods and apparatus for enhancing the efficiency of the XeF 2  release etch are also described more fully below. As discussed in greater detail below, materials etchable by XeF 2  include materials comprising silicon, titanium, zirconium, hafnium, vanadium, tantalum, niobium, molybdenum, and tungsten. 
     A brief description of certain steps in the fabrication of an embodiment of an interferometric modulator follows, and is illustrated schematically in cross section in  FIG. 7A-FIG .  7 E. Some embodiments of the illustrated process use semiconductor manufacturing techniques known in the art, for example photolithography, deposition, masking, etching, and the like. Deposition steps include “dry” methods, for example, chemical vapor deposition (CVD), and “wet” methods, for example, spin coating. Etching steps include “dry” methods, for example, plasma etch, and “wet” methods. Those skilled in the art will understand that a range of methods are useful in the fabrication of the optical modulator, and that the process described below is only exemplary. 
       FIG. 7A  illustrates a stage in the fabrication of a interferometric modulator  700  in which an optical stack is formed on a substrate  720 . The optical stack comprises the fixed or primary mirror  714  discussed above. In some embodiments, the optical stack further comprises a transparent conductor, for example, an indium tin oxide layer, and/or a supporting layer, for example, a silicon oxide layer. Some embodiments comprise a metallic mirror, for example, chromium, aluminum, titanium, and/or silver. Other embodiments comprise a dielectric mirror. The optical stack is formed by methods known in the art, for example, deposition, patterning, and etching. 
     In  FIG. 7B , a supporting layer  740  has been formed over the optical stack and substrate  720 . In the illustrated embodiment, the supporting layer  740  comprises a lower or “bulk” portion  750  and an upper layer or “stop” portion  760 . The lower portion  750  comprises a material that is removable in a later etching step, for example, molybdenum, silicon, a silicon-containing material (e.g. silicon nitride, silicon oxide, etc.), tungsten, and/or titanium. The upper portion  760  comprises a material that resists the etchant used to etch the lower portion  750 , for example, a metal such as aluminum, silver, chromium, and/or titanium. In some embodiments, the upper portion  760  comprises a dielectric material, for example, a metal oxide and/or aluminum oxide. In some embodiments, the lower portion  750  and upper portion  760  is graded. Some embodiments do not comprise a supporting layer. 
       FIG. 7C  illustrates a stage in the fabrication of the device  700  in which the upper portion  760  of the supporting layer has been patterned and etched to form a variable thickness supporting layer  765 , as well as to expose sections of the lower portion  750  of the supporting layer. The patterning is performed using any method known in the art, for example, using a photoresist. In the illustrated embodiment, unmasked regions of the upper portion  760  of the supporting layer were etched, while substantial portions of the lower portion  750  were not. 
       FIG. 7D  illustrates a stage in which a sacrificial layer  710  has been deposited on the supporting layer  740 . The sacrificial layer was patterned, etched, and planarized, and support posts  718  formed therein. A second mirror/upper electrode assembly  716  was formed over the sacrificial layer  710  and posts  718  by deposition, patterning, and etching. The sacrificial layer  710  comprises a material that is selectively etchable relative to the other materials exposed to a selected etchant. Suitable materials and etchants are discussed in greater detail below. In some preferred embodiments, the sacrificial layer  710  comprises molybdenum and/or silicon. 
       FIG. 7E  illustrates the device  700  after etching the sacrificial layer  710 . This etch step is referred to herein as a “sacrificial etch” and/or a “release etch.” Methods and procedures for performing a release etch are discussed in greater detail below. In the illustrated embodiment, parts of the lower portion  750  of the supporting layer were also etched. In some embodiments, the lower portion  750  is partially etched or not etched at all. In other embodiments, the supporting layer  740  does not comprise a lower portion  750 . In the illustrated embodiment, removal of the sacrificial layer  710  and portions of the lower portion  750  of the supporting layer forms a cavity  722 . Suitable etchants are discussed in greater detail below. In some preferred embodiments, the etchant used in the sacrificial and/or release etch comprises xenon difluoride. Without being bound by any theory, XeF 2  is believed to be a source of F 2  gas, which is the active etching species. 
     At ordinary temperatures and pressures, XeF 2  is a crystalline solid that sublimes with a vapor pressure of about 3.8 Torr at room temperature (0.5 kPa at 25° C.). XeF 2  vapor etches certain materials without the need to generate a plasma. Materials etchable using XeF 2  vapor include silicon, molybdenum, and titanium, which are selectively etched over other materials including silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), aluminum, and chromium. At ambient temperature, XeF 2  has a vertical etch rate of about 50 Å/s for molybdenum and about 350 Å/s for silicon. In comparison, SiO 2 , Al, and Al 2 O 3  are substantially not etched by XeF 2 . Etch rates are known in the art, as disclosed, for example, in  IEEE J. Microelectromech. Syst.,  1996, 5(4), 262;  IEEE J. Microelectromech. Syst.,  1996, 12(6), 761. In some embodiments, the partial pressure of the XeF 2  is from about 0.1 torr (13 Pa) to about 10 torr (1.3 kPa). Process temperatures range from ambient temperature to about 100° C. 
       FIG. 8  illustrates an apparatus  800  useful for implementing a XeF 2  etching step. The apparatus  800  comprises a XeF 2  vessel  812  in which XeF 2  crystals are housed, an expansion chamber  814 , an etching chamber  816 , and a vacuum source  818 . The XeF 2  vessel  812  is fluidly connected to the expansion chamber  814  through a first conduit  820  and a first valve  822 . The expansion chamber  814  is in turn fluidly connected to the etching chamber  816  through a second conduit  824  and a second valve  826 . The etching chamber  816  is fluidly connected to the vacuum source  818  through a third conduit  828  and a third valve  830 . 
       FIG. 9  illustrates a method  900  for etching a substrate using XeF 2  with reference to the apparatus illustrated in  FIG. 8 . In step  910 , a substrate or batch of substrates to be etched (not illustrated), is loaded into the etching chamber  816 . 
     In step  920 , the second and third valves  826  and  830  are opened, fluidly connecting the expansion chamber  814  and etching chamber  816  to the vacuum source  818 , thereby evacuating the expansion chamber  814  and etching chamber  816 . In step  920 , the first valve  822  between the XeF 2  vessel  812  and the expansion chamber  814  remains closed. 
     In step  930 , the second valve  826  is closed, and the first valve  822  is opened. Opening the first valve  822  permits XeF 2  vapor to fill the expansion chamber  814  from the XeF 2  vessel  812 . 
     In step  940 , the second valve  826  between the expansion chamber  814  and the etching chamber  816  is opened, and the first and third valves  822  and  830  are closed. Opening the second valve  826  permits transfers XeF 2  from the expansion chamber  814  to the etching chamber  816 , which etches the substrate(s) therein. 
     Steps  910 - 930 , in which no etching occurs, take time, thereby reducing the throughput of the apparatus  800 . In some embodiments, the conduits ( 820 ,  824 , and  828 ) and valves ( 822 ,  826 , and  830 ) fluidly connecting the XeF 2  vessel  812 , expansion chamber  814 , etching chamber  816 , and vacuum source  818  also reduce one or more mass and/or fluid transport characteristics of the apparatus  800 . 
     An embodiment of an apparatus  1000  illustrated in  FIG. 10A-FIG .  10 C permits solid XeF 2  and the substrate-to-be-etched to reside in close proximity within the same chamber during the etching step.  FIG. 10A  illustrates an etching chamber  1010  comprising inner sidewalls  1012  defining a central or main cavity  1014  therein.  FIG. 10A  includes a cut-away view of the chamber  1010  showing a plurality of substrates-to-be-etched  1016  disposed on a substrate support  1018 , within the central cavity  1014 . In the illustrated embodiment, the etching chamber  1010  is substantially cylindrical; however, those skilled in the art will understand that the etching chamber  1010  can have any suitable shape. 
       FIG. 10D  illustrates a top view of an embodiment of an etching chamber  1010 ′ in which the inner sidewalls  1012 ′ of the etching chamber substantially match the size and shape of the substrate support  1018 ′, which is in turn, substantially similar to the size and shape of the substrate  1016 ′. In the illustrated embodiment, the substrate is substantially rectangular. Those skilled in the art will understand that other configurations are possible.  FIG. 10E  is a cross section view of the etching chamber  1010 ′. In the illustrated embodiment, the top of the etching chamber  1013 ′ along with the sidewalls  1012 ′ defines the central cavity  1014 ′. In some embodiments, the geometry of the central cavity  1014 ′ is configured to improve the efficiency of the etching step performed therein. For example, in the illustrated embodiment, if the distance between the top of the etching chamber  1013 ′ and the substrate  1016 ′ is relatively small, the volume of the etching chamber  1014 ′ is insufficient to hold a sufficient amount of etchant, for example, XeF 2  vapor, to efficiently etch the substrate  1016 ′. On the other hand, if the distance between the top of the etching chamber  1013 ′ and the substrate  1016 ′ is relatively large, XeF 2  vapor from near the top  1013 ′ will take a significant time to diffuse to the substrate  1016 ′. The etching chamber  1010 ′ illustrated in  FIG. 10D  and  FIG. 10E  is configured for etching a single substrate at a time. In other embodiments, the etching chamber is configured for processing a plurality of substrates simultaneously. Those skilled in the art will understand that the dimensions of the etching chamber will depend on factors including the sizes of the substrate or substrates, the amount of material to be etched, the nature of other processes that are performed in the etching chamber. In some embodiments, the lateral dimensions, e.g. the length and width, of the etching chamber are up to about 20% larger than the size of the substrate. For example, some embodiments provide an etching chamber  1010 ′ with a length and/or width of from greater than about 100 mm to about 120 mm for a 100-mm diameter substrate. Other embodiments provide for a 370-mm×470-mm substrate, an etching chamber  1010 ′ with dimensions of from greater than about 370 mm to about 450 mm, by from greater than about 470 mm to about 570 mm. In some embodiments, the lateral dimensions, e.g. the length and width, of the etching chamber are up to about 10% larger than the size of the substrate. 
     Referring back to  FIG. 10A-FIG .  10 C, the etching chamber  1010  optionally includes one or more other components useful for performing other processing tasks, for example, deposition, patterning, etching, testing, packaging, and the like (not illustrated). In some embodiments, the substrate holder  1018  includes optional features, including, for example, a heater, one or more translation stages, and/or other features known in the art useful in processing the substrate(s)  1016 . 
     In some embodiments, the inner sidewalls  1012  of the etching chamber  1010  and/or the components enclosed therein comprise one or more materials that are not etched or are minimally etched by XeF 2 . Such materials include without limitation, stainless steel, aluminum, nickel, nickel alloys, monel, hastelloy, glass, fused silica, alumina, sapphire, polymer resins, acrylic, polycarbonate, polytetrafluoroethylene (Teflon®), polychlorotrifluoroethylene (Kel-F®, Tefzel®), perfluoroelastomers (e.g., Kalrez®), and alloys, blends, copolymers, and composites thereof. Components include windows, the substrate stage  1018 , and other components that are described below. In some embodiments, other materials are used. For example, in some embodiments, one or more of the components is affected by XeF 2  and is disposable and/or replaceable. 
     Returning to  FIG. 10A , the illustrated apparatus  1000  also comprises a purge system  1020  fluidly connected to the etching chamber  1010  through a purge inlet  1022  and a purge outlet  1024 . A source of purge gas  1026  is fluidly connected to the purge inlet  1022  through line  1028  and an inlet valve  1030 . The purge gas is any suitable purge gas known in the art, for example, nitrogen, helium, argon, neon, and combinations thereof. The source of purge gas is any source known in the art, for example, a compressed gas cylinder, a gas generator, a liquefied gas, and the like. In some embodiments, the purge gas comprises another gas. The purge outlet  1024  is fluidly connected to a vacuum source (not illustrated) through outlet valve  1034  and line  1032 . In some embodiments, the purge system does not comprise a purge outlet. For example, in some of these embodiments, the inlet valve  1030  and the outlet valve  1034  are fluidly connected to a manifold (not illustrated), and the manifold is fluidly connected to the purge inlet  1022 . 
     The apparatus  1000  is also equipped with a opening (not illustrated) through which the substrates  1016  are loaded and unloaded from the apparatus  1000 . The opening is of any type known in the art, for example, a gate valve between the etching chamber  1010  and a handling chamber (not illustrated). 
     In the illustrated embodiment, a solid etchant, for example, solid XeF 2 , is held in an etchant holding unit  1035  mounted to the etching chamber  1010 . The illustrated apparatus  1000  comprises one etchant holding unit  1035 . Other embodiments comprise a plurality of etchant holding units. In the illustrated embodiment, etchant unit  1035  is equipped with a translation device  1036  that comprises rails  1040 , bellows  1042 , and a threaded shaft (not illustrated) engaging a threaded coupler (not illustrated) and a rotatable control  1044 . The illustrated translation device  1036  further comprises an arm (not illustrated) disposed within the bellows  1042 . Rotating the rotatable control  1044  rotates the threaded shaft in the threaded coupler, thereby translating (extending or retracting) the arm. In the illustrated embodiment, the bellows  1042  is compressed or expanded to accommodate the translation. Those skilled in the art will understand that other mechanisms are useful for the translation device  1036 , for example, a pantograph, a rack and pinion, a piston and cylinder, a rail, and the like. Other mechanisms include motors, stepper motors, solenoids, pneumatics, and/or hydraulic devices. In other embodiments, the motion is rotational, as described in greater detail below, or has another type of motion known in the art. In some embodiments, the translation device  1036  is automated, for example, controlled using a computer and/or microprocessor (not illustrated). In some embodiment, the computer and/or microprocessor controls also other functions of the apparatus, for example, the purge system, substrate loading, substrate unloading, and/or loading solid XeF 2 . 
     The etchant holding unit  1035  comprises an access port  1038 . The access port  1038  comprises a passageway therethrough that opens into an open inner region  1039  therein. In the illustrated embodiment, the access port  1038  also includes a door  1050  that provides access to the inner region  1039  of the access port. In some embodiments, the door  1050  is automated, thereby permitting automated loading of XeF 2 . In the illustrated embodiment, solid XeF 2  is loaded into the XeF 2  unit  1035  through the door  1050 . In some embodiments, the open inner region  1039  is fluidly connected to a purge system, for example, a source of purge gas and/or a vacuum source (not illustrated). The purge system is useful, for example, when solid XeF 2  is loaded into the XeF 2  unit  1035 . 
     Also illustrated in  FIG. 10A  through a cutaway in the access port  1038  is a module  1052  for supporting solid XeF 2 . An enlarged view of the module  1052  is provided in  FIG. 10B . In the illustrated embodiment, the module  1052  includes a platform  1056  that supports a solid XeF 2  sample  1054 . The platform  1056  is secured to a rod  1058 , which is in turn secured to the arm of the translation device  1036 . Accordingly, the translation device  1036  is capable of longitudinally positioning the module  1052  on which the solid XeF 2    1054  is supported. 
     In  FIG. 10A , the module  1052  is in a retracted position, within the inner region  1039  of the access port  1038 . The module  1052  is not disposed in the central cavity  1014  of the chamber  1010 . In the illustrated configuration, the access port  1038  is isolated from the central cavity  1014  of the chamber  1010  such that vapor from the solid XeF 2    1054  is substantially contained within the access port  1038  and does not enter the central cavity  1014  of the chamber  1010 . In the illustrated retracted position, solid XeF 2    1054  is loaded on the module  1052  through the door  1050 . 
     In an embodiment of the module  1052  illustrated in  FIG. 10B , the solid XeF 2    1054  is supported on the platform  1056 . In the illustrated embodiment, a faceplate  1060  is secured to the platform  1056 . The faceplate  1060  is sized and shaped to engage a matching opening (illustrated as part  1062  in  FIG. 10C ) in the sidewall  1012  of the chamber. In some embodiments, in the retracted position, the module  1052  is substantially sealed from the cavity  1014  of the chamber. For example, in some embodiments, the faceplate  1060  and/or the matching opening  1062  comprises a gasket and/or seal, which assists in substantially retaining XeF 2  and/or F 2  vapor from entering the chamber  1010  when the module  1052  is in the retracted position. In some embodiments, the module  1052  in the retracted position is not substantially sealed from the cavity  1014  of the chamber. In some embodiments, the module  1052  comprises a locking mechanism or mechanisms, useful for example, for maintaining the module in the retracted position and/or extended position. Suitable locking mechanisms are known in the art, for example, a latch between the faceplate  1060  and the sidewall  1012  of the chamber. In some embodiments, the locking mechanism is under automated control, for example, interlocked with the translation device  1036 . 
     The faceplate  1060  physically separates the inner region  1039  of the access port from the central cavity  1014  when the module  1052  is in the retracted position. In the illustrated embodiment, the inner region  1039  of the access port has a relatively small volume, and consequently, relatively poor mass transport characteristics. Even if the faceplate  1060  were absent, when the module  1052  is in the retracted position, XeF 2  vapor diffuses slowly into the central cavity  1014 . In the illustrated embodiment, the mass transport conditions translate into many minutes to hours for the partial pressure of XeF 2  to reach the equilibrium pressure of 3.8 Torr within the cavity  1014  with the module  1052  in the retracted position, even absent the faceplate  1060 . 
     In the embodiment illustrated in  FIG. 10B , the platform  1056  of the XeF 2  module  1052  does not include sidewalls or a backwall, thereby reducing the number of barriers between the solid XeF 2    1054  and the substrate  1016 . In other embodiments, the platform  1056  comprises one or more depressions and/or spoon-shaped areas in which the solid XeF 2  is placed. In some embodiments, the platform  1056  comprises one or more sidewalls and/or backwalls. In some embodiments, the platform  1056  comprises a grate and/or mesh, thereby providing improved mass transport through the platform  1056  by increasing the surface area of the solid etchant  1054  exposed the atmosphere. In some embodiments, the platform comprises a plurality of raised areas supporting the solid XeF 2    1054 , for example a surface with corrugations and/or a raised grid. In some embodiments, the platform  1056  comprises a heater. Those skilled in the art will understand that in other embodiments, the platform  1056  has different configurations. 
       FIG. 10C  is a cutaway view through the sidewall  1012  of the chamber  1010 , illustrating the XeF 2  module  1052  in an extended position. In the extended position, the XeF 2  module extends into the central cavity  1014  of the chamber. The translation stage  1036  is adjusted to extend the platform  1056  supporting the solid XeF 2    1054  through an opening  1062  in the sidewall  1012  and into the central cavity  1014  of the chamber, thereby exposing the substrate  1016  to XeF 2  vapor. 
     In some embodiments, in the extended position, the module  1052  is proximate to the substrate  1016 . In some embodiments, the distance between the module  1052  and the substrate  1016  is not more than from about 1 cm to about 10 cm. In other embodiments, the distance is not more than about 0.5 cm, 2 cm, 3 cm, 4 cm, 5 cm, 6 cm, 7 cm, 8 cm, or 9 cm. For example, in some embodiments in which the substrate-to-be-etched is not larger than about 300 mm (8″), the distance is not greater than about 2 cm. In some embodiments in which the substrate-to-be-etched is at least about 300 mm, the distance is greater than about 5 cm. In other embodiments, the distance between the module  1052  and the substrate  1016  has another value. In the illustrated embodiment, the faceplate  1060  is situated between the module  1052  and the substrate  1016 . In other embodiments, the relative positions of the module  1052  and the substrate  1016  are different, for example with the module  1052  above or below the substrates, or to one side, such that the faceplate  1060  is not between the module  1052  and the substrate  1016 . 
     The illustrated embodiment eliminates the conduits and/or pipes between the solid XeF 2  and the substrates-to-be-etched, thereby provided improved mass transport compared to the apparatus  800  illustrated in  FIG. 8 . Furthermore, the disposition of the solid XeF 2  within the cavity  1014  permits the vapor pressure of the XeF 2  in the cavity  1014  to equilibrate rapidly. 
       FIG. 11A  illustrates a side view of an embodiment of a module  1152  in which the faceplate  1160  is pivotably attached to the platform  1156  using hinge  1164 . When the module  1152  is in the extended position, the faceplate  1160  pivots downwards around the hinge  1164  as illustrated in solid lines in  FIG. 11A . When the module  1152  is retracted in direction y, the faceplate  1160  engages the opening in the sidewall (not illustrated), thereby pivoting the faceplate  1160  into the position illustrated in phantom in  FIG. 11A . 
       FIG. 11B  illustrates a top view of an embodiment of a module  1152 ′ that pivotably moves from an extended position (solid lines) to a retracted position (phantom lines). In the illustrated embodiment, the module  1152 ′ comprises a platform  1156 ′ mounted to a pivot point  1166 ′. A faceplate  1160 ′ is mounted to an edge of the platform  1156 ′. Solid XeF 2    1154 ′ is supported on the platform  1156 ′. In the extended position, the XeF 2    1154 ′ is positioned within the cavity  1114 ′ of the etching chamber. When the module  1152 ′ is pivoted into the retracted position, the faceplate  1160 ′ seals against an inner sidewall  1112 ′ of the chamber, thereby isolating the XeF 2    1154 ′ from the cavity  1114 ′. 
       FIG. 11C  illustrates a side view of an embodiment of a faceplate  1160 ″ pivotably mounted to the inner sidewall  1112 ″ of the chamber using hinges  1164 ″. In the illustrated embodiment, the module  1152 ″ does not comprise a faceplate. A spring  1168 ″ maintains the faceplate  1160 ″ in a closed position when the module  1152 ″ is in the retracted position. As the module  1152 ″ is extended, the platform  1156 ″ bears against and opens the faceplate  1160 ″, thereby permitting extension of the platform  1156 ″ and XeF 2    1154 ″ into the cavity  1114 ″. In other embodiments, the faceplate  1160 ″ is maintained in a closed position by another means, for example, a mechanism that works in concert and/or interlocks with the mechanism that extends and retracts the module  1152 ″. Those skilled in the art will understand that other arrangements between the faceplate and sidewall are possible, for example, pivoting around an axis normal to the faceplate and sidewall, or in which the faceplate seals against the outer sidewall of the etching chamber. In other embodiments, the faceplate blocks and exposes the opening in the sidewall by sliding rather than by pivoting. Some embodiments comprise a plurality of faceplates. In some embodiments, the module is installed on the top or bottom of the etching chamber. In some embodiments, the apparatus comprises a plurality of modules. 
       FIG. 11D  illustrates an embodiment comprising a turntable  1170 ″′ that comprises a plurality of platforms  1156 ″′ and faceplates  1160 ″′. The illustrated turntable  1170 ″′ comprises four platforms  1156 ″′ and faceplates  1160 ″′, although those skilled in the art will understand that more or fewer platforms and/or faceplates are possible. Those skilled in the art will also understand that the number of modules and faceplates need not be equal. The turntable is rotatable around an axis  1072 ″′. In use, a predetermined amount of solid XeF 2  is loaded on one or more of the platforms  1156 ″′. Rotating the turntable  1170 ″′ a predetermined angle around the axis  1072 ″′ moves one of the platforms  1156 ″′ into the cavity  1114 ″′ of the etching chamber. In the illustrated embodiment, the faceplate  1160 ″′ rotates into a position that occludes the opening  1162 ″′ in the sidewall. The embodiment illustrated in  FIG. 11D  is useful, for example, in processes that comprise a plurality of etching steps. Those skilled in the art will understand that the embodiments presented above are only exemplary and that any number of mechanisms are useful for moving a solid etchant into the etching chamber. 
       FIG. 12A  illustrates in cross section an apparatus  1200  comprising an etching chamber  1210 , wherein the etching chamber  1210  comprises a substrate support  1218  and a solid etchant holding area  1235 . Solid XeF 2    1254  is disposed in the solid etchant holding area  1235 . Disposed between the substrate support  1218  and the solid etchant holding area  1235  is a configurable partition  1260 . In the illustrated embodiment, the partition  1260  comprises a set of louvers. Closing the louvers substantially prevents XeF 2  vapor in the etchant holding area  1235  from reaching the substrate support  1218  and a substrate supported thereon  1216 . Opening the louvers permits XeF 2  vapor to etch the substrate  1216 . Those skilled in the art will understand that other mechanisms are useful for the configurable partition  1260 , for example, one or more shutters, gate valves, tambours and/or roll-tops, and the like. Those skilled in the art will understand that embodiments of the apparatus  1200  include other features described above. 
       FIG. 12B  illustrates an embodiment of an apparatus  1200 ′ in which the solid etchant holding area  1235 ′, the configurable partition  1260 ′, and solid XeF 2    1254 ′ are disposed below the substrate support  1218 ′. In the illustrated embodiment, the configurable partition  1260 ′ comprises a set of shutters. 
       FIG. 13  is a flowchart illustrating an embodiment of a method for processing a substrate with reference to the apparatus illustrated in  FIG. 10A-FIG .  10 C. Those skilled in the art will understand that other apparatus are also suitable for performing the method, including other apparatus disclosed herein. In step  1310 , the substrate  1016  is loaded into the chamber  1010 . Optionally, one or more processing steps not using XeF 2  are performed on the substrate  1016  in the etching chamber  1010 . The module  1052  is in the retracted position, thereby sealing the XeF 2    1054  within the inner region  1039  of the access port, and preventing the entry of XeF 2  vapor into the cavity  1014 . The particular processing step will depend on the particular device under fabrication, the configuration of the etching chamber  1010 , and the particular process flow. An example of a suitable processing step includes depositing a layer or film, for example, a sacrificial layer, a mask, and/or a structural layer, using any method compatible with the configuration of the etching chamber  1010 . Examples of suitable methods include spin-coating, sputtering, physical vapor deposition, chemical vapor deposition, atomic layer deposition, molecular beam epitaxy, and the like. Examples of other processing steps include etching using an etchant other than XeF 2 , cleaning, and the like. 
     Step  1320  is an etching step. In step  1320 , the XeF 2  module  1052  is extended into the central cavity  1014  of the etching chamber  1010  using the translation device  1036 , thereby exposing the substrate  1016  to XeF 2  vapor from the solid XeF 2    1054 . The XeF 2  vapor etches materials and/or structures formed on the substrate  1016 , for example, a sacrificial layer in the fabrication of a MEMS device. The module  1052  is then retracted into the access port  1038 . 
     In some embodiments, the material and/or structure is a sacrificial layer used in the fabrication of an interferometric modulator. In some embodiments, the XeF 2  etch comprises a release etch that releases the secondary mirror/conductor  16  as discussed above and illustrated in  FIG. 6A . In some embodiments, the XeF 2  vapor etches another material and/or structure used in the fabrication of a MEMS device, for example, an interferometric modulator. 
     Some embodiments use a predetermined amount of solid XeF 2    1054  in the etching step. The amount of solid XeF 2  is determined, for example, from the type and amount of material-to-be-etched. For example, in some embodiments, the volume of the sacrificial layer-to-be-removed is known. An amount of solid XeF 2    1054  is then selected sufficient to etch the sacrificial layer. In other embodiments, the thickness of the sacrificial layer is unknown. In some embodiments, the amount of solid XeF 2    1054  is selected based on previous experience or on experimentation. In other embodiments, an amount of solid XeF 2    1054  is selected such that substantially all of the solid XeF 2  sublimes, thereby filling the chamber with XeF 2  vapor at a partial pressure of about 3.8 Torr. Those skilled in the art will understand that amount of solid XeF 2  used in these embodiments depends on a variety of factors including the volume and temperature of the cavity. 
     In some embodiments, the progress of the release etch is monitored and the etching is terminated at a predetermined endpoint. In some embodiments, the monitoring is performed optically, for example, in the fabrication of an optical modulator. The monitoring is performed using any suitable device. In some embodiments, the monitoring is performed through a window in the etching chamber  1010 . In other embodiments, optical sensors are disposed within the etching chamber  1010 . In some embodiments, the reflectivity of the substrate is monitored. Those skilled in the art will understand that the reflectivity of the substrate will change as the release etch proceeds in the fabrication of an optical modulator. In some embodiments, the monitoring is performed at one or more wavelengths. 
     Some embodiments use another type of monitoring, for example, of the concentration of particular compounds in the etching chamber. For example, in some embodiments, the concentration of one or more etching byproducts is monitored. As discussed above, in some embodiments, the etching byproducts include MoF 6  and/or SiF 4 . Those skilled in the art will understand that the particular byproducts will depend on factors including the composition of the particular substrate, as well as the materials used in the construction of the etching apparatus  1000 . In some embodiments, the etching byproducts are monitored spectroscopically using any method known in the art, for example, using infrared spectroscopy, UV-visible spectroscopy, Raman spectroscopy, and the like. In some preferred embodiments, the etching byproducts are monitored by mass spectroscopy. In some embodiments, the etching byproducts are monitored chromatographically, for example, by gas chromatography, liquid chromatography, and the like. In some embodiments, the disappearance of XeF 2  vapor is monitored, as discussed above for the monitoring of etching byproducts. 
     In some embodiments, the solid XeF 2    1054  is monitored, for example, the weight, volume, and/or appearance. 
     Because XeF 2  is relatively expensive, in some embodiments, an amount of solid XeF 2    1054  is loaded in the etching chamber such that substantially all of the solid XeF 2    1054  is exhausted in the etching step  1320 . Moreover, unused solid XeF 2    1054  remaining after completion of the etching step  1320  is likely contaminated with byproducts of the etching process, for example, MoF 6  and/or SiF 4 , as well as contaminants entering the etching chamber  1010  in normal use, for example, organic contaminants. Consequently, in some embodiments, solid XeF 2  remaining after step  1320  is not reused. 
     In some embodiments, for example, where the amount of material-to-be-etched is relatively small, the material-to-be-etched is etched in a single exposure. The XeF 2  module  1052  is extended into the chamber  1010  and remains therein until the XeF 2  vapor etches the material-to-be-etched, for example, one or more sacrificial layers, from the substrate  1016 . As described above, in some embodiments, the amount of solid XeF 2    1054  is predetermined to perform the etch in a single step, and to be substantially exhausted in the etching step  1320 . Consequently, no additional portions of solid XeF 2  are added to the module  1052  in the etching of each batch of substrates in these embodiments. 
     In other embodiments, for example, where amount of material-to-be-etched is relatively large, the method  1300  comprises a plurality of etching steps  1320 , each of which comprises an extension of the XeF 2  module  1052  into the central cavity  1014  of the chamber and a retraction of the module  1052  into the access port  1038 . In some embodiments, the solid XeF 2    1054  is not replenished on the module  1052  between etching steps  1320 . 
     In other embodiments, in optional step  1330 , the solid XeF 2    1054  is replenished on the module  1052  between etching steps  1320 . In some embodiments, the module  1052  is retracted into the access port  1038  where additional solid XeF 2    1054  is added to the platform  1056 , for example, using door  1050 . The module  1052  is then reextended into the central cavity  1014  of the chamber, whereupon additional etching occurs. The etching and replenishment is repeated as needed until the desired degree of etching is achieved. As discussed above, in some embodiments, the total amount of solid XeF 2  is predetermined to reduce waste of XeF 2 . 
     In some embodiments, the etching step  1320  etches one layer from the substrate  1016 . In other embodiments, the etching step  1320  etches a plurality of layers from the substrate  1016 . For example, some embodiments of the fabrication of the device illustrated in  FIG. 6C  use a first sacrificial layer between the mirror  14  and  16 , and a second sacrificial layer above mirror  14 . In some embodiments, the layer or layers comprise substantially one material. In other embodiments, the layer or layers comprise a plurality of materials. In embodiments etching a plurality of layers, in some embodiments, the layers have substantially the same composition. In other embodiments, at least one of the layers has a different composition. 
     In some embodiments, the amount of solid XeF 2  used in step  1320  controls the degree of etching. Where the quantity of etchable material exceeds the amount of XeF 2 , etching proceeds until the XeF 2  is substantially depleted. In some embodiments, this method etches a predetermined thickness of an etchable material. 
     In step  1340 , the chamber  1010  is purged. In some embodiments, the purge removes byproducts of the etching step  1320  from the central cavity  1014  of the etching chamber using the purge system  1020 . The particular etching byproducts depend on the particular materials etched in step  1320 . In some embodiments, the etching byproduct is MoF 6  and/or SiF 4 . With reference to the etching chamber  1010  illustrated in  FIG. 10A , some embodiments use a pump/backfill method to purge the cavity  1014 . The outlet valve  1034  is opened, thereby fluidly connecting the cavity  1014  of the chamber to the vacuum source. After a predetermined point, for example, time or pressure, the outlet valve  1034  is closed and the inlet valve  1030  opened, thereby filling the cavity  1014  with the purge gas. In some embodiments, the pump/backfill procedure is repeated one or more times. In other embodiments, opening valves  1030  and  1034  causes a purge gas to flow from the source of purge gas  1026  into the etch chamber  1010  through purge inlet  1022 , then out of the etch chamber  1010  through the purge outlet  1024  to the vacuum source  1032 . Some embodiments do not comprise a vacuum source, and the purge gas is exhausted from the apparatus  1000  through the purge outlet  1024  at substantially ambient pressure. Suitable purge gases are known in the art and are selected based on factors including the particular etching byproduct(s), the process steps preceding and/or following the etching step, the particular process flow, cost of the gas, and the like. Particular examples of purge gases are discussed above. In some embodiments, the chamber  1010  is purged after all of the solid XeF 2    1054  in the module  1052  has been substantially exhausted. 
     Some embodiments comprise a single purge step  1340 . Other embodiments use a plurality of purge steps. In some embodiments, a plurality of purge steps  1340  are performed after the etching of the substrate is complete. As discussed above, some embodiments comprise a plurality of etching steps  1320 . Some of these embodiments comprise at least one purge step  1340  between two etching steps. Some embodiments comprise a purge step  1340  between each etching step. In some embodiments, a purge  1340  is performed substantially contemporaneously with step  1330  in which solid XeF 2  is replenished in the module  1052 . 
     For purposes of illustration, a description of method  1300  with reference to the apparatus in  FIG. 12A  is as follows. Because the method is substantially as described above, the following description focuses on differences. In optional step  1310 , the configurable partition  1260  is closed and the substrate  1216  is subjected to another processing step. In step  1320 , the configurable partition  1260  is opened and the substrate  1216  exposed to XeF 2  vapor formed by the solid XeF 2  in the etchant holding area  1235 . In optional step  1330 , the etchant holding area  1235  is replenished with solid XeF 2 . In step  1340 , the chamber  1210  is purged. 
     EXAMPLE 1 
     An array of modulators at the stage illustrated in  FIG. 7D  are fabricated according to the method described in U.S. Published Application 2004/0051929 on a 200-mm diameter glass substrate. The sacrificial layer is molybdenum. The substrate is loaded onto a fused silica substrate support in a stainless steel etching chamber with internal dimensions of 220 mm by 400 mm by 70 mm. The bottom of the etching chamber is equipped with a fused silica window. The etching chamber is also equipped with a port to a mass spectrometric (MS) detector and an etchant unit as illustrated in  FIG. 10A-FIG .  10 C. 
     The etching chamber is purged three times by evacuating to 10 −2  torr and backfilling with nitrogen gas at ambient pressure. XeF 2  (8.5 g, 50 mmol) is loaded onto the etchant unit and the unit purged with nitrogen. The module is then extended into the etching chamber. The progress of the etching is monitored optically through the window, as well as using the MS. The etching is complete when color of the substrate changes from grey to uniformly white and the concentration of MoF 6  as detected by the MS levels off. 
     Those skilled in the art will understand that changes in the apparatus and manufacturing process described above are possible, for example, adding and/or removing components and/or steps, and/or changing their orders. Moreover, the methods, structures, and systems described herein are useful for fabricating other electronic devices, including other types of MEMS devices, for example, other types of optical modulators. 
     Moreover, while the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others.