Abstract:
A method of forming a micromechanical structure comprising, forming a sacrificial layer on a surface and walls of a trench in a substrate; depositing a structural layer over the sacrificial layer, extending into the trench, selectively etching the structural layer to define a pattern having a boundary, at least a portion of the structural layer overlying a respective portion of the trench being removed and at least a portion of the structural layer extending into the trench being preserved at the boundary; and removing at least a portion of the sacrificial layer from underneath the structural layer, prior to removal of at least a portion of the sacrificial layer extending into the trench at the structural boundary. A micromechanical structure formed by the method is also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is a Continuation of U.S. patent application Ser. No. 14/935,771, filed Nov. 9, 2015, now U.S. Pat. No. 9,554,213, issued Jan. 24, 2017, which is a Division of U.S. patent application Ser. No. 14/039,149, filed Sep. 27, 2013, now U.S. Pat. No. 9,181,086, issued Nov. 10, 2015, which is a non-provisional of, and claims benefit of priority from, U.S. Provisional Patent Application Ser. No. 61/708,282, filed Oct. 1, 2012, the entirety of which are expressly incorporated herein by reference. 
     
    
     STATEMENT OF FEDERAL SUPPORT 
       [0002]    This invention was made with government support under awards R01 DC009859 and R01 DC009429 awarded by the National Institutes of Health. The government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates to the field of micro-electro-mechanical system (MEMS) devices. 
       BACKGROUND OF THE INVENTION 
       [0004]    Over the past twenty years researchers have created a nearly uncountable number of MEMS sensors and actuated structures using the tools of microfabrication. The scientific and engineering literature contains many examples of high-performance devices. There are, however, various barriers to manufacturing successful designs. While the fabrication methods MEMS designers have inherited from the microelectronics industry permit the construction of remarkably detailed and complex moving structures, fabrication-induced stresses and resulting fractures often result in a very poor yield. 
         [0005]    One type of device which can be produced using micro-electro-mechanical systems (MEMS) technology is a microphone. Microphones can be omnidirectional, meaning that the microphone is responsive to the dynamically changing pressure incident on the diaphragm from all directions, or directional, meaning that the microphone is selectively sensitive to sound waves of particular propagation axes. 
         [0006]    The advantages of MEMS microphones over electret condenser microphones (ECMs) are their size, performance, sound quality and suitability for mass production. The advantages of a microphone according to this design are its small size, low internal noise, low power consumption and high directionality. All of these features have been demonstrated during evaluation of a prototype and confirm its superior performance over existing commercially available MEMS directional microphones [7]. 
         [0007]    A particular type of directional microphone provides a “counterbalanced” diaphragm in which two adjacent portions, separated by a central pivot axis, are exposed to sound. The pivoting diaphragm is selectively responsive to acoustic waves depending on the inclination of the acoustic wave with respect to the plane of the diaphragm and pivot axis. This type of directional microphone is inspired by the acoustic sensory organ of  Ormia ochracea [ 42]. 
         [0008]    The pivoting diaphragm itself requires a rotating support, which concentrates stress at the pivoting support. The basic design of such a directional microphone is shown in  FIGS. 1A and 1B . These MEMS sensors are used, for example, to construct biomimetic directional microphones for hearing aid applications [1-13]. 
         [0009]    The fabrication of parallel plate electrodes is a common way to achieve capacitive sensing for microphones and other planar displacement sensors, and may be accomplished by depositing a sacrificial oxide film (typically having a thickness in the range of 5 to 10 microns) and then depositing a conductive material, such as silicon or metal, on top of the oxide. The space between the conducting electrode and the moving diaphragm may then be opened through a release etch, creating a diaphragm membrane which floats over a backplate, supported by a pair of opposed hinges. A disadvantage of employing parallel plate sensing is that the air between the diaphragm and the fixed electrode is squeezed as the diaphragm moves and must flow in the plane of the diaphragm. The viscous force due to the flow of air depends on the third power of the distance between the diaphragm and the stationary electrode, and can be a dominant source of damping in the system [19-26]. It is, of course, desirable to have this gap be as small as feasible to increase the amount of capacitance and the overall sensitivity of the device. While holes in the electrode can be designed to reduce the damping, it can be very difficult to design a system that has both high sensitivity and low damping, which is desired in a device that must move quickly. 
         [0010]    An additional substantial difficulty with the use of parallel plate electrodes is that the bias voltage that is required between the electrode and the diaphragm results in an attractive force that compels the two surfaces to reduce the size of the gap. If the mechanical restoring force provided by the hinge is not sufficiently high, the use of too large of a bias voltage will cause the gap between the diaphragm and electrode to collapse completely, with the rear of the diaphragm contacting the backplate. 
         [0011]    The use of interdigitated fingers at the periphery of the diaphragm can overcome many of the disadvantages of parallel plate electrodes, and can be fabricated in the same steps and mask as used to create the slits around the diaphragm. 
         [0012]    While the use of interdigitated fingers solves many difficult problems inherent in parallel plate MEMS devices, there are practical difficulties that have greatly impeded their use. The main difficulties are that the capacitance that can be achieved is lower than desired, due to the small surface area of the interdigitated fingers and therefore small change in charge to the movement of the fingers, and that fabrication-induced stresses can result in cracks in the polysilicon film resulting in poor fabrication yield. As a result of these difficulties, nearly all of the 2 billion microphones produced each year employ parallel plate electrodes to achieve capacitive sensing. 
         [0013]    Another example of a pivoting planar microstructure MEMS device technology is a so-called micromirror, which is an actuator driven device [14-18]. It is important that these devices be able to move as quickly as possible (thus implying maintaining a low inertia), and in many designs, they are designed to rotate about a hinge axis in a similar manner to the concept shown in  FIGS. 1A and 1B . Reducing the inertia along with the stiffness of the supporting hinge also reduces the voltage (and power) needed for capacitive actuation. These systems are thus subject to the same stress concentration at the pivot as the concept of  FIGS. 1A and 1B . 
         [0014]    In an interdigitated electrode finger structure, a series of N fingers having length  1  extend in the plane of the diaphragm into a corresponding set of fingers held in fixed position, overlapping by a distance h and separated by a gap d. The diaphragm is free to move normal to its plane in the x direction, and thus the respective sets of fingers have a displacement related to the amount of movement. It is noted that the capacitance of the structures is generally the sum of the respective capacitances of the fingers, and therefore if there is variation, the individual values may be calculated and summed. The total capacitance C of a microphone structure using the interdigitation technique may therefore be roughly estimated by: 
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         [0015]    If a bias voltage Vb is then applied between diaphragm and back plate, for example to permit sensing of the displacement, the resulting electrostatic force f (for small x, neglecting fringing effects) will be: 
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         [0016]    The bias voltage Vb does not reduce the stability of the diaphragm&#39;s motion in the x direction; a high bias voltage Vb may be used without a need to increase diaphragm stiffness, resulting in increased microphone sensitivity (for equal capacitance) without the diaphragm collapse problems. The applied static voltage results in an attractive force that acts to bring the moving sensing electrode toward the fixed electrode. In the case of the present comb-sense microphone, this attractive force acts to bring the microphone diaphragm toward its neutral position (i.e., x=0), in line with the fixed fingers. As a result, the bias voltage tends to stabilize the diaphragm rather than lead to instability. As long as the fingers are designed so that they themselves will resist collapsing toward each other, the diaphragm&#39;s compliance does not need to be adjusted to avoid collapse against the fixed electrodes. For example, the interdigital fingers may be provided on opposing sides of the diaphragm structure, so that the forces tending to displace it with respect to the finger gap balance each other. This means that the diaphragm may be designed to be highly compliant and thus very responsive to sound. 
         [0017]    The following U.S. patents are expressly incorporated herein in their entirety: U.S. Pat. Nos. 7,041,225; 7,318,349; 7,520,173; 7,724,417; 7,808,640; 7,825,484; 7,903,835; 8,129,803; 8,165,323; 7,542,580; 6,963,653; 7,471,798; 7,545,945; 7,826,629; 7,992,283; 8,073,167; 7,146,016; 7,146,014; and 6,788,796. 
       SUMMARY AND OBJECTS OF THE INVENTION 
       [0018]    The present technology provides an improved method of incorporating capacitive actuation and/or sensing in a microstructure that is intended to undergo rotational out of plane motions, as is used in sensors for sound or motion as well as micromirrors and oscillators. Likewise, coupled systems may include both actuation and sensing. 
         [0019]    A particular problem with creating a light-weight structure that is intended to rotate rapidly out of plane, is that the point or axis about which it rotates will comprise a stress concentration, which can adversely affect fabrication yield. This problem is addressed, by designing and creating a method of fabricating interdigitated comb-fin structures that serve the dual purpose of protecting the highly compliant rotational axis from fabrication-induced stresses while providing a highly effective mechanism for linear capacitive actuation and sensing. 
         [0020]    A particular feature of various embodiments is an exploitation of the potential for high depth aspect ratios in micromachined structures. The technology enables the construction of lighter-weight (and hence rapidly movable) rotational structures than are possible with existing micromachining technology that meets the same performance criteria. The fabrication of these devices typically requires the deposition of a structural film (such as polycrystalline silicon) onto a sacrificial film (such as thermal silicon oxide). The present technology generally eliminates the structural stress (and associated fracture failures) of the structural film that arise both prior to and during the removal of the sacrificial layer. This problem has hampered the creation and manufacture of a number of important microdevices. 
         [0021]    The present technology therefore provides, according to one embodiment, a micromechanical structure, comprising a substrate having an aperture, a sacrificial oxide layer peripheral to the aperture; and a polysilicon layer overlying the aperture patterned to have (a) a planar portion; (b) a supporting portion connecting the planar portion to a region of the polysilicon layer peripheral to the aperture; (c) a pattern of polysilicon stiffeners extending beneath the planar portion overlying the aperture; and (d) a pattern of polysilicon ribs surrounding the supporting portion, attached near a periphery of the planar portion, the polysilicon ribs extending from the planar portion to a depth beneath the polysilicon stiffeners, and extending laterally beyond an edge of the planar portion. The polysilicon ribs may form a set of fingers which interdigitate with a corresponding set of polysilicon fingers extending from the polysilicon layer peripheral to the aperture, such that when the planar portion moves, e.g., by a flexion or torsion of the supporting portion due to acoustic waves, a relative movement causes one set of fingers to move out of plane with respect to the other set of fingers, and thus define e.g., an interdigitated sensor. 
         [0022]    An exemplary application of this technology is the fabrication of a directional microphone having a diaphragm which rocks about a pivot axis, which may be used, e.g., for hearing aid applications. The technology provides improvements in cost, fabrication yield, and reliability of this microphone. Other devices that employ torsional structures or rotating planar microstructures may likewise be designed and fabricated using this technology. 
         [0023]    The technology provides an improved method of fabricating microstructures that are intended to undergo rotational out-of-plane motions, as are used in sensors for sound or motion as well as micro-mirrors, microoptoelectronic systems, microfluidic devices, valves, micro-motors, and oscillators, for example. A major problem with creating a light-weight structure that is intended to rotate rapidly out of plane is that the point or axis about which it rotates will concentrate stress, which can adversely affect fabrication yield and lead to eventual device failure. 
         [0024]    The present technology provides a method of fabricating interdigitated comb-fin structures that serve the dual purpose of protecting the highly compliant rotational axis from fabrication-induced stresses, while providing a highly effective mechanism for linear capacitive actuation and/or sensing, in particular by providing high depth aspect ratio structures. 
         [0025]    The present technology provides a practical method of implementing interdigitated electrodes in devices that deflect out-of-plane. This increases the capacitance, and thus improves the signal. Likewise, the processes and resulting structures reduce stress induced cracks proximate to the hinges. 
         [0026]    In any device that utilizes capacitance for either actuation or sensing, the overall system performance is a strong function of the nominal device capacitance and the rate at which the capacitance changes with the primary structure&#39;s deflection. In sensors that must detect extremely small motions (such as microphones), one typically desires a device capacitance that is on the order of 1 pF. This is because typical parasitic capacitances (due to bond pads, wiring, and the buffer amplifier input capacitance) often are also on the order of 1 pF. Because of a number of practical design constraints, this amount of source capacitance has been extremely difficult to achieve in practice. In addition, the variation in capacitance with the deflection of the diaphragm can prove to be nonlinear, which adversely affects the performance. While sensors with small source capacitance can be accommodated with careful design of the interface electronics, the performance and overall system cost will improve if the source capacitance is increased. 
         [0027]    The capacitance in the gap between the interdigitated fingers is roughly inversely proportional to the distance across the gap and approximately proportional to the depth of the gap. Unfortunately, there are practical limits on how small the distance across the gap can be formed using typical optical lithography. Creating a gap that is smaller than approximately 1 micron can be very difficult using readily available photolithographic tools. It is also difficult to increase the depth of the gap since this typically will be equal to the thickness of the material (polysilicon) used to form the diaphragm skin. Since it is desirable to have the skin be as thin (and as lightweight) as possible, the thickness is typically on the order of one to two microns. The gap is thus roughly as deep as it is wide. 
         [0028]    Because the gap between the interdigitated fingers is on the order of one micron deep, a typical initial deflection (due to static stresses occurring during fabrication) that is greater than one or two microns can result in a significant loss of capacitance, and hence, a reduction in sensitivity. The sensitivity of a fabricated structure can thus be very dependent on the initial static deflection. 
         [0029]    The present technology provides a new fabrication process that makes it practical to create interdigitated electrodes having substantially greater capacitance than has previously been achievable in thin diaphragm structures. The signal from the resulting electrodes is also predicted to vary in a much more linear manner with the deflection of the planar diaphragm than is found in conventional interdigitated fingers. 
         [0030]    It is therefore an object of this technology is to provide a micromechanical structure, comprising: a substrate having a through hole; a residual portion of a sacrificial oxide layer peripheral to the through hole formed on the substrate; and a polysilicon layer overlying the through hole, patterned to have: a planar portion; at least one supporting portion connecting the planar portion to a portion of the polysilicon layer on the residual portion of the sacrificial oxide layer peripheral to the through hole; a first pattern of polysilicon stiffeners formed extending beneath the planar portion overlying the through hole, configured to stiffen the planar portion; and a second pattern of polysilicon ribs selectively disposed surrounding the at least one supporting portion, attached near a periphery of the planar portion, wherein the polysilicon ribs extend from the planar portion to a depth beyond a depth of the polysilicon stiffeners, and extend laterally beyond an edge of the planar portion. 
         [0031]    At least a portion of the first pattern of polysilicon stiffeners may be configured as a set of fingers which interdigitate with a corresponding set of fingers, formed of polysilicon, extending from a portion of the polysilicon layer on the residual portion of the sacrificial oxide layer peripheral to the hole, wherein the planar portion is configured to move, by a flexion or torsion of the at least one supporting portion, to thereby cause a relative movement of the set of fingers with respect to the corresponding set of fingers out of a plane of the planar portion. The set of fingers and the corresponding set of fingers may be conductive and electrically isolated, and are configured to act as a capacitive displacement sensor. 
         [0032]    The planar portion may move in response to acoustic vibrations. The planar portion may be configured to rotate in response to acoustic vibrations. 
         [0033]    The planar portion may be supported by a pair of opposed supporting portions, and be configured to rotate in response to acoustic vibrations about an axis defined by the pair of opposed supporting portions. 
         [0034]    The planar portion may comprise a diaphragm of a directional microphone. 
         [0035]    The polysilicon ribs may have a height at least  10  times a thickness of the planar portion. 
         [0036]    The polysilicon stiffeners may intersect the polysilicon ribs. 
         [0037]    Another object provides a method of forming a micromechanical structure, comprising: etching at least one trench into a substrate in a first etch; forming a sacrificial layer on the substrate and walls of the at least one trench; depositing a structural layer over the sacrificial layer, extending into the at least one trench; etching a boundary of a structure formed from the structural layer, wherein at least a portion of the structural layer overlying portions of the at least one trench is removed in a second etch; etching a void through the substrate from beneath the structure in a rear etch; and removing at least a portion of the sacrificial layer, to preserve a portion of the structural layer separated from the substrate by a fluid space, wherein at least a portion of the sacrificial layer on the walls of the at least one trench is removed subsequent to removal of a portion of the sacrificial layer on the substrate. 
         [0038]    The portion of the structural layer separated from the substrate by a fluid space may be flexibly supported by a narrow portion of the structural layer, and the at least one trench may be formed proximate to the narrow portion, the sacrificial layer underlying the narrow portion being removed before the sacrificial layer on the walls of the at least one trench, such that the narrow portion remains held in position by the structural layer which extends into the at least one trench while the sacrificial layer underlying the structural layer in regions absent the at least one trench is removed before the sacrificial layer adjacent to the structural layer which extends into the at least one trench, the removing of the sacrificial layer proceeding until the structural layer which extends into the at least one trench is free to move into and out of the trench by a flexion of the narrow portion. 
         [0039]    The substrate may comprise silicon, and the sacrificial layer may comprise silicon dioxide formed by oxidizing a surface of the silicon substrate. The sacrificial layer may be removed by dissolution of the silicon dioxide sacrificial layer in hydrofluoric acid. 
         [0040]    The at least one trench may comprise at least two trenches, having different depths, wherein the deeper trench is formed proximate to the narrow portion, to thereby support the narrow portion while the sacrificial layer on the walls of the shallower trench is removed. 
         [0041]    The at least one trench may be formed by reactive ion etching. The second etch may also comprise reactive ion etching. The rear etch may comprise a through-wafer, deep reactive ion etch. 
         [0042]    The at least one trench may be provided as a pair of linear arrays of trenches to define a set of interdigitated fingers. The second etch may free a portion of the set of interdigitated fingers attached within the boundary to the structure to move with respect to a portion of the set of interdigitated fingers detached from the structure. The interdigitated fingers may be conductive, thus forming a capacitive displacement sensor. The interdigitated fingers may have a depth greater than about ten times a thickness of the structural layer. The interdigitated fingers may overly the void, and at least a portion of the set of interdigitated fingers may be free to move toward and away from the void. 
         [0043]    The sacrificial layer may be formed by chemically converting the substrate to another composition, wherein the set of interdigitated fingers is defined by trenches formed sufficiently close, and conversion is conducted to completely convert a wall between adjacent trenches to the other composition. 
         [0044]    The sacrificial layer may keep the structure from being fully released after the rear etch. 
         [0045]    The structure may comprise a diaphragm of a microphone, which displaces in response to acoustic vibrations. The diaphragm may pivot about a pair of narrow portions. The diaphragm may further symmetrically pivot about the pair of narrow portions, to provide a directional acoustic response. 
         [0046]    The at least one trench may comprise a first set of trenches which define a set of stiffeners configured to stiffen the structure, and a second set of trenches, deeper than the first set of trenches, configured to define supports for the structure which are released from the substrate by removal of the sacrificial layer after the stiffeners are released from the substrate. Respective ones of the first set of trenches may intersect ones of the second set of trenches. 
         [0047]    The method may further comprise defining third and fourth sets of trenches, configured to together form a set of interdigitated fingers, the third set of trenches defining a first set of fingers attached to the substrate, and the fourth set of trenches defining a second set of fingers attached to the structure and interleaved with the first set of fingers, and moveable with the structure with respect to the substrate. The second set of trenches may be deeper than the first set of trenches. 
         [0048]    The sacrificial layer may serve as an etch stop which limits an extent of the rear etch. 
         [0049]    A surface of the structure may remain fixed to the substrate through the sacrificial layer until the removing is conducted. The structural layer may comprise polysilicon, which covers the substrate and fills the at least one trench. 
         [0050]    A plurality of structures may be formed on the substrate, the method further comprising separating a plurality of dice each containing at least one structure prior to said removing. 
         [0051]    The method may further comprise rinsing the substrate after said removing, and then drying the substrate in a critical point drier to avoid stiction. 
         [0052]    It is understood that the deep trenches support the fragile portion during processing, and are freed as a late step in fabrication, avoid stress on the fragile portion, and may be used in various MEMS systems. The fragile portion may be used in flexion or torsion, and may be a supporting element, actuator, sensor, or the like. In general, it is desirable to free the protective structure from the substrate by removing the sacrificial layer which holds it in place; however, some devices do not require complete removal, and therefore a partial removal may be used, retaining a portion of the sacrificial oxide deep in the trench, for example, in regions of the substrate not aligned with the rear through hole. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0053]    The invention will now be explained by way of example, in which:  FIGS. 1A and 1B  show a top view of a diaphragm, and cross section view of a prior art planar microstructure that is supported on flexible hinges; the structure is fabricated on a silicon chip that supports the hinges, and the diaphragm is separated from the surrounding substrate by etching a slit around the perimeter while leaving the connection at the hinges intact. 
           [0054]      FIG. 2  shows the stiffened diaphragm of  FIGS. 1A and 1B , as seen through the hole in the backside of the chip. 
           [0055]      FIGS. 3A-3F  show a fabrication process according to the prior art for microphone diaphragm shown in  FIGS. 1A, 1B, and 2 , with interdigitated comb fingers at the perimeter of the diaphragm, in which: 
           [0056]      FIG. 3A  shows deep reactive ion etching (RIE); 
           [0057]      FIG. 3B  shows thermal oxide growth; 
           [0058]      FIG. 3C  shows polysilicon deposition; 
           [0059]      FIG. 3D  shows polysilicon smoothing and RIE to define diaphragm and comb fingers; 
           [0060]      FIG. 3E  shows backside RIE; and 
           [0061]      FIG. 3F  shows buffered hydrofluoric acid etching to remove the thermal oxide. 
           [0062]      FIG. 3G  shows an L-Edit image of the prior art silicon differential microphone diaphragm shown in  FIGS. 1A, 1B, and 2  with comb fingers, slits that define the diaphragm and the comb fingers, and stiffeners that reinforce the diaphragm. 
           [0063]      FIG. 4  shows a predicted capacitance of a diaphragm according to the present invention, as a function of the out of plane displacement of the planar diaphragm, for electrodes having thickness of one or five microns. 
           [0064]      FIG. 5  shows a mask design (showing only the region near the supporting hinge) of comb fins to protect the supporting hinge according to the present invention. 
           [0065]      FIGS. 6A-6F  show a fabrication process for interdigitated comb fins according to the present invention, in which stiffeners can be incorporated through the use of a separate trench etch as in the initial step, in which: 
           [0066]      FIG. 6A  shows trench etching; 
           [0067]      FIG. 6B  shows trench oxidation; 
           [0068]      FIG. 6C  shows polysilicon deposition; 
           [0069]      FIG. 6D  shows polysilicon patterning; 
           [0070]      FIG. 6E  shows backside etching; and 
           [0071]      FIG. 6F  shows buffered hydrofluoric acid release. 
           [0072]      FIG. 7  shows a flowchart of the fabrication process according to  FIG. 6 . 
           [0073]      FIGS. 8A-8C  show cross section views of micromechanical device during fabrication in which  FIG. 8A  corresponds to the stage represented in  FIG. 6E ,  FIG. 8C  corresponds to the stage represented in  FIG. 6F , and  FIG. 8B  is an intermediate stage. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0074]    According to one embodiment of the technology, a 1 mm×2 mm microphone diaphragm is made of polysilicon and has stiffeners and carefully designed hinge supports to ensure that it responds like a rigid body on flexible hinges. Larger microphone diaphragms, e.g., 1 mm×3 mm are also possible. The diaphragm is designed to respond to pressure gradients, e.g., due to acoustic waves in air, giving it a first-order directional response to incident sound. Both the diaphragm and stiffening ribs are made of LPCVD (low pressure chemical vapor deposition) polysilicon. The diaphragm is about 2 μm thick and the stiffening ribs are 4 μm wide and 40 μm tall. This structure provides a highly compliant differential microphone that responds to the differences in pressure on the two sides of the diaphragm that are separated by the hinges at the center [4, 20-22]. Interdigitated fingers, which consist of 100 μm long, 1.5 μm wide fingers with 6 μm periodicity, are incorporated at the perimeter of the two ends of the diaphragm, the locations with maximum deflection.  FIG. 3G  shows the L-Edit image for the microphone diaphragm with interdigitated comb sense fingers. 
         [0075]      FIGS. 1A and 1B  show a planar microstructure diaphragm  12  that is supported on flexible hinges  14 . The structure is fabricated on a silicon chip that supports the hinges  14 . The diaphragm  12  is separated from the surrounding silicon substrate  3  by etching a slit  15  around the perimeter while leaving the connection at the hinges  14  intact. The diaphragm  12  is intended to rotate like a rigid body without bending, so that its motion is dominated by rotation about its central axis  11 . In applications where the rotation of the structure is intended to occur rapidly, it is essential that the mass moment of inertia about the pivot axis be as small as possible. To achieve the stiff diaphragm  12 , stiffeners  2  may be provided on a thin membrane, to maintain its rigid body motion. 
         [0076]      FIG. 2  shows a stiffened diaphragm  12 , as seen from the backside, through the backside hole below the diaphragm  12 . This structure&#39;s motion is dominated by the rocking motion about the central hinge axis  11  as in the concept of  FIGS. 1A and 1B  [7].  FIG. 2  does not show the slit that surrounds the diaphragm at all locations other than at the pivots. 
         [0077]    The stiffeners  2  shown in  FIG. 2  are formed using the same polysilicon material that is used to make the planar portion (the skin) of the diaphragm  12 . The diaphragm  12  shown in  FIG. 2  is 2 mm long and 1 mm wide. The stiffeners  2  are 30 microns tall and 2 microns wide. The skin of the diaphragm is approximately 1.5 microns thick. The substrate chip is 500 microns thick, which is also the depth of the rectangular hole shown in  FIG. 2 . The planar microstructure is created by processing silicon wafers  3  using the tools commonly employed in silicon microfabrication. 
         [0078]      FIGS. 3A-3F  show a cross section view of the fabrication process flow for manufacturing a prior art microphone diaphragm  12 . The process begins in step  1  shown in  FIG. 3A , with a deep reactive ion etch of a bare silicon wafer  3  to create trenches  6  that are approximately 3 microns wide and 30 microns deep that act as the mold for the polysilicon stiffeners  10 . 
         [0079]    This is followed, in step  2  shown in  FIG. 3B , by a wet oxidation at 1100 degrees Celsius to grow a one-micron thick thermal oxide layer  4  on the wafer surface and in the trenches  6 . This oxide is used as an etch stop for a subsequent backside cavity etch shown in  FIG. 3E . 
         [0080]    The phosphorus-doped polysilicon is then deposited onto the thermal oxide  4  at 580 degrees Celsius through low-pressure chemical vapor deposition (LPCVD) and subsequently annealed to form polycrystalline silicon  5  at 1100 degrees Celsius in argon gas for  60  minutes in order to reduce intrinsic stress in the film in step  3  shown in  FIG. 3C . 
         [0081]    The next step  4  shown in  FIG. 3D  planarizes the annealed polysilicon to form a flat diaphragm surface  8  having stiffeners  10 , followed by reactive ion etching to define the interdigitated comb sense fingers  16  and slits  7  that separate the diaphragm  12  from the substrate  3  around the perimeter of the diaphragm  12  so that it will eventually be supported only on the flexible hinges  14  (shown in top view  FIG. 3G ). 
         [0082]    A through-wafer, deep reactive ion etch of the back cavity is then performed in step  5  shown in  FIG. 3E , to free the back side of the diaphragm  12 . This etch stops on the thermal oxide layer  4  film grown in step  2  shown in  FIG. 3B . 
         [0083]    Finally, the diaphragm  12  is released in step  6  shown in  FIG. 3F , using buffered hydrofluoric acid to remove the exposed portion of the thermal oxide layer  4 . 
         [0084]    The process of  FIGS. 3A-3F  schematically show a practical method to create light-weight, yet stiff planar structures that are supported on flexible hinges. To create a device that can be actuated (such as a mirror) or to create a sensor (such as a motion detector or microphone), one may incorporate capacitive electrodes into the fabrication process of  FIGS. 3A-3F . This could be accomplished either by constructing parallel plate-type capacitive electrodes at either end of the diaphragm or by modifying the slit etch to construct interdigitated fingers at the ends as shown in  FIG. 5 . The biologically-inspired microphone with interdigitated comb sense fingers  16 , see e.g., U.S. Pat. No. 7,545,945 and U.S. Pat. No. 8,073,167, expressly incorporated herein by reference, is fabricated on a silicon substrate using a combination of surface and bulk micromachining techniques. This fabrication technique uses deep-trench etching and sidewall deposition to create very lightweight, very stiff membranes with stiffening ribs at optimal locations. 
         [0085]    An analytical model predicts the capacitance of these interdigitated electrodes as a function of the out of plane displacement of the planar diaphragm, as shown in  FIG. 4 . This technology enables the creation of electrodes having thickness on the order of five to ten microns, leading to substantial improvements in performance as shown. The fabrication process enables fabrication of interdigitated fingers having substantially larger thickness (in the direction normal to the plane of the diaphragm) than has been practical previously. Rather than constructing fingers having a thickness that is equal to that of the diaphragm skin (on the order of one micron), this process allows construction of interdigitated fins having a thickness on the order of five to ten microns. This results in a dramatic increase in capacitance as shown in  FIG. 4 . In addition, the predicted capacitance is seen to vary much more linearly with the diaphragm deflection with this increased electrode thickness. 
         [0086]    The results shown in  FIG. 4  are for a device having dimensions that are appropriate for a MEMS microphone, and indicate that the nominal sensor capacitance is increased significantly by the use of five micron thick electrodes rather than one micron electrodes. The source capacitance is greater than the target minimum value of 1 pF, and is seen to vary linearly with the diaphragm deflection for a wide range of displacement. 
         [0087]    While the fabrication process exemplified in  FIGS. 3A-3F  easily permits the incorporation of interdigitated fingers, the resulting diaphragms can be subject to stress-induced fracture during the final release etch depicted in  FIG. 3F . The diaphragm material is subjected to substantial stresses after completion of the back side deep reactive ion etch ( FIG. 3E ) because the sacrificial thermal oxide layer  4  typically has approximately 350 MPa of compressive stress relative to that of the bulk silicon wafer  3 . This is in contrast to the polysilicon that comprises the diaphragm  12  which has approximately 50 MPa of stress (either compressive or tensile, depending on the anneal process, etc.). This substantial compressive stress on such a thin diaphragm  12  results in substantial deflection, typically in the negative direction (toward the back side hole). As the wet etch release of  FIG. 3F  commences, portions of the slit  7  may be released while sacrificial thermal oxide layer  4  remains over much of the diaphragm  12  surface, resulting in even greater deflections of the diaphragm  12 . This can result in excessive deformations and stresses at the hinge  14  support, which can produce cracks in this critical element of the system. 
         [0088]    The present technology creates interdigitated electrodes having significantly increased thickness (i.e., depth into the plane of the structure) which causes the electrodes to also have substantially increased bending stiffness, that resist flexure of the diaphragm during the fabrication process. The interdigitated electrodes are separated by sacrificial oxide having a thickness of approximately one micron. During the release process, this oxide is likely to remain longer than that on the planar portions of the diaphragm. This is because the oxide that separates the electrodes is contained within a space that is approximately five microns deep, one micron wide and having a length equal to that of the electrodes (typically 50 to 100 microns). The wet etch process will require considerably longer to remove this buried material than that which is covering the plane surface of the diaphragm. The electrodes will thus resist flexure (and the associated damaging stresses and strains) that occur around the perimeter of the diaphragm. If the electrodes are also utilized in the vicinity of the delicate hinge support, they would provide considerable protection from the oxide compressive stress. 
         [0089]    The thickened interdigitated electrodes may be configured to provide a substantial increase in resistance to flexure in the vicinity of the diaphragm&#39;s hinge support. It is believed that this increased stiffness will provide sufficient protection to prevent cracks in the hinge and diaphragm during the wet release. 
         [0090]    The technology provides a method for creating interdigitated fins that provide the dual benefits of substantially increased capacitance over what can be achieved with interdigitated fingers which are limited to the thickness of the diaphragm (itself limited in thickness due to mass issues), and a reduction in structural stress during the critical steps in the fabrication process. These benefits may be achieved together or independently, and thus the thick interdigitated fins do not require protection of a hinge structure, and a protected hinge structure does not also require thick fins. The fins essentially combine the structural functions of the stiffeners and the capacitive functions of the fingers, described above. This process enables the practical construction of interdigitated electrodes having a depth that can be substantially greater than the thickness of the diaphragm skin. For example, a fin depth can be 5 to 10 microns, rather than the approximately one micron depth of the skin. Since the bending stiffness of the fins is proportional to the cube of the depth, this structure provides a dramatic increase in resistance to flexure, which serves to protect the supporting hinge during the release process. In addition, the increased depth of the interdigitated electrodes provides an increase in capacitance as shown in  FIG. 4 . 
         [0091]      FIG. 5  shows a plan view of a mask design the interdigitated fins  26  near the supporting hinge  24  of the diaphragm, which serve to protect the supporting hinge  24 . The fins  21  are fixed to the substrate and the planar sheet on each side, and form an interdigitated comb fin  26  structure where they approach one another, the slit  23  etch is immediately lateral on each side of the supporting hinge  24 , and define the lateral edges of a torsional bridge between the substrate and planar sheet, and the stiffeners  25  extend perpendicular or diagonal to the comb fins  21 , as well as being provided parallel to the comb fins  21  adjacent to the slit  23  etch.  FIG. 5  shows only the region near the supporting hinge. The supporting hinge  24  in this case consists of a stiffener that spans the entire width of the approximately rectangular structure and terminates in the bulk silicon comprising the substrate.  FIG. 5  shows interdigitated fins  26  in the vicinity of the hinge  24  but they may also be incorporated around the entire diaphragm perimeter. The fins that are adjacent to the hinge  24  are primarily employed to protect the hinge  24  during fabrication; the limited motion of the diaphragm near the hinge  24  causes these nearby electrodes to have minimal use for sensing or actuation. 
         [0092]    The fabrication process to create the interdigitated comb fins is shown in  FIGS. 6A- 6  F and  FIG. 7 . Note that stiffeners can be incorporated through the use of a separate trench etch, as in step  801  of  FIG. 7 .  FIGS. 6A-6F  show cross section views of the wafer at the interdigitated fins, i.e., beyond the edge of the surface of the diaphragm. A plan view is provided in  FIG. 5 , which shows that the trenches  6  used to create the fins  26  have separate terminations for fins that move with the diaphragm and for those that are fixed to the surrounding substrate. 
         [0093]    The fabrication starts in step  801 , with a deep trench etch (Fig,  6  A) into the silicon wafer to create a mold (trench  6 ) for the diaphragm  33  and foundation stiffeners having a first depth  39  from a first etch, and for the two sets of comb-fins having a shallower second depth  37  from a second etch. See  FIG. 8C . The ends of the fin trenches intersect with the stiffener-trenches. According to alternate embodiments, the first etch may be the same or deeper than the second etch. In like manner, various structures may be formed at different, independently controlled depths to form a complex structure. 
         [0094]    A layer of sacrificial wet oxide  4  ( FIG. 6B ) is grown in step  802  to form an etch stop for the backside cavity etch (See  FIG. 6E ), and to form the gap between the fixed comb-fins and moving comb-fins. The oxide layer  4  should be thick enough such that the bulk silicon between the trenches  6  for the two sets of comb-fins is completely oxidized (as shown in  FIG. 6B ). The oxide layer  4  also keeps the fragile diaphragm  33  from being fully released following the backside cavity etches, as shown in  FIG. 8B . The oxide  4  will hold the diaphragm  33  firmly in place until it is etched away from between the fins  35  during the final step, as shown in  FIG. 8B , and thus serves to avoid stress and curling. 
         [0095]    The polysilicon is deposited in step  803  ( FIG. 6C ), such that the deposited polysilicon  5  fills the trenches  6  that will become the supporting stiffeners  36  and the comb-fins  35 . The polysilicon  5  on the surface of the wafer will form the diaphragm  33  ‘skin’. 
         [0096]    The polysilicon film  5  is then patterned ( FIG. 6D ) in step  804 , with a reactive ion etch, to separate the diaphragm  33  from the foundation  31  and remove the polysilicon layer  5  above the two sets of comb-fins  35  so that they can be displaced in and out of the plane of the diaphragm  33 . 
         [0097]    Step  805  consists of a through-wafer, deep reactive ion etch on the backside ( FIG. 6E ), to create a cavity  9  that defines the air chamber behind the diaphragm  33 . The wafer may be diced into chips, to provide separate microphones, or maintained as an integral substrate for a microphone array. 
         [0098]    Step  806  provides the final release, achieved by dissolving the sacrificial oxide layer  4  in buffered hydrofluoric acid ( FIG. 6F ,  FIGS. 8B and 8C ). The structures are rinsed, and placed in a critical point drier (to avoid stiction). 
         [0099]    In this description, several preferred embodiments were discussed. It is understood that this broad invention is not limited to the embodiments discussed herein, but rather is composed of the various combinations, subcombinations and permutations thereof of the elements disclosed herein. The invention is limited only by the following claims. 
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