Abstract:
A fluidic micro-electromechanical device includes a pressure compensating subsystem that enables the device to operate consistently in changing environmental pressure conditions. Such a fluidic micro-electromechanical device includes an actuator having an actuator cavity underneath an actuator membrane, the actuator membrane moving in response to a driving signal applied to an actuator electrode, and a pressure compensating chamber coupled to the actuator cavity.

Description:
BACKGROUND AND SUMMARY  
       [0001]     The systems described herein relate to micro-electromechanical systems (“MEMS”) and, more particularly, to MEMS having structures containing fluids.  
         [0002]     Micro-electromechanical systems are mechanical systems that are micromachined in silicon and may be optionally integrated with control electronic circuits. MEMS are generally categorized as either microsensor or microactuator systems, depending on the application. MEMS incorporate electrostatic, electromagnetic, thermoelastic, piezoelectric, or piezoresistive effects in the operations of the systems.  
         [0003]     Fluidic MEMS often include a closed chamber, sealed membrane, or other fluid passageway. A MEMS device with a closed chamber, a sealed membrane, or other fluid delivery system may be susceptible to differential pressure. This pressure variation can occur during various stages of a device&#39;s lifetime from processing, storage, or shipping of the device for operation at different locations. For example, the pressure variation can arise from operation at various altitudes, trapped pressure, temperature change, out-gassing of materials used in the device or active operation (such as pumping or priming). Differential pressure may cause undesirable membrane deflection, including bulging or collapsing membranes, trapped bubbles or fluids in the cavities behind the membranes or cracking or bursting resulting in a change of device performance and/or device failure.  
         [0004]     Fluidic MEMS are utilized in a variety of devices for achieving a variety of functions. Fluidic MEMS incorporating electrostatic actuators may be utilized for micro-pump, micro-mixer, micro-fluidic analysis, and inkjet print head applications.  
         [0005]     A sealed actuator cavity in a fluidic MEMS can be susceptible to the pressure variations. One source of pressure variation acting on a MEMS device arises from air pressure changes related to the altitude of particular locations. For example, the altitude above sea level of Rochester, N.Y. is approximately 300.0 feet resulting in a standard local atmospheric pressure of 0.99 atmospheres, while the altitude above sea level of Denver Colo. is approximately 5300.0 feet resulting in a standard local atmospheric pressure of 0.82 atmospheres. Thus when a device embodying a fluidic MEMS is transported from one location, such as a manufacturing location, to another location at a substantially different altitude, such as a user&#39;s location, the sealed cavity of the fluidic MEMS is subjected to pressure changes that may result in the fluidic MEMS operating outside of its design parameters.  
         [0006]     In many current designs of fluidic MEMS having sealed cavity actuators, a 0.2-0.3 atm reduction in atmospheric pressure requires an additional 4-5 volts of driving voltage to operate the device. Some fluidic MEMS devices that incorporate sealed actuator cavities trap gas as the result of contamination, chemical reaction, or outgassing of structural, residual sacrificial, or packaging materials. For example, some embodiments of fluidic MEMS having sealed actuator cavities are fabricated with a process that requires an actuator to be sealed by organic materials such as SU8 polymer. The internal pressure of the device in the vicinity of the actuator may be altered by the out-gassing of the sealing materials. In one particular application, ambient pressure changes or internal pressure changes may cause an inkjet print head that incorporates a fluidic MEMS having a sealed actuator cavity to experience degradation in the jetting speed, drop volume, directionality, or overall print quality produced by the print head. For all these reasons, reduced sensitivity of actuators in fluidic MEMS devices to pressure fluctuations is desirable.  
         [0007]     Some prior attempts have been made to reduce the sensitivity of actuators in fluidic MEMS devices to pressure variation. One attempt to address the pressure differential problems provides a micro-fluidic structure that is vented to atmosphere to allow pressure equalization to occur outside a normal operating cycle of the actuator chamber between the seal and the actuator electrode. This approach to addressing pressure differential problems may cause stiction concerns because humidity in the air may result in condensation that leads to capillary forces that cause stiction.  
         [0008]     A fluidic MEMS device is disclosed herein that exhibits a reduced sensitivity to pressure variations arising from one or more of the sources noted above. One such fluidic micro-electromechanical device includes a pressure compensating subsystem that enables the device to operate consistently in changing pressure conditions. The device includes an actuator having an actuator cavity underneath an actuator membrane, the actuator membrane moving in response to a driving signal applied to an actuator electrode, and a pressure compensating chamber that is coupled to the actuator cavity.  
         [0009]     In one embodiment of a fluidic MEMS device that compensates for changing pressure conditions, the pressure compensating chamber is covered with a flexible covering that is more responsive to pressure fluctuations than the actuator membrane. The flexing of the covering enables the fluid in the pressure compensating chamber to absorb the pressure differential before the actuator membrane responds. Thus, the effect of the changing pressure on the actuator cavity is negligible. The covering over the pressure compensating chamber may be rendered more flexible than the actuator membrane by constructing the flexible covering with a width and length relative to the width and length of the actuator membrane in a manner described in more detail below.  
         [0010]     In another embodiment of such a fluidic device, the pressure compensating chamber is covered with a plate that is coupled to the rigid walls to form the pressure compensating chamber. The pressure compensating chamber formed by the rigid walls and plate is much larger than the actuator cavity to which the pressure compensating chamber is coupled. For example, the pressure compensating chamber may be 1 to 2 orders of magnitude taller than the actuator cavity. The coupling of the larger pressure compensating chamber to the actuator cavity enables the gas in the actuator cavity to resist deformation by pressure fluctuations in the device. This embodiment, however, is not responsive to ambient pressure changes because the plate and rigid walls do not respond to ambient pressure changes as the flexible covering does in the embodiment described earlier.  
         [0011]     A print head for an inkjet printer may be constructed with such a fluidic electro-mechanical construction. Such a print head may comprise a substrate, a plurality of actuators formed over the substrate, the actuators being actuated by electrical signals, a plurality of actuator membranes and actuator cavities, each actuator membrane and actuator cavity in the plurality being formed over the substrate, each actuator membrane moving in response to excitation of the actuator about which the actuator membrane is mounted, a fluidic chamber having an inlet for drawing ink from an ink supply into the fluidic chamber in response to the actuator being excited, a nozzle in each fluidic chamber through which ink is expelled from the fluidic chamber in response to the actuator membrane returning to its position before excitation, a pressure compensating chamber being formed over the substrate, the pressure compensating chamber being in fluid communication with each actuator cavity in the plurality of actuator cavities, and a covering over the pressure compensating chamber to separate the pressure compensating chamber from ambient air.  
         [0012]     Additional features and advantages of the presently disclosed fluidic MEMS device will become apparent to those skilled in the art upon consideration of the following detailed description of embodiments embodying the pressure compensating subsystem discussed above. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     A more complete understanding of the disclosed apparatus can be obtained by reference to the accompanying drawings wherein:  
         [0014]      FIG. 1  is a schematic top-view diagram of a fluidic MEMS chip having of a pressure compensating subsystem covered with a flexible covering;  
         [0015]      FIG. 2  is a cross-sectional view of the MEMS chip shown in  FIG. 1  taken along the cross-sectional lines for  FIG. 2  shown in  FIG. 1 ;  
         [0016]      FIG. 3  is a cross-sectional view of the MEMS chip shown in  FIG. 1  taken along the cross-sectional lines for  FIG. 3  shown in  FIG. 1 ;  
         [0017]      FIG. 4  is a schematic top view diagram of a fluidic MEMS chip having of a pressure compensating subsystem having a pressure compensating chamber formed with rigid walls and a plate;  
         [0018]      FIG. 5  is a cross-sectional view of the MEMS chip shown in  FIG. 4  taken along the cross-sectional lines for  FIG. 5  shown in  FIG. 4 ; and  
         [0019]      FIG. 6  is a cross-sectional view of the MEMS chip shown in  FIG. 4  taken along the cross-sectional lines for  FIG. 6  shown in  FIG. 4 . 
     
    
       [0020]     Corresponding reference characters indicate corresponding parts throughout the several views. Like reference characters tend to indicate like parts throughout the several views.  
       DETAILED DESCRIPTION  
       [0021]     For the purposes of promoting an understanding of the principles of the disclosure, reference is now made to the embodiments illustrated in the drawings and described in the following written specification. No limitation to the scope of the disclosure is intended by these particular depictions and their descriptions.  
         [0022]     A top view of one embodiment of a fluidic MEMS device  10  is shown in  FIG. 1 . The device  10  includes a substrate  14 . There are three main MEMS structure regions on top of substrate  14 . There regions are a pressure compensating region  20 , an actuator area  24 , and an inlet area  28 . The substrate may be made from a suitable substrate material for a particular application, such as silicon, and the tall sidewalls  18  that are constructed on top of the silicon substrate may be made of a suitable material, such as nickel. Within actuator area  24  are a plurality of rigid walls that extend across the width of the area to divide the actuator area into a plurality of actuator areas  42  ( FIG. 2 ). Actuator membranes  30 A- 30 E are anchored by a known method around the perimeter of the actuator areas to form an actuator cavity  42  between each actuator membrane  54  and a portion of the underlying substrate  14  ( FIG. 2 ). A covering  26  is anchored around the perimeter of the pressure compensating area  24  to form a pressure compensating chamber  52  ( FIGS. 2 and 3 ) between the covering  26  and a portion of the substrate  14  underlying the covering. The layer separating the pressure compensating area  24  from the actuator area  28  has a plurality of passageways  22 A- 22 E etched in it for an extension from each actuator membrane into the pressure compensating area  20 . Therefore, the pressure compensating chamber  52  is in gas or liquid communication with each of the actuator cavities underlying the membranes  30 A- 30 E depending upon whether a gas, such as air, or a liquid is used to implement the actuator. Therefore, gas communication as used herein refers to both gas and liquid flow between the actuator cavities and the pressure compensating chamber depending upon the fluid used to implement the actuator. Although the pressure compensating area is shown as being adjacent to the actuator area in  FIG. 1 , other arrangements are possible. For example, the pressure compensating area  20  may underlie or be located over the actuator area  24  or the inlet area  28 .  
         [0023]     The passageways  22 A- 22 E are designed so almost no gas flow occurs between an actuator cavity  42  and the pressure compensating chamber  52  during high frequency operation of the actuator. Air is exchanged between the actuator cavity  42  and the pressure compensating chamber  52  during actuator idle time. To achieve this goal, each passageway  22  is relatively long with a small cross-sectional area. For example, a passageway may have a cross-sectional area of 3-5 μm by 0.5-2 μm and a length of 100-1000 μm. The gas sealed within the actuator cavities  22 A- 22 E and pressure compensating chamber  52  may be air. Alternatively, the gas may be nitrogen (N 2 ), sulfur hexafluoride (SF 6 ), or an inert gas to prevent humidity and contamination effects on the performance of the actuators.  
         [0024]     Because the rigid walls  18  extend above the actuator membranes  30 A- 30 E, a fluidic chamber exists over each of the actuator membranes  30 A- 30 E. The rigid wall  18  separating the actuator area  24  from the inlet area  28  is segmented so that a plurality of passageways provide fluid communication between the inlet area  28  and the fluidic chambers  34  overlying the actuator membranes  30 E- 30 E. Consequently, expansion of a fluidic chamber caused by the movement of an actuator membrane towards the substrate  14  results in the flow of fluid from the inlet area  28  into the fluidic chamber in which the actuator membrane moved. Return of the actuator membrane to its equilibrium position expels some of the fluid in the fluidic chamber out through a nozzle  36  associated with the fluidic chamber and some of the fluid flows back to the inlet region  28 .  
         [0025]     In further detail,  FIG. 2  shows a cross-section of the device  10  through the passageway  22 B. Deposited on the substrate  14  may be one or more insulating layers  40 . The insulating layer may be comprised of silicon dioxide and silicon nitride. A conductive layer, which may be comprised of polysilicon, may be deposited and etched to form a bottom electrode  44  for an actuator. Although the example of a device incorporating a pressure compensating subsystem is described with reference to an electrostatic actuator, devices incorporating a piezoelectric actuator or any actuator having a cavity underneath an actuator membrane may also utilize such a subsystem. A sacrificial layer, which may be comprised of silicon dioxide, may be deposited, patterned, and etched to form holes for anchoring a subsequent structural layer. The structural layer forms the flexible pressure compensating membrane  26  and the actuator membranes  30 A- 30 E. These structural layers may be comprised of polysilicon. The flexible membrane  26  anchored over the pressure compensating area  20  and the underlying substrate form a pressure compensating chamber  52  for pressure compensation in the device  10 . The actuator membranes  30 A- 30 E anchored over the actuator areas and the underlying substrate form a plurality of actuator cavities  42 . The structural layer comprising the flexible membrane  26  is optionally etched to decrease the thickness of the layer and its corresponding stiffness so it is more responsive to pressure fluctuations than the actuator membranes  30 A- 30 E. The thickness of the actuator membranes  30 A- 30 E may be approximately 1.5 times to approximately 3 times the thickness of the flexible membrane  26 .  
         [0026]     A seed layer (not shown), which may be comprised of gold or copper, may be deposited, patterned, and etched for electroless plating. A thick layer is then deposited and patterned to form a mold for a subsequent plating step. If the thick layer is a photoresist layer, then no etching is required. After the mold is formed, a metal wall made of nickel, for example, is electrolessly plated to form the rigid walls  18 . The mold is then removed. In some embodiments, the MEMS device  10  is formed on two wafers, an upper wafer and a lower wafer. In other embodiments, the walls may be formed at the desired height and then a cover is placed over the device. When the two wafer construction is used, the upper wafer has a seed layer of gold or copper, for example, deposited on it and then a thick layer to form a mold is deposited and patterned. The other halves of the rigid walls are formed with an electrolessly plated metal. A solder layer is electroplated on the ends of the walls and the mold material is removed.  
         [0027]     The two wafers are bonded together by holding them face-to-face and heating them so the solder forms a bond between adjoining metal walls. The top wafer  54  may be ground down to its desired height and deep reactive ion etching may be used to form nozzle holes  36  for the fluidic chambers in the top wafer  54 . The deep reactive ion etching may optionally be used to expose the flexible membrane  26  over the pressure compensating area to ambient air. The bottom wafer is also etched using the deep reactive ion etching to form an inlet  60  to the inlet area.  
         [0028]     As shown in  FIG. 2 , the inlet  60  provides access to the inlet area  28  for a fluid source. The fluid entering through the inlet flows into one of the fluidic chambers  34  overlying an actuator membrane  30 B. An electrical signal in one of the actuator electrodes  44  causes the actuator membrane  30 A- 30 E that overlies the actuator electrode coupled to an active signal to move towards the actuator electrode, although other types of excitation may be used to actuate other types of actuators. This deflection expands the fluidic chamber and decreases the pressure in the chamber  34  so fluid flows from the inlet area  28  into the fluidic chamber. When the signal in the actuator electrode  44  returns to an inactive state, the actuator membrane returns to its equilibrium position. This action causes a portion of the fluid in the fluidic chamber  34  to be expelled from the fluidic chamber through the nozzle  36 . When the MEMS device  10  is coupled to an ink source, it may be operated as an ink jet print head.  
         [0029]      FIG. 3  is a cross-sectional view of the MEMS device  10  through the layer adjacent the passageway  22 B. Thus,  FIGS. 2 and 3  demonstrate that a layer of material separates the pressure compensating chamber  52  from the actuator cavities  42  except for the passageways  22 A- 22 E. This separating layer is typically comprised of silicon as described earlier.  
         [0030]     The pressure compensating area under the flexible membrane  26  aids in immunizing the actuator membranes  30 A- 30 E from the effects of pressure fluctuations through at least two mechanisms. For one, as noted above, the flexible membrane  26  may be etched so it is thinner and, therefore, more responsive to pressure fluctuations. Thus, the flexible membrane  26  is likely to deflect in response to a pressure change and relieve the pressure differential before it affects any of the actuator membranes  30 A- 30 E. Secondly, the dimensions of the flexible membrane  26  are sized to provide a substantially larger volume under the flexible membrane than the sum of the volumes of the actuator cavities  42 . Even without etching, the larger area of the flexible membrane  26  would render the membrane  26  more flexible than any one of the actuator membranes  30 A- 30 E because they are smaller in surface area that the flexible membrane  26 .  
         [0031]     As shown in the figures, the actuator membranes  30 A- 30 E and the flexible membrane  26  are generally rectangular in shape. The flexibility of the actuator membranes and the flexible membrane may be described with the following equations:  
       D   =       Et   3       12   ⁢     (     1   -     v   2       )             
         Δ   ⁢           ⁢   P     =     Dy     0.0026   ⁢     w   4             
 
 Where y is the deflection of a membrane caused by a pressure differential ΔP, w is the short dimension or width of a rectangular membrane, D is the flexural rigidity of a membrane, E is Young&#39;s modulus, t is the thickness of the membrane, and v is Poisson&#39;s ratio. As the formulae show, when the width of the pressure compensating membrane  26  is five times the width of an actuator membrane, the flexible membrane is 3125 times more flexible than the actuator membrane. For acceptable pressure compensating characteristics, the short dimension of the flexible membrane  26  may be approximately 3 to approximately 10 times the width of the actuator membrane. These dimensions help ensure that environmental pressure changes are more likely to be absorbed by the flexible membrane rather than by one of the actuator membranes. 
 
         [0032]     The actual amount of deflection in the flexible pressure compensating membrane  26  may be calculated from the following equation: 
 
 P   i ( V   a1   +V   b1 )= P   2  ( V   a2   +V   b2 ) 
 
 Where P 1  is the initial pressure inside the membrane cavity, V a1  is the sum of the initial volumes of the actuator cavities covered by the actuator membranes  30 A- 30 E, V b1  is the initial volume of the pressure compensating chamber covered by the flexible covering  26 , P 2  is the final pressure, V a2  is the sum of the final volumes of the actuator cavities, V b2  is the final volume of the pressure compensating chamber  52 . Since the flexible membrane  26  is designed to be much more flexible than the actuator membranes  30 A- 30 E, either no or minimum deflection of the actuator membranes  30 A- 30 E should occur, i.e., V a1 =V a2 . Also, V b2 =V b1 +ΔV b , where ΔV b  is the volume change of the pressure compensating chamber  52 . Therefore: 
 
 P   1 ( V   a1   +V   b1 )= P   2 ( V   a1   +V   b1   +ΔV   b ) 
 
         [0033]     For illustration purposes, assume a print head incorporating a fluidic MEMS  10  having a sealed actuator cavity is manufactured in Rochester and shipped to Denver for use. As mentioned above, the altitudes and consequently standard pressures in Rochester and Denver are substantially different. The volumetric changes can be calculated for the pressure change due to altitude change between Rochester and Denver. In this case P1=1 atm, P2=0.82 atm.  
         Δ   ⁢           ⁢     V   b       =       0.18   0.82     ⁢     (       V     a   ⁢           ⁢   1       +     V     b   ⁢           ⁢   1         )           
 
 Further assume the length, L b , of the flexible membrane is equal to the total width of the actuator area  24 . Substituting the values for the actuator membrane length of 1000 μm, the membrane gap of 0.8 μm, and the flexible membrane width of 500 μm, the equation now reads:  
         y   *     L   b     *   500     =       0.18   0.82     ⁢     (       5   ⁢     L   b     *   0.8   *   1000     +     500   *     L   b     *   0.8       )           
 
 The flexible membrane  26  bulges no more than 0.5 μm in this case. 
 
         [0034]     An illustration of how the pressure compensating chamber  52  absorbs pressure variation caused by out gassing is now presented. If an additional pressure of 0.1 atm is accumulated inside the actuator cavities during the manufacturing process of plugging release or venting holes with SU8 polymer, the deflection of the pressure compensating membrane  26  can still be calculated. Deriving from the above discussed formulae: 
 
Δ V   b =−0.1( V   a1   +V   b1 ) 
 
 In this case, the pressure compensating membrane  26  is deflected downwardly by 0.24 μm. 
 
         [0035]     While the flexible membrane  26  discussed above is made from the same polysilicon membrane material as the actuator membranes  30 A- 30 E, the flexible membrane  26  may also be made from SU8 polymer or other silicon layers. Furthermore, the flexible membrane may be dimpled by known methods to reduce the possibility of stiction.  
         [0036]     The surface micro-machined actuator and flexible membranes need to be released. In one embodiment of a MEMS device, the release holes are formed in the top of the device and the release holes are made relatively narrow and long (typically 2-3 μm by 15 μm). These release holes are more feasibly plugged by depositing oxide in the holes as is known.  
         [0037]     While some embodiments are described with reference to an ink-jet printer, one ordinarily skilled in the art would understand that embodiments herein are not limited to ink jet printers. Rather, any MEMS device that uses sealed actuator cavities is contemplated by this disclosure, including, but not limited to a micro pump, or other fluid device. The example shown in the drawings has an active polysilicon membrane  30 , such as one of the membranes  30 A- 30 E, over an electrode  44 . This lower electrode (actuator)  44  can have a charge applied to it to attract the membrane  30  towards it. When the charge is released, the membrane  30  springs back to its natural position. When a fluidic cavity  34  is formed over the membrane  30 , the forces generated during the release of membrane  30  eject a droplet of ink through the nozzle opening  36  onto a piece of media. By varying the design of this type of structure, a wide variety of small pumps, chambers, and sensors may be constructed.  
         [0038]     If the pressure in an actuator cavity between an electrode  44  and a membrane is altered significantly, operation of the device could be affected. Too much or too little pressure can alter the deflection characteristics of the membranes  30 A- 30 E in an undesirable manner. The flexible membrane  26  and its underlying pressure compensating chamber  52  help maintain the pressure beneath the membranes  30 A- 30 E to make the operation of the device more reliable.  
         [0039]     A second embodiment of an improved fluidic MEMS device  410  is shown in  FIGS. 4-6 . The device  410  includes a substrate  414  from which tall side walls  418  extend upwardly to divide the substrate into three main regions. These regions are a pressure compensating region  420 , an actuator area  424 , and an inlet area  428 . The substrate may be made from a suitable substrate material for a particular application, such as silicon, and the rigid walls  418  may be made of suitable material, such as nickel. Within actuator area  424  are a plurality of rigid walls that extend across the width of the area to divide the actuator area into a plurality of actuator areas. Actuator membranes  430 A- 430 E are anchored by a known method around the perimeter of the actuator areas to form an actuator cavity  442  between each of the actuator membranes  430 A- 430 E and a portion of the underlying substrate  414 . The layer separating the pressure compensating area  424  from the actuator area  428  is etched to provide a plurality of passageways  422 A- 422 E for providing gas flow between the actuator cavities  442  and the pressure compensating area  424 . Although the pressure compensating area is shown as being adjacent to the actuator area in  FIG. 4 , other arrangements are possible. For example, the pressure compensating area  420  may underlie or be located over the actuator area  424  or the inlet area  428 .  
         [0040]     The passageways  422 A- 422 E are designed so almost no gas flow occurs between an actuator cavity  442  and the pressure compensating chamber  452  during high frequency operation of the actuator. Air is exchanged between the actuator cavity  442  and the pressure compensating chamber  452  during actuator idle time. To achieve this goal, each passageway  422  is relatively long with a small cross-sectional area. For example, a passageway may have a cross-sectional area of 3-5 μm by 0.5-2 μm and a length of 100-1000 μm. The gas sealed within the actuator cavities  422 A- 422 E and pressure compensating chamber  452  may be air. Alternatively, the gas may be nitrogen (N 2 ), sulfur hexafluoride (SF 6 ), or an inert gas to prevent humidity and contamination effects on the performance of the actuators.  
         [0041]     Because the side walls  418  extend above the actuator membranes  430 A- 430 E, a fluidic chamber exists over each of the actuator membranes  430 A- 430 E. The side wall  418  separating the actuator area  424  from the inlet area  428  is segmented so that a plurality of passageways provide fluid communication between the inlet area  428  and the fluidic chambers  434  overlying the actuator membranes  430 A- 430 E. Consequently, expansion of a fluidic chamber caused by the movement of an actuator membrane towards the substrate  414  results in the flow of fluid from the inlet area  428  into the fluidic chamber in which the actuator membrane moved. Return of the actuator membrane to its position before the downward displacement expels some of the fluid in the fluidic chamber out through a nozzle  436  and some of the fluid returns to the inlet region  428 .  
         [0042]     In further detail,  FIG. 5  shows a cross-section of the device  410  taken through passageway  422 B in  FIG. 4 . Deposited on the substrate  414  may be one or more insulating layers  440 . The insulating layer may be comprised of silicon dioxide and silicon nitride. A conductive layer, which may be comprised of polysilicon, may be deposited and etched to form a bottom electrode  444  for an actuator. A sacrificial layer, which may be comprised of silicon dioxide, may be deposited, patterned, and etched to form holes for anchoring a subsequent structural layer. The structural layer forms the actuator membranes  430 A- 430 E that extend past the rigid wall into the pressure compensating area  420 . At least a portion of the membranes  430 A- 430 E are not anchored to the substrate  414  within the pressure compensating area  420  so gas communication is provided between the actuator cavities  442  and a pressure compensating chamber  452 . The structural layer forming the actuator membranes may be comprised of polysilicon. The actuator membranes  430 A- 430 E anchored over the actuator areas and the underlying substrate form a plurality of actuator cavities  442 .  
         [0043]     A seed layer (not shown), which may be comprised of copper or gold, may be deposited, patterned, and etched for electroless plating. A thick layer is then deposited and patterned to form a mold for a subsequent plating step. If the thick layer is a photoresist layer, then no etching is required. After the mold is formed, a metal wall made of nickel, for example, is electrolessly plated to form the rigid walls  418 . The mold is then removed. In some embodiments, the MEMS device  410  is formed on two wafers, an upper wafer and a lower wafer. In other embodiments, the walls may be formed at the desired height and then a cover is placed over the device. When the two wafer construction is used, the upper wafer has a seed layer of copper or gold, for example, deposited on it and then a thick layer to form a mold is deposited and patterned. The other halves of the rigid walls are formed with an electrolessly plated metal. A solder layer is electroplated on the ends of the walls and the mold material is removed.  
         [0044]     The two wafers are bonded together by holding them face-to-face and heating them so the solder forms a bond between adjoining metal walls. The top wafer  454  may be ground down to its desired height and deep reactive ion etching may be used to form nozzle holes  436  for the fluidic chambers in the top wafer  454 . The bottom wafer is also etched using the deep reactive ion etching to form an inlet to the inlet area. Once the wafers have been bonded to one another, the pressure compensating chamber  452  is closed to ambient air. The major difference between this embodiment and the one described above with reference to  FIGS. 1-3  is that the pressure compensating chamber  452  in the second embodiment includes the tall chamber area between the nickel walls. In the earlier described embodiment, the chamber  52  is a relatively shallow cavity.  
         [0045]     As shown in  FIG. 5 , the inlet  460  provides access to the inlet area  428  for a fluid source. The fluid entering through the inlet flows into one of the fluidic chambers  434  overlying an actuator membrane  430 B. An electrical signal in one of the actuator electrodes  444  causes the actuator membrane  430 A- 430 E that overlies the actuator electrode coupled to an active signal to move towards the actuator electrode. This deflection expands the fluidic chamber and decreases the pressure in the chamber  434  so fluid flows from the inlet area  428  into the fluidic chamber. When the signal in the actuator electrode  444  returns to an inactive state, the actuator membrane returns to its equilibrium position. This action causes a portion of the fluid in the fluidic chamber  434  to be expelled from the fluidic chamber through the nozzle  436 . When the MEMS device  410  is coupled to an ink source, it may be operated as an ink jet print head.  
         [0046]      FIG. 6  is a cross-sectional view of the MEMS device  410  that is taken through the layer adjacent passageway  422 B. Thus,  FIGS. 5 and 6  demonstrate that a layer of material separates the actuator cavities  442  from the pressure compensating chamber  452  except for the passageways  422 A- 422 E in the layer. This separating layer is typically comprised of silicon as described earlier.  
         [0047]     The pressure compensating chamber  452  aids in immunizing the actuator membrane  430  from the effects of pressure fluctuations because the dimensions of the chamber  452  are sized to provide a substantially larger volume than the sum of the volumes of the actuator cavities  442 . This additional volume that is in gas communication with the actuator cavities  442  provides the actuator membranes  430 A- 430 E with resiliency to absorb pressure changes that may arise from internal pressure fluctuations.  
         [0048]     The pressure compensating chamber  452  creates a large pocket of air in the system that decreases the effect of pressure generated inside the system (due to outgassing, chemical reaction, arcing, etc.) because more volume means less pressure variation for a given amount of additional gas generated (PV-nRT). Because the pressure is acting on a larger volume, a smaller differential pressure is experienced by the actuator membranes  430 A- 430 E and there is less deflection of the membranes resulting from the fluctuation. Optionally, a rigid cover  454  means the pressure compensating chamber  452  does not absorb changing ambient (external) pressure, because the top wafer does not provide a pressure responsive interface as the flexible membrane  26  does in the MEMS device  10  discussed above. The air volume in the pressure compensating chamber  452  is illustratively much higher (approximately 50 times to approximately 100 times) than the total air volume under all the membranes  430 A- 430 E as the pressure compensating chamber  452  is about 80 μm in height, whereas the air under each of the membranes  430 A- 430 E is less than 1 μm in height.  
         [0049]     While the foregoing has been described in conjunction with various exemplary embodiments, it is to be understood that many alternatives, modifications, and variations would be apparent to those skilled in the art. Accordingly, Applicants intend to embrace all such alternatives, modifications and variations that follow in this spirit and scope.