Patent Publication Number: US-11035496-B2

Title: Three-way microvalve device and method of fabrication

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 14/872,202 filed Oct. 1, 2015, incorporated herein in its entirety by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under FA8651-13-C-0267 awarded by the Air Force. The government has certain rights in the invention. 
    
    
     FIELD OF INVENTION 
     The present invention is directed to a three-way (3-way) Micro-Electro-Mechanical Systems (MEMS)-based micro-valve device and a method of fabricating the device. The present invention involves a novel feature of using the fluid under control of the micro-valve to pressure balance the actuator and thereby enable small actuation forces to open and close the device. The present invention has a wide range of applications, including medical, industrial control, aerospace, automotive, consumer electronics and products, as well as any application(s) requiring the use of three-way micro-valves for the control of fluids. 
     BACKGROUND OF THE INVENTION 
     A number of MEMS-based micro-valves have been reported in the literature using a variety of actuation methods, including: pneumatic; electrostatic; thermo-pneumatic; shape-memory alloy (SMA); thermal bimetallic; piezoelectric; and electromagnetic. 
     All of these micro-valves previously reported in the literature have been 2-way devices that can merely “open” or “close” to allow the device to “turn on” or “turn off” the flow of fluid through the structure. Importantly, none of these devices can be operated as three-way micro-valves that can direct the flow of fluid in a preferred direction. This is partly due to the fact that MEMS is, in general, a relatively new technology, and specifically because MEMS-based micro-valves are even less mature. Consequently, the only available method for the implementation of a fluidic system wherein the fluid can be directed to a preferred direction has been to use at least a quantity of at least two (2) separate two-way micro-valves. However, this is an expensive solution that doubles the power required, size, weight and space, as well as reduces reliability, and therefore is not an optimal or preferred solution for many applications. 
     A major challenge for MEMS-based actuators in general, and micro-valves in particular, is the very low actuation forces that can be generated on the small dimensional size scales of the actuator elements. The resulting small actuation forces typically prevent these types of devices to be used where the actuator must overcome larger forces. For example, a typical electrostatically-actuated micro-valve will only generate less than a 1 psi (pound per square inch) of actuation pressure. Therefore, if the micro-valve actuator must overcome the fluid pressure in order to open and/or close the device to the flow of fluid, then the micro-valve would be restricted to applications where the fluid pressures are smaller than the actuation pressure, which is less than 1 psi. 
     Disclosed herein is a three-way micro-valve device and method of fabrication that can be tailored to the requirements of a wide range of applications. The disclosed 3-way micro-valve can use a number of different actuation methods, including actuation methods that have very small actuation pressures while being able to control fluid pressures much higher than the pressures that can be generated by the actuator. The micro-valve of the present invention employs a pressure balancing scheme so that it can be actuated while controlling fluid pressures much larger than the actuation pressure generated by the actuator. 
     SUMMARY OF INVENTION 
     The present invention is directed to a three-way (3-way) Micro-Electro-Mechanical Systems (MEMS)-based micro-valve device and method of fabrication for the implementation of a three-way MEMS-based micro-valve. The present invention has a wide range of applications, including medical, industrial control, aerospace, automotive, consumer electronics and products, as well as any application(s) requiring the use of three-way micro-valves for the control of fluids. 
     A major challenge for MEMS-based actuators in general, and micro-valves in particular, is the very low actuation forces that can be generated on the small dimensional size scales of the actuator elements. The resulting small actuation forces typically prevent these types of devices to be used where the actuator must overcome larger forces. For example, a typical electrostatically-actuated micro-valve will only generate less than a 1 psi (pound per square inch) of actuation pressure. Therefore, if the micro-valve actuator must overcome the fluid pressure in order to open and/or close, then the microvalve would be restricted to applications where the fluid pressures are smaller than the actuation pressure, that is, less than 1 psi 
     The present invention allows for the implementation of a three-way micro-valve device and method of fabrication that can be tailored to the requirements of a wide range of applications and fluid types. The 3-way micro-valve disclosed can also use a number of different actuation methods, including actuation methods that have very small actuation pressures and energy densities even at higher fluidic pressures. This is enabled by a novel pressure-balancing scheme wherein the fluid pressure balances the actuator mechanism so that only a small amount of actuation pressure (or force) is needed to switch the state of the actuator and device from open to closed, or closed to open. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1( a ) and 1( b )  are an illustration of a three-way micro-valve with two inlet ports and one outlet port, and showing two functional states of the device. 
         FIGS. 2( a ) and 2( b )  are an illustration of a three-way micro-valve with one inlet port and two outlet ports, and showing two functional states of the device. 
         FIG. 3  is a table showing the possible states of a three-way micro-valve having two inlet ports and one outlet port. These states are applicable to the three-way micro-valve of  FIGS. 1( a ) and 1( b ) . 
         FIG. 4  is a table showing the possible states of a three-way micro-valve having one inlet port and two outlet ports. These states are applicable to the three-way micro-valve of  FIGS. 2( a ) and 2( b ) . 
         FIGS. 5( a ) -5( c )  are cross sectional drawings of a pressure-balanced, normally-open, electrostatically-actuated, three-way micro-valve with two inlet ports and one outlet port. 
         FIGS. 6( a ) and 6( b )  are cross sectional drawings of a pressure-balanced, normally-closed, electrostatically-actuated, three-way micro-valve with two inlet ports and one outlet port. 
         FIGS. 7( a ) and 7( b )  are cross sectional drawings of a pressure-balanced, normally-closed, piezoelectrically-actuated, three-way micro-valve with two inlet ports and one outlet port. 
         FIGS. 8( a )-8( h )  are cross sectional drawings illustrating the fabrication process of the bottom substrate and the movable membrane used for implementation of the pressure-balanced, electrostatically-actuation three-way micro-valve. 
         FIGS. 9( a )-9( e )  are cross sectional drawings illustrating the fabrication process of the top substrate used for implementation of the pressure-balanced, electrostatically-actuated three-way micro-valve. 
         FIGS. 10( a )-10( b )  are cross sectional drawings illustrating the fabrication process for the implementation of process pressure-balanced, electrostatically-actuated three-way micro-valve, wherein the top and bottom substrates are joined together to form the micro-valve. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is directed to a three-way (3-way) Micro-Electro-Mechanical Systems (MEMS)-based micro-valve device and method of fabrication for the implementation of a three-way MEMS-based micro-valve. The present invention has a wide range of applications, including medical, industrial control, aerospace, automotive, consumer electronics and products, as well as any application(s) requiring the use of three-way micro-valves for the control of fluids. 
     The present invention allows for the implementation of a three-way micro-valve device and method of fabrication that can be tailored to the requirements of a wide range of applications and fluid types. The three-way micro-valve disclosed herein can also use a number of different actuation methods, including actuation methods that have very small actuation energy densities, and still be able to control the flow of fluids even at higher fluidic pressures. This is enabled by a novel pressure-balancing scheme wherein the fluid pressure balances the actuation so that only a small amount of actuation force or pressure is needed to switch the state of the actuator and device, even when the fluid pressure is much larger than the pressure that can be generated by the actuator. 
       FIGS. 1( a ) and 1( b )  and  FIGS. 2( a ) and 2( b )  illustrate the functionality of the three-way micro-valve of the present invention. The micro-valve has three (3) fluidic ports (that is, openings into or out of the device structure through which fluid that is either a gas or a liquid or a combination of gas and liquid, can flow) with two (2) different micro-valve device configurations, with the first device configuration shown in  FIGS. 1( a ) and 1( b )  and the second in  FIGS. 2( a ) and 2( b ) . 
     In one micro-valve device configuration  10 , as shown in  FIGS. 1( a ) and 1( b ) , the micro-valve device  11  has two inlet fluidic ports, inlet port one, numbered  12  in  FIGS. 1( a ) and 1( b ) , and inlet port two, numbered  13  in  FIGS. 1( a ) and 1( b ) , that are used as inlet ports, thereby allowing fluid to flow into the micro-valve  11 . That is, fluid can flow into the micro-valve device  11  through these inlet ports  12  and  13 , through the micro-valve  11 , and into, and through the remaining port, outlet port three, numbered  16  in  FIGS. 1( a ) and 1( b ) , if these ports are in an “open” state. 
     Inlet port 1 , numbered  12  in  FIGS. 1( a ) and 1( b ) , is connected to inlet fluid conduit  14 , which is a fluid pathway into inlet port one  12  and the micro-valve device  11  in  FIGS. 1( a ) and 1( b ) . Outlet port three, numbered  16  in  FIGS. 1( a ) and 1( b ) , is connected to outlet fluid conduit  17  that is a fluid pathway out of the micro-valve device  11 . Inlet port two, numbered  13  in  FIGS. 1( a ) and 1( b ) , is connected to fluid conduit  15  and is a fluid pathway into inlet port two  13  and the micro-valve  11  in  FIGS. 1( a ) and 1( b ) . 
     The micro-valve device  11  shown in  FIGS. 1( a ) and 1( b )  has a fluidic switching mechanism  18 , whereby either inlet port one, numbered  12  in  FIGS. 1( a ) and 1( b ) , or inlet port two, numbered  13  in  FIGS. 1( a ) and 1( b ) , is connected to outlet port three, numbered  16  in  FIGS. 1( a ) and 1( b ) . This fluidic switching mechanism  18  is shown in two of the switched states with the first switched state illustrated in  FIG. 1( a )  wherein inlet port one, numbered  12  in  FIGS. 1( a ) and 1( b ) , is fluidically connected to outlet port three, numbered  16  in  FIGS. 1( a ) and 1( b ) . That is, fluid can flow from conduit  14 , through inlet port one  12 , through the micro-valve  11 , through the outlet port  16 , and subsequently through conduit  17 . In the state shown in  FIG. 1( a ) , inlet port two, numbered  13  in  FIGS. 1( a ) and 1( b ) , is not connected to outlet port three, numbered  16  in  FIGS. 1( a ) and 1( b ) . That is, no fluid is allowed from conduit  15 , through the inlet port two  13  and into the micro-valve  11 , and therefore no fluid can flow through conduit  17  from inlet port two  13 . 
     In the second switched state shown in  FIG. 1( b ) , inlet port two, numbered  13  in  FIGS. 1( a ) and 1( b ) , is fluidically connected to outlet port three, numbered  16  in FIGS.  1 ( a ) and  1 ( b ) by the fluid switch  18  of the microvalve  11 . That is, fluid can flow from conduit  15 , through the inlet port two  13 , through the microvalve  11 , through the outlet port  16 , and subsequently through conduit  17 . In the state shown in  FIG. 1 b   , inlet port one, numbered  12  in  FIG. 1 , is not connected to outlet port three, numbered  16  in  FIGS. 1( a ) and 1( b ) . That is, no fluid is allowed from conduit  14 , through the inlet port one  12  and into the microvalve  11 , and therefore no fluid can flow through conduit  17  from inlet port one  12 . 
     In the second device configuration, as shown in  FIGS. 2( a ) and 2( b ) , the microvalve device  21  has one inlet fluidic port, inlet port one, numbered  26  in  FIGS. 2( a ) and 2( b ) , that is used as an inlet port, that is, fluid can flow into the microvalve device  21  through this inlet port  26 . There are two outlet fluidic ports, with outlet port one numbered  22  in  FIGS. 2( a ) and 2( b ) , and outlet port two numbered  23  in  FIGS. 2( a ) and 2( b ) . These outlet ports  22  and  23  are used as outlet ports  22  and  23 , whereby fluid can flow out of the microvalve device  21  that entered through inlet port one  26 . Inlet port one, which is numbered  26  in  FIGS. 2( a ) and 2( b ) , is connected to inlet fluid conduit  27  that is a fluid pathway into the microvalve device  21 . Outlet port one, which is numbered  22  in  FIGS. 2( a ) and 2( b ) , is connected to outlet fluid conduit  24  that is a fluid pathway out of the microvalve device  21 . Outlet port two, which is numbered  23  in  FIGS. 2( a ) and 2( b ) , is connected to outlet fluid conduit  25  that is a fluid pathway out of the microvalve device  21 . 
     The microvalve device  21  shown in  FIGS. 2( a ) and 2( b )  has a switching mechanism  28  whereby the inlet port, which is numbered  26  in  FIGS. 2( a ) and 2( b ) , is connected to either outlet port one, numbered  22  in  FIGS. 2( a ) and 2( b ) , or outlet port two, numbered  23  in  FIGS. 2( a ) and 2( b ) . 
     This switching mechanism  28  is shown in two states, with the first switched state illustrated in  FIG. 2( a )  wherein the inlet port, numbered  26  in  FIG. 2( a ) , is fluidically connected to outlet port one, numbered  22  in  FIG. 2( a ) . That is, fluid can flow from conduit  27 , through the inlet port  26 , through the micro-valve  21 , through the outlet port one  22 , and subsequently through conduit  24 . In the state shown in  FIG. 2( a ) , the inlet port, numbered  26  in  FIG. 2( a ) , is not connected to outlet port two, numbered  23  in  FIG. 2( a ) . That is, no fluid is allowed from conduit  27 , through the micro-valve  21 , and through conduit  25 . 
     In the second switched state of  FIGS. 2( a ) and ( b ) , shown in  FIG. 2( b ) , the inlet port, numbered  26  in  FIG. 2( b ) , is fluidically connected to outlet port two, numbered  23  in  FIG. 2( b )  by the micro-valve  21  switch mechanism  28 . That is, fluid can flow from conduit  27 , through the inlet port  26 , through the micro-valve  21 , through the outlet port two  23 , and subsequently through conduit  25 . In the state shown in  FIG. 2( b ) , inlet port one, numbered  26  in  FIG. 2( b ) , is not connected to outlet port one, numbered  22  in  FIG. 2( b ) . That is, no fluid is allowed from conduit  27 , through the microvalve  21 , and through conduit  24 . 
     As can be seen from  FIGS. 1( a ) and 1( b )  and  FIGS. 2( a ) and 2( b ) , the micro-valve is able to control the direction of the fluid from the inlet port(s) and conduit(s) to the outlet port(s) and conduit(s). 
     In general, the 3-way micro-valve of the device configuration of  FIGS. 1( a ) and 1( b ) , with two inlet ports and one outlet port, will have several possible states, as shown in the table  30  of  FIG. 3 , depending on which of the inlet ports and outlet port are either in an “on” or “off” state. As can be seen, five (5) of these states are essentially equivalent, in that no fluid is allowed to flow through the micro-valve device. Specifically, these are states  2 ,  3 ,  4 ,  5 , and  8 . Additionally, state  1 , wherein fluid flows through the device with all ports open, is not of much interest since this state can be obtained without the presence of a valve by just having a branching from port  1  to ports  2  and  3 . The two (2) states of primary interest are states  6  and  7  whereby the fluid can flow from inlet port  1  to outlet port  3  in State  6  and State  7 , where the fluid can flow from inlet port  2  to outlet port  3 . 
     Similarly, the 3-way micro-valve of the device configuration of  FIGS. 2( a ) and 2( b ) , with one inlet port and two outlet ports, also has several possible states, as shown in the table  40  of  FIG. 4 , depending on which of the inlet ports and outlet port are either in an “on” or “off” state. As in the previous case, there are five (5) states that allow no fluid to flow through the device. Specifically, these are states  3 ,  4 ,  5 ,  7 , and  8 . Additionally, state  1 , wherein fluid flows through the device with all ports open, is not of much interest since this state can be obtained without the presence of a valve by just having a branching from ports  1  and  2  to port  3 . The two (2) states of primary interest are states  2  and  6  whereby the fluid can from inlet port  1  to outlet port  2  in State  2  and State  6  where the fluid can flow from inlet port  1  to outlet port  3 . 
     It is important to note that valves in general, and micro-valves in particular, may not exhibit all of the states shown in  FIGS. 3 and 4 . The ability of these devices to exhibit specific states is dependent on the specific design of the device, method of actuation, as well as other factors. Nevertheless, as noted above, many of the states are redundant (e.g., the “no flow” states) or have no particular interest in applications (e.g., the state with all ports open) and therefore the ability of a micro-valve device to exhibit less than all possible states is not limiting in most applications. 
     Another differentiating element of 3-way micro-valves is whether they are “normally open” or “normally closed.” “Normally open” and “normally closed” describe the state or position of the valve when no actuation signal is applied to the device. That is, the natural or resting state of the device when no electrical power is applied to the device&#39;s actuator. Typically, a “normally closed” device would employ some kind of spring or mechanical force that results in the valve port or ports being closed when no power is applied to the micro-valve actuator. Conversely, a “normally open” micro-valve&#39;s ports are open when no power is applied. Whether the micro-valve is normally “open” or normally “closed” will depend on the exact design of the micro-valve, as well as the application requirements. The 3-way micro-valves of the present invention can be implemented in both the “normally open” or “normally closed” device configurations. 
     Typically, the actuation method employed in any micro-valve design is dictated by the requirements of the intended application. Typically these requirements would include: maximum flow rate, maximum pressure differential, operating temperatures, electrical power; size and weight; type of fluid to be controlled; as well as other factors. 
     The specific device requirements of a particular application will typically allow the number of viable actuation methods to be reduced. For example, in applications where the operational temperatures are relatively low or vary over a large range, the use of any type of thermally-initiated actuation methods such as thermal bimetallic, shape-memory alloy (SMA), and thermo-pneumatic may not be a good choice since all of these methods require heating of the actuator, and additionally, the actuator itself is temperature sensitive. 
     For example, shape-memory alloy and thermo-pneumatic actuators operate by heating an actuator material to induce a phase change, and therefore, the phase change temperature would have to be higher than the maximum operational temperature. Therefore, the operational temperatures are an important determiner of the choice of actuation method. Nevertheless, thermally-initiated actuation methods may have some significant advantages in some applications. For example, shape-memory alloy (SMA) actuators have several advantages compared to other actuation schemes, including: the actuation energy densities of SMA actuators are typically very high compared to other actuation methods and this allows the control of fluids at large pressure differentials; and the maximum allowable mechanical strains of SMA actuators are also very high (i.e., some SMA actuators have reported repeatable strain levels of around 8%) thereby enabling larger strokes and consequently larger flow rates at modest differential pressures. Thermo-pneumatic actuators also have very high actuation energy densities, but typically do not have large strokes since it is considered prudent to limit the strain levels of the materials used in the actuator to below 1%. 
     Often a very important criterion for selection of actuation method is the power requirements for the specific application. For example, for some applications the heating requirements of thermal-actuation methods may exceed the maximum desired device power requirements. 
     Additionally, pneumatic actuation approaches wherein an external pressure generator is required to provide pressures to actuate the device will increase the size (and power requirements) of the device considerably. Therefore, for some applications pneumatic actuation may not be an optimal approach. 
     Electromagnetic actuation is a popular method of actuation in macro-scale valves, but this type of actuation does not scale well to the MEMS size domain. Many MEMS-based electromagnetic actuation schemes require a meso-scale electromagnetic solenoid that must be attached to the valve mechanism and this increases the cost and size of the system considerably, and therefore, this actuation method may not be desirable for some applications. Alternatively, some MEMS-based electromagnetic actuation schemes attempt to integrate wire windings into the device structure, but this makes the fabrication very challenging and the maximum current that can be safely passed through small wires often limits the electromagnetic forces that can be generated using this approach. 
     Electrostatic and piezoelectric actuation methods are often employed for micro-valve devices. However, it is important to note that both of these approaches have small inherent strokes. That is, the amount of deflection of the actuator during actuation is relatively small. The resultant effect of a small stroke of the micro-valve is that the fluid flow pressure through the opening will be high in order to overcome flow resistance created by the small stroke, and therefore, this may limit the amount of fluid flow through the device when in an “open” state. 
     Another important point about electrostatic actuation is that the actuation energy densities or actuation pressures that can be generated using this actuation method are very small. The consequence of this is that a device using this actuation scheme may not be able to operate, that is actuate to open and/or close, at differential fluid pressures higher than can be generated by this type of actuator. 
     Piezoelectric actuation schemes, on the other hand, can generate very large actuation energy densities, and therefore, can be used in applications requiring operation at high differential fluid pressures. Typically, electrostatic actuation schemes are simpler to implement compared to piezoelectric actuation schemes. In fact, as a general rule, electrostatic-based actuation schemes will be the simplest to implement since it requires no additional or exotic materials such as in the case for shape-memory alloys, thermo-pneumatics, bimetallics and piezoelectrics. 
     The important point about actuation schemes for MEMS-based microvalves is that the requirements of the specific application will often dictate the type of actuator that can be used. The three-way micro-valve devices disclosed herein of the present invention can be used with any of the available actuation schemes. 
     Pressure-Balanced Three-Way Microvalve. 
     The first embodiment of a three-way micro-valve  50  of the present invention is shown in  FIGS. 5( a )-5( c ) . The three-way microvalve  50  shown in  FIGS. 5( a )-5( c )  is electrostatically-actuated and also pressure-balanced as described herein. In  FIG. 5( a ) , the micro-valve is shown an un-actuated state with both inlet port one and inlet port two both in an “open” state and connected to the outlet port so that fluid can flow through both of these ports and through the micro-valve outlet port. In  FIG. 5( b )  the device is shown in an actuated state with inlet port two in an “open” state and connected to the outlet port thereby allowing fluid to flow through inlet port two, through the micro-valve, and through the outlet port. In  FIG. 5( b ) , inlet port one is in a “closed” state and does not allow fluid to flow through this port. In  FIG. 5( c )  the micro-valve device is shown in the alternative actuated state with inlet port one “open” and connected to the outlet port whereby fluid is allowed to flow through inlet port one, through the micro-valve, and through the outlet port. In  FIG. 5( c ) , inlet port two is in a “closed” state and does not allow fluid to flow through this port. 
     The micro-valve  50  shown in  FIG. 5( a )  is shown in the un-actuated state, that is, with no power applied to the actuator. The micro-valve  50  has two inlet ports, inlet port one  54  and inlet port two  53 . There is one outlet port  57 . In the un-actuated state shown in  FIG. 5( a ) , the micro-valve  50  has both inlet ports,  53  and  54 , fluidically connected to the outlet port  57 . 
     The micro-valve  50  shown in  FIG. 5( a )  is composed of a bottom substrate layer  56  that is electrically conductive, a top substrate layer  55  that is also electrically conductive, and a middle substrate layer  62  that is electrically conductive. An electrically insulating layer  58  electrically insulates the top substrate layer  55  from the middle substrate layer  62 , and an electrically insulating layer  59  electrically insulates the bottom substrate layer  56  from the middle substrate layer  62 . The micro-valve device  50 , has a fluidic chamber  51  wherein the fluid to be controlled by the micro-valve  50  can pass through. Inside the flow chamber  51  of the micro-valve  50 , the middle substrate layer  62  has been made thinner and essentially is a membrane  64  that is mechanically compliant. That is, the membrane  64  can be deflected under the action of an actuation force of sufficient magnitude. The membrane  64  is also electrically conductive and is electrically connected to the electrically conductive middle substrate layer  62 . The membrane  64  has openings  52  that fluidically connect the top portion of the microvalve chamber  51  to the bottom portion of the microvalve chamber  51  and also fluidically connect the inlet ports,  53  and  54 , to the outlet port  57  when the micro-valve  50  is not actuated as shown in  FIG. 5( a ) . Additionally, depending on the exact details of the microvalve  50  design the membrane  64  may be patterned in various shapes and sizes in order to obtain specific design requirements. 
     As shown in  FIG. 5( a ) , the middle substrate layer  62  is connected to an electrical terminal  61 . Additionally, the top substrate layer  55  is connected to an electrical terminal  60 , and the bottom substrate layer is connected to an electrical terminal  63 . Importantly, in  FIG. 5( a ) , none of the electrical terminals, either  60 ,  61  or  63 , are connected to an applied voltage, since the micro-valve shown in  FIG. 5( a )  is in an un-actuated state. 
     The microvalve  50  shown in  FIGS. 5( a )-5( c )  has sealing rings (or surfaces or valve seats),  66  and  67 , the purpose of which is to reduce or eliminate leakage of fluid through the ports when the valve is in a closed position. The sealing rings  66  and  67  also help to reduce stiction effects, whereby the membrane  64  stays stuck to the sealing rings  66  or  67  when it is desired that the membrane  84  separate from the sealing rings  86  or  87 . 
     An actuated state of the micro-valve is shown in  FIG. 5( b ) . An applied voltage potential V  65  is applied across electrical terminal  60  connected to the top substrate layer  55  and electrical terminal  61  connected to the middle substrate layer  62 , which is also electrically connected to the membrane  64 . The polarity of the applied voltage  65  shown in  FIG. 5( b )  has the positive side of the voltage potential  65  applied to terminal  60  and the negative side of the voltage potential  65  applied to terminal  61 . However, the applied voltage  65  can be reversed with the same effect on actuation of the micro-valve  50 . Additionally, one side of the applied voltage potential  65  could be connected to ground also with the same effect on actuation of the micro-valve  50 . 
     When an electrical voltage potential is applied across the terminals  60  and  61  of the micro-valve  50 , as shown in  FIG. 5( b ) , electrostatic charges (not shown) will develop on the top substrate layer  55 . These electrical charges will be mirrored on the middle substrate layer  62  and the membrane  63 . That is, electrical charges of equal magnitude and opposite polarity (not shown) to the electrical charges on the top substrate layer  55  will develop in the middle substrate layer  62  and also the membrane  64 . 
     These electrostatic charges on the top substrate layer  55  and the middle substrate layer  62  that is electrically connected to the membrane  64 , result in an electrostatic force of attraction (not shown) to develop between the top substrate layer  55  and the membrane  64 . Since the membrane  64  is substantially mechanically compliant, the membrane  64  under the electrostatic forces of attraction will deflect towards the top substrate layer  55  if the electrostatic forces are larger than the mechanical stiffness of the membrane  64 . If the applied voltage potential  65  across electrical terminals  60  and  61  is sufficiently large in magnitude, the membrane  64  will be pulled toward and eventually touch the sealing ring  66 . This is the so-called “electrostatic pull-in phenomena.” The electrically insulating layer  58  will prevent electrical shorting of the electrostatically-charged membrane  64  and the electrostatically-charged top substrate layer  55 . The touching of the membrane  64  to the insulating layer  58  on the sealing ring  66  of the top substrate  55  is shown in  FIG. 5( b ) . When the membrane  64  makes sufficient contact to the sealing ring  66 , the micro-valve is in a fully actuated state, whereby the inlet port one  54  is closed to the flow of fluid, as shown in  FIG. 5( b ) . In this actuated state, inlet port two  53  is open and fluid can flow into this port, through the micro-valve  50  bottom part of the chamber  51 , through the openings  52  in the membrane  64 , through the top part of the micro-valve  50  chamber  51 , and through the outlet port  57 . Therefore, in this state, inlet port one  54  is closed to fluid flow, inlet port two  53  is open to fluid flow, and outlet port  57  is open to fluid flow. 
     An alternative actuated state of the micro-valve is shown in  FIG. 5( c ) . An applied voltage potential V  68  is applied across electrical terminal  63  connected to the bottom substrate layer  56  and electrical terminal  61  connected to the middle substrate layer  62  that is also electrically connected to the membrane  64 . The polarity of the applied voltage  68  shown in  FIG. 5( c )  has the positive side of the voltage potential  68  applied to terminal  63  and the negative side of the voltage potential  68  applied to terminal  61 . However, the applied voltage  68  can be reversed with the same effect on actuation of the micro-valve  50 . Additionally, one side of the applied voltage potential  68  could be connected to ground also with the same effect on actuation of the micro-valve  50 . 
     When an electrical voltage potential is applied across the terminals  63  and  61  of the micro-valve  50 , as shown in  FIG. 5( c ) , electrostatic charges (not shown) will develop on the bottom substrate layer  56 . These electrical charges will be mirrored on the membrane  64 . That is, electrical charges of equal magnitude and opposite polarity (not shown) to the electrical charges on the bottom substrate layer  56  will develop on the membrane  64 . 
     These electrostatic charges on the bottom substrate layer  56  and the middle substrate layer  62  that is electrically connected to the membrane  64 , result in an electrostatic force of attraction (not shown) to develop between the bottom substrate layer  56  and the membrane  64 . Since the membrane  64  is substantially mechanically compliant, the membrane  64  under the electrostatic force of attraction will deflect towards the bottom substrate layer  56  if the electrostatic forces are larger than the mechanical stiffness of the membrane  64 . If the applied voltage  68  across electrical terminals  63  and  61  is sufficiently large in magnitude, the membrane  64  will be pulled toward and eventually touch the bottom sealing ring  67 . This is the so-called “electrostatic pull-in phenomena.” The electrically insulating layer  59  will prevent electrical shorting of the electrostatically-charged membrane  64  and the electrostatically-charged bottom substrate layer  56 . The touching of the membrane  64  to the insulating layer  59  is shown in  FIG. 5( c ) . When the membrane  64  makes sufficient contact to the bottom sealing ring  67 , the micro-valve is in a fully actuated state, whereby the inlet port two  53  is closed to the flow of fluid. In this actuated state, inlet port one  54  is open and fluid can flow into this port, through the micro-valve  50  top part of the chamber  51 , through the openings  52  in the membrane  64 , through the top part of the micro-valve  50  chamber  51 , and through the outlet port  57 . Therefore, in this state, inlet port two  53  is closed to fluid flow, inlet port one  54  is open to fluid flow, and outlet port  57  is open to fluid flow. 
     As pointed out above, an important feature of the micro-valve  50  shown in  FIGS. 5( a )-5( c )  is the pressure-balancing scheme of the device  50 . Specifically, the inlet fluid pressure inside the micro-valve  50  chamber  51  is present on both sides of the membrane  64  and therefore applies equal fluid pressure over both surfaces of the membrane  64  with the result that the fluid pressure is balanced over both sides of the membrane  64 . Therefore, if the fluid pressure is balanced as shown in  FIG. 5( a ) , the micro-valve can be actuated, as shown in  FIGS. 5( b ) and 5( c ) , with an actuation pressure that is substantially less than the pressure of the fluid. This is useful when an actuation method has limited amounts or levels of actuation pressure that is available. This is particularly useful when electrostatic actuation is used that inherently has very limited amounts or levels of actuation pressure that can be generated with this actuation method. Noteworthy is that without the feature of pressure-balancing as shown in  FIG. 5( a ) , the actuation method would have to overcome the mechanical stiffness of the membrane  64  and the pressure of the fluid, which can be substantial. However, with pressure balancing, even if the pressure of the fluid is many times larger in magnitude than the electrostatic pressure that can be generated during actuation, the micro-valve  50  membrane can still be actuated as shown in  FIGS. 5( b ) and 5( c ) . 
     An alternative embodiment of a three-way micro-valve  70  of the present invention is shown in  FIGS. 6( a ) and 6( b ) . The three-way micro-valve  70  shown in  FIGS. 6( a ) and 6( b )  is electrostatically-actuated and also pressure-balanced, as described herein. In  FIG. 6( a ) , the micro-valve  70  is shown an un-actuated state with inlet port one in an “open” state and connected to the outlet port, thereby allowing fluid to flow through inlet port one, through the micro-valve, and through the outlet port. In this state, inlet port two is in a “closed” state and no fluid can flow through this port. In  FIG. 6( b ) , the micro-valve  70  is shown actuated state with inlet port two in an “open” state and connected to the outlet port, thereby allowing fluid to flow through inlet port two, through the micro-valve, and through the outlet port. In this alternative state, inlet port one is in a “closed” state and no fluid can flow through this port. 
     The micro-valve  70  shown in  FIG. 6( a )  is shown in the un-actuated state, that is, with no power applied to the actuator. The micro-valve  70  has two inlet ports, inlet port one  74  and inlet port two  73 . There is one outlet port  77 . In the un-actuated state shown in  FIG. 6( a ) , the micro-valve  70  has inlet port one  74  fluidically connected to the outlet port  77  and inlet port two  73  is closed to the flow of fluid through the port and consequently through the micro-valve  70 . 
     The micro-valve  70  shown in  FIG. 6( a )  is composed of a bottom substrate layer  76  that is electrically conductive, a top substrate layer  75  that is also electrically conductive, and a middle substrate layer  82  that is electrically conductive. An electrically insulating layer  78  electrically insulates the top substrate layer  75  from the middle substrate layer  82 , and an electrically insulating layer  79  electrically insulates the bottom substrate layer  76  from the middle substrate layer  82 . The micro-valve device  70 , has a fluidic chamber  71  wherein the fluid to be controlled by the micro-valve  70  can pass through. Inside the flow chamber  71  of the micro-valve  70 , the middle substrate layer  84  has been made thinner and essentially is a membrane  84  that is mechanically compliant. That is, the membrane  84  can be deflected under the action of an actuation force of sufficient magnitude. The membrane  84  is also electrically conductive and is electrically connected to the electrically conductive middle substrate layer  82 . The membrane  84  has openings  72  that fluidically connect the inlet port one  74  to the outlet port  77  when the micro-valve  70  is not actuated as shown in  FIG. 6( a ) . Additionally, depending on the exact details of the microvalve  70  design the membrane  84  may be patterned in various shapes and sizes in order to obtain specific design requirements. 
     As shown in  FIG. 6( a ) , the middle substrate layer  82  is connected to an electrical terminal  81 . Additionally, the top substrate layer  75  is connected to an electrical terminal  80  and the bottom substrate layer  76  is connected to an electrical terminal  83 . Importantly, in  FIG. 6( a ) , none of the electrical terminals, either  80 ,  81  or  83 , are connected to an applied voltage since the micro-valve shown in  FIG. 6( a )  is in the un-actuated state. 
     The micro-valve  70  shown in  FIGS. 6( a ) and 6( b )  has sealing rings (or surfaces or valve seats),  86  and  87 , the purpose of which is to reduce or eliminate leakage of fluid through the ports when the valve is in a closed position. The sealing rings  86  and  87  also help to reduce stiction effects, whereby the membrane  84  stays stuck to the sealing rings  86  or  87  when it is desired that the membrane  84  separate from the sealing rings  86  or  87 . 
     An actuated state of the micro-valve is shown in  FIG. 6( b ) . An applied voltage potential V  85  is applied across electrical terminal  80  electrically connected to the top substrate layer  75  and electrical terminal  81  electrically connected to the middle substrate layer  82  that is also electrically connected to the membrane  84 . The polarity of the applied voltage  85  shown in  FIG. 6( b )  has the positive side of the voltage potential  85  applied to terminal  80  and the negative side of the voltage potential  85  applied to terminal  81 . However, the applied voltage  85  can be reversed with the same effect on actuation of the micro-valve  70 . Additionally, one side of the applied voltage potential  85  could be connected to ground also with the same effect on actuation of the micro-valve  70 . 
     When an electrical voltage potential is applied across the terminals  80  and  81  of the micro-valve  70  as shown in  FIG. 6( b ) , electrostatic charges (not shown) will develop on the top substrate layer  75 . These electrical charges will be mirrored on the middle substrate layer  82  and the membrane  84 . That is, electrical charges of equal magnitude and opposite polarity (not shown) to the electrical charges on the top substrate layer  75  will develop in the middle substrate layer  82  and also the membrane  84 . 
     These electrostatic charges on the top substrate layer  75  and the middle substrate layer  82  that is electrically connected to the membrane  84 , result in an electrostatic force of attraction (not shown) to develop between the top substrate layer  75  and the membrane  84 . Since the membrane  84  is substantially mechanically compliant, the membrane  84  under the electrostatic force of attraction will deflect towards the top substrate layer  75  if the electrostatic forces are larger than the mechanical stiffness of the membrane  84 . If the applied voltage  85  across electrical terminals  80  and  81  is sufficiently large in magnitude, the membrane  84  will be pulled toward and eventually touch sealing ring  86 . This is the so-called “electrostatic pull-in phenomena.” The electrically insulating layer  78  will prevent electrical shorting of the electrostatically-charged membrane  84  and the electrostatically-charged top substrate layer  75 . The touching of the membrane  84  to the insulating layer  78  on the sealing ring  86  is shown in  FIG. 6( b ) . When the membrane  84  makes sufficient contact to the sealing ring  86 , the micro-valve  70  is in a fully actuated state whereby the inlet port one  74  is closed to the flow of fluid. In this actuated state, inlet port two  73  is open and fluid can flow into this port, through the micro-valve  70  bottom part of the chamber  71 , through the openings  72  in the membrane  84 , through the top part of the micro-valve  70  chamber  71 , and through the outlet port  77 . Therefore, in this state, inlet port one  74  is closed to fluid flow, inlet port two  73  is open to fluid flow, and outlet port  77  is open to fluid flow. 
     The micro-valve  70  shown in  FIG. 6( b )  can be returned to the un-actuated state, shown in  FIG. 6( a ) , by turning off or removing the applied voltage potential  85 . When the voltage potential  85  is removed, the electrostatic charges on the top substrate layer  75  and the membrane  84  dissipate and the force of attraction between the top substrate layer  75  and the membrane  84  diminishes and eventually goes to zero. In this condition, the mechanical stiffness of the membrane  84  will become larger than the electrostatic force of attraction as the electrostatic forces of attraction diminish and the membrane  84  will return to the un-deflected state as illustration in  FIG. 6( a ) . 
     As noted above, an important feature of the micro-valve  70  shown in  FIGS. 6( a )  and (b) is the pressure-balancing scheme of the device  70 . Specifically, the inlet fluid pressure inside the micro-valve  70  fluid chamber  71  is present on most of both sides of the membrane  84  with the exception of the area inside the sealing ring  87  of inlet port two  73 . Consequently, there is a nearly equal fluid pressure over both surfaces of the membrane  84  with the result that the fluid pressure (and force) is nearly balanced over both sides of the membrane  84 . Therefore, if the fluid pressure is balanced as shown in  FIGS. 6( a ) and 6( b ) , the micro-valve can be actuated, as shown in  FIG. 6( b )  with an actuation pressure that is substantially less than the pressure of the fluid. This is useful when an actuation method has limited amounts or levels of actuation pressure that is available. This is particularly useful when electrostatic actuation is used that inherently has very limited amounts or levels of actuation pressure that can be generated with this actuation method. Noteworthy is that without the feature of pressure-balancing as shown in  FIGS. 6( a ) and 6( b ) , the actuation method would have to overcome the mechanical stiffness of the membrane  84  and the pressure of the fluid, which can be substantial. However, with pressure balancing, even if the pressure of the fluid is many times larger in magnitude than the electrostatic pressure that can be generated during actuation, the microvalve  70  membrane can still be actuated as shown in  FIG. 6( b ) . 
     The important distinction between the micro-valve  50  shown in  FIGS. 5( a )-5( c )  and the micro-valve  70  shown in  FIGS. 6( a ) and 6( b ) , is that the micro-valve  50  in  FIGS. 5( a )-5( c )  is a normally open micro-valve  50 , wherein when in an un-actuated state or resting state, fluid is allowed to flow freely through either of the two inlet ports  53  and  54 , through the micro-valve  50  chambers  51 , and through the outlet port  57 . In contrast, the micro-valve  70  shown in  FIGS. 6( a ) and 6( b )  is a normally closed micro-valve  70  at least for inlet port two,  73 , when in an un-actuated state. 
     Additionally, as seen in  FIGS. 5( b ) and 5( c ) , actuation of the micro-valve  50  to close either of the inlet ports,  53  and  54 , requires the application of separate applied voltages  65  or  68 , whereas the micro-valve  70  shown in  FIGS. 6( a ) and 6( b ) , requires only one applied voltage potential  85  to actuate the micro-valve. Therefore, the micro-valve  70  shown in  FIGS. 6( a ) and 6( b )  has a less complicated applied voltage requirement compared to the micro-valve  50  shown in  FIGS. 5( a )-5( c ) . 
     As noted above, the micro-valve  50  and  70  devices shown in  FIGS. 5( a )-5( c )  and  FIGS. 6( a ) and 6( b )  employ electrostatic actuation as the method for actuating the devices  50  and  70 . However, other means can be employed for actuation in either design configuration including: piezoelectric; bimetallic; shape-memory alloy, and thermo-pneumatic. 
     For example, shown in  FIGS. 7( a ) and 7( b )  is a micro-valve  90  of the present invention having a similar design configuration as shown in  FIGS. 6( a ) and 6( b ) , but instead the micro-valve  90  uses piezoelectric actuation rather than electrostatic actuation. The 3-way micro-valve  90  shown in  FIGS. 7( a ) and 7( b )  is piezoelectrically-actuated and also pressure-balanced. In  FIG. 7( a ) , the micro-valve is shown an un-actuated state with inlet port one is an “open” state and connected to the outlet port, thereby allowing fluid to flow through inlet port one, through the micro-valve, and through the outlet port. In this state, inlet port two is in a “closed” state and no fluid can flow through this port. In  FIG. 7( b )  the device is shown actuated state with inlet port two in an “open” state and connected to the outlet port, thereby allowing fluid to flow through inlet port two, through the micro-valve, and through the outlet port. In this state, inlet port one is in a “closed” state and no fluid can flow through this port. 
     The micro-valve  90  shown in  FIG. 7( a )  is shown in the un-actuated state, that is, with no power applied to the actuator. The micro-valve  90  has two inlet ports that are inlet port one  94  and inlet port two  93 . There is one outlet port  97 . In the un-actuated state shown in  FIG. 7( a ) , the micro-valve  90  has inlet port one  94  fluidically connected to the outlet port  97  and inlet port two  93  is closed to the flow of fluid through this inlet port  93  and consequently through the micro-valve  90 . 
     The micro-valve  90  shown in  FIG. 7( a )  is composed of a bottom substrate layer  96 , a top substrate layer  95 , and middle substrate layers  110 . Middle substrate layers  110  may be composed of a plurality of layers as shown in  FIGS. 7( a ) and 7( b ) , so as to implement a configuration that allows the inclusion of a piezoelectric layer  105  in combination with electrode and electrical interconnection layers  106  and  107 . Additionally, depending on the exact design configuration details, middle substrate layers may also include one or more insulating layers  102  and  108  on either sides of the electrode and electrical interconnection layers  106  and  107 , and an insulating layer  109  to separate the electrode and electrical interconnection layers  106  and  107  where the piezoelectric layer  105  is not present. 
     An electrically insulating layer  98  may be present to electrically insulate the top substrate layer  95  from the middle substrate layers  110 , and an electrically insulating layer  99  may be present to electrically insulate the bottom substrate layer  96  from the middle substrate layers  110 . The micro-valve device  90 , has a fluidic chamber  91 , wherein the fluid to be controlled by the micro-valve  90  can pass through. Inside the flow chamber  91  of the micro-valve  90 , is located a mechanically-compliant membrane  112 . That is, the membrane  112  can be deflected under the action of an actuation force of sufficient magnitude. The membrane  112  may or may not be electrically conductive, and as shown in  FIGS. 7( a ) and 7( b ) , may be composed of a multiplicity of layers (one or more layers) including: a silicon layer  114 , one or more piezoelectric layers  105 , and electrode layers  106  and  107 . In other design and device configurations, the silicon layer  114  may be replaced with an alternative material layer or layers, or may be omitted completely using only a piezoelectric layer  105  and electrodes  106  and  107  in the membrane. Additionally, the silicon layer  114  may be composed of an alternative material layer and may also be in direct contact with the lower sealing ring  103  when the micro-valve  90  is un-actuated. That is, in an alternative configuration, the silicon layer  114  is below the piezoelectric layer  105  and the electrode layers  106  and  107 , rather than on top as shown in  FIGS. 7( a ) and 7( b ) . Additionally, depending on the exact details of the microvalve  90  design the silicon layer  114 , the piezoelectric layer  105 , as well as the electrode layers  106  and  107 , may be patterned in various shapes and sizes in order to obtain specific design requirements. 
     The membrane  112  has openings  92  that fluidically connect the inlet port  94  to the outlet port  97  when the micro-valve  90  is not actuated as shown in  FIG. 7( a ) . As shown in  FIG. 7( a ) , the electrode and electrical interconnection layer  107  is connected to an electrical terminal  101 . Additionally, electrode and electrical interconnection layer  106  connected electrical terminal  100 . Importantly, in  FIG. 7( a ) , the electrical terminals,  100  and  101 , are not connected to an applied voltage, since the micro-valve shown in  FIG. 7( a )  is in the un-actuated state. 
     The micro-valve  90  shown in  FIGS. 7( a ) and 7( b )  has sealing rings (or surfaces or valve seats),  103  and  104 , whose purpose is to reduce or eliminate leakage of fluid through the ports when the valve is in a closed position. The sealing rings  103  and  104  also help to reduce stiction effects, whereby the membrane  112  stays stuck to the sealing rings  103  or  104  when it is desired that the membrane  112  separate from the sealing rings  103  or  104 . 
     An actuated state of the micro-valve  90  is shown in  FIG. 7( b ) . An applied voltage potential  111 , V, is applied across electrical terminals  100  and  101  connected to the electrode and electrical interconnect layers  106  and  107  across the piezoelectric layer  105  and acts as the actuator on the membrane layers  112 . The polarity of the applied voltage  111  shown in  FIG. 7( b )  has the positive side of the voltage potential  111  applied to terminal  101  and the negative side of the voltage potential  111  applied to terminal  100 . However, the applied voltage  111  can be reversed with the same effect on actuation of the micro-valve  90 . Additionally, one side of the applied voltage potential  111  could be connected to ground also with the same effect on actuation of the micro-valve  90 . 
     When an electrical voltage potential is applied across the terminals  100  and  101  of the micro-valve  90 , as shown in  FIG. 7( b ) , the electrical field created by the applied voltage potential  111  results in a piezoelectric force (not shown), whereby a strain is produced in the piezoelectric material layer  105 . This strain in the piezoelectric layer  105  that part of the membrane layers  112 , also causes a strain the membrane layers  112  due to the mechanical coupling of the membrane layers  112  to the piezoelectric layer  105 . The consequence of this strain is that the layers of the membrane  112  deflect upwards so as to open the inlet port one  93  of the micro-valve  90  to the flow of fluid. That is, the membrane layers  112  deflect upwards under the action of the strain induced in the piezoelectric layer  105 . 
     Since the membrane layers  112  are substantially mechanically compliant, the membrane layers  112  under the piezoelectric force will deflect towards the top substrate layer  95  if the piezoelectric forces are larger than the mechanical stiffness of the membrane layers  112 . If the applied voltage  111  across electrical terminals  100  and  101  is sufficiently large in magnitude, the membrane layers  112  will deflect toward and eventually touch the sealing ring  104 . The touching of the membrane layers  112  to the sealing ring  104  is shown in  FIG. 7( b ) . When the membrane layers  112  make sufficient contact to the sealing ring  104 , the micro-valve  90  is in a fully actuated state, whereby the inlet port one  94  is closed to the flow of fluid. In this actuated state, inlet port two  93  is open and fluid can flow into this port, through the micro-valve  90  bottom part of the chamber  91 , through the openings  92  in the membrane layers  112 , through the top part of the micro-valve  90  fluid chamber  91 , and through the outlet port  97 . Therefore, in this state, inlet port one  94  is closed to fluid flow, inlet port two  93  is open to fluid flow, and outlet port  97  is open to fluid flow. 
     The micro-valve  90  shown in  FIG. 7( b )  can be returned to the un-actuated state, shown in  FIG. 7( a ) , by turning off or removing the applied voltage potential  111 . When the voltage potential  111  is removed, the piezoelectric forces on the membrane layers  112  dissipate and eventually go to zero. In this condition, the mechanical stiffness of the membrane layers  112  become larger than the piezoelectric forces and the membrane layers  112  will return to the un-deflected state, as illustrated in  FIG. 7( a ) . 
     An important different between the electrostatically-actuated micro-valve  50  and  70  shown in  FIGS. 5( a )-5( c ) and 6( a ) and 6( b ) , and the piezoelectrically-actuated micro-valve  90  shown in  FIGS. 7( a ) and 7( b ) , is that the electrostatic actuation phenomena is non-linear, whereby the deflection of the membranes  64  and  84  is a non-linear function of the applied voltage potential  68  and  85 . Additionally, once the membranes  64  and  84  have deflected a little over ½ of the total distance between the membrane  64  and  84  and substrate  55  and  75 , the membrane  64  and  84  snaps to the fully actuation position due to the electrostatic pull-in phenomena. In contrast, the piezoelectric actuator shown in  FIGS. 7( a ) and 7( b )  has a more linear deflection of the membrane with the applied voltage potential. 
     Another distinction of the micro-valve  90  shown in  FIGS. 7( a ) and 7( b )  compared to the micro-valves  50  and  70  shown in  FIGS. 5( a )-5( c ) and 6( a ) and 6( b )  is that the piezoelectric actuation of the micro-valve  90  will typically generate more actuation force than will an electrostatic actuator. The consequence of this higher actuation force is that the micro-valve  90  will have less probability of so-called “stiction” effects, whereby the membrane  112  stays stuck to the sealing ring  103  and  104 . 
     As noted above, an important feature of the micro-valve  90  shown in  FIGS. 7( a ) and 7( b )  is the pressure-balancing scheme of the device  90 . Specifically, the inlet fluid pressure inside the micro-valve  90  fluid chamber  91  is present on most of both sides of the membrane layers  112 , with the exception of the area inside the sealing ring of inlet port two  103 . Consequently, there is a nearly equal fluid pressure over both surfaces of the membrane layers  112  with the result that the fluid pressure is balanced over both sides of the membrane layers  112 . Therefore, if the fluid pressure is balanced, as shown in  FIGS. 7( a ) and ( b ) , the micro-valve  90  can be actuated, as shown in  FIG. 7( b ) , with an actuation pressure that is substantially less than the pressure of the fluid. 
     Noteworthy is that without the feature of pressure-balancing, as shown in  FIGS. 7( a ) and 7( b ) , the actuation method would have to overcome the mechanical stiffness of the membrane layers  112  and the pressure of the fluid, which can be substantial. However, with pressure balancing, even if the pressure of the fluid is many times larger in magnitude than the piezoelectric force that can be generated during actuation, the micro-valve  90  membrane layers  112  can still be actuated, as shown in  FIG. 7( b ) . 
     The micro-valve  90  shown in  FIGS. 7( a ) and 7( b )  is a normally-closed micro-valve  90 , wherein inlet port two  93  is closed to the flow of fluid when the micro-valve  90  is an un-actuated state or resting state. 
     It is important to note that while an embodiment of a normally-closed micro-valve  90  is shown in  FIGS. 7( a ) and 7( b ) , a normally-open design configuration of a pressure-balanced micro-valve, such as shown in  FIGS. 5( a )-5( c )  using piezoelectric actuation, is also readily possible with the use of one or more piezoelectric layers, so as to be part of the present invention. 
     While the micro-valves shown in  FIGS. 7( a ) and 7( b )  uses piezoelectric actuation, it is understood that other methods of actuation can be substituted for piezoelectric actuation including: bi-metallic actuation; shape-memory alloy actuation; thermo-pneumatic actuation; and others, and are covered under the present invention. 
     Method of Fabrication of 3-Way Pressure-Balanced Microvalve 
     The method of implementation of the pressure-balanced microvalve is illustrated in cross-sections of the substrate at various points in the fabrication process sequences  120 ,  150  and  160  of the micro-valve as shown in  FIGS. 8( a )-8( h ), 9( a )-9( e ) and 10( a ) and 10( b ) . The micro-valve fabrication process sequences  120 ,  150  and  160  shown in  FIGS. 8( a )-8( h ), 9( a )-9( e ) and 10( a ) and 10( b )  are for the implementation of an electrostatically-actuated pressure-balanced micro-valve, but can be modified so as to implement micro-valves using other methods of actuation such as piezoelectric-actuation, bimetallic actuation, shape-memory alloy actuation, thermo-pneumatic actuation, etc. 
     The micro-valve process sequence  120  begins in  FIG. 8( a )-8( h )  with a substrate  121 . The substrate  121  can be made of silicon, or alternatively any material compatible with the fabrication process and materials, such as other semiconductors, metal, glass, polymer, or ceramic. The surface of the substrate  121  may be doped using either diffusion or implantation so as to make the surface more electrically conductive (not shown). This doping may be masked in certain regions of the substrate  121  surface. This can be achieved by depositing a masking layer, performing photolithography on the masking layer, followed by etching of the masking layer, followed by introduction of the dopants into the unmasked regions of the surface of the substrate  121 . An alternative to introducing dopants into the surface of the substrate  121  is to deposit an electrically conductive material layer (not shown) onto the surface of the substrate  121 . This electrically conductive layer may be patterned using photolithography followed by etching so as to pattern the electrically conductive layer into the shape and pattern desired. 
     Subsequently, a layer of electrically insulating material  122  is deposited onto the surface of the substrate  121 . The electrically insulating material layer  122  can be made from low-stress silicon nitride (LSN) as well as other material layer alternatives such as silicon dioxide (SiO2), alumina, oxy-nitride, as well as any thin film material layer that is electrically insulating. The thickness of the electrically insulating material layer  122  can vary, depending on the exact device design, cost and time considerations, as well as technology considerations. This electrically insulating layer  122  may be patterned using photolithography and etching to open areas  124  in the layer  122  so as to make electrical contact to the underlying electrically conductive substrate  121  or the underlying electrically conductive layer previously deposited (not shown) and possibly patterned as described above. This is shown in  FIG. 8( b ) . 
     Next, a layer of material  123  that acts as a sacrificial material layer  123  is deposited on top of the electrically insulating layer  122  that was previously deposited onto the substrate  121 . This sacrificial material layer  123  can be composed of phosphosilicate glass (PSG) or any material type compatible as a sacrificial layer with the other materials used in fabrication and suitable for microfabrication, such as glasses, ceramics, metals, semiconductors, polymers, etc. The thickness of this sacrificial layer  123  can vary depending on the exact design, cost and time considerations, as well as technology considerations. Subsequently, this layer  123  has photolithography performed on it, followed by etching to pattern the layer  123  as shown in  FIG. 8( c ) . 
     Subsequently, a layer  125  of electrically conductive material is deposited that acts as a structural layer  125  of the micro-valve. This layer  125  can be composed of polycrystalline silicon (polysilicon), as well as any material type compatible with the other materials used in fabrication and suitable for microfabrication, such as semiconductors, metals, semi-metallic ceramics, etc. The thickness of this structural layer  125  can vary, depending on the exact design, cost and time considerations, as well as technology considerations. This structural layer  125  may be doped so as to make it higher in electrical conductivity. Subsequently, this structural layer  125  has photolithography performed on it, followed by etching to pattern the layer  125  as shown in  FIG. 8( d ) . During the patterning and etching of the structural layer  125 , holes  126  that fluidically connect the top and bottom sections of the micro-valve chambers may be made in this layer structural  125 . These holes  126  in the structural layer  125  enable the pressure balancing of the micro-valve membrane that will be made from the structural layer  125 . 
     Subsequently, a thin material layer  127  is deposited that will act as a strain-biasing layer  127  on the underlying structural layer  125 . This strain-biasing layer  127  will have an internal or residual stress that will cause the underlying structural layer  125  to slightly strain. This strain-biasing layer  127  can be made of Chromium (Cr), as well as any material layer type that is compatible with the other materials used in fabrication and suitable for microfabrication, such as semiconductors, metals, ceramics, etc. The thickness of this strain-biasing layer  127  can vary, depending on the exact design, cost and time considerations, as well as technology considerations. This stain-biasing layer  127  will be patterned as shown in  FIG. 8( e )  using photolithography followed by etching as are well known in the art. Alternatively, the strain-biasing layer  127  may be patterned using lift-off whereby a photosensitive polymer is first deposited and exposed to patterned the photosensitive polymer, then the strain-biasing layer  127  is deposited, and then, the photosensitive polymer is removed thereby only leaving the strain-biasing layer  127  in those areas where the photosensitive polymer was not present over the surface of the substrate. “Lift-off” is a well-known method for patterning material layers. 
     Next, an electrically conductive layer  128  of material will be deposited to form metal areas that can act as electrical bonding pads and also as a wafer-to-wafer bonding layer. This electrically conductive material layer  128  may be made of gold, as well as any material layer type that is compatible with the other materials used in fabrication, and that is suitable for microfabrication, electrically conductive, and can be used in wafer-to-wafer bonding. If gold is used as the electrically conductive layer  128 , then a very thin adhesion layer (not shown in  FIG. 8( f ) ), such as Chromium or any suitable adhesion material layer, may be deposited prior to the deposition of the layer  128  to ensure good adhesion of the layer  128  to the underlying layers  125  on the substrate  121 . The thickness of this electrically conductive layer  128 , and accompanying adhesion layer, if used, can vary, depending on the exact design, cost and time considerations, as well as technology considerations. The electrically conductive layer  128  (and accompanying adhesion layer, if used) is appropriately patterned as shown in  FIG. 8( f ) . This layer  128  will be patterned as shown in  FIG. 8( f )  using either photolithography followed by etching, or lift-off, both of which are well known in the art. 
     Note that an electrically conductive layer  128  is also deposited and patterned in the regions of the electrically insulating layer  122  that were opened up to expose either the electrically conductive substrate  121  or an electrically conductive material layer (not shown) that was deposited onto the substrate  121 . This layer  128  deposited directly onto the substrate  121  in these regions will allow electrical connection to the substrate  121  during micro-valve operations. 
     Photolithography is then performed on the backside of the substrate  121 , and then a through-wafer deep, high-aspect ratio etch, such as Deep, Reactive-Ion Etch (DRIE), is performed to form one or more micro-valve fluid ports  129 , as shown in  FIG. 8( g ) . Next, the layer  123  that acts as a sacrificial layer  123  is removed, thereby releasing the structural layer  125  as shown in  FIG. 8( h ) . Note that the strain-biasing layer  127  causes strain in the structural layer  125  upwardly. This upward deflection will enable the micro-valve to be normally closed when the fabrication is completed. 
     The process sequence  150  continues in  FIG. 9( a )-9( e )  on a second substrate  151 , as shown in  FIG. 9( a ) . The second substrate  151  to be used in the process sequence  150  can be made of silicon, or alternatively, any suitable substrate material compatible with the fabrication process  150  and materials, such as other semiconductors, metal, glass, polymer, or ceramic. 
     A thin layer of Chrome, or other appropriate material layer, is then deposited that acts as an adhesion layer (the adhesion layer is not shown in  FIG. 9( b )  since the layer is so thin). This is followed by the deposition of a layer of metal  152 , such as gold, that can act to help perform wafer-to-wafer bonding later in the process sequence. The metal and adhesion layers  152  are patterned as shown in  FIG. 9( b )  using photolithography, followed by etching, or are patterned using lift-off, as shown in  FIG. 9( b ) . Bother techniques are well known in the art. 
     Next, photolithography is performed on the substrate  151  followed by a deep, high-aspect ratio etch, such as DRIE, partially into the surface of the substrate to form part of the fluid chamber  153  of the micro-valve, as shown in  FIG. 9( c ) . 
     Subsequently, another photolithography and deep, high-aspect ratio etch, such as DRIE, is performed partially into the surface of the substrate  151  to form the valve seat or sealing ring  154  and complete the making of the fluid chamber  153  of the micro-valve as shown in  FIG. 9( d ) . Alternatively, a material layer for implementing the sealing ring  154  may be deposited, patterned and etched so as to form the shape and structure of the sealing ring  154 . 
     The purpose of the sealing ring  154  is two-fold: the first is to provide better fluid sealing when the micro-valve is closed; and the second is to reduce the stiction effects when the micro-valve is actuated (that is, opened by separating the micro-valve membrane from the sealing ring  154  surface). Typically, the shape of the sealing ring  154  is to have either a narrow width and/or a sharp edge on the surface. The reason for this preferred shape is that it will reduce leakage through the micro-valve when the device is closed to fluid flow and it will also reduce stiction effects between the sealing ring  154  and membrane that may make opening the micro-valve more difficult when the device is to be actuated. The substrate with the sealing rings  154  present is shown in  FIG. 9( d ) . 
     Photolithography is then performed on the backside of the second substrate  151 , followed by the performance of a deep, high-aspect ratio etch, such as DRIE, on the substrate  151  completely through the backside of the substrate  151  to form the microvalve ports  155  and  156 , as shown in  FIG. 9( e ) . An inlet port  155  and an outlet port  156  can be made with the same etching. Additionally, a through-substrate via or opening is made in the substrate  157  during this etch that will allow electrical connection to the substrate  121  for electrical actuation of the micro-valve. 
     The first substrate from  FIG. 8( h )  and the second substrate from  FIG. 9( e )  are then brought together and bonded as shown in  FIG. 10( a ) , by aligning the Gold bonding areas  128  and  152  on the two substrates  121  and  151  and performing a thermo-compression bond  162  thereby resulting in the completed micro-valve structure  163 , as shown in  FIG. 10( b ) . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.