US 20030002994 A1
A flow controller for use in microelectromechanical systems. The principal components of the controlled are a microvalve and sensor which are micromachined on one surface of a substrate that is formed with a fluid flow channel. The microvalve includes a shape memory alloy actuator element that is operated by a feedback signal from a control circuit. The sensor can be a fluid flow rate sensor or a fluid temperature sensor or a fluid pressure sensor. Conditions in the channel are sensed for generating the feedback signal.
1. A miniature flow controller for use in microelectromechanical systems, the flow controller comprising a substrate, the substrate being formed with a channel for confining a fluid flow, a thin film microvalve micromachined on the substrate, the microvalve comprising a valve actuator, the actuator having an operating element comprised of a shape memory alloy which undergoes a crystalline phase transformation and resulting shape change from a low temperature deformable phase to a high temperature memory phase when the element is heated through the alloy's phase change transformation temperature, the element being positioned for movement in the channel for contolling the fluid flow responsive to the shape change, a sensor micromachined on the substrate for sensing fluid conditions in the channel, the sensor being selected from the group consisting of a fluid flow sensor, a fluid temperature sensor and a fluid pressure sensor.
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 This application claims the benefit under 35 USC §119(e) of U.S. provisional application serial No. 60/273,621 filed Mar. 7, 2001.
 1. Field of the Invention
 This invention relates to microelectromechanical (MEMS) sytems, and more particularly to flow controllers for use in MEMS systems.
 2. Description of the Related Art
 Most flow controllers are made of discrete components and are single channel. This invention has fabricated multi-channel flow controllers integrated onto a single substrate using microfabrication (MEMS) processes. This approach integrates the normally separated functions of flow measurement, feedback, and control enabling miniaturization of these devices. The functions of sensing, feedback control, and communication with a host computer are performed by a dedicated microprocessor on the same substrate. Miniaturization of components allows for the reduction of dead volume and increased portability.
 Miniaturization of components also allows for the incorporation of multiple channels of various sizes within the footprint of a conventional single channel flow controller made from discrete components. A multi-channel flow controller enables changes in flow ranges without having to change out the flow controller itself as is now the practice in the semiconductor industry. This saves on labor costs and reduces the number of different ranges of flow controllers that must be stocked.
 Flow controllers require a valve. In the prior art, the lack of suitably fast, reliable valves produced by MEMS processes has prevented the fabrication of truly integrated MEMS-produced flow controllers.
 It is a general object of the invention to provide new and improved fluid flow controllers which are of sufficient minature size for enabling their use in MEMS applications.
 This invention provides minature proportional microvalves that can be configured in multiple valve arrays. These microvalves are much smaller than the smallest currently available solenoid valve. It is the use of this very small valve along with thin film sensors, which sense fluid flow and/or pressure and/or temperature, that enables a significant reduction in size.
FIG. 1 is a top plan view of a multiple valve array in accordance with one embodiment of the invention.
FIG. 2-A is an exploded isometric front view of an individual microvalve used in the array of FIG. 1.
FIG. 2-B is an exploded isometric rear view of the microvalve of FIG. 2-B.
FIG. 3 is a top plan view showing two types of ceramic valve substrates for use in the microvalve of FIGS. 2-A and 2-B.
FIG. 4 is a block diagram showing the overall control circuit used in the invention.
FIG. 5 is a circuit diagram for the first pressure sensor circuit in the invention.
FIG. 6 is a circuit diagram for the second pressure sensor circuit in the invention.
FIG. 7 is a circuit diagram for the temperature sensor circuit in the invention.
FIG. 8 is a circuit diagram for the flow sensor circuit in the invention.
FIG. 9 is a circuit diagram for the microprocessor circuit in the invention.
FIG. 10-A is a front view and left side view of a manifold for use in a dual range flow controller embodiment of the invention.
FIG. 10-B is a back view and right side view of the manifold of FIG. 10-A.
FIG. 11 is a top plan view of a dual channel flow controller incorporating the manifold of FIGS. 10-A and 10-B.
FIG. 12-A is a schematic plan view of a flow sensor in the invention.
FIG. 12-B is a schematic cross sectional view of the flow sensor of FIG. 12-A.
FIG. 12-C is a schematic isometric view of the flow sensor of FIG. 12-A.
FIG. 13 is a block diagram of a digital mass flow controller using a flow restrictor for use in the invention.
FIG. 14-A is a block diagram showing a mass flow controller that is responsive to a flow sensor that measures differential pressure.
FIG. 14-B is a block diagram showing a mass flow controller that is responsive to a flow sensor that measures absolute pressure.
FIG. 15 is a simplified flow chart of a flow control algorithm for the mass flow controller of FIG. 13.
FIG. 16 is a block diagram showing a dual range mass flow controller in accordance with the invention.
 In accordance with one preferred embodiment, the invention provides a flow controller 13 (FIG. 11) of minature size suitable for use in microelectromechanical (“MEMS”) systems that controls fluid flow.
 Flow controller 13 includes a thin film microvalve array, shown generally at 10 in FIG. 1, which is formed by micromaching on a chip substrate. Any desired number of microvalves can be employed in such an array, and in the illustrated embodiment the array comprises four microvalves 12, 12′ 12″ and 12′″. Each microvalve is comprised of a valve actuator 15 (shown only for valve 12) having a thin film operating element 17 formed of a shape memory alloy (SMA) to control fluid flow, a sensor or sensors (FIGS. 12-A, 12-B and 12-C) to measure flow rate and/or fluid temperature and/or fluid pressure, and channels 19, 21 (FIG. 11) connecting the sensor(s) and microvalve through which flow takes place.
 SMA operating element 17 in actuator die 22 (FIGS. 2-A and 2-B) preferably is comprised of nickel-titanium thin film shape memory alloy. The alloy undergoes a crystalline phase change transformation, and resulting shape change, from a low temperature deformable shape to a high temperature memory shape when the element is heated through the alloy's phase change transformation temperature. When so heated, the shape change can be used to perform work. In this invention the work occurs as the operating element changes shape so as to move within or across a valve channel to partially or completely open or close the fluid flow path. The actuator is proportional in operation. The thin film SMA actuator can be formed on a substrate by the methods set forth in U.S. Pat. No. 5,061,914 to Busch et. al., the disclosure of which is incorporated by this reference.
 The assembly of components of microvalve 12, which is typical in the invention, is shown in the exploded views of FIGS. 2-B and 2-B. The microvalve 12 is comprised of a housing 14, O-ring 16, orifice die 18, spacer 20, actuator die 22, bias spring 24 and cover 26. These components are assembled together as shown in the Figures by suitable means such as pogo pins 28 and screws 30. Bias spring 24 serves the function of applying a yieldable force against actuator element 15 so as to move it from the memory shape to the deformable shape when the element's temperature is below the transformation temperature. The force of element 15 when actuated toward its memory shape is sufficent to overcome the spring bias force.
 The SMA operated microvalves of the invention can provide proportional control, which is desirable for many applications. The flow sensor measures fluid mass passing by a point in a fixed time, or measures the pressure drop across a calibrated orifice. All of the components can be fabricated on a single small chip by known methods of photolithography and micromaching of silicon substrates.
 Particular advantages of the invention are that the SMA actuators are so small that multiple actuators may be placed in a space without increasing the package size. This has the important consequence that if several valves are packaged together with each having its own flow channel, then the dynamic range of the flow controller can be extended. Conventional sensors always have a range over. which they work best. A sensor that measures accurately to 0.1% in the range of 10 to 1000 ml/min is not accurate in the range of 0.001 to 1 ml/min. In the invention when a set-point is specified, the appropriate range is selected by software. An advantage is that the user need only buy one, not multiple, controllers. Then when flow rate must be changed, it can be done in the system software without opening a line and interrupting the process.
FIG. 3 shows a substrate 32 of a ceramic with wire-bonded electrical leads 34. Mounted on the substrate are two or more microvalves 12, 12′ of the valve array, a sensor 36, which can comprise either a flow, pressure or temperature sensor, and an electronic controller board 38. The microvalves can be fabricated in accordance with the methods of U.S. Pat. No. 5,325,880 to A. David Johnson et. al., the disclosure of which is incorporated by this reference. The channels for providing fluid flow paths between the valves, sensors and inlet/outlet ports are formed in layers (not shown) of a suitable material that can be sputter deposited over the substrate.
 The overall control circuit shown in FIG. 4 provides feedback to adjust each microvalve so that the measured flow matches a preset value, called the set-point. Feedback can be performed by a microprocessor 60 running on suitably programmed software. The circuit comprises first pressure sensor circuit 52, second pressure sensor circuit 54, temperature sensor circuit 56, flow sensor circuit 58 and microprocessor circuit 60.
 The first and second pressure sensor circuits 52 and 54 are shown in detail in FIGS. 5 and 6, respectively. Temperature sensor circuit 56 is shown in detail in FIG. 7, and flow sensor circuit 58 is shown in detail in FIG. 8. Microprocessor circuit 60 is shown in detail in FIG. 9. This circuit 60 comprises a suitable programmable microprocessor such as a PIC16C74A chip 61. First and second pressure sensor circuits 52 and 54 have respective leads 64 and 66 which connect with respective chip terminals 68 and 70. Temperature sensor circuit 56 has a lead 72 which connects with chip terminal 74. And flow sensor circuit 58 has a lead 76 which connects with chip terminal 78.
 A suitable MEMS flow sensor 36 is provided on at least one of the substrates to control flow rates. For this purpose, the microprocessor circuit 60 of FIG. 9 is programmed with suitable software. A feedback loop is incorporated in the software to minimize fluctuations. In the preferred embodiment the microprocessor chip is programmed to control the pulse width to the valve actuator via RS232 serial communication.
 The invention employs a graphical user interface (GUI) which is programmed in a suitable language such as Visual Basic ™. For purposes of illustration, dual channel operation has been chosen. Multi-channel operation can be achieved by expanding the GUI interface and increasing the number of channels on the controller hardware. The GUI interface can be displayed on a PC and is connected to the PIC16C74A via RS232 communication. The operator chooses a sensor supported by the software and selects the desired communications port. Then the operator selects the desired channel. The operator then enters the setpoint for each channel.
 FIGS. 10-A and 10-B show a manifold 80 for a dual channel flow controller embodiment of the invention. FIG. 11 shows the manifold 80 with flow restrictors 82 and 84 extending from opposite sides. This manifold enables a user to switch between various channel sizes without having to switch out the controller itself. The flow controller comprises first and second ceramic substrates mounted on the Delryn ® plastic manifold 80, with flow range selection achieved by the flow restrictors. The first substrate comprises two differential pressure sensors of the type which measure differential pressure drop across the restricor tubes. The flow restrictors 82 and 84 are formed of stainless steel tubing which are pinched at 85 and 87 to smaller effective cross sections while connecting to a flow meter. This flow controller has two ranges—from 1 to 100 SCCM and from 1 to 1,000 SCCM. The controller requires two valves for shut-off and two proportional valves. It also comprises a four-valve multiple microvalve array.
 As shown in FIG. 11, manifold 80 is formed with a pair of microvalves 89, 91 which are connected through channels 21, 21′ with respective flow restrictors 82, 84. Inlet ports 93, 95 and outlet ports 97, 99 direct flow to and from the respective microvalves. Pressure transducers 101 and 103 are provided in the flow paths for the respective microvalves.
 Components of sensor 86 are shown in schematically in the plan view of FIG. 12-A, the cross section view of FIG. 12-B and isometric view of FIG. 12-C. This sensor is comprised of three resistors H, T1 and T2 located at the middle of a layer of Si membrane formed by anisotroopic wet etching of the Si layer.
 The three resistors are formed by diffusing boron into the Si membrane. To configure the flow sensor, the resistor H is used as the heater element and resistors T1 and T2 are used as temperature sensors, with one temperature sensor located upstream related to the heater and the other downstream, as shown for T1 in FIG. 12-A where the flow is depicted as from right to left.
 When there is no fluid flow, the heat produced by the heater H is equally distributed to T1 and T2. When there is flow, there is an imbalance in the heat distribution which is detected by the circuit measuring the differential resistance of T1 and T2.
FIG. 13 is a block diagram showing a mass flow controller system for operating an SMA microvalve actuator 79 in the invention. The flow from the source is read by flow sensor 81 and converted from analog to digital signals by the microprocessor circuit. At step 83 the digital signal is compared to the set-point that has been specified by the user. The average current into valve 79 is then increased or decreased, thereby regulating the downstream flow. All flow sensor, valve and electronics components are micrfrabricated and attached to a common substrate.
 Flow sensor 81 can be of the type shown in the block diagram of FIG. 14-A which measures pressure differential across a restrictor, as in the embodiment explained in connection with FIG. 11. Alternatively, the flow sensor could be of the type shown in the block diagram of FIG. 14-B which is responsive to measurement of absolute pressure. In this case, the software would be altered to provide feedback as a pressure regulator. In both systems of FIGS. 14-A and 14-B, signals from the pressure sensor are compared with the set-point value to control a proportional valve by means of the microprocessor software.
FIG. 15 is a simplified flow chart of the flow control algorithm for the mass flow controller system of FIG. 13. Software resident in microprocessor 60 of FIG. 4 controls the average current to microvalve actuator 79 (FIG. 13) to bring measured flow into equality with the set-point flow. All functions of analog to digital conversion, timing, comparison of measured flow to set-point and communication with the host computer are embodied in the single chip.
FIG. 16 shows a block diagram for another embodiment providing a dual range mass flow controller. Conventional flow sensors have limited dynamic range. For example, a sensor which is accurate at medium flow rates will give inaccurate performance at high or very low flows. In existing equipment, it is standard practice to replace the restrictors to change the range. This requires opening the line while the change is being made, and recalibration is usually necessary. Normal practice is to provide factory restrictors which are not changed in the field.
 With the present invention, multiple flow paths can be fabricated in very small spaces so that the appropriate sensor/restrictor combination can be used for a desired flow range. This enables having separate valves for each flow range in a small package. The circuit of FIG. 16 has separate flow channel in limited space, thereby enabling a flow controller with much greater dynamic range and hence increased versatility without increasing cost. One micromachined flow controller may replace a series of conventional separate flow controllers.
 The circuit of FIG. 16 is controlled by the micropocessor software to determine from the preestablished set-point which of the two channel paths to open. A flow sensor feeds information back for proportional control. As flow increases, the upper limit of flow is reached, and a separate valve-sensor combination is opened. This system can be made without increasing the overall size significantly because most of the volume of the controller is the package; sensors and valves can be orders of magnitude smaller when they are integrated in such a package.
 The overall control circuit (FIG. 4) reads output from each sensor and gives feedback so that flow will remain at the desired setpoint. This control circuit has certain desirable features: a data acquisition port monitoring exciter currents, sensor outputs, amplifier outputs, actuator drivers and currents, and a single connector to connect the actuator/flow-sensor/press-sensor/temp-sensor package. This circuit also has an actuator-driver that can be scaled to drive up to four actuators, the selection of which is controlled by the program's operator screen. Besides being able to logically enable/disable this driver, its power is limited so that it should not be able to bum-up an actuator. This circuit also has an LED indicator to show that the PIC-chip has initialized and is ready: if the PIC-chip should be caused to reset, the LED will blink “on” during each initialization routine.