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THERMOPNEUMATIC MICROVALVE
RELATED APPLICATION
This application claims the benefit of U.S. provisional application entitled "A Latching Thermo-pneumatic Microvalve For Ultra-Low Power Applications," filed May 8, 2003, and having Ser. No. 60/468,868.
STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH OR DEVELOPMENT
This invention was made with government support under Contract No.: EEC-9986866 awarded by the National Science Foundation. The government has certain rights in the invention.
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Past microvalves have minimized power consumption through bistable designs, where power is required only during switching. A bistable micro valve taught by Wagner et al. relies upon electrostatic actuation to drive a pair of
5 buckled membranes acted upon pneumatically via a pair of linked cavities. When one membrane is pulled down electrostatically, the other membrane is pushed up pneumatically. Wagner et al., "Micromachined Bistable Valves for Implantable Drug Delivery Systems," 18'* Annual Interna
10 tional Conference of the IEEE Engineering in Medicine and Biology Society, pp 254-255 (1996). However, such approaches to bistable valves are burdened by complexity in both design and fabrication process, and are also unsuitable for high pressure applications.
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SUMMARY OF THE INVENTION
GENERAL FIELD
The disclosure relates generally to microfluidic devices and systems and, more specifically, to microvalves having ultra-low power consumption.
BACKGROUND
Valves and their miniature counterparts, microvalves, control the flow of fluids (e.g., gas or liquids) in fluidic systems. Microvalves have generally improved fluid flow control in applications where the significant power demands of a macro-scale valve would be unsuitable. Microvalves also generally avoid large dead volumes—the undesirable empty space in a valving or other fluidic device that must be pressurized when flow starts and depressurized when flow stops. Microvalves having a small dead volume present faster response times than macro-scale valves. Notwithstanding these improvements from the macro-scale regime, past microvalve designs have left room for improvement in both power consumption and response time, as well as in connection with other valve performance parameters and fabrication considerations.
A number of different microvalve designs and actuation schemes have been introduced. Electromagnetic microactuators have been demonstrated, although magnetic forces scale unfavorably for devices with small volume. Piezoelectric actuators have shown substantial actuation force and fast response times, but have also required large operating voltages and a complex, stacked hybrid construction in order to achieve substantial actuation displacement.
Many commercially available microvalves have relied upon some type of thermal actuation. Unfortunately, shape memory alloy (SMA) and bimetallic thermal actuators (i.e., bimorph structures) have tended to require significant power for actuation, typically hundreds to thousands of milliwatts.
Thermopneumatic microvalves have also been reported, but the typical overall power consumption of such valves has also been undesirably high.
In contrast, microvalves utilizing electrostatic actuation schemes have shown near-zero power consumption. However, such microvalves have been susceptible to particulate contamination and weak actuation force over large distances. Electrostatically actuated microvalves have been unsuitable for applications requiring long valve throw (i.e., large valve travel distances) and accordingly been designed with low-flow rate applications in mind. See, for example, Robertson et al., "A Nested Electrostatically-Actuated Microvalve for an Integrated Micro flow Controller," MEMS 1994 Proceedings, IEEE Workshop, pp. 7-12 (1994).
In accordance with one aspect of the invention, a microvalve device includes a semiconductor-based valve housing
20 that defines a flow passage and a valve face disposed within the valve housing and in fluid communication with the flow passage. The microvalve device further includes a thermal expansion actuator that drives movement of the valve face from a first position to a second position relative to the flow
25 passage, and a capacitor that holds the valve face in the second position.
According to one embodiment, the microvalve device may also include an insulating layer disposed on portions of the semiconductor-based valve housing. The capacitor may
30 include a first electrode and a second electrode, where the first electrode is disposed on the valve face and a voltage is applied across the first and second electrodes to electrostatically hold the valve face in the second position. The insulating layer may then be further disposed between the first
35 and second electrodes to prevent contact of the first and second electrodes when the valve face is moved from the first position to the second position.
The microvalve device may still further include a capacitance sensor coupled to the first and second electrodes
40 wherein the capacitance sensor monitors a capacitance level to determine when the valve face reaches the second position. The capacitance sensor generates a signal indicative of the valve face having reached the second position and utilized to discontinue application of power to a heating
45 element of the thermal expansion actuator.
In accordance with another aspect of the invention, a microvalve device includes a semiconductor-based valve housing, a valve face disposed within the valve housing, and an actuator having a heating element that displaces the valve
50 face to effect a transition from a first stable state to a second stable state. The microvalve device further includes a latch that maintains displacement of the valve face in the second stable state.
In one embodiment, the microvalve device further 55 includes a sensor that detects when the transition from the first stable state to the second stable state has occurred. The sensor generates a signal indicative of the transition which is utilized to discontinue heating by the heating element. The latch may include a capacitor where a voltage is applied to 60 a pair of electrodes of the capacitor to activate the latch and hold the microvalve in the second stable state electrostatically. The pair of electrodes may also be coupled to a capacitance level detection circuit of the sensor.
In accordance with yet another aspect of the invention, a 65 method is useful for fluid flow control through a passage in a microvalve having a valve face that rests in a first position. Application of power is provided to an actuator to displace
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the valve face from the first position to a second position, and the time at which the valve face reaches the second position is sensed. A latch is activated to maintain the valve face in the second position, and the application of power to the actuator is then discontinued. 5
BRIEF DESCRIPTION OF THE DRAWINGS
The features of this invention which are believed to be novel are set forth with particularity in the appended claims. io The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures, and in which:
FIG. 1 is a schematic representation of a microvalve in 15 accordance with one embodiment.
FIG. 2 is a schematic, cross-sectional illustration of the microvalve of FIG. 1 in greater detail.
FIGS. 3a-3i are schematic, cross-sectional illustrations of the fabrication sequence for a top substrate assembly of the 20 microvalve of FIG. 2.
FIGS. 4a-4d are schematic, cross-sectional illustrations of the fabrication sequence for a bottom substrate assembly of the microvalve of FIG. 2.
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DETAILED DESCRIPTION OF PREFERRED
EMBODIMENTS
Described herein is a microvalve design having an actuation mechanism complemented by a latch or hold such that 30 the microvalve consumes power only during valve displacement or transitions from one stable state to another stable state. More particularly, the advantages of thermal-based actuation are combined with the benefits of electrostatic latching to provide bi-stable microvalve functionality with 35 ultra-low power consumption. Further efficiencies are gained through the use of a sensor to determine accurately when power should no longer be applied to the thermalbased actuation mechanism such that power is only substantially consumed during the transitions. Power consumption 40 during the transitions may also minimized with an insulated cavity that prevents unnecessary heating of the device outside of the actuation mechanism as described in greater detail hereinbelow.
Thermal-based actuation such as thermopneumatic drive 45 mechanisms provide high force, small size, large valve throw, and relatively high speed (i.e., fast response times). These operational characteristics are augmented by the additional advantage of low power consumption brought about by an integrated sensor that determines when a latch 50 should be activated. This combination of high performance with low power consumption makes the disclosed microvalve design ideal for wireless applications, such as a lowpower wireless gas chromotagraphy system. The microvalve is, however, also well suited for any number of other 55 applications and operation in other contexts for the reasons already identified, as well as a robust design capable of batch fabrication via the fabrication sequences described below.
With reference now to FIG. 1, an integrated micro flow controller indicated generally at 10 includes a microvalve 12 60 for regulating flow through a flow passage 14. The microvalve 12 has a valve housing shown schematically at 16 that defines a portion of the flow passage 14 as it passes through the microvalve 12. The valve housing 16 and more generally the microvalve 12 may be integrated or coupled with the 65 flow passage 14 in any number of ways not critical to operation but well known to those skilled in the art. Fur
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thermore, the microvalve 12 is bi-directional such that operation is not limited by flow direction or a distinction between inlet or outlet.
The microvalve 12 has a thermal expansion actuator having a region 18 where heat is applied by a heating element, such as a resistive heater 20. The region 18 may be insulated as described in more detail below to improve performance and efficiency. Lines 22 and 24 supply current from a power source (not shown) that may act as the common source for all components of the microflow controller 10.
In this embodiment, the microvalve 12 rests in the open position, and current supplied to the resistive heater 20 displaces a valve face (see, e.g., FIG. 2) through thermal expansion of a thermal expansion substance disposed in the region 18. Such displacement then drives the microvalve 12 to the closed position.
Positioning in the closed state is stable because the microvalve of FIG. 1 also includes an electrostatic latch (see, e.g., FIG. 2) and a sensor circuit 26 that determines when to activate the latch. The sensor circuit 26 is depicted schematically to include a pair of lines 28, 30 coupling the circuit to a pair of electrodes 32, 34, respectively, which are, in general, coupled to elements of the microvalve 12 for measurement of valve position. With the electrodes 32 and 34 disposed on, for example, a valve face and seat (see, e.g., FIG. 2) of the microvalve 12, the sensor circuit 26 is a capacitance level sensor. In this case, the sensor circuit 26 monitors the capacitance to determine the distance between the two electrodes 32, 34, which decreases as the microvalve 16 is displaced by the thermal expansion actuator. As will be shown in greater detail below, the distance is indicative of the state of the microvalve 12, or the positioning of the valve face and, therefore indicative of whether the microvalve 12 has closed the flow passage 14.
In this embodiment, the same lines 28, 30 carrying a signal indicative of valve position may also apply a voltage across the electrodes 32, 34 to latch, or hold, the microvalve 12 in the closed position. Waiting until the microvalve 12 has reached the closed position to apply the voltage is advantageous because it avoids the contamination problems that plague other electrostatically based microvalves. That is, particles present in the flow channel may be electrostatically attracted and then attach to the electrodes of the valve. Such particles in the flow passage 14 will not be present between the electrodes 32, 34 because the microvalve 12 will already be closed.
A control 36 is responsive to the signal generated by the sensor circuit 26 to determine when to apply the voltage across the electrodes 32, 34. As is well known to those skilled in the art, the sensor 26 and the control circuit 36 may, but need not, be integrated to any greater or lesser extent. The sensor 26 and control circuits 36 may be incorporated, for instance, in a single circuit capable of sensing capacitance and sourcing voltage at the same time. Generally speaking, however, the sensor 26 and/or the control circuit 36 analyze the signal generated at the electrodes 32, 34 using feedback to sense the change in state. The same analysis may be utilized for calibration purposes.
In operation, fluid flow control is achieved by providing power to the actuation mechanism—in this case, the resistive heater 20—to displace the valve face from the stable rest position to the closed position. The sensor 26 may determine that the capacitance level has reached a threshold indicative of the closed position, at which time the latch is activated to maintain the valve face in that position. Application of power to the resistive heater 20 may then be discontinued
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once the latch is activated. From that point, the microvalve 12 resides in a stable state (i.e., the closed position) without any further significant consumption of power, if any.
The foregoing steps may be performed at the direction of the control 36 and, more generally, may be implemented 5 through hardware, software, firmware, or any combination thereof.
FIG. 2 depicts the microvalve 12 in greater detail as a microfabricated device assembly having a number of components, including multiple semiconductor (e.g., silicon) 10 microstructures indicated generally at 40 and a pair of glass caps 42, 44. The microvalve 12 is therefore semiconductorbased insofar as typical semiconductor device fabrication processes are used to fabricate the device from one or more silicon substrates. The silicon microstructures are anodi- 15 cally-bonded to the glass caps 42, 44 to define a valve housing or body 45 that includes a flow channel or passage indicated generally at 46 with a pair of fluidic ports 48, 50. Interior walls 52 of the valve housing also define a sealed actuation cavity indicated generally at 54, which may cor- 20 respond with the region 18 (FIG. 1) in which heat is applied by a resistive heater 56.
The resistive heater 56 transfers energy to a substance contained within the cavity 54 and depicted schematically in FIG. 2 as a liquid 58 in which the heater 56 is immersed. The 25 substance need not take the form of a liquid as-shown, but rather may start or reside as a solid or gas prior to heating. The substance is capable of thermal expansion and, in the specific cases of gases and solids, may not require or undergo a phase change to do so. In the general case of a 30 fluidic thermal expansion substance or a solid substance that will be vaporized, the resistive heater 56 increases the vapor pressure of the fluid in the cavity 54 to provide thermopneumatic actuation. More particularly, the sealed cavity 54 maintains the thermal expansion substance 58 in a saturated 35 liquid-vapor phase such that temperature determines the vapor pressure. A small quantity of working fluid may be all that is needed, inasmuch as elimination or reduction of excess working fluid decreases system thermal mass, thereby improving the transient response. In one embodi- 40 ment, the amount of fluid used would completely evaporate upon reaching the peak pressure within the actuation cavity 54, yet be sufficient to recoat the resistive heater 56 upon cool-down and re-condensation. Other thermal expansion schemes may be used. 45
In general, the thermal expansion substance drives or displaces a movable valve plate 60 relative to the flow passage 46. The valve plate 60 is suspended above the actuation cavity 54 via a corrugated silicon membrane or diaphragm 62. The corrugated nature of the membrane 62 50 provides a compliant structure with greatly increased deflection (>50 microns) capability but with relatively small actuation force as compared to other flat silicon or thin-film diaphragms. The valve plate 60 has a face or surface 64 that eventually occludes the flow passage 46 after displacement 55 or deflection brings the valve face 64 in contact with a valve seat 65. The membrane or diaphragm 62 need not include a corrugated portion if, for example, stiffness is reduced for other structural or geometric reasons.
Further information regarding the capabilities and char- 60 acteristics of the corrugated membrane and the deflection of the valve face 62 may be found in the microvalve literature directed to thermopneumatic valve actuation schemes and well known to those skilled in the art.
The resistive heater 56 may take the form of one or more 65 grids or other structures having shapes with large surface area for contact with the thermal expansion substance 58. In
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one exemplary embodiment, the grid may include a lattice (not shown) of 100 micron beam segments forming diamond-shaped pores. Each beam has a 4 micron wide crosssection, where each beam includes a bulk silicon (doped) frame that may also serve as the heating element. Other materials may be used either alone or in combination with the doped silicon, such as a dielectrically insulated polysilicon layer. Such polysilicon or other layers may provide for independent control of the heater resistance (rather than being limited to the frame doping level). As will be explained below in connection with one embodiment, the frame doping is set to ensure an adequate etch stop. More generally, the resistive heater 56 may be suspended or elevated above the lower glass cap 44 by a pair of posts 66, 68. Elevation of the heat generating components of the heater 56 (e.g., approximately, for example, 9 microns) helps to avoid direct heat conduction to the glass cap 44. In the case of a resistive grid of heating elements, heater electrode contacts 70 may spread out into a number of fingers (not shown), each of which is coupled to one of the posts 66, 68, which are in turn coupled to the grid. Such multiple contact points to the posts 66, 68 provide for reduced contact resistance and improve the likelihood of good contacts generally.
With continued reference to FIG. 2, the microvalve 12 further includes a capacitor having a top electrode 72 disposed on the valve face 64 and a bottom electrode 74 disposed on the valve seat 65. The electrodes 72, 74 of the capacitor are coupled to a power source (not shown) to latch electrostatically the microvalve 12 in the closed state or position. More particularly, a voltage is applied across the electrodes 72, 74 when the sensor 26 (FIG. 1) detects that the valve position has reached the closed state. To this end, the sensor 26 is coupled to the electrodes 72, 74 via lines (not shown) that eventually lead to contacts on the lower glass cap 44. The line for the bottom electrode 74 includes a lead transfer portion 76 to a contact 78 on the glass cap 44. The line for the top electrode 72 also includes a lead transfer portion 80 contained within the valve body 45 to a contact 81 on the glass cap 44.
A dielectric layer 83 of the capacitor is shown on the electrode 74 in FIG. 2 in the interest of clear illustration of the valve components. That said, the dielectric layer 83 is disposed between the electrodes 72, 74 and therefore may be disposed on either the valve face 64 or the valve seat 65, or both. When the microvalve reaches the closed position or state, the thickness of the dielectric layer 83 may correspond with the distance separating the electrodes 72, 74, as well as form the seal that occludes the flow passage 50. As a result, a thin dielectric may be desirable in certain cases in the interest of reducing the required latching voltage or increasing the strength of the latching force. A thin silicon dioxide layer may be deposited or grown to this end.
In one embodiment, an insulator is deposited in the cavity 54 to reduce heat loss to the valve body 45 and other device components outside of the cavity 54. The insulator may include an insulating layer 82 disposed on one or more of the interior walls 52 of the cavity 54. In one exemplary embodiment, the insulating layer 82 is a parylene layer deposited after the device components have been assembled and before the thermal actuation substance 58 is introduced or injected into the cavity 54. The parylene layer may have a thickness of approximately 10 microns. A parylene layer may also act as the dielectric layer 83 for the abovedescribed capacitor, and two separate depositions steps may be used to optimize thicknesses and other characteristics of the respective layers. More generally, performance benefits
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