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The design and fabrication of a magnetically actuated micromachined flow valve
Sensors and Actuators A, 24 (1990) 47-53
47
The Design and Fabrication of a Magnetically Actuated Micromachined Flow Valve*
R. L. SMITH, R. W. BOWER and S. D. COLLINS Department of Electrical Engineering and Computer Science, University of California, Davis, CA 95616 (U.S.A.) (Received December 11; 1989; accepted December 13, 1989)
Abstract In this paper, a magnetically actuated valve, manufactured by silicon and thin-film micromachining, is considered. An analysis of the magnetic circuit is made to determine the critical design parameters, and a fabrication sequence is described. An integrated array of these microvalves, when combined with chemical microsensors, is intended to form the basis of an automated flow distribution and chemical analysis system. The feasibility of magnetic actuation, an addressing scheme, and the fabrication process are discussed.
Introduction During the past two decades, the technology for micromachining three-dimensional structures in silicon has advanced to the stage where previously only imagined micromechanical systems are now becoming realities. These micromachined structures are setting new standards for size and precision of mechanical and electromechanical devices. Using this technology, a system of microvalves, integrated with an electronic control system, associated reservoirs, reaction chambers, sensors and interconnecting channels, can ultimately provide a chemical analysis system or processing plant the size of a postage stamp. There are several potential applications of such a miniature analysis sys-
*Based on a paper presented at the 5th International Conference on Solid-State Sensors and Actuators (Transducers '89), Montreux, Switzerland,June 25-30, 1989. 0924-4247/90/$3.50
tern, including systems for drug delivery and clinical blood analysis. Microfabrication reduces the size, and potentially the cost, of this system, which are matters of special concern in biomedical applications. If the system is disposable, the need for cleaning and resterilization is eliminated. In addition, small size is desirable, even for extracorporeal measuring, to reduce the volume of biological test specimens, and the volume of costly chemicals or bioactive materials. This paper considers the design and fabrication of the micromechanical elements, i.e., the valves, in such an analysis system. So far, actuated microfabricated valves which have appeared in the literature use pneumatic [1], piezoelectric [2] and thermal expansion [3] forces to control valve operation. These valve structures have been relatively large (cm dimensions) and are without a system for controlling flow direction through multiple ports. Another means of actuation, which can be integrated, scaled to less than millimeter dimensions, and addressable in an array configuration, leads one to consider electrostatic drives [4]. Much attention is currently being paid to electrostatic microactuators [5, 6]. However, this method of drive, used in several micromotor designs, is not directly applicable to controlling the flow of conducting fluids. In addition, if the valve dimensions are on the order of tens of microns, then the stored energy density which is achievable with a magnetic actuator is several orders of magnitude greater than with electrostatic actuators. Therefore, the dimensions and the conductive liquid environment make magnetic actuation a more viable choice for this microvalve design. © Elsevier Sequoia/Printed in The Netherlands
48
Analysis of the Magnetic Circuit The basic valve geometry [7] is shown in Fig. 1. The actuation of this valve can be accomplished by means of an external permanent magnet or electromagnet. A simplified equivalent magnetic circuit of the actuator is depicted in Fig. 2. Assuming that the relucFlow
~
'
]
~ ' ~ ' ~ "
Now
Flow
t
Pseal
Fig. 1. Basic geometry of the magnetic flow valve.
Magnetic Actuator
N~
S
(" (.~
" I~ A
connectinglow reluctancepath
/
N ~
gap ~
magnetic
Lg, Ag
Jill ~
magneticvalvestopper / Ls. As Valve
S
[;I LI
~mam Magnetic Circuit
Fig. 2. Simplified magnetic circuitof the magnetically actuated valve. The basic elements of the magnetic circuit of length Lj, and cross-sectionalarea Aj are shown.
t3
(I)
F = B2As/2#g
where #g, Bg and Ag are the magnetic susceptance, magnetic flux in the gap and the effective area of the gap respectively [9-11]. As one might intuitively expect, the force driving the valve stopper depends strongly upon the magnetic flux in the gap, Bg. The pressure with which the stopper seals is given by the closed circuit force, which is the maximum force, divided by that portion of the stopper which physically seals the valve opening:
.~, Magnetic Field
Metal Film
PolySilicon . ~ Stopper
tances of the connecting paths and magnetic stopper are low compared to the magnetic field inducer, and that the magnetic gap's reluctance is high compared to the rest of the circuit, the principle of virtual work [8] may be used to give a simple estimate of the force, F, driving the valve stopper:
m
fmax/Aseal
(2)
If the stopper is treated as a flat stiff plate, then the seal area, A~a~, is equal to the difference between the stopper area and the area of the valve opening. The behavior of the magnetic circuit of Fig. 2 is shown in Fig. 3(a), which depicts the hysteresis curve of a particular magnet, indicating the values of (residual) flux at zero field, Br, and the field at zero flux (coercivity), He. The relationship between the magnet flux, Bm, and magnetic field, Hm, in a circuit, depends on the geometry of the circuit and the magnet's hysteresis curve. The simplified magnetic circuit is to first order given by Lm As Bm = --Hm Lg -~/'/g ~mm (3)
Bm
Force ~ . t 3
tl tO
J !
Hc
(a)
0
(b)
Lg
~0
t1
t2
t3
t
(c)
Fig. 3. Plots of (a) the B - H curve, (b) the force exerted on the stopper vs. its position in the gap, and (c) the position of the stopper vs. time as the valve closes.
49 where Am is the magnet cross-sectional area, and Lm and Lg are the magnet and gap length respectively. This equation describes the load line for the circuit. The operating point of the magnetic circuit is the intersection point of this load line with the hysteresis curve. The gap flux, Bg, is related to the magnet flux according to B, = BmAm/Ag. If the magnet area and gap area are equal, the load line directly yields the gap flux to magnetic field relationship. In the region of the B - H curve where Hm is relatively constant and approximately equal to H~, the force driving the stopper (eqn. (1)) may be written F=~-
/-/~L,,] A,
(4)
When the area of the magnet clement, Am, is increased relative to the gap area, A,, the slope of the load line decreases. If the operating point remains in the region where Hm ~/arc, the gap flux and the force on the stopper remain relatively constant, but the magnet is demagnetized more for the same gap length. For an electromagnet, more current will be required to return Bm to B, as L,--+ 0, For a permanent magnet, the magnet will recoil to a maximum flux, which is less than B,, and which depends on the magnetic material and the amount of demagnetization. The dynamics of the magnetic circuit are illustrated by the set of load lines in Fig. 3(a), which correspond to increasing time from the initial position of the stopper, Lgma., at to through times h, t2 and t3 as the valve is closing. The slope of the load line at L, ma. (i.e., to) and H~ almost entirely determine the time required for the valve to close. This can be best explained with the plot of force versus gap length in Fig. 3(b). At Lgma., the force is minimum and changes very little with L,, so that the largest part of the closing time is taken to move the stopper in this region. Near the end of the closing process, the load line reaches the relatively flat portion of the B - H curve. There, the force equation depends only on the residual saturation flux, B,, of the magnet: Fm~,,----F(L, = O) ~ 2#--: B~ A,
(5)
The value of Br is determined by the magnet material, and has little effect on the closing
time of the magnet, since only a very small amount of closing time is spent in the high force region between t2 and t3. However, Br determines the final maximum force, and thus the maximum pressure of the stopper in the 'sealed position'. An expression which estimates the valve closing time has been found by solving the dynamic equations for a stopper in a vacuum. While this gives an optimistic estimate of valve closing time, it at least gives a lower bound and allows a first-order design consideration for a suitable magnetic circuit and shape for the B - H curve to be made. The closing time was estimated by setting the force (eqn. (4)) equal to the product of stopper mass, m, and acceleration: K L[ = mLg (6) Pt
where L : is the second time derivative of the gap separation, and K=(#g/2)As(LmHc) 2. This equation is analogous to the classical Coulomb potential problem, which when solved for these boundary conditions (velocity = 0 at t = 0, where L, = Ls,,ax ) has a solution of the form t=
{[y( 1 - y)] ~/2 + arcsin( 1 - y) ~/2}
(7) where y is the ratio Lg/Lgma x. Figure 4 illustrates unipolar and bipolar magnetic circuit configurations that may be employed to actuate the valve. Unipolar configurations, where only one magnet is provided per site, require a hard ferromagnetic stopper, while bipolar arrangements, with magnets at both the top and the bottom of the valve, can operate with a soft stopper, such as a ferrimagnetic material [ 12]. In order to drive the stopper from one valve opening to the other, it is necessary to overcome the pressure in the chamber of the valve, which is driving the flow. The maximum 'back pressure' which the valve stopper can overcome for a given magnetomotive force and Lgmax = 40 tim is plotted in Fig. 5. Comparison of the magnetomotive force (He x Lm) of a permanent magnet in the magnetically actuated valve and the electromotive force (voltage) for an electrostatically actuated valve can be made by determining their re-
50 The magnetic field is applied here Poly Silicon Stopper__
Magnet
In this case the stopper is repelled by the magnet
~ " ~...7 ~~ _ 1 ~ . ~" I\I~" ~ ~
The magnetic Magnet field is applied here r = = ~ H
Material
~
Either the magnetic ..~ field is applied here
P~lySilicon A eto_~'~ ~topper / V l . IV / I "~ ~ Flow
Flow
In this case the stopper is am'acted by the magnet
Polys.i0on
Magnet
| rmw][ • or the magnetic field ==~4[--- is applied here Magnet
/1 f
~w
//
Fig. 4. (a) Unipolar and (b) bipolar magnetic actuation schemes. In either case the magnetic field attracts the high-permeability magnetic stopper towards the magnetic dipole.
10000 ' 1000' 100' 10"
.01 .001 , .1
• 1"1'
A
......
;o
.....
"~"~o . . . . .
~ooo
M.M.F. [Ampere-turns]
Fig. 5. The maximum back pressure which can be overcome by the driving force of the magnet, vs. the mmf of the magnet, for a gap o f 40 #m. Points A and B are described in the text.
spective values for the same resultant energy stored in the gap. For an air gap, the maximum electric field is about 3 x 106 V/m, which produces a stored energy density of about 40 J/m. For the same energy density, a magnetic field of 0.01 Tesla is required. If the gap length is 40 #m, the maximum voltage is 120 V, which corresponds to a magnetomotive force of 0.32 Ampere-turns. Small permanent magnets with a magnetomotive force of 10 Ampere-turns are readily available, so that a force of more than 100 times that which is possible with electrostatic actuation is achievable.
A scheme by which a two-dimensional array of valves can be addressed and controlled electronically is now considered. Figure 6 illustrates a planar array of magnetic elements formed with crossed wire techniques. Each valve has a cross point of two wires which pass through the local magnetic circuit. When the currents add constructively, the magnetization in the local magnetic circuit may be switched into either of its two stable states. A magnetic shunt in each element allows one magnetic circuit to be turned on, while the other is turned off. Hence, this scheme can be used in the bipolar configuration where the magnetic stopper is a soft, possibly ferrimagnetic, material. A suitable magnetic material might be Cr steel, which can be formed with an Ho of 50 Oersteds and which has a residual magnetic field, Br, of greater than 10 000 Gauss [ 13]. Such a magnetic material can be switched between states with a total current of 200 mA. The performance of the cross point actuation scheme has been evaluated for a stopper of 50 # m on a side with a thickness of 1 #m, a gap length of 40/tm, a valve opening of 30 # m on a side, and an effective gap area equal to a quarter of the stopper area. The overall size of each valve, including its ports, is greater than 100 #m on a side. Therefore, a magnet length of 100/~m is chosen, so that
51 Top 1. Magnetic Studs formed to provide low reluctance magnetic path to stopper
3. cross point conductors deposited
Cross sectional view
View 2. Hard Magnetic Shunt deposited over Studs
4. Soft magnetic switch is deposited over conductors & connected to Studs
100, but would also increase the required switching current. Another possible means to increase the force is to increase the length of the magnet, i.e., increase the number of turns. The feasibility of this is the subject of ongoing investigation. Although the single turn, cross point actuation scheme produces very small forces at 40 #m, an electrostatic scheme cannot do any better (point B in Fig. 5), even with an air gap. However, small permanent magnets, employed as shown in Fig. 4, could provide more than enough force to overcome several psi of back pressure, and would have faster switching speed and higher seal pressure.
Elec~cal conductors
Process Description
Fig. 6. A cross point two-dimensional array selection scheme for actuating valves. This scheme could be entirely fabricated using batch microfabrication techniques. The sum of the currents in conductors 1 and 2 switches the soft magnetic material, causing net magnetization and pulling the stopper up or down.
the actuation scheme will not limit the pitch of the valve array. Using these dimensions and Cr steel as the magnetic material, a seal pressure of about 50 000 N/m 2, or 0.5 atmospheres, is obtained. The closing time, estimated from eqn. (6), is about 1 ms (in a vacuum). This projects a reasonable order of magnitude for valve switching time in gas and fluid flow control applications. Although the seal pressure obtained with this actuation scheme seems adequate for the intended application, the maximum back pressure allowable is unreasonably small (indicated by the point A in Fig. 5). Using a material with Hc = 500 Oersteds would increase the force and maximum back pressure by a factor of
The fabrication process flow is given below and cross-sectional diagrams o f the process at the stages numbered are shown in Fig. 7. (1) Grow thermal oxide on wafer 1. Deposit nitride on wafer 2. Pattern wafer 1 and open oxide window for anisotropic etch of the Vgroove channel. (2) Pattern wafer 2 and open nitride window (plasma etch) for anisotropic etch of via holes. Anisotropically etch wafers (KOH or EDP). Etch/clean and reoxidize wafer 1. Deposit polysilicon, followed by LPCVD nitride onto wafer 1. (3) Open window on back of wafer 1 (removes nitride and polysilicon leaving oxide etch mask). Anisotropically etch wafter 1 (KOH or EDP). Remove nitride, hot phosphoric acid. Deposit a ferro- or ferdmagnetic material (iron was used in the valve constructed for this paper). Pattern photoresist mask for electroplating. Electroplate magnetic material (no electroplating was done on the test devices). (4) Deposit and pattern low-reluctance material onto and into opening in wafer 2. Remove resist.
52 Wa~r
1
Wa~r
1.
2
3.
4.
2
Via
5. Bond wafers 1 and2
,L Fig. 7. Cross-sectional schematic of the device during processing at steps indicated in the process flow sequence.
(5) Repattern photoresist to define float. Etch iron (aqua regia). Etch polysilicon. (6) Bond wafer 1 and wafer 2. Release float by immersion in 5 wt.% HF. The process requires back to front alignment of mask patterns. This was accomplished using an infrared aligner (Optical Associates Inc., Hyperline 400). For the test devices the wafers were bonded together by coating them with photoresist. Photoresist was employed as the bonding material with the first-generation devices in order to establish an assembly process which was simple and non-permanent. Without baking the resist, both wafers were placed in the infrared aligner. One wafer was placed in the normal position for a wafer, while the other wafer was attached to the aligner at the normal position where a mask is held. Infrared light was then used to bring the patterned wafers into proper alignment, and the soft contact mode was used to press the wafers together with a controlled force. This same alignment technique will be used to bond wafers with either a low-temperature glass bond [ 14] or a silicon fusion bond in future work. Silicon
fusion bonding would be feasible if the magnetic material in the stopper can tolerate the high temperatures (700-1000°C) required. The device was prebaked with the aligner's IR source, then exposed to UV and developed. A five minute etch in HF released the stopper. The performance of these valves has not yet been evaluated, but the structure in Fig. 1 has been successfully fabricated.
Conclusions
This paper has described a magnetically actuated, microfabricated valve for controlling flow direction. A prototype of this valve has been fabricated to demonstrate the technological feasibility of this approach. This valve can control the flow of electrically conductive liquids. A cross point actuation scheme has been proposed which is integrable into a two-dimensional array of microvalves and could be controlled with an x - y electrical addressing scheme. Analysis of this scheme projects a closing time of a few milliseconds, and seal pressures of 0.5 atmospheres.
53
Acknowledgements We wish to thank Jeff Wong and TsengYen Chuang for their help in designing and fabricating these valves, James Fulmer for his assistance with the laboratory equipment, and Nova Sensors for their donation of the double-side polished wafers used in this work.
5
6
7
References 1 F. C. M. van de Pol, D. G. J. Wonnink, M. Elwenspoek and J. H. J. Fluitman, A thermopneumatic actuation principle for a microminiature pump and other micromechanical devices, Sensors and Actuators, 17 (1989) 139-143. 2 M. Esashi, S. Eoh, T. Matsuo, and S. Choi, The fabrication of integrated mass flow controllers, Tech. Digest, 4th Int. Conf. Solid-State Sensors and Actuators (Transducers '87), Tokyo, Japan, June 2 5, 1987, pp. 830-833. 3 M. J. Zdeblick and J. B. Angell, A microminiature electric-to-fluidic valve, Tech. Digest, 4th Int. Conf. Solid-State Sensors and Actuators (Transducers '87), Tokyo, Japan, June 2-5, 1987, pp. 827-829. 4 G. A. Racine and Ph. Krebs, Application of silicon micromachining to the fabrication of microvalves, from Research Project Reports, Microelectronics
8 9 10
11
12 13 14
and Microfabricated Transducers, The Institute of Microtechnology, University of Neuchfitel, Switzerland. W. S. N. Trimmer, Microrobots and micromechanical systems, Sensors and Actuators, 19 (1989) 267287. S. F. Bart, T. A. Lober, R. T. Howe, J. H. Lang and M. F. Schlecht, Design considerations for micromachined electric actuators, Sensors and Actuators, 14 (1988) 269-292. R. W. Bower, R. L. Smith, S. D. Collins and B. Higgins, Patent disclosure on magnetic valve and actuation schemes for individual and planar array actuation, UC Patent Office, May 1989. W. Hayt, Engineering Electromagnetics, McGrawHill, New York, 5th edn., 1989. M. A. Plonus, Applied Electromagnetics, McGrawHill, New York, 1978. L. W. Matsch, Capacitors, Magnetic Circuits and Transformers, Prentice-Hall, Englewood Cliffs, N J, 1964. R. J. Parker and R. J. Studders, Permanent Magnets and Their Applications, John Wiley, New York, 1962. G. F. Dionne, A review of ferrites for microwave applications, Proc. IEEE, (May) (1975) 777-789. B. D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, Reading, MA, 1972. L. A. Field and R. S. Muller, Fusing silicon wafers with low-melting-temperature glass, Sensors and Actuators, A21-A23 (1990) 935-938.
47
The Design and Fabrication of a Magnetically Actuated Micromachined Flow Valve*
R. L. SMITH, R. W. BOWER and S. D. COLLINS Department of Electrical Engineering and Computer Science, University of California, Davis, CA 95616 (U.S.A.) (Received December 11; 1989; accepted December 13, 1989)
Abstract In this paper, a magnetically actuated valve, manufactured by silicon and thin-film micromachining, is considered. An analysis of the magnetic circuit is made to determine the critical design parameters, and a fabrication sequence is described. An integrated array of these microvalves, when combined with chemical microsensors, is intended to form the basis of an automated flow distribution and chemical analysis system. The feasibility of magnetic actuation, an addressing scheme, and the fabrication process are discussed.
Introduction During the past two decades, the technology for micromachining three-dimensional structures in silicon has advanced to the stage where previously only imagined micromechanical systems are now becoming realities. These micromachined structures are setting new standards for size and precision of mechanical and electromechanical devices. Using this technology, a system of microvalves, integrated with an electronic control system, associated reservoirs, reaction chambers, sensors and interconnecting channels, can ultimately provide a chemical analysis system or processing plant the size of a postage stamp. There are several potential applications of such a miniature analysis sys-
*Based on a paper presented at the 5th International Conference on Solid-State Sensors and Actuators (Transducers '89), Montreux, Switzerland,June 25-30, 1989. 0924-4247/90/$3.50
tern, including systems for drug delivery and clinical blood analysis. Microfabrication reduces the size, and potentially the cost, of this system, which are matters of special concern in biomedical applications. If the system is disposable, the need for cleaning and resterilization is eliminated. In addition, small size is desirable, even for extracorporeal measuring, to reduce the volume of biological test specimens, and the volume of costly chemicals or bioactive materials. This paper considers the design and fabrication of the micromechanical elements, i.e., the valves, in such an analysis system. So far, actuated microfabricated valves which have appeared in the literature use pneumatic [1], piezoelectric [2] and thermal expansion [3] forces to control valve operation. These valve structures have been relatively large (cm dimensions) and are without a system for controlling flow direction through multiple ports. Another means of actuation, which can be integrated, scaled to less than millimeter dimensions, and addressable in an array configuration, leads one to consider electrostatic drives [4]. Much attention is currently being paid to electrostatic microactuators [5, 6]. However, this method of drive, used in several micromotor designs, is not directly applicable to controlling the flow of conducting fluids. In addition, if the valve dimensions are on the order of tens of microns, then the stored energy density which is achievable with a magnetic actuator is several orders of magnitude greater than with electrostatic actuators. Therefore, the dimensions and the conductive liquid environment make magnetic actuation a more viable choice for this microvalve design. © Elsevier Sequoia/Printed in The Netherlands
48
Analysis of the Magnetic Circuit The basic valve geometry [7] is shown in Fig. 1. The actuation of this valve can be accomplished by means of an external permanent magnet or electromagnet. A simplified equivalent magnetic circuit of the actuator is depicted in Fig. 2. Assuming that the relucFlow
~
'
]
~ ' ~ ' ~ "
Now
Flow
t
Pseal
Fig. 1. Basic geometry of the magnetic flow valve.
Magnetic Actuator
N~
S
(" (.~
" I~ A
connectinglow reluctancepath
/
N ~
gap ~
magnetic
Lg, Ag
Jill ~
magneticvalvestopper / Ls. As Valve
S
[;I LI
~mam Magnetic Circuit
Fig. 2. Simplified magnetic circuitof the magnetically actuated valve. The basic elements of the magnetic circuit of length Lj, and cross-sectionalarea Aj are shown.
t3
(I)
F = B2As/2#g
where #g, Bg and Ag are the magnetic susceptance, magnetic flux in the gap and the effective area of the gap respectively [9-11]. As one might intuitively expect, the force driving the valve stopper depends strongly upon the magnetic flux in the gap, Bg. The pressure with which the stopper seals is given by the closed circuit force, which is the maximum force, divided by that portion of the stopper which physically seals the valve opening:
.~, Magnetic Field
Metal Film
PolySilicon . ~ Stopper
tances of the connecting paths and magnetic stopper are low compared to the magnetic field inducer, and that the magnetic gap's reluctance is high compared to the rest of the circuit, the principle of virtual work [8] may be used to give a simple estimate of the force, F, driving the valve stopper:
m
fmax/Aseal
(2)
If the stopper is treated as a flat stiff plate, then the seal area, A~a~, is equal to the difference between the stopper area and the area of the valve opening. The behavior of the magnetic circuit of Fig. 2 is shown in Fig. 3(a), which depicts the hysteresis curve of a particular magnet, indicating the values of (residual) flux at zero field, Br, and the field at zero flux (coercivity), He. The relationship between the magnet flux, Bm, and magnetic field, Hm, in a circuit, depends on the geometry of the circuit and the magnet's hysteresis curve. The simplified magnetic circuit is to first order given by Lm As Bm = --Hm Lg -~/'/g ~mm (3)
Bm
Force ~ . t 3
tl tO
J !
Hc
(a)
0
(b)
Lg
~0
t1
t2
t3
t
(c)
Fig. 3. Plots of (a) the B - H curve, (b) the force exerted on the stopper vs. its position in the gap, and (c) the position of the stopper vs. time as the valve closes.
49 where Am is the magnet cross-sectional area, and Lm and Lg are the magnet and gap length respectively. This equation describes the load line for the circuit. The operating point of the magnetic circuit is the intersection point of this load line with the hysteresis curve. The gap flux, Bg, is related to the magnet flux according to B, = BmAm/Ag. If the magnet area and gap area are equal, the load line directly yields the gap flux to magnetic field relationship. In the region of the B - H curve where Hm is relatively constant and approximately equal to H~, the force driving the stopper (eqn. (1)) may be written F=~-
/-/~L,,] A,
(4)
When the area of the magnet clement, Am, is increased relative to the gap area, A,, the slope of the load line decreases. If the operating point remains in the region where Hm ~/arc, the gap flux and the force on the stopper remain relatively constant, but the magnet is demagnetized more for the same gap length. For an electromagnet, more current will be required to return Bm to B, as L,--+ 0, For a permanent magnet, the magnet will recoil to a maximum flux, which is less than B,, and which depends on the magnetic material and the amount of demagnetization. The dynamics of the magnetic circuit are illustrated by the set of load lines in Fig. 3(a), which correspond to increasing time from the initial position of the stopper, Lgma., at to through times h, t2 and t3 as the valve is closing. The slope of the load line at L, ma. (i.e., to) and H~ almost entirely determine the time required for the valve to close. This can be best explained with the plot of force versus gap length in Fig. 3(b). At Lgma., the force is minimum and changes very little with L,, so that the largest part of the closing time is taken to move the stopper in this region. Near the end of the closing process, the load line reaches the relatively flat portion of the B - H curve. There, the force equation depends only on the residual saturation flux, B,, of the magnet: Fm~,,----F(L, = O) ~ 2#--: B~ A,
(5)
The value of Br is determined by the magnet material, and has little effect on the closing
time of the magnet, since only a very small amount of closing time is spent in the high force region between t2 and t3. However, Br determines the final maximum force, and thus the maximum pressure of the stopper in the 'sealed position'. An expression which estimates the valve closing time has been found by solving the dynamic equations for a stopper in a vacuum. While this gives an optimistic estimate of valve closing time, it at least gives a lower bound and allows a first-order design consideration for a suitable magnetic circuit and shape for the B - H curve to be made. The closing time was estimated by setting the force (eqn. (4)) equal to the product of stopper mass, m, and acceleration: K L[ = mLg (6) Pt
where L : is the second time derivative of the gap separation, and K=(#g/2)As(LmHc) 2. This equation is analogous to the classical Coulomb potential problem, which when solved for these boundary conditions (velocity = 0 at t = 0, where L, = Ls,,ax ) has a solution of the form t=
{[y( 1 - y)] ~/2 + arcsin( 1 - y) ~/2}
(7) where y is the ratio Lg/Lgma x. Figure 4 illustrates unipolar and bipolar magnetic circuit configurations that may be employed to actuate the valve. Unipolar configurations, where only one magnet is provided per site, require a hard ferromagnetic stopper, while bipolar arrangements, with magnets at both the top and the bottom of the valve, can operate with a soft stopper, such as a ferrimagnetic material [ 12]. In order to drive the stopper from one valve opening to the other, it is necessary to overcome the pressure in the chamber of the valve, which is driving the flow. The maximum 'back pressure' which the valve stopper can overcome for a given magnetomotive force and Lgmax = 40 tim is plotted in Fig. 5. Comparison of the magnetomotive force (He x Lm) of a permanent magnet in the magnetically actuated valve and the electromotive force (voltage) for an electrostatically actuated valve can be made by determining their re-
50 The magnetic field is applied here Poly Silicon Stopper__
Magnet
In this case the stopper is repelled by the magnet
~ " ~...7 ~~ _ 1 ~ . ~" I\I~" ~ ~
The magnetic Magnet field is applied here r = = ~ H
Material
~
Either the magnetic ..~ field is applied here
P~lySilicon A eto_~'~ ~topper / V l . IV / I "~ ~ Flow
Flow
In this case the stopper is am'acted by the magnet
Polys.i0on
Magnet
| rmw][ • or the magnetic field ==~4[--- is applied here Magnet
/1 f
~w
//
Fig. 4. (a) Unipolar and (b) bipolar magnetic actuation schemes. In either case the magnetic field attracts the high-permeability magnetic stopper towards the magnetic dipole.
10000 ' 1000' 100' 10"
.01 .001 , .1
• 1"1'
A
......
;o
.....
"~"~o . . . . .
~ooo
M.M.F. [Ampere-turns]
Fig. 5. The maximum back pressure which can be overcome by the driving force of the magnet, vs. the mmf of the magnet, for a gap o f 40 #m. Points A and B are described in the text.
spective values for the same resultant energy stored in the gap. For an air gap, the maximum electric field is about 3 x 106 V/m, which produces a stored energy density of about 40 J/m. For the same energy density, a magnetic field of 0.01 Tesla is required. If the gap length is 40 #m, the maximum voltage is 120 V, which corresponds to a magnetomotive force of 0.32 Ampere-turns. Small permanent magnets with a magnetomotive force of 10 Ampere-turns are readily available, so that a force of more than 100 times that which is possible with electrostatic actuation is achievable.
A scheme by which a two-dimensional array of valves can be addressed and controlled electronically is now considered. Figure 6 illustrates a planar array of magnetic elements formed with crossed wire techniques. Each valve has a cross point of two wires which pass through the local magnetic circuit. When the currents add constructively, the magnetization in the local magnetic circuit may be switched into either of its two stable states. A magnetic shunt in each element allows one magnetic circuit to be turned on, while the other is turned off. Hence, this scheme can be used in the bipolar configuration where the magnetic stopper is a soft, possibly ferrimagnetic, material. A suitable magnetic material might be Cr steel, which can be formed with an Ho of 50 Oersteds and which has a residual magnetic field, Br, of greater than 10 000 Gauss [ 13]. Such a magnetic material can be switched between states with a total current of 200 mA. The performance of the cross point actuation scheme has been evaluated for a stopper of 50 # m on a side with a thickness of 1 #m, a gap length of 40/tm, a valve opening of 30 # m on a side, and an effective gap area equal to a quarter of the stopper area. The overall size of each valve, including its ports, is greater than 100 #m on a side. Therefore, a magnet length of 100/~m is chosen, so that
51 Top 1. Magnetic Studs formed to provide low reluctance magnetic path to stopper
3. cross point conductors deposited
Cross sectional view
View 2. Hard Magnetic Shunt deposited over Studs
4. Soft magnetic switch is deposited over conductors & connected to Studs
100, but would also increase the required switching current. Another possible means to increase the force is to increase the length of the magnet, i.e., increase the number of turns. The feasibility of this is the subject of ongoing investigation. Although the single turn, cross point actuation scheme produces very small forces at 40 #m, an electrostatic scheme cannot do any better (point B in Fig. 5), even with an air gap. However, small permanent magnets, employed as shown in Fig. 4, could provide more than enough force to overcome several psi of back pressure, and would have faster switching speed and higher seal pressure.
Elec~cal conductors
Process Description
Fig. 6. A cross point two-dimensional array selection scheme for actuating valves. This scheme could be entirely fabricated using batch microfabrication techniques. The sum of the currents in conductors 1 and 2 switches the soft magnetic material, causing net magnetization and pulling the stopper up or down.
the actuation scheme will not limit the pitch of the valve array. Using these dimensions and Cr steel as the magnetic material, a seal pressure of about 50 000 N/m 2, or 0.5 atmospheres, is obtained. The closing time, estimated from eqn. (6), is about 1 ms (in a vacuum). This projects a reasonable order of magnitude for valve switching time in gas and fluid flow control applications. Although the seal pressure obtained with this actuation scheme seems adequate for the intended application, the maximum back pressure allowable is unreasonably small (indicated by the point A in Fig. 5). Using a material with Hc = 500 Oersteds would increase the force and maximum back pressure by a factor of
The fabrication process flow is given below and cross-sectional diagrams o f the process at the stages numbered are shown in Fig. 7. (1) Grow thermal oxide on wafer 1. Deposit nitride on wafer 2. Pattern wafer 1 and open oxide window for anisotropic etch of the Vgroove channel. (2) Pattern wafer 2 and open nitride window (plasma etch) for anisotropic etch of via holes. Anisotropically etch wafers (KOH or EDP). Etch/clean and reoxidize wafer 1. Deposit polysilicon, followed by LPCVD nitride onto wafer 1. (3) Open window on back of wafer 1 (removes nitride and polysilicon leaving oxide etch mask). Anisotropically etch wafter 1 (KOH or EDP). Remove nitride, hot phosphoric acid. Deposit a ferro- or ferdmagnetic material (iron was used in the valve constructed for this paper). Pattern photoresist mask for electroplating. Electroplate magnetic material (no electroplating was done on the test devices). (4) Deposit and pattern low-reluctance material onto and into opening in wafer 2. Remove resist.
52 Wa~r
1
Wa~r
1.
2
3.
4.
2
Via
5. Bond wafers 1 and2
,L Fig. 7. Cross-sectional schematic of the device during processing at steps indicated in the process flow sequence.
(5) Repattern photoresist to define float. Etch iron (aqua regia). Etch polysilicon. (6) Bond wafer 1 and wafer 2. Release float by immersion in 5 wt.% HF. The process requires back to front alignment of mask patterns. This was accomplished using an infrared aligner (Optical Associates Inc., Hyperline 400). For the test devices the wafers were bonded together by coating them with photoresist. Photoresist was employed as the bonding material with the first-generation devices in order to establish an assembly process which was simple and non-permanent. Without baking the resist, both wafers were placed in the infrared aligner. One wafer was placed in the normal position for a wafer, while the other wafer was attached to the aligner at the normal position where a mask is held. Infrared light was then used to bring the patterned wafers into proper alignment, and the soft contact mode was used to press the wafers together with a controlled force. This same alignment technique will be used to bond wafers with either a low-temperature glass bond [ 14] or a silicon fusion bond in future work. Silicon
fusion bonding would be feasible if the magnetic material in the stopper can tolerate the high temperatures (700-1000°C) required. The device was prebaked with the aligner's IR source, then exposed to UV and developed. A five minute etch in HF released the stopper. The performance of these valves has not yet been evaluated, but the structure in Fig. 1 has been successfully fabricated.
Conclusions
This paper has described a magnetically actuated, microfabricated valve for controlling flow direction. A prototype of this valve has been fabricated to demonstrate the technological feasibility of this approach. This valve can control the flow of electrically conductive liquids. A cross point actuation scheme has been proposed which is integrable into a two-dimensional array of microvalves and could be controlled with an x - y electrical addressing scheme. Analysis of this scheme projects a closing time of a few milliseconds, and seal pressures of 0.5 atmospheres.
53
Acknowledgements We wish to thank Jeff Wong and TsengYen Chuang for their help in designing and fabricating these valves, James Fulmer for his assistance with the laboratory equipment, and Nova Sensors for their donation of the double-side polished wafers used in this work.
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