- Email: [email protected]
Capacitive depression sensor for microfluidic pneumatic networks
Sensors and Actuators A 173 (2012) 75–80
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Capacitive depression sensor for microfluidic pneumatic networks Francisco Perdigones ∗ , Antonio Luque, José A. Morales, José M. Quero Avenida de los Descubrimientos s/n, Departamento de Ingeniería Electrónica, Escuela Superior de Ingeniería, Sevilla, Spain
a r t i c l e
i n f o
Article history: Received 7 July 2011 Received in revised form 15 October 2011 Accepted 15 October 2011 Available online 24 October 2011 Keywords: Sensor Vacuum Microfluidics
a b s t r a c t This paper reports a capacitive depression sensor intended to make measurements in-line in microfluidics pneumatic networks which works with negative pressures in the order of hundreds of millibars. The principle of operation of the sensor is neither thermal nor magnetic, but purely mechanical, and does not need a servomechanism. The proposed device is composed by two circular membranes linked by a column. This structure presents a particular behavior when exerting negative pressures between the membranes, named negative behavior. The behavior is confirmed prior to fabrication using numerical simulations. The materials used are the negative epoxy photoresist SU-8, printed circuit board (PCB) and sputtered gold. The device has been experimentally tested in the laboratory with successful results, providing an increment of capacitance from 1.7 to 3 pF for applied depressions from 0 to 500 mbar and presenting good agreement with the electromechanical FEM simulations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There are recent microfluidic devices and circuits driven by partial vacuum, that is, negative pressures or depressions. Among others, computational microfluidic devices [1–3], and PDMS peristaltic micropumps [4] have been reported. The measurement of these negative pressures, relative to atmospheric pressure, in any location of the circuit is convenient to perform the control of the devices operation. The order of magnitude of these depressions is usually of the hundreds of millibar. For instance, depression from 320 to 870 mbar in [3,5], from 200 to 650 mbar in [4] or from 540 to 820 mbar in [1]. There are many vacuum sensors reported in the literature. The most common sensors work under a thermal principle [6–11]. For example, Pirani vacuum gauges are based on the principle that the heat transfer between surfaces is related to the quantity of molecules transferring the heat. Also, there are spinning rotor and resonant vacuum gauges [12]; friction vacuum gauge [13], and servo capacitive vacuum sensors [14]. The main field of application of these sensors is the vacuum packaging, being the silicon the typical material of fabrication. The majority of these sensors are intended to be used in high vacuum. Some of them measure pressures as low as 10−6 mbar [12,14]. The use of polymers is interesting in sensor technology [15], as they satisfy the need of non-silicon vacuum sensors with materials that can be consider low cost, and that can be easily integrated
∗ Corresponding author. E-mail address: [email protected] (F. Perdigones). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.10.020
in microfluidic networks in a monolithical manner. These devices do not need to operate in high vacuum because the polymers such as SU-8 [16] or PDMS [17] deform easily. The structure that allows the fabrication of the polymer capacitive sensor for microfluidic pneumatic networks presented in this paper is composed by two circular membranes linked by a column. The structure allows an asymmetric deformation due to the pressure between membranes. Some devices have been fabricated using this structure, for example, an SU-8 flow regulator [18] and an nMOS-like microfluidic transistor [19]. In addition, the structure has been previously fabricated with a different geometry using PDMS for a high integrable flow regulator with positive gain [20]. However, in all these cases the structure worked under positive pressures. This operation is named positive behavior. The use of negative pressures in this microstructure provides a different behavior. This new operation is named negative behavior and is presented here for the first time. In this paper, a capacitive depression sensor fabricated using the photoresist SU-8 is presented. Furthermore, the sensor does not need thermal or magnetic activation, and no servomechanism, so that the consumption of energy is only due to the electronic circuit included for signal processing. The rest of this paper is structured as follows. In Section 2, the sensor structure is presented, discussing the parts of the device. Then, in Section 3, the principle of operation is reported, where the new behavior of the structure and the materials are presented. This behavior is analyzed by simulations and compared with the typical behavior of the structure in Section 4. The fabrication process is presented step by step in Section 5. Then, in Section 6, the experimental results are presented and discussed. Finally, in the last section, the conclusions of the research are commented.
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Fig. 1. Cross-section sketch of the depression sensor.
Fig. 2. A three-dimensional sketch of the device with several sections can be seen. The figure is exaggerated in Z axis. The gap is the space between electrodes.
2. Sensor structure 3. Principle of operation and materials The capacitive sensor is composed by a fixed and a movable electrode, being the capacitance between them a function of the depression to be measured. It is usually desirable that in resting position there is a given distance between electrodes. When the depression is present, the electrodes approach each other. If instead of it the electrodes moved away, the capacitance would decrease and it would be eventually more difficult to measure. The easiest way of accomplishing this is by having the channel which depression is to be measured between both electrodes, one on the top cover of the microchannel and another on the bottom. A depression would make the movable electrode approach the fixed one. But the drawback of this design is that, at the same time that the capacitance is increasing, the channel cross-section would decrease, making it more difficult for the fluid to flow through it, and maybe even completely clogging the channel. A possible workaround is the use of an existing structure [18], which allows a membrane with an electrode to be deflected in the opposite direction to the applied pressure. The structure is composed of two circular membranes with the same thickness t, and different radii, R for the large membrane and r for the small one, rigidly linked by a column whose radius is B and height is H. The smaller membrane is coated by a conductive material and will act as the movable electrode. The depression to be measured is applied in the space between both membranes, and the variable capacitance is located between the small membrane and a fixed layer (see Fig. 1). The initial gap between electrodes is defined by the thickness of a deposited layer, also shown in Fig. 1. At fabrication time, the chamber between electrodes is set to room pressure, Pref = 1000 mbar. This is the pressure present beneath the large membrane as well. On the small membrane, electrical connections are made to measure the capacitance. A three-dimensional view of the structure is shown in Fig. 2. The electrical connections are then fed into a relaxation oscillator which uses an operational amplifier and three resistors in addition to the variable capacitance. The circuit can be seen in Fig. 3. The output of the circuit is a square wave whose frequency is a function of the capacitance, given by 1 f = 2RC ln 3
The behavior of the structure has been already reported in [18] when positive pressure was used. The positive behavior consists on exerting a positive pressure between membranes so that the column is moved in the negative direction of the vertical axis. Therefore the small membrane goes down in the figure because the force exerted over the large membrane is higher than over the small one due to the fact that the area of the large membrane is larger. This behavior is different if a negative pressure or vacuum are present between membranes. When a negative pressure is used, the large membrane deforms towards the small one. Therefore, the small membrane deforms in the same direction than the large one due to the rigid link imposed by the column, making the gap lower and the capacitance higher. The behavior of the structure for depressions, named negative behavior, is presented by simulations in the Section 4. Assuming a parallel plate capacitor, the expression that relates the capacitance, C, and the gap, g, is C=
A g
(2)
where is the air permittivity and A is the area of electrodes. The increment of capacitance is
C = A
1 1 − gf go
(3)
where gf and go are the final and initial gap, respectively.
R C −
Vo
+
R
(1)
Another reason for choosing this structure instead of a simple membrane is to place the capacitor outside the microchannel. If a simple membrane is used as electrode on top of the microchannel and the other electrode is placed on the bottom, the gap is imposed by the height of that microchannel. However, with the proposed approach, this gap can be defined in the fabrication process.
R
Fig. 3. Relaxation oscillator using an operational amplifier to convert capacitance to frequency. In the circuit, R = 1M.
F. Perdigones et al. / Sensors and Actuators A 173 (2012) 75–80
77
The equation that relates the pressure between membranes and the column displacement has been reported in [18] and it is valid for small displacements, when the behavior of the structure is linear. This relationship is yt = Pr 3 R3
r˛ı − Rˇ D(ıR3 + ˇr 3 )
(4)
where yt is the column displacement, P is the pressure in the microchannel, r and R are the small and large membrane radii, respectively. The parameters ˛, ˇ, and ı are constants which relate the column radius and the membranes radii [18]. Finally, D is the membrane flexural rigidity, given by D=
Et 3 12(1 − 2 )
(5)
where E is the Young modulus, t is the membrane thicknesses and is the Poisson ratio. The value of yt corresponds to the gap decrease, yt = go − gf
(6)
Fig. 4. Positive and negative behavior of the microstructure are presented. The displacements of the column, yt as a function of the pressure are shown for several values of H.
Using expressions (3) and (6) results in
A go
yt go − yt
(7)
Finally, using (4), (5) and (7) the increment of capacitance in the linear behavior is obtained as a function of the geometrical parameters and pressure. C =
A go
r˛ı − Rˇ (go Et 3 /Pr 3 R3 (1 − 2 ))(ıR3 + ˇr 3 )r˛ı − Rˇ
(8)
When the behavior turns into non linear, it is not possible to obtain an analytical expression, and the study must be performed using simulations, as commented in [18]. The microfluidic networks used in Lab-On-a-Chip (LOC) or computational pneumatic devices are commonly fabricated using polydimethilsiloxane (PDMS) or SU-8. Therefore, the choice of a polymeric material is justified due to the objective of an easy integration of the device in the microfluidic network. Unlike the majority of the fabricated vacuum silicon sensors, the material used for this proposed sensor structure is SU-8. The capacitor electrodes are fabricated using copper for one of them and gold for the other one. The technology of fabrication selected allows the fabrication of low cost devices if compared with silicon or polysilicon technology, because of the materials and equipment used in the fabrication process. Also, the use of polymers in moving parts improves the deformation if compared with high Young modulus materials. This technology allows the visual inspection inside the device due to the transparency of SU-8. This fact is interesting in microfluidics devices not only because the fluid flows can be seen, but also the motion of internal structures, such as microvalves or the presented structure. The fabrication material of the structure is not limited to be SU8. In fact, this structure has already been fabricated using PDMS, with different membrane shapes for a highly integrable positive gain microvalve [20]. In general, the structure can be fabricated in a reliable manner as long as the materials used in the fabrication process assure an irreversible bonding.
The expression that governs this negative behavior is the same that the positive behavior. This expression is (4) and is only valid in the linear case, that is, for small displacements. The curves obtained by simulations for linear and non linear cases are shown in Fig. 4, where the displacement of the column is depicted as a function of the pressure for several values of the dimension H, the column height. The changes of H do not appreciably affect the negative behavior. However, the positive behavior presents differences as can be seen in Fig. 4. The rest of the dimensions for the simulations are R = 3500 m, r = 2000 m, B = 1000 m, and t = 50 m. These are the dimensions for the fabricated structure described in Section 5, together with H = 300 m. The displacement of the column is depicted as a function of the absolute value of the pressure for positive and negative behavior curves and H = 100 m in Fig. 5. This figure demonstrates that the absolute values of the displacements of the column are similar for positive and negative behaviors in the linear case. However, these values are different in the non linear case and the difference is higher if the height of the column, H, is lower. In fact, the deformations of the membranes are different due to the column
80
Displacement of the column (µm)
C =
Positive behavior Negative behavior
70 60 50 40 30 20 10
4. Numerical simulations 0
The positive behavior of the structure was reported in [18] and studied by mechanical simulations using the software CoventorWare 2010 [21]. The negative behavior of the structure appears when negative pressure is exerted between the membranes. In this case, the column and the small membrane go upwards.
0
500
1000
1500
Absolute value of pressure between membranes (mbar) Fig. 5. Positive and negative behavior of the microstructure. The displacements of the column as a function of the absolute value of the pressure are shown for H = 100 m.
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Fig. 6. Cross-section of the deformed microstructure under depression of 400 mbar.
deformations. In addition, the column state of stress is traction in the positive behavior and compression in the negative one. A cross-section of a typical deformed structure under depression can be seen in Fig. 6. Finally, it is worth highlighting that the layer which defines the small membrane radius affects the device behavior. If this layer has a thickness smaller than the membrane, the ideal behavior presented before turns into a different one (Fig. 7). This figure is obtained by mechanical simulations and presents two zones. A linear zone until 200 mbar, approximately, and a saturation from 200 mbar, where the behavior is not linear. The more rigid this layer is the more the real behavior tends to the ideal one, i.e., the red solid line tends to the blue dashed one in Fig. 7. 5. Fabrication process The sensor structure is fabricated using SU-8 (MicroChem Corporation) as the base material and FR4 from a PCB as the substrate [22]. The steps of the fabrication process are shown in Fig. 8. A copper layer, 32 m-thick, in the PCB is patterned by photolithography and then etched away. It is then used to locate the reference points on the substrate (step 1). After that, the substrate is drilled in order to perform the connection port and define the location of the membranes (step 2). Once the substrate is drilled, the BETTS process [23] to transfer membranes with a thickness of 50 m is performed spinning SU-8 at 2700 rpm (step 3). Next, two 150-m-thick layers of SU-8 2050 are spin coated over the transferred membrane using two steps of deposition at 700 rpm for 60 s,
Displacement of the column (µm)
80
Ideal behavior Real behavior 60
40
Linear zone 20
Saturation zone 0 0
500
1000
1500
Depression between membranes (mbar) Fig. 7. The real behavior when the layer that defines the small membrane radius is not completely rigid is depicted together with the ideal behavior.
Fig. 8. Steps of the capacitive depression sensor fabrication process.
with a softbake at 65 ◦ C for 5 min, and then at 95 ◦ C for 40 min between the steps, and a bake at 65 ◦ C for 5 min and then at 95 ◦ C for 90 min after the second step. Next, the layer is exposed to UV light during 120 s, and the post-exposure bake (PEB) is performed at 95 ◦ C for 6 min, and finally, the uncrosslinked SU-8 is developed using Mr-600 developer (MicroChem Corporation) by immersion during 6 min (step 4). After that, the connections of the device are drilled, step 5. Then, in step 6, another SU-8 layer of 50 m is transferred using the BETTS process. This layer defines the thickness of the small membrane. This new transfer is made over the last structured SU-8 layer, achieving an SU-8 to SU-8 bonding. Once the second layer has been transferred, a new deposition of SU-8 is performed to achieve a thickness of 40 m (due to the tolerances of the process this value was actually 36 m). This thickness defines the initial gap of the capacitor. After that, softbake is performed at 65 ◦ C for 3 min and then at 95 ◦ C for 45 min, and the layer is exposed to UV light using an appropriate mask during 40 s. Next, the PEB is carried out at 95 ◦ C for 4 min, and finally, the uncrosslinked SU-8 is developed by immersion in Mr-600 for 4 min (step 7). After that, in step 8, 80 nm of gold are sputtered, and one electrode and the connections are created using a mask. In step 9, the typical process of printed circuit board is carried out in another PCB to fabricate the top electrode of the capacitor and the electronic connections. The positive photoresist of the last PCB is removed with acetone. Finally, in step 10 the substrate with the
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3
79
Experimental Results Electromechanical Simulation
2.8
Capacitance (pF)
2.6 2.4 2.2 2
Non linear behavior 1.8
Linear behavior
1.6 0
100
200
300
400
500
Depression (Pref−P) (mbar) Fig. 9. Bottom view of the sensor. The membranes, column, electrodes and the connection are shown.
structure and the last etched PCB are clamped using screws and bolts as can be seen in Fig. 8. A detail of the fabricated sensor is shown in Fig. 9. The large and small membranes, the column, the electrodes and the fluidic connection can be seen. The device dimensions are R = 3500 m, r = 2000 m, B = 1000 m, t = 50 m, H = 300 m. The gap is 36 m, the top electrode radius is 1500 m and the bottom electrode is a square with a 3000-m side. 6. Experimental results and discussion The experimental setup used to test the working of the device can be seen in Fig. 10. One of the fluid connections of the device is blocked in order to achieve negative pressures. The instruments used to perform the experiments are a syringe pump and a manometer to create the depression and to measure it, and a power supply and an oscilloscope to power the circuit and to obtain the value of the frequency, respectively. The experimental results are shown in Fig. 11, where the capacitance as a function of the depression is depicted. In Fig. 11 two zones can be seen. There is a linear zone, from 0 to 200 mbar, where the device presents a linear behavior. The second zone, named saturation zone, from 200 mbar of depression presents a non linear behavior due to the increments of the displacements are smaller when the depression is increased. Therefore, the capacitance increases as depression increases, until certain limit. This
Fig. 10. Experimental setup to test the sensor behavior.
Fig. 11. Capacitance as a function of the depression obtained experimentally and by simulations are shown.
limit is imposed by the negative real behavior of the structure, seen in Fig. 7. Therefore, the increments of capacitance are negligible when the structure saturates in displacement for high values of depressions in saturation zone. The results of the electromechanical simulations present good agreement with the experiments, as can be seen in Fig. 11. The two limits presented in Fig. 11 approximately fit with the ones obtained by mechanical simulations in Fig. 7 where the linear, and saturation zones were defined. The range of depression that can be measured is in the order of the hundreds of millibars. These values are suitable for microfluidic applications, for example the circuits and devices reported in [1,4]. The microstructure could be reconfigured changing their parameters if different values of depression are required, modifying the membrane diameters or thicknesses and column diameter. In addition, if other values for capacitance are needed, the thickness of the last deposited layer (step 7 in the fabrication process) can be modified by varying the spin speed of the spin coater. Also, the positive photoresist of the top PCB could be left on the electrodes in order to avoid the short circuit between them 7. Conclusion A capacitive depression sensor intended to make measurements in real time in microfluidic networks is reported. The principle of operation of the sensor does not need thermal or magnetic activation, or a servomechanisms. Therefore, the consumption of energy is minimized. This SU-8 device can be integrated in-line in microfluidic channels in order to measure the desired depression in any place of the network. This device can be fabricated using other materials as long as the bonding between membranes and column were irreversible. The sensor working is governed by a particular behavior of an existing structure, named negative behavior. This new behavior is also reported in this paper by using mechanical simulations and comparing it with the positive behavior. The proposed device presents linear and saturation zones due to the nature of the structure behavior. The experimental results provide an increment of capacitance from 1.7 to 3 pF when the depression varies from 0 to 500 mbar, presenting an average ratio of change of 2.6 fF/mbar. These results present good agreement with the electromechanical simulations. In addition, the parameters that need to be modified when a new value of the capacitance or depressions range are needed are also commented in this paper.
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References [1] H. Takao, M. Ishida, K. Sawada, A pneumatically actuated full in-channel microvalve with MOSFET-like function in fluid channel networks, Journal of Microelectromechanical Systems 11 (5) (2002) 421–426. [2] H. Takao, M. Ishida, Microfluidic integrated circuits for signal processing using analogous relationship between pneumatic microvalve and MOSFET, Journal of Microelectromechanical Systems 12 (4) (2003) 497–505. [3] E.C. Jensen, W.H. Grover, R.A. Mathies, Micropneumatic digital logic structures for integrated microdevice computation and control, Journal of Microelectromechanical Systems 16 (2007) 1378–1385. [4] J. Cui, T. Pan, A vacuum-driven peristaltic micropump with valved actuation chambers, Journal of Micromechanics and Microengineering 21 (2011) 065034 (7pp.). [5] W.H. Grover, R.H.C. Ivester, E.C. Jensen, R.A. Mathies, Development and multiplexed control of latching pneumatic valves using microfluidic logical structures, Lab on Chip 6 (2006) 623–631. [6] J. Mitchell, G.R. Lahiji, K. Najafi, An improved performance poly-Si Pirani vacuum gauge using heat-distributing structural supports, Journal of Microelectromechanical Systems 17 (1) (2008) 93–102. [7] E.S. Topalli, K. Topalli, S.E. Alper, T. Serin, T. Akin, Pirani vacuum gauges using silicon-on-glass and dissolved-wafer processes for the characterization of MEMS vacuum packaging, IEEE Sensors Journal 9 (3) (2009) 263–270. [8] M. Kimura, F. Sakurai, H. Ohta, T. Terada, Proposal of a new structural thermal vacuum sensor with diode-thermistors combined with a micro-air-bridge heater, Microelectronics Journal 38 (2) (2007) 171–176. [9] G. Bedö, W. Kraus, R. Müller, Comparison of different micromechanical vacuum sensors, Sensors and Actuators A: Physical 85 (1–3) (2000) 181–188. [10] F. Zhang, Z. Tang, J. Yu, R. Jin, A micro-Pirani vacuum gauge based on microhotplate technology, Sensors and Actuators A: Physical 126 (2) (2006) 300–305. [11] W. Jiang, X. Wang, J. Zhang, A single crystal silicon micro-Pirani vacuum gauge with high aspect ratio structure, Sensors and Actuators A: Physical 163 (1) (2010) 159–163. [12] F. Völklein, A. Meier, Microstructured vacuum gauges and their future perspectives, Vacuum 82 (4) (2007) 420–430. [13] D. Tenholte, S. Kurth, T. Geßner, W. Dötzel, A MEMS friction vacuum gauge suitable for high temperature environment, Sensors and Actuators A: Physical 142 (1) (2008) 166–172. [14] Y. Wang, M. Esashi, The structures for electrostatic servo capacitive vacuum sensors, Sensors and Actuators A: Physical 66 (1–3) (1998) 213–217. [15] G. Harsanyi, Polymer films in sensor applications: a review of present uses and future possibilities, Sensor Review 20 (2) (2000) 98–105. [16] H. Lorenz, M. Despont, N. Fahrni, N. LaBianca, P. Renaud, P. Vettiger, SU-8: a low-cost negative resist for MEMS, Journal of Micromechanics and Microengineering 7 (1997) 121–124. [17] F. Schneider, T. Fellner, J. Wilde, U. Wallrabe, Mechanical properties of silicones for MEMS, Journal of Micromechanics and Microengineering 18 (2008) 065008 (9pp.). [18] F. Perdigones, A. Luque, J.M. Quero, Novel structure for a pneumatically controlled flow regulator with positive gain, Journal of Microelectromechanical Systems 19 (2010) 1070–1078.
[19] F. Perdigones, A. Luque, J.M. Quero, Pneumatically actuated positive gain microvalve with n-channel metal-oxide semiconductor-like behaviour, Micro and Nano Letters 6 (2011) 363–365. [20] F. Perdigones, A. Luque, J.M. Quero, Highly integrable flow regulator with positive gain, Journal of Microelectromechanical Systems 20 (2011) 12–14. [21] Coventor, Inc., CoventorWare—architect and analyzer: users & reference guide. [22] F. Perdigones, J.M. Moreno, A. Luque, J.M. Quero, Characterisation of the fabrication process of freestanding SU-8 microstructures integrated in printing circuit board in microelectromechanical systems, Micro and Nano Letters 5 (2010) 7–13. [23] C. Aracil, F. Perdigones, J.M. Moreno, J.M. Quero, Betts: bonding, exposing and transferring technique in SU-8 for microsystems fabrication, Journal of Micromechanics and Microengineering 20 (2010) 035008 (7pp.).
Biographies Francisco Perdigones was born in Huelva, Spain. He received the M.Sc., and Ph.D. degrees in electronics engineering from the University of Seville, Seville, Spain, in 2006, and 2010, respectively. In 2006, he joined the Department of Aerospace Engineering and Fluid Mechanics, University of Seville. Since 2009, he has been with the Department of Electronics Engineering, University of Seville. His current research interests include 3D polymer (SU-8 and polydimethylsiloxane) microstructure fabrication, fluidic microdevices, printed circuit board microelectromechanical systems, sensors, and actuators. Antonio Luque received the M.Sc. and Ph.D. degrees in electrical engineering from University of Seville, Seville, Spain, in 2000 and 2005, respectively. He is currently an Associate Professor in the Department of Electronics Engineering, University of Seville. His research interests include microfluidics, micropower generation, BioMEMS and polymer microsystems. Dr. Luque was chairman of the IEEE Industrial Electronics Society Technical Committee on MEMS during 2008 and 2009. José Antonio Morales Sánchez was born in Córdoba, Spain. He received the M.Sc. degree in Engineering of Telecommunication from the University of Sevilla, Sevilla, Spain in 2011. In 2010 he joined the Department of Electronics Engineering, his research was focused in 3D Polymer (SU-8 and polydimethylsiloxane) microstructure fabrication, fluidic microdevices, printed circuit board microelectromechanical systems, sensors, and actuators. His current research interest includes printed circuit board for power electronics. José M. Quero received the M.Sc. and Ph.D. degrees in electrical engineering from the University of Seville, Seville, Spain, in 1988 and 1991, respectively. He is Full Professor in the Department of Electronic Engineering, University of Seville since 2000. He is also Senior Researcher within AICIA, a non-profit research organization. He is evaluator and reviewer for the European Commission in the Information Society Technology (IST) programme since 2002. Currently he is CEO of the spin-off company Solar MEMS Technologies. His research interests include MEMS sensors and actuators and their application in Microfluidics, RF and Space.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Capacitive depression sensor for microfluidic pneumatic networks Francisco Perdigones ∗ , Antonio Luque, José A. Morales, José M. Quero Avenida de los Descubrimientos s/n, Departamento de Ingeniería Electrónica, Escuela Superior de Ingeniería, Sevilla, Spain
a r t i c l e
i n f o
Article history: Received 7 July 2011 Received in revised form 15 October 2011 Accepted 15 October 2011 Available online 24 October 2011 Keywords: Sensor Vacuum Microfluidics
a b s t r a c t This paper reports a capacitive depression sensor intended to make measurements in-line in microfluidics pneumatic networks which works with negative pressures in the order of hundreds of millibars. The principle of operation of the sensor is neither thermal nor magnetic, but purely mechanical, and does not need a servomechanism. The proposed device is composed by two circular membranes linked by a column. This structure presents a particular behavior when exerting negative pressures between the membranes, named negative behavior. The behavior is confirmed prior to fabrication using numerical simulations. The materials used are the negative epoxy photoresist SU-8, printed circuit board (PCB) and sputtered gold. The device has been experimentally tested in the laboratory with successful results, providing an increment of capacitance from 1.7 to 3 pF for applied depressions from 0 to 500 mbar and presenting good agreement with the electromechanical FEM simulations. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There are recent microfluidic devices and circuits driven by partial vacuum, that is, negative pressures or depressions. Among others, computational microfluidic devices [1–3], and PDMS peristaltic micropumps [4] have been reported. The measurement of these negative pressures, relative to atmospheric pressure, in any location of the circuit is convenient to perform the control of the devices operation. The order of magnitude of these depressions is usually of the hundreds of millibar. For instance, depression from 320 to 870 mbar in [3,5], from 200 to 650 mbar in [4] or from 540 to 820 mbar in [1]. There are many vacuum sensors reported in the literature. The most common sensors work under a thermal principle [6–11]. For example, Pirani vacuum gauges are based on the principle that the heat transfer between surfaces is related to the quantity of molecules transferring the heat. Also, there are spinning rotor and resonant vacuum gauges [12]; friction vacuum gauge [13], and servo capacitive vacuum sensors [14]. The main field of application of these sensors is the vacuum packaging, being the silicon the typical material of fabrication. The majority of these sensors are intended to be used in high vacuum. Some of them measure pressures as low as 10−6 mbar [12,14]. The use of polymers is interesting in sensor technology [15], as they satisfy the need of non-silicon vacuum sensors with materials that can be consider low cost, and that can be easily integrated
∗ Corresponding author. E-mail address: [email protected] (F. Perdigones). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.10.020
in microfluidic networks in a monolithical manner. These devices do not need to operate in high vacuum because the polymers such as SU-8 [16] or PDMS [17] deform easily. The structure that allows the fabrication of the polymer capacitive sensor for microfluidic pneumatic networks presented in this paper is composed by two circular membranes linked by a column. The structure allows an asymmetric deformation due to the pressure between membranes. Some devices have been fabricated using this structure, for example, an SU-8 flow regulator [18] and an nMOS-like microfluidic transistor [19]. In addition, the structure has been previously fabricated with a different geometry using PDMS for a high integrable flow regulator with positive gain [20]. However, in all these cases the structure worked under positive pressures. This operation is named positive behavior. The use of negative pressures in this microstructure provides a different behavior. This new operation is named negative behavior and is presented here for the first time. In this paper, a capacitive depression sensor fabricated using the photoresist SU-8 is presented. Furthermore, the sensor does not need thermal or magnetic activation, and no servomechanism, so that the consumption of energy is only due to the electronic circuit included for signal processing. The rest of this paper is structured as follows. In Section 2, the sensor structure is presented, discussing the parts of the device. Then, in Section 3, the principle of operation is reported, where the new behavior of the structure and the materials are presented. This behavior is analyzed by simulations and compared with the typical behavior of the structure in Section 4. The fabrication process is presented step by step in Section 5. Then, in Section 6, the experimental results are presented and discussed. Finally, in the last section, the conclusions of the research are commented.
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Fig. 1. Cross-section sketch of the depression sensor.
Fig. 2. A three-dimensional sketch of the device with several sections can be seen. The figure is exaggerated in Z axis. The gap is the space between electrodes.
2. Sensor structure 3. Principle of operation and materials The capacitive sensor is composed by a fixed and a movable electrode, being the capacitance between them a function of the depression to be measured. It is usually desirable that in resting position there is a given distance between electrodes. When the depression is present, the electrodes approach each other. If instead of it the electrodes moved away, the capacitance would decrease and it would be eventually more difficult to measure. The easiest way of accomplishing this is by having the channel which depression is to be measured between both electrodes, one on the top cover of the microchannel and another on the bottom. A depression would make the movable electrode approach the fixed one. But the drawback of this design is that, at the same time that the capacitance is increasing, the channel cross-section would decrease, making it more difficult for the fluid to flow through it, and maybe even completely clogging the channel. A possible workaround is the use of an existing structure [18], which allows a membrane with an electrode to be deflected in the opposite direction to the applied pressure. The structure is composed of two circular membranes with the same thickness t, and different radii, R for the large membrane and r for the small one, rigidly linked by a column whose radius is B and height is H. The smaller membrane is coated by a conductive material and will act as the movable electrode. The depression to be measured is applied in the space between both membranes, and the variable capacitance is located between the small membrane and a fixed layer (see Fig. 1). The initial gap between electrodes is defined by the thickness of a deposited layer, also shown in Fig. 1. At fabrication time, the chamber between electrodes is set to room pressure, Pref = 1000 mbar. This is the pressure present beneath the large membrane as well. On the small membrane, electrical connections are made to measure the capacitance. A three-dimensional view of the structure is shown in Fig. 2. The electrical connections are then fed into a relaxation oscillator which uses an operational amplifier and three resistors in addition to the variable capacitance. The circuit can be seen in Fig. 3. The output of the circuit is a square wave whose frequency is a function of the capacitance, given by 1 f = 2RC ln 3
The behavior of the structure has been already reported in [18] when positive pressure was used. The positive behavior consists on exerting a positive pressure between membranes so that the column is moved in the negative direction of the vertical axis. Therefore the small membrane goes down in the figure because the force exerted over the large membrane is higher than over the small one due to the fact that the area of the large membrane is larger. This behavior is different if a negative pressure or vacuum are present between membranes. When a negative pressure is used, the large membrane deforms towards the small one. Therefore, the small membrane deforms in the same direction than the large one due to the rigid link imposed by the column, making the gap lower and the capacitance higher. The behavior of the structure for depressions, named negative behavior, is presented by simulations in the Section 4. Assuming a parallel plate capacitor, the expression that relates the capacitance, C, and the gap, g, is C=
A g
(2)
where is the air permittivity and A is the area of electrodes. The increment of capacitance is
C = A
1 1 − gf go
(3)
where gf and go are the final and initial gap, respectively.
R C −
Vo
+
R
(1)
Another reason for choosing this structure instead of a simple membrane is to place the capacitor outside the microchannel. If a simple membrane is used as electrode on top of the microchannel and the other electrode is placed on the bottom, the gap is imposed by the height of that microchannel. However, with the proposed approach, this gap can be defined in the fabrication process.
R
Fig. 3. Relaxation oscillator using an operational amplifier to convert capacitance to frequency. In the circuit, R = 1M.
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The equation that relates the pressure between membranes and the column displacement has been reported in [18] and it is valid for small displacements, when the behavior of the structure is linear. This relationship is yt = Pr 3 R3
r˛ı − Rˇ D(ıR3 + ˇr 3 )
(4)
where yt is the column displacement, P is the pressure in the microchannel, r and R are the small and large membrane radii, respectively. The parameters ˛, ˇ, and ı are constants which relate the column radius and the membranes radii [18]. Finally, D is the membrane flexural rigidity, given by D=
Et 3 12(1 − 2 )
(5)
where E is the Young modulus, t is the membrane thicknesses and is the Poisson ratio. The value of yt corresponds to the gap decrease, yt = go − gf
(6)
Fig. 4. Positive and negative behavior of the microstructure are presented. The displacements of the column, yt as a function of the pressure are shown for several values of H.
Using expressions (3) and (6) results in
A go
yt go − yt
(7)
Finally, using (4), (5) and (7) the increment of capacitance in the linear behavior is obtained as a function of the geometrical parameters and pressure. C =
A go
r˛ı − Rˇ (go Et 3 /Pr 3 R3 (1 − 2 ))(ıR3 + ˇr 3 )r˛ı − Rˇ
(8)
When the behavior turns into non linear, it is not possible to obtain an analytical expression, and the study must be performed using simulations, as commented in [18]. The microfluidic networks used in Lab-On-a-Chip (LOC) or computational pneumatic devices are commonly fabricated using polydimethilsiloxane (PDMS) or SU-8. Therefore, the choice of a polymeric material is justified due to the objective of an easy integration of the device in the microfluidic network. Unlike the majority of the fabricated vacuum silicon sensors, the material used for this proposed sensor structure is SU-8. The capacitor electrodes are fabricated using copper for one of them and gold for the other one. The technology of fabrication selected allows the fabrication of low cost devices if compared with silicon or polysilicon technology, because of the materials and equipment used in the fabrication process. Also, the use of polymers in moving parts improves the deformation if compared with high Young modulus materials. This technology allows the visual inspection inside the device due to the transparency of SU-8. This fact is interesting in microfluidics devices not only because the fluid flows can be seen, but also the motion of internal structures, such as microvalves or the presented structure. The fabrication material of the structure is not limited to be SU8. In fact, this structure has already been fabricated using PDMS, with different membrane shapes for a highly integrable positive gain microvalve [20]. In general, the structure can be fabricated in a reliable manner as long as the materials used in the fabrication process assure an irreversible bonding.
The expression that governs this negative behavior is the same that the positive behavior. This expression is (4) and is only valid in the linear case, that is, for small displacements. The curves obtained by simulations for linear and non linear cases are shown in Fig. 4, where the displacement of the column is depicted as a function of the pressure for several values of the dimension H, the column height. The changes of H do not appreciably affect the negative behavior. However, the positive behavior presents differences as can be seen in Fig. 4. The rest of the dimensions for the simulations are R = 3500 m, r = 2000 m, B = 1000 m, and t = 50 m. These are the dimensions for the fabricated structure described in Section 5, together with H = 300 m. The displacement of the column is depicted as a function of the absolute value of the pressure for positive and negative behavior curves and H = 100 m in Fig. 5. This figure demonstrates that the absolute values of the displacements of the column are similar for positive and negative behaviors in the linear case. However, these values are different in the non linear case and the difference is higher if the height of the column, H, is lower. In fact, the deformations of the membranes are different due to the column
80
Displacement of the column (µm)
C =
Positive behavior Negative behavior
70 60 50 40 30 20 10
4. Numerical simulations 0
The positive behavior of the structure was reported in [18] and studied by mechanical simulations using the software CoventorWare 2010 [21]. The negative behavior of the structure appears when negative pressure is exerted between the membranes. In this case, the column and the small membrane go upwards.
0
500
1000
1500
Absolute value of pressure between membranes (mbar) Fig. 5. Positive and negative behavior of the microstructure. The displacements of the column as a function of the absolute value of the pressure are shown for H = 100 m.
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Fig. 6. Cross-section of the deformed microstructure under depression of 400 mbar.
deformations. In addition, the column state of stress is traction in the positive behavior and compression in the negative one. A cross-section of a typical deformed structure under depression can be seen in Fig. 6. Finally, it is worth highlighting that the layer which defines the small membrane radius affects the device behavior. If this layer has a thickness smaller than the membrane, the ideal behavior presented before turns into a different one (Fig. 7). This figure is obtained by mechanical simulations and presents two zones. A linear zone until 200 mbar, approximately, and a saturation from 200 mbar, where the behavior is not linear. The more rigid this layer is the more the real behavior tends to the ideal one, i.e., the red solid line tends to the blue dashed one in Fig. 7. 5. Fabrication process The sensor structure is fabricated using SU-8 (MicroChem Corporation) as the base material and FR4 from a PCB as the substrate [22]. The steps of the fabrication process are shown in Fig. 8. A copper layer, 32 m-thick, in the PCB is patterned by photolithography and then etched away. It is then used to locate the reference points on the substrate (step 1). After that, the substrate is drilled in order to perform the connection port and define the location of the membranes (step 2). Once the substrate is drilled, the BETTS process [23] to transfer membranes with a thickness of 50 m is performed spinning SU-8 at 2700 rpm (step 3). Next, two 150-m-thick layers of SU-8 2050 are spin coated over the transferred membrane using two steps of deposition at 700 rpm for 60 s,
Displacement of the column (µm)
80
Ideal behavior Real behavior 60
40
Linear zone 20
Saturation zone 0 0
500
1000
1500
Depression between membranes (mbar) Fig. 7. The real behavior when the layer that defines the small membrane radius is not completely rigid is depicted together with the ideal behavior.
Fig. 8. Steps of the capacitive depression sensor fabrication process.
with a softbake at 65 ◦ C for 5 min, and then at 95 ◦ C for 40 min between the steps, and a bake at 65 ◦ C for 5 min and then at 95 ◦ C for 90 min after the second step. Next, the layer is exposed to UV light during 120 s, and the post-exposure bake (PEB) is performed at 95 ◦ C for 6 min, and finally, the uncrosslinked SU-8 is developed using Mr-600 developer (MicroChem Corporation) by immersion during 6 min (step 4). After that, the connections of the device are drilled, step 5. Then, in step 6, another SU-8 layer of 50 m is transferred using the BETTS process. This layer defines the thickness of the small membrane. This new transfer is made over the last structured SU-8 layer, achieving an SU-8 to SU-8 bonding. Once the second layer has been transferred, a new deposition of SU-8 is performed to achieve a thickness of 40 m (due to the tolerances of the process this value was actually 36 m). This thickness defines the initial gap of the capacitor. After that, softbake is performed at 65 ◦ C for 3 min and then at 95 ◦ C for 45 min, and the layer is exposed to UV light using an appropriate mask during 40 s. Next, the PEB is carried out at 95 ◦ C for 4 min, and finally, the uncrosslinked SU-8 is developed by immersion in Mr-600 for 4 min (step 7). After that, in step 8, 80 nm of gold are sputtered, and one electrode and the connections are created using a mask. In step 9, the typical process of printed circuit board is carried out in another PCB to fabricate the top electrode of the capacitor and the electronic connections. The positive photoresist of the last PCB is removed with acetone. Finally, in step 10 the substrate with the
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3
79
Experimental Results Electromechanical Simulation
2.8
Capacitance (pF)
2.6 2.4 2.2 2
Non linear behavior 1.8
Linear behavior
1.6 0
100
200
300
400
500
Depression (Pref−P) (mbar) Fig. 9. Bottom view of the sensor. The membranes, column, electrodes and the connection are shown.
structure and the last etched PCB are clamped using screws and bolts as can be seen in Fig. 8. A detail of the fabricated sensor is shown in Fig. 9. The large and small membranes, the column, the electrodes and the fluidic connection can be seen. The device dimensions are R = 3500 m, r = 2000 m, B = 1000 m, t = 50 m, H = 300 m. The gap is 36 m, the top electrode radius is 1500 m and the bottom electrode is a square with a 3000-m side. 6. Experimental results and discussion The experimental setup used to test the working of the device can be seen in Fig. 10. One of the fluid connections of the device is blocked in order to achieve negative pressures. The instruments used to perform the experiments are a syringe pump and a manometer to create the depression and to measure it, and a power supply and an oscilloscope to power the circuit and to obtain the value of the frequency, respectively. The experimental results are shown in Fig. 11, where the capacitance as a function of the depression is depicted. In Fig. 11 two zones can be seen. There is a linear zone, from 0 to 200 mbar, where the device presents a linear behavior. The second zone, named saturation zone, from 200 mbar of depression presents a non linear behavior due to the increments of the displacements are smaller when the depression is increased. Therefore, the capacitance increases as depression increases, until certain limit. This
Fig. 10. Experimental setup to test the sensor behavior.
Fig. 11. Capacitance as a function of the depression obtained experimentally and by simulations are shown.
limit is imposed by the negative real behavior of the structure, seen in Fig. 7. Therefore, the increments of capacitance are negligible when the structure saturates in displacement for high values of depressions in saturation zone. The results of the electromechanical simulations present good agreement with the experiments, as can be seen in Fig. 11. The two limits presented in Fig. 11 approximately fit with the ones obtained by mechanical simulations in Fig. 7 where the linear, and saturation zones were defined. The range of depression that can be measured is in the order of the hundreds of millibars. These values are suitable for microfluidic applications, for example the circuits and devices reported in [1,4]. The microstructure could be reconfigured changing their parameters if different values of depression are required, modifying the membrane diameters or thicknesses and column diameter. In addition, if other values for capacitance are needed, the thickness of the last deposited layer (step 7 in the fabrication process) can be modified by varying the spin speed of the spin coater. Also, the positive photoresist of the top PCB could be left on the electrodes in order to avoid the short circuit between them 7. Conclusion A capacitive depression sensor intended to make measurements in real time in microfluidic networks is reported. The principle of operation of the sensor does not need thermal or magnetic activation, or a servomechanisms. Therefore, the consumption of energy is minimized. This SU-8 device can be integrated in-line in microfluidic channels in order to measure the desired depression in any place of the network. This device can be fabricated using other materials as long as the bonding between membranes and column were irreversible. The sensor working is governed by a particular behavior of an existing structure, named negative behavior. This new behavior is also reported in this paper by using mechanical simulations and comparing it with the positive behavior. The proposed device presents linear and saturation zones due to the nature of the structure behavior. The experimental results provide an increment of capacitance from 1.7 to 3 pF when the depression varies from 0 to 500 mbar, presenting an average ratio of change of 2.6 fF/mbar. These results present good agreement with the electromechanical simulations. In addition, the parameters that need to be modified when a new value of the capacitance or depressions range are needed are also commented in this paper.
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Biographies Francisco Perdigones was born in Huelva, Spain. He received the M.Sc., and Ph.D. degrees in electronics engineering from the University of Seville, Seville, Spain, in 2006, and 2010, respectively. In 2006, he joined the Department of Aerospace Engineering and Fluid Mechanics, University of Seville. Since 2009, he has been with the Department of Electronics Engineering, University of Seville. His current research interests include 3D polymer (SU-8 and polydimethylsiloxane) microstructure fabrication, fluidic microdevices, printed circuit board microelectromechanical systems, sensors, and actuators. Antonio Luque received the M.Sc. and Ph.D. degrees in electrical engineering from University of Seville, Seville, Spain, in 2000 and 2005, respectively. He is currently an Associate Professor in the Department of Electronics Engineering, University of Seville. His research interests include microfluidics, micropower generation, BioMEMS and polymer microsystems. Dr. Luque was chairman of the IEEE Industrial Electronics Society Technical Committee on MEMS during 2008 and 2009. José Antonio Morales Sánchez was born in Córdoba, Spain. He received the M.Sc. degree in Engineering of Telecommunication from the University of Sevilla, Sevilla, Spain in 2011. In 2010 he joined the Department of Electronics Engineering, his research was focused in 3D Polymer (SU-8 and polydimethylsiloxane) microstructure fabrication, fluidic microdevices, printed circuit board microelectromechanical systems, sensors, and actuators. His current research interest includes printed circuit board for power electronics. José M. Quero received the M.Sc. and Ph.D. degrees in electrical engineering from the University of Seville, Seville, Spain, in 1988 and 1991, respectively. He is Full Professor in the Department of Electronic Engineering, University of Seville since 2000. He is also Senior Researcher within AICIA, a non-profit research organization. He is evaluator and reviewer for the European Commission in the Information Society Technology (IST) programme since 2002. Currently he is CEO of the spin-off company Solar MEMS Technologies. His research interests include MEMS sensors and actuators and their application in Microfluidics, RF and Space.