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A thermo-activated tactile micro-actuator for displays
Microelectronic Engineering 205 (2019) 6–13
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Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A thermo-activated tactile micro-actuator for displays a,b,⁎,1
a,1
a
Salvatore Puce , Tommaso Dattoma , Francesco Rizzi , Mohamed Emara Antonio Qualtieria, Massimo De Vittorioa,b a b
T a,b
,
Center for Bio-Molecular Nanotechnologies@Unile, Istituto Italiano di Tecnologia, Via Eugenio Barsanti 14, 73010 - Arnesano (LE), Italy Dipartimento di Ingegneria dell'Innovazione, Università del Salento - Complesso Ecotekne, edificio “Corpo O” - Via per Monteroni, 73100 Lecce, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Tactile display Taxel KMPR Electro-deposition Galinstan®
In this work we report the design, the fabrication and the characterization of an innovative soft tactile microactuator, also called TAXEL (TActile piXEL), which is developed to be integrated in a portable tactile display for providing text content and graphical information to visually impaired people through the sense of touch. It exploits a thermo-active approach, by taking inspiration from common thermometers: the actuator is activated by the thermal deformation of an active material, namely the metallic alloy Galinstan©, determined by heating the alloy through an underlying metallic resistor designed to work as a heater. The microfabrication of TAXELs is achieved in several steps consisting in heater fabrication, in SU8 micro chambers fabrication, in the deposition of Galinstan® inside and sealing by a PDMS membrane. Measurements of the TAXEL deformation have been accomplished by measuring the displacement of the PDMS sealing membrane, which is promoted by the expansion of the heated Galinstan® drop. These measurements have been achieved by using the Laser Doppler Vibrometer in the “topography mode “and revealed a total displacement of 50 μm when a tension of 2.4 V is applied at taxel terminals and, according to the Joule's law, a power converted from electrical energy to thermal energy of 7,2 W.
1. Introduction Visually impaired and blind people obtain information by earing and touch senses. They read computer screen and other electronic supports through a specific refreshable Braille based screen which, in addition to the representation of characters, but need to display also of graphic information, especially on portable devices such as tablets and smartphones by exploiting a number of characteristics of the display surface, such as roughness, shape and temperature. Since 2005, micromachined dots, called TAXELS (TActile piXEL), that can be easily read by visually impaired people have been demonstrated [1]. Several devices are nowadays commercially available and others will be produced in the near future, but, even though some advanced prototypes exist, current micro-actuation technologies are still affected by important drawbacks such as resolution, bandwidth, integration, compactness and power consumption [2], so that a standard technology for the production of tactile displays is not yet available. TAXELS are required to be sufficiently small and compact for ensuring both an easy identification of the actuated element by touching and a high density of active elements on a standard screen.
Psychophysical studies suggest a minimum taxel dimension of one mm2 and an order of magnitude of 100 μm seems to be a reasonable displacement. Finally, a taxel should be sufficiently persistent, quick in the change of state, compact to be practically used and possibly integrated with electronics [2,3]. With the intent of prototyping miniaturized devices with a high standard of portability, many research groups are involved in developing MEMS (Micro-Electro-Mechanical Systems) based tactile displays. A key point is that these products should be cheap and suitable for mass production. In the state of art of touchable displays, many technological solutions are proposed in which the display is fabricated by linear electromagnetic actuators (LEA) [4–6], piezoelectric actuators [7,8], shape memory alloy actuators [9–11], electroactive polymer (EAP) based actuators [12–15] and shape memory polymers [16], pneumatic [17–19] and surface acoustic wave actuators [20]. Each activation technology is characterized by advantages and disadvantages. For example, most of these devices are not able to realise a high density of active pins in the array or matrix due to the complexity of the assembling process. Both force and power consumptions are key design aspects, but also scalability and integration are central
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Corresponding author at: Center for Bio-Molecular Nanotechnologies@Unile, Istituto Italiano di Tecnologia, Via Eugenio Barsanti 14, 73010 - Arnesano (LE), Italy. E-mail addresses: [email protected] (S. Puce), [email protected] (F. Rizzi). 1 These authors equally contributed to the work. https://doi.org/10.1016/j.mee.2018.11.010 Received 7 June 2018; Received in revised form 8 October 2018; Accepted 18 November 2018 Available online 24 November 2018 0167-9317/ © 2018 Elsevier B.V. All rights reserved.
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structure of a low resistance thermo-active planar heater to investigate the resistor geometry which allows for the best actuation; Section 3, describes the microfabrication methods of the proposed actuators; in Section 4, results and measurements of the actuator displacement will be presented. Finally, conclusions are drawn in Section 5.
requirements. While a limit to matrix actuation is the crosstalk between single taxels, for example in an electromagnetic-based matrix disposition [5], a solution based on electroactive polymer, experiencing a strain if an electric field is locally applied, allows to pack many actuators into a small area without interference [15,16]. Alternatively, a pneumatic actuation is a promising candidate for the actuation of taxels due to its rapid response time and large force generated while maintaining simplicity, scalability, and low cost [20,21]. This activation technology becomes quite feasible with the emergence of microelectromechanical systems (MEMS) technology that offers batch fabrication, miniaturization, display's weight reduction, better resolution (number of actuators) and integration. In fact, due to to the fabrication of actuators and electronics on the same substrate, it is possible to reduce costs and assembly problems [22]. The combination of the multistable nature of Shape Memory Polymers (SMPs), varying their stiffness over a narrow temperature range, enables dense arrays of pneumatic actuators exhibiting simultaneously large strokes and high holding forces [23,24]. Each of the actuation technologies presented in this work can be more or less advantageous in terms of the tradeoff between tactile feedback effectiveness, system complexity, power consumption, and cost [25]. Table 1 shows the qualitative comparison among many actuation technologies that have been reported as both commercial or research devices. Pneumatic devices can generate high forces and strokes at a limited bandwidth [26,27] but, such as of displays based on LEA (Linear Electromagnetic Actuation), they needs a miniaturization based on advanced technologies because they are usually bulky too. [28] Another important parameter is the response time, especially for large screens, and piezoelectric actuators are very fast and have a large bandwidth; furthermore, their consumption once activated is very low [29–31]. On the other hand, displays based on SMA actuators have limited bandwidth, which is their main drawback [32–38] while EAP -based displays need high voltages (thousands of volts) and stacked actuators in order to obtain a significant stroke. [39,40] Our work aims at investigating the design and fabrication of a MEMS tactile actuator to be produced by using only a thermo-active approach. Here we propose an alternative thermo-active actuation method: the taxel is deformed by the thermal expansion of a liquid metal alloy, called Galinstan®, an eutectic alloy composed of Gallium, Indium and Tin, which is heated up by an integrated metallic (Au) resistor on which it is poured. Due to high surface tension, high electrical conductivity, low toxicity, and low viscosity, Galinstan, embedded in polydimethylsiloxane (PDMS) layers, has been one of the most popular liquid metal alloys used for microfluidic, strain gauges and pressure sensors [41]. Gold resistors are exploited as heaters by Joule effect when an electron current is applied at its terminals. The expansion of the alloy locally deforms a covering PDMS membrane producing a dome-like relief. The study starts from the design of a single thermoactive taxel with the goal of integrating a taxel matrix as a future perspective. This approach will be able to remove cross-talking among taxels and the energy consuming and cumbersome pneumatic apparatus for each single taxel. In contrast with pneumatic, the proposed thermoactive taxel has an instantaneous deformation and miniaturized dimensions. The paper will be as follows: in Section 2, we will introduce the design and the FEM (Finite Element Method) analysis of the
2. Design and simulation of a low resistance thermo-active planar heater In Fig. 1 it is shown the sketch of the analysed structure (Fig. 1(a)) and the expected working operation (Fig. 1(b)). As shown in Fig. 1, the simulated structure of each taxel is composed of a nonconductive substrate, a metal heater which works as a Joule-effect heat source and a metallic filler. The metallic filler is the active material, which is expected to deform in a dot-like shape as a result of the heating of the resistor (Fig. 1(b)). Materials are silicon nitride (Si3N4) on Silicon for the substrate and Gold (Au) for the heaters, while Galinstan® is the thermo-active material. The silicon nitride nonconductive substrate must guarantee the electric and thermal insulation with the overlying heaters and is a good candidate for the design and fabrication because it is very hard [42]. Gold (Au), the material chosen for the heaters, has a good conductivity and a general resistance to oxidation and corrosion [43]. Taxel deformation by the thermal expansion of the active material is obtained by a liquid metal alloy known as Galinstan® (composed of Gallium, Indium and Tin), which is properly heated up by the heating circuit. The metal alloy is filling a chamber, placed above each heater and fabricated by SU8 epoxy resin. By increasing the temperature, the active material total volume expands, therefore deforming a soft membrane sealing the chamber from above. The specific material used for the sealing membrane plays a fundamental role for satisfying the requirement of deformation. In particular, to have a very thin and flexible membrane a low Young's modulus material, such as polydimethylsiloxane (PDMS), was employed. As resistors are used as heaters, their shapes are designed in order to maximize the Joule effect heating. It is well known that the electric power dissipated in a resistor is directly proportional to the applied voltage times the flowing electronic current. In order to increase the power dissipation, if the applied voltage is kept constant, the flowing current in the heating circuit must be maximised or its resistance reduced as much as possible. By the general formula for the resistance of a metallic conductor the resistance is proportional to the length over section surface ratio l/S, where l is the total length of the resistor and S its transversal section area, and to the electric resistivity of the resistance material. Two different coil designs have been investigated: a 3 turns coilbased resistor of variable size and a resistor composed of an array of metallic rods connected in parallel as in Fig. 2(a) and Fig. 2(b), respectively. The first investigated geometry for the heater is the coilbased resistor. It is characterized by a spiral shape of 3 turns with two feeding contacts. In the rod-based geometry the conductive path consists in an array of metallic rods connected in parallel. In Fig. 2(a) the fundamental parameters for the design of the coilbased heater are shown: wm is the width of each turn; we is the separation between adjacent turns; th is the thickness of the resistor.
Table 1 Comparison of common static technologies for refreshable tactile displays. Actuation types
Bandwidth
System complexity
Miniaturization
Power consumption
Response time
Cost
Ref.
LEA Piezoelectric SMA EAP Pneumatic Thermo activated
Limited Large Narrow Limited Limited Limited
Good Moderate Poor Moderate Poor Good
Poor Moderate Good Good Poor Good
Moderate Low High High High High
Fast Fast Slow Slow Moderate Fast
High Low Limited Limited Low Limited
[4–6,28] [7,8,29,30,31] [9,10,11,32,33,34,35,36,37,38] [12–15,39,40] [17–19,26,27] [This work]
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Fig. 1. Elementary thermo-activated taxel structure. (a) OFF State: the supply is off and the active alloy is flat. (b) ON State: the supply is switched on and the heated active alloy expands its volume.
order to satisfy the technical requirement about the total area of the taxel, which was set to 1 mm2, the structure has been designed by fixing 3 turns, where 1 mm is the maximum size of the most external. In Fig. 2(b) the fundamental parameters for the design of the rodbased heater are highlighted. In particular, wm is the width of each rod and we is the separation between adjacent rods. As in the previous case, th is the thickness of the resistor. Another important parameter is the number of rods in the parallel arrangement. By varying the number of rods and the other parameters there are a lot of possible configurations; however, all simulated structures have been designed by fixing 8 rods of constant length l = 800 μm and we has been fixed at the constant value of we = 50 μm, consequently fixing the total area of the single heater equal to about 1 mm2 in order to satisfy the technical requirements about taxel area. Assuming the electrical resistivity of gold equal to ρ = 2.44•10−2 [(Ω•μm], which is a valid value at room temperature [44], in Fig. 3 two figures of merit (FoM) have been shown. In order to understand the best design, the comparison between the resistance of the two distinct kind of heaters, the series coil-shaped and the parallel rod-based resistor, is proposed in dependence with two different figures of merit, the total length of the resistive path over the heater wire cross section (l/S, Fig. 3(a)) and total exposed area to Galinstan (Fig. 3(b)), respectively. Both configurations fit the same maximum taxel area, therefore the l/S range of values, dependent on geometrical parameters such as length, width and thickness, is automatically limited by the heater geometry. From the plot, it emerges that, at the fixed maximum occupied surface of 1 mm2, rod-based resistors exhibit smaller resistances. In particular, lowest values are obtained for rods thicker than 50 μm. The efficient heating of the Galinstan is also directly dependent on the resistor
Fig. 2. Heater structure geometry and parameters. (a) Section of the heater coil: width (wm), inter-coils empty space (we) and coils thickness (th). (b) Section of the heater rods: width (wm), inter-rods empty space (we) and rods thickness (th).
Moreover, another important parameter is the number of turn of the coil. These parameters play a role for the definition of the total length and section area of the coil. By varying the number of turns and the other parameters there are many possible configurations; however, in 8
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order to obtain the highest taxel deformation. For a fixed supply voltage (Vin = 2 V and Vin = 5 V) the deformation of the structures has been investigated. The alloy thermal displacement caused by coil-based taxel is more influenced by the thickness of heater coils than the width of the wire, although a width close to 45 μm seems the most effective value for maximizing the displacement. In contrast to the coil-based heater, in rod-based heater the displacement increases with the rod width, whose role is consequently more important then wire thickness. Finally, in both cases, for a fixed thickness (th = 50 μm and th = 100 μm) the deformation of the taxel tends to increase with the applied electric potential. From the comparison of simulations, a general remark can be stressed: for comparable thickness and applied voltages, displacement obtained in rod-based taxel are one order of magnitude higher than coil-based taxel. Thus, for rod-based heaters, a sufficient deformation in the range of the 100 μm can be obtained for lower supply voltages and thinner thicknesses with respect to coil-based structures with identical geometrical parameters, while the same goal for coil-based structures could be satisfied only if thicker coils and higher voltages are applied. As a result, on the basis of the previous theoretical analysis about the total resistance and the FOMs (Fig. 3), the better behaviour of the rodbased was expected. In conclusion, from theoretical and simulated results, a suitable design for the rod-based structure has been accomplished. In fact, through a rod-based design, a displacement value aroud one hundreds of microns at a lower applied voltage would be obtained with a thickness of the heater of both 50 μm and 70 μm.
Fig. 3. Figure of Merit: Comparison between resistances of coil and rod-based heaters. (a) as a function of geometrical parameters. (b) as a function of the exposed area to Galinstan for different thickness values (th = 20,70,50, 80).
surface exposed to the deformable alloy. In fact, the resistor determines a better heating of Galinstan if the contact surface is greater. Because the Galinstan fills the space separating adjacent turns or rods in both structures, the exposed surface consists of all resistor faces, except that touching the substrate. Therefore, a valid FoM for the structures is the total resistance of the specific resistor geometry in relation to the exposed surface. It shows, at a fixed exposed surface, which of the two structure has the lower resistance. Rod-based heaters expose larger areas to Galinstan as emerges from Fig. 3(b), which reports the behavior of resistances as a function of the conductor area globally covered by Galinstan. The numerical analysis of the micro-actuator in terms of the highest displacement of the Galinstan alloy is accomplished by a Multiphysics Finite Element Method (FEM). The substrate is geometrically defined by setting its thickness at 30 μm whereas the area is a parameter depending on the size of heaters, which are built on it. The heaters have been parameterized by the section area: while the space between the heaters branch (we) is fixed at 45 μm for the coils and 50 μm for the rods, the geometry has been parameterized for different values of heater thickness (th) and width (wm). Thus, by comparing the two structures, the most reliable configuration of heaters to be used for fabrication can be chosen and compared with the two FOMs described above. To complete the geometrical model, a further deformable metallic alloy is added on the top of the heater. The alloy thickness has been fixed equal to 100 μm for all computations while its diameter is equal to that of the heater so its base area is the same of the underlying structure. Supposing the chamber walls don't undergo lateral deformation with temperature, the structure guarantees a deformation only in the orthogonal direction, as wished for the correct taxel behaviour. Therefore, only the upper surface of the alloy may expand while the other surfaces have fixed constraints. The multiphysics employed for this study combined structural mechnics and electromagnetism for accounting of actuation with heat transfer for Joule heating and thermal deformation. It is worth underlining that electric resistivity of materials changes as a function of the temperature and, to take into account different temperatures, temperature-dependent resistivity data has been employed for gold [45]. Finally the substrate temperature is fixed at 28 °C. To evaluate the structure's behaviour, a parametric study has been employed on heater section and on applied voltage (Vin), looking for a good combination of the devices parameters and applied voltages in
3. Microfabrication methods of a thermo-active taxel The microfabrication of taxels, outlined in Fig. 4, is achieved in three main steps consisting in heater fabrication, chamber definition by SU8 photoresist, in the deposition of Galinstan inside and a PDMS sealing membrane, respectively. Heaters are fabricated on a 390 μm-thick silicon substrate, coated by a Si3N4 insulating layer 300 nm thick. As in Fig. 4(a), a thin seed layers for electrodeposition, consisting of chromium (10 nm) and gold (30 nm), is thermally evaporated on the insulating layer. For heater shape definition (Fig. 4(b)), a photolithography processing by spin coating a 70 μm thick KMPR® 1050 negative resist, suitable for electrodeposition, is then carried out. To promote a stronger adhesion between the seed layer and KMPR resist, a MCC Primer 80/20, composed of a combination of 20% Hexamethyldisilazane (HMDS) and 80% Propylene Glycol Monomethyl Ether Acetate (PM Acetate), was applied. Subsequently, a processing of gold electrodeposition is accomplished to build metallic resistors (Fig. 4(c)). A suitable solution for gold electrodeposition (99,9% Au and 0,1% Co) was used to fabricate the heaters. The process allowed the growth of pure gold structure with a good conductivity and corrosion/abrasion resistance. The electrodeposition processing was carried on for 8 h by applying a supply current of 40 mA at the temperature of 65 °C. After completing the electrodeposition, the photoresist mask was stripped and the metal seeding layer was wet-etched in order to remove shortcircuits among rods. The 30 nm-thick layer of gold was wet etched in a potassium iodide/iodine solution [46] (KI: 4 g I2: 1 g H2O: 40 ml) while the underlying 10 nm-thick layer of chromium by a solution called Ammonium Cerium (IV) Nitrate (CAN), that is an inorganic compound represented by the chemical formula (NH4)2Ce(NO3)6. In Fig. 5(a)-(c) Au electrodeposited rod-based heaters on a Si3N4 substrate are represented. The measured thickness gold of electrodeposition was approximately 66 μm. A 100 μm thick and 2 mm diameter SU-8 2100 epoxy resist -based spacers (Fig. 4(d)) were perfectly aligned on each specific heater by UV photolithography (Fig. 5(d)). Spacers were filled by Galinstan bump from the top by using the dispenser of a “Multi-purpose Die Bonder FINEPLACER® pico ma” – “flip chip” with a controlled pressure. As a final step in the fabrication processing, a sealing deformable membrane 9
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Fig. 4. Work flow of the fabrication processing. (a) Substrate and Surface Preparation (b-c) Optical lithography for electrodeposition (d-e) Gold electrodeposition and patterning. (f) Lithography of SU8 spacers (g) Filling spacers with Galinstan (h) PDMS membrane covering. The Silicon substrate is not depicted in the figure.
Fig. 5. Fabrication of a rod-based heaters. (a) Picture of the fabricated 70 μm-thick rod-based heaters by means of Au electro-deposition on a substrate of Si3N4. (b)-(c) Scanning electron micrographs (SEM) of the fabricated heater; (d) Picture of the heaters equipped with the SU8 spacers. 10
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The resistor presents a measured resistance of 0.8 Ω. The thermal deformation of the taxel was tested by current injection in the 66 μm thick heater demonstrating the working principle (Fig. 7(a)for the OFF State andFig. 7(b) for the ON State respectively). Membrane deformation measurement was achieved by means of the Micro System Analyser MSA-500 by Polytec, used in the “topography mode“, through a Coherence Scanning Interferometry (CSI) on the same structure used in the heating test, namely wm = 80 μm and we = 50 μm (Fig. 7(c)). The supply current, obtained by a Keithley DMM meter, is 3 A with a tension of 2.4 V for an electrical power of approximately 7.2 W. When the supply was applied at taxel terminals a vertical displacement of the PDMS membrane has been obtained. Repeated tests have highlighted a rise of the PDMS membrane and an equally shutdown when the power supply was interrupted in approximately 1.5 s. Areal surface thickness measurements have produced a 3D representation of a surface of the PDMS (see Fig. 8(a)). The displacement increases towards the center of the taxel (blue central zone in Fig. 8(a)) while is lower on the edges (red edge zone in Fig. 8(a)). The 3D surface topography measured by CSI interferogram of the tested heater shown a displacement between the edge and the centre of approximately 50 μm. A height function with respect lateral displacement, z(x), can mathematically represent a profile measurement through a line across the surface. As the deformation measurement was achieved by an areal optical instrument, the profile has been extracted by software [47]. The profile measurement from the areal measurement of Fig. 8(a) is shown in Fig. 8(b). This profile measurement have a higher level of about 50 μm at about 180 μm from the zero starting point. Even if the experimental total displacement is lower than the expected calculated value around 100 μm, there are many factors that could be taken into account. Firstly, the PDMS membrane, which applies a mechanical resistance to deformation, was not inserted in the FEM simulations, therefore lowering the calculated maximum value of the displacement. Moreover, the oxidization of Galinstan and its conductivity could add some additive resistance to the circuitry. While the equivalent resistance of the simulated heaters has been evaluated taking into account only the conductive paths in the rods, in the fabricated device the Galinstan can conduct current, becoming a heater itself. Finally, the adhesion force of the Galinstan to the spacers wall has not been evaluated in the simulation and certainly affect the measurements. From these data we can compare the thermal active taxel technology with respect the previous state of art introduced in Table 1. Compared to other activation technologies, thanks to the microfabrication process, our proposal does not have the dimensional problems of both pneumatic and electromagnetic that are usually bulky.
Fig. 6. Current-temperature characteristc: comparison between two heaters characterized by identical sizes but different thicknesses.
of PDMS was stretched out on the surface (Fig. 4(e)), that results in a completely cured, insoluble and hydrophobic structure. The membrane was originally cured on a sacrificial layer coated substrate and then transferred, after lift off from the substrate, on the taxel sample as a covering and sealing membrane. 4. Experimental results and discussion To test the rod-based resistance as an efficient heater to verify the ability to increase their temperature, a current flow was applied to the power supply terminals. The chosen heater for the temperature measurements was composed by 8 rods with wm = 80 μm and we = 50 μm. Two thicknesses have been tested: th = 66 μm and th = 41 μm, respectively. The temperature detection in dependence with the flowing electronic current was measured by a Platinum Resistance Temperature Detector (RTD) interfaced to an Arduino® programmable platform for the visualization of the measurement results. From the measurements, it emerges that the temperature tends to increase linearly with the applied current. However, the temperature of a resistor depends on its thickness. In Fig. 6 the comparison between the temperature behaviour of two heaters characterized by identical sizes, but different thicknesses, namely th = 66 μm and th = 41 μm, respectively is showed. Finally, as expected, the heating in the thicker device is definitely more efficient.
Fig. 7. Membrane deformation measurement under current driving. (a) OFF state; (b) ON state; (c) scheme of the experimental “topography” setup. 11
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Fig. 8. Areal surface thickness measurements. (a) result of an areal surface texture measurement of the PDMS membrane; (b) Surface profile measurement with the lowest displacement in the left bottom corner.
References
Furthermore, the response time once activated is very low and have a limited bandwidth. However, as a main drawback, displays based on micro thermal actuators require an high power (~ tens of W) to achieve a significant stroke.
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5. Conclusions The micro-actuator proposed in this work, also called TAXEL for its possible application as a Tactile pixel, exploits a thermo-active actuation principle based on thermal deformation of Galinstan®, an alloy of Gallium, Indium and Tin. The thermal expansion is determined by heating the alloy by means of an underlying metallic resistor, which is designed to work as a heater. Heaters characterized by two different geometries (coil-based and rod-based) has been simulated and their yield has been evaluated by appropriate Figure of Merits (resistance versus total length over section and resistance versus exposed area). The most efficient rod-based heater design was inserted in a PDMS membrane-sealed Galinstan®-filled chamber for actuation tests and fabricated by means of standard micromachining techniques. TAXELs were characterized by measuring the heater temperature by Platinum Resistance Temperature Detector and membrane deformation by Coherence Scanning Interferometry. The profile measurement, achieved by using an actuation current for the taxel of 3A and a supply voltage of 2,4 V, revealed a total displacement of about 50 μm in line with the expected value given by the simulations. Stray capacitance and current reduction, very low operating voltages and suppression of leakage currents are some key techniques for drastic reduction of power dissipation. Therefore, power consumption optimazation requires a geometry and technology improvement that can be achieved, for example, using a pulse mode supply with a proper duty cycle in order to eliminate any DC currents. A pulse mode supply will allow to perform a life cycle test to study the stability of the taxel actuator. Moreover, a latching mechanisms will strengthen the displacement in order to to move the TAXEL from a stable OFF position to a stable ON position. Finally, a mechanical amplification of displacement can be added to avoid the direct contact of finger tips on the soft PDMS membrane. Eventually, a future direcction will be the integration of thousands of taxels in an array or matrix to build a working portable tactile display.
Acknowledgements Special thanks are addressed to Gianmichele Epifani and Roberto Giannuzzi for help in the realization of these studies.
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[21] X. Wu, G. Yuan, Y.K. Yoon, M.G. Allen, Kinematically stabilized microbubble actuator arrays, J. Microelectromech. Syst. 17 (1) (Feb. 2008) 124–132. [22] L. Yobas, D.M. Durand, G.G. Skebe, F.J. Lisy, M.A. Huff, A Novel Integrable Microvalve for Refreshable Braille Display System, J. Microelectromech. Syst. 12 (3) (June 2003) 252–263. [23] X. Wu, S.H. Kim, H. Zhu, C. Ji, M.G. Allen, A Refreshable Braille Cell based on Pneumatic microbubble actuators, J. Microelectromech. Syst. 21 (4) (August 2012) 908–916. [24] N. Besse, S. Rosset, J.J. Zarate, H. Shea, “Flexible active skin: large reconfigurable arrays of individually addressed shape memory polymer actuators” Advanced Mater. Technol. 2 (10). [25] X. Xie, S. Liu, C. Yang, Z. Yang, T. Liu, J. Xu, C. Zhang, X. Zhai, “A review of smart materials in tactile actuators for information delivery”, December 2017, C J. Carbon Res., 3, 38, doi.https://doi.org/10.3390/c3040038. [26] D.G. Caldwell, N. Tsagarakis, C. Giesler, An integrated tactile/shear feedback array for stimulation of finger mechanoreceptor, Proc. IEEE Int. Conf. Robotics Automation, 1999, pp. 287–292. [27] G. Moy, C. Wagner, R.S. Fearing, A compliant tactile display for teletaction, Proc. IEEE Int. Conf. Robotics Automation, 2000, pp. 3409–3415. [28] M.B. Khoudja, M. Hafez, J.M. Alexandre, A. Kheddar, Electromagnetically driven high-density tactile interface based on a multi-layer approach, Int. Symp. Micromechatronics Human Sci. (2003) 147–152. [29] F.J. Tieman, K. Zeehuisen, Tactile Relief Display Device and Method for Manufacture it, (Jul. 1988). [30] J.G. Linvill, J.C. Bliss, A direct translation reading aid for the blind, Proc. IEEE 54 (1966) 40–50. [31] I.R. Summers, C.M. Chanter, A.L. Southall, A.C. Brady, Results from a tactile array on the fingertip, Proc. EUROHAPTICS, 2001, pp. 26–28, 2001. [32] W.J. Peine, P.S. Wellman, R.D. Howe, Temporal bandwidth requirements for tactile shape displays, Proc. 6th Haptics Symp. ASME IMECE, 1997, pp. 107–113. [33] R.D. Howe, D.A. Kontarinis, W.J. Peine, Shape memory alloy actuator controller design for tactile displays, Proc. 34th IEEE Conf. Decision Control, vol. 4, 1995, pp. 3540–3544. [34] P.M. Taylor, A. Moser, A. Creed, A sixty-four element tactile display using shape
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Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
A thermo-activated tactile micro-actuator for displays a,b,⁎,1
a,1
a
Salvatore Puce , Tommaso Dattoma , Francesco Rizzi , Mohamed Emara Antonio Qualtieria, Massimo De Vittorioa,b a b
T a,b
,
Center for Bio-Molecular Nanotechnologies@Unile, Istituto Italiano di Tecnologia, Via Eugenio Barsanti 14, 73010 - Arnesano (LE), Italy Dipartimento di Ingegneria dell'Innovazione, Università del Salento - Complesso Ecotekne, edificio “Corpo O” - Via per Monteroni, 73100 Lecce, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords: Tactile display Taxel KMPR Electro-deposition Galinstan®
In this work we report the design, the fabrication and the characterization of an innovative soft tactile microactuator, also called TAXEL (TActile piXEL), which is developed to be integrated in a portable tactile display for providing text content and graphical information to visually impaired people through the sense of touch. It exploits a thermo-active approach, by taking inspiration from common thermometers: the actuator is activated by the thermal deformation of an active material, namely the metallic alloy Galinstan©, determined by heating the alloy through an underlying metallic resistor designed to work as a heater. The microfabrication of TAXELs is achieved in several steps consisting in heater fabrication, in SU8 micro chambers fabrication, in the deposition of Galinstan® inside and sealing by a PDMS membrane. Measurements of the TAXEL deformation have been accomplished by measuring the displacement of the PDMS sealing membrane, which is promoted by the expansion of the heated Galinstan® drop. These measurements have been achieved by using the Laser Doppler Vibrometer in the “topography mode “and revealed a total displacement of 50 μm when a tension of 2.4 V is applied at taxel terminals and, according to the Joule's law, a power converted from electrical energy to thermal energy of 7,2 W.
1. Introduction Visually impaired and blind people obtain information by earing and touch senses. They read computer screen and other electronic supports through a specific refreshable Braille based screen which, in addition to the representation of characters, but need to display also of graphic information, especially on portable devices such as tablets and smartphones by exploiting a number of characteristics of the display surface, such as roughness, shape and temperature. Since 2005, micromachined dots, called TAXELS (TActile piXEL), that can be easily read by visually impaired people have been demonstrated [1]. Several devices are nowadays commercially available and others will be produced in the near future, but, even though some advanced prototypes exist, current micro-actuation technologies are still affected by important drawbacks such as resolution, bandwidth, integration, compactness and power consumption [2], so that a standard technology for the production of tactile displays is not yet available. TAXELS are required to be sufficiently small and compact for ensuring both an easy identification of the actuated element by touching and a high density of active elements on a standard screen.
Psychophysical studies suggest a minimum taxel dimension of one mm2 and an order of magnitude of 100 μm seems to be a reasonable displacement. Finally, a taxel should be sufficiently persistent, quick in the change of state, compact to be practically used and possibly integrated with electronics [2,3]. With the intent of prototyping miniaturized devices with a high standard of portability, many research groups are involved in developing MEMS (Micro-Electro-Mechanical Systems) based tactile displays. A key point is that these products should be cheap and suitable for mass production. In the state of art of touchable displays, many technological solutions are proposed in which the display is fabricated by linear electromagnetic actuators (LEA) [4–6], piezoelectric actuators [7,8], shape memory alloy actuators [9–11], electroactive polymer (EAP) based actuators [12–15] and shape memory polymers [16], pneumatic [17–19] and surface acoustic wave actuators [20]. Each activation technology is characterized by advantages and disadvantages. For example, most of these devices are not able to realise a high density of active pins in the array or matrix due to the complexity of the assembling process. Both force and power consumptions are key design aspects, but also scalability and integration are central
⁎
Corresponding author at: Center for Bio-Molecular Nanotechnologies@Unile, Istituto Italiano di Tecnologia, Via Eugenio Barsanti 14, 73010 - Arnesano (LE), Italy. E-mail addresses: [email protected] (S. Puce), [email protected] (F. Rizzi). 1 These authors equally contributed to the work. https://doi.org/10.1016/j.mee.2018.11.010 Received 7 June 2018; Received in revised form 8 October 2018; Accepted 18 November 2018 Available online 24 November 2018 0167-9317/ © 2018 Elsevier B.V. All rights reserved.
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structure of a low resistance thermo-active planar heater to investigate the resistor geometry which allows for the best actuation; Section 3, describes the microfabrication methods of the proposed actuators; in Section 4, results and measurements of the actuator displacement will be presented. Finally, conclusions are drawn in Section 5.
requirements. While a limit to matrix actuation is the crosstalk between single taxels, for example in an electromagnetic-based matrix disposition [5], a solution based on electroactive polymer, experiencing a strain if an electric field is locally applied, allows to pack many actuators into a small area without interference [15,16]. Alternatively, a pneumatic actuation is a promising candidate for the actuation of taxels due to its rapid response time and large force generated while maintaining simplicity, scalability, and low cost [20,21]. This activation technology becomes quite feasible with the emergence of microelectromechanical systems (MEMS) technology that offers batch fabrication, miniaturization, display's weight reduction, better resolution (number of actuators) and integration. In fact, due to to the fabrication of actuators and electronics on the same substrate, it is possible to reduce costs and assembly problems [22]. The combination of the multistable nature of Shape Memory Polymers (SMPs), varying their stiffness over a narrow temperature range, enables dense arrays of pneumatic actuators exhibiting simultaneously large strokes and high holding forces [23,24]. Each of the actuation technologies presented in this work can be more or less advantageous in terms of the tradeoff between tactile feedback effectiveness, system complexity, power consumption, and cost [25]. Table 1 shows the qualitative comparison among many actuation technologies that have been reported as both commercial or research devices. Pneumatic devices can generate high forces and strokes at a limited bandwidth [26,27] but, such as of displays based on LEA (Linear Electromagnetic Actuation), they needs a miniaturization based on advanced technologies because they are usually bulky too. [28] Another important parameter is the response time, especially for large screens, and piezoelectric actuators are very fast and have a large bandwidth; furthermore, their consumption once activated is very low [29–31]. On the other hand, displays based on SMA actuators have limited bandwidth, which is their main drawback [32–38] while EAP -based displays need high voltages (thousands of volts) and stacked actuators in order to obtain a significant stroke. [39,40] Our work aims at investigating the design and fabrication of a MEMS tactile actuator to be produced by using only a thermo-active approach. Here we propose an alternative thermo-active actuation method: the taxel is deformed by the thermal expansion of a liquid metal alloy, called Galinstan®, an eutectic alloy composed of Gallium, Indium and Tin, which is heated up by an integrated metallic (Au) resistor on which it is poured. Due to high surface tension, high electrical conductivity, low toxicity, and low viscosity, Galinstan, embedded in polydimethylsiloxane (PDMS) layers, has been one of the most popular liquid metal alloys used for microfluidic, strain gauges and pressure sensors [41]. Gold resistors are exploited as heaters by Joule effect when an electron current is applied at its terminals. The expansion of the alloy locally deforms a covering PDMS membrane producing a dome-like relief. The study starts from the design of a single thermoactive taxel with the goal of integrating a taxel matrix as a future perspective. This approach will be able to remove cross-talking among taxels and the energy consuming and cumbersome pneumatic apparatus for each single taxel. In contrast with pneumatic, the proposed thermoactive taxel has an instantaneous deformation and miniaturized dimensions. The paper will be as follows: in Section 2, we will introduce the design and the FEM (Finite Element Method) analysis of the
2. Design and simulation of a low resistance thermo-active planar heater In Fig. 1 it is shown the sketch of the analysed structure (Fig. 1(a)) and the expected working operation (Fig. 1(b)). As shown in Fig. 1, the simulated structure of each taxel is composed of a nonconductive substrate, a metal heater which works as a Joule-effect heat source and a metallic filler. The metallic filler is the active material, which is expected to deform in a dot-like shape as a result of the heating of the resistor (Fig. 1(b)). Materials are silicon nitride (Si3N4) on Silicon for the substrate and Gold (Au) for the heaters, while Galinstan® is the thermo-active material. The silicon nitride nonconductive substrate must guarantee the electric and thermal insulation with the overlying heaters and is a good candidate for the design and fabrication because it is very hard [42]. Gold (Au), the material chosen for the heaters, has a good conductivity and a general resistance to oxidation and corrosion [43]. Taxel deformation by the thermal expansion of the active material is obtained by a liquid metal alloy known as Galinstan® (composed of Gallium, Indium and Tin), which is properly heated up by the heating circuit. The metal alloy is filling a chamber, placed above each heater and fabricated by SU8 epoxy resin. By increasing the temperature, the active material total volume expands, therefore deforming a soft membrane sealing the chamber from above. The specific material used for the sealing membrane plays a fundamental role for satisfying the requirement of deformation. In particular, to have a very thin and flexible membrane a low Young's modulus material, such as polydimethylsiloxane (PDMS), was employed. As resistors are used as heaters, their shapes are designed in order to maximize the Joule effect heating. It is well known that the electric power dissipated in a resistor is directly proportional to the applied voltage times the flowing electronic current. In order to increase the power dissipation, if the applied voltage is kept constant, the flowing current in the heating circuit must be maximised or its resistance reduced as much as possible. By the general formula for the resistance of a metallic conductor the resistance is proportional to the length over section surface ratio l/S, where l is the total length of the resistor and S its transversal section area, and to the electric resistivity of the resistance material. Two different coil designs have been investigated: a 3 turns coilbased resistor of variable size and a resistor composed of an array of metallic rods connected in parallel as in Fig. 2(a) and Fig. 2(b), respectively. The first investigated geometry for the heater is the coilbased resistor. It is characterized by a spiral shape of 3 turns with two feeding contacts. In the rod-based geometry the conductive path consists in an array of metallic rods connected in parallel. In Fig. 2(a) the fundamental parameters for the design of the coilbased heater are shown: wm is the width of each turn; we is the separation between adjacent turns; th is the thickness of the resistor.
Table 1 Comparison of common static technologies for refreshable tactile displays. Actuation types
Bandwidth
System complexity
Miniaturization
Power consumption
Response time
Cost
Ref.
LEA Piezoelectric SMA EAP Pneumatic Thermo activated
Limited Large Narrow Limited Limited Limited
Good Moderate Poor Moderate Poor Good
Poor Moderate Good Good Poor Good
Moderate Low High High High High
Fast Fast Slow Slow Moderate Fast
High Low Limited Limited Low Limited
[4–6,28] [7,8,29,30,31] [9,10,11,32,33,34,35,36,37,38] [12–15,39,40] [17–19,26,27] [This work]
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Fig. 1. Elementary thermo-activated taxel structure. (a) OFF State: the supply is off and the active alloy is flat. (b) ON State: the supply is switched on and the heated active alloy expands its volume.
order to satisfy the technical requirement about the total area of the taxel, which was set to 1 mm2, the structure has been designed by fixing 3 turns, where 1 mm is the maximum size of the most external. In Fig. 2(b) the fundamental parameters for the design of the rodbased heater are highlighted. In particular, wm is the width of each rod and we is the separation between adjacent rods. As in the previous case, th is the thickness of the resistor. Another important parameter is the number of rods in the parallel arrangement. By varying the number of rods and the other parameters there are a lot of possible configurations; however, all simulated structures have been designed by fixing 8 rods of constant length l = 800 μm and we has been fixed at the constant value of we = 50 μm, consequently fixing the total area of the single heater equal to about 1 mm2 in order to satisfy the technical requirements about taxel area. Assuming the electrical resistivity of gold equal to ρ = 2.44•10−2 [(Ω•μm], which is a valid value at room temperature [44], in Fig. 3 two figures of merit (FoM) have been shown. In order to understand the best design, the comparison between the resistance of the two distinct kind of heaters, the series coil-shaped and the parallel rod-based resistor, is proposed in dependence with two different figures of merit, the total length of the resistive path over the heater wire cross section (l/S, Fig. 3(a)) and total exposed area to Galinstan (Fig. 3(b)), respectively. Both configurations fit the same maximum taxel area, therefore the l/S range of values, dependent on geometrical parameters such as length, width and thickness, is automatically limited by the heater geometry. From the plot, it emerges that, at the fixed maximum occupied surface of 1 mm2, rod-based resistors exhibit smaller resistances. In particular, lowest values are obtained for rods thicker than 50 μm. The efficient heating of the Galinstan is also directly dependent on the resistor
Fig. 2. Heater structure geometry and parameters. (a) Section of the heater coil: width (wm), inter-coils empty space (we) and coils thickness (th). (b) Section of the heater rods: width (wm), inter-rods empty space (we) and rods thickness (th).
Moreover, another important parameter is the number of turn of the coil. These parameters play a role for the definition of the total length and section area of the coil. By varying the number of turns and the other parameters there are many possible configurations; however, in 8
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order to obtain the highest taxel deformation. For a fixed supply voltage (Vin = 2 V and Vin = 5 V) the deformation of the structures has been investigated. The alloy thermal displacement caused by coil-based taxel is more influenced by the thickness of heater coils than the width of the wire, although a width close to 45 μm seems the most effective value for maximizing the displacement. In contrast to the coil-based heater, in rod-based heater the displacement increases with the rod width, whose role is consequently more important then wire thickness. Finally, in both cases, for a fixed thickness (th = 50 μm and th = 100 μm) the deformation of the taxel tends to increase with the applied electric potential. From the comparison of simulations, a general remark can be stressed: for comparable thickness and applied voltages, displacement obtained in rod-based taxel are one order of magnitude higher than coil-based taxel. Thus, for rod-based heaters, a sufficient deformation in the range of the 100 μm can be obtained for lower supply voltages and thinner thicknesses with respect to coil-based structures with identical geometrical parameters, while the same goal for coil-based structures could be satisfied only if thicker coils and higher voltages are applied. As a result, on the basis of the previous theoretical analysis about the total resistance and the FOMs (Fig. 3), the better behaviour of the rodbased was expected. In conclusion, from theoretical and simulated results, a suitable design for the rod-based structure has been accomplished. In fact, through a rod-based design, a displacement value aroud one hundreds of microns at a lower applied voltage would be obtained with a thickness of the heater of both 50 μm and 70 μm.
Fig. 3. Figure of Merit: Comparison between resistances of coil and rod-based heaters. (a) as a function of geometrical parameters. (b) as a function of the exposed area to Galinstan for different thickness values (th = 20,70,50, 80).
surface exposed to the deformable alloy. In fact, the resistor determines a better heating of Galinstan if the contact surface is greater. Because the Galinstan fills the space separating adjacent turns or rods in both structures, the exposed surface consists of all resistor faces, except that touching the substrate. Therefore, a valid FoM for the structures is the total resistance of the specific resistor geometry in relation to the exposed surface. It shows, at a fixed exposed surface, which of the two structure has the lower resistance. Rod-based heaters expose larger areas to Galinstan as emerges from Fig. 3(b), which reports the behavior of resistances as a function of the conductor area globally covered by Galinstan. The numerical analysis of the micro-actuator in terms of the highest displacement of the Galinstan alloy is accomplished by a Multiphysics Finite Element Method (FEM). The substrate is geometrically defined by setting its thickness at 30 μm whereas the area is a parameter depending on the size of heaters, which are built on it. The heaters have been parameterized by the section area: while the space between the heaters branch (we) is fixed at 45 μm for the coils and 50 μm for the rods, the geometry has been parameterized for different values of heater thickness (th) and width (wm). Thus, by comparing the two structures, the most reliable configuration of heaters to be used for fabrication can be chosen and compared with the two FOMs described above. To complete the geometrical model, a further deformable metallic alloy is added on the top of the heater. The alloy thickness has been fixed equal to 100 μm for all computations while its diameter is equal to that of the heater so its base area is the same of the underlying structure. Supposing the chamber walls don't undergo lateral deformation with temperature, the structure guarantees a deformation only in the orthogonal direction, as wished for the correct taxel behaviour. Therefore, only the upper surface of the alloy may expand while the other surfaces have fixed constraints. The multiphysics employed for this study combined structural mechnics and electromagnetism for accounting of actuation with heat transfer for Joule heating and thermal deformation. It is worth underlining that electric resistivity of materials changes as a function of the temperature and, to take into account different temperatures, temperature-dependent resistivity data has been employed for gold [45]. Finally the substrate temperature is fixed at 28 °C. To evaluate the structure's behaviour, a parametric study has been employed on heater section and on applied voltage (Vin), looking for a good combination of the devices parameters and applied voltages in
3. Microfabrication methods of a thermo-active taxel The microfabrication of taxels, outlined in Fig. 4, is achieved in three main steps consisting in heater fabrication, chamber definition by SU8 photoresist, in the deposition of Galinstan inside and a PDMS sealing membrane, respectively. Heaters are fabricated on a 390 μm-thick silicon substrate, coated by a Si3N4 insulating layer 300 nm thick. As in Fig. 4(a), a thin seed layers for electrodeposition, consisting of chromium (10 nm) and gold (30 nm), is thermally evaporated on the insulating layer. For heater shape definition (Fig. 4(b)), a photolithography processing by spin coating a 70 μm thick KMPR® 1050 negative resist, suitable for electrodeposition, is then carried out. To promote a stronger adhesion between the seed layer and KMPR resist, a MCC Primer 80/20, composed of a combination of 20% Hexamethyldisilazane (HMDS) and 80% Propylene Glycol Monomethyl Ether Acetate (PM Acetate), was applied. Subsequently, a processing of gold electrodeposition is accomplished to build metallic resistors (Fig. 4(c)). A suitable solution for gold electrodeposition (99,9% Au and 0,1% Co) was used to fabricate the heaters. The process allowed the growth of pure gold structure with a good conductivity and corrosion/abrasion resistance. The electrodeposition processing was carried on for 8 h by applying a supply current of 40 mA at the temperature of 65 °C. After completing the electrodeposition, the photoresist mask was stripped and the metal seeding layer was wet-etched in order to remove shortcircuits among rods. The 30 nm-thick layer of gold was wet etched in a potassium iodide/iodine solution [46] (KI: 4 g I2: 1 g H2O: 40 ml) while the underlying 10 nm-thick layer of chromium by a solution called Ammonium Cerium (IV) Nitrate (CAN), that is an inorganic compound represented by the chemical formula (NH4)2Ce(NO3)6. In Fig. 5(a)-(c) Au electrodeposited rod-based heaters on a Si3N4 substrate are represented. The measured thickness gold of electrodeposition was approximately 66 μm. A 100 μm thick and 2 mm diameter SU-8 2100 epoxy resist -based spacers (Fig. 4(d)) were perfectly aligned on each specific heater by UV photolithography (Fig. 5(d)). Spacers were filled by Galinstan bump from the top by using the dispenser of a “Multi-purpose Die Bonder FINEPLACER® pico ma” – “flip chip” with a controlled pressure. As a final step in the fabrication processing, a sealing deformable membrane 9
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Fig. 4. Work flow of the fabrication processing. (a) Substrate and Surface Preparation (b-c) Optical lithography for electrodeposition (d-e) Gold electrodeposition and patterning. (f) Lithography of SU8 spacers (g) Filling spacers with Galinstan (h) PDMS membrane covering. The Silicon substrate is not depicted in the figure.
Fig. 5. Fabrication of a rod-based heaters. (a) Picture of the fabricated 70 μm-thick rod-based heaters by means of Au electro-deposition on a substrate of Si3N4. (b)-(c) Scanning electron micrographs (SEM) of the fabricated heater; (d) Picture of the heaters equipped with the SU8 spacers. 10
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The resistor presents a measured resistance of 0.8 Ω. The thermal deformation of the taxel was tested by current injection in the 66 μm thick heater demonstrating the working principle (Fig. 7(a)for the OFF State andFig. 7(b) for the ON State respectively). Membrane deformation measurement was achieved by means of the Micro System Analyser MSA-500 by Polytec, used in the “topography mode“, through a Coherence Scanning Interferometry (CSI) on the same structure used in the heating test, namely wm = 80 μm and we = 50 μm (Fig. 7(c)). The supply current, obtained by a Keithley DMM meter, is 3 A with a tension of 2.4 V for an electrical power of approximately 7.2 W. When the supply was applied at taxel terminals a vertical displacement of the PDMS membrane has been obtained. Repeated tests have highlighted a rise of the PDMS membrane and an equally shutdown when the power supply was interrupted in approximately 1.5 s. Areal surface thickness measurements have produced a 3D representation of a surface of the PDMS (see Fig. 8(a)). The displacement increases towards the center of the taxel (blue central zone in Fig. 8(a)) while is lower on the edges (red edge zone in Fig. 8(a)). The 3D surface topography measured by CSI interferogram of the tested heater shown a displacement between the edge and the centre of approximately 50 μm. A height function with respect lateral displacement, z(x), can mathematically represent a profile measurement through a line across the surface. As the deformation measurement was achieved by an areal optical instrument, the profile has been extracted by software [47]. The profile measurement from the areal measurement of Fig. 8(a) is shown in Fig. 8(b). This profile measurement have a higher level of about 50 μm at about 180 μm from the zero starting point. Even if the experimental total displacement is lower than the expected calculated value around 100 μm, there are many factors that could be taken into account. Firstly, the PDMS membrane, which applies a mechanical resistance to deformation, was not inserted in the FEM simulations, therefore lowering the calculated maximum value of the displacement. Moreover, the oxidization of Galinstan and its conductivity could add some additive resistance to the circuitry. While the equivalent resistance of the simulated heaters has been evaluated taking into account only the conductive paths in the rods, in the fabricated device the Galinstan can conduct current, becoming a heater itself. Finally, the adhesion force of the Galinstan to the spacers wall has not been evaluated in the simulation and certainly affect the measurements. From these data we can compare the thermal active taxel technology with respect the previous state of art introduced in Table 1. Compared to other activation technologies, thanks to the microfabrication process, our proposal does not have the dimensional problems of both pneumatic and electromagnetic that are usually bulky.
Fig. 6. Current-temperature characteristc: comparison between two heaters characterized by identical sizes but different thicknesses.
of PDMS was stretched out on the surface (Fig. 4(e)), that results in a completely cured, insoluble and hydrophobic structure. The membrane was originally cured on a sacrificial layer coated substrate and then transferred, after lift off from the substrate, on the taxel sample as a covering and sealing membrane. 4. Experimental results and discussion To test the rod-based resistance as an efficient heater to verify the ability to increase their temperature, a current flow was applied to the power supply terminals. The chosen heater for the temperature measurements was composed by 8 rods with wm = 80 μm and we = 50 μm. Two thicknesses have been tested: th = 66 μm and th = 41 μm, respectively. The temperature detection in dependence with the flowing electronic current was measured by a Platinum Resistance Temperature Detector (RTD) interfaced to an Arduino® programmable platform for the visualization of the measurement results. From the measurements, it emerges that the temperature tends to increase linearly with the applied current. However, the temperature of a resistor depends on its thickness. In Fig. 6 the comparison between the temperature behaviour of two heaters characterized by identical sizes, but different thicknesses, namely th = 66 μm and th = 41 μm, respectively is showed. Finally, as expected, the heating in the thicker device is definitely more efficient.
Fig. 7. Membrane deformation measurement under current driving. (a) OFF state; (b) ON state; (c) scheme of the experimental “topography” setup. 11
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Fig. 8. Areal surface thickness measurements. (a) result of an areal surface texture measurement of the PDMS membrane; (b) Surface profile measurement with the lowest displacement in the left bottom corner.
References
Furthermore, the response time once activated is very low and have a limited bandwidth. However, as a main drawback, displays based on micro thermal actuators require an high power (~ tens of W) to achieve a significant stroke.
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5. Conclusions The micro-actuator proposed in this work, also called TAXEL for its possible application as a Tactile pixel, exploits a thermo-active actuation principle based on thermal deformation of Galinstan®, an alloy of Gallium, Indium and Tin. The thermal expansion is determined by heating the alloy by means of an underlying metallic resistor, which is designed to work as a heater. Heaters characterized by two different geometries (coil-based and rod-based) has been simulated and their yield has been evaluated by appropriate Figure of Merits (resistance versus total length over section and resistance versus exposed area). The most efficient rod-based heater design was inserted in a PDMS membrane-sealed Galinstan®-filled chamber for actuation tests and fabricated by means of standard micromachining techniques. TAXELs were characterized by measuring the heater temperature by Platinum Resistance Temperature Detector and membrane deformation by Coherence Scanning Interferometry. The profile measurement, achieved by using an actuation current for the taxel of 3A and a supply voltage of 2,4 V, revealed a total displacement of about 50 μm in line with the expected value given by the simulations. Stray capacitance and current reduction, very low operating voltages and suppression of leakage currents are some key techniques for drastic reduction of power dissipation. Therefore, power consumption optimazation requires a geometry and technology improvement that can be achieved, for example, using a pulse mode supply with a proper duty cycle in order to eliminate any DC currents. A pulse mode supply will allow to perform a life cycle test to study the stability of the taxel actuator. Moreover, a latching mechanisms will strengthen the displacement in order to to move the TAXEL from a stable OFF position to a stable ON position. Finally, a mechanical amplification of displacement can be added to avoid the direct contact of finger tips on the soft PDMS membrane. Eventually, a future direcction will be the integration of thousands of taxels in an array or matrix to build a working portable tactile display.
Acknowledgements Special thanks are addressed to Gianmichele Epifani and Roberto Giannuzzi for help in the realization of these studies.
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