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4-inch transparent plates based on thin-film AlN actuators for haptic applications
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4-inch transparent plates based on thin-film AlN actuators for haptic applications F. Casset a,b,∗, JS. Danel a,b, P. Renaux a,b, C. Chappaz c, F. Bernard d, T. Sednaoui c, S. Basrour d, B. Desloges a,b, S. Fanget a,b a
University of Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles, France d TIMA (CNRS-Grenoble INP-UJF), 38000 Grenoble, France b c
a r t i c l e
i n f o
Article history: Received 24 September 2015 Revised 6 April 2016 Accepted 29 May 2016 Available online xxx Keywords: Piezoelectric Actuator Haptic Transparent design Characterization
a b s t r a c t Numerous applications require tactile interfaces today. In particular, many customers’ applications such as automotive, Smartphone, tablet PC or touch pad can be concerned by high performances, low voltage haptic interfaces which allow the user to interact with its environment by the sense of touch. This technology is already used but with limitations such as high power consumption and limited feedback effect because today a simple vibration is commonly obtained. We chose to work on the squeeze-film effect. It consists in changing the friction between the finger and a plate resonator. It provides high granularity level of haptic sensation. This paper deals with the design, realization and characterization of high performances actuators in order to promote the squeeze-film effect on a 4-inch transparent plate (diagonal of the plate). Using Finite Element Method (FEM) models, we select the best design, able to generate the highest plate displacement amplitude as possible. We built demonstrators using a generic technology based on thin-film Aluminum Nitride (AlN) actuators on glass substrate. Electromechanical characterizations prove that it is possible to obtain the focused substrate vibration amplitude using only 35 V in amplitude. The integration of the thin-film actuator plate in a haptic demonstrator is now ongoing. © 2016 Published by Elsevier Ltd.
1. Introduction Recent demand in new tactile interfaces in many customers’ application such as Smartphones or tablet PCs, has focused research efforts towards developing high performances transparency haptic interfaces. Among the different haptic solutions [1–4], squeeze-film effect is one of the most promising ones. It provides high granularity level of haptic sensation, playing with the variable friction between a finger and a resonant plate when the plate displacement amplitude (PDA) reaches about 1 μm in a flexural anti-symmetric Lamb mode [5–8]. To promote the desired mode, we use piezoelectric actuators and bimorph effect. We developed Finite Element Method (FEM) models and proved the concept using thin-film PZT actuators deposited on silicon substrate [9–10]. Nevertheless, to address transparency, low temperature process was used, to deposit AlN actuators directly on transparent glass substrate [11]. This paper reports first on the design of thin-film AlN actuated haptic plates. Using our FEM models, we proposed an actu-
∗
Corresponding author. E-mail address: [email protected] (F. Casset).
ator design able to promote the required PDA to a 4-inch transparent plate (diagonal of the transparent area of the plate) [12]. Then the technological stack used to build demonstrators is presented. Finally, electromechanical characterizations prove that we meet the micrometric substrate displacement amplitude specification for only 35 V. The haptic plate will be integrated in a thin-film haptic demonstrator, in order to obtain complex haptic effect in a close future.
2. Design We designed high performances thin-film AlN actuated haptic plates using FEM approach and CoventorWare® tool. Our model consists in the study of unclamped 700 μm thick plates made of glass with 2 μm thick AlN actuators. Thus, as it will be explained in Section 3, no clamping condition is applied to the glass plate. Previous work on PZT actuated silicon plates validates this hypothesis [13]. We neglected top and bottom electrodes on both sides of the AlN layer due to their low impact on the PDA. Indeed, as it will be detailed later (Section 3), top and bottom electrode thicknesses (220 nm) are negligible compared to the glass substrate thickness
http://dx.doi.org/10.1016/j.mechatronics.2016.05.014 0957-4158/© 2016 Published by Elsevier Ltd.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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F. Casset et al. / Mechatronics 000 (2016) 1–6 Table 1. Material properties.
Material
Young’s modulus (GPa)
Poisson’s ratio
Density (g/cm3 )
e31,f (C/m²)
AlN Glass
300 73.6
0.3 0.23
3.27 2.38
-1.1 -
Fig. 2. . Schematic diagram of the FEM model.
Fig. 1. . Modal simulation on a 110 × 65 mm² glass plate for actuator column positioning – View of the selected mode, f = 24.68 kHz.
(700 μm). Thus we suppose that their contribution to the system stiffness is negligible. The material properties used in the FEM model are given in Table 1. The AlN mechanical material properties are coming from picosecond ultrasonics measurement (details on this measurement technique can be found in [14–15]), whereas the Glass material properties are coming from the data sheet of this commercial material (EAGLE XG® ). The piezoelectric coefficient e31, f is extracted from electromechanical measurements performed on an AlN-actuated cantilever, in particular using an AixACCT tool [16]. The simulation procedure consists of a modal simulation to select the adequate mode to promote the squeeze-film effect, and then of harmonic simulations to design actuators leading to the highest substrate displacement amplitude. First, modal simulation was performed on a 110 × 65 mm² plate to determine the frequency of the flexural anti-symmetric Lamb modes known to promote the squeeze-film effect [5–8]. Among these modes we select a mode presenting a frequency beyond audible frequencies. We chose to work with a mode at 24.68 kHz as shown in Fig. 1. We accurately positioned an actuator column (AC1) by taking the deformed shape of the selected mode into account, and matching the actuators’ position with the maximum PDA. The actuator column consists in 5 individual actuators (each actuator length, LAC = 12,208 μm in the y direction). Fig. 2 gives a schematic diagram of this first FEM model. Then, harmonic simulations were performed in order to define the optimum actuator column width and localization. We applied 60 V to the top electrodes of the AlN actuators, whereas 0 V is applied to the bottom ones. We introduce a damping parameter of 7.10−8 to predict the absolute PDA of our plate under 60 V. This damping parameter is coming from the comparison between PDAs measurement and post simulation on 60 × 40 and 40 × 30 mm² haptic plates [9]. This previous calibration step of our FEM model led us to estimate the adequate damping parameter for the 110 × 65 mm² plate study. An actuator width WAC = 7175 μm, located at B = 6110 μm from the end of the plate exhibit the best performances. The actuator width study can be observed in Fig. 3 and the optimal position of the actuator is determined in Fig. 4.
Fig. 3. Optimum actuator width for a 110 × 65 mm² glass plate determined using harmonic simulations (60 V and B = 6110 μm).
Fig. 4. Optimum actuator localization for a 110 × 65 mm² glass plate determined using harmonic simulations (60 V and WAC = 7175 μm).
A second actuator column at the opposite plate end (AC2), as well as actuators localized along the plate length ends, were also positioned (Actuators along the plate length: AL actuators) taking into account the vibration mode shown in Fig. 1. The complete implementation can be observed in Fig. 5 and the main dimensions are given in Table 2. As expected, the 110 × 65 mm² plate presents a transparent area, between the actuators, of about 4-inch in diagonal (9.06 × 4.9 mm²). One can note that several actuator configurations can be used: AC1 and/or AC2, AC1-in-phase AL actuators and/or AC1-out-ofphase AL actuators, or actuators.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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F. Casset et al. / Mechatronics 000 (2016) 1–6
Fig. 5. . Optimum design for the 24.68 kHz Lamb mode obtained on the 110 × 65 mm² glass plate and schematic cross section of the 24.68 kHz mode shape.
3
Fig. 6. . Harmonic simulations of the maximum PDA of a 110 × 65 mm² glass plate based on thin-film AlN actuators regarding the actuation configuration (± 60 V, damping parameter = 7.10−8 ).
Table 2. 110 × 65 mm² haptic plate based on thin-film AlN actuators on glass main dimensions. Dimension Plate size AC1, AC2
AL
Value
Actuator Actuator Actuator Actuator
Actuator Actuator Actuator Actuator Transparent area
length, L AC width, WAC column position, B column number width, W length, L spacing number
110 × 65 mm² 12,208 μm 7175 μm 6110 μm 2
Fig. 7. . AlN-on-glass schematic technological stack cross section.
50 0 0 μm 7125 μm 1714 μm 2 × 10 9.06 × 4.9 mm²
Different actuation configurations were studied through harmonic simulations. We applied 60 V at AC1 and AC1-in-phase AL actuators and –60 V to AC2 and AC1-out-of-phase AL actuators. Maximum PDA regarding these different actuation configurations are reported in Fig. 6. Micrometric PDAs were predicted using various actuator configurations. Using all the actuators, it’s interesting to note that only 30 V were necessary to meet specifications (substrate displacement amplitude at less higher than 1 μm), and thus to obtain haptic effects. 3. Technology AlN-based devices were already manufactured out of 200 mm glass substrates. We used standard 700 μm thick plates made of glass (EAGLE XG® ). First we deposited a 250 nm thick silicon oxide (SiO2 ) layer. The piezoelectric stack consists of 2 μm thick AlN in between 220 nm thick Molybdenum bottom electrode and 220 nm thick Molybdenum top electrode [17]. The AlN is deposited using reactive sputtering from an aluminum target under a nitrogen atmosphere. Then we deposited and patterned a 300 nm thick silicon oxide (SiO2 ) passivation layer. Finally, we deposited and etched gold lines and pads. It consists in a 500 nm thick gold (Au) layer above a 20 nm thick Titanium (Ti) adhesive layer. Fig. 7 gives a schematic view of the technological stack. The following Fig. 8 gives photography of the final glass wafer before sawing. We can observe two 110 × 65 mm² haptic plates based on thin-film AlN actuators. Two 40 × 30 mm² haptic plates
Fig. 8. . AlN-on-glass wafer with two AlN actuated 110 × 65 mm² haptic plates and with an opacifying layer necessary for optical electromechanical characterizations.
and various test dies were also implemented in order to perform complementary characterizations. An opacifying layer was also deposited on the glass substrate in order to make possible the future electromechanical characterization using laser vibrometry. We observed the cross section of an AlN actuator using SEM observations. Fig. 9 shows the different thicknesses of the final stack. In particular, bottom and top electrodes as well as the AlN actuator thicknesses were measured. An acceptable discrepancy between measured and focused thicknesses was obtained. An over etch of the AlN side, under the Molybdenum top electrode is noticed. It can lead to a reduction of the reliability of the actuator.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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Fig. 9. AlN-on-glass SEM cross-section in order to measure the thickness of the different layers of the piezoelectric stack.
Fig. 10. . AlN-on-glass individual haptic plate with flexible electrical connectors.
Fig. 11. . AlN-on-glass 110 × 65 mm² haptic plate reported on a mechanical carrier and flexible electrical connectors connected on a PCB.
Finally, we sawed the glass substrate to obtain individual plate. We also stuck flexible electrical connectors in order to electrically connect the metal pads of the haptic plate with the electronic board. Fig. 10 shows a picture of a haptic plate with its flexible connectors and Fig. 11 gives a view of the plate reported on a mechanical support. To maintain the plate on the support, adhesive tapes were used, located at nodal points of the plate vibration mode. It corresponds to the unclamped hypothesis retained for simulation. 4. Electrical and electromechanical characterizations We performed electrical characterizations to check the AlN quality. We selected a frequency range from 10 to 100 kHz, thus
Fig. 12. . AlN-on-glass capacitance measurement at 10 V (Cp) versus frequency on a 5 × 5 mm² capacitance presenting an AlN thickness of 2 μm.
Fig. 13. . AlN-on-glass losses measurement at 10 V (tanδ ) versus frequency on a 5 × 5 mm² capacitance presenting an AlN thickness of 2 μm.
in the kilohertz range as the considered mechanical resonant frequency of the plate. In particular, through the measurement of the capacitance (parallel capacitance Cp ) versus frequency (Fig. 12), a relative dielectric constant of 10 was obtained, as expected. Indeed, as shown on Fig. 12, a capacitance value of 1.12 nF was measured on a 5 × 5 mm² capacitance presenting an AlN thickness of 2 μm. Moreover, we also extracted a reasonable losses factor (tan δ ) lower than 10−3 as shown in Fig. 13. It proves the good quality of our piezoelectric material. To excite our device, two generators (33250A) and two amplifiers (HAS4011 and FALCO) were used in order to applied, when necessary, two out-of-phase actuation voltages. It corresponds to the +60 V applied to AC1 and AC1-in-phase AL actuators and to the –60 V applied to the AC1-out-of-phase actuators (AL actuators and AC2). Fig. 14 shows the vibrating haptic plate covered with sugar while excited at its resonant frequency. We evidenced the Lamb mode of the plate because the grains of sugar move towards the vibration nodes when the plate is actuated. We measured the focus anti-symmetric lamb mode at 23.02 kHz. The discrepancy with the simulated frequency (6.7 %) can be explained by the inaccuracy of the final dimensions of the plate caused by the sawing of the substrate to obtain individual plates. The harmonic measurement performed on the plate, which confirm this frequency, is reported in Fig. 15. Optical measurements of 110 × 65 mm² AlN-actuated vibrating plates were also performed using laser vibrometry (POLYTEC®). The apparatus uses interferometry to measure the velocity of vibrating
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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Fig. 16. . Comparison of the measured and the simulated parameter = 7.10−8 ) PDA of the 110 × 65 mm² plate under ± 60 V.
5
(damping
Fig. 14. . Highlight of the vibration mode shape on the AlN-on-glass plate using sugar.
Fig. 17. . Measurement using the laser vibromètre of the PDA of the 110 × 65 mm² plate regarding the actuation voltage. Fig. 15. . Harmonic measurement on the 110 × 65 mm² plate to determine the resonant frequency of the Lamb mode (+60 V applied to AC1 and AC1-in-phase AL actuators and –60 V applied to the AC1-out-of-phase actuators).
points of the plate surface. In particular, the PDA regarding the actuation configuration was measured and compared to simulation. A micrometric displacement can be obtained using various actuator configurations, as expected. In particular, a PDA of 1.7 μm is measured applying 60 V at AC1 and AC1-in-phase AL actuators and –60 V to AC2 and AC1-out-of-phase AL actuators (all actuators actuated) as shown in Fig. 15. Fig. 16 shows that an acceptable agreement is observed between simulation and measurement (discrepancy ranging from 6 to 20 %). It indicates that the damping parameter used to predict the electromechanical behavior is correct. Using laser vibromètre, we also measured the linear dependence of the PDA function of the actuation voltage as shown in Fig. 17. It proves that a smaller actuation voltage can be used in order to obtain a micrometric PDA. For example, we calculated that only ± 35 V is necessary to obtain a PDA of 1 μm using all the actuators (+35 V at AC1 and AC1-in-phase AL actuators and –35 V to AC2 and AC1-out-of-phase AL actuators). A weak but clearly perceptible haptic feedback effect was felt with the finger on this plate by applying an actuation signal of 60 V at AC1 and AC1-in-phase AL actuators and –60 V to AC2 and AC1-out-of-phase AL actuators, modulated at 10 Hz in amplitude. No haptic sensations were noticed for smaller actuation voltage and thus smaller PDA. This point must be investigated to go further on haptic interfaces based on squeeze-film effect. In particu-
lar, it can indicate that plate displacement amplitude higher than 1 μm is necessary to promote a haptic affect, contrary to our initial specification. Finally, the power consumption of thin-film AlN actuators was studied. It consists in the use of a current probe. A power consumption of 1 W was measured under 60 V on the AC1 or on the AC2 columns whereas 0.5 W was measured on the AC1-In-phase or on the AC1-out-of-phase AL actuators. It indicates that 3 W were necessary to actuate all the actuators under 60 V. 5. Conclusion Transparent haptic interfaces based on AlN-actuators deposited above glass substrate were designed in order to obtain a 110 × 65 mm² plate with a 4-inch transparent area. FEM simulations predict micrometric PDA using +30 V at AC1 and AC1-inphase AL actuators and –30 V to AC2 and AC1-out-of-phase AL actuators. We build haptic plate using a generic AlN technology above glass wafers. Measurement results indicate that we can obtain micrometric PDA under +35 V at AC1 and AC1-in-phase AL actuators and –35 V to AC2 and AC1-out-of-phase AL actuators, thus in good agreement with simulation. Finally, a weak but clearly perceptible haptic feedback effect was felt with the finger when all the actuators were actuated under ± 60 V, whereas no haptic sensations were noticed for smaller actuation voltage. This small haptic sensation is under investigation to make possible the 4-inche transparent plate integration in a thin-film haptic demonstrator in a close future.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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ACKNOWLEDGEMENTS The authors wish to acknowledge all participants at the MINALOGIC project Touch It and the BPI funding to give us the opportunity to work on this subject. Moreover we would like to acknowledge Karine Saxod from STMicroelectronics for the flexible electrical connector integration on the glass haptic plate. References [1] Hudin C, Lozada J, Hayward V. Localized tactile feedback on a transparent surface through time-reversal wave focusing. IEEE Trans Haptics 2015;8(April-June (2)):188–98. [2] Kim SC, Israr A, Poupyrev I. Tactile rendering of 3D feature on touch surfaces. In: Proceedings of the 26th annual ACM symposium on user interface software and technology; October 2013. [3] Watanabe J, Ishikawa H, Arouette X, Matsumoto Y, Miki N. Artificial tactile feeling displayed by large displacement MEMS actuator array. In: The 25th international conference on micro electro mechanical systems (MEMS); 2012. p. 1129–32. [4] Crommentuijn K, Hermes DJ. The effect of Coulomb friction in a haptic interface on positioning performance. In: Haptics: generating and perceiving tangible sensations. Berlin: Springer; 2010. p. 398–405. [5] Sergeant P, Giraud F, Lemaire-Semail B. Geometrical optimization of an ultrasonic tactile plate. Sensors Actua A 2010;161:91–100. [6] Zheng T, Giraud F, Semail B, Amberg M. Analysis of a new haptic display coupling tactile and kinesthetic feedback to render texture and shape. In: EuroHaptics 2010, Part II. Springer; 2010. p. 87–93. [7] Biet M, Giraud F, Lemaire-Semail B. Squeeze film effect for the design of an ultrasonic tactile plate. IEEE Trans Ultrason Ferroelectr Frequency Control 2007;54(December (12)):2678–88. [8] Wiertlewski M, Colgate JE. Power optimization of ultrasonic friction-modulation tactile interfaces. IEEE Trans Haptics, 2015;8(January-March (1)):43–53.
[9] Casset F, Danel JS, Chappaz C, Civet Y, Amberg M, Gorisse M, Dieppedale C, Le Rhun G, Basrour S, Renaux P, Defaÿ E, Devos A, Semail B, Ancey P, Fanget S. Low voltage actuated plate for haptic applications with PZT thin-film. In: International conference on solid-state sensors, actuators and microsystems (Transducers); 2013. p. 2733–6. [10] Casset F, Danel JS, Renaux P, Chappaz C, Le Rhun G, Dieppedale C, Gorisse M, Basrour S, Fanget S, Ancey P, Devos A, Defaÿ E. Characterization and post simulation of thin-film PZT actuated plates for haptic applications. In: International conference on thermal, mechanical and multi-physics simulation and experiments in microelectronics and microsystems (Eurosime); 2014. p. 1–4. [11] Bernard F, Gorisse M, Casset F, Chappaz C, Basrour S. Design, Fabrication and characterization of a tactile display based on AlN transducers. Eur Conf Solid-State Transd (Eurosensors) 2014;87:1877–7058. [12] Casset F, Danel JS, Chappaz C, Bernard F, Basrour S, Desloges B, Fanget S. Design of thin-film AlN actuators for 4-inch transparent plates for haptic applications. International conference on thermal, mechanical and multi-physics simulation and experiments in microelectronics and microsystems (Eurosime); 2015. To be published. [13] Casset F, Danel JS, Chappaz C, Civet Y, Amberg M, Gorisse M, Dieppedale C, Le Rhun G, Basrour S, Renaux P, Defay E, Devos A, Semail B, Ancey P, Fanget S. Low voltage actuated plate for haptic application with PZT thin-film. In: The 17th international conference on solid state sensors, actuators and microsystems (transducers); 2013. p. 2733–6. [14] Devos A, Emery P, Defay E, Ben Hassine N, Parat G. Ultrafast optical technique for measuring the electrical dependence of the elasticity of piezoelectric thin film: demonstration on AlN. Rev Sci Instrum 2013;84:015007. [15] Defay E, Ben Hassine N, Emery P, Parat G, Abergel J, Devos A. Tunability of aluminium nitride acoustic resonators: a phenomenological approach. IEEE Trans Ultrason Ferroelectr Frequency Control 2011;58(December (12)):2516–20. [16] Prume K. Measurement of d33,f and e31,f of piezo cantilever with a 4-point bending sample holder. Tutorial AixACCT, November 23; 2006. [17] Billard C, Buffet N, Reinhardt A, Parat G, Joblot S, Bar P. 200 mm manufacturing solution for coupled resonateir filters. In: Proceedings of the 39th European solid-state device research conference; 2009. p. 133–6.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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[m5G;June 4, 2016;2:33]
Mechatronics 0 0 0 (2016) 1–6
Contents lists available at ScienceDirect
Mechatronics journal homepage: www.elsevier.com/locate/mechatronics
4-inch transparent plates based on thin-film AlN actuators for haptic applications F. Casset a,b,∗, JS. Danel a,b, P. Renaux a,b, C. Chappaz c, F. Bernard d, T. Sednaoui c, S. Basrour d, B. Desloges a,b, S. Fanget a,b a
University of Grenoble Alpes, F-38000 Grenoble, France CEA, LETI, MINATEC Campus, F-38054 Grenoble, France STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles, France d TIMA (CNRS-Grenoble INP-UJF), 38000 Grenoble, France b c
a r t i c l e
i n f o
Article history: Received 24 September 2015 Revised 6 April 2016 Accepted 29 May 2016 Available online xxx Keywords: Piezoelectric Actuator Haptic Transparent design Characterization
a b s t r a c t Numerous applications require tactile interfaces today. In particular, many customers’ applications such as automotive, Smartphone, tablet PC or touch pad can be concerned by high performances, low voltage haptic interfaces which allow the user to interact with its environment by the sense of touch. This technology is already used but with limitations such as high power consumption and limited feedback effect because today a simple vibration is commonly obtained. We chose to work on the squeeze-film effect. It consists in changing the friction between the finger and a plate resonator. It provides high granularity level of haptic sensation. This paper deals with the design, realization and characterization of high performances actuators in order to promote the squeeze-film effect on a 4-inch transparent plate (diagonal of the plate). Using Finite Element Method (FEM) models, we select the best design, able to generate the highest plate displacement amplitude as possible. We built demonstrators using a generic technology based on thin-film Aluminum Nitride (AlN) actuators on glass substrate. Electromechanical characterizations prove that it is possible to obtain the focused substrate vibration amplitude using only 35 V in amplitude. The integration of the thin-film actuator plate in a haptic demonstrator is now ongoing. © 2016 Published by Elsevier Ltd.
1. Introduction Recent demand in new tactile interfaces in many customers’ application such as Smartphones or tablet PCs, has focused research efforts towards developing high performances transparency haptic interfaces. Among the different haptic solutions [1–4], squeeze-film effect is one of the most promising ones. It provides high granularity level of haptic sensation, playing with the variable friction between a finger and a resonant plate when the plate displacement amplitude (PDA) reaches about 1 μm in a flexural anti-symmetric Lamb mode [5–8]. To promote the desired mode, we use piezoelectric actuators and bimorph effect. We developed Finite Element Method (FEM) models and proved the concept using thin-film PZT actuators deposited on silicon substrate [9–10]. Nevertheless, to address transparency, low temperature process was used, to deposit AlN actuators directly on transparent glass substrate [11]. This paper reports first on the design of thin-film AlN actuated haptic plates. Using our FEM models, we proposed an actu-
∗
Corresponding author. E-mail address: [email protected] (F. Casset).
ator design able to promote the required PDA to a 4-inch transparent plate (diagonal of the transparent area of the plate) [12]. Then the technological stack used to build demonstrators is presented. Finally, electromechanical characterizations prove that we meet the micrometric substrate displacement amplitude specification for only 35 V. The haptic plate will be integrated in a thin-film haptic demonstrator, in order to obtain complex haptic effect in a close future.
2. Design We designed high performances thin-film AlN actuated haptic plates using FEM approach and CoventorWare® tool. Our model consists in the study of unclamped 700 μm thick plates made of glass with 2 μm thick AlN actuators. Thus, as it will be explained in Section 3, no clamping condition is applied to the glass plate. Previous work on PZT actuated silicon plates validates this hypothesis [13]. We neglected top and bottom electrodes on both sides of the AlN layer due to their low impact on the PDA. Indeed, as it will be detailed later (Section 3), top and bottom electrode thicknesses (220 nm) are negligible compared to the glass substrate thickness
http://dx.doi.org/10.1016/j.mechatronics.2016.05.014 0957-4158/© 2016 Published by Elsevier Ltd.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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F. Casset et al. / Mechatronics 000 (2016) 1–6 Table 1. Material properties.
Material
Young’s modulus (GPa)
Poisson’s ratio
Density (g/cm3 )
e31,f (C/m²)
AlN Glass
300 73.6
0.3 0.23
3.27 2.38
-1.1 -
Fig. 2. . Schematic diagram of the FEM model.
Fig. 1. . Modal simulation on a 110 × 65 mm² glass plate for actuator column positioning – View of the selected mode, f = 24.68 kHz.
(700 μm). Thus we suppose that their contribution to the system stiffness is negligible. The material properties used in the FEM model are given in Table 1. The AlN mechanical material properties are coming from picosecond ultrasonics measurement (details on this measurement technique can be found in [14–15]), whereas the Glass material properties are coming from the data sheet of this commercial material (EAGLE XG® ). The piezoelectric coefficient e31, f is extracted from electromechanical measurements performed on an AlN-actuated cantilever, in particular using an AixACCT tool [16]. The simulation procedure consists of a modal simulation to select the adequate mode to promote the squeeze-film effect, and then of harmonic simulations to design actuators leading to the highest substrate displacement amplitude. First, modal simulation was performed on a 110 × 65 mm² plate to determine the frequency of the flexural anti-symmetric Lamb modes known to promote the squeeze-film effect [5–8]. Among these modes we select a mode presenting a frequency beyond audible frequencies. We chose to work with a mode at 24.68 kHz as shown in Fig. 1. We accurately positioned an actuator column (AC1) by taking the deformed shape of the selected mode into account, and matching the actuators’ position with the maximum PDA. The actuator column consists in 5 individual actuators (each actuator length, LAC = 12,208 μm in the y direction). Fig. 2 gives a schematic diagram of this first FEM model. Then, harmonic simulations were performed in order to define the optimum actuator column width and localization. We applied 60 V to the top electrodes of the AlN actuators, whereas 0 V is applied to the bottom ones. We introduce a damping parameter of 7.10−8 to predict the absolute PDA of our plate under 60 V. This damping parameter is coming from the comparison between PDAs measurement and post simulation on 60 × 40 and 40 × 30 mm² haptic plates [9]. This previous calibration step of our FEM model led us to estimate the adequate damping parameter for the 110 × 65 mm² plate study. An actuator width WAC = 7175 μm, located at B = 6110 μm from the end of the plate exhibit the best performances. The actuator width study can be observed in Fig. 3 and the optimal position of the actuator is determined in Fig. 4.
Fig. 3. Optimum actuator width for a 110 × 65 mm² glass plate determined using harmonic simulations (60 V and B = 6110 μm).
Fig. 4. Optimum actuator localization for a 110 × 65 mm² glass plate determined using harmonic simulations (60 V and WAC = 7175 μm).
A second actuator column at the opposite plate end (AC2), as well as actuators localized along the plate length ends, were also positioned (Actuators along the plate length: AL actuators) taking into account the vibration mode shown in Fig. 1. The complete implementation can be observed in Fig. 5 and the main dimensions are given in Table 2. As expected, the 110 × 65 mm² plate presents a transparent area, between the actuators, of about 4-inch in diagonal (9.06 × 4.9 mm²). One can note that several actuator configurations can be used: AC1 and/or AC2, AC1-in-phase AL actuators and/or AC1-out-ofphase AL actuators, or actuators.
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Fig. 5. . Optimum design for the 24.68 kHz Lamb mode obtained on the 110 × 65 mm² glass plate and schematic cross section of the 24.68 kHz mode shape.
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Fig. 6. . Harmonic simulations of the maximum PDA of a 110 × 65 mm² glass plate based on thin-film AlN actuators regarding the actuation configuration (± 60 V, damping parameter = 7.10−8 ).
Table 2. 110 × 65 mm² haptic plate based on thin-film AlN actuators on glass main dimensions. Dimension Plate size AC1, AC2
AL
Value
Actuator Actuator Actuator Actuator
Actuator Actuator Actuator Actuator Transparent area
length, L AC width, WAC column position, B column number width, W length, L spacing number
110 × 65 mm² 12,208 μm 7175 μm 6110 μm 2
Fig. 7. . AlN-on-glass schematic technological stack cross section.
50 0 0 μm 7125 μm 1714 μm 2 × 10 9.06 × 4.9 mm²
Different actuation configurations were studied through harmonic simulations. We applied 60 V at AC1 and AC1-in-phase AL actuators and –60 V to AC2 and AC1-out-of-phase AL actuators. Maximum PDA regarding these different actuation configurations are reported in Fig. 6. Micrometric PDAs were predicted using various actuator configurations. Using all the actuators, it’s interesting to note that only 30 V were necessary to meet specifications (substrate displacement amplitude at less higher than 1 μm), and thus to obtain haptic effects. 3. Technology AlN-based devices were already manufactured out of 200 mm glass substrates. We used standard 700 μm thick plates made of glass (EAGLE XG® ). First we deposited a 250 nm thick silicon oxide (SiO2 ) layer. The piezoelectric stack consists of 2 μm thick AlN in between 220 nm thick Molybdenum bottom electrode and 220 nm thick Molybdenum top electrode [17]. The AlN is deposited using reactive sputtering from an aluminum target under a nitrogen atmosphere. Then we deposited and patterned a 300 nm thick silicon oxide (SiO2 ) passivation layer. Finally, we deposited and etched gold lines and pads. It consists in a 500 nm thick gold (Au) layer above a 20 nm thick Titanium (Ti) adhesive layer. Fig. 7 gives a schematic view of the technological stack. The following Fig. 8 gives photography of the final glass wafer before sawing. We can observe two 110 × 65 mm² haptic plates based on thin-film AlN actuators. Two 40 × 30 mm² haptic plates
Fig. 8. . AlN-on-glass wafer with two AlN actuated 110 × 65 mm² haptic plates and with an opacifying layer necessary for optical electromechanical characterizations.
and various test dies were also implemented in order to perform complementary characterizations. An opacifying layer was also deposited on the glass substrate in order to make possible the future electromechanical characterization using laser vibrometry. We observed the cross section of an AlN actuator using SEM observations. Fig. 9 shows the different thicknesses of the final stack. In particular, bottom and top electrodes as well as the AlN actuator thicknesses were measured. An acceptable discrepancy between measured and focused thicknesses was obtained. An over etch of the AlN side, under the Molybdenum top electrode is noticed. It can lead to a reduction of the reliability of the actuator.
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Fig. 9. AlN-on-glass SEM cross-section in order to measure the thickness of the different layers of the piezoelectric stack.
Fig. 10. . AlN-on-glass individual haptic plate with flexible electrical connectors.
Fig. 11. . AlN-on-glass 110 × 65 mm² haptic plate reported on a mechanical carrier and flexible electrical connectors connected on a PCB.
Finally, we sawed the glass substrate to obtain individual plate. We also stuck flexible electrical connectors in order to electrically connect the metal pads of the haptic plate with the electronic board. Fig. 10 shows a picture of a haptic plate with its flexible connectors and Fig. 11 gives a view of the plate reported on a mechanical support. To maintain the plate on the support, adhesive tapes were used, located at nodal points of the plate vibration mode. It corresponds to the unclamped hypothesis retained for simulation. 4. Electrical and electromechanical characterizations We performed electrical characterizations to check the AlN quality. We selected a frequency range from 10 to 100 kHz, thus
Fig. 12. . AlN-on-glass capacitance measurement at 10 V (Cp) versus frequency on a 5 × 5 mm² capacitance presenting an AlN thickness of 2 μm.
Fig. 13. . AlN-on-glass losses measurement at 10 V (tanδ ) versus frequency on a 5 × 5 mm² capacitance presenting an AlN thickness of 2 μm.
in the kilohertz range as the considered mechanical resonant frequency of the plate. In particular, through the measurement of the capacitance (parallel capacitance Cp ) versus frequency (Fig. 12), a relative dielectric constant of 10 was obtained, as expected. Indeed, as shown on Fig. 12, a capacitance value of 1.12 nF was measured on a 5 × 5 mm² capacitance presenting an AlN thickness of 2 μm. Moreover, we also extracted a reasonable losses factor (tan δ ) lower than 10−3 as shown in Fig. 13. It proves the good quality of our piezoelectric material. To excite our device, two generators (33250A) and two amplifiers (HAS4011 and FALCO) were used in order to applied, when necessary, two out-of-phase actuation voltages. It corresponds to the +60 V applied to AC1 and AC1-in-phase AL actuators and to the –60 V applied to the AC1-out-of-phase actuators (AL actuators and AC2). Fig. 14 shows the vibrating haptic plate covered with sugar while excited at its resonant frequency. We evidenced the Lamb mode of the plate because the grains of sugar move towards the vibration nodes when the plate is actuated. We measured the focus anti-symmetric lamb mode at 23.02 kHz. The discrepancy with the simulated frequency (6.7 %) can be explained by the inaccuracy of the final dimensions of the plate caused by the sawing of the substrate to obtain individual plates. The harmonic measurement performed on the plate, which confirm this frequency, is reported in Fig. 15. Optical measurements of 110 × 65 mm² AlN-actuated vibrating plates were also performed using laser vibrometry (POLYTEC®). The apparatus uses interferometry to measure the velocity of vibrating
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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Fig. 16. . Comparison of the measured and the simulated parameter = 7.10−8 ) PDA of the 110 × 65 mm² plate under ± 60 V.
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(damping
Fig. 14. . Highlight of the vibration mode shape on the AlN-on-glass plate using sugar.
Fig. 17. . Measurement using the laser vibromètre of the PDA of the 110 × 65 mm² plate regarding the actuation voltage. Fig. 15. . Harmonic measurement on the 110 × 65 mm² plate to determine the resonant frequency of the Lamb mode (+60 V applied to AC1 and AC1-in-phase AL actuators and –60 V applied to the AC1-out-of-phase actuators).
points of the plate surface. In particular, the PDA regarding the actuation configuration was measured and compared to simulation. A micrometric displacement can be obtained using various actuator configurations, as expected. In particular, a PDA of 1.7 μm is measured applying 60 V at AC1 and AC1-in-phase AL actuators and –60 V to AC2 and AC1-out-of-phase AL actuators (all actuators actuated) as shown in Fig. 15. Fig. 16 shows that an acceptable agreement is observed between simulation and measurement (discrepancy ranging from 6 to 20 %). It indicates that the damping parameter used to predict the electromechanical behavior is correct. Using laser vibromètre, we also measured the linear dependence of the PDA function of the actuation voltage as shown in Fig. 17. It proves that a smaller actuation voltage can be used in order to obtain a micrometric PDA. For example, we calculated that only ± 35 V is necessary to obtain a PDA of 1 μm using all the actuators (+35 V at AC1 and AC1-in-phase AL actuators and –35 V to AC2 and AC1-out-of-phase AL actuators). A weak but clearly perceptible haptic feedback effect was felt with the finger on this plate by applying an actuation signal of 60 V at AC1 and AC1-in-phase AL actuators and –60 V to AC2 and AC1-out-of-phase AL actuators, modulated at 10 Hz in amplitude. No haptic sensations were noticed for smaller actuation voltage and thus smaller PDA. This point must be investigated to go further on haptic interfaces based on squeeze-film effect. In particu-
lar, it can indicate that plate displacement amplitude higher than 1 μm is necessary to promote a haptic affect, contrary to our initial specification. Finally, the power consumption of thin-film AlN actuators was studied. It consists in the use of a current probe. A power consumption of 1 W was measured under 60 V on the AC1 or on the AC2 columns whereas 0.5 W was measured on the AC1-In-phase or on the AC1-out-of-phase AL actuators. It indicates that 3 W were necessary to actuate all the actuators under 60 V. 5. Conclusion Transparent haptic interfaces based on AlN-actuators deposited above glass substrate were designed in order to obtain a 110 × 65 mm² plate with a 4-inch transparent area. FEM simulations predict micrometric PDA using +30 V at AC1 and AC1-inphase AL actuators and –30 V to AC2 and AC1-out-of-phase AL actuators. We build haptic plate using a generic AlN technology above glass wafers. Measurement results indicate that we can obtain micrometric PDA under +35 V at AC1 and AC1-in-phase AL actuators and –35 V to AC2 and AC1-out-of-phase AL actuators, thus in good agreement with simulation. Finally, a weak but clearly perceptible haptic feedback effect was felt with the finger when all the actuators were actuated under ± 60 V, whereas no haptic sensations were noticed for smaller actuation voltage. This small haptic sensation is under investigation to make possible the 4-inche transparent plate integration in a thin-film haptic demonstrator in a close future.
Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014
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ACKNOWLEDGEMENTS The authors wish to acknowledge all participants at the MINALOGIC project Touch It and the BPI funding to give us the opportunity to work on this subject. Moreover we would like to acknowledge Karine Saxod from STMicroelectronics for the flexible electrical connector integration on the glass haptic plate. References [1] Hudin C, Lozada J, Hayward V. Localized tactile feedback on a transparent surface through time-reversal wave focusing. IEEE Trans Haptics 2015;8(April-June (2)):188–98. [2] Kim SC, Israr A, Poupyrev I. Tactile rendering of 3D feature on touch surfaces. In: Proceedings of the 26th annual ACM symposium on user interface software and technology; October 2013. [3] Watanabe J, Ishikawa H, Arouette X, Matsumoto Y, Miki N. Artificial tactile feeling displayed by large displacement MEMS actuator array. In: The 25th international conference on micro electro mechanical systems (MEMS); 2012. p. 1129–32. [4] Crommentuijn K, Hermes DJ. The effect of Coulomb friction in a haptic interface on positioning performance. In: Haptics: generating and perceiving tangible sensations. Berlin: Springer; 2010. p. 398–405. [5] Sergeant P, Giraud F, Lemaire-Semail B. Geometrical optimization of an ultrasonic tactile plate. Sensors Actua A 2010;161:91–100. [6] Zheng T, Giraud F, Semail B, Amberg M. Analysis of a new haptic display coupling tactile and kinesthetic feedback to render texture and shape. In: EuroHaptics 2010, Part II. Springer; 2010. p. 87–93. [7] Biet M, Giraud F, Lemaire-Semail B. Squeeze film effect for the design of an ultrasonic tactile plate. IEEE Trans Ultrason Ferroelectr Frequency Control 2007;54(December (12)):2678–88. [8] Wiertlewski M, Colgate JE. Power optimization of ultrasonic friction-modulation tactile interfaces. IEEE Trans Haptics, 2015;8(January-March (1)):43–53.
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Please cite this article as: F. Casset et al., 4-inch transparent plates based on thin-film AlN actuators for haptic applications, Mechatronics (2016), http://dx.doi.org/10.1016/j.mechatronics.2016.05.014