Low stiffness tactile transducers based on AlN thin film and polyimide

Abstract

In this paper, we propose a flexible piezoelectric MEMS transducer based on aluminum nitride thin film grown on polyimide soft substrate and developed for tactile sensing purposes. The proposed device consists of circular micro-cells, with a radius of 350 lm, made of polycrystalline c-axis textured AlN. The release of compressive stress by crystalline layers over polymer substrate allows an enhanced transduction response when the cell is patterned in circular dome-shaped geometries. The fabricated cells show an electromechanical response within the full scale range of 80 mN ('200 kPa) both for dynamic and static load. The device is able to detect dynamic forces by exploiting both piezoelectric and flexoelectric capabilities of the aluminum nitride cells in a combined and synergistic sensing that occurs as voltage generation. No additional power supply is required to provide the electrical readout signals, making this technology suitable candidate when low power consumption is demanding. Moreover a capacitance variation under constant stress is observed, allowing the detection of static forces. The sensing ability of the AlN-based cells has been tested using an ad hoc setup, measuring both the applied load and the generated voltage and capacitance variation. V C 2015 AIP Publishing LLC. [http://dx.Artificial tactile systems for pressure and force measurements are important in many applications as automated assembling, machining, sorting, and stacking production; in minimal invasive medical procedures (MIS) for the evaluation of tissues stiffness and damages; in prosthetic and orthotic devices where artificial skin restores loss of tactile sensations. Similarly, humanoid robots need tactile interface devices for a safe interaction with humans in assisting activities. Equipping a robot with specific sensors and transducers is a way to confer him sufficient autonomy to perform tasks in unstructured environments. Unlike other senses, tactile sensory system provides information on physical properties of different nature such as shape, texture, and stiffness (to control the manipulation with optimal forces), which is not easily achieved by vision alone. Also during grasping and manipulation of objects, the right tactile perception prevents slippage and damage (ensuring the manual dexterity in a robotic system). An artificial tactile system that emulates these abilities and performs advanced in-hand manipulation tasks should detect dynamic forces, such as normal and tangential contact for gathering spatial and geometrical information from surfaces exploration, as well as static forces. To this aim, arrays of pressure-sensitive sensors should be integrated on the robot fingertips as an electronic skin and flexibility and stretchability are desirable properties to increase the anthropomorphism.In the last decades, several technological approaches have been used to realize a low-cost, large-area-compatible artificial skin, suitable for object manipulation, and with sufficient sensitivity in a wide pressure range (10-100 kPa...

Review—Investigation and Review of the Thermal, Mechanical, Electrical, Optical, and Structural Properties of Atomic Layer Deposited High-kDielectrics: Beryllium Oxide, Aluminum Oxide, Hafnium Oxide, and Aluminum Nitride

“…125,126 We do note that the above mentioned film thicknesses are significantly higher than those typically utilized in traditional Si CMOS high-k dielectric applications where thicknesses of <10 nm are more common. [2][3][4][5][6][7][8] However, many of the applications involving these materials as diffusion barriers, 27,31,127 nano-resonators, 80,81 and piezoelectric transducers 43,87 can require significantly higher thicknesses of 20-1000 nm. Also, use of film thicknesses >100 nm minimize substrate 128 and interfacial thermal boundary resistance 129 effects that can complicate the mechanical and thermal property measurements, respectively.…”

“…85,86 The piezoelectric properties of AlN 87 have additionally made it of interest as a transducer material in surface acoustic wave and NEM devices. 43,88 The high resistance of AlN to fluorinated plasmas 89 has further lead to its use as a plasma etch stop, 33 hard mask, 90,91 and Cu capping layer 33 in microelectronic device applications.…”

“…[39][40][41] The mechanical properties of high-k dielectrics have also become a key consideration for the implementation of these materials in various nanoelectromechanical (NEM) 42,43 and flexible micro-/nanoelectronic devices. 44,45 For such devices, knowledge of properties such as Young's modulus and film stress are critical for predicting the flexure and resonance frequencies of bridged and cantilevered switches and sensors, [46][47][48][49] stretched transistor device performance, 50,51 buckling failures in nanopatterned structures, 52,53 and macroscale buckling and nanoscale wrinkling effects for stiff high-k films deposited on compliant polymeric substrates.…”

Nitride-Based Materials for Flexible MEMS Tactile and Flow Sensors in Robotics

“…For dome fabrication, a 25 $\mathrm{sans-serif\mu }\mathrm{normalm}$-thick polyimide layer is laminated on a silicon substrate, and subsequently, a sputtering growth process follows with the deposition of 120 $\mathrm{normaln}\mathrm{normalm}$-thick Mo followed by an 800–1000 $\mathrm{normaln}\mathrm{normalm}$-thick AlN layer. In an analogous manner, AlN/Mo-based multilayers grown on a flexible polyimide substrate release tensile stress by an upward deflection reaching approximately 40 $\mathrm{sans-serif\mu }\mathrm{normalm}$ for a diameter of 350 $\mathrm{sans-serif\mu }\mathrm{normalm}$ [27]. Consequently, structural elements based on AlN/Mo or Mo/AlN/Mo multilayers can be used for bent structures, such as beams and membranes [26,27,28].…”

“…In an analogous manner, AlN/Mo-based multilayers grown on a flexible polyimide substrate release tensile stress by an upward deflection reaching approximately 40 $\mathrm{sans-serif\mu }\mathrm{normalm}$ for a diameter of 350 $\mathrm{sans-serif\mu }\mathrm{normalm}$ [27]. Consequently, structural elements based on AlN/Mo or Mo/AlN/Mo multilayers can be used for bent structures, such as beams and membranes [26,27,28]. …”