Chih-Fan Hu

Development of patterned carbon nanotubes on a 3D polymer substrate for the flexible tactile sensor application

This study reports an improved approach to implement a carbon nanotube (CNT)-based flexible tactile sensor, which is integrated with a flexible print circuit (FPC) connector and is capable of detecting normal and shear forces. The merits of the presented tactile sensor by the integration process are as follows: (1) 3D polymer tactile bump structures are naturally formed by the use of an anisotropically etched silicon mold; (2) planar and 3D distributed CNTs are adopted as piezoresistive sensing elements to enable the detection of shear and normal forces; (3) the processes of patterning CNTs and metal routing can be easily batch fabricated on rigid silicon instead of flexible polymer; (4) robust electrical routing is realized using parylene encapsulation to avoid delamination; (5) patterned CNTs, electrical routing and FPC connector are integrated and transferred to a polydimethylsiloxane (PDMS) substrate by a molding process. In application, the CNT-based flexible tactile sensor and its integration with the FPC connector are implemented. Preliminary tests show the feasibility of detecting both normal and shear forces using the presented flexible sensor.

Development of 3D carbon nanotube interdigitated finger electrodes on polymer substrate for flexible capacitive sensor application

This study reports a novel approach to the implementation of 3D carbon nanotube (CNT) interdigitated finger electrodes on flexible polymer, and the detection of strain, bending curvature, tactile force and proximity distance are demonstrated. The merits of the presented CNT-based flexible sensor are as follows: (1) the silicon substrate is patterned to enable the formation of 3D vertically aligned CNTs on the substrate surface; (2) polymer molding on the silicon substrate with 3D CNTs is further employed to transfer the 3D CNTs to the flexible polymer substrate; (3) the CNT-polymer composite (~70 μm in height) is employed to form interdigitated finger electrodes to increase the sensing area and initial capacitance; (4) other structures such as electrical routings, resistors and mechanical supporters are also available using the CNT-polymer composite. The preliminary fabrication results demonstrate a flexible capacitive sensor with 50 μm high CNT interdigitated electrodes on a poly-dimethylsiloxane substrate. The tests show that the typical capacitance change is several dozens of fF and the gauge factor is in the range of 3.44-4.88 for strain and bending curvature measurement; the sensitivity of the tactile sensor is 1.11% N(-1); a proximity distance near 2 mm away from the sensor can be detected.

A novel stress isolation guard ring design for the improvement of three-axis piezoresistive accelerometer

This study designs and implements a stress isolation guard-ring structure to improve the performances of the existing single proof-mass 3-axis piezoresistive accelerometer. Thus, the environment disturbances (temperature variation and force/deflection transmittance) for such packaged 3-axis piezoresistive accelerometer are significantly reduced. Measurements demonstrate the guard-ring design successfully reduce the false signals (induced by temperature variation and force/displacement transmittance) for one order of magnitude. Moreover, the proposed accelerometer still maintains the advantages of existing design, such as single proof-mass for 3-axis acceleration sensing, and better linearity. Sensitivities of present accelerometer range 0.127∼1.77mV/G/V and non-linearity<1.02%VFSS.

Development of a 3D distributed carbon nanotubes on flexible polymer for normal and shear forces measurement

This study reports a simple approach to implement a flexible CNTs tactile sensor to enable the detecting of normal and shear forces. The merits of the presented tactile sensors are as follows, (1) embedded patterned CNTs into polymer using simple Si-substrate molding process, (2) 3D distributed CNTs enable the detection of shear and normal forces, and (3) 3D polymer structure by molding as a tactile-bump. In applications, the CNTs are grown and patterned on bulk-micromachined Si substrate with 3D surface profile. After polymer molding, the 3D distributed CNTs are successfully transferred onto a flexible PDMS with 3D tactile-bump. With proper CNTs pattern designs, the tactile sensor has a sensitivity (linearity) of up to 20.95%/N (R2=0.93) for normal load, and up to 95.24%/N (R2=0.95) for shear load.

Development of 3D CNTS interdigitated finger electrodes on flexible polymer for bending strain measurement

This study reports a novel approach to implement the CNTs (carbon nanotubes)-based flexible capacitive strain sensor and the detection of bending strain is demonstrated. The merits of presented design are as follows, (1) employ the aligned CNTs (50µm∼100µm in height) as interdigital finger electrodes to increase sensing area and initial capacitance, (2) the 3D CNTs electrodes and electrical routings, and flexible polymer substrate are batch fabricated on Si substrate and polymer molding, and (3) the CNTs-polymer composites as sensing electrodes and electrical routing can avoid the problem of delamination. Preliminary fabrication results demonstrate a flexible capacitive sensor with 50 µm high CNTs interdigital electrodes on PDMS, the tests show its typical capacitance change is several dozens of fF and the gauge factor ranges 4.6∼6.5.

The integration of CNTs-based flexible tactile sensor and flexible circuit by polymer molding process

This study reports an improved approach to implement a carbon nanotubes (CNTs)-based flexible tactile sensor, which is integrated with flexible print circuit (FPC) and is capable of the detecting of normal and shear forces. The merits of the presented integration process are as follows, (1) patterned CNTs, electrical routing, and FPC are integrated and transferred to PDMS substrate by molding process; (2) more robust electrical routing is realized using the parylene encapsulation to avoid delamination; (3) the process can be easily batch fabricated on rigid silicon instead of flexible polymer. In application, the CNTs-based flexible tactile sensor and its integration with FPC are implemented. Preliminary tests show the feasibility of detecting both normal and shear forces using the presented flexible sensor.

Study and characterization of plastic encapsulated package for a three-axis piezoresistive acceleromete with guard-ring structure

This study reports the performance of a three-axis piezoresisitvie accelerometer with a guard-ring structure after plastic packaging. The accelerometers, with and without guard-ring structure, are capped with glass substrate to form the glass/Si/glass sandwich and then encapsulated in plastic package. The testing results on these packaged accelerometers have shown that the guard-ring structure successfully suppresses the performance shift caused by plastic package for one order of magnitude. The glass/Si/glass sandwich structure adds air damping to the accelerometer, hence its resonant frequency is slightly reduced (less than 0.5%). Moreover, the guard-ring structure has relatively large stiffness and is considered as a rigid body, and its influence on the sensor dynamics can be ignored. In conclusion, the guard ring structure significantly reduces the performances variation of packaged sensor. Thus, the inexpensive plastic encapsulated package for accelerometers can be implemented on the real products.

Piezoresistive pressure sensor with Ladder shape design of piezoresistor

In this study, a novel piezoresistor (PZR) design to improve the sensitivity of the piezoresistive type pressure sensor is reported. In this design, the PZR combined with various doping concentrations and doping depths is proposed and implemented. According to the design concept, the combined PZR which includes multiple piezoresistance coefficients would be arranged in a unique shape, just like a ladder. Theoretically, the sensitivity improvement could boost up to 15% based on this PZR design, even under the conditions of membrane edge offset due to fabrication processes. As a result, the sensitivity of the pressure sensor which adopted the Ladder shape PZR design is 0.058 mV/V/kPa. Moreover, the doping profile of the Ladder shape PZR is also extracted by SIMS. The serial measurements successfully prove the feasibility of this design concept. In the future, the presented PZR design can be further extended to various piezoresistive sensors, such as accelerometer, gyro, etc.

A flexible, highly-sensitive, and easily-fabricated carbon-nanotubes tactile sensor on polymer substrate

A flexible, highly-sensitive, and easily fabricated carbon nanotubes (CNTs) tactile sensor is reported in this paper. CNTs are grown and patterned on bulk-micromachined silicon substrate with 3-dimensional surface profile. After polymer molding, the CNTs with 3-dimensional distribution are successfully transferred onto a flexible PDMS with 3-dimensional tactile-bump. Advantages of presented tactile sensor are (1) exactly determine loading force by resistance change due to almost linearly current-voltage (I–V) characteristics, (2) embedded patterned CNTs into polymer using simple silicon-substrate molding process, (3) 3-dimensional distributed CNTs enable the detection of shear and normal forces, and (4) 3-dimensional polymer structure by molding as a tactile-bump. One of anisotropic-type patterned CNT approach behaves good sensing sensitivity of both normal (23%/N) and shear (18%/N) forces loading. With proper CNTs designs, tactile sensor has normal and shear forces sensitivities of up to 23%/N and 95%/N, respectively.

Method for performance improvement and size shrinkage of a three-axis piezoresistive accelerometer with guard-ring structure

A stress isolation guard-ring to reduce the unwanted signals induced by the environmental disturbances for a packaged three-axis piezoresistive accelerometer is proposed [1-2]. This study further reports an optimum design to shrink the size of guard-ring, yet maintain the performance of accelerometer. The commercial finite element analysis (FEA) software, CoventorWare, is employed to evaluate the candidate designs of sensors. The Taguchis optimum design method is further employed to improve the sensor performances under environment disturbances and to shrink the size of the stress isolation structure. Based on the results, the performance of the accelerometer is improved both in offset shift and sensitivity shift for one order of magnitude, and the size of the stress isolation structure (the sum of guard-ring length/width and connection bridge length) has been shrunk for 29% (from 138 μm to 98 μm). Moreover, the unwanted higher vibration modes are far away from the first three modes. The proposed accelerometer design keeps the advantages of the original three-axis accelerometer design.