All-Printable Real-time Airframe Monitoring System (ARAMS) S. Joshi 1, S. Bland 1, R. DeMott 1, N. Anderson 2, G. Jursich 2, Y. Zhang 3, D. Gamota 4 1 NextGen Aeronautics, Inc., 2 University of Illinois at Chicago, 3 University of Southern California, 4 Jabil
Outline of Talk Printed Transistors for multiplexing Strain Sensor Arrays Device Integration: Hybrid Approach
Motivation Large area strain mapping of aerospace vehicles is key to enhancing reliability and safety Technology is sought for continuous realtime diagnosis and prognosis of complex structural components during operation The monitoring system should be flexible to conform to curved surfaces of critical regions of the structure Low-cost manufacturing is required NextGen/UIUC FLXSS Array
PRINTED TRANSISTORS
Challenges in Printing of Thin Film Transistors (TFTs) Printing requirements for optimal TFTs hindered by state-of-the-art screen printing capabilities in terms of resolution, thickness, smoothness, and repeatability Ink formulations require variety of active components (metal, semiconductor, insulator) with good uniformity and fluid properties well-tuned for the specific printing process Multi-layer printing requires sharp layer-layer interface without mixing of adjacent layers Typically, printed inks leave particle-polymer binder films which can reduce electrical performance
Transistor Development: Printing PET Substrate NanoIntegris IsoSol-S100 S-SWCNT Solution Applied Ink Solutions Ag-530 Silver Ink, and BT-101 Dielectric Ink
Transistor Development: Printed TFTs CNT percolating network Printed TFTs CNT channel width ~ 100 μm Thickness Gate ~ 5.7 μm BTO ~ 10 μm S/D ~ 4.2 μm Top View of a TFT Bottom View of a TFT
Reducing Short Circuits Roughness of conductive layers and protruding particles can lead to short circuits through insulating layers Mechanical flattening compresses these conductive layers, reducing the frequency of short circuits Transistor Cross-sectional Image Transistor Cross-sectional Image Gate Dielectric Source/ Drain Unflattened Source/Drain Layer Mechanically Flattened Source/Drain Layer
Transistor Development: I-V Curves Alumina Coated PET Uncoated PET 1E-4 TG-TFT - 100 µm Channel 1E-9 TG-TGT - 150 µm Channel Length V DS = -1V V DS = +1V V DS = Gate 1E-5 1E-6 1E-10 I D [Amps] 1E-7 1E-8 I D [Amps] 1E-11 1E-9 V DS = -1V 1E-12 1E-10 V DS = +1V V DS = Gate 1E-11-20 -10 0 10 20 1E-13-20 -10 0 10 20 V G [Volts] V G [Volts]
Transistor Development: I-V Curves Untreated PET 1E-7 Plasma Treated PET 1E-9 1E-10 V DS = +1V V DS = Gate 1E-8 1E-9 V DS = +1V V DS = Gate I D [Amps] 1E-11 1E-12 I D [Amps] 1E-10 1E-11 1E-12 1E-13 1E-13 1E-14-25 -20-15 -10-5 0 5 10 15 20 25 V G [Volts] 1E-14-25 -20-15 -10-5 0 5 10 15 20 25 V G [Volts]
Summary of Transistor Development In general, we found dimension requirements of printing were challenging and required evolving optimization of the printing process. Commercial inks required dilution to reduce film thickness but excess dilution degraded resolution Large particles (of the size of film thickness) in conductive inks can lead to shorting. This effect has been reduced by mechanical rolling of the printed film. Substrate strongly influences transistor performance as seen in effect of Al 2 O 3 coating and plasma surface treatment of PET Printed TG-TFTs still lacking sufficient on-current and reliability. Further research on this is continuing
Use Discrete Transistors Abdinia et. al (2013) mentioned that current OTFTs have low yield, and a high rate of hard and soft faults (unusable devices and low-performance devices) limits designs that use only ~100 transistors are viable (the 4x4 design uses 192 transistors) A 2016 OTFT article using CNTs [Cao et. al], produced ~45 TFTs with On/off ratios ranging from ~10 2 10 5 using nearly identical methods we re using. These OTFTs were printed over an area of 5x5 cm (~2x2 in) We are currently using discrete transistors to develop multiplexed strain sensor arrays
STRAIN SENSORS ARRAYS
4x4 Sensor Arrays: Strain Sensitivity 0.06 R/R 0 vs Microstrain R/R 0 0.04 0.02 0.00 GF: 18.8 ± 0.7 0 1000 2000 3000 Microstrain Four separate gauges Materials DuPont Carbon 7102 Gauge Factor 18.8 ± 0.7 Constantan (Cu-Ni) 2 Linear response to strain Gauge factors of ~19 using conductive carbon ink
R/R 0 0.08 0.06 0.04 0.02 4x4 Sensor Arrays: Temperature Linear Portion Sensitivity Temp. Coeff. Of Resistance Materials DuPont Carbon 7102 (25-85 o C) αα = RR RR 0 TT Temperature coefficient of resistance (10-3 K -1 ) 0.26 Silver 3.8 0.00 20 40 60 80 100 120 140 Temperature ( o C) Copper 3.9 Gold 3.4 Aluminum 3.9 Constantan (Cu-Ni) 8 x 10-3
4x4 Sensor Arrays: Ink Adhesion Carbon Ink Silver Ink Dielectric Ink Carbon Ink Polyimide Silver Ink Polyimide Dielectric Ink Silver Ink Polyimide Adhesion is essential for devices which undergo strain No de-bonding/gaps between polyimide substrate and inks No de-bonding/gaps between inks
4x4 Sensor Arrays: Repeatability of Sensor Resistances Bad print (2-1) Good print (14-1) a b Average Resistance [kω] Standard Deviation [%] Deviation of Minimum from the Mean [%] Deviation of the Maximum from the Mean [%] a 8.11 20.52 37.66 56.66 b 13.61 10.07 18.88 21.85 Resistance tolerance of ~±20% achieved using a manual process and a ~7 x10 pattern
DEVICE INTEGRATION: HYBRID APPROACH
4x4 Sensor Arrays: Printing T Polyimide substrate DuPont 7102C Carbon Ink Applied Ink Solutions Ag-530 Silver Ink, and UV-2530 Dielectric Ink
Dummy Gauge 4x4 Sensor Arrays: Hybrid Device Discrete Transistors DAQ Connector 45 Rosette
4x4 Sensor Arrays: DAQ Designed to control and measure a 4x4 array of printed strain gauge rosettes Can accommodate a wide range of gauge resistances and transistor types Programmable voltage and current source capability for strain gauge excitation Supported current range: 6.25uA to 35.2mA Supported voltage range: 0 to 30V Programmable gate drive voltage for multiplexing transistors Gate voltage range: 0 to 30V; supports both n-type and p-type Programmable fully differential amplifiers for strain gauge signal conditioning TI PGA280: supports gains ranging from 0.125 to 176 Precision, low-drift, 24-bit analog-to-digital converters for gauge resistance measurements TI ADS1259: 0.4ppm INL, 2ppm/*C Reference Drift, 0.5ppm/*C Gain Drift USB interface for real-time data collection
Strain Measurement 4x4 Rosettes (64 gages) The sample is loaded to achieve strains of 2000με on R1C1 gage - Cycled three times and the response strain gage was recorded Change in resistance of the gage was roughly linear - Larger variations in response as the strain increased - Believed to be due to random noise which is present primarily due to the differential measurement approach Loading frame 4x4 sensor array Aluminum Test Plate DAQ systems
Overall Summary A large area strain sensing application utilizing screen printing technologies discussed to highlight technical challenges and interim solutions The variation in characteristics of printed sensors is large, especially in manual screen printing some of which can be reduced by automation Measurement approach that relaxes manufacturing tolerance requirements, allowing a wide range of gage resistance changes is being pursued Inks used for printing strain gages are sensitive to temperature In order to compensate for it, an additional strain sensor is used in the rosette configuration, which is decoupled from strain Screen printed transistors are not currently reliable use discrete transistors Discrete multiplexing electronics created a new challenge related to bonding with screen printed traces as well as flexible substrate Our team continues to understand and solve these technical challenges
Acknowledgements Office of Naval Research: Contract # N00014-14-C-0028 Professor Steve Nutt, M.C. Gill Chair in Composite Materials, USC UIC Nanotechnology Core Facility: Dr. Antonio DiVenere and Dr. Seyoung An UIC Micro/Nanoscale Fluid Transport Laboratory: Professor C.M. Megaridis UIC Advanced Material Research Group: Professor Christos Takoudis