The MADRAS project focuses on developing a highly secure, high resolution, mechanically stable fingerprint readers for user authentication that will be integrated into the hard plastics of scooters. These biometric sensors in shared scooters are expected to increase the user convenience for unlocking, as well as the security level. Furthermore, it is expected to lead toa decreased amount of vandalism by the renter, which is an important factor impacting negatively the social mobility companies, both financially and environmentally.
In MADRAS, TNO – Holst Centre applies its years of expertise in image sensor technology in developing, adapting and improving the following:
- IGZO based Thin Film Transistor (TFT) backplane
- High-performance thin-film encapsulation (TFE)
- Thermoforming methods of plastic processing
- Characterization of the OPD devices with materials developed in MADRAS project
- Integration and readout of the fingerprint sensors using state-of-the-art readout integrated circuit (ROIC)
Figure 1. High resolution, high contrast & high speed images captured by a 200ppi resolution image sensor
Dual-Gate Self-Aligned IGZO TFTs monolithically integrated with high-temperature bottom moisture barrier
Amorphous InGaZnO (a-IGZO) is a well-known n-type oxide semiconductor composed of In2O3, Ga2O3 and ZnO. a-IGZO TFTs are of great interest for flexible imaging and display applications. Within the self-Aligned (SA) TFT architecture, both the gate electrode and gate insulator are patterned in a single lithography mask step, resulting a better definition of the TFT channel. Next, the gate electrode is used as a mask for doping of the a-IGZO region connected to the source and drain upon inter-metal dielectric deposition. The SA-TFT architecture has demonstrated to yield superior performance. The pursuit of even higher performance has led to the implementation of the dual-gate TFT architecture. A dual-gate transistor consists of a single bottom-gate field-effect transistor with an additional second top-gate and second dielectric, enabling,
Figure 2. Cross-section of the realized dual-gate self-aligned TFT monolithically integrated on bottom
- higher current drive
- steeper sub-threshold slope
- better determined VON close to Vg = 0V
- tuning of the TFT onset voltage
In MADRAS, TNO-Holst Centre designs and builds large-area flexible TFT backplanes on a ultra-thin polyimide plastic film, protected by a bottom moisture barrier stack that can withstand higher processing temperatures of up to 350°C. The resulting high resolution 500ppi (50×50 µm pixels) TFT fingerprint reader has been newly designed enabling thermoforming processes and simple routing of the electronics inside the scooter through an integrated injected moulded part.
High-performance thin-film encapsulation (TFE)
Figure 3. Schematic of a fingerprint reader stack with thin film encapsulation
TNO-Holst Centre has developed previously a high-performance inorganic based thin-film encapsulation (TFE) to protect organic light-emitting diodes (OLED) devices from degradation in ambient conditions. The organic photodetector (OPD) used in the MADRAS fingerprint sensor has to be protected in a similar way.
This hybrid TFE stack consists of two inorganic barrier layers of silicon nitride deposited at low temperature with an organic layer in between. In MADRAS, TNO-Holst Centre evaluates the TFE stack, with a water vapor transmission rate (WVTR) << 10-6 g/m2/day, aiming for a device stability of >300 hours under relevant environmental conditions (temp. and RH). The TFE process compatibility with OPD materials and transport layers developed in the MADRAS project is being assessed.
About the authors
Holst Centre is an independent research and innovation centre, was founded in 2005, jointly operated by imec and TNO. TNO-Holst Centre’s clients benefit from its valuable expertise in sensor technologies and flexible electronics and its ability to develop demonstrators and prototypes. Building on this proof-of-concept, they turn Holst Centre’s technologies into new products and new manufacturing processes. Employing over 180 specialists from 28 nations, Holst Centre develop technology that responds to the global societal challenges of tomorrow and contributes to a healthier and more sustainable world.
Santhosh Shanmugam, Senior Process Architect and Project Lead at TNO-Holst Centre since 2012:
- Tech in Electronics and Communication Engineering from SRM University, India
- MSc in Microtechnology from Chalmers University of Technology, Sweden
- Several years of R&D experience in varied thin-film semiconductor device applications, including light-absorbing and light-sensing devices
- Expertise in architecting processes and leading frontplane fabrication for flexible electronics
Suzanne de Winter, Senior Process Engineer and Project Leader at TNO-Holst Centre since 2007:
- BSc in Chemistry at Fontys University of Applied Science in Eindhoven, the Netherlands
- Process engineer in chemistry and thin film depositions
- Project manager the field of OLED, thin-film barriers and TFT
