Thin-Film Technology for Large-Area / CMOS Hybrid Systems
Authors: Wagner Sigurd , Moy Tiffany , Sanz-Robinson Josue , Rieutort-Louis Warren , Afsar Yasmin , Hu Yingzhe , Huang Liechao , Sturm James , Verma Naveen 
Published: February 2020

1. Hybrid large-area thin-film / CMOS systems with wireless power transfer

We have been developing demonstrations of large-area electronic systems that combine thin-film components and circuits with CMOS. While only thin-film technology can cover large areas economically at a high device density, it lacks the speed needed for computation and communication. Thin-film technology also can exhibit substantial device-to-device variations. CMOS, on the other hand, has the speed required for control, computation and communication but cannot be applied economically as sensor arrays over large surfaces areas.
The multitude of sensor applications envisaged for large-area electronics draws from diverse materials technologies, which are difficult to integrate monolithically. Therefore we fabricate subsystem sheets and integrate them to complete systems by lamination.
In this presentation we discuss requirements that passive and active film components must meet for this hybrid technology, in particular wireless contacts, thin-film transistors for oscillator circuits, and thin-film blocking diodes and rectifiers. Also, we will show how thin-film circuits can be designed, and signals from thin-film sensors processed in the thin-film domain, to overcome drawbacks of thin-film transistors, such as noise and device-to-device variability.

2. Passive backplane, inductive and capacitive coupling

We fabricate subsystems on separate substrates, which are then laminated. Interconnects are made by inductive or capacitive coupling, instead of via-type metallurgy. Such wireless interconnects are robust against variations in sheet and adhesive thickness. In addition, inductive interfaces enable voltage step-up from low-voltage CMOS to higher-voltage a-Si TFTs. Capacitive interfaces have low parasitic power consumption and high power efficiency at low frequency.
The large surfaces that are available offer the freedom of designing inductors and capacitors for optimal circuit performance. The typically largest-area substrate is a passive backplane that also carries in-plane conductor interconnects.

3. Thin-film LC oscillator for inductive power transfer

For power transfer from a solar power harvesting subsystem to a load on another sheet the DC power from the solar cell is converted to AC. The need to obtain positive feedback for meeting the oscillation condition identifies the importance of the parasitic capacitance CPar, which originates in the gate/source-drain overlap capacitances of the TFTs; reducing CPar is not practical when fabricating LAE on plastic substrates. To compensate the effect of CPar the TFT gate resistance RGate can be reduced effectively by sandwiching Al in a Cr/Al/Cr tri-layer gate electrode.
Because a cross-coupled oscillator circuit resonates with its CPar, its frequency can exceed the TFTs’ cutoff frequency fT . This is important, as the fT of a-Si TFTs is ~ 1 MHz, while the inductive power transfer efficiency reaches its maximum at several MHz.

4. Thin-film rectifier

Besides diode-connected TFTs, we use Cr/a-Si and Cr/a-Si/n+Si Schottky diodes. In both devices the barrier is formed at the Cr/a-Si contact; the n+ layer is introduced to minimize the series resistance of the diode. Because of its low reverse leakage, the Cr/a-Si Schottky is preferred as the blocking diode that prevents the thin-film battery (not shown) from discharging. On the other hand, power-efficient demodulation in the LAE domain, of ac signals received from the CMOS domain, relies on Cr/a-Si/n+Si Schottky diodes. Given their higher current density these can be made smaller, leading to a lower capacitance, and a lower forward voltage drop, than Cr/a-Si Schottkys.

5. Thin-film transistors

For the majority of applications the transconductance gm, hence the field-effect mobility of the charge carrier, is the decisive measure of TFT performance. It is worth noting, though, that other parameters may also be of importance. In an image detection system based on classifiers, TFT variability rather than gm is found to be the more important parameter for system performance. Flicker noise in the 1 Hz to 10 kHz range can bury signals generated by body-worn sensors.

6. Acknowledgements

Funding provided by Systems on Nanoscale Information fabriCs (SONIC) sponsored by MARCO & DARPA, and NSF grants ECCS-1202168 and CCF-1218206.