The OMI-based mechanical deformation model (OMI-MD) extends standard SPICE. TFT models with the extra instance and model parameters defining ...
Mechanical Deformation-Aware TFT Modeling for Highly Flexible Wearable Electronics Design Slobodan Mijalković*, Ahmed Nejim*, Pedro Barquinha** and Ian Scott* *Silvaco Europe, Cambridge, UK / **CENIMAT, I3N, FCT-UNL, Caparica, Portugal
Introduction Mechanical strain induces undesirable variations in electrical responses of the flexible TFT circuits and could potentially disturb the intended functionality of target applications [1,2,3,4].
Mechanical Deformation Effects on TFT Device Geometry ➢ Characteristic device dimensions (channel length and width, gate insulator thickness). ➢ Shape of thin films layers (stretching, bending, twisting, wrinkling)
Modeling of TFT Parameter Variations
Modeling for Technology Design Mechanical Deformation Effects on TFT Concave and convex bending along the channel
Structure oscillation (wrinkling)
Electronic Properties ➢ Carrier mobility ➢ Density of States (DOS) distributions (band-gap, tail and deep states) ➢ Interface states density ➢ Piezoresistivity and piezoelectricity.
The externally applied plane strain to the flexible TFT substrate is generally defined as a plain strain tensor
The variations of a TFT model parameter P with the externally applied strain, is controlled by a polynomial meta-model:
where P0 is a reference value of the parameter P, obtained for the device in its natural state, without externally applied strain, and M is the order of the polynomial approximation. The coefficients a ijk of the polynomial meta-model can be evaluated by a linear least-squares regression analysis from the parameter P values obtained from device characterization under different externally applied strain conditions.
Mechanical Deformation - Aware Modeling for Circuit Design
The linear (M=1) and quadratic (M=2) metamodels are typically sufficient to accurately describe weakly nonlinear SPICE TFT model parameter dependence on the externally applied strain. The resulting polynomial meta-model may not include all the input strain components and the corresponding polynomial coefficients.
The main objective of this project is to develop new modeling methodologies and design tools extensions to support mechanical deformation-aware design of highly flexible wearable electronics.
Motivating Technology
Implementation in Circuit Simulators
The research team at CENIMAT has been developing a process for IGZO based devices on thin PEN strips in order to integrate into advanced weaved fibres. These devices will be at the heart of logic circuits that drive a myriad of fiber based smart applications including sensors and woven active matrix displays.The TFT circuits on e-fibers should sustain extremely small bending radii during weaving process and during smart-textile handling.
An effective and generic way to implement the metamodel for the variation of TFT model parameters with externally applied strain in circuit simulators is to employ the Open Model Interface (OMI). It is a new standard released recently by Si2 Compact Model Coalition (CMC) [8].
Schematic layout of IGZO TFT transistors on PEN strip printed on 1mm wide highly flexible tapes.
SmartSpice Standard Models: ● RPI a-Si ● RPI p-Si ● UOTFT ● MOTFT ● ...
OMI Custom Models: ● Aging ● Statistical Variations ● Mechanical Deformation (MD) ● ...
When implemented in a circuit simulator, OMI could be used as an environment to build add-on wrappers around the standard SPICE TFT models. It allows to enhance standard SPICE models with model parameter dependencies on layout, process variation, aging effects and, as proposed here, the effects of externally applied mechanical strain. The pictures of the first batch of fabricated IGZO TFTs.
Using numerical transformation, it is possible to create sinusoidal deformation of the planar TFT structures based on the IGZO material. Thin film transistor (TFT) structures based on the IGZO material were simulated using Technology Computer Aided Design (TCAD) software. This is based on the drift diffusion models with a definition of complex defect density of states (DOS) in the band gap. Field mediated mobility as well as contact work functions were invoked to simulate charge injection and collection from the contacts. Crucially novel meshing has been developed to enable the bending of the structure. Here we present the transfer curve data for the same channel lengths but different bending radii.
DC characteristics of the sputtered TFT device: (a) output characteristics, (b) transfer characteristics at Vg=5V (Initial measurements reveal hysteresis in the characteristics).
Conclusions ➢ This project paves the way for fibre based smart wearable electronics. ➢ Form factor and high substrate flexibility is key to this technology. ➢ Advanced 3D TCAD software and mechanical deformation-aware SPICE models is being developed in this project. ➢ 3D TCAD tools are needed to account for geometrical modifications effects due to fiber weaving and its subsequent usage. ➢ The extended SPICE TFT models will permit, for the first time, simulation and design of flexible TFT circuits with dynamic, layout-aware mechanical deformation input.
A wrinkled thin film geometry created by the buckling process [3] significantly enhances the TFT stretchability.
in the local coordinate system aligned with the TFT channel length and width directions.
The TFT devices and circuits in forthcoming wearable electronics applications are required to sustain significant mechanical deformation (e.g. bending radii below 50 µm while stretched to more than 5% of its nominal dimensions). The success of future wearable electronics applications strongly depends upon availability of mechanical deformation-aware modeling for predictive technology and circuit design.
Technology Design
In order to bypass the complexity of the physical relationship between the externally applied strain intensity and the corresponding shifts in the TFT characteristics, a metamodeling approach, based on multivariate polynomials is used to control TFT parameter variations with applied external mechanical strain.
Metamodel Generation
There appears to be a significant geometrical effect on the drain current in the small radius of curvature structures.
The OMI-based mechanical deformation model (OMI-MD) extends standard SPICE TFT models with the extra instance and model parameters defining externally applied strain along TFT channel length and width. The implementation of the polynomial metamodel (OMI-MD) does not affect the original SPICE TFT model formulation. implementation.
The meshing engine captures these deformations with minimal obtuse elements to ensure convergence of the device simulation calculations. Such wrinkling takes place in stretched printed substrates for the wearable electronics technology. Taking into account the geometrical effect of the field across the channel, there appears to be a significant degradation of the current between the planar and the three oscillation structure.
Customized mechanical deformation aware SPICE TFT models can be also used for coupled electro-mechanical simulations.
Careful consideration were taken to ensure the same channel lengths were used for the comparison.
References 1. P. Heremans, et al., “Mechanical and Electronic Properties of Thin-Film Transistors on Plastic, and Their Integration in Flexible Electronic Applications,” Advanced Materials, 28, 4266–4282 (2016). 2. C. Zysset, N. Münzenrieder, T. Kinkeldei, K. Cherenack, G. Tröster, “Woven Active-Matrix Display”, IEEE Trans. Electron Devices, 59, 721–727 (2012). 3. G. Cantarella, et al., “Buckled Thin-Film Transistors and Circuits on Soft Elastomers for Stretchable Electronics,” ACS Appl. Mater. Interfaces, 9, 28750−28757 (2017). 4. R. Amalraj, S. Sambandan, ”Influence of curvature on the device physics of thin film transistors on flexible substrates”, Journal of Applied Physics, 116, 164507 (2014). 5. M. Asadirad, et al., “Numerical Simulation for Operation of Flexible Thin-Film Transistors With Bending”, IEEE Electron Device Letters, 38, 217-220 (2017). 6. S. Mijalković, “MOS Compact Modelling for Flexible Electronics”, Chapter 21 in Ultra-thin Chip Technology and Applications, Springer, 259-270 (2011). 7. G. Cantarella, et al., “Flexible In–Ga–Zn–O Thin-Film Transistors on Elastomeric Substrate Bent to 2.3% Strain,” IEEE Electron Device Letters, 36, pp. 781-783 (2015). 8. “Open Model Interface (OMI) Version 1.0.0,” Technical Report, OMI API Working Group, Si2 Compact Model Coalition (2018).
Acknowledgements This work is partially funded by the EU H2020-NMP-22-2015 1D-NEON project. This enabled the software development effort and the valuable collaboration to access the all important measurement data.
An example of generating a first order metamodel. The red squares annotate available measured values [7] for the TFT parameters: (a) threshold voltage, (b) band mobility and (c) subthreshold slope (c). The re-flattening of the wrinkles in either the channel length or the channel width direction requires ~2.3% of externally applied tensile strain.
