Fabrication and modeling of short cannel OTFTs by self-aligned techniques

Project leaders

Luigi Mariucci, Magalì Estrada Del Cueto

Agreement

MESSICO - CINVESTAV - Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional

Call

CNR-CINVESTAV 2017-2018

Department

Physical sciences and technologies of matter

Thematic area

Physical sciences and technologies of matter

Status of the project

New

Research proposal

Since the discovery of the conduction properties in polymeric materials, the development of electronic devices on their basis has been an important objective, in which direction, intensive research is done
Among organic electronic devices, organic thin film transistors, OTFTs have found commercial applications as Radio Frequency Identifier, RFID tags or as sensors. However, other promising applications, such as high-resolution video displays with integrated row and column drivers, or high-frequency conformable amplifiers, are still pendant due to the relatively low value of TFT mobility, as well as the necessity to improve device stability. This application also requires high operation frequency, in the order of MHz,, which is difficult to achieve with standard organic devices. Other important problems, not yet solve, are the lack of a fabrication technology that can be implemented in industry, as well as the lack of appropriate models that provide means for the design of applications on their basis.
Increase of operation frequency can be achieved by decreasing the OTFT channel length and by reducing the parasitic capacitances with a careful design of the devices. However, the implementation of OTFTs with short channel and low parasitic capacitance results a quite difficult task because of several issues, including the high series resistance at the drain and source contact, the low mobility values of the available organic materials and the technological difficulties related to the fabrication process.
Therefore, it is important from both the scientific results and potential economical application, to develop both the technological process for fabricating short channel OTFTs, able to achieve high operation frequency. Furthermore, in order to design of applications based on these devices, it is mandatory to implement a compact model that includes the specific characteristics that can appear in these devices as their channel length is reduced.
At the Institute of Microelectronics and Microsystems (IMM-CNR), researchers have acquired experience in specific technologies for organic device fabrication on flexible plastic substrates. In particular, they developed fabrication process for organic thin film transistors with evaporated and solution processed organic semiconductor and, more recently, they also developed a process for fully printed OTFTs. Furthermore, they investigated in detail the issue of parasitic contact resistance in OTFTs, achieving a deep understanding of the mechanisms influencing the contact resistance in these devices and their connection with the device structures.
At the Sección de Electrónica del Estado Sólido of Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional (SEES-CINVESTAV-IPN), researchers have achieved experience in developing compact AC and DC models for different types of TFTs. The core model they used for all TFTs is the Unified Model and extraction Method, UMEM, which will be used as stating point in this project.
For the above reasons, we are proposing a joint project CNR-CINVESTAV which main objective is to fabricate short channel organic thin film transistors, OTFTs and to develop a DC and AC compact model that describes efficiently the behavior of these devices. The model will be described in Verilog-A, so it can be included in a commercial simulation program to be used in the design of new applications based on these devices.
IMM-CNR will develop a self-aligned process for OTFTs fabrication on flexible plastic substrate, including the patterning of source drain electrodes by a back exposure technique, the evaporation of high mobility organic semiconductor and spin-coated gate dielectric. In order to reduce the device threshold voltage, several strategies will be tested, as the reduction of dielectric thickness, cross-linking of polymer, hybrid inorganic/polymer dielectric layer with higher permittivity. Simple circuits (inverter and ring oscillators) will be fabricated by using these devices in order to test the operation frequency .
SEES-CINVESTAV will develop the specific compact model required to proceed with circuit design, using devices obtained with this technology, suitable for implementation in circuit simulation software environments. The models will allow for both DC as well as AC analysis: in order to extend the range of validity of the models in the high frequency range, where the short channel devices are likely to be operated in advanced applications. The non quasi static (NQS) regime of the OTFTs will be addressed.
During all the duration of the project, the continuous contact between centers will provide that researchers and students at SEES-CINVESTAV become familiar with the technology developed at IMM-CNR. At the same time, researchers at IMM will become familiar with the model developed at CINVESTAV, representing satisfactorily the devices fabricated at IMM. As a result, designers at both institutions can develop applications using the fabricated OTFTs.
As can be seen from the CVs of the involved researchers, for many years, both institutions have been working in the same field. For this reason, the present joint project gives the possibility of using the previous experience at each of the participating institutions and with their joint efforts, obtain new important results, which due to their complexity will be difficult to achieve only by one of the institutions.

Research goals

The main objectives of the project is the development of a technological process for the fabrication of high frequency organic thin film transistors, on flexible substrate, and the development of a specific DC and AC compact model in order to extend the range of validity of the models in the high frequency range. Organic devices with MHz operation frequencies will be achieved by developing short channel (down to 2 micron) coplanar OTFTs with self-aligned structure, that minimizes the parasitic capacitance. CNR will develop a self-aligned process for OTFTs fabrication on flexible plastic substrate, including the patterning of source drain electrodes by a back exposure technique and the evaporation of high mobility organic semiconductor. Compact models, specifically studied for these devices and suitable for implementation in circuit simulation software environments, will be developed at CINVESTAV. The models will allow both DC as well as AC analysis, considering non quasi static (NQS) regime of the OTFTs. Contact effects and short channel effects (SCE) will be investigated and accounted in the models, also by using 2D numerical simulations. The compact models will be modified and/or extended in order to include the SCE. Organic circuits (inverter and ring oscillators) will be designed and fabricated by using the high frequency devices in order to test the operation frequency. The experimental and simulated results will be compared in order to validate the model.

Last update: 09/06/2025