Silicon based Flexible electronics – e-FlexSi

Fore more than 20 years, Silicon Large Area Electronics (LAE) on glass has developed an important background. Development was mainly pushed by its application in flat panel displays. Flat panel market covers now nearly all the display needs so that LAE takes an important part of the market of electronics. The main developments and production fabrications are now mainly concentrated in far-east Asia. However, first developments were conducted in Europe. Indeed, Europe maintained a leadership during a long time. As a consequence, European Laboratories have a great experience in this field, and some of them still maintain it now. Particularly, partners of the present project, LETI (CEA Grenoble), PICM (Ecole Polytechnique Palaiseau) and IETR (Université Rennes 1), still maintain this activity in France LAE suffers from a development devoted mainly and even only towards the flat panel displays application. Our conviction is that flat panel displays application is the past and the present of LAE. Future consists in the need to expand LAE to other fields. Indeed, many electronic, mechanical, chemical and biological functions need to be integrated with electronic treatment in-vivo on the same substrate. The goal is to develop a new silicon technology as it was done with CMOS, SOI or Bipolar technologies. It has to be fabricated at temperatures compatible with the fabrication of these functions. Nowadays, organic materials, transparent oxides, or silicon compete in the development of this technology. The present proposal focuses on the use of deposited silicon on flexible substrate at low temperature. For high performance in terms of electrical parameters and stability, silicon films with crystallized structure are targeted. For technological reasons linked mainly to the low temperature requirement, as-deposited micro (or nano) crystalline silicon is preferred in comparison with amorphously deposited and laser post-crystallized silicon. The present proposal consists in the development of basic CMOS electronic functions on flexible plastic substrate using as-deposited micro(nano)-crystalline silicon. Basic electronic functions (inverters, ring oscillators, current mirrors, amplifiers, logic gates') constitute the first pieces of a CMOS technology. Choosing plastic substrate is a real challenge. It requires the development of low temperature process leading to both N-type and P-type thin film transistors with interesting and stable performances. Experience of present partners leads to use heat stabilized PEN (polyethylene naphthalate) (Teonex Q65) produced by DuPont Teijin Films. With this film the maximum targeted temperature during the process is 180°C. Interesting performances need to define some objectives in terms of functioning frequency, field effect mobility and threshold voltage values. Targeted values are a frequency of some hundred kilo-Hertz, electron mobility of 5 to 10 cm2/V.s and hole mobility of 2 to 5 cm2/V.s, threshold voltage of 2 to 3 volts. Targeted stability is that of present very low mobility Bottom Gate Microcrystalline silicon TFTs with silicon nitride as gate insulator. Work programme is divided in 5 work packages, and each of ones in different tasks. Work programme begins by the fabrication of N-type and P-type TFTs on PEN sheets with previously defined performances. Study of the effects of the silicon microstructure, the type of gate insulator and its interface with silicon active layer, will be done. Then integrated P-type and N-type TFTs with similar performance than individual ones will be obtained that leads to develop some basic functions as CMOS inverters, ring oscillator and numerical gates. Particular effort is given to the characterization of devices and circuits and their behaviour under electrical as well as mechanical (due to the flexibility) stresses. Special work package is devoted to this characterization. Compact modelling is the purpose of another work package. Indeed, as-deposited silicon based TFTs need particular modelling due to particular microstructure. Moreover, the design of complex electronic circuits containing µc-Si TFTs requires an efficient and fast computing behavioural model to reproduce the input/output behaviour of the components. The objective is to develop a specific SPICE-like model for µc-Si TFTs. This model must take into account the static and dynamic response and ageing issue, e.g. response under thermal mechanical and electrical stress Main parameter of the success of any project and particularly the present one is the definition of an efficient coordination. Strong and continuous collaboration, closer than usual, between partners is needed all along the project. The longstanding collaboration between the 3 partners will help to this need. Moreover, common post-docs are planed. Finally, the importance of the expected technological results pushed to devote particular task to the exploitation and dissemination.