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					     Fully transparent zinc oxide based thin film              and transparent electrodes using ITO are considered. Our
  transistors deposited at low temperature: choice of          zinc oxide based TFTs, deposited without intentional
         gate dielectric and effect of bias stress             substrate heating, have field effect mobility of 10 cm2 V-1
                                                               s-1, and on-off current ratio of 107.
     F. M. Li, M. Mann, A. J. Flewitt, W. I. Milne
 Electrical Engineering Division, Cambridge University,        The electrical characteristics of the various metal oxide
    J J Thomson Avenue, Cambridge, CB3 0FA, UK                 TFTs based on different transparent semiconductor and
                                                               gate dielectric combinations will be presented. The
     J. D. Dutson, S. J. Wakeham, M. J. Thwaites               relative advantages of various semiconductor-dielectric
 Plasma Quest Ltd., Unit 1B, Rose Estate, Osborn Way,          combinations will be discussed, together with the results
           Hook, Hampshire, RG27 9UT, UK                       of bias-stressing of the metal-oxide TFTs to allow
                                                               assessment of the device stability.

Metal oxide semiconductors have emerged as a possible          In summary, this investigation demonstrates the
alternative technology to existing thin film transistor        feasibility of using the HiTUS system to produce metal
(TFT) materials such as hydrogenated amorphous silicon         oxide TFTs at low deposition temperatures and at high
and organic semiconductors. Metal oxide semiconductors         growth rates, rending this technique to be particularly
are attractive owing to their unique characteristics           promising for manufacturing of transparent TFTs on
including high optical transparency attributed to their        plastic substrates for flexible electronic applications.
wide band-gap structure, higher carrier mobility,
mechanical robustness, and improved stability under bias
stress. These properties present metal oxide TFTs as an        1.   K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.
attractive candidate for application in backplanes for next-        Hirano, and H. Hosono, Science 300, 1269 (2003).
generation flat panel displays, and more interestingly for     2.   K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.
use in transparent displays and other innovative see-               Hirano, and H. Hosono, Nature 432, 488 (2004).
through electronics.                                           3.   H. Yabuta, M. Sano, K. Abe, T. Aiba, T. Den, H.
                                                                    Kumomi, K. Nomura, T. Kamiya, and H. Hosono,
Techniques that are commonly employed for the                       Appl. Phys. Lett. 89, 112123 (2006).
deposition of metal oxide thin films include: pulsed laser     4.   P. F. Carcia, R. S. McLean, and M. H. Reilly, Appl.
deposition1-3, atomic layer deposition from metalorganic            Phys. Lett. 88, 123509 (2006).
precursors4, and rf magnetron sputtering5,6, as well as        5.   E. Fortunato, A. Pimentel, L. Pereira, A. Goncalves,
solution-based processing techniques such as spray                  G. Lavareda, H. Aguas, I. Ferreira, C. N. Carvalho,
pyrolysis7. All of these techniques have been                       and R. Martins, J. Non-Cryst. Solids 338-40, 806
demonstrated to produce metal oxide TFTs with good                  (2004).
device mobility. However, the best performance metal           6.   E. M. C. Fortunato, P. M. C. Barquinha, A. Pimentel,
oxide TFTs reported-to-date required depositions at                 A. M. F. Goncalves, A. J. S. Marques, R. F. P.
elevated substrate temperatures, which are not suited for           Martins, and L. M. N. Pereira, Appl. Phys. Lett. 85,
plastic substrates. In light of the emergence of flexible           2541 (2004).
electronics, low deposition temperatures are desirable to      7.   A. Bashir, P. H. Wöbkenberg, J. Smith, J. M. Ball, G.
ensure compatibility with plastic substrates. Another               Adamopoulos, D. D. C. Bradley, T. D. Anthopoulos,
major attraction of low temperature deposition is the               Adv. Mater. 21, 2226 (2009).
possibility of depositing material onto plastic substrates
using a reel-to-reel processing technique, as this would
significantly reduce the cost of producing large area
electronic backplanes.
                                                                                                            8        7
                                                                                        Ar Gas
In this study, we report the deposition of metal oxide
                                                                                                            8        7
TFTs at low temperature (with no intentional substrate                                  O2 Gas
heating) using a novel High Target Utilisation Sputtering                              99.999%
(HiTUS) system at very high growth rates. A schematic                                                                                   8   8

diagram of the HiTUS system is shown in figure 1. The                              5      6

HiTUS system features a remote plasma chamber, in                                             13
which the plasma generation is separated from the
substrate. This results in reduced ion bombardment of the            1   Sputter target
                                                                     2   Earth shield
substrate and allows for more precise control over                   3   Gas ring
deposition parameters and thus the film properties.                  4   Rotating sample stage     11           3
                                                                         13.56 MHz rf power supply
Deposition rates of ~ 50 nm min-1 can be achieved whilst
                                                                     5                                                   1
                                                                     6   Matching network
retaining excellent material properties (including low film          7   Mass flow controllers

                                                                     8   Valves                                          14
stress, high optical transparency, and high carrier                  9   Turbo pump
mobilities) even without intentional substrate heating,             10
thus compatible with plastic substrates.                            12
Metal oxide TFTs based on zinc oxide (ZnO), zinc tin                15   Shutter
oxide (ZnSnO) and indium zinc oxide (InZnO)
semiconductor channels, deposited using the HiTUS               Figure 1. Schematic illustration of a HiTUS sputtering
system at low temperature, have been demonstrated. To              system, with a remote plasma chamber where the
implement fully transparent TFTs, transparent gate              substrate/sample is removed from the sputtering plasma.
dielectrics including aluminium oxide and hafnium oxide