Potential of Hot Wire CVD for active matrix TFT manufacturing
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Potential of Hot Wire CVD for active matrix TFT manufacturing R.E.I. Schropp, Z.S. Houweling, V. Verlaan1 Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science, Department of Physics and Astronomy, Section Nanophotonics – Physics of Devices, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands 1 Present address: Eindhoven University of Technology, Eindhoven, The Netherlands ABSTRACT Hot Wire Chemical Vapor Deposition (HWCVD) is a fast deposition technique with high potential for homogeneous deposition of thin films on large area panels or on continuously moving substrates in an in-line manufacturing system. As there are no high-frequency electromagnetic fields, scaling up is not hampered by finite wavelength effects or the requirement to avoid inhomogeneous electrical fields. Since 1996 we have been investigating the application of the HWCVD process for thin film transistor manufacturing. It already appeared then that these Thin Film Transistors (TFTs) were electronically far more stable than those with Plasma Enhanced (PE) CVD amorphous silicon. Recently, we demonstrated that very compact SiNx layers can be deposited at high deposition rates, up to 7 nm/s. The utilization of source gases in HWCVD of a-Si3N4 films deposited at 3 nm/s is 75 % and 7 % for SiH4 and NH3, respectively. Thin films of stoichiometric a-Si3N4 deposited at this rate have a high mass-density of 3.0 g/cm3. The dielectric properties have been evaluated further in order to establish their suitability for incorporation in TFTs. Now that all TFT layers, namely, the SiNx insulator, the a-Si:H or µc-Si:H layers, and the n-type doped thin film silicon can easily be manufactured by HWCVD, the prospect of “all HWCVD” TFTs for active matrix production is within reach. We tested the 3 nm/s SiNx material combined with our protocrystalline-Si:H layers deposited at 1 nm/s in ‘all HW’ TFTs. Results show that the TFTs are state of the art with a field-effect mobility of 0.4 cm2/Vs. In order to assess the feasibility of large area deposition we are investigating in-line HWCVD for displays and solar cells. INTRODUCTION High deposition rates are required to reduce production costs of large-area applications (e.g., displays and solar cells) in mass fabrication. The common approach in the development of plasma sources for high deposition rate is to aim for high ionization rates and high gas utilization. Fundamentally, in plasma enhanced chemical vapor deposition (PECVD) high electron energy is required to fragment molecules while the probability of occurrence is low, since point collisions are required in a 3D space. Hot Wire Chemical Vapor Deposition (HWCVD) is fundamentally different since molecules are cracked on a 2D catalyst material. Because the source gases are catalytically decomposed, the method is often referred to as Thermo-Catalytic Chemical Vapor Deposition or Catalytic Chemical Vapor Deposition (CatCVD). A schematic view of our HWCVD reactors is given in figure 1.
Figure 1. Schematic drawing of the experimental
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