Potential of Hot Wire CVD for Active Matrix TFT Manufacturing

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.