AVS 50th International Symposium
    Thin Films Tuesday Sessions
       Session TF-TuM

Paper TF-TuM8
Interface and Bulk Charge in Low Temperature Silicon Nitride Dielectrics on Plastic Substrates

Tuesday, November 4, 2003, 10:40 am, Room 329

Session: Thin Films on Organic, Polymeric and Biological Substrates
Presenter: K.J. Park, North Carolina State University
Authors: K.J. Park, North Carolina State University
G.N. Parsons, North Carolina State University
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Silicon nitride is a common gate dielectric for thin film transistors (TFT's) on plastic substrates, but the effect of processing temperature on charged defects in the film bulk and at the semiconductor/dielectric interface is not well known. For this work silicon nitride was deposited using various NH@sub3@/SiH@sub4@ gas ratios at temperatures between 50° and 300°C and the effect of process conditions on current vs. voltage (IV) and capacitance vs. voltage (CV) measurements was evaluated. For some conditions, CV was measured as a function of film thickness, and values for bulk and interface charge were extracted from the measured trends. We find that the apparent leakage current decreased with increasing NH/SiH ratio, but CV showed that increasing NH/SiH also leads in an increase in the flat band voltage shift, consistent with fixed charge in the films. Thickness dependence of CV indicates that increasing NH/SiH results in an increase in positive fixed charge at the interface, and an increase in negative fixed charge in the film bulk. When the NH/SiH ratio is fixed at 10, changing the substrate temperature from 50 to 250°C results in an increase in positive interface charge, and an increase in negative bulk charge, leading to charge compensation at higher temperatures. Internal charge can lead to a built in field which opposes the applied field, leading to a decrease in leakage current under high internal field conditions. Fixed charge is important to control threshold voltage and may affect transconductance in TFT devices. Results of amorphous silicon TFT's fabricated on polyimide substrates at various temperatures will be discussed.