Plasma Nitride Hydrogen Source Encapsulation Method to Hydrogenate Polysilicon TFTs

摘要

A novel,commercially attractive process has been developed to hydrogenate polysilicon thin film transistors (poly-Si TFT).Active device regions of the TFTs are encapsulated by two nitride layers;one underneath the poly-Si channel but separated by a thin oxide,and the other capping the device (Fig.1). During the dopant activation anneal,hydrogen is driven from nitride layers,through intermediate oxide layers,and into the channel passivating the trap sites at poly-Si grain boundaries.Dopant activation was carried out in a furnace at 600 deg C in N_2 ambient for 4 hours or RTA furnace at 620 deg C in N_2 for only 20 minutes.On test samples,secondary ion mass spectroscopy (SIMS) suggests for furnace annealing this treatment introduces up to 10~(20) cm~(-3) hydrogen from the nitride layers into the poly-Si channel (Fig.2). The electrical characteristics of the TFTs after various modes of hydrogenation are shown in Fig.3.Double encapsulation of active region reduced the threshold more effectively than single topside or backside nitride,most likely because nitride encapsulation reduces hydrogen loss from the channel during processing.Hydrogenation by double nitride encapsulation using a short RTA anneal results in electrical characteristics very close to that of the furnace process providing a possible alternative low-thermal budget process.After double nitride hydrogenation,the threshold voltage of the TFTs were reduced by about 40 V and the saturation on-current improved,closely approaching the performance of electron cyclotron resonance (ECR) hydrogenated TFTs.