Poly-Silicon Deposition by Evaporation for TFTs
- PDF / 263,586 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 49 Downloads / 196 Views
POLY-SILICON DEPOSITION BY EVAPORATION FOR TFTs
W.Schmolla, J.Diefenbach, G.Blang and W.Senske AEG Research Institute, Goldsteinstrasse 235, D-6000 Frankfurt 71, FRG Abstract Poly-Si films were e-gun evaporated onto glass substrates. The Hall-mobility for holes was found to be about 2 cm2 /Vs in undoped poly-Si films deposited at 4000 C and 9 cm 2 /Vs at 500'C. TFTs were fabricated on the base of poly-Si evaporation technique on borosilicate glass at a highest process temperature of 550'C without ion implantation. The electrical TFT characteristics yield electron field-effect mobilities higher than 10 cm 2 /Vs as4 well as threshold voltages less than IV and an on/off current ratio in excess of 10 Introduction Poly-silicon is becoming a very attractive candidate for TFT (thin film transistor) applications in large area electronics, such as e.g. AMLCDs (active matrix liquid crystal displays) /1/, /2/, /3/, /4/. More general, the fabrication of active electronic devices with Si on special substrates, e.g. glass, large areas etc., necessitates the use of cheap thin film technology because this technology is more economic than bonding of separately fabricated circuits onto the substrate later. Substrate temperatures below 600'C or 450*C offer the advantage of using borosilicate glass or even soda-lime-glass respectively. Ion implantation technique has to be avoided in the case of soda-lime glass because the postannealing temperatures above 500°C are not compatible with soda-lime glass. The deposition rate for poly-Si should be high even at low substrate temperatures. Therefore the evaporation technique is a very favourable deposition process for poly-Si as the deposition rate dependence on the substrate temperature can be neglected. This is in favourable contrast to the LPCVD method where the thermal decomposition of silane determines the lower limit of the substrate temperature. Poly-Si We started investigations in order to get information about TFT characteristics obtainable by a poly-Si evaporation technique under HV (high vacuum) conditions. The ESCA analysis of poly-Si films deposited indicated impurities of carbon and oxygen: carbon at.%
oxygen at.%
10
5
at 550 0 C substrate
temperature
crystallite size/nm (x-ray) 70-100
Presumably these impurities originate from the residual gas in the HV system. The films of 0.5 pm thickness exhibit a column-like growth structure. The average crystallite size is about 70-100 nm.
Mat. Res. Soc. Symp. Proc. Vol. 106. ' 1988 Materials Research Society
330
We fabricated undoped poly-Si films of 0.5 pm thickness by e-gun evaporation on different kinds of substrates. N-type doped poly-Si films were obtained by coevaporation of Sb. The film quality was electrically characterized by Hall-mobility measurements before a complete TFT fabrication process had to be performed. The results are summarized in table 1. The starting evaporation material was high resistivity 800A cm p-type monocrystalline Si. In Hall measurements the majority carriers of the undoped poly-Si are holes. T
Data Loading...
