High-mobility low-temperature ( 600 • C) unhydrogenated in situ doped polysilicon thin film transistors (TFTs) are made. Polysilicon layers are grown by a low pressure chemical vapour deposition (LPCVD) technique and crystallized in a vacuum by thermal annealing. The source and drain regions are in situ doped. The gate insulator is made of an atmospheric pressure chemical vapour deposition (APCVD) silicon dioxide. Hydrogen passivation is not performed on the transistors. One type of transistor is made of two polysilicon layers, the other one is fabricated from a single polysilicon layer. The electrical properties are better for transistors made of a single polysilicon layer: a low threshold voltage (1.2 V), a subthreshold slope S = 0.7 V/dec, a high field effect mobility (≈100 cm 2 V −1 s −1 ) and an on/off-state current ratio higher than 10 7 for a drain voltage V ds = 1 V. At low drain voltage, for both transistors, the off-state current results from a pure thermal emission of trapped carriers. However, at high drain voltage, the electrical behaviour is different: in the case of single polysilicon TFTs, the current obeys the field-assisted (Poole-Frenkel) thermal emission model of trapped carriers while for TFTs made of two polysilicon layers, the higher off-state current results from a field-enhanced thermal emission.

The influence of the silane (SiH 4 ) pressure on the dopant incorporation during LPCVD silicon deposition at 550 • C using silane and phosphine (PH 3 ) or diborane (B 2 H 6 ) is examined, for a range of pressure from 1 to 100 Pa. We conclude that different deposition and dopant incorporation mechanisms occur according to the deposition pressure. It is shown that, under low-pressure conditions, silane remains the preponderant host species, while it is silylene (SiH 2 ) at high pressure. At low pressure, SiH 4 and PH 3 or B 2 H 6 are separately but not independently adsorbed. At high pressure, the presence of silylene promotes the formation of monosilylphosphine and monosilylborane which are found to be the adsorbed dopant species. The usual change of the growth rate caused by the addition of the dopant, i.e. a reduction with phosphine and an increase with diborane, is a function of the silane pressure; the dopant content of the solid films causes a significant variation of the growth rate only when it is superior to a threshold of about 10 19 cm −3 in both cases.

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Abstract. Silicon films, typically 1 pm thick are deposited by low pressure chemical vapor deposition using pure silane at 550°C and 3 deposition rates : 13, 23 and 45 h. Using numerous physical, optical and electrical characterization techniques, we show an evident amorphous character of these as-deposited ftlms. Films deposited at high rate correspond more likely to the relaxed amorphous network The quality of the polysllicon produced by annealing these high deposition rate Nms at 600°C is largely enhanced The crystallization time, dehed from the in-situ conductivity measuements at 600°C, is about 4 h for the high deposition rate amorphous film. Such time is very attractive in the attempt to obtain simultaneously cc acceptable n crystallization time and high quality polysilicon. This assertion is emphasized if we consider the total time t of the process (deposition time and crystallization time).

Our project is the realization of monolithic loaded line phase shifters for hyperfrequency signals supplying planar antennas. Phase shifters include poly-Si microswitches taking the place of the usual PIN diodes. To this aim, undoped polysilicon cantilever beams were produced by surface micromachining. Results on the stress non uniformity and the stiction problem encountered during the process are reported.