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A simplified technique for characterizing crystallites in polysilicon films has been demonstrated based on use of the atomic force microscope (AFM). The crystallization of films deposited by two different techniques was examined.
Undoped 20 μm thick polycrystalline silicon (polysilicon) films were epitaxially grown on 0.5 μm thick polysilicon seed layers prepared by cycles of CVD depositions and oxidations. The resistivity of the epitaxial films was found to vary by a factor of 4, depending on the number of deposition/oxidation cycles used to prepare the initial seed layer. Exposing the films to a hydrogen plasma for as little as 5 seconds decreased the resistivity of n-i-n resistors by four orders of magnitude and changed their activation energy from 0.4 eV to 0.001 eV. Whereas, the resistivity of p-i-p resistors only decreased by a factor of two after a 2 hour hydrogenation with no change in the activation energy. The initial high resistivity value of the n-i-n resistors was restored by either removing 0.5 μm of polysilicon from the surface of the resistors or by a 15 minute anneal in argon at 550 °C.
Hydrogen diffusion in as-deposited and in oxidation-annealed polycrystalline silicon films was investigated using n-type accumulation-mode MOSFET's. The diffusion was studied by measuring the reduction in the grain boundary trap density with hydrogenation time. The number of traps in fully hydrogenated as-deposited films fell to about 45% of the initial trap state density and fell to about 20% in the oxidized-annealed films. Concurrently, the mobility increased about 95 % to 5cm2/Vs in the as-deposited films and by about 55 % to 25 cm2/Vs in the oxidized polysilicon devices. The effective preexponential diffusion coefficient and activation energy for hydrogen diffusion in the two different films were Do= 5.4×10−10 cm2/s and EA= 0.37 eV for the as-deposited polysilicon and Do=2.1×10−10 cm2/s and EA= 0.36 eV for the oxidized polysilicon.
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