Aluminum-Induced Crystallization of PECVD Amorphous Silicon

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Aluminum-Induced Crystallization of PECVD Amorphous Silicon Kenneth Jenq*, Shawn S. Chang*, Yaguang Lian*, Grant Z. Pan* and Yahya Rahmat-Samii**, Microfabrication Laboratory, **Antenna Research, Analysis and Measurement Laboratory, Department of Electrical Engineering, University of California at Los Angeles, Los Angeles, CA 90095-1594. *

ABSTRACT Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) in a conventional furnace with N2 protection has been studied at reaction temperatures ranging from 200 to 500 °C by using optical microscopy, and transmission and scanning electron microscopy. The a-Si and Al layers were deposited with plasma-enhanced chemical vapor deposition (PECVD) and electron beam evaporation, respectively. The structures in the study are Al/a-Si and a-Si/Al on Si or glass wafers coated with 3000 Å PECVD SiO2. It was found that Al induces crystallization of a-Si for both Al/a-Si and a-Si/Al structures by exchanging positions of Al and Si layer through diffusion of Si into Al and the grain size of crystallized Si (c-Si) increases with the decrease of AIC temperature. AIC for Al/a-Si structures starts at a temperature as low as 200 °C, which is 100 °C lower than that for a-Si/Al structures. Kinetics analysis found that the activation energies are 1.76 eV and 1.65 eV for both Al/a-Si and a-Si/Al structures, respectively. The quality of AIC c-Si depends on the order, thickness and thickness ratio of a-Si to Al. Microstructural observations indicated that the c-Si for Al/a-Si structures is better and more suitable for use in fabrication of thin film transistors (TFTs) than that for a-Si/Al structures. INTRODUCTION Polycrystalline silicon is desirable for TFTs because of its high carrier mobility compared to that of amorphous silicon. Formation of polycrystalline silicon at a low temperature is vital for low temperature large area microelectronics. Laser crystallization, though effective in transforming a-Si to c-Si, is an expensive and complicated process compared to metal-induced crystallization. Solid phase crystallization, which involves annealing at temperatures above 600 °C, is too high for many semiconductor substrates [1]. In the present study, we investigate aluminum-induced crystallization of PECVD a-Si in a conventional furnace at reaction temperature from 200 to 500 °C for both Al/a-Si and a-Si/Al structures. EXPERIMENTAL DETAILS At first a-Si and Al with thickness from 200-2000 Å were deposited by PECVD in a PlasmaTherm 790 system and E-beam evaporation in a CHA Mark 40 evaporator, respectively, on top of 4 inch Si and 1737 corning glass wafers coated with 3000 Å PECVD SiO2. The layers were structured as Al/a-Si (i.e. Al on top) and a-Si/Al (i.e. Al on bottom). To prevent the first aSi (or Al) layer from oxidation before deposition of the second Al (or a-Si) layer, the wafers were immediately loaded into the e-beam evaporator (or PECVD) chamber for Al/a-Si (or a-


Si/Al) structures. The a-Si was deposited at 275 ºC with a power of 300 W in He-diluted SiH4 u