Effects of Power Density and Thickness On Aluminum-Induced Crystallization of PECVD Amorphous Silicon

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Effects of Power Density and Thickness On Aluminum-Induced Crystallization of PECVD Amorphous Silicon Kendrick S Hsu1, Jeremy Ou-Yang1, Li P. Ren2, and Grant Z. Pan1 1 Microfabrication Laboratory, University of California at Los Angeles, Los Angeles, CA, 90095 2 Nanoelectronics and Nanophotonics Laboratory, Global Nanosystems, Inc., Los Angeles, CA, 90025 ABSTRACT The effect of power density and thickness on aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) was studied by using N2-protected conventional furnace reaction and optical microscopy. The a-Si with a thickness ranging from 500 to 5000 ≈ was formed by plasma enhanced chemical vapor deposition (PECVD) at a power density from 0.05 to 1.00 W/cm2, followed by an E-beam evaporation of 300 ≈ Al. It was found that a low power density as well as a large a-Si thickness could result in a decrease of activation energy and therefore a significant reduction of the AIC reaction temperature. Scanning and transmission electron microscopy and X-ray diffraction were used to check the crystallinity and quality of the AIC thin films. Polysilicon thin films consisting of submicron-scale Si crystallites within large AIC networks of about 10 µm were achieved at an AIC reaction temperature as low as 120 °C. INTRODUCTION Large-area microelectronics on flexible substrates such as flexible displays on plastic substrates requires novel thin film transistors (TFTs) made of high carrier mobility materials and fabricated at temperatures below 150 ∞C [1]. It is well known that Al can induce the crystallization of a-Si at a temperature much lower than the eutectic temperature of silicon with Al [2-4]. This is the so-called aluminum-induced crystallization (AIC). It was recently reported that polysilicon (c-Si) could be achieved from PECVD a-Si by AIC reaction at a temperature as low as 200 ∞C if the Al layer is placed on top of the a-Si before the AIC reaction [5]. In order to further reduce the AIC temperature and to achieve device grade c-Si for use in large area microelectronics on flexible substrates, the effects of power density and a-Si thickness on the AIC reaction and mechanism of PECVD a-Si were investigated extensively. In this investigation, polysilicon thin films consisting of submicron crystalline silicon were achieved at an AIC temperature as low as 120 °C. EXPERIMENTAL DETAILS On top of a 4 inch Si or 1737 corning glass wafer coated with 3000 ≈ PECVD SiO2, a-Si with a thickness ranging from 500-5000 ≈ was deposited by PECVD in a PlasmaTherm790 system at a power density from 0.05 to 1.00 W/cm2, a temperature of 150 ˚C, and a vacuum of 500 mTorr with a He-diluted SiH4 gas. This PECVD Si was confirmed to be amorphous through X-ray diffraction testing. Within a maximum of 1 h right after the a-Si deposition, the wafers were transferred into a CHA Mark40 evaporator and a thin Al layer of 300 ≈ was evaporated at

room temperature with a predeposition vacuum of 2x10-7 Torr. After deposition, the wafers were first cut into 1 x 1 cm squares and stored in