Aluminum metallization for flat-panel displays using ion-beam-assisted physical vapor deposition
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Aluminum metallization for flat-panel displays using ion-beam-assisted physical vapor deposition Zhenqiang Ma and Gary S. Was Department of Nuclear Engineering and Radiological Sciences, The University of Michigan, Ann Arbor, Michigan, 48109 (Received 30 July 1998; accepted 22 July 1999)
Failures in aluminum interconnects in display control devices are often caused by the formation of hillocks during postdeposition annealing. Ion-beam-assisted deposition was used to create a (110) out-of-plane texture in aluminum films to suppress hillocking. X-ray diffraction was used to quantify the (110)/(111) out-of-plane texture ratio, and scanning electron microscopy and atomic force microscopy were used to characterize the surface topology. Results show that no hillocks were observed on (110)-textured aluminum films following annealing for 30 min at 450 °C. Following annealing, the resistivity of the films made by ion-beam-assisted deposition recovered to within a factor of 2 of the physical-vapor-deposition films. Results show that ion-beam-assisted deposition can effectively modify the aluminum out-of-plane texture in such a way that hillock suppression can be achieved without significant change in resistivity.
I. INTRODUCTION
The flat panel display market has experienced a tremendous growth over the past 15 years. Among all display types, thin film transistor–active matrix liquid crystal display (TFT-AMLCD) accounts for the majority of the market.1 For the next generation of this type of display, larger display area, higher resolution, and faster response time have been identified as the most important goals.2 The improvement in performance depends largely on further development of the manufacturing process and the selection of appropriate metallization materials. Among all the current metallization materials, such as pure Al, Al alloy, Ta, etc., pure Al appears to be the most promising metallization material for next generation TFT-AMLCD because of its low resistivity. With pure Al as the gate line metallization, the parasitic resistance × capacitance (RC) delay time (which causes the distortion of the gate pulse as it travels down the line) will be minimized and the panel size can thus be expanded without an increase in the response time.3 A major disadvantage of pure aluminum as gate metallization material is the high hillock density when the deposited aluminum film is subjected to a high temperature for subsequent dielectric layer deposition.2 Hillocks can short circuit gate lines, interfere with the deposition of subsequent layers, or prevent complete encapsulation of the gate line. It is believed that thermal hillocks are caused by compressive stresses in the metal film arising from a mismatch in thermal expansion coefficients between the aluminum film and the substrate.4 The formaJ. Mater. Res., Vol. 14, No. 10, Oct 1999
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tion of hillocks provides a means of stress relief. An analysis of hillocking thermodynamics
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