Electrical Behavior of Spin-on Silicon Dioxide (SOX) in a Metal-Oxide-Semiconductor Structure
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ELECTRICAL BEHAVIOR OF SPIN-ON SILICON DIOXIDE (SOX) IN A METAL-OXIDE-SEMICONDUCTOR STRUCTURE N. Lifshitz, G. Smolinsky, and J.M. Andrews AT&T Bell Laboratories, Murray Hill, NJ 07974-2070
ABSTRACT We studied the behavior of mobile charges, which we believe are hydrogen ions, in a novel spin-on oxide, SOX. Our analysis is based on the Triangular Voltage Sweep (TVS) technique, which is normally used to detect the presence of mobile alkali ions in silicon dioxides. We found that the TVS traces of protons exhibit several unique features that allow us to distinguish a proton signal from that of other common contaminants, such as sodium. We suggest a model to explain these unique features. We show that the presence of hydrogen in the SOX material is due to the high water retention of this porous oxide. INTRODUCTION At the present stage of MOS VLSI integration, one avenue for future miniaturization is a three-dimensional integration, wherein conducting interconnects are formed in more than one level. Isolation of the different metallic interconnect levels is one of the most important problems in VLSI manufacturing. An important constraint on the intermediate dielectric is the limitation on the deposition temperature: the dielectric is deposited on the first layer of aluminum metallization. Therefore, the deposition temperature cannot exceed 450 Another requirement, especially important for submicron design rules, is good step coverage. In this respect, spin-on silicon dioxides appear particularly attractive since they can be used in combination with better quality oxides, such as low-temperature plasma-deposited oxides from tetraethoxysilane (PE-TEOS).
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In this report, we discuss the electrical behavior of a novel spin-on oxide SOX in the Metal-OxideSemiconductor (MOS) system. The main tool of our study is the Triangular Voltage Sweep (TVS) technique. This method was developed by Kuhn and Silversmith [1] to study ionic contamination in thermally grown Sit 2 of MOS structures. The technique is based on the measurement of the ionic displacement current in an MOS capacitor driven by a slow linear-voltage ramp. Usually the measurements are carried out at elevated temperatures (up to 300 C), because the mobility of the ions in oxides is sufficiently high. The ionic motion is revealed as a current peak on the displacement-current-versus-voltage curve; the concentration of the mobile charge is calculated from the area under the peak. A typical TVS trace of thermal oxide contaminated with sodium is shown in Fig. 1. While the shape of the peaks obtained during the forward and reverse directions of the sweep and their displacement from zero bias may differ, the area under the peaks, which reflects the total mobile charge, should be the same for both sweep directions. For the MOS structures used in the present study, the composite dielectric consisted of 500 A of thermal oxide, 2000 A of SOX, and 1500 A of PE-PTEOS (phosphorus-doped PE-TEOS). The thermal oxide prevented ohmic leakage through the structure. The PE-PTEOS layer
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