Physical and Electrical Properties of Yttrium Silicate Thin Films
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Physical and Electrical Properties of Yttrium Silicate Thin Films James J. Chambers and Gregory N. Parsons Department of Chemical Engineering, North Carolina State University, Raleigh, NC 27695 ABSTRACT This article reports on the physical and electrical properties of yttrium silicate, which is a possible high-k replacement for the SiO2 gate dielectric in CMOS devices. The yttrium silicate (Y-O-Si) films are formed by sputtering yttrium onto clean silicon, annealing in vacuum to form yttrium silicide and then oxidizing in N2O to form the silicate. Shifts in the Y 3d, Si 2p and O 1s photoelectron spectra with respect to Y2O3 and SiO2 indicate that the films are fully oxidized yttrium silicate. FTIR results that reveal a Si-O stretching mode at 950 cm-1 and Y-O stretching modes in the far-IR are consistent with XPS. XPS and FTIR results are in accordance with the donation of electron density from the yttrium to the Si-O bond in the silicate. The yttrium silicate films contain a fixed charge density of ∼9x1010 cm-2 negative charges as calculated from measured C-V behavior. The properties of ultra-thin yttrium silicate films with an equivalent silicon dioxide thickness (electrical) of ∼1.0 nm will be discussed elsewhere. INTRODUCTION The continuous scaling of complementary-metal-oxide-semiconductor (CMOS) devices toward smaller dimensions will require replacing the gate SiO2 with an alternate high dielectric constant (high-k) material. Metal silicates are appealing materials, since they are expected to posses a greater dielectric constant than SiO2 and crystallize at higher annealing temperatures than metal oxides. Yttrium silicate possesses desirable thermodynamic, dielectric and structural properties that make it attractive as a high k candidate. The Y-O bond is quite strong, since the free energy of formation1 (at 25 °C, per O-atom) is –2.40x10-22 kcal compared to –1.70x10-22 kcal for SiO2. Previous researchers demonstrated MOS capacitors with an Al/Y2.45Si0.55O5/Si structure and obtained a dielectric constant for the silicate layer of ∼12,2 which should be suitably large to obtain EOT < 1.0 nm with low tunneling. Figure 1a depicts the mineral keiviite (Y2O3⋅2SiO2), which consists of a Si2O76- structural unit with yttrium connecting the two corner sharing SiO4 tetrahedra.3 Figure 1b shows the yttrium orthosilicate (Y2O3⋅SiO2) structure where four of the oxygen atoms are bound in a silicon tetrahedron with each corner joining two yttrium octahedron and the fifth oxygen atoms, which are not involved with the silicon tetrahedron, are shared between four yttrium octahedron in a rod-like chain.4 Y2O3⋅2SiO2 and Y2O3⋅SiO2 exhibit low lattice mismatch with silicon, since the lattice constants a(Y2O3⋅2SiO2) = 0.554 nm and a(Y2O3⋅SiO2) = 1.041 nm are closely match with a (Si) = 0.543 and a(Si)x2 = 1.086 nm, respectively. Also, yttrium silicate films exhibit low oxygen permeability and should provide excellent passivation.5 These advantageous properties make yttrium silicate worthy of study as a dielectric for CMOS technology.
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