Limiting Native Oxide Regrowth for High-k Gate Dielectrics
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Limiting Native Oxide Regrowth for High-k Gate Dielectrics K. Choi, H. Harris, S. Gangopadhyay, and H. Temkin Nano Tech Center, Texas Tech University, Lubbock, TX 79409 ABSTRACT A cleaning process resulting in atomically smooth, hydrogen-terminated, silicon surface that would inhibit formation of native silicon oxide is needed for high-k gate dielectric deposition. Various cleaning methods thus need to be tested in terms of resistance to native oxide formation. Native oxide re-growth is studied as a function of exposure time to atmospheric ambient using ellipsometry. Hafnium dioxide film (k~23) is deposited on the as-cleaned substrates by electron beam evaporation and subsequently annealed in hydrogen. The difference in the effective oxide thickness re-grown on surfaces treated with the conventional RCA and modified Shiraki cleaning methods, after one-hour exposure, can be as large as 2 Å. This is significant in device applications demanding equivalent oxide thickness less than 20 Å. The degree of hydrogen passivation, surface micro-roughness and organic removal capability are considered to be the main factors that explain the differences between the cleaning methods. Data derived from capacitance-voltage analysis of test capacitors verified the trend observed in the native oxide thickness measurements. An increase of 10~15 % in accumulation capacitance is observed in the samples treated by the new cleaning method.
INTRODUCTION The presence of thin native oxide on the silicon surface, before gate dielectric deposition, is critical in high-k dielectric process since the low dielectric constant (~ 3.9) of native oxide limits dielectric constant of the dielectric stack. Furthermore, the non-uniform nature of the native oxide growth1 may results in non-uniform transistor properties. The initial oxidation mechanism of the hydrogen-passivated Si surface in air has been studied by numerous researchers2 but a comprehensive understanding is lacking since the actual growth behavior depends on many parameters such as humidity or residual surface contaminants. Hydrogen passivated Si surface obtained by etching in HF/H2O solution is considered to be inert against re-oxidation in ambient air3. However, the reported native oxide re-growth rates vary, with some studies4,5 reporting relatively quick regrowth (2 ~ 4 Å) in the first couple of hours after cleaning; comparable to typical delay times encountered on fabrication lines. Since the equivalent oxide thickness of less
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than 20 Å is needed for advanced devices, re-growth of even a monolayer of SiO2 (2.7 Å) is undesirable. The advantage of Modified Shiraki over RCA cleaning6, followed by HF/H2O in the pre-gate cleaning for HfO2 based MOS device, has been reported elsewhere7. The main difference is the final etch step, whi
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