Etching of Zirconium Oxide, Hafnium Oxide, and Hafnium Silicates in Dilute Hydrofluoric Acid Solutions
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Oxides and silicates of zirconium and hafnium are actively being considered and tested to replace SiO2 as the gate material. Though these materials have the high-dielectric constant (k ∼ 20–25) needed to provide a larger equivalent oxide thickness, they are very refractory and difficult to etch by wet and dry methods. In this paper, work done on wet etching of ZrO2, HfO2, and HfSixOy in dilute hydrofluoric acid (HF) solutions is presented and discussed. Experiments were done on various high-k films deposited by metalorganic chemical vapor deposition. It was found that the as-deposited high-k films can be dissolved with a good selectivity over SiO2 in dilute HF solutions, but heat-treated high-k films are difficult to etch with good selectivity over SiO2 under the same conditions.
I. INTRODUCTION
The primary means by which the semiconductor industry has achieved unprecedented gains in productivity and performance is by device scaling. However, continued downscaling of feature sizes is putting great demands on some existing materials such as SiO2 and has spurred interest in the introduction of new materials. In no area is this issue more clear or urgent than in the fabrication of metal oxide semiconductor devices. Here, the challenge is to reduce the gate oxide (currently SiO2) thickness to less than 1.5 nm without suffering from high leakage current.1 The use of a material with a dielectric constant (k) higher than conventional SiO2 as a gate oxide would allow thicker films to be used in the fabrication of devices.2 For example, an 8.6-nm layer of ZrO2 of dielectric constant k ∼ 23 is equivalent to a 1.5-nm layer of SiO2 of dielectric constant k ∼ 4. Many high-k materials have been investigated for gate dielectric application. High-k materials that are thermally stable in contact with Si are the most attractive. Oxides and silicates of rare earth metals Zr and Hf, such as HfO2, ZrO2, and Hf silicates, fall in this category. Both HfO2 and ZrO2 have similar material characteristics; have moderate dielectric constants of approximately 20–25, as well as low lattice mismatch and good thermal expansion matching with silicon.3–7 One of the drawbacks with HfO2 and ZrO2 and their silicates is that they are difficult to etch due to their refractory nature. For the deposition of ZrO2 and HfO2 films, both physical vapor deposition (sputtering) and chemical vapor
DOI: 10.1557/JMR.2004.0149 J. Mater. Res., Vol. 19, No. 4, Apr 2004
http://journals.cambridge.org
Downloaded: 11 Sep 2014
deposition (CVD) [metalorganic chemical vapor deposition (MOCVD) and ALCVD] techniques are used. Due to the fact that sputtering results in unwanted interfacial states, CVD techniques are more commonly used. In the MOCVD technique, organic precursors such as zirconium/hafnium trifluoroacetylacetone and zirconium/ hafnium t-butoxide are used. The films deposited by ALCVD technique generally use inorganic precursors such as chloride and nitrate salts of Zr and Hf. The films deposited by ALCVD technique are denser than those deposited by MOCVD tec
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