Study of sputtered Hafnium oxide Films for Sensor Applications
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Study of sputtered Hafnium oxide Films for Sensor Applications H. Grüger, C. Kunath, E. Kurth, S. Sorge, W. Pufe Fraunhofer IPMS, Grenzstr. 28, 01109 Dresden, Germany
Abstract In this paper results of the deposition and annealing of hafnium oxide thin films are reported. Due to the sensor application in mind, thicknesses between 30 and 150 nm have been deposited by r.f. sputtering of a high purity oxide target. Annealing has an important influence on the layer structure, stress and application correlated properties. A detailed understanding of the layer preparation is necessary to adjust deposition and annealing. After deposition the layers are predominately amorphous, annealing leads to textured layers with monocline or orthorhombic phases. Besides gas sensor applications optimized layers may serve as protective coating or combined with a second material to multi layer stacks as high reflective dielectric mirror.
Introduction Transition metal oxides have gathered importance for different applications. Some examples are surface coatings, dielectric layers for semiconductors or as sensitive material for thin film sensors [1, 2]. Important for these applications are the manifold exceptional material properties of these oxides like high ε, good temperature stability or mechanical and chemical inertness. Due to the wide range of applications, a broad variety of thickness has to be deposited. Atomic layer deposition (ALD) [3] and metal organic chemical vapor deposition (MOCVD) [4] has been used for thin layers. Physical vapor deposition (PVD) like sputtering is suitable for thicker layers. Today the main focus are ultra thin layers for semiconductor devices [5] but nevertheless sensor applications use the same material for its favorable electrochemical properties.
Experimental Hafnium oxide films with a thickness range between 30 and 150 nm have been deposited by r.f. sputtering from a high purity oxide target in Argon / Oxygen mixtures [6, 7]. 150mm wafers served as substrate. During deposition no additional substrate heating was applied. Thicker layer were deposited continuously or in multi step deposition without interruption of the vacuum. After deposition thermal treatment up to 1000°C was possible either by RTA with 50 K/s heating rate or in a standard CMOS oven process with 3K/min. The total annealing time was set to 30 min typically 10 min in Oxygen and 20 min in Nitrogen or Argon. After finalization of the process the stress was measured by bow measurements. Surface analysis was applied by AFM and critical dimension SEM. For further characterization wafers were cut into pieces. X-ray diffraction and TEM have been used to determine crystalline fraction, structure and orientation. The chemical and electrochemical characterization was done on samples in the laboratory.
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Layer properties From earlier work [6, 7] it is well known, that the layer properties are strongly influenced by deposition and annealing parameters. First of all, the layer stress has been evaluated. After deposition typically c
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