Elastic modulus of low- k dielectric thin films measured by load-dependent contact-resonance atomic force microscopy
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Sean W. King Portland Technology Development, Intel Corporation, Hillsboro, Oregon 97124
Robert F. Cook Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 15 May 2009; accepted 17 June 2009)
Correlated force and contact resonance versus displacement responses have been resolved using load-dependent contact-resonance atomic force microscopy (AFM) to determine the elastic modulus of low-k dielectric thin films. The measurements consisted of recording simultaneously both the deflection and resonance frequency shift of an AFM cantilever probe as the probe was gradually brought in and out of contact. As the applied forces were restricted to the range of adhesive forces, low-k dielectric films of elastic modulus varying from GPa to hundreds of GPa were measurable in this investigation. Over this elastic modulus range, the reliability of load-dependent contact-resonance AFM measurements was confirmed by comparing these results with those from picosecond laser acoustic measurements.
I. INTRODUCTION
At the core of technology advances in modern nanoelectronics is the knowledge and advantageous use of material properties at the nanoscale. Mastering both the electrical and mechanical properties of materials has proven to be crucial in successful fabrication of new integrated electronic systems. Since the invention of atomic force microscopy (AFM),1 interrogation of mechanical properties at the nanoscale for electronics and other technologies has been a propelling factor in developing various dynamic AFM-based techniques: contactresonance AFM (CR-AFM) (which includes atomic force acoustic microscopy2 and ultrasonic atomic force microscopy3), ultrasonic force microscopy,4 and torsional harmonic dynamic force microscopy,5 among others. In this work, we propose a novel procedure for measuring the elastic modulus of nanoscale volumes probed by AFM. The procedure is based on recording real-time contact-resonance frequency versus force curves in the range of small applied contact forces. The benefit of working at small applied forces is that the mechanical properties of materials in the form of samples of reduced thickness (e.g., nanostructures6 and thin films7) can be probed. The drawback is that controlling the applied force in the range of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0357
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 9, Sep 2009 Downloaded: 02 Feb 2016
adhesion forces can be a difficult and deceiving task in CR-AFM measurements. However, much of the uncertainty can be eliminated when measurements are performed not simply at a single applied force, but over a wide force range, so that the force dependence of contactresonance frequencies is measured. Moreover, by correlating the measurements on a test material with those on a reference material, the need for accurate measurements of some parameters (e.g., cantilever stiffness and tip radius) is eliminated.8,9 A similar frequency shi
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