Elastic modulus of low- k dielectric thin films measured by load-dependent contact-resonance atomic force microscopy

Gheorghe Stan, Sean W. King, Robert F. Cook
2009 Journal of Materials Research  
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
more » ... 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 contactresonance AFM measurements was confirmed by comparing these results with that from picosecond laser acoustics measurements. 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 microscopy 2 and ultrasonic atomic force microscopy 3 ), ultrasonic force microscopy 4 , torsional harmonic dynamic force microscopy 5 amongst 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., nanostructures 6 and thin films 7 ) can be probed. The drawback is that controlling the applied force in the range of adhesion forces can be a difficult and deceiving task in CR-AFM measurements. However, much of the unknown error can be eliminated when measurements are performed not simply at a single applied force but over a force range, such that the force dependence of contact-resonance frequencies is measured. Moreover, by correlating the measurements on a test material with those on a reference, the need for accurate measurements of some parameters (e.g., cantilever stiffness and tip radius) is eliminated. 8, 9 We have tested the applicability of the proposed method by performing load-dependent CR-AFM measurements on low dielectric constant (low-k) materials: amorphous hydrogenated silicon carbide (a-SiC:H) and oxycarbide (a-SiOC:H) films. Mechanical properties of * Electronic mail: gheorghe.stan@nist.gov low-k dielectric films 10,11 are vital for fabricating robust architectures in copper interconnection-based electronics. CR-AFM measurements were made on films of elastic modulus in the range of GPa (compliant materials) to hundreds of GPa (stiff materials) and thickness around 500 nm. The CR-AFM results were compared with those from picosecond laser acoustics (PLA) 12,13 measurements made on samples of the same thickness but larger area. All films used in these experiments were deposited on 300 mm Si(100) wafers using a high volume manufacturing PECVD system at temperatures on the order of 400 • C. The precursors used for deposition consisted of various combinations of SiH 4 , methylsilanes, H 2 , He, and oxidizing gases. Young's modulus for these films was first determined by PLA. This ultrasonic technique requires knowledge of the film density as well as Poisson's ratio. The film density for these films was determined using an X-ray reflectivity technique 14 and a Poisson's ratio of 0.25 was assumed. For the SiOC:H films, the presence of porosity was checked using solvent diffusivity measurements described elsewhere. 15 All film deposition and subsequent measurements were performed in high volume manufacturing, class 10 microelectronic fabrication clean rooms with relative humidity controlled to 40 ± 1 %. CR-AFM exploits the sensitivity of AFM cantilever resonances to the elastic properties of materials probed. The shifts experienced by the resonance frequencies of a cantilever when the AFM probe is brought from air into contact are converted into the elastic modulus of the material tested. First, a clamped-spring coupled beam model 2 is used to determine the contact stiffness from the measured cantilever dynamics and, second, an adequate contact mechanics model is needed to convert contact stiffness into elastic modulus. Nominally, CR-AFM measurements are performed at a fix applied force, a few times greater than the adhesion force between the probe and material. With these precautions, (i) the applied force can be easily controlled with a precision better than 10% even with a stiff cantilever (20 Nm −1 to 40 Nm −1 ) and (ii) the contact can be described by simple contact
doi:10.1557/jmr.2009.0357 fatcat:emgsj65bxjhj3mnfeu5r2pfr54