The protective coating for hard disks, namely a diamond-like carbon (DLC) film, is now targeted for thickness less than 3 nm because of the reduced spacing between the magnetic layer and the read/write head . The mechanical properties become very important for reliability of the devices. The chemical structure of DLC significantly depends on the deposition process and influences the mechanical properties such as elasticity and hardness. Especially the Young's modulus E drastically varies with a content of sp3-bonds, which form three-dimensional interlinks in the amorphous network of carbons (E ≈ 100–800 GPa) [2–4]. Therefore, the modulus is useful to identify the chemical structure of films.
Various approaches for the determination of the elastic properties of thin films have been previously used, including nanoindentation , laser spectroscopic methods , and removed substrate methods . However, it is still a challenging problem to evaluate ultrathin films like DLC films with thickness less than 10 nm.
Atomic force acoustic microscopy (AFAM)  is a promising method, which belongs to a family of dynamic techniques of atomic force microscope (AFM) such as micro-deformation microscopy  and ultrasonic atomic force microscopy . AFAM measures the resonant frequency f of an AFM cantilever whose sensor tip is in contact with a sample oscillated by a piezoelectric device. If an appropriate order of the vibration mode is selected, f varies with the contact stiffness k*, namely the interactive force gradient between a tip and a sample. The effective Young's modulus of a sample, defined as (Es: the Young's modulus, νs: the Poisson's ratio), is evaluated using contact mechanics relating k* to .
Characterization of a 50-nm-thick Ni film deposited on a Si substrate was demonstrated in AFAM, where f was observed without the substrate effects . In regard to DLC thin films, only relative evaluation was performed . These studies required a blunt tip with a radius of about 200 nm and a stiff cantilever of spring constant kc ≈ 50 N/m to realize reproducible measurements. However, the requirement reduced the spatial resolution and the sensitivity in detection of the contact force.
When attempting to analyze difficult samples like a DLC film with thickness less than 10 nm, higher performance of AFAM is required on the detection of k* and the spatial resolution. We previously proposed a concentrated-mass (CM) cantilever as a way of enhancing the sensitivity in k*-detection without trade-offs . A CM cantilever assures the maximum sensitivity for any sample material. Also, a flat tip with ductile-metal coating, keeps a stable contact area of a radius less than 5 nm and drastically simplifies the relation between k* and .
The method we previously developed, termed sensitivity-enhanced AFAM , is extended in this letter to the determination of the elastic modulus of ultrathin films. The demonstration was carried out for DLC films with thickness of 6 and 10 nm, deposited on a hard disk. A curve relating f to was determined from multiple measurements on reference samples. The uncertainty was discussed by error analysis. In the evaluation of the DLC-coated samples, the substrate effect was taken into account by using an analytical model for indentation of a layered half-space .