Nanoscale Mechanical Property Measurements
Shefford P. Baker, Cornell University
The National Nanotechnology Initiative (NNI) seeks to realize new technologies based
on materials, properties, and functional densities that become available when the
structure of a material or dimensions of an object are controlled at nanometer length
scales. The potential for progress in this area has been richly demonstrated by
developments in a wide range of devices including microelectronics,
microelectromechanical systems, microfluidics, microphotonics, and so on. As we
continue to improve our ability to manipulate and control materials at smaller length
scales, and to apply these methods to new materials (“soft-“ and “bio-materials),
powerful new technologies can be expected.
One of the fundamental requirements for any engineered device, regardless of whether its
primary function is electrical, mechanical, optical, chemical, magnetic, or biological, is
mechanical stability. In short, devices must retain mechanical integrity in order to remain
useful. Thus, understanding the mechanical properties of nanofabricated materials and
objects will be critical to understanding and improving the reliability of nanofabricated
devices. However, two aspects of the nanoscale conspire to make this difficult. First, the
mechanical properties of materials often deviate from bulk scaling laws as key
dimensions become small, and furthermore, many materials exist only on this length
scale (e.g. carbon nanotubes, protein molecules). Thus mechanical properties must be
measured at the nanoscale. Second, however, such measurements are difficult. For
example, one must impose and measure displacements that are some orders of magnitude
smaller than the object being measured. Further complications are provided by the fact
that nanofabricated devices are typically composites of dissimilar materials, and that new
materials, particularly soft materials, are being used.
Here are three areas in which progress in metrology must be made in order for the NNI to
reach its full potential:
Dynamic mechanical nanocontact measurements and analysis: By bringing a sharp point
in contact with a material, it is, in principle, possible to deform an arbitrarily small
volume of material. By recording the load on and displacement of the tip as it is pressed
into and retracted from the material, and using an appropriate model for interpreting these
data, it is possible to obtain mechanical properties such as the hardness and modulus of
the sampled volume. Both depth-sensing indentation (DSI, also known as
nanoindentation) and scanning force microscopy (SFM) methods have been developed
based on this principle. However, to date, accurate, quantitative mechanical property
measurements can only be reliably obtained from quasi-static measurements of linear
elastic materials using DSI. As more and more “soft” materials (e.g. polymers,
biomaterials) are developed and incorporated into nanodevices, it will become more
and more important to be able to obtain quantitative information about dynamic
properties, such as loss modulus, damping, and viscoplasticity.1,2,3 As a metrology effort,
this will require both the development of testing equipment and methods (the dynamic
behavior of the testing machine must be very well understood) and analysis models.
Stress measurement methods for nano-objects: Nanofabricated devices typically consist
of many dissimilar materials patterned into layers, lines, and/or dots. Due to thermal
expansion mismatch, lattice mismatch, and volume changes due to microstructural
evolution, these components typically experience very high stresses, which may affect
the performance of the part, and can result in failure of the device due to deformation or
fracture. In order to understand the mechanical behavior of such devices, it is necessary
to interrogate the internal structures to determine stress levels, and, if possible,
deformation. This can be accomplished without the need to section the sample (thereby
creating artificial free surfaces) using energetic beam methods. For example, synchrotron
x-ray diffraction methods have been used to determine strains and stresses in differentlyoriented grains in thin metal films.4,5 Metrology needs in order for this type of work to be
extended include development of smaller high-intensity beams, specialized sample
stages, faster goniometers with access to a wide range of geometries, and methods for
interrogating amorphous materials.
Adhesion and interface phenomena: Because they include many dissimilar materials,
nanofabricated device typically include a high density of interfaces. These interfaces
include a wide variety of materials combinations, and affect mechanical behavior in a
number of ways. One of the most obvious, of course, is de-adhesion.6,7 But interfaces can
also determine how much load is transferred to an individual nanocomponent, the rate of
diffusion (and accompanying deformation) in the material, crack propagation (or
deflection), and constraints on plasticity. Interface contributions to mechanical behavior
are only poorly understood and represent one of the areas where detailed study at the
nanoscale would pay off handsomely. At the moment, progress is being made in adhesion
and deformation studies in thin films. This effort should be extended to 1- and 0dimensional structures (lines an dots, respectively). Metrology needs include
standardized methods and analyses of all kinds.
1
Syed Asif, S.A., K.J. Wahl, and R.J. Colton, Nanoindentation and contact stiffness measurement using
force modulation with a capacitive load-displacement transducer. Rev. Sci. Inst., 1999. 70(5): p. 24082413.
2
Burnham, N.A., S.P. Baker, and H.M. Pollock, Model for mechanical properties nanoprobes. J. Mater.
Res., 2000. 15(9): p. 2006-2014.
3
Oyen, M.L. and R.F. Cook, Load-displacement behavior during sharp indentation of viscous-elasticplastic materials. J. Mater. Res., 2003. 18(1): p. 139-150.
4
Baker, S.P., A. Kretschmann, and E. Arzt, Thermomechanical Behavior of Different Texture Components
in Cu Thin Films. Acta Mater., 2001. 49: p. 2145-2160.
5
Spolenak, R., et al. High Resolution Microdiffraction Studies Using Synchrotron Radiation. in Sixth
International Workshop on Stress Induced Phenomena in Metallization. 2002. Ithaca, NY: American
Institute of Physics.
6
Dauskardt, R.H., et al., Adhesion and debonding of multi-layer thin film structures. Eng. Fract. Mech.,
1998. 61: p. 141-162.
7
Pang, M., X. Wang, and S.P. Baker, Quantitative measurement of subcritical debonding of Cu films from
glass substrates. in Materials, Technology, and Reliability for Advanced Interconnects and Low-k
Dielectrics Mat. Res. Soc. Symp. Proc 766 2003.