In microscopy, scanning joule expansion microscopy (SJEM) is a form of scanning probe microscopy heavily based on atomic force microscopy (AFM) that maps the temperature distribution along a surface. Resolutions down to 10 nm have been achieved and 1 nm resolution is theoretically possible. Thermal measurements at the nanometer scale are of both academic and industrial interest, particularly in regards to nanomaterials and modern integrated circuits.
Basic Principles
Scanning joule expansion microscopy is based on the contact operation model of atomic force microscopy. During the operation, the tip on the cantilever is brought into contact with the surface of the sample. AC or pulsed electrical signal is applied to the sample creating Joule heating and resulting in periodic thermal expansion. At the same time, the laser, which is focused on the top surface of the cantilever and the photodiode of the equipment, detects the displacement of the cantilever. The detecting photodiode is composed of two segments, which normalizes the incoming signal deflected from the cantilever. This differential signal is proportional to the cantilever deflection.
The deflection signals are caused not only by sample topography, but also by the thermal expansion caused by Joule heating. Since AFM has feedback controller with a bandwidth, for example 20 kHz (different AFM may have different bandwidths), the signal below 20 kHz is captured and processed by the feedback controller which then adjusts the z-piezo to image surface topography. Joule heating frequency is kept well above 20 kHz to avoid feedback response and to separate topological and thermal effects. The upper limit of the frequency is limited by the decrease of thermoelastic expansion with the inverse power of the modulation frequency and the frequency characteristics of the cantilever arrangement. A lock-in amplifier is specially tuned to the Joule heating frequency for detecting only the expansion signal and provides the information to an auxiliary Atomic Force Microscopy channel to create the thermal expansion image. Usually expansion signals approximately 0.1 Angstroms start to be detected, although the resolution of SJEM highly depends on the whole system (cantilever, sample surface, etc.).
By comparison, Scanning Thermal Microscopy (SThM) has coaxial thermocouple at the end of sharp metal tip. The spatial resolution of SThM critically depends on the thermocouple sensor size. Much effort has been dedicated to reducing sensor size to sub-micrometre scales. The quality and resolution of the images are very dependent on the nature of the thermal contact between tip and the sample; hence it is quite difficult to control in a reproducible way. The fabrication also becomes very challenging particularly for thermocouple sensor size below 500 nm. With optimization on the design and the fabrication, it was possible to achieve resolution around 25 nm. Scanning Joule Expansion Microscopy, however, has the potential of achieving similar to AFM resolution of 1~10 nm. In practice, however, the spatial resolution is limited to the size of the liquid film bridge between the tip and the sample, which is typically about 20 nm. The microfabricated thermocouples used for Scanning Thermal Microscopy are rather expensive and more importantly very fragile. Scanning Joule Expansion Microscopy has been used to measure the local heat dissipation of an in-plane gate (IPG) transistor to study hot spots in semiconductor devices, and thin-film alloy like cobalt-nickel silicide.
Signal acquisition and analysis
Signal obtained by the AFM (and captured by lock-in amplifier) are actually representations of the cantilever deflection at a specific frequency. However, besides thermal expansion, several other sources may also result in cantilever deflection.
Thermally induced cantilever bending
This is usually due to the mismatch in thermal expansion of two cantilever materials, for instance, silicon cantilever coated with a thin layer of metal (to increase the deflection). When heated, materials with higher expansion coefficient will expand more than the material with lower expansion coefficient. In this case, two materials, one in tensile strain, the other in compression strain, will induce substantial bending. However, this mechanism can be excluded for two reasons; first, cantilever coatings have been stripped experimentally and no change in signal was observed; second, the calculated thermal diffusion length in SiNx and Si cantilevers at the SJEM working frequency (typically 10 kHz~100 kHz) is small, much smaller than the length of the cantilever (typically 100 um).