Scanning joule expansion microscopy

Scanning Joule Expansion Microscopy is a form of scanning probe microscopy heavily based on atomic force microscopy 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. Scanning Joule Expansion Microscopy is a form of scanning probe microscopy heavily based on atomic force microscopy 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. Scanning Joule Expansion Microscopy (SJEM) is based on the contact operation model of Atomic Force Microscopy (AFM). 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.