Our supplier Filmetrics provides a family of sheet resistance measurement instruments which can measure conductive sheets and thin films deposited on various substrates. These include Semiconductor wafers, glass or plastic substrates, PCB patterned features, solar cells, metal foils and flat panel display layers and patterned features.
We offer two four-point-probe measurement systems, both suitable for samples up to 200mm. One has a motorized x-y stage with a 100mm travel range, the other with a range of 200mm. We also offer two systems in the same configurations, for non-contact eddy curry probe measurements. Typical applications for these excellent research tools include, but are not limited to the following: Metal film thickness, Substrate thickness, Sheet Resistance, Resistivity, Conductivity and Stacked films
All systems are equipped with an advanced mapping tool, the RsMapper software, which visualizes results in 2D or 3D to help you evaluate the results. The software also provides measurement automation via a built-in map pattern, which allows you to easily generate spot patterns to measure the relevant data of your samples.
Sheet resistance is a measure of the resistivity of a system considered to be two-dimensional, like a thin film with uniform thickness. Sheet resistance is independent of the distance between contacts, and can therefore be used to compare the electrical properties of devices that are of different size.
Sheet resistance is measured along the plane of the sheet, rather than perpendicular to it. As the sheet resistance is calculated from the thickness of the film and its resistivity, a sheet resistance measurement can also be used to calculate the thickness of a film if its resistivity is known.
Sheet resistance is usually determined by a four-point probe measurement. The four electrodes are spaced in a straight line at equal distances. A current is applied between the two outer electrodes, and the voltage is measured between both inner electrodes. This configuration means both the wire resistance and the contact resistance (between electrode and sample) do not contribute to any decrease in voltage between. Thus, this decrease arises entirely from the sample resistance.
A non-contact alternative for this set up uses eddy currents, which are induced in the sample using a coil through which an AC current is passed. This creates a magnetic field which induces a current in the sample. This current is measured using a second coil, or by measuring changes in current in the primary coil.
First, as mentioned above, sheet resistance does not depend on the size of the sample, so it can be used to compare devices of different sizes. The measurement is also used to assess the electrical properties of doping variation in semiconductors or doped polysilicon film, or thin and thick metal film. It is also possible to asses epitaxial layer thickness resistivity for process control in the production of silicon wafers.
Furthermore, the processes associated with annealing also change the sheet resistance, so the technique can be used for annealing monitoring. Diffusion inside a thin film or other sample can also change the resistivity, so sheet resistance can be used to for diffusion measurements.
The non-contact eddy current method is well suited to inspect complex shapes and detect small cracks in a sample, even if is not perfectly even. It is also well suited to study the electrical conductivity or the thickness of coatings.
Aliyu Kabiru Isiyaku et al. use four-point probe measurements to study the effects of post-heat (annealing) treatment on the structural, optical and electrical properties of ITO/AgAl/ITO (indium tin oxide/silver-aluminium) multilayer films at different temperatures. The sheet resistance of the as-deposited film is about 7.85 Ω/sq and falls down to 3.23 Ω/sq by heating, which shows that the electrical properties of post annealed multilayer films have been successfully enhanced.
Reference: Aliyu Kabiru Isiyaku, Ahmad Hadi Ali and Nafarizal Nayan: The effects of post heat treatment on ITO/AgAl/ITO/p-Si multilayer films. Journal of Physics: Conference Series.
Optimisation of doped metal thin film production
Transparent and conductive thin films are important for a large number of applications, including but not limited to touch screens, solar cells, and light emitting diodes. Eugen Stamate performed spatially resolved optoelectronic parameters, including four-probe sheet resistance measurement, of aluminium-doped zinc oxide (AZO) thin films deposited by RF magnetron sputtering without intentional substrate heating, to narrow down the process parameters that give the best film properties, which include a low resistivity below 3×10-3 Ωcm.
Reference: Eugen Stamate: Spatially Resolved Optoelectronic Properties of Al-Doped Zinc Oxide Thin Films Deposited by Radio-Frequency Magnetron Plasma Sputtering Without Substrate Heating. Nanomaterials 2020