A handheld eddy current instrument for measuring thickness of thin metallic films using SARK-110
Recently, Melchor.Varela, SARK-110 Antenna Analyzer designer, did a collaboration application with a company for the use of the SARK-110 Antenna Analyzer along their software package for measuring the thickness of thin film sheets -Z-Scope*SRK. From his side, he created a custom firmware according their specifications.
An overview about the measuring technique using SARK-110 Antenna Analyzer is provided below.
INTRODUCTION
Thin films have become increasingly important in physics and engineering. In industry, there are applications in areas including communications, the automotive industry, energy generation, energy conservation, and a variety of coatings in electric appliances. Thin films are also heavily used in microelectronics and semiconductor devices. The thin film industry is growing as quickly as scientists and engineers can find applications for it. This field will become even more vibrant as we try to make appliances and systems smaller and thinner.
One of the most important application areas of thin film is energy stocking. The current trend is to stock energy in supercapacitors. The advantages of supercapacitors are their ability to stock a high amount of electricity in a small volume and their short charging time. These are very important for applications such as electric vehicles, remote controlled systems, or portable devices.
Carbon or metalized film is used intensively in the supercapacitor manufacturing (see picture below). The thickness profile of the metal deposit is carefully measured in order to enhance quality and to avoid wasting materials. This thickness profile is a determining factor for the product quality and sometimes belongs to the trade secret.
Schematic construction of a wound supercapacitor (source: https://en.wikipedia.org/wiki/Supercapacitor)
1.Terminals, 2.Safety vent, 3.Sealing disc, 4.Aluminum can, 5.Positive pole, 6.Separator, 7.Carbon electrode, 8.Collector, 9.Carbon electrode, 10.Negative pole
TECHNIQUES FOR MEASURING THICKNESS OF THIN FILMS
There are several techniques for measuring the thickness of thin films. Here, we deal with metallic films only. The simplest one is to cut the film in order to measure the metallic thickness profile. Another technique is to determine the metal weight and divide it by the considered surface in order to obtain the thickness. These are absolute techniques and are generally trustworthy, but they are destructive and time consuming.
Among the nondestructive methods for measuring the thickness of a thin metallic film, optical methods are often used. They are based on spectral reflectance, ellipsometry, or transparency measurement.
As shown in the photos below, some optical instruments are desktop equipment and are destined for laboratory use.
Sciensoria has developed a new instrument for measuring thin metallic films, the Z-Scope*SRK. It is a handheld instrument based on the eddy current principle. The measurement is carried out from one side of the film, thus enabling measurement of wide sheets. The result is obtained in less than one second, and is very stable even while being held manually.
The eddy current measurement principle is well-known: an alternative magnetic field is applied to the metallic sheet and generates eddy currents inside it. The eddy currents serve as a probe and reveal the sheet properties: thickness, electrical conductivity, distance, etc. When properly set, an eddy current instrument is very efficient for measuring thin metallic layer thickness. The example below shows how the eddy current method is well correlated with several optical methods.
EXPERIMENTAL RESULTS
The photo below shows 3 metalized film samples. From left to right, we can see that they are more or less transparent. This is due to the thickness of the metallic layer: the film is more transparent when the metallic layer is thin, and inversely.
When the metallic layer is thin, optical methods can be applied and get a good result. However, when the metallic layer attains 1 micrometer, it becomes opaque and the optical methods are no longer sensitive to the metal thickness. For instance, the 1st sample from the left is totally opaque with laser light.
Unlike the optical methods, the eddy current method still is sensitive to a metallic layer thickness up to several millimeters. In general, the thickness of a thin metallic deposit layer is under 1 micrometer. That is why eddy current and optical methods are complementary and can be used together to cover a wide range of thicknesses.
The obtained eddy current signals are strongly correlated with the images on figure 2. As we can see, the 1st sample from the left is totally opaque; it corresponds to the high amplitude signal. The next 2 samples are less opaque; they correspond to the next weaker signals. For these 2 samples, the right half is more transparent which reveals a lower thickness. This corresponds to a decrease in amplitude in each signal. The first signal also has a decreasing amplitude; this corresponds perfectly to the thickness profile desired by the manufacturer, but we cannot see it on the image. Indeed, the metallic layer is too thick and does not let the light get through, so we cannot see the decrease in metallic thickness. On the contrary, the eddy current method is sensitive to this thickness and allows observing the thickness variation. In this case the eddy current method shows its superiority compared to the optical methods.
The eddy current method is an indirect method and therefore needs to be calibrated. That is why Sciensoria provides a software package, the WinEC(™), with each Z-Scope*SRK. The software helps users to calibrate the instrument with respect to standard gauges and to suppress lift-off effect.
CONCLUSION
The eddy current method is an efficient alternative means for measuring the thickness of thin metallic deposit layers. It is not only quicker and easier to use, but also more efficient than optical methods when the measured layer is opaque. This makes the Z-Scope*SRK an ideal complementary measuring instrument to the existing standard tools.
This post is originally posted on this linkedin post from Minh-Quang Sciensoria
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