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Giovanni D’Amore discussed the use of impedance analyzers and professional fixtures to characterize dielectric and magnetic materials.
We are accustomed to thinking about technological progress from mobile phone model generations or semiconductor manufacturing process nodes.These provide useful shorthand but obscure advances in enabling technologies (such as the field of materials science).
Anyone who has taken apart a CRT TV or turned on an old power supply will know one thing: You cannot use 20th century components to make 21st century electronics.
For example, rapid advances in materials science and nanotechnology have created new materials with the characteristics needed to build high-density, high-performance inductors and capacitors.
The development of equipment using these materials requires accurate measurement of electrical and magnetic properties, such as permittivity and permeability, over a range of operating frequencies and temperature ranges.
Dielectric materials play a key role in electronic components such as capacitors and insulators.The dielectric constant of a material can be adjusted by controlling its composition and/or microstructure, especially ceramics.
It is very important to measure the dielectric properties of new materials early in the component development cycle to predict their performance.
The electrical properties of dielectric materials are characterized by their complex permittivity, which consists of real and imaginary parts.
The real part of the dielectric constant, also called the dielectric constant, represents the ability of a material to store energy when subjected to an electric field.Compared with materials with lower dielectric constants, materials with higher dielectric constants can store more energy per unit volume, which makes them useful for high-density capacitors.
Materials with lower dielectric constants can be used as useful insulators in signal transmission systems, precisely because they cannot store large amounts of energy, thereby minimizing the signal propagation delay through any wires insulated by them.
The imaginary part of the complex permittivity represents the energy dissipated by the dielectric material in the electric field.This requires careful management to avoid dissipating too much energy in devices such as capacitors made with these new dielectric materials.
There are various methods of measuring the dielectric constant.The parallel plate method places the material under test (MUT) between two electrodes.The equation shown in Figure 1 is used to measure the impedance of the material and convert it to a complex permittivity, which refers to the thickness of the material and the area and diameter of the electrode.
This method is mainly used for low frequency measurement.Although the principle is simple, accurate measurement is difficult due to measurement errors, especially for low-loss materials.
The complex permittivity varies with frequency, so it should be evaluated at the operating frequency.At high frequencies, the errors caused by the measurement system will increase, resulting in inaccurate measurements.
The dielectric material test fixture (such as Keysight 16451B) has three electrodes.Two of them form a capacitor, and the third provides a protective electrode.The protective electrode is necessary because when an electric field is established between the two electrodes, part of the electric field will flow through the MUT installed between them (see Figure 2).
The existence of this fringe field can lead to erroneous measurement of the dielectric constant of the MUT.The protection electrode absorbs the current flowing through the fringe field, thereby improving the measurement accuracy.
If you want to measure the dielectric properties of a material, it is important that you only measure the material and nothing else.For this reason, it is important to ensure that the material sample is very flat to eliminate any air gaps between it and the electrode.
There are two ways to achieve this.The first is to apply thin film electrodes to the surface of the material to be tested.The second is to derive the complex permittivity by comparing the capacitance between the electrodes, which is measured in the presence and absence of materials.
The guard electrode helps to improve the measurement accuracy at low frequencies, but it may adversely affect the electromagnetic field at high frequencies.Some testers provide optional dielectric material fixtures with compact electrodes that can extend the useful frequency range of this measurement technique.Software can also help eliminate the effects of fringing capacitance.
Residual errors caused by fixtures and analyzers can be reduced by open circuit, short circuit and load compensation.Some impedance analyzers have built-in this compensation function, which helps to make accurate measurements over a wide frequency range.
Evaluating how the properties of dielectric materials change with temperature requires the use of temperature-controlled rooms and heat-resistant cables.Some analyzers provide software to control the hot cell and heat-resistant cable kit.
Like dielectric materials, ferrite materials are steadily improving, and are widely used in electronic equipment as inductance components and magnets, as well as components of transformers, magnetic field absorbers and suppressors.
The key characteristics of these materials include their permeability and loss at critical operating frequencies.An impedance analyzer with a magnetic material fixture can provide accurate and repeatable measurements over a wide frequency range.
Like dielectric materials, the permeability of magnetic materials is a complex characteristic expressed in real and imaginary parts.The real term represents the material’s ability to conduct magnetic flux, and the imaginary term represents the loss in the material.Materials with high magnetic permeability can be used to reduce the size and weight of the magnetic system.The loss component of magnetic permeability can be minimized for maximum efficiency in applications such as transformers, or maximized in applications such as shielding.
The complex permeability is determined by the impedance of the inductor formed by the material.In most cases, it varies with frequency, so it should be characterized at the operating frequency.At higher frequencies, accurate measurement is difficult due to the parasitic impedance of the fixture.For low-loss materials, the phase angle of the impedance is critical, although the accuracy of the phase measurement is usually insufficient.
Magnetic permeability also changes with temperature, so the measurement system should be able to accurately evaluate temperature characteristics over a wide frequency range.
The complex permeability can be derived by measuring the impedance of magnetic materials.This is done by wrapping some wires around the material and measuring the impedance relative to the end of the wire.The results may vary depending on how the wire is wound and the interaction of the magnetic field with its surrounding environment.
The magnetic material test fixture (see Figure 3) provides a single-turn inductor that surrounds the toroidal coil of the MUT.There is no leakage flux in the single-turn inductance, so the magnetic field in the fixture can be calculated by electromagnetic theory.
When used in conjunction with an impedance/material analyzer, the simple shape of the coaxial fixture and the toroidal MUT can be accurately evaluated and can achieve a wide frequency coverage from 1kHz to 1GHz.
The error caused by the measurement system can be eliminated before the measurement.The error caused by the impedance analyzer can be calibrated through three-term error correction.At higher frequencies, low-loss capacitor calibration can improve phase angle accuracy.
The fixture can provide another source of error, but any residual inductance can be compensated for by measuring the fixture without the MUT.
As with dielectric measurement, a temperature chamber and heat-resistant cables are required to evaluate the temperature characteristics of magnetic materials.
Better mobile phones, more advanced driver assistance systems and faster laptops all rely on continuous advancements in a wide range of technologies.We can measure the progress of semiconductor process nodes, but a series of supporting technologies are developing rapidly to enable these new processes to be put into use.
The latest advances in materials science and nanotechnology have made it possible to produce materials with better dielectric and magnetic properties than before.However, measuring these advances is a complicated process, especially because there is no need for interaction between the materials and the fixtures on which they are installed.
Well-thought-out instruments and fixtures can overcome many of these problems and bring reliable, repeatable and efficient dielectric and magnetic material property measurements to users who do not have specific expertise in these fields.The result should be a faster deployment of advanced materials throughout the electronic ecosystem.
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