Quality Assurance using Eddy Current Testing

Process

The eddy current testing method is a nondestructive evaluation method. It is widely used for crack detection as cracks cause very large local conductivity changes. However, there are many other applications in which highly sensitive and spatially resolved conductivity analysis can help to solve various inspection tasks. The basic principle is shown below.

Non-contact testing solution for different applications:

• Sheet resistance
• Resistivity
• Metal thickness
• Electrical Anisotropy
• Distance (to conductive materials) / paint thickness
• Carbon fiber testing 
• Composition monitoring

Characteristics

• Contactless and nondestructive
• extremely fast / high sample rates (k samples / second)
• good automation abilities
• high sensitivity

Eddy Current Density

Since the sensitivity of eddy current inspection depends on the eddy current density at the defect location, it is important to consider the strength of the induced eddy currents at the defect location. Typically, a setup / frequency / sensor is selected which places the expected defect within a one standard penetration depth. This assures that the strength of the eddy currents would be sufficient to produce a flaw indication.

Penetration depth depends on to the permeability, conductivity of the material as well as the frequency.

Standard penetration depth

The standard penetration depth is a term that is relevant to select a good testing setup for the characterization bulk materials. The depth that eddy currents penetrate into a material is affected by the frequency of the alternating current, the electrical conductivity and magnetic permeability of the sample. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d or 1d) and is used as criteria of ideal measurement for the investigation of bulk materials. At three standard depth of penetration (3d), the eddy current density is down to only 5% of the surface density. So, defects or variation in deeper depth than this do not add recognizable to the measurement effect and therefore those are hardly detectable. Thus, a setup that achieves a standard penetration depth suitable (1d) to the depth of the characteristics of interest enables the best testing result. SURAGUS offers a wide range of sensors with different frequency ranges for optimum testing of materials with different properties.

Eddy Current Impedance

• X is the real part
• Y is the imaginary part
• Z is the magnitude
• Φ is the phase
• Rotated phase plot

At a given frequency and position the eddy current value is at P0. When bringing a sample into the field or moving it to a different location the complex value shifts towards P1.

Capacitive and resistive effects can be separated by interpreting the data. For example by projecting on x- or y-axis.

Measurement of Encapsulated Layers

One of the biggest advantages of the eddy current method compared to the four-point method is the possibility to measure hidden layers. A hidden layer is a layer that lies beneath another and is therefore not accessible from the outside. This is possible due to the characteristic of the eddy current method, which does not require direct contact with the layer under investigation. It works very effectively even at a distance of several millimeters from the layer or material under investigation.

For example, a non-conductive substrate might have an electrically conductive layer deposited underneath an insulating layer. In such cases, the eddy current method can characterize the conductive layer. It is even possible to separate different conductive layers from each other as long as their conductivities differ significantly. For a precise assessment of whether your particular application can be measured, please contact our team.

Eddy Current Sensor

Types of Eddy Current Sensors

• Sender: inductive coils
• Receiver: inductive coils, hall sensors, fluxgate sensors, GMR, SQUIDs

Types of Eddy Current Testing

• Single frequency
• Multi-frequency
• Spectral
• Impulse eddy current
• etc.

Setup for Layer Characterization

• Frontside reflection mode
• Backside reflection mode
• Transmission mode

Eddy current measurement setups include single-sided and double-sided setups. Both are widely used but there are advantages and disadvantages to each approach.

Double side transmission mode sensors and tools operate in large distances to the wafer and excel with high tolerance to vertical wafer surface positions. This means the same set can be used for immediate measurement of thin and thick wafers without requiring time consuming set up changes. With large sensor elements gaps up to 100 mm makes double sided transmission ideal for robot handling and the integration into process tools without requiring additional tool space or processing time.

Reflection mode (single side resp. frontside or backside) tools are best for small spot sizes down to 1 mm. The tradeoff is that precise distance control and comes with limitations in sensitivity. This enables penetration depth modification, which allows focusing on resistivity measurements to the surface regions of SiC, GaAs and Si materials on ingot and boule level.

Benefits of Wide and High Frequency Testing

Relevance of Frequency

• The frequency determines the signal strength

• Low sheet resistance materials require lower frequencies
• High sheet resistance materials require higher frequencies

Flexibility in the frequency range supports wide measurement range setups enabling the measurement over 6 decades with the same setup