Eddy Current Thickness Measurement Research Articles

A Dual Function Coating Thickness Measuring Device

In engineering applications, the harsh external environment leads to the erosion of the surface of the tower. The existence of the coating layer provides better protection for the tower, but the uniformity of the thinness and thickness of the coating layer has a great impact on the performance of the material. In this paper, a dual-function tower coating thickness measurement device is designed. The device can automatically measure the thickness of the coating layer by magnetic induction or eddy current thickness measurement principle for the towers with magnetic or non-magnetic materials. The small prototype experiments show that the average error of the measurement of the non-magnetic substrate is only 0.74%, and the average error of the measurement of the magnetic substrate is only 6%, and it has the characteristics of low cost and long service life.

Edge Effects of an Eddy-Current Thickness Sensor During Chemical Mechanical Polishing

Accurate and in-situ thickness measurements of copper films on Si-based wafers are employed to improve global planarization, adjust polishing pressure, and avoid over-polishing and under-polishing during chemical mechanical polishing (CMP). However, edge effects cause inaccurate thickness measurements at the wafer edge, and greatly decrease the effective area of integrated circuit (IC) manufacturing. In this study, edge effects were studied using both a trans-dimensional simulation model of electromagnetic fields and circuit coupling in eddy-current thickness measurement and an experimental nanoscale thickness-measurement sensor system with a resolution of 2.07 Å /mV. The simulation and experimental results were in close agreement when the sensor coil was located at different edge positions. The influence of different factors such as copper film thickness, edge positions, calibrated thickness, lift-off distance, and excitation voltages on thickness-measurement performance in the edge region were also examined. Copper film surface current density was calculated to clearly evaluate eddy-current distortion at different edge positions. The determined influences of these parameters on edge effects could be used to improve the edge-measurement performance of sensor systems by adjusting coil and circuit parameters or propose an edge signal compensation algorithm during CMP.

Thickness Measurement Based on Eddy Current Sensor With Coaxial Double Coil for Chemical Mechanical Polishing

Real-time and accurate thickness measurements of copper films on 300-mm silicon wafers are utilized to improve global planarization, control polishing pressure, and avoid dishing and erosion caused by under-polishing and over-polishing during chemical mechanical polishing (CMP) of metals. Nevertheless, the requirements of nanometer-scale measurement sensitivity, wide measurement range from tens of nanometers to 2 μm, and high edge measurement performance make eddy-current thickness measurement extremely challenging. Therefore, we proposed a concise and effective theoretical model to analyze the measurement range and sensitivity in both the amplitude- and phase-based methods. The advantages and disadvantages of the two methods are compared and discussed systematically through simulations and experiments. The effects of the lift-off distance and frequency of excitation signal on measurement sensitivity, range, and edge measurement performance are also examined. The results show that the proposed theory can be used to improve measurement range and sensitivity. And the double-sensor system demonstrated better edge measurement performance than a single coil. Finally, a double-coil eddy current thickness measurement system designed with an average sensitivity of 1.9 mV/nm, a measurement range of 60 nm to 2 μm, and an edge attenuation position of 141.5 mm at a lift-off distance of 3.5 mm can be utilized in the amplitude-based method for advanced Cu-CMP.

The Use of Projection Methods of Multivariate Analysis in Eddy Current Thickness Measurement

Multifrequency eddy current measurements of the thickness of non-magnetic metallic materials with dielectric coatings have been carried out. Based on the principal component analysis, the influence of competing factors such as electrical conductivity, thickness of the metal substrate and thickness of the dielectric layer is separated. Using the projection method on latent structures, eddy current measurements have determined the numerical values of the thicknesses of aluminum and copper plates and dielectric coatings.

Adjusting LOI for Enhancement of Pulsed Eddy Current Thickness Measurement

The liftoff point of intersection (LOI) is a desirable signal feature in the pulsed eddy current (PEC) testing. It is immune to liftoff effect caused by the change of liftoff distance between a probe and a sample. However, adjusting LOI remains an open problem for enhancement of PEC evaluation or extends the application of LOI to thicker samples. In this paper, LOI point could be adjusted by varying the rising time of pulse excitation to extend the measurement range or increase sensitivity for enhancement of the PEC evaluation. The simulations and experiments demonstrate that the difference PEC signal increases and decreases significantly slowly when the rising time of pulse excitation increases. When the rising time of pulse excitation increases, the measurement range of LOI amplitude increases, and the measurement range of LOI time decreases, whereas the sensitivity of LOI amplitude decreases, and the sensitivity of LOI time increases. This investigation helps us to optimize the rising time for the improvement of specific PEC evaluation.

Multi-frequency method of control for eddy current thickness measurement

When eddy-current thickness measurement is carried out, one of the disturbing factors leading to an error in determining the thickness of the conductive coating on a conducting ferromagnetic or non-ferromagnetic substrate are the variations of the electromagnetic parameters of the coating and the substrate observed when the transducer moves from point to point along the surface of the controlled product, when moving from one product to another, at presence of heat treatment or other thermal effects on the controlled product after coating. The paper presents the results of experimental studies of the influence of variations in the electromagnetic parameters of a conducting ferromagnetic substrate on the phase of the emf, introduced into the superimposed transducer. It is shown, when the minimum influence of such variations on the specified phase is achieved. As a result, it was suggested to use the multi-frequency method to reduce the influence of variations of electromagnetic parameters on the accuracy of determining the coating thickness during application of the phase control method. It consists in the fact that the frequency of the excitation current of the transducer, mounted on the monitored product, is discretely reduced from a certain maximum to a certain minimum frequency during measurements. At the high frequency, the specific electric conductivity of the coating material is taken into account, with decreasing frequency, such a value is determined when the electromagnetic parameters of the substrate begin to affect the phase formation. Then, using the calibration curve obtained from samples from the same coating material and substrate as the controlled article and having a known coating thickness, the desired coating thickness on the product to be tested is determined.

Application of projection methods for multivariate analysis in eddy current thickness measurement

Application of projection methods for multivariate analysis in eddy current thickness measurement

INFLUENCE OF VARIATIONS OF THE CONDUCTIVITY OF UPPER LAYER OF TWO-LAYER SAMPLE ON OF PHASE INTRODUCED ELECTROMOTIVE FORCE OF SUPERIMPOSED TRANSDUCER OF EDDY CURRENT THICKNESS METER

In carrying out eddy current thickness measurement of two-layer conductive objects one from the interfering factors is the presence of variations in the value of the electrical conductivity of the material of the upper layer (coating) when moving from point to point on the surface of object of control or when passing from one object of control to another. The aim of this work is to evaluate the accuracy of determining the thickness of the conductive coating disposed on a conducting ferromagnetic basis, using the phase method of eddy current testing. The reason of the error is variation of the electrical conductivity of the material of coating. Determination of the error is based on calculations using known analytical expressions for the loop with current of sinusoidal form arranged over the infinite half space with a covering as a thin layer. Selected in calculating electromagnetic parameters of coating and substrate approximately correspond to the case -chromium layer on a nickel base. Calculations are performed for different frequencies of current passed through coil. It is shown that at reduction of frequency of the current passes through the coil the error is reduced. The value of the lowest possible operating frequency of the excitation current is determined by the condition of absence influence on the phase introduced into the superimposed transducer emf variations in the thickness of the basis. To reduce the indicated error it is proposed to determine, on the basis of phase method at a relatively high frequency transducer current excitation, conductivity of the material of coating. After this, at a low frequency excitation current and using phase method, the coating thickness is determined, taking into consideration the previously determined value of the conductivity of coating. Also discussed ways to improve the accuracy of phase measurements in the MHz region of the excitation current frequency.

Pulsed eddy current thickness measurement using phase features immune to liftoff effect

Conventionally, peak value and peak time are extracted from pulsed eddy current (PEC) response as features for thickness measurement. However, they suffer from liftoff variations. In this work, the phase of spectral PEC response from a Hall sensor are proposed to serve as robust features for thickness evaluation. The presented novel features are immune to liftoff effect, because phase signals remain nearly constant against liftoff variations. An analytical model was formulated, and simulations were performed to uncover the physics of the characteristics of the phase feature and establish the relationship between the phase feature and sample thickness. Experiments were carried out to validate the proposed phase feature. Eventually, the proposed phase feature was evaluated for accurate thickness measurement and some key factors were discussed.

Pulsed eddy current thickness measurement of conductive layers over ferromagnetic substrates

The focus of this study is the behaviour of the lift-off point of intersection (LOI) under various test conditions: lift-off, thickness and conductivity of metallic layers with and without a ferromagnetic substrate. The pulsed eddy current (PEC) signal response is analyzed and a lift-off independent method to measure the thickness of conductive layers over ferromagnetic substrates is presented. This approach is tested by scanning specimens with varying lift-off and conductive layer thickness. The test articles have different substrate permeabilities, but identical lift-off and conductive layer conditions. The results demonstrate that the LOI feature can be used to measure the thickness of conductive layers over ferromagnetic substrates.