Using XRF Spectrometry to Analyze the Film Thickness of Multi-Layered Plating Used on Cards

The contact areas of IC chips and electronic devices are normally coated with a three-layer plating of nickel (Ni), gold (Au), and copper (Cu). Non-destructive measurement of the amount of plating material deposited (film thickness) can be done using X-ray fluorescence (XRF) spectrometry.

This article describes a simple quantitative analysis of Au, Ni, and Cu film of three-layer plating by using the thin-film fundamental parameter (FP) technique without using standard samples is established.

Sample

  1. SIM card, IC chip
  2. Certified Reference Material: NMIJ CRM 5208-a, 20 mm × 20 mm

IC chip (left) and SIM card (right).

Figure 1. IC chip (left) and SIM card (right).

Elements and Layers of Plating

Figure 2 shows the layers and elements of the plating.

Elements and layers of plating.

Figure 2. Elements and layers of plating.

Sample Pretreatment

Without any pretreatment, the samples were directly placed on the sample stage.

Quantitative Analysis of Film Thickness and Amount of Deposition

The thin-film FP technique was used to quantitatively analyze the layer of each metal — Cu, Ni, and Au. The analysis diameter was fixed to 1 mmϕ.

1. Certified Reference Material NMIJ

Table 1 shows the analysis results of central point of each layer. An excellent result was obtained in that the error of the quantitative value of each layer was less than 5% of the certified value.

Table 1. Results of the quantitative analysis of NMIJ CRM 5208-a

[μg/cm2]

Element/Layer Au Ni Cu
Quantitative Value 192 862 852
Certified Value 184 869 880
(Uncertainty) (5) (17) (14)

 

2. IC Chip (IC) and SIM Card (SIM)

Table 2 shows the analysis results of one central point on the IC and the SIM.

Table 2. Quantitation results of IC and SIM

[μg/cm2]

Element/Layer Au Ni Cu
IC 71.0 1,700 25,275
SIM 76.3 1,673 23,941

 

Formula for the Amount of Deposition and Film Thickness

In XRF spectrometry, the analysis result is estimated as the amount of deposition. After that, the following formula is used for calculating the film thickness with an assumed density.

In this measurement test, the densities (g/cm3) of Ni, Au, and Cu were assumed to be 8.90, 19.3, and 8.94, respectively. The film thicknesses computed using the values of Table 2 are given in Table 3.

Table 3. Film thickness of IC and SIM

[μm]

Element/Layer Au Ni Cu
IC 0.037 1.91 28.3
SIM 0.040 1.88 26.8

 

Of the three layers, the Cu layer was the thickest, with a thickness of about 30 μm. In such a thick area of Cu layer, it is difficult to achieve adequate quantitation accuracy. Therefore, the film thickness of the Ni and Au layers were re-calculated by assuming that the thickness of the Cu layer was infinite. Table 4 shows the final results of the film thickness and the amount of deposition.

Table 4. Amount of deposition and film thickness at a central point with the Cu layer having an infinite thickness

Element/Layer Au Ni Cu
Amount of deposition [μg/cm2]
IC 70.9 1,782
SIM 76.3 1,756
Film thickness [μm]
IC 0.037 2.00
SIM 0.040 1.97

 

3. Spectra

Figure 3 illustrates the spectra of the analytical lines of each layer. The peak was clear when the thickness of the Au layer was thin, only about a few dozen nanometers, signifying the high sensitivity of the analysis.

Standard X-ray emission lines were used for analytical lines: NiKα, AuLα, and CuKα. When analytical lines of the element of each layer are close to each other, Kβ and Lβ lines could be used.

Spectra of analytical lines.

Figure 3. Spectra of analytical lines.

Repeatability

A repeatability test was carried out by testing the IC chip continually for 10 times with an analysis diameter of 1 and 3 mmϕ. The amount of deposition of Ni and Au layer was assessed by assuming that the thickness of Cu layer was infinite. Table 5 shows the results of the repeatability test.

Table 5. IC chip quantitation repeatability

[μg/cm2]

  Au Ni Analysis Diameter
Average 70.0 1,709 1 mmϕ
Standard Deviation 0.38 3.2
Coefficient of Variation [%] 0.55 0.19
Average 69.5 1,723 3 mmϕ
Standard Deviation 0.41 3.0
Coefficient of Variation [%] 0.59 0.17

 

Relationship Between Theoretical X-ray Intensity and Thickness of Cu Film

Figure 4 shows the relationship between the Cu film thickness (μm) and the theoretical X-ray intensity. The upper quantitation limit is roughly 18 μm when the intensity of the saturation thickness is determined to be 90% of the saturation intensity achieved when the Cu film has an infinite thickness (JIS H 8501).

Relationship between X-ray Intensity and Thickness [μm] of Cu Film.

Figure 4. Relationship between X-ray Intensity and Thickness [μm] of Cu Film.

Conclusion

Analysis of the film thicknesses of a three-layer plating of nickel, gold, and copper was performed in a simple manner with high precision and sensitivity in the order of nanometers to micrometers using small analysis diameters of 1 and 3 mmϕ. Using the standard analysis diameter of 10 mmϕ, it might be possible to quantitate even thinner areas. As shown in this study, XRF spectrometry is useful for film thickness measurement.

Moreover, XRF spectrometry can also easily assess the elements and their amounts used in the material. For instance, it can be used to manage the used amount and seize the recovered amount during recycling of precious metals such as palladium (Pd), gold, rhodium (Rh), and platinum (Pt) that are used in films of plating and physical vapor deposition.

Measurement Conditions

Instrument : EDX-8000 / (7000)
Element - Analytical Line : AuLα, NiKα, CuKα
Analysis Method : Thin-film FP method
X-Ray Tube : Rh Target
Detector : SDD
Tube Voltage - Current : 50 [kV] - Auto [μA]
Collimator : 1, 3 [mmϕ]
Primary Filter : None
Measurement Atmosphere : Air
Integration time : 100 [s]
Dead Time : Max. 30 [%]

 

This information has been sourced, reviewed and adapted from materials provided by Shimadzu Scientific Instruments.

For more information on this source, please visit Shimadzu Scientific Instruments.

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