Cu Seed Composition and Thickness

Application Note RSMD-20101936

PCB panel

Introduction

Seed copper is deposited during the PCB manufacturing process, often over other Cu layers, a process that requires accuracy in the thickness of this thin layer. Measurement of the seed layer thickness for quality assurance is available using Energy Dispersive X-ray Fluorescence (EDXRF), which overcomes the challenge of differentiating seed copper from the Cu layer beneath.

XRF Analysis

  • XRF results of sample surface
  • The following elements were detected: Cu, Al, Si, S, Fe, Ni, Cu
  • Further analysis examples:
    • Light element detection
    • What other elements should be expected?

RSMD20101936 Figure 1 Acquisition time: 30 sec


RSMD20101936 Figure 2
Wet etch

  • Etch performed by dipping the sample (for one minute) in a solution containing:
  • H₂0₂ (3%)
  • Citric acid
  • NaCl

Measurement objective

To measure the thickness of the top, electroless-deposited Cu seed layer at several sample locations.

Electroless Cu seeding (1 µm)

RSMD20101936 Figure 3

  • ABF, Ajinmoto Build-up Film, is a carbon-based epoxy film developed by Ajinmoto Co.
  • FR-4, flame retardant class 4, is a type of laminated substrate material used in the manufacture of printed circuit boards (PCBs).

Top Cu layer isolation

  • The top Cu layer was removed from one of the samples using wet etch.
  • XRF comparison was made between etch and non-etch samples.
  • Cu Lα comes only from the top Cu layer.

RSMD20101936 Figure 4

Cu Lα saturation test

  • For the top Cu layer, 1 µm is much below the saturation depth.
  • Saturation in XRF is a specific depth of the layer whereby intensity no longer changes beyond this layer thickness.
  • Thickness changes correlate to changes in the current.
  • XRF measurement was performed at the same location using varying currents in order to determine the ideal linear section of the intensity vs. current curve.

Cu thickness sample comparison

  • XRF scan was performed over six points (1 mm step) on three different samples.
  • Cu Lα peak intensity was compared.
  • The average intensity of Samples B and C were used as reference with thickness of 1 µm.
  • Sample A has significantly thinner top Cu layer than Samples B and C.
  • The top Cu layer of Samples B and C has uniform thickness, unlike Sample A.

Cu intensity

RSMD20101936 Cu intensity


Intensity vs. current

RSMD20101936 Intensity vs current
Across-wafer variation

RSMD20101936 across-wafer variation
Cu layer thickness [µm]

Point Sample A Sample B Sample C
1 0.42 1.00 1.00
2 0.52 1.01 1.01
3 0.64 1.01 1.01
4 0.65 1.01 0.98
5 0.74 0.97 1.01
6 0.78 0.99 0.99
Average 0.62 1.00 1.00

Cu seed thickness measurement by direct and indirect methods

  • Cu layer thickness is measured by an automated algorithm using two methods.
  • Direct method (0–0.7 µm) measuring the Cu Lα peak and calculating Cu layer thickness using linear regression
  • Indirect method (0.7–1.4 µm) measuring the Si Kα peak and calculating Cu layer thickness using exponential regression
  • Cu Lα peak intensity is saturated due to its low energy (0.93 keV), so the Cu layer thickness is measured indirectly using the Si from the ABF layer.

RSMD20101936 Figure 5

  • Cu Lα and Si Lα were clearly detected by the EDXRF.
  • Cu upper layer thickness can be monitored by using both peaks independently – Cu Lα or Si Kα peaks.
  • Acquisition time: 12 sec
Nominal [µm] Direct Method [µm] Indirect Method [µm]
0.4 0.398 0.382
0.5 0.496 0.462
0.6 0.613 0.571
0.7 0.689 0.704
0.8 - 0.788
0.9 - 0.888
1.0 - 0.960
1.1 - 1.083
 

Regression curves

  • Linear regression for the direct method
  • Exponential regression for the indirect method

RSMD20101936 Figure 6
Roughness

  • 3D scanner was used for measuring roughness by measuring surface height variations.
  • Height variation up to 2 µm

3D scan imaging

RSMD20101936 3D Scan Imaging


2D microscope image

RSMD20101936 2D Microscope image


RSMD20101936 Figure 7Summary

  • XRF is very sensitive to thickness variation up to 0.1 nm. Many elements can be analyzed, qualitatively and quantitatively.
  • Top Cu layer thickness can be measured using Cu Lα peak intensity changes.
  • For accurate results, standards are required.
  • The 3D scanner can monitor surface roughness.

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