Local variation in element concentration

Surface specific element quantification by small spot XPS

The analysis of an optically discernible discoloration often poses a particular challenge for the analyst, since even the smallest local concentration changes can lead to significant optical changes. A suitable analysis technique is therefore ideally:

  •      quantitative
  •      surface-sensitive
  •      sensitive
  •      focusable
  •      specific for elements and molecules

Modern XPS devices meet the majority of the requirements listed here and are therefore the method of choice when it comes to root cause investigations in the field of discoloration and stains on surfaces. The following example illustrates the possibilities:

 

Bond pad

Quantitative elemental analysis of small semiconductor structures

Optical discoloration of a bond pad

On a structured wafer, a microscopically visible light reflection was observed in the area of a bond pad (diameter: 100 μm) (see figure 1). To clarify the cause, small-spot XPS measurements were performed in the region of this reflex (Pos. 2) and on an inconspicuous reference position (Pos. 1). The analyzed spots each had a diameter of 20 μm. Based on the overview ­ spectra generated on these measurement spots, the elemental compositions in the area of the reference point and the defect area could be determined quantitatively. The results are listed in table 1.

 

The comparison of the XPS data listed in table 1 shows a local accumulation of aluminum as the cause of the optical disturbance. The quantitative composition of small surface areas provided by XPS analysis was of crucial importance to solve this problem.

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