After Removing ~8 nm (80 angstroms)

Argon Ion (Ar⁺) Etching
can be used for Sample Preparation

Argon (Ar⁺) Ion Etching the surface, while sample is inside the analysis chamber, is the Standard Method to Remove Material from the surface of any Metal Surface or Inorganic Surface. Polymer Surfaces require a Cluster beam of Ar⁺ ions.

Effects of Argon Ion Etching are readily visible from SEM images (below) which are images of the top 50 angstroms of a piece of highly polished super alloy. This sample was cut from the bulk of a jet engine blade and was mapped using Auger. These images are used here to demonstrate the cleaning effect of Argon ion etching.

The Ar+ ion gun at 2 kV was turned on for 0 seconds, 10 seconds, 60 seconds, and 120 seconds to reveal and demonstrate the visible effects of sequential ion etching. After a total of 120 seconds of ion etching a comparison shows the progressive removal of the surface contamination, the metal oxides that had formed during polishing, and the actual alloy.

The Ar+ ion etching at 2 kV is ~1 angstrom/second for SiO2 (the reference standard for ion etching). The standard ion etched crater is 4 x 2 mm which is ~3X bigger than the largest X-ray spot (0.8 x 1,4 mm). After 120 seconds of argon ion etching the actual grain structure starts to appear.

The 120 seconds of 2 kV ion etching is estimated to remove ~120 angstroms of surface materials. This is a rough estimate because polish remnants, oxides due to polishing, and the alloy itself have different ion etch rates. The actual final depth could be 100 angstroms or 140 angstroms. It is possible that some of the elements in the super alloy were preferentially ion etched which affects the atom% composition.

Graphic displays a depiction of Argon Ion Etching of a Surface, and displays
4 SEM images that show the progressive effect of Argon Ion Etching

p-Si <111> Sample As-Received – NO Ion Etching
raw data

p-Si <111> Sample As-Received – NO Ion Etching
With Labels

p-Si <111> Sample – After Ar⁺ Ion Etching for 20 sec
raw data

p-Si <111> Sample – After Ar⁺ Ion Etching for 20 sec
With peak labels

Summary Atom % Table for Elements
Shows Change in Atom %s after “As-Received” Sample was “Ar+ Ion Etched”

Information about the sample received from the customer.

The sample is a polished Silicon wafer having a <111> crystalline structure that has been in a multi-wafer sample rack inside a sealed plastic bag for 1 year after being produced.
The final treatment of the polished wafer was a rinse with clean DI water, followed by rapid drying with warm Nitrogen gas, and then placed in the rack.
After the rack was filled with 50 identical wafers, the rack was placed inside a plastic bag, that was shrink-wrapped to exclude air while pulling a slight vacuum.
The surface is expected to be pure Silicon and has a trace level dopant to make it behave as a p-type semi-conductor.

Based on the atom%s of the elements detected by XPS, we make the following observations and interpretations.

By analyzing in the “As-Received” condition before any ion etching, we know that there was roughly 16 angstroms of Carbon on the as-received surface.
We also know that the amount of Oxygen decreased to almost zero (0) atom% because Ar+ ions removed roughly 31 angstroms of a layer that has Oxygen.
The amount of Silicon greatly increased after the ion etching by roughly 2X.
The ratio of Si : O in the as-received condition is roughly 1.7 : 1.0.
The amount of Nitrogen is very low, but it does increase by ~3X after the ion etching.
The Argon signal appears because the sample was ion etched with Ar+ ions which become implanted and trapped within the pure silicon lattice.
If the top 150 angstroms (15 nm) of this surface was pure SiO₂, then the ratio of Si : O would be roughly 1 : 2. Because the atomic ratio is far from 1 : 2, we know from experience, that top surface (~10 nm) the Silicon wafer, that should be pure Silicon, is actually a natice oxide of Silicon on top of the pure silicon wafer.
From experience we also know that pure silicon wafers often easily form a 20-25 angtrom thick layer of Silicon native oxide.
Because XPS measures the top 10 nm we can know that the atom% results are due to the presence of pure silicon and silicon native oxides.
To learn more, we run high energy resolution spectra that show multiple chemical state peaks from all of the main elements.
The Silicon native oxides usually include 4 different silicon oxides: Si₂O, SiO, and Si₂O₃, and SiO₂ which are due to Si ¹⁺, Si ²⁺, Si ³⁺ and Si ⁴⁺.

Summary Atom % Table for Elements