edax - VIP: Variable Pressure Quantitative Analysis

ViP Quant is an EDAX-exclusive software package, designed to improve the accuracy of standardless quantification under low vacuum or variable pressure (VP) SEM conditions.

Variable Pressure vs. Conventional High-Vacuum Microanalysis

The Variable Pressure (VP) microscope can provide the advantage of working with samples in a more natural, uncoated condition compared to conventional, high vacuum instruments which require conductive samples or coatings. The primary problem for X-ray analysis associated with VP arises from beam spread, or skirting. Without EDAX ViP Quant, getting reliable results from complex samples is virtually impossible when operating at higher pressures in the VP mode.

Key Variables For EDS Quantitative Analysis

The analyst needs to be aware of the relationship and effects that parameters such as working distance (WD), accelerating voltage (kV) and VP conditions will have on electron beam and resulting EDS data.

High-Vacuum

Working Distance (WD)
DU Geometry
Accelerating Voltage (kV)
SEM Parameters (mag, etc.)

Low-Vacuum

Same as high-vacuum
Gas Pressure
Gas Type
Gas Path Length

We can examine the effects of VP in more detail using a special version of the Electron Flight Simulator from Small World. The program can model electron beam spread under a variety of conditions.

For instance, the following table shows the percentages of electrons and the distances scattered by varying chamber pressure while holding all other variables constant. Notice that even at pressures less than one Torr, the radius of the beam has already exceeded one millimeter.

Pressure (Torr)	% Unscattered Electrons	10% - 90% Radius (um)
0.1	91.3	81
0.2	82.9	429
0.4	70.7	769
0.8	48.9	1202
1.6	26.7	1619
3.2	8.4	2519

Electron beam spread is also a function of working distance or, more accurately, gas path beam length. The pictures show the percentage of electrons and the distance scattered at two different working distances, while holding all other variable constant.

Low-Vacuum at 15 kV, 0.8 Torr. 10mm vs 20mm Working Distance

10 mm: 71% Unscattered, 10-90% = 263 um

20 mm: 49% Unscattered, 10 - 90% = 1488 um

Simple Scatter Experiment

This example looks at the simple situation where a particle has a known composition.

This shows a small piece of MgSO4 on carbon tape. The blue circles represent the increased area being sampled as pressure increases in the SEM chamber.

Elements present in the area of interest remain constant with increasing pressure. Elements not present in the area of interest increase with increasing pressure.

When analyzing large / known samples:

Minimize impact of variables with shortest WD, largest kV and lowest Torr
Remove artifact / unwanted elements (if they are present) and perform quantification routine
Quantification results of standardless routine are nearly as accurate as high-vacuum conventional analyses

Complex Case Experiment

Here, we look at an example where the elements in the area of interest are also present outside the area.

The composition at our "spot" #1 is FeSi. However, due to beam skirting, we will be collecting X-ray data from the surrounding area, which contain additional quantities of Fe and Si, along with Mg and Ca.

The red spectrum shows the results obtained under "high" vacuum. The blue overlay shows the results obtained at lower vacuum. The data has been normalized to the largest peak (Si) for display purposes.

The quant results from ViP. Notice the Mg and Ca have been "zeroed" out. The empircal results for Fe and Si are very close to the theoretical values of 66.5% and 33.5%, respectively.

Analysis of small / unknown samples:

Minimize impact of variables with shortest WD, largest kV and lowest Torr
Verify artifact / unwanted elements (if they are present) and perform the ViP quantification analysis
Quantification results of standardless routine are nearly as accurate as the high-vacuum conventional analyses