Problems with the carbon or conductive coating
There are two main problems caused by differences in the thickness (or absence) of the carbon or conductive coating that can affect the emitted intensity of the analyzed x-rays. The first is from differential absorption of low energy x-rays from different thicknesses of carbon coat. The second is from retardation of electrons from differences in thickness (or absence) of the carbon or conductive coating. The first is mainly a problem only for very low energy x-rays such as oxygen or nitrogen, the second is mainly a problem for high energy x-ray at low overvoltages,
In the first, soft x-rays emitted from the sample are absorbed by the coating. Hence if the absorption is significant enough, then differences in the thickness of the coating between the standard and the unknown will produce a difference in the intensity of the x-ray detected from the sample and standards. Due to the non-linear and complex nature of the absorption (absorption edges) it is not possible to make a general statement regarding the magnitude of the effect. The following table gives several examples for absorption of several commonly measured soft x-rays in carbon coats of three different thicknesses:
Percent x-ray transmission (assume density of carbon is 2.7 gm/cm3):
|
|
10 nm (carbon) |
20 nm (carbon) |
40 nm (carbon) |
|
Ti Kα (19.76) |
0.99994 |
0.99989 |
0.99978 |
|
Si Kα (356.8) |
0.99903 |
0.99807 |
0.99615 |
|
Al Kα (557.2) |
0.99849 |
0.99699 |
0.99400 |
|
Mg Kα (904.8) |
0.99756 |
0.99512 |
0.99027 |
|
Na Kα (1534) |
0.99586 |
0.99175 |
0.98356 |
|
F Kα (6366) |
0.98295 |
0.96620 |
0.93355 |
|
O Kα (12,380) |
0.96712 |
0.93533 |
0.87484 |
|
N Kα (25,490) |
0.93349 |
0.87140 |
0.75935 |
Another way in which the carbon coat can affect the emitted intensities is due to the slowing down (retardation) of the primary beam electrons in the coating. This slowing down of the primary electrons results in an effective loss in ionization efficiency by the incident electrons. For x-rays with a moderate to high overvoltage this reduction in ionization efficiency is negligible, but for elements with an over voltage less than 1.5 (for example Ni Ka excited at 10 keV), not accounting for the differences in the thickness (or absence) of a carbon or conductive coating could affect the generated intensity significantly.