SIMS analysis: which analysis conditions should be chosen?

1. Detection limit

The detection limit of an element is the smallest concentration of this element that can be detected for a given material and for given analysis conditions. In a SIMS concentration profile, several factors can be the source of a detection limit:

• the signal absence,
• the residual atmosphere (for elements H, C, O and N),
• a mass interference,
• a contamination of the analysis chamber.

The best analysis conditions to choose are different for each one of these cases. It is important to note that detecting a signal does not mean that the sample contains the searched element.

1.1 Limit due to a signal absence

The first issue, of course, is to choose the most appropriate analysis mode: Cs⁺ or O₂⁺ primary ions, secondary ions polarity, detected species. The factors that should be taken into account to make the right choice are described in « Physical description of SIMS analysis ». An optimal set of parameters is a compromise between advantages and drawbacks:

• low number of elements analyzed during one analysis (high data density),
• a large crater (low sputtering rate but possibility of high signals or good depth resolution),
• a large image diaphragm (high signals but low depth resolution or low sputtering rate),
• the largest cross-over diaphragm and slit (high signals but low mass resolution),
• an intense primary beam or a long integration time (high signals but low data density),
• …

A low sputtering rate results sometimes in a degraded detection limit due to in situ organic contamination. The use of a large crater (with a large image diaphragm) is useful to preserve a sufficient data density as a function of depth when thin layers are analyzed.

1.2 Limit due to the residual atmosphere

Atmospheric elements (H, C, N, O) are present in any analysis chamber residual atmosphere, even in ultra-vacuum. When the concentrations of these elements in a sample are too low, the detected signals do not come from atoms located inside the sample: in fact, these signals come from atmospheric atoms adsorbed at the sample surface. Then, the detection limits are the concentrations below which this phenomenon becomes prevalent. When the concentrations are lower than the detection limits, the analysis can only provide an upper bound of these concentrations (in other words, the true concentrations are lower than the concentrations indicated by the profiles).The detection limits can be lowered by using:

• a long pumping time before analysis,
• a high sputtering rate (at the expense of depth resolution).

1.3 Limit due to mass interference

A mass interference occurs when different ions that have very similar masses are simultaneously detected. Examples:

• the masses of ³¹P and ³⁰Si¹H are very close to 31amu,
• the masses of ²⁸Si, ²⁷Al¹H and ¹⁴N₂ are very close to 28amu,
• the masses of ⁷⁵As and ³⁰Si²⁹Si¹⁶O are very close to 75amu.

Several solutions exist:

• detecting other isotopes or even another species,
• using high mass resolution settings,
• using the energy filtering method,
• decreasing the interference signal: if an atmospheric element is involved in the interference, as for SiH or Si₂O, it can be considered that the detection limit is due to the residual atmosphere and then the approach described above is required.

1.3.1 Detecting other isotopes

When the analyzed element has several isotopes and when its natural isotopic abundances are respected, several isotopes can be advantageously detected. As analysis data are treated, the isotopes signals ratios are compared to the natural isotopic abundances. If one isotope have its signal higher t han expected, it means that a mass interference exist with this isotope. On the contrary, if natural abundances are perfectly respected, it is nearly sure that the detected signals indicate a real presence of the element.

1.3.2 High mass resolution

When interfering species masses are not too close to each other, they can be separated by the mass slit thanks to high mass resolution settings.

1.3.3 The energy filtering method

This method uses the energy slit as a filter. As monoatomic and molecular ions have different energy distributions, secondary voltage and energy slit are tuned so as to discriminate monoatomic ions versus molecular ions (figure 1).

2. Depth resolution

The depth resolution is usually given as « nm/decade » (number of depth nanometers which are necessary to obtain a 10 factor signal variation from a perfectly abrupt interface). Several features may decrease a concentration profile depth resolution:

• the sample surface roughness,
• the crater side effects,
• the collisional mixing effect,
• the crater bottom inclination.

A good quality of the sample surface is essential to obtain a good depth resolution. The next sections describe conditions that optimize the depth resolution when the surface is perfectly flat. With perfect conditions, the depth resolution can reach approximately 1nm/decade and enables to detect details such as quantum wells. With bad conditions (for instance when the detection limit is privileged against any other performance), the depth resolution can reach several tens of nm/decade. With a very rough surface, the depth resolution is limited anyway by this roughness.

2.1 Side effects

The primary beam digs a flat bottomed crater, but its sides can be angled. When the primary beam bombards the crater borders, it ejects ions which do not come from the crater bottom. If big quantities of such ions are detected, the concentration profile « depth » information is distorted. The solution consists in preserving only the ions emitted close to the crater centre. It can be done with the image diaphragm. A diaphragm diameter much smaller than the crater size is usually used so as to have a margin near the edges. The signal decreases in the same proportion as the aperture surface.

2.2 Collisional mixing

The collisional mixing phenomenon consists in a superficial atomic layers mixing due to primary ions implantation. It can be minimized by using a low primary ions impact energy so as to decrease primary ions penetration depth.

2.3 Crater bottom inclination

During an analysis, as long the primary beam digs the crater, the crater bottom may gradually form a small angle relatively to the surface plan. This feature is illustrated in broad outline on figure 2. On this figure, the angles are exaggerated: the real angles can reach approximately 0.02°, then generating a few tens nanometers depth difference between one side and the opposite side.

This phenomenon occurs when the primary beam incidence angle relatively to the surface is small, which is the case when impact energy is low. In this case, the incidence angle significantly changes during the scanning. The depth difference from one crater side to the opposite side degrade the depth resolution. This deterioration is as weak as:

• the image diaphragm is narrow,
• the crater depth is small (at a few tens nanometers depth, the resolution remains quite good; at higher depths, it is gradually degraded).