JP2605630B2 - Secondary ion mass spectrometry - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary ion mass spectrometry (SIMS) widely used for analyzing impurities such as ICs and LSIs.

[0002]

2. Description of the Related Art In a SIMS, a predetermined portion (measured region) of a sample is irradiated with an ion beam (primary ion beam) such as Cs ⁺ , O ₂ ⁺ , Ga ⁺ or a neutral particle beam such as Ar as a primary beam. This is an analysis method for evaluating the types and concentrations of elements contained in the surface of a sample by mass analysis of ions (secondary ions) generated by the method.
Since the density distribution of the primary ion beam is widened as described in JP-A-195954, the periphery of the measured area of the sample is also sputter-etched during the irradiation of the primary ion beam. Therefore, multiple ions (molecular ions or multi-charged ions) are generated in the surrounding area that contain the element to be measured in a concentration equal to or higher than the area to be analyzed (measurement area) or have a mass-to-charge ratio almost equal to that. In such a case, the obtained analysis result actually includes information on the surrounding area, and the lower detection limit is significantly deteriorated.

[0003]

One technique for solving such a problem is disclosed in the above-mentioned publication.
That is, as shown in FIG.
The groove 103 is formed by subjecting the periphery of the area to be measured to sputter etching with a focused beam. By irradiating a primary in-on beam to the remaining columnar measurement area 102, information from the surrounding area is not taken in. Although this method does not prevent the primary ion beam from hitting the peripheral region, the evaluation of the semiconductor material by SIMS is effective because it is an analysis in the depth direction such as an impurity profile.

However, when the columnar measured region 102 is sputter-etched with the primary ion beam, the electric field distribution becomes non-uniform, and therefore, as shown in FIG. Since the ion beams are gathered and etched unevenly, it is difficult to obtain an accurate distribution in the depth direction.

An object of the present invention is to provide a secondary ion mass spectrometry method capable of reducing interference information from a peripheral area and uniformly etching a measured area with a primary ion beam.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a secondary ion mass spectrometric method for irradiating a predetermined portion of a sample with a predetermined primary ion beam to detect secondary ions generated and performing elemental analysis of the sample. After coating with a conductive film less than the predetermined location of the sample, the periphery of the predetermined location may be composed of an element different from the element to be measured and may generate multiple ions having the same or close mass-to-charge ratio as the element ion to be measured. The primary ion beam is applied to the predetermined location. The formation of the conductive film is preferably performed by a focused ion beam deposition method.

[0007]

When a primary ion beam is irradiated, even if the periphery of a predetermined portion (measured region) is sputter-etched, the detection lower limit is improved because there is little generation of multiple ions which cause interference information. In addition, the uniformity of the etching of the area to be measured is not impaired.

[0008]

FIG. 1A is a plan view of a sample for explaining a first embodiment of the present invention, and FIG.
It is an X-ray enlarged sectional view.

[0009] First SiO ₂ film 2 having a thickness of about 0.5μm on the surface of the silicon substrate 1 is formed as a sample, SiO ₂
A film is prepared in which a polycrystalline silicon film 3 is formed in which an opening of about 1 μm × 1 μm provided in the film 2 contains As atoms at a peak concentration of about 1 × 10 ²⁰ / cm ³ . A case where the concentration distribution of As atoms in the polycrystalline silicon film 3 is measured will be described.

The thickness around the polycrystalline silicon film 3 is 200 to
It is covered with a carbon film 4 of 400 nm. That is, a sample is put into a focused ion beam apparatus, and a Ga ⁺ beam accelerated to 20 to 40 keV is irradiated while blowing a styrene (C ₈ H ₈ ) gas around the polycrystalline silicon film 3 which is an area to be measured. The film 4 is formed. Such focused ion beam deposition methods are known and are described, for example, in RESEARCH STUDIES.
PRESS LTD. ), 1991, “Focused Ion Beams from Liquid Metal Ion Sources” (Focused Ion Beams)
from Liquid MetalIon Sour
ces).

Next, the sample is transferred to a sample holder (not shown) of the SIMS apparatus. At this time, the silicon substrate 1 and the carbon film 4 are connected. If there is a material similar to the polycrystalline silicon film 3 shown outside the region to be measured, the surface of the film should be extended and covered with the carbon film 4 on the region to be measured, and if it is fixed to the sample holder, it must be connected again. There is no.

Next, for example, Cs having a beam diameter of 0.1 μm
The surface of the polycrystalline silicon film 3 is scanned with a ⁺ beam to perform sputter etching to detect As ⁺ . When the carbon film 4 is irradiated with the Cs ⁺ beam, as shown in FIG. 2, the edge 4a is easily etched, but the polycrystalline silicon film 3 itself is uniformly etched. When the SiO ₂ film 2 is irradiated with a Cs ⁺ beam, molecular ions having the same mass number as As
^Although interference information due to ²⁹ Si ³⁰ Si ¹⁶ O is generated, in this embodiment, there is no danger since the SiO ₂ film 2 is covered with the carbon film 4. Molecular ion C ₆ H ₃ generated by hydrogen and carbon contained as impurities in the carbon film 4
However, the probability of occurrence of the molecular ion C ₆ H ₃ is considered to be extremely small because the total number of atoms is large. Since the carbon film 4 is conductive and is connected to the silicon substrate, there is no possibility of being charged by the Cs ⁺ beam. FIG. 3 shows the measurement results. Curve 1 in which the lower limit of detection is improved by about two orders of magnitude compared to curve 11 in the case where carbon film 4 is not formed
2 (As concentration profile) was obtained.

Next, a second embodiment will be described.

A tungsten film is formed in place of the carbon film 4 in the first embodiment. SiO ₂ around the area to be measured
Tungsten hexacarbonyl (W (C
O) ₆ ) Irradiating a Ga ⁺ beam accelerated to, for example, 40 keV while blowing a gas to a thickness of 200 to 400 nm
Is deposited. Then, as in the first embodiment, the surface of the polycrystalline silicon film is sputter-etched with a Cs ⁺ beam to detect As ⁺ . Thus, the concentration profile of As shown by the curve 13 in FIG. 3 was obtained. Although the detection lower limit of As is further improved as compared with the first embodiment, in the present embodiment, almost no multiple ions confusing to As ⁺ are generated.

In the above, an example has been described in which the formation of the conductive film and the secondary ion mass spectrometry are performed using different devices. However, the secondary ion mass spectrometer is provided with two primary ion irradiation systems, If a system is provided, it is possible to carry out continuously in the same apparatus.

[0016]

As described above, according to the present invention, the periphery of the area to be measured is covered with a conductive film which is less likely to generate multiple ions having the same or close mass-to-charge ratio as the element ions to be measured, and then the secondary Since ion mass spectrometry is performed, interference information due to multiple ions can be reduced and the lower detection limit can be improved. Further, even if there is a conductive film, the uniformity of the etching by the primary ion beam of the measurement area is not impaired, so that accurate analysis in the depth direction is possible.

[Brief description of the drawings]

FIG. 1 is a plan view (FIG. 1A) of a sample used in a first embodiment of the present invention and a cross-sectional view taken along line XX of FIG. 1A (FIG. 1B).

FIG. 2 is a plan view for explaining the first embodiment.

FIG. 3 is a graph showing a density profile for explaining the present invention, wherein curves 11, 12 and 13 show measurement results according to the conventional example, the first embodiment and the second embodiment, respectively.

FIGS. 4A and 4B are cross-sectional views in the order of steps, which are separately shown in FIGS.

[Explanation of symbols]

1 silicon plate 2 SiO ₂ film 3 polycrystalline silicon film 4,4a carbon film 101 semiconductor substrate 102 under test region 103 trench

Claims (3)

(57) [Claims] 1. A secondary ion mass spectrometry method for irradiating a predetermined portion of a sample with a predetermined primary ion beam to detect secondary ions generated and performing elemental analysis of the sample, wherein the periphery of the predetermined portion is covered. The possibility of generating multiple ions having the same or close mass-to-charge ratio as the element ions to be measured and composed of elements different from the element to be measured is such that the primary ion beam is applied to the predetermined area after coating with a conductive film less than the predetermined area of the sample. Secondary ion mass spectrometry. 2. The secondary ion mass spectrometric method according to claim 1, wherein the conductive film is formed by a focused ion beam deposition method. 3. The method according to claim 1, wherein the conductive film is a carbon film or a tungsten film.