JPH11295247A - Method for analyzing sample surface - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen background influences by impurity light elements adsorbed to a surface of a sample, by processing secondary ions of a compound consisting of an impurity and elements constituting the sample in mass spectroscopy, and calculating the impurity from a difference of masses of the secondary ions obtained by the analysis and impurity elements. SOLUTION: Ion beams projected from an excitation beam generation source 5 illuminate a sample to be analyzed in a sample chamber 3. A surface of the sample is sputtered, emitting neutral particles and charged particles. The charged particles are taken into a mass spectrograph by an electrostatic lens 19. The spectrograph comprises a mass spectrometric part 21 and a secondary ion detection part 23. Light elements adsorbed to the sample surface are also sputtered simultaneously and taken into the spectrograph at the same time. Since electrons from an electron beam generation device 6 are irradiated to the sample surface at the time of analyzing an insulation substance, recoil electrons from the sample surface are taken into the spectrograph at the same time. The ions and recoil electrons are separated in terms of a mass, so that solely ions of a target mass are counted by the detector 23.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for analyzing a surface, and more particularly, to a method for analyzing a sample surface for analyzing a trace amount of light elements contained in a sample.

[0002]

2. Description of the Related Art In a conventional analytical method, when measuring a light element, the light element in the atmosphere of a sample chamber adsorbed on the sample surface and
The light element in the sample could not be distinguished, and the lower limit of detection was not good. In addition, not only the measurement of light elements, but also the secondary ion yield is small, so that it was not possible to measure with a lower detection limit by the conventional analysis method.

[0003]

In the conventional method, since the light element in the atmosphere of the sample chamber is adsorbed on the surface of the sample, the lower limit of detection of impurities in the sample is not good due to the influence of the light element in the sample chamber. Molecular ions are detected using molecules formed by impurities in the film and constituent elements of the sample, and background (hereinafter referred to as BG) by light elements of impurities adsorbed on the sample surface.
And reduce the effect of light impurities present in the sample with a lower detection limit. Another object of the present invention is to provide a method for analyzing a sample in which the yield of secondary ions is increased by detecting a molecule having a high electron affinity, and the detection lower limit is improved.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a method for analyzing the surface of a sample, which comprises irradiating the surface of the sample with charged particles and detecting secondary ions to be excited to measure the amount of light element impurities in the sample. Mass spectrometry of a secondary ion of a compound consisting of the impurity and the constituent elements of the sample, and a step of calculating an impurity from a difference between the mass of the secondary ion by the analysis and the mass of the impurity element. This is a method for analyzing a sample surface.

That is, the gist of the present invention is to carry out measurement of mass and the like and ion detection in the form of a molecule in which a composition element of a sample and an impurity in the sample are combined in order to achieve the above object.

More specifically, according to the present invention, in order to detect molecular ions between sample constituent elements and impurities in a film,
It is possible to separate the light impurity element present in the sample chamber atmosphere adsorbed on the sample surface from the light impurity element already present in the film. For this reason, the BG of the light element generated by being adsorbed on the sample surface is reduced, and the impurity light element in the film can be measured down to a low concentration. In addition, depending on the molecular ion species to be detected, some elements have a higher electron affinity in the case of a molecule than in the case of an element alone and are easily ionized. By using such a molecular ion, the ion yield increases, and measurement with a lower detection limit can be performed.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. The following embodiments are examples embodying the present invention, and do not limit the technical scope of the present invention.

FIG. 1 is a diagram schematically showing the configuration of the mass spectrometer of the present embodiment. In FIG. 1, the entire surface analyzer 1 can be made of a member made of, for example, stainless steel or an aluminum alloy. Further, an excitation beam generating means 5 for generating an excitation beam for exciting the surface of the sample S is connected to the sample chamber 3 in which the sample S is disposed via an excitation beam waveguide 7 and is also transmitted from the sample chamber 3. The secondary ion mass spectrometer 20 is connected via a beam waveguide 17, and a vacuum pump 15 for evacuating the inside of the surface analyzer 1 and maintaining a high vacuum is attached.

As the excitation beam generated by the excitation beam generating means 5, any beam can be used as long as it can excite the surface of the sample S and sputter non-analytical elements from the surface of the sample S. For example, an ion beam, a neutral particle beam, a laser beam, or the like can be used. An excitation beam guide 7 between the excitation beam generating means 5 and the sample chamber 3
Is provided with an electrostatic lens 9 for converging the excitation beam. Further, an electron beam generating means 6 is provided so as to be able to measure even an insulator, and when the excitation beam is irradiated on the sample, the same place can be irradiated with the electron beam.

The mass spectrometer 20 has mass spectrometers 21 and 2 for mass spectrometric analysis of secondary ions sent from the sample chamber 3.
And a secondary ion detector 23 for detecting secondary ions.
The beam waveguide 17 between the mass analyzer 20 and the sample chamber 3 is provided with an electrostatic lens 19 for taking in secondary ions generated in the sample chamber 3 into the mass analyzer 20.

Hereinafter, the apparatus will be further embodied, and its operation will be described with reference to the surface analyzer shown in FIG. The ion beam having a specific monochromatic energy emitted from the excitation beam source 5 is applied to the sample to be analyzed which is set on the sample setting table S in the sample chamber 3. During irradiation, the sample surface is sputtered, and neutral particles and charged particles are emitted. The charged particles generated in the sample chamber 3 are formed by an electrostatic lens 19 between the sample chamber 3 and the beam waveguide 17.
It is taken into the mass analyzer 20. At this time, the light element adsorbed on the sample surface is simultaneously sputtered and simultaneously taken into the mass spectrometer. In addition, since electrons are emitted from the electron beam generator 6 to the sample surface during the insulator analysis, recoil electrons from the sample surface are also taken into the analyzer at the same time. The ions and recoil electrons taken into the mass spectrometer are separated by mass, and only ions having a target mass are counted by the detector 23. However, in the case of a quadrupole mass analyzer, electrons having a mass of 0 are taken into the detector without mass separation. In addition, ions that collide with the mass analyzer generate secondary electrons, which are also captured by the detector. Since these electrons have a peak at mass number 0 and draw a tail toward the high mass side, they become a background particularly during light element analysis. As described above, the background of the light element adsorbed on the sample surface and the secondary electrons generated by the irradiation of the electron neutralizing gun causes the background of the impurity light element in the sample. To reduce these background counts,
Secondary ions are detected not in the form of light element simple ions but in the form of molecular ions of the constituent elements of the matrix and the light elements of impurities in the film.

FIG. 2 shows the secondary ion coefficient ratios of Ga, H and HN when the H analysis in the GaN film is performed, and FIG.
^-, HN ^- shows the results of H concentration by detection. Compared with the case of detecting a simple H ion, the molecular ion detection of HN has a lower secondary ion yield, but the detection lower limit is improved about twice. This is because in single ion detection, both H in the sample and H adsorbed on the sample surface are similarly ionized by sputtering, whereas in molecular ion detection, H in the film and N as a constituent element of the matrix are mainly This is because the bound molecules are ionized. In some cases, hydrogen adsorbed on the sample surface is injected into the film and bonds with N atoms, resulting in molecular ionization.
It is lower than the probability that H adsorbed on the surface is ionized and becomes the background of H. Such a method can be applied not only to the analysis of a GaN film but also to the analysis of H in a film other than a nitride such as a GaAs film, and the H analysis can be performed at a lower detection limit by detecting AsH.

FIGS. 4 and 5 show the case where C analysis in the GaN film is performed. When detection is performed with molecular ions as in the case of H, the secondary ion yield is improved by about two orders of magnitude, and the lower limit of detection is improved by about two orders of magnitude. This is because, similarly to the case of H, the background due to C of surface adsorption is reduced. The CN molecule has a high electron affinity of 3.82 eV and is a molecule that is easily ionized. For this reason, C bonded to N in the GaN film is ionized to approximately CN ^- and the secondary ion yield is high. As shown in FIG. 4, the secondary ion yield increases by more than two orders in the case where CN ^- molecular ions are detected compared to the case where C ^- simple ion is detected. Accordingly, the lower detection limit is improved by two digits or more as shown in FIG.

Further, when H in SiO ₂ is detected by OH molecular ions in the same manner as in the case of H in nitride or GaAs, measurement with good secondary ion yield is possible. One of the reasons is that the background due to H of surface adsorption is reduced as in the case of H in the GaN film. Further, since the mass number of the ions to be detected is increased, it is difficult to receive the background due to the secondary electrons. OH molecules have a high electron affinity of 1.83 eV and are easily ionized. Therefore, H bonded to O in the SiO ₂ film is efficiently converted to OH
^- are detected well secondary ion yields are ionized. Since the electron affinity of H alone is 0.754 eV, H alone has H ⁻
It's easier than becoming an ion. As a result, the secondary ion yield is more than two orders of magnitude higher when OH ^- molecular ions are detected than when only H-single ions are detected. However, in the case of OH ^- ions, the sum of the mass numbers of ¹⁶ O and ¹ H is 1
7, which overlaps with the mass of ¹⁷ O which is an isotope of oxygen. The molecules of ¹⁷ O, ¹⁸ O, and H can also be a hindrance to measurement because elements of the same mass, such as ¹⁸ O and ¹⁹ F, are present. Therefore, it is necessary to perform mass separation using a sector type mass spectrometry system having good mass resolution. Since the existing apparatus has a mass resolution M / ΔM of 10,000 or more, these molecules and elements can be mass-separated. ¹⁶ O presence ratio is large because ¹⁶ O + H can best yield better measurements. As a result, the detection lower limit is improved about twice when the measurement is performed in the high mass resolution mode using OH ions. As described above, even if there is a similar mass interference line when molecular ions are used, mass separation can be performed by high mass resolution measurement if the secondary ion yield is sufficient, and only the molecular ions to be detected can be detected. It is possible to do.

The enhancement of the sensitivity of H 2 by the detection of molecular ions as described above is not limited to the oxide film. For example, in ZnSe
The same applies to the analysis of H. The SeH molecule is also a molecule having a large electron affinity and easily ionized like OH.
Therefore, when analyzing H 2 in ZnSe, it is possible to measure the secondary ion yield better by detecting the molecular ion of SeH than by detecting H alone, and SeH has a mass number of 80.
Because it is before and after, it is not affected by secondary electrons. Therefore, H can be measured with a good detection lower limit. Se has many isotopes, but when Se having a mass number of 80 is used, ⁸¹ Se does not exist and the abundance ratio is as large as 49.6%, so that measurement can be performed with a lower detection limit.

These techniques are not limited to oxide films and ZnSe films, but may be applied to other materials depending on the combination, as long as the electron affinity of the molecule consisting of the constituent element of the sample and the impurity is larger than the electron affinity of the impurity alone. It is.

Light-weight elements such as H are susceptible to the background of zero-mass electrons, but detection in the form of such molecular ions increases the mass number.
There is also an effect of being less affected by the background of electrons. For example, even if the electron affinity of a molecular ion is smaller than the electron affinity of a simple ion, there is a case where the lower limit of detection is improved because it is hardly affected by the background of electrons. One example is the analysis of H in a thermal oxide film. The analysis of H in the thermal oxide film includes the detection of simple H ions and the detection of OH molecule ions described above, and other methods include SiH detection. The SiH molecule has a smaller electron affinity than OH molecules and the like, and the secondary ion yield is not large. However, since it is hardly affected by the background of secondary electrons, the lower limit of detection of H when converted to a concentration is as follows. It is possible to improve about twice. As described above, in other samples, the combination of the constituent elements of the sample and the impurity elements can increase the sensitivity of analysis and reduce the background during light element analysis.

[0018]

As described above, according to the present invention,
BG caused by light elements in the sample room atmosphere adsorbed on the sample surface
And the concentration of light impurity elements present in the sample can be measured to a low concentration. In addition, CN ^-
And OH ^- large electron affinity such as typified by detecting easy molecules ionized increase the secondary ion yield, it is possible to improve the detection limit.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a surface analyzer.

FIG. 2 is a diagram showing secondary ion calculation rates of Ga, H and N.

FIG. 3 is a diagram showing a result of H concentration by H ⁻ and HN ⁻ detection.

FIG. 4 is a view showing secondary ion yields of H ions and OH ions in a SiO ₂ film.

FIG. 5 is a diagram showing a lower limit of detection of H by detection of H ions and OH ions in a SiO ₂ film.

[Explanation of symbols]

Reference Signs List 1 surface analyzer, 3 sample chamber 5 excitation beam generating means, 6 electron gun, 7 excitation beam guide, 9, 19 electrostatic lens, 15 vacuum pump, 17 beam guide, 21 mass analyzer, 23 secondary ion detection parts, 31 69Ga ^- 2 ion count rate, 32 H ^- secondary ion count rate, 33 HN ^- secondary ion count rate, 34 H ^- concentration of H by detecting, 35 HN ^- concentration of H by the detection, 36 C ^- 2 ion count rate, 37 CN ^- 2 ion count rate, 38 C ^- concentration of H by detecting, 39 CN ^- concentration of H by detecting

Claims (1)

[Claims] 1. A sample surface analysis method for measuring the amount of light element impurities in a sample by irradiating charged particles to the sample surface and detecting secondary ions to be excited, comprising: Mass spectrometry of a secondary ion of a compound consisting of an element and a mass of the secondary ion by the analysis and a step of calculating an impurity from a difference between the mass of the impurity element and a step of calculating an impurity. Analysis method.