JP2011029043A - Mass spectroscope and mass spectrometry - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a mass spectroscope capable of improving damages of a sample without lowering a secondary ion yield. <P>SOLUTION: The mass spectroscope for analyzing secondary ions and neutral secondary particles ionized afterward to be analyzed is provided with an ion source for making primary ion beams for generating secondary particles by irradiating the sample, and an analyzing unit for mass spectrometry of the secondary particles, and is also provided with a function capable of controlling kinetic energy per atom structuring a primary ion in a range of 20 eV or less. As the primary ions, gas cluster ions formed of gas atoms condensed by Van der Waals' forces are preferably used. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a secondary ion mass spectrometer and a mass spectrometry method for analyzing the types of atoms or molecules present on a sample surface.

  Secondary ion mass spectrometry (hereinafter abbreviated as SIMS) is a method for measuring the mass of secondary ions released from the sample surface when the sample is irradiated with primary ions. It is a method of analyzing the type of molecule. The release of secondary ions is due to a sputtering phenomenon caused by collision between the primary ions and atoms or molecules constituting the sample. For this reason, the molecule | numerator of a sample may dissociate by collision with a primary ion, or the crystal structure of a sample may be destroyed. These are called sample damage by primary ions.

Conventionally, ions obtained by ionizing atoms (monoatomic ions) have been used as SIMS primary ions. On the other hand, there are some expectations that collisions can be stopped in the extremely shallow region of the surface and the damage to the sample can be reduced by using ions of molecules or clusters with large mass as primary ions. The use of molecular ions and cluster ions was studied. However, in the ions that have been tried so far, the number of atoms constituting the cluster is about 10, so the kinetic energy per constituent atom was on the order of 100 eV. For this reason, damage to the sample by primary ions has not been improved sufficiently (FIG. 1, conventional method). Therefore, the molecular weight that can be detected without dissociating the sample molecules is about 500 at present. This is a major obstacle to the application of SIMS to biological materials such as proteins.
For example, the following is a technical document related to conventional SIMS.

A. Brunelle, D. Negra, J. Depauw, D. Jacquet, L. Beyee, M. Pautrat, Phys. Rev. A 63, 022902 (2001). G. Gillen, A. Fahey, Appl, Surf. Sci. 203/204, 209 (2003).

  In order to improve the damage of the sample due to the primary ions, it is necessary to sufficiently reduce the kinetic energy of the primary ions. However, since the sputtering rate of the sample is lowered by lowering the kinetic energy of the primary ions, the secondary ion yield (the number of secondary ions that can be detected per primary ion) is lowered. Therefore, it is a problem to suppress the dissociation of sample molecules without reducing the secondary ion yield as much as possible.

  The present invention solves the above-described problems in SIMS and provides means for improving sample damage without reducing the secondary ion yield.

  The inventors have advanced development such as flattening of a rough surface at an atomic level and generation of a high-quality thin film by utilizing sputtering characteristics by irradiation of gas cluster ions in which a large number of gas atoms are aggregated. In this development, the sputtering characteristics of gas cluster ions became clear, and the development of a secondary ion mass spectrometry method utilizing these characteristics was newly started. The present invention was created in the development of a secondary ion mass spectrometry method using gas cluster ions.

The mass analyzer of the invention according to claim 1 is a mass analyzer for analyzing secondary ions and neutral secondary particles ionized later, and generates secondary particles by irradiating a sample. With an ion source that generates a primary ion beam and an analysis unit for mass analysis of secondary particles, and the ability to control the kinetic energy per atom constituting the primary ion in a region of 20 eV or less It is characterized by having.
When such a mass analyzer is used, damage to the sample can be improved by lowering and adjusting the kinetic energy per constituent atom of the SIMS primary ion to the molecular dissociation energy.

The average value of the binding energy of the sample molecules is obtained by dividing the kinetic energy transferred to the sample molecules when the primary ions collide with the sample molecules by the number of chemical bonds contained in the sample molecules. It is preferable to prescribe the irradiation conditions for suppressing dissociation by adjusting the acceleration voltage of primary ions and the number of constituent atoms so that the sample is irradiated as follows.
If the kinetic energy of the accelerated primary ion (mass m ₁ ) is E ₁ and an elastic collision with a stationary sample molecule is assumed, the kinetic energy E ₂ transferred to the sample molecule (mass m ₂ ) is αE ₁ (α = 4 m ₁ m ₂ / (m ₁ + m ₂ ) ² ). Divided by the chemical bonding number N _b contained the E ₂ to a sample molecule is a kinetic energy that can be used to cut one chemical bond. In practice, a part of this kinetic energy is consumed for molecular dissociation. Therefore, a sufficient condition for suppressing the dissociation of molecules is expressed by the following relational expression, where E _d is the average dissociation energy of chemical bonds of sample molecules.
αE ₁ / N _b <E _d (1.1)
Here, N and Ea are used from the relationship of E ₁ = NE _a (where N is the number of constituent atoms of the primary ion and E _a is the kinetic energy per constituent atom of the primary ion, ie E ₁ / N). (1.1) is rewritten as follows.
αNE _a / N _b <Ed (1.2)
Furthermore, if the mass of the cluster ion and the number of constituent atoms are the same as the mass of the sample molecule and the number of chemical bonds, that is, m ₁ ≒ m ₂ and N ≒ N _b , (1.2) can be simplified as follows: The
Ea <Ed (1.3)
(1.3) shows that dissociation can be suppressed if the kinetic energy per constituent atom of the primary ion is smaller than the dissociation energy of the sample molecule.

  In order to solve the problem, it is particularly important to secure the secondary ion strength while suppressing the dissociation of the sample molecules due to the irradiation of the primary ions. For this purpose, cluster ions having a larger number of constituent atoms than in the past are used as primary ions. The gas cluster ions used as primary ions in the invention according to claim 3 are those in which gas atoms are aggregated by van der Waals force or the like, and those having more than 10,000 constituent atoms can be stably generated. As a result, when accelerating an argon cluster ion with 2000 atoms at a practical acceleration voltage, eg 5 kV, the kinetic energy per atom is 2.5 eV (5000/2000), and the chemical bond energy of molecules ( Generally lower than 3-5eV). Thereby, it is possible to perform sputtering while suppressing dissociation of the sample molecules (FIG. 1). Furthermore, since the number of constituent atoms is more than two orders of magnitude higher than that of the conventional cluster, it is possible to avoid a decrease in the sputtering rate of the sample molecules due to multi-body collisions between these constituent atoms and the sample molecules.

  If gas cluster ions composed of rare gas atoms such as argon and krypton are used as primary ions, the possibility that these clusters evaporate immediately after colliding with the sample and remain in the sample is extremely low. This is also advantageous from the viewpoint of contamination on the sample. On the other hand, the use of oxygen cluster ions can be expected to increase the secondary ionization rate of a metal sample or the like. In addition, if water cluster ions are used for the biological sample, the adhesion of hydrogen ions to the biomolecule is promoted, and the secondary ion yield can be improved.

  According to the present invention, it is possible to solve the above-described problems in SIMS and improve the damage to the sample without reducing the secondary ion yield.

SIMS conceptual diagram of conventional method and method of the present invention SIMS equipment diagram using gas cluster ion beam as primary ion beam Kinetic energy distribution per Ar atom adjusted by time-of-flight method

Example 1
FIG. 2 is a SIMS apparatus diagram in which a gas cluster ion beam according to the present invention is used as a primary ion beam.

The SIMS device consists of four vacuum chambers, each capable of differential pumping. The cluster source chamber 1 and the ionization chamber 2 are evacuated by turbo molecular pumps (TMP) with exhaust speeds of 1000 l / s and 1400 l / s, respectively. The cluster size sorting chamber 3 and the ion beam irradiation chamber 4 are evacuated by TMP with a pumping speed of 350 l / s. The ultimate vacuum in the source chamber 1, ionization chamber 2, size selection chamber 3, and irradiation chamber 4 is 2 × 10 ⁻⁶ , 2 × 10 ⁻⁶ , 1 × 10 ⁻⁶ , and 2 × 10 ⁻⁵ Pa, respectively. The cluster beam is generated as a supersonic continuous beam by adiabatic expansion of gas in a vacuum from a brass conical nozzle 6 having an orifice diameter of 0.1 mm and a diffusion length of 60 mm. The nozzle 6 is attached to the XYZ stage, and the alignment of the beam and the distance between the nozzle 6 and the skimmer 7 can be adjusted. The generated neutral cluster beam is cut out by a nickel skimmer 7 having a diameter of 0.5 mm and introduced into the ionization chamber 2. In the ionization chamber 2, it is ionized by the thermoelectrons emitted from the tungsten filament 8 and accelerated to several keV by the acceleration electrode 9.

Since the cluster breaks due to collision with the residual gas and the cluster size (number of constituent atoms) and kinetic energy change, the vacuum level of the beam transport section must be kept at ~ 10-3 Pa or less. The degree of vacuum under the beam irradiation condition with a nozzle pressing pressure of about 1 MPa is 6 × 10 ⁻⁴ , 6 × 10 ⁻⁴ , and 5 × 10 ⁻⁵ Pa in the ionization chamber 2, the size selection chamber 3, and the irradiation chamber 4, respectively. High vacuum sufficient for cluster ion beam transport can be maintained.

The neutral cluster beam generated in the cluster source chamber 1 is ionized by an electron impact method with an ion source installed in the ionization chamber 2. The ion source includes a ground electrode portion, an extraction electrode portion, and an acceleration electrode 9 each having an opening diameter of 20 mm. The acceleration electrode 9 is composed of three plate electrodes and a grid. Each electrode is at the same potential (acceleration voltage Va + ionization electron voltage V _e ). In this apparatus, the distance between the first and second deflection electrodes 10-12 is set to 633.2 mm in order to obtain a sufficient size resolution by time-of-flight separation (TOF). For this reason, a 30 ° taper angle is provided on the extraction electrode and the ground electrode to suppress divergence of the beam during flight. In addition, the distance between the electrodes and the extraction electrode voltage can be adjusted, so that the optimum beam focal length can be adjusted. The ground electrode is at ground potential.

  Electrons generated by thermionic emission from the tungsten filament 8 having a diameter of 0.3 mm are extracted in the direction of the beam center axis by the potential difference (Ve) between the grid 8 provided between the electrodes of the acceleration electrode 9 and the filament 8, and a cluster is formed by electron impact. Ionize. The ionized cluster is accelerated by the potential difference between the acceleration electrode 9 and the ground electrode. The acceleration voltage can be applied up to 20 kV. A first deflection electrode 10 is attached 10 mm downstream of the ground electrode, and a beam is deflected by applying a voltage of 1.0 kV between the electrodes of the deflection electrode 10. Further, a magnet 11 having a magnetic flux density of 0.3 T is installed on the downstream side of 50 mm to remove monoatomic ions contained in the cluster ion beam.

The clusters generated by the conical nozzle 6 have a wide size distribution of several to thousands. In this embodiment, in order to realize SIMS with low damage to the sample, the goal was to control the kinetic energy per atom with an accuracy of about 0.1 eV. For this purpose, a cluster size resolution (m / Δm) of 10 or more is required. In this example, a time-of-flight separation method (TOF method) was used for cluster size selection. The cluster ions contained in the cluster ion beam pulsed by the first deflection electrode 10 directly under the ion source have different flight speeds depending on the mass of the clusters, and the arrival times at the second deflection electrode 12 are different. After the beam is pulsed by the first deflection electrode 10, an appropriate delay time (flight time) is provided, a pulse voltage is applied to the second deflection electrode 12, and the beam is chopped again, thereby selecting a cluster of a specific size. Then, the sample 15 can be irradiated. The size resolution depends on the distance between the first and second deflection electrodes 10 and 12 and the pulse width of the voltage applied to the deflection electrodes 10 and 12. If the pulse width through which ions can pass is shortened, the resolution is improved, but the pulse width needs to be set to a time during which the cluster ions sufficiently pass through the electrode portion. For example, when a cluster ion (Ar ₂₀₀₀ ⁺ ) composed of 2000 argon atoms is accelerated at 5 keV, it takes 5.7 ms to completely pass through a deflection electrode section having a width of 20 mm. . The electrode length of the second deflection electrode 12 can be changed in 5 steps from 5 mm to 20 mm according to the required resolution and beam intensity. In this apparatus, the distance between the first and second deflection electrodes 10 and 12 is set to 633.2 mm in order to obtain a sufficient size resolution by the TOF method. For this reason, a 30 ° taper angle is provided on the extraction electrode and the ground electrode to suppress divergence of the beam during flight. In addition, the distance between the electrodes and the extraction electrode voltage can be adjusted, so that the optimum beam focal length can be adjusted. The ground electrode is at ground potential. The ion beam was pulsed by the first deflection electrode 10 with a width of 10 μs. The ion current is detected by a microchannel plate (MCP) installed 80 mm downstream of the second deflection electrode 12, and the time of flight is measured by a digital oscilloscope (Iwatsu-LeCroy LT322). The size was determined.

  In this embodiment, a case where an argon (Ar) cluster ion beam is generated and used as primary ions will be described. The Ar gas nozzle back pressure is 1.21 MPa, the ionization electron voltage is 100 V, and the ion acceleration voltage is 5 kV. Before the size selection, the cluster size had a very wide distribution of several hundred to several thousand atoms / cluster, and the half width was 2300 atoms / cluster. By applying an appropriate delay time to the second deflection electrode 12 and applying a pulse voltage, it was possible to select a cluster of an arbitrary size from a cluster ion beam having a wide distribution with a half-width of several hundred atoms / cluster. . FIG. 3 shows a distribution of kinetic energy per one cluster constituent atom (Ar) adjusted by the above method. By setting a delay time (time of flight) for opening the two deflection electrodes 10-12, the kinetic energy could be controlled in the range of 1 to 5 eV.

  Mass spectrometry of secondary ions emitted from the sample 15 is performed by a reflectron type TOF tube 5 (drift distance 1800 mm) attached to the irradiation chamber 4. The size-selected cluster ion pulse has a time width of about 10 μs at the sample position. Therefore, in order to ensure the TOF resolution necessary for secondary ion mass spectrometry, a secondary ion gate electrode (comb gate device) 13 is provided immediately before the reflectron type TOF 5. This device 13 deflects an ion beam by stretching ultrafine tungsten wires having a diameter of 0.05 mm at intervals of 1 mm and alternately applying positive and negative potentials to the wires. The secondary ions released from the sample 15 are drawn into the TOF tube by a potential of -1 to -2 kV (drawing potential) applied to the tip of the TOF tube, and then pulsed at a width of 100 to 200 ns by a comb gate. It becomes. Then, the time of flight from the comb gate position to the secondary ion detector (MCP) 14 installed at the end of the TOF tube is measured and subjected to mass spectrometry. At the drawing potential of −1 kV and the comb-shaped gate width of 200 ns, the mass resolution (m / Δm) of the secondary ions is 200 at m / z = 700.

The sample 15 was irradiated with Ar cluster ions of 1000 atomic atoms at an acceleration of 5 kV using a thin film of insulin (molecular weight: 5808) (film thickness: 100 nm), and secondary ions were measured. Insulin molecular ions could be measured. The irradiation conditions are as follows: kinetic energy E ₁ = 5000 eV of cluster ion (molecular weight 40000) with 1000 Ar atoms, kinetic energy transfer rate α≈0.4 to insulin molecule (molecular weight 5808) due to collision, number of chemical bonds of insulin molecule When N _b ≈1000 (estimated as 50 amino acids) and average dissociation energy E _d ≈4 eV, it is confirmed that the relational expression (1.1) is satisfied.

(Example 2)
In this embodiment, a case where an oxygen cluster ion beam is generated and used as a primary ion beam will be described. Oxygen clusters are generated by injecting a mixed gas (dilution ratio (O ₂ / He) = 5 to 20%) obtained by diluting oxygen gas (O ₂ ) with helium gas (He) from a nozzle at a pressure of about 5 to 10 atm. . Oxygen clusters are generated by the cooling effect due to the adiabatic expansion of oxygen gas. Hereinafter, ionization of oxygen clusters, adjustment of kinetic energy per cluster constituent atom, and irradiation of the sample are the same as in Example 1.

(Example 3)
In this embodiment, a case where a water cluster ion beam is generated and used as a primary ion beam will be described. In the generation of the water cluster, water vapor generated by bubbling pure water with He is injected from a nozzle at a pressure of about 1 to 5 atm. Water clusters are generated by the cooling effect due to the adiabatic expansion of water vapor. Hereinafter, ionization of water clusters, adjustment of kinetic energy per cluster constituent atom, and irradiation of the sample are the same as in Example 1.

  The present invention is a method for analyzing a minute amount of atoms and molecules present on the surface of a wide range of materials such as semiconductors, metals, inorganic materials, organic substances, and biological substances, and further analyzing the distribution within these materials, The present invention provides a method for measuring the molecular weight of a molecule without dissociating it.

1: Cluster source room 2: Ionization room 3: Cluster size sorting room 4: Ion beam irradiation room 5: Reflectron type TOF
6: Nozzle 7: Skimmer 8: Filament 9: Acceleration electrode 10: First deflection electrode 11: Magnet 12: Second deflection electrode 13: Comb gate device 14: Secondary ion detector (MCP)
15: Sample

Claims (11)

  A mass analyzer for analyzing secondary ions and neutralized secondary particles that are ionized later, an ion source that generates a primary ion beam for generating secondary particles by irradiating a sample; Mass spectrometer with an analysis unit for mass spectrometry of secondary particles, and a function that can control the kinetic energy per atom constituting the primary ion in a region of 20 eV or less .   Acceleration of the primary ion so that the value obtained by dividing the kinetic energy transferred to the sample molecule when the primary ion collides with the sample molecule by the number of chemical bonds contained in the sample molecule is less than the average value of the binding energy of the sample molecule The mass spectrometer according to claim 1, wherein the sample is irradiated with the voltage and the number of constituent atoms adjusted.   The mass spectrometer according to claim 1 or 2, wherein a gas cluster ion in which gas atoms are aggregated by van der Waals force is used as the primary ion.   4. The mass analyzer according to claim 3, wherein the kinetic energy per constituent atom of the cluster ion of the gas cluster ion which is a primary ion is controlled by adjusting the acceleration voltage of the cluster ion and the number of cluster constituent atoms.   The mass analyzer according to claim 4, wherein the number of constituent atoms of the cluster ions is adjusted by a time-of-flight separation method of ions.   The mass spectrometer according to any one of claims 1 to 5, further comprising a time-of-flight measuring unit for analyzing secondary ions and neutral secondary particles ionized later.   In order to accurately perform time-of-flight measurement for analyzing secondary ions and neutral ions that are ionized later, secondary ions and neutral ions that are ionized later are placed in front of the time-of-flight measurement unit. 7. The mass analyzer according to claim 6, further comprising a gate device for pulsing the beam of secondary particles.   The time until the secondary ion generated from the sample reaches the gate device differs depending on the mass of the secondary ion, and thus the time-of-flight measurement is performed while shifting the timing of opening the gate device. Item 8. The mass analyzer according to Item 7.   The gate device stretches a plurality of fine metal wires at equal intervals, and alternately applies high and low potentials to the fine wires to deflect the secondary ion beam to block the passage. The mass analyzer according to claim 7, wherein switching is performed so that   The mass spectrometer according to claim 1, wherein the sample is irradiated with rare gas atoms, oxygen molecules, or gas cluster ions in which water molecules are aggregated as primary ions. A mass spectrometry method for analyzing secondary ions emitted by irradiating a sample with a primary ion beam and neutral secondary particles ionized later,
A mass spectrometry method characterized in that kinetic energy per atom constituting a primary ion is set to 20 eV or less, and gas cluster ions in which gas atoms are aggregated by van der Waals force are used as the primary ions.

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