JP2008135308A - Polarized ion beam generating method, and polarized ion beam generating device used for its implementation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and device for generating a polarized ion beam having a high polarization rate exceeding 18% so that highly precise measurement which has not be conventionally available becomes possible by shortening a measuring time. <P>SOLUTION: In the structure of this polarized ion beam generating method, circularly polarized light the wavelength of which is adjusted to D<SB>0</SB>rays corresponding to transition to 2<SP>3</SP>P<SB>0</SB>and linearly polarized light are irradiated to a metastable helium atom from a mutually perpendicular irradiating direction to carry out optical pumping. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a polarized ion beam generating method in an inspection apparatus for examining the magnetism of the surface and interface of a material using a polarized ion beam, and a polarized ion beam generating apparatus used for the method.

  In this specification, the following words are used in the following meanings.

Polarization rate is defined as (n ↑ -n ↓) / (n ↑ + n ↓), where n ↑ and n ↓ are the numbers of ions with upward and downward spin, respectively. In this specification, this is multiplied by 100 and expressed as%.

Spin polarization of metastable helium atom 2 ³ S ₁ by symbolic optical pumping such as D ₀ line, D ₁ line, D ₂ line, 2 ³ S ₁ uses transition between 2 ³ S ₁ and 2 ³ P . This 2 ³ P level has a fine structure due to spin-orbit interaction, and the transition from 2 ³ S ₁ to these is called D ₀ , D ₁ , D ₂ line from the one with the larger transition energy. Yes. (See Figure 4)

In inspection methods that examine the magnetic properties of material surfaces and interfaces with polarized ion beams, polarized ion beams are extremely sensitive to surfaces, so the measurement time must be reduced as much as possible to prevent surface contamination during measurement. There is.
Conventionally, as shown in Non-Patent Document 1, by optical pumping using circularly polarized light having a wavelength of D ₁ line corresponding to the transition of metastable helium atom 2 ³ S ₁ to 2 ³ P ₁ , Helium ion polarization has been made. The polarization rate of helium ions according to this prior art was 18%, and it was impossible to generate polarized helium ions having a polarization rate higher than that.
For this reason, it is difficult to avoid surface contamination during the measurement, and the measurement results are naturally limited.
Review of Scientific Instruments, 70 (1999) 240, Bixler

  In view of such circumstances, the present invention provides a polarized ion having a high polarization rate exceeding 18% so that measurement time can be shortened and highly accurate measurement not possible in the past is possible. It is an object to provide a method and apparatus for generating a beam.

In order to solve the above-mentioned problem, the polarized ion beam generation method of the invention 1 quasi-circulates the circularly polarized light and the linearly polarized light that are wavelength-adjusted to the D ₀ line corresponding to the transition to 2 ³ P ₀ from the mutually perpendicular irradiation directions. Light pumping is performed by irradiating stable helium atoms.

  Invention 2 is a polarized ion beam generator for carrying out the polarized ion beam generation method of Invention 1, wherein the high frequency helium discharge tube, the laser oscillator, and the laser from the laser oscillator are branched into two, One of them is a circularly polarized light and the other is a linearly polarized light, and is composed of an optical polarizer that irradiates the high-frequency helium discharge tube with a 90 ° difference in irradiation angle.

According to the invention 1, a polarization ratio of 1.5 times or more as compared with the conventional one was obtained, and in the measurement with the same accuracy as the conventional one, the measurement time could be reduced to 2/3 or less.
In addition, Invention 2 can realize the generation of such a polarized helium ion beam having a high polarization rate.

  The following examples show an example of the present invention, and modifications can be made based on the prior art as long as they do not differ from the gist of the present invention.

FIG. 1 shows an irradiation device for optical pumping.
The output light having a wavelength of 1083 nm of the optical fiber laser (1) was input to the optical fiber amplifier (2) via the optical fiber.
This input light was amplified by the optical fiber amplifier (2) and emitted from the optical fiber connector (3) into the space.
The optical fiber amplifier (2) was adjusted so that the output was 3W.
In addition, using a polarizer installed in the optical fiber laser (1), the emission light was adjusted in advance so as to be linearly polarized light.
The direction of polarization of the light emitted into the space was adjusted using the half-wave plate (5), and the path of the light having about half the intensity was changed using the half mirror (6).
Next, the light whose direction was changed was converted into circularly polarized light using a quarter-wave plate (7) and irradiated to the high frequency helium discharge tube (12).
The irradiation direction of this circularly polarized light was adjusted to be parallel to the magnetic field created by the coil (13).
The DC power source (14) was adjusted so that the magnetic field generated by the coil (13) was about 1 Gauss.
On the other hand, the mirror (9) and the concave mirror (11) were adjusted so that the light passing through the half mirror (6) irradiates the discharge tube (12).
The irradiation direction of this linearly polarized light was adjusted to be perpendicular to the magnetic field created by the coil (13).
Further, the polarization direction was adjusted by the half-wave plate (10) so that the polarization component of the linearly polarized light was parallel to the magnetic field generated by the coil (13).
The wavelength of the irradiation light of the optical pumping was adjusted to the D ₀ line corresponding to the transition of the metastable helium atom 2 ³ S ₁ to 2 ³ P ₀ .
At that time, while observing the polarization rate of the metastable helium atom in the plasma by the method described in Document 1, the polarization rate was finely adjusted so as to maximize.

FIG. 2 shows a system for evaluating the polarization rate of ions generated in a polarized helium ion source.
First, in the high frequency helium discharge tube (15), helium plasma was generated using a high frequency power source or the like (16 to 19).
Next, the metastable helium atom 2 ³ S ₁ in this plasma was spin-polarized by the optical pumping shown in FIG.
Polarized helium ions were generated using the Penning ionization reaction of this polarized metastable helium atom.
This polarized helium ion is converted into O / Fe / MgO magnetism using the repeller electrode (20), extraction electrode (17), lenses (21, 22, 24), deflectors (23, 26), and speed reducer (25). It was transported to the body substrate.
The O / Fe / MgO magnetic substrate was obtained by the following production method.
・ First, grow an Fe thin film of about 50 nm on an MgO (001) single crystal substrate
・ Heat this in vacuum at about 600 ℃ for 10 minutes,
After exposing this substrate to 100 Langmuir oxygen atmosphere,
・ Heat the substrate in vacuum at about 500 ° C for 10 minutes,
・ Pulse magnetized in vacuum.
Most of the polarized helium ions that have reached the O / Fe / MgO substrate are neutralized in the interaction with the substrate surface to become ground state helium atoms.
In this interaction, electrons are emitted from the O / Fe / MgO substrate.
As a function of the kinetic energy of this electron, the electrostatic analyzer (28), the secondary electron multiplier (29), the preamplifier (30), the multichannel scaler (31) and the personal are classified according to the spin direction of the polarized ions. Measurement was performed using a computer (32).
The direction of helium ion polarization (upward or downward) was controlled by the direction of the quarter-wave plate (7) in FIG.
If the emitted electron intensity due to helium ions having an upward and downward spin component is Ip and Iap, respectively, the spin asymmetry is defined as (Ip−Iap) / (Ip + Iap).
Since the measured spin asymmetry is proportional to the polarization rate of the polarized ions, the change in the polarization rate of the polarized ions can be obtained by measuring this spin asymmetry with the apparatus shown in FIG.

The absolute value of the polarized ion is obtained by calculating the polarization rate of the metastable helium atom in advance using a Stern-Gerlach analyzer, and then the emission electron spectrum of the metastable helium atom (spin-polarized metastable atom deexcitation spectroscopy). And the emission electron spectrum (spin-polarized ion neutralization spectroscopy) of polarized ions on the same surface. By this method, the polarization rate of helium ions when the helium pressure was 20 Pa was previously determined to be 16.6%. For details, refer to Proceedings of the 53rd Joint Conference on Applied Physics, 2 (2006) 782, Taku Suzuki.

FIG. 3 shows the polarization ratio of helium ions as a function of helium gas pressure in the discharge tube.
The plot is made on the assumption that the polarization rate when the helium pressure is 15 Pa is equal to the above-mentioned polarization rate of 16.6% when the pressure is 20 Pa.
The spin asymmetry was determined by measuring electrons with a kinetic energy of 7.7 to 9.4 eV.
Optical pumping according to the prior art, in terms of the irradiation light of the linearly polarized light by tilting the mirror of FIG. 1 (9) is in a state not irradiated discharge tube (12), on a wavelength of the irradiated light was adjusted to ₁ line D It was.
FIG. 3 shows that the maximum value of the polarization rate according to the present technology is 1.5 times or more that of the conventional technology.
FIG. 3 was measured with the high frequency power supply adjusted to 1 W.

In magnetoresistive elements widely used in the industrial field, it is often required to elucidate the magnetic structure of the interface between the magnetic material and the nonmagnetic material. By using a highly polarized ion beam made possible by the present invention as a probe, it is expected that detailed elucidation will be possible. On the other hand, as represented by ion implantation techniques, material modification and shaping using an ion beam are widely performed. By using a highly polarized ion beam made possible by the present invention, it is expected that more advanced materials can be created by newly controlling spin. In addition, since this technology has made it possible to increase the polarization of metastable helium atoms, applications in the field of using polarized metastable helium atoms, such as magnetic resonance imaging with polarized helium atoms, are also expected.

BRIEF DESCRIPTION OF THE DRAWINGS Schematic which shows the polarized ion beam generator of an Example. The schematic of the system which evaluates the polarization rate of the polarized ion beam of an Example. The graph which shows the change of the polarization rate accompanying the change of helium pressure about an Example and a prior art Diagram explaining energy levels involved in optical pumping of metastable helium atom 2 ³ S ₁

Explanation of symbols

1. 1. Optical fiber laser 2. Optical fiber amplifier Optical fiber connector 4. Lens 5. 1/2 wavelength plate 6. Half mirror 7. 1/4 wave plate 8. Lens 9. Mirror 10.1 / 2 wavelength plate 11. Concave mirror 12. High frequency helium discharge tube 13. Coil 14. DC power supply 15. High frequency helium discharge tube 16. High-frequency electrode 17. Extraction electrode 18. Matching unit 19. High frequency power supply 20. Repeller electrode 21. Condenser lens 22. Focusing lens 23. Deflector 24. Einzel lens 25. Reducer 26. Deflector 27. O / Fe / MgO magnetic substrate 28. Electrostatic analyzer 29. Secondary electron multiplier 30. Preamplifier 31. Multi-channel scaler 32. Personal computer

Claims (2)

A method of generating a polarized ion beam in an inspection apparatus that examines the magnetism of the surface and interface of a material by using a polarized ion beam, which corresponds to a D ₀ line corresponding to a transition from a metastable helium atom 2 ³ S ₁ to 2 ³ P ₀ A polarized ion beam generating method characterized in that optically pumping is performed by irradiating metastable helium atoms with wavelength-adjusted circularly polarized light and linearly polarized light from mutually perpendicular irradiation directions.   A polarized ion beam generator for carrying out the polarized ion beam generating method according to claim 1, wherein the high frequency helium discharge tube, the laser oscillator, and the laser from the laser oscillator are branched into two, A polarized ion beam generator comprising: an optical polarizer that irradiates the high-frequency helium discharge tube with a difference in irradiation angle of 90 ° relative to each other, wherein