RU2539828C1 - Spectral ellipsometer with device of magnetodynamic measurements - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to the field of in situ monitoring of production under conditions of ultrahigh vacuum of nanosized magnetic structures and may be used in magnetic nanoelectronics for characterisation of heterogeneous magnetic elements in memory devices, in sensor devices, etc. The spectral ellipsometer additionally comprises a magnetodynamic module made of axial coils of various diameters, which performs measurements based on non-linearity of film magnetisation characteristic. Therefore, film thickness may be determined on the basis of independent measurements.

EFFECT: increased functionality and accuracy of measurements due to usage of an additional optically non-correlated method.

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Description

The invention relates to the field of in situ monitoring of the production of nanoscale magnetic heterostructures using optical methods.

Ellipsometric parameters are very sensitive to dielectric functions and layer thicknesses, but Ellipsometric measurements are not direct. In the general case, it is necessary to solve the inverse problem, which can be significant difficulties. Here, a very important role is played by the construction of an initially adequate model, which is already adjusted using experimental methods to fit experimental results. At the same time, spectral ellipsometry has significant advantages, since in this case the experimental data are taken at many wavelengths. At the modeling stage, the test sample is presented in the form of a multilayer system, the layers of which are characterized by certain model optical functions and thicknesses. The response from such a system is calculated based on the Fresnel equations. It should be noted that the ellipsometric parameters may be sensitive to changes in optical constants by a fraction of a percent. Thus, even small changes can lead to errors. Therefore, it is advisable to supplement ellipsometric methods with other control methods.

For example, ellipsometric measurements can be performed in the presence of variable modulating excitation: light, electric, and magnetic. Suitable methods are known as photoellipsometry, electroellipsometry, and magnetoellipsometry. So, in the analogue (PCT Application / WO 1998028606 A1 In-situ monitoring of electrical properties by ellipsometry, Carline Roger Timothy, Pickering Christopher) a method for monitoring material parameters in real time during production using ellipsometry and modulation of the internal electric field is proposed. Considering changes in ellipsometric parameters, it is possible to determine the optical and electrical parameters of materials. Modulation is carried out using a source of electromagnetic radiation, such as laser exposure.

In the open press there is also a description of the joint use of ellipsometry and magnetooptics. These measurements are well combined instrumentally based on the static spectral ellipsometer described in the patent and adopted as the prototype of the ellipsometer (see Patent No. 16314, priority date September 28, 2005, Spesivtsev E.V., Rykhlitsky S.V., Shvets V.A. . Ellipsometer.)

This is a static photometric scheme of an ellipsometer in which parallel reading of reflected signals is realized for two mutually orthogonal polarizations of light. The development uses spatial separation of the light beam, rather than amplitude division. Part of the luminous flux is split into the phase channel using the Fresnel rhombus of the original design, which simultaneously performs the function of a phase shifter.

The disadvantages of these methods are the use of only optical schemes that are equally limited by the sensitivity of the optical components (switching speed of the diffraction grating of a monochromator or polarizer, light intensity during reflection, etc.), which introduces additional errors in determining the optical constants and layer thickness. The elimination of such errors requires the use of measurements independent of the optical system (but compatible with it).

To monitor the growth process of magnetic heterostructures, it is proposed to supplement ellipsometry with magnetodynamic analysis, which is also a non-contact measuring method based on the nonlinearity of magnetization of magnetic structures and the generation of higher harmonics. The prototype of such a device is missing. Functional diagram of the device shown in figure 1.

The list of elements of the functional diagram:

1 - light source with a monochromator,

2 - polarizer,

3 - sample

4 - analyzer,

5 - magnetodynamic measurement device,

6 - detector with controller,

7 - computer.

A distinctive feature of the nonlinear system is the absence of superposition, which leads to a qualitative distortion of the initial exciting signal. So, if the excitation can be decomposed into elementary signals, then the response of a nonlinear system cannot be represented as the sum of the responses to these elementary perturbations. Powerful research methods and applications of nonlinear systems in optoelectronics, mobile communications, microwave technology, etc. are based on this fundamental property. Recently, due to the needs of nanobiotechnology, this method was proposed for registration and tomography of superparamagnetic particles with sizes of the order of hundreds of nanometers, which are used as biological labels. An implementation of this method proposed in (see EUROPEAN PATENT, International publication number: WO 01/067062 Analysis of biological and / or chemical mixtures using magnetic particles, P.I. Nikitin, P.M. Vetoshko) is used as an analogue.

In this development, specific excitation of magnetic nanoparticles by a magnetic field containing two frequencies is used.

The work of the analogue is described by the following scheme. The excitation signal of the generators creates an alternating magnetic field in the inductive unit (a system of magnetic coils or a single coil) with the frequencies of the excitation signal f ₁ and the modulation signal f ₂ . Since the process of interaction of a magnetic field with ferromagnetic particles is nonlinear, especially at a sufficiently high intensity of the acting magnetic field, Raman spectral components appear in the frequency spectrum of the response of the measured sample to the field. In the development of the prototype, the signal was detected at a frequency f = f ₁ + 2f ₂ . The case when the modulating signal has the lowest frequency and largest amplitude has certain technical benefits associated with the simplicity of the technical implementation and the technical result achieved. Specifically, this case relates to the specific implementation of the proposed device with frequencies f ₁ = 100 kHz and f ₂ = 100 Hz.

Since the power of the indicated combination components is unambiguously related to the number of ferromagnetic particles in the sample, it is possible to obtain information about this quantity by selecting the combination frequency signal with a filter. For this, the receiver is manufactured according to any known scheme used in the technique of noise-resistant reception of low-power signals against the background of noise and interference, and performs the functions of amplification, detection and accumulation.

For in situ use, the method of the analogue invention has several disadvantages.

A sample with particles is placed inside the inductive unit, which imposes restrictions on the geometry of the test sample, and also limits the access of light.

The technical result of the present invention is to increase the functional flexibility and sensitivity of the spectral ellipsometer by using the magnetodynamic measurement module and in the possibility of its application for in situ characterization of the growth of magnetic films.

The magnetodynamic module includes the following main elements shown in figure 2.

8 - generator signal excitation and modulation,

9 - low-frequency excitation coil

10 - high-frequency coil with a detection coil,

11 - measured sample

12 - receiving unit,

13 - control microprocessor.

14 - optical system

The excitation signal of the generator 8 creates in the excitation coil 9, which has a larger radius, an alternating magnetic field of low frequency f ₁ with a sufficiently large amplitude. In a high-frequency coil 10 with a smaller radius, which is partially located inside the excitation coil 9, a high frequency magnetic field f ₂ with a small amplitude is created. Due to the nonlinearity of the magnetization process, Raman spectral components appear in the frequency spectrum of the response of the measured sample to the influence of such a field. In the general case, they are linear combinations of the indicated frequencies: f _i = mf ₁ + nf ₂ , where m, n are integers other than zero. At a sufficiently high frequency f _{2, it is} proposed to measure the signal in the vicinity of 2f ₂ . In principle, a larger number of frequencies of the spectral components of the acting magnetic field can also be included in the linear combination. Various ratios between the amplitudes of the indicated frequency components of the magnetic field can also be used. The amplitude of the output signal in the vicinity of f ₂ is determined in the receiving unit 12, where a phase-sensitive circuit tuned to a frequency of 2f ₂ can be used. The magnitude of the signal is related to the type of magnetic nonlinearity and is proportional to the amount of ferromagnet. Using calibration curves, it is possible to determine the thickness of the growing ferromagnetic film of sample 11. The study of the waveform can provide additional information about the magnetic parameters: the coercivity field, anisotropy, etc. The operation of the entire system is controlled using a microprocessor 13, which also synchronizes the magnetic unit with the optical system 14.

Example

The inductive unit consists of two concentric coils. A coil with a large diameter creates an exciting magnetic field with a lower frequency (100 Hz), and a small coil is used both to create high-frequency excitation (of the order of 100 KHz) and to record the response at a combinatorial frequency (f ₁ + 2f ₂ ). This coil can be offset along the axis with respect to the larger coil to optimize their mutual influence. The optimal position is determined from a sensitivity measurement using control samples. Figure 3 shows the dependence of the output signal on the thickness of the Fe films in the zone of maximum sensitivity. With a film size of 1 cm × 1 cm, the resolution of the method is 2 nm.

Thus, a new development is proposed that supplements the in situ optical method with the non-correlating magnetodynamic method, with which the thickness of the magnetic layers of a multilayer system can be independently determined. The device of the magnetodynamic block also has the following advantages compared to the analog of the magneto-inductive sensor:

1) the configuration of the inductive unit allows measurements on an open surface and does not impose restrictions on the shape of the test sample;

2) the method can be used in conjunction with other characterization methods, such as optical and magneto-optical.

Claims (1)

A spectral ellipsometer containing a radiation source with a monochromator, shoulders of a polarizer and an analyzer, which implements parallel reading of reflected signals for two mutually orthogonal polarizations of light using spatial separation of the light beam by a Fresnel rhombus, which also performs the function of a phase-shifting device, characterized in that it contains a magnetodynamic module, consisting of axial coils of different diameters, creating an alternating magnetic field of high and low frequency, and the coils smaller diameter creates a high-frequency field is simultaneously detecting coil, an excitation signal generator, a receiving unit and a control microprocessor.

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