SU1191981A1 - Ion microanalyzer - Google Patents

Description

The invention relates to mass spectrometry, in particular to mass spectrometric methods for studying solids, and. can be used when creating installations for the analysis of solid bodies with high sensitivity. by secondary-ion mass spectrometry.

The invention aims to ^{increase, 0} chenie sensitivity and resolving power analyzer vtorichngh ions by increasing the collection ratio of the secondary ions, reducing losses of particles on the electrodes ¹⁵ and opta- minimization condition input to the mass spectrometer.

FIG. 1 schematically shows the proposed microanalyzer; in fig. 2 and 3 - theoretical curves, 20 confirming the efficiency of the proposed device.

FIG. .2 shows plots of resolution R and ££ aperture ratio for different values of the geometrical factor of the torus ²⁵ in the first lens at a fixed position of the point source with respect to its center, and the power factor curve P. Here A corresponds to y = 0.5; curve In y = 2,0; .thirty

curve C - y = oo.

FIG. Figure 3 shows the dependence of the efficiency of the ion-optical system, defined as the product of the particle collection coefficient and resolution, depending on the ratio of the potentials on the second cylindrical electrode and the ring electrode of the first lens.

The proposed device includes 40 primary ion source 1, the test sample on the holder 2, focusing the secondary ion-optical ions 2

A system made of two axially symmetric hyperboloid lenses, the first lens acting as an energy analyzer includes input 3 and output 4 end electrodes and an annular electrode 5, and the second lens performing the role of forming an ion flux with given element dimensions also includes input 6 and output 7 end electrodes and the ring electrode 8.

In addition, the device contains an annular gap 9 in the input end electrode 3 and an annular gap 10 in the output end electrode 4, as well as cylindrical holes 11 and 12 for the entrance and exit of the beam in the input 6 and output 7 end electrodes of the second hyperboloid lens, additional cylindrical electrodes (axes) 13 and 14, quadrupole mass filter 15, ion detector 16, recording system 17 and display system 18.

All electrodes 3-8 are rotation hyperboloids with surface equations: for ring electrodes 5 and 8

g ² = g „ ² + 2g ² ,

for end electrodes 3,4 and 6,7: ² g ² = g ² +2 <3 ² ,

where GD - the minimum distance from

ring electrode to the center of the system;

ά - the minimum distance from

end electrode to the center of the system.

The values of r _i and ά for the first and second lenses in the general case may be different. Moreover, in order to increase the sensitivity of the system, it is necessary to choose Td, where r _a ^ is the minimum distance from the annular electro3 1191981 4

yes to the center / for the first lens;

g „- for the second lens. Dotted

the point is removed from the center of the first lens by a distance of ζ ₅ , the distance between the centers of the lenses is ζ ₂ . five

The proposed device works as follows.

The high-energy flow of primary ions, formed by the source 1 into a small-diameter beam, falls on the sample under study, fixed in the holder 2, while selecting secondary ions carrying information about the surface composition. The secondary ions from a point source spread out in a large solid angle approximately according to the cosine law and fall into the ion-optical system made of two axially symmetric hyperboloid lenses. 20

The ion-optical system in the ion microanalyzer provides accord-. combining a point source with input characteristics of a mass spectrometer; serves to improve analytics. 25 device characteristics, sensitivity, resolution and signal / noise ratio improvement; provides a large collection rate of charged particles, which at-30 leads to an increase in sensitivity and accuracy. analysis; selection of charged particles by energy to form a monoenergetic ion flux at the entrance to the mass spectrometer, which significantly improves the resolution of the device according to the analyzed masses; optical isolation of areas of formation of secondary ions and their registration, which allows ₄ θ

reduce detector background light and improve signal to noise ratio.

The purpose of meeting all the requirements listed above, in the proposed device, the ion-optical collection system $ _{4 is} made of two hyperbolic axisymmetric lenses, and the functions of these lenses are different: the lens closest to the sample ensures efficient collection of secondary ions and the conversion of a point source of charged particles into a converging stream with their simultaneous selection by energies, the second hyperboloid lens is used to form a flux of ions 55 with given geometrical dimensions and angles of convergence for effective insertion and the analyzer used

mass spectrometer i.e. without additional losses on the analyzer electrodes.

The selection of charged particles by the energies of the hyperboloid lens closest to the target is as follows. The secondary ions through the annular slit 9 in the input end electrode 3 enter the field of the lens, the potential and _{K1 of the} annular electrode 5 of which coincides in polarity with the charge sign of the analyzed particles, and leave it through the annular slit 10 in the output end electrode 4. Equation of motion of charged particles in the field of such a lens

• T = r ₀ sod ^ + pD2 * y ² 5ϊη о £;

2 = -Ζ ₀ 1 + 0.5 / cos βί syya,

where g _in , ζ ₆

'Mon

the initial coordinates of the entry corresponding to the input end electrode; the angle of entry of particles into the lens;

the coefficient characterizing the input energy of the particles,

potential of the annular electrode of the first lens; potential of the incoming part

potential in the center of the sensor.

For particles entering the lens from the same source at the same angles, depending on their energy, the trajectories of motion in the field of the lens will be different. By appropriate selection of the entry angle ο подбор for given Ρ, ζ, it is possible to obtain in the plane of the output face, electrode, the fulfillment of the condition of focusing charged particles in a first-order angle, i.e.

 Y ™ ”

If an annular slot 10 is made in the output end electrode, then this ensures the maximum collection rate of charged particles and their effective selection according to. energies. Efficiency of selection of charged particles by energies and influence5

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6

A view of this effectiveness of lens geometry is illustrated in FIG. 2, where 'the dependences of the energy resolution K _P5 · and the aperture power of lens 52 are given for three values.' Aperture characterizes the collection rate of charged particles and is determined by the magnitude of the solid angle, the particles from which pass through the lens, according to the formula

Р ~ 5ΐη << _С р 100%, where is the half-solution of the input angles

flying particles; e4 _cf - the average angle of entry of particles.

Of the dependencies shown in FIG. 2, it can be seen that the hyperboloidal axisymmetric lens makes it possible to obtain energy resolutions up to 200-, 300 with aperture ratio up to 5-8%. Comparison of parameters of energy analysis of lenses with different y. while it is indicated that it is more preferable for these purposes to use lenses with large y and in the limit, lenses whose end electrodes degenerate into their asymptotes with surface equations

The ion flux, after selection by energy at the exit of the lens closest to the sample, has large relative sizes and the angle of convergence of the beam; therefore, the second hyperboloidal axisymmetric lens is used to form a parallel beam of small diameter. The potential of the annular electrode 8 and _k 'has the opposite sign to the potential of the annular electrode 5. To enter and pull out charged particles, the end electrodes 6 and 7 of the second lens have axial cylindrical holes 11 and 12. The geometry of the second lens may be different from the geometry of the first lens It is more preferable to use a lens with a smaller one to increase the efficiency of forming a stream of charged particles. The potentials of the end electrodes of both lenses are equal to each other.

To increase the collection rate of charged particles in the region between the output end electrode 5 of the first lens and the input end electrode 6 of the second lens can be placed

35

40

45

50

Additional electrodes, for example, in the form of “cylinders 13 and 1.4 arranged in series on one axis with corresponding potentials. In this case, the potential of the input end electrode of the second lens 6 can be chosen different from zero.

From FIG. 3 shows that the introduction of additional electrodes in the area between the lenses allows significantly (3-5 times) to increase the efficiency of the ion-optical collection system, and the device as a whole.

Example. The ion-optical system is made of hyperboloidal axisymmetric lenses with _rh = 7.8 m, the lens closest to the target has angular end electrodes, the second lens has ^ = Υ2. The distance from the ion source to the center of the lens is ζ ^ = 27.5 mm, the distance between the centers of the lenses is ζ _ζ = 39.5 mm. The range of input angles from which the secondary ions are collected by ion optics is up to 6 = · 0.285-0.425. At the same time, the following potentials are applied to the lens electrodes with respect to the positive potential of the ring electrode near the lens target, which correspond to the condition for the analysis of positive secondary ions: υ _Κι = + 1; υ _γ = -1,5i _k ; υ _Κι = -θ, ΐυκ _Λ ; and _m ^ = o.

The potentials of the end electrodes 3 and 4 and _T and u and _t are zero (the electrodes are grounded).

In the space between the end electrodes of the lenses there are two cylindrical electrodes with a diameter of 12.16 mm and 7.6 mm, each having potentials of 15 ^ = 0 and υ ^ ₂ = + 0.75υ , _ι , the whole system is placed in an electrostatic screen with a diameter of 70 mm, length 80 mm systems. The developed ion-optical system makes it possible to obtain an ion collection ratio of up to 5%, an energy resolution of 20 at a level of 0.5, an output ion current diameter of 2 mm with an angular divergence of flow no more than 10 °. When the potential υχ ₁ changes, it is possible to tune the passage of any ions. energy, the parameters of ion optics collection does not change.

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Claims (2)

1. ION MICROANALIZER, including a source of primary ions, located coaxially and sequentially a sample holder, focusing secondary ions optics, mass spectrometer, ion detector, recording and display systems, characterized in that, in order to increase the sensitivity and resolution of the device, focusing the secondary The optics ions are made of two hyperboloidal axisymmetric lenses, the ring electrodes of which are connected to the opposite poles of the power source, the ring electrode being closest to The lens sample holder is connected to a power supply pole that matches polarity with the charge sign of the ions being analyzed, and the end electrodes of each lens are connected to the same poles of the respective power sources, and the condition ^g a, 7 ^g " _g " where g _and - the minimum distance from the annular electrode closest to the sample holder of the lens to its center; g _Kg - the minimum distance from the ring electrode to the center of the second lens, The end electrodes and proximal to Tieliu The holder _with the sample lens are formed as tapered surfaces angled electrodes described by the equation τ = ± Υ2ζ ", where g is the distance from the axis; ζ - longitudinal coordinate coinciding with the axis of the microanalyzer, The channels for input and output of charged particles in the end electrodes of the hyperboloid lenses are made in the form of coaxial annular slots for the lens closer to the sample holder and axial coaxial holes for the second lens. (ABOUT „1191981 > 1191981 2. The microanalyzer according to claim 1 ,. characterized in that between the end electrodes of the hyperboloid lenses are additional electrodes, made in the form of two coaxial cylinders arranged one after the other on the axis of the system, while the middle The electrode holder is galvanically connected to the sample holder with the output end electrode of the first lens, and the second cyclic electrode is connected to the pole of the power source that coincides in polarity with the charge sign of the analyzed ions. one