SU1396174A1 - Method of mass-separation of charged particles - Google Patents

Abstract

This invention relates to the field of quadrupole mass spectrometry. The method of mass separation of charged particles is implemented in the device. The charged particles are directed to quadrupole capacitors (CC) 3 and 6, register them at the output of CC 6, an alternating voltage is applied to one of the CCs and an electric field is formed in it with the Mattie parameter qo) corresponding to the second region stability of the trajectories of charged particles, and an alternating and constant voltage is applied to the other QC and form an electric field in it with the parameter q corresponding to the first region of stability of the trajectories of charged particles, with 8, 773 q / q, 10.643, q, 4eV , / m- u) r% q, 4eVi TT where e, m is h ar d and mass m oj J of the ion, respectively; V.., V - amplitudes of the alternating voltage at QC; U), the frequencies of the mentioned voltages; r,, r; j are the radii of the span space of the spacecraft. Increases permission-. the ability of quadrupole mass spectro (etra. 5 ill. S (L 00 (UD 05 4il. cosivtuHcamit "utt

Description

 This invention relates to quadrupole mass spectrometry and is intended for secondary ion mass spectrometry (SIMS),

The mode of separation of ions in a quadrupole capacitor when applied to it in an appropriate combination of high-frequency and constant voltage is carried out in a number of parameter areas a-q, where stable ion trajectories are realized.

The first stability region can be characterized by a change in the parameter q in the interval 0-0.9080, and the second - b the interval 7.5136-7.5797,

The disadvantage of a quadrupole mass spectrometer is the small (I-20 eV) energy range of the analyzed ions, which is due to its design, which allows only iKo to operate in the first region of stability. the disadvantage does not allow the effective separation and collection, for example, of secondary ions, which reduces the sensitivity of the device,

The use of the second region of SUSTAINABILITY allows satisfactory separation of ions with energies up to 300 eV for ions of average rliacc (30-100 amu). ; The aim of the invention is to increase the resolution of a quadrupole mass spectrometer. I The essence of the invention consists in the realization of the joint use of the first and second stability regions of the trajectories of charged particles in two 1 consecutively located quadrupole capacitors, with the magnitudes of the parameters of the Mathieu equation q and 42 of both capacitors with the first and second regions respectively mi stability choose satisfactory ratio

, 773 .10,643,

4eV "1

V is the amplitude of the alternating voltage, V;

uJ is the frequency of alternating voltage, Hz;

g - the radius of the span of the quadrupole capacitor, m

Moreover, based on the definition of q, this ratio will be achieved

five

0 5

0 d

five

0

five

then when choosing either amplitudes V, and V, satisfying the condition

8,773g Jl 10,643, or frequencies CJ, and w, provided

U).

2, 3.262, or radii g, and g, provided

2,962 - 3,262.

The invention is illustrated with graphic materials.

FIG. 1 shows a flowchart implementing the proposed method of the device, in which the relation connecting the parameters q Hsiq ,, is achieved by an appropriate choice

g, and g i. 2,: 3.262; on

7

FIG. 2 is a circuit for connecting supply voltages to the electrodes of quadrupole capacitors; in fig. 3 shows a typical stability diagram of the first stability region of the first quadrupole capacitor; in fig. 4 — ion transmission region in the parameter q of the second quadrupole capacitor using the second stability region; in fig. 5 is an ion transmission diagram of the mass spectrometer as a whole.

The mass spectrometer consists of an ion source I, an input diaphragm 2, a first quadrupole capacitor with electrodes 3, a focusing lens 4, an accelerating electrode 5, a second quadrupole capacitor with electrodes 6, an ion collector 7, a recording system 8, power sources for the ion source, and an input aperture 9, a source of 10 constant and alternating high-frequency voltages connected to capacitors with electrodes 3 and 6, a lens power supply unit P, an accelerating electrode power source 12, an ion collector power unit 3, C - blocking ovine capacity; .

R - resistor.

I

FIG. Figure 2 shows a circuit for connecting electrodes (rods) to the output of an RF generator and a constant voltage source. The quadrupole capacitor assembly is shown in section, 3 and 6 are electrodes of the first and second quadrupole capacitors with a radius

3139617

mi of the span of g, and g, respectively. C — blocking capacitors, which divide the first and second quadrupole capacitors by constant voltage; R — high-resistance resistors (10 MOhm), which serve to isolate the RF generator and the power source 12 (Fig. 1), as well as to provide galvanic communication of electrodes 6 to the source 12 (Fig. 1).

FIG. 3 shows the first stability region of the first quadrupole capacitor, where a and q are dimensionless parameters, defined as is

8mey 4eV yy

ion charge and mass, respectively; . 20 circular frequency alternating voltage with amplitude V applied to opposite electrodes of quadrupole capacitors;

the magnitude of the constant component on the electrodes of the first quadrupole capacitor;

the radius of the span space of a quadrupole capacitor with the first region of stability;

the abscissas of the points of intersection of .jc or direct scanning () with the boundaries of stability of a- (ion oscillations along the y axis) and a X (ion oscillations along the x axis);

q parameter corresponding to vergain M of stability diagram; masses corresponding to

parameters q, q, q. 4 shows regions I and II of ions according to the parameter q of the horn capacitor; without constant, the parameter q of FIG. 4 oprede

50

4eV mlyirl

where e and m are the charge and mass of the ion;

V and 1Л - amplitude and frequency of alternating voltage applied to the electrodes

and

second quadrupole capacitor with a radius of the span

the boundary parameter of the first stability region of the second quadrupole capacitor;

IQ, 5136 and

is

, 5797 are the values of the parameters q, which determine the second stability region; and 1. g ion mass, corresponding &

20 25

zo

 . jc to

to

50

these parameters when scanning by mass as the amplitude of the high-frequency voltage changes.

FIG. Figure 5 shows the idealized transmittance diagram of the mass spectrometer analyzer, where S is the filter transmittance in the mass and m range (marked by double hatching); m and m, is the mass range transmitted by the first quadrupole capacitor; mass m corresponds to the parameter q d of the second quadrupole capacitor; t, is the smallest mass of the ion, which is passed by the second analyzer due to the manifestation of the first stability region.

The choice of the ratios q / q, or in this case, the radii r, / g is made from the following considerations.

The purpose of the capacitor with the first stability region is to cut off the heavy masses of the ions corresponding to parameter q 0.908 of the second capacitor (in which the separation conditions are realized in the second stability region), as well as to adjust the transmission window d (Fig. 5) by changing the first capacitor.

The stability limit in the region of the parameters aq of the oscillations of the ion along the Y axis (the axis passing through the centers of the electrodes to which a negative constant voltage is applied; the mode of analysis of positive ions) is given by the curve a (q) (Fig. 3), which can be represented in the form of a polynomial in powers of q q:

55

 q 7q 29q 68687q "

VRT TTITT T YYY Y - - - - T -

128 2304 188774368

+ ...

(3)

Similarly for ion oscillations along the X axis:

171

, 4eV /

do:

(Fig. 3) there is a vertex of the diagram of stability hell, and the coordinate along the q axis is equal to, 7060, q, 0.9080 is the limiting value of q when, q, and q are the projections of the intersection points of the scan line with curves ay and aj respectively. The filtering window m f-m at the first capacitor can be expressed through the parameters q and q in accordance with (1)

as

4eV

qv

(five;

Thus, it can be said that

parameters q and

q .. match gras j

the ionic mass m y and m y.

To find the permissible range of change, for example, the ratios g / g we will proceed from the fact that the tuning to the mass m of the first analyzer (at), corresponding to the value of m d, was to the right of the Year-Shd transmission window (Fig. 4) of the second analyzer. When reducing the angle of inclination. (FIG. 3) the corresponding value m x falls into the filtering window m — Shd (FIG. 5). The location of t (, to the right of the Fr-Shd interval along the AISI m (Fig. 5). Was chosen from the condition of obtaining a steeper mass peak from the side of light masses by more

steep border

X

not a (fig. 3), (6)

The second condition is that with maximum transmission of the first analyzer, the following relations should occur (Fig. 5):

m, m g; m m ,,

(7)

the setting of the vertex M (fig.Z) will correspond to the parameter q,.,

defined by the ratio

q 4eU / t, and g,

: 2):

(eight)

Thus, based on the specified requirements, we obtain

4eV / q

4eV / q

(9)

from (9) we get

, / rj

.c

(eleven)

1-shi

q./q q / q,

IE -lo To determine the value of q, we use, in accordance with (), the system of equalities:

five

0

m jj m .j

 1 L.

12)

where mx 4eV / q,; m.4eV / q w rj;

0 - ,. P1l

Of

m mg

(2

- 4eV / q wV; ha, 4eV / q (13) r 4eV / a toV: m- 4eV / a u; r:

: 4eV / q to

I follows that

qx qa „„, r- - Z l 5Ioop -0.348

q, 0.9080

(14)

The values of q and q V are uniquely COOT T

each other, they can be found using the form of the dependences a (q) and aY (q) (3) and (4), taking into account that. Numerical solution of the transcendental equation (14) gives a limit value, 8640.

Finally, from (12) we find

 - -if5br- - .г 3; %

In this case, from (II) follows 8,773 i 32 10,643

q 1

or

 g,

2,., 262

0.7060

0.7060

(sixteen)

five

0

five

The method is implemented as follows.

Ions, exit from ion source 1 (Fig. 1) ,. through the diaphragm 2, the first capacitor with electrodes 3 enters, in the alternating and constant fields of which the ions are separated. At the entrance of a capacitor with electrodes 3, an ion beam with masses my is formed. Further, the ions are focused by the lens L into a beam of smaller diameter at the entrance of the second capacitor with a smaller radius of the transit space r. In this case, in the field of the accelerating electrode, the ions receive additional energy proportional to the applied potential - L c.

7I3

When the analyzed ions are positive, negative potential -U ,, is applied to the accelerating electrode 5, otherwise + Uo,

In order for the ions at the exit from the field of the accelerating electrode 5 not to slow down and not to dissipate, it is necessary to raise the potential of the four electrodes of the second capacitor with electrodes 6 to the value U,. Development of the RF signal through the source 10 - accelerating electrode 5 (Fig.)) Occurs due to high-resistance resistors (H 10 MΩ) connected between the electrode 5 and the opposite pair of capacitor electrodes 6 (Fig. 2).

In the high-frequency field of a capacitor with electrodes 6, ions are cut off with masses greater than Sd (Fig. 5), and ions with masses lying in the interval m – j j (Fig. 5, marked by double hatching) fall onto the ion collector. A signal proportional to the number of ions with masses from the output of the ion collector enters the registration system 8. Thus, the resolution increases due to the joint action of two areas of stability.

In this case, alternating voltages of one frequency and amplitude are applied to the electrodes of quadrupole capacitors with electrodes 3 and 6 to ensure the combined filtering effect of two quadrupole capacitors.

The purpose of the first capacitor with electrodes 3, having a larger radius g, is to increase the resolution

mass spectrometer capacity

In general, that is achieved by cutting off heavy masses, as well as by matching the phase of the input of ions into the capacitor with electrodes 6, since the RF signal from the source 10 is immediately fed to the electrodes of the two capacitors, i.e. in-phase. Mass scanning is performed by varying the amplitude V of the output voltage of source 10. The potential also varies linearly with increasing mass of the analyzed ion. it

it is necessary to achieve a good transmission of the capacitor with electrodes 6, since the energy E of the ion entering the edge region of the capacitor with electrodes 6 must be sufficient to overcome it in 0.2 period of the applied field. Therefore, E is chosen to satisfy the condition

74

 / J 7 /, 4 E, .2. (G; 2-T - 2 0: 2,

where f u / 2 i is the cyclic frequency;

 - period. Let E, be the ion energy of a beam entering a capacitor with electrodes 3. For BliMC, the half-width of the energy distribution of the secondary ions is E, 10-60 eV. Then, in the case of using an accelerating electrode 5 (Fig. 1), the energy of the ions is full, the input of a second capacitor with electrodes 6 (more precisely, its component corresponding to the ion motion along the axis of the system) will be determined by

t / fr h 2

eUp + E, 7, (d- |)

(18)

On the other hand, the transmission of the mass m ion through a quadrupole mass spectrometer occurs at an amplitude V equal to

 - (nineteen)

From (Yu) and (P) we find that the ratio of the constant component U on the accelerating electrode 5 and the amplitude V of the RF voltage on the electrodes 3 and 6 of the quadrupole capacitors should satisfy

V mrU2irf)

Uc

-fEiF: - T .. (

ppl 1

OS 5

since the maximum resolution of the second mass filter with electrodes 6 takes place at the energy of ions E. eV, this makes it possible to calculate for SIMS,.

Thus, the time-varying, when scanning by masses, the quantities V and UQ for the case in question are related by the relation

,

(21)

A more accurate value of the V / UP ratio is chosen experimentally and from the specific conditions of the analysis.

A feature of ion filtering in a capacitor with electrodes 6 (Fig. 1) operating in the stability region II is the rapid exit of the ion to an unstable trajectory at appropriate masses due to an increase in the intensity of the alternating field, therefore the required number of filtering periods decreases dramatically in comparison with the usual the filtering mode in the T. region. Thus, a rapid passage of an ion through a quadrupole capacitor with simultaneous filtration is possible. This allows satisfactory filtration of particles in the extended energy range of the analyzed ions, which is important for secondary ion mass spectrometry.

The proposed method is intended for use in a SIMS installation operating in the microprobe mode for layer-by-layer analysis of large integrated circuits.

1396174 O

An alternating and constant voltage is applied to another quadrupole capacitor, and an electric field is formed in it with the parameter q, corresponding to the first region of stability of the trajectories of the charged particles, differing from that by increasing the resolution, the values of the parameters of the Mathieu equation q and q 2 of both quadrupole capacitors with, respectively, the first and second regions of stability are chosen to satisfy the condition

8,773 - 2 10,643,

Claims (1)

Invention Formula The method of mass separation, charging particles, is that the particles are successively directed to the first and second quadrupole capacitors and register them at the output of the second quadrupole capacitor: an alternating voltage is applied to one of the quadrupole capacitors and an electric field with a parameter of the Mathieu equation q, corresponding to number 1 in the second region of stability of the trajectories of charge f / g. 8,773 - 2 10,643, Where 4eVi . tsg. e is the charge of the ion. 4eV2 gash | g | CL; m is the mass of ions with a stable trajectory, amu V, V - amplitudes of alternating voltage on quadrupole capacitors, B; 00, and u) are the frequencies of the mentioned voltages, Hz; r i - the radii of the span of quadrupole capacitors, m; Wei FIG S mark fl d / T7x J Fig.Z M p- / tta four / 77u / T / .m