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PRECISE MEASUREMENT OF ENERGY DISTRIBUTION IN Ga LMIS
M. Komuro, T. Kato
To cite this version:
M. Komuro, T. Kato. PRECISE MEASUREMENT OF ENERGY DISTRIBUTION IN Ga LMIS.
Journal de Physique Colloques, 1987, 48 (C6), pp.C6-141-C6-146. �10.1051/jphyscol:1987623�. �jpa-
00226826�
PRECISE MEASUREMENT OF ENERGY DISTRIBUTION IN Ga LMIS
M. Komuro and T. ~ a t o *
Electrotechnical Laboratory, 1-1-4 Urnezono, Sakura-mura, Niihari-gun, Ibaraki 305, Japan
*ANEEVA Co., 5-8-1 Yotsuya, Fuchu-shi, Tokyo 183, Japan
ABSTRACT
Temperature dependence of energy distribution-for Gaf ion from Ga liquid metal ion source(LM1S) is measured in the range of more than 3 digits in ion current by using a single focusing sector magnet with an energy resolution of less than 2 eV. As increasing the source temperature at a fixed ion current of 1, PA, the distribution curve changes from a single Gaussian peak to three peaks including 1)the Gaussian, 2)the peak at the energy position about 7 eV lower than it, and 3)the peak at about 32 eV. From the calculation of the energy deficit based on free space field ionizattion of neutrals coming from the outer space, it is shown that the experimental energy deficit agrees with the calculated for the liquid apex radius of 1 nm and the field strength of 16 V/nm. The rate equation representing variation of ion flux is proposed and it is shown that the ratio of the ion fluxes produced by field evaporation, free space field ionization, and charge tranfer collision can be quntitatively explained.
INTRODUCTION
Energy distribution of ions emitted from liquid metal ion source(LM1S) is one of the most important factors in finely focused ion probe forming, because a chromatic aberration commonly limits the ion probe diameter and the current density 1)
.
In addition, it gives us more information on ion emission mechanism of LMIS. So far three2-4)
kinds of ion emission processes have been proposed ; 1)field evaporation(FEV), 2)field ionization(FI), and 3)charge transfer collision between ion and neutral atom(CT). Swanson et al. 2) identified the field evaporation process from the measurement of energy deficit for several kinds of LMISs at low emission current.
However, there is no distinct evidence for other two processes.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987623
C6-142 JOURNAL DE PHYSIQUE
In this paper, we describe the experimental results on precise measurement of energy distribution for Ga LMIS and the two processes are identified from a calculation of energy deficit and a semi- quantitativie treatment for the ion flux.
EXPERIMENTS
The measurement of energy distribution curves was carried out by use of a single focusing sector magnet, where the magnetic field was sweeped in narrow range near the specified mass peak. The expected voltage resolution for this instrument was less than 1.8 V with an accelerating voltage of 3 kV, which was ascertained from the actually measured full width at half maximum(FWHM) of 7 eV for ~ e + + from AuSiBe alloy source. The Ga ion source with an underlying tungsten needle apex radius (tip radius) oE about 0 . 5 pm was prepared. This was installed in the ion gun assembly, where the ion source was maintained at 3 kV and the total ion emission current was controlled by changing the voltage applied to the cathode. The detected ion current was logarithmically amplified and recorded in the range of more than 3 digits in intensity. The source temperature above 5 0 0 OC was determined by spot thermometer and the lower temperature was deduced from the extraported curve by assuming that the temperature is proportional to the square of heater current.
RESULTS AND DISCUSSION
Temperature dependence of energy distribution curve are shown in Fig.1 where the total ion current It is fixed at 1 pA. The profile
at 3 0 ° C is nearly equal to a Gaussian with FWHM of 5 eV and as rising
the temperature, the main peak shifts to the higher energy position of about 2 eV. Andalso the two shoulders appear at the energy postion lower than that for the main peak.
Such a result has been already reported by Swanson et a1. 4 , except for appearance of the third peak. lo-3h I
ION ENERGY
Since the energy broad- Fig.1 Temperature dependence of energy ening due to stochastic spectrum for Ga LMIS at 1 pA.
Boersch effect is at most about 5 eV as observed for the spect- rum at 3OnC, the two shoulders appearing at higher tempera-
c
v,tures should be due to ioniza-
2
+Z
tion processes other than the
-
lo-' field evaporation and hence9
the curves are divided into
a
three peaks P I , P2 and P3 as
E
0 Z
shown in Fig.2. From this figure, it is seen that the energy deficit which is
-40 -20 0 20 '40 measured from the main peak
ION ENERGY (eV)
position is about 7 eV for the
p2 and about 32 e~ for the Fig.2 Separation of the spectrum into three peaks.
P3. These values are not so
dependent on the source temperature and the emission current, though the intensity for these three peaks considerably varys. Figure 3 shows a temperature dependence of the ratio of the ion flux for the P
2 and P3 to the total ion flux. Neutral atoms generated at the liquid apex surface where the FEV occurs
may be immediately field-ionized and can not reach the space far away from the liquid surface. So it is conjectured, as pointed out by Swanson et ala4), that the neutral atoms produced at low field region are attracted into the high field space by the force of polarization. Part of the atoms approaching the liquid apex surface from the outer space may be ionized by the CT process at low field space and all the rest may be field-ionized at the higher field space than that for
the CT process. SOURCE TEMP. ("C)
Fig.3 Temperature dependence of In order to treat the above ratio of ion fluxes for p2 and conjecture quantitatively, we P3 to total ion flux at 1 P A -
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represent the ion fluxes by FEV, F I and CT processes at a distance x from the liquid apex surface as fFEV(x), fFI(x), and fCT(x), respecitvely, and the neutral atom flux as fN(x). For these ion fluxes, the following equations are derived.
where is the electron's frequency of Ga atom, v the velocity of Ga atom, D(x) the electron's tunneling probability in an electric field,
0 the cross section for charge transfer collision, and n(x) the spatial density of neutral atom at x. Notice that fN has a negative value.
At first, the energy deficit for the F I process is computed from Eqs.(Z) and (4) in which the term of the CT process is neglected and the D(x) is derived from VKB approximation by assuming a trapezoid for the potential barrier of the electron tunneling. In order to determine an appropriate value of V/V, the calculation for H2 gas/^
tip system is performed by use of a hyperboloid model for the electric field distribution so as to coincide with the experimental result by - .
I
, , , ,I
Table 1. Calculated energy deficit '30 40 50 for FI process of Ga-atom at ~,=16 V/nm
,
L)=l. 5x101*/s, and a kineticELECTRIC FIELD (Vlnm) energy of 0.25 eV.
Fig.4 Comparison between ra (nm) 0.5 1.0 2.0 calcul.ated and experimental
energy deficits gas/W Energy deficit(eV) 4.4 9.1 18.6 tip system.
5 1
Sakurai and Muller
.
Comparison between the present calculation for Y=1.5x10 /s and a kinetic energy of 0.25 eV is shown in Fig.4. 14' By using these values , the energy deficit computed for Ga atom is sur~~~aarized in Table 1 for several values of the liquid apex radius ra and for the electric field strength of 16 V/nm at the liquid surface. The experimental value of the energy deficit in Fig.2 is in good agreement with the calculated result for ra=l nm which is nearly equal to the value of r determined from the electrona
microscope observation6) and predicted 600-
500-
z
400- 9 !&
300-
>
;
200-5
100-H, gas on w
o Exp. results by Sakurai & Miilter -Present calculation
denoted by P2 is produced by free space field ionization of neutral atoms coming from the outer space.
From the result in Fig.2, it is considered that the neutral atom density n(x) approaches to zero at the energy deficit less than 20 eV which is corresponding to x =5 nm for r =I nm, because the field
c a
ionization abruptly occurs at xcxc. So the CT process occurs only at x>xc where the initial ion flux of fFEV(xc) and fFI(xc) decreases monotonously and is converted to fCT. When we assume the neutral atoms are approaching the liquid apex surface in normal direction, the n(x) is expressed as
where Q is the solid angle within which the ions are emitted. Then Eqs.(l)-(3) can be readily solved at x>xc for a constant flux of
fN.
Thus we can obtain the ratio of each ion flux at x=m to the total ion flux ft as follows.
where we used the relationships of ft=fFEV(xc)+fFI(xc)=fFEV(m) +fFI(m)+fcT(m)r fFI(xc)=-fN(m)-fcT(m)
,
andwhere dI/dQ is the angular.current intensity and e the electric charge. When 0.1 0-l6 cm2, dI/dR=10 pA/str, xc+ra=6 nm, and v=l0 5 cm/s corresponding to a kinetic energy of 0.25 eV, Eqs. (7) and (8) gives us fFI(m)/ft=0.261 and fCT(-)/ft=0.031 for fN(m)/ft=-0.3.
These values agree with the present results at the temprature higher than 200'~ in Fig.3.
SUMMARY
The precise measurement of energy distribution for Ga LMIS reveals that the sepctrum at higher temperatura consists of three kinds of peaks. For a quantitative expianation on the origin of
C6-146 JOURNAL DE PHYSIQUE
these peaks, the rate equations for the ion fluxes and the neutral flux are proposed by assuming field evaporation, field ionization, and charge transfer collision. Then the second peak is identified as free space field ionization from the calculation of the energy decifit by use of this equation. And it is shown that the intensity ratio of the three peaks can be quantitatively explained by these equations.
The authors wish to thank Dr. H. Arimoto of Fujitu Laboratory for useful discussions.
REFERENCES
1)M. Komuro; Thin Solid Films,
92
(1982) 1552)L. W. Swanson; Proc. Ion Engineering ~ o n g r e s s - I S 1 ~ ~ ' 8 3 & I P A T ' ~ ~ , (Kyoto University, Kyoto, 1983)325
3)A. R. Waugh; J. Phys. D:Appl. Phys.,
13
(1980) L2034)L. W. swanson; G. A. Schwind, and A. E. Bell; J. Appl. Phys.,
51
(1980) 3453
5)T. Sakurai and E. W. Muller; Phys. Rev. Lett.,
30
(1973) 5326 ) G . Benasayag, P. Sudraud, and B. Jouffrey; Ultramicroscopy,
16
(1985) 1
7)D. K. Kingham and L. W. Swanson; Appl. Phys.,
