3 SIMS results for polyatomic ion bombardment
6. Experimental Setup
6.2 Vacuum system
6.3.6 Second version of the source
In the new design there are three sputter targets installed on a rotating platform. The rotation is transferred through a feedthrough on the flange. With these modifications, it is possible rapidly to change the target and, hence the type of ions within seconds during operation.
The length of the cathode insulator is prolonged to 70 mm and shielded by a sheet metal screen from cesium. As a consequence, a several weeks of intensive exploitation did not produce any leak current across cathode insulator. In addition, operation and maintenance (reloading of cesium, changing targets, etc) of the sputter source is easier with the new design.
In the new design the same type of cesium ion source was used. The only difference is that the tantalum tube which is used as a vapor conductor of the Cs vapor was bent by 90 degrees. Moreover, the whole cesium source is closed in a stainless steel box to reduce contamination of the first lens insulators.
Innovative improvements were utilized in the design of the cesium ionizer. Due to the fact that the design of the cesium ion source is stricter in the space dimensions as compared to
experience in the routine use has shown that the new design of ionizer does produce the same performance as the old construction and, moreover, has additional advantages like lower power consumption at the same produced cesium ion currents in combination with simpler power supply circuit, since it does not requires an additional HV power supply to accelerate electrons. During operation producing a cesium current of 100 µA, the ionizer was consuming a power of 5.4x4 = 21.6 W which is nearly a factor of two lower in comparison with the previous design. The higher efficiency can be explained by the fact that in the latest design the ionizer and heater were higher are combined more closely together. Therefore the working temperature of the ionizer can be maintained at greatly reduced electrical heating power. The only drawback in the current design of the ionizer is that its surface has a roughness of about 1 mm. However, simulations performed with SIMION proved that in the current design of the ion optics the influence of this roughness influence on the cesium ion trajectories is negligible.
Good performance of the ion source means the productions of the ion beam with the high value of the brightness having small solid angle extension. The only possibility to increase brightness of the beam is to reduce the area from which beam is issuing, or in other words to reduce the cross-section of the cesium beam at the sputter target, if the Cs+ current and beam energy are not to be changed.
After operation of the source the erosion craters on the target show that the Cs+ beam spot size on the target has the dimensions of 2.4 and 1.4 mm with area 3.3 mm2 which is much lower in comparison with previous model of the source having sputter area ≈ 9 mm2.
Fig. 6.12. Spectrum of − (m=1÷3) negative ions.
m Au
Fig. 6.13. Spectrum of − (m=1÷5) negative ions.
m Ag
Fig. 6.14. Spectrum of Cu (m=1− ÷5) negative ions.
m
Like in the design of the first version of the source there are used two slits in the ion column. The first slit is placed at the crossover of the sputtered ions (fig. 6.11). This diaphragm determines the source size of the beam and it has a size of 0.4 mm. The second slit is placed at a distance 19 mm after the first one and has a size 3 mm. The second slit restricts the emittance of the beam to 0,2 mm⋅π⋅rad. These beam properties were required to achieve the following conditions. The first is to ensure that the first electrostatic (decelerating) lens is filled to less than 30% of its diameter (14 mm) in order to reduce its aberrations. The second is that source size of the beam must be less than 0.5 mm in order to be focused on the exit slit of the Wien filter. The current density
In the target holder of the source three types of target (gold, silver, copper) have been loaded. In the fig. 6.12, 6.13 and 6.14 the spectra of gold, silver and copper negative ions produced at the Cs+ current of 60 Aµ are shown. Like of gold projectiles copper and silver negative ions show strong odd-even alternations. The intensity of atomic ions in the case of copper and silver targets is significantly lower in comparison with gold target. Though the yields of triatomic projectiles are at the same order of magnitude, the intensity of silver and copper ions is 13 and 12 times less in comparison with the intensity of the gold atomic ions. Appearance of Ag and Cu heteroclusters containing chloride may be explained by Cl flooding from cesium ion source which uses CsCl salt.
These data can be compared to published data on secondary ion yields that have been measured under Cs+ ion bombardment. As an example, the results of Storms obtained for a Cs impact energy 10.5 keV are depicted in fig. 6.15 [St77].
Fig.6.15. Relative negative yields for 16.5 keV Cs+, φ=0°. From Storms et al. [St77.] In their results the ratio between intensity of the gold and silver ions is two orders of magnitude.
On the other hand, the data sheet from Peabody scientific on their PS-120 Negative Sputter Ion Source which is reproduced in table 6.1 shows ratio between gold and silver ions of only factor 2. Between gold and copper ions this factor is about 3.
Ion Species Beam Current (microamps) H 200 Li 1 B 80 C 350 O 200 Al 10 Si 400 P 40 S 100 Cl 100 Fe 20 Ni 60 Cu 60 Ag 100 Cs 1 Pt 100 Au 225
Table. 6.1. Beam performance of the PS-120 Negative Sputter Ion Source from Peabody scientific at the primary ion current 8 mA.
In order to discuss the possible reason for this
In the works [No87] and [Lu78] there is proposed the model of ionization for negative ions in which it described by equation
α+
(
(
)
)
n e A ε ϕ / exp − − ∝ (6.7)where Ae is the electron affinity, ϕ the work function of the emitting surface, and εn- is a
characteristic energy depending on the particular model description employed. Possible interpretations of εn may be include energies of local equilibrium (thermal) or non- equilibrium electronic excitation as well as quantum mechanical non-adiabatic passage, electron tunneling, etc.
From this equation its clear that a reduce of the work function ϕ , including one by means of alkali metal adsorption on the surface, must lead to a significant increase of negative ionization efficiency. In fact, this is the reason why cesium ions are often used as projectiles in practical surface analysis application using negative secondary ions.
The difference between results on the efficiency in the ionization of silver atoms in comparison with gold ones obtained by Storms and ratios produced from the ion source may be explained by different concentration of the cesium on the surface and , therefore, the value of the workfunction ϕ. The equilibrium concentration of the cesium on the metal surface in the case of cesium irradiation under high vacuum conditions depends on the sputtering rate of the metal. In the conditions of sputter negative ion source, in
In the dynamic equilibrium conditions of sputtering process the primary flux of Cs+ ions
in addition with flux of Cs neutrals is equal to the sputtered flux of Cs from uppermost layer of the surface:
p tot Cs p Cs dep p j Y j C Y j j + = ⋅ = ⋅ ⋅ (6.8)
here jp flux of Cs+ ions, flux of Cs neutrals, Y partial sputtering yield of Cs, concentration of Cs on the surface, Y total sputtering yield from the surface.
dep
j Cs CCs
tot
From Eq. (6.8) concentration of the cesium is derived as
tot p dep Cs Y j j C =1+ (6.9)
if we assume jdep<< jp, Eq. (6.9) transforms to
tot Cs
Y
C = 1 (6.10)
Sputtering yields Y may be obtained from computer simulation using program SRIM. For 90
tot
0 degrees incident angle with energy 10 keV sputtering by Cs projectiles SRIM
predicts the following sputtering yields: 16.9, 9.5, 8.2 for Au, Ag and Cu respectively. From Eq. (6.10) and dependence of the change of the work function ∆ on the cesium ϕ concentration [St93] the following ∆ for Au –0.55 eV, Ag –1 eV, and Cu –1.2 eV are ϕ obtained.
From [Be87] parameterεn is estimated to be 0.26, 0.25, 0.24 eV for Au, Ag, and Cu respectively.
where Eb is binding energy of the surface, m is atomic mass of ion.
From Eq. (6.7) and (6.11), using estimated data on ∆ one can calculate ratio between ϕ the ionization probabilities in the case, for instance, of gold and silver cesium bombardment:
(
)
( )
(
( )
)
−∆ − + − ∆ − − ≅ − − Ag A Au A n Ag Ag Ag n Au Au Au Ag Au ε ϕ ϕ ε ϕ ϕ α α exp (6.12)The following ratios: 11 for −
− Ag Au α α and 3.3 for − − Cu Au α α
have been obtained. For Ag target the obtained ratio is in the good agreement with experimental ratio measured from the source, though for Cu theory predicts ratio by factor about 3 higher.
In order to estimate how neutral cesium flooding from an Cs ion source may change an ion yields (rations), we can assume that Cs ion and neutral flows are equal. Using Eq. (6.9) and data from [St93] we the following ∆ for Au –1.4. eV, Ag –2.2 eV, and Cu –ϕ 2.4 eV are obtained. Evaluation of −
− Ag Au α α and − − α α Cu
Au in this case produce the 2.8 and 2.25
respectively. These ratios are in the good agreement with data from Peabody (Table 6.1). As it was described earlier, in their design of ion source sputter target is exposed to the strong flooding of Cs vapor (fig. 6.2).
Therefore, to improve the yields of negative ions is desirable to produce a high vapor pressure of Cs at the sputter target.