- Email: [email protected]
Investigations on the MCs2+ formation in secondary ion mass spectrometry
Mass Spectrometry and ion Processes
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
Investigations on the MCs formation in secondary ion mass spectrometry T. Mootz*, A. Adriaens, F. A d a m s University of Anm'erp, Department of Chemist~,, Universiteit~plein 1, B-2610 Wilrijk, Belgium Received 8 February 1996; accepted 18 June 1996
Abstract
The formation of MCs~ secondary ions under Cs ÷ bombardment of several elemental surfaces M was studied. In conjunction with their respective mass signals the energy distributions of the secondary MCs +, Cs +, Cs~ ions and the neutral M ° atoms corroborate an MCs~ formation mechanism via recombination of an already built MCs ° molecule with a Cs ÷ ion. Evaluation of the mass signals reveals that the spatial and temporal correlation between two partners forming the MCs~ molecule is strongly enhanced in comparison with the respective correlation of M ° and Cs ÷, which is generally agreed to form the MCs ÷ molecule. The surprisingly broad energy distributions of Cs ~ are determined by the broad distribution of the neutral M o atoms which are essentially implicated in the Cs~ formation process.
Kevwords. Secondary ion mass spectrometry; Cesium bombardment; Metal cesium ion formation; Molecule formation; Energy distributions; Spatial and time correlated emission
1. Introduction It has recently been shown that MCs ÷ secondary ion mass spectrometry (SIMS) using Cs ÷ primary bombardment and detecting MCs ÷ secondary ions for the analysis of element M minimizes the SIMS matrix effect enormously [1-5]. The MCs + ion formation was proposed to proceed via recombination of independently sputtered neutral M ° atoms and Cs + ions [2-8]. In this case the emission and ionization processes are decoupled in analogy to secondary neutral mass spectrometry (SNMS) allowing subsequently an almost matrix independent analysis * Corresponding author.
of element M. The remaining MCs + matrix effect can be attributed to the efficiency of molecule formation which is related to the density of molecule partners available above the surface [6] as well as their probability of recombination, The latter parameter surveyed by the yield of the MCs ÷ molecules exhibits a roughly quadratic relationship to the polarizability of the individual M atom [7]. Furthermore, the MCs ÷ yield depends strongly on the emission energy of the emitted neutral M ° atoms which is governed by the surface binding energy of the sample. The yield is enhanced by decreasing the mean M ° emission energy [8]. This indicates the importance of a spatial and time correlated emission of the two partners forming the molecule.
0168-1176/96/$15.1)0 Copyright ~ 1996 Elsevier S zience B.V. All rights reserved PII S 0 1 6 8 - 1 1 7 6 ( 9 6 ) 0 4 4 1 0 - 2
2
T. Mootz et al./lnternational Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
The MCs ÷ yield depends on the density of Cs ÷ ions and neutral M ° atoms available above the surface. In many cases the Cs + bombardment can be expected to lead to an equilibrium state in which the number of primary Cs + ions striking the surface is equal to the number of resputtered Cs + ions [5]. However, recent studies [4,9,10] show that the bombardment-induced surface concentration of Cs can lower the electronic work function of the sample below a critical value which is found to be slightly below the Cs ionization energy [11]. As a consequence the ionization probability for resputtered Cs + and the correlated MCs ÷ signal manifest a drastic decrease which can be understood in the framework of the electron tunneling model for secondary ion formation [11]. In order to rule out a significant decrease in the Cs ÷ ionization probability and a subsequent decrease of the corresponding MCs ÷ signal, in the present work elemental targets (Zn, Cu, Ge, Ni, Mo) were chosen by which the mass signals were found to be insensitive to the Cs surface concentration and the samples' work function [4,12]. Despite all this, MCs ÷ SIMS reduces the SIMS matrix effect drastically and as a consequence eases quantification. The application of this technique is sometimes hampered by its low useful MCs ÷ yield which is in the range 1 0 - 5 - 1 0 -6 for electronegative elements [13]. This drawback can be circumvented by analyzing the MCs~ molecules offering a useful yield which for most electronegative elements is more than one order of magnitude higher than the respective MCs + yield [13]. The mechanism of MCs~ formation is not yet well understood. The present paper intends to gain more insight into this formation process. Special emphasis is laid on evaluating measured energy distributions of the emitted molecules with respect to the temporal and spatial correlation of the constituents implicated in the molecule formation process. A comparison of the energy distributions of the components forming the MCs + molecules
with their respective counterparts for the MCs~ leads to the conclusion that among the different MCs~ formation processes proposed in the literature [10,13] the recombination of a Cs ÷ ion with an already built MCs ° seems to be the most probable one. Additionally, in the MCs~ formation also the spatial and temporal correlation of the two partners forming the molecule is enhanced compared to the MCs + formation process.
2. Experimental The experiments were carried out using a Cameca 4f ion microprobe [14] with 5.5 keV Cs ÷ primary ions at an incidence angle of 42 ° with respect to the surface normal. Beam currents in the range 1-150 nA were raster-scanned across an area of 250 ~m × 250 gin. The mass resolution of the ins~ument was M / A M = 300. The energy distribution measurements were performed by ramping the target potential and keeping the experimental parameters unchanged. For measuring the energy distributions the instruments' energy bandpass of 130 eV was reduced to approximately 3 eV. Sample preparation was performed by diamond paste polishing, ultrasonic bath cleaning and sputtering subsequent to the introduction into the sample chamber. The working pressure in the sample chamber was in the low 10 -9 mbar region.
3. Results and discussion Fig. l(a)-(d) show energy spectra of Cs ÷, Cs~, MCs + and MCs~ emitted from several elemental targets. It is striking that for all elements investigated the energy distributions of Cs~ are broader than the respective Cs ÷ distributions. This is even more surprising considering that neutral dimers generally exhibit a smaller distribution than the corresponding monomers [15-18] due
Z Mootz et al.,/lnternattonal Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10 r
r
10
I
~
O6
'
08
,,,06
CS +
dao
04
1°I
[] Zn ,~ Cu o Ge Ni o Mo
~S
08
c Z
3
Z
02
n Zn I ~. C u o Gel
%
04
~n
[]
o Mo'l
/,,
02
C 00
00(
I
8
i
i
L
,
5
1C
15
20
(a)
(eV)
Energy
I
1
I
-5
(b)
r
'~-]v',~
°
[]zn 0
~
"....u
t o
Ge NJ
'7
[] Zn ,~ Cu o Ge
,... 08
\
86 v 5= Z
,,i q~o
0,4
J (C)
C,
i
I
,
I
5
"C: Energy
,
I
i
(eV)
q~.n
-5
!5
o
o
J5
(d)
Mo
~w~,R""'S2+
vOz o o
°2aN4 5
20
f
,~:':':~&o ~ s~° o ~z~ ~o o
08
I
15
Energy (eV)
10
-aqa~x~5 o
i
10
5
0
O ::)
110
O
115
20
Energy (eV)
Fig. 1. Normalized energy spectra of (a) Cs +, (b) Cs~, (c) MCs + and (d) biCs~ due to a 5 keY Cs + bombardment for various targets.
to the fact that dimer formation becomes more and more hampered on increasing the emission energy of the involved monomers. By analogy, for the same reason, the MCs~ trimer energy distributions are expected to be smaller than their respective dimer distributions. In fact, it is seen that the MCs,+ energy distributions appear to be smaller than both the MCs* and the Cs~ distributions. This result corresponds to an MCs~ formation process via recombination of an MCs ° dimer with a Cs + ion or alternatively via recombination of M ° with Cs~. In contrast, for the MCs + molecule a comparison of the MCs + and the Cs +energy spectra reveals that only for Zn the MCs + distribution is significantly smaller than the respective Cs + distribution. For Mo the MCs + distribution appears to be even broader than in the case of Cs +.
3.1. M C s + formation
It has previously been demonstrated [8] that the energy distribution of MCs ÷ is dominated by the corresponding distribution of the neutral M ° atoms, which is mainly determined by the surface binding energy U0 of the sample. The energy distribution of the neutral sputtered atoms N(E) can be calculated assuming a primary ion induced isotropic collision cascade inside the target [19]: E
y(E) (E + U0)3
(1)
Here E denotes the emission energy of the ejected atom. In order to enlighten the role played by the surface binding energy the energy spectra of the neutral M ° for several elements
4
7", Mootz el al ,'International Journal of Mass Spectromet~ and Ion Processes 156 (1996) 1-10
characterized by different U0 (Fig. 2) are compared with the respective spectra of MCs ÷ and Cs ÷ (Fig. l(a) and (c)) respectively. For comparison all spectra are normalized to their maximum intensity. While the energy spectra of Cs ÷ appear to be independent of the investigated elemental targets, and hence independent of the corresponding surface binding energies, the MCs ÷ distributions are obviously correlated with the energy distributions of the sputtered neutrals. A broader M ° distribution leads also to a broader MCs ÷ distribution. Qualitatively this finding is in agreement with the generally agreed MCs ÷ molecule formation process in which independently sputtered M ° and Cs ÷ recombine above the surface according to [2-8] M ° + Cs + ~ MCs ÷
(2)
This process can proceed in the interaction range of the solid of a few ~ngstrOms [20,21]. The surface thereby acts as a third interaction partner taking away some excess kinetic energy. The relative energy of the two partners forming the molecule should not exceed the dissociation energy of the molecule. As the relative energy between the two partners forming the molecule may not exceed the critical value of the molecules' dissociation 11
-
energy, the probability of molecule formation is decreased by enhancing the emission energy of the M ° atom.
3.2. MCs~formation Recently several mechanisms for the formation of MCs~ molecules were proposed. Among them there are the following recombination processes [10,13]: A • M ° + Cs~ ---* MCs~
(3)
B" MCs ° + C s + ~ M C s ]
(4)
In order to discuss the MCs~ formation process in more detail, Fig. 3 and Fig. 4 display the mean emission energy of the molecular components involved in formation process A and B respectively. The mean emission energy E, characterizing the shape and broadness of the energy distribution, is defined as follows: : N(E)E aE N(E) dE
From Fig. 3 and Fig. 4 it is obvious that the mean emission energy of MCs~ is always smaller than the respective energy of its components. Since 25
--
(5)
r. . . .
i
,
n
10.
0,9 o,8 -
a
Zn
Z~ Cu o Ge
~7~
20
0 ¸
• •
15
0,6
o
M°
/ r
CS 2
+ •
•
LM 10
o3
~o
MCs2 ÷
M°
T-v 01
•
•
,i
O0 -01
-,o
o
;o
2~o
,o
so
60
;o
80
9o
~oo 11o
Energy (eV)
Fig. 2. Calculated energy distributions for sputtered neutral M atoms for elements with different surface binding energies. The spectra are normalized to their maxima,
1
2
3
I
I
I
4
5
fi
U o (eV)
Fig. 3. Mean emission energy of M °, Cs~ and MCs~ as a function of the surface binding energy for several elemental targets: U0, surface binding energy.
T. Mootz et al./International Journal of Mass Spectrometry and 1on Processes 156 (1996) 1-10 F
r
I
I
I
]
i
6
5
•
4
nl
•
m
v ~
3
•
• •
•
2
i ,
I
0
•
MCs*
•
MCs2+'
• I I
i
CS +
I
I
d
,
i
i
2
3
4
5
5
7
U o (eV)
Fig. 4. Mean emission energy of MCs +. Cs* and MCs~ as a function of the surface binding energy for several elemental targets: Uc~,surface binding energy.
this behavior can be seen in the energy distributions underlying formation process A and alternatively also in those underlying formation process B, in principle both formation mechanisms are in agreement with a recombination of a monomer and a dimer subsequent to their sputter ejection. This is due to the fact that by analogy with the MCs ÷ formation an increased emission energy of one of the partners forming the trimer molecule increases the molecules' dissociation probability. Hence the energy distributions of the trimers show a strong decrease toward high emission energies, resulting consequently in a decrease in its mean emission energy. In order to distinguish between formation processes A and B it is useful to compare the MCs2 formation directly with the MCs ÷ formation. This is done with special emphasis on the energy distributions. In the case of the MCs ÷ dimer which is generally agreed to be formed according to Eq. (2) one has to deal with energy distributions of M °, Cs ÷ and MCs ÷. On the other hand, energy distributions of M °, Cs~+ and MCs~ are involved in trimer formation via process A. As stated above, the experimentally determined energy distribution of Cs~ was found to be broader than the Cs ÷ distribution while the MCs~ distribution is smaller than the corresponding distribution of
5
MCs ÷ (Fig. 3 and Fig. 4). A comparison of Eqs. (2) and (3), respective to the dimer formation process with the trimer formation process A, reveals a contradiction in the case that both of these molecule formation processes are valid. This discrepancy arises because the energy distribution of Cs~ is broader than the Cs + distribution and hence would have demanded that also the MCs~ distribution is broader than the distribution of MCs ÷. However, the measured energy spectra can not confirm this demand. In contrast, comparing Eqs. (2) and (4) (MCs~ formation process B) no such discrepancy between the dimer and trimer formation is found. The energy distributions of neutral diatomic dimers are generally considered to be smaller than the energy distributions of their neutral constituents [15-18]. Hence, in accordance with the experiment, the resulting MCs~ molecule in formation process B is smaller than the MCs ÷ molecule related to Eq. (2). Therefore an MCs~ formation process being performed by process B seems to be the most probable one.
3.3. Temporal and spatial correlation of the constituents forming the MCs~ molecule A qualitative evaluation of the energy spectra of the components involved in MCs ÷ and MCs~ formation respectively reveals that the overlap of the energy distributions of M ° and Cs ÷, both taken as a basis for MCs ÷ formation, is lower than the corresponding overlap of MCs ÷ and Cs ÷ (Fig. 5(a)-(e)). Hence, it is reasonable that the efficiency of molecule formation is lower in the case of the MCs ÷ molecule, because the relative energy of the molecule constituents is preferably lower when both partners are ejected with comparable emission energy. As stated above, a stable molecule requires that this relative energy is lower than the molecules' dissociation energy. In order to investigate this qualitative finding in more detail, attention was focused on the equations underlying the formation processes of M °, Cs +, Cs~, MCs ÷ and MCs~.
6
T. Mootz et al./'lnternatiorull Journal of Mass Spectrometo' and Ion Processes 156 (1996) 1-10 r
i
:
1,0
i
,
I
O•
i
,
i
i
10
3-
D u i 0
0,8
i
O~ z~ []
A
A
z&
0
ZnCs+l
A
Cs2 ÷
0,8
~"
q•
~,~,,
[]¢ zx 0.6
°
W
o/
~zo4
\
,~.
DV
IA
[]
0.2 I
n o
A
5 10 Energy (eV)
(a) i
15
2O
/,, I
0,4-
~7
t3
[]
&
/,,
A ~
/',
[] O
m a
0,2-
~ 15
2O
Energy (eV)
1,0 -
~7
A
0S2 +
v
Ge °
I
o
LIJ v
v v ~
E] ,
02]
A
D
"
[]
V
D' O
/" "7
[]O
Ig /'
z0,4 -
Ge
t ~-~
A E1 )&
~0,6-
A
[] °°Co
o g ~ z ~
o8
mo z~ O
I0
i
0,8-
,
A
V ~
~ 5
(b)
[] Cs + o GeCs +
"
~
!
-5
1,0-
ao
0
i
0 , 0 ~ ~'
0
Cu
a V
~o
i~
,3.0P
Cu °
A
DD O0 ~7
V
v
O
m_
/'J
-5
~
[] o
'~A
,,.;
A
[] 0
A
[]
02
0,6
,,,
A @ o
0
[]
~
.~
0.6
Zn
A
O ~7
Cs +
NiCs
A.
Cs2 +
'7
Ni °
+
/,,
Ni
~7
u°o
Cs ÷
0
v~ v
v
O/,,
~\
o,o,~ -5
1
I
0
5 10 Energy (eV)
(c) '
'
lo
r
,
|O
0,6 .
~
O
~^0 | ~ IZ~V
•1"__
[]
i
•
[]
o 5 [
02 / -5
O ~
[]o "
MOCS*
~7 M c V
o°
A v O
D[] []
20
z~ cs/ O
O0
~ 0,4
lh5
Energy (eV)
,
©
A
,7¢ O 0
1~0
-5
[] cs.
27
o
20
(d)
~ 2 & ~
0,8-
(e)
15
M• <7~
Oo ,~"
vv~7
~ oo~ 0
. . . . . . . . 5 10 Energy (eV)
'
4
A
I 15
20
Fig. 5. Overlap of the energy spectra of Cs +, MCs" Cs_t and M ° for various elemental targets: (a) Zn; (b) Cu; (c) Ge; (d) Ni; (e) Mo.
Taking into account that, apart from a few exceptions, the overwhelming fraction of all sputtered particles consists of neutral species [22] the intensity of the mass signal for these
sputtered neutral particles I(M °) is given by I(M °) IpCMY'rlMO~0 =
(6)
where Ip is the primary ion current, CM is the
12 Mootz et al./lnternational Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
surface concentration of element M, Yis the total sputter yield, ~TM describes the geometry, transmission and amplification factor for element M and o~° is the post-ionization factor for neutral sputtered particles M. The mass signal of the secondary Cs + ions can be described by I(Cs + ) = IpccsY[3~Ocs.
(7)
the polarizability of M ° [7]) and a factor describing the temporal and spatial correlation of the two partners forming the m o l e c u l e o~tS(M°;Cs+). Hence * Cs = o l e ° l ( M ° ) ' o f f ( M ° ; "J/M-
Cs + )
(12)
Assuming that r/MC~, and ~/cs* are nearly equal and using Eqs. (7), (11) and (12) we obtain
Here ccs denotes the Cs surface concentration and fl~s is the ionization probability of a sputtered Cs atom. Assuming a Cs~ formation process via recombination of Cs ° and Cs + atoms in the near-surface region of the target the corresponding I(Cs~) mass signal is given by
I(MCs + ) = CMYo~P°l(M°)o~tS(M°; C s + )I(Cs + )
I(Cs+)=Ipccsr2(l - fl+ )fl + r/Cs; "YCs,, Cs÷
I(MCs + ) =IpCMYCc~* 2 Y 2/3 + ~'M0_c~* 3,MCs0_cs*
(8)
7Cso-cs* is a factor describing the recombination probability between two independently sputtered particles which here are Cs ° and Cs +. The MCs + signal can be considered to depend linearly on the density of Cs + and element M above the surface [6]. Hence we may write I( MCs+ ) =IpnMnCs'YM° -Cs* ~MCs*
(9)
rt M and ncs describe the density of M ° and Cs + above the surface. The latter can be written as follows [6]:
CMY
ccsYfl+
(10)
n M = (~Mo) and ncs = (VCs.-~ ~
where (VMO) and (vcs.) are the mean ejection velocities of M ° and Cs ÷ respectively. Combining Eqs. (9) and (10) and considering that (VMO)and (Vc~+) influence the molecule formation probability and consequently transform 3'MO_C~. into 7M"-Cs', one obtains I(MCs + ) =IpCMY2Ccsfl+ "YM0 • Cs.7/MCs+
(11)
With respect to further evaluations we separate the factor describing the molecule formation probability into a factor taking into account the polarizability of M, otP°I(M°) (the MCs ÷ yield was found to have a quadratic dependence on
(13) By analogy, considering the MCs~ molecule formation process which is predominately performed by recombination of MCs ° with Cs ÷ the /(MCs~) signal can be expressed as
'qMCs7
(14)
From Eq. (13) an expression for the factor c~tS(M°;Cs+) describing the temporal and space correlation between M ° and Cs + can be derived: I(MCs ÷ )/I(Cs + ) = o tS(M0; Cs + )Y CMCeP°I(M0)
(15)
As stated above, in the present work elemental samples were chosen by which the probability of ionization of the sputtered Cs atoms is largely independent of the bombardment-induced surface concentration of Cs. Since thereby the factor of ionization can be assumed to be nearly 1, the largest fraction of resputtered Cs is ejected in ionic form. As a consequence, the recombination of a sputtered neutral M ° atom with a predominately ionic Cs + results also in an ionic MCs +. Therefore the MCs~ formation which is proposed to be performed via recombination of a neutral MCs ° with a Cs + needs additionally an electron in order to neutralize the MCs +. Since this neutralization can be assumed to occur subsequent to the sputter ejection, with negligible momentum transfer between electron and molecule, the emission energy of MCs ° and consequently the respective energy distribution remain unchanged. Taking into account the effect of neutralization
T. Mootz et al./lnternational Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
we may define the fraction of neutralized MCs ° and the originally formed MCs ÷ by introducing a factor k: k = I(MCs°)/I(MCs
(16)
÷)
Under the assumption that the polarizability of M ° equals the polarizability of MCs ° and further that yM0_cso and ' ~ M* {} - Cs + as well as ~ MCs~ and ~/cs: are approximately in agreement, the cornbination of Eqs. (11), (12), (14) and (16) yields I(MCs='+ ) = kcxt~(MCs°; Cs + )Y I(MCs + )CcsoeP°l(M)
(17)
A further evaluation of Eq. (17) is hampered by the unknown factor k. As the MCs~ formation mechanism A was found to be contradictory to the experimentally determined energy spectra the conclusion was drawn that MCs~ formation according to process A is less probable than the corresponding process B.
On the other hand, Gao et al. [13] favors, particularly for electronegative elements, MCs~ formation via process A. According to this process, I(MCs~) is given by I(MCs~ ) = I p C M Y 3C ~~ s ( l
- fl+)fl+T(Cs°;
"T[,M o,; CS 2+ )7/MCs:
Cs
+
)
where n is the density of the target, l the depth of the sputter crater, e0 the elementary charge, j the primary ion density and t the sputter time. Both methods of the sputter yield determination agree within a deviation of 25%. Fig. 6 shows ottS(M°;Cs +) and ottS(M°;Cs~), derived from Eqs. (15) and (19), for several elements as a function of their surface binding energy. It is seen that ottS(M°;Cs~) is more than one order of magnitude higher than c~tS(M°;Cs+). This finding is expected qualitatively from the energy spectra in which the overlap of the M ° and Cs ÷ energy spectra was found to be lower than the corresponding overlap of M ° and Cs~. With increasing overlap the efficiency of molecule formation increases too, due to an improved spatial and temporal correlation of the partners involved in the molecule formation process. A second topic of Fig. 6 is that both ottS(M°;Cs+) a n d o t t S ( M ° ; C s ~ ) s h o w a decrease with increasing surface binding energy specific for each element M. This finding can also be related to a decreasing overlap of the energy spectra of the corresponding components forming the molecule. While the energy spectra of Cs~ and Cs ÷ are found to be nearly independent of the elements investigated and hence of the surface binding energy U0, the energy distributions of
(18)
From Eqs. (8) and (18) we can obtain an expression for a ts,, ,-, ,, Uvl,0 ;L~S2):
10 -1
[
T
i
i
,
i
•
M -Cs"
0
M-
Cs2 *
10 "2
I(MCs; )
=
o~tS(M;C s
+
)Y
(19)
I(Cs~ )CMo~P°I(M0) 1 0 ~3
Eq. (19) can be evaluated using the measured mass signals. Data for the polarizability otp°l(M°) are taken from Ref. [23]. Further, the total sputter yield is determined by using the SUSPRE code [24] based on the sputter theory of Sigmund [25]. For comparison, the sputter yield is additionally determined by crater depth measurements according to Y = nle____2o
jt
10 -4
10 .5
10 -6
I
I
I
I
J
I
J
1
2
3
4
5
6
7
U o (eV)
Fig. 6. Temporal and spatial correlation of M 0 and Cs + as well as M ° and Cs~ as a function of the surface binding energy for several elemental targets.
T. Mootz et al./International Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
the sputtered neutrals M ° are strongly influenced by U0. It follows from Eq. (1), and as seen in Fig. 2 an increase in U0 leads to a broader energy distribution. As a consequence, the overlap of the energy spectra of M ° with Cs ÷ and M ° with MCs ÷ respectively decreases. Therefore the corresponding factors describing the spatial and temporal correlation of the molecule partners prior to their recombination also decrease, resulting in a reduced probability of molecule formation.
3.4. Cs~ formation Surprisingly, the energy distributions of the Cs~ molecules were found to be broader than the Cs ÷ monomers. As stated above, this contradicts a molecule formation process in which the Cs~ molecules are formed by two partners each exhibiting the energy distribution of Cs ÷. Conversely, direct emission and subsequent dissociation of Cs~ into Cs ° and Cs ÷ appears to be an improbable process too. The direct emission of dimers was found to work efficiently only by large mass differences and strong bonds between the constituent atoms [15,16,26]. Therefore the formation of monoatomic dimers is generally described by atomic combination of independently sputtered atoms. As the ionization factor of the resputtered Cs particles can be expected to be nearly 1 the overwhelming fraction of resputtered Cs is ionized. This supports the relatively low intensity of Cs~ (l(Cs~)/I(Cs ÷) ~ 10 -3) because Cs~ requires for its formation both the ionized as well as the neutral Cs atom. The energy spectra of Cs ÷ and Cs~ can be understood by taking into account that the energy spectra of the emitted Cs ÷ and neutral Cs ° particles show distinct differences. It can be assumed that the mean emission energy of Cs ° is determined by the surface binding energy of the analyzed element. Since the mean emission energy of the emitted Cs ° neutrals is generally higher than the corresponding Cs ÷ energy, the recombination of Cs + with Cs ° results
in a Cs~ molecule having a higher mean emission energy than the related Cs ÷ ion. Further, while the Cs + have been found to be nearly independent of the elements' surface binding energy, the corresponding distribution of the neutrals show a strong U0 dependence (Eq. (1), Fig. 2). An increase in the surface binding energy leads to an increase in the mean emission energy of the sputtered neutrals. As a consequence the energy overlap between the sputtered Cs ° and Cs ÷ decreases and the probability of Cs~ formation is hampered due to an increase in the relative energy between the two independently sputtered constituents forming the molecule. It follows that the mean emission energy of Cs~ decreases on increasing the surface binding energy.
4. Conclusion It is shown that the energy distribution of the MCs ÷ molecules in Cs-assisted SIMS is mainly determined by the energy distribution of the sputtered neutrals M. While the energy distribution of resputtered Cs ÷ is nearly independent of the elemental targets investigated, an increase in the surface binding energy increases the mean emission energy of the sputtered neutrals M ° and the subsequently formed MCs + molecules respectively. This result confirms an MCs ÷ formation process in which a neutral sputtered M ° atom recombines with a sputtered Cs ÷ above the surface. Furthermore, the energy distribution of the MCs~ molecules is found to depend also on the surface binding energy, which provides evidence that the energy distribution of the neutrals is also responsible for the MCs~* distributions. The energy spectra of the trimers are smaller than their respective dimers' distribution. This can be qualitatively understood in the framework of a recombination of the constituents of the trimers subsequent to their sputter process. Among the trimer formation mechanisms which are in agreement with these results the recombination of
I0
T. Mootz et al./lnternational Journal of Mass Spectrometry and hm Processes 156 (1996) 1-10
MCs ° with Cs ÷ seems to be the most favorable one. This conclusion is reached by comparing the energy distributions of the components involved in the dimer formation process with the energy distributions of components which might be responsible for the trimer formation. In molecule formation processes in which the constituents of the molecule recombine after their sputter ejection above the samples' surface, these constituents have to be emitted mandatorily in an appropriate spatial and temporal correlation. A qualitative measure for this correlation is the overlap of the respective energy spectra. A more quantitative evaluation of the respective mass signals, with respect to the spatial and temporal correlation of the components forming the molecule, corroborates that in the case of the trimer formation this space/time correlation is more than one order of magnitude higher than in the dimer formation process. In both cases, the dimer as well as the trimer formation, the space/time correlation decreases with increasing surface binding energy. This is due to a decrease of the overlap of the respective energy spectra. Particularly the molecule constituents containing element M exhibit energy spectra which broaden by increasing U0, resulting in a smaller overlap with the Cs* partner. Compared with Cs ÷ the respective Cs~ distributions are surprisingly broad. This can be attributed to the dominating influence of the relatively broad Cs ° distributions.
Acknowledgements This work was supported by FKFO/IIKW and the Belgian program of Interuniversity Attraction Poles initiated by the Belgian State, Prime Minister's Office, Science Policy Programming.
References [1] Y. Gao, J. Appl. Phys., 64 (1988) 3760. [2] H. Gnaser and H. Oechsner, Fresenius J. Anal. Chem., 341 (1991) 54. [3] H. Gnaser and H. Oechsner, Surf. Interface Anal., 17 (1991) 646. [4] K. Wittmaack, Nucl. Instrum. Methods B, 64 (1992) 621. [5] C.W. Magee, W.L. Harrington and E.M. Botnick, Int. J. Mass Spectrom. Ion Processes, 103 (1990) 45. [6] H. Gnaser and H. Oechsner, Surf. Interface Anal., 21 (1994) 257. [7] H. Gnaser and H. Oechsner, Surf. Sci. Lett., 302 (1994) L289. [8] T. Mootz and F. Adams, Int. J. Mass Spectrom. Ion Processes, 152 (1996) 209. [9] K. Wittmaack, Nucl. Instrum. Methods B, 85 (1994) 374. [10] Y. Marie. Y. Gao, F. Saldi and H.N. Migeon, Surf. Interface Anal., 23 (1995) 38. [11] M.L. Yu and N.D. Lang, Phys. Rev. Lett., 50(2) (1983) 127. [12] K. Wittmaack, in A. Benninghoven, K.T.F. Janssen, J. T ~impner and H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS VIII, Wiley, Chichester, 1992, p. 91. [13] Y. Gao, Y. Marie, F. Saldi and H.N. Migeon, in A. Benninghoven, Y. Nihei, R. Shimizu and H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS IX, Wiley, Chichester, 1994, p. 382. [14] H.N. Migeon, C. Le Pipec and J.J. Le Goux, in A. Benninghoyen, R.J. Colton, D.S. Simons and H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS V, Springer, Berlin, 1986, p. 155. I15] H. Oechsner and W. Gerhard, Surf. Sci., 44 (1974) 480. [16] G.P. Koennen, A. Tip and E. de Vries, Radiat. Eft., 21 (1974) 269. [17] N. Winograd, D.E. Harrison, Jr., and B.J. Garrison, Surf. Sci., 78 (1978) 467. [18] A. Wucher and B.J. Garrison, Surf. Sci., 260 (1992) 257. [19] M.W. Thompson, Philos. Mag., 18 (1968) 377. [20] B.J. Garrison, N. Winograd and D.E. Harrison, Jr., J. Vac. Sci. Technol., 16 (1979) 789. [21] B.J. Garrison, N. Winograd and D.E. Harrison, Jr., Surf. Sci., 78 (1978) 467. [22] H.W. Werner, in L. Fiermans, F. Vennik and W. Dekeyzer (Eds.), Electron and Ion Spectroscopy of Solids, Plenum Press, New York, 1978, p. 324. [23] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 69th edn., CRC Press, Boca Raton, FL, 1989. [24] R,P. Webb, SUSPRE, V1.3 Manual Nov 86, V.G. Ionex, 1986. [25] P. Sigmund, Phys. Rev, 184 (1969) 383. [26] W. Gerhard and H. Oechsner, Z. Phys., B22 (1975) 41.
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
Investigations on the MCs formation in secondary ion mass spectrometry T. Mootz*, A. Adriaens, F. A d a m s University of Anm'erp, Department of Chemist~,, Universiteit~plein 1, B-2610 Wilrijk, Belgium Received 8 February 1996; accepted 18 June 1996
Abstract
The formation of MCs~ secondary ions under Cs ÷ bombardment of several elemental surfaces M was studied. In conjunction with their respective mass signals the energy distributions of the secondary MCs +, Cs +, Cs~ ions and the neutral M ° atoms corroborate an MCs~ formation mechanism via recombination of an already built MCs ° molecule with a Cs ÷ ion. Evaluation of the mass signals reveals that the spatial and temporal correlation between two partners forming the MCs~ molecule is strongly enhanced in comparison with the respective correlation of M ° and Cs ÷, which is generally agreed to form the MCs ÷ molecule. The surprisingly broad energy distributions of Cs ~ are determined by the broad distribution of the neutral M o atoms which are essentially implicated in the Cs~ formation process.
Kevwords. Secondary ion mass spectrometry; Cesium bombardment; Metal cesium ion formation; Molecule formation; Energy distributions; Spatial and time correlated emission
1. Introduction It has recently been shown that MCs ÷ secondary ion mass spectrometry (SIMS) using Cs ÷ primary bombardment and detecting MCs ÷ secondary ions for the analysis of element M minimizes the SIMS matrix effect enormously [1-5]. The MCs + ion formation was proposed to proceed via recombination of independently sputtered neutral M ° atoms and Cs + ions [2-8]. In this case the emission and ionization processes are decoupled in analogy to secondary neutral mass spectrometry (SNMS) allowing subsequently an almost matrix independent analysis * Corresponding author.
of element M. The remaining MCs + matrix effect can be attributed to the efficiency of molecule formation which is related to the density of molecule partners available above the surface [6] as well as their probability of recombination, The latter parameter surveyed by the yield of the MCs ÷ molecules exhibits a roughly quadratic relationship to the polarizability of the individual M atom [7]. Furthermore, the MCs ÷ yield depends strongly on the emission energy of the emitted neutral M ° atoms which is governed by the surface binding energy of the sample. The yield is enhanced by decreasing the mean M ° emission energy [8]. This indicates the importance of a spatial and time correlated emission of the two partners forming the molecule.
0168-1176/96/$15.1)0 Copyright ~ 1996 Elsevier S zience B.V. All rights reserved PII S 0 1 6 8 - 1 1 7 6 ( 9 6 ) 0 4 4 1 0 - 2
2
T. Mootz et al./lnternational Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
The MCs ÷ yield depends on the density of Cs ÷ ions and neutral M ° atoms available above the surface. In many cases the Cs + bombardment can be expected to lead to an equilibrium state in which the number of primary Cs + ions striking the surface is equal to the number of resputtered Cs + ions [5]. However, recent studies [4,9,10] show that the bombardment-induced surface concentration of Cs can lower the electronic work function of the sample below a critical value which is found to be slightly below the Cs ionization energy [11]. As a consequence the ionization probability for resputtered Cs + and the correlated MCs ÷ signal manifest a drastic decrease which can be understood in the framework of the electron tunneling model for secondary ion formation [11]. In order to rule out a significant decrease in the Cs ÷ ionization probability and a subsequent decrease of the corresponding MCs ÷ signal, in the present work elemental targets (Zn, Cu, Ge, Ni, Mo) were chosen by which the mass signals were found to be insensitive to the Cs surface concentration and the samples' work function [4,12]. Despite all this, MCs ÷ SIMS reduces the SIMS matrix effect drastically and as a consequence eases quantification. The application of this technique is sometimes hampered by its low useful MCs ÷ yield which is in the range 1 0 - 5 - 1 0 -6 for electronegative elements [13]. This drawback can be circumvented by analyzing the MCs~ molecules offering a useful yield which for most electronegative elements is more than one order of magnitude higher than the respective MCs + yield [13]. The mechanism of MCs~ formation is not yet well understood. The present paper intends to gain more insight into this formation process. Special emphasis is laid on evaluating measured energy distributions of the emitted molecules with respect to the temporal and spatial correlation of the constituents implicated in the molecule formation process. A comparison of the energy distributions of the components forming the MCs + molecules
with their respective counterparts for the MCs~ leads to the conclusion that among the different MCs~ formation processes proposed in the literature [10,13] the recombination of a Cs ÷ ion with an already built MCs ° seems to be the most probable one. Additionally, in the MCs~ formation also the spatial and temporal correlation of the two partners forming the molecule is enhanced compared to the MCs + formation process.
2. Experimental The experiments were carried out using a Cameca 4f ion microprobe [14] with 5.5 keV Cs ÷ primary ions at an incidence angle of 42 ° with respect to the surface normal. Beam currents in the range 1-150 nA were raster-scanned across an area of 250 ~m × 250 gin. The mass resolution of the ins~ument was M / A M = 300. The energy distribution measurements were performed by ramping the target potential and keeping the experimental parameters unchanged. For measuring the energy distributions the instruments' energy bandpass of 130 eV was reduced to approximately 3 eV. Sample preparation was performed by diamond paste polishing, ultrasonic bath cleaning and sputtering subsequent to the introduction into the sample chamber. The working pressure in the sample chamber was in the low 10 -9 mbar region.
3. Results and discussion Fig. l(a)-(d) show energy spectra of Cs ÷, Cs~, MCs + and MCs~ emitted from several elemental targets. It is striking that for all elements investigated the energy distributions of Cs~ are broader than the respective Cs ÷ distributions. This is even more surprising considering that neutral dimers generally exhibit a smaller distribution than the corresponding monomers [15-18] due
Z Mootz et al.,/lnternattonal Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10 r
r
10
I
~
O6
'
08
,,,06
CS +
dao
04
1°I
[] Zn ,~ Cu o Ge Ni o Mo
~S
08
c Z
3
Z
02
n Zn I ~. C u o Gel
%
04
~n
[]
o Mo'l
/,,
02
C 00
00(
I
8
i
i
L
,
5
1C
15
20
(a)
(eV)
Energy
I
1
I
-5
(b)
r
'~-]v',~
°
[]zn 0
~
"....u
t o
Ge NJ
'7
[] Zn ,~ Cu o Ge
,... 08
\
86 v 5= Z
,,i q~o
0,4
J (C)
C,
i
I
,
I
5
"C: Energy
,
I
i
(eV)
q~.n
-5
!5
o
o
J5
(d)
Mo
~w~,R""'S2+
vOz o o
°2aN4 5
20
f
,~:':':~&o ~ s~° o ~z~ ~o o
08
I
15
Energy (eV)
10
-aqa~x~5 o
i
10
5
0
O ::)
110
O
115
20
Energy (eV)
Fig. 1. Normalized energy spectra of (a) Cs +, (b) Cs~, (c) MCs + and (d) biCs~ due to a 5 keY Cs + bombardment for various targets.
to the fact that dimer formation becomes more and more hampered on increasing the emission energy of the involved monomers. By analogy, for the same reason, the MCs~ trimer energy distributions are expected to be smaller than their respective dimer distributions. In fact, it is seen that the MCs,+ energy distributions appear to be smaller than both the MCs* and the Cs~ distributions. This result corresponds to an MCs~ formation process via recombination of an MCs ° dimer with a Cs + ion or alternatively via recombination of M ° with Cs~. In contrast, for the MCs + molecule a comparison of the MCs + and the Cs +energy spectra reveals that only for Zn the MCs + distribution is significantly smaller than the respective Cs + distribution. For Mo the MCs + distribution appears to be even broader than in the case of Cs +.
3.1. M C s + formation
It has previously been demonstrated [8] that the energy distribution of MCs ÷ is dominated by the corresponding distribution of the neutral M ° atoms, which is mainly determined by the surface binding energy U0 of the sample. The energy distribution of the neutral sputtered atoms N(E) can be calculated assuming a primary ion induced isotropic collision cascade inside the target [19]: E
y(E) (E + U0)3
(1)
Here E denotes the emission energy of the ejected atom. In order to enlighten the role played by the surface binding energy the energy spectra of the neutral M ° for several elements
4
7", Mootz el al ,'International Journal of Mass Spectromet~ and Ion Processes 156 (1996) 1-10
characterized by different U0 (Fig. 2) are compared with the respective spectra of MCs ÷ and Cs ÷ (Fig. l(a) and (c)) respectively. For comparison all spectra are normalized to their maximum intensity. While the energy spectra of Cs ÷ appear to be independent of the investigated elemental targets, and hence independent of the corresponding surface binding energies, the MCs ÷ distributions are obviously correlated with the energy distributions of the sputtered neutrals. A broader M ° distribution leads also to a broader MCs ÷ distribution. Qualitatively this finding is in agreement with the generally agreed MCs ÷ molecule formation process in which independently sputtered M ° and Cs ÷ recombine above the surface according to [2-8] M ° + Cs + ~ MCs ÷
(2)
This process can proceed in the interaction range of the solid of a few ~ngstrOms [20,21]. The surface thereby acts as a third interaction partner taking away some excess kinetic energy. The relative energy of the two partners forming the molecule should not exceed the dissociation energy of the molecule. As the relative energy between the two partners forming the molecule may not exceed the critical value of the molecules' dissociation 11
-
energy, the probability of molecule formation is decreased by enhancing the emission energy of the M ° atom.
3.2. MCs~formation Recently several mechanisms for the formation of MCs~ molecules were proposed. Among them there are the following recombination processes [10,13]: A • M ° + Cs~ ---* MCs~
(3)
B" MCs ° + C s + ~ M C s ]
(4)
In order to discuss the MCs~ formation process in more detail, Fig. 3 and Fig. 4 display the mean emission energy of the molecular components involved in formation process A and B respectively. The mean emission energy E, characterizing the shape and broadness of the energy distribution, is defined as follows: : N(E)E aE N(E) dE
From Fig. 3 and Fig. 4 it is obvious that the mean emission energy of MCs~ is always smaller than the respective energy of its components. Since 25
--
(5)
r. . . .
i
,
n
10.
0,9 o,8 -
a
Zn
Z~ Cu o Ge
~7~
20
0 ¸
• •
15
0,6
o
M°
/ r
CS 2
+ •
•
LM 10
o3
~o
MCs2 ÷
M°
T-v 01
•
•
,i
O0 -01
-,o
o
;o
2~o
,o
so
60
;o
80
9o
~oo 11o
Energy (eV)
Fig. 2. Calculated energy distributions for sputtered neutral M atoms for elements with different surface binding energies. The spectra are normalized to their maxima,
1
2
3
I
I
I
4
5
fi
U o (eV)
Fig. 3. Mean emission energy of M °, Cs~ and MCs~ as a function of the surface binding energy for several elemental targets: U0, surface binding energy.
T. Mootz et al./International Journal of Mass Spectrometry and 1on Processes 156 (1996) 1-10 F
r
I
I
I
]
i
6
5
•
4
nl
•
m
v ~
3
•
• •
•
2
i ,
I
0
•
MCs*
•
MCs2+'
• I I
i
CS +
I
I
d
,
i
i
2
3
4
5
5
7
U o (eV)
Fig. 4. Mean emission energy of MCs +. Cs* and MCs~ as a function of the surface binding energy for several elemental targets: Uc~,surface binding energy.
this behavior can be seen in the energy distributions underlying formation process A and alternatively also in those underlying formation process B, in principle both formation mechanisms are in agreement with a recombination of a monomer and a dimer subsequent to their sputter ejection. This is due to the fact that by analogy with the MCs ÷ formation an increased emission energy of one of the partners forming the trimer molecule increases the molecules' dissociation probability. Hence the energy distributions of the trimers show a strong decrease toward high emission energies, resulting consequently in a decrease in its mean emission energy. In order to distinguish between formation processes A and B it is useful to compare the MCs2 formation directly with the MCs ÷ formation. This is done with special emphasis on the energy distributions. In the case of the MCs ÷ dimer which is generally agreed to be formed according to Eq. (2) one has to deal with energy distributions of M °, Cs ÷ and MCs ÷. On the other hand, energy distributions of M °, Cs~+ and MCs~ are involved in trimer formation via process A. As stated above, the experimentally determined energy distribution of Cs~ was found to be broader than the Cs ÷ distribution while the MCs~ distribution is smaller than the corresponding distribution of
5
MCs ÷ (Fig. 3 and Fig. 4). A comparison of Eqs. (2) and (3), respective to the dimer formation process with the trimer formation process A, reveals a contradiction in the case that both of these molecule formation processes are valid. This discrepancy arises because the energy distribution of Cs~ is broader than the Cs + distribution and hence would have demanded that also the MCs~ distribution is broader than the distribution of MCs ÷. However, the measured energy spectra can not confirm this demand. In contrast, comparing Eqs. (2) and (4) (MCs~ formation process B) no such discrepancy between the dimer and trimer formation is found. The energy distributions of neutral diatomic dimers are generally considered to be smaller than the energy distributions of their neutral constituents [15-18]. Hence, in accordance with the experiment, the resulting MCs~ molecule in formation process B is smaller than the MCs ÷ molecule related to Eq. (2). Therefore an MCs~ formation process being performed by process B seems to be the most probable one.
3.3. Temporal and spatial correlation of the constituents forming the MCs~ molecule A qualitative evaluation of the energy spectra of the components involved in MCs ÷ and MCs~ formation respectively reveals that the overlap of the energy distributions of M ° and Cs ÷, both taken as a basis for MCs ÷ formation, is lower than the corresponding overlap of MCs ÷ and Cs ÷ (Fig. 5(a)-(e)). Hence, it is reasonable that the efficiency of molecule formation is lower in the case of the MCs ÷ molecule, because the relative energy of the molecule constituents is preferably lower when both partners are ejected with comparable emission energy. As stated above, a stable molecule requires that this relative energy is lower than the molecules' dissociation energy. In order to investigate this qualitative finding in more detail, attention was focused on the equations underlying the formation processes of M °, Cs +, Cs~, MCs ÷ and MCs~.
6
T. Mootz et al./'lnternatiorull Journal of Mass Spectrometo' and Ion Processes 156 (1996) 1-10 r
i
:
1,0
i
,
I
O•
i
,
i
i
10
3-
D u i 0
0,8
i
O~ z~ []
A
A
z&
0
ZnCs+l
A
Cs2 ÷
0,8
~"
q•
~,~,,
[]¢ zx 0.6
°
W
o/
~zo4
\
,~.
DV
IA
[]
0.2 I
n o
A
5 10 Energy (eV)
(a) i
15
2O
/,, I
0,4-
~7
t3
[]
&
/,,
A ~
/',
[] O
m a
0,2-
~ 15
2O
Energy (eV)
1,0 -
~7
A
0S2 +
v
Ge °
I
o
LIJ v
v v ~
E] ,
02]
A
D
"
[]
V
D' O
/" "7
[]O
Ig /'
z0,4 -
Ge
t ~-~
A E1 )&
~0,6-
A
[] °°Co
o g ~ z ~
o8
mo z~ O
I0
i
0,8-
,
A
V ~
~ 5
(b)
[] Cs + o GeCs +
"
~
!
-5
1,0-
ao
0
i
0 , 0 ~ ~'
0
Cu
a V
~o
i~
,3.0P
Cu °
A
DD O0 ~7
V
v
O
m_
/'J
-5
~
[] o
'~A
,,.;
A
[] 0
A
[]
02
0,6
,,,
A @ o
0
[]
~
.~
0.6
Zn
A
O ~7
Cs +
NiCs
A.
Cs2 +
'7
Ni °
+
/,,
Ni
~7
u°o
Cs ÷
0
v~ v
v
O/,,
~\
o,o,~ -5
1
I
0
5 10 Energy (eV)
(c) '
'
lo
r
,
|O
0,6 .
~
O
~^0 | ~ IZ~V
•1"__
[]
i
•
[]
o 5 [
02 / -5
O ~
[]o "
MOCS*
~7 M c V
o°
A v O
D[] []
20
z~ cs/ O
O0
~ 0,4
lh5
Energy (eV)
,
©
A
,7¢ O 0
1~0
-5
[] cs.
27
o
20
(d)
~ 2 & ~
0,8-
(e)
15
M• <7~
Oo ,~"
vv~7
~ oo~ 0
. . . . . . . . 5 10 Energy (eV)
'
4
A
I 15
20
Fig. 5. Overlap of the energy spectra of Cs +, MCs" Cs_t and M ° for various elemental targets: (a) Zn; (b) Cu; (c) Ge; (d) Ni; (e) Mo.
Taking into account that, apart from a few exceptions, the overwhelming fraction of all sputtered particles consists of neutral species [22] the intensity of the mass signal for these
sputtered neutral particles I(M °) is given by I(M °) IpCMY'rlMO~0 =
(6)
where Ip is the primary ion current, CM is the
12 Mootz et al./lnternational Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
surface concentration of element M, Yis the total sputter yield, ~TM describes the geometry, transmission and amplification factor for element M and o~° is the post-ionization factor for neutral sputtered particles M. The mass signal of the secondary Cs + ions can be described by I(Cs + ) = IpccsY[3~Ocs.
(7)
the polarizability of M ° [7]) and a factor describing the temporal and spatial correlation of the two partners forming the m o l e c u l e o~tS(M°;Cs+). Hence * Cs = o l e ° l ( M ° ) ' o f f ( M ° ; "J/M-
Cs + )
(12)
Assuming that r/MC~, and ~/cs* are nearly equal and using Eqs. (7), (11) and (12) we obtain
Here ccs denotes the Cs surface concentration and fl~s is the ionization probability of a sputtered Cs atom. Assuming a Cs~ formation process via recombination of Cs ° and Cs + atoms in the near-surface region of the target the corresponding I(Cs~) mass signal is given by
I(MCs + ) = CMYo~P°l(M°)o~tS(M°; C s + )I(Cs + )
I(Cs+)=Ipccsr2(l - fl+ )fl + r/Cs; "YCs,, Cs÷
I(MCs + ) =IpCMYCc~* 2 Y 2/3 + ~'M0_c~* 3,MCs0_cs*
(8)
7Cso-cs* is a factor describing the recombination probability between two independently sputtered particles which here are Cs ° and Cs +. The MCs + signal can be considered to depend linearly on the density of Cs + and element M above the surface [6]. Hence we may write I( MCs+ ) =IpnMnCs'YM° -Cs* ~MCs*
(9)
rt M and ncs describe the density of M ° and Cs + above the surface. The latter can be written as follows [6]:
CMY
ccsYfl+
(10)
n M = (~Mo) and ncs = (VCs.-~ ~
where (VMO) and (vcs.) are the mean ejection velocities of M ° and Cs ÷ respectively. Combining Eqs. (9) and (10) and considering that (VMO)and (Vc~+) influence the molecule formation probability and consequently transform 3'MO_C~. into 7M"-Cs', one obtains I(MCs + ) =IpCMY2Ccsfl+ "YM0 • Cs.7/MCs+
(11)
With respect to further evaluations we separate the factor describing the molecule formation probability into a factor taking into account the polarizability of M, otP°I(M°) (the MCs ÷ yield was found to have a quadratic dependence on
(13) By analogy, considering the MCs~ molecule formation process which is predominately performed by recombination of MCs ° with Cs ÷ the /(MCs~) signal can be expressed as
'qMCs7
(14)
From Eq. (13) an expression for the factor c~tS(M°;Cs+) describing the temporal and space correlation between M ° and Cs + can be derived: I(MCs ÷ )/I(Cs + ) = o tS(M0; Cs + )Y CMCeP°I(M0)
(15)
As stated above, in the present work elemental samples were chosen by which the probability of ionization of the sputtered Cs atoms is largely independent of the bombardment-induced surface concentration of Cs. Since thereby the factor of ionization can be assumed to be nearly 1, the largest fraction of resputtered Cs is ejected in ionic form. As a consequence, the recombination of a sputtered neutral M ° atom with a predominately ionic Cs + results also in an ionic MCs +. Therefore the MCs~ formation which is proposed to be performed via recombination of a neutral MCs ° with a Cs + needs additionally an electron in order to neutralize the MCs +. Since this neutralization can be assumed to occur subsequent to the sputter ejection, with negligible momentum transfer between electron and molecule, the emission energy of MCs ° and consequently the respective energy distribution remain unchanged. Taking into account the effect of neutralization
T. Mootz et al./lnternational Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
we may define the fraction of neutralized MCs ° and the originally formed MCs ÷ by introducing a factor k: k = I(MCs°)/I(MCs
(16)
÷)
Under the assumption that the polarizability of M ° equals the polarizability of MCs ° and further that yM0_cso and ' ~ M* {} - Cs + as well as ~ MCs~ and ~/cs: are approximately in agreement, the cornbination of Eqs. (11), (12), (14) and (16) yields I(MCs='+ ) = kcxt~(MCs°; Cs + )Y I(MCs + )CcsoeP°l(M)
(17)
A further evaluation of Eq. (17) is hampered by the unknown factor k. As the MCs~ formation mechanism A was found to be contradictory to the experimentally determined energy spectra the conclusion was drawn that MCs~ formation according to process A is less probable than the corresponding process B.
On the other hand, Gao et al. [13] favors, particularly for electronegative elements, MCs~ formation via process A. According to this process, I(MCs~) is given by I(MCs~ ) = I p C M Y 3C ~~ s ( l
- fl+)fl+T(Cs°;
"T[,M o,; CS 2+ )7/MCs:
Cs
+
)
where n is the density of the target, l the depth of the sputter crater, e0 the elementary charge, j the primary ion density and t the sputter time. Both methods of the sputter yield determination agree within a deviation of 25%. Fig. 6 shows ottS(M°;Cs +) and ottS(M°;Cs~), derived from Eqs. (15) and (19), for several elements as a function of their surface binding energy. It is seen that ottS(M°;Cs~) is more than one order of magnitude higher than c~tS(M°;Cs+). This finding is expected qualitatively from the energy spectra in which the overlap of the M ° and Cs ÷ energy spectra was found to be lower than the corresponding overlap of M ° and Cs~. With increasing overlap the efficiency of molecule formation increases too, due to an improved spatial and temporal correlation of the partners involved in the molecule formation process. A second topic of Fig. 6 is that both ottS(M°;Cs+) a n d o t t S ( M ° ; C s ~ ) s h o w a decrease with increasing surface binding energy specific for each element M. This finding can also be related to a decreasing overlap of the energy spectra of the corresponding components forming the molecule. While the energy spectra of Cs~ and Cs ÷ are found to be nearly independent of the elements investigated and hence of the surface binding energy U0, the energy distributions of
(18)
From Eqs. (8) and (18) we can obtain an expression for a ts,, ,-, ,, Uvl,0 ;L~S2):
10 -1
[
T
i
i
,
i
•
M -Cs"
0
M-
Cs2 *
10 "2
I(MCs; )
=
o~tS(M;C s
+
)Y
(19)
I(Cs~ )CMo~P°I(M0) 1 0 ~3
Eq. (19) can be evaluated using the measured mass signals. Data for the polarizability otp°l(M°) are taken from Ref. [23]. Further, the total sputter yield is determined by using the SUSPRE code [24] based on the sputter theory of Sigmund [25]. For comparison, the sputter yield is additionally determined by crater depth measurements according to Y = nle____2o
jt
10 -4
10 .5
10 -6
I
I
I
I
J
I
J
1
2
3
4
5
6
7
U o (eV)
Fig. 6. Temporal and spatial correlation of M 0 and Cs + as well as M ° and Cs~ as a function of the surface binding energy for several elemental targets.
T. Mootz et al./International Journal of Mass Spectrometry and Ion Processes 156 (1996) 1-10
the sputtered neutrals M ° are strongly influenced by U0. It follows from Eq. (1), and as seen in Fig. 2 an increase in U0 leads to a broader energy distribution. As a consequence, the overlap of the energy spectra of M ° with Cs ÷ and M ° with MCs ÷ respectively decreases. Therefore the corresponding factors describing the spatial and temporal correlation of the molecule partners prior to their recombination also decrease, resulting in a reduced probability of molecule formation.
3.4. Cs~ formation Surprisingly, the energy distributions of the Cs~ molecules were found to be broader than the Cs ÷ monomers. As stated above, this contradicts a molecule formation process in which the Cs~ molecules are formed by two partners each exhibiting the energy distribution of Cs ÷. Conversely, direct emission and subsequent dissociation of Cs~ into Cs ° and Cs ÷ appears to be an improbable process too. The direct emission of dimers was found to work efficiently only by large mass differences and strong bonds between the constituent atoms [15,16,26]. Therefore the formation of monoatomic dimers is generally described by atomic combination of independently sputtered atoms. As the ionization factor of the resputtered Cs particles can be expected to be nearly 1 the overwhelming fraction of resputtered Cs is ionized. This supports the relatively low intensity of Cs~ (l(Cs~)/I(Cs ÷) ~ 10 -3) because Cs~ requires for its formation both the ionized as well as the neutral Cs atom. The energy spectra of Cs ÷ and Cs~ can be understood by taking into account that the energy spectra of the emitted Cs ÷ and neutral Cs ° particles show distinct differences. It can be assumed that the mean emission energy of Cs ° is determined by the surface binding energy of the analyzed element. Since the mean emission energy of the emitted Cs ° neutrals is generally higher than the corresponding Cs ÷ energy, the recombination of Cs + with Cs ° results
in a Cs~ molecule having a higher mean emission energy than the related Cs ÷ ion. Further, while the Cs + have been found to be nearly independent of the elements' surface binding energy, the corresponding distribution of the neutrals show a strong U0 dependence (Eq. (1), Fig. 2). An increase in the surface binding energy leads to an increase in the mean emission energy of the sputtered neutrals. As a consequence the energy overlap between the sputtered Cs ° and Cs ÷ decreases and the probability of Cs~ formation is hampered due to an increase in the relative energy between the two independently sputtered constituents forming the molecule. It follows that the mean emission energy of Cs~ decreases on increasing the surface binding energy.
4. Conclusion It is shown that the energy distribution of the MCs ÷ molecules in Cs-assisted SIMS is mainly determined by the energy distribution of the sputtered neutrals M. While the energy distribution of resputtered Cs ÷ is nearly independent of the elemental targets investigated, an increase in the surface binding energy increases the mean emission energy of the sputtered neutrals M ° and the subsequently formed MCs + molecules respectively. This result confirms an MCs ÷ formation process in which a neutral sputtered M ° atom recombines with a sputtered Cs ÷ above the surface. Furthermore, the energy distribution of the MCs~ molecules is found to depend also on the surface binding energy, which provides evidence that the energy distribution of the neutrals is also responsible for the MCs~* distributions. The energy spectra of the trimers are smaller than their respective dimers' distribution. This can be qualitatively understood in the framework of a recombination of the constituents of the trimers subsequent to their sputter process. Among the trimer formation mechanisms which are in agreement with these results the recombination of
I0
T. Mootz et al./lnternational Journal of Mass Spectrometry and hm Processes 156 (1996) 1-10
MCs ° with Cs ÷ seems to be the most favorable one. This conclusion is reached by comparing the energy distributions of the components involved in the dimer formation process with the energy distributions of components which might be responsible for the trimer formation. In molecule formation processes in which the constituents of the molecule recombine after their sputter ejection above the samples' surface, these constituents have to be emitted mandatorily in an appropriate spatial and temporal correlation. A qualitative measure for this correlation is the overlap of the respective energy spectra. A more quantitative evaluation of the respective mass signals, with respect to the spatial and temporal correlation of the components forming the molecule, corroborates that in the case of the trimer formation this space/time correlation is more than one order of magnitude higher than in the dimer formation process. In both cases, the dimer as well as the trimer formation, the space/time correlation decreases with increasing surface binding energy. This is due to a decrease of the overlap of the respective energy spectra. Particularly the molecule constituents containing element M exhibit energy spectra which broaden by increasing U0, resulting in a smaller overlap with the Cs* partner. Compared with Cs ÷ the respective Cs~ distributions are surprisingly broad. This can be attributed to the dominating influence of the relatively broad Cs ° distributions.
Acknowledgements This work was supported by FKFO/IIKW and the Belgian program of Interuniversity Attraction Poles initiated by the Belgian State, Prime Minister's Office, Science Policy Programming.
References [1] Y. Gao, J. Appl. Phys., 64 (1988) 3760. [2] H. Gnaser and H. Oechsner, Fresenius J. Anal. Chem., 341 (1991) 54. [3] H. Gnaser and H. Oechsner, Surf. Interface Anal., 17 (1991) 646. [4] K. Wittmaack, Nucl. Instrum. Methods B, 64 (1992) 621. [5] C.W. Magee, W.L. Harrington and E.M. Botnick, Int. J. Mass Spectrom. Ion Processes, 103 (1990) 45. [6] H. Gnaser and H. Oechsner, Surf. Interface Anal., 21 (1994) 257. [7] H. Gnaser and H. Oechsner, Surf. Sci. Lett., 302 (1994) L289. [8] T. Mootz and F. Adams, Int. J. Mass Spectrom. Ion Processes, 152 (1996) 209. [9] K. Wittmaack, Nucl. Instrum. Methods B, 85 (1994) 374. [10] Y. Marie. Y. Gao, F. Saldi and H.N. Migeon, Surf. Interface Anal., 23 (1995) 38. [11] M.L. Yu and N.D. Lang, Phys. Rev. Lett., 50(2) (1983) 127. [12] K. Wittmaack, in A. Benninghoven, K.T.F. Janssen, J. T ~impner and H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS VIII, Wiley, Chichester, 1992, p. 91. [13] Y. Gao, Y. Marie, F. Saldi and H.N. Migeon, in A. Benninghoven, Y. Nihei, R. Shimizu and H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS IX, Wiley, Chichester, 1994, p. 382. [14] H.N. Migeon, C. Le Pipec and J.J. Le Goux, in A. Benninghoyen, R.J. Colton, D.S. Simons and H.W. Werner (Eds.), Secondary Ion Mass Spectrometry, SIMS V, Springer, Berlin, 1986, p. 155. I15] H. Oechsner and W. Gerhard, Surf. Sci., 44 (1974) 480. [16] G.P. Koennen, A. Tip and E. de Vries, Radiat. Eft., 21 (1974) 269. [17] N. Winograd, D.E. Harrison, Jr., and B.J. Garrison, Surf. Sci., 78 (1978) 467. [18] A. Wucher and B.J. Garrison, Surf. Sci., 260 (1992) 257. [19] M.W. Thompson, Philos. Mag., 18 (1968) 377. [20] B.J. Garrison, N. Winograd and D.E. Harrison, Jr., J. Vac. Sci. Technol., 16 (1979) 789. [21] B.J. Garrison, N. Winograd and D.E. Harrison, Jr., Surf. Sci., 78 (1978) 467. [22] H.W. Werner, in L. Fiermans, F. Vennik and W. Dekeyzer (Eds.), Electron and Ion Spectroscopy of Solids, Plenum Press, New York, 1978, p. 324. [23] R.C. Weast (Ed.), CRC Handbook of Chemistry and Physics, 69th edn., CRC Press, Boca Raton, FL, 1989. [24] R,P. Webb, SUSPRE, V1.3 Manual Nov 86, V.G. Ionex, 1986. [25] P. Sigmund, Phys. Rev, 184 (1969) 383. [26] W. Gerhard and H. Oechsner, Z. Phys., B22 (1975) 41.