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Chemical enhancement effects in SIMS analysis
Nuclear Instruments and Methods in Physics Research B15 (1986) 151-158 North-Holland, Amsterdam
Section
V. SIMS
CHEMICAL
151
and analysis of sputtered neutrals
ENHANCEMENT
EFFECTS
IN SIMS ANALYSIS
Ming L. YU IBM
T.J. Watson Research Center, Yorktown
Heights, NY 10598, USA
This paper summarizes some of the recent advances in the understanding of the secondary ion emission process, and relates them to the important cesium and oxygen enhancement effects in secondary ion mass spectrometry (SIMS). The oxygen enhancement of ionization results from local breaking of oxide bonds and its magnitude is related to the microscopic distribution of oxygen atoms in the target. The Cs enhancement effect is found to be connected with the work function change induced by the Cs on the sample surface, and the ionization process is best described by an electron tunneling model. dependence, alloying effect and the optimum operating conditions in SIMS analysis.
1. Introduction An important analytical application of secondary ion mass spectrometry (SIMS) is to measure depth distributions of elements in solid samples. Sensitivity and depth resolution are the two major issues in this mode of SIMS analysis. The sensitivity of the technique is related to the mechanisms of secondary ion formation. On the other hand SIMS shares many of the physical limitations on the achievable depth resolution of other sputter profiling techniques [l]. In this paper, we shall concentrate on the sensitivity issue. The emission of secondary ions during ion bombardment of solid surfaces is the result of a two step process. At first, the impact of an energetic primary ion provides, through a collision cascade, the momentum for the sputtering of a surface atom. Then the charge state of the sputtered atom is determined by the electron exchange between the atom (or ion) and the surface. In general, the charge state of the sputtered atom is not related to the charge of the incoming primary ion. The ionization phenomenon is usually caused by the charge exchange between the valence levels of the sputtered atom or ion and those of the solid. The secondary ion yield I; (counts/s) of element A is given by the following equation: Ii
= pPA*CAYip,
(1)
where /3 = instrument transmission factor, PA’ = ionization probability of element A, C, = atomic fraction of A in the sample, Y = matrix sputtering yield (atoms/ion), ‘P = primary ion beam current (ions/s). The sensitivity is very much dictated by PAi of the atomic species. It has been well established [2] that for many metallic elements PAi is less than lop4 when 0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
These
differences
affect
the concentration
high purity samples are sputtered by rare gas ions. It was observed however that the presence of electronegative atoms (e.g. oxygen) on the sample surface can greatly increase the formation of positive secondary ions, with P+ approaching unity. On the other hand the presence of electropositive atoms (e.g. cesium) on a sample surface can greatly increase P- by three to four orders of magnitude [3]. It is now a routine practice in SIMS analysis to utilize these enhancement phenomena to achieve high sensitivity.
2. Mechanisms of ionization Many advances have been made in the past few years on the physical understanding of these eahancement effects. Although there were attempts to establish a phenomenlogical “unified” model [4], recent experimental and theoretical studies indicate that microscopically these two enhancement effects due to oxygen and cesium are quite different. Reviews on these phenomena are available in the literature [5-91. Here we shall briefly highlight some of the recent progress that are relevant to our discussion below. 2.1. The oxygen enhancement
effict
Although many electronegative elements (e.g. N, 0, Cl, F) can cause an enhancement of the positive secondary ion yields, oxygen is being used most frequently. The prevailing explanation of this enhancement effect is the bond-breaking mechanism. The ionization of the sputtered atom is believed to be the result of breaking of the chemical bond with oxygen (or other electronegative atom). The Landau-Zener-Stueckelberg theory of curve-crossing for the ionic dissociation of diatomic molecules has been frequently used to describe qualitaV. SIMS AND SPUTTERED NEUTRALS
152
M. L. Yu / Chemical enhancement
effcts
tively
the secondary ion emission from a solid [6,8], yet a useful microscopic description for the oxygen enhancement effect is still not available. The major difficulties are the uncertainties of the energy surfaces involved and the treatment of the relaxation of the broken bond. Phenomenlogically the secondary ion yield IL is the sum of the contributions from different bonding configurations (i) involving A [lo]: (2)
in SIMS
4
anal,vsis
I 0 - Si (100) InA
c 4 z
t
z 0’ 0
500eV
I
Ar+
/
0 o-
(x5)
I 7eV,
0”
.
/-
/ 2-
,’ 1)
P
5 v,
/
lSi+
3-
2
where f, = amount of the surface A atoms bonded to oxygen in the i th configuration = ionization probability of A when it is sputtered P,’ from the i th configuration y = partial sputtering coefficient of A in the ith configuration. Eq. (2) however is difficult to apply since the usual experimental observable is c,, the oxygen concentration (or that of other electronegative species). The relation between co and f, depends very much on the oxide growth kinetics in the individual experiment. Recently Mann and Yu [ll] and Yu et al. [12] have reported experimental data from several model systems that supports the linear relation between IL and f,. These SIMS experiments were performed with in situ X-ray photoemission (XPS) to monitor the coverage and the chemical state of the surface. The surface reactions (e.g. oxidation, nitridation) were induced thermally and sputtering was kept in the static mode to minimize ion beam induced chemistry. The coverage of electronegative species was kept close to the monolayer range so that the information from both SIMS and XPS originate from the same topmost reacted layer. The simplest system they have studied is the enhancement of the sputtered Si+ yield by nitridation of the Si (100) surface at 1100°C [II]. XPS showed that only Si,N, was formed on the Si surface. i.e. f (for Si,N,) is simply proportional to the nitrogen coverage. Indeed the enhanced Si+ yield was found to be directly proportional to the nitrogen coverage up to a monolayer [II]. A more complicated illustration of eq. (2) is shown in fig. 1 where the oxygen coverage dependences of the Si+ yield during the oxidation of a Si (100) surface at 6OO’C shows two linear regions [II]. At oxygen coverages below 7 x 10’4/cm2, the position of the Si 2p peak in the XPS spectra indicates the formation of islands of silicon suboxide. f, for the suboxide is simply proportional to the oxygen coverage c,, giving the linear dependence of Si+ on oxygen XPS signal in the low coverage range of fig. 1. With further thermal oxidation, the amount of suboxide formed (f,) stays practically constant but the Si2p peak shows an increase in the formation of SiO,. Hence I+(Si) has an additional contribution f2 from SiO, which is also linear with the
I
600°C
l
/”
’ /
.
0 Is PEAK
, ,! o,o,oP=
/A0
AREA
(ARB.
UNITS)
Fig. 1. Sputtered Si+ and O- yields from oxygenated Si (100) surface as a function of oxygen coverage. Oxygen was incorporated by thermal oxidation at 600°C. The oxygen coverage was measured by the area of the 0 1s XPS peak. The maximum coverage was estimated to be about 1.2 X 10’5/cm2.
oxygen coverage and that gives the second linear region in fig. 1. This is verified by actually decomposing the Si 2p X-ray photoemission spectra to measure the amount of SiO, and suboxides formed. It was found that the ratio ( Pf Y)s102/(P+ Y)5uboxide - 3.5 for Sit emitted with 8 eV. Hence the ion yield increases with the oxygen coordination number as expected. This ratio decreases with increasing emission energy. At 16 eV, the ratio is only 2.4. This energy dependence probably originates from the velocity dependence of the curve crossing transition [13]. These are specially selected systems with well defined stages of reaction to illustrate the validity of eq. (2). In general the functional dependences of f, on cO are determined by the oxide growth kinetics which may change with the experimental conditions. This is an important aspect in SIMS analysis and is discussed in sect. 3.2. It has been proposed [14] that the formation of oxide bandgap on a solid surface can inhibit the neutralization of positive ions by electron tunneling. This may be an applicable explanation for the oxygen matrix effect for a uniform oxide surface. However a large matrix effect has also been observed during the chemisorption of oxygen [12] at which no large bandgap is formed. Recent velocity selected measurement [II] also do not show a simple dependence of the ionization probability on the normal component of the emission velocity cL as predicted by the electron tunneling model. There is also no clear correlation between the enhancement of the positive secondary ion yields with the work
153
M. L. YU / Chemrccrlenhancement ejfects rn SIMS mtilys~s
function change caused by the presence of oxygen on the sample surface [7]. All these data favor a local interaction model like bond-breaking rather than the electron tunneling model. Recently Gerhard and Plog [15] proposed that the oxygen enhancement effect originates from the dissociation of sputtered oxide molecules with excess internal energy at a distance from the surface where electronic influence of the surface, such as surface bond breaking or electron tunneling, is negligible. They derived a sputtering theory for inhomogeneous targets to explain the mass dependence of the monometal ions Mt yield f(,r metal oxides and chlorides, and quoted the constant M+/Oratios during oxidation of Sn and Si [16] as supporting evidence. This “molecular dissociation in vacuum” scheme has also being considered by others [l&18]. But as was pointed out by Wittmaack [16], the bond-breaking model is also consistent with the proportionality between Si+ and O- yields. To discriminate bond-breaking at the surface and molecular dissociation in vacuum is difficult with just this piece of information. Mann and Yu [ll] also studied the Si+/Oratio during the oxidation of Si (100) and the data is shown in fig. 1. The oxygen enhanced Si+ yield was found to be proportional to O- only during the growth of silicon suboxides. The Si+/Oratio for SiO, is about 2.9 times higher and is increased in a direction opposite to the change of the stoichiometry. An important feature of the molecular dissociation in vacuum model is that the hI+/Oratio should be some simple fraction. For example, Si+/O- should be unity for the dissociation of SiO. Absolute yield measurements are difficult to obtain and is not reported for the 0-Si system. Since P+ of Si is believed to be close to unity in this system [2], Pm for oxygen should also have the same magnitude. However by depositing Cs or Li on partially oxidized Si surfaces, we observed an enhancement of O- by over two orders of magnitude while suppressing the Si+ yield [19]. It suggests that the value of P- for oxygen in the original oxidized Si surface is probably less than 10e2. Judging fi om these observations, it does not appear that molecular dissociation in vacuum is a major channel in secondary ion formation. 2.2. The Cs induced enhancement
effect
Significant advancement in the understanding of the Cs enhancement effect has been attained in the last few years. These experimental studies have led to the development of an electron tunneling model for the surface ionization of sputtered atoms. By vapor depositing Cs or other electropositive elements on sample surfaces in ultrahigh vacuum and followed by sputtering in the static mode, it was found that there is a strong correlation between the secondary ion yields and the work function Qi of the sample surface [20,21]. Submono-
.
SitIll) .
\
5.2eV
Si+
.
0 Li 00
.
cs
‘0
i ‘0 .
0
\
00
\
0
\
il
\
d
1 I
IO -3.0
I
I
I
-2.0
I
O\’
-1.0
0.0
A$ (eV) Fig. 2. Work-function dependence of the Si-yield at 5.2 eV emission energy. 1 nA of 500 eV Ne+ primary ions were used to sputter
in the static mode. Counting
time was 30 s.
layers of alkali atoms (e.g. Li, Na, Cs) deposited on a sample surface in general induce a lowering of the work function by as much as 2 to 3 eV. The negative secondary ion yield I- increases exponentially with the induced decrease @ frequently for over three orders of magnitude (fig. 2). The presence of alkali atoms on sample surfaces in general suppresses the positive ion yields [19,22]. Simple exponential relations between the positive secondary ion yield and @ were also observed in some systems. If there are no electronegative species (e.g. oxygen) on the sample surface, the @ dependence of the secondary ion yield is not sensitive to the choice of the electropositive element used to induce the change in @ [23]. In fact for secondary ions sputtered from adsorbed layers, the ion yield is also affected by the work function change induced by the adatoms themselves [24]. The work function is the minimum energy required to remove an electron from the solid surface. It is a global, non-local quantity of the surface. The strong correlation of the ionization probability with @ suggests that the ionization process involves interaction with electrons coming from atoms beyond the immediate neighbors of the sputtered atom on the surface. It V. SIMS AND SPUTTERED
NEUTRALS
154
M. L. Yu /
Chemical
enhancement
suggests a non-local interaction picture. This is significantly different from the local bond-breaking model for the oxygen matrix effect. The most successful non-local interaction theory proposed is the electron tunneling model. This theory has been discussed in various forms in detail by Blandin et al. [25], Blaise and Nourtier [7], Norskov and Lundqvist [26], Brako and Newns [27], Lang and Norskov [29], Yu and Lang [20]. Readers can consult the above references for more details. For a reasonable range of @ it can be shown that [26,28,29]: P-=
exp[-(a-A)/C_u,]
and a similar gives: P+=
whenA<@,
derivation
exp[ -(I-
for positive
@)/C+v,]
secondary
when I> @.
(3) ions
(4)
Here A and Z are the electron affinity and the ionization potential respectively of the sputtered atom. C, and C_ are constants of the system. In spite of its simplicity, the theory is in good agreement with the experimental data. The dependence of P+ on v1 was also verified in experiments with velocity and angular selectivity [31].
3. Relation to SIMS analysis 3. I. The experimental
conditions
In SIMS analysis, we usually have to encounter a dynamic sputtering condition where oxygen or cesium atoms are constantly being introduced and sputterd off at the same time. The surface concentration of these species depends on the exact experimental condition. Energetic oxygen and cesium ion beams can be used as primary ion beams for sputtering and implanting oxygen or cesium atoms for ion yield enhancement. The rate of implantation of these species m per unit area is just &(l - r~) where jr is the primary ion current density (atoms/cm2s), and n the backscattered fraction. Simultaneously, primary species are sputtered away from the sample with a rate per unit area given by c,,,u,j,, where em is the sputtering cross section of m, counting also all the molecular species containing m, and c, is the surface concentration of the implanted species. For simplicity we assume that the sputtered particles originate only from the topmost atomic layers. At steady state, there is a balance of the incoming and outgoing flux of m so that c, = a,-‘(1
- ?J).
(5)
In reality, sputtered atoms originate within a certain escape depth of the sample. c, and u, should be replaced by the escape function weighted averages [32]. Unfortunately both u, and the escape function are in
effects in
SIMS
analysis
general difficult to measure. According to eq. (5) the concentration of the implanted atoms on the surface should be self-limiting and independent of the beam current density. Ideally this mode of operation is simple and highly reproducible. To change c,, we can make use of the fact that u,,, depends on the energy and angle of the primary ion beam. For example, c, can be increased if the incident beam is more normal to the sample surface. The second common mode of operation is to introduce oxygen or cesium by an oxygen jet or a Cs effusion source while an Ar+ beam is used for sputtering. The rate of incorporation of m on the surface is s,,,pJ (2mMkT)“’ where pm, M and T are the partial pressure, mass and temperature of the gaseous species m is the sticking coefficient of m on the ion [331. sin bombarded surface. If the primary ion beam is not being scanned, the steady state c, is given by the flux balance, c, = s,,,pm/om jP(2vMkT)1’2.
(6)
According to eq. (6) c, can easily be adjusted by changing pm within experimental limits and steady state can be reached rapidly while c, is self-limited in implantation. However c, is sensitive to the tuning of the primary ion beam through the dependence on jr. In actual analysis, the primary beam is usually well focussed and scanned to provide a flat bottomed crater. The secondary ions is either electronically gated or apertured to discriminate against contributions from crater edges. Eq. (5) is still valid but eq. (6) is only qualitatively useful since no dynamic equilibrium would be established. The surface concentration c, is now described by dc,/dt
= s,p,/(2?rMkT)“2
- c,,,q,, jP( t).
(7)
To see the salient features, we can approximate the scanning cycle by a very short “beam on” t,, period where the sputtering term c,,,u,,,j,, dominates and c,(t)
= c,(O)
exp( -j,u&).
(8)
We also assume that the overall oxygen enhancement depends on the time averaged oxygen concentration on the surface during the sputtering period t,,
(L(t)>“” = $!I = &
s,I””exp(
-jr,umt)
dt
[I - exp( -.&J,,)]~
Then it is followed by a long “beam off” period t,rr where the sputtering term is zero and c, builds up by adsorption. Let us examine the various limiting cases.
155
M. L. Yu / Chemrcal enhuncement effects in SIMS anabsis
For fast adsorption,
e.g., large srnprnr
For slow adsorption, e.g., small, SOPS, and if the adsorbed oxygen is removed by sputtering every cycle, c,,(O) can be approximated by as,p,t,rr. Here (Y is a proportionality constant. Then
Hence the degree of oxygen enhancement not only depends on the oxygen partial pressure and sticking coefficient, it depends on the scanning speed and the current density of the primary ion beam in general. The secondary ion signals do not usually scale with i,. 7 hese are important considerations in optimizing the experimental conditions for sensitivity and when data from different instruments are compared. Up to now we have not considered any surface or bulk diffusion of the incorporated oxygen or cesium atoms. Oxygen is able to form a strong oxide bond and diffusion is rarely a problem. Cs which forms only weak metallic bond may cause some concern. For example it has been reported that oxygen contamination can cause the pile-up of implanted Cs near the surface in the case of a Si target [34]. But due to the lack of knowledge on these diffusion process a rigorous treatment is not available at present. It should contribute like an additional adsorption (or desorption) term and can be checked by going to different scanning rates when using a Cs+ beam. The effect of such diffusion processes in SIMS analysis have not been fully addressed. 3.2. The oxygen induced enhancement
Z(A+)=Z,A[A]+Z;[B],
(13)
IO
effect
According to the bond-breaking model, the oxygen enhanced ion yield is linearly related to the distribution f, of various oxide species as shown in eq. (2). In the dynamic sputtering condition, there is usually a wide range of oxides formed. As a result the ion yield is rarely found to be linear with the oxygen coverage. For example let us assume a completely random distribution of the oxygen atoms in the target with a range of oxides formed. As the oxygen concentration increases, the distribution would skew towards higher oxygen coordination per substrate atom. Following Oechsner and Sroubek [35], if the ion yield enhancement factor R per oxygen neighbor is constant, the averaged ionization probability Pt is related to the oxygen concentration c, by: P’-=P,+[c,(R-l)+l]N
oxygen neighbors permissible by the chemical system. Hence a very nonlinear dependence on c, is obtained. In general the distribution of the various oxides formed depends on the experimental conditions thus producing the diverse functional dependences on the oxygen concentration. Even for the well studied case of Si+ emission from silicon surfaces in the presence of oxygen, the reported oxygen coverage dependences ranged from linear, at very low oxygen coverages (I 3 at.%) [16], to power law dependences [16,36,37] to exponential dependences [38,39]. The major difficulty with these experiments is that there is still no easy way, even with electron spectroscopies, to determine the exact chemical composition of the oxidized layer from which the secondary ions originate under dynamic sputtering conditions. The dependence of the oxygen effect on the nature of the oxide bonding is important in the SIMS analysis of alloys. Pivin et al. [40] studied the secondary ion emission from three binary alloys (Fe-Ni, Fe-Cr and Cr-Ni), in the presence of an 0, jet. They observed that the oxygen enhanced ion yield is affected by the presence of the second alloy component. They found that the ion yield Z(A+) of A for alloy A-B is linearly related to the atomic concentrations [A] and [B]:
(12)
where Pa+ is the ionization probability at zero oxygen concentration and N is the maximum number of nearest
-A 0
20
A
A/
40
/
60
00
Ni
at. % Ni
Fig. 3. The ionization probability P+ of Ni as a function of the composition in the Cr-Ni. Fe-Ni, and Cu-Ni alloys, normalized to the P+ value of pure Ni. V. SIMS AND SPUTTERED
NEUTRALS
156
M. L. Yu / Chemical enhancement
where I,” and I: are the ionization yields of A+ ions per unit concentration respectively for a pure A sample and a sample of nearly pure B containing A as a dilute impurity. These linear relations again suggest that the effect of B on the emission of A+ is local. To understand these alloy effects more fully, Yu and Reuter [41] have studied the alloy effects for six different alloys, using both 0: bombardment and Art bombardment with 0, adsorption. They also monitored the oxide chemistry on the sample surfaces with in situ XPS measurements. They did not observe simple linear relations with the alloy composition as reported by Pivin et al. For a reasonably large sticking coefficient for oxygen, oxidation can proceed practically to completion by using higher 0, partial pressure in the 0, jet. But with 0: bombardment oxidation is self-limiting, depending on a,,, of the oxygen atoms on the material. This was indeed verified by XPS measurements. The general rule for alloys is as follows: For an alloy A-B where A forms a stronger oxide bond than B, the presence of A enhances the emission of B+ while the presence of B suppresses the emission of A+. Fig. 3 shows the data from three different Ni alloys: CuNi, Fe-Ni and Cr-Ni under 0; bombardment. The presence of Cr and Fe significantly enhances the Ni+ ionization probability while the presence of Cu suppresses it. Since the oxide bond energy per metal atom is 15.5 eV, 12.5 eV, 9.5 eV and 5.7 eV for Cr, Fe, Ni and Cu respectively, the general rule is obeyed. In situ XPS measurements, indicated that the large alloy effect on Nit was not from a change in the oxygen concentration. In fact the interaction between Fe,O, and NiO, for example, strengthens the nickel-oxygen bond as in ferrite. XPS spectra of the 0: bombarded Fe-Ni alloy show an increase in the binding energy of the Ni 2p,,z peak. This illustrates again the sensitivity of the ionization probability to the state of the oxide bond. It is also found that the general rule works best when the sample surfaces are heavily oxidized. For alloys that have a high sputtering cross-section a,,, such that oxidation is far from complete, deviations can occur. In such cases, the use of an 0, jet with high pm can correct the situation. It has been observed that the secondary ion energy distribution in general sharpens with stronger oxidation (e.g. chemisorption to oxide formation) [41]. This is consistent with the discussion in sect. 2.1 that secondary ions emitted with a smaller energy (velocity) are more sensitive to chemical bond changes. Hence the oxygen matrix enhancement effect is larger for secondary ions in the low energy end of the energy distribution. The net effect is that the energy distribution is sharpened and the peak shifts towards the lower energy side. Yu and Reuter [41] also observed that in the case of secondary ion emission from an alloy A-B where A forms a stronger oxide bond than B, the presence of A sharpens
effects in SIMS
ana!ysls
the energy distribution of B+ while the presence of B broadens the energy distribution of A+. This observation is in line with the sharpening of the energy distribution as the oxide bond gets stronger. It should be mentioned that in some cases, e.g. heavily oxidized Ba, a significant amount of sputtered atoms may be taken up to form molecular species, e.g. BaO. The velocity distribution of the atomic species can be distorted by the oxidation [42]. Oxygen incorporation by 0, jet and by oxygen ion beam are complementary. An 0, jet can provide high oxygen coverage by using a high partial pressure but an oxygen primary ion beam is more effective for samples with very low oxygen sticking coefficients, for example germanium and gallium arsenide. It is sometimes useful to use an oxygen primary ion beam with an 0, jet. This technique combines the advantages of the two modes of oxygen introduction. Steady state condition can be reached rapidly and signal enhancement is maximum. The dependence on jp may make the tuning of the primary ion beam more critical than just using 0:. Usually it is advantageous to oxidize the sample completely to achieve the maximum secondary ion yield and sensitivity. However oxidation can in many cases interfere with the analysis, for example, by inducing a large alloying effect. Such alloy effect can cause changes in the SIMS sensitivity at interfaces where there is a rapid variation in the composition [43]. The alloy effect can be minimized by reducing the oxygen concentration. Reuter and Yu have found that this can be achieved by going to a shallower angle of incidence for the 0; beam [44]. The resulting increase in the sputtering coefficient reduces the steady state oxygen concentration on the sample surface. Very recently it was also found that oxidation can cause the segregation of dilute implants during SIMS analysis [45,46]. Again reducing the oxygen coverage by using a more glancing incidence oxygen primary ion beam improves the recovery of the true profile. There have been several systematic studies on the effect of the angle of incidence of an oxygen ion beam. It was found for example for 15 keV 0; incident on Ni [44], the steady state surface oxygen concentration as measured by Auger spectroscopy decreases by an order of magnitude when the angle of incidence increases from 0” (normal) to 70”. The corresponding Ni partial sputter yield also increased by a factor of 14. This strong angular dependence is only partially a result of the angular dependence expected from the collision cascade sputtering theory (set 70” = 3). The major reason is that the oxide formation changes the surface binding energy in sputtering. The incidence angle not only affects the steady state oxygen concentration c,, it also changes the kind of oxides formed. Similar studies have been made on silicon [47-501 and germanium [49,50]. At normal inci-
M. L. Yu / Chemrcul enhancement
dence, silicon is fully oxidized to SiO,, whereas at more oblique angle of incidence silicon is only partially oxidized (suboxides). In contrast, XPS shows that germanium is only weakly oxidized even under normal incidence bombardment without the formation of GeO, [50]. This again correlates well with the much larger sputter coefficient of Ge (greater than four times that of silicon) even at normal incidence. The use of the angle of incidence as a parameter in SIMS analysis is still quite scarce. This is partially due IO the usually fixed beam/sample geometry in SIMS equipment. But still it is a crucial parameter when data f’rom different SIMS facilities are compared. .‘.3. The Cs induced
enhancement
effect
Cs can be introduced either by using a Cs+ primary ion beam or by vapor deposition on the target during sputtering. As discussed in sect. 3.1, the use of a Cs+ primary ion beam is simpler and more reproducible. The reproducibility is especially critical at low Cs coverages where the work function decreases roughly linearly with the increase in coverage while the negative ion ),ield increases exponentially. The major disadvantages ii that the steady state surface Cs coverage is controlled by 0,‘. The limitation can be overcome by Cs vapor deposition. For example it is even possible to build multilayers of Cs on a metallic substrate although coverages on semiconductors may be self-limiting to a monolayer [51]. Cs being quite reactive is permeable to oxygen contamination from the residual gas. The incorporation of oxygen in the Cs layer can cause further lowering of the work function [52,53]. It can also cause the segregation of implanted Cs to the surface [34]. This is similar to the oxygen adsorption induced surface segregation (of the reactive component) in an alloy [54]. Bernheim et al. 121.551 have studied the Cs matrix effect by depositing Cs with a Cs vapor jet during dynamic Ar+ bombardment. They inferred the work function change from the shift of the secondary ion energy distribution. In spite of some concern raised on this method of work function measurement [56]. their result shows a strong correlation between the negative secondary ion yield and the work function change, with segments of their data showing an exponential dependence of the negative ion yield on A@ [8]. Particularly interesting is that as the Cs coverage was increased to obtain the minimum work function, the negative secondary ion yield reached a maximum, similar to observations with static mode sputtering. This point of minimum work function is probably the optimum both for sensitivity and stability against fluctuation in Cs flux. The work function @ varies relatively slowly with C‘s coverage at this point. Also many experimental studies have shown [20,23,31] that the negative ion yield usually deviates from the steep exponential dependence
effects in SIMS
analysis
151
on @ as @ approaches the minimum value. An example has been shown in fig. 2. It has been shown theoretically [28] that the exponential dependence of Y- on @ is only an approximation for small ranges of A@. Such deviation is expected. The higher coverages of Cs in this region probably also contributes to a smaller rate of enhancement by reducing the sputtering yield from the sample. The exponential dependence of the negative ion yield on the electron affinity was also observed in dynamic sputtering conditions [3,4,57], which is consistent with eq. (3). Emission energy (velocity) dependence of Y- has also been observed by Wittmaack [58]. Since the chemical bonding between Cs and metallic substrates are much weaker than an oxide bond, Cs is expected not to induce a large alloying effect. Bernheim et al. [21.55,57] have studied the negative secondary ion emission from binary alloys. By exposing the alloy surfaces to a jet Cs vapor they found that the maximum Cs enhanced negative ion yields are simply porportional to the concentration in the alloys. Also it is of interest to note that Cs also enhances negative secondary molecular ions and suppresses positive ions [58.59]. Hence Cs can cause a significant alternation in the molecular fingerprint spectrum of a compound without affecting the stoichiometric composition of the substrate. The use of Cs vapor has an advantage over the use of a Csf beam. With a Cs+ beam, the Cs coverage may be limited [eq. (2)] to a value where the enhancement factor is very sensitive to changes in the work function. This is important when there is a variation in the sputtering coefficient in the sample either through a gradual change in the alloy composition or when going through an interface. A related effect has been observed when analyzing the interfacial oxygen between amorphous and crystalline silicon [60]. With a Cs vapor jet, the cesium coverage can be adjusted to the minimum work function where both sensitivity and stability are maximum.
4. Conclusions There has been significant progress in delineating the physics of secondary ion emission in the last five years. In spite of the fact that our basic understanding of the phenomena has to rely on data obtained from static mode sputtering, our knowledge can easily be applied to situations in SIMS analysis. We also learn that the quantitation of SIMS through theoretical treatment is at present impractical since many required detailed material and experimental parameters are hard to obtain or not within our reach in the near future. Nevertheless SIMS has already acquired a strong enough basis to support analytical applications and further development. V. SIMS
AND
SPUTTERED
NEUTRALS
M. L. Yu / Chemical enhancement effects in SIMS analyxs
158
The author is delighted to acknowledge the collaborations with W. Reuter, N. Lang, K. Mann and J. Clabes, and the very useful discussions with K. Wittmaack. The research was partially supported by the Office of Naval Research.
References
VI K. Wittmaack, Vacuum 34 (1983) 119. PI A. Benninghoven, Surf. Sci. 53 (1975) 596. [31 C.A. Andersen, Int. J. Mass Spectrom. Ion Phys. 2 (1969) 61 and 3 (1970) 413.
[41 V.R. Deline, CA. Evans, Jr., and P. Williams, Appl. Phys. Lett. 33 (1978) 578.
[51 K. Wittmaack, Appl. Surf. Sci. 9 (1981) 315. [61 P. Williams, Surf. Sci. 90 (1979) 588. (71 G. Blaise and A. Nourtier. Surf. Sci. 90 (1979) 495. PI G. Slodzian, Phys. Scripta T6 (1983) 54. 191 M.L. Yu and N.D. Lang, Nucl. Instr. and Meth.
B14 (1986) in press. Springer Series in [lOI P. Sigmund, SIMS IV Conference, Chemical Physica, eds., Benninghoven, Okano, Shimizu and Werner vol. 36 (1984) p. 2. IllI K. Mann and M.L. Yu, presented at SIMS V, Washington DC (1985), to be published. [=I M.L. Yu, D. Vitkavage and J. Clabes, J. Vat. Sci. Technol. A3 (1985) 1316. 1131 J.B. Delos, Rev. Mod. Phys. 53 (1981) 287. [I41 P.J. Martin, A.R. Bayly, R.J. MacDonald, N.H. Tolk, G.J. Clark and J.C. Kelly, Surf. Sci. 60 (1976) 349. [I51 W. Gerhard and C. Plog, Z. Phys. B54 (1983) 59 and 71; Surf. Sci. 152/153 (1985) 127. [I61 K. Wittmaack, Surf. Sci. 112 (1981) 168. P71 G. Blaise, Surf. Sci. 60 (1976) 65. [I81 G.E. Thomas, Radiat. Eff. 31 (1977) 185. 1191 M.L. Yu, Phys. Scripta T6 (1983) 67. WI M.L. Yu, Phys. Rev. Lett. 40 (1978) 574. PI M. Bernheim and G. Slodzian, J. Micros. Spectrosc. Electron 6 (1981) 141. PI M.L. Yu, Phys. Rev. B24 (1981) 1147. ~31 M.L. Yu, Phys. Rev. B26 (1982) 4731. 1241 M.L. Yu, Phys. Rev. B29 (1984) 2311. PI A. Blandin, A. Nourtier and D.W. Hone, J. Phys. 37 (1976) 369. [26] J.K. Norskov and B.I. Lundqvist, Phys. Rev. B19 (1979) 5661. [27] R. Brako and D.M. Newns, Surf. Sci. 108 (1981) 253. [28] N.D. Lang, Phys. Rev. B27 (1983) 2019. [29] N.D. Lang and J.K. Norskov, Phys. Scripta T6 (1983) 15. [30] M.L. Yu and N.D. Lang, Phys. Rev. Lett. 50 (1983) 127. [31] M.L. Yu, Phys. Rev. Lett. 47 (1981) 1325.
[32] K. Wittmaack, Nucl. Instr. and Meth. B7/8 (1985) 779. [33] G. Blaise and M. Bernheim, Surf. Sci. 47 (1975) 324. [34] N. Menzel and K. Wittmaack, Nucl. Instr. and Meth. 191 (1981) 235. [351 H. Oechsner and Z. Sroubek, Surf. Sci. 127 (1983) 10. [361 V.R. Deline, W. Katz, C.A. Evans, Jr., and P. Williams, 4ppl. Phys. Lett. 33 (1978) 832. [371 H. Gnaser, Nucl. Instr. and Meth. 218 (1983) 312. H.W. [381 A.E. Morgan, H.A.M. de Grefte, N. Warmoltz, Werner and H.J. Tolle, Appl. Surf. Sci. 7 (1981) 372. [391 P. Sanda, V. Kaiser, R. Jede, D. Lipinsky, 0. Ganschow and A. Benninghoven, J. Vat. Sci. Technol. A3 (1985) 1946. and G. Slodzian, Int. J. [401 J.C. Pivin, C. Roques-Carmes Mass. Spectrom. Ion. Phys. 26 (1978) 219. (411 M.L. Yu and W. Reuter, J. Appl. Phys. 52 (1981) 1478 and 1489. M.L. Yu and A.C. Balant, Surf. Sci., ~421 D. Grischkowsky, 127 (1983) 315. [431 M.L. Yu and W. Reuter, J. Vat. Sci. Technol. 17 (1980) 36. [441 W. Reuter and M.L. Yu, J. Appl. Phys. 53 (1982) 3784. [451 V.R. Deline, W. Reuter and R. Kelly, Proc. Materials Research Society Meeting, San Francisco (1985) to be published. [461 S.M. Hues and P. Williams these Proceedings (IBA ‘85) Nucl. Instr. and Meth. B15 (1986) 206; W. Vandervorst, M.L. Swanson, H.H. Plattner, O.M. Westcott, I.V. Mitchell and R.R. Shepherd, ibid. p. 201. [471 N. Warmoltz, H.W. Werner and A.E. Morgan, Surf. Interf. Anal. 2 (1980) 47. [481 W. Reuter and K. Wittmaack, Appl. Surf. Sci. 5 (1980) 221. I491 K. Wittmaack, Nucl. Instr. and Meth. 218 (1983) 307. [501 W. Reuter, these Proceedings (IBA ‘85) Nucl. Instr. and Meth. B15 (1986) 173. [511 P. Wagner, K. Miller and K. Heinz, Surf. Sci. 68 (1977) 189. [521 B. Goldstein, Surf. Sci. 35 (1973) 227. Surf. Sci. 92 [531 J.L. Desplat and C.A. Papageorgepoulos, (1980) 97. [54] CR. Brundle. E. Silverman and R.J. Madix, J. Vat. Sci. Technol. 16 (1979) 474. [55] M. Bernheim and G. Slodzian, J. de Phys. (Lettres), 38L (1977) 325, and SIMS III Conference, Springer Series in Chemical Physics vol. 19 (1982) p. 151. [56] K. Wittmaack, Phys. Scripta T6 (1983) 71. [57] M. Bernheim, J. Rebiere and G. Slodzian, SIMS II Conference, Springer Series in Chemical Physics, vol. 9 (1979) p. 40. [58] K. Wittmaack, Surf. Sci. 126 (1983) 573. [59] M.L. Yu, Phys. Rev. B24 (1981) 5624. [60] K. Wittmaack, Nucl. Instr. and Meth. B7/8 (1985) 750.
Section
V. SIMS
CHEMICAL
151
and analysis of sputtered neutrals
ENHANCEMENT
EFFECTS
IN SIMS ANALYSIS
Ming L. YU IBM
T.J. Watson Research Center, Yorktown
Heights, NY 10598, USA
This paper summarizes some of the recent advances in the understanding of the secondary ion emission process, and relates them to the important cesium and oxygen enhancement effects in secondary ion mass spectrometry (SIMS). The oxygen enhancement of ionization results from local breaking of oxide bonds and its magnitude is related to the microscopic distribution of oxygen atoms in the target. The Cs enhancement effect is found to be connected with the work function change induced by the Cs on the sample surface, and the ionization process is best described by an electron tunneling model. dependence, alloying effect and the optimum operating conditions in SIMS analysis.
1. Introduction An important analytical application of secondary ion mass spectrometry (SIMS) is to measure depth distributions of elements in solid samples. Sensitivity and depth resolution are the two major issues in this mode of SIMS analysis. The sensitivity of the technique is related to the mechanisms of secondary ion formation. On the other hand SIMS shares many of the physical limitations on the achievable depth resolution of other sputter profiling techniques [l]. In this paper, we shall concentrate on the sensitivity issue. The emission of secondary ions during ion bombardment of solid surfaces is the result of a two step process. At first, the impact of an energetic primary ion provides, through a collision cascade, the momentum for the sputtering of a surface atom. Then the charge state of the sputtered atom is determined by the electron exchange between the atom (or ion) and the surface. In general, the charge state of the sputtered atom is not related to the charge of the incoming primary ion. The ionization phenomenon is usually caused by the charge exchange between the valence levels of the sputtered atom or ion and those of the solid. The secondary ion yield I; (counts/s) of element A is given by the following equation: Ii
= pPA*CAYip,
(1)
where /3 = instrument transmission factor, PA’ = ionization probability of element A, C, = atomic fraction of A in the sample, Y = matrix sputtering yield (atoms/ion), ‘P = primary ion beam current (ions/s). The sensitivity is very much dictated by PAi of the atomic species. It has been well established [2] that for many metallic elements PAi is less than lop4 when 0168-583X/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
These
differences
affect
the concentration
high purity samples are sputtered by rare gas ions. It was observed however that the presence of electronegative atoms (e.g. oxygen) on the sample surface can greatly increase the formation of positive secondary ions, with P+ approaching unity. On the other hand the presence of electropositive atoms (e.g. cesium) on a sample surface can greatly increase P- by three to four orders of magnitude [3]. It is now a routine practice in SIMS analysis to utilize these enhancement phenomena to achieve high sensitivity.
2. Mechanisms of ionization Many advances have been made in the past few years on the physical understanding of these eahancement effects. Although there were attempts to establish a phenomenlogical “unified” model [4], recent experimental and theoretical studies indicate that microscopically these two enhancement effects due to oxygen and cesium are quite different. Reviews on these phenomena are available in the literature [5-91. Here we shall briefly highlight some of the recent progress that are relevant to our discussion below. 2.1. The oxygen enhancement
effict
Although many electronegative elements (e.g. N, 0, Cl, F) can cause an enhancement of the positive secondary ion yields, oxygen is being used most frequently. The prevailing explanation of this enhancement effect is the bond-breaking mechanism. The ionization of the sputtered atom is believed to be the result of breaking of the chemical bond with oxygen (or other electronegative atom). The Landau-Zener-Stueckelberg theory of curve-crossing for the ionic dissociation of diatomic molecules has been frequently used to describe qualitaV. SIMS AND SPUTTERED NEUTRALS
152
M. L. Yu / Chemical enhancement
effcts
tively
the secondary ion emission from a solid [6,8], yet a useful microscopic description for the oxygen enhancement effect is still not available. The major difficulties are the uncertainties of the energy surfaces involved and the treatment of the relaxation of the broken bond. Phenomenlogically the secondary ion yield IL is the sum of the contributions from different bonding configurations (i) involving A [lo]: (2)
in SIMS
4
anal,vsis
I 0 - Si (100) InA
c 4 z
t
z 0’ 0
500eV
I
Ar+
/
0 o-
(x5)
I 7eV,
0”
.
/-
/ 2-
,’ 1)
P
5 v,
/
lSi+
3-
2
where f, = amount of the surface A atoms bonded to oxygen in the i th configuration = ionization probability of A when it is sputtered P,’ from the i th configuration y = partial sputtering coefficient of A in the ith configuration. Eq. (2) however is difficult to apply since the usual experimental observable is c,, the oxygen concentration (or that of other electronegative species). The relation between co and f, depends very much on the oxide growth kinetics in the individual experiment. Recently Mann and Yu [ll] and Yu et al. [12] have reported experimental data from several model systems that supports the linear relation between IL and f,. These SIMS experiments were performed with in situ X-ray photoemission (XPS) to monitor the coverage and the chemical state of the surface. The surface reactions (e.g. oxidation, nitridation) were induced thermally and sputtering was kept in the static mode to minimize ion beam induced chemistry. The coverage of electronegative species was kept close to the monolayer range so that the information from both SIMS and XPS originate from the same topmost reacted layer. The simplest system they have studied is the enhancement of the sputtered Si+ yield by nitridation of the Si (100) surface at 1100°C [II]. XPS showed that only Si,N, was formed on the Si surface. i.e. f (for Si,N,) is simply proportional to the nitrogen coverage. Indeed the enhanced Si+ yield was found to be directly proportional to the nitrogen coverage up to a monolayer [II]. A more complicated illustration of eq. (2) is shown in fig. 1 where the oxygen coverage dependences of the Si+ yield during the oxidation of a Si (100) surface at 6OO’C shows two linear regions [II]. At oxygen coverages below 7 x 10’4/cm2, the position of the Si 2p peak in the XPS spectra indicates the formation of islands of silicon suboxide. f, for the suboxide is simply proportional to the oxygen coverage c,, giving the linear dependence of Si+ on oxygen XPS signal in the low coverage range of fig. 1. With further thermal oxidation, the amount of suboxide formed (f,) stays practically constant but the Si2p peak shows an increase in the formation of SiO,. Hence I+(Si) has an additional contribution f2 from SiO, which is also linear with the
I
600°C
l
/”
’ /
.
0 Is PEAK
, ,! o,o,oP=
/A0
AREA
(ARB.
UNITS)
Fig. 1. Sputtered Si+ and O- yields from oxygenated Si (100) surface as a function of oxygen coverage. Oxygen was incorporated by thermal oxidation at 600°C. The oxygen coverage was measured by the area of the 0 1s XPS peak. The maximum coverage was estimated to be about 1.2 X 10’5/cm2.
oxygen coverage and that gives the second linear region in fig. 1. This is verified by actually decomposing the Si 2p X-ray photoemission spectra to measure the amount of SiO, and suboxides formed. It was found that the ratio ( Pf Y)s102/(P+ Y)5uboxide - 3.5 for Sit emitted with 8 eV. Hence the ion yield increases with the oxygen coordination number as expected. This ratio decreases with increasing emission energy. At 16 eV, the ratio is only 2.4. This energy dependence probably originates from the velocity dependence of the curve crossing transition [13]. These are specially selected systems with well defined stages of reaction to illustrate the validity of eq. (2). In general the functional dependences of f, on cO are determined by the oxide growth kinetics which may change with the experimental conditions. This is an important aspect in SIMS analysis and is discussed in sect. 3.2. It has been proposed [14] that the formation of oxide bandgap on a solid surface can inhibit the neutralization of positive ions by electron tunneling. This may be an applicable explanation for the oxygen matrix effect for a uniform oxide surface. However a large matrix effect has also been observed during the chemisorption of oxygen [12] at which no large bandgap is formed. Recent velocity selected measurement [II] also do not show a simple dependence of the ionization probability on the normal component of the emission velocity cL as predicted by the electron tunneling model. There is also no clear correlation between the enhancement of the positive secondary ion yields with the work
153
M. L. YU / Chemrccrlenhancement ejfects rn SIMS mtilys~s
function change caused by the presence of oxygen on the sample surface [7]. All these data favor a local interaction model like bond-breaking rather than the electron tunneling model. Recently Gerhard and Plog [15] proposed that the oxygen enhancement effect originates from the dissociation of sputtered oxide molecules with excess internal energy at a distance from the surface where electronic influence of the surface, such as surface bond breaking or electron tunneling, is negligible. They derived a sputtering theory for inhomogeneous targets to explain the mass dependence of the monometal ions Mt yield f(,r metal oxides and chlorides, and quoted the constant M+/Oratios during oxidation of Sn and Si [16] as supporting evidence. This “molecular dissociation in vacuum” scheme has also being considered by others [l&18]. But as was pointed out by Wittmaack [16], the bond-breaking model is also consistent with the proportionality between Si+ and O- yields. To discriminate bond-breaking at the surface and molecular dissociation in vacuum is difficult with just this piece of information. Mann and Yu [ll] also studied the Si+/Oratio during the oxidation of Si (100) and the data is shown in fig. 1. The oxygen enhanced Si+ yield was found to be proportional to O- only during the growth of silicon suboxides. The Si+/Oratio for SiO, is about 2.9 times higher and is increased in a direction opposite to the change of the stoichiometry. An important feature of the molecular dissociation in vacuum model is that the hI+/Oratio should be some simple fraction. For example, Si+/O- should be unity for the dissociation of SiO. Absolute yield measurements are difficult to obtain and is not reported for the 0-Si system. Since P+ of Si is believed to be close to unity in this system [2], Pm for oxygen should also have the same magnitude. However by depositing Cs or Li on partially oxidized Si surfaces, we observed an enhancement of O- by over two orders of magnitude while suppressing the Si+ yield [19]. It suggests that the value of P- for oxygen in the original oxidized Si surface is probably less than 10e2. Judging fi om these observations, it does not appear that molecular dissociation in vacuum is a major channel in secondary ion formation. 2.2. The Cs induced enhancement
effect
Significant advancement in the understanding of the Cs enhancement effect has been attained in the last few years. These experimental studies have led to the development of an electron tunneling model for the surface ionization of sputtered atoms. By vapor depositing Cs or other electropositive elements on sample surfaces in ultrahigh vacuum and followed by sputtering in the static mode, it was found that there is a strong correlation between the secondary ion yields and the work function Qi of the sample surface [20,21]. Submono-
.
SitIll) .
\
5.2eV
Si+
.
0 Li 00
.
cs
‘0
i ‘0 .
0
\
00
\
0
\
il
\
d
1 I
IO -3.0
I
I
I
-2.0
I
O\’
-1.0
0.0
A$ (eV) Fig. 2. Work-function dependence of the Si-yield at 5.2 eV emission energy. 1 nA of 500 eV Ne+ primary ions were used to sputter
in the static mode. Counting
time was 30 s.
layers of alkali atoms (e.g. Li, Na, Cs) deposited on a sample surface in general induce a lowering of the work function by as much as 2 to 3 eV. The negative secondary ion yield I- increases exponentially with the induced decrease @ frequently for over three orders of magnitude (fig. 2). The presence of alkali atoms on sample surfaces in general suppresses the positive ion yields [19,22]. Simple exponential relations between the positive secondary ion yield and @ were also observed in some systems. If there are no electronegative species (e.g. oxygen) on the sample surface, the @ dependence of the secondary ion yield is not sensitive to the choice of the electropositive element used to induce the change in @ [23]. In fact for secondary ions sputtered from adsorbed layers, the ion yield is also affected by the work function change induced by the adatoms themselves [24]. The work function is the minimum energy required to remove an electron from the solid surface. It is a global, non-local quantity of the surface. The strong correlation of the ionization probability with @ suggests that the ionization process involves interaction with electrons coming from atoms beyond the immediate neighbors of the sputtered atom on the surface. It V. SIMS AND SPUTTERED
NEUTRALS
154
M. L. Yu /
Chemical
enhancement
suggests a non-local interaction picture. This is significantly different from the local bond-breaking model for the oxygen matrix effect. The most successful non-local interaction theory proposed is the electron tunneling model. This theory has been discussed in various forms in detail by Blandin et al. [25], Blaise and Nourtier [7], Norskov and Lundqvist [26], Brako and Newns [27], Lang and Norskov [29], Yu and Lang [20]. Readers can consult the above references for more details. For a reasonable range of @ it can be shown that [26,28,29]: P-=
exp[-(a-A)/C_u,]
and a similar gives: P+=
whenA<@,
derivation
exp[ -(I-
for positive
@)/C+v,]
secondary
when I> @.
(3) ions
(4)
Here A and Z are the electron affinity and the ionization potential respectively of the sputtered atom. C, and C_ are constants of the system. In spite of its simplicity, the theory is in good agreement with the experimental data. The dependence of P+ on v1 was also verified in experiments with velocity and angular selectivity [31].
3. Relation to SIMS analysis 3. I. The experimental
conditions
In SIMS analysis, we usually have to encounter a dynamic sputtering condition where oxygen or cesium atoms are constantly being introduced and sputterd off at the same time. The surface concentration of these species depends on the exact experimental condition. Energetic oxygen and cesium ion beams can be used as primary ion beams for sputtering and implanting oxygen or cesium atoms for ion yield enhancement. The rate of implantation of these species m per unit area is just &(l - r~) where jr is the primary ion current density (atoms/cm2s), and n the backscattered fraction. Simultaneously, primary species are sputtered away from the sample with a rate per unit area given by c,,,u,j,, where em is the sputtering cross section of m, counting also all the molecular species containing m, and c, is the surface concentration of the implanted species. For simplicity we assume that the sputtered particles originate only from the topmost atomic layers. At steady state, there is a balance of the incoming and outgoing flux of m so that c, = a,-‘(1
- ?J).
(5)
In reality, sputtered atoms originate within a certain escape depth of the sample. c, and u, should be replaced by the escape function weighted averages [32]. Unfortunately both u, and the escape function are in
effects in
SIMS
analysis
general difficult to measure. According to eq. (5) the concentration of the implanted atoms on the surface should be self-limiting and independent of the beam current density. Ideally this mode of operation is simple and highly reproducible. To change c,, we can make use of the fact that u,,, depends on the energy and angle of the primary ion beam. For example, c, can be increased if the incident beam is more normal to the sample surface. The second common mode of operation is to introduce oxygen or cesium by an oxygen jet or a Cs effusion source while an Ar+ beam is used for sputtering. The rate of incorporation of m on the surface is s,,,pJ (2mMkT)“’ where pm, M and T are the partial pressure, mass and temperature of the gaseous species m is the sticking coefficient of m on the ion [331. sin bombarded surface. If the primary ion beam is not being scanned, the steady state c, is given by the flux balance, c, = s,,,pm/om jP(2vMkT)1’2.
(6)
According to eq. (6) c, can easily be adjusted by changing pm within experimental limits and steady state can be reached rapidly while c, is self-limited in implantation. However c, is sensitive to the tuning of the primary ion beam through the dependence on jr. In actual analysis, the primary beam is usually well focussed and scanned to provide a flat bottomed crater. The secondary ions is either electronically gated or apertured to discriminate against contributions from crater edges. Eq. (5) is still valid but eq. (6) is only qualitatively useful since no dynamic equilibrium would be established. The surface concentration c, is now described by dc,/dt
= s,p,/(2?rMkT)“2
- c,,,q,, jP( t).
(7)
To see the salient features, we can approximate the scanning cycle by a very short “beam on” t,, period where the sputtering term c,,,u,,,j,, dominates and c,(t)
= c,(O)
exp( -j,u&).
(8)
We also assume that the overall oxygen enhancement depends on the time averaged oxygen concentration on the surface during the sputtering period t,,
(L(t)>“” = $!I = &
s,I””exp(
-jr,umt)
dt
[I - exp( -.&J,,)]~
Then it is followed by a long “beam off” period t,rr where the sputtering term is zero and c, builds up by adsorption. Let us examine the various limiting cases.
155
M. L. Yu / Chemrcal enhuncement effects in SIMS anabsis
For fast adsorption,
e.g., large srnprnr
For slow adsorption, e.g., small, SOPS, and if the adsorbed oxygen is removed by sputtering every cycle, c,,(O) can be approximated by as,p,t,rr. Here (Y is a proportionality constant. Then
Hence the degree of oxygen enhancement not only depends on the oxygen partial pressure and sticking coefficient, it depends on the scanning speed and the current density of the primary ion beam in general. The secondary ion signals do not usually scale with i,. 7 hese are important considerations in optimizing the experimental conditions for sensitivity and when data from different instruments are compared. Up to now we have not considered any surface or bulk diffusion of the incorporated oxygen or cesium atoms. Oxygen is able to form a strong oxide bond and diffusion is rarely a problem. Cs which forms only weak metallic bond may cause some concern. For example it has been reported that oxygen contamination can cause the pile-up of implanted Cs near the surface in the case of a Si target [34]. But due to the lack of knowledge on these diffusion process a rigorous treatment is not available at present. It should contribute like an additional adsorption (or desorption) term and can be checked by going to different scanning rates when using a Cs+ beam. The effect of such diffusion processes in SIMS analysis have not been fully addressed. 3.2. The oxygen induced enhancement
Z(A+)=Z,A[A]+Z;[B],
(13)
IO
effect
According to the bond-breaking model, the oxygen enhanced ion yield is linearly related to the distribution f, of various oxide species as shown in eq. (2). In the dynamic sputtering condition, there is usually a wide range of oxides formed. As a result the ion yield is rarely found to be linear with the oxygen coverage. For example let us assume a completely random distribution of the oxygen atoms in the target with a range of oxides formed. As the oxygen concentration increases, the distribution would skew towards higher oxygen coordination per substrate atom. Following Oechsner and Sroubek [35], if the ion yield enhancement factor R per oxygen neighbor is constant, the averaged ionization probability Pt is related to the oxygen concentration c, by: P’-=P,+[c,(R-l)+l]N
oxygen neighbors permissible by the chemical system. Hence a very nonlinear dependence on c, is obtained. In general the distribution of the various oxides formed depends on the experimental conditions thus producing the diverse functional dependences on the oxygen concentration. Even for the well studied case of Si+ emission from silicon surfaces in the presence of oxygen, the reported oxygen coverage dependences ranged from linear, at very low oxygen coverages (I 3 at.%) [16], to power law dependences [16,36,37] to exponential dependences [38,39]. The major difficulty with these experiments is that there is still no easy way, even with electron spectroscopies, to determine the exact chemical composition of the oxidized layer from which the secondary ions originate under dynamic sputtering conditions. The dependence of the oxygen effect on the nature of the oxide bonding is important in the SIMS analysis of alloys. Pivin et al. [40] studied the secondary ion emission from three binary alloys (Fe-Ni, Fe-Cr and Cr-Ni), in the presence of an 0, jet. They observed that the oxygen enhanced ion yield is affected by the presence of the second alloy component. They found that the ion yield Z(A+) of A for alloy A-B is linearly related to the atomic concentrations [A] and [B]:
(12)
where Pa+ is the ionization probability at zero oxygen concentration and N is the maximum number of nearest
-A 0
20
A
A/
40
/
60
00
Ni
at. % Ni
Fig. 3. The ionization probability P+ of Ni as a function of the composition in the Cr-Ni. Fe-Ni, and Cu-Ni alloys, normalized to the P+ value of pure Ni. V. SIMS AND SPUTTERED
NEUTRALS
156
M. L. Yu / Chemical enhancement
where I,” and I: are the ionization yields of A+ ions per unit concentration respectively for a pure A sample and a sample of nearly pure B containing A as a dilute impurity. These linear relations again suggest that the effect of B on the emission of A+ is local. To understand these alloy effects more fully, Yu and Reuter [41] have studied the alloy effects for six different alloys, using both 0: bombardment and Art bombardment with 0, adsorption. They also monitored the oxide chemistry on the sample surfaces with in situ XPS measurements. They did not observe simple linear relations with the alloy composition as reported by Pivin et al. For a reasonably large sticking coefficient for oxygen, oxidation can proceed practically to completion by using higher 0, partial pressure in the 0, jet. But with 0: bombardment oxidation is self-limiting, depending on a,,, of the oxygen atoms on the material. This was indeed verified by XPS measurements. The general rule for alloys is as follows: For an alloy A-B where A forms a stronger oxide bond than B, the presence of A enhances the emission of B+ while the presence of B suppresses the emission of A+. Fig. 3 shows the data from three different Ni alloys: CuNi, Fe-Ni and Cr-Ni under 0; bombardment. The presence of Cr and Fe significantly enhances the Ni+ ionization probability while the presence of Cu suppresses it. Since the oxide bond energy per metal atom is 15.5 eV, 12.5 eV, 9.5 eV and 5.7 eV for Cr, Fe, Ni and Cu respectively, the general rule is obeyed. In situ XPS measurements, indicated that the large alloy effect on Nit was not from a change in the oxygen concentration. In fact the interaction between Fe,O, and NiO, for example, strengthens the nickel-oxygen bond as in ferrite. XPS spectra of the 0: bombarded Fe-Ni alloy show an increase in the binding energy of the Ni 2p,,z peak. This illustrates again the sensitivity of the ionization probability to the state of the oxide bond. It is also found that the general rule works best when the sample surfaces are heavily oxidized. For alloys that have a high sputtering cross-section a,,, such that oxidation is far from complete, deviations can occur. In such cases, the use of an 0, jet with high pm can correct the situation. It has been observed that the secondary ion energy distribution in general sharpens with stronger oxidation (e.g. chemisorption to oxide formation) [41]. This is consistent with the discussion in sect. 2.1 that secondary ions emitted with a smaller energy (velocity) are more sensitive to chemical bond changes. Hence the oxygen matrix enhancement effect is larger for secondary ions in the low energy end of the energy distribution. The net effect is that the energy distribution is sharpened and the peak shifts towards the lower energy side. Yu and Reuter [41] also observed that in the case of secondary ion emission from an alloy A-B where A forms a stronger oxide bond than B, the presence of A sharpens
effects in SIMS
ana!ysls
the energy distribution of B+ while the presence of B broadens the energy distribution of A+. This observation is in line with the sharpening of the energy distribution as the oxide bond gets stronger. It should be mentioned that in some cases, e.g. heavily oxidized Ba, a significant amount of sputtered atoms may be taken up to form molecular species, e.g. BaO. The velocity distribution of the atomic species can be distorted by the oxidation [42]. Oxygen incorporation by 0, jet and by oxygen ion beam are complementary. An 0, jet can provide high oxygen coverage by using a high partial pressure but an oxygen primary ion beam is more effective for samples with very low oxygen sticking coefficients, for example germanium and gallium arsenide. It is sometimes useful to use an oxygen primary ion beam with an 0, jet. This technique combines the advantages of the two modes of oxygen introduction. Steady state condition can be reached rapidly and signal enhancement is maximum. The dependence on jp may make the tuning of the primary ion beam more critical than just using 0:. Usually it is advantageous to oxidize the sample completely to achieve the maximum secondary ion yield and sensitivity. However oxidation can in many cases interfere with the analysis, for example, by inducing a large alloying effect. Such alloy effect can cause changes in the SIMS sensitivity at interfaces where there is a rapid variation in the composition [43]. The alloy effect can be minimized by reducing the oxygen concentration. Reuter and Yu have found that this can be achieved by going to a shallower angle of incidence for the 0; beam [44]. The resulting increase in the sputtering coefficient reduces the steady state oxygen concentration on the sample surface. Very recently it was also found that oxidation can cause the segregation of dilute implants during SIMS analysis [45,46]. Again reducing the oxygen coverage by using a more glancing incidence oxygen primary ion beam improves the recovery of the true profile. There have been several systematic studies on the effect of the angle of incidence of an oxygen ion beam. It was found for example for 15 keV 0; incident on Ni [44], the steady state surface oxygen concentration as measured by Auger spectroscopy decreases by an order of magnitude when the angle of incidence increases from 0” (normal) to 70”. The corresponding Ni partial sputter yield also increased by a factor of 14. This strong angular dependence is only partially a result of the angular dependence expected from the collision cascade sputtering theory (set 70” = 3). The major reason is that the oxide formation changes the surface binding energy in sputtering. The incidence angle not only affects the steady state oxygen concentration c,, it also changes the kind of oxides formed. Similar studies have been made on silicon [47-501 and germanium [49,50]. At normal inci-
M. L. Yu / Chemrcul enhancement
dence, silicon is fully oxidized to SiO,, whereas at more oblique angle of incidence silicon is only partially oxidized (suboxides). In contrast, XPS shows that germanium is only weakly oxidized even under normal incidence bombardment without the formation of GeO, [50]. This again correlates well with the much larger sputter coefficient of Ge (greater than four times that of silicon) even at normal incidence. The use of the angle of incidence as a parameter in SIMS analysis is still quite scarce. This is partially due IO the usually fixed beam/sample geometry in SIMS equipment. But still it is a crucial parameter when data f’rom different SIMS facilities are compared. .‘.3. The Cs induced
enhancement
effect
Cs can be introduced either by using a Cs+ primary ion beam or by vapor deposition on the target during sputtering. As discussed in sect. 3.1, the use of a Cs+ primary ion beam is simpler and more reproducible. The reproducibility is especially critical at low Cs coverages where the work function decreases roughly linearly with the increase in coverage while the negative ion ),ield increases exponentially. The major disadvantages ii that the steady state surface Cs coverage is controlled by 0,‘. The limitation can be overcome by Cs vapor deposition. For example it is even possible to build multilayers of Cs on a metallic substrate although coverages on semiconductors may be self-limiting to a monolayer [51]. Cs being quite reactive is permeable to oxygen contamination from the residual gas. The incorporation of oxygen in the Cs layer can cause further lowering of the work function [52,53]. It can also cause the segregation of implanted Cs to the surface [34]. This is similar to the oxygen adsorption induced surface segregation (of the reactive component) in an alloy [54]. Bernheim et al. 121.551 have studied the Cs matrix effect by depositing Cs with a Cs vapor jet during dynamic Ar+ bombardment. They inferred the work function change from the shift of the secondary ion energy distribution. In spite of some concern raised on this method of work function measurement [56]. their result shows a strong correlation between the negative secondary ion yield and the work function change, with segments of their data showing an exponential dependence of the negative ion yield on A@ [8]. Particularly interesting is that as the Cs coverage was increased to obtain the minimum work function, the negative secondary ion yield reached a maximum, similar to observations with static mode sputtering. This point of minimum work function is probably the optimum both for sensitivity and stability against fluctuation in Cs flux. The work function @ varies relatively slowly with C‘s coverage at this point. Also many experimental studies have shown [20,23,31] that the negative ion yield usually deviates from the steep exponential dependence
effects in SIMS
analysis
151
on @ as @ approaches the minimum value. An example has been shown in fig. 2. It has been shown theoretically [28] that the exponential dependence of Y- on @ is only an approximation for small ranges of A@. Such deviation is expected. The higher coverages of Cs in this region probably also contributes to a smaller rate of enhancement by reducing the sputtering yield from the sample. The exponential dependence of the negative ion yield on the electron affinity was also observed in dynamic sputtering conditions [3,4,57], which is consistent with eq. (3). Emission energy (velocity) dependence of Y- has also been observed by Wittmaack [58]. Since the chemical bonding between Cs and metallic substrates are much weaker than an oxide bond, Cs is expected not to induce a large alloying effect. Bernheim et al. [21.55,57] have studied the negative secondary ion emission from binary alloys. By exposing the alloy surfaces to a jet Cs vapor they found that the maximum Cs enhanced negative ion yields are simply porportional to the concentration in the alloys. Also it is of interest to note that Cs also enhances negative secondary molecular ions and suppresses positive ions [58.59]. Hence Cs can cause a significant alternation in the molecular fingerprint spectrum of a compound without affecting the stoichiometric composition of the substrate. The use of Cs vapor has an advantage over the use of a Csf beam. With a Cs+ beam, the Cs coverage may be limited [eq. (2)] to a value where the enhancement factor is very sensitive to changes in the work function. This is important when there is a variation in the sputtering coefficient in the sample either through a gradual change in the alloy composition or when going through an interface. A related effect has been observed when analyzing the interfacial oxygen between amorphous and crystalline silicon [60]. With a Cs vapor jet, the cesium coverage can be adjusted to the minimum work function where both sensitivity and stability are maximum.
4. Conclusions There has been significant progress in delineating the physics of secondary ion emission in the last five years. In spite of the fact that our basic understanding of the phenomena has to rely on data obtained from static mode sputtering, our knowledge can easily be applied to situations in SIMS analysis. We also learn that the quantitation of SIMS through theoretical treatment is at present impractical since many required detailed material and experimental parameters are hard to obtain or not within our reach in the near future. Nevertheless SIMS has already acquired a strong enough basis to support analytical applications and further development. V. SIMS
AND
SPUTTERED
NEUTRALS
M. L. Yu / Chemical enhancement effects in SIMS analyxs
158
The author is delighted to acknowledge the collaborations with W. Reuter, N. Lang, K. Mann and J. Clabes, and the very useful discussions with K. Wittmaack. The research was partially supported by the Office of Naval Research.
References
VI K. Wittmaack, Vacuum 34 (1983) 119. PI A. Benninghoven, Surf. Sci. 53 (1975) 596. [31 C.A. Andersen, Int. J. Mass Spectrom. Ion Phys. 2 (1969) 61 and 3 (1970) 413.
[41 V.R. Deline, CA. Evans, Jr., and P. Williams, Appl. Phys. Lett. 33 (1978) 578.
[51 K. Wittmaack, Appl. Surf. Sci. 9 (1981) 315. [61 P. Williams, Surf. Sci. 90 (1979) 588. (71 G. Blaise and A. Nourtier. Surf. Sci. 90 (1979) 495. PI G. Slodzian, Phys. Scripta T6 (1983) 54. 191 M.L. Yu and N.D. Lang, Nucl. Instr. and Meth.
B14 (1986) in press. Springer Series in [lOI P. Sigmund, SIMS IV Conference, Chemical Physica, eds., Benninghoven, Okano, Shimizu and Werner vol. 36 (1984) p. 2. IllI K. Mann and M.L. Yu, presented at SIMS V, Washington DC (1985), to be published. [=I M.L. Yu, D. Vitkavage and J. Clabes, J. Vat. Sci. Technol. A3 (1985) 1316. 1131 J.B. Delos, Rev. Mod. Phys. 53 (1981) 287. [I41 P.J. Martin, A.R. Bayly, R.J. MacDonald, N.H. Tolk, G.J. Clark and J.C. Kelly, Surf. Sci. 60 (1976) 349. [I51 W. Gerhard and C. Plog, Z. Phys. B54 (1983) 59 and 71; Surf. Sci. 152/153 (1985) 127. [I61 K. Wittmaack, Surf. Sci. 112 (1981) 168. P71 G. Blaise, Surf. Sci. 60 (1976) 65. [I81 G.E. Thomas, Radiat. Eff. 31 (1977) 185. 1191 M.L. Yu, Phys. Scripta T6 (1983) 67. WI M.L. Yu, Phys. Rev. Lett. 40 (1978) 574. PI M. Bernheim and G. Slodzian, J. Micros. Spectrosc. Electron 6 (1981) 141. PI M.L. Yu, Phys. Rev. B24 (1981) 1147. ~31 M.L. Yu, Phys. Rev. B26 (1982) 4731. 1241 M.L. Yu, Phys. Rev. B29 (1984) 2311. PI A. Blandin, A. Nourtier and D.W. Hone, J. Phys. 37 (1976) 369. [26] J.K. Norskov and B.I. Lundqvist, Phys. Rev. B19 (1979) 5661. [27] R. Brako and D.M. Newns, Surf. Sci. 108 (1981) 253. [28] N.D. Lang, Phys. Rev. B27 (1983) 2019. [29] N.D. Lang and J.K. Norskov, Phys. Scripta T6 (1983) 15. [30] M.L. Yu and N.D. Lang, Phys. Rev. Lett. 50 (1983) 127. [31] M.L. Yu, Phys. Rev. Lett. 47 (1981) 1325.
[32] K. Wittmaack, Nucl. Instr. and Meth. B7/8 (1985) 779. [33] G. Blaise and M. Bernheim, Surf. Sci. 47 (1975) 324. [34] N. Menzel and K. Wittmaack, Nucl. Instr. and Meth. 191 (1981) 235. [351 H. Oechsner and Z. Sroubek, Surf. Sci. 127 (1983) 10. [361 V.R. Deline, W. Katz, C.A. Evans, Jr., and P. Williams, 4ppl. Phys. Lett. 33 (1978) 832. [371 H. Gnaser, Nucl. Instr. and Meth. 218 (1983) 312. H.W. [381 A.E. Morgan, H.A.M. de Grefte, N. Warmoltz, Werner and H.J. Tolle, Appl. Surf. Sci. 7 (1981) 372. [391 P. Sanda, V. Kaiser, R. Jede, D. Lipinsky, 0. Ganschow and A. Benninghoven, J. Vat. Sci. Technol. A3 (1985) 1946. and G. Slodzian, Int. J. [401 J.C. Pivin, C. Roques-Carmes Mass. Spectrom. Ion. Phys. 26 (1978) 219. (411 M.L. Yu and W. Reuter, J. Appl. Phys. 52 (1981) 1478 and 1489. M.L. Yu and A.C. Balant, Surf. Sci., ~421 D. Grischkowsky, 127 (1983) 315. [431 M.L. Yu and W. Reuter, J. Vat. Sci. Technol. 17 (1980) 36. [441 W. Reuter and M.L. Yu, J. Appl. Phys. 53 (1982) 3784. [451 V.R. Deline, W. Reuter and R. Kelly, Proc. Materials Research Society Meeting, San Francisco (1985) to be published. [461 S.M. Hues and P. Williams these Proceedings (IBA ‘85) Nucl. Instr. and Meth. B15 (1986) 206; W. Vandervorst, M.L. Swanson, H.H. Plattner, O.M. Westcott, I.V. Mitchell and R.R. Shepherd, ibid. p. 201. [471 N. Warmoltz, H.W. Werner and A.E. Morgan, Surf. Interf. Anal. 2 (1980) 47. [481 W. Reuter and K. Wittmaack, Appl. Surf. Sci. 5 (1980) 221. I491 K. Wittmaack, Nucl. Instr. and Meth. 218 (1983) 307. [501 W. Reuter, these Proceedings (IBA ‘85) Nucl. Instr. and Meth. B15 (1986) 173. [511 P. Wagner, K. Miller and K. Heinz, Surf. Sci. 68 (1977) 189. [521 B. Goldstein, Surf. Sci. 35 (1973) 227. Surf. Sci. 92 [531 J.L. Desplat and C.A. Papageorgepoulos, (1980) 97. [54] CR. Brundle. E. Silverman and R.J. Madix, J. Vat. Sci. Technol. 16 (1979) 474. [55] M. Bernheim and G. Slodzian, J. de Phys. (Lettres), 38L (1977) 325, and SIMS III Conference, Springer Series in Chemical Physics vol. 19 (1982) p. 151. [56] K. Wittmaack, Phys. Scripta T6 (1983) 71. [57] M. Bernheim, J. Rebiere and G. Slodzian, SIMS II Conference, Springer Series in Chemical Physics, vol. 9 (1979) p. 40. [58] K. Wittmaack, Surf. Sci. 126 (1983) 573. [59] M.L. Yu, Phys. Rev. B24 (1981) 5624. [60] K. Wittmaack, Nucl. Instr. and Meth. B7/8 (1985) 750.