=

=

at higher energies and L1L2,3V transitions at the low energy side of the spectrum. Detailed analysis and identification of the different transitions is difficult due to the complexity of the silicon valence band. However, some of the characteristic features of the valence band are reflected in the LVV series of Auger transitions. Using Kane's density of state calculations [9], by assigning the binding energies V1 3 eV, V2 4 eV, V3 = 7 eV, and V4 = 10 eV to the four major peaks in the density of states, and using the binding energies EL1 148.7 eV and EL2.3 99.2 eV, we are able to give a crude identification of the features of LVV Auger spectrum of Si. This is the same approach as that of Maguire [10]. The major peak in Figure 2b, at 88 eV, is due to L2,3V1V1 transitions, while the minor peak is due to L2,3V4V4 transitions at 72 eV. The shoulder

=

=

between these peaks is largely L2,3V3V3 transitions at 79 eV. The two small pieces of structure on the low energy side are

L1L2,31,2 transitions at ~ 41 eV and L1L2,3V3,4 transitions at ~ 35 eV. The details of the LVV spectrum are amplified by differentiation w.r.t. energy as shown in Figure 2a.

In Figure 3, we show a series of Si LVV spectra of silicon oxides (a), and oxygen KLL spectra (b) for the corresponding compounds. The stoichiometry of the samples was determined using the amount of oxygen present in the films, as indicated by the peak-to-peak heights of Figure 3 (b). The oxygen spectra of Figure 3 (b) indicate that oxygen is present in only one chemical state, namely as a bridge between two silicon atoms [12]. Furthermore, the coordination of oxygen determines the major features of the LVV spectrum through the density of states in the valence band of silicon oxides. The two major peaks in the LVV spectra of silicon at 64 eV and 78 eV are associated with transitions from bonding and non-bonding molecular orbitals of the Si-O-Si (Si20) quasi-molecule. The transition probabilities are apparently determined by the oxygen 2p predominance of these orbitals. The peak appearing

at 92 eV is due to the presence of "free" silicon, i.e., silicon coordinated by four nearest silicon neighbors. A small amount

of "free" silicon in silicon oxides is in this case produced by ion sputter cleaning. In fact, 500 A of material had been removed before the AES spectra were taken.

It is important to understand that the ratio of the peak-to-peak heights of the 78 eV and 92 eV peaks in the spectra does not give, directly, the ratio of the concentrations of Si bound to 0 to the concentration of Si bound to Si ("free" Si). Observation of the N (E) spectrum shows that the peaks at 64 eV and 78 eV are very broad whereas the 92 eV peak is narrow, with a very sharp high energy edge. This, of course, results in a large dN negative excursion on the 92 eV peak without the corresponding large number of electrons in the peak. In fact, N (E) spectra corresponding to a condition like that shown in the spectra of Figure 3 will show the 92 eV peak to be only a small, although sharp, step on the high energy side of the large, broad 78 eV peak. This does not, however, prohibit the calibration of the LVV spectrum to determine the relative concentrations of these two states of Si. On the other hand, the KLL spectra give much more obvious and direct information about the relative concentrations of the different chemical states of Si.

The local atomic arrangement around the silicon atoms is shown by the Si KLL spectra in Figure 4, for silicon oxides of varying stoichiometry. = For X 2 (SiO2) there appears a well defined KL2,3L2,3 peak at 1611 eV, associated with Si-784) "coordination. For X 1.8 there appears a small shoulder above 1611 eV, and for X ♬ 1.3 a broad peak occurs at 1616 eV. This peak may be associated with silicon tetrahedra of the type Si (Siy 04-y), where y 0, 4. However, upon electron exposure a single peak emerges at 1618 eV corresponding to Si - (Si4) tetrahedra (or "free" silicon).

Silicon Nitride

In Figure 5a and b, we show silicon nitride Auger spectra in the energy range between 25 eV and 525 eV. Figure 5a is the spectrum from a sample exposed to atmosphere. Note the presence of carbon and oxygen KLL transitions, the large silicon nitride LVV peak at 85 eV, and the small silicon oxide peak at 64 eV. Removal of carbon and oxygen by ion-sputter etching results in the clean silicon nitride spectrum shown in Figure 5b. It is interesting to note that oxygen is associated with silicon oxide in the

contaminated surface, and that removal of silicon oxide results in a "free" silicon peak at 92 eV, while there is no observable change in the 382 eV N KLL peak. This indicates that nitrogen exists only in one chemical state associated with a silicon nitride quasi-molecule (Si3N), where the Si-N bonds have less ionic character than the Si-0 bond. This observation is confirmed by the Si KLL spectrum of the nitride which has a well defined peak at 1618 eV, apparently identical to the "free" silicon Si-(Si4) coordination, indicating a negligible charge transfer from Si to N in silicon nitride. We are not aware of X-ray photoemission spectra of Si3N4 or valence band structure calculations so an analysis beyond this mere descriptive one is not pursued here.

We have shown above that the Si3N4 LVV spectrum is strongly influenced by oxygen adsorbed on the surface. Oxygen adsorption completely removed the 92 eV "free" silicon peak. This phenomena suggests that amorphous silicon nitride is a microscopic mixture of Si and Si3N4•

ELECTRON STIMULATED DESORPTION EFFECTS

We have already mentioned in the previous section that the stoichiometry of silicon oxides may be changed by electron exposure. The effect is to reduce the oxygen content in the sample. This phenomenon is commonly known as electron stimulated desorption (ESD), and is thoroughly reviewed in the literature [13]. We have investigated electron beam exposure effects on Auger spectra of Si0x (1.2 < X ≤ 2) and Si3N4 films on Si (100) single crystals. Si3N4 appears to be stable to electron exposure at energies less than 2 keV. Apart from desorption of surface contaminants (carbon and oxygen) the LVV and KLL spectra appear unchanged upon prolonged exposure. Silicon oxides on the other hand are more volatile, and we will restrict our discussion to these compounds.

Si02 is known to decompose under electron irradiation. It has been shown that oxygen desorption from SiO2 is accompanied by the formation of "free" silicon [14]. Our findings are similar for Si0. However, where the stoichiometry of silicon oxide films is Siox with 0 < x < 2, this effect is much more pronounced. In Figure 6a and b, we show the growth of the 92 eV Si LVV peak as a function of exposure time for Si01.4 (a) and SiO2 (b), the parameter is the electron flux density, and the primary electron energy is 2 keV. There appears to be a linear increase at small exposure times followed by a saturation region. The linear slope and

In Figure 8, we show a typical depth profile through a 1080 A thick layer of Si0x (X = 1.4) vacuum deposited on Si(100). Only the initial and the final part of this particular profile are shown. The initial part of the curve shows how adsorbed carbon is removed, and how a balance between bound oxygen and "free" silicon is reached by oxygen removal. The saturation level gives a measure of the stoichiometry of the sample. The final part of the profile shows the interface between film and substrate, and the finite transition region between the two. In this case, the width is 29 Å (defined as distance between 80% and 20% of the oxygen peak-to-peak height). The question is now what factors control and determine the observed width or depth resolution. A recent paper by Ishitani et.al. [15] deals with these problems in model calculations of knock-on atomic mixing. They find a depth resolution decreasing with increasing ion energy, and increasing with incidence angle of the ion beam between 0° and 60°. An experimental investigation of Ta205 on Ta [16] has shown a minimum transition width of

~ 60-70 Å for ion energies between 6 and 7 deV. We have employed A+ ions at 3 keV of varying current density. In Figure 9, we show the transition with AY as a function of ion power density for silicon oxides and silicon nitride on Si(100). The general trend is that AY decreases with increasing

power density for both compounds, towards a broad minimum or saturation level of the order of 30 to 40 A. This value is roughly 50% of that reported for Ta205 on Ta [16]. Figure 9 also shows that the nitride interface is sharper than the oxide interface at low power densities. We find that the sputtering yield for silicon nitride is 0.5 molecule/ion, and for silicon oxide ~ 1.5 molecule/ion, and an ion energy of 3 keV. We find our experimental results in qualitative agreement with the knock-on mixing model [15], when we take into account the lower ion energy and the angle of incidence we have used. The lower limit of 30 - 40 Å interface width may be reduced even further by reducing the ion energy. However, other factors such as surface roughness, preferential sputtering, and inhomogeneities may set an upper limit to the depth resolution. The ultimate limit is set by the escape depth of the Auger electrons, which will be discussed in the next paper. CONCLUSION

The results presented in the preceeding sections may be summarized in the following conclusions. The electronic structure of silicon compounds is largely determined by the molecular bonds and the local atomic arrangement. Thus, in silicon oxides it is the Si-0 bond and in silicon nitride it is the Si-N bond that gives the major contribution to the electronic structure. The Si-0 bond energy is larger than Si-Si bond*. The charge transfer associated with the Si-0 bond gives rise to a chemical shift in the binding energy of the Si L2,3 level of 3.9 eV [18]. This gives rise to a shift in the Si KLL transition energy of 7 eV from Si-(S14) coordination to Si-(04) coordination [18]. We have observed varying amounts of these two states in electron irradiated Si0x films, X ≤ 2. Similar chemical shifts have not been observed in silicon nitride. We believe that this is due to a much smaller charge transfer associated with the Si-N bond.

Electron distortion of silicon oxides by dissociation and desorption of oxygen can be reduced to a negligible level by keeping the total electron dose below a limit of 1019 el/cm2 at 2 keV electron energy. Silicon nitride is stable to electron irradiation within the experimental conditions of this investigation. Why this is so is not fully understood. It may be associated with the higher ionization potential of nitrogen compared with oxygen.

We have observed knock-on mixing of elements caused by energetic ions during depth