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Submitted on 1 Jan 1995
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Surface Potential Control on Thin Oxide Films with Respect to Electron Stimulated Desorption Studies
Marc Bernheim, Gilles Rousse
To cite this version:
Marc Bernheim, Gilles Rousse. Surface Potential Control on Thin Oxide Films with Respect to Electron Stimulated Desorption Studies. Journal de Physique III, EDP Sciences, 1995, 5 (9), pp.1407- 1424. �10.1051/jp3:1995200�. �jpa-00249390�
Classification Physics Abstracts
07.77 07.80 41.80D 79.20K
Surface Potential Control on Thin Oxide Films with Respect to
Electron Stimulated Desorption Studies
Marc Bemheim and Gilles Rousse
Laboratoire de Physique des Solides, A8soc16 au CNRS, Bht.510 Urdversit6 Paris-Sud, 91405
Orsay, France
(Received 30 March 1995, accepted 19 June 1995)
R4sum4. Des collisions dlectroniques sur des surfaces peuvent provoquer une 4jection d'ions
ndgatifs formds h partir des mo14cules adsorb4es suivant un processus rdsonnant h trks basse
dnergie. Pour 4tendre
ces 4tudes exp4rimentales aux surfaces isolantes, un contr61e prdcis du po- tentiel superficiel devient indispensable tant pour fixer l'4nergie finale des 41ectrons sur la surface que pour effectuer la spectrom6trie des ions d4sorb4s. L'examen de surfaces de silice en couche mince montre comment une conduction tunnel intervient pour fixer le potentiel des surfaces ex- arnin4es. Ces travaux exp4rimentaux effectu4s dans une configuration de miroir 41ectrostatique reposent principalement sur l'enregistrement des variations de l'intensitd des 41ectrons transmis
au travers de l'oxyde et de celle des 41ectrons r4trodiffusds par la surface lorsque les potentiels
appliqu4s sont modifi4s. La comparaison des distributions dnerg4tiques des ions O~ 4mis par l'oxyde fournit un contr61e direct de ces rdsultats. Ces ions r4sultent d'une d4composition de la silice provoqu4e par le bombardement 41ectronique. Mars notre montage n'a nullement per-
mis de reproduire les variations rapides de rendement de "d4sorption" au voisinage des seuils d'excitation Auger du silicium rapport4s par d'autres auteurs. A notre avis, diff4rents artefacts
exp4r1nlentaux ont tr4s vraisemblablement perturb4 leurs investigations.
Abstract. These experiments deal with the study of desorption of negative ions stimulated by low energy electron collisions on insulating surfaces covered with various adsorbates. For
such investigation a careful control of the sample surface potential is required to set the incident electron energy accurately as well
as to identify the desorbed species by mass spectrometry.
Most of the reported experiments concern this surface potential control. We show that, for very thin Si02 films thermally grown on silicon substrates, a tunnel conduction might set the surface potential accurately. This assertion mainly relies on recording the intensity of electrons
transmitted through thin oxides as well as the intensity of backscattered and secondary electrons re-emitted from the surface. A direct comparison of the O~ ion energy distributions confirms
the correct control of the surface potential for a large range of incident electron energies. In
such condition we noticed that the O~ desorption yields just swiftly varied with incident electron energy. In particular no modification could be detected
as the incident electron energy passed the various Auger excitation levels. The discrepancy between this last result and the published
data is discussed in the last part of this paper.
Q Les Editions de Physique 1995
1. Introduction
Electron impact on surfaces may cause the ejection ofions and neutrals from adsorbed molecules
according to various DIET (Desorption Induced by Electronic Transition) processes iii. The incident electron energy is kept above 100 eV in most of the experimental studies. However very low energy collisions (I.e. below 12 eV) may already trigger intense desorptions of negative
ions provided the adsorbed elements and radicals have electron affinities. Here a bonding-anti- bonding transition (valence-level excitation) gives rise to a resonant desorption process very
similar to the dissociative electron attachments well known for gas phase collisions [2].
Up to now these studies at very low energy essentially concerned physisorbed molecules with weak molecule surface couplings [3-5]. Obviously the same resonant desorption process works also for directly chemisorbed molecules. But now the initial dissociation (dissociation of either intra-molecular bonds or bonds to the substrate) may be followed by additional electron transfer to the surface leading to a significant neutralisation of the desorbed species. Presumably the ion survival probability is influenced by the surface work function, the ion initial velocity and the electron affinity.
A comparison of negative ion yields for various molecule species chemisorbed on different metallic crystals supports such assumption directly [6,7]. Indeed, following a water vapour
chemisorption, the measured desorption yield stays about 3 orders of magnitude higher for ions H~ than for O~ and OH~ ions whereas consideration of the electron affinities alone [8]
suggest just the opposite behaviour. Ejected with similar energies but consequently with higher velocities, the H~ light ions experience a much lower neutralisation rate than more massive O~ ions, for instance.
The large variations in H~ desorption yield for (100) Al and Ni crystal saturated with water molecules were mainly attribuated to difference in surface work functions. Indeed, for both samples the adsorption was followed by similar changes in surface work function. So we consequently assumed that the surface concentrations of water molecules and hydroxyl radicals
were comparable(~).
Variation in molecule or atom distance to the surface may also affect the ion survival proba- bility so that presumably the neutralisation rate for O~ ions should be much higher for oxygen
atoms dissociatively chemisorbed on a surface than for CO molecules coupled via a carbon-
metal bond and standing normal to the surface.
Studying insulating substrates covered with various adsorbed molecules may provide a new
way for reduction of the neutralisation process leading to increase the negative ion desorption yields. This was our main motivation to start investigations on insulating surfaces.
Let us add some other incentives.
. Electron bombardment of oxides may cause depletion in oxygen surface concentration [9]
followed by progressive degradation of surface electrical properties. Presumably a direct
study of such surface modifications for various insulating materials might help to increase the long term stability of MOS components exposed to particle bombardment.
. Insulating surfaces like metal oxides are known as very efficient catalysts. Therefore it
seems very important to investigate desorption processes stimulated for instance by the
(~) Water vapour can't chemisorb on clean nickel crystal surfaces, but reacts with oxygen superficial impurities [28]. Our investigated surfaces were sputtered by a mass filtered argon ion beam. Such preparation leaves the surface free of oxygen (control by secondary ion emission), well ordered (control by Ion Scattering Spectrometry [29,30]). But here the large number of surfaces vacancies could play
a significant role during the fixation of water vapour molecules.
low energy electron impact. Here according to the different ionic character of the sub- strate material [10-12] both chemisorption and desorption processes might be influenced by microscopic electrostatic forces, especially when the adsorbed molecules have dipole
moments. Here we particularly plan to compare resonant effects affecting the High Reso-
lution Electron Energy Loss Spectrometry (HREELS) [13] with possible resonant negative
ion desorption processes.
Obviously for such studies the insulating nature of the sample implies a very careful control of the surface potential both for the incident electron energy adjustment and for the analysis of desorbed ion beams. The main goal of the experiments reported here was the preliminary but necessary study about the surface potential control. We chose to start with studying
well controlled thin Si02 films thermally grown on (100) silicon wafers with a low impurity
concentration (3 Q cm) [14].
2. Experimental Set-up and Experimental Conditions
The experimental set-up is an ion emission microscope equipped with an ultra high vacuum target chamber(~ and an energy dispersive mass spectrometer [15] in which a coaxial "electron
gun", a set of correction magnetic prisms and a transfer optics were added recently.
The coaxial electron gun is nearly identical to the system developed for regulating the surface potential of insulating samples during analysis by negative secondary ion mass spectrometry [16,17],
Let us first summarize the working conditions for conducting samples with a well defined surface potential. The surface to be studied serves as the cathode of an immersion objective lens where two beams (incident electron and desorbed ion beams) pass along the main optical
axis but in opposite directions (See Fig. 1).
Electron Beam
The optical system works almost as an electrostatic mirror. Let I§
= -4000 V be the con- stant potential of the electron source filament, l~ the variable negative voltage applied to the
substrate and Wf and Ws the filament and sample work function respectively. The electrons reach the sample surface with the final kinetic energy [6]:
ib = e(I~ l§) + Wf Ws in electron volts, neglecting the electron initial energy.
When an image (50 pm in diameter) of the electron source filament is projected on the center of the aperture stop di set at the image focal plane of the objective lens Li, the electron beam reaches the cathode under normal mean incidence and covers a final area (about 300 pm in diameter) exclusively controlled by the aperture angle independently of the electron final energy. As an outstanding consequence, the electron beam density remains really constant for any final electron energy which can varied between a few tenths and several tens of electron- volts. Let us add that the usual perturbations by space charge effects and residual magnetic fields get also minimized because the electron deceleration occurs just in front of the sample
surface.
Ion Optics
Since the ion beam gets slightly deflected passing through the electron injection magnetic prism,
a double magnetic correction was added to re-align the ion beam with the main optical axis.
Once being adjusted, this correction works for any ion mass in a wide range of energies. An
(~) A 15 K cryogenic trap set around the investigated surface induced a local reduction of the residual pressure (5x10~~° torr).
~
10 V Electron Source
Vi " -4oo0 v
Lo Condenser
Illjeetion Maglletie Prism
Sample
Vi
j I.
Lj d
j negathe
~ *~ ~°°'~ ~2 ~
2
~ Immersion Objective Lens
~ ~ ~
R~emitted Electrons
Fig. 1. Incident electron beam optics. A variation of repulsive voltage v = 1~ i§ changes the final electron energy but keeps very constant the bombardment density provided an image of the electron
source is focused on the centre of stop di (image focal plane of the immersion objective lens). The
correction magnetic prisms [6] are not shown here. The fixed high voltage supply (Vo = I§ 10 V) is connected to the electron source through a constant additional supply (+10 V).
enlarged image of the sample surface is focussed by the transfer lens on a field stop D. Besides the combined effects of both di and d2 aperture stops are to define well controlled angular and energy discriminations for the ions admitted in the energy dispersive mass spectrometer. Over the whole imaged area (125 ~Jm, which is directly fixed by the diameter D), the ion collection
efficiency stays fully constant.
The complete set-up is especially convenient for recording the variations of desorption yield with incident electron energy for any ion species as well as for comparing the energy distribu- tions of ions desorbed by various incident electron energies.
In Figure 1 is also shown how the connection of a fixed negative high voltage supply l§
(-4000 V) to a variable supply v = l~ l§ (-10 <
v < +90 V) controls the sample bias and consequently the electron final energy. Since the v voltage modification also influences the final ion energy, the spectrometer settings (magnetic and electrostatic prisms) should be
continuously adjusted by using a watchful computer control [6]. Let us add that all yield
measurements were performed without any assumptions about the ion initial energies.
Intensities in VA
0.40
0.35
~~"~~~
0.30
0.25
1.
0.20
o.15
o.io
lr
5.oo o.oo 5,oo io.oo
V,-V, In Volts
Fig. 2. Intensities of the collected is and re-enlitted ir electron beams versus increasing difference in applied potentials Vs I§. The surface oxide from Si (100) sample was eliminated by classical
treatment (5% HF solution in ethyl alcohol) just before introduction into the UHV chamber.
A specifically built nano-ammeter was also inserted in the sample high voltage circuit to record the variation of electron intensity is trapped by the conducting sample (or eventually by the underlying conducting substrate when thin oxides are investigated). A Faraday cup
was also used to measure the intensity ir of backscattered and secondary electrons leaving the
surface.
3. Electron Capture on Various Surfaces
Initially, for conducting samples, the measurements of is and ir intensity were made for an in situ determination of the surface work function so that the incident electron energy could be calibrated accurately [6]. This determination is based on the retarded potential method
being especially reliable due to the constant size and to the normal incidence angle of the electron beam. Hereafter it will be shown how the very same intensity measurements may also determine the surface potential for thin insulating samples and may give information about the electron absorption and reflection as well as about the ejection of secondary electron at the
sample surface.
3.I. CONDUCTING SAMPLES. Figure 2 gives an example of ir and is intensity recordings
for the (100) silicon crystal:
. For negative difference in applied potential, I~ I§ < -2 V, the entire electron beam is reflected on the equipotential l§ without any direct interaction with the sample surface;
therefore after passing back the objective lens, aperture stop di and injection magnetic prism, the beam is collected in the Faraday cup so that the measured intensity ir just
corresponds to the constant incident beam current io.
. When the negative repulsive potential is reduced about l~ I§ > -2V(~), is increases and simultaneously ir decreases progressively as soon as the electrons get collected by the sample. Due to the energy dispersion of the incident beam, the first electrons collected by the sample surface are the electrons emitted with the highest initial energies. Indeed
a variable upper limit for the energy of the particles reaching the surface is set by the
objective lens acting as a high-pass energy filter. Besides, even at very low energy, the incident beam might partly be backscattered by the surface. According to intensity
conservation io
" is + ir(~) with ir = air (where factor o depends on both the substrate
nature and crystal orientation, on the nature and surface concentration of adsorbed
species and on the electron energy). For -2 < l~ l§ < 0, the derivation of either is or ir variations provides the energy distribution of the incident electron beam. The rather large energy spread about 1.3 eV found here may be attributed to mutual longitudinal
interactions between electrons, "Boersch effect" [18], because of the low electron energy in the whole optical system.
Afterwards, as I~ l§ and consequently the incident electron energy further increase, the ejection of secondary electrons grows progressively so that is intensity decreases and changes
the sign as soon as the secondary emission coefficient 7e reaches unity (ib > 90 eV for a clean Si (100) surface). Note that low energy diffraction effects as well as various surface
excitation processes (plasmons excitation, population of unoccupied states just as in inverse
photo-emission studies, etc.) may give rise to complementary fine structures on both is and ir recordings [6,7, 18,20-22].
3.2. THICK INSULATING SAMPLES. Figure 3 summarizes the practical conditions used
during insulating material analyses by erttission of negative ions (SIMS). Let us add that the
same conditions can be used when any primary particles (neutral, photon, electron, ion,..) eject negative particles (electrons or ions) out of an insulating surface:
The surface potential of either small aggregates embedded in a conducting matrix, thin
insulating films or even bulk insulating materials (I.e. thickness up to several millimeters)
can be regulated by an auxiliary electron source accurately. The sample back surface bias is I~ slighly lower than the electron source filament I§ and the auxiliary electron beam should
always be started before primary particles are directed toward the surface.
When the electron beam is on, some electrons get trapped on the surface(~) setting the
sample surface voltage at l§ potential of the electron source precisely and immediately the electron beam is reflected by the surface. As soon as energetic primary particles (ions for
instance) reach the surface, the usual positive charging-up, due to high secondary electron yield ~fj » 1, gets eliminated immediately by the electrons available in the intense swarm
above the surface. These electrons impinge on the surface with an energy sufficiently low to minimize the effect of further secondary electron emission (7e < 1). Once the surface voltage
is returned to I§, the electron-surface interaction disappears until a new primary particle reaches the surface. Therefore, by this self regulating process, the electron beam builds a kind of virtual conducting surface on insulating samples; the surface potential get stabilized with
(~) Value fixed by the difference in work functions and by offset voltages for the electric supplies.
(~) This relation supposes that the aperture stop di does not intercept the re-emitted electrons; this is true when the electrons leave the surface with low ejection angles and low kinetic energies.
(~) The external electrostatic field E = a/£o being 1 V/pm, the charge density ao is about 55 elemental charges per square micrometer. The mean distance between elementary surface charges 0.14 pm is
very large in comparison with the lattice parameters and consequently the adsorption process is not
significantly influenced by the sample surface charge distribution.
Win metallic grid E~
~~ 3
Vs
<
nducting sulating
substrate material
High energy
Incident particles
Fig. 3. Schematic of the conditions used to control the superficial potential of thick insulating sample. The electron source sets the superficial voltage to I§. The back surface of the sample is biased at 1~ voltage, 1~ < VF. A large metallic grid (about 300 ~m step) is usually deposited on the sample surface. This grid made by a double evaporation is either biased at the potential 1~ applied to the
sample holder or rather kept "floating" to reach the potential l§ of the electron source. Obviously the measurements are made on the surface free area.
an accuracy of few tenths of eV largely sufficient to carry out the sample analysis using the negative secondary ion emission. Let us add that the difference between the surface potential l§ and the substrate bias l~ can be low enough to minimize the internal electrostatic field and
consequently the eventual redistribution due to ion drifts.
3.3. THIN INSULATING FILMS. Generally speaking the best conditions for studying electron interactions with insulating samples, ESD for example, should consist in using a double electron beam, one being used for setting the sample surface potential as describded previously and the
other with an ajustable energy for triggering the various excitations and the ion desorption
itself. Since such conditions could not be accomplished easily, we chose in the meanwhile to
investigate thin film samples, taking advantage of a tunnel conduction sufficient to stabilize the surface potential:
For instance Figure 4 shows is and ir variations recorded on a thin silicon oxide (55 I),
which is thick enough to avoid local defects(~). The onset of the growth of intensity is and the
corresponding decay for ir now occur at l~ I§ m +5.5 V, in comparison with -2V for the bare silicon surface. As soon as l~ l§ reaches 8.8 V, most of the incident electrons (80il)
pass through the oxide-Then is starts to decrease progressively until an outstanding steep fall
occurs when the difference in applied potentials reaches 14.5 V; a complementary variation is
also observed for ir.
According to Figure 5 a slight decrease in the oxide thickness (45 I) is followed by a reduction to 3.5 V for the onset of the conduction and by a similar shift for the step transition.
(~)Below 30 h thicknesses, the oxide may exhibit local heterogeneities affecting the conduction process.
