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YIELD OF PARENT AND FRAGMENT IONS SPUTTERED FROM ORGANIC OVERLAYERS BY
LOW-VELOCITY ARGON : VARIATION WITH IMPACT ANGLE AND COVERAGE
W. Szymczak, K. Wittmaack
To cite this version:
W. Szymczak, K. Wittmaack. YIELD OF PARENT AND FRAGMENT IONS SPUTTERED FROM ORGANIC OVERLAYERS BY LOW-VELOCITY ARGON : VARIATION WITH IM- PACT ANGLE AND COVERAGE. Journal de Physique Colloques, 1989, 50 (C2), pp.C2-75-C2-78.
�10.1051/jphyscol:1989214�. �jpa-00229411�
JOURNAL DE PHYSIQUE
Colloque C2, supplgment au n02, Tome 50, fgvrier 1989
Y I E L D OF PARENT AND FRAGMENT I O N S SPUTTERED FROM ORGANIC OVERLAYERS BY LOW- VELOCITY ARGON : VARIATION WITH IMPACT ANGLE AND COVERAGE
W. SZYMCZAK and K. WITTMAACK
GSF, Institut fiir Strahlenschutz, D-8042 Neuherberg, F.R.G.
Resum6 : Pour diverses multicouches organiques deposees sur Au, n o u s a v o n s mesure les rendements d'ions Y+ d'ions parents et d e fragments en fonction d e l'angle d'impact 8 d'ions argon d e 30 keV. Les resultats furent enregistres a p r h s avoir determine la d&pendance laterale d e 1'efficacitC relative d u dbtecteur. P o u r 6' jusqu'a 70" les resultats p!euvent e t r e represent4s par Y * ( H > = cos-"B. L a puissance n r&vBle qu'il existe u n e variation prononcee, avec n allant d e
0,s
h 2, qui depend d ela r i b l e (type d e depi3t et nature). P o u r d e faibles recouvrements et d e s masses d e molecules en dessous 90 u, les valeurs d e n sont p l u s petites
(n < 1 1 . P a r c o n t r e o n trouve n r 2 pour d e s couches e p a i s s e s d e
vitamines 81 et d e s masses > 150 n.
Abstract - For different organic overlayers deposited on gold substrates we have measured secondary ion yields, Y+, of parent and fragment ions as a function of the impact angle
e
of 30 kev ~ r + ions. The data were recorded after determining the lateral dependence of the relative detector efficiency. Fore
up to 70' the data can be approximated by Y+(B) = C O S - ~ ~ . The power n reveals a pronouncedspecies and coverage dependence with n-values ranging from 0.5 to 2. For low coverages and molecular masses below 80 u, the n-values are smallest (n < 1). By contrast n = 2 for thick layers of vitamin 81 and molecular masses > 150 u.
Accurate secondary ion yield measurements over a wide range of impact angles are difficult, specifically near glancing incidence. In time-of-flight systems additional problems arise from the presence of the accelerating grid in front of the target /1/. Another aspect of concern is the angular spread of the secondary ions. In order to obtain reliable data one has to make sure that at all angles of beam incidence the postaccelerated ions hit the active area of the detector. Moreover, the detection efficiency should be independent of the point of secondary ion impact on the active area.
we have implemented a deflection system along the drift tube of our time-of-flight mass spectrometer (Fig. 1). Using two pairs of deflection plates we were able to displace the bunch of secondary ions by 5 25 nun in the entrance plane of the multichannel plate (diameter of the active area 40 nun). secondary ions ejected by a single primary pulse were recorded by means of a signal registration scheme which was deligned for single particle counting in combination with a transient recorder /2/.
In order to derive reliable data we first measured ion yields versus the radial displacement of the beam of secondary ions at the detector entrance. Apertures of different size were mounted in front of grid G2, see Fig. 1. With a small aperture (diameter D = 10 nun) centered to the active detector area, the yield showed a unimodal distribution (Fig. 2, open
circles). This result is in accordance with measurements of the yield of molecular ions of insulin released by 18 keV CS+ bombardment / 3 / . For a postacceleration voltage of 4 kV (Vd = 0) we measured a full width at half maximum (FWHM) W = 15 nun, from which we derive the FWHM of the angular distribution of the secondary ion beam, Wo = W
-
D = 5 nun.Next we placed the small aperture near the rim of the detector area (radial displacement 10 nun). we observed a somewhat asymmetric distribution (Fig. 2, full circles). This asymmetry may be attributed to geometrical effects introduced by the increasing bending of the secondary ion beam. The maximum yield at the rim was found to be more than a factor 8 higher than in the center of the multichannel plate. Measurements with a wide aperture (D =
35 nun) also revealed this drastic decrease in the yield at the central part of the detector (Fig. 3). The effect was found to be particularly significant for molecules containing a relatively large number of constituents, as shown in panel (a) of Fig. 3. For H+ the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989214
JOURNAL DE PHYSIQUE
TRANSIENT
SPECTRA
CHEVRON-
WLSED I O N BEAM
Fig. 1. Schematic illustration of the basic time-of-flight unit. G1 and G2 are grids with 85% transparency (MCP = multichannel plate). The signal registration circuit is set up for single particle counting. 8, is the tilt angle
effect is rather small, cf. panel (b). Note that, within experimental accuracy, the widths of the angular distributions do not depend on the impact angle of the primary ions.
We ascribe the radial dependence of the secondary ion yields seen in Figs. 2 and 3(a) to a severe degradation of the detection efficiency in the central part of the multichannel plate. This degradation is probably due to the large secondary ion
fluence to which the detector was exposed at the initial stage of performance
evaluation of our TOF system, carried out in the analog mode. According to Figs. 2 and 3 this degradation is only evident beyond a certain secondary ion velocity (i-e.
hardly evident for 4 kev H+).
we also observed an azimuthal dependence of the detection efficiency. This effect could be due to different angles of incidence of the secondary ions with respect to the axis of the microchannels (tilt angle 8O with respect to the spectrometer axis).
With our deflection geometry the impact angle of the secondary ions at the detector varied between O0 (normal incidence) and 19' (maximum deflection).
28 si +
0.1
-
30 keV Ar* -- VI Si (etched)
3 a Vt = L keV
.
6-30' -NOMINAL RADIAL DEFLECTION 1 mm I
Fig. 2. Ion yields versus radial displacement of the secondary ion beam, measured with a 10-nun aperture being placed at different locations in front of grid 02, as shown schematically by the insets
The targets used for yield measurements were prepared by the micro syringe technique.
We deposited 6 pl of a 3 x 10-3 molar solution of vitamin B1, dissolved in a 90/10 mixture of ethanol and water, on a 50 run thick Au layer which had been
evaporated on a polished Si substrate. Control samples prepared without the addition of vitamin 61 or without any deliberate deposition were also investigated. Under conditions of well-defined detection efficiency, i.e. in the peaks of Fig. 3, secondary ion yields Y+ of parent and fragment ions were determined as a function of the true impact angle e (Fig. 4). For 8 up to about 70° the data can be
0.6 I I I I
~ ' 1 1 2 2 ~ 1 30 keV Art
- lOML Vitamin 0, -
(gold backing) 0.L -
-
--
ma
0.2-.
V)- m "
-
-
8 -a 0...,. ':'..
,.,. ...
'7 ....,. y..
0 2
0
-20 -10 0 10 M
NOMINAL RADIAL DEFLECTION ( mm I
Fig. 3. The same as fig. 2 but for a large, on-axis aperture (D = 35 mm).
Parameter is the impact angle 9
IMPACT ANGLE 0 ( d e g l
>
n
n no intentional deposition lQML Vitarn~n B, (gold bocking I 1 6 ~
1 2 5 1 0 1 2 5 X)
c a j ' 9
X)ML Vitamin B, (gold backing)
Fig. I . Secondary ion yields of low mass fragments emitted from difEerent layers on a gold backing versus the inverse cosine of the impact angle. The yields are corrected for differences in primary ion transmission through the acceleration grid. The dashed lines represent yield variations corresponding to Y+ = C O S - ~ ~ ,
the power n being indicated on each line (Vt = 4 kV, and Vd = -- 2 kV)
C2-78 JOURNAL DE PHYSIQUE
approximated by ~'(8) o: cos-"8. The exponent n reveals a pronounced species and coverage dependence with n-values in the range 0.5 < n < 2 (Fig. 5). For the C,H& - fragments and the most intense fragment (M = 122 u) emitted from vitamin Bl the power n is more or less constant (n = 1). A cos-l8 - dependence has also been reported for -100 MeV ~ r l ~ + bombardment of a layer of phenaline (M = 165 u) /4/. For a gold surface with a non-intentional, low-density coverage of hydrocarbons the impact-angle dependence of the yield is sensitive ta the number of constituents of the fragment (Figs. 4 and 5). The n-values pass through a mass-dependent minimum located at mass numbers between 40 and 50.
Differences in the yield variation for different surfaces might be attributed to changes in the binding conditions /5/. Deviations from the 'standard' C O S - ~ ~ - dependence could be indicative of a depth dependence of the energy deposition /6/. The observed mass dependence of n, on the other hand, is not understood yet.
0
0 20 LO 60 80 100 200 300
MASS l u l
I I I I I " l I I l
30 keV Ar' -
n no ~ntentional depos~tion m Ethanol
- 10 ML Vitarn~n (gold back~ngl B,
1 .--.-
--
=---
1 --
-i
-- -. \n.-.-9 -
Y a co8"O + c,.H;-
I I I I I O I I I I
Fig. 5. Power n of the initial cos-"8 yield variation for different deposits on a gold backing
Another aspect of the data in Fig. 4 is that the yields of H+ and light hydrocarbon ions exhibit a maximum in the range 80° < 8 < 85'. or possibly even beyond 85'. By contrast, the yields of secondary ions originating from the vitamin B1 layer already arrive at a plateau for 8 = 75O. In order to maximize yields as well as to discriminate against
low-mass fragments it appears to be advisable, therefore, to operate at impact angles between about 60' and 75O.
In conclusion, we have shown that yield measurements as a function of the impact angle can be performed not only in the electronic but also in the nuclear stopping regime.
Investigations on a larger variety of samples and under improved experimental conditions (e.9.. better control of the quality and the lateral uniformity of the adsorbed layer) are necessary in order to substantiate the trends observed in this study.
REFERENCES
/1/ W. szymczak and K. Wittmaack, Secondary Ion Mass Spectrometry, SIMS VI, ed. by
A. Benninghoven, A.M. Huber and H.W. Werner (Wiley, New York 1988) p. 243
/2/ W. szymczak and K. Wittmaack, Proc. of llth Mass Spectrometry Conference, to be published /3/ W. Ens, B.U.R. Sundqvist, P. Hakanson. A. Hedin and G. Jonsson, Proc. of llth Mass
Spectrometry Conference, to be published
/4/ S. Della-Negra, Y. LeBeyec, B. Monart. K. Standing and K. Wien, Nucl. Instrum. Methods 832 (1988) 360
/5/ G. Bolbach, R. Beavis, S. Della-Negra, C. Deprun, W. Ens, Y. LeBeyec, D.E. Main, B. Schueler and K. Standing, Nucl. Instrum. Methods B30 (1988) 74
/6/ R.E. Johnson. B. Sundqvist. P. Hakanson. A. Hedin, M. Salehpour and G. Save, Surf. Sci.
179 (1987) 187
