The use of synchrotron radiation to measure electron attenuation lengths in condensed molecular solids

820

Nuclear Instruments and Methods in Physics Research A246 (1986) 820 824 North-Holland, Amsterdam

THE USE OF SYNCHROTRON RADIATION TO MEASURE LENGTHS IN CONDENSED MOLECULAR SOLIDS Roger STOCKBAUER,

R i c h a r d L. K U R T Z ,

Noriaki USUKI

ELECTRON

ATTENUATION

*, a n d T h e o d o r e E. M A D E Y

Surface Science Division, National Bureau of Standards, Gaithersburg, MD 20899, USA

We describe a method for using synchrotron radiation to measure accurately electron attenuation lengths in condensed molecular solids as a function of electron energy. It consists of measuring the attenuation of photoelectrons from a well characterized, relatively inert, cooled surface as a condensable overlayer is deposited. As photoelectrons from the substrate escape they pass through and are scattered in the overlayer. This scattering appears as a decrease in the intensity of the substrate photoelectron peak. The measurement of this decrease as a function of layer thickness gives the electron attenuation lengths directly. By using monochromatized synchrotron radiation for the photoemission excitation source, one can tune the photon energy and, hence, obtain the attenuation lengths as a function of electron kinetic energy. The techniques developed for obtaining a uniform overlayer film and for determining its thickness are given in detail. These techniques are applicable to most condensable samples that can be introduced into the vacuum system as a gas.

1. Introduction

2. Experiment

Electron a t t e n u a t i o n lengths are i m p o r t a n t in quantitative surface analysis for such methods as Auger electron spectroscopy a n d X-ray photoelectron spectroscopy. They are also critically i m p o r t a n t in understanding a n d predicting the effect of ionizing radiation on biological materials a n d the radiation h a r d e n i n g of integrated circuits. Measurements of electron attenuation lengths in organic materials have been largely limited to polymer films at electron energies above 1 keV and below 10 eV [1]. It has been convenient to interpolate between such values using a universal curve which is an empirical fit to available data [1]. This works rather well for metals and inorganic c o m p o u n d s where there is data for a wide range of elements a n d c o m p o u n d s s p a n n i n g a large energy range. Because of the paucity of data, however, it is not k n o w n how accurately a universal curve describes electron m e a n free path data for organic molecules or if one curve fits all classes of organic compounds. In order to expand the measurements to a b r o a d e r range of electron energies and to a b r o a d e r class of organic compounds, we present here a technique to measure electron m e a n free paths in condensed molecular films.

The basic m e t h o d used to measure electron m e a n free paths has been developed previously a n d is termed the overlayer technique [2]. It involves depositing a uniform film of the substance to be measured on top of a well characterized substrate. Electron emission from the substrate is m o n i t o r e d as the overlayer film thickness is increased. Since the electrons from the substrate must pass through the overlayer to be detected, they are attenuated b y scattering in the overlayer. This a t t e n u a t i o n as a function of overlayer thickness is a measure of the electron scattering, i.e., the electron a t t e n u a t i o n length. In the technique developed here, dispersed radiation from the NBS S U R F - I I s y n c h r o t r o n light source is used to generate photoelectrons in an ultraviolet photoemission experiment (UPS). These are energy analyzed by a double pass, cylindrical mirror analyzer so that photoelectrons emitted b y the substrate can be distinguished from those emitted by the overlayer. Since the relatively thin molecular films employed here are t r a n s p a r e n t to the incident light, we assume that the ph0toemission from the substrate is i n d e p e n d e n t of the overlayer thickness. Thus, we have a c o n s t a n t source of monoenergetic electrons whose energy can be varied by varying the energy of the incident photons. The choice of substrate is important. It must have a feature in its U P S spectra which is clearly separated from those of the overlayer. It must also be relatively inert to avoid dissociation of the overlayer molecules as they are adsorbed and it should be relatively easy to clean to o b t a i n a reproducible surface. We have chosen

* NBS visiting scientist. Permanent address: Sumitomo Metal Industries Co., Ltd, Central Research Laboratories, 3, 1Chome, Hishinagasu-Hondori, Amagasaki, Japan.

R. Stockbauer et aL / Electron attenuation lengths in condensed molecular solids a copper single crystal with a (100) face which has the desired characteristics. The Cu(100) substrate was cooled to - 90 K using liquid nitrogen so that we were able to condense many organic compounds which could be introduced as a gas into the vacuum system. Water, methanol, and cyclohexane were chosen as representative compounds. The vapor pressure of these molecules at liquid nitrogen temperature is low enough that the films did not show significant evaporation during a measurement. To obtain a uniform overlayer of known thickness, we employed a microcapillary array gas doser. This is a hexagonal array of - 105 small tubes 25/zm in diameter and 1 mm long. The flow from any single tube is highly directional [3] which produces a high flux of molecules on the surface of the substrate while maintaining a low background pressure in the vacuum system. The large number of parallel tubes produces a uniform flux over the area of the sample. In the configuration used here (12.7 mm diameter doser, 5 mm square Cu(100) sample at a distance of 20 ram), model calculations showed that the uniformity of the flux was better than 4% [4]. The precision of the electron attenuation length measurement depends on how precisely one knows the thickness of the film. The dosing system allows us to determine the number of molecules which adsorb on the

(a)

I (b) "~ 1.0 C

,4

~o.9 from

doser

of

doser

g move sample

o 0 II1

Time

Fig. 1. (a) Schematic diagram of the doser system: (A) the microcapillary array; (C) the flow limiting capillary; (V) the gas storage vessel. (b) Change in the signal from the residual gas analyzer as the sample is placed in front of the doser to measure the percentage of the flux that is intercepted.

821

substrate. This determination requires two measurements. The first is a measure of the total number of molecules entering the vacuum system trough the doser. A schematic of the microcapillary array dosing system is shown in fig. la. It consists of the microcapillary array, a low conductance capillary to limit the gas flow, and the gas storage vessel. By knowing the volume (V) of the vessel and measuring the pressure drop (d P / d t) in it as the sample gas is dosed, one can calculate the number of molecules per unit time entering the vacuum system through the doser using the relationship

d n / d t = ( V / R T ) ( d P / d t ), where R is the gas constant, and T is the temperature. In practice, the pressure in the vessel is maintained near 0.5 Tort level so that it can be easily monitored with a capacitance manometer. Of the molecules entering the vacuum chamber through the doser, only a small percentage are adsorbed on the surface. To determine this percentage, the change in the relative partial pressure of the sample gas in the vacuum chamber is measured with the surface in front of the doser and with the surface removed. This percentage change in pressure, monitored with a quadrupole residual gas analyzer (QMS) tuned to a mass peak characteristic of the sample gas, is a direct measure of the percentage of sample gas entering the vacuum chamber that adheres to the surface. The results from such a measurement are shown in fig. lb. This number is typically 6% and agrees closely with the number obtained from model calculations of the doser [4]. The substrate was exposed to the gas effusing from the doser for varying times to build up various thicknesses. The dosing rate times the time gives the number of molecules adsorbed on the substrate. Once the number of molecules on the surface is known, it is a simple matter to calculate the overlayer thickness from the known area of the substrate and the density of the condensed gas. The most critical element in this measurement is the thickness of the condensed molecular overlayer film. Two additional experiments and a model calculation were performed to verify the film thickness. The experiments rely on the observation that the bonding interaction between an adsorbed molecule and the substrate is different from the bonding interaction between neighboring molecules in the condensed film. It is possible, therefore, to distinguish experimentally a monolayer from a multilayer due to this difference in bonding interaction. The first experiment involves observing the binding energy of the valence electrons of the overlayer molecule using UPS. A definite shift in the binding energy is observed between the monolayer and multilayer. This is illustrated in fig. 2a where the UPS spectra are shown for increasing thicknesses of methanol on the Cu(100) V. RESEARCH APPLICATIONS

822

R. Stoekbauer et al. / Electron attenuation lengths in condensed molecular solids

Methanol

on Cu(lO0)

i

i

,

i

I

I

I

I

i

i

i

i

i

I

I

I

i

I

,

.6

20

10

I

0

(b) E

v

c:

I

I

I

I

I

I

1o Initial Energy (eV)

i

I

a n d multilayer, UPS difference spectra are shown in fig. 2b. The b o t t o m three curves are U P S spectra for fractional m o n o l a y e r thicknesses from which has been subtracted the spectrum of the clean surface. The remaining curves are spectra from increasing multilayer thicknesses from which the monolayer spectra has been subtracted. This latter set of difference curves shows the c o n t r i b u t i o n to the UPS of only the multilayer. In effect, this process separates the monolayer from the multilayer spectra. The results show a definite shift in the binding energy between the m o n o l a y e r and multilayer, and illustrate that the film we h a d identified as a monolayer from the doser calibration is indeed a m o n o layer. Had the doser calibration been incorrect, the shift would have occurred at a different film thickness. The second experimental verification comes from thermal desorption spectroscopy (TDS). In this experiment, after the overlayer is formed at low temperature, the substrate is heated at a constant rate and the desorption of the overlayer molecules monitored using a quadrupole residual gas analyzer t u n e d to a characteristic mass peak of the overlayer molecule. The difference in binding energy between the molecule and the substrate as opposed to that between the molecules is

TDS of CH3O H on

I

5

Fig. 2. (a) UPS energy distribution curves (EDSs) for the adsorption of CH3OH on Cu(100) at 90 K for a photon energy of 45 eV. The clean Cu(100) spectrum, the dotted curve, shows that the d-electron band exhibits maximal intensity at 3 eV and becomes attenuated with increasing methanol exposure, the solid curves. The thicknesses are 0.2, 0.6, 1.0 (dashed curve), 2.0, 2.9, 3.9, and 5.7 layers. (b) sequential difference spectra of the data shown in (a) obtained by: 1. subtracting the clean surface spectrum from those obtained for exposures up to one monolayer (lower three curves) and by 2. subtracting the monolayer exposure spectrum from those obtained for exposures greater than one monolayer (top three curves).

Cu(100)

t-

5

MOnolayer erage

.J

2.0

03

substrate. The dotted curve is the spectrum from the clean Cu(100) substrate. The b i n d i n g energy of the m e t h a n o l peaks appear to remain c o n s t a n t for thicknesses less t h a n a monolayer (the dashed curve). A b o v e a monolayer, the peak positions gradually shift toward higher binding energy with increased thickness. The shift is gradual because there is still a UPS c o n t r i b u t i o n from the monolayer, each peak actually being a superposition of two peaks, one from the monolayer a n d one from the multilayer. As the thickness of the film increases, the c o n t r i b u t i o n from the multilayer increases while that from the monolayer decreases. To separate these two contributions and d e m o n s t r a t e the distinct difference in b i n d i n g energy between the m o n o l a y e r

I J 80 120

I

I 160

Temperature

ii

I 180

(K)

Fig. 3. Thermal desorption spectra of CH3OH on Cu(100) at 90 K. The peak near 150 K is due to the first monolayer of methanol and the peak near 130 K is from methanol multilayers. The horizontal scale is linear with time.

823

R. Stockbauer et al. / Electron attenuation lengths in condensed molecular solids

observed as a difference in the desorption rate, i.e, the molecules in the monolayer will desorb in a different temperature range from those in the multilayer. Typical T D S results are shown in fig. 3 for methanol on Cu(100) surface. At thicknesses of a monolayer or less, only 1 peak is observed. This corresponds to desorption from molecules which are bonded directly to the substrate. At thicknesses greater than a monolayer, a second peak is observed at lower temperature. The intensity of this peak increases with increasing thickness while the intensity of the monolayer peak remains constant. The appearance of the low temperature peak at thicknesses above a monolayer again verifies that our calibration of the microcapillary array doser is, in fact, correct. A third verification was performed using a model calculation of the effusion from the doser [4]. The percentage of the gas flux intercepted by the sample in the geometry used predicted by this model was in excellent agreement with our measurement result.

3. Results The electron attenuation results from our experiments with methanol deposited on the Cu(100) substrate are shown as the points plotted in fig. 4. This is Methanol I

I

on C u ( I O 0 ) I

I

Electron

I

Kinetic

I

Energy

t.oo : ~ "

~ Cu (3d) Intensity-

• 20 eV

~.

\

", Lk'~\~

0.1o

o.ot

~

I

5

,

,

o

28 eV

•

68 eV

simply the intensity of the Cu(3d) photoemission peak (which is well separated from the methanol UPS peaks as shown in fig. 2a) plotted as a function of methanol layer thickness on a semilogarithmic scale. The data were obtained over a range of photon and hence a range of electron energies. The straight lines are linear-leastsquares-fits to the data. The linearity of the plots shows that the layers grow uniformly, i.e., island or cluster growth is at a minimum. Similar data were obtained for water and cyclohexane.

4. Discussions The slopes of the fitted lines to the UPS intensity data in fig. 4 are directly proportional electron attenuation lengths. Two correction factors have been applied to the data. The first corrects for the adsorption of the sample gas from the ambient background in the vacuum chamber. The second is applied to account for the acceptance geometry of the cylindrical mirror analyzer. This arises from the fact that the analyzer accepts electrons over a large solid angle so that the path length of an escaping electron is, on the average, larger than the film thickness. We have used the correction factors derived by Shelton [5] and N o r m a n and Woodruff [6] for this situation. The corrected data have been plotted as a function of electron energy in fig. 5 over the energy range available to us at the S U R F - I I light source. Also plotted as the dashed line in this region is the universal curve derived by Seah and Dench [1] for organic material. This curve was derived using an equation of the form X = A E - 2 + BEO.5,

where ~ is the electron mean free path, E is the electron energy, and A and B are the constants to be determined. The data set to which Seah and Dench fitted this curve consisted of measurements taken near 10 eV and I keV. Our values are higher than the Seah and Dench fit and are different for the three different compounds. It is apparent that the universal curve for

\

,

10 15 20 Layer Thickness (~)

+![++

+= +F c.°.

I

25

30

Fig. 4. Attenuation of Cu(3d) electron emission intensity as a

function of CH3OH overlayer thickness; corrections for background adsorption and experimental geometry have not been applied. The electron energy is referred to E v = 0 and the range of electron kinetic energies is obtained by varying the photon energy in the UPS measurements. The lines are the least-squares fits to the data at 20 eV ( ), 28 eV (. . . . . ), and 68 eV (-- - - --).

0

,~

i

~

t0

20

+

t I I [ L 30 40 50 60 70 Electron Kinetic Energy (eV)

1 80

Fig. 5. Results of the electron attenuation length measurements for condensed CH3OH using the overlayer technique. Error bars are given as maximum estimated error. The semiempirical electron mean free path curve for organics calculated in ref. [1] is also plotted. V. RESEARCH APPLICATIONS

824

R. Stockbauer et al. / Electron attenuation lengths in condensed molecular solids

organic c o m p o u n d s derived from a limited n u m b e r of samples over a limited energy range m a y not apply to different materials at other energies. A more detailed account of this research as well as results for water a n d cyclohexane are given elsewhere [71. We would like to acknowledge Drs. C.J. Powell a n d D.L. Doering for helpful discussions. We would also like to t h a n k the staff of the NBS s y n c h r o t r o n light source.

References [1] M.P. Seah and W.A. Dench, Surf. and Interf. Anal. 1 (1979) 2. [21 C.J. Powell, Surface Sci. 44 (1974) 29; C.J. Powell, in: Scanning Electron Microscopy IV (SEM Inc., AMF O'Hare, Chicago, 1984) p. 1649. [3] P. Clausing, Z. Physik 66 (1930) 471. [4] R.L. Kurtz, unpublished results. [51 J.C. Shelton, J. Electron Spectrosc. Relat. Phenom. 3 (1974) 417. [6] D. Norman and D.P. Woodruff, Surface Sci. 75 (1978) 179. [7] R.L. Kurtz, N. Usuki, R. Stockbauer and T.E. Madey, J. Electron Spectrosc. Relat. Phenom., to be published.