US3742227A - Process and apparatus for the mass spectrometric analysis of surfaces of solids - Google Patents

Abstract

Surfaces of solids, in particular the uppermost monolayer, are analyzed by means of secondary ion spectrometry. The primary ion currents used for bombarding the surface of the solid are transmitted at such low current densities that the time required for disintegration of a monolayer, in particular the uppermost monolayer of the solid, is long compared with the time taken for recording a spectrum.

Description

D United States Patent 91 [11 1 3,742,227 Benninghoven 1 June 26, 1973 [54] PROCESS AND APPARATUS FOR THE MASS 3,5l7,l9l 6/1970 Liebl 250/495 SPECTROMETRIC ANALYSIS OF SURFACES 0F SOLIDS OTHER PUBLICATIONS Inventor: Alfred Benninghoven, C l gn Mass-Spectrometric Micro-Surface Analysis by erm ny Geophysics Corporation of America, Bedford, Mass.

[73] Assigne'e: Bayer Aktieugesellschaft,

Level'kusen, Germany Primary Examiner-William F. Lindquist [22] Filed: No 12, 1970 Att0rneyBurgess, Dinklage & Sprung [2]] App]. No.: 88,747

[57] ABSTRACT 3 F A Ii P 01 oreign pp canon nomy Data Surfaces of solids, in particular the uppermost mono- NOV. I4, Germany P layer, a e analyzed means of seconda y ion Spec V trometry. The primary ion currents used for bombardfi gii ing the surface of the solid are transmitted at such low E 9 SEn49 g P current densities that the time required for disintegra- I 0 can l tion of a monolayer, in particular the uppermost monolayer of the solid, is long compared with the time taken [56] References Cned for recording a spectrum.

UNITED STATES PATENTS 9 Claims, 4 Drawing Figures 2,772,363 11/1956 Robinson 250/419 X, Y RECORDER HALL -PROBE IN TE GRATOR 37 PULSE COOLING 9 TRAP 8 DIFFUSION PUMPS FORMING X23 DEVICE i--- I 00; I DISCRIMINATOR 2 7 4 PREA MPLIFIER MULTIPLIER 25 PAIENIEDJUNZG I975 SHEET 2 0F 4 FIG. 2

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, '1 PROCESS AND APPARATUS FOR THE MASS SPECTROMETRIC ANALYSIS OF SURFACES F SOLIDS The invention relates to aprocess for the mass spectrometric analysis of surfaces of solids by means of secondary ion spectrometry. The process according to the invention is especially suitable for the analysis of the uppermost monolayer of a solid.

Numerous chemical and physical properties of a solid, in particular its catalytic properties, depend upon the chemical and physical nature of its surface. Accurate knowledge of the chemical composition of the first monolayers which form the surface is therefore of decisive importance, for the control of phenomena of activation and poisoning in catalysts, for example. Since such an analysis should only cover the first monolayers, whose composition is generally completely different from that of the interior of the solid, the known processes which include electron microprobe, mass spec-' trometry with a spark ion source, and neutron activation analysis, cannot be used because the information supplied by any of these analytical process extends over much too great a depth of the solid.

When the surface of asolid is bombarded with ions of sufficiently highenergy, it emitsnot only electrons and neutral particles but also positive and negative secondary ions. The composition ofthese secondary ion currents is characteristic of the elements and compounds which'make up the surface of thesolid. Mass spectrometric analysis of the emitted secondary ions therefore supplies information on the chemical composition of the surface of the solid. I

The surface of the solid is generallyformed by the uppermost monolayer of the solid itself and an adsorption layer lying above it, which is in most cases monomolecular. The distinction between these two layers will now be explained. The uppermost monolayer of the solid may have the same composition as the interior of the solid but it may also contain chemically bound foreign substances or even consist entirely of such substances. The adsorption layer, which is not'chemically bound and which is usually'monomolecular, is generally in a state of equilibrium with the surrounding gaseous phase, in other words with the residual gas in the target chamber. The composition of the originally uppermost monolayer of the solid is generally completely different from the interior of the solid, especially in the case of a catalyst. v

Degradation of the surface of the solid by ionic bombardment proceeds in accordance with the following equation:

a (t) wherein T= li/1 '7 wherein i? (t) denotes the proportion still present after a degradation time t of a monolayer which is a complete monolayer at the time t 0;

N denotes the number of particles in the closed monolayer;

j denotes the primary ion current density on the surface of the solid;

e denotes the elementary charge; and

y denotes the sputtering rate, i.e. the average number of particles of the monolayer removed per incident primary ion.

Analysis of the uppermost monolayer of the solid cannot be carried out with the process of secondary ion mass spectrometry hitherto employed (see e.g. i) be cause the primary ion beams are employed at such high current densities that degradation of a monolayer takes place within a period of time which is substantially less than 1 second. Since with the given extremely low streams of secondary ions and the recording instruments available recording times of several hundred seconds' are required to cover the mass range of about 1 lOO in a mass spectrometric analysis of secondary ions, analysis of the originally uppermost monolayer of the solid, which is already disintegrated before the first second expires, is not possible by this method; The analytical results obtainedwith the usual primary ion current densities therefore always apply to at least several hundred such monolayers which are disintegrated in the course of a single analysis.

it is the object of the invention to obtain information about the composition of the upper monolayers of the solid by means of secondary ion mass spectroscopy.

According to the invention, this problem is solved by using such low primary ion current densities for bombardingthe surface of the solid that the time required for sputtering a monolayer of the solid is long compared with the time-for recording a spectrum.

Since in .many cases it is the composition of the uppermost monolayer about which information is required, the sputtering time of the uppermost monolayer must be long compared with the time taken for recording a spectrum. in theprocesses hitherto employed, the

' conditions observed were precisely the reverse so that the analysis was dynamic. The process according to the invention, on the other hand, relates to a static analysis.

' dividual foreign atoms and molecules which is of great interest. The process according to the invention is eminently suitable for this purpose if the variation of intensity of the individual types of ions with time is measured during bombardment of the surface of the solid with a constant primary ion density."

Itfisprecisely static-analysis which enables the variation in intensity of the individual types of ions with time to be followed while sputtering of the uppermost monolayer is still taking place.

The process described here has the following advantages against the previous processes of secondary ion mass spectroscopy: t

1. Instead of primary ion current densities of the order of 10' Acm", densities of the order of less than H)" amp cm are used, with the result that the sputtering time for the uppermost monolayer of the solid is increased to about 10 seconds. Because of this increase in the degradation time the originally uppermost monolayer of the solid for the first time becomes accessible to analysis by secondary ion emission (see FIG. 2). 1

2'. During the time required for recording a spectrum (several hundred seconds), only such a small frac' tion of the layer which is to be analyzed enters into interaction with the beam. of primary ions that the analysis virtually gives information about the unchanged original layer. High primary ion current densities were employed in the previous process in order to keep the surface of the solid which had to be analyzed free from adsorption layers. This measure was intended both to keep the proportion of surface impurities low compared with the impurities in the volume and to ensure that the adsorption layer would not prevent sputtering of the solid body itself.

When using the method according to the invention, it was suprisingly found that although a superficial monomolecular adsorption layer leads to the occurrence of characteristic mass lines in the secondary ion spectrum, it does not interfere with analysis of the underlying uppermost monolayer of the solid. It is therefore sufficient, even with the long degradation times T which occur in these circumstances, simply to keep down to a low value the proportion of those components in the residual gas which could lead to the build up of more than monoatomic layers (e.g. hydrocarbons) or to surface reactions. A vacuum of below mm Hg which was free from hydrocarbons and had an oxygen partial pressure of below 10 mm Hg was generally found to be sufficient. More preferable an ultra high vacuum range of 10"" l0 mm Hg is employed.

When analyzing metal surfaces by the process according to the invention, it is found to contain numerous surface compounds in which an ionic bond exists between a positive metal atom and a negatively charged radical with which it may form a compound (e.g. S0,, N0 C], etc). These radicals appear as negative secondary ions in the secondary ion spectrum. The measurements carried out by the method described here led to the surprising result that the degree of ionization a n of these radicals is independent of the surface nature of the solid. The degree of ionization 'y n is understood to be the ratio of the number of emitted negatively charged particles (negative secondary ions) to the total number of emitted particles in a given time interval for a particular type of particle n. Thus in accordance with a further development of the process according to the invention, metal salts and oxides present in the uppermost and next following monolayers are analyzed quantitatively.

The process according to the invention will now be described more fully with reference to drawings in which:

FIG. 1 shows an apparatus for carrying out the process according to the invention,

FIG. 2 shows a secondary ion spectrum of the uppermost monolayer of a silver catalyst,

FIG. 3 shows the secondary ion intensities of individual types of ions during degradation of the upper monolayers of the silver catalyst,

FIG. 4 shows a secondary ion spectrum of the silver catalyst after degradation of the poisoned upper monolayers.

The method of carrying out the analysis of the uppermost and next following monolayers of a solid will now be described with the aid of the following example of a silver catalyst. The catalyst l which was to be investigated, was mounted on an easily interchangeable target support 2 and introduced into a target chamber in which the analysis took place. The entire vacuum apparatus inclusive the target chamber was made of stainless steel. The flange connections 2.47 between the various elements of the apparatus were sealed with gold wire so that the apparatus could be heated to 450C. The vacuum was generated and maintained by means of two mercury diffusion pumps 8 connected in series. Between the target chamber and the first diffusion pump a cooling trap 9 was inserted which was cooled with liquid nitrogen. According to an alternative embodiment, a turbo-molecular pump was used instead of the mercury diffusion pumps which has a much higher pumping speed in the ultra high vacuum range especially for noble gas. The analysis was carried out at a total pressure below 10' mm Hg. The main components of the residual gas were H H O, CO and CO Hydrocarbons were practically undetectable. By employing a turbo-molecular pump, an ultra high vacuum of 10 10'" mm Hg could be obtained.

The surface of the catalyst was bombarded with a beam of Ar ions 1 10 amp at an angle of from the perpendicular to the target. The intensity of primary ion current density can best be in the range of about 10'" 10 amp cm", preferably about I0 to 10 amp cm". The degradation times can best be in the range of about 500 h sec, preferably about 50 h 5 h. The ratio of degradation time and the spectrum reading time can best be in the range of about 5,000 0,5, preferably 500 50. The Ar ions are generated by a plasma-type ion source similar to that described by Finkelstein in Rev. Sci. Instr. Il,95(l940).

Electrons emitted from a thermionic cathodeof the Wehnelt- type 10,12 are accelerated to about 150 volt by the cylinder shaped anode 13. In the region of the anode an axial magnetic field is applied, generatedby the dc-controlled coil 14. At an argon pressure of about 10" mm Hg in the ion source a plasma is created within the region of the anode. The density of this plasma is to be controlled either by the intensity of the emitted electrons adjusting the heating current of the filament or by the voltage between the filament l0 and the Wehnelt control grid 12.

By applying suitable voltages at the electrostatic lenses 15-17 an ion beam is extracted from the plasma and accelerated onto the diaphragm 18. A part of this ion beam emerging from the diaphragm 18 hit the target I- with an energy of about 3 kev. The surface area of the catalyst bombarded by the ion beam was about 0.1 cm. The current density of the primary ion beam was therefore 1 l0" Amp.cm'. The average degradation time of a monolayer (reduction to T0 He) at this density of the ion beam'found, according to the equation T= (N e lj 'y) already mentioned above, to have a value of about 5 10 sec, i.e. approximately 14 hours (N= 1.5 l0 y 5). A uniform surface density of the beam of primary ions was produced on the catalyst surface by cutting the ionic beam 5 which was originally 1 cm in width down to a narrow beam of about 0.2 mm in diameter by means of the diaphragm 18 just in front of the target.

The positive and negative secondary ions emitted from the surface of the catalyst during the bombardment of the primary ions were focused by an electrostatic lens system 19, 20, 21 onto the entrance slit 22 of the mass spectrometer. In the magnetic field 23 of the single focusing 60 mass spectrometer the secondary ions were separated according to their different elm ratios. Ions with a different e/m ratio emerged from the exit slit 24 into an open magneticmultiplier 25. The current pulses, created by this way were fed capacitively into a preamplifier 26. The amplified pulses then passed a pulse discriminator 27 and a pulse forming device 28 and were then integrated by the integrator 29 plier 25 to the preamplifier 26 provides for a floating of the multiplier output. Thus it is possible to receive also negative ions with a grounded preamplifier.

Instead of the 60-sector field mass spectrometer alternatively a quadrupole mass filter has been employed. A primary advantage of this quadrupole mass filter is its independency of the resolution power from the primary energy of the ions. A further advantage results from the fact that the quadrupole mass filter is available as a compact and relative small unit, which facilitates its arrangement within the analyzing system.

FIG. 2 shows by way of example the negative secondary ion spectrum of the catalyst surface in the mass range of 12-130. The time required for recording the spectrum was 400 seconds, i.e. less than 1 percent of the uppermost monolayer of the solid was disintegrated in the course of the analysis. This layer therefore remained practically unchanged during the analysis.

The lines of the spectrum in FIG. 2 can be associated with certain types of ions and these again with certain compounds. This association may be carried out on the basis of known isotopic compositions, as in the case of C1, or by use of separately measured secondary ion spectra of certain compounds. The spectrum represented in FIG. 2 shows that hydroxides, cyanides, chlorides, thiocyanates, nitrates, sulphates and the compounds of various carboxylic acids were present on the surface of the silver catalyst.

By observing the variation of the intensity of individual types of ion with time during bombardment of the catalyst surface with primary ions of constant surface density, it is possible to decide whether the components which cause them occur only in the uppermost monolayer of the solid or whether they occur also in deeper layers. The basis on which such a decision can be made is as follows. The intensity of the secondary ions which are caused by components which are present only in the uppermost monolayer of the solid must decrease with time t according to the function e' if one assumes a constant ionization probability of a It will be seen from FIG. 3 that this holds good e.g. in the case of the ions S N0 SCN and CH CHOOO', and the corresponding compounds were therefore only present in the uppermost monolayer of the solid. Additional evidence for this is the very close agreement between the measured time values (fall to He of the original intensity) and the values calculated according to the equation T= (N e /j 7).

When the catalyst surface had been bombarded with an ion dose of IO" A.s.cm corresponding to sputtering of about monolayers, only ions which correspond to components which were also present in deeper layersremained. The secondary ion spectrum shown in FIG. 4 was obtained.

One monolayer after another is therefore disintegrated in the course of the analysis. It is thereby possible to determine the distribution in depth ofa certain component in the region of individual monolayers and to make a continuous transition to pure volumetric analysis. I

When a primary ion density of 10 A.cm and a residual gas pressure of several 10 mm Hg were employed for recording the spectrum shown in FIG. 2, the surface of the solid was certainly covered with an almost monomolecular adsorption layer. The analytical results (spectrum FIG. 2, variation with time FIG. 3) show unequivocally that neither the degradation of the uppermost monolayer of the solid nor detection of the components present in it are hindered by this adsorption layer.

The intensity of some types of secondary ions recorded in FIG. 3 )NO{, SO W.) decreases with time t strictly according to the term e" for T values of about 1,800 seconds. These T-values agree very closely with the values calculated according to the equation T (N e lj 'y).

This agreement alone is evidence for the fact that the components which are responsible for these secondary .ions occur only in the uppermost monolayer of the solid. The strictly exponential course (e) of the intensity decrease of these types of ions also proves that the ionization probability y n for the components which cause them is independent of the nature of the surface and that absolutely quantitative analyses of the corresponding components are possible. The same observation was during the disintegration of monomolecular layers of oxides on metal surfaces.

The process according to the invention is not limited in its application to the investigation of catalysts but can in principle be used successfully wherever problems of boundary surface physics or chemistry have to be investigated. Thus the process is also very suitable, for example, for the investigation of semi-conductor surfaces and semi-conductor boundary layers.

I claim:

1. In a process for the analysis of surfaces of solids which comprises:

a. generating a primary ion beam and directing primary ions of the beam onto the surface of the solid, and thereby causing sputtering of the surface for emission of-secondary ions,

b. analyzing the secondary ions by mass spectrometry to identify components of the sputtered/ions,

the improvement which comprises:

c. expanding the generated primary ion beam and thereafter extracting a portion of expanded beam, said extracted portion being of a uniform density of less than 10" amp. cm' and composing the primary ions directed onto the surface, whereby sputtering time of monolayers at the surface of the solid is prolonged and thereby the time for analyzing the sputtered ions is enlarged.

2. Process according to claim 1, and measuring the variation in intensity of individual types of sputtered ions with time while maintaining the current density constant.

3. Process according to claim 1, metal salt or oxide being present in the surface of the solid, and said analyzing comprises quantitatively analyzing the metal salt or oxide.

4. Process according to claim I, said current density being 10' to 10 amp. emf, and the degradation time being 5 50 hours.

5. Process according to claim 1, and maintaining a vacuum during said analysis of below mm of mercury.

6. Apparatus for analyzing monolayers at the surfaces of solids comprising:

a. a closed chamber having target support means for mounting therein, a specimen of the solid to be analyzed,

b. vacuum means for maintaining a vacuum in the chamber,

means for generating a primary ion beam and directing primary ions of the beam toward the surface of a specimen mounted in the target supportmeans,

d. interposed between the generating means and the target support means, means for expanding the generated ion beam and means for thereafter extracting a portion of the expanded beam, for providing an extracted primary ion beam having a uniform current density less than the density'of the primary ion beam and less than 10 amp. cm, for

I connected in series to the open pulse multiplier, a pulse discriminator, a pulse forming device and an integrator.

9. Apparatus according to claim 6, the expanding means and the extracting means providing an extracted primary ion beam having a uniform current density of 10' to 10 amp. cmf resulting in a degradation time of 5 50 hours.

UNITE!) was m'tz'crw (WFHJI!) CIVIRTE FRI/3'1 ;5 OF C0.BLREQC'HQN Patent No. 35742 ,227 Dated June 26, 1973 lnwntofls) Alfred Benninghoven It is certified that error appears in the nbove-identified patent and that said Letters Patent are hereby corrected as shown below:

1. Col. 4, line 45, cancel "T0",

Signed and sealed this 27th day of November 1973.

(SEAL) Attestz EDWARD M. FLETCHER,JR. RENE D 'lfEGTMEYER I Attesting Officer Acting Commissioner of Patents

Claims (9)

1. In a process for the analysis of surfaces of solids which comprises: a. generating a primary ion beam and directing primary ions of the beam onto the surface of the solid, and thereby causing sputtering of the surface for emission of secondary ions, b. analyzing the secondary ions by mass spectrometry to identify components of the sputtered/ions, the improvement which comprises: c. expanding the generated primary ion beam and thereafter extracting a portion of expanded beam, said extracted portion being of a uniform density of less than 10 7 amp. cm 2 and composing the primary ions directed onto the surface, whereby sputtering time of monolayers at the surface of the solid is prolonged and thereby the time for analyzing the sputtered ions is enlarged.

2. Process according to claim 1, and measuring the variation in intensity of individual types of sputtered ions with time while maintaining the current density constant.

3. Process according to claim 1, metal salt or oxide being present in the surface of the solid, and said analyzing comprises quantitatively analyzing the metal salt or oxide.

4. Process according to claim 1, said current density being 10 10 to 10 9 amp. cm. 2, and the degradation time being 5 - 50 hours.

5. Process according to claim 1, and maintaining a vacuum during said analysis of below 10 8 mm of mercury.

6. Apparatus for analyzing monolayers at the surfaces of solids comprising: a. a closed chamber having target support means for mounting therein, a specimen of the solid to be analyzed, b. vacuum means for maintaining a vacuum in the chamber, means for generating a primary ion beam and directing primary ions of the beam toward the surface of a specimen mounted in the target support means, d. interposed between the generating means and the target support means, means for expanding the generated ion beam and means for thereafter extracting a portion of the expanded beam, for providing an extracted primary ion beam having a uniform current density less than the density of the primary ion beam and less than 10 7 amp. cm 2, for bombarding the surface of a specimen mounted in the target support means, to cause sputtering of monolayers of the surface for emission therefrom of secondary ions, e. a mass spectrometer connected to the closed chamber for receiving secondary ions emitted by the specimen for analyzing said secondary ions.

7. Apparatus according to claim 6, the mass spectrometer being outfitted with an open pulse multiplier as an ion detector.

8. Apparatus according to claim 7, and operatively connected in series to the open pulse multiplier, a pulse discriminator, a pulse forming device and an integrator.

9. Apparatus according to claim 6, the expanding means and the extracting means providing an extracted primary ion beam having a uniform current density of 10 10 to 10 9 amp. cm. 2, resulting in a degradation time Of 5 - 50 hours.

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