Method and apparatus for surface diagnostics

Method and apparatus for mass spectral analysis of unknown species of matter present on a surface even in extremely low concentrations. A probe beam such as an ion beam, electron beam or laser is directed to the surface under examination to remove a sample of material. An untuned, high-intensity laser is directed to a spatial region proximate to the surface. The laser has sufficient intensity to induce a high degree of nonresonant, and hence non-selective, photoionization of the sample of material within the laser beam. The non-selectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species.

BACKGROUND OF THE INVENTION 
The invention relates to techniques for local analysis of surfaces; more 
particularly, it is directed to techniques for mass spectral analysis of 
species of matter on, or forming a part of, a surface. 
In surface analysis by mass spectroscopic techniques the specimen to be 
examined is placed in a high-vacuum chamber and bombarded by a probe beam 
of one kind or another. The probe beam removes a sample of matter from the 
surface, at least a portion of which is ionized in the process of removal 
or subsequent thereto. The ionized sample is then subjected to mass 
spectral analysis. 
Examples of known techniques for surface mass spectral analysis include 
laser microprobe methods and secondary ion mass spectrometry. Both of 
these methods contain undesirable limitations. Laser microprobe mass 
spectrometry, for example, uses a focused high-intensity laser to 
irradiate a surface directly and blow off large amounts of material, only 
a small fraction of which is ionized as it departs from the surface. The 
high-intensity laser results in highly destructive sampling of the 
surface; the intense laser pulse used in typical instruments forms craters 
of 0.1 to 1.0 micrometers in depth and is therefore not truly 
surface-sensitive. The laser microprobe technique is also difficult to 
model because ionization efficiencies depend sensitively on the various 
collisional processes occurring in the laser-generated plasma at the 
surface. 
In secondary ion mass spectrometry (SIMS) the surface under examination is 
bombarded with an ion beam or fast neutral beam, which sputters a sample 
of ionized and neutral matter from the surface. In general, quantitative 
analytical information is difficult to derive from the SIMS method because 
the physical processes that determine the ionization probability of the 
sputtered matter are not, as a rule, well understood and because 
ionization probabilities depend sensitively on surface composition 
(so-called matrix effects) and cleanliness (so-called chemical enhancement 
effects). As a practical matter, quantitative analytical information can 
be extracted from the SIMS method only by using especially prepared 
standards for comparison. 
In many of the commonly practiced techniques, such as the laser microprobe 
or SIMS methods, only a small portion of the sample removed from the 
surface is ionized. One attempt to provide a highly ionized sample is 
disclosed in U.S. Pat. No. 4,001,582 in the name of Castaing, et al. In 
the Castaing method, particles sputtered from the surface are introduced 
into a chamber at high temperatures and are subjected to successive 
adsorptions and desorptions on the walls of the chamber. This process 
efficiently ionizes atoms of suitably low ionization potential with a high 
probability. However, molecules (as opposed to atoms) are generally 
dissociated in the hot ionizing chamber, so that the Castaing method 
yields information only on atomic species. Additionally, many atoms have 
ionization potentials too large to be ionized and detected by this method. 
Contamination and material degradation problems can also be severe in the 
extreme environment of the high temperature detector. 
Another approach presenting enhanced ionization efficiency is disclosed by 
N. Winograd, J. P. Baxter and F. M. Kimock, Chemical Physics Letters, Vol. 
88, No. 6, 1982 pp. 581-84. In this approach a laser is directed to a 
sample of neutral atoms, which have been sputtered from the surface under 
examination. The laser is tuned to a predetermined wavelength 
corresponding to an excited state of a preselected atom of interest known 
or expected to be present in the sample. The laser has sufficient 
intensity to induce resonance multiphoton ionization of the preselected 
atom. This method has the obvious drawback that it is necessary to tune 
the laser to a predetermined wavelength. Thus, the method is applicable 
only to certain species of matter which have known excitation spectra with 
excitation wavelengths accessible to the available laser and which are 
already known or strongly suspected to be present on the surface. 
SUMMARY OF THE INVENTION 
The present invention provides an extremely reliable and versatile method 
and apparatus for detecting and distinguishing unknown species of matter 
at extremely low surface concentrations. It has been discovered that the 
process of non-resonant photoionization may be utilized to make a 
practical instrument capable of highly sensitive surface diagnostics and 
free of the restrictions imposed by the above-mentioned methods. 
Briefly, under high vacuum a probe beam such as an ion beam, electron beam, 
or laser beam is directed to the surface under examination to cause a 
sample of material to be removed from the surface. A beam of 
electromagnetic radiation, which may be provided by an untuned, 
high-intensity laser, is then directed to a spatial region proximate to 
the surface causing non-resonant ionization of the removed surface sample 
within the beam of radiation. It has been discovered that within the 
practical limits of readily achievable laser intensities this beam may be 
given sufficient intensity to induce a high degree of non-resonant, and 
hence non-selective, photoionization of the sample. The ionized sample is 
then subjected to mass spectral analysis to determine the nature of the 
species included therein. 
Apparatus according to the invention for practicing the above method 
includes an evacuation chamber, in which is disposed a means for mounting 
the specimen under examination. A probe beam means is provided for 
directing a probe beam at the surface of the specimen so as to cause a 
sample to be ejected into the evacuation chamber. Ionizing beam means 
provides a non-resonant ionizing beam of radiation which is directed at a 
spatial region (referred to as the ionization region) above the surface. 
The ionizing beam has an intensity sufficient to induce non-resonant 
photoionization of the sample found in the ionizing region. Means is 
provided within the evacuation chamber for accelerating the ionized sample 
into a region including means for mass analysis of the ions. A preferred 
mass analysis means compatible with pulsed laser ionization provides an 
ion drift region, so as to enable time-of-flight analysis of the ionized 
sample. For greater mass resolution a preferred embodiment of the 
apparatus includes ion reflector means disposed within the drift region 
for effectively compensating for undesirable spreading of the arrival 
times caused by the initial ion velocity distributions. Detection means is 
disposed within the evacuation chamber for detecting the ions emerging 
from the drift region. 
The invention provides a number of advantages which have not previously 
been found in a single instrument. The method and apparatus according to 
the invention are not restricted to the investigation of atomic species or 
simple molecular species having known excitation and ionization spectra. 
The invention may be applied to a wide variety of compounds of interest 
including contaminated or doped substrates, adsorbed or reacted 
overlayers, or even biological samples which have been precipitated onto 
or otherwise applied to a surface. All masses can be investigated 
simultaneously with a single (untuned) laser wavelength. Every atom is 
accessible to ionization by the present technique. 
The probe beam intensity may be adjusted so as to produce minimal damage to 
the surface under examination. Notwithstanding the reduced probe beam 
intensity, the nonresonant ionizing beam produces sufficient ionization of 
the sample that even extremely low concentrations of matter can be 
detected. 
The method and apparatus of the invention are sufficiently reliable and 
accurate, and sufficiently nondestructive in the surface-removal step, 
that it is possible to monitor changes in concentrations of species in the 
course of chemical reactions taking place on the surface. 
A further understanding and appreciation of the nature and advantages of 
the invention may be gained by reference to the remaining portion of the 
specification and to the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The nonresonant photoionization method of the present invention requires a 
probe beam for removing a sample of material from the surface under 
examination and a separate high-intensity nonresonant ionizing beam of 
radiation. As the term "nonresonant ionizing beam of radiation" is used 
herein, it means a beam of electromagnetic radiation which has not been 
tuned to a predetermined wavelength associated with the specimen under 
examination. The great advantage to be derived from a high-intensity 
nonresonant ionizing beam, which has not been achieved by photoionization 
methods employed in the past, is that efficient ionization is achievable 
without the necessity of knowing in advance the nature of the species of 
matter under examination. Therefore, it is also not necessary to know the 
excitation spectra of the various components of the species under 
examination. 
The probe beam for removing the sample from the surface may be provided, 
for example, by an electron beam, an ion beam, a fast atom beam, or a 
laser beam. The separation of the sample-removal step from the ionization 
step allows independent control of the sample-removal and ionization 
processes. Thus the probe beam can be directed to a localized region of 
the surface and adjusted in intensity so as to scan the depth from which 
the sample is removed (so-called depth profiling) or to scan the surface 
area over which the sample is removed (so-called microscopy). In this 
manner monolayers of particles adsorbed on the surface may be sampled from 
extremely localized regions. 
Ionization of the sample under examination may occur through single-photon 
and multi-photon processes. Ionization occurs by means of a multi-photon 
mechanism at a lower photon energy than by means of a single-photon 
mechanism, but the multi-photon approach requires higher overall 
intensities. In fact, it has generally been believed that a resonance 
condition is required in the multi-photon approach to achieve sufficient 
ionization for subsequent mass spectral analysis. In the present invention 
it has been discovered that ionization of the sample removed from the 
surface can be saturated in the ionizing region with nonresonant 
multi-photon photoionization. This discovery has eliminated the serious 
drawbacks of the resonance multi-photon ionization method: namely, the 
need to know the components of the species of matter under examination in 
advance; the need to know their excitation spectra so that the laser may 
be tuned to each predetermined multi-photon resonance; and the resultant 
limitation of detecting only one mass-to-charge ratio at a time. Although 
a laser with sufficient intensity to induce nonresonant multi-photon 
ionization may also coincidently produce an occasional resonant ionization 
of some species present in the sample, the knowledge of the existence of 
such species is not necessary beforehand. The possibility of such 
incidental resonant excitations has not been found to interfere with the 
nonresonant photoionization of the bulk of the sample, nor has the 
presence of an occasional resonant excitation been found to detract from 
the advantages derived from nonresonant photoionization. 
The ionizing beam for nonresonant multi-photon ionization may be provided 
by a laser having a power density in the range of 10.sup.6 to 10.sup.12 
W/cm.sup.2. The laser is advantageously pulsed for time-of-flight mass 
spectrometry, the pulses having a period of about 10.sup.-8 seconds. For 
nonresonant single-photon ionization less light intensity is needed; 
generally though, pulses with at least about 10.sup.12 photons per pulse 
are needed for efficient ionization. 
By way of example, a mass spectrum was derived from a sputter-cleaned 
surface of an NBS standard copper specimen of known bulk composition. With 
a pulsed 3-keV Ar.sup.+ ion probe beam hitting the surface over an area of 
about 0.1 cm.sup.2 and a 10 pulse/sec, 20 millijoule/pulse KrF (248 nm) 
laser ionizing beam focused by a 40 cm focal length lens to a spot 1 mm 
above the specimen surface, impurity isotopes at concentrations from 1 to 
10 ppm (parts per million) were easily observed in an experiment that 
removed a total of about 10.sup.-10 g, equivalent to 0.01 monolayer, from 
the copper sample. In this experiment the detector was estimated to 
collect one ion for every 10.sup.4 -10.sup.5 surface atoms removed by the 
probe beam. These results are cited by way of example only, and it should 
be readily appreciated by those skilled in the art that conventional 
techniques can be used to obtain a larger collection solid angle in the 
ionization zone both through better focusing of the ion beam onto the 
surface, allowing smaller distances from the ionization zone to the 
surface, and through larger ionization zones derived from more intense 
pulsed lasers or larger laser beam focal waists. Impurity measurements 
well below the level of 1 ppm should be readily accessible. 
In this copper sample, sputtered neutral dimers are also observed as dimer 
ions after photoionization by the laser. These include, of course, 
Cu.sub.2 but also CuAg even though each of the two isotopes of Ag is only 
approximately 50 ppm of the Cu sample. For various metal samples doubly 
charged ions, e.g. Pt.sup.++, W.sup.++, Ta.sup.++, Cu.sup.++, have also 
been observed, which indicates strongly saturated conditions for the 
singly ionized entities. In another example, hydrocarbon molecules 
adsorbed on catalytically active metal surfaces have been observed. 
Apparatus for practicing the above method will now be described with 
reference to the overall view shown in FIG. 1 and the detail of FIG. 2. 
The specimen of interest 10 is mounted in fixed position by specimen 
holder 11, which exposes the surface 12 under examination. Probe beam 
means 13 provides a probe beam which is directed to a localized area of 
the surface 12. The probe beam means 13 is illustrated schematically in 
FIG. 1. It will generally include a beam source 14 and means 15 for 
directing the probe beam generated by source 14 onto the localized area of 
the surface 12 under examination. 
Beam source 14 may be provided, for example, by an electron gun or ion gun 
or by a laser. The construction of such beam sources is conventional and 
is well known to those skilled in the art, for example, from electron beam 
desorption, ion beam sputtering, or laser microprobe techniques. When the 
probe beam is a charged particle beam, means 15 for directing the beam 
onto the surface 12 may be provided by an assembly comprising an 
electrostatic or magnetic lens and horizontal and vertical electrostatic 
deflection plates. When a laser provides the probe beam, means 15 will 
generally be provided by a system of mirrors and lenses. In focusing the 
ion beam or laser beam onto a localized region of the surface 12, the 
means 15 enhances the power density of the beam at the surface, which 
allows a lower intensity ion beam or laser to be used for the beam source 
14. 
Means 17 provides an ionizing beam of radiation which is directed to a 
region proximate to the surface 12 so as to irradiate a substantial 
portion of the sample removed by the probe beam. The means 17 will 
generally include a high-intensity light source 18, a focusing lens system 
19 and an iris 20 for use in defining the position of the ionizing beam. 
Beam source 18 will typically be provided by a high-intensity laser or an 
assembly of lasers and optical materials and components coupled together 
to achieve sufficiently high power for saturation of the ionization of the 
sample under investigation. 
Generally, use of a shorter wavelength of light will permit saturation of 
ionization at lower light intensities than for longer wavelengths. 
Quantitative analysis of relative amounts of desorbed or sputtered atomic 
species can be achieved by measuring the signal levels at the saturation 
power density for ionization of each chemical species. The dependence of 
the mass spectrum on laser power is a useful diagnostic tool both for 
assessing relative degrees of ionization of different species and for 
evaluating the importance of dissociation processes in molecular 
components. For examination of molecules, especially complex entities, 
high laser powers may cause extensive molecular fragmentation in addition 
to the ionization; use of lower laser powers will often yield spectra with 
lower degrees of fragmentation. 
The specimen under investigation is housed within an evacuation chamber 22 
which is provided with pumping means, e.g., a port 23 adapted for 
connection to a vacuum pump (not shown). When the probe beam is provided 
by a laser beam, the laser is mounted outside of chamber 22, which is 
provided with an additional window (not shown) through which the beam 
enters the chamber. When probe beam source 14 is provided by an ion gun, 
the source 14 may be contained in an adjacent chamber evacuated by 
additional vacuum pumps (not shown) and connected to chamber 22 by a small 
aperture through which the ion beam enters. 
In the embodiment of FIG. 1 evacuation chamber 22 is provided with 
diametrically opposed windows 24 and 25, through which the ionizing beam 
of radiation is projected. The ionizing beam means 17 is mounted outside 
evacuation chamber 22, and the ionizing beam is directed through window 24 
to the ionization region proximate to the surface 12 under investigation. 
The ionizing beam continues through window 25, passes through 
position-defining iris 26, and is received by detector 27, which serves to 
monitor the light intensity. For convenience mounting means 28 is provided 
within evacuation chamber 22 for mounting a plurality of specimens. In 
this way several specimens may be investigated without having to break the 
vacuum within chamber 22. Mounting means 28 is connected to a position 
manipulator (not shown) and is accessible through an associated specimen 
introduction system (not shown). 
Evacuation chamber 22 also includes mass spectrometer means, indicated 
generally at 31, for mass analysis of the sample ionized by the ionizing 
beam. The preferred embodiment utilizes a time-of-flight mass 
spectrometer. Other types of mass spectrometers, for example, the 
Mattauch-Herzog focal plane mass spectrometer, may be employed as well. As 
illustrated in FIGS. 1 and 2, evacuation chamber 22 includes gridded 
electrostatic extraction network 32, electrostatic focusing lens 33, 
deflection plates 34, field-free region 36, ion reflection means 37, and 
particle detector 38. The arrangement of FIGS. 1 and 2 provides for two 
alternative placements of the ionizing laser beam. When the specimen under 
examination has an electrically conducting surface, the beam may be 
directed close to the surface at the position indicated at 39 in FIG. 2. 
In this arrangement the electrostatic extraction network 32 is not 
energized. Instead, the surface of the specimen is floated at a high 
potential to repel the photo-ions through the network 32 and into the 
time-of-flight drift region 36. When the surface of the specimen under 
examination is not electrically conducting, the ionizing laser beam is 
directed between two grids of the electrostatic extraction network 32, as 
illustrated in FIG. 2 at 40. In this arrangement it is the network 32 
which repels the photo-ions into drift region 36. Windows 24 and 25 are 
large enough to allow the laser beam to be directed to either of the 
positions 39 or 40. After extraction from the ionizing region, the 
photo-ions are focused by electrostatic focusing lens 33 and aligned by 
deflection plates 34. In an alternative embodiment, immersion lens means 
could be used to accelerate and direct the photo-ions. 
In a typical time-of-flight mass spectrometer the ions exiting from the 
drift region would be received directly by a detector. In the preferred 
embodiment of the invention ions leaving the drift region are reflected by 
ion reflection means 37 and traverse the field-free drift region 36 once 
again to enter detector 38. The detector is a particle multiplier, for 
example, a microchannel plate which is apertured so that only ions coming 
from the direction of reflecting means 37 will be detected. As illustrated 
in FIG. 1, the ion reflector 37 comprises outer reflector grid 41 at 
ground potential, middle reflector grid 42, providing a decelerating 
potential, an assembly of electrostatic guard rings 43 connected by 
resistors to insure a uniform field in the reflector region, and a back 
reflector grid 44, providing a reflecting potential. The embodiment of 
FIG. 1 also includes an auxiliary particle detector 46 of like 
construction to particle detector 38 positioned to detect particles 
passing entirely through reflector means 37. When the ionization region is 
located at position 39 near the surface under examination, ions produced 
directly at the surface by the probe beam (the so-called secondary ions) 
will have a higher energy than the photo-ions produced by the ionizing 
laser. These ions can be separated by adjusting the relative potential of 
the surface of specimen 10 and the back reflecting grid 44 of reflecting 
means 37 so that the higher-energy secondary ions pass through the back 
reflector grid 44 and are detected by particle detector 46 while the 
lower-energy photo-ions are reflected and detected by detector 38. When 
the ionizing beam passes between the plates of the electrostatic 
extraction network 32, the secondary ions are prevented from entering 
spectrometer 31 by the repelling plate potential of network 32. 
The use of an ion reflector in time-of-flight mass spectrometry is known in 
the art. The construction of such an ion reflector is disclosed, for 
example, by D. M. Lubman et al., Analytical Chemistry, Vol. 55, No. 8 
(1983) pp. 1437-40 and by G. S. Janes in U.S. Pat. No. 3,727,047. The ion 
reflector provides for high resolution over a wide range of charge-to-mass 
ratios by compensating for the spread in the times at which ions would 
otherwise arrive at detector 38 due to their initial velocity 
distribution. The ion reflector is especially advantageous in the present 
invention to provide greater resolution of the many components which may 
be non-selectively ionized by the nonresonant beam. A further advantage of 
the ion reflector in the present invention is the ability it affords for 
distinguishing metastable molecular ions that dissociate along the path in 
drift region 36 before entering the reflector. The potential of the back 
reflector grid 44 can be adjusted so that the parent ions pass through 
while the lower-energy decay products of the metastable ions are reflected 
for detection by detector 38. Thus, the use of the ion reflector enables 
the instrument to take greater advantage of the nonresonant beam's 
capacity for nonselective ionization. A further advantage of the ion 
reflector in the present invention, which has not been appreciated in 
surface analysis techniques, is the discrimination between photo-ions and 
secondary ions described in the preceding paragraph. 
In many applications of the invention it is desirable to operate the probe 
beam and the ionizing beam in a pulsed mode. A block schematic diagram of 
electronic circuitry for operation of the invention is shown in FIG. 3. 
Pulse generator 51 provides a master repetition-rate signal, for example, 
a ten Hertz signal for controlling the pulse repetition rate of the probe 
and/or ionizing beams. The master signal from pulse generator 51 is 
applied to three-channel delay generator 52. Channel 1 of delay generator 
52 provides a trigger signal to probe beam pulsing means 53. The pulsing 
means 53 is operatively associated with the probe beam means for producing 
a probe beam pulse having a desired width. Channel 2 of delay generator 52 
provides a trigger signal to ionizing beam pulsing means 54, which is 
operatively associated with the ionizing beam source to provide a desired 
pulse delay. Suitable pulsing means are well known to those skilled in the 
art from their use in other pulsed ion beam or pulsed electron beam 
applications and pulsed laser applications and will not be described 
further. Channel 3 of delay generator 52 provides a trigger signal to 
transient recorder 56. The trigger signal from channel 3 sets the time 
position for the initial channel of the transient recorder in a 
time-of-flight measurement. Block 57 represents the signal-generating 
portion of the particle detector, for example, the anode of a multichannel 
plate particle multiplier. The signal from block 57 is passed through a 
variable high-frequency response signal attenuator 58, which may be of 
conventional design, and is amplified by fast linear amplifier 59. The 
amplified signal is applied to transient recorder 56, which may be 
provided by a fast A/D converter. With a laser pulse width of 5-10 
nanoseconds, attenuator 58 and amplifier 59 should have bandwidths greater 
than 150 megahertz to achieve acceptable resolution. The time-of-flight 
measurement data registered by transient recorder 56 are entered into 
computer 60 for storage and analysis. Methods of analysis of 
time-of-flight measurement data are well known to those skilled in the art 
and do not form a part of the present invention. 
Signal attenuator 58 is helpful in comparing different components present 
in greatly differing concentrations. For a sufficiently large signal 
associated with a component present in high concentration, amplifier 59 
will tend to saturate, which will bias the ratio of components present. 
Attenuator 58 reduces large signals by a known factor so as to allow the 
normalization of small signals to large ones. 
The circuitry of FIG. 3 is presented only by way of illustration and 
numerous alternative arrangements could be used. For example, the trigger 
signal indicating the time zero for a time-of-flight measurement may 
alternatively be provided by a pulse suitably delayed after initiation by 
a pulse from a photo diode positioned to detect light from the laser beam. 
Start and stop pulses generated by the photo diode and by the particle 
detector, respectively, can be applied with suitable time delay of the 
start pulse to a time-to-digital converter for time-of-flight measurement 
in an alternative approach for examining mass ranges where the ion arrival 
rate is low (less than 1 ion per laser pulse). 
The apparatus and method of the present invention may be utilized in a 
variety of surface and diagnostic applications. In addition to detecting 
components present in low concentrations on a surface, the invention 
provides sufficient resolution and is sufficiently non-destructive that it 
may be used to monitor chemical reactions taking place on the surface. The 
invention is also suited for in situ diagnostics of integrated circuit 
components. For example, the method described herein may be combined with 
standard ion milling techniques to achieve depth profiling of IC chips. 
With a focused probe beam the method and apparatus can also be used as a 
microscope to reveal the composition of topographical features. Adsorbed 
molecules, including radical species, can be detected in circumstances as 
diverse as heterogeneous catalysis, chemical agents monitoring, and 
analysis of biological molecules. In this regard, the ionizing laser power 
density may be varied over a range to examine fragmentation patterns of 
biological or other complex molecules. Angular distributions of atoms and 
molecules removed from the surface by the probe beam may be monitored by 
controlling the position at which the probe beam strikes the surface and 
its relation to the location of the ionization volume. In addition, the 
pulsed mode of operation may be used to monitor kinetic energy 
distributions of desorbing or sputtered species by varying the time delay 
between channel 1 and 2 of delay generator 52 in FIG. 3. In this 
circumstance, the velocity of a particle removed from the surface and 
traveling toward the ionization region will be inversely proportional to 
the time of flight from the localized region under examination to the 
ionization region. With the present invention the probe beam pulse width 
and the width of the ionization region may be made sufficiently small to 
achieve good resolution of the kinetic energy distribution. 
While the above provides a full and complete disclosure of the preferred 
embodiments of the invention, various modifications, alternate 
constructions, and equivalents may be employed without departing from the 
true spirit and scope of the invention. Therefore, the above description 
and illustration should not be construed as limiting the scope of the 
invention, which is defined by the appended claims.