Process for operating a time-of-flight secondary-ion mass spectr

Details Process for operating a time-of-flight secondary-ion mass spectr Process for operating a time-of-flight secondary-ion mass spectr

1996-02-09

1997-05-27

Berman, Jack I.

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

250309, H01J 4940, H01J 37252, G01N 23225

Patent

active

056334958

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention concerns a method of operating a time-of-flight secondary-ion mass spectrometer for the purpose of analyzing mass spectra wherein several finely structured ranges of mass appear in isolation and widely separated, whereby
a) the surface of a sample of material is bombarded at regular intervals (cycle times t.sub.z) with primary-ion pulses,
b) secondary ions of different mass are thereby released from the surface and are accelerated to the same level of energy,
c) their mass-dependent time t of flight over a path 1 is measured and their mass determined therefrom.
Time t of flight is proportional to the mathematical square root of the mass (t proportional .sqroot.m) in this situation. The number of secondary ions equivalent to a particular mass m yield within a specified cycle time t.sub.z fine-structure maxima within "nominal ranges". Each nominal range corresponds to a whole-number atomic or molecular weight of elemental or molecular ions. The amplitudes of the fine-structure maxima allow qualitative and quantitative analyses of the composition of the sample's surface.
Time-of-flight secondary-ion mass spectrometer (TOF-SIMS) is known (from e.g. Analytical Chemistry 64 (1992), 1027 ff and 65 (1993), 630 A ff). It is employed for the chemical analysis of solid surfaces.
The surface of a sample is bombarded with a pulsed beam of primary ions at a pulse duration t.sub.p. The beam releases secondary ions from the surface. The free secondary ions are accelerated to the same level of energy E (a few KeV) in an extraction field and then travel along a flight path 1. At the other end of the path they are detected by a time-resolving detector. The great majority of secondary ions are simply charged.
The secondary ions' time of flight can be represented by
The precise mass of a secondary ion can accordingly be calculated at constant energy from the detected time t of flight.
The secondary ions are registered in accordance with the desired range of masses within a specific interval, cycle time t.sub.z, subsequent to the impact of a primary-ion pulse. From equation (1), primary-ion pulses can impact the sample once cycle time t.sub.z has lapsed. Times of flight t are accordingly measured at a frequency of repetition f=1/t.sub.z. Very few secondary ions, typically 0.1 to 10, are released and detected per cycle. A mass spectrum of adequate dynamics over several orders of magnitude, meaning an adequate ratio between the highest and lowest intensities, can be obtained by accumulating the counting events over a large number of cycles. The measurements typically take 100 to 1000 seconds.
Both elemental and molecular ions are released from the surface of the probe. The precise mass of a secondary-ion species, which can be either elemental or molecular, equals the sum of its atomic weights. Since the individual atomic weights deviate slightly from integral values due to the binding energy of the atomic nuclei, each aforesaid nominal-mass range will be found on each side of an integral value. The precise masses of elemental and molecular ions differ only slightly. One example of a secondary-ion species is 27 u: aluminum.sub.+ : 26.99154 u: C.sub.2 H.sub.3.sup.+ : 27.023475 u. The various species of secondary ions can be separated and resolved into fine-structure maxima, that is, if the mass resolution is high enough, and elements and compounds can be detected separately. The separation of such species is an essential prerequisite for demonstrating traces of compounds and elements. The mass resolution m/DELTAm employed in time-of-flight secondary-ion mass spectrometry relates to the mass difference DELTAm at which a mass m can still be separated into fine-structure maxima at. It depends decisively on primary-ion pulse duration t.sub.p. Other factors involved in the separation are the resolution capacity of the time-of-flight analyzer and the time resolution of the detector and recording electronics. Improving these factors are not, however, an objective of the present invention.

REFERENCES:
patent: 5396065 (1995-03-01), Myerholtz et al.
Schwieters et al, Journal of Vacuum Science & Technology A 9 (6), Nov./Dec. 1991, pp. 2864-2871.

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