Radiant energy – Ionic separation or analysis – Methods
Reexamination Certificate
1999-01-08
2001-09-18
Berman, Jack (Department: 2881)
Radiant energy
Ionic separation or analysis
Methods
C250S287000, C250S309000
Reexamination Certificate
active
06291820
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to secondary ion mass spectrometry using slow, highly charged ions.
2. Description of Related Art
The requirements on surface analytical techniques are becoming more stringent, particularly as the feature size of semiconductor devices continues to decrease. Among the currently available techniques, secondary ion mass spectroscopy (SIMS) is highly favored because it offers in-depth information, low detection limits, and high depth resolution. In standard SIMS, a primary beam of energetic singly charged ions strikes a sample surface, which releases electrons and secondary ions. Typically, the sputter yield is about 2-10 sample atoms per incident ion, and the secondary ion yield per incident ion is often less than 10
−2
. The number of secondary ion counts per unit of sample consumption primarily determines the sensitivity limit of SIMS. In the case of surface analysis by static SIMS, values for sensitivity limits are on the order of 10
9
atoms/cm
2
.
The atomic and molecular secondary ions emanating from the sample are introduced into a mass analyzer; both positive and negative ion mass spectra of the species present in the surface can be measured. While the molecular ions can dissociate, these fragment ions are usually not distinguished and give rise to some background. The SIMS spectrum contains secondary ions that are stable to dissociation and the ionic fragments of those that are not. The composition of a microscopic region on the surface of the solid sample can thus be elucidated. Instruments for conducting SIMS are broadly classified into two types: a scanning type that scans an analyzed region with a sharply focused primary beam to obtain an ion image, and a direct imaging type that bombards the whole analyzed region with a primary beam of a relatively large diameter and obtains an ion image on the principle of an ion microscope.
Limitations in standard SIMS are becoming apparent in more advanced applications. Development of the next generations of semiconductor devices will require much improved characterization techniques. Thus, a need exists to develop SIMS with at least an order of magnitude greater sensitivity. The present invention addresses the limitations of conventional SIMS by using enhanced sputtering by slow, highly charged ions.
SUMMARY OF THE INVENTION
The present invention is a secondary ion mass spectrometry system using slow, highly charged ions. The system comprises a ion source producing a primary ion beam of highly charged ions (HCI), which are directed at a target surface, and a time-of-flight mass analyzer and a detector of the secondary ions that are sputtered from the target surface after interaction with the primary beam. Highly charged ions create extreme densities of electronic excitations on surfaces; thus, yields of secondary ions per incident ion are increased by two to three orders of magnitude compared to singly charged ions, which allows a 10 to 100-fold improvement in the sensitivity of the quantitative surface analysis. Examples of highly charged ions include Xe
12−52+
and Au
44−69+
.
The present invention further improves on standard SIMS by applying coincidence counting. The high secondary ion yield and the secondary ion emission from a small area as a result of HCI-SIMS make the coincidence technique very powerful. In coincidence counting, the secondary ion stops are detected in coincidence with a start signal. There may be the additional requirement that a particular secondary ion is present. To detect secondary ions in coincidence with a required secondary ion on a practical time scale (e.g., hours vs. days), the secondary ion yield must be on the order produced by HCI-SIMS, i.e., 1-20 secondary ions per primary ion, in contrast with conventional SIMS, which on average provides less than 0.01 secondary ions per primary ion. Highly charged ion excitation is well suited to coincidence time-of-flight secondary ion mass spectrometry.
The present invention offers high mass resolution and mass accuracy for surface characterization and unambiguous identification of organic compounds and inorganic elemental contamination on surfaces, in particular, the quantification of metal trace contaminants on silicon wafer surfaces. The present invention is useful for determining dopant concentrations in semiconductor materials. In addition, HCI-SIMS is extremely surface sensitive and analyzes over a mass range of one to >3000 amu. The high potential energy excitation in the surface by the highly charged ions leads to the desorption of molecular cluster ions. HCI-SIMS is a novel cluster source and enables the investigation of cluster stability of a large number of materials. Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.
REFERENCES:
Beam Interactions with Materials & Atoms, T. Schenkel et al., Nuclear Instruments and Methods in Physics Research B 125 (1997) 153-158.
Emission of Secondary Particles from Metals and Insulators at Impact of Slow Highly Charged Ions, T. Schenkel et al., Nuclear Instruments and Methods in Physics Research B 00 (1996) 1-6.
Electronic Sputtering of Thin Conductors by Neutralization of Slow Highly Charged Ions, T. Schenkel et al., The American Physical Society (1997) 1-4.
Electronic Sputtering and Desorption Effects in TOF-SIMS Studies Using Slow Highly Charged Ions Like AU69+, T. Schenkel et al., Materials Science Applications of Ion Beam Techniques, (1997) 1-5.
Quantification of Metal Trace Contaminants on Wi Wafer Surfaces by Laser-SNMS and TOF-SIMS Using Sputter Deposited Submonolayer Standards, A. Schnieders et al., American Vacuum Society, (1996) 2712-2724.
McGraw-Hill Yearbook of Science & Technology, (1996) 25-29.
The Electron-Beam Ion Trap, Roscoe E. Marrs et al., American Institute of Physics, Oct. (1994) 27-34.
Barnes Alan V.
Hamza Alex V.
Schenkel Thomas
Schneider Dieter H.
Berman Jack
Carnahan L. E.
Fernandez Kalimah
The Regents of the University of California
Thompson Alan H.
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