Radiant energy – Inspection of solids or liquids by charged particles – Positive ion probe or microscope type
Reexamination Certificate
1999-03-02
2001-09-11
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Positive ion probe or microscope type
C250S42300F, C250S492300, C250S492210, C250S281000, C250S287000, C315S111810
Reexamination Certificate
active
06288394
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to emission microscopes, and more specifically to a high spatial resolution microscope using slow, highly charged ions as the excitation source.
2. Description of Related Art
The need for highly sensitive surface analytical techniques has increased, particularly due to the demands of the semiconductor industry, which routinely operates at critical dimensions on the nanometer scale. A variety of instruments and techniques are available for high resolution lateral imaging of materials, such as transmission electron microscopy, scanning electron microscopy, scanning tunneling microscopy, and atomic force microscopy. In addition, many techniques are available to determine material composition of the imaged materials, including Auger electron spectroscopy, photoelectron spectroscopy, and secondary ion mass spectroscopy. 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.
Emission microscopes, which are a special class of electron microscopes, accelerate and image low energy electrons and/or other charged particles emitted or reflected from a planar sample surface. One type of conventional emission microscope is an Ar
+
ion beam induced electron emission microscope, where the kinetic emission of electrons from the surface forms an image. In a second type of emission microscope, a secondary ion emission microscope, an ion beam of energetic singly charged ions is used to sputter secondary ions that are then imaged. It would be advantageous to combine the capabilities of both microscopes to image both electrons and the secondary ions, and to determine the mass of the secondary ions by time-of-flight (TOF) so that all of the secondary ions are detected simultaneously. An emission microscope for imaging and mass spectrometry to acquire spatially-resolved, compositional information with high sensitivity would have unique capabilities.
However, the use of singly charged ions for sputtering limits the technique due to the low secondary ion yield per incident ion. Most of the energy transferred to the sample surface comes from the kinetic energy of the projectile ion. Typically, the sputter yield is about 2-10 sample ions 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.
The low secondary ion yield may be overcome by the use of highly charged ions for sputtering. The technique of SIMS using highly charged ions is described in a co-pending patent application (Ser. No. 09/227,997). The use of highly charged ions has many advantages, since these ions bring considerable energy to the sample surface which is released in a few femtoseconds of the surface interaction. This release causes highly localized energy deposition, which results in very high electron yields (hundreds of electrons per incident ion) and dramatically enhanced, high secondary ion yields (greater than one). The ratio of the secondary ion yield to the secondary neutral yield gives ionization probabilities of 10% for highly charged ion excitation. The high electron yield allows very high spatial resolution.
The present invention addresses the limitations of conventional emission microscopes by providing an emission microscope that images both electrons and secondary ions, and uses slow, highly charged ions as the sputtering source to achieve at least an order of magnitude greater sensitivity. The present invention provides highly sensitive compositional information at high spatial resolution.
SUMMARY OF THE INVENTION
The present invention is a high spatial resolution microscope using slow, highly charged ions as the excitation source. The present microscope images both electrons and secondary ions, and the mass of the secondary ions are determined by time-of-flight (TOF) so that all of the secondary ions are detected simultaneously. A large variety of contrast imaging modes are available. The highly charged ion source provides several advantages: a large secondary electron yield, high secondary ion yields, high ionization probability of the secondary emission, and high molecular ion yields. The high secondary ion yield, particularly the high secondary molecular ion yield, and the high ionization probability with highly charged ion excitation allows the present invention to achieve compositional information at high spatial resolution, although there is a trade-off between sensitivity and resolution. Depending on the information desired, either highly sensitive compositional information (about 0.1 ppm) can be achieved, or very high spatial resolution (about 10 nm).
The system comprises a ion source producing a primary ion beam of highly charged ions that are directed at a sample surface, and a mass analyzer and a detector of the emitted electrons and secondary ions that are sputtered from the sample 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 emission microscopes by applying coincidence counting. The high secondary ion yield and the secondary ion emission from a small area make the coincidence technique very powerful. In coincidence counting, the secondary ion stops are detected in coincidence with a start signal, and there can 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., minutes vs. hours), the secondary ion yield must be on the order produced by highly charged ions, i.e., 1-20 secondary ions per primary ion, in contrast with singly charged ions, which on average provide less than 0.01 secondary ions per primary ion. Highly charged ion excitation is well suited to coincidence time-of-flight techniques.
The present microscope offers extremely high mass resolution and mass accuracy for surface characterization and unambiguous identification of organic compounds and inorganic elemental contamination on surfaces. In particular, this microscope can be used to image and quantify metal trace contaminants and dopants on silicon wafer surfaces and other thin films. The present invention may be useful for determining particulate contamination present in semiconductor fabrication areas. The present invention has many other capabilities, including compositional imaging of corrosion features on metal surfaces with high sensitivities for all species present, imaging biological materials like DNA and other macromolecules for structural determination, and compositional and isotopic analyses of samples collected for environmental monitoring or forensic studies. Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.
REFERENCES:
patent: 3819941 (1974-06-01), Carrico
patent: 5849093 (1998-12-01), Andra
patent: 6002128 (1999-12-01), Hill et al.
Barnes Alan V.
Doyle Barney
Hamza Alex V.
Schenkel Thomas
Schneider Dieter H.
Anderson Bruce
Carnahan L. E.
The Regents of the University of California
Thompson Alan H.
Wells Nikita
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