Radiant energy – Ionic separation or analysis – With sample supply means
Patent
1994-03-09
1996-01-23
Berman, Jack I.
Radiant energy
Ionic separation or analysis
With sample supply means
250282, H01J 4914
Patent
active
054866960
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to mass spectrometry. More particularly this invention relates to secondary neutral-particle mass spectrometry.
2. Description of the Invention
In the known procedures concerned with the secondary neutral-particle mass spectrometry, SNMS, neutral atoms and molecules are released from the surface of a solid body by ion bombardment, are ionized by the electron components of a low-pressure gas-discharge plasma and are then analyzed by a mass spectrometer. In an especially advantageous design of SNMS, ions from the ionizing plasma are accelerated in accordance with the direct bombardment technique toward the specimen by which method, in conjunction with an extremely planar adjustment of the equal potential planes in the ion acceleration path between plasma and specimen, a laterally highly homogeneous ion bombardment of the specimen surface can be achieved. If by lowering the voltage between the specimen and a reference electrode in the plasma, the ion energy is reduced to values around 100 eV any of the atomic mixing processes in the specimen typical for higher energies can almost completely be avoided. When employed for determining deep profiles of concentrations, this method, in conjunction with a laterally highly homogeneous ion removal, yields an extremely high depth resolution in the order of less than 1 nm.
These methods characteristic of the direct bombardment method of SNMS could, so far, be employed only on specimens whose electric conductivity was sufficiently high such that a dissipation of the imposed ion bombardment current could occur. It is the objective of the present invention to enable using this method also on completely or partially electrically non-conducive specimens.
SUMMARY OF THE INVENTION
The invention is based on the following principle:
When a negative voltage is applied to the underside of an isolating specimen or of a specimen that is mounted on an isolating surface (see (1) in FIG. 1a), this same voltage will also appear at the topside of the specimen through electrostatic induction. If the specimen is located within a plasma, the constant ion saturation current flowing onto the specimen will compensate this surface voltage until the floating voltage is reached (FIG. 1b). During this `discharge time`, .DELTA.t.sub.1, the specimen is bombarded by ions with ever decreasing energy. The determining factors of .DELTA.t.sub.1 are the specimen's capacity and the ion saturation current.
When a positive voltage is applied to the specimen (see (2) in FIG. 1a) the process described above repeats itself, now, however, with electrons flowing onto the specimen instead of ions. The corresponding `discharge time` .DELTA.t.sub.2 is shorter than .DELTA.t.sub.1 due to the comparatively much higher electron current (FIG. 1b).
If, instead, a rectangular-wave alternating voltage is applied with a frequency where the discharge time is large compared to the duration of the negative half period, the specimen surface cannot discharge itself during the negative half period (FIG. 2). During this period the specimen is bombarded by highly monoenergetic ions and, thereby, eroded (sputtering). The length ratio depends on the requirements of the corresponding analyses. It lies between 1 to 10.sup.2 and 1 to 10.sup.5 and is typically (normal case) 1 to 10.sup.3.
By choosing a proper ratio of the positive voltage phase duration to the negative voltage phase duration, .DELTA.T(-):.DELTA.T(+) in the high-frequency period T (FIG. 2), the portion of T during which the specimen is bombarded by ions and which, therefore, is equal to the duration of analysis or the counting rate for the mass-spectrometric signals, can be optimized. The requirement always remains that .DELTA..sub.1 has to remain large in comparison to .DELTA.T(-). .DELTA.T(+) may be shortened to as low as .DELTA.t.sub.2.
With the method described, the `direct bombardment mode (DBM)` of SNMS, formerly applicable only to electrically conductive specimens, can now also be appli
REFERENCES:
patent: 4670651 (1987-06-01), Meier et al.
patent: 4734579 (1988-03-01), Lucatorto et al.
patent: 4968888 (1990-11-01), Appelhans et al.
Muller Karl-Heinz
Oechsner Hans
Berman Jack I.
Nguyen Kiet T.
SPECS Gesellschaft fur Oberflachenanalvtik and Computertechnolog
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